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Autor(in)
Datum
2020Typ
- Doctoral Thesis
ETH Bibliographie
yes
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Abstract
The diverse functional properties of transition-metal oxides have given rise to entire fields of research spanning fundamental aspects such as quantum phenomena to the highly applied field of oxide electronics. In the latter, electrically ordered oxides, such as ferroelectrics, stand out as prominent materials for implementation in low-energy-consuming oxide applications due to their characteristic response under the application of mechanical stress, electric field or optical irradiation.
The significant progress in oxide thin-film engineering over the last 20 years has enabled studies of such oxide functionality to go beyond bulk crystals and include characterization under confinement of the materials to the nanoscale. However, in epitaxially grown ultrathin films, the manifestation of electric polarization specifically and functionality in general can greatly differ from the behavior of the corresponding bulk crystals. In order to make use of the exotic functionality of ultrathin oxide films, it is therefore essential to understand when and how the polar states are set with respect to the thin-film synthesis and, upon implementation in electronic devices, with respect to device operation. This is however a challenging task. So far, detailed understanding of the ultrathin limit of polarity in oxide thin films remains restricted to a few model systems.
In this thesis, we present an approach to probe such polar states in ultrathin oxide layers with high sensitivity and in a nondestructive manner by nonlinear optics. We demonstrate the of use optical second harmonic generation both in situ, during thin-film synthesis, and operando, during device operation, to provide unique insight into the evolution of polar states in oxide films in these highly dynamic environments. Here, we focus on a set of oxide materials whose polar architecture becomes particularly involved in the nanometer limit. We first establish the sensitivity of our optical probe to distinguish between differently oriented polar states with a complex nanoscale microstructure in a single-phase material, and its evolution under electric-field application. We next exploit this sensitivity to follow the phase coexistence of emergent polar phases in an epitaxially strained system during the thin-film synthesis process. We thus obtain novel insight into an unusually robust, yet metastable high-temperature polar phase. We further investigate the peculiar coupling between polarization and structural order in a so-called improper ferroelectric in the ultrathin regime. Here, a combination of nonlinear optics and electron microscopy could reveal the crucial impact of epitaxial interfaces on the evolution of polarization. Finally, we reveal emergent symmetry-breaking in layered oxides during epitaxial growth, allowing the nanoscale design of symmetry and functionality beyond polar compounds.
Common to all of these systems, we find that the mechanical and electrostatic boundary conditions set during the thin-film synthesis itself dictates the emergence of electric polarity in the oxide films, which can even result in the emergence of new material phases that are unique to the thin-film geometry and do not have a bulk counterpart. The results presented in this thesis hence point to the many possibilities of designing ultrathin oxides with unique, yet robust, polar properties of interest for both oxide electronics applications and the emergent field of thin-film quantum materials. Mehr anzeigen
Persistenter Link
https://doi.org/10.3929/ethz-b-000446702Publikationsstatus
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Beteiligte
Referent: Fiebig, Manfred
Referent: Trassin, Morgan
Referent: Noheda, Beatriz
Referent: Dörr, Kathrin
Verlag
ETH ZurichOrganisationseinheit
03918 - Fiebig, Manfred / Fiebig, Manfred
Zugehörige Publikationen und Daten
Is supplemented by: http://hdl.handle.net/20.500.11850/453645
ETH Bibliographie
yes
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