Bulk growth and characterization of cadmium zinc telluride crystals for mercury cadmium telluride infrared detector applications

Ergunt, Hasan Yasin
HgCdTe (MCT) infrared (IR) photodetectors have been used for various military and civilian applications including thermal imaging, medical imaging, and astronomy. These detectors are commonly fabricated on MCT layers grown on a foreign substrate epitaxially using delicate growth techniques such as Molecular Beam Epitaxy (MBE). The crystal quality of epitaxial layers grown on a substrate critically depends on the quality of the substrate. One of the stringent requirements in choosing the substrate material is the lattice match between grown layer and the underlying substrate. With perfectly matching lattice structure, CdZnTe has been the major substrate material for the MCT growth. The production of defect free single crystal CdZnTe bulk crystal has then attracted great attention among the research and industrial community of MCT based photodetectors. This thesis focuses on the growth and characterization of Cd1-xZnxTe crystals with the main objective of obtaining high-quality, CZT bulk crystal with large crystal sizes. To be compatible with the subsequent MCT growth, we aimed to obtain Cd0.96Zn0.04Te crystals with (211) crystal surface orientation. CdZnTe bulk crystal growths were performed in three-zone vertical Bridgman furnace by a high temperature melt process called “Modified Bridgman Technique”. Difficulties in both growth and characterization are presented and discussed in this report. Characterization of the grown CdZnTe crystal was performed to determine the crystallographic orientation, crystal quality, Zn distribution, IR transmission, resistivity, polarity, etch pit density, and surface properties. For this purpose, electron microscopy with analytical diagnostic tools like EDS and EBSD, XRD, optical transmission spectroscopy, and electrical measurement systems have been employed. We demonstrated the successful growth of single crystal CZT crystals using our simple Bridgman furnace. Physical properties of the grown crystal were very promising and encouraging for future applications. Crystal pieces having sizes larger than 5 x 5 mm2 with uniform Zn distribution and (211) surface orientation were obtained. IR transmission of nearly 60% which is as good as that of the commercial substrates was achieved. The electrical resistivity was much better (higher) than generally accepted values. However, the XRD results indicated the presence of defects and/or micro grains in the bulk crystal. These structures seemed to have prevented obtaining good FWHM values, which are the measure of crystal quality, in the XRD analysis.