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Abstract This thesis studies the factors that govern transverse cracking during continuous casting of low carbon, niobium microalloyed and boron microalloyed steels. Crack susceptibility in the thick slab, billet and thin slab casting processes are compared by using typical conditions in laboratory hot ductility tests. There is limited published literature on hot ductility in aluminium-killed and siliconkilled boron microalloyed steels and the proposed mechanisms of failure by transverse cracking are contradictory. Few published papers specifically compare hot ductility behaviour of any steels between thick slab, billet and thin slab continuous casting processes. Thus, the basis of this research is to assess the influence of casting parameters and compositional variations on hot ductility behaviour in low carbon steels, niobium microalloyed steels, aluminium-killed boron microalloyed steels and silicon-killed, boron microalloyed steels. The typical temperature ranges, cooling rate and strain rate conditions of the continuous casting processes were used in reheated and in situ melted hot tensile tests performed on steel specimens. Solidification, transformation and precipitation temperatures were calculated using solubility equations and modelled using the Thermo-CalcTM thermodynamics program. Scanning electron microscopy and transmission electron microscopy were used to determine the modes of failure in the tested specimens. In the low carbon steels, hot ductility was improved by increasing the strain rate; by calcium treatment, which minimises copper sulphide and iron sulphide formation; and by maintaining a nickel to copper ratio of 1:1. It was shown that thin slab casting conditions provided the best hot ductility results for the low carbon steels. All the niobium steels showed poor ductility in the single-phase austenite temperature region, indicating that intergranular precipitation of fine niobium carbonitrides was the cause of the poor ductility. It was shown that the hot ductility was greatly improved by calcium treatment, by decreasing the cooling rate and by increasing the strain rate. Slow iv thin slab and thick slab casting conditions provided the best hot ductility results for the niobium steels. Hot ductility was substantially improved in the aluminium-killed boron steels by increasing the boron to nitrogen ratio from 0.19 to 0.75. The results showed that, at cooling rates generally associated with thick slab, bloom and slow thin slab casting, a boron to nitrogen ratio of ≥0.47 was sufficient to avoid a ductility trough altogether. However, under conditions typically experienced in fast thin slab and billet casting, a boron to nitrogen ratio of 0.75 was required to provide good hot ductility. The mechanism of the ductility improvement with increasing boron to nitrogen ratio was found to be enhanced precipitation of boron nitride, leading to a decrease in nitrogen available for aluminium nitride precipitation. In the silicon-killed boron steels, it was found that the boron to nitrogen ratio had the overriding influence on hot ductility and hence on crack susceptibility. Excellent hot ductility was found for boron to nitrogen ratios above 1. Additionally, analysis of industrial casting data showed that the scrap percentage due to transverse cracking increased significantly at manganese to sulphur ratios below fourteen. An exponential decay relationship between the manganese to sulphur ratio and the average scrap percentage due to transverse cracking was determined as a tool to predict scrap levels in the casting plant.

Thin film coatings are used to improve the properties of components and products in such diverse areas as tool coatings, wear resistant biological coatings, miniature integrated electronics, micro-mechanical systems and coatings for optical devices. This thesis focuses on understanding the development of intrinsic stress and microstructure in coatings of the technologically important materials of aluminium nitride (AlN) and titanium vanadium nitride (TiVN) deposited by filtered cathodic arc deposition. Thin films of AlN are fabricated under a variety of substrate bias regimes and at different deposition rates. Constant substrate bias was found to have a significant effect on the stress and microstructure of AlN thin films. At low bias voltages, films form with low stress and no preferred orientation. At a bias voltage of -200 V, the films exhibited the highest compressive stress and contained crystals preferentially oriented with their c axis in the plane of the film. At the highest bias of -350 V, the film forms with low stress yet continue to contain crystallites with their c axis constrained to lie in the plane of the film. These microstructure changes with bias are explained in terms of an energy minimisation model. The application of a pulsed high voltage bias to a substrate was found to have a strong effect on the reduction of intrinsic stress within AlN thin films. A model has been formulated that predicts the stress in terms of the applied voltage and pulsing rate, in terms of treated volumes known as thermal spikes. The greater the bias voltage and the higher the pulse rate, the greater the reduction in intrinsic stress. At high pulsing and bias rates, a strong preference for the c axis to align perpendicular to the substrate is seen. This observation is explained by dynamical effects of the incident ions on the growing film, encouraging channelling and preferential sputtering. For the first time, the effect of the rate of growth on AlN films deposited with high voltage pulsed bias was investigated and found to significantly change the stress and microstructure. The formation of films with highly tensile stress, highly compressive stress and nano-composites of AlN films containing Al clusters were seen. These observations are explained in terms of four distinct growth regions. At low rates, surface diffusion and shadowing causes highly porous structures with tensile stress; increased rates produced Al rich films of low stress; increasing the growth rate further led to a dense AlN film under compressive stress and the highest rates produce dense, low stress, AlN due to increased levels of thermal annealing. Finally this thesis analyses the feasibility of forming ternary alloys of high quality TiVN thin films using a dual cathode filtered cathodic arc. The synthesised films show exceptional hardness (greater than either titanium nitride or vanadium nitride), excellent mixing of the three elements and interesting optical properties. An optimum concentration of 23% V content was found to give the highest stress and hardness.

Le travail présenté ici est une contribution à l’étude des propriétés de photoluminescence (PL) d’ions de terre rare (Erbium) insérés dans des matrices grand gap (AlN) de différentes morphologies et déposées sous forme de films minces. Une méthode de dépôt PVD magnétron rf a été utilisée pour obtenir des dépôts minces de nitrure d’aluminium dont les morphologies cristallines couvrent une gamme allant de la morphologie colonnaire classique jusqu’à l’état nano cristallisé et amorphe. On montre comment, plus la puissance magnétron est élevée plus les cristallites colonnaires d’AlN sont de grande dimension et comment l’application d’une polarisation négative sur les substrats de silicium permet l’obtention de dépôts nano cristallisés. Différents taux de dopage, de 0.1 à 6 at. %, sont obtenus avec une cible composite Al+Er.La PL à 1.54 [micro]m de l’atome d’Er a été étudiée en fonction des valeurs des paramètres « procédé » et donc en fonction des morphologies de AlN. Il a été montré que le maximum d'émission de PL a lieu pour un dopage de 1 at. %. L’étude montre que l'intensité de PL augmente avec la puissance magnétron et diminue avec l’intensité de polarisation des substrats. Ces deux résultats montrent que l'intensité PL est fortement corrélée à la morphologie des films. Plus les cristallites sont importantes, plus l'émission de PL est efficace. Cette corrélation entre la PL et la morphologie des matrices a permis de mettre en évidence le rôle des champs cristallins des défauts non radiatifs dans les cristallites. Le rôle des défauts a été confirmé par des mesures de luminescence résolue en temps, des mesures sur dépôts recuits et des mesures de PL à basse température. L’effet de diminution de la PL avec la température est très faible ce qui rend le matériau très prometteur pour des applications en optoélectronique et en photonique / The work presented here is a contribution to the study of the photoluminescence (PL) properties of a rare earth ions (Erbium) inserted inside wide gap matrices (AlN) of different morphologies and deposited as thin films. A physical vapour deposition magnetron rf technique has been used to obtain thin layers of aluminium nitride whose crystalline morphologies are ranging from the classical columnar morphology to the nanocrystalline state or amorphous. One shows how, the higher the magnetron power, the larger are the columnar crystallites and how the use of a negative polarization on the silicon substrates allows obtaining nano crystallized layers. Different doping rates (from 0.1 to 6 at. %) have been achieved using a Al+Er composite target.The PL of the Er atom at 1.54 [micro]m has been studied versus the process parameters and so as a function of the different AlN morphologies. It was shown that the maximum of PL emission is achieved for a rate of 1 atomic %. PL intensity was shown to increase with the magnetron power and decrease with the polarization intensity of the substrates. These two results demonstrate that PL intensity is strongly correlated to the matrix morphology. The larger the crystallites, the most efficient are the PL emission allows evidencing the role of the non radiative defects crystalline fields in the crystallites. The role of the defects was confirmed by time resolved photoluminescence measurements and by PL measurements performed on annealed samples or at low temperature. The decrease of PL with temperature is very weak, making this way the material very promising for optoelectronic and photonic applications

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The global energy market is continuously changing due to changes in demand and fuel availability. Amongst the technologies considered as capable of fulfilling these future energy requirements, Photovoltaics (PV) are one of the most promising. Currently the majority of the PV market is fulfilled by crystalline Silicon (c-Si) solar cell technology, the so called 1st generation PV. Although c-Si technology is well established there is still a lot to be done to fully exploit its potential. The cost of the devices, and their efficiencies, must be improved to allow PV to become the energy source of the future. The surface of the c-Si device is one of the most important parts of the solar cell as the surface defines the electrical and the optical properties of the device. The surface is responsible for light reflection and charge carrier recombination. The standard surface finish is a thin film layer of silicon nitride deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). In this thesis an alternative technique of coating preparation is presented. The HiTUS sputtering tool, utilising a remote plasma source, was used to deposit the surface coating. The remote plasma source is unique for solar cells application. Sputtering is a versatile process allowing growth of different films by simply changing the target and/or the deposition atmosphere. Apart from silicon nitride, alternative materials to it were also investigated including: aluminium nitride (this was the first use of the material in solar cells) silicon carbide, and silicon carbonitride. All the materials were successfully used to prepare solar cells apart from the silicon carbide, which was not used due to too high a refractive index. Screen printed solar cells with a silicon nitride coating deposited in HiTUS were prepared with an efficiency of 15.14%. The coating was deposited without the use of silane, a hazardous precursor used in the PECVD process, and without substrate heating. The elimination of both offers potential processing advantages. By applying substrate heating it was found possible to improve the surface passivation and thus improve the spectral response of the solar cell for short wavelengths. These results show that HiTUS can deposit good quality ARC for silicon solar cells. It offers optical improvement of the ARC s properties, compared to an industrial standard, by using the DL-ARC high/low refractive index coating. This coating, unlike the silicon nitride silica stack, is applicable to encapsulated cells. The surface passivation levels obtained allowed a good blue current response.

Cette thèse se place dans le contexte des recherches menées sur l'élaboration de support de haute qualité cristalline pour des applications optoélectronique et piézoélectrique. Les nitrures d'aluminium, AlN, et de bore, BN sont deux matériaux présentant des propriétés physiques intéressantes pour leurs utilisations en tant que substrat et partiellement comme couche active dans de telles applications. Les objectifs de cette thèse étaient de continuer les travaux en cours sur l'hétéroépitaxie d'AlN (avec le mélange H2 – NH3 – AlCl3 en phase gazeuse) sur substrat saphir et silicium, et d'explorer la croissance de BN par dépôt chimique en phase vapeur (CVD) à haute température avec une chimie chlorée (mélange.H2 – NH3 – BCl3 en phase gazeuse). Des études thermodynamiques ont été menées pour évaluer les équilibres ayant lieu entre la phase gazeuse et les matériaux en présence sur une large gamme de température. Ces premiers résultats ont permis d'en déduire des conditions opératoires favorables afin d'éviter toutes réactions parasites qui nuiraient à la croissance des nitrures. Plusieurs études expérimentales ont été effectuées sur les réacteurs du SIMaP. Une étude de l'influence du ratio N/Al dans la phase gazeuse sur la croissance d'AlN a été entreprise. Par la suite les mécanismes de croissance de ces couches sont expliqués afin de comprendre l'effet de ce paramètre. Suite à cela, des dépôts avec plusieurs étapes de croissances à différente température ont permis l'obtention de couches d'AlN peu fissurées, peu contraintes et avec des qualités cristallines satisfaisantes. Concernant le dépôt de BN, des essais ont été menés sur substrats AlN et métalliques (chrome et tungstène). À haute température (1600 °C), le dépôt sur AlN a permis l'obtention de couche turbostratique peu désorientée. La croissance sur substrats métalliques a été effectuée à basse température, ne favorisant pas l'épitaxie de BN sur ces substrats. Enfin, des comparaisons ont été menées entre température de dépôt, vitesse de croissance des couches et sursaturation de la phase gazeuse, permettant la délimitation de domaine de conditions opératoire où l'épitaxie est favorisée. / This work takes place in the context of the development of high crystalline quality supports for optoelectronic and piezoelectric fields. Aluminum and boron nitrides (AlN, and BN) are both materials with interesting physical properties that are used like substrate or active layers in such devices. The aims of this thesis were to continue the work in progress about AlN epitaxy in SIMaP, and to explore the growth of BN by chemical vapor deposition (CVD) with halide chemistry at high temperature. Thermodynamical studies were lead in order to evaluate the equilibrium between the gas phase and the materials in a wide range of temperature. The results were used to choose operating conditions in order to avoid parasitic reactions that could decrease the nitrides growth quality. Several experimental studies were done to evaluate the influence of the N/Al ratios in the gas phase. Growth mechanisms of these layers are explained and consequences of the growth are linked to crystal quality and strain states of the films. Next, multi-steps growth with several temperatures was lead and shows an interesting improvement of the crystal quality and strain state. BN deposits were done on AlN and metallic substrates (chromium and tungsten). High temperature growth was performed on AlN and lead high quality turbostratic films. For lower temperature, BN deposits were done on metallic substrates and lead to the growth of the turbostratic phase. Finally, a comparison between deposition temperature, the growth speed of the films and supersaturation of the gas phase allow to estimate operating conditions domains were the epitaxy of the nitrides are predominant.

Les ondes acoustiques, démontrées théoriquement en 1885 par le scientifique anglais Lord Rayleigh, constituent de nos jours un sujet de recherches très intéressant. Elles sont devenues indispensables à la fabrication des systèmes de télécommunication miniatures et performants, tels que par exemple les filtres, les oscillateurs ou encore les capteurs. Les dispositifs fonctionnant grâce aux ondes acoustiques sont connus sous le nom de « dispositifs piézoélectriques » puisqu’ils transforment les signaux RF en ondes acoustiques, et vice versa, grâce au phénomène piézoélectrique direct. Le développement de ces dispositifs piézoélectriques a été indispensable pour répondre aux exigences particulières et extrêmes des systèmes de télécommunication actuels (sélectivité, miniaturisation, faible coût, facilité de fabrication et d’intégration). Cette thèse s’inscrit dans une démarche générale de développement des dispositifs piézoélectriques, notamment des micro-résonateurs piézoélectriques qui en constituent la dernière génération. Deux axes principaux ont été développés au cours de ces travaux de recherches : l’étude théorique des micro-résonateurs piézoélectriques à travers une modélisation électrique d’une part, et, d’autre part, la description des procédés de fabrication réalisés en salle blanche du laboratoire d’XLIM. / The acoustic waves, theoretically demonstrated in 1885 by the English scientist Lord Rayleigh, are nowadays an interesting research subject. It became essential to the fabrication of miniature and efficient systems of telecommunication, such as filters, oscillators or sensors. Devices using the acoustic waves are known as piezoelectric devices, because they transform RF signal into acoustic waves, and vice versa, thanks to the direct piezoelectric phenomenon. The development of these piezoelectric devices was essential to meet the particular and extreme requirements of the current systems of telecommunication (selectivity, miniaturization, low cost, ease of manufacturing and integration). This thesis is part of a global approach to develop the piezoelectric devices, notably the piezoelectric micro-resonators which constitute the latest generation. Two main axes have been developed during the research work: the theoretical study of piezoelectric micro-resonators through an electric modelling, on one hand, and, on the other hand, the description of the manufacturing processes accomplished in clean room of XLIM’s laboratory.