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Physics Faculty Publications

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Articles 1801 - 1830 of 1942

Full-Text Articles in Physical Sciences and Mathematics

Infrared-Absorption Of Deep Defects In Molecular-Beam-Epitaxial Gaas-Layers Grown At 200°C - Observation Of An El(2)-Like Defect, M. O. Manasreh, David C. Look, K. R. Evans May 1990

Infrared-Absorption Of Deep Defects In Molecular-Beam-Epitaxial Gaas-Layers Grown At 200°C - Observation Of An El(2)-Like Defect, M. O. Manasreh, David C. Look, K. R. Evans

Physics Faculty Publications

Infrared optical absorption and Hall-effect techniques were employed to study deep defects in As-rich molecular-beam-epitaxial GaAs layers grown at very low temperature (200°C). A large ir absorption band was observed between 0.55 eV and the band edge. This band is composed of photoquenchable and photounquenchable components. Photoquenching, thermal recovery from the metastable state, and ir absorption properties of the quenchable defect, of estimated concentration ~3x1018 cm-3, are identical to those of EL2. On the other hand, the unquenchable defect, of estimated concentration ~3x1019 cm-3, resembles the isolated AsGa, antisite observed in …

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Polymerization On The Diamond Hierarchical Lattice: The Migdal-Kadanoff Renormalization-Group Scheme, Miron Kaufman, J. E. Touma Apr 1990

Polymerization On The Diamond Hierarchical Lattice: The Migdal-Kadanoff Renormalization-Group Scheme, Miron Kaufman, J. E. Touma

Physics Faculty Publications

The thermodynamics of the equilibrium polymerization model (grand-canonical ensemble of self-avoiding walks) in two dimensions is worked out by means of the Migdal-Kadanoff renormalization-group technique. This method involves renormalization-group flows in an eight-dimensional parameter space. At the critical point the number of relevant fields (positive exponents) is four. The leading exponent value differs by less than 1% from the (presumed) exact value. The results are exact for the polymerization problem defined on the diamond hierarchical lattice. Some results are peculiar to this lattice and are not expected to hold for Bravais lattices. For instance, the polymerized phase (infinite polymerization index) …

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Surface And Interface Free-Carrier Depletion In Gaas Molecular Beam Epitaxial Layers: Demonstration Of High Interface Charge, David C. Look, C. E. Stutz, K. R. Evans Feb 1990

Surface And Interface Free-Carrier Depletion In Gaas Molecular Beam Epitaxial Layers: Demonstration Of High Interface Charge, David C. Look, C. E. Stutz, K. R. Evans

Physics Faculty Publications

Molecular beam epitaxial GaAs layers of electron concentration 1.69×1017 cm−3, and various thicknesses d=0.25, 0.50, 1.00, and 2.00 μm, have been grown on semi‐insulating GaAs substrates and characterized by the Hall effect and capacitance‐voltage (C‐V) techniques. A plot of sheet Hall concentration ns vs d gives accurate values of (ND−NA) and (ws+wi), the sum of the surface and interface free‐carrier depletion widths, respectively. The C‐V measurements verify the value of ND−NA …

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Surface And Interface Free-Carrier Depletion In Gaas Molecular-Beam Epitaxial Layers: Demonstration Of High Interface Charge, David C. Look, C. E. Stutz, K. R. Evans Feb 1990

Surface And Interface Free-Carrier Depletion In Gaas Molecular-Beam Epitaxial Layers: Demonstration Of High Interface Charge, David C. Look, C. E. Stutz, K. R. Evans

Physics Faculty Publications

Molecular beam epitaxial GaAs layers of electron concentration 1.69×1017 cm−3, and various thicknesses d=0.25, 0.50, 1.00, and 2.00 μm, have been grown on semi‐insulating GaAs substrates and characterized by the Hall effect and capacitance‐voltage (C‐V) techniques. A plot of sheet Hall concentration ns vs d gives accurate values of (ND−NA) and (ws+wi), the sum of the surface and interface free‐carrier depletion widths, respectively. The C‐V measurements verify the value of ND−NA …

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Mirrorless Optical Bistability In A Nonlinear Absorbing Dielectric Film, Kevin L. Stokes, Ashok Puri Jan 1990

Mirrorless Optical Bistability In A Nonlinear Absorbing Dielectric Film, Kevin L. Stokes, Ashok Puri

Physics Faculty Publications

The optical transmissivity of a mirrorless, nonlinear, absorbing dielectric thin film is investigated numerically. The dielectric function in the film region is dependent on the intensity of the electromagnetic field. Multivalued solutions of transmissivity as a function of incident power are calculated for the steady-state wave equation. The numerical solution is applied to two different model dielectric functions. As the absorption parameter is increased, larger values of incident intensity are required to switch the systems between stable output states. Also, the peak values of transmissivity are reduced as the absorption is increased.

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Review Of Hall Effect And Magnetoresistance Measurements In Gaas Materials And Devices, David C. Look Jan 1990

Review Of Hall Effect And Magnetoresistance Measurements In Gaas Materials And Devices, David C. Look

Physics Faculty Publications

The use of magnetic fields in the electrical characterization of semiconductor materials is familiar to everyone in the form of Hall‐effect measurements. However, there is another magnetic‐field‐based phenomenon, magnetoresistance (MR), which is highly useful but not nearly so familiar to the majority of workers. One of the unique features of MR measurements is their applicability to common device structures, in particular, field‐effect transistors (FETs) and contact‐resistance patterns. We will show how channel mobility information can be extracted from the MR data in metal‐semiconductor FETs (MESFETs) and modulation‐doped heterostructure FETs (MODFETs), and also how the material under ohmic contacts can be …

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Transition Probabilities And Franck-Condon Factors For The Second Negative Band System Of O2+, Jane L. Fox, Alexander Dalgarno Jan 1990

Transition Probabilities And Franck-Condon Factors For The Second Negative Band System Of O2+, Jane L. Fox, Alexander Dalgarno

Physics Faculty Publications

Transition probabilities for the second negative band system of O2+ are computed using the dipole transition moment presented by Wetmore et al. [1984]. Vibrational levels υ″ = 0 − 54 of the X²Πg ground state and υ′ = 0 − 33 of the excited A²Πu state are included. Franck-Condon factors for ionization-excitation of O2 (X³Σg−; υ = 0 − 25) to O2+(A²Πu;υ′ = 0 − 33) are also presented.

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A Signature Of Auroral Precipitation In The Nightside Ionosphere Of Venus, Jane L. Fox, H. A. Taylor Jr. Jan 1990

A Signature Of Auroral Precipitation In The Nightside Ionosphere Of Venus, Jane L. Fox, H. A. Taylor Jr.

Physics Faculty Publications

We show here that the densities of mass-28 ions measured by the Pioneer Venus Orbiter ion mass spectrometer (OIMS) on the nightside of Venus are highly variable and show little correlation with the values of the O+ densities. We have determined the total production rates of mass-28 ions in the chemical equilibrium region and find that this production rate cannot be explained by known chemical production reactions. We propose that the “excess” production is due to precipitation of electrons into the nightside thermosphere.

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Theoretical Intensity-Dependent Response Of Nonlinear Periodic Structures, Paul A. Gohman, Gust Bambakidis, Robert J. Spry Jan 1990

Theoretical Intensity-Dependent Response Of Nonlinear Periodic Structures, Paul A. Gohman, Gust Bambakidis, Robert J. Spry

Physics Faculty Publications

We have modeled the response of a nonlinear periodic structure by means of the Abelés 2×2 matrix method. Our structure differs from the usual rejection‐band filter designs, in that we have chosen the filter elements to be index matched in the absence of radiation, providing a rejection band that both grows and shifts as a function of incident intensity. The intensity output function of the model not only directly demonstrates optical bistability, but also limiting, switching, self‐pulsing, and chaos.

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In Situ Hall-Effect System For Real-Time Electron-Irradiation Studies, B. S. Ziebro, David C. Look, Joseph W. Hemsky, J. Rice Jan 1990

In Situ Hall-Effect System For Real-Time Electron-Irradiation Studies, B. S. Ziebro, David C. Look, Joseph W. Hemsky, J. Rice

Physics Faculty Publications

A unique system capable of taking in situ Hall‐effect measurements during electron irradiation has been developed. The key element is a small, powerful rare‐earth magnet. Measurements can be taken while the electron beam is on, resulting in a considerable time savings and eliminating problems associated with mounting and demounting the sample. High resolution electron concentration and mobility versus fluence data are quickly and easily obtained, making possible detailed defect production rate studies as functions of energy and flux.

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Dielectric Response Spectrum Of A Damped One- Dimensional Double-Well Oscillator, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer Dec 1989

Dielectric Response Spectrum Of A Damped One- Dimensional Double-Well Oscillator, John W. Flocken, R. A. Guenther, John R. Hardy, L. L. Boyer

Physics Faculty Publications

The characteristic features of the dielectric response spectrum associated with the condensation of a "soft" phonon mode during a ferroelectric phase transition can be reproduced by an oscillator moving in a damped linear double-well potential. The behavior of the response function below the transition temperature T, can be simulated by introducing a linear mean-field coupling which destroys the symmetry of the well. Potentials derived from physically realistic parameters are shown to result in dielectric responses which are in order-of-magnitude agreement with experimental values.

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Observability Of Atmospheric Glories And Supernumerary Rainbows, James A. Lock Dec 1989

Observability Of Atmospheric Glories And Supernumerary Rainbows, James A. Lock

Physics Faculty Publications

The finite spatial coherence width of sunlight at the Earth imposes restrictions on the production of scattering phenomena based on the interference of light waves. With the spatial coherence properties of sunlight taken into account, the visibility of the supernumerary rainbow sequence adjacent to the primary rainbow and the radii of the water droplets that produce the optimum glory intensity were calculated. A substantial reduction was found in the contrast of all the supernumeraries beyond the first few, and the peak observability of the glory occurred for water droplets with radii between 10 and 20 µm.

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Deviations From Bulk Transport Measurements In Semi-Insulating Gaas, N. C. Halder, David C. Look Nov 1989

Deviations From Bulk Transport Measurements In Semi-Insulating Gaas, N. C. Halder, David C. Look

Physics Faculty Publications

Because of the high bulk resistivity of semi‐insulating GaAs, surface or near‐surface effects can change the apparent magnitudes of resistivity, mobility, and carrier concentration. We consider the following causes: (1) above‐surface conduction, such as that due to impurities in a porous oxide; (2) subsurface conduction, due to sawing and polishing damage; (3) tunneling conduction in surface states; and (4) changes in near‐surface conduction due to the modification of surface potential by surface states or absorbates. The most important of these effects appear to be subsurface damage and surface potential changes.

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Hall-Effect Depletion Corrections In Ion-Implanted Samples: Si29 In Gaas, David C. Look Sep 1989

Hall-Effect Depletion Corrections In Ion-Implanted Samples: Si29 In Gaas, David C. Look

Physics Faculty Publications

The sheet free‐carrier concentration in a thin, conducting layer on an insulating substrate is lower than the net, sheet‐dopant concentration because of free‐carrier depletion in the surface and interface regions. Here we develop an algorithm to give the true, net sheet‐donor concentration from the measured sheet‐Hall concentration under the assumption of a Gaussian donor profile, which is usually sufficiently accurate for ion‐implanted samples. Correction curves are generated for Si29 ions implanted into GaAs at energies of 60, 100, 130, 150, and 200 keV, and at doses of 1×1011–2×1014 ions/cm2. Also, the …

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Silicon Crystallite Formation In Ion-Implanted Quartz, U. B. Ramabadran, H. E. Jackson, Gary C. Farlow Sep 1989

Silicon Crystallite Formation In Ion-Implanted Quartz, U. B. Ramabadran, H. E. Jackson, Gary C. Farlow

Physics Faculty Publications

Rapid thermally annealed silicon‐implanted x‐cut α‐quartz samples have been examined by Rutherford backscattering and Raman microprobe spectroscopy. The data indicate that the silicon has diffused at 1200 °C to form a buried layer of crystallites of size 1–10 μm. The crystallites are preferentially oriented and under substantial stress.

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Uniformity Of 3-In, Semi-Insulating, Vertical-Gradient-Freeze Gaas Wafers, David C. Look, D. C. Walters, M. G. Mier, J. S. Sewell, J. S. Sizelove, A. Akselrad, J. E. Clemans Jul 1989

Uniformity Of 3-In, Semi-Insulating, Vertical-Gradient-Freeze Gaas Wafers, David C. Look, D. C. Walters, M. G. Mier, J. S. Sewell, J. S. Sizelove, A. Akselrad, J. E. Clemans

Physics Faculty Publications

We have evaluated the uniformity in [EL2], dislocation (or etch‐pit) density (EPD), resistivity, mobility, and carrier concentration for 3‐in., semi‐insulating GaAs wafers grown by the vertical‐gradient‐freeze (VGF) technique. Although slight W or U patterns were observed in [EL2] and EPD along the 〈110〉 directions, for the first time, nevertheless the overall uniformity was excellent, and comparable to that in the best In‐doped and whole‐boule‐annealed ingots grown by the liquid‐encapsulated Czochralski (LEC) technique. Based on results from implant‐activation studies on LEC wafers, it is estimated that the measured nonuniformities in EPD and [EL2] for the VGF wafers would contribute only about …

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An Improved Correction Algorithm For Number Density Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac Jun 1989

An Improved Correction Algorithm For Number Density Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

A correction factor to the number density measured by the Forward Scattering Spectrometer Probe (FSSP) which compensates for dead time and coincidence errors was determined by calculating the probabilities of and the average number of particles in the six possible types of dead time and coincidence events. These probabilities and averages were calculated by means of a probabilistic model based on Poisson statistics. A Monte Carlo computer simulation of the FSSP operation was also carried out and the number density correction factor was compared with the Monte Carlo data. For an actual number density of 2000/cm3, it was …

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A Correction Algorithm For Particle-Size Distribution Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac Jun 1989

A Correction Algorithm For Particle-Size Distribution Measurements Made With The Forward-Scattering Spectrometer Probe, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

Multiparticle coincidence events in the scattering volume of the forward-scattering spectrometer probe (FSSP) cause the instrument to bias the measurement of the particle size distribution of atmospheric aerosols toward large diameters. We employ a probabilistic model based on Poisson statistics to determine the average diameter and rms width of the actual size distribution as functions of the average diameter and rms width of the measured distribution. We compare our predictions to a Monte Carlo simulation of the FSSP operation

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Ferroelectricity In Perovskitelike Nacaf3 Predicted Ab Initio, P. J. Edwardson, L. L. Boyer, R. L. Newman, D. H. Fox, John R. Hardy, John R. Flocken, R. A. Guenther, Wai-Ning Mei May 1989

Ferroelectricity In Perovskitelike Nacaf3 Predicted Ab Initio, P. J. Edwardson, L. L. Boyer, R. L. Newman, D. H. Fox, John R. Hardy, John R. Flocken, R. A. Guenther, Wai-Ning Mei

Physics Faculty Publications

The ability of zero-stress simulations, using Gordon-Kim pair potentials, to describe the structures and transformations of known fluoride-based perovskites is demonstrated for the case of KCaF3. When K+ is replaced by Na+ a new ferroelectric crystal isomorphous with LiNbO3 is predicted. The equivalent relationships of the ferroelectric lithium niobate structure with the perovskite and antiperovskite structures are examined. A polarization of 21 jµC/cm2 at room temperature and a transition temperature of 550 K are predicted for NaCaF3. Surface effects are examined in simulations of a 1080-ion cluster.

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An Alternative Approach To The Teaching Of Rotational Dynamics, James A. Lock May 1989

An Alternative Approach To The Teaching Of Rotational Dynamics, James A. Lock

Physics Faculty Publications

An alternative approach to the stability of torque-free rotations and to top nutation that emphasizes conservation laws and physical reasoning is examined. It is felt that such an approach, which is largely free of sophisticated mathematics, would be useful in teaching these topics in rotational motion at the introductory physics level.

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Determination Of Shallow Minority-Acceptor Concentration In Multiply Doped Silicon, Gust Bambakidis, G. J. Brown Apr 1989

Determination Of Shallow Minority-Acceptor Concentration In Multiply Doped Silicon, Gust Bambakidis, G. J. Brown

Physics Faculty Publications

A method is presented, based on photothermal ionization spectroscopy (PTIS), for determining the shallow minority‐acceptor concentration in multiply doped silicon, over the concentration range 1013/cm3–1015/cm3. The method is an extension of a model developed previously for the PTIS response in a multiply doped semiconductor, which accounts for the experimentally observed change in signature, from negative to positive, of the lower‐energy lines of the deeper acceptor as the temperature is increased. It uses a calculated curve of the dependence on the shallow‐acceptor concentration of the temperature at which the change in …

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Photodissociation Of Co In The Thermosphere Of Venus, Jane L. Fox Apr 1989

Photodissociation Of Co In The Thermosphere Of Venus, Jane L. Fox

Physics Faculty Publications

Recent investigations of CO photoabsorption demonstrate that photodissociation longward of the ionization threshold at 88.5 nm occurs primarily through line absorptions rather than continuous processes. We have constructed high resolution photoabsorption cross sections for CO at rotational temperatures near 250 K from the improved data on dissociating transitions. We examine the effects of the new cross sections on the rate of solar photodissociation of CO in the thermosphere of Venus and compare the results to values obtained with the lower resolution cross sections available previously. We find that the photodissociation profile peaks slightly higher in the atmosphere and the peak …

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Equilibrium Polymerization On The Equivalent-Neighbor Lattice, Miron Kaufman Apr 1989

Equilibrium Polymerization On The Equivalent-Neighbor Lattice, Miron Kaufman

Physics Faculty Publications

The equilibrium polymerization problem is solved exactly on the equivalent-neighbor lattice. The Flory-Huggins entropy of mixing is exact for this lattice. We verify the discrete version of the n-vector model when n→0 is equivalent to the equal reactivity polymerization process in the whole parameter space, including the polymerized phase. The polymerization processes for polymers satisfying the Schulz distribution exhibit nonuniversal critical behavior. A close analogy is found between the polymerization problem of index r and the Bose-Einstein ideal gas in d=-2r dimensions, with the critical polymerization corresponding to the Bose-Einstein condensation.

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An Analysis Of Two Unusual Reflection Caustics, James A. Lock, Judith R. Woodruff Mar 1989

An Analysis Of Two Unusual Reflection Caustics, James A. Lock, Judith R. Woodruff

Physics Faculty Publications

A reflection caustic consisting of an x or a cross surrounded by a circle was observed using two different reflector geometries. Double-glazed insulating windows that have become bowed, and circularly symmetric cookie trays and cake covers possessing two different curvatures at different distances from the symmetry axis, produce similar caustics when plane waves are incident far off-axis. For the case of the insulating windows, it was found that the concavely bowed outer pane produced the x portion of the caustic and the convexly bowed inner pane produced the circle portion. For the case of the circularly symmetric reflectors, the x …

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A New Technique For Whole-Wafer Etch-Pit Density Mapping In Gaas, David C. Look, D. C. Walters, J. S. Sewell, S. C. Dudley, M. G. Mier, J. S. Sizelove Feb 1989

A New Technique For Whole-Wafer Etch-Pit Density Mapping In Gaas, David C. Look, D. C. Walters, J. S. Sewell, S. C. Dudley, M. G. Mier, J. S. Sizelove

Physics Faculty Publications

The pits formed on an etched GaAs surface, due to the anisotropic etching around dislocations, are efficient light scatterers, and thus reduce transmission. We have derived a quantitative relationship between the fractional transmission and the etch‐pit density (EPD) and have shown that the same absorption apparatus which is commonly used to obtain a whole‐wafer [EL2] map can also be used to generate an EPD map. The technique is verified by comparing the fractional transmission with the actual EPD count at 166 points on a three‐inch, low‐pressure, liquid‐encapsulated Czochralski wafer. Also, [EL2] and EPD maps, with more than 3500 points each, …

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Non-Debye Enhancements In The Mie Scattering Of Light From A Single Water Droplet, James A. Lock, Judith R. Woodruff Feb 1989

Non-Debye Enhancements In The Mie Scattering Of Light From A Single Water Droplet, James A. Lock, Judith R. Woodruff

Physics Faculty Publications

The glare spots usually seen on a single water droplet which has been illuminated by a plane wave are produced by geometrical rays which correspond to the different terms of the Debye series expansion of the Mie scattered field. Recently other glare spot enhancements have been predicted which correspond to scattering resonances coupling to the orbiting rays associated with high-order geometrical rainbows. We observed the non-Debye enhancement of the eleventh-order rainbow glare spot at an observation angle of 90° on a 3.5-mm water droplet illuminated by polarized He-Ne laser light.

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On The Size Distribution Of Newly Formed Grains In Red Supergiant Atmospheres, C Gregory Seab, Theodore P. Snow Jan 1989

On The Size Distribution Of Newly Formed Grains In Red Supergiant Atmospheres, C Gregory Seab, Theodore P. Snow

Physics Faculty Publications

Theoretical ultraviolet extinction curves have been calculated for comparison with observed curves for circumstellar dust in M supergiants. The theoretical curves assumed a silicate grain composition, because silicate grains are expected in the oxygen-rich environments that are observed. Calculations were performed with and without the inclusion of scattering into the beam, with largely similar results. A comparison of the computed curves with the observed ultraviolet extinction curve for circumstellar dust in Scorpii indicates that the size distribution of the circumstellar grains must cut off near 800 Å that is, there are few or no grains smaller than this. Our conclusion …

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Predicting Peculiar Interstellar Extinction From Gaseous Abundances, Charles L. Joseph, Theodore P. Snow, C Gregory Seab Jan 1989

Predicting Peculiar Interstellar Extinction From Gaseous Abundances, Charles L. Joseph, Theodore P. Snow, C Gregory Seab

Physics Faculty Publications

Molecular and atomic abundances are examined for 19 lines of sight through dense clouds, each with a peculiar selective extinction curve. The interstellar clouds in the present study appear to fall into two distinct categories: CN-rich, with relatively small amounts of neutral iron, or CN-poor, with large amounts of neutral iron. Lines of sight, having a CN/(Fe i) abundance ratio about two (~ 0.3 dex) or greater, are found to have a shallow (2.57 ± 0.55 mag) 2175 Å feature relative to the underlying extinction, while the strength of the bump is 3.60 ± 0.36 for the other dense clouds …

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A Search For Interstellar And Circumstellar C60, T P. Sno, C G. Seab Jan 1989

A Search For Interstellar And Circumstellar C60, T P. Sno, C G. Seab

Physics Faculty Publications

It has recently been suggested that the diffuse interstellar bands may be formed by ionized polyhedral carbon molecules such as C60(+). While specific laboratory measurements of absorption bands of this molecular ion have not been made, a feature due to the neutral molecule C60 has been discovered at 3860A. Examination of spectra of several reddened stars, as well as one star known to have circumstellar carbonaceous dust, shows no sign of the feature, leading to upper limits of the order of 10 to the 14th/sq cm for the column density of C60. These limits are not yet sensitive enough to …

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Cooperative Effects Among Partial Waves In Mie Scattering, James A. Lock Dec 1988

Cooperative Effects Among Partial Waves In Mie Scattering, James A. Lock

Physics Faculty Publications

Observations of an illuminated water droplet at a close distance are described mathematically by the Fourier transform of the Mie-scattering amplitude convolved with the aperture function of the observer’s eye. Most of the sharp enhancements found in the Fourier transform correspond to geometrical rays associated with the various terms in the Debye-series expansion of the Mie amplitude. However, there are some enhancements that cannot be ascribed to any individual Debye-series term. Instead, they arise from a constructive interference cooperation of the phase of a scattering resonance in a single partial wave with the region of the stationary phase corresponding to …

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