Die vorliegende Arbeit befasst sich mit dem Einfluss der elektrischen Polarisation auf Eigenschaften freier Träger in ZnO basierten Halbleiterheterostrukturen. Dabei werden insbesondere Transporteigenschaften freier Träger sowie deren Rekombinationsdynamik untersucht. Die Arbeit behandelt vier inhaltliche Schwerpunkte. Der erste Schwerpunkt liegt auf den physikalischen Eigenschaften der verwendeten Materialen, hier wird der Zusammenhang der Bandlücke und der Gitterkonstanten von MgZnO Dünnfilmen und deren Magnesiumgehalt beschrieben. Weiterhin wird die Morphologie solcher Filme diskutiert. Auf unterschiedliche Substrate und Abscheidebedingungen wird dabei detailliert eingegangen. Der zweite Schwerpunkt behandelt die Eigenschaften undotierter und phosphordotierter ZnO und MgZnO Dünnfilme. Die strukturellen, Transport- und Lumineszenzeigenschaften werden hier verglichen und Rückschlüsse auf die Züchtungsbedingungen gezogen. Im dritten Schwerpunkt werden Quanteneffekte an ZnO/MgZnO Grenzflaechen behandelt. Hierbei wird insbesondere auf den Einfluss der elektrischen Polarisation eingegangen. Die Präsenz eines zweidimensionalen Elektronengases wird nachgewiesen, und die notwendigen Bedingungen zur Entstehung des sogenannten qunatum confined Stark-effects werden dargelegt. Insbesondere wird hier auf züchtungsrelevante Parameter eingegangen. Den vierten Schwerpunkt stellen Kopplungsphänomene in ZnO/BaTiO3 Heterostrukturen dar. Dabei werden zuerst die experimentell beobachten Eigenschaften verschiedener Heterostrukturen die auf unterschiedlichen Substraten gezüchtet wurden aufgezeigt. Hier stehen strukturelle und Transporteigenschaften im Vordergrund. Ein Modell zur Beschreibung der Ausbildung von Raumladungszonen in derartigen Heterostrukturen wird eingeführt und zur Beschreibung der experimentellen Ergebnisse angewandt. Die Nutzbarkeit der ferroelektrischen Eigenschaften des Materials BaTiO3 in Kombination mit halbleitendem ZnO wurden untersucht. Hierzu wurden ferroelektrische Feldeffekttransistoren unter Verwendung beider Materialien hergestellt. Die prinzipielle Eignung der Bauelemente als nichtflüchtige Speicherelemente wurde nachgewiesen.

Applying ferroelectric materials for nanoelectronic circuits opens, next to exploiting completely new functionalities, the possibility of improving resource efficiency in electronic circuits. Due to its defined and easy-to-manipulate domain structure, lithium niobate (LiNbO3, LNO) is a promising candidate to realize such circuits. As a prerequisite, a detailed understanding of the underlying conduction mechanisms is required for a future large scale application. The main field of attention of this thesis is the domain wall conductivity in lithium niobate, investigated with temperature-dependent dc conductivity measurements as well as higher-harmonic current analysis under alternating-voltage excitation. Thereby the parameters of the electric field are of special interest, comprising the static dc field and both the amplitude and the frequency of the ac excitation voltage. Prior to the analysis of the experimental results, the setups are characterized in depth and a theoretical framework to calculate higher-harmonic current contributions generated by non-ohmic conduction models is derived. In case of high static offset voltages, an ohmic-like conductance is observed, which is ascribed to the intrinsic conductivity of the domain wall. For lower static offset fields, a diode-like current-voltage characteristic is found, originating from the junction of the domain wall and the metallic contact electrode. The results are compared to measurements at an industrial Schottky diode taken under the same conditions. Based on the theory of metal-semiconductor junctions, the effective donor density within the conducting domain wall is estimated to be of the order of 1019/cm3, which agrees well with theoretical calculations in the literature. An equivalent circuit based on two diodes and two resistors is proposed to model the observed nonohmic conductance. For all experimental techniques, a good agreement between this model and the experimental data is observed, proving especially the non-ohmic conductivity to be of Schottky-type.

Second harmonic (SH) microscopy represents a powerful tool for the investigation of crystalline systems, such as ferroelectrics and their domain walls (DWs). Under the condition of normal dispersion, i.e., the refractive index at the SH wavelength is larger as compared to the refractive index at the fundamental wavelength, n(2ω) . n(ω), bulk crystals will generate no SH signal. Should the bulk, however, contain DWs, an appreciable SH signal will still be detectable at the location of DWs stemming from the Čerenkov mechanism. In this work, we demonstrate both how SH signals are generated in bulk media and how the Čerenkov mechanism can be inhibited by using anomalous dispersion, i.e., n(ω) . n(2ω). This allows us to quantitatively estimate the relative strength of the Čerenkov compared to other SH contrast mechanisms in DWs, such as the interference contrast. The results are in agreement with previous experiments based on the geometric separation of the signals. Due to the observed, strong Čerenkov contrast, such signal contributions may not be neglected in polarimetry studies of ferroelectric DWs in the future.

The formation of the micro- and nanodomain structures during polarization reversal has been studied in single crystal lithium niobate with inhomogeneously modified conductivity. It is well known that the vacuum annealing and plasma-source ion irradiation of the lithium niobate crystals leads to sufficient increase of the bulk conductivity due to out-diffusion of the oxygen from the sample surface. Creation of layers with modified conductivity leads to inhomogeneous distribution of applied electric field in bulk of ferroelectric crystals. Polarization reversal in such a crystals permits to localize charged domain walls in the bulk. Methods of creation of charged domain walls can be used for the construction of the waveguide structures, optical modulators and resonators. The main conclusions of the work: 1) It was shown that plasma-source ion irradiation and vacuum annealing leads to inhomogeneous change of absorption and increase of the conductivity, which can be attributed to the out-diffusion of oxygen and lithium segregation in the surface layer of crystal. 2) Inhomogeneous distribution of electric field in the bulk of modified crystals leads to significant decrease of switched layer thickness. 3) Effect of formation and growth of non-through domains with charged domain walls has been revealed. 4) Analysis of switching current by modified Kolmogorov-Avraami formula permit to determine mobility of domain wall and threshold filed of polarization reversal. / Целью работы являлось экспериментальное исследование формирования микро- и нанодоменных структур в монокристаллах ниобата лития с неоднородно модифицированной проводимостью. Известно, что обработка воздействием низкоэнергетичного ионно-плазменного облучения и восстановительного отжига приводит к резкому увеличению проводимости кристаллов за счёт аут-диффузии кислорода с поверхности. Создание слоёв с измененной проводимостью приводит к неоднородному распределению электрического поля в объёме сегнетоэлектрических кристаллов, что позволяет создавать внутри-объёмные заряженные доменные структуры при переключении поляризации. Методы создания заряженных доменных структур используются для создания элементов интегральных оптических устройств: волноводных структур, оптических модуляторов и резонаторов. Основные выводы работы: 1) Показано, что ионно-плазменное облучение и восстановительный отжиг кристаллов приводят к неоднородному изменению поглощения и увеличению проводимости, что может быть отнесено за счёт аут-диффузии кислорода и сегрегации лития в поверхностном слое. 2) Установлено, что неоднородное распределение электрического поля в объёме модифицированных кристаллов приводит к значительному понижению порогового поля переключения поляризации, за счёт уменьшения толщины переключаемого слоя. 3) Впервые обнаружен и изучен эффект формирования и роста несквозных доменов с заряженными доменными стенками. 4) Анализ тока переключения модифицированной формулой Колмогорова-Аврами позволил определить подвижность доменной стенки и пороговое поле начала переключения.

No description available.

The Rayleigh law describes the linear dependence of the permittivity of a ferroelectric on the applied ac electric field amplitude due to irreversible motions of domain walls. We show that this gives rise to a new equivalent-circuit element predestined to fit the impedance spectra of ferroelectrics based on an accepted physical model. Such impedance spectroscopy is a powerful tool to obtain a dielectric and resistive representation of the entire sample structure. The superiority of the Rayleigh analysis based on impedance spectroscopy compared to the common single-frequency approach is demonstrated for a ferroelectric Si : HfO₂ thin film

<p dir="ltr">Thin film materials are used in many areas of our daily lives. From memory storage chips to optical coatings, these thin films are essential to the technologies on which we rely. Multiferroic thin films, a group of materials that simultaneously exhibit ferromagnetism and ferroelectricity, are of particular interest because of the new opportunities that they enable in memory storage and sensors. Bismuth-based oxide materials have proven to be excellent candidates for these applications, with multiferroic properties and anisotropic structures. This novel self-assembled structure found in layered supercell systems has applications in optical devices, such as isolators and beamsplitters. Throughout this study, thin film strain and epitaxy must be tended to as the fundamentals of film growth, adding to the complexity of these challenges.</p><p dir="ltr">In this dissertation, bismuth-based oxides, and more specifically the Bi₃Fe₂Mn₂O_x (BFMO) layered supercell phase, are studied from three perspectives. First, BFMO is integrated onto silicon substrates for commercialization using a complex buffer layer stack to mediate the differences in the crystal lattice. This allows for a demonstration of device fabrication with this film. Second, the growth and impact of strain are examined through geometric phase analysis, discovering that strain is essential for the growth of the supercell phase in BFMO. This strain can be tuned through buffer layer addition to optimize the growth of this phase. Third, two methods are demonstrated to free the BFMO material from the typical film-substrate lattice matching requirements. The process of transferring the film from the original substrate onto a different substrate removes these restrictions, allowing virtually unlimited access to applications that were previously not possible. The two methods demonstrate different solutions to the specific challenges of transferring the highly strained BFMO thin film. These findings pave a practical way to integrate multiferroic layered oxide thin films onto chips for the next generation of devices.</p>

The new generation of non-linear optical devices, based on periodically-poled lithium niobate thin film platforms, aims at achieving record-high conversion efficiencies and a formerly unknown capability for integration into modern quantum optical systems. It thus not only suits the demands of high-end telecommunication applications, but also provides striking potential for further performance enhancement. However, most of these record-setting new developments lack in in-depth analysis of their domain grid structures to determine whether the key requirements for efficient non-linear optical conversion are ideally fulfilled. Established analysis techniques for the exploration of bulk lithium niobate, like Raman spectroscopy, have not been adapted for these thin film structures, because the low interaction volumes require long acquisition times. In this work, the importance of Raman spectroscopy analysis for thin film lithium niobate device optimization is demonstrated in cross-correlated co-imaging with second-harmonic imaging and piezoresponse force microscopy. Key performance indicators of quasi-phase matching are identified and specifically investigated. The experiments show that Raman spectroscopy is capable of detecting all these indicators and is unique in its ability to identify performance-inhibiting mechanical and electrical stress fields. In an attempt to establish high-speed Raman imaging on lithium niobate structures, the coherent four-wave mixing method of broadband coherent anti-Stokes Raman scattering (B-CARS) is, for the first time, systematically introduced for fundamental solid state analysis by theoretically and experimentally addressing all special implications of crystalline material systems via the model material lithium niobate, e.g. phase matching conditions, phase retrieval, and complex selection rules. It is shown that the resonant B-CARS signal can be retrieved in post-processing and allows a direct comparison with spontaneous Raman spectroscopy. The predicted CARS selection rules are experimentally confirmed, and phonons are assigned to their respective B-CARS peaks. Furthermore, B-CARS signals are shown to be predominantly generated by scattering in forward direction. The insights of these fundamental investigations are applied for lithium niobate domain wall imaging via B-CARS. Hyperspectral spontaneous Raman spectroscopy and B-CARS images are compared with regard to imaging speed, signal-to-noise ratio, domain wall contrast mechanism, and signature width. The experiments prove that B-CARS allows at least a 20-fold speed increase with an improved signal-to-noise ratio as compared to spontaneous Raman spectroscopy. The domain wall signature is of similar nature for both techniques, and is not changed via phase retrieval, thus allowing high-speed B-CARS domain wall imaging in lithium niobate without post-processing. The massive B-CARS domain wall signal is attributed to a Čerenkov-like effect analogous to second-harmonic imaging. This is the first time that such an effect has been shown by a Raman scattering technique. These findings show the importance of Raman scattering investigations for the optimization of modern non-linear optical devices, and outline a way to massively increase the speed of Raman crystal analysis via B-CARS. Thus, with further studies that quantify the effects detailed qualitatively in this work and take B-CARS analyses to a broader range of crystalline samples, this work can form the basis towards establishing the high-speed and in-depth analysis of modern non-linear optical platforms via coherent Raman imaging.:1. Motivation .......... 1 2. Fundamentals .......... 4 2.1 Lithium Niobate .......... 4 2.1.1 Ferroelectric Structures .......... 4 2.1.2 Domain Engineering .......... 6 2.1.3 Thin Film Lithium Niobate (TFLN) .......... 8 2.2 Spontaneous Raman Spectroscopy .......... 10 2.2.1 Raman Spectroscopy of Lithium Niobate Crystals .......... 14 2.2.2 Domain Imaging with Spontaneous Raman Spectroscopy .......... 18 2.3 Coherent anti-Stokes Raman Scattering (CARS) .......... 20 2.3.1 Signal Generation .......... 21 2.3.2 Broadband Coherent anti-Stokes Raman Scattering (B-CARS) .......... 24 2.3.3 The Non-Resonant Background .......... 25 2.3.4 Selection Rules .......... 27 2.3.5 Phase Matching .......... 30 3. Experimental Methods .......... 32 3.1 Spontaneous Raman Spectroscopy (SR) Setup .......... 32 3.2 B-CARS Setup .......... 34 3.3 Raman Data Analysis .......... 35 3.4 B-CARS Phase Retrieval .......... 36 3.5 Second-Harmonic Generation Microscopy .......... 38 3.6 Piezoresponse Force Microscopy .......... 40 4. Thin Film Lithium Niobate Co-Imaging .......... 42 4.1 TFLN Sample and Co-Imaging Approach .......... 45 4.2 Piezoresponse Force Microscopy Imaging .......... 47 4.3 Second-Harmonic Generation Imaging .......... 49 4.4 Hyperspectral Raman Imaging .......... 53 4.5 Conclusions on Cross-Correlated Co-Imaging .......... 58 5. Fundamental Aspects of B-CARS on Lithium Niobate .......... 61 5.1 Phase Matching .......... 62 5.2 Phase Retrieval .......... 65 5.3 Selection Rules and Phonon Assignment .......... 67 5.4 Conclusions .......... 71 6. B-CARS Domain Wall Analysis in Lithium Niobate .......... 73 6.1 Preliminary Considerations .......... 73 6.2 Sample Preparation .......... 74 6.3 Spectral Analysis .......... 75 6.4 Determination of Acquisition Times .......... 78 6.5 Domain Wall Signatures .......... 79 6.6 Domain Imaging .......... 82 6.7 Conclusions .......... 83 7. Summary and Outlook .......... 85 Appendix .......... I References .......... XIII Own Publications .......... XXIV Acknowledgements .......... XXV

Les systèmes d’information actuels reposent fortement sur les technologies micro-ondes utilisées pour les communications hertziennes. L’amélioration des performances des MEMS radio fréquence aptes à fonctionner dans la bande X (8 GHz et 12 GHz) est un enjeu important pour des applications de télécommunications mais aussi pour les applications radar. Pour y parvenir l’intégration de matériaux ferroélectriques à haute constante diélectrique est requise. Les matériaux qui possèdent de telles propriétés et qui sont les plus adaptés, sont les composés qui dérivent de la structure pérovskite. Intégrer ce type de matériaux dans des commutateurs radio-fréquence (MEMS-RF) pose de nouveaux chalenges en termes de maîtrise du matériau et de compatibilité avec les technologies MEMS existantes. Cette thèse s’est portée sur le composé PMN-PT avec la composition 65/35 qui possède une permittivité relative supérieure à 10000 sous forme de matériau massif.Ce travail de thèse a été consacré à l’étude de l’intégration du composé PMN-PT dans des composants passifs que sont les commutateurs MEMS. Dans la gamme de fréquence d’intérêt, de 500 MHz jusqu’à 20 GHz, les propriétés de ces matériaux ont été peu étudiées sur les matériaux massifs et encore moins sous forme de films minces. L’objectif de cette thèse était de réaliser les couches minces ferroélectriques et de tester leur compatibilité dans l’ensemble du fonctionnement d’un composant MEMS mais aussi de mener une étude réciproque : l’analyse des FeMEMS (MEMS basé sur les ferroélectriques) permettant de compléter les connaissances de ces matériaux dans cette gamme de fréquence. Ce travail est d’intérêt pour l’industrie de la technologie MEMS mais aussi pour la science des matériaux ferroélectriques mais aussi par la compréhension des mécanismes physiques gouvernant aux propriétés diélectriques en termes de pertes notamment dans ce domaine de fréquences.Les caractérisations des MEMS-RF présentées dans cette thèse ont démontré la compatibilité du MEMS PMN-PT dans la gamme de fréquence entre 500MHz et 10 GHz avec de très bonnes performances. En utilisant cette adaptation, la technologie actuelle est ainsi capable de couvrir tous les bandes de fréquence les plus importantes : la bande de télécommunication civile de 1 GHz à 5 GHz en utilisant le PMN-PT, la bande X pour les satellites entre 5 GHz et 15 GHz avec PZT et la bande de haute fréquence de 15 GHz à 40 GHz pour la défense avec les diélectriques traditionnels (Si3N4). / The current information systems depend strongly on the microwave technology used for wireless communications. The enhanced performance of MEMS radio frequency capacity in X-band (8 GHz and 12 GHz) is an important issue not only for Telecom applications but also for Radar applications. The integration of ferroelectric materials with high-k t is highly demanded to replace the traditional dielectrics. This high-k property is accessible for compounds derived from the perovskite structure. Incorporating such materials in switches radio-frequency (RF-MEMS) impose however new chalenges in terms of the compatibility with the existing MEMS technologies. This thesis is focused on the compound PMN-PT with composition 65/35, which has a relative permittivity greater than 10,000 in the form of bulk material.This thesis has been devoted to the study of the integration of PMN-PT thin films in passive components such as MEMS switches. In the frequency range of interest, 500 MHz to 20 GHz, the properties of these materials have not been studied in bulk materials and even less in the form of thin films. The aim of this thesis was to fabricate the ferroelectric thin films and test their compatibility in the overall operation of a MEMS component. This study provides a reciprocal analysis FeMEMS (MEMS based on ferroelectrics) to complete knowledge of these materials in this frequency range. This work makes interest to both the industry and MEMS ferroelectric materials science who is trying to understand the physical mechanisms governing the dielectric properties in terms of losses in this particular range of frequencies.The characterizations of RF-MEMS presented in this thesis have demonstrated the compatibility of MEMS PMN-PT in the frequency range between 500MHz to 10 GHz with very good performance. Using this adaptation, the current technology is able to cover the most important frequency bands: the civil band telecommunication 1 GHz to 5 GHz using the PMN-PT, the X-band satellites between 5 GHz and 15 GHz with PZT and high frequency band of 15 GHz to 40 GHz for the defense with traditional dielectric (Si3N4).

Made available in DSpace on 2017-07-24T19:38:04Z (GMT). No. of bitstreams: 1 Alan Rogerio Ferreira Lima.pdf: 4210754 bytes, checksum: 82a7a7898a00633ad3aa474df848f207 (MD5) Previous issue date: 2011-02-28 / This work aimed the study of the effect of transition metals (Ni, Fe, Co) on the spontaneous polarization of the ferroelectric oxides of tetragonal tungsten bronze (TTB)-type structure of potassium strontium niobate (KSr2Nb5O15) and of the KSr2(FeNb4)O15-d, KSr2(NiNb4)O15-d and KSr2(CoNb4)O15-d solid solutions. All systems investigated were prepared by mechanical mixture of oxide/carbonates by high-energy ball milling. The calcination temperature was performed to obtain nanoparticles and the nanostructured particles were evaluated. The evolution of mass loss, chemical bonds, crystalline structure and the electrical behavior were evaluated using the thermal analysis (thermogravimetric Analysis (TGA) and Differential Scanning Calorimeter (DSC), infrared absorption spectroscopy (FTIR), X-ray diffraction (XRD) and impedance spectroscopy, respectively. From the impedance spectroscopy technique were studied the dielectric properties of the nanoparticles and the dielectric permittivity of the KSr2Nb5O15, KSr2(FeNb4)O15-d, KSr2(NiNb4)O15-d and KSr2(CoNb4)O15-d systems was determined. The highest permittivity value was obtained for KSr2(FeNb4)O15-d. The determination of crystallographic parameters of nanopowders was performed using the Rietveld method. From the crystallographic parameters was simulated the crystalline structure and determined the spontaneous polarization for KSr2Nb5O15, KSr2(FeNb4)O15-d, KSr2(NiNb4)O15-d and KSr2(CoNb4)O15-d systems based on the atomic displacement (Dz) of Nb (niobium) atom, in the z plane, from the central position of the [NbO6] octahedron. The variation of spontaneous polarization of the nanoparticles with the addition of transition metals in the KSr2Nb5O15 host structure confirmed the existence of ferroelectricity in the systems investigated. / O objetivo deste trabalho foi estudar o efeito de metais de transição (Ni, Fe, Co), sobre a polarização espontânea, dos óxidos ferroelétricos de estrutura tungstênio bronze (TB) de niobato de potássio e estrôncio (KSr2Nb5O15) e das soluções sólidas de KSr2(FeNb4)O15-d, KSr2(NiNb4)O15-d e KSr2(CoNb4)O15-d. Todos os sistemas investigados foram preparados por mistura de óxidos/carbonatos pelo do método de ativação mecânica por moagem de alta energia. A temperatura de calcinação necessária à obtenção de nanopartículas foi otimizada e as partículas nanoestruturadas foram avaliadas. A evolução dos parâmetros de perda de massa, ligações químicas, estrutura cristalina e o comportamento elétrico, foram avaliados utilizando as técnicas de análise térmicas (análise termogravimétrica (ATG) e calorimetria diferencial exploratória (DSC), espectroscopia de absorção na região do infravermelho, difração de raios X e espectroscopia de impedância, respectivamente. A partir da técnica de espectroscopia de impedância foram estudadas as propriedades dielétricas das nanopartículas em suspensão, determinando a permissividade dielétrica dos sistemas KSr2Nb5O15, KSr2(FeNb4)O15-d, KSr2(NiNb4)O15-d e KSr2(CoNb4)O15-d. O maior valor de permissividade foi obtido para o KSr2(FeNb4)O15-d. A determinação dos parâmetros cristalográficos dos nanopós foi realizada utilizando o método de Rietveld de refinamento e/ou ajuste de estrutura cristalina. A partir dos parâmetros cristalográficos foi simulada a estrutura cristalina e determinada a polarização espontânea para os sistemas KSr2Nb5O15, KSr2(FeNb4)O15-d, KSr2(NiNb4)O15-d e KSr2(CoNb4)O15-d com base no deslocamento atômico (Dz) do átomo de Nb (nióbio), no plano z, da posição central do octaedro [NbO6]. A variação da polarização espontânea das nanopartículas, com a adição de metais de transição na estrutura hospedeira do KSr2Nb5O15, confirmou a existência de ferroeletricidade nos sistemas investigados.