Operando chemistry and electronic structure of electrode/ferroelectric interfaces
In the past decade, oxide-based heterostructures have been studied extensively as potentially attractive systems for applications in nanoelectronics. Among them, ferroelectric materials raised interest as potential support for those technological applications. Indeed, their spontaneous electric polarization easily switched by applying an electric field makes them a good basis for non-volatile data storage. Switching the polarization requires a metallic contact with an electrode, thus heterostructures of ferroelectric thin films with metallic electrodes have been widely studied. At the interface between those two materials, free charges of the electrode help screening the polarization induced surface charges detrimental to maintaining proper polarization in the ferroelectric thin film. With metallic oxide electrodes, an ionic displacement at the electrode/ferroelectric interface will help the screening, bringing this interface at the core of the screening process. However, despite important theoretical discoveries, direct experimental data is scarce and the behavior of the electrode/ferroelectric interface is still only partially understood. Further understanding is crucial for a proper integration of ferroelectric films in functioning nanometer-sized devices. In this thesis, photoemission spectroscopy based techniques are used to probe the buried interface of an electrode/BaTiO3/electrode heterostructure, for two different top electrodes: the metallic oxide SrRuO3 and the Co metal. Combining operando hard X-ray photoemission spectroscopy, hard X-ray photoemission electron microscopy and time-resolved experiments, we acquired information on the behavior of the interface and its response to polarization switching. The work presented is a new step towards a complete understanding on the behavior of the interface between electrodes and ferroelectric materials, in the case of electrode / BaTiO3 / electrode heterostructures, in terms of electronic properties, kinetic, and fatigue. The three experiments presented combined state of the art characterization techniques, where the use of hard X-rays and in situ bias application made it possible to resolve the difficult task of probing buried interfaces in working conditions.