Second-order nonlinear optical spectroscopies reveal midgap interface states at buried metal/GaAs junctions, and demonstrate that these states are sensitive to interface preparation. In As-rich (Ga-rich) Au/GaAs $n$-type samples one (two) midgap resonance was observed. Similar resonances were exhibited in As/GaAs $n$-type samples, but were not present in metal/GaAs $p$-type systems. The sharp spectral features provide compelling evidence for the existence and symmetry of atomic displacement-induced defect states just below the buried interface.

• Received 9 June 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.3174