Resonantly excited photoluminescence in porous silicon

We report on the photoluminescence (PL) spectra of porous silicon excited resonantly by laser lines within the luminescence band. Measurements have been performed for different excitation energies, temperatures and delay times. At low temperatures, the known step-like phonon structure in the PL spectra of porous silicon and a gap of the few millielectronvolts between the laser line and the onset of the luminescence are observed. As the temperature is increased, the onsets of both the PL spectra and the step features shift towards higher energies whereas the peak of the spectrum moves towards lower energies by an amount which depends on the delay time after excitation. Furthermore, the gap disappears and simultaneously an exponential tail of the spectrum occurs on the high energy side of the laser line, which broadens proportionally to kT. These results are discussed in light of the existing theories for the luminescence mechanism in porous silicon and for the origin of the step features in the PL spectra.