ИСТИНА

Ultrafast laser irradiation is a perspective tool for modification of thin amorphous silicon (a-Si) films’ properties to improve their performance in thin-film photovoltaics and optics [1]. Application of femtosecond laser pulses allows achieving simultaneous crystallization in the bulk and “ripples” or laser-induced periodic surface structures (LIPSS) formation on the surface of the a-Si film. The LIPSS formed due to the excitation of surface plasmon-polaritons (SPP), induce electrical anisotropy of a-Si film [2], as well as birefringence and dichroism, which can be potentially applied in polarization-sensitive devices [3]. In this work we irradiated a-Si films with various thickness (400–1200 nm) and doping type (phosphorous-doped n-a-Si or boron-doped p-a-Si) by femtosecond laser pulses (λ = 1250 nm, τ = 150 fs, ν = 10 Hz) in scanning mode at various moving speeds. The laser fluence was 0.15 – 0.3 J/cm2. In all cases on the irradiated surfaces, we observed formation of LIPSS orthogonal to the laser polarization. The LIPSS period was close to λ and decreased from 1100±100 to 840±70 nm with decreasing scanning speed. Such effect can be caused by increase of the relief height from 150±50 to 300±100 nm which was simultaneously observed when the film was modified at lower scanning speeds. The higher surface relief leads to a shift in the value of the SPP resonant period [4]. An additional contribution to this effect may be given by excessive electron emission from the surface, assuming that the a-Si film is heated more at low scanning speed. The Raman spectra demonstrated formation of crystalline silicon (c-Si) phase within irradiated films with the volume fraction up to 82±13% for p-a-Si, and up to 19±3% for n-a-Si. Dark conductivity of irradiated a-Si films increased by up to 7 orders (up to 1.2·10–2 S/cm) compared to initial films, due to the crystalline Si phase formation. The conductivity dependence of irradiated a-Si films was nonlinear due to nonuniform c-Si phase distribution within film depth, which was confirmed by Raman measurements. Electrophysical anisotropy induced by LIPSS formation was observed in all samples: the dark conductivity was up to 10 times higher along the LIPSS ridges. Observed anisotropy may be explained by LIPSS depolarizing influence, ablated surface relief and uneven crystalline phase distribution within a-Si films. The investigation was funded by the Russian Science Foundation (grant 22-19-00035), https://rscf.ru/project/22-19-00035/