Silicon hydride composition of plasma-deposited hydrogenated amorphous and nanocrystalline silicon films and surfaces

In situ attenuated total reflection Fourier transform infrared spectroscopy was used to study the H bonding on the surfaces of $a-Si:H$ and $nc-Si:H$ during plasma enhanced chemical vapor deposition from $SiH4/H2/Ar$ containing discharges. Well-resolved $SiHx$ $(1⩽x⩽3)$ absorption lines that correspond to the vibrational frequencies commonly associated with surface silicon hydrides were detected. During deposition of $a-Si:H$ films using $SiH4$ without $H2$ dilution, the surface coverage was primarily di- and trihydrides, and there are very few dangling bonds on the surface. In contrast, during deposition of $nc-Si:H$ using $SiH4$ diluted with $H2,$ the amount of di- and trihydrides on the surface is drastically reduced and monohydrides dominate the surface. Furthermore, the vibrational frequencies of the monohydrides on $nc-Si:H$ film surfaces match well with the resonant frequencies of monohydrides on H terminated Si (111) and Si (100) surfaces. The decrease of higher hydrides on the surface upon $H2$ dilution is attributed to increased dissociation rate of tri- and dihydrides on the surface through reaction with dangling bonds created by increased rate of H abstraction from the surface. Results presented are consistent with $SiH3$ being at least one of the precursors of $a-Si:H$ deposition.