Surface Smoothening Mechanism of Plasma-Deposited Amorphous Silicon Thin Films

An important concern in the plasma deposition of thin hydrogenated amorphous silicon (a-Si:H) films is to obtain smooth surfaces. Under conditions that lead to device-quality a-Si:H films, the dominant deposition precursor is the SiH$_{3}$ radical. In this presentation, we report results of molecular-dynamics simulations combined with first-principles density functional theory calculations to elucidate the smoothening mechanism of plasma deposited a-Si:H thin films. We show that SiH$_{3}$ may diffuse rapidly on the a-Si:H film surface via overcoordinated surface Si atoms and incorporate into the film preferentially in surface valleys, after H atom transfer and formation of two Si-Si backbonds, with activation barriers for incorporation dependent on the local surface morphology. Experimental data on smoothening and SiH$_{3}$ diffusion are accounted for.