bluuebunny/crossref_metadata_2025 huggingface dataset:
Processing and characterization of oval piezoelectric actuators
Authors: Jadidian B. , Allahverdi M. , Mohammadi F. , Safari A.
Published: July 2002

The processing and characterization of piezoelectric actuators with oval geometry are presented. The monolithic actuators were fabricated using the fused deposition of ceramic process. The minor diameter of the ovals varied between 2 and 14 mm and their major diameter, wall thickness, and width were 20, 0.85, and 7 mm, respectively. When driven under electric field, the actuators expanded along their minor diameter. The static and dynamic displacements of ∼7 and ∼5.6 μm were observed at 850 V(dc) and 100 V(ac). The static displacement of the ovals varied almost linearly with voltage and did not change under the application of external load in the range of 1–15 N. However, both dynamic displacement and resonant frequency of the ovals varied, with a maximum of 42 μm and 38 Hz, respectively, under 13 N load.
High resolution pulsed infrared cavity ringdown spectroscopy: Application to laser ablated carbon clusters
Authors: Casaes Raphael , Provençal Robert , Paul Joshua , Saykally Richard J.
Published: July 2002

We report the design and performance of a tunable, pulsed high resolution mid infrared cavity ringdown spectrometer. Stimulated Raman scattering in H2/D2 is used to downconvert the output of a SLM Alexandrite ring laser (720–800 nm) to the mid infrared (3–8 μm). The infrared frequency bandwidth was determined to be 90±5 MHz from measurements of Doppler broadened OCS transitions at 5 μm. The minimum detectable per pass fractional absorption is 1 ppm. We observe a frequency dependent ringdown cavity transmission of ±5 ppm due to spatial variations of the mirror reflectivity. The υ6 band of linear C9 formed by laser ablation of graphite in a He molecular beam was measured, showing a factor of 2 improvement in sensitivity relative to previous IR diode laser experiments. Based on calculated IR intensities, the number density of C9 in the molecular beam is 1.3*1011 molec/cm3 and the minimum detectable density is 1*109 molec/cm3. We expect this spectrometer to be a powerful tool for the study of transient species formed in molecular beams.
Shift of the photonic band gap in two photonic crystal/liquid crystal composites
Authors: Mertens Guido , Röder Thorsten , Schweins Ralf , Huber Klaus , Kitzerow Heinz-S.
Published: July 2002

We have investigated the photonic band gap of normal and inverted colloidal crystals which were infiltrated with a nematic liquid crystal. In both systems, the wavelength of the photonic band gap can be tuned by adjusting the temperature. In agreement with recent theoretical predictions, the spectral shift is larger (≈14 nm) in the inverted structure.
Effects of ion temperature gradients on the formation of drift-Alfvén vortex structures in dusty plasmas
Authors: Onishchenko O. G. , Pokhotelov O. A. , Sagdeev R. Z. , Pavlenko V. P. , Stenflo L. , Shukla P. K. , Zolotukhin V. V.
Published: July 2002

A set of equations describing the nonlinear dynamics of drift-Alfvén waves in a dusty plasma accounting for the nonzero ion temperature gradients is derived. It is shown that these new equations yield a solution in the form of two-scale dipolar vortex structures propagating with velocities close to the ion-drift velocity in a narrow cone centered around the direction perpendicular to both the external magnetic field and the plasma gradient directions. The typical scales, characteristic vortex velocities as well as the relevant conditions for their existence are discussed. It is shown that nonzero ion temperature gradients substantially enlarge the range of possible propagation directions and characteristic scales of the vortex structures.
The role of clustering effects in interpreting nondiffusive transport measurements in tokamaks
Authors: Graves J. P. , Dendy R. O. , Hopcraft K. I. , Jakeman E.
Published: July 2002

Recent measurements in tokamak plasmas provide clear evidence for rapid nondiffusive transport and non-Gaussian fluctuations, and have been widely interpreted in terms of the sandpile and self-organized criticality (SOC) paradigms. Many of the statistical physics inferences that can be drawn from observations of, for example, avalanching transport remain to be explored. This paper will show that the statistical characterization of both experimentally observed and simulated avalanching transport phenomena reveals several points of contact with existing stochastic process models that have seldom been deployed in a plasma physics context. It will be shown that statistical physics techniques developed to model clustering of events can be used to characterize microscopic fluctuations in both local density and flux, as well as the global transport properties to which they give rise. This provides a fresh interpretation for some of the key aspects of observed critical gradient-driven transport phenomenology in tokamaks. In particular it provides new evidence for scale-free correlations in the fluctuations which drive the transport, and quantifies their distribution in terms of few-parameter non-Gaussian models. The correlation properties of density fluctuations can be interpreted in terms of random walk models, whereas flux fluctuations cannot: instead they can be described by the discrete negative binomial distribution, which again indicates clustering. Some of the spatio–temporal correlations considered emulate multichannel measurements in tokamaks, and it is shown how these can be used to characterize the transport of naturally arising coherent structures.
Semiconductor nanostructures defined with self-organizing polymers
Authors: Haupt M. , Miller S. , Ladenburger A. , Sauer R. , Thonke K. , Spatz J. P. , Riethmüller S. , Möller M. , Banhart F.
Published: July 2002

We describe a technique to create very small semiconductor nanostructures, with sizes far beyond the limit of conventional optical lithography processes, by the use self-assembling diblock copolymers as nanolithographic masks. Quantum structures with very high aspect ratio of 1:10 were fabricated by dry etching. In a first step, so-called diblock copolymer micelles were generated in a toluene solution. These micelles were loaded by a noble-metal salt. After dipping a substrate into this solution, a monolayer of ordered micelles is generated, covering almost the complete surface. After treatment in a hydrogen plasma all of the organic components are removed and only crystalline metal clusters of ≈12 nm size remain. This metal cluster mask can be used directly in a chlorine dry etching process to etch cylinders in GaAs and its alloys of In and Al. It is also possible to etch through a quantum well layer underneath the surface in order to produce quantum dots. The resulting nanostructures were investigated by scanning force microscopy, by high resolution transmission electron microscopy, and also by low temperature photoluminescence spectroscopy.
A 118 nm vacuum ultraviolet laser/time-of-flight mass spectroscopic study of methanol and ethanol clusters in the vapor phase
Authors: Shi Y. J. , Consta S. , Das A. K. , Mallik B. , Lacey D. , Lipson R. H.
Published: July 2002

Clusters of methanol and ethanol formed above neat liquid samples were entrained in a supersonic jet of helium and probed in the expansion using 118 nm vacuum ultraviolet laser single-photon ionization/time-of-flight (TOF) mass spectrometry. Almost every cluster ion observed in the TOF mass spectra could be represented by the formula H(ROH)n+, where R=CH3 or C2H5, and n=1–5. Formation of these species is attributed to a well-established ionization pathway where each protonated (n−1)-mer originates from its n-mer neutral parent. Signals in the TOF mass spectra due to the protonated trimers H(CH3OH)3+ and H(CH3CH2OH)3+ were found to be the most intense and provides direct evidence that these particular cluster ions are “magic-number” structures. The possible relationships between the observed ion data and the neutral cluster vapor phase distributions are discussed. In this context, methanol and ethanol vapor cluster distributions at 298.15 K and at several pressures⩾the equilibrium vapor pressure were computed using the grand canonical Monte Carlo and molecular dynamics techniques. Lastly, differences between these experiments and the results of bimolecular reaction studies are discussed.
Nonlocal theory of drift type waves in a collisionless dusty plasma
Authors: Sarmah D. , Chakravorty M. , Sen S.
Published: July 2002

A nonlocal theory is formulated to study drift waves in a collisionless multicomponent (dusty) plasma in a sheared slab geometry. The dynamics of dust particles and ions are treated by fluid models, whereas the electrons are assumed to follow the Boltzmann distribution. It is found that the usual stability of drift waves in a sheared slab geometry is destroyed by the presence of dust particles. A drift wave is excited which propagates with a new characteristic frequency modified by dust particles. This result is similiar to our earlier work for the collisional dusty plasma [Chakraborty et al., Phys. Plasmas 8, 1514 (2001)].
Diffusion barrier properties of amorphous TiB2 for application in Cu metallization
Authors: Pelleg Joshua , Sade G.
Published: July 2002

Amorphous TiB2 films were obtained by magnetron cosputtering and conditions for obtaining good diffusion barrier properties against penetration by Cu were evaluated by Auger electron spectroscopy. Controlling the B/Ti ratio is of major importance. It was found that the best diffusion barrier properties were obtained in stoichiometric or overly stoichiometric titanium boride. Almost no penetration of Cu occurred at 953 K for 30 min. Films of TiBx where x<2.0 are of inferior barrier quality. At B/Ti≈1.63 complete deterioration of the barrier property occurs. The effect of composition is explained in terms of the packing density of the amorphous film. Preliminary diffusion studies by secondary ion mass spectroscopy in the amorphous titanium boride of near-stoichiometric composition (B/Ti≈2.07) indicate that two mechanisms seem to operate in the TiB2 film. The low temperature mechanism occurs in the amorphous disordered structure, whereas at higher temperature diffusion occurs in an amorphous film in which short-range order exists. An activation energy of ≈2.0 eV was derived from the steep slope of the Arrhenius curve that represents diffusion in the short-range ordered region.
Investigation of the state-to-state rotational relaxation rate constants for carbon monoxide (CO) using infrared double resonance
Authors: Phipps Steve P. , Smith Tony C. , Hager Gordon D. , Heaven Michael C. , McIver J. K. , Rudolph W. G.
Published: July 2002

State-to-state rotational relaxation of carbon monoxide (CO) has been studied using an ir double resonance technique. Individual rotational lines of the (2-0) vibrational overtone band were pumped by a pulsed tunable ir laser and the subsequent evolution of the v=2 rotational population distribution was monitored by the absorption of a tunable cw ir laser via the (3-2) band transitions. Both the excitation and probe lasers were linearly polarized, with linewidths that were narrower than the CO Doppler width. Consequently, alignment and velocity relaxation effects were observed in these measurements. A data set consisting of 54 time-dependent rotational state population profiles was acquired. The full CO–CO rotational relaxation matrix, which consists of state-to-state rate constants for rotational levels up to J=29, was deduced from computer simulations of the data. Scaling and fitting laws were used to provide parametric representations of the rate constants. The three most common models, modified exponential gap, statistical polynomial-exponential gap (SPEG), and energy corrected sudden with exponential-power gap (ECS-EP) were investigated. We concluded that the SPEG law best reproduced the CO–CO rotational energy transfer data. A propensity to preserve the CO parity in rotational energy transfer was observed for collisions where the amount of energy transferred was small. Hence even ΔJ processes were favored for transitions between levels with low J values. For near-single collisions events a correlation was noted between the amount of rotational energy transferred and the degree of velocity distribution relaxation. This correlation yielded insights regarding the energy transfer dynamics.
Multichannel scaler for general statistical analysis of dynamic light scattering
Authors: Sprik Rudolf , Baaij Edwin
Published: July 2002

A four channel scaler for counting applications has been designed and built using a standard high transfer rate parallel computer interface bus parallel data card. The counter section is based on standard complex programmable logic device integrated circuits. With a 200 MHz Pentium based host PC a sustained counting and data transfer with channel widths as short as 200 ns for a single channel is realized. The use of the multichannel scaler is demonstrated in dynamic light scattering experiments. The recorded traces are analyzed with wavelet and other statistical techniques to obtain transient changes in the properties of the scattered light.
The embedding of space–times in five dimensions with nondegenerate Ricci tensor
Authors: Dahia F. , Romero C.
Published: July 2002

We discuss and prove a theorem which asserts that any n-dimensional semi-Riemannian manifold can be locally embedded in an (n+1)-dimensional space with a nondegenerate Ricci tensor which is equal, up to a local analytic diffeomorphism, to the Ricci tensor of an arbitrary specified space. This may be regarded as a further extension of the Campbell–Magaard theorem. We highlight the significance of embedding theorems of increasing degrees of generality in the context of higher dimensional space–times theories and illustrate the new theorem by establishing the embedding of a general class of Ricci-flat space–times.
A method for increasing depth of field during droplet imaging
Authors: Saylor J. R. , Jones B. K. , Bliven L. F.
Published: July 2002

The measurement of water drops using direct optical imaging involves a tradeoff between the camera depth of field and the accuracy of the measured droplet size. A large depth of field increases the length of that portion of the optical axis that is in focus. However, since drops closer to the camera appear larger than those farther away, increasing the depth of field also increases errors in the measured drop size unless a priori knowledge exists concerning the location of the drop along the optical axis. Herein a method is presented for ascertaining drop location using a single camera. The method uses a characteristic of the droplet image which is observed when droplets are illuminated from behind. Once the location of the droplet is obtained, the appropriate magnification ratio is applied, permitting an accurate droplet size measurement. This method has been tested on glass spheres of various size. The relevance of this work to precipitation science is discussed.
Fiber Bragg gratings with enhanced thermal stability
Authors: Brambilla Gilberto , Rutt Harvey
Published: July 2002

Gratings written in tin-doped silica fibers by using a 248 nm excimer laser exhibit extremely high thermal stability. Isothermal measurements up to 1100 K demonstrate significant advantages over conventional fiber gratings. Extrapolations from experimental data indicated that gratings operating at 500 K for ten years will retain more than 99% of the initial strength.
Wavelet approximation of correlated wave functions. I. Basics
Authors: Flad Heinz-Jürgen , Hackbusch Wolfgang , Kolb Dietmar , Schneider Reinhold
Published: July 2002

We suggest an alternative approach to electronic structure calculations based on numerical methods from multiscale analysis. By this we are aiming to achieve a better description of the various length- and energy-scales inherently connected with different types of electron correlations. Taking a product ansatz for the wave function Ψ=FΦ, where Φ corresponds to a given mean-field solution like Hartree–Fock or a linear combination of Slater determinants, we approximate the symmetric correlation factor ℱ in terms of hyperbolic wavelets. Such kinds of wavelets are especially adapted to high dimensional problems and allow for local refinement in the region of the electron–electron cusp. The variational treatment of the ansatz leads to a generalized eigenvalue problem for the coefficients of the wavelet expansion of ℱ. Several new numerical features arise from the calculation of the matrix elements. This includes the appearance of products of wavelets, which are not closed under multiplication. We present an approximation scheme for the accurate numerical treatment of these products. Furthermore the calculation of one- and two-electron integrals, involving the nonstandard representation of Coulomb matrix elements, is discussed in detail. No use has been made of specific analytic expressions for the wavelets, instead we employ exclusively the wavelet filter coefficients, which makes our method applicable to a wide class of different wavelet schemes. In order to illustrate the various features of the method, we present some preliminary results for the helium atom.
Spin–lattice relaxation in the bcc phase of phase-separated He3–He4 solid mixtures
Authors: Mikhin N. P. , Maidanov V. A. , Polev A. V.
Published: July 2002

The spin–lattice relaxation time in two samples of He3–He4 solid mixtures with initial concentrations of 0.5% He3 in He4 and 0.5% He4 in He3 is measured by the pulsed NMR method. As a result of phase separation, in both cases two-phase crystals form, having the same helium concentration in the concentrated bcc phase. However, in the first sample the bcc phase forms as small inclusions in an hcp matrix, while in the second sample the bcc phase is the matrix. It is established that in the second case the spin–lattice relaxation occurs in the same way as in pure bulk He3, while in the first case one observes anomalous behavior of the spin–lattice relaxation time at low temperatures. Experiments have shown that this anomaly is due not to the possible influence of the small He4 impurity but to the small dimensions of the inclusions of the of the bcc phase. In this case the main contribution to the relaxation is apparently due to defects formed at the boundaries of the bcc inclusions and the hcp matrix.
The dynamics of ND4+ and NH3D+ groups in ammonium persulphate studied by deuteron nuclear magnetic resonance. I. Tunneling and stochastic reorientations at low temperatures
Authors: Olejniczak Z. , Lalowicz Z. T. , Schmidt T. , Zimmermann H. , Haeberlen U. , Schmitt H.
Published: July 2002

We report on a single-crystal deuteron nuclear magnetic resonance (NMR) spectroscopy study of the low-temperature dynamics of ND4+ and NH3D+ ions in the title compound. The most prominent feature of the dynamics of ND4+ ions is uniaxial rotational (primary) tunneling of three deuterons about the N–D bond of the fourth. At T<25 K, the latter deuteron gets localized on the time scale of 10−3 s. We identify the direction of this unique N–D bond. The low-temperature limit of the primary tunneling frequency is 1.6 MHz. In the form of splittings of NMR resonances, our spectra also contain clear evidence of secondary tunneling which, again, is uniaxial. Again we say about which of the other three N–D bonds it takes place. The secondary tunneling frequency is only 4.5 kHz. The deuteron of NH3D+ ions gets localized at T<25 K. It can reside in any of the four sites available to the hydrogens of the ion. The dynamics of the three protons depends strongly on which site the deuteron occupies. If it is the site which was identified as unique for ND4+ ions, the protons reorient stochastically with a rate k>106 s−1. Very likely, they also undergo tunneling but the stochastic reorientations erase any tunneling features from the spectra. By contrast, if the deuteron occupies any other site, stochastic reorientations of the protons are slow and (proton) tunneling on the scale of 105 Hz can be identified. Finally, isotopic ordering is observed. The single deuteron of NH3D+ ions goes preferentially into the site identified as unique. Energetically, the preference amounts to 1 meV.
A high temperature probe operating in the variable temperature insert of a commercial superconducting magnet system
Authors: Leighton C. , Kern John , English Suzanne R.
Published: July 2002

We describe the design, construction, and performance of an oven insert for performing magnetotransport measurements, which operates inside the variable temperature environment of commercial superconducting magnet systems. The probe provides a high temperature environment up to 800 K and is completely compatible with magnetic fields of at least 10 T. The system is simple to construct, easy to operate and has a relative temperature stability of better than 100 mK at 800 K. We illustrate the operation of the system via high temperature magnetotransport measurements on the spin–state transition compound LaCoO3.
Effect of annealing on dielectric constant of boron carbon nitride films synthesized by plasma-assisted chemical vapor deposition
Authors: Sugiyama Tomohiko , Tai Tomoyoshi , Sugino Takashi
Published: July 2002

Polycrystalline boron carbon nitride (BCN) films are synthesized by plasma-assisted chemical vapor deposition. The dielectric constant is estimated from the accumulation region of capacitance–voltage characteristics of Cu/BCN/p-Si samples. It is found that the dielectric constant of the BCN film is reduced by annealing at 400 °C. The BCN film before and after annealing is characterized by Fourier transform infrared absorption measurements. The infrared absorption intensities due to the C–H and C=C bonds decrease after annealing. It is suggested that the C–H and C=C bonds are undesirable to obtain a low dielectric constant. A dielectric constant as low as 2.1 is achieved for the BCN film which is treated by annealing process.
Fabrication of Si single-electron transistors having double SiO2 barriers
Authors: Ito Yuhei , Hatano Tsuyoshi , Nakajima Anri , Yokoyama Shin
Published: July 2002

We fabricated Si single-electron transistors (SETs) having double SiO2 barriers and a polycrystalline Si (poly-Si) dot. The fabrication method of this device is completely compatible with the complementary metal–oxide–semiconductor technology, and the position of the poly-Si dot is self-aligned between the source and drain regions. The device exhibits drain current (Id) oscillation against gate voltage. From the dot size dependence of the electrical characteristics, the Id oscillation is considered to be due to the Coulomb blockade effect caused by poly-Si grains in the poly-Si dot. The self-alignment of the poly-Si dot in the fabrication process also means that the SET is promising for practical use.
Phase synchronization of chaotic oscillations in terms of periodic orbits
Authors: Pikovsky Arkady , Zaks Michael , Rosenblum Michael , Osipov Grigory , Kurths Jürgen
Published: July 2002

We consider phase synchronization of chaotic continuous-time oscillator by periodic external force. Phase-locking regions are defined for unstable periodic cycles embedded in chaos, and synchronization is described in terms of these regions. A special flow construction is used to derive a simple discrete-time model of the phenomenon. It allows to describe quantitatively the intermittency at the transition to phase synchronization.
The sharkskin instability of polymer melt flows
Authors: Graham Michael D.
Published: July 2002

Flows of polymeric liquids undergo instabilities whose origins are quite different from those of Newtonian flows, due to their elastic character and the complexity of the fluid/solid boundary condition. This article reviews recent studies of one such instability, the sharkskin phenomenon observed during extrusion of many linear polymers. Key experimental observations are summarized; one important fact that has become clear is the importance of the interaction between the molten polymer and the solid walls of the flow channel, especially near the contact line at the exit of the channel. Recent developments in understanding the relationship between wall slip and disentanglement of wall-adsorbed polymers from the bulk flow are briefly described, and putative heuristic mechanisms relating the instability to slip and contact line motion are presented. Finally, we review mathematical analyses of the stability of viscoelastic shear flows with slip boundary conditions. Some recent analyses yield instability predictions that are consistent with experiments, but further work is required to discriminate between the various mechanisms that have been proposed.
Extremely low contact resistances for AlGaAs/GaAs modulation-doped field-effect transistor structures
Authors: Ketterson A. , Ponse F. , Henderson T. , Klem J. , Morkoç H.
Published: July 2002

We report contact resistances as low as 0.035 Ω mm between the ohmic contact metal and the two-dimensional electron gas channel in an AlGaAs-GaAs modulation-doped field-effect transistor structure. The contact resistances are achieved by transient annealing of AuGe/Ni/Au, and are to our knowledge the smallest values reported to date for these structures at room temperature. Assuming no change in the semiconductor sheet resistivity under the metal contact, the calculated specific contact resistivity would be as low as 5×10−8 Ω cm2, however, the true figure is not known. Details of our contact study include varying temperature-time cycles and heterojunction structures. Current-voltage measurements at 77 K show ohmic behavior, revealing the destruction of the heterojunction barrier in the contact region. Preliminary Auger energy spectroscopy profiles support an intermixing of the metal and semiconductor constituents down through the original AlGaAs-GaAs interface. We, therefore, propose a model where the current flows laterally from grains (formed during annealing) into the two-dimensional electron gas.
Hopping conduction in polycrystalline semiconductors
Authors: Sharma R. P. , Shukla A. K. , Kapoor A. K. , Srivastava R. , Mathur P. C.
Published: July 2002

Measurements of dc conductivity (σ) on polycrystalline semiconductors, viz., InSb, Si, and CdTe, have been reported in the temperature range 77–300 K. The conduction mechanism near liquid-nitrogen temperature has been identified as the hopping of charge carriers from the charged trap centers to empty traps near the Fermi level.
Modeling of hard and soft magnetic materials in devices (invited; abstract)
Authors: Campbell Peter
Published: July 2002

In the calculation of magnetic fields within electromagnetic devices, modeling of the magnetic materials themselves usually relies upon data acquired from major magnetization curves. Most of the materials used in such devices as rotating electrical machines are anisotropic and a single curve is insufficient for accurate modeling of the material. This paper will review the approximations that are commonly made to account for anisotropy and by considering the analytical derivation of anisotropy, a simple technique is suggested by which directional magnetic properties may be more accurately represented in a field computation. A permeability tensor has sometimes been used to solve the field in regions containing laminations, but for convenience the transverse term has been assumed identical to the normal term, and even to be constant. By considering rotation of the magnetization vector, a new permeability tensor is derived, in which no such assumptions are made, but the terms are found from measured parameters. This tensor is used to improve the accuracy of field computation.
Magnetic response of type-II superconductors subjected to large-amplitude parallel magnetic fields varying in both magnitude and direction
Authors: Perez-Gonzalez Antonio , Clem John R.
Published: July 2002

Starting from our general critical-state theory, which includes the effects of both flux-line cutting and flux pinning, we develop a numerical method for finding the magnetic response of type-II superconductors subjected to parallel magnetic fields that change in both magnitude and orientation. We describe model calculations of the time-evolving magnetic profiles when the direction of an applied magnetic field of fixed magnitude oscillates with large amplitude, and we compare our results with related experiments by Cave and LeBlanc. We also report model calculations for the ac behavior when the sample is subjected to a dc bias field and a large-amplitude ac field at various angles (0°, 45°, and 90°) relative to this field.
Improved performance of a closed-cycle self-sustained discharge-excited cw CO laser
Authors: Sato Shun-ichi , Kiyota Mamoru , Fujioka Tomoo , Saito Hideaki
Published: July 2002

Detailed performance characteristics of a closed-cycle subsonic cw CO laser excited by a transverse self-sustained dc glow discharge are described. Using an electrode configuration which consists of a hollow-type cathode and a planar anode, the discharge characteristics have been measured under various gas conditions. The measurements show that the discharge instability, and hence the available discharge input, are strongly influenced not only by gas temperature and total static pressure but also by gas composition. From a discharge volume of about 480 cm3, a laser output power of 406 W is extracted with an electrical conversion efficiency of 11.6% for a mixture of CO/N2/He/O2=6/16/78/0.19 at an entrance gas temperature of 156 K. Furthermore, by applying an additional cathode array, the discharge uniformity and the specific input energy have been enhanced. As a result, the maximum extracted laser output reaches as high as 729 W or 1820 W per m of discharge length, with the corresponding conversion efficiency and specific output energy of 16.3% and 125 J/g, respectively. This performance is comparable to or higher than that which has been obtained with an electron-beam controlled discharge, a pulser-sustained discharge, etc.
Two-photon-ionization coefficients of CS2, SO2, and (CH3)3N
Authors: Wang W. C. , Lee L. C.
Published: July 2002

Electrons produced by two-photon ionization of CS2, SO2, and (CH3)3N in N2 and CH4 buffer gases at 193 nm were investigated using a parallel-plate drift-tube apparatus. At a low charge density, the transient voltage induced by electron motion between the electrodes is proportional to the gas pressure and the square of laser power. The two-photon-ionization coefficients measured from the number of electrons produced are 3.3×10−27, 8.3×10−30, and 1.7×10−27 cm4/W for CS2, SO2, and (CH3)3N, respectively. The coefficient for (CH3)3N agrees with the earlier value measured by ion current. At a high charge density, the number of electrons observed deviates from the square dependence of laser power. The numbers of ions and electrons are greatly reduced by charge recombination whose reaction rate is enhanced in the presence of space charge.
Photon correlation spectroscopic measurement of the bulk compliance of poly(vinyl acetate) above T
g
Authors: Wang C. H. , Hagenah J.-U. , Meier G. , Fytas G. , Fischer E. W.
Published: July 2002

The technique of determining the longitudinal compliance over a wide dynamic range (from 10−1 to 106 Hz) using Rayleigh–Brillouin scattering has been studied. The method of computing the longitudinal compliance spectrum from the time correlation of density fluctuations of poly(vinyl acetate), as determined by the photon correlation spectroscopic technique, is illustrated.
Preparation effects on the UV optical properties of GeO2 glasses
Authors: Jackson J. M. , Wells M. E. , Kordas G. , Kinser D. L. , Weeks R. A. , Magruder III R. H.
Published: July 2002

The fusion temperature (Tφ), the oxygen partial pressure (PO2) during melting and the quenching rate from Tφ all influence the optical absorption in the 5.06-eV (245-nm) region. These observations are consistent with the assumption that the defect responsible for the optical absorption is an oxygen vacancy or complex of vacancies. The activation energy for the formation of the defect in GeO2 glasses which is repsonsible for the optical absorption is 2.3 eV which is larger than the activation energy for oxygen diffusion (∼1 eV).
Phase-sensitive detection on Lorentzian line shape and its application to frequency stabilization of lasers
Authors: Nakazawa Masataka
Published: July 2002

A formulation to describe the general expression for the modulation broadening of Lorentzian profile is given analytically by using the Fourier expansion and the residue theorem. It is shown that the specific nth derivative signal can be obtained in a most straightforward manner. The present method is applied to a study of the maximum sensitivity (amplitude versus frequency detuning) of phase-sensitive detection as a frequency discriminator. For the first derivative signal, maximum discrimination sensitivity can be obtained when the modulation width is set to be 1/2 times of the full width at the half maximum (FWHM) of the profile, while the maximum amplitude (signal-to-noise ratio) is obtained when the width is exactly equal to FWHM.
Real-time measurement of crystallization process of polymers by pulsed nuclear magnetic resonance
Authors: Tanaka Hajime , Nishi Toshio
Published: July 2002

Real-time measurement of the crystallization process of polymers [poly(ε-caprolactone) and its blend with poly(vinyl methyl ether) and polystyrene] by pulsed nuclear magnetic resonance has been made. The measurements on both the spin-spin relaxation time T2 and the spin-lattice relaxation time in the rotating frame T*1 ρ have been performed for the first time. From the data, we can obtain information on the temporal change of the crystallinity as well as the change of the mobility in each phase. Furthermore, the temporal change of the intermediate phase corresponding to the interface between crystalline and amorphous part has been successfully detected.
Electronic structure of silicon nitride and amorphous silicon/silicon nitride band offsets by electron spectroscopy
Authors: Iqbal A. , Jackson W. B. , Tsai C. C. , Allen J. W. , Bates C. W.
Published: July 2002

The film thickness, chemical state, and polarization screening for a-SiN1.4 :H films deposited by glow discharge over hydrogenated amorphous silicon (a-Si:H) were determined by x-ray photoelectron spectroscopy (XPS) and Auger spectroscopy. The nitride films were observed to be single phase and the escape depth for 1400-eV electrons in the a-SiN1.4 :H film was determined to be 30 Å. The band offsets for the a-Si:H/a-SiN1.4 :H interface were determined by XPS and Bremsstrahlung isochromat spectroscopy (BIS) to be 1.2 eV for the valence band and 2.2 eV for the conduction band, while the band gap for a-SiN1.4 :H was found to be 5.3 eV in accordance with the optical gap. By combining optical absorption measurements with the valence-band density of states and conduction-band density of states determined by electron spectroscopy, a semiquantitative estimate of the band tailing within the nitride gap was obtained. Correlation of the defect absorption with the electron spin resonance measurements suggest that the dangling bond defect level in a-SiN1.4 :H lies approximately 2.5 eV from the nitride valence band and is, therefore, somewhat below the a-Si:H conduction band. This study has demonstrated that BIS is an effective technique for determining the conduction-band offsets in heterostructures without relying on valence-band offset or band-gap measurements.
Silane pyrolysis rates for the modeling of chemical vapor deposition
Authors: Meyerson Bernard S. , Jasinski Joseph M.
Published: July 2002

The pyrolysis of silane (SiH4) is the initial step in the silicon component of film formation for a wide variety of electronic materials. Modeling of such film-deposition processes requires the use of appropriate rate constants for this reaction. We report in this communication the widely ignored yet extremely important corrections that must be employed to properly account for the pressure dependence of this reaction. The Rice, Ramsperger, Kassel, and Marcus theory is employed to calculate rate constants, in the manner elaborated in the communication that follows, and quantitative examples are given for the errors introduced by the use of incorrect reaction rates.
Effects of perpendicular stresses on the magnetization processes of polycrystalline magnetic materials
Authors: Le Floc’h M. , Globus A.
Published: July 2002

Effects of perpendicular stresses on the magnetization mechanisms of polycrystalline ceramics are studied. It is shown that the magnetic behavior depends strongly upon the state of the domain-wall configuration before the application of the uniaxial stress. To this end, a general study of the magnetization laws is made on two different types of samples according to the well-established fact that the domain-wall configuration is first conditioned by the microstructure. The first type, called T, is assigned to samples with a homogeneous structure of very pure grains (without defect). The second type, called D, is assigned to samples with a heterogeneous structure of grains containing apparent defects as micropores, for example.
Energy-band diagrams and current-voltage characteristics of single-barrier tunnel structures
Authors: Bonnefoi A. R. , Chow D. H. , McGill T. C.
Published: July 2002

Energy-band diagrams of single-barrier heterostructures are calculated by solving Poisson’s equation self-consistently. The implications of these energy-band profiles for the electronic properties of a few tunnel structures of current interest are discussed. The voltage drops across the cladding layers are found to be significant, in contrast with the commonly made assumption that all of the applied voltage drops linearly across the barrier layer. Furthermore, energy-band diagrams have substantial effects upon calculated tunneling current-voltage characteristics, which are extremely sensitive to the actual shape of the barrier through which the charge carriers tunnel. It is found that, in some cases, the dominant current transport mechanisms may be quite different from those expected when band bending is neglected.
Ellipsometric and optical reflectivity studies of reactively sputtered NbN thin films
Authors: Tanabe K. , Asano H. , Katoh Y. , Michikami O.
Published: July 2002

The optical parameters of NbN thin films reactively sputtered mainly at room temperature are investigated by in situ ellipsometry using a He-Ne laser source. Systematic correlations between the ellipsometric parameters and the film properties are observed. In particular, the Tc film shows an approximately linear relation with the parameter ψ over the wide range of 7–17 K. Optical reflectivity measurements are carried out in the wavelength range of 0.2–2.5 μm for these films. Each spectrum is well fitted by the curve calculated using the Drude model with a screened plasma frequency of 25 000–30 000 cm−1. The Tc film also shows a stronger correlation with the damping constant of plasma excitation than with the reflectivity. This is considered to be a universal degradation curve for this system, similar to those observed in high -Tc A15 compounds. The relation of Tc to ψ is found to be equivalent to this correlation.
Transient transport in bulk Ga0.47ln0.53As and the two-dimensional electron gas in Ga0.47ln0.53As/Al0.48ln0.52As
Authors: Yoon K. S. , Stringfellow G. B. , Huber R. J.
Published: July 2002

Transient electronic transport properties of the bulk Ga0.47In0.53As and the two-dimensional electron gas (2DEG) at the Ga0.47In0.53 As/Al0.48In0.52As heterointerface for various electric fields are investigated by ensemble Monte Carlo simulations. The average electron velocity during transient transport in the 2DEG at the Ga0.47In0.53As/Al0.48In0.52As interface is about 8 times the steady-state velocity for E=20 kV/cm at room temperature and 30% higher than that in the intrinsic bulk Ga0.47In0.53As because of a higher peak velocity and a shorter transient time. This transient velocity enhancement in conjunction with higher 2DEG densities may significantly improve the performance of submicron-gate and even near-micron-gate Ga0.47In0.53As/Al0.48In0.52As high electron mobility transistors.
Polarity determination of CdTe crystals by electron diffraction
Authors: Hewat E. A. , DiCioccio L. , Million A. , Dupuy M. , Gailliard J. P.
Published: July 2002

The polarity of cadmium telluride crystals has been determined by a novel electron diffraction technique. This technique employs an electron microscope in normal operating conditions, and is applicable to any noncentrosymmetric crystal, provided that the deviation from centrosymmetry is not too small. We have thus identified the best {111} face for epitaxy on CdTe as the (1̄1̄1̄)Te face. This is in agreement with recent x-ray results and Rutherford backscattering results. Chemical etching characteristics of {111} faces were used to compare our results with previous polarity determinations. One commonly used etch is found to be unreliable for polarity identification.
Interface studies and electrical properties of plasma sulfide layers on n-type InP
Authors: Klopfenstein P. , Bastide G. , Rouzeyre M. , Gendry M. , Durand J.
Published: July 2002

The growing conditions and the basic interface properties of InP-sulfide-metal structures formed by direct and indirect plasma-enhanced sulfidation were investigated. The grown-in sulfide layer is an admixture of InPS4 and In2S3. The relative concentration of the low-gap indium sulfide is responsible for the high leakage currents (10−3 A cm−2) in the direct plasma layers. In the indirect plasma case the leakage currents are reduced to 10−6 A cm−2 with breakdown voltages about 7×106 V cm−1. The current transport and the frequency dependence of the capacitance of these diodes are consistently attributed to traps located in the sulfide near the semiconductor (SC) surface with a concentration in the 1011 cm−2 range. Detailed measurements of the capacitance-voltage characteristics reveal that the Fermi level in InP is swept through the upper half portion of the band gap and that accumulation and strong depletion regimes are reached. These measurements also reveal that the room-temperature hysteresis-free C-V plots result from the compensative effects of mobile charges in the sulfide and of charges trapped on the interface states. Separation is made by freezing the mobile charges either in the nearby InP or Au electrode region and the interface states in the empty or in the filled state. Induced shifts in the C-V characteristics allow a direct access to both concentrations which are in the high 1011 cm−2 range. The energy density of interface states is calculated in two ways from the C-V plots and directly measured by deep-level transient spectroscopy. The energy density is equal to ∼1.1012 cm−2 eV−1 from Ec to Ec−0.6 eV and then increases to 7–8×1012 cm−2 eV−1 near Ec−0.7 eV.
A novel microwave technique for detection of superconductivity
Authors: Kim B. F. , Bohandy J. , Moorjani K. , Adrian F. J.
Published: July 2002

A new technique for detection of superconductivity which is based upon microwave resistivity is described. This method is similar in both its implementation and execution to the technique of electron-spin resonance, and, as a consequence, exhibits high sensitivity. An additional benefit of this method is the fact that metal-insulator phase transitions of materials are recorded only if they have a magnetic field dependence.
The interaction of gold with gallium arsenide
Authors: Weizer Victor G. , Fatemi Navid S.
Published: July 2002

Gold and gold-based alloys, commonly used as solar-cell contact materials, are known to react readily with gallium arsenide. Experiments designed to identify the mechanisms involved in these GaAs-metal interactions have yielded several interesting results. It is shown that the reaction of GaAs with gold takes place via a dissociative diffusion process. It is shown further that the GaAs-metal reaction rate is controlled to a very great extent by the condition of the free surface of the contact metal, an interesting example of which is the previously unexplained increase in the reaction rate that has been observed for samples annealed in a vacuum environment as compared to those annealed in a gaseous ambient. A number of other hard-to-explain observations, such as the low-temperature formation of voids in the gold lattice and crystallite growth on the gold surface, are also explained by invoking this mechanism.
Electron-transport, ionization, attachment, and dissociation coefficients in SF6 and its mixtures
Authors: Phelps A. V. , Van Brunt R. J.
Published: July 2002

An improved set of electron-collision cross sections is derived for SF6 and used to calculate transport, ionization, attachment, and dissociation coefficients for pure SF6 and mixtures of SF6 with N2, O2, and Ne. The SF6 cross sections differ from previously published sets primarily at very low and high electron energies. At energies below 0.03 eV the attachment cross section is adjusted to fit recent electron swarm experiments, while the elastic momentum transfer cross section is increased to the theoretical limit. At high energies an allowance is made for the excitation of highly excited levels as observed in electron beam experiments. The cross-section sets used for the admixed gases have previously been published. Electron kinetic energy distributions computed from numerical solutions of the electron-transport (Boltzmann) equation using the two-term, spherical harmonic expansion approximation were used to obtain electron-transport and reaction coefficients as functions of E/N and the fractional concentration of SF6. Here E is the electric field strength and N is the gas number density. Attachment rate data for low concentrations of SF6 in N2 are used to test the attachment cross sections. Particular attention is given to the calculation of transport and reaction coefficients at the critical E/N=(E/N)c at which the ionization and attachment rates are equal.
Effect of process parameters on stress development in two-dimensional oxidation
Authors: Hu S. M.
Published: July 2002

A simplified viscoelastic analysis has been made of the stress evolution during two-dimensional (2D) oxidation of silicon substrates, with the objective of learning the effect of process parameters such as temperature and steam pressure. A cylindrical silicon surface was chosen for simplicity of analysis, and yet it still has most of the essential elements pertinent to practical problems such as, e.g., the oxidation of trench corners in silicon integrated circuits. With correlations between the viscosity and the hydroxyl content of SiO2, and between the hydroxyl content and the steam pressure, the analysis shows that stress reduction can be achieved by carrying out oxidation at high steam pressures. However, stresses remain rather high if the oxidation temperature is as low as 800 °C. For a linear-parabolic oxidation kinetics, both the oxide and the substrate stresses do not increase indefinitely with the increase of oxide thickness, but reach their respective peaks at oxide thicknesses that are dependent on process parameters. The present results should be useful in serving as guidelines in the selection of 2D oxidation conditions. The accuracy of a previous 2D oxidation model based on the viscous flow of an incompressible fluid has also been assessed with reference to the viscoelastic model. The incompressible-fluid model is found to be quite accurate at high temperatures ≳900 °C.
Tungsten/titanium nitride low-resistance interconnections durable for high-temperature processing
Authors: Nakasaki Y. , Suguro K. , Shima S. , Kashiwagi M.
Published: July 2002

Newly developed W/TiN bilayer interconnections, able to withstand high-temperature processing after metalization, are described. Minimum TiN thickness was found to be 100 Å in order to retain adhesion between W and SiO2, while being a reaction barrier between W and Si. Silicidation rate in the W/TiN/Si system was reduced by about 4.5 orders of magnitude, in comparison with a W/Si system at 950 °C. TiN/p+-Si contact resistivity on the order of 10−6 Ω cm2 was realized by boron ion implantation through a TiN layer and rapid thermal annealing for boron activation at temperatures above 1000 °C.