Optical properties and luminescence mechanism of oxidized free-standing porous silicon films

Abstract

We have systematically studied the evolution of the optical properties of free-standing porous silicon (PS) films during thermal oxidation at 200 °C in air by measuring of the PL, IR, optical absorption and Raman scattering spectra. After thermal oxidation for 200 h, the PL peak energies of free-standing PS films focus on a small energy range centered around 1.61 eV. In this case, a conclusion that the sizes of nanometer silicon particles (NSPs) decrease with increasing time of thermal oxidation is obtained by theoretical fitting for Raman scattering spectra. The evolution of transmission curve is quite complicated (which redshifts first and then blueshifts during thermal oxidation), and can be explained by a model including the quantum confinement effect in the NSPs and the influence of the Si–O bonds on the surface of NSPs. Meanwhile, no focusing of the optical absorption edge of free-standing PS films is found. Experimental results clearly indicate that there is no simple correlation between the PL energies and the sizes or the energy gaps of NSPs. These results can be explained by the quantum confinement/luminescence center model.