Polarization effects on the Raman and photoluminescence spectra of porous silicon layers

The effect of different polarization orientations of the excitation source on the Raman and photoluminescence (PL) spectra of a porous silicon (PS) layer within two different micro-regions was investigated. Two micro-regions (2 mu m) 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 tbe 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 differences in the Raman linewidths and different 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 effects are discussed from a critical point of view by taking into account quantum confinement and chemical composition of the nano-crystallite surface, since the SiH2 stretching mode was also observed in the Raman spectra. Excitation with different 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. Copyright (C) 1999 John Wiley & Sons, Ltd.