Strain and Doping Study of Lead-free Antiferroelectric Thin Films

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Recently, the interest in antiferroelectric (AFE) materials has increased due to envisaged applications in sustainable energy supply. Thin films serve as ideal model systems to study intrinsic solid-state properties, as they can be grown single-crystalline, with controlled epitaxial strain and substitutional doping. Especially the clarification of the relation between applied strain and the AFE properties holds the promise to improve relevant performance indicators such as energy storage capacity. This is due to the fundamental connection between strain and polarization in (anti)ferroelectrics in general. Despite the relevance of this issue, no clear results on antiferroelectrics have been uncovered so far, notably due the low amount of thin film investigations, which would allow for a well-defined and revisable way to control strain and doping. One reason for this lack of research originates from the difficulty to synthesize these materials with suitable crystalline quality including a bottom electrode for capacitive measurements. Based on the first time achievement of an unambiguously confirmed antipolar AFE phase in a pure NaNbO3 thin film, we propose the following experiments: • Growth of high-quality thin films of the AFE perovskites NaNbO3 and AgNbO3 by advanced oxide molecular beam epitaxy (ADOMBE) with characterization of structural (by X-ray diffraction (XRD)) and compositional (by X-ray photoelectron spectroscopy (XPS)) materials properties with suitable dopants to stabilize the AFE phase; • In-situ top and bottom contact fabrication for electrical characterization via an already established route using highly conducting perovskite oxides; and • Investigation of the interplay between polar and antipolar phases to optimize AFE properties via strain engineering using structurally compatible substrates with varying lattice constants. Based on the obtained experimental results, we will develop a clear understanding of the influence of strain and dopants on the AFE nature of lead-free materials. These results will be a valuable guideline to calibrate modelling predictions and will also help to define experimental routes towards optimization of energy storage density via doping and strain engineering.