High-quality fs-laser structured Si in air for optoelectronic applications

• Rapid thermal annealing removes femtosecond-laser–induced extended crystalline defects in black silicon. • Annealing drives oxygen toward the surface, forming an oxide-rich layer revealed by STEM and ToF-SIMS. • Buffered oxide etching selectively removes the oxygen-rich decorations without altering the microstructure. • The combined RTA and BOE processes yield high absorptance with minimized defect-related optical losses. In this work, we describe a methodology to highly improve the quality of micro-structured silicon obtained by femtosecond-laser irradiation in ambient air atmosphere. Optimum femtosecond laser irradiation conditions have been selected to obtain a high aspect ratio spike morphology at the surface of silicon. Due to the aggressive fs-laser process, the crystalline structure of the material presents extended defects, which can be removed by rapid thermal annealing. Moreover, thermal annealing triggers diffusion of oxygen to the surface, which was incorporated from the air atmosphere into the bulk upon repetitive fs-laser irradiation. This oxygen binds with silicon atoms within the amorphous surface layer, giving rise to SiOx, obtaining irregular decorations at the surface of the spike structures. These decorations can be removed by a buffered hydrofluoric acid solution, which etches the silicon oxide but not the crystalline silicon. Using this strategy we have obtained a high crystalline quality with: high absorption of around 95 % in the wavelength region from 200 to 1100 nm, minimized defect-related absorption from 1100 to 2500 nm; a crystalline lattice free of defects and oxygen; and a high aspect-ratio micro-spike morphology formed by c-Si free of surface SiO x nano-decorations that can be useful to fabricate optoelectronic devices.