A study of the role of various reactions on the density distribution of hydrogen, silylene, and silyl in SiH4/H2 plasma discharges

Abstract

Fluid model has been used to study the effect of pressure on the distribution of hydrogen (H), silylene (SiH2), and silyl (SiH3) in hydrogen silane plasma discharges used for deposition of hydrogenated microcrystalline silicon (μc-Si:H) or hydrogenated amorphous silicon (a-Si:H) thin films for solar cells. Time averaged reaction rates have been calculated to study the influence of various reactions on the density distributions of hydrogen, silylene, and silyl. Change in the distributions of hydrogen and silylene from bell shaped distribution at low pressure (1 Torr) to double humped distribution at high pressure (5 Torr) is explained with the help of time averaged reaction rates. Important reactions have been identified that contribute to the production and consumption of hydrogen (H), silylene (SiH2), and silyl (SiH3). The hydrogen consumption reactions SiH4 + H → SiH3 + H2 and SiH3 + H → SiH2 + H2 are found to play a central role in deciding the distribution of hydrogen. On the other hand, silylene consumption reaction SiH2 + H2 → SiH4 is found to play a central role in determining the distribution of silylene. The distribution of these species at high pressure has been explained by using time averaged continuity equation. The code has been optimized by identifying 33 reactions (out of 53 reactions which contribute in the production and consumption of H, SiH2, and SiH3) that have no net effect on the density and distribution of these species. It is observed that dropping of 33 reactions has insignificant effect on the density of all the thin film deposition precursors such as Si, SiH, SiH2, SiH3, and Si2H5. This reduced set of 20 reactions can be used instead of 53 reactions to calculate the density and distribution of H, SiH2, and SiH3 in the fluid simulation of SiH4/H2 plasma discharges.