Micro-Raman study of free-standing porous silicon samples

Micro-Raman spectroscopy has been employed for the characterization of a set of free-standing porous silicon samples with different degrees of porosity, fabricated by electrical anodization of n+ Si wafers. A broadening and a shift toward lower energy of the transverse optical silicon phonon mode have been clearly observed, indicating the presence of nanometer-sized crystalline structures. In particular, higher porosity samples reveal an inhomogeneous structure characterized by a spread of the nanocrystal size as a function of the region under investigation. The photoluminescence properties of samples produced with similar fabrication parameters, displaying both a blue and a red band, seem to be in agreement with the nanocrystal dimensions derived through micro-Raman spectroscopy.