  A theoretical framework incorporating atomic-level interfacial details is
derived to include the electronic structure of buried interfaces and describe
the behavior of charge carriers in heterostructures in the presence of finite
interfacial broadening. Applying this model to ultrathin heteroepitaxial
(SiGe)m/(Si)m superlattices predicts the existence of localized energy levels
in the band structure induced by sub-nanometer broadening, which provides
additional paths for hole-electron recombination. These predicted interfacial
electronic transitions and the associated absorptive effects are confirmed
experimentally at variable superlattice thickness and periodicity. By mapping
the energy of the critical points, the optical transitions are identified
between 2 and 2.5 eV thus extending the optical absorption to lower energies.
This phenomenon enables a straightforward and non-destructive probe of the
atomic-level broadening in heterostructures.
