Anisotropy of optical absorption in birefringent porous silicon

Strong polarization-sensitive anisotropy of the optical absorption in the visible and infrared spectral range is observed in porous silicon (PSi) layers formed from bulk (110) Si wafers. It is shown that the anisotropy of the absorption via band-to-band and intraband (Drude-like) electronic transitions is consistent with the strong birefringence of these layers. Both phenomena are described by a model considering the form anisotropy in the (110) PSi layer assembled from nonspherical Si nanocrystals. We show that the anisotropic shape of the nanocrystals results in polarization-sensitive absorption by their surface silicon-hydrogen bonds. This allows an exact assignment of the stretching modes of silicon monohydride in the infrared absorption spectra of the PSi layers.