Synthesis and Properties of “Black Silicon” Formed by Pd-Catalyst Etching in NH₄HF₂/H₂O₂Solution

Abstract

An etching system NH4HF2/H2O2 was developed to fabricate porous columnar structure on Si surfaces. The Pd catalyst was electroless-deposited on Si wafers in aqueous HF (5%) solution containing PdCl2. The PdCl2 concentration was varied from 0.001 to 0.04 mol/L and various etching conditions were optimized. The structural and optical properties of the catalytic-etched samples were investigated by scanning electron microscopy, wettability observation, Fourier-transform infrared spectroscopy analysis, near-IR−visible transmittance, photoluminescence (PL), and Raman scattering measurements. Vertically well-aligned columnar structure was formed in the limited conditions of electroless Pd deposition and subsequent catalytic etching. Such columns also formed a “macroporous” structure, in direct contrast to the case of the HF/AgNO3 etching (i.e., “dense” nanowires). No strong PL emission was observed from the present samples. The Raman scattering intensities were much stronger with more asymmetric and broader spectral features than that for the bulk Si wafer. Such Raman spectra were well interpreted by a modified Lorentzian line-shape model. Passive HF etching of the catalytic-etched surface changed wettability from superhydrophilic (∼0°) to highly hydrophobic (∼140°). The catalytic-etched Si samples were also found to exhibit “black silicon” in nature.