In situ photoluminescence and photomodulated infrared study of porous silicon during etching and in ambient

Porous silicon etched in HF without air exposure after preparation does not exhibit the well-known red photoluminescence, but a weak yellow-green emission is observed instead. The electron states whose population is affected by light excitation have been investigated using photomodulated infrared spectroscopy. For the yellow-green luminescent samples in HF, the photoinduced infrared absorption has a shape typical of free carriers; it is very large and appears correlated with photoluminescence intensity, suggesting that the yellow-green photoluminescence is due to direct recombination of the photocreated free carriers. For air-exposed red luminescent samples, the photoinduced infrared absorption is much weaker, and its shape exhibits an extra absorption in the 1500-3000 cm^-1 region, suggesting that the red luminescence occurs through carriers trapped in localized states.