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, 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, 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, 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, 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

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

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

The behavior of ferromagnetic alloys of manganese and iron with gallium when coupled with different magnetic and/or non-magnetic systems is investigated. The studies explore how the structural and electronic/magnetic properties vary with thickness and composition, probing systems in the sub-monolayer, ultra-thin, and thin film regimes. Molecular beam epitaxy is used to prepare clean sample surfaces that are subsequently investigated in-situ down to atomic level using scanning tunneling microscopy and Auger electron spectroscopy. A variety of ex-situ methods are also utilized to obtain information about the overall system properties, with additional theoretical calculations accompanying the experimental findings for two of the investigated systems. The first study refers to L10-structured ferromagnetic MnGa(111) ultra-thin films grown on semiconducting GaN(0001) substrates under lightly Mn-rich conditions. Room temperature scanning tunneling microscopy investigations reveal smooth and reconstructed terraces, with the surface structure consisting primarily of a hexagonal-like 2 x 2 reconstruction. Theoretical calculations are carried out using density functional theory, revealing that a Mn-rich 2 x 2 surface structure gives the best agreement with the observed experimental images and Auger electron spectroscopy surface composition investigations. It is found that under such growth conditions, the Mn atoms incorporate at different rates: surfaces become highly Mn-rich, while the bulk remains stoichiometric, making the MnGa system very sensitive to the ratio of elements in its structure. Such behavior reveals a potential recipe for tuning, for example, magnetic properties by carefully controlling the surface reconstruction during growth. With the aim of understanding how the properties change as the growth conditions are varied, we also investigate the structure, surface, and magnetism of ferromagnetic Ga-rich MnGa thin and ultra-thin films grown again on GaN(00 (open full item for complete abstract)

Committee: Arthur Smith (Advisor) Subjects: Condensed Matter Physics; Physics

Master of Science (MS), Ohio University, 2015, Physics and Astronomy (Arts and Sciences)

A trench-like structure has been observed after sub-monolayer Mn is deposited onto N-polar gallium nitride (GaN) surfaces. It is reminiscent of the well-documented formation of the vacancy line structures on Si(001) surfaces. However, little is known about what really causes this trench-like structure to form. Although it was previously hypothesized that it may be due to the Mn atoms, it may in fact be present before Mn deposition, and possibly caused by annealing or some other variations in the growth conditions. This study investigates the causes of the appearance of this trench-like structure on N-polar w-GaN surfaces by optimizing the growth recipe to obtain reproducible trench structures. Additionally, surface structure after Mn deposition onto w-GaN (0001 ¯) was observed and the role of the Mn was analyzed. In the current study, GaN samples were grown on sapphire (0001) substrates using ultra high vacuum molecular beam epitaxy (UHV-MBE). The growth was monitored using a reflection high energy electron diffraction (RHEED) system. For room temperature scanning tunneling microscopy (RT-STM) studies, the samples were transferred in-situ to the analysis chamber, where STM images were taken using a tungsten tip under a constant current condition. In the first attempt, the RHEED patterns of the GaN growth showed the well-known 3×3 reconstruction, which was further confirmed by atomic resolution STM. The sample was annealed at a high temperature range of ~ 600°C to 700°C but no trench-like structure was observed. Another GaN sample was grown under highly Ga-rich conditions and then annealed at a higher temperature, ~ 800°C, for ~ 6 minutes. These growth conditions resulted in the trench-like formation often appearing nearby the c(6×12) reconstruction. The trench-like structure is composed of two sub-units of the c(6×12) reconstruction because of the observable similarities between the trench-like formation and the c(6×12) reconstruction. After depositing ~ 0.15 (open full item for complete abstract)

Committee: Arthur Smith (Advisor) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2014, Materials Science and Engineering

Over the last two decades, group III-nitride compound semiconductor materials have revolutionized modern optoelectronics and high frequency devices. In this work, III-nitride based compound semiconductor nanostructures with tailor-made optoelectronic and magnetic functionalities are investigated. The first research vector concerns design, synthesis and characterization of novel ferromagnetic materials based on III-nitrides involving manipulation of magnetic dopants as well as heteroepitaxy of ferromagnetic materials. Synthesis of III-nitride-GdN epitaxial, ferromagnetic nanocomposites is developed using the technique of plasma assisted molecular beam epitaxy. Magnetic, structural and optical characteristics of these materials are tailored to yield nanocomposites which preserve the structural and semiconducting characteristics of GaN while integrating the ferromagnetic compound GdN. In the second part of this work, the growth, characterization and development of self-assembled III-nitride nanowire based ultraviolet light emitting diodes is explored. These devices are formed by a novel heterostructure which utilizes synthetic gradients in dipole moment per unit volume to mitigate many of the shortcomings of traditional thin film wide bandgap light emitting diode (LED) device designs for deep ultraviolet wavelengths. The optical and electronic characteristics of these devices are investigated by a number of spectroscopic methods. Combination of this heterostructure with the epitaxy of GdN on III-nitrides is found to yield a unique electrical device which allows electrical modulation of narrow linewidth, ultraviolet Gd intra-f-shell fluorescence at significantly lower voltages compared to existing technology. During the course of this work, a number of unique scientific instruments were developed to aid research efforts in the Myers group. The design, construction and operation of a wide spectral bandwidth, ultrafast semiconductor photoluminescence characterization syst (open full item for complete abstract)

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

Doctor of Philosophy, The Ohio State University, 2013, Materials Science and Engineering

In the past twenty years, III-nitride devices have had an enormous impact on semiconductor-based technologies. This impact is seen in both optoelectronic and electronic devices. The aim of this dissertation is to take advantage of III-nitride nanowires grown by plasma-assisted molecular beam epitaxy to form heterostructures that are difficult or impossible to achieve in traditional, thin films. To do this, it is first necessary to establish the growth phase diagrams that correlate the characteristics of GaN nanowires to MBE growth conditions. By using the information in these growth maps we can control growth kinetics and the resulting nanowire structures by making strategic, timely changes to growth conditions. Using this control electronic and optoelectronic III-nitride nanowire devices are created. First, coaxially-oriented AlN/GaN nanowire resonant tunneling diodes are formed on Si substrates. Second, polarization-induced nanowire light emitting diodes (PINLEDs) are fabricated that exhibit electroluminescence at wavelengths from the deep UV into the visible. Because these PINLEDs utilize polarization doping, they can be formed with and without the use of dopants. Device and structural characterization are provided, including a detailed investigation of the mixed material polarity in these nanowires. Finally, the dissertation closes with a discussion of recent work and future ideas for optimizing the PINLED design.

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

PhD, University of Cincinnati, 2009, Engineering : Electrical Engineering

Rare Earth (RE) doped III-nitrides are being widely investigated for potential applications in optical communication and displays, due to the wide and direct energy bandgap of GaN resulting in low thermal quenching of RE ion sharp emission from ultraviolet (UV) through visible to infrared (IR) region. The UC Nanolab has been conducting RE doped GaN research for more than 10 years and many achievements were obtained, ranging from material growth to device fabrication. This dissertation studied RE emission in GaN material, focusing on the effects of electronic impurity (Si) co-doping on RE luminescence. Advanced RE doped GaN electroluminescent devices (ELDs) were also designed and fabricated. Detailed device characterization was carried out and the effect of co-dopant was investigated. Eu-doped GaN thin films were grown on sapphire wafers by molecular beam epitaxy (MBE) technique and the growth conditions were optimized for the strongest Eu luminescence. It was found that GaN thin film quality and Eu doping concentration mutually affected Eu luminescence. High quality GaN:Eu thin films were grown under Ga rich condition (III/V>1), but the strongest Eu luminescence was obtained under slightly N rich condition (III/V<1). The optimum Eu doping concentration is ~0.1-1.0at.%, depending on the GaN:Eu thin film quality. Higher growth temperature (>750°C) was also found to enhance Eu luminescence intensity (~10x) and efficiency (~30x). The effect of Si co-doping in GaN:RE thin films was investigated. Eu photoluminescence (PL) was enhanced ~5-10x by moderate Si co-doping (~0.05at.%) mostly due to the increase of Eu PL lifetime, but decreased very fast at high Si co-doping concentration (>0.08at.%). The increase of Eu PL lifetime is possibly due to the incorporation of Si uniformly distributing Eu ions and shielding Eu-Eu interactions. Combined with the increase in excitation cross section and carrier flux, there is a significant enhancement on Eu PL intensity. The elec (open full item for complete abstract)

Committee: Andrew J. Steckl (Committee Chair); Joseph T. Boyd (Committee Member); Jason C. Heikenfeld (Committee Member); Kenneth P. Roenker (Committee Member); John M. Zavada (Committee Member) Subjects: Electrical Engineering

Master of Science, The Ohio State University, 2012, Electrical and Computer Engineering

In this study Ba0.5Sr0.5TiO3 (BST) thin films were grown on (100) oriented SrTiO3 (STO) substrates using molecular beam epitaxy. The BST films were produced under varying conditions with substrate temperature ranging from 700 °C to 900 °C, nominal oxygen background pressures ranging from ~0 Torr to 1E-6 Torr, and with an oxygen environment ranging from a 200 W plasma to an ambient O2 atmosphere. The films were analyzed using a multitude of techniques including depth resolved cathodoluminescence spectroscopy (DRCLS) surface photovoltage spectroscopy (SPS), atomic force microscopy (AFM), and x-ray diffraction (XRD). The DRCLS spectra obtained from the films display well-resolved emissions at 1.9 eV, 2.1 eV, 2.26 eV, 2.95 eV, and 3.5 eV. Surface photovoltage spectroscopy results correlate well with the emissions observed in the DRCLS and allow for the assignment of the positions of these defect states within the BST band gap. Furthermore, several trends are observed correlating the nature of the 2.9 eV emission to an oxygen related defect.

Committee: Leonard Brillson (Advisor); Siddharth Rajan (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2007, Electrical Engineering

Anion-based InAsP metamorphic step-graded buffers are a novel solution to the lattice-mismatch problem of devices with the lattice constant between InP and InAs grown on InP substrates. Compared to mixed-cation step-graded buffers, recent work shows InAsP step-graded buffers have achieved smoother surface morphology and lower threading dislocation density. The focus of this dissertation will be to investigate the fundamental growth science of InAsP metamorphic step-graded buffers using solid source molecular beam epitaxy. The structural and electronic properties of InAsP step-graded buffers will be explored by various characterization approaches, including high resolution X-ray diffraction, electron microscopy, Hall measurements, etc. and compared to those of mixed-cation step-graded buffers. Based on the characterization results, the buffer design of InAsP step-graded buffers will be optimized, aiming to achieve high-quality material for device applications. This research is expected to provide a further understanding of the growth, material properties and device application of InAsP metamorphic step-graded buffers.

Committee: Steven Ringel (Advisor) Subjects: