Electrical & Computer Engineering Theses and Dissertations
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Item NOVEL GRAPHENE HETEROSTRUCTURES FOR SENSITIVE ENVIRONMENTAL AND BIOLOGICAL SENSING(2024) Pedowitz, Michael Donald; Daniels, Kevin; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The COVID-19 pandemic has underscored the need for rapid, mobile, and adaptable sensing platforms to respond swiftly to pandemic-level emergencies. Additionally, smog and volatile organic compounds (VOCs), which posed significant health risks during last year’s wildfires, highlight the critical need for environmental air quality monitoring. Graphene, with its high sensitivity and fast response times, shows promise as a powerful sensing platform. However, it faces challenges related to low selectivity and the complexities of device fabrication using conventional chemical vapor-deposited graphene grown on metal foil, which requires exfoliation and transfer to suitable substrates.This dissertation explores the use of epitaxial graphene, which is graphene grown from the sublimation of silicon from silicon carbide, and forming heterostructures with legacy functional materials, such as transition metal oxides and selective capture probes like antibodies and aptamers to develop rapid, ultrasensitive, and selective sensors to address critical environmental and public health challenges. Epitaxial graphene provides a single-crystal, lithography-compatible graphene substrate that retains the desirable electronic properties of graphene without the drawbacks associated with transferred materials. This work focuses on creating heterostructures using traditional functional materials, such as manganese dioxide and antibodies, to develop high-quality, selective sensors for both biological and environmental applications. The practical applications of these sensors are demonstrated and validated using techniques such as Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and electrical characterization. Additionally, detailed material analysis on producing these heterostructures is provided, emphasizing their ability to be modified without damaging the underlying graphene surface. This highlights epitaxial graphene's robust and versatile nature and its potential for creating high-quality devices with relatively simple designs. Finally, these biosensors are applied to alternate antibody-antigen systems, including influenza, to enhance disease-tracking capabilities. We also explore advanced functional materials, such as protease-peptide systems, which enable the creation of on-chip chemistry systems previously unattainable with current material systems.Item NOVEL QUASI-FREESTANDING EPITAXIAL GRAPHENE ELECTRON SOURCE HETEROSTRUCTURES FOR X-RAY GENERATION(2024) Lewis, Daniel; Daniels, Kevin M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene, the 2D allotrope of carbon, boasts numerous exceptional qualities like strength, flexibility, and conductivity unmatched for its scale, and amongst its lesser-known capabilities is electron emission at temperatures and electric fields too low to allow for conventional thermionic or field emission sources to function. Driven by the mechanism of Phonon-Assisted Electron Emission (PAEE), planar microstructures fabricated from quasi-freestanding epitaxial graphene (QEG) on silicon carbide have exhibited emission currents of up to 8.5 μA at temperatures and applied fields as low as 200 C and 1 kV/cm, orders of magnitude below conventional electron source requirements.These emission properties can be influenced through variations in microstructure design morphology, and performance is controllable via device temperature and applied field in the same manner as thermionic or field emission sources. As 2D planar devices, graphene microstructure electron emitters can also be encapsulated with a thermally evaporated oxide, granting electrical isolation and environmental resistance, and can even exhibit emission current enhancement under these conditions. Graphene electron emitters expressed as heterostructure material stacks could see implementation as electron emission sources in environments or devices where conventional thermionic or field emission sources can’t be supported due to thermal, power system, or physical size limitations, the presence of contaminants, or even poor vacuum containment. An explorable application could see an oxide-encapsulated graphene electron source paired with a layered interaction-emission anode to create a micron-scale vertical alignment x-ray source with no need of vacuum containment. We investigate these properties with using hydrogen-intercalated quasi-freestanding bilayer epitaxial graphene, a rare and difficult to manufacture formulation that allows the graphene to behave as if it were a freestanding structure, while still benefiting from the macro-scale mechanical strength and fabrication process compatibility afforded by its silicon carbide substrate. The quasi-freestanding nature of the graphene limits substrate phonon interactions, allowing the graphene phonon-electron interactions to dominate, in turn empowering the PAEE mechanic. Our devices benefit from an ease of interaction that is untenable for processes not employing QEG, with the speed and simplicity of fabrication being a hallmark of our investigations. We begin our exploration of how the PAEE mechanism itself can be influenced in our designs, and how process and fabrication optimizations can be leveraged for device applications. Graphene’s role in the fields of microelectronics, condensed matter physics, and materials science is still novel, and rapidly expanding, and our investigations explore a unique facet of this wonder material’s capabilities.Item MONOCRYSTALLINE SUPERSATURATED ALUMINUM LAYERS BURIED IN EPITAXIAL SILICON(2019) Kim, Hyun soo; Iliadis, Agis; Pomeroy, Joshua M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum information device performance in semiconductors and superconductors is limited by the quality of materials and interfaces, particularly the interface to the oxide layer. By merging semiconductors and superconductors in a single crystal material, the oxide layer can be eliminated, and the advantage of both systems can be realized. To explore interface free circuits in quantum computing, I have synthesized and studied a new two-dimensional hole gas in silicon using aluminum layers sandwiched between single crystal Si layers. At high enough Al density, this system is expected to behave as a superconductor in single crystal Si. The samples were fabricated by low temperature molecular beam epitaxy (MBE) with modulation doping of elemental Al. Scanning tunneling microscope (STM) and scanning transmission electron microscope (STEM) images show epitaxial Si layers with low surface defects and no crystalline defects in the Al enriched region. Electrical measurement shows that holes are the dominant carrier in this system with charge carrier densities of $\approx$ 1.39 x 10$^{14}$ cm$^{-2}$, and Hall mobilities of $\approx$ 20 cm$^{2}$/(Vs). The charge carrier density corresponds to $\approx$ (0.93 $\pm$ 0.1) hole per Al dopant atom. Unfortunately, no superconductivity was observed down to 300 mK. The likely reason for this is found to be re-distribution of Al dopants over $\approx$ (17 to 25) nm due to thermal annealing up to 550 $^{\circ}$C, which decreases the peak Al concentration in Si below the critical density for superconductivity. Al has not been well studied as a dopant in Si due to its low solid solubility, low vapor pressure, and tendency to segregate. To better understand Al as a dopant, the structures and electrical properties of incorporated Al in Si(100) are studied using STM. The scanning tunneling spectroscopy (STS) spectra show shifts of band edges on incorporated Al compared to (2x1) Si(100) dimers. To test the compatibility of elemental Al for STM lithography with a hydrogen resist layer, a standard experimental protocol is tested. Elemental Al is evaluated using 3 different metrics: 1) sticking coefficient contrast, 2) effective enthalpy of sublimation contrast, and 3) surface diffusivity by deposition rates. Elemental Al is shown incompatible with STM lithography and hydrogen masking. Our study suggests that other dopants may overcome this difficulty. Finally, a new method using ion implanted wires is a promising technique for making electrical contacts to devices in Si fabricated with STM lithography. Here, I report a new in situ method for detecting ion implanted wires using STM and STS with a novel lock-in technique. Using the ion implanted wires, a-first-of-its-kind STM-patterned nano-wire made of P dopants is demonstrated.Item Radio Frequency Superconducting Quantum Interference Device Meta-atoms and Metamaterials: Experiment, Theory, and Analysis(2016) Zhang, Daimeng; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metamamterials are 1D, 2D or 3D arrays of articial atoms. The articial atoms, called "meta-atoms", can be any component with tailorable electromagnetic properties, such as resonators, LC circuits, nano particles, and so on. By designing the properties of individual meta-atoms and the interaction created by putting them in a lattice, one can create a metamaterial with intriguing properties not found in nature. My Ph. D. work examines the meta-atoms based on radio frequency superconducting quantum interference devices (rf-SQUIDs); their tunability with dc magnetic field, rf magnetic field, and temperature are studied. The rf-SQUIDs are superconducting split ring resonators in which the usual capacitance is supplemented with a Josephson junction, which introduces strong nonlinearity in the rf properties. At relatively low rf magnetic field, a magnetic field tunability of the resonant frequency of up to 80 THz/Gauss by dc magnetic field is observed, and a total frequency tunability of 100% is achieved. The macroscopic quantum superconducting metamaterial also shows manipulative self-induced broadband transparency due to a qualitatively novel nonlinear mechanism that is different from conventional electromagnetically induced transparency (EIT) or its classical analogs. A near complete disappearance of resonant absorption under a range of applied rf flux is observed experimentally and explained theoretically. The transparency comes from the intrinsic bi-stability and can be tuned on/ off easily by altering rf and dc magnetic fields, temperature and history. Hysteretic in situ 100% tunability of transparency paves the way for auto-cloaking metamaterials, intensity dependent filters, and fast-tunable power limiters. An rf-SQUID metamaterial is shown to have qualitatively the same behavior as a single rf-SQUID with regards to dc flux, rf flux and temperature tuning. The two-tone response of self-resonant rf-SQUID meta-atoms and metamaterials is then studied here via intermodulation (IM) measurement over a broad range of tone frequencies and tone powers. A sharp onset followed by a surprising strongly suppressed IM region near the resonance is observed. This behavior can be understood employing methods in nonlinear dynamics; the sharp onset, and the gap of IM, are due to sudden state jumps during a beat of the two-tone sum input signal. The theory predicts that the IM can be manipulated with tone power, center frequency, frequency difference between the two tones, and temperature. This quantitative understanding potentially allows for the design of rf-SQUID metamaterials with either very low or very high IM response.Item MEMS SENSOR PLATFORMS FOR IN SITU CHARACTERIZATION OF LI-ION BATTERY ELECTRODES(2016) Jung, Hyun; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lithium-ion batteries provide high energy density while being compact and light-weight and are the most pervasive energy storage technology powering portable electronic devices such as smartphones, laptops, and tablet PCs. Considerable efforts have been made to develop new electrode materials with ever higher capacity, while being able to maintain long cycle life. A key challenge in those efforts has been characterizing and understanding these materials during battery operation. While it is generally accepted that the repeated strain/stress cycles play a role in long-term battery degradation, the detailed mechanisms creating these mechanical effects and the damage they create still remain unclear. Therefore, development of techniques which are capable of capturing in real time the microstructural changes and the associated stress during operation are crucial for unravelling lithium-ion battery degradation mechanisms and further improving lithium-ion battery performance. This dissertation presents the development of two microelectromechanical systems sensor platforms for in situ characterization of stress and microstructural changes in thin film lithium-ion battery electrodes, which can be leveraged as a characterization platform for advancing battery performance. First, a Fabry-Perot microelectromechanical systems sensor based in situ characterization platform is developed which allows simultaneous measurement of microstructural changes using Raman spectroscopy in parallel with qualitative stress changes via optical interferometry. Evolutions in the microstructure creating a Raman shift from 145 cm−1 to 154 cm−1 and stress in the various crystal phases in the LixV2O5 system are observed, including both reversible and irreversible phase transitions. Also, a unique way of controlling electrochemically-driven stress and stress gradient in lithium-ion battery electrodes is demonstrated using the Fabry-Perot microelectromechanical systems sensor integrated with an optical measurement setup. By stacking alternately stressed layers, the average stress in the stacked electrode is greatly reduced by 75% compared to an unmodified electrode. After 2,000 discharge-charge cycles, the stacked electrodes retain only 83% of their maximum capacity while unmodified electrodes retain 91%, illuminating the importance of the stress gradient within the electrode. Second, a buckled membrane microelectromechanical systems sensor is developed to enable in situ characterization of quantitative stress and microstructure evolutions in a V2O5 lithium-ion battery cathode by integrating atomic force microscopy and Raman spectroscopy. Using dual-mode measurements in the voltage range of the voltage range of 2.8V – 3.5V, both the induced stress (~ 40 MPa) and Raman intensity changes due to lithium cycling are observed. Upon lithium insertion, tensile stress in the V2O5 increases gradually until the α- to ε-phase and ε- to δ-phase transitions occur. The Raman intensity change at 148 cm−1 shows that the level of disorder increases during lithium insertion and progressively recovers the V2O5 lattice during lithium extraction. Results are in good agreement with the expected mechanical behavior and disorder change in V2O5, highlighting the potential of microelectromechanical systems as enabling tools for advanced scientific investigations. The work presented here will be eventually utilized for optimization of thin film battery electrode performance by achieving fundamental understanding of how stress and microstructural changes are correlated, which will also provide valuable insight into a battery performance degradation mechanism.Item Engineering thin films of magnetic alloys and semiconductor oxides at the nanoscale(2016) Xie, Ting; Gomez, R. D.; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The thesis aims to exploit properties of thin films for applications such as spintronics, UV detection and gas sensing. Nanoscale thin films devices have myriad advantages and compatibility with Si-based integrated circuits processes. Two distinct classes of material systems are investigated, namely ferromagnetic thin films and semiconductor oxides. To aid the designing of devices, the surface properties of the thin films were investigated by using electron and photon characterization techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and energy-dispersive X-ray spectroscopy (EDS). These are complemented by nanometer resolved local proximal probes such as atomic force microscopy (AFM), magnetic force microscopy (MFM), electric force microscopy (EFM), and scanning tunneling microscopy to elucidate the interplay between stoichiometry, morphology, chemical states, crystallization, magnetism, optical transparency, and electronic properties. Specifically, I studied the effect of annealing on the surface stoichiometry of the CoFeB/Cu system by in-situ AES and discovered that magnetic nanoparticles with controllable areal density can be produced. This is a good alternative for producing nanoparticles using a maskless process. Additionally, I studied the behavior of magnetic domain walls of the low coercivity alloy CoFeB patterned nanowires. MFM measurement with the in-plane magnetic field showed that, compared to their permalloy counterparts, CoFeB nanowires require a much smaller magnetization switching field , making them promising for low-power-consumption domain wall motion based devices. With oxides, I studied CuO nanoparticles on SnO2 based UV photodetectors (PDs), and discovered that they promote the responsivity by facilitating charge transfer with the formed nanoheterojunctions. I also demonstrated UV PDs with spectrally tunable photoresponse with the bandgap engineered ZnMgO. The bandgap of the alloyed ZnMgO thin films was tailored by varying the Mg contents and AES was demonstrated as a surface scientific approach to assess the alloying of ZnMgO. With gas sensors, I discovered the rf-sputtered anatase-TiO2 thin films for a selective and sensitive NO2 detection at room temperature, under UV illumination. The implementation of UV enhances the responsivity, response and recovery rate of the TiO2 sensor towards NO2 significantly. Evident from the high resolution XPS and AFM studies, the surface contamination and morphology of the thin films degrade the gas sensing response. I also demonstrated that surface additive metal nanoparticles on thin films can improve the response and the selectivity of oxide based sensors. I employed nanometer-scale scanning probe microscopy to study a novel gas senor scheme consisting of gallium nitride (GaN) nanowires with functionalizing oxides layer. The results suggested that AFM together with EFM is capable of discriminating low-conductive materials at the nanoscale, providing a nondestructive method to quantitatively relate sensing response to the surface morphology.Item An Integrated Gas Sensing System Based on Surface-Functionalized Gallium Nitride Nanowires with Embedded Micro-Heaters(2015) Liu, Guannan; Peckerar, Martin C; Motayed, Abhishek; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the last few decades, significant improvements have been made in gas sensor technologies. Metal-oxide sensors have been used for low-cost detection of combustible and toxic gases. However, hurdles relating to sensitivity, stability and selectivity still remain. Recently, nanotechnology has helped tremendously through the introduction of nano-engineered materials like nanowires and nanoclusters. Nanowire sensors have much better sensitivity as compared with thin-film devices due to the larger detecting surface-to-volume ratio. But clearly, improvements are still needed. For real-world applications, selectivity between different classes of compounds, such as combustible and toxic gases, is highly desirable. An ideal chemical sensor should distinguish between the individual analytes from a single class of compounds. For example, in detection of benzene or toluene, a good sensor will not be disturbed by other aromatic compounds present in the environment. This is a huge challenge for semiconductor based metal-oxide sensors, such as TiO2, SnO2, Fe2O3 and ZnO, which have inherent non-selective surface adsorption sites. Recently, a new class of nanowire-nanocluster (NWNC) based gas sensors has gained interest. This type of sensor represents a new method of functionalizing the surface for selective adsorption and detection. The adjustable sensitivity can be achieved by tuning the density, size or composition of the nanoparticles that decorate the nanowires. These advantages make the NWNC sensors a good alternative to conventional thin-film sensors. So far, research into NWNC sensors has demonstrated the potential in sensing many important classes of compounds. However, most of these NWNC devices require elevated working temperatures. They also have long response/recovery times and must function in an inert atmosphere. All these limitation will be the obstacles in real-world usage for domestic, environmental or industrial applications. And finally, the sensors thus developed must be manufacturable. That is, they must be batch fabricated with high yield. To remedy these problems, my thesis was divided into the following tasks, 1. Develop dry etching techniques to fabricate horizontally aligned GaN nanowires (NW), combining these techniques with wet etching treatment for surface damages removal. I call this a “top-down approach” using a subtractive process that fabricates NWs from thin-films and adding sensitive nanocrystals after the initial NW definition. This is to be compared to the additive “bottom-up” nanowire growth by MBE/HVPE/Sol-gel, in which NWs are grown, harvested from the growth surface and subsequently re-attached to a new surface. The top-down approach enhances the yield and homogeneity of the NW and it is mass-production oriented. 2. Study the metal-oxide nanoclusters (NCs) deposition method by physical vapor deposition (PVD) and rapid thermal annealing (RTA) for TiO2, SnO2, WO3, Fe2O3, etc. Develop the metal nanoparticle deposition method by PVD for Au, Ag, Pt, Pd, etc. 3. Study the crystalline phases and gas adsorption sites formed by the method and establish a database connecting metal-oxide bonding sites with different target chemicals. 4. Utilize Si doped n-type and unintentionally doped GaN nanowires functionalized with different metal-oxide and metal-oxide/metal composite nanoclusters to create a series of highly selective and sensitive gas sensing nanostructure devices. 5. Develop a low-cost micro-heater (MH) for local high temperature generation with low power consumption. This allows the rapid chemical desorption cycles as we anticipate frequently re-use or reset of the sensor. It also enables the use of these NWs in high temperature sensor applications. 6. Integrate the NW, NCs and MH into one working sensor, and integrate multiple types of gas sensors on a single chip. The chip can simultaneously sense many types of gases without interference. In this study, the potential of multicomponent NWNC based sensors for developing the next-generation of ultra-sensitive and highly selective chemical sensors was explored. We have achieved uA and nA levels of baseline detector current and we have shown that low UV illumination enhances sensitivity for some cases. These sensors have low power consumption making them suitable for portable devices.Item Optical and Thermal Properties of Nanoporous Material and Devices(2015) Kim, Kyowon; Murphy, Thomas E; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we investigate the optical and thermal properties of porous silicon and its applications. In first part, porous silicon's optical properties and application as a highly sensitive refractive index sensor is studied. An integrated Mach-Zehnder interferometer waveguide fabricated from nanoporous silicon is shown to exhibit high sensitivity and measurement stability that exceeds previously demonstrated porous sensors. In second part, we discuss experimental methods to characterize the thermal conductivity of nanoporous silicon films. We use the 3-ω method to characterize the exceptionally low thermal conductivity of porous silicon. Finally, we employ an improved heat conduction analysis method for the 3-ω method to measure the anisotropy in thermal conductivity. Our measurement show that porous silicon has very low in-plane thermal conductivity compared to cross-plane conductivity. We confirmed this anisotropy using direct numerical simulation of the anisotropic heat equation.Item INTEGRATED MODELING OF RELIABILITY AND PERFORMANCE OF 4H-SILICON CARBIDE POWER MOSFETS USING ATOMISTIC AND DEVICE SIMULATIONS(2015) Perinthatta Ettisserry, Devanarayanan; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)4H-Silicon Carbide (4H-SiC) power MOSFET is a promising technology for future high-temperature and high-power electronics. However, poor device reliability and performance, that stem from the inferior quality of 4H-SiC/SiO2 interface, have hindered its development. This dissertation investigates the role of interfacial and near-interfacial atomic defects as the root cause of these key concerns. Additionally, it explores device processing strategies for mitigating reliability-limiting defects. In order to understand the atomic nature of material defects, and their manifestations in electrical measurements, this work employs an integrated modeling approach together with experiments. Here, the electronic and structural properties of defects are analyzed using first-principles hybrid Density Functional Theory (DFT). The insights from first-principles calculations are integrated with conventional physics-based modeling techniques like Drift-Diffusion and Rate equation simulations to model various device characteristics. Subsequently, the atomic-level models are validated by comparison with experiments. From device reliability perspective, this dissertation models the time-dependent worsening of threshold voltage (Vth) instability in 4H-SiC MOSFETs operated under High-Temperature and Gate-Bias (HTGB) conditions. It proposes a DFT-based oxygen-vacancy hole trap activation model, where certain originally ‘electrically inactive’ oxygen vacancies are structurally transformed under HTGB stress to form electrically ‘active’ switching oxide hole traps. The transients of this atomistic process were simulated with inputs from DFT. The calculated time-evolution of the buildup of positively charged vacancies correlated well with the experimentally measured time-dependence of HTGB-induced Vth instability. Moreover, this work designates near-interfacial single carbon interstitial defect in SiO2 as an additional switching oxide hole trap that could cause room-temperature Vth instability. This work employs DFT-based molecular dynamics to develop device processing strategies that could mitigate reliability-limiting defects in 4H-SiC MOSFETs. It identifies Fluorine treatment to be effective in neutralizing oxygen vacancy and carbon-related hole traps, unlike molecular hydrogen. Similarly, Nitric Oxide passivation is found to eliminate carbon-related defects. From device performance perspective, this dissertation proposes a methodology to identify and quantify channel mobility-limiting interfacial defects by integrating Drift-Diffusion simulations of 4H-SiC power MOSFET with DFT. It identifies the density of interface trap spectrum to be composed of three atomically distinct defects, one of which is potentially carbon di-interstitial defect.Item Silicon-based terahertz waveguides(2015) Li, Shanshan; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present the design, fabrication and application of two types of silicon-based terahertz waveguides. The first is anisotropically etched highly doped silicon surface for supporting terahertz plasmonic guided wave. We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with subwavelength structures. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of structures. The second is silicon dielectric ridge waveguide used to confine terahertz pulses and study silicon's terahertz intensity-dependent absorption. We observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanisms underlying this nonlinear effect. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation.