Browsing by Subject "Defects"
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Item Atomic Scale Details of Defect-Boundary Interactions(2014-12-18) Chen, DiThe study is aimed to understand atomic scale details of defect-boundary interactions, which are critical to develop radiation tolerant fuel cladding materials for harsher neutron environments. By means of molecular dynamics simulations, we addressed the key questions of (1) how defects are trapped by a grain boundary, (2) how defect are annihilated at a grain boundary, and (3) what are upper limits of radiation tolerance of boundary-engineered metals. The modeling is performed by using large-scale atomic/molecular massively parallel simulator (LAMMPS) code and pure Fe is selected as the model material. For mechanism of defect tapping towards a grain boundary, we find that, instead of the general consensus that the trapping is caused by biased defect diffusions due to relatively lower defect formation energies at a grain boundary, long range defect migration is realized by creation of chain like defects. A chain is induced by the stress field around a defect, and is formed by pushing its immediate lattice atom neighbor into an interstitial site. This newly formed interstitial can induce formation of another vacancy-interstitial pair along the chain direction. The process is repeated or simultaneously occurs along the chain. Thus, a chain consists of alternately positioned interstitials and vacancies. The subsequent defect annihilation between neighboring defects on the chain leads to the defect transport. We identify three types of defect transport models which involve different chains. For mechanism of defect annihilation on a grain boundary, we find that both defect transport and interstitial-vacancy recombination are realized by formation of similar chain-like defects. The vacancy and interstitial along the chain correspond to the sites of their corresponding formation energy minima, thus the capability to form such chains is determined by the patterns of boundary defect formation energies. For a boundary of small misorientation angle, chain formation is allowed to occur in one direction only and all chains are parallel to each other. At large angles, however, chains are so close to each other that new allowable chain directions are created by linking patterns from different chains. This suggests that large angle boundaries are more efficient to move and recombine defects. The modeling further calculates the energy barriers for chain-mediated defect recombination under different boundary configurations. These findings lead to the conclusion that defect sink strengths of grain boundaries are determined by not only the efficiency to transport to boudnaries, but also the efficiency to recombine boundary defects. Otherwise, the difficulty to remove defects will quickly turn of the sink property. This is confirmed by comparing the width of defect denuded zone created around a boundary, in a cell randomly bombarded by Fe self-ions to different damage levels. A large angle boundary is more preferred to achieve maximum radiation tolerance.Item Data-mining the Ubuntu Linux Distribution for bug analysis and resolution(2012-08) Arges, Christopher John; Stewart, Kate; Ghosh, JoydeepThe Ubuntu Linux Distribution represents a massive investment of time and human effort to produce a reliable computing experience for users. To accomplish these goals, software bugs must be tracked and fixed. However, as the number of users increase and bug reports grow advanced tools such as data mining must be used to increase the effectiveness of all contributors to the project. Thus, this report involved collecting a large amount of bug reports into a database and calculating relevant statistics. Because of the diversity and quantity of bug reports, contributors must find which bugs are most relevant and important to work on. One study in this report created an automatic way to determine who is best fit to solve a particular bug by using classification techniques. In addition, this report explores how to initially classify if a bug report will be eventually marked invalid or not.Item MRI volumetric analysis of the Anterior Cingulate in families with and without a reading disorder(2008-08) Wellington, Tasha McMahon; Semrud-Clikeman, Margaret; Carlson, Caryn L.The current study is the first to demonstrate that structural deficits in the Anterior Cingulate Cortex (ACC) of the human brain may play a role in reading ability. Recent imaging work has indicated that the ACC is activated by tasks involving modulation of the fronto-temporal networks during language processing tasks and may be involved in anticipatory reactions and response preparation during reading. This study investigated the relationship between ACC volumetric measurements and reading ability in a sample of 68 individuals nested within 24 families with and without reading disorders. This sample allowed for examination of the effect of the volume of the ACC on reading, while controlling for normally occurring fluctuations in the size of the ACC due to heredity and shared environment. Forty-five linear models were conducted in SPSS on all 68 participants using the brain measurements (ACC, ACC with Paracingulate (PaC), and Putamen, separately) as well as control variables (gender, FSIQ, family membership) as predictors of the outcomes variables related to reading achievement (GORT Passage, rate, and accuracy) and reading processes (CTOPP phonological awareness and rapid naming). The use of family membership as a random effect predictor together with the specific brain volume as a predictor allowed for the effect of family on reading outcomes to be accounted for while, explicitly accounting for any relationships that may exist between family and brain volume. Additional sets of measurements, with PaC, were included in the final analyses to address the inconsistent inclusion of this tertiary structure in earlier research. Finally, a control region (putamen) was included to rule out whole brain effects and improve the specificity of the findings. The most significant findings were that the results varied systematically with inclusion or exclusion of the PaC. Measurements including the PaC were statistically significant for reading achievement for the left side of the ACC as expected. However, for the ACC volume without PaC, it was the right side that was related to reading measures. Neither set of measurements of the ACC were predictive of group membership. The current study supported a role for the ACC in reading and suggests a standardized method for inclusion of the PaC in the volumetric analysis of the ACC.Item Optical characterization of high-[Kappa] dielectric structures(2009-12) Price, James Martin, 1980-; Downer, Michael Coffin; Demkov, Alexander A.; Fink, Manfred; Ekerdt, John G.; Shih, Chih-KangCharge trapping dynamics in Si/SiO2/Hf(1-x)SixO2 and III-V film stack systems are characterized using spectroscopic ellipsometry (SE) and second harmonic generation (SHG). For the first time, discrete absorption features within the bandgap of the SiO2 interfacial layer are identified using SE, and their relation to both intrinsic and process-induced defects is proposed. Sensitivity of the absorption features to process conditions is demonstrated and evidence that these defects contribute to Vfb roll-off is presented. Defects in the Hf(1-x)SixO2 films are probed with fs laser-induced internal multi-photon photo-excitation (IMPE) and time dependent electrostatic field induced second harmonic (TD-EFISH) generation. For the as deposited HfO2 films, a unique TD-EFISH response is identified and explained by resonant two photon ionization of a specific point defect and subsequent tunneling of the photoelectrons to the Si substrate. Charge trapping kinetics for all Hf(1-x)SixO2 films are investigated. Two characteristic trap cross sections are identified and found to be insensitive to dielectric film and process conditions, and associated with a surface “harpooning” mechanism. EFISH from non-centrosymmetric III-V media, including GaAs and In0.53Ga0.47As, is also studied. The anisotropic and time dependent SHG response from different chemically treated In0.53Ga0.47As surfaces is clearly distinguishable and associated with a process-induced change in the surface depletion field.Item Scanning tunneling microscopy of Bi₂Se₃ and CuxBi₂Se₃(2013-08) Mann, Christopher William; Shih, Chih-KangRecently, Bi₂Se₃ was added to a new class of materials known as topological insulators. While several studies have provided tantalizing hints towards novel physical properties, such as backscatter suppression and spin-polarized transport, several concerns remain in actual materials. In particular, high defect densities, strong surface band bending, and potential fluctuations have been observed. Here, scanning tunneling microscopy and spectroscopy are used to reveal surface effects in Bi₂Se₃ and CuxBi₂Se₃. First, a detailed examination of defects in bulk-grown samples is described. Then, I provide an analysis of molecular beam epitaxy results, done in collaboration with colleague Yuxuan Chen. Following this, I provide a detailed study of individual point defects in Cu-doped Bi₂Se₃ and examine how Cu is incorporated into the Bi₂Se₃ lattice. Finally, through spectroscopic analysis, a novel depth-sensitive measurement of the local band bending field is developed. Furthermore, for the first time, fluctuations of the Dirac point can be correlated to specific near-surface defects, namely Se vacancies. These analyses provide valuable insights into the preparation of future samples for the investigation of topological insulators.Item A study of the ferroelectric properties of neutron irradiated lead zirconate titanate(2013-08) Graham, Joseph Turner; Landsberger, Sheldon; Ferreira, Paulo J. (Paulo Jorge)Lead zirconate titantate (PZT) is an electroceramic material with many important technological applications in sensing and computer memory. Some of these applications require the PZT based devices to operate in radiation fields where they will be exposed to a high flux of energetic, heavy and light, charged and uncharged particles. The risk to any device exposed to ionizing radiation is the accumulation of displacement and ionization damage. Significant damage accumulation over time can lead to property drifts and, in some cases, failure of the device to perform properly. The goal of the undertaking recounted in this dissertation was to study changes in the ferroelectric properties of PZT exposed to the neutron field of a research nuclear reactor and to help develop an understanding for the type of radiation induced defects that play a dominant role in the degradation process. Thin film PZT capacitors were prepared using a wet chemical technique. The capacitors were then irradiated in a 1 MW TRIGA research nuclear reactor at the University of Texas at Austin up to a maximum 1 MeV equivalent neutron flux of 5.2 x 10¹⁵ cm⁻². Following irradiation, electronic characterization of polarization-electric field hysteresis loops, first order reversal curves, and small-signal permittivity were performed to ascertain tendencies between irradiation dose and ferroelectric properties. The measurements indicate a drop in remanent polarization, a loss of domain wall mobility, shifts in local switching fields and the formation of dipolar defects. These effects are all attributed to the introduction of defects into the material through displacement damage cascades. Numerical models of the damage cascades were performed to determine the displacement concentration. Comparison of those values and the primary recoil spectrum with typical survival rates found in the literature suggest that both free point defects as well as defect clusters are produced in comparable if not larger concentrations. It is proposed that defect clusters play a more significant role in ferroelectric property change than previously believed.