Kinetics and reaction mechanisms of laser-assisted chemical vapor deposition of polycrystalline silicon dots from silane

Abstract

Polycrystalline silicon dots have been deposited on silicon-coated SiO2 substrates from silane irradiated with a continuous wave (cw) argon ion laser and a cw CO2 laser. Various experimental configurations were adopted for laser-assisted chemical vapor deposition of Si dots. The cw argon ion laser was focused either directly on the surface of absorbent polycrystalline Si films acting as substrates (front side visible laser illumination) or penetrated through the transparent SiO2 plate and focused at the SiO2–Si interface (rear side visible laser illumination). The cw CO2 laser beam penetrated through the transparent polycrystalline Si film and was absorbed into the SiO2 plate (front side IR laser illumination). The deposition rate of polycrystalline Si dots was investigated as a function of the laser power (or surface temperature which was proportional to the laser power) and silane pressure. With the direct visible laser irradiation of Si/SiO2 substrates (front side laser illumination), at a surface temperature in the range 700–1000 °C, the growth kinetics of polycrystalline Si dots was found to be limited by surface reactions. The reaction mechanism proposed for chemical vapor deposition of Si films from pyrolysis of silane in conventional reactors can be invoked to explain the kinetic data, i.e., the rate-limiting step is the release of hydrogen from the Si surface. Visible photon interactions with reactive species were found to promote the release of hydrogen since this kinetic regime was not observed with other experimental configurations investigated. The deposition rate of polycrystalline Si dots produced at a surface temperature in the range 1100–1410 °C was proportional to the laser power or surface temperature and silane pressure. In addition, the deposition rate was independent of the experimental configuration used. In other words, only pyrolytic effects are involved in the laser-induced deposition of Si dots even with the IR laser light which is absorbed by silane molecules. With the gas phase in the fluid regime, the rate-limiting step was probably the diffusion of reactive species through the boundary layer developed in the vicinity of the surface of the deposition zone. Silylene radicals, SiH2, formed in the gas phase from inelastic collisions between excited silane molecules seem to be the major reactive species. A reaction mechanism of laser-assisted chemical vapor deposition of polycrystalline Si dots involving the formation and diffusion of silylene radicals through silane which acts as a buffer gas with respect to silylene radicals is proposed and discussed in this article.