bluuebunny/crossref_metadata_2025 huggingface dataset:
Visible cathodoluminescence of Er-doped amorphous AlN thin films
Authors: Gurumurugan K. , Chen Hong , Harp G. R. , Jadwisienczak W. M. , Lozykowski H. J.
Published: July 2002

In this report, we demonstrate the observation of visible cathodoluminescence (CL) of Er-doped amorphous AlN films produced by sputtering. Optical transmission studies point out that the films are highly transparent and the band gap is about 5.61 eV. Elemental analysis by Rutherford backscattering spectrometry shows that AlN films are stoichiometric. X-ray diffraction measurements reveal that the films retain the amorphous structure even after annealing in N2 ambient at 1000 °C. The sharp characteristic emission lines in the CL spectra correspond to Er3+ intra-4fn shell transitions and are observed over the temperature range of 9–330 K. The results indicate the suitability of Er-doped amorphous AlN films for light-emitting device applications.
Nature and elimination of yellow-band luminescence and donor–acceptor emission of undoped GaN
Authors: Li G. , Chua S. J. , Xu S. J. , Wang W. , Li P. , Beaumont B. , Gibart P.
Published: July 2002

The nature of yellow-band luminescence (YL) and donor–acceptor (DA) emissions of undoped GaN grown on sapphire or laterally overgrown on patterned Si3N4 was investigated using low-temperature photoluminescence and spatially resolved photoluminescence. The states, producing the levels responsible for the YL and DA emissions, arise from complexes of extended defects and native-point defects (most likely Ga vacancies) or impurities (such as carbon). For GaN directly grown on a low-temperature-grown GaN buffer layer, the YL and DA emissions can be eliminated by simply increasing the buffer-layer growth temperature as the result of enlarging hexagonal crystallites, and consequently, reducing the density of extended defects. For laterally overgrown GaN, a much lower density of extended defects substantially suppresses the YL emission.
Stability investigation of cubic GaN films grown by metalorganic chemical vapor deposition on GaAs (001)
Authors: Sun X. L. , Yang Hui , Zheng L. X. , Xu D. P. , Li J. B. , Wang Y. T. , Li G. H. , Wang Z. G.
Published: July 2002

The thermal stability of cubic-phase GaN (c-GaN) films are investigated by photoluminescence (PL) and Raman scattering spectroscopy. C-GaN films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition. PL measurements show that the near-band-edge emissions in the as-grown GaN layers and thermally treated samples are mainly from c-GaN. No degradation of the optical qualities is observed after thermal annealing. Raman scattering spectroscopy shows that the intensity of the E2 peak from hexagonal GaN grains increases with annealing temperature for the samples with poor crystal quality, while thermal annealing up to 1000 °C has no obvious effect on the samples with high crystal quality.
Preparation of rare-earth manganite-oxide thin films by metalorganic aerosol deposition technique
Authors: Moshnyaga V. , Khoroshun I. , Sidorenko A. , Petrenko P. , Weidinger A. , Zeitler M. , Rauschenbach B. , Tidecks R. , Samwer K.
Published: July 2002

A chemical deposition technique based on the use of solutions of metal-chelate coordination compounds has been applied to prepare rare-earth–manganite-oxide thin films. La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 thin films have been grown epitaxially on MgO(100) substrates and characterized by structural (x-ray diffraction analysis, atomic force microscopy) and magnetotransport (T=4.2–300 K, and B=0–5 T) measurements.
Solid-state amorphization at tetragonal-Ta/Cu interfaces
Authors: Kwon Kee-Won , Lee Hoo-Jeong , Sinclair Robert
Published: July 2002

This letter describes the formation of a thin amorphous layer at the tetragonal-Ta/Cu interfaces, which appear in copper metallization structures of microelectronic devices. The disordered layer grows up to 4 nm when annealed at between 400 and 600 °C. Since Ta and Cu are immiscible according to thermodynamic data, this is an unusual observation. A mechanism for the amorphous phase formation is proposed using both physical and chemical considerations. A high content of Cu is detected in the Ta layer up to 5 nm from the interface when annealed at 600 °C. Although the adhesion is promoted by the interface reaction, a sufficiently thick Ta underlayer is recommended for efficient blocking of Cu diffusion. Neither solid-state amorphization nor Cu diffusion into Ta is observed at bcc-Ta/Cu interfaces.
Influence of epitaxial growth and substrate-induced defects on the breakdown of 4H–SiC Schottky diodes
Authors: Wahab Q. , Ellison A. , Henry A. , Janzén E. , Hallin C. , Di Persio J. , Martinez R.
Published: July 2002

Morphological defects and elementary screw dislocations in 4H–SiC were studied by high voltage Ni Schottky diodes. Micropipes were found to severely limit the performance of 4H–SiC power devices, whereas carrot-like defects did not influence the value of breakdown voltage. The screw dislocation density as determined by x-ray topography analysis under the active area of the diode was also found to directly affect the breakdown voltage. Only diodes with low density of screw dislocations and free from micropipes could block 2 kV or higher.
Gap symmetry and critical current of YBa2Cu3O7 step-edge Josephson junctions
Authors: Lee Soon-Gul , Hwang Yunseok
Published: July 2002

Effects of d-wave symmetry on the critical current of high Tc step-edge Josephson junctions were studied theoretically by using the concept of grain-boundary junctions, and the theory was compared with experimental results of YBa2Cu3O7 step-edge junctions on SrTiO3 (100) substrates. A step-edge junction contains two grain boundaries, and each grain boundary has both in-plane and out-of-plane misorientation angles at the same time. The critical current of the step-edge junction as a function of the in-plane misorientation angle has a periodic modulation with maxima when the step line is parallel to one of the major axes of the SrTiO3 (100) substrate. Experimental results of c-oriented YBa2Cu3O7 step-edge junctions with various in-plane misorientation angles were qualitatively in good agreement with the theory. The out-of-plane misorientation angle is formed between two grains usually with the c axes perpendicular to each other and is normally not controllable.
Doping screening of polarization fields in nitride heterostructures
Authors: Di Carlo Aldo , Della Sala Fabio , Lugli Paolo , Fiorentini Vincenzo , Bernardini Fabio
Published: July 2002

Using self-consistent tight-binding calculations, we show that modulation doping can be used to screen macroscopic polarization fields in nitride quantum wells. The blue-shift of photoluminescence peak as well as the reduction of radiative recombination lifetime at increasing doping density is explained and correlated to polarization-field screening. The field-induced ionization of the dopants and its relation with alloy composition in the heterostructure barriers is also analyzed.
Analysis of the activation energy of the subthreshold current in laser- and solid-phase-crystallized polycrystalline silicon thin-film transistors
Authors: Pichon L. , Mercha A. , Carin R. , Bonnaud O. , Mohammed-Brahim T. , Helen Y. , Rogel R.
Published: July 2002

Analysis of the thermal and gate-voltage dependences of the current in the subthreshold region is performed on both low-temperature laser-crystallized and solid-phase-crystallized polycrystalline silicon (polysilicon) thin-film transistors (TFTs). Temperature measurements are made at first in order to extract the variations of the activation energy EA of the drain current with the gate voltage. The plot of the subthreshold current versus the measured activation energy leads to an apparent activation energy EA/n, where the n factor is extracted from the slope of this plot. The n factor is close to 1 for laser-crystallized polysilicon TFTs while it is rather close to 2 for solid-phase-crystallized ones. These two values can be attributed to a different defect distribution in the two differently crystallized TFTs polysilicon active layers.
Charge ordering in Pr0.5Ca0.5MnO3 thin films: A new form initiated by strain effects of LaAlO3 substrate
Authors: Haghiri-Gosnet A. M. , Hervieu M. , Simon Ch. , Mercey B. , Raveau B.
Published: July 2002

The growth of [101] epitaxial films of Pr0.5Ca0.5MnO3 on LaAlO3 substrates has been achieved. The structure, nanostructure, and magnetotransport properties are reported and compared to those of the bulk material. The electron diffraction study evidences, at RT, a monoclinic cell with a≈c≈5.4 Å, b≈7.6 Å, and β≈91°. The monoclinic distortion is assumed to be due to strain effects. At 92 K, a system of extra reflections is observed which implies an incommensurate modulated structure. The component q of the modulation vector along a* ranges between 0.35 and 0.40. The lattice images show that the modulation is established throughout the whole film. The q value observed in the film is significantly lower than the one of the bulk material. This effect is correlated to strain effects of the substrate, limiting the cell distortion induced by charge ordering. An unusual insulating-ferromagnetic phase is observed with a critical temperature of 240 K, close to the TCO.
Carbon rings and cages in the growth of single-walled carbon nanotubes
Authors: Kiang Ching-Hwa
Published: July 2002

We present a growth model for single-walled carbon nanotubes (SWNTs) based on experimental observation and energetic considerations. Carbon rings, observed experimentally and suggested to be fullerene precursors, are the templates for SWNTs nucleation. SWNTs grow by addition of C2 radicals to catalytically induced reactive sites. This model is consistent with known experimental results and explains the characteristic diameter distribution of SWNTs as resulting from the gas-phase distribution of carbon ring isomers.
Experimental and theoretical study of line mixing in methane spectra. IV. Influence of the temperature and of the band
Authors: Pieroni D. , Nguyen-Van-Thanh  , Brodbeck C. , Hartmann J.-M. , Gabard T. , Champion J.-P. , Bermejo D. , Domenech J.-L. , Claveau C. , Valentin A.
Published: July 2002

Line-mixing effects are studied in infrared bands of CH4 perturbed by N2 at various pressures. The effects of temperature are investigated in the ν3 region whereas spectral shapes of the ν2, ν4, and ν3 bands are compared at room temperature. The theoretical approach proposed in preceding papers is used in order to model and analyze the influence of collisions on the spectral shape. All model parameters are now fixed to values determined in the previous studies. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They show that our approach satisfactorily accounts for the effects of temperature, pressure, and of rotational quantum numbers on the absorption by the ν3 band. Furthermore, the effects of collisions on spectra in the ν4 region at room temperature are also correctly calculated. On the other hand, the proposed approach fails in modeling the evolution with increasing pressure of absorption in the spectral range containing the ν2 band. This result is attributed to the Coriolis coupling between the ν2 and ν4 vibrational states and to a contribution whose physical origin remains unclear. The latter, which is negligible when CH4–He mixtures are considered, behaves as collision-induced absorption.
Broadband optical limiting based on excited state absorption in Pt:ethynyl
Authors: Staromlynska J. , McKay T. J. , Wilson P.
Published: July 2002

Analytical, numerical, and experimental studies of the nonlinear transmission of a Pt:ethynyl compound have been carried out. Based on a model for the electronic transitions derived from earlier work, optical limiting behavior that is both broadband across the visible and effective over a range of pulse lengths, is predicted. Detailed experimental results are presented which exhibit many of the theoretically predicted characteristics, including a broadband nonlinear response from 450 to 700 nm.
Deposition and characterization of nanocrystalline diamond films prepared by ion bombardment-assisted method
Authors: Gu C. Z. , Jiang X.
Published: July 2002

This article reports a process for preparing nanocrystalline diamond films by ion bombardment of different energies induced by applying a negative substrate bias voltage in microwave plasma-assisted chemical-vapor-deposition. The dependencies of the film morphology, grain size, growth rate, and average surface roughness on the substrate bias voltage were studied. The influences of substrate temperature, total gas pressure, and CH4 concentration on the grain size and growth rate were investigated. The results indicate that a high bias voltage, substrate temperature, CH4 concentration, and low total gas pressure are required to obtain high frequency of secondary nucleation so that smooth nanocrystalline diamond films can be prepared. High bias voltage, substrate temperature, total gas pressure, and CH4 concentration lead to high growth rate. Diamond films with a grain size and average surface roughness of several nanometers can be synthesized at a bias potential of −140 V. The micro-Raman spectroscopy shows a broad peak at around 1140 cm−1 which can be used to characterize nanocrystalline diamond films. The broad peak intensity from 1400 to 1600 cm−1 increases with decreasing grain size.
Detection of stochastic waves in plasma monolayer crystals from video images
Authors: Melandsø Frank , Bjerkmo Åge , Morfill Gregor , Thomas Hubertus , Zuzic Milenko
Published: July 2002

The motion of dust particles confined in plasma monolayer crystals is analyzed from video images, under conditions dominated by dust-neutral collisions. In these crystals, dust-neutral collisions will act as a random driving force, exciting phonons with a stochastic nature. The phonons are investigated using standard statistical tools, including both single- and multiparticle correlation functions. Single-particle correlations as obtained from the velocity autocorrelation function yield oscillations in a very narrow frequency band. Similar behaviors have previously been reported for strongly coupled one-component plasmas, and for trapped Brownian particles. Spatial correlations in the crystal lattice are studied from multiparticle correlation functions, suggesting an average wavelength slightly larger than the dimension of the crystal. Throughout the crystal, the dust velocity amplitude and polarization vary significantly, with the main variation in the radial direction out of the crystal center. This suggests the observed wave feature is a standing wave with a stochastic amplitude, dominated by its lowest eigenfrequency.
Nonlinear response and power handling capability of ferroelectric BaxSr1−xTiO3 film capacitors and tunable microwave devices
Authors: Kozyrev A. , Ivanov A. , Samoilova T. , Soldatenkov O. , Astafiev K. , Sengupta L. C.
Published: July 2002

The nonlinear response of ferroelectric BaxSr1−xTiO3 films to microwave electric field intensity up to ∼3×106 V/m was studied. Two techniques were used for this investigation: (i) 10 GHz pulsed power measurements, and (ii) 4 GHz intermodulation distortion (IMD) measurements. The nonlinear distortion of the resonant curve under microwave pulsed power and generation of the third-order IMD products in microwave resonators using ferroelectric film planar capacitors were measured. The use of microwave pulses and continuous signals enabled the separation of the nonlinear dielectric response from the heating response of the ferroelectric films and the microwave nonlinear parameters of the ferroelectric films to be determined. It is shown that up to a specified value of microwave voltage amplitude the nonlinear response of BaxSr1−xTiO3 film capacitors can be predicted from the small signal capacitance–voltage characteristics. Formulas to estimate power handling capability connected with the field dielectric nonlinearity and the film overheating are derived for the tunable microwave devices based on ferroelectric films.
A summation procedure that improves the accuracy of the fourth-order Mo/ller–Plesset perturbation theory
Authors: Goodson David Z.
Published: July 2002

A procedure is demonstrated for summing the Mo/ller–Plesset many-body perturbation expansion based on the ability of quadratic summation approximants to locate branch point singularities in the complex plane of the perturbation parameter. Accuracy comparable to that from CCSDT coupled-cluster calculations is obtained using fourth-order perturbation theory.
Micromagnetics of the dynamic susceptibility for coupled Permalloy stripes
Authors: Gérardin O. , Youssef J. Ben , Le Gall H. , Vukadinovic N. , Jacquart P. M. , Donahue M. J.
Published: July 2002

The dynamic susceptibility of arrays of narrow Permalloy stripes (9 mm × several μm × 200 nm) has been investigated using a single-coil broadband susceptibility spectrometer. Disagreement is observed between experimental results and the macroscopic Landau-Lifshitz model. This model does not take into account the dipolar interaction between magnetic stripes. We have performed micromagnetic calculations that include these dipolar interactions, and have found the resulting frequency dependence of the dynamic susceptibility in the linear regime to be in good agreement with our experimental data.
Hydrolysis of the amorphous silica surface. II. Calculation of activation barriers and mechanisms
Authors: Walsh Tiffany R. , Wilson Mark , Sutton Adrian P.
Published: July 2002

Using a previously derived model of the dry, amorphous, hydrophilic SiO2 surface, the reactivity of generic defect sites on the surface with respect to water, and the local network rearrangement that accompanies hydrolysis at these sites, is investigated using cluster models. Ab initio methods are used to calculate reaction barriers and reaction pathways. Consequences of the various types of hydrolysis product found are discussed with reference to potential sites for polymer chemisorption on the hydrolyzed, amorphous SiO2 surface.
Electron paramagnetic resonance of ferrite nanoparticles
Authors: Koksharov Yu. A. , Pankratov D. A. , Gubin S. P. , Kosobudsky I. D. , Beltran M. , Khodorkovsky Y. , Tishin A. M.
Published: July 2002

Three types of iron-based oxide nanoparticles (weight compositions Fe2O3, BaFe2O4, and BaFe12O19) embedded in a polyethylene matrix are studied using the electron paramagnetic resonance technique. All nanoparticles are found to be multiphase. Thermal variations of electron paramagnetic resonance spectra reveal the presence of two phases in the Fe2O3 nanoparticles. One such phase undergoes an antiferromagnetic-like transition near 6 K. Nanoparticles of BaFe2O4 demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. Reduced magnetic anisotropy in BaFe12O19 nanoparticles may be related to either structural imperfection or particle smallness (effective diameter of less than 10 nm). Our data clearly show that low temperature experiments are desirable for the correct identification of nanoparticles by means of the electron paramagnetic resonance technique.
Growth of nanocrystalline diamond films by biased enhanced microwave plasma chemical vapor deposition: A different regime of growth
Authors: Sharda T. , Umeno M. , Soga T. , Jimbo T.
Published: July 2002

Hard and smooth nanocrystalline diamond films are grown on mirror polished silicon substrates by biased enhanced growth (BEG) in microwave plasma chemical vapor deposition at lower temperatures. Hardness of the films varies with deposition condition and can be defined by the relative concentration of nanocrystalline diamond in the films, as measured by the Raman intensity ratio of the feature near 1150 cm−1 to the intensity of graphitic G band. The hardness of the films approaches the hardness of diamond at conditions giving maximum concentration of nanocrystalline diamond while still having a low amount of stress (1–2 GPa). A different regime of growth appears to exist in the films deposited by the BEG process that may, however, be a combination of surface and subsurface processes.
Complete femtosecond linear free induction decay, Fourier algorithm for dispersion relations, and accuracy of the rotating wave approximation
Authors: Albrecht Ferro Allison , Hybl John D. , Jonas David M.
Published: July 2002

A complete (both amplitude and phase) measurement of the femtosecond linear free induction decay on a Raman active dye in solution (IR144 in methanol) is demonstrated. For weak femtosecond pulses passing through a homogeneous material, Beer’s Law can be used to predict the spectral amplitude changes, and dispersion relations can be used to calculate the spectral phase change. A modified fast Fourier transform algorithm calculates the phase change by applying dispersion relations to a symmetrical absorption spectrum spanning positive and negative frequencies. A reformulation of the rotating wave approximation in the frequency domain shows that related limits on the accuracy of the rotating wave approximation in nonlinear optics arise from the width of the linear spectrum relative to the center frequency, and not from the excitation pulses.
Low-temperature dynamics and spectroscopy in exohedral rare-gas C60 fullerene complexes
Authors: Ruiz A. , Hernández-Rojas J. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

The adatom dynamics in exohedral C60 fullerene complexes of rare-gas atoms are studied with a three degrees of freedom model. The eigenvalue problem of the corresponding quantum Hamiltonian is solved and the electric-dipole spectra for ArC60, NeC60, and HeC60 in the low-temperature range from 5 to 40 K are simulated. The most important spectral features are related to the degree of angular anisotropy in the adatom–C60 interaction. The ArC60 and NeC60 complexes present very simple spectra which can be assigned in terms of three-mode oscillators; the corresponding motion takes place in the deep hexagon wells (also in the pentagon wells for NeC60) of the interaction potential. On the contrary, the HeC60 complex shows more complicated spectra with important tunneling effects due to the smaller angular anisotropy of the interaction. The onset of almost free internal rotation takes place in this complex at rather low energies, and this gives rise to a low-frequency rotational band in the spectra at temperatures above T∼15 K.
Coupled guide and cavity in a two-dimensional photonic crystal
Authors: Smith C. J. M. , De La Rue R. M. , Rattier M. , Olivier S. , Benisty H. , Weisbuch C. , Krauss T. F. , Houdré R. , Oesterle U.
Published: July 2002

We demonstrate, in a planar two-dimensional (2D) configuration, in the optical regime a clear association of two photonic crystal elements and the ability to produce a low-loss coupled system. A channel waveguide is brought to between two and five crystal rows (450 to 1126 nm) from a 2D microcavity fabricated in a GaAs/AlGaAs waveguide. We probe these two elements individually and explore their interaction.
Temperature profile of GaInAs/AlInAs/InP quantum cascade-laser facets measured by microprobe photoluminescence
Authors: Spagnolo Vincenzo , Troccoli Mariano , Scamarcio Gaetano , Gmachl Claire , Capasso Federico , Tredicucci Alessandro , Sergent A. Michael , Hutchinson Albert L. , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

The local temperature of quantum-cascade lasers operating in continuous wave mode is reported. This information is extracted from the thermal shift of the band-to-band photoluminescence peaks in the AlInAs and InP cladding layers of quantum-cascade laser facets using a high-resolution microprobe setup. Interpolation by means of a two-dimensional heat diffusion model allows to obtain the temperature profile and the thermal conductivity in the waveguide core. Comparison between substrate and epilayer-side mounted lasers shows the superior thermal dissipation capability of the latter, and explains their better performance with respect to threshold current and maximum operating temperature.
Flattening of organic molecules under pressure
Authors: Zhuravlev K. K. , McCluskey M. D.
Published: July 2002

The different conformations of molecular compounds play important roles in biochemistry and organic solid-state technology. Hydrostatic pressure has been applied to para-quaterphenyl to probe its molecular structure at liquid-helium temperatures. The molecules transform from a twisted to a planar conformation at a critical pressure of 0.9 GPa. This conformational change results in the abrupt disappearance of five infrared-absorption peaks. A group-theoretical analysis shows that the peaks, which correspond to out-of-plane hydrogen bending modes, disappear due to the transition from a low to a high symmetry.
Phase envelopes for variable width square well chain fluids
Authors: Cui Jingyu , Elliott J. Richard
Published: July 2002

Discontinuous Molecular Dynamics (DMD) and Thermodynamic Perturbation Theory (TPT) have been used to study square-well (SW) chain molecules with variable well-width SW potentials. Well widths of 1.5, 1.8, and 2.0 are considered for united atom models of ethane, n-hexane, and n-octane. The properties studied are the acentric factor, vapor pressure, and liquid density. DMD of purely repulsive potentials was applied to record the number of interaction sites in different wells, giving estimates of the TPT contributions from the attractive potential. DMD simulations of the complete potential near the coexistence condition were used to refine estimates of the derivative quantities related to the compressibility factor. Evaluations of this approach indicate that it is accurate and efficient at βε>0 and η>0.28. Phase diagrams of pure fluids also indicate quantitative accuracy for DMD/TPT at reduced temperatures less than 0.9. The results show that wider wells improve the representation of thermodynamic properties for longer chains. The well width becomes a function of the molecular weight, however.
Magnetic flux evolution in highly shaped plasmas
Authors: Strand P. I. , Houlberg W. A.
Published: July 2002

The resistive evolution of magnetic flux in toroidal devices is studied. The formulation is applicable to general nonaxisymmetric (three-dimensional) toroidal configurations. In particular, it can treat highly shaped, high β, three-dimensional stellarator configurations, as well as two-dimensional (axisymmetric) tokamak plasmas. The time evolution of the poloidal magnetic flux is posed in terms of the rotational transform, ι̸, and allows for a transparent inclusion of stellarator specific current-free contributions to ι̸. Strong diamagnetic and paramagnetic contributions to toroidal magnetic flux, as evident in spherical tokamaks and similar concepts, are calculated by direct iteration with an equilibrium solver. The nonlinear evolution equation is derived using a susceptance matrix formulation originally introduced by Grad and co-workers [Bateman, Nucl. Fusion 13, 227 (1973)]. Here, it is extended to general, nonstraight field line coordinate systems. The basic equations are described, explicit expressions for the susceptance matrix are given, and example applications using the stand-alone code, THRIFT (THRee-dimensional Inductive Flux evolution in Toroidal devices), are discussed.
Switching dynamics of suspended mesogenic polymer microspheres
Authors: Cairns Darran R. , Sibulkin Merwin , Crawford Gregory P.
Published: July 2002

Ordered polymer microspheres were produced by photo-polymerizing reactive nematic liquid crystal droplets suspended in glycerol, thus capturing the mesogenic order indefinitely. The electro-optic switching of these structures suspended in a fluid is investigated and described by a simple phenomenological model.
A common-path heterodyne interferometer for surface profiling in microelectronic fabrication
Authors: Klein Eric J. , Ramirez W. Fred , Hall John L.
Published: July 2002

We describe the design of a common-path heterodyne laser interferometer for the surface profiling of micron-sized photopatterned features during the microelectronic fabrication process. The common-path design of the interferometer’s reference and measurement arms effectively removes any path length difference in the measurement which can be attributed to the movement of the target surface. It is shown that repeated surface profiling during the ion milling process allows the difference in etch rates between the photoresist layer and the exposed portions of the underlying substrate layer to be monitored online. A prototype apparatus has been assembled and results demonstrating the usefulness of the device are reported. The surface profiles of both a photopatterned nickel–iron trench and an unmasked aluminum trench are measured and compared to those obtained using a stylus-based scanning profiler and an atomic force microscope.
Determination of the thickness of Al oxide films used as barriers in magnetic tunneling junctions
Authors: Zhu W. , Hirschmugl C. J. , Laine A. D. , Sinkovic B. , Parkin S. S. P.
Published: July 2002

Aluminum films oxidized in oxygen atmosphere are widely used in magnetic tunneling junctions (MTJs). We have determined the oxidation depth of these films by Fourier transform infrared (FTIR) spectroscopy in reflection-absorption mode. The oxide/Al interfaces are found to be abrupt and the thickness of the oxide layer increases slowly with exposure to oxygen, after a rapid onset. These results provide a physical picture of observed changes in the magnetotransport of MTJs as a function of Al thickness and oxidation time. The results also agree well with the empirically found optimum oxide growth condition which yields a maximum magnetoresistance.
Reducing photorefractive effect in periodically poled ZnO- and MgO-doped LiNbO3 wavelength converters
Authors: Asobe Masaki , Tadanaga Osamu , Yanagawa Tsutomu , Itoh Hiroki , Suzuki Hiroyuki
Published: July 2002

The photorefractive effect in annealed proton-exchanged waveguides in periodically poled nondoped, MgO- and ZnO-doped LiNbO3 was evaluated by monitoring the quasiphase matching (QPM) wavelength shift induced by a 0.784-μm-irradiating light. The QPM wavelength shift was reduced at room temperature by a factor of 3–6 in ZnO- and MgO-doped samples compared with the nondoped samples within a 104–105-W/cm2-irradiation intensity range. The doped samples exhibited no significant wavelength shifts when the temperature was raised to slightly above room temperature (50–60 °C).
Energetic ion loss diagnostic for the Wendelstein 7-AS stellarator
Authors: Darrow D. S. , Werner A. , Weller A.
Published: July 2002

A diagnostic to measure the loss of energetic ions from the Wendelstein 7-AS (W7-AS) stellarator has been built. It is capable of measuring losses of both neutral beam ions and energetic ions arising from ion cyclotron resonant heating. The probe can measure losses of both clockwise and counterclockwise-going energetic ions simultaneously, and accepts a wide range of pitch angles in both directions. Initial measurements by the diagnostic are reported.
Multiple-ion-beam time-of-flight mass spectrometer
Authors: Rohrbacher Andreas , Continetti Robert E.
Published: July 2002

An innovative approach to increase the throughput of mass spectrometric analyses using a multiple-ion-beam mass spectrometer is described. Two sample spots were applied onto a laser desorption/ionization target and each spot was simultaneously irradiated by a beam of quadrupled Nd:YLF laser radiation (261.75 nm) to produce ions by laser-desorption ionization. Acceleration of the ions in an electric field created parallel ion beams that were focused by two parallel einzel lens systems. After a flight path of 2.34 m, the ions were detected with a microchannel plate-phosphor screen assembly coupled with a charge coupled device camera that showed two resolved ion beams. Time-of-flight mass spectra were also obtained with this detector. Experiments were performed using both metal atom cations (Ti+ and Cr+) produced by laser desorption/ionization and the molecular ions of two different proteins (myoglobin and lysozyme), created by matrix assisted laser desorption/ionization using an excess of nicotinic acid as matrix.
Hyper-Rayleigh scattering in the Fourier domain for higher precision: Correcting for multiphoton fluorescence with demodulation and phase data
Authors: Wostyn Kurt , Binnemans Koen , Clays Koen , Persoons André
Published: July 2002

An improved experimental technique for the suppression of the multiphoton fluorescence contribution in hyper-Rayleigh scattering experiments for the determination of the first hyperpolarizability of molecules in solution is presented. This improvement allows for a better correction for the fluorescence artifact, so as to eliminate any overestimation for the value of the first hyperpolarizability. The measurement of the demodulation only of the fluorescence as a function of modulation frequency [Olbrechts et al., Rev. Sci. Instrum. 69, 2233 (1998)] is now complemented by the measurement of the phase lag between the intermediate scattering and the time-delayed fluorescence. From the simultaneous data reduction of demodulation and phase shift toward the hyperpolarizability, fluorescence contribution, and fluorescence lifetime, an improvement in precision of 1 order of magnitude is demonstrated. This level of precision has been used to show the relative impact of f-orbital filling and ligands on the molecular second-order nonlinear optical response of lanthanide complexes containing a hemicyanine chromophore.
Free energy of solvation for the reference interaction site model: Critical comparison of expressions
Authors: Ten-no Seiichiro
Published: July 2002

We investigate expressions of excess chemical potential in the reference interaction site model (RISM) integral equation theory. In addition to the previous expressions from the Gaussian density fluctuation theory and from the extended RISM (XRISM) theory, we examine a new free energy functional from the distributed partial wave expansion of molecular correlation functions, using the embedded site model and alcohols with different parameter sets. The results clearly show that the free energy of solvation in the XRISM theory includes a serious error, which is related to the number of interaction sites and the geometry of a solute molecule.
Comparison between simulations and calibrations of a high resolution electrostatic analyzer
Authors: Vilppola J. H. , Tanskanen P. J. , Barraclough B. L. , McComas D. J.
Published: July 2002

The ion beam spectrometer (IBS) is one of the three spectrometers in the Cassini plasma spectrometer instrument aboard the Cassini/Huygens spacecraft. The IBS is a very high energy resolution hemispherical electrostatic analyzer. The design values of the IBS are analyzer gap 2.5 mm, and middle radius 100 mm. Because of the high energy resolution required, special care had to be used in the design and manufacturing of the instrument. A simulation was developed in order to aid the designing process. Here we show that the best fit to the laboratory calibration of the IBS flight model is obtained using the simulation model, where the inner hemisphere is misaligned by about 20 μm, where a maximum asymmetry of 75 μm is included in the inner hemisphere, and where the analyzer gap is increased from the design value by about 0.1 mm. We show here that geometric factors postulated in the theoretical model resulted in a better agreement between calibrations and simulations than the geometric factors calculated from calibration data alone. We find a value of (2.5±0.1)×10−4 cm2 sr for the geometric factor of the flight model. Finally, we also show that the energy and angle responses of the IBS are somewhat lower than the values calculated directly from the calibration data: (2.0±0.1)% is the value of the full width at half maximum (FWHM) of the energy distribution, and (3.1±0.1)° is the value of the FWHM of the azimuth angle distribution.
Polarization, hosing and long time evolution of relativistic laser pulses
Authors: Naumova N. M. , Koga J. , Nakajima K. , Tajima T. , Esirkepov T. Zh. , Bulanov S. V. , Pegoraro F.
Published: July 2002

The effect of the pulse polarization and of the accelerated fast electrons on the propagation anomalies (“hosing” and “snaking”) of a high-intensity laser pulse in an underdense plasma is investigated with two dimensional particle in cell simulations. The pulse deflection and the type of plasma modes (solitons, vortices) into which the laser pulse energy is eventually deposited, are shown to depend on the pulse polarization.
Equilibrium and stability properties of self-organized electron spiral toroids
Authors: Chen C. , Pakter R. , Seward D. C.
Published: July 2002

A cold-fluid model for a self-organized electron spiral toroid (EST) is presented. In the present model, the electrons are assumed to undergo energetic spiral motion along a hollow torus with a fixed ion background, the electron mean free path is assumed to be long compared with the torus size, and the minor radius of the EST is assumed to be small compared with the major radius. Using this model, the equilibrium and stability properties of the electron flow in the self-organized EST are analyzed. It is found that a class of self-organized EST equilibria exists with or without an externally applied toroidal magnetic field. It is shown that in the absence of any applied toroidal magnetic field, the EST equilibria are stable at high electron densities (i.e., at high toroidal self-magnetic fields), although they are unstable at low electron densities (i.e., at low toroidal self-magnetic fields).
Geometric edge barrier in the Shubnikov phase of type-II superconductors
Authors: Brandt E. H.
Published: July 2002

In type-II superconductors the magnetic response can be irreversible for two different reasons: vortex pinning and barriers to flux penetration. Even without bulk pinning and in the absence of a microscopic Bean–Livingston surface barrier for vortex penetration, superconductors of nonellipsoidal shape can exhibit a large geometric barrier for flux penetration. This edge barrier and the resulting irreversible magnetization loops and flux-density profiles are computed from continuum electrodynamics for superconductor strips and disks of constant thickness, both without and with bulk pinning. Expressions are given for the field of first flux entry Hen and for the reversibility field Hrev above which the pin-free magnetization becomes reversible. Both fields are proportional to the lower critical field Hc1 but otherwise depend only on the specimen shape. These results for rectangular cross section are compared with the well-known reversible magnetic behavior of ideal ellipsoids.
The local free volume, glass transition, and ionic conductivity in a polymer electrolyte: A positron lifetime study
Authors: Bamford D. , Dlubek G. , Reiche A. , Alam M. A. , Meyer W. , Galvosas P. , Rittig F.
Published: July 2002

The size of free-volume holes in neat poly[(ethylene glycol)23dimethacrylate] [poly((EG)23DMA)] and in the same polymer doped with 0.6 mol/kg LiCF3SO3 have been studied as a function of temperature in the range between 100 and 370 K using positron annihilation lifetime spectroscopy. The results are compared with differential scanning calorimetry and ionic conductivity measurements. In both systems, the hole volume νh shows a typical glass-transition behavior, i.e., a small linear increase with temperature below the glass transition temperature Tg and a steeper increase above Tg. From these measurements Tg was estimated to be 233 K (neat polymer) and 240 K (polymer with salt) and the coefficients of the thermal expansion of the hole volume were determined. The fractional free volume (f=0.080) and the number density of holes (Nh=0.6 nm−3) were also estimated. Below Tg the average hole volume of the polymer electrolyte is larger than in the neat polymer. This is consistent with the bulky character of the CF3SO3− anion. Above Tg the salt-doped system shows the lower hole volume of the two systems, probably caused by a reduced segmental mobility as a consequence of the interactions of the Li+ ions with the ethylene oxide units of the polymer. Based on the free-volume theory of Cohen–Turnbull the ionic conductivity σ is correlated with the mean hole volume νh. A linear relation between log(σT 0.5) and 1/νh was observed to be valid for variations of the conductivity over several orders of magnitudes. From these plots critical hole sizes of γν*=0.65 nm3 (neat polymer) and 0.87 nm3 (polymer-salt system) were estimated. The parameters B and T0 of the Vogel–Tamman–Fulcher equation were also determined, as well as the apparent activation volume ΔVapp by pressure-dependent conductivity measurements. The cationic transference number in the polymer-salt system was determined by pulsed field gradient-nuclear magnetic resonance to be t+≈0.3.
Effect of annealing atmosphere on domain structures and electromechanical properties of Pb(Zn1/3Nb2/3)O3-based ceramics
Authors: Fan Huiqing , Park Gun-Tae , Choi Jong-Jin , Kim Hyoun-Ee
Published: July 2002

Annealing atmosphere effects on domain structures and electrical properties of Pb[(Zn1/3Nb2/3)0.5(Zr0.47Ti0.53)0.5]O3 (PZN–PZT) ferroelectric materials were investigated. The PZN–PZT specimens were annealed in argon, air, oxygen, and PbZrO3 atmospheres after being sintered at 1100 °C in air. The as-sintered specimens were composed of large plate-like domains. When the specimen was annealed in flowing oxygen atmosphere for 8 h at 960 °C, the domains were refined into fine twin-like domains; whereas when the specimen was annealed in argon atmosphere, the domains were modified into needle-like structures. These modifications to the domain structures have a strong effect on the electromechanical properties of this material. The formation and redistribution of lead and oxygen vacancies during thermal annealing were responsible for those variations in domain structures and electrical properties.
Photodissociation of the dibromomethane cation at 355 nm by means of ion velocity imaging
Authors: Huang Jianhua , Xu Dadong , Fink William H. , Jackson William M.
Published: July 2002

The photodissociation dynamics of the dibromomethane cation, CH2Br2+, have been studied by means of ion velocity imaging and time-of-flight mass spectroscopy methods at 355 nm. The dibromomethane cation is produced through the direct ionization of the neutral molecule with a pulsed 118 nm laser. The translational energy distribution shows that the CH2Br+ fragment is formed in highly vibrationally excited states with two distinguished dissociation channels following a parallel excitation from 2b2 to 3b2 of the parent ion. The broad fast speed distribution is fit with two Gaussian functions, from which a branching ratio of Br*(2P1/2) to Br(2P3/2) is determined as 2.2:1. The sharp peak with very slow speed was modeled with a Boltzmann distribution with a temperature of 300 K. This channel contributes ∼4.5% to the reaction and is proposed to proceed on the ground state surface following internal conversion. Ab initio calculations for both the parent and the fragment ions have been performed that strongly support the proposed dissociation mechanisms.
Late time behavior of bubbles at unstable interfaces in two dimensions
Authors: Sohn Sung-Ik , Zhang Qiang
Published: July 2002

The late time behavior of bubbles at unstable interfaces plays a crucial role in chaotic interfacial fluid mixing. Several theoretical studies are based on potential flow models with or without a point source in the velocity potential. Our main purpose in this paper is to determine whether these models give the same predictions for the late time behavior of bubbles at unstable interfaces. We show that the predictions from these two models (with and without a point source) are qualitatively the same, but quantitatively different.
Engineering acoustic band gaps
Authors: Lai Yun , Zhang Xiangdong , Zhang Zhao-Qing
Published: July 2002

By using a perturbative approach, we propose a simple, systematic, and efficient method to engineer acoustic band gaps. A gap can be enlarged or reduced by altering the microstructure according to the field-energy distributions of the Bloch states at the band edges as well as their derivatives. Due to the structure of the acoustic wave equation, the engineering of acoustic band gaps is much more efficient than that of photonic band gaps. The validity of the proposed method is supported by multiple-scattering calculations. Our method makes the acoustic band gap “designable.”
The physical basis for the magnetic field dependence of proton spin-lattice relaxation rates in proteins
Authors: Korb Jean-Pierre , Bryant Robert G.
Published: July 2002

The magnetic field dependence of the proton spin-lattice relaxation rate in polymeric materials and biological macromolecules may report important dynamical information. In the case where the system is dynamically heterogeneous as in plasticized polymeric systems or hydrated biopolymers, the spin-lattice relaxation of the liquid-proton population is generally coupled to the spin-relaxation behavior of the solid spins that often dominate the observable response of the liquid. In many of these systems the magnetic field dependence of the proton spin-lattice relaxation rate may be represented as a power law: 1/T1(ω)=Aω−b where a is a constant and b is usually found to be in the range of 0.5 to 0.8. We have shown that this power law may arise naturally from localized structural fluctuations along the backbone of chain molecules that modulate the proton dipole–dipole couplings, which form a network described by a fractal dimension that may be less than the Euclidean dimension. When the model for the solid spin-lattice relaxation rate constant is incorporated into the relaxation equations that couple the liquid and solid spin population responses, the composite model accounts quantitatively for proton spin-lattice relaxation rates measured in immobilized and hydrated protein systems and provides a fundamental basis for understanding the parametric dependence of proton spin-lattice relaxation rates in dynamically heterogeneous systems such as hydrated biopolymers, proteins, and biological tissues.
Tailoring detection bands of InAs quantum-dot infrared photodetectors using InxGa1−xAs strain-relieving quantum wells
Authors: Kim Eui-Tae , Chen Zhonghui , Madhukar Anupam
Published: July 2002

We report on tailoring detection bands of InAs quantum-dot infrared photodetectors (QDIPs) using InxGa1−xAs strain-relieving capping layers that also act as quantum wells (QWs). QDIPs with InAs QDs capped by a 20 ML In0.15Ga0.85As QW show a sharp photoresponse at ∼9 μm, while the counterpart QDIPs without QWs show broad photoresponse in the 5–7 μm range. The excited states involved in the intraband transitions in QDIPs with the In0.15Ga0.85As QW appear to be coupled QD and QW electron excited states.
Annealing behavior of vacancies and Z1/2 levels in electron-irradiated 4H–SiC studied by positron annihilation and deep-level transient spectroscopy
Authors: Kawasuso A. , Redmann F. , Krause-Rehberg R. , Weidner M. , Frank T. , Pensl G. , Sperr P. , Triftshäuser W. , Itoh H.
Published: July 2002

Annealing behavior of vacancies and the Z1/2 levels in n-type 4H–SiC epilayers after 2 MeV electron irradiation has been studied using positron annihilation and deep-level transient spectroscopy. Isochronal annealing studies indicate that silicon vacancy-related defects are primarily responsible for positron trapping. The Z1/2 levels are the predominant deep centers after irradiation and subsequent annealing at 1200 °C. Both the positron-trapping rate at vacancies and the Z1/2 concentration decrease in a similar manner while annealing from 1200 to 1500 °C. It is thus proposed that the Z1/2 levels originate from silicon vacancy-related defects.