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 orig ins of this phenomenon are still not clearly assessed, Two main models have been proposed: a quantum confinement of charge carriers in quant um sized Si crystallites found in porous silicon (PS) and the intrinsi c luminescence of direct band gap species (such as polysilanes or poly siloxenes) suggested to form during various stages of the PS fabricati on. These Si based compounds, of unknown well-defined nature, eventual ly may constitute caplayers, passivating the Si nanocrystallites. To c larify this point, highly PS obtained from lightly doped (p(-)) substr ates was investigated by electron energy loss spectroscopy and by high resolution electron microscopy in order to determine its local chemic al microstructure. Our results lead to a microstructural model for the investigated PS samples composed mostly by nanocrystallites of silico n surrounded by a passivated layer of amorphous hydrogenated silicon, A fine study of the low loss region of the spectra was performed detai ling the contributions of different peaks observed in the collective p lasma oscillation energy range, A bulk Si plasmon was recorded at cong ruent to 17 eV. An interface plasmon, between Si nanocrystallites and their caplayer mainly formed by amorphous silicon passivated with hydr ogen (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 capl ayer. The respective intensities of these peaks imply very large surfa ce/volume ratios and suggest also a significant contribution of the su rface a-Si-H caplayer. Electron beam irradiation induces the disappear ance of these surface and interface peaks and enhances the bulk Si sec ond order plasmon. This provides us with a clear evidence on the desor ption of hydrogen from the a-Si-H passivated layer and on its recrysta llisation. The disappearance of this passivated layer under electron b eam irradiation is correlated to the photoluminescence degradation upo n annealing or under illumination.