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, 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, 2022, Electrical and Computer Engineering

Gallium nitride (GaN) represents a wide bandgap semiconductor material that has been widely utilized in optoelectronic and electronic devices. As silicon (Si) based power devices are quickly approaching their fundamental material limits, GaN exhibits great potential for next generation power electronic devices due to its wide bandgap (3.4 eV), high breakdown field (3.3 MV/cm), and high electron mobility (1500 cm2/(V∙s)). With the availability of free-standing GaN substrates, GaN vertical power devices with superior performances have been demonstrated. One of the key challenges to further advance the GaN vertical power devices lies in the development of high quality, thick GaN epitaxy with low background doping and high mobility. This dissertation focuses on the development of high-quality GaN metal-organic chemical vapor deposition (MOCVD) growths and vertical GaN PN diodes for high performance power electronic applications. The sources and incorporation mechanisms of typical compensations in GaN MOCVD are investigated. The pre-growth wafer cleaning process and growth susceptor are identified as two major sources of iron (Fe) impurity incorporation. The (EC-0.6) eV defect peak is confirmed to be associated with Fe impurity. Fe impurity can be suppressed with proper wafer cleaning and full coverage of susceptor pocket. An optimized MOCVD GaN growth condition with a typical growth rate (GR) of 2 µm/hr is established as the baseline with low controllable doping (Nd-Na) at 4×1015 cm-3. Background carbon (C) impurity typically increases monotonically with GR. The high background C impurity in MOCVD GaN is related to the low pyrolysis efficiency of NH3. Laser-assisted MOCVD (LA-MOCVD) growth technique is proposed to address this issue using a 9.219 µm wavelength carbon dioxide (CO2) laser. The LA-MOCVD shows higher effective V/III ratios via efficient NH3 decomposition. The background [C] in LA-MOCVD GaN films decreases monotonically as the laser power increases. A lo (open full item for complete abstract)

Committee: Hongping Zhao (Advisor); Wu Lu (Committee Member); Anant Agarwal (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering; Materials Science

Doctor of Philosophy (PhD), Ohio University, 2000, Chemical Engineering (Engineering)

MOCVD of GaN on silicon substrate is a very complicated process with the coupling of heteroepitaxy, fluid dynamics, and chemical mechanisms. This dissertation presents the effect of fluid dynamics and reactor design, specifically the susceptor geometry, susceptor rotation, growth pressure, inlet configuration, and flow distribution, on the epitaxial growth of GaN on Si (111) in a new commercial MOCVD reactor. The properties of the GaN/Si films were characterized using XRD, PL, FT-IR, SEM, AFM, and Hall effect. The fluid dynamics within the reactor was simulated using CFD codes. High quality GaN/Si films were achieved through this work. The geometry of the susceptor greatly affects the structural quality of GaN/Si films. For the manufacturer-supplied susceptor geometry, though single crystal GaN films could be obtained with an AlN buffer layer, the films were dark/gray in appearance with a rough morphology and a broad x-ray rocking curve (FWHM>2°). After modifying the susceptor geometry, smooth, mirror-like, single crystal GaN films were obtained with the best x-ray rocking curve FWHM of 0.2°. Simulation results showed that the geometry of the susceptor had significant effect on the vertical velocity distribution. A higher vertical velocity gradient over the modified susceptor was probably the main reason for the film quality improvement. The inlet configuration and flow distribution have a significant effect on the growth rate, uniformity, morphology, and structural quality of GaN/Si films. The availability of TMG determines the film growth rate and uniformity, but the relationship between TMG mass flux and growth rate is complicated. Adduct formation has very little effect on the film growth rate and reaction efficiency. GaN growth rate decreases with increasing pressure. Moderate pressure changes have little effect on the flow pattern but have a significant effect on the temperature distribution. The temperature gradient over the susceptor increases with the incre (open full item for complete abstract)

Committee: Daniel Gulino (Advisor) Subjects: Engineering, Chemical

Master of Science (MS), Ohio University, 2000, Chemical Engineering (Engineering)

Residual stress in gallium nitride films grown on silicon substrates by metalorganic chemical vapor deposition

Committee: Daniel Gulino (Advisor) Subjects: Engineering, Chemical