Scan-path dependence of surface ripples on Si thin films induced by laser irradiation with an asymmetric Gaussian beam

Abstract

A Nd:YAG linearly polarized nanosecond laser emitting at a wavelength of 355 nm was used to irradiate amorphous-Si thin film (50 nm thickness) on a glass substrate at a repetition rate of 14 kHz and a pulse duration of 5.5 ns. By scanning the laser beam with an asymmetric Gaussian shape in the lateral and vertical directions with various scan pitches at maximum fluences of 55 mJ/cm2 and 90 mJ/cm2, uniform surface textures were obtained when the laser was scanned in the vertical direction at a scan pitch of ∼35–40 μm along the long beam axis. The surface textures were composed of nanoparticle arrays embedded in the Si layer, whose height gradually increased up to 100–150 nm as the number of pulses increased. However, when over-irradiated, the nanoparticles with a rounded shape were transformed to an irregular shape, and the surface roughness also increased. These results would enable application of a 355-nm nanosecond laser to achieve uniform surfaces as a result of the laser crystallization process for organic light-emitting diode displays or photovoltaic devices.