Enhancement of ultra-violet light absorption of surface-textured silicon induced by nanosecond laser irradiations

Abstract

The process of inducing crystallization on the surface of a 50-nm-thick amorphous Si (a-Si) thin film was analyzed by applying a Nd:YAG (λ = 355 nm) nanosecond laser with an asymmetric Gaussian shape at a pulse repetition rate of 14 kHz and pulse duration of 5.5 ns. Single-laser-pulse irradiation with a peak fluence of 124 mJ/cm2 resulted in the formation of circular Si nanoparticles on the surface, which was observed to have multiple domains with varying degrees of surface roughness and crystallinity. Additionally, the light absorption significantly increased at the center of irradiation, where the density of the nanoparticles was maximized. Specifically, at the ultraviolet wavelength of 380 nm, the absorption was 85%, which is approximately twice that for the non-laser-treated a-Si (i.e., 42%). These results are attributable to the anti-reflection effect of the Si nanoparticles, and have been validated by numerical simulations. Although the intensity and absorption were observed to be heterogeneous, it is suggested that uniform crystallinity can be achieved by optimizing the scan pitch and laser fluence; these conditions would enable application of a 355-nm nanosecond laser to the laser crystallization process for organic light-emitting diode displays or photovoltaic devices.