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Siegl, Manuel. "Atomic-scale investigation of point defect interactions in semiconductors." Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10043636/.
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Miniaturisation of computer hardware has increased the transistor density in silicon devices significantly and is approaching the ultimate physical limit of single atom transistors. A thorough understanding of the nature of materials at the atomic scale is needed in order to increase the transistor density further and exploit more recent technology proposals. Moreover, exploring other materials with more desirable characteristics such as wide band gap semiconductors with a higher dielectric strength and optical addressability are paramount in the effort of moving to a post-silicon era. Scanning Tunnelling Microscopy (STM) has been shown to be a suitable tool for the investigation of the technologically important material surface properties at the atomic scale. In particular, Scanning Tunnelling Spectroscopy (STS) – and its spatial extension Current Imaging Tunnelling Spectroscopy (CITS) – can reveal the electronic properties of single atom point defects as well as quantum effects caused by the confinement of energetic states. Nanoscale device performance is governed by these effects. In order to control and exploit the quantum effects, they firstly need to be understood. In this thesis, three systems have been investigated with STM and STS/CITS to broaden the comprehension of confined quantum states and material surface properties. The first data chapter concentrates on the interaction of confined quantum states of dangling bonds (DB) on the Si(111)-(√ 3 × √ 3)R30◦ surface. The site dependent interaction between neighbouring bound states is investigated by changing the distance and crystallographic direction between two DB point defects, revealing a non-linear constructive interference of the bound states and an antibonding state in resonance with the CB. In the second data chapter we explore subsurface bismuth dopants in silicon, a system relevant to recent information processing proposals. Bismuth was ion-implanted in the Si(001) surface and hydrogen passivated before the STM study. The bismuth dopants form a bismuth-vacancy (Bi+V) complex, which acts as an acceptor and lowers the Fermi level. The Bi+V complex further induces in-band gap states, which appear as square-like protrusions with a round depression in the centre. Interference of these states is energy dependent and the antibonding state is found at a lower energy than the bonding state due to the acceptor-like nature of the Bi+V defect complex. The third investigated system concerns the silicon face of the wide band gap semiconductor Silicon Carbide (SiC(0001)). The influence of atomic hydrogen on the 4HSiC(0001)-3 × 3 surface was investigated and found to result in a surface etching at the lower and upper end of the passivation temperature range. The electronic structure of two different surface defects of the 3 × 3 reconstruction is presented and a new superstructure consisting of silicon atoms on top of the 4H-SiC(0001)-(√ 3 × √ 3)R30◦ surface was discovered. A Schottky barrier height study of different surface reconstructions finds a nearly optimal power device fabrication value for the (√ 3× √ 3)R30◦ prepared surface. In summary, I have found a quantum interference that results in bonding and antibonding states for DB bound states on the Si(111):B surface and Bi+V complex states in the Si(001):H surface. Additionally, a new silicon superstructure on the SiC surface and a silicon reconstruction dependent Schottky barrier height are found.
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Valikova, Irina, and Andrei Nazarov. "Pressure effects on point defect diffusion features in cubic metals: atomic simulation." Diffusion fundamentals 6 (2007) 48, S. 1-2, 2007. https://ul.qucosa.de/id/qucosa%3A14227.
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Lee, Donghun. "Atomic Scale Gate Electrode Formed by a Charged Defect: Scanning Tunneling Microscopy of Single Impurities in GaAs Semiconductors." The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1274913629.
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Uppal, Hasan Javed. "Nanoscale performance, degradation and defect analysis of mos devices using high-k dielectric materials as gate stacks by atomic force microscopy." Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.509394.
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Gilbert, Mark R. "BCC metals in extreme environments : modelling the structure and evolution of defects." Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:d972d28d-5d2d-4392-8cf5-fc5728dc74f6.
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Designing materials for fusion applications is a very challenging problem, requiring detailed understanding of the behaviour of materials under the kinds of extreme conditions expected in a fusion environment. During the lifetime of fusion-reactor components, materials will be subjected to high levels of neutron irradiation, but must still perform effectively at high operating temperatures and under significant loading conditions. Body-centred cubic (bcc) transition metals are some of the most promising candidates for structural materials in fusion because of their relatively high density, which allows for effective neutron-shielding with the minimum volume and mass of material. In this work we perform atomistic simulations on two of the most important of these, Fe and W. In this thesis we describe atomic-scale simulations of defects found in bcc systems. In part I we consider the vacancy and interstitial loop defects that are produced and accumulated as a result of irradiation-induced displacement cascades. We show that vacancy dislocation loops have a critical size below which they are highly unstable relative to planar void defects, and thus offer an explanation as to why they are so rarely seen in TEM observations of irradiated bcc metals. Additionally, we compare the diffusion rates of these vacancy loops to their interstitial counterparts and find that, while interstitial loops are more mobile, the difference in mobility is not as significant as might have been expected. In part II we study screw dislocations, which, as the rate limiting carriers of plastic deformation, are significantly responsible for the strength of materials. We present results from large-scale finite temperature molecular dynamics simulations of screw dislocations under stress and observe the thermally-activated kink-pair formation regime at low stress, which appears to be superseded by a frictional regime at higher stresses. The mobility functions fitted to the results are vital components in simulations of dislocation networks and other large-scale phenomena. Lastly, we develop a multi-string Frenkel-Kontorova model that allows us to study the core structure of screw dislocations. Subtle changes in the form of the interaction laws used in this model demonstrate the difference between the non-degenerate and degenerate core structures. We provide simple criteria to guarantee the correct structure when developing interatomic potentials for bcc metals.
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Jin, Xin. "Combining RBS/Channeling, X-ray diffraction and atomic-scale modelling to study irradiation-induced defects and microstructural changes." Thesis, Limoges, 2021. http://www.theses.fr/2021LIMO0017.
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Les particules énergétiques sont souvent impliquées dans les activités de la société moderne. Ils ont contribué à l'essor de l'industrie des semi-conducteurs et pourront à l'avenir jouer un rôle important dans la mise en forme des matériaux de manière contrôlée. Cependant, leur nature énergétique pose de grands défis. Ainsi, il est essentiel d'avoir une compréhension globale des mécanismes sous-jacents des défauts induits par l'irradiation et des changements microstructuraux associés. Expérimentalement, les effets induits par l'irradiation peuvent être suivis par des techniques de caractérisation telles que la rétrodiffusion de Rutherford en mode canalisé (RBS/C) et la diffraction des rayons X (XRD), pour ne citer que ces deux car elles sont extrêmement sensibles aux perturbations au sein des cristaux. Cependant, il n'est pas aisé d'établir un lien clair entre le résultat de la mesure et la quantité et la nature des défauts, et ce lien est généralement fait à partir de modèles phénoménologiques. Dans ce travail de thèse, afin de faire face à ce problème, nous avons couplé modélisations à l'échelle atomique et simulations de signaux de RBS/C et XRD. La première étape a consisté à améliorer un code de simulation RBS/C récemment développé qui peut générer des signaux à partir de structures atomiques. En modifiant les algorithmes décrivant les interactions ion-solide et en ajoutant de nouvelles fonctionnalités, nous avons amélioré la flexibilité du code et son applicabilité à différents types de matériaux. Par la suite, nous avons utilisé le code RBS/C amélioré avec un code pour la DRX, lui aussi utilisant les données de structures atomiques. Avec ces signaux, nous avons extraits des paramètres de désordre et de déformation élastique et nous avons déterminé les cinétiques d'évolution associées et ce, pour un matériau modèle, à savoir UO2. Les défauts d'irradiation ont été générés par dynamique moléculaire (MD) avec la technique de l'accumulation de paires de Frenkel. Les cinétiques issues des modélisations présentent un accord qualitativement étroit avec celles déterminées expérimentalement, indiquant la validité de la méthodologie utilisée. La décomposition des cinétiques modélisées a permis de décrire de façon quantitative l'évolution des différents de types de défauts. Enfin, nous avons calculé les signaux RBS/C et XRD à partir de cellules modèles de Fe produites par MD et contenant chacune un type de défauts à une concentration donnée, les deux informations étant connues. Une comparaison claire du désordre et de la déformation élastique induits par les différents types de défauts dans Fe a été faite. La relation entre le rendement RBS/C et l'énergie des ions sonde a également été étudiée et la dépendance en énergie, fonction de la nature des défauts, a été établie. L'approche globale utilisée dans ce travail doit désormais être étendue et testée dans d'autres matériauxEnergetic particles are involved in many activities of modern society. They constitute a significant aspect of the semiconductor industry and may play important role in shaping materials in a controllable way in the future. However, their energetic nature also poses grand challenges, especially in the nuclear industry. Thus, it is crucial to have a comprehensive understanding of the underlying mechanisms of irradiation-induced defects and the associated microstructural changes. Experimentally, irradiation-induced effects can be monitored by characterization techniques including, but not limited to, Rutherford backscattering spectrometry in channeling mode (RBS/C) and X-ray diffraction (XRD), because they are extremely sensitive to changes in the crystalline structure. However, it is not straightforward to establish a clear link between the characterization results and the defect quantity and nature, and this connection is usually made according to simple phenomenological models. In this thesis work, in order to cope with this problem, we performed RBS/C and XRD atomic-scale modelling. The first step was to improve a recently developed RBS/C simulation code that can generate RBS/C signals from arbitrary atomic structures. By modifying the algorithms describing ion-solid interactions and adding new features, we enhanced the flexibility of the code and its applicability to different types of materials. Subsequently, we employed the improved RBS/C code with a XRD program to compute disordering and elastic strain kinetics of a model material, namely UO2, as a function of irradiation fluence. Radiation defects in UO2 were simulated by molecular dynamics (MD) calculations. Both the strain and disordering kinetics exhibit qualitatively close agreement with those determined experimentally, indicating the validity of the used methodology. The decomposition of the kinetics was performed in order to study the effect of each defect separately, which enables a quantitative description of the disordering and strain build-up processes. Finally, we computed RBS/C and XRD signals from Fe MD cells, each of which contains one single type of defects. A clear comparison of disorder and elastic strain induced by different types of defects in Fe was made. The relation between RBS/C yield and He energy was also studied using the Fe MD cells, which shows dependency with defect types. The global approach used in this work has the hope to be extended and tested in more materials
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CARUSO, FRANCESCO. "Study of electrical conduction and defects in high-permittivity metal oxides: experiments and simulation." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2022. http://hdl.handle.net/10281/382298.
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Inizialmente studiati dall’industria elettronica per sostituire lo strato isolante di SiO2, gli ossidi metallici sono ora ampiamente utilizzati come strati attivi o isolanti in una moltitudine di dispositivi elettronici. Le proprietà elettriche sono fortemente correlate ai difetti atomici, che generano stati elettronici localizzati all'interno del band gap che fungono da trappole di carica. La comprensione dei meccanismi fisici e del ruolo dei difetti che regolano il trasporto di carica negli ossidi metallici è pertanto della massima importanza per l'ottimizzazione dei dispositivi nanoelettronici. Tuttavia, il trasporto di carica e il ruolo dei difetti negli ossidi metallici è ancora oggetto di dibattito e non è stata raggiunta una comprensione completa e autoconsistente in ampi regimi di spessore, temperatura e tensione.
In questa tesi ho studiato i meccanismi di conduzione in condensatori metallo-isolante-metallo (MIM) che incorporano tre materiali modello Al2O3, HfO2 e HfO2 drogato con Al (AlHfO) depositati mediante deposizione di strati atomici (ALD), in tre diversi spessori 5, 10 e 20nm. Inoltre, sono stati analizzati gli ossidi a base di Hf depositati utilizzando acqua o ozono come fonte di ossigeno ALD, nonché AlHfO a due concentrazioni di Al (5% e 17%). Lo scopo di questo studio è identificare le proprietà delle trappole di carica di ciascun materiale e studiare il percorso che gli elettroni percorrono all'interno dei dielettrici di ossidi metallici sotto l’azione del campo elettrico applicato. Viene inoltre discusso l'impatto dei diversi processi di produzione e dello spessore dei film sulle proprietà del materiale.
Le proprietà delle trappole sono estratte dalla caratteristica sperimentale corrente-tensione dei condensatori MIM, in un ampio regime di temperatura e tensione, utilizzando un modello di trasporto di carica completo implementato nel software di simulazione Ginestra (Applied Materials). I difetti del Al2O3 sono caratterizzati da un'energia di ionizzazione termica ET~3.5eV e da un'energia di rilassamento EREL~1eV, in accordo con i calcoli ab-initio per le vacanze di ossigeno riportati in letteratura. In ogni ossido spesso 10 e 20nm a base di Hf sono identificati due tipi di difetti, caratterizzati da ET~1.8eV per le trappole "superficiali" e ET~3eV per le trappole "profonde". L'uso dell'acqua durante la deposizione ALD introduce cariche positive fisse nell'ossido. L'introduzione di atomi di Al nel HfO2 aumenta il band gap dell'ossido, senza influire sulla densità e sulle proprietà dei difetti.
L'analisi ha permesso di identificare la posizione delle trappole maggiormente coinvolte nella conduzione e il meccanismo di trasporto dominante in ossidi spessi 20nm, ad ogni campo elettrico applicato. Nonostante le diverse proprietà, in ciascun materiale si verificano correnti di spostamento transitorie a bassi campi elettrici, originate dall'intrappolamento e dall'emissione di elettroni in/da trappole vicine all'interfaccia metallo/ossido. Il trasporto di elettroni attraverso l'ossido avviene solo a campi elettrici più elevati, in due modi diversi. Se una grande densità di trappole è localizzata energeticamente vicino al livello di Fermi degli elettrodi (come nel caso del HfO2), gli elettroni passano da una trappola all'altra fino a raggiungere l'anodo. Altrimenti, quando le trappole sono più vicine alla banda di conduzione (come nel Al2O3 e AlHfO), gli elettroni passano dal catodo in una trappola e poi nella banda di conduzione dell'ossido, interagendo solo con trappole vicino al catodo.
Questi risultati potrebbero avere profonde implicazioni per l'ottimizzazione dei futuri dispositivi nanoelettronici. Inoltre, poiché negli ossidi metallici l'intrappolamento, la generazione di difetti e i processi di rottura sono fortemente correlati, i risultati presentati possono fornire nuove indicazioni sul processo di rottura degli ossidi metallici, con un impatto sull'affidabilità dei dispositivi.Originally investigated in the electronic manufacturing to replace the SiO2 insulating layer, metal oxides are now extensively used as insulating or active layers in a multitude of electronics devices. It is known that the electrical properties are strongly correlated to atomic defects, which generate localized electronic states inside the band gap that act as charge traps. Therefore, the understanding of the physical mechanisms and the role of defects governing the charge transport in metal oxide stacks is of utmost importance for the optimization of nano-electronic devices. However, the charge transport and role of defects in metal oxides is still under debate and a complete and self-consistent understanding over large thickness, temperature and voltage regimes is not reached. In this thesis I investigated the conduction mechanisms in metal-insulator-metal (MIM) capacitors incorporating three model materials Al2O3, HfO2 and Al-doped-HfO2 (AlHfO) deposited by atomic layer deposition (ALD), in three different thicknesses 5, 10, and 20 nm. Furthermore, Hf-based oxides deposited using either water or ozone as ALD oxygen source, as well as AlHfO at two Al concentrations (5% and 17%) were analyzed. The aim of this study is to identify the charge traps properties of each material and investigate the path that electrons take within metal oxide dielectrics under applied electric field. Moreover, the impact of different manufacturing processes and film thicknesses on the material properties is discussed. Traps properties are extracted from experimental current-voltage characteristics of MIM capacitors, over a broad temperature and voltage regime, using a comprehensive charge transport model implemented in the Ginestra® (Applied Materials, Inc.) simulation software. Defects in Al2O3 are characterized by a thermal ionization energy ET≈3.5 eV below the dielectric conduction band minimum (CBM) and a relaxation energy EREL≈1 eV, in agreement with the ab-initio calculations of oxygen vacancies reported in literature. Two kinds of defects are identified in each 10 and 20 nm-thick Hf-based oxide, characterized by ET≈1.8eV for "shallow" traps, and ET≈3eV for "deep" traps. The use of water as oxygen source during the oxide ALD introduces fixed positive charges in the oxide. The introduction of Al atoms in HfO2 increases the oxide energy band gap, without significantly impacting on the density and properties of defects. The analysis allowed to identify the location of traps most involved in the conduction and the dominant transport mechanism in 20 nm-thick oxides, at each applied electric field. Despite the different properties, in each material transient displacement currents occur at low electric fields, originating from electron trapping and emission at traps near the metal/oxide interface. The transport of electrons through the oxide occurs only at higher electric fields, in two different ways. If a large density of traps is energetically located near the electrodes Fermi level (as in HfO2), the electrons tunnel from trap to trap until they reach the anode. Otherwise, when traps are closer to the conduction band (as in Al2O3 and AlHfO), the electrons tunnel from the cathode into one trap and then into the oxide conduction band, interacting only with traps near the cathode. These findings may have profound implications for the functional optimization of future nano-electronics devices. Furthermore, since in metal oxides trapping, defects generation and breakdown processes are strongly related, results can provide new insight in the breakdown process of metal oxides, impacting on device reliability.
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Bagués, Salguero Núria. "Atomic and electronic structure of self-organized defects in epitaxial films of functional perovskite-type oxides." Doctoral thesis, Universitat Autònoma de Barcelona, 2017. http://hdl.handle.net/10803/405668.
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Les capes fines epitaxials d’òxids funcionals tipus perovskita (ABO3) presenten mecanismes d’acoblament d’intercara i de relaxació del desajust governats per un joc complex de graus de llibertat químics, electrònics i estructurals. Aquests mecanismes poden acomodar defectes, tals com dislocacions de desajust i parets de macla, que presenten una gran tendència a l’auto-organització amb escales característiques de desenes de nanòmetres. L’estructura de la xarxa en el nucli d’aquests defectes és diferent de la major part del material, i per tant pot ser considerat com una nano-fase amb propietats físiques probablement diferents, portant a la formació de nano-estructures funcionals. La correlació entre l’estructura del defecte i la funcionalitat, juntament amb la capacitat d’aquests defectes per auto-organitzar-se, ofereix una oportunitat única per l’elaboració bottom-up de nano-dispositius d’òxids funcionals complexes.
Aquesta Tesi es centra en la caracterització de la microestructura, l’intercara i els defectes auto-organitzats de capes epitaxials i nanoestructures funcionals de materials d’òxids mitjançant l’ús de Microscòpia Electrònica de Transmissió. Es presta especial atenció a l’estructura atòmica i electrònica de les intercares i defectes generats, tals com dislocacions, parets de macla i segregacions de fases, així com als camps de deformació i la seva relació amb les homogeneïtats químiques. En aquesta direcció, dos sistemes diferents compostos per manganites de Lantà són considerats: nanocompost de LaMnO3:MnOx crescut sobre substrats de (001)SrTiO3 i de (001)LaAlO3; i capes de La0.7Sr0.3MnO3 amb defectes auto-organitzats crescudes sobre substrats de (001)SrTiO3 i de (001)LaAlO3. Els materials estudiats en aquest treball poden ser considerats com capes nanoestructurades resultants de l’autoorganització de defectes que relaxen el desajust: nanoinclusions de MnOx (defectes de volum) en el LaMnO3; parets de macla entre dominis de macla (defectes planars) en La0.7Sr0.3MnO3/SrTiO3 i dislocacions de desajust (defectes lineals) en La0.7Sr0.3MnO3 /LaAlO3.
En el nanocompost de LaMnO3:MnOx, s’analitza la formació regular de nanoinclusions d’òxid de manganès verticalment alienades dins d’una capa de LaMnO3 a través d’una caracterització microestructural. Aquestes anàlisis inclouen la determinació de la microestructura del LaMnO3 respecte el substrat conjuntament amb la identificació de la fase de l’òxid de manganès i d’una fase secundària, una capa rica en La a prop de l’intercara LaMnO3-SrTiO3. En el cas del La0.7Sr0.3MnO3/SrTiO3, es realitza una anàlisi detallada de les parets de macla i de les implicacions d’aquestes en les propietats funcionals. Els canvis locals en les propietats físiques i estructurals de les parets de macla permeten veure una capa amb macles com a una estructura auto-organitzada consistint en nano-lamines verticals de La0.7Sr0.3MnO3 fortament comprimides dins d’una matriu tensionada de La0.7Sr0.3MnO3. En el cas de les capes ultrafines de La0.7Sr0.3MnO3/LaAlO3, s’analitza el mecanisme de relaxament d’aquestes capes, les quals alleugen la tensió de desajust a través de la formació de dislocacions per sobre d’un gruix de crític de capa de 2.5nm. Es realitza un estudi detallat dels canvis estructurals, químics i electrònics associats amb les dislocacions, posant especial atenció a la influència dels camps de deformació en la composició química a la nanoescala. En aquesta direcció, s’observa una reorganització química a la regió del nucli de la dislocació, la qual té lloc per acomodar la deformació. També s’explora la dependència de l’organització de les dislocacions amb el gruix de les capes. Finalment, són analitzades les implicacions del camp de deformació de les dislocacions en la topografia i el transport elèctric a la superfície de les capes, demostrant que la naturalesa multi-escala de les dislocacions és de gran potencial per la creació de nanoestructures funcionals organitzades espontàniament en la superfície de capes fines d’òxids complexes.
Els principals resultats i conclusions obtinguts en aquesta Tesis obren una nova perspectiva per al desenvolupament d’estructures funcionals auto-organitzades basades en defectes que relaxen tensions.The epitaxial thin films of functional perovskite-type oxides (ABO3) present interfacial coupling and misfit relaxation mechanisms governed by a complex interplay of chemical, electronic and structural degrees of freedom. The relaxation mechanisms of strained films may accommodate defects, such as misfit dislocations or twin walls, which exhibit a strong tendency towards self-organization with characteristic length scales of tens of nanometres. The core lattice structure of these defects is different from the bulk of the material and thus may be considered as a nano-phase with likely different physical properties, leading to the formation of functional nanostructures. The correlation between defect structure and functionality, together with the capacity of these defects to self-organize, offers a unique opportunity for the bottom-up elaboration of functional complex oxides nanodevices. This thesis focuses on the characterization of the microstructure, interface and self-organized defects of epitaxial films and functional nanostructures of oxide materials by using advanced transmission electron microscopy. Special emphasis is put on the atomic and chemical structure of the interfaces and generated defects, such as dislocations, twin walls and phase segregations, as well as on the strain fields and their correlation with chemical heterogeneities. In this regard, two different systems composed of lanthanum manganites are considered: LaMnO3:MnOx nanocomposite grown on (001)SrTiO3 and on (001)LaAlO3 substrates; and La0.7Sr0.3MnO3 films with self-organized defects grown on (001)SrTiO3 and on (001)LaAlO3. The materials studied in this work may be regarded as nanostructured films resulting from the self-organization of misfit relieving defects as follows: nanoinclusions of a MnOx phase (volume defects) in LaMnO3; twin walls between twin domains (planar defects) in La0.7Sr0.3MnO3 on SrTiO3; and misfit dislocations (line defects) in La0.7Sr0.3MnO3 on LaAlO3. In the LaMnO3:MnOx nanocomposite, the formation of regular vertically aligned nanoinclusions of a manganese oxide (MnOx) embedded in an LaMnO3 film is analysed via microstructural characterization. This analysis includes the determination of the LaMnO3 matrix microstructure with respect to the substrate together with the identification of the manganese oxide phase and a secondary phase: a La-rich layer close to LaMnO3-substrate interface. In the case of La0.7Sr0.3MnO3 on (001)SrTiO3 substrates, a detailed analysis of twin walls and their implications on the functional properties is performed. Local changes in the physical and structural properties of the TWs lead to the view of a twinned film as a self-organized nanostructure consisting of vertical nano-sheets of strongly compressed La0.7Sr0.3MnO3 embedded in a matrix of tensile strained La0.7Sr0.3MnO3. In the case of La0.7Sr0.3MnO3 ultrathin films grown on (001)LaAlO3, the relaxation mechanism of this films is analysed. These films relieve the misfit strain by the formation of misfit dislocations above a critical film thickness of 2.5 nm. A detailed study of structural, chemical and electronic changes associated with the dislocation is also performed paying particular attention to the influence of strain fields on chemical composition at the nanoscale. A chemical reorganization occurs to accommodate the strain at the dislocations core region. The dependence of the degree of order of the dislocation pattern on film thickness is also explored. Finally, the implications of the dislocation strain field on surface topography and electrical transport are analysed, demonstrating that the multiscale nature of dislocations holds great promise for the creation of spontaneous surface ordered functional nanostructures in complex oxide thin films.The results and main conclusions obtained in this work open new perspectives for the development of functional self-organized nanostructures based on strain relieving defects.
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Khazaka, Rami. "From atomic level investigations to membrane architecture : an in-depth study of the innovative 3C-SiC/Si/3C-SiC/Si heterostructure." Thesis, Tours, 2016. http://www.theses.fr/2016TOUR4023/document.
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Le polytype cubique du carbure de silicium (3C-SiC) est un matériau très prometteur pour les applications MEMS. En plus de sa tenue mécanique et chimique, il peut être épitaxié sur des substrats Si de faible coût. De plus, l'hétéroépitaxie multiple, c’est-à-dire quand on empile plusieurs couches Si et 3C-SiC peut ouvrir des pistes pour de nouveaux dispositifs à base de 3C-SiC. Vue la complexité de développer de telles hétérostructures, nous avons procédé à l'amélioration de la qualité de chaque couche séparément. De plus, nous avons mené une étude approfondie sur la nature des défauts dans chaque couche. Après le développement de l'hétérostructure complète, nous avons procédé à la fabrication de microstructures à base de cet empilement. Nous présentons une méthode inédite pour former des membranes de 3C-SiC auto-supportées. Cette technique simplifie considérablement le procédé de fabrication de membranes tout en réduisant le temps de fabrication et le coût. En outre, elle aide à surmonter plusieurs problèmes techniquesDue to its outstanding physico-chemical properties, the cubic polytype of silicon carbide (3C-SiC) gained significant interest in several fields. In particular, this material emerged as a potential candidate to replace Si in MEMS devices operating in harsh environment. The development of 3C-SiC/Si/3C-SiC heterostructures on top of Si substrate can pave the road towards original and novel MEMS devices profiting from the properties of the 3C-SiC. However, such epitaxial system suffers from wide range of defects characterizing each layer. Thus, we first tried to improve the quality of each layer in this heterostructure. This was achieved relying on two levers; (i) the optimization of the growth parameters of each layer and (ii) the understanding of the nature of defects present in each layer. These two key points combined together allowed an in-depth understanding of the limit of improvement of the overall quality of this heterostructure. After the development of the complete heterostructure, the fabrication of 3C-SiC microstructures was performed. Furthermore, we presented an unprecedented method to form free-standing 3C-SiC membranes in-situ during its growth stage. This novel technique is expected to markedly simplify the fabrication process of suspended membranes by reducing the fabrication time and cost
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Drain, John Frederick. "Development of magnetic bond-order potentials for Mn and Fe-Mn." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:68a01493-4a20-4d78-ad4a-6c3c2fe072d6.
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While group VII 4d Tc and 5d Re have hexagonally close-packed (hcp) ground states, 3d Mn adopts the complex chi-phase which exhibits non-collinear magnetism. Density functional theory (DFT) calculations have shown that without magnetism the chi-phase remains the ground state of Mn implying that magnetism is not the critical factor, as is commonly believed, in driving the anomalous stability of the chi-phase over hcp. Using a tight-binding (TB) model it is found that while harder potentials stabilise close-packed hcp, a softer potential stabilises the more open chi-phase. By analogy with the structural trend from open to close-packed phases down the group IV elements, the anomalous stability of the chi-phase in Mn is shown to be due to 3d valent Mn lacking d states in the core which leads to an effectively softer atomic repulsion between the atoms than in 4d Tc and 5d Re. Subsequently an analytic Bond-Order Potential (BOP) is developed to investigate the structural and magnetic properties of elemental Mn at 0 K. It is derived within BOP theory directly from a new short-ranged orthogonal d-valent TB model of Mn, the parameters of which are fitted to reproduce the DFT binding energy curves of the five experimentally observed phases of Mn, alpha, beta, gamma, delta, and epsilon-Mn. Not only does the BOP reproduce qualitatively DFT binding energy curves of the five different structure types, it also predicts the complex collinear antiferromagnetic (AFM) ordering in alpha-Mn, the ferrimagnetic (FiM) ordering in beta-Mn and the AFM ordering in the other phases that are found by DFT. A BOP expansion including 14 moments is sufficiently converged to reproduce most of the properties of the TB model with the exception of the elastic shear constants which require further moments. Magnetic analytic BOPs are also developed for Fe and Fe-Mn. The Fe model correctly reproduces trends in the structural stabilities of the common metallic structures except that AFM hcp is overstabilised. Reproduction of the elastic constants with a 9-moment BOP is reasonable although as is found for the Mn BOP the elastic shear constants require more moments to converge. Vacancy formation energies are close to those determined by experiment and DFT and the relative stabilities of self-interstitial atom (SIA) defects in ferromagnetic bcc Fe are correctly reproduced. The SIA formation energies are found to be better than those calculated with existing BOP models. The Fe-Mn TB and BOP models were challenging to fit and nonmagnetic face-centred cubic (fcc) structures are overstabilised. Furthermore within BOP an incorrect magnetic solution is predicted for one fcc structure resulting in poor reproduction of the DFT stacking fault energies. Refitting the bond integrals might help to better reproduce the nonmagnetic hcp-fcc energy differences while an environment-dependent Stoner parameter could help provide the flexibility needed to correctly capture the magnetic energy differences.
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Xu, Shuo. "A study of irradiation damage in iron and Fe-Cr alloys." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:fec4b2d0-411e-4c20-862d-e5fd884367f3.
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Irradiation damage structures induced in pure Fe and Fe-Cr (up to 14%Cr) alloys by 2 MeV Fe+ ion irradiations in the temperature range 300-460°C were investigated by transmission electron microscopy. Specimens were irradiated in bulk to doses of 1.5 x 1019 Fe+/m2 (about 2.5 displacements per atom: dpa) and 4.5 x 1019 Fe+/m2 (about 7 dpa). In most cases, damage took the form of dislocation loops with diameters from 2-100nm; the loops were distributed uniformly within all the samples. At higher irradiation temperatures (400°C, 460°C), complex microstructures such as finger loops (50nm in width and 1 micron in length) and perpendicular <100> loop clusters, were observed in both pure Fe and Fe-Cr samples. Loop sizes and densities were seen to change as a function of irradiation temperature and dose. Loop sizes were seen to increase as the increase of irradiation temperatures and doses, while loop densities only increased with increasing doses and decreased as increasing temperatures. Loops with both types of Burgers vectors (<100> and ½<111>) were observed in all the samples. The proportion of <100> loops was higher in Fe than that in Fe-Cr alloys at the same irradiation condition, which has can be attributed to the high mobility of ½<111> loops in Fe, so that a large proportion of them will escape to the (001) foil surface. A transition in loop Burgers vectors as a consequence of increasing temperature was observed. In Fe, the proportion of <100> loops increased with increasing irradiation temperature from 40% at 300°C to 60% at 460°C. A similar trend was found in the Fe-Cr alloys, but due to the higher proportion of ½<111> loops in these alloys, the increase of <100> loops was not that obvious, being from 30% at 300°C to 45% at 460°C(Fe-11Cr). The effects of irradiation dose rate on the formation of dislocation loops by 2 MeV Fe+ ions were also investigated. These irradiations were carried out at 300°C with two different implantation dose rates: 6 x 10-4 dpa/s and 3 x 10-5 dpa/s. The implantation dose for both implantations was 0.38 x 1019 Fe+/m2 (0.5 dpa). Both the average loop size and loop densities for the Fe-Cr specimens subjected to the high dose rate irradiation were higher than that in the low dose rate irradiations. Take Fe-14Cr as an example, that the loop densities in high dose rate irradiation increased about 90% compared to that in low dose rate, and the average loop size in high dose rate irradiation was 30% larger than that in low dose rate irradiation. The ‘inside-outside contrast’ method was applied to determine the loop nature in all the samples. It was found that all the large loops (>5nm) are of interstitial type. Any vacancies are believed to exist in the form of small dislocation loops (<5nm) or sub-microscopic voids.
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Liu, Qiong. "Mechanical properties of Ti-o based ceramic nanowires." Thesis, Queensland University of Technology, 2020. https://eprints.qut.edu.au/201954/1/Qiong_Liu_Thesis.pdf.
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This thesis thoroughly investigated the mechanical properties of Ti-O based ceramic nanowires. It revealed that elastic bending properties including elastic strain and elastic moduli of different kinds of Ti-O based ceramic nanowires were related to their crystalline structures, defects in the structures, and defect activities during bending deformation. These findings help to provide more opportunities for strain engineering on Ti-O NWs and promote potential applications of Ti-O NW-based devices.
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Bouillaud, Pascal. "Irradiation aux ions lourds de films minces nanocristallins d'alliages FeCo et FeAl." Grenoble 2 : ANRT, 1988. http://catalogue.bnf.fr/ark:/12148/cb37612171v.
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De, Souza Maria Merlyne. "Atomic level diffusion mechanisms in silicon." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319817.
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Zhang, Shu Yan. "High energy white beam X-ray diffraction studies of strains in engineering materials and components." Thesis, University of Oxford, 2008. http://ora.ox.ac.uk/objects/uuid:957786c6-114c-40f1-8ee7-649c8b2522bc.
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The primary aim of this research was to develop and improve the experimental method and data interpretation for strain measurements using diffraction methods to gain a better understanding of micromechanical deformation and anisotropy of lattice strain response. Substantial part of the research was devoted to the development of the laboratory high energy X-ray diffractometer (HEXameter) for bulk residual strain evaluation. White beam energy dispersive X-ray diffraction was chosen as the principal diffraction mode due to its extreme efficiency in utilising X-ray flux and its ability to capture large segments of diffraction patterns. The specimens that have been examined were real engineering components, mechanically deformed specimens and thermally treated specimens, ranging from dynamic in-situ measurements to ex-situ materials engineering. For the real engineering components, a wedge coupon from the trailing edge of a Ti64 wide fan blade and a turbine combustion casing were examined. Among the mechanically deformed specimens that have been measured were shot-peened steel plates, elasto-plastically bent bars of Mg alloy and cold expanded Al disks. Amongst the thermally deformed specimens, laser-formed steel plates, thermal spray coatings, a manual inert gas weld of Al plates, a friction stir weld of Al plates and Ni tubes and a quenched Ni superalloy cylinder used for strain tomography were studied. In-situ loading experiments have also been carried out, such as experiments on pointwise mapping of grain orientation and strain using the 3DXRD microscope at the ESRF and in-situ loading experiments on titanium alloy, rheo-diecast and high pressure diecast Mg alloy, IN718 Ni-base superalloy and Al2024 aluminium alloy. Experimental results from X-Ray diffraction and strain tomography were used to achieve a better understanding of the material properties. Some results were compared with polycrystal Finite Element model predictions. Amongst the most prominent research achievements are the development on the HEXameter laboratory instrument, including: (i) the development of special collimation systems for the detectors and the source tube; (ii) the development of a twin-detector setup (that allows for simultaneous determination of strain in two mutually orthogonal directions); (iii) improved alignment procedures for better performance; and (iv) the adaptation of instrumentation for efficient scanning of both large and small components, that included choosing and adapting translation devices, programming of the translation system and designing sample mounting procedures. In this research several approaches to data treatment were investigated. Quantitative phase analysis, single peak fitting (using custom Matlab routines and GSAS) and full pattern fitting (with individual pattern data refined by GSAS and batch refinement done by invoking GSAS via a Matlab routine) were applied. Different Matlab routines were written for specific experimental setups; and various analysis methods were selected and used for refinement depending on the requirements of the measurement results interpretation. 16 papers were published, ensuring that the results of this thesis are readily available to other researchers in the field.
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Robertson, Alexander William. "Synthesis and characterisation of large area graphene." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:aee750dd-41b8-4462-9efa-4e89e06e0ed7.
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The pursuit of high quality, large area graphene has been a major research focus of contemporary materials science research, in the wake of the discovery of the multitude of exceptional properties exhibited by the material. The DPhil project was undertaken with the objective of developing an understanding of the growth of large graphene sheets by chemical vapour deposition (CVD), and also in the subsequent characterisation of their material properties. By conducting atmospheric pressure CVD growth at high methane flow rates, it was found that few-layered graphene (FLG) could be deposited on a copper catalyst. It is demonstrated that the self-limiting property of a copper catalyst is not universal to all deposition conditions, and shown that FLG grows in a terrace-like configuration. In depth transmission electron microscopy (TEM) studies were carried out on FLG. By selective image reconstruction from the inverse power spectrum of the TEM images, it was possible to elucidate the inter-grain connectivity of few-layer graphenes. It was determined that there were two possible inter-grain configurations possible; specifically an overlap of graphene layers or a discrete atomic bonding edge. The perturbation of the few-layer structure when subject to an out of plane distortion was found to incur a shift in the conventional AB-Bernal stacking of FLG. By utilising the aberration corrected TEM (AC-TEM) at Oxford it was possible to resolve atomic detail in CVD synthesised monolayer films, including atomic bond rotations and vacancies. The use of a high current density at low accelerating voltage (80 kV) was demonstrated to allow for the controlled defect creation in graphene sheets. This permitted the creation of monovacancies and iron doped vacancy complexes suitable for further study. The behaviour of these two defect types under electron beam irradiation was subsequently studied.
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Atkinson, Kurt James William. "Atomic scale simulation of defects in bulk materials and monolayer surfaces." Thesis, Imperial College London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397160.
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FILHO, HENRIQUE DUARTE DA FONSECA. "SEMICONDUCTOR NANOSTRUCTURE FABRICATION IN MECHANICAL DEFECTS PRODUCED BY ATOMIC FORCE MICROSCOPY." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2008. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=12973@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOA combinação de alta densidade, locais seletivos de nucleação e controle da distribuição de tamanho de nanoestruturas semicondutoras tem acelerado o desenvolvimento de dispositivos ópticos e eletrônicos. Para construir estruturas satisfazendo essas necessidades, várias combinações de técnicas deposição de pontos quânticos e nanolitografia foram desenvolvidas. A nanolitografia por AFM foi aplicada em diversos materiais abrindo uma possibilidade para fabricar dispositivos opto-eletrônicos.Nesta tese de Doutorado, apresentamos um estudo sistemático de crescimento de nanoestruturas de InAs em buracos produzidos na superfície (100) de substratos de InP por nanoindentação com o AFM. Para isto, a ponta precisa exercer uma força no InP que produz deformações plásticas na superfície. A pressão aplicada entre a extremidade da ponta de AFM e a superfície da amostra pode ser variada de modo controlado através do ajuste de alguns parâmetros operacionais do microscópio tais como setpoint, raio da ponta e constante de mola do cantilever. A habilidade para controlar a forma do padrão indentado assim como a natureza dos defeitos cristalinos permite controlar o crescimento seletivo de InAs por epitaxia em fase de vapor de metais orgânicos. Também é apresentada a fabricação de nanoestruturas de InAs/InP alinhadas em uma dimensão. A nanoindentação é produzida pelo arraste da ponta do AFM sob força constante ao longo das direções <100> e <110> do InP. Observamos que o número e o tamanho das nanoestruturas nucleadas são dependentes da distância entre as linhas litografadas. Esses resultados sugerem que o mecanismo de crescimento das nanoestruturas de InAs não é governado por degraus atômicos gerados durante a indentação. Os dados sugerem que, a densidade de defeitos induzidos mecanicamente, tais como discordâncias e fraturas, é o responsável pelo número de nanoestruturas nucleadas.
The combination of high density, site selective nucleation, and size distribution control of semiconductor nanostructures has become a challenge in the development of effective optical and electronic devices. In order to build structures satisfying these requirements, various combinations of quantum dot deposition and nanolithography techniques have been developed. The AFM nanolithography technique has been applied on several materials opening a possibility to fabricate opto-electronic devices. In this Phd Thesis, we present a systematic study of growth of InAs nanostructures on pits produced on (100) InP by nanoindentation with the AFM. For that purpose, the AFM tip needs to exert a force on the InP that produces plastic deformation on the surface. The applied pressure between the very end of the AFM tip and the sample surface may be varied in a controlled way by adjusting some of the microscope operational parameters like set point, tip radius and cantilever normal bending constant. The ability to control the shape of the indentation pattern as well as the nature of the crystalline defects allows control of the selective growth of InAs by metal organic vapor phase epitaxy. We also report the fabrication of one-dimensional arrays of InAs/InP nanostructures. The nanoindentation is produced by dragging the AFM tip under constant force of the substrate, along the <100> and <110> InP crystallographic directions. We have observed that the number and the size of nucleated nanostructures are dependent on the distance between the lithographed lines. These results suggest that the growth mechanism of the InAs nanostructures on the pits produced by AFM on InP is not governed by the number of atomic steps generated during the scratching. Instead, the data suggests that, the density of mechanically induced defects, like dislocations and cracks, are responsible for the number of nucleated nanostructures.
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Chen, Qu. "Structural studies of defects in two-dimensional materials with atomic resolution." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:392f2b1d-0488-4d10-96d9-817def04db2a.
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Defective structures in two-dimensional (2D) materials have been proved to have significant influences on the materials' properties. Understanding structural defects in 2D materials at atomic scale is therefore required. With the use of advanced imaging techniques, one of the main approaches applied in this project was aberration-corrected transmission electron microscopy (AC-TEM), the structures are able to be resolved with single-atom sensitivity with the reduction of both spherical aberration and the influence of chromatic aberration. This laid the foundation for the first two experiments, which involve the bond length measurement of each C-C bond within three types of divacancies and Si-C bonds at graphene edges. The former explains the tendency of bond rotations within the divacancies from the perspective of strain inside the defective areas and surrounding lattice; the latter revels the interactions between isolated Si atoms and zigzag/armchair graphene edges. The use of in-situ heating holder in the AC-TEM makes the direct visualization of structures and their dynamics at elevated temperatures possible. The Si-graphene edge interactions, as well as the following two experiments are all designed to study the high-temperature performances for different systems. Gold nanoclusters are introduced to monolayer graphene by thermal evaporation to study the interaction between gold and graphene at elevated temperature. Due to the strong interaction between gold and graphene, gold crystals are able to adapt to planar configurations with two different crystalline forms, and an epitaxial relationship was found for planar gold crystals and graphene. Atomically flat and long line defects and zigzag edges in monolayer molybdenum disulfide (MoS2) are successfully created by in-situ thermal annealing. The relationship between S vacancy mobility and defect forms are revealed based on the experiment. High-temperature atomic configurations of line defects and edge terminations are resolved in the first time. Their electronic properties are also explored with the support of density functional theory calculations.
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Watson, Stuart. "Structural relaxation at defects by Ab initio molecular dynamics." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320648.
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Xu, Tao. "Low-dimensional atomic-scale multiferroics in nonmagnetic ferroelectrics from lattice defects engineering." Kyoto University, 2017. http://hdl.handle.net/2433/227622.
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Grinolds, Michael Sean. "Nanoscale Magnetic Resonance Imaging and Magnetic Sensing Using Atomic Defects in Diamond." Thesis, Harvard University, 2014. http://dissertations.umi.com/gsas.harvard:11544.
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Magnetic resonance imaging (MRI) has revolutionized modern medicine by providing non-invasive, chemically selective, three-dimensional imaging of living organisms. Industrial-scale MRI has the capability to image with millimeter-scale spatial resolution and has the sensitivity to detect as few as \(10^{14}\) nuclear spins. Increasing spatial resolution to the atomic scale and sensitivity to the single-spin level would enable a wide array of applications most notably including imaging molecular structur. However, conventional MRI methods are already highly optimized, and further order-of-magnitude-scale improvements cannot be reasonably expected without employing fundamentally different technologies.Physics
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Dudda, Bruna. "Morphology of leds by atomic force microscopy." Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/6647/.
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The concern of this work is to present the characterization of blue emitting GaN-based LED structures by means of Atomic Force Microscopy. Here we show a comparison among the samples with different dislocation densities, in order to understand how the dislocations can affect the surface morphology. First of all we have described the current state of art of the LEDs in the present market. Thereafterwards we have mentioned in detail about the growth technique of LED structures and the methodology of the characterization employed in our thesis. Finally, we have presented the details of the results obtained on our samples studied, followed by discussions and conclusions.
L'obiettivo di questa tesi é quello di presentare la caratterizzazione mediante Microscopia a Forza Atomica di strutture di LED a emissione di luce blu a base di nitruro di gallio (GaN). Viene presentato un confronto tra campioni con differente densità di dislocazioni, allo scopo di comprendere in che modo la presenza di dislocazioni influisce sulla morfologia della superficie. Innanzitutto, viene descritto il presente stato dell'arte dei LED. Successivamente, sono forniti i dettagli riguardanti la tecnica di crescita delle strutture dei LED e il metodo di caratterizzazione adottato. Infine, vengono mostrati e discussi i risultati ottenuti dallo studio dei campioni, seguiti dalle conclusioni.
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Bottomley, Joseph Anthony. "Novel application of atomic force microscopy to the analysis of barrier film defects." Thesis, University of Birmingham, 2012. http://etheses.bham.ac.uk//id/eprint/3672/.
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As a result of numerous advantages over glass based devices, flexible displays continue to gather momentum. Many problems associated with fabrication of devices on polymers have been overcome, but the detrimental effect of water permeation through these substrates due to defects is still prevalent despite attempts to block pinholes with thin film barrier materials. A rigorous study of film defects using atomic force microscopy (AFM) plus novel application of the calcium test, with additional scanning electron microscopy and white light interferometry data has been undertaken. While pursuing this study, new methods of displaying defect data were discovered, allowing the problem to be visualised better. In addition, the results demonstrated that the major cause of device failure was point defects in the barrier films caused by airborne dust which adheres to the film surface before application of the barrier. The calcium test was applied in new ways to demonstrate the damaging influence of these film defects. To explain the results, a new model of permeation was devised through quantative analysis of the defects present on industrially produced films and confirmed using model substrates and a synthetic model, to explain the behaviour observed in the literature, but never previously explained. The conclusion was that the combination of AFM study, calcium testing and theoretical modelling gave improved understanding of the defect problems in flexible display devices. It is expected that the findings of this work will help DuPont Teijin Films to eliminate such flaws from their manufacturing process, ultimately allowing the films to be applied to commercial applications.
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Rahman, Shams ur. "Probing the effect of oxygen vacancies in strontium titanate single crystals." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:0b153fd2-3aa8-4fe5-b811-8ac42b4457fa.
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This thesis describes investigations into the role of non-stoichiometry in the surface and bulk properties of SrTiO3 single crystals. A family of (n×n) reconstructions, where n = 2, 3, 4, 5, 6 are produced by argon ion sputtering of the SrTiO3 (111) single crystals and subsequent annealing in UHV or in an oxygen rich environment. The sputtering process introduces defects or oxygen vacancies in the surface region of the sample, whilst the annealing gives rise to surface reconstructions. The surface preparation conditions such as sputtering time, annealing temperature and environment are optimized to obtain various reconstructions in a controlled and reproducible manner. High resolution STM images of these reconstructions are also obtained and utilized in the investigation of the surface reactivity. Fullerene molecules are deposited on the reconstructed surfaces to elucidate the surface reactivity through template assisted growth. Fullerene molecules are first deposited with substrate surfaces held at room temperature. Being the most highly reduced among the (n×n) family, the 5×5 reconstruction significantly influenced the growth of fullerenes. Both C60 and C70 adsorb as individual molecules and produce clusters with magic numbers. The 4×4 and 6×6 reconstructed surfaces encourage the formation of close-packed structures upon the deposition at room temperature. When the surface covered with fullerenes is heated to a temperature of around 200 °C, epitaxial islands are observed. The 6×6 reconstructed surface appeared to be less reactive than the 4×4. Electrical transport, cathodoluminescence (CL) and electron spin resonance (ESR) experiments are also carried out to investigate the effect of oxygen vacancies on the bulk properties of UHV annealed SrTiO3 single crystals. Thermal reduction leads to carrier doping of the material, which not only gives rise to electrical conduction but also induces room temperature luminescence. Both the electrical conductivity and CL intensity increases with annealing time. The work presented in this thesis provides insight into the defect driven properties in both the surface and bulk of SrTiO3 single crystals, which could play an important role in the development of oxide-based electronic devices.
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Tuesday, Andrew J. "Modeling Atomic Defects in a Two-Dimensional Lennard-Jones Lattice Using Molecular Dynamics Simulations." University of Akron / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1334579948.
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Söngen, Hagen [Verfasser]. "Atomic force microscopy at mineral-water interfaces : hydration, chemical identification and point defects / Hagen Söngen." Mainz : Universitätsbibliothek Mainz, 2018. http://d-nb.info/1160611017/34.
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Gong, Chuncheng. "Atomic structure and dynamics study of defects in graphene by aberration-corrected transmission electron microscope." Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:53bd9a04-71ad-4da8-b982-cb45a005e791.
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Graphene has grabbed enormous research attention due to its multiple unique properties. These properties, however, can be strongly influenced by lattice imperfections. Aberration corrected transmission electron microscopy (AC-TEM) is one of the leading methods to image two-dimensional materials at the atomic level. This thesis addresses the issue of structure and dynamics characterization of dislocations and grain boundaries (GBs) in graphene with single atom sensitivity using the state-of-the-art AC-TEM in Department of Materials, University of Oxford. My first goal is to understand the interaction between dislocation and the edge of graphene. When a dislocation is located near an edge, a decrease in the rippling and increase of the in-plane rotation occurs relative to the dislocations in the bulk. The increased in-plane rotation near the edge causes bond rotations at the edge of graphene to reduce the overall strain in the system. Dislocations are highly stable and remain fixed in their position even when located within a few lattice spacings from the graphene edge. With the aid of an in situ heating holder, the high temperature behavior of dislocations is then investigated. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into annealing processes in graphene and the behavior of defects with increased heat. The in situ heterogeneous nucleation and growth of graphene are also studied within the AC-TEM. The growth mechanism consists of alternating carbon cluster attachment and indentation filling to maintain a uniform growth front of lowest energy. The highly polycrystalline graphene seed is found to evolve with time into a higher order crystalline structure. The motion of GBs is discontinuous and mediated by both bond rotation and atom evaporation. These results provide insights into the formation of crystalline seed domains that are generated during bottom-up graphene synthesis. Finally, the formation, reconfiguration and annihilation of GB loops are demonstrated. It is shown that the GB loop cannot fully relaxed under electron beam irradiation with its terminal state being isolated dislocations far apart from each other. Line defects composed of several adjacent excess-atom defects can be found during the reconfiguration process. This work gives detailed information about the stability and behavior of large GB loops in two dimensional materials.
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Johnson, Jared M. "Atomic Scale Characterization of Point Defects in the Ultra-Wide Band Gap Semiconductor β-Ga2O3." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1577916628182296.
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Bîlteanu, Liviu. "Atomic scale simulation of hydrogen related defects in hydrogen implanted silicon - Smart Cut™ technology." Paris 11, 2010. http://www.theses.fr/2010PA112293.
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Le sujet de cette thèse est lié à l'étape d'implantation de la technologie Smart Cut™. Cette technologie utilise l'hydrogène pour transférer des couches de silicium sur des substrats isolants. Le transfert se fait par une fracture induite par la formation des défauts bidimensionnels appelés dans la littérature des plaquettes (en anglais « platelets »). Plus précisément, nous avons étudié dans le cadre de cette thèse les défauts qui apparaissent dans l'état post implantation et leur évolution de l'endommagement d'implantation vers l'état qui contient des plaquettes. L'étude est organisée en deux parties : une première partie qui contient les résultats obtenus par simulation atomistique et une deuxième partie qui contient l'étude par spectroscopie infrarouge de l'évolution des concentrations des défauts suites à des recuits à différentes températures. Les simulations atomistiques ont été effectuées dans le cadre de la théorie de la fonctionnelle densité et ont permis de calculer des énergies de formation et de migration/recombinaisons. Les défauts étudiés sont les interstitiels d'hydrogène atomique et moléculaire, des lacunes et multi-lacunes hydrogénées et finalement des différents modèles de plaquettes. Ces énergies ont permis réaliser un schéma hiérarchique de stabilité des défauts. Ce schéma a été confronté avec des analyses infrarouge sur des échantillons de silicium implanté par hydrogène (à 37 keV) dans le régime dit de «sous-dose» qui ne permets habituellement la formation immédiate des plaquettes lors de l'étape de l'implantation. Ces analyses ont permis de discriminer des évolutions des concentrations de défauts déduites des comportements lors des recuits des pics correspondants aux défauts. La comparaison entre ces évolutions et le schéma énergétique a permis de valider un scénario d'évolution des défauts vers l'état plaquetteThe topic of this thesis is related to the implantation step of the Smart Cut™ technology. This technology uses hydrogen in order to transfer silicon layers on insulating substrates. The transfer is performed through a fracture induced by the formation of bidimensional defects well known in Iiterature as "platelets". More exactly, we have studied within this thesis work the defects appearing in the post implant state and the evolution of the implantation damage towards a state dominated by platelets. The study is organised into two parts: in the first part we present the results obtained by atomic scale simulations while the second part we present an infrared spectroscopy study of the evolution of defects concentrations after annealing at different temperatures. The atomic scale simulations have been performed within the density functional theory and they allowed us to compute the formation energies and the migration and recombination barriers. The defects included in our study are: the atomic and diatomic interstitials, the hydrogenated vacancies and multivacancies and the several platelets models. The obtained energies allowed us to build a stability hierarchy for these types of defects. This scheme has been confronted with some infrared analysis on hydrogen implanted silicon samples (37 keV) in a sub-dose regime which does not allow usually the formation of platelets during the implantation step. The analysis of the infrared data allowed the detailed description of the defects concentration based on the behaviour of peaks corresponding to the respective defects during annealing. The comparison between these evolutions and the energy scheme obtained previously allowed the validation of an evolution scenario of defects towards the platelet state
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Messina, Luca. "Multiscale modeling of atomic transport phenomena in ferritic steels." Doctoral thesis, KTH, Reaktorfysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-177525.
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Defect-driven transport of impurities plays a key role in the microstructure evolution of alloys, and has a great impact on the mechanical properties at the macroscopic scale. This phenomenon is greatly enhanced in irradiated materials because of the large amount of radiation-induced crystal defects (vacancies and interstitials). For instance, the formation of nanosized solute clusters in neutron-irradiated reactor pressure vessel (RPV) ferritic steels has been shown to hinder dislocation motion and induce hardening and embrittlement. In Swedish RPV steels, this mechanical-property degradation is enhanced by the high content of manganese and nickel impurities. It has been suggested that the formation of Mn-Ni-rich clusters (which contain also Cu, Si, and P) might be the outcome of a dynamic process, where crystal defects act both as nucleation sites and solute carriers. Solute transport by point defects is therefore a crucial mechanism to understand the origin and the dynamics of the clustering process. The first part of this work aims at modeling solute transport by point defects in dilute iron alloys, to identify the intrinsic diffusion mechanisms for a wide range of impurities. Transport and diffusion coefficients are obtained by combining accurate ab initio calculations of defect transition rates with an exact mean-field model. The results show that solute drag by single vacancies is a common phenomenon occurring at RPV temperature (about 300 °C) for all impurities found in the solute clusters, and that transport of phosphorus and manganese atoms is dominated by interstitial-type defects. These transport tendencies confirm that point defects can indeed carry impurities towards nucleated solute clusters. Moreover, the obtained flux-coupling tendencies can also explain the observed radiation-induced solute enrichment on grain boundaries and dislocations. In the second part of this work, the acquired knowledge about solute-transport mechanisms is transferred to kinetic Monte Carlo (KMC) models, with the aim of simulating the RPV microstructure evolution. Firstly, the needed parameters in terms of solute-defect cluster stability and mobility are calculated by means of dedicated KMC simulations. Secondly, an innovative approach to the prediction of transition rates in complex multicomponent alloys is introduced. This approach relies on a neural network based on ab initio-computed migration barriers. Finally, the evolution of the Swedish RPV steels is simulated in a "gray-alloy" fashion, where impurities are introduced indirectly as a modification of the defect-cluster mobilities. The latter simulations are compared to the experimental characterization of the Swedish RPV surveillance samples, and confirm the possibility that solute clusters might form on small interstitial clusters. In conclusion, this work identifies from a solid theoretical perspective the atomic-transport phenomena underlying the formation of embrittling nanofeatures in RPV steels. In addition, it prepares the ground for the development of predictive KMC tools that can simulate the microstructure evolution of a wide variety of irradiated alloys. This is of great interest not only for reactor pressure vessels, but also for many other materials in extreme environments.QC 20151123
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Ghorbani, Nohadanimoghaddam Elaheh [Verfasser]. "Atomic scale defects in thin film solar cell materials from ab initio calculations / Elaheh Ghorbani Nohadanimoghaddam." Mainz : Universitätsbibliothek Mainz, 2016. http://d-nb.info/111234246X/34.
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Gehrmann, Jan. "Transferable reduced TB models for elemental Si and N and binary Si-N systems." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:002b0c99-0e9d-4d8c-a0dc-ad07383f083f.
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Silicon nitride is a bulk and a coating material exhibiting excellent mechanical properties. The understanding of the complex processes at the nanometre scale gained through experimental research will be enhanced by the existence of a computationally efficient and accurate model that is able to describe the mechanical properties of silicon nitride. Such a model has yet to be proposed. In this thesis we present a transferable reduced tight-binding (TB) model for the silicon nitride system. More precisely, this model consists of a reduced TB model for elemental silicon, a reduced TB model for elemental nitrogen, and a reduced TB model for silicon nitride. These models are developed within the framework of coarse-graining the electronic structure from density functional theory (DFT) to tight binding (TB) to bond-order potentials (BOPs), and can therefore be used in the future as the stepping stone to develop BOPs for the application in large scale simulations. The bond integrals employed in the reduced TB models are obtained directly from mixed-basis DFT projections of wave functions onto a minimal basis of atom-centred orbitals. This approach reduces the number of overall parameters to be fitted and provides models which are transferable through the different coarse-graining levels. We provide an example by using the same bond integrals in the reduced TB model for silicon and the preliminary bond-based BOP for silicon. DFT binding energies of ground state and metastable crystal structures are used as the benchmark to which the TB and BOP repulsive parameters are fitted. In addition to model development, we present an improved methodology when going from TB to reduced TB. By weighting all four σ TB bond integrals equally, we provide a new parameterisation (Eqs. (2.73) and (2.74)) and show that the quality of the silicon reduced TB model can be increased by choosing one of the reduced TB parameters to be distance invariant. The ingredients, the development methodology, and the quality of each of the four models are discussed in a separate chapter. The quality of the reduced TB models and BOP is demonstrated by comparing their predictions for the binding energies, heats of formation, elastic constants, and defect energies with DFT and experimental values.
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Chow, Ernest Ho Hin. "The study of organic crystals by atomic force microscopy." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/245569.
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Organic crystals are found in everyday goods such as foods, drugs, dyes, and agricultural products. To better understand the solid-state behaviour of organic crystals, the study of their surfaces is crucial, as several reactions occur at the interface between the crystal surface and its immediate environment. While atomic force microscopy (AFM) is a useful tool for studying surfaces, it is not a common technique for studying organic crystals. The rst part of this report aims to address problems of imaging organic crystals by AFM which arise from the nature of the imaging technique and the material property of organic crystals. Methods of detecting and predicting the likelihood of the problems encountered in imaging organic crystals are suggested in order for a more accurate interpretation of the information obtained by this technique. The e ect of humidity on aspirin crystal surfaces is then investigated by AFM. The growth of new features on the surface is believed to be a result of the hydrolysis of aspirin molecules. Mechanisms are suggested based on the observed surface response of aspirin, where surface defects and the mobility of surface molecules are believed to be important factors a ecting reactivity. The last section investigates the solid-state photochemical reaction of anthracene, which is a reaction that should not occur according to the topochemical postulate. The surface response of anthracene crystals to UV light was studied, and the results indicate strong reactivity at sites of surface defects, which is likely due to photodimerisation. A similar mechanism that described the behaviour of aspirin surfaces was suggested for this reaction. In summary, both reactions that were studied provided a better insight towards understanding the solid-state reactivity of organic crystals. The proposed surface mechanisms imply that surface defects and the presence of humidity or solvent vapour are very likely to play a role in determining reactivity. Further studies on the origin of defects are suggested in order to better control the behaviour of organic crystals in the solid-state.
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Evans, Geraint Andrew. "Characterisation of point defects in SiC by microscopic optical spectroscopy." Thesis, University of Bristol, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.391196.
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Loth, Sebastian. "Atomic Scale Images of Acceptors in III-V Semiconductors." Doctoral thesis, Göttingen Univ.-Verl. Göttingen, 2007. http://hdl.handle.net/11858/00-1735-0000-0006-B466-2.
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Rashetnikava, Alena, Alexander Germanov, Irina Valikova, and Andrei Nazarov. "Molecular dynamics simulation of atomic structure in the vicinity of point defects in FCC and BCC metals." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-190156.
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Rashetnikava, Alena, Alexander Germanov, Irina Valikova, and Andrei Nazarov. "Molecular dynamics simulation of atomic structure in the vicinity of point defects in FCC and BCC metals." Diffusion fundamentals 11 (2009) 52, S. 1-2, 2009. https://ul.qucosa.de/id/qucosa%3A14014.
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Chen, Jhewn-Kuang. "The role of defects during precipitate growth in a Ni-45wt% Cr alloy." Diss., This resource online, 1995. http://scholar.lib.vt.edu/theses/available/etd-06062008-162241/.
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Nedelkoski, Zlatko. "The atomic and spin-electronic structure of interfaces and extended structural defects in the Co-based full Heusler alloys." Thesis, University of York, 2017. http://etheses.whiterose.ac.uk/16736/.
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The atomic and spin-electronic structure of interfaces and extended structural defects in the Co-based full Heusler alloys is studied. Interfaces between a half-metallic Heusler alloy and metal or semiconductor are fundamental and determine the performance of spintronic devices such as spin valves or devices for spin injection applications. It is shown that for the Co2MnSi/Ag bulk-like terminated interfaces, the interfacial spin-polarisation significantly depends on the atomic plane termination. In addition, on the example of experimentally realised interface, part of a spin valve, it is demonstrated that there is an additional monolayer at the interface, which as shown by the density functional theory calculations can create significantly negative local spin-polarisation, detrimental for the device performance. It is demonstrated that the interfaces between the Heusler alloy and Si, suffer from large interfacial interdiffusion which leads to a gradual decrease of magnetic moment over 2-3 nm region in which the spin-polarisation is also significantly affected. It is shown that even sharp interfaces are not desirable since they lead to reversed spin-polarisation. However, it is demonstrated that the addition of thermodynamically stable Si-Co-Si monolayer provides very high spin-polarisation across all interface layers. An ideal candidate for spin injection applications is found to be the Co2FeAl0.5Si0.5/Ge interface which shows very minor and atomic plane selective interdiffusion that does not affect film’s half-metallic properties, absence of formation of any secondary phases and almost no interfacial strain. Based on models derived from electron microscopy observations, it is demonstrated that this interface retains very high interfacial spin-polarisation. Finally, the atomic structure of an extended structural defect observed in Co2FeAl0.5Si0.5 thin film is revealed by electron microscopy. The performed density functional theory modelling shows that these boundaries reverse the sign of spin-polarisation, hence their presence has to be minimised in order to achieve films with better properties.
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Hardie, Christopher David. "Micro-mechanics of irradiated Fe-Cr alloys for fusion reactors." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:a3ac36ba-ca6f-4129-8f37-f1278ef8a559.
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In the absence of a fusion neutron source, research on the structural integrity of materials in the fusion environment relies on current fission data and simulation methods. Through investigation of the Fe-Cr system, this detailed study explores the challenges and limitations in the use of currently available radiation sources for fusion materials research. An investigation of ion-irradiated Fe12%Cr using nanoindentation with a cube corner, Berkovich and spherical tip, and micro-cantilever testing with two different geometries, highlighted that the measurement of irradiation hardening was largely dependent on the type of test used. Selected methods were used for the comparison of Fe6%Cr irradiated by ions and neutrons to a dose of 1.7dpa at a temperature of 288°C. Micro-cantilever tests of the Fe6%Cr alloy with beam depths of 400 to 7000nm, identified that size effects may significantly obscure irradiation hardening and that these effects are dependent on radiation conditions. Irradiation hardening in the neutron-irradiated alloy was approximately double that of the ion-irradiated alloy and exhibited increased work hardening. Similar differences in hardening were observed in an Fe5%Cr alloy after ion-irradiation to a dose of 0.6dpa at 400°C and doses rates of 6 x 10-4dpa/s and 3 x 10-5dpa/s. Identified by APT, it was shown that increased irradiation hardening was likely to be caused by the enhanced segregation of Cr observed in the alloy irradiated with the lower dose rate. These observations have significant implications for future fusion materials research in terms of the simulation of fusion relevant radiation conditions and micro-mechanical testing.
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Baimpas, Nikolaos. "'Hybrid' non-destructive imaging techniques for engineering materials applications." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:1aa00fed-34e6-4a5e-951b-c710e21ac23c.
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The combination of X-ray imaging and diffraction techniques provides a unique tool for structural and mechanical analysis of engineering components. A variety of modes can be employed in terms of the spatial resolution (length-scale), time resolution (frequency), and the nature of the physical quantity being interrogated. This thesis describes my contributions towards the development of novel X-ray “rich” imaging experimental techniques and data interpretation. The experimental findings have been validated via comparison with other experimental methods and numerical modelling. The combination of fast acquisition rate and high penetration properties of X-ray beams allows the collection of high-resolution 3-D tomographic data sets at submicron resolution during in situ deformation experiments. Digital Volume Correlation analysis tools developed in this study help understand crack propagation mechanisms in quasi-brittle materials and elasto-plastic deformation in co-sprayed composites. For the cases of crystalline specimens where the knowledge of “live” or residual elastic strain distributions is required, diffraction techniques have been advanced. Diffraction Strain Tomography (DST) allows non-destructive reconstruction of the 2-D (in-plane) variation of the out-of-plane strain component. Another diffraction modality dubbed Laue Orientation Tomography (LOT), a grain mapping approach has been proposed and developed based on the translate-rotate tomographic acquisition strategy. It allows the reconstruction of grain shape and orientation within polycrystalline samples, and provides information about intragranular lattice strain and distortion. The implications of this method have been thoroughly investigated. State-of-the-art engineering characterisation techniques evolve towards scrutinising submicron scale structural features and strain variation using the complementarity of X-ray imaging and diffraction. The first successful feasibility study is reported of in operando stress analysis in an internal combustion engine. Finally, further advancement of ‘rich’ imaging techniques is illustrated via the first successful application of Time-of-Flight Neutron Diffraction Strain (TOF-NDST) tomography for non-destructive reconstruction of the complete strain tensor using an inverse eigenstrain formulation.
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Guttman, Jeremy. "Polymer-based Tunnel Diodes Fabricated using Ultra-thin, ALD Deposited, Interfacial Films." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1469125487.
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Jackson, Christine M. "Correlations of Electronic Interface States and Interface Chemistry on Dielectric/III Nitride Heterostructures for Device Applications." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu15257361319909.
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Willke, Philip [Verfasser], Martin [Akademischer Betreuer] Wenderoth, Martin [Gutachter] Wenderoth, Konrad [Gutachter] Samwer, and Rolf [Gutachter] Möller. "Atomic-scale transport in graphene: the role of localized defects and substitutional doping / Philip Willke ; Gutachter: Martin Wenderoth, Konrad Samwer, Rolf Möller ; Betreuer: Martin Wenderoth." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2017. http://d-nb.info/1149958421/34.
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Reiche, Rainer. "Elektronenspektroskopie und Faktoranalyse zur Untersuchung von ionenbeschossenen Metall (Re, Ir, Cr, Fe)-Silizium-Schichten." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2000. http://nbn-resolving.de/urn:nbn:de:swb:14-994160101234-34968.
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Pohl, Johan [Verfasser], Karsten [Akademischer Betreuer] Albe, and Hans-Werner [Akademischer Betreuer] Schock. "Structure and properties of defects in photovoltaics absorber material: Atomic scale computer simulations of Si and Cu(In,Ga)Se2 / Johan Pohl. Betreuer: Karsten Albe ; Hans-Werner Schock." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2013. http://d-nb.info/1112268480/34.
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Newman, Nicholas C. "Traffic Related Air Pollution Exposure in the First Year of Life and Hyperactivity at Age Seven in a High Risk Atopic Birth Cohort." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1312293718.
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Isik, Mehmet. "Optical And Electrical Characterization Of Ga0.75in0.25se Layered Single Crystals." Phd thesis, METU, 2013. http://etd.lib.metu.edu.tr/upload/12615314/index.pdf.
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In the present thesis, optical and electrical properties of Ga0.75In0.25Se layered single crystals have been studied.
The optical properties of the crystals have been investigated by means of visible and infrared reflectivity and transmittance, ellipsometry, Raman spectroscopy, photoluminescence (PL) and thermoluminescence (TL) measurements. The analysis of the absorption data at room temperature revealed the existence of indirect transitions in the crystal. Moreover, the rate of change of the band gap energy with temperature was calculated from the analysis of the temperature dependence of transmission measurements. The spectroscopic ellipsometry measurements on Ga0.75In0.25Se crystals were also performed to get detailed information about the real and imaginary parts of the pseudodielectric function, pseudorefractive index and pseudoextinction coefficient. The critical point analysis of the second derivative spectra of the dielectric function was done to reveal the interband transition energies. The vibrational spectra of Ga0.75In0.25Se crystals were studied by means of infrared reflectivity and transmittance and Raman scattering. The refractive and absorption indices, the frequencies of transverse and longitudinal optical modes, high- and low-frequency dielectric constants were obtained from the analysis of the IR reflectivity spectra. PL experiments were carried out as a function of temperature and excitation laser intensity to get detailed knowledge about the recombination levels in Ga0.75In0.25Se crystals. The observed emission bands in PL spectra were interpreted as the transitions from donor levels to an acceptor level.
Electrical characterization of the crystal have been performed using dark electrical conductivity, space charge limited current, photoconductivity and thermally stimulated current (TSC) measurements. The detailed information about the localized levels in the band gap has been obtained from the analysis. The photoconductivity measurements were performed to determine the dominant recombination mechanism in the crystal.
Defect centers in the crystal were characterized from TSC and TL measurements accomplished in the low temperature range. The activation energies, attempt-to-escape frequencies, concentrations and capture cross sections of the traps were calculated from the analysis of the experimental data.
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Reiche, Rainer. "Elektronenspektroskopie und Faktoranalyse zur Untersuchung von ionenbeschossenen Metall (Re, Ir, Cr, Fe)-Silizium-Schichten." Doctoral thesis, Technische Universität Dresden, 1999. https://tud.qucosa.de/id/qucosa%3A24738.
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