Photoluminescence quenching effect on porous silicon films for gas sensors application

Porous silicon (PS) films were investigated by Raman, and photoluminescence (PL) spectroscopies using different laser excitations: 488.0, 514.5, 632.8, and 782.0 nm. The analysis of the first-order and second-order Raman spectra have shown that the band gaps of the PS films are indirect as in the bulk c-Si. The Raman phonon and the PL spectra as well as the spectral distribution of the linear polarisation degree (LPD) of PS layers have shown to be dependent on the laser excitation energy. This dependence cannot be explained within the quantum confinement model. A mechanism for the PL emission in PS layers is presented in which the radiative recombination of electron-hole pairs occurs in localised centres (the Si-O-SiR moieties) at the pore/crystallite interface. These quasi-molecular centres are Jahn-Teller active, i.e. the radiative recombination is a phonon-assisted phenomena. The adsorption of gas molecules on the porous silicon surface was studied throughout photoluminescence quenching effect. The adsorption experiments were performed at 10(-6) bar of pressure using gas molecules of organic solvents. In all these cases, the PL intensity was recovered after gas desorption. The PL quenching effect was explained in the sense of electron transfer mechanism (ET).