Gas Phase and Surface Kinetic Processes in Hot-Wire Chemical Vapor Deposition
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Gas Phase and Surface Kinetic Processes in Hot-Wire Chemical Vapor Deposition J. K. Holt, M. Swiatek, D. G. Goodwin, and Harry A. Atwater Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, CA 91125, U.S.A. ABSTRACT One- and two-dimensional numerical simulations have been used to determine the parameters critical to high rate growth of high quality polycrystalline silicon via hot-wire chemical vapor deposition at silane partial pressures of 1-70 mTorr and a wire temperature of 2000oC. The Direct Simulation Monte Carlo method [1] was used, including gas-phase chemistry relevant for growth. Model predictions agree both qualitatively and quantitatively with experimental measurements. INTRODUCTION Synthesis of large-grained polycrystalline silicon at low temperatures with high throughput is critical to enabling a future thin-film silicon photovoltaics technology. A promising approach for low temperature, high throughput film growth is hot-wire chemical vapor deposition (HWCVD). To this end, we are carrying out numerical simulations to explore the fundamental gas-phase and surface interactions of importance in HWCVD and to optimize growth conditions for growth rate, crystal quality, and process uniformity. The simulations use the Direct Simulation Monte Carlo (DSMC) technique, a particle-based method that is the most appropriate simulation method in transitional pressure regimes where gas mean free paths are larger than wire dimensions, but smaller than wire-to-substrate distances. Critical to any simulation of the HWCVD environment is the inclusion of gasphase chemistry, since the atomic silicon that leaves the wire is highly reactive and must be converted via gas-phase reactions to a less reactive precursor for high quality films (amorphous or polycrystalline) to result at low temperatures (under 400oC) [2]. Wire and surface chemistries were handled approximately by use of dissociative and reactive sticking probabilities, as described previously [2,3]. The present work aims to understand the relative roles in film growth of different gasphase species in different growth regimes. Species often cited as leading to “high-quality film growth”, such as SiH3, may only be effective in a certain range of film growth temperatures, for example. Another goal of this work is to characterize the effect of H2 dilution on the gas-phase and film surface morphology. Hydrogen itself is involved in a number of the gas phase reactions of interest, suggesting that it may impact the distribution of potential growth species. In addition, recent results [4,5] suggest an increase in grain size and reduction in nucleation density with the addition of H2. This is desirable for the particular photovoltaic applications in mind, since a lower grain boundary density will reduce the density of potential minority carrier trap sites.
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MODEL DESCRIPTION The DSMC technique is a direct, physical simulation technique whereby particles that each represent a large number of real molecules move, collide, re
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