Time-resolved Fourier-transform infrared and visible luminescence spectroscopy of photoexcited porous silicon

In order to identify the luminescent centers in aged porous silicon, both the time-resolved vibration spectra in the 750–${2000-\mathrm{c}\mathrm{m}}^{\mathrm{-}1}$ region and the visible luminescence spectra are studied for the same sample in the 0–440-μs time domain. Laser excitation gives rise to emission characteristics of the porous silicon as well as strong bleach in the infrared spectrum in the region of the ground-state silicon-oxygen stretching vibration frequencies at 1100 and 1235 ${\mathrm{cm}}^{\mathrm{-}1}$. In addition, new transient absorption bands appear in this region as a result of the laser excitation. Three characteristic lifetimes in the decay of the bleach band at 1235 ${\mathrm{cm}}^{\mathrm{-}1}$ are observed on the 1-, 10-, and 100-μs time scale. These are similar to the observed decay times of the photoluminescence from porous silicon. These results suggest that the emitting centers in aged porous silicon are trapped excitation sites in an inhomogeneously defected oxidized silicon surface.