Ab initio chemical kinetics for SiH₃ reactions with Si _xH_2x+2 (x = 1-4)

Gas-phase kinetics and mechanisms of SiH₃ reactions with SiH₄, Si₂H₆, Si₃H₈, and Si₄H₁₀, processes of relevance to a-Si thin-film deposition, have been investigated by ab initio molecular orbital and transition-state theory (TST) calculations. Geometric parameters of all the species involved in the title reactions were optimized by density functional theory at the B3LYP and BH&HLYP levels with the 6-311++G(3df,2p) basis set. The potential energy surface of each reaction was refined at the CCSD(T)/6-311++G(3df,2p) level of theory. The results show that the most favorable low energy pathways in the SiH₃ reactions with these silanes occur by H abstraction, leading to the formation of SiH₄ + Si_xH_2x+1 (silanyl) radicals. For both Si₃H ₈ and n-Si₄H₁₀ reactions, the lowest energy barrier channels take place by secondary Si-H abstraction, yielding SiH ₄ + s-Si₃H₇ and SiH₄ + s-Si ₄H₉, respectively. In the i-Si₄H₁₀ reaction, tertiary Si-H abstraction has the lowest barrier producing SiH ₄ + t-Si₄H₉. In addition, direct SiH ₃-for-X substitution reactions forming Si₂H₆ + X (X = H or silanyls) can also occur, but with significantly higher reaction barriers. A comparison of the SiH₃ reactions with the analogous CH₃ reactions with alkanes has been made. The rate constants for low-energy product channels have been calculated for the temperature range 300-2500 K by TST with Eckart tunneling corrections. These results, together with predicted heats of formation of various silanyl radicals and Si ₄H₁₀ isomers, have been tabulated for modeling of a-Si:H film growth by chemical vapor deposition.