Structural and electronic impact of SrTiO₃ substrate on TiO₂ thin films

Abstract

We demonstrate that the atomic structures, electronic states, and bonding nature of the interface between SrTiO₃ substrate and anatase TiO₂ thin films could be related and technologically manipulated at the atomic level. Applying advanced transmission electron microscopy, the grown anatase TiO₂ thin films are found to make a clean and direct contact to the SrTiO₃ substrates in an epitaxial, coherent, and atomically abrupt way. The atomic-resolution microscopic images reveal that the interface comprises SrO-terminated SrTiO₃ and Ti-terminated TiO₂ with the interfacial Ti of TiO₂ sitting above the hollow site, which is confirmed theoretically to be the most energetically favorable. Quantitatively, the first-principles calculations predict that the oxygen sublattice at the interface undergoes a notable reconstruction, i.e., the interfacial O atoms of TiO₂ are displaced largely toward the SrO plane of the SrTiO₃, flattening the originally zigzag TiO₂ atomic chains. Consequently, the interfacial layers suffer a remarkable modification in the charge accumulation and also a deviation in the density of states from their bulk counterparts, indicating that the substrate can have an impact on the deposited thin films electronically. Using several analytic methods, the SrTiO₃/TiO₂ interface is found to take on a metallic nature, and the interfacial bonding is determined to be of a mixed covalent and ionic character. This combined experimental and theoretical investigation gains insight into the complex atomic and electronic structures of the buried interface, which are fundamental for relating the atomic-scale structures to their properties on a quantum level.