Atomic-Scale Analysis of SiH3 Diffusion on Surfaces of Plasma-Deposited Amorphous Si Thin Films

Under conditions of low SiH$_{4}$ dissociation during plasma-assisted deposition of hydrogenated amorphous silicon (a-Si:H) thin films, the dominant deposition precursor is the SiH$_{3}$ radical. Device-quality a-Si:H films grown under these conditions are remarkably smooth as the SiH$_{3}$ radical is very mobile on the films' surfaces and fills surface valleys during its diffusion. In this presentation, we discuss atomic-scale mechanisms of SiH$_{3}$ diffusion on a-Si:H surfaces based on molecular-dynamics simulations of SiH$_{3}$ radical impingement on a-Si:H film surfaces. The computed average activation barrier for radical diffusion on the a-Si:H surface 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. The mechanisms and energetics of SiH$_{3}$ migration reported for the a-Si:H surface are consistent with our density functional theory calculations on crystalline Si surfaces. The diffusing SiH$_{3}$ radical also incorporates preferentially into surface valleys on the a-Si:H film when it transfers an H atom and forms a Si-Si backbond.