<title>Effect of twin boundaries on the spectroscopic performance of CdZnTe detectors</title>

Most single grains in cadmium zinc telluride (CdZnTe) grown by the high-pressure Bridgman (HPB) technique contain multiple twin boundaries. As a consequence, twin boundaries are one of the most common macroscopic material defects found in large area (400 to 700 mm z) CdZnTe specimens obtained from HPB ingots. Due to the prevalence of twin boundaries, understanding their effect on detector performance is key to the material selection process. Twin boundaries in several 2 mm thick large area specimens were first documented using infrared transmission imaging. These specimens were then fabricated into either 2 mm pixel or planar detectors in order to examine the effect of the twin boundaries on detector performance. Preliminary results show that twin boundaries, which are decorated with tellurium inclusions, produce a reduction in detector efficiency and a degradation in resolution. The extent of the degradation appears to be a function of the density of tellurium inclusions. In order to support these programs, the Materials Engineering Branch at NASA Goddard Space Flight Center has developed a process for mining large area specimens from HPB CdZnTe produced by eV Products, Inc. This mining process is based on the results of a correlation study between bulk material defects and detector performance) The process has yielded numerous single grain large area (>700 mm z) bulk specimens, of which approximately 70 percent contain one or more twin boundaries. Previous work using a x-ray spectral mapping system indicated that twins in several of the mined specimens did not produce a detectable degradation in detector performance. This result was obtained with detectors configured with large anode pixels (12.5 by 12.5 mm) and a collimated, but relatively non-monochromatic x-ray source. The contention within the CdZnTe community was that our inability to detect the twins was due to the large pixel size used for spectral mapping. The theory was that distortions in the electric field created by a twin would not show up in a large pixel device, as all the induced charge would still be collected. Whereas in a small pixel device, distortions in the electric field would produce charge channeling to adjacent pixels and result in an area of poorer performance. In this paper we describe our efforts to determine the effect of twins on detector performance and to gain insight into whether charge is channeled or trapped by twins.