Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

Not available / text

abstract: Off-axis electron holography (EH) has been used to characterize electrostatic potential, active dopant concentrations and charge distribution in semiconductor nanostructures, including ZnO nanowires (NWs) and thin films, ZnTe thin films, Si NWs with axial p-n junctions, Si-Ge axial heterojunction NWs, and Ge/LixGe core/shell NW. The mean inner potential (MIP) and inelastic mean free path (IMFP) of ZnO NWs have been measured to be 15.3V±0.2V and 55±3nm, respectively, for 200keV electrons. These values were then used to characterize the thickness of a ZnO nano-sheet and gave consistent values. The MIP and IMFP for ZnTe thin films were measured to be 13.7±0.6V and 46±2nm, respectively, for 200keV electrons. A thin film expected to have a p-n junction was studied, but no signal due to the junction was observed. The importance of dynamical effects was systematically studied using Bloch wave simulations. The built-in potentials in Si NWs across the doped p-n junction and the Schottky junction due to Au catalyst were measured to be 1.0±0.3V and 0.5±0.3V, respectively. Simulations indicated that the dopant concentrations were ~1019cm-3 for donors and ~1017 cm-3 for acceptors. The effects of positively charged Au catalyst, a possible n+-n--p junction transition region and possible surface charge, were also systematically studied using simulations. Si-Ge heterojunction NWs were studied. Dopant concentrations were extracted by atom probe tomography. The built-in potential offset was measured to be 0.4±0.2V, with the Ge side lower. Comparisons with simulations indicated that Ga present in the Si region was only partially activated. In situ EH biasing experiments combined with simulations indicated the B dopant in Ge was mostly activated but not the P dopant in Si. I-V characteristic curves were measured and explained using simulations. The Ge/LixGe core/shell structure was studied during lithiation. The MIP for LixGe decreased with time due to increased Li content. A model was proposed to explain the lower measured Ge potential, and the trapped electron density in Ge core was calculated to be 3×1018 electrons/cm3. The Li amount during lithiation was also calculated using MIP and volume ratio, indicating that it was lower than the fully lithiated phase. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

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Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaf 99).

published_or_final_version / Physics / Master / Master of Philosophy

Here it is demonstrated that nanofabricated structures can be used as electron optical elements in new types of electron interferometers. This enables novel investigations with electrons analogous to experiments in light and atom optics. Far field diffraction from a single nanograting is used to examine the force on a charge moving in close proximity to a surface. Near field diffraction from the nanograting is investigated in a Talbot interferometer. It is found that electron waves form replicas of the grating in free space, and these replicas can be de-magnified using illumination by a converging beam. An electron Lau interferometer has the same grating configuration as the Talbot interferometer, but uses spatially incoherent beams that give rise to drastically different interference behavior. A single optical theory is developed to efficiently model a variety of grating interferometers under a diverse set of illumination conditions, and it is used to understand the experiments described here. Applications for these new interferometers are discussed, as well as possible directions for future research.

abstract: This dissertation research has involved microscopic characterization of magnetic nanostructures using off-axis electron holography and Lorentz microscopy. The nanostructures investigated have included Co nanoparticles (NPs), Au/Fe/GaAs shell/core nanowires (NWs), carbon spirals with magnetic cores, magnetic nanopillars, Ni-Zn-Co spinel ferrite and CoFe/Pd multilayers. The studies have confirmed the capability of holography to describe the behavior of magnetic structures at the nanoscale. The phase changes caused by the fringing fields of chains consisting of Co NPs were measured and calculated. The difference between chains with different numbers of Co NPs followed the trend indicated by calculations. Holography studies of Au/Fe/GaAs NWs grown on (110) GaAs substrates with rotationally non-uniform coating confirmed that Fe was present in the shell and that the shell behaved as a bar magnet. No fringing field was observed from NWs with cylindrical coating grown on (111)B GaAs substrates. The most likely explanation is that magnetic fields are confined within the shells and form closed loops. The multiple-magnetic-domain structure of iron carbide cores in carbon spirals was imaged using phase maps of the fringing fields. The strength and range of this fringing field was insufficient for manipulating the carbon spirals with an external applied magnetic field. No magnetism was revealed for CoPd/Fe/CoPd magnetic nanopillars. Degaussing and MFM scans ruled out the possibility that saturated magnetization and sample preparation had degraded the anisotropy, and the magnetism, respectively. The results suggested that these nanopillars were not suitable as candidates for prototypical bit information storage devices. Observations of Ni-Zn-Co spinel ferrite thin films in plan-view geometry indicated a multigrain magnetic domain structure and the magnetic fields were oriented in-plane only with no preferred magnetization distribution. This domain structure helps explain this ferrite's high permeability at high resonance frequency, which is an unusual character. Perpendicular magnetic anisotropy (PMA) of CoFe/Pd multilayers was revealed using holography. Detailed microscopic characterization showed structural factors such as layer waviness and interdiffusion that could contribute to degradation of the PMA. However, these factors are overwhelmed by the dominant effect of the CoFe layer thickness, and can be ignored when considering magnetic domain structure. / Dissertation/Thesis / Doctoral Dissertation Engineering 2015

In paleomagnetism, practical measurements are rarely made using perfect, isolated, single-phase, ferromagnetic crystals. Experimental observations are typically made using magnetic materials formed by a variety of natural processes. In this thesis, we will look at bridging the gap between current numerical modelling capability and experimental observations. First, we work towards micromagnetic modelling of multi-phase magnetic materials, including magnetostriction, embedded in a rocky matrix, along with crystal defects. We present a derivation of the Boundary Element Method formulation used by the micromagnetics package, MERRILL, and provide an extension of this from single-phase materials to multi-phase. After discussing issues with previous approaches to modelling magnetostriction, we derive and present a more robust and flexible approach. This model of magnetostriction is suitable for non-uniformmagnetizations, for multi-phase materials, and for arbitrary boundary conditions, and can be incorporated into MERRILL.We then outline a method for extending our model to materials embedded in an infinite elastic matrix of arbitrary elasticity. Finally, we present a method for modelling the magnetic response of a material due to crystal defects, along with a concrete example of a magneto-dislocation coupling energy at a magnetite-ilmenite boundary where stress due to lattice misfit is eased by regular edge dislocations. Second, we work towards being able to verify micromagnetic models against nano-scale experimental data. To do this, we present two techniques for simulating electron holograms from micromagnetic modelling results, a technique capable of imaging magnetic structures at the nano-scale. We also present example electron holograms of commonly occurring magnetic structures in nano-scale rock and mineral magnetism, and highlight some distinguishing features, which may be useful for interpreting experimental electron holography data.

<p>The work presented in this thesis is concerned with the microstructure of zinc oxide varistor materials used in surge protecting devices. This class of material has been characterised with special emphasis on the functional microstructure and the development of the microstructure during sintering. Several different techniques have been used for the analysis, especially scanning electron microscopy (SEM) in combination with electron beam-induced current (EBIC) analysis and <i>in-situ</i> studies of heat-treatment experiments and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectrometry (EDS) and electron holography. </p><p>Detailed TEM analyses using primarily centred dark-field imaging of grain boundaries, especially triple and multiple grain junctions, were used to reveal the morphological differences between the various Bi₂O₃ phases. The triple and multiple grain junctions were found to exhibit distinct differences in morphology, which could be attributed the difference in structure of the crystalline Bi₂O₃ polymorphs present in the junctions. </p><p>Electrical measurements were performed on individual ZnO/ZnO grain boundaries using EBIC in the SEM. The EBIC signal was found to depend strongly on the geometric properties of the interface and also on the symmetry of the depletion region at the interface. A symmetric double Schottky barrier was never observed in the experiments, but instead barriers with clear asymmetry in the depletion region. Experimental results together with computer simulations show that reasonably small differences in the deep donor concentrations between grains could be responsible for this effect.</p><p>Electron holography in the TEM was used to image the electrostatic potential variation across individual ZnO/ZnO interfaces. The sign of the interface charge, the barrier height (about 0.8 eV) and the depletion region width (100 to 150 nm) were determined from holography data. Asymmetries of the depletion region were also found with this technique. </p><p>The full sintering process of doped ZnO powder granules was studied <i>in-situ</i> in the environmental SEM. The densification and grain growth processes were studied through the sintering cycle. The formation of a functional microstructure in ZnO varistor materials was found to depend strongly on the total pressure.</p>