Anisotropic Femtosecond Laser-Induced Modification of Phosphorus- and Boron-Doped Amorphous Silicon

One-dimensional femtosecond laser-induced periodic surface structures (LIPSS) were formed on amorphous silicon (a-Si) films doped with phosphorus (n-a-Si) and boron (p-a-Si). The formed LIPSS ridges are directed orthogonally to the laser polarization and their period decreases from 1.1 ± 0.1 µm to 0.84 ± 0.07 µm for p-a-Si and from 1.06 ± 0.03 to 0.98 ± 0.01 for n-a-Si when the number of laser pulses per unit area increases from 30 to 120. Raman spectra analysis indicated nonuniform nanocrystallization of the irradiated films, which have a composite structure of amorphous matrix containing nanocrystalline Si phase with volume fraction decreasing with depth from ~80 to ~40% for p-a-Si and from ~20 to ~10% for n-a-Si. Observed in plane conductivity anisotropy of up to 1 order for irradiated films may be explained by the LIPSS depolarizing effect, excessive ablation of the film between LIPSS ridges, as well as anisotropic crystalline phase distribution within the film.