Ultra-high aspect ratio functional nanoporous silicon via nucleated catalysts
Nanoporous silicon (NPSi) has drawn recent interest because of its potential in a range of applications such as battery anodes, photocatalysis, thermoelectrics, and filtration membranes. However, the inexpensive and scalable manufacturing of high aspect ratio porous structures on the nanometer scale has been difficult due to the reliance of current methods on complex and expensive equipment used for techniques such as anodization or photolithography. Here, we report a method of producing NPSi with sub-10 nm pore sizes and aspect ratios as high as 400:1 by leveraging the nucleation of sputtered noble metals on the Si surface, followed by metal-assisted chemical etching (MACE). The technique is capable of producing NPSi in an intrinsically scalable manner. Samples are characterized with SEM and TEM, along with vertical and horizontal FIB cross-sectional milling to elucidate the porous structure at several μm of depth within the substrate. Following preparation of the NPSi, it is functionalized with Al2O3 and TiO2 via atomic layer deposition (ALD). TiO2-functionalized NPSi exhibits reflectivity of 6–8% for visible wavelengths, and 2–3% in the infrared – showing its promise as a robust and functional porous substrate. The developed approach of employing MACE with sputtered nucleated catalysts facilitates the scalable fabrication of functional ultra-high aspect-ratio nanopores in silicon.