We have measured micro-potoluminescence (PL) and micro-Raman spectra on the cross section of porous silicon multilayers to sample different layer depths. We find noticelable differences in the spectra of layers with different porosity, as experted from the the quantum confinement of electrons and phonons in silicon nanocrystals with different average sizes. The PL emission band gets stronger, blue shifts, and narrows at the high porosity layers. The average isze can be estimated from the shift. The Raman phonon band at 520 cm~(-1) weakens and broadens asymmetrically to-wards the low energy side. The line shape can be related quantitatively with the average size by the phonon confinement model. To get a good agreement with the model we add a band at around 480 cm~(-1), which has been attributed to amorphous silicon. We also have to leave as free parameters the bulk solicon phonon frequency and its line width, which depend on temperature and stress. We reduced laser power to eliminate heating effects. Then we use the change of frequency with depth to monitor the stress. At the interface with the substrate we find a compressive stress in excess of 10 kbar, which agrees with the reported lattice mismatch. FInally, average sizes are laeger than those estimated from PL.
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