Advances in Compound Semiconductor Radiation...
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Paul Sellin, Radiation Imaging Group
Advances in Compound Semiconductor RadiationDetectors
a review of recent progress
P.J. SellinRadiation Imaging GroupDepartment of Physics
University of Surrey
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Paul Sellin, Radiation Imaging Group
CZT/CdTe
Review of recent developments in compound semiconductordetectors:
r 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
r good results from CdTe Schottky diodes CdTe from a number of suppliers (eg. Acrotech, Eurorad, Freiburg)
r CZT/CdTe pixel array detectors under development: hard X-ray astronomical imaging gamma cameras for nuclear medicine
r custom ASICs for CZT/CdTe starting to appear
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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 50InP 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
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Paul Sellin, Radiation Imaging Group
Detection Efficiency
Vast majority of compund semiconductor detector development isdriven by improved photoelectric absorption for hard X-rays andgamma rays:
Exceptions are radiation hard detector programmes - SiC and DiamondPhoton energy (keV)
20 40 60 80 100 120 140
Det
ectio
n E
ffici
ency
(%)
0
20
40
60
80
100
SiGaAsCdTeInP
Calculated efficiencies for 500µm thick material
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Paul Sellin, Radiation Imaging Group
Material Quality in CdZnTe
High Pressure Bridgman CdZnTe is the new material of choice for medium resolution X-ray andgamma ray detection
Material suffers from mechanical defects - monocrystalline pieces are selected from wafers - nowhole-wafer availability
CZT material grown by High Pressure Bridgman from eV Products (Growth and properties ofsemi-insulating CdZnTe for radiation detector applications, Cs. Szeles and M.C. Driver SPIEProc. 2 (1998) 3446).
New growth methods have developed very recently - eg. Low Pressure Bridgman CZT fromYinnel Tech (US) and Imarad (Israel)
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Paul Sellin, Radiation Imaging Group
‘Hole tailing’ in a 5mm thickCdZnTe detector
Poor hole transport causes position-dependent charge collection efficiency
⇒ ‘hole tailing’ characteristic of higher energygamma rays in CdZnTe
GF Knoll, Radiation Detection andMeasurement, Ed. 3
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Paul Sellin, Radiation Imaging Group
Scanning of CCE vs depth using lateral Ion-beam induced charge microscopy
400 Vca
thod
e-400 V
cathode
Pulse height spectra as a function of depth+400 V -400V
Image of CCEusing 1µm
resolution 2MeVscanning proton
beam
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Paul Sellin, Radiation Imaging Group
Induced signals due to charge drift
In a planar detector thedrifting electrons andholes generate equaland opposite inducedcharge on anode andcathode
In CZT the holes arequickly trapped:
• hole component ismuch reduced
• interactions close tothe anode have lowCCEReviewed in Z. He et al,NIM A463 (2001) 250
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Paul Sellin, Radiation Imaging Group
The coplanar griddetector
Z
Coplanar electrodes produceweighting fields maximisedclose to the contacts
The subtracted signal fromthe 2 sets of coplanarelectrodes gives a weightingfield that is zero in the bulk
The subtracted signal is onlydue to electrons - generallyholes do not enter the sensitiveregion
First applied to CZT detectorsby Luke et al. APL 65 (1994)2884
cathode
anode 1
anode 2
holes electrons
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Paul Sellin, Radiation Imaging Group
Depth sensing
Coplanar CZT detectors provide depth position information:r signal from planar cathode ∝ distance D from coplanar anodes
and event energy Eγ :SC ∝ D x Eγ
r signal from coplanar anode is depth independent:SA ∝ Eγ
r 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:r γ-ray interaction depth allows correction to be made for residual
electron trappingr 3D position information is possible, for example useful for
Compton scatter cameras
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Paul Sellin, Radiation Imaging Group
Interaction Depth position resolution from CZT
Position resolution of ~1.1 mm FWHM achieved at 122 keVCollimated 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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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:
r the pixel signal is mainly insensitive to hole transportr depth dependent hole trapping effects are minimisedr the pixel signal decreases dramatically when the interaction
occurs close to the pixel - the ‘missing’ hole contributionbecomes important:
A. Shor et al, NIM A458 (2001) 47
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Paul Sellin, Radiation Imaging Group
Correcting for electron trapping
Knowing the depth of the interaction, spectral degradation due toelectron trapping can be compensated for:
Energy vs positionplot for 133Ba
spectrum:
Resolution @356keVimproves from 1.7%
FWHM to 1.1%FWHM
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Paul Sellin, Radiation Imaging Group
3D pixel array detectors
A 3D sensitive CZT pixel array has beendeveloped:• non-collecting guard rings plus small pixelsform a single-polarity sensing device• depth information allows pulse heightcorrections due to trapping and non-uniformity
Z. He et al., NIM A422 (1999) 173
The ‘coplanar grid’ detector acts as aform of 2D strip detector - with allelectrodes on one side of the device:• small pixel anodes are connectedorthogonally across ‘guard ring’ anodestrips• relatively complex design
V.T. Jordanov et al., NIM A458 (2001) 511
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Paul Sellin, Radiation Imaging Group
CZT/CdTe pixel array detectors
Outstanding issues:r CZT-compatible flip-chip bonding: low temperature indium or polymerr material uniformity and cost for large area arrays - requirement for large area
mono-crystalline CZT or CdTer motivation is astronomical X-ray imaging and nuclear medicine gamma ray
imaging
Goal for astronomy: 20x20mm active area with <1mm spatial resolution
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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 EnergyFocussing Telescope (HEFT):r 600 µm pixel pitch, 500 µm pixel sizer 8 x 7 x 2 mm CZT from eV productsr low power ASIC, < 300 µW per pixelSpectral response:r achieved 670 eV FWHM @ 59.5 keV
(1.1%) operated at -10°Cr reduced CCE in inter-pixel gap
causes peak broadeningr pixel leakage current slightly
higher than expected
W.R. Cook et al, Proc SPIE 3769 (1999) 92
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Paul Sellin, Radiation Imaging Group
Leicester/Surrey prototype CZT pixel array
reference 5120
5040
5120
4800
5120
5040Quadrant Q4:12x12 pixels400µm pitch
Quadrant Q1:32x32 pixels160µm pitch
Quadrant Q3:16x16 pixels320µm pitch
Quadrant Q2:21x21 pixels240µm pitch
A prototype pixel detector for 10 - 100 keVX-ray imaging - based on the Rockwell ASIC
Low noise current integrating ASIC, alreadyavailable bonded to Si and MercuricCadmium Telluride (MCT)
Pixel Pitch 40 µm
Pixel integration capacity 2 x 105 C
Pixel noise <20 electrons
Readout rate 2 MHz
Chip power dissipation <1 mW
ASIC pixel pitch
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Paul Sellin, Radiation Imaging Group
Other CZT pixel arrays
Marshall Space Centre - prototype 4x4 CZT pixel arrays wirebonded to discrete preamplifiers
r CZT is 5 x 5 x 1 mm from eV productsr 750 µm pixel pitch, 650 µm pixel sizer ~ 2% FWHM at 59.5 keV
BICRON / LETI - aimed at 140 keV medical imagingr CZT from BICRON has 4.5 mm pixel size, 4 x 4 pixel moduler module is 18 x 18 mm, 6 mm thick CZTr motherboard is 10 x 12 modules,
18 x 21.5 cm (1920 pixels)r motherboard is edge-buttable, up to
8 boards giving 43 x 72 cm active area
B. Ramsey et al, NIMA458 (2001) 55
C. Mestais et al, NIMA458 (2001) 62
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Paul Sellin, Radiation Imaging Group
CdTe Schottky diode detectors
r Improved qualitymono-crystalline CdTematerial from Acrotecof Japan
r In/p-type CdTe Schottycontact gives ~100xlower leakage thanohmic Pt/CdTe contact
r High electric fieldminimises charge loss
Spectrum is 0.5mm thickCdTe at 800V, +5°C:
r 1.4 keV FWHM @ 122keV (1.1%)
r 4 keV FWHM @ 511keV (0.8%)
1 T. Takahashi et al, NIM A436 (1999) 111
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Paul Sellin, Radiation Imaging Group
Stack of CdTe detectors
0.5mm CdTe Schottky detectors offer <1% resolution at severalhundred keV
Requires: charge drift time << charge trapping timedrift 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 5x 0.5mm. 400V bias on each detector,at +5°C
Separate readout of each layer - use asa Compton scatter detector
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Paul Sellin, Radiation Imaging Group
‘CdTe stack’ spectra from 133Ba
top layer
sum oflayers 1-8
layer 2
layer 6
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Paul Sellin, Radiation Imaging Group
Other materials
A number of materials other than CZT/CdTe continue to develop:
r very high-Z materials TlBr and HgI2 are of interest for hard X-rayand nuclear medicine imaging
r intermediate-Z materials GaAs and InP have seen dramaticimprovements in the purity of thick epitaxial material: fano-limited performance has been shown in a small number of
epitaxial GaAs detectors
r diamond continues to make progress with increasing CCE -improvements in SiC material also look promising
r a number of other materials have short term potential:for example, GaN, PbI2, and GaSe
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Paul Sellin, Radiation Imaging Group
InP detectors
Electric Field (kV/cm)
0 5 10 15 20 250.0
5.0e+6
1.0e+7
1.5e+7
2.0e+7
2.5e+7 GaAs electronsInP electrons
0.65 eV
1.35 eV
shallow donor impurity states
Fe deep acceptor
• InP is a direct bandgap semi-conductor - similar properties to GaAs• 2-3x high stopping power, and higherelectron drift velocities than GaAs.• Compensation is achieved using Feas a deep acceptor: 0.65 eV below theconduction band edge.
Electron drift velocity
Semi insulating InP grown by:• Fe dopant added to liquid melt(crystal doping)• Fe dopant diffused into eachwafer from surface deposition(MASPEC process)R. Fornari et al,JAP 88/9 (2000) 5225-5229
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Paul Sellin, Radiation Imaging Group
ESTEC InP detectors
InP performance is limited by leakage current and charge trapping: benefitfrom cooled operation:ESTEC 180µm thick InP detectors, grown by Fe-doped Czochralski:
T = -60°C T = -170°CFuture developments need a blocking contact technology, and better
material purity
A. Owens et al., NIMA487 (2002) 435-440.
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Paul Sellin, Radiation Imaging Group
Epitaxial GaAs
Epitaxial GaAs can be grown as high purity thick layers usingchemical Vapour Phase Epitaxy (Owens - ESTEC, Bourgoin - Paris).
Photoluminescence mapping clearly shows the uniformity ofepitaxial GaAs compared to semi-insulating bulk material:
H. Samic et al., NIM A 487 (2002) 107-112.
Epitaxial GaAs Bulk GaAs
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Paul Sellin, Radiation Imaging Group
GaAs pixels array detectors
GaAs pixel arrays have been flip-chip bonded and tested withseveral ASICs: Medipix (CERN), MPEC (Freiberg), Cornell.
C. Schwarz et al., NIM A 466 (2001) 87M. Lindner et al., NIM A 466 (2001) 63
LEC semi-insulating GaAs suffersfrom poor CCE due to low electricfield close to the ohmic contact,and material non-uniformity
Software gain matching cancorrect for some pixel-to-pixelvariations
Various commercial flip-chipbonding processes are compatiblewith GaAs, eg. tin-lead reflow
Future tests with thick epitaxialGaAs are more promising Medipix pixel pitch is 170 µm, the inter-pixel
gap is10 µm and bond pad size is 20 µm.
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Paul Sellin, Radiation Imaging Group
Epitaxial GaAs detectors
Epitaxial GaAs (lightly n type) is generally grown on a n+ GaAswafer substrate:
A Schottky contact is deposited on the front surfaceThe n+ substrate acts as the ohmic contact
C. Erd et al., NIM A 487 (2002) 78-89.
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Paul Sellin, Radiation Imaging Group
High resolution GaAs spectrometers
Best results to date are from ESTEC with 400µm thick GaAs devicesdepleted to ~100µm, achieving as low as 465 eV FWHM at 59.5 keV:
A. Owens, JAP 85 (1999) 7522-7527
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Paul Sellin, Radiation Imaging Group
Spatial uniformity and Fano limit
The measured resolution of 468 eV FWHM is close to the intrinsicFano noise limit (F=0.14) of 420 eV FWHM:
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Paul Sellin, Radiation Imaging Group
Conclusions
r Prototype CZT pixel array detectors are becoming available:
sub-millimetre resolution X-ray imaging detectors for astronomy
4-5 millimetre resolution medical gamma cameras
r Significant recent improvements in the supply of HPB/LPB CZT andCdTe is providing better quality large-area mono-crystalline material
r Novel trapping-correction and 3D depth sensing techniques continueto develop for CZT and CdTe
r Excellent spectral performance has been seen in a small number ofsamples of epitaxial GaAs, InP and TlBr from the ESTEC programme:
new sources of high purity epitaxial material is the key for futuredevelopment
r Excellent medium-term future for compound semiconductor imagingdetectors
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Paul Sellin, Radiation Imaging Group
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Paul Sellin, Radiation Imaging Group
Acknowledgements
I am grateful to the many authors of published papers and privatecommunications that have made this review possible