bluuebunny/crossref_metadata_2025 huggingface dataset:
An improved Helmholtz coil and analysis of its magnetic field homogeneity
Authors: Wang Jian , She Shouxian , Zhang Sijiong
Published: July 2002

We attempt to improve the Helmholtz coil by adding a third coil with the same radius connected in series. In this three-coil array, the distance from the outside coils to the central coil is 0.760 times radius of the coils, and the ratio of the number of turns, central to the outside, is exactly 59 to 111. Numerical results demonstrate that the uniform region of magnetic field in the improved Helmholtz coil is larger than that of the Helmholtz coil and of Maxwell’s tricoil.
Chaotic and fractal properties of deterministic diffusion-reaction processes
Authors: Gaspard P. , Klages R.
Published: July 2002

We study the consequences of deterministic chaos for diffusion-controlled reaction. As an example, we analyze a diffusive-reactive deterministic multibaker and a parameter-dependent variation of it. We construct the diffusive and the reactive modes of the models as eigenstates of the Frobenius–Perron operator. The associated eigenvalues provide the dispersion relations of diffusion and reaction and, hence, they determine the reaction rate. For the simplest model we show explicitly that the reaction rate behaves as phenomenologically expected for one-dimensional diffusion-controlled reaction. Under parametric variation, we find that both the diffusion coefficient and the reaction rate have fractal-like dependences on the system parameter.
Two-dimensional global manifolds of vector fields
Authors: Krauskopf Bernd , Osinga Hinke
Published: July 2002

We describe an efficient algorithm for computing two-dimensional stable and unstable manifolds of three-dimensional vector fields. Larger and larger pieces of a manifold are grown until a sufficiently long piece is obtained. This allows one to study manifolds geometrically and obtain important features of dynamical behavior. For illustration, we compute the stable manifold of the origin spiralling into the Lorenz attractor, and an unstable manifold in ζ3-model converging to an attracting limit cycle.
Mass densities of amorphous Co-rich FeCo-SiB alloys
Authors: Kohmoto Osamu , Ohya Kazuo
Published: July 2002

The mass density D, the atom number per unit volume ρ0 and the packing fraction η were studied for amorphous (FeCo) 100−y(SizB1−z)y alloys having zero magnetostriction in the range of 20≤y≤30 and 0≤z≤0.6. The variations of D and ρ0 with y and z are understood in terms of the atomic weight and the atomic radius of each atom. η is observed to increase with decreasing Si/(Si+B) ratio. This is attributed to the difference of the site occupation between B and Si atoms.
Low-temperature heat capacities of Ni3Al
Authors: Ho James C. , Liang Royce C. , Dandekar D. P.
Published: July 2002

Heat capacities of two single-crystal Ni3Al samples have been measured between 2 and 14 K. The data can be well fitted to the sum of a lattice and an electronic term, without a magnetic cluster contribution as previously reported by de Dood and de Chatel. The resulting Debye temperature values are comparable to those derived from elastic constant measurements, but the calorimetrically determined density of states at the Fermi level differs appreciably from that based on band-structure calculations.
One-dimensional diffusion model for extended solid solution in laser cladding
Authors: Kar A. , Mazumder J.
Published: July 2002

A mathematical model is presented for determining the composition of extended solid solution formed due to rapid cooling in laser cladding. This model considers a diffusion mechanism for mass transport in a one-dimensional semi-infinite molten pool of the cladding material from which heat is removed by conduction through a one-dimensional semi-infinite solid substrate. The rate of solidification has been obtained by modeling the cooling process as a composite medium heat transfer problem. The discontinuity of the concentration field has been simulated using a nonequilibrium partition coefficient, and then a nonsimilar exact solution for the mass transport equation has been obtained using a set of similarity variables which has been derived using Lie group theory.
Characterization of Si-SiO2 interface traps in p-metal-oxide-semiconductor structures with thin oxides by conductance technique
Authors: Hung K. K. , Cheng Y. C.
Published: July 2002

There has been a substantial effort made on the application of Nicollian–Goetzberger’s conductance technique to probe the Si-SiO2 interface traps on n-type substrates. However, it was reported that conductance measurement on the p-type substrate was impossible due to the strong surface potential fluctuations. By using metal-oxide-semiconductor (MOS) capacitors with thin (88–434 Å) oxides to damp out the fluctuations arising from the interface charge inhomogeneities, it is possible to carry out an accurate conductance measurement on as-oxidized p-MOS capacitors. A systematic dependence of the interface trap density on the oxide thickness and oxidation temperature is observed. The hole capture cross sections have no obvious dependence on the process conditions, but show an exponential dependence on the energy. Both the magnitude and bias dependence of the measured time-constant dispersion parameters are found to be much larger than those theoretically predicted. Results of numerical simulation show that the anomalous width of the conductance peaks observed for p-MOS structures is caused by the strong energy dependence of the hole capture cross sections, which has been overlooked in the conventional theory of MOS conductance.
Silicon surface cleaning by low dose argon-ion bombardment for low-temperature (750 °C) epitaxial deposition. II. Epitaxial quality
Authors: Garverick L. M. , Comfort J. H. , Yew T. R. , Reif R. , Baiocchi F. A. , Luftman H. S.
Published: July 2002

Argon-ion bombardment has been optimized as a method to clean silicon substrates prior to epitaxial deposition at low temperatures (750 °C) and very low pressures (<1×10−2 Torr). Structural characterization of subsequently deposited films has been performed to establish the relations between cleaning conditions and film properties. When cleaning conditions are insufficient, a residue of native oxide remains at the interface and disrupts the transfer of crystalline structure from the substrate to the epitaxial layer. When cleaning conditions are excessive, the substrate is damaged and the initial stages of film nucleation and growth are disrupted. Using an elevated substrate temperature of 750 °C, a window in ion energy and dose has been found in which native oxide is removed without net damage being produced in the substrate. Under these conditions, epitaxial films of excellent structural quality may be deposited, as evidence by transmission electron microscopy and defect etching.
Characterization of metal-oxide-semiconductor capacitors with a fast-ramp technique
Authors: Lou L. F. , Tettemer G. L.
Published: July 2002

The displacement current in a metal-oxide-semiconductor (MOS) capacitor under fast-ramp conditions has been measured in the accumulation, depletion, and avalanche regimes. From the experimental displacement current, a self-consistent numerical method has been developed to extract electrical parameters, including total MOS capacitance and stored charge as well as capacitances and voltages across the oxide and the semiconductor. These electrical parameters provide insight regarding dynamic charge properties. The fast-ramp technique is demonstrated to be a tool for obtaining an enhanced understanding of MOS device physics. The displacement current (i) is discussed in terms of the voltage ramp rate (dV/dt) and the rate of change of the surface potential (dφs/dt), i.e., i=CoxdV/dt−Coxdφs/dt, where Cox is the oxide capacitance. An analysis is performed to show that an MOS device can experience the three situations of undershielding, overshielding, and perfect shielding. This analyis leads to an explanation of experimentally obtained fast-ramp MOS properties.
Mapping the orthorhombic-to-tetragonal transition at ambient temperature in YBa2Cu3Oy ceramics
Authors: Kistenmacher Thomas J.
Published: July 2002

The orthorhombic-to-tetragonal transition at ambient temperature in YBa2Cu3Oy ceramics is empirically mapped as a function of oxygen content (y) based on published neutron and x-ray diffraction studies. On approaching the transition region (y∼6.5) from the orthorhombic phase, there is a noticeable anomaly in the c-axis length, a smooth evolution of the b axis, and a rather abrupt transition of the a axis (the axis along which oxygen deficiency obtains at larger values of y in the orthorhombic phase). Finally, the empirical method developed here for ascertaining oxygen content from crystallographic data is compared to neutron, hydrogen reduction, and iodometric titration results for several metastable YBa2Cu3Oy ceramics and Fe- and Co-doped materials.
Evidence for the electron traps at dislocations in GaAs crystals
Authors: Wosiński Tadeusz
Published: July 2002

A new electron trap, labeled ED1, with an activation energy of 0.68 eV has been found to appear in the deep-level transient spectroscopy spectrum of n-type GaAs crystals as a result of their plastic deformation at 400 °C. The trap has been systematically investigated taking into account broadening of its energy level. The trap concentration was proportional to the dislocation density and not affected by the post-deformation annealing at 500 °C. The concentration of electrons captured at the trap was found to depend logarithmically on the duration time of the filling pulse and an acceptor character of the trap was established. The results are analyzed in a model involving barrier-limited capture rate, and it is concluded that the ED1 traps are most probably located in the dislocation core. Possible types of dislocations responsible for the traps are discussed.
Large Kerr effects in transparent encapsulated liquid crystals
Authors: Sansone Michael J. , Khanarian Garo , Leslie Thomas M. , Stiller Marc , Altman Joseph , Elizondo Philip
Published: July 2002

The encapsulation of liquid crystals in a polymer matrix produces a material with interesting and potentially useful optical properties. The earliest efforts generated a dispersed phase whose dimensions were comparable to optical wavelengths and therefore afforded materials which were highly scattering. These materials have already been found useful for display applications. We have found that when these materials are fabricated so that the dimensions of the dispersed phase are substantially smaller than the incident radiation wavelengths, the material is far less scattering. dc Kerr measurements on these liquid-crystal composites yield quite large values, ranging as high as 30 000 times CS2 This activity is shown to be a strong function of the liquid-crystal loading and the temperature.
Indium phosphide on silicon heteroepitaxy: Lattice deformation and strain relaxation
Authors: Wuu D. S. , Horng R. H. , Lee M. K.
Published: July 2002

Lattice deformation and strain relaxation in epitaxial InP directly on (001) Si are studied as a function of layer thickness using x-ray diffraction and photoluminescence (PL) techniques. The heteroepilayers were grown by low-pressure organometallic vapor-phase epitaxy and showed good quality. We find that mismatch-induced compressive strains are still present in InP layers with a thickness less than 1 μm. The rate of strain release is much lower than the prediction based on the equilibrium theory. With increasing thickness above 1.1 μm, the InP/Si layers suffer in-plane tensile strains as a result of differential thermal contraction during the cooling process after growth. Fairly good agreement is found between the PL and x-ray data for the strain variations in the InP/Si heterostructures.
Growth of pinhole-free epitaxial yttrium silicide on Si(111)
Authors: Siegal Michael P. , Graham William R. , Santiago-Aviles Jorge J.
Published: July 2002

This paper reports the growth of pinhole-free epitaxial YSi2−x layers on Si(111) as thin as 30 Å. This has been accomplished by depositing both Y and Si at room temperature and then annealing to 500–900 °C. Use of the template method allows for the growth of thicker films also free of pinholes. Deposition of yttrium metal only onto Si(111) requires a temperature ∼300 °C for nucleation of the silicide reaction between the Y overlayer and Si substrate. Such a process creates small pinholes ∼500 Å in diameter, randomly distributed throughout the film. These pinholes increase in size with higher annealing temperature, resulting from a raised interface free energy intrinsic to the nucleation controlled growth.
Novel low-dimensional spin 1/2 antiferromagnets: Two-halide exchange pathways in A2CuBr4 salts
Authors: Zhou P. , Drumheller J. E. , Rubenacker G. V. , Halvorson K. , Willett R. D.
Published: July 2002

Magnetic susceptibility studies of several A2CuBr4 salts, where A is an organoammonium cation, are reported. These salts contain tetrahedrally distorted CuBr2−4 anions. Magnetic exchange pathways exist through interanionic Br⋅⋅⋅Br pathways. In this manner, various types of new antiferromagnetic spin 1/2 magnetic systems have been obtained. As examples, a one-dimensional chain, a coupled pair of chains or ‘‘ladder,’’ a two-dimensional square lattice, and a distorted ‘‘honeycomb’’ lattice are discussed.
Propagation of ultrasonic Lamb waves in piezoelectric plates
Authors: Joshi S. G. , Jin Y.
Published: July 2002

The propagation of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb wave modes in a piezoelectric plate is investigated both theoretically as well as experimentally. The characteristics of the modes and their relation to the surface acoustic wave is modified by electrical boundary conditions on the plate surfaces. It is found that when both surfaces are either metallized or unmetallized, the surface acoustic wave is obtained by a linear superposition of the A0 and S0 modes. On the other hand, if only one surface is metallized, then the surface acoustic wave is equivalent to just one of modes (either the A0 or S0 mode, depending on which surface is metallized). The beating phenomenon, whereby wave energy launched on one surface transfers periodically back and forth between opposite plate surfaces, is present when both surfaces are either metallized or unmetallized, but is absent if only one surface is metallized. Experimental measurements performed on Lamb wave devices fabricated on Y-cut, Z-propagating lithium niobate plates are found to be in good agreement with theoretical calculations.
Phase formation in Cu-Si and Cu-Ge
Authors: Hong Stella Q. , Comrie Craig M. , Russell Stephen W. , Mayer James W.
Published: July 2002

Phase formation and growth kinetics have been investigated with lateral diffusion couples in Cu-Si and Cu-Ge systems. Analytical electron microscopy was used to determine the crystal structures and chemical compositions of the growing phases. Cu3Si is found to be the dominant phase in the Cu-Si system. The growth of the silicide follows a (time)1/2 dependence with an activation energy of 0.95 eV in the temperature range of 200–260 °C. Cu3Ge is the only phase observed in Cu-Ge lateral diffusion couples with its length up to 20 μm. The growth of Cu3Ge is a diffusion controlled process at a rate similar to that of Cu3Si. The activation energy of Cu3Ge growth is 0.94 eV at 200–420 °C. In Cu-silicide or Cu-germanide formation, Cu appears to be the dominant diffusing species.
A high-power two stage traveling-wave tube amplifier
Authors: Shiffler D. , Nation J. A. , Schachter L. , Ivers J. D. , Kerslick G. S.
Published: July 2002

Results are presented on the development of a two stage high-efficiency, high-power 8.76-GHz traveling-wave tube amplifier. The work presented augments previously reported data on a single stage amplifier and presents new data on the operational characteristics of two identical amplifiers operated in series and separated from each other by a sever. Peak powers of 410 MW have been obtained over the complete pulse duration of the device, with a conversion efficiency from the electron beam to microwave energy of 45%. In all operating conditions the severed amplifier showed a ‘‘sideband’’-like structure in the frequency spectrum of the microwave radiation. A similar structure was apparent at output powers in excess of 70 MW in the single stage device. The frequencies of the ‘‘sidebands’’ are not symmetric with respect to the center frequency. The maximum, single frequency, average output power was 210 MW corresponding to an amplifier efficiency of 24%. Simulation data is also presented that indicates that the short amplifiers used in this work exhibit significant differences in behavior from conventional low-power amplifiers. These include finite length effects on the gain characteristics, which may account for the observed narrow bandwidth of the amplifiers and for the appearance of the sidebands. It is also found that the bunching length for the beam may be a significant fraction of the total amplifier length.
Relationships between the plasma environment and the composition and optical properties of plasma-polymerized thin films produced in rf discharges of C2H2-SF6 mixtures
Authors: Durrant Steven F. , Mota Rogério P. , Bica de Moraes Mário A.
Published: July 2002

Polymer films were grown in rf discharges containing different proportions of C2H2 and SF6. Quantitative optical emission spectrometry (actinometry) was used to follow the trends in the plasma concentrations of the species H and F, and more tentatively, of CH, CF, and CF2, as a function of the feed composition. Infrared spectroscopy revealed the density of CH and CF bonds in the deposited material. As the partial pressure of SF6 in the feed was increased, the degree of fluorination of the polymer also rose. The form of the dependency of the deposition rate on the proportion of SF6 in the feed was in good qualitative agreement with the activated growth model. From transmission ultraviolet visible spectroscopy data the refractive index and the absorption coefficient of the polymers were calculated as a function of the deposition parameters. Since the optical gap depended to some extent upon the degree of fluorination, it could, within limits, be determined by a suitable choice of the proportion of SF6 in the feed. A qualitative explanation of this relationship is given.
Interface states generated by heat treatment in Au/InGaP Schottky diodes
Authors: Chae H. J. , Kim C. H. , Kwon S. D. , Lee J. B. , Choe B. D. , Lim H. , Lee Hyung Jae
Published: July 2002

This work is a study of the formation of interface traps in Au/n−In0.5Ga0.5P contacts. The effects of heat treatment near the ohmic alloying temperature on the characteristics of the Schottky diodes are studied using current-voltage (I-V), capacitance-voltage (C-V), and deep level transient spectroscopy measurements. New interface states that are distributed around 0.73 eV below the conduction band minimum were generated by heat treatment above 350 °C before metallization. In a sample that was heat treated at 400 °C for 30 min, the maximum density of generated interface states was estimated to be approximately 2×1011 cm−2 eV−1. The origin of these interface states is attributed to the transformation of a phosphorus vacancy that is generated by the vaporization of phosphorus from the surface of In0.5Ga0.5P.
The formation of jets from shaped charges in the presence of asymmetry
Authors: Brown J. , Curtis J. P. , Cook D. D.
Published: July 2002

The jet from a nominally axisymmetric shaped charge is formed by the collapse of the typically conical liner under the high pressures resulting from detonation of the explosive around it. Where asymmetries are present the velocities imparted to the elements of the liner at a given axial distance from the cone tip will vary as a function of the azimuthal angle among other variables. In general these elements will not meet at all. Conversely a pair of liner elements that do meet will in general have started from different axial positions, and will meet away from the nominal axis of symmetry of the charge. This effect renders the formation process asymmetrical. Consequently the jet particles have an off-axis velocity component which can substantially degrade the penetrative capability of the charge. A complete analysis of asymmetrical liner collapse and jet formation is presented for the case where the liner is axisymmetric, but there is an azimuthal variation in liner projection velocity. The classical analysis of symmetric liner collapse and jet formation is extended to include this type of asymmetry. Earlier work on linear shaped charges, particularly a model for asymmetric jet formation, is drawn upon to consider plane sections of the liner individually. A fully three-dimensional analysis is then achieved by the combination of the results for these sections. This analysis is implemented in a computational model and example results are presented and compared in as far as is possible and meaningful with previous theoretical and experimental results. Good agreement with the previous theoretical work is obtained, and the values of the off-axis velocity components predicted are of the orders observed in the limited experimental data as yet available.
Effects of anneal ambients and Pt thickness on Pt/Ti and Pt/Ti/TiN interfacial reactions
Authors: Olowolafe J. O. , Jones R. E. , Campbell A. C. , Hegde R. I. , Mogab C. J. , Gregory R. B.
Published: July 2002

Using x-ray diffraction, Rutherford backscattering spectrometry, Auger electron spectroscopy, and scanning electron microscopy, effects of ambient anneal and Pt thickness on the interdiffusion of Pt/Ti bilayers deposited on SiO2/Si and on reactively sputtered TiN/Ti/Si substrates have been investigated. The Pt layer was 2000 or 930 Å thick while the Ti thickness was fixed at 1000 Å. The wafers were annealed in either O2 or N2 ambients or in N2 followed by O2, with temperatures ranging from 600 to 800 °C for 30 min. The anneal ambients and the thickness of Pt relative to the Ti layer had tremendous effects on the interdiffusion processes, the reaction products, and the surface morphology of the multilayer structures. Samples annealed in O2 were generally rough, with bumps of the order of 1000 Å, while those annealed in N2 were relatively smooth. With lead zirconium titanate on top, the surface of Pt/Ti electrode annealed in O2 at 650 °C was relatively smooth.
Photoacoustic investigation of the thermal properties of layered materials: Calculation of the forward signal and numerical inversion procedure
Authors: Glorieux C. , Fivez J. , Thoen J.
Published: July 2002

The structure of layered materials or substances with depth-dependent properties is often difficult to investigate with conventional optical or acoustical methods. Since the photoacoustic technique is based on thermal waves, which have other transmission and reflection properties than optical or acoustical waves, it can provide information on samples for which other methods fail. In this work, a straightforward numerical calculation of the photoacoustical signal for samples with a one-dimensional layered optical and thermal structure is described. An inversion procedure, based on the nonlinear least-squares fit routine minuit is shown to be very effective for obtaining depth information from the frequency dependence of the photoacoustic signal.
Origin of biquadratic exchange in magnetic multilayers (invited)
Authors: Slonczewski J. C.
Published: July 2002

We postulate localized-electron states with unpaired spin located within or at the interfaces of an otherwise nonmagnetic metallic spacer layer. Loose exchange coupling of these spins to two ferromagnets mediates a non-Heisenberg exchange coupling between them which includes a biquadratic term. A particular version of the model assumes that each interfacial layer of magnetic atoms is weakly exchange coupled to the remainder of the ferromagnets. This model permits interpretation of the biquadratic-coupling data of Gutierrez et al. for Fe/Al/Fe trilayers and of Fuss et al. for Fe/Au/Fe. According to this interpretation, the observed biquadratic coupling is intrinsic to the ideal multilayer structure rather than due to impurities or structural defects.
Quantitative study of the contribution of deep and shallow levels to the compensation mechanisms in annealed InP
Authors: Hirt G. , Wolf D. , Müller G.
Published: July 2002

We have studied the effects of heat treatment under phosphorus atmosphere on the balance of electronic levels by capacitance-voltage and deep level transient spectroscopy measurements. A series of special samples was annealed under the conditions which we are normally using for the processing of nominally undoped semi-insulating (S.I.) InP. It is shown explicitly that in this annealing process the reduction of the free-carrier concentration is predominantly caused by a reduction of the net concentration of defects related to shallow levels. Furthermore, we have identified in the annealed material two defects related to electron traps with activation energies of 400 and 600 meV, which are created or incorporated during the annealing with limited concentrations of about (0.5–1)×1015 cm−3. On the basis of these results we conclude that for the compensation mechanism in the annealed nominally undoped S.I. InP only a concentration below 1015 cm−3 of defects with a midgap energy level is necessary.
Effect of local flaws in polymeric permeation reducing barriers
Authors: Rossi Giuseppe , Nulman Mark
Published: July 2002

Thin polymeric coating layers are often used to improve the barrier properties of packaging structures and container walls. We study how local flaws in the coating layers affect the overall permeation properties of such systems. We find that many small ‘‘holes’’ in a barrier layer are much more effective in compromising the system barrier properties than a few large ‘‘holes’’ with the same total area.
Monte Carlo simulation of electron transport in gallium nitride
Authors: Gelmont B. , Kim K. , Shur M.
Published: July 2002

The results of an ensemble Monte Carlo simulation of the electron transport in gallium nitride (GaN) are presented. The calculation shows that intervalley electron transfer plays a dominant role in GaN in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. An analytic expression for the polar optical momentum relaxation time for phonon energies larger than the thermal energy is also derived. This expression applies to many wide-gap semiconductors, such as GaN and SiC, at room temperature since these semiconductors have large polar optical-phonon energies (on the order of 100 meV). The calculated mobility agrees well with the results of the Monte Carlo calculation.
Second-order optical nonlinearity in corona-poled glass films
Authors: Okada A. , Ishii K. , Mito K. , Sasaki K.
Published: July 2002

Several experiments were performed to elucidate the origin of the second-order optical nonlinearity χ(2) induced in the corona-poled Corning 7059 glass films. The values and the time decay of the second-harmonic coefficient d33 were measured for the glass films poled on several different kinds of substrates. The value of d33 was found to be dependent on the kinds of substrates. It was also observed that the glass films poled on a soda-lime substrate showed faster time decay of d33 than those poled on a Pyrex glass substrate. The glass films were also poled at room temperature, and the values of d33 were measured as a function of poling time. As a result, the value of d33 increased with the poling time and approached a value which was close to that obtained for the glass films poled at 100 °C. From the experimental results, it is speculated that the space charge formed in the vicinity of the interface between the glass film and the substrate contributes to the creation of the χ(2). The planar charge density at the glass-film–substrate interface is derived, and the experimental results are discussed.
Energetic model of ferromagnetic hysteresis
Authors: Hauser Hans
Published: July 2002

Calculations of a total energy function are used to describe a theory of hysteresis effects in crystalline, ferromagnetic materials. This model is applied on the cubic structure of FeSi crystals, which are oriented in a (110)[001] texture. The magnetic energy of the crystal is separated in reversible and irreversible parts: The reversible energy is expressed by magnetocrystalline anisotropy and shape anisotropy. They are responsible for the rotation of the domain magnetization at strong fields. At weak fields the reversible interaction of the domain wall motion with the stray fields of pinning centers (nonmagnetic inclusions, grain boundaries or inner strains) is described by a probability function of statistic domain behavior. The irreversible energy is caused by these pinning centers, too, and can be explained by the interaction losses of the magnetic moments in the Bloch wall with the crystal lattice during an irreversible Barkhausen jump. The energy of the applied field is added to these two parts and the magnetic state of the material is represented by the minima of the total energy function. The results show all features of hysteresis such as initial magnetization curve and major and minor hysteresis loops in good agreement with measurements on grain oriented 3,5% silicon-steel with Goss texture.
Large Kerr effects in transparent encapsulated liquid crystals. II. Frequency response
Authors: Sansone Michael J. , Khanarian Garo , Kwiatek Mark S.
Published: July 2002

The encapsulation of liquid crystals in polymer matrices produces materials with interesting electro-optical and scattering properties. When the dispersed phase dimensions are substantially smaller than the incident radiation wavelengths, the material is optically clear with a large Kerr effect. These materials had been shown to behave as orientational Kerr media with Kerr constants as high as 30 000 times CS2 [J. Appl. Phys. 67, 4253 (1990)]. In this paper, the frequency response is studied with sinusoidal and pulsed electric fields. The electroactivity rolls off at higher frequencies with the 3 dB point in the range of 2000–12 000 Hz depending on liquid crystal loading and temperature. This indicates response times are in the microsecond range which is consistent with the pulsed field data.
Theoretical analysis for diffusion of interstitial atoms in crystal
Authors: Wu Hongcai
Published: July 2002

A new theory for analysis of diffusion of interstitial atoms in crystal is presented. The foundation of the theory is based on the fact that an interstitial atom experiences the action of a cyclic potential of lattice atoms when it moves along a channel of crystal. An equation for the motion of the interstitial atom is derived from this hypothesis. According to the characteristics of solutions of the equation, the diffusing process of interstitial atoms along the crystal axis 〈hkl〉 are described. Formulas for calculation of the openness of a channel in crystal for diffusion of interstitial atoms and diffusivity are obtained. The calculated results obtained by using this theory are close to those of experiments.
Relaxation and crystallization kinetics of amorphous germanium films by nanosecond laser pulses
Authors: Vega F. , Serna R. , Afonso C. N. , Bermejo D. , Tejeda G.
Published: July 2002

Relaxation and crystallization of amorphous germanium films on silicon are induced by nanosecond laser pulses. Real time reflectivity measurements and Raman spectroscopy show that amorphous regrowth occurs upon melting and rapid solidification of the film because the thermal conductivity of the silicon substrate is high enough to extract the laser energy absorbed by the film in a very efficient way. The amorphous regrown film is in a relaxed state when compared to the as-grown amorphous material. Further pulses induce fast crystallization of the film. An increase of the melting threshold is found upon relaxation and crystallization of the film.
Analytical studies on multiple delta doping in diamond thin films for efficient hole excitation and conductivity enhancement
Authors: Kobayashi Takeshi , Ariki Takuya , Iwabuchi Mamoru , Maki Tetsuro , Shikama Shozo , Suzuki Sei
Published: July 2002

Beneficial features of boron multiple delta doping (MDD) in synthetic diamond thin films are studied analytically and compared with MDD in GaAs. In spite of a deep boron level (∼0.3 eV), MDD greatly helps the thermal excitation of holes via elevation of the Fermi level toward the acceptor boron level. Thus, a hole excitation 6–7 times higher than that of the uniformly doped one is obtained. Furthermore, more than 90% of holes are in the spacer layer (i-diamond) where the mobility is high, resulting in a film conductance of the MDD structure more than 20 times higher than that of the conventional one when the same amount of boron is uniformly doped.
Effective-medium theory of piezoelectric composites
Authors: Nan Ce-Wen
Published: July 2002

Based on the developed multiple-scattering theory [C.-W. Nan and F. S. Jin, Phys. Rev. B 48, 8578 (1993)], an effective-medium theory (EMT) is proposed to treat coupled electromechanical behavior in composite media. The explicit relations for determining effective behavior of piezoelectric composites are derived. To illustrate the technique, numerical results of piezoelectric ceramic/epoxy composites for various particle shapes are presented over the whole range of concentrations. The EMT estimates are shown to be in good agreement with available experimental results. Porous piezoelectric ceramics are also discussed. The EMT predicts a similar critical behavior for heterogeneous piezoelectric materials to recent experiments.
Raman scattering from hydrogenated amorphous carbon films
Authors: Vuppuladhadium Rama , Jackson Howard E. , Wu Richard L. C.
Published: July 2002

Amorphous hydrogenated carbon films (a-C:H), as-deposited and modified by rapid thermal annealing, by implantation, and by laser irradiation were studied by Raman scattering. The ratio of carbon to hydrogen in each of the samples was determined by Rutherford backscattering (RBS) and proton recoil spectra. The hydrogen content was found to decrease with increasing annealing temperatures, with ion implantation, and with laser irradiation. The Raman spectra were analyzed to obtain peak position and linewidth of the D and G lines of the disordered graphite and the intensity ratio (ID/IG) of these lines correlated with the C/H ratio obtained from RBS and proton recoil spectra. These measurements indicate that with increasing annealing temperature, with ion implantation, and with the laser radiation, the a-C:H films are modified from less tetrahedrally bonded to more trigonally bonded; the a-C:H films become more graphitic.
Free electron laser gain: Approximant forms and inclusion of inhomogeneous broadening contributions
Authors: Dattoli G. , Ottaviani P. L. , Segreto A. , Altobelli G.
Published: July 2002

The free electron laser gain formulae for low-, intermediate-, and high-gain domains are parametrized by means of a simple expression reproducing linear and exponential regimes. The proposed reformulation provides a useful tool of analysis because of its remarkable simplicity and because it offers the possibility of deriving comparably simple gain formulae that include inhomogeneous broadening effects.
Graphoepitaxy of CeO2 on MgO and its application to the fabrication of 45° grain boundary Josephson junctions of YBa2Cu3O7−x
Authors: Copetti C. A. , Schubert J. , Klushin A. M. , Bauer S. , Zander W. , Buchal Ch. , Seo J. W. , Sanchez F. , Bauer M.
Published: July 2002

We communicate a detailed study of the epitaxial growth of CeO2 on MgO. The key feature of the growth is the dependence of the in-plane orientation of the CeO2 epitaxial layer on the MgO surface morphology. Atomic force microscopic (AFM) measurements, x-ray analyses, as well as high-resolution transmission electron microscopy (HRTEM) investigations reveal that on rough substrates a cube-on-cube growth of CeO2 on MgO occurs while on smooth substrates the CeO2 unit cell is rotated around the surface normal by 45° with respect to the MgO unit cell when the deposition rate is low (∼0.3 Å/s) during the first stages of growth. This growth mechanism can be used for a defined fabrication of 45° grain boundaries in the CeO2 layer by controlling the surface roughness of the MgO substrate. This report demonstrates that these 45° grain boundaries may be used to fabricate YBa2Cu3O7−x Josephson junctions.
Role of alloying elements in the stability of nitrides in nitrogen- implanted α-Fe
Authors: Kopcewicz M. , Jagielski J. , Gawlik G. , Grabias A.
Published: July 2002

The role of alloying elements, such as Cr, Al, Ti, and Mn, in the formation and stability of the nitride phases formed due to nitrogen ion implantation into metallic α-iron was studied by conversion electron Mössbauer spectroscopy. The thermal stability of the nitride phases of α-Fe implanted with N was greatly increased by coimplanting with N the alloying elements. For instance, the stability of α′-martensite at low N-dose implantation is strongly enhanced and also that of γ′-Fe4N at high N dose. Titanium stabilizes the ε-Fe3−xN-type nitride preventing the formation of the γ′ phase. The presence of alloying elements influences the thermal stability of nitrides by increasing their binding energy and thus preventing the dissociation of nitrogen and also by forming traps that lead to the formation of TiN, AlN, or CrN nitrides. The thermal stability of nitrides, especially of ε-Fe3−xN and γ′-Fe4N ones, is improved in decreasing order by the presence of Ti, Al, Cr, and Mn.
Rear surface passivation of high-efficiency silicon solar cells by a floating junction
Authors: Altermatt Pietro P. , Heiser Gernot , Dai Ximing , Jürgens Jörn , Aberle Armin G. , Robinson Steven J. , Young Trevor , Wenham Stuart R. , Green Martin A.
Published: July 2002

The passivated emitter, rear locally diffused (PERL) cells, fabricated in our laboratory, reach an efficiency of 24.0%, the highest value for any silicon-based solar cell under terrestrial illumination. In an attempt to improve the rear surface passivation, which is usually obtained by a thermally grown oxide, we add a floating (i.e., noncontacted) p–n junction at the rear surface, resulting in the passivated emitter, rear floating p–n junction (PERF) cell design. Although these cells exhibit record 1-sun open-circuit voltages of up to 720 mV, their efficiency is degraded by nonlinearities (‘‘shoulders’’) in the logarithmic I–V curves. In order to understand and manipulate such nonlinearities, this paper presents a detailed investigation of the internal operation of PERF cells by means of numerical modelling based on experimentally determined device parameters. From the model, we derive design rules for optimum cell performance and develop a generalized argumentation that is suitable to compare the passivation properties of different surface structures. For example, the oxidized rear surface of the PERL cell is treated as an electrostatically induced floating junction in this approach and analogies to the diffused floating p–n junction are drawn. Our simulations indicate that optimum rear surface passivation can be obtained in three different ways. (i) The floating junction of the PERF cell should be very lightly doped, resulting in a sheet resistivity of 5000 Ω/⧠, and losses due to shunt leaking paths between the p–n junction and the rear metal contacts must be avoided. (ii) The rear surface of the PERL cell should be passivated by chemical vapor deposition of a silicon nitride film containing a larger positive interface charge density than exists in thermally grown oxides. (iii) An external gate can be added at the rear with low leakage currents and gate voltages of around 15 V.
Optical properties of GaN epilayers on sapphire
Authors: Tchounkeu Magloire , Briot Olivier , Gil Bernard , Alexis Jean Paul , Aulombard Roger-Louis
Published: July 2002

The optical properties of GaN epilayers grown by metal-organic vapor-phase epitaxy on (0001)-oriented sapphire are investigated by means of photoluminescence, reflectance, and differential spectroscopy. We obtain quantitative information about the intrinsic or extrinsic nature of the 2 and 300 K photoluminescence features. From detailed investigations of the reflectance properties of these layers we can quantify the residual strain field in these layers and determine the GaN deformation potentials. Comparison of these values with quantities measured on other semiconductors with wurtzite symmetry is also addressed. Last we utilize photoreflectance spectroscopy to measure exciton binding energies.
Hydrogen in amorphous and microcrystalline silicon films prepared by hydrogen dilution
Authors: Kroll U. , Meier J. , Shah A. , Mikhailov S. , Weber J.
Published: July 2002

Hydrogen incorporation in silicon layers prepared by plasma-enhanced chemical-vapor deposition using silane dilution by hydrogen has been studied by infrared spectroscopy (IR) and elastic recoil detection analysis (ERDA). The large range of silane dilution investigated can be divided into an amorphous and a microcrystalline zone. These two zones are separated by a narrow transition zone at a dilution level of 7.5%; here, the structure of the material cannot be clearly identified. The films in/near the amorphous/microcrystalline transition zone show a considerably enhanced hydrogen incorporation. Moreover, comparison of IR and ERDA and film stress measurements suggests that these layers contain a substantial amount of molecular hydrogen probably trapped in microvoids. In this particular case the determination of the total H content by IR spectroscopy leads to substantial errors. At silane concentrations below 6%, the hydrogen content decreases sharply and the material becomes progressively microcrystalline. The features observed in the IR-absorption modes can be clearly assigned to mono- and/or dihydride bonds on (100) and (111) surfaces in silicon crystallites. The measurements presented here constitute a further indication for the validity of the proportionality constant of Shanks et al. [Phys. Status Solidi B 110, 43 (1980)], generally used to estimate the hydrogen content in ‘‘conventional’’ amorphous silicon films from IR spectroscopy; additionally, they indicate that this proportionality constant is also valid for the microcrystalline samples.
Modeling the optical dielectric function of GaAs and AlAs: Extension of Adachi’s model
Authors: Rakić Aleksandar D. , Majewski Marian L.
Published: July 2002

Optical dielectric function model of Ozaki and Adachi [J. Appl. Phys. 78, 3380 (1995)] is augmented by introducing Gaussian-like broadening function instead of Lorentzian broadening. In this way a consistent and comparatively simple analytic formula has been obtained, which accurately describes the optical dielectric function of GaAs and AlAs in a wide spectral range between 0.1 and 6 eV. The acceptance-probability-controlled simulated annealing technique was used to fit the model to experimental data.
Structure and magnetism of hcp-Co fine particles
Authors: Sato H. , Kitakami O. , Sakurai T. , Shimada Y. , Otani Y. , Fukamichi K.
Published: July 2002

We have synthesized hcp-Co fine particles with the average diameter of less than 50 nm by sputtering Co in a somewhat high inert gas pressure. A nearly perfect hcp phase can be stabilized by increasing sputtering gas pressure and decreasing sputtering power in contrast to conventional preparation conditions. Magnetic measurements have revealed the clear relation of coercivity with the volume ratio of hcp phase. That is, the increase in the hcp phase considerably enhances the coercivity. Appearance of the hcp phase seems to be related to the promotion of allotropic phase transformation in Co initiated by lattice defects within each particle.
Magnetic behavior of nanostructured glass covered metallic wires
Authors: Chiriac H. , Óvári T. A. , Pop Gh. , Barariu Firuţa
Published: July 2002

We present a study of the evolution of the magnetic properties and behavior of Fe73.5Cu1Nb3Si13.5B9 glass covered wires and wires after glass removal with the annealing temperature up to 600 °C starting from the amorphous state. The changes induced in the magnetic properties of these wires are determined by the stress relief process occurring at temperatures below 550 °C, and by the appearance of the nanosized α-FeSi crystalline grains after annealing for 1 h at 550 °C. The nanocrystalline phase formation leads to an improvement of the soft magnetic properties of these wires—increase of permeability and decrease of the coercive force—but also determines the disappearance of the large Barkhausen effect presented by these wires in the amorphous state. Annealing at temperatures over 550 °C determines a depreciation of the soft magnetic properties of both glass covered wires and wires after glass removal. The magnetic behavior of such wires can be fully explained by taking into account the relaxation of the internal stresses with increasing the annealing temperature as well as the changes in the magnetostriction constant due to the appearance of the nanocrystalline grains.
Influence of Ba content on grain size and dynamics of crystallization in barium ferrite thin films
Authors: Chen Yingjian , Laughlin David E. , Ma Xiaoding , Kryder Mark H.
Published: July 2002

The effects of Ba content on the dynamics of the crystallization process, which ultimately determines the grain size, were studied in barium ferrite thin films. Rapid thermal annealing was used to crystallize the amorphous as-deposited barium ferrite films. The annealing time and temperature dependent crystalline volumes were determined from the magnetization of the films, and were analyzed by means of the Johnson–Mehl–Avrami relation. Microstructural studies showed that the films with higher Ba content had higher nucleation rates and lower growth rates; consequently, they had finer grain size and smaller grain aspect ratio. Magnetic measurements showed that the higher Ba content films also had higher coercivities and smaller magnetic switching volumes.
Analysis of the photothermal deflection technique in the surface reflection scheme: Theory and experiment
Authors: Bertolotti M. , Liakhou G. L. , Li Voti R. , Paoloni S. , Sibilia C.
Published: July 2002

The photothermal deflection technique has been usually applied, for the thermal diffusivity measurements, in the transverse skimming scheme. To overcome some limitations of the skimming, a surface reflection scheme (i.e., bouncing scheme) has been introduced in which the probe beam is reflected from the sample surface. In this configuration the probe beam deflection is obtained as a result of two different mechanisms: the thermal gradient in the gas near to the heated sample (mirage) and the sample surface deformation due to the thermal expansion (displacement). The superposition of these two effects must be taken into account when deriving the thermal diffusivity. In this article the mirage and the displacement have been studied from a theoretical and experimental point of view, and a new method for the measurement of thermal diffusivity in the bouncing scheme is presented. A special setup is described to obtain separately the mirage and the displacement signals from which the thermal diffusivity and the thermal expansion coefficient can be derived. The experimental values for different samples obtained by applying our method are in agreement with the literature values.
Sputter process diagnostics by negative ions
Authors: Zeuner Michael , Neumann Horst , Zalman Jan , Biederman Hynek
Published: July 2002

We measured the energy distributions of negative ions during reactive sputtering of silicon in oxygen. Various oxygen containing negative ions are formed in the cathode sheath or directly at the sputter target, respectively. These negative ions are accelerated away from the cathode by the electrical field, and can be detected using a mass spectrometer facing the sputter magnetron. The origin of each ion can be determined from peak structures in the energy distribution. Additionally the flux of different negative ions provides information on poisoning of the target by oxide films.