bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution patterning of high T
c superconductors
Authors: Harriott L. R. , Polakos P. A. , Rice C. E.
Published: July 2002

We have used a 20 keV Ga focused ion beam to pattern superconducting submicrometer bridge structures in thin films of Ba2YCu3O7 material by physical sputtering. The technique can produce structures down to 0.5 μm or less in epitaxial films with no degradation in superconducting transition temperature (Tc) or critical current density (Jc). Photolithography was used to define a coarse pattern of 20-μm-wide and 50-μm-long strips, each wired for four-terminal resistance measurements. Submicrometer constrictions were then milled by the focused ion beam to form weak-link junctions with roughly 0.3 μm separating the superconducting banks. We have demonstrated that focused ion beam micromachining is capable of producing submicrometer-sized superconducting structures.
Scanning tunneling microscopy on rough surfaces: Deconvolution of constant current images
Authors: Reiss G. , Schneider F. , Vancea J. , Hoffmann H.
Published: July 2002

This letter critically discusses the topographical information obtained by scanning tunneling microscopy (STM) on surfaces with a mesoscopic roughness, i.e., in the range of some nm’s. In a foregoing publication [J. Appl. Phys. 67, 1156 (1990)], we already treated the evaluation of constant current images based on the knowledge of the real surface and the shape of the tunneling tip (‘‘tip shape limited resolution’’). Now we deal with the invers problem: the reconstruction of the real surface topography based on the corresponding STM image and the tip shape, using a simple, straightforward formalism.
Suppression of Al-Ga interdiffusion by a WNx film on an AlxGa1−xAs/AlAs superlattice structure
Authors: Allen E. L. , Pass C. J. , Deal M. D. , Plummer J. D. , Chia V. F. K.
Published: July 2002

Undoped AlAs/AlxGa1−xAs superlattice structures were grown by molecular beam epitaxy and annealed under Si3N4, SiO2, or WNx encapsulant films, both with and without the presence of a 120Sn implant. Enhancement of the Al-Ga interdiffusion coefficient occurred under the Si3N4 and SiO2 films due to indiffusion of Si from the films. Enhancement was even greater during diffusion of the Sn implant. In both cases intermixing enhancement was due to the Fermi level effect. Underneath the WNx film, however, interdiffusion was suppressed even in the presence of the Sn dopant.
Etching characteristics of AlxGa1−xAs in (NH4)2Sx solutions
Authors: Seo Jong-Wook , Koker Terry , Agarwala Sambhu , Adesida Ilesanmi
Published: July 2002

The etching of GaAs and AlxGa1−xAs have been characterized in (NH4)2Sx solution with various excess sulfur concentrations and for temperatures ranging from 20 to 60 °C. The etch rate varies with the concentration of excess sulfur and is highest at 60 °C using a 4% excess sulfur solution. The etch rate of AlxGa1−xAs increases exponentially with increasing Al mole fraction. Activation energies of 19.8 and 15.9 kcal/mole are obtained for GaAs and Al0.3Ga0.7As in 4% (NH4)2Sx, respectively. These high values and the linear time dependence of etch rates signify that the etching process of AlxGa1−xAs in (NH4)2Sx solutions is predominantly reaction-rate limited. Possible chemical processes involved in the etching and the formation of a passivating sulfur layer are discussed.
In situ transmission electron microscopy studies of silicide-mediated crystallization of amorphous silicon
Authors: Hayzelden C. , Batstone J. L. , Cammarata R. C.
Published: July 2002

The silicide-mediated phase transformation of amorphous to crystalline silicon was observed in situ in the transmission electron microscope. Crystallization of nickel-implanted amorphous silicon occurred at ∼500 °C. Nickel disilicide precipitates were observed to migrate through an amorphous Si film leaving a trail of crystalline Si. Growth occurred parallel to 〈111〉 directions. High resolution electron microscopy revealed an epitaxial NiSi2/Si(111) interface which was Type A. A diffusion-controlled mechanism for the enhanced crystallization rate was determined.
New principle for optical filters
Authors: Magnusson R. , Wang S. S.
Published: July 2002

A theoretical characterization of the guided-mode resonance properties of planar dielectric waveguide gratings is presented. Efficient exchange of energy between forward and backward propagating diffracted waves is shown to be possible with smooth line shapes. The linewidths of the resonances can be controlled by the grating modulation amplitude. Due to the inherent separation between the TE and TM modes associated with the waveguide grating, these elements can provide polarization separation. Applications to polarization sensitive filtering and to electro-optic switching are suggested. The guided-mode resonance filter represents a basic new optical element.
Segregation and defect termination of fluorine at SiO2/Si interfaces
Authors: Ono Y. , Tabe M. , Sakakibara Y.
Published: July 2002

Segregation of fluorine in a SiO2/Si structure has been examined with respect to depth profile and paramagnetic defects at the SiO2/Si interface. The fluorine distribution was found to be restricted to a narrow (less than 35 Å) interface region on the oxide side. Furthermore, a greater amount of fluorine segregation was observed for (111) than for (100) interfaces. These observations suggest that fluorine segregation is caused by intrinsic defects in the interface region and that fluorine atoms terminate the defects. In fact, electron spin resonance measurements provide direct confirmation that fluorine atoms terminate the trivalent Si dangling bonds at the interface.
Novel epitaxial growth mechanism of magnesium oxide/titanium oxide ceramics superlattice thin films observed by reflection high-energy electron diffraction
Authors: Imai Fumikazu , Kunimori Kimio , Nozoye Hisakazu
Published: July 2002

Magnesium oxide/titanium oxide ceramics superlattice thin films were synthesized by means of a novel reactive evaporation method; a pulsed molecular beam evaporation method. Reflection high-energy electron diffraction (RHEED) intensity oscillation was observed from the beginning to the end of the synthesis of the superlattice. From the analysis of the RHEED intensity oscillation, the detailed deposition process was analyzed and a novel epitaxial growth mechanism, rough–smooth mechanism, was inferred. This mechanism was proved by high-resolution transmission electron microscopy (HRTEM).
Growth of SnO2 films on micromachined hotplates
Authors: Cavicchi R. E. , Suehle J. S. , Kreider K. G. , Shomaker B. L. , Small J. A. , Gaitan M. , Chaparala P.
Published: July 2002

Arrays of micromachined hotplates have been used for materials processing on a microscopic scale. The temperature of individual elements ‘‘micro-hotplates’’ of an array is controlled by addressing a given element with a specified current and measuring the temperature from a resistance change. This unique temperature control capability has been exploited to deposit SnO2 overlayers onto micro-hotplates with individually controlled temperatures using reactive sputter deposition and organometallic chemical vapor deposition. Post-deposition heating in vacuum was used to alter the stoichiometry of films. The result is an array of separately, but simultaneously, processed films. The micro-hotplates have excellent thermal isolation from other devices (transistors, logic elements) on the chip. Electrical contact pads allow for in situ electrical characterization of the films. The use of micro-hotplates allows high-temperature growth to occur on portions of a silicon substrate, while other portions remain at room temperature. This is especially important for materials such as SnO2, used as the sensing layer in thin film gas sensors. With large arrays of micro-hotplates, efficient growth and processing surveys are possible.
Low-pass filters for signal averaging
Authors: Voigtman Edward , Winefordner James D.
Published: July 2002

Detailed comparison of the settling time-noise bandwidth products of 31 types of low-pass filters demonstrates that the settling time-noise bandwidth product is the figure of merit for such filters when the goal is averaging. Common filters such as Butterworth, elliptic, and Chebyshev are found to be unusable for such purposes while others, such as Bessel filters, offer only moderate figures of merit. The best reported analog low-pass filter differs from ideality by only about 11%. The optimum analog low-pass filter, having continuous, rational transfer function, is unknown.
Neutral-beam performance analysis using a CCD camera
Authors: Hill D. N. , Allen S. L. , Pincosy P. A.
Published: July 2002

We have developed an optical diagnostic system suitable for characterizing the performance of energetic neutral beams. An absolutely calibrated CCD video camera is used to view the neutral beam as it passes through a relatively high-pressure (10−5 Torr) region outside the neutralizer: collisional excitation of the fast deuterium atoms produces Hα emission (λ=6561 Å) that is proportional to the local atomic current density, nearly independent of the species mix of accelerated ions over the energy range 5–20 keV. Digital processing of the video signal provides profile and aiming information for beam optimization.
Highly doped ruby filters for stray light reduction in SCATPAK II
Authors: Thomas C. E. , Lazarus E. A. , Kindsfather R. R. , Painter S. , Hays P. , Murakami M.
Published: July 2002

One of the major problems in Thomson scattering measurement of electron temperature is the suppression of stray light. Filters of AR-coated ruby with a high-percentage doping of Cr have been produced for use on SCATPAK II. The filters have negligible attenuation of the signal above 7000 Å and attenuate light at the laser line by a factor of ≂9. Experimental details of the filter performance, design, and fabrication are presented. The filter performance is compared with experimental measurements of the Cr-in-sapphire absorption cross section at the R1 line. Ruby filters present a cost-effective method of stray light suppression for experiments using ruby lasers and the red-shifted portion of the scattered spectrum.
Neutron fluctuation measurements on TFTR
Authors: Heidbrink W. W.
Published: July 2002

Measurements of fluctuations in the neutron yield are made on the tokamak fusion test reactor (TFTR) with plastic scintillators. Light from the scintillators is coupled through acrylic rods or fiber-optic cables to photomultipliers operated in current mode. Discrimination against hard x rays is accomplished through comparison with the signal from a ZnS(6Li) scintillator. These measurements are useful in studies of deuterium pellet deposition, of the acceleration of beam ions during major radial compression, and of MHD instabilities. Techniques for measuring the neutral beam density profile and Qequivdt using neutron fluctuation measurements during pellet injection also have been proposed.
A high impedance mega-ampere generator for fiber z-pinch experiments
Authors: Mitchell I. H. , Bayley J. M. , Chittenden J. P. , Worley J. F. , Dangor A. E. , Haines M. G. , Choi P.
Published: July 2002

At Imperial College a mega-ampere generator for plasma implosion experiments has been designed, built, and commissioned. With a final line impedance of 1.25 Ω this terawatt class generator has been designed primarily to drive a maximum current of 1.8 MA with a rise time of 150 ns into high inductance z-pinch loads of interest to radiative collapse studies. This article describes the design and tests of the generator which has a novel configuration of lines and a new design of a magnetically insulated transmission line (MITL). In summary, the generator consists of four Marx generators each of the Hermes III type (2.4 MV, 84 kJ), each connected to 5 Ω pulse forming lines and trigatron gas switches. The power is fed into the matched 1.25 Ω vertical transfer line which feeds a diode stack and a short conical MITL in vacuum which concentrates the power into the z-pinch load. At 80% charge a current rising to 1.4 MA in 150 ns has been measured in a 15 nH inductive short. Similar results are obtained when using a plasma load.
Characterization of micromachined piezoelectric PZT force sensors for dynamic scanning force microscopy
Authors: Lee Chengkuo , Itoh Toshihiro , Maeda Ryutaro , Suga Tadatomo
Published: July 2002

The key component for the piezoelectric scanning force microscopy (SFM) is the piezoelectric force-sensing microcantilever. In this article, the resonance frequency and quality factor of Pb(Ti,Zr)O3 (PZT) microcantilevers; effective Young’s modulus and piezoelectric Pb(Ti,Zr)O3 property of PZT layer of piezoelectric microcantilevers are discussed. The PZT microcantilevers after poling have quality factors such as 204 to 248 in air, and 608 to 1094 in vacuum. The calculated spring constants are in the range of 1.0–30.0 N/m for cantilevers with 0.64–1.7-μm-thick PZT layer and 125–300 μm length. The force sensitivity and resolution of the PZT microcantilever are studied when it is oscillated by an external oscillator and applied to the cyclic contact SFM. Then the real transverse piezoelectric coefficient d31 of piezoelectric PZT thin films is derived from the measured sensitivity and effective Young’s modulus. The derived effective Young’s moduli of PZT layers slightly increases from 53.5 to 58 GPa as the thickness of the PZT layer increases. The d31 is around −35 pC/N which is about 8 times larger than the value of ZnO, and half the value of bulk PZT ceramics. The sensitivity of 0.98 fC/nm can be obtained by PZT force sensors. The longitudinal resolution obtained is 1.5 Å at a bandwidth of 125 Hz. Cantilevers with proper characteristics for different applications can be predicted, designed, and fabricated via the adjustment of the thickness ratio of each layer and the total thickness of the cantilever. Very clear images of an evaporated Au film on a smooth glass plate are obtained by an SFM with a PZT force sensor.
Solubility of solids in supercritical fluids from the measurements of the dielectric constant: Application to CO2–naphthalene
Authors: Hourri A. , St-Arnaud J. M. , Bose T. K.
Published: July 2002

A new method for the measurement of solid solubilities in supercritical fluids is described in detail. The method is based on the measurement of the dielectric constant of the saturated supercritical solution solvent–solute and that of the supercritical solvent as a function of pressure along different isotherms. Our method is simple, absolute, easy to use, and permits precise measurement of the solubility of solids in supercritical fluids. Solubilities of naphthalene in supercritical CO2 in the pressure range 12–30 MPa are reported along the isotherms 313.15, 318.15, and 323.15 K.
Electron-electron interaction in linear arrays of small tunnel junctions
Authors: Likharev K. K. , Matsuoka K. A.
Published: July 2002

We have calculated the spatial distribution of the electrostatic potential created by an unbalanced charge q in one of the conducting electrodes of a long, uniform, linear array of small tunnel junctions. The distribution describes, in particular, the shape of a topological single-electron soliton in such an array. An analytical solution obtained for a circular cross-section model is compared with results of geometrical modeling of a more realistic structure with square cross section. These solutions are very close to one another, and can be reasonably approximated by a simple phenomenological expression. In contrast to the previously accepted exponential approximation, the new result describes the crossover between the linear change of the potential near the center of the soliton to the unscreened Coulomb potential far from the center, with an unexpected ‘‘hump’’ near the crossover point.
Nonlinear optical properties of photoresists for projection lithography
Authors: Kewitsch Anthony S. , Yariv Amnon
Published: July 2002

Optical beams are self-focused and self-trapped upon initiating crosslinking in photoresists. This nonlinear optical phenomenon is apparent only for low average optical intensities and produces index of refraction changes as large as 0.04. We propose using the self-focusing and self-trapping phenomenon in projection photolithography to enhance the resolution and depth of focus.
High critical current density superconducting tapes by epitaxial deposition of YBa2Cu3Ox thick films on biaxially textured metals
Authors: Goyal A. , Norton D. P. , Budai J. D. , Paranthaman M. , Specht E. D. , Kroeger D. M. , Christen D. K. , He Q. , Saffian B. , List F. A. , Lee D. F. , Martin P. M. , Klabunde C. E. , Hartfield E. , Sikka V. K.
Published: July 2002

A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high-temperature superconductors is reported. The technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.
Correlation of the dispersed particles surface properties with the electrorheological response
Authors: Hao Tian , Xu Yuanze
Published: July 2002

A set of oxidized polyacrylonitrile materials of different surface situations were employed to examine how the dispersed particles surface properties influence the electrorheological (ER) response. The shear stresses of these ER fluids were found to peak for the surface tension and the surface polarity of the dispersed particles, which could be qualitatively understood with our recent finding that the large dielectric loss, along with the large dielectric constant, are argued for good ER performance. Our findings have the direct implication that the ER effect would be most likely improved by means of certain surface modification methods, especially some chemical methods like the molecular design method, which perhaps can provide a good approach to develop the expected ER solid materials. Our results would also be significant for selecting ER solid materials and understanding the ER mechanism. A further theoretical work should be performed in the future.
Zone plates with high efficiency in high orders of diffraction described by dynamical theory
Authors: Schneider G.
Published: July 2002

The coupled wave theory is applied to calculate the diffraction efficiency of zone plates used in high orders of diffraction. In contradiction to the existing understanding about 30%–50% of the incident intensity can be diffracted by a single high order if the line-to-space ratio of the zone structures is in the range of 0.1–0.5 and the aspect ratios are larger than 30:1. Highly efficient diffractive transmission x-ray optics with high numerical apertures and resolving power can be developed without manufacturing extremely small zones which are necessary for zone plates used in the first order.
Properties of interface-engineered high Tc Josephson junctions
Authors: Moeckly B. H. , Char K.
Published: July 2002

We have created YBCO thin film ramp edge Josephson junctions by modification of the edge surface prior to counterelectrode deposition. No deposited interlayer or barrier layer is employed. These devices are uniform and reproducible, and they display resistively shunted junction current-voltage characteristics with excellent magnetic field modulation. IcRn values over the range 0.5–3 mV and corresponding RnA values of 6×10−8–1.2×10−9 Ω cm2 at 20 K are easily attained by varying the process. We believe these junctions offer significant promise as the building blocks of a high Tc electronics technology.
Length control of individual carbon nanotubes by nanostructuring with a scanning tunneling microscope
Authors: Venema Liesbeth C. , Wildöer J. W. G. , Tuinstra H. L. J. Temminck , Dekker C. , Rinzler A. G. , Smalley R. E.
Published: July 2002

We present a technique to control the length of carbon nanotubes. Individual carbon nanotubes can be locally cut by applying a voltage pulse to the tip of a scanning tunneling microscope (STM). Topographic imaging and STM spectroscopy are subsequently used to investigate the result. The electronic properties of a nanotube can be strongly changed by reducing the size. Current-voltage curves obtained by STM spectroscopy on a 30 nm short tube created from a longer nanotube show a stepwise increase of the current, which is attributed to quantum size effects.
Direct observation of large-scale nonuniformities in hydride vapor-phase epitaxy-grown gallium nitride by cathodoluminescence
Authors: Goldys E. M. , Paskova T. , Ivanov I. G. , Arnaudov B. , Monemar B.
Published: July 2002

Hydride vapor phase epitaxial GaN films grown on sapphire without a buffer are found to contain large-scale regions with high electron concentration located close to the interface. These regions are composed of individual columns forming an irregular but quasicontinuous layer, while the rest of the film has a much lower carrier concentration. The highly doped regions are easily visualized using cathodoluminescence. The coexistence of regions with low and high electron concentration allows us to explain the concurrent evidence of high film quality in photoluminescence, Raman spectroscopy and x-ray diffraction, and a high electron concentration measured in transport studies.
Direct evidence for oxygen stabilization of icosahedral phase during crystallization of Zr65Cu27.5Al7.5 metallic glass
Authors: Murty B. S. , Ping D. H. , Hono K. , Inoue A.
Published: July 2002

The present letter reports the formation of the icosahedral phase during crystallization of amorphous Zr65−xCu27.5Al7.5Ox (x=0.43% and 0.82%) alloys. Three-dimensional atom probe studies have revealed that the quasicrystalline phase is enriched in oxygen. This, together with the fact that no icosahedral phase forms at low oxygen level (0.14%), clearly establishes that the quasicrystalline phase in Zr-based metallic glasses is stabilized by oxygen.
Catalytically active nickel {110} surfaces in growth of carbon tubular structures
Authors: Kuang M. H. , Wang Z. L. , Bai X. D. , Guo J. D. , Wang E. G.
Published: July 2002

Recent interest in the growth of aligned carbon nanotube films using transition metal catalysts has led to questions concerning the growth mechanism involved. In our experiment carbon tubules grown using Ni catalysts exhibit a preferred orientation relative to the catalytically active surfaces of Ni. The axial directions of the tubular structure are mainly parallel to the 〈110〉 and 〈042〉 directions of Ni. The faceted shape of the Ni particles determines the intrinsic structure of the tubules. A mechanism involving spiral growth is proposed to explain the nucleation and growth of such tubules.
Energy distribution of ions and fast neutrals in microdischarges for display technology
Authors: Hagelaar G. J. M. , Kroesen G. M. W. , Klein M. H.
Published: July 2002

In this work we present simple theoretical predictions as well as full Monte Carlo calculations of the energy distribution of the ion and fast neutral fluxes impinging on the materials that surround the microdischarges in plasma display panels and plasma addressed liquid crystal displays. We consider various rare gas ion species in different microdischarge designs. Often simple theoretical distribution functions are in good agreement with the results of Monte Carlo calculations. Under the influence of symmetric charge transfer collisions the ion energy distribution is essentially different from a Maxwellian distribution, and the ion motion is strongly orientated along the electric field. The flux of the fast neutrals formed by symmetric charge transfer is usually even larger than the ion flux itself.
An experimental study of the correlation between wear and entropy flow in machinery components
Authors: Doelling K. L. , Ling F. F. , Bryant M. D. , Heilman B. P.
Published: July 2002

Based on first principles, a hypothesis was made on the potential correlation between entropy and degradation of machinery components in an earlier investigation of stochastic characterization of degradation dynamics. This paper reports on an experimental study in which degradation in the form of wear of model machinery component pairs was made on an accelerated testing basis. Concomitant measurement of entropy flow was made by means of a simple calorimeter. Results show a strong correlation between the referenced wear and the production of entropy flow.
Molecular weight dependence of the conformational phase transition and electroluminescence of diarylpolysilane diodes
Authors: Hoshino Satoshi , Furukawa Kazuaki , Ebata Keisuke , Breyl Ingo , Suzuki Hiroyuki
Published: July 2002

We investigated the molecular weight dependence of the conformational phase transition behavior of thin films of poly[bis(p-n-butylphenyl)silane] (PBPS), which is a near-ultraviolet (NUV) electroluminescence (EL) material for polysilane-based polymer light-emitting diodes (LEDs). A low molecular weight PBPS with efficient NUV EL exhibited a phase transition from a disordered to an ordered backbone conformation when we increased the temperature. By contrast, the opposite transition behavior was observed for a high molecular weight PBPS. The photoluminescence (PL) quantum yield of the PBPS films changed in association with the thermal phase transition behavior; the PL quantum yield of the low molecular weight PBPS was improved by increasing the temperature, whereas that of the high molecular weight polymer deteriorated. The time dependence of the EL investigation suggests that, although the EL intensity continued to increase for the first few hours, insufficient Joule heat was generated to induce a phase transition in the entire PBPS layer of an LED made of the low molecular weight PBPS during continuous operation . We conclude that the conformational phase transition was not a key factor as regards efficient NUV EL generation, but may have occurred at the molecules in contact with the external electrodes of the LED, and may have been partially responsible for the improvement in the carrier injection efficiency of the external electrodes.
Analysis of photon recycling in light emitting diodes with nonuniform injection
Authors: Tsutsui N. , Khmyrova I. , Ryzhii V. , Ikegami T.
Published: July 2002

We studied the effect of photon recycling in double heterostructure light emitting diodes (LEDs) with relatively small area contact providing nonuniform injection of electrons. A simple phenomenological model of the electron and photon transport in the LED is used to calculate the spatial distributions of electrons and output radiation as well as the external quantum efficiency as functions of device parameters. It is shown that photon recycling is the governing factor of the operation of LEDs with nonuniform injection.
Viscous drag of a solid sphere straddling a spherical or flat surface
Authors: Danov Krassimir D. , Dimova Rumiana , Pouligny Bernard
Published: July 2002

The aim of this paper is to compute the friction felt by a solid particle, of radius a, located across a flat or spherical interface of radius R, and moving parallel to the interface. This spherical interface can be a molecular film around an emulsion or aerosol droplet, the membrane of a vesicle or the soap film of a foam bubble. For simplicity, the acronym VDB is used to refer to either vesicle, drop, or bubble. The theory is designed as a tool to interpret surface viscosimetry experiments involving spherical probes attached to films or model membranes, taking care of the finite-size effects when the film encompasses a finite fluid volume. The surface of the VDB is a two-dimensional fluid, characterized by dilational (ηsdil) and shear (ηssh) surface viscosities. The particle intercepts a circular disc in the interface, whose size depends on the particle penetration inside the VDB. The three-dimensional fluids inside and outside the interface may be different. The analysis holds in the low Reynolds number and low capillary number regime. A toroidal (x1,x2,φ) coordinate system is introduced, which considerably simplifies the geometry of the problem. Then the hydrodynamic equations and boundary conditions are written in x1,x2,φ. The solution is searched for the first-order Fourier component of the velocity field in the radial angle φ. Reformulating the equations in “two-vorticity-one-velocity” representation, one basically ends up with a set of equations in x1,x2 only. This set is numerically solved by means of the Alternating-Direction-Implicit method. Numerical results show that the particle friction is influenced both by the viscosity and by the finiteness of the VDB volume. Finite-size effects have two origins: a recirculation effect when a/R is not very small, and an overall rotation of the VDB-particle complex when ηs is very large. In principle, the theory allows for a quantitative determination of ηs whatever a/R, including the limit a/R=0 (flat interface).
Hole blocking in carbon nanotube–polymer composite organic light-emitting diodes based on poly (m-phenylene vinylene-co-2, 5-dioctoxy-p-phenylene vinylene)
Authors: Woo H. S. , Czerw R. , Webster S. , Carroll D. L. , Ballato J. , Strevens A. E. , O’Brien D. , Blau W. J.
Published: July 2002

In order to investigate the role of carbon nanotubes in a polymer matrix, organic light-emitting diodes were fabricated from a polymer composite composed of poly (m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene) (PmPV) and dispersed single-wall carbon nanotubes (SWNTs). Tris-(8-hydroxyquinolinolato) aluminum (Alq3) doped by Nile Red was used as an emissive material between the polymer composite and cathode. The device fabricated without SWNTs dispersed in the PmPV shows a dominant emission near red at 600 nm, which is in the range of the characteristic emission of Nile Red-doped Alq3, with a small amount of green emission from the PmPV. However, the devices fabricated with the polymer composite show an increase in the oscillator strength of the green emission with a dominant emission peak near 500 nm, the characteristic emission of PmPV. This was observed for SWNT concentrations up to 0.1 wt %. The shift in the emission indicates that the SWNTs in the PmPV matrix act as a hole-blocking material that results in a shifting of the recombination region from the Nile Red-doped Alq3 layer to the PmPV composite layer.
Growth of hexagonal GaN on Si(111) coated with a thin flat SiC buffer layer
Authors: Wang D. , Hiroyama Y. , Tamura M. , Ichikawa M. , Yoshida S.
Published: July 2002

Hexagonal GaN films were grown on Si(111) covered with a thin flat SiC buffer layer under both N- and Ga-rich growth conditions. A flat 2.5-nm-thick SiC layer was an effective buffer layer for GaN growth. The growth mode and microstructure of GaN depended strongly on the Ga/N flux ratios. Under N-rich growth conditions, the growth mode was three dimensional; GaN showed statistical roughening of the surface and a characteristic columnar structure. Under Ga-rich conditions, the GaN growth mode was two dimensional; GaN films with a flat surface and an almost stacking-fault-free microstructure were obtained. The two-dimensional growth mode was facilitated by strong wetting between Ga and SiC(111) at the first Ga-layer deposition on SiC.
Helium scattering structure analyses of the chemisorbed hydrogen phases on Co(101̄0)
Authors: Patting M. , Farı́as D. , Rieder K.-H.
Published: July 2002

The adsorption of hydrogen on the Co(101̄0) surface has been studied by means of helium atom scattering. In agreement with previous results, three ordered adsorbate structures have been identified at 85 K: a c(2×4)4H, a (2×1)p2mg, and a (1×2) saturation phase. The surface structures were determined from a quantitative analysis of the corresponding He-diffraction spectra. The corrugation functions derived from the calculations reveal that in the c(2×4)4H phase the hydrogen atoms from zigzag and zagzig chains along every second close-packed row of the substrate, although the kind of occupied threefold sites cannot be determined from our data. In the (2×1)p2mg phase, hydrogen atoms are found to occupy the hcp-like sites, whereas the substrate’s reconstruction in the (1×2) phase is of the pairing-row type. This means that hydrogen forms analogous structures on the Co(101̄0) rather than on the structurally similar Ni(110) surface.
Admittance spectroscopy of efficient CuInS2 thin film solar cells
Authors: Kneisel Joachim , Siemer Kai , Luck Ilka , Bräunig Dieter
Published: July 2002

Thermal admittance spectroscopy is applied to characterize the trap properties of high-efficiency CuInS2/CdS/ZnO thin film solar cells. To that aim, the capacitance spectrum is examined in detail. Nontrap capacitance contributions like freeze-out of free carriers and series resistance are discussed. The dependence of the resistance on the sample area is used to identify its physical origin. Simulation results of a transmission line model are in good agreement with the spreading resistance of the ZnO window layer. Defect spectra, i.e., the distribution of the deep trapping states in the band gap, are extracted from the admittance spectra, using a method established by T. Walter, R. Herberholz, C. Müller, and H. W. Schock [J. Appl. Phys, 80, 4411 (1996)]. Arrhenius data of the traps are drawn best from the spectrum of conductance versus temperature. The defect spectra do not depend on the buffer layer. Generally, they show a high constant background trap density with some broad peaks. Two new trap levels at 0.3 and 0.5 eV are found, confirmed by deep level transient spectroscopy and identified as majority carrier traps in the bulk of the p-type CuInS2 absorber. An increasing density of the trap at 0.5 eV correlates with a decreasing open-circuit voltage.
The valence band structures of BeO, MgO, and CaO
Authors: Sashin Vladimir A. , Dorsett Helen E. , Bolorizadeh Mohammad A. , Ford Michael J.
Published: July 2002

We have performed direct measurements of the valence band structures of the light alkaline earth oxides BeO, MgO, and CaO using electron momentum spectroscopy (EMS). From these measurements, we have determined the band dispersions, valence bandwidths, and O(2s)–O(2p) intervalence bandgaps at the Γ point. For comparison we have also performed Hartree–Fock (HF) and density-functional (DFT) calculations in the linear combination of atomic orbitals (LCAO) approximation. Intervalence bandgaps compare reasonably well with the DFT calculations and previous experimental and theoretical studies. Our measured bandwidths, however, are significantly smaller. In particular, we find that contrary to conventional wisdom, the local density approximation of DFT overestimates the valence bandwidths of these ionic solids.
Low-frequency noise in cadmium-selenide thin-film transistors
Authors: Deen M. Jamal , Rumyantsev S. L. , Landheer D. , Xu D.-X.
Published: July 2002

Low-frequency noise in cadmium-selenide (CdSe) thin-film transistors (TFTs) has been studied over a wide range of gate and drain biases, temperatures, and gate areas. The dependencies of the noise on the gate voltage and the gate length indicate that the 1/f noise originates from the bulk sources homogeneously distributed in the channel. The value of Hooge parameter α lies within the usual range 10−3<α<2×10−2 for Si TFTs and amorphous Si.
Ion fluxes and energies in inductively coupled radio-frequency discharges containing CHF3
Authors: Wang Yicheng , Misakian M. , Goyette A. N. , Olthoff J. K.
Published: July 2002

Measurements of ion energy distributions, relative ion intensities, and absolute total ion current densities were made at the grounded electrode of an inductively coupled Gaseous Electronics Conference (GEC) radio-frequency reference cell for discharges generated in CHF3 and its mixtures with argon. In general, the dominant ion species detected were not due to direct ionization of the CHF3 feed gas. Results are presented for plasmas generated with and without a confining quartz annulus that has recently been used to extend the operating parameter range of inductively coupled GEC cells for certain etching gases. Compared to similar plasmas generated without the annulus, the presence of the ring increases the ion flux density by approximately a factor of 2, and increases the mean ion energies. The presence of the ring does not significantly affect the measured relative ion intensities.
Accurate partition function and thermodynamic data for water
Authors: Vidler Marcus , Tennyson Jonathan
Published: July 2002

The partition function, Q, of H216O is calculated by explicit summation of about 10 500 experimental vibration-rotation energy levels and very high accuracy estimates are obtained for the specific heat capacity (Cp), the Gibbs enthalpy function (gef), the Helmholtz function (hcf) and the entropy (S) of gas phase water as a function of temperature. For temperatures above 600 K it is necessary to augment the sum with theoretical estimates of the energy levels. These are obtained from high accuracy variational calculations which are extended to dissociation using a model for rotational levels based on a Padé approximant. Estimates for the partition function and other thermodynamic quantities are obtained for temperatures up to 6000 K and temperature dependent error bars presented. All estimates are highly accurate with the exception of Cp for T>5000 K, for which further work is required.
Small-angle x-ray and neutron scattering studies of the volume phase transition in thermosensitive core–shell colloids
Authors: Seelenmeyer S. , Deike I. , Rosenfeldt S. , Norhausen Ch. , Dingenouts N. , Ballauff M. , Narayanan T. , Lindner P.
Published: July 2002

The volume transition in thermosensitive colloidal core–shell particles is investigated by small-angle x-ray scattering (SAXS), small-angle Neutron scattering (SANS), and dynamic light scattering (DLS). The latex particles are dispersed in water and consist of a solid poly(styrene) core with a diameter of 100 nm. The thermosensitive shell is made up of poly(N-isopropylacrylamide) (PNIPA) chains crosslinked by 2.5 mol % N,N’-methylenbisacrylamide (BIS). Water is a good solvent for PNIPA at room temperature but becomes a poor solvent above 32 °C. The PNIPA network of the shell undergoes a volume transition at this temperature. As a result the diameter of the particle shrinks. The scattering intensities of the particles measured by SAXS and SANS as a function of temperature may be decomposed into a part deriving from the overall structure and a part originating from the fluctuations within the network. The analysis of the overall structure leads to the volume fraction of the swollen network at different temperatures. SANS in conjunction with contrast variation demonstrates that the network is confined in a well-defined shell. SAXS and SANS data therefore allow the phase diagram of the network in the shell of the particles to be derived, i.e., the average volume fraction of the network in the shell can be determined as a function of temperature. DLS corroborates this result but demonstrates that there is a small fraction of chains exceeding the outer radius derived from SAXS and SANS. The static intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. SAXS data show that this part can be described by a Lorentzian both below and above the volume transition. The analysis demonstrates that critical fluctuations of the network around the transition temperature are fully suppressed. This finding is explained by the strong steric constraint of the network by its confinement within a shell of colloidal dimension. The swelling and shrinking can only take place along the radial direction and the chains are bound to the solid surface of the cores which remains constant during the transition.
Intermediate Hamiltonian Fock-space coupled cluster method in the one-hole one-particle sector: Excitation energies of xenon and radon
Authors: Landau Arie , Eliav Ephraim , Ishikawa Yasuyuki , Kaldor Uzi
Published: July 2002

The intermediate Hamiltonian Fock-space coupled cluster method developed recently is applied to excitations in the one-hole one-particle sector, taking xenon and radon atoms as test cases. Virtual orbitals are modified to yield better approximations to orbitals occupied in excited states. The usual Fock-space coupled cluster scheme diverges for these systems, but the intermediate Hamiltonian approach converges for large P spaces and yields excitation energies in very good agreement with experiment. The average error in the calculated values for the lowest excitation energies (about 20 for each atom) is 0.6%. Predictions are made for the unobserved 8s Rydberg states of Rn.
Electrical activation of B in the presence of boron-interstitials clusters
Authors: Mannino Giovanni , Solmi Sandro , Privitera Vittorio , Bersani Massimo
Published: July 2002

Boron marker-layer structures have been used to analyze the evolution of boron-interstitial clusters (BICs) formed during transient enhanced diffusion. Our approach is based on the measure of B activation by spreading resistance profiling after annealing of Si implantation damage. We investigated a wide range of implant conditions in terms of defect densities below and above the amorphization threshold of Si. We found a common behavior of BICs in terms of trapping and release processes of B atoms. The BICs density as a function of time for different concentration ratios of I and B has been determined.
Characterization of heterojunction diodes with hydride vapor phase epitaxy grown AlGaN on 4H–SiC
Authors: Danielsson E. , Zetterling C.-M. , Östling M. , Tsvetkov D. , Dmitriev V. A.
Published: July 2002

AlGaN/4H–SiC heterojunction diodes with varying composition of Al have been fabricated. Five different compositions were investigated, GaN, Al0.1Ga0.9N, Al0.15Ga0.85N, Al0.3Ga0.7N, and Al0.5Ga0.5N, along with a 4H–SiC homojunction diode for comparison. The turn on voltage was around 1 V, and the ideality factor between 1 and 2 for all heterojunction diodes except for the Al0.3Ga0.7N diode. This diode had an ideality factor between 2 and 3, and also showed a much lower series resistance, indicating a change in transport mechanism across the junction. A tunnel assisted recombination model was analyzed and compared to the extracted values of the GaN diode. The model agreed well with both current–voltage and capacitance–voltage measurements for this diode. This model was not applied to the other samples, since their characteristics could not be explained by a simple mechanism.
Invariant algebraic surfaces of the Lorenz system
Authors: Llibre Jaume , Zhang Xiang
Published: July 2002

In this paper we provide a complete classification of the Darboux invariants, of the irreducible Darboux polynomials, of the rational first integrals and of the algebraic integrability for the classical Lorenz system ẋ=s(y−x), ẏ=rx−y−xz, ż=−bz+xy. In the proofs, we use the weight homogeneous polynomials and the method of characteristic curves for solving linear partial differential equations.
Computational designing of graphitic silicon carbide and its tubular forms
Authors: Miyamoto Yoshiyuki , Yu Byung Deok
Published: July 2002

By employing density-functional theory calculations we predict graphitic and tubular forms of silicon carbide (SiC) and proposes their synthesis. The present calculations suggest that the metastable SiC tubes can be synthesized by using a technique of extreme hole injection [J. M. Schön, Ch. Kloc, and B. Batlogg, Nature (London) 406, 702 (2000); 408, 549 (2000)]. This method is in contrast to the idea to synthesize new-tubular forms with a usage of a carbon nanotube as nucleation seed. Furthermore, the strain energies of SiC nanotubes are lower than those of the carbon nanotubes. The calculated band gaps of the investigated SiC tubes are either direct or indirect depending on the helicities. We expect that the SiC tubes with direct band gaps can be applied as nanoscale optoelectronics devices with strong oscillator strengths.
Velocity field statistics in homogeneous steady turbulence obtained using a high-resolution direct numerical simulation
Authors: Gotoh Toshiyuki , Fukayama Daigen , Nakano Tohru
Published: July 2002

Velocity field statistics in the inertial to dissipation range of three-dimensional homogeneous steady turbulent flow are studied using a high-resolution DNS with up to N=10243 grid points. The range of the Taylor microscale Reynolds number is between 38 and 460. Isotropy at the small scales of motion is well satisfied from half the integral scale (L) down to the Kolmogorov scale (η). The Kolmogorov constant is 1.64±0.04, which is close to experimentally determined values. The third order moment of the longitudinal velocity difference scales as the separation distance r, and its coefficient is close to 4/5. A clear inertial range is observed for moments of the velocity difference up to the tenth order, between 2λ≈100η and L/2≈300η, where λ is the Taylor microscale. The scaling exponents are measured directly from the structure functions; the transverse scaling exponents are smaller than the longitudinal exponents when the order is greater than four. The crossover length of the longitudinal velocity structure function increases with the order and approaches 2λ, while that of the transverse function remains approximately constant at λ. The crossover length and importance of the Taylor microscale are discussed.
Hot exciton transport in ZnSe quantum wells
Authors: Zhao Hui , Moehl Sebastian , Wachter Sven , Kalt Heinz
Published: July 2002

The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Fröhlich coupling, the initial kinetic energy of the excitons is well controlled by choosing the excess energy of the excitation laser. When increasing the laser excess energy, we find a general trend of increasing transport length and more importantly a pronounced periodic quenching of the transport length when the excess energy corresponds to multiples of the LO-phonon energy. Such features show the dominant role of the kinetic energy of excitons in the transport process. Together with the excitation intensity dependence of the transport length, we distinguish the phonon wind driven transport of cold excitons and defect-limited hot exciton transport.
Carbon cage-like materials as potential low work function metallic compounds: Case of clathrates
Authors: Timoshevskii V. , Connétable D. , Blase X.
Published: July 2002

We present an ab initio calculation of the electronic affinity of the hypothetical C-46 clathrate by studying its bare and hydrogenated (100) surfaces. We show that such a system shares with the diamond phase a small electronic affinity. Furthermore, contrary to the diamond phase, the possibility of endohedrally doping these cage-like systems allows to significantly raise the position of the Fermi level, resulting in a true metal with a small work function. This is illustrated in the case of the Li8@C-46 doped compound. Such a class of materials might be of much interest for the design of electron-emitting devices.