Fabrication of vertical silicon nanowire arrays on three-dimensional micro-pyramid-based silicon substrate

Recently, a lot of attention is being paid to combine silicon nanowires (SiNWs) and the conventional micro-pyramid silicon (µT-Si) structures leading to SiNWs/µT-Si binary structures for efficient light harvesting. We report large-area fabrication of vertical SiNWs array over three-dimensional micro-pyramid structured silicon substrate through silver (Ag)-assisted electroless wet chemical etching in aqueous HF and AgNO3 solution. The influence of AgNO3 concentration has been investigated over the formation of the SiNWs. Vertical SiNWs formation takes place for lower AgNO3 concentrations. For higher concentrations, random formation of NWs and depletion of the micro-pyramids is observed. The present study reveals strong influence of local morphology of micro-pyramids over NWs formation. The formation mechanism of such binary structures has been discussed in co-relation with dimensional features of the micro-pyramids and surface free energy. Such hierarchically textured binary structured silicon surfaces exhibit excellent light trapping properties reducing reflectance to as low as <3 % in broad spectral range. Further, such surfaces also have enhanced Raman spectra (~15-fold enhancement) as compared to only micro-pyramid Si surfaces, thereby having potential applications for photovoltaic devices as well as surface enhanced Raman spectroscopic investigations.