IN-SITU PHOTOLUMINESCENCE AND PHOTOMODULATED INFRARED STUDY OF POROUSSILICON DURING ETCHING AND IN AMBIENT

Porous silicon etched in HF without air exposure after preparation doe s not exhibit the well-known red photoluminescence, but a weak yellow- green emission is observed instead. The electron states whose populati on is affected by light excitation have been investigated using photom odulated infrared spectroscopy. For the yellow-green luminescent sampl es in HF, the photoinduced infrared absorption has a shape typical of free carriers; it is very large and appears correlated with photolumin escence 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 absorpt ion is much weaker, and its shape exhibits an extra absorption in the 1500-3000cm(-1) region, suggesting that the red luminescence occurs th rough carriers trapped in localized states.