J. Toney

Cadmium zinc telluride and its use as a nuclear radiation detector material

We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

Optimal bandgap variants of Cd1−xZnxTe for high-resolution X-ray and gamma-ray spectroscopy

We show that the trade-off between noise and charge generation statistics in Cd1−xZnxTe leads to an optimal band gap of approximately 2.0 eV at room temperature. This implies a ZnTe fraction of approximately 0.7–0.8. We show that for X-rays and relatively low energy gamma-rays Cd0.2Zn0.8Te theoretically offers a significant potential improvement in energy resolution over Cd0.9Zn0.1Te even if compensation of shallow levels is less complete and carrier lifetimes are an order of magnitude lower for the higher x variant. We also show that these calculations are consistent with observed detector performance reported by many workers over a large period of time.

Uniformity of Cd1 − xZnxTe grown by high-pressure Bridgman

We have employed both low-temperature photoluminescence (PL) spectroscopy and high-spatial-resolution, room-temperature PL mapping to determine composition variation in Cd1 − xZnxTe grown by high-pressure Bridgman. Composition variations Δx of approximately 5–10% are observed between tip and heel of an approximately 13 cm long boule, while fluctuations of 1–2% are observed within a single, detector-scale sample. We also show that there are great discrepancies in the calculated zinc concentration, depending on which expression for Eg(x) is chosen from the literature. We have performed high-resolution, triaxial X-ray analysis on selected samples to determine which of the published relations is most accurate for our material. We have also examined the relationship between the low-temperature PL spectrum and detector performance, and found that the line width of the donor-bound-exciton peak can be used as a predictor of performance.

CHARACTERIZATION OF LEAD IODIDE FOR NUCLEAR SPECTROMETERS

Abstract We report on the results of a number of investigations into the material properties of lead iodide and their relation to x- and gamma-ray spectrometers. The effectiveness of zone refining as determined by inductively coupled plasma optical emission spectroscopy is demonstrated. We show that zone refining is effective in producing lead iodide with a higher degree of purity in terms of extrinsic dopants and we determine the segregation coefficients for a number of these impurities. Low temperature photoluminescence also indicates an improvement in the material properties of the purified lead iodide. The chemical etching characteristics, including etch rates, of lead iodide are presented for a number of etching solutions. Triple axis x-ray diffraction measurements have been employed to determine the structural perfection of the lead iodide after diamond sawing and etching and recovery of the crystallinity of the material is seen after Nal etching.

Growth and characterization of p-type Cd1 − xZnxTe (x = 0.2, 0,3, 0.4)

Abstract Single crystals of p-type semi-insulating Cd1 − xZnxTe (CZT) with x = 0.2, 0.3 and 0.4 were grown using the high-pressure vertical Bridgman method. The crystals showed a high degree of axial homogeneity over most of the length of the ingot. The crystals were characterized by measuring the specific resistivity, X-ray spectral analysis, triaxial rocking curves, photoluminescence, etch-pit density and photocurrent kinetics. Despite the high homogeneity the photocurrent kinetics are worse than n-type CZT (x = 0.1 and 0.2) also grown by high-pressure vertical Bridgman.

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.

Material inhomogeneities in Cd 1−x Zn x Te and their effects on large volume gamma-ray detectors

Cadmium zinc telluride (Cd1−x ZnxTe or CZT) has shown great promise as a material for room temperature x-ray and gamma-ray detectors. In particular, polycrystalline material grown by the high pressure Bridgman method with nominal Zn fraction (x) from 0.1 to 0.2 has been used to fabricate high resolution gamma-ray spectrometers with resolution approaching that of cooled high-purity Ge. For increased sensitivity, large areas (> 1 cm2) are required, and for good sensitivity to high energy gamma photons, thick detectors (on the order of 1 cm) are required. Thus, there has been a push for the development of CZT detectors with a volume greater than 1 cm3. However, nonuniformities in the material over this scale degrade the performance of the detectors. Variations in the zinc fraction, and thus the bandgap, and changes in the impurity distributions, both of which arise from the selective segregation of elements during crystal growth, result in spectral distortions. In this work, several materials characterization techniques were combined with detector evaluations to determine the materials properties limiting detector performance. Materials measurements were performed on detectors found to have differing performance. Measurements conducted include infrared transmission, particle induced x-ray emission, photoluminescence, and triaxial x-ray diffraction. To varying degrees, these measurements reveal that “poor-performance” detectors exhibit higher nonuniformities than “spectrometer-grade” detectors. This is reasonable, as regions of CZT material with different properties will give different localized spectral responses, which combine to result in a degraded spectrum for the total device.

Homogeneity of CdZnTe detectors

Abstract We describe the current state of nuclear radiation detectors produced from single crystals of Cd 1− x Zn x Te(CZT), with 0.04 x

Modeling and simulation of uniformity effects in Cd/sub 1-x/Zn/sub x/Te gamma-ray spectrometers

Monte Carlo simulations of pulse height spectra for Cd/sub 1-x/Zn/sub x/Te detectors are used to investigate the effect of variations in alloy composition and carrier drift lengths on energy resolution. The results, which are based on a simple phenomenological model, show that these nonuniformities can have significant detrimental effects on spectrometer performance. For the case of Bridgman-grown material, the orientation of the growth axis relative to the detector axis is shown to be an important consideration, especially for crystals which come from the heel end of a boule, where the composition gradient due to zinc segregation is large. Other effects which we have simulated include growth striations, zinc segregation at grain boundaries, and trapping by inclusions and grain boundaries; each of these effects is detrimental to energy resolution. We conclude that material nonuniformity is a major obstacle to achieving statistically limited energy resolution in cadmium zinc telluride detectors.

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.