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 SiH_x (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.