Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

No description available.

A relatively simple and reliable gasket insulation technique for electrical feedthroughs in the Diamond Anvil Cell has been successfuly developed. This involved the use of, easy to use components such as epoxy resins which can be easily obtained, and, the use of simple equipment which are part of any reasonably equiped laboratory. Photoconductivity, demonstrator, experiments on strained layer quantum well material were successfully used to prove the feasibility of these techniques. The pressure coefficient values obtained for strained InGaAs/GaAs, InGaAs/AlGaAs and unstrained InGaAs/InGaAsP were in agreement with that obtained with a photoluminescence technique. Unlike photoluminescence it is also possible to obtain the pressure coefficients of higher energy states with the photoconductivity experiment. In the process, laminated gaskets for electrical measurements were also developed. This has led to a tremendous improvement in the experimental reliability to such an extent that the electrical feedthroughs have become almost as routine as the optical experiments. Further, it is now possible to use ordinary optically cut anvils to conduct electrical feedthroughs without resorting to specially designed ones, such as rounded or multiple bevelled anvils, or, even depositing permanent conducting strips onto the diamond culets. The benefits of this is that obtaining ordinary cut anvils is much simpler than obtaining specialist bevelled or rounded edge anvils.

Electron transport in relaxed InxGa1-x As grown on GaAs by MBE and doped with Si to a carrier density of approximately 1016cm-3 has been investigated as a function of temperature (4.2 - 300K) and pressure (0-8 kbar) for a wide range alloy compositions. A dramatic decrease in both the mobility and carrier concentration in a limited alloy composition range, x=0.30-0.60, has been observed. The results, analyzed using an Iterative Solution of Boltzmann equation (ISBE) based on the usual scattering mechanisms, indicate that in general very good agreement can be obtained at high temperatures for the alloy compositions where the electron scattering from deep centres is unimportant. Both the measured temperature and pressure dependence of mobility and carrier concentration together with the ISBE calculations indicate that the deep levels are responsible for the sharp decrease both in the mobility and carrier concentration in the alloy composition range, x=0.30-0.60. For the x=0.51 and 0.60 layers an additional scattering mechanism having temperature dependence ~T2.6 has been observed, which is attributed to electron scattering from the deep levels generated in association with layer relaxation. The resistivity analysis at low temperatures shows that the Mott transition occurs in the limited alloy composition range x=0.30-0.50 at these doping densities. The temperature and pressure dependence of the low temperature resistivity can be well described in terms of the thermally-activated hopping conduction law for alloy compositions x0.72.

No description available.

In this project, the formation and characterisation of CdS nanoparticles embedded within Langmuir-Blodgett (LB) films have been studied systematically. CdS nanoparticles were formed within calix[8]arene (CA) and stearic acid (SA) Cd-salt LB films by exposure to H[2]S gas at room temperature. The AFM images of the treated SA and CA bilayers show CdS clusters with lateral dimensions in the range of 20-30 nm and 10-15 nm, respectively. These particles are pseudo two-dimensional and have a shape of hexagonal platelets which is most likely consequence of their wurtzite structure. Calculations of the cluster thickness Lz yield the value of 2 nm (Photoluminescence excitation-PLE) and 1.9 nm (UV-visible) for CdS clusters in SA matrix and two values of 1.2 nm and 1.6 nm (PLE) and 1.08 nm and 1.42 nm (UV-visible) for CA LB films. The PL spectra demonstrate a large Stokes shift, indicating the formation of "dark excitons" in the platelet CdS clusters. The transformations of the absorption spectra caused by ageing yield a monotonic increase in CdS cluster size with the tendency of saturation. This behaviour can be explained by the model of two dimensional (2D) diffusion. According to this model, CdS molecules in the SA films, initially evenly distributed within each LB bilayer, aggregate and form "large" particles with an average size of 1.9 nm and smaller particles with their size distributed over a wide range down to single molecules. The Surface Plasmon Resonance (SPR) and ellipsometry measurements confirmed that the H2S treatment to the LB films results both in the increase in thickness and refractive index of the LB films due to formation of CdS nanoparticles inside. SPR kinetics measurements during H[2]S treatment show that the formation of CdS clusters happened very fast and almost reached the saturation in about 30 minutes. The I-V characteristics of the sandwich structures of Aluminium/LB films/ Aluminium show exponential behaviour and weak temperature dependence, indicating a combination of electron-tunnelling and hopping mechanism. The formation of CdS clusters in the film matrix did not result in either an enhancement of the electron transfer or an additional polarisation at a small (50 mV) AC signal. Most likely, the observed changes in DC conductivity and AC impedance are caused by the film disorder introduced by CdS nanoparticles.

We have presented evidence that the blue emission band, which is dominant at room temperature in ZnSe made under a wide variety of conditions, arises from a transition between a free hole and an electron bound to a donor, but not all donors contribute. There is a cutoff energy within the spread of donor levels above which there is negligible contribution to the emission. It is suggested that the cutoff corresponds to a localization edge of the same nature as the mobility edge. The line shape calculation based on a simple model agrees well with the experimental data. The origin of the blue emission seen at room temperature in the electroluminescence is examined to be the same. We have also discussed the injection mechanism of minority carriers in the ZnSe MIS diodes. It has been shown that annealing as-grown MOCVD ZnSe in the temperature range 300-400 °C can lead to large increases in resistivity. The effect is large for annealing in air or selenium and smaller for annealing in vacuum or zinc vapour. The process involved has an activation energy of only 0.26 eV and appears to be caused by a lattice defect acting as an acceptor. The photocapacitance spectra show that the acceptor is likely to be the so-called M-centre in ZnSe. We have shown that the attribution of the M-centre to copper-red centre is by no means conclusive. The possibility is still open that the M-centre is a lattice defect. We have made double light source steady-state photocapacitance measurements on ZnS single crystals. The Schottky diodes were made by evaporating a metal contact onto a chemically cleaned ZnS surface. Levels were found at 0.9 eV and 2.0 eV below the conduction band and 0.8 eV above the valence band in both melt-grown and iodine-transported material. These centres might be due to lattice defects. An additional level at 1.6 eV below the conduction band occurred in the iodine-transported material.

Neste trabalho iremos apresentar a teoria autoconsistente k. p aplicada a heteroestruturas dopadas tipo-p que resolve a Hamiltoniana 6 x 6 de Luttinger-Kohn, generalizada para tratar diferentes materiais, juntamente com a equação de Poisson na representação de ondas planas. A generalização do termo de energia cinética é inserido explicitamente na Hamiltoniana de Luttinger-Kohn levando em conta os efeitos de diferentes materiais, através da inclusão de conjuntos distintos de parâmetros de Luttinger e considerando os elementos adicionais na matriz, da Hamiltoniana de Luttinger-Kohn e que não aparecem naquela para homoestruturas. O método é aplicado ao estudo pioneiro da estrutura de sub-bandas e minibandas de valência de heteroestruturas com dopagem tipo-p envolvendo semicondutores nitretos do grupo-III cúbicos. Heteroestruturas de compostos III-V derivados do GaAs também são estudadas, com o objetivo principal de testar o método generalizado e sua implementação no programa. Especificamente, iremos obter soluções auto consistentes da equação da massa efetiva de multibandas e da equação de Poisson para poços quânticos e super-redes de AlxGa1-xN/GaN e GaN/InxGal-xN na fase cúbica, bem como para heteroestruturas de GaAs/lnxGal-xAs e AlxGa1-xAs/GaAs, variando a concentração de aceitadores, o perfil de dopagem, o período da super-rede e a composição da liga x. Os efeitos de troca-correlação, dentro da aproximação da densidade local, são levados em conta para o gás de buracos bidimensional formado dentro do poço. Analisaremos os efeitos na banda de valência das heteroestruturas quando consideramos diferentes parâmetros de Luttinger (associados a materiais distintos). Mostraremos que para descrever corretamente os sistemas envolvendo nitretos é fundamental incluir a banda de split-ojJ, os efeitos de tensão devidos ao descasamento de parâmetros de rede e os efeitos de troca-correlação. Iremos verificar que quando utilizamos um conjunto de parâmetros adequados que especificam a heteroestrutura, podemos obter uma densidade alta de gás de buracos bidimensional, particularmente para os sistemas de poços quânticos e super-redes baseados nos nitretos. Apresentaremos também comparações de nossos resultados com dados experimentais obtidos de fotoluminescência para sistemas de GaN/InxGal_xN e de fotoluminescência e absorção para AlxGal-xAs/InxGal-xAs observando um bom acordo e mostrando que este trabalho serve como um guia para futuras experiências. / In this work we present a self-consistent k p theory of p-doped semiconductor heterostructures whích solves the full six-band Luttinger-Kohn Hamiltonian, generalized to treat different materials, in conjunction with the Poisson equation in a plane-wave representation. The generalization of the kinetic energy term is performed by taking into account explicitly in the Luttinger-Kohn Hamiltonian the effects of different materials through the inclusion of distinct sets of Luttinger parameters and by considering the corresponding additional Luttinger-Kohn matrix elements, not present in the one-material based homostructures. The method is applied to the pioneering study of the valence sub-bands and minibands structures of p-type doped heterostructures involving cubic nítride semiconductors of the group-III. Heterostructures of I III-V compounds derived from GaAs are also studied, with the main aím of testing the generalized method and its implementation in the programo Self-consistent solutions of the multiband effectivemass-Poisson equations are obtained for several AlxGa1-xN/GaN and GaN/InxGa1-xN quantum wells and superlattices in the cubic phase, as well as for GaAs/InxGa1-xAs and AlxGa1-xAs/GaAs heterostructures, in which acceptor doping concentration and its profile, SL period, and the alloy content x are varied. Exchange-correlation effects within the formed two-dimensional hole gas are taken into account in the local density approximation. The role played by the use of different Luttinger parameters (associated with the distinct materials) in the hole bands of the heterostructures is analyzed. The inclusion of the spin-orbit split-off band and strain, as well as the exchange-correlation effects are shown to be fundamental for the correct description of the systems. Particularly for the nitride-derived quantum wells and superlattices, it is shown that high-density twodimensional hole gases can be formed if adequate design parameters are employed. We also compare our results with the experimental data obtained from photoluminescence spectra for the GaN/InxGa1-xN and photoluminescence and absorption spectra for the AlxGal-xAs/InxGal-xAs, observing a good agreement and showing that this work can be used as a guide for future experiments.

This thesis presents the results of a study of the thermal stability of InGaAs and InGaAsP material systems fabricated on InP substrates. We have used photoluminescence, coupled with repetitive annealing to follow the diffusion of a single, 100A, quantum well, of either InGaAs or InGaAsP. Structures were designed such that in each sample only atoms on one sublattice were diffusing, so that any changes in the photoluminscence would be solely due to a single sublattice. We concluded that the interdiffusion process was Fickian with a diffusion coefficient that was concentration independent. Contrary to some of the data in the literature, the substrate type had no effect upon the intermixing, neither did zinc incorporated during MOCVD growth. It was shown that the interdiffusion of the group III and group V atoms were identical and could be described by an Arrhenius expression with EA = 3.49 +/-0.15 eV and DO = 0.12 +0.66 -0.02 cm2 s-1.

No description available.