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Physics Faculty Publications

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Articles 1801 - 1830 of 2178

Full-Text Articles in Physical Sciences and Mathematics

Intercomparison Of Retrospective Radon Detectors, R. William Field, Daniel J. Steck, Mary Ann Parkhurst, Judy A. Mahaffey, Michael C.R. Alavanja Nov 1999

Intercomparison Of Retrospective Radon Detectors, R. William Field, Daniel J. Steck, Mary Ann Parkhurst, Judy A. Mahaffey, Michael C.R. Alavanja

Physics Faculty Publications

We performed both a laboratory and a field intercomparison of two novel glass-based retrospective radon detectors previously used in major radon case-control studies performed in Missouri and Iowa. The new detectors estimate retrospective residential radon exposure from the accumulation of a long-lived radon decay product, 210Pb, in glass. The detectors use track registration material in direct contact with glass surfaces to measure the α-emission of a 210Pb-decay product, 210Po. The detector's track density generation rate (tracks per square centimeter per hour) is proportional to the surface α-activity. In the absence of other strong sources of α-emission in …

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Velocity Distributions Of C Atoms In Co+ Dissociative Recombination: Implications For Photochemical Escape Of C From Mars, Jane L. Fox, Aleksander Hać Nov 1999

Velocity Distributions Of C Atoms In Co+ Dissociative Recombination: Implications For Photochemical Escape Of C From Mars, Jane L. Fox, Aleksander Hać

Physics Faculty Publications

We have carried out Monte Carlo calculations to determine the velocity distributions of C atoms produced by dissociative recombination of CO+ using recent data for the branching ratios of various allowed channels and ion and electron temperatures appropriate to the Martian thermosphere. We find that the fractions of 12C atoms with velocities greater than the escape velocity are ∼ 0.66 and ∼ 0.62, and those for 13C are ∼ 0.47 and ∼ 0.48 at the plasma temperatures characteristic of the exobases at low and high solar activities, respectively. The ratio of the escape fractions of 13C …

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Strain Splitting Of The Gamma(5) And Gamma(6) Free Excitons In Zno, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, T. C. Collins, M T. Harris, M. J. Callahan, J. S. Bailey Nov 1999

Strain Splitting Of The Gamma(5) And Gamma(6) Free Excitons In Zno, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, T. C. Collins, M T. Harris, M. J. Callahan, J. S. Bailey

Physics Faculty Publications

High-quality ZnO crystals have been grown by vapor-phase techniques and by the hydrothermal method. Depending on the surface preparation technique, some hydrothermally grown crystals contain strain. These strains result in energy shifts of the free excitons as well as relaxation of the selection rules. The Γ6 unallowed exciton is observed in these samples without the application of a magnetic field. The Γ6 exciton is also observed to split in a strain field, consistent with the Γ9 symmetry for the top valence band in ZnO. The Γ5 and Γ6 excitons have been observed to split in …

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Low-Frequency Noise In N-Gan With High Electron Mobility, M. E. Levinshtein, S. L. Rumyantsev, David C. Look, Richard J. Molnar, M. A. Khan, G. Simin, V. Adivarahan, M. S. Shur Nov 1999

Low-Frequency Noise In N-Gan With High Electron Mobility, M. E. Levinshtein, S. L. Rumyantsev, David C. Look, Richard J. Molnar, M. A. Khan, G. Simin, V. Adivarahan, M. S. Shur

Physics Faculty Publications

We report on the results of measurements of low frequency noise in n-type gallium nitride (GaN) grown on sapphire with 300 K electron mobility of 790 cm2/V s. The noise spectra have the form of 1/f noise with a Hooge parameter α of approximately 5×10−2. This value of α is two orders of magnitude smaller than that observed before in n-GaN. The obtained results show that the level of flicker noise in GaN, just like that in GaAs and Si, strongly depends on the structural perfection of the material (the amplitude of the …

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Configurational Entropy And Collective Modes In Normal And Supercooled Liquids, Ulrich Zürcher, T. Keyes Aug 1999

Configurational Entropy And Collective Modes In Normal And Supercooled Liquids, Ulrich Zürcher, T. Keyes

Physics Faculty Publications

Soft vibrational modes have been used to explain anomalous thermal properties of glasses above 1 K. The soft-potential model consists of a collection of double-well potentials that are distorted by a linear term representing local stress in the liquid. Double-well modes contribute to the configurational entropy of the system. Based on the Adam-Gibbs theory of entropically driven relaxation in liquids, we show that the presence of stress drives the transition from Arrhenius to Zwanzig-Bässler temperature dependence of relaxation times. At some temperature below the glass transition, the energy scale is dominated by local stress, and soft modes are described by …

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Production And Annealing Of Electron Irradiation Damage In Zno, David C. Look, D. C. Reynolds, Joseph W. Hemsky, R. L. Jones, J. R. Sizelove Aug 1999

Production And Annealing Of Electron Irradiation Damage In Zno, David C. Look, D. C. Reynolds, Joseph W. Hemsky, R. L. Jones, J. R. Sizelove

Physics Faculty Publications

High-energy (>1.6 MeV) electrons create acceptors and donors in single-crystal ZnO. Greater damage is observed for irradiation in the [0001] direction (Zn face) than in the [000] direction (O face). The major annealing stage occurs at about 300–325 °C, and is much sharper for defects produced by Zn-face irradiation, than for those resulting from O-face irradiation. The defects appear to have a chain character, rather than being simple, near-neighbor vacancy/interstitial Frenkel pairs. These experiments suggest that ZnO is significantly more “radiation hard” than Si, GaAs, or GaN, and should be useful for applications in high-irradiation environments, such as electronics …

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Valence-Band Ordering In Zno, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, G. Cantwell, W. C. Harsch Jul 1999

Valence-Band Ordering In Zno, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, G. Cantwell, W. C. Harsch

Physics Faculty Publications

The emission and reflection spectra of ZnO have been investigated in the intrinsic region and the data have been interpreted in terms of the wurtzite crystal band structure. Free-exciton emission is observed for the first time. Both the Γ5 and Γ6 state excitons associated with top valence band have been identified. This identification has established the valence-band symmetry ordering in ZnO.

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Particle Sizing In Strongly Turbid Suspensions With The One-Beam Cross-Correlation Dynamic Light-Scattering Technique, Anthony J. Adorjan, James A. Lock, Thomas W. Taylor, Padetha Tin, William V. Meyer, Anthony E. Smart May 1999

Particle Sizing In Strongly Turbid Suspensions With The One-Beam Cross-Correlation Dynamic Light-Scattering Technique, Anthony J. Adorjan, James A. Lock, Thomas W. Taylor, Padetha Tin, William V. Meyer, Anthony E. Smart

Physics Faculty Publications

The utility of the one-beam cross-correlation dynamic light-scattering system for sizing small particles in suspension was previously limited by its small-intensity signal-to-baseline ratio for strongly turbid suspensions. We describe three improvements in the optical system and sample cell that raise the ratio to a value comparable with that of other cross-correlation dynamic light-scattering systems. These improvements are (i) using a square cross-sectional sample cell to minimize the attenuation of the incident beam and singly scattered light, (ii) placing a 200-mu m-wide slit between the sample cell and the detector fibers to mask off the region of weak single scattering and …

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Excitation Of Morphology-Dependent Resonances And Van De Hulst's Localization Principle, James A. Lock Apr 1999

Excitation Of Morphology-Dependent Resonances And Van De Hulst's Localization Principle, James A. Lock

Physics Faculty Publications

When a laser beam scatters from a microparticle whose shape deviates from that of a sphere, a number of partial waves of the incident beam couple to a given partial wave of the scattered and interior fields. As a result, partial-wave coupling caused by small surface irregularities of a liquid droplet provides the mechanism for exciting low-radial-order morphology-dependent resonances. (C) 1999 Optical Society of America.

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Performance Of Sinusoidally Deformed Hydrophone Line Arrays, Deanna M. Caveny, Donald R. Del Balzo, James H. Leclere, George E. Ioup Apr 1999

Performance Of Sinusoidally Deformed Hydrophone Line Arrays, Deanna M. Caveny, Donald R. Del Balzo, James H. Leclere, George E. Ioup

Physics Faculty Publications

It is well known that array deformations can distort beam patterns and introduce bearing errors if the beamformer assumes linearity. It is also known that deformed arrays can resolve left–right ambiguities, provided the shape is known. In this work, these two effects are studied for undamped and damped sinusoidally deformed arrays with small deformation amplitudes in the horizontal (x,y) plane only. By use of fixed arc-length separations along the array, the hydrophone (x,y) coordinates are determined numerically and the error in assuming equal x spacing is summarized for a sample array. Array-response …

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Residual Native Shallow Donor In Zno, David C. Look, Joseph W. Hemsky, J. R. Sizelove Mar 1999

Residual Native Shallow Donor In Zno, David C. Look, Joseph W. Hemsky, J. R. Sizelove

Physics Faculty Publications

High-energy electron irradiation in ZnO produces shallow donors at about EC-30meV. Because the production rate is much higher for Zn-face (0001) than O-face (0001̅ ) irradiation, the donor is identified as a Zn-sublattice defect, most likely the interstitial ZnI or a ZnI-related complex. The donor energy is quite close to that of the unirradiated sample, and of other samples discussed in the literature, strongly suggesting that ZnI (and not VO) is the dominant native shallow donor in ZnO. An exceptionally high displacement threshold energy (∼1.6MeV) is quantitatively explained in terms of a …

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Exposure To Atmospheric Radon, Daniel J. Steck, R. William Field, Charles F. Lynch Feb 1999

Exposure To Atmospheric Radon, Daniel J. Steck, R. William Field, Charles F. Lynch

Physics Faculty Publications

We measured radon (222Rn) concentrations in Iowa and Minnesota and found that unusually high annual average radon concentrations occur outdoors in portions of central North America. In some areas, outdoor concentrations exceed the national average indoor radon concentration. The general spatial patterns of outdoor radon and indoor radon are similar to the spatial distribution of radon progeny in the soil. Outdoor radon exposure in this region can be a substantial fraction of an individual's total radon exposure and is highly variable across the population. Estimated lifetime effective dose equivalents for the women participants in a radon-related lung cancer …

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Dislocation Scattering In Gan, David C. Look, J. R. Sizelove Feb 1999

Dislocation Scattering In Gan, David C. Look, J. R. Sizelove

Physics Faculty Publications

A theory of charged-dislocation-line scattering is developed within the framework of the Boltzmann transport equation. A fit of the theory to temperature-dependent Hall-effect data in GaN gives dislocation densities which are in excellent agreement with those measured by transmission electron microscopy. This work shows that threading edge dislocations in GaN indeed are electrically active, in agreement with recent theoretical predictions.

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Search For Exchange-Antisymmetric Two-Photon States, D. Demille, D. Budker, N. Derr, Edward Deveney Jan 1999

Search For Exchange-Antisymmetric Two-Photon States, D. Demille, D. Budker, N. Derr, Edward Deveney

Physics Faculty Publications

Atomic two-photon J = 0↔J′ = 1 transitions are forbidden for photons of the same energy. This selection rule is related to the fact that photons obey Bose-Einstein statistics. We have searched for small violations of this selection rule by studying transitions in atomic Ba. We set a limit on the probability v that photons are in exchange-antisymmetric states: v<1.2×10-7.

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Role Of Spontaneous Emission In Ultracold Two-Color Optical Collisions, C. I. Sukenik, T. Walker Jan 1999

Role Of Spontaneous Emission In Ultracold Two-Color Optical Collisions, C. I. Sukenik, T. Walker

Physics Faculty Publications

We have observed violet photon emission resulting from energy-pooling collisions between ultracold Rb atoms illuminated by two colors of near-resonant infrared laser light. We have used this emission as a probe of doubly excited state ultracold collision dynamics. By varying the detuning of the lasers, we have clearly identified the effect of spontaneous emission on the collision process.

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Factorization And High-Energy Effective Action, Ian Balitsky Jan 1999

Factorization And High-Energy Effective Action, Ian Balitsky

Physics Faculty Publications

I demonstrate that the amplitude for high-energy scattering can be factorized as a convolution of the contributions due to fast and slow fields. The fast and slow fields interact by means of Wilson-line operators—infinite gauge factors ordered along the straight line. The resulting factorization formula gives a starting point for a new approach to the effective action for high-energy scattering in QCD.

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Digital Signal Propagation In Dispersive Media., P. M. Jordan, Ashok Puri Jan 1999

Digital Signal Propagation In Dispersive Media., P. M. Jordan, Ashok Puri

Physics Faculty Publications

In this article, the propagation of digital and analog signals through media which, in general, are both dissipative and dispersive is modeled using the one-dimensional telegraph equation. Input signals are represented using impulsive, Heaviside unit step, Gaussian, rectangular pulse, and both unmodulated and modulated sinusoidal pulse type boundary data. Applications to coaxial transmission lines and freshwater signal propagation, for both digital and analog signals, are included. The analysis presented here supports the finding that digital transmission in dispersive media is generally superior to that of analog. The boundary data (input signals) give rise to solutions of the telegraph equation which …

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Mössbauer Effect Determination Of Relative Recoilless Fractions For Iron Oxides, Sei J. Oh, Desmond C. Cook Jan 1999

Mössbauer Effect Determination Of Relative Recoilless Fractions For Iron Oxides, Sei J. Oh, Desmond C. Cook

Physics Faculty Publications

The relative recoilless fraction (F-value) of each of six iron oxides, defined as the ratio of the recoil-free fractions of two different materials, was experimentally determined relative to hematite at 300 K and 77 K by Mössbauer spectroscopy. Using the relative recoil-free fractions compared to that of hematite, the relative recoilless fractions between all pairs of the seven iron oxides were determined. The F-values can allow conversion of Mössbauer subspectral areas to the relative atomic, molecular, or weight fractions of each iron oxide present in a mixed oxide phase sample.

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Relaxational Mode Structure For Optical Probe Diffusion In High Molecular Weight Hydroxypropylcellulose, Kiril A. Streletzky, George D.J. Phillies Dec 1998

Relaxational Mode Structure For Optical Probe Diffusion In High Molecular Weight Hydroxypropylcellulose, Kiril A. Streletzky, George D.J. Phillies

Physics Faculty Publications

We studied translational diffusion of dilute monodisperse spheres (diameters 14 < d < 455 nm) in aqueous 1 MDa hydroxypropylcellulose (0 ≤ c ≤ 7 g/L) at 25°C using quasielastic light scattering. Spectra are highly bimodal. The two spectral modes (“slow,” “fast”) have different physical properties. Probe behavior differs between small (d < Rh) and large (d ≥ Rg) probes; Rh and Rg are the matrix polymer hydrodynamic radius and the radius of gyration, respectively. We examined the dependences of spectral lineshape parameters on d, c, scattering vector q, and viscosity η for all four probe-size and mode-type combinations. We find three time scale-separated modes: (1) a large-probe slow mode has properties characteristic of particle motion in a viscous medium; (2) a large-probe fast mode and small-probe slow modes share the same time scale, and have properties characteristic of probe motion coupled to internal chain dynamics; and (3) a small-probe fast mode has properties that can be attributed to the probe sampling local chain relaxations. In the analysis, we also attempted to apply the coupling/scaling (CS) model of Ngai and Phillies [Ngai, K. L., Phillies, G. D. J. J. Chem. Phys.,105, 8385 (1996)] to analyze our data. We find that the second mode is described by the coupling/scaling model for probe diffusion; the first and third modes do not follow the predictions of this model.

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Excitation Efficiency Of A Morphology-Dependent Resonance By A Focused Gaussian Beam, James A. Lock Dec 1998

Excitation Efficiency Of A Morphology-Dependent Resonance By A Focused Gaussian Beam, James A. Lock

Physics Faculty Publications

The excitation efficiency of a morphology-dependent resonance (MDR) by an incident beam is defined as the fraction of the beam power channeled into the MDR. The efficiency is calculated for a focused Gaussian beam of arbitrary width incident on either a spherical particle or a cylindrical fiber located at an arbitrary position in the plane of the beam waist. In each case a simple formula for the efficiency is derived by use of the localized approximation for the beam-shape coefficients in the partial-wave expansion of the beam. The physical interpretation of the efficiency formulas is also discussed. (C) 1998 Optical …

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Effect Of Ammonia Flow Rate On Impurity Incorporation And Material Properties Of Si-Doped Gan Epitaxial Films Grown By Reactive Molecular Beam Epitaxy, Wook Kim, A. E. Botchkarev, H. Morkoç, Z-Q. Fang, David C. Look, David J. Smith Dec 1998

Effect Of Ammonia Flow Rate On Impurity Incorporation And Material Properties Of Si-Doped Gan Epitaxial Films Grown By Reactive Molecular Beam Epitaxy, Wook Kim, A. E. Botchkarev, H. Morkoç, Z-Q. Fang, David C. Look, David J. Smith

Physics Faculty Publications

Effect of ammonia flow rate on the impurity incorporation and material properties of Si-doped GaN films grown by reactive molecular beam epitaxy (RMBE) process is discussed. It appears that the ammonia flow rate has a marginal effect on the incorporation of impurities into the Si-doped GaN films except there was a little decrease in O and Si with increasing ammonia flow rate when the Si concentration in the film is higher than 1018 cm−3. Electron Hall mobility of Si-doped GaN films grown by RMBE varies with ammonia flow rate used during film growth. From deep level transient …

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Development Of A Generalized Theoretical Model For The Response Of A Phase/Doppler Measurement System To Arbitrarily Oriented Fibers Illuminated By Gaussian Beams, Scott A. Schaub, James A. Lock, Amir A. Naqwi Nov 1998

Development Of A Generalized Theoretical Model For The Response Of A Phase/Doppler Measurement System To Arbitrarily Oriented Fibers Illuminated By Gaussian Beams, Scott A. Schaub, James A. Lock, Amir A. Naqwi

Physics Faculty Publications

We present a generalized theoretical model for the response of the phase/Doppler (P/D) measurement system to light scattered by cylindrical fibers. This theoretical model is valid for arbitrary fiber diameters and refractive indices, for Gaussian incident beams, and it accounts for arbitrary fiber orientations, fiber positions, and effects that are due to the two-dimensional receivers. The generalized P/D computer model (GPDCM) is the extension of an earlier study by the authors, combining past P/D simulation methodology with recent developments in modeling light scattering by tilted cylindrical fibers. A FORTRAN computer program that implements the GPDCM theoretical development was written and …

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Comment On A Critique Of The Instantaneous Normal Mode (Inm) Approach To Diffusion, T. Keyes, Wu Xiong Li, Ulrich Zurcher Sep 1998

Comment On A Critique Of The Instantaneous Normal Mode (Inm) Approach To Diffusion, T. Keyes, Wu Xiong Li, Ulrich Zurcher

Physics Faculty Publications

A critique of the instantaneous normal mode (INM) theory of diffusion by Gezelter, Rabani, and Berne (GRB) [J. Chem. Phys. 107, 4618 (1997)] is analyzed. GRB assert that imaginary-frequency INM are corrupted with modes unrelated to barrier crossing, that proposals for removing such nondiffusive (ND) modes are inadequate, and thus that INM cannot be used to predict the self-diffusion constant, D. In rebuttal it is argued that Lennard-Jones, the system studied by GRB, is anamolously rich in ND modes. INM in molecular liquids are shown to behave as excellent indicators of barrier crossing. Even in LJ ND-INM, while plentiful, do …

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Optical Implementation Of The Wavelet Transform By Using A Bacteriorhodopsin Film As An Optically Addressed Spatial Light Modulator, Joby Joseph, F. J. Aranda, D.V.G.L.N. Rao, B. S. Decristofano, B. R. Kimball, M. Nakashima Sep 1998

Optical Implementation Of The Wavelet Transform By Using A Bacteriorhodopsin Film As An Optically Addressed Spatial Light Modulator, Joby Joseph, F. J. Aranda, D.V.G.L.N. Rao, B. S. Decristofano, B. R. Kimball, M. Nakashima

Physics Faculty Publications

An optical system utilizing the photoinduced dichroism in a bacteriorhodopsin film has been demonstrated for the optical implementation of wavelet transforms. The dichroism, induced by the image of a wavelet filter on a bacteriorhodopsin film leads to polarization rotation of the Fourier components of an image. The polarization-rotated Fourier components of an input scene are analyzed with a polarizer to give the wavelet transform components. The dichroism is induced with beams whose profiles are determined by wavelet filters in order to perform the optical wavelet transform.

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Spiral Morphology-Dependent Resonances In An Optical Fiber: Effects Of Fiber Tilt And Focused Gaussian Beam Illumination, Andrew W. Poon, Richard K. Chang, James A. Lock Jul 1998

Spiral Morphology-Dependent Resonances In An Optical Fiber: Effects Of Fiber Tilt And Focused Gaussian Beam Illumination, Andrew W. Poon, Richard K. Chang, James A. Lock

Physics Faculty Publications

Spiral morphology-dependent resonances have been observed in a tilted optical fiber. The polarization-preserving and the cross-polarized elastic-scattering spectra for plane-wave illumination show that the wavelengths of the resonances are blueshifted quadratically as the fiber tilt angle increases. When a focused Gaussian beam illuminates the fiber at its edge, the resonances are blueshifted and broadened as the detector is offset from the scattering plane with the maximum scattering intensity. The blueshift with focused beam illumination is also a consequence of the spiral resonances. (C) 1998 Optical Society of America.

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Low Saturation Intensities In Two-Photon Ultracold Collisions, C. I. Sukenik, D. Hoffman, S. Bali, T. Walker Jul 1998

Low Saturation Intensities In Two-Photon Ultracold Collisions, C. I. Sukenik, D. Hoffman, S. Bali, T. Walker

Physics Faculty Publications

We have observed violet photon emission resulting from energy-pooling collisions between ultracold Rb atoms illuminated by two colors of near-resonant infrared laser light. We have used this emission as a probe of doubly excited state ultracold collision dynamics. We have observed the lowest saturation intensity for light-induced ultracold collisions seen to date which we identify as due to depletion of incoming ground state flux. We have also varied the detuning of the lasers which allows us to clearly identify the effect of spontaneous emission and optical shielding.

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Translational Diffusion Of Small And Large Mesoscopic Probes In Hydroxypropylcellulose-Water In The Solutionlike Regime, Kiril A. Streletzky, George D.J. Phillies Jun 1998

Translational Diffusion Of Small And Large Mesoscopic Probes In Hydroxypropylcellulose-Water In The Solutionlike Regime, Kiril A. Streletzky, George D.J. Phillies

Physics Faculty Publications

Quasi-elastic light scattering spectroscopy was used to study the translational diffusion of monodisperse spheres in aqueous 1 MDa hydroxypropylcellulose (HPC) at 25 °C. Probe diameters d spanned 14–455 nm; HPC concentrations were 0⩽c⩽7g/L. Light scattering spectroscopy consistently found spectra having the form g(1)(t)=(1−Af)exp(−θtβ)+Af exp(−θftβf). Here θf and βf refer to the “fast” mode; θ and β describe the “slow” mode. We examine the dependence of θ, β, θf, βf, and Af on d, c, scattering vector q, and viscosity η. β=1 for large probes; elsewise, β and βf are ∈(0,1). The slow mode, with short-lived memory function, is diffusive; for …

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Deep Centers In N-Gan Grown By Reactive Molecular Beam Epitaxy, Z-Q. Fang, David C. Look, W. Kim, Z. Fan, A. Botchkarev, H. Morkoç May 1998

Deep Centers In N-Gan Grown By Reactive Molecular Beam Epitaxy, Z-Q. Fang, David C. Look, W. Kim, Z. Fan, A. Botchkarev, H. Morkoç

Physics Faculty Publications

Deep centers in Si-doped n-GaN layers grown by reactive molecular beam epitaxy have been studied by deep-level transient spectroscopy as a function of growth conditions. Si-doped GaN samples grown on a Si-doped n+-GaN contact layer at 800 °C show a dominant trap C1 with activation energy ET = 0.44 eV and capture cross-section σT = 1.3×10−15 cm−2, while samples grown at 750 °C on an undoped semi-insulating GaN buffer show prominent traps D1 and E1, with ET = 0.20 eV and σT = 8.4×10−17 cm …

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Neutral-Donor-Bound-Exciton Complexes In Zno Crystals, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, T. C. Collins, W. C. Harsch, G. Cantwell May 1998

Neutral-Donor-Bound-Exciton Complexes In Zno Crystals, D. C. Reynolds, David C. Look, B. Jogai, C. W. Litton, T. C. Collins, W. C. Harsch, G. Cantwell

Physics Faculty Publications

Neutral-donor–bound-exciton transitions have been observed in ZnO. The isolated neutral donors are made up of defect pair complexes. The neutral-donor nature of these pair complexes was determined from magneticfield measurements and from two-electron transitions. Excited states of the neutral-donor bound excitons were observed in the form of rotator states analogous to rotational states of the H2 molecule.

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High Mobility Algan/Gan Heterostructures Grown By Gas-Source Molecular Beam Epitaxy, L. K. Li, J. Alperin, W. I. Wang, David C. Look, Donald C. Reynolds May 1998

High Mobility Algan/Gan Heterostructures Grown By Gas-Source Molecular Beam Epitaxy, L. K. Li, J. Alperin, W. I. Wang, David C. Look, Donald C. Reynolds

Physics Faculty Publications

We report on the growth of high electron mobility AlGaN/GaN heterostructures on sapphire substrates by gas-source molecular beam epitaxy (GSMBE) using ammonia as the nitrogen source. Improvements in structural, electrical, and optical properties of GaN and AlGaN layers have been made to achieve this goal. For the growth of AlGaN layers, the reflection high-energy electron diffraction revealed a twofold surface reconstruction, indicative of atomic smoothness of the film surface. High mobility two-dimensional electron gas has been achieved in both unintentionally doped (by piezoelectric effect induced by lattice mismatch strain) and modulation doped AlGaN/GaN heterostructures. The modulation-doped n+-Al0.2Ga …

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