bluuebunny/crossref_metadata_2025 huggingface dataset:
Short-wave, localized disturbances in jets, with applications to flows on a beta plane with topography
Authors: Benilov E. S.
Published: December 2003

We examine the stability of jets over topography on the so-called barotropic beta plane (which models oceanic currents in mid-latitudes). Attention is focused on disturbances with a large wave number, for which an asymptotic solution of the normal-mode eigenvalue problem is presented. It is demonstrated that short-wave modes, if they exist, are localized in narrow strips “centered” at local extrema of the velocity profile U(y) (y is the transverse variable). It is further shown that an extremum, say y=yl, can support a short-wave mode only if the vorticity gradient and potential vorticity (PV) gradient at yl are of opposite signs. If they are, the leading-order solution of the eigenvalue problem describes a mode with a phase speed slightly larger than U(yl), if yl is a maximum; or slightly smaller than U(yl), if yl is a minimum. In other words, the mode does not have critical levels in the vicinity of yl, although it may have them elsewhere, at a distant point. If that indeed happens, an additional condition is required to guarantee the existence of the mode: namely, the PV gradient at yl and that at the critical level must have opposite signs. If they do, the mode exists and is weakly unstable (the phase speed has a small imaginary part). Thus, a change of sign of the PV gradient does not necessarily destabilize the flow; and in order to guarantee instability, the PV gradient should have opposite signs at the “important” points, i.e., the localization point and critical level. The asymptotic results are tested against the numerical solution of the exact normal-mode eigenvalue problem. The former and the latter are in good agreement, and not only for large wave numbers, but also for moderate ones. It is also demonstrated that our approach can be applied to other cases of jets in fluids and plasmas.
Experimental visualization of Lagrangian coherent structures in aperiodic flows
Authors: Chrisohoides A. , Sotiropoulos F.
Published: December 2003

A technique is introduced for extracting the coherence time scale of Lagrangian coherent structures (LCS) in aperiodic flows from experimental, light intensity time series. The technique employs digital photography to record the transport of passive tracers in a chaotic instantaneous flow. Coherent eddies are detected by time-averaging the instantaneous light intensity field on finite-size temporal windows. The optimal size of the time-average window (the coherence time scale of the LCS) is extracted from the light intensity field using statistical scaling arguments based on the central limit theorem. The technique is applied to visualize LCS in the vicinity of a surface-piercing rectangular block mounted at one corner of a rectangular open channel.
Effect of a “slip” splitter plate on vortex shedding from a cylinder
Authors: Mittal S.
Published: December 2003

The effect of placing a “slip” splitter plate in the wake of a circular cylinder along the line of symmetry is studied. Such a hypothetical plate allows slip of velocity along itself but prevents any flow normal to it. Unlike the conventional splitter plate the slip plate does not have a wake of its own. The objective of the present study is to increase our understanding of vortex shedding by tracking down that part of the wake that needs to be constrained to suppress it completely. Computations for various configurations of the plate for the Re=100 flow are carried out using a finite element formulation. It is found that the shortest length of the plate required to suppress vortex shedding is two cylinder diameters, approximately, and needs to be located in the latter part of the wake bubble of the basic unperturbed flow. In this region the vertical component of velocity in each of the two standing vortices is towards the centerline. The upstream edge of the optimal plate is located very close to the region in the wake bubble where the vertical velocity component changes direction. As compared to the downstream edge, the location of the upstream edge of the plate has a much more significant effect on the unsteadiness of the flow. This study establishes that the latter part of the wake bubble of the basic unperturbed solution plays an important role in vortex shedding.
Wall-pressure-array measurements beneath a separating/reattaching flow region
Authors: Hudy Laura M. , Naguib Ahmed M. , Humphreys William M.
Published: December 2003

A database of wall-pressure-array measurements was compiled for studying the space–time character of the surface-pressure field within a separating/reattaching flow region. The experimental setup consisted of a long splitter plate located within the wake of a fence and instrumented with an array of flush-mounted microphones. Data were acquired for a Reynolds number of 7900, based on the fence height above the splitter plate. Two distinctive regions, defined based on their location relative to the position of the mean reattachment point (xr) of the shear layer, emerged from this investigation. Upstream, from the fence to 0.25xr, the surface-pressure signature was dominated by large time scale disturbances and an upstream convection velocity of 0.21U∞. Beyond 0.25xr, turbulent structures with smaller time scales and a downstream convection velocity of 0.57U∞ generated most of the pressure fluctuations. Interestingly, the low-frequency wall-pressure signature typically associated with the flapping of the separated shear layer was found to be composed of standing and downstream/upstream propagating wave components. The latter seemed to originate from a point near the middle of the reattachment zone, suggesting the existence of an absolute instability of the recirculation bubble, which may be the cause of the flapping of the shear layer.
Dispersion of particle pairs and decay of scalar fields in isotropic turbulence
Authors: Thomson David J.
Published: December 2003

The decay of homogeneous scalar fields in isotropic turbulence is addressed by considering the dispersion of particle pairs. The evolution of the variance is studied by considering the dispersion backwards in time from the measurement time to the source time. The decay is shown to depend on the scalar field’s low wave number spectral exponent and a critical exponent of 8/3 is derived. This critical value separates cases where the dominant contribution to the variance at large time comes from the initial large scales from cases where the dominant contribution comes from the initial variance-containing scales. This confirms the results obtained for the particular case of the Kraichnan white noise model by Eyink and Xin and shows that the value of the critical exponent is not an artifact of the white noise model. The evolution of the scalar spectrum and correlation function has also been considered and results have been derived for the backtransfer of variance to low wave numbers, the permanence of large (scalar) eddies, the asymptotic convergence for different initial conditions and the self-similarity of the evolution. In particular it is shown that the form of the backtransfer of variance to low wave numbers previously found in models (i.e., the proportionality of the backtransfer to the fourth power of the wave number) is a direct consequence of the finite variance of particle separations and can be derived without modelling assumptions. Finally the particular case of the “mandoline” source geometry often used in wind tunnel studies is addressed.
Force on a spinning sphere moving in a rarefied gas
Authors: Borg Karl I. , Söderholm Lars H. , Essén Hanno
Published: December 2003

The force acting on a spinning sphere moving in a rarefied gas is calculated. It is found to have three contributions with different directions. The transversal contribution is of opposite direction compared to the so-called Magnus force normally exerted on a sphere by a dense gas. It is given by F=−ατξ23πR3mnω×v, where ατ is the accommodation coefficient of tangential momentum, R is the radius of the sphere, m is the mass of a gas molecule, n is the number density of the surrounding gas, ω is the angular velocity, and v is the velocity of the center of the sphere relative to the gas. The dimensionless factor ξ is close to unity, but depends on ω and κ, the heat conductivity of the body.
Thin front propagation in steady and unsteady cellular flows
Authors: Cencini M. , Torcini A. , Vergni D. , Vulpiani A.
Published: December 2003

Front propagation in two-dimensional steady and unsteady cellular flows is investigated in the limit of very fast reaction and sharp front, i.e., in the geometrical optics limit. For the steady flow, a simplified model allows for an analytical prediction of the front speed vf dependence on the stirring intensity U, which is in good agreement with numerical estimates. In particular, at large U, the behavior vf∼U/log(U) is predicted. By adding small scales to the velocity field we found that their main effect is to renormalize the flow intensity. In the unsteady (time-periodic) flow, we found that the front speed locks to the flow frequency and that, despite the chaotic nature of the Lagrangian dynamics, the front evolution is chaotic only for a transient. Asymptotically the front evolves periodically and chaos manifests only in its spatially wrinkled structure.
Transient impact of a liquid column on a miscible liquid surface
Authors: Kersten B. , Ohl C. D. , Prosperetti A.
Published: December 2003

The flow induced by a liquid column falling on an undisturbed liquid surface is studied with the aid of a high-speed camera. The falling liquid spreads over the receiving liquid forming a cavity which eventually pinches off due to the action of gravity. It is only at this point that the normal flow pattern in which the impacting liquid penetrates below the free surface is established. The same process—at a scale smaller by four orders of magnitude—is encountered in the jetting behavior of collapsing cavitation bubbles. It is also observed that the cavity dynamics is strikingly similar to that found when a disturbance is induced on a steady jet falling on a liquid. This observation supports a generic mechanism for air entrainment hypothesized in an earlier paper.
Free-surface evolution due to an impulsive bottom sink at uniform depth
Authors: Haugen Kjetil B. , Tyvand Peder A.
Published: December 2003

The free-surface evolution due to an impulsively started point sink at the bottom of a uniform horizontal layer of inviscid and incompressible fluid is investigated analytically. A third-order small-time expansion of the full nonlinear problem is performed. The dip formation above the sink depends on the Froude number of the sink. An initially critical Froude number 0.1376 is found for the sign change of the third-order elevation above the point sink. A physically critical Froude number 0.109 is identified for the threshold for the dip to be swallowed into the point sink. The same problem is solved in two dimensions, where a uniform line sink is turned on impulsively at the bottom.
Three-dimensional numerical simulation of Marangoni flow instabilities in floating zones laterally heated by an equatorial ring
Authors: Lappa M.
Published: December 2003

Instability of Marangoni convection in floating zones (full zone configuration) of a low Prandtl number fluid under microgravity conditions is investigated by parallel supercalculus and direct three-dimensional and time-dependent simulation of the problem. A parametric analysis (still absent in literature) of the influence of the aspect ratio of the liquid column on the features of the three-dimensional bifurcation of Marangoni flow is carried out. A novel distribution is introduced for the surface heat flux corresponding to the radiative flux generated by a ring heater positioned around the equatorial plane of the full zone at a distance h from the free interface. Axisymmetric computations are used to obtain the steady basic state, then the three-dimensional Navier–Stokes equations are solved to investigate the evolution of azimuthal disturbances. These disturbances always exhibit antisymmetric behavior with respect to the equatorial plane. The mirror symmetry with respect to this plane is broken. Strong interaction occurs in fact between the toroidal convection rolls located in the upper part and lower part of the liquid column. This leads for some values of the aspect ratio to a heretofore unseen “apparent” doubling or quadrupling of the azimuthal wave number of the azimuthal velocity distribution in the midplane. The present analysis points out that the instability of the half zone flow is not relevant for the full zone configuration.
Vortex shedding in subcritical conditions
Authors: Buffoni E.
Published: December 2003

An expressly developed apparatus has been applied to the study of the phenomenon of vortex shedding at Reynolds numbers inferior to the critical value. During previous research it was discovered that vortex shedding could be triggered under subcritical conditions by imparting low-amplitude transversal vibrations at specific frequencies to a cylinder positioned in the flow.
Convection-induced enhancement of mass transfer through an interface separating two immiscible liquids in a two-layer horizontal annulus
Authors: Gelfgat A. Yu. , Yarin A. L. , Bar-Yoseph P. Z.
Published: December 2003

Two-fluid natural-convection flow in the horizontal cylindrical annulus and its effect on mass transfer through the liquid-liquid interface of two immiscible fluids are studied numerically. The liquids are stratified by gravity, with the denser one occupying the lower part of the annulus. The convective motion is driven by heating of the inner or outer cylindrical boundary. It is shown that the mass transfer of a passive scalar (say, a protein) through the interface can be significantly enhanced by the convective flow. Varying the radii ratio from 0.1 to 0.5, it is found that the mass transfer is more intensive in annuli with smaller radii ratio. No significant difference in the mass transfer rates was found between the heating of either inner or outer cylinder. A possibility of further mass transfer enhancement using more complicated temperature distribution on the boundaries is demonstrated. The problem is related to the search for novel bioseparator devices.
Mechanisms of particle deposition in a fully developed turbulent open channel flow
Authors: Narayanan Chidambaram , Lakehal Djamel , Botto Lorenzo , Soldati Alfredo
Published: December 2003

Particle dispersion and deposition in the region near the wall of a turbulent open channel is studied using direct numerical simulation of the flow, combined with Lagrangian particle tracking under conditions of one-way coupling. Particles with response times of 5 and 15, normalized using the wall friction velocity and the fluid kinematic viscosity, are considered. The simulations were performed until the particle phase reached a statistically stationary state before calculating relevant statistics. For both response times, particles are seen to accumulate strongly very close to the wall in the form of streamwise oriented streaks. Deposited particles were divided into two distinct populations; those with large wall-normal deposition velocities and small near-wall residence times referred to as the free-flight population, and particles depositing with negligible wall-normal velocities and large near-wall residence times (more than 1000 wall time units), referred to as the diffusional deposition population. Diffusional deposition (deposition induced by the small residual turbulent fluctuations near the wall) is found to be the dominant mechanism of deposition for both particle response times. The free-flight mechanism is shown to gain in importance only for τp+=15 particles. For τp+=5 particles only 10% deposit because of free flight, whereas the fraction is around 40% for τp+=15 particles. This result runs counter to the widely held opinion that free flight is the dominant mechanism of deposition in wall-bounded flows and clearly quantifies the relative importance of the two mechanisms. A simple relationship between the particle wall-normal velocity on deposition and the residence time for free-flight particles is presented. Particle deposition locations over the period of the entire simulation reveal that, while diffusional deposition occurs mostly along streamwise oriented lines below the near-wall particle accumulation patterns, free-flight particles deposit more evenly over the wall.
Inapplicability of the dynamic Clark model to the large eddy simulation of incompressible turbulent channel flows
Authors: Kobayashi Hiromichi , Shimomura Yutaka
Published: December 2003

The inapplicability of the dynamic Clark model to the large eddy simulation of incompressible turbulent channel flows is proved both analytically and numerically. The reason is neither a negative subgrid-scale eddy viscosity nor an incorrect near-wall scaling, but a negative effective viscosity in the viscous sublayer for the tensor diffusivity term that models the sum of the Leonard and the cross terms.
Ultrasonic absorption in ultra-low carbon steel
Authors: Bolognini S. , Moreau A.
Published: December 2003

The laser-based reverberant technique is used to measure ultrasonic absorption spectra in the 2 to 45 MHz frequency range. This technique, being contactless, allows measurements at high temperature. The absorption spectra of ultra-low carbon steel samples are studied at room temperature in a magnetic field (in order to suppress the magnetoelastic contribution) and in a high temperature furnace (20–1200 °C) without magnetic field. Small steel samples (about 10×10×1 mm3) are used. At room temperature, two main contributions to the ultrasonic absorption are identified: microeddy currents (magnetoelastic contribution) and absorption caused by dislocations (deformation contribution). A typical microeddy current peak is observed and yields a reasonable estimate of the magnetic domain size. Above 10 MHz, the nonmagnetic contribution to the total absorption follows the classical vibrating string model. However, other phenomena also contribute to the absorption spectra. Below 10 MHz, an amplitude-independent damping background is observed. In addition, a small frequency-independent contribution to the absorption is observed at room temperature and is attributed to a thin surface layer. The absorption at high temperature is dominated below the Curie point by the magnetoelastic contribution. Two internal friction peaks are also detected. The first one, at 100 °C, is related to the dislocation kink motion. The second one, measured at 330 °C and 10 MHz, is attributed to the Snoek relaxation of carbon and/or nitrogen in α-iron. The Curie transition as well as the ferrite-austenite transition strongly affect the internal friction spectra.
Effect of gases on the field emission properties of ultrananocrystalline diamond-coated silicon field emitter arrays
Authors: Hajra M. , Hunt C. E. , Ding M. , Auciello O. , Carlisle J. , Gruen D. M.
Published: December 2003

We performed studies of electron emission from ultrananocrystalline diamond (UNCD)-coated, ungated silicon field emitters as a function of in situ exposure to various gases during current versus voltage and current versus time measurements. The emitter arrays were fabricated by a subtractive tip fabrication process and coated with UNCD films using microwave plasma chemical vapor deposition with a CH4/Ar plasma chemistry. The emission characteristics of the coated tip arrays were studied in the diode configuration; using a 2 mm diameter anode with rounded edges to suppress arcing. Significant enhancement of the electron emission was observed, increasing from 35% to 100%, after the emitting surface was exposed to H2 at pressures in the 10−5 and 10−4 Torr range. Upon termination of the H2 exposure, the current decreased to the initial value of 2 μA. The emission current subsequently remained stable at 2 μA upon continued evacuation down to the base pressure below 10−9 Torr. The emission current variation is repeatable with ensuing hydrogen exposure, indicating that the enhancement is due to the hydrogen exposure. Negligible emission current variations are observed at pressures less than 10−5 Torr. Exposure to either Ar or N2 resulted in a reduction of the emission current for ambients of up to 10−5 Torr. This effect is reversible. The effect of the investigated gases on the emission characteristics of UNCD-coated Si tip arrays is attributed to a modification of the effective work function at the localized sites from where electrons are being emitted.
Effects of classical and quantum charge fluctuations on sequential electron tunneling in multiple quantum wells
Authors: Huang Danhong , Cardimona D. A.
Published: December 2003

A previous theory [M. Ershov et al., Appl. Phys. Lett. 67, 3147 (1995)] for studying the distribution of nonuniform fields in multiple-quantum-well photodetectors under an ac voltage is generalized to include nonadiabatic space-charge-field effects. From numerical results calculated by the generalized theory, it is found that field-domain effects are only important at high temperatures or high voltages, where both injection and sequential-tunneling currents are expected to be large. On the other hand, field-domain effects become negligible at low temperatures and low voltages, but nonadiabatic effects included in this extended theory are enhanced for small sequential-tunneling currents. The time duration for nonadiabatic effects is determined by the quantum capacitance. By using the generalized theory, a differential capacitance is calculated for a non-steady state, and a negative conduction current is predicted under a positive voltage in this case due to charge accumulation around the collecting contact.
Time and real space dependence of impact ionization events in low noise impact avalanche transit time diodes
Authors: Lim K. P. D. , Childs P. A. , Herbert D. C.
Published: December 2003

Impact avalanche transit time (IMPATT) diodes are an important source of radio-frequency power at millimeter and submillimeter wavelengths. However, exploitation of these devices has been restricted, as they are commonly believed to suffer from high noise levels. In this article, we demonstrate that a heterostructure IMPATT diode has the potential for almost noise-free operation. Our analysis is based on a Monte Carlo simulation of oscillating IMPATT devices.
Ferroelectric properties of (Ba,Sr)TiO3 thin films grown on YBa2Cu3O7 layers
Authors: Tao K. , Hao Z. , Xu B. , Chen B. , Miao J. , Yang H. , Zhao B. R.
Published: December 2003

Ferroelectric and superconductor bilayers of Ba1−xSrxTiO3 (BST)/YBa2Cu3 O7 (YBCO) are grown on (001) SrTiO3 substrates by magnetron sputtering and pulsed laser deposition. The BST thin films exhibit typical ferroelectric behavior in their hysteresis loops. Capacitance–voltage curves are measured. From the capacitance, a dielectric constant of 1250 is obtained. The current–voltage curve is fitted to investigate the mechanism of leakage. The Schottky barrier height at the Ag/BST interface is calculated to be 0.521 eV. The trapped level Et in BST is estimated to be 0.335 eV below the conduction-band edge. An energy band diagram of the Ag/BST/YBCO structure is proposed to explain the experimental results.
Control parameters for the tunable refractive index of magnetic fluid films
Authors: Hong Chin-Yih , Yang S. Y. , Horng H. E. , Yang H. C.
Published: December 2003

In this work, we systematically investigate the influence of control parameters, including the concentration of magnetic particles, film thickness, external field direction, and field sweep rate, on the magnetically tunable refractive index of magnetic fluid films. We find that the variation in the refractive index of the magnetic fluid film with rising field strength is enhanced by increasing either the concentration or the film thickness, or by applying the field along the film surface, whereas it is independent of the sweep rate of the field. The results clearly reveal how to control the tunable refractive index of magnetic fluid films. Furthermore, they provide valuable information that may assist in the development of magnetic-fluid-based index-tunable devices.
Magnetic coupling in epitaxial TM/MgO/Fe(001) (TM=FeCo, Fe/Co, Fe) macroscopic and microscopic trilayers
Authors: Martı́nez Boubeta C. , de Teresa J. M. , Costa-Krämer J. L. , Anguita J. , Serrate D. , Arnaudas J. I. , Ibarra M. R. , Cebollada A. , Briones F.
Published: December 2003

Multilayered TM/MgO/Fe (001) heterostructures (TM: FeCo, Co/Fe, and Fe) are grown epitaxially, to study the dependence of the magnetic coupling between the two ferromagnetic electrodes on the insulating MgO barrier width and the lateral dimensions of the structures. The crystal quality is investigated by reflection high-energy electron diffraction in situ at different growth stages of the TM/MgO/Fe(001) heterostructures. Magnetic characterization by superconducting quantum interference device magnetometry (macroscopic structures) and transverse Kerr effect (microscopic structures) shows clearly independent switching of top and bottom electrodes at large (above 20 Å) spacer thicknesses for macroscopic films. This independent switching is also observed on patterned structures. For very thin barriers, decreasing the size of the elements in patterned arrays decreases the number of junctions coupled via pinholes.
Efficient, stable, corona discharge 172 nm xenon excimer light source
Authors: Salvermoser M. , Murnick D. E.
Published: December 2003

We demonstrate that corona discharges in xenon gas can be an efficient source of 172 nm Xe2* excimer vacuum ultraviolet (VUV) radiation. Conversion efficiencies of electrical power into VUV light greater than 50% have been observed. A model describing the light production mechanism in the discharge region including the influence of water vapor content in the 10 ppm region is presented. A prototype large area lamp consisting of 21 corona discharges operating in parallel has been built with 38 mW/cm2 VUV output power per area at the lamps surface. Based on the model and experimental results achieved, a continuous wave large area Xe2* 172 nm excimer light source with 130 mW/cm2 output and a possible wall plug efficiency close to 48% is proposed.
Anisotropic liquid absorption in homogeneous two-dimensional nonwoven structures
Authors: Mao N. , Russell S. J.
Published: December 2003

The anisotropy of liquid absorption is influenced by the fiber orientation distribution in the fabric plane. A unique theoretical analysis of planar fluid transport using D’Arcy’s law with two-dimensional directional permeability in nonwoven structures is presented which takes into account the fiber orientation distribution of the structure.
Electrical and chemical characterization of the Schottky barrier formed between clean n-GaN(0001) surfaces and Pt, Au, and Ag
Authors: Tracy K. M. , Hartlieb P. J. , Einfeldt S. , Davis R. F. , Hurt E. H. , Nemanich R. J.
Published: December 2003

Platinum, gold, and silver formed abrupt, unreacted, smooth, and epitaxial metal–semiconductor interfaces when deposited from the vapor onto clean, n-type GaN(0001) films. The Schottky barrier heights, determined from data acquired using x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, capacitance–voltage, and current–voltage measurements agreed to within the experimental error for each contact metal and had the values of 1.2±0.1, 0.9±0.1, and 0.6±0.1 eV for Pt, Au, and Ag, respectively. The band bending and the electron affinity at the clean n-GaN surface were 0.3±0.1 and 3.1±0.1 eV, respectively. The barrier height is proportional to the metal work function, indicating that the Fermi level is not pinned at the GaN surface. However, discrepancies to the Schottky–Mott model were found as evidenced by a proportionality factor of 0.44 between the work function of the metal and the resulting Schottky barrier height. The sum of these discrepancies constitute the interface dipole contributions to the Schottky barrier height which were measured to be ∼1.4, 1.3, and 0.7 eV for Pt, Au, and Ag, respectively.
Electrically active sulfur-defect complexes in sulfur implanted diamond
Authors: Kalish R. , Uzan-Saguy C. , Walker R. , Prawer S.
Published: December 2003

Single crystal type IIa 〈100〉 diamonds were implanted with sulfur, phosphorus, and argon ions under different implantation and annealing conditions. Shallow (sub-MeV) as well as deep (MeV) implantations into samples held at low (liquid nitrogen) ambient (room temperature) and high (400 °C) temperatures were employed. The implanted samples were subjected to postimplantation annealing up to 1000 °C. Following each processing step the samples were subjected to (i) Raman spectroscopy, in order to investigate the implantation related residual defects, and (ii) electrical (resistivity and sometimes Hall effect) measurements as function of temperature. The correlation between the results of these structural and electrical measurements and the comparison of results obtained under identical processing conditions for possible n-type dopant ion-implantations (S and P) and inert (Ar) ion-implantations, as controls, leads to the following conclusions: (a) Sulfur implanted samples always exhibit at least one order of magnitude higher conductivity than Ar control implanted samples. The activation energy associated with the S related conductivity is 0.32–0.37 eV whereas that of the Ar control is 0.5 to 0.6 eV. Hall effect shows, for selected cases, n-type conductivity with low carrier concentration and mobility. (b) Although the presence of some residual defects (mainly split interstitials) seems to accompany the appearance of the S related electrical activity, the level of residual damage in the S implanted samples is always less than that of the Ar control. (c) The electrical effects due to the implantation of S vanish upon annealing at temperatures in access of 800 °C. (d) No significant difference in the electrical properties between P and control Ar implantations are evident. It is concluded that a sulfur-defect related complex, which decomposes at T>800 °C, is responsible for the electrical effects in S implanted diamond. The presence of B contamination which has complicated the interpretation of experiments involving S doping during chemical vapor deposition diamond growth is excluded in the present work in which ion-implantation doping was employed.
Micropolar mixture theory of porous media
Authors: Eringen A. Cemal
Published: December 2003

A continuum theory is developed for a mixture of a micropolar elastic solid and a micropolar viscous fluid. Soil with grains and tortuous rock formations containing dirty fluids may be modeled with this theory. The micropolar model allows the rotational degrees of freedom that give rise to a nonsymmetric stress tensor and couple stresses. Balance laws are given. The second law of thermodynamics is used to develop constitutive equations. Linear constitutive equations are constructed for a heat conducting mixture. Field equations are obtained, boundary and initial conditions presented. As a special case, the field equations of a mixture of an elastic solid and a Newtonian fluid are obtained, by dropping the micropolar effects.
Electron paramagnetic resonance characterization of impurity Gd3+ ions in a PbWO4 single crystal
Authors: Yeom T. H. , Lee S. H. , Choh S. H. , Han T. J. , Kim T. H. , Ro J. H.
Published: December 2003

Lead tungstate single crystals doped with Gd2O3 were grown by the Czochralski method in Ar atmosphere. The electron paramagnetic resonance of the Gd3+ ion in a PbWO4 single crystal has been investigated at 9.4 GHz. The spectroscopic splitting tensor g and zero-field splitting parameters Bkq were determined using the effective spin Hamiltonian. The rotation patterns in the crystallographic planes together with spin Hamiltonian parameters of Gd3+ ion show that the local site symmetry of the Gd3+ ion in PbWO4 crystal is tetragonal and that the Gd3+ ion substitutes for the Pb2+ ion without local charge compensation at the nearby surroundings.
Band line-up determination at p- and n-type Al/4H-SiC Schottky interfaces using photoemission spectroscopy
Authors: Kohlscheen J. , Emirov Y. N. , Beerbom M. M. , Wolan J. T. , Saddow S. E. , Chung G. , MacMillan M. F. , Schlaf R.
Published: December 2003

The band lineup of p- and n-type 4H–SiC/Al interfaces was determined using x-ray photoemission spectroscopy (XPS). Al was deposited in situ on ex situ cleaned SiC substrates in several steps starting at 1.2 Å up to 238 Å nominal film thickness. Before growth and after each growth step, the sample surface was characterized in situ by XPS. The analysis of the spectral shifts indicated that during the initial deposition stages the Al films react with the ambient surface contamination layer present on the samples after insertion into vacuum. At higher coverage metallic Al clusters are formed. The band lineups were determined from the analysis of the core level peak shifts and the positions of the valence bands maxima (VBM) depending on the Al overlayer thickness. Shifts of the Si 2p and C 1s XPS core levels occurred to higher (lower) binding energy for the p-(n-)type substrates, which was attributed to the occurrence of band bending due to Fermi-level equilibration at the interface. The hole injection barrier at the p-type interface was determined to be 1.83±0.1 eV, while the n-type interface revealed an electron injection barrier of 0.98±0.1 eV. Due to the weak features in the SiC valence bands measured by XPS, the VBM positions were determined using the Si 2p peak positions. This procedure required the determination of the Si 2p-to-VBM binding energy difference (99.34 eV), which was obtained from additional measurements.
Electronic structure of thin film silicon oxynitrides measured using soft x-ray emission and absorption
Authors: McGuinness Cormac , Fu Dongfeng , Downes James E. , Smith Kevin E. , Hughes Gregory , Roche Jason
Published: December 2003

The elementally resolved electronic structure of a thin film silicon oxynitride gate dielectric used in commercial device fabrication has been measured using soft x-ray emission and absorption spectroscopies. The SiOxNy was grown by annealing SiO2 in NH3. Soft x-ray emission and soft x-ray absorption were used to measure the valence and conduction band partial density of states in the interfacial region of both the nitrogen and oxygen states. The elementally specific band gap for the O 2p states was measured to be 8.8 eV in the interfacial region, similar to that of pure SiO2. The elementally specific band gap for the N 2p states in the interfacial region was measured to be approximately 5 eV.
Exciton capture by nanocrystals in a polymer matrix
Authors: Shik A. , Konstantatos G. , Sargent E. H. , Ruda H. E.
Published: December 2003

A model for the capture of nonequilibrium excitons by semiconductor nanocrystals in a polymer matrix via dipole–dipole interactions is developed. The transition probability W is found by calculating Joule energy losses in the nanocrystal induced by an external oscillating dipole. To find the net capture rate, the problem is solved simultaneously with the diffusion equation describing generation and recombination of excitons in the polymer matrix and their transport to the nanocrystal interface. In real conducting polymers the capture rate is often limited not by dipole interactions but by diffusion transport so that the key condition for high efficiency of light-emitting devices based on organic–inorganic nanocomposites is that the diffusion length of excitons in the polymer matrix exceeds the average internanocrystal spacing.
Stress current calculation of stacked dielectrics in time dependent dielectric breakdown
Authors: Yang Wenwei , Cheng Xinhong , Xing Yumei , Li Wenjun , Yu Yuehui
Published: December 2003

This article presents a simple method for calculating the stress current in accelerated test of time dependent dielectric breakdown of stacked dielectrics. By replacing an energy barrier of arbitrary shape with a series of rectangular energy barriers and resolving rigorously the Schrödinger equation in each regions, interfaces and variations of electron effective masses in different dielectrics have been analyzed. Our calculations fit experimental data well. The method is applicable in this reliability study aimed at failure mechanism detection and prediction.
Band parameters for nitrogen-containing semiconductors
Authors: Vurgaftman I. , Meyer J. R.
Published: December 2003

We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III–V semiconductors that have been investigated to date. The two main classes are: (1) “conventional” nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) “dilute” nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III–V material, e.g., GaAsN and GaInAsN). As in our more general review of III–V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature and alloy-composition dependences are also recommended wherever they are available. The “band anticrossing” model is employed to parameterize the fundamental band gap and conduction band properties of the dilute nitride materials.
Pulsed laser deposition of Bi2Te3-based thermoelectric thin films
Authors: Makala Raghuveer S. , Jagannadham K. , Sales B. C.
Published: December 2003

Thin films of p-type Bi0.5Sb1.5Te3, n-type Bi2Te2.7Se0.3, and n-type (Bi2Te3)90(Sb2Te3)5(Sb2Se3)5 (with 0.13 wt % SbI3) were deposited on substrates of mica and aluminum nitride (on silicon) using pulsed laser ablation at substrate temperatures between 300 °C to 500 °C. The films were characterized using x-ray diffraction and transmission electron microscopy for crystalline quality and epitaxial growth on the substrates. The surface morphology and microstructure were examined using scanning electron microscopy. X-ray mapping and energy-dispersive spectroscopy were performed to determine nonstoichiometry in the composition and homogeneity. The quality of the films, in terms of stoichiometric composition and crystal perfection, was studied as a function of growth temperature and laser fluence. The values of the Seebeck coefficient, electrical resistivity, and Hall mobility in the thin films were measured and compared with those in the bulk. Thermoelectric figure of merit of the films was evaluated from the measured parameters. Correlation of the thermoelectric properties, with the crystalline quality and stoichiometric composition of the films, showed the advantages of pulsed laser deposition of the multicomponent thermoelectric thin films. The results illustrate that laser physical vapor deposition is a suitable choice for deposition of multicomponent thermoelectric films. However, optimization of target composition, substrate temperature, and annealing of the films after deposition were found necessary to maintain the desired stoichiometry and low defect density. AlN/Si substrates provided better quality films compared to substrates of mica. Poor adhesion and cracking of the films on mica were found to be detrimental factors. Films deposited on AlN/Si substrates were found to show higher carrier mobility and higher values of Seebeck coefficient.
Electron plasma ion trap/source
Authors: Ordonez C. A.
Published: December 2003

A theoretical study is presented on a type of Penning trap configuration referred to as an electron plasma ion trap/source. Ions in the configuration are confined within a three-dimensional electric potential well, which is produced by a combination of the electric field generated by the trap electrodes and the electric field generated by a trapped electron plasma. The ion density is not limited by the Brillouin ion density limit. Instead, the ion charge density must be smaller than the electron charge density. Various mechanisms that may limit the electron charge density are identified. Example calculations are used to find that the most restrictive limit on the electron charge density is likely to be the voltage difference that must be applied to trap the electron plasma parallel to a magnetic field. For confinement of low-charge-state ions, the ion temperature must be smaller than the electron temperature. Relatively long ion confinement times are found to be possible, however, because the equilibration of the ion temperature and the electron temperature is a slow collisional process due to the disparate masses involved. The ions can be easily extracted before the ion temperature reaches a value such that ion loss to the electrode walls becomes a significant source of impurities. Thus, since ion–wall interactions can be minimal, high purity ion plasmas may be generated. A self-consistent finite-differences computation is used to predict a possible plasma equilibrium.
Spatially resolved lifetime imaging of silicon wafers by measurement of infrared emission
Authors: Schubert Martin C. , Isenberg Joerg , Warta Wilhelm
Published: December 2003

The measurement of infrared absorption of excess carriers is a successful technique by which images of the excess free carrier density and recombination lifetime in silicon can be generated. Carrier density imaging (CDI) has recently been developed as a powerful tool by which one obtains such images. This article analyzes both the effect due to absorption and emission of infrared radiation by excess carriers in silicon. The former effect is the basis of the existing CDI technique whereas this article describes a lifetime measurement technique based on the latter effect. Both methods allow one to obtain lifetime images with high spatial resolution on the order of seconds and measure actual lifetimes. These techniques are contactless and nondestructive. The emission CDI mode facilitates the measurement of low-lifetime wafers on the order of seconds.
Asymptotic study of film thinning process on a spinning annular disk
Authors: Dandapat B. S. , Daripa Prabir , Ray P. C.
Published: December 2003

We consider an axisymmetric flow of a thin liquid film on a rotating annular disk. The effects of surface tension and gravity terms are included. An asymptotic solution for the free surface of the thin film is found using an expansion for the film thickness in powers of a small parameter characterizing the film thickness in comparison to the inner disk radius, and then applying the method of matched asymptotic expansion. The asymptotic solution is capable of predicting several features of spin-coating processes. For example, we find that the surface tension enhances film thinning at the central region whereas the gravity does not. The results show that the final film thickness does not depend on the initial distribution of the film thickness (be it uniform or nonuniform) and on the initial amount of liquid deposited. We find that most of the liquid initially deposited on the disk flows out of the disk in a very short time at the initial stages of spinning, regardless of the type of initial distribution of the film thickness. The retention of fluid for nonuniform initial distribution is more than that for uniform distribution at the early stages of the spreading of the thin film.
Random field models for two-phase microstructures
Authors: Grigoriu Mircea
Published: December 2003

Level-cut filtered Poisson fields are defined by cuts of filtered Poisson fields above prescribed levels and used to model two-phase microstructures. The filtered Poisson fields are sums of deterministic functions h scaled by random variables and centered at random points generated by a Poisson field N. The structure of the level-cut filtered Poisson fields depends on the functional form of h, the properties of the random variables scaling h, and the intensity of N. Level-cut Gaussian fields used extensively to represent two-phase microstructures are special cases of level-cut filtered Poisson fields. A Monte Carlo algorithm was developed for generating two-phase composites based on level-cut filtered Poisson fields. It is shown by examples that level-cut filtered Poisson fields can produce a broad range of two-phase microstructures.
Photogenerated charge carrier transport in p-polymer n-polymer bilayer structures
Authors: Manoj A. G. , Alagiriswamy A. A. , Narayan K. S.
Published: December 2003

We investigate polymer bilayer structures that can be utilized effectively for efficient photovoltaic properties. The transport characteristics of photogenerated free charge carriers across the interface of a bilayer consisting of p-type polymer poly(3-octyl thiophene) (P3OT) or poly(2-methoxy, 5-(2-ethylhexoxy)-1, 4-phenylene vinylene) (MEHPPV) and n-type polymer poly(benzamidazobenzo phenanthroline) (BBL) are studied using photocurrent spectral response, intensity modulated photocurrent spectra, current-voltage, and thermally stimulated current measurements. The parameters governing the device efficiency are controlled by the interfacial defect characteristics and electric field redistribution in the device. A clear evidence of these defect states at the polymer–polymer interface is revealed using the time and frequency domain photocurrent measurements. We compare the figure of merits of the P3OT/BBL and MEHPPV/BBL structures and correlate these properties to the interfacial processes.
Early stage of plastic deformation in thin films undergoing electromigration
Authors: Valek B. C. , Tamura N. , Spolenak R. , Caldwell W. A. , MacDowell A. A. , Celestre R. S. , Padmore H. A. , Bravman J. C. , Batterman B. W. , Nix W. D. , Patel J. R.
Published: December 2003

Electromigration occurs when a high current density drives atomic motion from the cathode to the anode end of a conductor, such as a metal interconnect line in an integrated circuit. While electromigration eventually causes macroscopic damage, in the form of voids and hillocks, the earliest stage of the process when the stress in individual micron-sized grains is still building up is largely unexplored. Using synchrotron-based x-ray microdiffraction during an in-situ electromigration experiment, we have discovered an early prefailure mode of plastic deformation involving preferential dislocation generation and motion and the formation of a subgrain structure within individual grains of a passivated Al (Cu) interconnect. This behavior occurs long before macroscopic damage (hillocks and voids) is observed.
X-ray reciprocal space mapping of strain relaxation in GaAs1−xNx on GaAs [100] by molecular-beam epitaxy
Authors: Cheah W. K. , Fan W. J. , Yoon S. F. , Wang S. Z. , Loke W. K.
Published: December 2003

The effect of strain on GaAs1−xNx grown on GaAs is observed using two-dimensional [115] high-resolution x-ray diffraction rocking curves. The instance when the epilayer changes from a highly strained to a relaxed state, is captured and a change in peak profile is observed. The increase of growth rate is found to have an effect of maintaining the crystal quality and surface reconstruction. Interstitial N complex lowers the tensile strain due to its compressive nature, thereby increasing the critical thickness at high N%. We demonstrated that GaAs0.952N0.048 can be coherently grown to 100 nm on GaAs, exceeding the critical thickness by 4.7 times after an optimization of III–V–N growth by means of higher rf power compensation under an increased growth rate.