Digital Commons Network^™

Ground And Excited State Exciton Spectra From Gan Grown By Molecular-Beam Epitaxy, D. C. Reynolds, David C. Look, W. Kim, Ö. Aktas, A. Botchkarev, A. Salvador, H. Morkoç, D. N. Talwar

Physics Faculty Publications

The emission and reflection spectra of GaN have been investigated in the intrinsic region and the data have been interpreted in terms of the wurtzite crystal band structure. Three intrinsic exciton transitions have been observed, one associated with each of the valence bands. Exciton excited states associated with the two top valence bands were also observed. The exciton binding energies, the band-gap energies, and the exciton Bohr radii are all reported along with the dielectric constant and the spin-orbit and crystal-field parameters for GaN.

Binary Nucleation Kinetics. Iii. Transient Behavior And Time Lags, Barbara Ellen Wyslouzil, Gerald Wilemski

Physics Faculty Research & Creative Works

Transient binary nucleation is more complex than unary because of the bidimensionality of the cluster formation kinetics. To investigate this problem qualitatively and quantitatively, we numerically solved the birth-death equations for vapor-to-liquid phase transitions. Our previous work [J. Chem. Phys 103, 1137 (1995)] showed that the customary saddle point and growth path approximations are almost always valid in steady state gas phase nucleation and only fail if the nucleated solution phase is significantly nonideal. The current work demonstrates that in its early transient stages, binary nucleation rarely, if ever, occurs via the saddle point. This affects not only the number …

Charge-Dipole Model For The Universal Field Dependence Of Mobilities In Molecularly Doped Polymers, David H. Dunlap, Paul Ernest Parris, Vasudev M. Kenkre

Physics Faculty Research & Creative Works

Using the general result that the mobility μ, of charge carriers driven in a spatially correlated random potential by an electric field E can be expressed in terms of the Laplace transform of a particular correlation function related to the random potential, we demonstrate that the exponential dependence of μ on √E universally observed in molecularly doped polymers arises naturally from the interaction of charge carriers with randomly distributed permanent dipoles.

Structure, Dynamics, And Phase Transitions In The Fullerene Derivatives C_{60}O And C_{61}H_{2}, C. Meingast, G. Roth, L. Pintschovius, R. H. Michel, C. Stoermer, M. M. Kappes, Paul A. Heiney, Laurent Brard, Robert M. Strongin, Amos B. Smith Iii

Chemistry Faculty Publications and Presentations

The effect of perturbing the icosohedral symmetry of C₆₀ by the addition of the side groups -O and -CH₂ upon orientational order-disorder and glass transitions in solid C₆₀ has been studied by a combination of high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives C₆₀O (epoxide) and C₆₁H₂ (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure C₆₀, i.e., a first-order orientational ordering transition just below room temperature followed by an orientational glass transition at lower temperatures. Although the …

Nickel Doping Of Boron Carbide Grown By Plasma Enhanced Chemical Vapor Deposition, Seong-Don Hwang, N.B. Remmes, Peter A. Dowben, D.N. Mcilroy

Peter Dowben Publications

We have nickel doped boron carbide grown by plasma enhanced chemical vapor deposition. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to grow the boron carbide, while nickelocene [Ni(C₅H₅)₂] was used to introduce nickel into the growing film. The doping of nickel transformed a p-type, B₅C material, relative to lightly doped n-type silicon, to an n-type material. Both n-n heterojunction diodes and n-p heterojunction diodes were constructed, using as substrates n- and p-type Si(111), respectively. With sufficient partial pressures of nickelocene in the plasma reactor, …

A Study Of The Numerical Implementation Of Multidisciplinary Design Optimization Strategies, Rodney A. Taylor

Mechanical & Aerospace Engineering Theses & Dissertations

Design optimization provides an organized, systematic, and efficient method for obtaining design improvements based upon a specified measure of design performance. In the multidisciplinary design optimization (MDO) environment efficient optimization techniques allow complex designs to be improved by managing conflicting disciplinary design goals so as to improve the global design. This thesis studies the implementation issues surrounding MDO techniques and strategies. To facilitate this investigation two testbed problems have been selected for study. These problems consider two approaches to the conceptual design of an aeroelastic wing. The first approach is based on an idealized, two-dimensional representation of an airfoil under …

Two-Dimensional Drift-Diffusion Simulations Of Silicon Avalanche Shaper (Sas) Devices For High Power Applications, Hamid Jalali

Electrical & Computer Engineering Theses & Dissertations

Silicon Avalanche Shaper devices have been projected as being important components of an inexpensive, semiconductor-based technology for high power switching applications. The primary advantage of this technology is that it is based on Silicon material which is easy to fabricate and has a well established processing technology. Unlike other high power technologies, the SAS devices do not rely on external optical triggering which eliminates the need for lasers and related optical circuitry.

The SAS based high power switching technology has been pioneered and tested by a Russian group. Though preliminary results have been very encouraging, the device reliability and its …

Light Front Qcd In (1+1)-Dimensions Coupled To Chiral Adjoint Fermions, David G. Robertson, Stephen S. Pinksy

Physics Faculty Scholarship

We consider SU(N) gauge theory in 1+1 dimensions coupled to chiral fermions in the adjoint representation of the gauge group. With all fields in the adjoint representation the gauge group is actually , which possesses nontrivial topology. In particular, there are N distinct topological sectors and the physical vacuum state has a structure analogous to a θ vacuum. We show how this feature is realized in light-front quantization for the case N = 2, using discretization as an infrared regulator. In the discretized form of the theory the nontrivial vacuum structure is associated with the zero momentum mode of the …

Evidence For Binary Orbital Motion Of A Quasar Broad-Line Region, C. Martin Gaskell

C. Martin Gaskell Publications

Analysis of spectra of the quasar 3C 390.3 covering a period of over 20 yr shows that the blueshifted peak of H beta has been changing its radial velocity at an almost constant rate during this time. The radial velocity has increased by over 1500 km/s. The lower limit to the period of radial velocity changes is 210 yr. Although very long periods cannot be excluded by the radial velocity curve alone, other considerations suggest that the period is about 300 yr. If the radial velocity changes are due to orbital motion, the radius of the orbit is about 0.3 …

Dynamics Of Conversion Of Conduction Electrons Into A Collective Charge-Density-Wave Current, Alexander S. Kovalev, Yuriy V. Pershin Dr, Alexander S. Rozhavsky

Faculty Publications

The exactly solvable model which describes the dynamics of transformation of conduction electrons into nonlinear charge-carrying excitations of charge-density waves in quasi-one-dimensional Peierls-Frohlich conductors is formulated and studied by the inverse scattering transformation method. The pair of self-trapped conduction electrons transform into a charged 2π kink localized in a single conducting chain and surrounded by dipoles in neighboring chains.

Structural And Magnetic Studies On Prba2fe3o8, V. P.S. Awana, Latika Menon, S. K. Malik, Apurva Mehta, Sanjay Mishra, William B. Yelon

Physics Faculty Research & Creative Works

It is known that Fe moments order antiferromagnetically with Néel temperature, TN, above 700 K in RBa2Fe3O7-δ with R=Y and Eu, and Pr moments order antiferromagnetically in PrBa2Cu3,O7-δ, with TN of 17 K. We have synthesized PrBa2Fe3O8 to study the ordering of Pr and Fe moments. From X-ray and neutron diffraction studies, this compound is found to crystallize in the tetragonal structure (space group P4/mmm). 57Fe Mössbauer studies show a complex hyperfine split pattern at 77 K, but a doublet superimposed on a six-line pattern at 300 K implying that some of the Fe moments order magnetically between 77 and …

Optical Properties Of Boron Carbide (B5C) Thin Films Fabricated By Plasma-Enhanced Chemical-Vapor Deposition, Ahmad A. Ahmad, N.J. Ianno, P.G. Snyder, D. Welipitiya, D. Byun, Peter A. Dowben

Peter Dowben Publications

Variable angle of incidence spectroscopic ellipsometry was used to determine the optical constants near the band edge of boron carbide (B₅C) thin films deposited on glass and n-type Si(111) via plasma-enhanced chemical-vapor deposition. The index of refraction n, the extinction coefficient k, and the absorption coefficient are reported in the photon energy spectrum between 1.24 and 4 eV. Ellipsometry analysis of B₅C films on silicon indicates a graded material, while the optical constants of B₅C on glass are homogeneous. Line shape analyses of absorption data for the films on glass indicate …

Static-Electric-Field Effects On High Harmonic Generation, Min-Qi Bao, Anthony F. Starace

Anthony F. Starace Publications

High harmonic generation in the presence of a static electric field is analyzed theoretically by an extension of the zero-range potential model treatment of Becker et al [Phys. Rev. A 50, 1540 (1994)]. For H- and a CO₂ laser of 5 × 10¹⁰ W/cm² intensity, a static electric field of 100 kV/cm is shown to produce a plateau from the 3rd to the 23rd harmonic in which neighboring even and odd harmonics are of comparable intensity. Furthermore, odd harmonics on the plateau are one or more orders of magnitude more intense than in the absence of the …

Charge Collection And Trapping In Low‐Temperature Silicon Detectors, M. J. Penn, Brian L. Dougherty, Blas Cabrera, R. M. Clarke, Betty A. Young

Physics

Charge collection efficiency measurements in silicon detectors at low temperature (T < 0.5 K) and low applied electric field (E=0.1–100 V/cm) were performed using a variety of high‐purity, p‐type silicon samples with room‐temperature resistivity in the range 2–40 kΩ cm. Good charge collection under these conditions of low temperature and low electric field is necessary for background suppression, through the simultaneous measurement of phonons and ionization, in a very low event rate dark matter search or neutrino physics experiment. Charge loss due to trapping during drift is present in some samples, but the data suggest that another charge–loss mechanism is also important. We present results which indicate that, for 60 keV energy depositions, a significant fraction of the total charge loss by trapping occurs in the initial electron‐hole cloud near the event location which may briefly act as a shielded, field‐free region. In addition, measurements of the lateral size, transverse to the applied electric field, of the initial electron‐hole cloud indicate large transverse diffusion lengths. At the lowest fields a lateral diameter on the order of 1 mm is found in a detector ∼5 mm thick.

Observation Of A Large Parity Nonconserving Analyzing Power In Xe, J J. Szymanski, W M. Snow, J D. Bowman, B Cain, Bret E. Crawford, P P J. Delheij, R D. Hartman, T Haseyama, C D. Keith, J N. Knudsen, A Komives, M Leuschner, L Y. Lowie, A Masaike, Y Matsuda, G E. Mitchell, S I. Penttila, H Postma, D Rich, N R. Roberson, S J. Seestrom, E I. Sharapov, Sharon L. Stephenson, Y-F Yen, V W. Yuan

Physics and Astronomy Faculty Publications

A large parity nonconserving longitudinal analyzing power was discovered in polarized-neutron transmission through Xe. An analyzing power of 4.3±0.2% was observed in a p-wave resonance at E_n=3.2 eV. The measurement was performed with a liquid Xe target of natural isotopic abundance that was placed in the polarized epithermal neutron beam, flight path 2, at the Manuel Lujan Neutron Science Center. This apparatus was constructed by the TRIPLE Collaboration, and has been used for studies of parity symmetry in compound nuclear resonances. Part of the motivation of the experiment was to discover a nucleus appropriate for a sensitive test …

Multicomponent Diffusion In Two-Temperature Magnetohydrodynamics, John D. Ramshaw, C. H. Chang

Physics Faculty Publications and Presentations

A recent hydrodynamic theory of multicomponent diffusion in multitemperature gas mixtures [J. D. Ramshaw, J. Non-Equilib. Thermodyn. 18, 121 (1993)] is generalized to include the velocity-dependent Lorentz force on charged species in a magnetic field B. This generalization is used to extend a previous treatment of ambipolar diffusion in two-temperature multicomponent plasmas [J. D. Ramshaw and C. H. Chang, Plasma Chem. Plasma Process. 13, 489 (1993)] to situations in which B and the electrical current density are nonzero. General expressions are thereby derived for the species diffusion fluxes, including thermal diffusion, in both single- and two-temperature multicomponent magnetohydrodynamics (MHD). It …

Thermally Stimulated Current Trap In Gan, David C. Look, Z-Q. Fang, W. Kim, Ö. Aktas, A. Botchkarev, A. Salvador, H. Morkoç

Physics Faculty Publications

A thermally stimulated current peak, occurring at 100 K for a heating rate of 0.4 K/s, has been found in semi‐insulating GaN grown by molecular beam epitaxy. This peak has contributions from two traps, with the main trap described by the following parameters: emission thermal activation energy E≂90±2 meV, effective capture cross‐section σ≂3±1×10⁻²² cm⁻², and Nμτ≂3±1 × 10¹⁴ cm⁻¹ V ⁻¹, where N is the trap concentration, μ the mobility, and τ the free‐carrier lifetime. This trap is much deeper than the typical shallow donors in conducting GaN, but shallower than any …

Electrical Characterization Of Gasb Based Semiconductors For 2-4 Micrometers Diode Laser Applications, Daniel K. Johnstone

Theses and Dissertations

Deep Level Transient Spectroscopy (DLTS) was used to characterize the band offsets and deep levels in MBE grown GaSb-based semiconductors that are used in 2-4 µm laser diode structures. One of several deep level traps found in Al_xGa_1-xAs_ySb_1-y (x=0, 0.5, 0.6, 1.0) is a GaSb double acceptor trap. Progress is also made in establishing the model for the DX center in this material. The degree of compensation of the donor related DX center by GaSb affect where donors are situated, giving preference to one configuration over the many other possible configurations. One minority …

Ionization And Charge Changing In 0.5 - 8 Qmev Li^Q+ + He (Q=1,2,3) Collisions And Setup Of An Electron Spectrometer Control System, Oliver Woitke

Dissertations

Projectile charge changing and target ionization cross sections are measured for 0.5 to 8 qMeV Li^q+ + He (q=1,2,3) collisions. Total cross sections for single-electron capture (for L i^1,2,3+) and single-electron loss (for L i^1,2+) are obtained and compared to existing experimental and theoretical data. Also, single and double target ionization of He associated with specific outgoing projectile charge states are identified using coincidence techniques. Cross sections for target ionization associated with no projectile charge change (direct ionization), single-electron capture, and single-electron loss are measured and compared to existing experimental data and to available calculations. Double-to-single target ionization ratios R …

Design Of Gradient Index Optical Thin Films, Jeffrey J. Druessel

Theses and Dissertations

Gradient index thin films provide greater flexibility for the design of optical coatings than the more conventional 'layer' films. In addition, gradient index films have higher damage thresholds and better adhesion properties. This dissertation presents an enhancement to the existing inverse Fourier transform gradient index design method, and develops a new optimal design method for gradient index films using a generalized Fourier series approach. The inverse Fourier transform method is modified to include use of the phase of the index profile as a variable in rugate filter design. Use of an optimal phase function in Fourier-based filter designs reduces the …

Design And Fabrication Of A Miniature Pressure Sensor Head Using Direct Bonded Ultra-Thin Silicon Wafers, Chad Eugene Statler

Theses

A miniature pressure sensor head is designed and fabricated using an ultra-thin silicon membrane directly bonded to an excimer laser micromachined substrate. The pressure sensor head has its intended implementation as part of an optically interrogated device with sensitivity to pressures ranging from 0.5 to 4.0 MPa. The pressure range design is shown to be easily adjusted by tailoring the thickness of the membrane wafer. The fabrication process features numerous advantages over existing pressure sensor construction technology including a maskless procedure and no chemical etching or mechanical thinning necessary to form the membrane after bonding. An optic lever is constructed …

Degradation And Breakdown In Ultra-Thin Silicon Oxides, Christopher Franck

Theses

Emerging trends in the semiconductor device industry call for detailed knowledge of the properties of devices whose dimensions are small enough to exploit Quantum Mechanical effects. This thesis presents a complete picture of oxide degradation in MOS direct tunnel diodes (t0 <3.4 nm). It is demonstrated that for structures fabricated at different facilities and stressed with either gate or substrate injection, a universal degradation mode is revealed which is manifested as the build up of positive charge in the oxide. The data gathered demonstrates that the positive charging phenomena in sub-3.5 nm oxides is independent of oxide thickness, and is characterized by a voltage threshold and two-regime temperature dependence. Further, the catastrophic failure, or breakdown, of these oxides was studied and the strongest evidence to date is presented which links the positive charging phenomena to the oxide breakdown. This thesis concludes with the presentation of a novel device design which can exploit the properties of degradation and breakdown, in thin oxides, to achieve an EEPROM memory cell of superior endurance.

Supermassive Binaries And Extragalactic Jets, C. Martin Gaskell

Department of Physics and Astronomy: Faculty Publications

Some quasars show Doppler shifted broad emission line peaks. I give new statistics of the occurrence of these peaks and show that, while the most spectacular cases are in quasars with strong jets inclined to the line of sight, they are also almost as common in radio-quiet quasars. Theories of the origin of the peaks are reviewed and it is argued that the displaced peaks are most likely produced by supermassive binaries. 3C 390.3 shows precisely the change in radial velocity predicted by the supermassive binary model. The separations of the peaks in 3C 390.3-type objects are consistent with orientation-dependent …

The Flat Phase Of Crystalline Membranes, Mark Bowick, Simon Catterall, Marco Falcioni, Gudmar Thorleifsson, Konstantinos N. Anagnostopoulos

Physics - All Scholarship

We present the results of a high-statistics Monte Carlo simulation of a phantom crystalline (fixed-connectivity) membrane with free boundary. We verify the existence of a flat phase by examining lattices of size up to $128^2$. The Hamiltonian of the model is the sum of a simple spring pair potential, with no hard-core repulsion, and bending energy. The only free parameter is the the bending rigidity $\kappa$. In-plane elastic constants are not explicitly introduced. We obtain the remarkable result that this simple model dynamically generates the elastic constants required to stabilise the flat phase. We present measurements of the size (Flory) …

The Flat Phase Of Crystalline Membranes, Simon Catterall, Mark Bowick, Konstantinos N. Anagnostopoulos

Physics - All Scholarship

We present the results of a high-statistics Monte Carlo simulation of a phantom crystalline (fixed-connectivity) membrane with free boundary. We verify the existence of a flat phase by examining lattices of size up to 128^2. The Hamiltonian of the model is the sum of a simple spring pair potential, with no hard-core repulsion, and bending energy. The only free parameter is the the bending rigidity \kappa. In-plane elastic constants are not explicitly introduced. We obtain the remarkable result that this simple model dynamically generates the elastic constants required to stabilise the flat phase. We present measurements of the size (Flory) …

The Structure Of Alkali Halide Dimers: A Critical Test Of Ionic Models And New Ab Initio Results, T. Törring, S. Biermann, J. Hoeft, Richard J. Mawhorter, Robert J. Cave, C. Szemenyei

All HMC Faculty Publications and Research

In semiempirical ionic models a number of adjustable parameters have to be fitted to experimental data of either monomer molecules or crystals. This leads to strong correlations between these constants and prevents a unique test and a clear physical interpretation of the fit parameters. Moreover, it is not clear whether these constants remain unchanged when the model is applied to dimers or larger clusters. It is shown that these correlations can be substantially reduced when reliable information about dimers is available from experiments or ab initio calculations. Starting with Dunham coefficients of the monomer potential determined from microwave measurements, we …

A Classical Analysis Of Proton Collisions With Ground-State And Excited, Aligned Sodium Targets, C. J. Lundy, Ronald E. Olson

Physics Faculty Research & Creative Works

The dependence of electron capture and ionization processes on the pre-collision electronic distribution is examined for the systems H+ + Na(3s, 3pΣ, 3pΠ) primarily in the energy range 1-15 keV. Populations of the capture and ionization channels as well as the coherence of the post-collision electronic distribution of the capture product are formulated using the classical trajectory Monte Carlo (CTMC) method. Total capture and ionization cross sections arising from initially aligned Na(3p) targets exhibit transient behaviour near the velocity-matching energy of 5.7 keV. This observation is described in terms of anisotropy parameters; anisotropy effects are also analysed for the capture …

State-Selective Electron Capture In Low Velocity Multiply Charged Ion, Helium Collisions, A. Cassimi, S. Duponchel, X. Flechard, P. Jardin, P. Sortais, D. Hennecart, Ronald E. Olson

Physics Faculty Research & Creative Works

Recoil ion momentum spectroscopy has been applied to study low energy, state-selective single electron capture by highly charged projectiles. The specific systems investigated are fully stripped 6.82 keV/u Ne10+ 6.82 keV/u Ar18+ on He. Measurements of the He+ recoil ion longitudinal momenta are used to determine the final n-state dependence of the captured electron. Simultaneous measurement of the transverse momenta of the recoil ion yields information about the impact parameter dependence of the reaction and the reaction window for electron capture. © 1996 The American Physical Society.

Vortex Dynamics And Defects In Simulated Flux Flow, Alan Middleton, Michael Chance Faleski, M. C. Marchetti

Physics - All Scholarship

We present the results of molecular dynamic simulations of a two-dimensional vortex array driven by a uniform current through random pinning centers at zero temperature. We identify two types of flow of the driven array near the depinning threshold. For weak disorder the flux array contains few dislocation and moves via correlated displacements of patches of vortices in a {\it crinkle} motion. As the disorder strength increases, we observe a crossover to a spatially inhomogeneous regime of {\it plastic} flow, with a very defective vortex array and a channel-like structure of the flowing regions. The two regimes are characterized by …

Distribution Of Ionized And Neutral Halogenated Phenols In An Octanol-Water Membrane Model System, Robert Word

Dissertations and Theses

The accumulation of xenobiotics, such as halogenated phenols, in soils, sediments, and living organisms is primarily a partitioning process between an aquatic and organic phase. It is traditional to use a bulk octanol-water system to model the partitioning of a compound between complex biological lipid membranes and aqueous media. The octanol-water partition coefficient K_OW successfully approximates the lipid membrane-water partition coefficient K_LipW of neutral compounds. However, the ionized species of substituted phenols were shown to have a higher affinity for lipid membranes than predicted from octanol-water results.

Data on the octanol-water partition coefficients of molecular ions is limited. …