We report local-density ab-initio pseudopotential calculations designed to link structural properties of real interfaces to carrier dynamics and recombination observed in spectroscopic studies of Si-Ge and similar heterostructures. We find novel interface-related effects of deep localisation which may provide a fresh basis for explaining the optical and scattering phenomena reported in these systems. In particular, the electronic structure of Si-Ge interfaces is significantly modified by the presence of new well-localised levels due to dopants such as Sb which are commonly used to improve molecular beam epitaxy growth.