Ce travail de thèse porte sur la diffusion des porteurs de charge en excès dans deux semiconducteurs à large bande interdite: l’alliage ZnMgO et le diamant 13C. Il est basé sur l’étude d’hétérostructures ZnMgO/ZnO/ZnMgO et 13C/12C/13C à puits de collecte ZnO ou 12C. Sur leurs sections transverses et avec la résolution nanométrique en excitation par cathodoluminescence (CL), nous avons étudié l’évolution de l’intensité de l’émission issue du puits en ZnO ou 12C en fonction de la distance entre l’impact de l’excitation et le puits. Cela nous a permis de mesurer directement les longueurs de diffusion effectives dans ZnMgO et le diamant.Dans ZnMgO, la valeur de 55 nm à 300 K, mesurée sur section transverse clivée, est proche de celle du matériau massif. Elle correspond à une diffusion mixte excitons/porteurs libres. Avec l’utilisation de lames minces érodées par faisceau d’ions, une diminution de a été observée jusqu’à 8 nm dans les parties les plus fines. Cet effet est attribué aux recombinaisons non radiatives de surface. Les lames minces apparaissent alors d’un grand intérêt pour améliorer la résolution spatiale des images CL.Dans le diamant, la diffusion excitonique à basse température montre une faible dépendance de avec l’énergie incidente des électrons. Cela indique que ≈ 15 µm à 20 K dans le diamant massif 13C. Une diminution de jusqu’à 3,3 µm à 118 K est observée en fonction de la température.Enfin, nous avons mis en évidence la formation de polyexcitons dans le diamant en augmentant la densité des paires électron-trou, soit par la puissance d’excitation, soit par le confinement spatial des excitons dans des puits de diamant 12C de faible épaisseurs. / This work focuses on the determination of the carrier diffusion length in two wide bandgap semiconductors: the ternary alloy ZnMgO and diamond. This determination has been achieved by using of ZnMgO/ZnO/ZnMgO and 13C/12C/13C heterostructures containing ZnO or 12C collecting wells. Their transverse section was scanned by CL spectroscopy with a nanometer scale resolution in excitation. The effective excess carrier diffusion length is deduced from the evolution of the well emission intensity with the distance between the excitation impact and the well.In ZnMgO, the value at 300 K is 55 nm, obtained from a cleaved cross section. It is close to the bulk material diffusion and is attributed to a mixed diffusion of excitons/free carriers. A decrease of down to 8 nm is observed in the thinnest portions of cross sections shaped by focused ion beam (FIB). This effect is attributed to non-radiative surface recombinations. These thin slabs appear of great interest to enhance the spatial resolution of CL images.In diamond, the exciton diffusion at 20 K exhibits a slight dependence on the incident electron energy. This indicates that the exciton diffusion length is around 15 µm in 13C bulk diamond. The values decrease down to 3.3 µm at 118 K.Finally, we highlighted the formation of polyexcitons in diamond by increasing the electron-hole pairs density either by the excitation power, or by the spatial confinement of excitons in thin 12C wells.

Die vorliegende Arbeit beschreibt den Verlauf der Forschung von der Entwicklung einer neuartigen Methode der gepulsten Laser-Plasmaabscheidung (PLD) über die Untersuchung der ternären In- und Al-Legierungssysteme von metastabilem orthorhombischen κ-Ga2O3 auf der Basis dieser Methode hin zu Multi-Quantengraben (QW) Supergitter (SL) Heterostrukturen für transparente Quantengrabeninfrarotphotodetektoren (QWIPs). Im ersten Teil wird die Methode, welche vertical continuous composition spread (VCCS) PLD genannt wird, eingeführt und am MgxZn1−xO Legierungssystem erprobt. Die Methode erlaubt die Kontrolle der Komposition von Dünnfilmen über die radiale Position des PLD Laserspots auf der Targetoberfläche. Das ist eine wichtige Voraussetzung für die Bestimmung der kompositionsabhängigen Eigenschaften der Legierungssysteme und für präzise Profile der physikalischen Eigenschaften in Wachstumsrichtung für das Design von Bauelementen. Die Dünnfilme mit 0 ≤ x ≤ 0.4 zeigen die gleichen Eigenschaften wie solche, die mit Standard-PLD abgeschieden wurden. Numerische Modelle werden präsentiert, welche die Dünnfilmkomposition exakt vorhersagen. Im zweiten Teil werden κ-Ga2O3 Dünnfilme durch die Beigabe von Zinn während des PLD Prozesses stabilisiert. Die Dünnfilme weisen hohe kristalline Qualität, glatte Oberflächen und große Bandlücken (Eg ≈ 4.9 eV) auf. Ein Wachstumsmodell wird präsentiert, welches Zinn als Oberflächenschicht beschreibt. Im dritten Teil werden die In- und Al-Legierungssysteme von κ-Ga2O3 mittels VCCS PLD untersucht. Die Löslichkeitsgrenzen xIn <~ 0.35 und xAl <~ 0.65 sind die höchsten bislang berichteten. In- und out-of-plane Gitterkonstanten wurden in Abhängigkeit der Zusammensetzung bestimmt und Eg konnte von 4.1 eV bis 6.4 eV variiert werden. Die Position des Valenzbandmaximums wird als unabhängig von der Komposition gezeigt, womit die Variation in Eg den Leitungsbandunterschieden gleicht und Detektionsbereiche vom fernen IR bis in das Sichtbare für QWIP-Anwendungen bedeutet. Berechnungen anhand dieser Ergebnisse ergeben Polarisationsladungsdichten an Grenzflächen von Heterostrukturen gleich oder höher derer im etablierten AlGaN/GaN System, welche wichtig zur Polarisationsdotierung zur Besetzung des Grundzustandes in QWIPs sind. Dies bestätigt das große Potential der κ-Phase. Im letzten Teil werden erste kohärent gewachsene κ-(AlxGa1−x)2O3/Ga2O3 SL Strukturen untersucht. Glatte Grenzflächen im Bereich weniger Monolagen werden gezeigt und es konnten kritische Dicken für die κ-Ga2O3 QW Schichten bestimmt werden, die für QWIP-Anwendungen genügen. / The presented thesis describes the research path from the development of a novel pulsed laser deposition (PLD) technique over the exploration of the ternary In- and Al-alloy systems of metastable orthorhombic κ-Ga2O3 employing this technique towards multi-quantum well (QW) superlattice (SL) heterostructures for solar-blind quantum well infrared photodetector (QWIP) applications. In the first part, the PLD technique called vertical continuous composition spread (VCCS) PLD employing radially-segmented targets is established and tested on the well-known MgxZn1−xO alloy system. The technique enables direct control of the chemical composition of thin films by a variation of the radial position of the PLD laser spot on the target surface. This is a prerequisite for a discrete compositional screening of alloy properties and the exact tailoring of physical parameters in growth direction for heterostructure device design. The resulting thin films with 0 ≤ x ≤ 0.4 exhibit the same quality as thin films deposited by standard PLD and numerical models are presented that precisely predict the thin film composition. In the second part, κ-Ga2O3 thin films are stabilized by the addition of tin in the PLD process. The thin films show a high crystalline quality, smooth surfaces and large bandgaps (Eg ≈ 4.9 eV). A growth model is proposed based on tin acting as surfactant. In the third part, the In- and Al-alloy systems of κ-Ga2O3 are explored by VCCS PLD. Solubility limits of xIn <~ 0.35 and xAl <~ 0.65 are the highest reported to date. In- and out-of-plane lattice constants were determined as function of alloy composition and bandgap engineering from 4.1 eV to 6.4 eV is feasible within these limits. The energetic position of the valence band maximum was found independent on chemical composition such that the change in bandgap equals the conduction band offset rendering wavelength ranges from far IR to the visible spectral range in QWIP applications possible. Calculations based on these results found polarization charge densities at the interfaces of corresponding heterostructures on par or larger than for the established AlGaN/GaN system important for polarization doping to populate the ground state in QWIPs. This corroborates the high potential of the κ-phase. In the last part, first coherently grown κ-(AlxGa1−x)2O3/Ga2O3 SL heterostructures are presented. Smooth interfaces of the order of a few monolayers are confirmed and critical thicknesses for coherent growth of the Ga2O3 QW layer are found to be sufficient for QWIP applications.

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<p>Recent developments in semiconductor device technology have seen the advent of wide-bandgap (WBG) based devices that enable operation at high switching frequencies. These devices, such as silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs), are becoming a favored choice in inverters for electric drive systems because of their lower switching losses and higher allowable operating temperature. However, the fast switching of such devices implies increased voltage edge rates (high <i>dv/dt</i>) that give rise to various undesirable effects including large common-mode currents, electromagnetic interference, transient overvoltages, insulation failure due to the overvoltages, and bearing failures due to</p> <p>microarcs. With increased use of these devices in transportation and industrial applications, it is imperative that accurate models and efficient simulation tools, which can predict these high-frequency effects and accompanying system losses, be established. This research initially focuses on establishing an accurate wideband model of a surface-mount permanent-magnet</p> <p>ac machine supplied by a WBG-based inverter. A new multirate simulation framework for predicting the transient behavior and estimating the power losses is then set forth. In this approach,</p> <p>the wideband model is separated into high- and low-frequency models implemented using two different computer programs that are best suited for the respective time scales. Repetitive execution of the high-frequency model yields look-up tables for the switching losses in the semiconductors, electric machine, and interconnecting cable. These look-up tables are then incorporated into the low-frequency model that establishes the conduction</p> <p>losses. This method is applied to a WBG-based electric drive comprised of a SiC inverter and permanent-magnet ac machine. Comparisons of measured and simulated transients are provided.</p>

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With the development of solar energy, power conversion systems responsible for energy delivering from photovoltaic (PV) modules to ac or dc grid attract wide attentions and have significantly increased installations worldwide. Modular power conversion system has the highest efficiency of maximum power point tacking (MPPT), which can transfer more solar power to electricity. However, this system suffers the drawbacks of low power conversion efficiency and high cost due to a large number of power electronics converters. High-power density can provide potentials to reduce cost through the reduction of components and potting materials. Nowadays, the power electronics converters with the conventional silicon (Si) based power semiconductor devices are developed maturely and have limited improvements regarding in power conversion efficiency and power density. With the availability of wide bandgap devices, the power electronics converters have extended opportunities to achieve higher efficiency and higher power density due to the desirable features of wide bandgap devices, such as low on-state resistance, small junction capacitance and high switching speed. This dissertation focuses on the application of wide bandgap devices to the dc-dc power conversion for the modular PV applications in an effort to improve the power conversion efficiency and power density. Firstly, the structure of gallium-nitride (GaN) device is studied theoretically and characteristics of GaN device are evaluated under testing with both hard-switching and soft-switching conditions. The device performance during steady-state and transitions are explored under different power level conditions and compared with Si based devices. Secondly, an isolated high-efficiency GaN-based dc-dc converter with capability of wide range regulation is proposed for modular PV applications. The circuit configuration of secondary side is a proposed active-boost-rectifier, which merges a Boost circuit and a voltage-doubler rectifier. With implementation of the proposed double-pulse duty cycle modulation method, the active-boost-rectifier can not only serve for synchronous rectification but also achieve the voltage boost function. The proposed converter can achieve zero-voltage-switching (ZVS) of primary side switches and zero-current-switching (ZCS) of secondary side switches regardless of the input voltages or output power levels. Therefore, the proposed converter not only keeps the benefits of highly-efficient series resonant converter (SRC) but also achieves a higher voltage gain than SRC and a wide range regulation ability without adding additional switches while operating under the fixed-frequency condition. GaN devices are utilized in both primary and secondary sides. A 300-W hardware prototype is built to achieve a peak efficiency of 98.9% and a California Energy Commission (CEC) weighted efficiency of 98.7% under nominal input voltage condition. Finally, the proposed converter is designed and optimized at 1-MHz switching frequency to pursue the feature of high-power density. Considering the ac effects under high frequency, the magnetic components and PCB structure are optimized with finite element method (FEM) simulations. Compared with 140-kHz design, the volume of 1-MHz design can reduce more than 70%, while the CEC efficiency only drops 0.8% at nominal input voltage condition. There are also key findings on circuit design techniques to reduce parasitic effects. The parasitic inductances induced from PCB layout of primary side circuit can cause the unbalanced resonant current between positive and negative half cycles if the power loops of two half cycles have asymmetrical parasitic inductances. Moreover, these parasitic inductances reflecting to secondary side should be considered into the design of resonant inductance. The parasitic capacitances of secondary side could affect ZVS transitions and increase the required magnetizing current. Because of large parasitic capacitances, the dead-time period occupies a large percentage of entire switching period in MHz operations, which should be taken into consideration when designing the resonant frequency of resonant network. / Doctor of Philosophy / Solar energy is one of the most promising renewable energies to replace the conventional fossils. Power electronics converters are necessary to transfer power from solar panels to dc or ac grid. Since the output of solar panel is low voltage with a wide range and the grid side is high voltage, this power converter should meet the basic requirements of high step up and wide range regulation. Additionally, high power conversion efficiency is an important design purpose in order to save energy. The existing solutions have limitations of narrow regulating range, low efficiency or complicated circuit structure. Recently, the third-generation power semiconductors attract more and more attentions who can help to reduce the power loss. They are named as wide band gap devices. This dissertation proposed a wide band gap devices based power converter with ability of wide regulating range, high power conversion efficiency and simple circuit structure. Moreover, this proposed converter is further designed for high power density, which reduces more than 70% of volume. In this way, small power converter can merge into the junction box of solar panel, which can reduce cost and be convenient for installations.