Theoretical study of the interactions of SiH₂ radicals with silicon surfaces

Silylene (SiH₂) radicals created by electron impact dissociation of silane in reactive gas discharges can play an important role in plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH₂ radicals with a variety of crystalline and amorphous silicon surfaces based on atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds is shown to exert the strongest influence on the radical-surface reaction mechanisms and the corresponding reaction probabilities. The SiH₂ radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si-Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is found to react with a probability of approximately 50%. The SiH₂ radical attaches itself to the surface either by forming two bonds with Si atoms of adjacent dimers in the same dimer row or through Si-Si bonds with one or both atoms of a surface dimer. In addition, the SiH₂ radical can attach itself in the trough between dimer rows, forming two Si-Si bonds with second-layer Si atoms. The energetics and dynamics of these surface reactions are analyzed in detail. A reaction probability of approximately 70% is calculated for SiH₂ radicals impinging on surfaces of hydrogenated amorphous silicon (a-Si:H) films with varying concentrations of hydrogen. Recent experimental measurements have reported a 60% loss probability for the SiH₂ radical on the reactor walls through laser induced fluorescence. The experimentally obtained reaction probability falls within the range for the sticking coefficients on the H-terminated and amorphous film surfaces as determined by our atomistic calculations. Molecular-dynamics (MD) simulations of a-Si:H film growth by repeated impingement of SiH₂ radicals have revealed adsorption reactions at early stages to occur with similar energetics as the corresponding reactions of isolated radicals on crystalline surfaces. The reaction probability of SiH₂ on a-Si:H films deposited through MD simulations is approximately 30%. Finally, it is found that the SiH₂ radical is much more mobile on surfaces of a-Si:H films than on crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.