Metallicity at interphase boundaries due to polar catastrophe induced by charge density discontinuity

The electronic properties of interphase boundaries are of basic importance for most materials, particularly when those properties deviate strongly from the bulk behavior. We introduce a mechanism that can result in metallicity at stoichiometric interphase boundaries between semiconductors based on the idea of polar catastrophe, which is usually considered only in the context of heterostructures. To this end, we perform ab initio calculations within density functional theory to investigate the electronic states at stoichiometric SnO/SnO2 (110) interphase boundaries. In this system, one would not expect polar catastrophe to have a role according to state-of-the-art theory because the interface lacks formal charge discontinuity. However, we observe the formation of a hole gas between the semiconductors SnO and SnO2. To explain these findings, we provide a generalized theory based on the idea that the charge density discontinuity between SnO and SnO2, a consequence of lattice mismatch, drives a polar catastrophe scenario. As a result, SnO/SnO2 (110) interphase boundaries can develop metallicity depending on the grain size. The concept of metallicity due to polar catastrophe induced by charge density discontinuity is of general validity and applies to many interphase boundaries with lattice mismatch. Tin oxide semiconductors can turn metallic at customized interfaces between two crystal phases, a theoretical study has found. Inspired by recent discoveries of surprisingly conductive regions formed when different insulators are sandwiched together, Udo Schwingenschlögl and two colleagues from KAUST in Saudi Arabia investigated whether similar properties could emerge at phase boundaries inside tin oxide materials. Density functional calculations of tin monoxide-dioxide interfaces indicate that certain polar arrangements generate desirable, high-mobility quantum gases even though the two sides of the boundary have the same formal charge – typically, abrupt charge changes are needed to create two-dimensional metallic states. The team proposed that this unexpected behavior arises from charge density discontinuities due to the mismatched lattices of tin monoxide and dioxide. We introduce a mechanism that can result in metallicity at stoichiometric interphase boundaries between semiconductors, based on the idea of polar catastrophe induced by charge density discontinuity. As an example, we demonstrate metallicity at the stoichiometric (110) interphase boundary of the semiconductors SnO and SnO2. The proposed mechanism is of general validity and expected to have an important role at interphase boundaries between polar materials.