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Physics Faculty Publications

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Articles 1741 - 1770 of 1942

Full-Text Articles in Physical Sciences and Mathematics

Interpretation Of Electron Correlation In Local-Density Approximation For Exchange, Viraht Sahni, Marlina Slamet Jul 1993

Interpretation Of Electron Correlation In Local-Density Approximation For Exchange, Viraht Sahni, Marlina Slamet

Physics Faculty Publications

We provide a new interpretation of how electron correlations are represented within the local-density approximation for exchange by deducing via Harbola-Sahni theory the corresponding pair-correlation density. The expression differs from that in the literature and contains, in addition to the uniform electron-gas result, a term proportional to the gradient of the density. Thus, the nonuniformity of the electron density is explicitly incorporated in the approximation, and constitutes thereby the reason underlying its success.

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Upper Limits To The Nightside Ionosphere Of Mars, Jane L. Fox Jul 1993

Upper Limits To The Nightside Ionosphere Of Mars, Jane L. Fox

Physics Faculty Publications

The nightside ionosphere of Mars could be produced by electron precipitation or by plasma transport from the dayside, by analogy to the Venus, but few measurements are available. We report here model calculations of upper limits to the nightside ion densities on Mars that would be produced by both mechanisms. For the auroral model, we have adopted the downward traveling portions of the electron spectra measured by the HARP instrument on the Soviet Phobos spacecraft in the Martian plasma sheet and in the magnetotail lobes. For the plasma transport case, we have imposed on a model of the nightside thermosphere, …

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High-Voltage Mosfet Bipolar Square-Wave Generator, T. D. Usher, Grant A. Mcauley Jul 1993

High-Voltage Mosfet Bipolar Square-Wave Generator, T. D. Usher, Grant A. Mcauley

Physics Faculty Publications

he construction of a high‐voltage (up to 1000 V) bipolar metal‐oxide‐semiconductor field‐effect transistor square‐wave generator is described. This generator is capable of producing both positive and negative going square waves with variable amplitude, repetition rate, and width. The circuit was designed for ferroelectrics research, however other applications are possible. The rise time of the prototype was 200 ns which was quite satisfactory for the present ferroelectric research project however the rise time can be decreased to 50 ns if necessary. The reader with a modest knowledge of electronics should be able to construct the circuit. Possible pitfalls and critical points …

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Prominent Thermally Stimulated Current Trap In Low-Temperature-Grown Molecular-Beam Epitaxial Gaas, Z-Q. Fang, David C. Look Jul 1993

Prominent Thermally Stimulated Current Trap In Low-Temperature-Grown Molecular-Beam Epitaxial Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

By far, the largest thermally stimulated current trap in molecular beam epitaxial GaAs grown at 200–250 °C is T5, with an activation energy of 0.27 eV and most likely related to VGa. After an anneal at 300–350 °C, another trap T6☒ appears, with an activation energy of 0.14 eV and closely identified with VAs or the complex, VAs‐AsGa. Proposed defect reactions in this As‐rich material include VGa+AsAs→VAs‐AsGa, and VGa+AsGa→VGa‐AsGa.

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Annealing Dynamics Of Molecular-Beam Epitaxial Gaas Grown At 200°C, David C. Look, D. C. Walters, G. D. Robinson, J. R. Sizelove, M. G. Mier, C. E. Stutz Jul 1993

Annealing Dynamics Of Molecular-Beam Epitaxial Gaas Grown At 200°C, David C. Look, D. C. Walters, G. D. Robinson, J. R. Sizelove, M. G. Mier, C. E. Stutz

Physics Faculty Publications

By separating a 2‐μm‐thick molecular‐beam‐epitaxial GaAs layer grown at 200 °C from its 650‐μm‐thick substrate, we have been able to obtain accurate Hall‐effect and conductivity data as functions of annealing temperature from 300 to 600 °C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. …

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Analytical Two-Layer Hall Analysis - Application To Modulation-Doped Field-Effect Transistors, David C. Look, C. E. Stutz, Christopher A. Bozada Jul 1993

Analytical Two-Layer Hall Analysis - Application To Modulation-Doped Field-Effect Transistors, David C. Look, C. E. Stutz, Christopher A. Bozada

Physics Faculty Publications

The classical magnetic‐field‐dependent Hall coefficient and conductivity equations are inverted to give the mobilities μ1 and μ2 and carrier concentrations n1 (or p1) and n2 (or p2) in two degenerate bands. The two‐band solution holds for arbitrary magnetic‐field strength as long as quantum effects can be ignored (i.e., kT≳ℏeB/m∗), and it is argued that the analysis can also be applied to two separate layers up to reasonable field strengths. The results are used to determine the two‐dimensional electron gas mobility and carrier concentration in a modulation‐doped field‐effect …

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Donor And Acceptor Concentrations In Molecular-Beam Epitaxial Gaas Grown At 300-Degrees-C And 400-Degrees-C, David C. Look, G. D. Robinson, J. R. Sizelove, C. E. Stutz Jun 1993

Donor And Acceptor Concentrations In Molecular-Beam Epitaxial Gaas Grown At 300-Degrees-C And 400-Degrees-C, David C. Look, G. D. Robinson, J. R. Sizelove, C. E. Stutz

Physics Faculty Publications

The first Hall‐effect measurements on molecular beam epitaxial GaAs layers grown at the low temperatures of 300 and 400 °C are reported. Two independent methods were used to determine donor ND and acceptor NA concentrations and activation energy ED0, with the following combined results: ND≂3±1×1018, NA≂1.5±1×1017 cm−3, and ED0=0.645±0.009 eV for the 300 °C layer; ND≂2±1×1017, NA≂7±3×1016 cm−3, and ED0=0.648±0.003 eV for the 400 °C layer. Thus, the deep donor is …

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Thermally Activated Escape Over Fluctuating Barriers, Ulrich Zürcher, Charles R. Doering Jun 1993

Thermally Activated Escape Over Fluctuating Barriers, Ulrich Zürcher, Charles R. Doering

Physics Faculty Publications

We investigate the thermally activated escape of a Brownian particle over a potential barrier whose height fluctuates with a rate α between the values E+ and E−. We are mainly interested in the low-temperature behavior where E+/T≫E−/T. We calculate the mean exit time as a function of the rate of the barrier fluctuations for the piecewise linear and the piecewise constant barrier, τ=τ(α). For the piecewise constant potential we find three different regimes: τ∼τ+ for α<τ−1+=exp(-E+/T), τ∼2τ− for α>τ−1−=exp(-E−/T), and τ∼α−1 for τ−1+<α<τ−1−. The mean exit time for the piecewise linear potential has a different behavior for fast barrier fluctuations, α>τ−1−; τ(α) is a monotonously increasing function that approaches the asymptotic value τ∼ √τ+τ− for α→∞. We show that …

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Photoquenching And Thermal Recovery Of A Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look May 1993

Photoquenching And Thermal Recovery Of A Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

A prominent thermally stimulated current peak T5 appearing in semi‐insulating GaAs is shown to photoquench under infrared illumination, and then thermally recover at a rate r=2.0×108 exp(−0.26 eV/kT) s−1, exactly the same as that observed for EL2, within experimental error. Two possible explanations exist: (1) T5 and EL2 are microscopically very similar, probably each with an AsGa core; or (2) T5 is an electron trap that only appears to quench and recover with EL2 because EL2 controls the electron lifetime. Several other traps show similar quenching and recovery behavior.

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Contribution Of High-Order Rainbows To The Scattering Of A Gaussian Laser Beam By A Spherical Particle, James A. Lock Apr 1993

Contribution Of High-Order Rainbows To The Scattering Of A Gaussian Laser Beam By A Spherical Particle, James A. Lock

Physics Faculty Publications

I review the theory of the scattering of a Gaussian laser beam by a dielectric spherical particle and give the details for constructing a computer program to implement the theory. Computational results indicate that if the width of the laser beam is much less than the diameter of the particle and if the axis of the beam is incident near the edge of the particle, the fifth-, sixth-, and ninth-order rainbows should be evident in the far-field scattered intensity. I performed an experiment that yielded tentative evidence for the presence of the sixth-order rainbow.

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The Effect Of Ionizing And Displacive Radiation On The Thermal Conductivity Of Alumina, D. P. White Apr 1993

The Effect Of Ionizing And Displacive Radiation On The Thermal Conductivity Of Alumina, D. P. White

Physics Faculty Publications

The effects of ionizing and displacive radiation on the thermal conductivity of alumina at high temperatures have been studied. The phonon scattering relaxation times for several scattering mechanisms have been used to determine the effect on the thermal conductivity. The scattering mechanisms considered are scattering by electrons excited into the conduction band, vacancies, aluminum precipitates, and voids. It is found that under irradiation conditions where the electrical conductivity and dielectric loss tangent are greatly increased there is not a significant decrease in the thermal conductivity due to phonon-electron scattering. The conditions under which the scattering due to vacancies, aluminum precipitates, …

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Measurement Of The Casimir-Polder Force, C. I. Sukenik, M. G. Boshier, S. Cho, V. Sandoghdar, E. A. Hinds Feb 1993

Measurement Of The Casimir-Polder Force, C. I. Sukenik, M. G. Boshier, S. Cho, V. Sandoghdar, E. A. Hinds

Physics Faculty Publications

The authors have studied the deflection of ground-state sodium atoms passing through a micron-sized parallel-plate cavity by measuring the intensity of a sodium atomic beam transmitted through the cavity as a function of cavity plate separation. This experiment provides clear evidence for the existence of the Casimir-Polder force, which is due to modification of the ground-state Lamb shift in the confined space of a cavity. The results confirm the magnitude of the force and the distance dependence predicted by quantum electrodynamics.

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On The System Of Diffuse Interstellar Bands At 5844 And 5850 Å, J Krelowski, Theodore P. Snow, J Papaj, C G. Seab, B Wszolek Jan 1993

On The System Of Diffuse Interstellar Bands At 5844 And 5850 Å, J Krelowski, Theodore P. Snow, J Papaj, C G. Seab, B Wszolek

Physics Faculty Publications

Two neighboring diffuse interstellar bands (DIBs) at 5844 and 5850 Å are shown to be of different origin. The presence or absence of the 5844 DIB is related to the ratio of the two prominent DIBs at 5780 and 5797 Å as well as to the nature of the ultraviolet extinction curve. The 5844 Å DIB is very sensitive to the extinction, being completely absent in cases where the far-UV extinction is low. This suggests that the carrier is destroyed by photons having energies above 11 or 12 eV. The 5850 Å DIB correlates very well with the well-studied DIB …

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The Production And Escape Of Nitrogen Atoms On Mars, Jane L. Fox Jan 1993

The Production And Escape Of Nitrogen Atoms On Mars, Jane L. Fox

Physics Faculty Publications

We have computed the production rates and densities of odd nitrogen species in the Martian atmosphere using updated rate coefficients and a revised ionosphere-thermosphere model. We find that the computed densities of NO are somewhat smaller than those measured by Viking 1, but reasonable agreement can be obtained by assuming that the rate coefficient for loss of odd nitrogen in the reaction of N with NO is smaller at temperatures that prevail in the lower Martian thermosphere (about 130–160 K) than the standard value, which applies to temperatures of 200–400 K. We have also modeled the escape fluxes of N …

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Recovery Of Quenched Hopping Conduction In Gaas-Layers Grown By Molecular-Beam Epitaxy At 200-Degrees-C, David C. Look, Z-Q. Fang, J. R. Sizelove Jan 1993

Recovery Of Quenched Hopping Conduction In Gaas-Layers Grown By Molecular-Beam Epitaxy At 200-Degrees-C, David C. Look, Z-Q. Fang, J. R. Sizelove

Physics Faculty Publications

The dark current at 82 K, in GaAs layers grown by molecular-beam epitaxy at 200 °C and annealed at 550 °C, is reduced by a factor 350 after 5 min of IR (hν<~1.12 eV) light illumination. As temperature is swept upward at 0.2 K/s, the current recovers rapidly near 130 K. A numerical analysis of the current recovery, based on hopping conduction, gives an excellent fit to the data for a thermal recovery rate r=3×108 exp(-0.26/kT), very close to the rate observed for EL2 (AsGa). This proves that the conduction below 300 K in this material is due to hopping between AsGa-related centers in their ground states. Variable-range hopping [exp-(T0/T)1/4] gives a slightly better fit to the data than nearest-neighbor hopping [exp(-ɛ3/kT)] in the range T=82-160 K, but the fitted recovery …

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New Asga Related Center In Gaas, David C. Look, Z-Q. Fang, J. R. Sizelove, C. E. Stutz Jan 1993

New Asga Related Center In Gaas, David C. Look, Z-Q. Fang, J. R. Sizelove, C. E. Stutz

Physics Faculty Publications

A new center related to AsGa has been found at relatively high concentrations (1017 cm-;3) in semi‐insulating (2×10;7 Ω cm) molecular beam epitaxial GaAs grown at 400 °C. Although the ir photoquenching and thermal recovery characteristics are nearly identical to those of ;EL2, the thermal activation energy is only 0.65±0.01 eV, much lower than the ;EL2 value of 0.75±0.01 eV. Other properties which are different include the electron‐capture barrier energy, hyperfine constant, and magnetic circular dichroism spectrum.

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Determination Of The Neutron Spin Structure Function, P. L. Anthony, R. G. Arnold, H. R. Band, H. Borel, P.E. Bosted, V. Breton, G. D. Cates, T. E. Chupp, F. S. Dietrich, J. Dunne, R. Erbacher, J. Fellbaum, H. Fonvielle, R. Gearhart, R. Holmes, E. W. Hughes, J. R. Johnson, D. Kawall, C. Keppel, S. E. Kuhn, R. M. Lombard-Nelsen, J. Marroncle, T. Maruyama, W. Meyer, Z. E. Meziani, H. Middleton, J. Morgenstern, N. R. Newbury, G. G. Petratos, R. Pitthan, R. Prepost, Y. Roblin, S. E. Rock, S. H. Rokni, G. Shapiro, T. Smith, P. A. Souder, M. Spengos, F. Staley, L. M. Stuart, Z. M. Szalata, Y. Terrien, A. K. Thompson, J. L. White, M. Woods, J. Xu, C. C. Young, G. Zapalac, E142 Collaboration Jan 1993

Determination Of The Neutron Spin Structure Function, P. L. Anthony, R. G. Arnold, H. R. Band, H. Borel, P.E. Bosted, V. Breton, G. D. Cates, T. E. Chupp, F. S. Dietrich, J. Dunne, R. Erbacher, J. Fellbaum, H. Fonvielle, R. Gearhart, R. Holmes, E. W. Hughes, J. R. Johnson, D. Kawall, C. Keppel, S. E. Kuhn, R. M. Lombard-Nelsen, J. Marroncle, T. Maruyama, W. Meyer, Z. E. Meziani, H. Middleton, J. Morgenstern, N. R. Newbury, G. G. Petratos, R. Pitthan, R. Prepost, Y. Roblin, S. E. Rock, S. H. Rokni, G. Shapiro, T. Smith, P. A. Souder, M. Spengos, F. Staley, L. M. Stuart, Z. M. Szalata, Y. Terrien, A. K. Thompson, J. L. White, M. Woods, J. Xu, C. C. Young, G. Zapalac, E142 Collaboration

Physics Faculty Publications

The spin structure function of the neutron g1n has been determined over the range 0.03 < x < 0.6 at an average Q2 of 2 (GeV/c)2 by measuring the asymmetry in deep inelastic scattering of polarized electrons from a polarized 3He target at energies between 19 and 26 GeV. The integral of the neutron spin structure function is found to be f-10 gn1(x)dx = -0.022 ± 0.011. Earlier reported proton results together with the Bjorken sum rule predict f-10 gn1(x)dx = -0.059 ± 0.019.

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²H (D,Γ) ⁴He Polarization Observables At 20, 30, And 50 Mev, R. M. Whitton, H. R. Weller, E. Hayward, W. R. Dodge, S. E. Kuhn Jan 1993

²H (D,Γ) ⁴He Polarization Observables At 20, 30, And 50 Mev, R. M. Whitton, H. R. Weller, E. Hayward, W. R. Dodge, S. E. Kuhn

Physics Faculty Publications

Tensor and vector analyzing powers for the ²H (d,γ) ⁴He reaction have been obtained as a function of angle at Ed(lab) = 20, 30, and 50 MeV. Differential cross sections were extracted at 30 and 50 MeV. Data for Ayy(θ) and Ay(θ) were obtained at all three energies, while T20(θ) was also measured at Ed(lab) = 50 MeV. A direct capture calculation was performed and compared to the data. This calculation assumes point deuterons and that the reaction proceeds primarily via E2 radiation in this energy region. A value of …

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The Structure Of A Complex Of Bovine &-Thrombin And Recombinant Hirudin At 2.8-A Resolution, Jacqueline Vitali, Philip D. Martin, Michael G. Malkowski, William D. Robertson, Jerome B. Lazar, Richard C. Winant, Paul H. Johnson, Brian F.P. Edwards Sep 1992

The Structure Of A Complex Of Bovine &-Thrombin And Recombinant Hirudin At 2.8-A Resolution, Jacqueline Vitali, Philip D. Martin, Michael G. Malkowski, William D. Robertson, Jerome B. Lazar, Richard C. Winant, Paul H. Johnson, Brian F.P. Edwards

Physics Faculty Publications

Crystals of the complex of bovine alpha-thrombin with recombinant hirudin variant 1 have space group C222(1) with cell constants a = 59.11, b = 102.62, and c = 143.26 A. The orientation and position of the thrombin component was determined by molecular replacement and the hirudin molecule was fit in 2 magnitude of Fo - magnitude of Fc electron density maps. The structure was refined by restrained least squares and simulated annealing to R = 0.161 at 2.8-A resolution. The binding of hirudin to thrombin is generally similar to that observed in the crystals of human thrombin-hirudin. Several differences in …

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Shifted X-Ray Photoelectron Peak In Molecular-Beam Epitaxial Gaas Grown At 200 Degrees C, David C. Look, J. T. Grant, J. R. Sizelove Sep 1992

Shifted X-Ray Photoelectron Peak In Molecular-Beam Epitaxial Gaas Grown At 200 Degrees C, David C. Look, J. T. Grant, J. R. Sizelove

Physics Faculty Publications

X‐ray photoelectron spectroscopic results show that molecular beam epitaxial GaAs grown at 200 °C has a reduced effective surface potential energy, about 0.5 eV, compared with the usual 0.7 eV. A Poisson analysis of the data, using parameters from Hall effect and absorption measurements, requires that the Fermi‐level‐controlling defect in this material must have a significantly lower activation energy than that of EL2, an unexpected result.

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Photoquenching Of Hopping Conduction In Low-Temperature-Grown Molecular-Beam-Epitaxial Gaas, Z-Q. Fang, David C. Look Sep 1992

Photoquenching Of Hopping Conduction In Low-Temperature-Grown Molecular-Beam-Epitaxial Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

We have observed IR photoquenching of the hopping conduction in GaAs samples grown by molecular beam epitaxy at the low temperature of 250 °C and annealed at temperatures from 300 to 600 °C. A key element in the success of this study is removal of the layers from their substrates. The hopping conduction recovers at about 140 K, with a thermal activation energy of about 0.3 eV.

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Traps In Semi-Insulating Inp Studied By Thermally Stimulated Current Spectroscopy, Z. Q. Fang, David C. Look, J. H. Zhao Aug 1992

Traps In Semi-Insulating Inp Studied By Thermally Stimulated Current Spectroscopy, Z. Q. Fang, David C. Look, J. H. Zhao

Physics Faculty Publications

Traps in Fe‐doped semi‐insulating InP samples have been studied by thermally stimulated current spectroscopy with IR (hν≤1.12 eV) excitation at 81 K. The possible involvement of native defects in determining the compensation mechanisms is suggested based on the observation of other than the usual 0.64 eV Fe‐related activation energy for the dark current in one of the four samples supplied from different sources. A metastable behavior of traps in another sample was found and explained by a charge‐controlled defect reaction model. Three out of the six traps observed are suggested to be electron traps and one among the …

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Defect Models In Electron-Irradiated N-Type Gaas, B. Ziebro, Joseph W. Hemsky, David C. Look Jul 1992

Defect Models In Electron-Irradiated N-Type Gaas, B. Ziebro, Joseph W. Hemsky, David C. Look

Physics Faculty Publications

1 MeV electron irradiation has been performed in degenerate, n‐type (n≂2×1017 cm-3), molecular beam epitaxial GaAs layers, and Hall effect measurements have been carried out during the irradiation in order to get accurate defect production data. The results have been fitted with statistical models, and are most consistent with the usual E1 (EC-0.045 eV) and E2 (EC-0.15 eV) levels being the (-/0) and (0/+) transitions of the As vacancy, respectively. Also, an acceptor well below EC-0.15 eV is produced at a much higher rate than that of E1 and E2.

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Mechanisms For Gaas Surface Passivation By A Molecular-Beam Epitaxial Cap Layer Grown At 200-Degrees-C, David C. Look, D. C. Walters, C. E. Stutz, K. R. Evans, J. R. Sizelove Jun 1992

Mechanisms For Gaas Surface Passivation By A Molecular-Beam Epitaxial Cap Layer Grown At 200-Degrees-C, David C. Look, D. C. Walters, C. E. Stutz, K. R. Evans, J. R. Sizelove

Physics Faculty Publications

A thin, undoped, molecular beam epitaxial (MBE) GaAs cap layer grown on top of an n‐type conductive layer significantly reduces the free‐electron depletion from the latter. By analyzing electron transfer to surface, interface, and bulk acceptor states in the cap, as a function of cap thickness, we show that either (1) the usual EC−0.7 eV surface states are absent, (2) a dense donor near EC−0.4 eV exists or (3) a high donor interface charge (∼5×1012 cm−2) is present. Any of these conclusions constitutes an important new aspect of low‐temperature MBE GaAs.

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Native Donors And Acceptors In Molecular-Beam Epitaxial Gaas Grown At 200 Degrees C, David C. Look, D. C. Walters, M. Mier, C. E. Stutz, S. K. Brierley Jun 1992

Native Donors And Acceptors In Molecular-Beam Epitaxial Gaas Grown At 200 Degrees C, David C. Look, D. C. Walters, M. Mier, C. E. Stutz, S. K. Brierley

Physics Faculty Publications

Absorption measurements at 1.1 and 1.2 μm were used along with the known electron and hole photoionization cross sections for EL2 to determine deep donor (EL2‐like) and acceptor concentrations ND=9.9×1019 and NA=7.9×1018 cm−3, respectively, in a 2‐μm‐thick molecular‐beam epitaxial GaAs layer grown at 200 °C on a 2‐in.‐diam semi‐insulating wafer. Both lateral and depth uniformities of ND over the wafer were excellent as was also the case for the conductivity. Band conduction was negligible compared to hopping conduction at 296 K as evidenced by the lack of a measurable Hall coefficient.

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Direct Measurement Of The Van Der Waals Interaction Between An Atom And Its Images In A Micron-Sized Cavity, V. Sandoghdar, C. I. Sukenik, E. A. Hinds, S. Haroche Jun 1992

Direct Measurement Of The Van Der Waals Interaction Between An Atom And Its Images In A Micron-Sized Cavity, V. Sandoghdar, C. I. Sukenik, E. A. Hinds, S. Haroche

Physics Faculty Publications

The authors have measured by laser spectroscopy the energy of interaction between a sodium atom and its images in the walls of a micron-sized cavity. This cavity-QED study is the first direct quantitative test of the Lennard-Jones van der Waals interaction as a function of controlled atom-surface separation and mean-square electric dipole moment.

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Rayleigh-Brillouin Scattering To Determine One-Dimensional Temperature And Number Density Profiles Of A Gas Flow Field, James A. Lock, Richard G. Seasholtz, W. Trevor John May 1992

Rayleigh-Brillouin Scattering To Determine One-Dimensional Temperature And Number Density Profiles Of A Gas Flow Field, James A. Lock, Richard G. Seasholtz, W. Trevor John

Physics Faculty Publications

Rayleigh-Brillouin spectra for heated nitrogen gas were measured by imaging the output of a Fabry-Perot interferometer onto a CCD array The spectra were compared with the theoretical 6-moment model of Rayleigh-Brillouin scattering convolved with the Fabry-Perot instrument function. Estimates of the temperature and a dimensionless parameter proportional to the number density of the gas as functions of position in the laser beam were calculated by least-squares deviation fits between theory and experiment.

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Incorporation Of Si And Al In Low Temperature Mbe Gaas, M. O. Manasreh, K. R. Evans, C. E. Stutz, David C. Look May 1992

Incorporation Of Si And Al In Low Temperature Mbe Gaas, M. O. Manasreh, K. R. Evans, C. E. Stutz, David C. Look

Physics Faculty Publications

The localized vibrational modes (LVMs) of silicon donor (SiGa) and aluminum isovalent (AlGa) impurities in molecular beam epitaxial GaAs layers grown at various temperatures are studied using the infrared absorption technique. It is found that the total integrated absorption of these impurities LVMs is decreased as the growth temperature decreases. This finding suggests a nonsubstitutional incorporation of Si and Al in GaAs layers grown at 200 °C. On the other hand, a subtitutional incorporation is obtained in GaAs layers grown at temperatures higher than 350 °C. A recovery of the SiGa LVMs in GaAs layers …

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Incorporation Of Silicon And Aluminum In Low-Temperature Molecular-Beam Epitaxial Gaas, M. O. Manasreh, K. R. Evans, C. E. Stutz, David C. Look, Joseph Hemsky May 1992

Incorporation Of Silicon And Aluminum In Low-Temperature Molecular-Beam Epitaxial Gaas, M. O. Manasreh, K. R. Evans, C. E. Stutz, David C. Look, Joseph Hemsky

Physics Faculty Publications

The localized vibrational modes (LVMs) of silicon donor (SiGa) and aluminum isovalent (AlGa) impurities in molecular beam epitaxial GaAs layers grown at various temperatures are studied using the infrared absorption technique. It is found that the total integrated absorption of these impurities LVMs is decreased as the growth temperature decreases. This finding suggests a nonsubstitutional incorporation of Si and Al in GaAs layers grown at 200 °C. On the other hand, a subtitutional incorporation is obtained in GaAs layers grown at temperatures higher than 350 °C. A recovery of the SiGa LVMs in GaAs layers …

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Dephasing Processes In Glasses With Strong Strain Interactions, Ulrich Zürcher, R. Silbey May 1992

Dephasing Processes In Glasses With Strong Strain Interactions, Ulrich Zürcher, R. Silbey

Physics Faculty Publications

Spectral diffusion decay is calculated for a glass modeled by two level systems which are strongly coupled to phonons. The spin-phonon interaction induces an effective spin-spin interaction which dominates the energy scale. We show that spectral diffusion is a property of macroscopic local fields which fluctuate on time scales that are much longer than the spin-phonon relaxation time T1. We assume for the spectral diffusion a Gaussian distribution and derive a self-consistent equation for its variance which is nonlocal in time. At high temperatures, the variance grows linearly with time while at low temperatures, we find strong deviations from simple …

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