Stabilization of Photoluminescence of Porous Silicon with Nonaqueous Anodic Oxidation

In order to prevent the reduction of photoluminescence from porous silicon, we tried to form stable Si–O bonds by anodization in nonaqueous electrolyte at room temperature. Extremely strong, stable, and blue-shifted photoluminescence was obtained in porous silicon that was prepared in 1:1 solution of 49% HF and EtOH and subsequently anodized in KNO3-ethylene glycol electrolytes. The optimum condition was anodization at 20 mA cm-2 for 5 min in 0.02 M KNO3-ethylene glycol. For the formation of Si–O bonds on nanostructured surfaces, it is suggested that the electric field across the surface/electrolyte plays an important role because oxidation becomes more effective when the electrolyte is more resistive. Si–OH, which is formed as a precursor at subsequent anodization, stabilizes the nanostructured surface, converting itself to more stable Si–O bond with photoexcitation. The high current density introduced destroys nanostructured silicon by clustering with surrounding SiO2. With this clustering, wall sizes of the nanostructured silicon remain constant with anodization.