ИСТИНА

Femtosecond laser pulses are a powerful tool for modifying the amorphous silicon (a-Si) films on micro- and nanoscale. Nanocrystallization of a-Si films by femtosecond laser radiation increases the conductivity and reduces the photodegradation of this material which is perspective in photovoltaics [1]. Additionally, due to the excitation of plasmon polaritons, the laser-induced periodic surface structures (LIPSS) [2] possessing birefringence and dichroism on amorphous silicon can be formed and potentially applied in polarization optics [3]. In this work, a-Si films with thickness from 400 to 1200 nm and different types of doping (phosphorous-doped n-a-Si or boron-doped p-a-Si) were irradiated by femtosecond laser pulses (λ = 1250 nm, τ = 150 fs, ν = 10 Hz) with fluence 0.15 – 0.3 J/cm2 in scanning mode at various moving speeds. In all cases LIPSS orthogonal to laser polarization on the irradiated surfaces were observed. The LIPSS period was close to λ and varied from 840±70 to 1100±100 nm with increasing scanning speed from 50 to 200 μm/s. Simultaneously the LIPSS relief increased from 100 nm up to 400 nm. 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 was induced by LIPSS formation on all sample’s surfaces: 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 work was supported by the Russian Science Foundation (project 22-19-00035).