In this paper we show that the lineshapes of the dielectric functions of porous p-doped silicon layers are strongly correlated to their original bulk doping concentrations. Features in the dielectric functions of layers formed from heavily doped bulk material correspond to bulk critical points, with an additional feature in the visible that results from the electronic structure of silicon that has been changed by the presence of inner surfaces. These results suggest that the layers remain highly connected. For lightly doped starting material the bulk-related features are much broader, with some samples exhibiting only an E2 interband critical point feature that is shifted to higher energies. This shift, which can be understood in terms of strong confinement of the carriers, suggests reduced connectivity in these layers.
Rossow, U., Frotscher, U., Pietryga, C., Aspnes, D. E., & Richter, W. (1996). Porous silicon layers as a model system for nanostructures. Applied Surface Science, 104–105, 552–556. https://doi.org/10.1016/S0169-4332(96)00201-2