Master of Science (MS), Bowling Green State University, 2023, Physics

Gallium oxide has been recognized as a promising Ultrawide-Bandgap(UWBG) semiconductor with a band gap ranging from 4.3-5.3eV. Beta gallium oxide, a polymorph of Gallium Oxide, has an ultra-wide bandgap of 4.9eV making it a great candidate for next-generation power electronics. In addition, UWBG semiconductors are capable of large breakdown voltage and has the potential of becoming a high-effciency switching device. The electrical characteristics are then analyzed using a Hall effect. Some samples are often annealed or doped to improve their electrical properties. The work presented in this thesis shows the process of recently developed gallium oxide flms with a focus on indium gallium oxide samples. The result of our research shows that signifcant changes to the electrical properties of gallium oxide films can be achieved through doping and annealing. In order to utilize the powerful properties of gallium oxide, this work incorporated indium to grow indium gallium oxide. Homogeneous and heterogeneous samples of indium gallium oxide were grown on the MOCVD with a unique recipe that allowed for controlled precursor injection. Furthermore, growth parameters were varied to test their effectiveness in the interplay between indium oxide and gallium oxide growth. The samples were then characterized using X-ray Diffraction(XRD) and Hall effect system. The XRD system allows for further understanding between the growth parameters and the resulting structure of the material. The combination of the XRD system and Hall effects allows for a great correlation between the MOCVD growth parameters and the resulting film qualities. Samples are annealed in hydrogen to enhance their electrical properties. Our experiments demonstrate enhanced electrical characteristics and even p-type conductivity can be produced with effective growth processing.

Committee: Farida Selim Ph.D (Committee Chair); Marco Nardone Ph.D (Committee Member); Alexey Zayak Ph.D (Committee Member) Subjects: Physics

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2021, Photochemical Sciences

This dissertation work is focused on the deposition of gallium oxide (Ga2O3) thin films by metal organic chemical vapor deposition (MOCVD) method. This material belongs to a special group of wide bandgap oxide semiconductors with high optical transmittance and high levels of conductivity. The importance of this material is generated by the wide range of applications of electronic and optoelectronic devices such as MOSFET's, photo diodes, solar cells, LED's, laser diodes, sensors etc. Through MOCVD technique, an implementation of a Si4+ dopant was incorporated in the monoclinic β-Ga2O3 crystal structure on homoepitaxial and heteroepitaxial β-Ga2O3 single crystal wafers. The MOCVD process allowed us to deposit at a growth rate of 1 μm/hour while controlling the electrical transport properties with this dopant. These films were carefully characterized by surface morphology, crystal structure, levels of conductivity and trapping defects. The work shows that the electron density and conductivity of MOCVD Ga2O3 films are mainly governed by the interplay between dopant concentration, C concentration and the presence of trapping defects in the films, which is most likely applicable for other oxide films grown by MOCVD. Conductive films of Ga2O3 with resistivity in the order of 0.07 Ω.cm were successfully grown. The electron density in most of these films was in the range of 1019 cm−3 but the mobility was limited to 1.5 cm2/V⋅s. Higher mobility of 30 cm2/V⋅s was obtained in some films at the expense of carrier concentration by reducing Si doping level resulting in resistivity in the order of 0.3 Ω.cm. This range of conductivity and mobility is relevant for field-effect transistors (FET) and the applications of Ga2O3 as transparent FET in Deep Ultra-Violet (DUV) technology. The second part of this work focuses on investigating the electronic and crystal structure properties of an indium gallium oxide alloy (IGO) doped with Si4+ ions through MOCVD technique on c-sapph (open full item for complete abstract)

Committee: Farida Selim Dr. (Advisor); Marco Nardone Dr. (Committee Member); Alexander Tarnovsky Dr. (Committee Member); Ellen Grosevski Dr. (Other) Subjects: Materials Science; Physics

Master of Science (MS), Bowling Green State University, 2018, Physics

Gallium Oxide (Ga2O3) is a highly resistive material because of its wide band gap. However, the conductivity of Edge-defined film-fed growth (EFG) Ga2O3 crystal increased by three orders of magnitude when doped with Sn and the conductivity decreased when doped with Fe. This work presents the first study of photoconductivity in Ga2O3. It has been carried out on Ga2O3 bulk single crystals grown by Czochralski (CZ) method. Illumination by sub-band gap light of 400 nm led to 5 orders of magnitude persistent photoconductivity and 8 orders of magnitude increase in carrier density. Such levels are higher than any reported photoconductivity in bulk materials. However, photoconductivity was only observed in undoped CZ grown bulk crystals. Fe and Mg doping led to an opposite effect and decrease in conductivity upon illumination. Gamma Induced Positron Spectroscopy (GIPS) and Digital Coincidence Doppler Broadening of Positron Annihilation Spectroscopy (CDBPAS) measurement was carried to study the defects in the crystal. Both lifetime and S-parameter values indicated that all Ga2O3 single crystals have the same level of defects regardless of the method of growth.

Committee: Farida Selim Ph.D. (Advisor); Marco Nardone Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) 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 (Ph.D.), Bowling Green State University, 2023, Photochemical Sciences

Transparent Semiconducting Oxides (TSOs) represent a special category of materials known for their unique combination of high transparency and electrical conductivity. These materials hold significant importance due to their wide range of applications, including use in electronic and optoelectronic devices such as MOSFETs, solar cells, photodiodes, gas sensors, and LEDs, among others. In recent years, Gallium Oxide (Ga2O3) has garnered substantial attention due to its promising physical and chemical properties. The most stable polymorph of Ga2O3 is known as β-Ga2O3, characterized by a relatively wide bandgap of 4.9 eV and a high breakdown voltage, making it a suitable candidate for high-power electronic devices. However, the full understanding of the optical and electrical properties of this material is still under exploration. One approach to enhancing the optical and electrical properties of β-Ga2O3 is to alloy it with another Group III metal, such as indium. This alloying process induces changes in the defect states within the material, resulting in improved film properties. Doping β-Ga2O3 with shallow donor and acceptor states is another strategy to modify the material's properties. Silicon (Si) is a commonly used donor impurity in β-Ga2O3, while magnesium (Mg) is employed to create shallow acceptor states in the material. The Metal-Organic Chemical Vapor Deposition (MOCVD) technique is employed to grow these films. Trimethylindium (TMI) serves as a precursor to alloy indium oxide with gallium oxide. For donor doping, a separate SiH4 tank is utilized, and an ion implantation process is carried out in some cases to investigate the shallow acceptor levels. Throughout this research, a specialized instrument known as Cryogenic-Thermally Stimulated Emission Spectrometry (C-TSES) is used to provide critical information about the bandgap levels of the materials. The Hall Effect method is employed to gather essential electrical properties data about the films. The fi (open full item for complete abstract)

Committee: Farida Selim Ph.D. (Committee Chair); Howard Cromwell Ph.D. (Other); Joseph Furgal Ph.D. (Committee Member); Marco Nardone Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2023, Materials Science and Engineering

Beta-gallium oxide (β-Ga2O3) has emerged as a highly promising ultrawide bandgap semiconductor with unique advantages, capturing significant attention. The presence of point and extended defects in β-Ga2O3 plays a crucial role in shaping the performance of devices based on this material, as they can either decrease or increase the net doping. However, the field has been lacking direct and detailed experimental information about the atomic-level structure of these defects. Bridging this knowledge gap is crucial to establish connections between the measured material properties and the observed atomic structure of defects in β-Ga2O3. To address this, atomic scale scanning transmission electron microscopy (STEM) was employed in this research to investigate the formation and impact of point and extended defects in β-Ga2O3. In the earlier works, we have used quantitative analysis of atomic and nanoscale defects from STEM images to understand the formation of various types of defects in β-Ga2O3, such as interstitial-divacancy complexes and planar defects. Furthermore, phase transformations in (AlxGa1−x)2O3, directly correlating them with Al incorporation into the lattice, were also extensively studied. Based on the findings, we began to tackle bigger scientific questions regarding the fundamental atomic scale mechanisms driving the phase transition from the β phase to the γ phase in Ga2O3. Additionally, it was also required to gain a deeper understanding of the effects of defect incorporation, such as by Sn doping, Al alloying, Si ion implantation, and Ir metal diffusion, on this phase transformation process. In order to address these questions, an atomic scale investigation was conducted to examine the effect of ion implantation on β-Ga2O3 materials, aiming to unravel the atomic scale mechanism behind structural changes, lattice relaxation, and phase transformation in Si-implanted β- Ga2O3 as a function of Si dose. Furthermore, by combining the secondary ion mass spectrom (open full item for complete abstract)

Committee: Jinwoo Hwang (Advisor); Ezekiel Johnston-Halperin (Committee Member); Siddharth Rajan (Committee Member); Hongping Zhao (Committee Member) Subjects: Materials Science

Master of Science (M.S.), University of Dayton, 2023, Electrical Engineering

Development of high permittivity, or high-k dielectrics for ultra-wide bandgap (UWBG) materials, such as B-Ga2O3, is critical to the electric field management in devices made with UWBG semiconductor materials [1]. Typically, high-k dielectrics are deposited using high-temperature processes, such as pulsed laser deposition (PLD), which poses a significant challenge in the development of B-Ga2O3 devices. This thesis is focused on studying the quality of high-k dielectric Ba{x}Sr{1-x}TiO3 (BST) deposited using PLD on ALD-grown SiO2 on Sn-doped (010) B-Ga2O3 substrates. The PLD deposition parameters of thickness, target composition, and deposition temperature were varied to study multiple variations of Ba{x}Sr{1-x}TiO3 MOSCAP devices. BST deposition conditions were identified to produce a device with an effective breakdown field Eeff,BD ≥ 30 MV/cm within the BST/SiO2 dielectric stack with a current leakage ≤ 10^-8 A/cm^2 at Eeff < 5.8-7.6 MV/cm. Interface defect density was analyzed for these devices using the conductance method [2] and photo-assisted capacitance-voltage (C-V) measurements [3]. The devices with the largest Eeff,BD showed a shallow-level defect density Dit ≤ 1012 cm^-2-eV^-1 and deep-level defect density Nit ~ 2x10^12 cm^-2. The extracted shallow and deep-level interface defect densities indicated a high defect density at the dielectric/semiconductor interface, affirming the need for further interface optimization. This work demonstrates successful fabrication of a BST on B-Ga2O3 MOSCAP device, offering supporting motivation for further investigation into the use of high-k BST for B-Ga2O3 devices.

Committee: Guru Subramanyam (Advisor) Subjects: Electrical Engineering; Engineering; Materials Science; Nanotechnology

Master of Science (M.S.), University of Dayton, 2023, Chemistry

Beta-gallium oxide (BGO) has gained attention in recent years because of its known ultra-wide bandgap, high breakdown electric field, and appropriate mobility, which makes it a promising semiconductor for high-efficiency device applications. In the fabrication process, etching is a critical step in device completion and performance. BGO is widely known to be difficult to etch, due to the strength of its ionic bonds. This difficulty limits the reagents that can etch BGO, which can lead to surface damage and low-aspect ratio architecture. A better etching technique is needed to increase device performance for this semiconductor material. We prove that BGO can be etched without plasma assistance and without the introduction of HF using microwave chemistry. Samples were characterized via atomic force microscopy (AFM) and energy dispersive X-ray spectrometry (EDS).

Committee: Judit Beagle (Advisor); Andrew Green (Committee Member); Shawn Swavey (Committee Member) Subjects: Chemistry; Materials Science

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, The Ohio State University, 2023, Physics

Wide bandgap semiconductors (WBGS) are an exceptionally important class of materials for next-generation microelectronics that exhibit a diverse set of physical phenomena spanning piezo- and ferroelectricity, ferromagnetism, superconductivity, and 2-dimensional electron and hole gases. Oxide and nitride WBGS are particularly well-positioned to accelerate the advancement of renewable energy technologies and improve power electronic efficiencies and life-cycle costs. The identification and capacity for control of optically and electrically active point defects are central in transitioning these materials from research laboratory to practical applications. This dissertation investigates the optoelectronic property-structure relationship of native point defects in wide bandgap semiconductors. By combining spatially resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy (SPS) with surface sensitive x-ray photoelectron spectroscopy (XPS), native anionic point defects and cation defect complexes are identified in ternary ZnGeN2 and binary ScN. The control and refinement of these defects with material growth is essential for coherent integration with complementary GaN architectures. Direct control of point defects on the nanoscale is demonstrated in oxide semiconducting Ga2O3 vertical devices and ZnO nanowire structures by applying electric fields to stimulate the migration of intrinsic defect species. Redistribution of oxygen vacancies (VO) in high power Ga2O3 is demonstrated for the first time by nanoscale hyperspectral imaging after strong reverse biasing while manipulation of oxygen vacancies in suspended ZnO nanowires is selectively driven by applied bias through nanoscale contacts. The density of positively charged interfacial VO is tuned via electric fields to control Schottky barrier heights and reversibly convert metal-ZnO interfaces between Ohmic and Schottky behavior, highlighting the need to consider intrinsic point defect migration i (open full item for complete abstract)

Committee: Leonard Brillson (Advisor); Ciriyam Jayaprakash (Committee Member); Andrew Heckler (Committee Member); Jay Gupta (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2023, Materials Science and Engineering

Ultra-wide-bandgap (UWBG) materials-based power electronic devices suffer from unexpected and uncertain locations of non-uniformity, and high fields degrade these devices, limiting their lifetimes. It is a challenge to identify the exact locations of breakdown (hot spots), and often destructive processes are used, which are costly, time- consuming, and often not realistic. The work presented here is an attempt to demonstrate a non-destructive and reliable photocurrent spectroscopy technique based on the exciton Franz-Keldysh effect in probing the local electric field (F). By including the excitonic effect in quantitatively modeling the absorption lineshape (and, in turn, photocurrent responsivity), F values are estimated while exploring a wide range of physics. A probe that measures F locally is an extremely useful tool for mapping out the F distribution, performing reliability testing, locating hot and cold spots, validating, or refining electrostatic models, and optimizing device geometry. Analyzing the F-dependent responsivity has shed insight into the contributions of self-trapped excitons and self- trapped holes in 𝛽 − 𝐺𝑎2𝑂3 to the photocurrent-production pathway. Polarization- dependent photocurrent spectroscopy is also performed to verify various excitonic transitions, identify the crystallographic axes, and understand their behavior with the applied bias. For solar-blind photodetectors, light polarization could help to make PDs more selective to deep UV waveleng

Committee: Roberto Myers (Advisor); Andrea Serrani (Committee Member); Tyler Grassman (Committee Member); Wolfgang Windl (Committee Member) Subjects: Electrical Engineering; Engineering; Materials Science

Master of Science in Mechanical Engineering (MSME), Wright State University, 2022, Mechanical Engineering

β-Ga2O3 is a robust semiconductor material set with a large band gap of ~4.8 eV, low intrinsic carrier concentration, and high melting point that offers a stable platform for operating electronic devices at high temperatures and extreme environments. The first half of this thesis will cover the fabrication of a fixture and packaging to test electronic components at high temperatures. Then it will highlight the characterization of β-Ga2O3 field effect transistors from room temperature (RT) up to 500 °C. The devices, fabricated with Ni/Au and Al2O3 gate metal-oxide-semiconductor (MOS), demonstrate stable operation up to 500 oC. The tested device shows no measured current degradation in the ID-VD characteristics up to 450 oC. Improvements to the drain current, ID within this temperature range are due to activation carriers from dopants/traps and the negative push in threshold voltage, VT. The device exhibits a drop in ID at 500 °C; however, device characteristics recover once the device returns to RT. Even after 20 hours of device operation at 500 °C, the device shows negligible degradation. Device characteristics such as gate leakage, ION/IOFF ratio, gm, Ron, and contact resistance show monotonic variation with temperature. The experimental results suggest that an optimized choice of metals and gate dielectrics β-Ga2O3 will provide a platform for device operation at high temperatures and extreme environments. The second half of the thesis focuses on creating an electrostatic model of a metal-oxide-semiconductor field effect transistor with COMSOL finite element analysis software to understand the physics behind semiconductor technology.

Committee: Hamed Attariani Ph.D. (Advisor); James Menart Ph.D. (Committee Member); Weisong Wang Ph.D. (Committee Member) Subjects: Electrical Engineering; Mechanical Engineering; Nanotechnology

Doctor of Philosophy, The Ohio State University, 2022, Electrical and Computer Engineering

Gallium oxide in its beta phase (β-Ga2O3) has compelling material properties that have generated an intense worldwide research interest for applications in high-voltage and high-power electronics, radio frequency (RF) electronics, and ultraviolet optoelectronics. The ultra-wide bandgap (UWBG) of 4.8 eV and large breakdown field of 8 MV/cm lead to a potentially superior performance compared to contemporary wide bandgap materials (SiC, GaN) in high-power and high-frequency applications, as well as in harsh radiation environments, due to the predicted improved radiation hardness. The potentially transformative performance advantages of β-Ga2O3 are further enhanced by the ability to grow low-cost, large-area native substrates through melt-based growth methods. This enables high-quality homoepitaxially grown layers for improved device reliability compared to non-native substrates from the significant reduction in mismatch-related defects such as dislocations. This dissertation is focused on accelerating β-Ga2O3 material and device technology through the characterization and understanding of how atomic-level defects impact the performance, reliability, and radiation hardness of cutting-edge β-Ga2O3 transistors and provide pathways to reduce their effects by a combined understanding of growth, defect engineering, and device engineering. This is done by systematically evaluating the effects of the Fe deep acceptor in molecular beam epitaxy (MBE) grown devices, which is then compared with the use of the Mg deep acceptor in metalorganic chemical vapor deposition (MOCVD) grown devices. Furthermore, the exploration of high energy particle irradiation effects and the implications of the evolving defect spectrum from irradiation are studied for materials and devices. The defects causing dispersion in MBE grown δ-doped MESFETs are determined to be located in the buffer. The close proximity of the Fe from the substrate that also surface rides into the epitaxially grown buffer (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Siddharth Rajan (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering; Nanotechnology

Doctor of Philosophy, The Ohio State University, 2022, Physics

As in all physical models, the response of a system to external perturbations is only approximately linear and with greater driving force, becomes nonlinear. The study of light interactions in solids is no different and with the development of laser technology, this can be explored with great detail. The use of femtosecond, and even few-cycle pulse lasers, allows for the study of very intense, localized interactions, which can range from frequency generation and spectral broadening of the incident pulse to photoionization of electrons and optical damage of the material itself. This work presents a trio of experiments that investigate the interaction of strong laser pulses with crystalline solids using femtosecond pulses. The first experiment characterizes the supercontinuum generation (SCG) in undoped, single-crystal yttrium aluminum garnet (YAG) optical fibers. Pump wavelengths near the zero-dispersion wavelength of YAG (1600 nm) are used to study SCG in both the normal and anomalous group-velocity dispersion (GVD) regimes. The output spectra of the fiber over many input pulse energies are collected for each pump wavelength and presented in 2D “energy scans.” Three different criteria are developed to help characterize the SCG in each energy scan. The second experiment looks at laser damage of gallium nitride (GaN) and gallium oxide (Ga2O3) using few-cycle pulses (FCPs) with a central wavelength near 760 nm. These experiments report on the damage thresholds as well as examine the crater morphology of single and multi-shot damage. Simulations using the Keldysh photoionization model and finite-difference time-domain method coupled to ionization and plasma effects are also employed to understand the carrier dynamics for single-shot exposure and possible mechanisms that differentiate FCP from longer femtosecond mechanisms. The final experiment returns to YAG in order to study damage under FCPs using a pump-probe technique known as time-resolved surface micro (open full item for complete abstract)

Committee: Enam Chowdhury (Advisor); Jay Gupta (Committee Member); Gregory Lafyatis (Committee Member); Douglass Schumacher (Committee Member) Subjects: Physics

Master of Science, The Ohio State University, 2021, Electrical and Computer Engineering

Power electronic devices represent one of the commonly used devices in a wide range of applications. With the increasing demand for electrical power, applying voltage on the electronic devices goes up drastically, which means that the new generation of power device requires good performance under high voltages. Nowadays, silicon carbide (SiC), and gallium nitride (GaN) are two of the commonly used materials for power electronic devices because of their wide bandgap, high breakdown field, and high mobility properties. For future high voltage applications, many researchers focus on the beta phase gallium oxide (β-Ga2O3) which has a much wider bandgap and much higher breakdown field comparing with SiC and GaN. However, one of the main obstacles is that β-Ga2O3 has low thermal conductivity, which means that the device temperature will result in degradation of output performance in high voltage applications. In this thesis, I have studied three parameters that are temperature-dependent and designed an Sn-doped β-Ga2O3 MOSFET using β-Ga2O3, sapphire, 4H-SiC, and diamond as the substrate. By analyzing the thermal performance and output performance of the device with different substrates, I found that if only consider the device temperature reduction in low drain bias, using the sapphire substrate, 4H-SiC substrate, and diamond substrate could reduce the increasing lattice temperature by 14% to 67%. At the same time, the peak drain current could increase 7.5% to 22%. Because of the temperature decrease, the degradation of output characteristics is not severe under higher voltages. In this thesis, the device structure is simpler as compared with the commercialized product, but the result of the thesis could provide an approach to solve the self-heating effect for β-Ga2O3 power devices.

Committee: Hongping Zhao (Advisor); Wu Lu (Committee Member) Subjects: Engineering

Doctor of Philosophy, The Ohio State University, 2021, Physics

Metal oxides are of tremendous interest since they offer a wide span of physical properties, including tunable dielectric constants, ferroelectricity, ferromagnetism, ionic conductivity, catalytic activity, and superconductivity. They are critical materials for next generation electronic and energy-related applications including microelectronics, photovoltaic devices, data storage, magnetoelectronic, and fuel cells. While the versatility of metal oxides' electronic applications is exciting, key to the performance of many of these applications is the electronic property-structure relationship. This relationship is fundamentally determined by the electronic structures of metal oxides, which are sensitive to compositions, impurities, surface/interfaces, grain boundaries and, in particular, defects. Defects are commonly believed to play a key role in metal oxides, where they can compensate free carriers, change carrier mobilities, “pin” Fermi level, and create trap states that initiate dielectric breakdown. This Ph.D. thesis describes my work using a combination of growth, processing, and characterization techniques to understand and control native point defects in metal oxides and their devices/nanostructures. By altering growth conditions, or implementing treatments including thermal anneal, plasma, mechanical strain, neutron irradiation, and high electric field, the atomic-scale processes to control electronic defects are characterized in nanometer scale by a complement of depth-resolved cathodoluminescence spectroscopy (DRCLS), Kelvin probe force microscopy (KPFM), surface photovoltage spectroscopy (SPS), and scanning electron microscope (SEM). It is evident in our results that the creation/passivation, redistribution, and configuration changes of defect complexes are critical in affecting the electronic properties of metal oxide materials, i.e., initiating the dielectric breakdown phenomenon. Based on four of my previously published journal articles, this thesis d (open full item for complete abstract)

Committee: Leonard Brillson (Advisor); Enam Chowdhury (Committee Member); Ilya Gruzberg (Committee Member); Fengyuan Yang (Committee Member) Subjects: Condensed Matter Physics; Electrical Engineering; Materials Science; Nanoscience; Physics; Solid State Physics

Doctor of Philosophy, The Ohio State University, 2021, Nuclear Engineering

Additive manufacturing (AM) facilitates rapid prototyping, development of novel designs and replacement of older fabrication techniques. This technique, while still expanding, is being researched to replace current electronic and sensor fabrication methods. AM offers many advantages including fast production times, on hand production capabilities, and customizability. The nuclear industry has begun exploring the possibility of using additive manufacturing for future and current reactor designs and construction. Sensor development is one area within the nuclear realm, where AM could make tremendous improvements. Rapid prototyping of future instrumentation would allow for a faster and better design process where ideas can be tested and modified easily. This paper details the development of an additive manufactured sensor for radiation detection purposes for the nuclear industry. Instrumentation serves many purposes in the nuclear industry from sensors to monitor power that must withstand a harsh reactor environment to precise energy spectroscopy of samples for isotope identification and quantification. Some devices, such as those used in alpha spectroscopy like solid-state semiconductor detectors, require a cleanroom and many different tools and procedures to fabricate completely. AM offers a unique opportunity to fabricate these devices while minimizing the required cleanroom work and equipment and replacing it with fast and easy to use AM machines. Aerosol inkjet deposition is a type of 3D printing that enables the deposition of functional material onto a substrate. This technique can be used for metals, biological material, and dielectrics. Fabrication of radiation and temperature sensors can be achieved rapidly and easily through the deposition of metal nanoparticle inks onto a semiconductor wafer. These devices offer a simple, yet effective device configuration capable of high energy resolution alpha and gamma spectroscopy depending on the semiconductor material. (open full item for complete abstract)

Committee: Raymond Cao (Advisor); Thomas Blue (Committee Member); Anant Agarwal (Committee Member); Pooran Joshi (Committee Member) Subjects: Nuclear Engineering

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2021, Photochemical Sciences

The growth mechanism of PbSe nanorods is confirmed as one-dimensional oriented attachment and the nano-dipole is the confirmed as the driven force to align the oriented direction along (100). By tuning the length of ligand, the strength of nano-dipolar interaction can be tuned, and this will turn on/off the one-dimensional attachment. Meanwhile, acetic acid is confirmed as the trouble for the branching formation where acetic acid will affect the growth of (111) facet which will change the dipole direction which may changed from (100) to (111) or (110) direction. Beside one-dimensional material, two-dimensional materials (PbS nanosheets) also studied. Oriented attachment growth of PbS nanosheets has been confirmed and chloroalkane is used as co-solvent to initial the oriented attachment. In our further study, chlorine ions can also play a role like chloroalkane and it can form single exponential and long decay lifetime PbS nanosheets. Our post-synthesis treatment of PbS nanosheets by trioctylphosphine can enhance the photoluminescence quantum yield of PbS nanosheets from 6% to over 50%. This high quantum yield nanosheets is applied for a down-conversion LED device and a 13% quantum yield is achieved. The up-conversion properties of carbon quantum dots and the low temperature spectrum of gallium oxide with doped by titanium also been studied.

Committee: Liangfeng Sun Ph.D (Advisor); Irina Stakhanova Ph.D (Other); Mikhail Zamkov Ph.D (Committee Member); Peter Lu Ph.D (Committee Member) Subjects: Materials Science; Nanoscience; Optics

Doctor of Philosophy, The Ohio State University, 2020, Electrical and Computer Engineering

β-Ga2O3 has recently attracted attention as an ultra-wide bandgap (4.7 eV) semiconductor that can be controllably doped, and grown directly from the melt in single crystal form. The ease of n-type doping with tetravalent cations, a wide variety of bulk single crystal and epitaxial film growth techniques have triggered worldwide interest in β-Ga2O3. The predicted breakdown electric field (6-8 MV/cm) is higher than that of GaN or SiC (~3 MV/cm), which when combined with electron mobility (predicted ~250 cm2/Vs) and electron velocity (1.2×107 cm/s) yields higher figures of merits than SiC and GaN devices. This thesis presents theoretical and experimental investigations of β-Ga2O3 device designs to achieve high-performance RF and power electronics for future applications. The first part of the thesis analyzes the potential device advantages that can be derived from the fundamental β-Ga2O3 material parameters (electron mobility, saturation velocity, and breakdown field). The theoretical β-Ga2O3 device output power density is calculated and compared to GaN HEMTs to find the potential RF applications of β-Ga2O3 devices. The potential of β-Ga2O3 for power devices is also calculated and discussed. Through detailed 2-D device simulation, device design strategies for utilizing the high breakdown field and mitigating low electron mobility effects are proposed. The thesis then focuses on the experimental demonstration and progress on lateral β-Ga2O3 device designs, including (AlGa)2O3/Ga2O3 MODFETs and delta-doped MESFFETs. (AlGa)2O3/Ga2O3 MODFET is one suitable device structure for high-performance β-Ga2O3 electronics because of the 2-D electron gas (2DEG) channel with high electron mobility. The electrical properties of the grown film and the MODFET device characteristics are studied. MBE-grown Ohmic contact is developed for the (AlGa)2O3/Ga2O3 MODFETs to improve their device performance. The high mobility 2DEG channel and low-resistance Ohmic contact enable the direct (open full item for complete abstract)

Committee: Siddharth Rajan (Advisor); Robert Coffie (Committee Member); Steven Ringel (Committee Member); Wu Lu (Committee Member); Xiaoxue Wang (Committee Member) Subjects: Electrical Engineering; Solid State Physics

Master of Science in Renewable and Clean Energy Engineering (MSRCE), Wright State University, 2019, Renewable and Clean Energy

Due to the large band gap of β-Ga2O3 and recent improvements toward high quality native substrates and the ability to shallow dope epitaxial β-Ga2O3 it is an attractive material for applications in power electronic devices. Such devices require advances in the areas of thin film growth and carrier concentration control to deliver high mobility films appropriate for the device structures. Transmission electron microscopy (TEM) analysis can provide information concerning doping, crystal structure, and internal strain which will be valuable to assess the role of defects and impurities on the transport properties for feedback to optimize the bulk and epitaxial growth processes. The objective of this work is to fabricate high-quality TEM specimens with the help of dual-beam Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to study the quality of Si-doped β-Ga2O3 homoepitaxial films grown by pulsed laser deposition and to analyze the nanoscale features in the films. Final thinning procedures were developed to enable observation of tensile strain attributed to the Si dopant level. These results are important for improving the thin film growth towards nanoscale device design and fabrication

Committee: Hong Huang Ph.D. (Advisor); Gregory Kozlowski Ph.D., D.Sc. (Committee Member); John Boeckl Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering