Advances in Compound Semiconductor Radiation Detectors a review of recent progress

1. Advances in Compound Semiconductor Radiation Detectorsa 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 14 1.12 3.6 ~104 Ge 32 0.66 2.9 50 InP 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

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 

7. Scanning of CCE vs depth using lateral Ion-beam induced charge microscopy A region of reduced CCE only seen near one contact. SEM and PL microscopy show a defect region only at one contact.A region of reduced CCE only seen near one contact. SEM and PL microscopy show a defect region only at one contact.

8. Induced signals due to charge drift

9. The coplanar grid detector

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 - 1.5 mm slit pitch:

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:

13. Correcting for electron trapping Knowing the depth of the interaction, spectral degradation due to electron trapping can be compensated for:

14. 3D pixel array detectors

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

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 FWHM @ 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

17. Leicester/Surrey prototype CZT pixel array

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

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 FWHM @ 122 keV (1.1%) 4 keV FWHM @ 511 keV (0.8%) 1

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

21. �CdTe stack� spectra from 133Ba

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 Electron drift velocity

24. ESTEC InP detectors

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:

26. GaAs pixels array detectors GaAs pixel arrays have been flip-chip bonded and tested with several ASICs: Medipix (CERN), MPEC (Freiberg), Cornell.

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

28. High resolution GaAs spectrometers

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