ИСТИНА

Ultrafast laser irradiation is a powerful 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 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 is perspective for polarization-sensitive devices [3]. In this work we irradiated phosphorous-doped n-a-Si and boron-doped p-a-Si films with thickness 400–1200 nm 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. We demonstrated LIPSS formation in all cases on the irradiated surfaces. The LIPSS were orthogonal to the laser polarization. Their period was close to λ (from 1100±100 to 840±70 nm) and the relief height varied from 150±50 to 300±100 nm. Observed LIPSS period decrease with increasing the total irradiation dose of the film can be explained by a shift in the value of the SPP resonant period, caused by growth of the relief height [4]. The Raman spectra demonstrated formation of nonuniformly distributed crystalline silicon (c-Si) phase within irradiated films with the volume fraction maximum at the surface 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. We also observed nonlinearity of the conductivity dependence for the irradiated a-Si films, which was caused by nonuniform c-Si phase distribution within film depth. LIPSS formation induced electrophysical anisotropy 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/