EELS investigation of luminescent nanoporous p-type silicon

Although a great deal of work has been performed on the understanding of luminescence mechanisms in highly porous silicon, the physical origins of this phenomenon are still not clearly assessed. Two main models have been proposed: a quantum confinement of charge carriers in quantum sized Si crystallites found in porous silicon (PS) and the intrinsic luminescence of direct band gap species (such as polysilanes or polysiloxenes) suggested to form during various stages of the PS fabrication. These Si based compounds, of unknown well-defined nature, eventually may constitute caplayers, passivating the Si nanocrystallites. To clarify this point, highly PS obtained from lightly doped (p−) substrates was investigated by electron energy loss spectroscopy and by high resolution electron microscopy in order to determine its local chemical microstructure. Our results lead to a microstructural model for the investigated PS samples composed mostly by nanocrystallites of silicon surrounded by a passivated layer of amorphous hydrogenated silicon. A fine study of the low loss region of the spectra was performed detailing the contributions of different peaks observed in the collective plasma oscillation energy range. A bulk Si plasmon was recorded at ≅ 17 eV. An interface plasmon, between Si nanocrystallites and their caplayer mainly formed by amorphous silicon passivated with hydrogen (a-SiH) was recorded around 9 eV and finally a surface plasmon, recorded at about 12 eV, was propagating along the a-SiH surface caplayer. The respective intensities of these peaks imply very large surface/volume ratios and suggest also a significant contribution of the surface a-SiH caplayer. Electron beam irradiation induces the disappearance of these surface and interface peaks and enhances the bulk Si second order plasmon. This provides us with a clear evidence on the desorption of hydrogen from the a-SiH passivated layer and on its recrystallisation. The disappearance of this passivated layer under electron beam irradiation is correlated to the photoluminescence degradation upon annealing or under illumination.