Advances in Compound Semiconductor Radiation Detectors. a review of recent progress

Transcription

1 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 E G W ρ i at RT (ev) (ev/ehp) (Ω) Si ~10 4 Ge InP 49/ GaAs 31/ CdTe 48/ CdZn 0.2 Te 48/ HgI 2 80/ TlBr 81/ Diamond >10 13 Also: SiC, PbI 2, GaSe

4 Detection Efficiency Vast majority of compund semiconductor detector development is driven by improved photoelectric absorption for hard X-rays and gamma rays: 100 Calculated efficiencies for 500µm thick material Detection Efficiency (%) Si GaAs CdTe InP Photon energy (kev) 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 positiondependent charge collection efficiency hole tailing characteristic of higher energy gamma rays in CdZnTe GF Knoll, Radiation Detection and

7 Scanning of CCE vs depth using lateral Ionbeam induced charge microscopy cathode cathode 400 V -400 V Image of CCE using 1µm resolution 2MeV scanning proton beam +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 anode 1 Coplanar electrodes produce weighting fields maximised close to the contacts cathode 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 holes electrons anode 2 First applied to CZT detectors by Luke et al. APL 65 (1994) 2884

10 Depth sensing Coplanar CZT detectors provide depth position information: signal from planar cathode distance D from coplanar anodes and event energy E γ : S C D x E γ signal from coplanar anode is depth independent: S A E γ so the depth is simply obtained from the ratio: D = S C / S A Z. He et al, NIM A380 (1996) 228, NIM A388 (1997) 180 Benefits of this method: γ-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 133 Ba 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 Outstanding issues: CZT/CdTe pixel array detectors 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 µm pixel pitch, 500 µm pixel size 8 x 7 x 2 mm CZT from ev products low power ASIC, < 300 µw 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 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) ASIC pixel Pixel pitch Pitch 40 µm Pixel integration capacity 2 x 10 5 C Pixel noise <20 electrons Readout rate 2 MHz Chip power dissipation <1 mw Quadrant Q4: 12x12 pixels 400µm pitch Quadrant Q2: 21x21 pixels 240µm pitch 5 Quadrant Q1: 32x32 pixels 160µm pitch Quadrant Q3: 16x16 pixels 320µm pitch 5 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 µm pixel pitch, 650 µm pixel size ~ 2% FWHM at 59.5 kev B. Ramsey et al, NIM A458 (2001) 55 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 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 133 Ba top layer layer 6 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 HgI 2 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, PbI 2, and GaSe

23 InP detectors InP is a direct bandgap semiconductor - 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 ev 1.35 ev shallow donor impurity states Fe deep acceptor e+7 GaAs electrons InP electrons e+7 e+7 e+7 e Electron drift velocity 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)

24 ESTEC InP detectors np performance is limited by leakage current and charge trapping: benefit rom cooled operation: STEC 180µm thick InP detectors, grown by Fe-doped Czochralski: T = -60 C A. Owens et al., NIM A487 (2002) T = -170 C 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 µm, the inter-pixel gap is10 µm and bond pad size is 20 µm. 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 400µm thick GaAs devices depleted to ~100µm, 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

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