1. Radiation Imaging Group
Advances in Compound Semiconductor Radiation Detectors a review of recent progress
P.J. Sellin
Radiation Imaging Group
Department of Physics
University of Surrey

2. CZT/CdTe
Review of recent developments in compound semiconductor detectors:
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
good results from CdTe Schottky diodes
CdTe from a number of suppliers (eg. Acrotech, Eurorad, Freiburg)
CZT/CdTe pixel array detectors under development:
hard X-ray astronomical imaging
gamma cameras for nuclear medicine
custom ASICs for CZT/CdTe starting to appear

3. Material Properties Summary of some material properties:
Z EG W ri at RT
(eV) (eV/ehp) (W)
Si ~104 Ge
InP 49/
GaAs 31/
CdTe 48/
CdZn0.2Te 48/
HgI2 80/
TlBr 81/
Diamond >1013
Also: SiC, PbI2, GaSe

4. Detection Efficiency
Vast majority of compund semiconductor detector development is driven by improved photoelectric absorption for hard X-rays and gamma rays:
Exceptions are radiation hard detector programmes - SiC and Diamond

5. Material Quality in CdZnTe
High Pressure Bridgman CdZnTe is the new material of choice for medium resolution X-ray and gamma ray detection
Material suffers from mechanical defects - monocrystalline pieces are selected from wafers - no whole-wafer availability
CZT material grown by High Pressure Bridgman from eV Products (Growth and properties of semi-insulating CdZnTe for radiation detector applications, Cs. Szeles and M.C. Driver SPIE Proc. 2 (1998) 3446).
New growth methods have developed very recently - eg. Low Pressure Bridgman CZT from Yinnel Tech (US) and Imarad (Israel)

6. ‘Hole tailing’ in a 5mm thick CdZnTe detector
Poor hole transport causes position-dependent charge collection efficiency
 ‘hole tailing’ characteristic of higher energy gamma rays in CdZnTe
GF Knoll, Radiation Detection and Measurement, Ed. 3

7. Image of CCE using 1mm resolution 2MeV scanning proton beam
Scanning of CCE vs depth using lateral Ion-beam induced charge microscopy
400 V
cathode
-400 V
cathode
Image of CCE using 1mm resolution 2MeV scanning proton beam
A region of reduced CCE only seen near one contact. SEM and PL microscopy show a defect region only at one contact.
+400 V
-400V
Pulse height spectra as a function of depth

8. Induced signals due to charge drift
In a planar detector the drifting electrons and holes generate equal and opposite induced charge on anode and cathode
In CZT the holes are quickly trapped:
hole component is much reduced
interactions close to the anode have low CCE
Reviewed in Z. He et al, NIM A463 (2001) 250

9. The coplanar grid detector
cathode
anode 1
anode 2
The coplanar grid detector
Coplanar electrodes produce weighting fields maximised close to the contacts
The subtracted signal from the 2 sets of coplanar electrodes gives a weighting field that is zero in the bulk
The subtracted signal is only due to electrons - generally holes do not enter the sensitive region
First applied to CZT detectors by Luke et al. APL 65 (1994) 2884 Z

10. Depth sensing
Coplanar CZT detectors provide depth position information:
signal from planar cathode  distance D from coplanar anodes and event energy E :
SC  D x E
signal from coplanar anode is depth independent:
SA  E
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:
g-ray interaction depth allows correction to be made for residual electron trapping
3D position information is possible, for example useful for Compton scatter cameras

11. Interaction Depth position resolution from CZT
Position resolution of ~1.1 mm FWHM achieved at 122 keV
Collimated gamma rays were irradiated onto the side of a 2cm CZT detector mm slit pitch:
Z. He et al, NIM A388 (1997) 180

12. CZT pixel detectors
In a pixel detector, the weighting field from the ‘small pixel effect’ acts similarly to a coplanar structure:
the pixel signal is mainly insensitive to hole transport
depth dependent hole trapping effects are minimised
the pixel signal decreases dramatically when the interaction occurs close to the pixel - the ‘missing’ hole contribution becomes important:
A. Shor et al, NIM A458 (2001) 47

13. Correcting for electron trapping
Knowing the depth of the interaction, spectral degradation due to electron trapping can be compensated for:
Energy vs position plot for 133Ba spectrum:
improves from 1.7% FWHM to 1.1% FWHM

14. 3D pixel array detectors
A 3D sensitive CZT pixel array has been developed:
non-collecting guard rings plus small pixels form a single-polarity sensing device
depth information allows pulse height corrections due to trapping and non-uniformity
Z. He et al., NIM A422 (1999) 173
The ‘coplanar grid’ detector acts as a form of 2D strip detector - with all electrodes on one side of the device:
small pixel anodes are connected orthogonally across ‘guard ring’ anode strips
relatively complex design
V.T. Jordanov et al., NIM A458 (2001) 511

15. CZT/CdTe pixel array detectors
Outstanding issues:
CZT-compatible flip-chip bonding: low temperature indium or polymer
material uniformity and cost for large area arrays - requirement for large area mono-crystalline CZT or CdTe
motivation is astronomical X-ray imaging and nuclear medicine gamma ray imaging
Goal for astronomy: 20x20mm active area with <1mm spatial resolution

16. Caltech HEFT CZT pixel array
8x8 CZT pixel array flip-chip bonded to custom ASIC - Caltech, Pasedena
For focal plane imaging of High Energy
Focussing Telescope (HEFT):
600 mm pixel pitch, 500 mm pixel size
8 x 7 x 2 mm CZT from eV products
low power ASIC, < 300 mW per pixel
Spectral response:
achieved 670 eV 59.5 keV
(1.1%) operated at -10°C
reduced CCE in inter-pixel gap
causes peak broadening
pixel leakage current slightly
higher than expected
W.R. Cook et al, Proc SPIE 3769 (1999) 92

17. Leicester/Surrey prototype CZT pixel array
ASIC pixel pitch
A prototype pixel detector for keV X-ray imaging - based on the Rockwell ASIC
Low noise current integrating ASIC, already available bonded to Si and Mercuric Cadmium Telluride (MCT)
reference

18. Other CZT pixel arrays
Marshall Space Centre - prototype 4x4 CZT pixel arrays wire bonded to discrete preamplifiers
CZT is 5 x 5 x 1 mm from eV products
750 mm pixel pitch, 650 mm pixel size
~ 2% FWHM at 59.5 keV
BICRON / LETI - aimed at 140 keV medical imaging
CZT from BICRON has 4.5 mm pixel size, 4 x 4 pixel module
module is 18 x 18 mm, 6 mm thick CZT
motherboard is 10 x 12 modules,
18 x 21.5 cm (1920 pixels)
motherboard is edge-buttable, up to
8 boards giving 43 x 72 cm active area
B. Ramsey et al, NIM A458 (2001) 55
C. Mestais et al, NIM A458 (2001) 62

19. CdTe Schottky diode detectors
Improved quality mono-crystalline CdTe material from Acrotec of Japan
In/p-type CdTe Schotty contact gives ~100x lower leakage than ohmic Pt/CdTe contact
High electric field minimises charge loss
Spectrum is 0.5mm thick CdTe at 800V, +5°C:
1.4 keV 122 keV (1.1%)
4 keV 511 keV (0.8%) 1
T. Takahashi et al, NIM A436 (1999) 111

20. Stack of CdTe detectors
0.5mm CdTe Schottky detectors offer <1% resolution at several hundred keV
Requires: charge drift time << charge trapping time
drift 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 5 x 0.5mm. 400V bias on each detector, at +5°C
Separate readout of each layer - use as a Compton scatter detector

21. ‘CdTe stack’ spectra from 133Ba
layer 6
top layer
layer 2
sum of layers 1-8

22. Other materials
A number of materials other than CZT/CdTe continue to develop:
very high-Z materials TlBr and HgI2 are of interest for hard X-ray and nuclear medicine imaging
intermediate-Z materials GaAs and InP have seen dramatic improvements in the purity of thick epitaxial material:
fano-limited performance has been shown in a small number of epitaxial GaAs detectors
diamond continues to make progress with increasing CCE - improvements in SiC material also look promising
a number of other materials have short term potential: for example, GaN, PbI2, and GaSe

23. InP detectors
InP is a direct bandgap semi-conductor - similar properties to GaAs
2-3x high stopping power, and higher electron drift velocities than GaAs.
Compensation is achieved using Fe as a deep acceptor: 0.65 eV below the conduction band edge.
Semi insulating InP grown by:
Fe dopant added to liquid melt (crystal doping)
Fe dopant diffused into each wafer from surface deposition (MASPEC process)
R. Fornari et al,
JAP 88/9 (2000)
Electron drift velocity

24. ESTEC InP detectors
InP performance is limited by leakage current and charge trapping: benefit from cooled operation:
ESTEC 180mm thick InP detectors, grown by Fe-doped Czochralski:
T = -60°C
T = -170°C
A. Owens et al., NIM A487 (2002)
Future developments need a blocking contact technology, and better material purity

25. Epitaxial GaAs
Epitaxial GaAs can be grown as high purity thick layers using chemical Vapour Phase Epitaxy (Owens - ESTEC, Bourgoin - Paris).
Photoluminescence mapping clearly shows the uniformity of epitaxial GaAs compared to semi-insulating bulk material:
Epitaxial GaAs
Bulk GaAs
H. Samic et al., NIM A 487 (2002)

26. GaAs pixels array detectors
GaAs pixel arrays have been flip-chip bonded and tested with several ASICs: Medipix (CERN), MPEC (Freiberg), Cornell.
LEC semi-insulating GaAs suffers from poor CCE due to low electric field close to the ohmic contact, and material non-uniformity
Software gain matching can correct for some pixel-to-pixel variations
Various commercial flip-chip bonding processes are compatible with GaAs, eg. tin-lead reflow
Future tests with thick epitaxial GaAs are more promising
Medipix pixel pitch is 170 mm, the inter-pixel gap is10 mm and bond pad size is 20 mm.
C. Schwarz et al., NIM A 466 (2001) 87
M. Lindner et al., NIM A 466 (2001) 63

27. Epitaxial GaAs detectors
Epitaxial GaAs (lightly n type) is generally grown on a n+ GaAs wafer substrate:
A Schottky contact is deposited on the front surface
The n+ substrate acts as the ohmic contact
C. Erd et al., NIM A 487 (2002)

28. High resolution GaAs spectrometers
Best results to date are from ESTEC with 400mm thick GaAs devices depleted to ~100mm, achieving as low as 465 eV FWHM at 59.5 keV:
A. Owens, JAP 85 (1999)

29. Spatial uniformity and Fano limit
The measured resolution of 468 eV FWHM is close to the intrinsic Fano noise limit (F=0.14) of 420 eV FWHM:

30. Conclusions
Prototype CZT pixel array detectors are becoming available:
sub-millimetre resolution X-ray imaging detectors for astronomy
4-5 millimetre resolution medical gamma cameras
Significant recent improvements in the supply of HPB/LPB CZT and CdTe is providing better quality large-area mono-crystalline material
Novel trapping-correction and 3D depth sensing techniques continue to develop for CZT and CdTe
Excellent spectral performance has been seen in a small number of samples of epitaxial GaAs, InP and TlBr from the ESTEC programme:
new sources of high purity epitaxial material is the key for future development
Excellent medium-term future for compound semiconductor imaging detectors

32. Acknowledgements
I am grateful to the many authors of published papers and private communications that have made this review possible