Additive manufacturing (AM) is a novel fabrication technique capable of producing highly complex parts. Nevertheless, a major challenge is improving the quality of the fabricated parts. While there are several ways of approaching this problem, developing data-driven methods that use AM process signatures to identify these part anomalies can be rapidly applied to improve the overall part quality during the build. The objective of this dissertation is to model multiple processes within the AM to quantify the quality of the parts and reduced the uncertainty due to variation in input process parameters. The objective of first study is to build a new layer-wise process signature model to characterize the thermal-defect relationship. Based on melt pool images, we propose novel layer-wise key process signatures, which are calculated using multilinear principal component analysis (MPCA) and are directly correlated with layer-wise quality of the part. Second study broadens the spectrum of the dissertation to include mechanical properties, where a novel two-phase modeling methodology is proposed for fatigue life prediction based on in-situ monitoring of thermal history. In final study, our objective is to pave the way toward a better understanding of the uncertainty in the process-defect-structures relationship using an inverse robust design exploration method. The method involves two steps. In the first step, mathematical models are developed to characterize and model the forward flow of information in the intended additive manufacturing process. In the second step, inverse robust design exploration is carried out to investigate satisfying design solutions that meet multiple AM goals.

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We have used deep level transient spectroscopy (DLTS) to characterise four defects with shallow levels in ZnO grown by pulsed laser deposition (PLD). These defects all have DLTS peaks below 100 K. From DLTS measurements and Arrhenius plots we have calculated the energy levels of these defects as 31 meV, 64 meV, 100 meV and 140 meV, respectively, below the conduction band. The 100 meV defect displayed metastable behaviour: Annealing under reverse bias at temperatures of above 130 K introduced it while annealing under zero bias above 110 K removed it. The 64 meV and 140 meV defects exhibited a strong electric field assisted emission, indicating that they may be donors.

We have used deep level transient spectroscopy (DLTS) to characterise four defects with shallow levels in ZnO grown by pulsed laser deposition (PLD). These defects all have DLTS peaks below 100 K. From DLTS measurements and Arrhenius plots we have calculated the energy levels of these defects as 31 meV, 64 meV, 100 meV and 140 meV, respectively, below the conduction band. The 100 meV defect displayed metastable behaviour: Annealing under reverse bias at temperatures of above 130 K introduced it while annealing under zero bias above 110 K removed it. The 64 meV and 140 meV defects exhibited a strong electric field assisted emission, indicating that they may be donors.

In der vorliegenden kumulativen Arbeit werden Untersuchungen am transparenten p-Typ Halbleiter Kupferiodid (CuI) beschrieben. Die Dünnschichtherstellung erfolgte mittels gepulster Laserabscheidung (PLD). Der Einfluss der Wachstumsbedingungen auf die strukturellen, morphologischen und elektrischen Eigenschaften wird dargelegt hinsichtlich des Ziels der Fabrikation funktioneller Schichten auf Basis von CuI. Dazu wird im ersten Teil der Arbeit der generelle Einfluss der PLD-Parameter auf das Schichtwachstum beschrieben. Ein dominierender Einfluss der Wachstumstemperatur auf die kristalline Qualität, Oberflächenrauhigkeit sowie Ladungsträgerdichte und Mobilität wurde beobachtet. Die 250nm dicken Dünnschichten zeigen im sichtbaren Bereich eine Transmittanz von bis zu 90%. Exzitonische Absorptions- und Lumineszenzcharakteristika konnten an solchen Schichten nachgewiesen werden. Eine schützende Deckschicht aus Al2O3 erwies sich als elementar zur Stabilisierung der elektrischen Eigenschaften und erlaubte die Fabrikation nicht-degenerierten Kupferiodids. Im zweiten Teil werden detail- lierte Untersuchungen des elektrischen Langzeitverhaltens von CuI in Abhängigkeit des Wachstums der Al2O3 Deckschichten diskutiert. Für Sauerstoff-defizientes Al2O3 zeigte das CuI komplexe elektrische Degradationsmechanismen, welche auf die Diffusion von Sauerstoff innerhalb der Heterostruktur zurückgeführt wurden. Die Ergebnisse beweisen einen dominierenden Einfluss extrinsischer Akzeptoren auf die elektrischen Eigenschaften von PLD gewachsenem CuI. Im Kontrast zum bisherigen Stand der Literatur spielen intrinsische Defekte für die Erklärung eine untergeordnete Rolle. Um eine Stabilisierung der Ladungsträgerdichten im Zusammenspiel mit Sauerstoff-defizientem Al2O3 zu er- reichen, wurden die CuI Dünnfilme mit Selen dotiert. Die Grenze zwischen Dotierung und Legierung, welche sich durch reduzierte Bandkantenenergien und eine einsetzende Phasenseparation definiert, wurde bestimmt. Weiterhin konnte die Bindungsenergie des Selen-Akzeptors in CuI bestimmt werden. Der finale Teil der Arbeit fokussierte sich auf die Unterdrückung der Bildung von Rotationsdomänen als dominierende Defekte von PLD gewachsenem CuI. Dieses Ziel wurde mittels PLD gewachsenen Natriumbromid (NaBr) Zwischenschichten auf kommerziellen Strontiumfluorid Substraten erreicht. Die Oberflächenrauigkeit von entsprechenden Dünnschichten ist deutlich reduziert. Unter Ausnutzung der hohen Wasserlöslichkeit von NaBr wurde ein epitaktischer Ablöseprozess entwickelt, der zur Herstellung frei stehender CuI Schichten geeignet ist. Das Wachstum von einkristallinen Volumenkristallen ist jedoch beschränkt, da bei Schichtdicken > 2 μm Defekthäufungen beobachtet werden und Rotationsdomänen auftreten.:1 Introduction 2 Theoretical Descriptions 2.1 Copper Iodide 2.1.1 Conductivity and Defect Chemistry 2.2 Epitaxy of Crystalline Thin Films 2.2.1 Stress and Strain 2.2.2 Heteroepitaxy of Copper Iodide 2.3 Electronic Defect States in Semiconductors 3 Experimental Methods 3.1 Sample Preparation by Pulsed Laser Deposition-PLD 3.1.1 Combinatorial and Eclipse PLD 3.2 Characterization Techniques 3.2.1 X-ray Diffraction-XRD 3.2.2 Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectrometry(EDX) 3.2.3 Atomic Force Microscopy-AFM 3.2.4 Laser Scanning Microscopy-LSM 3.2.5 Time-of-Flight Secondary Ion Mass Spectrometry - ToF-SIMS 3.2.6 Rutherford Backscattering-RBS 3.2.7 Hall-Effect 3.2.8 Spectroscopic Ellipsometry 3.2.9 Photoluminsescence Spectroscopy-PL 3.2.10 Transmission Spectroscopy 4 Cumulative Part 4.1 Optimization of Copper Iodide Thin Film Growth by Pulsed Laser Deposition 4.2 Origin of Free Charge Carriers and p-Doping of PLD Copper Iodide 4.3 Suppression of Rotational Domains, Volume Crystals and Epitaxial Lift-Off 5 Summary and Outlook Bibliography List of Abbreviations List of Publications Author Contributions Zusammenfassung nach Promotionsordnung §11(4) Selbstständigkeitserklärung

<p>The growth of CdTe thin films on Al₂O₃(0001) substrates by pulsed laser deposition from undoped pressed powder targets was studied. Thin film crystal structure was investigated by x-ray texture analysis as a function of plume flux, growth temperature and film thickness. Crystal texture increased for a decrease in plume flux. Single crystal CdTe (111) films were obtained by optimizing the plume flux. Increasing the growth temperature demonstrated a reduction in twin density. An optimum temperature of 300°C minimized the twin density without adverse desorption effects. The twin density decreased as an inverse squared function of film thickness. Single crystal CdTe films with comparable structural quality to Bridgeman single crystal wafers were grown under optimal conditions.</p> <p>The optoelectronic properties of CdTe films were investigated by photoluminescence and photoreflectance spectroscopy. The room temperature bandgap energy of 1.51 eV was consistent between spectroscopic measurements. Broadening parameters for spectra were consistent with reference high quality material. Low temperature photoluminescence spectra had a dominant emission consistent with bound excitons found in bulk CdTe. Emissions consistent with self-compensation or doping were not found. Hall effect and conductivity measurements at 300 K demonstrated high resistivity for undoped material and electron mobilities comparable to bulk CdTe for lightly doped films. Spectroscopic and electrical measurements of high structural quality CdTe films were consistent with high optoelectronic quality.</p> <p>An as-grown ability of the films to detach from their substrate was discovered. X-ray texture analysis and photoluminescence spectroscopy of films released onto rigid secondary carriers demonstrated that they maintained their structural and optoelectronic quality proceeding lift-off. Substrates having films released from them were found to be suitable for repeated growth. The technological relevance of this discovery is likely to drive further study into the lift-off phenomena and controlled doping of CdTe thin films.</p> / Master of Applied Science (MASc)

Advancements in nanoscale engineering of oxide interfaces and heterostructures have led to discoveries of emergent phenomena and new artificial materials. Reactive molecular-beam epitaxy (MBE) and pulsed-laser deposition (PLD) are the two most successful growth techniques for epitaxial heterostructures of complex oxides. PLD possesses experimental simplicity, low cost, and versatility in the materials to be deposited. Reactive MBE employing alternately-shuttered elemental sources (atomic layer-by-layer MBE, or ALL-MBE) can control the cation stoichiometry precisely, thus producing oxide thin films of exceptional quality. There are, however, major drawbacks to the two techniques. Reactive MBE is limited to source elements whose vapor pressure is sufficiently high; this eliminates a large fraction of 4- and 5-d metals. In addition, the need for ozone to maintain low-pressure MBE conditions increases system complexity in comparison to conventional PLD. On the other hand, conventional PLD using a compound target often results in cation off-stoichiometry in the films. This thesis presents an approach that combines the strengths of reactive MBE and PLD: atomic layer-by-layer laser MBE (ALL-Laser MBE) using separate oxide targets. Ablating alternately the targets of constituent oxides, for example SrO and TiO2, a SrTiO3 film can be grown one atomic layer at a time. Stoichiometry for both the cations and oxygen in the oxide films can be controlled. Using Sr1+xTi1-xO3, CaMnO3, BaTiO3 and Ruddlesden–Popper phase Lan+1NinO3n+1 (n = 4) as examples, the technique is demonstrated to be effective in producing oxide films with stoichiometric and crystalline perfection. By growing LaAl1+yO3 films of different stoichiometry on TiO2-terminated SrTiO3 substrate at high oxygen pressure, it is shown that the behavior of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface can be quantitatively explained by the polar catastrophe mechanism. / Physics

This work presents a study on stoichiometry and structure in perovskite-type oxide thin films and investigates the role of growth–induced defects on the properties of materials. It also explores the possibility to grow thin films with properties close or similar to the ideal bulk parent compound. A novel approach to the growth of thin films, atomic layer-by-layer (ALL) laser molecular beam epitaxy (MBE) using separate oxide targets is introduced to better control the assembly of each atomic layer and to improve interface perfection and stoichiometry. It also is a way to layer materials to achieve a new structure that does not exist in nature. This thesis is divided into three sections. In the first part, we use pulsed laser deposition (PLD) to grow LaAlO3 (LAO) thin films on SrTiO3 (STO) and LAO substrates in a broad range of laser energy density and oxygen pressure. Using x-ray diffraction (θ-2θ scan and reciprocal space mapping), transmission electron microscopy (TEM) and x-ray fluorescence (XRF) we studied stoichiometry and structure of LAO films as a function of growth parameters. We show deviation from bulk–like structure and composition when films are grown at oxygen pressures lower than 10-2 Torr. We conclude that the discussion of LAO/STO interfacial properties should include the effects of growth–induced defects in the LAO films when the deposition is conducted at low oxygen pressures, as is typically reported in the literature. In the second part, we describe a new approach to atomically layer the growth of perovskite oxides: (ALL) laser MBE, using separate oxide targets to grow materials as perfectly as possible starting from the first atomic layer. We use All laser MBE to grow Ruddlesden–Popper (RP) phase Lan+1NinO3n+1 with n = 1, 2, 3 and 4 and we show that this technique enables us to construct new layered materials (n=4). In the last and main section of this thesis, we use All laser MBE from separate oxide targets to build the LaNiO3 (LNO) films as near perfectly as possible by depositing one atomic layer at a time. We study the thickness dependent metal-insulator transition (MIT) in ultrathin LNO films on an LAO substrate. In LNO, the MIT occurs in thin films and superlattices that are only a few unit cells in thickness, the understanding of which remains elusive despite tremendous effort devoted to the subject. Quantum confinement and structure distortion have been evoked as the mechanism of the MIT; however, first-principle calculations show that LaNiO3 remains metallic even at one unit cell thickness. Here, we show that thicknesses of a few unit cells, growth–induced disorders such as cation stoichiometry, oxygen vacancies, and substrate-film interface quality will impact the film properties significantly. We find that a film as thin as 2 unit cells, with LaO termination, is metallic above 150 K. An oxygen K-edge feature in the x-ray absorption spectra is clearly inked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies in LNO ultrathin films. Dimensionality, strain, crystallinity, cation stoichiometry, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strongly–correlated materials. / Physics

Multiferroics have attracted lots of research interest due to their potential in numerous multifunctional applications. The multiferroic materials could simultaneously exhibit two or more ferroic order parameters, and the coupling effects between ferroelectricity and ferromagnetism are named as magnetoelectric (ME) effect. Recently, with the development of thin film growth techniques, the multiferroics magnetoelectric composite heterostructures exhibit a very promising future prospects. This dissertation focused on the design, fabrication and characterization of new multiferroics magnetoelectric composite heterostructures. First, based on the specific phase architectures in BFO-CFO self-assembled thin films grown on variously oriented STO substrates and the epitaxial film growth knowledge, I designed two kinds of new film heterostructures: (i) I utilized self-assembled BFO nanopillars in a BFO-CFO two phase layer on (111) STO as a seed layer on which to deposit a secondary top BiFeO3 layer. The growth mechanism and multiferroic properties of these new heterostructures were investigated. (ii) I demonstrated the formation of a new quasi-(0-3) heterostructure by alternately growing (2-2) and (1-3) layers within the film. I proposed a new concept to overcome limitations of both the (2-2) and (1-3) phase connectivities and identified an indirect ME effect by the switching the characteristics of the piezoresponse for the new heterostructure. Second, for the option for candidates thin film materials with a high piezoelectric coefficient, which is a critical factor for ME composite films, I utilized the simple compositional BaSn0.11Ti0.89O3 bulk ceramic material as a target to grow films with the large piezoelectric properties. The grown high qualify lead-free epitaxial thin films had a chemical constituent similar to the reported giant piezoelectric ceramics near the MPB and with the QP. Both coherent and incoherent regions were observed in the interface and a larger piezoelectric coefficient d33 was achieved in this film. Finally, with respect to their characteristics and potential, I redirected from two-dimensional thin film materials to one-dimensional nanowire materials. By utilizing vertically aligned templates, I fabricated a new type of coaxial two-phase composite nanowires. Multiferroic properties of these new one-dimensional materials have been investigated. All these multiferroics magnetoelectric composite herterostructures would provide lots of potential in applications. / PHD