Matching Items (54)
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- All Subjects: Materials Science
- Creators: Jiao, Yang
- Member of: ASU Electronic Theses and Dissertations
Description
Amorphous oxide semiconductors are promising new materials for various optoelectronic applications. In this study, improved electrical and optical properties upon thermal and microwave processing of mixed-oxide semiconductors are reported. First, arsenic-doped silicon was used as a model system to understand susceptor-assisted microwave annealing. Mixed oxide semiconductor films of indium zinc oxide (IZO) and indium gallium zinc oxide (IGZO) were deposited by room-temperature RF sputtering on flexible polymer substrates. Thermal annealing in different environments - air, vacuum and oxygen was done. Electrical and optical characterization was carried out before and after annealing. The degree of reversal in the degradation in electrical properties of the thin films upon annealing in oxygen was assessed by subjecting samples to subsequent vacuum anneals. To further increase the conductivity of the IGZO films, Ag layers of various thicknesses were embedded between two IGZO layers. Optical performance of the multilayer structures was improved by susceptor-assisted microwave annealing and furnace-annealing in oxygen environment without compromising on their electrical conductivity. The post-processing of the films in different environments was used to develop an understanding of mechanisms of carrier generation, transport and optical absorption. This study establishes IGZO as a viable transparent conductor, which can be deposited at room-temperature and processed by thermal and microwave annealing to improve electrical and optical performance for applications in flexible electronics and optoelectronics.
ContributorsGadre, Mandar (Author) / Alford, Terry L. (Thesis advisor) / Schroder, Dieter (Committee member) / Krause, Stephen (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2011
Description
This thesis discusses the use of low temperature microwave anneal as an alternative technique to recrystallize materials damaged or amorphized due to implantation techniques. The work focuses on the annealing of high-Z doped Si wafers that are incapable of attaining high temperatures required for recrystallizing the damaged implanted layers by microwave absorption The increasing necessity for quicker and more efficient processing techniques motivates study of the use of a single frequency applicator microwave cavity along with a Fe2O3 infused SiC-alumina susceptor/applicator as an alternative post implantation process. Arsenic implanted Si samples of different dopant concentrations and implantation energies were studied pre and post microwave annealing. A set of as-implanted Si samples were also used to assess the effect of inactive dopants against presence of electrically active dopants on the recrystallization mechanisms. The extent of damage repair and Si recrystallization of the damage caused by arsenic and Si implantation of Si is determined by cross-section transmission electron microscopy and Raman spectroscopy. Dopant activation is evaluated for the As implanted Si by sheet resistance measurements. For the same, secondary ion mass spectroscopy analysis is used to compare the extent of diffusion that results from such microwave annealing with that experienced when using conventional rapid thermal annealing (RTA). Results show that compared to susceptor assisted microwave annealing, RTA caused undesired dopant diffusion. The SiC-alumina susceptor plays a predominant role in supplying heat to the Si substrate, and acts as an assistor that helps a high-Z dopant like arsenic to absorb the microwave energy using a microwave loss mechanism which is a combination of ionic and dipole losses. Comparisons of annealing of the samples were done with and without the use of the susceptor, and confirm the role played by the susceptor, since the samples donot recrystallize when the surface heating mechanism provided by the susceptor is not incorporated. Variable frequency microwave annealing was also performed over the as-implanted Si samples for durations and temperatures higher than the single frequency microwave anneal, but only partial recrystallization of the damaged layer was achieved.
ContributorsVemuri, Rajitha (Author) / Alford, Terry L. (Thesis advisor) / Theodore, David (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2011
Description
Microwave (MW), thermal, and ultraviolet (UV) annealing were used to explore the response of Ag structures on a Ge-Se chalcogenide glass (ChG) thin film as flexible radiation sensors, and Te-Ti chalcogenide thin films as a material for diffusion barriers in microelectronics devices and processing of metallized Cu. Flexible resistive radiation sensors consisting of Ag electrodes on a Ge20Se80 ChG thin film and polyethylene naphthalate substrate were exposed to UV radiation. The sensors were mounted on PVC tubes of varying radii to induce bending strains and annealed under ambient conditions up to 150 oC. Initial sensor resistance was measured to be ~1012 Ω; after exposure to UV radiation, the resistance was ~104 Ω. Bending strain and low temperature annealing had no significant effect on the resistance of the sensors. Samples of Cu on Te-Ti thin films were annealed in vacuum for up to 30 minutes and were stable up to 500 oC as revealed using Rutherford backscattering spectrometry (RBS) and four-point-probe analysis. X-ray diffractometry (XRD) indicates Cu grain growth up to 500 oC and phase instability of the Te-Ti barrier at 600 oC. MW processing was performed in a 2.45-GHz microwave cavity on Cu/Te-Ti films for up to 30 seconds to induce oxide growth. Using a calibrated pyrometer above the sample, the temperature of the MW process was measured to be below a maximum of 186 oC. Four-point-probe analysis shows an increase in resistance with an increase in MW time. XRD indicates growth of CuO on the sample surface. RBS suggests oxidation throughout the Te-Ti film. Additional samples were exposed to 907 J/cm2 UV radiation in order to ensure other possible electromagnetically induced mechanisms were not active. There were no changes observed using XRD, RBS or four point probing.
ContributorsRoos, Benjamin, 1990- (Author) / Alford, Terry L. (Thesis advisor) / Theodore, David (Committee member) / Kozicki, Michael (Committee member) / Arizona State University (Publisher)
Created2013
Description
Rapid processing and reduced end-of-range diffusion effects demonstrate that susceptor-assisted microwave annealing is an efficient processing alternative for electrically activating dopants and removing ion-implantation damage in ion-implanted semiconductors. Sheet resistance and Hall measurements provide evidence of electrical activation. Raman spectroscopy and ion channeling analysis monitor the extent of ion implantation damage and recrystallization. The presence of damage and defects in ion implanted silicon, and the reduction of the defects as a result of annealing, is observed by Rutherford backscattering spectrometry, moreover, the boron implanted silicon is further investigated by cross-section transmission electron microscopy. When annealing B+ implanted silicon, the dissolution of small extended defects and growth of large extended defects result in reduced crystalline quality that hinders the electrical activation process. Compared to B+ implanted silicon, phosphorus implanted samples experience more effective activation and achieve better crystalline quality. Comparison of end-of-range dopants diffusion resulting from microwave annealing and rapid thermal annealing (RTA) is done using secondary ion mass spectroscopy. Results from microwave annealed P+ implanted samples show that almost no diffusion occurs during time periods required for complete dopant activation and silicon recrystallization. The relative contributions to heating of the sample, by a SiC susceptor, and by Si self-heating in the microwave anneal, were also investigated. At first 20s, the main contributor to the sample's temperature rise is Si self-heating by microwave absorption.
ContributorsZhao, Zhao (Author) / Alford, Terry Lynn (Thesis advisor) / Theodore, David (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2013
Description
This work focuses on simulation of electrical resistivity and optical behaviors of thin films, where an Ag or Au thin layer is embedded in zinc oxide. Enhanced conductivity and transparency were earlier achieved with multilayer structured transparent conducting oxide (TCO) sandwich layer with metal (TCO/metal/TCO). Sputtering pattern of metal layer is simulated to obtain the morphology, covered area fraction, and the percolation strength. The resistivity as a function of the metal layer thickness fits the modeled trend of covered area fraction beyond the percolation threshold. This result not only presents the robustness of the simulation, but also demonstrates the influence of metal morphology in multilayer structure. Effective medium coefficients are defined from the coverage and percolation strength to obtain simulated optical transmittance which matches experimental observation. The coherence of resistivity and optical transmittance validates the simulation of the sputtered pattern and the incorporation of percolation theory in the model.
ContributorsFang, Chia-Ling (Author) / Alford, Terry L. (Thesis advisor) / Crozier, Peter (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
The object of this study is to investigate and improve the performance/stability of the flexible thin film transistors (TFTs) and to study the properties of metal oxide transparent conductive oxides for wide range of flexible electronic applications. Initially, a study has been done to improve the conductivity of ITO (indium tin oxide) films on PEN (polyethylene naphthalate) by inserting a thin layer of silver layer between two ITO layers. The multilayer with an optimum Ag mid-layer thickness, of 8 nm, exhibited excellent photopic average transmittance (~ 88 %), resistivity (~ 2.7 × 10-5 µ-cm.) and has the best Hackee figure of merit (41.0 × 10-3 Ω-1). The electrical conduction is dominated by two different scattering mechanisms depending on the thickness of the Ag mid-layer. Optical transmission is explained by scattering losses and absorption of light due to inter-band electronic transitions. A systematic study was carried out to improve the performance/stability of the TFTs on PEN. The performance and stability of a-Si:H and a-IZO (amorphous indium zinc oxide) TFTs were improved by performing a systematic low temperature (150 °C) anneals for extended times. For 96 hours annealed a-Si:H TFTs, the sub-threshold slope and off-current were reduced by a factor ~ 3 and by 2 orders of magnitude, respectively when compared to unannealed a-Si:H TFTs. For a-IZO TFTs, 48 hours of annealing is found to be the optimum time for the best performance and elevated temperature stability. These devices exhibit saturation mobility varying between 4.5-5.5 cm2/V-s, ION/IOFF ratio was 106 and a sub-threshold swing variation of 1-1.25 V/decade. An in-depth study on the mechanical and electromechanical stress response on the electrical properties of the a-IZO TFTs has also been investigated. Finally, the a-Si:H TFTs were exposed to gamma radiation to examine their radiation resistance. The interface trap density (Nit) values range from 5 to 6 × 1011 cm-2 for only electrical stress bias case. For "irradiation only" case, the Nit value increases from 5×1011 cm-2 to 2×1012 cm-2 after 3 hours of gamma radiation exposure, whereas it increases from 5×1011 cm-2 to 4×1012 cm-2 for "combined gamma and electrical stress".
ContributorsIndluru, Anil (Author) / Alford, Terry L. (Thesis advisor) / Schroder, Dieter (Committee member) / Krause, Stephen (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2011
Description
Miedema's plot is used to select the Cu/metal barrier for Cu metallization.The Cu/metal barrier system selected should have positive heat of formation (Hf) so that there is no intermixing between the two layers. In this case, Ru is chosen as a potential candidate, and then the barrier properties of sputtered Cu/Ru thin films on thermally grown SiO2 substrates are investigated by Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), and electrical resistivity measurement. The Cu/Ru/SiO2 samples are analyzed prior to and after vacuum annealing at various temperatures of 400, 500, and 600 oC and at different interval of times of 0.5, 1 and 2 hrs for each temperature. Backscattering analysis indicate that both the copper and ruthenium thin films are thermally stable at high temperature of 600 oC, without any interdiffusion and chemical reaction between Cu and Ru thin films. No new phase formation is observed in any of the Cu/Ru/SiO2 samples. The XRD data indicate no new phase formation in any of the annealed Cu/Ru/SiO2 samples and confirmed excellent thermal stability of Cu on Ru layer. The electrical resistivity measurement indicated that the electrical resistivity value of the copper thin films annealed at 400, 500, and 600 oC is essentially constant and the copper films are thermally stable on Ru, no reaction occurs between copper films and Ru the layer. Cu/Ru/SiO2 multilayered thin film samples have been shown to possess good mechanical strength and adhesion between the Cu and Ru layers compared to the Cu/SiO2 thin film samples. The strength evaluation is carried out under static loading conditions such as nanoindentation testing. In this study, evaluation and comparison is donebased on the dynamic deformation behavior of Cu/Ru/SiO2 and Cu/SiO2 samples under scratch loading condition as a measure of tribological properties. Finally, the deformation behavior under static and dynamic loading conditions is understood using the scanning electron microscope (SEM) and the focused ionbeam imaging microscope (FIB) for topographical and cross-sectional imaging respectively.
ContributorsVenkatesh, Srilakshmi Hosadurga (Author) / Alford, Terry L. (Thesis advisor) / Krause, Stephen (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2010
Description
Electromigration, the net atomic diffusion associated with the momentum transfer from electrons moving through a material, is a major cause of device and component failure in microelectronics. The deleterious effects from electromigration rise with increased current density, a parameter that will only continue to increase as our electronic devices get smaller and more compact. Understanding the dynamic diffusional pathways and mechanisms of these electromigration-induced and propagated defects can further our attempts at mitigating these failure modes. This dissertation provides insight into the relationships between these defects and parameters of electric field strength, grain boundary misorientation, grain size, void size, eigenstrain, varied atomic mobilities, and microstructure.First, an existing phase-field model was modified to investigate the various defect modes associated with electromigration in an equiaxed non-columnar microstructure. Of specific interest was the effect of grain boundary misalignment with respect to current flow and the mechanisms responsible for the changes in defect kinetics. Grain size, magnitude of externally applied electric field, and the utilization of locally distinct atomic mobilities were other parameters investigated. Networks of randomly distributed grains, a common microstructure of interconnects, were simulated in both 2- and 3-dimensions displaying the effects of 3-D capillarity on diffusional dynamics.
Also, a numerical model was developed to study the effect of electromigration on void migration and coalescence. Void migration rates were found to be slowed from compressive forces and the nature of the deformation concurrent with migration was examined through the lens of chemical potential. Void migration was also validated with previously reported theoretical explanations. Void coalescence and void budding were investigated and found to be dependent on the magnitude of interfacial energy and electric field strength.
A grasp on the mechanistic pathways of electromigration-induced defect evolution is imperative to the development of reliable electronics, especially as electronic devices continue to miniaturize. This dissertation displays a working understanding of the mechanistic pathways interconnects can fail due to electromigration, as well as provide direction for future research and understanding.
ContributorsFarmer, William McHann (Author) / Ankit, Kumar (Thesis advisor) / Chawla, Nikhilesh (Committee member) / Jiao, Yang (Committee member) / McCue, Ian (Committee member) / Arizona State University (Publisher)
Created2022
Description
Applications such as heat exchangers, surface-based cellular structures, rotating blades, and waveguides rely on thin metal walls as crucial constituent elements of the structure. The design freedom enabled by laser powder bed fusion has led to an interest in exploiting this technology to further the performance of these components, many of which retain their as-built surface morphologies on account of their design complexity. However, there is limited understanding of how and why mechanical properties vary by wall thickness for specimens that are additively manufactured and maintain an as-printed surface finish. Critically, the contributions of microstructure and morphology to the mechanical behavior of thin wall laser powder bed fusion structures have yet to be systematically identified and decoupled. This work focuses on elucidating the room temperature quasi-static tensile and high cycle fatigue properties of as-printed, thin-wall Inconel 718 fabricated using laser powder bed fusion, with the aim of addressing this critical gap in the literature. Wall thicknesses studied range from 0.3 - 2.0 mm, and the effects of Hot Isostatic Pressing are also examined, with sheet metal specimens used as a baseline for comparison. Statistical analyses are conducted to identify the significance of the dependence of properties on wall thickness and Hot Isostatic Pressing, as well as to examine correlations of these properties to section area, porosity, and surface roughness. A thorough microstructural study is complemented with a first-of-its-kind study of surface morphology to decouple their contributions and identify underlying causes for observed changes in mechanical properties. This thesis finds that mechanical properties in the quasi-static and fatigue framework do not see appreciable declines until specimen thickness is under 0.75 mm in thickness. The added Hot Isostatic Pressing heat treatment effectively closed pores, recrystallized the grain structure, and provided a more homogenous microstructure that benefits the modulus, tensile strength, elongation, and fatigue performance at higher stresses. Stress heterogeneities, primarily caused by surface defects, negatively affected the thinner specimens disproportionately. Without the use of the Hot Isostatic Pressing, the grain structure remained much more refined and benefitted the yield strength and fatigue endurance limit.
ContributorsParadise, Paul David (Author) / Bhate, Dhruv (Thesis advisor) / Chawla, Nikhilesh (Committee member) / Azeredo, Bruno (Committee member) / Jiao, Yang (Committee member) / Arizona State University (Publisher)
Created2022
Description
Microstructure refinement and alloy additions are considered potential routes to increase high temperature performance of existing metallic superalloys used under extreme conditions. Nanocrystalline (NC) Cu-10at%Ta exhibits such improvements over microstructurally unstable NC metals, leading to enhanced creep behavior compared to its coarse-grained (CG) counterparts. However, the low melting point of Cu compared to other FCC metals, e.g., Ni, might lead to an early onset of diffusional creep mechanisms. Thus, this research seeks to study the thermo-mechanical behavior and stability of hierarchical (prepared using arc-melting) and NC (prepared by collaborators through powder pressing and annealing) Ni-Y-Zr alloys where Zr is expected to provide solid solution and grain boundary strengthening in hierarchical and NC alloys, respectively, while Ni-Y and Ni-Zr intermetallic precipitates (IMCs) would provide kinetic stability. Hierarchical alloys had microstructures stable up to 1100 °C with ultrafine eutectic of ~300 nm, dendritic arm spacing of ~10 μm, and grain size ~1-2 mm. Room temperature hardness tests along with uniaxial compression performed at 25 and 600 °C revealed that microhardness and yield strength of hierarchical alloys with small amounts of Y (0.5-1wt%) and Zr (1.5-3 wt%) were comparable to Ni-superalloys, due to the hierarchical microstructure and potential presence of nanoscale IMCs. In contrast, NC alloys of the same composition were found to be twice as hard as the hierarchical alloys. Creep tests at 0.5 homologous temperature showed active Coble creep mechanisms in hierarchical alloys at low stresses with creep rates slower than Fe-based superalloys and dislocation creep mechanisms at higher stresses. Creep in NC alloys at lower stresses was only 20 times faster than hierarchical alloys, with the difference in grain size ranging from 10^3 to 10^6 times at the same temperature. These NC alloys showed enhanced creep properties over other NC metals and are expected to have rates equal to or improved over the CG hierarchical alloys with ECAP processing techniques. Lastly, the in-situ wide-angle x-ray scattering (WAXS) measurements during quasi-static and creep tests implied stresses being carried mostly by the matrix before yielding and in the primary creep stage, respectively, while relaxation was observed in Ni5Zr for both hierarchical and NC alloys. Beyond yielding and in the secondary creep stage, lattice strains reached a steady state, thereby, an equilibrium between plastic strain rates was achieved across different phases, so that deformation reaches a saturation state where strain hardening effects are compensated by recovery mechanisms.
ContributorsSharma, Shruti (Author) / Peralta, Pedro (Thesis advisor) / Alford, Terry (Committee member) / Jiao, Yang (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2022