Le polytype cubique du carbure de silicium (3C-SiC) est un matériau très prometteur pour les applications MEMS. En plus de sa tenue mécanique et chimique, il peut être épitaxié sur des substrats Si de faible coût. De plus, l'hétéroépitaxie multiple, c’est-à-dire quand on empile plusieurs couches Si et 3C-SiC peut ouvrir des pistes pour de nouveaux dispositifs à base de 3C-SiC. Vue la complexité de développer de telles hétérostructures, nous avons procédé à l'amélioration de la qualité de chaque couche séparément. De plus, nous avons mené une étude approfondie sur la nature des défauts dans chaque couche. Après le développement de l'hétérostructure complète, nous avons procédé à la fabrication de microstructures à base de cet empilement. Nous présentons une méthode inédite pour former des membranes de 3C-SiC auto-supportées. Cette technique simplifie considérablement le procédé de fabrication de membranes tout en réduisant le temps de fabrication et le coût. En outre, elle aide à surmonter plusieurs problèmes techniques. / Due to its outstanding physico-chemical properties, the cubic polytype of silicon carbide (3C-SiC) gained significant interest in several fields. In particular, this material emerged as a potential candidate to replace Si in MEMS devices operating in harsh environment. The development of 3C-SiC/Si/3C-SiC heterostructures on top of Si substrate can pave the road towards original and novel MEMS devices profiting from the properties of the 3C-SiC. However, such epitaxial system suffers from wide range of defects characterizing each layer. Thus, we first tried to improve the quality of each layer in this heterostructure. This was achieved relying on two levers; (i) the optimization of the growth parameters of each layer and (ii) the understanding of the nature of defects present in each layer. These two key points combined together allowed an in-depth understanding of the limit of improvement of the overall quality of this heterostructure. After the development of the complete heterostructure, the fabrication of 3C-SiC microstructures was performed. Furthermore, we presented an unprecedented method to form free-standing 3C-SiC membranes in-situ during its growth stage. This novel technique is expected to markedly simplify the fabrication process of suspended membranes by reducing the fabrication time and cost.

En premier lieu, des couches minces épitaxiales ont été obtenues sur des substrats de silicium grâce à l’utilisation de l’épitaxie par jets moléculaire et de couches tampons de titanate de strontium SrTiO3. Une technique de croissance par co-déposition a été développée de manière à obtenir un rapport Ba:Ti proche de la stoechiométrie, et ce afin d’éviter la formation de défauts cristallins dans la couche de BaTiO3. Le matériau déposé cristallise dans une structure de symétrie quadratique, ce qui est unpré-requis pour l’obtention de propriétés électro-optiques. De plus, selon les conditions de croissance, l’axe c de la maille élémentaire quadratique a pu être ajusté de manière à être aligné parallèlement ou perpendiculairement à la surface du substrat. L’utilisation d’une mince couche tampon de nucléation a également permis de croitre des films mincesBaTiO3 épitaxiées par pulvérisation, technique largement répandue en milieu industriel.Un coefficient de Pockels élevé a par la suite été obtenu sur de tellescouches épitaxiées. La valeur mesurée de 148pmV est clairement supérieure aux valeurs admises dans la littérature pour d’autres matériaux nonlinéairestels que le niobate de lithium, pour lequel un coefficient de31pmV est rapporté. La méthode de caractérisation électro-optique développée à cette occasion révèle également le caractère ferroélectrique des couches de BaTiO3, observé pour la première fois dans de tels matériaux épitaxiés sur silicium.Finalement, ces couches minces électro-optiquement actives ont été intégrées dans des dispositifs photoniques sur silicium. Dans cette optique,une structure de guide d’onde à fente a été utilisée en insérant 50nm deBaTiO3 entre deux couches de silicium. Dans ce type de structure, le confine mentoptique est 5 fois supérieur à celui obtenu pour des guides d’onde en silicium avec une gaine à base de BaTiO3. Des guides d’ondes rectilignesont tout d’abord été fabriqués, pour lesquels des pertes optiques del’ordre de 50−100 dB/cm ont été mesurées. Par la suite, des composants passifs fonctionnels ont été fabriqués, tels que des interféromètres typeMach-Zehnder, des résonateurs circulaires et des coupleurs. Finalement,la fonctionnalité de composants actifs a été démontrée pour la première fois, en se basant notamment sur des résonateurs ayant un facteur de qualité Q d’environ 5000, et pour lequel la résonance varie en fonction du champ électrique transverse. L’origine physique de cette variation n’a cependant pas pu être expliquée sur la seule base de l’effet Pockels. Cette thèse démontre que l’utilisation de nouveaux matériaux électro optiquement actifs au coeur de dispositifs photoniques sur silicium créede nouvelles opportunités pour la conception et l’ingénierie de circuitsphotoniques. L’intégration d’oxydes tels que barium titanate permet d’envisager de nouveaux concepts de dispositifs pour ajuster, moduler ou commuter la lumière au sein de circuits photoniques denses. De nouveaux défis et perspectives s’ouvrent également aux scientifiques pour modifier artificiellement les propriétés électro-optiques de ces matériaux, que ce soit par contrainte, dopage ou par l’ingénierie de multicouches. De telles avancées pourront sans aucun doute fortement améliorer les performances des dispositifs. / A novel concept of utilizing electro-optical active oxides in silicon photonic devices is developed and realized in the frame of this thesis. The integration of such oxides extends the silicon photonics platform by non-linear materials, which can be used for ultra-fast switching or low-power tuning applications. Barium titanate is used as active material as it shows one of the strongest Pockels coefficients among all oxides. Three major goals are achieved throughout this work: First, thin films of BaTiO3 are epitaxially grown on silicon substrates via molecular beam epitaxy (MBE) using thin SrTiO3 buffer layers. A shuttered co-deposition growth technique is developed in order to minimize the formation of defects in the BaTiO3 films by achieving a 1:1 stoichiometry between barium and titanium. The layers show a tetragonal symmetry and are therefore well-suited for electro-optical applications. The orientation of the long c -axis of the BaTiO3 crystal can be tuned to point perpendicular or parallel to the film surface, depending on the growth conditions. In addition, thin MBE-grown seed layers are combined with rf-sputter deposition. With this hybrid growth approach, rather thick ( > 100 nm), epitaxial BaTiO3 layers on silicon substrates are obtained with a commercially available, wide spread deposition technique. As a second goal, a strong Pockels coefficient of reff = 148 pm/V is determined in the epitaxial BaTiO3 films. This first experimental result on the electro-optical activity of BaTiO3 layers on silicon shows a clear enhancement compared to alternative non-linear materials such as lithium niobate with reff = 31 pm/V. By means of the electro-optical characterization method, also the presence of ferroelectricity in the films is demonstrated. Third, the electro-optical active BaTiO3 layers are embedded into silicon photonic devices. For this purpose, a horizontal slot-waveguide structure with a ~50 nm-thick BaTiO3 film sandwiched between two silicon layers is designed. With this design, the optical confinement in the active BaTiO3 layer is enhanced by a factor of 5 compared to Si-waveguide structures with a standard cross section and BaTiO3 as cladding. Straight BaTiO3 slot-waveguides with propagation losses of 50 − 100 dB/cm as well as functional passive devices such as Mach-Zehnder-interferometers, couplers, and ring resonators are experimentally realized. Additionally, first active ring resonators with Q-factors of Q~5000 are fabricated. The physical origin of the observed resonance shift as a function of the applied bias voltage, however, can not be conclusively clarified in the present work. The combination of high-quality, functional BaTiO3 layers with silicon photonic devices as demonstrated in this thesis offers new opportunities by extending the design palette for engineering photonic circuits with the class of electro-opticalactive materials. The integration of oxides such as BaTiO3 enables novel device concepts for tuning, switching, and modulating light in extremely dense photonic circuits. The integration also opens exciting challenges for material scientists to tailor the electro-optical properties of those oxides by strain engineering or fabrication of superlattice structures, which could ultimately lead to another boost of their electro-optical properties.

Made available in DSpace on 2014-10-09T12:35:01Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:05Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:42:13Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:34Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2015-03-26T13:35:12Z (GMT). No. of bitstreams: 1 texto completo.pdf: 4918198 bytes, checksum: d8295a3e9f68661e7fe4f267ae5aa01d (MD5) Previous issue date: 2010-04-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The presence of nuclear radiation detectors is extremily important in various industries such as medical, astronomy and of national security. There are many types of detectors. However, the detector constructed with CdTe and CdZnTe semiconductor films has become very popular due to some characteristics as convenience, density, energy resolution and for having the possibility of operating at room temperature. In this work, a review of nuclear radiation detectors is made, especially those built with semiconductor. Here are also presented structural, superficial and electric characterization methods to inform which type of sample is the most viable for such purpose. We also present in this work the results of the of CdTe films growth using HotWall Epitaxy technique (HWE) in temperatures from 150 C and 250 C over Si (111), simple glass and glass covered with tin oxide with fluorine . It is also presented the results ofcharacterization of CdTe films by x-ray diffraction and electrical characterization by curves I x V. / A presença de detectores de radiação nuclear é de extrema importância em várias indústrias como, por exemplo, a médica, a astronômica e de segurança nacional. Existem inúmeros tipos de detectores. Um deles, o detector construído com ligas semicondutoras de CdTe e CdZnTe, tem se tornado bastante popular devido às características peculiares como: praticidade, densidade, resolução energética e pela possibilidade de operarem a temperatura ambiente. Neste trabalho, faremos uma revisão de detectores de radiação nuclear, especialmente dos construídos com semicondutores. Apresentamos também métodos de caracterização estrutural, superficial e elétrica de amostras a fim de informar qual tipo de amostra é a mais viável para tal finalidade. Mostramos os resultados do crescimento de filmes espessos de CdTe, utilizando a técnica de Epitaxia de Paredes Quentes (HWE) nas temperaturas de 150 C e 250 C sobre Si (111), vidro simples e vidro coberto com óxido de estanho dopado com flúor. São também apresentados os resultados de caracterização dos filmes de CdTe por difração de raios-X e caracterização elétrica através de curvas I x V do filme.

Made available in DSpace on 2015-03-26T13:35:22Z (GMT). No. of bitstreams: 1 texto completo.pdf: 1879397 bytes, checksum: c16c14d3a2ba0d1c37de13950124ee9d (MD5) Previous issue date: 2013-03-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In the last decade, the growth of quantum dots has been studied with great interest in order to learn more about the basic physics of zero dimensional confinement and also due to their application in optoelectronic devices. The quantum dot energy levels can be controlled by changing the size, shape and material used, therefore its morphology plays an important role in determination of optical and electronic properties. In this study we used the molecular beam epitaxial technique (MBE) for producing CdMnTe quantum dots on silicon (111) substrates. To produce the samples, substrate temperature, growth time and manganese concentration were varied. The morphological and structural properties of the samples were analyzed by Atomic Force Microscopy (AFM) at the Laboratório Nacional de Nanotecnologia and by x- ray diffraction using the XRD2 beam line of the Laboratório Nacional de Luz Síncrontron (LNLS), in Campinas. The results showed that the substrate temperature influences the size dispersion of the islands, the growth time changes the size and the quality and the concentration of manganese affects mainly the quality of the quantum dots. / Na última década, tem-se estudado com grande interesse o crescimento de pontos quânticos com o intuito de conhecer mais sobre a física básica do confinamento zero dimensional e também devido a sua aplicação em dispositivos óptico-eletrônicos. Os níveis de energia podem ser controlados mudando o tamanho, a forma e o material utilizado, sendo assim a morfologia do ponto quântico desempenha um papel importante na determinação das propriedades ópticas do material estudado. Neste trabalho foi utilizada a técnica de crescimento epitaxial por feixe molecular (MBE) para a produção de pontos quânticos do semicondutor magnético diluído CdMnTe sobre substratos de silício (111). Para a produção das amostras, foram variados a temperatura do substrato, o tempo de crescimento e concentração de manganês. As propriedades morfológicas e estruturais das amostras foram analisadas através de Microscopia de Força Atômica (AFM) no Laboratório Nacional de Nanotecnologia, e através de difração de raios-x utilizando a linha XRD2 do Laboratório Nacional de Luz Síncrotron (LNLS), em Campinas. Os resultados mostraram que a temperatura do substrato influencia na dispersão de tamanhos das ilhas, o tempo de crescimento altera o tamanho e a qualidade e a concentração de manganês influencia principalmente a qualidade dos pontos quânticos.

abstract: This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is to use II-VI (MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe superlattices are proposed as a potential candidate for a subcell in the MJ solar cell designs using this material system, and therefore the material properties of the superlattices are studied. The high structural qualities of the superlattices are obtained from high resolution X-ray diffraction measurements and cross-sectional transmission electron microscopy images. The effective bandgap energies of the superlattices obtained from the photoluminescence (PL) measurements vary with the layer thicknesses, and are smaller than the bandgap energies of either the constituent material. Furthermore, The PL peak position measured at the steady state exhibits a blue shift that increases with the excess carrier concentration. These results confirm a strong type-II band edge alignment between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused by the band bending effect based on self-consistent solutions of the Schrödinger and Poisson equations. Secondly, the design of the contact grid layout is studied to maximize the power output and energy conversion efficiency for concentrator solar cells. Because the conventional minimum power loss method used for the contact design is not accurate in determining the series resistance loss, a method of using a distributed series resistance model to maximize the power output is proposed for the contact design. It is found that the junction recombination loss in addition to the series resistance loss and shadowing loss can significantly affect the contact layout. The optimal finger spacing and maximum efficiency calculated by the two methods are close, and the differences are dependent on the series resistance and saturation currents of solar cells. Lastly, the accurate measurements of external quantum efficiency (EQE) are important for the design and development of MJ solar cells. However, the electrical and optical couplings between the subcells have caused EQE measurement artifacts. In order to interpret the measurement artifacts, DC and small signal models are built for the bias condition and the scan of chopped monochromatic light in the EQE measurements. Characterization methods are developed for the device parameters used in the models. The EQE measurement artifacts are found to be caused by the shunt and luminescence coupling effects, and can be minimized using proper voltage and light biases. Novel measurement methods using a pulse voltage bias or a pulse light bias are invented to eliminate the EQE measurement artifacts. These measurement methods are nondestructive and easy to implement. The pulse voltage bias or pulse light bias is superimposed on the conventional DC voltage and light biases, in order to control the operating points of the subcells and counterbalance the effects of shunt and luminescence coupling. The methods are demonstrated for the first time to effectively eliminate the measurement artifacts. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012

abstract: Multiple quantum well (MQW) structures have been employed in a variety of solid state devices. The InGaAs/GaAs material system is of special interest for many optoelectronic applications. This study examines epitaxial growth and defect creation in InGaAs/GaAs MQWs at its initial stage. Correlations between physical properties, crystal perfection of epitaxial structures, and growth conditions under which desired properties are achieved appear as highly important for the realization and final performance of semiconductor based devices. Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature T_dep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature T_dep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2014

Neste trabalho, foi estudado a aplicação de novas heteroestruturas semicondutoras para detecção de radiação na região do infravermelho médio. Pontos quânticos de submonocamada, detectores de cascateamento quântico e pontos quânticos de InAlAs foram testados como opção para corrigir as deficiências em responsividade, corrente de escuro e temperatura de operação, comuns nas heteroestruturas convencionais baseadas em poços quânticos e pontos quânticos de InAs obtidos no regime de crescimento Stranski-Krastanov. Também foi projetado, fabricado e testado um circuito eletrônico de leitura de sinal misto para integração com matrizes de sensores e produção de imagens. Esse tipo de circuito possui uma série de vantagens em relação aos dispositivos convencionais que costumam ser completamente analógicos. / In this work, we studied the application of new types of semiconductor heterostructures for mid-infrared detection. Submonolayer quantum dots (SML-QDs), quantum-cascade detectors (QCDs) and InAlAs quantum dots were tested as an option to circumvent the common shortcomings of responsivity, dark current and operating temperature of the usual heterestructures based on quantum wells (QWs) and InAs Stranski-Krastanov quantum dots. We also designed, fabricated and tested a mixed-signal read-out circuit aiming the fabrication of focalplane arrays (FPAs) for applications to infrared imaging. This kind of architecture has several advantages over a fully analog design.

Made available in DSpace on 2014-10-09T12:35:01Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:05Z (GMT). No. of bitstreams: 0 / Este trabalho teve como objetivo o desenvolvimento de sistemas dosimétricos baseados em diodos especiais de Si, resistentes a danos de radiação, para monitoração online de processos de irradiação com elétrons de 1,5 MeV de energia e para dosimetria relativa e escaneamento de feixe de elétrons clínicos dentro de uma faixa de energia de 6 a 21 MeV. Os diodos utilizados foram produzidos pelos métodos de fusão zonal padrão (FZ), Czochralski em presença de um campo magnético (MCz) e crescimento Epitaxial (EPI). Para utilizar os diodos como detectores, eles foram fixados em uma base de alumina permitindo a ligação dos eletrodos de polarização e de extração de sinais. Após a montagem na base, cada diodo foi fixado em uma sonda acrílica preta dotada de uma janela de Mylar® aluminizado e de conector do tipo LEMO®. Com os dispositivos operando em modo fotovoltaico, a integração dos sinais de corrente em função do tempo de irradiação permitiu obter a carga produzida no volume sensível de cada diodo irradiado. O acelerador de elétrons utilizado para as irradiações de doses altas foi o DC 1500/25/4 - JOB 188 de 1,5 MeV instalado no Centro de Tecnologia das Radiações do IPEN/CNEN-SP. Foram estudados o perfil da corrente em função do tempo de exposição, a repetibilidade de resposta, a sensibilidade em função da dose absorvida e a curva resposta de cada dispositivo. Foi observada uma queda na sensibilidade mais acentuada para o diodo MCz do que para o diodo FZ e uma boa repetibilidade nos dois casos. Ainda, o aumento da carga com a dose absorvida obedeceu a uma função polinomial de segunda ordem. Na caracterização do diodo EPI, ele exibiu melhor repetibilidade que a obtida por dosímetros CTA, rotineiramente aplicados em processamento por radiação. Os resultados acima descritos indicam a potencial utilização desses diodos de Si resistentes a danos de radiação em dosimetria online para aplicações envolvendo elevadas doses. Para as irradiações de doses baixas foram utilizados os Aceleradores Lineares KD2 e Primus, ambos fabricados pela Siemens e instalados no Hospital Sírio-Libanês. A resposta dos diodos foi avaliada para energias de 6 a 21 MeV. Foram estudados: a repetibilidade de resposta, a curva dose-resposta em função da dose absorvida, a sensibilidade em carga com a energia do feixe de elétrons, a porcentagem de dose profunda (PDP) e o perfil transversal de dose. Apesar da resposta dos diodos FZ, MCz e EPI serem levemente dependentes da energia do feixe de elétrons, a resposta dosimétrica, em todo o intervalo de energia de feixe estudado, mostrou-se linear. Ainda, em relação aos diodos epitaxiais, os dispositivos estudados mostraram excelente acordo com simulações de Monte Carlo e medições realizadas com MatriXX®, demonstrando que os dispositivos podem ser usados como dosímetros em elétrons radioterápicos para escaneamento de varredura de feixe, mapeamento de distribuições de dose de feixes, monitoramento rotineiro da constância do fator calibração e dosimetria relativa. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP