Andreas Müller (PhD thesis)

Andreas Müller (PhD thesis): Towards functional oxide heterostructures

Abstract

Oxide heterostructures attract a lot of attention as they display a vast range of physical phenomena like conductivity, magnetism, or even superconductivity. In most cases, these effects are caused by electron correlations and are therefore interesting for studying fundamental physics, but also in view of future applications. This thesis deals with the growth and characterization of several prototypical oxide heterostructures.
Fe₃O₄ is highly ranked as a possible spin electrode in the eld of spintronics. A suitable semiconductor for spin injection in combination with Fe₃O₄ is ZnO due to its oxide character and a sufficiently long spin coherence length. Fe₃O₄ has been grown successfully on ZnO using pulsed laser deposition and molecular beam epitaxy by choosing the oxygen partial pressure adequately. Here, a pressure variation during growth reduces an FeO-like interface layer. Fe₃O₄ films grow in an island-like growth mode and are structurally nearly fully relaxed, exhibiting the same lattice constants as the bulk materials. Despite the presence of a slight oxygen off- stoichiometry, indications of the Verwey transition hint at high-quality film properties. The overall magnetization of the films is reduced compared to bulk Fe₃O₄ and a slow magnetization behavior is observed, most probably due to defects like anti-phase boundaries originating from the initial island growth.
LaAlO₃/SrTiO₃ heterostructures exhibit a conducting interface above a critical film thickness, which is most likely explained by an electronic reconstruction. In the corresponding model, the potential built-up owing to the polar LaAlO₃ overlayer is compensated by a charge transfer from the film surface to the interface. The properties of these heterostructures strongly depend on the growth parameters. It is shown for the first time, that it is mainly the total pressure which determines the macroscopic sample properties, while it is the oxygen partial pressure which controls the amount of charge carriers near the interface. Oxygen-vacancy-mediated conductivity is found for too low oxygen pressures. A too high total pressure, however, destroys interface conductivity, most probably due to a change of the growth kinetics. Post-oxidation leads to a metastable state removing the arbitrariness in controlling the electronic interface properties by the oxygen pressure during growth.
LaVO₃/SrTiO₃ heterostructures exhibit similar behavior compared to LaAlO₃/SrTiO₃ when it comes to a thickness-dependent metal-insulator transition. But in contrast to LaAlO₃, LaVO₃ is a Mott insulator exhibiting strong electron correlations. Films have been grown by pulsed laser deposition. Layer-by-layer growth and a phase-pure pervoskite lattice structure is observed, indicating good structural quality of the film and the interface. An electron-rich layer is found near the interface on the LaVO₃ side for conducting LaVO₃/SrTiO₃. This could be explained by an electronic reconstruction within the film. The electrostatic doping results in a band-filling-controlled metal-insulator transition without suffering from chemical impurities, which is unavoidable in conventional doping experiments.

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