Low voltage electron emission from ferroelectric materials [Elektronische Ressource] / vorgelegt von Oliver Mieth

Low voltage electron emission from ferroelectric materials [Elektronische Ressource] / vorgelegt von Oliver Mieth

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Institut fur Angewandte PhysikFachrichtung PhysikFakultat Mathematik und NaturwissenschaftenTechnische Universitat DresdenLow Voltage Electron Emissionfrom Ferroelectric MaterialsDissertationzur Erlangung des akademischen GradesDoctor rerum naturalium(Dr. rer. nat.)vorgelegt vonOliver Miethgeboren am 17. Februar 1981 in Mei enDresden 2010Eingereicht am 11.03.20101. Gutachter: Prof. Dr. Lukas M. Eng2. Gutachter: Prof. Dr. Ramamoorthy RameshVerteidigt am 26.10.2010AbstractElectron emission from ferroelectric materials is initiated by a variation of thespontaneous polarization. It is the main focus of this work to develop ferroelec-tric cathodes, which are characterized by a signicantly decreased excitationvoltage required to initiate the electron emission process. Particular attentionis paid to the impact of the polarization on the emission process. Two materialsare investigated. Firstly, relaxor ferroelectric lead magnesium niobate - leadtitanate (PMN-PT) single crystals are chosen because of their low intrinsic5 2coercive eld. Electron emission current densities up to 5 10 A=cm areachieved for excitation voltages of 160 V. A strong enhancement of the emis-sion current is revealed for the onset of a complete polarization reversal. Sec-ondly, lead zirconate titanate (PZT) thin lms are investigated.

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Institut für Angewandte Physik Fachrichtung Physik Fakultät Mathematik und Naturwissenschaften Technische Universität Dresden
Low Voltage Electron Emission from Ferroelectric Materials
Dissertation
zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.)
vorgelegt von Oliver Mieth geboren am 17. Februar 1981 in Meißen
Dresden 2010
Eingereicht am 11.03.2010
1. Gutachter: Prof. Dr. Lukas M. Eng
2. Gutachter: Prof. Dr. Ramamoorthy Ramesh
Verteidigt am 26.10.2010
Abstract
Electron emission from ferroelectric materials is initiated by a variation of the spontaneous polarization. It is the main focus of this work to develop ferroelec tric cathodes, which are characterized by a significantly decreased excitation voltage required to initiate the electron emission process. Particular attention is paid to the impact of the polarization on the emission process. Two materials are investigated. Firstly, relaxor ferroelectric lead magnesium niobate  lead titanate (PMNPT) single crystals are chosen because of their low intrinsic 5 2 coercive field. Electron emission current densities up to 510 A/cm are achieved for excitation voltages of 160 V. A strong enhancement of the emis sion current is revealed for the onset of a complete polarization reversal. Sec ondly, lead zirconate titanate (PZT) thin films are investigated. A new method to prepare top electrodes with submicrometer sized, regularly patterned aper tures is introduced and a stable electron emission signal is measured from these structures for switching voltages<Furthermore, a detailed analysis of20 V. the polarization switching process in the PMNPT samples is given, revealing a spatial rotation of the polarization vector into crystallographic easy axes, as well as the nucleation of reversed nanodomains. Both processes are initiated at field strengths well below the coercive field. The dynamics of the polar ization reversal are correlated to the electron emission measurements, thus making it possible to optimize the efficiency of the investigated cathodes.
Kurzfassung
Die Ursache für Elektronenemission aus ferroelektrischen Materialien ist eine Veränderung des Zustandes der spontanen Polarisation. Gegenstand der vor liegenden Arbeit ist eine Verringerung der dafür nötigen Anregungsspannung, wobei besonderes Augenmerk auf die Rolle der ferroelektrischen Polarisation innerhalb des Emissionsprozesses gelegt wird. Es werden zwei verschiedene Materialien untersucht. Das RelaxorFerroelektrikum Bleimagnesiumniobat  Bleititanat (PMNPT) wurde aufgrund seines geringen Koerzitivfeldes aus 5 2 gewählt. Es konnten Emissionsstromdichten von bis zu 510 A/einercm bei Anregungsspannung von 160 V erreicht werden. Bei Einsetzen eines vollständi gen Umschaltens der Polarisation wurde eine deutliche Verstärkung des Emis sionsstromes festgestellt. Desweiteren werden Untersuchungen an Bleizirko niumtitanat (PZT) Dünnfilmen gezeigt. Eine neue Methode, eine Elektrode mit periodisch angeordneten Aperturen im Submikrometerbereich zu präpari eren, wird vorgestellt. Diese Strukturen liefern ein stabiles Emissionssignal für Anregungsspannungen<Eine detailierte Analyse des Schaltverhaltens20 V.
der Polarisation der PMNPT Proben zeigt sowohl eine Rotation des Polari sationsvektors als auch eine Nukleation umgeschaltener Nanodomänen. Beide Prozesse starten bei Feldstärken unterhalb des Koerzitivfeldes. Die ermittelte Zeitabhängigkeit des Schaltprozesses erlaubt Rückschlüsse auf den Emissions prozess und erlaubt es, die Effizienz der untersuchten Kathoden weiter zu optimieren.
Contents
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3
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Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferroelectricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Basic Properties of Ferroelectrics . . . . . . . . . . . . . . . . 2.1.1 Ferroelectric Domains . . . . . . . . . . . . . . . . . . 2.1.2 Switching the Ferroelectric Polarization . . . . . . . . . 2.1.3 Applications of Ferroelectric Materials . . . . . . . . . 2.2 Relaxor Ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . 2.3 Ferroelectric Thin Films . . . . . . . . . . . . . . . . . . . . . 2.4 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Lead Magnesium Niobate  Lead Titanate . . . . . . . 2.4.2 Lead Zirconate Titanate (PZT) . . . . . . . . . . . . .
Ferroelectric Electron Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Weak Ferroelectric Electron Emission . . . . . . . . . . . . . . 3.3 Strong Ferroelectric Electron Emission . . . . . . . . . . . . . 3.4 Thin Film Ferroelectric Electron Emission . . . . . . . . . . . 3.4.1 “PlanetoPlane” Electrode Geometry . . . . . . . . . . 3.4.2 Structured Top Electrodes . . . . . . . . . . . . . . . . 3.5 Applications of Ferroelectric Electron Emission . . . . . . . .
Instrumentation and Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Electron Emission Measurements . . . . . . . . . . . . . . . . 4.2 Ferroelectric Domain Imaging and Manipulation . . . . . . . . 4.2.1 Domain Imaging . . . . . . . . . . . . . . . . . . . . . 4.2.2 Domain Manipulation . . . . . . . . . . . . . . . . . .
7
9
13 13 14 16 17 17 20 21 21 23
25 25 28 32 33 33 35 36
39 39 41 42 46
8
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Electron Emission from PMNPT Single Crystals . . . . . . . . . . . . . . . 5.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Dependence on Voltage Amplitude and Frequency . . . . . . . 5.2.1 Single Electron Detector Measurements . . . . . . . . . 5.2.2 Emission Current Measurements . . . . . . . . . . . . . 5.2.3 Nature of the Electron Emission Process . . . . . . . . 5.3 Influence of Ferroelectric Polarization . . . . . . . . . . . . . . 5.3.1 Polarization Orientation . . . . . . . . . . . . . . . . . 5.3.2 Polarization Reversal . . . . . . . . . . . . . . . . . . . 5.4 Energy Distribution . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Surface Conductivity of Emitter Structures . . . . . . . . . . .
47 47 48 48 53 56 56 56 59 63 64
Electron Emission from PZT Thin Films . . . . . . . . . . . . . . . . . . . . . . . . 71 6.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . 71 6.2 Electron Emission Measurements . . . . . . . . . . . . . . . . 73 6.3 Influence of Ferroelectric Polarization . . . . . . . . . . . . . . 75
Switching of Ferroelectric Polarization in PMNPT Single Crystals 7.1 Initial Domain Structure . . . . . . . . . . . . . . . . . . . . . 7.2 Macroscopic Polarization Reversal . . . . . . . . . . . . . . . . 7.3 Rotation of Ferroelectric Polarization . . . . . . . . . . . . . . 7.3.1 Configurations of PFM Measurements . . . . . . . . . 7.3.2 Complete Polarization Rotation Path . . . . . . . . . . 7.3.3 Incomplete Polarization Rotation Path . . . . . . . . . 7.3.4 Multiphase Coexistence . . . . . . . . . . . . . . . . . 7.4 Polarization State in Electron Emission Structures . . . . . . .
77 77 80 85 86 88 93 97 99
General Conclusions and Future Perspectives . . . . . . . . . . . . . . . . . . . 105 8.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 8.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Erklärung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
1
Introduction
th Since the discovery of cathode rays in the 19 century [1], different kinds of electron sources have been invented. This development not only changed most peoples’s everyday life, but also stimulated scientific progress in many ways. The invention of television based on cathode ray tubes brought the “world” into millions of living rooms. The electron was discovered as a particle based on experiments on cathode rays [2] and the successful explanation of electron emission due to the photoelectric effect [3] represents a breakthrough in quantum physics being finally rewarded with the Nobel Prize. The wave character of emitted electrons is exploited in electron microscopes [4], which opened the door to the nanoworld. There have always been attempts to improve existing cathodes as well as to develop new types of electron emitters, but it was not before 1974 when the idea of ferroelectric emitters has been reported [5]. Ferroelectricity was discov ered in 1921 [6]. Since then scientific interest in this materials has increased drastically. In the 1940s, the possible use in sonar applications for subma rine detection of recently discovered ferroelectric oxides initiated a new era of research in this field. Nowadays, ferroelectrics are particularly promising can didates for nonvolatile memory devices [7], but also finding increased interest in functional metamaterial applications [8, 9]. Accordingly, after decades of research, the preparation of highquality single crystals and thin films is well established and has been extended to a wide variety of materials. The mechanism of ferroelectric electron emission (FEE) is different from any other kind of cathodes developed so far. The crystal lattice provides not only a reservoir of electrons but is directly involved in the emission process [10]. A variation of the ferroelectric polarization, which is induced by distortions of the crystal structure, initiates the emission of surface charges that screened the depolarization field. The first period of research on ferroelectric electron emission was charac 7 2 terized by small emission current densities (<10 A/cm ) achieved by py roelectric, piezoelectric or electric field induced polarization variations from
10
1
Introduction
2 single crystal ferroelectrics. Much higher current densities of up to 100 A/cm have been found in 1989 [11], which was accomplished by a new kind of top electrodes being prepared on the ferroelectric emitters. A metal layer with patterned apertures allowed the formation of a surface flashover plasma that enhanced the emission current tremendously. Inspired by this, many groups joined this topic and investigations were performed on almost any known fer roelectric material using different electrode patterns. It was shown that ferro electric cathodes can be used as electron sources in microwave tubes [12] or in flat panel displays [13]. However, since the emission efficiency is poorer in thin films compared to bulk materials, most presented cathodes were single crystals or ceramics with thicknesses of>100µm, and corresponding operation voltages of several kV. Moreover, most investigated materials exhibit quite large coercive fields. As a result, high voltages are required to switch the spontaneous polarization and to induce a polarization variation sufficient to initiate an emission of electrons. Lower operation voltages of ferroelectric electron sources are desired not only for reasons of convenience and applicability but also to reduce the power con sumption for this kind of cathodes. It is the objective of the present work to proove that ferroelectric electron emission is possible at drastically reduced excitation voltages and to clarify the role of ferroelectric polarization within the emission process. A brief introduction into the phenomenon of ferroelectricity (see Chapter 2) is followed by a description of the basic principles of ferroelectric electron emission in Chapter 3. The experimental setup and methods are introduced in Chapter 4. Two different ferroelectric systems were investigated. Lead magnesium nio bate  lead titanate (PMNPT) single crystals and lead zirconate titanate (PZT) thin films both promise low voltage electron emission. PMNPT is a relaxor ferroelectric material that recently attracted great interest due to its outstand ing piezoelectric properties [14]. The spontaneous polarization can be reversed 2 with low applied electric fields (2 kV/which is about one order of magcm ), nitude smaller than for most other ferroelectrics, and thus a large polarization variation can be induced at moderate voltage amplitudes<100 V even for sin gle crystals with a thickness of several hundreds ofachieved resultsm. The are presented and discussed in Chapter 5. A different approach to reduce the operation voltage for electron emission was performed for PZT thin films. As can be seen in Chapter 6, a reduced emitter thickness enables the polarization switching and subsequently, the on set of electron emission at lower voltages.
11
In Chapter 7 the reversal of the ferroelectric polarization in the emitter structures is investigated in detail and correlations to the emission process are revealed. Finally, a summary of the presented results and conclusions is given in Chapter 8, as well as suggestions of a further optimization of the investigated electron emitters.