Physics Commons^™

Self-Compensation In Semiconductors: The Zn Vacancy In Ga-Doped Zno, David C. Look, K. D. Leedy, L. Vines, B. G. Svensson, A. Zubiaga, F. Tuomisto, Daniel R. Doutt, L. J. Brillson

Physics Faculty Publications

Self-compensation, the tendency of a crystal to lower its energy by forming point defects to counter the effects of a dopant, is here quantitatively proven. Based on a new theoretical formalism and several different experimental techniques, we demonstrate that the addition of 1.4 × 10²¹-cm⁻³ Ga donors in ZnO causes the lattice to form 1.7 × 10²⁰-cm⁻³ Zn-vacancy acceptors. The calculated V_Zn formation energy of 0.2 eV is consistent with predictions from density functional theory. Our formalism is of general validity and can be used to investigate self-compensation in any degenerate semiconductor material.

Thermal Annealing Effect On Spin Coherence In Zno Single Crystals, Z. Yang, David C. Look, H. M. Zhou, W. V. Kawakami, R. K. Kawakami, P. K. L. Yu, J. L. Liu

Physics Faculty Publications

The spin coherence time (T₂^*) in ZnO single crystals at 8.5 K decreases significantly from ∼11.2 ns to ∼2.3 ns after annealing at 500 °C, as indicated by time-resolved Kerr-rotation pump-probe magneto-optical spectroscopy. The annealing-induced spin coherence degradation in ZnO arises neither from crystallinity degradation during the annealing process, as confirmed by x-ray rocking curves; nor from reflection variations of the probe laser beam induced by surface roughness changes during the annealing process, as confirmed by atomic force microscopy. Temperature-dependent Hall-effect studies indicate that decreased mobility and increased shallow-donor concentration in the annealing-induced surface conducting layer …

Growth Of Low-Dimensional Carbon Nanomaterials, John J. Boeckl, Weijie Lu, William Mitchel

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Nano-Based Optical Chemical Sensors, Aleksandra Lobnik

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

High Ammonia Yield Using Magnetic Induction Method, Noorhana Yahya

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Nonlinear Optical Properties Of Monolayer Films Deposited On Ir Transparent Substrates, Ivan V. Kityk, Marek Matusiewicz, Gregory Kozlowski, Jacek Kasperczyk, Krzysztof Koziol

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Understanding Irreducible And Reducible Oxides As Catalysts For Carbon Nanotubes And Graphene Formation, Alicja Bachmatiuk

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Ultrasonics And Carbon Nanotubes, Marian Nowak, Krzysztof Koziol, Gregory Kozlowski, Jan Berdowski, Jacek Kasperczyk, Ivan V. Kityk, Krystian Mistewicz, Marcin Jesionek

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Dynamics Of Fingering Convection I: Small-Scale Fluxes And Large-Scale Instabilities, Adrienne L. Traxler, Stephan Stellmach, Pascale Garaud, T. Radko, N. Brummell

Physics Faculty Publications

Double-diffusive instabilities are often invoked to explain enhanced transport in stably stratified fluids. The most-studied natural manifestation of this process, fingering convection, commonly occurs in the ocean's thermocline and typically increases diapycnal mixing by 2 orders of magnitude over molecular diffusion. Fingering convection is also often associated with structures on much larger scales, such as thermohaline intrusions, gravity waves and thermohaline staircases. In this paper, we present an exhaustive study of the phenomenon from small to large scales. We perform the first three-dimensional simulations of the process at realistic values of the heat and salt diffusivities and provide accurate estimates …

Dynamics Of Fingering Convection Ii: The Formation Of Thermohaline Staircases, Stephan Stellmach, Adrienne L. Traxler, Pascale Garaud, N. Brummell, T. Radko

Physics Faculty Publications

Regions of the ocean's thermocline unstable to salt fingering are often observed to host thermohaline staircases, stacks of deep well-mixed convective layers separated by thin stably stratified interfaces. Decades after their discovery, however, their origin remains controversial. In this paper we use three-dimensional direct numerical simulations to shed light on the problem. We study the evolution of an analogous double-diffusive system, starting from an initial statistically homogeneous fingering state, and find that it spontaneously transforms into a layered state. By analysing our results in the light of the mean-field theory developed in Part 1 (Traxler et al., J. Fluid …

Ga-Doped Zno Grown By Pulsed Laser Deposition In H-2: The Roles Of Ga And H, David C. Look, T. C. Droubay, J. S. Mccloy, Z. H. Zhu, S. A. Chambers

Physics Faculty Publications

Highly conductive thin films of ZnO doped with Ga were grown by pulsed laser deposition with 10 mTorr of H₂ in the growth chamber. Compared with a more conventional method of producing conductive films of ZnO, i.e., growth in O₂ followed by annealing in forming gas (5% H₂ in Ar), the H₂ method requires no postgrowth anneal and also produces higher carrier concentrations and lower resistivities with better depth uniformity. As an example, a 65-nm-thick sample had a room-temperature mobility of 32 cm²/V s, a concentration of 6.8×10²⁰ cm⁻³, and a …

Propagation Dynamics Of Controlled Cross-Talk Via Interplay Between Chi((1)) And Chi((3)) Processes, Paul S. Hsu, George R. Welch, James R. Gord, Anil K. Patnaik

Physics Faculty Publications

We investigate theoretically and experimentally the propagation dynamics of a nonlinear cross-talk effect between two probe channels in a double-ladder system and show that an interplay between χ⁽¹⁾ and χ⁽³⁾ processes leads to the control of cross-talk. We derive analytical solutions to describe the propagation dynamics of the probe fields with the cross-talk effect built in. From the analytical results we identify and examine the regimes of interest where contributions of either χ⁽¹⁾ or χ⁽³⁾ or both are significant. The control of cross-talk is demonstrated experimentally, and good quantitative agreement is found between the analytical solutions …

Turbulent Mixing And Layer Formation In Double-Diffusive Convection: 3d Numerical Simulations And Theory, Erica Rosenblum, Pascale Garaud, Adrienne L. Traxler, Stephan Stellmach

Physics Faculty Publications

Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux criterion but unstable according to the Schwarzschild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct …

Episodic Tremors And Slip In Cascadia In The Framework Of The Frenkel-Kontorova Model, Naum I. Gershenzon, Gust Bambakidis, Ernest Hauser, Abhijit Ghosh, Kenneth C. Creager

Physics Faculty Publications

The seismic moment for regular earthquakes is proportional to the cube of rupture time. A second class of phenomena, collectively called slow earthquakes, has very different scaling. We propose a model, inspired from the phenomenology of dislocation dynamics in crystals, that is consistent with the scaling relations observed in the Cascadia episodic tremor and slip (ETS) events. Two fundamental features of ETS are periodicity and migration. In the northern Cascadia subduction zone, ETS events appear every 14.5 months or so. During these events, tremors migrate along-strike with a velocity of 10 km/day and simultaneously zip back and forth in the …

Numerically Determined Transport Laws For Fingering ("Thermohaline") Convection In Astrophysics, Adrienne L. Traxler, Pascale Garaud, Stephan Stellmach

Physics Faculty Publications

We present the first three-dimensional simulations of fingering convection performed in a parameter regime close to the one relevant for astrophysics, and reveal the existence of simple asymptotic scaling laws for turbulent heat and compositional transport. These laws can straightforwardly be extrapolated to the true astrophysical regime. Our investigation also indicates that thermocompositional "staircases," a key consequence of fingering convection in the ocean, cannot form spontaneously in the fingering regime in stellar interiors. Our proposed empirically-determined transport laws thus provide simple prescriptions for mixing by fingering convection in a variety of astrophysical situations, and should, from here on, be used …

Growth And Characterization Of Carbon Nanomaterials, Jay Patel

Browse all Theses and Dissertations

In this thesis, single and multi-layered graphene films were epitaxially grown on either Si-face or C-face of SiC single crystal substrates. The film growth conditions, such as decomposition temperatures and pressures, and their surface morphologies were optimized. These films were then characterized by using surface analysis tools including SEM, TEM, AFM evanescent wave microscopy and electron educed spectroscopy. In addition to studying graphene decomposed from SiC crystals, carbon nanotube material was fabricated using a floating catalyst technique. These carbon nanotube material was then studied for potential cathode applications in this thesis. Field emission properties of these cathodes was measured and …

Computed Tomography Reconstruction: Investigating The Effect Of Varying Circle Diameter, William F. Sanders Iv

Browse all Theses and Dissertations

In Computed Tomography (CT) reconstruction, several methods for determining the intensity of individual pixels from the back-projection of the scanned profiles exist. The standard reconstruction method uses linear interpolation between ray values to determine pixel intensity. This study quantifies the effects of varying circle diameter on the accuracy of an alternative method where the pixel is approximated as a circle and the area contributions calculated.

A library of 104 scans in 3 image families was created by a synthetic CT scanner and reconstructed with circle radii from 0.1 to 1.0 pixel in 0.1 pixel steps. Images were compared against a …

Defects At Oxygen Plasma Cleaned Zno Polar Surfaces, Y. F. Dong, Z-Q. Fang, David C. Look, Daniel R. Doutt, G. Cantwell, J. Zhang, J. J. Song, L. J. Brillson

Physics Faculty Publications

Depth-resolved cathodoluminescence spectroscopy (DRCLS) reveals the evolution of surface and near surface defects at polar surfaces with remote oxygen plasma (ROP) treatment. Furthermore, this evolution exhibits significant differences that depend on surface polarity. ROP decreased the predominant 2.5 eV defect emission related to oxygen vacancies on the O face, while creating a new 2.1 eV defect emission on the Zn face that increases with ROP time. The surface-located 2.1 eV emission correlates with carrier profiles from capacitance-voltage measurements and a shift of the E3 trap to higher binding energy from deep level transient spectroscopy (DLTS). This result suggests that ROP …

Deep Traps In Algan/Gan Heterostructures Studied By Deep Level Transient Spectroscopy: Effect Of Carbon Concentration In Gan Buffer Layers, Z-Q. Fang, B. Claflin, David C. Look, D. S. Green, R. Vetury

Physics Faculty Publications

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed …

Highly Conductive Zno Grown By Pulsed Laser Deposition In Pure Ar, Robin C. Scott, Kevin D. Leedy, Burhan Bayraktaroglu, David C. Look, Yong-Hang Zhang

Physics Faculty Publications

Ga-doped ZnO was deposited by pulsed laser deposition at 200 °C on SiO₂/Si, Al₂O₃, or quartz in 10 mTorr of pure Ar. The as-grown, bulk resistivity at 300 K is 1.8×10⁻⁴ Ω cm, three-times lower than that of films deposited at 200 °C in 10 mTorr of O₂ followed by an anneal at 400 °C in forming gas. Furthermore, depth uniformity of the electrical properties is much improved. Mobility analysis shows that this excellent resistivity is mostly due to an increase in donor concentration, rather than a decrease in acceptor concentration. Optical …

Magnetotransport Properties Of High Quality Co:Zno And Mn:Zno Single Crystal Pulsed Laser Deposition Films: Pitfalls Associated With Magnetotransport On High Resistivity Materials, John S. Mccloy, Joseph V. Ryan, Timothy C. Droubay, Tiffany C. Kasper, Scott A. Chambers, David C. Look

Physics Faculty Publications

The electrical resistivity values for a series of pure and doped (Co, Mn, Al) ZnO epitaxial films grown by pulsed laser deposition were measured with equipment designed for determining the direct current resistivity of high resistance samples. Room-temperature resistances ranging from 7 x 10(1) to 4 x 10(8) Omega/sq were measured on vacuum-reduced cobalt-doped ZnO, (Al,Co) co-doped ZnO, pure cobalt-doped ZnO, Mn-doped ZnO, and undoped ZnO. Using a four-point collinear geometry with gold spring-loaded contacts, resistivities were measured from 295 to 5 K for resistances of < approximately 10(12) Omega/sq. In addition, magnetoresistance and Hall effect were measured as a function of temperature for select samples. Throughout the investigation, samples were also measured on commercially available instrumentation with good agreement. The challenges of transport measurements on high resistivity samples are discussed, along with some offered solutions to those challenges.

Mobility Analysis Of Highly Conducting Thin Films: Application To Zno, David C. Look, K. D. Leedy, D. H. Tomich, B. Bayraktaroglu

Physics Faculty Publications

Hall-effect measurements have been performed on a series of highly conductive thin films of Ga-doped ZnO grown by pulsed laser deposition and annealed in a forming-gas atmosphere (5% H₂ in Ar). The mobility as a function of thickness d is analyzed by a simple formula involving only ionized-impurity and boundary scattering and having a single fitting parameter, the acceptor/donor concentration ratio K = N_A/N_D. For samples with d = 3–100 nm, K_avg = 0.41, giving N_D = 4.7×10²⁰ and N_A = 1.9×10²⁰ cm⁻³. Thicker samples require a …

Structural Investigations And Magnetic Properties Of Sol-Gel Ni0.5zn0.5fe2o4 Thin Films For Microwave Heating, Pengzhao Z. Gao, Evgeny V. Rebrov, Tiny M. W. G. M. Verhoeven, Jaap C. Schouten, Richard Kleismit, Gregory Kozlowski, John S. Cetnar, Zafer Turgut, Guru Subramanyam

Physics Faculty Publications

Nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation …

Experimental Comparison Of Acr And Icamrl Magnetic Resonance Imaging Accreditation Protocols, Brock Andrew Prater

Browse all Theses and Dissertations

Two primary accrediting bodies exist for magnetic resonance imaging systems: the American College of Radiology (ACR) and the Intersocietal Commission for the Accreditation of Magnetic Resonance Laboratories (ICAMRL), each of which defines specific standards for specific image quality criteria at which MRI images must be produced. An MRI clinic that wishes to show a commitment to image quality may do so by becoming accredited by one of these organizations of their choosing. The limits of these image criteria were compared to demonstrate the standards of each accrediting body. Images were produced that fell well within the standards of both accrediting …

Determining Metastable Densities In An Argon Discharge Through Optical Emission Spectroscopy, Jared A. Miles

Browse all Theses and Dissertations

A plasma diagnostic technique has been experimentally demonstrated where optical emission measurements of relative intensities of spectral lines in the violet range were combined with available electron-impact cross sections to yield absolute Ar metastable species concentration. An enabling factor of this analysis was that the electron excitation pattern was quite different between the Ar ground state and the metastable state. The result of this pattern was that the optical spectrum was unique depending on whether the emission was generated by direct excitation from the ground state, or by stepwise excitation from one of the metastable states. This study has shown …

Ostwald Ripening Of Iron (Fe) Catalyst Nanoparticles On Aluminum Oxide Surfaces (Al2O3) For The Growth Of Carbon Nanotubes, Roberto I. Acosta

Browse all Theses and Dissertations

Theoretical models have proposed that the nucleation and growth mechanism of carbon nanotubes (CNTs) has been affected by the catalytic activity of transition metals. The catalyst behavior during growth has been mainly associated as the responsible mechanism for the termination of CNT growth. Although several hypotheses have been developed to explain this mechanism, is still today an unresolved phenomenon. It was recently shown that the Ostwald ripening of iron (Fe) nanoparticles played a dominant role in the termination of CNT growth. The Ostwald ripening mechanism was further investigated as a function of thermal annealing in Hydrogen (H₂) for …

Atmospheric Effects On The Propagation Of Mmw And Sub-Mmw Radiation, John S. Cetnar

Browse all Theses and Dissertations

This thesis is a study of the propagation of millimeter wavelength (MMW) and submillimeter wavelength (sub-MMW) electromagnetic radiation (a.k.a. THz radiation) through the Earth's atmosphere. THz radiation is electromagnetic radiation that exists between the microwave and far infrared regions of the electromagnetic spectrum. It is nonionizing radiation but can penetrate through materials that are opaque to visible light so therefore has many new and useful applications. Unfortunately, THz radiation is heavily attenuated by the Earth's atmosphere as it propagates through it. This therefore represents a challenge to communications and sensing applications at millimeter and sub-millimeter wavelengths. In this work, the …

Proximity And Thickness Estimation Of Aluminum 3003 Alloy Metal Sheets Using Multi-Frequency Eddy Current Sensor, Sunil S. Kamanalu

Browse all Theses and Dissertations

The research work is focused on conducting a feasibility study on a new "non-contact" single probe dual coil inductive sensor for sensing the proximity and thickness of Aluminum (Al) 3003 alloy metal sheets, which is a non-magnetic metal. A bulk of the research and development work has already been done in the area of non-destructive testing (NDT) using eddy current technology targeted to various applications like corrosion detection, material thickness, material conductivity, etc. The research work presented in this thesis uses the prior research and development work completed in NDT as a platform for conducting this study to estimate proximity …

Respiratory-Gated Imrt Quality Assurance With Motion In Two Dimensions, Michael Todd Massie

Browse all Theses and Dissertations

Intensity modulated radiation therapy (IMRT) plans can be further customized to each patient with the use of a four-dimensional (4D) respiratorygated computed tomography (CT), with time being the fourth dimension. The 4D respiratory-gated CT allows for the internal margin (IM), the expansion of the tumor volume that accounts for physiologic motions, to be addressed in the treatment planning process and no longer assumes that the treatments will be delivered to a fixed or rigid patient anatomy. Delivering the IMRT plan with a gated technique limits the treatment to a duty cycle when the target motion is at a minimum.

The …

The Derivation And Testing Of Three-Dimensional Line Equations That Predict The Location Of Brachytherapy Sources, Rhett Ellis Lindsey

Browse all Theses and Dissertations

In High Dose Rate brachytherapy, a catheter is placed inside the body and a radioactive source is allowed to dwell at specific positions to treat a tumor. In normal usage, anatomical images acquired before the treatment are used to plan dwell positions, and then the plan is executed without further verifying source locations during treatment. However, slight errors in catheter positioning and shifts in internal anatomy cause variations in source position. In this study, a general method for determining dwell positions during treatment is evaluated. For this method, the treatment source exposes the tumor to radiation and creates an image …