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Full-Text Articles in Physics

Evidence For Day-To-Night Ion Transport At Low Solar Activity In The Venus Pre-Dawn Ionosphere, J. F. Brannon, Jane L. Fox, H. S. Porter Dec 1993

Evidence For Day-To-Night Ion Transport At Low Solar Activity In The Venus Pre-Dawn Ionosphere, J. F. Brannon, Jane L. Fox, H. S. Porter

Physics Faculty Publications

Periapsis of the Pioneer Venus spacecraft dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre-dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re-entry period. In particular, we examine here the atomic ion density profiles ...


Recombination Of 2-Dimensional Electrons With Free Light And Heavy Holes In Alxga1-Xas-Gaas Single Heterojunctions In A Magnetic-Field, D. C. Reynolds, David C. Look, B. Jogai, C. E. Stutz Dec 1993

Recombination Of 2-Dimensional Electrons With Free Light And Heavy Holes In Alxga1-Xas-Gaas Single Heterojunctions In A Magnetic-Field, D. C. Reynolds, David C. Look, B. Jogai, C. E. Stutz

Physics Faculty Publications

Landau-level oscillations are observed in the photoluminescence from an AlxGa1-xAs-GaAs single heterojunction in an applied magnetic field. Extrapolating the oscillations back to zero field gives the energy of the transition from the two-dimensional (2D) electrons to the free heavy holes (hh1 and hh2) and free light holes (lh) weakly confined in the GaAs active layer. The measured energy separation between hh1 and lh is 1.8 meV, which agrees very well with the calculated value of 2.2 meV. The measured energy separation between hh1 and hh2 is 3.8 meV, in good agreement with the ...


On The Escape Of Oxygen And Hydrogen From Mars, Jane L. Fox Sep 1993

On The Escape Of Oxygen And Hydrogen From Mars, Jane L. Fox

Physics Faculty Publications

Escape rates of oxygen atoms from dissociative recombination of O2+ above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured by ...


Semi-Insulating Nature Of Gas Source Molecular Beam Epitaxial Ingap Grown At Very Low Temperatures, David C. Look, Y. He, J. Ramdani, N. Elmasry, S. M. Bedair Aug 1993

Semi-Insulating Nature Of Gas Source Molecular Beam Epitaxial Ingap Grown At Very Low Temperatures, David C. Look, Y. He, J. Ramdani, N. Elmasry, S. M. Bedair

Physics Faculty Publications

InxGa1−xP lattice matched to GaAs (x≂0.51) has proven to be useful in many device applications. Here we show that undoped, semi‐insulating InGaP is possible by growing with gas source molecular beam epitaxy at very low temperatures, 150–250 °C. The material grown at about 200 °C is n‐type with a 296‐K resistivity of 9×105 Ω cm, a mobility of 120 cm2/V s, and a donor activity energy of 0.48 eV. When annealed at 600 °C for 1 h, the resistivity increases to greater than 10 ...


Electron-Beam Modification Of Gaas Surface-Potential - Measurement Of Richardson Constant, S. M. Lindsay, Joseph W. Hemsky, David C. Look Aug 1993

Electron-Beam Modification Of Gaas Surface-Potential - Measurement Of Richardson Constant, S. M. Lindsay, Joseph W. Hemsky, David C. Look

Physics Faculty Publications

The surface potential of GaAs is strongly modified in the presence of a high‐energy electron beam due to the creation of electron‐hole pairs in the depletion region and the subsequent drift of the holes to the surface where they neutralize surface states. This effect is modeled in terms of a parameter K=AT2/Ib(dE/dz)η, where Ib is the beam current density, A∗ is the effective Richardson constant, dE/dz is the beam energy loss per unit length, and η−1 is the average energy required to create an electron‐hole pair ...


Electron-Drift Velocities In Argon-Boron Trichloride Gas-Mixtures, Donald L. Mosteller Jr., Merrill L. Andrews, Jerry D. Clark, A. Garscadden Aug 1993

Electron-Drift Velocities In Argon-Boron Trichloride Gas-Mixtures, Donald L. Mosteller Jr., Merrill L. Andrews, Jerry D. Clark, A. Garscadden

Physics Faculty Publications

Electron drift velocities were measured for argon gas mixtures containing 25 ppm to 0.5% boron trichloride additive using a pulsed-Townsend drift tube. These results show a marked sensitivity to mixture ratio and the gas mixtures also show negative differential conductivity and strong attachment at low electric field/gas number density.


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 ...


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 ...


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 ...


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+AsAsVAs‐AsGa, and VGa+AsGaVGa‐AsGa.


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 ...


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 ...


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.


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 ...


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 ...