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In a hypothetical Grand Unified Theory, magnetic monopoles are a particle which would act as a charge carrier for the magnetic force. Evidence of magnetic monopoles has yet to be found and based off of their relatively high mass (4-10 TeV) will be difficult to find with current technology. The goal of my thesis is to mathematically model the magnetic monopole by finding numerical solutions to the equations of motion. In my analysis, I consider four cases: kinks, cosmic strings, global monopoles, and magnetic monopoles. I will also study electromagnetic gauge fields to prepare to include gauge fields in the magnetic monopole case. Numerical solutions are found for the cosmic string and global monopole cases. As expected, the energy is high at small distance r and drops off as r goes to infinity. Currently numerical solutions are being worked towards for electromagnetic gauge fields and the magnetic monopole case.
The effects of different growth conditions on the structural quality were thoroughly investigated. Lattice-matched condition was successfully achieved and material of exceptional quality was demonstrated.
After growth optimization had been achieved, structural defects could hardly be detected, so different characterization techniques, including etch-pit-density (EPD) measurements, cathodoluminescence (CL) imaging and X-ray topography (XRT), were explored, in attempting to gain better knowledge of the sparsely distributed defects. EPD revealed the distribution of dislocation-associated pits across the wafer. Unfortunately, the lack of contrast in images obtained by CL imaging and XRT indicated their inability to provide any quantitative information about defect density in these InAs/InAsSb T2SLs.
The nBn photodetectors based on mid-wave infrared (MWIR) and long-wave infrared (LWIR) InAs/InAsSb T2SLs were fabricated. The significant difference in Ga composition in the barrier layer coupled with different dark current behavior, suggested the possibility of different types of band alignment between the barrier layers and the absorbers. A positive charge density of 1.8 × 1017/cm3 in the barrier of MWIR nBn photodetector, as determined by electron holography, confirmed the presence of a potential well in its valence band, thus identifying type-II alignment. In contrast, the LWIR nBn photodetector was shown to have type-I alignment because no sign of positive charge was detected in its barrier.
Capacitance-voltage measurements were performed to investigate the temperature dependence of carrier densities in a metal-oxide-semiconductor (MOS) structure based on MWIR InAs/InAsSb T2SLs, and a nBn structure based on LWIR InAs/InAsSb T2SLs. No carrier freeze-out was observed in either sample, indicating very shallow donor levels. The decrease in carrier density when temperature increased was attributed to the increased density of holes that had been thermally excited from localized states near the oxide/semiconductor interface in the MOS sample. No deep-level traps were revealed in deep-level transient spectroscopy temperature scans.
The 21cm emission from the hyperfine splitting of neutral Hydrogen in the early universe is predicted to provide precise information about the formation and evolution of cosmic structure, complementing the wealth of knowledge gained from the CMB.
21cm cosmology is a relatively new field, and precise measurements of the Epoch of Reionization (EoR) have not yet been achieved. In this work I present 2σ upper limits on the power spectrum of 21cm fluctuations (Δ²(k)) probed at the cosmological wave number k from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) 64 element deployment. I find upper limits on Δ²(k) in the range 0.3 < k < 0.6 h/Mpc to be (650 mK)², (450 mK)², (390 mK)², (250 mK)², (280mK)², (250 mK)² at redshifts z = 10.87, 9.93, 8.91, 8.37, 8.13 and 7.48 respectively
Building on the power spectrum analysis, I identify a major limiting factor in detecting the 21cm power spectrum.
This work is concluded by outlining a metric to evaluate the predisposition of redshifted 21cm interferometers to foreground contamination in power spectrum estimation. This will help inform the construction of future arrays and enable high fidelity imaging and
cross-correlation analysis with other high redshift cosmic probes like the CMB and other upcoming all sky surveys. I find future
arrays with uniform (u,v) coverage and small spectral evolution of their response in the (u,v,f) cube can minimize foreground leakage while pursuing 21cm imaging.
A microstructural study of tin selenide and tin manganese selenide thin films grown by molecular beam epitaxy (MBE) on GaAs (111)B substrates with different Se:Sn flux ratios and Mn concentrations was carried out. Low flux ratios lead to highly defective films, mostly consisting of SnSe, whereas higher flux ratios gave higher quality, single-phase SnSe2. The ternary (Sn,Mn)Se films evolved quasi-coherently, as the Mn concentration increased, from SnSe2 into a complex lattice, and then into MnSe with 3D rock-salt structure. These structural transformations should underlie the evolution of magnetic properties of this ternary system reported earlier in the literature.
II-VI/III-V compound semiconductor heterostructures have been characterized for growth in both single- and dual-chamber MBE systems. Three groups of lattice-matched materials have been investigated: i) 5.65Å materials based on GaAs, ii) 6.1Å materials based on InAs or GaSb, and iii) 6.5Å materials based on InSb. High quality II-VI materials grown on III-V substrates were demonstrated for ZnTe/GaSb and CdTe/InSb. III-V materials grown on II-VI buffer layers present additional challenges and were grown with varying degrees of success. InAsSb quantum wells in between ZnTe barriers were nearly defect-free, but showed 3D island growth. All other materials demonstrated flat interfaces, despite low growth temperature, but with stacking faults in the II-VI materials.
Femtosecond laser-induced defects (LIDs) in silicon solar cells were characterized using a variety of electron microscopy techniques. Scanning electron microscope (SEM) images showed that the intersections of laser lines, finger and busbar intersections, exhibited LIDs with the potential to shunt the contacts. SEM and transmission electron microscope (TEM) images correlated these LIDs with ablated c-Si and showed these defects to come in two sizes ~40nm and ~.5µm. The elemental profiles across defective and non-defective regions were found using energy dispersive x-ray spectroscopy.