B. A. Brunett

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.

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.

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.

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.

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.

Exploratory search for improved oxidizing agents used in the reduction of surface leakage currents of CdZnTe detectors

We have studied the ability of different oxidizing agents, other than H2O2 to reduce the surface leakage current of CdZnTe devices. All chemical treatments were performed in aqueous solutions, at room temperature, with weight percent concentrations of 2.5g/25ml. Before and after I-V curves were obtained. It was found that by increasing the basicity of the chemical treatment, greater reduction in surface leakage current occurred. The result show that these alternative chemical treatments reduced the surface leakage current as well as or better than H2O2 chemical treatment.

Ion microbeam studies of cadmium zinc telluride radiation detectors by IBICC

Abstract Ion beam induced charge collection (IBICC) and time resolved IBICC (TRIBICC) techniques were used for imaging electronic properties of cadmium zinc telluride (CZT) room temperature radiation detectors. The detectors were bombarded with a scanned 5.4 MeV He microbeam and the detector response was analyzed at each point. The electron mobility ( μ e ) and lifetime ( τ e ), and charge collection efficiency maps were calculated from the data. In order to determine the radiation damage to the detectors, the signal deterioration was measured as the function of dose.

Material Requirements for a Boron Phosphide Thermal Neutron Counter

Electrical characterization (current versus voltage and capacitance versus voltage) of nonstoichiometric amorphous boron phosphide Schottky diodes for neutron detection is presented. These results are incorporated in a Monte Carlo model of detector response to determine material requirements for a boron phosphide neutron counter.