The line shape of the one phonon Raman peak has been used extensively
in the literature to estimate the crystallite size in porous silicon.
However it has been shown that the line shape obtained on top surface
experiments depends on the excitation wavelength. Because the porosity
depends on depth, previous results are masked by the change in penetr
ation depth. In this communication we report depth-resolved micro-Rama
n spectra at 514.5 and 632.8 nm. The spectra were measured at differen
t points along across section of porous silicon films. We show that ev
en when the same layer and, therefore, the same porosity is probed the
Raman peak is broader at shorter wavelengths. To explain the results
we suggest a contribution of indirect gaps to the resonant Raman cross
section induced by quantum confinement. (C) 1998 American Institute o
f Physics. [S0021-8979(98)00216-3].