ELECTRONIC STRUCTURE OF TIO2 THIN FILMS AND LAALO3-SRTIO3 HETEROSTRUCTURES: THE ROLE OF TITANIUM 3D1 STATES IN MAGNETIC AND TRANSPORT PROPERTIES

The scope of this Thesis is the study of the electronic structure of two Ti-based oxide systems, TiO2 thin films and the ultra-thin LaAlO3-SrTiO3 (LAO-STO) heterojunctions, which display remarkable physical phenomena, so far not completely understood. In both cases, the titanium-related electronic states are expected to play a fundamental role and thus have been probed by means of X-ray photoemission spectroscopies. A weak room-temperature ferromagnetism (FM) has been recently detected in slightly reduced TiO2 thin film and in other oxides, such as HfO2 and CaO; since these materials are insulating closed-shell systems, this phenomenon has been classified as "d0 magnetism". Magnetism in these compounds seems to be related to the growth methodology and ultimately to the presence of structural defects, such as oxygen vacancies. Therefore, a thorough analysis of titanium electronic states, and especially of the defect-related Ti3+ energy levels, is needed in order to understand the origin of magnetic interactions. In the first part of this Thesis, a comprehensive magnetic characterization of a set of TiO2 samples is given, together with the analysis of Ti 3d-related states carried out with X-ray photoemission (XPS) and resonant photoemission (ResPES). A set of N-doped TiO2 thin films have also been grown, in order to verify the effect of doping on the TiO2 magnetism. The hypothesis of a clustered oxygen-vacancies origin of FM is then discussed in the light of the experimental and theoretical results. Another interesting oxide system in which the stoichiometry of Ti ions play a fundamental role is the LAO-STO interface. LAO and STO, separately, are two band insulators, with an empty shell electronic structure (3d0 for STO, 4f0 for LAO) and a similar perovskite structure; however, the interface created by growing LAO on the top of STO (001) has found to become metallic, hosting a 2D electron gas. This heterostructure becomes conductive only when the STO is terminated with a TiO2 layer; therefore, the Ti-related electronic states are expected to host the metallic states. The second part of this Thesis is devoted to the study of conductive and insulating LAO-STO interfaces, carried out by XPS, X-ray absorption (XAS) and with ResPES techniques. The stoichiometry of each atomic species has been evaluated through a comparison with LAO and STO single crystals. A resonance enhancement of the conductive Ti states, associated to a small fraction of Ti3+ ions is reported and compared to theoretical calculations. On the basis of these results, the origin of metallic states in ultra-thin LAO-STO interfaces is properly addressed. In addition, a characterization of the intermixing and the disorder at the LAO-STO interface has been done through angle-resolved XPS, providing important information on the intermixing of light cations (Al, Ti) otherwise missed by X-ray diffraction techniques.