Recent advances in compound semiconductor radiation detectors

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  • A region of reduced CCE only seen near one contact. SEM and PL microscopy show a defect region only at one contact.
  • Recent advances in compound semiconductor radiation detectors

    1. 1. Paul Sellin, 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. 2. Paul Sellin, Radiation Imaging Group 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. 3. Paul Sellin, Radiation Imaging Group 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 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. 4. Paul Sellin, Radiation Imaging Group 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. 5. Paul Sellin, Radiation Imaging Group 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. 6. Paul Sellin, Radiation Imaging Group ‘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. 7. Paul Sellin, Radiation Imaging Group Scanning of CCE vs depth using lateral Ion- beam induced charge microscopy 400 Vcathode -400 V cathode Pulse height spectra as a function of depth +400 V -400V Image of CCE using 1µm resolution 2MeV scanning proton beam
    8. 8. Paul Sellin, Radiation Imaging Group 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. 9. Paul Sellin, Radiation Imaging Group The coplanar grid detector Z 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 cathode anode 1 anode 2 h o l e s e l e c t r o n s
    10. 10. Paul Sellin, Radiation Imaging Group Depth sensing Coplanar CZT detectors provide depth position information:  signal from planar cathode ∝ distance D from coplanar anodes and event energy Eγ : SC ∝ D x Eγ  signal from coplanar anode is depth independent: SA ∝ Eγ  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:  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. 11. Paul Sellin, Radiation Imaging Group 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: Z. He et al, NIM A388 (1997) 180
    12. 12. Paul Sellin, Radiation Imaging Group 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. 13. Paul Sellin, Radiation Imaging Group 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: Resolution @356keV improves from 1.7% FWHM to 1.1% FWHM
    14. 14. Paul Sellin, Radiation Imaging Group 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. 15. Paul Sellin, Radiation Imaging Group 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. 16. Paul Sellin, Radiation Imaging Group 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 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 W.R. Cook et al, Proc SPIE 3769 (1999) 92
    17. 17. Paul Sellin, Radiation Imaging Group Leicester/Surrey prototype CZT pixel array reference A prototype pixel detector for 10 - 100 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 pitch
    18. 18. Paul Sellin, Radiation Imaging Group 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 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. 19. Paul Sellin, Radiation Imaging Group 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 T. Takahashi et al, NIM A436 (1999) 111
    20. 20. Paul Sellin, Radiation Imaging Group 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. 21. Paul Sellin, Radiation Imaging Group ‘CdTe stack’ spectra from 133 Ba top layer sum of layers 1-8 layer 2 layer 6
    22. 22. Paul Sellin, Radiation Imaging Group 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. 23. Paul Sellin, Radiation Imaging Group InP detectors Electric Field (kV/cm) 0 5 10 15 20 25 0.0 5.0e+6 1.0e+7 1.5e+7 2.0e+7 • 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. 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) 5225-5229
    24. 24. Paul Sellin, Radiation Imaging Group ESTEC InP detectors InP performance is limited by leakage current and charge trapping: benefit from cooled operation: ESTEC 180µm thick InP detectors, grown by Fe-doped Czochralski: T = -60°C T = -170°C Future developments need a blocking contact technology, and better material purity A. Owens et al., NIM A487 (2002) 435-440.
    25. 25. Paul Sellin, Radiation Imaging Group 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: H. Samic et al., NIM A 487 (2002) 107-112. Epitaxial GaAs Bulk GaAs
    26. 26. Paul Sellin, Radiation Imaging Group GaAs pixels array detectors GaAs pixel arrays have been flip-chip bonded and tested with several ASICs: Medipix (CERN), MPEC (Freiberg), Cornell. C. Schwarz et al., NIM A 466 (2001) 87 M. Lindner et al., NIM A 466 (2001) 63 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.
    27. 27. Paul Sellin, Radiation Imaging Group 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) 78-89.
    28. 28. Paul Sellin, Radiation Imaging Group 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) 7522-7527
    29. 29. Paul Sellin, Radiation Imaging Group 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. 30. Paul Sellin, Radiation Imaging Group 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
    31. 31. Paul Sellin, Radiation Imaging Group
    32. 32. Paul Sellin, Radiation Imaging Group Acknowledgements I am grateful to the many authors of published papers and private communications that have made this review possible

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