<p>Silicon carbide is a promising wide bandgap semiconductormaterial for high-temperature, high-power, and high-frequencydevice applications. However, there are still a number offactors that are limiting the device performance. Among them,one of the most important and critical factors is the formationof low resistivity Ohmic contacts and high-temperature stableSchottky diodes on silicon carbide.</p><p>In this thesis, different metals (TiW, Ti, TiC, Al, and Ni)and different deposition techniques (sputtering andevaporation) were suggested and investigated for this purpose.Both electrical and material characterizations were performedusing various techniques, such as I-V, C-V, RBS, XRD, XPS,LEED, SEM, AFM, and SIMS.</p><p>For the Schottky contacts to n- and p-type 4H-SiC, sputteredTiW Schottky contacts had excellent rectifying behavior afterannealing at 500 ºC in vacuum with a thermally stableideality factor of 1.06 and 1.08 for n- and p-type,respectively. It was also observed that the SBH for p-type SiC(Φ_Bp) strongly depends on the choice the metal with alinear relationship Φ_Bp= 4.51 - 0.58Φ_m, indicating no strong Fermi-level pinning.Finally, the behavior of Schottky diodes was investigated byincorporation of size-selected Au nano-particles in Ti Schottkycontacts on silicon carbide. The reduction of the SBH isexplained by using a simple dipole layer approach, withenhanced electric field at the interface due to the small sizeof the circular patch (Au nano-particles) and large differenceof the barrier height between two metals (Ti and Au) on both n-and p-SiC.</p><p>For the Ohmic contacts, titanium carbide (TiC) was used ascontacts to both n- and p-type 4H-SiC epilayers as well as onAl implanted layers. The TiC contacts were epitaxiallydeposited using a co-evaporation method with an e-beam Tisource and a Knudsen cell for C₆₀, in a UHV system at low substrate temperature(500 ºC). In addition, we extensively investigatedsputtered TiW (weight ratio 30:70) as well as evaporated NiOhmic contacts on both n- and p-type epilayers of SiC. The bestOhmic contacts to n-type SiC are annealed Ni (>950ºC)with the specific contact resistance of ≈ 8× 10^-6Ω cm²with doping concentration of 1.1 × 10^-19cm^-3while annealed TiW and TiC contacts are thepreferred contacts to p-type SiC. From long-term reliabilitytests at high temperature (500 ºC or 600 ºC) invacuum and oxidizing (20% O₂/N₂) ambient, TiW contacts with a platinum cappinglayer (Pt/Ti/TiW) had stable specific contact resistances for>300 hours.</p><p><b>Keywords</b>: silicon carbide, Ohmic and Schottky contacts,co-evaporation, current-voltage, capacitance-voltagemeasurement, power devices, nano-particles, Schottky barrierheight lowering, and TLM structures.</p>

Cette thèse s’inscrit dans le cadre du développement d’une filière de transistors de puissance à base de nitrure de Gallium au CEA-LETI. Ces transistors, en particulier les HEMT utilisant l’hétérostructure AlGaN/GaN, présentent des propriétés très utiles pour les applications de puissance. L’essor de cette technologie passe notamment par le développement de contacts ohmiques peu résistifs. Cette thèse a pour objectif d’approfondir la compréhension des mécanismes de formation du contact ohmique sur une structure AlGaN/GaN. Dans un premier temps, une étude thermodynamique sur une dizaine de métaux de transition utilisables comme base de l’empilement métallique du contact a été menée, ce qui a permis de retenir une métallisation Ti/Al. Puis, les différentes réactions physico-chimiques de cet empilement avec des substrats nitrurés ont été étudiées en faisant varier la composition et les températures de recuit de formation du contact ohmique. Enfin, plusieurs études sur structure AlGaN/GaN couplant caractérisations électriques et physico-chimiques ont permis d’identifier des paramètres décisifs pour la réalisation d’un contact ohmique, peu résistif et nécessitant une faible température de recuit. / This PhD is part of the development of Gallium nitride based power transistors at the CEA-LETI. These transistors, especially those based on AlGaN/GaN heterostructure, are very promising for power electronics applications. The goal of this PhD is to increase the knowledge of the mechanisms responsible for the ohmic contact formation on a AlGaN/GaN structure. First, a thermodynamic study of several transition metals has been performed, leading us to select Ti/Al metallization. Then, the multiple physico-chemical reactions of this stack with nitride substrates have been studied depending on the stack composition and the annealing temperature. Finally, several studies on AlGaN/GaN structure coupling both physico-chemical and electrical characterizations reveal different decisive parameters for the formation of an ohmic contact with a low-resistance and a low annealing temperature.

Les phases MAX sont des carbures ou nitrures ternaires dont les propriétés sont généralement décrites comme la combinaison exceptionnelle des meilleures propriétés des métaux et des céramiques. Sous forme de couches minces, ces matériaux sont prometteurs en tant que contact ohmique sur des substrats de SiC pour la microélectronique de puissance. Des approches originales dédiées à l'obtention de films minces épitaxiés des phases MAX Ti2AlN, Ti3SiC2 et Ti3(Si,Ge)C2 sont développées dans ce travail. Des recuits à 750°C de systèmes multicouches (Ti+Al)/AlN permettent ainsi de former des couches de Ti2AlN fortement texturées sur des substrats de SiC ou Al2O3. La seconde approche consiste à recuire à 1000°C des couches de TixAly ou TixGey, déposés sur 4H-SiC, pour obtenir des films minces épitaxiés de Ti3SiC2 et Ti3(Si,Ge)C2. Ces derniers présentent les caractéristiques d'un contact ohmique sur SiC. / MAX phases are a family of ternary carbides or nitrides which properties are generally described as an exceptional combination of the best properties of metals and ceramics. Thin films of MAX phases being considered as good candidates for ohmic contacts on SiC substrates for power microelectronics devices, thin films of Ti2AlN and Ti3(Si,Ge)C2 were synthesized by using original approaches. Highly textured Ti2AlN thin films were so obtained by thermal annealing at 750°C of (Ti+Al)/AlN multilayers whereas epitaxial thin films of Ti3SiC2 on 4H-SiC were achieved after an annealing at 1000°C of TixAly or TixGey layers. Good ohmic contact behaviors of Ti3SiC2 layers were confirmed in this work whereas Ti2AlN thin films behave as Schottky barriers.

In 1972 Fujishima and Honda conceptualised a photoelectrochemical cell for hydrogen generation via PEC water splitting. Hydrogen as a clean energy carrier provides environmentally friendly energy storage solutions or can fuel certain applications. This idea has since then been further built upon with new materials and combinations with the aim of improving efficiency. In this project n-type cubic silicon carbide thick layers were grown by a sublimation method and characterised for water splitting performance. A generated photo-current density of 0.45 mA/cm2 was measured with no bias between the working and counter electrodes.

L'eau de coco, liquide transparent situé à l'intérieur de la noix de coco (Cocos nucifera L.), est une boisson tropicale rafraîchissante dont le marché est en pleine expansion. Ce jus de fruit possède des propriétés originales dues à sa composition en sels minéraux, sa faible teneur en sucres solubles et son arôme. Cependant, ses propriétés physico-chimiques ont été peu étudiées et sa stabilisation reste un challenge technologique, notamment en raison de la présence de deux enzymes PPO et POD et de la thermosensibilité des composés aromatiques. Aussi ce travail s'attache, premièrement, à décrire les caractéristiques physicochimiques et le profil aromatique de l'eau de coco de cinq variétés récoltées à trois stades de maturité. A partir de ces données, un indice global de qualité de la matière première a été défini et permet de prédire le potentiel de transformation en boisson d'une variété de cocotier. Deuxièmement, une étude de la dégradation de l'eau de coco à température ambiante tropicale (30°C) a permis de détecter les phases critiques du phénomène. Enfin, des essais de stérilisation de l'eau de coco par deux technologies différentes ont été réalisés : dans un réacteur batch de traitement ohmique et dans un pilote d'ultrafiltration avec des membranes de 10, 20, 50 et 100 nm. Le chauffage ohmique a démontré sa capacité à inactiver les enzymes PPO et POD (barème de traitement optimal de 5s à 140°C) mais il provoque des modifications du profil aromatique. L'ultrafiltration avec une membrane de 20 nm permet d'obtenir des densités de flux intéressantes d'un point de vue économique, de retenir la totalité des enzymes mais il semble qu'une partie non négligeable des esters soit retenue par la membrane sélectionnée. / Coconut water, the clear liquid located into the coconut (Cocos nucifera L.) is a refreshing tropical beverage driving a valuable market expansion. This fruit juice exhibits original properties due to its specific mineral composition, its low content in soluble sugars, and its aroma. However, its physicochemical properties have been rarely investigated. Coconut water stabilization remains a technological challenge, in particular due to the presence of the two enzymes PPO and POD, and to the thermo-sensitivity of aromatic compounds. Thus, this work aimed first at describing the physicochemical characteristics and the aromatic profile of the coconut water of five varieties being collected at three stages of maturity. From these data, an overall raw material quality index was defined which will favor the prediction of the processing potential into beverage of a coconut variety. Secondly, an investigation of the coconut water degradation at tropical ambient temperature (30°C) highlighted the critical stages of the phenomenon. Finally, some coconut water sterilization trials were carried out using two different techniques: an ohmic heater batch reactor, and an ultrafiltration pilot with 10, 20, 50, and 100nm membranes. The ohmic heating favored the inactivation of both POD and PPO enzymes (with an optimal 5s/140°C treatment), while inducing some modification of the aromatic profile. The 20nm ultrafiltration membrane permitted to obtain valuable densities of flux from an economic point of view, the full retention of the enzymes, whereas a significant amount of ester compounds seemed to be retained by the selected membrane.

AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) are promising candidates for next generation high-efficiency and high-voltage power applications. The excellent physical properties of GaN-based materials, featuring high critical electric field and large carrier saturation velocity, combined to the high carrier density and large mobility of the two-dimensional electron gas confined at the AlGaN/GaN interface, enable higher power density minimizing power losses and self-heating of the device. However, the advent of the GaN-based MIS-HEMT to the industrial production is still hindered by technological challenges that are being faced in parallel. Among them, one of the biggest challenge is represented by the insertion of a gate dielectric in MIS-HEMTs compared to Schottky-gate HEMTs, which causes operational instability due to the presence of high-density trap states located at the dielectric/III-nitride interface or within the dielectric. The development of a gold-free ohmic contact technology is another important concern since the high-volume and cost-effective production of GaN-based transistors also depends on the cooperative manufacturing of GaN-based devices in Si production facilities, where gold represents an undesidered source of contamination. In fact, even though over the past years there have been multiple attemps to develop gold-free ohmic contacts, there is still no full understanding of the contact formation and current transport mechanism. The first objective of this work was the investigation of a gold-free and low-resistive ohmic contact technology to AlGaN/GaN based on sputtered Ta/Al-based metal stacks annealed at low temperatures. A low contact resistance below 1 Ω mm was obtained using Ta/Al-based metal stacks annealed at temperatures below 600 °C. The ohmic behavior and the contact properties of contact resistance, optimum annealing temperature and thermal stability of Ta/Al-based contacts were studied. The nature of the current transport was also investigated indicating a contact mechanism governed by thermionic field emission tunneling through the AlGaN barrier. Finally, gold-free Ta/Al-based ohmic contacts were integrated in MIS-HEMTs fabricated on a 150 mm GaN-on- Si substrate, demonstrating to be a promising contact technology for AlGaN/GaN devices and revealing to be beneficial for devices operating at high temperatures. The optimization of the MIS-gate structure in terms of trap states at the dielectric/III-nitride interface and inside the dielectric in MIS-HEMTs using atomic layer deposited (ALD) Al2O3 as gate insulator was the second focus of this work. First, the MIS-gate structure was improved by an O2 plasma surface preconditioning applied before the Al2O3 deposition and by an N2 postmetallization anneal applied after gate metallization, which significantly reduced trap states at the Al2O3/GaN interface and within the dielectric. Afterwards, the effectiveness of these treatments was demonstrated in Al2O3-AlGaN/GaN MIS-HEMTs by pulsed current–voltage measurements revealing improved threshold voltage stability. Lastly, it was shown that also the lower annealing temperatures used for the formation of Ta/Al-based ohmic contacts, processed before gate dielectric deposition, are beneficial in terms of trap states at the ALD-Al2O3/GaN interface, representing a new aspect to be considered when using an ohmic first fabrication approach.

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Le gaz électrogénéré dans les réacteurs électrochimiques est un phénomène à la fois électrochimique et hydrodynamique. La chute ohmique dans la solution électrolyte est l'un des paramètres importants à évaluer pour l'optimisation des réacteurs électrochimiques. Elle est due à la résistance de la solution, donc, à sa conductivité électrique et la distance entre les deux électrodes. Pour réduire la consommation énergétique de la cellule de réduction électrolytique de particules d'hématite en fer métallique, on a étudié la conception des anodes, sièges de la production des bulles d'oxygène, dans deux cellules équivalentes d'électrolyse d'eau dans un milieu alcalin. Les résultats ont montré que seulement 25% de l'anode est réellement active et que le taux de rétention augmente le long de l'anode et les bulles atteignent leur vitesse terminale dès 50% de la hauteur de l'anode. Ceci nous a permis de formuler des recommandations qui permettent d'avoir les meilleures conditions de désengagement des bulles électrogénérées, pour une consommation énergétique plus faible du procédé électrochimique / Electrogenerated gas in electrochemical reactors is considered as an electrochemical and hydrodynamic phenomenon. The ohmic drop in the electrolyte solution is one of important parameter to evaluate for the optimization design of electrochemical reactors. It is due to the resistance of the solution, therefore, its electrical conductivity and of the distance between the two electrodes. To reduce the energy consumption of the electrolytic reduction cell of hematite particles to metallic iron, we studied the design of anode, the location of oxygen bubbles production, in two equivalent cells for water electrolysis in an alkali media. The results showed that the gas hold up increases along the anode and only 25% of the initial anode height is actually active. Moreover the bubbles reach their terminal velocity after 50% of the initial anode height. This allowed us to formulate recommendations that allow the best conditions of bubbles electrogenerated disengagement and low energy consumption