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An irreversible deterioration of CdTe and CdZnTe detectors after heat treatments in the temperature range of 150 - 200 degrees Celsius was reported by several authors; however, the nature of the processes responsible for the detector degradation and increased dark currents is not fully understood. In this study we have prepared CdTe and CdZnTe detectors equipped with Au contacts. The detectors were tested before and after thermal annealing under vacuum. Using combined measurements of current voltage characteristics, low temperature photoluminescence and nuclear spectroscopic measurements, we have attempted to differentiate between the various possible contributions to the detector degradation and elucidate the defect formation process involved.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Vertical high pressure Bridgman (VHPB) was considered until now to be the most successful crystal growth method to produce CZT, for x- and gamma-ray detector crystals. Recently horizontal Bridgman (HB) CZT crystals produced by IMARAD Co. have also been successfully fabricated into nuclear spectroscopic radiation detectors. In view of our database of many years' study of the electrical properties of VHPB CZT grown and obtained from various sources we also studied the HB CZT crystals in order to compare the defects present in both different kinds of crystals grown by different methods. The VHB-grown samples were examined using thermoelectric emission spectroscopy (TEES), x- and gamma- ray spectroscopy and laser induced transient charge technique (TCT). The surface and the bulk crystalline homogeneity were mapped using triaxial double crystal x-ray diffraction and IR transmission spectroscopy. We have found a correlation between crystallinity, IR transmission microstructure an trapping times. Spectrometer grade VHPB CZT crystal exhibit trapping times of 20 microsecond(s) for electrons and 7 microsecond(s) for holes, however, regions, which were opaque to IR transmission, had trapping times shorter by one order of magnitude. The trapping times of HB CZT for electrons, were 10-15 micrometers . A similar trend has been observed on VHPB CZT crystals with poor crystallinity. The HB CZT crystals that we measured in this study had a crystallinity that was inferior to that of the best spectroscopic grade VHPB crystals.

We show that the finite size of pixels employed in arrays of the type produced by IMARAD Imaging Systems Ltd. determines the performance of these devices in terms of energy resolution and efficiency. The geometrical considerations are similar to those discussed in the context of small pixels. Based on an understanding of this geometrical effect we show that the performance of the arrays may be optimized for particular device dimensions. In addition we also so that material grown by IMARAD Imaging Systems Ltd. by the modified horizontal Bridgman method is quite uniform and indeed well suited for the fabrication of imaging systems.