Zatterin, Edoardo M.; (2025) Superdomains in ferroelectric thin films. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Below a critical temperature, Tc, ferroelectric materials develop a spontaneous electric polarization, which can be switched by an external electric field. Most ferroelectrics are also ferroelastic, so that below Tc a spontaneous strain also appears. The resulting longrange elastic and electric fields are minimized by the formation of domains, regions of material with uniform order parameter (polarisation or strain) that changes sign or direction across a domain wall. When ferroelectrics are grown as epitaxial thin films, the mechanical clamping to the substrate induces a biaxial stress in the film, which favours the formation of complex, dense nanodomain structures. Peculiar functional properties of technological interest emerge depending on the exact domain arrangement and crystallography, which can be controlled by appropriate processing parameters. In some cases this leads to a hierarchical arrangement of periodic polytwin structures into bundles, or “superdomain”. Different superdomains contain twins of different crystallographic orientations and sizes. Compared to the more commonly observed “cellular polytwin” case, the presence of such hierarchical bundles is seen to enhance the response of the overall domain structure to external stimuli. Deterministic re-configuration of domain arrangements under applied fields is crucial for the exploitation of novel functionalities found at domain walls in potential future nanoelectronic devices, making these systems excellent candidates for the task. Most studies to date have however focussed on certain consequences of the presence of hierarchical domain arrangements, without dwelling on the unconventional crystallography of such structures or the exact nature of its response to applied field or temperature. In this thesis, the structure of a prototypical superdomain ferroelectric thin film is investigated using a combination of laboratory and synchrotron diffraction and piezoresponse force microscopy (PFM) in device-like conditions, and the peculiar crystallography of superdomains is elucidated both in the as-grown case, as well as following external excitations (electric field and temperature). Chapter 1 gives a general overview of the fundamental properties of ferroic materials and domains, while Chapter 2 focusses specifically on perovskite ferroelectric oxide thin films, introducing the mechanisms that lead to the formation of dense polytwin structures and superdomains in these systems. Chapter 3 describes the experimental and data analysis techniques used in this work, providing the tools to correctly interpret the complicated features of the diffraction patterns and PFM images shown in this thesis. The complex crystallography of superdomains is investigated in detail in Chapter 4; their stability is subsequently assessed in Chapter 5, which discusses the results of a temperature-dependent study. Finally, Chapter 6 demonstrates that reversible and non-volatile ferroelastic superdomain switching occurs in different ways depending on the boundary conditions imposed to the system.

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