Impact of wavelength, intensity and polarization on the morphology of femtosecond laser-induced structures on crystalline silicon surface

Abstract

The features of laser structuring of the n-doped crystalline silicon surface achieved by femtosecond laser irradiation with different wavelengths, namely fundamental wavelength of 800 nm, the second (400 nm) and the third (266 nm) laser harmonics, have been studied. Morphology analysis of the processed Si surface reveals laser-induced periodic surface structures (LIPSS) perpendicular to the laser beam polarization for all the applied laser wavelengths. The laser-processed Si surfaces also contain nanoparticles and defects in periodic structure (strip breaks, bifurcations, nodes, etc.). In the case of circularly polarized laser irradiation, the irregular granular structure with the short-range order distance of about 600 nm has been observed. The 2D-FFT analysis of SEM images of LIPSS generated by the fundamental laser wavelength detects structures with two periods simultaneously: near-subwavelength ripples and weaker deep-subwavelength ripples. The latter are attributed to the second harmonic generation of the laser radiation at the surface. For 400 nm laser irradiation the ripple period is observed in the range of 290–310 nm, for 266 nm – 240–250 nm. The effect of the pulse irradiation energy density on the LIPSS morphology has been investigated for linearly polarized femtosecond laser radiation with the wavelength of 800 nm. The main period of LIPSS has been observed to decrease slightly with the decrease of laser pulse energy density that is in agreement with theoretical model of LIPSS formation.