Si + SiH 4 Reactions and Implications for Hot-Wire CVD of a-Si:H: Computational Studies
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Si + SiH4 Reactions and Implications for Hot-Wire CVD of a-Si:H: Computational Studies Richard P. Muller1, Jason K. Holt2, David 1 Materials and Process Simulation Center, 2 Department of Chemical Engineering, 3
G. Goodwin3, and William A. Goddard, III1
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, 91125 ABSTRACT Gas phase chemistry is believed to play an important role in hot-wire CVD of amorphous silicon, serving to convert the highly-reactive atomic Si produced at the wire into a less-reactive species by reaction with ambient SiH4. In this paper, we use quantum chemistry computations (B3LYP/cc-pvTZ) to examine the energetics and rates of possible gas-phase reactions between Si and SiH4. The results indicate that formation of disilyne (Si2H2) is energetically favorable. Unlike other products of this reaction, Si2H2 does not require collisional stabilization, and thus this species is the most likely candidate for a benevolent precursor that participates in the growth of high-quality Si films. INTRODUCTION Hot-wire chemical vapor deposition (HWCVD) is a technique for growing device-quality amorphous silicon films with low hydrogen content1-5. In HWCVD, a tungsten wire at ≈2000 K cracks silane (SiH4) to produce atomic silicon and hydrogen. The role of atomic H is well understood: H can abstract another H from SiH4 to produce H2 gas and SiH3, a species known to have good film growth properties4. The role of atomic Si is much more puzzling. Si has a very high sticking coefficient, and thus produces very low-quality films. The fact that HWCVD produces high-quality films indicates that atomic Si reacts with some other species in transit from the wire to the substrate4. The only species present in appreciable quantities in the HWCVD growth chamber is SiH4. Consequently, we are investigating the gas phase reactions of Si and SiH4 in an attempt to understand the identity of another species, a benevolent precursor responsible for the high-quality surfaces seen in the HWCVD procedure6. COMPUTATIONAL DETAILS The calculations reported in this paper use density functional theory (DFT)7,8 using the B3LYP functional9 and the Dunning cc-pVTZ basis set10. For computational efficiency, we exclude f-functions from the basis set, a simplification we have found to have negligible impact on the energies or geometries resulting from these calculations. We adjust these calculations with zero-point energy (ZPE) corrections and Gibbs free energy adjustments. These adjustments are obtained from the vibrational frequencies using a harmonic approximation. We use a temperature of 1200 K to estimate free energies for the reactions considered here. This temperature is between the wire temperature (~1900-2300 K) and the substrate temperature (~300 K)3. These calculations are performed using the Jaguar program suite11. RESULTS We will divide our study of the gas phase chemistry of Si and SiH4 into several phases: bimolecular reactions of Si and SiH4; rearrangements of Si2H4 compounds; elimin
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