Femtosecond and Nanosecond Laser Structuring of Crystalline Silicon Surface by Soft Ablation: Morphology and Optical Properties

Abstract

Laser-induced periodic surface structuring (LIPSS) is a unique method for micro-nanoscale texturing and surface functionalization of solids. Here we investigate the roles of laser pulse duration and background pressure in the fabrication of surface structures on crystalline silicon. 100 fs and 7 ns laser pulses having fluences near the ablation threshold, which produce a weak plasma, are employed. fs pulses lead to the formation of nanoscale periodic patterns on the silicon surface, with the periodicity dependent on the laser wavelength. Clean structures are obtained when the silicon wafer is irradiated in high vacuum (5 × 10-5 Torr), but the structures get decorated by silicon nanoparticles when the irradiation is under atmospheric pressure. However, irradiation using ns laser pulses produces irregular microscale structures on the surface. Raman measurements show that the crystallinity of the silicon lattice is not affected by soft ablation. Substantial reduction in surface reflectivity (surface blackening) is seen in both fs and ns structuring, with the effect more pronounced in fs structuring. Silicon surfaces blackened by fs-LIPSS can have a variety of applications in photovoltaics, silicon photonics, electro-optics devices and sensors.