Thermal Annealing: A Technique to Improve the Performance of Cadmium Zinc Telluride (CZT) Material for Semiconductor Radiation Detector Applications

Extensive research was undertaken over the past 20 years to investigate the suitability of cadmium zinc telluride (CZT) crystals as a material for room-temperature nuclear-radiation detectors. Large-volume CZT crystals, with thicknesses up to 2 cm and large effective areas of roughly 5–10 cm2, are needed to fabricate efficient detectors that meet the working requirements of federal agencies, such as the DOE/NNSA (Department Energy National Nuclear Security Administration), Department of Homeland Security (DHS), and the Department of Defense (DOD). However, because of the imperfect methods for growing crystals, the resulting large-volume crystals most often are not perfect single ones, and contain structural defects such as voids, pipes, impurities from source materials, tellurium inclusions and precipitates, vacancies, and vacancy-impurity complexes generated during the process of their production. Other extended defects that may be present include grain boundaries, micro twins, and walls of dislocations (sub-grain boundaries). Identifying these defects, controlling their occurrence and eliminating them from the bulk CZT material currently are important tasks that will improve the yield of detector-grade crystals from ingots, and ultimately better their performance. In this study, we used a post-growth thermal annealing technique to remove the performance-limiting defects caused by tellurium inclusions and associated impurities in the CZT crystals. We realized a 66% ± 16% reduction in the size of the inclusions, with an overall elimination of 17% ± 2% of them. We believe that our experimental results offer a better understanding of the optimal annealing parameters, and of the dynamic properties of post-growth annealing processes.