RED-LIGHT AND BLUE-LIGHT EMISSION FROM FREESTANDING POROUS SILICON

We report the observation of strong red- and blue-light emission in fr ee-standing porous-silicon samples prepared from n(+) substrates at di fferent anodization current densities. The surface morphology of the f ree-standing samples has been analyzed by means of atomic-force micros copy. 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 processe s and is consistent. with models based on exciton diffusion through th e interconnected silicon nanocrystals (quantum dots) skeleton. The rel atively fast blue-band decay time (approximate to 400 ns) is shown to be related to non-radiative recombination through trap states at the s ilicon nanocrystal surface.