Advances in Compound Semiconductor Radiation Detectors a review of recent progress

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Advances in Compound Semiconductor Radiation
Detectorsa review of recent progress

• P.J. Sellin
• Radiation Imaging Group
• Department of Physics
• University of Surrey

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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

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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 gt1013
• Also SiC, PbI2, GaSe

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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

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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)

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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
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Scanning of CCE vs depth using lateral Ion-beam
induced charge microscopy
Image of CCE using 1mm resolution 2MeV scanning
proton beam
400 V
-400V
Pulse height spectra as a function of depth
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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

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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
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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

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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
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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
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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
Resolution _at_356keV improves from 1.7 FWHM to
1.1 FWHM
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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

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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 lt1mm
spatial resolution
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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, lt 300 mW per pixel
• Spectral response
• achieved 670 eV FWHM _at_ 59.5 keV
• (1.1) operated at -10C
• 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
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Leicester/Surrey prototype CZT pixel array
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)
reference
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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
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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, 5C
• 1.4 keV FWHM _at_ 122 keV (1.1)
• 4 keV FWHM _at_ 511 keV (0.8)
• 1

T. Takahashi et al, NIM A436 (1999) 111
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Stack of CdTe detectors

• 0.5mm CdTe Schottky detectors offer lt1
  resolution at several hundred keV
• Requires charge drift time ltlt 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 5C Separate
readout of each layer - use as a Compton scatter
detector
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CdTe stack spectra from 133Ba
layer 6
top layer
sum of layers 1-8
layer 2
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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

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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) 5225-5229

• Electron drift velocity

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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
A. Owens et al., NIM A487 (2002) 435-440.
Future developments need a blocking contact
technology, and better material purity
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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) 107-112.
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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
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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.
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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) 7522-7527
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Spatial uniformity and Fano limit

• The measured resolution of 468 eV FWHM is close
  to the intrinsic Fano noise limit (F0.14) of 420
  eV FWHM

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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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Acknowledgements

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