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.
Cadmium zinc telluride (CdZnTe) is being developed for room- temperature x-ray and gamma ray detectors. Identification and control of point defects and charge compensators are currently important issues. We have used electron paramagnetic resonance (EPR) and photo-induced EPR to evalute shallow-donor defects in CdZnTe crystal grown by two different techniques. Samples grown by the high-pressure Bridgman technique and a crystal grown by horizontal Bridgman at IMARAD and doped with indium were included in this study. Prior to the EPR investigations, we performed liquid-helium photoluminescence (PL) in order to examine the radiation recombination paths and identify the presence of other defects in these crystals. Spectra were obtained showing sharp excitonic lines, shallow and deep DAP emission bands, and a deeper 1.1 eV emission. The PL data help define the optical excitation range used in photo-EPR measurements. The photo-EPR data obtained from our samples is used to determine the concentration of isolated donor centers, while the EPR signal present under no illumination gives a measure of the net compensation. We also report the excitation wavelength dependence of the isotropic EPR signal from the shallow donors.
Various types of precipitates and grain boundaries have been studied in Cd1-xZnxTe (CZT). In this study we used elemental analysis methods such as scanning electron microscopy (SEM), microprobe analysis, inductively coupled plasma mass spectroscopy (ICP/MS) and the new laser ablation ICP/MS methods. Transient charge technique was applied for the first time of CZT crystals for evaluating the electrical transport properties of semiconductors. Another method, IR transmission spectroscopy, enables us to evaluate the microstructure defects and then to correlate this with impurity level and electrical properties in order to have a better understanding of the requirements to improve the yield for large volume CZT spectrometers. We have evaluated crystals from the former Soviet Union, which have high concentration of defects. Precipitates and grain boundaries rich with carbon were observed in CZT crystals. Electrical transport properties such as (mu) (tau) (mobility-lifetime product) were measured and correlated with the chemical physical defects, as observed by IR transmission. On crystals rich with many microstructures, as shown by IR transmission, lifetimes below 1 microsecond(s) were measured, compared with 5 - 15 microsecond(s) on the detector grade materials. SEM and microprobe analysis performed on the precipitates gave high values of carbon. However, using laser ablation ICP/MS, a value in the range of 200 - 800 ppm for carbon was measured.
Conference Committee Involvement (1)
Hard X-Ray and Gamma-Ray Detector Physics V
4 August 2003 | San Diego, California, United States
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