High Quality CdTe Growth by Gradient Freeze Method
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HIGH QUALITY CdTe GROWTH BY GRADIENT FREEZE METHOD A.Tanaka, Y. Masa, S. Seto and T. Kawasaki Electronics Materials Laboratory, Sumitomo Metal Mining Co., Ltd. Suehiro-cho, Ohme-shi, Tokyo 198, Japan ABSTRACT The vertical Bridgman method, a gradient freeze technique, is feasible for the growth of high quality, large CdTe crystals. The equi-composition contour of Zn in doped crystals has been used to reveal the solid-liquid interface shape in the growth process. A slow cooling rate is necessary to obtain a convex interface shape. A low temperature gradient at the solid-liquid interface, down to 2 0 C/cm, and Zn doping are found to be effective to reduce the etch pit density to 5x103 /cm2 (minimum) to 10 4/cm2 (average). The crystallographic quality has been evaluated by means of X-ray diffraction, X-ray topography, etch pit delineation with the Nakagawa etchant, infrared measurements, photoluminescence and Hall effect measurements. CdTe crystals are found to be free from subgrain structure, Te precipitates and deep levels, and have high electron mobility. INTRODUCTION Cadmium telluride is one of the ideal substrates for HgCdTe (MCT) epitaxial layers because of the good lattice matching and wide band gap of CdTe. However, it is reported that CdTe crystals usually have a low angle subgrain structure, and a dislocation density of 10 5-10 6/cm2 [1-21. The minority carrier lifetime in MCT epitaxial layers decreases with increasing dislocation density of substrate when the density is above 10 5/cm2 [3]. Low dislocation density, subgrain-free, large CdTe substrates are therefore necessary to obtain devices of MCT epitaxial layers [4]. The vertical Bridgman (VB) method has been found to be most suitable to grow high-quality, large CdTe crystals because: 1) a crystal can be grown using a low temperature gradient near the growth interface; 2) stoichiometry control of the melt and the crystal are possible using a Cd or Te reservoir; 3) the growth rate is fast; 4) large crystals can be grown; and 5) the growth system is simple. The first reason is particularly important because a -low temperature gradient is one of the key factors in reducing dislocation density. In this paper, we report on the crystal growth of CdTe by the gradient freeze (GF) technique, a method to measure the shape of the growth interface, the factors influencing the dislocation density, and the properties of the crystals. CRYSTAL GROWTH OF CdTe A schematic diagram of the growth system is shown in Fig. 1. The starting materials, Cd and Te, were refined by sublimation and zone melting. The nominal purities of these materials were over 99.9999%. Charge materials were sealed in an evacuated quartz ampoule under a pressure of 10-6 torr. The inside wall of the ampoule was coated with pyrolytic carbon to avoid reaction of residual Cd-oxide with the quartz. CdTe poly crystals were synthesized from metallic Cd and Te in the ampoule. Growth conditions used in this investigation are summarized in Table I. The average growth rate was kept in the range of 0.6-5 mm/hr.
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