Theoretical investigations of elementary processes in the chemical vapor deposition of silicon from silane. Unimolecular decomposition of SiH4

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Abstract
The rates and mechanism for the unimolecular decomposition of SiH4 have been investigated using quasiclassical trajectory methods to follow the dynamics and Metropolis sampling procedures to average over the initial SiH4 phase space. The semiempirical potential‐energy surface has been fitted to scaled SCF calculations and to a variety of experimental data. It gives the correct SiH4 equilibrium structure, reaction endothermicities, and bond energies for SiH4, SiH3, and SiH2. All hydrogen atoms are treated in an equivalent fashion. Excellent first‐order decay plots are obtained for the microcanonical rates for the total SiH4 decomposition as well as for the separate decomposition channels. The low‐energy pathway is found to be a three‐center elimination to form SiH2+H2. The decomposition channel forming SiH3+H becomes important only at internal SiH4 energies in excess of 5.0 eV. Comparison of computed falloff curves with RRKM calculations fitted to experimental results indicates that the critical threshold energy for the three‐center reaction lies in the range 2.10<E0k(T,∞)=(1.08±0.02)×1013 exp(−49 600±1200/RT) s−1. The calculated distributions of relative translational energies for SiH2 and H2 reflect the fact that there is no barrier to the back reaction. Both concerted three‐center eliminations and processes that resemble ‘‘half‐collisions’’ of SiH3+H are found to be important decomposition pathways.