Femtosecond Laser Fabrication of Anisotropic Structures in Phosphorus- and Boron-Doped Amorphous Silicon Films

Femtosecond laser-modified amorphous silicon (a-Si) films with optical and electrical anisotropy have perspective polarization-sensitive applications in optics, photovoltaics, and sensors. We demonstrate the formation of one-dimensional femtosecond laser-induced periodic surface structures (LIPSS) on the surface of phosphorus-(n-a-Si) and boron-doped (p-a-Si) amorphous silicon films. The LIPSS are orthogonal to the laser polarization, and their period decreases from 1.1 +/- 0.1 mu m to 0.84 +/- 0.07 mu m 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 indicates nonuniform nanocrystallization of the irradiated films, with the nanocrystalline Si phase volume fraction decreasing with depth from similar to 80 to similar to 40% for p-a-Si and from similar to 20 to similar to 10% for n-a-Si. LIPSS' depolarizing effect, excessive ablation of the film between LIPSS ridges, as well as anisotropic crystalline phase distribution within the film lead to the emergence of conductivity anisotropy of up to 1 order for irradiated films. Current-voltage characteristic nonlinearity observed for modified p-a-Si samples may be associated with the presence of both the crystalline and amorphous phases, resulting in the formation of potential barriers for the in-plane carrier transport and Schottky barriers at the electric contacts.

Automated summary

Femtosecond laser-modified amorphous silicon films demonstrated the formation of one-dimensional femtosecond laser-induced periodic surface structures, resulting in conductivity anisotropy. The nonuniform nanocrystallization of the irradiated films, depolarizing effect of LIPSS, and anisotropic crystalline phase distribution within the film contributed to the emergence of conductivity anisotropy. The nonlinearity observed in current-voltage characteristics of modified p-a-Si samples may be associated with the presence of both crystalline and amorphous phases.