bluuebunny/crossref_metadata_2025 huggingface dataset:
Some effects of localized injection on the turbulence structure in a boundary layer
Authors: Krogstad Per-Åge , Kourakine Anatoli
Published: July 2002

The effects of localized injection through a porous strip on a turbulent boundary layer at zero pressure gradient conditions were examined experimentally. The magnitude of the injection velocity was kept very small (less than 1% of the free-stream velocity) to prevent separation near the injection strip and to keep the perturbations small. It was found that the injection increases all the Reynolds stresses and that this perturbation dies out very slowly as the affected layer is sandwiched between the outer edge of the incoming boundary layer and a new layer that develops at the wall. A study of the anisotropy tensor indicated no effects of the blowing rate on the flow anisotropy downstream of the injection region, although a quadrant analysis showed that the contributions to the shear stresses from the various quadrants were affected.
Domain cooperation in ferromagnetic hysteresis
Authors: Schneider Carl S.
Published: July 2002

We present precise experimental data suggesting that domains cooperate in ferromagnetic hysteresis. This model has potential applications in magnetostrictive transducers, transformer design, magnetoelastic effects and hysteresis in general. For an annealed 3% nickel steel alloy, we measured magnetic induction from the demagnetized state to near saturation, the reversal to near saturation and return branches from both these curves. Modeling variables include the coercive field, saturation magnetization, initial susceptibility and the hysteresis effective field. Magnetization is scaled to its saturation value and magnetic field to its coercive value in the direction of field change, and susceptibility is scaled to its reversible function. Scaled differential susceptibility increases from its reversible value exponentially with magnetization change from any reversal and decreases exponentially with decreasing slope to its reversible value at saturation. We define the slope of this exponential to be a hysteresis constant divided by the scaled effective field, and this dimensionless field is the square root of 2 for hysteresis loops of polycrystalline domains with fully nucleated walls. Rayleigh’s hysteresis constant, valid for heavily worked steels over a small region, transforms to our cooperative hysteresis constant at larger fields. For the saturate curve, the scaled effective field reaches a minimum less than 1 near the coercive field as domain interactions increase through wall nucleation and their local fields. Variation in the effective field causes hysteresis loops to creep upon cycling.
Triggering and guiding high-voltage large-scale leader discharges with sub-joule ultrashort laser pulses
Authors: Pépin H. , Comtois D. , Vidal F. , Chien C. Y. , Desparois A. , Johnston T. W. , Kieffer J. C. , La Fontaine B. , Martin F. , Rizk F. A. M. , Potvin C. , Couture P. , Mercure H. P. , Bondiou-Clergerie A. , Lalande P. , Gallimberti I.
Published: July 2002

The triggering and guiding of leader discharges using a plasma channel created by a sub-joule ultrashort laser pulse have been studied in a megavolt large-scale electrode configuration (3–7 m rod-plane air gap). By focusing the laser close to the positive rod electrode it has been possible, with a 400 mJ pulse, to trigger and guide leaders over distances of 3 m, to lower the leader inception voltage by 50%, and to increase the leader velocity by a factor of 10. The dynamics of the breakdown discharges with and without the laser pulse have been analyzed by means of a streak camera and of electric field and current probes. Numerical simulations have successfully reproduced many of the experimental results obtained with and without the presence of the laser plasma channel.
Formation of Liesegang patterns: Simulations using a kinetic Ising model
Authors: Antal T. , Droz M. , Magnin J. , Pekalski A. , Rácz Z.
Published: July 2002

A kinetic Ising model description of Liesegang phenomena is studied using Monte Carlo simulations. The model takes into account thermal fluctuations, contains noise in the chemical reactions, and its control parameters are experimentally accessible. We find that noisy, irregular precipitation takes place in dimension d=2 while, depending on the values of the control parameters, either irregular patterns or precipitation bands satisfying the regular spacing law emerge in d=3.
Properties of carbon-doped GaN
Authors: Tang H. , Webb J. B. , Bardwell J. A. , Raymond S. , Salzman Joseph , Uzan-Saguy C.
Published: July 2002

The properties of carbon-doped GaN epilayers grown by molecular-beam epitaxy have been studied by temperature-dependent resistivity, Hall-effect measurements, x-ray diffraction, and by photoluminescence spectroscopy. Carbon doping was found to render the GaN layers highly resistive (>108 Ω cm) and quench the band edge excitonic emissions. Yellow luminescence is still present in carbon-doped GaN layers. The highly resistive state is interpreted as being caused by direct compensation by the carbon acceptors and by the consequently enhanced potential barrier at the subgrain boundaries. Evidence of dislocations joining to form potential barriers along the subgrain boundaries was observed in photoassisted wet etching experiments on electrically conducting GaN layers. GaN films grown on insulating carbon-doped base layers are of excellent transport and optical properties.
Radiographic and spectroscopic studies of X-pinch plasma implosion dynamics and x-ray burst emission characteristics
Authors: Shelkovenko T. A. , Sinars D. B. , Pikuz S. A. , Hammer D. A.
Published: July 2002

Experimental results are presented from studies of the dynamics of X-pinch plasmas, formed using two fine wires that cross and touch at a single point (in the form of an X) as the load of a high current pulser. High-resolution (sub-ns in time and 2–3 μm in space) x-ray radiographs of X pinches driven by current pulses that rise to 200–250 kA peak current in 40 ns show that ⩽300 μm long Z pinches form in the region of the original wire cross-point a few ns prior to the first sub-ns intense x-ray bursts that are characteristic of an X pinch. The Z pinches implode to ⩽10 μm diam and then appear to develop gaps in the radiographic images in one or two places, coincident in time with the x-ray burst(s). The emission spectra of the intense x-ray bursts from different wire materials indicate electron temperatures of 500–1300 eV and densities in excess of 1022/cm3.
Integrated multi-scale model for ionized plasma physical vapor deposition
Authors: Arunachalam V. , Rauf S. , Coronell D. G. , Ventzek P. L. G.
Published: July 2002

In order to aid process development and address extendibility of ionized physical vapor deposition (IPVD) technology to future integrated circuit generations, an integrated model capable of simulating phenomena across the various length scales characteristic of these systems has been developed. The model is comprised of a two-dimensional equipment simulation, which relates process variables to characteristics of material fluxes to the wafer, and a three-dimensional Monte Carlo based feature scale model. The ion-surface interaction data required to model the surface processes is generated by a molecular dynamics based simulation. The integrated model is used to study the effect of various IPVD process parameters such as wafer bias, coil power, target power, and buffer gas composition on copper film profile inside a trench. Variations in film profile across the wafer are also examined. It is found that increasing the wafer bias results in an increase in the mean ion energy and the amount of sputtering inside the feature. This results in material transfer from the bottom of the feature to the sidewalls and faceting of the upper corners of the trench. Two variables, namely the total ion to Cu flux ratio (RI/N) and the mean ion energy, are found to play a crucial role in determining the effects of coil power and target power. Increasing the coil power enhances RI/N and slightly decreases the mean ion energy. This leads to more sputtering, and therefore a thicker film on the sidewalls relative to that on the bottom. Increase in target power causes RI/N to decrease, which decreases sputtering within the feature. Film profiles generally show evidence of enhanced sputtering as buffer gas ionization threshold decreases (He→Ne→Ar→Xe) for the gases considered. These variations can be explained in terms of two factors: Cu flux ionization fraction, which decreases with buffer gas ionization threshold, and mean ion energy, which increases with ionization threshold.
Hydrogen bonding in benzonitrile–water complexes
Authors: Kryachko Eugene S. , Nguyen Minh Tho
Published: July 2002

The hydrogen bonding between benzonitrile and one and two water molecules is studied ab initio by means of a hybrid B3LYP density functional in conjunction with 6-31+G(d,p) and aug-cc-pVDZ basis sets. The strength of such hydrogen bonding is analyzed in terms of a magnitude of “window region” appearing in the infrared spectra of the studied complexes and experimentally measurable and also in terms of the shifts of the stretching mode of the triple CN bond of benzonitrile. The lower-energy portions of the potential energy surfaces in the neighborhoods of the stable complexes of benzonitrile with one and two water molecules are fully determined, and implications of their landscape on the large-amplitude dynamics of the water moiety around benzonitrile and comparison with experiments are elaborated.
Onset of slip in silicon containing oxide precipitates
Authors: Jurkschat K. , Senkader S. , Wilshaw P. R. , Gambaro D. , Falster R. J.
Published: July 2002

We report a study of the behavior of dislocations at oxide precipitates in (001) Czochralski silicon wafers for different oxide-precipitate sizes (100–600 nm), densities (108−1011 cm−3), and background oxygen concentrations (7.7×1017−10.35×1017 cm−3) using a bending technique with annular knife edges causing a biaxial stress distribution in the samples. The main advantage of the method we use is the possibility of detecting single slip events that may be caused by precipitates with special properties. We found that the stress level at which dislocation movement can be detected around precipitates depends mainly on the mean-precipitate diameter. The stress threshold at which dislocations begin to move can be increased by a thermal treatment prior to application of an external stress. This effect is due to the diffusion of oxygen to the dislocations causing a locking effect and shows that the dislocations are associated with the oxide precipitates prior to any external stress being applied. It has been shown that such heat treatments can lead to a mechanical strengthening of the wafers in certain circumstances.
Thin films of the spinel Cd1+xIn2−2xSnxO4 transparent conducting oxide solution
Authors: Kammler D. R. , Mason T. O. , Young D. L. , Coutts T. J.
Published: July 2002

Thin films of a transparent conducting oxide solid solution Cd1+xIn2−2xSnxO4 (x=0.15, 0.45, and 0.70) were deposited via rf magnetron sputtering. X-ray diffraction indicated the films consisted of a polycrystalline spinel phase. Atomic force microscopy measurements revealed a surface root mean square roughness between 1.3 and 6.0 nm. Optical absorption was 10% or less in the visible for x=0.15, 0.45, and 0.70. Optical gaps averaged near 3.5, 3.70, and 3.65 eV for films annealed in Ar/CdS of compositions corresponding to x=0.15, 0.45, and 0.70. Conductivity exceeded 2000 S/cm for x=0.15 and 4000 S/cm for x=0.45 and 0.70. Mobilities of 43, 50, and 56 cm2/V s were measured for films annealed in Ar/CdS of compositions corresponding to x=0.15, 0.45, and 0.70, respectively. Composition data obtained via electron probe microanalysis indicate the films are becoming Cd deficient during the annealing process. This suggests an excess of In+3 and/or Sn+4 on Cd+2 sites may play a role in carrier production in these films. The Cd volatilization may also inhibit crystallization and decrease mobility.
Current deep-level transient spectroscopy investigation of acceptor levels in Mg-doped GaN
Authors: Nakano Yoshitaka , Kachi Tetsu
Published: July 2002

The current deep-level transient spectroscopy (I-DLTS) technique was used to investigate acceptor levels in Mg-doped GaN grown by metalorganic vapor phase epitaxy. For activation of the Mg dopants, rapid thermal annealing was performed with a SiO2 encapsulation layer at 850 °C in N2. I-DLTS measurements on the Schottky diode fabricated revealed a discrete deep level located ∼112 meV above the valence band, corresponding to the energy level measured by conventional thermal admittance spectroscopy. This energy level is also in good agreement with the frequency dependence of the capacitance in view of the characteristic frequency. Therefore, this energy level can most probably be attributed to the Mg acceptor state itself.
Order-automorphisms of the set of bounded observables
Authors: Molnár Lajos
Published: July 2002

Let H be a complex Hilbert space and denote by Bs(H) the set of all self-adjoint bounded linear operators on H. In this article we describe the form of all bijective maps (no linearity or continuity is assumed) on Bs(H) which preserve the order ⩽ in both directions.
Double- to single-hump shape change of secondary electron emission curve for thermal SiO2 layers
Authors: Yu SeGi , Jeong Taewon , Yi Whikun , Lee Jeonghee , Jin Sunghwan , Heo Jungna , Kimb J. M. , Jeon D.
Published: July 2002

Secondary electron emission yields (SEEYs) were measured for silicon oxides which were thermally grown on doped silicon substrates. Generally, SEEY curves can be described by the so-called universal curve, i.e., one hump with a monotonic increase (decrease) before (after) the hump. However, we found that our thick oxide layers exhibited double-hump shaped SEEY curves instead of single-hump shaped curves. Additionally, we were able to change the shape of a SEEY curve with two humps to a curve with one hump, or vice versa, by varying the experimental parameters. This change in curve shape can be explained if we consider the competition between the oxide layer thickness and the electron’s penetration depth, the charge accumulation due to emission of secondary electrons, and charge traps created during thermal oxidation at the same time.
Detection of collapsed and ordered polymer structures by fluorescence resonance energy transfer in stiff homopolymers: Bimodality in the reaction efficiency distribution
Authors: Srinivas Goundla , Bagchi Biman
Published: July 2002

Distribution of fluorescence resonance energy transfer (FRET) efficiency between the two ends of a stiff homopolymer chain is calculated by Brownian dynamics simulations. We show that the various collapsed and/or ordered conformations (spherical, rod, and toroidal) of a stiff polymer chain can be effectively distinguished by using the fluorescence resonance energy transfer technique. In particular, FRET efficiency distribution is unimodal at low chain stiffness (where collapsed structure is disordered) but becomes bimodal with increasing chain stiffness, when ordered structures are formed. We have also presented a simple theoretical analysis for the evaluation of the structural parameters.
Energy growth of initial perturbations in two-dimensional gravitational jets
Authors: de Luca Luigi , Costa Michela , Caramiello Ciro
Published: July 2002

The potential for transient growth of small perturbations to plane liquid jets falling under gravity in a still gas atmosphere is studied extending concepts thus far exclusively applied to one phase shear flows. The relevant initial value problem in time is formulated keeping into account the effect of the liquid viscosity, besides those of surface tension and gravity. The numerical counterpart of the differential operator, obtained by means of a spectral discretization, is examined focusing attention on both eigenvalues and pseudospectra with the aim of highlighting its non-normal character. The trend of an opportunely defined energy of perturbations is computed as a function of time in different flow regimes. The far field behavior is always recovered according to the classical temporal modes analysis, whereas non-normality is shown to amplify perturbations energy over short time scales even in situations where the spectrum is entirely confined to the stable half-plane.
Hydrogen desorption properties of multiwall carbon nanotubes with closed and open structures
Authors: Lee Ho , Kang Youn-Seon , Kim Seoung-Hoe , Lee Jai-Young
Published: July 2002

The multiwall carbon nanotubes (MWNTs) synthesized by microwave plasma enhanced chemical vapor deposition in 0.1% CH4 (H2 dilution) reaction gas were shaped in curly structures with the blocked nanoholes and closed caps. On the contrary, the carbon nanotubes with different structures could be obtained in oxygen loaded reaction gas, showing aligned structures with connected holes and open caps. The hydrogen desorption properties of the carbon nanotubes (CNTs) with these closed and open structures were compared by thermal desorption technique. The MWNTs with closed structure desorbed hydrogen at two different temperature ranges such as 290–330 and ∼420 K, where the evolved hydrogen amount were ∼0.64 and ∼0.03 wt %, respectively. In case of aligned and open MWNTs, hydrogen as high as ∼1.97 wt % was released at ambient temperature. The hydrogen desorption of MWNTs with open and closed structure at ambient temperature showed the hydrogen desorption activation energies of −16.5 kJ/mol H2 and −18.5 kJ/mol H2, respectively. The high temperature hydrogen desorption observed only in CNTs with closed and highly defective structure was high desorption activation energy of −124.4 kJ/mol.
Magnetocaloric effect in bulk amorphous Pd40Ni22.5Fe17.5P20 alloy
Authors: Shen T. D. , Schwarz R. B. , Coulter J. Y. , Thompson J. D.
Published: July 2002

We have studied the temperature (T) and field (H) dependence of the magnetocaloric effect in a bulk amorphous Pd40Ni22.5Fe17.5P20 alloy. With decreasing T, and depending on H, the alloy exhibits superparamagnetic, field-induced ferromagnetic, and spin-glass behavior. The temperature-dependence of the magnetic entropy change, ΔS=S(H)−S(H=0), exhibits extreme values: a minimum at the superparamagnetic-to-ferromagnetic transition and a maximum at the ferromagnetic-to-spin-glass transition. At 80 K, and for H=50 kOe, the entropy change is −0.029 kB per Fe atom in the alloy. This value compares favorably with that measured in other crystalline and amorphous Fe-based alloys, but it is lower than that measured in some rare-earth elements.
Elucidation of the surface passivation role on the photoluminescence emission yield of silicon nanocrystals embedded in SiO2
Authors: López M. , Garrido B. , Garcı́a C. , Pellegrino P. , Pérez-Rodrı́guez A. , Morante J. R. , Bonafos C. , Carrada M. , Claverie A.
Published: July 2002

The ability of surface passivation to enhance the photoluminescence (PL) emission of Si nanocrystals in SiO2 has been investigated. No significant increase of the average nanocrystal size has been detected for annealings at 1100 °C between 1 min and 16 h. In contrast, the PL intensity steadily increases and reaches saturation after 3–4 h of annealing time. Such behavior shows an inverse correlation with the amount of Si dangling bonds (Pb centers) at the interface between Si nanocrystals and the SiO2 matrix. A postannealing at 450 °C in forming gas enhances the PL intensity and lifetime, due to a reduction in Pb concentration, without modifying the spectral shape of the PL emission.
Monte Carlo, density functional theory, and Poisson–Boltzmann theory study of the structure of an electrolyte near an electrode
Authors: Boda Dezsö , Fawcett W. Ronald , Henderson Douglas , Sokołowski Stefan
Published: July 2002

Monte Carlo (MC) and density functional theory (DFT) results are reported for an electrolyte, consisting of charged hard spheres of diameter 3 Å with the solvent modeled as a dielectric continuum, near a charged flat uniformly charged electrode. These results are more interesting than the earlier MC results of Torrie and Valleau [J. Chem. Phys. 73, 5807 (1980); J. Phys. Chem. 86, 3251 (1982)] for 4.25 Å spheres because the popular Gouy–Chapman (GC) theory is less successful for this system. The DFT results are in good agreement with the MC results. Both the MC and DFT results show particularly interesting features when the counterions are divalent. For such divalent counterions, the diffuse layer potential passes through a maximum magnitude, then declines, and ultimately has a sign that is opposite to that of the electrode charge. The consequences of this behavior are discussed. In contrast, the well-known GC theory consistently overestimates the magnitude of the diffuse layer potential, does not have any unusual behavior, and is in poor agreement with the simulation results.
Energy transient growth in the Taylor–Couette problem
Authors: Meseguer Álvaro
Published: July 2002

This work is devoted to the study of transient growth of perturbations in the Taylor–Couette problem due to linear mechanisms. The study is carried out for a particular small gap case and is mostly focused on the linearly stable regime of counter-rotation. The exploration covers a wide range of inner and outer angular speeds as well as axial and azimuthal modes. Significant transient growth is found in the regime of stable counter-rotation. The numerical results are in agreement with former analyses based on energy methods. Similarities with transient growth mechanisms in plane Couette flow and in Hagen–Poiseuille flow are discussed. This is reflected in the modulation of the basic circular Couette flow by the presence of azimuthal streaks as a result of the nonmodal growth of initial axisymmetric perturbations. This study might shed some light on the subcritical transition to turbulence which is found experimentally in Taylor–Couette flow when the cylinders rotate in opposite directions.
Optical recording using smectic layer rotation in ferroelectric liquid crystal
Authors: Nakayama Keizo , Ohtsubo Junji , Ozaki Masanori , Yoshino Katsumi
Published: July 2002

Optical recording in a ferroelectric liquid crystal using smectic layer rotation induced by the application of asymmetric voltage pulses has been proposed. This recording method is based on the temperature dependence of the rotation rate and the fact that the rate in the smectic A (SmA) phase is considerably smaller than that in the chiral smectic C (SmC*) phase. The transition from the SmC* to the SmA phase can be induced by the photothermal effect. The application of asymmetric voltage pulses during partial laser irradiation results in the patterning of the layer alignment. This recording method can erase and invert the stored pattern and can handle gray-level patterns.
Dielectric properties of Mn-substituted Ni–Zn ferrites
Authors: Singh Amarendra K. , Goel T. C. , Mendiratta R. G. , Thakur O. P. , Prakash Chandra
Published: July 2002

The dielectric properties of MnxNi0.5−xZn0.5Fe2O4 ferrites with x varying from 0.05 to 0.4 synthesized by the citrate precursor method have been investigated as a function of frequency, temperature, composition, and sintering temperature. An increase in the dielectric constant is observed with the increase in Mn concentration except for x=0.3. Dispersion in the dielectric constant with frequency in the range of 100 Hz–1 MHz is observed. Resonance peaks were observed in tan δ∈ versus frequency curves for all samples. A shift in the resonance frequency towards higher frequency is observed with increase in temperature. The peak height also increases with increase in temperature. Possible mechanisms contributing to these processes have been discussed. From the temperature variation of the dielectric relaxation, activation energies for various samples have been calculated and compared with those obtained from dc resistivity.
An analytical solution for microtip field emission current and effective emission area
Authors: Zuber J. D. , Jensen K. L. , Sullivan T. E.
Published: July 2002

Classical field emission theory relates the material work function and applied electric field at the emitter surface to tunneling current density, based upon the one-dimensional, planar Fowler–Nordheim equation. The extension of this theory to nonplanar, tip-anode geometries is complicated by the spatial variation of the electric field and resultant current density along the microtip surface. To relate this spatially dependent electric field to the applied voltage in this tip-to-plane geometry, experimenters have adopted empirically determined field enhancement factors (i.e., F=βV) and emission area terms to explain quantitative measurements of microtip current. In this work, the nature of the field enhancement and area terms are defined analytically by employing an exact three-dimensional electric field solution in prolate spheroidal coordinates and examining the relationship between total tip current and current density. The analytical and numerical results based on this model provide a physical context for understanding the validity of the generally used field enhancement factor β.
Discrete surface state related to nitrogen-vacancy defect on plasma-treated GaN surfaces
Authors: Hashizume Tamotsu , Nakasaki Ryusuke
Published: July 2002

Detailed studies on the defect-related surface states of plasma-exposed n-GaN surfaces were carried out. An anomalous flat portion appeared in the metal–insulator–semiconductor capacitance–voltage characteristics for the sample exposed to H2 plasma, corresponding to a localized peak at EC−0.5 eV in the surface state density distribution. Atomic-force microscope and x-ray photoemission studies revealed the formation of Ga droplets on H2-plasma-treated GaN surfaces, caused by the desorption of nitrogen atoms in the form of NHx. These results suggested that a nitrogen-vacancy-related state near the conduction-band edge was introduced on the H2-plasma-treated GaN surface. No such effects took place on the N2-plasma-treated GaN surfaces.
Unstable periodic orbits and templates of the Rössler system: Toward a systematic topological characterization
Authors: Letellier C. , Dutertre P. , Maheu B.
Published: July 2002

The Rössler system has been exhaustively studied for parameter values (a∈[0.33,0.557],b=2,c=4). Periodic orbits have been systematically extracted from Poincaré maps and the following problems have been addressed: (i) all low order periodic orbits are extracted, (ii) encoding of periodic orbits by symbolic dynamics (from 2 letters up to 11 letters) is achieved, (iii) some rules of growth and of pruning of the periodic orbits population are obtained, and (iv) the templates of the attractors are elaborated to characterize the attractors topology.
Silicon etching mechanisms in a CF4/H2 glow discharge
Authors: Oehrlein Gottlieb S. , Williams Holly L.
Published: July 2002

Fluorocarbon film deposition onto Si and its influence on the measured Si etch rate in CF4/H2 reactive ion etching in a symmetric two electrode reactor has been studied as a function of CF4/H2 feed gas composition, total gas flow, and applied rf power. For reactive ion etching, the fluorocarbon film thickness on Si increases as the percentage x of H2 in CF4/x% H2 is increased. The fluorocarbon film thickness depends on the total gas flow and is greater for greater gas flows. The observed Si etch rate is controlled by the fluorocarbon film. The Si etch rate is directly proportional to the inverse of the F,C-film thickness for fluorocarbon films thicker than ∼10 Å, which may indicate a diffusion-limited mechanism. Both in-diffusion of fluorine and out-diffusion of SiF4 etch product through the fluorocarbon film are consistent with the decrease of the Si etch rate. The relative importance of the lowering of the atomic F concentration in the gas phase on the Si etch rate, e.g., by the H scavenging mechanism, has also been studied. This reaction can be important for conditions where either the inner walls of the etching apparatus can be maintained free of C,F film throughout the etching experiment, and/or a low hydrogen concentration (≤20%) in the CF4/H2 feed gas is used. For a fixed gas composition of CF4/40% H2, the rf-power dependence of the C,F-film thickness and of the Si etch rate was studied. In reactive ion etching, i.e., if rf power is supplied to the bottom (substrate) electrode, at first a monotonic rise in deposited fluorocarbon film thickness with increasing rf power is observed; at high rf-power levels a dramatic decrease in the C,F-layer thickness occurs, which is concomitant with a greater intensity of near surface lattice disorder (from ion channeling studies) and Si etching. Silicon etching is not observed for lower rf powers. In cases where rf power was supplied to the top electrode only, C,F-film deposition has been observed and no Si etching. These data are consistent with a recombinant model of etch anisotropy.
Acoustoelectric interaction of plate modes with solutions
Authors: Niemczyk T. M. , Martin S. J. , Frye G. C. , Ricco A. J.
Published: July 2002

Acoustic plate mode (APM) devices operating in liquids have an associated electric field which extends several micrometers into the liquid and is capable of interacting with ions and dipoles in solution. Experiments have been performed using quartz APM devices which demonstrate these acoustoelectric interactions. Changes in the dielectric constant or conductivity of solutions in contact with the device were found to perturb the propagation velocity of the plate mode. An equivalent circuit is proposed to describe the interaction of the electric field with ions and dipoles. From this simple model we obtain the velocity shift and attenuation which should occur as a result of this interaction and compare the model predictions to the experimentally observed trends.
Characterization of gold surfaces for use as substrates in scanning tunneling microscopy studies
Authors: Emch R. , Nogami J. , Dovek M. M. , Lang C. A. , Quate C. F.
Published: July 2002

We have used scanning tunneling microscopy to characterize the surface of epitaxial gold on mica in air. We find that these surfaces are simple to prepare, are relatively inert to exposure to air or to water, and have atomically flat terraces extending for up to several hundred angstroms. The observed topography is consistent with the Au(111) surface. It is possible to produce bumps on the surface of less than 100 Å in size in a controlled manner by pulsing the tip voltage while scanning. Self-diffusion of gold is observed in the decay of written features and well as in the movement of existing terrace edges. In some cases, a periodicity in both the geometry of terrace edges and the spatial variation of surface diffusion rates suggest the presence of the 22×1 surface reconstruction.
Improved corrosion resistance and magnetic properties of Nd-Fe-B-type sintered magnets with Mo and Co
Authors: Hirosawa Satoshi , Mino Shuji , Tomizawa Hiroyuki
Published: July 2002

A few atomic percent addition of Mo considerably improves the intrinsic coercivity (HcJ) of Nd-Dy-Fe-Co-B sintered magnets if the Mo content is optimized against B content. It is shown that upgrading of the magnet can be realized by decreasing both Mo and B contents, keeping the optimum Mo-B relation. Dependence of corrosion resistance of both uncoated and aluminum-coated magnets on Co and Nd contents are studied and significant improvement of the corrosion resistance is attributed to formation of an Nd-Co compound, probably Nd3Co.
Misfit dislocations and critical thickness in InGaAs/GaAs heterostructure systems
Authors: Zou J. , Cockayne D. J. H. , Usher B. F.
Published: July 2002

The onset of misfit dislocation generation is investigated and the critical thickness is determined by transmission electron microscopy using the epitaxial lift-off technique for InGaAs/GaAs single heterostructures and single quantum wells. The observed geometries of the dislocations in both cases are in good agreement with the predicted models [J. Appl. Phys. 41, 3800 (1970) and J. Cryst. Growth 27, 118 (1974)]. However, each dislocation undergoes the predicted elongation mechanism [J. Appl. Phys. 41, 3800 (1970)] at different strained-layer thicknesses. A comparison of the predicted and the experimental critical thicknesses is given.
Soft magnetic properties of composite-anisotropy CoNbZr/ceramics multilayers (invited)
Authors: Sugawara E. , Yamaguchi K. , Furukawa S. , Matsumoto F. , Fujimori H. , Masumoto T.
Published: July 2002

CoNbZr/ceramics multilayered films, the metal layers of which have sequentially changed anisotropy directions, were investigated. Such newly processed film is called ‘‘composite-anisotropy multilayer film.’’ The effects of this new processing on soft magnetic properties were studied. In the case where the shift angle in the easy magnetization direction between neighboring CoNbZr layers is 45°, a smooth B-H loop with low coercivity was obtained. Furthermore, annealing from 200 to 400 °C resulted in high permeability (μ′) of 1000–1500 and excellent frequency responses of μ′ above 100 MHz in all directions in the plane of the film. These results could be explained in terms of magnetic interactions of the CoNbZr magnetic layers which are in close contact with one another across the intervening ceramics layers. These findings indicate that such composite film has many advantages over conventional uniaxial multilayers in applications to magnetic devices. Since these characteristics are thought to be very significant for microdevices composed of disc-shaped magnetic cores, we fabricated a planar microtransformer composed of circular spiral coils and disk-shaped magnetic cores, 2.4 mm in size, using photolithographic technology. The input–output voltage ratio, VOUT/VIN, was measured in the frequency range of 2–40 MHz and showed a high value of 0.83 at 40 MHz.
On the relationship between interfacial defects and Schottky barrier height in Ag, Au, and Al/n-GaAs contacts
Authors: van de Walle R. , Van Meirhaeghe R. L. , Laflère W. H. , Cardon F.
Published: July 2002

The variation of the Schottky barrier height in Ag, Au, and Al/n-GaAs contacts has been investigated as a function of the annealing temperature of the GaAs substrate. Angle resolved x-ray photoemission spectroscopy measurements show that the substrate surface changes from As-rich into Ga-rich over the applied annealing range. Two distinct barrier heights (about 0.85 and 0.65 eV) are found, depending on the metal and the annealing temperature. These values correspond to the Fermi level pinning positions expected for amphoteric defect reactions involving the AsGa and GaAs antisites, respectively. Changes in the barrier height are found at annealing temperatures associated with the removal of these defects.
Microwave interferometry of shock waves. I. Unreacting porous media
Authors: Krall Albert D. , Glancy Brian C. , Sandusky Harold W.
Published: July 2002

Microwave interferometry appears to be a promising method for the study of wave and mass velocity in shocked dielectric materials. This paper discusses the mathematics concerning the frequencies and amplitudes in the microwave reflections from the shock wave and from an impacting piston which drives the shock into a nonreacting porous solid. Methods for the determination of the state variables in the compressed region between the shock wave and the piston are given. In this paper, these methods have been confirmed by experimental measurements in nonenergetic materials in which there is no ionization from chemical reaction. The microwave reflection from the shock wave traveling through an inert media is shown to result from a dielectric discontinuity at the wave front. Studies on energetic materials (explosives and propellants) are considered in a companion paper where ionization associated with the chemical reactions is required to explain the increased reflection from the shock front.
Thermionic refrigeration
Authors: Mahan G. D.
Published: July 2002

A refrigerator is described that is based upon thermionic emission. Room-temperature refrigeration is efficient when the work function of the anode is about 0.3–0.4 eV but those low values are unattainable. The refrigerator only operates at higher temperatures.
Particle charging in low-pressure plasmas
Authors: Matsoukas Themis , Russell Marc
Published: July 2002

Particles embedded in a plasma acquire a net charge as a result of collisions with electrons and ions. Due to the stochastic nature of encounters between particle and charged species, the instantaneous charge fluctuates. The static properties of the charge fluctuations are quantified for particles surrounded by an undisturbed plasma in orbital motion limit. For particles that satisfy the condition e2/4πε0RkTe≪1 the charge distribution is a Gaussian function whose average and variance is related to the ion and electron currents toward the particle. For a Maxwellian plasma, in particular, analytical solutions are developed for the average charge and the variance as a function of the parameters of the plasma ne/ni, Te/Ti, and Me/Mi. Finally, the methodology is extended to non-Maxwellian plasmas using the Druyvesteyn as an example.
Electrical breakdown theories applied to polyethylene terephthalate films under the combined effects of pressure and temperature
Authors: Zebouchi N. , Bendaoud M. , Essolbi R. , Malec D. , Ai Bui , Giam Hoang The
Published: July 2002

On the basis of experimental results for the electrical breakdown field Fb in polyethylene terephthalate films, investigated previously under high hydrostatic pressure, P, from 50 to 500 bar at temperatures, T, of 35, 70, and 100 °C, the electromechanical and thermal breakdown models were used to explain variations in Fb. It was found by numerical calculations that the thermal breakdown model agreed better qualitatively and quantitatively with the negative temperature dependence and the positive pressure dependence of Fb than did the electromechanical model.
Influence of ohmic grain boundaries in ZnO varistors
Authors: Bartkowiak M. , Mahan G. D. , Modine F. A. , Alim M. A.
Published: July 2002

A realistic model of transport properties of zinc oxide varistors is constructed from two-dimensional Voronoi networks and studied via computer simulations. In agreement with experimental microcontact measurements made on individual junctions, the networks are assumed to contain randomly distributed microjunctions of two types: (1) electrically active with highly nonlinear current-voltage (I-V) characteristics and (2) ohmic, i.e., with linear I-V characteristics. Effects of the ohmic grain boundaries in the network are simulated for various concentrations and resistivities. Shapes of the simulated I-V characteristics and current dependence of the coefficient of nonlinearity of the network are in good agreement with those experimentally observed for thin varistor samples and in the measurements employing various surface electrode patterns. It is found that the breakdown voltage of the networks increases with the number of the ohmic grain boundaries, except when their resistivity is so low that it becomes comparable with that of the ZnO grains. The maximal value of the coefficient of nonlinearity of the network is shown to be insensitive to the presence of the ohmic grain boundaries, regardless of their resistivity and concentration.
Raman spectroscopy on amorphous carbon films
Authors: Schwan J. , Ulrich S. , Batori V. , Ehrhardt H. , Silva S. R. P.
Published: July 2002

The origin and interpretation of the Raman features of amorphous (hydrogenated) carbon films deposited at room temperature in the region of 1000–1700 cm−1 is discussed in this paper. Possible interpretations of the linewidths, positions of the ‘‘G’’ graphite peak and ‘‘D’’ disordered peak, and their intensity ratios are examined using results obtained from magnetron sputtered and magnetic field enhanced plasma deposited films. It is shown that even small ‘‘clusters’’ of condensed benzene rings (cluster size below 20 Å) in carbon films can explain the observed Raman scattering. Besides the care that should be taken in the correct interpretation of Raman results, the utility of Raman scattering in obtaining an estimate of cluster sizes in amorphous (hydrogenated) carbon films is discussed. Carbon films prepared by magnetron sputtering show two additional Raman features at 1180 and 1490 cm−1 in addition to the G and D peaks. It is shown that a correlation exists between the 1180 cm−1 peak and the sp3 content in the films.
Selenium molecules and their possible role in deep emission from glasses doped with selenide nanocrystals
Authors: Su Zhenpeng , Rodrigues P. A. M. , Yu Peter Y. , Risbud Subhash H.
Published: July 2002

We report the observation of Raman scattering from a vibration mode with frequency of 320 cm−1 and its higher-order harmonics in silicate glasses doped with selenide semiconductor nanocrystals such as CdSe and ZnSe. Comparison with Raman spectra of glasses and alkali halides doped with Se suggests that these modes are caused by the presence of selenium molecules in the glasses. When excited in the blue and green by an Ar+ laser, glasses containing Se only are found to emit strong near-infrared luminescence whose peak and line shape are very similar to the so-called ‘‘deep emission’’ observed frequently from selenide-doped glasses. Possible effects of Se molecules on the linear and nonlinear optical properties of glasses containing selenide nanocrystals are discussed.
Defects, doping, and conduction mechanisms in nitrogen-doped tetrahedral amorphous carbon
Authors: Stumm P. , Drabold D. A. , Fedders P. A.
Published: July 2002

First principles methods are used to study N doping of diamondlike amorphous carbon. A structural model containing 216 atoms is introduced, whose properties are in agreement with the available experimental data. The topological and electronic properties for different N doping concentrations are investigated. We find that N occurring in tetrahedral sites or chains of an even number of π bonded sites results in an increase of the Fermi energy, while N incorporation in strained network sites induces structural changes that lead to an increase in the sp2 fraction of the material. The prevalent conduction mechanisms are identified and discussed. While the Fermi energy increases upon N doping, the localization of the conduction-band-tail states limits extended state conduction. These results are compared to the recent experimental reports on N doping of ta-C and we find that the nondoping threefold N incorporation (N30) is energetically most likely, which explains the low doping efficiency seen in experiments.
Numerical study of the etch anisotropy in low-pressure, high-density plasma etching
Authors: Tuda Mutumi , Nishikawa Kazuyasu , Ono Kouichi
Published: July 2002

Evolution of etched profiles has been numerically studied during low-pressure, high-density (LPHD) plasma etching of Si in Cl2. The surface etch rates were calculated using a reaction model of synergism between incoming ions and neutral reactants, including the spread of ion angular distributions due to their thermal motions and the transport of neutrals arising from the reemission on surfaces in a microstructure. Etched profiles were then simulated using a so-called two-dimensional string algorithm to examine the effects of ion temperature kTi and energy (or sheath voltage) eVs on the etch anisotropy for different neutral-to-ion flux ratios Γn/Γi toward the substrate. Numerical results indicated that in typical Cl2 LPHD plasma etching environments, where the neutral-to-ion flux ratio is Γn/Γi∼1 and the ratio of sheath voltage to ion temperature is eVs/kTi∼100, the chlorinated surface coverage is microscopically nonuniform in etched features: The coverage is very low at the bottom (α∼0.1), whereas the sidewall surface (α∼1) is almost saturated with neutrals. This microscopic nonuniformity of the coverage in etched features is the proposed mechanism responsible for the inversely tapered profiles that often occur in LPHD plasma etching. Additionally, the decrease in vertical etch rate in microstructures or the reactive-ion-etching lag due to neutral shadowing effects is also found to become significant in LPHD plasma etching. At such a low flux ratio of Γn/Γi∼1, more directional ions with a higher ratio of eVs/kTi≳500 are required for the anisotropic etching; e.g., for an ion energy (or sheath voltage) of eVs=50 eV, the ion temperature in a plasma is required to be kTi≲0.1 eV.
A Preisach model with a temperature and time-dependent remanence maximum
Authors: Mitchler P. D. , Roshko R. M. , Dahlberg E. Dan
Published: July 2002

We present a simple physical model which is capable of generating a maximum in the field dependence of both the isothermal remanent magnetization and the thermoremanent magnetization, with temperature and time-dependent systematics which are identical to those observed experimentally in spin glasses. The model uses an output dependent (moving) Preisach formalism to calculate the response to an applied field of a collection of interacting two-level subsystems with intrinsic anisotropy barriers, which are allowed to relax to equilibrium by thermal over-barrier activation. The maxima occur when the distribution of subsystem anisotropies has an output dependence which reduces the mean activation barrier in proportion to the magnetization induced in the system, so that the thermal relaxation processes become progressively more effective as the system approaches saturation.
Improved calibration of the SrB4O7:Sm2+ optical pressure gauge: Advantages at very high pressures and high temperatures
Authors: Datchi F. , LeToullec R. , Loubeyre P.
Published: July 2002

This paper presents a new calibration of the pressure shift of the D07−F05 fluorescence line of SrB4O7:Sm2+, in hydrostatic medium (helium) up to 124 GPa and in nonhydrostatic medium (H2O) up to 130 GPa. There is no quenching of the luminescence, in contrast to that observed in similar compounds. We show that this line permits more accurate pressure measurements at very high pressure than the commonly used R1 emission line of ruby because it remains intense, sharp, well isolated from the other lines, and weakly dependent on nonhydrostaticity. The wavelength pressure shift of the D07−F05 line is found to be very well represented by the nonlinear relation: P=4.032Δλ(1+9.29×10−3Δλ)/(1+2.32×10−2Δλ) with P in GPa and Δλ=λ−685.41 in nm. High temperature experiments up to 900 K at room pressure were also performed. The negligible temperature dependence and the small thermal line broadening are further advantages for high pressure–high temperature studies in diamond anvil cells. A metrology for in situ measurements of pressure and temperature, based on the combined use of this compound with ruby is also presented, along with a new calibration of the ruby R1 frequency shift with temperature from 300 to 800 K.
Persistent photoconductivity in n-type GaN
Authors: Chen H. M. , Chen Y. F. , Lee M. C. , Feng M. S.
Published: July 2002

Results of photoconductivity measurements in undoped n-type and Se-doped GaN epitaxial thin films are presented. Similar to the photoluminescence spectra, the photoconductivity also shows a broad yellow band centered around 2.2 eV. It is found that persistent photoconductivity (PPC) does exist in all the studied thin films. In addition, the PPC effects can be observed for the pumping photon energy down to the yellow band. The results reveal that the origin of the PPC effect and yellow luminescence may arise from the same intrinsic defect. It is shown that the most probable candidate of the intrinsic defect is nitrogen antisite.