Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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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 THE EFFICIENT CONTROL OF TWO-DIMENSIONAL MAGNETISM BY MULTIPLE EXTERNAL STIMULI(2023) Xie, Ti; Gong, Cheng; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetism has played a crucial role in both fundamental research and technological advancement, from ancient compasses to modern spintronics. With the advent of artificial intelligence and the increasing demand for high-volume data storage, there have been significant efforts to reduce the dimensionality of memory materials. Recently, the discovery of two-dimensional magnetic van der Waals materials has enabled the observation of long-range magnetic order in monolayer crystals, which exhibit high sensitivity to external stimuli such as optical incidence, mechanical strain, and chemical functionalization. Our systematic work focuses on the efficient control of two-dimensional magnetism through multiple external stimuli, including chemical, optical, electrical, and mechanical means. These works achieved the effective control of a wide range of magnetic properties of two-dimensional magnets, such as Curie temperatures, magnetic coercivities, domain profiles, and magnetic phases. These research achievements will provide valuable insights into the fundamentals of two-dimensional magnetism and its interplay with external stimuli, paving the way for advancing the nanoscale spintronic and photonic devices in ultrathin platforms.Item MONOLAYER MOLYBDENUM DISULFIDE IN SEMICONDUCTOR ELECTRONICS(2023) Mazzoni, Alexander; Daniels, Kevin M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-dimensional (2D) semiconductors are a new class of materials being researched due to their unique electrical, optical, and mechanical properties compared to their bulk counterparts. Here I investigate the use of the 2D semiconductor molybdenum disulfide (MoS2) as the active channel material in various electronic devices and circuits. Motivation is provided for 2D materials in general and monolayer MoS2 in particular, followed by an overview of the material properties of MoS2 and a relevant literature review. Back-gated field-effect transistors (FETs) were fabricated and characterized to investigate the impact of growth conditions on material properties, and to study the performance of different contact metals. A top-gated fabrication process was developed to make RF transistors and simple amplifier circuits on rigid and flexible substrates. Finally, device operating characteristics were modeled using simple transistor current-voltage equations, and Monte Carlo electron transport simulations were performed to demonstrate the importance of device operating temperature and intervalley separation in the conduction band.Item SPECTROSCOPY OF TWO LEVEL DEFECTS & QUASIPARTICLES IN SUPERCONDUCTING RESONATORS(2021) Kohler, Timothy; Osborn, Kevin D; Anlage, Steven; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducting films are inherently limited by losses due to two-level system (TLS) defects within the amorphous oxide layers surrounding them and from quasiparticles in the film. In this thesis I will discuss novel theoretical and experimental methods toward understanding superconducting resonator loss from deleterious surface TLS defects as well as a loss transition from non-equilibrium quasiparticles in granular TiN. I will show using finite element solver software that a resonator with submicron linewidth and linespacing can be used to better characterize and simulate surface TLS as part of a circuit QED system. I have observed individual surface TLS and found coupling values in the range of g/2π =50 kHz -280 kHz with a maximum dipole moment pz-max = 4.5 Debye (.93 eÅ). I have found in in simulation of experiment that over 80% of the strongly coupled TLS reside within 50 nm of the corner between the Metal-Substrate (MS) and Substrate-Air (SA) interface. Additionally I have studied a loss transition from non-equilibrium quasiparticles in TiN films. These films exhibit an anomalous loss dependence on substrate treatment and film thickness. The films of interest are ones grown thin on oxidized substrates, which exhibit an order of magnitude decrease in internal quality factor (Qi) relative to either thicker ˝films or films grown without the oxidized substrate. These films additionally exhibit a grain size on average of 7.5 nm, a higher inhomogeneous gap, a transition to lower stress and a preference for the [111] crystal growth. The temperature dependence of the conductivity is fit and a factor of two difference in quasiparticle lifetime is found between the two films where the thinner film has a shorter lifetime. A two gap quasiparticle trapping model is fit to the temperature dependent loss data. The data is consistent with a model where non-equilibrium quasi-particles are trapped in low gapped grains on the inside of the films. From these works and others presented in my thesis the understanding of TLSs on surfaces and non-equilibrium quasiparticles in TiN has improved. This will help illuminate some of the most important absorption mechanisms plaguing superconducting qubits and resonators.Item Microwave Photos in High Impedance Transmission line: Dispersion, Disorder and Localization(2017) Mehta, Nitish Jitendrakumar; Murphy, Thomas E; Manucharyan, Vladimir E; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we will describe the theoretical and experimental studies of a TEM on-chip superconducting transmission line with a wave impedance as high as 20 $\mathrm{k}\Omega$, phase and group velocity of waves simultaneously reduced by a factor of 100 in a broad range of frequencies from 0 to about 10 $\mathrm{GHz}$. A conventional microwave coaxial transmission line gets its inductance and capacitance from magnetic and electric fields stored in the space between its inner and outer conductors. This in turn limits its impedance to around 50 $\Omega$ and group velocity of waves very close to the speed of light in vacuum. In this work we are able to increase the impedance by over two orders of magnitude and reduce the group and phase velocity of waves by over two orders of magnitude as well, by constructing a coplanar transmission line out of a pair of long Al/AlOx/Al Josephson tunnel junction chains. A Josephson junction gets its inductance not from the magnetic energy but rather from the much larger kinetic energy of tunneling Cooper pairs, which is unrelated to the electromagnetic properties of vacuum. In this work we present a design of such a transmission line and low-temperature measurement of its dispersion relation. We then study and characterize the disorder present in the circuit parameters of our system and using this, we conclude that for frequencies up to 12 GHz, there is no evidence of Anderson localization of waves, even for chains exceeding 30,000 junctions. Low dissipation and absence of localization make this transmission line ideal for use in various experiments where high impedance can enable strong coupling between light and matter.Item Nanophotonic quantum interface for a single solid-state spin(2016) Sun, Shuo; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to store and transmit quantum information plays a central role in virtually all quantum information processing applications. Single spins serve as pristine quantum memories whereas photons are ideal carriers of quantum information. Strong interactions between these two systems provide the necessary interface for developing future quantum networks and distributed quantum computers. They also enable a broad range of critical quantum information functionalities such as entanglement distribution, non-destructive quantum measurements and strong photon-photon interactions. Realizing spin-photon interactions in a solid-state device is particularly desirable because it opens up the possibility of chip-integrated quantum circuits that support gigahertz bandwidth operation. In this thesis, I demonstrate a nanophotonic quantum interface between a single solid-state spin and a photon, and explore its applications in quantum information processing. First, we experimentally realize a spin-photon quantum phase switch based on a strongly coupled quantum dot and photonic crystal cavity system. This device enables coherent light-matter interactions at the fundamental limit, where a single spin controls the polarization of a photon and a single photon flips the spin state. Furthermore, we theoretically propose a way to deterministically generate spin-photon entanglement based on the spin-photon quantum interface, which is an important step towards solid-state implementations of quantum repeaters and quantum networks. Next, we show both theoretically and experimentally, a new method to optically read out a solid-state spin based on the same cavity quantum electrodynamics (QED) system. This new method achieves significant improvement in spin readout fidelity over typical approaches using fluorescence light detection. In the end, we report efforts to realize tunable and robust quantum dot based cavity QED systems. We present a technique for tuning the frequency of a quantum dot that is strongly coupled to a photonic crystal cavity by applying strain. This tuning technique enables us to accurately control the detuning between a quantum dot and a cavity without affecting other emission properties of the dot, which is essential for lots of applications associated with cavity QED systems, including non-classical light generation, photon blockade, single photon level optical switch, and also our major focus, the spin-photon quantum interface.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 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 Topological Edge States in Silicon Photonics(2014) Mittal, Sunil; Hafezi, Mohammad; Migdall, Alan; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Under the influence of a magnetic field, at low temperatures, charged particles confined in two-dimensional systems exhibit a remarkable range of macroscopic quantum phenomena such as the quantum Hall effects. A hallmark of these phenomena is the presence of unidirectional, topologically robust edge states - states which are confined to the edge of the system. It is, in principle, possible to engineer a synthetic magnetic field for photons and hence achieve photonic analogs of the robust electronic edge states. Investigating photonic edge states is interesting from a fundamental perspective of studying photonic transport in the presence of a gauge field and also for its application in classical and quantum information processing. In this thesis, we present the implementation of a synthetic magnetic field for photons and our observation of topological edge states in a two-dimensional lattice of coupled ring resonators, fabricated using CMOS-compatible silicon-oninsulator technology. We qualitatively show the robustness of edge states against deliberately induced lattice defects. We then analyze the statistics of transport measurements (transmission and delay) made on a number of different devices and quantitatively verify the robustness of edge states against lattice disorder. Using Wigner delay-time distribution, we show that localization is suppressed in the edge states. Furthermore, to unequivocally establish the non-trivial topological nature of edge states, we compare their transmission to a topologically trivial one dimensional system of coupled ring resonators and demonstrate that the edge states achieve higher transmission. Moreover, for photonic analogs of the quantum Hall effect, the winding number - a topologically invariant integer which characterizes edge states - is quantized, analogous to quantization of conductivity in electronic systems. We measure the winding number of the edge states in our system. Finally, we investigate the effect of nonlinear interactions in silicon ring resonators, on the stability of edge states. We show that the presence of a strong pump can result in a significant decrease in the transmission through edge states.Item WAVE CHAOTIC EXPERIMENTS AND MODELS FOR COMPLICATED WAVE SCATTERING SYSTEMS(2013) Yeh, Jen-Hao; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wave scattering in a complicated environment is a common challenge in many engineering fields because the complexity makes exact solutions impractical to find, and the sensitivity to detail in the short-wavelength limit makes a numerical solution relevant only to a specific realization. On the other hand, wave chaos offers a statistical approach to understand the properties of complicated wave systems through the use of random matrix theory (RMT). A bridge between the theory and practical applications is the random coupling model (RCM) which connects the universal features predicted by RMT and the specific details of a real wave scattering system. The RCM gives a complete model for many wave properties and is beneficial for many physical and engineering fields that involve complicated wave scattering systems. One major contribution of this dissertation is that I have utilized three microwave systems to thoroughly test the RCM in complicated wave systems with varied loss, including a cryogenic system with a superconducting microwave cavity for testing the extremely-low-loss case. I have also experimentally tested an extension of the RCM that includes short-orbit corrections. Another novel result is development of a complete model based on the RCM for the fading phenomenon extensively studied in the wireless communication fields. This fading model encompasses the traditional fading models as its high-loss limit case and further predicts the fading statistics in the low-loss limit. This model provides the first physical explanation for the fitting parameters used in fading models. I have also applied the RCM to additional experimental wave properties of a complicated wave system, such as the impedance matrix, the scattering matrix, the variance ratio, and the thermopower. These predictions are significant for nuclear scattering, atomic physics, quantum transport in condensed matter systems, electromagnetics, acoustics, geophysics, etc.