ИСТИНА

Modification of amorphous hydrogenated silicon (a-Si:H) by high-power femtosecond laser pulses allows to achieve anisotropy of their structural, electrical and optical properties due to formation of laser-induced periodic surface structures (LIPSS) [1]. Such modified material can possess conductivity anisotropy [1] and dichroism [2], which can be used in thin-film photovoltaics and optoelectronics. In this work we experimentally observed formation of LIPSS with the period close to the wavelength of laser pulses (1250 nm). The orientation of such structures is parallel or perpendicular to the laser radiation polarization depending on the pulse number used during irradiation. According to the theoretical modeling [3], the observed structural changes were caused by different concentrations of nonequilibrium electrons excited by various numbers of high-power femtosecond laser pulses, which leads to variation of the dielectric constant real part of photoexcited a-Si:H film from negative to positive during the laser processing. The threshold values of the nonequilibrium electrons concentration required for turning the LIPSS direction were calculated. [1] Shuleiko D. V., Potemkin F. V., Romanov I. A., et al. Laser Phys. Lett. 15:056001 (2018) [2] R. Drevinskas, M. Beresna, M. Gecevičius, et al. Appl. Phys. Letters 106:171106 (2015) [3] J. Bonse, M. Munz, H. Sturm. J. Appl. Phys. 97:013538 (2005)