Doctor of Philosophy in Materials Science and Engineering, Youngstown State University, 2022, Department of Mechanical, Industrial and Manufacturing Engineering

Ni-Mn-Ga thin films have attracted significant attention over the past two decades due to its multifunctional properties, leveraging these characteristics in applications such as actuators, sensors, and micro-electromechanical systems (MEMS). The most favorable deposition technique for making Ni-Mn-Ga thin films is magnetron sputtering where the target used is near stoichiometric Ni2MnGa alloy. Ni-Mn-Ga alloy target manufacturing has been challenging and costly due to design constraints, process optimization issues and inefficient target utilization resulting in compounded negative economics. To address these problems, this research aimed at investigating and demonstrating the viability of a cost-effective, modern technology known as binder jetting additive manufacturing (BJAM) technique to produce targets with excellent target consumption efficiency based on proposed target design. The additive manufactured Ni-Mn-Ga alloy target process began with ball-milled Ni-Mn-Ga powder having bimodal particle distribution to ensure an increased packing density and mechanical strength after the determination of optimized 3D printing parameters. The printed targets were post-processed through curing, de-binding and sintering. Sintering was conducted in an inert/argon atmosphere to safeguard essential material properties which were benchmarked through characterization. Backscattered electron (BSE) micrographs showed AM targets were homogenous with martensitic twin microstructures necessary for shape memory behavior. The XRD results showed that the martensitic twin microstructures were mostly tetragonal and monoclinic crystal structures. The martensitic transformation temperatures for Ni-Mn-Ga targets ranged from 79.3 to 148.1°C and possessed a maximum density of 87.18%. Using the direct current (DC) magnetron sputtering, Ni-Mn-Ga thin films were deposited on Si (100) substrates at discharge currents ranging from 0.05 to 0.15 A and substrate temperatures 20°C to 700°C. The effect (open full item for complete abstract)

Committee: Solomon Virgil PhD (Advisor); Clovis Linkous PhD (Committee Member); Tom Oder PhD (Committee Member); Donald Priour PhD (Committee Member); Matthew Caputo PhD (Committee Member) Subjects: Materials Science

Doctor of Philosophy (Ph.D.), University of Dayton, 2015, Materials Engineering

This work details two specific research thrusts exploring the deposition and characterization of mixed valent oxide systems. The first of these thrusts investigated the effect of the oxygen content, during reactive sputter deposition, on the optical, chemical, and structural properties of oxides of molybdenum, germanium, and rhenium. Exploration of the Mo-O system was conducted using a technique known as modulated pulse power magnetron sputtering (MPPMS), while the Ge-O and Re-O systems were deposited via direct current magnetron sputtering (DCMS). Films deposited under poisoned mode conditions were shown to be highly transparent with refractive index (n) values of n550=1.60 for GeO2, and n550=1.97 for MoO3, similar to values reported for bulk constituents. The Re-O system, unlike Ge-O and Mo-O, displayed a significantly high sensitivity to ambient moisture. Chemical analysis via XPS indicated the presence of instability as a result of the moisture induced decomposition of Re2O7 into HReO4, and catalytic disproportionation of Re2O3 into Re and hydrous ReO2. The second research thrust within this project was focused on the deposition of three component mixed oxide systems with multiple valence states. This effort, which utilized the results from individual material depositions mentioned previously, required the use of stable and thermodynamically compatible material systems, namely Mo-O and Ge-O (ΔfHo(MoO2)= -588 kJ/mol and ΔfHo(GeO2)= -580 kJ/mol). Note that Re-O was not explored as part of the ternary deposition effort due to the aforementioned chemical instability. To achieve the goal of depositing mixed valent thin films with tailorable optical absorption, an industrially scalable co-deposition method was devised in order to deposit molybdenum cations within a dielectric GeO2 matrix. The high power densities associated with the MPPMS process were systematically varied in order to control the oxygen partial pressure via gettering, allowing for control over the (open full item for complete abstract)

Committee: P. Terrence Murray Ph.D. (Committee Chair); Dean R. Evans Ph.D. (Advisor); John T. Grant Ph.D. (Committee Member); Daniel P. Kramer Ph.D. (Committee Member); Andrew M. Sarangan Ph.D. (Committee Member) Subjects: Materials Science

Master of Science, Miami University, 2024, Physics

The Magnetic Heusler Weyl Semimetals Co2MnGa and Co2MnSi are topological materials with promising spintronic applications through the exploitation of their rich transport properties, such as the anomalous Hall and Nernst Effects. In this study, we performed a series of growths via radio-frequency (RF) magnetron sputtering onto MgO(001) substrates under systematically varied conditions of substrate temperature and sputtering power and conducted a series of measurements to ascertain trends in film composition, morphology, crystallographic characteristics, and demagnetized domain structure dependent upon these growth conditions. We combine degree of crystallinity computations with multi-material texture coefficient analyses to provide a complete picture of sample contents, including amorphous contribution and the proportions of each orientation of the crystalline portions. We reveal categorically distinct morphologies and crystalline growth preferences with varying sputtering power and temperature, including novel observation of Co2MnGa on MgO(001) of highly-textured (311) and (533) orientations with large flat grains and a complete absence of the (100) orientation when grown at 630 °C, 50 WRF. We also produce polycrystalline Co2MnSi when grown at 330 °C and confirm the presence of the L21 phase.

Committee: Joseph Corbett (Advisor); Imran Mirza (Committee Member); Mahmud Khan (Committee Chair) Subjects: Physics

Doctor of Philosophy in Materials Science and Engineering, Youngstown State University, 2024, Materials Science

Semiconductor materials have played a huge role in advancing today's technology through the electronic and photonic devices ushered in over the years. The advancement has been driven in part by society's growing need for electronic devices capable of handling higher power, higher temperature, and higher frequency. Current research efforts are expanding to ultra-wide bandgap semiconductors such as gallium oxide Ga2O3). The principal goal of this dissertation is to obtain high quality β-Ga2O3 films with controlled conductivity by magnetron sputtering deposition. The specific objectives are the following: To grow β-Ga2O3 films on sapphire substrates (section 5.2) and on native β-Ga2O3 by rf sputtering (section 5.3), to produce doped and undoped β-Ga2O3 films (Section 5.4). Additionally, to grow Lu2O3/ Ga2O3 and B2O3/Ga2O3 alloy films on (-2 0 1) UID or Sn-doped Ga2O3 and Al2O3 substrates to tune Ga2O3 original bandgap (Section 5.5). To obtain microstructural, morphological, compositional, and optical data from XRD, AFM, SEM, EDS, and UV-Vis characterization methods for all the experiments mentioned above. From this data, correlate the effects of the varying parameters for the optimization of the films, to use the developed films to fabricate Schottky barrier diodes and proceed with the electrical characterization of the fabricated devices (section 5.6).

Committee: Tom Oder PhD (Advisor); Clovis Linkous PhD (Committee Member); Constantin Solomon PhD (Committee Member); Michael Crescimanno PhD (Committee Member); Donald Priour PhD (Committee Member) Subjects: Electrical Engineering; Engineering; Experiments; Materials Science; Optics; Physics; Technology

Doctor of Philosophy, The Ohio State University, 2023, Physics

The ability to control magnetic material and devices has been of interest for many application, in particular data storage is a important motivator. This has been done in the past with ferromagnets in hard drives that use a spinning platter where the data can be read of. Limitations to this design is the use of moving parts, data density, and speed of reading and writing. Research interest into various magnetic materials and heterostructures has expanded from ferromagnetic systems to chiral magnetic systems and antiferromagnetics. Of particular interest are magnetic skyrmions that have stablility due to their structure and whose size can range from a few nanometers to hundreds of nanometers. Another type of magnetic materials that has recently gain more attention are antiferromagnetics which were known to be difcult to work due to the lack a net magnetic moment. However, recent developments in the feld have made the ability to measure and manipulate magnetic domains more accessible which can make antiferromagnetic materials an attractive candidate for data storage devices. ii

Committee: Fengyuan Yang (Advisor); Jay Gupta (Committee Member); Micha Elsner (Committee Member); Mohit Randeria (Committee Member); Annika Peter (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2023, Physics

Current manufacturing techniques allow for mass production of high-efficiency cadmium telluride (CdTe) photovoltaic (PV) modules at a low cost per watts. The robust nature of the materials and the high optical absorption coefficient with a suitable band gap for optimal photon power conversion made CdTe more attractive. Today's CdTe solar cells hold a record efficiency of 22.1%. However, the CdTe device efficiency is below the theoretical limits due to the recombination of photo-generated carriers in front/back interfaces and in the bulk of the absorber. Such recombination reduces the open circuit voltage (Voc) of the devices. Understanding the role of the different defects and defects complexes formed during absorber preparation and after post-deposition treatments is necessary to minimize carrier recombination. Also, using the front/back buffer layers for proper band alignment at interfaces are needed to reduce interfacial recombination. This dissertation focuses on materials engineering and control to minimize carrier recombination and hence improve devices performance. We fabricated high-quality CdTe absorbers with a new approach to CdTe deposition using a high vacuum close space sublimation (CSS) system. Reorganization of the defect complexes associated with Cu ion migration during light soaking of CdSe/CdTe devices is studied. A minority carrier lifetime of 656.5 ns is reported with a high-quality CdTe absorber and passivated back surface with a back buffer layer of copper aluminum oxides (CuxAlOy), resulting in ~ 860 mV Voc and ~ 17.5 % device efficiency. A problem with low-doped magnesium zinc oxide (MZO) as a front emitter layer in CdSe/CdTe devices has been resolved by increasing the doping density of MZO films with high vacuum annealing. To minimize front interfacial recombination, a new wide bandgap front emitter layer of indium gallium oxide (InxGa1-x)2O3 (IGO) has been introduced to tune the bandgap and conduction band offset (CBO) with absorber at th (open full item for complete abstract)

Committee: Michael Heben Dr. (Advisor); Michael Heben Dr. (Committee Chair); Alvin Compaan Dr. (Committee Member); Randy Ellingson Dr. (Committee Member); Richard Irving Dr. (Committee Member); Yanfa Yan Dr. (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2022, Physics

In this dissertation the structural properties of bulk alloys and thin-films are studied using a variety of di erent techniques including density functional theory (DFT) and accelerated dynamics. The first part of this dissertation involves the use of DFT calculations. In particular, in Chapter 3 the stability and mechanical properties of 3d transitional metal carbides in zincblende, rocksalt, and cesium chloride crystal structures are studied. We find that the valence electron concentration and bonding configuration control the stability of these compounds. The filled bonding states of transition metal carbides enable the stability of the compounds. In the second part of this dissertation we use a variety of accelerated dynamics techniques to understand the properties of growing and/or sublimating thin-films. In Chapter 4, the results of temperature-accelerated dynamics (TAD) simulations of the submonolayer growth of Cu on a biaxially strained Cu(100) substrate are presented. These simulations were carried out to understand the e ects of compressive strain on the structure and morphology. For the case of 4% compressive strain, stacking fault formation was observed in good agreement with experiments on Cu/Ni(100) growth. The detailed kinetic and thermodynamic mechanisms for this transition are also explained. In contrast, for smaller (2%) compressive strain, the competition between island growth and multi-atom relaxation events was found to lead to an island morphology with a mixture of open and closed steps. In Chapter 5, we then study the general dependence of the diffusion mechanisms and activation barriers for monomer and dimer diffusion as a function of strain. The results of TAD simulations of Cu/Cu(100) growth with 8% tensile strain are also presented. In this case, a new kinetic mechanism for the formation of anisotropic islands in the presence of isotropic diffusion was found and explained via the preference for monomer diffusion via exchange over hopp (open full item for complete abstract)

Committee: Jacques Amar Professor (Advisor) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2020, Physics

Energy is considered one of the top problems facing humanity, and climate change clearly require3s growth in the fraction of energy provided by renewable sources in order to minimize the use of fossil fuels. Solar energy is an environmentally friendly technology which avoids nearly all generation of greenhouse gases. It is more available than hydro, wind, and other renewable energy sources. As science and technology advance, various types of solar cells are being produced to harvest solar energy. Silicon technology dominates the photovoltaic (PV) industry. Thin film solar electric technologies are evolving to reduce the cost of energy ($/Watt). Cadmium telluriude (CdTe) is one of the leading thin film technologies with a reduced energy cost and the shortest energy payback time. CdTe is a direct band gap II-VI material and has been investigated for decades to improve device performance, stability, and conversion efficiency. However, the open-circuit voltage (VOC) of CdTe devices is lower than for GaAs solar cells, despite having similar energy band gaps. Simulation and theoretical study show that the voltage deficit can be improved by making an ohmic contact to CdTe, or by reducing the back-contact potential barrier and increasing the doping level of CdTe. In this dissertation, I report on nanomaterials-based back contact interface layers, surface etching, the doping of CdTe using copper (II) chloride (CuCl2), and on the opto-electronic properties of CdTe double heterostructure samples. Nanocrystals (NCs) of FeSe2, FeTe2, NixFe1-xS2, and CuFeS2 were synthesized using hot injection colloidal methods. These materials were then characterized using X-ray diffraction, electron microscopy, Raman and UV-Vis-NIR spectroscopies. The charge transport properties of these nanomaterial-based thin films were tested in CdTe photovoltaics. Nickel iron pyrite (NixFe1-xS2) NCs showed composition-controlled conductivity, and where x = 0.05 (5%), they showed optimal device performanc (open full item for complete abstract)

Committee: Randy Ellingson (Committee Chair); Michael Heben (Committee Member); Robert Collins (Committee Member); S. Thomas Megeath (Committee Member); Mikhail Zamkov (Committee Member) Subjects: Nanoscience; Nanotechnology; Physical Chemistry; Physics

Doctor of Philosophy, The Ohio State University, 2020, Physics

Magnetic data storage has proven to be a viable technique to hold non-volatile memory in which the direction of magnetization is utilized to represent the 0 or 1 state of a computer bit. While various forms of magnetic data storage have been implemented in devices for decades, recent advances in thin film deposition and magnetic characterization techniques have shed light on new approaches to engineering advantageous magnetic properties in novel magnetic materials that can improve their performance. The work reported in this dissertation demonstrates the ability to produce and tune several magnetic properties in two types of material systems that are incorporated into thin film heterostructures. The first system is insulating rare earth garnets, where the first measurement of topological magnetic textures in an insulator at room temperature is reported as well as the detection of a novel and significant interfacial anisotropy due to non-magnetic capping layers. The second system is CoxFe1-x, where the magnetic damping is reduced to the lowest value ever reported for a metallic ferromagnet and the perpendicular magnetic anisotropy is significantly enhanced. All of these material properties and magnetic interactions are improved through epitaxial growth on various substrates. These results highlight the unique properties exhibited by these two material systems, which are beneficial for their potential integration into future spintronic applications.

Committee: Fengyuan Yang (Advisor); Jay Gupta (Committee Member); Andrew Heckler (Committee Member); Ralf Bundschuh (Committee Member) Subjects: Physics

Master of Science (M.S.), University of Dayton, 2019, Electro-Optics

The next generation of electronic and optical devices require high quality, crystalline materials in order to obtain relevant properties for novel devices. Two classes of materials offer unique material properties that can satisfy the requirements for next generation devices. These two classes of materials are the transition metal nitrides (TMNs) and transition metal oxides (TMOs). These materials offer electronic properties that range from conductive, metallic, materials to semiconducting and insulating materials. However, for many optical and electronic applications, the band structure and crystalline symmetries must be preserved. This work examines the growth and characterization of the TMN materials AlN and ScN as well as the TMO materials VO2 and TiO2. In all these materials, the crystalline structure plays and extremely important role in the desired properties. In addition, incorporation of other impurities can detrimentally impact the functionality of these film materials. In order to minimize the impurity incorporation and maintain the crystalline structure, the growth of AlN, ScN, VO2, and TiO2 films by various deposition techniques were examined and optimized. This allowed growth of high quality TMN and TMO materials that resulted in characterization and optimization of the relevant optical, electronic, and structural properties and, somewhat, the degree to which these properties could be tuned through growth conditions.

Committee: Andrew Sarangan PhD (Advisor); Amber Reed PhD (Committee Member); Partha Banerjee PhD (Committee Member) Subjects: Engineering; Materials Science; Optics

Doctor of Philosophy, University of Toledo, 2019, Physics

Thin film solar cells are promising candidates for generation of low cost and pollution-free energy. The materials used in these devices, mainly the active absorber layer, can be deposited in a variety of industry-friendly ways, so that the cost associated with manufacturing is generally lower than for competing technologies such as crystalline silicon. This dissertation will focus on the fabrication and characterization of nanocrystalline hydrogenated silicon (nc-Si:H) and polycrystalline cadmium telluride (CdTe) thin films by industrially scalable, non-toxic, and comparatively simple magnetron sputtering. The performance of the solar cells incorporating these films as an active absorber layers are discussed. In this work, spectroscopic ellipsometry is used as the primary tool for the characterization of optical and structural properties of thin films and bulk material. As a first case study, the anisotropic optical properties of single crystal strontium lanthanum aluminum oxide (SrLaAlO4) in the form of birefringence and dichroism is obtained from Mueller matrix ellipsometry. SrLaAlO4 exhibit uniaxial anisotropic optical properties and the indirect optical band gap of 2.74 eV. A parametric model consisting of parabolic band critical points (CPs) for electronic transitions and a gap function is used to describe the complex dielectric function spectra in both the ordinary and extra-ordinary directions. The modeling in this case study has applications to both nc-Si:H, an indirect band gap semiconductor, and CdTe which may exhibit microstructural anisotropy depending upon the deposition method. Fabrication and characterization of hydrogenated silicon (Si:H) thin films produced by reactive magnetron sputtering is the second case in this study. RTSE and a virtual interface analysis (VIA) are used to track the growth evolution of sputtered Si:H. From these studies, growth evolution diagrams depicting the nucleation of nanocrystallites from the amorphous phase and (open full item for complete abstract)

Committee: Nikolas Podraza (Committee Chair); Robert Collins (Committee Member); Yanfa Yan (Committee Member); Michael Cushing (Committee Member); Sylvain Marsillac (Committee Member) Subjects: Energy; Materials Science; Optics; Physics

Doctor of Philosophy (PhD), Ohio University, 2019, Physics and Astronomy (Arts and Sciences)

Quantum confinement of charge carriers in semiconductor nanostructures have garnered considerable attention in the past few decades. With new materials being discovered and advanced growth techniques allowing them to be engineered into nanoscale devices with atomic precision, the localization of charge carriers is becoming easier to control. The focus of this dissertation is to highlight the employed growth techniques and the characterization of the device structures studied in our lab.In the first project of the dissertation we examined the temporal dynamics of the Optically Generated Electric Field (OGEF) within a CQD device. We demonstrated the potential of using the interdot transition as a sensitive probe to measure electric fields by using photovoltaic band flattening in a Schottky diode structure. A modulated high energy laser was used to create the OGEF leading to photovoltaic band flattening. A CW laser with energy required to create the interdot transition was used to monitor the electric field in the device and characterize the temporal behavior of the field to determine rise time and decay time as well as to show how they depend on different variables.In the Second project we report on monolayer TMD metal semiconductor metal photodetectors produced using a CVD process. The photodetectors showed maximum responsivity of up to 15 A/W. The response time of the devices is found to be on the order of 1 µs, an order of magnitude faster than previous reports. The main project in this dissertation involved using the CVD growth technique employed in developing TMD devices to create deterministic single photon emitters (SPEs) by carrying out the growth on etched substrates. While we have seen successful growth with TMDs growing over perturbations, SPEs are yet to be found. However, in the process of developing these devices we were able to address several challenges in our technique.As highlighted in previous work in the group while the growth technique employed do (open full item for complete abstract)

Committee: Eric Stinaff Ph.D. (Advisor); Sergio Ulloa Ph.D. (Committee Member); David Tees Ph.D. (Committee Member); Wojciech Jadwisienczak Ph.D. (Committee Member) Subjects: Condensed Matter Physics; Physics; Solid State Physics

Master of Science in Materials Science and Engineering (MSMSE), Wright State University, 2019, Materials Science and Engineering

Graphene, a 2-dimensional single layer of carbon, has high carrier mobility, strength and electrical conductivity. Due to the absence of a band gap and chemical reactivity, pristine graphene has less competitiveness in semiconductors and sensors. Functionalizing graphene is imperative in the development of advanced applications. Among various wet chemical or physical vapor deposition, magnetron sputtering is cost-effective, minimum maintenance, user-friendly, and can be used to rapidly deposit nano-particulates or thin films with less contaminations on any substrates surface. This study is to investigate the morphology evolution of the deposited films using magnetron sputtering and to find appropriate conditions for nanoparticulate deposition towards graphene surface functionalization and device fabrication. Experimentally, highly conductive copper and silicon substrate are the choice of the materials for their low-cost and all deposition was performed at preset power and room temperature. The deposition was conducted by varying chamber pressures and times, but the supplied power was held constant. In addition, the post-thermal treatment as applied to study its impacts on morphological changes. Deposition of nonconductive silicon oxide and copper/silicon oxide composite films are also explored. High-resolution scanning electron microscopy (Hi-Res SEM), electron dispersive spectroscopy (EDS), and ImageJ software were utilized to analyze the morphologies of the deposit films, such as the size, density, coverage of nanograin and/or nanoisland, as a function of deposition time, pressure and post treatment temperatures. The mechanism of the film evolution was proposed. The copper nanoparticulates was successfully deposited at high pressure for short time with post annealing.

Committee: Hong Huang Ph.D. (Advisor); Yan Zhuang Ph.D. (Committee Member); Shin Mou Ph.D. (Committee Member) Subjects: Morphology; Nanoscience; Nanotechnology

Master of Science in Engineering, Youngstown State University, 2018, Department of Electrical and Computer Engineering

Gallium oxide (Ga2O3) belongs to the family of transparent conducting oxides (TCOs) which have emerged as attractive semiconductor material due their excellent properties. TCOs offer the combination of high conductivity along with excellent transparency in the visible region and a large direct band gap of 4.9 eV. These open the scope for applications for deep UV optical and high power/high voltage electronic device applications. The objective of this research was to fabricate high quality Ga2O3 thin films by magnetron sputtering which would be used to fabricate optoelectronic devices. The thin films were deposited on double polished c-plane sapphire substrates. Four investigations were conducted in order to optimize the quality of the thin films. First, the effect of using different Ar/O2 mixture for deposition was investigated. Second, the post deposition annealing was investigated where the films were annealed in vacuum and in different gas environments. Third, the effect of different substrate temperature from 20 ℃ to 800 ℃ was investigated. The fourth investigation was where different amounts of tin were introduced in order to perform n-type doping of the films. The structural and elemental compositional properties of the films were determined using x-ray diffraction and energy dispersive spectrometry measurements. ( ¯2 0 1 ) oriented ß-Ga2O3 single crystal thin films were obtained when deposited using 100 % Ar at 500 ℃. The optical characteristics obtained by UV-VIS spectroscopy measurements showed excellent transmission of 90 - 95% and optical bandgaps of 4.7- 5.0 eV. Addition of tin dopants in the films produced a decrease in the optical bandgaps with increasing concentration of tin to the films.

Committee: Tom Nelson Oder PhD (Advisor); Faramarz Mossayebi PhD (Committee Member); Eric MacDonald PhD (Committee Member) Subjects: Engineering; Materials Science; Solid State Physics

Doctor of Philosophy, The Ohio State University, 2017, Physics

Spintronics research over the past two decades has focused on developing an understanding of spin transport in materials currently used in the semiconductor industry for potential spin-based applications. Recently, a surge of interest in spin transport in antiferromagnetic materials and spin interactions in novel materials have led to many promising discoveries. The work in this dissertation explores discrepancies in past discoveries, provides evidence to support recent theories on antiferromagnetic (AFM) spin transport, and develops the capabilities to further explore spin physics in novel states of matter. This dissertation focuses on four primary topics. First, a thickness dependence of the spin Hall angle in Au is discussed as a potential explanation for a large variance in the previously reported values. Second, evidence supporting a highly efficient mode of spin transport mediated by AFM fluctuations is found in the temperature dependence of the spin pumping signal in Pt/NiO/Y3Fe5O12 trilayers. Third, a new low damping metallic ferromagnet is developed and characterized as a potential platform for future spintronic research. Finally, a molecular beam epitaxy system is established with the capabilities to prepare topologically insulating materials that are predicted to host many novel phenomena.

Committee: Fengyuan Yang (Advisor) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2017, Physics and Astronomy (Arts and Sciences)

This dissertation presents a study on the fabrication of Indium Gallium Nitride (InGaN) based heterojunction solar cells using RF magnetron sputtering method. The goal of the study includes improving the electrical, optical and structural properties of InGaN thin films and examining their potential for photovoltaic applications and to reduce the parasitic resistive loses in solar cells. Reactive radio-frequency (RF) magnetron and Direct Current (DC) sputtering are deposition methods for thin films. The characterization techniques include Hall Effect measurement system for electrical properties, UV-Visible Spectroscopy for optical properties, X-Ray Diffraction (XRD) and Energy Dispersive X-Ray Spectroscopy (EDXS) for structural properties and AM 1.5 G irradiance spectrum to measure current-voltage (IV curves) and photovoltaic measurements. Copper Oxide thin films and Beryllium Zinc Oxide thin films are fabricated and their properties are examined for their potential to pair with n-InGaN to form a p-n junction. We conclude Silicon (111) wafer has better electrical properties than RF deposited Copper oxide and BeZnO and used as p-type layer. Aluminum (Al) and Indium Tin Oxide (ITO) are used as back and front metallic contacts respectively. In this study, we present a simple method for optical bandgap tuning of Indium Gallium Nitride (InGaN) thin films by controlling the growth conditions in magnetron RF sputtering. Thin films with different Indium (In) atomic compositions, x = 0.02 to 0.57 are deposited on high temperature aluminosilicate glass and Silicon (111) substrates. Substrate temperature is varied from 35 oC to 450 oC. Total pressure of sputtering gas mixture is kept constant at 12 mTorr but partial pressures of Ar and N2 are varied. Ar partial pressure to total pressure ratio is varied from 0 to 0.75. Optical bandgap values from 1.4 eV to 3.15 eV, absorption coefficient values of ~ 104 /cm to ~ 7 x 105 /cm and critical film thickness values of 0. (open full item for complete abstract)

Committee: Martin Kordesch (Advisor) Subjects: Condensed Matter Physics; Physics

PhD, University of Cincinnati, 2017, Engineering and Applied Science: Materials Science

Microfabricated solid oxide fuel cells (mSOFCs) have recently gained attention as a promising technology, with the potential to offer a low temperature (as low as 300°C), reduced start-up time, and improved energy density for portable power applications. At present, porous Pt is the most common cathode being investigated for mSOFCs. However, there are significant technical challenges for utilizing pure metallic electrodes at the operating temperatures of interest due to their tendency towards Ostwald ripening, as well as no bulk ionic conductivity. Nanocomposite materials (e.g. Pt/YSZ) are a promising alternative approach for providing both microstructural and electrochemical stability to the electrode layer. The overall objective of this research was to explore the processing of nanocomposite metal / metal oxide materials (i.e. Pt/YSZ) for use as a high performance cathode electrode for mSOFCs. The Pt/YSZ nanocomposite cathodes were deposited through a co-sputtering process and found to be stable up to 600°C in air for extended periods of time through an exhaustive materials and electrochemical study. A percolation theory model was utilized to guide the design of the Pt/YSZ composition, allowing for a networked connection of ionic- and electron-conduction through the membrane, leading to an extension of the triple phase boundary (TPB). The Pt/YSZ composite deposition pressure was found to be a key in helping to stabilize the morphology of the film. By increasing the deposition pressure, this led to the formation of intergranular void spacing, or porosity, as well as a reduction of film strain in the post-annealed film. Surface analyses of the composite film demonstrated that the lower film strain led to a minimization of Pt hillock grain coarsening and de-wetting, even after exposure to high temperatures (600°C) for extended periods of time (tested up to 24hrs) in air. Analyses of the Pt/YSZ composite microstructure and composition by TEM confirmed an int (open full item for complete abstract)

Committee: Raj Singh Sc.D. (Committee Chair); Relva Buchanan Sc.D. (Committee Member); Hong Huang Ph.D. (Committee Member); Rodney Roseman Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2016, Physics

Since 1997, spintronics has had applications in magnetic memory storage. Future spintronics technologies may continue to reduce the cost and increase the performance of logic operations. This could be achieved by replacing charge currents with lower energy spin currents and discovering novel phases of matter brought by spin-orbit coupling and spin-spin interactions such as topological insulators, Weyl semi metals, and skyrmion particles. The theory behind these effects is well understood and materials with these properties have been discovered; however, more research is needed to determine the best material for these applications. This dissertation covers the synthesis and spintronic properties of three different materials proposed for such applications. First, pyrochlore iridate thin films are predicted to be topological insulators or Weyl semimetals which may serve as a graphene analog enabling electronic and spin transport at relativistic velocities. Second, yttrium iron garnet (Y3Fe5O12 - YIG) has been proven to be an excellent material for generating spin currents with the objective to optimize heterostructure composition for spin current generation and transport. Third, B20 phase FeGe is a candidate for memory storage using skyrmion spin structures.

Committee: Fengyuan Yang (Advisor); Leonard Brillson (Committee Member); Ralf Bundshuh (Committee Member); Richard Furnstahl (Committee Member) Subjects: Physics

Master of Sciences (Engineering), Case Western Reserve University, 2015, EECS - Electrical Engineering

This thesis reports the development of a direct write, microplasma-based sputtering instrument and associated process for the fabrication of metallic microstructures on rigid and flexible substrates. The process capitalizes on a physical vapor that is generated within a small capillary by Ar ion bombardment of a small diameter metal wire. Forced Ar flow ejects the sputtered vapor through the orifice and onto the substrate. Integration with an x-y-z stage enables the direct patterning of structures without the need for masks. As-deposited, 40 nm-thick Au structures fabricated on glass substrates exhibit a resistivity that is only 20% higher than that of bulk Au. Deposition has also been demonstrated on liquid crystal polymer and PDMS substrates. The process is performed at atmospheric pressure, thereby addressing one of the most significant limitations associated with conventional magnetron sputtering. The direct-write capability and room temperature deposition make this process a potential alternative to ink-jet printing.

Committee: Christian Zorman Dr. (Advisor); Mohan Sankaran Dr. (Committee Member); Philip Feng Dr. (Committee Member) Subjects: Electrical Engineering; Engineering; Materials Science; Plasma Physics

Doctor of Philosophy (Ph.D.), University of Dayton, 2015, Materials Engineering

Zinc oxide (ZnO) is an emerging thin film transistor (TFT) material for transparent flexible displays and sensor technologies, where low temperature synthesis of highly crystallographically ordered films over large areas is critically needed. This study maps plasma assisted synthesis characteristics, establishes polycrystalline ZnO growth mechanisms and demonstrates for the first time low-temperature and scalable deposition of semiconducting grade ZnO channels for TFT applications using reactive high power impulse magnetron sputtering (HiPIMS). Plasma parameters, including target currents, ion species and their energies were measured at the substrate surface location with mass spectroscopy as a function of pressure and applied voltage during HiPIMS of Zn and ZnO targets in O2/Ar. The results were correlated to film microstructure development investigated with x-ray diffraction, atomic force microscopy, scanning electron microscopy and transmission electron microscopy which helped establish film nucleation and growth mechanisms. Competition for nucleation by (100), (101) and (002) oriented crystallites was identified at the early stages of film growth, which can result in a layer of mixed crystal orientation at the substrate interface, a microstructural feature that is detrimental to TFT performance due to increased charge carrier scattering in back-gated TFT devices. The study revealed that nucleation of both (100) and (101) orientations can be suppressed by increasing the plasma density while decreasing ion energy. After the initial nucleation layer, the microstructure evolves to strongly textured with the (002) crystal plane oriented parallel to the substrate surface. The degree of (002) alignment was pressure-dependent with lower deposition pressures resulting in films with (002) alignment less than 3.3°, a trend attributed to less energy attenuation of the low energy (2- 6 eV) Ar+, O+, and O2+ ions observed with mass spectrometry measurements. At pressur (open full item for complete abstract)

Committee: Andrey Voevodin Ph.D. (Committee Chair); Christopher Muratore Ph.D. (Committee Member); Paul Murray Ph.D. (Committee Member); Kevin Leedy Ph.D. (Committee Member); Charles Browning Ph.D. (Committee Member) Subjects: Materials Science