bluuebunny/crossref_metadata_2025 huggingface dataset:
Alignment of InAs quantum dots on a controllable strain-relaxed substrate using an InAs/GaAs superlattice
Authors: Kim Kwang Moo , Park Young Ju , Park Young Min , Hyon Chan Kyeong , Kim Eun Kyu , Park Jung Ho
Published: October 2002

We fabricated InAs self-assembled quantum dots on a strained layer using molecular beam epitaxy. The controllable strained layer consisted of an InAs/GaAs superlattice and a GaAs spacer layer on a GaAs (001) substrate. We formed two-dimensional arrays of quantum dots along the 〈110〉 directions on the partially strain-relaxed layer that is formed using the superlattice system. The increase in the thickness of the partially strain-relaxed layer resulted in stronger alignment of the quantum dots. The aligned quantum dots are applicable to quantum devices, because they confine carriers well, in spite of the existence of dislocation networks. Strongly aligned quantum dots have a lower carrier transition energy because of their larger size and increased relaxation.
High-mobility polymer gate dielectric pentacene thin film transistors
Authors: Klauk Hagen , Halik Marcus , Zschieschang Ute , Schmid Günter , Radlik Wolfgang , Weber Werner
Published: October 2002

We have fabricated pentacene organic thin film transistors with spin-coated polymer gate dielectric layers, including cross-linked polyvinylphenol and a polyvinylphenol-based copolymer, and obtained devices with excellent electrical characteristics, including carrier mobility as large as 3 cm2/V s, subthreshold swing as low as 1.2 V/decade, and on/off current ratio of 105. For comparison, we have also fabricated pentacene transistors using thermally grown silicon dioxide as the gate dielectric and obtained carrier mobilities as large as 1 cm2/V s and subthreshold swing as low as 0.5 V/decade.
Coarse bifurcation analysis of kinetic Monte Carlo simulations: A lattice-gas model with lateral interactions
Authors: Makeev Alexei G. , Maroudas Dimitrios , Panagiotopoulos Athanassios Z. , Kevrekidis Ioannis G.
Published: October 2002

We present a computer-assisted study of “coarse” stability/bifurcation calculations for kinetic Monte Carlo simulators using the so-called coarse timestepper approach presented in A. G. Makeev, D. Maroudas, and I. G. Kevrekidis, J. Chem. Phys. 116, 10083 (2002). Our illustrative example is a model of a heterogeneous catalytic surface reaction with repulsive adsorbate–adsorbate interactions and fast diffusion. Through numerical continuation and stability analysis, we construct one- and two-parameter coarse bifurcation diagrams. We also discuss several computational issues that arise in the process, the most important of which is the “lifting” of coarse, macroscopic initial conditions (moments of adsorbate distributions) to fine, microscopic initial conditions (distributions conditioned on these moments).
Molecular dynamics simulations of a ferroelectric nematic liquid under shear flow
Authors: McWhirter J. Liam , Patey G. N.
Published: October 2002

We perform nonequilibrium molecular dynamics simulations to examine the change in orientational order of a ferroelectric nematic liquid crystal under shear flow, specifically planar Couette flow. The orientational order is found to decrease with increasing shear rate. This drop in orientational order is not due to structural changes but is rather a result of the director dynamics being flow unstable. Employing both nonequilibrium and equilibrium molecular dynamics with the director orientation fixed, we determine the shear and twist viscosities which relate the pressure tensor to the shear or strain rate tensor in a nematic liquid crystal under flow. The Miesowicz viscosities are then obtained from linear combinations of the shear and twist viscosities. The short-range spatial structure of the ferroelectric nematic liquid crystal is similar to that of a ferroelectric tetragonal I lattice. The relative magnitudes of the Miesowicz viscosities can be understood by examining the shear stress response of this lattice to a shear deformation.
Fragmentation processes following core excitation in acetylene and ethylene by partial ion yield spectroscopy
Authors: Piancastelli M. N. , Stolte W. C. , Öhrwall G. , Yu S.-W. , Bull D. , Lantz K. , Schlachter A. S. , Lindle D. W.
Published: October 2002

Partial ion yield spectroscopy provides a very detailed picture of fragmentation processes following core excitation in isolated molecules. We exploit this potential in the analysis of decay processes following C1s→π and C1s→Rydberg excitations in ethylene and acetylene. We show that the relative intensity of spectral features related to the excitation to empty molecular orbitals or to Rydberg states is a function of the time variation of the fragmentation process. Namely, we see an intensity increase in the Rydberg states compared to the molecular orbitals as the fragmentation process becomes more extensive, a result attributable to the diffuse nature of the Rydberg virtual orbitals, which makes spectator decay more likely than participator decay. Therefore, the number of dissociative final states that can be reached from Rydberg excitation is higher than for excitation to empty molecular orbitals. In acetylene, we obtain the first direct evidence of the presence of a σ* excitation embedded in the Rydberg series. The formation of the fragment H2+ in ethylene occurs following a recombination process, while in acetylene it is related to excess vibrational energy stored in the intermediate state. Furthermore, we can use the enhancement in channels corresponding to doubly charged species as an indication for the presence of shape resonances.
Structure of XeN+ clusters (N=3–30): Simulation and experiment
Authors: Gascón José A. , Hall Randall W. , Ludewigt Christoph , Haberland Hellmut
Published: October 2002

We present an experimental and computational study of the photoabsorption line shape of XeN+ clusters. Positively charged xenon clusters have a unique feature not seen in lighter rare gas cation clusters: there are two families of isomers whose ground states consist of neutral atoms surrounding either a linear trimer ion core or a linear tetramer ion core. Interconversion of these two isomers is possible at temperatures as low as 60 K. The combination of simulation and experiment demonstrates the existence of these two families of isomers and their manifestation in the photoabsorption spectra. Clusters present, in general, either of the two cores depending on the number of atoms along the axis that contains the ion core.
The solvation of ions in acetonitrile and acetone. II. Monte Carlo simulations using polarizable solvent models
Authors: Fischer R. , Richardi J. , Fries P. H. , Krienke H.
Published: October 2002

Structural properties and energies of solvation are simulated for alkali and halide ions. The solvation structure is discussed in terms of various site–site distribution functions, of solvation numbers, and of orientational correlation functions of the solvent molecules around the ions. The solvent polarizability has notable effects which cannot be intuitively predicted. In particular, it is necessary to reproduce the experimental solvation numbers of small ions. The changes of solvation properties are investigated along the alkali and halide series. By comparing the solvation of ions in acetone to that in acetonitrile, it is shown that the spatial correlations among the solvent molecules around an ion result in a strong screening of the ion–solvent direct intermolecular potential and are essential to understand the changes in the solvation structures and energies between different solvents. The solvation properties derived from the simulations are compared to earlier predictions of the hypernetted chain (HNC) approximation of the molecular Ornstein–Zernike (MOZ) theory [J. Richardi, P. H. Fries, and H. Krienke, J. Chem. Phys. 108, 4079 (1998)]. The MOZ(HNC) formalism gives an overall qualitatively correct picture of the solvation and its various unexpected findings are corroborated. For the larger ions, its predictions become quantitative. The MOZ approach allows to calculate solvent–solvent and ion–solvent potentials of mean force, which shed light on the 3D labile molecular and ionic architectures in the solution. These potentials of mean force convey a unique information which is necessary to fully interpret the angle-averaged structural functions computed from the simulations. Finally, simulations of solutions at finite concentrations show that the solvent–solvent and ion–solvent spatial correlations at infinite dilution are marginally altered by the introduction of fair amounts of ions.
Shear velocity stabilization of reactive trapped electron modes
Authors: Davydova T. A. , Weiland J. , Yakimenko A. I.
Published: October 2002

One of the main candidates for the explanation of anomalous electron transport in tokamaks is the reactive trapped electron mode driven by compressibility (curvature) and electron temperature gradient. The influence of flow shear on this mode is studied by solving the radial eigenvalue problem. It is found that the Waltz rule stabilization criterion usually gives too strong a stabilization of this mode. The spatial-temporal evolution of unstable trapped electron modes is also investigated. It is found that it is strongly influenced by the continuum part of the spectrum. It is shown that the modes propagate with an acceleration from the source region, thereby widening the region of instability.
Pressure-gradient-driven current induced by a magnetic island in one-dimensional equilibrium plasmas and its implications
Authors: Shaing K. C.
Published: October 2002

It is shown that a magnetic island in a one-dimensional equilibrium plasma, such as a cylindrical plasma or a slab plasma, can induce a pressure-gradient-driven current parallel to the magnetic field similar to the bootstrap current in toroidal plasmas. This current flows on both sides of the island in opposite directions. It modifies the current density profile that drives the island growth in the vicinity of the island and may provide a mechanism for island saturation in such plasmas. The magnetic island induced plasma viscosity damps the helical component of the plasma flow velocity. This alters the polarization drift. The polarization drift can evolve nonlinearly as the island grows. It has opposite signs on both sides of the island. The plasma electrical conductivity is also reduced due to the existence of trapped particles when the island width is finite.
Regrowth of high-quality Al0.3Ga0.7As/GaAs superlattices on laterally oxidized digital AlxGa1−xAs (x>0.80) alloys
Authors: Pickrell G. W. , Chang K. L. , Epple J. H. , Hsieh K. C. , Cheng K. Y.
Published: October 2002

Digital AlxGa1−xAs alloys consisting of a repeated structure of alternating layers of AlAs and GaAs were grown between layers of GaAs. Lateral oxidation characteristics of these digital AlxGa1−xAs (x>0.8) alloys were determined. Decreasing aluminum compositions resulted in slower oxidation rates, similar to true AlxGa1−xAs alloys. Further investigations of the oxidation characteristics of a digital Al0.98Ga0.02As alloy indicate that shorter periods (24.5 monolayers (ML) AlAs/0.5 ML GaAs) showed slower oxidation rates than the longer period alloys (49 ML AlAs/1 ML GaAs or 98 ML AlAs/2 ML GaAs). As the GaAs layers become closer together (that is a digital alloy with a shorter period), the material more closely resembles a true alloy showing slower oxidation rates. Regrowth of an Al0.3Ga0.7As/GaAs superlattice on the laterally oxidized samples, capped with a thin GaAs layer, was performed. The high-temperature regrowth process caused delamination problems for the oxidized conventional AlAs material, resulting in a rough surface morphology causing a decrease in photoluminescence (PL) intensity. Oxidized digital alloys allowed for a large increase in useable surface area and higher PL intensities, indicating the superiority of the digital AlxGa1−xAs alloys for regrowth applications.
Suppression of beam fanning in near-stoichiometric lithium niobate crystal by ultraviolet light irradiation
Authors: Liu Youwen , Jayavel Ramasamy , Nakamura Masura , Kitamura Kenji , Yamaji Takashi , Hatano Hideki
Published: October 2002

A method has been proposed and demonstrated to suppress the beam-fanning effect induced by a green laser beam of 532 nm in near-stoichiometric lithium niobate crystal with an ultraviolet (UV) beam of 350 nm. The beam-fanning effect at different intensities of UV and green beams has been investigated qualitatively and quantitatively. The results show that the beam-fanning effect can be suppressed almost perfectly with the UV light intensity of only about one seventh of the intensity of green light. Suppression of beam fanning is attributed to the large increase of photoconductivity by UV light irradiation.
Kinematics of mechanical and adhesional micromanipulation under a scanning electron microscope
Authors: Saito Shigeki , Miyazaki Hideki T. , Sato Tomomasa , Takahashi Kunio
Published: October 2002

In this paper, the kinematics of mechanical and adhesional micromanipulation using a needle-shaped tool under a scanning electron microscope is analyzed. A mode diagram is derived to indicate the possible micro-object behavior for the specified operational conditions. Based on the diagram, a reasonable method for pick and place operation is proposed. The keys to successful analysis are to introduce adhesional and rolling-resistance factors into the kinematic system consisting of a sphere, a needle-shaped tool, and a substrate, and to consider the time dependence of these factors due to the electron-beam (EB) irradiation. Adhesional force and the lower limit of maximum rolling resistance are evaluated quantitatively in theoretical and experimental ways. This analysis shows that it is possible to control the fracture of either the tool-sphere or substrate-sphere interface of the system selectively by the tool-loading angle and that such a selective fracture of the interfaces enables reliable pick or place operation even under EB irradiation. Although the conventional micromanipulation was not repeatable because the technique was based on an empirically effective method, this analysis should provide us with a guideline to reliable micromanipulation.
Theory of surface-plasmon resonance optical-field enhancement at prolate spheroids
Authors: Calander Nils , Willander Magnus
Published: October 2002

The optical-field enhancement from plasmon resonance at spheroids is studied by solving Maxwell equations using spheroidal vector wave functions. This treatment is an extension of the Mie theory for spheres. The phase retardation or dephasing effects, as studied by finite-element methods in a previous article, are confirmed. Nevertheless, the optical-field enhancement is shown to be substantial under certain resonance conditions. It is suggested that the positions of the resonances in parameter space are determined by global antenna properties and the magnitude of the field enhancement by local plasmon resonance.
Investigation of Ne IX and Ne X line emission from dense plasma using Ross-filter systems
Authors: Kroupp E. , Starobinets A. , Klodzh E. , Ralchenko Yu. V. , Maron Y. , Bogatu I. N. , Fisher A.
Published: October 2002

We report on the application of well-balanced Ross-filter systems for the diagnostics of x-ray emission from a Z-pinch plasma. The composition and thicknesses of the filter layers were so selected to yield the intensities of neon H- and He-like emission lines separately with relatively good accuracy. The systems provide convenient absolute and time-dependent measurements of the emission intensities, and they are particularly useful for relatively low source-light intensities. The applicability of the systems is examined with the aid of time-dependent modeling of the stagnating plasma, using collisional-radiative and radiation transport calculations. The data and modeling are used to yield information on time evolution of the plasma density and temperature at stagnation.
Combined effects of argon addition and substrate bias on the formation of nanocrystalline diamond films by chemical vapor deposition
Authors: Yang Tien-Syh , Lai Jir-Yon , Wong Ming-Show , Cheng Chia-Liang
Published: October 2002

The article reports combined effects of Ar addition and substrate bias on the grain size, microstructure, and growth rate of the diamond films prepared in microwave plasma-enhanced chemical vapor deposition. The nanocrystalline diamond (NCD) films with grain size of 50–100 nm, characterized by Raman spectra, scanning and transmission electron microscopy, were produced at 90–99 vol % Ar concentration under −50 V substrate biasing. The growth rate of the NCD films was 0.7–0.8 μm h−1, larger apparently than those grown by only Ar addition or by substrate bias effect alone. The NCD formation by various mechanisms is discussed, and a revised C2 insertion mechanism by the promotion of H+ ions is proposed to interpret the higher growth rate of the NCD films.
Improved diffusion barriers for copper metallization obtained by passivation of grain boundaries in electroless deposited cobalt-based films
Authors: Kohn A. , Eizenberg M. , Shacham-Diamand Y.
Published: October 2002

The mechanism for improved barrier properties against copper diffusion in electroless deposited Co0.9W0.02P0.08 and Co0.9P0.1 thin films compared to physical vapor deposition (PVD) of cobalt is quantitatively explained. Secondary ion mass spectrometry depth profile measurements were performed on the films deposited on copper substrates after subjecting them to thermal anneals at approximately half the melting temperature of cobalt. A steady-state mode was observed in the form of concentration plateaus which originate from a combined contribution of grain-boundaries’ saturation and copper solubility in the grains. The difference in plateau heights between the samples is assigned to the varying degrees of grain-boundaries’ passivation. For Co0.9W0.02P0.08, the copper concentration in the grain boundaries is negligible and the solubility in the temperature region between 550 and 700 °C may be described as CS≃6×102×exp(−0.52 eV/kT) at. %. The higher-copper concentration plateaus in the Co0.9P0.1 films are a result of a low, non-negligible copper concentration at the grain boundaries, estimated at less than 10 at. %. The copper concentration plateaus in PVD cobalt were significantly larger due to saturation of the grain boundaries.
YSi
2−x
formation in the presence of interfacial SiO2 layer
Authors: Cho M.-H. , Ko D.-H. , Choi Y. G. , Lyo I. W. , Jeong K. , Whang C. N.
Published: October 2002

YSi 2−x films were grown by ion-assisted evaporation in an ultrahigh vacuum (UHV) on Si(111) through a thin SiO2 layer. The films grown on the oxidized Si were changed from a polycrystalline structure with various phases of oxides and silicides into a single-crystalline silicide structure as the annealing temperature was increased in the UHV chamber. The structural change with the annealing temperature implied that various Y2O3 phases formed by the reaction between Y and SiO2 were decomposed and transformed into YSi2−x under the UHV environment. Rutherford backscattering spectroscopy/channeling showed that, although the interfacial crystallinity of the film grown on the SiO2 layer was poorer quality than the film grown on a clean Si surface, a single crystalline YSi2−x layer with high crystallinity (χmin=8%) was grown. These results showed that the difference of the thermal energy and the formation energy between the oxides of Y2O3−SiO2 and yttrium silicide determined the evolution of the silicide layer formation and its crystal structure.
Fluid flow in nanopores: Accurate boundary conditions for carbon nanotubes
Authors: Sokhan Vladimir P. , Nicholson David , Quirke Nicholas
Published: October 2002

Steady-state Poiseuille flow of a simple fluid in carbon nanopores under a gravitylike force is simulated using a realistic empirical many-body potential model for carbon. Building on our previous study of slit carbon nanopores we show that fluid flow in a nanotube is also characterized by a large slip length. By analyzing temporal profiles of the velocity components of particles colliding with the wall we obtain values of the Maxwell coefficient defining the fraction of molecules thermalized by the wall and, for the first time, propose slip boundary conditions for smooth continuum surfaces such that they are equivalent in adsorption, diffusion, and fluid flow properties to fully dynamic atomistic models.
Bonding and ordering of decacyclene molecules on Cu(110) studied by scanning tunneling microscopy
Authors: Schunack M. , Lægsgaard E. , Stensgaard I. , Besenbacher F.
Published: October 2002

We have studied the adsorption of decacyclene (DC) molecules on a Cu(110) surface. The results are compared to previous studies of hexa-tert-butyl decacyclene (HtBDC) also on Cu(110). Opposite to HtBDC, the DC molecules do not induce any restructuring of the Cu(110) surface and no ordered structures are observed at low coverages. This difference is discussed in terms of molecule–substrate and molecule–molecule interactions. Close to full monolayer coverage, DC molecules are forced into ordered arrangements. The ordered structures with different molecule density are described in a model.
A Monte Carlo simulation for the micellization of ABA- and BAB-type triblock copolymers in a selective solvent. II. Effects of the block composition
Authors: Kim Seung Hyun , Jo Won Ho
Published: October 2002

The micellization behaviors of two types of ABA and BAB triblock copolymers with different block compositions in a solvent selective for block A are investigated by using the grand canonical Monte Carlo simulation combined with the multiple histogram method. The block composition and sequence are found to significantly control the micelle formation and the association behavior for both types of triblock copolymers. The thermodynamic analysis shows that ΔH0 and ΔS0 decreases as the length of insoluble block B decreases, indicating that the driving force toward micellization decreases with decreasing the B block length for both types of copolymers. On the other hand, BAB copolymers show less negative ΔH0 and more negative ΔS0 than ABA at all the compositions except at fB=0.20, where the reduced capability of BAB copolymer to form micelle leads to increase the amount of dangling chains in the micelle, yielding less negative ΔS0. The basic micellar parameters such as the micellar size, the micellar shape, and the distribution of segments in a micelle are examined as a function of block composition and sequence. It is evident that the micelle formation of triblock copolymer follows the closed association mechanism, irrespective of the block composition and sequence.
Methodology of an optical pump-terahertz probe experiment: An analytical frequency-domain approach
Authors: Němec H. , Kadlec F. , Kužel P.
Published: October 2002

Time-resolved optical pump-terahertz (THz) probe experiments are currently used to obtain information about the ultrafast dynamics of photoexcited carriers in semiconductors and about the far-infrared nonlinear response during solvation in liquids. The THz dynamics in such photoexcited systems is fully characterized by a two-dimensional nonlinear susceptibility. We have developed a frequency-domain analytical method for the direct extraction of this susceptibility from the experimental data. Following effects are taken into account: dispersive propagation of radiation in a photoexcited medium, refraction on its surfaces, THz sensor responsivity, and spatio-temporal transformations of the THz pulses. Strategies for possible experiments are discussed.
Observation and characterization of intermolecular homonuclear single-quantum coherences in liquid nuclear magnetic resonance
Authors: Chen Zhong , Chen Zhiwei , Zhong Jianhui
Published: October 2002

Intermolecular zero-quantum and double-quantum coherences (iZQCs and iDQCs) are frequently discussed in literature since they may provide novel contrast mechanisms in magnetic resonance imaging and possibilities for high-resolution spectra in an inhomogeneous and unstable magnetic field. In a previous paper [J. Chem. Phys. 115, 10769 (2001)], we have studied both theoretically and experimentally the properties of iZQC and iDQC nuclear magnetic resonance (NMR) signals related to intermolecular dipolar interactions in two-component systems. In this paper, the investigation is extended to homonuclear intermolecular single-quantum coherences (iSQCs) from the second-order spin interactions, which have not been observed and studied previously. Selective excitation was used to suppress the strong conventional single-spin single-quantum signals. A combination of dipolar field treatment and Torrey equation was used to derive a general theoretical expression for the time evolution of spins with arbitrary flip angles of rf pulses. The expression was used to predict the optimal conditions for iSQCs among highly polarized spins in liquid. Dependence of the iSQC signals on the experimental parameters was measured and analyzed to verify the theoretical predictions. For the first time, signals from pure homonuclear two-spin iSQCs free of much larger conventional single-spin single-quantum signals, and intermolecular iSQC cross peaks in homonuclear pulsed-field gradient COSY experiments were observed and characterized, in one- and two-dimensional (1D and 2D) experiments, respectively. The use of coherence-selection gradients tilted at the magic angle results in the suppression of iSQC cross peaks. It provides strong evidence that the observed signals originate from distant dipolar interactions. Relaxation and diffusion properties of iSQCs in multiple-component samples were characterized and analyzed as well as the optimal rf flip angles. Theoretical and experimental results presented herein demonstrate that the signals from the homonuclear second-order iSQCs not only have a similar signal intensity as iZQCs or iDQCs, all of which are much stronger than that from three-spin iSQCs reported previously, but also provide spatial information related to dipolar correlation scales similar to iZQCs and iDQCs, which is not present in conventional SQC experiments. All 1D and 2D NMR experimental observations based on single- and multiple-component samples are in excellent agreement with the theoretical predictions. The quantitative study of iSQCs provides a better understanding of their unique mechanisms, and may find useful applications in NMR analyses such as sample purification and/or preparation of metabolites, biofluids, and natural compounds dissolved in nondeuterated solvents.
Vector correlations in dissociative photoionization of O2 in the 20–28 eV range. II. Polar and azimuthal dependence of the molecular frame photoelectron angular distribution
Authors: Lafosse A. , Brenot J. C. , Guyon P. M. , Houver J. C. , Golovin A. V. , Lebech M. , Dowek D. , Lin P. , Lucchese R. R.
Published: October 2002

A combined experimental and theoretical study of the polar and azimuthal dependence of the molecular frame photoelectron angular distributions (MFPADs) for inner-valence-shell photoionization of the O2 molecule into the O2+(B 2Σg−,3 2Πu,c 4Σu−) states is reported. The measured MFPADs, for each orientation of the molecular axis with respect to the linear polarization of the synchrotron radiation, are derived from the spatial analysis of the (VO+,Ve,P) vector correlation, where the nascent ion and electron velocity vectors VO+ and Ve are determined for each dissociative photoionization (DPI) event using imaging and time of flight resolved coincidence technique as described in the companion paper of this series [J. Chem. Phys. 114, 6605 (2001)]. Expressed in the general form of four FLN(θe) functions which contain all the dynamical information about the photoionization processes, they are compared with the MFPADs computed using the multichannel Schwinger configuration interaction method. A very satisfactory agreement is found. When the lifetime of the O2+ ionic states is a significant fraction of the rotational period, the rotational motion of the molecule is included in the quantal derivation of the MFPADs. Measured MFPADs are also reported for the additional DPI process identified in Paper I, and for DPI involving the excitation of the neutral (3 2Πu,4sσg) Rydberg state.
Vibrational eigenstates of NO2 by a Chebyshev-MINRES spectral filtering procedure
Authors: Bäck Andreas
Published: October 2002

It is shown that the minimum residual algorithm (MINRES) is able to generate spectral filters sharp enough to obtain bound vibrational eigenstates of NO2 (J=0) by direct access in the most dense part of the spectrum even for the worst near-degeneracy cases. The same is not true for filters constructed as an expansion of the spectral density operator via Chebyshev polynomials. The best performance is obtained in a progressively restarted scheme in which the sharpness of the filter is increased between subsequent restarts to accompany the refinement of the state. Best efficiency and most convenient handling is obtained with early restarts using the Chebyshev-filter, whereas the MINRES-filter is more efficient later in the filtering procedure, where sharp peaking of the filter action is necessary.
Stabilization of tokamak plasma by lithium streams
Authors: Zakharov Leonid E.
Published: October 2002

The theory of stabilizing free-boundary magnetohydrodynamic instabilities in tokamaks by liquid lithium streams driven by magnetic propulsion is formulated. It is shown that even for uniform current distribution lithium streams open stability windows. Also, it was found that while the conventional, wall-locked, resistive wall mode can be well suppressed by the flow, a new, stream-locked mode determines the limits of the flow stabilization.
Electrical characterization of acceptor levels in Mg-doped GaN
Authors: Nakano Yoshitaka , Jimbo Takashi
Published: October 2002

Thermal admittance and current deep-level transient spectroscopy techniques have been applied to Schottky diodes fabricated on Mg-doped GaN grown by metalorganic chemical vapor deposition to investigate the dependence of the Mg acceptor levels on the annealing temperature. Both measurement techniques revealed two deep acceptor levels with activation energies at ∼135 and ∼160 meV above the valence band. The former level was only seen when the samples were annealed at temperatures between 650 and 700 °C, and its presence corresponds with a significant increase in effective acceptor concentration, as confirmed by capacitance–voltage measurements. Therefore, this acceptor level is considered to dominate the electrical activation of Mg in GaN.
Photoelectrical properties of sprayed In2−2xAl2xS3−3yO3y alloys
Authors: Bhira L. , Belgacem S. , Bernède J. C.
Published: October 2002

In 2−2x Al 2x S 3−3y O 3y alloys have been prepared on Pyrex™ glass substrates by the spray pyrolysis technique. The shape of the photoconductivity spectrums Iph (hν) and the variations Iph (f ) and Iph (V) allow us to understand the conduction mechanism and the photocarriers’ recombination. For low compositions (x⩽0.2), Iph(V) parabolic variation shows that this conduction is limited by the space charge zone in accordance with Child’s law (Iph∝V2) [N. F. Mott and R. W. Gurney, Electronics Processes in Ionic Crystals (Oxford, New York, 1940), p. 463]. The exploitation of such variation shows that the density of the trap centers increases with the composition. For x⩾0.4, the electrical conduction instead follows Ohm’s law. In the same way, the analysis of an extension of Devore’s model [Phys. Rev. 102, 86 (1956)] as (Iphhν)2 versus photon energy hν shows an increase of the band gap energy Eg according to a parabolic profile. On the other hand, for low compositions (x=0,0.05,0.1,0.2), the conductivity study as a function of the temperature presents a deviation to Arrhenius’s law in the intermediate temperature domain ranging from 80 to 330 K. Moreover, in this domain, the study of the activation energy Ea(T) according to Werner’s model [Solid State Phenom. 37, 214 (1994)] supposing potential fluctuations at the grain boundaries yields the values of the barrier high φb and the standard deviation σφ. From these results, we see that Eg increased versus x and that the electrical properties are essentially preserved for low aluminum concentration films. This may be due to a minor presence of an Al2O3 phase for such deposits.
Microcathodoluminescence spectroscopy of defects in Bi2O3-doped ZnO grains
Authors: Sun X. L. , Brillson L. J. , Chiang Y.-M. , Luo J.
Published: October 2002

Low-temperature (10 K) microcathodoluminescence spectroscopy with a scanning electron microscope has been used to probe shallow and deep levels within the grains of Bi2O3-doped ZnO powders prepared under different process conditions. These studies reveal strong electronic state variations between the bulk and boundaries of these submicron ZnO grains, including new emissions at ∼3.329 eV at grain boundaries attributed to Bi phase segregation and/or the formation of new electrically active defects. The characteristic deep level defect emission at 2.4 eV due to nonstoichiometric point defects varies spatially, appearing much stronger in the grain bulk than at the boundaries. The 3.329 eV emissions become more pronounced with higher doping concentration and annealing temperature, indicating a strong dependence on processing. The strong temperature and composition dependence of these boundary-specific features are consistent with the known influence of process conditions on the varistor properties of ZnO powder.
Theory of surface noise under Coulomb correlations between carriers and surface states
Authors: Kochelap V. A. , Sokolov V. N. , Bulashenko O. M. , Rubı́ J. M.
Published: October 2002

We present a theory of the surface noise in a nonhomogeneous conductive channel adjacent to an insulating layer. The theory is based on the Langevin approach which accounts for the microscopic sources of fluctuations originated from trapping–detrapping processes at the interface and intrachannel electron scattering. The general formulas for the fluctuations of the electron concentration, electric field as well as the current-noise spectral density have been derived. We show that due to the self-consistent electrostatic interaction, the current noise originating from different regions of the conductive channel appears to be spatially correlated on the length scale correspondent to the Debye screening length in the channel. The expression for the Hooge parameter for 1/f noise, modified by the presence of Coulomb interactions, has been derived.
How to calculate the neoclassical viscosity, diffusion, and current coefficients in general toroidal plasmas
Authors: Sugama H. , Nishimura S.
Published: October 2002

A novel method to obtain the full neoclassical transport matrix for general toroidal plasmas by using the solution of the linearized drift kinetic equation with the pitch-angle-scattering collision operator is shown. In this method, the neoclassical coefficients for both poloidal and toroidal viscosities in toroidal helical systems can be obtained, and the neoclassical transport coefficients for the radial particle and heat fluxes and the bootstrap current with the nondiagonal coupling between unlike-species particles are derived from combining the viscosity-flow relations, the friction-flow relations, and the parallel momentum balance equations. Since the collisional momentum conservation is properly retained, the well-known intrinsic ambipolar condition of the neoclassical particle fluxes in symmetric systems is recovered. Thus, these resultant neoclassical diffusion and viscosity coefficients are applicable to evaluating accurately how the neoclassical transport in quasi-symmetric toroidal systems deviates from that in exactly symmetric systems.
Bonding characterization, density measurement, and thermal diffusivity studies of amorphous silicon carbon nitride and boron carbon nitride thin films
Authors: Chattopadhyay S. , Chen L. C. , Chien S. C. , Lin S. T. , Chen K. H.
Published: October 2002

Thermal diffusivity (α) of amorphous silicon carbon nitride (a-SiCxNy) and boron carbon nitride (a-BCxNy) thin films on crystalline silicon has been studied as a function of the carbon content and thickness of the films using the traveling wave technique. The thermal diffusivity showed a steady fall from ∼0.35 to about 0.15 cm2/s for a-SiCxNy films as the carbon content increased from 30 to ∼70 at. %. This decrease in thermal diffusivity was also accompanied by a decrease in the film density from 3.35 to ∼2.3 g/cm3 as a function of the carbon content of the a-SiCxNy films. In case of a-BCxNy, a peak in thermal diffusivity (0.6 cm2/s) was detected at a carbon concentration of ∼25 at. % which reduced to 0.2 cm2/s for a carbon concentration of ∼60 at. % in the films. The value of the density also showed a peak (∼2 g/cm3) at a carbon concentration of 25 at. % before decreasing in the a-BCxNy films. A study of bonding characterization revealed a dominant lower coordinated C(sp)–N phase at higher carbon concentrations that played a detrimental role in the film properties observed. A critical issue of the thickness dependence of thermal diffusivity in a layered structure of a-SiCxNy and a-BCxNy on silicon is addressed with information extracted from aluminum thin films on different substrates. An empirical model is proposed which can explain the reported thickness and substrate dependence of the thermal diffusivity data.
Growth of CrSi2 by radiation enhanced diffusion at room temperature
Authors: Shannon J. M. , Hoyland J. D. , Gerstner E. G.
Published: October 2002

Chromium disilicide layers have been formed by ion bombardment through a chromium layer into silicon at room temperature. It was found that the thickness of the silicide formed under the chromium layer was similar for hydrogenated amorphous silicon, laser crystallized silicon, and crystalline silicon substrates. The growth was consistent with a radiation enhanced diffusion mechanism in which the rate limiting process was the diffusion of silicon or chromium through the silicide. It was found that the amount of chromium in the silicide and its thickness were proportional to the square root of the total amount of energy dissipated by the ion beam at the interface between the chromium and the substrate.
Influence of cluster-assembly parameters on the field emission properties of nanostructured carbon films
Authors: Ducati C. , Barborini E. , Piseri P. , Milani P. , Robertson J.
Published: October 2002

Supersonic cluster beam deposition has been used to produce films with different nanostructures by controlling the deposition parameters such as the film thickness, substrate temperature and cluster mass distribution. The field emission properties of cluster-assembled carbon films have been characterized and correlated to the evolution of the film nanostructure. Threshold fields ranging between 4 and 10 V/μm and saturation current densities as high as 0.7 mA have been measured for samples heated during deposition. A series of voltage ramps, i.e., a conditioning process, was found to initiate more stable and reproducible emission. It was found that the presence of graphitic particles (onions, nanotube embryos) in the films substantially enhances the field emission performance. Films patterned on a micrometer scale have been conditioned spot by spot by a ball-tip anode, showing that a relatively high emission site density can be achieved from the cluster-assembled material.
Efficient simulation of quantum state reduction
Authors: Brody Dorje C. , Hughston Lane P.
Published: October 2002

The energy-based stochastic extension of the Schrödinger equation is a rather special nonlinear stochastic differential equation on Hilbert space, involving a single free parameter, that has been shown to be very useful for modeling the phenomenon of quantum state reduction. Here we construct a general closed form solution to this equation, for any given initial condition, in terms of a random variable representing the terminal value of the energy and an independent Brownian motion. The solution is essentially algebraic in character, involving no integration, and is thus suitable as a basis for efficient simulation studies of state reduction in complex systems.
Magnetotransport in variable-coupling one-dimensional ballistic constrictions
Authors: Roddaro Stefano , Piazza Vincenzo , Beltram Fabio , Wegscheider Werner , Liang Chi-Te , Pepper Michael
Published: October 2002

A scheme to produce closely spaced high-quality one-dimensional electronic channels is presented. It is based on standard nanolithography fabrication applied to a single, wide quantum well AlGaAs/GaAs heterostructure where the spatial separation of the wire modes is given by Coulomb repulsion. The lack of a composition-induced barrier in the double layer leads to electron mobility of 5×106 cm2/Vs in the starting double two-dimensional system. This in turn yields high-quality one-dimensional channels following nanolithography, as shown by our detailed analysis of the magnetotransport properties of the system. Variable coupling is achieved by split-gate polarization and external magnetic-field intensity and orientation. The relevance for the implementation of coherent nanodevices of this fabrication approach is discussed.
Characteristics of ZrO2 gate dielectric deposited using Zr t–butoxide and Zr(NEt2)4 precursors by plasma enhanced atomic layer deposition method
Authors: Kim Yangdo , Koo Jaehyoung , Han Jiwoong , Choi Sungwoo , Jeon Hyeongtag , Park Chan-Gyung
Published: October 2002

Scaling down of the microelectronic devices feature size is demanding high-quality ultra thin high-k gate dielectric as a potential replacement for SiO2 gate dielectric. Among the high-k materials, ZrO2 is considered as a potential alternative to SiO2 gate dielectric below 2 nm due to its thermodynamical stability as well as excellent electrical properties. For these reasons, we investigated ZrO2 films prepared by the normal atomic layer deposition (ALD) and plasma enhanced ALD (PEALD) techniques using Zr t–butoxide and Zr(NEt2)4 metal organic precursors. This study showed that ZrO2 films deposited by the PEALD method showed generally improved film qualities with relatively low-leakage current, negligible hysteresis, and low-carbon incorporation compared to those of the films deposited by the conventional ALD method. Also, ZrO2 films deposited using Zr(NEt2)4 precursor showed better film qualities than those of films deposited using Zr t–butoxide. Especially, ZrO2 films deposited using Zr(NEt2)4 with oxygen plasma showed the leakage current as low as about 2.5×10−9 A/cm2 with an equivalent oxide thickness value of about 1.4 nm. This study demonstrated the possible application of the PEALD technique for the high-quality ultra thin high-k gate dielectric film deposition.
Control over exciton confinement versus separation in composite films of polyfluorene and CdSe nanocrystals
Authors: Tseng Y. C. , Tzolov M. , Sargent E. H. , Cyr P. W. , Hines M. A.
Published: October 2002

Composite films of polyfluorene derivative poly(9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl) and cadmium selenide nanocrystals were investigated using photomodulation spectroscopy exciting only the nanocrystal phase. Efficient charge separation resulting in hole injection into the polymer was observed in films in which the nanocrystals had been stripped of surface trioctylphosphine oxide passivating groups. The resulting induced absorption band centered at 1.2 eV was assigned to bipolarons or π-dimers formed on the polymer in the near vicinity of the polymer/nanocrystal interface. The intensity dependence of this band suggests bimolecular recombination, supporting the interpretation of the band as due to charge separation. The measured wide distribution of lifetimes for the photogenerated states confirms the glassy nature of the polymer.
Near infrared to visible photon upconversion in Re4+ doped Cs2ZrBr6
Authors: Aebischer Annina , Salley G. Mackay , Güdel Hans U.
Published: October 2002

Two visible luminescence bands are observed in Re4+:Cs2ZrBr6 under NIR excitation at temperatures between 10 and 300 K. We perform absorption, luminescence, excitation, and time resolved spectroscopic studies at various temperatures to characterize this upconversion luminescence and its driving mechanism. From the time dependent measurements we assign a ground state absorption/energy transfer upconversion type mechanism to the upconversion process. The visible to near-infrared photon ratio is 0.008 at T=50 K with a laser power density of 415 W/cm2. Most of the visible luminescence is quenched by a strong cross-relaxation process out of the highest emitting level as well as intense radiative interexcited state transitions. The ReBr62− unit is one of very few transition metal complexes with three emitting electronic levels in the near-infrared and visible range of the electromagnetic spectrum.
Cavity ring down spectroscopy measurements of absolute CN concentrations during flame deposition of diamond
Authors: Stolk R. L. , ter Meulen J. J.
Published: October 2002

Cavity ring down spectroscopy (CRDS) was used for measuring absolute concentration profiles of the CN (cyano) radical during oxyacetylene flame deposition of diamond. Profiles were measured for three different nitrogen additions to the flame. Novel ways for the determination of the lateral position of the laser beam and its height above the deposition substrate are presented. Measured column density profiles show that the influence of added nitrogen is limited to a central area within a radius of 2.7 mm. Outside this area the profiles are hardly influenced by the added nitrogen flow. Comparison with work of S. J. Firchow and K. L. Menningen [J. Phys. D: Appl. Phys. 32, 937 (1999)] shows a good correspondence. Absolute concentration profiles were obtained by applying Abel inversion processing and using a temperature distribution resulting from numerical simulations. The resulting profiles show an off-axis maximum for all three nitrogen additions. With an increasing nitrogen addition the maximum and central concentrations also increased. Upon inspection of the profiles it follows that the gas phase above the deposition area can be divided into three sections: a central one in which the added nitrogen is the dominant nitrogen source, an outer one where the nitrogen from the ambient plays a dominant role, and an intermediate zone where both nitrogen sources have a significant influence. The latter is the area in which the maximum of the CN concentration is situated. Previous laser-induced fluorescence (LIF) profiles of CN, measured under very similar experimental conditions, show similar distributions. Differences between the CRDS and LIF results are discussed.
Effects of finite radial excursion on the slowing-down distribution of toroidally circulating energetic ions produced by tangential neutral beam injection
Authors: Wang Shaojie , Ozeki Takahisa , Xie Jikang , Hayashi Nobuhiko
Published: October 2002

Slowing-down of the toroidally circulating energetic ions produced by tangential neutral beam injection is described by the reduced drift kinetic equation, in terms of three constants of motion (the energy, the magnetic moment and the canonical toroidal momentum), to take into account the effects of finite radial excursion of guiding-centers of the energetic ions. An approximate analytical solution to the slowing-down equation is obtained to describe the equilibrium slowing-down distribution of the toroidally circulating energetic ions in the core region of a large-aspect-ratio tokamak, with the effects of finite radial excursion included.
Short wavelength temperature gradient driven modes in tokamaks. II. Electron mode
Authors: Hirose A. , Elia M.
Published: October 2002

A short wavelength electron mode driven by toroidicity and electron temperature gradient has been found in stability analysis of tokamaks based on a fully kinetic, electromagnetic integral equation code. The mode exists in the regime (k⊥ρi)2>1 and propagates in the electron diamagnetic direction (electron mode with ωr>0). The instability vanishes in negative magnetic shear (s<0). In positive shear regime, the growth rate increases with s almost linearly, γ∝s. Neither electrons nor ions are adiabatic.