ИСТИНА

Amorphous hydrogenated silicon films (a-Si:H) modified by femtosecond laser radiation are of great interest for thin-film photovoltaics. Using ultrashort laser pulses allows to obtain silicon nanocrystals that are uniformly distributed over the entire volume of a-Si:H film. Such treatment can be used to reduce photodegradation of the a-Si:H film electric properties under long-term light irradiation (Staebler-Wronski effect) and also enhance light absorption. In addition, femtosecond laser modification leads to anisotropy of the structural, optical and electrical properties of irradiated surfaces. In particular, periodic structures can be formed on the a-Si: H surface by femtosecond laser pulses due to plasmon-polariton excitation. In this paper, a-Si:H films were irradiated by femtosecond laser pulses (1250 nm, 125 fs) in the raster mode. Scanning electron microscopy revealed presence of various laser-induced periodic surface structures (LIPSS) on the treated surface of a-Si:H, including one-dimensional gratings with a period of 1.20 ± 0.02 μm. The shape and orientation of the observed LIPSS is determined by incident beam polarization and changes depending on the number of femtosecond laser irradiated spots Ns that were overlapping during rastering of the a-Si:H film. The observed LIPSS evolution with increasing Ns is due to the emerging of a feedback between the formed surface relief and the electronic processes in the film. Raman spectroscopy revealed the formation of silicon nanocrystals in the a-Si:H films caused by ultrashort laser pulses of high power. The calculated nanocrystalline silicon phase volume fraction ranges from 15 to 67% depending on processing conditions of the a-Si:H film. In addition, the presence of silicon polymorphs Si-III and Si-XII was detected in the a-Si:H films processed with Ns ≥ 500. The proposed explanation of the observed Raman signal anisotropy is related to the ordered crystallographic axes orientation of silicon polymorphous nanocrystals formed in the film as a result of pulsed laser modification. Thus the considered anisotropic structures based on a-Si:H can be used to create polarization-sensitive elements of optics and photovoltaics. This work was supported by the Russian Foundation for Basic Research (project 17-52-04062) and Belarussian Republican Foundation for Fundamental Research (project F17RM-079)