Cadmium zinc telluride (CZT) is one of the most promising materials for the production of large-volume X-ray and gamma-ray spectrometers and imaging arrays operable at room temperature.  The performance of CZT devices, the global capacity for growth of detector-grade crystals, and the size of the commercial market have progressed steadily over the past 10 years.  Because of deficiencies in the quality of the material, commercial high-resolution CZT spectrometers are still limited to relatively small dimensions (< 5 cm³), which makes them inefficient at detecting high photon energies (> 1 MeV) and somewhat ineffective for weak radiation signals except in proximity to the source.  The detectors are very attractive for a much broader potential range of spectroscopic and imaging applications; however, increases in their efficiency are needed without sacrificing the ability to spectrally resolve gamma-ray energies.  To increase the detector efficiency for medical and space applications, the most common method has been to tile separate high-energy-resolution CZT detectors into a suitable mosaic array, although this approach comes at the expense of system cost and complexity. Achieving the goal of low-cost, efficient CZT detectors requires progress in the following areas: better uniformity of detector response, growth of large uniform single crystals, and improved device fabrication procedures.  For some medical applications such as digital X-ray radiography, it will also be necessary to understand and reduce the detrimental effects of device polarization, which is present only under high-excitation conditions. Despite the current material constraints, several types of electron-transport-only detectors have been developed: pixel, coplanar-grid, cross-strip, drift-strip, orthogonal coplanar strip, and virtual Frisch-grid, some of which are poised to address important applications. This talk summarizes the factors limiting the performance of CZT detectors, and it relates the defects observed in the crystals to growth and doping processes. It provides new insight into the critical role of small-scale defects on the energy resolution and efficiency of detectors. One of the main tools in these material and device characterizations is collimated synchrotron X-ray radiation provided by the National Synchrotron Light Source (NSLS). Data from the NSLS has helped to elucidate, in detail, the roles of non-uniformity and extended defects on the performance of CZT detectors, as well as the root cause of device polarization during exposure to a high flux of incident X-rays. Measurements of carrier traps within different detectors will be reported, including their nature and relationships to four growth methods (conventional vertical Bridgman, high-pressure Bridgman, traveling heater, and floating zone methods) and post-growth thermal processing. Most findings will be correlated with the performance of spectrometer-grade CdZnTe X-ray and gamma detectors, and new directions to resolve the material deficiencies will be offered.

Brief Summary (Invited – Michael Schieber Session): This presentation will summarize recent progress to understand the factors limiting the performance of current cadmium zinc telluride (CZT) gamma-ray detectors and discuss ways to overcome the material defects through appropriate corrections in the growth and post-growth thermal annealing processes.

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1. Material properties limiting the use of cadmium zinc telluride X- and gamma detectors
2. Factors Limiting the Performance of Large-Volume CZT Detectors