A comprehensive study is presented of the interactions of SiH radicals originating in silane containing plasmas with crystalline and amorphous silicon surfaces based on a detailed atomic-scale analysis. The hydrogen concentration on the surface is established to be the main factor that controls both the surface reaction mechanism and the reaction probability; other important factors include the location of impingement of the radical on the surface, as well as the molecular orientation of the radical with respect to the surface. On the ordered crystalline surfaces, the radical reacts in such a way as to maximize the number of Si–Si bonds it can form even if such bond formation requires dissociation of the radical and introduction of defects in the crystal structure. The radical is established to be fully reactive with the pristine Si(001)-(2×1) surface. This chemical reactivity is reduced significantly for the corresponding H-terminated surface with a hydrogen coverage of one monolayer. SiH is found to be highly reactive with surfaces of hydrogenated amorphous silicon films, independent of radical orientation and the location of impingement. Our simulations predict an average reaction probability of 95% for SiH with a-Si:H film surfaces, which is in excellent agreement with experimental data.

SiH 3 radicals created by electron impact dissociation of SiH4 in reactive gas discharges are widely believed to be the dominant precursor for plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH3 radicals with a variety of crystalline and amorphous silicon surfaces through atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds has the strongest influence on the radical–surface reaction mechanisms and the corresponding reaction probabilities. The SiH3 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 much less reactive; the SiH3 radical was reflected back into the gas phase in all but two of the 16 simulations of radical impingement designed to sample the high-symmetry adsorption sites on the surface. When SiH3 reacts on the H-terminated surface, it either inserts into the Si–Si dimer bond or returns to the gas phase as SiH4 after abstracting H from the surface. The insertion into the Si–Si bond occurs through a dissociative adsorption reaction mechanism that produces two surface SiH2 species after transfer of one of the H atoms from SiH3 to one of the dimer Si atoms. The energetics and dynamics of the surface reactions are analyzed in detail. During simulations of a-Si:H film growth, adsorption onto a dangling bond, dissociative insertion, and H abstraction reactions also were observed to occur with similar energetics as the corresponding reactions on crystalline surfaces. The radical is much more mobile on surfaces of a-Si:H films than crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.

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The relationship between the structure, H coverage, morphology, and reactivity of plasma deposited hydrogenated amorphous silicon (a-Si:H) film surfaces was investigated using molecular-dynamics simulations. Surfaces of a-Si:H films grown with SiH3 as the sole deposition precursor are found to be remarkably smooth due to a valley-filling mechanism where mobile precursors, such as SiH3 and Si2H6, diffuse and react with dangling bonds in the valleys on the surface. Surface valleys are reactive due to the increased concentration of dangling bonds and decreased H coverage in these regions. The previously speculated physisorbed configuration, where SiH3 is weakly bound to the surface through a H atom, is highly unlikely to be the mobile precursor state.

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.