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CdTe/CdZnTe in X- and g -ray Detection

CdTe/CdZnTe in X- and g -ray Detection. Risto Orava University of Helsinki and Helsinki Institute of Physics. San Diego 27. February 2002 GE- iXi. CdTe/CdZnTe is an attractive choice.

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CdTe/CdZnTe in X- and g -ray Detection

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  1. CdTe/CdZnTe in X- and g-ray Detection Risto Orava University of Helsinki and Helsinki Institute of Physics San Diego 27. February 2002 GE- iXi

  2. CdTe/CdZnTe is an attractive choice. Direct conversion  High signal and high spatial resolution Z(48/52) d=6 g/cm3 High stopping power low ionisation energy =4.43 eV r = 109 - 1011 cm  Room temperature operation band gap at 300K = 1.52 eV (mt)e- > 10-3 cm2/V  Detector grade material available DE(FWHM) of 3.5 keV@122 keV Moreover: II-IV infrared experience No. of presentations in X-/g-ray meetings 1991-2000: CdTe + CdZnTe: 1%  80% Si+PbI2+GaAs+ZnSe+ZnS+HgI2: 100%  20%

  3. Requirements in X-/g-ray imaging • applications include: • Large detector volumes ( several cm3’s) • Good homogenity of: • electrical properties • e- & hole transportation properties • crystallinity • Manufacturing technologies: • material processing • signal processing

  4. Challenges in Growing Detector Grade CdTe/CdZnTe Crystals: • high resistivity (109 - 1011cm): • residual impurities • compensation • hole transport mechanisms: • reduced mobility • crystallinity: • twins & grain boundaries • control of the growth process (size)

  5. Challenges in Integrating CdTe/CdZnTe into a Detector System: • processing: • metallisation, passivation • pixelisation • read-out contacts (flip chip bonding) • device technologies: • tiling (alignment) • read-out electronics

  6. CdTe vs. CdZnTe Growth Technique THM HPB Dopant Cl - Resistivity 3x109cm (3-9)x1010cm Structure large polycr large polycr Electron mobility (me) 1150cm2/Vs 1000cm2/Vs Hole mobility (mh) 110cm2/Vs 50cm2/Vs Electron lifetime(te) 2.0ms 1.0ms Hole lifetime(th) 2.0ms 1.0ms mete 2.3x10-3cm2/V 1.0x10-3cm2/V mhth 2.2x10-4cm2/V 5.0x10-5cm2/V Availability Eurorad,Acrotec eV prod, Imarad Imarad... ...

  7. Challenges in Growing CdTe/CdZnTe Crystals: • Te inclusions (THM) • Polycrystalline structure • Zn segregation • Inhomogenities • Special contact structure (CdZnTe) • Twins and grains • Impurity concentrations (HPB) • Supercooling effects (HPB)

  8. Need to select the detector grade material: • (1) Electrical properties: • resistivity mapping (variation <15%) • (2) Photoluminescence (dopant segregation): • constant Zn composition • low variation of deep levels, impurities, shallow levels • (3) X-ray diffraction: • crystallinity • conductivity • (4) X-ray photoelectron spectroscopy& Raman • spectroscopy • surface studies

  9. Challenges and Cures: Schottky CdTe detectors exhibit polarization effects when operated in room temperatures - these are removed by using ohmic contacts, higher bias voltage (>200V), and/or modest cooling. CdZnTe detectors exhibit charge/hole trapping phenomenon , this can cause reduction in signal amplitudes and afterglow - this is reduced by material selection and matching CdTe vs. CdZnTe according to the modalities. A hole trapping time dependent calibration can also be applied.

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