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Bulklike band-offset mystery solved through energy minimization: Lessons from perovskite oxide heterojunctions

The formation of band offset (BO) at isovalent semiconductor heterojunctions has been branded ``bulklike'' because of an insensitivity of the BO to the interface orientation and atomic structure and a transitivity in BOs. Even though tunability and nontransitivity of BO are frequently found for heterovalent interfaces, empirical theories with built-in bulklike characteristics have thus far dominated the explanation of experimental BOs. Presently, the distribution of charge density and the formation of BO at a large number of interfaces between lattice-matched perovskite oxides are studied in detail using density functional theory. Ionic screening is found to dominate the formation of the BO, as a sharp dependence of the (apparently tunable) BO on atomic structure for unrelaxed interfaces is essentially washed out upon lattice relaxation. Numerical experimentation with deliberate embedding of dipolar layers in perovskite oxides and their interfaces corroborates the effectiveness of ionic screening. The relaxed, converged (bulklike) BOs are found to be in good agreement with the prediction of the neutral polyhedra theory (NPT). The success of the NPT, presently for ionic interfaces and previously for covalent zinc-blende interfaces, unmasks a possible connection between the partition into neutral symmetric cells and the energy-minimization requirement on the interface charge distribution. The independence of the BO on interface specifics, i.e., bulklike behavior, is shown to stem directly from such a property of charge distribution with minimized electrostatic energy. As energy minimization governs the formation of charge distribution in general, NPT is expected to describe the band offset of a wide variety of material interfaces.