Effect of Cl/H input ratio on the growth rate of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates from SiCl4–H2 precursor gases

Under chemical vapor deposition (CVD) conditions of Si on Mo substrate limited by mass transport of reactant gas species through a gas boundary layer, the effect of the Cl/H input ratio on the growth rate of MoSi2 coating at 1200 °C was investigated using a horizontal hot-wall reactor and SiCl4–H2 gas mixtures. The growth kinetic of the MoSi2 coating obeyed a parabolic rate law irrespective of Cl/H input ratios. The growth rate of MoSi2 coating initially increased with increasing Cl/H input ratio until the maximum growth rate was reached, and then decreased inversely proportional to the Cl/H input ratio at higher Cl/H input ratio. This suggests that there is a limit to the increase in growth rate for MoSi2 coating due to the etching effect of Si with respect to the input ratio of Cl/H. The etching effect of the Cl/H input ratio on the growth rate of the MoSi2 coating was explained by thermodynamic calculations based on the variation of silicon activity on the surface of the MoSi2 coating. The mass balance between the Si flux supplied by the mass transport step and that consumed by solid-state diffusion to form MoSi2 coating was considered to be responsible for the activity of silicon.