Liquid‐Assisted Laser Nanotexturing of Silicon: Onset of Hydrodynamic Processes Regulated by Laser‐Induced Periodic Surface Structures

Abstract Upon systematic studies of femtosecond‐laser processing of monocrystalline Si in oxidation‐preventing methanol, it is shown that the electromagnetic processes dominating at initial steps of the progressive morphology evolution define the onset of the hydrodynamic processes and morphology upon subsequent multi‐pulse exposure. Under promoted exposure quasi‐regular subwavelength laser‐induced periodic surface structures (LIPSSs) are justified to evolve through the template‐assisted development of the Rayleigh‐Plateau hydrodynamic instability in the molten ridges forming quasi‐regular 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 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, this experiments also demonstrated that resulting quasi‐regular and random spiky morphology can be tailored by the intensity/polarization distribution of incident laser beam allowing on‐demand surface nanotexturing with diverse hierarchical surface morphologies exhibiting reduced reflectivity at visible and shortwave‐IR wavelengths. Finally, the practical attractiveness of the suggested approach for improving near‐IR photoresponse and expanding operation spectral range of vertical p‐n junction Si photodetector operating under room temperature and zero‐bias conditions via single‐step annealing‐free nanopatterning is highlighted.