Le développement de LED à base de nitrures représente un enjeu important tant sur le plan scientifique qu’industriel et sociétal. De par leur large bande interdite, les matériaux semi-conducteurs à base de nitrures d’éléments III (composés III-N) tels que le GaN et ses alliages sont de très bons candidats pour la réalisation de dispositifs optoélectroniques nouveaux. Néanmoins, ces systèmes présentent bon nombre de limitations, principalement dues à l’évolution des propriétés de l’InGaN lorsque la concentration d’indium augmente. Les effets de contrainte et de polarisation affectent la qualité du matériau et donc l’émission spontanée de la LED en général. De plus, dans un contexte de raréfaction des ressources naturelles, l’utilisation de l’indium, matériau rare et cher, doit se faire de manière raisonnée. Or les systèmes actuels (micro-écran, dispositifs portatifs, ...) requièrent des LED toujours plus puissantes et riches en Indium. Le but est aujourd’hui d’obtenir des LED haute performance, avec un bon rendu de couleurs et surtout à moindre coût en utilisant des matériaux alternatifs. C’est dans ce contexte que s’inscrit ce sujet de thèse qui consiste en l’étude théorique du matériau ZnGeN2 et de son introduction au sein d’une structure LED. L’idée est ici de créer un puits quantique de type II InGaN-ZnGeN2 afin d’augmenter l’efficacité des zones d’actives et ainsi de réaliser des LED pouvant opérer sur une large gamme de longueurs d’ondes allant de l’IR à l’UV. Cette approche permet de diminuer la quantité d’indium dans les LED et ainsi de créer des structures moins onéreuses avec un matériau de meilleure qualité. Le ZnGeN2 dérive des nitrures d’éléments III en remplaçant le groupe III alternativement par un élément du groupe II (Zn) et du groupe IV (Ge). Les énergies de gap et le paramètre de maille de ZnGeN2 sont très proches de ceux du GaN. De plus, les organisations cristallographiques sont similaires et le large décalage de bande entre InGaN et ZnGeN2 autorise la formation d’une hétérostructure du type II InGaN/ZnGeN2. L’insertion d’une couche de ZnGeN2 dans une structure classique de puits quantique GaN/InGaN aboutit à des modifications significatives : le fort confinement des trous dans la couche de ZnGeN2 autorise l’utilisation d’une quantité moindre d’indium dans le puits. Dans le puits quantique de type II InGaN/ZnGeN2 une fine couche d’AlGaN est utilisée comme barrière pour un meilleur confinement. L’ensemble permet d’obtenir un meilleur recouvrement des fonctions d’ondes électron-trou comparé à celui obtenu dans le cas d’une LED classique. Au cours de la thèse nous présenterons les résultats des simulations des différentes structures LED avec puits quantique de type-II. Nous étudierons des structures LED pour des émissions dans le vert et le rouge. Différentes géométries de LED seront développées en faisant varier la position et l’épaisseur de la couche de ZnGeN2. Nous utiliserons ici le logiciel de simulation SILVACO/ATLAS avec le modèle k.p à six bandes pour le calcul de la structure de bandes, qui prend en compte les effets de tension, l’enchevêtrement des bandes de valence ainsi que les polarisations spontanées et piézoélectriques / Nitride LEDs development presents significant scientific and societal issues. The aim is to get low-cost, high efficiency LEDs with accurate color-rending (typically the Color Rending Index has to be higher than 90). Due to their large band gap (from 0.8 to 6.2 eV), III-N materials, as GaN and alloys, are still used for LEDs development. Nevertheless, they present several huge limitations mainly due to the evolution of InGaN properties for higher Indium concentrations. Strain and polarization effects affect then the LED quality through the reduction of the spontaneous emission. New high-performance devices require the development of new materials and the introduction of ZnGeN2 layers could be an alternative solution. We report here on a new green and red-emitting light emitting device (LED) architecture containing only 16% of indium. The structure is based on the use of a new type-II ZnGeN2/In0.16Ga0.84N quantum well. Type II InGaN-ZnGeN2 quantum wells (QWs) were proposed for the improvement of efficiency in active regions and realizing then devices operating in a large wavelength range from UV to IR. The zinc germanium nitride (ZnGeN2) is a new promising semiconductor for optoelectronic devices such as LED or photovoltaic cells due to its large, direct, and adjustable band gap, most particularly considered to overcome the green-gap issue in LED technology. ZnGeN2 derives from the III-nitride elements by replacing the III-group alternatively by a group II (Zn) and a group IV (Ge). Both the energy band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The crystallographic organizations are similar and the recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II InGaN-ZnGeN2 heterostructure. Studies of ZnGeN2 based quantum well behaviors are scarce and no information on the overall electro-optical operation of such LED is available. We simulate here with SILVACO/ATLAS the complete behavior of a green and red LED structures in which the active region is a type-II ZnGeN2/In0.16Ga0.84N quantum well. A thin AlGaN layer is used as a barrier for a better carrier confinement. The position and the thickness of the ZnGeN2 layer are parameters used to examine the luminous and electrical behavior as well as the external quantum efficiency of this LED compared to a standard InGaN-based LED emitting at the same wavelength. Inserting a ZnGeN2 layer in a conventional type-I InGaN QW structure yields significant modifications. The strong confinement of holes in the ZnGeN2 layer allows the use of a lower In-content InGaN QW with uniform In content. We demonstrate a significant enhancement of the spontaneous emission and the possibility to reach both a better quantum efficiency and light output when using the type-II structure. The self-consistent 6-band k.p method is used to perform the band structure calculations, which consider the effect of strain, the valence band mixing, and the spontaneous and piezoelectric polarizations

A semiconductor optical source monolithically integrated with a surface acoustic wave (SAW) Bragg-cell to operate as a functional device is proposed in this thesis. The practical structure of such an integrated device is demonstrated and design guidelines are presented. Compared with conventional optical beam processed devices, this functional integrated semiconductor optical source (FISOS) is revised to be compact in size, flexible in function and potentially robust in performance. <br /> The FISOS is analyzed as two sub-divisions, optical source and acoustic processor, which have the common substrate structure. The optical beams excited from the optical source part of the device undergoes a scattering in the Bragg grating formed by SAWs that are generated by an IDT positioned on top of the acoustic processing part of device. By altering the property (power, frequency, etc.) of the SAW, versatile functionalities such as modulation, filtering, beam steering and so on of the optical beams can be realized in this optical source device. <br /> A multilayer structure based on GaN/InGaN MQWs grown on sapphire is designed for the FISOS to be blue light emitting and efficiently launching SAWs. An etch-down technique employed in the SAW processing part is taken to improve the overlap between the optical and acoustic waves and then the interaction efficiency. Optimizations to the geometrical dimensions of the FISOS, such the width of the ridge waveguide, the position of the IDT and the etching depth, etc., are discussed in the given structure. <br /> Numerical models are investigated to access the operational characteristics and then to provide design guidelines for the proposed integrated device. The Bragg diffraction of optical waves occurring within the acoustic waves in the proposed structure are simulated as a two-dimensional interaction between two guided optical modes and an acoustic surface wave. <br /> The modal distributions and propagation velocities of SAWs in a multilayer system are calculated using Adler’s matrix method. The electrical characteristics of an IDT, such as impedance, insertion loss, electromechanical constant and so on are also discussed. <br /> Transverse and lateral optical modes in the given multilayer structure are analyzed by the transfer matrix method. The interaction of optical waves and acoustic waves are modeled using the rigorous grating diffraction theory. Starting from Floquet’s theory, the well-known coupled-wave method and modal method can both be derived from the rigorous grating diffraction theory. Discussions of some useful approximate methods are also presented. In this thesis, the simulations of the acoustooptic interaction are performed using the coupled-wave method. <br /> From the simulation results, the angular distribution profile and spatial profile of the output of the FISOS are evaluated. An improvement to the expression of the diffraction efficiency in such an integrated device is proposed. The so-called beam diffraction efficiency gives a more complete measure to the acoustooptic diffraction and is used to investigate the features of FISOS different from conventional acoustooptic devices. Contour plots of the beam efficiency varying with acoustic frequency and power in a FISOS is demonstrated to be a convenient and powerful approach in the device design. <br /> The operational performances of an integrated deflector and a modulator in FISOS are analyzed to investigate the feasibility of FISOS. The trade-off of the efficiency-resolution in an integrated deflector design is discussed. Short interaction length, high acoustic frequency and narrow ridge are proved to be helpful for a larger number of resolvable spots with a fairly high efficiency. In the case of the integrated modulator, given that the figure of merit Q is fixed, it is demonstrated that the smaller the Q, the longer the interaction length, larger ridge width and lower acoustic frequency will give rise to a larger bandwidth, though the highest efficiency might appear at a higher frequency. <br /> Some practical issues such as the misalignment of planar elements on the device and the incoherence of the integrated optical source are also discussed. A modified working frequency can be used to compensate the efficiency loss in the former case; in the latter case, it is demonstrated that a distortion of beam diffraction efficiency versus acoustic power with an incoherent optical source arises due to the wide spectrum of the incident optical waves.

Orientador: Leandro Russovski Tessler / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-01T20:54:28Z (GMT). No. of bitstreams: 1 Bosco_GiacomoBizinotoFerreira_D.pdf: 9507140 bytes, checksum: 4980b29f48f98f8ff97e8a0a37b7577e (MD5) Previous issue date: 2017 / Resumo: Este trabalho fornece caracterização ótica e estrutural de filmes finos compostos por nitreto de silício amorfo hidrogenado dopado com térbio (a-SiNx:H) ¿ crescidos por deposição química a vapor assistida por plasma gerado através de ressonância ciclotrônica de elétrons (ECR PECVD) e por pulverização catódica reativa em radiofrequência (reactive RF-Sputtering) ¿ com o propósito de avançar a investigação em fabricação de novos materiais e dos mecanismos da emissão de luz de íons de Tb quando diluídos em materiais baseados em silício. A fotoluminescência (PL) atribuída aos filmes de a-SiNx:H foi investigada em termos das condições de deposição e correlacionadas com suas propriedades estruturais e de recozimento pós-deposição. Entre as propriedades caracterizadas estão: estequiometria, taxa de deposição, índice de refração, coeficiente de extinção, bandgap ótico E04, concentração de térbio e vizinhança química presente ao redor de íons Tb3+. Concentrações de Tb da ordem de 1.8 at.% ou 1.4×?10?^21 at/cm^3 foram obtidas em amostras crescidas por Sputtering enquanto que concentrações de 14.0 at.%, ou da ordem ?10?^22 at/cm^3, puderam ser obtidas em amostras crescidas por ECR PECVD. Em Sputtering, a incorporação de Tb varia linearmente com a área recoberta por pastilhas de Tb4O7 em pó, enquanto que em PECVD, a incorporação de Tb é inversamente proporcional e pode ser ajustada sensivelmente pelo fluxo de gás SiH4. Forte emissão de luz, atribuída às transições eletrônicas em Tb3+ (PL do Tb), foi obtida em filmes não-recozidos que possuíam bandgap estequiométrico (E04 = 4.7 ± 0.4 eV and x = 1.5 ± 0.2). Espectros de PL do Tb não mostraram mudanças significativas no formato e na posição dos picos de emissão devido a alterações na temperatura de recozimento, nas condições de deposição ou entre amostras crescidas por diferentes técnicas de deposição. Entretanto, esses parâmetros influenciaram fortemente a intensidade da PL do Tb. Estudos da estrutura fina de absorção de raios-X (XAFS) em filmes crescidos por sputtering mostraram a estabilidade da vizinhança química ao redor dos íons Tb3+ mesmo em altas temperaturas (1100ºC). Investigações por sonda atômica tomográfica (APT) não encontraram formação de nanoclusters envolvendo ou não Tb, mesmo após recozimentos em altas temperaturas. Isso sugere que a excitação de Tb3+ deve ocorrer através da própria matriz hospedeira amorfa e não por mudanças no campo cristalino e, portanto, na força de oscilador das transições eletrônicas do Tb3+. Caracterização da densidade de ligações Si-H por espectroscopia infravermelha a transformada de Fourier (FTIR) em filmes recozidos em diferentes temperaturas foi relacionada com a intensidade da PL do Tb. Ela mostra que um decréscimo na densidade das ligações Si-H, que está relacionada a um aumento na concentração de ligações pendentes de Si (Si-dbs), resulta em filmes com maior intensidade na PL do Tb. Portanto, isso sugere que a excitação de Tb3+ parece acontecer através de transições envolvendo Si-dbs e estados estendidos, o que é consistente com o modelo de excitação Auger por defeitos (DRAE) / Abstract: This work offers optical and structural characterization of terbium (Tb) doped hydrogenated amorphous silicon nitrides thin films (a-SiNx:H) grown by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and reactive RF-Sputtering with the purpose of advancing the investigation in fabrication of novel materials and the mechanisms of light emission of Tb ions when embedded in Si-based materials. Photoluminescence (PL) of a-SiNx:H films were investigated and correlated with the deposition conditions, structural properties, and post-deposition thermal treatments (isochronal annealing under flow of N2). Among the characterized properties are: film stoichiometry, deposition rate, refractive index, extinction coefficient, optical bandgap, terbium concentration, and the chemical neighborhood around Tb ions. Tb concentrations of about 1.8 at.% or 1.4×?10?^21 at/cm^3 have been achieved in Sputtering system while concentrations of 14.0 at.%, or about ?10?^22 at/cm^3, could be achieved in ECR PECVD samples. In Sputtering, Tb incorporation varies linearly with the covered area of the Si target by Tb4O7 powder pellets, while in PECVD, Tb incorporation is inversely proportional to and can be sensitively adjusted through SiH4 gas flow. Bright PL attributed to Tb3+ electronic transitions (Tb PL) were obtained in as-deposited films with stoichiometric bandgaps (E04 = 4.7 ± 0.4 eV and x = 1.5 ± 0.2). The Tb PL spectra did not show any significant change in shape and in PL peak positions due to alterations in annealing temperature, deposition conditions or due to the used deposition method. However, these parameters strongly affected Tb PL intensity. Studies of X-ray absorption fine structure (XAFS) in Sputtering grown films show the stability of the chemical neighborhood around Tb3+ under annealing conditions even after thermal treatments at temperatures as high as 1100ºC. Atom probe tomography (APT) investigation also found no formation of nanoclusters of any type (involving Tb ions or not) after high temperature annealing treatments suggesting that Tb3+ excitation should come from the amorphous host matrix itself and not by changes in crystal field and thus in oscillator strength of Tb3+ electronic transitions. Fourier transform infrared spectroscopy (FTIR) characterization of Si-H bond density in films treated atin different annealing temperatures were crossed correlated with Tb PL intensity. It shows that a decrease in Si-H bond density, related to increase in Si dangling bonds (Si-dbs) concentration, results in greater Tb PL intensity. Thus, it suggests that excitation of Tb3+ happens through transitions involving silicon dangling bonds and extended states, consistent with the defect related Auger excitation model (DRAE) / Doutorado / Física / Doutor em Ciências / 142174/2012-2 / 010308/2014-08 / CNPQ / CAPES

Cette thèse porte sur l'interaction lumière-matière au sein de nanostructures placées dans des cavités optiques à base de semi-conducteurs nitrures. A l'aide d'expériences de micro-photoluminescence dans l'ultra-violet, nous étudions les propriétés optiques de boîtes quantiques GaN/AlN dans des microcavités planaires et celles de puits quantiques GaN/AlN insérés dans des microdisques AlN.Afin d'améliorer la collection du faible signal de photoluminescence de boîtes quantiques uniques, nous utilisons des microcavités planaires pour modifier le diagramme d'émission d'une boîte quantique. Le dessin des microcavités est optimisé grâce à des simulations numériques basées sur la méthode des matrices de transfert en présence d'un émetteur. Nous montrons que, pour une microcavité nitrure à base de miroirs de Bragg AlN/AlGaN, la collection des photons émis par une boîte quantique peut être théoriquement améliorée d'un ordre de grandeur, ce qui est confirmé par nos mesures sur boîtes quantiques uniques, ouvrant ainsi la voie à des études avancées de corrélations de photons dans l'UV.La seconde partie des travaux est dédiée à la réalisation d'un micro-laser opérant dans l'UV profond et à température ambiante. En utilisant des puits quantiques GaN/AlN de 2,8 mono-couches, crûs sur substrat silicium et insérés dans des microdisques AlN, nous observons une émission laser à 275 nm sous pompage optique impulsionnel. Cette démonstration montre le fort potentiel des semi-conducteurs nitrures pour la nano-photonique UV sur silicium. / This thesis addresses the light-matter interaction in nitride nanostructures embedded in optical microcavities. By using micro-photoluminescence experiments, we study the optical properties of GaN/AlN quantum dots embedded in planar microcavities and those of GaN/AlN quantum wells in AlN microdisks.By placing quantum dots in planar microcavities, we are able to modify the emission diagram and increase the collection efficiency. The design of the microcavities is optimized by using numerical simulations based on transfer matrix method with an internal emitter. For an AlN microcavity with AlN/AlGaN Bragg mirrors, we show that the collection efficiency could be theoretical increase by one order of magnitude, which is confirmed by our micro-photoluminescence experiments on single quantum dots. This observation opens the way for advanced studies such as photon correlations experiments in the UV range.The second part of our work is devoted to the realization of a micro-laser operating in the deep-UV range at room-temperature. By using thin GaN/AlN quantum wells (2.8 monolayers), grown on silicon substrate and embedded in AlN microdisks, we observe a laser emission at 275 nm under pulsed optical pumping. This demonstration shows the strong potentiality for future developments of nitride-on-silicon nano-photonics.

[pt] Pós nanoestruturados de nitreto de silício (Si3N4) foram sintetizados a 300 graus Celsius por precipitação a partir da reação em fase vapor entre o cloreto de silício (SiCl4) e a amônia (NH3). O argônio (Ar) foi utilizado como gás de arraste. Além do pó de nitreto de silício amorfo, o cloreto de amônio sólido (NH4Cl) é formado como subproduto. Os pós Si3N4 quando expostos à atmosfera são facilmente oxidados a oxi-nitreto de silício. As fases cristalinas do Si3N4 foram obtidas por tratamento térmico em uma atmosfera de argônio a 1500 graus Celsius por 2 horas. Caracterizações por Difração de Raios-X e Espectroscopia no Infravermelho com Transformada de Fourier (FTIR) revelaram as fases alfa-Si3N4 e beta-Si3N4, dióxido de silício e oxinitretos de silício. A Microscopia Eletrônica de Varredura por Emissão de Campo (MEV) e Microscopia Eletrônica de Transmissão (MET) mostrara diversas morfologias nas nanoestruturas tais como bastões, cristais facetados, fitas e fios amorfos. O padrão de difração de área selecionada (SADP) indica a natureza cristalina das partículas colunares e as imagens HRTEM revelaram que o espaçamento interplanar da rede é 0,67 nm, que se relaciona com o plano de rede (100) do alfa-Si3N4. A maior superfície específica determinada dos pós, por BET, foi de 96,56m(2)/g. / [en] Nanostructured silicon nitride powders (Si3N4) were synthesized at 300 Celsius degrees by precipitation from the vapor phase reaction between silicon chloride (SiCl4) and ammonia (NH3). Argon (Ar) was used as carrier gas. Solid ammonium chloride (NH4Cl) is formed as by-product, in addition to silicon nitride powder. When exposed to the atmosphere these powders are readily oxidized to silicon oxynitride. Crystalline phases of Si3N4 were obtained by heat treatment in an argon atmosphere at 1500 Celsius degrees for 2 hours. Characterization by X-ray Diffraction and Infrared Spectroscopy with Fourier Transform (FTIR) revealed formation of the alpha-Si3N4 and beta-Si3N4 phases, silicon dioxide and silicon oxynitrides. Field emission scanning electron microscopy (SEM-FEG) and Transmission Electron Microscopy (MET) showed different morphologies such as nano sticks, faceted crystals, ribbons and whiskers. The selected area diffraction pattern (SADP) indicates the crystalline nature of the columnar particles and the HRTEM images reveal that the lattice fringe spacing is 0.67 nm, which match with the (100) plane of alpha-Si3N4. The highest specific surface area of the powders determined, by BET, was 96.56 m(2)/g.

Machine learning is a branch of artificial intelligence that uses data to automatically build inferences and models designed to generalise and make predictions. In this thesis, the use of machine learning in materials science is explored, for two different problems: the optimisation of gallium nitride optoelectronic devices, and the prediction of material failure in the setting of laboratory earthquakes. Light emitting diodes based on III-nitrides quantum wells have become ubiquitous as a light source, owing to their direct band-gap that covers UV, visible and infra-red light, and their very high quantum efficiency. This efficiency originates from most electronic transitions across the band-gap leading to the emission of a photon. At high currents however this efficiency sharply drops. In chapters 3 and 4 simulations are shown to provide an explanation for experimental results, shedding a new light on this drop of efficiency. Chapter 3 provides a simple and yet accurate model that explains the experimentally observed beneficial effect that silicon doping has on light emitting diodes. Chapter 4 provides a model for the experimentally observed detrimental effect that certain V-shaped defects have on light emitting diodes. These results pave the way for the association of simulations to detailed multi-microscopy. In the following chapters 5 to 7, it is shown that machine learning can leverage the use of device simulations, by replacing in a targeted and efficient way the very labour intensive tasks of making sure the numerical parameters of the simulations lead to convergence, and that the physical parameters reproduce experimental results. It is then shown that machine learning coupled with simulations can find optimal light emitting diodes structures, that have a greatly enhanced theoretical efficiency. These results demonstrate the power of machine learning for leveraging and automatising the exploration of device structures in simulations. Material failure is a very broad problem encountered in a variety of fields, ranging from engineering to Earth sciences. The phenomenon stems from complex and multi-scale physics, and failure experiments can provide a wealth of data that can be exploited by machine learning. In chapter 8 it is shown that by recording the acoustic waves emitted during the failure of a laboratory fault, an accurate predictive model can be built. The machine learning algorithm that is used retains the link with the physics of the experiment, and a new signal is thus discovered in the sound emitted by the fault. This new signal announces an upcoming laboratory earthquake, and is a signature of the stress state of the material. These results show that machine learning can help discover new signals in experiments where the amount of data is very large, and demonstrate a new method for the prediction of material failure.

Zinc nitride and magnesium nitride are examples of the relatively unexplored II3V2 group of semiconductor materials. These materials have potential applications in the electronics industry due to their excellent optical and electrical properties. This study mainly focuses on the growth and characterization of the new semiconductor materials: zinc nitride, magnesium nitride, and their alloys. The (100) oriented zinc nitride thin films were grown on both (110) sapphire substrates and (100) MgO substrates by plasma-assisted molecular beam epitaxy (MBE). The typical growth rate is in the range of 0.02-0.06 nm/s, the growth temperature is in the range of 140-180 oC, and background nitrogen pressure is around 10-5 Torr. The growth process was monitored by in-situ: reflection high energy electron diffraction (RHEED) and optical reflectivity. The RHEED and X-ray diffraction patterns of the zinc nitride indicate that the film is a single crystal material. The in-situ optical reflectivity pattern of the zinc nitride shows interference oscillations, and these oscillations are damped out as the thickness increases. The reflectivity as a function of time was accurately simulated by an optical equation. The optical constants of the thin films, the growth rate, and the thickness were derived from the simulation of the in-situ reflectance. The X-ray diffraction shows that (400) oriented zinc nitride thin films were grown on both A-plane (110) sapphire substrates and (100) MgO substrates. Optical transmittance measurements were performed on the zinc nitride thin films. The spectrum of the zinc nitride transmittance indicates that zinc nitride has a high optical absorption in the visible light region. The absorption coefficient was calculated from the transmittance spectrum, and the optical band gap of the zinc nitride thin film was found to be 1.25-1.28 eV. Ellipsometry measurements suggested that the refractive index of zinc nitride is 2.3-2.7, and the extinction coefficient is ~0.5-0.7 in the energy range 1.5-3.0 eV. The electron transport measurement shows that the single crystal zinc nitride has a mobility as high as 395 cm2 /Vs. A plasma-assisted MBE system was employed for magnesium nitride growth. The growth temperature was in the range of 300-350 oC. RHEED and laser reflectivity were employed during growth. The RHEED and X-ray diffraction patterns indicated that the epilayers are single crystal films. The optical laser reflectivity was well fitted by a modified optical equation. The optical constants and growth rate were derived from the simulation. X-ray diffraction showed that (400) oriented single crystal magnesium nitride films were grown on (100) MgO substrates. The optical transmittance spectra show that the magnesium nitride has a high absorption below 500 nm. The calculated absorption coefficient is as high as 4x10-4 cm-1 in the range of ~2.5-3.0 eV. The optical band gap was estimated to be ~2.5 eV. Ellipsometry measurements showed that the refractive index of the magnesium nitride is 2.3-2.75 and the extinction coefficient is less than 0.3 in the energy range of 1.5-3.0 eV. Zinc nitride-magnesium nitride (Zn3-3xMg3xN2) alloys were grown on (100) YSZ substrates by sputtering. The bandgap ranged from 1.2 eV to 2.1 eV for Mg content x in the 0-0.59 range. One film with a bandgap of ~1.4 eV and Mg content of 0.18 has the relatively high mobility of 47 cm2 /Vs which was expected for photovoltaics application. / Graduate

Graphene has emerged as an appealing material for a variety of optoelectronic applications due to its unique electrical and optical characteristics. In this thesis, I will present recent advances in integrating graphene and graphene-boron nitride (BN) heterostructures with confined optical architectures, e.g. planar photonic crystal (PPC) nanocavities and silicon channel waveguides, to make this otherwise weakly absorbing material optically opaque. Based on these integrations, I will further demonstrate the resulting chip-integrated optoelectronic devices for optical interconnects. After transferring a layer of graphene onto PPC nanocavities, spectral selectivity at the resonance frequency and orders-of-magnitude enhancement of optical coupling with graphene have been observed in infrared spectrum. By applying electrostatic potential to graphene, electro-optic modulation of the cavity reflection is possible with contrast in excess of 10 dB. And furthermore, a novel and complex modulator device structure based on the cavity-coupled and BN-encapsulated dual-layer graphene capacitor is demonstrated to operate at a speed of 1.2 GHz. On the other hand, an enhanced broad-spectrum light-graphene interaction coupled with silicon channel waveguides is also demonstrated with ∼0.1 dB/μm transmission attenuation due to graphene absorption. A waveguide-integrated graphene photodetector is fabricated and shown 0.1 A/W photoresponsivity and 20 GHz operation speed. An improved version of a similar photodetector using graphene-BN heterostructure exhibits 0.36 A/W photoresponsivity and 42 GHz response speed. The integration of graphene and graphene-BN heterostructures with nanophotonic architectures promises a new generation of compact, energy-efficient, high-speed optoelectronic device concepts for on-chip optical communications that are not yet feasible or very difficult to realize using traditional bulk semiconductors.

The increased use of DC-consuming electronics in many applications relevant to everyday life, necessitates significant improvements to power conversion and distribution methodologies. The surge in mobile electronics created a new power application space where high efficiency, size, and reduced complexity are critical, and at the same time, many computational tasks are relegated to centralized cloud computing centers, which consume significant amounts of energy. In both those application spaces, conversion and distribution efficiency improvements of even a few-% proves to be more and more challenging. A lot of research and development efforts target each source of loss, in an attempt to improve power electronics such that it serves the advances in other fields of electronics. Non-isolated DC-DC converters are essential in every electronics system, and improvements to efficiency, volume, weight and cost are of utmost interest. In particular, increasing the operation frequency and the conversion ratio of such converters serves the purposes of reducing the number or required conversion steps, reducing converter size, and increasing efficiency. The aforementioned improvements can be achieved by using superior technologies for the components of the converter, and by implementing higher level of integration than most present-day converters exhibit. In this work, Gallium Nitride (GaN) high electron mobility transistors (HEMT) are utilized as switches in a half-bridge buck converter topology, in conjunction with fine-line 180nm complementary metal oxide semiconductor (CMOS) driver circuitry. The circuits are integrated through a face-to-face bonding technique which results in significant reduction in interconnects parasitics and allows faster, more efficient operation. This work shows that the use of GaN transistors for the converter gives an efficiency headroom that allow pairing converters with state-of-the-art thin-film inductors with magnetic material, a task that is currently usually relegated to air-core inductors. In addition, a new "core-clad" structure for thin-film magnetic integrated inductors is presented for the use with fully integrated voltage regulators (IVRs). The core-clad topology combines aspects from the two popular inductor topologies (solenoid and cladded) to achieve higher inductance density and improved high frequency performance.

Zircônia 3YTZP apresenta propriedades excelentes à temperatura ambiente, mas estas propriedades são afetadas pelo aumento da temperatura pois esta age negativamente sobre o mecanismo de transformação de fase induzida por tensão, que fortalece a tenacidade da matriz. A adição de Si3N4 e SiC em uma matriz de 3YTZP é muito interessante porque conduz à formação de oxinitreto de silício, melhorando as propriedades mecânicas tais como dureza e tenacidade, mas esta adição está limitada por várias dificuldades que se apresentam durante o processamento e sinterização destes materiais. Neste trabalho foi estudada a obtenção, por sinterização sem pressão, do compósito Y-TZP/Si2N2O, partindo-se da adição de 20vol%Si3N4-SiC em uma matriz de zircônia dopada com 3mol% de Y2O3 - 3YTZP, utilizando-se Al2O3 e Y2O3 como aditivos de sinterização. A mistura foi moída e moldada por prensagem isostática a frio. Amostras foram sinterizadas a 1500º, 1600º e 1700ºC por 2h sem pressão e em atmosfera ambiente, utilizando-se um leito de nitreto de silício. Após sinterização, as amostras foram caracterizadas por difração de raios-X. Foram medidas a densidade, tenacidade, dureza e resistência mecânica à flexão em temperatura ambiente. A estrutura do material foi observada em microscopia eletrônica de varredura e de transmissão, com mapeamento químico, para verificar a homogeneidade e morfologia das fases do compósito. A formação de Si2N2O foi observada no material sinterizado devido à reação entre os pós adicionados. O material obtido apresentou aumento de tenacidade e dureza com o aumento de temperatura de sinterização. As amostras apresentaram boa resistência à oxidação a 1000ºC. / Zirconia 3YTZP presents excellent properties at room temperature. These properties decrease as the temperature increases because high temperature acts negatively over the stress induced transformation toughening in the matrix. The addition of Si3N4 and SiC in a Y-TZP matrix is very interesting because leads to formation of silicon oxynitride and it increases the mechanical properties like toughness and hardness. Certainly the mechanical properties increment is limited by several difficulties which have appeared during processing and heating of these materials. This paper studies the Y-TZP/Si2N2O pressureless sintered composite, under different temperatures, showing the behavior of 20vol%Si3N4-SiC when added in YTZP matrix and heated under no pressure system. Al2O3 and Y2O3 were used as sintering aids. The mixture was milled and molded by cold isostatic pressure. Samples were heated at 1500º, 1600º and 1700ºC x 2h without pressure under atmospheric conditions using Si3N4 bed-powder. Samples were characterized by XRD and density, hardness, toughness, bending strength were measured. The structure of the material was observed in SEM/TEM/EPMA to verify the distribution and composition of the materials in the composite and the contact between filler surface and the matrix. The formation of SiON2 was observed in the sintered material due to reaction between both nitride and carbide with Y-TZP matrix. Furthermore the material showed an increment of both hardness and toughness as temperature increases. The samples presented considerable resistance to oxidation below 1000ºC.