bluuebunny/crossref_metadata_2025 huggingface dataset:
Density functional study of wetting by polymers. I. Effects of polymer length and surface potential
Authors: Forsman J. , Woodward C. E. , Freasier B. C.
Published: July 2002

Wetting by a polymer fluid at an attractive surface is investigated using density functional theory. It is shown that the qualitative behavior of the wetting temperature, Tw, as a function of the polymer length, depends on the strength of the surface potential. Defining Tc as the bulk critical temperature, we find that the normalized wetting temperature, Tw/Tc, shows nonmonotonic dependence on polymer length. For weak surface potentials, Tw/Tc increases with polymer length and, in some cases, wetting seems to be suppressed. It is conjectured that this may occur via an encroaching drying transition.
Comparison of wear characteristics of etched-silicon and carbon nanotube atomic-force microscopy probes
Authors: Larsen T. , Moloni K. , Flack F. , Eriksson M. A. , Lagally M. G. , Black C. T.
Published: July 2002

The resolution and wear properties of carbon nanotube and etched-silicon atomic force microscopy probes are compared in intermittent-contact mode. Carbon nanotube probes have at least 20 times the life of etched-silicon probes and provide better resolution at all stages. Sample wear is minimized with carbon nanotube probes.
Thermal conductivity of Si/SiGe and SiGe/SiGe superlattices
Authors: Huxtable Scott T. , Abramson Alexis R. , Tien Chang-Lin , Majumdar Arun , LaBounty Chris , Fan Xiaofeng , Zeng Gehong , Bowers John E. , Shakouri Ali , Croke Edward T.
Published: July 2002

The cross-plane thermal conductivity of four Si/Si0.7Ge0.3 superlattices and three Si0.84Ge0.16/Si0.76Ge0.24 superlattices, with periods ranging from 45 to 300 and from 100 to 200 Å, respectively, were measured over a temperature range of 50 to 320 K. For the Si/Si0.7Ge0.3 superlattices, the thermal conductivity was found to decrease with a decrease in period thickness and, at a period thickness of 45 Å, it approached the alloy limit. For the Si0.84Ge0.16/Si0.76Ge0.24 samples, no dependence on period thickness was found and all the data collapsed to the alloy value, indicating the dominance of alloy scattering. This difference in thermal conductivity behavior between the two superlattices was attributed to interfacial acoustic impedance mismatch, which is much larger for Si/Si0.7Ge0.3 than for Si0.84Ge0.16/Si0.76Ge0.24. The thermal conductivity increased slightly up to about 200 K, but was relatively independent of temperature from 200 to 320 K.
Co-occurrence of magnetic and structural transitions in the Heusler alloy Ni53Mn25Ga22
Authors: Jiang Chengbao , Feng Gen , Xu Huibin
Published: July 2002

A co-occurrence of magnetic and structural transitions was observed in a nonstoichiometric Ni53Mn25Ga22 alloy which undergoes a reverse martensitic transformation from ferromagnetic martensite to paramagnetic austenite at a magnetic transition temperature (134 °C) higher than the Curie temperature of the stoichiometric Ni2MnGa alloy (103 °C). The effect of the magnetic field on the phase transition temperature was found to be two orders of magnitude greater in the present alloy than in Ni2MnGa due to the absence of the ferromagnetic state in austenite. This may open the possibility of utilizing NiMnGa alloys at a low magnetic field.
Volume-preserving maps with an invariant
Authors: Gómez A. , Meiss J. D.
Published: July 2002

Several families of volume-preserving maps on R3 that have an integral are constructed using techniques due to Suris. We study the dynamics of these maps as the topology of the two-dimensional level sets of the invariant changes.
An approach to transport measurements of electronic molecules
Authors: Amlani Islamshah , Rawlett Adam M. , Nagahara Larry A. , Tsui Raymond K.
Published: July 2002

We present a hybrid assembly technique to facilitate the transport measurements of electronic molecules. The technique consists of forming a self-assembled monolayer of the investigated molecule on prepatterned electrodes and then bridging the electrodes with nanoparticles using an alternating electric field. This technique can potentially provide a quick and simple way to screen a large number of electronic molecules. As an example, we report preliminary transport measurements of 1-nitro-2,5-di(phenylethynyl-4*-thioacetyl)benzene as a test molecule. The data show qualitative agreement with previously published results for a similar molecule.
Noncontact potentiometry of polymer field-effect transistors
Authors: Bürgi L. , Sirringhaus H. , Friend R. H.
Published: July 2002

We report on high-resolution potentiometry of operating organic thin-film field-effect transistors by means of scanning Kelvin probe force microscopy. It is demonstrated that the measured potential reflects the electrostatic potential of the accumulation layer at the semiconductor/insulator interface. We present data revealing gate bias and lateral electric field dependence of the field-effect mobility in poly(hexylthiophene) at temperatures from 50 to 300 K.
Translational dynamics of a cold water cluster in the presence of an external uniform electric field
Authors: Vegiri Alice
Published: July 2002

Molecular dynamics simulations for a TIP4P water cluster consisting of 32 molecules at T=200 K, under the influence of a broad range of constant electric fields (0.5–7.0×107 V/cm), are presented. This work focuses on the evolution of the single particle translational dynamics, mainly along the field axis as the field is progressively increased, by means of mean-square-displacement curves, the self-part of the van Hove distribution functions and the intermediate scattering functions. Two critical fields have been identified, the one, (EC1=1.5×107 V/cm) assigned to the onset of the dipole alignment and the second one (EC2=5.0×107 V/cm) to the onset of crystallization. These transitions are marked by an abrupt increase of the corresponding structure relaxation times, which remain nearly constant for electric fields between EC1 and EC2. Structure relaxation has been found to obey stretched exponential dynamics, whereas the Q dependence of the relaxation times, for all fields, followed a power law. Fields weaker than EC1 have been found to induce a weakening of the molecular interactions. In this case, the system develops a dynamic behavior similar to that met in the liquid.
A dynamical systems approach to spiral wave dynamics
Authors: Barkley Dwight , Kevrekidis Ioannis G.
Published: July 2002

A simple system of five nonlinear ordinary differential equations is shown to reproduce many dynamical features of spiral waves in two-dimensional excitable media.
The evolution of chemical patterns in reactive liquids, driven by hydrodynamic instabilities
Authors: Avnir D. , Kagan M. L.
Published: July 2002

We summarize our activity in unveiling a very wide phenomenon: When a chemical reaction takes place at a liquid interface, spectacular patterns of product form (see Plate 1). The pattern formation phenomenon is general, and is observed in reactions between liquids separated by a membrane, in liquids subjected to gaseous reactants, and in photoreactive liquids. We have demonstrated the phenomenon on over 100 different reactions of all types, thus discovering what we believe to be one of the widest macroscopic pattern formation processes known to chemistry. As can be seen in the accompanying pictures, the richness, beauty, and variations in types of patterns can be breathtaking. Two important aspects of these patterns are noted: First, the patterns are true far-from-equilibrium structures, which are maintained only as long as reactants are available, or only as long as light energy is supplied to the system; and second, the chemical products that form the patterns are not precipitates, but are entirely soluble in the liquid in which they form. Thus, if the containers in which the patterns form are shaken or stirred, a homogeneous solution results. Our research of this phenomenon concentrated on three main aspects. The first one was phenomenological. Here we explored the scope and generality of the phenomenon, motivated both by the aesthetic appeal of the phenomenon, and by the puzzle of how is it that such a wide-scope, experimentally simple phenomenon, has by and large, escaped the attention of the scientific community.

The second aspect was devoted to the understanding of the underlying general mechanism. Of the many mechanisms we analyzed and tested, some very complex, others quite trivial, the one that fits the majority of the physical and chemical observations is the following: By performing a reaction through a liquid interface, a concentration gradient of the product forms near the interface. We have shown that in many cases, these gradients lead to hydrodynamic instabilities, which then break nonlinearly into a pattern which onsets slow convections. In other words, we found that these patterns mark the route along which a chemical instability relaxes. The third aspect of our research was theoretical. Here we concentrated in depth on one of the reactions (the Fe+2/Fe+3 photoredox reaction), determined all its important physical parameters, and modeled its behavior theoretically. Our model, which was based on the instability buildup described above, was solved numerically, and its results compared with computerized image analysis of the evolving patterns; very good agreement between theory and experiment, was obtained.
Entropy estimation of symbol sequences
Authors: Schürmann Thomas , Grassberger Peter
Published: July 2002

We discuss algorithms for estimating the Shannon entropy h of finite symbol sequences with long range correlations. In particular, we consider algorithms which estimate h from the code lengths produced by some compression algorithm. Our interest is in describing their convergence with sequence length, assuming no limits for the space and time complexities of the compression algorithms. A scaling law is proposed for extrapolation from finite sample lengths. This is applied to sequences of dynamical systems in non-trivial chaotic regimes, a 1-D cellular automaton, and to written English texts.
Stable algorithm for the computation of Mie coefficients for scattered and transmitted fields of a coated sphere
Authors: Kaiser T. , Schweiger G.
Published: July 2002

The Mie coefficients and the electric-field vector inside a spherical, layered particle are presented in a form which is more suitable for computations. Difficulties in the computation occur especially for large, absorbing spheres, due to numerical instabilities in the computation of Besselfunctions with complex arguments. This can be avoided by rearranging the Mie coefficients, so that only ratios and logarithmic derivatives of Besselfunctions are used. A suitable analytical expression for the coefficients of the scattered and transmitted fields and the electric-field vector is given.
High-field electron drift velocity measurements in gallium phosphide
Authors: Johnson R. H. , Eknoyan O.
Published: July 2002

The drift velocity of electrons in gallium phosphide has been measured for electric fields ranging from 10 to 280 kV/cm. A novel variation on the conductivity technique was used. The resulting saturated drift velocity, ∼1.25×107 cm/sec, agrees with theoretical predictions.
Interfacial conditions for thermomechanical equilibrium in two-phase crystals
Authors: Johnson William C. , Alexander J. Iwan D.
Published: July 2002

A virtual variational approach is used to deduce the interfacial conditions for thermomechanical equilibrium in two-phase crystals separated by a curved interface. The interface is modeled as a Gibbsian dividing surface and the excess quantities which include the deformation gradients, entropy, and number density of components are accounted for explicitly. Conditions necessary for equilibrium are obtained for both a coherent and incoherent interface that incorporate the effects of surface stress. An example illustrating the use of these new interfacial conditions in establishing boundary conditions is given.
Studies of total ionization in gases/mixtures of interest to pulsed power applications
Authors: Reinking G. F. , Christophorou L. G. , Hunter S. R.
Published: July 2002

The energy required to produce an electron-ion pair (i.p.), W, for the α-particle irradiated gases CF4, C2F6, C3F8, and n-C4F10 has been measured and found to be 34.3, 34.5, 34.4, and 34.2 eV/ i.p., respectively. The large values of W for these perfluorocarbons (PFCs) are attributed to their large cross sections for electron impact-induced dissociation. The constancy of W for this group of molecules is discussed with respect to the constancy of the ratio (∼0.68) of the respective (energy integrated) total ionization cross section to the total (energy integrated) inelastic scattering cross section. W values are also reported for the binary gas mixtures Ar/CF4, Ar/C2F6, Ar/C3F8, and CH4/CF4 which have conduction and insulation properties suitable for use in diffuse discharge opening switches. The W values for these binary mixtures (and for C2H2/CF4, C2H2/C2F6, and C2H2/C3F8) increase with the amount of PFCs in the mixture and (contrary to the Ar/C2H2 mixture) exhibit no Jesse effect. The ternary gas mixtures Ar/C2H2/PFC (=CF4, C2F6, C3F8) containing ≲10% PFC were found to exhibit a pronounced Jesse effect which is a function of the content of the PFC in the mixture and the electron scattering cross section of the PFC itself. Substantial increases in ionization can be achieved by adding to the Ar/PFC binaries 1–3% of C2H2. An analysis of the W data yielded information on the partition of the α particle’s energy among the gas components and indicated that the rate constant for energy transfer from excited argon atoms (Ar*) to the PFC is smaller than that from Ar* to C2H2; it showed, also, that the rate constant for energy transfer from Ar* to C2H2 is ∼104 larger than that for collisional deexcitation of Ar* by ground-state Ar atoms. The quantitative measurements reported herewith and the parameters deduced from a simple modeling of the data permit the tailoring of ternary gas mixtures with substantial current increases over those achieved in the binary Ar/PFC (=CF4, C2F6, C3F8) mixtures which possess the desirable conduction and insulation characteristics for pulsed power opening switches.
Photothermal waves in anisotropic media
Authors: Iravani M. Vaez , Nikoonahad M.
Published: July 2002

The coupled 3-D thermoacoustic equations, describing wave propagation in anisotropic solids, are derived. While solutions to these equations predict the behavior of both thermal and acoustic waves, a specific formulation is presented for the analysis of thermal wave generation, and propagation, in the decoupled mode. The approach is based on a transfer function formalism, and can easily be adapted to accommodate external sources with arbitrary spatial distribution. Simulations pertaining to the case of thermal wave propagation in a semi-infinite sample, as well as a slab of material are presented. Results predict anisotropy-induced asymmetric field formation and thermal beam-steering effects.
Forward stimulated Brillouin scattering
Authors: Corvo Antonio , Gavrielides Athanasios
Published: July 2002

Forward stimulated Brillouin scattering characteristics are formulated by an extension of the current theory. Angle-dependent gain functions are derived in collimated and focused pump geometries. The time of growth for forward scattering based on the derived gain functions is calculated by comparing the forward and backward intensity time scales. The forward intensity is shown to grow in times on the order of thousands of phonon lifetimes and its behavior is that of an expanding cone reaching a maximum angle varying inversely as the square root of the pump f number.
Quantum spin chains and the conformal anomaly
Authors: Bonner Jill C. , Parkinson John B.
Published: July 2002

The conformal anomaly c determines the universality class of a model system in statistical mechanics. The value of c characterizes both 2D classical models and their 1D quantum counterparts. The conformal anomaly may therefore be determined numerically for quantum spin chains using the relation: E0(N)≂E0(∞)−(NΔE/12)c(1/N2), where E0 (N) is the ground-state energy of an N-spin finite system, E0 (∞) is the ground-state energy in the thermodynamic limit, and ΔE is the energy gap between the ground state at k=0 and the first excited state of the dispersion curve at k=2π/N. The numerical approach is highly successful when tested on the integrable s= 1/2 Heisenberg antiferromagnetic XXZ chain and the integrable s=1 SU(2) model. The method gives c=1 to within 2% accuracy for the s=1 and (3)/(2) XY chains, placing them in the universality class of the 2D XY model. The result c=1 (2% accuracy) is obtained for the s= (3)/(2) Heisenberg antiferromagnetic chain, in agreement with the Haldane prediction. The s=1 pure antiferromagnetic biquadratic chain and the s=1 XXZ model with uniaxial anisotropy in the vicinity of the critical point Δ=Δ2 ∼1.15 −1.18 have also been studied.
New ion trap for frequency standard applications
Authors: Prestage J. D. , Dick G. J. , Maleki L.
Published: July 2002

We have designed a novel linear ion trap which permits storage of a large number of ions with reduced susceptibility to the second-order Doppler effect caused by the rf confining fields. This new trap should store about 20 times the number of ions as a conventional rf trap with no corresponding increase in second-order Doppler shift from the confining field. In addition the sensitivity of this shift to trapping parameters, i.e., rf voltage, rf frequency, and trap size, is greatly reduced.
The role of gas adsorption in ion-beam-induced deposition of gold
Authors: Dubner A. D. , Wagner A.
Published: July 2002

Gold films were deposited on quartz crystal microbalances (QCM) by decomposing C7H7F6O2Au [dimethyl gold hexafluoroacetylacetonate, or DMG(hfac)] with a 5-keV argon-ion beam. A model for ion-beam-induced deposition is presented which relates the net deposition yield to the gas adsorption, the decomposition cross section, and the sputtering yield. The deposition rate was measured in situ as a function of ion current, gas pressure, and substrate temperature using the QCM. The deposition yield (mass deposited per incident ion) increased with increasing gas pressure and decreasing substrate temperature. The QCM was also used to measure the adsorption of DMG(hfac). The results demonstrated that the variation in deposition yield with temperature and pressure was proportional to the number of DMG(hfac) molecules adsorbed per cm2. Based on the observed correlation between deposition yield and adsorption, a decomposition cross section for 5-keV argon ions of 2×10−13 cm2 was estimated.
Levitation force and magnetic stiffness in bulk high-temperature superconductors
Authors: Chang P.- Z. , Moon F. C. , Hull J. R. , Mulcahy T. M.
Published: July 2002

Levitation forces between a small permanent magnet and a disk of bulk high-temperature superconductor at 77 K were measured as a function of vertical separation for disks of composition Y-Ba-Cu-O, Ag/Y-Ba-Cu-O, (Pb,Bi)-Sr-Ca-Cu-O, and Tl-Ba-Ca-Cu-O. The forces were highly hysteretic; however, for all samples, on the initial descent of the magnet toward the disk, the force was unique, independent of magnet speed, and varied approximately as the negative exponential of the separation distance. Magnetic stiffness, associated with minor hysteresis loops, was found to be approximately proportional to the levitation force, and nearly independent of magnet configuration and superconductor composition.
Influence of CaO addition on the electrical properties of BaTiO3 ceramics
Authors: Lin Tsai-Fa , Hu Chen-Ti , Lin I-Nan
Published: July 2002

The microstructure and electrical properties of calcium-modified barium titanate ceramics of compositions (Ba1−xCax)TiO3 have been investigated. From the influence of the CaO content and stoichiometry on the said characteristics of the materials, it is concluded that the cationic ratio, α=(Ba+Ca)/Ti, is the predominant factor affecting the resistance of materials against the reducing sintering atmosphere. The electrical properties, including resistivity and dielectric dispersion, can be completely preserved when the sintering atmosphere is switched from air to H2 /N2 , but only for samples with values of α greater than unity. The formation of a hexagonal BaTiO3−δ phase, which consumes the oxygen vacancies, is presumed to be the factor that improves the resistance of these materials against the reducing sintering atmosphere.
Variation of deep electron traps created by γ irradiation of GaAs
Authors: Hashizume Tamotsu , Hasegawa Hideki
Published: July 2002

The effect of γ irradiation on deep electron states in liquid encapsulated Czochralski (LEC) grown GaAs has been investigated by deep-level transient spectroscopy (DLTS) and photocapacitance measurements. With γ rays of 2×108 R, EL6 was reduced in concentration by a factor of 3–5, whereas EL3 was increased about one order of magnitude, as compared with those in as-grown material. In addition to E traps that were previously reported in electron-irradiated material, two new traps were observed near the surface region. From their concentration profiles and annealing behavior, the new traps were most likely created by the interaction of the primary irradiation-induced defects with the grown-in defects. In contrast to these results, neither the DLTS spectrum nor the metastable behavior of EL2 was affected by γ irradiation.
Secondary ion mass spectroscopy determination of oxygen diffusion coefficient in heavily Sb doped Si
Authors: Pagani M.
Published: July 2002

The diffusion coefficient of oxygen in heavily antimony doped Czochralski Si was measured in the temperature range 950–1100 °C by using secondary ion mass spectroscopy (SIMS). The diffusion coefficient, obtained from SIMS oxygen concentration profiles in samples submitted to out diffusion, shows no dependence on antimony concentration. The combined data give an activation energy of 2.68 eV, which is in good agreement with published results.
Light scattering from polymer-dispersed liquid crystal films: Droplet size effects
Authors: Montgomery G. Paul , West John L. , Tamura-Lis Winifred
Published: July 2002

The effects of droplet size on light scattering by polymer-dispersed liquid crystal (PDLC) films have been studied theoretically and experimentally. Rayleigh–Gans calculations predict that, provided the droplets scatter independently, backscattering will be maximum at wavelength λ when the droplet radius is about λ/7. However, correlation effects become noticeable for droplet volume fractions η above about 0.2 and shift the radius for maximum backscattering to about λ/5 as η increases to 0.5. These calculations are consistent with hemispherical transmittance and reflectance measurements on PDLC films in which multiple scattering is known to be important. These measurements indicate that solar attenuation by PDLC films will probably be maximized by choosing droplet size to maximize backscattering of visible radiation. This choice also increases solar attenuation by absorption of radiation trapped inside a PDLC film by total internal reflection.
Analysis of Lagrangian gauge measurements of simple and nonsimple plane waves
Authors: Aidun John B. , Gupta Y. M.
Published: July 2002

Analysis of multiple, embedded gauge measurements of large-amplitude plane waves is addressed. Although many of the developments are general, the emphasis is on the determination of stress and density fields from particle velocity measurements. Practical considerations for embedded particle velocity and stress gauge measurements are discussed. Difficulties with existing analysis methods are described. The use of wave similarity as a criterion for determining the preferred analytic approach is emphasized. Simple waves are easily analyzed using the approaches suggested by Fowles, Cowperthwaite, and Williams J. Appl. Phys. 41, 360 (1970); 42, 456 (1971)]. Nonsimple waves, of broader interest, are best analyzed using surfaces explicitly fit to measured histories and directly integrating the conservation equations. Analytic and numerical examples of the surface-fitting procedure are presented. Uniqueness and verification of calculated results in past and present work are discussed.
Positive holes in magnesium oxide. Correlation between magnetic, electric, and dielectric anomalies
Authors: Batllo F. , LeRoy R. C. , Parvin K. , Freund F. , Freund M. M.
Published: July 2002

Magnetic susceptibility measurements of high purity MgO single crystals (<50-wt. ppm transition metals) by means of a vibrating-sample magnetometer shows an anomaly at 800 K. At the same temperature the electric conductivity increases anomalously, the static dielectric constant ■ increases from 9 to ≊150, a pronounced positive surface charge appears, and Fe2+ in the MgO matrix oxidizes to Fe3+. The data are consistent with O2−2 (peroxy) defects, representing self-trapped, spin-paired positive holes at Mg2+ vacancy sites. Diamagnetic at low temperatures, the holes start to decouple their spins ≳600 K, probably forming at first V0 centers (two O− at an Mg2+ vacancy), then V− centers (single O− at an Mg2+ vacancy), and releasing mobile O− states. These O− represent itinerant charge carriers on acceptor levels near the O 2p-dominated valence band and conduct by O−/O2− valency fluctuations. The O− concentration is of the order of 8 × 1019 cm−3.
Corrosion resistance of Nd-Fe-B sintered magnets
Authors: Tokuhara Koki , Hirosawa Satoshi
Published: July 2002

The corrosion mechanism of Nd-Fe-B sintered magnets has been investigated. A correlation between the lifetime of epoxy-coated magnets in a humidity test (80 °C, 90% RH) and the rate of weight loss (ΔW/Δt) of the bare magnets in the pressure cooker test (PCT: 125 °C; 85% RH; 2 atm) has been established. In this system, corrosion is attributed to the selective oxidation of grain boundaries (the Nd-rich phase). The value of ΔW/Δt is closely related to the concentration of neodymium and it decreases when cobalt or nickel is added. Consistency of corrosion resistance of the surface-coated Nd-Fe-B sintered magnets can be achieved by controlling ΔW/Δt below a certain critical value.
Thermal conductivity of amorphous alloys above room temperature
Authors: Choy C. L. , Tong K. W. , Wong H. K. , Leung W. P.
Published: July 2002

The thermal conductivity of ten amorphous alloys has been measured between 280 and 500 K. The thermal conductivity, K, can be separated into the electronic (Ke) and phonon (Kph) contributions. The electronic thermal conductivity, deduced from the Wiedemann–Franz law, varies almost linearly with temperature, whereas the phonon thermal conductivity shows a slower increase. At 300 K, Kph accounts for 34–49% of K. The phonon mean free path l is 12.5 Å for the binary alloy Fe80B20, but l decreases as the number of chemical components increases, reaching 7 Å for the five-component alloys Fe32Ni36Cr14P12B6 and Co66Fe4Mo2B12Si16. The metal-metal glasses, Cu70Zr30 and Cu45Zr55, have l values slightly larger than 11 Å, indicating that they have short-range order similar to that of Fe80B20.
Characterization of FeS2 thin films prepared by thermal sulfidation of flash evaporated iron
Authors: Ferrer I. J. , Sánchez C.
Published: July 2002

FeS2 thin films were prepared by thermal sulfidation of iron layers evaporated on glass substrates. Experiments were carried out to optimize the sulfidation parameters. For this purpose, morphological, compositional, crystallographic, optical, and electrical properties of the produced films and their relation with the parameters involved in the sulfidation process (temperature, sulfur pressure) are presented. From x-ray spectra and energy dispersive analyses of x ray it is concluded that FeS2 starts to be formed at sulfidation temperatures about 200 °C. X-ray patterns show that crystallinity and grain size of the films improve with the sulfidation temperature with a ‘‘critical value’’ which varies with the sulfur pressure. On the other hand, both the film resistivity and the optical absorption coefficient change abruptly at temperatures higher than 350 °C. Above 500 °C polycrystalline aggregates known as ‘‘framboidal pyrite’’ are observed.
The role of the ion-solid interaction in ion-beam-induced deposition of gold
Authors: Dubner A. D. , Wagner A. , Melngailis J. , Thompson C. V.
Published: July 2002

Gold films were deposited on quartz-crystal microbalances by decomposing C7H7F6O2Au (dimethyl gold hexafluoroacetylacetonate) with 2–10-keV Xe+, Kr+, Ar+, Ne+, or He+ ion beams. The number of molecules decomposed per incident ion (i.e., the total decomposition yield) was determined as a function of ion mass and energy. The total decomposition yield increases with increasing ion mass and ion energy, and is approximately proportional to the nuclear stopping power. A binary collision model and a thermal spike model are developed that relate the energy deposited by the ion, at the substrate surface, to the total number of molecules decomposed. Both models predict total decomposition yields that are in reasonable agreement with the experimental measurements; however, the variation of total yield with changes in ion mass and energy are best described by the binary collision model. The success of both models demonstrates that the energy deposited into the substrate surface through the ion-solid interaction is responsible for the decomposition of adsorbed molecules.
Formation and magnetic properties of nanocrystalline mechanically alloyed Fe-Co
Authors: Kuhrt Ch. , Schultz L.
Published: July 2002

Fe100−xCox powders were prepared by mechanical alloying of the elements in a planetary ball mill. They were investigated with respect to phase formation and magnetic properties using x-ray diffraction, differential scanning calorimetry, and measurements of the saturation magnetization and the coercivity. The measurement of the saturation magnetization proved the true formation of the bcc (x≤80) and fcc (x=90) solid solutions by mechanical alloying. A nonequilibrium microstructure originates from a grain-size reduction to minimum 20–30 nm and the introduction of internal strain up to 1% (root-mean-square strain). An improvement in the soft magnetic properties by the nanocrystalline state, as hoped for, does not occur, because the high amount of internal strain together with the high saturation magnetostriction of the Fe-Co alloys causes relatively high coercivities of 5–40 A/cm. Grain growth and strain relaxation induced by controlled heat treatment of the as-milled powders allowed the separation of the influence on coercivity of the nanocrystalline structure from that of the internal strain: Below about 30 nm grain size, grain growth results in a pronounced coercivity increase superimposed on the general coercivity decrease caused by strain relaxation. This effect can be explained by the random anisotropy model describing the magnetization process in nanocrystalline ferromagnets.
Passivation effects of polyphenylene sulphide on the surface of GaAs
Authors: Bhide R. S. , Bhoraskar S. V. , Rao V. J.
Published: July 2002

Vacuum-evaporated thin films of polyphenylene sulphide have been used as an insulating overlayer on n-GaAs(110). Sulphur present in the polymer is seen to passivate the dangling bonds of GaAs. The interface of n-GaAs/polyphenylene sulphide was studied using grazing-angle x-ray diffraction at various angles for different annealing temperatures. The electronic properties of the interface are studied using electron-beam-induced-current measurements for determining the minority-carrier diffusion length and surface recombination velocity. The x-ray-diffraction analysis indicates the formation of arsenic sulphide at the interface and is expected to reduce the dangling bond density. The treated surface shows an increase in diffusion length and reduction in the surface recombination velocity. The polymer-semiconductor interface shows stability against annealing up to a temperature of 300 °C.
Magnetic anisotropies in Co/Ni(111) multilayers
Authors: Bloemen P. J. H. , de Jonge W. J. M. , den Broeder F. J. A.
Published: July 2002

The magnetic anisotropy and the saturation magnetization have been investigated in several series of Co/Ni multilayers, with relatively thick Co and Ni layers, grown in ultrahigh vacuum (UHV) and high vacuum (HV) by vapor deposition. The volume and interface contributions to the magnetic anisotropy were determined from magnetization measurements. The Co/Ni interface contribution was found to favor perpendicular anisotropy. A series of 2 Å Co/4 Å Ni multilayers, grown in HV, exhibited an easy axis perpendicular to the layer planes, as evidenced both by polar Kerr rotation and ferromagnetic resonance (FMR) measurements. The first- and second-order anisotropy constants were determined from FMR. Indications were found that the presence of FMR signals in (perpendicular easy) samples with low magnetic sublayer thicknesses depends on the magnitude of the surface anisotropy and on the amplitude of fluctuations in the magnetic layer thicknesses.
Photoluminescence of high-quality GaSb grown from Ga- and Sb-rich solutions by liquid-phase epitaxy
Authors: Wu Meng-Chyi , Chen Chi-Ching
Published: July 2002

The temperature dependence of photoluminescence from the high-quality unintentionally doped GaSb layers grown by liquid-phase epitaxy has been studied. The epitaxial layers grown at temperatures above 590 °C, from the Ga- or Sb-rich solutions, reproducibly have a low-carrier concentration of 6–8×1015 cm−3 and exhibit p-type conduction. But it gives n type as grown from the Ga-rich solution at 360 °C. For the samples grown from the Ga-rich solutions, the 16 K photoluminescence spectrum is dominated by the partially resolved lines related to the transitions of excitons bound to donors and neutral acceptors. The acceptor-related band (777.8 meV) which is always presented in the GaSb material due to the native lattice defects has been much reduced as compared to the exciton-related lines. Especially for the n-type samples grown at low temperatures (360 °C), the ratio of the emission intensity from the exciton-related lines to that from the acceptor-related band is 10. For the samples grown from the Sb-rich solutions, the 16 K photoluminescence spectrum is also dominated by a bound exciton-related line with a full width at half maximum of 4.2 meV. In addition, the free-exciton transition with a full width at half maximum of 0.4 meV is also observed. As the temperature is increased, the intensity of the lines associated with the bound excitonic transitions for all the GaSb samples rapidly quenches off and the free-electron-to-free-hole transition becomes dominant. The temperature dependent band gap in GaSb layers determined from the photoluminescence peak energy varies as 0.813–[1.08×10−4 T2/(T−10.3)] eV.
Reduction of droplet emission and target roughening in laser ablation and deposition of metals
Authors: Riet E. van de , Nillesen C. J. C. M. , Dieleman J.
Published: July 2002

The droplet concentration in laser-deposited metal, Si, and alloy thin films is studied. It is found for these materials that the number of droplets is strongly dependent on the laser fluence and is low at high laser fluences. This behavior is contradictory to what is usually observed for oxidic materials. It is also found that the amount and average size of the droplets correlate closely with the surface roughness. Rough surfaces generally emit more droplets. The target used in laser ablation and deposition experiments usually roughens during ablation. By intelligently varying the azimuthal angle of incidence, the roughening, and thereby the emission of droplets, can be greatly reduced.
Fluid saturation-dependent nuclear magnetic resonance spin-lattice relaxation in porous media and pore structure analysis
Authors: Chen Songhua , Liaw Hsie-Keng , Watson A. Ted
Published: July 2002

Nuclear-magnetic-resonance spin-lattice relaxation measurements were conducted in a Bentheimer sandstone sample for drainage experiments involving gas-liquid fluid phases with ten different saturation levels ranging from complete water saturation (Sw=1) down to Sw=0.14. A monotonic decrease in relaxation times was observed as the water saturation was lowered over this broad saturation range. This phenomenon is explained by considering that for a drainage process the liquid (wetting) phase was drained from different-sized pores at different saturations with larger pores being drained first. The relaxation decay curves corresponding to each saturation state were analyzed using both stretched exponential and discrete multiexponential functions. In particular, the effect due to bulk fluid relaxation was eliminated so that a more appropriate relationship between pore size distribution and the relaxation rate is obtained. From these analyses, the relative variation of pore size distributions corresponding to different saturation levels was obtained. A power-law dependence of saturation with relaxation times is observed which indicates that the relaxation analysis can be used to characterize fluid saturations.
Excitonic photoluminescence in a CuAlSe2 chalcopyrite semiconductor grown by low-pressure metalorganic chemical-vapor deposition
Authors: Chichibu Shigefusa , Matsumoto Satoru , Shirakata Sho , Isomura Shigehiro , Higuchi Hirofumi
Published: July 2002

Excitonic photoluminescence (PL) in a CuAlSe2 chalcopyrite semiconductor was observed. High-quality CuAlSe2 epilayers were grown by the low-pressure metalorganic chemical-vapor deposition technique. Based on photoreflectance measurements, the PL peak at 2.739 eV was assigned to a free exciton emission. The PL peak at 2.677 eV was tentatively assigned to a bound exciton emission.
Schottky barrier height of metal contacts to p-type alpha 6H-SiC
Authors: Waldrop J. R.
Published: July 2002

A survey of metal (Pd, Ni, Au, Ag, Mg, Ti, and Al) Schottky barrier contact formation to p-type Si-face (0001) and C-face (0001̄) 6H-SiC by using x-ray photoemission spectroscopy is reported. The Schottky barrier height φB ranges from 1.17 to 2.56 eV and is influenced by the contact metal work function and the 6H-SiC crystal face. A comparison with prior φB values for n-type material indicates that for similarly prepared metal/6H-SiC interfaces (including those which have been annealed) φBp and φBn sum to the 6H-SiC band gap.
Plasma-charging damage: A physical model
Authors: Cheung K. P. , Chang C. P.
Published: July 2002

An entirely physical model is proposed to explain a wide range of seemingly conflicting observations of plasma-charging damage. Unlike other authors who largely ignored the role of substrate potential, we carefully track both the gate and the substrate potentials to explain the origin of the electric field developed across the thin oxide during plasma exposure. Central to this model is the fact that the surface floating potential tracks the plasma potential. Thus a nonuniform plasma drives the gate potential to a nonuniform distribution. Another important idea of this model is the continued adjustment by both the gate and the substrate of their potentials to satisfy the charge balance requirement of plasma system. The interaction between saturated ion-current, asymmetric electron-current, and the Fowler–Nordheim tunneling current produces a complex dynamic for the movements of the gate and the substrate potentials. This complex behavior of the gate and the substrate potential allows many of the reported observations in the literature to be explained logically. We explored three types of charging effect and their damage characteristics. They are dc effect, ac effect, and transient effect. The separation of dc and ac effect is artificial. They exist simultaneously and add to each other to cause more serious damage than by themselves. The model predicts antenna effects from all three types of charging effects. Unique to the ac antenna effect are saturation behavior, the oxide thickness ratio dependence, and the rf bias-frequency dependence. The effect of ON/OFF transient is explored quantitatively using the concept of effective exposed substrate area. The combination of large antenna ratio and rapid turn-off of plasma causes severe damage to gate-oxide. Magnetic field in general will worsen the charging damage. The reason for that is explained. Plasma uniformity is the most important factor governing charging damage, but is not the only factor. Plasma uniformity extending beyond the edge of the wafer is important. Mere showing that the plasma is uniform across the most part of the wafer is misleading.
Growth studies of erbium-doped GaAs deposited by metalorganic vapor phase epitaxy using novel cyclopentadienyl-based erbium sources
Authors: Redwing J. M. , Kuech T. F. , Gordon D. C. , Vaartstra B. A. , Lau S. S.
Published: July 2002

Erbium-doped GaAs layers were grown by metalorganic vapor phase epitaxy using two new sources, bis(i-propylcyclopentadienyl)cyclopentadienyl erbium and tris(t-butylcyclopentadienyl) erbium. Controlled Er doping in the range of 1017–1018 cm−3 was achieved using a relatively low source temperature of 90 °C. The doping exhibits a second-order dependence on inlet source partial pressure, similar to behavior obtained with cyclopentadienyl Mg dopant sources. Equivalent amounts of oxygen and Er are present in ‘‘as-grown’’ films indicating that the majority of Er dopants probably exist as Er-O complexes in the material. Er3+ luminescence at 1.54 μm was measured from the as-grown films, but ion implantation of additional oxygen decreases the emission intensity. Electrical compensation of n-type GaAs layers codoped with Er and Si is directly correlated to the Er concentration. The compensation is proposed to arise from deep centers associated with Er which are responsible for a broad emission band near 0.90 μm present in the photoluminescence spectra of GaAs:Si, Er films.
The connection between hydrodynamic stability of gas flow in spin coating and coated film uniformity
Authors: Öztekin Alparslan , Bornside David E. , Brown Robert A. , Seidel Philip K.
Published: July 2002

The thickness uniformity of a spin-cast film is governed by the air flow through the spin coater, particularly the boundary layer flow above the surface of the spinning wafer, which controls solvent evaporation from the dry film. Laser Doppler velocimetry (LDV) and hot wire anemometry (HWA) are used to map the flow field throughout an industrial spin coater and to study flow instabilities in the boundary layer for various combinations of wafer spin speed and exhaust flow rate. The flow field measured by LDV compares well with a numerical simulation of laminar, axisymmetric, and steady air flow throughout the coating bowl. However, Ekman spiral flow instabilities of both type I (positive spiral angle) and type II (negative spiral angle) were found by HWA in the boundary layer near the surface of the spinning wafer. The type-II spirals form at Reynolds number in the range 2000–2500 and the type-I spirals form at Reynolds number in the range 80 000–85 000. It is the type-II spirals that are responsible for disrupting the air flow in the boundary layer flow and that cause nonuniform drying of spin-cast films.
Second order optical nonlinearity of surface crystallized glass with lithium niobate
Authors: Ding Yong , Osaka Akiyoshi , Miura Yoshinari , Toratani Hisayoshi , Matsuoka Yoshihiko
Published: July 2002

Significant second harmonic generation was observed for a surface crystallized but transparent glass of 35Li2O⋅30Nb2O5⋅35SiO2 (in mol %), for which the crystalline layer (about 0.35 μm in thickness) was composed of partially c-axis oriented and connected LiNbO3 fine grains. The effective second order nonlinear optical susceptibility of the film deff reached ∼5.25 pm/V after adjusting the incident angle. The tensor components of the film were d33=6.9 pm/V and d31=d21=d24=d15=1/3d33=2.3 pm/V if the partially c-axis oriented thin film was described by a space group C∞V.
Comparison of the electrical properties of simultaneously deposited homoepitaxial and polycrystalline diamond films
Authors: Malta D. M. , von Windheim J. A. , Wynands H. A. , Fox B. A.
Published: July 2002

The electrical transport properties of simultaneously deposited, B-doped homoepitaxial and polycrystalline diamond thin films have been evaluated by Hall-effect and resistivity measurements over a temperature range of 80–600 K. The same films were later characterized by scanning electron microscopy, secondary-ion-mass spectroscopy, and an oxidation defect etch. The study involved four sets of chemical-vapor-deposited diamond films with individual B concentrations ranging from 1.5×1017 to 1.5×1020 cm−3. In each of the four cases the mobility of the polycrystalline film was lower than that of the homoepitaxial film by 1–2 orders of magnitude over the entire temperature range. Polycrystalline films also incorporated 2–4 times more B, had 3–5 times higher compensation ratios, and displayed activation energies that were 0.05–0.09 eV lower than in the homoepitaxial films. Hopping conduction was observed in both types of films at low temperatures, but was enhanced in polycrystalline films as evident by higher transition temperatures. Preliminary efforts have been made to evaluate these results by considering the possible effects of crystal structure, compensation, impurity segregation, grain-boundary trapping, and impurity conduction.
Nucleation of oriented diamond particles on cobalt substrates
Authors: Liu Wei , Tucker Denise A. , Yang Peichun , Glass Jeffrey T.
Published: July 2002

Nucleation of oriented diamond particles on cobalt substrates has been achieved by a multistep, hot-filament chemical vapor deposition process, which involves seeding, annealing, nucleation, and then growth. The substrates were seeded with either diamond powders, graphite powders, or gaseous carbon species. Scanning electron microscopy showed that 〈111〉 oriented diamond particles were obtained on 〈0001〉 oriented single crystal cobalt substrates. Micro-Raman indicated that the quality of the diamond grown on the cobalt substrate was high, with a full width at half maximum of 4.3 cm−1. A very weak graphitic peak was observed on regions of the substrate not covered by the diamond particles, indicating that graphite codeposition was significantly suppressed. Scanning Auger depth profile analysis was done to characterize the diamond nucleation. Based on the experimental observations, a nucleation model is proposed.
Chiral quantum spin solitons
Authors: Braun Hans-Benjamin , Loss Daniel
Published: July 2002

The concept of chirality is introduced in the quantum dynamics of spin solitons. For half-integer spin chains we find as universal properties that the Brillouin zone is halved and that two subsequent band minima have opposite chirality. This effect occurs in quasi-one-dimensional (1D) magnets in the semiclassical limit, in anisotropic spin-1/2 chains and for holes propagating in a two-dimensional (2D) antiferromagnetic background.
Carbon nanocapsules and single-layered nanotubes produced with platinum-group metals (Ru, Rh, Pd, Os, Ir, Pt) by arc discharge
Authors: Saito Yahachi , Nishikubo Keishi , Kawabata Kenichiro , Matsumoto Takehisa
Published: July 2002

Encapsulation of platinum-group metals (Ru, Rh, Pd, Os, Ir, Pt) within carbon nanocapsules and synthesis of single-layered (SL) carbon nanotubes by arc evaporation of metal/carbon composites have been studied. Transmission electron microscopy revealed that all the platinum-group metals, forming small particles (10–200 nm in diameter), were encapsulated within multilayered graphitic cages. Particles trapped in the cages were single-domain crystallites in normal metallic phases. Rh, Pd, and Pt showed catalytic activity for growing SL carbon tubes, but the other metals did not. Bundles of dense SL tubes with diameter 1.3–1.7 nm were extruding radially from metal particles for Rh and Pd; the sizes of core particles were 20–30 nm for Rh, and 50–200 nm for Pd. In the case of Pt, one or a few SL tubes (typically 1.3–2.0 nm in diameter and sometimes ∼3 nm) grew from a tiny particle (∼10 nm).