Growth and Characterization of High Quality, Low Defect, Subgrain Free Cadmium Telluride by a Modified Horizontal Bridgm
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GROWTH AND CHARACTERIZATION OF HIGH QUALITY, LOW DEFECT, SUBGRAIN FREE CADMIUM TELLURIDE BY A MODIFIED HORIZONTAL BRIDGMAN TECHNIQUE W. P. ALLRED , A. A. KHAN, C. J. JOHNSON, N. C. GILES AND J. F. SCHETZINA++ * Galtech Inc., North State Street, Mt. Pleasant, UT 84647 ** Washington State University, Dept. of ECE, Pullman, WA 99164-2752 + II-VI Inc., Saxonburg Boulevard, Saxonburg, PA 16506 ++ North Carolina State University, Dept. of Physics, Box 8202, Raleigh, NC 27695-8202 ABSTRACT A low stress modified horizontal Bridgman technique has been developed and used to grow low defect, large area, subgrain free CdTe crystals for use as substrates in the epitaxial growth of HgCdTe and related IR detector materials. CdTe wafers cut from horizontal Bridgman grown boules exhibit, resistivities in the 10 ohm-cm range. Etch pit counts are in the 10 cm-' range. Etch pit patterns as well as x-ray topographs indicate the absence of low-angle grain boundaries. Double crystal x-ray rocking curves are single peaked and very narrow with FWHM(333) as low as 9 arc-sec. Rocking curves of FWHM(333) = 9 to 15 arc-sec, measured at several different laboratories, have been obtained for CdTe wafers cut from several boules. This is in contrast to standard vertical Bridgman grown CdTe samples, which generally show broader x-ray rocking curves sometimes with multiple peaks as a result of subrgrain structure. Low temperature (1.6-4.5 K) photoluminescence (PL) measurements on these low defect samples reveal bright edge emission lines which are the main feature of the spectrum. Additional bound exciton lines and other sharp features associated with donor and acceptor impurities are also present. The very weak defect band luminescence (1.40-1.46 eV) provides additional evidence of sample quality. INTRODUCTION Cadmium Telluride is a technologically important member of the JI-VI family of compound semiconductors. It is used in the fabrication of electrooptic modulators, gamma ray detectors, laser optics and as epitaxial substrate for the growth of epitaxial mercury cadmium telluride. Cadmium telluride has been successful in these applications because of a unique combination of characteristics such as large mean atomic number (50) and reasonably high band gap (1.44) eV) [1]. It also has other characteristics such as low thermal conductivity, large bond lengths, low hardness (2.75 on Moh's scale [2]), which makes its bulk processing a very delicate task. The bulk crystals are normally polycrystalline and the cutting and polishing damage extends several microns beneath the polished surface. The single crystal substrates exhibit intensive dislocation networks and subgrain s~ructyre. The etch pit counts of commercially available CdTe is in the low 10' cm-' range. This material is generally grown by the vertical Bridgman (VB) method. We have investigated the growth of single crystal CdTe by the horizontal Bridgman (HB) technique. The early results of these growth efforts have been published elsewhere [3]. These preliminary results clearly indicated that i
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