Les films minces luminescents dopés terres rares offrent des propriétés intéressantes pour la conversion spectrale UV-bleu, en particulier pour une meilleure adaptation du spectre solaire aux cellules solaires en silicium. Deux matériaux luminophores ont été étudiés dans cette thèse : Y2O3 dope Eu3+ et CaYAlO4 codopé Ce3+, Pr3+. L'utilisation de ces luminophores pour les applications sous forme de films minces est limitée car ils possèdent un faible coefficient d'absorption. Deux pistes ont été examinées pendant cette thèse pour pallier ce problème : (i) l'effet plasmonique a été étudié en réalisant des films avec une architecture multicouche couplant les films luminophores et des nanostructures métalliques d'aluminium et d'argent qui possèdent une résonnance plasmon dans la gamme UV et bleu respectivement ; (ii) l'effet photonique a été évalué en réalisant une structuration du film luminophore via une croissance sur des membranes macro-poreuses. La méthode de synthèse choisie est le dépôt par ablation laser pulsé / Rare earth luminescent thin film offers attractive properties for down shifting application, particularly for a better adaptation of the solar spectrum to silicon solar cells. In this thesis, we studied two phosphor materials : Y2O3 doped Eu3+ and CaYAlO4 codoped Ce3+, Pr3+. One issue identified for the use of these phosphors as thin films is their low absorption coefficient. We examined two tracks during this thesis to meet these needs : (i) the plasmonic effect was studied by making films with a multilayer architecture coupling the phosphor films and aluminium or silver metallic nanostructures which have a plasmon resonance in the UV range and blue respectively ; (ii) the photonic effect was evaluated by structuring the phosphor layer by self-organization growth on macroporous membranes. We did the syntheses by pulsed laser deposition

Cette thèse présente l'élaboration et l'étude de nanofils ferromagnétiques de cobalt, nickel et d'alliages cobalt-nickel épitaxiés en matrice de titanate de strontium et de baryum. Les fils sont élaborés par auto-assemblage lors de dépôts séquentiels par ablation laser pulsé. Tout d'abord, les paramètres de croissance permettant de contrôler le diamètre des fils et leur densité sont mis en évidence en modélisant la croissance de l'hétéro-structure par simulations Monte-Carlo cinétique. Ensuite, on montre que les fils sont dilatés axialement et relaxés radialement. L'origine de l'état dilaté est expliquée en adaptant le modèle de Frenkel-Kontorova à notre situation et les inhomogénéités de déformation des nanofils sont décrites en analysant des cartographies de l'espace réciproque. La dilatation crée une anisotropie magnétique, par couplage magnéto-élastique, qui, dans le cas du nickel, peut compenser l'anisotropie de forme des fils. Enfin, pour des fils de Co0.4Ni0.6 de diamètre supérieur à quatre nanomètres, la température de blocage de l'assemblée de fils est supérieure à la température ambiante et la barrière d'énergie du renversement magnétique est de l'ordre d'un électronvolt, ce qui est intéressant pour d'éventuelles applications, par exemple en enregistrement de données. / In this PhD thesis, we study the growth and the properties of ferromagnetic nanowires made of cobalt, nickel and cobalt-nickel, embedded in a matrix made of of strontium and baryum titanate. The nanowires are grown taking advantage of self-assembly processes occurring during sequential pulsed laser deposition. First, we model the growth with a kinetic Monte-Carlo code to highlight the parameters that control the diameter and the density of the nanowires. Then, it is shown that the nanowires are strained along their axis, and relaxed perpendicular to it. The origin of the strained state is explained in the framework of the Frenkel-Kontorova model, and its inhomogeneities are described through analysis of mappings of the reciprocal space. Furthermore, it is shown that the strain is high enough to shift the magnetic easy axis of the nickel nanowires, through magneto-elastic coupling. Finally, for Co0.4Ni0.6 nanowires with a diameter greater than four nanometers, the blocking temperature of the assembly is above room temperature and the energy barrier for the magnetic reversal of the nanowires is of the order of one electronvolt. This is interesting for potential applications in data storage, for example.

<p>This study is focused on the prediction of residual stresses and microstructure development of steel and aluminum alloys during laser-based manufacturing processes by means of multi-physics numerical modeling.</p> <p>A finite element model is developed to predict solid-state phase transformation, material hardness, and residual stresses produced during laser-based manufacturing processes such as laser hardening and laser additive manufacturing processes based on the predicted temperature and geometry from a free-surface tracking laser deposition model. The solid-state phase transformational model considers heating, cooling, and multiple laser track heating and cooling as well as multiple layer tempering effects. The residual stress model is applied to the laser hardening of 4140 steel and to laser direct deposition of H13 tool steel and includes the effects of thermal strain and solid-state phase transformational strain based on the resultant phase distributions. Predicted results, including material hardness and residual stresses, are validated with measured values.</p> <p>Two dendrite growth predictive models are also developed to simulate microsegregation and dendrite growth during laser-based manufacturing processes that involve melting and solidification of multicomponent alloys such as laser welding and laser-based additive manufacturing processes. The first model uses the Phase Field method to predict dendrite growth and microsegregation in 2D and 3D. It is validated against simple 2D and 3D cases of single dendrite growth as well as 2D and 3D cases of multiple dendrite growth. It is then applied to laser welding of aluminum alloy Al 6061 and used to predict microstructure within a small domain. </p> The second model uses a novel technique by combining the Cellular Automata method and the Phase Field method to accurately predict solidification on a larger scale with the intent of modeling dendrite growth. The greater computational efficiency of the this model allows for the simulation of entire weld pools in 2D. The model is validated against an analytical model and results in the literature.

This thesis deals with the synthesis and characterization of Ge-Te-Sb (GST) thin films. The films were deposited using a Pulsed Laser Deposition (PLD) method and mainly characterized with XRD, SEM, AFM and TEM. For amorphous and polycrystalline films, un-etched Si(100) was used. The amorphous films showed a similar crystallization behavior as films deposited with sputtering and evaporation techniques. When depositing GST on un-etched Si(100) substrates at elevated substrate temperatures (130-240°C), polycrystalline but highly textured films were obtained. The preferred growth orientation was either GST(111) or GST(0001) depending on if the films were cubic or hexagonal. Epitaxial films were prepared on crystalline substrates. On KCl(100), a mixed growth of hexagonal GST(0001) and cubic GST(100) was observed. The hexagonal phase dominates at low temperatures whereas the cubic phase dominates at high temperatures. The cubic phase is accompanied with a presumed GST(221) orientation when the film thickness exceeds ~70 nm. Epitaxial films were obtained with deposition rates as high as 250 nm/min. On BaF2(111), only (0001) oriented epitaxial hexagonal GST films are found, independent of substrate temperature, frequency or deposition background pressure. At high substrate temperatures there is a loss of Ge and Te which shifts the crystalline phase from Ge2Sb2Te5 towards GeSb2Te4. GST films deposited at room temperature on BaF2(111) were in an amorphous state, but after exposure to an annealing treatment they crystallize in an epitaxial cubic structure. Film deposition on pre-cleaned and buffered ammonium fluoride etched Si(111) show growth of epitaxial hexagonal GST, similar to that of the deposition on BaF2(111). When the Si-substrates were heated directly to the deposition temperature films of high crystal-line quality were obtained. An additional heat treatment of the Si-substrates prior to deposition deteriorated the crystal quality severely. The gained results show that PLD can be used as a method in order to obtain high quality epitaxial Ge-Sb-Te films from a compound target and using high deposition rates.

UV and deep-UV selective photodiodes from visible-blind to solar-blind were realized based on a Si-doped (InxGa1–x)2O3 thin film with a monotonic lateral variation of 0.0035<x<0.83. Such layer was deposited by employing a continuous composition spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. The photo response signal is provided from a metal-semiconductor-metal structure upon backside illumination. The absorption onset was tuned from 4.83 to 3.22 eV for increasing x. Higher responsivities were observed for photodiodes fabricated from indium-rich part of the sample, for which an internal gain mechanism could be identified. VC 2016 AIP Publishing LLC.

In recent years, ultra-wide bandgap semiconductors have increasingly moved into scientific focus due to their outstanding material properties, making them promising candidates for future applications within high-power electronics or solar-blind photo detectors. The group-III-sesquioxides can appear in various polymorphs, which influences, for instance, the energy of the optical bandgap. In gallium oxide, the optical bandgap ranges between 4.6 and 5.3 eV depending on the polymorph. For each polymorph it can be increased or decreased by alloying with aluminum oxide (8.8 eV) or indium oxide (2.7–3.75 eV), respectively, enabling bandgap engineering and thus leading to an extended application field. For this purpose, an overview of miscibility limits, the variation of bandgap and lattice constants as a function of the alloy composition are reviewed for the rhombohedral, monoclinic, orthorhombic and cubic polymorph. Further, the effect of formation and desorption of volatile suboxides on growth rates is described with respect to chemical trends of the discussed ternary materials.

In dieser Arbeit werden das Wachstum und die Charakterisierung der Heterostruktur eines FeFET auf der Grundlage von BaSnO3, LaInO3 und (K,Na)NbO3 Schichten untersucht. Für jedes Material wurden die Wachstumsbedingungen bestimmt und im Hinblick auf die strukturellen und elektrischen Eigenschaften optimiert. Epitaktische BaSnO3 Filme, die auf SrTiO3 Substraten gewachsen sind, weisen eine hohe Dichte an Versetzungen auf, die ihre elektrischen Eigenschaften beeinträchtigen. Die Verwendung von NdScO3 Substraten und Einführung einer SrSnO3 Pufferschicht verbesserten die strukturellen und elektrischen Eigenschaften der BaSnO3 Schichten. Dies ermöglichte schließlich Untersuchungen an der LaInO3/BaSnO3 Grenzfläche. Schon eine geringe La-Dotierung der BaSnO3 Schicht von 0,3 % führte zur Bildung eines 2DEG nach der Grenzflächenbildung und damit zum Einschluss von Elektronen an der Grenzfläche. Dies konnte durch C-V, Van-der-Pauw und Hall-Effekt-Messungen eindeutig nachgewiesen werden. Eine deutliche Verbesserung der strukturellen und elektrischen Eigenschaften der BaSnO3 Schichten wurde durch die Verwendung von LaInO3:Ba Substraten erreicht. Diese sind gitterangepasst an BaSnO3, sodass zum ersten Mal vollständig verspannte Schichten ohne Versetzungen gewachsen werden konnten. Strukturelle und elektrische Eigenschaften von (K,Na)NbO3 Schichten wurden auf SrRuO3/DyScO3 und SrTiO3:Nb-Substraten untersucht. Auf diese Weise wurden der Einfluss der Gitterdehnung auf die kritische Schichtdicke und die Prozesse der plastischen Relaxation des Gitters bestimmt. Die elektrische Charakterisierung ergab einen hohen Leckstrom, der durch strukturelle Defekte verursacht wird. Die gesamte FeFET Heterostruktur wurde auf LaInO3:Ba Substraten gewachsen und untersucht. BaSnO3 und LaInO3 Schichten wuchsen kohärent, während (K,Na)NbO3 Schichten eine plastische Gitterrelaxation aufwiesen. Das führte zur Bildung von Strukturdefekten und zu einer Verschlechterung der ferroelektrischen Eigenschaften. / In this thesis, the design, growth and characterisation of the heterostructure of a FeFET based on BaSnO3, LaInO3 and (K,Na)NbO3 thin films are investigated. For each material, the growth conditions were determined and optimised with respect to their structural and electrical properties. Epitaxial BaSnO3 thin films grown on SrTiO3 substrates exhibit a high density of threading dislocations, which degrade their electrical properties. The use of NdScO3 substrates and the introduction of a SrSnO3 buffer layer improved the structural and electrical properties of the BaSnO3 thin films. This finally allowed investigations on the LaInO3/BaSnO3 heterointerface. Even a low La doping of the BaSnO3 layer of 0.3 % led to the formation of a 2DEG after interface formation and thus to the confinement of electrons at the interface. This could be clearly demonstrated by C-V, Van-der-Pauw and Hall effect measurements. A significant improvement of the structural and electrical properties of the BaSnO3 thin films was achieved by using LaInO3:Ba substrates. These are lattice-matched to BaSnO3 so that, for the first time, fully strained thin films could be grown without dislocations. Structural and electrical properties of (K,Na)NbO3 thin films were investigated on SrRuO3/DyScO3 and SrTiO3:Nb substrates. In this way, the influence of lattice strain on the critical film thickness and plastic lattice relaxation were determined. Their electrical characterisation revealed a high leakage current caused by structural defects. Therefore, the entire FeFET heterostructure was grown and investigated on LaInO3:Ba substrates. The BaSnO3 and LaInO3 thin films were grown coherently, while the (K,Na)NbO3 thin films exhibited plastic lattice relaxation. This led to the formation of structural defects and consequently to a deterioration of their ferroelectric properties.

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