Ce mémoire est consacré à l'étude de deux composés à structure chalcopyrite que sont le thiogallate et le sélénogallate d'argent. La présentation de leur diagramme de phases et de leurs propriétés physiques a permis de mieux comprendre la complexité de leur cristallogénèse. L'objectif de cette étude a donc été d'améliorer chacune des différentes étapes intervenant dans leur élaboration afin d'optimiser leur qualité cristalline. Des mesures d'absorption résiduelle ainsi que des expériences de doublage laser à 10.6 µm ont permis de confirmer l'intérêt porté aux cristaux d'AgGaX2 (X = S ou Se), en vue de leur intégration dans des systèmes optiques non linéaires. Parallèlement à cette étude, une approche numérique de l'hydrodynamique de la phase fluide, a été effectuée afin de définir plus précisement les phénomènes inhérents à la cristallogénèse de ces composés.

Single crystals of GaSe were grown from the melt using 3-zone vertical Bridgman-Stockbarger system. In order to determine the doping effect, nitrogen and silicon ions were implanted to the grown crystals. Surface morphology and stoichiometry were examined using scanning electron microscope equipped with EDAX and structure properties were examined by x-ray diffraction technique. It was observed that the resulting ingot was stoichiometric and the structure was hexagonal. To identify the effects of ion implantation on the physical properties of the samples depending on annealing / electrical conductivity, hall measurements, current-voltage characteristics, photoconductivity and photoluminescence measurements were carried out in the temperature range of 100-450 K. Also spectral transmission measurements were carried out for all the samples at room temperature. It was observed that both N- and Si- implantation followed by annealing process decreased the resisitivity values from 107 to 103 (&amp / #61527 / -cm). Temperature dependent conductivity measurements were analyzed to deduce the dominant transport mechanisms. The trap levels were also investigated by the space charge limited currents (SCLC) measurements. The temperature dependence of hole concentrations showed that as-grown, N- and Si-implanted samples behave as partially compensated p-type semiconductors. Using suitable statistical method, transport parameters such as acceptor level, donor and acceptor concentrations were extracted from the experimental data. Trapping centers and recombination mechanisms were determined from the temperature dependent photoconductivity measurements by investigating the relation between photocurrent and illumination intensity. N- and Si- implantation effects on GaSe were also examined by spectral photoconductivity and transmission measurements. And lastly, radiative recombination mechanisms in as-grown GaSe were investigated through photoluminescence (PL) measurements and the information related to the structural defects, the exciton levels and the structure of the forbidden gap were investigated.

In this work, structural, electrical and optical characterization of as-grown, Ge-implanted, and annealed GaSe single crystals grown by using 3-zone vertical Bridgman-Stockbarger system, have been studied by carrying out X-ray Diffraction (XRD), electrical conductivity, Hall effect, photoconductivity, and spectral transmission measurements. The temperature dependent electrical conductivity of these samples have been measured between 100 and 400 K. As a result, it was observed that upon implanting GaSe with germanium following annealing process, the resistivity is reduced from 2.1x109 to 6.5x105 &amp / #937 / -cm. Also it was found that Ge-implantation followed by annealing at 500 oC increases the conductivity in exponential fashion. From the temperature dependent conductivities, the activation energies have been found to be 4, 34 and 314 meV for as-grown, 36 and 472 meV for as-implanted, and 39 and 647 meV for implanted and annealed at 500 oC GaSe single crystals. Using XRD measurements it was observed that there is an increase in peak intensities at specific annealing temperatures (300 and 500 C) and a decrease in higher annealing temperatures (700 C). Temperature dependent carrier concentration and Hall mobility measurement were performed in the temperature range of 230 - 410 and 100 - 400 K for as-grown and Ge-implanted and annealed GaSe samples, respectively. All of the samples in this study were found to be p-type with the help Hall measurements. In addition, the density of donor and acceptor atoms are found for each sample and results are compared with each other. In addition, using photoconductivity measurement the relation between photocurrent and illumination intensity and the character of photoconduction were determined. As a result it was found that while at specific temperature intervals impurity scatterings are dominant, in other intervals phonon scatterings start to dominate. Finally, in order to determine annealing dependent change of band gap of unimplanted and Ge-implanted GaSe samples at room temperature, the transmission measurement have been carried out as a optical characterization part of our study. As a consequence of this measurement it was observed that there is almost no considerable change in optical band gap of samples with increasing annealing temperatures for as-grown GaSe samples and a slight shift of optical band gap toward to high energy for Ge-implanted samples with annealing process.

No description available.

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties / QC 20100901

Pour étudier l'influence de la gravité sur la croissance par T.H.M. du semi-conducteur CdTe dopé indium (programme EURECA), nous avons effectué une analyse comparative d'expérience spatiales et terrestres, complétée par une simulation numérique de l'hydrodynamique de la phase fluide utilisant un code 3D. Notre deuxième objectif a été de prépérer des monocristaux de CdTe dopé vanadium par la technique Bridgman présentant de bonnes propriérés photoréfractives. Pour une meilleure compréhension du mécanisme de l'effet photoréfractif, les matériaux ont été caractérisés par R.P.E., D.C.M. et P.I.C.T.S. afin de déterminer le degré de valence de V et la signature des niveaux vanadium dans la bande interdite. Des mesures optiques (absorption, photoconductivité...) et d'effet photoréfractif ont été effectuées. Enfin, pour évaluer la pertinence spatiale de cette croissance, nous avons étudié par simulation numérique l'influence de la gravité sur la configuration hydrodynamique de la phase fluide.

In der vorliegenden Arbeit werden wissenschaftlich-technologische Untersuchungen zur wandabgelösten VGF-Züchtung von Germanium präsentiert. Dazu wurden zwei Varianten mit Hinblick auf die Etablierung und Stabilisierung bestimmter Druckverhältnisse angewendet: (i) Beim passiven dVGF-Verfahren erfolgt die Erzeugung der Druckdifferenz über die thermische Beeinflussung des Inertgases Ar, und (ii) beim erstmals gezeigten aktiven dVGF-Verfahren über eine temperaturkontrollierte, separate Zn-Dampfdruckquelle. Ge-Einkristalle mit einem Durchmesser bis zu 3 Zoll wurden nahezu vollständig ohne Tiegelkontakt gezüchtet. Der Effekt der Wandablösung wird anhand der mikroskopischen Charakterisierung der Kristalloberfläche, der Durchbiegung der Phasengrenze sowie der etch pit density (EPD), ein Maß für die Versetzungsdichte gezeigt. Im Vergleich zu konventionell gezüchteten VGF-Kristallen zeigen die detached gezüchteten Kristalle eine hohe strukturelle Qualität. Dies wird auf reduzierte thermische und thermo-mechanische Spannungen bei der wandabgelösten Züchtung zurückgeführt.

Bei der Vertical Gradient Freeze-Züchtung von Halbleiterkristallen bis zu einem Durchmesser von 2″ wurde in einer speziell entwickelten Anlage die Beeinflussung des Wärme- und Stofftransports in der Schmelze durch ein rotierendes Magnetfeld (RMF) untersucht. An GaAs- und Ge-Kristallen konnte bei Einwirkung des RMF eine verminderte Durchbiegung der Phasengrenze aufgrund geringerer radialer Temperaturgradienten festgestellt werden. Die Analyse der Segregationsprozesse ergab unter RMF-Einfluss signifikante Änderungen der Ausdehnung der Diffusionsgrenzschicht vor der Phasengrenze. Dies spiegelte sich in einer verbesserten axialen und radialen Homogenität der Ladungsträgerverteilung in den Kristallen wider. Die kritische magnetische Taylor-Zahl, die den Übergang von stationärer zu instationärer RMF-Konvektion charakterisiert, konnte für verschiedene Geometrieverhältnisse der Schmelze experimentell bestimmt werden.

In der vorliegenden Arbeit werden wissenschaftlich-technologische Untersuchungen zur wandabgelösten VGF-Züchtung von Germanium präsentiert. Dazu wurden zwei Varianten mit Hinblick auf die Etablierung und Stabilisierung bestimmter Druckverhältnisse angewendet: (i) Beim passiven dVGF-Verfahren erfolgt die Erzeugung der Druckdifferenz über die thermische Beeinflussung des Inertgases Ar, und (ii) beim erstmals gezeigten aktiven dVGF-Verfahren über eine temperaturkontrollierte, separate Zn-Dampfdruckquelle. Ge-Einkristalle mit einem Durchmesser bis zu 3 Zoll wurden nahezu vollständig ohne Tiegelkontakt gezüchtet. Der Effekt der Wandablösung wird anhand der mikroskopischen Charakterisierung der Kristalloberfläche, der Durchbiegung der Phasengrenze sowie der etch pit density (EPD), ein Maß für die Versetzungsdichte gezeigt. Im Vergleich zu konventionell gezüchteten VGF-Kristallen zeigen die detached gezüchteten Kristalle eine hohe strukturelle Qualität. Dies wird auf reduzierte thermische und thermo-mechanische Spannungen bei der wandabgelösten Züchtung zurückgeführt.

No description available.