T.E. Schlesinger

Mapping high-pressure Bridgman Cd/sub 0.8/Zn/sub 0.2/Te

Single crystals of Cd/sub 0.8/Zn/sub 0.2/Te grown at the Institute of Solid State Physics, Chernogolovka, Russia, by the high-pressure vertical Bridgman method (HPVB) were mapped using X-ray fluorescence (XRF), X-ray diffraction (XRD), photoluminescence (PL), and leakage current measurements, most of the Russian samples which we refer to as p-type CZT were more uniform in Zn composition than U.S. commercially produced material. The Russian material had a poorer crystallinity and, in the best case, could only count nuclear radiation. Differences in the material properties between Russian (p-type) and U.S. (n-type) material will be described.

Lead Iodide X-Ray and Gamma-Ray Spectrometers for Room and High Temperature Operation

In this study the authors report on the results of the investigation of lead iodide material properties. The effectiveness of a zone refining purification method on the material purity is determined by ICP-MS and ICP-OES and correlated to the electrical and physical material properties. They show that this zone refining method is very efficient in removing impurities from lead iodide, and they also determine the segregation coefficient for some of these impurities. Triple axis X-ray diffraction (TAD) analysis has been used to determine the crystalline perfection of the lead iodide after applying various cutting, etching and fabrication methods. The soft lead iodide crystal was found to be damaged when cleaved by a razor blade, but by using a diamond wheel saw, followed by etching, the crystallinity of the material was much improved, as observed by TAD. Low temperature photoluminescence also indicates an improvement in the material properties of the purified lead iodide. Electrical properties of lead iodide such as carrier mobility, were calculated based on carrier-phonon scattering. The results for the electrical properties were in good agreement with the experimental data.

Correlation of nuclear spectrometer performance with uniformity and resistivity in cadmium zinc telluride

We have used low-temperature photoluminescence (PL) spectroscopy and room-temperature photoluminescence mapping as a measure of composition variation in cadmium zinc telluride grown by high-pressure Bridgman. We have correlated the uniformity, as measured by the line width of the bound exciton peak in the low-temperature spectrum and the degree of variation in the peak position in the room-temperature map, with the peak-to-valley ratio of the 59.5 keV photopeak in the pulse-height spectrum of /sup 241/Am. For detectors having active areas of 10-30 mm/sup 2/, the PL measurements can be combined with resistivity measurements to give a strong predictor of detector performance. For arrays of 1 mm/sup 2/ detectors, the correlation between these material parameters and detector performance is much weaker. Our measurements show that material parameters and detector performance can vary substantially between points on a 1 cm sample.

The investigation of custom grown vertical zone melt semi-insulating bulk gallium arsenide as a radiation spectrometer

Vertical zone melt (VZM) bulk GaAs boules have been zone refined (ZR) and zone leveled (ZL) to reduce EL2 deep donor levels and impurity concentrations with the intent of improving properties for gamma ray detectors. ZR and ZL GaAs boules had background impurity levels and deep donor EL2 concentrations near or below detectable limits. The crystal mosaic of the material at locations near the seed end was slightly superior to commercial liquid encapsulated Czochralski (LEC) material, and nearly equivalent to commercial vertical gradient freeze (VGF) material. The crystal mosaic in ZL material degraded towards the tail end. The homogeneity of the electrical properties for the ZL and ZR VZM material was inferior compared to commercially available bulk GaAs material. Post growth annealing may help to homogenize some electrical properties of the material. The charge collection efficiency of the ZR GaAs detectors was only 30% maximum, and only 25% maximum for the ZL GaAs detectors. Resulting gamma ray spectra was poor from detectors fabricated with the ZL or ZR VZM material. Detectors fabricated from material that was both ZR and ZL did not demonstrate gamma ray resolution, and operated mainly as counters. The poor spectroscopic performance is presently attributed to the inhomogeneity of the electrical properties of the ZR and ZL GaAs materials. Comparisons are made with detectors fabricated from VGF SI bulk GaAs.

Multi-parameter high-resolution spatial maps of a CdZnTe radiation detector array

Resistivity results from a 48/spl times/48 pixelated CdZnTe (CZT) radiation detector array are presented alongside X-ray topography and detector mapping with a collimated gamma-ray beam. By using a variety of measurements performed on the same sample and registering each data set relative to the others, the spatial dependence of relationships between them was examined. The local correlations between resistivity and one measure of detector performance were strongly influenced by the positions of grain boundaries and other gross crystal defects in the sample. These measurements highlight the need for material studies of spatially heterogeneous CZT to record position information along with the parameters under study.

Optical and electrical characterization of copper- and chlorine-doped cadmium zinc telluride

The authors have used low-temperature photoluminescence spectroscopy and photo-induced current transient spectroscopy to study the properties of copper-doped Cd{sub 1{minus}x}Zn{sub x}Te with x {approx} 0.1 and chlorine-doped Cd{sub 1{minus}x}Zn{sub x}Te with x = 0.2, 0.35 and 0.5. The current-voltage characteristics and detector response were also measured. They observed variations in charge collection and resistivity in the Cu-doped samples which was correlated with variations in PICTS spectra. The Cl-doped material was found to have insufficient resistivity for detector operation.

Elementary analysis of line shapes and energy resolution in semiconductor radiation detectors

The authors have used an elementary statistical technique to derive a closed-form expression for the hole-tailing line shape produced by photoelectric absorption of monoenergetic radiation in a semiconductor X-ray/{gamma}-ray detector. In the case of compound semiconductors, where the drift length for electrons is much greater than that for holes, the line shape is given by a type of power law, except for a small region very near the photopeak. This analytical result agrees well with Monte Carlo simulations and is used to extract approximate {mu}{tau} products from a {sup 57}Co pulse height spectrum. They also present an expression for the maximum obtainable energy resolution of a semiconductor detector in the presence of leakage current noise and intrinsic statistical fluctuations as a function of material parameters, along with a chart of the optimal band gap as a function of temperature and photon energy. Based on these considerations, the optimal band gap for room-temperature operation is approximately 2.0 eV.

Mapping high pressure Bridgman Cd/sub 0.8/Zn/sub 0.2/Te from Russia

Mapping of single crystals of Cd/sub (1-x/)Zn/sub x/Te (x=0.2) grown in Russia by the high pressure vertical Bridgman method, (HPVBM) under argon pressure of 2 to 10 MPa was performed using X-ray fluorescence (XRF), X-ray diffraction (XRD), photoluminescence (PL) and leakage current uniformity. The crystals were more uniform in Zn composition than US produced material but had a poorer crystallinity and in the best case could only count nuclear radiation. Differences in the material properties between Russian and US material will be described.

Defects and impurities in mercuric iodine processing

In the fabrication of mercuric iodide HgI{sub 2} room temperature radiation detectors, as in any semiconductor process, the quality of the final device is very sensitive to the impurities and defects present. Each process step can change the effects of existing defects, reduce the number of defects, or introduce new defects. In HgI{sub 2} detectors these defects act as trapping and recombination centers, thereby degrading immediate performance and leading to unstable devices. In this work we characterized some of the defects believed to strongly affect detector operation. Specifically, we studied impurities that are known to be present in typical HgI{sub 2} materials. Leakage current measurements were used to study the introduction and characteristics of these impurities, as such experiments reveal the mobile nature of these defects. In particular, we found that copper, which acts as a hole trap, introduces a positively charged center that diffuses and drifts readily in typical device environments. These measurements suggest that Cu, and related impurities like silver, may be one of the leading causes of HgI{sub 2} detector failures.