bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth of large-scale GaN nanowires and tubes by direct reaction of Ga with NH3
Authors: He Maoqi , Minus Indira , Zhou Piezhen , Mohammed S. Noor , Halpern Joshua B. , Jacobs Randy , Sarney Wendy L. , Salamanca-Riba Lourdes , Vispute R. D.
Published: July 2002

Large-scale wurtzite GaN nanowires and nanotubes were grown by direct reaction of metal gallium vapor with flowing ammonia in an 850–900 °C horizontal oven. The cylindrical structures were as long as 500 μm with diameters between 26 and ∼100 nm. Transmission electron microscopy, scanning electron microscopy, and x-ray diffraction were used to measure the size and structures of the samples. Preliminary results show that the size of the nanowires depends on the temperature and the NH3 flow rate. The growth mechanism is discussed briefly. The simple method presented here demonstrates that GaN nanowires can be grown without the use of a template or catalyst, as reported elsewhere.
Classical mapping approaches for nonadiabatic dynamics: Short time analysis
Authors: Golosov Andrei A. , Reichman David R.
Published: July 2002

A systematic approach to improve the short time dynamics for classical mapping treatments of nonadiabatic dynamics is developed. This approach is based on the Taylor expansion of time-dependent observables around t=0. By sampling initial conditions in a manner that renders accurate static moments of the electronic population, it is shown that the short time electronic population dynamics described by classical mapping approaches for nonadiabatic dynamics can be greatly improved. The approach is illustrated on the example of the spin-boson model. For this problem, the analysis of the expansion coefficients reveals why classical mapping approaches to nonadiabatic dynamics often perform much worse for energetically biased reactions than they do for reactions with zero bias. The analysis presented here not only allows for the improvement of short time (and often long time) behavior, but also points to a systematic way of accessing how accurate a given classical mapping approach should be for a given problem.
Carrier capture into InAs/GaAs quantum dots via multiple optical phonon emission
Authors: Feldmann J. , Cundiff S. T. , Arzberger M. , Böhm G. , Abstreiter G.
Published: July 2002

We have performed nondegenerate optical pump–probe experiments with subpicosecond laser pulses to investigate the dynamics of carrier capture into self-assembled InAs/GaAs quantum dots. For low excitation intensities the capture rate depends only slightly on excitation density, whereas a clear dependence on crystal temperature is observed. This temperature dependence can be explained by assuming that the emission of four longitudinal optical (LO) phonons and one longitudinal acoustic phonon is the dominant capture process for electrons. This assumption is consistent with the finding that, for electrons in the conduction band, the energetic separation between the single quantized quantum dot state and the onset of the two-dimensional states of the InAs wetting layer is slightly more than the energy of four LO phonons.
Stretch dynamics of flexible dendritic polymers in solution
Authors: Biswas Parbati , Kant Rama , Blumen Alexander
Published: July 2002

We study the stretch dynamics of flexible dendritic polymers (dendrimers and stars) under external forces. We work in the framework of the bead-spring model with hydrodynamic interactions (HI) and take spacers of different length into account. The applied fields may, e.g., be of mechanical or electrical origin. We study the motion of a specific monomer, the time evolution of the stretch (the mean distance of the monomer on which the force acts from the center of mass of the polymer) and also the elastic moduli. We analyze how these dynamic properties depend on the underlying topology, i.e., on the number of generations for dendrimers and the length and number of branches for stars. As a special point we assess in how far the HI method utilized here (the Kirkwood–Riseman scheme) is stable for dendritic structures. Characteristic for the topology is the intermediate dynamics (between short and long times). It turns out that, different from stars, for dendrimers the stretch dynamics is for intermediate times close to logarithmic; hence the crossover in behavior at intermediate times is characteristic of the polymer’s topology.
On the radiated noise computed by large-eddy simulation
Authors: Seror C. , Sagaut P. , Bailly C. , Juvé D.
Published: July 2002

This paper addresses the problem of the estimation of the noise radiated by forced isotropic turbulence using an hybrid large-eddy simulation/Lighthill analogy approach. The scale separation associated with the LES approach leads to splitting the acoustic source term as the sum of several contributions. The subgrid scale and high frequency contributions to radiated acoustic spectrum are first evaluated on the ground of filtered direct numerical simulations. The parametrization of subgrid scale effects based on a scale similarity model is addressed. Both a priori and a posteriori tests demonstrate the efficiency of the proposed model.
X-ray photoelectron spectroscopy and micro-Raman analysis of conductive RuO2 thin films
Authors: Bhaskar S. , Dobal P. S. , Majumder S. B. , Katiyar R. S.
Published: July 2002

Ruthenium oxide (RuO2) was synthesized in thin film and powder forms using the solution chemistry technique. The oxide electrodes on Si substrates were characterized in terms of their structure, composition, stoichiometry, and conductivity. X-ray lattice parameter calculations and micro-Raman analysis revealed the rutile structure in the material. Both films and powders exhibited an unassigned Raman band at about 477 cm−1 in their Raman spectra. Performing peak frequency calculations for B2g and A1g modes of RuO2 using the rigid-ion model, which ruled out the possibility that this band originated from disorder induced symmetry, allowed silent mode. Based on the x-ray photoelectron spectroscopy (XPS) and temperature dependent Raman studies, this band was assigned to hydrated RuO2. XPS characterizations of our samples revealed minute surface contamination of oxygen and chlorine, probably due to the film preparation and high temperature deposition processes. Films with uniform microstructure, low surface roughness, and good electrical properties meet the requirements for serving as the bottom electrode for the ferroelectric capacitor.
High-electron-mobility AlGaN/GaN heterostructures grown on Si(111) by molecular-beam epitaxy
Authors: Semond F. , Lorenzini P. , Grandjean N. , Massies J.
Published: July 2002

We report on the growth of high-electron-mobility AlGaN/GaN heterostructures on silicon (111) substrates by molecular-beam epitaxy using ammonia as the nitrogen source. Crack-free GaN layers up to 3 μm are obtained. Their optical properties are similar to those commonly obtained for films grown on sapphire, but photoluminescence spectra indicate that GaN on Si(111) is in a tensile strain state which increases with the epitaxial layer thickness. Such uncracked GaN buffer layers grown on Si(111) have been used to achieve undoped AlGaN/GaN heterostructures having electron mobilities exceeding 1600 cm2/V s at room temperature and 7500 cm2/V s at 20 K.
Substrate-dependent crystallization and enhancement of visible photoluminescence in thermal annealing of Si/SiO2 superlattices
Authors: Khriachtchev Leonid , Kilpelä Olli , Karirinne Suvi , Keränen Jaakko , Lepistö Toivo
Published: July 2002

We study annealing of Si/SiO2 superlattices on fused quartz and crystalline Si substrates. Under annealing at 1200 °C, the superlattices on Si undergo partial crystallization involving clusterization of Si layers through ultrathin (1 nm) oxide, and visible photoluminescence (∼2.1 eV) strongly increases for the samples with thinner Si layers (⩽2.5 nm). The annealed superlattices on quartz exhibit a higher disorder, tensile stress, and weaker visible photoluminescence. The results do not support assignment of the observed visible photoluminescence to quantum confinement in Si crystallites but rather indicate that it originates from Si=O bonds stabilized in the Si/SiO2 network.
Initial physics results from the National Spherical Torus Experiment
Authors: Kaye S. M. , Bell M. G. , Bell R. E. , Bialek J. , Bigelow T. , Bitter M. , Bonoli P. , Darrow D. , Efthimion P. , Ferron J. , Fredrickson E. , Gates D. , Grisham L. , Hosea J. , Johnson D. , Kaita R. , Kubota S. , Kugel H. , LeBlanc B. , Maingi R. , Manickam J. , Mau T. K. , Maqueda R. J. , Mazzucato E. , Menard J. , Mueller D. , Nelson B. , Nishino N. , Ono M. , Paoletti F. , Paul S. , Peng Y-K. M. , Phillips C. K. , Raman R. , Ryan P. , Sabbagh S. A. , Schaffer M. , Skinner C. H. , Stutman D. , Swain D. , Synakowski E. , Takase Y. , Wilgen J. , Wilson J. R. , Zhu W. , Zweben S. , Bers A. , Carter M. , Deng B. , Domier C. , Doyle E. , Finkenthal M. , Hill K. , Jarboe T. , Jardin S. , Ji H. , Lao L. , Lee K. C. , Luhmann N. , Majeski R. , Medley S. , Park H. , Peebles T. , Pinsker R. I. , Porter G. , Ram A. , Rensink M. , Rognlien T. , Stotler D. , Stratton B. , Taylor G. , Wampler W. , Wurden G. A. , Xu X. Q. , Zeng L. ,   NSTX Team
Published: July 2002

The mission of the National Spherical Torus Experiment (NSTX) is to extend the understanding of toroidal physics to low aspect ratio (R/a≃1.25) in low collisionality regimes. NSTX is designed to operate with up to 6 MW of high harmonic fast wave (HHFW) heating and current drive, 5 MW of neutral beam injection (NBI) and co-axial helicity injection (CHI) for noninductive startup. Initial experiments focused on establishing conditions that will allow NSTX to achieve its aims of simultaneous high βt and high-bootstrap current fraction, and to develop methods for noninductive operation, which will be necessary for Spherical Torus power plants. Ohmic discharges with plasma currents up to 1 MA and with a range of shapes and configurations were produced. Density limits in deuterium and helium reached 80% and 120% of the Greenwald limit, respectively. Significant electron heating was observed with up to 2.3 MW of HHFW. Up to 270 kA of toroidal current for up to 200 ms was produced noninductively using CHI. Initial NBI experiments were carried out with up to two beam sources (3.2 MW). Plasmas with stored energies of up to 140 kJ and βt=21% were produced.
Operation of a three-junction single-electron pump with on-chip resistors
Authors: Lotkhov S. V. , Bogoslovsky S. A. , Zorin A. B. , Niemeyer J.
Published: July 2002

We report on the operation of a single-electron pump (comprising three Al/AlOx/Al tunnel junctions and two gates) connected to the bias electrodes through the compact on-chip Cr resistors, R≈60 kΩ>Rk=h/e2≈26 kΩ. The function of the resistors in this so-called R pump was to suppress electron cotunneling, the process which otherwise severely deteriorates the performance of few-junction single-electron devices. When a harmonic ac drive of frequency f of several MHz was applied to the gates, the current–voltage curve of the R pump exhibited remarkably horizontal current steps at I=ef. We show that the use of the resistors is capable of substantially increasing the accuracy of the pump in comparison to operation of the pump without resistors.
Coherent effects of ion–electron collisions in a strong laser field
Authors: Fraiman G. M. , Balakin A. A. , Mironov V. A.
Published: July 2002

Results of numerical and theoretical investigations of electron–ion collision processes in plasma in a strong laser field are represented. It is shown both numerically and analytically that the existing knowledge about e–i collisions in a rather strong field should be revised. Moreover, it was found that in such fields, along with the efficient growth of effective cross section, the effective bunching of electrons at phases takes place simultaneously. Effects are observed both at the linear and the circular laser field polarization. The quasiphenomenological approach for the electron–ion collision integral derivation for the case of strong fields is suggested.
Ion cyclotron range of frequency heating experiments on the large helical device and high energy ion behavior
Authors: Kumazawa R. , Mutoh T. , Seki T. , Watari T. , Saito K. , Torii Y. , Shimpo F. , Nomura G. , Yokota M. , Kato A. , Hartmann D. A. , Zhao Y. , Fukuyama A. , Okada H. , Ohkubo K. , Sato M. , Kubo S. , Shimozuma T. , Idei H. , Yoshimura Y. , Notake T. , Takita Y. , Kobayashi S. , Itoh S. , Mizuno Y. , Kaneko O. , Takeiri Y. , Oka Y. , Tsumori K. , Osakabe M. , Ikeda K. , Yamamoto S. , Kawamoto T. , Asano E. , Ohyabu N. , Kawahata K. , Komori A. , Yamada H. , Akaishi K. , Ashikawa N. , Emoto M. , Funaba H. , Goto M. , Ida K. , Inagaki S. , Inoue N. , Isobe M. , Krasilnikov A. , Masuzaki S. , Minami T. , Miyazawa J. , Morisaki T. , Morita S. , Murakami S. , Muto S. , Nakamura Y. , Nagayama Y. , Nakanishi H. , Narihara K. , Narushima Y. , Nishimura K. , Noda N. , Kobuchi T. , Ohdachi S. , Ozaki T. , Peterson B. J. , Sagara A. , Sakakibara S. , Sakamoto R. , Sasao H. , Sasao M. , Sato K. , Shoji M. , Suzuki H. , Tanaka K. , Toi K. , Tokuzawa T. , Yamada I. , Yamaguchi S. , Yokoyama M. , Watanabe K-Y. , Watanabe T. , Matsuoka K. , Itoh K. , Sudo S. , Yamazaki K. , Hamada Y. , Motojima O. , Fujiwara M. ,   LHD Experimental Group
Published: July 2002

Ion cyclotron range of frequency (ICRF) heating experiments on the Large Helical Device (LHD) [O. Motojima et al. Fus. Eng. Des. 20, 3 (1993)] achieved significant advances during the third experimental campaign carried out in 1999. They showed significant results in two heating modes; these are modes of the ICH-sustained plasma with large plasma stored energy and the neutral beam injection (NBI) plasma under additional heating. A long-pulse operation of more than 1 minute was achieved at a level of 1 MW. The characteristics of the ICRF heated plasma are the same as those of the NBI heated plasma. The energy confinement time is longer than that of International Stellarator Scaling 95. Three keys to successful ICRF heating are as follows: (1) an increase in the magnetic field strength, (2) the employment of an inward shift of the magnetic axis, (3) the installation of actively cooled graphite plates along the divertor legs. Highly energetic protons accelerated by the ICRF electric field were experimentally observed in the energy range from 30 to 250 keV and the tail temperature depended on the energy balance between the wave heating and the electron drag. The transfer efficiency from the high energy ions to the bulk plasma was deduced from the increase in the energy confinement time due to the high energy ions in the lower density discharge, which agrees fairly well with the result obtained by the Monte Carlo simulation. The transfer efficiency is expected to be 95% at an electron density of more than ne=5.0×1019 m−3 even in the high power heating of 10 MW. The accumulation of impurities, e.g., FeXVI and OV was not observed in high rf power and long pulse operation. The well-defined divertor intrinsic to LHD is believed to be useful in reducing the impurity influx.
A comparative study of dissociative adsorption of NH3, PH3, and AsH3 on Si(001)–(2×1)
Authors: Miotto R. , Srivastava G. P. , Miwa R. H. , Ferraz A. C.
Published: July 2002

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH3, PH3, and AsH3 on the Si(001)–(2×1) surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH3 molecules considered in this work is as follows: the gas phase XH3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si–Si dimers that are (almost) symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH3 and XH2 systems retain the pyramidal geometry observed for the gas molecules. Our calculated vibrational spectra further support the dissociative model for the XH3 molecules considered here.
Heterojunction wavelength-tailorable far-infrared photodetectors with response out to 70 μm
Authors: Perera A. G. U. , Matsik S. G. , Yaldiz B. , Liu H. C. , Shen A. , Gao M. , Wasilewski Z. R. , Buchanan M.
Published: July 2002

Results are presented on the performance of a heterojunction interfacial workfunction internal photoemission (HEIWIP) wavelength-tailorable detector. The detection mechanism is based on free-carrier absorption in the heavily doped emitter regions and internal emission across a workfunction barrier caused by the band gap offset at the heterojunction. The HEIWIP detectors have the high responsivity of free-carrier absorption detectors and the low dark current of quantum well infrared photodector type detectors. For a 70±2 cutoff wavelength detector, a responsivity of 11 A/W and a D*=1×1013 cmHz/W with a photocurrent efficiency of 24% was observed at 20 μm. From the 300 K background photocurrent, the background limited performance (BLIP) temperature for this HEIWIP detector was estimated to be 15 K. This HEIWIP detector provides an exciting approach to far-infrared detection.
Raman scattering test of single-wall carbon nanotube composites
Authors: Hadjiev V. G. , Iliev M. N. , Arepalli S. , Nikolaev P. , Files B. S.
Published: July 2002

Raman spectroscopy is used to infer elastic properties of single-wall carbon nanotubes (SWNTs) in composites. This letter presents strain-induced frequency shift of tangential Raman active modes of SWNTs embedded in epoxy resin subjected to bending. Epoxy curing and sample extension in the tensile strength test are found to create residual strains on the SWNT ropes. We demonstrate that specimen compression in combination with the Raman microprobe technique provides a means for determining of these strains and hence load transfer effectiveness.
Screen-printed passive matrix displays based on light-emitting polymers
Authors: Birnstock J. , Blässing J. , Hunze A. , Scheffel M. , Stößel M. , Heuser K. , Wittmann G. , Wörle J. , Winnacker A.
Published: July 2002

Due to their outstanding properties, e.g., good contrast, wide viewing angle, low power consumption, and self-emission organic light-emitting (OLE) displays on the basis of conjugated polymers are on the verge of commercialization. Two major disadvantages of the current processing technique for the polymers—spin coating—are the material waste and the difficulties involved in patterning multichrome or even full-color displays. Therefore, we investigated the screen-printing technique for the production of OLE displays. In this letter, we present performance data and images of screen-printed OLE diodes. They are already comparable to spin-coated ones. We observed luminance of 10 000 cd/m2 at 8 V and peak efficiencies exceeding 10 cd/A for green diodes. These data indicate that printed organic displays have the potential to replace “classical” spin-coated devices.
Multichannel motional Stark effect diagnostic discriminating radial electric field in the JFT-2M tokamak
Authors: Kamiya K. , Kimura H. , Hoshino K. , Isei N. , Kawashima H. , Ogawa H. , Ogawa T. , Tsuzuki K. , Uehara K. ,   JFT-2M Group
Published: July 2002

A multichannel motional Stark effect polarimeter system, which is capable of simultaneous measurement of a radial electric field, has recently been developed on the JFT-2M. The diagnostic can measure the polarization angle at 18 radial locations, which cover a region between just inside from the magnetic axis and the outboard edge of the plasma. By viewing two neutral beam lines (one is co-parallel to the plasma current and the other is counter parallel) simultaneously and near tangentially to the toroidal magnetic field from only one spectroscopic instruments, it provides the best sensitivity in radial electric field measurements with good spatial resolution. The magnetic field pitch angle is also measured with the smallest uncertainty. Preliminary data for L-mode plasma has been obtained. It is found that the statistical uncertainty of the magnetic field pitch angle and radial electric field is about 0.1° and 4 kV/m, respectively, with a time resolution of 10 ms.
Optical phonons of hexagonal AlxGa1−xN: Simulation and experiment
Authors: Liu Ming S. , Bursill Les A. , Prawer Steven
Published: July 2002

The behavior of longitudinal and transverse optical phonons in hexagonal AlxGa1−xN is derived theoretically and experimentally as a function of the concentration x(0⩽x⩽1). The theoretical approach is based on a modified random element isodisplacement model which considers the interactions with the nearest neighbor and second neighbor atoms. We find “one-mode” behavior in AlxGa1−xN in which the phonon frequency in general varies continuously and approximately linearly with x. The theoretical simulations are in good agreement with Raman scattering experiments, which also reveal that both the linewidth and intensity of the optical phonons strongly depend on the concentration.
Dielectric loss anomaly and polaron hopping conduction of Gd1/3Sr2/3FeO3
Authors: Jung Woo-Hwan
Published: July 2002

In this study, the dielectric, magnetic, and transport properties of Gd0.33Sr0.67FeO3 have been analyzed. The temperature dependence of resistivity exhibits the adiabatic small polaron hopping conduction; i.e., ρ/T=A exp(E/kBT), E being the small polaron hopping energy. The dielectric loss anomaly was found to be around 170 K. The activation energy corresponding to the relaxation process of this dielectric anomaly was 0.16 eV. A dielectric relaxation process shows up with an activation energy nearly equal to that of dc conduction. The dielectric anomaly and resistivity of Gd0.33Sr0.67FeO3, which arises due to the presence of Fe4+ and Fe3+ ions, has been analyzed in the light of the small polaron hopping conduction mechanism.
Statics and dynamics of model dendrimers as studied by molecular dynamics simulations
Authors: Karatasos K. , Adolf D. B. , Davies G. R.
Published: July 2002

Molecular dynamics simulations were performed for a series of AB2 dendrimer models, in explicit-solvent solutions where the ratio Rg/L (Rg is the radius of gyration and L the size of the simulation box) is kept between 0.15⩽Rg/L⩽0.2. Results on static properties (size, shape, density profiles) are in good agreement with recent theoretical and experimental studies. Dynamic properties are systematically investigated on the local and entire molecule length scale. The dynamic characteristics of the examined models capture the qualitative behavior observed experimentally in dendrimer molecules. The systematic and comparative nature of this study affords detailed insight into the origin and the relative contribution of different relaxational mechanisms in the observed dynamic spectra.
Quantitative carrier profiling in ion-implanted 6H–SiC
Authors: Giannazzo Filippo , Calcagno Lucia , Raineri Vito , Ciampolini Lorenzo , Ciappa Mauro , Napolitani Enrico
Published: July 2002

Dopant profiles in n-type 6H–SiC samples implanted with N+ ions have been measured by scanning capacitance microscopy on cross-sectioned samples. The obtained carrier profiles have been accurately quantified by calculating a complete set of capacitance-to-voltage curves by simulation of the measurement setup, followed by the extraction of the system response characteristic as a function of the local carrier concentration. The discrepancy observed to occur between the scanning capacitance microscopy data and the corresponding secondary ions mass spectroscopy profiles is explained by the fact that the scanning capacitance microscopy is sensitive on the local concentration of free carriers, which is on the local concentration of electrically activated dopant ions.
Localized photothermal infrared spectroscopy using a proximal probe
Authors: Bozec L. , Hammiche A. , Pollock H. M. , Conroy M. , Chalmers J. M. , Everall N. J. , Turin L.
Published: July 2002

A near-field thermal probe, as used in scanning thermal microscopy, is used to obtain photothermal Fourier transform infrared (FT-IR) spectra of polymers, as a first step toward developing FT-IR microscopy at a spatial resolution better than the diffraction limit. The signal from the probe after amplification provides an interferogram, and the resultant spectra are consistent with those obtained by means of the established technique of attenuated total reflection FT-IR spectroscopy. We have extended this technique to the analysis of “real-world” industrial samples, both solid (a fungicide in a fine powder form) and liquid (a concentrated surfactant solution). The overall shapes of the main peaks or bands reflect the fact that the spectrum is a convolution of different contributions from both optical and thermal properties. To confirm the feasibility of subsurface detection of polymers, we demonstrate the ability of the technique to perform spectroscopic detection of a model polymeric bilayer system, polyisobutylene on top of polystyrene. A quantitative analysis of the variation of peak height with coating thickness allows values of thermal diffusion length to be derived. This investigation provides a preliminary result for the understanding of the depth sensitivity of the current setup. Relative intensity distortions are seen, and are attributed to photothermal saturation. A complementary technique has been developed that uses tunable monochromatic radiation, using an optical parametric generator as the infrared source. Spectra have successfully been obtained using the same localized photothermal detection principle.
First-principles molecular dynamics study of a polymer under tensile stress
Authors: Röhrig Ute F. , Frank Irmgard
Published: July 2002

We present first-principles simulations of cis-polyacetylene fragments under tensile stress. The barrier of a cis-trans isomerization of conjugated carbon–carbon double bonds is lowered by tensile stress. However, it does not seem likely that the barrier can be sufficiently reduced to achieve a cis-trans isomerization in an atomic force microscopy experiment. In particular, the transition state of the reaction cannot be converted into a stable or metastable minimum by mechanical stress. We devise a general model for the calculation of bond rupture probabilities.
Electromigration critical length effect in Cu/oxide dual-damascene interconnects
Authors: Lee Ki-Don , Ogawa Ennis T. , Matsuhashi Hideki , Justison Patrick R. , Ko Kil-Soo , Ho Paul S. , Blaschke Volker A.
Published: July 2002

Electromigration tests at temperatures between 340 and 400 °C and current densities between 1.0 and 3.0 MA/cm2 have been performed to determine the temperature dependence of the critical length effect in 0.5-μm-wide Cu/oxide dual-damascene interconnects with 0.1 μm silicon nitride (SiNx) passivation. A focused-ion-beam-induced contrast imaging technique is used to locate failure sites of critical length test structures. Statistical analysis [E. T. Ogawa et al., Appl. Phys. Lett. 78, 18 (2001)] yields a threshold-length product (jL)c, of 3700 A/cm, and a temperature dependence is not observed within the temperature range 340–400 °C.
An optimal adiabatic-to-diabatic transformation of the 1 2A′ and 2 2A′ states of H3
Authors: Abrol Ravinder , Kuppermann Aron
Published: July 2002

Molecular reaction dynamics in the adiabatic representation is complicated by the existence of conical intersections and the associated geometric phase effect. The first-derivative coupling vector between the corresponding electronically adiabatic states can, in general, be decomposed into longitudinal (removable) and transverse (nonremovable) parts. At intersection geometries, the longitudinal part is singular, whereas the transverse part is not. In a two-electronic-state Born–Huang expansion, an adiabatic-to-diabatic transformation completely eliminates the contribution of the longitudinal part to the nuclear motion Schrödinger equation, leaving however the transverse part contribution. We report here the results of an accurate calculation of this transverse part for the 1 2A′ and 2 2A′ electronic states of H3 obtained by solving a three-dimensional Poisson equation over the entire domain U of internal nuclear configuration space 𝒬 of importance to reactive scattering. In addition to requiring a knowledge of the first-derivative coupling vector everywhere in U, the solution depends on an arbitrary choice of boundary conditions. These have been picked so as to minimize the average value over U of the magnitude of the transverse part, resulting in an optimal diabatization angle. The dynamical importance of the transverse term in the diabatic nuclear motion Schrödinger equation is discussed on the basis of its magnitude not only in the vicinity of the conical intersection, but also over all of the energetically accessible regions of the full U domain. We also present and discuss the diabatic potential energy surfaces obtained by this optimal diabatization procedure.
Calculating molecular electric and magnetic properties from time-dependent density functional response theory
Authors: Autschbach Jochen , Ziegler Tom
Published: July 2002

Simple working equations are presented which allow for the treatment of time-dependent first-order electric and magnetic properties based on the linear response of the Kohn–Sham density matrix. They are generally applicable to existing nonrelativistic molecular density functional code implementations. This extends the formulas for frequency-dependent polarizabilities previously reported by other authors to include the cases of frequency-dependent optical rotation parameters and magnetic susceptibilities. An expression for the magnetic transition dipole moment is extracted from the results. Existing codes for excitation energies and polarizabilities can be easily extended to compute the optical rotation parameter and rotatory strengths. We further discuss possible extensions towards time-dependent current-density response.
Theory and simulation of the triplet structure factor and triplet direct correlation functions in binary mixtures
Authors: Jorge S. , Lomba E. , Abascal J. L. F.
Published: July 2002

In this work we present structure factors and triplet direct correlation functions extracted from extensive Monte Carlo simulations for a binary mixture of hard spheres. The results are compared with the predictions of two integral equation theories, namely, a recently proposed extension to mixtures of Attard’s inhomogeneous integral equation approach, and Barrat, Hansen, and Pastore’s factorization ansatz. In general, both theories yield acceptable estimates for the triplet structure functions, though, by construction, the inhomogeneous integral equation theory is more suited to furnish triplet distribution function results, whereas the factorization ansatz provides a more handy approach to triplet direct correlation functions.
Commissioning results from the REXEBIS charge breeder
Authors: Wolf B. H. , Wenander F. , Cederkäll J. , Forstner, and the O. ,   REX-ISOLDE Collaboration, Liljeby L. , Skeppstedt Ö. , Rensfelt K.-G. , Jonson B. , Nyman G. , Ames F. , Kester O. , Reisinger K.
Published: July 2002

The REXEBIS is an electron-beam ion source for charge breeding of radioactive isotopes. Together with a Penning trap it constitutes a unique system for beam preparation for consecutive post-acceleration to a few MeV/u in the REX-ISOLDE linac. The Penning trap accumulates, cools, and bunches the 60 keV semicontinuous 1+radioactive beam from the on-line mass separator ISOLDE at CERN. Thereafter, the beam is injected into the EBIS for charge breeding to a charge-to-mass ratio of ∼1/4 within less than 20 ms (in a first phase for ion masses A<50). After extraction and mass analysis in a Nier-type spectrometer, the ions are injected into a radio-frequency quadrupole, the first stage of the 2.2 MeV/u linac. The Penning trap/EBIS unit will provide an efficient and rapid beam preparation, capable of handling low ion intensities with half-lives down to some 10 ms. The REXEBIS has now been completed and the commissioning is in progress. We will report on the results from the electron-beam tests, the emittance measurements, and the injection and extraction tests of stable ions. Furthermore, the vacuum experience of the room-temperature inner structure is presented.
Phonon spectra and electro-optic properties of crystalline phases in proton-exchanged LiTaO3 wave guides
Authors: Kostritskii S. M. , Korkishko Yu. N. , Fedorov V. A. , Maring D. B. , Tavlykaev R. F. , Ramaswamy R. V.
Published: July 2002

Annealed proton-exchanged wave guides in X-cut LiTaO3 were characterized by infrared reflection and Raman scattering spectroscopy. The measured spectra were related to the multiple crystalline phases of HxLi1−xTaO3 as identified by direct wave guide index profiling. Unique relationships were found to exist between the crystalline phases and corresponding spectra, which thus can be used to identify phase contents in a given wave guide. Vibrational bands of the studied spectra were correlated with the electro-optic properties of HxLi1−xTaO3 phases. Phase-dependent anisotropy of protonic conductivity was predicted based on the observed polarization dependence of OH libration bands. Finally, the electro-optic coefficient r33 was theoretically estimated for all phases and compared with previously reported experimental data.
Transcritical flow of a stratified fluid: The forced extended Korteweg–de Vries model
Authors: Grimshaw R. H. J. , Chan K. H. , Chow K. W.
Published: July 2002

Transcritical, or resonant, flow of a stratified fluid over an obstacle is studied using a forced extended Korteweg–de Vries model. This model is particularly relevant for a two-layer fluid when the layer depths are near critical, but can also be useful in other similar circumstances. Both quadratic and cubic nonlinearities are present and they are balanced by third-order dispersion. We consider both possible signs for the cubic nonlinear term but emphasize the less-studied case when the cubic nonlinear term and the dispersion term have the same-signed coefficients. In this case, our numerical computations show that two kinds of solitary waves are found in certain parameter regimes. One kind is similar to those of the well-known forced Korteweg–de Vries model and occurs when the cubic nonlinear term is rather small, while the other kind is irregularly generated waves of variable amplitude, which may continually interact. To explain this phenomenon, we develop a hydraulic theory in which the dispersion term in the model is omitted. This theory can predict the occurrence of upstream and downstream undular bores, and these predictions are found to agree quite well with the numerical computations.
Simulated current response in avalanche photodiodes
Authors: Hambleton P. J. , Plimmer S. A. , David J. P. R. , Rees G. J. , Dunn G. M.
Published: July 2002

The time dependent current response to an impulse of injected carriers is calculated for an avalanche photodiode using Monte Carlo simulation. For low electric fields and long avalanche regions the results agree with the conventional model, which assumes that carriers travel always with their saturated drift velocities. However, while diffusion remains unimportant, for high fields and short avalanche regions, the conventional model underestimates the device speed. Monte Carlo simulations show that the mean downstream average velocity of ionizing carriers is significantly enhanced at high electric fields and agreement is restored if we allow for this effect in the conventional model.
Probabilistic immortality of Cu damascene interconnects
Authors: Hau-Riege Stefan P.
Published: July 2002

We have studied electromigration short-line effects in Cu damascene interconnects through experiments on lines of various lengths L, stressed at a variety of current densities j, and embedded in different dielectric materials. We observed two modes of resistance evolution: Either the resistance of the lines remains constant for the duration of the test, so that the lines are considered immortal, or the lines fail due to abrupt open-circuit failure. The resistance was not observed to gradually increase and then saturate, as commonly observed in Al-based interconnects, because the barrier is too thin and resistive to serve as a redundant current path should voiding occur. The critical stress for void nucleation was found to be smaller than 41 MPa, since voiding occurred even under the mildest test conditions of j=2 MA/cm2 and L=10.5 μm at 300 °C. A small fraction of short Cu lines failed even at low current densities, which deems necessary a concept of probabilistic immortality rather than deterministic immortality. Experiments and modeling suggest that the probability of immortality is described by (jL2/B), where B is the effective elastic modulus of the metallization scheme. By contrast, the immortality of Al-based interconnects with shunt layers is described by (jL) if no voids nucleate, and (jL/B) if voids do nucleate. Even though the phenomenology of short-line effects differs for Al- and Cu-based interconnects, the immortality of interconnects of either materials system can be explained by the phenomena of nucleation barriers for void formation and void-growth saturation. The differences are due solely to the absence of a shunt layer and the low critical stress for void nucleation in the case of Cu.
A model for gate oxide wear out based on electron capture by localized states
Authors: Bersuker Gennadi , Korkin Anatoli , Jeon Yongjoo , Huff Howard R.
Published: July 2002

A model is proposed which addresses the effects of the oxide electric field and anode bias as well as the role of hydrogen in the trap generation process. The oxide wear-out phenomenon is considered as a multistep process initiated by the capture of injected electrons by localized states in SiO2. The captured electron significantly weakens the corresponding Si–O bond, which becomes unstable with respect to the applied electric field and temperature. The hydrogen presented in the oxide (due to anode hydrogen release process) prevents restoration of the broken bond that leads to the generation of a neutral E′ center. The model describes the charge-to-breakdown dependence on the electron fluence and energy, electric field, temperature, and oxide thickness.
Thermal stability of exchange bias in FeMn based bilayers
Authors: Pan Minghu , You Biao , Zhao Yanling , Lu Mu , Hu An , Zhai Hongru , Zhou Shiming
Published: July 2002

The thermal stability of the exchange bias field HE in samples of Cu (30 nm)/[ferromagnetic (FM) FeMn]/Cu (30 nm) and Cu (30 nm)/(FeMn/FM)/Cu (30 nm) prepared with an ultrahigh vacuum magnetron sputtering system was studied experimentally and by model calculations. The samples were heated to various temperatures and then kept for 10 min in an applied field of 1000 Oe antiparallel to the cooling field. It was found that HE decreased when the annealing temperature was increased and became zero after annealing at about 100 °C. When the annealing temperature was further increased, HE changed sign. Although the exchange bias field and the coercivity are strongly dependent on various factor, e.g., the stacking order of the layers, the thickness and the magnetization of FM layer, the thermal stability of the exchange bias field is influenced only by the thickness of antiferromagnetic (AFM) layer. A magnetic training effect appeared and became more apparent after annealing in a field antiparallel to the cooling field, especially for the samples with a thin AFM layer.
Vibrational dynamics of large hot molecules in the collisionless gas phase
Authors: Stromberg C. , Myers D. J. , Fayer M. D.
Published: July 2002

Infrared ps pump-probe experiments are presented for the P, Q, and R rotational branches of the asymmetric CO stretching mode of tungsten hexacarbonyl (1997 cm−1) in the collisionless gas phase. The pump-probe decays are tri-exponentials (140 ps, 1.3 ns, and >100 ns) in contrast to single exponential decays observed in supercritical fluids and liquid solvents. The 1.3 ns decay component is the vibrational energy relaxation (VER) time. The long component occurs following intramolecular VER into a distribution of low-frequency modes. After VER is complete, the R signal is 48%, the Q signal is 29%, and the P signal is −10% (absorption increase) compared to the t=0 signal. These long-lived signals result from an increase in the occupation numbers of low-frequency modes (internal heating) that causes a shift of the vibrational spectrum. The fastest decay is produced by spectral diffusion. The spectrally narrow pump pulse burns a hole in the inhomogeneous ground state spectrum and generates a narrow spectral population in the excited state (excited state peak). The inhomogeneity arises from the distribution of occupation numbers of the low-frequency modes that produces different combination band spectral shifts. Spectral diffusion is caused by the time evolution of the complex low-frequency thermal vibrational wave packet. Two possible models for the spectral diffusion are evaluated, one in which spectral diffusion reduces the pump-probe signal by hole filling and broadening of the excited state peak, and one in which the spectral diffusion only broadens the excited state peak.
A Zener pair effect in lanthanum rutheno manganite
Authors: Sahu Ranjan K. , Manoharan S. Sundar
Published: July 2002

A strong correlation between the Curie temperature and the metal–insulator transition is displayed in Ru doped La0.6Pb0.4Mn1−xRuxO3, where an equiatomic ratio of Ru and Mn is present in the Mn–O–Ru sublattice. The magnetic pair making effect of Mn with Ru is facilitated by the variable valency of Ru (IV)/Ru (V) which is similar in ionic size to the Zener pair Mn (III)/Mn (IV). Further Ru, being a 4d metal with itinerant t2g electrons, facilitates the exchange coupling interaction. The spin-wave excitation and the hole carrier density dominate the magnetization and the electron transport for T<0.5 TC for a higher Ru doped composition.
Control of p- and n-type conductivity in sputter deposition of undoped ZnO
Authors: Xiong Gang , Wilkinson John , Mischuck Brian , Tüzemen S. , Ucer K. B. , Williams R. T.
Published: July 2002

Recent theoretical studies have concluded that the low formation enthalpies of intrinsic donor defects should preclude achievement of p-type conductivity in undoped ZnO grown in thermal equilibrium with a molecular oxygen reservoir. This letter demonstrates that reactive sputtering can produce intrinsic p-type ZnO, controlled by adjusting the oxygen partial pressure in the sputtering plasma. We report the properties of p–n homojunctions fabricated in this way, and characterize transport in the films by Hall measurements. Our finding of p-type conductivity in undoped ZnO grown with dissociated oxygen is qualitatively consistent with the effect of higher chemical potential of atomic oxygen reactant on defect formation enthalpies. This parallels to some degree the recent attention to nitrogen acceptor incorporation by means of dissociating nitrogen source gases.
A new reciprocal space based treatment of long range interactions on surfaces
Authors: Mináry Peter , Tuckerman Mark E. , Pihakari Katianna A. , Martyna Glenn J.
Published: July 2002

A new formalism designed to treat long range interactions on surfaces, systems which are infinitely replicated in two spatial directions but have finite extent in the third, is developed. The new formalism is based in reciprocal space and, thus, permits the facile extension of standard plane-wave based density functional theory, Ewald summation, and smooth particle-mesh Ewald methods to handle surfaces efficiently. The method is tested on both model (body centered cubic lattices) and realistic problems (an ice surface with a defect and the 2×1 surface reconstruction of silicon) and found to be accurate, efficient, and a marked improvement on existing formulations in speed, accuracy, and utility.
Giant change in infrared light transmission in La0.67Ca0.33MnO3 film near the Curie temperature
Authors: Sukhorukov Yu. P. , Gan'shina E. A. , Belevtsev B. I. , Loshkareva N. N. , Vinogradov A. N. , Rathnayaka K. D. D. , Parasiris A. , Naugle D. G.
Published: July 2002

Transport, magnetic, magneto-optical (Kerr effect) and optical (light absorption) properties have been studied in an oriented polycrystalline La0.67Ca0.33MnO3 film which shows colossal magnetoresistance. The correlations between these properties are presented. A giant change in infrared light transmission (more than a thousand-fold decrease) is observed on crossing the Curie temperature (about 270 K) from high to low temperature. Large changes in transmittance in a magnetic field were observed as well. The giant changes in transmittance and the large magnetotransmittance can be used for development of IR optoelectronic devices controlled by thermal and magnetic fields. Required material characteristics of doped manganites for these devices are discussed.
Ab initio study of the reaction mechanism of CO2 with Ti atom in the ground and excited electronic states
Authors: Hwang Der-Yan , Mebel Alexander M.
Published: July 2002

Density functional B3LYP/6-311+G(3df )//B3LYP/6-31G* calculations of potential energy surfaces (PES) have been performed for the Ti+CO2→TiO+CO reaction in the triplet, quintet, and singlet electronic states. The results indicate that in the ground triplet state the most favorable reaction mechanism involves insertion of the Ti atom into a CO bond [via a η2-C,O coordinated t-(TiOC)O complex] to produce a triplet t-OTiCO molecule with the energy gain of 43.9 kcal/mol and the latter can further dissociate to TiO(3Δ)+CO with the total reaction exothermicity of ∼30 kcal/mol. The addition mechanism leading to the same TiO(3Δ)+CO products via a metastable η2-O,O complex t-cyc-TiCO2 is also feasible at ambient temperatures since the highest barrier on the reaction pathway is only 4.7 kcal/mol. The reaction mechanisms in excited singlet and quintet electronic states have many similar features with the ground state reaction but also exhibit some differences. In the singlet state, the reaction can follow A″1 and A′1 pathways, of those the insertion via a s-(TiOC)O (1A′) complex leading to s-OTiCO (1A′) and then to TiO(1Δ)+CO does not have an activation barrier. The insertion mechanism on the A″1 PES depicts a low barrier of 1.8 kcal/mol and leads to s-OTiCO (1A″), which dissociates into TiO(1Δ)+CO. The addition pathways via η2-O,O coordinated complexes require to overcome significant barriers, 7.8 and 34.9 kcal/mol for the A″1 and A′1 states, respectively. In the quintet state, the reaction at low and ambient temperatures can proceed only by coordination of Ti(5F) toward CO2 with formation of η2-C,O q-(TiOC)O, η2-O,O q-cyc-TiCO2, and η1-O q-TiOCO bound by 9.7, 6.1, and 4.6 kcal/mol, respectively, relative to the reactants. The η2-C,O and η1-O coordinations occur without barriers, while the η2-O,O coordination has an entrance barrier of 4.2 kcal/mol. The calculated PESs show that the carbon dioxide reforming into CO in the presence of Ti atoms should take place spontaneously.
Convective and periodic motion driven by a chemical wave
Authors: Kitahata Hiroyuki , Aihara Ryoichi , Magome Nobuyuki , Yoshikawa Kenichi
Published: July 2002

The generation of convective flow by a chemical wave was studied experimentally on a mm-sized droplet of Belousov–Zhabotinsky (BZ) reaction medium. A propagating chemical wave causes a transient increase in interfacial tension, and this local change in interfacial tension induces convection. The observed flow profile was reproduced with a numerical simulation by introducing the transient increase in interfacial tension to a modified Navier–Stokes equation coupled with a chemical kinetic equation; a modified Oregonator. We also observed the periodic motion of a BZ droplet floating on an oil phase. Such periodic motion is attributed to the rhythmic change in interfacial tension. The observed periodic convective motion coupled with a chemical reaction is discussed in relation to chemo-mechanical energy transduction under isothermal conditions.