The role of SiH3 diffusion in determining the surface smoothness of plasma-deposited amorphous Si thin films:: An atomic-scale analysis

Device-quality hydrogenated amorphous silicon (a-Si:H) thin films grown under conditions where the SiH3 radical is the dominant deposition precursor are remarkably smooth, as the SiH3 radical is very mobile and fills surface valleys during its diffusion on the a-Si:H surface. In this paper, we analyze atomic-scale mechanisms of SiH3 diffusion on a-Si:H surfaces based on molecular-dynamics simulations of SiH3 radical impingement on surfaces of a-Si:H films. The computed average activation barrier for radical diffusion on a-Si:H is 0.16 eV. This low barrier is due to the weak adsorption of the radical onto the a-Si:H surface and its migration predominantly through overcoordination defects; this is consistent with our density functional theory calculations on crystalline Si surfaces. The diffusing SiH3 radical incorporates preferentially into valleys on the a-Si:H surface when it transfers an H atom and forms a Si-Si backbond, even in the absence of dangling bonds.