Doctor of Philosophy, The Ohio State University, 2018, Electrical and Computer Engineering

III-V multijunction solar cells (MJSC) are capable of the highest conversion efficiencies among all solar cell classifications. These devices are thus of major interest for both terrestrial and space applications. However, the economics of the terrestrial and space markets leads to significantly different design requirements for III-V MJSCs to become more economically viable in each market. In the terrestrial market, despite their high efficiency, the high manufacturing cost of III-V MJSCs currently limits their applicability in a market that is currently dominated by crystalline silicon. Thus, lower cost III-V MJSC approaches must be developed for them to become more competitive. This intuitively leads to the concept of merging III-V MJSCs with Si solar cells to demonstrate III-V/Si MJSCs. Such an approach simultaneously takes advantage of the high conversion efficiency of III-V MJSCs and the low-cost manufacturing of Si. In the space market, III-V MJSCs are already the dominant technology due to their high efficiency, radiation hardness, and reliability in extreme conditions. However, new III-V MJSC approaches must be developed if they are to push the boundary of conversion efficiency even further. An approach to improve the efficiency and thus economic viability is through the use of additional high-performance sub-cells at optimal bandgaps to more ideally partition the solar spectrum. Although the design requirements for improving the economic viability of III-V MJSCs in the terrestrial and space markets differ drastically, the design of III-V MJSCs can be altered to meet the design requirements for both markets by using the versatile technique of III-V metamorphic epitaxy. This is the growth of relaxed (i.e. unstrained) III-V compounds at a lattice constant that differs from that of the substrate. The major advantage of III-V metamorphic epitaxy is that it provides an additional degree of freedom for III-V MJSC device design. Traditional lattice-matche (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Tyler Grassman (Committee Member); Sanjay Krishna (Committee Member); Lei Cao (Committee Member) Subjects: Electrical Engineering

Master of Science, The Ohio State University, 2024, Electrical and Computer Engineering

Selective area epitaxy in III-V compound semiconductors has been used for decades in applications such as patterned quantum dots, strain blocking in metamorphic growth, and substrate removal. Recent work in integrated photonic structures, photonic crystal lasers, and metamaterials have led to a renewed interest in patterned and selective epitaxy. The ability to embed dielectric materials into an epitaxial layer or to create regions of complete selectivity (no growth) allow for flexibility in device design and monolithic integration. Advances in lithography and fabrication techniques offer opportunities to explore selective epitaxy and length scales that were not previously accessible. This work focuses on nanoscale selective epitaxy using hydrogen silsesquioxane (HSQ) as the dielectric mask. HSQ becomes a silicon oxide, nearly identical to SiO2, after electron beam exposure and development. HSQ patterns smaller than 20 nm wide, 90 nm tall on a 10 um grid on GaAs (100) offcut 6° toward the nearest (111)A were used in selective epitaxy by both molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (MOCVD). Addionally, on-axis GaAs (100) substrates were used in MOCVD as a comparison to the 6° offcut. The grid lines were aligned to the [011] and [01¯1]. The goal of this work was to understand the interaction between the HSQ “nanowalls” and the epitaxy. The specific orientation of HSQ nanowalls had the most significant impact on local epilayer morphology for both MBE and MOCVD growths. Interestingly, the two growth methods yielded effectively opposite effects, with vast differences in epitaxial wetting, growth initiation/inhibition, lateral overgrowth, and type/number/propagation/direction of material imperfections. For both on-axis and off-cut substrates, the MOCVD growths possess highly faceted trenches along the [011] direction that effectively extend down to the substrate surface; no GaAs growth is observed over or adjacent to the HSQ lines. (open full item for complete abstract)

Committee: Tyler Grassman Ph. D. (Advisor); Steven Ringel Ph. D. (Committee Co-Chair) Subjects: Electrical Engineering; Materials Science

Doctor of Philosophy (PhD), Wright State University, 2023, Electrical Engineering

Since their invention in 1958, Integrated Circuits (ICs) have become increasingly more complex, sophisticated, and useful. As a result, they have worked their way into every aspect of our lives, for example: personal electronic devices, wearable electronics, biomedical sensors, autonomous driving cars, military and defense applications, and artificial intelligence, to name some areas of applications. These examples represent both collectively, and sometimes individually, multi-trillion-dollar markets. However, further development of ICs has been predicted to encounter a performance bottleneck as the mainstream silicon industry, approaches its physical limits. The state-of-the-art of today's ICs technology will be soon below 3nm. At such a scale, the short channel effect and power consumption become the dominant factors impeding further development. To tackle the challenge, projected by the ITRS (International Technology Roadmap for Semiconductors) a thinner channel layer seems to be the most viable solution. This dissertation will discuss the feasibility of using 2D (two-dimensional) materials as the channel layer. The success of this work will lead to revolutionary breakthroughs by pushing silicon technology to the extreme physical limit. Starting from graphene in 2004, 2D materials have received a lot of attention associated with their distinct optical, electrical, magnetic, thermal, and mechanical properties. In the year 2010, IBM demonstrated a graphene-based field effect transistor with a cut-off frequency above 100 GHz. The major challenge of applying graphene in large-scale digital circuits is its lack of energy bandgap. Other than carbon, a variety of graphene-like 2D materials have been found in various material systems, like silicene, germanene, phosphorene, MoS2, WS2, MoSe2, HfS2, HfSe2, GaS, and InS, etc. Among all the 2D materials, silicene appears to be the most favored option due to its excellent compatibility with standard silicon technology. Simil (open full item for complete abstract)

Committee: Yan Zhuang Ph.D. (Advisor); Ray Siferd Ph.D. (Committee Member); Junghsen Lieh Ph.D. (Other); Marian K. Kazimierczuk Ph.D. (Committee Member); Saiyu Ren Ph.D. (Committee Member); Henry Chen Ph.D. (Committee Member) Subjects: Chemical Engineering; Chemistry; Electrical Engineering; Engineering; Materials Science; Nanoscience; Nanotechnology; Packaging; Physics; Quantum Physics; Solid State Physics

Master of Science, The Ohio State University, 2005, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science (MS), Ohio University, 2024, Physics and Astronomy (Arts and Sciences)

Kagome materials are defined as such due to a similarity between their lattice structure and the Japanese basket weaving that features interlaced triangles. They have recently become a popular topic of condensed matter research because they have properties that may advance technologies a great deal and develop understandings of fundamental physics due to the presence of these interlaced triangles of atoms. These materials have potential in quantum computing and beyond. One potential application of kagome materials is in magnetoresistive random access memory, or MRAM, a technology that when paired with kagome materials, has potential to revolutionize devices due to it being a non-volatile memory with reduced energy usage as a result of the kagome materials, like Mn3Sn. To do this however, Mn3Sn needs reliable growth procedures that give precise control of the lattice's orientation, and ideally lend themselves to being made into heterostructures. In current work, we present a growth procedure of Mn3Sn on GaN (0001) that allows for control of the lattice orientation by changing only the growth temperature of the surface we are nucleating onto. The GaN films are first nucleated onto nitrided Al2O3 (0001) , then annealed. Mn3Sn is then deposited onto the GaN surface. The growths are done in an ultra-high vacuum environment using molecular beam epitaxy and monitored in-situ using reflection high-energy electron diffraction. The samples were then analyzed using x-ray diffraction to determine out-of-plane lattice parameters, confirm the material was successfully nucleated, and determine the orientations present in the sample. We observed in our experiments that the lower growth temperatures supported growth of majority single oriented films. Additionally, we were successful in growing a epitaxial film with majority c-plane Mn3Sn orientation at temperature 300 ◦C. The c-plane Mn3Sn films were observed to rotate on the GaN (000-1) which is explained in detail in this wor (open full item for complete abstract)

Committee: Arthur Smith (Advisor); Martin Kordesch (Committee Member); David Ingram (Committee Member) Subjects: Condensed Matter Physics; Physics; Solid State Physics

Doctor of Philosophy (PhD), Ohio University, 2024, Physics and Astronomy (Arts and Sciences)

In this dissertation, the growth, surface and interface dynamics of kagome Mn3Sn film is investigated in detail. The studies explore how the structural, orientation and interface dynamics vary with temperature for bulk films. Molecular beam epitaxy is used to prepare the clean sample surfaces that are investigated in-situ using reflection high energy electron diffraction to monitor the growth. A variety of ex-situ techniques are used to obtain information of the morphology, composition and crystallinity of the film. Additionally, theoretical calculations are carried out to support the experimental findings. The first study refers to the deposition of c-plane Mn3Sn on Al2O3 (0001) at 524 ± 5 C. The experimental findings indicates that the resulting film is predominantly c-plane oriented. The samples prepared in this way were found to be discontiguous, showing a 3-dimensional morphology. According to first-principles calculations, Mn3Sn exhibits a displaced Kagome structure in the very first stages of growth, for 2ML and 4ML growth on Al2O3 (0001). This result is then corroborated by calculating the surface formation energies which shows that the Mn3Sn kagome structure is thermodynamically unstable during the initial stages of growth with a separation of the Mn and Sn sub-lattices. This separation helps to explain the observable RHEED pattern. With the aim of understanding of how the orientation would be affected by temperature, the second study discusses the deposition of a-plane Mn3Sn on Al2O3 (0001) at 453 ± 5 C. The experimental analysis shows the in-plane lattice constants of cM = 4.117 ± 0.027 A and bM = 4.943 ± 0.033 A, which is a very unexpected result indicating that the film is strained. Additionally, the deposition under these conditions appear to have 4 an overall 3D island morphology. Furthermore, in an effort to explain these results, two possible orientation relationships between the film and the substrates are proposed, from whic (open full item for complete abstract)

Committee: Arthur Smith (Advisor); Jixin Chen (Committee Member); Martin Kordesch (Committee Member); Nancy Sandler (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2024, Physics

The concept of “topology” provides a new merit for categorizing materials based on the properties that are not changed under continuous transforms. The introduction of this concept into condensed matter physics has led to the prediction and realization of many exotic quantum states in both momentum space and real space. In momentum space, two topologically distinct insulating states are expected to have a well-defined boundary where a metallic state emerges. In terms of material selection, the kagome lattice inherently manifests Dirac cones and flat bands, which becomes topologically nontrivial in the presence of spin-orbit coupling. In real space, the concept of topology can be used to classify different distributions of order parameters, and the ones with nonzero winding numbers are expected to be more stable since they are topologically protected. The magnetic skyrmions, as prototypical spin textures with a winding number of ±1, are of particular interest for next-generation memory and logic devices. This thesis aims to demonstrate that the nontrivial topology can be realized in thin films via epitaxial growth, where molecular beam epitaxy (MBE) plays a crucial role in controlling the sample structure at the atomic scale. Using MBE, we have synthesized thin films of kagome materials with different magnetic orderings: ferromagnetic Fe3Sn2 (Chapter 3), paramagnetic CoSn (Chapter 4). and ferrimagnetic RMn6Sn6 (Chapter 5), and the magnetic properties of these materials are studied using a combination of the magneto-optical Kerr effect (MOKE) and the superconducting quantum interference device (SQUID) magnetometer. In CoSn, we have directly observed topologically non-trivial flat bands using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). We have also established a quantitative connection between the band structures and the transport properties of CoSn by a semiclassical transport theory. In Chapter 6, we show that the real-space topology can (open full item for complete abstract)

Committee: Roland Kawakami (Advisor); Louis DiMauro (Committee Member); Jay Gupta (Committee Member); Yuanming Lu (Committee Member) Subjects: Condensed Matter Physics; Physics

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, 2023, Electrical and Computer Engineering

The objective of this thesis is to create highly efective light emitters for LiDAR technology by utilizing gain materials that are based on semiconductor quantum dots (QDs). The primary component of LiDAR technology is the light source, which is typically a laser. In order to function efectively under various conditions and with optimal efciency, the laser must meet specifc criteria: it should be safe for the eyes, provide high output power, exhibit thermal stability, and be insensitive to back-refections. QD materials possess advantageous properties such as three-dimensional carrier confnement and atom-like gain, resulting in discrete density of states. These properties contribute to an ultralow linewidth enhancement factor ‘α', leading to improved thermal stability, reduced sensitivity to back-refection, narrow spectral linewidth, and low-chirp characteristics under modulation. This study focuses on the development of a light source using diode lasers for two specific LiDAR wavelengths: 905 nm and 1.55 μm. The choice of these wavelengths is based on their respective advantages. The 905 nm wavelength has the lowest absorption in the atmosphere, making it an excellent option for topographic LiDARs used in navigation and environmental sensing. On the other hand, the 1.55 μm wavelength is considered eye-safe because it is blocked by retinal water and protects the cornea from potential damage. The 905 nm emitting QDs are based on the GaAs material platform, while the 1.55 μm wavelength utilizes InP-based materials. The work conducted in this thesis starts with material design and progresses to optimizing growth conditions to achieve high-density and uniform QD ensembles. Subsequently, these QDs are implemented into the epitaxial structure of diode lasers, which are then fabricated and characterized to ensure thermal stability and insensitivity to back-refection. The wavelength regime commonly referred to as the ‘Telecom band' is centered at 1.55 μm has well-establi (open full item for complete abstract)

Committee: Shamsul Arafin (Advisor); Joshua Goldberger (Committee Member); Ronald M Reano (Committee Co-Chair); Sanjay Krishna (Committee Chair) Subjects: Electrical Engineering; Engineering; Nanoscience; Nanotechnology

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

Highly sensitive avalanche photodiodes (APDs) are crucially used to support the receivers in optical communication, free-space communication, single-photon detection, and light detection and ranging (LiDAR) systems due to their internal gain, which improves the sensitivity of the receivers. LiDAR system, especially, requires an extremely high- sensitivity receiver rather than a high speed one to achieve high-accuracy data acquisition for 3D imaging and data transfer. Currently, prevalent LiDAR systems use 905 nm lasers as sources and Silicon (Si) APDs as receivers. Si APDs are low-cost and reliable, but the bandgap of Silicon limits the wavelength range of these LiDAR systems to less than 1150 nm. LiDAR at longer wavelengths promises significant advantages. In particular, 1.55 μm LiDAR is eye-safe and has superior atmospheric transmission. To enable LiDAR at this longer wavelength, a highly sensitive 1.55 μm receiver with low-cost is required. Commercial APDs for 1.55 μm wavelength detection are developed on the InP substrate platform because of their reasonable substrate cost and large production capability. The commercial APDs use separate absorption, charge, and multiplication (SACM) architecture for a device structure in which a high electric field lies in the multiplication region to achieve high gain, while a low electric field locates in the absorption region to reduce tunneling leakage current. These APD technologies typically consist of an In0.53Ga0.47As (InGaAs) absorber and InP or Al0.48In0.52As (AlInAs) multiplier. The commercial InGaAs/InP and InGaAs/AlInAs APDs typically have a gain (M) of 10-40 and an excess noise (F) of 3.5-5 at M=10 depending on what multipliers are used. Although those APDs show sufficient gain to be commercially valuable, the sensitivity of the APDs is limited by high F originating from the similar α/β ratio of InP and AlInAs multipliers (k = α/β, here, α and β are impact ionization coefficients for electron and hole, respective (open full item for complete abstract)

Committee: Sanjay Krishna (Advisor); John. P. R David (Committee Member); Steven A. Ringel (Committee Member); Siddharth Rajan (Committee Member) Subjects: Electrical Engineering; 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, 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, The Ohio State University, 2021, Physics

Advancements in spintronics rely on materials which exhibit stable magnetization or long-lived spin states that can be easily manipulated or probed. One class of two-dimensional (2D) materials, the monolayer transition metal dichalcogenides (TMDs) have gained significant interest due to their large spin-orbit coupling which leads to the existence of inequivalent, spin-split valleys in the band structure and predictions of long-lived spin-valley lifetimes. Furthermore, the TMDs exhibit unique optical selection rules which allow for selective control and manipulation of spin and valley polarization by helicity of light. In addition, due to their weak van der Waals coupling, these materials can be easily picked up and stacked into heterostructures with other 2D or three dimensional (3D) materials without the need to match lattice constant or growth conditions. Here, the strength of one material can surmount the weaknesses of another system, and such heterostructures have been utilized to control exciton diffusion, enhance spin/valley lifetimes, and even induce proximity spin-orbit coupling and ferromagnetism. In this thesis, we focus on measuring the spin/valley dynamics in TMD monolayers and heterostructure devices. Using time-resolved Kerr rotation (TRKR) microscopy, we observe long-lived (>5 ns) spin/valley lifetimes and a complex spatial dependence of spin/valley density in monolayer WS2 flakes. Comparisons to photoluminescence (PL) microscopy allow us to elucidate the roles that resident carriers and dark trion formation play in this spatial dependence and the stabilization of spin-valley lifetime. We extend these techniques to heterostructure devices of monolayer WSe2 and graphene where we reveal an ultrafast quenching of spin-valley signal at the WSe2/graphene heterojunctions. Complementary measurements of PL and photoconductivity demonstrate an efficient, electron dominated charge transfer into graphene, from which we conclude that the quenching of the spin- (open full item for complete abstract)

Committee: Roland Kawakami (Advisor); Ezekiel Johnston-Halperin (Committee Member); Yuan-Ming Lu (Committee Member); Andrew Heckler (Committee Member) Subjects: Condensed Matter Physics; Optics; Physics

Master of Science, The Ohio State University, 2020, Electrical and Computer Engineering

Capacitance measurements are commonly conducted for many types of semiconductor devices. Because they can detect separated charges, they are able to characterize doping concentration, built-in voltage, speed, and more for p/n junctions. This information provides feedback to the crystal growers and help them grow higher quality materials. However, these measurements do have limitations. The complex circuit model used to calculate capacitance from a measured impedance requires assumptions and simplifications. This thesis reviews analyses and best practices for capacitance measurements and presents two innovations that expand their applications. These new approaches use double-mesa p-i-n devices and the dependence of capacitance upon area to characterize important semiconductor properties. This work is especially relevant to infrared detectors based on narrow gap antimonide semiconductors such as Type-II superlattices. One analysis determines the doping polarity (p-type or n-type) of the intrinsic layer in p-i-n devices, and the other provides a more thorough analysis of the components in the circuit model, reducing the number of error-inducing simplifications. These analyses were applied to GaSb and/or 10 monolayer by 10 monolayer InAs/AlSb superlattice p-i-n and n-i-p devices.

Committee: Sanjay Krishna Dr. (Advisor); Siddharth Rajan Dr. (Committee Member); Daniel Jardine Dr. (Committee Member) Subjects: Electrical Engineering

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

With the current CMOS technology reaching its physical scaling limits, new device topologies and materials are being explored to continue its advancement in performance and power consumption for microelectronic chips now found in most consumer products. And with the advent of artificial intelligence (AI), the need for new advanced computing technologies is ever more important nowadays. This puts challenges upon the semiconductor materials therein, and the advanced devices derived from these materials. Germanium, as an alternative to the silicon semiconductor, has garnered much interest in recent years as a high mobility channel replacement material in p-MOS. However, the epitaxial growth of germanium atop a silicon substrate is non-trivial due to the ~4% lattice mismatch between the two crystals. There are multiple avenues to growing low defect density Ge layers on Si; such as growing thick buffer layers, graded buffers, low-temperature growth, and surfactant mediated growth. Various techniques are explored in this dissertation to reduce the density of a specific type of defect (threading dislocation defect) caused by the relaxing of grown Ge layer on Si after the critical thickness limit is passed. Advanced tunneling-based devices offer the highest room temperature oscillation (~2 THz) to date, far beyond any transistors. These tunneling devices are being reported in a wide variety of topics: memory, logic, optoelectronics, high-speed switching, etc. Monolithic integration of tunneling devices with the Si-based technology platform is a challenge. SiGe-based resonant interband tunneling diodes (RITD), incorporating Ge for judicious band offsets, overcome this challenge, and therefore have an advantage over other device topologies. Theoretical simulations are performed to gain a better understanding of the design issues affecting the performance of such devices and a very interrelated device, backwards diodes. Further, InGaAs/AlAs, a semiconductor material com (open full item for complete abstract)

Committee: Paul R. Berger (Advisor); Betty Lise Anderson (Committee Member); Nima Ghalichechian (Committee Member); Roger Loo (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2019, Materials Science and Engineering

Self-assembled nanowires are attractive because of their innate ability to effectively strain relax without the creation of extended defects. This allows for interesting heteroepitaxial growths and extreme heterostructures. III-Nitride nanowires are of particular interest because of the wide range of direct bandgaps available in the material system, spanning form the infrared to the deep ultraviolet finding uses in sensors, photovoltaics, lasers and LEDs. The work presented here will be focused on nanowire LEDs with emission in the ultraviolet grown by molecular beam epitaxy. The first part of this work will discuss the possible inhomogeneities present in self-assembled nanowires and how these manifest themselves in ensemble devices. The effect of nonuniformities (specifically shorts) on the current spreading in devices where many individual diodes are wired in parallel is then addressed, and the use of a short-term-overload bias is shown as a way to reduce the presence of nonuniformities, increasing the efficiency of ensemble devices. Next, alternative substrates are investigated, with the growth of high-quality GaN nanowires being demonstrated on polycrystalline foils, the fabrication of the first UV LED grown directly on metal foil follows. The final portion of this work begins by addressing the grain-dependent uniformity issues present with growth on bulk polycrystalline foils through the use of thin nanocrystalline metal films and amorphous metals. Finally, a different nanowire LED structure is discussed in which the upper portion of the nanowires is coalesced to form a “thin-film” transparent conductive layer, enabling the substitution of the traditional fully conformal thin metal top contact with only a current spreading grid.

Committee: Myers Roberto (Advisor); Siddharth Rajan (Committee Member); Tyler Grassman (Committee Member) Subjects: Electrical Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2018, Physics

The magnetic proximity effect (MPE) has generated a lot of interest recently due to its ability to introduce magnetic order into otherwise non-magnetic systems. It can be realized in 2D material stacks as well as inside 3D material thin film heterostructure. The work in this thesis explores MPE inside a variety of heterostructures using multiple measurement techniques. It demonstrates the first realization of MPE inside a thin film of Pt from a ferrimagnetic insulator CoFe2O4. Next, it experimentally demonstrates a novel growth method for synthesizing high quality thin films of topological Dirac semimetal Na3Bi on Al2O3 substrate, and further extends this growth method to synthesize the first topological Dirac semimetal/magnetic insulator heterostructure of Na3Bi/CoFe2O4. Finally, it lays the groundwork for ambitious studies of MPE inside 2D material/magnetic insulator heterostructures using angle-resolved photoemission spectroscopy. This is accomplished by a new process of transferring flakes of 2D materials on top of freshly deposited thin films while inside an ultra-high vacuum environment.

Committee: Roland Kawakami (Advisor); Yuanming Lu (Committee Member); Jay Gupta (Committee Member); Enam Chowdhury (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2017, Physics

Skyrmions are magnetic spin textures that have a non-zero topological winding number associated with them. They have attracted much interest recently since they can be as small as ~1 nm and could be the next generation of magnetic memory and logic. First, we grow epitaxial films of FeGe by molecular beam epitaxy and characterized the skyrmion properties. This had led us to image skyrmions in real-space with Lorentz transmission electron microscopy for the first time in the United States. Next, from an extensive series of thin and thick films, we have experimentally shown the existence of a magnetic surface state in FeGe and, consequently, any skyrmion material for the first time. Complementary theoretical calculations supported the existence of chiral bobbers—a surface state only predicted in 2015. Next, we fabricated for the first time a new class of skyrmion materials: B20 superlattices. These novel heterostructures of [FeGe/MnGe/CrGe] have now opened the door for tunable skyrmion systems with both Dresselhaus and Rashba Dzyaloshinskii-Moriya interactions. Additionally, we perform resonant soft x-ray scattering to image magnetic spin textures in reciprocal space for FeGe thin films in transmission. We have accomplished the removal of substrate and left an isolated single-crystal FeGe film. Lastly, SrO is grown on graphene as a crystalline, atomically smooth, and pinhole free tunnel barrier for spin injection.

Committee: Roland Kawakami (Advisor); Chris Hammel (Committee Member); Nandini Trivedi (Committee Member); John Beacom (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Ohio University, 2017, Individual Interdisciplinary Program

The interfacial and magnetic properties of the metal-rich phases and reconstructions of MnxNy and GaN are investigated. Thin films of the two most metal-rich phases of MnxNy (ε and ζ) are grown on MgO(001) using a custom-built ultra high vacuum molecular beam epitaxy growth system. By the same means, thin films of GaN with the two most metal-rich reconstructions [c(6×12) and psuedo-1×1+1⁄12] are grown on GaN(0001) and Al2O3(0001). The interfacial properties of these materials such as surface structure and local density of states are investigated in situ with low temperature scanning tunneling microscopy and reflection high energy electron diffraction. The bulk structure of these thin films are investigated using x-ray diffraction. Measurements of the morphology and magnetism of these thin films are made ex situ using atomic/magnetic force microscopy, spin-polarized scanning tunneling microscopy, scanning electron microscopy, vibrating sample magnetometry, and superconducting quantum interference device magnetometry techniques. Measurements of the chemical composition of the samples are made using back scattered electron scanning electron microscopy, energy dispersive x-ray spectroscopy, and Rutherford backscattering spectrometry. These techniques reveal that growth temperature heavily influences the quality of the ε-Mn4N grown. A nucleation temperature below 480 °C is observed to result in the growth of substantial antiferromagnetic η-Mn3N2 grains alongside ferrimagnetic ε-Mn4N grains. The most significant component of perpendicular magnetic anisotropy in ε-Mn4N thin films (9 to 300 nm thick) grown on MgO(001) is attributed to the shape induced partial overlap of Ising domains out-of-plane. Spin-polarized scanning tunneling microscopy and magnetic force microscopy measurements are consistent with the interpretation that these ε-Mn4N thin films form single material out-of-plane spin valves due to this out-of-plane overlapping of Ising domains. The ζ-Mn1 (open full item for complete abstract)

Committee: Arthur Smith (Advisor); Wojciech Jadwisienczak (Committee Member); Savas Kaya (Committee Member); David Ingram (Committee Member) Subjects: Electrical 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