Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Boron nitride (BN) is a compound formed by covalent bonds between boron and nitrogen atoms. In the crystalline phase it can be found in different structures, such cubic (c-BN), simgle hexagonal (h-BN), wurtzite (w-BN) and rhombohedral (r- BN). Similarly to graphite h-BN can form two dimensional structures, which can be cut to form tubes. Otherwise, c-BN has hardness similar to diamond, which is retained up to 2000 °C while diamond turns into to graphite at about 900 °C. These properties make BN a promissor material for nanoelectronics in a hot environment. Here, we investigate structural and electronic properties of BN nanotubes. Our study is directed not only to BN nanotubes in the pristine form (no defects), but also when topological defects (vacancies) and dopant impurities (Carbon) are present. The first principles calculations are based on the density functional theory with the generalized gradient approximation for the exchange-correlation term. The calculations were performed using the SIESTA computer code using gaussian functions to expand the Khon-Sham orbitals. We observe that C impurities have low formation energies when compared to vacancies have high formation energies. All the pristine BN nanotubes studied are non magnetic semiconductor with a band gap energy around 3.5 eV, which is almost independent of the tube chirality and diameter. Carbon impurities introduce localized electronic levels into the band gap while vacancies give magnetic moments to the BN nanotubes. Calculations for complex defects (carbon impurities and vacancies) show that the formation of these complex defects have lower formation energies when compared to the sum of the formation energies for isolated defects, indicating that these defects have higher probability of occurring. Double vacancies have formation energies close to those for single vacancies and in the equilibrium geometry, occurs a reconstruction where a pentagon-octagon-pentagon (5−8−5) structure is present. In the optimal geometry dangling bonds are not present and the magnetic moment is zero, but non-spin-polarized electronic levels are present in the band gap. / O nitreto de boro (BN) é formado por ligações covalentes entre B e N. Na fase cristalina pode ser encontrado nas diferentes estruturas: cúbica (c-BN), hexagonal (h-BN), wurtzita (w-BN) e romboédrica (r-BN). Similar ao grafite, o h-BN pode formar estruturas bidimensionais (planos ou camadas), as quais podem ser cortadas formando tubos. O c-BN tem uma dureza semelhante à do diamante, que é mantida até 2000 °C, enquanto que o diamante se desfaz em grafite a cerca de 900 °C. Estas propriedades fazem do BN um material promissor para nanoeletrônica em ambientes abrasivos. Neste trabalho, estudamos as propriedades estruturais e eletrônicas de nanotubos de BN. Nosso estudo é dirigido não só para os nanotubos de BN na forma pristina (sem defeitos), mas também quando defeitos topológicos (vacâncias) e impurezas (carbono) estão presentes. Os cálculos de primeiros princípios são baseados na teoria do funcional da densidade (TFD) com a aproximação do gradiente generalizado para o termo de troca-correlação (AGG). Utilizamos o código computacional SIESTA com funções gaussianas para expandir os orbitais de Khon-Sham. Observamos que as energias de formação para impurezas de carbono (C) são menores que para vacâncias. Os nanotubos de BN na fase pristina estudados são semicondutores não magnéticos com um gap de energia de cerca de 3,5 eV e praticamente independente do diâmetro e da quiralidade do tubo. Impurezas de carbono introduzem níveis eletrônicos localizados no gap enquanto que vacâncias geram momentos magnéticos para os nanotubos de BN. Cálculos para defeitos complexos (impurezas de C e vacâncias) mostram uma menor energia de formação quando comparados com defeitos isolados, indicando maior probabilidade de ocorrer. Vacâncias duplas têm energias de formação similares as de vacâncias simples e na geometria de equilíbrio ocorre uma reconstrução onde uma estrutura pentágono-octógono-pentágono (5−8−5) está presente. Não existem ligações pendentes e o momento magnético é zero, mas níveis eletrônicos de spin não polarizados estão presentes no gap.

Master of Science / Department of Chemical Engineering / James H. Edgar / Hexagonal boron nitride has been used as an inert, refractory material with excellent resistance to thermal decomposition and oxidation for more than fifty years. In the past few years, hBN has been targeted for potential electrical and optical devices such as neutron detectors, ultraviolet light emitters, deep ultraviolet light detectors, and substrates for graphene and other atomically-thin two-dimensional materials. All of these potential applications benefit from high quality, single crystals, with thicknesses varying from nanometers to microns. This research was undertaken to investigate four aspects of hBN crystal growth and recovery. (1) In an effort to optimize hBN crystal growth from a nickel-chromium flux, a series of stepped cooling experiments were undertaken. The temperature profile was stepped in a way as to promote growth in both the a and c directions, at their optimal growth conditions. Crystals were found to be typically 100-500 µm across and thickness of approximately 20-30 µm with a pyramid-like crystal habit. (2) A method for the removal of hBN crystals prior to freezing of the metal flux was demonstrated using a specialized hot pressed boron nitride crucible capable of removing hBN crystals from the flux in situ. (3) Growth of isotopically pure hBN crystals was undertaken. By modifying the crucible material for solution growth, enrichment of hBN crystals over 90% was accomplished. (4) Exfoliation of hBN has many potential applications, specifically as graphene-hBN heterostructures where layers approaching thicknesses of single atoms are most effective surface to interact with graphene as an electronic device. Several methods were tested toward exfoliating a single crystal resulting in few-layered hexagonal boron nitride nanosheets. As a result of these investigations a greater understanding of hBN bulk growth, its isotopic enrichment, its recovery, and its exfoliation was obtained.

abstract: Group III-nitride semiconductors have been commercially used in the fabrication of light-emitting diodes and laser diodes, covering the ultraviolet-visible-infrared spectral range and exhibit unique properties suitable for modern optoelectronic applications. InGaN ternary alloys have energy band gaps ranging from 0.7 to 3.4 eV. It has a great potential in the application for high efficient solar cells. AlGaN ternary alloys have energy band gaps ranging from 3.4 to 6.2 eV. These alloys have a great potential in the application of deep ultra violet laser diodes. However, there are still many issues with these materials that remain to be solved. In this dissertation, several issues concerning structural, electronic, and optical properties of III-nitrides have been investigated using transmission electron microscopy. First, the microstructure of In_xGa_1-xN (<italic>x</italic> = 0.22, 0.46, 0.60, and 0.67) films grown by metal-modulated epitaxy on GaN buffer /sapphire substrates is studied. The effect of indium composition on the structure of InGaN films and strain relaxation is carefully analyzed. High luminescence intensity, low defect density, and uniform full misfit strain relaxation are observed for <italic>x</italic> = 0.67. Second, the properties of high-indium-content InGaN thin films using a new molecular beam epitaxy method have been studied for applications in solar cell technologies. This method uses a high quality AlN buffer with large lattice mismatch that results in a critical thickness below one lattice parameter. Finally, the effect of different substrates and number of gallium sources on the microstructure of AlGaN-based deep ultraviolet laser has been studied. It is found that defects in epitaxial layer are greatly reduced when the structure is deposited on a single crystal AlN substrate. Two gallium sources in the growth of multiple quantum wells active region are found to cause a significant improvement in the quality of quantum well structures. / Dissertation/Thesis / Doctoral Dissertation Physics 2014

abstract: Semiconductor nanowires have the potential to emerge as the building blocks of next generation field-effect transistors, logic gates, solar cells and light emitting diodes. Use of Gallium Nitride (GaN) and other wide bandgap materials combines the advantages of III-nitrides along with the enhanced mobility offered by 2-dimensional confinement present in nanowires. The focus of this thesis is on developing a low field mobility model for a GaN nanowire using Ensemble Monte Carlo (EMC) techniques. A 2D Schrödinger-Poisson solver and a one-dimensional Monte Carlo solver is developed for an Aluminum Gallium Nitride/Gallium Nitride Heterostructure nanowire. A GaN/AlN/AlGaN heterostructure device is designed which creates 2-dimensional potential well for electrons. The nanowire is treated as a quasi-1D system in this work. A self-consistent 2D Schrödinger-Poisson solver is designed which determines the subband energies and the corresponding wavefunctions of the confined system. Three scattering mechanisms: acoustic phonon scattering, polar optical phonon scattering and piezoelectric scattering are considered to account for the electron phonon interactions in the system. Overlap integrals and 1D scattering rate expressions are derived for all the mechanisms listed. A generic one-dimensional Monte Carlo solver is also developed. Steady state results from the 1D Monte Carlo solver are extracted to determine the low field mobility of the GaN nanowires. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2017

Nous avons étudié les propriétés structurales et optiques d'hétérostructures de nitrures d'éléments III. Les croissances ont été réalisées par épitaxie par jets moléculaires assistée plasma d'azote. L'étude a été en particulier axée sur des structures émettant dans l'UV, à des longueurs d'onde sub-300 nm. Pour atteindre cette longueur d'onde à l'aide de couches bidimensionnelles, il est nécessaire d'utiliser des alliages ternaires d'AlGaN à forte teneur en Al (supérieure à 50 %). Nous avons donc tout d'abord corrélé les résultats de techniques de caractérisation aux conditions de croissance. Ainsi, il est possible de contrôler en partie l'homogénéité de l'alliage grâce à la température de croissance et au rapport de flux métal/azote. Nous avons ensuite tenté d'appliquer ces résultats à des hétérostructures à puits quantiques AlGaN/AlGaN. D'autre part, nous avons réalisé des alliages digitaux (super-réseaux à courte période de puits quantiques GaN/AlN). Ceux-ci se sont montré une alternative intéressante aux couches 2D pour la réalisation de dispositifs émetteurs de lumière. Enfin, une part importante du manuscrit s'attache à l'étude des effets d'un recuit après la croissance et à haute température. Cette étude a été menée sur des couches d'AlGaN ainsi que sur des boites quantiques GaN/AlN. Pour les couches d'AlGaN, nous avons constaté une augmentation de l'inhomogénéité des couches après un recuit à 950 °C. Pour les boîtes quantiques GaN, nous avons constaté dès 1000 °C, un décalage vers le bleu de la luminescence. A 1300-1500 °C, des images TEM ont clairement montré la diffusion de Ga dans la matrice AlN au-dessus des boîtes quantiques. / We studied the structural and optical properties of III-nitride heterostructures. The growth were achieved by plasma-assisted molecular beam epitaxy. The study was particularly focused on structures emitting in the UV range at sub-300 nm wavelengths. To achieve this wavelength using two-dimensional layers, it is necessary to use ternary AlGaN alloys with high Al content (above 50%). So we first correlated the results of characterization techniques to growth conditions. As a result, we find that it is possible to partially control the alloy homogeneity with the growth temperature and metal/nitrogen flux ratio. Then, we tried to apply these results to AlGaN/AlGaN quantum well heterostructures . On the other hand, we have made digital alloys (short-period GaN/AlN quantum well superlattices). They appeared as an interesting alternative to 2D layers for the realization of light emitting devices. Finally, an important part of the manuscript focuses on studying the effects of post growth annealing, at high temperature. This study was conducted on AlGaN layers and on GaN/AlN quantum dots. For AlGaN layers, we found an increase in the inhomogeneity of the alloy after annealing at 950 °C. For GaN quantum dots, a blue shift of the luminescence is observed for annealig temperature as low as 1000 °C. At 1300-1500 ° C, TEM images clearly showed the diffusion of Ga in the AlN matrix above the quantum dots.

Dans le cadre de la fabrication de pièces structurales pour l’industrie aéronautique, de nouveaux matériaux composites à matrice céramique sont envisagés par le motoriste Safran. Lors de l’utilisation dans des environnements sévères de combustion, et sous chargement mécanique, la matrice pourra s’endommager par fissuration et la capacité du matériau à s’auto-protéger sera réduite. Les travaux présentés dans cette thèse ont pour but d'identifier les mécanismes prépondérants de ruine de ce type de matériaux composites en fonction de différentes sollicitations thermiques, mécaniques et environnementales. Une démarche multi-échelle a été adoptée, de manière à considérer les phénomènes à une échelle microscopique (chaque constituant) et macroscopique (synergie entre les constituants au sein du matériau). Le comportement en oxydation/corrosion du matériau composite et de chacun de ses constituants a été caractérisé puis modélisé pour être extrapolé à des environnements de combustion. Différents domaines de protection du matériau ont ainsi été mis en évidence. Dans cette même optique, le comportement thermomécanique des constituants matriciels et du composite a fait l’objet d’essais de fluage pour déterminer les paramètres d’une première modélisation, et analyser les dégradations associées. En couplant les différents résultats obtenus, il est ainsi possible d’évaluer la criticité des différents phénomènes de ruine. Des scenarii de ruine sont donc présentés. / To design and manufacture CMC structural components for aeronautics, Safran develops a new CMC grade, provided to resist severe combustion environments. Understanding this new composite material damaging and failure mechanisms is essential: environmental conditions, mechanical loading, and matrix damages, can lead to a decrease in its self-protective properties. The aim of this work is to identify prevailing high temperature degradation mechanisms of those MI SiC/SiC CMCs, depending of different thermal, mechanical and environmental stresses. To fulfill that outcome, a multi-scale approach was considered, by examining phenomena from single constituents to complex composite architecture (effects of constituents’ synergy). High temperature oxidation/corrosion behaviors of the composite material and each of its constituents were characterized, and modeled to meet representative combustion environment. Different operating areas of efficient self-protection of the composite material were therefore highlighted. In the same perspective, themomechanical behaviors of both matrix constituents and composite were experimentally explored, enabling the determination of behavior laws. Related degradations in the composite material were also analyzed. Crossing all results, it was thereby possible to evaluate kinetics and criticality of failure mechanisms: different damaging scenarios are thus proposed, depending on environmental conditions.

Made available in DSpace on 2015-05-14T12:14:08Z (GMT). No. of bitstreams: 1 parte1.pdf: 2147467 bytes, checksum: 06d61abcd760c3d3a8cc6b1d61af88fa (MD5) Previous issue date: 2008-08-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work, we performed an analysis of the structural and electronic stability of nanostructures of Boron Nitride (BN), such as layers, tubes and cones, when doped with Carbon, through first-principles calculations as implemented in code SIESTA. We found that substitutional doping of Carbon for either a single Boron or a single Nitrogen atom produces significant changes in the conductive properties of each material. Such replacement process transforms the formerly insulating material, in a n-type conductor, donnor of electrons, for Boron replaced by Carbon, and the p-type, acceptor of electrons, for the Nitrogen substitution. Furthermore, we also performed calculations with spin-polarization and found occurrence of spontaneous magnetization of 1μB for all doped structures, except for tube (6,0), which presented a magnetic moment of 0;2μB. This magnetization is attributed to the unpaired electron located in the pz orbital of carbon. It was also noted that the doped layers become more energetically stable as the number of atoms increases. To the tubes, increased stability occurs with the increase in diameter combined with the consequent increase in the number of atoms. At the cones, stability energy is reduced with the increase in the angle of disclination. These effects are the result of a combination of percentage concentration of the defect and the greater or lesser degree of hybridization. / Neste trabalho, fizemos uma análise da estabilidade estrutural e eletrônica de nanoestruturas de Nitreto de Boro (BN), tais como planos, tubos e cones, quando dopadas com carbono, através do uso de cálculos de primeiros princípios como implementado no código SIESTA. Encontramos que a dopagem substitucional de carbono por boro ou nitrogênio provoca mudanças significativas nas propriedades condutoras de cada material. Tais substituições transformam o material que antes era isolante, em condutor do tipo n, doador de elétrons, para o boro substituído pelo carbono, e do tipo p, receptor de elétrons, para a substituição do nitrogênio. Além disso, realizamos cálculos com polarização de spin e verificamos a ocorrência de um momento magnético de 1μB para todas as estruturas dopadas, com exceção do tubo (6;0) que apresentou um momento magnético de 0;2μB. Essa magnetização é atribuída ao elétron desemparelhado localizado no orbital pz do carbono. Foi verificado também que os planos dopados se tornam mais estáveis energeticamente à medida que o número de átomos aumenta. Para os tubos, o aumento da estabilidade ocorre com o aumento do diâmetro combinado com o consequente aumento do número de átomos. Já nos cones, a estabilidade energética é diminuída com o aumento do ângulo de disclinação. Esses efeitos são fruto de uma combinação entre porcentagem de concentração do defeito e do maior ou menor grau de hibridização.

O presente trabalho avalia a deposição de filmes de TiN com caráter decorativo sobre substratos de AISI 304. Os filmes produzidos neste estudo são compostos por uma intercamada de Ti entre o substrato e o filme de TiN, produzindo assim um sistema substrato/Ti/TiN. Foram estudados os efeitos da tensão de polarização aplicada ao substrato durante a deposição da intercamada de Ti e a influência da temperatura de deposição do filme, com variação de temperatura entre 25°C e 200°C. Os filmes foram depositados através da tecnologia PVD por arco catódico em um processo de deposição reativa em escala industrial, partindo de cátodos de Ti e gases argônio e nitrogênio. Os filmes tiveram sua composição química analisada através da técnica de espectroscopia por descarga luminescente, as fases cristalinas foram determinadas por difração de raios-X e a microestrutura e espessura das camadas foram avaliadas através de microscopia eletrônica de varredura (MEV). Também foi medida a dureza em nanoescala dos filmes e, como forma de complementar os resultados obtidos, a carga crítica para início de deformação plástica dos filmes foi medida através de ensaios de esclerometria linear. Os resultados mostraram que ambos os parâmetros de processos analisados exercem influência significativa nas propriedades e características dos filmes formados, principalmente no teor de contaminação por oxigênio presente nos filmes e na carga crítica para deformação plástica. Além disso, também se observou que o filme depositado à 200°C apresenta uma mudança significativa na sua orientação cristalográfica e composição química em comparação com os depositados em temperaturas inferiores. Como resultado do trabalho, pôde-se determinar os parâmetros ideais para a deposição de filmes finos de TiN decorativos, possibilitando o desenvolvimento e a aplicação da tecnologia em escala industrial. / Submitted by Ana Guimarães Pereira (agpereir@ucs.br) on 2015-02-11T13:10:46Z No. of bitstreams: 1 Dissertacao Gustavo Caberlon Lain.pdf: 2832428 bytes, checksum: 9f93177004f259851cf5825bdbafb606 (MD5) / Made available in DSpace on 2015-02-11T13:10:46Z (GMT). No. of bitstreams: 1 Dissertacao Gustavo Caberlon Lain.pdf: 2832428 bytes, checksum: 9f93177004f259851cf5825bdbafb606 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior. / This work studies the deposition of titanium nitride (TiN) thin films on stainless steel AISI 304 substrate for decorative applications. The films studied here are composed of a layer of Ti working as an interlayer between the substrate and the TiN thin film, creating a system substrate/Ti/TiN. The BIAS applied on the substrate during the deposition of the Ti interlayer and the deposition temperature of the films between 25°C and 200°C were studied. The films were deposited by a PVD cathodic arc evaporation technique in a reactive deposition process, working with Ti cathodes and argon and nitrogen as gases. The equipment utilized for these depositions was an industrial PVD coating equipment. The chemical composition of the TiN films was analyzed by glow-discharge optical emission spectroscopy technique (GD-OES) and the crystalline structure by X-ray diffraction (XRD) and the microstructure and thickness by scanning electron microscopy (SEM). Also, the hardness at the nanoscale of the films was measured and scratch analyses were made to determine the critical load for the beginning of plastic deformation of the films. The results show that both analyzed process parameters significantly affect the properties and characteristics of the films, especially on the oxygen contamination content and the critical load for plastic deformation. Furthermore, it was also observed that the film deposited at 200°C presents a significant change on its crystallographic orientation and chemical composition compared with those deposited at lower temperatures. As a result of this research, it was possible to determine the optimized process conditions for the deposition of TiN thin films for decorative purposes, enabling the development and application of this technology in industrial scale.

Made available in DSpace on 2014-12-17T15:14:52Z (GMT). No. of bitstreams: 1 EduardoMD_DISSERT.pdf: 1460594 bytes, checksum: 3bdc57732c952c77e8ca0b42292d1e86 (MD5) Previous issue date: 2011-01-27 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / In this work we deposit via non-reactive magnetron sputtering of radio-frequency nanofilmes of nitreto of aluminum(AlN). The nanofilms aluminum nitride are semiconductors materials with high thermal conductivity, high melting point, piezoelectricity and wide band gap (6, 2 eV) with hexagonal wurtzite crystal structure, belonging to the group of new materials called III-V nitrides in which together with the gallium nitride and indium nitride have attracted much interest because they have physical and chemical properties relevant to new technological applications, mainly in microelectronic and optoelectronic devices. Three groups were deposited with thicknesses nanofilms time dependent on two substrates (glass and silicon) at a temperature of 25 ? C. The nanofilms AlN were characterized using three techniques, X-ray diffraction, Raman spectroscopy and atomic force microscopy (AFM), examined the morphology of these. Through the analysis of X-rays get the thickness of each sample with its corresponding deposition rate. The analysis of X-rays also revealed that nanofilms are not crystalline, showing the amorphous character of the samples. The results obtained by the technique, atomic force microscopy (AFM) agree with those obtained using the technique of X-rays. Characterization by Raman spectroscopy revealed the existence of active modes characteristic of AlN in the samples / Neste trabalho depositamos via magnetron sputtering de r?dio-frequ?ncia n?o reativo nanofilmes de nitreto de alum?nio (AlN). Os nanofilmes de nitreto de alum?nio s?o materiais semicondutores com alta condutividade t?rmica, elevado ponto de fus?o, piezoeletricidade e largo "bandgap"(6;2 eV) com estrutura cristalina wurtz?tica hexagonal, pertencentes ao grupo de novos materiais denominados nitretos III-V que em conjunto com o nitreto de g?lio e o nitreto de ?ndio t?m despertado muito interesse por possu?rem propriedades f?sico-qu?micas relevantes para novas aplica??es tecnol?gicas, principalmente em microeletr?nica e dispositivos optoeletr?nicos. Foram depositados tr?s grupos de nanofilmes com as espessuras depend?ntes do tempo, sobre dois tipos de substratos (vidro e sil?cio) a uma temperatura de 25?C. Os nanofilmes de AlN foram caracterizados usando tr?s t?cnicas, a difra??o de raios-X, espectroscopia Raman e microscopia de for?a at?mica (AFM), analisado-se a morfologia desses. Atrav?s da an?lise dos raios-X obtemos a espessura de cada amostra com sua respectiva taxa de deposi??o. A an?lise dos raios-X tamb?m revelou que os nanofilmes n?o s?o cristalinos, evidenciando o car?ter amorfo das amostras. Os resultados obtidos atrav?s da t?cnica, microscopia de for?a at?mica (AFM) concordam com os obtidos usando a t?cnica de raios-X. A caracteriza??o por espectroscopia Raman evidenciou a exist?ncia de modos ativos caracter?sticos do AlN nas amostras analisadas

abstract: This work is focused on modeling the reliability concerns in GaN HEMT technology. The two main reliability concerns in GaN HEMTs are electromechanical coupling and current collapse. A theoretical model was developed to model the piezoelectric polarization charge dependence on the applied gate voltage. As the sheet electron density in the channel increases, the influence of electromechanical coupling reduces as the electric field in the comprising layers reduces. A Monte Carlo device simulator that implements the theoretical model was developed to model the transport in GaN HEMTs. It is observed that with the coupled formulation, the drain current degradation in the device varies from 2%-18% depending on the gate voltage. Degradation reduces with the increase in the gate voltage due to the increase in the electron gas density in the channel. The output and transfer characteristics match very well with the experimental data. An electro-thermal device simulator was developed coupling the Monte Caro-Poisson solver with the energy balance solver for acoustic and optical phonons. An output current degradation of around 2-3 % at a drain voltage of 5V due to self-heating was observed. It was also observed that the electrostatics near the gate to drain region of the device changes due to the hot spot created in the device from self heating. This produces an electric field in the direction of accelerating the electrons from the channel to surface states. This will aid to the current collapse phenomenon in the device. Thus, the electric field in the gate to drain region is very critical for reliable performance of the device. Simulations emulating the charging of the surface states were also performed and matched well with experimental data. Methods to improve the reliability performance of the device were also investigated in this work. A shield electrode biased at source potential was used to reduce the electric field in the gate to drain extension region. The hot spot position was moved away from the critical gate to drain region towards the drain as the shield electrode length and dielectric thickness were being altered. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013