ИСТИНА

The present work describes main features of laser-induced periodic surface structures (LIPSS) formation at amorphous silicon surfaces under femtosecond laser action with the wavelength of 1030 nm or 1250 nm when the pulses number are varied in the range of 50–1000 at different energy fluences. It allows to fabricate ripples with the period from 880 to 1250 nm and the orientation perpendicular or parallel relative to the laser radiation polarization. Using mechanical translators and a galvanometric scanner we obtained relatively large areas (1 mm х 1 mm and more) with LIPSS and a good quality for the applications. To explain the observed ripple evolution, we used the model proposed by J.E. Sipe where the efficacy factor depends on the real and imaginary parts of the dielectric constant and defines the LIPSS wave vector on the irradiated surface. In turn, according to the Drude model the dielectric constant complex value is varied due to concentration change for the nonequilibrium electrons excited by different number of high-power femtosecond laser pulses. The modified surfaces are characterized by the high level of nanocrystallization. According to Raman spectra analysis the volume fraction of nanocrystals in the irradiated films ranges from 17% to 70% depending on the treatment conditions. The silicon nanocrystal presence leads to growth of the specific conductivity up to 3 orders for the irradiated samples in comparison with nonirradiated ones. The in-plane conductivity anisotropy was revealed also. This result is in a good agreement with the Bruggeman model and spectra of absorption for the surface possessing form anisotropy.