Defect Thermodynamics and Engineering in Transferable Oxide Thin Films and Bilayer Heterostructures

Free-standing oxides based on the delamination of atomically defined epitaxial thin films provide new opportunities to combine functional complex oxides with semiconductor (silicon) electronics. At the same time, the nanoscale confinement of these transferable lamellae enables to synthesize unique defect structures and functionalities, that follow from the structural boundary conditions and limited dimensions of the functional units.<br/>Here we discuss the synthesis of transferable perovskite oxide lamellae via the all-perovskite sacrificial-layer route as compared to the graphene-buffered remote epitaxy approach. Due to the generally high oxygen pressure and kinetics during oxide epitaxy by pulsed laser deposition (PLD), the latter approach bears a high potential for the damage of graphene-interlayers, which can be partially prevented by the use of inert gas atmospheres.<br/>We then explore the defect structure of transferred lamellae based on the example of SrTiO₃, serving as a model system for ionic-electronic phenomena. As we elaborate, the confinement of the lamella facilitates the overall formation of oxygen vacancies as compared to the bulk oxide. Exploiting the ion-transfer between SrTiO₃ and LaAlO₃ (deposited under low oxygen pressure), we address the local redox-activity and demonstrate defect concentrations beyond the thermodynamic limit in the transferred lamella.<br/>Finally, we demonstrate how the transferred SrTiO₃ lamellae can be used as a template for the growth of functional bilayer structures hosting ion-driven magnetic-electronic phase transitions and electrocatalytic properties. Achieving a layer-by-layer growth mode, we show that bilayer structures with sufficiently defined interfaces and morphology can be obtained on a non-epitaxial substrate, enabling a detailed analysis of magnetic and chemical depth profiles. Notably, despite the lack of an epitaxial relationship between substrate and bilayer, the lateral strain state of the bilayers depends on the expected bond-type (ionic vs. covalent) between the as-transferred lamella and substrate, allowing to achieve novel strain states that are inaccessible in standard epitaxy.<br/>These examples showcase that the integration of complex transferable oxides heterostructures with semiconductors will come with new opportunities but also new challenges to tailor and design the properties of functional oxides in their new environment.