The functional and mechanical properties of a material are often determined by the defects in them. A thorough understanding of the relationship between the defects and the properties allows for tailoring a material’s properties into the desired combinations. Amongst the different classes of defects, experimental identification of point defects is typically difficult and indirect, usually requiring an ingenious combination of different techniques. In this context, first-principles calculations, complemented with experiments, offer insights into the formation of defects and their role in properties. This was demonstrated in this thesis through investigations on the effect of calcium vacancies on structure, vibrational and elastic properties hydroxyapatite (HAp), and oxygen vacancies on elastic properties of zinc oxide (ZnO) using first-principles calculations based on density functional theory (DFT). Our results confirm a considerable reduction in the elastic constants of HAp—the inorganic constituent of bone—due to Ca-deficiency, which was experimentally reported earlier. Elastic anisotropic behavior of stoichiometric and Ca-deficient HAp is analyzed, which will be useful in understanding the effects of crystal orientation in designing synthetic bone. Local structural stability of HAp and Ca-deficient HAp structures is assessed with full phonon dispersion studies and the specific signatures in the computed vibrational spectra for Ca deficiency in HAp can be utilized in experimental characterization of different types of defected HAp. In ZnO, formation energies of oxygen vacancies in different types of oxygen deficient structures are analyzed to ascertain their stability. Our results show considerable degradation of some of the elastic moduli due to the presence of such vacancies. Further, the charge state of the defect structure is found to influence the shear elastic constants. Evaluation of elastic anisotropy of stoichiometric and oxygen deficient ZnO indicates the significant anisotropy in elastic properties and stiff c-axis orientation. The second part of the thesis deals with developing some understanding of the pressure-induced phase transformations (PIPT) in an inorganic material, titanium nitride (TiN), and in a metal-organic framework (MOF), erbium formate crystal. PIPT, which is a common phenomenon in many materials, is of great interest in materials science as the properties of the transformation product can diverge significantly from those of the parent phase. Hence, it is important to understand the pressure induced changes so to improve the component reliability and to enhance service life of materials used in high pressure applications. TiN undergoes PIPT from NaCl to CsCl structure. On the basis of our DFT calculations, we propose a new transformation path, which shows that the stress required for this transformation is substantially lower when it is deviatoric in nature than that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. Further, we provide a quantitative comparison of electronic structure of TiN in NaCl structure with that of high pressure phase with implication to electrical conduction properties. Next, we investigate the PIPT associated with bond rearrangement in erbium formate framework. Phase transition pressure is estimated and the corresponding changes in bonding characteristics are analyzed. Estimated lattice constants for both the phases agree well with the earlier experimental results. While the transformation pressure of the framework is overestimated with respect to experiment, our calculations confirm PIPT, and thus provide a theoretical evidence for the experimental finding.

No description available.

(U,Pu)O2 (aussi appelé MOX) est actuellement utilisé comme combustible dans les réacteurs nucléaires à eau pressurisée (REP) avec une teneur massique en Pu d’environ 10 %. Il est également envisagé comme combustible de référence pour les réacteurs à neutrons rapides à caloporteur sodium, avec une teneur massique en Pu d’environ 25 %. En conditions opérationnelles, (U,Pu)O2 est soumis à des réactions de fission qui génèrent une grande quantité de défauts et de produits de fission. Par migration, ces défauts et produits de fission gazeux peuvent s'agréger en nano-cavités, dislocations et bulles de gaz, conduisant à une modification de la microstructure. Une meilleure description du comportement du combustible à l’échelle atomique, notamment des mécanismes élémentaires impliqués dans la diffusion des défauts et des produits de fission, est donc nécessaire pour affiner les modèles utilisés dans les codes de performance des combustibles. Pour l’étude des propriétés de (U,Pu)O2, nous avons effectué des calculs de structure électronique basés sur la méthode DFT+U combinée au contrôle des matrices d’occupation des orbitales corrélées. Des minimisations d’énergie ainsi que la dynamique moléculaire ab initio ont été utilisées. Nous avons étudié dans un premier temps les propriétés du cristal de (U,Pu)O2 pour différentes teneurs en Pu. Nous avons ensuite étudié la stabilité des défauts ponctuels ainsi que les modifications structurales et électroniques induites par ces défauts ponctuels dans (U,Pu)O2 et (U,Ce)O2, matériau utilisé comme simulant de (U,Pu)O2. Enfin, nous avons étudié le piégeage et la solubilité des gaz de fission (Kr, Xe) et de l’hélium dans la matrice de (U,Pu)O2 / (U,Pu)O2 (commonly called MOX) is currently used as nuclear fuel in pressurized water reactors with a Pu content of around 10 wt.%, and is envisaged as the reference fuel in Generation IV sodium fast reactors (SFR) with a Pu content of around 25 wt.%. Under operation, (U,Pu)O2 is submitted to fission reactions which generate a large quantity and variety of point defects, as well as fission products. By migrating, point defects and gaseous fission products can aggregate into nano-voids, dislocations and fission gas bubbles, which lead to the modification of the fuel microstructure. Therefore, a better description of the fuel behaviour at the atomic scale, and especially of the elementary mechanisms involved in the diffusion of point defects and fission products, is necessary to refine the models used in the fuel performance codes used to simulate the behaviour of fuels at the macroscopic scale. We use electronic structure calculations based on the DFT+U method combined with the occupation matrix control scheme (OMC) to investigate (U,Pu)O2 properties for various Pu contents. Static energy minimizations and ab initio molecular dynamics were used. We have first determined bulk structural, electronic and thermodynamics properties of (U,Pu)O2. We then studied the stability of point defects in (U,Pu)O2 and (U,Ce)O2, as well as the structural and electronic modifications induced by these point defects, in (U,Pu)O2 and the common experimental surrogate (U,Ce)O2. Finally, the fission gas (Kr and Xe) and helium (He) trapping and solubility in (U,Pu)O2 matrix are investigated

Ein fragment-basierter Ansatz zur Berechnung von vertikalen Anregungsenergien in periodischen Systemen wurde entwickelt. Das Ziel war eine wellenfunktions-basierte Hierarchie von lokalen post-Hartree-Fock Methoden, welche über das weitverbreitete Ein-Elektronen Bild der Bandlücke hinausgehen und eine Möglichkeit zur systematischen Verbesserung der Ergebnisse liefern. Darüber hinaus sollte durch die Verwendung von lokalen Orbitalen eine nahtlose Einbettung des Fragments ermöglicht und eine effektive Methode für die Untersuchung von Defekten in periodischen Systemen geschaffen werden. Als erster Schritt wird das fragment-basierte Configuration Interaction Singles (CIS) Model vorgestellt. Im Anschluss erfolgt der Wechsel zum fragment-basierten lokalen algebraic-diagrammatic construction Modells zweiter Ordnung (DF-LADC(2)). Beide Methoden wurden für ein neutrales Farbzentrum in Magnesiumoxid (MgO) getestet. Dabei wurden Fragmente mit bis zu 57 Atomen verwendet. Eine Konvergenz mit der Fragmentgröße, der Größe der Superzellen und des K-mesh konnte erreicht werden. Dennoch wurde eine erste Anregungsenergie von 5.9 eV erhalten, was 0.9 eV über dem veröffentlichten experimentellen Wert liegt. Mit hoher Wahrscheinlichkeit rührt die Abweichung vom Basissatzvollständigkeitsfehler her. ”Finite-Cluster”-Berechnungen bestätigen entsprechende Basissatzfehler. Interessanterweise stimmt die erste Anregungsenergie für ein Oberflächenfarbzentrum in MgO mit einigen experimentellen Werten überein. Allerdings decken die experimentellen Werte für diese Systeme einen weiten Bereich ab (1.15 - 4.2 eV). / An embedded-fragment approach for calculation of vertical excitation energies in periodic systems has been developed. The aim is a wave-function-based hierarchy of local post-Hartree-Fock models, which goes beyond the very common one-electron picture of the band gap and offers a way for systematic improvability of the results. The use of local occupied and virtual orbitals allows for a seamless embedding model for the fragment and becomes especially effective in studying defects in solids. As a first step in the hierarchy an embedded-fragment Configuration Interaction Singles (CIS) model is presented. The second step is an embedded-fragment local algebraic diagrammatic construction scheme of second order (DF-LADC(2)). Both methods are tested for an neutral color center in bulk and surface magnesium oxide (MgO). Different fragments with up to 57 atoms were studied. A convergence with fragment size, super-cell size and k-mesh has been achieved. However a first excitation energy of 5.9 eV is obtained for the bulk MgO, which is 0.9 eV above the reported experimental value. The deviation most likely originates from the basis set incompleteness error, which, according to finite cluster studies, can be sizable. Interestingly for a surface color center in MgO the observed first excitation energy of 4.1 eV agrees with some of the experimental values (4.2 eV). However for the surface color centers in MgO the scatter of the experimental results is very large (1.15 eV - 4.2 eV).

<p dir="ltr">Advancements in material science are accelerating technological evolution, driven by initiatives like the Materials Genome Project, which integrates computational and experi- mental strategies to expedite material discovery. In this work, we focus on the reliability of advanced materials under extreme conditions, a critical area for enhancing their technological applications.</p><p dir="ltr">Multi-principal component alloys (MPEAs) exhibit remarkable properties under extreme conditions. However, their vast compositional space makes a brute-force exploration of potential alloys prohibitive. We address this challenge by employing a Bayesian approach to explore the oxidation resistance of hundreds of alloys, applying computational techniques to accurately calculate and quantify errors in the melting temperatures of MPEAs, and investigating the compositional biases and short-range order in their nucleation behaviors.</p><p dir="ltr">Furthermore, we scrutinize the role of wide bandgap semiconductors, which are essential in high-power applications due to their superior breakdown voltage, drift velocity, and sheet charge density. The lack of lattice-matched substrates often results in strained films, which enhances piezoelectric effects crucial for device reliability. Our research advances the pre- diction of piezoelectric and dielectric responses as influenced by biaxial strain and doping in gallium nitride (GaN). Additionally, we delve into how various common defects affect the formation of trap states, significantly impacting the electronic properties of these materials. These studies offer significant advancements in understanding MPEAs and wide bandgap semiconductors under extreme conditions. We also provide foundational insights for developing robust and efficient materials essential for next-generation applications.</p>

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2019-03-14T19:25:14Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Renan Augusto Pontes Ribeiro.pdf: 9570923 bytes, checksum: a291ba63c045a11cb0a642a480367e27 (MD5) / Made available in DSpace on 2019-03-14T19:25:14Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Renan Augusto Pontes Ribeiro.pdf: 9570923 bytes, checksum: a291ba63c045a11cb0a642a480367e27 (MD5) Previous issue date: 2019-02-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O desenvolvimento da spintrônica tem motivado a busca por novos materiais multiferróicos devido à multifuncionalidade desses compostos associada ao acoplamento entre diferentes ordens ferróicas em uma estrutura cristalina. No presente estudo, propomos a investigação teórica, baseada na Teoria do Funcional de Densidade, dos materiais ATiO3 (A = Mn, Fe, Ni) na estrutura R3c com objetivo de esclarecer o efeito da substituição do cátion A sobre as propriedades estruturais, magnéticas e eletrônicas, bem como descrever diferentes mecanismos de controle das propriedades multiferróicas baseados em arquiteturas de filmes-finos, morfologia e controle de defeitos intrínsecos. Para uma maior compreensão dos efeitos envolvidos nos materiais ATiO3, diferentes funcionais de troca e correlação foram investigados e o funcional PBE0 apresentou os menores desvios, consequentemente, a melhor representação comparado aos resultados experimentais. Com objetivo de investigar as propriedades conectadas a filmes-finos dos materiais ATiO3, propomos uma metodologia inovadora que permite descrever as deformações uni- e biaxial que se originam na região de interface entre o filme e o substrato. Nesse caso, os resultados obtidos indicam que as distorções estruturais induzem uma transição magnética para o NiTiO3, originando ordenamento ferromagnético a partir de um critério magneto-estrutural associado a deformação dos clusters [MO6] que reproduz satisfatoriamente os resultados experimentais reportados na literatura. De modo análogo, para elucidar a relação entre o magnetismo e a morfologia dos materiais ATiO3, combinamos cálculos de Energia de Superfície, Construção de Wulff e um formalismo avançado para descrever o magnetismo superficial considerando a existência de spins não compensados ao longo dos planos polares (100), (001), (101), (012), (111) e apolares (110). Os resultados indicam que a redução do número de coordenação dos metais A e Ti para os planos (001) e (111) resulta na transferência de carga entre os cátions A2+ e Ti4+, originando espécies Ti3+ magnéticas que aumentam o magnetismo superficial ao longo desses planos. Além disso, esse efeito é capaz de induzir uma alteração do caráter eletrônico para esses materiais, permitindo indicar que a clivagem das superfícies contribui para o controle das propriedades eletrônicas, reduzindo o valor de band-gap ou gerando comportamento meio-metálico. Os mapas morfológicos obtidos indicam que o controle da exposição majoritária do plano (001) para obtenção de discos hexagonais induz um aumento do magnetismo superficial para os materiais ATiO3 em acordo com resultados experimentais, além de predizer diferentes morfologias acessíveis com interessantes propriedades magnéticas. Ademais, o efeito de defeitos intrínsecos como vacâncias de oxigênio no bulk e superfície apolar (110) dos materiais ATiO3 foi investigado indicando que a redução do número de coordenação na região do defeito induz que os elétrons remanescentes sejam localizados, principalmente, nos orbitais 3d vazios dos cátions Ti vizinhos, gerando espécies [TiO5]ꞌ e [TiO4]ꞌ (3d1 ) que possibilitam uma interação ferromagnética nos materiais MnTiO3 e FeTiO3. A combinação entre os diferentes mecanismos investigados permitiu estabelecer um guia científico para o estudo teórico de materiais multiferróicos, contribuindo para descrever as potencialidades dos diferentes materiais bem como predizer novos candidatos. / The development of spintronic has motivated the search for new multiferroic materials due to the multifunctionality of these materials that are associated with the coupling of different ferroic orders into a single crystalline structure. In the present study, we propose a theoretical investigation, based on Density Functional Theory, of ATiO3 (A = Mn, Fe, Ni) materials in the R3c structure in order to clarify the effect of A-site cation replacement on the structural, magnetic and electronic properties, as well as to describe a different mechanism to control the multiferroic properties based on thin-film architectures, morphology and point defects. For a more comprehensive overview of the main effects involved on the ATiO3 materials several exchange-correlation functionals were investigated, being the PBE0 the functional with smallest deviations and, consequently, the best representation in comparison to the experimental results. Aiming to describe the main fingerprints related with the creation of ATiO3 thin-films, we propose an innovative methodology that allows to describe the uniaxial and biaxial deformations originated in the interface region between the film and the substrate. In this case, the results indicate that structural distortions induce a magnetic transition for the NiTiO3, originating ferromagnetic ordering from magneto-structural criteria, which is associated to the deformation of the [MO6] clusters that reproduces satisfactorily the experimental results reported in the literature. Similarly, in order to elucidate the relationship between the magnetism and the morphology of the ATiO3 materials, we combined Surface Energy, Wulff Construction, and an advanced formalism to describe surface magnetism by considering the existence of uncompensated spins along the polar planes (100), (001), (101), (012), (111) and non-polar (110). The results indicate that the reduction of the coordination for both A and Ti metals along the (001) and (111) planes induces a charge transfer between the A 2+ and Ti4+ cations, resulting in magnetic Ti3+ species that increase the superficial magnetism along such planes. Moreover, this effect allowed a change in the electronic structure for these materials, allowing to point out that the cleavage of the surfaces contribute to the control of the electronic properties reducing the band-gap value or generating half-metallic behavior. The morphological maps indicated that the control of the major exposure for the (001) surface to obtain hexagonal discsinduces an increase of the superficial magnetism for the ATiO3 materials according to experimental results, besides predicting different accessible morphologies with interesting magnetic properties. In addition, the effect of intrinsic defects such as oxygen vacancies on the bulk and non-polar (110) surface of the ATiO3 materials were investigated, indicating that the reduction of coordination in the defect region induces the localization of the remaining electrons in the empty 3d orbitals of neighboring Ti cations, generating [TiO5]'and [TiO4]' (3d1 ) species that allow a ferromagnetic interaction for MnTiO3 and FeTiO3 materials. The combination of the different mechanisms investigated has allowed to stablish a scientific guide for the theoretical study of multiferroic materials, contributing to describe the potentialities of the different materials as well as to predict new candidates.

Les défauts ponctuels dans les semi-conducteurs sont étudiés pour la réalisation de bits quantiques d’information (Qubit). A ce jour, le système le plus développé est le centre NV dans le diamant. Récemment, les défauts ponctuels du carbure de silicium (SiC) ont été identifiés comme prometteurs pour la réalisation de Qubit en raison de leur long temps de cohérence de spin et du fonctionnement à température ambiante. Dans ce contexte, nous étudions la formation, la caractérisation optique et magnétique des défauts ponctuels dans SiC, ainsi que l’amélioration de la collection de leur luminescence. Nous commençons par une description des différents critères qui font du SiC un matériau clé pour les applications Qubit. Ensuite, nous présentons une étude bibliographique sur les principaux défauts ponctuels dans SiC en nous focalisant sur les centres : VSi, VSiVC, NV. Nous portons par la suite notre étude sur les conditions optimales d’irradiation ioniques/électroniques et de recuit post-irradiation pour la formation de défauts ponctuels luminescents dans le polytype cubique de SiC. Nous avons identifié les différents types de défauts dans le visible. Dans l’infra-rouge, nous n’avons détecté que le centre VSiVC en trouvant les conditions optimales de sa luminescence dans le cas d’implantation par les protons (dose 1016 cm-2 et le recuit à 750 °C). Puis, nous avons comparé les résultats obtenus par des irradiations aux électrons à ceux obtenus avec les protons en précisant les différents types de défauts ponctuels détectés par deux méthodes: la photoluminescence et la résonance paramagnétique électronique. Enfin, nous avons développé un processus technologique qui consiste en la fabrication de nano-piliers en SiC-4H. Nous avons montré les avantages de leur réalisation sur l’efficacité de la collection de PL des défauts ponctuels comme VSi et VSiVC. Une amélioration d’un facteur 25 pour le centre VSi et d’un facteur 50 pour le centre VSiVC a été obtenue. / Point defects in semiconductor materials are studied for the realization of quantum information bits (Qubit). Nowadays, the most developed system is based on the NV center in diamond. Recently, point defects in silicon carbide (SiC) have been identified as promising for the realization of Qubit due to the combination of their long spin coherence time and room temperature operation. In this context, this thesis studies the formation, optical and magnetic characterization of point defects in SiC, as well as the improvement of their luminescence collection. We begin with a general introduction to SiC in which we describe the different criteria that make SiC a key material for Qubit applications. Next, we present a bibliographical study on the main point defects in SiC, focusing on the centers: VSi, VSiVC, NV. We have studied the optimal conditions of ionic/electronic irradiation and post-irradiation annealing for the formation of luminescent point defects in the cubic polytype of SiC. We have identified the different types of visible range defects. In the infra-red range, we detected only the Ky5 center (VSiVC) by finding the optimal luminescence conditions of this center in the case of implantation by protons (dose 1016 cm-2 and annealing at 750 °C). Then, we compared the results obtained by electron irradiations with those obtained with protons specifying the different types of point defects detected by two methods: photoluminescence and electronic paramagnetic resonance. Finally, we have developed a technological process that consists of nano-pillars fabrication in SiC-4H. We have shown the advantages of realizing these pillars on the efficiency of the PL collection of point defects like VSi and VSiVC : an improvement of a factor of 25 for the VSi center and a factor of 50 for the VSiVC center was obtained.

No description available.

Les imageurs CMOS représentent un outil d’avenir pour de nombreuses applications scientifiques de haut vol, tellesque l’observation spatiale ou les expériences nucléaires. En effet, ces imageurs ont vu leurs performancesdémultipliées ces dernières années grâce aux avancées incessantes de la microélectronique, et présentent aussi desavantages indéniables qui les destinent à remplacer les CCDs dans les futurs instruments spatiaux. Toutefois, enenvironnement spatial ou nucléaire, ces imageurs doivent faire face aux attaques répétées de particules pouvantrapidement dégrader leurs performances électro-optiques. En particulier, les protons, électrons et ions présents dansl’espace ou les neutrons de fusion nucléaire peuvent déplacer des atomes de silicium dans le volume du pixel et enrompre la structure cristalline. Ces effets de déplacement peuvent former des défauts stables introduisant des étatsd’énergie dans la bande interdite du silicium, et ainsi conduire à la génération thermique de paires électron-trou. Parconséquent, ces radiations non-ionisantes produisent une augmentation permanente du courant d’obscurité despixels de l’imageur et donc à une diminution de leur sensibilité et de leur dynamique. L’objectif des présents travauxest d’étendre la compréhension des effets de déplacement sur l’augmentation du courant d’obscurité dans lesimageurs CMOS. En particulier, ces travaux se concentrent sur l’étude de la forme de la distribution de courantd’obscurité en fonction du type, de l’énergie et du nombre de particules ayant traversé l’imageur, mais aussi enfonction des caractéristiques de l’imageur. Ces nombreux résultats permettent de valider physiquement etexpérimentalement un modèle empirique de prédiction de la distribution du courant d’obscurité pour une utilisationdans les domaines spatial et nucléaire. Une autre partie majeure de ces travaux consiste à utiliser pour la première foisla technique de spectroscopie de courant d’obscurité pour détecter et caractériser individuellement les défautsgénérés par les radiations non-ionisantes dans les imageurs CMOS. De nombreux types de défauts sont détectés etdeux sont identifiés, prouvant l’applicabilité de cette technique pour étudier la nature des défauts cristallins généréspar les effets de déplacement dans le silicium. Ces travaux avancent la compréhension des défauts responsables del’augmentation du courant d’obscurité en environnement radiatif, et ouvrent la voie au développement de modèles deprédiction plus précis, voire de techniques permettant d’éviter la formation de ces défauts ou de les faire disparaître. / CMOS image sensors are envisioned for an increasing number of high-end scientific imaging applications such asspace imaging or nuclear experiments. Indeed, the performance of high-end CMOS image sensors has dramaticallyincreased in the past years thanks to the unceasing improvements of microelectronics, and these image sensors havesubstantial advantages over CCDs which make them great candidates to replace CCDs in future space missions.However, in space and nuclear environments, CMOS image sensors must face harsh radiation which can rapidlydegrade their electro-optical performances. In particular, the protons, electrons and ions travelling in space or thefusion neutrons from nuclear experiments can displace silicon atoms in the pixels and break the crystalline structure.These displacement damage effects lead to the formation of stable defects and to the introduction of states in theforbidden bandgap of silicon, which can allow the thermal generation of electron-hole pairs. Consequently, nonionizingradiation leads to a permanent increase of the dark current of the pixels and thus a decrease of the imagesensor sensibility and dynamic range. The aim of the present work is to extend the understanding of the effect ofdisplacement damage on the dark current increase of CMOS image sensors. In particular, this work focuses on theshape of the dark current distribution depending on the particle type, energy and fluence but also on the imagesensor physical parameters. Thanks to the many conditions tested, an empirical model for the prediction of the darkcurrent distribution induced by displacement damage in nuclear or space environments is experimentally validatedand physically justified. Another central part of this work consists in using the dark current spectroscopy techniquefor the first time on irradiated CMOS image sensors to detect and characterize radiation-induced silicon bulk defects.Many types of defects are detected and two of them are identified, proving the applicability of this technique to studythe nature of silicon bulk defects using image sensors. In summary, this work advances the understanding of thenature of the radiation-induced defects responsible for the dark current increase in space or nuclear environments. Italso leads the way to the design of more advanced dark current prediction models, or to the development ofmitigation strategies in order to prevent the formation of the responsible defects or to allow their removal.