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.