Kinetics of SiHCl₃ and SiCl₄ evolution in Si(s)-HCl(g) system simulated by ab-initio MO

A reaction model for a HCl(g)-Si(s) system is proposed in this study as follows: first, SiCl₂(g) and H₂(g) are produced by a heterogeneous surface reaction, and then SiHCl₃(g) and SiH₂Cl₂(g) are produced by the homogeneous reaction between SiCl₂(g) and HCl(g) and between SiCl₂(g) and H₂(g), respectively; finally, SiCl₄(g) and SiHCl₃(g) are produced by the homogeneous reaction between SiHCl₃(g) and HCl(g) and between SiH₂Cl₂(g) and HCl(g), respectively. The mechanism in the first heterogeneous surface reaction step has been studied extensively in the past. However, the detail of the subsequent homogeneous reactions has not yet been understood. It is important to get information of the chemical kinetics of the reactions derived from the reaction intermediate of SiCl₂(g) in the SiCl₂(g)-HCl(g)-H₂(g) system, so the most plausible elementary reactions of reversible unimolecular fission and chain reaction are proposed here. The structures of the molecules formed in these reactions are optimized using the minimization principle of energy calculated by ab-initio molecular orbital method and also their rate constants are calculated by conventional transition state theory. Using these rate constants, the ordinary differential equations are solved on mass balance, and the time dependent concentration profile of respective chemical species and the prior reaction paths are discussed in this study.