Optical characterization of porous materials

In this work the application of spectroscopic ellipsometry in the characterization of porous materials is discussed. Porous Si serves hereby as a model system for this important class of materials. The effective dielectric functions < epsilon > measured by spectroscopic ellipsometry exhibits features corresponding to highly broadened interband critical points (E-1,E'(0)) and E-2 of bulk silicon, which indicates that porous Si is fundamentally crystalline. From lineshape analysis we can determine layer thickness and porosity. Furthermore, the dielectric function and hence the electronic properties are not only affectcd by surface effects, but are dominated by these effects. It seems also that the optical response is affected by the connectivity of the structure. Finally, the homogeneity in depth can be monitored (and controlled) by ellipsometry applied during electrochemical etching. However, for a quantitative analysis of the measured data a thorough understanding of the surface and interface contributions to the optical response and the modified response of nanocrystallites is necessary. This is not yet been achieved and is a formidable task.