TiO2(100)/Al2O3(0001) interface: A first-principles study supported by experiment

Abstract

We present the results of combined theoretical and experimental investigations of the TiO${}_{2}$/Al${}_{2}$O${}_{3}$ interface. High-quality rutile films are grown on sapphire substrates and characterized by XRD and pole figures analysis. The epitaxial relations established by experiment agree well with those reported in literature and are used as input for ab initio modeling. In order to cope with the problem of lattice mismatch, we introduce a stress balancing approach, which takes into account the elastic tensors of the bulk materials forming the interface. Applying this approach, optimized interface models are obtained and used to evaluate the work of separation of interfaces with various stoichiometries. It is shown that the work of separation of both, oxygen-rich (0.5 J/m${}^{2}$) and oxygen-poor (1.6 J/m${}^{2}$) interfaces, is significantly smaller than the one of the metal-oxygen-metal stacked interface close to stoichiometry (8.8--9.3 J/m${}^{2}$). The influence of atomic relaxation on the work of separation is discussed. Analysis of electronic structure and bonding reveals for the oxygen-rich interface, localized oxygen $2p$ states above and below the the valence-band region of the other types of interfaces. The oxygen-poor one turns out to be metallic, whereas the interfaces close to bulk stoichiometry remain insulating. It is shown that energetically favorable interfaces are formed when the two sides are most similar to each other in coordination and stacking.