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The ideal operation of CdZnTe devices entails having a uniformly distributed internal electric field. Such uniformity especially is critical for thick long-drift-length detectors, such as large-volume CPG and 3-D multi-pixel devices. Using a high-spatial resolution X-ray mapping technique, we investigated the distribution of the electric field in real devices. Our measurements demonstrate that in thin detectors, < 5 mm, the electric field-lines tend to bend away from the side surfaces (i.e., a focusing effect). In thick detectors, > 1 cm, with a large aspect ratio (thickness-to-width ratio), we observed two effects: the electric field lines bending away from or towards the side surfaces, which we called, respectively, the focusing field-line distribution and the defocusing field-line distribution. In addition to these large-scale variations, the field-line distributions were locally perturbed by the presence of extended defects and residual strains existing inside the crystals. We present our data clearly demonstrating the non-uniformity of the internal electric field. Index Terms--CdZnTe, crystal defects, radiation detectors.

Te inclusions existing at high concentrations in CdZnTe (CZT) material can degrade the performance of CZT detectors. These microscopic defects trap the free electrons generated by incident radiation, so entailing significant fluctuations in the total collected charge and thereby strongly affecting the energy resolution of thick (long-drift) detectors. Such effects were demonstrated in thin planar detectors, and, in many cases, they proved to be the dominant cause of the low performance of thick detectors, wherein the fluctuations in the charge losses accumulate along the charge's drift path. We continued studying this effect using different tools and techniques. We employed a dedicated beam-line recently established at BNL's National Synchrotron Light Source for characterizing semiconductor radiation detectors, along with an IR transmission microscope system, the combination of which allowed us to correlate the concentration of defects with the devices' performances. We present here our new results from testing over 50 CZT samples grown by different techniques. Our goals are to establish tolerable limits on the size and concentrations of these detrimental Te inclusions in CZT material, and to provide feedback to crystal growers to reduce their numbers in the material. Index Terms--CdZnTe, radiation detectors, Te inclusions.

We studied the effects of small, Index Terms--CdZnTe, semiconductor detectors, Te inclusions.

High-performance semi-insulating single crystals of n-type [Cd.sub.0.9][Zn.sub.0.1] Te (CZT) were grown using the traveling heater method (THM). X-ray and [gamma]-ray detector configurations fabricated from this material have a room-temperature mean energy resolution of 4.3% FWHM for a [sup.57]Co source (122 keV) and uniform pixel-to-pixei response on monolithic 20 x 20 x 5 [mm.sup.3] pixeilated detectors. Energy resolution of Index Terms--CdZnTe, CZT, homeland security, medical imaging, Optical Deep Level Transient Spectroscopy (ODLTS), pixellated detectors, Te inclusion, Traveling Heater Method.

Commercial 15 x 15 x 7.5 [mm.sup.3] coplanar grid CdZnTe detectors were studied on the micron-scale using a collimated high-energy X-ray beam provided by Brookhaven's National Synchrotron Light Source. This powerful tool enables simultaneous studies of detector response uniformity, electronic properties of the material, and effects related to the device's contact pattern and electric field distribution. The availability of a front-end Application Specific Integrated Circuit, developed at Brookhaven's Instrumentation Division, providing low noise amplification of grids and cathode signals, corresponding timing signal and adjustable relative gain, allowed to correlate performance mapping and fluctuations in collected charge. We observed the effect of the strip contacts comprising the coplanar grids on the energy resolution of the coplanar-grid device.

The dependence of the energy gap in Bridgman-grown [Cd.sub.1-x][Mn.sub.x]Te crystals on the Mn mole fraction and temperature is measured. New equations are formulated that have expressed the energy gap as a function of Mn mole fraction and temperature, where the energy gap has increased linearly with an increase in the Mn content in the crystal and with a decrease in temperature.

The analysis of photoconductive decay curves has helped in comparing the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. The studies have shown that not only the size but also the spatial configuration of the Te precipitates has modulated the carrier-transport properties.

Larger volume cadmium zinc telluride (CZT) crystals are developed by using high-resolution radiation detectors based on the traveling heater method. The results have shown that the theoretical energy resolution limit can be achieved with careful material selection by using infrared and high-quality fabrication processes.