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

We have systematically studied the evolution of the optical properties of f ree-standing porous silicon (PS) films during thermal oxidation at 200 degr ees C in air by measuring of the PL, IR, optical absorption and Raman scatt ering spectra. After thermal oxidation for 200 h, the PL peak energies of f ree-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 th eoretical fitting for Raman scattering spectra. The evolution of transmissi on curve is quite complicated (which redshifts first and then blueshifts du ring thermal oxidation), and can be explained by a model including the quan tum confinement effect in the NSPs and the influence of the Si-O bonds on t he surface of NSPs. Meanwhile, no focusing of the optical absorption edge o f free-standing PS films is found. Experimental results clearly indicate th at 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 conf inement/luminescence center model. (C) 1999 American Institute of Physics. [S0021-8979(99)06816-4].