Characterization of Extended Defects Observed in Cadmium Zinc Telluride (CZT) Crystal
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Characterization of Extended Defects Observed in Cadmium Zinc Telluride (CZT) Crystal
Samuel Uba1, Stephen Babalola1, 1
Alabama A&M University, Normal, AL 35811, U.S.A
Anwar Hossain2, Ralph James2, 2
Nonproliferation and national security, Brookhaven National Laboratory, Upton, NY 11973,
U.S.A
ABSTRACT Cadmium Zinc Telluride (CZT) semiconductor crystal properties have been studied extensively with a focus on correlations to their radiation detector performance. The need for defect-free CZT crystal is imperative for optimal detector performance. Extended defects like Tellurium (Te) inclusions, twins, sub-grain boundaries, and dislocations are common defects found in CZT crystals; they alter the electrical properties and, therefore, the crystal's response to high energy radiation. In this research we studied the extended defects in CZT crystals from two separate ingots grown using the low-pressure Bridgman technique. We fabricated several detectors cut from wafers of two separate ingots by dicing, lapping, polishing, etching and applying gold metal contacts on the main surfaces of the crystals. Using infrared (IR) transmission microscope we analyzed the defects observed in the CZT detectors, showing three dimensional scans and plot size distributions of Te inclusions, twins and sub-grain boundaries observed in particular regions of the CZT detectors. We characterized electrical properties of the detectors by measuring bulk resistivity and detector response to gamma radiation. We observed that CZT detectors with more extended defects showed poor opto-electrical properties compared to detectors with fewer defects.
INTRODUCTION CZT semiconductor detectors have diverse applications such as high energy scientific research in astrophysics, and nuclear physics, medical imaging in medicine, high energy radiation detector in portal monitoring stations for nonproliferation and homeland security, and microelectronics in industrial applications [1]. Its ability to detect high energy radiations is attributed to the properties of the CZT semiconducting material such as a high band gap energy value of 1.5ev, high density of 5.7 g/cm3 and high effective atomic number of the constituent atoms (Zeff ≈50). Despite these attractive properties, the performance of CZT detectors is impeded by structural heterogeneities like pipes, twinning, Te inclusions, and grain boundaries [2]. For crystal growers the task of growing CZT crystals is aimed at minimizing these structural heterogeneities during the growth process. Achieving this task we result in production of high-quality, and defect-free crystals which promote complete charge transport over a large distance [1].
EXPERIMENTAL DETAILS CZT crystals in the forms of ingots and wafers were provided by Brookhaven National Laboratory (BNL) and grown by Northrop Grumman Corporation (NGC). We selected wafers in different regions of two separate ingots 10-1 and 12-2 respectively and studied their defects. Wafers were prepared by mechanical polishing of the surfaces with silicon carbide abra
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