Red- and blue-light emission from free-standing porous silicon

We report the observation of strong red- and blue-light emission in free-standing porous-silicon samples prepared from n(+) substrates at different anodization current densities. The surface morphology of the free-standing samples has been analyzed by means of atomic-force microscopy. Upon excitation with nanosecond pulses at room temperature, both blue and red luminescence bands appear, peaked around 3.2 and 2.0 eV, respectively. An extensive study of the time-resolved behavior of the photoluminescence signal reveals different dynamical features for the two spectral regions. The observed long decay time (several mu s) of the red band reflects the predominant effect of non-radiative processes and is consistent. with models based on exciton diffusion through the interconnected silicon nanocrystals (quantum dots) skeleton. The relatively fast blue-band decay time (approximate to 400 ns) is shown to be related to non-radiative recombination through trap states at the silicon nanocrystal surface.