This thesis studies different Eu optical centers in MgS:Eu and CaS:Eu thin films produced by Chemically Controlled Pulse Laser Deposition (CCPLD) and evaluates their suitability for the development of spectral storage devices of the future. The produced thin films consist of one or more optically active layer(s), MgS:Eu, CaS:Eu or a similar material, and a corresponding ZnS capping layer that functions as a protecting barrier for the other layers and preserves their composition and integrity. Given that the synthesis of the materials used to produce the multilayer structures in this work proved a great challenge, careful attention was given to the optimization of all fabrication parameters. Mass Spectrometry was used during the deposition of the thin films and the data obtained resulted on improvements and optimization of the deposition process. Scanning electron microscopy studies of these thin films were conducted to study degradation upon long-term storage. Microscopy results show that the morphology of the produced thin films is correlated to the growth environment during deposition and deterioration of the deposited materials could be initiated by nano-gaps and cracks in the capping layer of the thin films. In addition to optical centers in MgS:Eu and CaS:Eu, new centers were created by changing the thin film growth environment inside a hi-vacuum chamber, modifying the composition of the ablation target material, or both. For example, introducing O2, or alternatively HCl, inside the CCPLD chamber while producing MgS:Eu thin films results in the formation of impurity associated centers across lattice sites throughout the deposited structures. In another method of impurity doping studied, Cl- and Na+ were introduced into the MgS:Eu and CaS:Eu lattices by mixing trace amounts of the impurity ions into these materials in polycrystalline form and making this mixture a deposition target by hi-pressure cold compression technique. The introduction of these impurity ions will alter the crystal field environment around the Eu ions thus creating new optical centers with a shift in energy of their characteristic Zero Phonon Line. After extensive characterization of the optical properties of the thin films produced, laser-induced fluorescence spectroscopy and absorption spectroscopy measurements confirm that they are suitable candidates to be used in conjunction with power-gated spectral holeburning technique and could potentially provide ultrahigh, terabits per square inch, storage densities. / Physics

Experiments with polarized "ultracold neutrons" (UCN) offer a new way to measure the decay correlations of neutron beta decay; these correlations can be used to test the completeness of the Standard Model and predict physics beyond it. Ultracold neutrons are very low energy neutrons that can be trapped inside of material and magnetic bottles. The decay correlations in combination with the neutron and muon lifetimes experimentally find the first element (Vud) of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. The CKM matrix is a unitary transform between the mass and weak eigenstates of the d, s and b quarks; if the matrix is not unitary this would imply that the Standard Model is not complete. Currently the first row of the CKM matrix is over 2 sigma from unitarity and Vud is the largest component of the row. The UCNA experiment looks at the correlation between the polarization of the neutron and the momentum of the electron resulting from the beta decay of the neutron (the A-correlation). The keys to making a high precision measurement of A-correlation are a near 100% polarization of the neutrons that decay, low"backscatter electron detectors, and small, well characterized backgrounds. UCN can be 100% polarized by passing them through a seven Telsa magnetic field. The key to the UCNA experiment is keeping them polarized until they decay or are lost. This dissertation covers the development of guides that are minimally depolarizing and efficient transporters of UCN and their use in the UCNA experiment. The entire guide development process is covered from conception to manufacturing and testing. This process includes development of a pulsed laser deposition, diamond-like carbon coating system and materials studies of the resulting coatings. After the initial studies of the guide coating, meter"long sections of guide are tested with UCN to determine their depolarization and transport properties. The guide technology developed in this dissertation has been used in the entire UCNA experiment. Also, this technology is currently the state of the art for polarized and non-polarized UCN guide systems and it is being implemented in several new UCN experiments. / Ph. D.

Barium Titanate (BaTiO3 or BTO) has attracted an ever increasing research interest because of its wide range of potential applications. Nano-sized or nanostructured BTO has found applications in new, useful smart devices, such as sensors and piezoelectric devices. Not only limited to one material, multi-layers or multi-phases can lead to multifunctional applications; for example, nanocomposites can be fabricated with ferrite or metal phase with BTO. In this study, I synthesized various BTO-ferrites, ranging from nanoparticles, nanowires to thin films. BTO-ferrite coaxial nanotubes, BTO-ferrite self-assemble thin films, and BTO single phase films were prepared by pulsed laser deposition (PLD) and sol-gel process. BTO-ferrite nanocomposites were grown by solid state reaction. Furthermore, BTO-metal nanostructures were also synthesized by solid state reaction under hydrogen gas which gave us a great inspiration to fabricate metal-ceramic composites. To understand the relationship between metal and BTO ceramic phase, I also deposited BTO film on Au buffered substrates. A metal layer can affect the grain size and orientation in BTO film which can further help us to control the distribution of dielectric properties of BTO films. After obtaining different nanomaterials, I am interested in the applications of these materials. Recently, many interesting electric devices are developed based on nanotechnology, e.g.: memristor. Memristor is a resistor with memory, which is very important in the computer memory. I believe these newly-synthesized BTO based nanostructures are useful for development of memristor, sensors and other devices to fit increasing needs. / Ph. D.

𝛼-Ga₂O₃ ist ein Halbleiter mit extrem großer Bandlücke, der als solar-blinder Ultraviolett (UV) Photodetektor eingesetzt werden kann. Seine rhomboedrische Kristallstruktur ermöglicht es Legierungen mit isostrukturellem 𝛼-Al₂O₃ und verschiedenen Übergangsmetall-Sesquioxiden wie z.B. 𝛼-Cr₂O₃ und 𝛼-V₂O₃ zu bilden. Damit kann eine Variation der Bandlücke über einen noch nie dagewesenen Spektralbereich vom Infraroten bis zum tiefen UV erreichtwerden. In der vorliegenden Arbeit wird demonstriert, wie räumlich adressierbare 𝛼-(Ga,Al,Cr,V,Ti)₂O₃ Materialbibliotheken durch kombinatorische gepulste Laserabscheidung (c-PLD) realisiert und ihre physikalischen Eigenschaften mittels anschließender Hochdurchsatzmessungen bestimmt werden können. Im ersten Teil dieser Arbeit wird die Entwicklung eines umfassenden Modells für die numerische Beschreibung der lateralen Kompositions- und Schichtdickenverteilung, die bei der c-PLD entstehen, vorgestellt. Durch Identifikation und Korrektur eines Fehlers in einem etablierten Modell zur adiabatischen Expansion des Plasmas, wird erstmals eine realistische Beschreibung von PLD-Prozessen ermöglicht und das Modell für verschiedene Materialien verifiziert. Im zweiten Teil dieser Arbeit wird das Wachstum von 𝛼-Ga₂O₃ Dünnschichten mittels PLD vorgestellt. Dazu wird ein umfassendes Phasendiagramm für das Wachstum von Ga₂O₃ auf m-Saphir erstellt, das ein ausgeprägtes Wachstumsfenster für die metastabile 𝛼-Phase mit hoher struktureller Qualität aufzeigt. Darauf basierend wurden Materialbibliotheken aus (CrₓGa₁₋ₓ)₂O₃, (VₓGa₁₋ₓ)₂O₃ und (TiₓGa₁₋ₓ)₂O₃ mit kontinuierlicher Kompositionsverteilung durch c-PLD hergestellt. Ihre physikalischen Eigenschaften wurden durch lateral aufgelöste Röntgenbeugung (XRD), energiedispersive Röntgenspektroskopie und Transmissionsmessungen erfasst. Es werden Vergleiche zwischen der gemessenen Verteilung der Zusammensetzung der Dünnschichten und dem zuvor entwickelten c-PLD-Modell erörtert, die Aufschluss über die Wachstumskinetik der verschiedenen Sesquioxide geben. Sowohl für (CrₓGa₁₋ₓ)₂O₃ als auch für (VₓGa₁₋ₓ)₂O₃ wurde das phasenreineWachstum in der rhomboedrischen 𝛼-Phase durchXRD-Messungen über den untersuchten Zusammensetzungsbereich von 0,08< x(Cr) <0,54 und 0,07 < x(V)<0,62 bestätigt. Die Absorptionsenergie der 𝛼-(VₓGa₁₋ₓ)₂O₃-Dünnfilme zeigte eine systematische Verschiebung bei steigendem x(V) von 5,3 eV auf 2,9 eV. Damit wird zum ersten Mal eine Bandlückenverschiebung hin zu niedrigeren Energien innerhalb des rhomboedrischen Sesquioxid-Material-systems über einen derart breiten Spektralbereich demonstriert. Diese Ergebnisse sind vielversprechend für mögliche Anwendungen des Materialsystems, z.B. als wellenlängenselektiver Photodetektor,:1. Introduction 2. Theoretical Background 2.1 Material Properties 2.1.1 Rhombohedral 𝛼-Ga2O3 2.1.2 Other Ga2O3 Polymorphs 2.1.3 Ternary (Me,Ga)2O3 Alloys 2.1.4 Growth mechanisms of Ga2O3 2.2 Layer Thickness Distributions for PLD Growth 2.2.1 Approaches to Layer Thickness Distributions 2.2.2 Adiabatic Plasma Plume Expansion Model for Pulsed Laser Deposition 3. Experimental Methods 3.1 Growth methods and sample preparation 3.1.1 Pulsed Laser Deposition 3.1.2 Photolithography 3.2 Characterization Techniques 3.2.1 X-Ray Diffraction 3.2.2 Spectroscopic Ellipsometry 3.2.3 Transmission Measurements 3.2.4 Atomic Force Microscopy 3.2.5 Profilometry 3.2.6 Energy-Dispersive X-Ray Spectroscopy 4. Analysis of PLD-Thickness Distributions and Applications to High-Throughput Combinatorial PLD 4.1 Analytical Description of PLD Thickness Distributions 4.2 Analysis of Lateral Thickness Distributions of Sesquioxide thin films 4.3 Modelling of combinatorial PLD for arbitrary target segmentations 5 A Novel PLD-Control Software and FAIR-Data Management 5.1 Digital Data Management 5.2 A Digital Twin for PLD 5.3 cPLD - Software 6. Growth of Phase-Pure, Highly Crystalline 𝛼-Ga2O3 Thin Films by PLD 6.1 Influence of Substrate Orientation 6.2 Influence of Growth Temperature 6.3 Influence of Layer Thickness 6.4 Comprehensive 𝑝(O2)-𝑑-𝑇g-phase diagram for PLD of Ga2O3 on m-plane sapphire 6.5 Achieving thick 𝛼-Ga2O3 layers on m-plane sapphire 6.6 Structuring 𝛼-Ga2O3 by Sacrificial ZnO Layers 6.7 High-Throughput Electrical Property Screening 6.8 Intermediate Summary 7 Ternary Alloys of 𝛼-Ga2O3 and Transition Metal Sesquioxides 7.1 Preliminary investigations of binary Cr2O3 and Ti2O3 thin films 7.2 Investigations of ternary (Ga,TM)2O3 alloys 7.2.1 Characterization of (Ga,Cr)2O3 thin films 7.2.2 Characterization of (Ga,V)2O3 thin films 7.2.3 Characterization of (Ga,Ti)2O3 thin-films 7.3 Intermediate Summary 8 Summary and Outlook 8.1 Summary 8.2 Outlook Abbreviations List of Symbols List of Electronic Lab Book References List of Own and Contributed Articles Collaborations and third-party services Supervisors Institutes Bibliography Appendix Acknowledgement / 𝛼-Ga₂O₃ is an ultra-wide bandgap semiconductor, with potential applications as a solar blind ultraviolett (UV) photodetector. Due to its rhombohedral crystal structure, alloying to isostructural 𝛼-Al₂O₃ and various transition metal sesquioxides like e.g., 𝛼-Cr₂O₃ and 𝛼-V₂O₃, enables bandgap engineering over an unprecedented large spectral range from the infrared to the deep-UV. In the present work, the realization of spatially-addressable 𝛼-(Ga,Al,Cr,V,Ti)₂O₃ material libraries by combinatorial pulsed laser deposition (c-PLD) and the subsequent high-throughput screening of their physical properties are discussed. In the first part of this thesis the development of a comprehensive model for the numerical description of lateral composition and thickness distributions arising during c-PLD are presented. An error in a well-established adiabatic plasma plume expansion model is identified and corrected, such that a real-world description of PLD processes is feasible from now on. In the second part of this thesis, the growth of 𝛼-Ga2O3 thin films by PLD is presented. Therefore, for the first time a comprehensive phase diagram for the growth of Ga2O3 on m-plane sapphire is constructed, exhibiting a distinct growth window for metastable 𝛼-Ga₂O₃ thin films with up to now unprecedented structural quality. Based on optimized process parameters, material libraries of (CrₓGa₁₋ₓ)₂O₃, (VₓGa₁₋ₓ)₂O₃ and (TiₓGa₁₋ₓ)₂O₃ with continuous composition spread were deposited by c-PLD. Their physical properties were mapped by high throughput laterally resolved X-ray diffraction, energy-dispersive X-ray spectroscopy and transmission measurements. Comparisons of the measured compositional distribution of the thin films to the c-PLD model developed earlier are discussed, revealing insight into the growth kinetics of the different sesquioxides. For both, (CrₓGa₁₋ₓ)₂O₃ and (VₓGa₁₋ₓ)₂O₃, phase-pure growth in the rhombohedral 𝛼-phase was confirmed by XRD measurements over the investigated composition range of 0. 08<𝑥(Cr) <0. 54 and 0. 07<𝑥(V) <0. 62. The absorption onset energy of the 𝛼-(VₓGa₁₋ₓ)₂O₃ thin films showed a systematic shift for increasing 𝑥(V) from 5.3 eV to 2.9 eV. With that, bandgap engineering within the rhombohedral sesquioxide material system towards lower energies over a wide spectral range is demonstrated for the first time. These results are promising for possible applications of the material system as e.g., a wavelength selective photodetector.:1. Introduction 2. Theoretical Background 2.1 Material Properties 2.1.1 Rhombohedral 𝛼-Ga2O3 2.1.2 Other Ga2O3 Polymorphs 2.1.3 Ternary (Me,Ga)2O3 Alloys 2.1.4 Growth mechanisms of Ga2O3 2.2 Layer Thickness Distributions for PLD Growth 2.2.1 Approaches to Layer Thickness Distributions 2.2.2 Adiabatic Plasma Plume Expansion Model for Pulsed Laser Deposition 3. Experimental Methods 3.1 Growth methods and sample preparation 3.1.1 Pulsed Laser Deposition 3.1.2 Photolithography 3.2 Characterization Techniques 3.2.1 X-Ray Diffraction 3.2.2 Spectroscopic Ellipsometry 3.2.3 Transmission Measurements 3.2.4 Atomic Force Microscopy 3.2.5 Profilometry 3.2.6 Energy-Dispersive X-Ray Spectroscopy 4. Analysis of PLD-Thickness Distributions and Applications to High-Throughput Combinatorial PLD 4.1 Analytical Description of PLD Thickness Distributions 4.2 Analysis of Lateral Thickness Distributions of Sesquioxide thin films 4.3 Modelling of combinatorial PLD for arbitrary target segmentations 5 A Novel PLD-Control Software and FAIR-Data Management 5.1 Digital Data Management 5.2 A Digital Twin for PLD 5.3 cPLD - Software 6. Growth of Phase-Pure, Highly Crystalline 𝛼-Ga2O3 Thin Films by PLD 6.1 Influence of Substrate Orientation 6.2 Influence of Growth Temperature 6.3 Influence of Layer Thickness 6.4 Comprehensive 𝑝(O2)-𝑑-𝑇g-phase diagram for PLD of Ga2O3 on m-plane sapphire 6.5 Achieving thick 𝛼-Ga2O3 layers on m-plane sapphire 6.6 Structuring 𝛼-Ga2O3 by Sacrificial ZnO Layers 6.7 High-Throughput Electrical Property Screening 6.8 Intermediate Summary 7 Ternary Alloys of 𝛼-Ga2O3 and Transition Metal Sesquioxides 7.1 Preliminary investigations of binary Cr2O3 and Ti2O3 thin films 7.2 Investigations of ternary (Ga,TM)2O3 alloys 7.2.1 Characterization of (Ga,Cr)2O3 thin films 7.2.2 Characterization of (Ga,V)2O3 thin films 7.2.3 Characterization of (Ga,Ti)2O3 thin-films 7.3 Intermediate Summary 8 Summary and Outlook 8.1 Summary 8.2 Outlook Abbreviations List of Symbols List of Electronic Lab Book References List of Own and Contributed Articles Collaborations and third-party services Supervisors Institutes Bibliography Appendix Acknowledgement

In dieser Arbeit werden das Wachstum und die Charakterisierung der Heterostruktur eines FeFET auf der Grundlage von BaSnO3, LaInO3 und (K,Na)NbO3 Schichten untersucht. Für jedes Material wurden die Wachstumsbedingungen bestimmt und im Hinblick auf die strukturellen und elektrischen Eigenschaften optimiert. Epitaktische BaSnO3 Filme, die auf SrTiO3 Substraten gewachsen sind, weisen eine hohe Dichte an Versetzungen auf, die ihre elektrischen Eigenschaften beeinträchtigen. Die Verwendung von NdScO3 Substraten und Einführung einer SrSnO3 Pufferschicht verbesserten die strukturellen und elektrischen Eigenschaften der BaSnO3 Schichten. Dies ermöglichte schließlich Untersuchungen an der LaInO3/BaSnO3 Grenzfläche. Schon eine geringe La-Dotierung der BaSnO3 Schicht von 0,3 % führte zur Bildung eines 2DEG nach der Grenzflächenbildung und damit zum Einschluss von Elektronen an der Grenzfläche. Dies konnte durch C-V, Van-der-Pauw und Hall-Effekt-Messungen eindeutig nachgewiesen werden. Eine deutliche Verbesserung der strukturellen und elektrischen Eigenschaften der BaSnO3 Schichten wurde durch die Verwendung von LaInO3:Ba Substraten erreicht. Diese sind gitterangepasst an BaSnO3, sodass zum ersten Mal vollständig verspannte Schichten ohne Versetzungen gewachsen werden konnten. Strukturelle und elektrische Eigenschaften von (K,Na)NbO3 Schichten wurden auf SrRuO3/DyScO3 und SrTiO3:Nb-Substraten untersucht. Auf diese Weise wurden der Einfluss der Gitterdehnung auf die kritische Schichtdicke und die Prozesse der plastischen Relaxation des Gitters bestimmt. Die elektrische Charakterisierung ergab einen hohen Leckstrom, der durch strukturelle Defekte verursacht wird. Die gesamte FeFET Heterostruktur wurde auf LaInO3:Ba Substraten gewachsen und untersucht. BaSnO3 und LaInO3 Schichten wuchsen kohärent, während (K,Na)NbO3 Schichten eine plastische Gitterrelaxation aufwiesen. Das führte zur Bildung von Strukturdefekten und zu einer Verschlechterung der ferroelektrischen Eigenschaften. / In this thesis, the design, growth and characterisation of the heterostructure of a FeFET based on BaSnO3, LaInO3 and (K,Na)NbO3 thin films are investigated. For each material, the growth conditions were determined and optimised with respect to their structural and electrical properties. Epitaxial BaSnO3 thin films grown on SrTiO3 substrates exhibit a high density of threading dislocations, which degrade their electrical properties. The use of NdScO3 substrates and the introduction of a SrSnO3 buffer layer improved the structural and electrical properties of the BaSnO3 thin films. This finally allowed investigations on the LaInO3/BaSnO3 heterointerface. Even a low La doping of the BaSnO3 layer of 0.3 % led to the formation of a 2DEG after interface formation and thus to the confinement of electrons at the interface. This could be clearly demonstrated by C-V, Van-der-Pauw and Hall effect measurements. A significant improvement of the structural and electrical properties of the BaSnO3 thin films was achieved by using LaInO3:Ba substrates. These are lattice-matched to BaSnO3 so that, for the first time, fully strained thin films could be grown without dislocations. Structural and electrical properties of (K,Na)NbO3 thin films were investigated on SrRuO3/DyScO3 and SrTiO3:Nb substrates. In this way, the influence of lattice strain on the critical film thickness and plastic lattice relaxation were determined. Their electrical characterisation revealed a high leakage current caused by structural defects. Therefore, the entire FeFET heterostructure was grown and investigated on LaInO3:Ba substrates. The BaSnO3 and LaInO3 thin films were grown coherently, while the (K,Na)NbO3 thin films exhibited plastic lattice relaxation. This led to the formation of structural defects and consequently to a deterioration of their ferroelectric properties.

The thesis deals with the preparation, characterization and, in particular, with the switching properties of phase-change material (PCM) thin films. The films were deposited using the Pulsed Laser Deposition (PLD) technique. Phase transformations in these films were triggered by means of thermal annealing, laser pulses, and electrical pulses. The five major physical aspects structure transformation, crystallization kinetics, topography, optical properties, and electrical properties have been investigated using XRD, TEM, SEM, AFM, DSC, UV-Vis spectroscopy, a custom-made nanosecond UV laser pump-probe system, in situ resistance measurements, and conductive-AFM. The systematic investigation of the ex situ thermally induced crystallization process of pure stoichiometric GeTe films and O-incorporating GeTe films provides detailed information on structure transformation, topography, crystallization kinetics, optical reflectivity and electrical resistivity. The results reveal a significant improvement of the thermal stability in PCM application for data storage. With the aim of reducing the switching energy consumption and to enhance the optical reflectivity contrast by improving the quality of the produced films, the growth of the GeTe films with simultaneous in situ thermal treatment was investigated with respect to optimizing the film growth conditions, e.g. growth temperature, substrate type. For the investigation of the fast phase transformation process, GeTe films were irradiated by ns UV laser pulses, tailoring various parameters such as pulse number, laser fluence, pulse repetition rate, and film thickness. Additionally, the investigation focused on the comparison of crystallization of GST thin films induced by either nano- or femtosecond single laser pulse irradiation, used to attain a high data transfer rate and to improve the understanding of the mechanisms of fast phase transformation. Non-volatile optical multilevel switching in GeTe phase-change films was identified to be feasible and accurately controllable at a timescale of nanoseconds, which is promising for high speed and high storage density of optical memory devices. Moreover, correlating the dynamics of the optical switching process and the structural information demonstrated not only exactly how fast phase change processes take place, but also, importantly, allowed the determination of the rapid kinetics of phase transformation on the microscopic scale. In the next step, a new general concept for the combination of PCRAM and ReRAM was developed. Bipolar electrical switching of PCM memory cells at the nanoscale can be achieved and improvements of the performance in terms of RESET/SET operation voltage, On/Off resistance ratio and cycling endurance are demonstrated. The original underlying mechanism was verified by the Poole-Frenkel conduction model. The polarity-dependent resistance switching processes can be visualized simultaneously by topography and current images. The local microstructure on the nanoscale of such memory cells and the corresponding local chemical composition were correlated. The gained results contribute to meeting the key challenges of the current understanding and of the development of PCMs for data storage applications, covering thin film preparation, thermal stability, signal-to-noise ratio, switching energy, data transfer rate, storage density, and scalability.

Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4). Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen. Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen.

The phase diagram of complex oxides is very diverse due to the strong interaction between electrons in the electronic structure. It is possible to probe those interactions by changing electrostatically the carrier density, the main concept behind the Field-Effect Transistors (FET) which is the building blocks of nanoelectronics devices. In the case of high-TC superconductor copper oxides, it is possible to use this concept to switch between superconducting and insulator phases, for example using an adjacent liquid electrolyte layer to inject charges in a superconducting film. With that in mind, the objective of this work was to establish protocols to grow superconductor and ferroelectric films for future fabrication of superconducting FET devices. We optimized the deposition conditions for the growth of a single layer of superconductor YBa2Cu3O7&ndash;x and the ferroelectric barium titanate on SrTiO3 substrates by pulsed laser deposition (PLD). Several techniques were employed to study the properties of the thin films, such as X-ray diffraction, atomic force microscope, X-ray photoelectron spectroscopy, resistance vs temperature and ferroelectric hysteresis. Regarding the superconductors thin films, we observed several relations between the superconducting features and the growth parameters. For instance, lower growth temperatures contribute to the nucleation of a-axis oriented grains meanwhile higher growth temperature tends to be c-axis oriented. Regarding the frequency of the laser (proportional to the growth rate), it seems that lower frequency is related to higher surface roughness and the presence of non-superconducting contributions. As it increases, the roughness decrease and the sample presents a sharper superconducting transition. Finally, we also did the first steps towards the field effect device by growing a heterostructure thin film consisting of a superconductor and ferroelectric material. The sample grew c-axis oriented on strontium titanate substrate, though with a high value of surface roughness. / O diagrama de fase dos óxidos complexos é muito diverso devido à forte interação entre os elétrons na estrutura eletrônica. É possível sondar essas interações alterando eletrostaticamente a densidade da portadores, o principal conceito por trás dos transistores de efeito de campo (FET), que é o elemento fundamental dos dispositivos nanoeletrônicos. No caso de supercondutores de alta temperatura a base de óxidos de cobre, é possível usar este conceito para alternar entre fases supercondutoras e isolantes, por exemplo utilizando uma camada adjacente de eletrólito líquido para injetar cargas no filme supercondutor. Com isso em mente, o objetivo desse trabalho foi estabelecer protocolos para crescer filmes supercondutores e ferroelétricos para fabricações futuras de dipositivos FET supercodutores. Nós optimizamos as condições de deposição para o crescimento de uma única camada do supercondutor YBa2Cu3O7&ndash;x e do ferroeléctrico titanato de bário em substratos SrTiO3 por deposição de laser pulsado (PLD). Diversas técnicas foram empregadas para estudar as propriedades dos filmes finos, como difração de raios-X, microscopia de força atômica, espectroscopia de fotoelétrons de raios-X, resistência vs temperatura e histerese ferroelétrica. Em relação aos filmes finos supercondutores, observamos várias relações das propriedades supercondutoras com os parâmetros de crescimento. Por exemplo, temperaturas de crescimento mais baixas contribuem para a nucleação de grãos orientados no eixo a, enquanto a temperatura de crescimento mais alta tende a ser orientada para o eixo c. Em relação à frequência do laser (proporcional à taxa de crescimento), há um indício que valores menores de frequência está relacionada à maior rugosidade superficial e à presença de contribuições não supercondutoras. À medida que aumenta a frequência, a rugosidade diminui e a amostra apresenta uma transição supercondutora mais nítida. Por fim, também fizemos os primeiros passos em direção ao dispositivo de efeito de campo, desenvolvendo um filme fino de heteroestrutura com um material supercondutor e ferroelétrico. A amostra cresceu orientada no eixo c em substrato de titanato de estrôncio com alto valor de rugosidade superficial.

A rapid, mask-less deposition technique for writing metal tracks has been developed. The technique was based on laser-induced chemical vapour deposition. The novelty in the technique was the usage of pulsed ultrafast lasers instead of continuous wave lasers in pyrolytic dissociation of the chemical precursor. The motivation of the study was that (1) ultrafast laser pulses have smaller heat affected zones thus the deposition resolution would be higher, (2) the ultrashort pulses are absorbed in most materials (including those transparent to the continuous wave light at the same wavelength) thus the deposition would be compatible with a large range of materials, and (3) the development of higher frequency repetition rate ultrafast lasers would enable higher deposition rates. A deposition system was set-up for the study to investigate the ultrafast laser deposition of tungsten from tungsten hexacarbonyl chemical vapour precursors. A 405 nm laser diode was used for continuous wave deposition experiments that were optimized to achieve the lowest track resistivity. These results were used for comparison with the ultrafast laser track deposition. The usage of the 405 nm laser diode was itself novel and beneficial due to the low capital and running cost, high wall plug efficiency, high device lifetime, and shallower optical penetration depth in silicon substrates compared to green argon ion lasers which were commonly used by other investigators. The lowest as-deposited track resistivity achieved in the continuous wave laser experiments on silicon dioxide coated silicon was 93±27 µΩ cm (16.6 times bulk tungsten resistivity). This deposition was done with a laser output power of 350 mW, scan speed of 10 µm/s, deposition pressure of 0.5 mBar, substrate temperature of 100 °C and laser spot size of approximately 7 µm. The laser power, scan speed, deposition pressure and substrate temperature were all optimized in this study. By annealing the deposited track with hydrogen at 650 °C for 30 mins, removal of the deposition outside the laser spot was achieved and the overall track resistivity dropped to 66±7 µΩ cm (11.7 times bulk tungsten resistivity). For ultrafast laser deposition of tungsten, spot dwell experiments showed that a thin film of tungsten was first deposited followed by quasi-periodic structures perpendicular to the linear polarization of the laser beam. The wavelength of the periodic structures was approximately half the laser wavelength (λ/2) and was thought to be formed due to interference between the incident laser and scattered surface waves similar to that in laser-induced surface periodic structures. Deposition of the quasi-periodic structures was possible on stainless steel, silicon dioxide coated silicon wafers, borosilicate glass and polyimide films. The thin-films were deposited when the laser was scanned at higher laser speeds such that the number of pulses per spot was lower (η≤11,000) and using a larger focal spot diameter of 33 µm. The lowest track resistivity for the thin-film tracks on silicon dioxide coated silicon wafers was 37±4 µΩ cm (6.7 times bulk tungsten resistivity). This value was achieved without post-deposition annealing and was lower than the annealed track deposited using the continuous wave laser. The ultrafast tungsten thin-film direct write technique was tested for writing metal contacts to single layer graphene on silicon dioxide coated silicon substrates. Without the precursor, the exposure of the graphene to the laser at the deposition parameters damaged the graphene without removing it. This was evidenced by the increase in the Raman D peak of the exposed graphene compared to pristine. The damage threshold was estimated to be 53±7 mJ/cm2 for a scanning speed of 500 µm/s. The deposition threshold of thin-film tungsten on graphene at that speed was lower at 38±8 mJ/cm2. However, no graphene was found when the deposited thin-film tungsten was dissolved in 30 wt% H2O2 that was tested to have no effect on the graphene for the dissolution time of one hour. The graphene likely reacted with the deposited tungsten to form tungsten carbide which was reported to dissolve in H2O2. Tungsten carbide was also found on the tungsten tracks deposited on reduced graphene oxide samples. The contact resistance between tungsten and graphene was measured by both transfer length and four-point probe method with an average value of 4.3±0.4 kΩ µm. This value was higher than reported values using noble metals such as palladium (2.8±0.4 kΩ µm), but lower than reported values using other metals that creates carbides such as nickel (9.3±1.0 kΩ µm). This study opened many potential paths for future work. The main issue to address in the tungsten ultrafast deposition was the deposition outside the laser spot. This prevented uniform deposition in successive tracks close to one another. The ultrafast deposition technique also needs verification using other precursors to understand the precursor requirements for this process. An interesting future study would be a combination with a sulphur source for the direct write of tungsten disulphide, a transition metal dichalcogenide that has a two-dimensional structure similar to graphene. This material has a bandgap and is sought after for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. Initial tests using sulphur micro-flakes on silicon and stainless-steel substrates exposed to the tungsten precursor and ultrafast laser pulses produced multilayer tungsten disulphide as verified in Raman measurements.

Complex-oxides have seen an enormous amount of attention in the realm of Condensed Matter Physics and Materials Science/Engineering over the last several decades. Their ability to host a wide variety of novel physical properties has even caused them to be exploited commercially as dielectric, metallic and magnetic materials. Indeed, since the discovery of high temperature superconductivity in the “Cuprates” in the late 1980’s there has been an explosion of activity involving complex-oxides. Further, as the experimental techniques and equipment for fabricating thin films and heterostructures of these materials has improved over the last several decades, the search for new and more exotic properties has intensified. These properties stem from the interfaces formed by depositing these materials onto one another. Whether it be interfacial strain induced by the mismatch between the crystal structures, modified exchange interactions, or some combination of these and other interactions, thin films and heterostuctures provide an invaluable tool the modern condensed matter community. Simply put, a “complex-oxide” is any compound that contains Oxygen and at least two other elements; or one atom in two different oxidation states. Transition Metal Oxides (TMO’s) are a subset of complex-oxides which are of particular interest because of their strong competition between their charge, spin and orbit degrees of freedom. As we progress down the periodic table from 3d to 4d to 5d transition metals, the crystal field, electron correlation and spin-orbit energies become more and more comparable. Therefore, TMO thin films and heterostructures are indispensable to the search for novel physical properties. KTaO3 (KTO) is a polar 5d TMO which has been investigated for its high-k dielectric properties. It is a band insulator with a cubic perovskite crystal structure which is isomorphic to SrTiO3 (STO). This is important because non-polar STO is famous for forming a highly mobile, 2-Dimensional Electron Gas (2DEG) at the hetero-interface with polar LaAlO3 (LAO) as a result of the so-called “polar catastrophe”. Here, I use this concept of polarity to ask an important question: “What happens at hetero-interfaces where two different polar complex oxides meet?” From this question we propose that a hetero-interface between two polar complex-oxides with opposite polarity (I-V/III-III) should be impossible because of the strong Coulomb repulsion between the adjacent layers. However, we find that despite this proposed conflict we are able to synthesize KTO thin films on (110) oriented GdScO3 (GSO) substrates and the conflict is avoided through atomic reconfiguration at the hetero-interface. SrRuO3 (SRO) is a 4d TMO, and an itinerant ferromagnet that is used extensively as an electrode material in capacitor and transistor geometries and proof-of-concept devices. However, in the thin film limit the ferromagnetic transition temperature, TC, and conductivity drop significantly and even become insulating and lose their ferromagnetic properties. Therefore, we ask “Are the transport properties of SRO thin films inherently inferior to single crystals, or is there a way to maintain and/or enhance the metallic properties in the thin film limit?” We have fabricated SRO thin films of various thickness on GSO substrates (tensile strain) and find that all of our samples have enhanced metallic properties and even match those of single crystals. Finally, we ask “Can these enhanced metallic properties in SRO thin films allow us to observe evidence of a topological phase without the complexity of off-stoichiometry and/or additional hetero-structural layers?” Recent reports of oxygen deficient EuO films as well as hetero-structures and superlattices of SRO mixed with SrIrO3 or La0.7Sr0.3MnO3 have suggested that a magnetic skyrmion phase may exist in these systems. By measuring the Hall resistivity, we are able to observer a topological Hall effect which is likely a result of a magnetic skyrmion. We find that of the THE exists in a narrow temperature range and the proposed magnetic skyrmions range in size from 20-120 nm. Therefore, the SRO/GSO system can provide a more viable means for investigating magnetic skyrmions and their fundamental interactions.