Physics Commons^™

Dosimetric Effects Near Implanted Vascular Access Ports Under External Electron Beam Radiation, David Coll Segarra

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Previous studies on dosimetry show important effects for metal vascular access ports for x-rays and electron beams and moderate to no effects for plastic ports for x-ray beams when ports are in the path of the beam. No previous studies exist regarding the effects of electron beams on vascular access ports other than for those made of metal although it has been suggested that electron beam attenuation through non-metal ports may be possible.

Measurements of relative ionization through the device and adjacent to the device anteriorly and laterally were taken. A clinical particle accelerator delivered typical clinical electron beams of …

Temperature And Frequency Dependent Conduction Mechanisms Within Bulk Carbon Nanotube Materials, John Simmons Bulmer

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The resistance of three types of bulk carbon nanotube (CNT) materials (floating catalyst CNT yarn, forest grown CNT yarn, and super acid spun CNT fiber) was measured from room temperature to 900 C. Fitting the curves to established conduction equations for disordered materials, competing conduction mechanisms pertaining to the material could be determined. Floating catalyst CNT yarn displayed both semiconductive and metallic isotropic behavior with a resistance minimum, similar to the behavior of crystalline graphite. It was found that, at room temperature, the semiconducting contribution-most likely junctions between CNTs-accounted for 99.99% of the overall resistance. The resistance of forest grown …

Morphology Of The Dayside Ionosphere Of Mars: Implications For Ion Outflows, Jane L. Fox

Physics Faculty Publications

Significant fluxes of tailward streaming ions have been detected in the Martian wake by instruments on spacecraft. Imposing outward fluxes at the top of a model will produce dayside ion density profiles that are characterized by smaller scale heights than those of diffusive equilibrium. We determine the maximum outward fluxes of ions, and those implied by radio occultation data, by constructing ∼180 models, with upward velocity boundary conditions in the range from 0 to (7–8) × 10⁵ cm s⁻¹ in small increments. As the upward velocity is increased, the topside ion or electron densities decrease until eventually the …

Electric Field Enhancement Of Electron Emission Rates From Z(1/2) Centers In 4h-Sic, A. O. Evwaraye, S. R. Smith, William C. Mitchel, Gary C. Farlow

Physics Faculty Publications

Z_1/2 defect centers were produced by irradiating 4H-SiC bulk samples with 1 MeV electrons at room temperature. The emission rate dependence on the electric field in the depletion region was measured using deep level transient spectroscopy and double-correlation deep level transient spectroscopy. It is found that the Z_1/2 defect level shows a strong electric field dependence with activation energy decreasing from E_c−0.72 eV at zero field to E_c−0.47 eV at 6.91×10⁵ V/cm. The phonon assisted tunneling model of Karpus and Perel [Sov. Phys. JETP 64, 1376 (1986) ] completely describes the …

Persistent Photoconductivity Studies In Nanostructured Zno Uv Sensors, Shiva Sullavarad, Nilima Hullavarad, David C. Look, Bruce Claflin

Physics Faculty Publications

The phenomenon of persistent photoconductivity is elusive and has not been addressed to an extent to attract attention both in micro and nanoscale devices due to unavailability of clear material systems and device configurations capable of providing comprehensive information. In this work, we have employed a nanostructured (nanowire diameter 30–65 nm and 5 μm in length) ZnO-based metal–semiconductor–metal photoconductor device in order to study the origin of persistent photoconductivity. The current–voltage measurements were carried with and without UV illumination under different oxygen levels. The photoresponse measurements indicated a persistent conductivity trend for depleted oxygen conditions. The persistent conductivity phenomenon is …

Effects Of Electron-Irradiation On Electrical Properties Of Algan/Gan Schottky Barrier Diodes, Z-Q. Fang, Gary C. Farlow, B. Claflin, David C. Look, D. S. Green

Physics Faculty Publications

Effects of 1 MeV electron-irradiation at room temperature on the electrical properties of AlGaN/GaN heterostructures, including leakage currents, threshold voltages, and electron traps, have been investigated using Schottky barrier diodes (SBDs) fabricated on the AlGaN. The SBDs, before and after the irradiation with a dose of 5×10¹⁵ cm⁻², were characterized by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy. It is found that the irradiation causes (i) significant increase in leakage currents, dominated by tunneling conduction, at both reverse and low-forward biases; (ii) a clear negative shift in threshold voltage in the pinch-off region; …

Effects Of Collisions On Electronic-Resonance-Enhanced Coherent Anti-Stokes Raman Scattering Of Nitric Oxide, Anil K. Patnaik, Sukesh Roy, James R. Gord, Robert P. Lucht, Thomas B. Settersten

Physics Faculty Publications

A six-level model is developed and used to study the effects of collisional energy transfer and dephasing on electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) in nitric oxide. The model includes the three levels that are coherently coupled by the three applied lasers as well as three additional bath levels that enable inclusion of the effects of electronic quenching and rotational energy transfer. The density-matrix equations that describe the evolution of the relevant populations and coherences are presented. The parametric dependencies of the ERE-CARS signal on collisional energy transfer and dephasing processes are described in terms of both a steady-state analytical …

Spontaneously Generated Atomic Entanglement In Free Space Reinforced By Incoherent Pumping, Ling Zhou, Gou Hui Yang, Anil K. Patnaik

Physics Faculty Publications

We study spontaneously generated entanglement (SGE) between two identical multilevel atoms in free space via vacuum-induced radiative coupling. We show that the SGE in two-atom systems may initially increase with time but eventually vanishes in the time scale determined by the excited-state lifetime and radiative coupling strength between the two atoms. We demonstrate that steady-state SGE can be established by incoherently pumping the atoms to their excited states. We have shown that an appropriate rate of incoherent pump can help in producing optimal steady-state SGE. The multilevel systems offer us more channels to establish entanglement. The system under consideration can …

Stochastic Simulation And Analysis Of Biomolecular Reaction Networks, John M. Frazier, Yaroslov Chushak, Brent Foy

Physics Faculty Publications

Background: In recent years, several stochastic simulation algorithms have been developed to generate Monte Carlo trajectories that describe the time evolution of the behavior of biomolecular reaction networks. However, the effects of various stochastic simulation and data analysis conditions on the observed dynamics of complex biomolecular reaction networks have not recieved much attention. In order to investigate these issues, we employed a a software package developed in out group, called Biomolecular Network Simulator (BNS), to simulate and analyze the behavior of such systems. The behavior of a hypothetical two gene in vitro transcription-translation reaction network is investigated using the Gillespie …

Metal Contacts On Bulk Zno Crystal Treated With Remote Oxygen Plasma, Z-Q. Fang, B. Claflin, David C. Look, Y. F. Dong, L. Brillson

Physics Faculty Publications

To study the quality of thin metal/ZnO Schottky contacts (SCs), temperature-dependent current-voltage (I-V), capacitance-voltage, deep level transient spectroscopy, and photoluminescence measurements were performed using bulk, vapor-phase ZnO, treated by remote oxygen plasma (ROP). Au/ZnO and Pd/ZnO contacts on both O and Zn faces are compared as a function of the ROP processing sequence and duration. We find that (i) as the duration of ROP treatment increases from 2 to 4 h, Au/ZnO contacts on the Zn face, deposited before ROP treatment, become rectifying, while those on the O face remain Ohmic; (ii) with long-term ROP …

Uv Light-Induced Changes To The Surface Conduction In Hydrothermal Zno, B. Claflin, David C. Look

Physics Faculty Publications

High quality, bulk ZnO crystals grown by Tokyo Denpa using the hydrothermal process typically exhibit a room temperature carrier concentration in the 10¹³–10¹⁴ cm⁻³ range and a low mobility, conductive surface layer, observed at low temperature, with a sheet concentration on the order of 10¹²–10¹³ cm⁻². In the sample discussed here, bulk conduction is controlled by two donor levels at 50 and 400 meV with concentrations of 1.2×10¹⁶ and 1.5×10¹⁶ cm⁻³, respectively. Temperature-dependent photo-Hall-effect measurements, using blue/UV light, in vacuum show an increase in the surface sheet …

Polarity-Related Asymmetry At Zno Surfaces And Metal Interfaces, Y. F. Dong, Z-Q. Fang, David C. Look, Daniel R. Doutt, M. J. Hetzer, L. J. Brillson

Physics Faculty Publications

Clean ZnO (0001) Zn- and (000(/1)) O-polar surfaces and metal interfaces have been systematically studied by depth-resolved cathodoluminescence spectroscopy, photoluminescence, current-voltage and capacitance-voltage measurements, and deep level transient spectroscopy. Zn-face shows higher near band edge emission and lower near surface defect emission. Even with remote plasma decreases of the 2.5 eV near surface defect emission, (0001)-Zn face emission quality still exceeds that of (000(/1))-O face. The two polar surfaces and corresponding metal interfaces also present very different luminescence evolution under low-energy electron beam irradiation. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O₂/He plasma-cleaned surfaces display not …

Strain Waves, Earthquakes, Slow Earthquakes, And Afterslip In The Framework Of The Frenkel-Kontorova Model, Naum I. Gershenzon, V. G. Bykov, Gust Bambakidis

Physics Faculty Publications

The one-dimensional Frenkel-Kontorova (FK) model, well known from the theory of dislocations in crystal materials, is applied to the simulation of the process of nonelastic stress propagation along transform faults. Dynamic parameters of plate boundary earthquakes as well as slow earthquakes and afterslip are quantitatively described, including propagation velocity along the strike, plate boundary velocity during and after the strike, stress drop, displacement, extent of the rupture zone, and spatiotemporal distribution of stress and strain. The three fundamental speeds of plate movement, earthquake migration, and seismic waves are shown to be connected in framework of the continuum FK model. The …

In-Implanted Zno: Controlled Degenerate Surface Layer, David C. Look, Gary C. Farlow, F. Yaqoob, L. H. Vanamurthy, M. Huang

Physics Faculty Publications

In was implanted into bulk ZnO creating a square profile with a thickness of about 100 nm and an In concentration of about 1×10²⁰ cm^-3. The layer was analyzed with Rutherford backscattering, temperature-dependent Hall effect, and low-temperature photoluminescence measurements. The implantation created a nearly degenerate carrier concentration n of about 2×10¹⁹ cm^-3, but with a very low mobility μ, increasing from about 0.06 cm²/V s at 20 K to about 2 cm²/V s at 300 K. However, after annealing at 600 °C for 30 min, n increased to about 5×10 …

Ga-Related Photoluminescence Lines In Ga-Doped Zno Grown By Plasma-Assisted Molecular-Beam Epitaxy, Z. Yang, David C. Look, J. L. Liu

Physics Faculty Publications

Low-temperature photoluminescence (PL) and temperature-dependent Hall-effect (T-Hall) measurements were carried out in undoped and Ga-doped ZnO thin films grown by molecular-beam epitaxy. As the carrier concentration increases from 1.8×10¹⁸ to 1.8×10²⁰ cm⁻³, the dominant PL line at 9 K changes from I₁ (3.368–3.371 eV) to I_DA (3.317–3.321 eV), and finally to I₈ (3.359 eV). The dominance of I₁, due to ionized-donor bound excitons, is unexpected in n-type samples but is shown to be consistent with the T-Hall results. We also show that I_DA has characteristics of …

Towards Medical Terahertz Sensing Of Skin Hydration, Michael Lee, Rahul S. Singh, Martin O. Culjat, Shyam Natarajan, Brian P. Cox, Elliott R. Brown, Warren S. Grundfest, Hua Lee

Physics Faculty Publications

Terahertz imaging has shown promise as a tool for noninvasive in-vivo detection of skin abnormalities, including skin cancer, burns, scars, and wounds due to its low non-ionizing photon energy and ability to penetrate clothing and gauze. This study examines whether low-level bulk differences in the water content between hyperhydrated and dehydrated skin can be detected using a scanning, reflective THz imaging system. Our results show an 8.7x difference in the THz reflectivity between hyperhydrated and dehydrated specimens of chicken skin. The results provide further evidence that water concentration is the primary contrast mechanism in reflective THz biomedical imaging.

Sub-Mm Wave Imaging And Waveguiding Techniques For Non-Destructive Materials Evaluation, Izaak Vincent Kemp

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Sub-mm wave technology (also known as Terahertz/THz) is a rapidly developing field of electro-optics which has great potential in a variety of applications. Generally defined as the band of the electro-magnetic spectrum with frequencies spanning from 300 GHz to 30 THz, sub-mm waves are non-ionizing and are capable of passing through a wide range of dielectric substances making them well suited for imaging purposes such as non-destructive materials evaluation (NDE). Because of their high sensitivity to metals and ability to penetrate surface obscurants such as paint, oil, and epoxy sub-mm waves are capable of imaging corrosion damage on metallic surfaces …

Morphology Of The Dayside Ionosphere Of Venus: Implications For Ion Outflows, Jane L. Fox

Physics Faculty Publications

The nightside ionosphere of Venus is formed mostly by day-to-night transport of ions below the ionopause, with a small contribution from precipitation of energetic electrons from the wake. This nightward flux of ions should result in dayside ionospheres that are characterized by smaller electron density scale heights at high altitudes than those that are characteristic of diffusive equilibrium. In order to determine both the maximum possible values of the upward fluxes and the upward fluxes implied by comparison of our computed ion and electron density profiles to measurements, we have constructed more than 60 models of the high solar activity …

Nonlinear Magneto-Optic Polarization Rotation With Intense Laser Fields, Paul S. Hsu, Anil K. Patnaik, George R. Welch

Physics Faculty Publications

We have studied the nonlinear Faraday effect with intense linear polarized light in an optically thick atomic rubidium vapor. We demonstrate that the polarization rotation rate (rotation angle per unit magnetic field, in the limit of low field) has a maximum value as the intensity and density are increased. We also show that the optimal sensitivity of an optical magnetometer based on this system reaches a saturation value as the intensity and density are increased.

Metal Modulation Epitaxy Growth For Extremely High Hole Concentrations Above 10(19) Cm(-3) In Gan, Gon Namkoong, Elaissa Trybus, Kyung Kuen Lee, Michael Moseley, W. Alan Doolittle, David C. Look

Physics Faculty Publications

The free hole carriers in GaN have been limited to concentrations in the low 10¹⁸ cm⁻³ range due to the deep activation energy, lower solubility, and compensation from defects, therefore, limiting doping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to ∼ 10% in GaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrations grown by periodically modulating Ga atoms and Mg dopants were over ∼ 1.5×10¹⁹ cm⁻³.

Variation Of The Martian Ionospheric Electron Density From Mars Express Radar Soundings, D. D. Morgan, D. A. Gurnett, D. L. Kirchner, Jane L. Fox, E. Nielsen, J. J. Plaut

Physics Faculty Publications

The Mars Advanced Radar for Subsurface and Ionospheric Sounding aboard Mars Express has been in operation for over 2 years. Between 14 August 2005 and 31 July 2007, we obtain 34,492 ionospheric traces, of which 14,060 yield electron density profiles and 12,291 yield acceptable fits to the Chapman ionospheric model. These results are used to study the Martian ionosphere under changing conditions: the presence or absence of solar energetic particles, solar EUV flux, season, solar zenith angle, and latitude. The 2-year average subsolar maximum electron density n ₀ is 1.62 × 10⁵ cm⁻³, the average subsolar electron …

Surface Traps In Vapor-Phase-Grown Bulk Zno Studied By Deep Level Transient Spectroscopy, Z-Q. Fang, B. Claflin, David C. Look, Y. F. Dong, H. L. Mosbacker, L. J. Brillson

Physics Faculty Publications

Deep level transient spectroscopy, current-voltage, and capacitance-voltage measurements are used to study interface traps in metal-on-bulk-ZnO Schottky barrier diodes (SBDs). c-axis-oriented ZnO samples were cut from two different vapor-phase-grown crystals, and Au- and Pd-SBDs were formed on their (0001) surfaces after remote oxygen-plasma treatment. As compared to Au-SBDs, the Pd-SBDs demonstrated higher reverse-bias leakage current and forward-bias current evidently due to higher carrier concentrations, which might have been caused by hydrogen in-diffusion through the thin Pd metal. The dominant traps included the well-known bulk traps E₃ (0.27 eV) and E₄ (0.49 eV). In addition, a surface-related trap, …

Active And Passive Imaging In The Thz Spectral Region: Phenomenology, Dynamic Range, Modes, And Illumination, Douglas T. Petkie, Corey Casto, Frank C. De Lucia, Steven R. Murrill, Brian Redman, Richard L. Espinola, Charmaine C. Franck, Eddie L. Jacobs, Steven T. Griffin, Carl E. Halford, Joe Reynolds, Sean O'Brien, David Tofsted

Physics Faculty Publications

The useful compromise between resolution and penetration power of the submillimeter or terahertz (THz) spectral region has long made it attractive for a variety of imaging applications. However, many of the demonstrations of imaging in this spectral region have used strategically oriented targets, especially favorable concealment materials, proximate imaging geometries, etc. This paper reports the results of studies aimed at better understanding the phenomenology of targets, the impact of this phenomenology on various active and passive imaging strategies, and most importantly, the development of imaging strategies that do not require the aforementioned special circumstances. Particular attention is paid to the …

Two-Layer Hall-Effect Model With Arbitrary Surface-Donor Profiles: Application To Zno, David C. Look

Physics Faculty Publications

A complete two-layer Hall-effect model, allowing arbitrary donor and acceptor profiles, is presented and applied to the problem of conductive surface layers in ZnO. Temperature-dependent mobility and carrier concentration data in the temperature range of 20–320 K are fitted with an efficient algorithm easily implemented in commercial mathematics programs such as MATHCAD. The model is applied to two ZnO samples, grown by the melt (MLT) and hydrothermal (HYD) processes, respectively. Under the assumption of a “square” surface-donor profile, the fitted surface-layer thicknesses are 48 and 2.5 nm, respectively, for the MLT and HYD samples. The surface-donor concentrations are 7.6×10^{17 …}

Zn- And O-Face Polarity Effects At Zno Surfaces And Metal Interfaces, Yufeng Dong, Z-Q. Fang, David C. Look, G. Cantwell, J. Zhang, J. J. Song, L. J. Brillson

Physics Faculty Publications

Depth-resolved cathodoluminescence spectroscopy, current-voltage, capacitance-voltage, and deep level transient spectroscopy of ZnO (0001) Zn- and (000) O-polar surfaces and metal interfaces show systematically higher Zn-face near band edge emission and lower near-surface defect emission. Even with remote plasma decreases of the 2.5 eV near-surface defect emission, (0001)-Zn face emission quality still exceeds that of (000)-O face. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O₂/He plasma-cleaned surfaces display a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. A comprehensive model accounts for the polarity-dependent transport properties and their correlations with carrier concentration profiles.

Very High Channel Conductivity In Low-Defect Aln/Gan High Electron Mobility Transistor Structures, A. M. Dabiran, A. M. Wowchak, A. Osinsky, J. Xie, Brian Hertzog, David C. Look, P. P. Chow

Physics Faculty Publications

Low defect AlN/GaN high electron mobility transistor (HEMT) structures, with very high values of electron mobility (>1800 cm²/V s) and sheet charge density (>3×10¹³ cm⁻²), were grown by rf plasma-assisted molecular beam epitaxy (MBE) on sapphire and SiC, resulting in sheet resistivity values down to ∼ 100 Ω/◻ at room temperature. Fabricated 1.2 μm gate devices showed excellent current-voltage characteristics, including a zero gate saturation current density of ∼ 1.3 A/mm and a peak transconductance of ∼ 260 mS/mm. Here, an all MBE growth of optimized AlN/GaN HEMT structures plus the results …

Reproducible Increased Mg Incorporation And Large Hole Concentration In Gan Using Metal Modulated Epitaxy, Shawn D. Burnham, Gon Namkoong, David C. Look, Bruce Claflin, W. Alan Doolittle

Physics Faculty Publications

The metal modulated epitaxy (MME) growth technique is reported as a reliable approach to obtain reproducible large hole concentrations in Mg-doped GaN grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates. An extremely Ga-rich flux was used, and modulated with the Mg source according to the MME growth technique. The shutter modulation approach of the MME technique allows optimal Mg surface coverage to build between MME cycles and Mg to incorporate at efficient levels in GaN films. The maximum sustained concentration of Mg obtained in GaN films using the MME technique was above 7×10²⁰ cm⁻³, leading …

Infrared Simulations Derived From Submillimeter Wave Analyses, Douglas T. Petkie, Rebecca A. H. Butler, Paul Helminger, Zbigniew Kisiel, Kenneth W. Jucks, Brenda P. Winnewisser, Manfred Winnewisser, Frank C. De Lucia

Physics Faculty Publications

No abstract provided.

The Rotational Spectrum Of Chlorine Nitrate (Ciono2): V6 And The V5/V6V9 Dyad, Zbigniew Kisiel, Ewa Bialkowska-Jaworska, Rebecca A. H. Butler, Douglas T. Petkie, Paul Helminger, Frank C. De Lucia

Physics Faculty Publications

No abstract provided.

Effects Of Annealing In N(2) Ambient On Traps And Persistent Conduction In Hydrothermally Grown Zno, Z-Q. Fang, B. Claflin, David C. Look

Physics Faculty Publications

Thermally stimulated current (TSC) spectroscopy and temperature–dependent dark current (DC) measurements have been applied to study traps and photoinduced persistent surface conduction in two hydrothermally grown bulk ZnO samples, as-grown, and annealed at 600 °C in N₂ ambient for 30 min, respectively. The as-grown sample had a room-temperature (RT) resistivity of 1.6×10³ Ω cm, mobility of 2.1×10² cm²/V s, and carrier concentration of 1.8×10¹³ cm⁻³, while the annealed sample was highly resistive, with RT resistivity of 3.6×10⁶ Ω cm, mobility of 4.4 cm²/V s, and carrier concentration of 3.9×10 …