Browsing by Subject "Defects"
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Item CuInSe₂ nanocrystal photovoltaics : device physics, defects, and ligand chemistry(2019-09-24) Houck, Daniel William; Korgel, Brian Allan, 1969-; Ekerdt, John G; Milliron, Delia; Vandenbout, David ANanocrystal based photovoltaics (PVs) have the potential to be printed at room temperature on a variety of substrates with the performance and stability required to enable low-cost, lightweight portable power applications. CuInSe₂ nanocrystal (PVs), fabricated using methods like those developed for polycrystalline CuIn [subscript x] Ga [subscript 1-x] Se₂ (CIGS) PVs, have exhibited some success in this direction, but the efficiency of the devices has been limited. Simulations of CuInSe₂ nanocrystal photovoltaic devices using Solar Cell Capacitance Simulator (SCAPS) software indicate that higher performance is not likely to be achieved though ligand exchange procedures that influence the nanocrystals’ electron affinity, as reported for PbS nanocrystal photovoltaics. Higher PV performance requires a combination of increased carrier mobility and longer carrier lifetimes. Both are influenced by the capping ligand chemistry and the presence of defects in the nanocrystals. The nanocrystal synthesis conditions are critical for controlling the defects and phase of the nanocrystal products. X-ray diffraction (XRD) shows that CuInSe₂ tends to form the tetragonal chalcopyrite structure while under fast nucleation conditions CuIn [subscript x] Ga [subscript 1-x] Se₂ nanocrystals form the hexagonal wurtzite phase. Raman spectroscopy has shown that the CuInSe₂ nanocrystals contain a significant concentration of InC [subscript u]+2V [subscript Cu] pair defects that depends on the synthesis conditions. These pair defects tend to order under light excitation and can be eliminated by high temperature annealing. Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy indicates that the oleylamine and diphenylphosphine capping ligands used in a typical CuInSe₂ nanocrystal synthesis bind strongly to the nanocrystal surface and exhibit very limited desorption. The addition of I₂ induces ligand desorption and facilitates ligand exchange on these nanocrystals. Furthermore, the tightly bound ligands do not evaporate when heated under inert conditions, like the free molecule would. The bound ligands are found to form disordered sp² carbon that vaporizes over tens of minutes. These studies provide insight for improving the performance of CuInSe₂ nanocrystal photovoltaics by showing that the ligand chemistry and defects in CuInSe₂ are relatively unique compared to other types of nanocrystalsItem 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 Effect of Defects on the Mechanical Properties of Laser Powder Bed Fused Ti-6Al-4V(2022) Muhammad, Muztahid; Shao, Shuai; Shamsaei, NimaProcess-induced volumetric defects are inherent to additively manufactured parts. This study investigates the effect of volumetric defects on the tensile properties of the laser powder bed fused (L- PBF) Ti-6Al-4V specimens fabricated with large variations in process parameters (a total of six sets of process parameters). Cylindrical rods of L-PBF Ti-6Al-4V specimens were stress-relieved before removal from plates and machined to tensile specimens. The defect distribution of specimens resulting from each set of process parameters was analyzed using a high-resolution X-ray computed tomography machine. Quasi-static tensile tests were performed at room temperature using a servo-hydraulic MTS machine. Tensile results were correlated with defect statistics. No apparent difference was observed in the yield strength of the L-PBF Ti-6Al-4V specimens despite the large variations in the process parameters resulting in significant differences in defect content. However, a considerable drop in ductility was observed for the specimens fabricated with insufficient energy.Item Engineering design quality defects : causal factors, interactions between causal factors, and mitigation strategies(2024-02-05) Woo, Jeyoung; O'Connor, James Thomas; Bagchi, Uttarayan; Caldas, Carlos H; Leite, Fernanda LEngineering design quality defects in engineering design deliverables have negative impacts on overall project performance. The author’s previous study identified the 11 most problematic engineering design deliverables, 73 significant engineering design defects with their 351 causal factors, and 266 completeness inclusion items for the 11 problematic engineering design deliverables. However, in-depth analysis of the causal factors for the engineering design defects could significantly enhance overall project performance and reduce design changes or construction phase rework. While previous studies have identified the causal factors and assessed the negative impacts of engineering design defects in terms of cost growth, engineering design defects have remained an unsolved problem. As an in-depth extension of a previous study, this dissertation compiles three journal article manuscripts that address major issues regarding engineering design quality management. These articles stand as the three chapters of the dissertation. The objective of Chapter 2 is to explore the causality of engineering design quality defects by identifying associations between causal factors and their impacts in terms of relative frequency, relative impact severity, and defect impact types. It also explains how these metrics form the basis for calculating priority scores for the causal factors. This study shows that human resource-related factors are associated with higher priority scores. The objective of Chapter 3 is to identify all two-causal-factor pairings for the 11 most problematic engineering design deliverables, to characterize the interactions between these causal factors and to assess associated impacts. The objective of Chapter 4 is to identify mitigation strategies that can be implemented easily and effectively to mitigate engineering design defects. The chapter presents 40 mitigation strategies and assesses survey respondents' opinions on the expected ease of implementation and on expected mitigation effectiveness. From the analysis, the author identified both deliverables and defects for which mitigation strategies are the easiest to implement and the most effective. This study contributes to both the body of knowledge and to practice in engineering design quality management. This contribution allows project practitioners to better understand the causality of engineering design defects, and to prioritize causal factors based on the causal factor pairings. Moreover, the research identified a set of deliverable defects that can be mitigated. The information and mitigation strategies developed through this research will help construction project stakeholders eliminate or reduce engineering design defects.Item Investigating the effect of defects on the crack initiation of additively manufactured IN718 using crystal plasticity simulations(2022) Nandi, Indrajit; Shamsaei, Nima; Shao, ShuaiThis study aims to analyze the effect of defects location on the fatigue behavior of additively manufactured IN718. A competing mechanism exists between persistent slip bands (PSBs) and volumetric defects on the initiation of fatigue cracks in AM IN718 under cyclic loading. Crystal plasticity simulations were performed to reveal the relative importance in crack initiation due to defects and PSBs. Cyclic loading was applied on a defect laden polycrystalline aggregate and the defect locations were systematically varied. The crack nucleation cycles and locations were captured using a strain contrast-based crack initiation criterion. The findings suggest that the presence of large defects affects the cyclic strain localization and crack initiation behavior of the AM IN718 material.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 Strain tuning of thermal, electrical and optical properties of semiconductors(2019-12-09) Meng, Xianghai; Wang, Yaguo; Lin, Jung-Fu; Shi, Li; Zhou, Jianshi; Lu, NanshuThe discovery of graphene by mechanical exfoliation has opened a new realm of research. Compared to traditional 3D crystal structures, 2D materials are characterized by strong in-plane covalent bond and weak interlayer van der Waals force, giving them unique 2D crystal structure. In the past ten years, various 2D materials have been explored with very different electronic properties, ranging from wide band-gap insulators to conductors. Owing to the different bond strength, 2D materials behave differently in their electrical, thermal properties along the cross-plane and in-plane direction. In addition, the in-plane electrical, optical and thermal properties are also found to be anisotropic for some particular 2D materials due to the asymmetric crystal structure. Both the in-plane and cross-plane anisotropic properties of 2D materials give rise to a possibility to design the micro/nano devices in various applications. The intrinsic properties of TMDs can be further adjusted by external factors, such as electrical fields, temperature, magnetic field, et al. Among all the external stimulations, strain has been shown an effective method to control the electronic, thermal, optical properties of semiconductors. With the discovery of 2D materials, the application of strain tuning has been growing since the reduced dimensional structures can sustain much larger strains than bulk crystals. In this dissertation, in-plane anisotropic nonlinear optical nonlinearity is studies with an Intensity-scan spectroscopy at ambient conditions. Then a diamond anvil cell (DAC) device is employed to generate large strain on MoS₂. With our home-built pico-second Transient Thermoreflectance technique, ~7x enhancement in cross-plane is observed due to the pressure/strain modified interlayer interaction. Moreover, photoluminescence and Raman spectroscopy are used to probe the impurity levels in BAs crystal. Pressure/Strain modified impurity level change will also have significant effect on this high thermal conductivity material. Lastly, a modified pico-second Transient Thermoreflectance system is developed to achieve simultaneous measurement on thermal conductivity and specific heat of materials.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.