Optical and electrical properties of pulsed laser annealed thin Si films

Abstract

Thin Si films used for solar energy purposes are commonly treated by relatively slow thermal annealing on a time scale of seconds to obtain the proper electrical behavior. We investigated a different approach, in which the films are annealed and/or molten by a frequency doubled Q-switched Nd:YAG laser pulse on a nanosecond time scale. We studied thin polycrystalline Si films of thickness between 43 nm and 250 nm on fused silica and on sapphire substrates. The different thermal conductivities of these substrates lead to different quench rates for the molten Si films. The optical and electrical properties of the Si films were systematically characterized during, respectively after the various annealing conditions. In addition we monitored the solidification process in situ by time-resolved optical measurements. At low energy densities the film is not completely molten by the laser pulse and resolidification takw place at the moving liquidsolid interface. Above a thickness-dependent threshold energy density complete melting is observed and nucleation in the supercooled melt prevails. In the latter case Sameshima and Usui showed that amorphization can be observed far Si films on fused silica up to thicknesses of 36 nm. We found that Si films on sapphire even with a thickness of 80nm can be amorphized. The reproducible threshold values suggest the possibilty of lateral structuring.