Annealing as grown large volume CZT single crystals increased spectral resolution

The spectroscopic performance of current large-volume Cadmium 10% Zinc Telluride, Cd{sub 0.9}Zn{sub 0.1}Te, (CZT) detectors is impaired by cumulative effect of tellurium precipitates (secondary phases) presented in CZT single-crystal grown by low-pressure Bridgman techniques(1). This statistical effect may limit the energy resolution of large-volume CZT detectors (typically 2-5% at 662 keV for 12-mm thick devices). The stochastic nature of the interaction prevents the use of any electronic or digital charge correction techniques without a significant reduction in the detector efficiency. This volume constraint hampers the utility of CZT since the detectors are inefficient at detecting photons >1MeV and/or in low fluency situations. During the project, seven runs CZT ingots have been grown, in these ingots the indium dopant concentrations have been changed in the range between 0.5ppm to 6ppm. The I-R mapping imaging method has been employed to study the Te-precipitates. The Teprecipitates in as-grown CZT wafers, and after annealing wafers have been systematically studied by using I-R mapping system (home installed, resolution of 1.5 {micro}m). We employed our I-R standard annealing CZT (Zn=4%) procedure or two-steps annealing into radiation CZT (Zn=10%), we achieved the 'non'-Te precipitates (size 10{sup 9-10} {Omega}-cm. Wemore » believe that the Te-precipitates are the p-type defects, its reducing number causes the CZT became n+-type, therefore we varied or reduced the indium dapant concentration during the growth and changed the Te-precipitates size and density by using different Cd-temperature and different annealing procedures. We have made the comparisons among Te-precipitates size, density and Indium dopant concentrations, and we found that the CZT with smaller size of Te-precipitates is suitable for radiation uses but non-Te precipitates is impossible to be used in the radiation detectors, because the CZT would became un-dopant or 'intrinsic' with non radiation affection (we have studied before). We used 3 weeks annealing time for 3-5 mm thickness CZT wafres, if the thickness increased to 10-15mm, the annealing time would be increased to many months, which is very unpractical and very difficult to control the CZT property. We have obtained as-grown CZT by using adding the extra Cd before growth, which showed the smaller size of Te-precipitates and excellent radiation performance. These CZT has very high {micro}{tau}(e) >1 x 10{sup -2}cm{sup 2}/V, {rho} > 2 x 10{sup 10} {Omega}-cm, and the thickness could be up to 80-100mm. The energy resolution of the detector (thickness>10mm) at 662 keV is about 1.2% without any correction (2) and according to Aquila, the 0.5-0.8% resolution at 662 keV would be expected by using appropriated electronic correction.« less