Size effects in epitaxial oxide thin films
Prof. dr. ing. A.J.H.M. Rijnders
The perovskite oxide material class comprises a vast range of interesting physical properties. The common oxygen backbone in such ABO3 perovskites and the similar lattice parameters allow for the creation of epitaxial oxide heterostructures. Materials with different intrinsic properties can be combined in order to find new or enhanced properties. Upon reducing the size of these complex oxide materials down to a few atomic layers, properties are often found to be altered with respect to their bulk counterparts.
This thesis is focused on reducing the size of complex oxide heterostructures and subsequently studying the effects on the material properties. A method is introduced for creating self-organized epitaxial SrRuO3 nanowires by pulsed laser deposition (PLD). Using this method, thin film nanopatterns are prepared on DyScO3 (110) substrates which have structural and physical properties similar to continuous thin films. For this purpose the surface structure of DyScO3 single crystal substrates is studied in great detail. This form of self-organization is studied experimentally and the time evolution of the atomistic growth is modeled by a Monte Carlo type growth simulation.
The effects of size reduction on the material properties and crystal structures are studied in two material systems, SrCuO2 and SrRuO3. In both cases a structural transition was predicted in literature to occur upon reducing the film thickness to only a few atomic layers. This transition was confirmed experimentally for SrCuO2 by using X-ray photoelectron diffraction (XPD). In the case of SrRuO3, the ferromagnetic transition temperature in thin films was enhanced to nearly bulk values by means of adding a SrTiO3 capping layer. This enhancement is explained by the effect of oxygen octahedral tilts and rotations in the SrRuO3 layer induced by the capping layer.