Femtosecond laser modification of amorphous silicon films for photovoltaic and polarization optics applications

Femtosecond laser structuring is a promising method for obtaining amorphous-crystalline silicon (a-Si/c-Si) heterojunction in a-Si thin films, as well as surface structures with optical anisotropy. Depth-resolved Raman spectroscopy of a-Si film irradiated at laser fluence of 0.1 J/cm2, which is below a-Si ablation threshold, revealed its surface crystallization with crystallized layer characteristic depth of 45 ± 5 nm. As a result of such laser irradiation, the electric current rectification coefficient in the film, determined from electrophysical measurements, increased from 2.7 to 13.6 indicating possible formation of a-Si/c-Si heterojunction. The presence of 10-nm-thick Al coating decreases the number of pulses per unit area required for a-Si crystallization by 2.5 times. Optical anisotropy of laser-crystallized a-Si films is manifested in their optical retardance of 280 ± 40 nm, caused by the formation of one-dimensional surface relief with 1100 ± 50 nm period.