The role of defects at functional interfaces between polar and non-polar perovskite oxides

Analogous to semiconductor heterostructures, interfaces between polar and non-polar perovskite oxides can exhibit novel electronic properties. Complex oxide heterostructures thus have potential for electronic applications not only due to the inherent material properties of oxides but also due to the properties of their interfaces. Metallic conductivity was found at the interface between the band gap insulators SrTiO3 (STO) and LaAlO3 (LAO). This particular interface effect has sparked enormous academic and technological interest. However, a better understanding of the conduction mechanism is required for future technological applications. This thesis investigates the specific influence of defects on conductive interfaces in various complex oxide heterostructures. In the context of defect chemistry models, both extrinsic doping and intrinsic defects (such as oxygen vacancies and cation vacancies) are taken into account. The measurement of the high temperature conductance in thermodynamic equilibrium as a function of ambient oxygen partial pressure serves as central characterization method. From the gained high temperature conductance characteristics, the decisive defect equilibria and the associated defect concentrations at the interface are determined. In addition, the influence of the cation stoichiometry of the involved perovskite materials is examined by systematically varying the process parameters during sample fabrication. On the basis of the resulting stoichiometry effects, the correlations between cationic defects and electrical properties of conducting oxide interfaces are discussed in detail. In high temperature equilibrium, a thermally stable conductance contribution of the LAO/STO interface is observed. A donor-type conduction mechanism is concluded from the characteristic interface behavior according to the defect chemistry model of STO. The presence of donor-type states at the interface can either be explained by an interdiffusion of lanthanum, or by a generation of trapped oxygen vacancies, or by electronic reconstructions. An involvement of donor-type ionic defects is supported by the results of high resolution transmission electron microscopy studies. Furthermore, the high temperature conductance measurements show that the donor states at the interface can be compensated by acceptor-type strontium vacancies formed under high oxygen partial pressures via the Schottky-equilibrium. In comparison with the STO single crystal, the incorporation of strontium vacancies is facilitated at the interface, so that ionic charge compensation mechanisms are observed already at temperatures below 1100 K. At the end of this thesis, a defect chemistry model is achieved describing the complex defect structure and the resulting electronic properties of interfaces between polar and non-polar perovskites. The properties of the involved polar oxide considerably contribute to the stabilization of the interface's defect structure. Simplified scenarios, which only take a reduction or a donor doping of STO into account, may therefore be excluded as origin of the interface conduction