bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for the onset of breakdown in an axisymmetric compressible vortex
Authors: Mahesh Krishnan
Published: July 2002

A simple inviscid model to predict the onset of breakdown in an axisymmetric vortex is proposed. Three problems are considered: the shock-induced breakdown of a compressible vortex, the breakdown of a free compressible vortex, and the breakdown of a free incompressible vortex. The same physical reasoning is used in all three problems to predict the onset of breakdown. It is hypothesized that breakdown is the result of the competing effects of adverse pressure rise and streamwise momentum flux at the vortex centerline. Breakdown is assumed to occur if the pressure rise exceeds the axial momentum flux. A formula with no adjustable constants is derived for the critical swirl number in all three problems. The dependence of the critical swirl number on parameters such as upstream Mach number, excess/deficit in centerline axial velocity, and shock oblique angle is explored. The predictions for the onset of shock-induced breakdown and free incompressible breakdown are compared to experiment and computation, and good agreement is observed. Finally, a new breakdown map is proposed. It is suggested that the adverse pressure rise at the vortex axis be plotted against the axial momentum flux to determine the onset of breakdown. The proposed map allows the simultaneous comparison of data from flows ranging from incompressible breakdown to breakdown induced by a shock wave.
Similarity scaling of acceleration and pressure statistics in numerical simulations of isotropic turbulence
Authors: Vedula Prakash , Yeung P. K.
Published: July 2002

The scaling properties of one- and two-point statistics of the acceleration, pressure, and pressure gradient are studied in incompressible isotropic turbulence by direct numerical simulation. Ensemble-averaged Taylor-scale Reynolds numbers (Rλ) are up to about 230 on grids from 323 to 5123. From about Rλ 40 onwards the acceleration variance normalized by Kolmogorov variables is found to increase as Rλ1/2. This nonuniversal behavior is traced to the dominant irrotational pressure gradient contributions to the acceleration (whereas the much weaker solenoidal viscous part is universal). Longitudinal and transverse two-point correlations of the pressure gradient differ according to kinematic constraints, but both (especially the latter) extend over distances of intermediate scale size large compared to the Kolmogorov scale. These extended-range properties essentially provide the Eulerian mechanism whereby (as found in recent work) the accelerations of a pair of fluid particles can remain significantly correlated for relatively long periods of time even as they move apart from each other. Although a limited inertial range is attained in the energy spectrum, little evidence for classical inertial scaling is found in acceleration correlations and pressure structure functions. The probability density function (PDF) of pressure fluctuations has negatively skewed tails that exhibit a stretched-exponential form. Pressure gradient statistics show a rapid increase in intermittency with Reynolds number, characterized by widening tails in the PDF and large flatness factors. The practicality of computing acceleration correlations from velocity structure functions is also assessed using direct numerical simulations (DNS); within some resolution limitations good agreement is obtained with experimental data in grid turbulence at comparable Reynolds number.
A model for Marangoni drying
Authors: Thess A. , Boos W.
Published: July 2002

We propose a simple theoretical model for the movement of a thin liquid film on a flat plate that is vertically withdrawn from a large liquid reservoir in the presence of a surface tension gradient. This problem is relevant to Marangoni drying—a technique that is used in the semiconductor industry for the purpose of water removal from wafers. Due to the smallness of the capillary number the fluid flow can be described by the classical Landau–Levich equation with an additional term accounting for the Marangoni effect. A numerical solution of this equation shows that the thickness of the residual film is a monotonically decreasing function of the surface tension gradient, thereby providing an explanation of the Marangoni drying process.
Simulation of nonquasilinear diffusion
Authors: Helander P. , Kjellberg L.
Published: July 2002

The velocity diffusion coefficient for particles in a field of a large number of randomly phased electrostatic waves is calculated numerically by iterating a set of finite-difference equations. For moderate values of Chirikov’s overlap parameter K, the diffusion is found to be significantly faster (up to a factor of 2.2 at K≊17) than suggested by quasilinear theory, which confirms recent findings by Cary et al. [Phys. Rev. Lett. 65, 3132 (1990)].
The impact of a biasing radial electric field on the scrape-off layer in a divertor tokamak
Authors: Rozhansky V. , Tendler M.
Published: July 2002

The issue of transport on open field lines is addressed using a fluid approach invoking anomalous inertia and viscosity. The emphasis of the model is on the transport of the toroidal momentum, which is notoriously anomalous. The model suggests that tokamaks with divertors may benefit from a significant amplification of the electric field. It is shown that biasing of the scrape-off layer (SOL) with respect to the grounded first wall results in fundamental phenomena, affecting the performance of a divertor. The theoretical model appears to be consistent with experimental results obtained on Tokamak de Varennes [Phys. Plasmas 1, 1485 (1994)].
A heuristic model for the nonlinear Rayleigh–Taylor instability in fast Z pinches
Authors: Hussey T. W. , Roderick N. F. , Shumlak U. , Spielman R. B. , Deeney C.
Published: July 2002

A simple, heuristic model for the early nonlinear phase of the Rayleigh–Taylor instability (RTI) in thin-cylindrical-shell Z-pinch implosions has been developed. This model is based on the fact that, as the field–plasma interface is deformed, there is a component of the applied force that acts to move mass from the low mass per unit area bubble region into the higher mass per unit area spike region. The resulting reduced mass per unit area of the bubble causes it to be preferentially accelerated ahead of the spike. The pinch begins to radiate as the bubble mass first reaches the axis, and it continues to radiate while the mass that is entrained within the spikes and within unperturbed parts of the shell also arrives on axis. This model relates the time at which the bubble arrives on axis to an initial wavelength and amplitude of a single mode of the RTI. Then, by comparing this to the time at which the unperturbed mass reaches the axis, one estimates pinch thermalization time, a quantity that is determined experimentally. Experimental data, together with analytic models, have been used to choose appropriate initial wavelength and amplitude both for foils and for certain gas puff implosions. By noting that thermalization time is a weak function of these parameters, it is argued that one may use the same values for an extrapolative study of qualitatively similar implosions.
Convergence, electrostatic potential, and density measurements in a spherically convergent ion focus
Authors: Thorson T. A. , Durst R. D. , Fonck R. J. , Wainwright L. P.
Published: July 2002

Unique measurements of the basic plasma-flow characteristics in a low pressure (⩽53 mPa H2) spherically convergent ion focus are obtained using high-voltage (⩽5 kV) emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4–5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density (nic∼1015 m−3) are consistent with simple flow convergence models.
The split-weight particle simulation scheme for plasmas
Authors: Manuilskiy Igor , Lee W. W.
Published: July 2002

An efficient numerical method for treating electrons in magnetized plasmas has been developed. The scheme, which is based on the perturbative (δf) gyrokinetic particle simulation, splits the particle electron responses into adiabatic and nonadiabatic parts. The former is incorporated into the gyrokinetic Poisson’s equation, while the latter is calculated dynamically with the aid of the charge conservation equation. The new scheme affords us the possibility of suppressing unwanted high-frequency oscillations and, in the meantime, relaxing the Courant condition for the thermal particles moving in the parallel direction. It is most useful for studying low-frequency phenomena in plasmas. As an example, one-dimensional drift wave simulation has been carried out using the scheme and the results are presented in this paper. This methodology can easily be generalized to problems in three-dimensional toroidal geometry, as well as those in unmagnetized plasmas.
Long implosion time tungsten wire array Z pinches on the Saturn generator
Authors: Douglas M. R. , Deeney C. , Spielman R. B. , Coverdale C. A. , Roderick N. F. , Haines M. G.
Published: July 2002

Recent success on the Saturn [C. Deeney et al., Phys. Rev. E 56, 5945 (1997)] and Z [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] accelerators at Sandia National Laboratories have demonstrated the ability to scale Z-pinch parameters to increasingly larger current pulsed power facilities. Next generation machines will require even larger currents (>20 MA), placing further demands on pulsed power technology. To this end, experiments have been carried out on Saturn operating in a long pulse mode, investigating the potential of lower voltages and longer implosion times while still maintaining pinch fidelity. High wire number, 25 mm diam tungsten arrays were imploded with implosion times ranging from 130 to 240 ns. The results were comparable to those observed in the Saturn short pulse mode, with rise times on the order of 4.5–6.5 ns. Experimental data will be presented, along with two-dimensional radiation magnetohydrodynamic simulations used to explain and reproduce the experiment.
Study of the kinetics and mechanism of the thermal nitridation of SiO2
Authors: Vasquez R. P. , Madhukar A. , Grunthaner F. J. , Naiman M. L.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) has been used to study the nitridation time and temperature dependence of the nitrogen distribution in thermally nitrided SiO2 films. The XPS data show that the maximum nitrogen concentration near the (SiOxNy)/Si interface is initially at the interface, but moves 20–25 Å away from the interface with increasing nitridation time. Computer modeling of the kinetic processes involved is carried out and reveals a mechanism in which diffusing species, initially consisting primarily of nitrogen, react with the substrate, followed by formation of the oxygen-rich oxynitride due to reaction of the diffusing oxygen displaced by the slower nitridation of the SiO2. The data are consistent with this mechanism provided the influence of the interfacial strain on the nitridation and oxidation kinetics is explicitly accounted for.
Role of surface kinetics and interrupted growth during molecular beam epitaxial growth of normal and inverted GaAs/AlGaAs(100) interfaces: A reflection high-energy electron diffraction intensity dynamics study
Authors: Madhukar A. , Lee T. C. , Yen M. Y. , Chen P. , Kim J. Y. , Ghaisas S. V. , Newman P. G.
Published: July 2002

Measurements of the time-dependent intensity in reflection high-energy electron diffraction during [100] growth of AlxGa1−x As on GaAs (normal interface) and of GaAs on AlxGa1−x As (inverted interface) are reported for such growth on static, dynamic, and relaxing surfaces. It is shown that the growth kinetics imply that interruption of growth for an optimized duration after deposition of each alternating layer can result in structurally and chemically improved interfaces, including the possibility of comparably good normal and inverted interfaces, in heterojunctions, multiple quantum wells, and superlattices.
Electric field induced decrease of photoluminescence lifetime in GaAs quantum wells
Authors: Kash J. A. , Mendez E. E. , Morkoç H.
Published: July 2002

Time-resolved photoluminescence measurements of excitons in GaAs-Ga1−xAlxAs quantum wells subject to an electric field perpendicular to the well plane have been made. With increasing field, both integrated luminescence (as previously reported) and luminescence lifetime decrease. Thus the electric field increases the exciton nonradiative decay rate. Estimates of several possible mechanisms suggest that Fowler–Nordheim tunneling is responsible for the quenching. With increasing pump laser intensities, larger electric fields are required to quench the lifetime because of exciton screening of the field.
Identification of a vanadium-related level in liquid encapsulated Czochralski-grown GaAs
Authors: Brandt C. D. , Hennel A. M. , Pawlowicz L. M. , Dabkowski F. P. , Lagowski J. , Gatos H. C.
Published: July 2002

We present the first positive identification of a vanadium-related electron trap in V-doped GaAs crystals grown by the liquid encapsulated Czochralski technique in pyrolytic boron nitride crucibles. Detailed deep level transient spectroscopy and capacitance transient analysis yielded a trap energy of 0.15±0.01 eV below the conduction band and an electron capture cross section of about 2×10−14 cm2. Optical absorption and mobility data show that this level corresponds to the ionized acceptor state V2+(3d3) of substitutional vanadium. No midgap levels other than EL2 could be detected in these V-doped crystals showing that doping with vanadium plays no direct role in the compensation process in semi-insulating GaAs crystals.
GexSi1−x strained-layer superlattice waveguide photodetectors operating near 1.3 μm
Authors: Temkin H. , Pearsall T. P. , Bean J. C. , Logan R. A. , Luryi S.
Published: July 2002

Properties of GexSi1−x strained-layer p-i-n detectors, in which the strained-layer superlattice itself was used as an absorption region, have been studied for the first time. These devices were grown on (100)Si by molecular beam epitaxy. Using waveguide geometry we have obtained internal quantum efficiencies on the order of 40% at 1.3 μm in superlattices with the Ge fraction x=0.6. The superlattice detectors show the frequency response bandwidth of over 1 GHz and uniformly excellent electrical characteristics for values of x as large as 0.8.
High-quality pyrographite films
Authors: Murakami Mutsuaki , Watanabe Kazuhiro , Yoshimura Susumu
Published: July 2002

High-temperature heat treatment of a heat-resistant, condensation polymer poly ( ρ-phenylene-1,3,4-oxadiazole) yielded a large-area, flexible film composed of highly oriented and nearly ideal graphite crystallites. The graphitic behavior was exemplified by both the (002) lattice spacing of 3.354 Å and extremely small full width at half-maximum intensity of the (002) reflection, 0.16–0.17°, for the films heat treated above 2800 °C.
Ordered structures in GaAs0.5Sb0.5 alloys grown by organometallic vapor phase epitaxy
Authors: Jen H. R. , Cherng M. J. , Stringfellow G. B.
Published: July 2002

Electron diffraction measurements on (100) GaAs1−xSbx layers with x≊0.5 grown by organometallic vapor phase epitaxy indicate that ordered phases are formed during growth. Two ordered phases are observed. The simple, tetragonal AuCu-I type phase consists of alternating {100} oriented GaAs and GaSb layers. Only the two variants with the tetragonal c axes perpendicular to the growth direction are observed. At least two variants are observed for the chalcopyrite E11 structure with alternating {210} oriented GaAs and GaSb layers.
Vapor phase epitaxial growth and characterization of InP on GaAs
Authors: Teng S. J. J. , Ballingall J. M. , Rosenbaum F. J.
Published: July 2002

Crystal growth of InP on GaAs by vapor phase epitaxy is reported. It is demonstrated that good quality InP epitaxial layers with featureless surface morphology can be grown on GaAs substrate. Carrier concentration profile and Hall mobility measurements from as-grown n-type InP layers show that its doping behavior and mobility are similar to those grown on InP substrates. The results are encouraging for the development of devices utilizing InP/GaAs heterojunctions and the use of bulk GaAs as an alternative substrate to bulk InP for the epitaxial growth of InP and related compounds.
Anomalous diffusion and gettering of transition metals in silicon
Authors: Sparks D. R. , Chapman R. G. , Alvi N. S.
Published: July 2002

By using metallic haze to detect the presence of transition metals in silicon, and rapid thermal annealing to thermally process wafers, anomalous diffusion and gettering of transition metals in silicon have been observed. Gettering is observed over a wide temperature range (300 –1100 °C) and anneal duration (1–300 s). The rapid initial gettering is found to occur over mechanical and laser damaged areas but not over polycrystalline silicon backsealed regions. Surface diffusion is found to dominate over anomalous bulk diffusion at the lower temperatures investigated.
Low-temperature ion beam enhanced etching of tungsten films with xenon difluoride
Authors: Bensaoula A. , Ignatiev A. , Strozier J. , Wolfe J. C.
Published: July 2002

The spontaneous and ion enhanced etching of sputter deposited tungsten films has been studied. The etch rates have been measured as a function of various etch parameters with striking differences found as a function of sample temperatures. The spontaneous etch rate is found to be totally suppressed at temperatures below 170 K, and concomitantly the ion enhanced etch rate is increased. With these two points in mind, it is shown that ion beam enhanced etching of W with XeF2 at moderately low temperatures could be used to greatly improve the ratio of the ion enhanced etch rate over the spontaneous etch rate. This unique condition could be used to achieve high aspect ratio structures with vertical walls and opens up the possibility of direct ion beam pattern definition.
Low-temperature chemical vapor deposition of SiO2 at 2–10 Torr
Authors: Bennett B. R. , Lorenzo J. P. , Vaccaro K.
Published: July 2002

We discuss a new low-pressure and low-temperature process for the chemical vapor deposition (CVD) of silicon dioxide. The process differs from conventional low-pressure CVD in that lower temperatures (150–300 °C) and a unique pressure window (2–10 Torr) provide the conditions for the reaction of silane (SiH4) and oxygen. In this thermal process, activation energies of 0.15–0.18 eV and deposition rates of 100 Å/min at 250 °C are achieved. This technique is approximately 15 times less sensitive to the O2:SiH4 ratio than atmospheric pressure CVD. The deposition conditions are compatible with both low-temperature silicon and III-V technologies. Preliminary current-voltage and capacitance-voltage measurements on Si indicate dielectric field strength of 3–8×106 V/cm and fixed oxide charge density (Qss) less than 1011 cm−2.
Secondary ion mass spectrometry study of oxygen accumulation at GaAs/AlGaAs interfaces grown by molecular beam epitaxy
Authors: Achtnich T. , Burri G. , Py M. A. , Ilegems M.
Published: July 2002

The accumulation of oxygen at GaAs/AlGaAs interfaces grown by molecular beam epitaxy has been established by secondary ion mass spectrometry profiling of GaAs/AlGaAs multilayer structures. An enhanced oxygen peak was observed at the boundary between GaAs and AlxGa1−xAs layers with x=0.35 and x=1 when the binary layer is deposited on top of the ternary layer. The segregation of oxygen may be a contributing factor responsible for the lower luminescence reported in the first GaAs well of multilayer quantum well structures and for the difference between normal and inverted interface high electron mobility devices.
Nonlinear absorption in AlGaAs/GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition
Authors: Lee H. C. , Hariz A. , Dapkus P. D. , Kost A. , Kawase M. , Garmire E.
Published: July 2002

We report the study of growth conditions for achieving the sharp exciton resonances and low intensity saturation of these resonances in AlGaAs-GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition. Low growth temperature is necessary to observe this sharp resonance feature at room temperature. The optimal growth conditions are a tradeoff between the high temperatures required for high quality AlGaAs and low temperatures required for high-purity GaAs. A strong optical saturation of the excitonic absorption has been observed. A saturation density as low as 250 W/cm2 is reported.
Electric field induced refractive index changes in GaAs-AlxGa1−xAs quantum wells
Authors: Hiroshima Tohya
Published: July 2002

The refractive index changes near the excitonic absorption edge in a GaAs-AlxGa1−xAs quantum well structure due to an external electric field have been calculated. Calculated maximum variation in the refractive index is approximately −0.07 for an electric field of 80 kV/cm for both a 60-Å-thick and a 100-Å-thick GaAs-Al0.4Ga0.6As quantum wells. Also calculated results indicate that the forbidden excitonic transition, which comes to be allowed in the presence of an external electric field, has much greater effect for relatively thick quantum wells.
Stress analysis of encapsulated fine-line aluminum interconnect
Authors: Jones Robert E. , Basehore Michael L.
Published: July 2002

Mechanical stresses are known to cause hillock formation and creep voids in aluminum based interconnect on integrated circuits. While the effects of differential thermal expansion between an aluminum film and a silicon substrate are well known, the effects on an encapsulated narrow aluminum line are not. We report here an analysis of the stresses generated in such a line upon cooling from 400 °C after passivation deposition. At 25 °C the principal stresses in the aluminum are all tensile with magnitudes several times the yield strength. The aluminum need not fail, but a high driving force for creep remains. High stresses also are generated in the passivation which could lead to cracking.
Role of atomic tellurium in the growth kinetics of CdTe (111) homoepitaxy
Authors: Cheung J. T.
Published: July 2002

Very high surface temperature can be reached by exposing CdTe to intense laser radiation. Under this condition, the evaporation is dissociative to form a mixture of Cd, Te, and Te2. The fraction of atomic Te can be varied by changing the laser power, thus allowing a systematic elucidation on the role of atomic Te in the growth kinetics of (111) CdTe homoepitaxy. In comparison with Te molecules, the use of Te atoms improves the surface morphology of layers grown on the (111)A face and enhances the growth rate on both the (111)A and (111)B faces. Results suggest that the atomic Te is more reactive.
Atomic layer epitaxy of the Ga-As-In-As superalloy
Authors: McDermott B. T. , El-Masry N. A. , Tischler M. A. , Bedair S. M.
Published: July 2002

Ga-As-In-As superalloy has been grown by atomic epitaxy on InP substrates. This has been achieved by sequential exposure of the substrate to trimethylgallium, arsine, triethylindium, and arsine. The thickness of the deposited film is in excellent agreement with the predicted value based on the number of exposure cycles. These results demonstrate that atomic layer epitaxy offers the ultimate control for depositing thin films. The superalloy films have been characterized by transmission electron microscopy and photoluminescence.
Interface disorder in GaAs/AlGaAs quantum wells grown by molecular beam epitaxy on 0.5°-misoriented (111)B substrates
Authors: Hayakawa T. , Kondo M. , Morita T. , Takahashi K. , Suyama T. , Yamamoto S. , Hijikata T.
Published: July 2002

The interface disorder in quantum wells (QW’s) grown by molecular beam epitaxy on 0.5°-misoriented (111)B GaAs substrates was characterized by low-temperature photoluminescence and by transmission electron microscopy. It was found that the abruptness of the heterointerface of (111)B QW’s is as sharp as that of (100) QW’s. The effect of growth interruption at a high substrate temperature of 720 °C on the interface disorder was also studied and was found to be detrimental.
Resonant tunneling through single layer heterostructures
Authors: Liu H. C.
Published: July 2002

A new resonant tunneling process is discussed theoretically. The process relies on elastic intervalley transfers between different band minima, e.g., between Γ and X minima in a GaAs-AlAs system. Single layer GaAs-AlAs-GaAs heterostructures are analyzed. An effective mass envelope function approach is used, and a delta-function transfer potential at heterointerfaces is employed. A resonance in the transmission coefficient is clearly seen, which gives rise to a negative differential resistance region in the current-voltage characteristic.
Microstructural characterization of patterned gallium arsenide grown on 〈001〉 silicon substrates
Authors: Matyi R. J. , Shichijo H. , Moore T. M. , Tsai H-L.
Published: July 2002

The microstructure of patterned GaAs grown on Si substrates by molecular beam epitaxy has been examined with both transmission and scanning electron microscopies. The GaAs was found to be single crystal with excellent morphology to the limit of the plasma oxide defining mask. In samples where the native oxide was not completely desorbed from the silicon substrate, the GaAs surface morphology was observed to degrade significantly within 20 μm of the single crystal to polycrystalline transition. Even in the region exhibiting a high density of surface defects, the underlying GaAs remained single crystal. Transmission electron microscopy showed a very low defect density in the center of the patterned growth region. The transition from polycrystalline to single-crystal growth occurred directly above the termination point of the oxide mask.
Evidence of the role of defects near the injecting interface in determining SiO2 breakdown
Authors: Olivo P. , Ricco B. , Nguyen Thao N. , Kuan T. S. , Jeng S. J.
Published: July 2002

Experimental results on oxide breakdown in thin insulator metal-oxide-semiconductor structures are presented to show that at a microscopic level breakdown is related to defects located near the injecting interface. In addition, breakdown is found to be almost independent of electron fluence.
Elimination of interface defects in mismatched epilayers by a reduction in growth area
Authors: Fitzgerald E. A. , Kirchner P. D. , Proano R. , Pettit G. D. , Woodall J. M. , Ast D. G.
Published: July 2002

We have eliminated interface defects from the mismatched In0.05Ga0.95As/ (001)GaAs interface by controlling the size of the growth area. 2-μm-high pillars with different lateral shapes and dimensions were defined within the GaAs substrate before the molecular beam epitaxial growth of 3500 Å of In0.05Ga0.95As, greater than four times the critical thickness. On the pillars, the linear density of misfit dislocations was reduced from >5000 dislocations/cm for large (several hundred μm lateral dimensions) growth areas to nearly zero for 25 μm lateral dimensions. The dislocation density remains less than 800 dislocations/cm for lateral dimensions up to 100 μm. We find that there is also a decrease in dislocation density in narrow channels between the pillars; therefore, the pillars also block the glide of misfit dislocations.
High-mobility variable-density two-dimensional electron gas in inverted GaAs-AlGaAs heterojunctions
Authors: Meirav U. , Heiblum M. , Stern Frank
Published: July 2002

Inverted heterointerfaces (GaAs on AlGaAs), which are basic constituents of all quantum wells and superlattices, have been significantly improved using electron diffraction and a refined molecular beam epitaxy growth procedure. Utilizing them in a novel structure allowed the variation of the electron density over a wide range, with peak mobilities of 4×105 cm2/V s. The continuously variable electron density allowed comparison to a theoretical analysis of the low-temperature scattering mechanisms, and their relation to the growth process, establishing the importance of interface charges and roughness. High-mobility samples were used to observe the quantum Hall effect with varying carrier concentrations in a single structure.
Correlation between electron trap density and hydrogen concentration in ultrathin rapidly reoxidized nitrided oxides
Authors: Hori Takashi , Iwasaki Hiroshi , Yoshioka Yoshiaki , Sato Minoru
Published: July 2002

Ultrathin nitrided oxides (8 nm) were reoxidized for the first time by lamp-heated rapid thermal annealing in dry oxygen at 900–1150 °C for 15–200 s. Extensive measurements of hydrogen concentration [H] using secondary ion mass spectroscopy were performed. Electron trapping was monitored by flatband voltage shift ΔVfb under high-field stress. Both [H] and the ΔVfb decrease monotonically as reoxidation proceeds in the present experimental conditions. We have found for the first time that the ΔVfb is correlated with the [H] by a single proportional relation regardless of the fabrication condition. This fact gives evidence that the electron traps in the (reoxidized) nitrided-oxide system originate from hydrogen-containing species.
Origin and reduction of interfacial boron spikes in silicon molecular beam epitaxy
Authors: Iyer S. S. , Delage S. L. , Scilla G. J.
Published: July 2002

An interfacial boron spike is formed during the molecular beam epitaxial growth of Si. We show two possible sources for this unintentional spike. We have found that some boron contamination invariably occurs when silicon surfaces are exposed to air. A greater degree of contamination results when the sample is heated to temperatures greater than 800 °C, as required for creating an atomically clean surface prior to molecular beam epitaxial growth. A source of boron suboxides, internal to the ultrahigh-vacuum system, was detected by residual gas analysis. While anneals at 1000 °C or greater result in almost complete activation of the B, we observe that for a cleaning regimen at 850 °C, less than 10% of the boron is active. Our results are consistent with the oxidation of the suboxides on oxygen-contaminated surfaces and their subsequent reduction at higher temperatures by silicon, with the volatization of SiO. Subsequent incorporation is by indiffusion.
Effect of doping on the optical gain and the spontaneous noise enhancement factor in quantum well amplifiers and lasers studied by simple analytical expressions
Authors: Vahala Kerry J. , Zah C. E.
Published: July 2002

The maximum optical gain and the spontaneous noise enhancement factor in quantum well structures are expressed as extremely simple functions that are accurate over a wide range of carrier densities. These expressions are used to study the effect of doping on the optical gain and the noise enhancement factor in a 100 Å InGaAs/InP quantum well structure. n-type doping is most effective in reducing the transparency excitation level (laser threshold) and the noise enhancement factor (amplifier noise figure), whereas p-type doping enables increased gain at a given excitation level.
Resonant tunneling in Si/Si1−xGex double-barrier structures
Authors: Liu H. C. , Landheer D. , Buchanan M. , Houghton D. C.
Published: July 2002

Resonant tunneling of holes has been observed for the first time in double-barrier diodes with strained Si1−xGex quantum wells formed between unstrained Si barriers. Negative differential resistance with a peak-to-valley ratio in current of 1.8 at 77 K and of 2.2 at 4.2 K has been exhibited by a sample with a 3.3-nm-wide Si0.79Ge0.21 well between 6.0 nm Si barriers. The positions of the current peaks are in reasonable agreement with calculations of the positions of heavy-hole levels in the quantum well.
High T
c YBa2Cu3O7−x thin films on Si substrates by dc magnetron sputtering from a stoichiometric oxide target
Authors: Lee W. Y. , Salem J. , Lee V. , Huang T. , Savoy R. , Deline V. , Duran J.
Published: July 2002

Thin films of YBa2Cu3O7−x were deposited on Si substrates at 600–700 °C by dc magnetron sputtering from a stoichiometric oxide target. Resistivity measurement results indicate that these films are superconducting with a zero resistance Tc as high as 76 K, without further high-temperature post-annealing treatments. These films give both core and valence-band x-ray photoemission, and x-ray diffraction spectra similar to those for superconducting films prepared with a high-temperature post-annealing step. No significant diffusion of Si from the substrate into the film was detected for the films deposited at 650 °C or lower, according to depth profiles obtained using secondary ion mass spectrometry.
Noninvasive optical probe of free charge and applied voltage in GaAs devices
Authors: Keller U. , Diamond S. K. , Auld B. A. , Bloom D. M.
Published: July 2002

We have demonstrated an application of a noninvasive optical probe that can independently measure both free sheet charge density and applied voltage in GaAs devices. Large-signal direct-current measurements of voltage and charge were made on a Schottky diode without any assumption of the device parameters. The measurements are reproducible, and no adjustable parameters have been used to quantitatively measure charge and voltage. In addition to a lateral charge resolution, given by the beam spot size, we have observed a longitudinal resolution due to the standing wave in the probe beam.
Low-temperature silicon epitaxy by low-energy bias sputtering
Authors: Ohmi T. , Matsudo K. , Shibata T. , Ichikawa T. , Iwabuchi H.
Published: July 2002

Bias sputtering at low energies, i.e., comparable to typical crystal interatomic binding energies, has been utilized to control the kinetics of thin-film growth. It was found that the crystallographic structures of sputter-deposited silicon films are drastically changed by the energy of ions incident at the substrate. As a result, formation of high quality epitaxial silicon films on (100) silicon substrates has been realized at such low temperatures as 320–350 °C. At the same time, low-temperature impurity doping of the epitaxial layer has been also demonstrated. Furthermore, the low-energy bias sputtering process has made it possible to perform very effective substrate surface cleaning at extremely low temperatures without introducing any damage to the substrate.
High-detectivity D*=1.0×1010 cm √H̄z̄/W GaAs/AlGaAs multiquantum well λ=8.3 μm infrared detector
Authors: Levine B. F. , Bethea C. G. , Hasnain G. , Walker J. , Malik R. J.
Published: July 2002

We report the first high-detectivity (D*=1.0×1010 cm (Hz)1/2/W), high-responsivity (Rv =30 000 V/W) GaAs/AlxGa1−xAs multiquantum well detector, sensitive in the long-wavelength infrared band at λ=8.3 μm (operating at a temperature of T= 77 K). Because of the mature GaAs growth and processing technologies as well as the potential for monolithic integration with high-speed GaAs field-effect transistors, large focal plane arrays of these detectors should be possible.
Unified model for impurity diffusion in silicon
Authors: Orlowski M.
Published: July 2002

A new theoretical framework for impurity diffusion in silicon is proposed. The basic mechanism employed here is the point defect-impurity pair diffusion as presented by Mulvaney and Richardson [Appl. Phys. Lett. 51, 1439 (1987)] in a generalized description of the impurity-interstitial model by Morehead and Lever [Appl. Phys. Lett. 48, 151 (1986)]. The model consists of coupled equations for the impurities and point defects, in which all species including structural defects (major new process variables) are treated on the same footing. Among other things, the model accounts for long-range point defect mediated enhancement and retardation of the diffusion. The essential features of the present formalism are the new equations for interstitials and vacancies which provide the major coupling between the impurities apart from the coupling via the Fermi level. This approach allows, for the first time, a consistent analysis and exploration of the diffusion phenomena step by step on various levels of complexity.
T-DNA integration into the barley genome from single and double cassette vectors
Authors: Stahl Rainer , Horvath Henriette , Van Fleet Jennifer , Voetz Michael , von Wettstein Diter , Wolf Norbert
Published: July 2002

Patterns and sites of T-DNA integrations into the barley genome from single and double cassette vectors are of interest for the identification of cultivars with value added properties as well as for the production of selection marker-free transgenic lines that can be retransformed. T-DNA/Plant DNA junctions were obtained by capturing a single-stranded DNA with a biotinylated primer annealing to the vector adjacent to the border and an adaptor ligated to a restriction site overhang in the flanking barley DNA. The captured junction was converted into a double strand and sequenced. Fifty left and right border junctions from plants transgenic for one of five human genes were analyzed. Primers of 15–30 nucleotides designed from the genomic DNA at the insertion site can PCR amplify fragments that identify unequivocally any transformant. Adjacent transgene insertions with single cassette vectors were always in tandem direct repeat configuration. With regard to T-DNA integration the patterns were comparable to the variations found in dicotyledonous plants. Twelve of the 46 integrations characterized by
blast
searches were within different regions of the
BARE
-
1
retrotransposon element occurring with a frequency of 2 × 10
5
copies in the barley genome. The use of border junctions to identify number of copies and loci of integrates in transformants is discussed.
Computational method to reduce the search space for directed protein evolution
Authors: Voigt Christopher A. , Mayo Stephen L. , Arnold Frances H. , Wang Zhen-Gang
Published: July 2002

We introduce a computational method to optimize thein vitroevolution of proteins. Simulating evolution with a simple model that statistically describes the fitness landscape, we find that beneficial mutations tend to occur at amino acid positions that are tolerant to substitutions, in the limit of small libraries and low mutation rates. We transform this observation into a design strategy by applying mean-field theory to a structure-based computational model to calculate each residue's structural tolerance. Thermostabilizing and activity-increasing mutations accumulated during the experimental directed evolution of subtilisin E and T4 lysozyme are strongly directed to sites identified by using this computational approach. This method can be used to predict positions where mutations are likely to lead to improvement of specific protein properties.
Utility and distribution of conserved noncoding sequences in the grasses
Authors: Kaplinsky Nicholas J. , Braun David M. , Penterman Jon , Goff Stephen A. , Freeling Michael
Published: July 2002

Control of gene expression requires cis-acting regulatory DNA sequences. Historically these sequences have been difficult to identify. Conserved noncoding sequences (CNSs) have recently been identified in mammalian genes through cross-species genomic DNA comparisons, and some have been shown to be regulatory sequences. Using sequence alignment algorithms, we compared genomic noncoding DNA sequences of the
liguleless1
(
lg1
) genes in two grasses, maize and rice, and found several CNSs in
lg1.
These CNSs are present in multiple grass species that represent phylogenetically disparate lineages. Six other maize/rice genes were compared and five contained CNSs. Based on nucleotide substitution rates, these CNSs exist because they have biological functions. Our analysis suggests that grass CNSs are smaller and far less frequent than those identified in mammalian genes and that mammalian gene regulation may be more complex than that of grasses. CNSs make excellent pan-grass PCR-based genetic mapping tools. They should be useful as characters in phylogenetic studies and as monitors of gene regulatory complexity.
14-3-3 protein is a regulator of the mitochondrial and chloroplast ATP synthase
Authors: Bunney Tom D. , van Walraven Hendrika S. , de Boer Albertus H.
Published: July 2002

Mitochondrial and chloroplast ATP synthases are key enzymes in
plant metabolism, providing cells with ATP, the universal energy
currency. ATP synthases use a transmembrane electrochemical proton
gradient to drive synthesis of ATP. The enzyme complexes function as
miniature rotary engines, ensuring energy coupling with very high
efficiency. Although our understanding of the structure and functioning
of the synthase has made enormous progress in recent years, our
understanding of regulatory mechanisms is still rather preliminary.
Here we report a role for 14-3-3 proteins in the regulation of ATP
synthases. These 14-3-3 proteins are highly conserved
phosphoserine/phosphothreonine-binding proteins that regulate a wide
range of enzymes in plants, animals, and yeast. Recently, the presence
of 14-3-3 proteins in chloroplasts was illustrated, and we show here
that plant mitochondria harbor 14-3-3s within the inner
mitochondrial-membrane compartment. There, the 14-3-3 proteins were
found to be associated with the ATP synthases, in a
phosphorylation-dependent manner, through direct interaction with the
F
1
β-subunit. The activity of the ATP synthases in both
organelles is drastically reduced by recombinant 14-3-3. The rapid
reduction in chloroplast ATPase activity during dark adaptation was
prevented by a phosphopeptide containing the 14-3-3 interaction motif,
demonstrating a role for endogenous 14-3-3 in the
down-regulation of the CF
o
F
1
activity. We
conclude that regulation of the ATP synthases by 14-3-3 represents a
mechanism for plant adaptation to environmental changes such as
light/dark transitions, anoxia in roots, and fluctuations in nutrient
supply.
The 1.9-Å crystal structure of the noncollagenous (NC1) domain of human placenta collagen IV shows stabilization via a novel type of covalent Met-Lys cross-link
Authors: Than Manuel E. , Henrich Stefan , Huber Robert , Ries Albert , Mann Karlheinz , Kühn Klaus , Timpl Rupert , Bourenkov Gleb P. , Bartunik Hans D. , Bode Wolfram
Published: July 2002

Triple-helical collagen IV protomers associate through their N- and C-termini forming a three-dimensional network, which provides basement membranes with an anchoring scaffold and mechanical strength. The noncollagenous (NC1) domain of the C-terminal junction between two adjacent collagen IV protomers from human placenta was crystallized and its 1.9-Å structure was solved by multiple anomalous diffraction (MAD) phasing. This hexameric NC1 particle is composed of two trimeric caps, which interact through a large planar interface. Each cap is formed by two α1 fragments and one α2 fragment with a similar previously uncharacterized fold, segmentally arranged around an axial tunnel. Each monomer chain folds into two structurally very similar subdomains, which each contain a finger-like hairpin loop that inserts into a six-stranded β-sheet of the neighboring subdomain of the same or the adjacent chain. Thus each trimer forms a quite regular, but nonclassical, sixfold propeller. The trimer–trimer interaction is further stabilized by a previously uncharacterized type of covalent cross-link between the side chains of a Met and a Lys residue of the α1 and α2 chains from opposite trimers, explaining previous findings of nonreducible cross-links in NC1. This structure provides insights into NC1-related diseases such as Goodpasture and Alport syndromes.
Analysis of cell-cycle-specific gene expression in human cells as determined by microarrays and double-thymidine block synchronization
Authors: Shedden Kerby , Cooper Stephen
Published: July 2002

Microarray analysis of gene expression patterns for thousands of human genes has led to the proposal that a large number of genes are expressed in a cell-cycle-specific manner. The identification of cyclically expressed genes was based on Affymetrix microarray analysis of gene expression after double-thymidine block synchronization. A statistical reanalysis of the original data leads to three principal findings. (
i
) Randomized data exhibit periodic patterns of similar or greater strength than the experimental data. This finding suggests that all apparent cyclicities in the expression measurements may arise from chance fluctuations. (
ii
) The presence of cyclicity and the timing of peak cyclicity in a given gene are not reproduced in two replicate experiments. This fact suggests there is an uncontrolled source of experimental variation that is stronger than the innate variation of gene expression in cells over time. (
iii
) The amplitude of peak expression in the second cycle is not consistently smaller than the corresponding amplitude in the first cycle. This finding places doubt on the assumption that the cells are actually synchronized. We propose that the microarray results do not support the proposal that there are numerous cell-cycle-specifically expressed genes in human cells.
Dissociation of peripheral T cell responses from thymocyte negative selection by weak agonists supports a spare receptor model of T cell activation
Authors: McNeil Lisa K. , Evavold Brian D.
Published: July 2002

We have focused on stability of the peptide-MHC complex as a determining factor of ligand potency for thymocytes and peripheral CD4+T cell responses. MHC variant peptides that have low affinities and fast dissociation rates are different in that they stimulate proliferation and cytolysis of mature T cells (classifying the variant peptides as weak agonists) but do not induce thymocyte negative selection. The MHC variant weak agonists require significant receptor reserve, because decreasing the level of T cell receptor on mature T cells blocks the proliferative response. These results demonstrate that peripheral T cells are more sensitive to MHC variant ligands by virtue of increased T cell receptor expression; in addition, the data support a T cell model of the spare receptor theory.
Controlling small guanine–nucleotide-exchange factor function through cytoplasmic RNA intramers
Authors: Mayer Günter , Blind Michael , Nagel Wolfgang , Böhm Thomas , Knorr Thomas , Jackson Catherine L. , Kolanus Waldemar , Famulok Michael
Published: July 2002

ADP-ribosylation factor (ARF) GTPases and their regulatory
proteins have been implicated in the control of diverse biological
functions. Two main classes of positive regulatory elements for ARF
have been discovered so far: the large Sec7/Gea and the small
cytohesin/ARNO families, respectively. These proteins harbor
guanine–nucleotide-exchange factor (GEF) activity exerted by the
common Sec7 domain. The availability of a specific inhibitor, the
fungal metabolite brefeldin A, has enabled documentation of the
involvement of the large GEFs in vesicle transport. However, because of
the lack of such tools, the biological roles of the small GEFs have
remained controversial. Here, we have selected a series of RNA aptamers
that specifically recognize the Sec7 domain of cytohesin 1. Some
aptamers inhibit guanine–nucleotide exchange on ARF1, thereby
preventing ARF activation
in vitro
. Among them, aptamer
M69 exhibited unexpected specificity for the small GEFs, because it
does not interact with or inhibit the GEF activity of the related
Gea2-Sec7 domain, a member of the class of large GEFs. The inhibitory
effect demonstrated
in vitro
clearly is observed as well
in vivo
, based on the finding that M69 produces similar
results as a dominant-negative, GEF-deficient mutant of cytohesin 1:
when expressed in the cytoplasm of T-cells, M69 reduces stimulated
adhesion to intercellular adhesion molecule-1 and results in a dramatic
reorganization of F-actin distribution. These highly specific cellular
effects suggest that the ARF-GEF activity of cytohesin 1 plays an
important role in cytoskeletal remodeling events of lymphoid cells.