justin g. a. wehner be(hons) bsc€¦ · justin g. a. wehner i. ii. abstract infrared (ir)...

Investigation of

Resonant-Cavity-Enhanced

Mercury Cadmium Telluride

Infrared Detectors

By

Justin G. A. Wehner BE(Hons) BSc

This thesis is presented for the degree of

Doctor of Philosophy of

The University of Western Australia

School of Electrical, Electronic and Computer Engineering

The University of Western Australia

2007

Declaration of Published Work Appearing in Thesis.

This thesis contains published work and/or work prepared for publication, some

of which has been co-authored. The bibliographic details of the works and where

they appear in the thesis are set out in appendices E and F, respectively. The details of

contribution of each paper are outlined in appendix E.

Signature:............................. (Candidate)

Justin G.A. Wehner

Signature:............................. (Supervisor)

A/Prof. J.M. Dell

Signature:............................. (Supervisor)

Prof. L. Faraone

Justin Wehner

3/158 Broadway

Nedlands, Western Australia

Australia 6009

2007

Executive Dean,

Faculty of Engineering,

Computing and Mathematics,

The University of Western Australia, Crawley,

Western Australia, 6009

Australia

Dear Sir,

I am pleased to present this thesis entitled Investigation of Resonant-Cavity-

Enhanced Mercury Cadmium Telluride Infrared Detectors as required for a Doctor of

Philosophy Degree.

Yours sincerely,

Justin G. A. Wehner

i

Abstract

Infrared (IR) detectors have many applications, from homeland security and defense, to

medical imaging, to environmental monitoring, to astronomy, etc. Increasingly, the wave-

length dependence of the IR radiation is becoming important in many applications, not

just the total intensity of infrared radiation. There are many types of infrared detectors

that can be broadly categorized as either photon detectors (narrow band-gap materials

or quantum structures that provide the necessary energy transitions to generate free car-

riers) or thermal detectors. Photon detectors generally provide the highest sensitivity,

however the small transition energy of the detector also means cooling is required to limit

the noise due to intrinsic thermal generation. This thesis is concerned with the tech-

nique of resonant-cavity-enhancement of detectors, which is the process of placing the

detector within an optically resonant cavity. Resonant-cavity-enhanced detectors have

many favourable properties including a reduced detector volume, which allows improved

operating temperature, or an improved signal to noise ratio (or some balance between the

two), along with a narrow spectral bandwidth.

This thesis uses the HgCdTe material system as a vehicle for investigation of resonant-

cavity-enhanced (RCE) detectors. IR detectors based on HgCdTe currently give the

highest sensitivity and RCE devices based on HgCdTe represent an excellent candidates

for improved (higher) operating temperature devices or narrow optical bandwidth de-

vices. Modelling of RCE device performance is performed to illustrate the benefits of

resonant-cavity-enhancement. Growth of RCE detectors by molecular beam epitaxy is

also investigated. Firstly, the design and growth of staggered HgCdTe dielectric mirrors

on which absorber layers can be grown is investigated. This is followed by design and

growth of complete RCE detectors, proving that RCE detectors for infrared applications

can be realised.

Modelling of RCE detectors indicates that decreasing the thickness of a photoconductive

detector by d will result in an increase in the detectivity which corresponds to√d.

For photovoltaic detectors, reducing the detector thickness from 10 µm to 100 nm thick

will increase the device zero-bias dynamic resistance, which is directly proportional to

detectivity, by approximately 2 orders of magnitude. These gains can result in a detector

that is able to operate at background limited performance at a temperature of 240K for

a 30 field of view (f/# = 1.86), which is well above the background limited temperature

of current generation detectors.

Mirror technology for fabricating resonant-cavity-enhanced detectors was investigated,

with the Hg(1−x)Cd(x)Te/CdTe material system used to provide the surface on which the

absorber layer is to be grown. Staggered dielectric mirrors are used to broaden the mirror

response of Hg(1−x)Cd(x)Te/CdTe mirrors from a few hundred nanometers for a quarter-

wave-stack to approximately 1 µm for a 17 layer mirror. The reflectivity of such a mirror

is reduced from ≈ 0.95 to ≈ 0.7. Mirror stacks were grown by molecular beam epitaxy and

exhibit strong reflectivity. In order to obtain good agreement between modelled response

iii

and measured mirror response, the refractive index of the CdTe layers had to be reduced

significantly. This is shown to be due to the presence of voids within the CdTe, with a

volume concentration of ≈ 10%.

The mirror layers were also investigated after annealing using conditions similar to those

required for preparation of MBE grown HgCdTe layers for device fabrication. The mirrors

were found to remain reflective after a typical annealing cycle of 20 hours at 250C in a

Hg atmosphere, with minimal degradation. The annealed layers were investigated using

secondary-ion mass-spectroscopy, to measure composition as a function of depth. The

profiles illustrated that there was minimal grading between the CdTe and Hg(1−x)Cd(x)Te

layers even after extended annealing, which for the CdTe on Hg(1−x)Cd(x)Te layers was

in agreement with interdiffusion modelling data. Grading of Hg(1−x)Cd(x)Te layers on

CdTe was greater than expected from model data, possibly due to the presence of voids

in the CdTe layer increasing the diffusion coefficient of Hg in CdTe.

Resonant-cavity-enhanced detector structures based on photoconductors were designed,

resulting in a proof-of-concept structure that was subsequently grown by molecular beam

epitaxy (MBE). A sample which was annealed in-situ in the MBE chamber at the growth

temperature (185C) for 30 minutes under a Hg flux to reduce the Hg vacancy concen-

tration, exhibited resonant-cavity performance with peak responsivity of 1×104 V/W for

a 75 nm thick 80 µm × 500 µm photoconductor at 80K, with a reasonable fit to model

data. Noise measurements were inconclusive, but a worst-case detectivity was calculated

to be 3.09×109 cm Hz1/2 W−1, while detectivity at 200K was calculated to be 4.48×108

cm Hz1/2 W−1. Varying the temperature resulted in a shifting cut-off, in agreement with

model data. The minority carrier lifetime extracted for this sample was 14 ns. Adding

a Ge/SiO mirror to complete the resonant cavity resulted in a reduction of response at

shorter wavelengths, in agreement with model results.

Responsivity of another sample annealed for 20 hours at 250C in a Hg atmosphere

(ex-situ) also shows resonant performance, but indicates significant shunting due the

mirror layers. There is good agreement with model data, and the peak responsivity

due to the absorber layer is 9.5×103 V/W for a 100 µm × 100 µm photoconductor at

80K. An effective lifetime of 50.4 ns is extracted for this responsivity measurement. The

responsivity was measured as a function of varying field, and sweepout was observed for

bias fields greater than 50 V/cm. The effective lifetime extracted from this measurement

was 224 ns, but is an over estimate.

Photodiodes were also fabricated by annealing p-type Hg(1−x)Cd(x)Te for 10 hours at

250C in vacuum and type converting in a CH4/H2 reactive ion etch plasma process

to form the n-p junction. There is some degradation to the mirror structure due to

the anneal in vacuum, but a clear region of high reflection is observed. Measurements

of current-voltage characteristics at various temperatures show diode-like characteristics

with a peak R0 of 10 GΩ measured at 80K (corresponding to an R0A of approximately

104 Ωcm2. There was significant signal from the mirror layers, however only negligible

signal from the absorber layer, and no conclusive resonant peaks.

iv

Table of contents

Abstract iii

Table of Contents v

Acknowledgments xi

List of Acronyms xiii

List of Symbols xv

1 Introduction 19

1.1 Infrared Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.1.1 Blackbody Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.2 Applications of Infrared Sensing . . . . . . . . . . . . . . . . . . . . . . . . 22

1.2.1 Infrared Sensing Devices . . . . . . . . . . . . . . . . . . . . . . . . 24

1.2.2 Broadband Infrared Photon Detectors . . . . . . . . . . . . . . . . 25

1.2.3 Two-colour Detectors . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.2.4 Multi- and Hyper-spectral Sensors . . . . . . . . . . . . . . . . . . 26

1.2.5 Methods for Improving Sensors . . . . . . . . . . . . . . . . . . . . 28

1.3 Thesis Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.3.1 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

v

2 Infrared Detectors 31

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2 Detector Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.1 Thermal Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.2 Photon Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3 Material/Structures for Photon Detectors . . . . . . . . . . . . . . . . . . 36

2.3.1 Bulk Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.3.2 Band-gap Engineered Structures . . . . . . . . . . . . . . . . . . . 37

2.4 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.4.1 Absorption Co-efficient . . . . . . . . . . . . . . . . . . . . . . . . 38

2.4.2 Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.4.3 This Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.5 Metrics and Detector Figures of Merit . . . . . . . . . . . . . . . . . . . . 47

2.5.1 Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.5.2 Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

2.5.3 Depletion Region Width . . . . . . . . . . . . . . . . . . . . . . . . 51

2.5.4 Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

2.5.5 Dark Currents and Noise . . . . . . . . . . . . . . . . . . . . . . . 52

2.5.6 Noise Equivalent Power and Detectivity . . . . . . . . . . . . . . . 55

2.5.7 Specific Detectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

2.6 Experimental Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

2.6.1 Material Characterisation . . . . . . . . . . . . . . . . . . . . . . . 57

2.6.2 Device Characterisation . . . . . . . . . . . . . . . . . . . . . . . . 58

3 Theory of Resonant-cavity-enhanced Detectors 61

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.2 Methods of Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.2.1 Heterostructure Devices . . . . . . . . . . . . . . . . . . . . . . . . 62

3.2.2 Multi-junction Devices . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.2.3 Resonant-cavity-enhanced Devices . . . . . . . . . . . . . . . . . . 63

vi

3.3 Fabry-Perot Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.3.1 Figures of Merit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3.3.2 Energy Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.3.3 Fabry-Perot Cavities with Absorption . . . . . . . . . . . . . . . . 67

3.3.4 Effect of Mirror Phase . . . . . . . . . . . . . . . . . . . . . . . . . 72

3.4 Examples of RCE Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3.5 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3.5.1 Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3.5.2 Improved Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . 74

3.5.3 Reduced Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3.6 Technologies for Growing RCE Structures . . . . . . . . . . . . . . . . . . 80

4 Staggered Dielectric Mirrors 83

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.2 Modelling of Mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

4.2.1 Quarter-wave Stack . . . . . . . . . . . . . . . . . . . . . . . . . . 84

4.2.2 Staggered Dielectric Mirrors . . . . . . . . . . . . . . . . . . . . . . 85

4.2.3 Phase Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.2.4 Final Mirror Design . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4.3 HgCdTe/CdTe Mirror Growth . . . . . . . . . . . . . . . . . . . . . . . . 92

4.3.1 Substrate Preparation . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.3.2 Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.3.3 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

4.3.4 Interdiffusion Modelling . . . . . . . . . . . . . . . . . . . . . . . . 96

4.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.4.1 Mirror-MCT75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.4.2 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

4.4.3 Refractive Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

vii

5 Realisation of Resonant-cavity-enhanced Detectors 117

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

5.2 RCE Detector Design and Modelling . . . . . . . . . . . . . . . . . . . . . 117

5.2.1 RCE Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

5.2.2 Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

5.3 MBE Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5.4 Photoconductor Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 129

5.4.1 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

5.4.2 Device Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.5.1 MCT-79 - Without Ge/SiO Mirror . . . . . . . . . . . . . . . . . . 136

5.5.2 MCT-79 - Complete Structure . . . . . . . . . . . . . . . . . . . . 144

5.5.3 Noise Measurements - MCT-79 . . . . . . . . . . . . . . . . . . . . 147

5.5.4 Contact Issues - MCT-79 . . . . . . . . . . . . . . . . . . . . . . . 150

5.5.5 MCT-92 - Without Ge/SiO Mirror . . . . . . . . . . . . . . . . . . 150

5.5.6 Contact Issues - MCT-92 . . . . . . . . . . . . . . . . . . . . . . . 155

5.6 Proceeding on to Photovoltaic Detectors . . . . . . . . . . . . . . . . . . . 156

5.6.1 Processing - MCT-95 . . . . . . . . . . . . . . . . . . . . . . . . . . 157

5.6.2 Results - MCT-95 . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

6 Summary and Conclusions 165

6.1 Thesis Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

6.2 Outcomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

6.3 Original Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

6.5 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

References 169

viii

Appendices

A Properties of Mercury Cadmium Telluride 185

A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

A.2 Crystal Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

A.3 Energy Band-gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

A.4 Intrinsic Carrier Concentration . . . . . . . . . . . . . . . . . . . . . . . . 186

A.4.1 Majority and Minority Carrier Concentration . . . . . . . . . . . . 187

A.5 Effective Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

A.6 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

A.7 Carrier Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

A.7.1 Shockley-Read-Hall Recombination . . . . . . . . . . . . . . . . . . 192

A.7.2 Auger Recombination . . . . . . . . . . . . . . . . . . . . . . . . . 193

A.7.3 Radiative Recombination . . . . . . . . . . . . . . . . . . . . . . . 195

A.7.4 Surface and Interface Recombination Effects . . . . . . . . . . . . . 195

A.8 Diffusion Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

A.9 Refractive Index of HgCdTe . . . . . . . . . . . . . . . . . . . . . . . . . . 196

A.9.1 Refractive Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

A.9.2 Extinction Co-efficient . . . . . . . . . . . . . . . . . . . . . . . . . 196

A.10 Refractive Index of CdTe . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

B Optical Properties and Modelling 199

B.1 Optical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

B.1.1 Characteristic Matrix - An Assembly of Films . . . . . . . . . . . 199

B.1.2 Reflectance, Transmittance, and Absorptance . . . . . . . . . . . . 200

B.1.3 Potential Transmittance . . . . . . . . . . . . . . . . . . . . . . . . 200

B.1.4 Backside Reflection Correction . . . . . . . . . . . . . . . . . . . . 200

C Molecular Beam Epitaxy 203

ix

D Processes Used 209

D.1 Photoconductor Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 209

D.1.1 Wafer Clean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

D.1.2 Mesa Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

D.1.3 CdTe Cap Etch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

D.1.4 Anodisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

D.1.5 Oxide Etch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

D.1.6 Metallisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

D.2 Photodiode Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

D.2.1 Wafer Clean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

D.2.2 ZnS Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

D.2.3 Windows in ZnS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

D.2.4 Etch Contact Vias . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

D.2.5 RIE Etch/Type Conversion . . . . . . . . . . . . . . . . . . . . . . 215

D.2.6 ZnS Etch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

D.2.7 Window for P Contact . . . . . . . . . . . . . . . . . . . . . . . . . 216

D.2.8 Metallisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

E Author’s Publications List 219

E.1 Journal Publications: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

E.2 Conference Publications: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

F Details of Contributions 225

x

Acknowledgements

While the majority of work in this thesis is work done by myself, there are a number of

people who have contributed. I would firstly like to thank my supervisors Assoc. Prof.

John M. Dell and Prof. Lorenzo Faraone for giving me the encouragement, support and

opportunity that have allowed this work to be undertaken. I would like to thank Dr.

Charles A. Musca for being a first port of call for discussion on devices, many hours of

proof reading annoyingly short sentences, and for taxi-cab directions. Many thanks also

to the group secretary Sabine Betts, who keeps everything in order and running like a

well oiled machine.

I would also like to thank the members of the Microelectronics Research Group at The

University of Western Australia, who have provided much support, knowledge, and a

wonderful working environment. In particular I would like to thank Dr. Richard H.

Sewell who is the group grower, and provided all the MBE growing support that allowed

this work to proceed. Thanks also to Gordon Tsen for SIMS measurements on samples

at ANSTO.

I would like to thank the staff in the workshops at the School of Electrical and Electronic

Engineering at The University of Western Australia. Thanks go especially to Ken Fogden,

George Voight, and Brian Cowling of the general workshop, who always got the job done.

Finally I would like to thank my family and friends for support and encouragement (Jia

You!) during my time undertaking this project. Without their support and friendship

this project would have been a much more arduous task. Special thanks to my Mum and

Dad, who always kept me positive, especially while writing.

xi

Acronyms and Abbreviations

Abbreviation Words

ANSTO Australian Nuclear Science and Technology Organisation

BLIP Background Limited Performance

CCD Charge Coupled Device

DBR Distributed Bragg Reflector

EBIC Electron Beam Induced Current

FESEM Field Emission Scanning Electron Microscope

FIR Far Infrared

FOV Field Of View

FPA Focal Plane Array

FSR Free Spectral Range

FTIR Fourier Transform Infrared

FWHM Full-width Half-maximum

HgCdTe Mercury Cadmium Telluride

HOT High Operating Temperature

IR Infrared

LBIC Laser Beam Induced Current

LPE Liquid Phase Epitaxy

LWIR Long Wavelength Infrared Region

MBE Molecular Beam Epitaxy

MCT Mercury Cadmium Telluride

MEMS Micro-electro-mechanical System

MOCVD Metal Organic Chemical Vapour Deposition

MWIR Medium Wavelength Infrared Region

MRG Microelectronics Research Group

xiii

Abbreviation Words

NEP Noise Equivalent Power

NIR Near Infrared

PCB Printed Circuit Board

QDIP Quantum-dot Infrared Photodetectors

QMSA Quantitative Mobility Spectrum Analysis

QWIP Quantum Well Infrared Photodetectors

QWS Quarter-wave Stack

RCE Resonant-cavity-enhanced

RHEED Reflection High Energy Electron Diffraction

RIE Reactive Ion Etch

RMS Root Mean Square

RF Radio Frequency

SCR Signal-to-clutter Ratio

SEM Scanning Electron Microscope

SIMS Secondary Ion Mass Spectroscopy

SLM Scanning Laser Microscope

SNR Signal-to-noise Ratio

SRH Shockley-Read-Hall

SWIR Short Wavelength Infrared Region

UHV Ultra-high Vacuum

UWA University of Western Australia

VCSEL Vertical Cavity Surface Emitting Laser

VLWIR Very Long Wavelength Infrared Region

xiv

Symbols

Symbol Description Units

A Optical area of detector cm2

AMJ Area of metallurgical junction cm2

A Absorptance (unitless)

Cd Common difference (unitless)

Cn Capture coefficient for electrons cm3s−1

Cp Capture coefficient for holes cm3s−1

Cr Common ratio (unitless)

c Speed of light: 3.00 × 108 ms−1

D∗ Detectivity cmHz1/2/W

De Diffusion coefficient of electrons in p-type material cm2 s−1

Dh Diffusion coefficient of holes in n-type material cm2 s−1

d Detector thickness cm

Eb Electric bias field V cm−1

Eg Band-gap energy eV

Et Trap energy eV

F Finesse (unitless)

G Photoconductor conductance S

GR Generation rate variable s−1

h Planck’s constant: 6.63 × 10−34 J s

Je Drift current density of electrons in p-type material A cm−2

JGRCurrent density due to generation and recombination

in the space charge regionA cm−2

Jh Drift current density of holes in n-type material A cm−2

k Boltzmann’s constant: 8.62 × 10−5 e V K−1

k imaginary part of refractive index (unitless)

Ld Thickness of absorber in RCE structure µm

Le Diffusion length for electrons in p-type material cm

Lh Diffusion length for holes in n-type material cm

l Photoconductor detector length µm

ℓ Cavity Length µm

m Atomic mass kg

m0 Rest mass of electron: 9.11 × 10−31 kg

m∗e Electron effective mass kg

m∗lh Light hole effective mass kg

m∗hh Heavy hole effective mass kg

xv


N Total number of electrons (unitless)

NA Acceptor concentration cm−3

ND Donor concentration cm−3

Nc Density of SRH centers within conduction band. cm−3

Nt Density of SRH centers within material. cm−3

Nv Density of SRH centers within valence band. cm−3

n Real part of refractive index (unitless)

n0 Thermal equilibrium concentration of electrons cm−3

ni Intrinsic carrier density cm−3

nHRefractive index of layers with high refractive index in

a Bragg mirror(unitless)

nLRefractive index of layers with low refractive index in

a Bragg mirror(unitless)

ns Refractive index of the cavity (unitless)

P Total number of holes (unitless)

P Gas pressure Torr

p0 Thermal equilibrium concentration of holes cm−3

Qs Photon flux cm−2µm−1

q Charge on electron: 1.602 × 10−19 C

R Mirror reflectivity (unitless)

R0 Zero bias dynamic resistance Ω

Rλ Responsivity V W−1

rd Photoconductor Resistance Ω

SnSurface recombination velocity at the surface of the

n-type materialcm s−1

SpSurface recombination velocity at the surface of the

p-type materialcm s−1

T Absolute temperature K

T Transmittance (unitless)

tHThickness of layers with high refractive index in a

Bragg mirror(unitless)

tLThickness of layers with low refractive index in a Bragg

mirror(unitless)

tO Optical thickness µm

V Applied voltage V

VBG Background flux generated noise voltage V

Vbi Built in voltage V

VJ Johnson noise voltage V

VTh Thermally generated carrier noise voltage V

xvi


Wdep Width of the depletion region cm

Wλ Spectral radiant emittance cm−2µm−1

w Photoconductor detector width µm

x Cadmium mole fraction of Hg(1−x)Cd(x)Te (unitless)

ε0 Permittivity of free space: 8.885 × 10−10 F cm−1

∆f Electrical bandwidth of detector Hz

εs Relative permittivity (unitless)

η Quantum efficiency (unitless)

λ Wavelength µm

λco Cut-off wavelength µm

µe Electron mobility cm2 V−1 s−1

µh Hole mobility cm2 V−1 s−1

ν0 Optical resonator frequency Hz

νF Mode spacing Hz

φa,b Phase change on reflection of mirror (unitless)

φB Background flux cm−2µm−1

τ Excess carrier lifetime s

τA Auger recombination lifetime s

τbulk Carrier lifetime in bulk s

τeff Effective lifetime s

τR Radiative recombination lifetime s

τSRH Shockley-Read-Hall recombination lifetime s

θs Angle of incidence

xvii

Chapter 1Introduction

1.1 Infrared Radiation

Infrared (IR) radiation was first discovered by Herschel in 1800 using white light dispersed

by a prism and thermometers located beyond the visible red part of the spectrum [1].

Figure 1.1.1 illustrates the full electromagnetic spectrum with the IR occupying that part

of the spectrum having wavelength longer than visible light and shorter than microwaves.

Infrared radiation includes radiation in the range from 0.75 µm up to 1000 µm, and

is further divided into sub-bands. Table 1.1.1 defines the various sub-bands of infrared

radiation as used in IR imaging applications. The submillimeter region can be further

divided to include terahertz radiation or T-rays, with a frequency of 0.1 THz to 30 THz,

or roughly 1 mm to 300 µm in wavelength.

Table 1.1.1: Sub-bands of infrared radiation.

Sub-band name Sub-band range (µm)

NIR 0.75 - 1.1

SWIR 1.1 - 3

MWIR 3 - 6

LWIR 6-18

VLWIR 18 - 50

FIR 50-100

Submillimeter 100-1000

20 1.1. Infrared Radiation

10 nm-6

10 nm-5

10 nm-4

10 nm-3

10 nm-2

10 nm-1

1 nm

10 nm

100 nm

1 mm

1 m

100 m

10 m

1 km10 km

1 mm

10 mm

100 mm

1 cm

10 cm

700nm

300nmVioletBlueGreenYellowOrangeRed

Gamma Rays

X Rays

Infrared

Microwaves

Radiowaves

UV

Wa

vele

ng

th

Figure 1.1.1: The electromagnetic spectrum.

1.1.1 Blackbody Radiation

All bodies emit electromagnetic radiation that is dependent on their temperature. A

body with an emissivity of unity is said to be a blackbody, or a perfect radiator, and has

a radiated spectrum given by Planck’s law for spectral radiant emittance in Watts cm−2

µm−1 expressed by [2, 3]:

Wλ =2πhc2

λ5

1

exp(

hcλkT

)

− 1

(1.1.1)

where h is Planck’s constant, c is the speed of light, λ is the wavelength, k is Boltz-

man’s constant, and T is the body temperature. Figure 1.1.2 illustrates the spectral

radiant emittance curves of a blackbody at various temperatures. A body at 5000K has

a peak emittance in the visible region, and corresponds to radiation from the sun. As

the blackbody temperature decreases, the peak spectral emittance occurs at longer and

longer wavelengths. This shift is given by Wein’s displacement law, which relates the

wavelength of peak spectral emittance (λmax) to the blackbody temperature (T ) through

b = 2.897 × 10−3 m K, a constant of proportionality:

λmax =b

T(1.1.2)

Infrared radiation is absorbed and scattered by gases and aerosols in the atmosphere.

Figure 1.1.3 shows the transmission through 1 km of atmosphere, as well as the molecules

CHAPTER 1. Introduction 21

0.1 1 1010-3

10-2

10-1

100

101

102

103

Spe

ctra

l Rad

iant

Em

ittan

ce (W

atts

cm

-2

m-1)

Wavelength ( m)

5000 K 2000 K 1000 K 750 K 500 K 300 KV

isib

le

Figure 1.1.2: The spectral radiant emittance of an ideal blackbody at tempera-

tures of 5000K, 2000K, 1000K, 750K, 500K, and 300K as a function

of wavelength. Note that a body at room temperature (300K) has

a peak emission centered around λ = 10µm.

that are responsible for the absorption at various wavelengths. There are a number of

“windows” in this transmission spectrum corresponding to regions where the transmission

through the atmosphere is high. The window located in the shortwave IR (1.5-2.5µm) is

important for reflected light imaging applications such as LIDAR and night glow, while the

windows in the midwave IR (3-5µm) and longwave IR (8-14µm) are important for thermal

imaging applications. These imaging windows are important because they correspond to

temperatures of approx 500-700K and 300K, respectively. Temperatures of 500-700K

correspond to objects such as hot exhaust gasses from missiles or engines, other heating

electrical components, and hot engine blocks and fairings, for example. Temperatures

of 300K correspond to room temperature, and therefore allow for imaging of humans

and their surrounding environment. The spectral radiant emittance of bodies at these

temperatures is plotted in Fig. 1.1.2. The peaks of the 500 and 750K curves fall at the

edges of the MWIR transmission window, indicating that bodies with temperatures in this

range are able to be imaged. Also shown in Fig. 1.1.2 is the spectral radiant emittance

for a body at room temperature (300K), which has a peak emittance at approximately

10 µm. While the peak for this curve is in the LWIR, there is still emittance in the

MWIR, which can be imaged. This can be beneficial, as detectors sensitive to MWIR

radiation suffer less from intrinsic generation of thermal carriers, and hence can offer a

better signal to noise than some LWIR detectors, despite the much weaker signal from a

300K blackbody in the MWIR.

22 1.2. Applications of Infrared Sensing

Figure 1.1.3: Transmission through 1km of atmosphere at sea level [3].

1.2 Applications of Infrared Sensing

Infrared sensing has found application in very diverse fields. Present applications can

broadly be split into imaging and spectral sensing. Imaging applications generally em-

ploy focal plane array (FPA) technology to view a scene in various IR spectral windows.

Examples of this include thermal imaging systems that are commonly used by the mil-

itary, homeland security, maintenance, medical imaging and astronomy. Figures 1.2.1,

1.2.2, and 1.2.3 illustrate some applications of IR imaging. These are typically broad-

band sensors tuned to one IR transmission window, and provide a grey-scale intensity

image based on the number of photons impinging on a given element in the array. The

detecting element provides a signal that represents the integrated photons over the de-

tected wavelength range, which is often displayed as a false-colour image, improving the

human readability of the display. As hot objects emit more photons, the intensity can be

used to give an indication of an objects temperature. A typical use of this is illustrated

by the transformer in Fig. 1.2.2, where the brighter colour of the transformer section

indicates that maintenance is needed on the overheating element, as the oil level is not

sufficient to keep the fins (much colder, and darker) filled and cooling the unit properly.

Spectral sensing provides spectral information about the impinging radiation. This is

achieved by refraction, diffraction or filtering techniques. Spectral information has appli-

cations in a number of fields including process monitoring, pollution monitoring, chemi-

cal/biological sensing, medical imaging and astronomy. Camera systems that discriminate

spectral content are referred to as multi- or hyper-spectral imaging systems. Currently

there is significant research effort in the defence area to fuse imaging with spectral sensing

in order to create imaging systems that provide better target detection [7].

The main impediment to large-scale commercialisation of high sensitivity IR sensing sys-

tems based on photon detectors is the high cost of these systems, which includes the very


Figure 1.2.1: IR image of a jet engine undergoing maintenance. The exhaust is

clearly visible [4].

Figure 1.2.2: IR image of an electrical power grid transformer. The transformer

section that is hotter is malfunctioning [5].

Figure 1.2.3: IR image of a human hand holding a lizard. The cold blooded lizard

appears darker than the warm blooded human [6].


high cooling budget that narrow band-gap infrared materials require. In order to decrease

noise due to thermally generated carriers, these devices must be operated at cryogenic

temperatures. While this is not an issue for thermal detectors, this is achieved at the cost

of lower sensitivity, lower signal bandwidth, and decreased spectral selectivity.

1.2.1 Infrared Sensing Devices

Infrared detectors produce an electric output in the presence of infrared radiation. There

are two main families of detectors; thermal detectors and photon detectors. Thermal

detectors produce an output based on changing device temperature, resulting in a change

in some other parameter. An example of a thermal detector is a bolometer, which changes

resistance due to a change in temperature. Thermal detectors are not investigated in this

thesis. Photon detectors produce electrical charge carriers directly from incoming infrared

radiation. There are two types of photon detectors that will be discussed in this thesis,

photovoltaic and photoconductive. Photovoltaic detectors contain a p-n junction or other

electric field containing band structure which separates electron-hole pairs generated by

incoming radiation that are then detected by an external circuit as a current or voltage.

A photoconductive detector changes conductivity due to carriers generated by incoming

radiation. This change in conductivity is then measured by an external circuit.

1.2.1.1 Application Driven Sensors

Each type of detector has advantages and disadvantages associated with it, and there

is no single style of detector that meets all performance criteria for all applications.

Thermal detectors are able to operate at room temperature, but do not have very high

sensitivity and are not able to operate at very high frequency (frame-rates of < 50 Hz are

typical for these detectors [8], with faster frame-rates resulting in degraded sensitivity).

Comparatively, photon detectors for MWIR and longer wavelength operation are able to

achieve a higher sensitivity, and can operate at much higher speeds, but usually require

significant cooling in order to inhibit thermally generated carriers, which are a problem

due to the narrow band-gap of these systems. This added cooling requirement can have

implications at the system level in terms of cost/portability/battery life etc.

Examples of photon detector applications that require high operating frequency include

missile tracking and target detection, while space-based sensing or narrow-band spectro-

scopic sensing requires high sensitivity and, hence, photon detectors. Thermal detectors

are best placed to take advantage of the price/performance trade-off as they are generally

cheaper than photon detectors, but offer reduced sensitivity or operating speed. For ex-

ample, applications such as civilian security imaging that cannot support the extra cost

that comes with the higher sensitivity photon detectors, often opt for thermal imaging

systems.


Re

lativ

e S

igna

l

Wavelength, l

lco

Figure 1.2.4: The theoretical spectral response for an ideal photodetector with

cutoff wavelength λco for a constant incident energy across all wave-

lengths.

1.2.2 Broadband Infrared Photon Detectors

A typical IR photon detector has a spectral response similar to the ideal response illus-

trated in Fig. 1.2.4. Therefore, any signal up to the cutoff wavelength that is transmitted

through the atmosphere is detected, and any spectral information is lost since the detector

does not discriminate between different wavelengths. Broadband semiconductor detectors

have been fabricated since the 1950s [9, 10], during which time IR imaging devices have

progressed from single element photoconductors over which a scene was scanned [11], to

linear arrays in the 1970s and 1980s, and finally to staring two-dimensional arrays in the

1990’s [12].

1.2.3 Two-colour Detectors

Two-colour detectors provide more information and can therefore assist in target detec-

tion and reduce false alarm rates. Two-colour detectors are only now entering active

service, however, they will be superseded by multi- and hyper-spectral detectors for most

applications, as discussed in the next section. Two-colour detectors are formed by bring-

ing together two broadband absorbers either next to each other spatially, or optically

aligned on top of each other, as illustrated in Fig. 1.2.5. In the vertically integrated case,

one detector absorber layer filters the other, hence MWIR-2 has a shorter cut-off wave-

length than MWIR-1. This will produce a spectral response similar to Fig. 1.2.6 [13].

Two-colour detectors for IR systems have been developed in a number of combinations

including MW/MW, MW/LW and LW/LW. There are various readout schemes for these

systems in which the signal from each detector is either sequentially read out or simul-

taneously read out. Two-colour detectors have met with some success, particularly in

missile detection, however the limited benefit of only two colours, issues with deep etches


Substrate

N-typeMWIR-2

ContactArrayCommon

P-typeContact

N-typeMWIR-1 Contact

IR Radiation

Figure 1.2.5: Schematic of a two-colour detector after [13].

Re

lativ

e R

esp

onse

pe

r Pho

ton

Wavelength ( m)m

Band 1 (MWIR1)

Band 2 (MWIR2)

Figure 1.2.6: Spectral response of a two colour detector [13]. The two bands

correspond to the different absorber layers (MWIR1 and MWIR2)

in Fig. 1.2.5.

required for device isolation, and methods for junction formation make this technology

difficult.

1.2.4 Multi- and Hyper-spectral Sensors

Multispectral detectors are detectors with 10-20 spectral channels with a spectral reso-

lution of δλλ ≤ 0.1, while hyperspectral detectors have 100-200 spectral channels, with

δλλ ≤ 0.01 [14]. There are various methods for realising multi- and hyperspectral imag-

ing systems, including refractive and diffractive spectrometers as well as filtering spec-

trometers. Some examples of these systems that have been developed are the HYDICE


(a) (b)

Figure 1.2.7: Model examples of a strong signal in a cluttered background. (a)

Multispectral case, the target is the green square in the center of

a cluttered correlated background. SCR of this scene is 33.2 (b)

Broadband case, the target is the white square in the center of a

cluttered correlated background. SCR of this scene is 4.1

system which has 210 spectral channels over the wavelength range 401 - 2504 nm in a

refractive prism spectrometer pushbroom configuration [15, 16, 17]. Other examples of

hyper-spectral sensors include COMPASS, a diffractive optic imager [18], THRIFTI, a

Fourier transform interferometric imager [19], SPIRIT, a filter based imager [20], and

DOIS, a diffractive optic image spectrometer [21].

The benefit of multi- and hyper-spectral imaging systems accrues mostly from improved

acquisition and identification using spectral information to identify pixel and even sub-

pixel targets, and the ability to identify objects that are heavily obscured or in cluttered

environments [22]. The improved acquisition and identification is brought about by being

able to compare signals in different spectral channels. Broadband sensors cannot discern

targets in a cluttered background, or targets that are camouflaged or concealed, due to

the inability to make this comparison. The algorithms for analysing multi- and hyper-

spectral images have been determined [23, 24, 25] and utilise the spectral correlation of

signals across multiple channels [26], thereby increasing the signal-to-clutter ratio (SCR)

and making the probability of detection and identification higher. Figure 1.2.7 illustrates

the concept of SCR, and shows in a very simple three-channel example how multispectral

data can increase the SCR. The two images both consist of a target in a cluttered, yet

correlated, background. In Fig. 1.7(a) the green target is much more visible than the

black and white only target in Fig. 1.7(b). Mathematically, the multispectral image in

Fig. 1.7(a) has a SCR of 33.2, while the SCR of the broadband black and white image

is only 4.1, making positive target detection much more likely in the multispectral case.

Significant effort is currently being invested in both the hardware and the image processing

algorithms, since multi- and hyper-spectral systems are seen as the next generation of IR

imaging technology. As these systems produce large volumes of data, research is also


Figure 1.2.8: Schematic showing the MEMS spectrometer. The top DBR (three

layers of Ge (silver) and two layers of SiO (pink)) is suspended

above an air gap by a SiN membrane (blue). The cavity length can

be changed by applying voltage across the air gap. The arms will

deform, allowing the membrane to be pulled down by the capacitive

forces induced by the applied field.

being directed into algorithms for determining the best IR system bands for yielding the

lowest false-alarm rate and highest probability of detection [27].

Finally, fusion of spectroscopic sensors with focal plane arrays is illustrated by recent

work focused on integrating a focal-plane array with micro-electro-mechanical systems

(MEMS)-based spectrometers. The MEMS spectrometer (shown in Fig. 1.2.8) is a tun-

able Fabry-Perot cavity consisting of a DBR mirror mounted on a broadband detector

[28], with a moveable mirror suspended above an air gap. The mirror is actuated capaci-

tively and allows tuning of the resonant wavelength of the Fabry-Perot cavity by changing

the cavity (air gap) length. This device has applications as a multi- or hyper-spectral sen-

sor, but also allows novel wavelength agile applications, such as a detector that can avoid

counter-measures by changing the sensing wavelength to avoid the operating wavelengths

of the counter measures [29].

1.2.5 Methods for Improving Sensors

The ideal infrared sensing device is a photon detector device that is operated at (or close

to) room temperature, while maintaining a very high sensitivity. There are a number of

technologies that can assist in increasing the operating temperature of photon detectors

(producing a higher operating temperature (HOT) device), including band-gap engineer-

ing and detector volume reduction.

Band-gap engineering can reduce certain noise mechanisms by lowering the carrier con-

centration in certain areas of the device. However, this results in a significant increase

in low-frequency noise and hence a reduction in signal-to-noise ratio at typical operating

frequencies. Reduction in detector thickness, and hence reduction of thermally generated


noise, can also be used to increase operating temperature. However, these devices suf-

fer from reduced absorption of photons and hence suffer from poor quantum efficiency,

making them less attractive. Resonant-cavity-enhanced (RCE) detectors reduce volume

while maintaining high quantum efficiency. This is achieved by placing the absorber

layer within an optical resonant cavity, which in effect allows multiple passes of radiation

through the absorbing layer. Hence, RCE detectors have improved signal-to-noise ratio

at a given operating temperature, possibly a faster operating frequency, and a narrower

optical bandwidth, only showing high quantum efficiency at wavelengths close to the cav-

ity resonance. This may be a hinderance for broad-band imaging, but is acceptable for

multi- and hyper-spectral imaging or spectral sensing. The fact that not only high op-

erating temperature can be achieved, but that higher signal-to-noise ratio and operating

frequency are also achievable, makes RCE detectors an interesting area of research.

1.3 Thesis Objectives

Improving the present generation of infrared imaging systems requires increasing the

operating temperature of photon detectors, while maintaining noise performance, and also

increasing device functionality by introducing features such as multi- and hyper-spectral

imaging. Resonant-cavity-enhanced (RCE) detectors are devices that can achieve all of

these outcomes. Therefore, this thesis will:

• Investigate resonant-cavity-enhancement and the benefits of RCE photon detectors.

• Model RCE devices and design a structure to prove the concept of resonant cavity

enhancement using the Hg(1−x)Cd(x)Te material system.

• Fabricate mirror structures and RCE detector material structures, and separately

characterise their optical performance.

• Fabricate devices from the RCE detector material structures and characterise device

performance, showing that resonant-cavity-enhanced performance is possible for

HgCdTe-based IR detectors.

1.3.1 Thesis Structure

The six chapters of this thesis begin with this introductory chapter. Chapter 2 investi-

gates the materials used for infrared sensing and photon detecting devices that can be

fabricated from these materials. It summarises performance metrics and figures of merit

for comparing detectors, as well as techniques for measuring these metrics. Chapter 3

introduces the concept of resonant-cavity-enhanced detectors, provides modelling results,

and discusses the advantages and disadvantages of this technique for improving device

performance. Also presented in chapter 3 is a discussion on the techniques used to grow

RCE structures, in particular molecular beam epitaxy (MBE), which is used to grow the

30 1.3. Thesis Objectives

mirrors and absorbing layers discussed in chapter 4 and chapter 5. Mirror design, fabri-

cation, and characterisation are discussed in chapter 4, including characterisation before

and after annealing, and characterisation of the refractive index of the CdTe material

used in the mirror structure. Design of resonant-cavity-enhanced detectors is discussed

in chapter 5, as well as results of fabrication and characterisation of RCE detectors, in-

cluding optical cut-off measurements, responsivity measurements, lifetime extraction, and

spatial photoresponse. The outcomes of this thesis are summarised in chapter 6. Mod-

eling details are given in the appendices, along with processing techniques, and a list of

the authors publications that have resulted from this work.

Chapter 2Infrared Detectors

2.1 Introduction

As discussed in section 1.2.1, infrared photon detectors give the highest performance (in

terms of speed, signal, etc.) at the cost of increased system overhead due to the strict

cooling requirements . This chapter investigates different detector types, device structures

for photon detectors and material systems used to create infrared detectors. It introduces

important material properties such as absorption co-efficient and lifetime, device perfor-

mance metrics including quantum efficiency, cut-off wavelength, dark current and noise,

and a variety of figures of merit used to evaluate and compare device performance.

2.2 Detector Types

2.2.1 Thermal Detectors

Thermal detectors operate by absorbing thermal radiation, causing a change in the tem-

perature of the device, which can then be sensed. There are a number of different types

of thermal detectors, including bolometers, thermocouples, and pyroelectric detectors.

Bolometers are the general name given to a large range of thermal detectors where in-

cident radiation is used to heat an absorbing material connected to a heat sink. This

temperature is then measured [30]. There are various ways of achieving this, frequently

with temperature dependent resistors (TDRs), a material which changes resistance with

temperature. The earliest bolometers had a simple design with two strips of platinum

covered with lampblack and arranged in a wheatstone bridge configuration [30]. State of

the art micro-bolometers use vanadium oxide as the TDR material, suspended above a

low-Q micro-machined optically resonant cavity to increase sensitivity [31].

Thermocouples and thermopiles rely on the thermoelectric effect, which occurs when

two different metals or semiconductors experience a temperature gradient, generating a

32 2.2. Detector Types

substrate

Contactpads

d

wl

mesaisolation

Figure 2.2.1: Isometric schematic of a typical photoconductor.

voltage [32]. Pyroelectric detectors rely on materials that develop a charge in the presence

of a temperature gradient: as the material heats, charges move towards opposite surfaces

generating an electric potential. Thermal detectors have a severe trade-off between speed

and sensitivity; with highly sensitive devices requiring significant thermal isolation which,

in turn, results in slow response.

2.2.2 Photon Detectors

Photon detectors work by absorbing incoming photons of infrared wavelength light, con-

verting these photons to free carriers (electrons, holes, or both electrons and holes), and

then sensing the resulting electrical signal. There are two types of photon detectors, pho-

tovoltaic and photoconductive. Photovoltaic detectors contain a p-n junction (or other

field-generating band structure), and carriers generated by incoming radiation are sepa-

rated by the built-in electric field of the junction and detected by an external circuit as

a current or voltage. A photoconductive detector relies on changes in the conductivity

of an absorbing material due to carriers generated by incoming radiation, which is then

measured by an external circuit.

2.2.2.1 Photoconductive Detectors

Photoconductors are the simplest of photon detectors. They consist of an absorbing

volume that is isolated from other devices (usually by mesa isolation) and contacts on

either side of the absorbing volume. Figure 2.2.1 illustrates a typical photoconductive

device. The optically active area of the device is between the two contacts and is defined

as the length l times the width w, with d indicating absorber layer thickness.

The conductance of a photoconductor is given by [10]:

G =

(

q

l2

)

(µeN + µhP ) (2.2.1)

where q is the charge of an electron, l is the detector length, µe and µh are the electron

and hole mobilities, respectively, N is the total number of electrons, and P is the total

number of holes. The change in conductance due to signal flux, Qs, is measured by an

external circuit and is given by:

CHAPTER 2. Infrared Detectors 33

∆G =

(

q

l2

)

(µe∆N + µh∆P ) (2.2.2)

=

(

q

l2

)

µhτ

[∫ ∞

0Qs (λ) η (λ)A

]

[1 + b] (2.2.3)

where:

b =µe

µh(2.2.4)

The number of excess carriers under steady state illumination are denoted by ∆N and

∆P , Qs is the signal photon flux, η is the quantum efficiency, and τ is the effective excess

carrier lifetime. The expression for b is only valid if ∆N = ∆P , which holds in the absence

of significant trap mediated recombination [10].

As photoconductors have such a simple internal band structure, there are few avenues

to improve device performance by band-structure engineering. Most focus has been on

improving performance by adjusting the band structure at the contacts. Blocking contacts

engineer the band structure in order to prevent minority carriers from easily reaching the

contacts, thereby increasing the effective lifetime. Another method of improving device

performance is by grading the composition of the structure through the thickness. This

can keep carriers away from imperfect surface layers which reduce lifetime [33], and is

used as an adjunct to surface passivation. Materials such as CdTe, ZnS and anodic oxide

[34] are used as surface passivants.

Photoconductive devices are used in this work because the fabrication processes and

electrical behaviour are simpler, and are therefore more easily controlled and modelled.

However, photoconductive devices are not practical for use in focal plane array type

applications, as the bias voltage needed for device operation leads to a large static power

dissipation. Furthermore, photoconductors cannot physically realise the high fill factors

that are required for focal plane arrays. Finally, as will be shown later in this thesis,

photoconductors are not ideal devices for resonant-cavity-enhanced detectors due to the

increased impact of surface recombination on thin photoconductor structures.

2.2.2.2 Photovoltaic Detectors

Photovoltaic devices incorporate a built-in field to separate carriers, which can be created

at some form of metallurgical junction, such as when a p-type semiconductor is brought

into intimate contact with an n-type semiconductor, making a p-n junction, or metal and

semiconductor with different work functions are brought together, creating a Schottky

barrier. In terms of a p-n junction, if the material for both the p-type material and the

n-type material is the same (i.e. has the same band-gap, electron affinity, etc.), then

the junction is said to be a homojunction. If the two materials have different band-gaps

and/or work functions, then the junction is a heterojunction.

Photodiodes can have either a horizontal junction geometry or a vertical junction geom-

etry. Figure 2.2.2(a) illustrates the horizontal junction geometry for a photodiode, in

34 2.2. Detector Types

AOpt

p pnn contact

p contact

a

dthickness

(a)

AOpt

p pn

n contactp contact

ad thickness

(b)

Figure 2.2.2: (a) Geometry for a horizontal junction diode (b) Geometry for a

vertical junction diode.

which the contacts are above and below the junction (or a remotely located common as

shown in Fig. 2.2.2(a)). Figure 2.2.2(b) illustrates the vertical junction geometry for

a photodiode, in which the junction extends through the entire thickness of the layer.

The contacts are located centrally and remotely, as illustrated, and these geometries are

generally circular and provide a toroidal absorption region.

2.2.2.3 Absorbing Regions

Photoconductors absorb over the entire optical area of the device, and rely on an applied

bias field to sweep generated majority carriers to the contacts for detection. Photovoltaic

detectors, on the other hand, rely on the built-in field to separate the carriers. This leads

to two regions where absorption takes place, those carriers which are generated within

the depletion region, and those which are generated in the neutral region and diffuse to

the depletion region. These two absorption regions lead to two types of detectors: those

that absorb mainly in the depletion region and those that absorb mainly in the neutral

region.

Figure 2.2.3 shows a cross-sectional view of a horizontal junction photovoltaic detector.

Absorption can occur in the neutral n-region, the depletion region or the neutral p-region.

For highest quantum efficiency and highest speed, absorption should primarily occur in the

depletion region, where photo-generated electron-hole pairs can be immediately separated

and swept out of the junction by the electric field. The signal from photons absorbed

in the neutral regions relies on the diffusion of the photo-generated minority carriers

to the junction before any signal can be detected, resulting in slow response and the

possibility of recombination before collection. Short wavelength detectors invariably are


AOpt

p

nn contact

p contactremote

Mesa isolation

DepletionRegion

AbsorbingRegion

(a)

AOpt

p

nn contact

p contactremote

Mesa isolation

DepletionRegion

AbsorbingRegion

(b)

Figure 2.2.3: Cross section showing absorbing region: (a) a photovoltaic detector

where most absorption occurs in the depletion region (b) a photo-

voltaic detector where most absorption occurs in the neutral region.

designed for absorption in the depletion region, with the wider band-gap allowing p-i-n

structures to be used to increase the extent of the depletion region. However, because of

maximum electric field constraints and low absorption in narrow band-gap materials,the

vast majority of photo-generated signal in standard IR photodiodes is due to absorption

in the neutral regions of the device, resulting in lower operating speeds and lower quantum

efficiencies. As will be shown later, RCE IR photodiodes can potentially overcome these

problems for horizontal geometry devices, as the junction is perpendicular to the incident

photon flux it is possible to design the cavity such that the region of highest energy

density is very narrow and coincides with the depletion region, which for a standard

MWIR Hg(0.7)Cd(0.3)Te detector at 80 K with doping densities NA = 5 × 1016 cm−3 and

ND = 5×1015 cm−3 is 300 nm thick. As the depletion region is so thin, vertical geometry

devices are generally only useful if there is significant contribution to the signal from

the neutral regions. Figure 2.2.4(a) illustrates a schematic top-down view of a vertical

junction geometry photovoltaic detector where the majority of the absorption is occurring

in the depletion region, similar to the situation in Fig. 2.2.3(a). This results in a very

small optical area device, which can only be addressed by structural design, such as a

p-i-n structure or multiple junctions [35] to increase the depletion region width.

Detectors in which the majority of the photo-generated signal comes from the neutral

regions are illustrated in Figs. 2.2.4(b) and 2.2.3(b). This is usually the case for material

systems with high mobilities and/or long lifetimes and therefore long diffusion lengths.

Bulk materials used in infrared detection generally have relatively long lifetimes and

high mobilities and are often used in this type of detection mode. An example of this is

Hg(0.4)Cd(0.3)Te, which can have lifetimes on the order of 10 µs and diffusion lengths in the

10’s of micrometers. For these types of detectors the lifetime and mobility of the material

becomes quite important in determining the detector performance, hence it would be very

beneficial to realise a narrow-band detector in which the majority of signal comes from

absorption within the depletion region, which can be realised using RCE structures.

36 2.3. Material/Structures for Photon Detectors

n-type regionn contact

depletion region

p-type region

opticalarea

(a)

p-type region

opticalarea n-type region

n contact

depletion region

(b)

Figure 2.2.4: Schematic of the optical area for: (a) a photovoltaic detector where

most absorption occurs in the depletion region (b) a photovoltaic

detector where most absorption occurs in the neutral region.

2.3 Material/Structures for Photon Detectors

2.3.1 Bulk Material

As the energy of photons in the infrared region of the spectrum is low, infrared photon

detectors must operate with small energy transitions. For example, MWIR 3-5 µm radi-

ation corresponds to photons with energies of 0.41 - 0.25 eV. Materials absorbing these

photons by promoting an electron from the valence band to the conduction band therefore

need to have a narrow band-gap.

There are a number of narrow band-gap materials suitable for IR detectors, including

indium antimonide (InSb), lead-chalcogenide and other lead-salts, and mercury cadmium

telluride (Hg(1−x)Cd(x)Te). Indium antimonide has a fixed band-gap suitable for detec-

tors operating at wavelengths of < 5.5 µm, while the band-gap of Hg(1−x)Cd(x)Te can

be tuned from 1.6 eV (x = 1) to -0.2eV (x = 0 corresponding to a semi-metal) by vary-

ing the mole fraction, x, of CdTe to HgTe. Both InSb and Hg(1−x)Cd(x)Te have high

electron mobilities and long lifetimes, and make excellent detectors with very high re-

sponsivity. However, as the band-gap is so narrow, carriers are easily generated due to

thermal processes. This has a number of negative effects on devices fabricated from these

materials. Firstly, noise due to these thermally generated carriers becomes the dominant

performance-limiting mechanism, often requiring cryogenic cooling to overcome this lim-

itation. Secondly, even for very low doping densities, these materials become degenerate

and exhibit a Burstein-Moss shift in the optical band edge [36, 37]. Further inhibiting

commercial market penetration, Hg(1−x)Cd(x)Te is especially difficult to work with. The

raw materials are all relatively harmful, and the Hg(1−x)Cd(x)Te crystal structure is very

fragile and susceptible to damage from very slight mis-handling, which can result in very

low device yields. InSb requires more cooling than Hg(1−x)Cd(x)Te and has therefore also

struggled with commercial market penetration.

It is also possible to use Si or Ge with suitable dopants (e.g. In, Ga, Sb, P, Be) as a

bulk infrared detecting material [38]. Generally these materials suffer from slower oper-

ating speeds, memory effects, and increased noise when the bias field becomes too great.


n stacklayers

InAs QD s

GaAsn+

Contact

GaAsBarrier GaAs

n+Contact

Al Ga As

Barrier0.3 0.7

Figure 2.3.1: Schematic of the conduction band profile of an n layer InAs/GaAs

QDIP stack under bias, after [43].

Furthermore, these extrinsically doped semiconductors must be operated at very low tem-

peratures [39]. Therefore, these materials have only found use in specialty applications

such as astronomy or satellite-based, very long wavelength missile warning systems.

2.3.2 Band-gap Engineered Structures

2.3.2.1 QWIPS/QDIPS

An alternate method of absorbing photons with small energies associated with IR radi-

ation is to utilize sub-band transitions. Quantum-well infrared photodetectors (QWIPs)

and quantum-dot infrared photodetectors (QDIPs) achieve this by promoting electrons

from one energy level to another in the quantum well or dot, or from one energy level

to the continuum. A schematic representation of the band structure for QWIP or QDIP

is shown in Fig. 2.3.1. The allowable energy levels illustrated in the quantum wells or

quantum dots are functions of the well or dot dimensions, allowing tuning during growth

and to a limited extent by bias, of the transition energies between filled states and empty

states or between filled states and the conduction band [40]. The most common material

system used for QWIPs and QDIPs is based around the (In,Ga) As / (Al,Ga) As material

family. Other material systems attracting interest include HgTe/CdTe quantum wells and

InAsP/InP/InGaAs multiple quantum well structures [41]. These structures are grown by

metal-organic chemical vapour deposition (MOCVD) or MBE growth techniques. Quan-

tum dots have a number of advantages compared to QWIPs; firstly, dots are intrinsically

sensitive to normal incidence light, as the dot always has confinement in the direction

of the E field [42], making coupling into QDIPs easier compared to QWIPs, which are

not sensitive to normal incidence light, for which the optical E has no component in the

direction of confinement of the quantum well. Secondly, QDIPs have longer lifetime of

photo-excited electrons and reduced electron-phonon scattering compared to QWIPs [43].

State of the art QDIPs are reaching D∗ = 1011 cmHz1/2/W at 100K for devices with a

cut-off wavelength in the MWIR [43].

38 2.4. Material Properties

QWIPs and QDIPs are also affected by thermal generation of carriers and require cryo-

genic cooling. Furthermore, quantum efficiency of detectors made from these materials

is poor due to a low absorption co-efficient. Quantum efficiencies are typically limited to

around 10-20%, which is low compared to the quantum efficiency of detectors fabricated

using direct narrow band-gap materials, which approaches 100%, due to larger absorp-

tion co-efficients and long diffusion lengths. This has restricted QWIPs and QDIPs from

becoming dominant for IR applications.

2.3.2.2 Superlattices

While superlattices are similar in structure to QWIPs, in that they consist of alternating

layers of wide band-gap material and a narrower band-gap material, the principle of op-

eration of these devices is quite different. When the wider band-gap material thickness

is reduced below a critical thickness electrons may tunnel through the barrier so that

electrons may then behave in a fashion similar to electrons in a crystal lattice, effec-

tively creating an engineered bulk material band structure that can be controlled by the

thickness of the layers of the superlattice. Material systems attracting interest include

HgTe/CdTe superlattices [44], and InAs/GaInSb strained layer superlattices [45] to list a

few.

2.4 Material Properties

2.4.1 Absorption Co-efficient

Absorption in materials occurs when a photon imparts its energy to the material. This

often takes the form of an electron being promoted from one energy level to another

energy level. The absorption co-efficient of a material represents how much absorption

occurs per unit thickness. A simplified expression for absorption coefficient in the case of

direct band-to-band transitions with the Fermi level a few kT away from the conduction

and valence bands is given by [46]:

α (ν) =

√2c2m

3/2r

τr

1

(hν)2(hν − Eg)

1/2 (2.4.1)

1

mr=

1

m∗e

+1

m∗h

(2.4.2)

where mr is the reduced mass of an electron-hole pair (with masses m∗e and m∗

h, respec-

tively, Eg is the band gap of the material and τr is the radiative lifetime. Figure 2.4.1

shows model results for the absorption co-efficient of GaAs using Eqn. 2.4.1. The model

shows that for energies lower than the band-gap there is no absorption, at the band

edge there is strong absorption, and as energy increases, there is still absorption, though

the absorption co-efficient decreases with increasing wavelength, as the probability of an

electron making a transition to these higher energy states is lower.


-1 0 1 20

2

4

6

8

103 2 1

(cm

-1x1

03 )

h -Eg (eV)

Wavelength ( m)0.5

Figure 2.4.1: Absorption co-efficient modelling a direct band-to-band transition

as a function of hν − Eg, plotted for Eg = 1.42 eV, τR = 0.4 ns,

me = 0.07m0 and mh = 0.5m0, which corresponds to GaAs.

The absorption co-efficient represents the inverse of the depth of penetration, i.e. the

inverse of the absorption coefficient is the depth that the incident radiation reaches when

its power is reduced to 1/e its initial intensity. The power intensity is given by Lambert’s

law as P = P0e−αx, where P0 is the initial power intensity, and x is the depth. Therefore,

a high absorption coefficient is desired for optical detectors, in order to absorb all incident

radiation (i.e. P → 0), in the minimum thickness.

Typical bulk infrared materials do not necessarily have an absorption coefficient described

by Eqn. 2.4.1, Hg(1−x)Cd(x)Te for example, while having a direct band-gap, for values of

x = 0.2−0.3 the bands are not parabolic (an inherent assumption in the derivation of Eqn.

2.4.1), besides which it is unlikely to have a Fermi level a few kT away from the conduction

or valence band, as the band gap is only a few kT for temperatures between 80K and

300K! Despite this the general trend of the absorption co-efficient is still prevalent, and

the absorption co-efficient relatively high, requiring relatively thin layers to absorb all

incident power. Comparatively, engineered structures such as QWIPs and QDIPs have

a much lower absorption co-efficient (by a number of orders of magnitude), meaning to

achieve close to 100 % absorption a QWIP/QDIP structure must be much thicker than a

bulk infrared material.


2.4.2 Lifetime

Carrier lifetime is the average period of time that a carrier exists before recombining

and should be represented by a probability density function. Interest is usually only in

minority carrier lifetimes, because minority carrier density due to injection or optical

generation may be considerably above the thermal equilibrium value. This is compared

with the majority carrier concentration, which is not appreciably changed, compared to

the thermal equilibrium value [47]. Excess minority carrier lifetimes in the bulk of a

semiconductor are affected by three dominant mechanisms, Shockley Read Hall recombi-

nation (SRH), Auger recombination, and radiative recombination, as given in Eqn. 2.4.3.

SRH recombination is material quality dependent, with higher quality material reducing

SRH recombination. Auger recombination and radiative recombination are fundamental

recombination processes where rates are determined by the band structure and doping of

the material.

1

τbulk=

1

τA+

1

τR+

1

τSRH(2.4.3)

where:τbulk is the effective minority carrier lifetime.

τA is the Auger lifetime.

τR is the radiative recombination lifetime.

τSRH is the Shockley Read Hall lifetime.

2.4.2.1 Shockley-Read-Hall Recombination

Shockley-Read-Hall recombination occurs via Shockley-Read-Hall centers. These centers

are defects, which create energy states in the energy band-gap [48]. Figure 2.4.2 shows

recombination via these centers.

The steady-state lifetime of excess holes due to SRH recombination via SRH centers

located at an energy Et below the conduction band is given by [49]:

τp =τp0 (n0 + n1) + τn0 (n0 + n1) τp0Nt

(

1 + n0

n1

)−1

n0 + p0 +Nt

(

1 + n0

n1

)−1 (

1 + n1

n0

)−1 (2.4.4)

The steady-state lifetime of excess electrons is similarly:

τn =τp0 (n0 + n1) + τn0 (n0 + n1) τn0Nt

(

1 + p0

p1

)−1

n0 + p0 +Nt

(

1 + p0

p1

)−1 (

1 + p1

p0

)−1 (2.4.5)

where:

τn0 =1

CnNt

τp0 =1

CpNt


n1 = Nc exp

(− (Eg − Et)

kT

)

(2.4.6)

p1 = Nv exp

(− (Et − Ev)

kT

)

(2.4.7)

Nc = 2

(

2πm∗ekT

h2

)1.5

Nv = 2

(

2πm∗hkT

h2

)1.5

p0 =1

2

[

NA +(

N2A + 4n2

i

)0.5]

and

n0 =n2

i

p0

The trap density Nt, and capture coefficients for electrons and holes (Cn, Cp) are all

dependent on the material quality. The effective electron and hole masses (m∗e, m

∗h) are

material dependent. Equations 2.4.6 and 2.4.7 are given by Nemirovsky et al. [50], which

also have approximated the trap energy Et to be

Et =Eg

2+ kT ln

(

m∗h

m∗e

)0.75

− kT ln

(

NA

ni

)

(2.4.8)

2.4.2.2 Auger Recombination

Auger recombination is a direct recombination mechanism, in which the energy of the

recombining carriers is taken by a third carrier, which then usually loses its excess energy

through thermal vibrations. There are a number of different combinations that result

in Auger recombination. For example: Auger1 is direct band-to-band recombination of

an electron with a heavy hole and excitation of another electron in the conduction band

[10, 48], and is shown in Fig. 2.4.3. Auger7 is direct band-to-band recombination leading

to excitation of electron from the light hole to heavy hole band [48]. For narrow band-gap

semiconductors Auger1 and Auger7 are the dominant Auger recombination mechanisms.

The lifetime due to Auger1 recombination is given by:

τA1 =2τA1in

2i

n0 (n0 + p0)(2.4.9)

while for Auger7 recombination the lifetime is given by:

τA7 =2τA7in

2i

p0 (n0 + p0)(2.4.10)

where: τA7i = γτA1i are the intrinsic Auger lifetimes, and are material dependent, and γ

is the ratio between Auger1 and Auger7 intrinsic lifetimes. Combining Auger1 and Auger7

gives the complete Auger lifetime expression as:

1

τA=

1

τA1+

1

τA7(2.4.11)


x x x x

EcEc

Ev

Figure 2.4.2: Shockley-Read-Hall recombination via SRH centers. (a) electron

capture (b) electron emission from center (c) hole capture (d) hole

emission from center.

E

k

E

k

ConductionBand

Heavy HoleBand

Light HoleBand(a) (b)

Figure 2.4.3: Auger Recombination (a)Auger1 (b)Auger7 [10].

EcEc

Ev

Photon

Figure 2.4.4: Radiative Recombination


Thicknessd

RecombinationVelocity S1

Absorber

RecombinationVelocity S2

Figure 2.4.5: Schematic illustrating structure with recombination at the top and

bottom interfaces.

2.4.2.3 Radiative Recombination

Radiative recombination is recombination of an electron hole pair in which a photon is also

emitted. Radiative recombination can be stimulated by a photon of wavelength similar

to the energy of the recombining electron. Figure 2.4.4 shows a radiative recombination

process. The Radiative recombination lifetime is dependent on the absorption coefficient

of the material and the generation rate variable GR. It is given by [51]:

τR =n2

i

GR (n0 + p0)(2.4.12)

2.4.2.4 Surface and Interface Recombination Effects

Recombination occurs at surfaces and interfacial layers due to defects and other recom-

bination centers, and the rate of recombination in these regions is usually greater than

the rate of recombination in the bulk. This recombination affects the lifetime of minority

carriers as given by [52]:

τeff =

D0

L0

S2 [cosh d/L0 − 1] + D0

L0sinh d/L0

D0

L0(S1 + S2) cosh d/L0 +

(

D20

L20

+ S1S2

)

sinh d/L0

(2.4.13)

where:

d is the absorber layer thickness (see Fig 2.4.5).

S1,2 are the surface or interface recombination velocities for the front and back

surfaces of the layer (see Fig 2.4.5).

D0 is the ambipolar diffusion coefficient of the carriers.

D0 =n+ p

(p/De) + (n/Dh)(2.4.14)

L0 =√D0τ is ambipolar diffusion length (see section 2.4.2.5).

De and Dh are the diffusion coefficients of electrons and holes respectively (see

section 2.4.2.5).


With simplifying assumptions such as device thickness d being much less than the diffusion

length L0, and the recombination velocity being the same at both the front and back

surfaces and equal to S, Eqn. 2.4.13 simplifies to:

1

τeff=

1

τbulk+

2S

d(2.4.15)

The surface recombination velocity is a function of the interface trap density, Dit, and is

given by [53]:

S =kTDit

q

√

CnCp (p0 + n0)

2ni

[

cosh(

(Et−Ei)kT − u0

)

+ cosh

(

(qφs+Ef−Ei)kT − u0

)] (2.4.16)

where Cn, Cp are the electron and hole capture coefficients, respectively, φs is the surface

potential, no and p0 are the equilibrium electron and hole concentrations, respectively,

Ei is the intrinsic energy level, Et is the trap energy level (relative to the intrinsic en-

ergy level), Ef is the fermi energy level, ni is the intrinsic carrier concentration and

u0 = ln (Cn/Cp).

2.4.2.5 Mobility and Diffusion Length

Under the influence of an electric field, the free carriers in a material are accelerated by

the field but also scattered by the lattice. The net result is that for bulk semiconductors of

dimension larger than the mean free path between scattering events, the carriers achieve a

constant average velocity proportional to the electric field. The proportionality constant

is called the mobility and is an important material parameter. In cases where one carrier

is dominant, the conductivity of the material will be directly proportional to the mobility.

Diffusion length is the average distance a carrier travels in the absence of an electric field

before it recombines. The diffusion length is related to the carrier lifetime τ [54]:

L = (Dτ)1/2 (2.4.17)

where:L is the diffusion length (cm).

D is the diffusion coefficient (cm2 s−1).

τ is the minority carrier lifetime (s).

For non degenerate1 materials Einstein’s relation describes the diffusion coefficient as:

D =µkT

q(2.4.18)

1A degenerate semiconductor is one in which the electron concentration in the conduction band, or hole

concentration in the valence band, is comparable with the density of states in the band. Consequently,

the Pauli exclusion principle is significant and Fermi-Dirac statistics must be used. The Fermi level is

either in the conduction band for a n+ type degenerate or in the valence band for a p

+ type degenerate

semiconductor.


2.4.2.6 Important Material Properties Affecting Devices

The material properties so far discussed strongly influence device performance. First and

foremost is the absorption co-efficient, which determines how much material is needed

to effectively absorb all the incident photons. Noise is generated throughout the entire

volume of the detector material (through various processes), and hence the smaller the

volume of the detector, the lower the noise.

Lifetime is an important property in determining the performance of a device, and its

importance varies with the device type (although a longer lifetime is usually preferred).

For a photoconductor, Eqn. 2.2.3 gives the change in conductance due to illumination. As

can be seen, a longer lifetime will yield a larger change in conductance (and, as discussed in

section 2.5.4, likewise a larger responsivity), leading to better detectors. A high mobility

also increases the change in conductance, but as the conductance depends on the mobility,

increased mobility doesn’t increase responsivity. However, the ratio of electron mobility

to hole mobility µe/µh is important, and a higher ratio will result in a higher responsivity.

A high mobility can lead to sweepout, however, which will limit device performance by

limiting effective lifetime.

Photovoltaic detectors also benefit from a larger lifetime, but for different reasons. Any

charges that are generated within the depletion region, or within a diffusion length of

the depletion region, will be separated and contribute to the photocurrent. Within the

depletion region, there is negligible impact of lifetime on this process, but outside the

depletion region the lifetime controls the diffusion length (along with the mobility), so a

longer lifetime can lead to a longer diffusion length and a larger collection area/volume.

More importantly, however, is the effect lifetime has on noise in photovoltaic detectors.

A longer lifetime leads to a reduced dark current density (or a larger zero-bias dynamic

resistance) from both the neutral region and from the depletion region [55], which improves

the detectivity of the device.

2.4.3 This Work

Mercury cadmium telluride (HgCdTe) is the infrared material that is used in this study

of resonant-cavity-enhanced detectors because it is the highest performing material sys-

tem for IR detectors. Mercury cadmium telluride has a long lifetime, very high electron

mobility and a tuneable band-gap, as well as a high absorption coefficient. The very long

minority carrier lifetime and high mobility of Hg(1−x)Cd(x)Te give rise to long diffusion

lengths. Its superiority as an IR absorber coupled with the fact that lattice constant

is almost constant for all compositions from HgTe to CdTe, makes this material system

ideal for molecular beam epitaxial (MBE) growth of RCE detectors. It should be noted,

however, that detectors fabricated from any material system would benefit from resonant

cavity enhancement. Detectors based on bulk material systems benefit mostly from re-

duced detector volume leading to reduced thermal generation of carriers, while maintain-

ing quantum efficiency. QWIPs and QDIPs can benefit from resonant-cavity-enhancement


as a result of increased quantum efficiency, as well as some possible reduction in thermal

generation of carriers.

2.4.3.1 Photoconductors

As Auger recombination is the dominant mechanism for state-of-the- art high-quality

Hg(1−x)Cd(x)Te [56], n-type material is usually used for photoconductors. The longer

lifetime and diffusion length of n-type material leads to a higher ∆G, as indicated by

Eqn. 2.2.3. Furthermore, the high electron mobility in p-type material leads to carrier

sweepout at very low bias fields, which inhibits performance. Sweepout is the process

whereby minority carriers generated by the signal flux are swept to the contacts by the bias

field. The contacts are regions of high recombination, so any minority carriers reaching

the contacts recombine immediately, reducing the effective bulk lifetime. High quality

n-type Hg(1−x)Cd(x)Te material is easier to produce than p-type material when using

MBE, although this is not necessarily the case for LPE and MOCVD grown material.

2.4.3.2 Photovoltaic Detectors

Hg(1−x)Cd(x)Te homojunctions are formed in a variety of ways. The most common is the

method similar to that used for all monolithic semiconductor processing: starting with

a bulk or epitaxial layer of one type and implanting dopants to create a region that is

of the opposite type. A common dopant for n-type materials is indium, while gold and

copper are common p-type dopants. Hg vacancies can also act as acceptors, and therefore

vacancy doped material is p-type. Arsenic is of interest since, depending on the lattice

site, it can act as both an acceptor or a donor. Interestingly, Hg(1−x)Cd(x)Te can be type

converted from p-type to n-type by exposing the material to a CH4/H2 plasma [57, 58].

Because of the variable band-gap of Hg(1−x)Cd(x)Te, and the very small change in lat-

tice constant as the composition is varied, it is very easy to grow heterostructures and

other band-gap engineered device improvements, such as compositionally graded surfaces

for keeping carriers away from surfaces. Device structures such as p-i-n and avalanche

photodetectors have been investigated. Avalanche detectors are of particular interest for

Hg(1−x)Cd(x)Te because the large difference between electron and hole ionisation con-

stants and unique band structure allows the avalanche diode to exhibit noise-free gain

[59].


2.5 Metrics and Detector Figures of Merit

2.5.1 Quantum Efficiency

Perhaps one of the most important detector metrics is quantum efficiency, η (0 ≤ η ≤ 1).

The quantum efficiency of a detector represents the probability that a single photon in-

cident on the device will generate an electron-hole pair that contributes to the signal

from the detector [46]. Quantum efficiency also represents the ratio of the concentration

of generated electron-hole pairs to the incident photon flux when many photons are im-

pinging on the device. Factors affecting the quantum efficiency are the reflection from

the incident surface of the detector (R), the fraction of electron-hole pairs generated by

photon absorption that successfully contribute to the signal (ζ), and the proportion of

photons that are absorbed (which depends on the absorption co-efficient and the detector

thickness). The quantum efficiency is given by [46]:

η = (1 −R) ζ [1 − exp (−αd)] . (2.5.1)

The fraction of electron-hole pairs generated by photon absorption that successfully con-

tribute to the signal (ζ) is affected by a number of factors. Firstly, the internal quantum

efficiency determines the proportion of absorbed photons that generate electron-hole pairs

and secondly the proportion of generated electron-hole pairs that do not contribute to

the signal, for example carriers that do not diffuse to the junction in a photodiode. The

internal quantum efficiency of Hg(1−x)Cd(x)Te is generally considered to be one [10].

2.5.1.1 Cut-Off Wavelength

The lowest energy photon that is absorbed to create an electron-hole pair is called the

cut-off energy (and associated cut-off wavelength). For wide band-gap semiconductors

this energy is equal to the band-gap energy. For Hg(1−x)Cd(x)Te, and narrow band-gap

semiconductors in general, the cut-off energy can be greater than the band-gap energy.

This difference is due to the Burstein-Moss effect [36], which describes an absorption limit

based on Em, the lowest unfilled level in the conduction band. The optical energy band-

gap EO is the difference between Em and the corresponding level in the valence band (Fig.

2.5.1). The Burstein-Moss effect is more pronounced for degenerate semiconductors. Due

to the narrow band-gap and very low effective electron mass, the density of states in

the conduction band is low and hence degeneracy is easily achieved even at low electron

concentrations in Hg(1−x)Cd(x)Te [60].

For Hg(1−x)Cd(x)Te the measured optical band-gap is defined in a number of different

ways. Figure 2.5.2(a) illustrates a method where the 50% of the peak transmittance is

used. This method was used by Hansen et al. to determine band-gap [62]. Alternately,

the absorption coefficient is used to define the optical band-gap (not shown in Fig. 2.5.2),

in which case the cutoff is defined as the wavelength beyond which the absorption is less

48 2.5. Metrics and Detector Figures of Merit

Em

Ec

Ev

EF

EG

EO

Conduction Bandm* 0.02 me ~

Valence Bandm = 0.55 mLH

0

Ene

rgy

G8

L

G6

C

G7

G8

H

D0

k

Figure 2.5.1: Energy-momentum band structure of degenerate Hg(1−x)Cd(x)Te,

showing the lowest unfilled band Em and the optical energy band-

gap EO [36, 61].

than a predetermined value [63]. The final method for determining the optical band-gap

uses 50% of peak responsivity of a detector, as in Fig. 2.5.2(b), to define the cutoff.

For intrinsic (or otherwise non-degenerate) Hg(1−x)Cd(x)Te, EO equals Eg [36, 37]. As Eg

is a function of x (appendix A.3), the mole ratio determines the cutoff wavelength, λco,

and cutoff energy, Eco, as shown in Eqn. 2.5.2. Figure 2.5.3 illustrates this relationship.

By varying the mole ratio, the cutoff wavelength can be tuned to various atmospheric

transmission windows.

Eco = Eg = hν =hc

λ≈ 1.24 × 10−6

λco(2.5.2)

2.5.2 Resistance

Device resistance is important in both photoconductors and photovoltaic photon detec-

tors, as noise is a function of resistance in both types of device. For photoconductors,

as the device is a resistor, there is an associated Johnson noise, while for photovoltaic

detectors an increased device resistance means decreased device noise due to lower dark

current.


1000 1500 2000 25000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.910 9 8 7 6 5 4

CO

T50%

TMax

TMax

Tran

smittan

ce

Wavenumber (cm-1)

Very thick 10um epilayer

Wavelenth ( m)

(a)

2 3 4 5 60

2

4

6

8

10

Res

pons

ivity

(x10

5 V/W

)

Wavelength ( m)

RPeak

R50%

CO

(b)

Figure 2.5.2: Definitions of cutoff in Hg(1−x)Cd(x)Te. (a) modelled x = 0.3 at

T=80K 50% transmission cutoff. The oscillations in the epilayer

are due to interference fringes generated by the reflection from the

layer surface and the layer/substrate interface. (b) modelled x = 0.3

at T=80K 50% of peak responsivity cutoff.


0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

5

10

15

20

25C

ut-o

ff W

avel

engt

h (

m)

Composition x

LWIR

MWIR

Figure 2.5.3: Cut-off Wavelength vs. Mole Ratio, T=80K

2.5.2.1 Photoconductive Devices

The photoconductor has an associated resistance that depends on the mobility of the

carriers and the carrier concentrations (Eqn. 2.5.3). As Hg(1−x)Cd(x)Te has a relatively

high electron mobility (≈ 104 cm2 V−1s−1, x ≈ 0.3, 80K), the resistance for a square

device of thickness ≈ 10µm is 0.1-1 kΩ for doping in the range 1014 − 1015 cm−3 and

typical operating temperatures. For RCE devices, this can be dramatically increased to

10-100 kΩ, due to the reduced device thickness, which decreases the cross-sectional area

of the conductor (see Eqn. 2.5.3).

rd =1

q (n0µe + p0µh)

l

wd(2.5.3)

2.5.2.2 Photovoltaic Devices

For photovoltaic devices, zero-bias dynamic resistance is derived from the diffusion current

and generation current combined in parallel at zero bias. Equation 2.5.4 gives the overall

zero bias dynamic resistance, due to the dark currents described in Eqns. 2.5.16, 2.5.17

and 2.5.20 [10].:

R0 =

(

dJe

dV

∣

∣

∣

∣

0AMJ +

dJh

dV

∣

∣

∣

∣

0AMJ +

dJGR

dV

∣

∣

∣

∣

0AMJ

)−1

(2.5.4)

Furthermore, the zero-bias resistance-area product can be derived by multiplying through

by the area of the device. The zero-bias resistance-area product, R0A, can be used as a


figure of merit to compare detectors, because R0A is directly proportional to signal-to-

noise ratio. A high R0A corresponds to high SNR.

2.5.3 Depletion Region Width

When a p-n junction is formed by bringing an n-type region into intimate contact with a

p-type region there is flow of charge to neutralise the imbalance in charges between the

two regions. This creates a region that is depleted of carriers and is called the depletion

or space-charge region. For an abrupt junction, the width of this region is given by [64]:

Wdep =

√

2εsε0 (Vbi − V ) (NA +ND)

qNDNA(2.5.5)

where:

εs is the dielectric constant,

Vbi is the built in voltage,

V applied voltage, and

NA and ND are the concentration of acceptors and donors, respectively.

For practical material systems, where a region is implanted with dopants or a region is

type converted, then the abrupt junction model is inadequate to describe the width of the

depletion region. Instead a linear approximation to the distribution of dopants is used to

model the changing doping density at the interface [64]:

Wdep =

[

12εsε0 (Vbi − V )

qacg

]1/3

(2.5.6)

where:

Vbi =2kT

qlnacgW0

2ni(2.5.7)

acg =dC

dx

∣

∣

∣

∣

x=xj

(2.5.8)

V is the applied voltage,

W0 is the width of the zero-bias junction, and

acg is the impurity concentration gradient at the junction.


2.5.4 Responsivity

Responsivity, Rλ, is one of the fundamental figures of merit for a detector. It is defined

as the output voltage or current signal per unit input optical power:

Rλ =Vs, IsP

(2.5.9)

where:

P is the incident optical power impinging on the device,

Vs is the RMS voltage signal (for photoconductive detectors), and

Is is the RMS current signal (for photovoltaic detectors).

Responsivity is actually a function of the modulation frequency of the input optical signal.

For most imaging applications, however, the frequency at which this becomes important

is so high as to not be important. For photoconductive detectors, the low frequency, low

field responsivity is given by [10]:

RV λ =η

lwd

λ

hc

Vbτeff

n0(2.5.10)

This equation indicates that there is a linear relation between the responsivity and the bias

field, however, at high fields, more minority carriers are swept to the high recombination

contacts, resulting in a reduction in the effective bulk lifetime of carriers. This then limits

the responsivity via an effect known as sweepout. Sweepout is only an issue for carriers

with a long lifetime and high mobility.

For photovoltaic detectors the responsivity expression becomes [10]:

RIλ =λ

hcη (λ) q (2.5.11)

where q is the charge of an electron.

2.5.5 Dark Currents and Noise

2.5.5.1 Background Flux

Absorption of thermally generated photons from the background is a random process that

generates noise in the detector. The proportion of the background photon flux that can

potentially generate this noise depends on the field of view of the detector (θ) and the

temperature of the background (Tb), and is given by:

φB = sin2[

θ

2

] ∫ λoff

λon

2πc

λ4(

exp[

hcλkTb

]

− 1)dλ cm−2 s−1 (2.5.12)

The contribution of this noise source to the total noise depends on the detector type, and

will be given in the relevant section below.


2.5.5.2 Photoconductive Devices

There are three main noise mechanisms that contribute to the noise voltage for photo-

conductive devices [65]. These are:

• the generation of carriers due to background photon flux, VBG,

• the Johnson noise, VJ , of the device due to the resistance associated with the pho-

toconductor, and

• the thermal generation of carriers within the semiconductor volume, VTh.

The expressions for each of these noise sources are given by [10]:

VBG = 2qrdµeEb

lτeff

√

lwdφBη

df (2.5.13)

VJ =√

4kTrdf (2.5.14)

VTh = 2qrdµeEb

lτeff

√

lwdn0p0

τeff (n0 + p0)f (2.5.15)

The background noise voltage, VBG, is due to photon flux, φB, from the background

generating carriers. It is also dependent on the electron mobility, µe, the effective lifetime,

τeff , the electric bias field, Eb, and the device dimensions.

Johnson noise, VJ is due to blackbody electromagnetic energy in the frequency interval

f within the detector. It is present in every resistive element, and is dependant on

operating temperature, T , and resistance, rd. In conventional photoconductors this is

not the dominant noise source due to the low resistance of such devices; however, in

RCE photoconductive devices, as the absorber layer thickness is two orders of magnitude

thinner than in standard photoconductive detectors, Johnson noise can become dominant,

especially in the presence of surface recombination.

The noise due to random thermal generation and recombination, VTh, is dependent on

the carrier concentrations n0 and p0, as well as the device properties listed above.

2.5.5.3 Photovoltaic Devices

Diffusion is generally the mechanism responsible for noise in the regions outside of the

space-charge region of a p-n junction (noise from the series resistance is also present, but

is usually negligible). Carriers generated in the vicinity of the space-charge region (within

a few diffusion lengths) can diffuse to the edge of the space-charge region and are swept

across the junction. The arrival of carriers to the edge of the depletion region is a random

process and hence a source of noise.


The current densities due to diffusion of electrons and holes for a device with structure

shown in Fig. 2.2.2 are given by [10]:

Je = qni

NA

De

Le

(

expqV

kT− 1

)

(

1 + βp tanh dLe

βp + tanh dLe

)−1

(2.5.16)

Jh = qni

ND

Dh

Lh

(

expqV

kT− 1

)

(

1 + βe tanh dLh

βe + tanh dLh

)−1

(2.5.17)

βp =SpLe

De(2.5.18)

βe =SnLh

Dh(2.5.19)

where Je and Jh are the diffusion current densities of the minority carriers at the edge

of the depletion region in the p-type and n-type materials, respectively. The diffusion

lengths of p-type and n-type materials are Le and Lh, respectively, and can be replaced

by the distance to the contact (a or d) for a short base diode, while De and Dh are the

minority carrier diffusion coefficients for p-type and n-type material, respectively. The

carrier densities are given by NA, the acceptor density in p-type material, ND, the donor

density in n-type material, and ni is the intrinsic carrier concentration. The surface

recombination velocities Sp and Sn are the recombination velocities at the surface of the

p-type and n-type regions, respectively. The device is biased with voltage V measured

with respect to the n-type side.

Any carriers generated within the depletion region are swept apart and therefore create

a generation noise current across the junction. Other than photogeneration, carriers are

randomly generated by either imperfections within the space-charge region that act as

Shockley-Read-Hall (SRH) generation and recombination centers, or random thermal gen-

eration of electron-hole pairs with SRH generally more important than other generation-

recombination mechanisms. Centers with an energy level close to the intrinsic Fermi level

contribute significantly to the generation current, which leads to a generation current

that follows the temperature dependence of ni [64]. Sah et al. [66] derived an expression

for this noise current by integrating the rate of generation and recombination over the

space-charge region.


The expression derived by Sah et al. was modified by Ajisawa et al. [67] to include surface

recombination, and is given by:

JGR = J0GR

2 sinh(

qV2kT

)

(Vbi − V ) qkT

f (b) (2.5.20)

J0GR = J0GRb + J0GRs (2.5.21)

=niWdepkT

τ0Vbi+

4S0niWdepkT

dVbi(2.5.22)

f (b) =1√

1 − b2

arctan

(

b+(

τn

τp

)1/2exp

[

−q(V −Vbi)2kT

]

)

√1 − b2

−arctan

(

b+(

τn

τp

)1/2exp

[

q(V −Vbi)2kT

]

)

√1 − b2

(2.5.23)

b = exp

[−qV2kT

]

cosh

[

Et − Ei

kT+

1

2log

(

τnτp

)]

(2.5.24)

This expression takes into account carriers generated in the volume of the depletion region,

as well as generation where the depletion region intersects the surface for a circular device,

and takes slightly different forms for other device configurations [64]. The generation-

recombination current JGR is dependent on a zero-bias generation-recombination current,

J0GR, the device bias, V , and the built-in voltage, Vbi, as well as the function fgr (b). The

zero-bias g-r current is a function of the current generated in the bulk due to an evenly

distributed concentration of SRH centers and the current generated at the surface due

to SRH centers, which depend on the intrinsic carrier concentration, ni, the depletion

region width, Wdep, the carrier lifetime t0, and the surface recombination velocity where

the depletion region intersects the surface of the device, S0. The function fgr (b) depends

on the carrier lifetimes τn and τp as well as the trap level, Et, and the intrinsic Fermi

level, Ei. There is also a component of dark current due to trap assisted tunnelling which

can dominate the dark current under reverse bias conditions, but as the devices in this

work are tested at zero bias this component is not considered in this work.

2.5.6 Noise Equivalent Power and Detectivity

Noise Equivalent Power (NEP) is a measure of the sensitivity of the device. It corresponds

to a signal-to-noise ratio of one. That is, it is the optical power required to generate a

signal voltage, Vs, that is equal to the noise voltage, Vn, or a signal current, Is, that is

equal to the noise current, Is. Detectivity, D, is simply the reciprocal of NEP.

NEP =Vn

RV λ(W) (2.5.25)

or

NEP =InRIλ

(W) (2.5.26)

D =1

NEP(W−1) (2.5.27)


2.5.7 Specific Detectivity

Detectivity is not a good figure of merit for comparing different detector structures.

Instead, specific detectivity normalises the detectivity to the measurement bandwidth

and detector area, making it more useful for comparing performance of detectors with

different structures:

D∗ =(Af)1/2

NEP=RI,V λ

I, Vn(Af)1/2 (2.5.28)

where:

D∗ is the specific detectivity (cmHz1/2W−1),

A is the area of detector (cm2), and

f is the electrical measurement bandwidth (Hz).

When the expressions for responsivity and noise voltage for a photoconductor are substi-

tuted into Eqn. 2.5.28, the expression for specific detectivity can be found as [10]:

D∗λ = Rλ

√wlf

√

V 2J + V 2

BG + V 2Th

(2.5.29)

When the noise voltage due to thermal generation of carriers is below the noise voltage

due to generation of carriers from the background photon flux, the device is said to have

background limited performance (BLIP). The Johnson noise voltage is usually below the

other two noise sources for traditional photoconductive detectors, but this is not always

the case for RCE detectors.

Similarly, substituting the expressions for responsivity and noise current for a photovoltaic

detector into Eqn. 2.5.28, the expression for specific detectivity for a photovoltaic detector

becomes [10]:

D∗λ = RIλ

√wlf√

I2n

(2.5.30)

=λ

hcηq

1√

(4kT/R0A) + 2ηq2φB(2.5.31)

For sufficiently high R0A product, the background flux term will dominate the detectivity

expression, and the device is said have background limited performance (BLIP).

2.5.7.1 Specific Detectivity for Background Limited Performance

Optimal performance for IR imaging systems occurs when the device is limited by the

background and not by noise from the detector itself. When a detector fulfills this re-

quirement, it is said to have background limited performance (BLIP). Specific detectivity

of a background limited detector is given solely by the background flux and the efficiency

with which photons are converted to carriers:

D∗ =λ

hc

[

η (λ)

2φB

]1/2

(2.5.32)


The background flux depends on the background temperature, field of view (FOV) and

cutoff wavelength, and was discussed in section 2.5.5.1.

2.6 Experimental Techniques

In the original work presented in this thesis a number of standard and non-standard

experimental techniques were used. This section briefly describes the techniques used.

2.6.1 Material Characterisation

Crystal quality in this work was primarily characterised using X-ray analysis, and can also

be characterised by etch pit density tests to determine defect densities [68]. When the

Bragg condition is satisfied, the reflected X-ray signal undergoes constructive interference

and represents a maximum. By adjusting the angle of incidence, the peak will determine

the lattice spacing, while the peak shape and width will characterise crystalline quality.

By examining multiple crystal orientations, crystal stresses, such as those introduced by

the lattice mismatch in heterostructures, can be determined. Information such as material

composition and layer thicknesses can also be extracted from X-ray measurements.

Material composition was characterised by secondary ion mass spectrometry (SIMS) [69]

and inferred from the cut-off wavelength measured using Fourier Transform Infrared

(FTIR) spectroscopy. SIMS sputters the sample surface with ions (typically Cs+ and

O−). Sputtered secondary ions are then collected and measured by a mass spectrometer.

The composition of materials can be inferred from the number/presence of the various

secondary ions that are emitted. More importantly SIMS can yield information on donor

concentrations, as well as impurity concentrations.

The optical properties of a material were measured using ellipsometry, FTIR spectroscopy,

reflection and transmission spectroscopy [70, 71, 72]. These methods allowed measurement

of the absorption coefficient and refractive index. These methods can also be used to

calculate layer thickness and growth rates by using a known refractive index and using

layer thickness as a fitting parameter.

Carrier density and carrier mobility were determined using Hall measurements and quan-

titative mobility spectrum analysis (QMSA) [73]. These methods require measurement of

current-voltage combinations on test structures as a function of magnetic field. Carrier

lifetime can be measured in a number of ways. One method is to generate carriers in

a device using a scanning laser microscope (SLM) and then extract a lifetime from the

decay of the generated signal [33].

58 2.6. Experimental Techniques

G-R

Tunnelling

Diffusion

Ω

Figure 2.6.1: Photodiode resistance as a function of voltage on a logarithmic

scale, also showing the dominant noise mechanism. Also plotted is

the dark current.

2.6.2 Device Characterisation

Device characterisation methods are concerned primarily with measuring the device per-

formance for bench-marking, and to establish performance limiting mechanisms. The

current-voltage, or I-V, measurement of a device is perhaps one of the most important

measurements. The I-V curve can also be used to extract the dynamic resistance as a

function of bias:1

R=

δI

δV(2.6.1)

For photoconductors with ohmic contacts, the I-V curve is linear, and the dynamic resis-

tance is constant. For photovoltaic devices the I-V curve demonstrates the familiar high

current in forward bias above the diode turn-on voltage, and negligible current below

turn-on and for reverse bias. A typical variation of dynamic resistance with bias for a

photovoltaic device is plotted in Fig. 2.6.1 The mechanisms determining the dynamic

resistance for this device are also indicated. The zero-bias resistance (discussed in section

2.5.2.2) is the dynamic resistance at V = 0.

Performance characterisation includes measurement of device responsivity and noise,

yielding detectivity. Responsivity was measured using a monochromator, and a cali-

brated detector. Device responsivity is determined by comparing measured signal from

the device with the signal from a calibrated detector of known area. Device noise is mea-

sured using a spectrum analyser. However, care must be taken when performing noise

measurements to remove noise sources from the measurement system. Typically, devices


are placed in a Faraday cage, and all efforts are made to remove mains power induced

noise sources. Furthermore, noise measurements can be made with the device illuminated

by a 300K background, or with the device in the dark (effectively zero FOV).

The spatial optical response of a device was measured using a scanning laser microscope

(SLM). The spatial photo-response can yield information about which regions of the

device are active optically, and also information about defects due to processing. Another

technique of measurement using a SLM is laser beam induced current (LBIC) which can

be used to characterise diodes [74, 75].

60 2.6. Experimental Techniques

Chapter 3Theory of Resonant-cavity-enhanced

Detectors

3.1 Introduction

This chapter focuses on resonant-cavity-enhanced (RCE) detectors. The development of

resonant-cavity-enhancement and the advantages and disadvantages of RCE detectors are

investigated. The relationships between quantum efficiency, finesse, and absorber thick-

ness are outlined. Modelling of device performance is presented, illustrating the benefits

of RCE detectors. Finally, techniques for fabricating RCE structures are investigated.

3.2 Methods of Improvement

As the adage goes “if it ain’t broke, it doesn’t have enough features.” Research is always

ongoing to expand device function and improve device performance. For infrared pho-

ton detectors device performance focuses on a number of areas, mainly improving the

signal-to-noise performance and also raising the operating temperature to achieve higher

operating temperature (HOT) devices.

For Hg(1−x)Cd(x)Te, historically most of the issues related to improving noise performance

focus on improving material quality, as performance has not been restricted by physical

limits, but by poor material quality. Therefore, most research in the past has investigated

improving contacts, surface passivation and interfaces, and other growth factors such as

reducing dislocation densities. Research into HOT devices has proceeded in three main

areas: Heterostructure devices, multi-junction devices, and more recently resonant-cavity-

enhancement [35, 76, 77, 78, 79, 80].

62 3.2. Methods of Improvement

n+ n+

p-n p+Ev

EF

Ec

E

Figure 3.2.1: Energy gap profile of a HgCdTe heterostructure device. After [82].

p layer

p layer+

(+)(-)hn

n layer+

Figure 3.2.2: Schematic of a multi junction device. After [83].

3.2.1 Heterostructure Devices

By engineering the band structure of a device, the noise performance of an infrared detec-

tor can be improved. Figure 3.2.1 illustrates such a structure proposed by Ashley et al.

[81], which is able to suppress the Auger-1 recombination mechanism by extracting car-

riers from the active region under reverse bias [76, 77]. Therefore, as the noise sources

are reduced, the device can be operated at higher temperature while maintaining the

same performance. However, these devices suffer from performance degradation at low

frequency due to increased 1/f noise [82].

3.2.2 Multi-junction Devices

Multi-junction devices take a single pn junction and repeat the junctions as in Fig. 3.2.2.

The junctions are connected in series, using the fact that the p+n+ junction is effectively a

short, due to significant tunnelling in the high field region [83]. The benefit of the multi-

junction device accrues because they maintain a good quantum efficiency and a high

differential resistance, while allowing the absorber layers of each stack to be reduced, thus

reducing the volume that is responsible for generating thermal noise. Modelling results

show that as the number of cells is increased, performance increases and background

limited performance be reached at higher temperatures [35].

CHAPTER 3. Theory of Resonant-cavity-enhanced Detectors 63

Cavity Length l

Mirror 2

Mirror 1

Illumination

Figure 3.3.1: Schematic of a Fabry-Perot cavity.

3.2.3 Resonant-cavity-enhanced Devices

Detector noise performance can be improved by thinning the absorber layer. However,

beyond a critical thickness, the absorber layer is too thin to absorb all incident pho-

tons, creating a trade-off between quantum efficiency and absorber thickness. By placing

the absorbing layer within a Fabry-Perot resonant cavity, this trade-off can be circum-

vented. This benefit only occurs at wavelengths where the cavity resonates, and therefore

resonant-cavity-enhanced response is inherently narrowband. For applications such as

multi- and hyperspectral imaging or spectral detection, this narrowband response is not

an issue, and is even desired. The reduced volume of the absorbing layer allows reduced

thermal generation of carriers. This can allow higher operating temperature (HOT) de-

vices or alternately a higher signal-to-noise ratio for the same operating temperature,

assuming the device is limited by thermal generation and recombination noise [79, 80].

Furthermore, the reduced absorber volume can allow faster electrical operation, which

could allow applications such as active imaging to be realised. Quantum efficiency of

devices fabricated from low absorption co-efficient material can also be increased, as well

as other benefits, that will be discussed in greater detail later in this chapter.

3.3 Fabry-Perot Cavities

Fabry-Perot cavities, frequently used as filters, are commonly fabricated using metal

mirrors and a hard etalon material such as quartz to space them (Fig. 3.3.1) [84]. The

two mirrors are typically matched (have the same reflectivity), in order to provide the

maximum transmission through the Fabry-Perot cavity at the resonant wavelength.

Resonance occurs in the cavity when the cavity length is an integer multiple of half the

wavelength. Assuming no absorption in the cavity, and no phase change on reflection

from the mirrors, resonance only occurs when the following condition is satisfied:

m =δ

2π=

2nsℓ cos θs

λ(3.3.1)

64 3.3. Fabry-Perot Cavities

High indexLow index

High index

High indexLow index

Air

Substrate

IncidentLight

Reflected light=combination of manybeams

Multilayer

Figure 3.3.2: A schematic of a dielectric stack mirror after [86].

where the effective cavity refractive index is ns, the angle of incidence into the cavity is

θs, and m = 0,±1,±2, ... is the mode of the cavity.

For use in monolithic RCE semiconductor fabrication the use of hard etalons and metal

mirrors is impractical for a number of reasons. Firstly, incorporation of an absorbing layer

into such a structure is impossible. Secondly, the absorption in metal mirrors, especially

for IR wavelengths, significantly degrades the performance of the cavity. Dielectric stack

mirrors are therefore used instead of metal mirrors, and consist of alternating high refrac-

tive index (nH) and low refractive index (nL) dielectric materials. Figure 3.3.2 illustrates

the structure and also illustrates schematically how these mirrors function. Each layer

by itself has a reflectance that is much lower than a simple metal mirror. However, the

total reflectance for the system is the combination of the reflection from all the layers.

If all the reflections sum in phase, then the total reflectance can quite easily approach

unity. The simplest layer thickness to achieve in-phase reflections occurs if each layer

has an optical thickness equal to a quarter wavelength, and the stack is therefore called

a quarter-wave-stack (QWS) mirror or a distributed Bragg reflector (DBR) after W.H.

Bragg and S.L. Bragg, since the reflections interfere constructively when the Bragg phase

condition is satisfied [85]. The Bragg condition is satisfied when

nLtL = nHtH =mλ0

4m = 1, 3, 5... (3.3.2)

where nH is the index of refraction of the high refractive index dielectric material and

nL is the index of refraction of the low refractive index material. The thicknesses of the

high refractive index dielectric material and low refractive index dielectric material are

denoted by tH and tL, respectively, and λ0 is the wavelength. A detailed analysis of the

advantages and limitations of dielectric mirror stacks will be given in chapter 4.

Extending the dielectric stack principal, a complete Fabry-Perot cavity can be fabricated

out of dielectric material. Such a cavity is illustrated in Fig. 3.3.3. Again the reflectance

of the stack is composed of the sum of many reflections, as is the transmittance. The

conditions for resonance remain roughly the same, although now phase changes due to the

dielectric stack mirrors must be included (as will be discussed later). The performance of


Multilayer

Multilayer

Spacer Layer

Reflected light=combination of manybeams

Transmitted light=combination of manybeams

IncidentLight

Figure 3.3.3: A schematic of a dielectric stack cavity after [86].

Tra

nsm

itta

nc

e

Wavelength l0

1

F= 10

F= 2

FSR

FWHM

Figure 3.3.4: The transmittance of a Fabry-Perot filter as a function of wave-

length. Finesse, FWHM and FSR are illustrated.

dielectric stack mirrors and cavities can be modelled using characteristic matrix method-

ology [86], which is outlined in appendix B. Extending the dielectric stack to include the

absorber layer for a RCE detector is then relatively straight forward.

3.3.1 Figures of Merit

There are a number of figures of merit used to describe Fabry-Perot filters, and almost

all describe the spectral width of the cavity. Perhaps the most all-encompassing figure is

that of finesse, F . The finesse is the ratio of the free spectral range (FSR), which is the

separation between resonant peaks, and the full-width half-maximum (FWHM), which

is the spectral bandwidth at 50% of the transmission peak. Generally, the higher the

finesse, the narrower the peak, and the deeper the rejection region [87]. For spectroscopic

applications where narrow bandwidth is desired, a high finesse is required, illustrated in

Fig. 3.3.4. The finesse can also be expressed as a function of the reflectance of the mirrors.


For matched mirrors the finesse is given by [84]:

F =π√R

1 −R (3.3.3)

where R is the nominal reflectance of both mirrors.

Another figure of merit is the spectral resolution δλλ . The spectral resolution is also a

measure of FWHM, and is related to the finesse:

λ

δλ= 0.97mF (3.3.4)

where m is the mode of the cavity. Hence, for hyperspectral based detection a spectral

resolution of δλλ ≤ 0.01 implies that a finesse of at least 100 is required for the fundamental

(m = 1) mode. Finally, the quality factor, Q, of the cavity relates the energy within the

cavity to the incident energy, where a higher Q indicates a narrower spectral line width.

The Q of a cavity is related to the finesse by the optical resonator frequency, ν0, and the

mode spacing, νF :

Q =ν0

νFF (3.3.5)

3.3.2 Energy Density

The finesse of a cavity is also a measure of the ratio between the energy within the cavity

to the energy incident, which is given by [88]:

|E|2

|Ein|2=

(∣

∣1 −R21

∣

∣

|1 −R1R2 exp−j (2βℓ+ φ1 + φ2)|2

)

×[

1 +R22 + 2R2 cos [2β (ℓ− z) + φ2]

]

(3.3.6)

β = 2πn/λ0 (3.3.7)

where R1, R2 are the mirror reflectivities, φ1, φ2 are the phase changes on reflection from

the two mirrors, ℓ is the cavity length, z is the position in the cavity, n is the refractive

index of the material in the cavity, and λ0 is the wavelength of incident light. Figure 3.3.5

illustrates the energy density within a cavity that consists of two mirrors, separated by

1.665 µm using the refractive index of CdTe for the cavity, and mirrors of Ge/SiO quarter-

wave-stacks. The cavity is operating in the second order, and the energy density in the

center of the cavity can be seen to be a maximum of approximately 600 times greater than

the incident energy. Wavelengths other than the resonant wavelengths do not exhibit an

increased energy density, and so will not experience any gain in absorption. By placing

an absorbing layer at an anti-node in the energy density function, a vast increase in

absorption is achievable.

The position and number of the nodes and anti-nodes within a cavity depend on the

operating mode. Figure 3.3.6 illustrates the first order mode of an air cavity that is 2

µm long and bounded by Ge/SiO Bragg mirrors centered at 4µm. There are two possible

arrangements for the mode profile, depending on the phase change on reflection occurring


Position within cavityZ ( m)m

Wavenumber1/ ( m)l m

|E|

/|E

|2

2

in

Mirror

Mirror

Z = 0 mm

Z = 1.665 mm

Position within cavityZ ( m)m

Figure 3.3.5: Energy density within a cavity as a function of wavelength and the

position within the cavity.

at the mirrors, which in turn depends on the order of the mirror layers, as there is a phase

change of π on reflection from low to high layers, and zero on reflection from high to low

layers. Therefore, quarter-wave mirrors beginning with the high refractive index layer

(i.e. HLH) have a pi phase change on reflection, while quarter-wave mirrors beginning

with the low refractive index (i.e. LHL) will have zero phase change on reflection.

3.3.3 Fabry-Perot Cavities with Absorption

Absorption within a thin detector can be enhanced by placing it within a Fabry-Perot

cavity, as illustrated in Fig. 3.3.7. Resonant-cavity-enhanced detectors work primarily

by confining light within the optical cavity. Despite the fact that an absorber layer is

very thin, and absorbs little of the light that passes through it in one pass, the multiple

passes through the absorber layer ensure high absorptance. Therefore, it is critical that

absorption only occurs within the absorbing layers, since small amounts of absorption

that occur in regions other than the absorbing layer will also experience multiple passes.

Maximum absorption occurs only when the cavity resonates, increasing the energy density

within the absorber region. As resonance depends on the cavity length, there are usually

narrow wavelength bands which satisfy this condition, making the technology inherently

narrowband. Wavelengths other than the resonant wavelength are rejected by the cavity,

which causes a decrease in absorption for an absorber layer within a detector compared

to a non-RCE absorber layer of similar thickness.


0

20

40

60

80

100

120

140

160

180

200

0

100

200

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.000

20

40

60

80

100

120

140

160

180

200

|E|2 /|

EIn|2

Position in Cavity ( m)

LHL - Air - LHL HLH - Air - HLH

Figure 3.3.6: Energy density within an air cavity at 4 µm wavelength. The re-

flectors are Ge/SiO Bragg mirrors, centered at 4 µm wavelength.

Cavity Length lL

L

t

b

Detector Thickness Ld

Mirror 2

Mirror 1

Illumination

Figure 3.3.7: Schematic of a resonant-cavity-enhanced detector


The quantum efficiency of a resonant-cavity-enhanced detector is given by [88]:

η =

(

1 + |Γb0|2 e−αeff Ld

) (

1 − |Γt0|2) (

1 − e−αeff Ld

)

1 − 2 |Γt0| |Γb0| e−αeff Ld cos Θ + |Γt0|2 |Γb0|2 e−2αeff Ld(3.3.8)

where:

Θ = 22π

λndLeff − Θt0 − Θb0 (3.3.9)

Leff =ncav (Lt + Lb) + ndLd

nd(3.3.10)

αeff = gα (3.3.11)

The round trip phase change within the cavity is Θ, which incorporates the phase change

at the reflectors (Θt0 and Θb0), nd and Ld are the refractive index of the detector material

and the thickness of the detector, respectively, Leff is the effective cavity length (Lt is

the length of cavity before the detector, Lb is the cavity length after the detector, see Fig.

3.3.7). The parameters |Γt0| and |Γb0| are the field based reflectivity of the top (first in the

optical path) reflector and the reflectivity of the bottom (last in optical path) reflector,

respectively. The reflection from the absorber layer is ignored in this equation, which is

reasonable if the cavity and absorber layer have similar refractive indices. The effective

absorption co-efficient, αeff , is the product of the detector material absorption, α, and

an optical field enhancement factor, g, that accounts for the standing wave effects in the

cavity and is dependent on the position of the detector within the cavity.

3.3.3.1 Optimum Reflectivity for 100% Absorption

Quantum efficiency will be maximised when certain conditions are satisfied. Murtaza [89]

gives the relation between the top and the bottom mirror reflectivities as:

|Γt0|2 = |Γb0|2 e−2αeff Ld (3.3.12)

Placing the detector within the cavity means that matched mirrors no longer give optimal

performance and, in fact, the first mirror must now have a lower reflectivity than that of

the last mirror. The difference depends on the amount of absorption within the detector,

with thinner detectors optimised using reflectors that are closer to being matched. Gen-

erally, the last reflector is as close to unity reflectivity as possible, as illustrated in Fig.

3.3.8. If the last mirror reflectivity is taken as unity, then the peak quantum efficiency

occurs for first mirror reflectivity less than unity. If the last mirror reflectivity is allowed

to vary, while the first mirror is optimised, then the peak quantum efficiency occurs at

unity reflectivity for the last reflector.


0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

Qua

ntum

Effi

cien

cy

Mirror Reflectivity

Back mirror R=1, varying front mirror R Varying back mirror R, front = optimised

Figure 3.3.8: Quantum efficiency as a function of mirror reflectivity. The last

mirror reflectivity is held at unity while the first mirror reflectivity

is varied, producing the peak quantum efficiency relation given in

Eqn. 3.3.12. Alternately, if the first mirror reflectivity is optimised,

then the peak quantum efficiency occurs at unity reflectivity for the

last mirror.

3.3.3.2 Effect on Line Width

The finesse of the cavity is also dependant on the absorbtion of the detector [89] where

increasing absorption decreases finesse and increases linewidth:

F =π√

|Γt0| |Γb0|e−(1/2)αeff Ld

1 − |Γt0| |Γb0| e−αeff Ld(3.3.13)

For given reflector parameters the finesse is reduced by having the detector within the

cavity, as illustrated in Fig. 3.3.9. The cavity is modelled with a unity reflectivity for

the bottom mirror and 0.9945 reflectivity for the top mirror, simulating 20.5 and 3.5

period Ge/SiO distributed Bragg reflectors, respectively, at λ = 4 µm. The thinner the

detector, the greater the finesse. Figure 3.3.9 also shows a plot of finesse in a cavity where

the reflectivity of the top mirror is optimised such that quantum efficiency is maximised

(as in Eqn. 3.3.13). As can be seen, the detector must be approximately 50 nm thick

to obtain the finesse required (≃ 100) for hyperspectral detection using constant mirror

reflectivities. For a finesse of 25 (for multi-spectral imaging), the required thickness

increases to 400 nm, which is more easily realisable. If maximum quantum efficiency is

required for the F = 100 case, then the detector must be approximately 30 nm thick


Constant Reflectivity

Optimised Reflectivity

Figure 3.3.9: Finesse as a function of detector thickness while holding mirror

reflectivity constant or optimising mirror reflectivity for maximum

quantum efficiency

to achieve hyperspectral detection, while for the F = 25 case the detector must be

approximately 200 nm thick.

Combining the requirements of reflector and finesse, the quantum efficiency can be plotted

for various cases, as shown in Fig. 3.3.10. All devices are modelled using x = 0.3 material

at 80K with a 2 µm cavity length, for the first mode resonance. The high finesse device

has an absorber thickness of 30nm, and top mirror has a reflectivity of 0.9945 (modelled

after a 3.5 period Ge/SiO distributed Bragg reflector), while the bottom mirror has unity

reflectivity. The low finesses device is 200nm thick, with a top mirror reflectivity of 0.9,

and unity bottom mirror reflectivity. The device is located at the center of the cavity,

and reflection from the absorber layer/spacer interface is ignored. As can be seen, both

devices have a quantum efficiency close to unity at the design wavelength, but the low

finesse cavity has a much broader line-width. It is important to note that at the design

wavelength, the resonant-cavity-enhanced device has a quantum efficiency very close to

one, whereas, a non-RCE device of the same thickness would have a quantum efficiency

significantly less than one. As shown in Fig. 3.3.10 for a 200nm thick detector operating

at 4 µm, the quantum efficiency of the non-RCE device is less than 0.2.

3.3.3.3 Effect of Absorber Position

The position of the absorber is important in a number of ways. From the standpoint

of improved performance, then the absorber layer must be located at a maxima in the

energy density. Careful design can ensure this, including such issues as reflection at the

interfaces of the absorber layer. More interesting is the location of the maxima, which


2 3 4 5 6

Wavelength ( m)m

0.2

0.4

0.6

0.8

1

Quantu

m E

ffic

iency,

h

First Order

SecondOrder

Figure 3.3.10: η as a function of Wavelength for thin detectors. For RCE high

finesse, |Γb0| = 1 and |Γt0| = 0.9945, with a 30nm thick detector.

For RCE low finesse, |Γb0| = 1 and |Γt0| = 0.9, with a 200nm

thick detector. Cavity lengths are 2µm. For non RCE detector

thickness is 200nm.

occur in the dielectric stack mirrors, as well as in the resonant cavity. The high energy

density in the mirror regions occur over a broader spectral range than within the cavity,

and is not ideal for narrow-band imaging.

3.3.4 Effect of Mirror Phase

Equation 3.3.1 is useful for describing the resonance condition in the most basic of Fabry-

Perot cavities, however, real mirrors introduce phase changes on reflection, and these

phase changes must be taken into account when designing a resonant cavity. The phase

change on reflection (φ1, φ2 for the optically first and second mirror, respectively) effec-

tively increases or decreases the optical path length, altering the resonant wavelength

[84]:

m =δ

2π=

2nsℓ cos θs

λ+φ1 + φ2

2π(3.3.14)

As the resonant wavelength is a function of phase change on reflection, the finesse is also

affected, with the line width of the resonance increased or decreased depending on the

sign of the phase change on reflection. Furthermore, the phase change on reflection affects

the position of the absorber layer.


3.4 Examples of RCE Devices

Chin and Chang [90] first proposed the use of a novel device structure which involved

InGaAs absorber layers grown by chemical vapour deposition on AlGaInAs and AlInAs

alternating layers. This first use of resonant-cavity-enhanced detectors was proposed to

overcome the tradeoff between electrical operating bandwidth and quantum efficiency

in optical communication systems without utilising waveguide detectors, which require

edge coupling and suffer from broad spectral response [90, 89]. The bandwidth-efficiency

tradeoff occurs as high-speed detection requires reduced depletion region width to reduce

transit time, as well as reduced area to minimise capacitance, while quantum efficiency

requires adequate detector thickness to ensure all photons are absorbed. The use of

resonant-cavity-enhanced detectors allowed high detector speed, without reducing quan-

tum efficiency. Other material systems targeting communications applications have also

been investigated to determine suitability for RCE detectors [91], for example Ge and Si

absorber layers with various mirror material systems [88].

The RCE detector concept has been extended for use in IR detectors [78, 92, 93, 94] al-

though the importance in reducing detector noise was not immediately recognized. While

Pautrat et al. [78] studied RCE structures primarily for the purpose of RCE vertical cav-

ity surface emitting lasers (VCSELs), they also noted that the structure used for the

VCSELs could be operated as a photoconductor or a photodiode. The spectral pho-

toconductivity measured by Pautrat shows strong agreement with model results, and

resonant detection is observable. Arnold et al. [92] also fabricated RCE detectors using

a lead-chalcogenide absorber layer coupled with a lead-chalcogenide and barium fluoride

spacer and dielectric stack mirror system. Arnold reported peak quantum efficiencies of

32% at a resonant wavelength of 4.4 µm with a FWHM of 0.037 µm, which equates toδλλ = 0.008, which is sufficient for hyperspectral imaging applications. Furthermore, in a

structure similar to Musca et al. [28], Zogg and Arnold [80, 94] have proposed a tuneable

resonant-cavity-enhanced detector.

Sioma et al. [93] proposed a structure for RCE detection in the LWIR window. The

material system was Hg(1−x)Cd(x)Te, and simulated results showed unity absorption at

the design wavelength. Both Sioma and Pautrat note that the resonance condition is

strongly dependant on the angle of incidence of the radiation to be detected.

3.5 Advantages

3.5.1 Speed

For photovoltaic detectors, as absorber layer thickness is reduced the electrical bandwidth

can be increased as it was for optical communications applications of RCE detectors

[90]. This could find applications in range-gated active imaging, where high electrical

bandwidth is a requirement.

74 3.5. Advantages

4 6 81E-5

1E-4

1E-3

0.01

0.1

1

Abs

orptan

ce

Wavelength ( m)

nonRCE QDIP RCE QDIP

Figure 3.5.1: Modelled absorptance of a QDIP with RCE compared with a QDIP

without RCE.

3.5.2 Improved Quantum Efficiency

Examining Eqn. 3.3.8, it can be seen that with an appropriately designed cavity, any

length of material Ld with some absorption co-efficient α can achieve 100% quantum ef-

ficiency. This has been illustrated already for a 200 nm thick absorber layer where the

quantum efficiency was increased from ≈ 15% to ≈ 100% (Fig. 3.3.10), but the principle

applies to any material. This is of importance to quantum well infrared photodetec-

tors (QWIPs) and quantum dot infrared photodetectors (QDIPs), as QWIPs and QDIPs

have very low absorption coefficients, which can lead to very low quantum efficiency.

Jiang et al. [95] calculated the quantum efficiency of their QDIPs to be η = 2.0 × 10−4

for one layer of dots. This poor quantum efficiency is primarily due to the poor absorp-

tion of photons, so that the use of a RCE structure can be used to increase absorption

without requiring more quantum dot layers. Figure 3.5.1 illustrates the improvement

in absorptance due to resonant-cavity-enhancement. This QDIP structure is based on

the structure outlined by Fu et al. [96], consisting of ten 50nm thick GaAs barrier layers

and In0.5Ga0.5As quantum dots. The absorption profile is broad due to dispersion in dot

dimensions. Absorptance at resonance is increased from ≈ 0.4% to ≈ 60%.


3.5.3 Reduced Volume

The interest in applying the principle of resonant cavity enhancement to IR detectors

stems from the fact that a reduced absorber layer volume produces less thermally gener-

ated carriers. If a device is dominated by background noise, then there is no improvement

however, this generally requires cooling of the device. For narrow optical band signals, it

is difficult to achieve BLIP. The resonant cavity is needed for reduced thickness devices

to maintain the quantum efficiency as the absorber layer thickness decreases in much the

same way as for the case in communications applications [90]. As the thermal generation

and recombination noise is reduced for a given operating temperature, the signal-to-noise

ratio is improved, which is useful for narrow-band sensors, where signal is limited. Alter-

nately, for a given operating noise level, the device operating temperature can be raised.

This allows for higher operating temperature (HOT) devices, which could lower the cost

of imaging systems considerably, as the detector cooling requirements are significantly

relaxed. Such benefits apply to both photovoltaic and photoconductive detectors. While

this work focuses on photoconductive detectors, focal plane arrays of photovoltaic detec-

tors could make use of RCE in similar ways.

3.5.3.1 Reduced Noise - Photoconductive Detectors

For photoconductors the relationship between the reduction in absorber layer thickness

and improved detectivity can be derived from Eqns. 2.5.15 and 2.5.10, assuming that

the thermal generation and recombination of carriers is the dominant noise mechanism.

Substituting Eqns. 2.5.15 and 2.5.10 into Eqn. 2.5.29 yields the relation

D∗λ ∝ 1√

d(3.5.1)

Therefore, while reducing the absorber layer thickness increases the voltage noise due to

thermal generation and recombination, the responsivity is increased to a greater degree,

resulting in an overall improvement in performance that is proportional to√d, e.g.

reducing the absorber layer thickness by two orders of magnitude will increase the detec-

tivity by one order of magnitude, assuming that thermal generation and recombination

of carriers is the dominant noise mechanism.

In order to illustrate the reduction in noise that occurs due to the reduction of absorber

volume, Hg(1−x)Cd(x)Te photoconductive devices are modelled in a two stage process.

Firstly, the optical performance of the device is modelled using characteristic matrix

methodology (see appendix B). The absorptance of an absorber layer is then used to

calculate the quantum efficiency of that layer. For Hg(1−x)Cd(x)Te, the internal quantum

efficiency is taken to be unity, and therefore the quantum efficiency of the absorber layer

is equal to the absorptance of that layer. The detectivity is then modelled using the

quantum efficiency, responsivity and noise, summarised by Eqns. 2.5.13 - 2.5.15, 2.5.10,

and 2.5.29. The material parameters used in this model are outlined in appendix A.

76 3.5. Advantages

4 6 8 10 121E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5300 250 200 150 100

Noi

se (V

Hz-1

/2)

1000/T (K-1)

total noise, Non-RCE J TH BG total noise, RCE J TH BG

Temperature (K)

Figure 3.5.2: Total noise and its three components as a function of temperature.

Comparison of the noise sources for a RCE device, compared to a

non-RCE device. The RCE device becomes limited by thermal g-r

noise at a higher temperature.

The various noise sources are plotted in Figs. 3.5.2 and 3.5.3 as a function of temperature.

Figure 3.5.2 illustrates the benefit of RCE devices based on a noise analysis. The overall

noise is higher in the RCE device (compensated for by a higher responsivity), but the

background noise voltage increases more rapidly than the thermal noise voltage (as does

the responsivity), so the device becomes limited by thermal g-r noise at higher temper-

ature. Also of note is the increased Johnson noise for the RCE device. As the thickness

of the device reduces, the resistance of the device increases by as much as two orders of

magnitude. Figure 3.5.3 illustrates the effect that surface recombination has on the noise

sources. Surface recombination becomes the dominant mechanism for these thin devices,

and drastically reduces the lifetime. This has the effect of lowering the thermal g-r and

background noise voltages (and also the responsivity), but not the Johnson noise volt-

age, which therefore dominates, so that the device is unable to reach background limited

performance.

Detectivity was modelled for Hg1−xCdxTe photoconductive devices with molar compo-

sition of x = 0.3 and a doping density of n0 = 1 × 1014 cm−3 at T = 80K, dimensions

of 100 µm × 100 µm and for absorber thicknesses of 10 µm and 75 nm. The mobility,

lifetime and intrinsic carrier concentration are calculated from the molar composition [56]

(see appendix A). The quantum efficiency of the 10 µm-thick non-RCE device was taken


4 6 8 10 121E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5300 250 200 150 100

Noi

se (V

Hz-1

/2)

1000/T (K-1)

total noise with S = 50 cms-1

J TH BG total noise with S = 0 cms-1

J TH BG

Temperature (K)

Figure 3.5.3: Total noise and its three components as a function of temperature.

Comparison of noise sources for a RCE device with no surface re-

combination (S = 0 cm s−1), compared with a RCE device with

surface recombination (S = 50 cm s−1). The device with surface re-

combination is limited by Johnson noise, for all temperatures while

the device with no surface recombination is background limited for

temperatures less than 160K.

as unity, while the quantum efficiency of the 75 nm-thick RCE device was calculated

based on the optical model (appendix B). For a Hg(1−x)Cd(x)Te RCE detector the ab-

sorptance of the absorber layer is determined using the optical model and the internal

quantum efficiency was assumed to be unity, resulting in a quantum efficiency of η = 0.96

at the resonant wavelength. The result of the model is given in Fig. 3.5.4 for surface

recombination velocity, S = 0 cm s−1. It can be seen in Fig. 3.5.4 that both devices are

background limited at low temperatures. As the temperature rises, both detectors suffer

from thermal noise: however, the non-RCE device is affected first. The RCE device can

operate at background limited performance at 200K, while the non-RCE device can only

sustain background limited performance up to 160K. This gain in performance is due to

the reduction in detector volume, which reduces the thermal generation and recombina-

tion noise mechanism, and the relationship between detectivity and absorber thickness is

discussed in section 3.4.

78 3.5. Advantages

4 6 8 10 121010

1011

1012

1013

1014300 250 200 150 100

D*

(cm

Hz1/

2 W-1)

1000/T (K-1)

D* 10 m thick non-RCE D* 10 m thick non-RCE, thermal g-r only D* BLIP D* 75nm thick RCE D* 75nm thick RCE, thermal g-r only

Temperature (K)

Figure 3.5.4: Modelled detectivity, thermal generation-recombination limited de-

tectivity and background limited performance (BLIP) detectivity

of a 10 µm-thick non-RCE and a 75 nm-thick RCE device.

3.5.3.2 Reduced Noise - Photovoltaic Detectors

Noise performance can be modelled for photodiodes by combining the dark currents in

Eqns. 2.5.16, 2.5.17 and 2.5.20, into a zero bias dynamic resistance as given in Eqn. 2.5.4.

Figure 3.5.5 illustrates the effect of thinning a vertical junction geometry (Fig. 2.2.2(b))

diode. A vertical geometry diode with ideal parameters in which Sn = Sp = S0 = 0 cm s−1

is seen to increase in zero bias dynamic resistance as the volume of the depletion region

reduces (due to decreasing thickness). The low S vertical diode represents a typical diode

with surface recombination velocities of Sn = Sp = 100 cm s−1 at the passivated surfaces

of the n-type and p-type region, respectively, and S0 = 2000 cm s−1 simulating surface

recombination at the surface of the depletion region which has damage due to junction

formation. The high S vertical diode represents a poor diode with surface recombination

velocities of Sn = Sp = 5000 cm s−1 and S0 = 20000 cm s−1. As can be seen the increase

in R0 as thickness decreases is limited by surface recombination. A critical thickness is

reached, beyond which further thinning results in the surface recombination becoming

dominant. The Low S curve, representing a well-passivated device, can be thinned to

40-50 nm before the surface effects start to dominate. Better passivation will result in

reduction of surface effects, thereby allowing the detector to be thinned further.


Hor. Low S

Vert. Ideal

Vert. Low S

Vert. High S

Figure 3.5.5: R0 for a horizontal (Low S, Sn = 105 cm s−1, Sp = 100 cm s−1

and S0 = 2000 cm s−1) and vertical diode with varying surface

recombination velocities (Low S, Sn = Sp = 100 cm s−1 and S0 =

2000 cm s−1, High S, Sn = Sp = 5000 cm s−1 and S0 = 20000

cm s−1).

Mirror 2

Mirror 1

Illumination

ModeProfile

Detector Thickness Ld

p

i

n

x=0.4

x=0.4x=0.3

Figure 3.5.6: Schematic of a RCE p-i-n structure.

80 3.6. Technologies for Growing RCE Structures

3.5.3.3 Reduced Auger Recombination

Increased operating frequency and improved quantum efficiency are both well known

advantages of RCE detectors. The improved noise characteristics is an interesting by-

product but recombination at surfaces and interfaces is an issue in this case (see Fig.

3.5.3). Perhaps the most non-traditional benefit of RCE detectors in IR imaging appli-

cations is the possibility of using the resonant cavity effect to confine the absorption of

photons to a very thin region that is totally within the space charge region of a photo-

voltaic detector. This is illustrated in Fig. 3.5.6, which shows a mode-profile maxima in

the absorber region, which is intrinsic. The p and n regions above and below the absorber

region will cause a built-in field, causing any carriers generated in the intrinsic region to

be swept apart and collected. Furthermore, only the absorber layer is sensitive to the

resonant wavelength, and hence a higher composition is used for the p and n regions, as

well as part of the intrinsic region. The advantage then is that there are very few free

carriers due to the combination of depletion region and wider band-gap material. The

Auger mechanisms and the radiative recombination mechanism are therefore suppressed,

in a way similar to the extracted heterostructure devices discussed in section 3.2.1, leaving

SRH recombination as the dominant mechanism, which can be reduced for high quality

material, resulting in lower dark current [55]. This is illustrated in Fig. 3.5.7, which

shows R0A as a function of 1000/T for the RCE structure of Fig. 3.5.6 compared with a

standard n-on-p diode structure shown in Fig. 2.2.2(a). The R0A of the RCE structure

is dominated by the SRH generation and recombination mechanism at low temperature

and is not limited by the diffusion current from the surrounding neutral material until a

much higher temperature than the standard structure. Both structures show similar per-

formance at low temperature as the thickness of the intrinsic region in the RCE structure

was matched to the thickness of the depletion region of the standard structure to illus-

trate the difference in performance. The RCE structure can allow the intrinsic region of

a p-i-n photodiode to be very short, or not required at all, depending only on the natural

depletion region, making fabrication easier, while still guaranteeing that all absorption

still occurs within the depletion region.

3.6 Technologies for Growing RCE Structures

There are a number of techniques for realising RCE detectors. However, methods which

involve thinning an absorber and hybridizing the thinned absorber layer into a resonant

cavity (for example methods similar to the HDVIP technology developed by DRS [97]

for thinning and hybridizing Hg(1−x)Cd(x)Te to silicon ROIC substrates) cannot thin the

absorber layer to the thicknesses required for high finesse applications such as multi-

and hyper-spectral sensing. Therefore, methods which grow thin layers are preferred.

Growth methods such as molecular beam epitaxy (MBE) and metal-organic chemical

vapour deposition (MOCVD) are ideal for this application as these methods produce


2 4 6 8 10 12 14100

101

102

103

104

105

106

107

108

109

R0A

( c

m2 )

1000/T (K-1)

RCE structure R0A

Diffusion component x=0.4 G-R component x=0.4 Standard R

0A

Diffusion component x=0.3 GR component x=0.3

Figure 3.5.7: R0A as a function of 1000/T for a RCE detector with structure

shown in Fig. 3.5.6 compared with a standard n-on-p diode struc-

ture shown in Fig. 2.2.2(a). Also shown are the various components.

high quality crystalline films that can range from a few nanometers to micrometers in

thickness.

Growth techniques such as MBE and MOCVD are also able to grow multiple material

systems in one chamber, allowing the possibility of growing dielectric mirror stacks, ab-

sorber layers and spacer layers all in one process, without needing to transfer samples

between tools. Any material system that is to be considered for growth of RCE detectors

needs to meet certain conditions. The most important condition for a material system

is that there is a good lattice match between the dielectric mirror layers, spacer and ab-

sorber layers. This reduces stress and decreases dislocation densities, resulting in higher

quality absorber layers. Another important consideration, is the refractive index of the

various materials to be used, especially for the dielectric mirror. The ratio of refractive

indexes nH

nLshould be large, if possible. This will allow a broader rejection range for

the Fabry-Perot filter, and also allow higher reflectivity using fewer layers. The material

system used for this work is Hg(1−x)Cd(x)Te. The dielectric mirror is fabricated using

CdTe and Hg(0.6)Cd(0.4)Te, while the spacer is CdTe and the absorber is Hg(0.7)Cd(0.3)Te.

All these layers were deposited by MBE. The top mirror can be deposited by MBE, or

can be subsequently added by other deposition techniques using more traditional optical

material systems such as Ge/SiO Bragg reflectors deposited by thermal deposition.

82 3.6. Technologies for Growing RCE Structures

Chapter 4Staggered Dielectric Mirrors

4.1 Introduction

In resonant-cavity-enhanced (RCE) structures the mirrors are a critical part of the design.

Generally, for IR applications the absorption in metal mirrors precludes their use for

Fabry-Perot cavities, and therefore dielectric mirrors are required. Figure 4.1.1 shows the

proposed RCE structure that will be realised in this work, indicating the absorber layer

between two mirrors. As the absorber is to be grown by MBE, one of the mirrors will

also need to be grown by MBE as a lattice matched template and to simplify fabrication,

as illustrated by the Hg(0.6)Cd(0.4)Te/CdTe mirror (mirror 1). For such RCE designs

in which the critical absorber layer is grown on top of one of the mirrors, the mirror

design becomes particularly important and must meet a number of requirements that are

sometimes mutually exclusive: the reflectivity for the mirror must be high enough to allow

resonance, and the spectral bandwidth of the reflector must be broad enough to cover

the window for the required application. The mirror must also provide a good crystalline

surface on which to grow the absorber layer. The material system needed to achieve a high

quality crystalline absorber determines the refractive indices available for the dielectric

mirrors, which can have implications for spectral bandwidth and reflectivity. This chapter

investigates the design of mirrors fabricated from the HgCdTe/CdTe material system,

their growth and also investigates how such mirrors will survive subsequent processing

steps.

84 4.2. Modelling of Mirrors

Cavity L

ength

l

SubstrateCdZnTe

BacksideIllumination

Absorber Hg Cd Te (x=0.3)(1-x) (x) d

Mirror 2

Mirror 1

SiOGe

SiO

Ge

CdTe

CdTeSpacer

Ge

Hg Cd Te(1-x) (x) (x=0.4)

Hg Cd Te(1-x) (x) (x=0.4)

Figure 4.1.1: Proposed structure for RCE HgCdTe detector. A x = 0.3 absorber

layer grown on a HgCdTe/CdTe dielectric mirror, with a Ge/SiO

DBR added after detector fabrication.

4.2 Modelling of Mirrors

4.2.1 Quarter-wave Stack

Dielectric mirrors can achieve very high (near unity) reflectivity with very minimal losses

due to absorption in the mirror material. The basic principles of a quarter-wave stack

(QWS) reflector was covered in section 3.3. Using the matrix methods outlined in ap-

pendix B, the peak reflectivity of a QWS reflector is given as [84]:

R2N =

1 − ns

ni

(

nH

nL

)2N

1 + ns

ni

(

nH

nL

)2N

2

(4.2.1)

where ni is the refractive index of the incident medium, ns is the refractive index of the

exit medium, and the alternating layers of the high and low refractive index materials of

the mirror having refractive indices nH and nL, respectively. The number of repetitions

of the alternating layers is given as N periods, so that the total number of layers in the

stack is given by 2N +1. The high reflectivity zones occur at integer multiples of g = λ0

λ ,

and are symmetric with full-width at half-maximum given by 2∆g with

∆g =2

πsin−1

(

nH − nL

nH + nL

)

(4.2.2)

CHAPTER 4. Staggered Dielectric Mirrors 85

As can be seen from Eqns. 4.2.1 and 4.2.2, the refractive indices of the dielectric materials

are critical in determining the reflectivity and optical bandwidth of the QWS reflector. If

the ratio of the refractive indices, nH

nL, is close to unity (i.e. the values of the high and low

refractive indices are close), then the reflectivity will be lower and the highly reflective

region will be narrower. The reflectivity can be increased by increasing the number of

layers, but for QWS reflectors, the width of the highly reflective region cannot be increased

unless the high-to-low refractive index ratio is increased and hence the material system

changed.

For the Ge/SiO material system the ratio of refractive indices is approximately 3. There-

fore, this mirror system produces high reflectivity over a wide region. In comparison, the

Hg(0.6)Cd(0.4)Te/CdTe material system has a refractive index ratio of approximately 1.2,

which results in a narrow reflective peak, and requires a large number of layers to attain

a highly reflective mirror. This is illustrated in Fig. 4.2.1 where a 15 layer mirror in both

material systems is modelled. The Ge/SiO mirror has near unity reflectivity for almost

the entire MWIR transmission window, whereas the Hg(0.6)Cd(0.4)Te/CdTe mirror only

reaches a reflectivity of 0.85 over a spectral region of a few hundred nanometers. Maximum

reflectivity can be improved by using more layers, but the narrow spectral width of QWS

mirrors fabricated in the Hg(0.6)Cd(0.4)Te/CdTe material system is an issue that must be

overcome to attain a RCE detector that is applicable over the whole MWIR region. For

wavelengths shorter than 3 µm the HgCdTe material in the Hg(0.6)Cd(0.4)Te/CdTe mirror

is absorbing at a temperature of 80K, which is the cause of the poor reflectivity in this

region and is indicated by the absence of side lobes. This effect can be used effectively as

a long pass filter, since the mirror will only function for wavelengths longer than 3 µm.

4.2.2 Staggered Dielectric Mirrors

In order to overcome the narrow spectral width issues associated with using low refractive

index ratio HgCdTe/CdTe quarter-wave Bragg reflectors outlined in section 4.2.1, varying

the individual layer thickness has been investigated. Heavens [98] proposed arithmetically

and geometrically varying layer thicknesses in order to increase the bandwidth of dielec-

tric mirrors fabricated from materials with a low ratio of refractive indices. Equation

4.2.3 describes a geometric progression in the optical thickness (tO) of each layer of the

dielectric mirror. In this design, the optical thickness of a layer depends on the initial

optical thickness (t), the common ratio (Cr) or common difference (Cd), the total number

of layers (n), and the layer number (Ln). Equation 4.2.4 describes an arithmetic progres-

sion. The common ratio (of layer thicknesses) of the geometric progression usually ranges

between 0.95 and 1.05, while the common difference varies between -0.05 and +0.05 for

the arithmetic progression [98].

tO = C(n−d)r t (4.2.3)

tO = t [1 + (n− d)Cd] (4.2.4)

d = n− Ln + 1 (4.2.5)


0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.500.0

0.2

0.4

0.6

0.8

1.08 7 6 5 4 3 2

Reflectivity

1/ ( m-1)

15 Layer Ge/SiO 15 Layer Hg

0.6Cd

0.4Te/CdTe

Wavelength ( m)

Figure 4.2.1: Calculated reflectivity as a function of inverse wavelength showing

comparison of 15 layer QWS reflectors fabricated from the Ge/SiO

(solid) and Hg(0.6)Cd(0.4)Te/CdTe material systems.

Staggering the mirror thicknesses with either arithmetic or geometric progressions can

increase the spectral width of the reflector as now there are layers within the stack that

are close to a quarter wavelength in optical thickness over a range of wavelengths. While

the quarter-wave condition is met for a larger range of wavelengths, the reflectivity is

reduced, as there is no single wavelength where the many layers meet the quarter-wave

condition.

Figure 4.2.2 illustrates the increase in bandwidth for a geometrically varying reflector of

31 layers of Hg(0.6)Cd(0.4)Te/CdTe with initial quarter-wave thickness corresponding to

a wavelength of 3.4 µm (to achieve reflection centered at 4 µm) and common ratio of

Cr = 1.01 spanning the quarter-wave range from 3.4 µm to 4.6 µm, compared with a

Hg(0.6)Cd(0.4)Te/CdTe quarter-wave stack of 31 layers with a design wavelength of 4 µm.

As the figure illustrates, the spectral bandwidth of the geometric reflector is greater than

that of the quarter-wave stack (1 µm compared to 500 nm). However, the reflectivity

is decreased substantially (≈ 0.7 compared to about≈ 0.95) and, despite the increased

spectral width, the entire MWIR transmission band (wavelengths from 3-5 µm) is still

not covered. One method to increase the spectral width of the reflector is to increase the

number of layers, since each layer is thicker than the previous layer. To cover the entire

MWIR spectral window with a common ratio of Cr = 1.01 more than 45 layers would be

required in a geometric series design.


0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.500.0

0.2

0.4

0.6

0.8

1.08 7 6 5 4 3 2

Reflectivity

1/ ( m-1)

31 Layer QWS 31 Layer Geometric stack

Wavelength ( m)

Figure 4.2.2: Reflectivity as a function of inverse wavelength, solid line: Quar-

ter wave stack using Hg(0.6)Cd(0.4)Te/CdTe, 31 layers. Dashed

line: Asymmetric geometrically varying dielectric stack mirror us-

ing Hg(0.6)Cd(0.4)Te/CdTe, 31 layers, initial thickness of 3.4 µm,

common ratio Cr = 1.01.

Rather than increase the number of layers to increase the spectral width, the common ratio

(difference) could be increased. As the common ratio (difference) increases, each layer in

the stack increases in thickness at a faster rate. This further reduces reflectivity, as there

are greater spectral ranges that are not near a quarter-wave layer thickness in the stack.

This is illustrated in Fig. 4.2.3. Using a geometrically varying Hg(1−x)Cd(x)Te/CdTe

mirror with a common ratio of Cr = 1.01, the wavelength range from approximately 3.6

to 4.4 µm can be covered. Using a common ratio of Cr = 1.02 the wavelength range

from approximately 3.2 to 5 µm can be covered. It should also be noted that a higher

common ratio also results in more fluctuations in reflectivity across the spectral band,

which is generally not desired. For the Hg(1−x)Cd(x)Te/CdTe material system a common

ratio higher than Cr = 1.02 results in a rapid decay in reflectivity and a rapid increase in

fluctuations in reflectivity across the spectral band. This essentially means that common

ratios are restricted to below 1.02 for this material system.


0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.88 7 6 5 4 3 2

Reflectivity

1/ ( m-1)

Common Ratio = 1.02 Common Ratio = 1.01

Wavelength ( m)

Figure 4.2.3: Reflectivity as a function of inverse wavelength, Geometrically vary-

ing dielectric stack mirror using Hg(0.6)Cd(0.4)Te/CdTe with 31 lay-

ers, initial thickness of 3.4 µm, solid line: common ratio Cr = 1.01,

dashed line: common ratio Cr = 1.02.

4.2.3 Phase Variations

There is a phase change associated with the reflection of light from a quarter-wave dielec-

tric stack mirror. For mirror stacks starting and ending with a layer of the high refractive

index material (i.e. of the form HLH) there is a phase change on reflection of π, while for

mirror stacks starting and ending with a layer of the low refractive index material (i.e.

of the form LHL) there is a phase change on reflection of zero. As the wavelength shifts

away from the design wavelength the phase change varies until at the wavelengths where

the stack acts as an antireflection coating (null reflection point in Fig. 4.2.1 for exam-

ple) the phase change of the stack is ±π/2. A property of a quarter-wave stack mirror

with a large ratio between the refractive indices of the layers is that the phase change

on reflection, as a function of wavelength, is close to π or zero for most of the reflective

region. For material systems with a low ratio between the two refractive indices (for

example Hg(1−x)Cd(x)Te/CdTe) the variation is much more pronounced, as illustrated in

Fig. 4.2.4, which shows more rapid phase changes in the Hg(1−x)Cd(x)Te/CdTe material

system than is seen in the Ge/SiO material system, which has a higher ratio of refractive

index.


1800 2000 2200 2400 2600 2800 3000 3200

2.0

2.5

3.0

3.5

4.0

4.5

6 5.5 5 4.5 4 3.5 3

Phase

Wavenumber (cm-1)

Ge/SiO QWS MCT QWS

Wavelength ( m)

Figure 4.2.4: Phase as a function of inverse wavelength, solid line: Quarter wave

stack using Hg(0.6)Cd(0.4)Te/CdTe with 17 layers. Dashed line:

Quarter wave stack using Ge/SiO with 17 layers.

1800 2000 2200 2400 2600 2800 3000 3200

2.0

2.5

3.0

3.5

4.0

4.5

6 5.5 5 4.5 4 3.5 3

Phase

Wavenumber (cm-1)

QWS Geometric Stack

Wavelength ( m)

Figure 4.2.5: Phase as a function of inverse wavelength, solid line: Quarter

wave stack using Hg(0.6)Cd(0.4)Te/CdTe with 17 layers. Dashed

line: Geometrically varying staggered dielectric reflector using

Hg(0.6)Cd(0.4)Te/CdTe with 17 layers.


The phase variations become more complex for dielectric stack mirrors with varying layer

thicknesses. Figure 4.2.5 illustrates the comparison between a quarter-wave stack reflector

and a staggered dielectric reflector. The separation between the nulls about the design

wavelength is increased (in accordance with the extended spectral bandwidth of the re-

flector), but there are complex phases variations introduced into the stack. These are

apparent in the ripples in the phase response at higher wave numbers. These ripples must

be taken into consideration when designing a staggered dielectric mirror for a Fabry-Perot

resonator. They can be beneficial, however, as they afford a wide range of phase changes

in a narrow spectral window, and can be responsible for resonances within the cavity

other than the simple modes due to the cavity spacing.

4.2.4 Final Mirror Design

It should also be noted that a quarter-wave stack mirror consisting of 31 layers of

Hg(1−x)Cd(x)Te/CdTe designed for a center wavelength in the MWIR transmission win-

dow would be approximately 20 µm thick. Applying this technology for longer wavelength

ranges or for varying layer thicknesses would require even thicker mirrors. Practical de-

position of such a thick Bragg reflector by MBE will be difficult due to variations during

growth such as variations in the growth rate, beam equivalent pressure (resulting in

compositional change) and substrate temperature. To maintain the reflector to practi-

cal thicknesses, a 17 layer staggered geometrically varying dielectric reflector is used for

modelling and proof of concept. The total thickness of this reflector is approximately 4

µm. This mirror was designed using the principles listed above using a common ratio of

Cr = 1.017, which is high enough to provide extended range, but still less than Cr = 1.02,

beyond which mirror performance deteriorates rapidly for the HgCdTe/CdTe material

system. The mirror layer thicknesses are illustrated in Fig. 4.2.6. The initial layer optical

thickness was also lowered to 3.0 µm, in-order to reduce the growth time, and thereby

decrease the variation of growth conditions as much as possible. The model reflectance

and phase response of the designed mirror are illustrated in Fig. 4.2.7. The layers are

modelled as Hg(0.6)Cd(0.4)Te/CdTe layers at 80K following the design listed in Fig. 4.2.6.

It should be noted that CdTe was used as the incident medium and exit medium, as

the mirror is to be used between the Cd(0.96)Zn(0.04)Te substrate (which has a similar

refractive index to CdTe) and the CdTe spacer layer. Due to the trade-offs used to make

the design more feasible to grow, the mirror reflectance isn’t high, and has a value of

approximately 0.7. There is some broadening exhibited in the spectral bandwidth, and

the phase response shows a number of variations throughout the high reflective region,

including one close to the design wavelength near 3.5 µm, however, the mirror represents

a fair trade-off between ease of growth and mirror reflectivity.


237.9 nm284.2 nm247.4 nm294.2 nm257.1 nm304.5 nm266.8 nm315.1 nm276.9 nm326.1 nm287.1 nm337.5 nm297.6 nm349.3 nm308.4 nm361.5 nm319.6 nm

Hg Cd Te0.6 0.4

CdTeHg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdZnTe Substrate

Figure 4.2.6: Layer thicknesses of the designed mirror: Geometrically varying

dielectric stack mirror with 17 layers, initial thickness of 3.0 µm,

common ratio Cr = 1.017

0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

0.00.10.20.30.40.50.60.70.80.9

1.5

2.0

2.5

3.0

3.5

4.0

4.5

6 5 4 3 2

Ref

lect

ance

Wavenumber 1/ ( m-1)

Pha

se

Wavelength ( m)

Figure 4.2.7: Modelled reflectance and phase of the designed mirror: Geometri-

cally varying dielectric stack mirror with 17 layers, initial thickness

of 3.0 µm, common ratio Cr = 1.017. Layers are Hg(0.6)Cd(0.4)Te

and crystalline CdTe modelled at 80K.

92 4.3. HgCdTe/CdTe Mirror Growth

4.3 HgCdTe/CdTe Mirror Growth

The Hg(1−x)Cd(x)Te/CdTe mirror outlined in section 4.2.4 was grown by molecular beam

epitaxy (MBE), which is discussed in appendix C. Before growth the substrate must be

prepared, first with wet processing and then an in-situ surface preparation. The growth

parameters used during growth of the mirror layers were determined during characterisa-

tion growths prior to mirror growth.

4.3.1 Substrate Preparation

The growth process starts with a CdZnTe substrate. The composition of the substrate

was selected so that the lattice spacing matches the lattice constant of Hg(0.7)Cd(0.3)Te

material. The substrate orientations used for this work were all (211)B orientation. The

CdZnTe substrates were acquired from Nikko Materials, Japan, and all substrates were 1

cm × 1 cm in area.

For each growth, the following substrate preparation was undertaken. The substrate was

first cleaned using two warm acetone baths. All acetone was then removed from the

substrate by a sequence of at least two methanol baths. This is essential as acetone

and bromine used in the next step can combine to leave a residue on the surface of the

substrate. The surface was then etched with a Br/Methanol dip etch. The concentra-

tion of the dip etch was 0.1% and was prepared by adding 0.1 mL of Br to 100 mL of

methanol and will remove approximately 100 nm in 10 s. The substrate was then washed

in methanol twice and then flushed in de-ionised water. The substrate was mounted onto

a molybdenum block, and was held on the block by liquid gallium. The block with sub-

strate was loaded into the MBE chamber, and out-gassed by heating to approximately

150C.

4.3.2 Growth

4.3.2.1 In-situ Substrate Surface Preparation

Before growth of Hg(1−x)Cd(x)Te commences, there is an initial preparation stage where

the substrate surface is prepared for nucleation. Initially the RHEED pattern for a freshly

loaded substrate is spotty, indicating poor crystallinity at the surface, due to an amor-

phous layer of tellurium left after the bromine etch. To remove this layer, the substrate

was heated to 190C to clean the substrate of excess tellurium. Once the RHEED pat-

tern indicates this was complete the substrate was further heated to 250-300C under a

tellurium flux to balance tellurium evaporating from the substrate. The bake at this step

has the effect of thermally cleaning the substrate and allowing further outgassing. There

is also some surface reconstruction providing a better surface for subsequent growth. The

substrate was left at this temperature for ten to fifteen minutes before being cooled to

the growth temperature of around 185C.


4.3.2.2 Flux Measurements and Growth Characterisation

The growth rate depends on the molecular beam fluxes of CdTe, Hg, and Te, which are

proportional to the beam equivalent pressure from the relevant effusion cell. The cells

used for the growth of Hg(1−x)Cd(x)Te for this work depend on the composition of the

layer to be grown. For the x = 0.4 mirror layers one tellurium cell, two cadmium telluride

cells and the mercury cell were used. For the CdTe mirror layers the tellurium cell shutter

is closed. This results in some incorporation of mercury into the CdTe mirror layers, but

as HgCdTe is grown on the tellurium limit, this incorporation is minimal [99], and results

in a composition higher than x = 0.95. The composition of all layers was measured for

these samples using SIMS (see section 4.4.2.2).

The absorber layer was grown using only one of the two cadmium telluride cells, the

tellurium cell and the mercury cell. Beam equivalent pressures were set so that one

cadmium telluride cell gave a composition of x = 0.3, while opening the second cell

increased the composition to x = 0.4. Thus the two compositions could be grown without

the need to change the cell temperatures during growth. This is a major advantage, as

there is a significant time required for cell temperatures to stabilise, during which time

no growth can proceed.

The beam equivalent pressures were initially calculated by growing a characterisation

sample. This sample was then analysed by FTIR transmission and the cut-off wavelength

determined in a similar manner to Fig. 2.5.2(a) and the layer thickness determined from

the interference fringes (by direct analysis or by fitting a thickness and using regres-

sion to determine the best thickness fit to the interference fringes). Using the thickness

calculated from the characterisation sample, the beam equivalent pressure measured for

the cell temperature used for growth, and the composition determined from transmission

measurements, a new set of growth conditions can be determined to achieve the desired

composition; as well as estimating the expected growth rate.

4.3.2.3 Mirror Growth

Several fifteen layer mirror stacks (mirror number 1 in Fig. 4.1.1) were grown at the

University of Western Australia using a Riber 32 molecular beam epitaxy system. The

stacks were grown on (211)B oriented CdZnTe substrates. The system was calibrated

to grow material with a composition of x = 0.4. During growth, the RHEED pattern

indicated good two-dimensional growth of HgCdTe layers in the mirror stack, however at

the start of each HgCdTe layer there were some indications that twin orientations were

present. The RHEED patterns for the CdTe layers of the stack were blurry, indicating

poor crystallinity or three-dimensional growth. This is to be expected, as the CdTe layers

were grown at ≈ 180C, which is lower than the optimum temperature for CdTe. The low

growth temperature was chosen as it is the optimum growth temperature for the HgCdTe

layers, and the lower temperature limits out diffusion of Hg, as well as interdiffusion of


Figure 4.3.1: SEM micrograph of as grown mirror structure taken using a Zeiss

1555 SUPRA Variable Pressure FESEM using an accelerating volt-

age of 1 kV. The brighter layers are the HgCdTe, while the darker

layers are CdTe.

Hg and Cd in the mirror layers. The HgCdTe layers still returned to crystalline growth

on these CdTe layers after the initial twinned regions discussed above.

Figure 4.3.1 is an SEM micrograph of a cross section of the as-grown layers, showing

sharp interfaces with the lighter stripes being the HgCdTe. The dark region at the top of

the image is the photo-resist that was used to protect the sample when it was cleaved for

the SEM micrograph. The thicknesses of the individual layers were measured from the

SEM micrograph. The general geometric trend in thickness was maintained, although the

common ratio between the layer thicknesses was less than expected, most likely due to

variations in the beam fluxes during growth, which resulted in a decreasing growth rate as

the growth progressed, and effectively making the stack appear closer to a quarter-wave

stack.


HgCdTeSample

QuartzTube

QuartzCaplet

QuartzRod

Hg Reservoir

Figure 4.3.2: Schematic of the apparatus used for annealing of HgCdTe samples

in a Hg atmosphere.

4.3.3 Annealing

Even extrinsically doped as-grown MBE layers of Hg(1−x)Cd(x)Te are generally of mixed

conduction type, typically requiring an anneal to convert the mixed type material to one

uniform type [100]. Furthermore, some types of damage introduced during fabrication

steps can be reduced by annealing [101, 102] so that annealing is a common process step in

Hg(1−x)Cd(x)Te detector fabrication. There are various methods for annealing; typically

the process is used to produce n-type Hg(1−x)Cd(x)Te, activate dopants or reduce damage,

and involves annealing in a Hg atmosphere to fill Hg vacancies and thereby producing

n-type material. In contrast, vacancy-doped p-type material is formed by annealing under

vacuum or low Hg overpressure conditions, resulting in the introduction of vacancies into

the material.

Annealing at UWA is performed in quartz ampoules, formed from two quartz tubes, one

placed inverted inside the other (see Fig 4.3.2). All quartz glass wear is cleaned in a

HF:H2O solution prior to use, and ensures that impurities are minimised. The ampoule is

evacuated by a turbo-molecular pump, which is separated from the ampoule by a liquid

nitrogen trap. Once the pressure inside the ampoule is sufficiently low, the ampoule is

sealed by melting the quartz tubes with an O2-H2 flame. Care is taken during the sealing

process to keep the sample temperature low. The sample is placed within two quartz

caplets (Fig. 4.3.2) to protect the sample and prevent mercury droplets from coming into

contact with the sample. The mercury reservoir is also placed within two quartz caplets,

and separated from the sample by a quartz rod.

The anneal is performed in an oven that is preheated to the desired annealing temperature.

The ampoule is placed into the oven with the sample end of the ampoule located at the

centre of the oven, which is the hottest region. The thermocouple controlling the oven


temperature is placed close to this end of the ampoule. The sample end of the ampoule

is also raised above the mercury reservoir, to prevent mercury droplets from forming on

the sample. The reservoir end is placed next to the oven door allowing the reservoir end

to cool first, which ensures that mercury condenses preferentially at the reservoir end.

The anneal temperatures used in this work varied from 200C to 250C for times up to

24 hours.

4.3.4 Interdiffusion Modelling

During annealing phases of the device fabrication, the Hg(1−x)Cd(x)Te/CdTe multi-layer

mirror structure will be subject to interdiffusion of Hg. At the interface between the

Hg(1−x)Cd(x)Te and CdTe layers of the mirror there is a flux of Hg atoms into the CdTe

layer, matched by a flux of Cd atoms flowing into the Hg(1−x)Cd(x)Te [103]. The rate of

this interdiffusion is a function of the composition (C) and self-diffusion coefficient of Hg

(D), and is given by Fick’s second law (Eqn. 4.3.1). For the case where the self-diffusion

coefficient is dependent on the composition (as in Hg(1−x)Cd(x)Te), Eqn. 4.3.1 expands to

Eqn. 4.3.2 [104], which gives the rate of change of composition as a function of the spatial

(z) compositional variation, and the change of inter-diffusion coefficient with changing

composition.

∂C

∂t=

∂

∂z

(

D∂C

∂z

)

(4.3.1)

∂C

∂t= D

∂2C

∂z2+∂D

∂C

(

∂C

∂z

)2

(4.3.2)

Using a finite difference numerical model [105], Eqn. 4.3.2 rearranges to a finite dif-

ference equation for the concentration as a function of position and time [106, 103]:

Cn+1j = rn

j

(

Cnj−1 + Cn

j+1 − 2Cnj

)

+ Cnj +

∆t

4∆z2

(

Cnj+1 − Cn

j−1

) (

Dnj+1 −Dn

j−1

)

(4.3.3)

rnj = Dn

j

∆t

∆z2(4.3.4)

∆t ≤ ∆z2

2Dnj

(4.3.5)

where Cnj is the composition at the jth position and the nth time step, Dn

j is the self-

diffusion co-efficient at position j in time period n, ∆t is the increment in time, and ∆z

is the size of the spatial step.

There has been much investigation into the diffusion coefficient of Hg in Hg(1−x)Cd(x)Te.

Shaw et al. [107] gives an overview of the results and describes models for the diffusion

coefficient:

D (x, T ) = D0 (x) exp−Q (x)

kT(4.3.6)

where x is the mole fraction, T is the temperature in K, andD0 (x) and Q (x) are functions

that have been determined empirically [107, 108, 109, 110]. Kim et al. [109] gives the


expressions for D0 (x) (in cm2s−1) and Q (x) (in eV) as:

D0 (x) = 24 exp−37.5x (4.3.7)

Q (x) = 1.82 − 1.50x (4.3.8)

It is interesting to note that there is a very wide variation in the self-diffusion coefficient

of Hg in Hg(1−x)Cd(x)Te reported in the literature. The largest cause of the variation

seems to be the growth method, with MBE resulting in diffusion coefficients that are 2-4

orders of magnitude lower than LPE and MOCVD [111, 109].

Using the finite difference model described by Eqn. 4.3.3, and the diffusion coefficient from

[109], the effects of annealing a Hg(0.578)Cd(0.422)Te/CdTe mirror stack under a mercury

atmosphere at 250C for two and twenty hours have been modelled. The results for

the two hour anneal are shown in Fig. 4.3.3(a), which shows that the interface exhibits

some grading, and a shift of the interfacial edge by a few nanometers. This is compared

to Fig. 4.3.3(b), which shows the results of annealing for 20 hours, indicating that the

interdiffused region expands from 2-5 nm after two hours to 10-20 nm after annealing for

20 hours. It should be noted that the graded region on the CdTe side of the interface

appears to be quite abrupt, which can be seen in the model results (Fig. 4.3.3(a), for

example). The non-symmetrical grading profile occurs because the interdiffusion is driven

by Hg atoms exchanging places with Cd [107] and therefore higher Cd molar compositions

result in lower diffusion, causing the Cd side of the interface to appear to have a more

abrupt grading profile.

The layer structure is generally unaffected (Fig. 4.3.4), but there is some movement

of Hg from the Hg(0.578)Cd(0.422)Te to the CdTe layer. As the self-diffusion coefficient

of Hg is greater in the lower x material, Hg is depleted from a wider region of the

Hg(0.578)Cd(0.422)Te layer, but has a greater effect over a narrow region of the CdTe layer,

making the grading in the CdTe appear more abrupt. This has the effect of appearing to

move the interface between the two layers, effectively broadening the Hg(0.578)Cd(0.422)Te

layer by approximately 10 nm. The grading of the Hg(0.578)Cd(0.422)Te layer results in

less of a change in composition, but this change occurs over a much wider area. The net

region of grading is approximately 45 nm in width after annealing for 20 hours. In regard

to the performance of a mirror that has been designed using a Hg(1−x)Cd(x)Te/CdTe

stack, this grading will reduce the reflectivity of each interface, resulting in reduced total

reflectance of the system. This is illustrated in Fig. 4.3.5, which shows the reflectance of

one interface of Hg(0.578)Cd(0.422)Te and CdTe under various annealing conditions. There

is a small decrease in reflectance after 10 and 20 hours annealing, however the change in

reflectance due to varying operating temperature is greater than the change in reflectivity

due to annealing. This is seen as a much larger change in reflectivity due to decreasing

the operating temperature from 300K to 80K than the change that would result from 20

hours annealing. Therefore, it is expected that the effects of annealing on reflectance will

be minimal.


-40 -20 0 20 400.0

0.2

0.4

0.6

0.8

1.0M

olar

ratio

x

Displacement z (nm)

As Grown 2 hours anneal

at 250o C

(a)

-40 -20 0 20 400.0

0.2

0.4

0.6

0.8

1.0

Mol

ar R

atio

x

Displacement z (nm)

As grown 20 hours anneal

at 250o C

(b)

Figure 4.3.3: Results of finite difference modelling, simulating an anneal of a

Hg(1−x)Cd(x)Te/CdTe structure at 250C under a Hg atmosphere.

(a) One interface of CdTe and Hg(0.578)Cd(0.422)Te, after 2 hours.

(b) One interface of CdTe and Hg(0.578)Cd(0.422)Te, after 20 hours.


0 50 100 150 200 250 300 3500.0

0.2

0.4

0.6

0.8

1.0

Mol

ar ra

tio x

Displacement z (nm)

As Grown 20 hours anneal

at 250o C

Figure 4.3.4: Results of finite difference modelling of one layer of

Hg(0.578)Cd(0.422)Te with CdTe on either side, simulating an

anneal at 250C under a Hg atmosphere for 20 hours.

0.15 0.20 0.25 0.30 0.350.0035

0.0040

0.0045

0.00508 7 6 5 4 3

Reflectan

ce

Wavenumber 1/ ( m-1)

As-grown at 300K As-grown at 80K Anneal 10 hours at 80K Anneal 20 hours at 80K

Wavelength ( m)

Figure 4.3.5: Modelled reflectance of a simple Hg(1−x)Cd(x)Te0.422/CdTe inter-

face. The reflectance of an abrupt interface at room temperature

is compared with the reflectance of an abrupt interface at 80K.

Also shown is the modelled reflectance at 80K of an interface after

annealing either 10 hours or 20 hours at 250C.

100 4.4. Experimental Results

Figure 4.4.1: Measured room temperature transmittance through mirror stack

MCT75 (dotted line) compared to modelled transmittance through

the stack using x = 0.422 (solid line).

4.4 Experimental Results

The samples grown for this work all follow the same preparation and growth steps outlined

in section 4.3.2. The sample design and growth parameters are outlined in table 4.4.1.

4.4.1 Mirror-MCT75

The mirror stack MCT-75 was grown as outlined in sections 4.3.1 and 4.3.2, and was

characterised on a Sopra GES-5 FTIR ellipsometer used in normal incidence mode for

measurements of transmission. Figure 4.4.1 shows the measured transmittance of the

stack at T = 300K, as well as modelling results for a stack having thicknesses as measured

by SEM (Fig 4.3.1) and using fitted molecular composition for the Hg(1−x)Cd(x)Te layers

of x = 0.422. This x value is very close to the target value of x = 0.4 used in the initial

design. A comparison of the measured spectral transmittance data with the model shows

that the transmittance drops substantially for wavelengths between 3 and 4 µm (3333 and

2500 cm−1). This is due to the mirror successfully reflecting these wavelengths. The other

main feature is the absorption edge at approximately 2.6 µm (3750 cm−1). Transmittance

drops to zero for wavelengths shorter than this, as the mirror structure is now absorbing.

CH

AP

TER

4.

Sta

ggere

dD

iele

ctric

Mirro

rs101

Table 4.4.1: Sample designations and growth conditions used in characterisation of mirror structure and CdTe refractive index.

Sample Designation Structure Design Substrate Temp (C) Cell Temps (C) Mirror Design Parameters

MCT75

Fifteen layers

Hg(0.6)Cd(0.4)Te/CdTe

(total ≈ 5µm) as in Fig. 4.2.6

183

Te - 312

CdTe - 527

Hg - 90.3

Cr = 1.017

initial thickness = 3.4

µm

MCT105Absorber layer Hg(0.7)Cd(0.3)Te

≈ 8µm, CdTe Cap ≈ 200 nm182

Te - 314

CdTe1 - 514

Hg - 96.9

In - 450

NA


The agreement between the measured data and the model is quite good at longer wave-

lengths, however this agreement deteriorates at shorter wavelengths. A possible explana-

tion for this disagreement is compositional change across the stack. The model assumes

uniform composition in all the x = 0.422 layers, and this may be inaccurate. Regions of

lower x can occur due to increased Hg uptake on transients when opening and closing

shutters at the interfaces between the CdTe and HgCdTe layers, resulting in increased

absorption in these regions and explaining the disagreement between measured and model

results at shorter wavelengths. Lastly, the shift in the position of maxima and minima

indicate that the refractive indices of the materials used in the stack differ from the model

values. The value of CdTe refractive index used in the model results of Fig. 4.4.1 was

based on crystalline CdTe, while the as-grown material has a lower refractive index (as the

maxima and minima are shifted to shorter wavelengths), which is investigated in section

4.4.3.

4.4.2 Annealing

As-grown MBE material generally contains mixed conduction regions. In order to fab-

ricate photoconductive devices on a mirror stack, these regions in the absorber must be

converted to n-type. This is traditionally done by annealing, as is described in section

4.3.3. However, the elevated temperatures used during annealing may cause interdiffusion

of the Cd and Hg at the HgCdTe/CdTe interfaces, as modelled in section 4.3.4. This will

cause graded interfaces, potentially degrading reflector performance. In order to inves-

tigate experimentally if the mirror stacks would still function after annealing, a sample

was annealed at 250C in a Hg atmosphere, and the FTIR transmittance was measured

after 2, 7 and 24 hours of annealing.

4.4.2.1 Transmittance

Figure 4.4.2 shows the measured transmittance after each annealing period. An important

thing to note from this figure is the dependence of the cutoff wavelength on annealing

time, near 2.66 µm (3750 cm−1). The band-gap of the x = 0.422 layers (the CdTe

has a wider band-gap) appears to be changing since the cutoff of the stack is changing.

As the sample is annealed for longer times, the absorption edge is shifting to shorter

wavelengths, suggesting that the composition, x, of the material is increasing. This is

reasonably explained by Hg diffusing from Hg(1−x)Cd(x)Te to CdTe, causing an increase

in the effective x values of the Hg(1−x)Cd(x)Te layers. Also, in the wavelength range 3 to

4 µm (3333 to 2500 cm−1), the transmittance is only marginally affected by the anneal,

and there is some increase in transmittance, suggesting that the reflectivity of the mirror

stack degrades as annealing time increases. The degradation in reflectance is likely due

to interdiffusion causing graded interfaces, the extend of which has been investigated

using SIMS (see section 4.4.2.2). Despite this degradation, the layers in this mirror are

still distinct after 24 hours of annealing, and the mirror still exhibits regions of high and


low refractive index, in agreement with the interdiffusion modelling results presented in

section 4.3.4. In terms of a mirror for resonant cavity applications, a 5% decrease in

reflectivity will result in approximately a 10% decrease in finesse, if the mirror reflectivity

was less than 0.8 to begin with. For reflectivities higher than 0.8, the decrease in finesse

is more dramatic.

Figure 4.4.2: Measured room temperature transmittance as a function of

wavenumber through mirror stack MCT75 before anneal and af-

ter 2, 7 and 24 hours annealing at 250C in a Hg atmosphere.

4.4.2.2 Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry measurements were performed on a Cameca IMS 5f

dynamic SIMS instrument, located at the Australian Nuclear Science and Technology

Organisation (ANSTO), Lucas Heights. All measurements were performed using Cs+ ions

to sputter the sample, at a beam current of 41nA. The mass spectrometer was calibrated

to detect the ions outlined in table 4.4.2

The raw results of SIMS consists of an ion yield (counts/second of each ion) for each

increment of sputtering time. This ion yield is first normalised to the amount of the Cs+

ions (in counts/second), resulting in the actual yield of the various ions under study (in

counts/second). The composition is then extracted from the ion yields by the ratio of the


Table 4.4.2: Ions detected during SIMS measurements

Ion Mass Charge State133Cs 132.905 Cs+

133Cs106Cd 238.811 CsCd+

133Cs114Cd 246.809 CsCd+

133Cs123Te 255.81 CsTe+

133Cs130Te 262.812 CsTe+

133Cs202Hg 334.876 CsHg+

yield of CsCd+ to the yield of CsTe+ [112]:

xSIMS = γsCsCd+

CsTe+ (4.4.1)

γs =0.96

(

CsCd+

sub

CsTe+sub

) (4.4.2)

where γs is a calibration factor, and is obtained from analysis of a known molar com-

position (in this case the substrate was used). The Cd and Te ion yields from the

Cd(0.96)Zn(0.04)Te substrate are denoted CsCd+sub and CsTe+

sub, respectively. It should

be noted that the ion yield was measured as a function of sputter time, which was then

associated with a sputter depth by measuring the total sputter depth (in this case the

mirror stack thickness was measured using SEM Fig. 4.3.1 and correlated to the sputter

depth). The sputter time is then converted to a depth based on this known depth. The

sputter rate is dependant on the molar composition, and the results of Sheng et al. [112]

were used to correct for the variation in sputter rate due to compositional variation.

The extracted composition of mirror stack MCT-75 as-grown is shown in Fig. 4.4.3. At

the surface the composition is initially x = 0.422, and as sputtering proceeds alternating

layers of CdTe and Hg(0.578)Cd(0.422)Te are encountered. There is some incorporation of

Hg in the CdTe layers, and this is apparent in the measured composition which, for the

nominally CdTe layers, causes the material to become Hg(1−x)Cd(x)Te with x ≈ 0.95.

Annealing mirror stack MCT-75 causes some interdiffusion as illustrated in Fig. 4.4.4,

which plots the composition as a function of sputter depth for sample MCT-75 after an-

nealing for 2,7 and 20 hours, as well as the as-grown composition. There is some variation

in the layer thicknesses, and also some grading of the interfaces between the layers, as ex-

pected from the interdiffusion modelling results (section 4.3.4). There is evidence of asym-

metric interdiffusion, which is more clearly illustrated in Fig. 4.4.5, showing the molar

composition of the top three layers of mirror stack MCT-75 as-grown and after 2, 7 and 20

hours annealing. There are two interfaces between CdTe and Hg(1−x)Cd(x)Te layers; the

CdTe on Hg(1−x)Cd(x)Te layer at approximately 625 nm depth, and the Hg(1−x)Cd(x)Te

on CdTe layer at approximately 325 nm depth. As growth proceeds from the substrate

(at approximately 4450 nm depth) the CdTe on Hg(1−x)Cd(x)Te layer is grown first. By

the end of the growth of the Hg(1−x)Cd(x)Te layer the crystal structure has recovered from


0 1000 2000 3000 4000 50000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

CompositionCd

mol

ar c

ompo

sitio

n

Sputter Depth (nm)

Compositions used in design

Figure 4.4.3: Molar composition of sample MCT75 as-grown determined by

SIMS.

0 1000 2000 3000 4000 50000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cd

mol

ar c

ompo

sitio

n

Sputter Depth (nm)

As-grown 2 hours anneal 7 hours anneal 20 hours anneal

Figure 4.4.4: Molar composition of sample MCT-75, as measured by SIMS. Plot-

ted are compositions measured after annealing for 2,7, and 20 hours,

as well as the as-grown composition.


previous growth issues such as twinning, outlined in section 4.3.2.3, and exhibits good

crystalline structure. The CdTe layer grown on this Hg(1−x)Cd(x)Te starts initially with

good structure. As growth proceeds, however, the crystal structure of the CdTe degrades

due to the non-optimum growth temperature, and the next Hg(1−x)Cd(x)Te layer starts

with twinning and perhaps also a high defect density. As the self-diffusion co-efficient

of Hg is dependant on defect density, there is expected to be more diffusion of Hg at

the Hg(1−x)Cd(x)Te on CdTe interface, than at the CdTe on Hg(1−x)Cd(x)Te interface,

and this is confirmed by the asymmetric molar composition profile in Fig. 4.4.5. The

self-diffusion co-efficient at the CdTe on Hg(1−x)Cd(x)Te interface is in good agreement

with the model results in section 4.3.4, although the SIMS data is not of sufficient reso-

lution to extract the actual self-diffusion co-efficient. The self-diffusion co-efficient at the

Hg(1−x)Cd(x)Te on CdTe interface is greater than the values reported by Kim et al. [109],

though it is still substantially lower than those reported for LPE and MOCVD material

[111]. Figure 4.4.6 shows the results of interdiffusion modelling, when the compositional

interdiffusivity is increased twenty fold, Eqn. 4.3.7 becomes

D0 (x) = 20 × 24 exp−37.5x (4.4.3)

There is good agreement between the model result and the measured result, in terms of

the amount of diffusion of Hg from the Hg(0.578)Cd(0.422)Te layer to the CdTe layer, but

this model still does not explain the grading within the CdTe layer. A more advanced

model of interdiffusion in the presence of varying defect densities is needed to establish

the actual self-diffusion co-efficient within this material.

4.4.3 Refractive Index

In order to further investigate the cause of the mismatch between the model transmission

spectrum and the measured transmission spectrum (Fig. 4.4.1) two further samples were

prepared consisting of an 8 µm Hg(0.63)Cd(0.37)Te layer capped with approximately 200

nm of CdTe, grown under the same conditions used for the mirror stack. The refrac-

tive index of samples was measured by ellipsometry on a Sopra (Bois-Colombes, France)

GES-5 FTIR ellipsometer, which is used for spectroscopic ellipsometric measurements at

various incident angles, with measurements taken at 65 and 70 incidence angles. The

tanψ and cos δ [113] results of measurements taken at 65 for sample MCT105 are shown

in Fig. 4.4.8. There are two distinct regions visible in both the tanψ and cos δ curves. For

wave numbers greater than 3500 cm−1, the Hg(0.63)Cd(0.37)Te layer is absorbing, and the

only interfaces that contribute to the reflection measurement are the top and bottom in-

terfaces of the CdTe layer. For wave numbers less than 3500 cm−1, the Hg(0.63)Cd(0.37)Te

layer is transparent, and the substrate interface is also measured in reflection, resulting

in interference fringes from the substrate reflection and the CdTe/Hg(0.63)Cd(0.37)Te re-

flection. Figure 4.4.8 also shows model results for tanψ and cos δ based on the values of

refractive index for Hg(0.63)Cd(0.37)Te and homogenous CdTe. There is a clear shift in the

model data with respect to measured data, suggesting that the refractive indices used in


0 100 200 300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 HgCdTeCdTe

Cd

mol

ar c

ompo

sitio

n

Sputter Depth (nm)

As-grown 2 hours anneal 7 hours anneal 20 hours anneal

HgCdTe

Growth Direction

Figure 4.4.5: Molar composition of one Hg(0.578)Cd(0.422)Te layer, and one CdTe

layer of sample MCT-75, as measured by SIMS.

the model are incorrect. In particular, the refractive index of homogenous CdTe at longer

wavelengths does not match experimental values, as the interference fringes show more

divergence from the model results at these wavelengths.

In order to determine the cause of the difference in refractive index between the as-grown

CdTe layers and the homogenous CdTe values, a Cauchy model was used to initially

determine the extent of the change in refractive index. The Cauchy model is a simplified

model on which the Sellmeier model is based [114], describing the dispersion in refractive

index as a function of wavelength as an inverse power series. It assumes that absorbtion

resonances are at much shorter wavelengths than the wavelength range of interest (i.e.

the material is transparent), which for CdTe is a valid assumption. The Cauchy model

calculates the real and imaginary parts of the dielectric function (εr and εi) using [114]:

n = A+B

λ2+C

λ4(4.4.4)

k =D

λ+E

λ3+F

λ5(4.4.5)

The refractive index of the HgCdTe layer was derived from previously published models

made up of the real part described by Rolland [115] and modified by Daraselia [116], and

the imaginary part derived from the absorption model of Price [63]. The refractive index

of the as-grown CdTe was extracted from the tanψ and cos δ data using WinElli software,

which is also produced by Sopra.


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 100 200 300 400 500 600

300 200 100 0 -100 -200C

d m

olar

com

posi

tion

Sputter Depth (nm)

Model As-grown Model 20 x D

MCT

Measured

Distance From Interface (nm)

Figure 4.4.6: Compositional grading of Hg(0.578)Cd(0.422)Te on CdTe interface af-

ter 20 hours annealing, compared with modelled interdiffusion re-

sults using a diffusion coefficient that is 20 times larger than re-

ported by Kim et al. [109].

8 mm

100-200 nm

Hg Cd Te1-x x

CdTe

CdZnTe Substrate

Figure 4.4.7: Schematic of the structure used to measure the refractive index of

the as-grown CdTe.

Based on initial results using the Cauchy model to extract the refractive index and

RHEED results during growth, a mixing model combining the refractive index of ho-

mogenous CdTe (supplied from a model fitted to published experimental measurements

[117]) with the refractive index of voids (nominally 1 for all wavelengths) was used to

provide a physically representative model. The effective medium approximation [118]

was used to combine the two refractive indices, and is the most common formula for

low concentrations of voids. The choice of the mixing model was based on several ob-

servations. RHEED patterns observed during growth of the CdTe layers suggest that

the layers have some three-dimensional structure. Two-dimensional growth gives rise to

streaky diffraction patterns, as recorded during growth of the HgCdTe layers, while large


1000 2000 3000 4000-1

0.00.10.20.30.40.50.60.70.80.91.0

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

1

Tan(

)

Wavenumber (cm-1)

Measured Tan( ) Model Tan( )

Cos

()

Measured Cos( ) Model Cos( )

Figure 4.4.8: Results of room temperature ellipsometry measurements taken on

sample MCT105

blurry spots were observed on RHEED patterns during growth of CdTe layers, indicating

three-dimensional structure. Secondly, cross-sectional images of mirror structures show

voids around 10-50 nm in size in the CdTe layers (discussed in section 4.4.3.1. While

these may be artifacts of the cleave, no such voids were seen in HgCdTe mirror layers.

Lastly, X-ray diffraction measurements indicate that the CdTe layer is a single crystal

epitaxial layer with nothing to indicate a crystallographic reason for the change in refrac-

tive index. It is assumed that the CdTe grows in some type of three dimensional growth

mode and is then subsequently overgrown by the HgCdTe layers (the RHEED pattern

becomes streaky again during HgCdTe growth, after initially exhibiting spottiness and

twin streaks), leaving voids in the material. This has been observed in other material

systems grown by MBE [119], however, further investigation is needed to determine if

this is the growth mechanism in this case.

The two different models for the CdTe layer are fitted to two functions, cos 2ψ and

sin 2ψ cos δ simultaneously (tanψ and cos δ functions produce better fits for thinner films).

The fitting was performed over the wavelength range from 2 µm to 7.7 µm and the fits

produced by these two models are shown in Figs. 4.4.9(a) and 4.4.9(b). The results of the

Cauchy model provide a better fit to the measured data than those of the mixing model,

based on the error between measured and extracted results, however both fits show some

deviation from the model data. At wavenumber 3800 cm−1 there is a discrepancy as the

band edge of the HgCdTe layer is modelled as being overly abrupt. Between wavenumbers

1500 and 3000 cm−1 there is divergence in the interference fringes, the amplitude of which

is due to a small difference in the refractive index of the HgCdTe layer (negligible at less


2000.0 3000.0 4000.0-1.0

-0.975

-0.95

-0.925

-0.9

-0.875

-0.85

-0.825

2000.0 3000.0 4000.0

-0.4

-0.3

-0.2

-0.1

0.0

Sin(2PSI)Cos(DELTA)

Measured Data Fit

Wavenumber.(cm-1)

Cos(2PSI)

Wavenumber.(cm-1)

Cauchy

(a)

2000.0 3000.0 4000.0

-0.975

-0.95

-0.925

-0.9

-0.875

-0.85

-0.825

2000.0 3000.0 4000.0

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

Sin(2PSI)Cos(DELTA)

Measured Data Fit

Wavenumber.(cm-1)

Cos(2PSI)

Wavenumber.(cm-1)

Mixing

(b)

Figure 4.4.9: (a) Measured results for ellipsometry on MCT105 (circles). Mea-

surement performed at 65 incidence angle. Model results (solid

line) using a Cauchy parametric model to fit the refractive index of

the CdTe layer. (b) Measured results for ellipsometry on MCT105

(circles). Measurement performed at 65 incidence angle. Model

results (solid line) using the mixing model for CdTe with 10.9%

volume of voids.

than 2 %) and an offset that is due to dispersion in the CdTe layer reducing the refractive

index further than has been modelled.

The Hg(0.63)Cd(0.37)Te layer thickness extracted from the fit to the Cauchy model is

8.45 ± 0.00578 µm and the CdTe layer is 0.142 ± 0.00265 µm thick, which is in good

agreement with the measured thickness of 135 nm obtained from X-ray analysis illustrated

in Fig. 4.4.10 [120]. The standard deviation of error of the Cauchy model is 9.835 ×10−4. The Hg(0.63)Cd(0.37)Te layer thickness extracted from the fit to the mixing model

is 8.41 ± 0.00325 µm thick and the CdTe layer is 0.153 ± 0.00216 µm thick, and the

concentration of voids by volume is 10.9% ± 0.6%. The standard deviation of error of

the mixing model is 1.174 × 10−3. For both models the fit for the sin 2ψ cos δ curve is

much better than the fit for the cos 2ψ curve. The regions above wave number 3500

cm−1 and below wave number 2000 cm−1 for the cos 2ψ curve show divergence from the

fit. The region above wave number 3500 cm−1 suffers from poor signal-to-noise during

measurement, though clearly there are problems with the result in this region as the band

edge of the Hg(0.63)Cd(0.37)Te has much lower dispersion in the measured results than in

the model fit, perhaps suggesting that this region of the HgCdTe refractive index model


35.50

1

10

counts

per

second

100

1000

35.60

135nm R=0

R=0.77

R=1

Theta angle (degrees)35.70 35.80

experiment

35.90

fit

Hg0.08Cd0.92Te

8.5µmHg0.63Cd0.37Te

800µmCd0.96Zn0.04Te

=> Thickness

CdTe

HgCdTe CdZnTe

Figure 4.4.10: 2θ − ω x-ray scan of MCT105 plotted over θ. Also shown is a fit

to the data using the parameters shown in the inset [120].

needs adjustment. The model used in this work was chosen in the interest of simplicity,

and a more detailed semiconductor oscillator model could be used for a better fit [121].

The refractive indices extracted from the various models are plotted in Fig. 4.4.11, and

compared with the refractive index of crystalline CdTe and Hg(0.63)Cd(0.37)Te. The re-

fractive indices extracted for both the Cauchy and mixing models are lower than the

refractive index for crystalline CdTe. The Cauchy model is more accurate as it has a

larger decrease in the refractive index at long wavelengths (6-10 µm) than the mixing

model, hence a more accurate mixing model is needed to account for the larger decrease

in refractive index at long wavelengths. Further investigation is required to establish the

relation between substrate temperature during MBE growth and refractive index of the

CdTe layers, and also to establish the growth mechanism of these layers. The reduced re-

fractive index of the CdTe layers is actually beneficial for dielectric mirrors. As illustrated

in Fig. 4.4.11, the ratio between the high refractive index (HgCdTe) and low refractive

index (CdTe) is increased, thereby improving mirror performance (increased reflectivity

over a wider spectral bandwidth), by lowering the refractive index of the CdTe layers

within the mirror.

The decrease in refractive index of the CdTe layers explains some of the differences be-

tween results of modelling using homogenous CdTe and measured results of the mirror

stack. Figure 4.4.12 illustrates the measured transmittance data compared with model

transmittance data using model homogenous CdTe refractive index and the reduced CdTe

refractive index. The layer thicknesses are as measured from SEM images, but have been

adjusted by 10 nm (within the measurement error, as the scale of the image used to es-

tablish the thickness is 5 nm per pixel) to create a better fit to the measured data. Using

refractive index extracted from the fit to the ellipsometric data using the mixing model


2 4 6 8 10

1.5

2.0

2.5

3.0

3.5R

efra

ctiv

e In

dex

Wavelength ( m)

Hg0.63

Cd0.37

Te model Homogenous CdTe model CdTe/Void mixing model CdTe Cauchy model

Figure 4.4.11: Comparison of the refractive indexes of Hg(0.63)Cd(0.37)Te, ho-

mogenous CdTe (based on the model), CdTe based on the Cauchy

fit, and CdTe based on the mixing of CdTe and 10.9 % voids.

1000 2000 3000 4000 50000.0

0.2

0.4

0.6

0.8

1.010 8 6 4 2

Tran

smittan

ce

Wavenumber (cm-1)

Measured transmission 15 layer mirror stack

CdTe mixed with voids Homogenous CdTe

Wavelength ( m)

Figure 4.4.12: 15 layer HgCdTe/CdTe mirror sample modelled with homogenous

CdTe and also CdTe mixed with voids, compared to measured

results.


clearly gives a much better fit to the measured transmittance for the 3-5 µm window. The

model transmission is still not in agreement with the measured transmission at shorter

wavelengths, most probably related to the band edge of the Hg(0.578)Cd(0.422)Te layers of

the mirror causing dispersion of the refractive index of the Hg(0.578)Cd(0.422)Te layers (see

Fig. 4.4.11), and also an over-estimation of the imaginary part of the refractive index

model of these layers at these wavelengths. The improvement in modelled mirror per-

formance is apparent in the decrease in transmission at the resonant peak (wavenumber

2750 cm−1), as the peak reflectivity of the mirror increases from the homogenous crystal

model (≈ 0.8) to that of the measured data and the model using the mixed CdTe/void

refractive index (≈ 0.9).

4.4.3.1 Scanning Electron Microscopy

The mirror layers were investigated by scanning electron microscopy (SEM), primarily

to extract as-grown layer thicknesses, but also to inspect layer quality. The samples

were prepared for SEM by scribing the back surface of the substrate and cleaving. The

samples were studied in a Zeiss (Oberkochen, Germany) 1555 SUPRA VP-FESEM. A

very low accelerating voltage of 0.5kV was used to minimise damage to the sample, but

this limited the resolution available. Figure 4.4.13 illustrates voids that appeared in the

CdTe layers (lighter) but not in the HgCdTe layers (darker) after cleaving. It is proposed

that these holes are a by-product of the voids that occur in the CdTe during growth, and

are subsequently overgrown by the HgCdTe layers. Further study is needed to confirm

this theory, for example an extensive investigation into the layer crystalline structure

using transmission electron microscopy.

The SEM images have been analysed using ImageJ software to determine the void density

by volume. The density of voids measured was between 5.3 and 5.9 % of the CdTe layers,

while the Hg(1−x)Cd(x)Te layers were free from voids. The average void size was between

0.01 µm2 and 0.012 µm2. The density of voids extracted by ellipsometry was ≈ 11%

(see section 4.4.3), which is on the order of those extracted by SEM. However, as the

sample measured by ellipsometry (MCT-105) was different to that examined by SEM

(MCT-91). Furthermore the ellipsometry measurement represents a volume, while the

SEM measurement an area, therefore there is no absolute comparison possible.

Finally, it should be noted that there are other explanations for the variation in refractive

index in the mirror stack. The refractive index of Hg(1−x)Cd(x)Te [116] used is one of the

most recent on MBE material, but could be a source of variation. The relatively greater

thickness of the Hg(1−x)Cd(x)Te layer compared to the CdTe layer could suggest that the

source of variation lies in the Hg(1−x)Cd(x)Te layer as well as the large discrepancies at the

band edge of the HgCdTe layer, but the extensiveness of the work by Daraselia et al. and

lends weight to the variation being in the CdTe. The smoothness of the interface between

the CdTe and Hg(1−x)Cd(x)Te layers in Fig. 4.4.13 suggests that the voids observed in the

SEM image are artifacts of the cleave, but CdTe grown at the temperature used in this


Figure 4.4.13: SEM micrograph showing voids in the CdTe layers in mirror layer

sample MCT92.

work has a columnar structure, which could be inducing voids on an atomic scale which

would only be visible using a TEM study. A last source of error would be interfacial

roughness itself, which was not considered as a source of variation in this study. Given

the weight of the refractive index of Hg(1−x)Cd(x)Te used, and the off-temperature growth

conditions of CdTe, the reduced CdTe refractive index seemed the most likely result, but

much more work, including a TEM study of the CdTe and the CdTe/Hg(1−x)Cd(x)Te

interface, is needed to verify that this is indeed the fact, or establish that the refractive

index of the HgCdTe layer was the cause of discrepancies.


4.5 Conclusions

This chapter has dealt with a number of concepts related to dielectric stack mirrors.

In particular, a dielectric stack mirror fabricated in the Hg(1−x)Cd(x)Te/CdTe material

system was investigated. Mirror design issues were taken into consideration and a mirror

design which broadened the reflectance region of the mirror was established. The mirror

response can be broadened by varying the thicknesses of the mirror layers according to a

geometric or an arithmetic progression. This increases the mirror spectral width at the

expense of the peak reflectivity, and also introduces complex phase variations.

Modelling of interdiffusion of mercury has been performed, and while typical annealing

conditions will cause grading of the interface between mirror layers, the reflectance of the

layer will not be dramatically lowered.

Growth of a Hg(1−x)Cd(x)Te/CdTe mirror by molecular beam epitaxy was also investi-

gated. Mirror performance shows reasonable agreement with model data. Annealing the

dielectric mirror stacks for 24 hours at 250C in a mercury atmosphere does not substan-

tially degrade performance. Investigation of the grading of the interface layers by SIMS

shows that the amount of grading is similar to the grading determined by interdiffusion

modelling. The interdiffusion of mercury shows a dependence on the lattice structure,

with a high self-diffusion co-efficient corresponding to regions of higher defect density.

A more detailed interdiffusion model is needed to further investigate the effects of this

asymmetric interdiffusion. The refractive index of the CdTe layers was also investigated.

The CdTe grown for the mirror stacks was found to have a reduced refractive index, and

resulted in improved mirror performance. The reduced refractive index of the CdTe is

caused by the incorporation of ≈ 10% voids in the material during growth. Voids have

also been observed in SEM images. When mirror performance is re-evaluated using the

new refractive index, very good agreement between model transmittance and measured

transmittance is observed.

116 4.5. Conclusions

Chapter 5Realisation of Resonant-cavity-enhanced

Detectors

5.1 Introduction

The operation of resonant-cavity-enhanced (RCE) detectors have been described in chap-

ter 3, with the technology for fabricating staggered dielectric mirrors investigated and

described in chapter 4. This chapter takes the theory of chapter 3 and investigates the

design, fabrication, and the measured performance of a mid-wave infrared (MWIR) RCE

detector. Measured performance is then compared to modelled performance. The struc-

ture used for the modelling is shown in Fig. 5.1.1, and was investigated with and without

the Ge/SiO mirror 2 using backside and frontside illumination, respectively. This corre-

sponds to the two stages of characterisation undertaken on photoconductors fabricated

using the same structure, which were tested before and after the addition of the Ge/SiO

mirror.

5.2 RCE Detector Design and Modelling

This section discusses the design of the RCE structures. It outlines the general design and

then examines the effects of the phase changes in the staggered dielectric mirror, optimum

cavity length, and optimum positioning of the absorber layer within the structure.

5.2.1 RCE Design

There are a number of trade-offs that must be considered in the design of a RCE detector.

As outlined in section 3.3, the quantum efficiency is linked to the absorption in the cavity,

and the reflectance of the two mirrors. Furthermore, the finesse is determined by the

reflectance of the two mirrors, so device performance is now linked to both the material

118 5.2. RCE Detector Design and Modelling

Cavity L

ength

l

SubstrateCdZnTe


Absorber Hg Cd Te (x=0.3)(1-x) (x) d

Mirror 2

Mirror 1

SiOGe

SiO

Ge

CdTe

CdTeSpacer

Ge

Hg Cd Te(1-x) (x) (x=0.4)

Hg Cd Te(1-x) (x) (x=0.4)

Figure 5.1.1: Schematic RCE device, complete with the ex-situ deposited Ge/SiO

mirror 2.

properties and device dimensions in a significantly more complex relationship than for

a standard detector. Typically, where the finesse is a limiting parameter, the absorber

layer thickness is adjusted to balance the mirror reflectances. Figure 5.2.1 illustrates the

effect of absorber layer thickness on finesse for cavities optimally designed to operate at 4

µm or at 3.4 µm wavelength. As the absorption is higher at 3.4 µm, the finesse is lower.

A finesse of 10 is sufficient for multispectral imagining, therefore 100nm is sufficiently

thin to realise multispectral imaging. However, hyperspectral imaging requires a finesse

of ≈ 100. Therefore, the Hg(1−x)Cd(x)Te absorber layer would need to be very thin, of

the order of 10 nm, or a different material with a lower absorption coefficient is required

in order to achieve a finesse of 100.

As the finesse is not critical for this proof of concept device, an absorber layer thickness of

75nm was chosen, resulting in a detector with a spectral bandwidth in the range required

for multispectral applications. The reflectance of the mirrors was then calculated using

Eqn. 3.3.12, assuming unity reflectance for the back mirror (mirror 2). Using a 75 nm

Hg(0.7)Cd(0.3)Te absorber layer, the required reflectance of the top mirror (mirror 1) for

maximum absorption is 0.835 and 0.777 for wavelengths of 4µm and 3.4µm, respectively.

If a reflectance of 0.777 is used then at 3.4µm wavelength all incident light will be absorbed

in the absorber layer at the resonant wavelength, as light reflected from the cavity under-

goes a π phase change, resulting in a cancellation of all reflected components, or 100% of

energy entering the structure (despite a highly reflective mirror 1). This is illustrated by

the modelled absorptance in Fig. 5.2.2, which shows the modelled absorptance as a func-

tion of wavelength for the structure illustrated in Fig. 5.1.1. Mirror 1 is 6.5 periods (13

layers) of Hg(0.6)Cd(0.4)Te/CdTe in a quarter-wave stack designed for a center wavelength

of 3.4µm, while mirror 2 (which needs to reach unity reflectivity) could either be 21.5

periods of Hg(0.6)Cd(0.4)Te/CdTe or 2.5 periods (5 layers) of Ge/SiO. It is impractical to

CHAPTER 5. Realisation of Resonant-cavity-enhanced Detectors 119

0 20 40 60 80 1000

20

40

60

80

100

120

140

160

180

200

Fine

sse

Absorber Layer Thickness (nm)

4 m wavelength 3.4 m wavelength

Figure 5.2.1: Finesse as a function of absorber thickness, where the absorber is

Hg(0.7)Cd(0.3)Te, for cavities that are designed for optical perfor-

mance at 4 µm and 3.4 µm wavelength.

grow 21.5 periods of MCT/CdTe, and therefore the Ge/SiO material system represents

a much better option for mirror 2, and is technologically feasible. Both material systems

reach unity absorptance at the design wavelength. However, since the MCT/CdTe mirror

(mirror 1) does not have a broad region with high reflectance, there are substantial ab-

sorption lobes at other wavelengths, particularly when using the Ge/SiO material system

for mirror 2. Therefore, the methods investigated in chapter 4 for broadening the spectral

response of the MCT/CdTe mirror need to be applied, resulting in the use of a staggered

mirror design.

5.2.1.1 Staggered Dielectric Mirrors in Resonant-cavity-enhanced Detectors

Staggered dielectric mirrors can be used for RCE detectors, but the design becomes more

convoluted as the complex and rapidly changing phase variations allow resonances to occur

at wavelengths other than integer mode wavelengths. As outlined in section 4.2.3, the

phase variations of staggered dielectric mirrors can result in narrow spectral bandwidth

resonances, which have a weaker dependence on cavity length than on the wavelength of

the phase variation. Furthermore, for a resonant cavity with QWS reflectors optically

designed for the operating wavelength, the resonance generally occurs when the cavity

contains an integer number of halfwaves (i.e. m is an integer for Eqn. 3.3.1). The resonant

condition expressed in Eqn. 3.3.1 must be modified to include the phase contribution of

the staggered dielectric mirror, assuming the QWS has been designed to be centered at


2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

Abs

orptan

ce

Wavelength ( m)

Hg0.6

Cd0.4

Te/CdTe QWSmirror 2

Ge/SiO mirror 2

Figure 5.2.2: Modelled absorptance of a RCE device with a 75 nm thick

Hg(0.7)Cd(0.3)Te absorber layer. Performance for a device using a

quarter-wave stack of Hg(0.6)Cd(0.4)Te/CdTe for mirror 2 (in Fig.

5.1.1) (dashed line) is compared with that of a device using a

quarter-wave stack using Ge/SiO for mirror 2 (solid line).

the resonant wavelength:

q =φ1 (λ) + φ2 (λ) − 2δ

2π(5.2.1)

where q = 0,±1,±2, ... is the resonant mode and φ1 (λ) and φ2 (λ) are the wavelength

dependent phase changes of the two mirrors, and δ is the phase change associated with

the cavity. Complex phase variations will allow a resonant condition to be established

without the cavity containing an integer number of half waves. Figure 5.2.3 shows the

mode profile at resonance for the structure of Fig. 5.1.1 without mirror 2 but with mirror

2 formed by the CdTe spacer-thin anodic oxide-air interface. The results are shown for

front side illumination, using the thicknesses for mirror 1 outlined in Fig. 4.2.6, at the

resonant wavelength. The modelled structure is similar to the complete RCE devices,

prior the addition of the Ge/SiO mirror layers. There are approximately 1.75 standing

waves in the CdTe spacer that forms the cavity, illustrating the effect of the phase change

in the mirror. This does not make the resonant cavity any less effective at confining the

incident energy, other than the reflectivity trade-off which has been discussed in section

5.2.1, but can decrease the spectral width of the resonance. Adding mirror 2 further

complicates the phase effects within the cavity, and careful design is needed to ensure the

resonant wavelength of the structure occurs at the correct wavelength.


4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.500

1

2

3

4

5

AirAnodic Oxid

e

CdTe

SpacerCdTeHg 0.7

Cd 0.3Te

AbsorberHg 0.6Cd 0.4

Te

E/E

in

Distance from substrate ( m)

Figure 5.2.3: Mode profile of a RCE detector using a staggered dielectric for one

mirror and an air-thin anodic oxide-spacer interface for the other

mirror.

5.2.1.2 Cavity Length

The cavity length determines the resonant wavelength for traditional Fabry-Perot cavities

(Eqn. 3.3.1). The cavity length of RCE detectors with staggered dielectric reflectors has

less of an effect on resonant wavelength (for resonances occurring at the phase variations

outlined in section 5.2.1.1), but can determine the effectiveness of the resonance, as the

phase variations result in ripples in the reflectivity of the staggered dielectric mirror.

Changes in cavity length will shift the spectral position of the resonant peak by a small

amount, and can cause the reflectivity to increase, without dramatically changing the

resonant wavelength. This is illustrated in Fig. 5.2.4, which plots the absorptance of

a 75 nm thick absorber layer for cavity lengths varying from 400 nm to 900 nm, for a

structure similar to that used in section 5.2.1.1 for Fig. 5.2.3. The absorptance is low,

as the Hg(1−x)Cd(x)Te/CdTe mirror 1 is the back reflector in this case, and the anodic

oxide layer on the CdTe spacer is the front mirror, resulting in non-optimum reflection

from both front and back mirrors. There is a negligible shift in resonant wavelength (a

difference of only 100 nm across all cavity lengths) while the cavity length changes by 400

nm, but an appreciable fluctuation in absorptance, as there are changes in the reflectivity

of the mirrors and position of the maximum electric field as the resonant wavelength

changes. Figure 5.2.5 illustrates the change in the mode profile associated with changing

the cavity length from 1 µm to 3 µm.


3.50 3.55 3.60 3.65 3.70 3.75 3.800.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40Abs

orptan

ce

Wavelength ( m)

450 500 550 600 700 750 800 850 900

Increasingcavitylength

Figure 5.2.4: Modelled absorptance of a RCE cavity with a cavity length that

varies from 450 nm long to 900 nm long. The structure used is that

of Fig. 5.2.3.

0 2 4 6 80

2

4

6

8

10

12

14

16

18

20

22

24

26

E/E

in

Distance from Substrate ( m)

1 m cavity length 3 m cavity length

Figure 5.2.5: Modelled mode profile of a RCE cavity with 1 µm long cavity and

a 3 µm long cavity. The structure used is that of Fig. 5.2.3.


5.2.1.3 Absorber Layer Position

The position of the absorber layer within the resonant cavity structure is important as

positioning the absorber at a node in the standing wave pattern results in dramatically

reduced absorption. Traditionally, the absorber layer is placed in the center of a first-order

resonant cavity. However, photoconductors require the spacer layer to be non-conducting

in-order to avoid shunting the absorber layer resistance by a lower resistance spacer layer.

Although Hg(0.6)Cd(0.4)Te material can be used for the spacer, the much greater thickness

of the spacer layer in comparison to the absorber layer will result in shunting. CdTe was

therefore used as the spacer, which in turn imposes a number of restrictions on the position

of the absorber layer.

Figure 5.2.6 shows three potential absorber layer locations within the RCE structure,

with the resulting spectral response for each case shown in Fig. 5.2.7. The three different

positions considered are: the center of the cavity (DP1), at the mirror 1-spacer interface

(DP2), and at the start of the last grown layer of the HgCdTe/CdTe mirror 1 (DP3). Tra-

ditional positions for the absorber layer in RCE structures are the center of the cavity, and

at the mirror/spacer interface, with the center position being preferred. However, since

the cavity is to be made from low refractive index CdTe which creates reflections within

the cavity, placing the absorber layer in the center of the cavity results in poor resonance

at the design wavelength of ≈ 3.5 µm (2800 cm−1). It would be possible to adjust the

cavity length in order to achieve better absorption at the desired wavelength. However,

the fact that growth of HgCdTe on CdTe (as discussed in section 4.3.2.3) is initially poor,

growing the absorber layer at the mirror/spacer interface (on x = 0.4 Hg(1−x)Cd(x)Te) is

more attractive. With the absorber layer at the mirror/spacer interface, the cavity is not

split and resonates as designed, as well as providing higher quality crystal on which the

absorber layer can be grown (x = 0.4). The varying phase response from arithmetically

varying reflectors produces a second resonance at ≈ 3.15 µm (3175 cm−1). Placing the

absorber layer within the reflector allows for stronger absorption, suggesting the peak

energy density of this resonance is within the reflector. This is shown in Fig. 5.2.8,

which illustrates that the absorber layer contains a local maximum in the energy density,

as a function of distance from the air/mirror interface. The energy density within the

absorber is clearly larger than Fig. 5.2.3 due to the addition of the Ge/SiO mirror 2, and

the absorber being positioned optimally at an anti-node in the standing wave pattern.

5.2.1.4 Final Design Structure

The final design used for the complete RCE structure uses the staggered geometric di-

electric mirrors discussed in chapter 4 and a 75 nm thick absorber layer, situated on the

staggered dielectric mirror (detector position 2 (DP2) in Fig. 5.2.6). The mirror, ab-

sorber and spacer are all grown by MBE. CdTe is used for the spacer material, as the

target device for this work is a photoconductor. The second mirror is made of Ge/SiO,

which is a more traditional mirror material system, and is thermally deposited ex-situ


Cavity L

ength

l

L

L

t

b

SubstrateCdZnTe


Detector LHgCdTe (x=0.3) d

Mirror 2

Mirror 1

SiOGe

SiOGe

CdTe

HgCdTe (x=0.4)

CdTe

Spacer

Spacer

CdTe

HgCdTe (x=0.4)

Ge

DetectorPosition 1

(center of cavity)

(a)

Ca

vity L

en

gth

l

SubstrateCdZnTe



Mirror 2

Mirror 1

SiOGe

SiOGe

HgCdTe (x=0.4)

CdTe

Spacer CdTe

HgCdTe (x=0.4)

Ge

DetectorPosition 2

(mirror 1 - spacerinterface)

(b)

Cavity L

ength

l

SubstrateCdZnTe



Mirror 2

Mirror 1

SiOGe

SiOGe

HgCdTe (x=0.4)

CdTe

Spacer CdTe

HgCdTe (x=0.4)

Ge

DetectorPosition 3

(within mirror 1)

(c)

Figure 5.2.6: Proposed structure for RCE HgCdTe detector, showing three pos-

sible absorber locations. A x = 0.3 absorber layer grown on a

HgCdTe/CdTe DBR, with a Ge/SiO DBR added after detector

fabrication. (a) DP1, (b) DP2, and (c) DP3.


DP 1

DP 2

DP 3

Figure 5.2.7: Absorptance of a 75 nm absorber layer in the middle of a 1.3 µm

cavity (DP1), at the spacer/mirror interface (DP2), and at the start

of the last grown mirror layer (DP3).

1.50 1.75 2.00 2.25 2.50 2.75 3.000

5

10

15

20

Ge Anodic

Oxid

e

CdTe

CdTe

Hg 0.7Cd 0.3

Te

E/E

in

Distance from air/mirror interface ( m)

Hg 0.6Cd 0.4

Te

Figure 5.2.8: Mode profile of a RCE detector on a staggered dielectric mirror,

with a Ge/SiO mirror.


237.9 nm284.2 nm247.4 nm294.2 nm257.1 nm304.5 nm266.8 nm315.1 nm276.9 nm326.1 nm287.1 nm337.5 nm297.6 nm349.3 nm308.4 nm361.5 nm244.6 nm

Hg Cd Te0.6 0.4

CdTeHg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdZnTe Substrate

75.0 nmHg Cd Te0.7 0.3

1.5 mm

GeSiOGeSiO

471.6 nmGe

CdTe

471.6 nm204.0 nm

471.6 nm204.0 nm

HgCdTe/CdTestaggereddielectricmirror 1

Spacer

Absorber

Ge/SiOQWSdielectricmirror 2

Figure 5.2.9: Layer thicknesses of the designed RCE detector structure (not to

scale).

to the MBE growth. The final design structure, including all layer thicknesses, is pre-

sented in Fig. 5.2.9. The absorptance of the absorber layer as a function of wavelength

of the designed structure is given in Fig. 5.2.10. Due to the design restraints (broader re-

sponse from the staggered dielectric mirror in a limited number of layers to reduce growth

time/thickness), the peak absorptance only reaches 90% as the mirror reflectivities are

not quite matched. The use of staggered dielectric mirrors using Hg(0.6)Cd(0.4)Te/CdTe

places the resonant peak close to the band edge of the Hg(0.6)Cd(0.4)Te material, so there

is a small amount of absorption in the mirror material. The resonance is still sufficient

to provide a proof-of-concept for Hg(1−x)Cd(x)Te RCE detectors.

5.2.2 Responsivity

The responsivity of RCE detectors are modelled in a two step process: Firstly, the ab-

sorptance of the absorber layer is calculated using characteristic matrix methods outlined

in appendix B, in particular the potential transmittance (appendix B.1.3) through the

absorber layer is used to determine the absorptance. The absorptance is then used as the

quantum efficiency to calculate the responsivity (Eqn. 2.5.10), assuming that the inter-


2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

Abs

orptan

ce

Wavelength ( m)

Figure 5.2.10: Modelled absorptance of the absorber layer as a function of wave-

length for the designed RCE structure shown in Fig. 5.2.9.

nal quantum efficiency of Hg(1−x)Cd(x)Te is unity [122]. Other Hg(1−x)Cd(x)Te material

properties used in modelling the responsivity are outlined in appendix A.

The responsivity of a RCE 50 × 50 µm2 photodetector, at 80K with no surface recombi-

nation and a voltage bias of 36 V/cm is given in Fig. 5.2.11. The RCE structure used is

that of the final design outlined in Fig. 5.2.9, making the absorber layer 75nm thick. The

main features of the responsivity match the absorptance of the structure (Fig. 5.2.10),

but are adjusted by the responsivity profile of the Hg(0.7)Cd(0.3)Te material (Fig. 2.2(b)

for example) with increasing responsivity as the wavelength approaches the cutoff of the

absorber layer (x = 0.3, λco = 5.1 µm at 80K).

5.3 MBE Growth

Preparation and growth of the RCE structures is very similar to the growth steps outlined

in sections 4.3.1 and 4.3.2 for the mirror test structures. The RCE structure was grown

using the same 17 layer mirror design determined in chapter 4, and is completed by

including the absorber layer ( Hg(0.7)Cd(0.3)Te) and the spacer layer (as illustrated in Fig.

5.2.2). The spacer layer was grown either (i) after a 30 minute anneal of the absorber

layer in-situ to the MBE growth chamber at the growth temperature (≈ 185C) in an

elevated mercury flux, or (ii) directly on the absorber layer with no in-situ anneal. The

samples with no in-situ anneal of the absorber layer were subsequently annealed ex-situ

for 20 hours in a process identical to that used for the mirror layers, and described in

128 5.3. MBE Growth

2 3 4 5 60

5

10

15

20

25R

espo

nsiv

ity (x

104 V

/W)

Wavelength ( m)

Figure 5.2.11: Modelled responsivity of a 50× 50 µm2 photoconductive detector,

at 80K with no surface recombination and a bias of 36 V/cm.

CdTe Spacer

CdZnTe Substrate

HgCdTe/CdTe

Staggered

Dielectric

Mirror 1

Photoresist

Figure 5.3.1: SEM micrograph of as grown mirror structure. The brighter layers

are the HgCdTe, while the darker layers are CdTe.


section 4.3.3. It should be noted that in both cases the spacer layer had a very spotty

RHEED pattern at the end of growth of this layer, indicating either three dimensional

crystal growth, or polycrystalline material.

The as-grown RCE structure was investigated using a Zeiss 1555 SUPRA Variable Pres-

sure FESEM at an accelerating voltage of 0.5 kV. The structure, shown in Fig. 5.3.1,

shows similar features as the SEM images of the mirror layer (Fig. 4.3.1), with the

addition of the spacer layer. The absorber layer is unfortunately not apparent as high

magnification imaging was not possible without inducing damage in the Hg(1−x)Cd(x)Te

due to the high accelerating voltage. Furthermore, the absorber layer is not apparent at

lower magnifications as there is negligible variation in the SEM imaged response between

x = 0.4 material and x = 0.3 material as the dielectric functions of the two compositions

are quite similar. A number of defects are visible in the image, which are most likely due

to process-induced damage while cleaving the sample to facilitate imaging.

5.4 Photoconductor Fabrication

5.4.1 Fabrication

A schematic cross-section of the as-grown structure is given in Fig. 5.4.1(a). The sub-

sequent processing steps are illustrated in Figs. 5.4.1(b) to 5.4.1(d). Firstly, the devices

were isolated by performing a mesa isolation etch in Br/HBr (Fig. 5.4.1(b)). Secondly,

the spacer layer was removed from the contacts in a second Br/HBr etch (Fig. 5.4.1(b)).

The depth of this etch was carefully controlled and monitored in order to stop the etch as

close to the absorber layer as possible, and certainly within the top Hg(0.6)Cd(0.4)Te layer

of the mirror. Thirdly, the surface of the sample was passivated with anodic oxide [34]

(Fig. 5.4.1(c)). Anodic oxide grows at different rates on Hg(1−x)Cd(x)Te, depending on

the composition, x, of the material, leading to variations in the colour of the thin anodic

oxide across the device. Figure 5.4.2 illustrates the colour variations and shows the grown

oxide. There are bands of purple (light grey in black and white image) corresponding to

the thicker oxide grown on x = 0.3 areas. These areas are just penetrating the spacer and

allow contacts to the absorber layer to be formed. The yellow (white in black and white

image) areas correspond to CdTe for the spacer and CdZnTe for the substrate in the mesa

isolation areas. The profile of an 80 µm long photoconductor after the spacer etch was

measured using a Dektak II scanning profilometer and is displayed in Fig. 5.4.3. The

spacer etch (Fig. 5.4.1(b)) was performed in 2 steps to obtain the right etch depth, hence

the double step in the profile of the spacer region. The valleys between the contact and the

spacer lead to the purple rings in Fig. 5.4.2, clearly indicating that the absorber layer is

just being exposed by the spacer layer etching process. Finally, the contacts were formed

by evaporating indium in a thermal deposition system (Fig. 5.4.1(d)). After deposition

of In contacts the devices were characterized. Subsequent to this initial characterisation,

the Ge/SiO DBR was subsequently added by thermal deposition of Ge and SiO. This


was done after bonding to allow characterisation of the device with and without the final

mirror, as well as simplifying the process. The final structure is shown in Fig. 5.4.1(e).

5.4.2 Device Layout

Photoconductors are usually square in area, enabling the device resistance to remain

constant, while varying the optical area. However, as has been outlined in section 2.5.2,

RCE photoconductors, with very thin absorber layers, will have a substantially larger

resistance than non-RCE photoconductors. While this is not an issue for ideal devices,

in the presence of surface/interface recombination the thermal noise due to the device

resistance can become the dominant noise mechanism. In an effort to study this, a

number of rectangular devices were designed, as well as square devices. The final layout

is shown in Fig. 5.4.4. The mask has a number of features that differ from common

photoconductor device layouts. Firstly, and most noticeably, are the remote contacts for

the photoconductors. Photoconductors usually have one common contact, and the other

contact is located at the other side of the photoconductor. However, as the devices are

to be characterised before the Ge/SiO mirror is deposited, there is a need to prevent the

gold balls used during bonding from shadowing the photoconductor during deposition

of the Ge/SiO mirror, hence the remote contacts. The large blank area on the right of

the patterned area is for electrically contacting (clamping) the sample during the growth

of anodic oxide. There are two types of devices on this layout, circular Van der Pauw

structures for Hall measurements, and rectangular photoconductors. There are seven

types of photoconductors, repeated 4 times. There are three devices with an optical area

of 4.0× 10−4 cm2, one with dimensions of 1000 µm × 40 µm (width × length), one with

dimensions of 500 µm × 80 µm, and one with dimensions of 200 µm × 200 µm. There

are two square photoconductors with dimensions of 100 µm × 100 µm and dimensions of

50 µm × 50 µm. Finally there are two other photoconductors that vary optical area as

well as resistance, one device with dimensions of 500 µm × 160 µm, and one device with

dimensions of 500 µm × 320 µm.

5.5 Experimental Results

The samples grown for this work all followed the same preparation and growth steps

outlined in section 4.3.2. The sample design and growth parameters are outlined in table

5.5.1. Structures were assessed using a variety of measurements tools: Transmission

was measured using FTIR spectroscopy, and cross sections examined using SEM. After

device fabrication the devices were characterised using current-voltage measurements,

responsivity measurements and probed using a scanning laser microscope (SLM).


Ca

vity L

en

gth

l

SubstrateCdZnTe

Mirror 1

SpacerCdTe

Detector Hg Cd Te L0.7 0.3 d

CdTe

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

(a)

SubstrateCdZnTe

Mirror 1

SpacerCdTe


CdTe

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

(b)

SubstrateCdZnTe

Mirror 1

SpacerCdTe


CdTe

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Spacer

Oxide

CdTe

Oxide

(c)

SubstrateCdZnTe

Mirror 1

Spacer

Oxide

ContactCdTe


CdTe

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

(d)

SubstrateCdZnTe


Mirror 1CdTe

Mirror 2SiOGe

SiOGe

Ge

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Spacer

Oxide

CdTe

(e)

Figure 5.4.1: Schematic of device cross-section after each processing step. (a) As-

grown structure showing 17 layer staggered dielectric reflector with

an x = 0.3 absorber and a CdTe spacer. (b) Mesa isolation and

CdTe spacer etch performed in Br/HBr. (c) Oxide growth: varying

colour in oxide represents different oxide thickness due to different

alloy composition. (d) Windows opened in oxide for indium con-

tacts: deposited by thermal deposition. (e) Final structure with

distributed Bragg reflector (mirror 2) added ex-situ.


Figure 5.4.2: 50×50µm2 photoconductor after anodic oxide is grown, and before

indium contacts are added. Anodic oxide on CdTe (and on CdZnTe

substrate) is yellow in colour (white), while anodic oxide grown on

Hg(0.7)Cd(0.3)Te is purple in colour (light grey).

0 50 100 150 2000

2

4

6

8Contact

Hei

ght (

m)

X position ( m)

Spacer Contact

Figure 5.4.3: Topographical profile of 80 × 500µm2 photoconductor after the

spacer layer has been etched.

CH

AP

TER

5.

Realisa

tion

ofR

eso

nant-c

avity

-enhanced

Dete

cto

rs133

8

1 2 3 4 5 6 7

Device: 1: 40 x 1000 4: 100 x 100 7: 320 x 500

2: 80 x 500 5: 50 x 50 8: Hall

3: 200 x 200 6: 160 x 500

Figure 5.4.4: Masks used to fabricate photoconductors. Device dimensions are listed in µm.

134

5.5

.Experim

enta

lR

esu

lts

Table 5.5.1: Sample designations and growth conditions used in this work.

Sample Designation Structure Design Substrate Temp (C) Cell Temps (C) Annealing conditions

MCT79

Nineteen layers



183

Te - 312C

CdTe - 527C

Hg - 90.4C

In-situ anneal under Hg

flux at 185C

MCT92

Nineteen layers



182.5

Te - 331C

CdTe1 - 473.5C

CdTe2 - 525.5C

Hg - 96.4C

Annealed 20 hours in

a Hg atmosphere at

250C

MCT95

Nineteen layers



182.5

Te - 331C

CdTe1 - 480C

CdTe2 - 522.5C

Hg - 95.8C

Annealed 10 hours in

vacuum at 250C


SiCGlow Bar

Mirrors

BeamPath

Device UnderTest

MonochromatorGratings

Chopper

Figure 5.5.1: Schematic of system used for responsivity measurements.

5.5.0.1 Responsivity Measurements

All responsivity measurements, unless stated, were performed on an Optronic Laborato-

ries Infrared Spectroradiometer 756. The system is outlined in Fig. 5.5.1, but is essen-

tially a light source, a monochromator, and collimating/focusing optics. The double pass

monochromator has two grating turrets, with each turret holding three gratings, one with

a blaze angle for 1 µm peak wavelength using 200 grooves per mm, one with a blaze angle

for 4 µm peak wavelength using 150 grooves per mm, and one with a blaze angle for 8

µm peak wavelength using 50 grooves per mm. The monochromator has slits at both the

beam entrance and beam exit. Unless stated, the slits were operated at the maximum

width of 5 mm. The light source is a SiC ceramic glow bar, with an operating current

of about 7 A. The output to the device under test is collimated and can exit either hori-

zontally or vertically. All measurements were taken using the horizontal exit. The signal

from the device under test is amplified with a Stanford Research Systems SR560 low noise

voltage amplifier, and then further amplified by an Optronic 756 lock-in amplifier. To

facilitate lock-in signal amplification there is a mechanical chopper inserted in the beam

path.

The dewar that the samples are placed in has a ZnSe window, through which the incident

light must pass. There is reflection at the surfaces of this window, which results in a

loss of light transmitted onto the sample. The transmission through the ZnSe window is

plotted in Fig. 5.5.2, and shows that the transmission across all wavelengths of interest

is constant, and can be taken as the loss due to Fresnel reflection at both surfaces of the

window, resulting in transmittance through this window of 0.7. The modelled responsivity

must therefore reflect this loss of incident light and this relationship is given by [123]:

Rλfitted =1

0.7Rλmeasured (5.5.1)


500 1000 1500 2000 2500 3000 3500 40000.0

0.2

0.4

0.6

0.8

1.020 15 10 5

Tran

smittan

ce

Wavenumber (cm-1)

Wavelength ( m)

Figure 5.5.2: Transmittance of the ZnSe window used on dewar for responsivity

measurements.

5.5.1 MCT-79 - Without Ge/SiO Mirror

Sample MCT-79 has the device structure given in section 5.2.1.4 and was grown using

techniques outlined in section 5.3. After growing the absorber layer, growth was paused

for an in-situ anneal at the growth temperature in an attempt to fill Hg vacancies in

the structure. Layer thicknesses were measured using SEM analysis, and were found to

be somewhat different from the design due to variations during MBE growth. Photo-

conductors were then fabricated, as outlined in section 5.4, and responsivity and noise

measurements performed. As there are a number of different devices with varying dimen-

sions, the results presented are either the most typical of the set of measurements, or the

most comprehensive.

5.5.1.1 Structure

After growth of sample MCT-79 the sample was diced in half and a small piece was

cleaved from one half. A cross-section of this cleaved portion was imaged using a Zeiss

1555 VP FESEM scanning electron microscope (SEM). The thicknesses of each layer were

extracted from the images taken and are shown in Fig. 5.5.3. There is some deviation

from the designed layer thicknesses shown in Fig. 5.2.9, but the asymmetric mirror

design is generally maintained. It should be noted that the total thickness of the last

Hg(0.6)Cd(0.4)Te mirror layer plus absorber layer was measured, and the absorber was


260 nm315 nm240 nm315 nm240 nm345 nm240 nm335 nm240 nm345 nm260 nm360 nm295 nm395 nm295 nm415 nm245 nm

Hg Cd Te0.6 0.4

CdTeHg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdZnTe Substrate

75 nmHg Cd Te0.7 0.3

1.55 mmCdTe


Spacer

Absorber

Figure 5.5.3: Layer thicknesses of MCT-79 as measured by SEM.

assumed to be 75nm thick in order to determine the thickness of the last mirror layer

(245nm).

The transmittance of the as-grown sample was measured on a Sopra GES-5 FTIR el-

lipsometer at normal incidence. Figure 5.5.4 shows the measured transmittance as well

as the results of optical modelling of the structure in Fig. 5.5.3. The layer thicknesses

are fixed based on the measured SEM thicknesses, but the refractive index of the CdTe

layers is fitted (in accordance with the decreased CdTe refractive index of section 4.4.3).

The refractive index of the CdTe spacer is fixed at 2.5, while the refractive index of the

CdTe mirror layers is 2.6. The two layers are modelled individually, as the spacer layer

is substantially thicker than the mirror layers, and could therefore have a higher void

concentration. There is good agreement with the fit, though at wavelengths longer and

shorter than the reflection band of the mirror, measured data and modelled results di-

verge. The differences between modelled results and measured results are possibly due to

dispersion in the refractive index of the Hg(0.6)Cd(0.4)Te material (which depends on the

oscillator strength and band-gap in a complex relationship) differing from the dispersion

suggested by the refractive index model. Furthermore, the dispersion in the refractive

index of the CdTe material, which is not included as the wavelength band of interest is

far removed from the band edge, will also have a minor contribution to the discrepancy

between measured and modelled results.


0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.010 8 6 4 2

Measured transmittance Model transmittance

Tran

smittan

ce

Wavenumber ( m-1)

Wavelength ( m)

Figure 5.5.4: Transmittance as a function of wavelength measured on the as-

grown MCT-79 wafer, compared with the modelled transmittance.

The measured data contains more points than displayed.

5.5.1.2 Responsivity

The responsivity as a function of wavelength measured at a temperature of 80K on a

80 µm × 500 µm photoconductor is shown in Fig. 5.5.5. The device structure is that

of Fig. 5.4.1(d), and the device was frontside illuminated at this point. Also shown in

Fig. 5.5.5 is the modelled performance of a 80 µm × 500 µm photoconductor with an

absorber layer thickness of d = 75 nm, surface recombination velocity of the front and

back surfaces S1,2 = 600 cm s−1, composition x = 0.3, and doping density ND = 3× 1014

cm−3. Other than the absorber layer, the layer thicknesses used for the modelled device

were taken from scanning electron microscopy measurements of the fabricated structure

(see Fig. 5.5.3). The general shape of both the measured and modelled data are in

good agreement, and both experiment and modelling clearly show resonant peaks at a

wavelength of approximately 2.75 µm. For wavelengths beyond 3.5 µm, the reflectivity of

the Hg(1−x)Cd(x)Te/CdTe mirror decreases, and the cavity will not reject signal for these

wavelengths. Hence the presence of signal at these wavelengths, which is similar to the

signal from a non-RCE 75 nm thick absorber layer.

Figure 5.5.5 shows that the model does not predict the broadening that is clearly visible in

the measured data: in particular broadening of the 2.75 µm resonant peak. This has been

investigated in detail, and has been found to be due the limited spectral resolution of the


Figure 5.5.5: Responsivity of an 80 µm × 500 µm photoconductor with single

mirror (after Fig. 5.4.1(d)) as a function of wavelength at bias

of Eb ≈ 50 V/cm and a temperature of 80K. Modelled respon-

sivity is also shown. Model parameters for the absorber layer are

T=80K, d = 75 nm, S1,2 = 600 cm s−1, x = 0.3, ND = 3.4 × 1014

cm−3, Eb = 50 V/cm. Also plotted is a non-RCE detector with

similar model parameters. Modelled responsivity of a 10 µm thick

photoconductive detector is plotted on the right axis, with model

parameters similar to the RCE case, except surface recombination

is neglected. Note: left-hand and right-hand scales are different.

monochromator used to take these measurements. Measurements taken using a different

system with narrower spectral bandwidth, in section 5.5.2.1, show a narrower peak. There

is also a difference between the longwave mirror cut-off at ≈ 3.5 µm of the measured data

compared to the modelled curve, which is most likely due to the refractive index of the

deposited CdTe layers differing from that of the model. As the CdTe was deposited at a

non-optimal temperature, it is very likely that the material optical properties will have

been affected, as was investigated in section 4.4.3. It is important to note that the surface

recombination velocity used to fit the curve is reasonable for MBE material. High quality

MOCVD and MBE grown Hg(1−x)Cd(x)Te material can achieve surface recombination

velocities as low as S = 50 cm s−1 [124]. Also plotted in Fig. 5.5.5 is the modelled

responsivity of a 75 nm thick photoconductor with similar parameters as the modelled

RCE detector. The resonance is clearly apparent, compared to the broadband response


of the non-RCE detector. Furthermore, the increase in performance for the RCE detector

is apparent as the resonant peak has a responsivity of 10×103 V/W, while the non-RCE

detector only achieves a peak responsivity of 4×103 V/W. Finally, plotted in Fig. 5.5.5 is

the modelled response of a bulk photoconductor, with the same parameters as the RCE

photoconductor, but neglecting surface recombination, which will have negligible effect

on the lifetime for a bulk photoconductor. The modelled peak responsivity of the bulk

photoconductor is approximately two orders of magnitude larger than that of the RCE

detector. This is due to the surface recombination dominating the lifetime of the RCE

detector and severely limiting performance. The resonant nature of the RCE detector is

also apparent when compared with the responsivity of a bulk photoconductor, which has

no resonant peaks.

5.5.1.3 Varying Temperature

By varying the measurement temperature, further material properties can be investigated.

Figure 5.5.6 illustrates the normalized responsivity of a 80 µm × 500 µm photoconductor

at temperatures of 80K and 250K. These are compared with the modelled normalized

responsivity for a 10 µm thick detector at 80K and 250K. There is very good agreement in

the shift in cut-off due to the changing band-gap of the Hg(1−x)Cd(x)Te with temperature.

This is apparent as the signal in the mirror roll-off region (3.5 - 5 µm) experiences a shift

in cut-off. The responsivity at 250K clearly drops to the noise floor of the measurement

set up after 4.4 µm. The cut-off at 80K is more difficult to determine, as a traditional

half-peak responsivity [62] analysis cannot be used since the signal does not cut out as

sharply as in the 250K case. Figure 5.5.5 shows good agreement between the modelled

responsivity and measured responsivity at longer wavelengths, and as the composition

used for the modelled RCE detector is the same for the bulk photoconductor and the

cutoff wavelength as a function of composition is very well defined, it can be inferred that

the cutoff at 80K is accurate as modelled. The peak responsivity shifts in wavelength

due to the change in refractive index with temperature, but this effect results in only a

100 nm change in the peak responsivity since the resonance is primarily controlled by the

optical length of the cavity and phase change in the mirrors, which are in turn controlled

by the refractive index of the materials. The effect of the change in refractive index as a

function of temperature is small compared to the change in energy gap.

Peak responsivity of a 50 µm × 50 µm RCE photoconductor was also measured for

varying temperatures for a fixed bias field of Eb ≈ 36 V/cm, with the results shown

in Fig. 5.5.7. Above 200K, the peak responsivity is highly dependant on the intrinsic

carrier concentration, as the carrier concentration is a function of temperature and the

narrow band-gap means that the intrinsic carrier concentration can become quite large

even at moderately low temperatures. Below 200K, the responsivity is independent of

temperature since the dominant lifetime mechanism is surface recombination. Despite

the relatively low value of fitted surface recombination of S1,2 = 200 cm s−1, surface

recombination becomes the dominant mechanism limiting the lifetime, and hence the


2.5 3.0 3.5 4.0 4.5 5.0 5.5

0.01

0.1

1

Normalised

Res

pons

ivity

Wavelength ( m)

80K Model 250K Model 250K Measured 80K Measured

Figure 5.5.6: Normalized responsivity of a 80 µm × 500 µm RCE photoconductor

with a single mirror (after Fig. 5.4.1(d)) as a function of wavelength,

at a bias of Eb ≈ 50 V/cm and temperatures of 80K and 250K.

Modelled normalized responsivity of a bulk photoconductor is also

shown. Model parameters for the absorber layer are T=80K and

T=250K, d = 10 µm, S1,2 = 0 cm s−1, x = 0.3, ND = 3.4 × 1014

cm−3, Eb = 50 V/cm.

responsivity, because the absorber layer is very thin. The measured and modelled peak

responsivity show good agreement at all temperatures.

Figure 5.5.7 also shows the steady-state effective lifetime extracted from the measured

responsivity. Once again, there is reasonable agreement between the modelled lifetime

and the extracted lifetime. The extracted lifetime of τeff ≈ 14 ns at 80K is very low

compared to bulk n-type material, which for high quality Hg(1−x)Cd(x)Te would be of the

order of microseconds [48]. A bulk defect density can be extracted assuming that bulk

Shockley-Read-Hall recombination is the limiting recombination mechanism in the lifetime

(Section 2.4.2.1). Alternatively, a surface defect density can be extracted assuming surface

recombination is the limiting recombination mechanism in the lifetime (Section 2.4.2.4).

Extracting the trap density based on bulk SRH as a limiting mechanism yields a bulk

trap density of 2.37 × 1014 cm−3, while extracting the trap density assuming surface

recombination is the limiting mechanism yields an interface trap density of 8.83 × 1011

cm−3. This indicates that surface recombination is the dominant mechanism, as a trap

density of approximately 10 × 1010 - 10 × 1011 cm−3 [125] would be typical for material

of reasonable quality. It is expected that the trap density of the RCE detectors under


2 4 6 8 10 12 1455

81010

20

40

60

80100100

400 300 200 100

5

10

15

20Res

pons

ivity

(x10

3 V

W-1)

1000/T (K-1)

Model Responsivity Measured Responsivity

Temperature (K)

Life

time

(ns) Model Lifetime

Measured Lifetime

Figure 5.5.7: Peak responsivity of a 50 µm × 50 µm photoconductor with single

mirror (after Fig. 5.1(d)) as a function of temperature at a bias of

Eb ≈ 36 V/cm. Modelled peak responsivity is also shown. Model

parameters for the absorber layer are d = 75 nm, S1,2 = 200 cm s−1,

x = 0.3, ND = 1 × 1015 cm−3, Eb = 36 V/cm. Extracted lifetime

and model lifetime (using model outlined in appendix A.7) are also

plotted.

study would be higher than a device fabricated from bulk or thicker epitaxial material,

primarily due to an increased defect density as a direct consequence of CdTe growth at a

non-optimum temperature. It should be noted that surface recombination was modelled

as being independent of temperature, which tends to be supported by the agreement

between extracted and modelled lifetime.

5.5.1.4 Varying Applied Field

As discussed in section 2.5.4, increasing the bias field can increase the responsivity, until

the onset of sweepout. The peak responsivity at a temperature of 250K of an 80 µm ×500 µm photoconductor with single mirror (after Fig. 5.1(d)) was measured as a function

of applied bias. The results are displayed in Fig. 5.5.8, along with the result of fitting a

linear trend, including the origin, through the data. The linear nature of the measured

data suggests that there is no evidence of sweepout for these bias fields. A bulk or epitaxial

device, where interface recombination is not the limiting mechanism, would usually show

evidence of sweepout at the higher fields (typically for fields greater than 30 V/cm). The


0 5 10 15 20 25 30 35 40 45 50 55 600

2

4

6

8

10

12

14

16

18

20

R2 = 0.96

Linear Fit

Res

pons

ivity

(x10

3 V/W

)

Bias Field (V/cm)

Measured Data

Figure 5.5.8: Responsivity of an 80 µm × 500 µm photoconductor with single

mirror (after Fig. 5.4.1(d)) as a function of bias at the peak wave-

length and a temperature of 250K. Linear fit, including the origin,

is also plotted

lack of sweepout effects in the RCE device is further evidence of the very short effective

carrier lifetime in the device being dominated by surface recombination.

Re-arranging Eqn. 2.5.10 to give the responsivity as a function of bias field, and then

using the slope of the fitted line in Fig. 5.5.8, yields:

τeff = RSlopewd

η

hc

λn0 (5.5.2)

The lifetime extracted from the slope of the responsivity (R = 367.1Eb) as a function

of bias field for the 80 µm × 500 µm photoconductor is 32.2 ns, compared to 21.9 ns

for the 50 µm × 50 µm photoconductor at the same temperature, which is in reasonable

agreement. Once again, the lifetime extracted is much lower than is typically found in

high quality bulk n-type Hg(0.7)Cd(0.3)Te material.

5.5.1.5 Varying Optical Area

Responsivity is a function of optical area (Eqn. 2.5.10), and as optical area increases the

responsivity decreases. In fact, as the responsivity is inversely proportional to the optical

area, the responsivity will exhibit a linear relationship on a log-log plot with a slope of -1.

Figure 5.5.9 shows the results for the devices measured on sample MCT-79. Due to low


1000 10000 1000001000

10000

100000

Measured Responsivity Linear Fit

Res

pons

ivity

(V/W

)

Area ( m2)

1/A

Figure 5.5.9: Responsivity as a function of optical area for various photoconduc-

tors at a temperature of 150K and bias field of 50 V/cm. A linear

fit to the measured data (dashed) is shown, as well the theoretical

fit with a slope of -1 (dotted).

yield, there is not a large number of devices, with some particular device areas having no

working devices, which lowers the value of statistics performed in this study. There is a

fairly large variation in measured responsivity between devices of the same optical area,

further suggesting problems with growth/fabrication across the sample. In spite of these

issues, there is a reasonable linear trend present in the data, though it only has a slope of

-0.75, compared to a theoretical value of -1. Unfortunately all the devices with the same

optical area and a differing perimeter-to-area ratio failed, so the effects of recombination

at edges/interfaces could not be investigated, which is likely to be one mechanism that

contributes to the deviation in slope for the responsivity as a function of area.

5.5.2 MCT-79 - Complete Structure

After characterisation of the incomplete structure (Fig. 5.4.1(d)), the processing of sample

MCT-79 was completed with deposition of the Ge/SiO mirror. Five alternating layers of

Ge (150 nm) and SiO (348 nm) were deposited in a thermal deposition system to cover

the entire wafer and the devices were re-characterised. The structure of the completed

device is shown in Fig. 5.4.1(e).


1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

3.0

Nor

mal

ised

Res

pons

ivity

Wavelength ( m)

Without Ge/SiO mirror With Ge/SiO mirror

Figure 5.5.10: Normalised (at 2.5 µm) responsivity of a 50 µm × 50 µm photocon-

ductor at 80K before and after the addition of the Ge/SiO mirror,

and at a bias of Eb ≈ 50 V/cm. The measured data contains more

points than displayed.


Responsivity as a function of wavelength was measured using backside illumination. Fig-

ure 5.5.10 shows the results for the same 50 µm × 50 µm photoconductor measured at a

temperature of 80K and a bias field of Eb = 50 V/cm, before and after the deposition of

the Ge/SiO mirror. Both data sets are normalised to the responsivity at a wavelength of

2.6 µm. Below 2.5 µm there is a marked difference between the two curves. This is due

to the fact that, as the device is now backside illuminated, and incident radiation must

pass through mirror 1 before entering the cavity. As the x = 0.4 material of mirror 1 is

absorbing at wavelengths below ≈ 2.5 µm, it prevents incident light from reaching the

x = 0.3 absorber layer or the top-most x = 0.4 layer of mirror 1 (since contact is also

made to this layer during processing). The measured data is confirmed with modelling

of the device before and after deposition of mirror 2, showing a decrease in absorptance

in this region (see Fig. 5.5.11). Wavelengths shorter than 2.7 µm show absorption in the

top x = 0.4 layer of mirror 1, as the cut-off for this layer is 2.7 µm at 80K. The resonant

peak of the cavity was designed to be at a longer wavelength (i.e. not so close to the

cut-off of the mirror layers): however, due to inadequate control of layer thickness during

MBE growth, the resonant peak occurs close to the cut-off of the mirror layer material.

For wavelengths longer than 3.6 µm, results both before and after deposition of mirror 2

agree with the results obtained after approximately one pass through a 75 nm absorber


1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.1

0.2

0.3

0.4

0.5

Abs

orptan

ce

Wavelength

Top mirror layer before Ge/SiO mirror is added Absorber layer before Ge/SiO mirror is added Top mirror layer after Ge/SiO mirror is added Absorber layer after Ge/SiO mirror is added

Figure 5.5.11: Modelled absorptance of the x = 0.3 absorber layer and the x =

0.4 top layer mirror 1 before and after the addition of the Ge/SiO

mirror.

layer. This is logical, as mirror 1 is not highly reflective at these wavelengths and, there-

fore, the cavity will not reject these wavelengths or resonate. There is, however, a resonant

peak at 3.4 µm that becomes more well defined after the Ge/SiO mirror is added, and is

in agreement with model absorptance (Fig. 5.5.11), which suggests that the peak should

become more well defined and stronger.

5.5.2.2 Varying Temperature

The spectral responsivity of a 50 µm × 50 µm photoconductor with the Ge/SiO mirror

added was measured under a bias field of Eb = 50 V/cm for various operating temper-

atures, and is shown in Fig. 5.5.12. As temperature increases the resonance at 3.35

µm decreases, in agreement with modelled results (Fig. 5.5.13), and eventually is no

longer present for temperatures above 200K. However, at a temperature of 240K there is

resonance at 3.2 µm which does not agree with modelled results, as the model only has

resonant peaks at 2.8 µm and 3.6 µm (even at 240K and 300K, which are not shown), and

no resonant peak in the middle of the rejection region at 3.2 µm. The differences between

modelled and measured results are possibly due to dispersion in the Hg(0.6)Cd(0.4)Te mate-

rial (which depends on the oscillator strength and band-gap, both of which are a function

of temperature), but further investigation is needed. Finally, the variation with temper-


ature of the responsivity peak at approximately 2.8 µm is not monotonically shifting to

shorter wavelengths, suggesting that the signal at 2.8 µm is due to resonant absorption

in the cavity by the Hg(0.7)Cd(0.3)Te absorber layer.

5.5.3 Noise Measurements - MCT-79

Noise measurements were performed using a HP 3665A dynamic spectrum anaylser, with

the sample and dewar enclosed in a Faraday cage as illustrated in Fig. 5.5.14. The

current source (for biasing the photoconductor) was situated external to the Faraday

cage, as it must be powered by the mains 240V/50Hz supply. The voltage amplifier

is battery operated, and can therefore be placed within the Faraday cage in order to

avoid introducing extra noise. Noise measurements were taken on a number of different

devices, and at different bias conditions and temperatures. The results were inconclusive,

however, since the current source that was connected to the mains power supply tended

to introduce extra noise. Furthermore, with the very short lifetime of sample MCT-79,

the expected noise levels were very low, and the noise from the current supply was found

to dominate the signal. Figure 5.5.15 shows a typical noise profile measured from a 80

µm × 500 µm photoconductor at a temperature of 200K with bias fields of 0 V/cm and

22.5 V/cm. As expected, increasing the bias field increases the noise and, in particular,

the 1/f component: however, under bias the 1/f component deviates significantly from a

slope of -1, which is an indication that the current source used to bias the photoconductor

is affecting the measurement, as power spectral density of low frequency noise is typically

of the form

S(f) ∝ 1

fa(5.5.3)

where 0 < a < 2, and is typically close to 1 for electronic devices. As can be seen from Fig.

5.5.15, the 1/f noise component at high bias has a slope of -3, which is much greater than

explainable as noise from a simple photoconductor. One possible way to circumvent the

excess noise from the current source would be to use a battery bias voltage and suitably

large resistor to bias the photoconductor.

Nevertheless, the detector noise at the chopping frequency (150 Hz) has been extracted,

and used to calculate the detectivity of the RCE photoconductors. Peak detectivity at

80K for an 80 µm × 500 µm photoconductor was calculated to be 3.09×109 cm Hz1/2

W−1, while peak detectivity at 200K was calculated to be 4.48×108 cm Hz1/2 W−1. These

values represent a worst-case detectivity, as the devices would usually be operated beyond

the 1/f knee (thus reducing noise, and increasing detectivity), and also operated without

a noisy current supply. The measured detectivity is compared with D∗BLIP = 4×1011

cm Hz1/2 W−1, which illustrates that this device is performing well below background

limited performance.


2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.20.0

0.2

0.4

0.6

0.8

1.0R

espo

nsiv

ity N

orm

alis

ed

Wavelength ( m)

80K 160K 200K 240K

Figure 5.5.12: Normalised measured responsivity of a 50 µm × 50 µm photocon-

ductor measured at various temperatures, and at a bias of Eb ≈ 50

V/cm.

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.20.0

0.1

0.2

0.3

0.4

0.5

0.6

Abs

orptan

ce

Wavelength ( m)

200K 160K 80K

Figure 5.5.13: Modelled absorptance of the x = 0.3 absorber layer at various

temperatures.


CurrentSource

VoltageAmp

Faraday Cage

Co-axsplitter

SpectrumAnalyser

Dewar

TempController

Figure 5.5.14: Schematic of the setup for noise measurements.

102 103 104 10510-8

10-7

10-6

10-5

10-4

a = -1.6

a = -2

a = -3

0 V/cm Bias 22.5 V/cm Bias

Noi

se V

olta

ge (V

Hz-1

/2)

Frequency (Hz)

a = -1

Figure 5.5.15: Noise voltage as a function of frequency of a 80 µm × 500 µm

photoconductor at a temperature of 200K at bias fields of 0 V/cm

and 22.5 V/cm. The measured data contains more points than

displayed. Also shown are slopes for 1/fa for a = 1, 2, and the

slopes of the measured data.


(a) (b)

Figure 5.5.16: Spatial photoresponse of a photoconductor (a) at 300K, and (b)

at 80K. Probe wavelength is 1.054 µm.

5.5.4 Contact Issues - MCT-79

Surface recombination is most likely the dominant recombination mechanism that is lim-

iting the minority carrier lifetime in these devices. However, other experimental results

indicate the presence of other non-ideal behaviour. For example, results of Hall mea-

surements to determine the doping density using van der Pauw [126] structures fabri-

cated alongside the photoconductors were inconclusive (hence the fitted doping density

of ND = 3.4 − 10 × 1014 cm−3), with non-symmetric Hall voltages observed, indicating

non-Ohmic contacts, or non-uniformities in the grown material. Spatial photoresponse

measurements (shown in Fig. 5.5.16) were undertaken using a Waterloo Scientific scan-

ning laser microscope (SLM) with a probe wavelength of 1.054 µm in order to determine

the cause of the non-symmetries in the Hall voltage. The strong signal surrounding the

contact in Fig. 5.5.16(b) suggests some issues with the doping in these areas, with the

indium possibly creating n+-n junctions. Alternatively, the annealing process may have

failed to fully convert the x = 0.3 layer to n-type and the indium is creating a compensated

region near the contact, such that a p-n junction is formed. This effect is not apparent

in Fig. 5.5.16(a), since at room temperature the material is intrinsic. These issues with

doping are likely to reduce the lifetime of the material: however, given that the extracted

effective lifetime is 14 ns, the most likely mechanism dominating this low lifetime value

is surface recombination. The carrier lifetime for compensated n-type material would be

on the order of many tens to hundreds of nanoseconds, which is much longer than the

lifetime observed in these devices [127].

5.5.5 MCT-92 - Without Ge/SiO Mirror

Sample MCT-92 has the device structure given in section 5.2.1.4 (similar to MCT-79) and

was grown by MBE using the techniques outlined in section 5.3. However, for this device

there was no in-situ anneal of the absorber layer. In contrast, the sample was annealed for

20 hours in a Hg atmosphere as outlined in section 4.3.3. Layer thicknesses were measured


340 nm321 nm293 nm321 nm302 nm331 nm330 nm311 nm311 nm331 nm311 nm360 nm437 nm408 nm340 nm360 nm236 nm

Hg Cd Te0.6 0.4

CdTeHg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdTe

CdZnTe Substrate

75 nmHg Cd Te0.7 0.3

0.91 mmCdTe


Spacer

Absorber

Figure 5.5.17: Layer thicknesses of MCT-92 as measured by SEM.

using SEM analysis, and were found to be somewhat different from the design due to

parameter variations during the MBE growth. Photoconductors were then fabricated, as

outlined in section 5.4, and responsivity and noise measurements undertaken.

5.5.5.1 Structure

After MBE growth of sample MCT-92, the sample was diced in half and a small piece was

cleaved from one half. This cleaved portion was measured using a Zeiss 1555 VP FESEM

scanning electron microscope (SEM). The thicknesses of the layers, with the exception of

the absorber layer, were extracted from the images taken and are given in Fig. 5.5.17.

There is some deviation from the designed layer thicknesses shown in Fig. 5.2.9, mostly

due to an increased growth rate of the Hg(0.6)Cd(0.4)Te layers. It should be noted that

the total thickness of the last mirror layer plus absorber was measured, and the absorber

was assumed to be 75 nm thick, due to the issues with imaging the layers mentioned

previously in section 5.3.


1000 1500 2000 2500 3000 3500 40000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.010 9 8 7 6 5 4 3

Tran

smittan

ce

Wavenumber (cm-1)

MCT-92 Before anneal MCT-92 After 20 hours

anneal at 250 oC in Hgatmosphere

Wavelength ( m)

Figure 5.5.18: Transmittance as a function of wavelength of sample MCT92 be-

fore and after annealing at 250C for 20 hours in a Hg atmosphere.

The measured data contains more points than displayed.

5.5.5.2 Annealing

In order to address the contact issues outlined in section 5.5.4 sample MCT92 was an-

nealed for 20 hours in a saturated Hg atmosphere at 250C, as outlined in section 4.3.3.

The transmission spectra through the sample were measured before and after annealing,

and are shown in Fig. 5.5.18. There is clearly very little degradation due to the anneal

as the transmission spectra before and after the anneal are very similar.


The results of responsivity measurements performed on a 100 µm × 100 µm photocon-

ductor at a temperature of 80K and a bias field of 36 V/cm are shown in Fig. 5.5.19.

There are two distinct regions of the curve, with the region consisting of wavelengths

shorter than 3.4 µm showing a much higher response from the top x = 0.4 layer of mirror

1, while the response for wavelengths longer than 3.4 µm is due solely to the response

from the absorber layer. This is confirmed by modelled absorption results given in Fig.

5.5.20, which show that the top x = 0.4 layer of mirror 1 has response below 3.4 µm, but

cuts off beyond this wavelength. The absorber layer, on the other hand, is the only layer


Figure 5.5.19: Measured responsivity as a function of wavelength of a 100 µm ×100 µm photoconductor fabricated on wafer MCT-92 at a temper-

ature of 80K and bias field of 36 V/cm.

in the device structure that is absorbing at wavelengths longer than 3.4 µm, and shows a

resonant peak at 3.6 µm, in agreement with the resonant peak in the measured data.

The response from the absorber layer can be modelled as in section 5.5.1.2, using the

responsivity equation given in Eqn. 2.5.10. However, since the whole structure has been

Hg-annealed, the mirror layers are also converted to n-type material and are therefore

conductive, with the thicker layers resulting in a resistance that is comparable to the

absorber layer. As the top x = 0.4 layer of mirror 1 is effectively electrically in parallel with

the absorber layer, the shunting effect of this layer must be included in the responsivity

model [128]:

RV λ =η

lwd

λ

hc

Vbτeff

n0

(

RShunt

RShunt +RAbs

)2

(5.5.4)

The only difference between Eqn. 2.5.10 and Eqn. 5.5.4 is the term containing the shunt

resistance (RShunt, due to the top x = 0.4 layer of mirror 1) and the absorber layer

resistance, RAbs. The model parameters used are for a 100 µm × 100 µm photoconductor

with an absorber layer of thickness d = 75 nm, surface recombination velocity of the

front and back surfaces S1,2 = 70 cm s−1, composition x = 0.3, and doping density

ND = 3.5×1015 cm−3. The resistance calculated using Eqn. 2.5.3 and material properties

outlined in appendix A, gives a value of 2.65 kΩ at 80K for a 100 µm × 100 µm device,

which compares favourably with the measured value of 2.4 kΩ.

The modelled responsivity is shown in Fig. 5.5.21, in addition to the measured results.

There is very good agreement between the measured responsivity and the model respon-


2 3 4 5 60.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Abs

orptan

ce

Wavelength ( m)

Top x=0.4 layer of Mirror 1 Absorber Layer

Figure 5.5.20: Modelled absorptance of the absorber layer and top x = 0.4 layer

of Mirror 1 as a function of wavelength.

sivity at the resonant wavelength and also at longer wavelengths. The carrier lifetime

used to model the data was 50.8 ns, which is significantly longer than the value of 14 ns

for the in-situ annealed sample, although surface recombination is still the performance-

limiting mechanism. The Hg anneal has reduced surface recombination and resulted in

an increased lifetime, but the annealing process has also annealed the mirror layers and

introduced a shunting resistance, which has compromised the performance of the device.

5.5.5.4 Varying Bias Field

The results of varying the bias field are shown in Fig. 5.5.22, which depicts the responsivity

of a 100 µm × 100 µm photoconductor at 80K for various bias fields. The responsivity

is linear for bias fields below 50 V/cm, in agreement with equation 5.5.4, but becomes

dominated by sweepout for higher bias fields. The fact that sweepout is occurring for

fields beyond 50 V/cm, which was not evident for sample MCT-79, is a consequence of

the longer carrier lifetime as a result of annealing sample MCT-92 at 250C for 20 hours

in a Hg atmosphere. The lifetime can be extracted from the sweepout knee field, Eb−k,

the device length, l and the ambipolar mobility, µ0 [129]:

τeff =l

2µ0Eb−k(5.5.5)

The effective lifetime using this method is 224 ns, which is substantially longer than the

50 ns which was extracted from fitting the model responsivity to the measured data.

This discrepancy can be explained by the fact that Eqn 5.5.5 assumes that all carriers are


3.0 3.2 3.4 3.6 3.8 4.00

2

4

6

8

10

Res

pons

ivity

(x 1

03 V/W

)

Wavelength ( m)

Measured Responsivity Modelled Responsivity

Figure 5.5.21: Measured and modelled responsivity as a function of wavelength

at a temperature of 80K. The model parameters used are extracted

from fitting the modelled responsivity to the measured responsiv-

ity. For a 100 µm × 100 µm photoconductor with an absorber

layer of thickness d = 75 nm and composition x = 0.3, the fitted

surface recombination velocity of the front and back surfaces was

S1,2 = 70 cm s−1 and the doping density was ND = 3.5 × 1015

cm−3.

generated in the center of the photoconductor, which clearly is not the case, resulting in

an over-estimation of the effective lifetime. The extracted lifetime of Hg-annealed sample

MCT-92 is still substantially improved when compared to sample MCT-79, which was

annealed in-situ at the growth temperature, and had an effective lifetime of 14 ns. The

improvement is most likely due to increased grading at the interfaces between the absorber

and the mirror and spacer, which will tend to repel carriers away from these interfaces,

thereby reducing surface recombination.

5.5.6 Contact Issues - MCT-92

As discussed in section 5.5.4, the in-situ anneal performed on sample MCT-79 resulted

in non-ohmic contacts. A study similar to that performed in section 5.5.4 was performed

on sample MCT-92, on a 100 µm × 100 µm photoconductor at 300K and at 80K, the

results of which are shown in Fig. 5.5.23. Both measurements show a peak in response

in the lower right of the photoconductor due to gold ball bonding. Since the indium

156 5.6. Proceeding on to Photovoltaic Detectors

0 20 40 60 80 100 120 1400

1

2

3

4

5

6

7

8

9

10

11

12

Res

pons

ivity

(x 1

03 V/W

)

Bias Field (V/cm)

Figure 5.5.22: Responsivity as a function of applied bias field for a 100 µm ×100 µm photoconductor from sample MCT-92 at a temperature of

80K. A linear fit including the origin and the first four data points

is also shown to illustrate the sweepout knee.

metal tracks contained discontinuities, the gold ball metal contacts were placed on the

photoconductor, one of which is right at the signal peak. However, note that the majority

of the photoconductor is uniform at both temperatures. This indicates that the anneal

performed on sample MCT-92 has succeeded in producing much more uniform material

than in sample MCT-79.

5.6 Proceeding on to Photovoltaic Detectors

Photoconductors are excellent structures for a proof of concept as they are simple to fab-

ricate and are governed by simple processes, which have fewer parameters that depend on

material properties. This makes device processing easier, and more forgiving to variations,

and also makes extracting the material parameters easier than for photodiodes. However,

for practical applications, photoconductors are not suitable. The bias field required to

operate the device produces a constant power drain, which for large arrays renders them

impractical. Furthermore, in a RCE photoconductor the effects of surface recombina-

tion velocity and Johnson noise outweigh the advantage of smaller volume. Photovoltaic

detectors are therefore required for most applications. Fortunately, all the principles in-

vestigated in terms of resonant cavity enhancement are applicable to any detector, not


(a) (b)

Figure 5.5.23: Spatial photoresponse of a photoconductor (a) at 300K, and (b)

at 80K. Probe wavelength is 1.054 µm.

just photoconductors. This means that the same layer structure devised for research into

photoconductors can be used to fabricate photovoltaic detectors. The device structure is

illustrated in Fig. 5.6.1, which is fabricated by etching a via for the n-contact, creating

a region of n-type material in the p-type absorber by either implantation [97] or type-

conversion [130], and adding contacts. For a photovoltaic device of this structure, the

absorber layer could be relocated to benefit from improved energy density in either the

mirror layers, or within the spacer. Alternatively, the structure could be re-designed in a

PIN structure (similar to existing structures in III-V material systems) outlined in section

3.5.3.3, but the design already established for this work can still provide a working proof

of concept, and so is used for simplicity.

5.6.1 Processing - MCT-95

Sample MCT-95 was grown by MBE following the design outlined previously and illus-

trated in Fig. 5.2.9. The sample was diced in half, but no SEM analysis was performed, so

the as-grown layer thicknesses of this sample are unknown. The sample was subsequently

annealed for 12 hours at 250C in vacuum in the loading chamber of the Riber 32 MBE

system to produce p-type material [131]. The transmission characteristics through the

as-grown stack and through the annealed stack were measured, and are shown in Fig.

5.6.2. The stack is still present after annealing, and displays evidence of resonance (in the

peak at 3.5 µm), but there is significant variation in the layer thicknesses before and after

the anneal (as evidenced by the shifting in the various interference and resonance peaks).

Furthermore, there is a significant shift in the cut-off of the mirror layers (from 3 µm to

2.5-2.6 µm), suggesting a decrease in Hg within the layers (i.e. due to out-diffusion).

Photodiodes were then fabricated by opening vias in the material using a 1% Br/HBr etch

and type converting laterally to form a vertical geometry junction (Fig. 2.2.2(b)). The

type conversion [132, 57] is performed in an Oxford Instruments reactive ion etching (RIE)

tool at a base pressure of 35 mT. The process pressure was 100mT, and the processes

gasses were H2 and CH4 with flow rates of 54 sccm and 10 sccm, respectively. The sample


Cavity L

ength

l

SubstrateCdZnTe

Mirror 1

SpacerCdTe

Ld

CdTe

Hg Cd Te0.6 0.4

Hg Cd Te0.6 0.4

Mirror 2SiOGe

SiOGe

Ge

Oxide

Vian – Contact

Typeconvertedregion

p-type

DetectorHg Cd Te0.7 0.3

p – Contactremote

Figure 5.6.1: Schematic of a fabricated photovoltaic RCE detector. n-contact is

at the wall of the via, while the p-contact is remote, and not shown

in this schematic.

1000 1500 2000 2500 3000 3500 40000

10

20

30

40

50

60

70

80

90

10010 9 8 7 6 5 4 3

Tran

smittan

ce

Wavenumber (cm-1)

As-Grown 10 hours anneal

in vacuum

Wavelength ( m)

Figure 5.6.2: Transmittance as a function of wavelength for sample MCT-95 be-

fore annealing compared with after annealing. The measured data

contains more points than displayed.


(a) (b)

Figure 5.6.3: Laser beam induced current of a via that has undergone type con-

version, but has no Cr/Au contact (a) at 300K, and (b) at 80K.

Probe wavelength is 1.054 µm.

was etched with RF power of 120W for two minutes. After type conversion, vias were

opened for the contact to the p-type material, and Cr/Au was deposited for both the

n-type and p-type contacts by thermal deposition.

5.6.2 Results - MCT-95

5.6.2.1 Scanning Laser Microscopy

The sample was probed optically in a Waterloo Scientific SLM, allowing measurement of

both the laser beam induced current (LBIC) and the spatial photoresponse using a probe

wavelength of 1.054 µm. Figure 5.6.3 shows the LBIC results measured at temperatures

of 300K and 80K. The junction is visible as a circle of signal due to the in-built field at the

junction separating carriers generated by the laser [75]. The scan shows signal collected

in a ring between 30 and 70 µm in width around the entire circular via of radius 300

µm. The variation in width of the signal is an artifact of the placement of the remote

contacts for LBIC, one contact is to the left, and one contact to the top of the device.

The neighbouring devices can be seen at the top and left of the scan. The junction is

visible at both 80K and 300K.

Spatial photoresponse measured on a loophole photodiode of radius 300 µm was performed

at temperatures of 300K and 80K, the results of which are shown in Fig. 5.6.4. At 80K

the photoresponse is clearly visible around the device in a ring similar to that observed

in the LBIC measurements (Fig. 5.6.3): however, there is also signal on the inside of

the via. The ring inside is likely due to the metal layer being thinner on the sidewall of

the diode, and light therefore penetrating the Hg(1−x)Cd(x)Te layers and being absorbed,

or alternately the metal on the via scattering the light. The signal at the center of the

diode is due to the gold ball and conductive epoxy used to connect the device scattering

light onto the Hg(1−x)Cd(x)Te material. The line in the image to the right of the gold

ball bonding is the gold wire leading to the chip carrier, while the absence of signal in


(a) (b)

Figure 5.6.4: Spatial photoresponse of a photodiode (a) at 300K, and (b) at 80K.

a ring around the device is due to the Cr/Au contact that crested over the lip of the

device. The p-contact is to the top of the image, hence the greater signal on this side of

the device. The device is still functional at 300K, but there is a much weaker signal at

this temperature.

The results from the scanning laser microscopy study point conclusively to the successful

formation of diodes. However, it does not differentiate between layers in the stack, as the

wavelength of the laser probe is 1.054 µm, which will be absorbed by both the mirror

layers and the absorber layer, all of which are contacted.

5.6.2.2 Current-Voltage Measurements

The current-voltage characteristics of a RCE photodiode were measured using a HP 4156

semiconductor device probe at temperatures varying from 80K to 300K. These measure-

ments are plotted in Fig. 5.6.5, and show that as the device temperature decreases,

the diode turn-on voltage increases. The band-gap has been estimated from the turn-on

voltage at 80K, and the composition extracted from this, resulting in a composition of

x = 0.389 for the curve at 80K, suggesting the mirror layers are dominating the system

(i.e. having the lowest resistance).

The dynamic resistance can also be plotted from I-V curves, and is plotted for temper-

atures varying from 80K to 300K in Fig. 5.6.6. At lower temperatures the resistance

is dominated in forward bias by diffusion current, while in reverse bias it is dominated

by generation-recombination current in the depletion region. This is an attribute of the

horizontal (loophole) geometry, as there are fewer dislocations (which usually propagate

through the crystal vertically, i.e. parallel to the junction) running through the junction.

At a temperature of 150K the device is starting to exhibit leakage current due to tun-

nelling in reverse bias, while at 80K tunnelling is clearly the dominant leakage current

mechanism in reverse bias.

A number of devices were measured, and the zero-bias resistances varied substantially

from 10 MΩ to 100GΩ. Assuming the junction area to be due to the Hg(0.6)Cd(0.4)Te


-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-5

0

5

10

15

20

25

30

Cur

rent

(A

)

Voltage (V)

300K 250K 200K 150K 80K

Figure 5.6.5: Diode current as a function of diode voltage for sample MCT-95,

measured at temperatures from 80K-300K.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6103

104

105

106

107

108

109

Res

istanc

e

Voltage (V)

300K 250K 200K 150K 80K

Figure 5.6.6: Diode dynamic resistance as a function of diode voltage for sample

MCT-95, measured at temperatures from 80K-300K.


2.5 3.0 3.5 4.0 4.510-5

10-4

10-3

10-2

10-1R

espo

nsiv

ity (A

/W)

Wavelength ( m)

Figure 5.6.7: Responsivity as a function of wavelength for a loophole photodiode

from sample MCT-95 with a radius of 300 µm at 80K.

layers in the mirror stack, the R0A product was calculated to be 3.2×105 Ωcm−2 for the

device with 100 GΩ resistance, which is quite reasonable for photodiodes of this x-value.


Responsivity was measured for one photodiode with a radius of 300 µm at a tempera-

ture of 80K. The responsivity plotted in Fig. 5.6.7 shows a clear signal for wavelengths

shorter than 2.9 µm, and then the signal drops substantially to a noise floor. The cut-off

wavelength of 2.9 µm at 80K corresponds to a composition of x = 0.41, which is in good

agreement with the composition extracted using the I-V data, further suggesting that

only the mirror layers are registering signal. More devices will need to be bonded and

measured, but it appears that this structure will be dominated by response from the much

larger area Hg(0.6)Cd(0.4)Te mirror layers (as opposed to the absorber layer).

5.7 Conclusions

Resonant-cavity-enhanced detectors have been modelled, fabricated and characterised.

The model results show that a resonant-cavity-enhanced detector can perform with stag-

gered dielectric mirrors, and furthermore can leverage the phase effects of the mirror for

improved performance. Devices fabricated from RCE structures show good agreement

with modelled results, but a degraded performance due to surface recombination, which

is the limiting recombination mechanism for photoconductors fabricated from these struc-


tures. The carrier lifetime of 14 ns extracted for sample MCT-79 is substantially below

the lifetime of bulk material. This low lifetime is apparent in results such as the absence

of sweep-out in the measurements of responsivity with varying applied field. The optical

properties of the absorber layer are in agreement with the as-grown model of x = 0.3

material, with regard to temperature variation. There are contact issues with samples

that are annealed in-situ at the MBE growth temperature.

These issues were overcome by annealing for 20 hours at 250C in a mercury atmosphere,

which resulted in much lower resistance photoconductors, due in part to shunting from

the underlaying mirror layers, which are also annealed and become more conductive. The

annealing did not greatly degrade the resonant performance of the structure, but did

result in reduced surface recombination velocity of 70 cm s−1 and an improved carrier

lifetime of between 50 and 224 ns.

Photodiode structures were investigated by annealing a RCE structure in vacuum to

produce p-type material and then type converting in a H2:CH4 plasma. This process did

produce diodes that exhibit good performance, but the optical response is dominated by

the mirror layers rather than the absorber layer, resulting in no signal in the MWIR.

Further work will need to be undertaken to confirm RCE performance in photovoltaic

structures.

Chapter 6Summary and Conclusions

6.1 Thesis Objectives

The principle objectives of this work were to investigate resonant-cavity-enhanced (RCE)

detectors, and to prove the concept of resonant-cavity-enhancement for IR detectors by

undertaking the following:

• Investigate the design and modelling of RCE devices and determine a structure to

prove the concept of RCE using the Hg(1−x)Cd(x)Te material system.

• Fabricate mirror structures and RCE detector structures, and characterise optical

performance.

• Fabricate devices from the RCE detector structure and characterise device perfor-

mance, showing that resonant-cavity-enhanced performance is possible for HgCdTe

based IR detectors.

6.2 Outcomes

The specific outcomes of this thesis are:

1. Design and modelling:

• The benefits of resonant-cavity-enhanced detectors have been modelled in terms

of increasing operating temperature and reducing the noise due to thermal gen-

eration and recombination of carriers in RCE devices.

• The Hg(1−x)Cd(x)Te material system has been investigated as a suitable system

for growing RCE structures. MBE growth of Hg(1−x)Cd(x)Te was investigated

as the growth method.

166 6.2. Outcomes

• Staggered dielectric mirrors have been investigated as a means of improving the

mirror response of Hg(1−x)Cd(x)Te/CdTe mirrors. Staggered dielectric mirrors

allow for a broader region of high reflectivity, at the expense of reducing re-

flectivity, for a given number of layers. This is useful for material systems such

as Hg(1−x)Cd(x)Te/CdTe, where the ratio of refractive indices is close to unity,

suggesting a narrow region of high reflectivity for a quarter-wave stack design.

• RCE structures were investigated and a proof-of-concept design balancing im-

proved performance, wide spectral range, absorber layer thickness and position,

target device, and growth times was derived.

• RCE performance was modelled for the Hg(1−x)Cd(x)Te material system, illus-

trating the improved performance.

2. Mirrors:

• Alternating layers of Hg(1−x)Cd(x)Te and CdTe have been grown by MBE at

a constant temperature to create mirror layers, showing that CdTe can be

grown on layers of Hg(1−x)Cd(x)Te, while maintaining a crystal structure that

supports growth of further mirror layers (without decreasing mercury content)

and absorber layers.

• Mirror performance has been characterised by FTIR transmission measure-

ments, showing good agreement with model results, except for a discrepancy

with the refractive index of CdTe in the mirror layers.

• Ellipsometry measurements indicate a discrepancy between the refractive index

model of crystalline CdTe and the as-grown CdTe in the mirror layers.

• Annealing is a critical step in device fabrication and, therefore, annealing was

performed on mirror stacks to characterize performance after annealing. Mirror

stacks have been annealed for up to 20 hours at 250 degrees in a mercury

atmosphere. Transmission measurements and interdiffusion modelling indicate

that the mirror stack have survived annealing at these temperatures for these

time periods.

• Secondary ion mass spectroscopy measurements of annealed mirror structures

also show that the mirror structures survived the annealing without any major

structural modifications.

3. Experimental RCE Detectors:

• RCE structures were grown by MBE, and characterised by FTIR transmission

measurements. There is good agreement with modelled results, with resonance

peaks, reflection peaks and transmission peaks all appearing in the model.

• Photoconductors were fabricated from the MBE grown RCE structures and

were characterised by responsivity measurements, noise measurements, and

using I-V measurements.

CHAPTER 6. Summary and Conclusions 167

• Effective carrier lifetime has been extracted from responsivity measurements,

as well as surface recombination velocity and interface trap density. The ef-

fective carrier lifetime for samples annealed in-situ was extracted to be 14 ns,

while samples that were Hg-annealed ex-situ had effective lifetimes of 50-224

ns. These lifetimes corresponded to surface recombination velocities of 300

cm s−1 and 70 cm s−1, respectively.

• Electric contact issues were investigated using laser beam induced current and

scanning laser microscopy techniques. Issues with the contacts were revealed,

which indicated problems with the in-situ annealing techniques used.

• The RCE structures were annealed and showed improved contact performance.

• RCE photodiodes were fabricated by p-to-n type converting vacuum annealed

p-type structures in a H2:CH4 plasma. These structures exhibited good elec-

trical diode performance: however, it was not possible to extract an optical

signal due to shunting by the mirror layers.

6.3 Original Results

Original results generated by this thesis include:

• Models were developed and utilised to accurately predict optical characteristics as

well as device performance and detectivity. Models were also used to predict the

lower refractive index of the low growth temperature CdTe, and to extract the

refractive index of this as-grown CdTe.

• Modelling of Hg(1−x)Cd(x)Te/CdTe mirror stacks and RCE structures yielded a

design that balances improved performance, wide spectral range, absorber layer

thickness and position, target device, and growth times. The design used 17 layers

for a staggered dielectric mirror with a small common ratio of 1.017 and a starting

wavelength of 3 µm. An absorber layer of 75 nm thickness was used, which is grown

directly on the top layer of the staggered dielectric mirror (to provide a better

crystalline surface on-which to grow). The spacer layer was also grown in-situ in

order to decrease surface recombination, and CdTe was used as a spacer material

to reduce shunting of the photoconductors.

• Mirrors consisting of a 17 layer staggered Hg(1−x)Cd(x)Te/CdTe dielectric design

and fabricated by MBE exhibit good 2-D crystallinity in the Hg(1−x)Cd(x)Te lay-

ers, after an initial period of poor growth on CdTe. CdTe layers exhibit degraded

crystal quality as a result of being grown at the optimum growth temperature for

the Hg(1−x)Cd(x)Te layers, which is substantially lower than required for crystalline

CdTe. The refractive index of the as-grown CdTe is reduced, and an incorporation of

10 % voids accounts for the reduction in refractive index to 2.4-2.5 in the wavelength

range of interest. Mirror layers fabricated by MBE can survive 20 hours anneal-

ing at 250 degrees in a mercury atmosphere. There is a degradation in reflectivity

168 6.3. Original Results

associated with annealing the sample due to interdiffusion of mercury between the

layers, causing grading of the interface but this is not enough to degrade the mea-

sured reflectivity by more than a few percent. SIMS measurements performed on

annealed samples show that the interdiffusion at the CdTe on Hg(1−x)Cd(x)Te inter-

face is very close to reported literature results. Interdiffusion at the Hg(1−x)Cd(x)Te

on CdTe interface is greater than reported in the literature, probably due to an

increased defect density. Mirror layers exhibit a change in the refractive index of

the CdTe layers. This is due to the incorporation of voids in the CdTe layers during

growth, as ellipsometry measurements suggest a void density of around ten percent

in these layers.

• RCE photoconductors fabricated using a 17 layer staggered dielectric mirror, a 75

nm absorber layer and a spacer layer of approximately 1 µm have been fabricated

and characterised. The measured responsivity shows resonance and good agreement

with modelled results. Peak responsivity was measured as 8×104 V/W for a 50

µm × 50 µm photoconductor at 200K, for a bias field of Eb = 36 Vcm−1. This

represents the first reported response from a RCE structure with a Hg(0.7)Cd(0.3)Te

absorber layer. For sample MCT-79 the surface recombination velocity is 300-600

cm s−1. This surface recombination, while satisfactory for thicker absorbing layers,

is substantial enough to significantly reduce the effective carrier lifetime in RCE

photoconductors as the absorber layer is so thin. The effective lifetime extracted

from the responsivity results is approximately 14ns. This is approximately 2 orders

of magnitude lower than the bulk lifetime for Hg(1−x)Cd(x)Te of similar composition

and doping density. Variable bias field measurements do not show evidence of sweep-

out, in accordance with the very low lifetime of the material.

• RCE photoconductors fabricated from material that was annealed for 20 hours in

a Hg atmosphere at 250C addressed the problems with the contacts of sample

MCT79. However, the anneal affected all layers of the structure, resulting in the

absorber layer being shunted by the Hg(0.6)Cd(0.4)Te layer on which the absorber

sits. The anneal did reduce the surface recombination velocity to between 50 and

70 cm s−1, which corresponds to an effective lifetime of 224 ns, and produced more

uniform material based on spatial photoresponse scans.

• RCE photodiodes have also been investigated. However, although the diodes ex-

hibited good electrical characteristics, they did not provide optical response in the

MWIR due to the mirror layers shunting the absorber.

CHAPTER 6. Summary and Conclusions 169

6.4 Conclusions

Resonant-cavity-enhancement is a technique that can be used to improve detector per-

formance by reducing noise due to thermally generated carriers, while maintaining high

quantum efficiency at the resonant wavelengths. This can be important for many types

of infrared detectors and has been investigated by fabricating RCE detectors using the

Hg(1−x)Cd(x)Te material system. The benefits of RCE detectors have been modelled, and

show improvement in detectivity, or allow higher operating temperature while maintaining

detectivity, due to the reduction of thermally generated carriers.

Mirrors required to facilitate the fabrication of RCE photoconductors using an absorber

layer of 75 nm thickness and Hg(0.7)Cd(0.3)Te material have been characterised and shown

to provide good mirror performance which agrees with modelled results. The mirror

structures have been shown not to degrade following annealing processes associated with

device fabrication.

RCE photoconductors have been fabricated which exhibit resonance, in agreement with

modelled results. These detectors indicate that RCE infrared detectors are a feasible

technology, although improvement is needed, including a reduced surface recombination

velocity (and defect density), before the detectors would be capable of approaching the

modelled performance increases.

6.5 Future Work

• The cause of the lowered refractive index of CdTe grown on Hg(1−x)Cd(x)Te at

the Hg(1−x)Cd(x)Te growth temperature needs further investigation. Initial results

suggest that the reduced refractive index is caused by voids incorporated during

growth, which proceeds in a three-dimensional or columnar growth, and subsequent

overgrowth by the next layer. This is supported by SEM results: however, this

theory would need an extensive study using a high resolution microscopy technique

such as transmission electron microscopy to verify the initial results.

• The mirror technology developed for this proof-of-concept work would need to be

extended to cover the entire MWIR transmission window for practical devices. Fur-

thermore, an alternate material system might be the only way to effectively cover

wider spectral windows, such as the LWIR window (8-14 µm). One possible tech-

nique would be to have buried oxide layers in a silicon substrate and then to grow

the absorber layer directly on the silicon substrate, using the buffer layer as the

spacer layer.

• Reduction of the surface recombination velocity of the absorber layer is a major

issue. It is likely that the primary contributor to the surface recombination is

interface defects introduced by the mirror layers, and by the poor quality CdTe.

170 6.5. Future Work

This could likely be overcome by migrating to a different mirror technology, as

discussed in the previous point.

• The material characteristics of the absorber layer require further investigation, in-

cluding determining doping density/carrier concentration, since no Hall measure-

ments could be performed in this work due to contact issues and poor sample yield.

The type conversion of the absorber layer for photodiodes is also an area where

more research is needed into the depth of the type conversion, the carrier profile of

the converted region, as well as many other material properties associated with the

type converted region.

• Finally, one of the most important issues that needs to be addressed is the migration

from the photoconductors used in this proof-of-concept study to more practical

photodiodes. The structure investigated in this work may still yield devices if the

issues with the mirror layers can be overcome, either by not contacting them, or by

improving processing/fabrication to the extent that these layers do not shunt the

absorber layer.

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Appendix AProperties of Mercury Cadmium Telluride

A.1 Introduction

Mercury Cadmium Telluride (Hg(1−x)Cd(x)Te) is a semiconductor with a band-gap at

80K that is tuneable from ≈ 1.6 eV to ≈ −0.2 eV, making this material very attractive

for use in infrared (IR) applications. The material properties important to IR device

performance are summarised in this appendix, including band-gap, mobility, and lifetime.

Noise models are also presented.

A.2 Crystal Structure

Hg(1−x)Cd(x)Te is a ternary compound of Mercury (Hg), Cadmium (Cd) and Tellurium

(Te). These elements are present in the ratio Hg1−xTe1−x to CdxTex, often shortened

to Hg(1−x)Cd(x)Te, which has a zinc-blende crystal lattice structure [133]. Figure A.2.1

shows this structure, with blue (smaller) elements representing Te, and red (larger) el-

ements either a Cd, or Hg atom depending on the mole ratio (or composition x). The

shape of the zinc-blende structure occurs as a result of the bonding of one group II and

one group VI element. This bonding occurs in the form of two interpenetrating cubes,

where each element is surrounded tetrahedrally by other elements. The bonds are mainly

covalent, although there is some ionic bonding between group II and group VI elements

[133].

A.3 Energy Band-gap

The energy band-gap of a semiconductor material represents the minimum energy required

to promote an electron from the valence band to the conduction band. Energies below

this minimum will not promote an electron to the conduction band, and hence the name

band-gap.

186 A.4. Intrinsic Carrier Concentration

Figure A.2.1: Atomic Structure.

In Hg(1−x)Cd(x)Te the energy band-gap is dependent on the mole ratio x. As x increases,

so does the energy gap. The energy gap is also a function of temperature. Eqn. A.3.1

gives the relation between band-gap energy and x and T [134].

Eg = −0.302 + 1.93x+ 5.35 × 10−4T (1 − 2x) − 0.810x2 + 0.832x3 (A.3.1)

Capper [134] determined that Eqn. A.3.1 was the preferred equation to use for x = 0.3

and above, and corresponded to work of Hansen [62] as x approached 0.2.

The energy band-gap varies with x from Eg = −0.173eV at x = 0 to Eg = 1.6eV at

x = 1 at a temperature of 80K. This variability of the energy band-gap is what makes

Hg(1−x)Cd(x)Te so attractive. The energy band-gap can be tuned to detect selected bands

of the infrared spectrum. The LWIR band responds to photon energies above 80meV. This

corresponds to a mole fraction of x ≈ 0.22, while MWIR responds to photon energy of

above 250meV, which corresponds to x ≈ 0.3 [134]. Another reason that Hg(1−x)Cd(x)Te

is so attractive as an optoelectronic material is that the energy band-gap is direct.

A.4 Intrinsic Carrier Concentration

The intrinsic carrier concentration represents the number of electrons that are able to

contribute to conductivity. The number of free holes equals the number of free electrons

in an intrinsic semiconductor. Hansen [135] gives the intrinsic carrier concentration as:

ni = (5.585−3.82x+1.753×10−4T−1.364×10−3xT )×1.0×1014E3/4g T 1/2 exp (−Eg/2kT )

(A.4.1)

APPENDIX A. Properties of Mercury Cadmium Telluride 187

2 4 6 8 10 12 14109

1010

1011

1012

1013

1014

1015

500 450 400 350 300 250 200 150 100

n i (cm

-3)

1000/T (K)

Temperature (K)

Figure A.4.1: Intrinsic carrier concentration ni for x = 0.3 HgCdTe as a function

of temperature.

Due to the exponential term, the intrinsic carrier concentration is strongly dependent on

temperature and energy gap, which is interdependent on the mole ratio x (Eqn. A.3.1).

ni increases with increasing temperature, and decreasing x. Figure A.4.1 illustrates the

strong temperature dependance of intrinsic carrier concentration on temperature, for

x = 0.3 material.

A.4.1 Majority and Minority Carrier Concentration

The majority and minority carrier concentrations for n-type Hg(1−x)Cd(x)Te are stan-

dard functions of the concentrations of uncompensated donors (ND − NA). For p-type

Hg(1−x)Cd(x)Te a similar relation holds.

n0 =ND −NA

2+

√

(ND −NA)2

4+ n2

i (A.4.2)

p0 =n2

i

n0(A.4.3)

188 A.5. Effective Mass

A.5 Effective Mass

For Hg(1−x)Cd(x)Te, the electron effective mass is determined to be [136]:

m∗e = m0 exp

[

4

3log

ni

3.126 × 1015T3

2 exp−Eg

2kT

]

(A.5.1)

Alternatively, the ratio of the rest mass of an electron to the effective electron mass is

given by [137]:

m0

m∗e

= 1 +8m0P

2π2

3h2 [(2/Eg) + (1/ (Eg + ∆))](A.5.2)

where P is the Kane momentum matrix element, related to inter-band energy (Ep), by

Ep = 8m0P2π2/h2, and can be assumed to be around 19eV.

The mass of the heavy hole is taken as [137]:

mh = 0.55m0 (A.5.3)

A.6 Mobility

Mobility (cm2/V s)is a measure of how fast or how easily carriers drift (how much they

are affected by an electric field), and is equal to the drift velocity per unit applied electric

field. Mobility is affected by scattering (lattice and impurity scattering are the main

types), which causes the carriers to lose kinetic energy imparted by the applied field. Hall

measurements by Scott et al. [138] lead to an approximation of µ for 0.2 ≤ x ≤ 0.6:

µn =9 × 108b

Z2a(A.6.1)

where:

b =(

0.2x

7.5)

a =(

0.2x

0.6)

for:

T > 50K;Z = T

T ≤ 50K; Z = 1.18×105

2600−|T−35|2.07

Mobility is dependent on temperature, as given in Eqn. A.6.1 and shown in Fig. A.6.1.

As can be seen in this figure, for temperatures above 40K the mobility decreases sharply

due to increased phonon scattering. For temperatures below 40K, the mobility is limited

by impurity scattering.


Figure A.6.1: Measured mobility vs. temperature, x = 0.2, ND − NA < 1015

cm−3.

Figure A.6.2: Mobility vs.Composition of Hg(1−x)Cd(x)Te.

Equation A.6.1 provides an approximation of mobility, indicating mobility depends on

the mole fraction x and temperature, T . Additionally, mobility depends on ND − NA,

as well as the doping densities ND and NA [139]. As Hg(1−x)Cd(x)Te becomes more

semiconductor like, and less semi-metal like (i.e. x increases), the mobility decreases, as

illustrated in Fig. A.6.2.

The hole mobility is ∼ 0.01 times that of the electron mobility given by Eqn. A.6.1 [140].

This is one reason why commercial photoconductor devices use n-type material, rather

190 A.7. Carrier Lifetimes

than p-type material. Because holes are 100 times less mobile than electrons, sweepout1

does not occur as readily at lower field strengths in p-type photoconductors. This issue

is of little importance photodiodes.

Electron mobility in p-type Hg(1−x)Cd(x)Te (µ′

e) is comparable to mobility in n-type for

low NA (≤ 1015 cm−3). For x = 0.2, µ′

e/µe = 0.5−0.7, and as net acceptor concentration

increases, µ′

e/µe decreases. That is, electron mobility in p-type MCT is lower, relative

to electron mobility in n-type. Higher mobility means longer minority carrier diffusion

lengths (see equations A.8.1 & A.8.2).

A.7 Carrier Lifetimes

Carrier lifetime is the average period of time that a carrier exists before recombining, and

should be represented by a probability density function. Interest is usually focused only

on minority carrier lifetimes, because minority carrier density due to electrical injection

or optical generation may be considerably above the thermal equilibrium value. This

is in contrast with the majority carrier concentration, which is not appreciably changed

compared to the thermal equilibrium value [47].

A.7.0.1 Minority Carrier Lifetimes

Minority carrier lifetimes in Hg(1−x)Cd(x)Te are affected by 3 dominant mechanisms,

Shockley-Read-Hall recombination (SRH), Auger recombination, and radiative recombi-

nation, as given in Eqn. A.7.1. SRH recombination is material dependent, with higher

quality material reducing SRH recombination. Auger recombination and radiative recom-

bination are fundamental recombination processes.

1

τeff=

1

τA+

1

τR+

1

τSRH+S1

d+S2

d(A.7.1)

where:

τeff is the effective minority carrier lifetime, as a result of bulk lifetime and

surface recombination.

τA is the Auger lifetime.

τR is the radiative recombination lifetime.

τSRH is the Shockley Read Hall lifetime.

S1,2 are the front and back surface recombination velocities, respectively.

d is the device thickness.

1Sweepout is the effect whereby increased recombination occurs at the contacts. This is caused by an

electric field across the photoconductor which results in an increase in the loss of minority carriers. It is

mainly evident in photoconductors, not photodiodes.


4 6 8 10 120.0

10.0µ

20.0µ

30.0µ

40.0µ

50.0µ

60.0µ

300 250 200 150 100

Life

time

(s)

1000/T (K-1)

SRH

Radiative

Auger 1

Effective

Temperature (K)

Figure A.7.1: Modelled lifetime vs. 1000/T. Model parameters are x = 0.3, Nd =

1 × 1015 cm−3, Nt = 3 × 1013 cm−3, Cp = 3 × 10−9 cm3 s−1,

Cn = 1.9 × 10−7 cm3 s−1, S1,2 = 0 cm s−1.

Figure A.7.1 illustrates the effect that varying temperature has on lifetime. Model pa-

rameters are x = 0.3, Nd = 1 × 1015 cm−3, Nt = 3 × 1013 cm−3, capture coefficients

of electrons and holes Cp = 3 × 10−9 cm3 s−1, Cn = 1.9 × 10−7 cm3 s−1, respectively,

S1,2 = 0 cm s−1.

Minority carrier lifetimes depend on doping in an inverse relationship. That is, the higher

the doping, the lower the minority carrier lifetime [141]. As well as this dependence

on doping, lifetime is related to the specific dopant type used. In general, impurity-

doped materials have higher lifetimes than vacancy-doped materials [141]. High quality

material (i.e. low concentration of defects < 1014 cm−3) will be dominated by radiative

recombination for x = 0.3 up to doping densities of ≈ 1015 cm−3. Auger recombination

is the dominant mechanism for higher doping densities.


A.7.1 Shockley-Read-Hall Recombination

Shockley-Read-Hall recombination occurs via Shockley Read Hall centers. These centers

are defects, which create energy states in the energy band-gap (section A.3) [48]. Figure

A.7.2 shows recombination via these centers.

The steady-state lifetime of excess holes due to SRH recombination via SRH centers

located Et below the conduction band is given by [49]:

τp =τp0 (n0 + n1) + τn0 (n0 + n1) τp0Nt

(

1 + n0

n1

)−1

n0 + p0 +Nt

(

1 + n0

n1

)−1 (

1 + n1

n0

)−1 (A.7.2)

The steady-state lifetime of excess electrons is similarly:

τn =τp0 (n0 + n1) + τn0 (n0 + n1) τn0Nt

(

1 + p0

p1

)−1

n0 + p0 +Nt

(

1 + p0

p1

)−1 (

1 + p1

p0

)−1 (A.7.3)

where:

τn0 = 1CnNt

(A.7.4)

τp0 = 1CpNt

(A.7.5)

n1 = Nc exp(

−(Ec−Et)kT

)

(A.7.6)

p1 = Nv exp(

−(Et−Ev)kT

)

(A.7.7)

Nc = 2(

2πm∗

ekTh2

)1.5(A.7.8)

Nv = 2(

2πm∗

hkT

h2

)1.5(A.7.9)

p0 = 12

[

NA +(

N2A + 4n2

i

)0.5]

(A.7.10)

n0 =n2

i

p0(A.7.11)

The trap density Nt, and capture coefficients for electrons and holes (Cn, Cp) are all

dependent on the material quality. The effective electron and hole masses (m∗e, m

∗h) are

also material dependent. Equations 2.4.6 and 2.4.7 are given by Nemirovsky et al. [50],

who also approximated the trap energy Et to be

Et =Eg

2+ kT ln

(

m∗h

m∗e

)0.75

− kT ln

(

NA

ni

)

(A.7.12)

SRH recombination can be simplified when the density of SRH centers is less than the

carrier concentration, yielding the SRH recombination lifetime of minority carriers in

HgCdTe as

τSRH =(n0 + n1) τp0 + (p0 + p1) τn0

n0 + p0(A.7.13)

SRH recombination is the dominant recombination mechanism that limits minority carrier

lifetimes in x = 0.3 material at 80K [142].


A.7.2 Auger Recombination

Auger recombination is a direct recombination mechanism. In Hg(1−x)Cd(x)Te Auger

recombination occurs in two dominant combinations, shown in Fig. A.7.3:

• Auger1: direct band-to-band recombination of electron with heavy hole and excite-

ment of another electron in conduction band. This is the dominant mechanism in

n-type material.

• Auger7: direct band-to-band recombination of electron and excitation of electron

from light hole to heavy hole band [48]. This is the dominant mechanism in p-type

material.

In high-quality n-type Hg(1−x)Cd(x)Te at 80K the lifetime is determined by Auger1 re-

combination [143]:

τA1 =2τA1in

2i

n0 (n0 + p0)(A.7.14)

τA1i = 3.8 × 10−18ε2∞m0

m∗e

(

1 +m0

mh

)2 (

1 + 2m0

mh

)(

Eg

kT

)3

2

× exp

(

1 + 2m0

mh

)

(

1 + m0

mh

)

Eg

kT× |f1f2|−2 (A.7.15)

where:

ε∞ is the high frequency permittivity and is taken as

ε∞ = 15.2 − 15.6 × x+ 8.2 × x2.

|f1f2| is the overlap function from the Bloch integral and for Hg(1−x)Cd(x)Te

a value of 0.15 is assumed.

Similarly, for Auger7 recombination the lifetime is given by:

τA7 =2τA7in

2i

p0 (n0 + p0)(A.7.16)

where:

τA7i = γτA1i

γ = ratio between Auger1 and Auger7 intrinsic lifetimes.

Combining Auger1 and Auger7 gives the complete Auger lifetime expression as:

1

τA=

1

τA1+

1

τA7(A.7.17)


x x x x

EcEc

Ev

Figure A.7.2: Shockley-Read-Hall recombination via SRH centers. (a) electron

capture (b) electron emission from center (c) hole capture (d) hole

emission from center.

E

k

E

k

ConductionBand

Heavy HoleBand

Light HoleBand(a) (b)

Figure A.7.3: Auger Recombination (a)Auger1 (b)Auger7 [10].

EcEc

Ev

Photon

Figure A.7.4: Radiative Recombination.


A.7.3 Radiative Recombination

Radiative recombination is recombination of an electron hole pair in which a photon is also

emitted. Radiative recombination can be stimulated by a photon of wavelength similar

to the energy of the recombining electron. Figure A.7.4 shows a radiative recombination

process.

The radiative recombination lifetime is dependent on the absorption coefficient of the

material and the generation rate variable GR, and is given by [51]:

τR =n2

i

GR (n0 + p0)(A.7.18)

where:GR = n2

i 5.8 × 10−13ε1/2∞

(

m0

me+mh

)3/2 (

1 + m0

me+ m0

mh

)

×(

300T

)3/2 (

E2g + 3kTEg + 3.75(kT )2

)

GR is the spontaneous generation rate.

ε∞ is the semiconductor permittivity (F/cm).

A.7.4 Surface and Interface Recombination Effects

Recombination at surfaces and interfaces can dominate over bulk recombination mech-

anisms. For a standard Hg(1−x)Cd(x)Te detector of thickness d ≈ 10 µm, a surface

recombination of less than S ≈ 100 cm s−1 will have negligible effect on effective carrier

lifetime [144].

A.8 Diffusion Length

Diffusion length is the average distance a carrier travels before it recombines. The diffusion

length is related to the carrier lifetime τ [54]:

L = (Dτ)1/2 (A.8.1)

where:

L = diffusion length (m).

D = diffusion coefficient (m2s−1).

τ = minority carrier lifetime (s).

For non-degenerate2 materials, Einstein’s relation describes the diffusion coefficient as:

D =µkT

q(A.8.2)

2A degenerate semiconductor is one in which the electron concentration in the conduction band, or hole

concentration in the valence band, is comparable with the density of states in the band. Consequently,

the Pauli exclusion principle is significant and Fermi-Dirac statistics must be used. The Fermi level is

either in the conduction band for a n+ type degenerate or in the valence band for a p

+ type degenerate

semiconductor.

196 A.9. Refractive Index of HgCdTe

A.9 Refractive Index of HgCdTe

A.9.1 Refractive Index

The real part of the refractive index used in this work is described by Capper [56] and

modified by Daraselia et al. [116]

n(λ)2 = a1 +a2a

23

(

a23 − 1

λ2

)

(

a23 − 1

λ2

)2+(

dfλ

)+

a4λ2

λ2 − a25

(A.9.1)

Table A.9.1: Fitting parameters for Eqn. A.9.1

Coefficient Ai Bi Ci

a1 19.76 -41.82 34.50

a2 0.373 -0.281 2.153

a3 -0.005 0.532 1.897

a1 18.79 -58.40 52.41

A.9.2 Extinction Co-efficient

The extinction co-efficient is derived from the absorption model of Price [63]:

α (E) =

α0 exp(

σ(E−E0)T+T0

)

α ≤ 500 + 5600x = αT

β√

E − Eg α > 500 + 5600x(A.9.2)

k (E) = α (E) ∗ λ

4π ∗ n (A.9.3)

Where :

ai = Ai +Bix+ Cix2 (i = 1, 2, 3, 4)

a5 = 73.25 µm

α0 = exp (−18.88 + 53.61x) (A.9.4)

σ = 3.267 × 104 (1 + x) (A.9.5)

E0 = −0.3424 + 1.838x (A.9.6)

Eg = log

(

αT

α0√e

)(

T + T0

σ

)

+ E0 (A.9.7)

β =αT

(

T + T0

2σ

)1/2(A.9.8)

T0 = 81.9 K


A.10 Refractive Index of CdTe

The refractive index of CdTe is assumed to be real (i.e. non-absorbing) for all wavelengths

of interest, and is given as:

Ev =hc

λ(A.10.1)

n =

√

√

√

√1 +1.031

πlog

(

(3.29q)2 − (Ev)2

(1.49q)2 − (Ev)2

)

· · ·

· · · + 20.567

πlog

(

(5.07q)2 − (Ev)2

(3.29q)2 − (Ev)2

)

+9.891e−3q2

(17.5e−3q)2 − (Ev)2 (A.10.2)

198 A.10. Refractive Index of CdTe

Appendix BOptical Properties and Modelling

B.1 Optical Model

B.1.1 Characteristic Matrix - An Assembly of Films

The optical response of the dielectric stack to an incident plane wave can be modelled

using characteristic matrices for each layer. Any layer has a characteristic matrix given

by Eqn. B.1.1 [86]:

Mj =

[

cos δr (i sin δr) /ηr

iηr sin δr cos (δr)

]

(B.1.1)

where:

δr =2πNrdr cosϑr

ληr = YNr cosϑr for s − polarisation(TE),

ηr = YNr/ cosϑr for p − polarisation(TM),

Y = (ǫ0/µ0)1/2 = 2.6544 × 10−3S,

ϑr is the angle of incidence at each layer,

Nr is the refractive index of each layer, and

dr is the thickness of each layer.

The characteristic matrix of an assembly of n films can be determined by multiplying the

characteristic matrices of all layers.[

Ea

Ha

]

=n∏

j=1

Mj

[

Eb

Hb

]

(B.1.2)

The electric and magnetic fields into and out of the stack are given by Ea, Ha and Eb,

Hb, respectively. By dividing through by Eb and utilizing optical admittance (η = H/E),

Eqn. B.1.1 and B.1.2 become:[

Ea/Eb

Ha/Eb

]

=

[

B

C

]

=

[

cos δr (i sin δr) /ηr

iηr sin δr cos (δr)

] [

1

ηb

]

(B.1.3)

200 B.1. Optical Model

B.1.2 Reflectance, Transmittance, and Absorptance

B and C now represent normalised electric and magnetic fields at the front of the thin

film assembly. These can then be used to calculate the reflectance (or reflectivity, R),

transmittance (T ) and absorptance (A) of an assembly of thin films:

R =

(

η0B − C

η0B + C

)(

η0B − C

η0B + C

)∗

(B.1.4)

T =4η0Re (ηm)

(η0B + C) (η0B + C)∗(B.1.5)

A =4η0Re (BC∗ − ηm)

(η0B + C) (η0B + C)∗(B.1.6)

B.1.3 Potential Transmittance

The absorptance of a single layer is calculated by determining the characteristic matrix

before and after the layer, and hence the electric and magnetic fields before and after the

layer. From this the BC matrix of the layer itself can be calculated, as well as the optical

admittance of the layer ηe, and the potential transmittance ψf :

B1 =E1

E2(B.1.7)

Cl =H1

E2(B.1.8)

ηe =H2

E2(B.1.9)

ψf =Re (ηe)

Re (B1C∗1 )

(B.1.10)

where E1,2 and H1,2 are the electric and magnetic fields at the front and back of the layer,

respectively.

B.1.4 Backside Reflection Correction

Film assemblies are assumed to be on infinite substrates for the purpose of modelling

using the characteristic matrix approach. The fact that real layers are on finite substrates

must be taken into account when modelling. If the substrate is backside rough, then the

assumption of an infinite substrate is valid, as the substrate scatters incident waves and

does not re-introduce waves to the thin film assembly. However, if the substrate is backside

polished, then the reflection from the back surface will be re-introduced to the assembly

of films. The reflectance and transmittance will then be affected [145]:

Ta = T 1 −Rs

1 −RRs(B.1.11)

APPENDIX B. Optical Properties and Modelling 201

Ra = R +R2Rs

1 −RRs(B.1.12)

Rs =

(

ns − ni

ns + ni

)(

ns − ni

ns + ni

)∗

(B.1.13)

where Ra and Ta are the adjusted reflectance and transmittance, R and T are the re-

flectance and transmittance of the thin film assembly, and Rs is the reflection of the

substrate/incident material interface at the back surface, and depends on the refractive

index of the substrate, ns, and incident media, ni.

202 B.1. Optical Model

Appendix CMolecular Beam Epitaxy

Molecular beam epitaxy (MBE) was first discovered in 1969 by Arthur [146] and Cho

[147]. It differs from methods such as vapour phase epitaxy and liquid phase epitaxy in

that it occurs under ultra high vacuum. Beams of the constituent molecules are incident

on a heated substrate and self arrange to form a single crystal. Fig. C.0.1 illustrates

the layout of a simple MBE chamber. The chamber itself is ultra high vacuum (UHV),

which is required to ensure that the molecular beams are not interrupted before they are

incident on the substrate. Hence, the mean free path of the molecular beams is longer

than the distance from the cell to the substrate. Furthermore, UHV is required to ensure

that the grown film remains free of impurities and defects due to reactions with any stray

gasses. The molecular beams come from effusion cells which are thermally isolated from

each other by liquid nitrogen cold shields. The cold shield also performs the task of

Figure C.0.1: Schematic of a simple MBE system after [148].

204

preventing stray molecules from contaminating the chamber, as molecules which are not

incorporated into the crystal growing on the substrate will stick to the walls of the cold

shield.

The flux of the beams is important for determining stoichiometry and growth rate. There

is a beam flux meter that can be rotated into the beam path in order to characterise the

beam flux. One method of determining flux is to measure the beam pressure (typically

with a cold cathode ion gauge) and relate the vapour pressure to the rate of atoms

impinging on the substrate in cm−2s−1 [148]:

dn

dt=

P√2πmkT

(C.0.1)

where P is the gas pressure in Torr, m is the atomic mass, k is Boltzmann’s constant

and T is the absolute temperature. As beam flux is proportional to beam pressure, it is

possible to use the beam pressure directly to calculate growth conditions if a previously

known operating point is established.

During growth the crystallinity of the epitaxial layer can be monitored by reflection high

energy electron diffraction (RHEED) measurements. RHEED is compatible with MBE

and in-situ monitoring can be performed during MBE growth, making it a powerful tool for

analysis of crystal arrangement near the surface. The smoothness of the substrate, buffer

layers and absorber layers can be assessed, and non-optimal growth conditions can be

observed and corrected [149]. Depending on the growth mechanism, the RHEED pattern

can produce an oscillation in intensity (depending on the alternating crystal layers), or

a streaky pattern that contains no oscillations for crystals that grow by the step-flow

method, which have the same average roughness over the whole wafer, and therefore the

same output signal (Hg(1−x)Cd(x)Te exhibits this type of RHEED pattern).

Typically, a RHEED system contains an electron gun, some deflecting plates, the sample

crystal, and a phosphor screen for viewing the diffracted electrons. Figure C.0.2 illus-

trates a typical RHEED system. The electron gun used in this work was operated at 20

kV with an emission current of 100 µA. The bias on the deflection plates is used to direct

the beam. The phosphor screen displays the RHEED pattern, which represents the

two dimensional surface in reciprocal space. If the RHEED pattern consists of elongated

streaks, then the crystal surface is smooth, with longer streaks representing a smoother

surface. Figure C.0.3 illustrates such a streaky RHEED pattern, with the figure illus-

trating the ideal pattern for the[

011]

azimuth during growth. Other azimuths will have

a similar pattern with the number of streaks visible depending on the azimuth. Points

or blobs in the RHEED pattern indicate that the surface is rough, or undergoing three

dimensional growth, while other RHEED features are short extra streaks beside the main

azimuth streaks, which indicate twin crystal plane growth, and angled streaks on the

main azimuth streaks, which indicate faceted growth. A RHEED pattern that consists of

concentric circles indicates polycrystalline growth [149].

Hg(1−x)Cd(x)Te grown by MBE uses liquid Hg, solid CdTe and solid Te. The solid ma-

terials are deposited using conical effusion cells [150]. However, special consideration is

APPENDIX C. Molecular Beam Epitaxy 205

SourceElectron Gun

Deflection Plates

Phosphor Screen

Sample

Figure C.0.2: Schematic of a RHEED system.

[211] Normal(Real Space)

[0 1] Normal1

Shadow

Ideal diffraction pattern(not to scale)

Figure C.0.3: A RHEED diffraction pattern of the[

011]

azimuth during growth.

206

MovableMercurytank

Chamberwall

Mercury cell

pumping

valves

Figure C.0.4: Schematic of a constant level Hg source for MBE growth of

HgCdTe.

needed for the Hg cell. Mercury has a very high vapour pressure (2×10−3 Torr at 300K),

and so Hg cannot be left in the chamber during bakeout or prior to growth. Therefore, a

special effusion cell is used for the Hg. Figure C.0.4 illustrates the reservoir system used

for the Hg effusion cell. The movable mercury reservoir is initially lowered so that all

mercury is drained from the effusion cell and below the valves. The growth chamber can

then be baked out, pumped and made ready for growth. During the growth, the movable

reservoir is raised so that the mercury level is filling the effusion cell as illustrated. The

cell is then heated and the reservoir maintains the volume of Hg within the cell using only

gravity. The reservoir can be height adjusted to maintain the volume of mercury within

the effusion cell.

The effusion cells are held at temperature using PID Eurotherm temperature controllers.

For the growth of Hg(1−x)Cd(x)Te in this work the two CdTe cells were maintained between

480C and 525C. The Te cell was maintained at approximately 330C and the Hg

cell was maintained at about 96C. During growth the mercury pressure dominates the

other source materials. The pressure measured during growth is 1.2×10−5 Torr. This

is compared with the measured beam equivalent pressure (BEP) for CdTe of 1.2×10−6

Torr and for Te of 1.9×10−6 Torr. The growth therefore proceeds under a mercury

overpressure with the limiting constituent being the Te.

Lattice matched CdZnTe substrates with a (211)B orientation are used for growing of

structures in this work. Substantial work is being undertaken to investigate using Si

as a substrate material [151, 152, 153, 154] in order to better integrate detectors onto

read-out integrated circuits (ROIC). Growth on silicon also provides a cheaper substrate

technology, and the larger area further increases economy. Growth on (211)B surfaces does

not proceed via 2D-nucleation growth typical of other crystal orientations such as (100)

[155]. Instead, growth proceeds through the step flow process, where molecules diffuse

along the surface of the crystal as they are deposited. Molecules tend to diffuse along the

terraces of the (211)B lattice structure and attach preferentially at the steps of the lattice

structure. This preferential attachment occurs because of the local minima in potential

APPENDIX C. Molecular Beam Epitaxy 207

V

Figure C.0.5: Potential energy diagram for attachment of atom at a step edge

after [156].

energy of attachment at a step edge [156]. Figure C.0.5 illustrates the potential energy

of attachment at a step edge. The local minima at the step edge causes the preferential

attachment at these edges, and growth therefore proceeds from these edges. Because

of this diffusion process, the substrate temperature is critical for obtaining crystalline

material. The substrate temperature for all samples in this work is held at around 185C.

This temperature provides a good crystal structure for x = 0.3 − 0.4 material. However,

the substrate temperature for good crystal quality CdTe is ≈ 300C. This substrate

temperature is too high for x = 0.3−0.4 material as the Hg atoms in the crystal lattice will

out-diffuse at this temperature. Therefore the layers of CdTe are grown with a substrate

temperature of around 185C. The reduced temperature causes the step flow process to be

less effective, and the CdTe material starts to become polycrystalline. Subsequent mirror

layers of Hg(0.6)Cd(0.4)Te material grown on CdTe layers initially exhibit poor crystallinity,

but good quality crystalline material is grown after approximately 100 nm of poor quality

growth. It should also be noted that there is a mismatch between the lattice spacing

of CdTe and the lattice spacing of Hg(0.7)Cd(0.3)Te (or the CdZnTe substrate, which is

matched to the spacing of the Hg(0.7)Cd(0.3)Te material). This further exacerbates the

problem, and results in the inability of growing thin Hg(0.7)Cd(0.3)Te absorber layers on

CdTe. Therefore, the Hg(0.7)Cd(0.3)Te absorber layer needs to be situated on one of the

Hg(0.6)Cd(0.4)Te mirror layers.

Appendix DProcesses Used

D.1 Photoconductor Fabrication

D.1.1 Wafer Clean

Soak in hot trichloroethylene 1 rinse @ 50 C, 5min

Soak in hot acetone 1 rinse @ 50 C, 5min

Soak in hot methanol 1 rinse @ 50 C, 5min

Soak in RT isopropyl alcohol 1 rinse, 5min

Blow Dry Dry N2

D.1.2 Mesa Isolation

D.1.2.1 Mask

Dry Oven 85C, 5min

Spin HPR photoresist 40s, 4000rpm

Prebake Hotplate, 100C, 1min

Relax 5 min

Align and Expose 30s

Develop MIF developer:DI Water, 1:3, 70s

Rinse DI Water

Blow Dry Dry N2

Postbake Oven 85C, 30mins

210 D.1. Photoconductor Fabrication

D.1.2.2 Etch

Etch1% Br/HBr (1mL Br in 100mL HBr),

90s (etch rate ≈ 3.9 µm/min)

Rinse DI water ≈10min

Blow Dry Dry N2

Soak in RT Acetone 1 rinse, 5min


Blow Dry Dry N2

D.1.3 CdTe Cap Etch

D.1.3.1 Mask

Dry Oven 85C, 5min





Relax 5 min



Rinse DI Water

Blow Dry Dry N2


APPENDIX D. Processes Used 211

D.1.3.2 Etch

To be done in steps:

EtchAgitated 0.5% Br/HBr (0.5mL Br in 100mL HBr),

8s (etch rate ≈ 3.9 µm/min)


Blow Dry Dry N2



Blow Dry Dry N2

Dektak Determine etch rate

Remask as per step D.1.3.1

EtchAgitated 0.5% Br/HBr (0.5mL Br in 100mL HBr),

≈ 10s (etch rate determined by Dektak)


Blow Dry Dry N2



Blow Dry Dry N2

D.1.4 Anodisation

Make electrolyte0.1m KOH in 10%, H2O 90% Ethylene glycol,

(1.12g KOH, 20mL H2O, 180mL Ethylene glycol)

Dip Etch 0.1% Br/Methanol (0.1mL Br in 100mL Methanol)

Flush Methanol, 5s

Rinse Methanol, 1min

Blow Dry Dry N2

Mount metal contact on part of sample

Submerge in Electrolyte Ensure metal contact is clear of electrolyte

Grow OxideCurrent Density 0.15 mA/cm2, Voltage limit 12 V,

Grow until current density decreases to 0.09mA/cm2

Rinse DI water, 5 minutes

Blow Dry Dry N2

212 D.1. Photoconductor Fabrication

D.1.5 Oxide Etch

D.1.5.1 Mask

Dry Oven 85C, 5min





Relax 5 min



Rinse DI Water

Blow Dry Dry N2


D.1.5.2 Etch

Dip Etch HCl:DI water, 1:3, 1 second

Rinse DI Water

Blow Dry Dry N2



Blow Dry Dry N2

D.1.6 Metallisation

D.1.6.1 Metal Mask

Dry Oven 85C, 5min

Spin AZ2035 photoresist 40s, 2000rpm

Softbake Hotplate, 95C, 1min

Relax 5 min


Hardbake Hotplate, 110C, 1min

Develop 300MIF developer, neat, 120s

Rinse DI Water

Blow Dry Dry N2



D.1.6.2 Metal Deposition and Liftoff

Clean and Load Metal 200mg In

Check thickness monitor

Load the sample

Evacuate metallisation chamber <1×10−6mbar

Evaporate 3000Aof In Rate ≈ 5 Aper sec.

Cool down 30min

Metal liftoff

Soak in acetone 30min

Squirt acetone gently(!) to accelerate metal detach-

ment

Soak for another 10-20min

Squirt acetone again (this time bit harder than be-

fore - but not too hard)

Soak/agitate for another 5min, then gently squirt

acetone again

Soak in RT Methanol 1 rinse, 5min


Blow Dry Dry N2

Inspect

D.2 Photodiode Fabrication

D.2.1 Wafer Clean

Soak in hot trichloroethylene 1 rinse @ 50 C, 5min

Soak in hot acetone 1 rinse @ 50 C, 5min

Soak in hot methanol 1 rinse @ 50 C, 5min


Blow Dry Dry N2

214 D.2. Photodiode Fabrication

D.2.2 ZnS Deposition

Clean chamber

Check ZnS crystals in boatEnsure no crystals in center

Check evap current

Check thickness monitor Monitor should turn on without a XTAL FAIL

Mount sample and place in chamber Ensure that the sample is directly over the boat

Attach thermocouple to plate Test temperature output

Attach current source to plate Test current connection

Close shutter

Chamber closed

Pump down <1×10−6mbar

Heat sample Turn off heater at 50C

Heat ZnS boat with shutter closedI=5A (pressure initially increases then should return

to previous value), 1 min

Open shutterIncrease current to 6A (30 to 60 second delay before

deposition rate changes)

Evaporate ZnS 2000A 0.2A/s

Cool down Until T<50C OR 2 hours in vacuum

D.2.3 Windows in ZnS

D.2.3.1 Mask

Dry Oven 85C, 5min



Relax 5 min



Rinse DI Water

Blow Dry Dry N2


D.2.3.2 ZnS Etch

Reduce underetch Soak DI water, 1 min

EtchHCl: DI water 2:1 (100mL HCl, 50mL H2O), 10s

(until colour change)


Blow Dry Dry N2


D.2.4 Etch Contact Vias

Etch 1% Br/HBr (1mL Br in 100mL HBr), 120s


Blow Dry Dry N2



Blow Dry Dry N2

D.2.5 RIE Etch/Type Conversion

Clean chamber with oxygen plasma

300W - 2min,

200W - 8min

100mTorr, 50sccm, 10min

Load sample

Establish chamber conditions

54 sccm H2 10 sccm CH4

Base pressure 35mT

Process pressure 100mT

Form junction

Power 120W

Etch time 2min

Approx etch rates: MCT 0.12um/min ZnS

0.04um/min

Unload sample

Pump out gases, purge, pump, purge

Unload sample

Flush gas lines

D.2.6 ZnS Etch

Dip EtchHCl: DI water 2:1 (100mL HCl, 50mL H2O), 10s

(until colour change)


Blow Dry Dry N2


D.2.7 Window for P Contact

D.2.7.1 Mask

Dry Oven 85C, 5min





Relax 5 min


Develop MIF developer : DI Water, 1:3, 70s

Rinse DI Water

Blow Dry Dry N2


D.2.7.2 Etch

Etch 1% Br/HBr (1mL Br in 100mL HBr), 120s


Blow Dry Dry N2



Blow Dry Dry N2

D.2.8 Metallisation

D.2.8.1 Metal Mask

Dry Oven 85C, 5min

Spin AZ2035 photoresist 40s, 2000rpm

Softbake Hotplate, 95C, 1min

Relax 5 min


Hardbake Hotplate, 110C, 1min

Develop 300MIF developer, neat, 120s

Rinse DI Water

Blow Dry Dry N2

Postbake Oven 85, 30mins


D.2.8.2 Metal Deposition and Liftoff

Load Cr and Au

Check thickness monitor

Load the sample

Evacuate metallisation chamber <1×10−6mbar

Evaporate 100 Aof Cr Rate ≈ 1 Aper sec.

Evaporate 3000Aof Au Rate ≈ 5 Aper sec.

Cool down 30min

Metal liftoff

Soak in acetone 30min

Squirt acetone gently(!) to accelerate metal detach-

ment

Soak for another 10-20min

Squirt acetone again (this time bit harder than be-

fore - but not too hard)

Soak/agitate for another 5min, then gently squirt

acetone again

Soak in RT Methanol 1 rinse, 5min


Blow Dry Dry N2

Inspect

Appendix EAuthor’s Publications List

The following is a listing of the authors publications, including a division of the contribu-

tion. There is naturally guidance and support (financial and in the form of manuscript

revision, discussion etc.) from supervisors estimated at approximately 15-20% of the ef-

fort. Of the remaining effort, the divisions of each contributor are indicated, as well as

the details of contribution.

E.1 Journal Publications:

[1] Wehner J.G.A., Nguyen T.N., Antoszewski J., Musca C.A., Dell J.M., Faraone

L., Resonant Cavity-Enhanced Mercury Cadmium Telluride Detectors, J.

Electron. Mat. 33, 6; p. 604-608, 2004.

The percentage contribution of each author is as follows:

• Wehner J.G.A. 80%, All, except -

• Nguyen T.N. 10%, technical discussions and coding assistance.

• Antoszewski J. 5%, technical discussions.

• Musca C.A. 5%, technical discussions.

• Dell J.M. Supervisor.

• Faraone L. Supervisor.

[2] Wehner J.G.A. Sewell R.H., Antoszewski J., Musca C.A., Dell J.M., Faraone,

L. Mercury Cadmium Telluride/Cadmium Telluride Distributed Bragg Reflec-

tors for Use with Resonant Cavity Enhanced Detectors, J. Electron. Mat., 34,

6, pp. 710-715, 2005.

220 E.1. Journal Publications:



• Sewell R.H. 15%, technical discussions and growth assistance.





[3] Wehner J.G.A., Musca C.A., Sewell R.H., Dell J.M., Faraone L., Mercury

Cadmium Telluride Resonant Cavity Enhanced Photoconductive Infrared De-

tectors, Appl. Phys. Lett. 87, pp 211104, 2005.




• Sewell R.H 15%, technical discussions and growth assistance.



[4] Wehner J.G.A., Musca C.A., Sewell R.H., Dell J.M., and Faraone L., Respon-

sivity and Lifetime of Resonant-Cavity-Enhanced HgCdTe Detectors, Solid

State Electronics, 50, pp 16401648, 2006.







APPENDIX E. Author’s Publications List 221

Accepted for Publication:

[5] Wehner J.G.A. Sewell R., Musca C.A., Dell J.M., Faraone, L. Refractive Index

Variations in MBE Grown CdTe for RCE Structures, Accepted 2/1/07 to J.

Electron. Mat..



• Sewell R.H 45%, x-ray measurements/writing , technical discussions and

all growth.




222 E.2. Conference Publications:

E.2 Conference Publications:

[1] Wehner J.G., Martyniuk M., Antoszewski J., Musca C.A., Dell J.M., Faraone

L. Optical and Membrane Modelling in a MEMS Hyper-spectral Imaging Sys-

tem, Conf. on Optoelectron. and Microelectron. Mat. And Dev.(COMMAD

2002), 11-13 Dec. 2002, UNSW, Sydney, Australia, IEEE Proc. 02EX 601,

pp. 579-582 (2002).



• Martyniuk M. 15%, Membrane images, technical discussion.





[2] Antoszewski J., Dell J.M., Shivakumar T., Martyniuk M., Winchester K.,

Wehner J., Musca C.A., Faraone L. Towards MEMS based infrared tun-

able micro-spectrometers, Smart Structures, Devices, and Systems, 16-18 Dec.

2002, RMIT, Melbourne, Australia, SPIE Proc. 4935, pp. 148-154 (2002).

The percentage contribution of this author is as follows:

• Wehner J.G.A. 5%, technical discussion.

[3] Wehner J.G.A., Sewell R., Antoszewski J., Musca C.A., Dell J.M., Faraone

L. Refractive Index Engineering for a Distributed Bragg Reflector for a Res-

onant Cavity Enhanced Detector, Design, Technology, and Packaging, Perth,

Australia, 10-12 Dec. 2003, SPIE Proc. 5277, pp. 138-145 (2004).



• Sewell R.H. 15%, technical discussions and modelling assistance.





[4] Wehner J.G.A, Sewell R.H., Musca C.A., Dell J.M., Faraone L. Responsivity

and Detectivity of Resonant Cavity Enhanced Mercury Cadmium Telluride

Infrared Detectors, Proceedings of Conf. on Optoelectron. and Microelec-

tron. Mat. and Dev. (COMMAD 2004), 8-10 Dec 2004, The University of

Queensland, Australia (2004).

APPENDIX E. Author’s Publications List 223







[5] Wehner J.G.A., Sewell R.H., Musca C.A., Dell J.M., Faraone L. Resonant Cav-

ity Enhanced HgCdTe Detectors, Oral presentation IEEE Laser and Electro-

optic Society symposium 2005, Sydney, 23-27 Oct 2005 (2005).







[6] Wehner J.G.A., Musca C.A., Sewell R.H., Dell J.M., Faraone L. Resonant

cavity enhanced HgCdTe photodetectors. Poster presentation at SPIE De-

fense and Security Symposium, Infrared Technology and Applications XXXII,

17-22 April 2006 at Gaylord Palms Resort and Convention Center in Orlando,

Florida, USA, SPIE Proc., pp. (2006).







[7] Wehner J.G.A., Musca C.A., Sewell R.H., Dell J.M., Faraone L. Responsivity

and Lifetime of Resonant Cavity Enhanced HgCdTe detectors, 2006 IEEE

Aerospace Conf., Big Sky, 3-4 March 2006 Montana, USA (2006).







224 E.2. Conference Publications:

Accepted Conference Presentations:

[8] Wehner J.G.A., Musca C.A., Sewell R.H., Dell J.M., Faraone L. Annealing

and Shunting in RCE HgCdTe Photoconductors, To be delivered: Conf. on

Optoelectron. and Microelectron. Mat. and Dev. (COMMAD 2006), 6-8 Dec

2006, The University of Western Australia, Australia (2006).







Appendix FDetails of Contributions

The following is a listing of the contributions to the work presented:

Chapters 1 and 2

These are introductory chapters and all work is adequately referenced. Contribution in

the form of draft revision and proof reading by supervisors is acknowledged.

Chapter 3

Chapter 3 is an introductory chapter, however, it also contains modelling results published

in journal publication 1 in appendix E.

Chapter 4

Chapter 4 discusses mirror design and experimentation. There are results published in

journal publications 2 and 5 in appendix E. The results presented in section 4.4.2.2 are

based on measurements performed at the Australian Nuclear Science and Technology

Organisation (ANSTO), Lucas Heights by Gordon Tsen. Figure 4.4.10 is the work of

R.H. Sewell, and is the only work from journal publication 5 included that is not the

authors work.

Chapter 5

Chapter 5 discusses resonant-cavity-enhanced device design and experimentation. There

are results published in journal publications 1, 3 and 4 in appendix E, as well as results

published in conference proceedings 8.

“A witty saying proves nothing.” – Voltaire

+ =

“Ah, beer. The cause of and the solution to all of life’s problems.” –Homer J. Simpson

228

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