The Influence of a Polar-Nonpolar Interfaces on Surface Segregation and Interface Termination: The Case of LaInO₃/BaSnO₃

Interfacial polar discontinuities are a unique way to manipulate charge states at interfaces and to create novel two-dimensional electron states. One recent example for this phenomenon is the interface between AlN and GaN, where a discontinuity in the spontaneous polarization leads to the formation of two-dimensional electron (2DEG) or hole gas (2DHG). The polar-nonpolar interface between ABO₃ perovskite oxides offers new degrees of freedom to tune the interface states by the accessibility of mixed valence states. The accepted model explaining the formation of a 2DEG is based on the concept of charge transfer between the layers terminating the polar-nonpolar interface. To realize a 2DEG or a 2DHG control of the interface is a prerequisite. When growing the heterostructures, it is a commonly assumed that the surface termination of the non-polar layer controls the interface properties. In this contribution we provide experimental evidence demonstrating that the compensation of the polar discontinuity can drive surface segregation and consequently, control the interface termination.<br/>We study the interface formation between the cubic wide band gap semiconductor BaSnO₃, and orthorhombic LaInO₃, by combining analytical scanning transmission electron microscopy (S)TEM, photoelectron spectroscopy and density functional theory (DFT) calculations. The samples were grown by plasma-assisted molecular beam epitaxy on DyScO₃ substrates at 835°C using a mixture of Sn and SnO₂ as a SnO source. While TEM experiments of BaSnO₃bulk crystals and DFT agree that BaO is the most stable surface termination of BaSnO₃ over wide range of chemical potentials, we find that the interface between BaSnO₃ and LaInO₃ is terminated by SnO₂. This is consistent with the presence of a 2DEG, but also with our DFT calculations, which show this interface to be the most energetically favorable. STEM and PES show the presence of BaO on the surface of thin LaInO₃ films indicating Ba surface segregation. Based on our DFT calculations we find that the driving force for Ba segregation is the compensation of the polar discontinuity at the interface. This compensation is an effect of the gradual reduction of the octahedral tilt from the orthorhombic LaInO₃ to the cubic BaSnO₃and the polar and non-polar distortions at the interface. In the case of the n-type SnO₂interface, this leads to an expansion of the out-of-plane lattice spacing at the interface, which most efficiently compensates for the discontinuity. At the p-type BaO terminated interface it leads to non-polar distortions in the BaSnO₃, while polar distortions remain mainly in the LaInO₃, which compensate the polar discontinuity less efficiently. This shows that in perovskites in addition to surface energy and strain, the response of the system to compensate for the polar discontinuity must be considered as an additional driving force for segregation that can control the interface termination.