ИСТИНА

Direct laser writing is an effective technique to fabricate surface structures with wavelength and subwavelength accuracy. One of realizations of such approach is the laser-induced periodic surface structure (LIPSS) formation via femtosecond laser irradiation where intensive surface photoexcitation results to surface plasmon-polariton generation not only in metals but also in semiconductors due to electron-hole plasma generation. Such surface electromagnetic waves give a contribution to the periodical relief modulation and corresponding ripple formation at the irradiated surfaces. The present work describes main features of the LIPPS formation at amorphous silicon surfaces under femtosecond laser action when the pulses number (50 – 1000) and laser wavelength (417, 625, 1250 nm) are varied. These parameters combination allows to fabricate ripples with the period from 125 to 1250 nm and the orientation perpendicular or parallel relative to the laser radiation polarization. To explain such evolution, we used the model proposed by J.E. Sipe where so-called 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, 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 40% 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. The specific conductivity along the ripples twofold – threefold exceeds this value for the perpendicular reciprocal orientation of the applied current and ripples. This result is in a good agreement with the Bruggeman model for the surface possessing form anisotropy. Thus, the amorphous silicon films modified by femtosecond laser pulses possess the valid in-plane anisotropy and are promising for design of new devices for electronics and photovoltaics.