Surface modification of an amorphous Si thin film crystallized by a linearly polarized Nd:YAG pulse laser beam

Abstract

Amorphous Si films of 60 and 10nm thick on glass substrates were irradiated by a linearly polarized Nd:YAG pulse laser with the wavelength λ=532nm at the incident angle θi=0. The surface of the irradiated 60-nm-thick film had both periodic ridges perpendicular to the electric field vector E and aperiodic ridges roughly parallel to E, where the spatial period of the periodic ridges was almost λ. From the continuous 10-nm-thick film, the separate rectangular Si islands were formed with a periodic distance of λ, with the edges parallel or perpendicular to E. When θi was increased from normal incidence of the s-polarized beam for a 60-nm-thick film, the aperiodic ridges were reduced while the periodic ridges were still formed. For a 10-nm-thick film, the Si stripes were formed perpendicular to E, using the s-polarized beam at θi=12°. In order to investigate the mechanisms of the surface modifications of, in particular, aperiodic ridges, islands, and stripes, we improved the previous theoretical model of the periodic distribution of the beam energy density (periodic E-D) generated by irradiation of the linearly polarized laser beam, taking account of the multireflection effect in the Si film which is semitransparent for λ. Further, the calculated E-D was corrected with respect to the thermal diffusion in the irradiated Si film. The calculation results show that the two-dimensional E-D consists of a constant or a dc term and a sinusoidal or an ac term which contains various spatial periods. The multireflection effect strongly influences the amplitude and phase of every ac term, which means that the amplitude and phase depend on the film thickness. The thermal diffusion during the heating of the irradiated film greatly reduces the amplitudes of the ac terms with periods below the thermal diffusion length. The theoretical calculation showed that, by increasing θi, the temperature distribution in the irradiated Si film was changed from two-dimensional toward one-dimensional, which can explain the above experimental results reasonably.