Matching Items (292)
Filtering by
- Resource Type: Text
Description
III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the progress of green emission light emitting devices has been limited by the incorporation of indium in the alloy, mainly due to phase separation. This difficulty could be addressed by studying the growth and thermodynamics of these alloys. Knowledge of thermodynamic phase stabilities and of pressure - temperature - composition phase diagrams is important for an understanding of the boundary conditions of a variety of growth techniques. In this dissertation a study of the phase separation of indium gallium nitride is conducted using a regular solution model of the ternary alloy system. Graphs of Gibbs free energy of mixing were produced for a range of temperatures. Binodal and spinodal decomposition curves show the stable and unstable regions of the alloy in equilibrium. The growth of gallium nitride and indium gallium nitride was attempted by the reaction of molten gallium - indium alloy with ammonia at atmospheric pressure. Characterization by X-ray diffraction, photoluminescence, and secondary electron microscopy show that the samples produced by this method contain only gallium nitride in the hexagonal phase. The instability of indium nitride at the temperatures required for activation of ammonia accounts for these results. The photoluminescence spectra show a correlation between the intensity of a broad green emission, related to native defects, and indium composition used in the molten alloy. A different growth method was used to grow two columnar-structured gallium nitride films using ammonium chloride and gallium as reactants and nitrogen and ammonia as carrier gasses. Investigation by X-ray diffraction and spatially-resolved cathodoluminescence shows the film grown at higher temperature to be primarily hexagonal with small quantities of cubic crystallites, while the one grown at lower temperature to be pure hexagonal. This was also confirmed by low temperature photoluminescence measurements. The results presented here show that cubic and hexagonal crystallites can coexist, with the cubic phase having a much sharper and stronger luminescence. Controlled growth of the cubic phase GaN crystallites can be of use for high efficiency light detecting and emitting devices. The ammonolysis of a precursor was used to grow InGaN powders with different indium composition. High purity hexagonal GaN and InN were obtained. XRD spectra showed complete phase separation for samples with x < 30%, with ~ 9% indium incorporation in the 30% sample. The presence of InGaN in this sample was confirmed by PL measurements, where luminescence from both GaN and InGaN band edge are observed. The growth of higher indium compositions samples proved to be difficult, with only the presence of InN in the sample. Nonetheless, by controlling parameters like temperature and time may lead to successful growth of this III-nitride alloy by this method.
ContributorsHill, Arlinda (Author) / Ponce, Fernando A. (Thesis advisor) / Chamberlin, Ralph V (Committee member) / Sankey, Otto F (Committee member) / Smith, David J. (Committee member) / Tsen, Kong-Thon (Committee member) / Arizona State University (Publisher)
Created2011
Description
The goal of this project was to design and create a genetic construct that would allow for <br/>tumor growth to be induced in the center of the wing imaginal disc of Drosophila larvae, the <br/>R85E08 domain, using a heat shock. The resulting transgene would be combined with other <br/>transgenes in a single fly that would allow for simultaneous expression of the oncogene and, in <br/>the surrounding cells, other genes of interest. This system would help establish Drosophila as a <br/>more versatile and reliable model organism for cancer research. Furthermore, pilot studies were <br/>performed, using elements of the final proposed system, to determine if tumor growth is possible <br/>in the center of the disc, which oncogene produces the best results, and if oncogene expression <br/>induced later in development causes tumor growth. Three different candidate genes were <br/>investigated: RasV12, PvrACT, and Avli.
ContributorsSt Peter, John Daniel (Author) / Harris, Rob (Thesis director) / Varsani, Arvind (Committee member) / School of Molecular Sciences (Contributor) / Department of Psychology (Contributor) / Barrett, The Honors College (Contributor)
Created2021-05
Description
Raman scattering from Ge-Si core-shell nanowires is investigated theoretically and experimentally. A theoretical model that makes it possible to extract quantitative strain information from the measured Raman spectra is presented for the first time. Geometrical and elastic simplifications are introduced to keep the model analytical, which facilitates comparison with experimental results. In particular, the nanowires are assumed to be cylindrical, and their elastic constants isotropic. The simple analytical model is subsequently validated by performing numerical calculations using realistic nanowire geometries and cubic, anisotropic elastic constants. The comparison confirms that the analytic model is an excellent approximation that greatly facilitates quantitative Raman work, with expected errors in the strain determination that do not exceed 10%. Experimental Raman spectra of a variety of core-shell nanowires are presented, and the strain in the nanowires is assessed using the models described above. It is found that all structures present a significant degree of strain relaxation relative to ideal, fully strained Ge-Si core-shell structures. The analytical models are modified to quantify this strain relaxation.
ContributorsSingh, Rachna (Author) / Menéndez, Jose (Thesis advisor) / Drucker, Jeffery (Committee member) / Ponce, Fernando (Committee member) / Tsen, Kong-Thon (Committee member) / Bennett, Peter (Committee member) / Arizona State University (Publisher)
Created2011
Description
Group III-nitride semiconductors have attracted much attention for applications on high brightness light-emitting diodes (LEDs) and laser diodes (LDs) operating in the visible and ultra-violet spectral range using indium gallium nitride in the active layer. However, the device efficiency in the green to red range is limited by quantum-confined Stark effects resulting from the lattice mismatch between GaN and InGaN. In this dissertation, the optical and micro-structural properties of GaN-based light emitting structures have been analyzed and correlated by utilizing cathodoluminescence and transmission electron microscopy techniques. In the first section, optimization of the design of GaN-based lasers diode structures is presented. The thermal strain present in the GaN underlayer grown on sapphire substrates causes a strain-induced wavelength shift. The insertion of an InGaN waveguide mitigates the mismatch strain at the interface between the InGaN quantum well and the GaN quantum barrier. The second section of the thesis presents a study of the characteristics of thick non-polar m-plane InGaN films and of LED structures containing InGaN quantum wells, which minimize polarization-related electric fields. It is found that in some cases the in-plane piezoelectric fields can still occur due to the existence of misfit dislocations which break the continuity of the film. In the final section, the optical and structural properties of InGaAlN quaternary alloys are analyzed and correlated. The composition of the components of the film is accurately determined by Rutherford backscattering spectroscopy.
ContributorsHuang, Yu (Author) / Ponce, Fernando A. (Thesis advisor) / Tsen, Kong-Thon (Committee member) / Treacy, Michael (Committee member) / Drucker, Jeffery (Committee member) / Culbertson, Robert (Committee member) / Arizona State University (Publisher)
Created2011
Raman spectroscopy characterization of anharmonicity and alloying effects in semiconductor materials
Description
The chemical sensitivity and spatial resolution of Raman spectroscopy, combined with the sensitivity of modern systems that can easily detect single atomic layers, have made this technique a preferred choice for the strain characterization of complex systems such as nanoscale complementary metal-oxide-semiconductor - CMOS - devices. A disadvantage of Raman spectroscopy, however, is that the shifts associated with strain are not related to the geometrical deformations in any obvious way, so that careful calibrations are needed to determine the anharmonic coefficients (p, q and r) that relate strain to Raman shifts. A new set of measurements of the Raman shift in strained Ge films grown on relaxed SiGe buffer layers deposited on Si substrates is presented, and thereby, a new consistent set of values for the parameters p and q for Ge has been proposed. In this dissertation the study of the vibrational properties of Ge1-xSnx alloys has also been reported. The temperature dependence of the Raman spectrum of Ge-rich Ge1-x Snx and Ge1-x-ySi xSny alloys has been determined in the 10 K - 450 K range. The Raman line shift and width changes as a function of temperature are found to be virtually identical to those observed in bulk Ge. This result shows that the anharmonic decay process responsible for the temperature dependence is extremely robust against the alloy perturbation.
ContributorsBagchi, Sampriti (Author) / Menéndez, Jose (Thesis advisor) / Treacy, Michael (Committee member) / Ponce, Fernando (Committee member) / Tsen, Kong-Thon (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)
Created2011
Description
The Cape Floral Region (CFR) in southwestern South Africa is one of the most diverse in the world, with >9,000 plant species, 70% of which are endemic, in an area of only ~90,000 km2. Many have suggested that the CFR's heterogeneous environment, with respect to landscape gradients, vegetation, rainfall, elevation, and soil fertility, is responsible for the origin and maintenance of this biodiversity. While studies have struggled to link species diversity with these features, no study has attempted to associate patterns of gene flow with environmental data to determine how CFR biodiversity evolves on different scales. Here, a molecular population genetic data is presented for a widespread CFR plant, Leucadendron salignum, across 51 locations with 5-kb of chloroplast (cpDNA) and 6-kb of unlinked nuclear (nuDNA) DNA sequences in a dataset of 305 individuals. In the cpDNA dataset, significant genetic structure was found to vary on temporal and spatial scales, separating Western and Eastern Capes - the latter of which appears to be recently derived from the former - with the highest diversity in the heart of the CFR in a central region. A second study applied a statistical model using vegetation and soil composition and found fine-scale genetic divergence is better explained by this landscape resistance model than a geographic distance model. Finally, a third analysis contrasted cpDNA and nuDNA datasets, and revealed very little geographic structure in the latter, suggesting that seed and pollen dispersal can have different evolutionary genetic histories of gene flow on even small CFR scales. These three studies together caution that different genomic markers need to be considered when modeling the geographic and temporal origin of CFR groups. From a greater perspective, the results here are consistent with the hypothesis that landscape heterogeneity is one driving influence in limiting gene flow across the CFR that can lead to species diversity on fine-scales. Nonetheless, while this pattern may be true of the widespread L. salignum, the extension of this approach is now warranted for other CFR species with varying ranges and dispersal mechanisms to determine how universal these patterns of landscape genetic diversity are.
ContributorsTassone, Erica (Author) / Verrelli, Brian C (Thesis advisor) / Dowling, Thomas (Committee member) / Cartwright, Reed (Committee member) / Rosenberg, Michael S. (Committee member) / Wojciechowski, Martin (Committee member) / Arizona State University (Publisher)
Created2013
Description
Group III-nitride semiconductors have wide application in optoelectronic devices. Spontaneous and piezoelectric polarization effects have been found to be critical for electric and optical properties of group III-nitrides. In this dissertation, firstly, the crystal orientation dependence of the polarization is calculated and in-plane polarization is revealed. The in-plane polarization is sensitive to the lateral characteristic dimension determined by the microstructure. Specific semi-polar plane growth is suggested for reducing quantum-confined Stark effect. The macroscopic electrostatic field from the polarization discontinuity in the heterostructures is discussed, b ased on that, the band diagram of InGaN/GaN quantum well/barrier and AlGaN/GaN heterojunction is obtained from the self-consistent solution of Schrodinger and Poisson equations. New device design such as triangular quantum well with the quenched polarization field is proposed. Electron holography in the transmission electron microscopy is used to examine the electrostatic potential under polarization effects. The measured potential energy profiles of heterostructure are compared with the band simulation, and evidences of two-dimensional hole gas (2DHG) in a wurtzite AlGaN/ AlN/ GaN superlattice, as well as quasi two-dimensional electron gas (2DEG) in a zinc-blende AlGaN/GaN are found. The large polarization discontinuity of AlN/GaN is the main source of the 2DHG of wurtzite nitrides, while the impurity introduced during the growth of AlGaN layer provides the donor states that to a great extent balance the free electrons in zinc-blende nitrides. It is also found that the quasi-2DEG concentration in zinc-blende AlGaN/GaN is about one order of magnitude lower than the wurtzite AlGaN/GaN, due to the absence of polarization. Finally, the InAlN/GaN lattice-matched epitaxy, which ideally has a zero piezoelectric polarization and strong spontaneous polarization, is experimentally studied. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1-xN layer. The V-grooves coalesce at 200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. The structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.
ContributorsWei, Qiyuan (Author) / Ponce, Fernando A. (Thesis advisor) / Tsen, Kong-Thon (Committee member) / Shumway, John (Committee member) / Menéndez, Jose (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
Triops (Branchiopoda: Notostraca) and Streptocephalus (Branchiopoda: Anostraca) are two crustaceans which cohabitate in ephemeral freshwater pools. They both lay desiccation resistant eggs that disperse passively to new hydrologically isolated environments. The extent of genetic distance among regions and populations is of perennial interest in animals that live in such isolated habitats. Populations in six natural ephemeral pool habitats located in two different regions of the Sonoran Desert and a transition area between the Sonoran and Chihuahuan Deserts were sampled. Sequences from Genbank were used for reference points in the determination of species as well as to further identify regional genetic distance within species. This study estimated the amount of within and between genetic distance of individuals from each region and population through the use of a neutral marker, cytochrome oxidase I (COI). We concluded that, although the method of passive dispersal may differ between the two genera, the differences do not results in different patterns of genetic distances between regions and populations. Furthermore, we only found the putative species, Triops longicaudatus "short", with enough distinct speciation. Although Triops longicaudatus "long" and Triops newberryi may be in the early stages of speciation, this study does not find enough support to conclude that they have separated.
ContributorsMurphy Jr., Patrick Joseph (Author) / Rutowski, Ronald (Thesis director) / Cartwright, Reed (Committee member) / Lessios, Nikos (Committee member) / School of Life Sciences (Contributor) / School of Human Evolution and Social Change (Contributor) / Barrett, The Honors College (Contributor)
Created2016-05
Description
The evolution of blindness in cave animals has been heavily studied; however, little research has been done on the interaction of migration and drift on the development of blindness in these populations. In this study, a model is used to compare the effect that genetic drift has on the fixation of a blindness allele for varying amounts of migration and selection. For populations where the initial frequency is quite low, genetic drift plays a much larger role in the fixation of blindness than populations where the initial frequency is high. In populations where the initial frequency is high, genetic drift plays almost no role in fixation. Our results suggest that migration plays a greater role in the fate of the blindness allele than selection.
ContributorsMerry, Alexandra Leigh (Author) / Cartwright, Reed (Thesis director) / Rosenberg, Michael (Committee member) / Schwartz, Rachel (Committee member) / Barrett, The Honors College (Contributor) / School of Life Sciences (Contributor)
Created2014-05
Description
Identifying disease biomarkers may aid in the early detection of breast cancer and improve patient outcomes. Recent evidence suggests that tumors are immunogenic and therefore patients may launch an autoantibody response to tumor associated antigens. Single-chain variable fragments of autoantibodies derived from regional lymph node B cells of breast cancer patients were used to discover these tumor associated biomarkers on protein microarrays. Six candidate biomarkers were discovered from 22 heavy chain-only variable region antibody fragments screened. Validation tests are necessary to confirm the tumorgenicity of these antigens. However, the use of single-chain variable autoantibody fragments presents a novel platform for diagnostics and cancer therapeutics.
ContributorsSharman, M. Camila (Author) / Magee, Dewey (Mitch) (Thesis director) / Wallstrom, Garrick (Committee member) / Petritis, Brianne (Committee member) / Barrett, The Honors College (Contributor) / College of Liberal Arts and Sciences (Contributor) / Virginia G. Piper Center for Personalized Diagnostics (Contributor) / Biodesign Institute (Contributor)
Created2012-12