A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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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 Artificial Kagome Spin Ice(2008-08-04) Qi, Yi; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Geometrical frustration is known to significantly modify the properties of many materials. Pyrochlore spin ice and hexagonal water ice are canonical systems that show the effects of frustration in both heat capacity and dynamical response. In both instances, microscopic ordering principles on the lattice lead to a macroscopic degeneracy of configurations. This degeneracy in spin ice may also be modified or lifted by lattice imperfections, external pressure, or magnetic field. Unfortunately, these effects are difficult to model or predict, because existing experimental techniques cannot directly observe the local ordering, near lattice defects or otherwise. To address this long outstanding problem, recent interest has focused on fabricating systems that allow the effects of frustration to be physically modeled and the resulting local configurations to be directly observed. In this dissertation, I present an artificial approach to kagome lattice. The kagome lattice is a two-dimensional structure composed of corner-sharing triangles and is an essential component of the pyrochlore spin ice structure. Our artificial kagome spin ice, constructed by magnetic nano-bar elements, mimics spin ice in 2D. The realized system rigorously obeys the ice rule (2-in 1-out or 1-in 2-out configuration at a vertex of three elements), thus providing a sought-after model system appropriate for further studies. To study the ground state of the artificial kagome system and to validate the artificial approach for spin ice study, we demagnetize the samples using rotating field and observe spin configurations using Lorentz TEM. The ice rule, short-range ordering and absence of long-range disorder, as well as the relatively low remnant magnetization are found in the system, which are signatures of spin ice materials in their ground states. To model our system and relate it to other spin study, we introduce magnetic charge model and Shannon entropy concept. The calculated charge correlation (charge ordering coefficient) and Shannon entropy suggest that the degeneracy of our lattice is lifted from a completely disordered kagome spin ice system, and close to a "true" ground state that is usually found as the kagome plateau in pyrochlore spin ice when applying a field in <111> direction. We also study the effects of external perturbations. When applying a magnetic field, chain-like spin flipping is found in the system, which can be explained by the magnetic charge model. When distorting the lattice by introducing an artificial strain, we observe partial ordering or symmetry breaking in the system, which is similar to the pressure effects in real spin ice. In the Appendix, I also introduce another study I have done, i.e. multiferroic thin film measurements. The focus of that chapter is the dielectric measurement for BaTiO3 (BTO) -CoFe2O4 (CFO) thin film material using a microwave microscope. The measurement has a quantitative spatial resolution of approximately 5 m, and it provides a method for film quality check and the basis for a proposed ME coupling measurement.Item Formation and Piezoelectricity of Self-Assembled PbTiO3-CoFe2O4 Nanostructural Films(2008-06-13) tan, zhuopeng; Roytburd, Alexander L; Levin, Igor; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Main tasks of our research include: (1) exploring optimum growth conditions for PLD deposition of self-assembled nanophase PbTiO3-CoFe2O4 films with different compositions and orientations; (2) analyzing morphologies and nanostructures of the two-phase films to clarify relative effects of elastic energy and interface energy on the self-assembled film formation; (3) investigating stress state and relaxation of stresses arising as a result of a paraelectric-ferroelectric transformation in PbTiO3; (4) exploring ferroelectric state in the confined PbTiO3 nanophase in the films with {110} and {111} orientations. Principal results of the research are: (1). Optimum PLD growth conditions to obtain high quality films with distinct separation of epitaxial PbTiO3 and CoFe2O4 nanophases are found after systematic studies. (2). Nano-facets along {111} plane between PbTiO3 and CoFe2O4 phases are found to be generic in addition to orientation dependent macroscopic interfaces. We have concluded that accounting of interface and surface energies is important for description of nano-faceting of interfaces and the near substrate zone of the films while the two-phase morphologies are determined by the elastic interactions; (3). The investigation of the stress state of the {001} film arising due to paraelectric-ferroelectric transition of PbTiO3 have discovered the polydomain nanostructure of the ferroelectric phase with ~50-60% c-domains. Piezoresponse of PbTiO3 should be reduced dramatically by combined effects of dissolution of Fe in PbTiO3, a domains and constraints. The relative large dzz from previous research must contain large extrinsic contribution due to movement of nano-domain walls. (4). Switching spectroscopy piezoresponse force microscopy (SS-PFM) is used to characterize local piezo- ferroelectric property of confined ferroelectrics in {110} and {111} films with composition of 1/3PbTiO3-2/3CoFe2O4. It is proved that PbTiO3 nano-inclusions exhibit ferroelectricity in both films. 180o domain switching is observed under measurement condition (<10V) for the {110} films but not for the {111} film. Quantitatively, both films yield a piezoresponse of about 15% compared to bulk single crystal PbTiO3. It is a reasonable value of intrinsic piezoeffect taking into account mechanical and electrical constraints (depolarizing field) as well as the effect of Fe dissolution and possible in-plane domainsItem Combinatorial Investigation of Magnetostrictive Materials(2007-08-24) Hattrick-Simpers, Jason Ryan; Takeuchi, Ichiro; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combinatorial materials synthesis is a research methodology, which allows one to study a large number of compositionally varying samples simultaneously. We apply this technique in the search for novel multifunctional materials. The work presented here will discuss the combinatorial investigation of novel magnetostrictive materials. In particular, binary Fe-Ga and the ternary Fe-Ga-Al, Fe-Ga-Pd systems are studied. Magnetron co-sputtered composition spread samples of the alloys have been fabricated to study composition dependent trends in magnetostriction. Magnetostriction measurements on all systems studied here have been carried out by optically measuring the deflection of micro-machined cantilever arrays. Measurements of the magnetostriction on binary Fe-Ga thin-films show similar compositional trends as had been reported in bulk systems. The maximum value of magnetostriction observed is 220 ppm, which is comparable to bulk values. A previously unreported minor maximum in magnetostriction as a function of composition has been found for Ga contents of about 4 at%. It is believed that the origin of this minor maximum is related to a peak in the magnetic moment of Fe atoms in Fe-Ga alloys at this composition. We have mapped the Fe-Ga-Pd and Fe-Ga-Al ternary systems. Large regions of the phase diagrams have been mapped out in a single experiment, and the observed magnetostrictive dependence on Ga content matches trends seen in bulk. It was found that the trend of magnetostriction deviated from that of bulk with the inclusion of as little as 1 at% Pd. The addition of up to 10 at % Al to Fe70Ga30 was possible without severe degradation of its magnetostriction.Item A Scaled Equation of State for the Liquid-Liquid Critical Point in Supercooled Water(2007-09-14) Fuentevilla, Daphne Anne; Anisimov, Mikhail A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The second-critical-point scenario is one of the most popular explanations for the anomalous behavior of supercooled liquid water. According to this scenario, liquid water at ambient conditions is a "supercritical" ?uid that separates into two types of liquid water in the supercooled region. However, experimental confirmation is challenging. In this work we developed a scaled parametric equation of state, based on the principle of critical-point universality, to examine the second-critical-point scenario from a new direction. The equation of state, built on the growing evidence for liquid-liquid water separation, is universal in terms of theoretical scaling fields and belongs to the Ising-model universality class. The theoretical scaling fields are postulated to be analytical combinations of the physical fields, pressure and temperature. The equation of state enables us to accurately locate the "Widom line" (locus of stability minima) and determine that the critical pressure is considerably lower than predicted by computer simulations.Item Evolution of patterned and unpatterned surfaces during high temperature annealing and plasma etching(2007-07-26) Kwon, Taesoon; Phaneuf, Raymond J.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we describe experiments designed to probe spontaneous and directed surface evolution during annealing and plasma etching of three materials of high technological interest: silicon, nanoporous silica and photoresist. Vicinal Si(111) surfaces provide a source of steps whose configuration we control via the introduction of a topographic pattern; this is done using combination of photolithography and reactive ion etching. We study the length scale dependence of self-organization of step bunches during annealing at ~ 1273 °C in ultrahigh vacuum (UHV), resulting from sublimation and diffusion, and the competition between effects due to the intrinsic stiffness of steps and their mutual interactions. We also show the results of numerical simulations on these surfaces based upon a simple model of step motion, which we compare with our experimental observations. Nanoporous silica (NPS) is a heterogeneous material which is of potential use in micro/nanoelectronic applications requiring an insulator with a small dielectric constant. We investigate the stability of the NPS-plasma interface during etching, comparing the tendency for spontaneous pattern formation with the persistence of patterned perturbations. We study samples with various porosity (0 ~ 50 vol.%) under low pressure C4F8/90%Ar plasma etching conditions. Our AFM characterization of unpatterned surfaces shows a monotonic increase in RMS roughness with etching time. Annealing etched NPS surfaces at temperatures over the range from 300 ~ 900 °C in UHV as well as in non-oxidizing environment produces no significant relaxation of etching-induced surface roughness. Statistical analysis using a height-height correlation function reveals that NPS surfaces do not show a simple scaling behavior during the technologically-relevant transient time regime. Etching of patterned surfaces reveals a persistent period of approximately 400 nm, which is ~ 4 times that which spontaneously appears during etching of unpatterned surfaces. Based upon this observation we investigate the possibility of period doubling, and find some evidence for it. Lastly, we present preliminary results of surface morphological and compositional evolution of model photoresists for lithography at 193 nm and 248 nm wavelengths during etching using plasma and ion beam sputtering under a range of conditions. Surprisingly, our AFM characterization shows that there is no significant difference in roughness evolution between resists whose chemical backbone is qualitatively different, i.e. benzene-ring based 248 nm and acrylate-admatane based 193 nm polymers. The surface roughening however varies strongly with the position of a methyl group on the polymer backbone. Fourier transform infrared spectroscopy (FTIR), Ellipsometry and XPS characterizations show that the polymers become dense at the early stages of plasma / ion beam exposure, possibly due to graphitization, cross-linking and hydrogen loss. We compare these observations with molecular dynamic (MD) simulations of Ar+ ion beam sputtering.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 TEMPERATURE DEPENDENCE OF THE GROWTH MODE DURING HOMOEPITAXY ON PATTERNED GALLIUM ARSENIDE (001); ATOMIC-SCALE MECHANISMS FOR UNSTABLE GROWTH.(2006-12-11) tadayyon-eslami, tabassom; Phaneuf, Raymond j; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)ABSTRACT Title of Document: TEMPERATURE DEPENDENCE OF THE GROWTH MODE DURING HOMOEPITAXY ON PATTERNED GALLIUM ARSENIDE (001); ATOMIC-SCALE MECHANISMS FOR UNSTABLE GROWTH. Tabassom Tadayyon-Eslami, Doctor of Philosophy, 2006 Directed By: Professor Raymond J. Phaneuf, Department of Materials Science and Engineering In this thesis we present an extensive investigation of instability in molecular beam epitaxial growth of GaAs(001) over a range of pattern periods, cell sizes, growth temperature and As2 flux. We find very good agreement with predictions of the continuum models of Sun, Guo and Grant [Phys. Rev. A 40, 6763(1989)] for the growth above ~540ºC and Lai and Das Sarma [Phys. Rev. Lett. 66, 2348 (1991)] for the growth below this temperature. Changing the growth temperature to lower than 540 ºC leads to the formation of ring-like protrusions in the [110] direction around pits patterned on the initial substrate, which are absent for growth at higher temperature. This change in growth mode occurs in the temperature range within in which both pre-roughening transition and surface reconstruction transition (β2(2x4) to c(4x4)) also occur. We rule out the possibility of preroughening and the change in surface reconstruction as the reason for this growth mode change, based on the As2 flux dependence of the growth mode transition temperature. Based on our atomic force microcopy characterization of the surface morphology during early the stage of growth, we propose a physically based model for the growth, which involves a competition between decreased adatom collection efficiency during growth on small terraces and a small anisotropic multiple step Ehrlich-Schwoebel barrier at the pit edge. This provides a physical basis for the nonlinear term in the continuum models proposed by Sun et. al., and Lai and Das Sarma, whose predictions qualitatively describe our experimental observations.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.