Changes in the porous silicon structure induced by laser radiation

Porous silicon (PS) films were investigated by Raman, and photoluminescence spectroscopies using different laser excitations at 488.0, 514.5, 632.8, a nd 782.0 nm. The exposure of PS layers to high laser powers causes an incre ase in the 480 cm(-1) Raman intensity and a shift and enhancement of the PL emission. A laser-assisted surface reaction is proposed to explain these o bservations. The analysis of the first- and second-order Raman spectra show ed that the band gaps of the FS films are indirect as in bulk c-Si. The Ram an phonon and the PL spectra and also the spectral distribution of the line ar polarization degree (LPD) of PS layers were shown to be dependent on the laser excitation energy. This dependence cannot be explained within the qu antum confinement model. A mechanism for the PL emission in PS layers is pr esented in which the radioactive recombination of electron-hole pairs occur s in localized centres (the Si-O-SiR moieties) at the pore/crystallite inte rface. These quasi-molecular centres are Jahn-Teller active, i.e. the radio active recombination is a phonon-assisted phenomenon. Copyright (C) 2001 Jo hn Wiley & Sons, Ltd.