Anomalous photoluminescence behavior of porous Si

We have investigated the photoluminescence (PL) behavior of porous Si and have found that the PL intensity rises with time under exposure to continuous laser light at high power densities (≳50 W/cm2). Samples exposed to lower power density laser light exhibit the well-known degradation of its PL intensity. As the power density is raised, the PL intensity increases with time and shows a ‘‘window’’ effect in which the PL intensities will again degrade at higher power density. Micro-Raman studies were performed on the sample and no apparent change in crystalline structure could be found for differing power densities. We correlate this anomalous PL effect with porous Si samples etched with different HF acid concentrations and show that this effect occurs only for porous Si samples etched with a relatively high acid concentration (≳30%).