Mechanisms and energetics of hydride dissociation reactions on surfaces of plasma-deposited silicon thin films.

We report results from a detailed analysis of the fundamental silicon hydride dissociation processes on silicon surfaces and discuss their implications for the surface chemical composition of plasma-deposited hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of hydride dissociation on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of adsorbed SiH(3) radical precursor dissociation on surfaces of MD-grown a-Si:H films. Our DFT calculations reveal that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.40-0.55 eV. The presence of dangling bonds (DBs) results in lowering the activation barrier for hydride dissociation to 0.15-0.20 eV, but such DB-mediated reactions are infrequent. Our MD simulations on a-Si:H film growth surfaces indicate that surface hydride dissociation reactions are predominantly mediated by fivefold coordinated surface Si atoms, with resulting activation barriers of 0.35-0.50 eV. The results are consistent with experimental measurements of a-Si:H film surface composition using in situ attenuated total reflection Fourier transform infrared spectroscopy, which indicate that the a-Si:H surface is predominantly covered with the higher hydrides at low temperatures, while the surface monohydride, SiH((s)), becomes increasingly more dominant as the temperature is increased.