Optical response of vertically aligned silicon nanowires fabricated via metal-assisted chemical etching

Vertical array of Si (100) nanowires (NWs) were fabricated by one step electroless metal-assisted chemical etching (MacEtch) of p-type Si (100) wafer in an aqueous solution comprising of AgNO₃ and HF at 60 °C for different time durations ranging from 15 to 120 min. The as-etched Si wafer surface was covered with vertically aligned NW arrays along with dense dendritic Ag structures. After dissolving the Ag dendrites using HNO₃ solution, vertical array of Si NWs became clearly visible. Straight Si NW bundles were uniformly distributed across the wafer surface, with NW diameters ranging from 100 to 150 nm. The NW sidewalls exhibited significant roughness due to the formation of Si nanocrystals (NCs), with mean diameter of 6 nm, on the nanowire surfaces. The lengths of the NWs were measured to be 3.8, 6.2, 18, and 24.5 μm for the etching durations of 15, 30, 60 min, and 120 min, respectively, indicating a nearly linear growth rate with the etching time duration. The NWs retained the crystalline orientation of the Si (100) wafer, confirming that they are single-crystalline with the Si (100) planes aligned perpendicular to the nanowire axis. The Si NWs exhibited strong visible photoluminescence, originating primarily from the embedded Si NCs as a result of quantum confinement effect. The Si NWs exhibited enhanced Raman scattering, primarily attributed to the partial localization of the excitation light resulting from multiple elastic scattering within the NW array. Additionally, plasmon-assisted Raman enhancement from the Ag dendrites further contributed to the overall signal amplification.