Optical spectroscopy studies of single layers and superstructures of porous silicon

In this paper, after a brief introduction describing the main properties of porous silicon (PS) that have attracted a great interest in view of possible applications in several scientific and technological fields, some recent studies by optical spectroscopies (Raman, IR, SHG) on PS systems, carried on by our group and collaborators, are reviewed. In the first part, dedicated to the study of single layers of PS, the spectra obtained by micro-Raman spectroscopy from some thin mesoporous samples at different laser power are reported. The thermal effects induced by the laser heating are discussed and it is shown how it is possible to evaluate, among other things, the thermal conductivity of the measured PS material. By FTIR reflection spectroscopy the oxidation processes of PS in humid air with and without pyridine vapor have been studied. The main results are the description of the time evolution of the oxidation processes and the discovery of the catalytic role played by the organic base that accelerates the oxidation owing mainly to the water present in the air. In the second part, concerning the works on PS superstructures (stacks of PS layers of different thickness and dielectric function), the study by FTIR spectroscopy of the dispersion of cavity-polaritons, due to the interaction of the Si-H absorption bands with the cavity mode in a PS Fabry-Perot is presented. Finally the enhancement of some order of magnitude of the signal (K Raman and SHG) in PS Fabry-Perot systems, due to the electric filed confinement in the cavity and to the resonance with the incident and/or the emitted light, is reported and discussed.