No description available.

No description available.

No description available.

No description available.

No description available.

Neste trabalho propomos um modelo molecular geral para o estudo da estrutura eletrônica de sólidos covalentes, utilizando o método do Espalhamento Múltiplo. Aplicamos o modelo ao cristal de GaAs com aglomeradosde 17 átomos. Analisamos os efeitos produzidos pelos elétrons pertencentes aos orbitais flutuantes, evidenciando a necessidade de tratarmos corretamente às condições de contôrno do aglomerado. Apresentamos os resultados para o gap de energia, largura da faixa de valência e a curva densidade de estados, resultados estes que concordam satisfatoriamente com a experiência. Utilizando o modelo proposto, estudamos ainda defeitos pontuais no cristal de GaAs tais como vacância de gálio, vacância de arsênio e impurezas substitucionais de cobre e selênio. A simulação de uma vacância de gálio no GaAs provocou o aparecimento de um nível de energia aceitador (profundo) e a vacância de arsênio provocou o aparecimento de 2 níveis doadores (profundos). Com a introdução de uma impureza de cobre no GaAs surgiu um nível aceitador energeticamente próximo ao nível produzido pela vacância de gálio. A impureza de selênio no GaAs provocou o aparecimento de um nível doador (raso) próximo a faixa de condução. Todos os resultados acima apresentados concordam perfeitamente com a evidência experimental. Por último incluímos o operador de massa de Ferreira-Leite para o estudo de níveis rasos de impureza. A partir dos resultados obtidos neste trabalho e da generalidade do modelo proposto abre-se um vasto campo para o estudo defeito em sólidos covalentes. / A molecular cluster model of tetrahedrally coordinated covalent semiconductors is proposed. The boundary condition at the cluster surface is the crucial problem. A general solution to this problem is suggested. The energy spectra of GaAs cluster with seventeen atoms are obtained within the framework of the selfconsistent field multiple scattering method. A critical analysis of the \"dangling bonds\" effects on the electronic structure of the cluster is performed, and a suitable simulation of the rest of the crystal at the cluster boundaries is discussed. lt is show that the main features of the perfect crystal electronic structure emerge from the cluster spectrum, when the proposed boundary condition is assumed. The model was applied to \"the study of the electronic structure of vacancies and substitutional impurities in GaAs. The proposed cluster model leads to the correct location of the electronic levels, introduced by the .impurity, with respect to the band edges. The agreement between the calculated results and the experimental ones suggests that the molecular cluster approach proposed is reliable to deal with a wide range of problems associated with the electronic structure of perfect and imperfect IV and III-V covalent semiconductors.

O trabalho apresenta um modelo para a descrição autoconsistente da polarização de spin em sistemas atômicos, onde o elétron e tratado como uma partícula de Dirac. O Hamiltoniano de interação de muitas partículas utilizado é obtido via teoria de perturbação covariante até primeira ordem em , constante de estrutura fina, e até termos da ordem (v/c)2, onde v representa a velocidade das partículas. Tal Hamiltoniano é uma boa descrição para sistemas de elétrons interagentes até uma densidade de aproximadamente 10.26 elétrons/cm3. No trabalho procura-se descrever os estados de uma partícula como \"orbitais atômicos\" e para tanto aproximações adicionais fazem-se necessárias. Além disso, a aproximação local por um gás de elétrons livres no tratamento dos termos de \"exchange\" e utilizada. Como aplicação do modelo é feito o cálculo da estrutura de multipletos para alguns sistemas atômicos. Obtém-se, também, um pequeno desdobramento em átomos com camadas fechadas (gases nobres) devido às interações magnética e de retardamento. / A relativistic, spin polarized, self-consistent, atomic system model is presented, the electrons being described as Dirac particles. The many-body interaction Hamiltonian used is correct until the orders and (v/c)2, with standing for the fine structure constant, v and c for the velocities of particles and light respectively. Such Hamiltonian may be used for the description of interacting electrons for densities lower than 10 26 electrons/cm3. The requirement for particles states as \"atomic orbitals\" implies some additional approximation. For exchange terms the local free electron gas approximation is used. As an application the multiplet structure for some atomic systems is presented. The model describes a break of degeneracy for closed shell atoms due to the magnetic and retarded interactions.

Estudamos neste trabalho a estrutura eletrônica de defeitos localizados em silício, responsáveis pela introdução de níveis profundos na faixa proibida do semicondutor. Utilizamos para tanto modelos de aglomerados moleculares dentro do formalismo do Espalhamento Múltiplo com aproximação local X para o potencial de troca (MS-X). Um tratamento adequado para os orbitais de superfície foi usado. Os defeitos estudados foram de dois tipos: defeitos simples (monovacância Si:V e oxigênio substitucional Si:O) e pares de defeitos vizinhos na rede (divacância Si : V IND 2) e par de átomos de fósforo Si: P IND 2). Os defeitos foram estudados em diferentes estados de carga (Si:O e Si : O POT -\', Si: P IND 2 e Si: P IND 2 POT + Si: V POT 0 IND 2, Si: V IND 2 POT + e Si: V POT IND 2) e no caso da divacância e do centro P IND 2 POT + foram incluídos efeitos de polarização de spin. Encontramos para todos os defeitos estudados indícios da ocorrência de efeitos Jahn-Teller. Incluímos análises das distorções dos primeiros vizinhos ao defeito nos sistemas Si: O POT , P IND 2 POT + e V POT IND 2. O modelo adotado mostrou-se capaz de descrever satisfatoriamente a estrutura eletrônica dos defeitos estudados, fornecendo resultados quantitativos que podem ser comparados diretamente com a experiência. / In this work we studied the electronic structure of deep-level defects in silicon. To do this we use molecular cluster models within the formalism of the Multiple Scattering method, in the local density functional approximation (X). The surface orbitals of the cluster are treated in a convenient way. The defects studied here were of two kinds: simple defects (single vacancy Si:V and oxygen substitutional Si:0), and pairs of defects occupying neighbouring sites in the lattice (divacancy Si :V2 and the pair of phosphoru- atoms Si :P2 ). The defects were studied in different charge states (Si:O and Si:O-, Si:P2 and Si:P+, Si:V20, SI:V2-) and for the divacancy and the center Si:P2+ the calculations were carried out to the spin-polarized limit. For all defects studied we found evidence as to the possible occurrence of Jahn-Teller effects. Analysis of nearest-neighbours distortions were included for the systems Si:O- , Si:V2+ and Si:V2-. The electronic structure of the defects studied here was satisfactorily described by the model we adopted, and quantitative results are given, that can be compared straightforwardly with experimental results.

本文运用化学计算软件Gaussian03以及基于赝势和平面波基组的从头计算分子软件VASP， 研究了阴离子簇合物M⁻L[subscript n], (M⁻ = Na⁻, Li⁻,; L = H₂O, NH₃ ; n = 1-10) 中的电子结构和溶剂化作用。 / 文中选用基于密度泛函理论的B3LYP方法和6-311++G** 基组, 从Na⁻ 离子和Li⁻ 离子分别与不同溶剂分子 (H₂O和NH₃) 的相互作用入手，探讨以这类气态团簇化合物作为载体的体系中，溶解于其中的两个电子之间的相互作用以及电子与溶剂分子之间的相互作用。对于因溶剂分子极性的差异，导致形成不同形态的簇合物，进而影响了溶解于其中的电子对状态的研究，为实验上进一步探究被溶解的电子对提供了有利的理论依据。 / 在阴离子簇合物Li⁻(H₂O)[subscript n]中, 锂负离子和氧原子的强相互作用使得锂负离子被水分子包围。当n = 10，锂负离子的两个外层电子转移到水分子团簇中，并且电子对可以以单重态或三重态的电子态稳定存在。与之相反，在阴离子簇合物Na⁻(H₂O)n中，钠负离子的两个外层电子没有向溶剂中转移，钠负离子更倾向于与水分子团簇表面的氢原子相互作用而稳定在团簇表面。 / 在以氨分子为溶剂的阴离子簇合物Na⁻(NH₃)n和Li⁻(NH₃)n中, 金属负离子Na⁻ 和Li ⁻的两个外层电子随着溶剂分子的增加逐渐从金属离子上转移到溶剂 中。事实上，当n = 10时，簇合物Na⁻(NH₃)[subscript n]和Li⁻(NH₃)[subscript n]形成了两个溶解中心：被溶剂分子包围的金属阳离子，及其附近分散在溶剂分子中的电子对。这对电子可以以单重态或三重态的电子态稳定存在。与以水为溶剂的簇合物Na⁻(H₂O)[subscript n]和Li⁻(H₂O)[subscript n]不同的是，在Na⁻(NH₃)[subscript n]和Li⁻(NH₃)[subscript n]中，被溶解电子对更加弥散，广泛分布于溶剂分子中。 / The solvation and electronic structures in M⁻Ln₂, M⁻ = Li⁻ and Na⁻, L = H₂O and NH₃, and n = 1-10 were explored computationally by quantum chemistry calculation using Gaussian 03 package and by density functional theory based ab initio molecular dynamics simulation (AIMD) method with pseudopententials and a plane wave basis set using Vienna Ab Initio Simulation package (VASP). / Interactions of Na⁻ and Li⁻ anions in different solvent molecules, H₂O and NH₃, were calculated by B3LYP method with a basis set of 6-311++G** in order to explore the possibility that these clusters could serve as gas-phase molecule models for the solvation of two electrons. Such models would capture the electron-electron interaction in a solution environment. / For Li⁻(H₂O)[subscript n], the Li⁻ is buried in the water cluster due to the strong Li-O interaction, and by n = 10, the two loosely bound s electrons are indeed detached from lithium, and they could in either the singlet (spin-paring) or the triplet (spin-coupling) state. In contrast, for Na⁻(H₂O)[subscript n], the electron pair stays on the sodium, and the Na⁻ stays on the cluter surface, which is solvated by no more than 4 hydrogen atoms. The solvent-solvent interactions are favored as the water molecules from structures similar to observed in water anions. The formation of a solvated electron pair and the variation in solvation structures make these two cluster series interesting subjects for further experimental investingation. / With two s electrons, the Na⁻(NH₃)[subscript n] and Li⁻(NH₃)[subscript n] clusters are unique in that they capture the important aspect of the coupling between two solvated electrons. By first principles calculations, we demonstrate that the two electrons are detached from the metal by n = 10, which produces a cluster with a solvated electron pair in the vicinity of a solvated alkali cation, and the two electrons could also exist in both singlet and triplet state. They are quite distinct from the hydrated anionic clusters Na⁻(H₂O)[subscript n] and Li⁻(H₂O)[subscript n], in that the solvated electrons are delocalized and widely distributed among the solvent ammonia molecules. The Na⁻(NH₃)[subscript n] and Li⁻(NH₃)[subscript n] series therefore provide another interesting type of molecular model for the investigation of solvated electron pairs. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Zhang, Han. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / TITLE PAGE --- p.I / ABSTRACT --- p.II / 论文摘要 --- p.IIV / ACKNOWLEDGEMENTS --- p.VI / CONTENTS --- p.VII / LIST OF TABLES --- p.X / LIST OF FIGURES --- p.XIII / Chapter Chapter One --- General Research Background / Chapter 1.1 --- General Introduction --- p.1 / Chapter 1.2 --- Electron in polar solvent --- p.1 / Chapter 1.2.1 --- Electron in water molecules --- p.1 / Chapter 1.2.2 --- Electron in ammonia molecules --- p.6 / Chapter 1.3 --- Experimental background of metal anion --- p.9 / Chapter 1.3.1 --- Li⁻(H₂O)[subscript n] and Na⁻(H₂O)[subscript n] --- p.9 / Chapter 1.3.2 --- Li⁻(NH₃)[subscript n] and Na⁻(NH₃)[subscript n] --- p.11 / Chapter 1.4 --- Scope of this thesis --- p.13 / Chapter 1.5 --- References --- p.14 / Chapter Chapter Two --- Theoretical Background / Chapter 2.1 --- Introduction --- p.18 / Chapter 2.2 --- The Schrödinger Equation and Born-Oppenheimer Approximation --- p.19 / Chapter 2.3 --- Hartree-Fock Theory --- p.20 / Chapter 2.4 --- Post-HF methods --- p.24 / Chapter 2.5 --- Density Functional Theory --- p.25 / Chapter 2.5.1 --- Kohn Sham (KS) scheme --- p.26 / Chapter 2.5.2 --- Local Density Approximation (LDA) --- p.29 / Chapter 2.5.3 --- Generalized Gradient Approximation (GGA) --- p.30 / Chapter 2.5.4 --- Hybrid Functionals --- p.30 / Chapter 2.6 --- DFT based ab initio Molecular Dynamics (AIMD) --- p.31 / Chapter 2.6.1 --- Plane Wave Basis Set --- p.34 / Chapter 2.6.2 --- Pseudopotentials and Projector Augmented Wave Method --- p.35 / Chapter 2.6.3 --- Molecular dynamics at constant temperature --- p.39 / Chapter 2.7 --- References --- p.40 / Chapter Chapter Three --- The identification of a solvated electron pair in the gaseous clusters of Na(H2O)n and Li(H2O)n / Chapter 3.1 --- Introduction --- p.42 / Chapter 3.2 --- Computaional methods --- p.45 / Chapter 3.3 --- Results and discussion --- p.46 / Chapter 3.3.1 --- Structure and energy for Na⁻(H₂O) and Li⁻(H₂O) --- p.46 / Chapter 3.3.2 --- H end structure for Na⁻(H₂O)[subscript n] --- p.52 / Chapter 3.3.3 --- O end structure for Li⁻(H₂O)[subscript n] --- p.58 / Chapter 3.3.4 --- Triplet structures for Li⁻(H₂O)[subscript n] --- p.64 / Chapter 3.3.5 --- Comparison between singlet and triplet Li⁻(H₂O)[subscript n] --- p.69 / Chapter 3.4 --- Conclusion --- p.73 / Chapter 3.5 --- References --- p.74 / Chapter Chapter Four --- The solvation of two electrons in the gaseous clusters of Na⁻(NH₃)n and Li⁻(NH₃)[subscript n] / Chapter 4.1 --- Introduction --- p.78 / Chapter 4.2 --- Computaional methods --- p.80 / Chapter 4.3 --- Results and discussion --- p.82 / Chapter 4.3.1 --- n = 1 --- p.82 / Chapter 4.3.2 --- H End Structures for Na⁻(NH₃)[subscript n] --- p.87 / Chapter 4.3.3 --- N End Structures for Li⁻(NH₃)n and Na⁻(NH₃)[subscript n] --- p.90 / Chapter 4.3.4 --- Triplet N End Structures for Li⁻(NH₃)[subscript n] --- p.97 / Chapter 4.3.5 --- Comparison between singlet and triplet Li⁻(NH₃)[subscript n] --- p.103 / Chapter 4.4 --- Conclusion --- p.105 / Chapter 4.5 --- References --- p.106

Aplicamos a técnica da função de Green em conjunto com o modelo de força de valência para estudar algumas propriedades vibracionais de redes perturbadas. Apresentamos uma descrição teórica dos modos locais de vibração de vacâncias e anti-sítios em GaAs. Incluímos efeitos de relaxação simétrica para as vacâncias. Baseados em nossos resultados, podemos concluir que o modo vibracional respiratório medido em 227 cm-1 não é devido a vacância de arsênio, conforme fora anteriormente sugerido. Aplicamos o mesmo método para o estudo da variação de densidade de estados vibracionais da vacância em silício. Efeitos de relaxação simétrica e distorção tetragonal são incluídos. Tais resultados são aplicados ao cálculo da entropia de formação deste defeito em Si. A partir destes cálculos verificamos que a vacância pode ser o defeito nativo responsável pela alta entropia de auto-difusão obtida experimentalmente. Finalmente, empregamos o modelo de aglomerado molecular e o método de Hartree-Fock para estudar a estrutura eletrônica da vacância neutra em Si. Calculamos a energia total destes sistemas quando submetidos aos efeitos simultâneos de relaxação e distorção. Verificamos que este modelo fornece resultados equivalentes aos obtidos por métodos de função de Green autoconsistente para a relaxação e apresenta a vantagem adicional de conseguir calcular a energia de distorção. Discutimos as consequências destes resultados sobre a entropia de formação de vacância. / The Green\'s-function technique within the framework of a valence-force field Hamiltonian has been used to study vibrational properties of intrinsic defects in gallium arsenide and silicon. This theoretical approach has been employed to predict the local vibrational modes for vacancies and antisites in GaAs. Symmetrical relaxation was included for vacancies. Our results indicate that the arsenjc vacancy can not explain the breathing mode recently found in 227 cm -1 by Raman studies. The same procedure has been applied to study the changes induced by an isolated vacancy to the local vibrational density of states in silicon. In order to treat the dangling bonds reconstruction, relaxation and tetragonal distortion effects have been included. These calculations have led to the formation entropy of a single vacancy in Si. It has been found that the single vacancy may play an important role in self-diffusion at high temperatures for this material. We also have performed cluster total-energy calculations to study relaxation and distortion around a silicon vacancy. These calculations were carried out under the Hartree-Fock approximation. Our results show that the cluster model, besides providing relaxation energies with the same accuracy of self consistent Green\'s-function method, has the additional advantage of calculating distortion energies. Consequences of these total-energy calculations on formation entropy of this defect have also discussed.