thermal and radiation exposure of graphene …hr475nc1100/...thermal and radiation exposure of...

THERMAL AND RADIATION EXPOSURE OF GRAPHENE-ENHANCED

GALLIUM NITRIDE ULTRAVIOLET PHOTODETECTORS FOR SPACE

EXPLORATION

A DISSERTATION

SUBMITTED TO THE DEPARTMENT OF

AERONAUTICS AND ASTRONAUTICS

AND THE COMMITTEE ON GRADUATE STUDIES

OF STANFORD UNIVERSTIY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Ruth A. Miller

March 2018

http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/hr475nc1100

© 2018 by Ruth Anne Miller. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

ii



http://purl.stanford.edu/hr475nc1100

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

Debbie Senesky, Primary Adviser


Fu-Kuo Chang


Sigrid Close

Approved for the Stanford University Committee on Graduate Studies.

Patricia J. Gumport, Vice Provost for Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.

iii

iv

Abstract

Space exploration, including gathering data and learning about Earth’s past and

possible future, sending humans to the surface of other planets, as well as searching for

life beyond Earth, requires sensors to operate reliably within the harsh environment of

space. Specifically, sensors operating within space need to withstand extreme

temperatures (from -229°C near Pluto to 460°C on the surface of Venus) and high levels

radiation (ultraviolet light, gamma rays, protons, electrons, neutrons, heavy ions, etc.)

as well as extreme pressures, high heat fluxes, and various forms of chemical and

physical corrosion. Currently, complex packaging schemes are used to keep

instrumentation at operable temperatures and shield against radiation at the cost of

added mass. By developing new sensor material platforms such as gallium nitride

(GaN), which is intrinsically radiation-hard and thermally stable, much of the extra

protective packaging can be removed. Additionally, due to its wide bandgap, GaN is

visible-blind and highly responsive to ultraviolet light making GaN an ideal material

platform for developing ultraviolet photodetectors for space-based applications.

Photodetectors that use traditional metal electrodes exhibit low quantum

efficiencies due to the metal electrodes blocking light from being absorbed by the

semiconductor. Here it is shown that graphene, a monolayer of carbon atoms, has high

v

transmission in the ultraviolet regime and creates a Schottky contact to GaN, resulting

in ultraviolet photodetectors with increased sensitivity. A microfabrication process for

manufacturing graphene-enhanced GaN MSM ultraviolet photodetectors as well as GaN

MSM photodetectors with semitransparent Ni/Au electrodes has been developed. Both

types of GaN photodetectors are characterized and compared while operating under

temperatures from room temperature up to 200°C. Thermionic, thermionic field, and

field emissions current transport models are examined and compared to the

experimental results to determine the temperature-dependent trends of GaN metal-

semiconductor-metal ultraviolet photodetectors. Additionally, the response of

graphene-enhanced and semitransparent Ni/Au GaN MSM photodetectors subjected to

2 MeV protons up to a fluence of 3.8 × 1013 cm-2 is examined through electrical

characterization and Raman spectroscopy of the graphene.

In addition to characterizing the response of GaN-based photodetectors

operating at high temperatures and when subjected to high levels of proton irradiation,

this thesis also details the implementation and testing of these sensors in flight-relevant

applications. A miniature sensor for detecting the orientation of incident ultraviolet light

was created with a focal plane consisting of a 3 × 3 array of microfabricated GaN-based

photodetectors. The photodetector array was integrated with an aperture mask to realize

a miniature sun sensor capable of determining incident light angles with a ±45° field of

view. The miniature sun sensor was integrated with a CubeSat to test its performance in

space. GaN photodetectors were also used for in situ measurements of ultraviolet shock-

layer radiation in a Titan simulant atmosphere in an Electric Arc Shock Tube in support

of atmospheric entry sensing technologies.

vi

Acknowledgments

First, I would like to acknowledge my advisor Professor Debbie Senesky. Thank

you for allowing me to join the Extreme Environment Microsystems Laboratory (XLab)

in its early days. Thank you for helping me navigate the interdisciplinary nature of my

interests and the XLab’s research goals. Thank you for pushing me to achieve more than

I ever thought I could (I didn’t think I’d be doing semiconductor device physics when I

entered grad school!) while allowing me to explore what I was interested in (Everything

and anything space and NASA related!). Thank you for being an amazing mentor and

friend on this crazy journey of grad school.

Thank you to Professor Sigrid Close and Professor Fu-Kuo Chang for your

support throughout my time at Stanford. Thank you serving on my committee and

providing your insightful comments as well and challenging questions.

I would like to thank the staff of the Stanford Nanofabrication Facility (SNF)

and Stanford Nano Shared Facilities (SNSF) for training me to use the microfabrication

equipment and helping develop my fabrication process. I would also like to express my

gratitude to Pauline Prather for her expert wirebonding and encapsulation without which

much of the harsh environment characterization would not have been possible.

vii

I would like to thank Professor Andrew Kalman for allowing me to incorporate

my miniature sun sensor on-board the QB50 CubeSat Discovery as well as teaching me

about CubeSats and developing flight software. I owe more thank yous than I can give

to David Wright and Monica Hew for working tirelessly to develop the electronics and

software to support the miniature sun sensor. I’m so grateful to have worked with both

of you. Also, many thanks to Dr. Nicolas Lee and Ana Tarano for managing the QB50

project and keeping me informed of all of the ups and downs.

I would like to thank Dr. Brett Cruden of NASA Ames research center for

teaching me about optical spectroscopy, allowing me to test my photodetectors in Ames’

Electric Arc Shock Tube (EAST) facility, and patiently answering my countless

questions. Your mentorship has been invaluable in my PhD and future career. Thank

you to Ramon Martinez for your willingness to help me conduct my experiments. Thank

you to Dr. David Hash for managing a fantastic internship program within the

Aerothermodynamics branch at Ames. Also, thanks to Mark McGlaughlin and Rick

Ryzinga for expert maintenance and operation of the EAST facility.

I would like to thank Yongqiang Wang, Joe Tesmer, and Bob Houlton of the Ion

Beam Materials Laboratory, Center for Integrated Nanotechnologies at Los Alamos

National Laboratory, who helped perform proton beam irradiation.

I would like to thank all of my XLab colleagues, past and present. I have learned

so much from all of you and I’m incredibly grateful to have been able to work with each

of you. Special thanks to Caitlin Chapin, Ateeq Suria, Minmin Hou, Ashwin Shankar,

and Karen Dowling who have been around since the beginning of the XLab. You have

all been a constant source of inspiration, knowledge, and made the XLab a wonderful

place to work. Also, the countless hours spent together in the cleanroom will not be

forgotten. Thank you to Dr. Heather Chiamori for introducing me to graphene and the

transfer process. Thank you to Dr. Hongyun So for your willingness to help as well as

reviewing and improving my papers.

viii

I am very grateful for all of the experiences I had during my time at Stanford,

inside and outside of the classroom and lab. One of the many reasons I was driven to

pursue a degree in aerospace engineering was to launch a rocket, which I was able to do

thanks to AA284. Designing, building, and launching a nitrox-paraffin hybrid rocket in

only two quarters was truly one of the most rewarding experiences. I would like to thank

everyone involved with AA284 especially my team. I will never forget the late night

testing on top of the parking structure and the bittersweet ending in the Mojave.

Participating in the young astronauts program, teaching science and engineering to local

grade school children, was another highly rewarding experience. Seeing their wide-eyed

wonderment and enthusiasm reminded me why I am here. I would also like to thank all

of my amazing friends who remind me there’s life outside of research.

Last and most important, I need to thank my family. I would not be here without

you, this degree is just as much yours as it is mine. Thank you for always encouraging

me to follow my dreams.

ix

Contents

1 Introduction 1

1.1 The Need for Space-Grade Instrumentation .................................................... 1

1.2 State-of-the-Art Sensor and Electronics .......................................................... 4

1.3 Emerging Material Platforms for Space Exploration ...................................... 5

1.4 Ultraviolet Photodetectors ............................................................................... 7

1.5 Graphene as a Space-Grade Material ............................................................. 15

1.6 Thesis Outline ................................................................................................ 16

2 GaN MSM Photodetector Operation and Modeling 18

2.1 Principal of Operation .................................................................................... 18

2.2 Semi-Analytical Model .................................................................................. 22

2.2.1 Dark Current Model ........................................................................ 22

Abstract iv

Acknowledgments vi

List of Tables xiii

List of Figures xv

x

2.2.2 Dark Current Results ....................................................................... 27

2.2.3 Photocurrent Model ......................................................................... 31

2.2.4 Photocurrent Results ....................................................................... 31

2.3 Conclusions .................................................................................................... 34

3 Fabrication and Spectral Characterization 36

3.1 Graphene-Enhanced GaN Photodetector Fabrication .................................... 36

3.1.1 Microfabrication Process ................................................................ 36

3.1.2 Graphene Transfer Process ............................................................. 38

3.2 Optical Properties and Spectral Response ..................................................... 40

3.2.1 Optical Properties of GaN Substrate ............................................... 40

3.2.2 Optical Properties of Graphene and Semitransparent Ni/Au .......... 41

3.2.3 Spectral Response Test Set-up ........................................................ 43

3.2.4 GaN Photodetector Spectral Response ............................................ 44

3.3 Conclusions .................................................................................................... 48

4 Extreme Environment Operation 50

4.1 High-Temperature Exposure .......................................................................... 50

4.1.1 Experimental Set-Up ....................................................................... 50

4.1.2 Results ............................................................................................. 51

4.2 Proton Irradiation Testing .............................................................................. 60

4.2.1 The Space Radiation Environment .................................................. 60

4.2.2 Test Set-Up ...................................................................................... 62

4.2.3 Electrical Characterization .............................................................. 63

4.2.4 Raman Spectroscopy ....................................................................... 65

4.3 Conclusions .................................................................................................... 68

5 Implementation 70

5.1 Miniature Sun Sensor ..................................................................................... 70

5.1.1 Current Sun Sensor Technology ..................................................... 70

xi

5.1.2 Principal of Operation ..................................................................... 73

5.1.3 Array Characterization .................................................................... 75

5.1.4 Miniature Sun Sensor Performance ................................................ 77

5.2 CubeSat Integration and Ground Testing ...................................................... 81

5.2.1 QB50 Mission ................................................................................. 81

5.2.2 Miniature Sun Sensor Integration ................................................... 82

5.2.3 Ground Test Results ........................................................................ 83

5.3 In Situ Shock-Layer Radiation Measurements .............................................. 86

5.3.1 Current Atmospheric Entry Sensing Technology ........................... 86

5.3.1 Shock Tube Test Set-up .................................................................. 89

5.3.3 Shock Tube Test Results ................................................................. 91

5.4 Conclusions .................................................................................................... 95

6 Conclusions and Future Work 97

6.1 Conclusions .................................................................................................... 97

6.2 Future Work ................................................................................................. 100

A MSM Photodetector Analytical Model Equations 103

B AlGaN/GaN-Based Photodetectors 108

B.1 Principal of Operation .................................................................................. 108

B.2 Fabrication and Spectral Characterization ................................................... 110

B.3 Extreme Environment Operation ................................................................. 115

B.3.1 AlGaN/GaN Phototransistor High-Temperature Testing .............. 115

B.3.2 AlGaN/GaN MSM Low-Temperature Testing ............................. 120

B.3.3 AlGaN/GaN MSM Proton Irradiation ........................................... 125

C Experiment Drawings 128

D Fabrication Run Sheet 139

xii

Bibliography 149

xiii

List of Tables

Table 1.1.1. Extreme environments experienced at planetary bodies. ........................... 3

Table 1.3.1. Semiconductor material properties comparison. ........................................ 6

Table 2.2.1. Parameter values for carrier mobility model of wurtzite GaN [94]. ........ 27

Table 2.2.2. Material properties of GaN used in MSM photodetector modeling. ........ 27

Table 4.1.1. Decay times of the graphene/GaN and semitransparent Ni/Au/GaN

photodetectors from 23°C to 200°C. ................................................................ 59

Table 4.2.1. Average pre- and 30 days post-irradiation PDCR and responsivity for

graphene and semi-transparent Ni/Au GaN-on-sapphire photodetectors. The

graphene electrodes were damaged post-irradiation during transport. ............. 65

Table 4.2.2. Proton implantation depth and material as a function of energy for the GaN-

on-sapphire substrate as calculated using the Stopping and Range of Ions in

Matter (SRIM) software. .................................................................................. 66

Table 5.1.1. Sun sensor technology comparison. ......................................................... 72

Table 5.1.2. Miniature sun sensor dimensions. ............................................................ 75

Table 5.1.3. Photodetector PDCR and responsivity at -1 V bias before attaching the

aperture mask. PD 1 was unintentionally scratched during electrical

measurements. .................................................................................................. 76

xiv

Table 5.2.1. Photodetector PDCR and responsivity at 5 V bias before attaching the

aperture mask to the QB50 miniature sun sensor. Three of the graphene

photodetectors (PD 4, PD 7 and PD 9) were not operational due to poor graphene

transfer. ............................................................................................................. 84

Table 5.2.2. “SUN ANGLE ARRAY” of the miniature sun sensor integrated with the

QB50 CubeSat with illumination angle of 0° and 5 V bias. ............................. 85

Table 5.3.1. Spacecraft to date that have or are planned to measure in-flight shock-layer

radiation during atmospheric entry. .................................................................. 88

Table 5.3.2. Shock test parameters. .............................................................................. 91

Table B.3.1. Measured thermocouple temperature versus chuck set temperature. .... 122

Table B.3.2. Average pre- and 30 days post-irradiation PDCR and responsivity for semi-

transparent Ni/Au AlGaN/GaN on GaN-on-sapphire photodetectors. ........... 127

xv

List of Figures

Figure 1.1.1. Temperature extremes and yearly radiation doses experienced at each

planet. Image adapted from discovery.com. ....................................................... 2

Figure 1.4.1. Black body radiation (Plank’s law) at 5800 K approximating the solar

irradiance spectrum outside Earth’s atmosphere with absorption

wavelengths/energies of GaN and Si indicated. ................................................. 8

Figure 1.4.2. Schematic structure of photoconductor, MSM, Schottky, p-n, p-i-n, and

avalanche semiconductor photodetectors. .......................................................... 9

Figure 1.4.3. Responsivity comparison of SiC, ZnO, GaN, and AlN photodetectors

as reported in literature with corresponding 50% and 100% quantum efficiency

values for each material indicated. ................................................................... 11

Figure 1.4.4. Maximum reported operational temperature comparison of Si, SiC,

ZnO, and III-nitride photodetectors. ................................................................. 12

Figure 2.1.1. Fundamental operating principal of semiconductor photodetectors. .. 19

Figure 2.1.2. Current transport mechanisms in metal-semiconductor junctions. ..... 20

Figure 2.1.3. MSM photodetector band diagram. .................................................... 22

Figure 2.2.1. GaN’s bandgap as a function of temperature. ..................................... 24

Figure 2.2.2. Intrinsic carrier concentration of GaN as a function of temperature. . 25

xvi

Figure 2.2.3. GaN (a) electron and (b) hole mobility as functions of temperature. . 26

Figure 2.2.4. Ni/GaN (a) electron and (b) hole barrier height as functions of

temperature. ...................................................................................................... 27

Figure 2.2.5. Semi-analytical model of GaN MSM photodetector dark current-

voltage from -100°C to 200°C for (a) thermionic emission, (b) thermionic field

emission, and (c) field emission. Note that the data shown in the left side and

right side of (a), (b), and (c) is the same. However, the figures on the right

employ a logarithmic current axis for clarity. .................................................. 29

Figure 2.2.6. Semi-analytical model of GaN MSM photodetector dark current-

voltage from -100°C to 200°C. ......................................................................... 30

Figure 2.2.7. Semi-analytical model of GaN MSM photodetector total dark current

vs. voltage for temperatures from -100°C to 200°C with (a) linear current axis

and (b) logarithmic current axis. ...................................................................... 30

Figure 2.2.8. GaN MSM photodetector dark and 365 nm illuminated current-voltage

response for temperatures from -100°C to 200°C with (a) linear current axis and

(b) logarithmic current axis. ............................................................................. 32

Figure 2.2.9. GaN MSM photodetector (a) PDCR and (b) responsivity as functions

of temperature at 200 mV bias. ........................................................................ 33

Figure 2.2.10. GaN MSM photodetector (a) PDCR and (b) responsivity as functions

of voltage at 23°C. ............................................................................................ 34

Figure 3.1.1. GaN-on-sapphire MSM photodetector microfabrication process: (a)

GaN-on-sapphire chip, (b) mesa etch for device isolation, (c) deposited SiO2

passivation layer, (d) passivation etch, (e) semitransparent Ni/Au or graphene

electrodes, and (f) deposited Ti/Au electrical contacts and interconnect.

Reprinted from [105], with the permission of AIP Publishing. ....................... 37

xvii

Figure 3.1.2. Top-view optical image of a GaN MSM photodetector with

semitransparent Ni/Au electrodes. Reprinted from [105], with the permission of

AIP Publishing. ................................................................................................. 38

Figure 3.1.3. Graphene transfer process: (a) graphene grown on copper foil via

chemical vapor deposition, (b) spin on PMMA, (c) oxygen plasma etch to

remove backside graphene, (d) etch copper foil, (e) rinse in deionized water, (f)

transfer graphene/PMMA stack to substrate, and (g) remove PMMA. ............ 39

Figure 3.1.4. (a) Top-view optical image and (b) top-view SEM of graphene-

enhanced GaN MSM photodetector. ................................................................ 40

Figure 3.2.1. Optical properties of the GaN-on-sapphire substrate: (a) absorbance and

reflectance spectra showing a cutoff wavelength around 370 nm and (b)

transmission spectrum and (αhν)2 versus hν with extrapolation of the linear

region (red dotted line) showing a bandgap of 3.34 eV. Reprinted from [105],

with the permission of AIP Publishing. ............................................................ 41

Figure 3.2.2. (a) Transmission spectra of graphene and semitransparent Ni/Au

(3 nm / 10 nm) on 1 mm thick glass and (b) corrected transmission spectra of

graphene and semitransparent Ni/Au. Reprinted from [117], with the permission

of AIP Publishing. ............................................................................................ 42

Figure 3.2.3. Cross-sectional schematic of (a) a graphene/GaN and (b) a

semitransparent/Ni/Au/GaN MSM photodetector. Reprinted from [117], with

the permission of AIP Publishing. .................................................................... 42

Figure 3.2.4. Benchtop test set-up for spectral response characterization of GaN-

based photodetectors with the applied optical power incident on the GaN-based

photodetectors shown in the inset. Reprinted from [118], with the permission of

AIP Publishing. ................................................................................................. 44

xviii

Figure 3.2.5. Current-voltage response of four semitransparent Ni/Au GaN-on-

sapphire MSM photodetectors on the same chip when unilluminated (dark) and

illuminated with wavelengths from 200 nm to 500 nm. ................................... 45

Figure 3.2.6. NPDR as a function of illumination wavelength for four

semitransparent Ni/Au GaN-on-sapphire MSM photodetectors. ..................... 47

Figure 3.2.7. Spectral responsivity of four semitransparent Ni/Au GaN-on-sapphire

MSM photodetectors. ....................................................................................... 48

Figure 4.1.1. Experimental test set-up for characterizing the elevated temperature

response of GaN-based photodetectors using a high-temperature probe

station…. .......................................................................................................... 51

Figure 4.1.2. (a) Graphene-enhanced and (b) semitransparent Ni/Au GaN-on-

sapphire MSM photodetector current-voltage curves under dark (dashed lines)

and 365 nm illuminated (solid lines) conditions from room temperature to

200°C… ............................................................................................................ 52

Figure 4.1.3. PDCR and responsivity as functions of voltage at temperatures from

23°C to 200°C for (a) and (b) a graphene/GaN photodetector and (c) and (d) a

semitransparent Ni/Au/GaN photodetector. ..................................................... 53

Figure 4.1.4. (a) PDCR and (b) responsivity as functions of temperature from room

temperature to 200°C for graphene-enhanced (shown in the inset) and

semitransparent Ni/Au GaN-on-sapphire MSM photodetectors. ..................... 55

Figure 4.1.5. Extracted barrier height as a function of temperature for (a)

graphene/GaN and (b) semitransparent Ni/Au/GaN photodetectors. ............... 56

Figure 4.1.6 GaN-based photodetectors reported in literature with responsivities

corresponding to quantum efficiencies greater than 100%. ............................. 57

Figure 4.1.7 Trapped photo-generated holes causing lowering or bending of the

Schottky barrier height resulting in an internal photo-gain. ............................. 58

xix

Figure 4.1.8. Transient current response of the (a) graphene-enhanced and (b)

semitransparent Ni/Au GaN-on-sapphire photodetectors from room temperature

to 200°C under a 365 nm UV intensity of 0.7 ± 0.1 mW/cm2. ........................ 59

Figure 4.2.1. One year trapped proton fluence as a function of energy and aluminum

shielding thickness for (a) low Earth orbit approximating the international space

station orbit and (b) Jupiter orbit approximating the science orbit of the Juno

spacecraft; calculated using the SPace ENVironment Information System

(SPENVIS) software. ....................................................................................... 61

Figure 4.2.2. (a) GaN-based photodetectors mounted inside the tandem ion

accelerator chamber at Los Alamos National Laboratory undergoing proton

irradiation and (b) test set-up for electrical characterization of proton irradiated

photodetectors. .................................................................................................. 62

Figure 4.2.3. Dark and illuminated (365 nm) scaled current-voltage response as a

function of proton irradiation fluence for GaN-on-sapphire photodetectors with

(a) graphene electrodes and (b) semitransparent Ni/Au electrodes. Reprinted

from [117], with the permission of AIP Publishing. ........................................ 64

Figure 4.2.4. (a) PDCR and (b) responsivity as a function of proton fluence for

graphene and semitransparent Ni/Au GaN-on-sapphire photodetectors.


Figure 4.2.5. SRIM ion distribution results for (a) 200 keV and (b) 2 MeV protons

impingent on graphene on a 5 μm GaN, 200 nm AlN and 500 μm sapphire

substrate. For 200 keV protons the stopping depth and thus majority of lattice

displacement damage occurs at approximately 2 μm which is within the GaN.

Whereas for 2 MeV protons, the stopping depth is in the sapphire substrate at

52.3 μm. ............................................................................................................ 67

xx

Figure 4.2.6. Raman spectroscopy comparison of graphene samples before and after

200 keV proton irradiation up to a fluence of 3.8 × 1013 cm-2. Reprinted from

[117], with the permission of AIP Publishing. ................................................. 68

Figure 5.1.1. Cross-sectional schematic of a GaN-based miniature sun sensor

depicting the principle of operation and design parameters using an aperture

mask, spacer, and focal plane. The inset shows the top view of a GaN-on-

sapphire MSM photodetector. Reprinted from [105], with the permission of AIP

Publishing. ........................................................................................................ 74

Figure 5.1.2. Optical image of 3 × 3 GaN-based MSM photodetector array. Reprinted

from [105], with the permission of AIP Publishing. ........................................ 75

Figure 5.1.3. Scaled current-voltage response of all nine photodetectors in the array

under dark and illuminated conditions before attaching the aperture mask.


Figure 5.1.4. Image of assembled miniature sun sensor with size comparison to a

penny. Reprinted from [105], with the permission of AIP Publishing. ............ 78

Figure 5.1.5. Scaled and normalized PDCR and responsivity comparisons of all nine

photodetectors when miniature sun sensor is illuminated (365 nm) at ((a) and

(b)) θsun = 0° and ((c) and (d)) θsun = 45°. Reprinted from [105], with the

permission of AIP Publishing. .......................................................................... 80

Figure 5.1.6. Miniature sun sensor operation at illumination angles (θsun) of (a) 0°

(illuminating PD 5) and (b) 45° (illuminating PD 2). The left-side images

indicate the illumination angle/orientation and the right-side images show the

illuminated array based on the normalized PDCR and responsivity at −1 V.


Figure 5.2.1. QB50 CubeSat Discovery CAD model with GaN-based miniature sun

sensor and finished CubeSat. ............................................................................ 83

xxi

Figure 5.2.2. Dark and illuminated (365 nm) scaled current-voltage response of (a)

graphene and (b) semitransparent Ni/Au photodetector arrays before attaching

the aperture mask and before QB50 implementation. Three of the graphene

photodetectors (PD 4, PD 7 and PD 9) were not operational due to poor graphene

transfer.. ............................................................................................................ 84

Figure 5.2.3. Ground test set-up for integration testing of GaN sun sensor with QB50

Discovery. ......................................................................................................... 85

Figure 5.2.4. Ground test performance of miniature sun sensor integrated with QB50

at 0° illumination angle and 5 V bias; (a) graphene photodetector array and (b)

semitransparent Ni/Au photodetector array. Three of the graphene

photodetectors (PD 4, PD 7, and PD 9) were not operational due to poor

graphene transfer. ............................................................................................. 86

Figure 5.3.1. Timeline of spacecraft that have or are planned to measure in-flight

shock-layer radiation during atmospheric entry. .............................................. 88

Figure 5.3.2. Test set-up for in situ UV shock radiance measurements in NASA

Ames’ electric arc shock tube. Reprinted from [118], with the permission of AIP

Publishing. ........................................................................................................ 90

Figure 5.3.3. Transient current response (8 Hz sampling frequency) for (a)

photodetector 1 during shock test 1 and (b) photodetector 2 during shock test 2.

Unilluminated pre- and post-shock current-voltage curves (device functionality

tests) are compared in the insets. ...................................................................... 92

Figure 5.3.4. Transient current response (5 MHz sampling frequency) during shock

test for (a) photodetector 1 and (b) photodetector 2. The filtered signal displays

a distinct increase in current after the shock passes the detector giving a direct

measure of the UV shock-layer radiation. Reprinted from [118], with the

permission of AIP Publishing. .......................................................................... 92

xxii

Figure 5.3.5. Photo-generated current (Iphoto - Idark) for the GaN-based photodetectors

as a function of applied optical power calculated as a square root of power fit

through the spectral response characterization data at 365 nm. By overlaying the

photo-generated current calculated from the shock test data for (a) photodetector

1/shock test 1 and (b) photodetector 2/shock test 2, the measured optical power

is found to be about 0.30 μW for both shock tests. Reprinted from [118], with

the permission of AIP Publishing. .................................................................... 93

Figure 5.3.6. GaN photodetector chip epoxied to header mounted in shock tube port

holder (a) before shock testing, (b) post shock test 1, and (c) post shock test

2……… ............................................................................................................ 95

Figure A.1.1. MSM photodetector band diagram. .................................................. 104

Figure B.2.1. AlGaN/GaN MSM photodetector fabrication process. (a) AlGaN/GaN

on GaN-on-sapphire substrate, (b) mesa etch, (c) SiO2 passivation, (d) etch SiO2

passivation, (e) deposit semitransparent Ni/Au interdigitated electrodes, and (f)

deposit Ti/Au electrical contacts and interconnect. From [193]; reprinted by

permission of the American Institute of Aeronautics and Astronautics, Inc. . 111

Figure B.2.2. Top-view optical image of an AlGaN/GaN MSM photodetector with

semitransparent Ni/Au electrodes. .................................................................. 111

Figure B.2.3. AlGaN/GaN phototransistor fabrication process. (a) AlGaN/GaN-on-

silicon substrate, (b) mesa etch, (c) deposit Ti/Al/Pt/Au source and drain

contacts, and (d) deposit Ti/Au gate contact. ................................................. 112

Figure B.2.4. Top-view optical image of an AlGaN/GaN phototransistor with (a)

200 μm × 100 μm rectangular gate and (b) 200 μm × 100 μm spine and rib

gate…… ......................................................................................................... 113

Figure B.2.5. Absorbance and reflectance spectra of the (a) AlGaN/GaN on GaN-on-

sapphire substrate and (b) AlGaN/GaN-on-silicon substrate. ........................ 113

xxiii

Figure B.2.6. Current-voltage response of two AlGaN/GaN-on-silicon MSM

photodetectors when illuminated with wavelengths from 200 nm to

500 nm……. ................................................................................................... 114

Figure B.2.7. PDCR as a function of illumination wavelength of two AlGaN/GaN-

on-silicon MSM photodetectors. .................................................................... 114

Figure B.2.8. Spectral responsivity of two AlGaN/GaN-on-silicon MSM

photodetectors. ................................................................................................ 115

Figure B.3.1. AlGaN/GaN phototransistor (200 μm × 100 μm, rectangular gate) (a)

drain current versus gate voltage and (b) drain current versus drain-to-source

bias under dark (dashed lines) and 365 nm illuminated (solid lines) conditions

at room temperature. ....................................................................................... 116

Figure B.3.2. AlGaN/GaN phototransistor (200 μm × 100 μm, spine and rib gate) (a)

drain current versus gate voltage and (b) drain current versus drain-to-source

bias under dark (dashed lines) and 365 nm illuminated (solid lines) conditions

at room temperature. ....................................................................................... 117

Figure B.3.3. AlGaN/GaN phototransistor (200 μm × 100 μm, rectangular gate) (a)

drain current versus gate voltage (VDS = 1 V) and (b) drain current versus drain-

to-source bias (VGS = 0 V) under dark (dashed lines) and 365 nm illuminated

(solid lines) conditions from room temperature to 300°C. ............................. 118

Figure B.3.4. AlGaN/GaN phototransistor (200 μm × 100 μm, spine and rib gate) (a)

drain current versus gate voltage (VDS = 1 V) and (b) drain current versus drain-

to-source bias (VGS = 0 V) under dark (dashed lines) and 365 nm illuminated

(solid lines) conditions from room temperature to 300°C. ............................. 118

Figure B.3.5. (a) Maximum PDCR as a function of temperature for four AlGaN/GaN

phototransistors with various gate configurations and (b) the corresponding gate

voltage as a function of temperature. ............................................................. 119

xxiv

Figure B.3.6. (a) Maximum responsivity as a function of temperature for four

AlGaN/GaN phototransistors with various gate configurations and (b) the

corresponding gate voltage as a function of temperature. .............................. 119

Figure B.3.7. (a) Overall experimental test set-up for characterizing the low-

temperature response of GaN-based photodetectors, (b) temperature controlled

stage (frozen over during testing), and (c) GaN-based photodetector chip

epoxied to a PCB. From [193]; reprinted by permission of the American Institute

of Aeronautics and Astronautics, Inc. ............................................................ 121

Figure B.3.8. AlGaN/GaN on GaN-on-sapphire MSM photodetector current-voltage

curves under dark (solid lines) and 365 nm illuminated (dashed lines) conditions

from room temperature to -138°C for (a) photodetector 1 and (b) photodetector

2. The inset in (a) shows the dark-current thermal recovery of photodetector 1

after exposure to -138°C. ................................................................................ 123

Figure B.3.9. (a) PDCR and (b) responsivity as a function of temperature at -8 V bias

for photodetectors 1 and 2. ............................................................................. 123

Figure B.3.10. Transient current response of photodetector 1 at different

temperatures under a 365 nm UV intensity of 0.7 ± 0.1 mW/cm2 and a bias of -

8 V. From [193]; reprinted by permission of the American Institute of

Aeronautics and Astronautics, Inc. ................................................................. 124

Figure B.3.11. Current-voltage response for photodetector 1 at 23.3°C and while in

liquid nitrogen vapor at -195.0°C. The inset shows the condensation on the GaN

chip and PCB from the liquid nitrogen. .......................................................... 125

Figure B.3.12. Dark and illuminated (365 nm) scaled current-voltage response as a

function of proton irradiation fluence for AlGaN/GaN on GaN-on-sapphire

photodetectors with semitransparent Ni/Au electrodes. ................................. 126

xxv

Figure B.3.13. (a) PDCR and (b) responsivity as a function of proton fluence for

three semitransparent Ni/Au AlGaN/GaN on GaN-on-sapphire

photodetectors… ............................................................................................. 127

Figure C.1. Full QB50 miniature sun sensor integration circuit diagram. .............. 129

Figure C.2. Drawings of spectrometer adapter for spectral response

characterization…. .......................................................................................... 130

Figure C.3. Shock tube assembly drawings. ............................................................ 135

1

Chapter 1

Introduction

1.1 The Need for Space-Grade Instrumentation

Sensors and electronics operating in space experience temperature extremes and

are exposed to high levels of radiation. Venus, with its runaway greenhouse gas effect,

has some of the highest temperatures seen in the solar system with an average surface

temperature of 460°C [1]. To date, the longest a Venus probe has survived on the surface

of Venus is only 2 hours and 7 minutes (Soviet Venera 13 in 1982) due to thermal failure

of the electronics [1]. On the other end of the temperature spectrum, spacecraft

exploring Mars can experience temperatures as low as -120°C and spacecraft exploring

Pluto can see temperatures as low as -229°C [2, 3]. In these very low-temperature

environments, some form of heating is required to keep electronics at operable

temperatures [4]. Future missions to Europa, one of Jupiter’s icy moons, would see

average surface temperatures of -160°C [5] while being exposed to one of the harshest

radiation environments in the solar system. Due to Jupiter’s strong magnetic field and

CHAPTER 1. INTRODUCTION 2

associated radiation belts, spacecraft orbiting the planet and its moons experience a dose

of about 40 krad/day of ionizing radiation [4, 6]. This high level of radiation is very

damaging to electronic components and thus extensive protective shielding is required.

The maximum and minimum temperatures as well as average yearly dose of radiation

for each planet is shown in Fig. 1.1.1. In addition to extreme temperatures and particle

radiation, sensors and electronics operating in space also need to withstand thermal

cycling, extreme pressures, high heat fluxes, high levels of electromagnetic radiation,

as well as various forms of chemical and physical corrosion. Some of the extreme

conditions experienced at several planetary bodies are listed in Table 1.1.1.

When mission designers plan a space mission, they may not need to consider

every harsh environmental condition. For example, high temperature and high pressure

operation is applicable to a Venus mission but for a Europa mission the ability to survive

in a low-temperature, radiation-rich environment is the primary concern. However, for

every space mission, reliable operation of sensors and electronics is important

throughout all phases of the mission including during ground testing (wind tunnels, arc

jets, shock tubes, vibration, thermal vacuum cycling, etc.), launch, in flight, during

atmospheric entry, and while at other planetary bodies (orbiters, rovers, landers, etc.).

Figure 1.1.1. Temperature extremes and yearly radiation doses experienced at each planet.

Image adapted from discovery.com.


Table 1.1.1. Extreme environments experienced at planetary bodies.

Average

Temp.

(°C)

High

Temp.

(°C)

Low

Temp.

(°C)

Radiation Dose

(krad/year)

Pressure

(bar) Corrosion

Mercury 167 [7]

425 to 426

[7, 8]

-193 to -

180 [7,

8]

100 to 200 [7] --- ---

Venus 460 to 470

[7, 8]

482 to 500

[4] 0 [4] 20 [7] 92 [7] H2SO4 [7]

Earth 17 [7] 70 [7] -89 [7]

LEO: 0.1 to 1

[7, 8]

MEO: 2 to 2,000

[7, 8]

GEO: 10 to 30 [7,

8]

--- ---

Moon 31 [4] 120 to 197

[4]

-233 to -

180 [4] --- --- Dust [7]

Mars -65 to -63

[4, 7]

20 to 27

[4, 7]

-143 to -

100 [4,

7]

5 to 10 [7, 8] 0.007 [7] Dust [7]

Jupiter -116 to -

110 [7, 9]

230 (Upper

Atm.) [4] to

1000’s

(Interior) [7]

-143 [7]

Orbit: 14,000

[7, 8]

Surface: 3,000 [7]

22 [7] ---

Europa -145 [8] --- -180 [4]

Orbit: 14,600 [4]

Surface: 7,300 [4]

--- ---

Saturn -147 to -

140 [7, 9]

1000’s

(Interior) [7] -140 [7] 30 [7] --- ---

Titan -179 [10] ---

-185 to -

178 [4,

10]

--- 1.5 [7] CH4 [7]

Uranus -197 [7] 1000’s

(Interior) [7] -224 [7] 10 [7] --- ---

Neptune -201 [7]

1000’s

(Interior) [7]

-218 [7] 10 [7] --- ---


Common spacecraft instrumentation includes temperature sensors, pressure sensors,

heat flux (convective and radiative) gauges, radiation (particle and electromagnetic)

detectors, and spectrometers (mass, absorption, emission, etc.). Data gathered by these

instruments are used to validate and improve models (aerodynamic,

aerothermodynamic, radiation, materials, mechanical, etc.) which enables improved

spacecraft design, furthers our scientific knowledge, and expands our understanding of

Earth and the solar system. If there was a one-size-fits-all material platform and

corresponding sensor package capable of withstanding the extreme temperatures, high

levels of radiation, and chemical attack experienced in space, the total footprint of

instrumentation would be reduced due to fewer packaging/shielding requirements.

Additionally, development and testing costs would be lower and more data could be

gathered.

1.2 State-of-the-Art Sensor and Electronics

Most sensors and electronics today are silicon (Si)-based due to well-established

manufacturing processes, easy circuit integration, and low cost. However, most

commercial-off-the-shelf (COTS) Si-based components are only rated to temperatures

up to 125°C [11] which is a limitation of the Si material platform. Si has a relatively

high intrinsic carrier concentration at room temperature (1010 cm-3) which increases

exponentially with temperature to reach 1015 cm-3 at 300°C [1]. Additionally, the

lightest doping concentrations of Si devices range from 1014 to 1017 cm-3 [12].

Therefore, when operating at high temperatures, the intrinsic carrier concentration will

overwhelm the doping concentration and the device will no longer function as intended.

Silicon-on-insulator (SOI) devices, which have a very thin active Si layer, have shown

operation up to 300°C as well as a higher tolerance for radiation than their bulk Si

counterparts [11]. However, many applications (especially space-based applications)

require much higher operating temperatures than these COTS components allow.


Another challenge electronics face when operating in the space environment is

radiation (electromagnetic and particle) which causes ionization damage (free electron-

hole pairs) and displacement damage (atoms displaced from their lattice sites) effects

within semiconductors [13]. Since Si has a narrow bandgap (1.12 eV) and a relatively

low atomic displacement energy (13 eV) [14], the energy required to displace an atom

from its lattice, Si-based electronics suffer from both ionization and displacement

damage effects and complete device failure occurs at low total doses. For low-Earth

orbit (LEO) CubeSats or other very short duration missions, COTS components work

well. However, for long duration and interplanetary type missions where electronics

failure is unacceptable, different solutions are needed. To avoid the cost and time

intensive nature of developing new technologies, the traditional solutions used by

spacecraft designers include incorporation of thick metal shielding to protect against

radiation (for example, the electronics on the Juno mission to Jupiter were housed inside

a titanium box [15, 16]), cooling/heating systems to keep electronics at operational

temperatures, and redundancy [4].

1.3 Emerging Material Platforms for Space Exploration

To extend the operating environments of electronics, wide bandgap materials

such as silicon carbide (SiC), zinc oxide (ZnO), and III-nitride materials (gallium nitride

(GaN), aluminum nitride (AlN), etc.) are being explored as material platforms for harsh

environment sensing applications. Table 1.3.1 lists relevant material properties of SiC,

ZnO, GaN, and AlN compared to Si. The wide bandgap and high melting point of these

materials indicates they will be able to operate at higher temperatures than Si. Similarly,

the high atomic displacement energies of the wide bandgap materials, compared to Si,

indicate they will be able to withstand higher doses of radiation before device failure.


Table 1.3.1. Semiconductor material properties comparison.

Si 4H-SiC ZnO GaN AlN

EG (eV) 1.12 [17] 3.2 [17] 3.35 [17] 3.39 [17] 6.2 [17]

Excitation

Wavelength (nm)

1108

(IR)

388

(UV)

370

(UV)

365

(UV)

200

(UV)

Melting Point (°C) 1410 [17] 2550 [17] 2248 [18] 2250 [19] 3000 [20]

Max Demonstrated

Operational

Temperature (°C)

300 (SOI)

[21, 11] 800 [22] ---

600 (AlGaN/GaN)

1000 (InAlN/GaN)

[23, 24]

---

Atomic Displacement

Energy (eV) 13 [14]

Si-35

C-22 [25]

Zn-18.5

O-41.4 [26]

Ga-20.5

N-10.8 [14, 27]

Al-42

N-38 [28]

III-nitride materials, their ternary alloys, and their associated heterostructures

are chemically, thermally, and mechanically stable [29]. Due to their robustness as well

as their excellent optical and electrical properties, III-nitride materials, similar to SiC

and ZnO, have found applications in power electronics, high-temperature electronics,

as well as sensing. AlGaN/GaN high electron mobility transistors (HEMTs) have been

shown operable at 600°C in air [23] and InAlN/GaN HEMTs have been shown operable

up to 1000°C in vacuum [24]. The higher operational temperatures of III-nitride-based

devices compared to Si, can be partially attributed to their much lower intrinsic carrier

concentrations (the intrinsic carrier concentration of GaN is about 105 cm-3 at 300°C)

[12]. In addition to high-temperature operation, III-nitride-based devices have been

shown to be robust to high levels of radiation [14, 30, 27, 31].

Due to the strong covalent bonds between silicon and carbon, SiC is very stable

at high temperatures [32]. Recently, SiC integrated circuits (ICs) have been shown

operable for over 1000 hours at 500°C in Earth-atmosphere and for over 500 hours in a

Venus simulate surface environment (460°C, 9.4 MPa, CO2, and SO2) [1]. Additionally,

SiC pressure sensors have demonstrated short term operation at 800°C and extended

(over 350 hours) operation at 600°C [22, 33]. SiC is also highly chemically stable [34]

(i.e. highly resistant to corrosion and biocompatible) making it an ideal material for use


as a chemical resistant coating in environments such as Venus with its sulfuric acid

clouds. However, from a sensor and electronic device microfabrication standpoint, the

high chemical stability of SiC (and GaN) means it is very difficult to etch.

Opposite to SiC (and GaN), ZnO is easily etched via wet chemical etching

making ZnO-based electronics easy to fabricate, but not well suited for use in corrosive

environments. However, ZnO-based devices have exhibited exceptionally high

tolerance to radiation; it is more resistant to radiation damage than Si, SiC, or GaN [35].

The theoretical atomic displacement energies for the Zn and O atoms have been

estimated to be 18.5 eV and 41.4 eV, respectively [26]. Experimentally, ZnO thin films

have shown minimal damage, in terms of electrical resistance and photoluminescence

intensity, when subject to 8 MeV protons with fluences as high as 5 × 1014 cm-2 [36].

Additionally, ZnO thin film transistors irradiated to 100 Mrad (gamma ray) showed only

a slight negative threshold voltage shift after irradiation which was completely

recovered with a one minute, 200°C anneal [37].

1.4 Ultraviolet Photodetectors

Due to their wide bandgap, SiC, ZnO, GaN, and AlN are sensitive to ultraviolet

(UV) light. On the electromagnetic spectrum, the UV region spans wavelengths from

400 nm to 10 nm and is typically divided into three regions: UV-A (400 - 320 nm), UV-

B (320 - 280 nm), and UV-C (280 - 10 nm) [17, 35, 38]. Outside the Earth’s

atmosphere, UV light makes up 9% of solar radiation. The approximate solar radiation

spectrum outside Earth’s atmosphere, calculated as black body radiation at 5800 K using

Plank’s law, is shown in Fig. 1.4.1 with the absorption wavelengths and energies of

GaN and Si indicated. The Earth’s ozone layer makes the planet habitable by humans

by almost completely absorbing UV-C and significantly attenuating UV-B, which

causes cataracts, burns, and skin cancer [17]. To assess the habitability of Mars for

future manned missions, the Mars Science Laboratory (MSL) was equipped with the


rover environmental monitoring station (REMS) which takes measurements of the UV

radiation reaching the surface of Mars [39]. In addition to understanding the UV

environment of other planets, there is a need to understand the UV radiative heating

during atmospheric entry in order to design effective and efficient spacecraft thermal

protection systems [40, 41]. For these applications, UV photodetectors which are highly

sensitive, visible-blind, and robust to harsh environmental conditions are needed.

Figure 1.4.1. Black body radiation (Plank’s law) at 5800 K approximating the solar

irradiance spectrum outside Earth’s atmosphere with absorption wavelengths/energies of

GaN and Si indicated.

There are different types of semiconductor photodetectors including:

photoconductor, metal-semiconductor-metal (MSM), Schottky, p-n, p-i-n, and

avalanche. The structure for each of these photodetectors is depicted in Fig. 1.4.2.

Regardless of the type, the fundamental operating principal of all semiconductor

photodetectors is the same. Photons with energy greater or equal to the bandgap of the

semiconductor are absorbed by the semiconductor and electron-hole pairs are generated.

Ideal photodetectors will generate one electron-hole pair per absorbed photon. The

photon excitation wavelength corresponding to the bandgap of the material is listed in


Table 1.3.1 for Si, SiC, ZnO, GaN, and AlN. These photo-generated carriers will be

separated by the electric field (electrons move to the conduction band and holes to the

valance band) and a photocurrent can be measured.

Figure 1.4.2. Schematic structure of photoconductor, MSM, Schottky, p-n, p-i-n, and

avalanche semiconductor photodetectors.

There are several key parameters used to characterize photodetector

performance, including: photocurrent-to-dark current ratio (PDCR), responsivity,

quantum efficiency, rise time, and decay time. PDCR, defined as

PDCR = 𝐼𝑝ℎ𝑜𝑡𝑜 − 𝐼𝑑𝑎𝑟𝑘

𝐼𝑑𝑎𝑟𝑘 (1.1)

where Iphoto is the photocurrent and Idark is the dark current, is a measure of photodetector

sensitivity [42, 43]. A second equally important sensitivity factor is responsivity (R)

which is defined as

R = 𝐼𝑝ℎ𝑜𝑡𝑜 − 𝐼𝑑𝑎𝑟𝑘

𝑃𝑜𝑝𝑡 (1.2)


where Popt is the applied optical power [17, 44]. Additionally, the external quantum

efficiency, η, (a measure of the number of charge carriers generated per incident

photon), which is directly proportional to the responsivity, is defined as

𝜂 = 𝑅ℎ𝑐

𝑞𝜆 (1.3)

where h is Plank’s constant, c is the speed of light, q is electron charge, and λ is

wavelength [44]. Lastly, the rise time (time in which photocurrent increases from 10%

to 90%) and decay time (time in which photocurrent drops from 90% to 10%) are

important parameters to consider especially in applications where a fast response time

is required.

Si and other narrow bandgap semiconductors are not able to directly measure

UV light due to their bandgaps corresponding to near infrared or visible wavelengths.

Since UV light is higher energy than the bandgap of Si and other narrow bandgap

materials, part of the energy will be lost to heating, resulting in low quantum efficiency.

To use these materials for UV photodetection, filtering devices that absorb UV light and

reemit lower energy light need to be incorporated [38]. Additionally, Si-based

photodetectors are limited to operational temperatures below 125°C due to the thermal

generation of charge carriers dominating the photo-generated response [42, 45]. Wide

bandgap materials should be used for UV detection, rather than Si, due to their ability

to directly measure UV light and intrinsically withstand harsh environmental conditions.

A comparison of SiC, ZnO, GaN, and AlN photodetector responsivities reported

in literature is shown in Fig. 1.4.3. Additionally, the 50% and 100% quantum efficiency

values for each of the four materials are indicated where the wavelength used for the

calculation was taken to be the absorption edge corresponding to the bandgap of the

material; these excitation values are as listed in Table 1.3.1. From Fig. 1.4.3, it is easy

to see that these wide bandgap materials have the potential for high performance (> 50%

quantum efficiency) but further development is required. There is not one type of


photodetector nor one material platform that appears to outperform the rest, in terms of

responsivity. Therefore, other factors such as dark current, response time, and

wavelength region of interest need to be considered. In general, photoconductors and

avalanche photodetectors have shown high internal gain, while Schottky photodetectors

have displayed very fast response times [44]. Furthermore, SiC photodetectors are

responsive to near-visible UV and AlN photodetectors are responsive to deep UV while

the absorption edge of GaN-based photodetectors is tunable for a wide range of

wavelengths (~200 nm to ~650 nm) [17].

Figure 1.4.3. Responsivity comparison of SiC, ZnO, GaN, and AlN photodetectors as

reported in literature with corresponding 50% and 100% quantum efficiency values for each

material indicated.

For space applications, the effect of extreme conditions on UV photodetector

performance is important to consider. As discussed above, Si-based photodetectors are

only able to operate up to 125°C, while wide bandgap photodetectors have shown

operational temperatures upwards of 200°C. A comparison of the maximum operational

temperatures reported in literature for Si, SiC, ZnO, and III-nitride photodetectors is


shown in Fig. 1.4.4. The benefits and limitations of each of these wide bandgap

materials for extreme environment UV photodetection will now be discussed.

Figure 1.4.4. Maximum reported operational temperature comparison of Si, SiC, ZnO, and

III-nitride photodetectors.

SiC is the most developed of the wide bandgap materials especially with regard

to operation in high ambient temperatures [12]. Thus it is not surprising to see from Fig.

1.4.4 that SiC-based photodetectors have demonstrated the highest operational

temperatures. SiC MSM photodetectors have demonstrated PDCRs as high as 1.3 × 105

at room temperature and up to 0.62 at 450°C [42]. However, at these high operational

temperatures, the wavelengths SiC photodetectors are responsive to has been shown to

shift significantly towards longer wavelengths due to the narrowing of the bandgap [46].

In addition to high-temperature operation, SiC UV detectors have been shown to retain

their operational characteristics after being exposed to 1 MeV neutrons with a fluence

of 8 × 1014 cm-2 [47], 2 MeV protons with a fluence of 1012 cm-2 [48], as well as under

167 MeV Xe heavy ions at a fluence of 6 × 109 cm-2 [49]. SiC and III-nitride materials

have theoretically and experimentally shown similar ability to operate at high


temperatures and when exposed to high levels of radiation. However, with further

development, III-nitride-based photodetectors may outperform SiC photodetectors due

to their direct and tunable bandgap as well as their ability to form heterostructures [50,

17].

GaN photodetectors have displayed exceptionally high responsivities

(> 103 A/W [51, 52]) which have been attributed to trapping of photo-generated holes

by surface states at the metal-semiconductor interface resulting in a lowering of the

Schottky barrier height and increased current [53, 54]. From spectral responsivity data,

the cut-off wavelength of AlGaN photoconductors has been shown to shift to shorter

wavelengths as the Al concentration increases from 0% (365 nm cut-off) to 35%

(325 nm cut-off) [17]. However, these devices show only a factor of 10 UV/visible

rejection due to material quality issues which remain to be surmounted in III-nitride

materials [12]. This limitation is readily evident in the very long fall times (on the order

of hours to days) for GaN and other III-nitride-based photodetectors which has been

attributed to impurities, dislocations, and deep-level defects trapping photo-generated

carriers for long times after the illumination source has been removed. This

phenomenon has been termed persistent photoconductivity [55, 56, 57, 58, 59] and

makes GaN photodetectors an unattractive solution for applications requiring fast

response times. Spectral responsivity measurements taken using a chopper with a lock-

in amplifier, in which all phenomena occurring at rates slower than the chopper

frequency are eliminated, results in UV/visible contrasts greater than three orders of

magnitude [17, 57, 60, 61]. Thus, GaN-based optoelectronic devices display great

potential but material quality and persistent photoconductivity issues need to be

overcome.

In terms of high-temperature characterization, GaN and AlGaN/GaN MSM

photodetectors have been shown operable up to 325°C [55] and InGaN Schottky

photodetectors have shown high responsivity (5.6 A/W) and high UV/visible rejection

(> 105) at 250°C [62]. Additionally, AlN MSM photodetectors have demonstrated

working temperatures up to 300°C [43]. At the other end of the temperature scale, there


have been very few studies characterizing the low-temperature operation of GaN

photodetectors which is an important environment to consider for space applications.

GaN p-i-n photodetectors have shown only a slight shift in the peak of the spectral

response curve from 362 nm at 7°C to 356 nm at -118°C but a 55% reduction in peak

magnitude due to a narrowing of depletion region at lower temperatures [63].

While III-nitride materials have high atomic displacement energies and III-

nitride-based devices have been shown to be robust to high levels of radiation [14, 30,

27, 31], there has been little work in understanding how radiation affects UV

photodetector performance. GaN MSM photodetectors irradiated to 750 krad (gamma

ray) show more than 60% decrease in PDCR over pre-irradiation values due to trapped

charges in a SiO2 passivation layer creating large leakage currents [64]. AlN MSM

photodetectors have demonstrated tolerance to 2 MeV proton irradiation up to a fluence

of 1013 cm-2, but at 1014 cm-2 PDCR drops to zero due to proton-induced displacement

damage. Clearly further work needs to be done to understand the operation of III-nitride

(as well as SiC and ZnO) photodetectors in harsh radiation (proton, neutron, gamma,

etc.) environments.

Compared to SiC and GaN, there have been relatively few reports on the

capability of ZnO photodetectors and even fewer reports on their operation in extreme

environments. ZnO photodetectors have exhibited a sharp cut-off at 365 nm with

UV/visible contrast of a few orders of magnitude [17] as well as operation up to 200°C

[65]. However, challenges in obtaining reliable p-type ZnO have limited the

development of p-n and p-i-n photodetectors [35, 66, 67]. Perhaps more attractive than

planar photodetectors on bulk ZnO, are ZnO-based nanostructures. ZnO nanostructures

enhance UV photodetector performance due to their large surface-to-volume ratio [35]

and ability to act as an antireflective coating [68, 69, 70] resulting in increased light

absorption. Compared to ZnO thin film photodetectors, ZnO nanorod arrays have shown

a much higher on/off ratio as well as faster response and decay times [71, 70]. Since

ZnO and GaN have low lattice mismatch (~1.8%) [35] and closely aligned bandgaps,

UV photodetectors with enhanced sensitivity can be created by integrating the two


materials. Indeed, ZnO nanorod arrays on GaN showed improved performance, in terms

of PDCR, at 300°C compared to bare GaN photodetectors [68].

While all of these wide bandgap materials have their benefits and further work

should be conducted to enable harsh environment UV photodetection by all materials,

this thesis will focus on the development, characterization, and applications of GaN-

based UV photodetectors within harsh space-like environments.

1.5 Graphene as a Space-Grade Material

Graphene is a two-dimensional material consisting of a monolayer of sp2-

bonded carbon atoms. Graphene is electrically and mechanically one of the strongest

materials and forms a Schottky contact to GaN [72, 73]. Graphene/GaN Schottky diodes

have been shown to retain rectifying properties up to 277°C and recover those properties

upon cooling from as high as 377°C [73]. In addition to thermal stability, graphene has

displayed resistance to radiation damage. Graphene has a high atomic displacement

energy of 18-22 eV [74] and has been shown to act as an effective barrier layer to

particle and high energy electromagnetic radiation [75, 76, 77, 78, 79]. By encapsulating

single layer molybdenum disulfide (MoS2) with graphene, damage from electron

irradiation was eliminated thus enabling high resolution, defect free transmission

electron microscopy (TEM) imaging [75]. Additionally, graphene was shown to protect

silver nanowire networks from high intensity (millions of W/cm2) UV laser irradiation

[76]. When exposed to low dose gamma rays, the carrier density of graphene increases

with little defect generation [77]. Due to the two-dimensional nature of graphene, the

probability of ion interaction is reduced and therefore high energy particle radiation

primarily creates defects in the substrate [77], underlying the importance of coupling

graphene with a radiation-hard substrate. These experimental results show that graphene

is an excellent material choice for use as a thermally stable and radiation-hard electrode

for GaN-based electronics.


Graphene is also well known as an optically transparent material across a broad

wavelength range while maintaining high electrical conductivity. Graphene alone is not

an excellent photodetector, but when coupled with a semiconductor material (such as

GaN) and used as a transparent electrode, highly responsive optoelectronics devices can

be created [80, 81, 72, 82, 83]. Graphene-based photodetectors exhibit fast response

times due to the high mobility of graphene [81]. Graphene/GaN photodetectors have

shown PDCRs as high as 56 at 10 V bias when illuminated with a 325 nm laser

(100 μW/μm2) [72] and Graphene/AlGaN/GaN Schottky photodetectors have

demonstrated photo-to-dark contrast ratios of more than three orders of magnitude [82].

To date, the response of Graphene/GaN UV photodetectors operating in extreme

temperature and radiation-rich environments has not been thoroughly investigated, if at

all. Since graphene has demonstrated excellent electrical and optical properties as well

as robustness to harsh environmental operating conditions, graphene-enhanced

photodetectors should be developed and characterized for space-based applications

which is the subject of study for this thesis.

1.6 Thesis Outline

Chapter 2 examines the principal of operation of GaN-based MSM

photodetectors and the development of a semi-analytical model based on thermionic

emissions is presented.

Chapter 3 describes the GaN photodetector microfabrication process including

the graphene transfer process. The detailed microfabrication run sheet is included in

Appendix D. Additionally, the optical properties of the GaN substrate are presented

along with a comparison of the transmission of semitransparent Ni/Au and graphene,

the two electrode materials examined in this thesis. The spectral response of the GaN

MSM photodetectors is also presented.


Chapter 4 delves into the characterization of GaN-based photodetectors when

operating in extreme environments including high temperatures and under proton

irradiation exposure. The performance of semitransparent Ni/Au electrodes is compared

to graphene electrodes for both extreme operating conditions.

Chapter 5 details the implementation and testing of GaN-based photodetectors

in practical aerospace applications. A miniature sun sensor was created using an array

of GaN-based photodetectors. The miniature sun sensor demonstrated operation under

0° and 45° illumination and has been integrated with a CubeSat enabling future in-space

evaluation. The GaN-based photodetectors were also implemented in a shock tube and

in situ measurements of UV shock-layer radiation were made.

Chapter 6 provides the conclusions and proposed future work to further the

development of GaN-based UV photodetectors for space applications.

18

Chapter 2

GaN MSM Photodetector Operation and Modeling

2.1 Principal of Operation

A photodetector is a device that converts light into a measurable electrical signal.

The fundamental operating principal of all semiconductor photodetectors is the same;

photons with energy greater or equal to the bandgap of the semiconductor are absorbed

by the semiconductor generating electron-hole pairs as shown in Fig. 2.1.1. When an

electric field is applied across the semiconductor, the photo-generated electron-hole

pairs separate and a photocurrent can be measured. Since the bandgap of GaN is 3.4 eV,

GaN-based photodetectors will be responsive to incident light with wavelengths of

365 nm or less. When there is no light or light with energy less than the bandgap of the

semiconductor shining on the photodetector, there will be a small leakage current

through the semiconductor film generated mostly due to thermal energy; this is termed

dark current. The magnitude of this dark current is determined by the metal-

semiconductor contact.

CHAPTER 2. GaN MSM PHOTODETECTOR OPERATION AND MODELING 19

Figure 2.1.1. Fundamental operating principal of semiconductor photodetectors.

When a metal and a semiconductor are brought into contact, a potential barrier

known as the Schottky barrier (φB) is formed. When charge carriers have sufficient

energy to overcome the Schottky barrier height, a current will be generated. This form

of current transport is known as thermionic emissions and is the dominate current

transport mechanism for metal-semiconductor devices with low doping (N < 1017 cm-3)

or operating at high temperatures [44, 84]. Thermally excited carriers that don’t have

quite enough energy to overcome the barrier height see a thinner barrier and are able to

tunnel through it. This form of current transport is termed thermionic field emissions

and is the dominate form of current transport for devices with intermediate doping (1017

< N < 1018 cm-3) operating at intermediate temperatures [44, 35, 62, 85]. For highly

doped devices direct tunneling is possible which is known as field emissions. Field

emissions becomes important for devices operating at very low temperatures [44] and

is also the basis for creating Ohmic contacts which have a linear current-voltage

response [44]. A band diagram depicting thermionic emissions, thermionic field

emissions, and field emissions is shown in Fig. 2.1.2.


Figure 2.1.2. Current transport mechanisms in metal-semiconductor junctions.

In the ideal case, the Schottky barrier height is the difference in the metal work

function and the electron affinity of the semiconductor [44]. In practice, the Schottky

barrier height is dependent on the fabrication process and can be modified by Fermi

level pinning, image force lowering, etc. [44]. However, experimental results for metal-

GaN junctions have shown that the barrier height is dependent on the metal work

function [86] and thus a metal with a large work function should be chosen to form a

Schottky contact. The electron affinity of GaN is 4.1 eV [86] and the work functions of

commonly used metal contacts to GaN are 5.65 eV for Pt, 5.1 eV for Au, 5.15 eV for

Ni, and 4.33 eV for Ti [87]. Rectifying behavior has been observed for Pt, Au, and Ni

contacts to GaN with reported barrier heights of 1.1 eV for Pt, 0.91 – 1.15 eV for Au,

and 0.66 – 0.99 eV for Ni [86]. Slightly rectifying behavior has been observed for Ti

contacts to GaN with a barrier height of 0.6 eV. However, Ti has been shown to alloy

with GaN forming TiN giving rise to nitrogen vacancy related donor states resulting in

the formation of a tunneling junction [88]. Thus, Pt, Ni, or Au should be used as a

Schottky contact to GaN rather than Ti.

For temperature tolerant photodetectors, the effect of temperature on the

Schottky barrier height (and thus the electrode material) needs careful consideration.

According to theory, the Schottky barrier height should decrease at higher temperatures


due to the higher kinetic energy of charge carriers at higher temperatures [89]. However,

experimental results have found the Schottky barrier height to increase with increasing

temperature [90, 91, 92]. This discrepancy has been attributed to the Schottky barrier

height inhomogeneity in which the Schottky barrier height is theorized to be inconsistent

across the metal-semiconductor junction [90, 91, 92]. At low temperatures, the current

will flow through the lower barrier regions and as temperature increases the charge

carriers will be able to overcome the higher barrier regions. The reported thermal limit

for Pt/GaN, Au/GaN, and Ni/GaN contacts are 400°C, 575°C and 600°C, respectively

[86, 93]. Thus, temperature tolerant photodetectors with low leakage current can be

formed using Pt, Au, or Ni electrodes on GaN.

There are several types of semiconductor photodetectors as shown in Fig. 1.4.2.

The specific type of semiconductor photodetector studied here is the metal-

semiconductor-metal (MSM) photodetector. MSM photodetectors operate by using

interdigitated Schottky electrodes (or comb fingers) on a semiconductor material [44,

84, 85]. A band diagram depicting the operation of an MSM photodetector is shown in

Fig. 2.1.3. Due to the metal-semiconductor contact, the dominate dark current transport

mechanism for MSM photodetectors is thermionic, thermionic field, and/or field

emissions. For traditional MSM photodetectors utilizing opaque metal electrodes,

photons can only be absorbed by the semiconductor region between the electrodes. Thus

the electrode width should be minimized in order to maximize the photodetector

efficiency. Another approach to maximize photodetector efficiency is use a semi-

transparent or transparent electrode which allows photon absorption underneath the

electrode. GaN-based MSM photodetectors with transparent graphene electrodes and

semitransparent Ni/Au electrodes are the subject of study in this thesis. The spectral

transmission of these two materials is presented in Chapter 3 and a comparison of the

response of GaN-based photodetectors using these two electrodes is detailed in Chapter

4. The remainder of Chapter 2 is dedicated to developing a semi-analytical model for

the temperature-dependent response of GaN-based MSM photodetectors based on

thermionic, thermionic field, and field emissions theories.


Figure 2.1.3. MSM photodetector band diagram.

2.2 Semi-Analytical Model

2.2.1 Dark Current Model

To simulate the temperature-dependent dark current-voltage response of GaN-

based MSM photodetectors, the current transport mechanisms of thermionic emissions,

thermionic field emissions, and field emissions will be considered. When a voltage is

applied to an MSM photodetector, one of the electrodes will be reverse biased and the

other forward biased. For this discussion, electrode 1 is reverse biased and electrode 2

is forward biased. Therefore, the total dark current for an MSM photodetector is the

electron current from electrode 1 added to the hole current from electrode 2. The three

bias voltage operational regions for MSM photodetectors are: (1) voltages less than

reach-through (voltage at which the sum of the depletion widths equals the electrode

spacing), (2) voltages greater than reach-through but less than flat band (voltage at

which the electric field at the forward biased electrode is zero), and (3) voltages greater

than flat band but less than breakdown. The reach-through voltage can be express as


𝑉𝑅𝑇 ≅𝑞𝑁𝑉𝐷𝐿2

2𝜀𝑠− √4𝑉𝐷

2 (2.1)

where q is elementary charge (1.602 × 10-19 C), N, is the net dopant concentration, VD

is the built-in potential, L is the electrode spacing, and εs is permittivity of the

semiconductor (relative permittivity multiplied by 8.854 × 10-12 F/m). It is easy to see

that with high net dopant concentration, the reach-through voltage will be large. For the

GaN-based MSM photodetectors fabricated (see Chapter 3) and characterized (see

Chapters 4 and 5) in this work, VRT > 1000 V, therefore only operational region (1),

voltages less than reach-through, will be considered.

For thermionic emission theory, there are three main assumptions: (1) the barrier

height is much larger than kT/q, where k is Boltzmann’s constant (1.381 × 10-23 J/K)

and T is absolute temperature, (2) the device is in thermal equilibrium, and (3) two

current fluxes can be superimposed (one from metal to semiconductor and the other

from semiconductor to metal) [44]. Additionally, a symmetric MSM photodetector will

be assumed, thus the current-voltage response will be symmetric and only the response

to positive bias voltages will be simulated. The detailed analytical equations for the

thermionic, thermionic field, and field emissions models implemented here are listed in

Appendix A. The total dark current can be expressed as the sum of the thermionic (ITE),

thermionic field (ITFE), and field emissions (IFE) currents, that is

𝐼𝑑𝑎𝑟𝑘 = 𝐼𝑇𝐸 + 𝐼𝑇𝐹𝐸 + 𝐼𝐹𝐸 . (2.2)

The four main temperature-dependent parameters incorporated in this

simulation are bandgap, intrinsic carrier concentration, mobility, and barrier height. The

temperature dependence of GaN’s bandgap (EG) is shown in Fig. 2.2.1 and can be

expressed as [94]


𝐸𝐺 = 𝐸𝐺(𝑇 = 0 𝐾) − 7.7 × 10−4 × 𝑇2

𝑇 + 600 . (2.3)

Figure 2.2.1. GaN’s bandgap as a function of temperature.

The temperature-dependent intrinsic carrier concentration (ni) is shown in Fig. 2.2.2 and

can be expressed as [94]

𝑛𝑖 = (𝑁𝑐𝑁𝑣)1/2 exp (−

𝐸𝐺

2𝑘𝑇) (2.4)

where Nc is the effective density of states in the conduction band, and Nv is the effective

density of states in the valence band. For wurtzite GaN, Nc and Nv are approximately

𝑁𝑐 ≅ (4.3 × 1014) 𝑇3/2

𝑁𝑣 ≅ (8.9 × 1015) 𝑇3/2.

(2.5)


Figure 2.2.2. Intrinsic carrier concentration of GaN as a function of temperature.

A semi-empirical analytical approximation for the carrier mobility of wurtzite GaN

considering temperature and concentration dependencies is shown in Fig. 2.2.3 and

given by [95]

𝜇𝑖(𝑁, 𝑇) = 𝜇𝑚𝑎𝑥,𝑖(𝑇0)𝐵𝑖(𝑁) (

𝑇𝑇0

)𝛽𝑖

1 + 𝐵𝑖(𝑁) (𝑇𝑇0

)𝛼𝑖+𝛽𝑖

(2.6)

where

𝐵𝑖(𝑁) =

[ 𝜇𝑚𝑖𝑛,𝑖 + 𝜇𝑚𝑎𝑥,𝑖 (

𝑁𝑔,𝑖

𝑁 )𝛾𝑖

𝜇𝑚𝑎𝑥,𝑖 − 𝜇𝑚𝑖𝑛,𝑖

]

||

𝑇=𝑇0

(2.7)

and the parameters μmax,i, μmin,i, Ng,i, γi, αi, and βi, as determined from a best fit to

experimental results, are listed in Table 2.2.1. For a metal electrode on n-type wurtzite


GaN, the temperature-dependent electron barrier height (φn) is estimated using a simple

Schottky model [96, 97, 89] as

𝜑𝑛 = 𝜑𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒(300 𝐾) + 𝜁𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒(𝑇 − 300) − 𝜒𝐺𝑎𝑁 (2.8)

where φelectrode is the work function of the electrode, ζelectrode is the electrode temperature

coefficient, and χGaN is the electron affinity of GaN. Note that the temperature

dependence of the electron affinity is assumed negligible compared to the variation of

the electrode work function. The hole barrier height can then be calculated as the

difference between the semiconductor bandgap and the electron barrier height [44], that

is

𝜑𝑝 = 𝐸𝐺 − 𝜑𝑛. (2.9)

Using a Ni electrode as an example where φNi = 5.15 eV and ζelectrode = 5.6 ×10-4 eV/K

[89], the electron and hole barrier heights as functions of temperature are shown in Fig.

2.2.4. The GaN material properties used in MSM photodetector modeling are listed in

Table 2.2.2.

Figure 2.2.3. GaN (a) electron and (b) hole mobility as functions of temperature.


Table 2.2.1. Parameter values for carrier mobility model of wurtzite GaN [94].

Type of Carrier μmax,i μmin,i Ng,i γi αi βi

Electrons 1000 55 2 × 1017 1.0 2.0 0.7

Holes 170 3 3 × 1017 2.0 5.0 ---

Figure 2.2.4. Ni/GaN (a) electron and (b) hole barrier height as functions of temperature.

Table 2.2.2. Material properties of GaN used in MSM photodetector modeling.

Property Variable Value Unit Reference

Bandgap (at 0 K) 𝐸𝐺(𝑇 = 0 𝐾) 3.47 eV [94]

Permittivity of Semiconductor 𝜀𝑠 10.4 F/m [44]

Electron Affinity 𝜒𝐺𝑎𝑁 4.1 eV [86]

Effective Mass of Electrons 𝑚𝑛 0.27 kg [44]

Effective Mass of Holes 𝑚𝑝 0.8 kg [44]

Net Dopant Concentration 𝑁 5 × 1017 cm-3 ---

Lattice Constant (at 300 K) a 0.3189 Nm [44]

2.2.2 Dark Current Results

The semi-analytical model results for the dark current-voltage response over

temperatures ranging from -100°C to 200°C are shown in Fig. 2.2.5a for thermionic

emissions, Fig. 2.2.5b for thermionic field emissions, and Fig. 2.2.5c for field emissions.

To easily visualize the temperature dependence of the current-voltage response, the


figures on the right side in Fig. 2.2.5 employ a logarithmic current axis. The current-

voltage responses from the three current transport models are plotted together in Fig.

2.2.6. From Fig. 2.2.5 and Fig. 2.2.6, it can be seen that field emissions is the dominate

form of current transport for temperatures below -100°C while thermionic field

emissions governs the response from -100°C up to 23°C. From 23°C to 200°C, the

current-voltage response is a combination of thermionic emissions and thermionic field

emissions. Lastly, thermionic emission is the dominate form of current transport for

temperatures higher than 200°C. The total dark current-voltage response (see Eq. 2.2)

of the modeled GaN MSM photodetector is shown in Fig. 2.2.7. From the above results

it can be concluded that thermionic field emissions is a good first approximation for the

dark current transport process for GaN MSM photodetectors operating at temperatures

between -100°C to 200°C. A similar result showing that thermionic field emissions was

the dominate current transport mechanism in InGaN Schottky diodes for temperatures

up to 250°C was found by Sang et al. [62]. A qualitative comparison of the semi-

analytical model results with experimental results is presented in Chapter 4.


Figure 2.2.5. Semi-analytical model of GaN MSM photodetector dark current-voltage

from -100°C to 200°C for (a) thermionic emission, (b) thermionic field emission, and (c) field

emission. Note that the data shown in the left side and right side of (a), (b), and (c) is the

same. However, the figures on the right employ a logarithmic current axis for clarity.


Figure 2.2.6. Semi-analytical model of GaN MSM photodetector dark current-voltage

from -100°C to 200°C.

Figure 2.2.7. Semi-analytical model of GaN MSM photodetector total dark current vs.

voltage for temperatures from -100°C to 200°C with (a) linear current axis and (b) logarithmic

current axis.


2.2.3 Photocurrent Model

The photo-generated current for GaN-based photodetectors has been shown to

be proportional to the square root of the applied optical power [61]. Thus, a simple

empirical model for the photo-generated current of GaN-based photodetectors can be

expressed as [61]

𝐼𝑜𝑝𝑡 = (𝛼𝑉 + 𝛽)√𝑃𝑜𝑝𝑡 (2.10)

where α and β are constants found by fitting experimental data, V is the applied bias

voltage, and Popt is the applied optical power. The total measured photocurrent is the

sum of the dark current and photo-generated current, that is

𝐼𝑝ℎ𝑜𝑡𝑜 = 𝐼𝑑𝑎𝑟𝑘 + 𝐼𝑜𝑝𝑡. (2.11)

2.2.4 Photocurrent Results

The simulated dark and 365 nm illuminated current-voltage response for a GaN

MSM photodetector operating at temperatures between -100°C to 200°C is shown in

Fig. 2.2.8 where α was taken as 0.079 and β was taken as 0.0 as suggested by Zhang et

al [61]. Since the photo-generated current model used in this simulation (Eq. 2.10) is

independent of temperature, it is not surprising that the total measured photocurrent is

of the same order of magnitude at each temperature. To easily visualize the effect of

temperature on photodetector performance, PDCR (see Eq. 1.1) and responsivity (see

Eq. 1.2) were computed as functions of temperature as shown in Fig. 2.2.9. In Fig.

2.2.9a, PDCR decreases with increasing temperature indicating thermal generation of

charge carriers (leakage current) dominates the photo-generation at elevated

temperatures which is a commonly reported result in literature for wide bandgap


photodetectors [62, 98, 42, 43]. The constant responsivity vs. temperature shown in Fig.

2.2.9b is due to the fact that photo-generated current model implemented does not have

a temperature-dependent component. Experimental results reported in literature for the

temperature-dependent responsivity of GaN-based photodetectors varies widely

showing responsivity to be approximately constant from 23°C to 200°C [98], constant

from 23°C to 150°C and then increasing [62], as well as increasing from 70°C to 130°C

then constant until 225°C and finally decreasing [55]. The inconsistencies for reported

responsivity vs. temperature can be attributed to traps at the metal-semiconductor

interface [62] as well as defects in the GaN [55] trapping photo-generated carriers at

lower temperatures and subsequently releasing the trapped carriers at higher

temperatures.

Figure 2.2.8. GaN MSM photodetector dark and 365 nm illuminated current-voltage

response for temperatures from -100°C to 200°C with (a) linear current axis and (b)

logarithmic current axis.

The PDCR and responsivity presented in Fig. 2.2.9 were taken at 200 mV bias.

To see the effect of bias voltage on PDCR and responsivity, both figures of merit were

plotted as functions of voltage at room temperature as shown in Fig. 2.2.10. From Fig.

2.2.10a, it can be seen that there is an optimal bias voltage to obtain the largest PDCR

whereas Fig. 2.2.10b shows responsivity continually increases with increasing voltage.


The responsivity increases linearly with voltage due to the linear relationship between

photo-generated current and voltage in the implemented model (Eq. 2.10). Physically,

the photo-generated carriers are collected more efficiently when operating under a

higher electric field resulting in enhanced responsivity at higher applied bias voltage. A

linear relationship between responsivity and voltage is consistent with experimental

results reported in the literature [61, 99, 100, 101, 102]. At higher bias voltages,

saturated carrier mobility and the sweep-out effect (all photo-generated carriers are

collected before they can recombine) may cause the responsivity-voltage relationship to

plateau [50, 101]. The linear relationship between photo-generated current and voltage

coupled with the non-linear relationship between dark current and voltage results in the

PDCR peaking at a relatively low voltage. Reported experimental results for PDCR as

a function of bias voltage are consistent with the analytical model results presented here

[103, 104, 62]. Lastly, the simulation results presented in Fig. 2.2.8, Fig. 2.2.9, and Fig.

2.2.10 are compared to experimental results for GaN-based MSM UV photodetectors in

Chapter 4.

Figure 2.2.9. GaN MSM photodetector (a) PDCR and (b) responsivity as functions of

temperature at 200 mV bias.


Figure 2.2.10. GaN MSM photodetector (a) PDCR and (b) responsivity as functions of

voltage at 23°C.

2.3 Conclusions

A semi-analytical model was developed to simulate the temperature-dependent

response of gallium nitride metal-semiconductor-metal ultraviolet photodetectors. The

current transport mechanisms of thermionic emissions (charge carriers have sufficient

energy to overcome the Schottky barrier height), thermionic field emissions (charge

carriers that do not have enough energy to overcome the Schottky barrier height but see

a thinner barrier and are able to tunnel through it), and field emissions (charge carriers

are able to directly tunnel though the Schottky barrier) were considered. For GaN MSM

photodetectors operating at temperatures between -100°C and 200°C, thermionic field

emissions was found to be the dominate form of current transport. The dark current was

seen to increase with increasing temperature due to increased thermal generation of

charge carriers. The effect of temperature on the photo-generated current was analyzed

in terms of the key device parameters of PDCR and responsivity. With increasing

temperature PDCR was found to decrease due to the thermal generation of charge

carriers dominating the photo-generation at elevated temperatures while responsivity


remained constant. The analytical model results presented here are fairly consistent with

experimental results for GaN MSM photodetectors to be discussed in detail in

Chapter 4.

36

Chapter 3

Fabrication and Spectral Characterization

3.1 Graphene-Enhanced GaN Photodetector Fabrication

3.1.1 Microfabrication Process

A cross-sectional view of the photodetector microfabrication process is shown

in Fig. 3.1.1. The GaN-on-sapphire substrate stack, from top to bottom, was composed

of 5 μm doped n-type GaN (< 5 Ω-cm resistivity), 200 nm AlN, and 500 μm sapphire

(Fig. 3.1.1a). The fabrication process began with a mesa etch of the GaN film via

reactive ion etching to isolate each device from their neighbors and mitigate

photodetector cross-talk (i.e., interaction between photodetectors) (Fig. 3.1.1b). Next,

50 nm of SiO2 was deposited via atomic layer deposition (Fig. 3.1.1c). The oxide layer

was then selectively etched to create the active area for each photodetector and allow

the interdigitated electrodes to be in direct contact with the GaN (Fig. 3.1.1d). Half of

the microfabricated photodetectors had semi-transparent Ni/Au (3 nm / 10 nm)

CHAPTER 3. FABRICATION AND SPECTRAL CHARACTERIZATION 37

electrodes formed using a standard metal lift-off procedure and the other half had

graphene electrodes formed using a poly(methyl-methacrylate) (PMMA)-mediated

transfer method (Fig. 3.1.1e). Lastly, 20 nm Ti / 200 nm Au films were deposited for

electrical contacts and interconnect (Fig. 3.1.1f). The photodetector active area is

250 μm × 250 μm with an electrode length of 240 μm, width of 5 μm, and spacing of

5 μm. Standard 1:1 contact photolithography processing was used for each step. A top-

view optical image of a typical microfabricated GaN-on-sapphire MSM photodetector

with semitransparent Ni/Au electrodes is shown in Fig. 3.1.2.

Figure 3.1.1. GaN-on-sapphire MSM photodetector microfabrication process: (a) GaN-

on-sapphire chip, (b) mesa etch for device isolation, (c) deposited SiO2 passivation layer, (d)

passivation etch, (e) semitransparent Ni/Au or graphene electrodes, and (f) deposited Ti/Au

electrical contacts and interconnect. Reprinted from [105], with the permission of AIP

Publishing.


Figure 3.1.2. Top-view optical image of a GaN MSM photodetector with semitransparent

Ni/Au electrodes. Reprinted from [105], with the permission of AIP Publishing.

3.1.2 Graphene Transfer Process

There are several methods for fabricating graphene including mechanical

cleavage, epitaxial growth, reduction of graphene oxide, and chemical vapor deposition

(CVD) [80, 106]. Mechanical cleavage methods such as the Scotch tape method provide

high quality and easily transferable graphene sheets, but are limited to micrometer sizes

[80, 107]. Graphene grown using epitaxial growth methods such as thermal

decomposition of SiC, where SiC is annealed at high temperature causing the silicon

atoms to evaporate leaving behind carbon atoms at the surface, cannot be transferred to

arbitrary substrates limiting its usefulness [80, 106]. Graphene formed via reduction of

graphene oxide contains a substantial amount of residual functionalities and thus the

resulting graphene has poor electrical properties [80]. The CVD method of graphene

growth involves decomposing gaseous carbon sources on transition metals [80, 106].

High-quality and large-area graphene sheets have been demonstrated using the CVD

growth and subsequent transfer method [80, 108, 109, 110, 111, 112, 81].


Figure 3.1.3. Graphene transfer process: (a) graphene grown on copper foil via chemical

vapor deposition, (b) spin on PMMA, (c) oxygen plasma etch to remove backside graphene,

(d) etch copper foil, (e) rinse in deionized water, (f) transfer graphene/PMMA stack to

substrate, and (g) remove PMMA.

The graphene transfer process used here began with graphene grown on copper

via CVD (Fig. 3.1.3a). Note that the CVD process results in graphene growth on both

sides of the copper. To enable transfer of the graphene, a layer of PMMA was spun onto

one side of the graphene on copper sheet (Fig. 3.1.3b). Next, the

PMMA/graphene/copper stack is flipped upside down and an oxygen plasma etch was

used to remove the backside graphene (Fig. 3.1.3c). The PMMA/graphene/copper stack

is flipped over again and floated in a bath of ferric chloride (FeCl3) to remove the copper

(Fig. 3.1.3d). Once the copper is etched away the PMMA/graphene stack is transferred

to a water bath to rinse off the FeCl3 (Fig. 3.1.3e). Lastly, the PMMA/graphene stack is

transferred to the GaN-on-sapphire substrate (Fig. 3.1.3f) and the PMMA is removed

using solvents (Fig. 3.1.3g). Standard photolithography and an oxygen plasma etch was

used to form the interdigitated electrodes. The detailed photodetector fabrication run

sheet including graphene transfer process is listed in Appendix D. A top-view optical

image of a typical graphene/GaN MSM photodetector is shown in Fig. 3.1.4a where the


2D graphene electrodes are not visible. The graphene electrodes can be seen in a

scanning electron microscope (SEM) image as shown in Fig. 3.1.4b.

Figure 3.1.4. (a) Top-view optical image and (b) top-view SEM of graphene-enhanced

GaN MSM photodetector.

3.2 Optical Properties and Spectral Response

3.2.1 Optical Properties of GaN Substrate

The optical properties of the GaN-on-sapphire substrate were measured using a

UV-Vis-NIR spectrophotometer (Cary 6000i, Agilent Technologies) and are shown in

Fig. 3.2.1. The absorbance and reflectance spectra in Fig. 3.2.1a show the intrinsic

visible blindness of GaN with a sharp cutoff wavelength near 370 nm. Additionally,

Fabry-Pérot oscillations (constructive interference of incident light with reflected light

in multilayer materials) in the visible light region confirm the good uniformity and

interface quality of the GaN-on-sapphire substrate [99, 113, 93]. Fig. 3.2.1b shows the

transmission spectrum and (αhν)2 versus incident photon energy (hν) where α is the

absorption coefficient, h is Plank’s constant, and ν is the photon frequency. By


extrapolating the linear region of the (αhν)2 curve (red dotted line) [99, 42, 114], the

bandgap of the GaN substrate is seen to be approximately 3.34 eV which, as expected,

is comparable to the wide bandgap of wurtzite crystallized GaN (~3.4 eV) [115, 116].

Figure 3.2.1. Optical properties of the GaN-on-sapphire substrate: (a) absorbance and

reflectance spectra showing a cutoff wavelength around 370 nm and (b) transmission

spectrum and (αhν)2 versus hν with extrapolation of the linear region (red dotted line)

showing a bandgap of 3.34 eV. Reprinted from [105], with the permission of AIP Publishing.

3.2.2 Optical Properties of Graphene and Semitransparent Ni/Au

A comparison of the transmittance spectra (measured using an Agilent Cary

6000i UV-Vis-NIR spectrophotometer) of semitransparent Ni/Au (3 nm / 10 nm) and

graphene on a 1 mm thick glass substrate is shown in Fig. 3.2.2a and the corrected

transmittance spectra for graphene and semitransparent Ni/Au accounting for the glass

substrate is shown in Fig. 3.2.2b. For wavelengths longer than 300 nm, the transmittance

of semitransparent Ni/Au is between 30 to 45% and only 20 to 30% for wavelengths

less than 300 nm. Since the glass substrate is strongly absorbing for wavelength less

than 300 nm, the signal is very low which is why the corrected data appears noisy at the

low wavelengths. The relatively low transmittance of the Ni/Au agrees with previously

reported values for 100 Å thick gold electrodes of 30 to 50% in the UV regime [17].

The transmittance of graphene is greater than 90% for wavelengths longer than 300 nm


and greater than 80% for wavelengths shorter than 300 nm. In particular, at 365 nm, the

transmittance of graphene is 94% compared to only 35% for semitransparent Ni/Au.

Therefore, photodetectors with graphene electrodes will allow more of the incident UV

light to be absorbed by the active GaN region as shown in Fig. 3.2.2 and thus will result

in more sensitive photodetectors.

Figure 3.2.2. (a) Transmission spectra of graphene and semitransparent Ni/Au

(3 nm / 10 nm) on 1 mm thick glass and (b) corrected transmission spectra of graphene and

semitransparent Ni/Au. Reprinted from [117], with the permission of AIP Publishing.

Figure 3.2.3. Cross-sectional schematic of (a) a graphene/GaN and (b) a

semitransparent/Ni/Au/GaN MSM photodetector. Reprinted from [117], with the permission

of AIP Publishing.


3.2.3 Spectral Response Test Set-up

To characterize the spectral response of the GaN-on-sapphire photodetectors, a

laser driven light source (EQ-1500, Energetiq) with nearly constant radiance across the

UV spectra and spectrometer (McPherson 218) with diffraction grating

(246.16 grooves/mm and 226 nm blaze) set-up was utilized as shown in Fig. 3.2.4. To

begin, the spectrometer count to power relationship was determined using a NIST-

traceable calibrated reference photodetector (918D-UV-OD3R, Newport) with a power

meter (841-P-USB, Newport). The inlet and exit slit widths of the spectrometer were

set to 2 mm and 0.75 mm, respectively, to maximize the applied optical power with a

reasonable wavelength resolution (10% of peak at ±20 nm). Next, the calibrated

photodetector was used to measure the power of the laser driven light source from

500 nm to 200 nm in increments of 10 nm. Above 500 nm, the laser driven light source

had low intensity and since wavelengths less than 200 nm (known as vacuum-

ultraviolet, VUV) are absorbed by oxygen, experimentation under vacuum is required

which was not implemented in this study. In order to reduce second order diffraction

affects, a long pass filter was used for wavelengths greater than 340 nm. Finally,

current-voltage measurements were taken for the GaN-based photodetectors under

illumination from 500 nm to 200 nm in 10 nm increments. The inset of Fig. 3.2.4 shows

the power incident on the GaN photodetectors as a function of wavelength as measured

at the imaging plane of the spectrometer by the reference detector with the difference in

detector size between the reference (0.75 mm × 1.13 cm) and GaN (250 μm × 250 μm)

photodetectors accounted for. Two separate measurements were taken above and below

340 nm with the filter in place. Differences in source alignment cause the applied power

to differ between the two measurements. All electrical measurements were taken using

a semiconductor parameter analyzer (B1500A, Agilent Technologies). Drawings of

spectrometer adapter that enabled spectral response characterization of the GaN-based

photodetectors are shown in Appendix C.


Figure 3.2.4. Benchtop test set-up for spectral response characterization of GaN-based

photodetectors with the applied optical power incident on the GaN-based photodetectors

shown in the inset. Reprinted from [118], with the permission of AIP Publishing.

3.2.4 GaN Photodetector Spectral Response

The current-voltage responses for four separate semitransparent Ni/Au GaN-on-

sapphire MSM photodetectors are shown in Fig. 3.2.5 under dark (unilluminated) and

illuminated (500 nm, 365 nm, and 200 nm wavelengths) conditions. All four

photodetectors were fabricated on the same GaN-on-sapphire chip. From the figures, it

is easily seen that the photodetectors are most responsive to 365 nm illumination which

directly corresponds to the bandgap of GaN-on-sapphire substrate (3.34 eV).


Figure 3.2.5. Current-voltage response of four semitransparent Ni/Au GaN-on-sapphire

MSM photodetectors on the same chip when unilluminated (dark) and illuminated with

wavelengths from 200 nm to 500 nm.

As described in Chapter 1, photodetector performance is typically quantified

using two figures of merit, PDCR (defined in Eq. 1.1) and responsivity (defined in Eq.

1.2). Since the photo-generated current (Iphoto - Idark) for GaN-based photodetectors is

proportional to the square root of the applied optical power [61], a more objective metric

for spectral characterization would be the normalized photocurrent-to-dark current ratio

(NPDR) [119, 108] defined as

NPDR = (𝐼𝑝ℎ𝑜𝑡𝑜 − 𝐼𝑑𝑎𝑟𝑘

𝐼𝑑𝑎𝑟𝑘)

√𝑃𝑜𝑝𝑡

. (3.1)


The NPDR as a function of illumination wavelength for the four semitransparent

Ni/Au/GaN photodetectors are shown in Fig. 3.2.6. The NPDR peaks around 365 nm

and quickly drops off in the visible wavelength region. The spectral responsivity (R)

can then be defined as,

R = 𝐼𝑝ℎ𝑜𝑡𝑜 − 𝐼𝑑𝑎𝑟𝑘

√𝑃𝑜𝑝𝑡

(3.2)

where, once again the square root dependence of photo-generated current with applied

optical power is taken into account. The spectral responsivities for the four characterized

GaN-on-sapphire photodetectors are shown in Fig. 3.2.7. The applied optical power

used for these calculations was that measured using the calibrated reference

photodetector as shown in Fig. 3.2.4.

The spectral responsivity of the GaN-based photodetectors peaks at 365 nm and

exhibits a 365 nm to visible rejection of about one order of magnitude while the

responsivity at 200 nm is about one-third the responsivity at 365 nm. The reduced

responsivity at shorter illumination wavelengths is unexpected from simply analyzing

the absorbance of the GaN-on-sapphire substrate. From Fig. 3.2.1, the absorbance of the

GaN-on-sapphire substrate is approximately constant for wavelengths shorter than

365 nm. The strong peak in the spectral responsivity has been previously observed in

GaN-based photodetectors and attributed to excitonic effects where excitons (bound

electron-hole pairs) with energy slightly less than GaN’s bandgap act as traps for photo-

generated carriers [120, 55, 121, 122, 44].

These results are consistent with other DC spectral responsivity measurements

of GaN-based photodetectors reported in literature [17, 60]. The relatively low

UV/visible rejection ratio can be attributed to persistent photoconductivity effects due

to impurities, dislocations, and defects trapping photo-generated carriers for long times

after the illumination source has been removed [55, 98, 17, 60, 57]. Most often, spectral

responsivity measurements are reported using a chopper with a lock-in amplifier, in


which all phenomena occurring at rates slower than the chopper frequency are

eliminated. Using this technique results in UV/visible contrasts greater than three orders

of magnitude. However, the chopping frequency affects the absolute value of the

responsivity, its variation with incident optical power, as well as its variation with the

excitation wavelength [17, 61, 60, 57]. For implementation in any other real-world

application without a pulsed illumination source, it is most important to understand the

DC characteristics of the device to accurately quantify the amount UV radiation present.

Figure 3.2.6. NPDR as a function of illumination wavelength for four semitransparent

Ni/Au GaN-on-sapphire MSM photodetectors.


Figure 3.2.7. Spectral responsivity of four semitransparent Ni/Au GaN-on-sapphire MSM

photodetectors.

3.3 Conclusions

The absorbance and reflectance spectra of the GaN-on-sapphire substrate

demonstrated the intrinsic visible blindness of GaN with a sharp cutoff wavelength near

370 nm. Additionally, the bandgap of the GaN film was shown to be 3.34 eV

comparable to the reported value of 3.4 eV for the wide bandgap of wurtzite crystallized

GaN. GaN-on-sapphire UV photodetectors with semitransparent Ni/Au (3 nm / 10 nm)

interdigitated electrodes were fabricated and shown to be highly responsive to UV

radiation, especially at 365 nm, demonstrating an order of magnitude contrast from


visible radiation. Semitransparent Ni/Au (3 nm / 10 nm) films demonstrated relatively

low transmittance between 30 - 40% in the UV regime while graphene was shown to

have a very high transmittance (> 80%) in the UV regime. In order to capitalize on this

high transmittance, GaN-on-sapphire UV photodetectors with graphene interdigitated

electrodes were fabricated. In the following chapters, the semitransparent Ni/Au/GaN

photodetectors and graphene/GaN photodetectors will be characterized and compared

while operating in high temperature and proton irradiation environments.

50

Chapter 4

Extreme Environment Operation

4.1 High-Temperature Exposure

4.1.1 Experimental Set-Up

The test set-up for characterizing the GaN-based photodetectors in air at elevated

temperatures using a high-temperature probe station (Signatone S-1060) is shown in

Fig. 4.1.1. Measurements began at room temperature (23°C) with dark current-voltage

measurements followed by photocurrent-voltage measurements. Testing proceeded in

50°C steps up to 200°C with dark current-voltage and photocurrent-voltage measured

at each temperature step. In addition to current-voltage measurements, transient current

measurements were taken at room temperature, 100°C, 150°C, and 200°C. Transient

current measurements were taken by measuring dark current for five minutes followed

by five minutes illuminated (365 nm) and then five minutes in the dark. The GaN-based

photodetector chip was held at each temperature for at least 10 minutes before

CHAPTER 4. EXTREME ENVIRONMENT OPERATION 51

measurements were taken. A semiconductor parameter analyzer (B1500A, Agilent

Technologies) was used for all electrical characterization and a 6 W, 365 nm UV lamp

(UVP, EL series UV lamp) was used for UV illumination. A UV light meter was used

to measure the incident UV intensity, which for all illuminated electrical measurements

was 0.7 mW/cm2. For this intensity, the optical power applied on the photodetectors is

approximately 438 nW.

Figure 4.1.1. Experimental test set-up for characterizing the elevated temperature response

of GaN-based photodetectors using a high-temperature probe station.

4.1.2 Results

One GaN-on-sapphire MSM photodetector with graphene electrodes and two

GaN-on-sapphire MSM photodetectors with semitransparent Ni/Au electrodes were

characterized during this testing. Dark (dashed lines) and 365 nm illuminated (solid

lines) current-voltage curves from room temperature up to 200°C are shown in Fig.

4.1.2a for a representative graphene/GaN photodetector and Fig. 4.1.2b for a


representative semitransparent Ni/Au/GaN photodetector. For both types of electrodes,

dark current increases as temperature increases as expected due to increased thermal

generation of carriers [44].

Figure 4.1.2. (a) Graphene-enhanced and (b) semitransparent Ni/Au GaN-on-sapphire

MSM photodetector current-voltage curves under dark (dashed lines) and 365 nm illuminated

(solid lines) conditions from room temperature to 200°C.

To quantitatively compare the performance of the graphene/GaN and

semitransparent Ni/Au/GaN photodetectors, PDCR (as defined in Eq. 1.1) and

responsivity (as defined in Eq. 1.2) as functions of voltage for temperatures from 23°C

to 200°C were calculated as shown in Fig. 4.1.3. Both types of photodetectors exhibit a

peak PDCR (30.8 for the graphene/GaN photodetector and 5.6 for the semitransparent

Ni/Au/GaN photodetectors at room temperature) at relatively low bias voltages which

is in good agreement with the analytical model developed and discussed in Chapter 2.

The responsivity for both types of photodetectors increases with increasing voltage

which is consistent with the analytical model results from Chapter 2. However, the room

temperature responsivity of the semitransparent Ni/Au/GaN photodetectors becomes

nearly constant for voltages greater than 2 V which can be attributed to the sweep-out

phenomenon where all photo-generated carriers are collected before they can recombine

resulting in responsivity being saturated above a certain bias voltage [123, 101, 50]. As


temperature increases, the voltage at which sweep-out occurs increases which can be

attributed to the increased recombination rate of photo-excited carriers at high

temperatures [56]. Since graphene has a higher UV transmittance than semitransparent

Ni/Au (see Fig. 3.2.2), the graphene/GaN device is able to absorb more incident photons

thus sweep-out would occur at a higher bias voltage.

Figure 4.1.3. PDCR and responsivity as functions of voltage at temperatures from 23°C to

200°C for (a) and (b) a graphene/GaN photodetector and (c) and (d) a semitransparent

Ni/Au/GaN photodetector.

To easily visualize the temperature-dependent trends, PDCR and responsivity as

functions of temperature are shown in Fig. 4.1.4. At room temperature, the


graphene/GaN photodetectors exhibit PDCRs four times greater than the

semitransparent Ni/Au/GaN photodetectors. The larger PDCR for the graphene/GaN

photodetectors is due to the higher transmission of the graphene electrodes allowing the

GaN to absorb more of the incident photons as discussed in Chapter 3, as well as the

lower dark current as seen in Fig. 4.1.2. For both electrode materials, PDCR decreases

as temperature increases up to 200°C which is in good agreement with the analytical

model results described in Chapter 2. For temperatures beyond 200°C, PDCR drops to

zero indicating the sensors no longer operate as photodetectors. The semitransparent

Ni/Au/GaN photodetectors showed 32 times greater room temperature responsivity

compared to the graphene/GaN photodetectors which can be attributed to the much

higher leakage (dark) current of the semitransparent Ni/Au/GaN devices, as will be

discussed further below. For both types of photodetectors, responsivity first increases

with temperature, but then decreases as temperature is further increased, reaching zero

above 200°C. This result is similar to that seen by De Vittorio et al. with the initial

increase attributed to thermal ionization of photo-generated carriers trapped in defect

sites resulting in increased responsivity at higher temperatures. The subsequent

reduction in responsivity at even higher temperatures is due to increased lattice

scattering and a reduced bandgap [55, 65]. From Fig. 2.2.1, the bandgap of GaN at 23°C

is 3.4 eV which corresponds to an absorption edge of 365 nm. The bandgap of GaN at

200°C is only 3.3 eV resulting in a shifted absorption edge of 376 nm. Assuming the

spectral responsivity curves shown in Fig. 3.2.7 shift by 11 nm toward longer

wavelengths, the responsivity of the GaN-based photodetectors will be reduced by

approximately 20% when illuminated by 365 nm light.


Figure 4.1.4. (a) PDCR and (b) responsivity as functions of temperature from room

temperature to 200°C for graphene-enhanced (shown in the inset) and semitransparent Ni/Au

GaN-on-sapphire MSM photodetectors.

The magnitude of the leakage current is much lower for the graphene/GaN

photodetectors (nanoamps at 23°C) compared to the semitransparent Ni/Au/GaN

photodetectors (milliamps at 23°C). To understand this, the barrier heights for both

types of photodetectors were extracted from the forward current-voltage characteristics

by fitting the current-voltage curves to the thermionic field emissions model described

in Chapter 2. At room temperature, the barrier height for the graphene/GaN

photodetectors was found to be 0.80 eV while that for the semitransparent Ni/Au/GaN

photodetectors was only 0.46 eV. Thus, the lower dark current in the graphene/GaN

photodetectors compared to the semitransparent Ni/Au/GaN photodetectors is due to the

superior contact of the graphene to the GaN creating a higher Schottky barrier height.

The extracted barrier heights from room temperature up to 200°C are shown in Fig.

4.1.5. As temperature increases, the barrier height increases which has been reported in

previous experimental results and attributed to the Schottky barrier height

inhomogeneity [90, 91, 92] as discussed in Chapter 2. Reported values for the barrier

height of graphene on GaN range from 0.49 eV to 0.74 eV [73, 72] and those for Ni on

GaN are between 0.66 - 0.99 eV [86]. The relatively low barrier height for Ni/Au on


GaN reported here could be due to the Ti/Au electrical contacts alloying with the GaN

forming TiN and generating leakage pathways as discussed in Chapter 2.

Figure 4.1.5. Extracted barrier height as a function of temperature for (a) graphene/GaN

and (b) semitransparent Ni/Au/GaN photodetectors.

The room temperature quantum efficiency (defined in Eq. 1.3) for the

graphene/GaN photodetector is 1.1 × 103 % while the semitransparent Ni/Au is

1.9 × 106 %. As shown in Fig. 4.1.6, quantum efficiencies greater than 100% are often

reported for GaN-based photodetectors and attributed to a photo-gain mechanism [124,

125, 51, 52, 126, 102]. The photo-gain is caused by photo-generated holes trapped at

negatively-charged surface states lowering or bending the Schottky barrier with respect

to the dark Schottky barrier as shown in Fig. 4.1.7. This change in the Schottky barrier

results in an enhanced leakage current. Since the Schottky barrier height of the

semitransparent Ni/Au/GaN photodetector was found to be lower than the

graphene/GaN photodetector, the additional lowering of the barrier height by trapped

photo-generated holes has a greater effect on the semitransparent Ni/Au/GaN

photodetectors resulting in a much larger responsivity. As the operating temperature is

increased, thermal energy releases the trapped carriers. The lowering or bending of the


Schottky barrier is lessened thus the extra leakage current and the enhancement of the

photocurrent is reduced. At 200°C, the quantum efficiencies are 1.0 × 103 % and

1.9 × 105 % for the graphene/GaN and semitransparent Ni/Au/GaN photodetectors,

respectively. Additionally, Zhang et al. reported that the internal photo-gain is much

higher when operating in DC mode [61], as opposed to using a chopper and lock-in

amplifier in which all phenomena occurring at rates less than the chopper frequency are

eliminated. This observation further supports the notion that trapped photo-generated

carriers, which effect the device response on long time scales, are creating the observed

internal photo-gain. Trapped photo-generated carriers also give rise to persistent

photoconductivity as mentioned in Chapter 1 and discussed further below.

Figure 4.1.6 GaN-based photodetectors reported in literature with responsivities

corresponding to quantum efficiencies greater than 100%.


Figure 4.1.7 Trapped photo-generated holes causing lowering or bending of the Schottky

barrier height resulting in an internal photo-gain.

The transient current response curves at 200 mV bias for graphene/GaN and

semitransparent Ni/Au/GaN photodetectors at temperatures between 23°C and 200°C

are shown in Fig. 4.1.8a and Fig. 4.1.8b, respectively. It is readily apparent from Fig.

4.1.8 that increasing temperatures result in a lower net photo-generated current (dark

current subtracted from illuminated current) which was the same result found above by

examining the current-voltage responses. From Fig. 4.1.8, it can also be seen that

increasing temperature results in faster photocurrent decay. Table 4.1.1 lists the 90% to

10% decay times for the graphene/GaN and semitransparent Ni/Au GaN photodetectors.

At room temperature the decay time for the graphene/GaN photodetector is 5900 s and

that for the semitransparent Ni/Au/GaN photodetector is 1250 s. Long photocurrent

decay times are characteristic of GaN photodetectors and have been attributed to

impurities, dislocations, and deep-level defects trapping photo-generated carriers for

long times after the illumination source has been removed [58, 59, 55, 56, 57]. This

phenomenon has been termed persistent photoconductivity. The photocurrent decay has

been modeled as a stretched exponential of the form

𝐼(𝑡) = 𝐼0exp [− (𝑡

𝜏)𝛽

] (4.1)


where I0 is the initial photocurrent, τ is the decay time constant, and β is the decay

exponent. At room temperature, reported values for β are between 0.2 and 0.4 [57, 59].

Values for β extracted from the experimental data range from 0.16 to 0.14 for the

graphene/GaN photodetectors and from 0.26 to 0.15 for the semitransparent Ni/Au/GaN

photodetectors from room temperature to 200°C. It has been shown that by increasing

the temperature of GaN-based photodetectors, photocurrent decay times are reduced and

thus persistent photoconductivity is suppressed [56, 57, 55, 98, 127]. Thus, at elevated

temperatures, thermal energy is able to release photo-generated carriers trapped at

defects sites.

Figure 4.1.8. Transient current response of the (a) graphene-enhanced and (b)

semitransparent Ni/Au GaN-on-sapphire photodetectors from room temperature to 200°C

under a 365 nm UV intensity of 0.7 ± 0.1 mW/cm2.

Table 4.1.1. Decay times of the graphene/GaN and semitransparent Ni/Au/GaN photodetectors

from 23°C to 200°C.

Temperature (°C)

23 100 150 200

Graphene/GaN 5900 s 2900 s --- ---

Semitransparent Ni/Au/GaN 1250 s 35 s 13 s 0.2 s


The rise time (10% to 90% rise time) of GaN-based photodetectors has been

shown to be on the order of milliseconds at room temperature [35]. While it is not

possible to accurately quantify the rise time of the GaN-based photodetectors due to the

slow turn-on time of the UV lamp, it is easy to visually see from Fig. 4.1.8 that rise time

decreases as temperature increases. Since charge carriers generated far from the

electrodes may recombine before they are collected [17], the faster rise time at higher

temperatures suggests that the recapture rate of photo-excited carriers becomes large

compared to the excitation rate as temperature increases [56].

4.2 Proton Irradiation Testing

4.2.1 The Space Radiation Environment

Electronic devices operating in space experience high levels of radiation in the

form of electromagnetic waves (ultraviolet light, gamma rays, etc.) and particles

(electrons, protons, neutrons, heavy ions, etc.), as discussed in Chapter 1. These high

levels of radiation can introduce atomic defects in the material lattice which alter or

even destroy electronic performance. Thick aluminum shielding (~10 mm) is often used

to protect spacecraft instrumentation from this damaging radiation [4]. Predictions for

the integral trapped proton fluence as a function of energy and aluminum shielding

thickness for one year in low-Earth orbit (LEO) (orbit approximating the international

space station orbit, perigee: 400.2 km, apogee: 409.5 km, inclination: 51.64°) computed

using the SPace ENVironment Information System (SPENVIS), a software package that

combines several models to determine the space environment and its effect on materials,

are shown in Fig. 4.2.1a. In particular, SPENVIS predicts a 2 MeV integral trapped

proton fluence of 4.04 × 109 cm-2 for unshielded electronics which is reduced to a

fluence of 6.76 × 107 cm-2 (98.3% reduction) with 10 mm of aluminum shielding. One

of the most severe radiation environments in the solar system is encountered near Jupiter


due to its strong magnetic field [4, 6]. SPENVIS predictions for the integral trapped

proton fluence as a function of energy and aluminum shielding thickness for one year

in orbit around Jupiter (orbit approximating the science orbit of the Juno spacecraft,

perijove: 4,200 km, apojove: 7,900 km, inclination: 90°) are shown in Fig. 4.2.1b.

SPENVIS predicts a 2 MeV integral trapped proton fluence of 3.35 × 1011 cm-2 for

unshielded electronics in this Jovian orbit which is an 83 times larger fluence compared

to one year in LEO. With 10 mm aluminum shielding the 2 MeV integral trapped proton

fluence is reduced to 1.60 × 1010 cm-2. However, for both orbits, the aluminum shielding

does little to protect instrumentation against higher energy protons as can be easily seen

in Fig. 4.2.1. Additionally, the aluminum shielding adds considerable mass and thus

cost to spacecraft.

The high atomic displacement energy of GaN makes it an intrinsically radiation-

hard material. Thus, GaN is an intriguing material platform for developing radiation-

tolerant sensors that do not require protective shielding. In the next sections, the

response of GaN UV photodetectors subjected to 2 MeV proton irradiation will be

discussed.

Figure 4.2.1. One year trapped proton fluence as a function of energy and aluminum

shielding thickness for (a) low Earth orbit approximating the international space station orbit

and (b) Jupiter orbit approximating the science orbit of the Juno spacecraft; calculated using

the SPace ENVironment Information System (SPENVIS) software.


4.2.2 Test Set-Up

Proton irradiation of the GaN-based photodetectors was conducted using the

NEC Pelletron Tandem Accelerator (9SDH-2) at Ion Beam Materials Laboratory

(IBML) at Los Alamos National Laboratory. The proton beam was scattered using a

very thin gold foil (~50 nm) with the photodetectors located at a forward angle of 45°

to the beam direction as shown in Fig. 4.2.2a. The proton flux was concurrently

monitored with a silicon surface barrier detector located at a backward angle of 167°.

After exposure to irradiation, the photodetectors were removed from the proton

irradiation chamber for electrical characterization at estimated average fluences of

8.9 × 1011, 1.3 × 1013, 2.7 × 1013, and 3.8 × 1013 cm-2 on the photodetectors. Current-

voltage measurements were taken using a semiconductor parameter analyzer (Agilent

Technologies B1500A) under dark and illuminated (365 nm, 1 mW/cm2) conditions as

shown in Fig. 4.2.2b. Two graphene/GaN and three semitransparent Ni/Au/GaN

photodetectors were characterized during this testing.

Figure 4.2.2. (a) GaN-based photodetectors mounted inside the tandem ion accelerator

chamber at Los Alamos National Laboratory undergoing proton irradiation and (b) test set-

up for electrical characterization of proton irradiated photodetectors.


4.2.3 Electrical Characterization

Current-voltage response curves (dark and 365 nm illuminated) at each

irradiation fluence are shown in Fig. 4.2.3a for a representative graphene/GaN

photodetector and Fig. 4.2.3b for a semitransparent Ni/Au/GaN photodetector. From

Fig. 4.2.3, it can be visually seen that 2 MeV proton irradiation up to a fluence of

3.8 × 1013 cm-2 does not greatly affect device performance. To quantitatively show this,

PDCR and responsivity as functions of proton fluence for both graphene/GaN and

semitransparent Ni/Au/GaN photodetectors are shown in Fig. 4.2.4. For both electrode

materials, the results in Fig. 4.2.4 show that proton irradiation up to a fluence of

3.8 × 1013 cm-2 has very little effect on the devices response to UV light. A similar result

was also shown by Khanna et al. using photoluminescence spectroscopy to study defect

formation in bare GaN films due to proton irradiation [128]. Additionally, the

graphene/GaN devices exhibit much larger PDCR (three times larger over all irradiation

fluences tested) and responsivity (five times larger over all irradiation fluences tested)

than the semitransparent Ni/Au/GaN devices which can be attributed to the higher

transmittance of the graphene electrodes as discussed in Chapter 3. The very high

responsivities exhibited by both devices has been previously reported in literature and

attributed to trapping of photo-generated holes by surface states at the metal-

semiconductor interface resulting in a lowering of the Schottky barrier height and

increased leakage current [53, 54], as discussed in the high-temperature test results and

shown in Fig. 4.1.7. For graphene/GaN photodetectors, this photoconductive gain is

further enhanced by the very short space distance at the graphene/GaN interface [72].


Figure 4.2.3. Dark and illuminated (365 nm) scaled current-voltage response as a function

of proton irradiation fluence for GaN-on-sapphire photodetectors with (a) graphene

electrodes and (b) semitransparent Ni/Au electrodes. Reprinted from [117], with the

permission of AIP Publishing.

Since pre- and 30 days post-irradiation current-voltage measurements were

taken using the high-temperature probe station (see Fig. 4.1.1) rather than the set-up

shown in Fig. 4.2.2b, a direct comparison of the pre- and post-irradiation data to the

measurements made during irradiation is not possible. However, an overall comparison

is still informative. The average pre- and 30 days post-irradiation PDCR and

responsivity for the graphene/GaN and semitransparent Ni/Au/GaN photodetectors are

listed in Table 4.2.1. The graphene electrodes were damaged post-irradiation during

transport, therefore no measurements of those devices could be made. The pre- and post-

irradiation PDCR and responsivity of the semitransparent Ni/Au/GaN photodetectors

are consistent with the measurements made during irradiation showing a slight increase

in PDCR and a slight decrease in responsivity post-irradiation.


Figure 4.2.4. (a) PDCR and (b) responsivity as a function of proton fluence for graphene

and semitransparent Ni/Au GaN-on-sapphire photodetectors. Reprinted from [117], with the


Table 4.2.1. Average pre- and 30 days post-irradiation PDCR and responsivity for graphene

and semi-transparent Ni/Au GaN-on-sapphire photodetectors. The graphene electrodes were

damaged post-irradiation during transport.

Electrode Material PDCR Responsivity (A/W)

Pre-Irradiation Graphene 0.69 3388

Semitransparent Ni/Au 0.06 351

Post-Irradiation Graphene --- ---

Semitransparent Ni/Au 0.09 310

4.2.4 Raman Spectroscopy

Predictions for the proton implantation depth as a function of energy computed

using the Stopping and Range of Ions in Matter (SRIM) software, a Monte Carlo

simulation which estimates the transport of ions in matter, are listed in Table 4.2.2. For

2 MeV protons impingent on graphene on GaN-on-sapphire, SRIM predicts a stopping

depth of 52 μm which is within the sapphire substrate, thus the majority of the

displacement damage occurs in the sapphire substrate, well below the active region of

the photodetector. To intentionally increase displacement damage within the active GaN


layer, 200 keV protons were used to irradiate graphene specimens based on SRIM

calculations which estimate that 200 keV protons stop at approximately 2 μm depth,

which is within the GaN. This irradiation may have a greater effect on device

performance since defects in the active semiconductor have been shown to influence the

graphene/semiconductor interface [77]. Additionally, previous reports have indicated

that defect formation in graphene increases as energy decreases [79]. SRIM ion

distributions for 200 keV and 2 MeV protons impingent on graphene on GaN-on-

sapphire are shown in Fig. 4.2.5.

Table 4.2.2. Proton implantation depth and material as a function of energy for the GaN-on-

sapphire substrate as calculated using the Stopping and Range of Ions in Matter (SRIM)

software.

Energy (MeV) Implantation Depth (μm) Implantation Material

0.1 1.0 GaN

0.2 2.0 GaN

0.5 6.2 Sapphire

1 17.5 Sapphire

2 52.4 Sapphire

5 242 Sapphire

7 432 Sapphire

Raman spectroscopy (HORIBA Scientific LabRAM HR Evolution

spectrometer, laser wavelength of 532 nm) was used to investigate the damage done to

graphene by 200 keV protons at a fluence of 3.8 × 1013 cm-2. Figure 4.2.6 shows the

Raman spectra of a graphene on a SiO2/Si sample before irradiation and 30 days after

irradiation (the spectra are offset for clarity). Before irradiation, the intensity of the 2D-

peak (2686 cm-1) compared to the G-peak (1596 cm-1) (I2D/IG) was 2.4 and the intensity

of the D-peak (1347 cm-1) compared to the G-peak (ID/IG) was 0.53. The presence of

two main peaks (G-peak and 2D-peak) and a negligible D-peak, which is the most

common method to detect defect formation in graphene, indicate a pristine graphene

sample [79, 129, 130]. Additionally, the full-width-at-half-maximum (FWHM) of the

2D-peak was 35.3 cm-1 which indicates monolayer graphene [131]. After irradiation,

I2D/IG decreased to 1.8 and ID/IG increased slightly to 0.56. The increased ID/IG ratio as


well as the appearance of the D'-peak (1621 cm-1) brought about by proton irradiation

indicate increased disorder in the graphene corresponding to vacancy related defects

[79, 129, 130]. Another indication of the damaging effect of proton irradiation is the

decrease in the 2D peak intensity. These changes in the Raman spectrum are quite small

considering the low energy (200 keV) and high fluence (3.8 × 1013 cm-2) proton

irradiation the graphene was exposed to. These results further confirm the radiation-

hardness of graphene and its potential to enhance other material platforms.

Figure 4.2.5. SRIM ion distribution results for (a) 200 keV and (b) 2 MeV protons

impingent on graphene on a 5 μm GaN, 200 nm AlN and 500 μm sapphire substrate. For

200 keV protons the stopping depth and thus majority of lattice displacement damage occurs

at approximately 2 μm which is within the GaN. Whereas for 2 MeV protons, the stopping

depth is in the sapphire substrate at 52.3 μm.


Figure 4.2.6. Raman spectroscopy comparison of graphene samples before and after

200 keV proton irradiation up to a fluence of 3.8 × 1013 cm-2. Reprinted from [117], with the


4.3 Conclusions

GaN UV photodetectors with graphene and semitransparent Ni/Au electrodes

were electrically characterized and compared when subjected to temperatures ranging

from 23°C to 200°C. The graphene/GaN photodetectors exhibited a higher PDCR

compared to the semitransparent Ni/Au/GaN photodetectors over all temperatures tested

which can be attributed to graphene’s very high transmittance (> 80%) in the UV regime

compared to semitransparent Ni/Au (> 30%). For both types of photodetectors, PDCR

was seen to decrease with increasing temperature eventually reaching zero above

200°C. The decreasing PDCR with increasing temperatures is due to the thermal

generation of charge carriers overwhelming the photo-generation. The responsivity of

both types of photodetectors was seen to decrease at high temperatures due to increased

lattice scattering as well as a reduced bandgap. From transient current response data, it

was shown that both graphene/GaN and semitransparent Ni/Au/GaN photodetectors


demonstrated faster rise and decay times at elevated temperatures. The faster rise times

at high temperatures were attributed to a higher recapture rate of photo-excited carriers

while the faster decay times were attributed to thermal energy releasing trapped photo-

generated carriers. Additionally, these temperature-dependent trends seen in the

experimental results match the predictions of the analytical model developed in Chapter

2.

The graphene/GaN and semitransparent Ni/Au/GaN photodetectors were also

characterized when subjected to 2 MeV proton irradiation up to a fluence of

3.8 × 1013 cm-2. Electrical measurements made throughout irradiation testing showed no

significant degradation in device performance in terms of PDCR and responsivity.

Additionally, Raman spectroscopy of 200 keV irradiated graphene showed only slight

increase of disorder in the form of an increased ID/IG ratio, the appearance of the D'-

peak, and a decrease in the 2D peak intensity. These results support the use of graphene

and GaN as temperature tolerant and radiation-hard material platforms for UV

photodetection with high sensitivity and responsivity.

70

Chapter 5

Implementation

5.1 Miniature Sun Sensor

5.1.1 Current Sun Sensor Technology

There are several sensor technologies used for spacecraft attitude determination

which include sun sensors, star trackers, gyroscopes, magnetometers, and the global

positioning system (GPS) [132, 133]. Sun sensors indicate the orientation of the

spacecraft relative to the sun while star trackers use multiple vector measurements to

different stars, resulting in a more accurate measurement at the cost of more mass and

higher power consumption [133]. Gyroscopes give relative angular measurements with

high accuracy and a fast response time, but must be coupled with an absolute reference

such as a star tracker. Magnetometers determine spacecraft attitude relative to the local

magnetic field, thus for Earth satellites can only be used below about 6,000 km. Lastly,

GPS can be used for attitude determination by Earth satellites in communication in GPS

CHAPTER 5. IMPLEMENTATION 71

satellites [132]. Of all of these technologies, sun sensors are the most commonly used.

Sun sensors have been used for aerospace applications since the early 1950’s and were

used before that for guidance of ground-based telescopes among other applications

[134]. Sun sensors have low mass (< 1 kg) and low power (< 0.2 W) requirements

compared to the other sensor technologies while maintaining decent accuracy (0.01°)

and a wide field of view (± 130°) [133]. The reliable operation of these sensors within

the harsh environment of space is critical for the success of space missions.

The two-axis sun sensors on-board the Voyager 1 and Voyager 2 spacecraft

(launched in 1977) used recrystallized photoresistive cadmium sulfide (CdS) detectors

for their sensing element and had a field of view of ± 20° [135, 136, 137]. These sun

sensors were relatively large and had additional packaging for radiation attenuation. Sun

sensor technology has advanced greatly since 1977; for example, the sun sensor on-

board the Curiosity rover (launched 2011) used a Si complementary metal-oxide-

semiconductor (CMOS) active pixel sensor (APS), had a field of view of ± 60°, and had

a mass of only 35 grams [138, 139, 140, 141]. Other two-axis sun sensors have been

reported including APS cameras [139, 142, 143], charge coupled device (CCD) cameras

[144], Si-based photodiodes [145, 146], and solar cells [147]. Additionally, three-axis

sun sensors utilizing a hemisphere [148] and the Doppler shift of the Sun’s photosphere

[132] have been reported.

Table 5.1.1 compares the sun sensors discussed above, from the CdS-based

detectors used on the early space missions through the Si-based APS and CCD detectors

being developed today. All of the sun sensors discussed are responsive to infrared and/or

visible light and most are based on Si technology. A GaN-based sun sensor will respond

to UV light which enhances sun sensing technology by expanding the operational

regime. Since GaN is a temperature tolerant and radiation-hard material platform, a

GaN-based sun sensor will be able to intrinsically withstand the harsh space

environment thus reducing extra protective packaging requirements. Therefore, for UV

photodetection and reliable space operation, a miniature sun sensor utilizing an array of


GaN-based photodetectors could address many technological challenges in one device

and will be discussed in the following sections.

Table 5.1.1. Sun sensor technology comparison.

Year Sensor

Element Field of View Accuracy Notes Reference

1971 Photoresistor ± 2.5° × ± 20° --- Cadmium sulfide

Flown on Mariner spacecraft [149]

1977 Photoresistor ± 20° ---

Cadmium sulfide

Flown on Voyager 1 and 2

[136, 137,

135]

2005 CMOS APS ~ ± 50° × ± 50° 0.1° Used on JAXA's small

satellites [150]

2009 CMOS APS 94° × 81° 1 arcmin

Uses a multi-aperture mask

to increase precision

[143]

2009 Solar Cell ---

0.1° with

12 bit A/D

converter

Low-cost coarse sun sensor

using already on-board solar

cells for Pico satellites

[147]

2011 CMOS APS ± 45° 0.01 pixels

Uses a multi-aperture mask

to increase precision

[151]

2011 CMOS APS ± 60° 1 arcmin Flown on Mars Curiosity

rover

[139, 138,

140, 141]

2012 Photodiode

Array ± 60° 20°

Uses several sensors to

extend the field of view while

maintaining high-precision

[152]

2014

TSL230R

Light-to-

Frequency

Converter

--- 5° Azimuth

1° Zenith

Uses commercial light

intensity sensors in a

hemispherical arrangement

[148]

2016 CCD ± 65° × ± 65° 0.1°

Three-axis sun sensor

Uses a V-shaped mask and

highly integrated control

circuit

[144]

2016

Magneto-

Optical

Filters and

CCD

25° with 200°/s

slew rate 0.1°

Images the Doppler shift of

the sun's photosphere to

measure the solar rotation

axis

[132]


5.1.2 Principal of Operation

A sun sensor composed of a focal plane, spacer, and aperture mask operates by

allowing light to shine through small openings in the opaque aperture mask. As the

sensor moves relative to the sun, the illumination pattern on the focal plane will change.

The position of the sensor relative to the sun can be determined by tracking the variation

in the illumination pattern. The principle of operation of a sun sensor is much like a sun

dial. A cross-sectional schematic of the sun sensor’s operating principle and design

parameters are shown in Fig. 5.1.1. For this design the optical focal plane consists of a

3 × 3 array of GaN-on-sapphire MSM photodetectors (as shown in Fig. 5.1.2) and the

spacer was made of glass (Pyrex, Corning Inc.). It should be noted that the upper left

photodetector in the array will be referred to as PD 1 with numbering continuing left to

right with the bottom right photodetector called PD 9. It should also be noted that due

to the scalability of microfabrication, larger array sizes could be utilized to increase field

of view and resolution. For reliable operation, several parameters had to be taken into

consideration in the design including the length (Lapp) and thickness (t) of the aperture,

length of the photodetector (LPD), spacing between the photodetectors (Lspace), and

height of the spacer (h), all labeled in Fig. 5.1.1. In brief, LPD, Lspace, and h determine

the resolution of the sun sensor (i.e. the closer together and smaller the photodetectors

are, the more precisely the incident light angle will be known), as well as the size of the

sun sensor (i.e., the taller the spacer, the farther apart the photodetectors need to be to

measure the same angle). Therefore a tall spacer with a larger array of closely spaced

photodetectors (achievable with today’s micro- and nano-fabrication techniques) would

give the maximum field of view with highest resolution.

The sun sensor was designed such that when the angle of the incident light (θsun)

is 45°, one of the edge photodetectors at the angle of θPD after refraction would be

illuminated. To accomplish this, Snell’s law of refraction was considered,


𝑛𝑎𝑖𝑟 sin(𝜃𝑠𝑢𝑛) = 𝑛𝑝𝑦𝑟𝑒𝑥 sin(𝜃𝑃𝐷) (5.1)

where the index of refraction of air (nair) is 1.00 and that of Pyrex (npyrex) was taken as

1.4734. The parameters of the final sun sensor are listed in Table 5.1.2. The ratio of the

wavelength of the incoming light to the size of the aperture determines how much a

wave spreads out behind an opening regardless of the shape of the opening. For 365 nm

wavelength light and the aperture size of 350 μm, this ratio calculates to be 0.001 and it

is easily seen that diffraction effects are negligible for this design. Since the spreading

due to diffraction is negligible to a first order approximation, the ray light model can be

employed and the light can be considered as traveling in straight lines [153].

Figure 5.1.1. Cross-sectional schematic of a GaN-based miniature sun sensor depicting the

principle of operation and design parameters using an aperture mask, spacer, and focal plane.

The inset shows the top view of a GaN-on-sapphire MSM photodetector. Reprinted from

[105], with the permission of AIP Publishing.


Figure 5.1.2. Optical image of 3 × 3 GaN-based MSM photodetector array. Reprinted

from [105], with the permission of AIP Publishing.

Table 5.1.2. Miniature sun sensor dimensions.

Parameter Designed Value

Lapp 350 μm

Lspace 300 μm

LPD 250 μm

t 110 nm

h 1.1 mm

5.1.3 Array Characterization

The electrical performance, in terms of dark and illuminated current versus

voltage, of the GaN MSM photodetectors was determined using a semiconductor

parameter analyzer (B1500A, Agilent Technologies) and a 6 W, 365 nm UV lamp

(UVP, EL series UV lamp). A UV light meter was used to measure the incident UV

intensity, which for all illuminated electrical measurements was 1.0 mW/cm2. For this

intensity, the optical power applied on the photodetector is approximately 625 nW.

The magnitude of the dark current and the response to UV illumination without

the aperture mask is about the same for each photodetector in the array with an

approximate array average PDCR (defined in Eq. 1.1) of 2.4 and responsivity (defined

in Eq. 1.2) of 3200 A/W at -1 V bias. PDCR values for GaN-based photodetectors

reported in literature range from less than one up to tens of thousands [98, 154, 155,


156]. The large PDCR values are obtained using high intensity UV sources (e.g., up to

1020 mW/cm2 using He-Cd laser [42]). Since the photo-generated current is highly

dependent on the incident light intensity, it is important to note that an increased PDCR

will be achieved with a higher intensity UV source. Reported responsivities range from

0.1 A/W well into the thousands [52, 51] with the large responsivity values attributed to

large internal gains. Since responsivity decreases with increasing optical power and

PDCR increases, these figure of merit are highly coupled and both need careful

consideration. From this, it can be concluded that the photodetector performance

reported here is on par with previously reported results for GaN-based photodetectors.

The PDCR and responsivity for each photodetector at -1 V bias is listed in Table

5.1.3. To show the nominal response of the devices, these values were calculated from

the raw pre-aperture mask current-voltage data. It is seen that PD 1 has a different

response than the rest of the array with a much lower PDCR and much lower

responsivity. This is mainly because PD 1 was unintentionally scratched by probe tips

during electrical measurements which gave it an altered response. However, this single

anomalous photodetector does not affect the sun sensor performance as a whole, as will

be described below.

Table 5.1.3. Photodetector PDCR and responsivity at -1 V bias before attaching the aperture

mask. PD 1 was unintentionally scratched during electrical measurements.

PDCR Responsivity [A/W]

PD 1 0.17 1785.7

PD 2 2.27 3952.3

PD 3 2.05 3409.4

PD 4 2.38 4102.9

PD 5 2.40 3647.0

PD 6 2.40 3696.9

PD 7 1.90 3600.6

PD 8 2.72 3042.4

PD 9 2.02 2182.0

The performance of each photodetector is slightly different, thus for accuracy

these slight variations in response from device to device need to be considered. To easily


visualize and compare the dark and illuminated current-voltage characteristics for all

nine photodetectors in the array, a scaling was performed so that the dark current versus

voltage curve is the same for each photodetector while maintaining each individual

photodetector’s PDCR. The result of this scaling for data taken before the aperture mask

was attached is shown in Fig. 5.1.3. From the figure it is seen that the nominal response

of all devices in the array is about the same except for PD 1 as discussed above.

Figure 5.1.3. Scaled current-voltage response of all nine photodetectors in the array under

dark and illuminated conditions before attaching the aperture mask. Reprinted from [105],

with the permission of AIP Publishing.

5.1.4 Miniature Sun Sensor Performance

With the photodetector array constituting the focal plane, a Pyrex spacer with

opaque gold aperture mask was attached with space-grade non-outgassing epoxy (EPO-

TEK 301-2). The aperture mask was fabricated using a standard metal lift-off procedure

with 10 nm Ti / 100 nm Au on a Pyrex wafer. As stated in Table 5.1.2, the size of the

aperture opening is 350 μm × 350 μm. For comparison purposes, two 3 × 3 arrays of

photodetectors were fabricated on a single 1 cm × 1 cm GaN-on-sapphire chip, resulting


in two sun sensors per chip. Optical images of the fabricated MSM photodetectors are

shown in Fig. 5.1.2 and the final miniature sun sensor is shown in Fig. 5.1.4.

To determine the “sun” (UV source) angle under simulated conditions (the UV

lamp was rotated and clamped at the specific test angle), a simple algorithm that

compares the PDCR and responsivity for each of the nine photodetectors was developed

as outlined below. In practice, a processing circuit could be used to implement this

simple algorithm.

Figure 5.1.4. Image of assembled miniature sun sensor with size comparison to a penny.

Reprinted from [105], with the permission of AIP Publishing.

1. Test all nine photodetectors in the 3 × 3 array before the aperture mask

is attached (dark and illuminated at θsun = 0°)

Calculate PDCR and responsivity for each device – call these the

“NOMINAL” values

2. Attach aperture mask

3. Test all devices in the array (dark and illuminated at θsun = 0°)

Calculate PDCR and responsivity for each device – call these the

“0° AP MASK” values


4. Normalize the “0° AP MASK” values by the “NOMIAL” values – call

these the “NORMALIZED ARRAYS”

5. Scale the “NORMALIZED ARRAYS” values to be between 0 and 1 –

call these the “SCALED and NORMALIZED ARRAYS”

6. Calculate the “SUN ANGLE ARRAY” by equally weighting the

PCDR and responsivity “SCALED and NORMALIZED ARRAYS”

values

7. Repeat steps 3 to 6 for θsun = 45° with normalizing the “45° AP

MASK” values by the “0° AP MASK” values

The results of this algorithm for two test cases are shown in Fig. 5.1.5 and Fig.

5.1.6. Fig. 5.1.5 shows the PDCR and responsivity “SCALED and NORMALIZED

ARRAYS” for illumination angles of θsun = 0° and 45° for a range of bias voltages.

From Fig. 5.1.5a and Fig. 5.1.5b, it is easily seen that PD 5 was the illuminated

photodetector (at θsun = 0°) and from Fig. 5.1.5c and Fig. 5.1.5d, it is PD 2 that was

illuminated (θsun = 45°). In Fig. 5.1.5c and Fig. 5.1.5d, it is seen that PD 3 has a slightly

higher scaled and normalized PDCR and responsivity than the other unilluminated

photodetectors. This anomaly was attributed to a misalignment during testing allowing

the UV light to leak around the edge of the aperture.

In Fig. 5.1.6, the “SUN ANGLE ARRAY” values at -1 V for the two test cases

shown in Fig. 5.1.5 have been overlaid on an optical image of the photodetector array

to provide visualization of the photodetector illumination pattern. Using the “SUN

ANGLE ARRAY” values calculated from the algorithm described above and shown in

Fig. 5.1.6, the minimum illuminated photodetector to dark photodetector (PDil/PDdark)

ratio is 1.6 with an average ratio of 2.4 for the 0° illumination case. For the 45°

illumination case, the minimum ratio is 1.4 with an average of 2.1. The overall sun

sensor performance is efficient and reliable considering the low 1.0 mW/cm2 intensity

(limitation of the UV lamp utilized). Since photo-generated current increases with

increasing intensity, it is obvious that the more UV light present, will lead to improved


performance (increased sensitivity) of the sun sensor. For comparison, the irradiance of

the sun at the earth’s mean distance from the sun outside the earth’s atmosphere is

137 mW/cm2 of which about 9% is UV [157]. Therefore, a sun sensor just outside the

earth’s atmosphere would experience a UV intensity of ~12 mW/cm2 which is 12 times

greater than that of the UV lamp used in this study. In addition to intensity, the

performance could be improved by further increasing the illuminated photodetector to

dark photodetector ratio by refining the aperture mask design to reduce the light leakage

and reflection around the edges and corners.

Figure 5.1.5. Scaled and normalized PDCR and responsivity comparisons of all nine

photodetectors when miniature sun sensor is illuminated (365 nm) at ((a) and (b)) θsun = 0°

and ((c) and (d)) θsun = 45°. Reprinted from [105], with the permission of AIP Publishing.


Figure 5.1.6. Miniature sun sensor operation at illumination angles (θsun) of (a) 0°

(illuminating PD 5) and (b) 45° (illuminating PD 2). The left-side images indicate the

illumination angle/orientation and the right-side images show the illuminated array based on

the normalized PDCR and responsivity at −1 V. Reprinted from [105], with the permission

of AIP Publishing.

5.2 CubeSat Integration and Ground Testing

5.2.1 QB50 Mission

The QB50 mission consists of a network of 50 satellites built by university teams

all over the world to collect in situ scientific data in Earth’s lower thermosphere. The

main payload on-board the QB50 CubeSat developed at Stanford (known as Discovery)

is an ion-neutral mass spectrometer (INMS). The INMS uses mass spectroscopy to

determine the composition of low mass ionized and neutral particles and has been

optimized for O, O2, NO, and N2, the major constituents in Earth’s lower thermosphere.


The GaN-based miniature sun sensor described in Chapter 5.1 was integrated on-board

CubeSat Discovery as a secondary payload. The integration and ground test results will

be described in the following sections. QB50 CubeSat Discovery is expected to be

launched off the international space station (ISS) in the near future.

5.2.2 Miniature Sun Sensor Integration

The GaN-based miniature sun sensor was epoxied and wirebonded to the

CubeSat’s sun-facing printed circuit board (PCB) near the CubeSat’s attitude

determination and control system (ADCS). In order to protect the very delicate

wirebonds from breaking, they were encapsulated in an epoxy (EPO-TEK 301-2) with

low-outgassing properties, high optical transmission (> 94% for wavelengths

> 320 nm), and a wide operating temperature range (-55°C to 200°C). The 1 cm × 1 cm

GaN-chip has two 3 × 3 arrays of photodetectors, one array of graphene photodetectors

and one array of semitransparent Ni/Au photodetectors, resulting in two miniature sun

sensors per chip. A computer-aided design (CAD) model of the CubeSat and miniature

sun sensor as well as a photo of the completed CubeSat are shown in Fig. 5.2.1.

To enable independent measurement of all 18 photodetectors, a circuit was

designed which powers only one column of photodetectors at once and measures the

voltage across one row. By switching which column is powered and which row is

measured, all 18 photodetectors can be read in quick succession and with post-

processing of the data, the angle of the incident light (i.e. the sun) can be determined.

The power supplied can be chosen ranging from 0 V to 5 V. The raw data returned by

the circuit is in bits which can be converted to volts by dividing by 4096 (the circuit

uses a 12 bit analog-to-digital converter) and multiplying by the bias voltage. This

voltage can be converted to current by dividing by the 40x amplifier gain. The full

circuit diagram for the miniature sun sensor integrated with the QB50 CubeSat is shown

in Appendix C. The CubeSat measures all 18 photodetectors of the miniature sun sensor


twice per orbit and stores it in a data packet that includes a timestamp, GPS location,

ADCS sun sensor data, and temperature measured by a thermocouple on the CubeSat’s

sun-facing PBC.

Figure 5.2.1. QB50 CubeSat Discovery CAD model with GaN-based miniature sun sensor

and finished CubeSat.

5.2.3 Ground Test Results

The GaN-based photodetector arrays were characterized before integrating them

with the CubeSat. The scaled current-voltage responses of the (a) graphene and (b)

semitransparent Ni/Au photodetector arrays under dark and illuminated (365 nm)

conditions before attaching the aperture mask are shown in Fig. 5.2.2. Three of the

graphene photodetectors (PD 4, PD 7, and PD 9) were not operational due to poor


graphene transfer. The corresponding PDCR and responsivity at 5 V bias are listed in

Table 5.2.1.

Figure 5.2.2. Dark and illuminated (365 nm) scaled current-voltage response of (a)

graphene and (b) semitransparent Ni/Au photodetector arrays before attaching the aperture

mask and before QB50 implementation. Three of the graphene photodetectors (PD 4, PD 7

and PD 9) were not operational due to poor graphene transfer.

Table 5.2.1. Photodetector PDCR and responsivity at 5 V bias before attaching the aperture

mask to the QB50 miniature sun sensor. Three of the graphene photodetectors (PD 4, PD 7

and PD 9) were not operational due to poor graphene transfer.

Graphene Semitransparent Ni/Au

PDCR Responsivity [A/W] PDCR Responsivity [A/W]

PD 1 0.15 3317.3 0.13 2302.5

PD 2 0.21 3605.3 0.15 2100.6

PD 3 0.12 2018.8 0.05 846.5

PD 4 X X 0.13 2579.2

PD 5 0.06 1099.7 0.06 911.3

PD 6 0.10 2195.2 0.09 1719.0

PD 7 X X 0.12 2352.0

PD 8 0.08 1569.5 0.09 1900.8

PD 9 X X 0.25 3465.7

The set-up for testing and debugging the miniature sun sensor circuit with the

QB50 CubeSat Discovery hardware is shown in Fig. 5.2.3. The miniature sun sensor

was tested at 5 V bias with the 365 nm UV lamp positioned at 0°. The simple algorithm

developed in Chapter 5.1 was used to determine the “SUN ANGLE ARRAY” with the

quantitative results listed in Table 5.2.2 and photodetector array visualization shown in


Fig. 5.2.4. The minimum PDil/PDdark ratio is 1.1 for the graphene photodetector array

and 1.2 for the semitransparent Ni/Au photodetector array. The average PDil/PDdark ratio

is 3.2 and 3.7 for the graphene and semitransparent Ni/Au photodetector arrays

respectively. Slight misalignment of the UV lamp could be contributing to the variation

of PDil/PDdark across the array. The performance of the miniature sun sensor is quite

good considering the low 1.0 mW/cm2 intensity of the UV lamp and is expected to be

much better with the higher intensity of the sun outside the Earth’s atmosphere.

Figure 5.2.3. Ground test set-up for integration testing of GaN sun sensor with QB50

Discovery.

Table 5.2.2. “SUN ANGLE ARRAY” of the miniature sun sensor integrated with the QB50

CubeSat with illumination angle of 0° and 5 V bias.

Graphene Semitransparent Ni/Au

PD 1 0.17 0.21

PD 2 0.18 0.29

PD 3 0.42 0.82

PD 4 X 0.42

PD 5 1 1

PD 6 0.88 0.49

PD 7 X 0.35

PD 8 0.87 0.40

PD 9 X 0.10


Figure 5.2.4. Ground test performance of miniature sun sensor integrated with QB50 at 0°

illumination angle and 5 V bias; (a) graphene photodetector array and (b) semitransparent

Ni/Au photodetector array. Three of the graphene photodetectors (PD 4, PD 7, and PD 9)

were not operational due to poor graphene transfer.

5.3 In Situ Shock-Layer Radiation Measurements

5.3.1 Current Atmospheric Entry Sensing Technology

Planetary exploration is constrained by the capabilities of atmospheric entry

technologies, including thermal protection systems (TPS). Spacecraft experience high

heat fluxes during atmospheric entry and thus require a TPS to protect the payload [4].

In-flight temperature and pressure measurements on the heatshields of the Flight

Investigation Reentry Environment II (FIRE II) vehicle, the Planetary Atmosphere

Experiments Test (PAET) vehicle, Apollo, Space Shuttle, and Mars Science Laboratory

(MSL), among other spacecraft, have enabled aerothermal model refinement and

improved TPS design [158, 159]. However, advancement of atmospheric entry sensing

technologies have been limited due to the high heat fluxes and complexity of

instrumenting the TPS. Current state-of-the-art seen on the Mars Entry Descent and

Landing Instrumentation (MEDLI) sensor suite on the MSL heatshield consists of

thermocouples embedded in a plug of TPS material (Phenolic-Impregnated Carbon


Ablator, PICA) and pressure transducers connected to pressure ports in the TPS [160,

161, 162].

Prediction of the entry heat load and subsequent sizing of the TPS is often

focused on convective heating. However, for large vehicles and high entry velocities,

shock-layer radiation can be a major contributor [163, 40, 164, 165]. As of now, few in-

flight shock-layer radiation measurements have been made. A timeline of spacecraft

that have or are planned to measure in-flight shock-layer radiation during atmospheric

entry is shown in Fig. 5.3.1 and listed in Table 5.3.1. To measure shock-layer radiative

heating a radiometer, consisting of a sensing element (often a thermopile) behind a

window, is embedded in the TPS. In the late 1960’s to early 1970’s, FIRE II, Apollo

4/6, and PAET, used on-board radiometers to measure radiation during Earth reentry

[166, 167, 158, 168]. Due to the window and sensor materials used by the radiometers

on-board Apollo and PAET, measurements were limited to visible to near infrared

radiation, neglecting UV radiation, which can account for half of the radiative heating

[166, 41]. Furthermore, shock-layer radiative heating in the VUV regime (< 200 nm),

which has never been measured in-flight, may contribute up to seven times the non-

VUV radiative flux on the backshell of Earth reentry vehicles [41]. More recently, the

sapphire window on the radiometer on-board the Orion Exploration Flight Test-1 (EFT-

1) vehicle enabled UV measurements down to 220 nm [41]. The first measurement of

shock-layer radiative heating during Mars entry was made by a broadband radiometer

on the heatshield of the European Space Agency’s ExoMars Schiaparelli lander with

data analysis forthcoming [169]. The next in-flight measurement of radiative heating is

slated to be NASA’s Mars Entry Descent and Landing Instrumentation II (MEDLI II)

sensor suite on the Mars 2020 heat shield [170].

Clearly there are gaps in our knowledge of shock-layer radiative heating seen in

the discrepancies between the in-flight data, ground test experiments, and simulations.

There are technological hurdles to overcome in order to advance atmospheric entry

sensing technology including developing high-temperature capable sensors, refining


sensor packaging for embedding in TPS so as to not degrade the integrity of the TPS,

and developing radiometers capable of UV radiation measurements.

Figure 5.3.1. Timeline of spacecraft that have or are planned to measure in-flight shock-

layer radiation during atmospheric entry.

Table 5.3.1. Spacecraft to date that have or are planned to measure in-flight shock-layer

radiation during atmospheric entry.

Vehicle Year Planet Sensing Element Window Material Location

FIRE II 1965 Earth --- Fused Quartz Fore and Aftbody

Apollo 4/6 1967/8 Earth Thermopile Quartz Fore and Aftbody

PAET 1971 Earth PIN Photodiode Fused Silica Forebody

EFT-1 2014 Earth Thermopile Sapphire Rod Forebody

ExoMars 2016 Mars --- --- Aftbody

MEDLI 2 2020 Mars Heat Flux Sensor Sapphire Aftbody

In this study, the feasibility of implementing GaN-based photodetectors to detect

shock-layer UV radiative heating through in situ measurements in a shock tube is

explored. GaN-based photodetectors offer many benefits over traditional solutions

including thermal and chemical stability enabling GaN-based devices to be placed

closer to the surface of the TPS allowing for more accurate measurements to be obtained

as well as the ability to measure the entire UV spectrum, as discussed in Chapter 1.

GaN-on-sapphire metal-semiconductor-metal (MSM) photodetectors were installed in


the Electric Arc Shock Tube (EAST) facility at NASA Ames Research Center and

characterized in a flight-relevant Titan entry environment (98% N2, 2% CH4). Titan,

Saturn’s largest moon, is the subject of many future exploration studies due to the

Cassini-Huygens mission revealing Titan’s Earth-like meteorology and geology as well

as its subsurface ocean [171]. Accurate characterization of the UV shock-layer radiation

is especially important for a Titan aeroshell as the peak radiative heat flux is predicted

to be four times greater than the peak convective heat flux [40, 164, 165]. Titan’s

atmosphere consists primarily of nitrogen with small amounts methane and argon. The

shock wave formed by the entering spacecraft dissociates the methane which then reacts

with the atmospheric nitrogen forming CN molecules. These CN radicals are strong

radiators especially in the UV and are responsible for the majority of the radiative

heating [40, 164, 165].

5.3.1 Shock Tube Test Set-up

The EAST facility at NASA Ames Research Center is a ground-based facility

that produces flight relevant radiation. The facility uses four separate spectrometers to

observe and characterize radiation from the shock wave from VUV (120 nm) to near

infrared (1700 nm) [163, 40, 172]. A schematic of the EAST facility is shown in Fig.

5.3.2 as well as the test set-up for characterizing the performance of the GaN-based

photodetectors in the shock tube. The 5 mm × 5 mm GaN-on-sapphire photodetector

chip was epoxied (Devcon, 5 Minute) and wirebonded to a transistor outline (TO)

header (Spectrum Semiconductor Materials, Inc., TO-8). To protect the wirebonds

during shock testing they were encapsulated in epoxy while leaving the photodetectors

exposed to the shock wave. The packaged photodetectors were soldered to wire leads in

the vacuum-tight shock port holder (see drawings in Appendix C) and installed in the

test section of the shock tube just downstream of the spectrometers used by the facility.

Two GaN-on-sapphire MSM photodetectors (referred to as photodetector 1 and


photodetector 2) were used to measure the UV radiance during two separate shock tests.

A source meter (2400 Series, Keithley) was used to take current-voltage measurements

pre- and post-shock as well as apply a voltage and measure the transient current response

during the shock test. Due to limitations of the equipment, the sampling rate during the

shock test was only about 8 Hz. Since the Titan entry conditions and test parameters

dictated that the shock speeds should be around 5-6 km/s, a separate high frequency

(5 MHz) data acquisition system (DAQ) was also used to acquire data during the shock

test. The DAQ was triggered to record 4 ms of data when the burst disk ruptured

whereas the source meter collected data for several minutes before and after the shock

event. The high frequency measurements were taken as voltage measurements (Vmeas)

across a resistor from which the photodetector current was calculated. For all tests, the

applied voltage (Vapp) was 200 mV.

Figure 5.3.2. Test set-up for in situ UV shock radiance measurements in NASA Ames’

electric arc shock tube. Reprinted from [118], with the permission of AIP Publishing.


5.3.3 Shock Tube Test Results

Table 5.3.2 lists the parameters for the two shock tests conducted, with

photodetector 1 active during the first shock test and photodetector 2 active during the

second. The first shock test melted the front edge of the TO header and broke the

wirebonds to photodetector 1, rendering the low frequency transient current

measurements useless as shown in Fig. 5.3.3a. The transient current response of

photodetector 2 during the second test, measured with the source meter (8 Hz sampling

frequency), is shown in Fig. 5.3.3b. The shock event is clearly visible in this data and

the slow current decay after the shock has past is consistent with the persistent

photoconductivity phenomena described in Chapter 1. The unilluminated (dark)

current-voltage response for photodetector 2 is unchanged from pre- to post-shock as

shown in the inset of Fig. 5.3.3b. A change in the dark current would indicate material

damage (displacements, dislocations, etc.) and/or electrical degradation (Schottky

barrier height). Since the unilluminated current-voltage response is unchanged, this

indicates that there is little degradation in the device performance after being subjected

to the shock wave.

Table 5.3.2. Shock test parameters.

Test 1 Test 2

Active Photodetector Photodetector 1 Photodetector 2

Test Gas 98% N2, 2% CH4 98% N2, 2% CH4

Driver Gas 93.2% He, 6.8% Ar 93.2% He, 6.8% Ar

Test Section Pressure (torr) 0.1 0.1

Shock Speed (km/s) 5.92 5.32

Equilibrium Temperature of Shocked Gas (K) 5589.53 5324.28

The 5 MHz sampling frequency transient current response during the shock test

is shown in Fig. 5.3.4a for photodetector 1 and Fig. 5.3.4b for photodetector 2. A moving

average filter with a window size of 100 samples was applied to the collected data to

reduce the noise. The filtered signal displays a distinct increase in current after the shock

passes the detector giving a direct measure of the UV shock-layer radiation. In


Fig. 5.3.4, the time when the shock and contact surface pass the detector is easily

identifiable due the large spikes in current. These spikes can be attributed to the exposed

contacts collecting free electrons in the partially ionized gases or forming a ground loop

between the measurement circuit and the facility.

Figure 5.3.3. Transient current response (8 Hz sampling frequency) for (a) photodetector

1 during shock test 1 and (b) photodetector 2 during shock test 2. Unilluminated pre- and

post-shock current-voltage curves (device functionality tests) are compared in the insets.

Figure 5.3.4. Transient current response (5 MHz sampling frequency) during shock test

for (a) photodetector 1 and (b) photodetector 2. The filtered signal displays a distinct increase

in current after the shock passes the detector giving a direct measure of the UV shock-layer

radiation. Reprinted from [118], with the permission of AIP Publishing.

For GaN-based photodetectors, the photo-generated current is proportional to

the square root of the applied optical power [61]. Applying this relation to the


measurements from the spectral response characterization at 365 nm (termed “Spectral

Response Data” in Fig. 5.3.5) results in the fits (termed “P1/2 Fit” in Fig. 5.3.5) shown

in Fig. 5.3.5. The photo-generated current from the shock tests can be calculated from

Fig. 5.3.4 as the average current before the shock subtracted from the average of the

maximum 100 filtered samples between the shock and contact surface. Overlaying this

photo-generated current from the shock tests (termed “Shock Test 1” and “Shock Test

2” in Fig. 5.3.5) on top of the square root of power fits in Fig. 5.3.5 results in a measured

optical power of about 0.30 μW for both shock tests. From the EAST facility

spectrometers, the average UV (190 nm to 370 nm) intensity during the two shock tests

were measured to be about 49.5 μJ/cm2 and 45.0 μJ/cm2. Multiplying these values by

the active area of the respective GaN-based photodetector and the characteristic time

gives 0.31 μW and 0.28 μW which are in excellent agreement with the 0.30 μW result

from the square root of power fit to the data from GaN-based photodetectors.

Figure 5.3.5. Photo-generated current (Iphoto - Idark) for the GaN-based photodetectors as a

function of applied optical power calculated as a square root of power fit through the spectral

response characterization data at 365 nm. By overlaying the photo-generated current

calculated from the shock test data for (a) photodetector 1/shock test 1 and (b) photodetector

2/shock test 2, the measured optical power is found to be about 0.30 μW for both shock tests.

Reprinted from [118], with the permission of AIP Publishing.

Using NASA’s chemical equilibrium with applications (CEA) software [173],

the temperatures behind the shocks were calculated to be, 5589.53 K and 5324.28 K for


tests 1 and 2, as listed in Table 5.3.2. These very high temperatures persist for less than

1 ms and thus the photodetectors are not heated substantially. Therefore it is reasonable

to neglect temperature effects in the proceeding analysis. However, the descent through

a planetary bodies’ atmosphere could be on the order of several minutes. To protect the

photodetectors from these very high temperatures over this relatively long duration, they

could be implemented behind a window or on the backside of the aeroshell, similar to

the implementation of previous heat shield sensors. However, due to the temperature

tolerance of GaN, these sensors can be placed closer to the surface of the TPS than

previously possible thus increasing the understanding of the UV radiative heat load.

Figure 5.3.6a shows the GaN photodetector chip epoxied to the TO header and

mounted in the shock tube port holder before shock testing. The damaged TO header

after the first shock test is shown in Fig. 5.3.6b. The upstream side of the TO header

experienced significant melting (which broke the wirebonds to photodetector 1) while

the downstream side was relatively unaffected. Since the packaged GaN chip does not

have the same curvature as the shock tube (i.e. it is flat), it sits just inside the diameter

of the shock tube and thus it is likely that an oblique shock formed over the leading edge

of the TO header. Thus, the damage to the TO header is attributed to the formation of

the oblique shock. The TO header incurred slightly more damage after the second shock

test, as shown in Fig. 5.3.6c. Since the GaN-based photodetectors were able to measure

the shock-layer radiation and showed no measurable signs of electrical degradation

post-shock testing, packaging appears to be the limiting factor and requires further

development.


Figure 5.3.6. GaN photodetector chip epoxied to header mounted in shock tube port holder

(a) before shock testing, (b) post shock test 1, and (c) post shock test 2.

5.4 Conclusions

GaN-based UV photodetectors were implemented and tested in space-relevant

applications including a miniature sun sensor for spacecraft orientation. A GaN-based

miniature sun sensor capable of operation in the harsh environment of space was

developed using scalable microfabrication techniques, which can be leveraged to create

larger device arrays for high-resolution sun angle detection. Photodetectors with an

average PDCR of 2.4 and responsivity of 3200 A/W at -1 V bias were microfabricated

and integrated with an opaque aperture mask to create the UV-sensitive sun sensor. The

successful operation of this instrument at 0° and 45° incident light angles with an

average illuminated photodetector to dark photodetector selectivity of 2.1 was

demonstrated using a simple sun angle determination algorithm. This algorithm was

able to correctly identify the illuminated photodetector for ten test cases with results

similar to those shown here. The repeatability of the measurements and results show the

feasibility of a space-environment capable UV sun sensor using an array of GaN-based

photodetectors. The GaN-based miniature sun sensor was integrated with the QB50

CubeSat Discovery which will be launched in the near future and will enable in-flight

evaluation of the GaN-based photodetectors.


In addition to sun sensing applications, GaN-based UV photodetectors were also

tested in a shock tube for atmospheric entry UV radiation sensing. Atmospheric entry

conditions for many planetary bodies have been predicted to have high heat fluxes

dominated by shock-layer radiation rather than thermal convection. In order to design

thermal protection systems to enable exploration missions to these places, this heat load

needs to be well understood. Since GaN-based photodetectors are highly responsive to

UV radiation and are temperature tolerant, it is worth exploring their ability to detect

shock-layer radiation in the UV regime. GaN-on-sapphire UV photodetectors were

successfully used to detect UV radiation behind a shock in a simulated Titan entry

environment in an electric arc shock tube. The calculated optical power from the

transient current response measurements made by the GaN photodetectors was

0.30 μW, which is in agreement with the UV radiance measured by EAST facility

spectrometers. Furthermore, the electrical response of the photodetectors was not

measurably degraded by the shock as well as there was no visible damage to the

photodetectors post-shock. These results demonstrate the feasibility of small-scale

GaN-based sensors operating in the harsh atmospheric entry environment.

97

Chapter 6

Conclusions and Future Work

6.1 Conclusions

Wide bandgap semiconductors, such as GaN, are intriguing material platforms

for developing space-based sensors and electronics. Compared to Si, GaN has a much

higher melting point and atomic displacement energy which allows for operation in high

temperature and radiation-rich environments. GaN has a wide and direct bandgap of

3.4 eV which enables ultraviolet photodetection. Photon absorption (and thus

sensitivity) can be increased in GaN-based photodetectors by using transparent

electrodes such as graphene. Graphene has a high atomic displacement energy and has

been shown to create a thermally stable Schottky contact to GaN. The properties of both

GaN and graphene are well suited for harsh environment operation and the

graphene/GaN material platform, specifically, GaN UV photodetectors with graphene

electrodes, should be characterized as a space-exploration enabling technology.

However, there are still many challenges to overcome before GaN-based sensors are

CHAPTER 6. CONCLUSIONS AND FUTURE WORK 98

ready for space-flight, including but not limited to, addressing material quality issues,

advancement of microfabrication processing, and development of extreme environment

packaging schemes.

Chapter 4 of this thesis delved into the characterization of the graphene/GaN and

semitransparent Ni/Au/GaN photodetectors when operating in extreme environments

including high temperatures and under proton irradiation exposure. The graphene/GaN

photodetectors exhibited a higher PDCR compared to the semitransparent Ni/Au/GaN

photodetectors over all temperatures tested (-100°C to 200°C) which can be attributed

to graphene’s very high transmittance (> 80%) in the UV regime compared to

semitransparent Ni/Au (> 30%) as detailed in Chapter 3. For both types of

photodetectors, PDCR was seen to decrease with increasing temperature eventually

reaching zero above 200°C. The decreasing PDCR with increasing temperatures is due

to the thermal generation of charge carriers overwhelming the photo-generation at

elevated temperatures. The responsivity of both types of photodetectors was seen to

decrease at high temperatures due to increased lattice scattering as well as a reduced

bandgap. From transient current response data, it was shown that both graphene/GaN

and semitransparent Ni/Au/GaN photodetectors demonstrated faster rise and decay

times at elevated temperatures. The faster rise times at high temperatures were attributed

to a higher recapture rate of photo-excited carriers while the faster decay times were

attributed to thermal energy releasing trapped photo-generated carriers. Additionally,

these temperature-dependent trends seen in the experimental results match the

predictions of the semi-analytical model based on thermionic emissions theory

developed in Chapter 2. The graphene/GaN and semitransparent Ni/Au/GaN

photodetectors were also characterized when subjected to 2 MeV proton irradiation up

to a fluence of 3.8 × 1013 cm-2. Electrical measurements made throughout irradiation

testing showed no significant degradation in device performance in terms of PDCR and

responsivity. Additionally, Raman spectroscopy of 200 keV irradiated graphene showed

only slight increase of disorder in the form of an increased ID/IG ratio, the appearance of

the D'-peak, and a decrease in the 2D peak intensity. These results support the use of


graphene and GaN as temperature tolerant and radiation-hard material platforms for UV

photodetection with high sensitivity and responsivity.

Chapter 5 detailed the implementation and testing of GaN-based photodetectors

in practical aerospace applications. An ultraviolet miniature sun sensor was created

using an array of GaN-based photodetectors coupled with an opaque aperture mask. The

miniature sun sensor demonstrated successful operation under 0° and 45° UV

illumination with an average illuminated photodetector to dark photodetector selectivity

of 2.1. The miniature sun sensor has been integrated with a CubeSat enabling future in-

space evaluation of the graphene/GaN material platform. The GaN-based

photodetectors were also implemented in an electric arc shock tube and in situ

measurements of UV shock-layer radiation in a Titan simulant environment were made.

The calculated optical power from the transient current response measurements made

by the GaN photodetectors was 0.30 μW, which is in agreement with the UV radiance

measured by EAST facility spectrometers. Furthermore, the electrical response of the

photodetectors was not measurably degraded by the shock as well as there was no visible

damage to the photodetectors post-shock. These results demonstrate the feasibility of

small-scale GaN-based sensors for future use in space sensing applications, specifically,

for spacecraft orientation and detection of radiative heating during atmospheric entry.

In summary, this thesis described the design and microfabrication of GaN UV

photodetectors with transparent graphene electrodes and the experimental test results

supporting the development of these devices for space-based sensing applications. The

graphene/GaN material platform overcomes the limitations of current Si-based sensor

technology by extending the operational temperature range and radiation exposure

limits. Sensors and electronics using the graphene/GaN material platform will require

less protective packaging which will increase the size of scientific payloads that can be

flown.


6.2 Future Work

Future work should be conducted to further understand the operation of and

improve the performance of GaN-based ultraviolet photodetectors operating in harsh

environments. In terms of photodetector device design, it is important to consider ways

to reduce the dark (leakage) current without sacrificing performance. A photodetector

architecture that incorporates an insulator such as a metal-insulator-semiconductor

(MIS) photodetector is an intriguing option since the insulator layer will increase the

Schottky barrier height thus reducing the dark current [62]. However, for space-based

applications, the insulator material needs to be chosen with care and needs to withstand

high levels of radiation and extreme temperatures. For example SiO2 thin films have

been shown to trap charges after exposure to gamma radiation resulting in large leakage

currents [64, 174]. Thin films made of HfO2, Al2O3, and h-BN have displayed

robustness to gamma radiation and may be good options [174], however difficulty in

wet etching HfO2 may prove to be limiting in terms of fabrication [175, 176]. In addition

to adding an insulating layer, further research needs to be conducted to develop reliable

high-temperature capable Schottky contacts to GaN [177, 178, 179, 180]. A

metallization stack that can operate at high temperatures while maintaining a high

barrier height would be highly desirable and would allow GaN-based photodetectors to

operate up to 600°C or above.

Since AlGaN/GaN transistors have demonstrated operation at 600°C in air [23],

photodetectors utilizing the AlGaN/GaN material platform may further increase the

temperature limit of GaN-based photodetectors. AlGaN/GaN photodetectors have

shown fast room temperature response times [181, 182], high UV sensitivity [181, 183,

182], and the ability to tune the absorption edge through the Al doping concentration

[17]. Additionally, self-heating of suspended AlGaN/GaN photodetectors has shown to

be an effective way to reduce the highly undesirable phenomenon of persistent

photoconductivity [127]. Future work should be conducted to realize the potential of

AlGaN/GaN photodetectors for harsh environment sensing. Further discussion and


preliminary experimental results for AlGaN/GaN MSM photodetectors are provided in

Appendix B.

Further testing is needed to fully understand the response of GaN-based

photodetectors operating in extreme environments (i.e. high temperature, low

temperature, radiation-rich, shock, corrosive, etc.). For example, it is important to

understand what happens to the microstructures of GaN, graphene, etc. when exposed

to extreme temperatures for long time periods and how that affects photodetector

sensitivity. Additionally, while operating in space, sensors and electronics can

experience extreme thermal cycling which may cause premature device degradation.

Also worth exploring is how long term exposure to UV light changes device

performance. It was noted above that the MIS photodetector architecture results in lower

leakage current than MSM photodetectors, however extended exposure to UV light

tends to damage oxide layers [17, 184]. Consequently, the MIS structure may help

reduce leakage current and with correct selection of oxide, can withstand particle

radiation, but further work needs to be conducted with regards to material degradation

in the presence of high intensity UV light. In terms of proton radiation exposure,

irradiating to higher fluences would be very informative to determine when

photodetector sensitivity degrades. Also, it is important to understand how GaN sensors

and electronics respond to electrons, neutrons, and heavy ions for successful operation

in space.

Future work to further the implementation work discussed in Chapter 5 includes

analysis of the data sent back from the miniature sun sensor integrated with the QB50

CubeSat. This data analysis will not only demonstrate the performance of the miniature

sun sensor when subjected to the sun’s high intensity UV light outside the Earth’s

atmosphere, but will also inform on how well the materials (GaN, graphene, etc.)

themselves withstand the space environment. To further improve the performance of the

miniature sun sensor, the GaN-based array should be scaled to include more closely

spaced photodetectors (achievable with today’s micro- and nano-fabrication techniques)

and a taller spacer should be implemented to achieve the maximum field of view with


the highest resolution. Another suggestion to improve the miniature sun sensor is to

consider alternate packaging methods. Wirebonding the GaN photodetector chip to a

PCB and encapsulating the wirebonds with epoxy worked well for proof of concept and

preliminary prototyping. However, as the number of photodetectors in the array is

increased the number of wirebonds will become overwhelming. Thus this is not a robust

solution. In lieu of GaN integrated circuit technology, which is currently being

researched and may be realized on a large scale in the near future [185, 186], it is

suggested to implement flip chip bonding [187, 188] and eliminate the excessive and

fragile wirebonds. In this packaging scheme, the sapphire substrate of the GaN-on-

sapphire wafer is the spacer in the miniature sun sensor and the photodetector array is

considered to be back-illuminated [99, 61].

The last suggestion for future work is to create a gratingless UV spectrometer

using an array GaN-based photodetectors. As discussed in Chapter 1, the absorption

edge of GaN-based photodetectors can be tuned through doping. Thus by selectively

doping the GaN film, an array of UV photodetectors that are sensitive to different

wavelengths can be created. One of the many applications for a gratingless UV

spectrometer is in detecting the UV spectra of shock-layer radiation during atmospheric

entry as discussed in Chapter 5.

103

Appendix A

MSM Photodetector Analytical Model Equations

To simulate the temperature-dependent dark current-voltage response of GaN-

based MSM photodetectors, the current transport mechanisms of thermionic emissions,

thermionic field emissions, and field emissions were considered in Chapter 2. The

thermionic, thermionic field, and field emission equations implemented in the analytical

model developed in Chapter 2 are detailed here.

When a voltage is applied to an MSM photodetector, one of the electrodes will

be reverse biased and the other forward biased. For this discussion, electrode 1 is reverse

biased and electrode 2 is forward biased, as shown in Fig. A.1.1. Therefore, the total

dark current for an MSM photodetector is the electron current from electrode 1 added

to the hole current from electrode 2. The three bias voltage operational regions for MSM

photodetectors: (1) voltages less than reach-through (voltage at which the sum of the

depletion widths equals the electrode spacing), (2) voltages greater than reach-through

but less than flat band (voltage at which the electric field at the forward biased electrode

APPENDIX A. MSM PHOTODETECTOR ANALYTICAL MODEL EQUATIONS 104

is zero), and (3) voltages greater than flat band but less than breakdown. The reach-

through voltage can be express as

𝑉𝑅𝑇 ≅𝑞𝑁𝑉𝐷𝐿2

2𝜀𝑠− √4𝑉𝐷

2 (A.1)

where q is elementary charge (1.602 × 10-19 C), N, is the net dopant concentration, VD

is the built-in potential, L is the electrode spacing, and εs is permittivity of the

semiconductor (relative permittivity multiplied by 8.854 × 10-12 F/m). It is easy to see

that with high net dopant concentration, the reach-through voltage will be large. For the

GaN-based MSM photodetectors fabricated (see Chapter 3) and characterized (see

Chapters 4 and 5) in this work, VRT > 1000 V, therefore only operational region (1),

voltages less than reach-through, will be considered.

Figure A.1.1. MSM photodetector band diagram.


For thermionic emission theory, there are three main assumptions: (1) the barrier

height is much larger than kT/q, where k is Boltzmann’s constant (1.381 × 10-23 J/K)

and T is absolute temperature, (2) the device is in thermal equilibrium, and (3) two

current fluxes can be superimposed (one from metal to semiconductor and the other

from semiconductor to metal) [44]. Additionally, a symmetric MSM photodetector will

be assumed thus the current-voltage response will be symmetric and only the response

to positive bias voltages will be simulated. For symmetric MSM photodetectors

operating at voltages less than the reach-through voltage, the current density according

to thermionic emissions is [84]

𝐽𝑇𝐸 = 𝐽𝑛𝑠𝑒𝛽∆𝜑𝑛1(1 − 𝑒−𝛽𝑉1)

+ [𝑞𝐷𝑝𝑝𝑛0 tanh[(𝑥1 − 𝑥2)/𝐿𝑝]

𝐿𝑝(1 − 𝑒−𝛽𝑉1)

+𝐽𝑝𝑠𝑒

−𝛽𝑉𝐷

cosh[(𝑥2 − 𝑥1)/𝐿𝑝](𝑒𝛽𝑉2 − 1)]

(A.2)

where 𝛽 = 𝑞/𝑘𝑇, V1 and V2 are the applied voltage at electrodes 1 and 2, x1 and x2 are

the locations of the depletion widths for electrodes 1 and 2, Dp is the hole diffusion

constant, Pn0 is the equilibrium hole density, and Lp is the diffusion length. The term

∆𝜑𝑛1 (= √𝑞𝐸𝑚1

4𝜋𝜀𝑠) is known as the image-force lowering or Schottky barrier lowering

and accounts for the reduction in the Schottky barrier height in the presence of an

electric field. The electron and hole saturation current densities are

𝐽𝑛𝑠 ≡ 𝐴𝑛∗ 𝑇2𝑒−𝑞𝜑𝑛1/𝑘𝑇

𝐽𝑝𝑠 ≡ 𝐴𝑝∗ 𝑇2𝑒−𝑞𝜑𝑝2/𝑘𝑇

(A.3)


where φn1 is the electron barrier height at electrode 1, and φp2 is the hole barrier height

at electrode 2. The effective Richardson constants for electrons and holes are [44]

𝐴𝑛∗ =

4𝜋𝑞𝑚𝑛𝑘2

ℎ3

𝐴𝑝∗ =

4𝜋𝑞𝑚𝑝𝑘2

ℎ3

(A.4)

where, mn is the effective mass of electrons, mp is the effective mass of holes, and h is

Planck’s constant (6.626 × 10-34 Js).

For symmetric MSM photodetectors, the current density according to thermionic

field emissions theory is [44]

𝐽𝑇𝐹𝐸 =𝐴𝑛

∗ 𝑇

𝑘√𝜋𝐸00𝑞 [𝑉1 +

𝜑𝑛1

𝑐𝑜𝑠ℎ2(𝐸00/𝑘𝑇)] 𝑒𝑥𝑝 (

−𝑞𝜑𝑛1

𝐸0) (𝑒𝑥𝑝 (

𝑞𝑉1

𝜀′) − 1)

+𝐴𝑝

∗ 𝑇√𝜋𝐸00𝑞(𝜑𝑝2 − 𝜑𝑛 − 𝑉2)

𝑘𝑐𝑜𝑠ℎ(𝐸00/𝑘𝑇)𝑒𝑥𝑝 (

−𝑞𝜑𝑛

𝑘𝑇

−𝑞(𝜑𝑝2 − 𝜑𝑛)

𝐸0) (𝑒𝑥𝑝 (

𝑞𝑉2

𝐸0) − 1)

(A.5)

where φn (= (𝐸𝑐 − 𝐸𝐹)/𝑞) is the fermi potential from conduction band edge in n-type

semiconductors. E0, E00, and ε’ are defined as follows where m* is mn or mp as

appropriate:

𝐸00 ≡𝑞ħ

2√

𝑁

𝑚∗𝜀𝑠, (A.6)


𝐸0 ≡ 𝐸00 coth (𝐸00

𝑘𝑇) , (A.7)

𝜀′ =𝐸00

(𝐸00/𝑘𝑇) − tanh(𝐸00/𝑘𝑇). (A.8)

For symmetric MSM photodetectors, the current density according to field

emissions theory is [44]

𝐽𝐹𝐸 = 𝐴𝑛∗ (

𝐸00

𝑘)

2

(𝜑𝑛1 + 𝑉1

𝜑𝑛1) exp(−

2𝑞𝜑𝑛13/2

3𝐸00√𝜑𝑛1 + 𝑉1

)

+𝐴𝑝

∗ 𝑇𝜋 exp(−𝑞(𝜑𝑝2 − 𝑉2)/𝐸00)

𝑐1𝑘 𝑠𝑖𝑛(𝜋𝑐1𝑘𝑇)

(A.9)

where

𝑐1 ≡1

2𝐸00log [

4(𝜑𝑝2 − 𝑉2)

−𝜑𝑛]. (A.10)

The total dark current can be expressed as the sum of the current densities

multiplied by the photodetector active area (A) that is

𝐼𝑑𝑎𝑟𝑘 = (𝐽𝑇𝐸 + 𝐽𝑇𝐹𝐸 + 𝐽𝐹𝐸)𝐴. (A.11)

108

Appendix B

AlGaN/GaN-Based Photodetectors

B.1 Principal of Operation

When AlGaN is grown on top of GaN, a highly conductive sheet of electrons is

formed known as the two-dimensional electron gas (2DEG). The 2DEG forms due to

spontaneous and piezoelectric polarization of the AlGaN and GaN films [189, 190].

Since GaN is a piezoelectric material (applied stress generates a voltage and vice versa),

the piezoelectric polarization is induced due to the lattice mismatch between the AlGaN

and GaN which puts the AlGaN in tension. The 2DEG formed by the AlGaN/GaN

heterojunction is often used to create high electron mobility transistors (HEMTs) and

has been used as the sensing element for various sensors (surface acoustic wave

resonators, pressure or strain sensors, photodetectors, etc.) [189, 182, 191, 183]. Here,

AlGaN/GaN-based UV photodetectors will be examined.

For AlGaN/GaN MSM photodetectors, it has been shown that majority carriers

dominate the response and their charge transport is through the 2DEG [182]. Photons

APPENDIX B. AlGaN/GaN-BASED PHOTODETECTORS 109

absorbed in the AlGaN, 2DEG, or GaN will create electron-hole pairs which will first

travel to the 2DEG and then to the electrical contacts due to the strong vertical electric

field. This is in contrast to GaN-based MSM photodetectors where photo-generated

charge carriers travel laterally between the interdigitated electrodes. Since the AlGaN

layer is very thin (on the order of 10’s of nm) the response time of AlGaN/GaN-based

photodetectors is much faster than their MSM GaN-based counterparts which are

response time limited by the spacing between electrodes (on the order of a few μm).

For AlGaN/GaN HEMT photodetectors (termed phototransistors), electron

transport is through the 2DEG, similar to AlGaN/GaN MSM photodetectors, while hole

transport, which can greatly affect the device response, is dependent on where the

electron-hole pair was generated [183]. Holes generated in the AlGaN will be swept to

the surface due to the strong piezoelectric polarization field and will combine with

trapped surface electrons. This produces a positive gate bias effect and increases the

2DEG concentration. Holes generated in the GaN will drift toward the substrate due to

the built-in electric field creating a positive back-gate which further increases the 2DEG

concentration. The increased 2DEG concentration from absorbed UV photons should

result in UV phototransistors with high responsivities.

Devices fabricated on the AlGaN/GaN material platform have demonstrated

operation in temperatures as low as -257°C [192] and as high as 600°C [23].

Additionally, AlGaN/GaN devices have shown the ability to operate when exposed to

high levels of radiation [14]. Therefore, by using the AlGaN/GaN material platform, a

temperature tolerant, radiation hard, highly responsive, and fast response time UV

photodetector can be created. In the following sections, the fabrication and spectral

characterization as well as the high temperature, low temperature, and proton irradiation

response of AlGaN/GaN MSM photodetectors and AlGaN/GaN phototransistors will

be explored.


B.2 Fabrication and Spectral Characterization

Fabrication of the AlGaN/GaN MSM photodetectors began by growing 3 nm

GaN, 30 nm AlGaN, and 1.5 µm of GaN via metal organic chemical vapor deposition

(MOVCD) on a GaN-on-sapphire template (5 µm GaN, 200 nm AlN, and 500 µm

sapphire). MSM photodetectors were also fabricated on an AlGaN/GaN-on-silicon

(1 nm GaN, 30 nm AlGaN, 1.5 µm GaN, 1.5 µm buffer, and 625 µm Si) substrate. The

AlGaN/GaN substrates were then mesa etched via reactive ion etching for device

isolation. Next, 50 nm of SiO2 passivation was deposited via atomic layer deposition

(ALD) and subsequently etched to create the active area for each photodetector. Semi-

transparent electrodes with 30 to 50% UV transmittance were formed using a standard

metal lift-off procedure with 3 nm Ni / 10 nm Au. Lastly, electrical contact metal was

deposited (20 nm Ti / 200 nm Au). The photodetector active area is 250 μm × 250 μm

with an electrode length of 240 μm, width of 5 μm, and spacing of 5 μm. The complete

fabrication process for the AlGaN/GaN on GaN-on-sapphire MSM photodetectors is

shown in Fig. B.2.1 and a top-view optical image of a typical AlGaN/GaN MSM

photodetector is depicted in Fig. B.2.2.

Fabrication of the AlGaN/GaN phototransistors began with a mesa etch via

reactive ion etching of an AlGaN/GaN-on-silicon (1 nm GaN, 30 nm AlGaN, 1.5 µm

GaN, 1.5 µm buffer, and 625 µm Si) substrate. Ohmic source and drain contacts were

formed using a standard metal lift-off procedure with 20 nm Ti / 100 nm Al / 40 nm Pt

/ 80 nm Au and annealed in N2 at 850°C for 35 seconds. Gate electrodes were formed

using a standard metal lift-off procedure with 20 nm Ti / 80 nm Au. In an attempt to

increase the external quantum efficiency, photodetectors utilizing spine and rib gates

were fabricated in addition to rectangular gates. For both gate geometries, two sizes

were fabricated with photodetector active areas of 200 μm × 100 μm and

200 μm × 50 μm. The complete fabrication process for the AlGaN/GaN

phototransistors is shown in Fig. B.2.3 and top-view optical images of typical


AlGaN/GaN rectangular gate and spine and rib gate phototransistor are depicted in Fig.

B.2.4a and Fig. B.2.4b, respectively.

Figure B.2.1. AlGaN/GaN MSM photodetector fabrication process. (a) AlGaN/GaN on

GaN-on-sapphire substrate, (b) mesa etch, (c) SiO2 passivation, (d) etch SiO2 passivation, (e)

deposit semitransparent Ni/Au interdigitated electrodes, and (f) deposit Ti/Au electrical

contacts and interconnect. From [193]; reprinted by permission of the American Institute of

Aeronautics and Astronautics, Inc.

Figure B.2.2. Top-view optical image of an AlGaN/GaN MSM photodetector with

semitransparent Ni/Au electrodes.


The absorbance and reflectance spectra of the AlGaN/GaN on GaN-on-sapphire

substrate shown in Fig. B.2.5a. show a sharp cutoff near 370 nm demonstrating the

intrinsic visible-blindness of GaN. The absorbance and reflectance spectra of the

AlGaN/GaN-on-silicon substrate shown in Fig. B.2.5b demonstrates a similar result.

The absorbance spectrum for both substrates shows a strong peak at 365 nm due to

photon absorption in the GaN (3.4 eV) and a plateau between 310 nm to 260 nm which

can be attributed to absorption in the AlGaN (~4 eV) and buffer layers. The oscillations

in the absorbance and reflectance data above 400 nm can be attributed to Fabry-Pérot

interference from light reflected off the interfaces of the different layers [99, 194, 195].

The relatively high absorbance in the visible region for the AlGaN/GaN-on-silicon

substrate is attributed to the silicon rather than the photo-sensitive AlGaN and GaN

layers.

Figure B.2.3. AlGaN/GaN phototransistor fabrication process. (a) AlGaN/GaN-on-silicon

substrate, (b) mesa etch, (c) deposit Ti/Al/Pt/Au source and drain contacts, and (d) deposit

Ti/Au gate contact.


Figure B.2.4. Top-view optical image of an AlGaN/GaN phototransistor with (a) 200 μm

× 100 μm rectangular gate and (b) 200 μm × 100 μm spine and rib gate.

Figure B.2.5. Absorbance and reflectance spectra of the (a) AlGaN/GaN on GaN-on-

sapphire substrate and (b) AlGaN/GaN-on-silicon substrate.

Similar to the spectral response characterization of the GaN-on-sapphire

photodetectors described in Chapter 3, AlGaN/GaN-on-silicon MSM photodetectors

were characterized using a laser driven light source (EQ-1500, Energetiq) with nearly

constant radiance across the UV spectra and spectrometer (McPherson 218) with

diffraction grating (246.16 grooves/mm and 226 nm blaze). For wavelengths longer

than 340 nm, a long pass filter was implemented to reduce second order diffraction

affects. The current-voltage response of two AlGaN/GaN-on-silicon MSM

photodetectors illuminated with wavelengths from 200 nm to 500 nm is shown in Fig.

B.2.6. PDCR (as defined in Eq. 1.1) and responsivity (as defined in Eq. 1.2) as functions

of the illumination wavelength are shown in Fig. B.2.7 and Fig. B.2.8 respectively. The

jumps in the spectral PDCR data at 340 nm can be attributed to misalignments after the


long pass filter was implemented resulting in a slightly higher light intensity. The

increase in PDCR for wavelengths shorter than 400 nm and the relatively constant

PDCR from 200 nm to 340 nm matches the results from the absorbance and reflectance

data in Fig. B.2.5b. However, the responsivity continually increases for wavelengths

less than 400 nm which is not as expected and could be the result of a number of things

including: absorption in the AlN-based buffer layers generating electron-hole pairs,

persistent photoconductivity, or misalignments in the test set-up. Further testing is

needed to confirm and fully understand this result.

Figure B.2.6. Current-voltage response of two AlGaN/GaN-on-silicon MSM

photodetectors when illuminated with wavelengths from 200 nm to 500 nm.

Figure B.2.7. PDCR as a function of illumination wavelength of two AlGaN/GaN-on-

silicon MSM photodetectors.


Figure B.2.8. Spectral responsivity of two AlGaN/GaN-on-silicon MSM photodetectors.

B.3 Extreme Environment Operation

B.3.1 AlGaN/GaN Phototransistor High-Temperature Testing

Since traditional rectangular metal gate electrodes transmit less than 30% of

incident UV light [17], a spine and rib gate was designed and the performance compared

to that of the traditional rectangular gate. The experimental set-up for comparing these

two gate geometries when exposed to high temperatures is described in Chapter 4 and

shown in Fig. 4.1.1. The drain current (ID) versus gate-to-source bias (VGS) and drain

current versus drain-to-source bias (VDS) under dark and 365 nm illuminated conditions

at room temperature for an AlGaN/GaN phototransistor with a 200 μm × 100 μm

rectangular gate are shown in Fig. B.3.1a and Fig. B.3.1b respectively. The same curves

are shown in Fig. B.3.2a and Fig. B.3.2b for a 200 μm × 100 μm spine and rib gate. For

both devices, increasing drain-to-source bias or increasing gate-to-source bias results in

an increased drain current. When the rectangular gated device is illuminated with

365 nm light, the drain current increases slightly and the threshold voltage (Vth) shifts

to more negative drain-to-source bias. For the spine and rib gated device, 365 nm

illumination results in a dramatic shift of the threshold voltage to more negative drain-


to-source bias. The maximum PDCR of the rectangular gated device was 4.6 at a gate-

to-source bias of -11.6 V while the spine and rib device exhibited a maximum PDCR of

13.5 at a gate-to-source bias of -12.4 V. The maximum responsivity of the rectangular

gated device was 0.8 A/W at a gate-to-source bias of -11.3 V and the responsivity of the

spine and rib device was 2.3 A/W at -12.8 V gate-to-source bias. The almost three times

increase in PDCR and responsivity for the spine and rib gate compared to the rectangular

gate can be partially attributed to the increased photon absorption by the AlGaN and

GaN. Additionally, the electric field is altered by using the spine and rib gate geometry

which slight changes the characteristics of device response. This can be easily seen by

comparing the slopes of the ID-VGS curves for the two different gate geometries.

Figure B.3.1. AlGaN/GaN phototransistor (200 μm × 100 μm, rectangular gate) (a) drain

current versus gate voltage and (b) drain current versus drain-to-source bias under dark

(dashed lines) and 365 nm illuminated (solid lines) conditions at room temperature.

The drain current versus gate bias (VDS = 1 V) and drain current versus drain-

to-source bias (VGS = 0 V) under dark (dashed lines) and 365 nm illuminated (solid

lines) conditions from room temperature to 300°C are shown in Fig. B.3.3a and Fig.

B.3.3b respectively for a 200 μm × 100 μm rectangular gate phototransistor. The same

curves are shown in Fig. B.4.4a and Fig. B.4.4b for a 200 μm × 100 μm spine and rib

gate phototransistor. For both devices, increasing temperature results in a decreased

drain current and a decreased on/off current ratio which can be attributed to the reduced


mobility of the 2DEG. Under dark conditions, the threshold voltage shifts only slightly

as temperature increases, while under 365 nm illuminated conditions the threshold

voltage dramatically shifts to less negative values as temperature increases.

The maximum PDCR as a function of temperature for four AlGaN/GaN

phototransistors with various gate configurations (200 μm × 100 μm rectangular gate,

200 μm × 50 μm rectangular gate, 200 μm × 100 μm spine and rib gate, 200 μm × 50 μm

spine and rib gate) is shown in Fig. B.3.5a. The gate-to-source bias corresponding to the

maximum PDCR as a function temperature is shown in Fig. B.3.5b. Similarly, the

maximum responsivity as a function of temperature for four AlGaN/GaN

phototransistors with various gate configurations and the corresponding gate-to-source

bias as a function of temperature are shown in Fig. B.3.6. For all four gate

configurations, it is seen that the maximum PDCR is relatively constant up to 200°C but

significantly diminishes at higher temperatures and reaches zero at 350°C. On the

contrary, the maximum responsivity continually decreases as temperature increases

reaching zero at 350°C. The gate-to-source bias where the maximum PDCR and

maximum responsivity occur generally decreases as temperature increases which is in

accordance with the positive shifting of the threshold voltage with temperature.

Figure B.3.2. AlGaN/GaN phototransistor (200 μm × 100 μm, spine and rib gate) (a) drain

current versus gate voltage and (b) drain current versus drain-to-source bias under dark

(dashed lines) and 365 nm illuminated (solid lines) conditions at room temperature.


Figure B.3.3. AlGaN/GaN phototransistor (200 μm × 100 μm, rectangular gate) (a) drain

current versus gate voltage (VDS = 1 V) and (b) drain current versus drain-to-source bias (VGS

= 0 V) under dark (dashed lines) and 365 nm illuminated (solid lines) conditions from room

temperature to 300°C.

Figure B.3.4. AlGaN/GaN phototransistor (200 μm × 100 μm, spine and rib gate) (a) drain

current versus gate voltage (VDS = 1 V) and (b) drain current versus drain-to-source bias (VGS

= 0 V) under dark (dashed lines) and 365 nm illuminated (solid lines) conditions from room

temperature to 300°C.

These results demonstrate that AlGaN/GaN HEMTs are highly responsive to

incident UV light up to temperatures of 300°C. The incident UV light increases the drain

current and causes the threshold voltage to shift to more negative drain-to-source bias.

Therefore, the AlGaN/GaN HEMT architecture can be used to create highly responsive

and high operating temperature capable UV phototransistors. However, careful


consideration needs to be made when selecting packaging materials in order to shield

AlGaN/GaN HEMTs from unwanted UV exposure which will drastically alter device

performance.

Figure B.3.5. (a) Maximum PDCR as a function of temperature for four AlGaN/GaN

phototransistors with various gate configurations and (b) the corresponding gate voltage as a

function of temperature.

Figure B.3.6. (a) Maximum responsivity as a function of temperature for four AlGaN/GaN

phototransistors with various gate configurations and (b) the corresponding gate voltage as a

function of temperature.


B.3.2 AlGaN/GaN MSM Low-Temperature Testing

Cryogenic tests on GaN-based HEMTs have shown improved performance (in

the form of slightly higher transconductance and electron mobility, among other

parameters) at low temperatures compared to room temperature [192, 196]. Since higher

mobility leads to the collection of more photo-generated carriers, AlGaN/GaN-based

MSM photodetectors should also demonstrate good performance at low temperatures.

However, the experimental response of AlGaN/GaN MSM photodetectors under

cryogenic conditions has not been thoroughly investigated, if at all. Here, two

AlGaN/GaN on GaN-on-sapphire MSM photodetectors (called photodetector 1 and

photodetector 2) are characterized when subject to temperatures as low as -138.0°C

through in situ monitoring of the dark-current and photo-generated current.

The test set-up for characterizing the GaN-based photodetectors at near-

cryogenic temperatures using a liquid nitrogen cooled pressure and temperature

controlled chamber (Linkam Scientific THMS600-PS) is shown in Fig. B.3.7. A close-

up view of the stage (iced over during testing due to condensation of water vapor in air)

is shown in Fig. B.3.7b. The GaN photodetector chip was epoxied onto a PCB as shown

in Fig. B.3.7c with a k-type thermocouple epoxied on top of the chip to measure the

photodetector temperature which differed from the chuck set temperature. The PCB was

secured in place on the temperature-controlled chuck inside the chamber with the

photodetectors visible through the small window on the cover. Once the chamber was

closed, an argon purge was used to remove any water from the air so that it did not

condense on the photodetectors during cooling. Dark current-voltage measurements

were then taken with the window covered to block any ambient light from affecting the

measurement. The UV lamp was then turned on and a photocurrent measurement taken.

After these initial reference measurements, the temperature of the chuck was decreased

until the thermocouple read 0°C. The chip was allowed to soak at this low temperature

for about 30 minutes and then dark and illuminated measurements were retaken. This

process was continued down to -138°C. In addition to current-voltage measurements,


transient current measurements were taken at room temperature, -50°C, and -98°C.

Transient current measurements were taken by measuring dark current for five minutes

followed by five minutes illuminated (365 nm) and finally five minutes in the dark. A

semiconductor parameter analyzer (B1500A, Agilent Technologies) was used for all

electrical characterization and a 6 W, 365 nm UV lamp (UVP, EL series UV lamp) was

used for UV illumination. A UV light meter (Sper Scientific UVA/UVB Light Meter)

was used to measure the incident UV intensity, which for all illuminated electrical

measurements was 0.7 mW/cm2. For this intensity, the optical power applied on the

photodetectors is approximately 438 nW.

The chuck set temperature and corresponding measured thermocouple

temperature (located on the GaN photodetector chip) are listed in Table B.3.1. This

difference in temperature is likely due to the fact that the photodetectors are not in direct

thermal contact with the chuck. Additionally, the argon purge is contributing to

convection across the surface. Thus, all results presented are with respect to the

thermocouple temperature.

Figure B.3.7. (a) Overall experimental test set-up for characterizing the low-temperature

response of GaN-based photodetectors, (b) temperature controlled stage (frozen over during

testing), and (c) GaN-based photodetector chip epoxied to a PCB. From [193]; reprinted by

permission of the American Institute of Aeronautics and Astronautics, Inc.


Table B.3.1. Measured thermocouple temperature versus chuck set temperature.

Thermocouple Temperature

(± 2.5°C)

Chuck Temperature

(± 0.15°C)

23.3 25.0

-0.2 -7.0

-49.9 -70.0

-98.0 -142.0

-138.0 -188.0

The current-voltage curves for two AlGaN/GaN on GaN-on-sapphire MSM

photodetectors under dark (solid lines) and 365 nm illuminated (dashed lines)

conditions from room temperature to -138°C are shown in Fig. B.3.8. As the

temperature decreases, the dark-current is reduced which is consistent with the idea that

thermal excitation of electrons is a dominate factor in the creation of dark-current. Thus

by lowering the temperature the dark-current is suppressed. However, this effect is not

permanent as can be seen in the dark-current thermal recovery of photodetector 1 after

exposure to -138°C shown in the inset in Fig. B.3.8a. Following the same trend as the

dark-current, the photocurrent also decreases with decreasing temperature; this is

expected since the photocurrent is generated by both thermal excitation and photo-

excitation of charge carriers. The PDCR and responsivity as functions of temperature at

-8 V bias for photodetectors 1 and 2 are shown in Fig. B.3.9a and Fig. B.3.9b,

respectively. From these figures, it can be said that PDCR and responsivity are not

significantly affected by temperature down to -138.0°C.


Figure B.3.8. AlGaN/GaN on GaN-on-sapphire MSM photodetector current-voltage

curves under dark (solid lines) and 365 nm illuminated (dashed lines) conditions from room

temperature to -138°C for (a) photodetector 1 and (b) photodetector 2. The inset in (a) shows

the dark-current thermal recovery of photodetector 1 after exposure to -138°C.

Figure B.3.9. (a) PDCR and (b) responsivity as a function of temperature at -8 V bias for

photodetectors 1 and 2.

The transient current response of photodetector 1 at -0.2°C, -49.9°C and -98.0°C

is shown in Fig. B.3.10. For these measurements, the device was biased at -8 V and

dark-current was measured for 5 minutes, then the UV light was switched on and

photocurrent (365 nm, 0.7 ± 0.1 mW/cm2) measured for five minutes, and finally the

UV light was turned off and dark-current measured for five minutes. Once again, the

reduction of dark-current and photocurrent with decreasing temperatures is readily


apparent. Also apparent is the increase in the decay time (time between 90% and 10%

steady-state photocurrent) with decreasing temperatures. For photodetector 1, at -0.2°C

the decay time was 132 seconds and for temperatures lower than -49.9°C the decay time

was greater than the five minute measurement time. The decay time is much longer than

the rise time (time between 10% and 90% steady-state photocurrent), which has been

seen in GaN-based and other III-V photodetectors and could be due to impurities,

dislocations, and deep-level defects creating a persistent photoconductivity [55, 56, 57,

58, 181, 59]. Defects act as traps for photo-generated carriers at low temperatures. At

higher temperatures, thermal energy is able to release these carriers. Therefore, the

increase in decay time experienced at low temperatures can be attributed to the electron-

hole recombination rate slowing down (decreasing recapture rate of photo-excited

electrons) at decreased temperatures [55, 56, 57].

Figure B.3.10. Transient current response of photodetector 1 at different temperatures under

a 365 nm UV intensity of 0.7 ± 0.1 mW/cm2 and a bias of -8 V. From [193]; reprinted by

permission of the American Institute of Aeronautics and Astronautics, Inc.

When the chuck set temperature was as low as it could be (-190°C), the on-chip

thermocouple read -138°C. In order to test the photodetectors at lower temperatures, the

PCB with AlGaN/GaN photodetector chip was held in liquid nitrogen vapor. Figure


B.3.11 shows the current-voltage response for photodetector 1 at 23.3°C and while in

liquid nitrogen vapor when the on-chip thermocouple read -195.0°C. This figure clearly

shows that AlGaN/GaN MSM photodetectors are still operational down to -195.0°C.

The PDCR and responsivity at -8 V bias were 0.06 and 173 A/W. The relatively poor

performance of the photodetector at this cryogenic temperature can be attributed to the

liquid nitrogen vapor blocking the UV light. Additionally, the inset of Fig. B.3.11 shows

the condensation on the GaN chip and PCB from the liquid nitrogen.

Figure B.3.11. Current-voltage response for photodetector 1 at 23.3°C and while in liquid

nitrogen vapor at -195.0°C. The inset shows the condensation on the GaN chip and PCB

from the liquid nitrogen.

B.3.3 AlGaN/GaN MSM Proton Irradiation

The experimental set-up for proton irradiation and electrical characterization of

GaN-based photodetectors is described in Chapter 4 and shown in Fig. 4.4.1. The dark

and illuminated (365 nm) scaled current-voltage response as a function of 2 MeV proton

irradiation fluence for an AlGaN/GaN on GaN-on-sapphire MSM photodetector with

semitransparent Ni/Au electrodes is shown in Fig. B.3.12. In an attempt to clearly show

the effect proton irradiation has on the photo-generated current, the current-voltage


curves were scaled such that the dark current is the same at each irradiation fluence.

From Fig. B.3.12 it is evident that there is little to no degradation, in terms of UV

response, of the AlGaN/GaN photodetectors when subjected to 2 MeV protons with

fluences up to about 3.8 × 1013 cm-2. The PDCR and responsivity as functions of proton

fluence for three semitransparent Ni/Au AlGaN/GaN on GaN-on-sapphire

photodetectors are shown in Fig. B.3.13. From examining how these figures of merit

change with increasing irradiation fluence, it is again apparent that the UV response

experiences minimal degradation. The average pre-irradiation PDCR and responsivity

were 1.13 and 4.88 A/W and the 30 days post-irradiation PDCR and responsivity were

0.75 and 8.59 A/W as listed in Table B.3.2.

Figure B.3.12. Dark and illuminated (365 nm) scaled current-voltage response as a function

of proton irradiation fluence for AlGaN/GaN on GaN-on-sapphire photodetectors with

semitransparent Ni/Au electrodes.

Compared to the proton irradiated GaN-on-sapphire photodetectors described in

Chapter 4, it is seen that the current is much lower for the Ni/Au AlGaN/GaN

photodetectors (about 1 mA at 5 V) compared to the Ni/Au GaN-on-sapphire

photodetectors (about 10 mA at 5 V). This intuitively makes sense since the Schottky

barrier height is higher for Ni on AlGaN (1.11-1.36 eV) than it is for Ni on GaN (0.84-

1.00 eV) [197]. The PDCR is larger for AlGaN/GaN-based photodetectors (about 1)


than GaN-based photodetectors (about 0.1). Conversely, the responsivity is much larger

for GaN-based photodetectors (about 300 A/W) than for AlGaN/GaN-based

photodetectors (about 13 A/W). The photo-generated current (dark current subtracted

from the photocurrent) is higher for the GaN-based photodetectors which once again

can be attributed to the lower Schottky barrier height. While the GaN-based

photodetectors have higher photo-generated current, they also have higher dark current

which results in lower PDCR and higher responsivity when compared to the

AlGaN/GaN photodetectors. Additionally, the high responsivities exhibited by both

devices has been previously reported in literature and attributed to trapping of photo-

generated holes by surface states at the metal-semiconductor interface resulting in a

further lowering of the Schottky barrier height and increased leakage current [53, 54].

Figure B.3.13. (a) PDCR and (b) responsivity as a function of proton fluence for three

semitransparent Ni/Au AlGaN/GaN on GaN-on-sapphire photodetectors.

Table B.3.2. Average pre- and 30 days post-irradiation PDCR and responsivity for semi-

transparent Ni/Au AlGaN/GaN on GaN-on-sapphire photodetectors.

Electrode Material PDCR Responsivity (A/W)

Pre-Irradiation Ni/Au 1.13 4.88

Post-Irradiation Ni/Au 0.75 8.59

128

Appendix C

Experiment Drawings

APPENDIX C. EXPERIMENT DRAWINGS 129

Fig

ure C

.1.

Full Q

B50 m

iniatu

re sun sen

sor in

tegratio

n circu

it diag

ram.


Fig

ure C

.2.

Draw

ings o

f spectro

meter ad

apter fo

r spectral resp

onse ch

aracterization

.


Figure C.2. Drawings of spectrometer adapter for spectral response characterization.


Fig

ure C

.2.

Draw

ings o

f spectro

meter ad

apter fo

r spectral resp

onse ch

aracterization

.


Fig

ure C

.2.

Draw

ings o

f spectro

meter ad

apter fo

r spectral resp

on

se characterizatio

n.


Fig

ure C

.3.

Shock

tube assem

bly

draw

ings.

139

Appendix D

Fabrication Run Sheet

Steps preformed in the Stanford Nano Shared Facility (SNSF) denoted, all other steps

preformed in Stanford Nanofabrication Facility (SNF).

1. Clean tweezers

a. Acetone, 10 mins

b. Methanol, 10 mins

c. IPA, 10 mins

2. Clean wafer(s) (including silicon dummy wafers)

a. 9:1 Piranha (H2SO4 (sulfuric acid): H2O2 (hydrogen peroxide)),

wbflexcorr2, 30 mins, 120°C

b. Water rinse, N2 dry

***************************** MESA ******************************

********************************************************************

APPENDIX D. FABRICATION RUN SHEET 140

LITHO – MESA ETCH

3. Dehydrate

a. 5 mins, 115°C hotplate

4. YES oven: dehydrate and prime with HMDS

5. Headway manual resist spinner

a. Shipley 3612, 60 sec, 5000 rpm

b. 1 min, 90°C hotplate

6. KarlSuss aligner

a. Clean mask: acetone, methanol, IPA, N2 dry

b. Hard contact, 40 um gap, expose 1.1 sec (Check exposure time on dummy

wafer first!)

c. 1 min, 115°C hotplate

7. Develop

a. MF-26A, 55 sec (Check development time on dummy wafer first!)

b. Water rinse and N2 dry

8. Inspect under microscope

9. Hard bake


MESA ETCH

10. Oxford III-V etcher, xlab_GaN_slow_etch (Check etch rate on dummy wafer

first!)

11. Clean

a. 9:1 Piranha (H2SO4 (sulfuric acid): H2O2 (hydrogen peroxide)),

wbflexcorr2, 30 mins, 120°C

b. Water rinse, N2 dry

***************************** OHMIC *****************************

********************************************************************


LITHO - OHMIC

12. Dehydrate




a. LOL 2000, 60 sec, 3000 rpm

b. 5 mins, 160°C hotplate

c. Shipley 3612, 60 sec, 5000 rpm

d. 1 min, 90°C hotplate

15. KarlSuss aligner


b. Hard contact, 40 um gap, expose 2.2 sec (check exposure time on dummy

wafer first)


16. Develop

a. MF-26A, 20 sec (check development time on dummy wafer first)



18. Hard bake


19. Drytek 2, descum

20. Weak ammonium hydroxide (1:20, ammonium hydroxide: water), 10 sec

OHMIC METAL

21. KJL e-beam evaporator - SNSF

a. 20 nm Ti / 100 nm Al / 40 nm Pt / 80 nm Au

22. Liftoff

a. 1165 overnight

b. Acetone, 5 mins


c. Methanol, 1 min

d. IPA, 1 min

e. N2 dry and inspect under microscope

f. Clean with PRS 1000, 20 mins, 40°C

g. Acetone, 5 mins

h. Methanol, 1 min

i. IPA, 1 min

j. N2 dry

23. Dehydrate


24. Rapid thermal anneal

a. awin_r, P_850_N2, 850°C, 35 sec

***************************** OXIDE *****************************

********************************************************************

OXIDE DEPOSITION

25. Atomic layer deposition (ALD) SiO2, 50 nm

LITHO – OXIDE ETCH

26. Dehydrate








b. Hard contact, 40 um gap, expose 1.1 sec



30. Develop

a. MF-26A, 55 sec



OXIDE ETCH

32. 50:1 hydrofluoric acid (HF) dip, 1 min 30 sec

33. Remove photoresist

a. Acetone, 5 mins

b. Methanol, 1 min

c. IPA, 1 min

d. N2 dry

******************** SEMI-TRANSPARENT Ni/Au ********************

*********************************************************************

LITHO – SEMI-TRANSPARENT Ni/Au

34. Dehydrate




a. LOL 2000, 60 sec, 3000 rpm






b. Hard contact, 40 um gap, expose 2.2 sec


38. Develop


a. MF-26A for 20 sec



40. Hard bake


41. Drytek 2, descum

SEMI-TRANSPARENT Ni/Au

42. Innotec e-beam evaporator

a. 3 nm Ni

b. 10 nm Au

43. Liftoff

a. 1165 overnight

b. Acetone, 5 mins

c. Methanol, 1 min

d. IPA, 1 min

e. N2 dry and inspect under microscope


g. Acetone, 5 mins

h. Methanol, 1 min

i. IPA, 1 min

j. N2 dry

**************************** BONDPAD ****************************

*********************************************************************

LITHO - BONDPAD

44. Dehydrate





a. LOL 2000, 30 sec, 3000 rpm




47. Karlsuss aligner


a. Hard contact, 40 um gap, expose 2.2 sec


48. Develop

a. MF-26A for 20 sec



50. Hard bake


51. DryTek2, descum

BONDPAD METAL

52. KJL e-beam evaporator - SNSF

a. 20 nm Ti

b. 200 nm Au

53. Liftoff

a. 1165 overnight

b. Acetone, 5 mins

c. Methanol, 1 min

d. IPA, 1 min

e. N2 Dry and inspect under microscope


g. Acetone, 5 mins


h. Methanol, 1 min

i. IPA, 1 min

j. N2 dry

****************************** DICE ******************************

********************************************************************

LITHO FOR DICING

54. Dehydrate





57. Hard bake


58. Dice


a. Acetone, 5 mins

b. Methanol, 1 min

c. IPA, 1 min

d. N2 Dry and inspect under microscope

*************************** GRAPHENE ****************************

*********************************************************************

GRAPHENE TRANSFER - SNSF

60. Start with graphene grown on copper through chemical vapor deposition (CVD)

61. Cut a piece of graphene slightly larger than sample size, using weigh paper as a

support while cutting

62. Flatten piece between two microscope slides

63. Attach to microscope slide using a tiny drop of water


64. Spin PMMA (495,000g/mol, 4% dissolved in anisole) onto top side of sample at 0

rpm for 10 sec and 1000 rpm for 45 sec

65. Let PMMA rest for 12 hours or bake at 100°C for 1 min

66. Flip piece over, etch away backside graphene, 30 sec, 3% oxygen, 300 W. Use a

glass slide to weigh down the edges so the pieces don’t get blown away.

67. Flip back over. Place in 1 M FeCl3 solution. Let sit until copper is etched away.

68. Scoop PMMA/graphene stack into DI water. Let sit for 10 mins then scoop into

2nd DI water. Let sit for 10 mins then scoop into 3rd DI water. Scoop

PMMA/graphene stack onto actual substrate. Absorb excess water with a clean

room wipe and let sit to dry

69. 15 mins, 150°C hotplate

70. Remove PMMA

a. Acetone, 5 mins

b. IPA, 5 mins

c. N2 dry

71. 10 mins, 200°C hotplate

LITHO – GRAPHENE ETCH






a. Hard contact, 40 um gap, expose 1.1 sec


74. Develop

a. MF-26A, 55 sec



GRAPHENE ETCH - SNSF

75. Etch graphene, 60 sec, 3% oxygen, 300 W


a. Acetone, 5 mins

b. Methanol, 1 min

c. IPA, 1 min

d. N2 dry

149

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