Structural anisotropy of amorphous silicon films modified by femtosecond laser pulses

It is demonstrated that the surface-relief orientation in the form of one-dimensional gratings with a period of 1.20 $\pm$ 0.02 $\mu$m formed under processing of hydrogenated-silicon films by femtosecond laser pulses (1.25 $\mu$m) with an energy density of 0.15 J/cm$^2$ is determined by the direction of the polarization vector of the radiation and total laser exposure. Based on the results of the analysis of Raman spectra, the presence of a nanocrystalline phase of silicon with a volume fraction between 15 and 67% (depending on processing conditions) is detected. The observed processes of micro- and nanostructuring are caused by excitation of the surface plasmon–polaritons and nanocrystallization in the near-surface region in the field of high-power femtosecond laser pulses, respectively. In addition, formation of polymorph modifications of silicon Si-III and Si-XII under femtosecond laser processing with a number of pulses exceeding 500 is discovered. Anisotropy of the Raman signal for the above polymorph modifications is revealed.