Photo-oxidation effects in porous silicon luminescence

We investigate the photoluminescence (PL) evolution of porous silicon samples when exposed to air in darkness and under illumination. Oxygen incorporation is monitored through ir spectroscopy. Fourier transform infrared measurements show that samples exposed to air in darkness exhibit low oxidation rates whereas its PL spectra remain unchanged. On the other hand, samples exposed to air under illumination show a remarkable change in its PL together with a drastic increase in the oxidation rate. A well defined PL peak starts to grow at the expense of a peak located at lower energies, which is present in the PL spectra of the as-prepared samples. The set of PL spectra obtained during the evolution shows an isostilbic point that suggests the existence of two emitting components kinetically correlated. The application of the factor analysis technique confirms this asseveration and enables us to obtain the spectrum of each single emitting component. The oxidation behavior, together with the shape and evolution of the PL spectra, are explained in terms of the quantum wires hypothesis combined with an oxidation mechanism with significant nucleation. The whole set of results demonstrates that quantum size effects may rule the photo-oxidation of the porous silicon structure, and gives strong support to a quantum confinement model for porous silicon PL.