MATERIAL PROPERTIES OF LARGE-VOLUME CADMIUM ZINC TELLURIDE CRYSTALS AND THEIR RELATIONSHIP TO NUCLEAR-DETECTOR PERFORMANCE

The material showing the greatest promise today for production of larg e-volume gamma-ray spectrometers operable at room temperature is cadmi um zinc telluride (CZT). Unfortunately, because of deficiencies in the quality of the present material, high-resolution CZT spectrometers ha ve thus far been limited to relatively small dimensions, which makes t hem inefficient at detecting high photon energies and ineffective for weak radiation signals except in near proximity. To exploit CZT fully, it will be necessary to make substantial improvements in the material quality. Improving the material involves advances in the crystallinit y, purity, carrier lifetimes, and control of the electrical compensati on mechanism. A more detailed understanding of the underlying material problems limiting the performance of CZT gamma-ray detectors is requi red; otherwise, problems with supply, delivery times, and unit cost of large-volume (>5 cm(3) active volume) CZT spectrometers are expected to continue. A variety of analytical and numerical techniques have bee n employed to quantify crystallinity, strain, impurities, compositiona l and stoichiometric variations, bulk and surface defect states, carri er mobilities and lifetimes, electric field distributions, and surface passivation. Data from these measurements were correlated with spatia l maps of the gamma-ray and alpha particle spectroscopic response, add feedback on the effectiveness of crystal growth and detector fabricat ion procedures has been generated. The results of several of these ana lytical techniques will be presented in this paper.