Creating Two-Dimensional Electron Gas in Polar/Polar Perovskite Oxide Heterostructures: First-Principles Characterization of LaAlO₃/A⁺B⁵⁺O₃

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) at the polar/polar (LaO)⁺/(BO₂)⁺ interface in the LaAlO₃/A⁺B⁵⁺O₃ (A = Na and K, B = Nb and Ta) heterostructures (HS). Unlike the prototype polar/nonpolar LaAlO₃/SrTiO₃ HS system where there exists a least film thickness of four LaAlO₃ unit cells to have an insulator-to-metal transition, we found that the polar/polar LaAlO₃/A⁺B⁵⁺O₃ HS systems are intrinsically conducting at their interfaces without an insulator-to-metal transition. The interfacial charge carrier densities of these polar/polar HS systems are on the order of 10¹⁴ cm^–2, much larger than that of the LaAlO₃/SrTiO₃ system. This is mainly attributed to two donor layers, i.e., (LaO)⁺ and (BO₂)⁺ (B = Nb and Ta), in the polar/polar LaAlO₃/A⁺B⁵⁺O₃ systems, while only one (LaO)⁺ donor layer in the polar/nonpolar LaAlO₃/SrTiO₃ system. In addition, it is expected that, due to less localized Nb 4d and Ta 5d orbitals with respect to Ti 3d orbitals, these LaAlO₃/A⁺B⁵⁺O₃ HS systems can exhibit potentially higher electron mobility because of their smaller electron effective mass than that in the LaAlO₃/SrTiO₃ system. Our results demonstrate that the electronic reconstruction at the polar/polar interface could be an alternative way to produce superior 2DEG in the perovskite-oxide-based HS systems