A presente dissertação aborda os movimentos teóricos no delineamento e estudo da problematização da subjetivação realizados por Michel Foucault. Considera-se, em um primeiro momento, que, na última etapa de sua obra, o autor dirige-se à questão da subjetivação, tecendo então o que define o chamado terceiro eixo de suas pesquisas os dois primeiros tratam da questão da subjetivação relacionada aos campos do saber e do poder, enquanto condição de sujeição e dominação. Mostra-se, em um segundo momento, que, depois de dedicar-se ao estudo da sexualidade, tendo como foco inicial o homem como sujeito de desejo, e de remeter-se às práticas discursivas, o autor deslocará o foco de sua investigação para a problematização histórica da subjetivação. Por fim, analisa-se, em um sentido, a questão das aphrodisia, enquanto substância ética que, através da prática ascética como campo da relação a si, viabiliza a constituição do sujeito de desejo; e, em outro, aborda-se a questão das técnicas ascéticas e do cuidado de si, em que o indivíduo é tomado em sua dimensão ética, referido à questão da verdade, a uma estética da existência e a uma hermenêutica de si. Neste último aspecto, retoma-se o sentido da transformação de si, enquanto prática de liberdade, e a ontologia histórica de si, contemplando com isso as práticas através das quais o indivíduo se torna sujeito de si mesmo.

Les circuits deviennent de plus en plus intégrés pour augmenter les performances des dispositifs microélectroniques. La formation de jonctions ultra courtes (USJs) est un challenge majeur pour la réalisation de la prochaine génération de transistors à effet de champ (MOSFET) ayant une longueur de grill inférieure à 25 nm. L’implantation ionique est la technique la plus utilisée pour fabriquer des jonctions dans du Si, mais elle génère des défauts étendus, des interstitiels (Is) et des lacunes (Vs), qui introduisent des effets néfastes dans les composants, comme l’effet d’un transitoire de diffusion accélérée (TED) du bore et la formation d’agrégats de bore et d’Is (BICs). Une ingénierie de défauts par triple implantation (He, Si et B) a été utilisée pour maîtriser ces effets. Le rôle de chaque implantation d’He et de Si sur la diffusion du B est présenté dans ce mémoire. Les échantillons ont été caractérisés par SIMS, TEM, effet Hall, PAS, NRA etc. Pour fabriquer des USJs, le meilleur procédé est dans un premier temps l’introduire des cavités par implantation d’He pour créer une barrière de diffusion aux Is. Puis une implantation Si est réalisée à une énergie telle que la couche de cavités soit située entre les couches de Vs et d’Is qui sont introduites par cette même implantation. Enfin les atomes de B sont introduits à une faible énergie par implantation ionique ou par immersion plasma (PIII) pour créer les USJs. Au cours du recuit rapide d’activation (RTA), les Vs introduites par implantation Si peuvent se recombiner avec les Is introduites par implantation du B pour augmenter l’activation du dopant et limiter la diffusion du B. Une jonction ayant une épaisseur Xj de (12 ± 1) nm et une Rs de (150 ± 10) O/? a été réalisée. / The circuit becomes more and more integrated. Ultra shallow junctions (USJs) formation is a key challenge for realization of the next sub-25nm generation of metal oxide semiconductor field-effect transistor (MOSFET). Ion implantation is now a basic technique in the manufacture of Si based junctions in microelectronic industry. But it generates damages such as interstitials (Is) and vacancies (Vs) and induces several drawback effects, including boron transient enhanced diffusion (TED) and boron-interstitial clusters (BICs). Defect engineering by triple-implantation (He, Si and B) used to overcome these drawback effects for the USJs formation has been studied in this thesis. Each effect of He or Si implantation on B diffusion has been presented in this rapport. The samples are mainly characterized by SIMS, TEM, Hall effect, PAS and NRA. The best solution is that the He implantation followed by an annealing is first performed to create cavities to stop the Is diffusion to the surface. Then Si implantation is realized with a appropriate energy to put the band of cavities located in the middle of the zone of Vs and Is introduced by itself. At last, the B is implanted in a low energy by ion implantation or plasma ion immersion implantation (PIII) to realize the USJ. With this structure, the Vs can recombine with the Is introduced by B implantation to raise the activation rate and to limit the B diffusion when rapidly annealed (RTA). A junction with a depth (Xj) of (12 ± 1) nm and a Rs of (15 0± 10) O/? is achieved.

As ligas de Al-Si são amplamente utilizadas na indústria em substituição ao aço e o ferro fundido nos setores de alta tecnologia. A importância comercial dessas ligas deve-se principalmente ao seu baixo peso, excelente resistência à abrasão e à corrosão, alta resistência em temperaturas elevadas, baixo coeficiente de expansão térmica e menor consumo de combustível que proporciona redução considerável de emissão de poluentes. No presente trabalho, uma liga de Al-Si utilizada na indústria automobilística para fabricação de pistões de motores de combustão interna, foi submetida a tratamentos superficiais de refusão a LASER (Nd:YAG, λ = 1,06 m, modo pulsado). A radiação laser possibilita diversas concentrações de energia com precisa transferência dela para o material sem contato físico. A transferência intensa de energia provoca a ocorrência de modificações estruturais na camada superficial do material. Experimentos com pulsos únicos e trilhas foram realizados sob diferentes condições de processamento LASER com o intuito de analisar as mudanças microestruturais resultantes dos tratamentos e seus efeitos sobre a dureza. Para a caracterização da camada endurecida foram utilizadas as seguintes técnicas: microscopia óptica (MO), microscopia eletrônica de varredura (MEV), espectroscopia por energia dispersiva (EDS), mapeamento de raios-X, ensaios de microdureza Vickers e de rugosidade máxima. Após os tratamentos superficiais de refusão a LASER da liga Al-Si, a alta taxa de resfriamento resultante causou mudança na microestrutura devido ao refinamento das partículas de silício eutético primário presentes na liga, resultando em aumento de dureza. / The Al-Si alloys are widely used in industry to replace the steel and gray cast iron in high-tech sectors. The commercial importance of these alloys is mainly due to its low weight, excellent wear (abrasion) and corrosion resistance, high resistance at elevated temperatures, low coefficient of thermal expansion and lesser fuel consumption that provide considerable reduction of emission of pollutants. In this work, Al-Si alloy used in the automotive industry to manufacture pistons of internal combustion engines, was undergone to surface treatments using LASER remelting (Nd:YAG, λ = 1.06 m, pulsed mode). The LASER enables various energy concentrations with accurate transfer to the material without physical contact. The intense energy transfer causes the occurrence of structural changes in the superficial layer of the material. Experiments with single pulses and trails were conducted under various conditions of LASER processing in order to analyze microestrutural changes resulting from treatments and their effects on the hardness. For the characterization of hardened layer was utilized the following techniques: optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), x-ray mapping, Vickers microhardness and maximum roughness tests. The high cooling rate caused a change in the alloy structure due to the refinement of the primary eutectic silicon particles, resulting in increase of the mechanical properties (hardness) of the Al-Si alloy.

O principal programa de pesquisa do Grupo de Diagrama de Fases e Termodinâmica Computacional do DEMAR/EEL-USP é o estudo de estabilidade de fases em ligas dos sistemas Me-Si-B (Me = Metal Refratário), os quais apresentam um interesse crescente na área de ligas com potencial para aplicações em altas temperaturas. Destes estudos, este Grupo está construindo uma base de informações termodinâmicas com a finalidade de extrapolá-las para sistemas de ordem superior. Diversos sistemas já foram ou estão sendo estudados. Dando continuidade a este programa, os objetivos principais deste trabalho são a avaliação experimental do sistema ternário Ta-Si-B em sua região rica em Ta e a modelagem termodinâmica dessa mesma região. A projeção liquidus e a seção isotérmica a 1900 ºC, ambas na região rica em Ta, de Ramos (2005), são as únicas informações experimentais para o sistema ternário Ta-Si-B encontradas na literatura. Ramos (2005) identifica em várias de suas amostras a presença de uma nova fase ternária, denominada φ, de estrutura ainda não determinada. A pesquisa de Ramos (2005) foi realizada sempre com amostras produzidas por fusão em forno a arco, a partir de pedaços de Ta, Si e B puros. Nessa condição de preparação das amostras, houve dificuldade em se alcançar o equilíbrio termodinâmico, mesmo após tratamentos térmicos prolongados (1900 oC por 48 horas). Já, neste trabalho, as amostras foram, em sua maioria, produzidas por tratamento térmico de pastilhas prensadas a partir de pós de Ta, Ta5Si3 e TaB. Com essa mudança na forma de confeccionar os experimentos conseguiu-se maior facilidade na obtenção da condição de equilíbrio termodinâmico das amostras durante os tratamentos térmicos, embora ainda com resultados não totalmente satisfatórios. De uma forma geral, os resultados de Ramos (2005) são confirmados no presente trabalho. Uma proposta de relação de fases a 2000 ºC é apresentada para a região envolvendo as fases TaSS, Ta2B, T2, Ta3B4 e TaB. A estabilidade da fase D88, proposta por Nowotny, Lux e Kudielka (1956), também é confirmada e os resultados sugerem que sua estequiometria seja Ta5Si3B. Nenhuma modelagem termodinâmica para este sistema ternário é encontrada na literatura. A otimização do sistema binário Ta-B é realizada por Kaufman (1991) e Chad (2003). A otimização do sistema binário Ta-Si é realizada por Vahlas, Chevalier e Blanquet (1989) e Kaufman (1991). A otimização do sistema binário Si-B é realizada por Fries e Lukas (1991), Zaitsev e Kodentsov (2001) e Fiorani e Coelho (2006). A única otimização adotada completamente é a de Fiorani e Coelho para o sistema binário Si-B. Para o sistema binário Ta-B é adotada a otimização de Chad (2003) com a inclusão de uma nova fase, denominada φ, de alta temperatura, a mesma do sistema ternário, que teria sua origem nesse sistema binário. Já, para o sistema binário Ta-Si é necessária uma nova otimização, por haver algumas inconsistências termodinâmicas nas otimizações anteriores, que poderiam levar a uma descrição inadequada das relações de fases no sistema ternário Ta-Si-B e em sistemas de ordem superior. Neste trabalho é apresentada uma proposta para a modelagem termodinâmica do sistema ternário, que reproduz satisfatoriamente os resultados experimentais disponíveis. / Currently, the main research program of the Phase Diagram and Computational Thermodynamics Group of DEMAR/EEL-USP is the study of phase stability in ternary Me-Si-B systems (Me = Refractory Metal), which presents an increasing interest in the area of alloys with potential for applications at high temperatures. It includes the development of a thermodynamic database to evaluate systems of higher order. Several systems have already been studied. Giving continuity to this program, the main objectives of this work are the experimental evaluation of the ternary Ta-Si-B system in the Ta-rich region and the thermodynamic modeling of this system. The liquidus projection and an isothermal section at 1900 ºC from Ramos (2005), both in the Ta-rich region, are the only experimental information found in literature. Ramos (2005) identifies in several of her samples the presence of a new ternary phase, named as φ, of structure not yet determined. The research of Ramos (2005) is carried out with samples produced from pure Ta, Si and B via arc melting. In this method of sample preparation, it is difficult to obtain thermodynamic equilibrium conditions during heat treatments at 1900 ºC for 48 h. In the present work, the majority of samples were produced via sintering of pressed pellets from Ta, Ta5Si3 and TaB powders. With this method of sample preparation, the attainment of equilibrium conditions was easier during the heat treatments, even though not completely satisfactory. In general, the results of Ramos (2005) were confirmed. A proposal for the phase relations at 2000 ºC is presented for the region involving the phases TaSS, Ta2B, T2, Ta3B4 and TaB. The stability of the D88 phase, proposed by Nowotny, Lux and Kudielka (1956), is also confirmed and the results suggest that its stoichiometry is near Ta5Si3B.No thermodynamic modeling for this ternary system is found in literature. The optimization of the binary Ta-B system is carried out by Kaufman (1991) and Chad (2003). The optimization of the binary Ta-Si system is carried out by Vahlas, Chevalier and Blanquet (1989) and Kaufman (1991). The optimization of the Si-B binary system is carried out by Fries and Lukas (1991), Zaitsev and Kodentsov (2001) and Fiorani and Coelho (2006). In this work the thermodynamic data from Fiorani and Coelho (2006) is used for the Si-B system. For the binary Ta-B system it was adopted the optimization from Chad (2003) with the inclusion of the high temperature φ phase, the same one of the ternary system, which has its origin in this binary system. In addition, for the binary system Ta-Si a new optimization is necessary, since some thermodynamic inconsistencies in the previous optimizations would lead to an inadequate description of the phase relations in the Ta-Si-B and systems of higher order. Thus, in this work presents for the first time a thermodynamic modeling for the Ta-Si-B system, which reproduces the available experimental results satisfactorily.

Les alliages Fe-Si sont connus pour combiner d’excellentes propriétés magnétiques avec de bonnes propriétés électriques (forte résistivité électrique). Dans ce contexte nous avons recherché à élaborer des matériaux à forte teneur en Si, souvent difficiles à obtenir et mettre en forme industriellement. Des alliages magnétiques doux de type Fe-Si avec une teneur élevée en Si (4,5%, 6,5%, 10% et 15% massique) ont été obtenus avec succès à l’état nanocristallin par broyage mécanique et recuit. La formation des alliages a été étudiée par diffraction X, spectroscopie Mössbauer et analyses thermomagnétiques. La stabilité thermique de la poudre a été analysée par DSC. Des mesures d’aimantation ont été réalisées pour caractériser les performances magnétiques. La durée de broyage nécessaire pour la formation de l’alliage a été déterminée pour chaque teneur en Si. Pour les faibles temps de broyage, le recuit conduit à la formation du composé Fe3Si. Après la formation de l’alliage par le broyage mécanique, l’effet du recuit est seulement de réduire les tensions internes du second ordre, induites dans la poudre par le broyage. L’addition de Si conduit à la diminution de la température de Curie de 770 °C pour le Fe pur, à 725 °C pour une teneur de 4,5% massique de Si et à 550 °C pour 15% massique de Si. Pour les temps faibles de broyage, l’écart entre l’aimantation de la poudre avant et après recuit est dû à la formation du composé Fe3Si pendant le recuit, lequel a une aimantation plus faible que la solution solide de Feα(Si). Pour les longs temps de broyage, le recuit à 400 °C pour 4 heures n’a pas d’effet sur la valeur de l’aimantation à saturation. En augmentant la teneur en Si, l’aimantation à saturation de l’alliage Fe-Si décroit.Les alliages Ni3Fe (aussi connus comme Permalloys) présentent de meilleures propriétés magnétiques, mais ils ont une résistivité inférieure à celles des Fe-Si. Une voie attractive semble la combinaison des propriétés des 2 classes de matériaux doux en formant un composite. Les alliages Fe-Si précédemment obtenus ont été utilisés pour l’élaboration des poudres composites de type Permalloy/Fe-Si par la mécanosynthèse. Le broyage mécanique conduit à la formation des particules composites avec un aspect stratifié. Quatre heures de broyage de l’alliage Fe-Si avec du Ni3Fe ne conduisent pas à la formation des nouvelles phases, mais la formation d’un alliage ternaire Ni-Fe-Si résulte d’un recuit ultérieur à 900 °C. L’aimantation à saturation du composite augmente avec la croissance de la teneur le d’alliage Fe-Si, mais le temps de broyage ne semble avoir aucun effet sur cela.Une étude préliminaire a été réalisée sur l’élaboration des compacts composites de type Ni3Fe/Fe-Si par frittage flash, dans le but de préserver l’état nanocristallin par de basses températures de frittage. L’influence de la température de frittage et de la durée de maintien sur la structure, et les propriétés physiques des compacts est discutée. Des températures allant jusqu'à 750 °C pour une durée de maintien minimale ou un palier de 2 minutes maximum à 700 °C ne conduisent pas à la diffusion des éléments des alliages. L'augmentation de la température ou de la durée de frittage conduit à des cristallites plus grandes, mais qui restent dans le domaine nano pour les températures étudiées. La densité des compactes augmente avec la température et le palier. En outre, la résistivité diminue en augmentant ces 2 paramètres. L'effet de la teneur en Fe-Si est de diminuer la densité et en même temps d'augmenter la résistivité des compacts. La perméabilité magnétique est réduite avec l'augmentation de la température et de la durée de frittage, ainsi que lors de la diminution du contenu de Ni3Fe. Une température élevée et un long temps de maintien à la température de frittage conduisent à l’augmentation des pertes magnétiques. Le champ coercitif est également influencé par les paramètres de frittage, via l'effet qu'ils ont sur la taille des cristallites. / Fe-Si alloys are known for combining excellent magnetic properties with good electric characteristics (high resistivity). In this context we sought to develop materials with a relatively high Si content, often difficult to obtain and shape industrially.In this thesis, soft magnetic Fe-Si alloys with high Si content (4.5, 6.5, 10 and 15 wt. %) were successfully obtained in nanocrystalline state by mechanical alloying and annealing. The formation of the alloy was studied by X-ray and neutron diffraction, Mossbauer spectroscopy and thermomagnetic analysis. DSC technique was used in order to study the powder’s thermal stability. Magnetisation measurements were also made in order to characterise their magnetic performances. The milling duration necessary for the formation of the alloy was determined for each Si content. For low milling times, annealing leads to the formation of the Fe3Si compound. Once the alloy is formed by mechanical milling, the effect of the annealing is only to reduce the second order stress induced in the powder by the milling process. Si addition leads to the decrease of the alloy’s Curie temperature from 770 °C for pure Fe to 725 °C for a 4.5 wt. % Si and down to 550 °C if the Si content increases to 15 wt. %. For low milling times, a gap between the magnetisation of the as-milled alloy and of the milled and subsequently annealed one is due to the formation of the Fe3Si compound during annealing which has a lower magnetisation than that of the αFe (Si) solid solution. For longer milling durations, annealing at 400 °C for 4 hours has no effect on the saturation magnetisation value. By increasing the Si content, the Fe-Si alloy’s saturation magnetisation decreases.Fe-Ni alloys whose composition is close to Ni3Fe (commonly known as Permalloys) have better magnetic properties, but a resistivity well inferior to that of Fe-Si alloys. Therefore, a combination of the properties of these 2 alloy classes of soft magnetic materials into a composite seems to be an attractive route. The previously obtained Fe-Si alloys were used for the preparation of Permalloy/Fe-Si composite powders by mechanical milling. Milling leads to the formation of composite powder particles with a stratified aspect. Milling of the Fe-Si and Ni3Fe alloys for 4 hours does not lead to the formation of new phases, but a subsequent annealing at 900 °C results in the formation of a Ni-Fe-Si alloy. Saturation magnetisation of the composite increases with increasing of the Fe-Si content, but milling duration seems to have no effect on it.A preliminary study was made on the elaboration of Ni3Fe/Fe-Si composite compacts obtained by spark plasma sintering, aiming to preserve the nanocrystalline state by lower sintering temperatures. The influence of the sintering temperature and temperature holding duration on the structure, density, resistivity and magnetic properties of the compacts is discussed. Temperatures of up to 750 °C for minimal holding duration or a maintain at the temperature of 700 °C for a duration of up to 2 minutes does not lead to a diffusion of the alloys’ elements. Increasing of the sintering temperature or duration leads to larger crystallite sizes, but they remain in the nano domain for the studied temperatures. The compacts’ density increases with temperature and sintering duration. Resistivity, on the other hand decreases when increasing the aforementioned parameters. The effect of the Fe-Si content is to decrease the density and at the same time increase the compacts’ resistivity. Magnetic permeability is reduced with increasing sintering temperature and duration, as well as when decreasing of the Ni3Fe content. High temperature and long maintaining duration leads to an increase of magnetic losses. Coercive field is also influenced by sintering parameters by the effect they have on the crystallite size.

De nos jours on utilise de plus en plus les aciers pré-revêtus dédiés aux applications pour emboutissage à chaud afin de protéger la surface de l’acier contre la décarburation et l’oxydation durant le traitement thermique. Le revêtement est déposé à chaud en continu par immersion de la bande d’acier dans un bain d’Al-Si. Pendant l’austénitisation le revêtement Al-Si se transforme par réactions d’inter-diffusion et de solidification. Ces réactions conditionnent la microstructure finale et en particulier la surface du revêtement, responsable des propriétés d’emploi telles que le soudage par point et l’adhérence peinture.A l’état de livraison le revêtement Al-Si contient les phases suivantes : des grains d’Al, l’eutectique ternaire Al-Fe-Si, une couche intermétallique ternaire Al-Fe-Si et une couche intermétallique binaire Al-Fe. Après austénitisation les phases se transforment en deux types d’intermétalliques : intermétalliques ternaires riches en Si et intermétalliques binaires pauvre en Si.Durant cette étude une identification complète des différentes phases a été établie. Les phases initialement riches en Si se transforment en phases fusibles par réactions eutectique ou péritectique. La présence de phase liquide accélère la diffusion locale de fer permettant aux phases riches en Si de garder leur contenu en Si pendant tout le traitement thermique. C’est la diffusion du fer qui stabilise les gradients en Si dans le revêtement, malgré les lois de diffusion classiques. En utilisant le diagramme de phases ternaire il a été démontré comment l’enrichissement en fer s’établit dans le revêtement tout en gardant l’équilibre aux interfaces entre les phases riches et pauvres en Si / Nowadays more and more pre-coated steels are applied in hot-stamping process to prevent the steel surface against iron oxidation and decarburization during the heat-treatment. The coating is deposited by hot-dipping the coil in an Al-Si bath. During the austenitization, the Al-Si coating transforms completely by inter-diffusion and solidification reactions, which define the final microstructure and particularly the top layer responsible for the in-use properties like spot welding and painting adhesion.At the delivery state, the Al-Si coating is a multiphase coating containing the following phases: Al-grains, Al-Fe-Si ternary eutectic phase, an Al-Fe-Si intermetallic layer and a binary Al-Fe interfacial layer. After the heat-treatment, all the phases transform into two types of intermetallic compounds: Si-rich ternary and Si-poor binary compounds.During this study, a complete identification of the different intermetallic phases has been conducted. Initial ternary Al-Fe-Si phases transform into fusible phases by eutectic or peritectic reaction. The presence of liquid state enables rapid local iron diffusion which allows to the Si-rich phases to keep their high Si content during the whole treatment. So the iron diffusion stabilizes the Si gradients in the coating despite the classical diffusion laws. Using the ternary phase diagram it was shown how iron enrichment in the coating proceeds by keeping thermodynamical equilibrium along the interfaces between Si-rich ternary and Si-poor binary phases. A phenomenological model of phase transformations explaining the formation of differente coating microstructures during the austenitization step is proposed

L’objectif de ce travail de thèse a été d’étudier une alternative à la cellule photovoltaïque à hétérojonction classique de silicium amorphe/cristallin avec un matériau (GaP) qui permettrait une amélioration de rendement grâce à ses propriétés optiques et électriques. L’étude du potentiel des hétérojonctions GaP/Si pour des applications PV nous a amené à étudier chacun des aspects critiques inhérents à leur réalisation. La préparation chimique de la surface des substrats et les mécanismes qui contrôlent la structuration de la surface de Si(100) ont été étudiés afin d’obtenir une surface de silicium mono-domaine (à marches diatomiques) et faiblement rugueuse par homoépitaxie (dépôts par UHV-CVD). Cette étude a été complétée par l’étude de l’influence de la préparation de surface (préparation chimique et homoépitaxie) du substrat sur la qualité cristalline du GaP déposé en deux étapes par MEE et MBE. La croissance de GaP par MEE a par la suite été effectuée sur des substrats de Si(100) ayant uniquement subi une préparation chimique de surface. Les paramètres de la séquence de croissances MEE ont été étudiés et ajustés afin d’optimiser la phase de nucléation du GaP. La qualité structurale des dépôts a été évaluée par des caractérisations par AFM et DRX. Les couches minces de faibles épaisseurs (20nm) présentent une faible rugosité de surface équivalente à une homoépitaxie et une fraction volumique de MTs inférieure à la limite de détection. La croissance MEE permet d’assurer une nucléation 2D. Cependant les caractérisations par TEM et STM révèlent la présence de parois d’antiphase. En parallèle, la simulation de structures HET GaP/Si (effectuée grâce au programme AFORS-HET) et la réalisation de diodes et de démonstrateurs cellules GaP/Si ont permis de démontrer les optimisations apportées par l’utilisation d’un émetteur de GaP. Ces composants ont été étudiés par caractérisations optiques et électriques. Nous avons constaté une limitation des performances due à la présence de pièges à l’interface et dans le volume. Ces différentes études ont donc permis d’identifier les verrous technologiques à lever pour exploiter pleinement les cellules à hétérojonctions GaP sur silicium afin d’obtenir des hauts rendements photovoltaïques. / The main objective of this thesis is to study an alternative to conventional amorphous/crystalline silicon heterojunction solar cell using gallium phosphide (GaP) as an emitter layer. This would allow a performance improvement because of its optical and electrical properties. The potential of GaP/Si heterojunction solar cells have been evaluated by studying each of the critical issues inherent to their fabrication process. The chemical preparation of the substrates surface and the mechanisms controling the structure of the Si (100) surface have been studied in order to obtain a single domain silicon surface (with diatomic steps) and slightly roughened by homoepitaxy (UHV-CVD). This work was completed by the study of the impact of surface preparation (chemical preparation and homoepitaxy) of the substrate on the crystalline quality of GaP deposited in two steps by MBE and MEE. The growth of GaP by MEE was subsequently carried out on Si(100) substrates having only undergone a chemical surface preparation. MEE growth sequence parameters were studied and adjusted to optimize GaP nucleation. The structural quality of the thin films was evaluated by AFM and XRD characterizations. Thin films of 20 nm have lower surface roughness equivalent to an homoepitaxy and a volume fraction of MTs below the detection limit. The MEE growth ensures a 2D nucleation. However, TEM and STM characterizations reveal the presence of antiphase boundaries. In parallel, simulations of the structure HET GaP/Si (with AFORS-HET) have been performed to evaluate the potential of the structure. First, diodes and demonstrator cells with GaP/Si junction have been fabricated and optically/electrically characterized. Limitations in performance due to the presence of traps at the interface and silicon volume degradation have been observed. All this work has allowed us to identify the technological issues to overcome in order to fully exploit the GaP/Si heterojunction cells to improve solar cell performance.

This thesis is devoted to the behavior of heavy metals in gettering multilayers made of polysilicon and silicon oxide.

Dans le contexte des cellules photovoltaïques (PV) à base de silicium cristallin (c-Si), le développement de structures de contacts dits « passivants », qui permettent de limiter les pertes par recombinaisons des porteurs de charge à l’interface entre le métal et le c-Si, est un des principaux leviers vers l’obtention de plus hauts rendements. Une approche de contacts passivés consiste à intégrer entre le métal et le c-Si une jonction composée d’une couche de silicium poly-cristallin (poly-Si) fortement dopée sur une mince couche d’oxyde de silicium (SiOx < 2 nm).Les objectifs de ce travail sont d’une part de développer une jonction poly-Si/SiOx compatible avec la fabrication industrielle des cellules PV, et d’autre part d’améliorer la compréhension des mécanismes de passivation et de transport des charges au niveau de la fine couche de SiOx située à l’interface entre le poly-Si et le c-Si.Dans ce travail, une jonction de poly-Si/SiOx dopée au bore a été développée, le dopage de la couche étant dans un premier temps réalisé in-situ pendant l’étape de dépôt chimique en phase vapeur assisté par plasma (PECVD) de la couche poly-Si. La méthode de dépôt PECVD est répandue dans l’industrie PV et permet la fabrication de la couche poly-Si d’un seul côté du substrat c-Si. Cependant, elle induit une forte concentration d’hydrogène dans la couche déposée, ce qui entraine la formation de cloques à l’interface avec le c-Si et tend à dégrader les propriétés de passivation de surface de la jonction après recuit de cristallisation. L’optimisation des conditions de dépôt (température de dépôt et ratio de gaz H2/SiH4) a permis d’obtenir des couches de poly-Si dopées in-situ intègres. Par la suite, une méthode de dopage alternative, par le biais du dépôt d’une couche diélectrique riche en bore sur le poly-Si, a été appliquée afin de réduire l’apport en hydrogène pendant le dépôt et d’obtenir des couches de poly-Si intègres plus épaisses. L’ajout d’une étape d’hydrogénation a permis d’obtenir des propriétés de passivation de surface au niveau de l’état de l’art pour les deux types de jonctions poly-Si/SiOx développées.A la suite du développement de la jonction poly-Si/SiOx, la caractérisation physico-chimique de la couche SiOx a été réalisée et a démontré une possible amélioration de la stœchiométrie de la couche vers SiO2 ainsi qu’une dégradation de son homogénéité en épaisseur sous l’effet du recuit de cristallisation à haute température. Ces phénomènes pourraient s’expliquer par une diffusion des atomes d’oxygène à l’interface. D’autre part, l’étude du transport des charges à travers le SiOx par C-AFM a mis en évidence les limites de cette technique quant à la détermination de nano-ouvertures au sein de la couche SiOx (qui favoriseraient le transport des charges). Enfin, une méthode de caractérisation des défauts recombinants à l’interface entre une jonction de poly-Si intrinsèque et le c-Si a été mise en œuvre. Cette méthode a permis de modéliser les recombinaisons à l’interface poly-Si/c-Si via deux défauts discrets apparents dont les niveaux d’énergie dans la bande interdite et les ratios de sections efficaces de capture des électrons et des trous ont été déterminés. / In the context of high efficiency solar cells (SCs) based on crystalline silicon (c-Si), the development of "passivating" contact structures to limit the recombination of charge carriers at the interface between the metal electrode and the c-Si has been identified as the next step to further improve the photovoltaic (PV) conversion efficiency. Passivating contacts consisting of a highly doped poly-crystalline silicon layer (poly-Si) on top of a thin layer of silicon oxide (SiOx ≤ 2 nm) are particularly sparking interest as they already demonstrated promising conversion efficiency when integrated in SCs.The objectives of this work are to develop a poly-Si/SiOx passivating contact compatible with the industrial production of c-Si SCs, and to investigate the passivation and charge transport mechanisms in the region of the thin SiOx layer located at the interface between the poly-Si and the c-Si.In this work, a boron-doped poly-Si/SiOx contact was fabricated. The doping of the layer was first performed in-situ during the deposition of a hydrogen-rich amorphous silicon (a-Si:H) layer by plasma-enhanced chemical vapor deposition (PECVD). The PECVD step was followed by an annealing step for crystallization of the poly-Si layer. The PECVD presents the advantages of being widespread in the PV industry and enabling the fabrication of the poly-Si contact on a single side of the c-Si substrate. However, it induces a high concentration of hydrogen in the deposited layer, which causes the formation of blisters at the interface with the c-Si and tends to degrade the surface passivation properties of the contact after annealing for crystallization. The optimization of the deposition conditions (temperature and H2/SiH4 gas ratio) enabled to obtain blister-free in-situ doped poly Si layers. An alternative doping method consisting of the deposition of a boron-rich dielectric layer on top of the poly-Si layer was applied to reduce the hydrogen content of the deposited layer. This approach enabled to obtain thicker blister-free poly-Si layers. The diffusion of hydrogen in the contact after annealing is known to provide a further chemical passivation of the poly-Si/c-Si interface. In this work, the addition of a hydrogenation step enabled to obtain state-of-the-art surface passivation properties for the two types of poly Si/SiOx contact fabricated.After developing the poly-Si/SiOx contact, a study of the effect of the annealing step on the chemical and structural properties of the SiOx layer was performed. Results indicated a possible improvement of the stoichiometry of the layer towards SiO2 as well as a degradation of its homogeneity at the poly-Si/c-Si interface after annealing at high temperature. These phenomena could be explained by a diffusion of the oxygen atoms content in the interfacial SiOx layer. The transport mechanism of charge carriers through the SiOx layer was conducted by C-AFM. This study revealed the limits of this technique to determine the presence of pinholes within the SiOx layer (that would help the transport of charge carriers). Finally, a method for characterizing recombinant defects at the interface between an intrinsic poly-Si junction and the c-Si has been developed. This method enabled to model the recombination phenomena at the poly-Si/c-Si interface via two apparent discrete defects. Their associated energy levels in the bandgap and ratios of electron and hole capture cross sections were estimated.

This diploma thesis presents experimental evidence of a theoretical concept which predicts the self-organization of delta-layers of silicon nanoclusters in the buried oxide of a MOS-like structure. This approach of "bottom-up" structuring might be of eminent importance in view of future semiconductor memory devices. Unconventionally, a 15nm thin SiO2 layer, which is enclosed by a 50nm poly-Si capping layer and the Si substrate, is irradiated with Si+ ions. Ion impact drives the system to a state far from thermodynamic equilibrium, i.e. the local composition of the target is modified to a degree unattainable in common processes. A region of SiOx (x<2) - where x is a function of depth - is formed which is not stable. During annealing, the system relaxes towards equilibrium, i.e. phase separation (via spinodal decomposition and nucleation) sets in. Within a certain time window of annealing, the structure of the system matches with a structure similar to the multidot non-volatile memory device, the principal character of which is a 2D layer of Si nanoclusters of ~3nm in diameter which is embedded in a 3D SiO2 matrix at a distance of ~3nm from the Si substrate. The physical mechanisms of ion mixing of the two Si-SiOx interfaces and subsequent phase separation, which result in the desired sample structure, are elucidated from the viewpoint of computer simulation. In addition, experimental evidence is presented based on various methods, including TEM, RBS, and SIMS. Of particular importance is a novel method of Si nanocluster decoration which applies Ge as contrast enhancing element in TEM studies of tiny Si nanoclusters.