bluuebunny/crossref_metadata_2025 huggingface dataset:
Atom-size gaps and contacts between electrodes fabricated with a self-terminated electrochemical method
Authors: Boussaad S. , Tao N. J.
Published: July 2002

We describe a method to fabricate atomic-scale gaps and contacts between two metal electrodes. The method uses a directional electrodeposition process and has a built-in self-termination mechanism. The final gap width and contact size are preset by an external resistor (Rext) that is connected in series to one of the electrodes. If 1/Rext is chosen to be much smaller than the conductance quantum (G0=2e2/h), a small gap with conductance determined by electron tunneling is formed. If 1/Rext is comparable or greater than G0, a contact with conductance near a multiple of G0 is fabricated.
Stable scattering-matrix method for surface acoustic waves in piezoelectric multilayers
Authors: Pastureaud Th. , Laude V. , Ballandras S.
Published: July 2002

A scattering matrix approach is proposed to avoid numerical instabilities arising with the classical transfer matrix method when analyzing the propagation of plane surface acoustic waves in piezoelectric multilayers. The method is stable whatever the thickness of the layers, and the frequency or the slowness of the waves. The computation of the Green’s function and of the effective permittivity of the multilayer is outlined. In addition, the method can be easily extended to the case of interface acoustic waves.
A coarse-graining procedure for flexible polymer chains with bonded and nonbonded interactions
Authors: Fukunaga Hiroo , Takimoto Jun-ichi , Doi Masao
Published: July 2002

Starting from the united atom model, we construct a coarse-grained model for a flexible polymer chain, in which some successive CH2 atoms are combined into an effective segment. To connect the coarse-grained model with the atomistic model, we propose a scheme to obtain the effective potentials acting between bonded and nonbonded segments from atomistic molecular dynamics simulation for a single isolated chain. We assume that the total effective potential is a sum of potential components for independent coarse-grained variables. The effective bond potentials are determined by simply taking the logarithm of the corresponding distribution functions calculated from the atomistic simulations. On the other hand, to consider the characteristic entropy effects of the polymer chain system, the effective nonbonded potentials are evaluated using the canonical ensemble average for fixed distance between the segments. We confirmed that the coarse-grained model using these effective potentials can reproduce the radii of gyration and various distribution functions of the coarse-grained variables over a wide temperature range. We also confirmed that the effective potentials obtained for the isolated chain system are applicable to the melt system. A detailed analysis of the distribution functions showed that the effective bond length and the effective torsion angle correlate strongly with the effective bond angle. In order to improve the quality of our coarse-grained potentials, these correlations should be taken into account.
High-speed near-field scanning optical microscopy with a quartz crystal resonator
Authors: Seo Yongho , Jhe Wonho
Published: July 2002

We demonstrate high-speed near-field scanning optical microscopy using a tapered fiber tip that is inserted in a perforated quartz-crystal resonator. With this high-frequency dithering probe, we have obtained near-field optical images at the scanning speed of 1.3 mm/s and it takes only 0.5 s to image the surface area of 100 μm2 for 64×64 pixels without any compromise of spatial lateral resolution. As an application, we have obtained sequential scanning images of the diffusing microspheres in water, which shows slow change of the sample configuration in the time scale of several minutes.
A high sensitivity electron momentum spectrometer with simultaneous detection in energy and momentum
Authors: Takahashi Masahiko , Saito Taku , Matsuo Motoaki , Udagawa Yasuo
Published: July 2002

A new apparatus for electron–electron coincidence experiments has been developed to examine molecular orbital patterns quantitatively by electron momentum spectroscopy. Using a spherical analyzer and position-sensitive detectors, it enables one to measure energy and angular correlations between the two outgoing electrons simultaneously. The design and performance of the apparatus is reported together with results on Ar to show extensive improvements in coincidence count rates and statistical precision, covering a wide range of binding energies and momenta.
“Coarse” stability and bifurcation analysis using stochastic simulators: Kinetic Monte Carlo examples
Authors: Makeev Alexei G. , Maroudas Dimitrios , Kevrekidis Ioannis G.
Published: July 2002

We implement a computer-assisted approach that, under appropriate conditions, allows the bifurcation analysis of the “coarse” dynamic behavior of microscopic simulators without requiring the explicit derivation of closed macroscopic equations for this behavior. The approach is inspired by the so-called time-stepper based numerical bifurcation theory. We illustrate the approach through the computation of both stable and unstable coarsely invariant states for kinetic Monte Carlo models of three simple surface reaction schemes. We quantify the linearized stability of these coarsely invariant states, perform pseudoarclength continuation, detect coarse limit point and coarse Hopf bifurcations, and construct two-parameter bifurcation diagrams.
Precision spectrometer for measurement of specular reflectance
Authors: Haapalinna Atte , Nevas Saulius , Manoochehri Farshid , Ikonen Erkki
Published: July 2002

The HUT reference spectrometer was modified for measuring specular reflectance in the wavelength range of 300 to 850 nm. The instrument is based on a diffraction-grating monochromator, reflecting optics, sample control mechanics and detection systems with linear responsitivities. Relative standard uncertainties between 0.14% and 0.32% were estimated for the reflectance measurements. For spectral reflectance between 1.5% and 15%, the results of test measurements using samples with known reflectances confirm that for all geometries the relative deviations are less than 0.36%. A set of ultraviolet (UV)-interference filters was measured in the UVB wavelength range, and the results are used as a part of filter radiometer characterization.
Optical tweezers system measuring the change in light momentum flux
Authors: Grange Wilfried , Husale Sudhir , Güntherodt Hans-Joachim , Hegner Martin
Published: July 2002

This article describes the design of a dual-beam optical tweezers (OT) instrument which, in contrast to conventional single-beam OT, directly measures the change in light momentum flux when a trapped object experiences a force. Consequently, no local calibration is needed to measure the force acting on a trapped particle. The instrument has a high trapping efficiency and forces up to 200 pN can be measured. In addition, the above-mentioned system operates in conjunction with a three-dimensional steerable single-beam OT.
System for automated magnetic resonance imaging of a superheated emulsion chamber for brachytherapy dosimetry
Authors: Pratt Ronald G. , Schmithorst Vincent J. , Lamba Michael , Dardzinski Bernard J. , d’Errico Francesco , Nath Ravinder , Holland Scott K.
Published: July 2002

This manuscript describes a pressure cycling and brachytherapy source insertion/removal system developed to automate magnetic resonance imaging (MRI) of a superheated emulsion chamber (SEC), a position-sensitive radiation dosimeter. Previous MRI studies of a SEC demonstrated its efficacy for obtaining two-dimensional relative radiation dose profiles from brachytherapy sources. The SEC detector is a 0.5 l chamber containing an emulsion of highly superheated halocarbon droplets suspended in a tissue-equivalent glycerin-based gel. When irradiated by a source, the halocarbon droplets vaporize into microbubbles. Relative dose profiles are determined by mapping, via MRI, the resulting microbubble distributions. During use the SEC is repeatedly cycled through a series of steps including source insertion, exposure, and source removal, MRI data collection, and chamber pressurization and depressurization. To acquire microbubble counts sufficient to derive dose profiles having good statistics, it is necessary to collect hundreds of MRI data sets as the SEC is cycled. Clearly, automation of SEC cycling is essential for its practical use as a radiation dosimeter. The operations required to use the SEC as a radiation dosimeter have been fully automated on a commercial 3.0 T MRI scanner. The hardware and software developed to achieve automated cycling of a SEC are described. Microbubble distributions and calculated microbubble density maps generated from MRI data collected while cycling a SEC are presented to demonstrate the utility of the automated system.
Improved solar-blind detectivity using an AlxGa1−xN heterojunction p–i–n photodiode
Authors: Collins C. J. , Chowdhury U. , Wong M. M. , Yang B. , Beck A. L. , Dupuis R. D. , Campbell J. C.
Published: July 2002

We report the improved detectivity of AlxGa1−xN-based solar-blind p–i–n photodiodes with high zero-bias external quantum efficiency. The zero-bias external quantum efficiency was ∼42% at 269 nm, and increased to ∼46% at a reverse bias of −5 V. In addition, the photodiodes exhibited a low dark current density of 8.2×10−11 A/cm2 at a reverse bias of −5 V, which resulted in a large differential resistance. The high quantum efficiency and large differential resistance combine to yield a high detectivity of D*∼2.0×1014 cm Hz1/2 W−1. These results are attributed to the use of an Al0.6Ga0.4N window n region, which allows improved transmission to the absorption region, and to improved material quality.
Shock wave response of a zirconium-based bulk metallic glass and its composite
Authors: Zhuang Shiming , Lu Jun , Ravichandran Guruswami
Published: July 2002

A zirconium-based bulk metallic glass, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1), and its composite, Zr56.3Ti13.8Cu6.9Ni5.6Nb5.0Be12.5 (β-Vit), were subjected to planar impact loading. A surprisingly low amplitude elastic precursor and bulk wave, corresponding to the elastic response of the “frozen structure” of the intact metallic glasses, were observed to precede the rate-dependent large deformation shock wave. A concave downward curvature after the initial increase of the Us–Up shock Hugoniots suggests that a phase-change-like transition occurred during shock compression. Further, compression damage occurred due to the shear localization. The spalling in Vit 1 was induced by shear localization, while in β-Vit, it was due to debonding of the β-phase boundary from the matrix. The spall strengths at strain rate of 2×106 s−1 were determined to be 2.35 and 2.11 GPa for Vit 1 and β-Vit, respectively.
Spin polarized tunneling through single-crystal GaAs(001) barriers
Authors: Kreuzer S. , Moser J. , Wegscheider W. , Weiss D. , Bichler M. , Schuh D.
Published: July 2002

We investigate spin-dependent transport through an epitaxial GaAs(001) barrier sandwiched between polycrystalline iron films. Electron transport through the barrier is dominated by quantum mechanical tunneling, demonstrated by a nonlinear I–V characteristic, an exponential dependence of the tunneling current on the barrier thickness and the temperature dependence of the current. Though small a clear tunneling magnetoresistance effect proves spin-dependent transport through the Fe–GaAs interface. The small size of the effect and the high-field magnetoresistance suggest that spin–flip scattering plays a decisive role in transport.
Chaos in a dynamic model of traffic flows in an origin-destination network
Authors: Zhang Xiaoyan , Jarrett David F.
Published: July 2002

In this paper we investigate the dynamic behavior of road traffic flows in an area represented by an origin-destination (O–D) network. Probably the most widely used model for estimating the distribution of O–D flows is the gravity model, [J. de D. Ortuzar and L. G. Willumsen, Modelling Transport (Wiley, New York, 1990)] which originated from an analogy with Newton’s gravitational law. The conventional gravity model, however, is static. The investigation in this paper is based on a dynamic version of the gravity model proposed by Dendrinos and Sonis by modifying the conventional gravity model [D. S. Dendrinos and M. Sonis, Chaos and Social-Spatial Dynamics (Springer-Verlag, Berlin, 1990)]. The dynamic model describes the variations of O–D flows over discrete-time periods, such as each day, each week, and so on. It is shown that when the dimension of the system is one or two, the O–D flow pattern either approaches an equilibrium or oscillates. When the dimension is higher, the behavior found in the model includes equilibria, oscillations, periodic doubling, and chaos. Chaotic attractors are characterized by (positive) Liapunov exponents and fractal dimensions.
The autowave modes of solid phase polymerization of metal-containing monomers in two- and three-dimensional fiberglass-filled matrices
Authors: Barelko V. V. , Pomogailo A. D. , Dzhardimalieva G. I. , Evstratova S. I. , Rozenberg A. S. , Uflyand I. E.
Published: July 2002

The phenomenon of autowave (frontal) solid phase polymerization of metal-containing monomers based on metal-acrylamide complexes is considered. The comparison of the features of autowave processes realized in both the single-component matrices of the monomer and the matrices filled by the fiberglass materials is performed. The unstable regimes of the polymerization wave as well as the conditions for the stabilization of the flat front in the filled matrices are described. The peculiarities of the frontal regimes in the three- and two-dimensional media are studied. Some possibilities for using of autowave polymerization in the fabrication of the polymer–fiberglass composites and composition prepregs are discussed.
Chaotic advection, diffusion, and reactions in open flows
Authors: Tél Tamás , Károlyi György , Péntek Áron , Scheuring István , Toroczkai Zoltán , Grebogi Celso , Kadtke James
Published: July 2002

We review and generalize recent results on advection of particles in open time-periodic hydrodynamical flows. First, the problem of passive advection is considered, and its fractal and chaotic nature is pointed out. Next, we study the effect of weak molecular diffusion or randomness of the flow. Finally, we investigate the influence of passive advection on chemical or biological activity superimposed on open flows. The nondiffusive approach is shown to carry some features of a weak diffusion, due to the finiteness of the reaction range or reaction velocity.
Parallel I/O ports for the IBM microchannel
Authors: Spencer C. D. , Herrmann E. J.
Published: July 2002

Eight-bit parallel input/output (I/O) ports for the IBM microchannel are described. First, the microchannel signals needed for the ports are identified and defined. Next, the states of the data and address lines and the controls during input and output sequences are described. Finally, ports circuitry which responds to the sequences is presented and explained.
A new method for visualizing data on a sphere
Authors: Hon David
Published: July 2002

A new method for visualizing data on a globe or unit sphere is described. Information that is distributed over a sphere—global oceanographic or geographic measurements, all-sky astronomy observations, or any quantities that are best represented in spherical coordinates—can benefit from this technique. Retaining a better sense of the geometry and information content of the data, 3-D graphics can provide an unobstructed view of the entire sphere, without undo deformation of its surface area. A ‘‘parametrized ray trace’’ produces look-up tables (LUTs) that can be used for all visualizations. The ray trace result shows one or more spheres with the data as a texture map and three reflecting rectangles that ‘‘mirror’’ the far sides of the sphere(s) into view. The LUTs need only be created once, and a general purpose computer will do. No special purpose hardware is required beyond a PC or workstation that supports color. Examples from astronomical and geophysical datasets, which are commonly displayed with an area deforming (2-D) projection, are presented.
A comparative study of temperature extremes analysis by regression methods and by an artificial neural network
Authors: Roy Swati , Lee W. L. (Nancy) , Mentele James W. , Nau Richard W.
Published: July 2002

This paper formulates and tests a mathematical model for the prediction of historical average daily maximum and minimum temperatures. First, the model is derived based on a physical understanding of the problem. Then some available weather data are analyzed by three traditional regression methods, namely, a nonlinear regression method, a linear regression method, and a Fourier transform method. Using the coefficients obtained by these analyses, temperatures are predicted for an independent test set and the root-mean-square errors (rms) determined. In an alternative approach, an artificial neural network (ANN) is trained using the same weather data. The trained ANN is made to predict the temperatures for the test set and its rms determined. The variation of rms values with the change in ANN structure is also observed. A comparative study shows that the ANN is a general tool for data analysis and capable of producing results comparable in accuracy to those obtained by the more conventional methods.
A fast, user-friendly code for calculating magnetohydrodynamic equilibria
Authors: Haney S. W. , Freidberg J. P. , Solomon C. J.
Published: July 2002

Using variational techniques, we have developed a fast, user-friendly code for computing approximate, but highly accurate fixed boundary magnetohydrodynamic equilibria for tokamak plasmas. The variational procedure simplifies the problem—a two-dimensional nonlinear partial differential equation—to a set of nonlinear algebraic equations. The reduced problem can be readily solved on workstations or personal computers. This allows us to exploit sophisticated graphical user interfaces that make supplying calculation data and viewing results easy. This ease-of-use, along with the semianalytic nature of our calculation, allows researchers to routinely incorporate equilibrium information into their work. It also provides a tool for educators teaching fusion theory. We describe the variational formulation, the speed and accuracy of the computer implementation, and the design and operation of a user-friendly graphical interface. © 1995 American Institute of Physics.
A new approach to an old algorithm for the Simulation of Ising-like Systems
Authors: Novotny M. A.
Published: July 2002

An n-fold-way algorithm for the Ising model in a square lattice with anisotropic interactions is discussed. This algorithm shows an improved computational performance at low temperature. (AIP)
Equilibrium polarization and piezoelectric and pyroelectric coefficients in poly(vinylidene fluoride)
Authors: Al-Jishi R. , Taylor P. L.
Published: July 2002

A calculation of the equilibrium polarization and of the piezoelectric and pyroelectric coefficients of β-phase poly(vinylidene fluoride) (PVF2) is presented. The effects of both the orthorhombic crystal structure and of the finite separation between the monopoles of the molecular dipole are taken into account. Good agreement is obtained with the experimentally measured polarization and piezoelectric and pyroelectric coefficients of β-PVF2 films.
Raman and photoluminescence spectra of GaAs1−xSbx
Authors: Cohen R. M. , Cherng M. J. , Benner R. E. , Stringfellow G. B.
Published: July 2002

The first experimental Raman and photoluminescence spectra are presented for the metastable alloy GaAs1−xSbx grown by organometallic vapor phase epitaxy throughout its miscibility gap extending from x=0.2 to x=0.75. The phonon peak halfwidths are found to broaden by nearly a factor of 2 over halfwidths found in the binary compounds GaAs and GaSb. Phonon line shapes become more asymmetric in the miscibility gap as the selection rules break down; in addition, a second peak appears for samples grown near the center of the miscibility gap. Line shapes are analyzed and the phonon coherence length is found to be reduced from several hundred angstroms in GaAs to approximately 60 Å in samples grown in the miscibility gap. The compositional dependence of the room-temperature band-gap energy has been found to closely follow earlier predictions.
Spall strength of shock-loaded glass
Authors: Rosenberg Z. , Yaziv D. , Bless S.
Published: July 2002

The spall strength of soda lime glass was measured with Manganin stress gauges, in planar impact experiments, below and above the Hugoniot elastic limit of the glass. It was found that while in its elastic range the glass can withstand very large tensions (over 30 kbars), its spall strength reduced to zero when shocked just above the HEL. This confirms the assumption that the elastic limit marks the onset of fracture in glass upon shock loading.
A universal coexistence curve for polymer solutions
Authors: Sanchez Isaac C.
Published: July 2002

Coexistence curves for binary polymer/solvent solutions are asymmetric when volume fraction is used as the concentration variable. Coexistence curves for polystyrene/methylcyclohexane solutions can be symmetrized by a simple transformation of variables; in the new dimensionless variable ψ, all of the data satisfy a universal scaling law: ‖ψ−ψc‖=ψ0(εN0.313)β; ε≡(Tc−T)/Tc, where N is the degree of polymerization and ψc and ψ0 are constants independent of N. The experimental value of the critical exponent β (0.327±0.002) agrees well with recent renormalization group calculations for the Ising model. The exponent on N(0.102±0.002) may be universal and should be compared with its mean-field value of 1/4; a calculation of its nonclassical value remains a challenging theoretical problem.
Mechanisms of interdiffusion in copper/nickel thin-film couples
Authors: Johnson B. C. , Bauer C. L. , Jordan A. G.
Published: July 2002

Mechanisms of interdiffusion in copper/nickel thin-film couples have been investigated in the temperature interval 573–777 K by in situ measurement of contact resistance, Auger depth profiling (ADP), and transmission electron microscopy. Correlation between evolution of contact resistance and measured Auger concentration profiles has been established and mechanisms incorporating rapid grain boundary diffusion, followed by defect-assisted diffusion into grain interiors (Type B kinetics), are proposed to explain the accelerated reactions observed. A modified Whipple model and two independent methods, based on ADP and contact resistance measurements, are used to calculate grain boundary and intragranular diffusion coefficients, respectively. The calculated grain boundary diffusion coefficient is (0.82 cm2/s) exp(−1.48eV/kT) for nickel in copper, and approximately 4×10−13 cm2/s for copper in nickel at 673 K. An average intragranular diffusion coefficient for nickel in copper is determined to be (2.6×10−6 cm2/s) exp(−1.38 eV/kT) by both methods, whereas ADP data yield a corresponding value of (5.2×10−8 cm2/s) exp(−1.51eV/kT) for copper in nickel. It is concluded that characterization of chemical composition and microstructure, combined with in situ measurement of concomitant electrical properties, provides a reliable description of interdiffusion mechanisms in this system.
Impact ionization across the conduction-band-edge discontinuity of quantum-well heterostructures
Authors: Chuang S. L. , Hess K.
Published: July 2002

Impact ionization across the band-edge discontinuity of quantum-well heterostructures is studied theoretically. We consider a heterolayer structure of alternating AlxGa1−xAs and GaAs layers where the GaAs layers are heavily doped with donors. Thus a large number of electrons is confined to the quantum-well region. Incident electrons are heated up by applied electric fields and collide with the electrons confined in the well regions. Both the ionization rate as a function of the incident energy, and average ionization rates are computed. Device applications of such multiple quantum-well structures and the possibility of a complete analog to the conventional photomultiplier are discussed.
Time-resolved x-ray monitoring of laser ablation of and plasma formation from Si
Authors: van Brug H. , Murakami K. , Bijkerk F. , van der Wiel M. J.
Published: July 2002

We present x-ray absorption measurements on silicon clusters and silicon plasmas produced by pulsed laser irradiation of bulk silicon. The results are compared with earlier time-resolved x-ray absorption measurements on amorphous silicon foils under pulsed laser irradiation. Clusters are formed at an irradiance as low as 3.0 J/cm2. At an irradiance of 14 J/cm2 ionized species up to Si4+ are formed. We find a removed Si layer thickness of 80 Å at an irradiance of ≊6 J/cm2, at 15 ns pulse duration.
Ion implantation of boron in germanium
Authors: Jones Kevin S. , Haller E. E.
Published: July 2002

The activation of boron implanted at room temperature into germanium has been studied. In sharp contrast to all other group III elements boron forms a p-type layer before any postimplant annealing steps. Variable temperature Hall effect measurements and deep level transient spectroscopy experiments indicate that all of the boron ions are electrically active as shallow acceptor centers over the entire dose range (5×1011/cm2 to 1×1014/cm2) and energy range (25–100 keV) investigated, without any postimplant annealing. The concentration of damage related acceptor centers is only 10% of the boron related, shallow accepted centers concentration for low-energy implants (25 keV), but becomes dominant at high energies (100 keV) and low doses (<1×1012/cm2). Three damage related hole traps are produced by ion implantation of 11B+. Two of these hole traps have also been observed in γ-irradiated Ge and may be oxygen-vacancy related defects, while the third trap may be divacancy related. All three traps anneal out at low temperatures (<300 °C). Room-temperature implantation of BF+2 into Ge, does not lead to substitutionally active boron without annealing. A thermal cycle of 350 °C for 30 min activates 100% of the boron.
Photoluminescence spectroscopy of ion-implanted 3C-SiC grown by chemical vapor deposition
Authors: Freitas J. A. , Bishop S. G. , Edmond J. A. , Ryu J. , Davis R. F.
Published: July 2002

Low-temperature photoluminescence (PL) spectroscopy has been used to characterize as-grown and ion-implanted 3C-SiC films grown by chemical vapor deposition on Si(100) substrates. The D1 and D2 defect PL bands reported previously in ion-implanted Lely-grown SiC were also observed in the as-grown chemical vapor deposited films, and the effects of annealing (1300–1800 °C) on these PL bands as observed in as-grown films and films implanted with B, Al, or P have been studied. As reported previously for Lely-grown SiC, the spectral details of the defect PL bands and their annealing characteristics were found to be independent of the particular implanted-ion species.
Effects of dielectric encapsulation and As overpressure on Al-Ga interdiffusion in AlxGa1−x As-GaAs quantum-well heterostructures
Authors: Guido L. J. , Holonyak N. , Hsieh K. C. , Kaliski R. W. , Plano W. E. , Burnham R. D. , Thornton R. L. , Epler J. E. , Paoli T. L.
Published: July 2002

Data are presented showing that the Al-Ga interdiffusion coefficient (DAl-Ga) for an AlxGa1−xAs-GaAs quantum-well heterostructure, or a superlattice, is highly dependent upon the crystal encapsulation conditions. The activation energy for Al-Ga interdiffusion, and thus layer disordering, is smaller for dielectric-encapsulated samples (∼3.5 eV) than for the case of capless annealing (∼4.7 eV). The interdiffusion coefficient for Si3N4-capped samples is almost an order of magnitude smaller than for the case of either capless or SiO2-capped samples (800≤T≤875 °C). Besides the major influence of the type of encapsulant, the encapsulation geometry (stripes or capped stripes) is shown, because of strain effects, to be a major source of anisotropic Al-Ga interdiffusion.
Reflection and transmission of ultrasound from a planar interface
Authors: Paranjape B. V. , Arimitsu N. , Krebes E. S.
Published: July 2002

Information about the local forces at an interface between two solids is of considerable technological interest. We have used a simple model of a homogeneous solid. We consider a cubic crystal with springs connecting the nearest and the next nearest neighbors. Two such crystals can be joined by a different set of springs at the interface. The force constants of the springs at the interface describe the local forces at the interface. If a longitudinal wave is incident on such an interface at an angle less than the critical angle, it will be partly reflected and transmitted as a longitudinal and transverse wave. We have derived the equations for the transmitted and reflected amplitudes. These can be expressed in terms of the macroscopic properties of solids on either side of the interface, and in terms of the force constants at the interface. It is shown that at low frequencies our equations reduce to those derived from macroscopic theory in which it is assumed that the bond at the interface is perfect and that the boundary atoms on both sides of the interface have equal displacements. If the force constants at the interface are weak compared with the force constants in the bulk media on either side of the interface, deviations from the macroscopic results occur. Thus, measurements of scattered wave amplitudes at frequencies where the deviation from macroscopic results occurs can provide valuable information about the nature of forces at the interface.
Characteristics of the Si-SiO2 interface states in thin (70–230 Å) oxide structures
Authors: Kar S. , Narasimhan R. L.
Published: July 2002

Important characteristics of the Si-SiO2 interface states, such as the interface state density distributions and the electron and the hole capture cross section versus energy profiles, were investigated in unannealed metal/silicon dioxide/silicon (MOS) structures with the gate oxide thickness in the range of 70–230 Å, using the optical MOS admittance technique. The experimentally obtained interface state density distribution, in case of p-Si/SiO2/Al structures, exhibited two peaked profiles, one near the valence-band edge Ev and the other near the conduction-band edge Ec, overlying a concave background. The state density at the peak was observed to undergo a maximum, preceded by a minimum, as the gate oxide thickness tox was increased. The peak energy versus tox and the capture cross section versus tox profiles also indicated strong features.
Induced anisotropy and magnetostriction in metallic glasses
Authors: Chen D.-X.
Published: July 2002

The temperature dependence of the saturation magnetostriction in (Fe,Co,Ni)-based metallic glasses can be explained rather satisfactorily in terms of contributions arising from single-ion and two-ion local magnetic anisotropy. The temperature dependence of the induced anisotropy, however, is found to be different. It is observed that induced anisotropy of Co-rich alloys originates mostly from single-ion contribution, while in the case of Fe-Ni-based alloys two-ion contribution seems to be the origin. Effects of annealing reveal a much more complicated situation. We suggest that these results can be interpreted in terms of the kinetics of local orientation and their general distribution on a macroscopic scale.
Optical properties of β-Ga2O3 and α-Ga2O3:Co thin films grown by spray pyrolysis
Authors: Kim Hyung-Gon , Kim Wha-Tek
Published: July 2002

Stable β-Ga2O3 and α-Ga2O3:Co thin films were grown on thoroughly cleaned borosilicate glass plates by the spray pyrolysis method. The substrate temperature during growth was maintained at 350 °C and the film was then annealed in air at 350 °C for 30 min. The optical energy gaps for these films at 295 K was given as 4.23 eV for β-Ga2O3 and 2.41 eV for α-Ga2O3:Co. The impurity absorption peaks for α-Ga2O3:Co thin film due to the transitions of 4A2(4F)→4T1(4P), 4A2(4F)→4T1(4F), and 4A2(4F)→4T2(4F) of Co2+ ions were observed at 14 925, 7142, and 4166 cm−1, with Racah parameters given as Dq=416 cm−1 and B=638 cm−1.
Transient thermoreflectance of thin metal films in the picosecond regime
Authors: Miklós András , Lőrincz András
Published: July 2002

The distributed nature of the reflection process is considered for pump-and-probe-type picosecond transient thermoreflectance (PTTR) measurements. A first-order approximation is given for the temporal dependence of the reflectivity for temperature distributions created by picosecond laser pulses. It was found that PTTR flashes back the temperature profile created by the pump pulse. The problem of determining the thermal transport properties from PTTR measurements is discussed.
The effect of debris formation on the morphology of excimer laser ablated polymers
Authors: Taylor R. S. , Leopold K. E. , Singleton D. L. , Paraskevopoulos G. , Irwin R. S.
Published: July 2002

A study of the distribution of the debris formed by the XeCl laser ablation of polyimide and polyethylene terephthalate and the KrCl laser ablation of polyimide shows that it is the redeposition of debris rather than impurities which accounts for the cone structures commonly seen in the surface morphology.
Sheath collision processes controlling the energy and directionality of surface bombardment in O2 reactive ion etching
Authors: Jurgensen C. W.
Published: July 2002

The effect of sheath collision processes on the energy and directionality of surface bombardment in reactive ion etching is modeled. Although the methods used are generally applicable, all the numerical examples are for a low-pressure high-frequency oxygen plasma. Charge transfer is shown to be the dominant process controlling bombardment energies. The effect of momentum-transfer collisions on ion bombardment energies is shown to be negligible. Equations are derived for the average energy of ions and neutrals, the average ion energy, the average neutral energy, and the ion energy distribution function. The ion drift velocity at a point in the sheath is related to the voltage distribution by an equation that provides a rigorous basis for a self-consistent theory of the sheath voltage distribution. These equations are generally applicable to high-frequency, low-pressure plasmas where charge transfer is the dominant collision process. The angular distribution of energetic species is modeled using elastic scattering theory. These angular distributions can be used as input to etching models that calculate profiles and process latitudes.
Picosecond transient reflectance of thin metal films
Authors: Miklós András , Bozóki Zoltán , Lörincz András
Published: July 2002

Picosecond transient reflectance (PTR) of metals is shown to depend strongly on temperature and strain coefficients of the complex dielectric constant. For short times the PTR is influenced by the strains, and byproducts of the heating process. Thermal transport properties may be determined after the strain edge moves farther than several optical penetration depths. Methods are suggested for the determination of temperature and strain coefficients of the complex dielectric constant as well as for the extraction of the thermal transport parameters of thin films.
Mechanical stress as a function of temperature for aluminum alloy films
Authors: Gardner Donald S. , Flinn Paul A.
Published: July 2002

Aluminum alloys have virtually replaced aluminum thin films for interconnections in very large-scale integration because of their improved reliability. Mechanical stress is a problem of growing importance in these interconnections. Stress as a function of temperature was measured for thin aluminum films on an oxidized silicon substrate and several aluminum alloys and layered films consisting of silicon, copper, titanium, tungsten, tantalum, vanadium, and TiSi2. Solid-state reactions of the aluminum with the additives and with the ambient during thermal cycling will occur, and depending on what compounds have formed and at what temperature, this will determine the morphology and reliability of the metallization. The measurement technique, based on determination of wafer curvature with a laser scanning device, directly measures the total film stress and reflectivity in situ as a function of temperature during thermal cycling. Changes in stress were detected when film composition and structure varied and were correlated using x-ray diffraction with the formation of aluminides. Other phenomena that contribute to stress changes include elastic behavior, recrystallization, grain growth, plastic behavior, yield strength, and film hardening from precipitates.
Numerical calculation of the height of velocity-matching step of flux-flow type Josephson oscillator
Authors: Zhang Y. M. , Wu P. H.
Published: July 2002

The height of the velocity-matching step (VM step) of a flux-flow type Josephson oscillator is studied systematically by numerical simulations. It is defined by the phase velocity of the propagating fluxons equaling that of the electromagnetic wave in the junction structure. The height of VM step (γm) is an important parameter for the practical application of such a device. The results presented on γm are important data in order to save computer time in the simulation of electromagnetic properties, such as the output voltage, at the velocity-matching condition. The detailed simulations were performed using the perturbed sine-Gordon equation (PSGE) with a uniformly distributed bias current γ. The PSGE was solved numerically by using an implicit finite-difference scheme. The distributions of electromagnetic fields in an overlap, long Josephson junction in the flux-flow state are mapped. The parameters influencing γm are discussed. We show that γm increases almost linearly with an external magnetic field βe or with the quasiparticle damping α, while it is less dependent on surface loss β. γm increases slowly with junction length above a minimum length, and it levels out at long lengths. We also present an approximate expression for γm which fits numerical results well.
Nonideal J-V characteristics and interface states of an a-Si:H Schottky barrier
Authors: Maeda Keiji , Umezu Ikuro , Ikoma Hideaki , Yoshimura Takahiro
Published: July 2002

A new theory is developed for nonideal J-V characteristics of Schottky barriers with an interfacial layer. This theory is based on the model that nonideal characteristics are due to changes of population in the interface states under applied bias and accompanying changes of the barrier height. The population in the interface states is expressed by the Fermi level, which can be determined by analyzing experimental results. The J-V characteristics are obtained from the flow of carriers into and out of the interface. Tunneling through the interfacial layer constitutes the bottleneck for the carrier flow. Under forward bias, the carrier concentration ns at the interface is proved to be in thermal equilibrium with the bulk. Under reverse bias, ns is in local thermal equilibrium with the interface states. This theory is applied to an undoped a-Si:H Schottky barrier without introducing any ambiguous quantities. The experimental ideality factor, its dependence on temperature and voltage, and current density are quantitatively explained. By analyzing experimental results, the following behaviors are disclosed. The Fermi level of the interface states is significantly lower than the bulk Fermi level at low forward bias, but it approaches the bulk Fermi level with increasing forward-bias voltage. As for the reverse characteristics, the decrease of the barrier height is proportional to √V in the present sample for applied voltage V. For electrons in the interface states, the probability of tunnel transition to the metal is small compared with that of communication with the conduction band.
Metalorganic chemical vapor deposition of BaTiO3 thin films
Authors: Kwak B. S. , Zhang K. , Boyd E. P. , Erbil A. , Wilkens B. J.
Published: July 2002

We report the results of a recent study on the in situ deposition of single phase BaTiO3 thin films by using the metalorganic chemical vapor deposition technique. The effects of growth parameters on the characteristics of the thin films were investigated in the substrate temperature range of 550–800 °C. Barium β-diketonate and titanium isopropoxide were used as the metal sources. Growth rates ranging from submicron to several microns per hour have been deposited on various substrates in an inverted vertical warm-wall reactor. X-ray diffraction data and Rutherford backscattering spectra provided the evidence for single BaTiO3 phase. Scanning electron microscopy was used to study the surface and the cross-sectional morphology of the films. Dielectric constant of the as-deposited film was around 250 at room temperature. The resistivity of the films were greater than 109 Ω cm.
Conductivity perpendicular to the plane of multilayered structures
Authors: Zhang Shufeng , Levy Peter M.
Published: July 2002

Due to the inhomogeneous nature of multilayered structures, the measured conductivities for currents parallel and perpendicular to the layers are different, even if the local conductivity is isotropic. The perpendicular conductivity is derived and compared to the parallel conductivity which has been calculated previously. When our formalism is applied to Fe/Cr superlattices, interesting features of the magnetoresistance are found that can be verified by experiments.
The nucleation and growth of germanium on (11̄02) sapphire deposited by molecular beam epitaxy
Authors: Godbey D. J. , Twigg M. E.
Published: July 2002

Single-crystal germanium on (11̄02) sapphire films are grown after a substrate preanneal of 1400 °C and at growth temperatures above 700 °C. At a growth temperature of 800 °C, the nucleation site density was ∼1011 cm−2. For thin germanium films, the isolated islands were singly oriented, with single-crystal films obtained for thicker grown films. A 400 °C growth temperature on sapphire was insufficiently high to get epitaxial growth and produced polycrystallites. Growth at 400 °C on an 800 °C grown germanium template did result in epitaxy. However, a high fraction of the twinned orientation was produced, resulting in a bicrystalline film.
The interaction of a high irradiance, subpicosecond laser pulse with aluminum: The effects of the prepulse on x-ray production
Authors: Cobble J. A. , Schappert G. T. , Jones L. A. , Taylor A. J. , Kyrala G. A. , Fulton R. D.
Published: July 2002

The conversion efficiency into kilovolt line radiation for 248-nm light at 1017 W/cm2 on an aluminum target is measured. The x-ray yield is found to increase with the scale length of the target plasma. The interaction is modeled as resonance absorption, and the plasma scale length is determined from the prelase energy and irradiance.
Strong magneto-optical enhancement in highly Ce-substituted iron garnet films prepared by sputtering
Authors: Gomi M. , Furuyama H. , Abe M.
Published: July 2002

Faraday rotation and ellipticity spectra were measured at photon energies hν=0.9–4.0 eV for R3−xCexFe5O12 (R=Y, Gd, Yb) films with substitutions up to x=2.5, expitaxially grown by rf sputtering. The Ce substitution prominently enhances the Faraday effect not only in the IR region at hν=1.4 eV but also in the UV region at hν=3.1 eV. Both transitions have a paramagnetic dispersion relation, which are nearly equal in magnitude but opposite in sign. Optical absorption is also induced by a new transition due to Ce3+ at hν=2.1 eV, which does not enhance Faraday rotation. These electronic transitions may be attributed to charge transfer transition Ce3+(4f)-Fe3+(tet.).
Microwave absorption by a lossy dielectric sphere in a rectangular cavity
Authors: Jackson H. W. , Barmatz M.
Published: July 2002

A new theory of absorption of microwave power by a lossy dielectric sphere in a resonant rectangular chamber is developed here. In this treatment, resonant modes of the cavity are resolved into traveling plane-wave fields, and Mie’s theory of scattering and absorption of a plane electromagnetic wave by a sphere is utilized. The fields in the cavity containing the sphere are found by superposition of results for the individual scattering problem for each traveling plane wave. Absorbed power is then calculated by evaluating the time-averaged Poynting vector and integrating over the surface of the sphere. Applications of microwave power absorption are discussed, with particular emphasis on processing of materials. Numerical studies based on the newly derived formulas are presented. The results exhibit the manner in which microwave absorption is influenced by a number of different experimentally controllable parameters. The examples considered were chosen with a view toward designing experiments and apparatus for containerless processing of materials in the microgravity environment of space.
Generation and laser diagnostic analysis of bismuth fluoride
Authors: Bohn M. J. , Berst D. C. , Dorko E. A.
Published: July 2002

Bismuth fluoride, BiF, a possible energy acceptor molecule in a chemically pumped donor–acceptor electronic transition laser has been produced at number densities exceeding 1013 molecules/cc in a flow tube reactor. A high voltage discharge was used to produce bismuth and fluorine atoms which combined in a three-body collision to form ground state BiF (X1). Laser induced fluorescence studies confirmed that BiF was formed following the discharge. A vibrational temperature of 581±35 K and rotational temperatures of 340 to 370 K were measured for BiF (X1). Laser absorption measurements were performed to obtain the number density. An absorption cross section, calculated from the physical properties of BiF, was used for the determination.
Vacancy supersaturation model for electromigration failure under dc and pulsed dc stress
Authors: Clement J. J.
Published: July 2002

Time-dependent vacancy concentration profiles are calculated numerically as solutions to the electromigration transport equation under dc and pulsed dc current stress conditions. An electromigration failure model based on the time to reach a critical vacancy supersaturation to initiate void formation agrees well with several reported experimental observations. In a line of finite length l carrying a constant current density j the electromigration-induced vacancy concentration buildup at a blocking boundary saturates with time which implies a threshold value of jl is required to initiate failure. The vacancy buildup as a function of time has a j−2 current density dependence below saturation. Under pulsed dc stress the level at which vacancy concentration saturates depends on duty ratio r; hence, there is a threshold value of r required to initiate failure as well. The vacancy buildup with time is found to be proportional to r−2 below saturation.
Processing and electrical properties of Pb (ZrxTi1−x)O3 (x=0.2–0.75) films: Comparison of metallo-organic decomposition and sol-gel processes
Authors: Klee M. , Eusemann R. , Waser R. , Brand W. , van Hal H.
Published: July 2002

The deposition of thin Pb(ZrxTi1−x)O3 films by sol-gel and by metallo-organic decomposition (MOD) processes has been studied. Powders obtained from different precursor solutions were analyzed with respect to their decomposition and crystallization. Using a spin-coating technique, Pb(ZrxTi1−x)O3 films with zirconium concentrations ranging from x=0.2 to 0.75 have been deposited on Pt-electroded Si wafers. The lattice constants of the perovskite films and their dielectric and ferroelectric properties (permittivity, remanent polarization, coercive field strength) have been measured as a function of the zirconium concentration. The results are compared to the data obtained on bulk ceramics. For Pb(ZrxTi1−x)O3 films with a composition located at the morphotropic phase boundary (x≊0.53) the influence of the processing and the lead excess of the starting solutions on the film properties was examined. First measurements on the resistance degradation of thin Pb(Zr0.53Ti47)O3 films deposited by the MOD process are reported.