A detailed analysis was made of the fundamental atomic-scale processes that determined the surface smoothness of hydrogenated amorphous Si (a-Si:H) thin films. The analysis was based on a synergistic combination of molecular-dynamics (MD) simulations of radical precursor migration on surfaces of a-Si:H films that were deposited computationally using MD simulation with first-principles density functional theory calculations on the hydrogen-terminated Si(001)-(2x1) surface. The surfaces of the MD-grown a-Si:H films were remarkably smooth, as the mobile precursor, the SiH3 radical, diffused fast and incorporates in surface valleys. Analysis of the MD simulations of SiH3 radical migration on a-Si:H surfaces yields an effective diffusion barrier of 0.16eV. The low diffusion barrier on the a-Si:H surface was attributed to SiH3 migration through over-coordinated surface Si atoms, where the radical remained weakly bonded to the surface at all times and did not break any strong Si-Si bonds along its migration pathway. Furthermore, the diffusing SiH3 radical incorporates into the a-Si:H film only when it transfers an H atom and forms a second Si-Si backbond. On rough a-Si:H films, such H transfer from diffusing SiH3 radicals was more likely to occur in surface valleys, even when the dangling bond density was low and DBs were not present in surface valleys. In addition, this H-transfer process was thermally activated with activation energy barriers (Ea) over the range 0.29–0.65eV; Ea was determined by the Si-Si interatomic distance between the Si of the SiH3 radical and the surface Si atom to which the H was transferred. The preferential incorporation in valleys was explained by both the increased residence time of the migrating precursor in valleys and the decreased activation barrier for incorporation reactions occurring in valleys. The mechanism and activation barrier for the H-transfer reaction on the a-Si:H surface were validated by performing first-principles density functional theory calculations on the crystalline Si surface.

Surface Smoothness of Plasma-Deposited Amorphous Silicon Thin Films - Surface Diffusion of Radical Precursors and Mechanism of Si Incorporation. M.S.Valipa, T.Bakos, D.Maroudas: Physical Review B, 2006, 74[20], 205324 (15pp)