Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item A SCANNING SQUID MICROSCOPE FOR IMAGING HIGH-FREQUENCY MAGNETIC FIELDS(2009) Vlahacos, Constantine Peter; Wellstood, Frederick C.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis examines the design and operation of a large-bandwidth scanning SQUID microscope for spatially imaging high frequency magnetic fields. Towards this end, I present results on a cryo-cooled 4.2 K scanning SQUID microscope with a bandwidth of dc to 2 GHz and a sensitivity of about 52.4 nT per sample. By using a thin-film hysteretic Nb dc-SQUID and a pulsed sampling technique, rather than a non-hysteretic SQUID and a flux-locked loop, the bandwidth limitation of existing scanning SQUID microscopes is overcome. The microscope allows for non-contact images of time-varying magnetic field to be taken of room-temperature samples with time steps down to 50 ps and spatial resolution ultimately limited by the size of the SQUID to about 10 micrometers. The new readout scheme involves repeatedly pulsing the bias current to the dc SQUID while the voltage across the SQUID is monitored. Using a fixed pulse amplitude and applying a fixed dc magnetic flux allows the SQUID to measure the applied magnetic flux with a sampling time set by the pulse length of about 400 ps. To demonstrate the capabilities of the microscope, I imaged magnetic fields from 0 Hz (static fields) up to 4 GHz. Samples included a magnetic loop, microstrip transmission lines, and microstrip lines with a break in order to identify and isolate electrical opens in circuits. Finally, I discuss the operation and modeling of the SQUID and how to further increase the bandwidth of the microscope to allow bandwidth of upwards of 10 GHz.Item SINGLE ELECTRON TRANSISTOR IN PURE SILICON(2009) HU, BINHUI; Yang, Chia-Hung; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As promising candidates for spin qubits, semiconductor quantum dots (QDs) have attracted tremendous research efforts. Currently most advanced progress is from GaAs QDs. Compared to GaAs, lateral QDs in 28silicon are expected to have a spin coherence time orders of magnitude longer, because 28Si has zero nuclear spin, and there is no hyperfine interaction between electron spins and nuclear spins. We have developed enhancement mode metal-oxide-semiconductor (MOS) single electron transistors (SETs) using pure silicon wafers with a bi-layer gated configuration. In an MOS-SET, the top gate is used to induce a two-dimensional electron gas (2DEG), just as in an MOS field effect transistor. The side gates deplete the 2DEG into a QD and two point contact channels; one connects the QD to the source reservoir, and the other connects the QD to the drain reservoir. We have systematically investigated the MOS-SETs at 4.2 K, and separately in a dilution refrigerator with a base temperature of 10 mK. The data show that there is an intrinsic QD in each point contact channel due to the local potential fluctuations in these SETs. However, after scaling down the SETs, we have found that the intrinsic QDs can be removed and the electrostatically defined dots dominate the device behavior, but these devices currently only work in the many-electron regime. In order to realize single electron confinement, it is necessary to continue scaling down the device and improving the interface quality. To explore the spin dynamics in silicon, we have investigated a single intrinsic QD by applying a magnetic field perpendicular to the sample surface. The magnetic field dependence of the ground-state and excited-state energy levels of the QD mostly can be explained by the Zeeman effect, with no obvious orbital effect up to 9 T. The two-electron singlet-triplet (ST) transition is first time directly observed in a silicon QD by excitation spectroscopy. In this ST transition, electron-electron Coulomb interaction plays a significant role. The observed amplitude spectrum suggests the spin blockade effect. When the two-electron system forms a singlet state in the dot at low fields, and the injection current from the lead becomes spin-down polarized, the tunneling conductance is reduced by a factor of 8. At higher magnetic fields, due to the ST transition, the spin blockade effect is lifted and the conductance is fully recovered.Item Measurements of doping-dependent microwave nonlinear response in cuprate superconductors(2007-04-25) Mircea, Dragos Iulian; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Near-field microwave techniques have been successfully implemented in the past for the local investigation of magnetic materials and high-temperature superconductors. This dissertation reports on novel phase-sensitive linear- and nonlinear response microwave measurements of magnetic thin films and cuprate superconductors and their interpretation.Item Near quantum limited measurement in nanoelectromechanical systems(2006-09-07) Naik, Akshay; Schwab, Keith; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanoelectromechanical systems have many potential applications in nanoelectronics as well as in fundamental studies of quantum mechanics in mesoscopic systems. Nanoelectromechanical systems have been touted as an extension of microelectromechanical systems which would operate at higher frequencies and consume far less power due their higher quality factors. Since these systems can be cooled close to their ground states with existing cryogenic techniques, they are useful tools to study the quantum effects like backaction, coherent states and superposition in mesoscopic mechanical systems. Also there have been proposals to use these systems as qubits and buses in quantum computing. In this thesis I discuss the effects of the backaction of a superconducting single electron transistor that measures the position of a radio frequency nanomechanical resonator. One of the novel effects of this backaction is the cooling of the nanomechanical resonator. The fact that a system can be cooled by merely coupling it to noisy non-equilibrium device is a counterintuitive phenomenon. Although backaction effects have been used to produce ultra-cold atoms, our results are the first demonstration of this cooling effect in a mesoscopic system. For a linear continuous position detection scheme, quantum mechanics places a lower limit on the product of position shot noise, Sx, and the backaction force noise, SF, which is given by, (S_x S_F)^(1/2)> hbar/2 As part of this work we demonstrate that our detection scheme is only 15 times away from this limit and only 4 times away from quantum limit for position sensitivity.Item Fabrication and Measurement of Regenerable Low Workfunction Photocathodes(2006-08-03) Moody, Nathan Andrew; O'Shea, Patrick G.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Laser-switched photoemitters are a source of electrons for high current applications such as free electron lasers. Laser-modulated photoemission permits rapid switching of the electron beam, far surpassing what can be achieved using electric-field gated emission. Photoinjector systems consist of a drive laser producing short bunches of photons and an efficient photocathode, which converts photon bunches into electron beam pulses. Development of both technologies is required, but the scope of this project is restricted to improvement of the photocathode. Most high-efficiency photocathodes employ cesium-based surface coatings to reduce work function and enable efficient electron emission in the visible range. Lifetime is severely limited by the loss of this delicate coating, which degrades rapidly in practical vacuum environments. More robust photocathodes exist, but have much lower efficiency, and place unrealistic demands on drive laser power and stability. This research proposes a novel dispenser concept that dramatically extends the lifetime of high efficiency cesium-based cathodes by continuously or periodically restoring the cesium surface monolayer during an in situ rejuvenation process. Sintered tungsten provides an interface between a cesium reservoir and the photoemitting surface. During temperature-controlled rejuvenation, cesium diffuses through and across the sintered tungsten to create and sustain a low-work function photocathode. The prototype dispenser cathode was fabricated and tested for two modes of operation: continuous and periodic near-room temperature rejuvenation. The data are compared with a photoemission model of partially covered surfaces under design for integration with existing beam simulations. Overall performance suggests that this cesium-delivery mechanism can significantly enhance the efficiency and operational lifetime of a wide variety of present and future cesium-based photocathodes. Also reported are surface characterization, ion beam cleaning, and fabrication techniques used to optimize performance of the dispenser photocathode.Item Quantum Transport in Nanoscale Semiconductor Devices(2006-08-02) Jones, Gregory Millington; Yang, Chia-Hung; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Because of technological advancement, transistor dimensions are approaching the length scale of the electron Fermi wavelength, on the order of only nanometers. In this regime, quantum mechanical phenomena will dominate electron transport. Using InAs single quantum wells, we have fabricated Y-shaped electron waveguides whose lengths are smaller than the elastic mean free path. Electron transport in these waveguides is ballistic, a quantum mechanical phenomenon. Coupled to the electron waveguide are two gates used to coherently steer the electron wave. We demonstrate for the first time that gating modifies the electron's wave function, by changing its geometrical resonance in the waveguide. Evidence of this alteration is the observation of anti-correlated, oscillatory transconductances. Our data provides direct evidence of wavefunction steering in a transistor structure and has applications in high-speed, low-power electronics. Quantum computing, if realized, will have a significant impact in computer security. The development of quantum computers has been hindered by challenges in producing the basic building block, the qubit. Qubit approaches using semiconductors promise upscalability and can take the form of a single electron transistor. We have designed, fabricated, and characterized single electron transistors in InAs, and separately in silicon, for the application of quantum computing. With the InAs single electron transistor, we have demonstrated one-electron quantum dots using a single-top-gate transistor configuration on a composite quantum well. Electrical transport data indicates a 15meV charging energy and a 20meV orbital energy spacing, which implies a quantum dot of 20nm in diameter. InAs is attractive due to its large electron Landé g-factor. With the silicon-based single electron transistor, we have demonstrated a structure that is similar to conventional silicon-based metal-oxide-semiconductor field effect transistors. The substrate is undoped and becomes insulating at low temperatures. There are two layers of gates that when properly biased define the single electron transistor potential profile. The measured stability chart at 4.2K indicates a charging energy of 18meV. Our silicon-based single electron transistor is promising, because spin coherence times in silicon are orders of magnitude longer than those in GaAs.Item Domain Wall Engineering of Nanoscale Ferromagnetic Elements and its Application for Memory Devices(2006-04-14) Florez, Sylvia Helena; Gomez, Romel D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis concerns the interaction of spin polarized electrons with the local magnetic moments in nanopatterned metallic systems. We study novel magnetic phenomena appearing in patterned thin film magnetic wires with length scales in the nanometer regime and in magnetic multilayers. The work has three mayor foci. The first is the interaction between magnetic domain walls and conduction electrons in single layer nanowires. We demonstrate the effect of using small constrictions as artificial traps for domain walls and use these structures to measure the contribution of a domain wall to the electrical resistivity. These measurements are correlated with the specific micromagnetic distribution induced by the constriction geometry. Similarly, we demonstrate and characterize the effect of spin current induced magnetization reversal in nanowires. This includes a measurement of the critical current/field phase space boundary between static and moving walls and an estimation of the intrinsic wall mobility. The second is focused on understanding the effects of spin currents on magnetoresistance and domain wall motion, in a multilayer nanostructure device exhibiting giant magnetoresistance (GMR). To demonstrate a potential application, we incorporate the effects of domain wall trapping and spin current induced domain wall motion into a nanometer scale spin-valve device. The device can be fully controlled through current and exhibits significant GMR response. This approach may be useful as a memory element in magnetoresistive random access memory (MRAM) technology, and the device serves as a proof of concept. The third focus is the understanding of the effect strain on the resistance of antiferromagnetically (AF) coupled giant magnetoresistive (GMR) multilayers containing highly magnetostrictive materials. Our measurements reveal that inverse magnetostriction effects lead to enhanced strain sensitivity in comparison to films made of the materials that compose the multilayer. A simple phenomenological model describing the measured field dependence of these effects is used to identify field-biasing values that optimize amplitude, linearity and reversibility of the effect.Item Fabrication and Measurement of Cesiated Metal Photocathodes(2004-11-30) Moody, Nathan Andrew; O'Shea, Patrick G.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A requirement for accelerator applications such as free electron lasers is a high current, high quality electron beam. This is achieved using a photoinjector, where a drive laser modulates the electron emission of a cathode in an electric field. Current photocathodes are plagued with limited efficiency and short lifetime, due to contamination or evaporation of a photosensitive surface layer. An ideal photocathode would have high efficiency in the visible range, long lifetime, and prompt emission. Cathodes with high efficiency typically have limited lifetime, and vice versa. A potential solution is the dispenser cathode, where limited lifetime is overcome by periodic in situ regeneration that restores the photosensitive surface. This project prepares for fabrication of dispenser cathodes by studying properties of cesiated tungsten. A test facility was constructed and used to fabricate and test cesiated tungsten cathodes, whose behavior closely agreed with recently developed photoemission theory at the Naval Research Laboratory.Item Analysis of fluctuations in semiconductor devices(2004-04-21) Andrei, Petru; Mayergoyz, Isaak D; Peckerar, Martin C; Ghodssi, Reza; Krishnaprasad, Perinkulam S; Electrical EngineeringThe random nature of ion implantation and diffusion processes as well as inevitable tolerances in fabrication result in random fluctuations of doping concentrations and oxide thickness in semiconductor devices. These fluctuations are especially pronounced in ultrasmall (nanoscale) semiconductor devices when the spatial scale of doping and oxide thickness variations become comparable with the geometric dimensions of devices. In the disseration, the effects of these fluctuations on device characteristics are analyzed by using a new technique for the analysis of random doping and oxide thickness induced fluctuations. This technique is universal in nature in the sense that it is applicable to any transport model (drift-diffusion, semiclassical transport, quantum transport etc.) and it can be naturally extended to take into account random fluctuations of the oxide (trapped) charges and channel length. The technique is based on linearization of the transport equations with respect to the fluctuating quantities. It is computationally much (a few orders of magnitude) more efficient than the traditional Monte-Carlo approach and it yields information on the sensitivity of fluctuations of parameters of interest (e.g. threshold voltage, small-signal parameters, cut-off frequencies, etc.) to the locations of doping and oxide thickness fluctuations. For this reason, it can be very instrumental in the design of fluctuation-resistant structures of semiconductor devices. Quantum mechanical effects are taken into account by using the density-gradient model as well as through self-consistent Poisson-Schrödinger computations. Special attention is paid to the presenting of the technique in a form that is suitable for implementation on commercial device simulators. The numerical implementation of the technique is discussed in detail and numerous computational results are presented and compared with those previously published in literature.