First-principles theoretical analysis of silyl radical diffusion on silicon surfaces

We report results from a detailed analysis of the fundamental radical precursor diffusion processes on silicon surfaces and discuss their implications for the surface smoothness of 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 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. Our DFT calculations yield activation energies for SiH3 migration that range from 0.18 to 0.89 eV 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 are highest, whereas hopping between nearest-neighbor overcoordinated surface Si atoms results in the lowest radical diffusion barrier of 0.18 eV; this low barrier is consistent with the activation barrier for SiH3 migration through overcoordinated 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 0.16 eV, 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. (c) 2006 American Institute of Physics.