Investigation on surface morphological and optical properties of black silicon fabricated by metal-assisted chemical etching with different etchant concentrations

In this study, the surface morphological and optical properties of black silicon (b-Si) fabricated by two-step metal-assisted chemical etching (MACE) process are investigated. The two-step MACE combines low-temperature annealing of silver (Ag) thin film to produce Ag nanoparticles (NPs) and short etching duration of crystalline silicon (c-Si) wafer. The etching is carried out in HF:H₂O₂:DI H₂O solution for 70 s with different etchant concentrations (represented in the form of volume ratio). The MACE process produces b-Si nanopores on the wafer. Compared with planar c-Si reference, broadband reflection (in 300-1100 nm wavelength region) of the b-Si is significantly lower. B-Si wafer with volume ratio of 1:5:10 exhibits the lowest broadband reflection of 3% at wavelength of 600 nm, which is believed to be due to refractive index grading which leads to enhanced light coupling into the b-Si wafer. The best b-Si wafer (with lowest reflection) shows 50 nm average pillar width and 300 nm height. The increased broadband light absorption results in the highest maximum potential short-circuit current density (J_sc(max)) of 40.9 mA/cm². This represents 55.4% enhancement, if compared with the planar c-Si reference wafer, assuming unity carrier collection. (paper)