Synthesis and characterization of (111)- oriented perovskite oxide heterostrctures

Abstract

The limits of silicon in semiconductor devices are fast approaching, with feature sizes reaching the realm where quantum effects start to dominate. A promising approach is to include materials with new functionality such as ferromagnetism and ferroelectricity, creating more energy efficient and tailored electronic devices. Perovskite oxides are interesting for this purpose due to their wide range of available functional properties and strong coupling between structure and functionality, opening for property control through depositing these materials as thin films on suitably chosen substrates. Although a significant amount of research has been devoted to exploring this class of materials, the effect of growing on different substrate crystal orientations is not well understood. In this study we focus on films and substrates cut along the pseudocubic (111)-facet. While the more commonly used (001)-orientation has a square symmetry, the (111)-orientation has a hexagonal symmetry. Additionally, employing the (111)-orientation results in altered oxygen octahedra connectivity across interfaces and different geometrical boundary conditions compared to the (001)-orientation. These properties of the (111)-orientation can induce interesting topological properties, novel phases such as polar metals, and enhanced interfacial exchange coupling. In this work, we study how the symmetry, strain, and interface morphology of the pseudocubic (111)-orientation affects the properties of ferromagnetic and ferroelectric perovskite oxides. Ferromagnetic La0.7Sr0.3MnO3 and ferroelectric BaTiO3 is chosen as model materials. Thin films are deposited on a variety of single crystal substrates using pulsed laser deposition. Both La0.7Sr0.3MnO3 and BaTiO3 are grown on SrTiO3, while La0.7Sr0.3MnO3 is additionally deposited on DyScO3 and NdGaO3 in order to study the effects of (111)-oriented strain on the magnetic properties. To achieve high quality thin film synthesis, control over the substrate surfaces is important. Treatments of SrTiO3, DyScO3, GdScO3, and NdGaO3 substrates are developed and optimized, resulting in atomically flat step-and-terrace surfaces, providing for layer-by-layer growth of perovskite thin films. While the step heights of the SrTiO3 surface correspond to one (111) interplanar distance due its cubic symmetry, the step heights of the orthorhombic substrates, DyScO3, GdScO3, and NdGaO3, can be tailored to be either one or two (111) interplanar distances depending on the specific (111)-orientation used. Analysis of the film growth indicates that the surfaces are reconstructed due to the polar stacking along the [111]-orientation, which can result in an intermixed layer at the substrate and thin film interface. The application of strain provides an avenue towards control of both structural and functional properties. A range of strain values are applied by employing DyScO3, SrTiO3, and NdGaO3 as substrates for deposition of epitaxial La0.7Sr0.3MnO3, enabling investigation into (111)-orientated strain. Moreover, the effects of different (111)-oriented in-plane symmetries are studied by exploiting the fact that SrTiO3 possesses a different crystal structure than DyScO3 and NdGaO3. We use various experimental techniques to probe the magnetic properties of the La0.7Sr0.3MnO3 films and find the respective easy and hard axes through the shape of the hysteresis curves. It is found through reciprocal space maps that La0.7Sr0.3MnO3 can assume rhombohedral, monoclinic, or triclinic unit cells depending on the applied strain and substrate symmetry. We link the observed anisotropy and structure through theoretical models of magnetic anisotropy. The step and terrace structure is shown to be important for the magnetic anisotropy of La0.7Sr0.3MnO3 on SrTiO3, where a uniaxial anisotropy is found at remanence. Through measuring the magnetic properties of La0.7Sr0.3MnO3, we find the direction of the anisotropy is dependent on the film thickness. The easy axis is aligned perpendicular to the step edges below a critical thickness while in thicker films the easy axis lies parallel to the step edges. The magnetic anisotropy is additionally sensitive to the structural phase transition of the SrTiO3 substrate at 105 K, and a sharp increase of the anisotropy constant in the tetragonal phase is observed. For both the strain and step and terrace induced anisotropies we hypothesize that octahedral rotations along with the crystal structure are necessary to explain the results. By creating theoretical phase diagrams of the magnetic anisotropy as a function of octahedral tilt, it is shown that induced octahedral rotations can explain the observed results, indicating that the octahedral response to (111)-orientated strain and morphology is different from films in the (001)-orientation. The growth of (111)-oriented BaTiO3 is found to be dominated by defect formation. Films of 10 nm and thicker are completely relaxed in-plane, with indications of higher defect density with increasing film thickness. Signs of ferroelectricity are observed in films of thickness down to 5 nm. As in La0.7Sr0.3MnO3, the properties of BaTiO3 is found to be sensitive to the structural phase transition of the SrTiO3 substrate, with an marked change in the measured pyroelectric current at 105 K. Through material synthesis, experimental characterization, and theoretical models, we have gained further insight into (111)-orientated perovskite material systems. We are successful in preparing high quality (111)-oriented substrates facilitating the deposition of thin films. The crystal structure and magnetic anisotropy of the films are found to be highly susceptible to (111)- oriented strain, substrate symmetry, and interface morphology. Lastly, we explore the growth and properties of (111)-oriented BaTiO3. The presented results demonstrate that utilization of the (111)-orientation is an interesting avenue in the development of functional material systems for use in future electronic devices.