J.M. Van Scyoc

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.

Evaluation of NH4F/H2O2 Effectiveness as a Surface Passivation Agent for Cd1-xZnxTe Crystals

Various passivating agents that reduce the surface leakage current of CZT crystals have been previously reported. In none of the studies, NH4F/H2O2 was identified as a promising passivation agent for CZT. We now present a study that includes the effect of NH4F/H2O2 treatment on the surface properties and detector performance. An elemental depth profile was obtained via Auger Electron Spectroscopy. Furthermore, X-ray Photoelectron Spectroscopy acquired at different processing times to identify the chemical states of the elemental species that composed the dielectric layer. It was found that the NH4F/H2O2 surface passivation significantly improved the sensitivity and energy resolution of CZT detectors. Furthermore, the NH4F/H2O2 treatment did not attack the Au electrodes, which eliminated the need to protect the contacts in the detector fabrication process.

Material uniformity of CdZnTe grown by low-pressure bridgman

Abstract We have employed Low-Temperature Photoluminescence (LTPL) and Room-Temperature Photoluminescence Mapping (RTPLM) to explore the crystalline quality and material uniformity of Cadmium Zinc Telluride (CZT) radiation detector material grown by the Low-Pressure Bridgman (LPB) technique. We report on the differences in crystalline quality and uniformity of material supplied by eV Products Inc. and IMARAD Imaging Systems Ltd. In addition, we have examined the general detector response of the material supplied by IMARAD. We report on the uniformity of the detector response and the temperature dependence of this response when used as a detector.

Material properties and room-temperature nuclear detector response of wide bandgap semiconductors

Several semiconductor materials for room-temperature X-ray and gamma-ray detectors, including HgI2, Cd1−xZnxTe (CZT), GaAs, and Pbl2 have been studied at Sandia National Laboratories, California. A comparison of the spectral response of these detectors will be given and related to material properties, such as charge carrier drift length, crystal purity, structural perfection, and material stoichiometry, as well as to the crystal growth techniques and device fabrication processes published elsewhere. Room-temperature detector spectral responses for each of these materials are presented, for photon energies in the range of 5.9 to 662 keV. CZT and HgI2 detectors demonstrate excellent energy resolution over the entire energy range, while PbI2 detectors exhibit reasonable response only up to about 30 keV. Some of the semi-insulating GaAs detectors fabricated from vertical gradient freeze materials show good spectral resolution for lower energies up to ∼60 keV, whereas other SI-GaAs detectors studied at Sandia function only as counters. Finally, some predictions on the future materials development of these wide bandgap semiconductors for room-temperature radiation detector applications will be discussed.

CdZnTe pixel array detectors and implications for producing large volume gamma-ray spectrometers

In this article we show that the behavior of new devices fabricated from horizontal Bridgman CdZnTe reported recently can be explained by the same conventional electrostatics arguments used to analyze detectors made from high pressure Bridgman material, and no qualitative differences in the material or contacts are necessary to explain the behavior of these new devices. Our work is an extension of the results obtained with a similar device geometry fabricated on high pressure Bridgman material. In addition we discuss the possibility of extending the design concepts learned here to fabricate large volume spectrometers based on a pixel array design.

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.

The spatial response of CdZnTe gamma-ray detectors as measured by gamma-ray mapping

Abstract We have developed a system to measure the spatial response of cadmium zinc telluride (CZT) radiation detectors. Using this system we have measured the response of several novel detector designs including several variations of the unipolar design. We have observed a wide range of energy resolution and efficiency among the different device designs. Each design has unique strengths and weaknesses which affect the device performance. In addition to design effects on performance, several instances of poor material uniformity degrading the device performance have been observed. In this paper we will discuss the spatial detector response focusing on the effects of the detector design. Where appropriate, we will also discuss the observed effects of material uniformity on device performance.

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.