Anisotropy in Patterned Perovskite Oxides
Prof. dr. ir. J.E. ten Elshof
The perovskite class encompasses a multiplicity of oxide materials with widely divergent properties and many potential applications. The similar unit cell dimensions and common oxygen octahedral backbone of the different perovskites allow stacking their building blocks on top of each other in a structurally ordered fashion. In the form of such heteroepitaxial thin films, properties can be fully exploited and manipulated under the act of strain or interfacial engineering. Pulsed laser deposition (PLD) is exceedingly suitable for growing artificial films of perovskite-type oxides, where single crystalline substrates with matching lattice parameters are required to completely dictate the crystal orientation. The technique allows manipulating material at a unit cell level in the direction of film growth, but lacks standardized approaches to control the deposition in other directions. Such lateral control is indispensable for future device integration, but particularly complicated by the elevated substrate temperatures that are required for the adatoms to reorganize into an energetically favorable configuration.
The intention of the thesis is to contribute to the ambitious objective to hold on to the unprecedented control of film growth that PLD offers, while at the same time reach for similar control in directions normal to that of growth. State-of-the-art lithographic techniques like those used for manufacturing logic or memory chips are incompatible with PLD, because they make use of organic polymers that degrade at elevated temperatures. Parallel patterning of epitaxial perovskite materials in arbitrary shapes is therefore still challenging even on micrometer length scales, illustrating the tremendous gap to reach atomic scale precision. No methods to achieve atomic accuracy are described in the thesis as well, but different routes are discussed to pattern epitaxial heterostructures on micrometer length scales and below. The structures and their properties are treated with a special emphasis on anisotropy, which may originate either from the shapes of the patterns, or the crystallographic order resulting from heteroepitaxy.