L’apparition de fibres de SiC de haute pureté au cours des années 1990 a permis de considérer leur utilisation dans les réacteurs nucléaires en tant que renfort des composites à matrice SiC, dits composites SiC/SiC. Envisagés en tant que matériau de cœur des réacteurs du futur (SFR, GFR) et en remplacement des gaines en zirconium des réacteurs REP, la compatibilité thermochimique du SiC et des composites SiC/SiC avec l’environnement de tels réacteurs a été examinée. Des traitements thermiques en systèmes ouvert et fermé entre le SiC et l’UO2 ont montré une réactivité marquée au-delà de 1200°C avec la formation principalement de CO et de siliciures d’uranium dans la phase condensée, voire l’apparition d’une phase liquide entre 1500 et 1650°C. L’exposition au sodium liquide à 550°C jusqu’à 2000h a été étudiée en fonction de la concentration en oxygène. Une amélioration des propriétés mécaniques des composites SiC/SiC fabriqués pour cette étude (déformations et contraintes à rupture plus importantes) a été mise en évidence après immersion quelle que soit la concentration en oxygène du sodium liquide. Enfin, les compositions et les cinétiques de croissance des zones de réaction du SiC avec le niobium et le tantale (métaux utilisés pour assurer l’étanchéité d’une gaine en composite SiC/SiC) ont été mesurées entre 1050 et 1500°C. / The development of high purity SiC fibers during the nineties has led to their consideration as nuclear reactors components through the use of SiC/SiC composites. SiC and SiC/SiC composites are considered as core materials of future nuclear reactors (SFR, GFR) and as a potential replacement for the zirconium cladding of PWR. Therefore, the thermochemical compatibility of these materials with typical environments of those nuclear reactors has been studied. The composition and the growth kinetics of the reaction zone of SiC towards niobium and tantalum (considered as materials to ensure the leak-tightness of a SiC/SiC cladding for GFR) have been studied between 1050 and 1500°C. High temperature heat treatments in open and closed systems between SiC and UO2 have shown a significant reactivity over 1200°C characterized by the formation of CO and uranium silicides. Moreover, a liquid phase has been detected between 1500 and 1650°C. The exposure of SiC/SiC to liquid sodium (550°C, up to 2000h) has been studied as a function of the oxygen concentration dissolved in liquid sodium. An improvement of the mechanical properties of the composites elaborated for this study (increase of the tensile strength and strain at failure) has been highlighted after immersion in the liquid sodium independently of its oxygen concentration. It is believed that this phenomenon is due to the presence of residual sodium in the material.

Silicon Carbide is a wide bandgap semiconductor with excellent physical and opto-electrical properties. Among these excellent properties are its radiation hardness, high temperature operation and high electric field breakdown. SiC can therefore be used in the fabrication of electronic devices capable of operating in harsh environments, e.g. radiation detectors. Like any other semiconductor, the success of SiC in device fabrication depends on elimination of defects that are detrimental to desired devices or controlled introduction of desired energy levels. The first step in so doing is understanding the defects that are either found in as grown material, introduced during device fabrication or introduced during device operation. In this study nickel ohmic and Schottky contacts were resistively fabricated on n-type 4H-SiC with a net doping density of 4 × 1014 cm-3. Current-Voltage (I-V), Capacitance-Voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and Laplace-DLTS measurement techniques were used to electrically characterize the fabricated Schottky diodes. The diodes were then irradiated with low energy electrons, alpha particles and protons. The characterization measurements were repeated after irradiation to evaluate the effect of irradiation on the electrical properties of SiC. It was observed from I-V measurements that electron, alpha particle and proton irradiations do not significantly affect the rectification of Ni/SiC Schottky contacts. C-V measurements indicated that the free carrier removal rate is higher for alpha particle irradiation as compared to electron irradiation. The irradiated diodes were annealed in argon ambient and significant recovery in the free carrier concentration was observed below 600 °C. The free carrier concentration of proton irradiated Schottky contacts, which was decreased to below detection levels was also partly recovered after heat treatment of up to 400 °C. DLTS and Laplace-DLTS measurements revealed the presence of four defect levels in as-grown 4H-SiC. These defects have been labelled E0.10, E0.12, E0.17 and E0.69 where the subscripts indicate the activation energies of the respective defects. Electron, alpha particle and proton irradiations were observed to induce three more defect levels with activation energies of 0.42 eV, 0.62 eV and 0.76 eV. Additionally, these irradiations were also observed to enhance the concentration of level E0.69. All the radiation induced defects were annealed out at temperatures below 600 °C. In proton irradiated diodes, another defect with activation energy of 0.31 eV was observed after annealing the irradiated diodes at 625 °C. / Dissertation (MSc)--University of Pretoria, 2014. / lk2014 / Physics / MSc / Unrestricted

Deep ultraviolet light emitting diodes (UV LEDs) are an important emerging technology for a number of applications such as water/air/surface disinfection, communications, and epoxy curing. However, as of yet, deep UV LEDs grown on sapphire substrates are neither efficient enough nor powerful enough to fully serve these and other potential applications. The majority of UV LEDs reported so far in the literature are grown on sapphire substrates and their design consists of AlGaN quantum wells (QWs) embedded in an AlGaN p-i-n junction with the n-type layer on the sapphire. These devices suffer from a high concentration of threading defects originating from the large lattice mismatch between the sapphire substrate and AlGaN alloys. Other issues include the poor doping efficiency of the n- and particularly the p-AlGaN alloys, the extraction of light through the sapphire substrate, and the heat dissipation through the thermally insulating sapphire substrate. These problems have historically limited the internal quantum efficiency (IQE), injection efficiency (IE), and light extraction efficiency (EE) of devices. As a means of addressing these efficiency and power challenges, I have contributed to the development of a novel inverted vertical deep UV LED design based on AlGaN grown on p-SiC substrates. Starting with a p-SiC substrate that serves as the p-type side of the p-i-n junction largely eliminates the necessity for the notoriously difficult p-type doping of AlGaN alloys, and allows for efficient heat dissipation through the highly thermally conductive SiC substrate. UV light absorption in the SiC substrate can be addressed by first growing p-type doped distributed Bragg reflectors (DBRs) on top of the substrate prior to the deposition of the active region of the device. A number of n-AlGaN films, AlGaN/AlGaN multiple quantum wells, and p-type doped AlGaN DBRs were grown by molecular beam epitaxy (MBE). These were characterized in situ by reflected high energy electron diffraction (RHEED) and ex situ by x-ray diffraction, scanning electron microscopy, atomic force microscopy, photoluminescence, and reflectivity. Using the primary elements of the proposed design, this research culminated in the MBE growth, fabrication, and characterization of prototype deep UV LED devices emitting below 300 nm.

This master’s thesis deals with design and construction of experimental flyback converter with utilization of novel lossless clamp circuit for switching transistor and with utilization of silicon carbide devices. The issue of flyback converter for higher power and possible control strategies are discussed. The thesis also describes power and control circuits design, design of PCB, construction of the converter and measured waveforms. End of the thesis contains technical documentation of designed board. Designed converter was successfully built and tested.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第17916号 / エネ博第288号 / 新制||エネ||60(附属図書館) / 30736 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 小西 哲之, 教授 星出 敏彦, 教授 木村 晃彦 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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Photoluminescence spectroscopy is one of the most efficient and sensitive non-contact techniques used to investigate defects in SiC. In this work, room temperature photoluminescence mapping is employed to identify different defects that influence material properties. The correlation of the distribution of these defects in n-type 4H-SiC substrates with electronic properties of SiC revealed connection between the deep levels acting as efficient recombination centers and doping in the substrate. Since deep levels are known to act as minority carrier lifetime killers, the obtained knowledge may contribute to our ability to control important characteristics such as minority carrier lifetime in SiC. In semi-insulating (SI) 6H-SiC, the correlation between room temperature infrared photoluminescence maps and the resistivity maps is used to identify deep defects responsible for semi-insulating behavior of the material. Different defects were found to be important in different families of SI SiC substrates, with often more than one type of defect playing a significant role. The obtained knowledge is expected to enhance the yield of SI SiC fabrication and the homogeneity of the resistivity distribution across the area of large SiC substrates.

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Global energy consumption continues to grow, driving the need for cheap, power-dense power electronics. Replacing the incumbent silicon insulated gate bipolar transistors with silicon carbide (SiC) metal oxide semiconductor field effect transistors (MOSFETs) has been proposed as a solution to increase the power densities of power converters in some applications. One such application is electric vehicles (EVs) where the efficiency and weight of the power electronics are critical; however, modern packaging technologies are still limiting the performance of SiC MOSFETs. One promising trend in power semiconductor packaging technologies is the use of printed circuit boards (PCBs) because the technology is mature—resulting in low costs—and the allowable stackups are ideal for integrating driving circuitry and power loop components—resulting in reduced manufacturing complexity. This thesis presents the design and analysis of two PCB-embedded 1.2 kV SiC MOSFET half-bridge packages and a hybrid PCB/DBC-based 1.2 kV SiC MOSFET full-bridge package for EV on-board charger applications. The first of the two PCB-embedded packages has integrated gate drive circuitry, less than 2.3 nH loop inductances, and dual-sided cooling with a total junction-to-case thermal resistance (RTH,JC) of 0.12 K/W. The second PCB-embedded package has only drain-side cooling to allow for surface mount terminals, has an area of 37.1 mm x 18.5 mm due to the removal of the gate drive circuitry, and has less than 2.4 nH loop inductances. The PCB/DBC-based full-bridge package has an RTH,JC of 0.65 K/W, less than 4.5 nH, and integrated gate drive circuitry. / M.S. / The continued increase in global energy consumption has led to concerns about sustainability, and as renewable energy generation is adopted more broadly, more efficient means of converting electrical energy from one form to another are required. Some applications, such as electric vehicles (EVs), also require a lightweight and a low volume from their converters in addition to high efficiency. The packaging of the semiconductors used in converters is important to the overall electrical efficiency of the converter and can also have an impact on the size of the converter as well. This thesis explores the design and analysis of three package structures for the semiconductors used in the on-board charger of an EV. These package structures are unified under the common theme of using printed circuit boards (PCBs) in the package itself. PCBs are commonly used to route the electrical connections between packaged semiconductors and other components in the converter, but they are not usually integrated into the package itself. The hope is that by integrating the PCB into the semiconductor package, higher-efficiency, lighter-weight, and smaller-volume converters will be possible.