Growth and transport of clusters in thermal plasma vapor deposition of silicon

Numerical simulation is conducted for Si cluster growth and transport processes during ultrafast deposition of microcrystalline Si for solar cell fabrication by thermal plasma chemical vapor deposition. Within the boundary layer just above the substrate, Si vapor is supersaturated during the quenching process, leading to the generation of Si clusters of various sizes under the effects of coagulation and evaporation. The transport process of these Si clusters within the plasma gas flow is investigated numerically by the continuum approach, taking into account convection and diffusion phenomena of clusters. The thermophoretic effects on cluster transport in terms of the deposition efficiency and the film growth rate are also investigated. It is found that the clusters in the nanosize range (<1 nm) are the dominant component among the deposition species and that the thermophoretic force FT is a highly crucial factor for achieving the high film growth rate of above 100 nm/s and high deposition efficiency of around 85%.