Characterization of the charge transport uniformity of CdZnTe crystals for large-volume nuclear detector applications

Mounting evidence indicates that charge transport nonuniformity is one of the most important obstacles limiting the energy resolution of large-volume (≥1 cm³) CdZnTe nuclear detectors. Better understanding of the defects responsible for charge transport nonuniformity is needed to develop advanced crystal growth processes producing CdZnTe crystals with more uniform defect distribution and, hence, more uniform charge transport. Better material screening techniques are also required to determine the charge transport uniformity of the CdZnTe crystals at the ingot or slice level in order to maximize the detector fabrication yields in manufacturing large-volume CdZnTe nuclear detectors. Here, we present results from a program aimed at implementing a defect and charge transport uniformity characterization process to provide feedback to crystal growth process development efforts and material selection for large-volume CdZnTe detector manufacturing. We first focused on recent results suggesting that structural nonuniformity of CdZnTe crystals around Te inclusions is responsible for electron transport nonuniformity and diminished energy resolution of large-volume CdZnTe coplanar-grid detectors. The spatial size and density distribution of Te inclusions was measured with infra-red microscopy. A charge transport mapping station employing a collimated alpha source was used to study the uniformity of carrier transport in CdZnTe crystals.