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We present a detailed atomic-scale analysis of the postdeposition treatment of hydrogenated amorphous silicon (a-Si:H) thin films with H2 plasmas. The exposure of a-Si:H films to H atoms from a H2 plasma was studied through molecular-dynamics (MD) simulations of repeated impingement of H atoms with incident energies ranging from 0.04to5.0eV. Structural and chemical characterizations of the H-exposed a-Si:H films was carried out through a detailed analysis of the evolution of the films’ Si–Si pair correlation function, Si–Si–Si–Si dihedral angle distribution, structural order parameter, Si–H bond length distributions, as well as film surface composition. The structural evolution of the a-Si:H films upon exposure to H atoms showed that the films crystallize to form nanocrystalline silicon at temperatures over the range of 500–773K, i.e., much lower than those required for crystallization due to thermal annealing. The MD simulations revealed that during H exposure of a-Si:H the reactions that occur include surface H adsorption, surface H abstraction, etching of surface silicon hydrides, dangling-bond-mediated dissociation of surface hydrides, surface H sputtering/desorption, diffusion of H into the a-Si:H film, and insertion of H into strained Si–Si bonds.

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 SiH(3) radical migration on the hydrogen-terminated Si(001)-(2 x 1) surface with molecular-dynamics (MD) simulations of SiH(3) radical precursor migration on surfaces of a-Si:H films. Our DFT calculations yield activation energies for SiH(3) migration that range from 0.18 to 0.89 eV depending on the local electronic environment on the Si(001)-(2 x 1):H surface. In particular, when no substantial surface relaxation (Si-Si bond breaking or formation) accompanies the hopping of the SiH(3) 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 SiH(3) migration through overcoordinated sites on the a-Si:H surface. Specifically, the analysis of the MD simulations of SiH(3) radical migration on a-Si:H surfaces yields an effective diffusion barrier of 0.16 eV, allowing for the rapid migration of the SiH(3) 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.