Effect of nonaqueous anodic oxidation on the intensity of photoluminescence of porous silicon

Porous silicon (PS) was anodized for short periods in 0.02 m KNO3–ethylene glycol electrolyte to improve the maximum intensity of its photoluminescence (PL) by changing surface –Si–H bonds to –Si–OH or –Si–O-related compounds. A PS sample prepared in 1:1 (49% HF:99.5% EtOH) electrolyte gave 15-fold PL intensity as well as stabilized luminescence with 5 min anodization. Prolonged anodization, however, peeled off the nano-ordered silicon particles and resulted in a decrease in PL intensity. The PL intensity of the PS sample prepared in 1:2 electrolyte decreased with 1min anodization but increased with 30 s anodization. During anodization, the nano-ordered silicon particles reacted with water, an impurity in ethylene glycol, to give Si–OH and Si–O-related compounds. Ethylene glycol proved to be the best anodization solvent for nano-ordered silicon particles because of its high resistivity, high viscosity, and good electrochemical stability. However, ethylene glycol had to be removed completely from the PS surface by rinsing with pure water, because polyhydroxy alcohols such as ethylene glycol behaved as quenchers for excited electrons formed in Si–OH-related compounds on the nano-ordered silicon as a result of illumination.