Atomic and electronic structure of self-organized defects in epitaxial films of functional perovskite-type oxides /

Resumen

The epitaxial thin films of functional perovskite-type oxides (ABO3) present interfacial coupling and misfit relaxation mechanisms governed by a complex interplay of chemical, electronic and structural degrees of freedom. The relaxation mechanisms of strained films may accommodate defects, such as misfit dislocations or twin walls, which exhibit a strong tendency towards self-organization with characteristic length scales of tens of nanometres. The core lattice structure of these defects is different from the bulk of the material and thus may be considered as a nano-phase with likely different physical properties, leading to the formation of functional nanostructures. The correlation between defect structure and functionality, together with the capacity of these defects to self-organize, offers a unique opportunity for the bottom-up elaboration of functional complex oxides nanodevices. This thesis focuses on the characterization of the microstructure, interface and self-organized defects of epitaxial films and functional nanostructures of oxide materials by using advanced transmission electron microscopy. Special emphasis is put on the atomic and chemical structure of the interfaces and generated defects, such as dislocations, twin walls and phase segregations, as well as on the strain fields and their correlation with chemical heterogeneities. In this regard, two different systems composed of lanthanum manganites are considered: LaMnO3:MnOx nanocomposite grown on (001)SrTiO3 and on (001)LaAlO3 substrates; and La0.7Sr0.3MnO3 films with self-organized defects grown on (001)SrTiO3 and on (001)LaAlO3. The materials studied in this work may be regarded as nanostructured films resulting from the self-organization of misfit relieving defects as follows: nanoinclusions of a MnOx phase (volume defects) in LaMnO3; twin walls between twin domains (planar defects) in La0.7Sr0.3MnO3 on SrTiO3; and misfit dislocations (line defects) in La0.7Sr0.3MnO3 on LaAlO3. In the LaMnO3:MnOx nanocomposite, the formation of regular vertically aligned nanoinclusions of a manganese oxide (MnOx) embedded in an LaMnO3 film is analysed via microstructural characterization. This analysis includes the determination of the LaMnO3 matrix microstructure with respect to the substrate together with the identification of the manganese oxide phase and a secondary phase: a La-rich layer close to LaMnO3-substrate interface. In the case of La0.7Sr0.3MnO3 on (001)SrTiO3 substrates, a detailed analysis of twin walls and their implications on the functional properties is performed. Local changes in the physical and structural properties of the TWs lead to the view of a twinned film as a self-organized nanostructure consisting of vertical nano-sheets of strongly compressed La0.7Sr0.3MnO3 embedded in a matrix of tensile strained La0.7Sr0.3MnO3. In the case of La0.7Sr0.3MnO3 ultrathin films grown on (001)LaAlO3, the relaxation mechanism of this films is analysed. These films relieve the misfit strain by the formation of misfit dislocations above a critical film thickness of 2.5 nm. A detailed study of structural, chemical and electronic changes associated with the dislocation is also performed paying particular attention to the influence of strain fields on chemical composition at the nanoscale. A chemical reorganization occurs to accommodate the strain at the dislocations core region. The dependence of the degree of order of the dislocation pattern on film thickness is also explored. Finally, the implications of the dislocation strain field on surface topography and electrical transport are analysed, demonstrating that the multiscale nature of dislocations holds great promise for the creation of spontaneous surface ordered functional nanostructures in complex oxide thin films. The results and main conclusions obtained in this work open new perspectives for the development of functional self-organized nanostructures based on strain relieving defects.