Unraveling the morphological evolution and etching kinetics of porous silicon nanowires during metal-assisted chemical etching

Many potential applications of porous silicon nanowires (SiNWs) fabricated with metal-assisted chemical etching are highly dependent on the precise control of morphology for device optimization. However, the effects of key etching parameters, such as the amount of deposited metal catalyst, HF–oxidant molar ratio (χ), and solvent concentration, on the morphology and etching kinetics of the SiNWs still have not been fully explored. Here, the changes in the nanostructure and etch rate of degenerately doped p-type silicon in a HF–H₂O₂ –H₂ O etching system with electrolessly deposited silver catalyst are systematically investigated. The surface morphology is found to evolve from a microporous and cratered structure to a uniform array of SiNWs at sufficiently high χ values. The etch rates at the nanostructure base and tip are correlated with the primary etching induced by Ag and the secondary etching induced by metal ions and diffused holes, respectively. The H₂O concentration also affects the χ window where SiNWs form and the etch rates, mainly by modulating the reactant dilution and diffusion rate. By controlling the secondary etching and reactant diffusion via χ and H₂O concentration, respectively, the fabrication of highly doped SiNWs with independent control of porosity from length is successfully demonstrated, which can be potentially utilized to improve the performance of SiNW-based devices.