Kinetics of Silane Decomposition in High-Pressure Confined Chemical Vapor Deposition of Hydrogenated Amorphous Silicon

We study the kinetics of silane pyrolysis via confined high-pressure chemical vapor deposition (HPCVD) at pressures of 20-33 MPa in a microcapillary of 5.9 μm inner diameter. We find the growth rate to be first order with respect to silane concentration, with an activation energy of 53.7 ± 2.9 kcal/mol and a pre-exponential factor of 1.5 × 10¹⁰ m/s. The obtained activation energy is in the range of activation energies reported for hydrogen desorption from c-Si surfaces, suggesting that hydrogen desorption from the surface is the rate-limiting step in film growth. To further investigate this finding, reactive molecular dynamics simulations of thermal decomposition of silane on clean and hydrogen-passivated c-Si were performed. Homogeneous reactions were not observed in any of the simulations, in support of the hypothesis that heterogeneous silane decomposition on the silicon surface is the dominant mechanism for film deposition. In silane pyrolysis simulations on clean c-Si surfaces, almost all available silicon surface sites (i.e., dangling bonds) were occupied by silicon-hydrides (mostly tri- and dihydrides) upon exposure to gas-phase silane, whereas no reaction was observed during silane decomposition simulations on the hydrogen-passivated c-Si. Therefore, the results of the reactive molecular dynamics simulations indicate that the availability of dangling bonds resulting from hydrogen desorption from the surface is the rate-limiting step in film growth at high pressure.