abstract: Point of Load (PoL) converters are important components to the power distribution system in computer power supplies as well as automotive, space, nuclear, and medical electronics. These converters often require high output current capability, low form factor, and high conversion ratios (step-down) without sacrificing converter efficiency. This work presents hybrid silicon/gallium nitride (CMOS/GaN) power converter architectures as a solution for high-current, small form-factor PoL converters. The presented topologies use discrete GaN power devices and CMOS integrated drivers and controller loop. The presented power converters operate in the tens of MHz range to reduce the form factor by reducing the size of the off-chip passive inductor and capacitor. Higher conversion ratio is achieved through a fast control loop and the use of GaN power devices that exhibit low parasitic gate capacitance and minimize pulse swallowing. This work compares three discrete buck power converter architectures: single-stage, multi-phase with 2 phases, and stacked-interleaved, using components-off-the-shelf (COTS). Each of the implemented power converters achieves over 80% peak efficiency with switching speeds up-to 10MHz for high conversion ratio from 24V input to 5V output and maximum load current of 10A. The performance of the three architectures is compared in open loop and closed loop configurations with respect to efficiency, output voltage ripple, and power stage form factor. Additionally, this work presents an integrated CMOS gate driver solution in CMOS 0.35um technology. The CMOS integrated circuit (IC) includes the gate driver and the closed loop controller for directly driving a single-stage GaN architecture. The designed IC efficiently drives the GaN devices up to 20MHz switching speeds. The presented controller technique uses voltage mode control with an innovative cascode driver architecture to allow a 3.3V CMOS devices to effectively drive GaN devices that require 5V gate signal swing. Furthermore, the designed power converter is expected to operate under 400MRad of total dose, thus enabling its use in high-radiation environments for the large hadron collider at CERN and nuclear facilities. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2018

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22447号 / 工博第4708号 / 新制||工||1735(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 山田 啓文, 准教授 船戸 充 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Microsized hexagonal gallium nitride pyramids show promise as next generation Light Emitting Diodes (LEDs) due to certain quantum properties within the pyramids. One metric for evaluating the efficiency of a LED device is by studying its Light Extraction Efficiency (LEE). To calculate the LEE for different pyramid designs, simulations can be performed using the FDTD method. Maximizing the LEE is treated as a black-box optimization problem with an interpolation method that utilizes radial basis functions. A simple heuristic is implemented and tested for various pyramid parameters. The LEE is shown to be highly dependent on the pyramid size, the source position and the polarization. Under certain circumstances, a LEE over 17% is found above the pyramid. The results are however in some situations very sensitive to the simulation parameters, leading to results not converging properly. Establishing convergence for all simulation evaluations must be done with further care. The results imply a high LEE for the pyramids is possible, which motivates the need for further research.

Le domaine du spectre électromagnétique correspondant aux fréquences térahertz est encore peu exploité, pourtant, les applications nécessitant la génération, l’amplification ou la détection d’un signal térahertz sont nombreuses et intéressantes. Dans ce travail, nous nous intéressons tout particulièrement aux détecteurs plasmoniques, qui constituent une alternative prometteuse à la montée en fréquence des capteurs électroniques et l’utilisation de capteurs thermiques pour les photons de faible énergie. Les capteurs plasmoniques fonctionnent grâce au couplage entre le photon térahertz et un plasmon au sein d’un gaz d’électrons bidimensionnel (2DEG). Le plasmon-polariton est ensuite transformé en un signal continu et détectable. Nous utilisons pour cela le 2DEG présent dans l’hétérostructure AlGaN/GaN. Le couplage entre le photon et le plasmon est réalisé par un réseau métallique déposé sur la structure semi-conductrice. Tout d’abord, l’étude du couplage photon/plasmon-polariton par des simulations électromagnétiques nous a permis de connaître les fréquences de résonance des plasmons-polaritons en fonction des dimensions du réseau. Le motif de réseau composé de deux bandes de métal de largeurs différentes a été plus particulièrement étudié. Ce motif permettant aux détecteurs d’atteindre une très haute sensibilité [Coquillat et al., 2010] et n’avait pas encore été étudié du point de vue de son efficacité de couplage. Des détecteurs, dimensionnés pour notre montage de test à 0,65 THz, ont ensuite été fabriqués puis mesurés avec un réseau non-polarisé, à température ambiante et refroidis à l’azote. La correspondance entre la variation de la sensibilité en fonction de la fréquence et les spectres d’absorption mesurés au spectromètre infrarouge à transformée de Fourier (FTIR) montre l’importance de l’étape de couplage dans le processus de détection. Contrôler la densité électronique dans le 2DEG permet de modifier la fréquence de résonance des plasmons-polaritons et d’augmenter la sensibilité des détecteurs. Nous avons mené des développements technologiques de manière à pouvoir contrôler la densité électronique du 2DEG en appliquant une tension sur le réseau. Cette étape constitue un défi technologique compte tenu de la surface très étendue des réseaux (plusieurs mm²). Nous avons finalement fabriqué des détecteurs pour lesquels la fréquence de résonance de couplage peut être contrôlée grâce à la tension appliquée sur le réseau. / The THz-domain of the electromagnetic spectrum is not frequently used, even if the generation, amplification and detection of THz-waves would open a wide range of interesting applications. In this work, we focus on plasmonic detectors as a promising alternative to the frequency-raising of high-frequency electronic detectors and to the use of thermic detectors for low-energy photons. The coupling between a THz-photon and a plasmon in a 2D electron gas (2DEG) gives birth to a plasmon-polariton, which is then turned into a continuous, measurable signal and explains the operation of the plasmonic detector. In this work, we use the 2DEG in the semiconductive heterostructure AlGaN/GaN. A metallic grating deposited on-top of the semiconductor realises the coupling between photon and plasmon. First, we used electromagnetic simulations to study the coupling between photon and plasmon and calculate the resonant coupling frequency with respect to the grating dimensions. We studied specifically a grating pattern made of two metal stripes of different widths. This pattern gives the highest sensitivity to the detectors [Coquillat et al., 2010] and had not been studied before in term of coupling efficiency. In a second time, we fabricated detectors designed to match our 0.65 THz experimental setup. These detectors have been measured at 77 K and at room-temperature. No voltage has been applied on the grating. We saw that the sensitivity variations with respect to the incident frequency correspond to the absorption spectra measured by Fourier Transform spectrometer (FTIR), what show the importance of the coupling for the detection. Monitoring the electronic density in the 2DEG is a way to monitor the plasmon-polariton resonant frequency and the detector sensitivity. We led technological development to monitor the electronic density in the 2DEG by applying a voltage on the grating. This has been a technological challenge because of the wide grating area (a few mm²). Finally, we fabricated detectors for which it was possible to monitor the resonant absorption frequency using the grating voltage.

abstract: The availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able to reach their true potential due to material growth related issues. Power devices typically have patterned p-n, and p-i junctions in lateral, and vertical direction relative to the substrate. Identifying the variations from the intended layer design is crucial for failure analysis of the devices. A most commonly used dopant profiling technique, secondary ion mass spectroscopy (SIMS), does not have the spatial resolution to identify the dopant distribution in patterned devices. The possibility of quantitative dopant profiling at a sub-micron scale for GaN in a scanning electron microscope (SEM) is discussed. The total electron yield in an SEM is shown to be a function of dopant concentration which can potentially be used for quantitative dopant profiling. Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

[pt] O nitreto de gálio (GaN) é um dos mais interessantes e promissores materiais para aplicação em dispositivos óptico- eletrônicos. GaN pode ser usado para a fabricação de diodos e lasers azuis. O desenvolvimento deste tipo de material está relacionado com três campos principais: 1) deposição de camadas de GaN cristalino; 2) produção de nano- filamentos a partir de reações confinadas no interior de nanotubos de carbono; 3) síntese de GaN em pó por diferentes métodos químicos. Recentemente, novas técnicas de deposição adotaram a sublimação de pós de GaN como fonte de gálio para a produção de nanofilamentos de GaN, filmes finos ou cristais. Estes métodos de sublimação mostram a necessidade do emprego de pós de GaN. No presente trabalho, é apresentada uma nova rota para a produção de pós de GaN a partir da reação gás-sólido entre Ga2O3 e NH3(g) utilizando o carbono como agente redutor no interior de um novo tipo de reator, disposto verticalmente. A partir desta rota obteve-se pós de GaN com conversões aproximadamente de 100% e com estrutura cristalina hexagonal. A quantidade de GaN obtida variou de acordo com os parâmetros experimentais adotados. Através de uma análise estatística foi possível determinar a influência da temperatura, razão molar de carbono/Ga2O3 e do tempo experimental sobre a taxa de produção de GaN. / [en] It is well known that gallium nitride (GaN) is one of the most interesting and promising materials for optoelectronic devices. GaN can be used for manufacturing blue light- emitting diodes and lasers. Development of this material is concerned with three main areas 1) deposition of GaN crystalline layers onto different substrates; 2) manufacturing of GaN nanorods from chemical reactions in the confined spaces provided by carbon nanotubes; 3) synthesis of GaN powders by different chemical methods. Recently, new deposition techniques have adopted sublimation of GaN powders as gallium source to produce GaN nanorods, thin films or bulk crystals. These sublimation methods rely on the supply of GaN powders. This thesis presents a new route to produce GaN powder from gas-solid chemical reaction between Ga2O3 and NH3 using carbon as reducing agent in a new reactor design. The GaN powder obtained from this route possesses a hexagonal crystal structure and was found to correspond to almost 100% conversion of Ga2O3. The amount of GaN present in the powders varied with experimental parameters. A statistical analysis showed the influence of temperature, carbon/Ga2O3 ratio and experimental time on the production of GaN powder.

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The assistance of thin film deposition with low-energy ion bombardment influences their final properties significantly. Especially, the application of so-called hyperthermal ions (energy <100 eV) is capable to modify the characteristics of the growing film without generating a large number of irradiation induced defects. The nitrogen ion beam assisted molecular beam epitaxy (ion energy <25 eV) is used to deposit GaN thin films on (0001)-oriented 6H-SiC substrates at 700 C. The films are studied in situ by reflection high energy electron diffraction, ex situ by X-ray diffraction, scanning tunnelling microscopy, and high-resolution transmission electron microscopy. It is demonstrated that the film growth mode can be controlled by varying the ion to atom ratio, where 2D films are characterized by a smooth topography, a high crystalline quality, low biaxial stress, and low defect density. Typical structural defects in the GaN thin films were identified as basal plane stacking faults, low-angle grain boundaries forming between w-GaN and z-GaN and twin boundaries. The misfit strain between the GaN thin films and substrates is relieved by the generation of edge dislocations in the first and second monolayers of GaN thin films and of misfit interfacial dislocations. It can be demonstrated that the low-energy nitrogen ion assisted molecular beam epitaxy is a technique to produce thin GaN films of high crystalline quality.