First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO₃/SrTiO₃ (A = Ca, Sr, and Ba) Heterostructures

By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) in nonpolar/nonpolar AHfO 3 /SrTiO 3 (A = Ca, Sr, and Ba) heterostructures. Two types of nonpolar/nonpolar interfaces, (AO) 0 /(TiO 2 ) 0 and (HfO 2 ) 0 /(SrO) 0, each with AO and HfO 2 surface terminations, are modeled, respectively. The polarization domain and resulting interfacial electronic property are found to be more sensitive to the surface termination of the film rather than the interface model. As film thickness increases, an insulator-to-metal transition is found in all the heterostructures with HfO 2 surface termination: for (AO) 0 /(TiO 2 ) 0 interfaces, predicted critical film thickness for an insulator-to-metal transition is about 7, 6, and 3 unit cells for CaHfO 3 /SrTiO 3, SrHfO 3 /SrTiO 3, and BaHfO 3 /SrTiO 3, respectively; for (HfO 2 ) 0 /(SrO) 0 interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the heterostructures with AO surface termination, CaHfO 3 /SrTiO 3 exhibits a much larger critical film thickness about 11–12 unit cells for an insulator-to-metal transition; while SrHfO 3 /SrTiO 3 and BaHfO 3 /SrTiO 3 do not show any polarization behavior even film thickness increases up to 20 unit cells. The strain-induced polarization behavior was well-elucidated from energy versus polarization profile. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO 3 /SrTiO 3 HS.