LIGHT-EMITTING POROUS SILICON - A STRUCTURAL INVESTIGATION BY HIGH-SPATIAL-RESOLUTION RAMAN-SPECTROSCOPY

The structure of light-emitting porous silicon layers, prepared by ano dization of p-type wafers, has been investigated as a function of dept h by microprobe Raman spectroscopy. The depth-profiles of the Raman sc attering from aged samples, several micrometres thick, were carried ou t in the region of the optical phonons for crystalline silicon. The Ra man data indicate the presence of both a predominant nanocrystalline p hase and of an amorphous silicon component at any depth within the por ous layers. A quantitative analysis of the experimental spectra is car ried out in the frame of phonon confinement models, by assuming both a spherical shape for the nanocrystals and an inhomogeneous distributio n of their sizes. When the probing laser spot approaches the region ne ar the interface with vacuum, a remarkable reduction of the average si ze of the nanocrystals is found, with a substantially unchanged a-Si c omponent.