The e โ ect of di โ erent polarization orientations of the excitation source on the Raman and photoluminescence (PL) spectra of a porous silicon (PS) layer within two di โ erent micro-regions was investigated. Two micro-regions (2 lm) along the (100) direction were studied, close to the crystalline silicon (c-Si)-PS layer interface and the PS layer-air interface. The Raman depolarization factors obtained in the 100-1200 cm-1 phonon spectra of these two micro-regions showed that the anisotropy presented by the PS layer is similar to that presented by the c-Si (100). Large di โ erences in the Raman linewidths and di โ erent emission maxima in the PL spectra were observed for the two micro-regions. These results were interpreted as the PS layer being inhomogeneous along the (100) direction. These e โ ects are discussed from a critical point of view by taking into account quantum conรnement and chemical composition of the nano-crystallite surface, since the stretching mode was also observed in the Raman SiH 2 spectra. Excitation with di โ erent polarizations causes a shift of the emission maxima. This shift was assigned to a polarization dichroism caused by a breakdown on the degeneracy as the nano-crystallites having a reduced symmetry in relation to c-Si. The results strongly support the hypothesis that the luminescence presented by the PS layer is due to the presence of Si-based compounds on the surface of Si nanocrystallites.