Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 191-206).

Universiẗat, Diss., 2002--Dortmund.

Le travail de la thèse s’articule sur le développement d’un nouveau type de capteurs de gaz à base des matériaux semi-conducteurs III-Nitrure (Les nitrures de gallium). Ces matériaux présentent de nombreux avantages qui pourraient être utilisées pour concevoir des capteurs NOx sensibles et sélectifs pour le contrôle des pollutions émises par la ligne d’échappement Diesel. Afin de limiter et déduire les gaz polluants émis par les moteurs à explosion en générale et les moteurs Diesel en particuliers (NO, NO2, NH3, CO, …), différentes normes européennes ont été établies. Pour respecter ces normes, plusieurs modifications sur les moteurs et les lignes d’échappement des véhicules ont été effectuées (filtres à particules, catalyseurs, capteurs NOx, …). Les capteurs NOx utilisés actuellement sont à base d’électrolyte solide. Ils sont basés dans leur fonctionnement sur la mesure de la concentration d’oxygène présente dans le gaz d’échappement qui permet de son tour l’estimation de la concentration totale des gaz NOx (mesure indirecte). Ces capteurs ne détectent pas le NH3 à la sortie de la ligne d’échappement, et ne donnent pas une information précise sur le rapport entre NO et NO2 (manque de sélectivité) qui est un facteur important pour le bon fonctionnement de catalyseur sélectif SCR (amélioration de rendement) ; d’où la nécessité d’un capteur de gaz plus performant et en particulier sélectif afin d’améliorer les systèmes de contrôle, de post-traitement et de diagnostic. Notre approche consiste à utiliser un transistor HEMT (High Electron Mobility Transistor) à gaz bidimensionnel d’électrons à base de nitrure de Gallium avec l’association d’une couche fonctionnelle à la place de la grille. L’interaction des molécules de gaz avec cette couche fonctionnelle donne une signature (variation de signal de sortie) spécifique pour chaque type de gaz qui aide à l’amélioration de la sélectivité. Le projet contient deux parties : l’optimisation de la structure choisie et l’optimisation de la couche fonctionnelle afin d’obtenir une détection sélective entre les différents gaz polluants. Cette technologie est intéressante pour développer des capteurs de gaz grâce aux possibilités de détecter des faibles variations de tensions et aux possibilités de fonctionnement dans des environnements sévères. La thèse de doctorat s’inscrit dans le cadre de l’OpenLab materials and processes en collaboration entre le laboratoire Georgia-Tech lorraine et l’entreprise Peugeot-Citroën PSA / The work of the thesis focuses on the development of a new type of gas sensors based III-Nitride semiconductor materials (gallium nitrides). These materials have many advantages that could be used to develop sensitive and selective NOx sensors for the control of pollution emitted by diesel exhaust line. To limit the polluting gases emitted by internal combustion engines in general and diesel in particular (NO, NO2, NH3, CO, ...), different European standards have been established. To meet these standards, anti-pollution systems (consisting of particle filters, catalysts, NOx sensors, ... etc) are used. NOx sensors currently used in automobiles are based on a solid electrolyte. Their operation is based on the measurement of the oxygen concentration. This enables an estimate of the total concentration of NOx gas (indirect measurement) after filtering NOx from O2 and decomposing NOx into O2. These sensors do not detect NH3 at the outlet of the exhaust line, and do not give accurate information on the relationship between NO and NO2 (lack of selectivity) which is important factor for an optimal functioning of selective catalyst (SCR performance improvement). Hence there exists a need for a more efficient and selective in particular gas sensor to improve the control systems, post-treatment and diagnosis. Our approach is to use a HEMT (High Electron Mobility Transistor) transistor based on gallium nitride with a combination of a functional layer instead of the gate. The interaction of the gas molecules with the functional layer gives a signature (output signal variation) specific for each type of gas that helps to improve the selectivity. The project contains two parts: the optimization of the chosen structure and the optimization of the functional layer in order to achieve selective detection between various gaseous pollutants. This technology is interesting for development of gas sensors through the possibility of detection low voltage variations and the possibility of operating in harsh environments. The thesis is part of OpenLab "Materials and Processes" in a collaboration between Georgia Tech-CNRS laboratory and the PSA Peugeot-Citroen Group

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.

Integration of sensors and silicon-based electronics for harsh environment applications is driven by the automotive industry and the maturity of semiconductor processes that allow embedding sensitive elements onto the same chip without sacrificing the performance and integrity of the electronics. Sensor devices post-processed on top of electronics by surface micromachining allow the addition of extra functionality to the fabricated ICs and creating a sensor system without significant compromise of performance. Smart sensors comprised of sensing structures integrated with silicon carbide-based electronics are receiving attention from more industries, such as aerospace, defense and energy, due to their ability to operate in very demanding conditions. This thesis describes the design and implementation of a novel, integrated thin film temperature sensor that uses a half-bridge arrangement to measure thin film platinum sensitive elements. Processes have been developed to fabricate temperature insensitive thin film tantalum nitride resistors which can be combined with the platinum elements to form the temperature transducing bridge. This circuit was designed to be integrated with an existing silicon carbide-based instrumentation amplifier by post-CMOS processing and to be initially connected to the bond pads of the amplifier input and output ports. Thin films fabricated using the developed TaN and Pt processes have been characterized using resistive test structures and crystallographic measurements of blanket thin film layer samples, and the relationship between the measurement results obtained has been analyzed. An initial demonstration of temperature sensing was performed using tantalum nitride and platinum thin film resistor element chips which were fabricated on passivated silicon substrates and bonded into high temperature packages. The bridge circuit was implemented by external connections through a printed circuit board and the bridge output was connected to a discrete instrumentation amplifier to mimic the integrated amplifier. The temperature response of the circuit measured at the output of the amplifier was found to have sensitivity of 844 μV·°C–1 over the temperature range of 25 to 100 °C. Two integrated microfabrication process flows were evaluated in this work. The initial process provided a very low yield for contact resistance structures between TaN and Pt layers, which highlighted problems with the thin film platinum deposition process. Multiple improvement options have been identified among which removal of the dielectric layer separating TaN and Pt layers and thicker Pt film were considered and a redesign of both layout and the process flow has resulted in improved yield of platinum features produced directly on top of TaN features. Temperature sensitivity of the integrated sensor devices was found to depend significantly on parasitic elements produced by thin film platinum step coverage, the values of which were measured by a set of resistive test structures. A new microfabrication design has enabled the production of a group of integrated temperature sensors that had a sensitivity of 150.84 μV·°C–1 in the temperature range between 25 and 200 °C on one of the fabricated wafers while the best fabricated batch of sensors had a sensitivity of 1079.2 μV·°C–1.

Cette thèse porte sur la réalisation de composants photoniques à base de fils de nitrures III-V. Les fils de GaN non-catalysés ont été élaborés de manière auto-assemblée par épitaxie en phase vapeur aux organométalliques (MOVPE) sur saphir. Un des axes de ce travail a porté sur la croissance organisée de ces fils à travers un réseau d’ouvertures défini par lithographie et gravure d’une couche de SiNx. Nous avons étudié en particulier l’influence des paramètres de croissance (température, pression, ratio V/III) et du motif sur l’homogénéité de la croissance sélective. Ces fils ont servi de substrat pour la croissance d’hétérostructures radiales cœur-coquille InGaN/GaN.D’autre part, la croissance, la fabrication et les propriétés physiques de trois types de composant ont pu être étudiées :-Des cellules solaires à fils uniques. Nous avons comparé l’efficacité de conversion de deux types d’hétérostructures : des coquilles épaisses d’In0.1Ga0.9N et des coquilles à 15 et 30 puits quantiques In0.18Ga0.82N/GaN. Après optimisation du contact électrique sur la coquille p-GaN, un rendement maximal de 0,33 % a été obtenu avec des fils à 30 puits quantiques sous éclairement équivalent à 1 soleil (AM1.5G). Le seuil d’absorption mesuré par spectroscopie de photocourant varie entre 400 et 440 nm.- Une plateforme émetteur-détecteur. Le système, qui fonctionne à 400 nm, comprend deux fils de GaN à hétérostructure radiale InGaN/GaN positionnés sur le même substrat et couplés par un guide d’onde en SiNx. La caractérisation électrique du dispositif a mis en évidence une durée de commutation inférieure à 0,25 s sans photocourant persistant.- Des diodes électroluminescentes (LED) flexibles. Ces diodes qui émettant dans le visible (400-470 nm) ont été réalisées en se basant sur une approche hybride organique/inorganique. Les fils émetteurs à puits quantiques InGaN/GaN sont encapsulés dans une matrice organique de PDMS puis détachés de leur substrat de croissance. Les contacts sont réalisés à partir de nanofils d’argent qui présentent l’avantage d’être à la fois flexibles, transparents et conducteurs. A partir de ce procédé, une LED bicolore flexible a été réalisée en combinant des émetteurs bleus et « verts ». / This thesis reports on the realization of photonic devices based on nitride wires. Self-assembled GaN wires were grown without catalyst by metal-organic vapor phase epitaxy (MOCVD) on sapphire substrates. Part of this work focused on the selective area growth of GaN wires through a dielectric SiNx mask with regular arrays of holes defined by lithography and dry etching. We studied the influence of the growth conditions (temperature, pressure, V/III ratio) and pattern geometry on the homogeneity of the selective area growth. These wires were used as templates for the growth of core-shell InGaN/GaN heterostructures. In addition, the growth, microfabrication process and properties of three types of devices were studied:- Single wire solar cells. We compared the efficiency of two type of heterostructures: shells composed of thick In0.1Ga0.9N layers and In0.18Ga0.82N/GaN quantum wells. After optimization of the electrical contact on the p-GaN shell, a maximal conversion efficiency of 0,33 % was obtained on single GaN wires with a shell of 30 quantum wells under 1 sun illumination (AM1.5G). Photocurrent spectroscopy revealed that the wire absorption edge varied between 400 and 440 nm.- An integrated photonic platform. The system, that operates around 400 nm, is composed of two GaN wires with radial InGaN/GaN heterostructures positioned on the same substrate and coupled with a SiNx waveguide. The electrical characterization of the platform revealed a switching speed inferior to 0.25 s without persistent photocurrent.- Flexible light emitting diodes (LED). The LED fabrication is based on a dual approach which associates inorganic InGaN/GaN emitters (400-470 nm) and a polymer. The wires are encapsulated in a PDMS matrix before being detached from their native substrate. Electrical contacts are made with silver nanowires which are flexible, highly conductive and transparent in the visible range. Based on this procedure a two-color LED was realized by stacking a blue and a “green” LED.

abstract: Cubic boron nitride (c-BN) has potential for electronic applications as an electron emitter and serving as a base material for diodes, transistors, etc. However, there has been limited research on c-BN reported, and many of the electronic properties of c-BN and c-BN interfaces have yet to be reported. This dissertation focused on probing thin film c-BN deposited via plasma enhanced chemical vapor deposition (PECVD) with in situ photoelectron spectroscopy (PES). PES measurements were used to characterize the electronic properties of c-BN films and interfaces with vacuum and diamond. First, the interface between c-BN and vacuum were characterized with ultraviolet PES (UPS). UPS measurements indicated that as-deposited c-BN, H2 plasma treated c-BN, and annealed c-BN post H2 plasma treatment exhibited negative electron affinity surfaces. A dipole model suggested dipoles from H-terminated N surface sites were found to be responsible for the NEA surface. Then, Si was introduced into c-BN films to realize n-type doped c-BN. The valence structure and work function of c-BN:Si films were characterized with XPS and UPS measurements. Measurements were unable to confirm n-type character, and it is concluded that silicon nitride formation was the primary effect for the observations. Finally, XPS measurements were employed to measure the band offsets at the c-BN/diamond interface. Measurements indicated the valence band maximum (VBM) of c-BN was positioned ~0.8 eV above the VBM of diamond. / Dissertation/Thesis / Doctoral Dissertation Physics 2016

abstract: In recent years, wide bandgap (WBG) devices enable power converters with higher power density and higher efficiency. On the other hand, smart grid technologies are getting mature due to new battery technology and computer technology. In the near future, the two technologies will form the next generation of smart grid enabled by WBG devices. This dissertation deals with two applications: silicon carbide (SiC) device used for medium voltage level interface (7.2 kV to 240 V) and gallium nitride (GaN) device used for low voltage level interface (240 V/120 V). A 20 kW solid state transformer (SST) is designed with 6 kHz switching frequency SiC rectifier. Then three robust control design methods are proposed for each of its smart grid operation modes. In grid connected mode, a new LCL filter design method is proposed considering grid voltage THD, grid current THD and current regulation loop robust stability with respect to the grid impedance change. In grid islanded mode, µ synthesis method combined with variable structure control is used to design a robust controller for grid voltage regulation. For grid emergency mode, multivariable controller designed using H infinity synthesis method is proposed for accurate power sharing. Controller-hardware-in-the-loop (CHIL) testbed considering 7-SST system is setup with Real Time Digital Simulator (RTDS). The real TMS320F28335 DSP and Spartan 6 FPGA control board is used to interface a switching model SST in RTDS. And the proposed control methods are tested. For low voltage level application, a 3.3 kW smart grid hardware is built with 3 GaN inverters. The inverters are designed with the GaN device characterized using the proposed multi-function double pulse tester. The inverter is controlled by onboard TMS320F28379D dual core DSP with 200 kHz sampling frequency. Each inverter is tested to process 2.2 kW power with overall efficiency of 96.5 % at room temperature. The smart grid monitor system and fault interrupt devices (FID) based on Arduino Mega2560 are built and tested. The smart grid cooperates with GaN inverters through CAN bus communication. At last, the three GaN inverters smart grid achieved the function of grid connected to islanded mode smooth transition / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

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

L’avenir des nouvelles caméras embarquées pour l’astrophysique spatiale semble passer par unaccroissement du nombre de pixels et un fonctionnement à très basse température (en dessous de 0,1 K).Avec cette évolution, le nombre important de fils en sortie du détecteur refroidi représente souvent lacharge thermique prédominante sur la source froide (cryostat).Dans ce contexte, l’isolation thermique entre les différents circuits de détection est un point crucial pources caméras. Une brique technologique innovante a été développée pour apporter une solution présentantune excellente conduction électrique couplée à une grande isolation thermique. Cette innovation,protégée par un brevet, permet de résoudre cet apparent paradoxe. La solution proposée consiste enl’empilement d’un grand nombre de couches minces de matériaux supraconducteurs dans lesinterconnexions.La résistance thermique à chaque interface est dépendante des propriétés élastiques des matériaux,de la qualité des interfaces et de la température à laquelle le système fonctionne. A très basse température,le modèle AMM, couplé aux mesures des caractéristiques des matériaux composants la multicouche,permettent une estimation théorique de la résistance thermique pour une interface. Les mesures effectuéesavec les liaisons supraconductrices à forte résistivité thermique concordent avec les estimationsthéoriques. Nous avons ainsi pu mesurer des résistances thermiques de l’ordre de 3,3.105 K/W à 200 mKpour une multicouche composée d’une succession (62 interfaces) de couches minces de nitrure de titaneet de niobium sur une surface de 16 mm2. Dans les conditions d’utilisation prévues pour une camérarayons X de 4000 pixels microcalorimétriques, l’utilisation de cette brique technologique devrait assurerune charge thermique sur la source froide (à 50 mK) très inférieure au μW pour plus de 8000 pointsde contact. Ce dispositif pourra être utilisé à l’avenir dans nombre de projets cryogéniques, lorsqu’une excellenteisolation thermique associée à une excellente conduction électrique sera recherchée. / Future of the next camera onboard space observatories implies a major enhancement in number of pixelsand a very low operative temperature (below 0.1 K). In this evolution, the large number of output wiresfrom the cool detector is often responsible of the most important thermal load onto the cold bath(cryostat).In this context, the thermal insulation between the different detection circuits is the bottleneck for thesecameras. An innovative technological component, protected by a patent, has been developed to tackle thisproblem. This device has both an excellent electrical resistivity and a very high thermal resistivity.The proposed solution is a stack of thin superconducting layers at electrical interconnections.The thermal resistance at each interface relies on the elastic properties of the materials used, the quality ofthe interfaces and temperature. The AMM model used in conjunction with the measured materialcharacteristics allows a theorical estimation of the thermal resistance per interface. The measurementsundertaken with superconducting connections with very high thermal resistivity are very well describedby this AMM model. We have measured thermal resistances as high as 3.3 105 K/W @ 200 mKfor a multilayer of 62 interfaces built with titaniun nitride and niobium alternatively on a 16 mm2 array.In the conditions foreseen for a 4000 micro-calorimeters camera operating at 50 mK in X-rays,this multilayer technique should allow a thermal load onto the cold bath that is much lower that 1 mWfor more than 8000 contacts.