(321ay) Atomic-Scale Analysis of Silicon Hydride Dissociation on Surfaces of Plasma-Deposited Amorphous Silicon Thin Films
Hydrogenated amorphous silicon (a-Si:H) thin films grown by plasma-assisted deposition from silane-containing discharges are technologically important semiconductor materials, used widely in the fabrication of solar cells and flat panel displays. Film properties, such as surface roughness and film crystallinity, affect device performance significantly. Development of systematic strategies for depositing silicon thin films with desirable properties requires a fundamental understanding of the interactions between reactive radicals originating in the plasma and the growth surface. The dominant precursor for deposition of smooth device-quality a-Si:H films is the SiH3 radical. Furthermore, atomic hydrogen plays a crucial role in low-temperature a-Si:H growth. In this context, the chemical state of the a-Si:H surface, in terms of its silicon hydride, SiHx (x = 1,2,3), composition, is of fundamental interest, as it can influence a-Si:H film growth by determining the interaction of gas-phase species with the a-Si:H surface. This presentation focuses on detailed atomic-scale analysis of the silicon hydride composition of the a-Si:H surface, during growth of the a-Si:H film. Using molecular-dynamics (MD) simulations of impingement of SiH3 radicals on growth surfaces of a-Si:H films, we studied the reactions with the growth surfaces of the SiH3 radical over the temperature (T) range 475 x (x = 1,2,3) surface dissociation pathways based on DFT calculations using crystalline Si surfaces as representative models of atomic bonding at film growth surfaces. In our DFT calculations, we used the generalized gradient approximation, plane-wave basis sets, ultra-soft pseudopotentials, slab supercells, and the nudged elastic band method for determining optimal surface reaction pathways. Our MD simulations reveal that the a-Si:H surface consists mostly of dihydride species (SiH2) at lower T and monohydride species (SiH) at higher T and that the surface trihydride concentration decreases with increasing T; a dangling-bond-mediated dissociation reaction was found to be the operative mechanism producing lower surface hydrides from higher surface hydrides. Both the formation of dangling bonds and the hydride dissociation reactions are thermally activated processes. At T > 500 K, surface trihydrides dissociate by transferring an H atom each in the presence of dangling bonds, leading to the formation of a-Si:H films covered mainly with Si dihydrides. At T > 650 K, the Si dihydrides formed after H transfer can dissociate further on the a-Si:H surface, leading to the formation of Si monohydrides. Such sequential hydride dissociation reactions have been observed in our MD simulations, and have been proposed also on the basis of experimental measurements using in situ attenuated total internal reflection Fourier transform infrared spectroscopy. The results of our DFT analysis for the mechanisms and energetics of SiHx (x = 1,2,3) dissociation are consistent with our MD simulations on the a-Si:H surface.