Ellipsometric analysis of silicon surfaces textured by ns and sub-ps KrF laser pulses

Abstract

Efficiency of solar cells can be enhanced by lowering their surface reflectance. A possible method to produce textured surfaces with low reflectance is by irradiating them with multiple low intensity nano-, pico- or femtosecond length laser pulses. In this study, the treatment of single crystal silicon surface by series of sub-ps and ns pulses from KrF excimer lasers was investigated. The change in surface properties was observed by spectroscopic ellipsometry, scanning electron microscopy and Raman spectroscopy. Spatial and temporal temperature distributions due to single laser pulse irradiations were numerically calculated and used for interpretation of experimental results. Ellipsometry and Raman spectroscopy clearly indicated that in the case of the sub-ps laser irradiation the first laser pulse caused the formation of a ~15nm thick amorphous silicon layer. In the case of the irradiation by ns laser pulses, the thickening of the top oxide layer and the appearance of a modified, less crystalline silicon layer, up to ~200nm thickness was observed. A series of ns laser pulses resulted in cycles of surface melting followed by recrystallization, which introduced partial degradation of crystalline structure. As indicated by scanning electron microscopy images and decreasing fitting quality of the ellipsometric spectra, the surface became structured after 5 and 100 laser shots in the case of sub-ps and ns length laser pulses, respectively.