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 (HS). Two types of interfaces, AO/TiO$_2$ and HfO$_2$/SrO, 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 (IMT) is found in all the HS with HfO$_2$ surface termination: for AO/TiO$_2$ interfaces, predicted critical film thickness for an IMT 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$_2$/SrO interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the HS with AO surface termination, only CaHfO$_3$/SrTiO$_3$ exhibits an IMT with a much larger critical film thickness about 11 - 12 unit cells. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO$_3$/SrTiO$_3$ HS.