Matching Items (22)

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Above 400-K robust perpendicular ferromagnetic phase in a topological insulator

Description

The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in

The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal–doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (T[subscript c]) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced T[subscript c] by exchange coupling TIs to Tm[subscript 3]Fe[subscript 5]O[subscript 12], a high-T[subscript c] magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced T[subscript c], absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.

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  • 2017-06-23

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Optical Response of a TiN Kinetic Inductance Detector (KID)

Description

This research compares shifts in a SuperSpec titanium nitride (TiN) kinetic inductance detector's (KID's) resonant frequency with accepted models for other KIDs. SuperSpec, which is being developed at the University

This research compares shifts in a SuperSpec titanium nitride (TiN) kinetic inductance detector's (KID's) resonant frequency with accepted models for other KIDs. SuperSpec, which is being developed at the University of Colorado Boulder, is an on-chip spectrometer designed with a multiplexed readout with multiple KIDs that is set up for a broadband transmission of these measurements. It is useful for detecting radiation in the mm and sub mm wavelengths which is significant since absorption and reemission of photons by dust causes radiation from distant objects to reach us in infrared and far-infrared bands. In preparation for testing, our team installed stages designed previously by Paul Abers and his group into our cryostat and designed and installed other parts necessary for the cryostat to be able to test devices on the 250 mK stage. This work included the design and construction of additional parts, a new setup for the wiring in the cryostat, the assembly, testing, and installation of several stainless steel coaxial cables for the measurements through the devices, and other cryogenic and low pressure considerations. The SuperSpec KID was successfully tested on this 250 mK stage thus confirming that the new setup is functional. Our results are in agreement with existing models which suggest that the breaking of cooper pairs in the detector's superconductor which occurs in response to temperature, optical load, and readout power will decrease the resonant frequencies. A negative linear relationship in our results appears, as expected, since the parameters are varied only slightly so that a linear approximation is appropriate. We compared the rate at which the resonant frequency responded to temperature and found it to be close to the expected value.

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  • 2018-05

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A Determination of the Partial Decay Width for the hb -> etab gamma Radiative Transition of Bottomonium

Description

Because of its massive nature and simple two-body structure, the heavy meson bottomonium (the flavorless bound state of the bottom quark and anti-quark) is among the simplest systems available for

Because of its massive nature and simple two-body structure, the heavy meson bottomonium (the flavorless bound state of the bottom quark and anti-quark) is among the simplest systems available for the study of the strong force and quantum chromodynamics (QCD)—a feature which has made it of special interest to particle physicists.

Despite being bound by the strong force, bottomonium exhibits a rich spectrum of resonances corresponding to excited states extremely analogous to that of positronium or even familiar atomic systems. Transitions between these levels are possible via the absorption or emission of either a photon, gluon, or gluons manifesting as light hadrons. The goal of this thesis was to establish a theoretical value for the currently unmeasured partial decay width for one such transition—the electromagnetic decay channel hb -> etab gamma. To this end, two methods were utilized.

The first approach relied on the presumption of a nonrelativistic constituent quark model interacting via a simple static potential, allowing for radial wave functions and energy eigenvalues to be obtained for the states of interest via the Schrödinger equation. Upon an application of the standard electromagnetic multipole expansion followed by a utilization of the electric dipole E1 decay width formula, a value of 57.7 ± 0.4 keV was obtained.

The second approach stemmed from the effective Lagrangian describing the bottomonium P to S electromagnetic transitions and relied on the presumption that a single coupling constant could be approximated as describing all nP to mS transitions regardless of spin. A value for this coupling constant could then be extracted from the 1P to 1S spin triplet data and used to predict the width for the singlet 1P to 1S transition. The partial decay width value found in this manner was 47.8 ± 2.0 keV.

Various other methods and models have established a predicted range of 35 to 60 keV for this partial decay width. As the values determined in this thesis fall within the expected range, they agree well with our current understanding of this electromagnetic transition and place further confidence on the expected range.

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  • 2016-12

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Quasi-two-dimensional electron gas at the epitaxial alumina/SrTiO3 interface: Control of oxygen vacancies

Description

In this paper, we report on the highly conductive layer formed at the crystalline γ-alumina/SrTiO[subscript 3] interface, which is attributed to oxygen vacancies. We describe the structure of thin γ-alumina

In this paper, we report on the highly conductive layer formed at the crystalline γ-alumina/SrTiO[subscript 3] interface, which is attributed to oxygen vacancies. We describe the structure of thin γ-alumina layers deposited by molecular beam epitaxy on SrTiO3 (001) at growth temperatures in the range of 400–800 °C, as determined by reflection-high-energy electron diffraction, x-ray diffraction, and high-resolution electron microscopy. In situ x-ray photoelectron spectroscopy was used to confirm the presence of the oxygen-deficient layer. Electrical characterization indicates sheet carrier densities of ∼1013 cm−2 at room temperature for the sample deposited at 700 °C, with a maximum electron Hall mobility of 3100 cm[superscript 2]V[superscript −1]s[superscript −1] at 3.2 K and room temperature mobility of 22 cm[superscript 2]V[superscript −1]s[superscript −1]. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator.

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  • 2015-03-07

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Carrier density modulation in a germanium heterostructure by ferroelectric switching

Description

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. Here we report a true ferroelectric field effect—carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in epitaxial c-axis-oriented BaTiO[subscript 3] grown by molecular beam epitaxy. Using the density functional theory, we demonstrate that switching of BaTiO[subscript 3] polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms BaTiO[subscript 3] tetragonality and the absence of any low-permittivity interlayer at the interface with Ge. The non-volatile, switchable nature of the single-domain out-of-plane ferroelectric polarization of BaTiO[subscript 3] is confirmed using piezoelectric force microscopy. The effect of the polarization switching on the conductivity of the underlying Ge is measured using microwave impedance microscopy, clearly demonstrating a ferroelectric field effect.

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  • 2015-01-01

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Magnetic domain structure in nanocrystalline Ni-Zn-Co spinel ferrite thin films using off-axis electron holography

Description

We report a study of the magnetic domain structure of nanocrystalline thin films of nickel-zinc ferrite. The ferrite films were synthesized using aqueous spin-spray coating at low temperature (∼90 °C) and

We report a study of the magnetic domain structure of nanocrystalline thin films of nickel-zinc ferrite. The ferrite films were synthesized using aqueous spin-spray coating at low temperature (∼90 °C) and showed high complex permeability in the GHz range. Electron microscopy and microanalysis revealed that the films consisted of columnar grains with uniform chemical composition. Off-axis electron holography combined with magnetic force microscopy indicated a multi-grain domain structure with in-plane magnetization. The correlation between the magnetic domain morphology and crystal structure is briefly discussed.

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  • 2014-08-28

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Polarization and electronic state configuration of III-N surfaces and plasma-enhanced atomic layer deposited dielectric interfaces

Description

GaN and AlGaN have shown great potential in next-generation power and RF electronics. However, these devices are limited by reliability issues such as leakage current and current collapse that result

GaN and AlGaN have shown great potential in next-generation power and RF electronics. However, these devices are limited by reliability issues such as leakage current and current collapse that result from surface and interface states on GaN and AlGaN. This dissertation, therefore, examined these electronic states, focusing on the following two points:

First, the surface electronic state configuration was examined with regards to the polarization bound 1013 charges/cm2 that increases with aluminum content. This large bound charge requires compensation either externally by surface states or internally by the space charge regions as relates to band bending. In this work, band bending was measured after different surface treatments of GaN and AlGaN to determine the effects of specific surface states on the electronic state configuration. Results showed oxygen-terminated N-face GaN, Ga-face GaN, and Ga-face Al0.25Ga0.75N surface were characterized by similar band bending regardless of the polarization bound charge, suggesting a Fermi level pinning state ~0.4-0.8 eV below the conduction band minimum. On oxygen-free Ga-face GaN, Al0.15Ga0.85N, Al0.25Ga0.75N, and Al0.35Ga0.65N, band bending increased slightly with aluminum content and thus did not exhibit the same pinning behavior; however, there was still significant compensating charge on these surfaces (~1013 charges/cm2). This charge is likely related to nitrogen vacancies and/or gallium dangling bonds.

In addition, this wozrk investigated the interface electronic state configuration of dielectric/GaN and AlGaN interfaces with regards to deposition conditions and aluminum content. Specifically, oxygen plasma-enhanced atomic layer deposited (PEALD) was used to deposit SiO2. Growth temperature was shown to influence the film quality, where room temperature deposition produced the highest quality films in terms of electrical breakdown. In addition, the valence band offsets (VBOs) appeared to decrease with the deposition temperature, which likely related to an electric field across the Ga2O3 interfacial layer. VBOs were also determined with respect to aluminum content at the PEALD-SiO2/AlxGa1-xN interface, giving 3.0, 2.9, 2.9, and 2.8 eV for 0%, 15%, 25%, and 35% aluminum content, respectively—with corresponding conduction band offsets of 2.5, 2.2, 1.9, and 1.8 eV. This suggests the largest difference manifests in the conduction band, which is in agreement with the charge neutrality level model.

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Date Created
  • 2015

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Microstructure of BAlN and InGaN epilayers for optoelectronic applications

Description

In this dissertation, various characterization techniques have been used to investigate many aspects of the properties of III-nitride materials and devices for optoelectronic applications.

The first part of this work

In this dissertation, various characterization techniques have been used to investigate many aspects of the properties of III-nitride materials and devices for optoelectronic applications.

The first part of this work is focused on the evolution of microstructures of BAlN thin films. The films were grown by flow-modulated epitaxy at 1010 oC, with B/(B+Al) gas-flow ratios ranging from 0.06 to 0.18. The boron content obtained from X-ray diffraction (XRD) patterns ranges from x = 0.02 to 0.09, while Rutherford backscattering spectrometry (RBS) measures x = 0.06 to 0.16. Transmission electron microscopy indicates the sole presence of the wurtzite crystal structure in the BAlN films, and a tendency towards twin formation and finer microstructure for B/(B+Al) gas-flow ratios greater than 0.15. The RBS data suggest that the incorporation of B is highly efficient, while the XRD data indicate that the epitaxial growth may be limited by a solubility limit in the crystal phase at about 9%. Electron energy loss spectroscopy has been used to profile spatial variations in the composition of the films. It has also located point defects in the films with nanometer resolution. The defects are identified as B and Al interstitials and N vacancies by comparison of the observed energy thresholds with results of density functional theory calculations.

The second part of this work investigates dislocation clusters observed in thick InxGa1-xN films with 0.07 ≤ x ≤ 0.12. The clusters resemble baskets with a higher indium content at their interior. Threading dislocations at the basket boundaries are of the misfit edge type, and their separation is consistent with misfit strain relaxation due the difference in indium content between the baskets and the surrounding matrix. The base of the baskets exhibits no observable misfit dislocations connected to the threading dislocations, and often no net displacements like those due to stacking faults. It is argued that the origin of these threading dislocation arrays is associated with misfit dislocations at the basal plane that dissociate, forming stacking faults. When the stacking faults form simultaneously satisfying the crystal symmetry, the sum of their translation vectors does add up to zero, consistent with our experimental observations.

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  • 2018

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Nanoscale Electronic Properties in GaN Based Structures for Power Electronics Using Electron Microscopy

Description

The availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able

The availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able to reach their true potential due to material growth related issues. Power devices typically have patterned p-n, and p-i junctions in lateral, and vertical direction relative to the substrate. Identifying the variations from the intended layer design is crucial for failure analysis of the devices. A most commonly used dopant profiling technique, secondary ion mass spectroscopy (SIMS), does not have the spatial resolution to identify the dopant distribution in patterned devices. The possibility of quantitative dopant profiling at a sub-micron scale for GaN in a scanning electron microscope (SEM) is discussed. The total electron yield in an SEM is shown to be a function of dopant concentration which can potentially be used for quantitative dopant profiling.

Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process.

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Date Created
  • 2019

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Synthesis, characterization, and application of hollow carbon nanostructures

Description

This dissertation describes fundamental studies of hollow carbon nanostructures, which may be used as electrodes for practical energy storage applications such as batteries or supercapacitors. Electron microscopy is heavily utilized

This dissertation describes fundamental studies of hollow carbon nanostructures, which may be used as electrodes for practical energy storage applications such as batteries or supercapacitors. Electron microscopy is heavily utilized for the nanoscale characterization. To control the morphology of hollow carbon nanostructures, ZnO nanowires serve as sacrificial templates. The first part of this dissertation focuses on the optimization of synthesis parameters and the scale-up production of ZnO nanowires by vapor transport method. Uniform ZnO nanowires with 40 nm width can be produced by using 1100 °C reaction temperature and 20 sccm oxygen flow rate, which are the two most important parameters.

The use of ethanol as carbon source with or without water steam provides uniform carbonaceous deposition on ZnO nanowire templates. The amount of as-deposited carbonaceous material can be controlled by reaction temperature and reaction time. Due to the catalytic property of ZnO surface, the thicknesses of carbonaceous layers are typically in nanometers. Different methods to remove the ZnO templates are explored, of which hydrogen reduction at temperatures higher than 700 °C is most efficient. The ZnO templates can also be removed under ethanol environment, but the temperatures need to be higher than 850 °C for practical use.

Characterizations of hollow carbon nanofibers show that the hollow carbon nanostructures have a high specific surface area (>1100 m2/g) with the presence of mesopores (~3.5 nm). The initial data on energy storage as electrodes of electrochemical double layer capacitors show that high specific capacitance (> 220 F/g) can be obtained, which is related to the high surface area and unique porous hollow structure with a thin wall.

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Date Created
  • 2016