ИСТИНА

Amorphous silicon (a-Si) modified by ultrafast laser pulses is a perspective material for applications in thin-film photovoltaics and optics. Femtosecond laser irradiation 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. While simple laser-induced crystallization of a-Si film only increases its conductivity, simultaneous formation of crystalline phase and LIPSS can induce anisotropy of their electrical and optical properties. Such modified material demonstrates conductivity anisotropy and dichroism. As initial samples we used undoped as well as doped by phosphorous (n-a-Si) and boron (p-a-Si) a-Si films with thickness from 400 to 1200 nm prepared by plasma-enhanced chemical vapor deposition method on glass substrates. The films were irradiated in scanning mode by the femtosecond laser pulses (λ = 1250 nm, τ = 150 fs, ν = 10 Hz) with the fluence from 0.15 J/cm2 to 0.5 J/cm2. The scanning speed varied from 2 to 200 μm/s, and as a result, number of irradiating femtosecond laser pulses per spot Ns changed from ~1000 to ~10. In all cases on the irradiated a-Si surface we observed the formation of one-dimensional LIPSS with the period close to the laser wavelength λ used, or smaller. The ridges of LIPSS were orthogonal or parallel to laser polarization depending on the Ns value. The observed structural changes were caused by the photoexcitation of nonequilibrium electrons by high-power femtosecond laser pulses, which leads to change of the dielectric constant real part of a-Si film from positive to negative at the laser processing. The Raman spectroscopy indicated crystallization of all irradiated samples. With increase of Ns the crystalline volume fraction decreased from 67% to 14% due to re-amorphization when the high number of laser pulses Ns is applied. Additionally, we demonstrated polymorph c-Si modifications formation at laser pulses number Ns > 500. The dark conductivity after the laser treatment increased by 3–7 orders of magnitude, up to (4.5±0.1) · 10−6 (Ω·cm)−1 for the undoped films, and up to (1.1±0.5) · 10−2 (Ω·cm)−1, for the doped ones. At that along the scan lines and periodic structures the dark conductivity was up to an order of magnitude higher than in the perpendicular direction. A possible explanation such anisotropy may be given by non-uniform crystallization of amorphous silicon and the electric field depolarization inside the LIPSS. The photoconductivity also demonstrated anisotropy, presumably caused by the difference of the charge carrier lifetime along and orthogonal to the LIPSS.