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, Electrical and Computer Engineering

Beta-phase gallium oxide (β-Ga2O3), with its ultrawide band gap energy (~4.8 eV), high predicted breakdown field strength (6-8 MV/cm), controllable n-type doping and availability of large area, melt-grown, differently oriented native substrates, has spurred substantial interest for future applications in power electronics and ultraviolet optoelectronics. The ability to support bandgap engineering by alloying with Al2O3 also extends β-(AlxGa1-x)2O3 based electronic and optoelectronic applications into new regime with even higher critical field strength that is currently unachievable from SiC-, GaN- or AlxGa1-xN- (for a large range of alloy compositions) based devices. However, the integration of β-(AlxGa1-x)2O3 alloys into prospective applications will largely depend on the epitaxial growth of high quality materials with high Al composition. This is considerably important as higher Al composition in β-(AlxGa1-x)2O3/Ga2O3 heterojunctions can gain advantages of its large conduction band offsets in order to simultaneously achieve maximized mobility and high carrier density in lateral devices through modulation doping. However, due to the relative immaturity of β-(AlxGa1-x)2O3 alloy system, knowledge of the synthesis and fundamental material properties such as the solubility limits, band gaps, band offsets as well as the structural defects and their influence on electrical characteristics is still very limited. Hence, this research aims to pursue a comprehensive investigation of synthesis of β-(AlxGa1-x)2O3 thin films via metal organic chemical vapor deposition (MOCVD) growth methods, building from the growth on mostly investigated (010) β-Ga2O3 substrate to other orientations such as (100), (001) and (-201), as well as exploring other polymorphs, such as alpha (α) and kappa (κ) phases of Ga2O3 and (AlxGa1-x)2O3 to provide a pathway for bandgap engineering of Ga2O3 using Al for high performance device applications. Using a wide range of material characterization techniqu (open full item for complete abstract)

Committee: Hongping Zhao (Advisor); Siddharth Rajan (Committee Member); Steven A. Ringel (Committee Member); Sanjay Krishna (Committee Member) Subjects: Condensed Matter Physics; Electrical Engineering; Engineering; Materials Science; Nanoscience; Nanotechnology; Physics

Doctor of Philosophy, University of Akron, 2022, Electrical Engineering

This research proposes low-frequency energy cycle optimization-based control algorithms for reducing the DC-link capacitance requirement and a high-frequency energy cycling-based regenerative output dV/dt filter for motor drives. The proposed methodologies improve the volumetric power density, efficiency, and reliability of the motor drives. Three low-frequency energy cycle optimization methods are proposed for the switched reluctance machine (SRM) drive to reduce the DC-link current ripple and DC-link capacitor requirements. The first method is based on an additional bi-directional converter-based active power decoupling stage, which processes the low-frequency content of the inverter's input power ripple. A wide voltage swing of the decoupling capacitor reduces the decoupling capacitor requirement. With this topology, the DC-link capacitor is used only to filter out the switching frequency ripple. Therefore, the capacitor requirement in the drive is significantly reduced. The second method reduces the low-frequency content of the DC-link current ripple in the SRM drive by injecting low-frequency harmonics on the phase currents. This control algorithm uses the ripple components of the DC-link current and generates an additional current reference on top of the original current reference. The controller ensures low-frequency ripple energy circulation between the phases of the electric machine and reduces the pulsating energy requirement from the DC source while maintaining the average torque, torque per ampere, and torque ripple performance. Thirdly, a phase collaborative interleaving fixed switching frequency predictive current control method is proposed where energy cycles for both incoming and outgoing phases are collaboratively together in each switching cycle. This technique moves the dominant low-frequency energy concentration towards the high-frequency region. The new energy spectrum is concentrated around twice the switching frequency (2fsw) and r (open full item for complete abstract)

Committee: Dr. Yilmaz Sozer (Advisor); Dr. Malik E. Elbuluk (Committee Member); Dr. Kevin Kreider (Committee Member); Dr. Alper Buldum (Committee Member); Dr. J. Alexis De Abreu Garcia (Committee Member) Subjects: Engineering

Doctor of Philosophy (PhD), Ohio University, 2011, Physics and Astronomy (Arts and Sciences)

The various thin film systems studied in this dissertation are outlined in Chapter one. Two groups of nitride and oxide thin film semiconductors were studied. The nitride group includes c-InxGa1-xN (x = 12, 19, and 55%), c-(In0.06Ga0.94N/GaN)-SLs, and a-In0.74B0.26N. The oxide group includes a-In0.06Ga0.47Zn0.03O0.45 and various compositions of a-InxGayZnzOβ alloys. All films were grown using reactive magnetron sputtering except c-(In0.06Ga0.94N/GaN)-SLs, which were grown using metal {organic chemical vapor deposition (MOCVD). For c-InxGa1-xaN thin films, two crystallographic orientations are observed, (0002) and (1011). The optical bandgap decreases with increasing In at.%. In addition, we characterize the electrical properties using Hall effect measurements,and relate them to the structure and composition. c-(In0.06Ga0.94N/GaN)-SLs are studied by X-ray diffraction. Simulation for the as-deposited SL was achieved in the frame work of fully strained SL structure. It was found that implantation with Eu3+ ions causes partial degradation of the SL structure, which can be highly recovered by annealing at high temperatures. For a-In0.74B0.26N thin films, we mainly characterize the optical properties using spectroscopic ellipsometry (SE). The measured SE spectra are described very well by a two-layer model structure, which consists of a transparent layer on top of an absorbing layer. We believe that oxidizing the upper layer is caused by formed voids due to alloying with boron, that creates paths for oxygen to diffuse into the upper layer and oxidize it. The optical and magnetic properties of a-In0.06Ga0.47Zn0.03O0.45 thin films were studied in depth. The bandgap determined by various methods has an average value of 3.85 eV, and the re ectivity shows that a-In0.06Ga0.47Zn0.03O0.45 could be a good candidate for anti-reflective coating. We prove that some or all of the indium, gallium, and oxygen ions exist in mixed oxidation states, which manifest an induced FM int (open full item for complete abstract)

Committee: Martin Kordesch PhD (Advisor); David Drabold PhD (Committee Member); Gang Chen PhD (Committee Member); Hugh Richardson PhD (Committee Member) Subjects: Engineering; Nanotechnology; Physics

Master of Science (M.S.), University of Dayton, 2024, Chemical Engineering

There is a continuous thrust for cleaner and more sustainable alternatives for energy conversion with the increasing global energy demand. Among them, photovoltaics, specifically thin film solar cells are highly promising and are one of the fastest growing clean energy technologies in the United States. This research presents the synthesis and characterization of a set of novel multinary copper chalcogenide semiconductor nanocrystals (NCs), CuZn2ASxSe4-x consisting primarily of earth-abundant elements for applications in photovoltaic devices. A modified hot-injection method was used to synthesize these semiconductor NCs containing both S and Se chalcogens. The novelty of the new semiconductor NCs lies in the incorporation of multiple cations as well as two different chalcogen anions within the crystal lattice, which is an achievement from the materials synthesis aspect. The composition-controlled optical and photoluminescence properties of the CuZn2ASxSe4-x NCs were investigated via multi-modal material characterization including x-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence spectroscopy (PL). The crystal structure, as determined from the XRD primarily consisted of the metastable wurtzite (P63mc) phase. The NCs exhibited direct band gap in the visible range that could be tuned both by varying the group III cation within the composition as well as the ratio of S/Se, based on the Tauc plot obtained from the UV-vis characterization. This work lays the groundwork for future investigations into the practical applications of copper chalcogenide NCs in solar energy conversion.

Committee: Soubantika Palchoudhury (Committee Chair); Guru Subramanyam (Committee Member); Robert Wilkens (Committee Member); Robert Wilkens (Committee Member); Guru Subramanyam (Committee Member); Kevin Myers (Advisor); Soubantika Palchoudhury (Committee Chair) Subjects: Aerospace Materials; Alternative Energy; Analytical Chemistry; Biochemistry; Chemical Engineering; Chemistry; Energy; Engineering; Environmental Science; Industrial Engineering; Information Science; Inorganic Chemistry; Materials Science; Nanoscience; Nanotechnology; Nuclear Chemistry; Nuclear Engineering

Doctor of Philosophy, The Ohio State University, 2024, Electrical and Computer Engineering

Contactless, nondestructive measurements of minority carrier lifetime by transient microwave reflectance (TMR) and photoluminescence are used to study the carrier dynamics of several ternary materials: InGaAs, GaAsSb, and InAsSb. As contactless measurements, TMR and photoluminescence can determine quality of as-grown wafers. The minority carrier lifetime is inversely proportional to the diffusion component of the dark current and can be used as an indicator of device performance, without the need for full device fabrication. The ability to yield useful information about wafer quality without the time and cost used for fabrication allows for quick feedback to growers. GaAsSb and InGaAs lattice-matched to InP are candidates for short-wave infrared (SWIR) detection at 1.5 μm, a wavelength used for eye safety and optical communication. The high speed or low signal applications at this wavelength benefit from the use of separate absorber, charge, and multiplier (SACM) avalanche photodiodes (APDs). In these devices, the absorber is optimized for detection at the wavelength of interest, and the multiplier is optimized for gain through impact ionization. InGaAs-based SACM APDs are a mature technology and are available commercially. The multipliers paired with InGaAs, however, typically have high noise. Research into low-noise multipliers has resulted in the demonstration of AlGaAsSb as a low noise material. When AlGaAsSb is paired with InGaAs, the grading material AlInGaAs creates a conduction band offset with AlGaAsSb, limiting bandwidth. GaAsSb lattice-matched to InP has similar properties to InGaAs and could be implemented without a conduction band offset due to the grading material being AlGaAsSb. When a GaAsSb/AlGaAsSb SACM APD was demonstrated, it was found to have higher dark current than commercial InGaAs-based devices. Because these materials are so similar, this was unexpected. As mentioned, the diffusion component of the dark current is inversely proportio (open full item for complete abstract)

Committee: Sanjay Krishna (Advisor); Steve Ringel (Committee Member); Preston Webster (Committee Member); Anant Agarwal (Committee Member); Shamsul Arafin (Committee Member) Subjects: Electrical Engineering

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

Doctor of Philosophy, The Ohio State University, 2023, Electrical and Computer Engineering

The aviation industry faces a growing imperative to decrease greenhouse gas emissions and transition towards more electric aircraft (MEA) and all-electric aircraft (AEA). One key requirement for a successful transition is increasing the power density and efficiency of onboard power converters. Wide-bandgap (WBG) power switching devices, such as silicon carbide (SiC), present a promising solution due to their high voltage capability and rapid switching speed. However, these devices also pose a challenge as their fast switching speed can adversely affect insulation systems. This dissertation aims to investigate and offer a deeper understanding of, as well as solutions to, the challenges related to partial discharge (PD) in aircraft power wiring under high dv/dt voltage pulses generated by SiC devices. The dissertation begins with an overview of the significant technical challenges related to PD that arise from using SiC device-based variable speed drives (VSD). It then discusses existing studies addressing these challenges and the failure of insulation systems due to PD. The literature review highlights disagreements in current research and the absence of comprehensive investigations into PD behavior caused by high dv/dt square-wave voltage pulses and their impact on the premature failure of insulation systems. Subsequently, the dissertation delves into the challenges associated with the PD phenomenon in aviation wires, explaining the factors that influence PD behavior, accurate PD detection methods, and the extraction of meaningful features to quantify PD intensity. This is followed by an experimental study of PD-induced aging under various conditions to explore the effects of different variables on PD behavior and the failure mechanism of aviation wires. The experimental conditions are chosen to examine voltage rise time, voltage amplitude, ambient pressure, ambient temperature, and the dielectric material of aviation wires. The experimental results reveal (open full item for complete abstract)

Committee: Jin Wang (Advisor); Julia Zhang (Committee Member); Anant Agarwal (Committee Member) Subjects: Electrical Engineering

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

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

Power electronics have been an integral part of any power system and high-power wide bandgap (WBG) devices are very attractive owing to their high switching frequency and high efficiency. Large bandgap and high breakdown field make WBG semiconductors very attractive for high-power devices. However, there are a lot of critical challenges when it comes to the efficiency and reliability of designing, fabricating, and packaging such high-power devices. Challenges range from the material properties such as mobility, saturation velocity, thermal conductivity, defect density etc. to the cost of the substrate or availability of the process for epi-layer growth, fabrication, or thermal packaging. This Ph D. research focuses on addressing some of these issues in the development of WBG semiconductor devices with proper thermal packaging for high-power applications. The key contributions of this work towards the high-power devices using WBG semiconductors are: (1) Designing the proper device structure for high-power vertical GaN PN diodes on bulk GaN structures. This includes analytical and numerical simulations to determine different attributes of the structure from the thickness of each layer to the doping concentration. Designing the device structure also includes determining the proper edge termination techniques such as mesa or guard rings for field management. Finally understanding the effects of different kinds of passivation dielectrics on a high-power vertical PN diode. A mix of thin high-k dielectric and a thicker low-k dielectric helps not only to reduce the leakage current but also with electric field management to increase the breakdown voltage. (2) Fabrication and Measurement of the vertical GaN PN diodes for various applications, ranging from low (1 kV) to high (>5 kV) voltage applications. A big issue that this work concentrates on is achieving ohmic contacts for the anode on the pGaN layer of the PN diode. This work reports the lowest p-contact resistance of mi (open full item for complete abstract)

Committee: Wu Lu (Advisor) Subjects: Electrical Engineering; Nanotechnology; Solid State Physics

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

In recent years, direct current power distribution and DC microgrids have gained popularity for a wide range of applications. However, unlike typical AC systems, DC systems must still deal with technical issues such as fault current management/protection, power flow control, power quality management, and the possibility of system instability. The T-type modular DC circuit breaker (T-Breaker) system utilization is proposed in this dissertation as a solution to some of the power quality problems thanks to its compensation capabilities. Inspired by the series and shunt compensation devices in AC transmission and distribution, the T-Breaker device can be utilized in a similar manner to improve the stability in DC grids. Utilizing its modularity feature allows it to be implemented in high voltage DC networks. Its use of locally integrated energy storage and a high tolerance for signal mismatch during quick network transients makes it a distinguished device. When its ancillary compensation functions (shunt, series and series-shunt) are combined with its current breaking function, it can be an all-in-one device that improves future DC grids. This dissertation starts with an overview of the power quality challenges of DC distribution covering the recently proposed solutions to each challenge. The main focus will be on the stability challenges under bus voltage and load power transients when constant power loads (CPLs) are present in the grid. Applications such as electric vehicles, ships, aircrafts and EV charging station contains power electronic converters (dc/dc, dc/ac) that tightly regulate the load, hence they act as CPLs. Due to CPLs' negative incremental impedance, when they interact with the DC system, they might destabilize the grid. Analysis of DC distribution systems's stability has been performed in preliminary studies, and passive stabilization and source/load converter level control strategies have been proposed to address the instability issue, but not (open full item for complete abstract)

Committee: Jin Wang (Advisor) Subjects: Electrical Engineering

Doctor of Philosophy, University of Akron, 2022, Electrical Engineering

This research proposes a direct voltage control approach for electric motors, including the single-stage converter topology and the control algorithms. The proposed motor drive system achieves smooth output voltage waveforms for phase excitations and utilizes them to extend the drive capability besides improving the torque ripple, noise, and vibration performance. Applicable to various motor types, the direct voltage control (DVC) is mainly investigated for driving switched reluctance motor (SRM) in the scope of this thesis. Different voltage regulation-based control algorithms are studied. Since the capability of shaping the phase voltage precisely allows control of any motor variables, this ability enables regulating the phase currents, flux linkages, and phase voltages to obtain superior performance. A finite element analysis (FEA) is performed to characterize the motor for building a dynamic simulation model for an SRM. The developed DVC and the conventional control are simulated using this machine model in comparison to each other. A new dual polarity power converter (DPC) is modeled, which can buck and boost the DC bus voltage and provides a variable voltage generation (VVG). The DPC can process power in both directions and provide a variable voltage in both positive and negative polarities at the motor windings. Following the DPC design process, power boards and gate driver boards are manufactured and populated as modular systems for individual motor phases. The developed converter model is customized and sized to construct a motor drive for the targeted operating conditions of the investigated SRM. It includes a control board to enable the 3-phase operation and a single DC bus as the power source for all three modular power converters. A resistive load setup is built to test the converter's performance. After verifying the DPC's performance for its designed load conditions and position-dependent dynamics, the motor tests are performed. The motor tests (open full item for complete abstract)

Committee: Yilmaz Sozer (Advisor); Patrick Wilber (Committee Member); Alper Buldum (Committee Member); Igor Tsukerman (Committee Member); J. Alexis De Abreu Garcia (Committee Member) Subjects: Aerospace Engineering; Alternative Energy; Electrical Engineering; Electromagnetics; Electromagnetism; Energy; Engineering; Technology

Master of Science, Miami University, 2022, Chemical, Paper and Biomedical Engineering

The effect of aluminum doping on the electrical and photoluminescent (PL) properties of flux-grown single crystal ZnGa2O4 was investigated. The undoped and Al-doped (0.55 at% Al) single crystal samples, with maximum dimensions of 6 mm and 4.5 mm (respectively), were both extremely electrically insulating. Low-temperature photoluminescence of single crystal ZnGa2O4 was measured for the first time. XRF, Raman, and photoluminescence analyses suggest the presence of a ZnO impurity phase in both the undoped and Al-doped single crystal samples, contributing to a strong UV-blue PL emission. A sharp PL peak at 500 nm and an atypical yellow-orange broadband emission, possibly related to impurities and either VO or Oi defects (respectively), were also observed for both samples. A PL peak at 621 nm unique to the Al-doped sample was observed. The effects of forming gas annealing, substrate material, and various PLD process conditions on the electrical properties of thin film ZnGa2O4 were also studied. Electrical resistivity decreased with oxygen partial pressure and increased with substrate temperature. The presence of zinc-rich impurity phases seemingly led to lower resistivities. Annealing in forming gas at 500°C generally led to a reduction in resistivity, while annealing at 700°C led to extreme electrical insulation for all samples.

Committee: Lei Kerr (Advisor); Kevin Leedy (Committee Member); Shashi Lalvani (Committee Member) Subjects: Chemical Engineering; Materials Science

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

The emerging utilization of SiC devices in VSDs for motor systems can lead to various advantages, with regards to the operation voltages, switching speeds, operation temperatures, etc. Therefore, compared with the conventional Si device-based VSDs, SiC device-based VSDs would lead to better efficiency and higher power density, which would be beneficial for the success of MEA applications. However, it also introduces new challenges, especially for motor winding insulation. PD behaviors of motor windings under the altered voltage stress are not clearly shown despite the presence of many studies. The impacts of rise time and frequency on PD behaviors have been reported differently in different publications, which is the initial motivation of the work in this dissertation. This dissertation mainly targets the following general challenges related to PD in random wound motors driven by SiC device-based VSDs: the realization of PD detection and measurement under square-wave excitations with ultra-short rise times, quantifications of rise time/pulse width/frequency/pressure impacts without the interference of uncontrolled voltage, and a PD-free design guideline for random wound motor winding. Accurate PD detection of twisted pair samples is realized through co-detection of conventional electrical and optical PD detection methods. The PD pulse currents detected by the former method have been reported to agree with the UV PD signals detected by the latter method very well. Measurement of the PD is realized through subtractions between waveforms captured with commercially available and custom-designed current sensors. The principles of these findings will be beneficial for future PD-related experimental studies. Utilizing the custom-designed pulse generator, square-wave voltage pulses with various parameters and well-controlled overshoot (less than 5%) are utilized to study the impacts of rise time/pulse width/frequency/pressure without the interference of voltage overshoot. (open full item for complete abstract)

Committee: Jin Wang (Advisor); Julia Zhang (Committee Member); Mahesh Illindala (Committee Member) Subjects: Electrical Engineering

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

Direct current power distribution and dc microgrids have been gaining momentum in recent years for various applications. However, compared to traditional ac systems, dc systems still need to address technical challenges such as fault current management/protection, power flow control, power quality management, and potential system instability. This dissertation proposes the T-Breaker system seeking to address these issues in an all-in-one device with modular multilevel converter functions. It is characterized by its use of locally integrated energy storage, high scalability, and high tolerance to control signal mismatch during the fast network transients. This is a paradigm shift from traditional solid-state circuit breakers as the proposed T-Breaker not only protects against faults, but can also function as an energy router with unparalleled ancillary functions for dc grids. This dissertation starts with a systematic overview of the remaining challenges of dc distribution covering the existing dc circuit breaker technologies, particularly on solid-state circuit breakers, as well as the needs and current status of compensation in dc networks. Taking inspiration from the series and shunt compensation devices in ac transmission and distribution systems, the T-Breaker system with half-bridge sub-modules is derived from traditional solid-state circuit breakers. The proposed circuit has modular multilevel converter functions with an increased number of active switches but no conduction loss penalty when compared with traditional solutions. The basic operation modes and the circuit analysis are carried out, and the limitations are identified for the half-bridge T-Breaker. In order to improve upon these drawbacks, the full-bridge T-Breaker is proposed, with improved performance in sub-module voltage injection to the line, and lowering of total heat flux for more compact thermal designs. Along with the basic functionalities of both T-Breaker topologies, the series and (open full item for complete abstract)

Committee: Jin Wang (Advisor); Vadim Utkin (Committee Member); Julia Zhang (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

This dissertation focuses on the development of chemical vapor deposition (CVD) of β-Ga2O3, an ultra-wide bandgap (UWBG) semiconductor representing one of the most promising semiconducting materials for next generation power electronics. Here, two types of CVD thin film deposition techniques were investigated, including the metalorganic chemical vapor deposition (MOCVD) and the low pressure chemical vapor deposition (LPCVD) methods. The main goal of this work aims to establish the fundamental understanding of this emerging UWBG semiconductor material through comprehensive mapping of the growth parameters combined with extensive material characterization. β-Ga2O3, with an ultra-wide bandgap of 4.5-4.9 eV and capability of n-doping, promises its applications for high power electronics. β-Ga2O3 is predicted to have a high breakdown field (~ 8 MV/cm) with room temperature mobility of ~200 cm2/Vs. The Baliga figure of merit (BFOM) of β-Ga2O3 for power electronics is predicted to be 2 to 3 times higher than that of GaN and SiC. One key advantage for β-Ga2O3 is from its availability of high quality and scalable native substrates synthesized via melt growth methods, which is critical to large-scale production with low cost. Thus, this UWBG material has great potential for future generation high power electronics as well as deep ultraviolet optoelectronics. The MOCVD growth window was explored for β-Ga2O3 thin films grown on native Ga2O3 substrates. Group IV Si was identified as an effective n-type dopant with a wide doping range from 1016-1020 cm-3. Under optimized growth conditions, β-Ga2O3 thin films grown on semi-insulating Fe-doped (010) Ga2O3 substrates demonstrated superior room temperature carrier mobilities of 184 and 194 cm2/V·s with and without intentional Si doping at charge concentration of 2.7×1016 cm-3 and 8.5×1015 cm-3, respectively. Temperature-dependent Hall measurements revealed a peak mobility of ~ 9500 cm2/V·s with extremely low compensation concen (open full item for complete abstract)

Committee: Hongping Zhao (Advisor); Steven Ringel (Committee Member); Siddharth Rajan (Committee Member); Hwang Jinwoo (Committee Member); Patrick Woodward (Committee Member) Subjects: Electrical Engineering; Materials Science

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

Transparent Semiconducting oxides (TSO) belong to a special group of wide bandgap oxide materials that have high optical transmittance and high conductivity at the same time. Wide bandgap semiconductors are extremely important for their use in numerous electronic/ optoelectronic devices including MOSFETs, Photo diodes, solar cell, LED, Laser diode, sensors, etc. Recently, wide bandgap oxide materials, especially Ga2O3 have attracted a great deal of attention from the scientific community. β-Ga2O3 is the most stable polymorphs of Ga2O3 with an ultra-wide bandgap of 4.9 eV, high breakdown voltage, and high Baliga's Figure of Merit (BFM) that make it an ideal candidate for the next generation high power devices. A comprehensive study of material properties of β-Ga2O3 is needed to fabricate high-performance devices. Unfortunately, our understanding of β-Ga2O3 as a semiconductor material is not comprehensive. Point defects (e.g., Cation or anion vacancies, interstitials, etc.) that significantly affect the electrical and optical properties of this material are not yet fully understood. Proper understanding, characterizing and modification of defects can lead to its application in semiconductor-based devices. Moreover, finding suitable donors and acceptors for β-Ga2O3 to tune its electrical conductivity is crucial for its use in electronic devices. In this thesis, different aspects of β-Ga2O3 are addressed as a semiconductor material. We have studied optical, electrical, and structural properties of β-Ga2O3 single crystals and epitaxial thin films grown by several techniques. Major point defects in β-Ga2O3 were investigated using several novel techniques. We have identified and characterized major electronic traps and investigated their effects on the optical, structural, and electrical properties of β-Ga2O3. We discovered an innovative way to dope β-Ga2O3 providing high free carrier density and good mobility while maintaining low defect concentration. A novel spectromete (open full item for complete abstract)

Committee: Farida Selim Ph.D. (Advisor); Amelia Carr Ph.D. (Other); Alexander Tarnovsky Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Chemistry; Materials Science; Physics

Doctor of Philosophy, Case Western Reserve University, 2021, Physics

The ever-increasing number of applications requiring semiconductor materials at their core is driving the need to understand certain oxide and nitride materials. In this thesis, we investigate two of such classes. The first of those is the class of wide band-gap oxides and includes materials like β-〖Ga〗_2 O_3 and the 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 alloy system. β-〖Ga〗_2 O_3 is the most stable of the five phases in which 〖Ga〗_2 O_3 is found to exist. With a significantly high experimentally measured band gap of 4.5-4.9 eV, it is touted to be an excellent material for high-power electronics and UV transparent optoelectronic applications. Using first-principles calculations, we study this material and present the electronic band structure calculations using the quasiparticle self-consistent GW method. Next, we extend this study to the alloy system 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 in which 〖Ga〗_2 O_3 is alloyed with an even higher band-gap material, 〖Al〗_2 O_3. We study the system in both the phases, α and β, present the electronic band structures for varying compositions of Al ranging from 0% to 100%, and predict the most favorable composition and phase for such an alloy to exist. The second class of materials in this thesis is the alloy system formed by the combination of group III- and II-IV nitrides, GaN and 〖ZnGeN〗_2, respectively. In particular, we study the vibrational properties of 〖ZnGeGa〗_2 N_4. 〖ZnGeGa〗_2 N_4, at 50% composition, is an octet-preserving and lowest energy superlattice of half a cell of 〖ZnGeN〗_2 and half GaN along the b-axis of 〖ZnGeN〗_2 in the 〖Pbn2〗_1 structure. Using Density Functional perturbation theory implemented in ABINIT, the phonon modes at the zone center, Γ allow us to calculate longitudinal optical-transverse optical splittings using Born effective charges. In addition, the IR and Raman spectra along with the phonon density of states, and the phonon band structure are presented. Lastly, we study the transition metal (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

Ultra-wide bandgap III-Nitride materials, such as AlxGa1-xN, possess many desirable material properties and have been shown to be suitable for a wide range of RF and power applications. These materials possess high electron saturation velocity and extremely high breakdown electric fields. The combination of these two properties makes them ideal for power density scaling at microwave and mm-wave frequencies. Due to the high critical breakdown electric fields in these devices, it is possible to fabricate ultra-scaled devices with these materials as well, which is important for both RF and power devices. Scaling for RF devices helps to significantly boost the electron saturation velocity but maintaining a high electric field profile, which can significantly cut down on the carrier transit time. For power devices, scaling is also important, as the miniaturization of the power circuitry is critically dependent of smaller and more efficient devices. In addition, the large critical breakdown field of these materials could enable more favorable device characteristics for normally-off devices used in power switching applications. It is critical to investigate AlGaN-based devices for high frequency and high-power applications as the necessity for long-range communications at higher frequencies are becoming a necessity. This thesis discusses into the fabrication, analysis and characterization of ultra-wide bandgap AlxGa1-xN channel devices using heterostructure engineered contact layers aimed at improving contact resistance to these devices. Two different approaches to contact formation to metal organic chemical vapor deposition (MOCVD) grown high Al-composition AlGaN films are discussed. Using this approach state of the art device results are also reported. Novel approaches to electric field management in ultra-wide bandgap nitrides have also been explored in this work. Integration of extreme dielectric constant materials like BaTiO3 as an effective solution to electr (open full item for complete abstract)

Committee: Siddharth Rajan (Advisor); Wu Lu (Committee Member); Anant Agarwal (Committee Member) Subjects: Electrical Engineering; Physics

Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

Silicon Carbide (SiC) power devices become popular in electric/hybrid vehicles, energy storage power converters, high power industrial converters, locomotive traction drives and electric aircrafts. Compared with its silicon counterparts, SiC metal oxide semiconductor field effect transistors (MOSFETs) feature higher blocking voltage, higher operating temperature, higher thermal conductivity, faster switching speed, and lower switching loss. This dissertation studies the medium voltage SiC power switch design, packaging, reliability testing and protection, aiming to achieve high power density low cost design with improved reliability. This work first investigates medium voltage SiC MOSFET short circuit capability and degradation under short circuit events. Lower short circuit energy is an effective approach to protect the medium voltage SiC MOSFET from catastrophic failure and slow down the device degradation under repeated over-current conditions. To ensure high efficiency operation under normal conditions and effective protection under short circuit condition, a three-step short circuit protection method is proposed. With ultra-fast detection, the protection scheme can quickly respond to the short circuit events and actively lower the device gate voltage to enhance its short circuit capability. Eventually, the conventional desaturation protection circuits confirm the faulty condition and softly turns off the device. Based on the 3300 V SiC MOSFET characteristic and circuit parameters, the protection circuit design guideline is provided. The exploration on the medium voltage SiC MOSFET packaging follows. To further increase the power density, the medium voltage SiC device packaging becomes a multi-disciplinary subject involving electrical, thermal, and mechanical design. Multi-functional package components are desired to deal with more than one concerns in the application. The relationship between electrical, thermal, and mechanical properties needs to be understo (open full item for complete abstract)

Committee: Jin Wang (Advisor); Longya Xu (Committee Member); Anant Agarwal (Committee Member); Selamet Ahmet (Committee Member) Subjects: Engineering