A novel process for low-temperature (LT) epitaxial growth of silicon carbide (SiC) by replacing the growth precursor propane with chloro-methane was recently developed at Mississippi State University. However, only limited information was available about the defects and impurity incorporation in the various types of epitaxial layers produced by this new method like blanket epitaxial layers, selectively grown epitaxial mesas, and highly doped epitaxial layers, prior to their comprehensive characterization in this work. Molten potassium hydroxide (KOH) etching, mechanical polishing and a variety of other characterizing techniques were used to delineate and identify the defects both in the epilayer and substrates. Under optimum growth conditions, the concentration of defects in the epitaxial layers was found to be less than that in the substrate, which established the good quality of the LT growth process. Defect concentrations, on selectively grown epitaxial layers, strongly depended on the crystallographic orientation of the mesa sidewall. The addition of HCl to the growth process, aimed at increasing the growth rate, caused a significant concentration of triangular defects (TDs) to be formed in the epitaxial layers. The TDs were traced down to the substrate by a combination of repeated polishing and molten KOH etching steps. The TDs were found not to originate from any substrate defects. Their origin was traced to polycrystalline silicon islands which form on the surface during growth and subsequently get evaporated away, which had made it impossible to detect them and suspect their influence on the TD generation prior to this work. The TDs were found to include single or multiple stacking faults bound by partial dislocations and, in some cases, inclusions of other SiC polytypes. Gradual degradation of the epitaxial morphology was found in heavily aluminum doped p+ layers, with an increase in the level of doping, followed by much steeper degradation when approaching the solubility limit of Al in 4H-SiC. Precipitates were the dominating defect at the highest levels of doping and were observed beyond a doping of 3.5x1020 cm-3. A dislocation generation model for heavily doped epitaxial layers was developed accounting for the stress in the lattice caused by Al doping.

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Continuous welding is one of the prominent techniques used in producing seamless piping used in many applications such as the mining and the oil and gas industries. Weld defects cause significant loss of time and money in the piping production industry. Therefore, there is a need for effective online weld defects detection systems. A laser-based weld defects detection (LBWDD) system has been developed by the industrial partner. However, the current LBWDD system can only detect some geometrically based weld defects, but not material inhomogeneity such as voids, impurities, inclusions, etc. The main objective of this study is to assess the predictability of a thermal imaging-based weld defects detection system (TIBWDD) using an IR camera that can be integrated with the current LBWDD system. The aim of the integrated detection system is to be able to detect a wider range of weld defects. A test rig has been designed and used to carry out a set of emissivity (ε) calculation experiments considering three different materials – Aluminum 5154 (Al), Stainless Steel 304L (SS), and Low Carbon Steel A131 (LCS) with two surface finishes 0.25 μm (FM) and 2.5 μm (RM), which are relevant to pipe welding operations. Al showed least change in ε varying from 0.162 to 0.172 for FM samples and from 0.225 to 0.250 for RM samples from 50°C to 550°C. LCS showed highest change in ε varying from 0.257 – 0.918 for FM samples and from 0.292 to 0.948 for RM samples. SS showed a consistent increase in ε for both FM and RM samples. Experimental and numerical analysis have been carried out mimicking two sets of possible weld defects investigating defect size, Dh, and distance between effect and sample surface, δ. Results showed that the δ based defects that are located within 3 mm can be detected by the IR camera. Defects with Dh = 1. 5 mm can be detected by the IR camera with and without glass wool. Laser welding simulations using 2D and 3D Gaussian heat source models have been carried out to assess the predictability of a set of possible weld defects. The heat source models have been validated using experimental data. Three sets of defects were considered representing material-based inhomogeneity, step and inclined misalignment defects. For material-based inhomogeneity in thin plates all defects located at 1.25 mm from the surface are found detectable as ΔT (temperature difference obtained on surface) > ΔTmin (detectability limit of TIBWDD system). For inhomogeneity defects in thick plates, except defects of 2.5 mm in square size all other defects were found detectable as ΔT > ΔTmin. All step misalignment defects were detected for thin and thick plates. In the case of inclined misalignment defects, for thin plates, the misalignment error in the thin plate had to be at least 0.275 mm to be detected. In the case of thick plates, the misalignment error had be at least 0.375 mm to be detected. Overall, results of the present study confirm that thermal imaging can be successfully used in detecting material-based and geometry-based weld defects. / Thesis / Master of Applied Science (MASc)

Nanoscale cerium oxide (nanoceria) have shown to possess redox active property , and has been widely studied for potential use in catalysis, chemical-mechanical planarization, biomedical and solid oxide fuel cell (SOFC), etc. The redox state of nanoceria can be tuned by controlling the defects within the lattice and thus its physical and chemical properties. Perfect ceria lattice has fluorite structure and the research in last decade has shown that oxide and mixed oxide systems with pyrochlore and fluorite have better structural stability under high energy radiation. However, the current literature shows a limited number of studies on the effect of high energy radiation on nanoceria. This dissertation aims at understanding the phenomena occurring on irradiation of nanoceria lattice through experiments and atomistic simulation. At first, research was conducted to show the ability to control the defects in nanoceria lattice and understand the effect in tailoring its properties. The defect state of nanoceria was controlled by lower valence state rare earth dopant europium. Extensive materials characterization was done using high resolution transmission electron microscopy (HRTEM), UV-Visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to understand the effect of dopant chemistry in modifying the chemical state of nanoceria. The defects originating in the lattice and redox state was quantified with increasing dopant concentration. The photoluminescence property of the control and doped nanoceria were evaluated with respect to its defect state. It was observed that defect plays an important role in modifying the photoluminescence property and that it can be tailored in a wide range to control the optical properties of nanoceria. iv Having seen the importance of defects in controlling the properties of nanoceria, further experiments were conducted to understand the effect of radiation in cerium oxide thin films of different crystallinity. The cerium oxide thin films were synthesized using oxygen plasma assisted molecular beam epitaxy (OPA-MBE) growth. The thin films were exposed to high energy radiation over a wide range of fluence (1013 to 1017 He+ ions/cm3 ). The current literature does not report the radiation effect in nanoceria in this wide range and upto this high fluence. The chemical state of the thin film was studied using in-situ XPS for each dose of radiation. It was found that radiation induced defects within both the ceria thin films and the valence state deviated further towards non-stoichiometry with radiation. The experimental results from cerium oxide thin film irradiation were studied in the light of simulation. Classical molecular dynamics and Monte Carlo simulation were used for designing the model ceria nanoparticle and studying the interaction of the lattice model with radiation. Electronic and nuclear stopping at the end of the range were modeled in ceria lattice using classical molecular dynamics to simulate the effect of radiation. It was seen that displacement damage was the controlling factor in defect production in ceria lattice. The simulation results suggested that nanosized cerium oxide has structural stability under radiation and encounters radiation damage due to the mixed valence states. A portion of the study will focus on observing the lattice stability of cerium with increasing concentration of the lower valence (Ce3+) within the lattice. With this current theoretical understanding of the role of redox state and defects during irradiation, the surfaces and bulk of nanoceria can be tailored for radiation stable structural applications

A study of defects in semiconductors is vital as defects tend to influence device operation by modifying their electrical and optoelectronic properties. This influence can at times be desirable in the case of fast switching devices and sometimes undesirable as they may reduce the efficiency of optoelectronic devices. ZnO is a wide bandgap material with a potential for fabricating UV light emitting diodes, lasers and white lighting devices only after the realization of reproducible p-type material. The realization of p-type material is greatly affected by doping asymmetry. The self-compensation behaviour by its native defects has hindered the success in obtaining the p-type material. Hence there is need to understand the electronic properties, formation and annealing-out of these defects for controlled material doping. Space charge spectroscopic techniques are powerful tools for studying the electronic properties of electrically active defects in semiconductors since they can reveal information about the defect “signatures”. In this study, novel Schottky contacts with low leakage currents of the order of 10-11 A at 2.0 V, barrier heights of 0.60 – 0.80 eV and low series resistance, fabricated on hydrogen peroxide treated melt-grown single crystal ZnO samples, were demonstrated. Investigations on the dependence of the Schottky contact parameters on fabrication techniques and different metals were performed. Resistive evaporation proved to produce contacts with lower series resistance, higher barrier heights and low reverse currents compared to the electron-beam deposition technique. Deep level transient spectroscopy (DLTS) and Laplace-DLTS have been employed to study the electronic properties of electrically active deep level defects in ZnO. Results revealed the presence of three prominent deep level defects (E1, E2 and E3) in the as-received ZnO samples. Electron-beam deposited contacts indicated the presence of the E1, E2 and E3 and the introduction of new deep level defects. These induced deep levels have been attributed to stray electrons and ionized particles, present in the deposition system during contact fabrication. Exposure of ZnO to high temperatures induces deep level defects. Annealing samples in the 300°C – 600°C temperature range in Ar + O2 induces the E4 deep level with a very high capture cross-section. This deep level transforms at every annealing temperature. Its instability at room temperature has been demonstrated by a change in the peak temperature position with time. This deep level was broad, indicating that it consists of two or more closely spaced energy levels. Laplace-DLTS was successfully employed to resolve the closely spaced energy levels. Annealing samples at 700°C in Ar and O2 anneals-out E4 and induces the Ex deep level defect with an activation enthalpy of approximately 160 – 180 meV. Vacuum annealing performed in the 400°C – 700°C temperature range did not induce any deep level defects. Since the radiation hardness of ZnO is crucial in space applications, 1.6 MeV proton irradiation was performed. DLTS revealed the introduction of the E4 deep level with an activation enthalpy of approximately 530 meV, which proved to be stable at room temperature and atmospheric pressure since its properties didn’t change over a period of 12 months. / Thesis (PhD)--University of Pretoria, 2013. / Physics / unrestricted

This thesis relates a technique for increasing the optical strength of NaCl and KCl single-crystal samples. The 1.06-μm pulsed laser damage thresholds were increased by factors as large as 4.6 for a bulk NaCl single-crystal sample. The bulk laser damage breakdown threshold (LDBT) of the crystal was measured prior to and after heat treatment at 800*C using a Nd:YAG laser operating at 1.06 μm. Bulk and surface LDBTs were also studied on samples annealed at 400° C. These samples showed differences in damage morphology on both cleaved and polished surfaces, and the cleaved surfaces had improved damage thresholds. However, neither the polished surfaces nor the bulk showed improved threshold at the lower annealing temperature.

<p>The solid oxide fuel cell (SOFC) technology has been under research since thelate 1950s, and most of the research has been on designs utilizing yttria stabilized zirconia (YSZ) as the electrolyte of choice. However, the SOFC technology has the major drawback of requiring high operation temperatures (up to 1000 degrees Celcius), so research of alternative materials have come into interest that would possibly require a lower working temperature without any significant loss of conductivity.One such material of interest for the electrolyte is compounds of ceriumdioxide (ceria). Ceria is well known for its ability to release oxygen by formingoxygen vacancies under oxygen-poor conditions, which increases its oxygen ionconductivity, and works at a lower temperature than the YSZ compounds whenproperly doped. Conversely, ceria is also able to absorb oxygen under oxygen-rich conditions, and those two abilities make it a very good material to use in catalytic converters for reduction of carbon monoxide and nitrogen oxide emission. The ability for the oxygen ions to easily relocate inbetween the different lattice sites is likely the key property of oxygen ion transportation in ceria. Also, in oxygen-rich conditions, the absorbed oxygen atom is assumed to join the structure at either the roomy octrahedral sites, or the vacant tetrahedral sites. Following that, the oxygen atom may relocate to other vacant locations, given it can overcome a possible potential barrier.</p><p>This thesis studies how those interstitial oxygen vacancies (defects) affect theenergy profile of ceria-based supercells by first principles calculations. The system is modeled within the density functional theory (DFT) with aid of (extended) local density approximation (LDA+U) using the software VASP. Furthermore, it is studied how those vacancies affect neighbouring oxygen atoms, and wether or not it is energetically benificial for the neighbouring atoms to readjust their positions closer or further away from the vacancy. The purpose of this thesis is to analyze wether or not it is theoretically possible that interstitial oxygen vacancies may cause neighbouring oxygen atoms to naturally relocate to the octahedral site in ceria, and how this affects the overall energy profile of the material.</p>

The solid oxide fuel cell (SOFC) technology has been under research since thelate 1950s, and most of the research has been on designs utilizing yttria stabilized zirconia (YSZ) as the electrolyte of choice. However, the SOFC technology has the major drawback of requiring high operation temperatures (up to 1000 degrees Celcius), so research of alternative materials have come into interest that would possibly require a lower working temperature without any significant loss of conductivity.One such material of interest for the electrolyte is compounds of ceriumdioxide (ceria). Ceria is well known for its ability to release oxygen by formingoxygen vacancies under oxygen-poor conditions, which increases its oxygen ionconductivity, and works at a lower temperature than the YSZ compounds whenproperly doped. Conversely, ceria is also able to absorb oxygen under oxygen-rich conditions, and those two abilities make it a very good material to use in catalytic converters for reduction of carbon monoxide and nitrogen oxide emission. The ability for the oxygen ions to easily relocate inbetween the different lattice sites is likely the key property of oxygen ion transportation in ceria. Also, in oxygen-rich conditions, the absorbed oxygen atom is assumed to join the structure at either the roomy octrahedral sites, or the vacant tetrahedral sites. Following that, the oxygen atom may relocate to other vacant locations, given it can overcome a possible potential barrier. This thesis studies how those interstitial oxygen vacancies (defects) affect theenergy profile of ceria-based supercells by first principles calculations. The system is modeled within the density functional theory (DFT) with aid of (extended) local density approximation (LDA+U) using the software VASP. Furthermore, it is studied how those vacancies affect neighbouring oxygen atoms, and wether or not it is energetically benificial for the neighbouring atoms to readjust their positions closer or further away from the vacancy. The purpose of this thesis is to analyze wether or not it is theoretically possible that interstitial oxygen vacancies may cause neighbouring oxygen atoms to naturally relocate to the octahedral site in ceria, and how this affects the overall energy profile of the material.

Depuis des années, le silicium est le semiconducteur principalement utilisé dansl’industrie électronique et photovoltaïque. Intensivement étudié depuis plusieursdécennies, ses propriétés sont essentiellement connues, mais de nouvelles questionsviennent se poser. En particulier, une meilleure connaissance des nombreux défauts etimpuretés ainsi que leurs propriétés et leur impact sur les performances des dispositifsà base de Si est souhaitable.Ce travail couvre un éventail de problèmes liés aux défauts ponctuels en interactionau moyen de calculs dits de premiers principes (Density Functional Theory).Une première partie est dédiée à l’impact du dopage sur la diffusivité de l’oxygèneinterstitiel. Les coefficients de diffusion obtenus en fonction de la température sonten très bon accord avec les résultats expérimentaux ce qui démontre la validité dela méthodologie appliquée. Nous avons montré que l’augmentation de la diffusivitédans le silicium dopé bore se produit par un mécanisme de transfert de charge depuisle dopant de type p.Une deuxième partie se rapporte aux différents complexes de défauts ponctuels etleur thermodynamique, leur cinétique, et leurs propriétés optiques. La formation de cescomplexes peut être induite expérimentalement par une irradiation par des électrons.Plus généralement, ils apparaissent aussi dans des environnements opérationnelsparticuliers comme le spatial. Ici, nous avons réalisé une étude expérimentale etthéorique combinée pour identifier l’impact du dopage isovalent (C, Ge) et du codopage(C-Ge, C-Sn, C-Pb) sur la production de différents complexes (VOi, CiOi,CiCs), qui sont électriquement et optiquement actifs.Enfin, une attention particulière a été portée à la paire de défaut carbone-carboneet ses propriétés. Récemment, il a été établi que le silicium fortement dopé en carboneprésente des propriétés d’émission laser. Ici nous avons cherché à étudier les formespossibles du complexe et leurs propriétés, afin de comprendre lequel est présentexpérimentalement. / Since many years, silicon is the primary semiconductor material in electronic andphotovoltaic industry. Intensively studied through decades, its properties are essentiallyknown, however new questions keep arising. We need to achieve deep insightinto the numerous possible defects and impurities properties as well as their impacton the performances of the Si based devices. This work covers a range of problemsrelated with point defects interaction of both types long range and short range bymeans of parameter free first principles calculations.The former refers to the impact of heavy doping on diffusivity of interstitialoxygen species. The obtained diffusion coefficients as a function of temperature arein a very good agreement with experimental results that demonstrates the validityof the applied methodology. We showed that the enhanced diffusivity in B-dopedsilicon occurs through a charge transfer mechanism from the p-type dopantThe latter accounts for the various point defect complexes and their thermodynamic,kinetic, and optical properties. Formation of these complexes can beinduced by electron irradiation of Czochralski silicon. This aspect is of extremeimportance for particular operational environment. Here, we performed a combinedexperimental-theoretical investigation to identify the impact of isovalent doping (C,Ge) and co-doping (C-Ge, C-Sn, C-Pb) on the production of different complexes(VOi, CiOi, CiCs, etc.), which are electrically and optically active.Finally, particular attention is addressed to the carbon-carbon defect pair and itsproperties. Recently, it was established that heavily carbon doped silicon elucidateslasing properties. Here we aimed to revisit the possible forms of the complex andtheir properties, in order to associate one of them with light emitting G-centre,observed in experiments.v

Neural tube defects (NTDs) are serious birth defects that affect 3,000 pregnancies in the United States annually. All women of childbearing age are recommended to consume 400 micrograms of folic acid daily for the prevention of NTDs. Women aged 18 to 24 years have multiple risk factors for having an NTD-affected pregnancy and should be targeted by efforts to promote folic acid consumption. Survey data capturing folic acid awareness, knowledge, and supplementation behavior of women aged 18 to 24 years who are not contemplating pregnancy were examined to identify the relationship of folic acid awareness and knowledge to daily supplementation with folic acid in an effort to predict the effectiveness of education-only interventions. Results of the study suggested that awareness and knowledge was not consistently related to daily supplementation. An evaluation of qualitative data using the Health Belief Model offers explanations for the findings and recommendations for targeting these at-risk women.