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, The Ohio State University, 2024, Physics

Ultrawide bandgap Ga2O3 has received increasing amounts of attention over the past decade due to its suitability for high power electronics (e.g. high Baliga Figure of Merit relative to competitors such a Si, SiC & GaN) and relatively low cost of manufacturing bulk substrates via melt growth methods. However, there remain several challenges that must be addressed for the Ga2O3 to fully manifest its potential in the devices of today. Among these challenges are the understanding of the electrical nature of impurities that are commonly found in bulk substrate growth, as well as a detailed description of the events that precede device failure. This PhD dissertation focuses on addressing these particular issues via the combination of optical spectroscopy, electrical measurements, scanning probe spectroscopy and atomic scale imaging. Ir crucibles are typically used in melt-grown substrate growth of Ga2O3, typically leading to significant impurity incorporation. Furthermore, optimized high power electronics is expected to reach high temperatures during use. To understand the impact of Ir on Ga2O3 electrical properties as a function of temperature, Ir contacts were deposited on Ga2O3 substrates and thermally processed at elevated temperatures. The deposition of Ir contacts reveal the presence of new gap states related to the incorporation of Ir into the Ga2O3 lattice, as confirmed by depth-resolved cathodoluminescence (DRCLS) and scanning transmission electron microscopy (STEM). These gap states evolve as a function of rapid thermal processing at elevated temperatures, correlating with Ir diffusion and complex formation. An activation energy and pre-exponential factor characteristic of the diffusion process was extracted using an Arrhenius plot for the first time, indicating that Ir diffuses relatively easily even at moderate temperatures. Depth resolved cathodoluminescence indicates changes in intrinsic defects under the Ir contact as a function of temperature, whi (open full item for complete abstract)

Committee: Leonard Brillson (Advisor); Ilya Gruzberg (Committee Member); Jay Gupta (Committee Member); Samir Mathur (Committee Member) Subjects: Condensed Matter Physics; Electrical Engineering; Materials Science; Physics

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 (MS), Bowling Green State University, 2023, Physics

Thin film metal oxide semiconductors have always been the materials of interest for various device applications due to their wide bandgap and interesting electrical properties. The role of defects in controlling these properties of the materials has also been widely known through extensive research. In order to use these materials for advanced device applications, it is necessary to characterize defects and understand their effect on the electrical properties. The main objective of this thesis is to study three different thin film metal oxide semiconductor samples of Gallium Indium Oxide named as depositions 55, 68 and 70 deposited by using the MOCVD technique. The samples were characterized to examine their electrical properties, and defects were studied to explain the origin of the electrical conductivity and resistivity shown by each of these samples. The samples were measured using Hall effect measurements for electrical characterization, and X-ray Diffraction (XRD) was used for structural characterization. Positron Annihilation Spectroscopy (PAS) technique was applied to study the vacancy type defects in these depositions. Hall effect results indicate that deposition 55 is the most conductive with high electron mobility and low value of sheet resistance, and it is found that this sample exhibits n-type conductivity. The electrical characterization data suggests that deposition 68 is the most resistive among the three depositions with high value of sheet resistance. Deposition 70 was found to exhibit n-type conductivity but showed higher value of sheet resistance as compared to deposition 55. Among the various techniques used for PAS, we applied Doppler Broadening Spectroscopy (DBS) for defects characterization of the depositions. DBS results show the presence of vacancy type defects in all of the three samples and indicate the crucial role of defects in the electrical properties exhibited by the thin films. The defects measurements were analyzed, and it (open full item for complete abstract)

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

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, 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 (Ph.D.), Bowling Green State University, 2019, Photochemical Sciences

Wide band gap oxide semiconductors exhibit a wide range of interesting electronic and optical properties that have been used in optoelectronic devices. Native defects play a significant role in oxide semiconductors and affect their optical and electrical properties. In order to use these materials in devices, understanding and controlling defects are crucial. The focus of this thesis is to study defects in two important wide band gap oxides, strontium titanate (SrTiO3) and gallium oxide (Ga2O3). A wide range of characterization techniques were employed to study the optical and electrical properties of both oxides. The first part of this thesis focuses on enhancing the understanding of photoconductivity and luminescence phenomena in bulk SrTiO3 single crystals. The measurements show that there is a strong correlation between photoconductivity and defect concentration when sub band gap visible light is used as an illumination source. Positron annihilation spectroscopy suggested that Ti-vancacy is behind the photoconductivity phenomena in SrTiO3 single crystals. We also observed an interesting luminescence behavior in SrTiO3 single crystals, that has never been observed in other systems. We believe that the unusual behaviors in SrTiO3 luminescence are associated with the relaxation and expansion of the lattice during phase transition of SrTiO3. The second part of this work aims to investigate defects and electronic properties of epitaxial β-Ga2O3 thin films grown by metal organic chemical vapor deposition (MOCVD) technique on c-sapphire substrate. Thermoluminescence spectroscopy in conjunction with positron annihilation spectroscopy were used to identify the nature of defects and their activation energies. We found that annealing in various environments populate different types of defects. The measurements revealed the presence of large vacancy clusters which has been substantially increased by annealing in argon atmosphere. However, annealing in air modified (open full item for complete abstract)

Committee: Farida Selim Dr. (Advisor); Malcolm Forbes Dr. (Committee Member); Alexey Zayak Dr. (Committee Member); Michael Geusz Dr. (Other) Subjects: Chemistry; Physics

Doctor of Philosophy, The Ohio State University, 2019, Electrical and Computer Engineering

Beta-phase gallium oxide (β-Ga2O3) is attracting significant interest for high-power electronics and ultraviolet optoelectronics due to its ~4.8 eV wide bandgap, large predicted breakdown field, ability to support β-(Al1-xGax)2O3/Ga2O3 heterojunctions, and the availability of large area, melt-grown native substrates for homoepitaxial growth. There is also continued interest for space-based applications due to its predicted high radiation hardness compared to contemporary wide bandgap materials (III-nitrides and SiC). However, the integration of β-Ga2O3 into prospective applications will largely depend on device design innovations as well as the availability of high quality and low defect-density materials. This is considerably important as crystalline defects can adversely affect material properties critical to device operation, output power, threshold voltage, and carrier mobility by causing carrier compensation, scattering, and trapping effects. Defect-induced degradation can also dictate the entire behavior of β-Ga2O3 devices in their intended space-based applications where exposure to energetic radiation particles is typical, leading to introducing defect states in the bandgap. Furthermore, there is an intense need of understanding of metal/ β-Ga2O3 contact and interface properties to ensure large Schottky barrier height and low leakage current for high power operations. However, despite remarkable early progress, the underlying knowledge of metal contact properties, dopants, electrically active defects, and their role on material properties is still very limited. Hence, this research aims to pursue a comprehensive investigation of defects in β-Ga2O3 bandgap, building from the native β-Ga2O3 substrate to subsequent homoepitaxial layers. Using deep level transient and optical spectroscopy (DLTS/DLOS) techniques, experiments have been undertaken to understand the formation, physical structure, electronic, and optical properties of defects in β-Ga2O3 bandgap, wit (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Wu Lu (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

This dissertation describes the design, fabrication and characterization of high-k dielectric enhanced Gallium Nitride (GaN)-based devices. Interface properties of atomic layer deposited (ALD) Aluminum Oxide (Al2O3) on GaN was initially investigated. The conduction band offset of Al2O3/GaN was experimentally found as 2.1 eV. High density of positive interface fixed charge (2.72x1013 cm-2) was observed in the Al2O3/GaN. These interface fixed charges can not only induce electrical field in the oxide which increases the reverse gate leakage, but shift the threshold voltage toward negative to prevent E-mode operation. A theoretical study using remote impurity scattering along with other scattering models showed that these interface fixed charges are able to degrade the electron mobility in the channel.

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