Neste trabalho é estudado o comportamento termodinâmico de um gás unidimensional de bosons sob a ação de uma impureza delta atrativa. O sistema apresenta o fenômeno da condensação de Bose- Einstein e a causa da transição é atribuida ao estado ligado introduzido pela impureza no espectro de partícula livre. A fase condensada é composta pelas partículas capturadas pela impureza, formando uma gota de partículas bem localizadas no espaço. Isto dá à condensação de Bose-Einstein apresentada pelo sistema a aparência da conhecida transição líquido-vapor. A ordem de transição é analisada pela equação de Clausius-Clayperon e interpretada como de primeira ordem. Deste modo, a semelhança entre a condensação de Bose-Einstein neste sistema ma e a transição líquido-vapor é reforçada.O cálculo do calor específico a comprimento constante, mostra a existência de uma descontinuidade finita na temperatura de transição. / The thermodynamic behavior of the one-dimensional Bose gas-attractive delta impurity system is studied in this work. The system is shown to undergo the Bose-Einstein condensation and the cause of the phase transition is attributed to the bound state introduced by the impurity in the free particle energy spectrum. The condensed phase is composed by particles captured by the impurity, forming a drop of particles well localized in space. This gives to the Bose-Einstein condensation in this system the appearance of the ordinary vapor-liquid phase transition. The order of the phase transition is analized with the aid of the Clausius-Clayperon equation wich allowed us to conclude that the transition is a first order one. This reinforce the interpretation of a vapor-liquid transition.The evaluation of the heat capacity at constant length shows the existence of a finite discontinuity at the transition temperature.

Neste trabalho estudamos as estruturas e as mobilidades eletrônicas em um sistema quase-bidimensional de poços quânticos parabólicos de AlxGa1-xAs dopados. Obtemos as auto-energias, as funções de onda e os perfis dos potencias de confinamento efetivo no sistema, através das soluções numéricas das equações de Schrödinger e de Poisson de forma autoconsistente. Em particular, estudamos as mobilidades quânticas e de transporte nestes sistemas. Devido a ocupação de várias subbandas nestes sistemas, as contribuições dos espalhamentos inter-subbandas para as mobilidades têm a mesma importância que os espalhamentos intra-subbandas. Obtemos as mobilidades de cada subbanda devido aos espalhamentos por impurezas doadoras e aceitadoras ionizadas e por potencial de liga. Analisamos os efeitos das distribuições de doadores dopados, de aceitadores de fundo e do potencial de gate externo / In this work we study the electronic structure and electron mobilities in doped wide parabolic quantum wells of AlxGa1-xAs. The subband energies, the wavefunctions, and the effective confining potential profile are obtained by studying selfconsistently the coupled Schrödinger and Poisson equations. Based on the numerical results of the electronic structure, we calculate the quantum and transport mobilities of the system. Usually several subbands are occupied in such systems and they are strongly coupled to each other, the intersubband interaction shows the same importance as the intrasubband one to the electronic transport. We study and analyze the electron mobility of each subband due to the ionized donor scattering and the alloy scattering. We also show the effect of ionized background acceptor impurity scattering

La compréhension du transport d’impuretés dans les tokamaks est cruciale. En effet, les noyaux lourds ne sont que partiellement ionisés dans le cœur du plasma, ils peuvent alors fortement rayonner et entraîner une diminution importante de la qualité du plasma. Une accumulation des impuretés au cœur du plasma est souvent observée au sein des tokamaks. Cette accumulation est souvent attribuée à la physique néoclassique mais le transport turbulent pourrait bien dominer dans la zone de gradient dans ITER. Jusqu’à récemment, le calcul des flux néoclassique et turbulent étaient réalisés de façon distincte, supposant implicitement que les deux canaux de transport sont indépendants. On peut se demander si cette hypothèse est valide.En effet, des simulations obtenues avec le code gyrocinétique GYSELA ont montré l’existence d’une synergie entre transports néoclassique et turbulent dans le cas des impuretés et un mécanisme permettant sa compréhension a été trouvé.La turbulence peut générer des asymétries poloidales. Un travail analytique permet de prédire le niveau et la structure de la partie axisymétrique du potentiel électrique. Deux mécanismes sont à l’origine des asymétries poloidales du potentiel électrique: la compressibilité du flot et le ballonnement de la turbulence.Une nouvelle prédiction du flux d’impureté néoclassique en présence d’asymétries poloidales et d’anisotropie de la pression a été réalisée. Un bon accord a été trouvé entre la nouvelle prédiction et une simulation réalisée avec GYSELA pour laquelle la turbulence est à l’origine des asymétries poloidales et de l’anisotropie de la pression. / Impurity transport is an issue of utmost importance for tokamaks. Indeed high-Z materials are only partially ionized in the plasma core, so that they can lead to prohibitive radiative losses even at low concentrations, and impact dramatically plasma performance and stability. On-axis accumulation of tungsten has been widely observed in tokamaks.While the very core impurity peaking is generally attributed to neoclassical effects, turbulent transport could well dominate in the gradient region at ITER relevant collisionality. Up to recently, first principles simulations of corresponding fluxes were performed with different dedicated codes, implicitly assuming that both transport channels are separable and therefore additive. The validity of this assumption is questionned. Simulations obtained with the gyrokinetic code GYSELA have shown clear evidences of a neoclassical-turbulence synergy for impurity transport and allowed the identification of a mechanism that underly this synergy.An analytical work allows to compute the level and the structure of the axisymmetric part of the electric potential knowing the turbulence intensity. Two mechanisms are found for the generation of poloidal asymmetries of the electric potential: flow compressibility and the ballooning of the turbulence. A new prediction for the neoclassical impurity flux in presence of large poloidal asymmetries and pressure anisotropies has been derived. A fair agreement has been found between the new theoretical prediction for neoclassical impurity flux and the results of a GYSELA simulation displaying large poloidal asymmetries and pressure anisotropies induced by the presence of turbulence.

Ce travail s’est intéressé à l’influence de contaminations par le cuivre (Cu) et le titane (Ti) sur les performances des cellules solaires photovoltaiques (PV) au silicium (Si) cristallin. Le Cu et le Ti sont des impuretés souvent présentes dans les plaquettes à l’issue de l’étape de croissance des lingots, et peuvent être considérées comme des impuretés modèles respectivement pour les difuseurs rapides et les diffuseurs lents. Des lingots monocristallins Czochralski (Cz) et multicristallins (mc), certains volontairement contaminés et d’autres sans contaminations volontaires ont été cristallisés. Leurs propriétés électriques et compositionnelles ont été caractérisées, et des cellules ont été fabriquées. Les influences du Cu et du Ti sur le rendement de conversion PV (η), sa stabilité sous éclairement, et la tension de claquage (Vbd) de la jonction, ont pu être évaluées. La contamination par le Ti, même à une concentration de 6 ppm pds ajoutée à la charge de Si, affecte significativement le η. Cela est lié au fort pouvoir recombinant des atomes de Ti (malgré un traitement d’hydrogénation) et au faible coefficient de diffusion du Ti, qui ne permet pas son extraction du volume par l’effet getter externe développé par la diffusion P. La contamination par le Ti cependant n’a pas d’influence significative sur la Vbd et est compatible avec des η stables sous éclairement. D’un autre côté, nous avons montré que la contamination par le Cu, pourtant importante (concentration de 90 ppm pds ajoutée au Si charge), n’affecte pas le η des cellules mc. Cela s’explique par le faible pouvoir recombinant des atomes de Cu, par ailleurs efficacement extraits du volume au cours de la diffusion P, en raison du coefficient de diffusion élevé du Cu. Par contre la contamination par le Cu influencerait de façon négative le claquage des jonctions. Un point important est que le η des cellules contaminées Cu diminue sous éclairement. Nous avons montré que cette dégradation est activée par l’étape de recuit rapide des métallisations. Cela confirme de façon indirecte que ces effets de dégradation sont liés à des mécanismes de précipitation du Cu sous éclairement. / This thesis focuses on the influence of copper (Cu) and titanium (Ti) contaminations on the photovoltaic (PV) performances of p-type crystalline silicon (Si) solar cells. These impurities are common in Si and can be considered as model elements since they behave differently and their properties are similar to those of other transition metals. For these studies, deliberately contaminated and uncontaminated single-crystalline Czochralski (Cz) and multicrystalline (mc) ingots were grown. The compositional and electrical properties of these ingots were extensively characterized. Then wafers from these ingots were transformed into solar cells in order to assess the impact of Cu and Ti on the PV conversion efficiency (η), its evolution under illumination, and the p-n junction breakdown voltage (Vbd). We showed that Ti strongly affects the η, for both Cz and mc Si solar cells (even if only 6 ppm wt of Ti were added into the feedstock). This is due to the high recombination strength of interstitial Ti atoms (even after an hydrogenation step), which are not efficiently extracted by the external gettering effect developped by the phosphorus (P) diffusion, due to their low diffusivity. However, the Ti contamination did not significantly influence the Vbd and was compatible with stable η under illumination. On the other hand, we unexpectedly showed that a strong contamination of the Si feedstock by Cu (90 ppm wt of Cu added to the feedstock) do not affect the η of the mc cells. This is essentially due to the lower recombination strength of Cu atoms, which are in addition efficiently extracted by external gettering effects due to their high diffusivity. Nevertheless, we showed that Cu could slighty enhance the junction breakdown. Above all, the mc Cu contaminated cells were affected by light-induced degradation (LID) effects. We showed that these Cu-related LID were activated by the metallization firing step (rapid annealing), which indirectely confirmed that these LID effects would be due to the precipitation of Cu atoms under illumination.

The present project has made a comprehensive assessment of the effect of Nb doping on various charge-transfer related properties of TiO2. Of particular focus, the electrical properties of Nb-doped TiO2 (0.65 at %) have been investigated using the simultaneous measurement of electrical conductivity and thermoelectric power. This investigation was undertaken at elevated temperatures (1073 K -- 1298 K) in equilibrium with a gas phase of controlled oxygen activity (10-10 Pa < p(O2) < 75 kPa). In addition, the effect of segregation on the surface versus bulk composition of Nb-doped TiO2 was also investigated at a function of temperature and oxygen activity. Specifically, the following determinations were undertaken: The effect of oxygen activity, p(O2) and temperature on both electrical conductivity and thermoelectric power The effect of Nb on the defect disorder and related electrical properties of TiO2 The determination of equilibration kinetics and the associated chemical diffusion data for Nb-doped TiO2 The determination of Nb bulk diffusion in TiO2 The effect of p(O2), temperature and dopant content on Nb segregation and the related surface composition of Nb-doped TiO2 The obtained electrical properties enable the determination of a defect disorder model for Nb-doped TiO2, which may be considered within the following p(O2) regimes: Strongly Reduced Regime. In this regime, the predominant ionic defect was anticipated to be oxygen vacancies compensated electronically by electrons. While the transition to this regime (from higher p(O2)) was clearly observed, the predominant defect disorder existing beyond this transition was not confirmed due to an inability to obtain sufficiently low oxygen activity. Metallic-type conductivity behaviour was observed within this transition region. Reduced Regime I. In this regime, the predominate defect disorder defined by the electronic compensation of incorporated Nb ions by electrons was clearly observed. Reduced Regime II. In this regime, the predominate defect disorder defined by the ionic compensation of incorporated Nb ions by quadruply-charged titanium vacancies, was clearly observed. The present project included the determination of diffusion data which included: Temperature dependence of 93Nb tracer diffusion in single crystal TiO2 over the temperature range 1073 K -- 1573 K Chemical diffusion coefficient over the temperature range 1073 K -- 1298 K and oxygen activity range, 10-10 Pa < p(O2) < 75 kPa These pioneering studies are significant as they enable the prediction of the processing conditions required to reliably 1) incorporate Nb into the TiO2 lattice, and 2) achieve equilibrium with the gas phase. Finally, the present project included investigations on the effect of Nb segregation on the surface composition of Nb-doped TiO2, with the following outcomes: Due to segregation, the surface can be significantly enriched in Nb compared to the bulk The extent of enrichment increases as the bulk Nb content or the oxygen activity is decreased Following enrichment, the surface Nb concentration could be sufficiently high to assume a unique surface phase The outcomes of the present project are significant as they can enable the processing of TiO2 with enhanced charge transport and controlled surface properties.

A primarily software based Fourier Deep Level Transient Spectroscope (FDLTS) is built. The raw capacitance transient is acquired and digitized using capacitance meter HP4280A whereas the signal analysis is done using a customized software module. The software module calculates both the conventional DLTS spectrum and the Fourier DLTS spectrum. This home-made FDLTS set up was compared to a commercial conventional box-car DLTS system (Sula Technology's DLTS) as well as to a commercial Fourier DLTS system (Bio-rad) and it was found to be either equivalent to the commercial systems or even better in some respects. In one case, Fourier analysis using the home-made setup, led to the detection of a trap completely undetected by the commercial conventional DLTS. The FDLTS system together with the commercial conventional DLTS were used to study possible gold contamination in an industrial process. The study was accomplished by comparing conventional and Fourier DLTS spectra and corresponding calculated trap properties using Schottky barrier diodes fabricated on the suspect wafers and an intentionally gold diffused reference sample wafer. During the investigation minority carrier emission in DLTS using Schottky barrier diodes was observed. The study revealed the presence of some possible gold-like contamination which trapped minority carriers (i.e. electrons) in p type silicon. / Graduation date: 2003

Two methods for refining metallurgical grade silicon to solar grade silicon have been investigated. The first method involved the reduction of impurities from metallurgical grade silicon by high temperature vacuum refining. The concentrations of analyzed elements were reduced several times. The main steps in the second refining method include alloying with copper, solidification, grinding and heavy media separation. A metallographic study of the Si-Cu alloy showed the presence of only two microconstituents, mainly pure silicon dendrites and the Cu3Si intermetallic. SEM analysis showed a distinct boundary between the silicon and the Cu3Si phases, with a large concentration of microcracks along the boundary, which allowed for efficient separation. After alloying and grinding, a heavy media liquid was used to separate the light silicon phase from the heavier Cu3Si phase. Cu3Si residues together with the remaining impurities were found to be located at the surface of the pure silicon particles, and should be efficiently removed by acid leaching. Thirty elements were analyzed by the Inductively Coupled Plasma Mass Spectrometry (ICP) chemical analysis technique. ICP revealed a several times higher impurity level in the Cu3Si intermetallic than in the pure silicon; furthermore, the amounts of 22 elements in the refined silicon were reduced below the detection limit where the concentrations of 7 elements were below 1ppmw and 6 elements were below 2ppmw. The results showed that the suggested method is efficient in removing impurities from metallurgical grade silicon with great potential for further development.

Two methods for refining metallurgical grade silicon to solar grade silicon have been investigated. The first method involved the reduction of impurities from metallurgical grade silicon by high temperature vacuum refining. The concentrations of analyzed elements were reduced several times. The main steps in the second refining method include alloying with copper, solidification, grinding and heavy media separation. A metallographic study of the Si-Cu alloy showed the presence of only two microconstituents, mainly pure silicon dendrites and the Cu3Si intermetallic. SEM analysis showed a distinct boundary between the silicon and the Cu3Si phases, with a large concentration of microcracks along the boundary, which allowed for efficient separation. After alloying and grinding, a heavy media liquid was used to separate the light silicon phase from the heavier Cu3Si phase. Cu3Si residues together with the remaining impurities were found to be located at the surface of the pure silicon particles, and should be efficiently removed by acid leaching. Thirty elements were analyzed by the Inductively Coupled Plasma Mass Spectrometry (ICP) chemical analysis technique. ICP revealed a several times higher impurity level in the Cu3Si intermetallic than in the pure silicon; furthermore, the amounts of 22 elements in the refined silicon were reduced below the detection limit where the concentrations of 7 elements were below 1ppmw and 6 elements were below 2ppmw. The results showed that the suggested method is efficient in removing impurities from metallurgical grade silicon with great potential for further development.

This thesis presents the analysis of how the social relations shape the situation ofwaste management in developing countries. The towns of Somanya and Agormanya,which are both located in the Eastern Region of Ghana, have been selected as casestudies for this thesis. Qualitative research methods were applied to collect theinformation during the fieldwork. The thesis applies different social science concepts,such as the concept of community, gender relations, corruption and space, to analyzethe social relations among the actors within waste management in the two study areas.The thesis aims to look at waste management from a new perspective and seeks tofind better solutions to deal with the waste situation in developing countries.The results obtained from the empirical work presents the relationships of differentactors in the private space and in the public domain. Relationships between membersof the family are analyzed through concepts of gender and cultural structure withinthe private space. Social relations in the public space involve several actors, such astraditional leaders, local officers, the church community and private companies. Theconcepts of state, community and decentralization are applied to explain how theactors relate to each other in waste management.

In this work , impurity free vacancy diffusion (IFVD) quantum well intermixing(QWI) technology by high thermal-expansion-induced stress is used to perform bandgap engineering. In this paper, 1530nm InGaAsP multiple QWs sandwiched by p-InP (2£gm thickeneess, top) and n-InP (bottom) material is used as testing material structure also laser fabrication material, where contact materials (InGaAs and InP) on p-InP are used for comparison. By the difference between thermal expansion coefficients of SiO2 on the different material (InGaAs, InP), large different behaviors of QWI are observed, while low different dependence on defects created by ion-implantation is found. Above 70nm photo luminance (PL) wavelength shift of InGaAsP MQW below 2£gm thick InP is realized in this method. Further more, CW in-plane laser structures are also successfully fabricated and demonstrated by such QWI, giving the same shift as PL. It shows that good qualify of material can be obtained in such QWI method. Using local deposition of SiO2 causes different bandgap materials, re-growth free processing for monolithic integration can be expected, offering a powerful scheme of QWI for bandgap engineering.