Ab Initio Studies of Functional Oxides for Thin Film Applications

Abstract

To continue the tremendous growth that the electronics industry has experienced over the last half-century, new energy efficient nanoscale devices are needed. One promising route towards this end is to introduce novel materials such as perovskite oxides. Perovskite oxides are known for their strong structure-property coupling and can host multiple device relevant properties, including magnetism and ferroelectricity. The strong structure-property coupling in these materials also enables controlling, enhancing and inducing such properties by external means, e.g. using a thin film geometry. However, as this coupling is often complex, and multiple phenomena occur simultaneously, theoretical studies are required to enable rational design of novel device concepts. Hence, in this thesis perovskite oxides are studied by density functional theory (DFT), with a focus on thin film applications. DFT is able to predict the atomic structure of materials with sub-ångström resolution. This includes the distortions of oxygen octahedra, which are central for the properties of perovskite oxides, but can be difficult to determine experimentally. Having established the ground state atomic structure, DFT can then predict several functional properties such as the ferroelectric polarization. In this work, the focus is on ordered oxygen vacancies and the effect of the crystallographic (111)-orientation as avenues to control the properties of perovskite oxide thin films. Oxygen vacancies are known to have a strong impact on the properties of these materials. When the concentration of vacancies becomes large, they start to interact, and can form structures with alternating layers of octahedra and tetrahedra. These layered structures are interesting as they enforce anisotropic properties and can decouple the oxygen octahedra. Furthermore, the (111)-orientation, is interesting due its trigonal symmetry, which can induce novel topological properties, and has a different interface octahedral connectivity compared to other orientations, such as the (001). Here we investigate both the effect of the strain from a (111)-substrate, and the effect of octahedral coupling at the (111)-interface. In this work, it is found that oxygen vacancies in (La,Sr)MnO3 thin films grown on (001)-oriented SrTiO3 can accumulate close to the interface. These vacancies are initially disordered, but will order in a brownmillerite structure given time. Furthermore, it is found that the layering in the brownmillerite family of structures gives rise to different magnetic ordering patterns in the octahedral and tetrahedral layers, which are nearly decoupled from each other. In addition, the electronic band gap is larger in the tetrahedral layers, and the band structure is almost completely flat perpendicular to the layers. Hence, ordered anion vacancies in (La,Sr)MnO3 enables spatially confined spin polarized conduction. Investigating strain in the (111)-plane, we find that it is considerably different compared to strain in the (001)-plane. Using LaAlO3 as a model system, we find an opposite splitting of in-plane and out-of-plane rotations compared to (001)- strain. Also in contrast to (001)-strain, there is no preferred in-plane rotation axis for (111)-strain, giving rise to Goldstone-like modes. Next, we analyse the strain-phonon coupling of twenty different perovskite oxides. We find that this coupling depends on the tolerance factor and the nominal charge of the A- and Bcations. This makes (111)-strained perovskite oxides interesting as it has multiple ways of engineering unstable phonons, while under (001)-strain the strain-phonon coupling is similar for all materials studied. Using this strain-phonon coupling, the crystal structure of LaFeO3 when strained to SrTiO3(111) is determined, and found to coincide with the antiferromagnetic domain structure. The octahedral coupling at (111)-oriented interfaces is established, and found to be significantly different compared to the coupling at (001)-oriented interfaces, due to the different interface symmetry. In the model system (La,Sr)MnO3/SrTiO3 it is found that the coupling length is similar for the two orientations. However, the way octahedral rotations couple from the SrTiO3 strongly depend on the facet of the interface. This difference in octahedral coupling can be used to control magnetic properties, as exemplified by the spatial distribution of the spin density. Finally we have shown that octahedral rotation mismatch between (111)-oriented LaFeO3 and (La,Sr)MnO3 has to reconstruct structurally. Concurrent with this octahedral reconstruction, an induced ferromagnetic moment in LaFeO3 is found. This moment can again be related to changes in electron correlation strength due to the octahedral mismatch. Using DFT, we have gained further insight into functional perovskite oxide thin films, on a level that would have been nearly impossible using experimental methods alone. Taken together, the results in this work show that both ordered oxygen vacancies and (111)-oriented interfaces are interesting avenues for tailoring material properties for novel electronic devices.