Entender os processos em superfície/interface de filmes e seus efeitos sobre as propriedades ópticas e elétricas de materiais orgânicos é de grande importância tecnológica. Esta pesquisa descreve a fabricação e caracterização de filmes poliméricos extremamente finos (espessura <10 nm) e homogêneos compostos por camadas de polímero/polieletrólitos e estruturas com modulação de energia ou poços quânticos. O objetivo principal foi o estudo dos processos de transferência de carga e energia em tais estruturas. Os polímeros luminescentes utilizados foram poli(9,9 dioctilfluoreno) (PFO) poli(p-fenileno vinileno (PPV). O PPV foi obtido a partir do precursor poli(cloreto de tetraidrotiofeno de xililideno) (PTHT). A técnica de deposição denominada deposição camada por camada assistida por spin (SA-LbL) foi utilizada para obtenção dos filmes. Medidas de absorbância confirmaram o crescimento linear das camadas para as interfaces polieletrólito/polieletrólito e polímero/polieletrólito. Com o objetivo de entender a transferência do elétron &pi; do polímero conjugado para o polieletrólito, as configurações das estruturas poliméricas foram alteradas através da deposição de diferentes monocamadas de polieletrólito sobre o filme polimérico. Observamos que os elétrons &pi; foram efetivamente transferidos para os polieletrólitos que possuem alta afinidades eletrônica. Este efeito interfere fortemente na absorção bem como nas características de condução do filme polimérico ultrafino. A absorção é restabelecida após a conversão de PTHT em PPV. Medidas de fotoluminescência (PL) em filmes PFO/PPV resultam em curvas de emissão com picos característicos de ambos os polímeros, o que confirma que a técnica SA-LbL permite a deposição de estruturas poliméricas multicamadas. As várias configurações de filmes obtidas elucidaram os processos de transferência que ocorrem em diferentes interfaces, tais como: mudança da sequencia de deposição do polieletrólito, número de camadas duplas PTHT/DBS e a introdução de camadas separadoras. Além disso, um único poço quântico, ou seja, estruturas formadas por uma camada PPV cercada por barreiras de PFO com 10 nm de espessura foram obtidas. Medidas de absorbâcia, PL e excitação mostraram uma eficiente migração estado excitado da barreira de PFO para o PPV (poço). A homogeneidade da imagem confocal, demonstrou um rigoroso controle da camada de cobertura ao nível de um única monocamada e sem contaminação pelos materiais depositados sequencialmente. A microscopia confocal de fluorescência (CFM) e espectroscopia de fluorescência resolvida no tempo (FLIM) foram utilizadas para caracterizar a dinâmica do exciton e o seu confinamento nos poços quânticos. As medidas de CFM demonstraram que excitons que são gerados na barreira de PFO são eficientemente transferidos para o PPV. Além disso, o tempo de decaimento da emissão PFO residual é fortemente reduzido devido a processos de migração concorrentes no poço. O tempo de decaimento de PPV diminui substancialmente para poços com espessuras abaixo de 5 nm como resultado da auto-aniquilação do exciton. Dessa forma, as estruturas de MQW obtidos pela técnica de SA-LbL podem ser usadas para estudar a transferência de energia, efeitos túneis e para a construção de novos dispositivos optoelectrónicos com maior eficiência. / The understanding of surface/interface processes and their effects on optical/electrical properties of organic materials is of strong technological importance. This research describes the fabrication and characterization of extremely thin (thickness <10 nm) and homogeneous multilayered polymeric structures including polymer/poly-electrolyte layers and structures with energy modulation such as quantum well. Our main purpose was the study of charge and energy transfer processes in such energy modulated structures. The luminescent polymers used were Poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and poly(p-phenylenevinylene) (PPV). PPV has been obtained from the poly(xylyliden tetrahydrothiophenium chloride) (PTHT) precursor. The so-called Spin Self-Assembly Layer-by-Layer deposition method (SA-LbL) was utilized to obtain the films. Absorption measurements confirmed the linear growth of layers using for polyelectrolyte/polyelectrolyte and polymer/polyelectrolyte interfaces. In order to understand the &pi;-electron transfer from the conjugated polymer to charged states of the polyelectrolyte, the configurations of the polymeric structures were modified by depositing different polyelectrolyte monolayer on the polymer film. We observed that &pi;-electrons were effectively transferred to polyelectrolytes that have high electron affinities. This effect strongly affects both absorption and conduction features of such very thin polymeric film. The absorption is restored after the conversion of PTHT in PPV. Photoluminescence measurements on PFO/PPV films result in emission curves with characteristic peaks of both polymers, confirming that SA-LbL technique allows deposition of multilayer polymeric structures. The various film configurations elucidates the transfer processes occurring at different interfaces like: change of polyelectrolyte deposition order, number of PTHT/DBS bilayers and introduction of spacers. In addition, Single Quantum Well (SQW), i.e., structures consisted of PPV layer surrounded of 10 nm thick PFO barriers were obtained. Optical absorption, PL and excitation spectroscopy showed an efficient excited state migration from the PFO barrier to the PPV well. The confocal image homogeneity demonstrated the layer coverage control at a monolayer level and without layer intermixing of the sequentially deposited polymeric materials. High resolution Confocal Fluorescence Microscopy (CFM) and Fluorescence Life spectroscopy and Imaging (FLIM) were used to characterize the exciton dynamics and confinement in quantum well. The CFM measurements demonstrated that excitons generated at the PFO barrier are efficiently transferred to the PPV well. Furthermore, the decay time of the residual PFO emission is strongly reduced due to the competing migration process in the well. The decay time of PPV decreases substantially for well thicknesses below 5 nm as a result of exciton self-annihilation. Thus, the MQW structures obtained by SA-LbL technique can be used to study energy transfer, tunneling effects and to build up new optoelectronic devices with greater efficiency.

This PhD thesis is devoted to study electro-optic properties of Gemanium/Silicon-Germanium (Ge/SiGe) multiple quantum wells (MQWs) for light modulation, detection, and emission on Si platform. It reports the first development of high speed, low energy Ge/SiGe electro-absorption modulator in a waveguide configuration based on the quantum-confined Stark effect (QCSE), demonstrates the first Ge/SiGe photodiode with high speed performance compatible with 40 Gb/s data transmission, and realizes the first Ge/SiGe light emitting diode based on Ge direct gap transition at room temperature. Extensive DC and RF measurements were performed on each tested prototype, which was realized using the same epitaxial growth and fabrication process. Simple theoretical models were employed to describe experimental properties of the Ge/SiGe MQWs. The studies show that Ge/SiGe MQWs could potentially be employed as a new photonics platform for the development of a high speed optical link fully compatible with silicon technology.

In my thesis I will discuss some aspects of collective dynamics of excitons and exciton-polaritons in nanoscale heterostructures. In the first Chapter I will make a brief introduction to the modern semiconductor physics and willdescribe the general elements and notions which will be used further. Other four chapters would be devoted to four works in which I participated, notably, in Chapter 2 I will speak about the coherent interactions between phonons and exciton orexciton-polariton condensates, in Chapter 3 I will discuss the quantum dots lasing and its amplification by an acoustic pulse. Chapter 4 and 5 will be devoted respectively to the polariton multistability and to the condensates of indirect excitons.

Στην εργασία αυτή, μελετάμε, τόσο με αναλυτικό όσο και με υπολογιστικό τρόπο, γραμμικά και μη γραμμικά οπτικά φαινόμενα σε συστήματα ημιαγώγιμων κβαντικών πηγαδιών GaAs/AlGaAs δύο ενεργειακών υποζωνών, όπου επάγονται διαϋποζωνικές μεταβάσεις, υπό την επίδραση ηλεκτρομαγνητικών πεδίων. Στο πρώτο κεφάλαιο, γίνεται μια θεωρητική περιγραφή των ημιαγώγιμων ετεροεπαφών. Ακολουθούν βασικά στοιχεία της στατιστικής και κβαντικής μηχανικής. Στο δεύτερο κεφάλαιο, εξάγονται οι γενικευμένες εξισώσεις Bloch για τις διαϋποζωνικές μεταβάσεις σε ημιαγώγιμα κβαντικά πηγάδια, στις οποίες ενυπάρχουν όροι που υπεισάγουν τις μη αμελητέες, λόγω εμπλουτισμού, αλληλεπιδράσεις μεταξύ των ηλεκτρονίων. Οι εξισώσεις αυτές αποτελούν τη βάση της μελέτης που ακολουθεί. Στα δύο επόμενα κεφάλαια, μελετούμε την αλληλεπίδραση μιας δομής διπλών συζευγμένων ημιαγώγιμων κβαντικών πηγαδιών με ένα ηλεκτρομαγνητικό πεδίο μεταβλητής γωνιακής συχνότητας, καταλήγουμε σε αναλυτικές εκφράσεις για τη οπτική επιδεκτικότητα πρώτης, τρίτης και πέμπτης τάξεως και αναλύουμε τα φάσματα διαφόρων οπτικών φαινομένων, ως προς τη γωνιακή συχνότητα του εξωτερικού πεδίου, για διάφορες τιμές της επιφανειακής ηλεκτρονιακής πυκνότητας της κβαντικής δομής. Επιπλέον, προσδιορίζουμε τις περιοχές όπου λαμβάνουν τιμή οι διάφορες παράμετροι, ούτος ώστε στο σύστημά μας να αναδυθεί η οπτική διστάθεια. Στα τρία τελευταία κεφάλαια, θεωρούμε ότι η ημιαγώγιμη κβαντική δομή αλληλεπιδρά ταυτόχρονα με ένα ισχυρό ηλεκτρομαγνητικό πεδίο (πεδίο άντλησης) καθορισμένης γωνιακής συχνότητας και ένα ασθενές (πεδίο ανίχνευσης) μεταβλητής συχνότητας και μελετούμε τα φάσματα γραμμικών και μη γραμμικών φαινομένων του πεδίου ανίχνευσης (μίξη τεσσάρων κυμάτων, απορρόφηση, διασπορά, μη γραμμικό οπτικό φαινόμενο Kerr), σε στάσιμη κατάσταση, καθώς και τη χρονική εξέλιξη αυτών. Περιγράφουμε τα φαινόμενα τόσο με αναλυτικές εκφράσεις που εξάγουμε, όσο και με την αριθμητική επίλυση των μη-γραμμικών διαφορικών εξισώσεων του πίνακα πυκνότητας που διέπουν τη δυναμική. Στη μελέτη των φαινομένων αυτών, εξετάζεται η επίδραση της επιφανειακής ηλεκτρονιακής πυκνότητας της κβαντικής δομής στις οπτικές ιδιότητες των κβαντικών πηγαδιών. / In this PhD thesis, we study analytically and numerically linear and nonlinear optical phenomena in intersubband transitions of a symmetric GaAs/AlGaAs double quantum well structure, with two energy subbands. In the first chapter, a theoretical description of the semiconductor heterostructures is presented. This is accompanied with a brief analysis of the basic elements of statistical and quantum mechanics follows, as far as this kind of structures is concerned. In the second chapter, we derive the generalised optical Bloch equations in intersubband transitions of semiconductor quantum well structures, which constitute the basis of the analysis that follows. These equations contain terms which owe their presence to the electron-electron interactions, because the quantum structure is doped with electron carriers. In the two following chapters, we consider the interaction of intersubband transitions of a double quantum well structure with an electromagnetic field of varying frequency, we derive analytical expressions for the first, third and fifth order optical susceptibility and, at last, we analyze the corresponding spectra, with respect to the frequency of the external field, for different values of electron sheet density of the structure. Furthermore, we identify the areas of values of the parameters used, in which the phenomenon of optical bistability arises. In the last three chapters, we consider the two quantum well subbands to be coupled to a strong pump electromagnetic field with fixed frequency and a weak probe electromagnetic field of varying frequency and study the spectra of various linear and nonlinear optical phenomena, which are due to the existence of the probe field. More specifically, we examine the spectra of four-wave mixing, absorption, dispersion and the nonlinear optical Kerr effect of the probe field as they evolve in time and in the steady state. Both analytical expressions are derived and numerical results are presented by solving the nonlinear differential density matrix equations that govern the dynamics of the system. In the study of the different kinds of optical phenomena, the influence of the electron sheet density on the spectral shapes is carefully examined.

Durante as últimas três décadas, a epitaxia por feixe molecular se estabeleceu como uma técnica excelente para o crescimento de camadas semicondutoras de alta qualidade, tanto para a construção de dispositivos quanto para a pesquisa básica. No entanto ainda não existe um método universalmente aceito para obter-se camadas dopadas do tipo p nesta técnica de crescimento. Neste trabalho, estudamos, otimizamos e comparamos três diferentes métodos para a dopagem de camadas de GaAs do tipo p. Dois desses métodos exploraram o caráter anfotérico do silício em substratos de GaAs(311)A (através da mudança das condições de crescimento) e GaAs(100) (aplicando uma nova técnica chamada epitaxia assistida por gotas (droplets) de gálio). O terceiro método foi baseado no uso do carbono, cujas propriedades como dopante do tipo p são bem conhecidas em outras técnicas de crescimento epitaxial, mas pouco estudadas na epitaxia por feixe molecular. Para verificar a qualidade das camadas dopadas obtidas, crescemos estruturas como poços quânticos parabólicos com alta mobilidade de buracos e dispositivos optoeletrônicos como diodos emissores de luz e laseres. / During the last three decades, molecular-beam epitaxy has emerged as an excellent technique for the growth of high-quality semiconductor layers both for device construction and for basic research. In spite of this fact, there is still a lack of a universally accepted method to obtain p-type doped layers by this growth technique. In this work, we studied, optimized and compared three different methods to get p-type GaAs layers. Two of these methods exploited the amphoteric behavior of silicon atoms both on GaAs(311)A (by changing the growth conditions) and on GaAs(100) (by employing a new growth mode called droplet-assisted epitaxy) substrates. The third method was based on the use of carbon, whose properties as a p-type dopant in GaAs layers are well known in other epitaxial techniques but scarcely investigated in molecular-beam epitaxy. In order to check the quality of the doped layers, we grew structures like high hole-mobility parabolic quantum wells and optoelectronic devices like light-emitting diodes and lasers.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / Os avanÃos nas tÃcnicas de crescimento tornaram possÃvel a fabricaÃÃo de estruturas semicondutoras quase-unidimensionais em escalas nanomÃtricas, chamadas pontos, fios, poÃos e anÃis quÃnticos. Interesse nessas estruturas tem crescido consideravelmente, nÃo sà devido Ãs suas possÃveis aplicaÃÃes em dispositivos eletrÃnicos e à sua manipulaÃÃo quÃmica fÃcil, mas tambÃm porque eles oferecem a possibilidade de explorar experimentalmente vÃrios aspectos de confinamento quÃntico, espalhamento e fenÃmenos de interferÃncia. Em particular, neste trabalho, investigamos as propriedades eletrÃnicas e de transporte em poÃos quÃnticos, fios e anÃis, cujas dimensÃes podem ser alcanÃados experimentalmente. Para isto, resolvemos a equaÃÃo de SchrÃdinger dependente do tempo utilizando o mÃtodo Split-operator em duas dimensÃes. Nesta tese, abordamos quatro trabalhos, sendo o primeiro uma analogia ao Paradoxo de Braess para um sistema mesoscÃpico. Para isso, utilizamos um anel quÃntico com um canal adicional na regiÃo central, alinhado com os canais de entrada e saÃda. Este canal extra faz o papel do caminho adicional em uma rede de trÃfego na teoria dos jogos, similar ao caso do paradoxo de Braess. Calculamos as auto-energias e a evoluÃÃo temporal para o anel quÃntico. Surpreendentemente, o coeficiente de transmissÃo para algumas larguras do canal extra diminuiu, semelhante ao que acontece com redes de trÃfego, onde a presenÃa de uma via extra nÃo necessariamente melhora o fluxo total. Com a analise dos resultados obtidos, foi possÃvel determinar que neste sistema o paradoxo ocorre devido a efeitos de interferÃncia e de espalhamento quÃntico. No segundo trabalho, foi feita uma extensÃo do primeiro, (i) aplicando-se um campo magnÃtico, onde foi possÃvel obter o efeito Aharonov-Bohm para pequenos valores do canal extra e controlar efeitos de interferÃncia responsÃveis pelo paradoxo mencionado, e (ii) fazendo tambÃm a aplicaÃÃo de um potencial que simula a ponta de um microscÃpio de forÃa atÃmica (AFM) interagindo com a amostra - este potencial à repulsivo e simula um possÃvel fechamento do caminho em que o pacote de onda se propaga. Assim, neste trabalho, realizamos uma contra-prova do primeiro, onde observamos que com o posicionamento da ponta do AFM sobre canal extra, se diminui o efeito de reduÃÃo de corrente devido ao paradoxo de Braess. No terceiro trabalho, realizamos uma anÃlise de tunelamento entre dois fios quÃnticos separados por uma certa distÃncia e calculamos qual a menor distÃncia para qual ocorre tunelamento significativo nesse sistema eletrÃnico. Este trabalho à de fundamental importÃncia para o manufaturamento de dispositivos nanoestruturados, porque nos permite investigar qual a distÃncia mÃnima para a construÃÃo de um circuito eletrÃnico sem que haja interferÃncias nas transmissÃes das informaÃÃes. No quarto e Ãltimo trabalho desta tese, investigamos a energia de ligaÃÃo do elÃtron-impureza em GaN/HfO2 para um poÃo quÃntico. Consideramos simultaneamente as contribuiÃÃes de todas as interaÃÃes das auto-energias devido ao descasamento das constantes dielÃtricas entre os materiais. Foram estudados poÃos largos e estreitos, comparando os resultados para diferentes posiÃÃes da impureza e a contribuiÃÃo da auto-energia para o sistema. / Advances in growth techniques have made possible the fabrication of quasi one-dimensional semiconductor structures on nanometric scales, called quantum dots, wires, wells and rings. Interest in these structures has grown considerably not only due to their possible applications in electronic devices and to their easy chemical manipulation, but also because they offer the possibility of experimentally exploring several aspects of quantum confinement, scattering and interference phenomena. In particular, in this work, we investigate the electronic and transport properties in quantum wells, wires and rings, whose dimensions can be achieved experimentally. For this purpose, we solve the time-dependent SchrÃdinger equation using the split-operator method in two dimensions. We address four different problems: in the first one, the electronic transport properties of a mesoscopic branched out quantum ring are discussed in analogy to the Braess Paradox of game theory, which, in simple words, states that adding an extra path to a traffic network does not necessarily improves its overall flow. In this case, we consider a quantum ringindex{Quantum ring} with an extra channel in its central region, aligned with the input and output leads. This extra channel plays the role of an additional path in a similar way as the extra roads in the classical Braess paradox. Our results show that in this system, surprisingly the transmission coefficient decreases for some values of the extra channel width, similarly to the case of traffic networks in the original Braess problem. We demonstrate that such transmission reduction in our case originates from both quantum scattering and interference effects, and is closely related to recent experimental results in a similar mesoscopic system. In the second work of this thesis, we extend the first system by considering different ring geometries, and by investigating the effects of an external perpendicular magnetic field and of obstructions to the electrons pathways on the transport properties of the system. For narrow widths of the extra channel, it is possible to observe Aharonov-Bohm oscillations in the transmission probability. More importantly, the Aharonov-Bohm phase acquired by the wave function in the presence of the magnetic field allows one to verify in which situations the transmission reduction induced by the extra channel is purely due to interference. We simulate a possible closure of one of the paths by applying a local electrostatic potential, which can be seen as a model for the charged tip of an atomic force microscope (AFM). We show that positioning the AFM tip in the extra channel suppresses the transmission reduction due to the Braess paradox, thus demonstrating that closing the extra path improves the overall transport properties of the system. In the third work, we analyze the tunneling of wave packets between two semiconductor quantum wires separated by a short distance. We investigate the smallest distance at which a significant tunneling between the semiconduting wires still occur. This work is of fundamental importantance for the manufacturing of future nanostructured devices, since it provides information on the minimum reasonable distances between the electron channels in miniaturized electronic circuits, where quantum tunnelling and interference effects will start to play a major role. In the last work of this thesis, we investigate the binding energy of the electron-impurity pair in a GaN/HfO2 quantum well. We consider simultaneously the contributions of all interactions in the self-energy due to the dielectric constant mismatch between materials. We investigate the electron-impurity bound states in quantum wells of several widths, and compared the results for different impurity positions.

Neste trabalho, apresentamos o resultado de nossas investigações em amostras de poços quânticos parabólicos (PQW) de AlGaAs crescidas em substratos de GaAs por MBE (Molecular Beam Epitaxy). Nossos estudos se concentram nas implicações da variação do fator g de Landé ao longo da estrutura dos PQW, a qual ocorre em virtude da dependência dessa grandeza com respeito ao conteúdo de Al na liga AlGaAs. Essas implicações são analisadas através de medidas de transporte eletrônico (medidas de Hall e do efeito Shubnikov-de Haas). As medidas de Subnikov-de Haas a temperaturas da ordem de dezenas a centenas de milikelvin com variação do ângulo de inclinação se mostram um eficiente método para a determinação do fator g. Distinguimos não só o fator g determinado pelas propriedades da liga, como também uma contribuição oriunda de efeitos de muitos corpos (contribuição de troca). Por outro lado, as medidas de Hall nos revelam um comportamento anômalo, que mostramos não ter origem no conhecido "efeito Hall anômalo" presente em materiais ferromagnéticos, nem em efeitos de ocupação de múltiplas sub-bandas. Atribuímos o fenômeno a um efeito "válvula de spin", conseqüente da variação espacial do fator g. Nossas observações nos permitem a idealização de um transistor "válvula de spin", prescindindo do emprego de materiais magnéticos. / We present the results of our investigations concerning MBE grown AlGaAs/GaAs parabolic quantum well (PQW) samples. We focused on the variation of the Landé g factor along the structure of the PQWs, which occur as a consquence of its dependence on the Al content on the alloy AlGaAs. The implications are studied by Hall and Shubnikov-de Haas measurements. Shubnikov-de Haas measurements at temperatures of the order of tenths to hundreds of milikelvin with variation of the tilt angle are shown to be an efficient method for the determination of the g factor. We could distinguish not only the alloy g factor, but its many body contribution (exchange contribution). On the other hand, Hall measurements exhibit an unusual behavior, which we prooved it has no relation neither to the well known "anomalous Hall effect", a characteristic of ferromagnetic materials, nor to a multi subband occupation effect. We atribute such behavior to a "spin valve effect", caused by the spatial variation of the g factor. Our observations allow us to idealize a "spin valve" transistor, without any ferromagnetic material in its structure.

Neste trabalho utilizamos sistemas modelos no desenvolvimento, implementação e análise de funcionais orbitais da densidade, focando, em particular, nas correções de autointeração de Perdew-Zunger (PZSIC) e Lundin-Eriksson (LESIC). Aplicamos as correções de auto-interação ao funcional local (LDA) do modelo de Hubbard e de poços quânticos semicondutores, ambos unidimensionais, no caso estático e dependente do tempo, respectivamente. Para o modelo de Hubbard unidimensional, comparamos a LDA, LDA+PZSIC e LDA+LESIC, identificando o desempenho para energias e densidades do estado fundamental, com e sem impurezas locais, além do gap fundamental de energia. Em adição, averiguamos o desempenho diante de cargas fracionárias, estabelecendo conexões com o erro de delocalização da LDA. Mostramos a possibilidade da correta descrição das freqüências das oscilações de Friedel no modelo de Hubbard, além de investigar como a falha da LDA em reproduzir esse aspecto pode estar relacionada com os erros de auto-interação e delocalização. Investigamos ainda as diferentes possibilidades de implementação autoconsistente de qualquer funcional orbital da densidade, analisando a relação entre funcionais aproximados e suas implementações aproximadas. Nos poços quânticos, sob o enfoque dependente do tempo, analisamos a descontinuidade do potencial de troca e correlação ao variarmos o número de partículas, em dois processos distintos: a ionização eletrônica em um poço simples e dissociação de um poço duplo assimétrico. No último caso, avaliamos os efeitos da descontinuidade no número total de partículas em cada poço, revelando os mecanismos que resgatam a neutralidade elétrica durante processos de dissociação, com a correta carga final inteira. / In this work we use model systems to develop, implement and analyse orbital-dependent density functionals, focusing, specifically, on the self-interaction corrections (SICs) of Perdew and Zunger (PZSIC) and of Lundin and Eriksson (LESIC). These self-interaction corrections are applied to the local-density approximation (LDA) for the one-dimensional Hubbard model and for semiconductor quantum wells, in one-dimensional static and time-dependent situations. For the one-dimensional Hubbard model we compare LDA, LDA+PZSIC and LDA+LESIC, and investigate the performance of these approaches for ground-state energies, densities and energy gaps, with and without impurities in the system. We also consider the case of fractional charges, where a connection to the delocalization error of the LDA can be made. We show that in principle a correct description of the frequences of Friedel oscillations in the Hubbard model can be obtained from DFT, and investigate how the failure of the LDA in reproducing this is related to the selfinteraction and delocalization errors. Moreover, we investigate different procedures for the selfconsistent implementation of any orbital-dependent functional, and analyse the question of the interplay between an approximate functional and its approximate implementation. For quantum wells sytems we analyse, in a time-dependent framework, the discontinuity of the exchange-correlation potential under variation of the particle number in two different processes: the ionization of a simple quantum well and the dissociation of an asymmetric double well. In the latter case, we also consider the effect of changes in the particle number in each subwell, thus revealing the mechanism that restores electric neutrality during dissociation, with correct final charge.

In this thesis I present and discuss the results of optical spectroscopy performed on InGaN/GaN single and multiple quantum well (QW) structures. I report on the optical properties of InGaN/GaN single and multiple QW structures, measured at high excitation power densities. I show a correlation exists between the reduction in PL efficiency at high excitation power densities, the phenomenon so-called ``efficiency-droop'', and a broadening of the PL spectra. I also show a distinct change in recombination dynamics, measured by time-resolved photoluminescence (PL), which occurs at the excitation power densities for which efficiency droop is measured. The broadening of the PL spectra at high excitation power densities is shown to occur due to a rapidly redshifting, short-lived high energy emission band. The high energy emission band is proposed to be due to the recombination of weakly localised/delocalised carriers occurring as a consequence of the progressive saturation of the local potential fluctuations responsible for carrier localisation, at high excitation power densities. I report on the effects of varying threading dislocation (TD) density on the optical properties of InGaN/GaN multiple QW structures. No systematic relationship exists between the room temperature internal quantum efficiency (IQE) and the TD density, in a series of nominally identical InGaN/GaN multiple QWs deposited on GaN templates of varying TD density. I also show the excitation power density dependence of the PL efficiency, at room temperatures, is unaffected for variation in the TD density between 2 x107 and 5 x109 cm-2. The independence of the optical properties to TD density is proposed to be a consequence of the strong carrier localisation, and hence short carrier diffusion lengths. I report on the effects of including an InGaN underlayer on the optical and microstructural properties of InGaN/GaN multiple QW structures. I show an increase in the room temperature IQE occurs for the structure containing the InGaN underlayer, compared to the reference. I show using PL excitation spectroscopy that an additional carrier transfer and recombination process occurs on the high energy side of the PL spectrum associated with the InGaN underlayer. Using PL decay time measurements I show the additional recombination process for carriers excited in the underlayer occurs on a faster timescale than the recombination at the peak of the PL spectrum. The additional contribution to the spectrum from the faster recombination process is proposed as responsible for the increase in room temperature IQE.

Neste trabalho utilizamos sistemas modelos no desenvolvimento, implementação e análise de funcionais orbitais da densidade, focando, em particular, nas correções de autointeração de Perdew-Zunger (PZSIC) e Lundin-Eriksson (LESIC). Aplicamos as correções de auto-interação ao funcional local (LDA) do modelo de Hubbard e de poços quânticos semicondutores, ambos unidimensionais, no caso estático e dependente do tempo, respectivamente. Para o modelo de Hubbard unidimensional, comparamos a LDA, LDA+PZSIC e LDA+LESIC, identificando o desempenho para energias e densidades do estado fundamental, com e sem impurezas locais, além do gap fundamental de energia. Em adição, averiguamos o desempenho diante de cargas fracionárias, estabelecendo conexões com o erro de delocalização da LDA. Mostramos a possibilidade da correta descrição das freqüências das oscilações de Friedel no modelo de Hubbard, além de investigar como a falha da LDA em reproduzir esse aspecto pode estar relacionada com os erros de auto-interação e delocalização. Investigamos ainda as diferentes possibilidades de implementação autoconsistente de qualquer funcional orbital da densidade, analisando a relação entre funcionais aproximados e suas implementações aproximadas. Nos poços quânticos, sob o enfoque dependente do tempo, analisamos a descontinuidade do potencial de troca e correlação ao variarmos o número de partículas, em dois processos distintos: a ionização eletrônica em um poço simples e dissociação de um poço duplo assimétrico. No último caso, avaliamos os efeitos da descontinuidade no número total de partículas em cada poço, revelando os mecanismos que resgatam a neutralidade elétrica durante processos de dissociação, com a correta carga final inteira. / In this work we use model systems to develop, implement and analyse orbital-dependent density functionals, focusing, specifically, on the self-interaction corrections (SICs) of Perdew and Zunger (PZSIC) and of Lundin and Eriksson (LESIC). These self-interaction corrections are applied to the local-density approximation (LDA) for the one-dimensional Hubbard model and for semiconductor quantum wells, in one-dimensional static and time-dependent situations. For the one-dimensional Hubbard model we compare LDA, LDA+PZSIC and LDA+LESIC, and investigate the performance of these approaches for ground-state energies, densities and energy gaps, with and without impurities in the system. We also consider the case of fractional charges, where a connection to the delocalization error of the LDA can be made. We show that in principle a correct description of the frequences of Friedel oscillations in the Hubbard model can be obtained from DFT, and investigate how the failure of the LDA in reproducing this is related to the selfinteraction and delocalization errors. Moreover, we investigate different procedures for the selfconsistent implementation of any orbital-dependent functional, and analyse the question of the interplay between an approximate functional and its approximate implementation. For quantum wells sytems we analyse, in a time-dependent framework, the discontinuity of the exchange-correlation potential under variation of the particle number in two different processes: the ionization of a simple quantum well and the dissociation of an asymmetric double well. In the latter case, we also consider the effect of changes in the particle number in each subwell, thus revealing the mechanism that restores electric neutrality during dissociation, with correct final charge.