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, Case Western Reserve University, 2021, Physics

The crystal structures and band structures of mixed ZnGeN2-GaN alloys, focusing on the 50-50 composition, ZnGeGa2N4, were investigated using first-principles calculations. Two 16-atom unit cell octet-rule-preserving structures were found. The Pmn21 phase was found to be the lowest-energy configuration for ZnGeGa2N4, with a band gap of 3.82 eV and a slightly negative energy of formation. It was found that other compositions can be rendered by random stacking of ZnGeN2 and GaN layers along the orthorhombic b axis of Pmn21 structure in octet-rule-preserving phases. This study has provided the first theoretical insights into the fundamental properties of the mixed ZnGeN2-GaN alloy composition at 50 at. %, and has revealed properties that encourage future experimental studies. Single-crystalline ZnGeN2-GaN films with near 50-50 compositions were synthesized by MOCVD. Within the range of conditions explored here, the optimal growth temperature and pressure were determined to be 670 oC and 550 torr, at which the highest growth rate, 3.46 μm/hr, was obtained for a film grown on r-sapphire. Insertion of a ZnGeN2 buffer layer improved the optical properties of the ZnGeGa2N4 film as measured by photoluminescence spectroscopy. It was found that improved crystal quality as evaluated by the crystal morphology and XRD linewidths is achieved at higher growth temperatures and closer-to-ideal Zn/Ge ratios. The study also revealed that the sensitivity of composition and morphology to growth parameters is specific to the desired alloy composition. This is the first study of epitaxial growth of this alloy phase. It has provided insights to future experiments in the search for possible device applications. Single-crystalline growth of ZnSnN2 by MOCVD was investigated. The films deposited on GaN/Sapphire substrate demonstrated superior crystalline quality, as evidenced by XRD analysis, compared to MBE-grown samples and RF-sputtered samples reported to date. The study reported revea (open full item for complete abstract)

Committee: Kathleen Kash (Committee Chair); Walter R. L. Lambrecht (Committee Member); Jesse Berezovsky (Committee Member); David Matthiesen (Committee Member); Hongping Zhao (Committee Member) Subjects: Condensed Matter Physics; Engineering; Physics

Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)

As a part of my Ph.D research, initially I was involved in construction and calibration of an ultra-high vacuum thin film facility, and later on I studied structural, electronic, and magnetic properties of GaN, CrN, Fe/CrN bilayers, and Fe islands on CrN thin films. All of these films were grown by molecular beam epitaxy and characterized with a variety of state-of-the-art techniques including variable temperature reflection high energy electron diffraction, low temperature scanning tunneling microscopy and spectroscopy, variable temperature vibrating sample magnetometry, variable temperature neutron diffraction and reflectometry, variable temperature x-ray diffraction, x-ray reflectometry, Rutherford backscattering, Auger electron spectroscopy, and cross-sectional tunneling electron microscopy. The experimental results are furthermore understood by comparing with numerical calculations using generalized gradient approximation, local density approximation with Hubbard correction, Refl1D, and data analysis and visual environment program. In my first research project, I studied Ga gas adatoms on GaN surfaces. We discovered frozen-out gallium gas adatoms on atomically smooth c(6×12) GaN(000¯1) surface using low temperature scanning tunneling microscopy. We identified adsorption sites of the Ga adatoms on c(6×12) reconstructed surface. Their bonding is determined by measuring low unoccupied molecular orbital level. Absorption sites of the Ga gas adatoms on centered 6$\times$12 are identified, and their asymmetric absorption on the chiral domains is investigated. In second project, I investigated magneto-structural phase transition in chromium nitride (CrN) thin films. The CrN thin films are grown by molecular beam epitaxy. Structural and magnetic transition are studied using variable temperature reflection high energy electron diffraction and variable temperature neutron diffraction. We observed a structural phase transition at the surface at 277±2 K, and a sharp (open full item for complete abstract)

Committee: Arthur Smith (Advisor); Sergio Ulloa (Committee Member); Tatiana Savin (Committee Chair); Eric Stinaff (Committee Member) Subjects: Condensed Matter Physics; Experiments; Low Temperature Physics; Nanoscience; Physical Chemistry; Physics

Doctor of Philosophy, The Ohio State University, 2013, Physics

Growth of BST films by MBE were carried out under a variety of oxidizing conditions and substrate temperatures. Typical oxygen pressures ranged from 0 Torr, with the substrate feeding atomic oxygen into the growing film, up to 1E-6 Torr with the oxygen either being introduced as an ambient oxygen background or as activated plasma. Films were characterized in-house by x-ray photoemission spectroscopy (XPS), depth resolved cathodoluminescence spectroscopy, and x-ray diffraction while the fabrication of interdigitated capacitor (IDC) structures and subsequent dielectric measurements we performed at Naval Research Laboratories. To facilitate XPS measurements of pristine sample surfaces, an ultra-high vacuum transfer line was designed and built to transfer samples from the MBE growth chamber into the XPS analysis chamber without exposing the sample to air. Depth resolved cathodoluminescence spectroscopy (DRCLS) measurements reveal a strong dependence of the material's electrically active defects on the growth parameters and chemical compositions. In particular, we see a large increase in the 2.55 eV and 2.95 eV emission intensity from the STO substrate for films grown with low oxygen pressures. It is known that the 2.55 eV and 2.95 eV emissions are related to oxygen vacancies, and it is shown that these defects are generated deep within the substrate by out diffusion of oxygen into the substrate. As we increase the oxygen pressure, we see an increase in the intensity of the 2.1 eV and 2.3 eV emission intensities, and we can understand this as the depopulation of a state within the bandgap of the material at 0.6 eV above the valence band allowing transitions from the oxygen vacancy and conduction band into this new state respectively. We then go on to show how an excess of Sr in reduced strontium titanate (STO) films can behave as an acceptor, depopulating this state and again leading to an increase in the 2.1 eV and 2.3 eV emissions. The utility of (open full item for complete abstract)

Committee: Leonard Brillson Dr. (Advisor); Fengyuan Yang Dr. (Committee Member); David Stroud Dr. (Committee Member); Evan Sugarbaker Dr. (Committee Member) Subjects: Physics