Ab Initio Chemical Kinetics for the Thermal Decomposition of SiH₄ ⁺ Ion and Related Reverse Ion–Molecule Reactions of Interest to PECVD of a-Si:H Films

The thermal unimolecular decomposition of SiH₄ ⁺ ion and its related reverse reactions, SiH₃ ⁺ + H and SiH₂ ⁺ + H₂, have been investigated by ab initio molecular orbital and quantum statistical variational RRKM theory calculations. The potential energy surface has been calculated at different levels of theory; the results at the highest level, CCSD(T)/CBS//CCSD(T)/6-311++G(3df,2p), show that the decomposition of SiH₄ ⁺ can mainly occur via a barrierless channel giving SiH₂ ⁺ + H₂ lying 11.8 kcal/mol above the reactant, or via a transition state forming SiH₃ ⁺···H complex to be followed by a barrierless decomposition yielding SiH₃ ⁺ + H lying 23.5 kcal/mol above the reactant. Barrierless processes were calculated using the CASPT2 and CASSCF methods with the 6-311++G(3df,2p) basis set and fitted with Morse potentials. The rate constants were predicted by solving master equations based on the RRKM theory at the E,J-resolved level; the results show that the channel SiH₄ ⁺ → SiH₂ ⁺ + H₂ is predominate under PEVCD conditions. For H- and H₂-capturing by SiH₃ ⁺ and SiH₂ ⁺ ions, respectively, the rate constants were found to be weakly dependent on temperature at the high-pressure limit and decrease rapidly with pressure. The calculated heats of formation of the SiH_x ⁺ (x = 2–4) ions are in close agreement with available thermochemical data.