Metallurgical Profile Modeling of Hg Corner LPE HgCdTe
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METALLURGICAL PROFILE MODELING OF Hg CORNER LPE HgCdTe BURT W. LUDINGTON Santa Barbara Research Center, 75 Coromar Dr.,
Santa Barbara,
CA
93117
INTRODUCTION Mercury cadmium telluride (HgI Cd Te) is an important material for infrared detector applications due to tde variable bandgap obtained through simply varying the mercury/cadmium ratio. Thus IR sensing systems with various wavelength requirements, typically from 2 to 12 wm, can potentially' all be supplied by one materials technology. Bulk crystal growth technology was originally pursued for these applications. However, large, high quality crystals are difficult to grow because of the high melting point of HgCdTe and the resulting high Hg vapor pressure. Liquid phase epitaxial (LPE) growth techniques can lower the growth temperature through the use of a solvent, and allow the deposition of thin films on large foreign substrates that are grown without any Hg. The HgCdTe alloy can be conceptualized as a binary compound of CdTe and HgTe. During the LPE growth process, changes in temperature cause the composition of the solid to change, because the segregation coefficients of the two alloys are different. Thus precipitate growth techniques that use cooling to drive growth, exhibit compositional variations in the grown layer. In addition to variations in x value due to the phase diagram, the HgCdTe system possess a large interdiffusion coefficient for Hg and Cd. This results in significant interdiffusion of Hg from the growing layer with Cd from the substrate at the growth times and temperatures commonly employed for LPE film growth. Interdiffusion will occur for any compositional gradient, and is important for heterojunction and superlattice based devices. The interdiffusion process cannot be modeled by simply error function solutions because the diffusion coefficient is
a strong function
of x value. Successful modeling of the growth process is important both as a test of scientific understanding, and as an engineering tool for material fabrication. Simulations of HgCdTe growth have been reported by both Shaw [I] and Zanio [2]. Shaw modeled an unstirred, Te rich melt with an approach that did not incorporate any interdiffusion. The model simulated the diffusion of solute to the growing layer, and could account for supercooled solutions. Zanio reported a model incorporating interdiffusion, that deposited material by the addition of sequential layers of pure CdTe and HgTe. The proper material composition was achieved by the addition of layers in the proper ratio, and by allowing interdiffusion to average out the composition variations. The model described here incorporates interdiffusion, and grows layers with the appropriate compositional value directly. SIMULATION Approach Due to the difficulty in solving the diffusion equations analytically when the diffusion coefficient is a function of concentration, a numerical approach was taken. The numerical approach forces the diffusion and growth processes to be treated as separate, sequential steps. The HgCdTe film i
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