Optical characterization of porous materials

In this work the application of spectroscopic ellipsometry in the character ization of porous materials is discussed. Porous Si serves hereby as a mode l system for this important class of materials. The effective dielectric fu nctions < epsilon > measured by spectroscopic ellipsometry exhibits feature s corresponding to highly broadened interband critical points (E-1,E'(0)) a nd E-2 of bulk silicon, which indicates that porous Si is fundamentally cry stalline. From lineshape analysis we can determine layer thickness and poro sity. Furthermore, the dielectric function and hence the electronic propert ies are not only affectcd by surface effects, but are dominated by these ef fects. It seems also that the optical response is affected by the connectiv ity of the structure. Finally, the homogeneity in depth can be monitored (a nd controlled) by ellipsometry applied during electrochemical etching. Howe ver, for a quantitative analysis of the measured data a thorough understand ing of the surface and interface contributions to the optical response and the modified response of nanocrystallites is necessary. This is not yet bee n achieved and is a formidable task.