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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.

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–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–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.

Hybridsolarzellen werden sowohl aus ZnO-Schichten als auch ZnO-Nanostrukturen und Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b'']dithiophen)-alt-4,7(2,1,3-benzothiadiazol)] (PCPDTBT) hergestellt. Das Wachstum der ZnO-Schichten und Nanostrukturen wird mittels gepulster Laserdeposition (PLD) auf Saphirsubstraten durchgeführt. Die Schichten weisen eine c-Achsenorientierung auf. Die Polarität einer ZnO-Schicht bestimmt die Morphologie der nachfolgend gewachsenen ZnO-Nanostrukturen. Dabei kann die Morphologie kontrolliert zwischen Nanostäbchen auf einer O-terminierten ZnO-Schicht und Nanowänden auf einer Zn-terminierten ZnO-Schicht eingestellt werden. Untersuchungen mittels konvergenter Elektronenbeugung zeigen, dass die Nanostrukturen immer Zn-terminiert sind. Die Grenzfläche zwischen ZnO und PCPDTBT wird mit Photoelektronenspektroskopie untersucht und ergibt eine Vakuumniveauangleichung zwischen beiden Materialien. Prinzipiell ist der Übergang für photovoltaische Aktivität geeignet, jedoch sind die erzielten Wirkungsgrade sehr niedrig. Die Ursache ist eine niedrige Exzitonendissoziationseffizienz, die durch die Benutzung von sol-gel ZnO, kleinen organischen Molekülen und einer niedrigeren Leitfähigkeit vom PLD-ZnO verbessert werden kann. Dennoch beträgt der maximale Wirkungsgrad der Hybridsolarzellen nur 0,21 %. / Hybrid solar cells are built from ZnO layers and ZnO nanostructures and Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b'']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT). The growth of the ZnO layers and nanostructures is performed with pulsed laser deposition (PLD) on sapphire substrates. The samples exhibit a c-axis orientation. The polarity of a ZnO layer determines the morphology of subsequently grown ZnO nanostructures. One can control the morphology between ZnO nanorods on an O-terminated layer and nanowalls on a Zn-terminated layer. Studies with convergent electron beam diffraction reveals that the ZnO nanostructures are always Zn-terminated. The interface between ZnO and PCPDTBT is studied with photoelectron spectroscopy and shows a vacuum level alignment between both materials. In principle, the interface is suitable for photovoltaic activity, however, the achieved power conversion efficiencies are very low. This is due to a low exciton dissociation efficiency, which can be improved by the use of sol-gel ZnO, small organic molecules, and a lower conductivity of the PLD ZnO. Nevertheless, the maximum power conversion efficiency amounts to 0.21 %, only.