Influence of wavelength on the Raman line shape in porous silicon

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 penetration depth. In this communication we report depth-resolved micro-Raman spectra at 514.5 and 632.8 nm. The spectra were measured at different points along across section of porous silicon films. We show that even 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 of Physics. [S0021-8979(98)00216-3].