Browsing by Author "Gerd Duscher, Committee Member"

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Characterization of Corrosion Pit Initiation in Aluminum Using Advanced Electron Microscopy Techniques
(2006-05-08) Elswick, Danielle Shea; Gerd Duscher, Committee Member; Nancy Missert, Committee Member; Peter Fedkiw, Committee Member; Paul Kotula, Committee Member; John Hren, Committee Chair
The resistance to pitting corrosion in aluminum is due to the presence of a compact thin, approximately 5 nm, oxide. Certain conditions locally attack this protective oxide layer leading to its breakdown and resulting in the formation of corrosion pits. Numerous studies have investigated the growth and propagation stages of pitting corrosion yet the initiation stage remains not clearly defined nor well understood. The presence of aggressive chemical species, such as chloride, plays a critical role in the pitting phenomenon and is explored in this investigation. This dissertation focuses on the localization of pitting corrosion in high purity aluminum in order to accurately predict where and when the pit initiation process will occur so that microstructural changes associated with pit initiation can be easily identified and characterized using electron microscopy. A comprehensive investigation into the corrosion initiation process was attempted utilizing advanced characterization techniques in the transmission electron microscope (TEM) coupled with high-resolution microanalysis. Localization of pitting was successful through use of different sample geometries that reduced the length scale for which pitting events occurred. Three geometries were investigated, each with unique features for pitting corrosion. Electropolished Al needles localized pitting to a sharp tip due to a geometric field enhancement effect, while other experiments employed an Al wire micro-electrode geometry. Both geometries minimized the area where corrosion pits initiated and were electrochemically tested using a solution that contained the chloride species. A third geometry included electron beam evaporated Al films implanted with chloride, which induced pitting corrosion in an otherwise chloride-free environment. Localization of pitting was successfully achieved using novel sample geometries that isolated the desired stages of pitting corrosion, i.e. metastable pitting, through controlled electrochemical tests. Potentiodynamic pitting experiments were performed on the different sample geometries and advanced TEM was utilized for characterization and microanalysis of the samples both prior to and following polarization. Automated eXpert Spectral Image Analysis (AXSIA) was one technique employed that allowed for spatial resolution of chloride in our material. Preliminary experiments using the Al needle and micro-electrode geometries aided in defining electrochemical parameters and sample properties. Results from these geometries will be presented. A more in-depth study was performed using the Al thin films. TEM samples were made from the Al films in both cross-sectional and plan-view, which provided more information into the size and distribution of the chloride species. Oxide thickness increased locally prior to pitting when local areas high in chloride concentration were present. Results obtained from advanced TEM characterization and sophisticated microanalysis are presented in this dissertation and provide striking information into sample morphology and structural changes that resulted from electrochemically induced pitting corrosion.
Creep-Rupture Study of Annealed Zircaloy 4: Stress and Temperature Effects
(2005-11-22) Marple, Brian Wesley; Gerd Duscher, Committee Member; Mohamed A. Bourham, Committee Member; K. Linga Murty, Committee Chair
Zircaloys are widely used as fuel rod cladding in light water reactors (LWRs) because of their low cross-section for absorption of thermal neutrons. Currently, the United States does not permit reprocessing of spent fuel so the primary barrier for the spent fuel in a repository will be the fuel rod cladding. Due to the decay heat of the spent fuel, creep rupture is considered to be the primary cause of failure in spent fuel cladding over the long period of time that it will be stored. A fundamental understanding of the creep mechanisms in Zircaloys is crucial to accurately predicting the integrity of the fuel cladding over long periods of time. Zirconium has a hexagonally close-packed crystal structure and because of this, exhibits creep anisotropy that is affected not only by the texture, but also by temperature, stress, and loading. Since the stress imposed on the spent fuel during long-term storage will be relatively low compared to service conditions, the low stress creep behavior must be characterized and mechanistically understood to avoid non-conservative estimates based on in-pile creep data. In addition, loading of spent fuel in a repository will be due to the internal pressure generated by fission product gasses and from the inert gas introduced at the time of fuel fabrication. This work focuses on the creep rupture behavior and microstructural characterization of annealed Zircaloy-4 at temperatures ranging from 250°C-600°C and stresses from 27 MPa-350 MPa. Typically, fuel assemblies that have been fabricated from Zircaloy-4 are not in the annealed condition. Instead, they are cold-worked and stress relieved (CWSR). Since low stress creep rupture testing would take years at low temperatures, high temperatures are used to observe the effects of low stress in a reasonable amount of time. At such high temperatures, the grain structure of the CWSR material would change drastically. Therefore the material was annealed prior to testing to avoid this complication. Testing on unirradiated material will yield higher strain rates because of irradiation hardening. Therefore, estimates based on unirradiated creep rupture data would be conservative. Prior to testing, optical metallographs were taken to characterize the grain structure. A limited texture study was performed to evaluate the texture coefficients for each direction in the rod. Transmission electron microscopy (TEM) was also performed to characterize the initial dislocation microstructure. After testing, diametric measurements were taken and the strain rate determined. From data at various stresses, the activation energy was derived along with equations predictive of rupture such as the Larson-Miller parameter and the Monkman-Grant relationship. Specimens of interest were selected for optical metallography and to obtain TEM micrographs of the dislocation microstructure. The activation energy deduced was in excellent agreement of that for self-diffusion. Optical metallography showed slight grain elongation in samples tested at high stresses while grains remained equiaxed at low stresses. TEM showed significant sub-grain formation at low stresses and random dislocation organization at higher stresses.
Defect and Impurity Distributions in Traditionally Cast Multicrystalline and Cast Monocrystalline Silicon for Solar Substrates
(2008-08-26) Witting, Ian Thomas; Gerd Duscher, Committee Member; Carlton Osburn, Committee Member; George Rozgonyi, Committee Chair
Electron and Ion-beam Characterization of Nitride Semiconductor Devices.
(2006-12-06) Parish, Chad Michael; Phillip Russell, Committee Chair; Dieter Griffis, Committee Member; Gerd Duscher, Committee Member; Michael Escuti, Committee Member
Gallium nitride (GaN) and its alloys are used to manufacture green-to-ultraviolet range light emitting diodes (LEDs) for the solid-state lighting industry. However, heteroepitaxial growth on substrates such as 6H-SiC or -Al2O3 results in LEDs with large densities of crystal defects; the optical and electronic properties of these defects, and their influences on LED device performance, are not yet well understood. Controlling and optimizing processing in modern optoelectronic materials, such as GaN, requires a high-resolution characterization technique to probe the localized bandstructure of the materials and their defects in order to relate properties to processing. The beam-injection modes of scanning electron microscopy (SEM) fulfill this need. When an SEM is used to examine a semiconductor, the electron beam injects electron-hole pairs (EHPs) into the semiconductor's band structure. Cathodoluminescence (CL) and electron-beam-induced current (EBIC) are SEM techniques that take advantage of this localized beam-injection of EHPs, and are used to directly probe the optoelectronic behavior of semiconductor materials, devices, and defects. This work examines the optoelectronic properties of defects in GaN-based LED devices. First, computer modeling of the polarization fields in quantum wells was performed, and quantitative predictions of cathodoluminescence peak shifts during electron injection, under varying conditions of electrical bias, were made. Experimental conditions and mathematical treatments for accurate EBIC quantification of the minority carrier diffusion length in GaN light-emitting diodes (LEDs) were developed and refined, as were combined CL and EBIC techniques for the study defect populations in GaN LEDs. Additionally, the effects of focused-ion-beam milling as a cross-sectional sample preparation technique for GaN were studied by CL and EBIC. Limitations and possible extensions to these techniques are also be discussed. By using SEM-EBIC/CL to pinpoint defects in LED devices, site-specific FIB microsampling has been used to prepare samples of defected areas for transmission electron microscopy (TEM). Analyses of these samples have shown how the identity of crystal defects within the devices directly relates to the optoelectronic behavior observed in EBIC and CL.
Electronic Defect Characterization of Strained-Si/SiGe/Si Heterostructure
(2007-08-22) Zhang, Renhua; George A. Rozgonyi, Committee Chair; Carl M. Osburn, Committee Member; Phillip Russell, Committee Member; Gerd Duscher, Committee Member
High-Rate Diamond Deposition by Microwave Plasma CVD
(2008-08-01) Li, Xianglin; Zlatko Sitar, Committee Chair; Ramon Collazo, Committee Member; Gerd Duscher, Committee Member; Carl Osburn, Committee Member
Lead Zirconate Titanate (PZT) Based Thin Film Capacitors For Embedded Passive Applications
(2003-08-21) Kim, Taeyun; Robert Croswell, Committee Member; Paul Franzon, Committee Member; Jon-Paul Maria, Committee Chair; Gerd Duscher, Committee Member; Angus Kingon, Committee Chair
Investigations on the key processing parameters and properties relationship for lead zirconate titanate (PZT, 52/48) based thin film capacitors for embedded passive capacitor application were performed using electroless Ni coated Cu foils as substrates. Undoped and Ca-doped PZT (52/48) thin film capacitors were prepared on electroless Ni coated Cu foil by chemical solution deposition. The effects of processing parameters on the phase evolution, microstructures, dielectric properties, and reliability were investigated. Electroless Ni coated Cu foil was selected as substrate for its low cost, oxidation resistance and lamination capability. When annealed at 450 °C, electroless Ni coated Cu foil showed transformation from amorphous Ni to crystalline phase of Ni-P (mostly Ni₃P) and Ni metal. For PZT (52/48) thin film capacitors on electroless Ni coated Cu foil, voltage independent (zero tunability) capacitance behavior was observed. Dielectric constant reduced to more than half of the identical capacitor processed on Pt/SiO₂/Si. Dielectric properties of the capacitors were mostly dependent on the crystallization temperature. Capacitance densities of almost 350 nF/cm² and 0.02-0.03 of loss tangent were routinely measured for capacitors crystallized at 575-600 °C. Leakage current showed dependence on film thickness and crystallization temperature. It is speculated that space charge limited conduction (SCLC) seems to be consistent with conduction mechanism in PZT thin films on electroless Ni. From a two-capacitor model, the existence of a low permittivity interface layer (permittivity -30) was suggested. Also it is suggested a high concentration of traps exist inside the PZT capacitor. Interface reaction between PZT thin film and electroless Ni was suggested to be responsible for measured electrical properties. The interfacial layer might be composed of unreacted oxide, phosphate, and phosphides possibly from phosphorous diffused from electroless Ni into PZT bulk. For Ca-doped PZT (52/48) thin film capacitors prepared on Pt, typical ferroelectric and dielectric properties were measured up to 5 mol%Ca doping. Further addition up to 10 mol % changed the lattice parameter of the unit cell, and reduced dielectric properties were observed. The possibility of Ca acceptor doping is suggested. When Ca-doped PZT (52/48) thin film capacitors were prepared on electroless Ni coated Cu foil, phase stability was influenced by Ca doping and phosphorous content. Dielectric properties showed dependence on the crystallization temperature and phosphorous content. Capacitance density of -400 nF/cm² was achieved, which is an improvement by more than 30% compared to undoped composition. Ca doping also reduced the temperature coefficient of capacitance (TCC) less than 10%, all of them were consistent in satisfying the requirements of embedded passive capacitor. Leakage current density was not affected significantly by doping. Interface control by controlled pO² crystallization was found to be not effective in interface layer mitigation. Phase purity, dielectric properties, surface microstructure, and pO² were found to have a correlated dependence. To tailor the dielectric and reliability properties, ZrO² was selected as buffer layer between PZT and electroless Ni. Only RF magnetron sputtering process could yield stable ZrO² layers on electroless Ni coated Cu foil. Other processes resulted in secondary phase formation, which supports the reaction between PZT capacitor and electroless Ni might be dominated by phosphorous component. Incorporation of ZrO² layers reduced maximum capacitance density by 10 %(- 350 nF/cm²) due to lower permittivity of ZrO² layer. Significantly improved leakage current densities were measured for PZT thin film capacitors on ZrO₂. For PZT thin film capacitors incorporating 100 nm thick ZrO₂ layer, leakage current density of 10⁻⁸ A/cm² was measured at 25 VDC, which is more than three orders of magnitude lower than those directly deposited on electroless Ni coated Cu foil. The complete set of experimental data provides validation and process conditions for the use of PZT thin films on low cost electroless Ni coated Cu foil substrate as embedded capacitors in high density printed circuit boards.
Light Element Impurities and Related Defects in Polycrystalline Silicon for Photovoltaic Application
(2004-11-04) Lu, Jinggang; David E. Aspnes, Committee Member; George A. Rozgonyi, Committee Chair; Carlton Osburn, Committee Member; Gerd Duscher, Committee Member
This thesis examines light element impurities and related defects in polycrystalline sheet and RGS (Ribbon Growth on Substrate) ribbon silicon. The interaction dynamics between oxygen, carbon, nitrogen, and intrinsic point defects, as well as the role of grain boundaries (GBs) on oxygen and carbon precipitation have been investigated. It is found that a high concentration of interstitial oxygen (Oi) will precipitate readily in polycrystalline sheet and ribbon silicon, and the precipitation of substitutional carbon (Cs) is mainly controlled by oxygen precipitation. By monitoring the Cs reduction by infrared absorption and the precipitate density by preferential etching, it is concluded that formation of interstitial carbon by trapping silicon self-interstitials is an indispensable step for the observed fast Cs precipitation. It is concluded that a low oxygen content is vital important to prevent extensive oxygen precipitation. On the contrary, a high carbon content (~ 1x 10¹⁸ cm⁻³) can be tolerated as long as the initial Oi concentration is low. The impact of GBs on oxygen precipitation in sheet silicon has been investigated. Infrared microspectroscopy shows nitrogen gettering at GBs and preferential etching reveals a precipitate denuded zone near GBs. The gettering of nitrogen at GBs is likely to be responsible for the denuded zone formation, considering the enhancement of nitrogen impurities on oxygen precipitation. The impact of GBs on carbon precipitation in RGS ribbons has also been studied. Infrared microspectroscopy indicates a higher remaining Cs concentration in the intra-grain region and preferential etching reveals a 20 to 30 μm wide precipitation band near GBs. Assuming that the tensile strain associated with carbon precipitates must be relaxed in order for the precipitation to proceed, it is shown that the precipitation band formation is mainly controlled by diffusion of vacancies from the intra-grain region to GBs.
Polarity Control in GaN Epilayers Grown by Metalorganic Chemical Vapor Deposition
(2008-08-21) Mita, Seiji; John Muth, Committee Member; Gerd Duscher, Committee Member; Ramon Collazo, Committee Member; Zlatko Sitar, Committee Chair
Polarity control of gallium nitride (GaN) on c-plane sapphire substrate was studied via low pressure Metalorganic Chemical Vapor Deposition (MOCVD). Under mass-transport-limited growth regime with a given process supersaturation, the polarities of GaN thin films (i.e. gallium (Ga) and nitrogen (N)-polarities) depended on specific treatments of the sapphire substrate prior to GaN deposition, in addition, identical growth rates for both polar films were obtained. This ability made the fabrication of lateral polar junction (LPJ) GaN structures possible. New designs of novel device structures utilizing the resulting polarity control scheme were developed. N-polar films were consistently obtained after exposing a H2-annealed sapphire substrate to an ammonia atmosphere at temperature above 950°C. Ga-polar films were obtained either by preventing any exposure of the substrate to ammonia prior to deposition or by depositing the film on a properly annealed low temperature aluminum nitride nucleation layer (LT-AlN NL) deposited on a previously ammonia annealed sapphire substrate. As-grown Ga-polar films were generally insulating and smooth surface morphology while N-polar films exhibited n-type conductivity with carrier concentration approaching 1x1019 cm-3 and a rougher surface morphology. Following the established polarity control scheme for GaN films, LPJ structures consisting Ga-polar and N-polar domains side-by-side on a single sapphire wafer were achieved by utilizing a prior patterned AlN⁄bare sapphire template. The two regions were separated by an inversion domain boundary (IDB), which did not hinder the current flow across it, i.e. no energy barrier for the charge carriers. This in principle showed the possibility for the fabrication of lateral junctions and lateral based devices within the GaN technology exploiting polar doping selectivity. Understanding the doping selectivity of the two different polar domains allowed us to fabricate a lateral p⁄n junction in GaN by the simultaneous growth of the p- and n-type regions. Identifying the basic characteristics of a p⁄n junction demonstrated that the fabricated structure was a functional p/n diode. For GaN based junctions, these characteristics were: current rectification, electroluminescence and the photovoltaic effect under UV excitation.
Self-assembled Magnetic Nanostructures: Epitaxial Ni on TiN (001) Surface
(2006-01-10) Zhou, Honghui; Gerd Duscher, Committee Member; Carl C. Koch, Committee Member; Robert M. Kolbas, Committee Member; Jagdish Narayan, Committee Chair
Systems that contain single domain magnetic particles have been receiving intensive attentions over recent years since they are possible candidates for applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this research is self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer-Weber mode. The growth was conducted by pulsed laser deposition (PLD) technique using epitaxial titanium nitride film as the template, which was in turn grown on silicon (100) substrate. The structural characterization includes X-ray diffraction and both cross-sectional and plan-view transmission electron microscopy. The results showed that the nickel islands formed exhibit a self-assembled nature, i.e., a certain degree of uniformity in orientation, shape, and size. The orientation relationship observed is Ni [100] // TiN [100] // Si [100], the so-called 'cube-on-cube' relationship. The islands are faceted, forming truncated pyramids with walls of (111) planes and a flat top of (100) plane. The base of islands is rectangular with the two principal edges parallel to two orthogonal 011 directions. The size distribution is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski-Krastanow (S-K) mode. A certain degree of self-organization was also found in the island lateral distribution: island chains were observed along the directions close to 011, which are also the edge directions. The island faceting could be explained by surface energy minimization. The interaction of island edge induced strain field between neighboring islands is believed to be responsible for the size uniformity and the lateral ordering. Magnetic measurements were also conducted on these crystallographically aligned nickel islands using superconducting quantum interference device (SQUID) magnetometer, and the results were compared with that obtained from the ensemble of randomly oriented nickel islands, which were grown on polycrystalline/amorphous Al2O3 matrix layer. It is found that both blocking temperature and coercivity of aligned nickel islands are significantly higher than that of the randomly oriented nickel islands. The enhancement in ferromagnetism is attributed to the increased collective effects resulting from the particle interactions in the ensemble of aligned islands, which are self-assembled and self-organized to some degree.
Spectroscopic Study of Hafnium Silicate Alloys prepared by RPECVD: Comparisons between Conduction/Valence Band Offset Energies and Optical Band Gaps
(2003-12-31) Hong, Joon Goo; Gerald Lucovsky, Committee Chair; Carlton Osburn, Committee Member; Gerd Duscher, Committee Member; Klaus Bachmann, Committee Member
Aggressive scaling of devices has continued to improve MOSFET transistor performance. As lateral device dimensions continue to decrease, gate oxide thickness must be scaled down. As one of the promising high k gate oxide material, HfO₂ and its silicates were investigated to understand their direct tunneling behavior by studying conduction and valence band offset energies with spectroscopy and electrical characterization. Local bonding change of remote plasma deposited (HfO₂)[subscript x](SiO[subscript 8322;)[subscript 1-x] alloys were characterized by Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) as a function of alloy composition, x. Two different precursors with Hf Nitrato and Hf-tert=butoxide were tested to have amorphous deposition. Film composition was determined off-line by Rutherford backscattering spectroscopy (RBS) and these results were calibrated with on-line AES. As deposited Hf-silicate alloys were characterized by off-line XPS and AES for their chemical shifts interpreting with a partial charge transfer model as well as coordination changes.Sigmoidal dependence of valence band offse energies was observed. Hf 5d state is fixed at the bottom of the conduction band and located at 1.3 ± 0.2 eV above the top of the Si conduction band as a conduction band offset by x-ray absorption spectroscopy (XAS). Optical band gap energy changes were observed with vacuum ultra violet spectroscopic ellipsometry (VUVSE) to verify compositional dependence of conduction and valence band offset energy changes. 1 nm EOT normalized tunneling current with Wentzel-Kramer-Brillouin (WKB) simulation based on the band offset study and Franz two band model showed the minimum at the intermediate composition matching with the experimental data. Non-linear trend in tunneling current was observed because the increases in physical thickness were mitigated by reduction in band offset energies and effective mass for tunneling. C-V curves were compared to each other, and more hysteresis was observed with increasing x. Localized Hf 5d state as a trap site was the reason for hysteresis and its reverse direction with temperature-dependent C-V curves. Temperature-dependent I-V study located Hf 5d state. For the integration issue, nitridation study was performed at the interface, surface and both. Interfacial nitridation gave more effective reduction in EOT.
Study of Mn Doped GaN for Spintronic Applications
(2006-11-20) Arkun, Fevzi Erdem; Gerd Duscher, Committee Member; Nadia El-Masry, Committee Chair; Salah Bedair, Committee Co-Chair; Mark Johnson, Committee Member
Spintronics is an emerging field in which the spin of carriers in addition to the charge of carriers can be used to achieve new functionalities in electronic devices. The availability of materials exhibiting ferromagnetism above room temperature is prerequisite for realizing such devices. Materials suitable for spintronic applications are desired to be compatible with conventional growth and fabrication techniques in addition to exhibiting above room temperature ferromagnetic properties. In this research the growth of GaMnN has been achieved on (0001) sapphire substrates by metal organic chemical vapor deposition using TMGa and (EtCp₂)Mn as organometallic precursors. Magnetic characterization of the grown films was performed by a Superconducting Quantum Interference Device (SQUID) at room temperature. Ferromagnetic properties were observed above room temperature for this material. Co-doping of ferromagnetic GaMnN by silicon and magnesium was performed and ferromagnetic properties of GaMnN have been found to depend on the Fermi level in the crystal itself. The mechanism of ferromagnetism in this material was proposed to be carrier mediated. The magnetic properties were also altered by carrier transfer at a heterointerface indicating that the electronic band structure of the crystal affects the magnetic properties of this material. Growth of GaN based blue light emitting diode structures were achieved by MOCVD using conventional organometallic sources. Fabrication of grown structures was performed in a clean room using standard fabrication techniques for III-Nitrides. Two spin-LEDs containing GaMnN injector layers were also grown to determine the polarization state of the emission from these spin-LEDs.
Synthesis and mechanical properties of two phase nanostructured Al based composites
(2007-08-22) Rajulapati, Koteswararao Venkata; Robert J Nemanich, Committee Member; Korukonda L (KL) Murty, Committee Member; Ronald O Scattergood, Committee Member; Carl C Koch, Committee Chair; Gerd Duscher, Committee Member