Measuring and Modeling the Optical Constants of Germanium Near the Band Gap

The standard theory of indirect gap optical absorption in semiconductors predicts an unphysical divergence when the photon energy reaches the direct band gap. Current theoretical efforts to eliminate this divergence require experimental validation with measurements covering a spectral region that exceeds the direct band gap. For transmittance/reflectance measurements of the absorption in a material like Ge, the needed film thicknesses are too small for the samples to be mechanically stable. On the other hand, very thin films can be grown epitaxially on silicon. In this presentation, we report on our efforts to extract the absorption coefficient from measurements of such epitaxial Ge layers. We observe sharp interference oscillations in the transmittance and reflectance data that are due to the optical contrast between Ge and Si. These interferences can introduce a large error in the extracted absorption if the unavoidable film thickness gradients in epitaxial layers are not carefully corrected for.