Effects of Te inclusions on the performance of CdZnTe radiation detectors

Abstract

Abstract-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.

Array of virtual Frisch-grid CZT detectors with common cathode readout and pulse-height correction

“…For wafer #2, the data points located at the bottom of the distribution indeed follow a straight line, in agreement with the predictions given in Ref. [12]. From this, we conclude that the peak broadening due to Te inclusions in the detectors fabricated from wafer #1 (with small inclusions) is less than 0.8%.…”

Study on thermal annealing of cadmium zinc telluride (CZT) crystals

“…CZT's unique properties include a high average atomic number, large bandgap, and good electron-transport properties, along with the availability of materials with acceptable cross-sectional areas and thicknesses. These merits support high detection-efficiency and good energy-resolution when operating at room temperature [1][2][3][4]. However, largely, the development of CZT detectors still is limited by material defects, typically Te inclusions and dislocations.…”

“…However, largely, the development of CZT detectors still is limited by material defects, typically Te inclusions and dislocations. Using a delicate X-ray response micro-scale mapping system developed at National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), we demonstrated, for the first time, that Te inclusions are the major impediment to good energy resolution in long-drift detectors [3]. More recently, we recognized that extended defects, e.g., dislocations, are another important performance-limiting problem, because they also act as trapping centers and lower the carrier-transport properties [4].…”

Extended defects in As-grown CdZnTe

“…CZT crystals are transparent to infrared light, but such defects scatter this wavelength, so we can visualize them from contrast images inside the crystal. However, using this long wavelength limits to 1-2 μm the minimum size of the feature that can be recognized [9]. In these images, Te inclusions larger than 10 μm appear as relatively sharp triangular-or diamondshaped objects, depending on crystal's orientation; those smaller than 5 μm typically appear as blurred objects with surrounding haloes [10], which can be suppressed by passing the illuminating beam through a ground-glass diffuser in our IR transmission microscopy system.…”