bluuebunny/crossref_metadata_2025 huggingface dataset:
Current–voltage characteristics of single molecular junction: Dimensionality of metal contacts
Authors: Wang Chuan-Kui , Luo Yi
Published: August 2003

Formalisms for current–voltage (I–V) characteristics of a single molecular junction with electrodes possessing one-, two-, and three-dimensional energy distributions are derived and compared in the framework of elastic scattering theory. Model calculations on a device constituted of 1,4 benzene-dithiolate molecule chemically bonded to two gold electrodes are carried out using hybrid density functional theory. It is shown that the dimensionality of metal contacts has a strong effect on the I–V characteristics of the molecular junction, in particular the shape of the conductance curves. Good agreement with experimental I–V curves has been found for the case of the three-dimensional electron system.
The correlation between film thickness and adsorbate line shape in surface enhanced infrared absorption
Authors: Priebe A. , Sinther M. , Fahsold G. , Pucci A.
Published: August 2003

Various experimental results on surface enhanced infrared absorption reveal asymmetric line shapes. Whereas the order of magnitude of the enhancement can be understood from electromagnetic field enhancement the unusual line shape remains without satisfactory explanation. An interaction with electron-hole pairs would lead to an asymmetric line but this should be restricted to the first monolayer. However, asymmetry is also observed for vibrations at larger distances from the metal-film surface. Here we show strongly asymmetric lines and their enhancement as a consequence of the interaction of adsorbate vibrations with surface plasmons of metal islands. Both the effects and also the baseline change can be estimated by a proper application of well established effective-media models.
Model experiments for direct visualization of grain boundary deformation in nanocrystalline metals
Authors: Van Vliet Krystyn J. , Tsikata Sedina , Suresh Subra
Published: August 2003

Recent experimental studies on nanocrystalline metals have shown that mechanical strength may decrease with decreasing grain size d for d<10–15 nm. The mechanisms underlying this trend are not understood, although such a relationship is in distinct contrast to that seen in microcrystalline metals whereby strength increases with decreasing grain size. Here, we present direct experimental observations of deformation via the bubble raft model, a two-dimensional analog to fcc crystals. We adopt nanoindentation as a means to introduce localized deformation, and quantify critical conditions for defect nucleation. We identify a transitional d of approximately 7 nm, at which further grain refinement leads to a decrease in the stress required to initiate plastic deformation. Further, we observe a concurrent transition in the primary deformation mechanism from discrete dislocation emission from grain boundaries (d>7 nm) to localized grain boundary migration (d<7 nm). Thus, these data suggest that both the onset and mechanisms of plasticity in nanocrystalline materials change markedly below a critical grain size.
Dynamics of distributed sources
Authors: Novikov E. A.
Published: August 2003

The dynamics of distributed sources is described by nonlinear partial differential equations. Lagrangian analytical solutions of these (and associated) equations are obtained and discussed in the context of Lagrangian modeling—from the Lagrangian invariants to dynamics. Possible applications of distributed sources and sinks to geophysical fluid dynamics and to the cosmology are indicated.
Onset of thermal convection in a vertical porous cylinder with conducting wall
Authors: Haugen Kjetil B. , Tyvand Peder A.
Published: August 2003

The onset of Rayleigh–Bénard convection in a finite circular porous cylinder with vertical axis is investigated analytically. The cylinder is heated from below, and its top and bottom are assumed impermeable and perfectly heat-conducting. The impermeable sidewall of the cylinder is assumed to be heat-conducting so that the temperature perturbation there is zero. The eigenvalue problem is split into two different Helmholtz equations. The resulting eigenmode is expressed in terms of Bessel functions. The preferred mode at the onset of convection is found to be axisymmetric. The critical Rayleigh number is a smooth function of the aspect ratio of the cylinder, in contrast to the standard case of an insulating sidewall.
Multimode decomposition of spatially growing perturbations in a two-dimensional boundary layer
Authors: Tumin Anatoli
Published: August 2003

Three-dimensional spatially growing perturbations in a two-dimensional incompressible boundary layer are considered within the scope of linearized Navier–Stokes equations. The Cauchy problem is solved under the assumption of a finite growth rate of the disturbances. It is shown that the solution can be presented as an expansion into a biorthogonal eigenfunction system. The result can be utilized for decomposition of flow fields derived from computational studies when pressure and all velocity components, together with their derivatives, are available. The method can be used also in a case where partial data are available when a priori information leads to consideration of a finite number of modes. In the case of a continuous spectrum, the problem of decomposition based on partial information is ill-posed, but the method might be applied under additional assumptions about the perturbations.
Diffuse interface modeling of the morphology and rheology of immiscible polymer blends
Authors: Keestra B. J. , Van Puyvelde P. C. J. , Anderson P. D. , Meijer H. E. H.
Published: August 2003

A diffuse interface model is used to simulate a step-shear experiment of a binary immiscible polymer blend. The gradient theory used in diffuse interface modeling makes it possible to incorporate interfacial tension and governs the process of coalescence and breakup without any additional decision criteria. The interface tensor q, a direct outcome of the model, is used to relate microstructural information to the first-normal stress difference N1. The results obtained are in qualitative agreement with experiments reported in the literature.
A coupled Landau model describing the Strouhal–Reynolds number profile of a three-dimensional circular cylinder wake
Authors: Sheard G. J. , Thompson M. C. , Hourigan K.
Published: August 2003

A known bifurcation scenario describing the development and interaction of Mode A and Mode B vortex shedding modes of a circular cylinder wake is extended to predict the Strouhal–Reynolds number profile over the three-dimensional transitions. The mode amplitudes are described by coupled Landau equations and, with frequency information being included by the addition of complex coefficients, the model predicts the discontinuous nature of the Strouhal–Reynolds number shedding profile of the circular cylinder wake throughout the laminar three-dimensional transition regime. The model coefficients are determined from computations of the three-dimensional modes of a circular cylinder wake.
Regularization of Grad’s 13 moment equations: Derivation and linear analysis
Authors: Struchtrup Henning , Torrilhon Manuel
Published: August 2003

A new closure for Grad’s 13 moment equations is presented that adds terms of Super-Burnett order to the balances of pressure deviator and heat flux vector. The additional terms are derived from equations for higher moments by means of the distribution function for 13 moments. The resulting system of equations contains the Burnett and Super-Burnett equations when expanded in a series in the Knudsen number. However, other than the Burnett and Super-Burnett equations, the new set of equations is linearly stable for all wavelengths and frequencies. Dispersion relation and damping for the new equations agree better with experimental data than those for the Navier–Stokes–Fourier equations, or the original 13 moments system. The new equations also allow the description of Knudsen boundary layers.
Velocity streak structure modifications induced by flow manipulation
Authors: Iuso Gaetano , Di Cicca Gaetano Maria , Onorato Michele , Spazzini Pier Giorgio , Malvano Riccardo
Published: August 2003

The present research is based on the belief that the observation of the response of near wall turbulent organized motions to external perturbations is a powerful technique for investigating the physics of turbulence regeneration. The low speed streak behavior in the buffer layer of a flat plate turbulent boundary layer under the action of transversal wall oscillation has been statistically analyzed by observing particle image velocimetry velocity fields in a plane parallel to the wall at a distance of 20 wall units. It was found that the observed reduction in turbulence activity (velocity variance and Reynolds stress) is accompanied by an increase of the averaged velocity streaks width, spacing and waviness and by a reduction of their strength. The increase in streak width and spacing has been attributed mainly to coalescence processes between neighboring streaks. The occurrence of velocity streaks strength reduction, associated with the consequent decrease of the number of unstable low speed streaks, is believed to be the main reason leading to the reduction of turbulent activity. The interpretation of the present results supports the concept that the so-called streak regeneration cycle plays an important role in the near wall turbulence reproduction process.
Lattice Boltzmann simulation of open flows with heat transfer
Authors: D’Orazio A. , Succi S. , Arrighetti C.
Published: August 2003

A thermal lattice Bhatnagar–Gross–Krook model with doubled populations, together with a new temperature boundary condition, is used to simulate thermal flows with viscous heating effects. Simulations of thermal Couette and Poiseuille flows show that the present model works over a wide range of parameters. Open flow configurations are discussed to demonstrate that basic thermal effects such as expansion cooling are correctly reproduced.
Instability of the interface between two inviscid fluids inside a rotating annulus in the absence of gravity
Authors: Dávalos-Orozco L. A. , Vázquez-Luis E.
Published: August 2003

In this paper we investigate the linear instability of two superposed inviscid fluids between two rotating concentric cylinders. In the hydrostatic state the two fluids rotate as a rigid body with the same angular velocity as the annulus. Two problems are investigated. First, calculations are made of the instability of the liquid layer coating the outside of a cylinder under rotation. Second, results are obtained of the instability of two stratified fluids between two concentric cylinders. The first case is the most unstable condition of the second case when the outer fluid is absent. Therefore, in the second case only the instability of a system with a heavier fluid located outside the interface was investigated. It was found that the introduction of the inner cylinder produces new and interesting results about the axial and azimuthal modes not previously published in the literature. In particular, when a liquid layer coats the outside of a cylinder it was found that the azimuthal mode, n=1, is the most important in a range of the nondimensional surface tension where the purely axial mode dominated in the absence of the inner cylinder. Additionally, it was found that some azimuthal spiral modes with a finite wavenumber appear as the most unstable in the presence of the inner cylinder. It was shown that the larger the centrifugal force is, the larger the nondimensional radius of the inner cylinder should be to make the instability more sensitive to the presence of the inner cylinder. An equation for the growth rate was obtained in the limit of very thin liquid layers and it was found that the maxima of growth rate agree very well with the experimental results of a fluid layer coating the outside of a rotating cylinder.
Database analysis of errors in large-eddy simulation
Authors: Meyers Johan , Geurts Bernard J. , Baelmans Martine
Published: August 2003

A database of decaying homogeneous, isotropic turbulence is constructed including reference direct numerical simulations at two different Reynolds numbers and a large number of corresponding large-eddy simulations at various subgrid resolutions. Errors in large-eddy simulation as a function of physical and numerical parameters are investigated. In particular, employing the Smagorinsky subgrid parametrization, the dependence of modeling and numerical errors on simulation parameters is quantified. The interaction between these two basic sources of error is shown to lead to their partial cancellation for several flow properties. This leads to a central paradox in large-eddy simulation related to possible strategies that can be followed to improve the accuracy of predictions. Moreover, a framework is presented in which the global parameter dependence of the errors can be classified in terms of the “subgrid activity” which measures the ratio of the turbulent to the total dissipation rate. Such an analysis allows one to quantify refinement strategies and associated model parameters which provide optimal total simulation error at given computational cost.
Optimization of steady suction for disturbance control on infinite swept wings
Authors: Pralits Jan O. , Hanifi Ardeshir
Published: August 2003

We present a theory for computing the optimal steady suction distribution to suppress convectively unstable disturbances in growing boundary layers on infinite swept wings. This work includes optimization based on minimizing the disturbance kinetic energy and the integral of the shape factor. Further, a suction distribution in a continuous control domain is compared to an approach using a number of discrete pressure chambers. In the latter case, the internal static pressures of these chambers are optimized. Optimality systems are derived using Lagrange multipliers. The corresponding optimality conditions are evaluated using the adjoint of the parabolized stability equations and the adjoint of the boundary layer equations. Results are presented for an airfoil designed for medium range commercial aircraft. We show that an optimal suction distribution based on a minimization of the integral of the shape factor is not always successful in the sense of delaying laminar-turbulent transition. It is also demonstrated that including different types of disturbances, e.g., Tollmien–Schlichting and cross-flow types, in the analysis may be crucial.
Free convection of binary liquid with variable Soret coefficient in thermogravitational column: The steady parallel base states
Authors: Labrosse G.
Published: August 2003

An exhaustive identification is made of the steady parallel base states which might occur in a thermogravitational column, with a constant horizontal thermal gradient, and a Soret coefficient depending both on temperature and composition, ST(T,C). It is shown that the horizontal dynamics is mainly governed by the Soret thermal sensitivity, and the vertical separation by its compositional one. Published ST(T,C) empirical laws are used to define a generic model which allows one to carry out a “normal mode” analysis of these flows. To account for the possible complete species vertical separation, the Boussinesq approximations are updated. To the leading order in the Soret coefficient amplitude, the horizontal dynamics is seen to be controlled by the solutal Péclet number, Pe, and a vertical stratification parameter, both drifting very slowly along the vertical. Stabilizing and potentially unstable vertical stratifications of the species are predicted, and, for each, the steady parallel flows are described for every thermal sensitivity of the Soret coefficient. Then an accurate determination of the vertical stratification profile is given for a constant and linear C-dependence of ST. The determinant peculiarity of the C-dependence is shown to be the number of zeroes that ST possesses in C∈]0,1[. Some suggestions are made regarding the way the Soret coefficient measurement could benefit from this first analysis. It is shown, in particular, that the vertical separation slope may be misleading in real molecular mixtures, such as the 0.5 N NaCl solution about T=12.09 °C, where the Soret term is mainly linear with temperature.
Second-order slip laws in microchannels for helium and nitrogen
Authors: Maurer Jean , Tabeling Patrick , Joseph Pierre , Willaime Herve
Published: August 2003

We perform gas flow experiments in a shallow microchannel, 1.14±0.02 μm deep, 200 μm wide, etched in glass and covered by an atomically flat silicon wafer. The dimensions of the channel are accurately measured by using profilometry, optical microscopy and interferometric optical microscopy. Flow-rate and pressure drop measurements are performed for helium and nitrogen, in a range of averaged Knudsen numbers extending up to 0.8 for helium and 0.6 for nitrogen. This represents an extension, by a factor of 3 or so, of previous studies. We emphasize the importance of the averaged Knudsen number which is identified as the basic control parameter of the problem. From the measurements, we estimate the accommodation factor for helium to be equal to 0.91±0.03 and that for nitrogen equal to 0.87±0.06. We provide estimates for second-order effects, and compare them with theoretical expectations. We estimate the upper limit of the slip flow regime, in terms of the averaged Knudsen number, to be 0.3±0.1, for the two gases.
Dual computational basis qubit in semiconductor heterostructures
Authors: Gilbert M. J. , Akis R. , Ferry D. K.
Published: August 2003

Advances in quantum computing have revealed computing capabilities that threaten to render many of the public encryption codes useless against the hacking potential for a quantum-mechanical-based computing system. This potential forces the study of viable methods to keep vital information secure from third-party eavesdropping. In this letter, we propose a coupled electronic waveguide device to create a qubit with two computational bases. The characteristics we have obtained by simulating such devices suggest a possible way of implementing quantum cryptography in semiconductor device architectures.
SiC-capped nanotip arrays for field emission with ultralow turn-on field
Authors: Lo H. C. , Das D. , Hwang J. S. , Chen K. H. , Hsu C. H. , Chen C. F. , Chen L. C.
Published: August 2003

Silicon nanotips with tip diameter and height measuring 1 nm and 1 μm, respectively, and density in the range of 109–3×1011 cm−2, were fabricated monolithically from silicon wafers by electron cyclotron resonance plasma etching technique at a temperature of 200 °C. Field emission current densities of 3.0 mA/cm2 at an applied field of ∼1.0 V/μm was obtained from these silicon nanotips. High-resolution transmission electron microscope and Auger electron spectroscopy analyses concluded that the nanotips are composed of monolithic silicon and nanometer-size SiC cap at the top. A 0.35 V/μm turn-on field to draw a 10 μA/cm2 current density was demonstrated, which is much lower than other reported materials. The excellent field emission property demonstrated by these nanotips, which were fabricated by a process integrable to the existing silicon device technology at low temperatures, is a step forward in achieving low-power field emission displays and vacuum electronic devices.
Optically tunable mechanics of microlevers
Authors: Vogel M. , Mooser C. , Karrai K. , Warburton R. J.
Published: August 2003

We show how the mechanical rigidity of a slightly detuned miniature Fabry–Pérot cavity can be modified with light. We use a microcavity in which one of the mirrors is a soft compliant microlever optimized to detect bolometric forces. The static compliance can either be decreased to zero or increased considerably depending on the detuning of the light with respect to the cavity resonance.
Effect of ordered arrays of native defects on the crystal structure of In- and Ga-rich Cu-ternaries
Authors: Rincón C. , Wasim S. M. , Marı́n G. , Delgado J. M. , Contreras J.
Published: August 2003

A comparative study of the unit cell parameters a and c, and volume V of the chalcopyrite-related of In- and Ga-rich ordered defect compounds of the ternary systems Cu-In-Se, Cu-In-Te, Cu-Ga-Se, and Cu-Ga-Te is presented. It is observed that these parameters decrease in the sequence 1:1:2→3:5:9→5:9:16→2:4:7→1:3:5→1:5:8. This behavior is attributed to the presence of arrays of ordered defects in the crystal lattice of these compounds. It is also found that values of a, c, and V in these systems vary linearly with the fraction m of [2 VCu−1+(In,Ga)Cu+2] defect pair for each unit of Cu(In,Ga)(Se2,Te2).
Two-dimensional tunable photonic crystal formed in a liquid-crystal/polymer composite: Threshold behavior and morphology
Authors: Escuti Michael J. , Qi Jun , Crawford Gregory P.
Published: August 2003

A two-dimensional (2D) transverse square lattice is formed using holographic methods in a liquid-crystal/polymer composite through a photoinduced diffusion process. The polymer morphology is investigated using scanning electron microscopy. The kinetics of formation is described within the framework of a 2D diffusion model, and a simple phenomenological description captures the nature of the Freedericksz transition, allowing the estimation of the surface anchoring strength W≈3×10−4 J/m2.
Third-order Lagrangian stochastic modeling
Authors: Reynolds A. M.
Published: August 2003

The formulation of third-order Lagrangian stochastic models for the hyper-accelerations (time derivative of acceleration) of fluid particles in turbulence is presented. Agreement between model predictions and data from direct numerical simulations [Yeung and Pope, J. Fluid Mech. 207, 531 (1989)] for the Lagrangian velocity structure function and Lagrangian acceleration autocorrelation function is excellent and significantly better than that obtained with a second-order Lagrangian stochastic model. The effects of third-order processes on the turbulent dispersion of fluid particles are found to be significant at Reynolds numbers typical of laboratory-scale experiments and direct numerical simulations and comparable in magnitude to the effects of second-order processes.
Effect of the InAlGaP matrix on the growth of self-assembled InP quantum dots by metalorganic chemical vapor deposition
Authors: Zhang X. B. , Heller R. D. , Noh M. S. , Dupuis R. D. , Walter G. , Holonyak N.
Published: August 2003

InP self-assembled quantum dots (QDs) were deposited on lattice-matched In0.5(AlxGa1−x)0.5P matrices grown on (001) GaAs substrates by using metalorganic chemical vapor deposition. We found that the Al concentration in the matrix has a great influence on the size of the InP QDs. For a fixed volume of deposited InP, the size of the QDs decreases with an increase in the Al concentration in the In0.5(AlxGa1−x)0.5P matrix. We suggest that this variation in QD size is a result of an alloying effect caused by atomic intermixing between the InP dots and the underlying matrix material. The stronger intermixing between InP and the Ga-rich matrices (relative to Al-rich matrices) results in larger QDs on these surfaces. The intermixing effect, as expected, reduces the lattice mismatch and, as a result, leads to a larger critical thickness of the wetting layer when the growth temperature is higher. The growth of InP QDs on an InAlP matrix with a thin GaP intermediate buffer layer also supports the interpretation as an intermixing effect.
Omnidirectional and compact guided light extraction from Archimedean photonic lattices
Authors: Rattier M. , Benisty H. , Schwoob E. , Weisbuch C. , Krauss T. F. , Smith C. J. M. , Houdré R. , Oesterle U.
Published: August 2003

We address the issue of extracting light from a waveguide towards air in a compact way for randomly oriented guided waves. The goal is to enhance the extraction efficiency of light-emitting diodes while retaining planar processing. For incidence-angle-independent extraction, preferred lattice designs appear to possess a ring-shaped Fourier transform. We demonstrate this property for an Archimedean lattice. This system is the outer part of a resonant-cavity light-emitting diode. Data suggest that ∼40% extraction efficiency is at hand in a planar top-emitting device retaining its substrate.
Influence of Si substrate orientation on stress development in Pd silicide films grown by solid-state reaction
Authors: Gergaud P. , Megdiche M. , Thomas O. , Chenevier B.
Published: August 2003

In situ real-time measurements of stress are performed during solid-state reaction of a palladium thin film with Si(001) or Si(111) single crystals. The stress in Pd2Si is compressive in both cases at variance with the sign of epitaxial misfit. A large difference in stress relaxation kinetics between fiber textured [on Si(001)] and epitaxial [on Si(111)] Pd2Si films is evidenced. This difference is correlated with a considerable variation in stress buildup during silicide growth. The microstructure of the growing phase is thus a key parameter for stress development during solid-state reaction.
Two-photon photoreduction of metallic nanoparticle gratings in a polymer matrix
Authors: Kaneko Koshiro , Sun Hong-Bo , Duan Xuan-Ming , Kawata Satoshi
Published: August 2003

We report femtosecond laser nanofabrication using a metal ion-doped polymer. Submicron grating lines arising from two-photon, two-beam interferential fringes were found consisting of photoreduced metallic nanoparticles with diameters ranging from tens to over a hundred of nanometers. The laser-hybrid material interaction was initialized with an in situ photochemical reaction, leading to particle protrusion out of the flat sample surface. A violent change of morphology occurred when the irradiation time exceeded a critical value, producing an abnormal laser ablation image: instead of local material removal, protruding lines are found created at bright fringes. The phenomenon was interpreted as arising from an indirect ablation process.
Influence of a GaN interfacial layer between n+-GaN and active layer on the characteristics of blue light-emitting diodes
Authors: Kim C. S. , Cho H. K. , Hong C.-H. , Lee H. J.
Published: August 2003

Influence of a GaN interfacial layer with graded growth rate and modulated Si-doping between n+-GaN and InGaN/GaN multiquantum well on device performance of blue light-emitting diodes (LEDs) was investigated. It was found that the introduction of a GaN interfacial layer leads to an improvement of current–voltage characteristics and also an enhancement of output power compared to a conventional LED. These could result from the removal of spiral growth hillocks and smoothened morphology at the interface, as confirmed by atomic force microscopy measurements, which might cause the leakage current to decrease and the current injection into the active layer to enhance.
Investigations on GaInNAsSb quinary alloy for 1.5 μm laser emission on GaAs
Authors: Li L. H. , Sallet V. , Patriarche G. , Largeau L. , Bouchoule S. , Travers L. , Harmand J. C.
Published: August 2003

GaInNAsSb quantum wells grown by molecular-beam epitaxy on GaAs substrates were investigated. Intricate incorporation mechanisms of the constituents in this quinary alloy were seen. In highly strained indium-rich alloys, antimony incorporation is strongly reduced, and a beneficial surfactant effect is observed. Due to this effect, high structural quality is preserved even for an uncompensated 2.67% strained multiquantum-well structure. Narrow luminescence linewidth (35 meV) could be achieved near 1.55 μm wavelength with these quantum wells. Laser emission is demonstrated at 1.50 μm with threshold current density of 3.5 kA/cm2.
Experimental and theoretical analysis of optically pumped semiconductor disk lasers
Authors: Zakharian A. R. , Hader J. , Moloney J. V. , Koch S. W. , Brick P. , Lutgen S.
Published: August 2003

We describe the experimental cw power scaling of optically pumped semiconductor disk lasers OPS-DLs and give a detailed insight into the physical mechanism of this type of high-power surface-emitting semiconductor laser with external cavity. Minimizing the thermal resistance between active region and heat sink enables improved efficiency and gives access to high power and excellent beam quality of OPS-DL at 1000 nm. Results from initial numerical modeling are in good agreement with the experimental data, and show that thermal management is a critical parameter for the temperature-driven power shutoff in such devices. The computations are based on the macroscopic thermal transport, spatially resolved in both the radial and longitudinal directions, and coupled to the carrier density rate equations. A quantitative microscopic approach is used for the quantum-well gain and absorption dependence on wavelength, carrier density, and lattice temperature. The dependence of the computed output power on the substrate thickness and detuning are discussed.
Pulse energy enhancement in passive Q-switching operation with a class of Nd:GdxY1−xVO4 crystals
Authors: Liu Junhai , Meng Xianlin , Shao Zongshu , Jiang Minhua , Ozygus Bernd , Ding Adalbert , Weber Horst
Published: August 2003

Passive Q-switching operation with a class of Nd:GdxY1−xVO4 crystals has been demonstrated. Compared to Nd:YVO4 and Nd:GdVO4, the pulse energy produced with Nd:Gd0.64Y0.36VO4 under certain conditions was found to be enhanced by factors of 6.0 and 2.5, while the peak power enhanced by factors of 14.7 and 3.6, respectively. At the incident pump power of 9.6 W, 1.3 W of average output power was obtained, with the pulse energy, peak power, and pulse repetition frequency being 166 μJ, 24.5 kW, and 7.7 kHz, respectively.
A simple model for the prediction of thermal conductivity in pure and doped insulating crystals
Authors: Gaumé Romain , Viana Bruno , Vivien Daniel , Roger Jean-Paul , Fournier Danièle
Published: August 2003

We present a simplified approach to evaluate the thermal conductivity of insulating crystals, taking into account chemical and structural properties. We complete the thermal conductivity evaluation in the case of point defects encountered in luminescent materials. Thermal conductivity coefficients have been determined by photothermal experiments and are in good agreement with the predicted values in Yb:Y3Al5O12 and Yb:Gd3Ga5O12 solid-state lasers.
Quantitative evaluation of stress-field attenuation in stacks of self-assembled Ge islands
Authors: Denker U. , Stoffel M. , Schmidt O. G.
Published: August 2003

We show that a reduced critical thickness observed in stacked layers of self-assembled Ge hut-cluster islands can be quantitatively related to the stress field originating from buried islands. The experimental data for stacks with up to five island layers and Si spacer layers between 3 und 9 nm thick are compared with a simple model for stress-induced reduction of critical thickness. While it is not possible to fit the experimental data using the inverse cubic stress-field scaling of a spherical island, a more accurate fit may be determined by using an inverse linear dependence derived for a pyramid. The best fit was obtained using numerical results calculated for the stress from a truncated pyramid. Si intermixing in stacked layers of hut clusters is experimentally observed and included in the model.
Formation of nanoscale voids and related metallic impurity gettering in high-energy ion-implanted and annealed epitaxial silicon
Authors: Kvit A. , Yankov R. A. , Duscher G. , Rozgonyi G. , Glasko J. M.
Published: August 2003

We have examined nanovoid formation, Fe gettering, and Fe clustering phenomena occurring in epitaxial silicon layers implanted with MeV Si ions. Insights into these phenomena as a function of depth have been gained from detailed analyses by Z-contrast imaging in conjunction with electron energy-loss spectroscopy. Our work has shown at the nanoscale structural and chemical levels that the defects produced by MeV self-ion implantation between the surface and the ion projected range Rp (i.e., in the so-called Rp/2 region) are voids, which provide extremely efficient and aggressive metallic impurity gettering. It has been proposed that the gettering does not occur via chemisorption or silicidation layering on the internal surface of the void walls, as in the well-known case of helium-induced cavities, but rather proceeds in a mode of metal–metal atom binding in the vicinity of the Rp/2 voids.
Ultrafast nonlinearity of antimony polyphosphate glasses
Authors: Falcão Filho E. L. , Bosco C. A. C. , Maciel G. S. , de Araújo Cid B. , Acioli L. H. , Nalin M. , Messaddeq Y.
Published: August 2003

We report on the ultrafast nonlinearity of antimony polyphosphate glasses measured using the Kerr shutter technique. The nonlinear refractive index, n2, was (1.1±0.2)×10−14 cm2/W at 800 nm, and enhancement of n2 by ≈80% was observed by adding 10% of lead oxide in the glass composition. The full width at half-maximum of the third-order correlation signal was 150 fs, which implies a fast response of the samples (⩽100 fs). Nonlinear absorption was negligible in the range of intensities used.
The role of a superconducting seed layer in the structural and transport properties of EuBa2Cu3O7−x films
Authors: Jia Q. X. , Foltyn S. R. , Arendt P. N. , Wang H. , MacManus-Driscoll J. L. , Coulter Y. , Li Y. , Maley M. P. , Hawley M. , Venkataraman K. , Maroni V. A.
Published: August 2003

We report on the structural and transport properties of EuBa2Cu3O7−x (Eu123) films on SrTiO3 substrates. A substrate temperature 100 °C higher than that used to deposit YBa2Cu3O7−x (Y123) is needed to grow high-performance Eu123 films directly on SrTiO3 substrates. However, this high-temperature restriction can be circumvented by inserting a seed layer (∼12 nm) of Y123, Gd123, or Dy123 between the substrate and the Eu123. A structural analysis reveals that a transition region that promotes the growth of the a-axis is first formed on the surface of the substrate if the Eu123 is deposited directly on SrTiO3 at a temperature of less than 800 °C. On the other hand, the use of a seed layer of another superconducting material restructures the Eu123 to the right phase without forming this transition region. Eu123 films with critical temperatures of 94 K and critical current densities of 5.8×106 A/cm2 at 75.5 K have been deposited under standard deposition conditions using such an approach.
A resonant spin lifetime transistor
Authors: Cartoixà X. , Ting D. Z.-Y. , Chang Y.-C.
Published: August 2003

We present a device concept for a spintronic transistor based on the spin relaxation properties a two-dimensional electron gas (2DEG). The device design is very similar to that of the Datta and Das spin transistor. However, our proposed device works in the diffusive regime rather than in the ballistic regime. This eases lithographical and processing requirements. The switching action is achieved through the biasing of a gate contact, which controls the lifetime of spins injected into the 2DEG from a ferromagnetic emitter, thus allowing the traveling spins to be either aligned with a ferromagnetic collector or randomizing them before collection. The device configuration can easily be turned into a memory and a readout head for magnetically stored information.
Direct access to the dispersion relations of multiple anisotropic surface acoustic modes by Fourier image analysis
Authors: Sugawara Y. , Wright O. B. , Matsuda O.
Published: August 2003

We present a method based on a combination of temporal and spatial Fourier image analysis, involving the incorporation of a time-reversed acoustic propagation component, that can be used to obtain the angular dispersion or general dispersion relations of multiple surface acoustic modes excited by a localized pulsed source. The method is applied to the study of acoustic propagation at the surface of the highly anisotropic single-crystal TeO2 coated with a thin gold film.
Curing temperature effects on liquid crystal gels
Authors: Du Fang , Wu Shin-Tson
Published: August 2003

The curing temperature is found to greatly affect the operating voltage, contrast ratio, hysteresis, and response time of the reversed-mode liquid crystal (LC) gels. For the gels using E48 LC host, the optimal curing temperature is around 40–50 °C. An LC gel with contrast ratio exceeds 2000:1 (at ∼2° collection angle), operating voltage lower than 7 Vrms, and response time of about 20 ms is demonstrated. Potential applications of such LC gels for optical switches, displays, and switchable polarizers are emphasized.
Properties of 4-dicyanomethylene-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran-doped Alq layers as optically pumped lasers
Authors: Jakabovič J. , Lengyel O. , Kováč J. , Wong T. C. , Lee C. S. , Lee S. T.
Published: August 2003

The optical properties of tris-(8-hydroxyquinoline) aluminum (Alq) doped with 4-dicyanomethylene-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) in solutions and in solid films were studied by measuring the steady-state excitation and spontaneous emission spectra. The emission peak in the solid films shifted from 628 to 659 nm as the doping concentration of DCM changed from 0.9% to 11%. The optically pumped waveguide lasers were fabricated by coevaporation of Alq films with 1.3 wt % DCM onto gallium arsenide (GaAs) substrates using silicon dioxide (SiO2) and magnesium fluoride (MgF2) as cladding layers. Both types of waveguide laser structures showed optical confinement and simulated emission at a threshold pumping energy near 1.4 μJ. The result suggests a thin MgF2 may be used as a confinement layer that has the advantage of preparation by low-temperature evaporation.
Pressure dependence of energetic (⩽160 keV) focused electron beams arising from heated or cooled (LiNbO3) pyroelectric crystals
Authors: Brownridge James D. , Shafroth Stephen M.
Published: August 2003

An interesting effect, “gas amplification of electron energy” is reported here; namely when a cylindrical pyroelectric crystal such as (LiNbO3) is contained in a concentric cylindrical chamber and is heated and then allowed to cool in a dilute gas the maximum energy of the resultant focused electron beam more than doubles as the pressure increases from 0.05 to 4 mTorr for seven different gases.
X-ray tomographic imaging of the complex refractive index
Authors: McMahon P. J. , Peele A. G. , Paterson D. , Nugent K. A. , Snigirev A. , Weitkamp T. , Rau C.
Published: August 2003

We present a quantitative three-dimensional reconstruction of the complex refractive index of boron clad tungsten fiber using 35 keV x rays. The reconstruction provides a quantitatively accurate measurement with a three-dimensional spatial resolution of approximately 2 μm.
Sensitive detection of plastic explosives with self-assembled monolayer-coated microcantilevers
Authors: Pinnaduwage L. A. , Boiadjiev V. , Hawk J. E. , Thundat T.
Published: August 2003

We report the detection of 10–30 parts-per-trillion levels of pentaerythritol tetranitrate and hexahydro-1,3,5-triazine within 20 s of exposure to a silicon microcantilever with its gold surface modified with a self-assembled monolayer of 4-mercaptobenzoic acid. These measurements correspond to a limit of detection of a few fg.
Liquid-metal-jet anode electron-impact x-ray source
Authors: Hemberg O. , Otendal M. , Hertz H. M.
Published: August 2003

We demonstrate an anode concept, based on a liquid-metal jet, for improved brightness in compact electron-impact x-ray sources. The source is demonstrated in a proof-of-principle experiment where a 50 keV, ∼100 W electron beam is focused on a 75 μm liquid-solder jet. The generated x-ray flux and brightness is quantitatively measured in the 7–50 keV spectral region and found to agree with theory. Compared to rotating-anode sources, whose brightness is limited by intrinsic thermal properties, the liquid-jet anode could potentially be scaled to achieve a brightness >100× higher than current state-of-the-art sources. Applications such as mammography, angiography, and diffraction would benefit from such a compact high-brightness source.
Bidirectional resonant tunneling spin pump
Authors: Ting David Z.-Y. , Cartoixà Xavier
Published: August 2003

We propose a mechanism for achieving bidirectional spin pumping in conventional nonmagnetic semiconductor resonant tunneling heterostructures under zero magnetic field. The device is designed specifically to take advantage of the special spin configuration described by the Rashba effect in asymmetric quantum wells. It induces the simultaneous flow of oppositely spin-polarized current components in opposite directions through spin-dependent resonant tunneling, and can thus generate significant levels of spin current with very little net electrical current across the tunnel structure, a condition characterized by a greater-than-unity current spin polarization. We also present modeling results on temperature dependence and finite device size effects.
Nanosecond exciton recombination dynamics in colloidal CdSe quantum dots under ambient conditions
Authors: Javier Artjay , Magana Donny , Jennings Travis , Strouse Geoffrey F.
Published: August 2003

In solution, CdSe quantum dot exciton recombination is composed of an intrinsic band edge exciton decay and a photoinduced charged exciton, which produces the observation of biexponential decay dynamics. The nearly identical radiative lifetimes of both intrinsic and extrinsic decays scale with the cube of the size, and the nonradiative rate for the intrinsic decay follows the energy gap law, while the charged exciton exhibits strong lattice perturbations arising from Frölich coupling to optical phonons. The charged exciton, which arises from recombination in a particle that has a photo-induced trapped carrier has a temporally resolved Stark shift of ∼20 meV.
Gain and temporal response of AlGaN solar-blind avalanche photodiodes: An ensemble Monte Carlo analysis
Authors: Sevik C. , Bulutay C.
Published: August 2003

Multiplication and temporal response characteristics of p+-n-n+ GaN and n-type Schottky Al0.4Ga0.6N avalanche photodiodes (APD) have been analyzed using the ensemble Monte Carlo method. Reasonable agreement is obtained with the published measurements for a GaN APD without any fitting parameters. In the case of AlGaN, the choice of a Schottky contact APD is seen to improve drastically the field confinement resulting in satisfactory gain characteristics. For the GaN APD, an underdamped step response is observed in the rising edge, and a Gaussian profile damping in the falling edge under an optical pulse with the switching speed degrading towards the gain region. In the AlGaN case, alloy scattering is seen to further slow down the temporal response while displacing the gain threshold to higher fields.
Influence of carrier localization on modulation mechanism in photoreflectance of GaAsN and GaInAsN
Authors: Kudrawiec R. , Sȩk G. , Misiewicz J. , Li L. H. , Harmand J. C.
Published: August 2003

GaAs 0.98 N 0.02 and Ga0.95In0.05As0.98N0.02 layers have been investigated by photoreflectance (PR) and photoluminescence in 10–300 K temperature range. A decrease in PR signal has been found when the temperature was lowered. This effect is attributed to a weakening of modulation efficiency, which is induced by carrier localization that has been evidenced in low temperature photoluminescence. The Kramers–Kronig analysis is proposed as a simple method to determine the evolution of transition intensity with temperature when the change in the PR line shape can take place.
Monolithically series-interconnected GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic devices with an internal backsurface reflector formed by wafer bonding
Authors: Wang C. A. , Huang R. K. , Shiau D. A. , Connors M. K. , Murphy P. G. , O’Brien P. W. , Anderson A. C. , DePoy D. M. , Nichols G. , Palmisiano M. N.
Published: August 2003

GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic (TPV) cells were monolithically interconnected in series to build open-circuit voltage Voc. GaInAsSb epitaxial layers were transferred to GaAs by wafer bonding with SiOx/Ti/Au, which provides electrical isolation of individual cells and forms an internal backsurface reflector. This configuration is compatible with monolithic series interconnection of TPV cells; can mitigate the requirements of filters used for front-surface spectral control; and has the potential to improve TPV device performance. Wafer-bonded GaInAsSb TPV cells exhibit nearly linear voltage building. At a short-circuit current density of 0.4 A/cm2, Voc of a single TPV cell is 0.2 V, compared to 0.37 and 1.8 V for 2- and 10-junction series-interconnected TPV cells, respectively.
Supergain transistors on high-purity float-zone silicon substrate
Authors: Han D. J. , Batignani G. , Guerra A. Del.
Published: August 2003

Since float-zone (FZ) silicon has lower contamination and longer minority carrier lifetime than those in Czochralski silicon and other semiconductor materials, it has potential advantages to fabricate a bipolar junction transistor on the FZ substrate to achieve high gain at very low current levels. In this report, the authors present preliminary experimental results on supergain bipolar junction transistors fabricated on unusual FZ refined high-resistivity silicon substrate and by ion implantation technology. A phosphorus-doped polycrystalline silicon backside gettering layer has been employed to preserve the long carrier lifetime of the high-purity FZ silicon. Bipolar junction transistors have demonstrated high current gain, more than 3300 for ultralow base current levels of 10 pA in this study. Possible applications of high-purity FZ silicon on some advanced semiconductor devices and circuits are discussed in this letter.
Large electric-field induced electron drift velocity observed in an InxGa1−xAs-based p–i–n semiconductor nanostructure at T=300 K
Authors: Liang W. , Tsen K. T. , Ferry D. K. , Wu Meng-Chyi , Ho Chong-Long , Ho Wen-Jeng
Published: August 2003

Transient subpicosecond Raman spectroscopy has been used to measure electron transport properties in an InxGa1−xAs-based semiconductor nanostructure under the application of an electric field. The deduced electron drift velocity has been found to be significantly larger than either GaAs or InP-based p–i–n nanostructures under similar experimental conditions. We attribute this finding to both the smaller electron effective mass and the larger Γ to L (X) energy separations in InxGa1−xAs. The experimental results are compared with ensemble Monte Carlo calculations.
Arsenic penetration behavior and electrical characteristics of As-doped n+ polycrystalline-silicon/high-k gate dielectric (HfO2 and Al2O3) films on Si (100) substrate
Authors: Lee Chihoon , Choi Jihoon , Cho Moonju , Park Jaehoo , Hwang Cheol Seong , Kim Hyeong Joon , Jeong Jaehack , Lee Wonshik
Published: August 2003

Arsenic (As)-doped polycrystalline-silicon gate/HfO2, HfO2–Al2O3, or Al2O3–HfO2–Al2O3/p-type Si (100) metal–oxide–semiconductor capacitors were fabricated using an atomic-layer-deposition technique to investigate the degree of As penetration and the electrical properties of various high-k gate dielectric stacks. The HfO2–Al2O3 stack film showed the highest resistance to As diffusion due to the presence of a rather thick amorphous interface layer. A flatband voltage shift of 100 mV, a leakage current density of −1.07×10−9 A/cm2 at −1 V, a hysteresis voltage <60 mV and excellent reliability characteristics were obtained from this capacitor stack due to the lowest As penetration, less generation of the interface state density, and the lowest surface roughness. Thin Al2O3 capping did not improve the As-diffusion barrier properties due to its island-like surface morphology.