advances in compound semiconductor radiation...

Paul Sellin, Radiation Imaging Group

Advances in Compound Semiconductor RadiationDetectors

a review of recent progress

P.J. SellinRadiation Imaging GroupDepartment of Physics

University of Surrey


CZT/CdTe

Review of recent developments in compound semiconductordetectors:

r CdZnTe (CZT) continues to dominate high-Z “room temperature”devices: a range of electrode configurations to overcome poor hole transport lack of monocrystalline whole-wafer material High Pressure Bridgman CZT from eV Products still the major

volume supplier HPB CZT also from Bicron (US), LETI (France), also LPB CZT

r good results from CdTe Schottky diodes CdTe from a number of suppliers (eg. Acrotech, Eurorad, Freiburg)

r CZT/CdTe pixel array detectors under development: hard X-ray astronomical imaging gamma cameras for nuclear medicine

r custom ASICs for CZT/CdTe starting to appear


Material Properties

Summary of some material properties:Z EG W ρi at RT

(eV) (eV/ehp) (Ω)

Si 14 1.12 3.6 ~104

Ge 32 0.66 2.9 50InP 49/15 1.4 4.2 107

GaAs 31/33 1.4 4.3 108

CdTe 48/52 1.4 4.4 109

CdZn0.2Te 48/52 1.6 4.7 1011

HgI2 80/53 2.1 4.2 1013

TlBr 81/35 2.7 5.9 1011

Diamond 6 5 13 >1013

Also: SiC, PbI2, GaSe


Detection Efficiency

Vast majority of compund semiconductor detector development isdriven by improved photoelectric absorption for hard X-rays andgamma rays:

Exceptions are radiation hard detector programmes - SiC and DiamondPhoton energy (keV)

20 40 60 80 100 120 140

Det

ectio

n E

ffici

ency

(%)

0

20

40

60

80

100

SiGaAsCdTeInP

Calculated efficiencies for 500µm thick material


Material Quality in CdZnTe

High Pressure Bridgman CdZnTe is the new material of choice for medium resolution X-ray andgamma ray detection

Material suffers from mechanical defects - monocrystalline pieces are selected from wafers - nowhole-wafer availability

CZT material grown by High Pressure Bridgman from eV Products (Growth and properties ofsemi-insulating CdZnTe for radiation detector applications, Cs. Szeles and M.C. Driver SPIEProc. 2 (1998) 3446).

New growth methods have developed very recently - eg. Low Pressure Bridgman CZT fromYinnel Tech (US) and Imarad (Israel)


‘Hole tailing’ in a 5mm thickCdZnTe detector

Poor hole transport causes position-dependent charge collection efficiency

⇒ ‘hole tailing’ characteristic of higher energygamma rays in CdZnTe

GF Knoll, Radiation Detection andMeasurement, Ed. 3


Scanning of CCE vs depth using lateral Ion-beam induced charge microscopy

400 Vca

thod

e-400 V

cathode

Pulse height spectra as a function of depth+400 V -400V

Image of CCEusing 1µm

resolution 2MeVscanning proton

beam


Induced signals due to charge drift

In a planar detector thedrifting electrons andholes generate equaland opposite inducedcharge on anode andcathode

In CZT the holes arequickly trapped:

• hole component ismuch reduced

• interactions close tothe anode have lowCCEReviewed in Z. He et al,NIM A463 (2001) 250


The coplanar griddetector

Z

Coplanar electrodes produceweighting fields maximisedclose to the contacts

The subtracted signal fromthe 2 sets of coplanarelectrodes gives a weightingfield that is zero in the bulk

The subtracted signal is onlydue to electrons - generallyholes do not enter the sensitiveregion

First applied to CZT detectorsby Luke et al. APL 65 (1994)2884

cathode

anode 1

anode 2

holes electrons


Depth sensing

Coplanar CZT detectors provide depth position information:r signal from planar cathode ∝ distance D from coplanar anodes

and event energy Eγ :SC ∝ D x Eγ

r signal from coplanar anode is depth independent:SA ∝ Eγ

r so the depth is simply obtained from the ratio:D = SC / SA

Z. He et al, NIM A380 (1996) 228, NIM A388 (1997) 180

Benefits of this method:r γ-ray interaction depth allows correction to be made for residual

electron trappingr 3D position information is possible, for example useful for

Compton scatter cameras


Interaction Depth position resolution from CZT

Position resolution of ~1.1 mm FWHM achieved at 122 keVCollimated gamma rays were irradiated onto the side of a 2cm CZT

detector - 1.5 mm slit pitch:

Z. He et al, NIM A388 (1997) 180


CZT pixel detectors

In a pixel detector, the weighting field from the ‘small pixel effect’acts similarly to a coplanar structure:

r the pixel signal is mainly insensitive to hole transportr depth dependent hole trapping effects are minimisedr the pixel signal decreases dramatically when the interaction

occurs close to the pixel - the ‘missing’ hole contributionbecomes important:

A. Shor et al, NIM A458 (2001) 47


Correcting for electron trapping

Knowing the depth of the interaction, spectral degradation due toelectron trapping can be compensated for:

Energy vs positionplot for 133Ba

spectrum:

Resolution @356keVimproves from 1.7%

FWHM to 1.1%FWHM


3D pixel array detectors

A 3D sensitive CZT pixel array has beendeveloped:• non-collecting guard rings plus small pixelsform a single-polarity sensing device• depth information allows pulse heightcorrections due to trapping and non-uniformity

Z. He et al., NIM A422 (1999) 173

The ‘coplanar grid’ detector acts as aform of 2D strip detector - with allelectrodes on one side of the device:• small pixel anodes are connectedorthogonally across ‘guard ring’ anodestrips• relatively complex design

V.T. Jordanov et al., NIM A458 (2001) 511


CZT/CdTe pixel array detectors

Outstanding issues:r CZT-compatible flip-chip bonding: low temperature indium or polymerr material uniformity and cost for large area arrays - requirement for large area

mono-crystalline CZT or CdTer motivation is astronomical X-ray imaging and nuclear medicine gamma ray

imaging

Goal for astronomy: 20x20mm active area with <1mm spatial resolution


Caltech HEFT CZT pixel array

8x8 CZT pixel array flip-chip bonded to custom ASIC - Caltech,Pasedena

For focal plane imaging of High EnergyFocussing Telescope (HEFT):r 600 µm pixel pitch, 500 µm pixel sizer 8 x 7 x 2 mm CZT from eV productsr low power ASIC, < 300 µW per pixelSpectral response:r achieved 670 eV FWHM @ 59.5 keV

(1.1%) operated at -10°Cr reduced CCE in inter-pixel gap

causes peak broadeningr pixel leakage current slightly

higher than expected

W.R. Cook et al, Proc SPIE 3769 (1999) 92


Leicester/Surrey prototype CZT pixel array

reference 5120

5040

5120

4800

5120

5040Quadrant Q4:12x12 pixels400µm pitch

Quadrant Q1:32x32 pixels160µm pitch



A prototype pixel detector for 10 - 100 keVX-ray imaging - based on the Rockwell ASIC

Low noise current integrating ASIC, alreadyavailable bonded to Si and MercuricCadmium Telluride (MCT)

Pixel Pitch 40 µm

Pixel integration capacity 2 x 105 C

Pixel noise <20 electrons

Readout rate 2 MHz

Chip power dissipation <1 mW

ASIC pixel pitch


Other CZT pixel arrays

Marshall Space Centre - prototype 4x4 CZT pixel arrays wirebonded to discrete preamplifiers

r CZT is 5 x 5 x 1 mm from eV productsr 750 µm pixel pitch, 650 µm pixel sizer ~ 2% FWHM at 59.5 keV

BICRON / LETI - aimed at 140 keV medical imagingr CZT from BICRON has 4.5 mm pixel size, 4 x 4 pixel moduler module is 18 x 18 mm, 6 mm thick CZTr motherboard is 10 x 12 modules,

18 x 21.5 cm (1920 pixels)r motherboard is edge-buttable, up to

8 boards giving 43 x 72 cm active area

B. Ramsey et al, NIMA458 (2001) 55

C. Mestais et al, NIMA458 (2001) 62


CdTe Schottky diode detectors

r Improved qualitymono-crystalline CdTematerial from Acrotecof Japan

r In/p-type CdTe Schottycontact gives ~100xlower leakage thanohmic Pt/CdTe contact

r High electric fieldminimises charge loss

Spectrum is 0.5mm thickCdTe at 800V, +5°C:

r 1.4 keV FWHM @ 122keV (1.1%)

r 4 keV FWHM @ 511keV (0.8%)

1 T. Takahashi et al, NIM A436 (1999) 111


Stack of CdTe detectors

0.5mm CdTe Schottky detectors offer <1% resolution at severalhundred keV

Requires: charge drift time << charge trapping timedrift time ∝ thickness / velocity

∝ thickness / mobility x electric field⇒ operation at high field and with thin detectors

For thicker detectors:bias voltage ∝ thickness 2

Stack of 12 CdTe detectors, each 5 x 5x 0.5mm. 400V bias on each detector,at +5°C

Separate readout of each layer - use asa Compton scatter detector


‘CdTe stack’ spectra from 133Ba

top layer

sum oflayers 1-8

layer 2

layer 6


Other materials

A number of materials other than CZT/CdTe continue to develop:

r very high-Z materials TlBr and HgI2 are of interest for hard X-rayand nuclear medicine imaging

r intermediate-Z materials GaAs and InP have seen dramaticimprovements in the purity of thick epitaxial material: fano-limited performance has been shown in a small number of

epitaxial GaAs detectors

r diamond continues to make progress with increasing CCE -improvements in SiC material also look promising

r a number of other materials have short term potential:for example, GaN, PbI2, and GaSe


InP detectors

Electric Field (kV/cm)

0 5 10 15 20 250.0

5.0e+6

1.0e+7

1.5e+7

2.0e+7

2.5e+7 GaAs electronsInP electrons

0.65 eV

1.35 eV

shallow donor impurity states

Fe deep acceptor

• InP is a direct bandgap semi-conductor - similar properties to GaAs• 2-3x high stopping power, and higherelectron drift velocities than GaAs.• Compensation is achieved using Feas a deep acceptor: 0.65 eV below theconduction band edge.

Electron drift velocity

Semi insulating InP grown by:• Fe dopant added to liquid melt(crystal doping)• Fe dopant diffused into eachwafer from surface deposition(MASPEC process)R. Fornari et al,JAP 88/9 (2000) 5225-5229


ESTEC InP detectors

InP performance is limited by leakage current and charge trapping: benefitfrom cooled operation:ESTEC 180µm thick InP detectors, grown by Fe-doped Czochralski:

T = -60°C T = -170°CFuture developments need a blocking contact technology, and better

material purity

A. Owens et al., NIMA487 (2002) 435-440.


Epitaxial GaAs

Epitaxial GaAs can be grown as high purity thick layers usingchemical Vapour Phase Epitaxy (Owens - ESTEC, Bourgoin - Paris).

Photoluminescence mapping clearly shows the uniformity ofepitaxial GaAs compared to semi-insulating bulk material:

H. Samic et al., NIM A 487 (2002) 107-112.

Epitaxial GaAs Bulk GaAs


GaAs pixels array detectors

GaAs pixel arrays have been flip-chip bonded and tested withseveral ASICs: Medipix (CERN), MPEC (Freiberg), Cornell.

C. Schwarz et al., NIM A 466 (2001) 87M. Lindner et al., NIM A 466 (2001) 63

LEC semi-insulating GaAs suffersfrom poor CCE due to low electricfield close to the ohmic contact,and material non-uniformity

Software gain matching cancorrect for some pixel-to-pixelvariations

Various commercial flip-chipbonding processes are compatiblewith GaAs, eg. tin-lead reflow

Future tests with thick epitaxialGaAs are more promising Medipix pixel pitch is 170 µm, the inter-pixel

gap is10 µm and bond pad size is 20 µm.


Epitaxial GaAs detectors

Epitaxial GaAs (lightly n type) is generally grown on a n+ GaAswafer substrate:

A Schottky contact is deposited on the front surfaceThe n+ substrate acts as the ohmic contact

C. Erd et al., NIM A 487 (2002) 78-89.


High resolution GaAs spectrometers

Best results to date are from ESTEC with 400µm thick GaAs devicesdepleted to ~100µm, achieving as low as 465 eV FWHM at 59.5 keV:

A. Owens, JAP 85 (1999) 7522-7527


Spatial uniformity and Fano limit

The measured resolution of 468 eV FWHM is close to the intrinsicFano noise limit (F=0.14) of 420 eV FWHM:


Conclusions

r Prototype CZT pixel array detectors are becoming available:

sub-millimetre resolution X-ray imaging detectors for astronomy

4-5 millimetre resolution medical gamma cameras

r Significant recent improvements in the supply of HPB/LPB CZT andCdTe is providing better quality large-area mono-crystalline material

r Novel trapping-correction and 3D depth sensing techniques continueto develop for CZT and CdTe

r Excellent spectral performance has been seen in a small number ofsamples of epitaxial GaAs, InP and TlBr from the ESTEC programme:

new sources of high purity epitaxial material is the key for futuredevelopment

r Excellent medium-term future for compound semiconductor imagingdetectors


Acknowledgements

I am grateful to the many authors of published papers and privatecommunications that have made this review possible

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