Liquid-assisted laser nanotexturing of silicon: onset of hydrodynamic processes regulated by laser-induced periodic surface structures

TL;DR

This study investigates femtosecond-laser processing of silicon in methanol, revealing how electromagnetic and hydrodynamic processes shape surface nanostructures, enabling tailored nanotexturing for improved photodetector performance.

Contribution

It uncovers the physical mechanisms behind morphology evolution during laser nanotexturing and demonstrates control over surface structures for enhanced silicon photodetectors.

Findings

LIPSS evolve via Rayleigh-Plateau instability in molten ridges.

Surface morphology can be tailored by laser intensity and polarization.

Resulting nanostructures reduce reflectivity and improve IR photoresponse.

Abstract

Here, upon systematic studies of femtosecond-laser processing of monocrystalline Si in oxidation-preventing methanol, we showed that the electromagnetic processes dominating at initial steps of the progressive morphology evolution define the onset of the hydrodynamic processes and resulting morphology upon subsequent multi-pulse exposure. In particular, under promoted exposure quasi-regular subwavelength laser-induced periodic surface structures (LIPSSs) were justified to evolve through the template-assisted development of the Rayleigh-Plateau hydrodynamic instability in the molten ridges forming quasi-regular surface patterns with a supra-wavelength periodicity and preferential alignment along polarization direction of the incident light. Subsequent exposure promotes fusion-assisted morphology rearrangement resulting in a spiky surface with a random orientation, yet constant inter-structure distance correlated with initial LIPSS periodicity. Along with the insight onto the physical picture driving the morphology evolution and supra-wavelength nanostructure formation, our experiments also demonstrated that the resulting quasi-regular and random spiky morphology can be tailored by the intensity/polarization distribution of the incident laser beam allowing on-demand surface nanotexturing with diverse hierarchical surface morphologies exhibiting reduced reflectivity in the visible and shortwave IR spectral ranges. Finally, we highlighted the practical attractiveness of the suggested approach for improving near-IR photoresponse and expanding operation spectral range of vertical p-n junction Si photo-detector operating under room temperature and zero-bias conditions via single-step annealing-free laser nanopatterning of its surface.