Theoretical study of the insulator/insulator interface:: Band alignment at the SiO2/HfO2 junction

While the physics of the Schottky barrier is relatively well understood, much less is known about the band alignment at the insulator/insulator interface. As a model problem we study theoretically the band alignment at the technologically important SiO2/HfO2 interface using density functional theory. We report several different atomic level models of this interface along with their energies and electronic properties. We find that the valence band offset increases near linearly with the interfacial oxygen coordination, changing from -2.0 eV to 1.0 eV. For the fully oxidized interface the Schottky limit is reached. We propose a simple model, which relates the screening properties of the interfacial layer to the band offset. Our results may explain a somewhat confusing picture provided by recent experiments.