Influence of HF concentration and specific resistivity of substrate Si on microstructure and luminescence behavior of porous Si

A porous silicon layer (PSL) was prepared on single crystal p-type Si (100) wafers with electrochemical etching in HF aqueous solutions of various concentrations to explore the optimum PSL preparation conditions for no exfoliation of the PSL from the substrate and for good reproducibility of visible luminescence with high intensity. The surface morphology and microstructure of the PSL were observed using various microscopic techniques (SEM, TEM and CLSM) and the surface compound of PSL was identified by FT-IR. In addition to the photoluminescence (PL), the electroluminescence (EL) emitted from the PSL during anodic oxidation in KNO₃ aqueous solution was measured to examine the luminescence mechanism. The microstructure of PSL prepared on Si with low specific resistivity (ρ=0.1 Ωm) was columnar, whereas that prepared on Si with high specific resistivity (ρ=1kΩm) was granular and randomly oriented. The microstructure of PSL also changed depending on the HF concentration. The EL and PL intensities of PSL were both higher for ρ=1kΩm than for ρ=0.1 Ωm and increased with decreasing HF concentration. The exfoliation of PSL from the substrate of ρ=1kΩm was prevented by using low current density in electrochemical etching. The optimum PSL preparation conditions were finally obtained when the specimen with specific resistivity of ρ=1kΩm was electrochemically etched in 10.4wt% HF aqueous solution under a galvanostatic condition of i=10Am^-2 for 15ks. The TEM images of PSL with high magnification (5 X 10⁵) suggest that the quantum confinement effect is operative in the visible luminescence.