Role of a universal branch-point energy at ZnO interfaces

Contemporary interface state models were tested using the barrier heights of laterally homogeneous ZnO Schottky contacts. A strong correlation was found between barrier height and metal Miedema electronegativity, indicating that interfacial chemical bonding may play an important role in the charge transfer at metal-ZnO interfaces. The charge neutrality level of metal-ZnO interface states was found to lie in the band gap, 0.90±0.05 eV below the conduction-band minimum. This is in contrast to the electrical nature of ZnO free surfaces and heterojunctions, and with quasiparticle band-structure calculations which have previously placed the branch-point energy of ZnO in the first conduction band. This large discrepancy indicates that we still lack a comprehensive predictive model for these interfaces.