ИСТИНА

Femtosecond laser irradiation is a promising technique for modification of amorphous hydrogenated silicon (a-Si:H) films, as it increases conductivity and optical absorption of such films due to crystallization and surface texturing. Also, micron-scale laser-induced periodic surface structures (LIPSS) can be formed in the a-Si:H films owing to excitation of surface plasmon-polaritons under effect of high-power femtosecond laser pulses. Such way structured a-Si:H films demonstrate birefringence, dichroism, and electrical anisotropy which can be used in design of polarization-sensitive optoelectronic and photovoltaic devices. However, these practical applications demand understanding the nature of the anisotropy arising in a-Si:H as a result of LIPSS formation. In our work a-Si:H films with 600 nm thickness were irradiated by femtosecond laser pulses (λ = 1250 nm, τ = 125 fs, f = 10 Hz, F = 0.5 J/cm2) in raster mode. The scan speed V varied from 2 to 50 μm/s during laser processing. After irradiation scanning electron microscopy revealed formation of LIPSS with periods close to the laser wavelength. At V = 50 μm/s one-dimensional gratings with 0.88 ± 0.03 μm period orthogonal to the laser polarization were formed. When V = 2 μm/s, the second LIPSS type formation was observed. The obtained ripples are directed along the laser radiation polarization and have the period of 1.12 ± 0.02 μm. Raman spectra indicate formation of crystalline silicon (c-Si) phase with the volume fraction from 17 to 30% inside the irradiated films. Electrical measurements showed increasing specific dark conductivity of modified films by 3 to 4 orders of magnitude, up to 3,8±0,2•10–5 (Ω∙cm)–1, due to formation of silicon nanocrystals by femtosecond laser pulses. Also, dark conductivity and photoconductivity of the modified a-Si:H films demonstrate anisotropy in the surface plane, when their values are higher along the LIPSS or the scan direction during laser irradiation. The ratios of these conductivities in mutually orthogonal directions differ up to 2.5 times. Observed electrical anisotropy is caused both by the LIPSS depolarizing effect in the effective medium theory framework and uneven c-Si phase distribution within the LIPSS and scan traces after laser irradiation. Additional analysis of photoconductivity spectral dependences and absorption coefficient spectra revealed anisotropy of the charge carrier lifetime along and orthogonal to the LIPSS. This difference of the charge carrier lifetimes explains the observed photoconductivity anisotropy.