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Domain formation and switching kinetics in thin ferroelectric films conditioned by flexoelectricity, surface screening and chemical strains

Vorotiahin, Ivan (2019):
Domain formation and switching kinetics in thin ferroelectric films conditioned by flexoelectricity, surface screening and chemical strains.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8606],
[Ph.D. Thesis]

Abstract

Polar and electromechanical properties of ferroelectric thin films including polarization domain formation and switching under the influence of flexoelectric effect, surface charges, and chemical strains have been studied using the phenomenological Landau-Ginzburg-Devonshire theory. The effects are inherent to a thin single-crystalline ferroelectric film containing lattice defects and were studied both separately and in competition with one another to determine their exclusive impacts as well as their interplay in the investigated system.

Ferroelectric films of barium titanate, lead titanate, and lead zirconate-titanate are best characterized, having the predictable behaviour at domain formation and polarization switching and possess the fullest set of measured parametric coefficients to use in phenomenological model in this thesis. Their behaviour is modelled with the inclusion of flexoelectricity, an electromechanical effect connecting polar properties with strain gradient, charges at the film surface, formed from surface states or a non-ideal electrode, and chemical strains resulting from inclusion of defects into the crystal lattice.

The first chapter gives an overview of ferroelectric research, applications, and challenges, as well as explains the origin and working principles of the effects of interest. Ferroelectrics are intensively studied materials that are invisibly omnipresent in modern electronic technology. Their phenomenological study is set to move forward the understanding of processes taking place in these substances. Previous experimental and theoretical works support the statement that the effects of interest indeed have influence upon solid-state substances, and particularly upon ferroelectrics. This study helps to understand how these effects can be represented in phenomenological models and most importantly, how they interact with each other.

The second chapter describes the theoretical background of this study. It is shown how properties of ferroelectrics are expressed through thermodynamic potentials and incorporated into the framework of the Landau-Ginzburg-Devonshire theory. All relevant effects taking place in the ferroelectric film are incorporated into the theoretical model, including ferroelastic properties and semiconductor properties, expressed via electromechanical and electrostatic terms, respectively. Among these properties, the effects of interest are introduced and underlined. A part of the chapter explains the basic principle of the finite-element method used in the modelling software to perform calculations with given precision.

The third chapter is dedicated to the specific problems studied in this thesis. The sections of the chapter follow this division, focusing first on the flexoelectric impact on the static and dynamic characteristics of the samples, then the surface charges influence on domain formation and switching dynamics, and finally the defect-driven chemical strains impacting domain properties of the ferroelectric in a competition with the two other mechanisms. The specific problems are first stated being followed by the publications where the investigations were made.

The fourth chapter discusses the results and makes conclusions on the contributions of each considered effect into the domain formation process and parameters and/or ferroelectric switching dynamics. The tunability of parameters and the prospects of usage and processing of ferroelectric films are discussed.

The final chapter gives an overview of the related scientific problems that may be addressed using methods or results described in the current work.

Item Type: Ph.D. Thesis
Erschienen: 2019
Creators: Vorotiahin, Ivan
Title: Domain formation and switching kinetics in thin ferroelectric films conditioned by flexoelectricity, surface screening and chemical strains
Language: English
Abstract:

Polar and electromechanical properties of ferroelectric thin films including polarization domain formation and switching under the influence of flexoelectric effect, surface charges, and chemical strains have been studied using the phenomenological Landau-Ginzburg-Devonshire theory. The effects are inherent to a thin single-crystalline ferroelectric film containing lattice defects and were studied both separately and in competition with one another to determine their exclusive impacts as well as their interplay in the investigated system.

Ferroelectric films of barium titanate, lead titanate, and lead zirconate-titanate are best characterized, having the predictable behaviour at domain formation and polarization switching and possess the fullest set of measured parametric coefficients to use in phenomenological model in this thesis. Their behaviour is modelled with the inclusion of flexoelectricity, an electromechanical effect connecting polar properties with strain gradient, charges at the film surface, formed from surface states or a non-ideal electrode, and chemical strains resulting from inclusion of defects into the crystal lattice.

The first chapter gives an overview of ferroelectric research, applications, and challenges, as well as explains the origin and working principles of the effects of interest. Ferroelectrics are intensively studied materials that are invisibly omnipresent in modern electronic technology. Their phenomenological study is set to move forward the understanding of processes taking place in these substances. Previous experimental and theoretical works support the statement that the effects of interest indeed have influence upon solid-state substances, and particularly upon ferroelectrics. This study helps to understand how these effects can be represented in phenomenological models and most importantly, how they interact with each other.

The second chapter describes the theoretical background of this study. It is shown how properties of ferroelectrics are expressed through thermodynamic potentials and incorporated into the framework of the Landau-Ginzburg-Devonshire theory. All relevant effects taking place in the ferroelectric film are incorporated into the theoretical model, including ferroelastic properties and semiconductor properties, expressed via electromechanical and electrostatic terms, respectively. Among these properties, the effects of interest are introduced and underlined. A part of the chapter explains the basic principle of the finite-element method used in the modelling software to perform calculations with given precision.

The third chapter is dedicated to the specific problems studied in this thesis. The sections of the chapter follow this division, focusing first on the flexoelectric impact on the static and dynamic characteristics of the samples, then the surface charges influence on domain formation and switching dynamics, and finally the defect-driven chemical strains impacting domain properties of the ferroelectric in a competition with the two other mechanisms. The specific problems are first stated being followed by the publications where the investigations were made.

The fourth chapter discusses the results and makes conclusions on the contributions of each considered effect into the domain formation process and parameters and/or ferroelectric switching dynamics. The tunability of parameters and the prospects of usage and processing of ferroelectric films are discussed.

The final chapter gives an overview of the related scientific problems that may be addressed using methods or results described in the current work.

Place of Publication: Darmstadt
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Materials Modelling
Date Deposited: 14 Apr 2019 19:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8606
URN: urn:nbn:de:tuda-tuprints-86061
Referees: Genenko, Prof. Dr. Yuri and Xu, Prof. Dr. Bai-Xiang
Refereed / Verteidigung / mdl. Prüfung: 18 January 2019
Alternative Abstract:
Alternative abstract Language
Polare und elektromechanische Eigenschaften dünner ferroelektrischer Schichten wie Domänenstruktur und Polarisationsumschaltung unter dem Einfluss der Flexoelektrizität, Oberflächenladungen und chemischen Dehnungen, wurden mithilfe der phänomenoligischen Theorie von Landau-Ginzburg-Devonshire erforscht. Die Effekte sind einem System der dünnen ferroelektrischen Einkristallschichte mit Defekten im Kristallgitter charakteristisch, und wurden sowohl getrennt, als auch in Kombination untersucht, um ihre einzelne Einflusse, sowie ihr Zusammenwirken im berücksichtigten System festzustellen. Untersucht wurden ferroelektrische BaTiO3 , PbTiO3 und Pb(Zr,Ti)O3 Schichten, da diese am ausführlichsten charakterisiert sind und sich vorhersagbar bei der Domänenformation und Polarisationsumschaltung verhalten, sowie die größte Anzahl der gemessenen parametrischen Koeffizienten zur Nutzung im phänomenologischen Modell besitzen. Ihr Verhalten wurde unter dem Einfluss von Flexoelektrizität (einem elektromechanischen Effekt, der die Polarisation mit dem Dehnungsgradient verbindet), von elektrischen Oberflächenladungen (die aus Oberflächenenergiezuständen entstehen oder durch eine imperfekte Elektrode verursacht werden) und von chemischen Dehnungen (die ein Resultat von Defekteinfügung sind) modelliert. Das erste Kapitel gibt eine Übersicht der ferroelektrischen Forschungen, Anwendungen und Herausforderungen und erklärt die Herkunft und Wirkung der berücksichtigten Effekten. Ferroelektrika gehören zu den intensiv erforschten Materialien und befinden sich unscheinbar in vielen modernen elektrotechnischen Geräten. Ihre Phänomenologishe Theorie ist auf die Weiterentwicklung des Verständnisses von Prozessen, die in diesen Stoffen auftreten, gezielt. Vorherige experimentelle und theoretische Werke haben gezeigt, dass die Flexoelektrizität, Oberflächenladungen und die chemishen Dehnungen wesentlich die Festkörpermaterialien, und besonders Ferroelektrika, beeinflussen. Diese Forschung hilft zu verstehen, wie diese Effekte in den phänomenologischen Modellen repräsentiert werden können und, was wichtiger ist, wie sie miteinander wechselwirken. Das zweite Kapitel beschreibt den theoretischen Hintergrund der Forschung. Es wird gezeigt, wie die Eigenschaften der Ferroelektrika durch thermodynamische Potentiale beschrieben werden können und in die Landau-Ginzburg-Devonshire-Theorie eingeführt sind. Alle relevanten Effekte, die in den ferroelektrischen Schichten stattfinden, sind auch im theoretischen Modell als Erscheinungen von elektromechanischen und Halbleitereigenschaften berücksichtigt. Ein Teil des Kapitels erklärt das Wirkungsprinzip der Finite-Elemente-Methode, die in der Modellierungssoftware benutzt wurde, um die Berechnungen mit der gegebenen Genauigkeit durchzuführen. Das dritte Kapitel ist den konkreten untergesuchten Problemen gewidmet. Die Abschnitte des Kapitels sind nach den spezifischen Forschungen geteilt, konzentrierend erst auf dem flexoelektrischen Einfluss auf die statischen und dynamischen Schichteigenschaften, dann auf Oberflächenladungen, die Domänenstruktur und Umschaltungsdynamik beeinflussen, und letztens - auf den defektgesteuerten chemischen Dehnungen und ihren Einfluss auf ferroelektrischen Domäneneigenschaften, in Zusammenwirkung mit den zwei anderen Mechanismen. Die formulierten Aufgaben sind von Veröffentlichungen mit den erforschten Ergebnissen gefolgt. Das vierte Kapitel ist die Schlussfolgerung über die Beiträge jedes untersuchten Effekts in die Domänenstrukturformation und/oder in die Polarisationsumschaltungsdynamik. Diese Ergebnisse werden angesichts der Anwendbarkeit der abstimmbaren Parameter in der Technologie diskutiert. Das letzte Kapitel gibt einen Überblick von wissenschaftlichen Problemen und Aufgaben, die mit in dieser Arbeit beschriebenen Methoden und Ergebnissen gelöst werden könnten.German
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