Neste trabalho, utilizamos o desenvolvimento recente do método DFT/LDA-1/2 para cálculos de estados excitados em materiais. Começamos com um resumo da teoria do funcional da densidade (DFT) e incluímos uma introdução ao método LDA-1/2 para cálculos de excitações em sólidos. Na compilação dos resultados esperamos ter demonstrado a utilidade do LDA-1/2 para cálculos de alinhamentos de bandas em junções semicondutor/semicondutor e semicondutor/isolante. A aplicação do método envolve o conhecimento da química básica dos sistemas. Para tanto, escolhemos sistemas importantes para diversas aplicações, e cujos modelos de simulação estão o limite ou fora do alcance de metodologias que envolvem alto custo computacional, mas que foram bem caracterizados experimentalmente. Concentramos nossas ações no estudo da capacidade preditiva do LDA-1/2 para alinhamentos de bandas, os chamados band offsets, particularmente importantes para a micro e optoeletrônica. Quando não foi possível compararmos nossos resultados com o experimento, procuramos a comparação com métodos estado-da-arte como GW. Bons resultados foram obtidos para band gaps e band offsets de interfaces A1As/GaAs, Si/SiO2, A1N/GaN e CdSe/CdTe, que representam os diferentes tipos de jun_c~oes poss__veis, com (e.g. A1As/GaAs, A1N/GaN) e sem (e.g. Si/SiO2, CdSe/CdTe) ^anions omuns, com (e.g. A1As/GaAs) e sem (e.g. CdSe/CdTe, Si/SiO2) casamento de parâmetros de rede e diferentes tipos de alinhamentos (\"straddling\", e.g. A1As/GaAs ou \"staggered\"e.g. CdSe/CdTe). Analisamos de maneira sistemática o comportamento do entorno do bandgap ao longo da interface, verificando plano a plano atômico o comportamento das bordas de valência e condução com LDA-1/2 em comparação com o LDA, ou comparando diferentes modelos dentro do LDA-1/2, como o caso do CdSe/CdTe e do Si/SiO2. Para o caso A1As/GaAs, aproveitamos o casamento de parâmetros de rede dos semicondutores constituintes e tentamos um modelo de interface de ligas A1xGa1-x As/GaAs para estudar a variação de valência, condução e bandgap em função da composição x. No AlN/GaN, estudamos também os offsets com as contribuições dos orbitais separadamente. Em todos os casos o LDA-1/2 levou-nos a resultados interessantes com modelos simples. A exploração de novas fronteiras de aplicação do método fez-se necessária com a diminuição da dimensionalidade dos sistemas, de 3D (bulk ) para 2D (interfaces) e depois para 1D, ou seja, _os quânticos (\"nanofios\"). Nosso material de estudo para os foi o ZnO que, além da motivação oriunda de conhecidas aplicações em optoeletrônica, apresenta desafios para simulações bulk com qualquer método, e que foi abordado com certo sucesso usando o LDA-1/2 anteriormente, sendo que para fios quânticos encontramos resultados interessantes em geometrias triangulares que facilitaram os modelos. Calculamos o bandgap ZnO bulk e de nanofios passivados e não passivados com hidrogênios usando LDA e LDA-1/2 sem polarização de spin. As estruturas de bandas e o bandgap como função do diâmetro do ano_o foram calculados e ajustes com funções de decaimento foram feitos para comparação, por extrapolação, dos bandgaps com valores experimentais. Foi possível comparar nossos resultados de fios com o bulk, e predizer uma faixa de variaação de bandgaps que os experimentais podem encontrar para nanofios triangulares de ZnO. Também foi feita análise de energias de confinamento em fios quânticos de ZnO, comparando o LDA com LDA-1/2. Finalmente, mostramos os resultados de uma oportunidade de aplicação do método a um material com defeitos, recentemente descoberto e promissor, e com enorme mercado potencial em fotocatálise, o Ti1-O4N. Nosso trabalho envolveu a aplicação do LDA-1/2 a um problema muito desafiador, e.g. a geração de energia limpa, especificamente a separação da molécula de água para produção de hidrogênio. O desafio maior vem da dificuldade de predição de bandgaps teoricamente, em particular para sistemas grandes como é o caso de modelos atomísticos com defeitos, devido aos altos custos computacionais envolvidos. Tais dificuldades forçam os pesquisadores a usarem parâmetros ajustáveis ou métodos semi-empíricos, ou modelos simplificados demais para descrever precisamente resultados experimentais. Isto dificulta o estudo dos sistemas fotocatalíticos potencialmente eficientes e que não foram ainda caracterizados ou otimizados. O LDA-1/2 é aqui validado para esta classe de materiais, abrindo assim a oportunidade para estudar sistemas mais realísticos e complexos para cálculos ainda mais precisos, particularmente para geração de energia limpa. Em particular, modelamos o TiO2 na estrutura rutile com nitrogênio substitucional, cuja estrutura eletrônica é ainda debatida. Foi a primeira aplicação do LDA-1/2 a sistemas com algum tipo de defeito, com ótimos resultados para o novo sistema Ti1- _O4N com vacâncias de Ti. / In this work, we used the recent development of DFT/LDA-1/2 method for calculations of excited states in materials. We begin with a summary of the density functional theory (DFT) and included an introduction to the method LDA-1/2 for calculations of excitations in solids. In compiling the results we hope to have demonstrated the usefulness of the LDA-1/2 for calculating alignments of bands at junctions semiconductor / semiconductor and semiconductor / insulator. The method involves the knowledge of basic chemical systems. To do this we chose systems important for several applications, and simulation models which are the limit or beyond the reach of methodologies involving high computational cost, but have been well characterized experimentally. We focus our actions in the study of the predictive capability of the LDA-1/2 for alignments of bands, the band called offsets, particularly important for micro and optoelectronics. When it was not possible to compare our results with experiment, we compared the methods with state of the art as GW. Good results were obtained for band gaps and band offsets of interfaces A1As/GaAs, Si/SiO2, A1N/GaN and CdSe / CdTe, which represent the different types of jun_c poss__veis-tions, with (eg A1As/GaAs, A1N/GaN) and without (eg Si/SiO2, CdSe / CdTe) ^ omuns anions with (eg A1As/GaAs) and without (eg CdSe / CdTe, Si/SiO2) matching network parameters and different types of alignments (\"straddling\" eg A1As/GaAs or \"staggered\" eg CdSe / CdTe). Systematically analyze the behavior of the environment along the interface bandgap, plane by plane scanning behavior of the edges atomic valence and conduction with LDA-half in comparison with LDA, or comparing templates within the LDA-1 / 2, as the case of CdSe / CdTe and Si/SiO2. For the case A1As/GaAs, we take the marriage of network parameters of semiconductor components and try an interface model alloys A1xGa1-x As / GaAs to study the variation of valence, conduction and bandgap as a function of composition x. In the AlN / GaN, we also studied the offsets with the contributions of the orbitals separately. In all cases the LDA-half led us to interesting results from simple models. The exploration of new frontiers of the method was necessary to decrease the dimensionality of the systems, the 3D (bulk) for 2D (interfaces) and then to 1D, ie, quantum _os (\"nanowires\"). Our study material for the ZnO was that, apart from the motivation coming from known applications in optoelectronics, presents challenges for bulk simulations with any method, and that was addressed with some success using the LDA-half earlier, and for wireless find interesting results in quantum triangular geometries that facilitated models. We calculate the bandgap and bulk ZnO nanowires passivated and not passivated with hydrogen using LDA and LDA-1/2 without spin polarization. The bandgap structures and strips as a function of the diameter of ano_o adjustments are calculated and decay functions for comparison were made by extrapolation of the bandgaps with experimental values. It was possible to compare our results with the bulk of wires, and predict a range of bandgaps that variaação can find experimental triangular ZnO nanowires. It was also made analysis of energy confinement in ZnO quantum wires, comparing LDA with LDA-1/2. Finally, we show the results of an opportunity to apply the method to a material with defects, newly discovered and promising, and with huge market potential in photocatalysis, the Ti1-O4N. Our work involved the application of LDA-1/2 to a very challenging problem, eg the generation of clean energy, specifically the separation of the water molecule for hydrogen production. The main challenge has been the difficulty of predicting bandgaps theoretically, in particular for large systems such as the model atomistic defects because of the high computational costs involved. These difficulties force the researchers to use adjustable parameters or semi-empirical methods, or other simplified models to accurately describe experimental results. This complicates the study of potentially efficient photocatalytic systems which have not yet been characterized or optimized. The LDA-1/2 is here validated for this class of materials, thus opening the opportunity to study more realistic and complex systems for more accurate calculations, particularly for clean energy generation. In particular, we modeled the structure of TiO2 in the rutile with substitutional nitrogen, whose electronic structure is still debated. It was the first application of the LDA-1/2 systems with some kind of defect, with excellent results for the new system Ti1-_O4N with Ti vacancies.

Orientador: Hugo E. Hernandez Figueroa / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-14T23:26:41Z (GMT). No. of bitstreams: 1 Sartori_EduardoJose_D.pdf: 11903812 bytes, checksum: 6e06f001155d33b841c61ae93464c897 (MD5) Previous issue date: 2009 / Resumo: Metamateriais são estruturas ou arranjos geométricos feitos a partir de materiais comuns, dielétricos, condutores, magnéticos ou por combinação destes. Os metamateriais caracterizam-se principalmente por apresentarem propriedades especiais de permissividade ( e) e permeabilidade ( µ) não encontradas nos materiais em estado natural, cujo principal efeito é o índice negativo de refração (n < 0). Essas características permitem seu emprego em diversos tipos de aplicações em eletromagnetismo e óptica, tais como filtros passa-faixa e rejeita-faixa, espelhos dielétricos, super lentes etc. Normalmente, o equacionamento envolvido no cálculo de parâmetros dos metamateriais são complexos e, na maioria das vezes, necessitam de apoio computacional. Por este motivo, o presente trabalho traz um estudo experimental sobre dois tipos de comportamento metamaterial, o de permissividade quase-zero e negativa, analisando seu desempenho sob vários aspectos geométricos e de características dos materiais envolvidos, além de propor uma metodologia de desenvolvimento, a qual possibilita um rápido dimensionamento de diversos tipos de grades metamateriais, baseada em cálculos simples ou consulta direta a tabelas e curvas de projeto. / Abstract: Metamaterials are structures or geometric arrangements made from common materials, dielectrics, conductors, magnetic or a combination of these. Metamaterials are characterized mainly because of their special characteristics of permittivity ( e) and permeability ( µ), not found in the materials at natural state, whose main effect is the negative index of refraction (n <0). These characteristics allow its use in several types of applications in electromagnetism and optics, such as band-pass and band-stop filters, dielectric mirrors, super lenses etc.. Typically, the equations involved in the calculation of parameters of metamaterials are complex and, in most cases, require high capability computational methods. For this reason, this work presents an experimental study on two types of metamaterial behavior, near-zero and negative permittivity, examining its performance in several geometric aspects and characteristics of the materials involved, and propose a development methodology, which allows a fast scaling of various types of metamaterials grids, based on simple calculations or direct consultation tables and curves design. / Doutorado / Telecomunicações e Telemática / Doutor em Engenharia Elétrica

In this dissertation, high-power ytterbium-doped fiber amplifiers designed with advanced waveguide concepts are characterized and power scaled. Fiber waveguides utilizing cladding microstructures to achieve wave guidance via the photonic bandgap (PBG) effect and a combination of PBG and modified total internal reflection (MTIR) have been proposed as viable single-mode waveguides. Such novel structures allow larger core diameters (>35 μm diameters) than conventional step-index fibers while still maintaining near-diffraction limited beam quality. These microstructured fibers are demonstrated as robust single-mode waveguides at low powers and are power scaled to realize the thermal power limits of the structure. Here above a certain power threshold, these coiled few-mode fibers have been shown to be limited by modal instability (MI); where energy is dynamically transferred between the fundamental mode and higher-order modes. Nonlinear effects such as stimulated Brillouin scattering (SBS) are also studied in these fiber waveguides as part of this dissertation. Suppressing SBS is critical towards achieving narrow optical bandwidths (linewidths) necessary for efficient fiber amplifier beam combining. Towards that end, new effects that favorably reduce acoustic wave dispersion to increase the SBS threshold are discovered and reported. The first advanced waveguide examined is a Yb-doped 50/400 µm diameter core/clad PBGF. The PBGF is power scaled with a single-frequency 1064 nm seed to an MI-limited 410 W with 79% optical-to-optical efficiency and near-diffraction limited beam quality (M-Squared < 1.25) before MI onset. To this author's knowledge, this represents 2.4x improvement in power output from a PBGF amplifier without consideration for linewidth and a 16x improvement in single-frequency power output from a PBGF amplifier. During power scaling of the PBGF, a remarkably low Brillouin response was elicited from the fiber even when the ultra large diameter 50 µm core is accounted for in the SBS threshold equation. Subsequent interrogation of the Brillouin response in a pump probe Brillouin gain spectrum diagnostic estimated a Brillouin gain coefficient, gB, of 0.62E-11 m/W; which is 4x reduced from standard silica-based fiber. A finite element numerical model that solves the inhomogenous Helmholtz equation that governs the acoustic and optical coupling in SBS is utilized to verify experimental results with an estimated gB = 0.68E-11 m/W. Consequently, a novel SBS-suppression mechanism based on inclusion of sub-optical wavelength acoustic features in the core is proposed. The second advanced waveguide analyzed is a 35/350 µm diameter core/clad fiber that achieved wave guidance via both PBG and MTIR, and is referred to as a hybrid fiber. The waveguide benefits mutually from the amenable properties of PBG and MTIR wave guidance because robust single-mode propagation with minimal confinement loss is assured due to MTIR effects, and the waveguide spectrally filters unwanted wavelengths via the PBG effect. The waveguide employs annular Yb-doped gain tailoring to reduce thermal effects and mitigate MI. Moreover, it is designed to suppress Raman processes for a 1064 nm signal by attenuating wavelengths > 1110 nm via the PBG effect. When seeded with a 1064 nm signal deterministically broadened to ~1 GHz, the hybrid fiber was power scaled to a MI-limited 820 W with 78% optical-to-optical efficiency and near diffraction limited beam quality of M_Squared ~1.2 before MI onset. This represents a 14x improvement in power output from a hybrid fiber, and demonstrates that this type of fiber amplifier is a quality candidate for further power scaling for beam combining.

Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30 mu m) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 mu m annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth similar to 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92x10(-11) m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers

The outstanding electrical and material properties of Graphene have made it a promising material for several fields of analog applications, though its zero bandgap precludes its application in digital and logic devices. With its remarkably high electron mobility at room temperature, Graphene also has strong potential for terahertz (THz) plasmonic devices. However there still are challenges to be solved to realize Graphene’s full potential for practical applications. In this dissertation, we investigate solutions for some of these challenges. First, to reduce the access resistances which significantly reduces the radio frequency (RF) performance of Graphene field effect transistors (GFETs), a novel device structure consisting of two additional contacts at the access region has been successfully modeled, designed, microfabicated/integrated, and characterized. The additional contacts of the proposed device are capacitively coupled to the device channel and independently biased, that induce more carriers and effectively reduce access resistance. In addition to that, in this dissertation, bandgap has been experimentally introduced to semi-metallic Graphene, by decorating with randomly distributed gold nano-particles and zinc oxide (ZnO) nano-seeds, where their interaction breaks its sublattice symmetry and opens up bandgap. The engineered bandgap was extracted from its temperature dependent conductivity characteristics and compared with reported theoretical estimation. The proposed method of device engineering combined with material bandgap engineering, on a single device, introduces a gateway towards high speed Graphene logic devices. Finally, THz plasmon generation and propagation in Graphene grating gate field effect transistors and Graphene plasmonic ring resonators have been investigated analytically and numerically to explore their potential use for compact, solid state tunable THz detectors.

abstract: Wurtzite (In, Ga, Al) N semiconductors, especially InGaN material systems, demonstrate immense promises for the high efficiency thin film photovoltaic (PV) applications for future generation. Their unique and intriguing merits include continuously tunable wide band gap from 0.70 eV to 3.4 eV, strong absorption coefficient on the order of ∼105 cm−1, superior radiation resistance under harsh environment, and high saturation velocities and high mobility. Calculation from the detailed balance model also revealed that in multi-junction (MJ) solar cell device, materials with band gaps higher than 2.4 eV are required to achieve PV efficiencies greater than 50%, which is practically and easily feasible for InGaN materials. Other state-of-art modeling on InGaN solar cells also demonstrate great potential for applications of III-nitride solar cells in four-junction solar cell devices as well as in the integration with a non-III-nitride junction in multi-junction devices. This dissertation first theoretically analyzed loss mechanisms and studied the theoretical limit of PV performance of InGaN solar cells with a semi-analytical model. Then three device design strategies are proposed to study and improve PV performance: band polarization engineering, structural design and band engineering. Moreover, three physical mechanisms related to high temperature performance of InGaN solar cells have been thoroughly investigated: thermal reliability issue, enhanced external quantum efficiency (EQE) and conversion efficiency with rising temperatures and carrier dynamics and localization effects inside nonpolar m-plane InGaN quantum wells (QWs) at high temperatures. In the end several future work will also be proposed. Although still in its infancy, past and projected future progress of device design will ultimately achieve this very goal that III-nitride based solar cells will be indispensable for today and future’s society, technologies and society. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

No description available.

Technologies based on alternative and sustainable energy sources present a vital solution in the present and for the future. These technologies are strongly driven by the increased global energy demand and need to reduce environmental issues created by fossil fuel. Solar energy is an abundant, clean and free-access resource, but it requires harvesting and storage for a sustainable future. Direct conversion and storage of solar energy using heterogeneous photocatalysts have been identified as parts of a promising paradigm for generating green fuels from sunlight and water. This thesis focused on developing semiconductor absorbers in a visible light region for photocatalytic hydrogen production reaction. In addition, theoretical studies are combined with experimental results for a deep understanding of the intrinsic optoelectronic properties of the obtained materials. The study presents a novel family of oxysulfide BiAgOS, produced by applying a full substitution strategy of Cu by Ag in BiCuOS. I was interested to address how the total substitution of Cu by Ag in a BiCuOS system affects its crystal structure, optical and electronic properties using experimental characterizations and theoretical calculations. Single-phase bismuth silver oxysulfide BiAgOS was prepared via a hydrothermal method. Rietveld refinement of the powder confirmed that BiAgOS is an isostructural BiCuOS. The diffraction peak positions of BiAgOS, relative to those of BiCuOS, were shifted toward lower angles, indicating an increase in the cell parameters. BiCuOS and BiAgOS were found to have indirect bandgaps of 1.1 and 1.5 eV, respectively. The difference in the bandgap results from the difference in the valence band compositions. The hybrid level of the S and Ag orbitals in BiAgOS is located at a more positive potential than that of S and Cu, leading to a widened bandgap. Both materials possess high dielectric constants and low electron and hole effective masses, making them interesting for photoconversion applications. BiAgOS has a potential for photocatalytic hydrogen evolution reaction in the presence of sacrificial reagents; however, it is inactive toward water oxidation. BiCuOS and BiAgOS can be considered interesting starting compositions for the development of new semiconductors for PV or Z-scheme photocatalytic applications. The second study investigates the synthesis and characterization of NaBiS2, this contains Bi3+, which belongs to the p-block electronic configuration Bi3+ 6s26p0, and NaLaS2, which contains La3+ with electronic configuration 6s05d0. Solid-state reactions from oxide precursor starting materials were applied for synthesis the materials. The sulfurization process was conducted by pressurizing a saturated vapor of CS2. The obtained black material of NaBiS2 has an indirect transition with high absorption coefficients in the visible region of the spectrum and the absorption edge is determined at 1.21 eV. However, NaBiS2 did not show photocatalytic activity toward hydrogen production. NaLaS2 is characterized by an indirect transition with a bandgap in the UV region at 3.15 eV and can drive the photocatalytic hydrogen evolution reaction in Na2S/Na2SO3 solution. Utilizing the solid solution NaLa1-xBixS2 strategy, the absorption properties and band edge position for photocatalytic hydrogen evolution reaction were optimized. The results indicated that the bismuth content is critical parameter for maintaining the photocatalytic activity. The incorporation of low Bi content up to 6% in NaLaS2 leads to extending the photon absorption from the UV to the visible region and enhancing the photocatalytic activity of hydrogen production. In contrast, all the solid solutions that have Bi content of more than 12% present absorption edges close to that of pure NaBiS2, and they are inactive for photocatalytic hydrogen production. Combining the experimental measurements with density functional theory calculations, such behavior can be explained by the degree of overlapping of Bi and La states on the conduction band minimum (CBM). Finally, self-assembly of Bi2S3 nanorods were grown on FG or FTO substrates. Bi2S3 thin films were prepared by sulfurization of Bi metal layer using the hydrothermal method. The results show that Bi2S3 has absorption up to 1.3 eV and has a moderate absorption coefficient in the visible region. The ultraviolet photoelectron spectroscopy and photoelectron spectroscopy in air results showed that the conduction band minimum of Bi2S3 is located slightly above the hydrogen redox potential. However, Pt/Bi2S3 did not evolve a detectable amount of hydrogen, suggesting the presence of surface states that can hinder the hydrogen reduction reaction.

In this thesis the principle of voltage reference especially bangap reference is described. Below are described two circuits of this type designed in ACMOS process. There is handmade evaluation of error analysis to identify main error contributors and also monte-carlo simulation. Also statistical analysis is made on the circuit. Results of all methods are compared. Error of reference voltage is compared for both circuits. Circuit with bigger error is optimized to achieve a better precision. Obtained results showed a good agreement of all methods, which evidences importance of hand error evaluation.

This diploma thesis deals with precise design of Brokaw BandGap voltage reference comparing with MOS references. There is STEP BY STEP separation and analysis of proposed devices, using Monte Carlo analysis. There are also presented the methods for achieving a lower deviation of the output voltage for yielding device, which needs no trimming.