Midgap states observed by nonlinear optical spectroscopy of metal:GaAs junctions

We present detailed second-order nonlinear optical spectroscopic studies of the interface electronic structure in metal:GaAs thin films and interfaces. The interface spectra from Au:GaAs, Ga-rich n-type systems exhibit two resonance features in the near infrared at 0.715 eV and 0.731 eV. A single resonance feature at 0.715 eV is observed in Au:GaAs, As-rich n-type interfaces. Similar single resonance features at 0.715 eV were observed in As:GaAs n-type samples. No resonances however were observed in oxide:GaAs and metal:GaAs p-type systems. All of the spectral features are sharp one-photon resonances. They provide compelling evidence for the existence and symmetry of atomic displacement-induced defect states just below the buried interface.