Characterization and evaluation of extended defects in CZT crystals for gamma-ray detectors

Material homogeneity is critical in achieving high-performance in all types of radiation detectors. This requirement is not inevitably satisfied in today's commercial detector-grade CdZnTe (CZT) material because it contains high concentrations of extended defects, in particular, Te inclusions, dislocation networks, and twin- and subgrain-boundaries that affect the energy resolution and the efficiency of the devices. Defects, such as grain boundaries and cracks that completely block charge-carrier transport are impermissible in CZT radiation-detectors at concentrations exceeding certain threshold values. Our group in Brookhaven National Laboratory (BNL) conducts systematic studies, detailing the roles of crystal defects in CZT detectors and the mechanisms underlying their formation and effects. We employ infrared transmission microscopy, white beam X-ray diffraction topography, and high-spatial-resolution X-ray response mapping to identify particular types of defects and reveal their relationship with the devices' performances. In this article, we summarize some of the most important results that our group obtained over the past 5 years.

► Material homogeneity is not satisfied in today's commercial detector-grade CdZnTe.
► Subgrain boundaries and secondary phases are the most critical defects in CZT detectors.
► They cause strong fluctuations in charge trapping and collected charge signals referred to as the material response non-uniformity.