Crystallographic and Metallurgical Characterization of Radiation Detector Grade Cadmium Telluride Materials
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CRYSTALLOGRAPHIC AND METALLURGICAL CHARACTERIZATION OF RADIATION DETECTOR GRADE CADMIUM TELLURIDE MATERIALS
C.J. JOHNSON*, E.E. EISSLER*, S.E. CAMERON* Y. KONG", S. FAN**, S. JOVANOVIC** AND K.G. LYNN** * eV PRODUCTS, div. of II-VI Incorporated, 375 Saxonburg Blvd., Saxonburg, PA 16056 ** Brookhaven National Laboratory, Department of Physics, Upton, NY 11973
ABSTRACT
Radiation detector grade CdTe crystals are characterized by several crystallographic and metallurgical techniques including infrared microscopy, dislocation etch pitting and X-ray diffraction. Results are presented for 50 detectors fabricated from an ingot produced by the high pressure Bridgman method. Data on the temperature dependence of leakage current and pulse height analysis are presented, along with measurements of room temperature charge transport properties. Attempts to relate crystal structure to detector performance will be discussed.
INTRODUCTION
The performance of cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) materials as room temperature radiation detectors is mainly limited by charge transport properties. These properties depend critically on the quality of crystals that can be grown. The purpose of this work is to investigate macro- and micro-structure relationships to charge transport characteristics and device performance. We have initiated this effort with the binary alloy CdTe and will progress to the ternary CdZnTe at a later date. Our ultimate objective is to better understand crystal growth and device fabrication, which in turn will support more predictable manufacturing and practical application of CdTe/CdZnTe radiation detectors. In the present work, we grew a CdTe crystal (HPA008) by the high pressure Bridgman (HPB) method. Crystallographic and metallurgical properties of the as-grown ingot and cut slices were characterized using optical microscopy, laser back reflection orientation, infrared (IR) microscopy, dislocation etch pitting and X-ray diffraction. A total of 51 detectors was fabricated from three slices and the performance of 50 units was evaluated (1 detector was broken prior to testing). Results of IV characterization, pulse height analysis and charge transport studies are under investigation in search of structure/performance correlations. IR microscopy has previously been used to identify numerous inclusions and precipitates in CdTe materials, particularly in the study of substrates for HgCdTe epitaxyl 2. Dislocation etch pitting has been extensively developed for this same application and some correlation has been established between specific dislocation features and epitaxial HgCdTe IR detector performance 3. Previous studies of CdTe/CdZnTe crystallinity by X-ray reflection or transmission topography and by X-ray double crystal rocking curve measurements have identified a number of structural defects but correlations to device performance have been elusive4,5. We intend to focus these characterization techniques, primarily applied to IR substrates in the past, on the assessment of CdTe/CdZnTe
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