Radical Species Distributions in Hot-Wire Chemical Vapor Deposition Probed Via Threshold Ionization Mass Spectrometry an
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Radical Species Distributions in Hot-Wire Chemical Vapor Deposition Probed Via Threshold Ionization Mass Spectrometry and Direct Simulation Monte Carlo Techniques Jason K. Holt, Maribeth Swiatek, David G. Goodwin, and Harry A. Atwater Thomas J. Watson Laboratories of Applied Physics R.P. Muller and W.A. Goddard, III Materials and Process Simulation Center California Institute of Technology Pasadena, CA 91125, U.S.A. ABSTRACT Monte Carlo simulations of hot-wire chemical vapor deposition (HWCVD) gas ambients indicate different flux ratios (SiH3/Si and H/SiHx) under conditions for amorphous or polycrystalline silicon growth. Gas-phase reactions of Si with ambient SiH4 studied using abinitio methods reveals that collisional stabilization of the adduct (H3SiSiH) is unlikely under typical HWCVD growth pressures, but an energetically favorable, low-pressure pathway has been identified that leads to the formation of Si2H2 and H2. Threshold ionization mass spectrometry has revealed significant quantities of the radical SiH2 at HWCVD growth pressures, indicative of heterogeneous pyrolysis. Mass spectrometry at low pressures suggests that incident silane dissociatively adsorbs at the wire and undergoes sequential H elimination to produce subhydrides. Disilicon species were not detected in significant quantities at HWCVD growth pressures. Finally, hot wire operation in a pure H2 ambient yields SiH4 as the dominant etching product from the silicon-coated walls of the growth chamber. INTRODUCTION Synthesis of large-grained polycrystalline silicon at low temperatures with high throughput is critical to enabling a future polycrystalline 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 have combined experiments and numerical simulations to explore the fundamental gas-phase and surface interactions of importance in HWCVD, with a final objective to optimize growth conditions for growth rate, crystal quality, and process uniformity. AB INITIO STUDIES OF THE REACTION OF SILICON WITH SILANE The possible reactions of a silicon atom with a silane molecule were investigated using density functional theory (DFT), as described in a previous paper [1]. To determine the steadystate rate coefficients as a function of temperature and pressure, microcanonical statistical theory (RRKM) was used, as implemented in the program ChemRate [2]. The objective of the ab-initio study was to identify energetically favorable pathways for the reaction of Si with SiH4 that do not require collisional stabilization, and therefore can be rapid even at very low pressure. Calculations have shown that the insertion of Si into the Si-H bond of SiH4 to form triplet state H3SiSiH is exothermic by 24.30 kcal/mole in energy, and slightly endothermic by 1.01 kcal/mole in free energy at 1200 K. The singlet state of the resulting H3SiSiH has been found to be even A3.2.1
0 triplet
H H
Si +
H H
Si
H
Si Si H H H -24
-39
H H H Si Si H H H
H
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