A detailed analysis was made of the fundamental radical precursor diffusion processes on Si surfaces and considered their implications for the surface smoothness of hydrogenated amorphous Si (a-Si:H) thin films. The analysis was based on a synergistic combination of first-principles density functional theory calculations of SiH3 radical migration on the hydrogen-terminated Si(001)-(2x1) surface with molecular-dynamics (MD) simulations of SiH3 radical precursor migration on surfaces of a-Si:H films. The density functional theory calculations yield activation energies for SiH3 migration that range from 0.18 to 0.89eV depending on the local electronic environment on the Si(001)-(2x1):H surface. In particular, when no substantial surface relaxation (Si–Si bond breaking or formation) accompanies the hopping of the SiH3 radical the activation barriers were highest, whereas hopping between nearest-neighbour over-coordinated surface Si atoms results in the lowest radical diffusion barrier of 0.18eV; this low barrier was consistent with the activation barrier for SiH3 migration through over-coordinated sites on the a-Si:H surface. Specifically, the analysis of the MD simulations of SiH3 radical migration on a-Si:H surfaces yields an effective diffusion barrier of 0.16eV, allowing for the rapid migration of the SiH3 radical prior to its incorporation in surface valleys; rapid migration and subsequent incorporation constitute the two-step mechanism responsible for the smoothness of plasma deposited a-Si:H thin films.

First-Principles Theoretical Analysis of Silyl Radical Diffusion on Silicon Surfaces. T.Bakos, M.S.Valipa, D.Maroudas: Journal of Chemical Physics, 2006, 125[10], 104702 (8pp)