Optical Characterization of Free-Standing Porous Silicon Films

We report here the optical characterization of two free-standing porous silicon (PS) films of porosity 70% and 85%, grown on a p- and a n-type Si. We determine the optical band gap in these films from the excitation wavelength dependence of the photoluminescence band. As the excitation wavelength is changed from red (800 nm) to UV (355 nm), a blue shift of the photoluminescence (PL) band is observed. We attribute the observed blue shift of the PL band to the emission due to the distribution of bandgap in PS. Both samples are oxidized in air and we believe that the observed bandgap in these films arises from the inhomogeneous distribution of Si particle sizes. However, we find that intrinsic defects play a dominant role in the process of luminescence. Electron spin resonance measurement indicates the presence of defects leading to the saturation of the optical absorption spectra and a decrease in intensity of the PL band. The lineshape of the PL band is modeled using a weighted asymmetric Gaussian that selects a gap distribution function at each excitation wavelength. From Raman measurements in these two films, the quantum confinement effect in Si nanostructures is clearly observed in both films.