abstract: In this dissertation, I described my research on the growth and characterization of various nanostructures, such as nanowires, nanobelts and nanosheets, of different semiconductors in a Chemical Vapor Deposition (CVD) system. In the first part of my research, I selected chalcogenides (such as CdS and CdSe) for a comprehensive study in growing two-segment axial nanowires and radial nanobelts/sheets using the ternary CdSxSe1-x alloys. I demonstrated simultaneous red (from CdSe-rich) and green (from CdS-rich) light emission from a single monolithic heterostructure with a maximum wavelength separation of 160 nm. I also demonstrated the first simultaneous two-color lasing from a single nanosheet heterostructure with a wavelength separation of 91 nm under sufficiently strong pumping power. In the second part, I considered several combinations of source materials with different growth methods in order to extend the spectral coverage of previously demonstrated structures towards shorter wavelengths to achieve full-color emissions. I achieved this with the growth of multisegment heterostructure nanosheets (MSHNs), using ZnS and CdSe chalcogenides, via our novel growth method. By utilizing this method, I demonstrated the first growth of ZnCdSSe MSHNs with an overall lattice mismatch of 6.6%, emitting red, green and blue light simultaneously, in a single furnace run using a simple CVD system. The key to this growth method is the dual ion exchange process which converts nanosheets rich in CdSe to nanosheets rich in ZnS, demonstrated for the first time in this work. Tri-chromatic white light emission with different correlated color temperature values was achieved under different growth conditions. We demonstrated multicolor (191 nm total wavelength separation) laser from a single monolithic semiconductor nanostructure for the first time. Due to the difficulties associated with growing semiconductor materials of differing composition on a given substrate using traditional planar epitaxial technology, our nanostructures and growth method are very promising for various device applications, including but not limited to: illumination, multicolor displays, photodetectors, spectrometers and monolithic multicolor lasers. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

Ce travail a porté sur la croissance par épitaxie par jets moléculaires de nanofils InGaN/GaNsur Si (111).Le dépôt d'InGaN en conditions riches azote sur des nanofi ls GaN pré-existants permet deconserver la structure colonnaire. La morphologie des nanofi ls s'est révélée dépendre fortementdu taux d'indium utilisé dans les fl ux. A faible taux nominal d'indium celui-ci se concentre dansle coeur du fi l, ce qui résulte en une structure coeur-coquille InGaN-GaN spontanée. Malgré letaux d'indium important dans le coeur, la relaxation des contraintes y est entièrement élastique.La luminescence est dominée par des eff ets de localisation de porteurs qui donnent lieu à unebonne tenue en température. Au contraire, à plus fort flux nominal d'indium il y a relaxationplastique des contraintes et aucune séparation de phase n'est observée.L'étude d'insertions InGaN permet de con firmer que, malgré le faible diamètre des nano fils, lacroissance est dominée par la nécessité de relaxation des contraintes, et la nucléation de l'InGaNse fait sous la forme d'un îlot facetté. Il en résulte une incorporation préférentielle de l'indiumau sommet de l'îlot, et donc un gradient radial de composition qui se développe en structurecoeur-coquille spontanée au cours de la croissance.Au contraire, la croissance en conditions riches métal entraîne une croissance latérale trèsimportante, nettement plus marquée dans le cas d'InGaN que de GaN : l'indium en excès a une ffet surfactant qui limite la croissance axiale et favorise la croissance latérale. / This work reports on the molecular-beam-epitaxial growth of InGaN/GaN nanowires on Si(111) substrates.The deposition of InGaN in nitrogen-rich conditions on preexisting GaN nanowires allows usto maintain the columnar structure. Wire morphology varies strongly with the indium concentrationin the fluxes. At low nominal In flux, it concentrates in the wire core, resulting in aspontaneous InGaN-GaN core-shell structure. In spite of the high indium content in the core,strain relaxation is purely elastic in these structures. On the other hand, using higher nominal Influxes lead to plastic relaxation and no phase separation is observed. Luminescence is dominatedby carrier localization phenomena, allowing for a low quenching of the emission up to roomtemperature.Studying InGaN insertions con firms that in spite of the small diameter of the wires, growthis dominated by strain relaxation e ffects, and InGaN nucleates as facetted islands. The incorporationof indium occurs preferentially at the top of the islands, resulting in a radial compositiongradient which leads to the spontaneous growth of a core-shell structure.Growth in metal-rich conditions results in a very strong lateral growth, far superior for InGaNthan for GaN : excess In has a surfactant e ffect limiting the axial growth rate and promotinglateral growth.

Orientador: José Roberto Cury Saad / Banca: Patrícia dos Santos Jardim / Banca: Luiz Alberto Plácido Penna / Banca: Osmir Batista de Oliveira Júnior / Banca: Marcelo Ferrarezi de Andrade / Resumo: O objetivo desse estudo foi avaliar a microdureza Vickers de um cimento resinoso químico (C&B Cement - Bisco) e um de ativação dual (RelyX ARC - 3M ESPE), fotopolimerizado por diferentes fontes de luz, em função de diferentes profundidades do canal radicular, na cimentação de pinos de fibra de vidro. Os aparelhos fotopolimerizadores empregados foram: Optilux 360 (Demetron) - luz halógena; Radii (SDI) - LED; UltraLume 5 (Ultradent) - LED. Nenhuma diferença estatisticamente significante foi observada nos valores de microdureza Vickers, dos terços cervical, médio e apical do canal radicular, para o cimento resinoso químico (G1), e os terços cervicais do cimento resinoso dual, para todos os aparelhos fotopolimerizadores (G2, G3 e G4) (p>0.05). Porém, o aparelho de luz halógena (G2) e o LED UltraLume 5 (G4) apresentaram comportamentos semelhantes, proporcionando uma diminuição gradativa nos valores médios de microdureza do cimento resinoso dual, do terço cervical para o apical (p<0.001). Os menores valores de microdureza foram encontrados nos terços apical do sistema RelyX ARC fotopolimerizado com luz halógena ou com LED UltraLume 5. / Abstract: This study evaluated the Vickers microhardness of a chemically cured resin cement (C&B Cement - Bisco) and a dual cure cement (RelyX ARC - 3M ESPE), light cured by different light sources, according to the different root canal depths, for luting of glass fiber posts. The light curing units employed were Optilux 360 (Demetron) - halogen lamp; Radii (SDI) - LED; UltraLume 5 (Ultradent) - LED. No statistically significant difference was observed in Vickers microhardness values at the cervical, middle and apical root thirds for the chemically cured resin cement (G1), and at the cervical third for the dual resin cement, for all light curing units (G2, G3 and G4) (p>0.05). However, the halogen lamp unit (G2) and LED UltraLume 5 (G4) presented similar performance, providing gradual reduction in mean microhardness values for the dual resin cement from the cervical to the apical third (p<0.001). The lowest microhardness values were found at the apical third for the system RelyX ARC light cured with halogen lamp or LED UltraLume 5. / Doutor

Polyfluorenes are polymers with outstanding properties: They are semi-conducting, relatively rigid, quite stable chemically and thermally, easily substituted and therefore potentially soluble in numerous solvents and more importantly, they exhibit blue electro- and photoluminescence. For all these reasons, these polymers are the subjects of numerous academic and industrial researches.<p>The first subject of this work deal with the design, the synthesis and the characterisation of polyfluorenes end-capped with perylene dicarboximide derivatives. These perylene moieties are able to interact by energy transfer under specific conditions of illumination, proximity and orientation. Their observation by single molecule spectroscopy permitted therefore to gain valuable information concerning the three-dimensional folding of single polyfluorene chains. To complete this study, the synthesis and characterisation of a perylene end-capped trimer of fluorene was performed. This structure being monodisperse, a finer analysis of the energy-transfer occurring between both perylene dyes could be accomplished, which confirmed the structural hypothesis made for the polymer. During these studies, it has been observed that, in addition to the energy transfer occurring between both perylene derivatives, another energy transfer occurs between the polyfluorene backbone and the perylene derivatives upon excitation of the first. This led to the idea of the synthesis of a polyfluorene bearing perylenes dicarboximide as side chains. This perylene-rich polyfluorene has been used to build a photovoltaic cell efficient in the wavelengths of both polyfluorene absorption and perylene carboximide absorption. <p>Another subject of this work was the design, synthesis and characterisation of polyfluorenes bearing bulky phenoxy groups as side-chains. These polymers, due to their lower tendency toward aggregation, exhibited a better stability of their emission colour upon annealing. Similarly, a series of homo- and copolymers of fluorene bearing bulky and hole accepting triphenylamine substituants was synthesised and characterised. In addition to their improved colour stability in comparison with dialkylpolyfluorenes, the LEDs build with these materials exhibited a very low turn on voltage. <p> / Doctorat en sciences, Spécialisation chimie / info:eu-repo/semantics/nonPublished

Ce travail de thèse porte sur l’étude de la croissance de nitrure d’éléments V sur des substrats de silicium (Si) (111) micro et nano-structuré. Son but est de simplifier l’hétéroépitaxie de GaN sur Si tout en gardant une qualité de matériau à l’état de l’art. L’originalité de ce travail repose sur la combinaison avantageuse de deux techniques de croissance : NH3-EJM et EPVOM. Nous avons d’abord évalué l’intérêt du silicium poreux (SiP) pour confiner les fissures et l’utiliser comme couche compliante. Malgré les changements structuraux s’opérant dans le SiP lorsqu’il est porté aux hautes températures nécessaires pour la croissance épitaxiale, nous avons pu démontrer la croissance de GaN de bonne qualité en ajoutant une couche de Si épitaxiée avant la croissance des nitrures d’éléments V. Puis, nous avons étudié la croissance de GaN dans des ouvertures réalisées dans un masque diélectrique déposé sur le substrat de Si. Cette approche a permis d’obtenir des motifs de GaN non fissurés et de bonne qualité cristalline en utilisant des conditions de croissance optimisées. L’analyse des contraintes sur les motifs de GaN indique une distribution en U où le maximum de la contrainte en tension est mesuré au centre des motifs ; cette contrainte se relaxe graduellement vers les bords libres des motifs. Enfin, une comparaison avec la croissance sur des mesas de Si gravé est proposée. Nous montrons que la croissance dans des ouvertures permet à la fois d’obtenir une couche de GaN uniformément épitaxiée sur les motifs ainsi que d’obtenir une plus faible courbure des substrats après croissance. Enfin, des LEDs ont pu être fabriquées à partir de GaN épitaxié sur substrat masqué. / This work deals with the growth of III-Nitrides on micro and nano-patterned silicon (111) substrates. The main goal is to simplify the heteroepitaxy of GaN on Si while keeping state of the art III-nitride materials. The originality of this work is to combine the advantages of both NH3-MBE and MOCVD growth techniques. We firstly evaluated the interest of porous silicon to confine cracks and to behave as a compliant substrate. Despite the issues regarding the structural changes of the porous silicon with the high temperatures necessary for the epitaxial growth of GaN, we demonstrated the growth of high quality GaN layers by growing a silicon layer of few tens of nanometers prior to III-nitride layers. Then, we studied the windowed growth of GaN on silicon substrates masked with dielectric films. We found that this approach can produce high quality crack free GaN (2µm thick) patterns with size up to 500x500 µm² with a dislocation density of few 108cm-2. Furthermore, crack statistics reveal that a large amount of crack free patterns can be obtained using optimized conditions. Stress analyses of GaN patterns demonstrate a “U-shape” stress distribution where the maximum tensile stress is found in the middle of the patterns and gradually decreases towards the pattern edges. Finally, a comparison with mesa patterned silicon substrates is proposed with identical grown structures. We found that windowed growth is more advantageous regarding growth uniformity and substrate bowing. As a result of this work, LEDs have been fabricated using GaN grown on masked substrates.

Os polímeros com propriedades luminescentes, e mais especificamente, os eletroluminescentes, tem atraído muita atenção devido a sua possível aplicação em dispositivos ópticos e eletrônicos do tipo LEDs (light-emitting diodes). Grande parte das pesquisas realizadas no campo da eletroluminescência se baseia em estudos sobre poli (arilenos-vinilenos), como o poli-(p-fenileno vinileno), PPV, e seus derivados. No processo de polimerização de tais polímeros, a influência de algumas variáveis são importantes em suas propriedades finais. Este trabalho descreve a exploração da influência de diversas variáveis no processo de polimerização do derivado do PPV, o poli-(2-metoxi-5-hexiloxi-p-fenileno-vinileno) MH-PPV. Além disso, com o intuito de verificar a influência da presença de aditivos na massa molar dos polímeros foram realizadas polimerizações utilizando-se os aditivos 4-metoxifenol, cloreto de tercbutila e cloreto de benzila durante o processo de polimerização. Também foram realizados estudos sobre o modo de adição e temperatura de polimerização utilizando o aditivo cloreto de benziIa. Todos os produtos foram caracterizados por HPSEC (Cromatografia de Exclusão por Tamanho) para determinação das massas molares dos polímeros. Observou-se que a temperatura e a velocidade de adição da base no meio reacional são variáveis que influenciam significativamente no processo de polimerização e, conseqüentemente, na distribuição na massa molar dos polímeros. Foi constatado que o uso de aditivos no processo de polimerização previne a formação de géis; fornece polímeros com baixa massa molar, aumentando a solubilidade dos polímeros em solventes orgânicos como clorofórmio e THF. A análise térmica dos polímeros revelou que além da Tg em tomo de 160oC, todos os polímeros apresentaram uma transição em tomo de 75oC. Os espectros de absorção e emissão não foram afetados de forma significativa dentro dos valores de massas molares médias obtidas. Foi possível também obter uma relação entre os valores de massa molar obtidos por HPSEC e valores de viscosidade relativa, que pode ser utilizada para estimar valores de Mw através de medidas de tempo de escoamento / Electroluminescent polymers have been widely investigated due to its potential applications in optoelectronic devices such as polymer light-emitting diodes (PLEDs). Most of the polymers suitable for application in PLEDs are related to the poly(arylenes-vinylene) polymers, including the poly(p-phenylene-vinylene) (PPV) and its derivatives, which are of special interest. Such materials can exhibit significant differences on their final properties upon changes on specific synthesis parameters. In this work it is described the influence of three different additives, viz. 4-methoxyphenol, terc-butyl chloride and benzyl chloride, on the polymerization process of the derivative poly(2-methoxy-5-hexyloxy-p-phenylenevinylene) MHPPV. Size Exclusion Chromatography analyses revealed that the type and rate of incorporation of the additives to the polymerization reaction largely affects the molecular weight of the polymers. In addition, it was observed that the use of additives leads to the synthesis of low molecular weight polymers, increasing their solubilities in common organic solvents, eg. chloroform and tetrahydrofuran. The glass transition temperature (Tg) of the polymers was determined by differential scanning calorimetry (DSC) to be ca. 160oC. Interestingly, all the polymers presented an additional transition point at ca. 75oC. Neither the electronic absorption nor the emission spectra were affected by the differences on the molecular weight of the polymers. Finally, it was possible to establish correlations between the molecular weight analyses (from HPSEC) and viscosity experiments, which can be useful for Mw determination

A monolithically integrated broadly tunable MQW laser that utilizes a combined impurity-free vacancy disordering (IFVD) of quantum wells and optical beam steering techniques is proposed and investigated experimentally. The device consists of a beam-steering section and an optical amplifier section fabricated on a GaAs/AlGaAs quantum well (QW) p-i-n heterostructure. The beam steering section forms a reconfigurable optical waveguide that can be moved laterally by applying separately controlled electrical currents to two parallel contact stripes. The active core of the gain section is divided in into selectively intermixed regions. The selective intermixing of the QW in the gain section results in neighboring regions with different optical bandgaps. The wavelength tuning is accomplished by steering the amplified optical beam through the selected region where it experiences a peak in the gain spectrum determined by the degree of intermixing of the QW. The laser wavelength tunes to the peak in the gain spectrum of that region. The IFVD technique relies on a silica (SiO2) capped rapid thermal annealing and it has been found that the degree of intermixing of the QW with the barrier material is dependent on the thickness of the SiO2 film. The QW sample is first encapsulated with a 400nm thick SiO2 film grown by plasma enhanced chemical vapor deposition (PECVD). In the gain section, the SiO2 film is selectively etched using multiple photolithographic and reactive ion etching steps whereas the SiO2 film is left intact in all the remaining areas including the beam-steering section. The selective area quantum well intermixing is then induced by a single rapid thermal annealing step at 975°C for a 20s duration to realize a structure with quantum well that has different bandgaps in the key regions. Optical characterizations of the intermixed regions have shown a blue shift of peak of the electroluminescence emission of 5nm, 16nm and 33nm for the uncapped, 100nm and 200nm iv respectively when compared to the as grown sample. The integrated laser exhibited a wavelength tuning range of 17nm (799nm to 816nm). Based on the same principle of QW selective intermixing, we have also designed and fabricated a monolithically integrated multi-wavelength light emitting diode (LED). The LED emits multiple wavelength optical beams from one compact easy to fabricate QW structure. Each wavelength has an independent optical power control, allowing the LED to emit one or more wavelengths at once. The material for the LED is the same AlGaAs/GaAs QW p-i-n heterostructure described above. The device is divided into selectively intermixed regions on a single QW structure using IFVD technique to create localized intermixed regions. Two different designs have been implemented to realize either an LED with multiple output beams of different wavelengths or an LED with a single output beam that has dual wavelength operation capabilities. Experimental results of the multiple output beams LED have demonstrated electrically controlled optical emission of 800nm, 789nm and 772nm. The single output LED has experimentally been shown to produce wavelength emission of 800nm and/or 772nm depending on electrical activation of the two aligned intermixed regions.

The unique properties, such as large direct bandgap, excellent thermal stability, high μH × ns, of III-nitrides make them ideal candidates for both optoelectronic and high-speed electronic devices. In the past decades, great success has been achieved in commercialization of GaN based light emitting diodes (LEDs) and laser diodes (LDs). However, due to the lack of native substrates, thin films grown on sapphire or SiC substrates have high defect densities that degrade the device performance and reliability. Conventional epitaxy lateral overgrowth (ELO) can reduce dislocation densities down to ∼10-6 cm-2 in the lateral growth area, but requires ex situ photolithography steps. Hence, an in situ method using a SiNx interlayer (nano-scale ELOG) has emerged as a promising technique. The GaN templates prepared by this method exhibit a very low dislocation density (low-10-7 cm-2) and excellent optical and electrical properties. As a cost, such high quality GaN templates containing SiN, nanonetworks are not suitable for heterojunction field effect transistor (HFET) applications due to degenerate GaN:Si layer which serves as parallel conduction channel. This dissertation discusses the growth of low dislocation density GaN templates, by using the in situ SiNx nanonetwork for conductive templates, and the AIN buffer for semi-insulating templates. On SiN x nanonetwork templates, double-barrier RTD and superlattice (SL) exhibited negative differential resistances. Moreover, the injection current of Blue LEDs (450 nm) was improved ∼30%. On semi-insulating GaN templates, nearly lattice matched AlInN/AIN/GaN HFETs were successfully demonstrated and exhibited ∼ 1600 cm2/Vs and 17 600 cm2/Vs Hall mobilities at 300 K and 10 K, respectively. Those mobility values are much higher than literature reports and indicate that high quality HFETs can be realized in lattice matched AlInN/AIN/GaN, thereby solving the strain related issue. The attempt to use InGaN as the 2DEG channel has also been successfully implemented. A Hall mobility (1230 cm2/Vs) was achieved in a 12 nm InGaN channel HFET with AlInGaN barrier, which demonstrates the viability of InGaN channel HFETs.

QC 20110831

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents the analysis and implementation of a connection between two power converters, applied to street lighting with LEDs. This connection is called optimized cascade. The main idea of this connection is the reduction of the processed energy by the second converter, increasing this way the system efficiency. Besides that, the electrolytic capacitor is eliminated, aiming the life-span increase of the converter. To do that, the capacitance is reduced, causing a low frequency ripple, which is compensated by the second converter. This way, it is possible to unify high eficiency with long life-span on this converter. Such characteristics are considered fundamentals on LED driver. To prove the idea, two prototypes were built, one based on feedback control scheme and the other on a feedback plus feedforward control scheme. Both had shown good results, however the feedback plus feedforward controller proved to be more effiective on the active compensation of the low frequency ripple. In the end, a comparison between the two most used topologies on LED driver with the optimized cascade was performed. The results had shown a possible application range, advantages e limitations of this idea. / Este trabalho apresenta a análise e implementação de uma conexão entre dois conversores de potência, com aplicação para iluminação pública com LEDs. Essa conexão foi chamada de cascata otimizada. A principal ideia dessa conexãoo é a minimização da energia processada pelo segundo conversor, aumentando assim a eficiência do sistema. Além disso, o capacitor eletrolítico é eliminado desta topologia com o intuito de aumentar a vida útil do sistema. Para tal, a capacitância é reduzida, sendo a ondulação de baixa frequência, proveniente de tal redução, compensada ativamente pelo segundo conversor. Com isso, é possível unir alta eficiência e durabilidade, características consideradas fundamentais em um driver para LEDs. Para provar a ideia, dois protótipos foram desenvolvidos, um baseado em controle feedback e o outro baseado na união de um controle feedback com feedforward. Ambos mostraram bons resultados, o entanto o controle feedback mais o feedforward mostrou-se mais eficaz na compensação ativa da ondulação de barramento. Por fim, uma comparação entre as duas topologias mais utilizadas em drivers para LED e a cascata otimizada foi realizada. Os resultados obtidos mostraram sua possível faixa de aplicação, bem como vantagens e limitações.