Atomic-scale analysis of fundamental mechanisms of surface valley filling during plasma deposition of amorphous silicon thin films

We present a detailed atomic-scale analysis of the fundamental processes that determine the surface smoothness of hydrogenated amorphous silicon (a-Si:H) thin films deposited computationally using molecular-dynamics (MD) simulation. The surfaces of these MD-grown films are remarkably smooth due to a valley-filling mechanism where the mobile precursor, SiH3, diffuses and passivates dangling bonds present in surface valleys or at the valley edges. The mechanisms of SiH3 precursor migration on the a-Si:H surface are studied placing special emphasis on elucidating the role of the surface bond strain in mediating the valley-filling phenomena. Surface transport of the SiH3 precursor is driven by the Si-Si bond strain distribution on the surface, which is strongly coupled with the surface morphology and reactivity. There is a strong driving force for SiH3 to migrate from a hill toward a valley, but not such a strong driving force for migration out of a valley. As a result, SiH3 radicals impinged on a surface hill eventually migrate to a nearby valley, regardless of the presence of dangling bonds at the hill, while radicals impinged in a valley remain confined within the valley's morphological well. Analysis of the MD trajectories for numerous SiH3 radical migration paths revealed the development of tensile strain bands along these paths, which typically lead to dangling bonds. Adsorbed SiH3 radicals follow these tensile strain bands, relieve strain along their migration paths, and passivate dangling bonds in valleys or at valley edges, thus leading to surface valley filling. During diffusion on the a-Si:H surface, SiH3 is observed to insert frequently into strained Si-Si bonds and relieve strain through an athermal and exothermic reaction.