Nós calculamos o efeito da interação elétron-fonons longitudinais óticos (LO) sobre a energia de transição ls &#8594 2p+ entre os níveis de uma impureza doadora, localizada em um poço quântico de GaAs-AlxGa1-xAs. Nossos resultados para a energia de transição em função do campo magnético aplicado mostram claramente, que a saturação da energia de transição (efeito pinning) ocorre na energia dos fônons LO, em boa concordância com recentes dados experimentais. Obtemos também a massa de cíclotron de polarons confinados em fios quânticos quase-unidimensionais, com potencial de confinamento parabólico. Observamos que o comportamento da massa é diferente daquele para sistemas bi-dimensionais e que esta diferença é maior quanto maior o potencial de confinamento. Para a heterojunção de GaAs-AlGaAs e GaAs-GaSb, investigamos a importância da interação elétron-fonons interfaciais sobre a massa de cíclotron. Verificamos que a contribuição dos fonons interfaciais é fundamental nas regiões próximas às resonâncias, onde domina o espectro. Finalmente, calculamos a energia de ligação de uma impureza hidrogenóide, localizada no centro de um ponto quântico circular de GaAs-AlGaAs. Na ausência de campo magnético aplicado, obtivemos uma expressão analítica para a função de onda do elétron ligado. Notamos, que a influência do campo magnético sobre a energia de ligação é fraca nas regiões de pequenos raios, devido ao forte potencial de confinamento. / We calculate the effects of the electron-longitudinal optical (LO) phonons interaction on the intra donor ls &#8594 2p+ transition energy in GaAs-AlGaAs quantum wells structures. Our results to the transition energy as a function of the magnetic Field strength, show that the pinning effect occur in the phonon LO energy in good agreement with recent experimental data. The cyclotron mass of polarons confined in quasi.one.dimensional quantum-well wires with parabolic confinement potential, is also obtained. The behavior of electrons effective mass with magnetic field is different, of the two-dimensional systems, and the difference increase when the confinement potential increase. To heterojunctions of GaAs-AlAs and GaAs-GaSb, we investigate the electroninterfacials optical (IO) phonons interactions on the effective cyclotron mass. We find that the electron-IO-phonons interaction is fundamental near the resonances, where they dominate the spectra. Finally, the ground state binding energy of donor impurity, placed in the center of a circular quantum dot is calculated. Without magnetic field, we obtained the analytic expression to the bound electron wave function. The influence of the magnetic field on the donor binding energy is weaker, when the radius of the quantum dot became smaller.

Des puits quantiques à base d'hétérostructures In0.53 Ga0.47 As/In0.52 Al0.48 As, fabriqués par épitaxie par jets moléculaires sur substrats InP(111)A, sont étudiés par microscopie et spectroscopie à effet tunnel à basse température et sous ultra-vide. La première partie est consacrée à une étude de la surface épitaxiée (111)A de In0.53 Ga0.47 As de type n. Il est découvert que le niveau de Fermi de surface est positionné dans la bande de conduction, à proximité du niveau de Fermi de volume, et peut être partiellement contrôlé en variant la concentration d'impuretés de type n dans le volume. Ce résultat est confirmé en déterminant la relation de dispersion de la bande de conduction en surface. Un tel dépiégeage partiel du niveau de Fermi de surface indique que la densité d'états de surface accepteurs est faible. Il est proposé que ces états proviennent de défauts ponctuels natifs localisés à la surface. La deuxième partie, basée sur les résultats obtenus dans la première partie, est consacrée à une étude de puits quantiques In0.53 Ga0.47 As de surface, déposés sur des barrières In0.52 Al0.48 As selon la direction (111)A. Les mesures sont conduites sur la surface épitaxiée (111)A du puits quantique In0.53 Ga0.47 As, de manière à pouvoir sonder à l'échelle du nanomètre la distribution de densité locale d'états électroniques dans le plan du puits quantique. Il est confirmé que des sous-bandes électroniques sont formées dans le puits quantique, et que la concentration d'électrons dans le puits peut être contrôlée du fait du dépiégeage partiel du niveau de Fermi de surface. Il est découvert qu'un phénomène de percolation d'états localisés survient dans la queue de chaque sous-bande, ce qui indique la présence d'un potentiel désordonné dans le puits quantique. Les seuils de percolation sont déterminés en utilisant un modèle semi-classique. L'origine du potentiel désordonné est attribuée à une distribution aléatoire des défauts ponctuels natifs à la surface du puits quantique. Il est également découvert qu'un état lié apparaît au bas de chaque sous-bande à proximité d'un défaut ponctuel natif de type donneur. L'énergie de liaison et le rayon de Bohr des états liés peuvent être directement déterminés. De plus, il est démontré que l'énergie de liaison et le rayon de Bohr sont fonctions de l'épaisseur du puits quantique, en accord quantitatif avec des calculs variationnels d'impuretés dans le modèle de l'atome d'hydrogène.

In the first two chapters we presented the basics of density functional theory and semiempirical LSD+U approximation, which was implemented in the full-potential local-orbital (FPLO) minimal-basis calculation scheme. In the third chapter we tested the implemented version of LSDA+U on 3d transitional metal monoxides. Essential improvement of the spectroscopic properties was obtained. A simple model describing the value and direction of the magnetic moment of a transition metal ion was presented. The model visualizes the interplay of the spin-orbit coupling and crystal field splitting. In the fourth chapter we calculated the electronic spectrum of the single Zn impurity in CuO2 plane considered as a vacancy in Cu 3d states. The analytic solution for the states of different symmetry was obtained. Depending on the strength of perturbation induced by the impurity on the neighboring Cu ions, the states are either resonant or localized. The critical values of the perturbation were computed. In the fifth chapter we presented the calculations for three novel vanadates: MgVO3, Sb2O2VO3 and VOMoO4. The tight-binding parameters and the exchange integrals were computed. The magnesium and antimony vanadates appeared to be spin-½ one-dimensional systems, the latter having much stronger one-dimensional character and being probably the best realization of inorganic spin-Peierls system. The molybdenum vanadate was found to be two-dimensional spin-½ system. The Mo 4d orbitals play an important role in the electronic transfer.

No description available.

No description available.

We investigate the manifestation of strong electronic correlations in the quasiparticle interference (QPI), arising from the scattering of conduction electrons from defects and impurities in an otherwise translationally-invariant host. The QPI may be measured experimentally as the Fourier transform of the spatial modulations in the host surface density of states that result, which are mapped using a scanning tunnelling microscope. We calculate the QPI for a range of physically relevant models, demonstrating the effect of strong local electronic correlations arising in systems of magnetic impurities adsorbed on the surface of non-interacting host systems. In the first instance the effect of these magnetic impurities is modelled via the single Anderson impurity model, treated via numerical renormalization group (NRG) calculations. The scattering of conduction electrons, and hence the QPI, demonstrate an array of characteristic signatures of the many-body state formed by the impurity, for example due to the Kondo effect. The effect of multiple impurities on the QPI is also investigated, with a numerically-exact treatment of the system of two Anderson impurities via state-of-the-art NRG calculations. Inter-impurity interactions are found to result in additional scattering channels and additional features in the QPI. The QPI is then investigated for the layered transition metal oxide Sr2RuO4, for which strong interactions in the host conduction electrons give rise to an unconventional triplet superconducting state at T_c ∼ 1.5K. The detailed mechanism for this superconductivity is still unknown, but electron-electron or electron-phonon interactions are believed to play a central role. We simulate the QPI in Sr₂RuO₄, employing an effective parametrized model consisting of three conduction bands derived from the Ru 4d t2g orbitals that takes into account spin orbit coupling and the anisotropy of the Ru t2g orbitals. Signatures of such interactions in the normal state are investigated by comparing these model calculations to experimental results. We also calculate the QPI in the superconducting state, and propose how experimental measurements may provide direct evidence of the anisotropy and symmetry of the superconducting gap, and thus offer insight into the pairing mechanism and the superconducting state.

No description available.

Nós calculamos o efeito da interação elétron-fonons longitudinais óticos (LO) sobre a energia de transição ls &#8594 2p+ entre os níveis de uma impureza doadora, localizada em um poço quântico de GaAs-AlxGa1-xAs. Nossos resultados para a energia de transição em função do campo magnético aplicado mostram claramente, que a saturação da energia de transição (efeito pinning) ocorre na energia dos fônons LO, em boa concordância com recentes dados experimentais. Obtemos também a massa de cíclotron de polarons confinados em fios quânticos quase-unidimensionais, com potencial de confinamento parabólico. Observamos que o comportamento da massa é diferente daquele para sistemas bi-dimensionais e que esta diferença é maior quanto maior o potencial de confinamento. Para a heterojunção de GaAs-AlGaAs e GaAs-GaSb, investigamos a importância da interação elétron-fonons interfaciais sobre a massa de cíclotron. Verificamos que a contribuição dos fonons interfaciais é fundamental nas regiões próximas às resonâncias, onde domina o espectro. Finalmente, calculamos a energia de ligação de uma impureza hidrogenóide, localizada no centro de um ponto quântico circular de GaAs-AlGaAs. Na ausência de campo magnético aplicado, obtivemos uma expressão analítica para a função de onda do elétron ligado. Notamos, que a influência do campo magnético sobre a energia de ligação é fraca nas regiões de pequenos raios, devido ao forte potencial de confinamento. / We calculate the effects of the electron-longitudinal optical (LO) phonons interaction on the intra donor ls &#8594 2p+ transition energy in GaAs-AlGaAs quantum wells structures. Our results to the transition energy as a function of the magnetic Field strength, show that the pinning effect occur in the phonon LO energy in good agreement with recent experimental data. The cyclotron mass of polarons confined in quasi.one.dimensional quantum-well wires with parabolic confinement potential, is also obtained. The behavior of electrons effective mass with magnetic field is different, of the two-dimensional systems, and the difference increase when the confinement potential increase. To heterojunctions of GaAs-AlAs and GaAs-GaSb, we investigate the electroninterfacials optical (IO) phonons interactions on the effective cyclotron mass. We find that the electron-IO-phonons interaction is fundamental near the resonances, where they dominate the spectra. Finally, the ground state binding energy of donor impurity, placed in the center of a circular quantum dot is calculated. Without magnetic field, we obtained the analytic expression to the bound electron wave function. The influence of the magnetic field on the donor binding energy is weaker, when the radius of the quantum dot became smaller.

Esta tese compõe-se de contribuições à física das propriedades eletrônicas das heteroestruturas semicondutoras. São investigadas propriedades eletrônicas das duas hetero­estruturas básicas: o poço quântico e a super-rede. Considera-se o poço quântico dopado com impurezas rasas e estudam-se as suas propriedades eletrônicas nos regimes de poço fraca e altamente dopado. No caso de baixa densidade de impurezas é feita uma simulação Monte Carlo. É utilizado um modelo semi-clássico de band de impureza. A interação elétron-elétron é incluída de forma exata e são calculadas as seguintes propriedades do estado fundamental à temperatura zero: densidade de estados de uma partícula, distribuição de carga, energia de Fermi e distribuição do campo elétrico sobre os doadores neutros, todas em função do grau de compensação, da densidade de impurezas e da largura do poço. É observada uma. grande dependência com a compensação. Os resultados são explicados à luz da competição entre os efeitos de desordem e confinamento. É observada a ocorrência de Coulomb Gap característico de sistemas bidimensionais. Mostra-se que a. distribuição de carga possui largura e constante de decaimento determinados independentemente pela compensação e pela concentração de impurezas, respectivamente. Tais resultados são importantes para a caracterização de poços quânticos puros. No limite altamente dopado parte-se de um modelo light-binding desordenado e calcula­se a densidade de estados de uma partícula formada devido ao overlapping entre os estados localizados; utiliza-se o método de Matsubara e Toyosawa. para a obtenção da média sobre configurações. Discutem-se os efeitos da desordem diagonal introduzida pelo potencial de confinamento os quais são comparados com os da. desordem não-diagonal. São apresentados resultados para a densidade de estados em função do grau de confinamento e concentração de impurezas para poços e fios quânticos. Sâo estudadas as propriedades eletrônicas das super-redes sob campo magnético transversal à direção de crescimento. Mostra-se que esta configuração é ideal para o estudo das características básicas das super-redes: a estrutura de mini bandas e o tunelamento. Calculam-se as sub-bandas de condução utilizando a teoria de massa efetiva de muitas bandas. Introduz-se a idéia de massa efetiva renormalizada para barreiras semicondutoras. Comparam-se os resultados com dados experimentais de ressonância ciclotrônica. A ótima concordância obtida demonstra a grande importância e a utilidade do conceito de massa efetiva renormalizada para barreiras semicondutoras, que é uma maneira nova e simples de lidar com as soluções evanescentes. / This thesis is composed of contributions to the theory of electronic properties of semicon­ ductor heterostructures. Electronic properties of the basic two heterostructures (quantum well and superlattice) are investigated. A quantum well doped with shallow impurities is considered and its electronic properties are studied in both limits: lightly and heavily doped. In the first case a Monte Carlo simula­ tion technique is used. A semiclassical impurity band model is used . The electron-electron interaction is included exactly and properties of the ground state such as the density of single particle states, the charge distribution, the Fermi energy and the electric field di tribution on the neutra/ donors are calculated, all of them as a function of the degree of compensation, the impurity concentration and the width of the well. A great dependency with the compensation is observed. The results are explained by the competition between the effects of disorder and confinement. The existence of a Coulomb Gap is verified . The charge distribution is shown to have a width and decay rate given by the degree of compensation and impurity concentration, in this order. Such results are important to characterize pure quantum wells. On the heavily doped limit, a disordered tight-binding model is used and the density of states that is formed by the overlapping of localized states is calculated by using the method of Matsubara and Toyosawa for the configuration average. The diagonal disord er effect introduced by the confinement potential is considered and compared to that of the non­ diagonal disorder. Results of the density of states as a function of the degree of confinement and impurity concentration for quantum wells and wires are presented. The electronic propertie s of a superlattice under a magnetic field which is transversal to the growth direction are studied. Jt is shown that this configuration is id eal for the study of the basic characteristics of the superlattices: the subband structure and the tunneling. The conduction subbands are calculated by using the theory of many bands effective mass. The idea of renormalized effective mass for barriers is introduced. The obtained level spacings are compared with cyclotron resonance experimental data (infrared absorption). The good agreement obtained demonstrates the importance and usefulness of the renormalized effective mass, which is a new and simple way to handle evanescent waves.

To investigate quantum confinement effects on silicon (Si) light source electroluminescence (EL) properties like quantum efficiency, external power efficiency and spectral emission, thin Si finger junctions with nanometre-scale dimensions were designed and manufactured in a fully customized silicon-on-insulator (SOI) semiconductor production technology. Since commonly available photolithography is unusable to consistently define and align nanometre-scale line-widths accurately and electron-beam lithography (EBL) by itself is too time-expensive to expose complete wafers, the wafer manufacturing process employed a selective combination of photolithography and EBL. The SOI wafers were manufactured in the clean-rooms of both the Carl and Emily Fuchs Institute for Microelectronics (CEFIM) at the University of Pretoria (UP) and the Georgia Institute of Technology’s Microelectronic Research Centre (MiRC), which made a JEOL JBX-9300FS electron-beam pattern generator (EPG) available. As far as is known this was the first project in South Africa (and possibly at the MiRC) that employed EBL to define functional nanometre-scale semiconductor devices. Since no standard process recipe could be employed, the complete design and manufacturing process was based on self-obtained equipment characterization data and material properties. The manufacturing process was unprecedented in both the CEFIM and MiRC clean-rooms. The manufacture of nanometre-scale Si finger junctions not only approached the manufacturing limits of the employed processing machinery, but also had to overcome undesirable physical effects that in larger-scale semiconductor manufacture usually are negligible. The device design, mask layout and manufacturing process therefore had to incorporate various material, equipment limitation and physical phenomena like impurity redistribution occurring during the physical manufacturing process. Although the complicated manufacturing process allowed many unexpected problems to occur, it was expected that at least the simple junction breakdown devices be functional and capable of delivering data regarding quantum confinement effects. Although due to design and processing oversights only 29 out of 505 measured SOI light sources were useful light emitters, the design and manufacture of the SOI light sources was successful in the sense that enough SOI light sources were available to conduct useful optical characterization measurements. In spite of the fact that the functional light sources did not achieve the desired horizontal (width) confinement, measured optical spectra of certain devices indicate that vertical (thickness) confinement had been achieved. All spectrometer-measured thickness-confined SOI light sources displayed a pronounced optical power for 600 nm < λ < 1 μm. The SOI light source with the highest optical power output emitted about 8 times more optical power around λ = 850 nm than a 0.35 μm bulk-CMOS avalanche light-source operating at the same current. Possible explanations for this effect are given. It was shown that the buried oxide (BOX) layer in a SOI process could be used to reflect about 25 % of the light that would usually be lost to downward radiation back up, thereby increasing the external power efficiency of SOI light sources. This document elaborates on the technical objectives, approach, chip and process design, physical wafer manufacture, production process control and measurement of the nanometre-scale SOI light sources. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Electrical, Electronic and Computer Engineering / unrestricted