On the Mechanism of Homogeneous Decomposition of the Chlorinated Silanes. Chain Reactions Propagated by Divalent Silicon Species

A mechanism for the homogeneous gas-phase decomposition of SiHCl3, SiH2Cl2, and SiH3Cl in hydrogen is derived from the results of ab initio molecular-orbital studies. It consists of 39 reversible elementary reactions among 25 species, including pressure-dependent unimolecular decomposition of the chlorinated silanes and secondary chemistry due to reactions of SiH2, SiHCl, and SiCl2 with one another and with the chlorinated silanes. Rate parameters in the mechanism have been calculated based on results of ab initio studies using transition-state theory and unimolecular rate theories. This allows us to construct a reasonably complete mechanism that provides qualitative explanations for several features of dichlorosilane decomposition that have been presented in the literature, including observations on the presence and concentrations of SiCl2, SiHCl, and Si atoms. Several chain reactions in which the chain carriers are divalent silicon species have been identified.