bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of temperature on electrical and microstructural changes of coevaporated Ir-Si alloy films
Authors: Weiss B. Z. , Tu K. N. , Smith D. A.
Published: July 2002

Electrical and microstructural changes of coevaporated Ir50Si50 have been studied as a function of temperature from room temperature to 970 °C. In situ resistivity measurements, in situ annealing and transmission electron microscopy, Rutherford backscattering spectroscopy and Seeman–Bohlin glancing angle incidence x-ray diffraction were applied. In the as-deposited state the alloy film is amorphous and exhibits semiconducting behavior. A two-stage phase transformation occurs in the process of heating. The first transition from amorphous to crystalline IrSi is nucleation controlled. It is accompanied by an abrupt decrease in resistivity and was found to be dependent on the heating rate and chemical composition. The second crystalline-to-crystalline transformation is controlled by nonstoichiometry of the alloy film. An excess of Ir atoms leads to formation of Ir2Si at ∼640 °C while an excess of Si atoms leads to formation of IrSi3 at ∼710 °C. The second transition is independent of the heating rate. The formation of both Ir2Si and IrSi3 decreases the electrical resistivity. The isothermal transformation from amorphous to crystalline structure follows a sigmoidal function. The apparent activation energy of this process is about 1.32 eV. The kinetic behavior implies random nucleation and interface-controlled two-dimensional growth of the crystalline phase.
The effect of induced air-flow on the spin coating of viscous liquids
Authors: Middleman Stanley
Published: July 2002

In the spin-coating process for producing thin films, air flow induced by disk rotation provides a shear stress at the liquid-air interface which enhances the rate of thinning of the liquid film on the disk. An approximate analysis using Cochran’s classical solution [Proc. Cambridge Philos. Soc. 30, 365 (1934)] for the induced air flow is presented. Regions wherein air flow significantly enhances thinning are delineated. As film thickness becomes very small, in accordance with a dimensionless criterion developed as part of the analysis, the external air flow dominates the centrifugal effect and controls the final rate of thinning. A set of data obtained under conditions appropriate to application of this new theory shows enhanced thinning, but not to the degree predicted.
Analytical electron microscopic studies and positron lifetime measurements in Al-doped MgO crystals
Authors: Pedrosa M. A. , Pareja R. , González R. , Abraham M. M.
Published: July 2002

MgO crystals intentionally doped with Al were characterized by analytical electron microscopic examinations and positron lifetime measurements. Large spinel (MgO Al2O3) precipitates were observed in samples with high contents of Al. A well-defined crystallographic relationship between the precipitates and the matrix was found. The characteristics of positron lifetime spectra appear to depend on the valence state of the different impurities in the MgO lattice suggesting that positrons are trapped by vacancy impurity complexes.
On the critical fields and current densities of YBa2Cu3O7 and La1.85Sr0.15CuO4 superconductors
Authors: Senoussi S. , Oussena M. , Hadjoudj S.
Published: July 2002

We report a detailed investigation of the magnetic hysteresis of YBa2Cu3O7 oxide in fields ranging from ∼0.2 Oe to 35 kOe and at different temperatures (4.2≤T≤100 K). It is shown that the magnetic behavior is that of a granular type-II superconductor in which strongly superconducting grains are coupled via less superconducting links. As a result, at low-H (H≲20 Oe at 4.2 K) the sample appears like a single superconducting filament while at high enough field (H≳5 kOe) it behaves like a granular superconductor with most of the weak links being broken by the applied field. Five threshold fields are identified charaterizing the above magnetic regimes and the transition between them. It is concluded that the critical current density within the individual grains could attain 107 A/cm2.
Molecular-beam epitaxial growth of InSb/CdTe heterojunctions for multilayer structures
Authors: Golding T. D. , Martinka M. , Dinan J. H.
Published: July 2002

We have used the technique of molecular-beam epitaxy to grow layers of CdTe on InSb and InSb on CdTe and have performed a detailed analysis of the layers and their interfaces using Auger depth profiling and reflection high-energy electron diffraction. We show that significant improvements in interfacial quality can be obtained by the proper choice of fluxes during growth. The use of a Cd/Te flux ratio of 3:1 (JCd/JTe=3) during the growth of CdTe has enabled epitaxy at a substrate temperature of 300 °C. Interfaces formed with this flux ratio are abrupt, in sharp contrast to those formed under stoichiometric flux conditions (JCd/JTe=1). Subsequent growth of InSb at a substrate temperature of 300 °C on thin CdTe epilayers (400 and 800 Å) is examined as a function of the InSb growth rate and Sb/In flux ratio. Quality of the interfaces shows a progressive improvement with increasing InSb growth rate.
Long wavelength (3–5 and 8–12 μm) photoluminescence of InAs1−xSbx grown on (100) GaAs by molecular-beam epitaxy
Authors: Yen M. Y. , People R. , Wecht K. W.
Published: July 2002

InAs1−xSbx films have been successfully grown by molecular-beam epitaxy on (100) GaAs substrates. Long wavelength photoluminescence spectra have been obtained reproducibly in the 3–5 and 8–12 μm spectral ranges in III-V compound semiconductors.
Ion implantation damage and annealing in InAs, GaSb, and GaP
Authors: Pearton S. J. , Von Neida A. R. , Brown J. M. , Short K. T. , Oster L. J. , Chakrabarti U. K.
Published: July 2002

The characteristics of ion implantation induced damage in InAs, GaSb, and GaP, and its removal by rapid thermal annealing have been investigated by Rutherford backscattering and transmission electron microscopy. There is relatively poor regrowth of these materials if they were amorphized during the implantation, leaving significant densities of dislocation loops, microtwins, and in the case of GaSb, polycrystalline material. For implant doses below the amorphization threshold, rapid annealing produces good recovery of the lattice disorder, with backscattering yields similar to unimplanted material. The redistribution of the implanted acceptor Mg is quite marked in all three semiconductors, whereas the donor Si shows no measurable motion after annealing of InAs or GaP. In GaSb, however, where it appears to predominantly occupy the group III site, it shows redistribution similar to that of Mg.
Surface copper contamination of as-received float-zone silicon wafers
Authors: Canham L. T. , Dyball M. R. , Barraclough K. G.
Published: July 2002

As-received float-zone Si wafers from two major suppliers are shown to have surface Cu contamination at a level of ∼5×1011 atoms cm−2, detectable by both low-temperature photoluminescence and room-temperature x-ray fluorescence. The effects of selective chemical removal of Cu from, or Cu adsorption onto, the front or rear surfaces of wafers have demonstrated where the majority of the contamination resides, thereby revealing its possible origin.
Enhanced electronic properties of GaAs surfaces chemically passivated by selenium reactions
Authors: Sandroff C. J. , Hegde M. S. , Farrow L. A. , Bhat R. , Harbison J. P. , Chang C. C.
Published: July 2002

We describe chemical modifications of GaAs surfaces which produce robust selenium layers that significantly enhance the electronic properties of the surface. The terminating layers are produced by depositing elemental selenium on GaAs surfaces under alkaline conditions followed by conversion to selenide and selenate using sodium sulfide. These selenium phases are more stable against photo-oxidation than their sulfide counterparts. On the chemically modified surface, photoluminescence is enhanced 400× and Raman spectroscopy indicates that surface band bending has been reduced to ∼0.1 eV. X-ray photoelectron spectroscopy reveals significant AsSe bond formation at the surface and a complicated interfacial structure with selenium present in oxidation states varying from 2− to 4+.
Electron paramagnetic resonance spectroscopy of titanium-ion-implanted silica
Authors: Whichard G. , Hosono H. , Weeks R. A. , Zuhr R. A. , Magruder III R. H.
Published: July 2002

Silica substrates were implanted with titanium ions, in the +1 charge state, to nominal doses of 1×1016, 3×1016, and 6×1016 ions/cm2 at an energy of 160 keV and a current of 2.5 μA/cm2. The implanted ion depth profiles were measured by backscattering techniques. Components in the vacuum ultraviolet absorption and electron paramagnetic resonance (EPR) spectra are attributed to a fraction of the implanted titanium in the Ti3+ state. The intensity of the Ti3+ EPR component has a Boltzmann temperature dependence between 490 and 5 K. The fraction of implanted titanium ions producing this EPR component ranges from 10% for doses of 6×1016 and 3×1016 ions/cm2 to 38% for a dose of 1×1016 ions/cm2. Based on the relative intensities of the Ti3+ charge transfer band resolved in optical absorption measurements, the fraction of Ti ions in the 3+ state is larger than the fraction estimated from the EPR spectral component. The Ti3+ ions not contributing to the EPR spectra are assumed to be antiferromagnetically (or speromagnetically) coupled. The linewidth of the EPR component decreases with increasing implantation dose. The value for the exchange integral for the paramagnetic fraction of implanted titanium ions is approximately (1.3±0.4)×10−6 eV. The fraction of Ti3+ ions antiferromagnetically coupled increases with increasing ion concentration. Thus a smaller fraction of Ti3+ ions are detected by EPR as the implantation dose is increased. A greater fraction of the titanium ions are incorporated in the SiO2 glass substrate in the Ti3+ state by ion implantation than are introduced in the 3+ state by other techniques such as fusion, reduction of Ti4+, and flame hydrolysis.
Resonant Raman scattering investigation of ion-irradiated hydrogenated amorphous carbon
Authors: Sela I. , Adel M. , Beserman R.
Published: July 2002

The effect of hydrogen removal by ion irradiation on the first- and second-order resonant Raman scattering of a-C:H are reported. We have measured the absolute first-order Raman intensity versus ion-irradiation dose and found a monotonic decrease in the absolute intensity with increasing ion dose. In addition, second-order Raman scattering also exhibited some resonance enhancement. The consequences of these results are discussed, particularly the important role of hydrogen in determining the vibrational properties of a-C:H films.
Thermal diffusivity measurements in solids by the ‘‘mirage’’ technique: Experimental results
Authors: Salazar A. , Sánchez-Lavega A. , Fernández J.
Published: July 2002

This work provides the experimental confirmation of a generalized theory for thermal diffusivity measurements of solid samples using the ‘‘mirage’’ technique presented by the authors in a previous paper [A. Salazar, A. Sanchez-Lavega, and J. Fernandez, J. Appl. Phys. 65, 4150 (1989)]. Moreover the influence of different effects on such determinations has been analyzed, and the limits of validity of the method have been established. The most important influence relies on the finite nature of two parameters: the height of the probe beam above the sample surface h and the radius of the exciting beam a. Such effects in all solids are discussed in detail according to their bulk thermo-optical properties and for two ways of measuring: ‘‘bouncing’’ (probe beam sent to the sample surface at a small angle) and ‘‘skimming’’ (probe beam parallel and grazing the surface). The first way of measuring gives reliable results while the second one must be managed with care. In any case, significant shifts in the thermal diffusivity measurement for nonzero values of h and a are specially observed when the thermal diffusivity of the sample is much lower than that of the deflecting medium.
Magnetic properties of a new family of ternary rare-earth iron nitrides R2Fe17N3−δ (invited)
Authors: Otani Y. , Hurley D. P. F. , Sun Hong , Coey J. M. D.
Published: July 2002

A series of interstitial ternary nitrides R2Fe17N3−δ has been prepared via a gas-solid reaction for R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y. These nitrides have structures related to the Th2Zn17 or Th2Ni17 structures of the R2Fe17 parent compounds, but the unit cell volumes are 6%–7% greater, and the Curie temperatures are approximately 400 K higher. The Fe-Fe exchange interactions are increased by a factor of 2.8 by nitrogenation, whereas the R-Fe exchange interactions are little changed. All compounds exhibit easy-plane anisotropy at room temperature, except for Sm2Fe17N3−δ, which shows strong uniaxial anisotropy and may be used to make permanent magnets. The Er and Tm compounds exhibit spin reorientations below room temperature. The anisotropy due to the iron sublattices is easy plane (K1 = −1.3 MJ/m3 at 4.2 K for Y2Fe17N3), but it changes sign to easy axis with cobalt substitution (K1 ≊ 1.0 MJ/m3 at 4.2 K for Y2(Fe1−xCox)17N3−δ when x≥0.2).
Effect of composition on the magnetic properties of (Ce1−xNdx)13.5 (Fe1−y−zCoySiz)80B6.5 sintered magnets
Authors: Li D. , Bogatin Y.
Published: July 2002

The influences of varying cerium, neodymium, cobalt, and silicon concentrations on the magnetic properties of Ce-Fe-B type magnets were investigated. The results indicated that magnets containing 13.5 at.% of cerium exhibits superior magnetic properties to those exhibited by magnets with higher rare-earth element compositions. High-temperature annealing of the cast alloy containing 13.5 at.% cerium at 1010 °C for 120 min results in the elimination of the α-Fe phase. Increasing the cobalt concentration from 0 to 17 at.% raises the Curie point from 174 °C to 324 °C. The addition of 1 at.% of silicon to the alloy improves intrinsic coercivity by up to 100%. The substitution of up to 5.4 at.% of cerium by neodymium increases magnetic properties of sintered magnets with a 13.5 at.% total rare-earth content. Neodymium contents exceeding 5.4 at.% decrease coercivity. The highest maximum energy product of 27.2 MGOe was exhibited by magnets with a composition of Ce8.1Nd5.4Fe62Co17SiB6.5.
Complex images of a ground electrode in layered soils
Authors: Chow Y. L. , Yang J. J. , Srivastava K. D.
Published: July 2002

In this paper, the complex image method is used to model ground electrodes in layered soils. The image locations and amplitudes are determined through a simple Prony method [R. W. Hamming, Numerical Methods for Scientists and Engineers (Dover, New York, 1973), pp. 620–622]. As an example, a toroidal electrode in a four-layer soil is modeled using one real image and four complex images. The results obtained are identical to those reported recently in the literature, given by more than 10 000 images using the conventional electrostatic image method.
Optical probing of laser induced surface waves in a transparent plate
Authors: Hrovatin R. , Možina J.
Published: July 2002

A method of determining pulsed laser generated Lamb wave dispersion is described, where the beam deflection technique is used to detect the first symmetric and antisymmetric mode. Waves were detected on a wedge shaped transparent plate which allows comparison between front and back surface distortions. Analysis of experimental signals allows the determination of wave dispersion relation.
Effect of a polymer network on the alignment and the rotational viscosity of a nematic liquid crystal
Authors: Jákli A. , Kim D. R. , Chien L. C. , Saupe A.
Published: July 2002

Polymer dispersions were made by photoinduced polymerization of 4,4′-bisacryloyl-biphenyl in liquid crystal 4-4′-cyano pentyl biphenyl as nematic solvent. Samples were prepared with 0.5, 1, and 1.5 wt % monomer. The polymerization was done in the isotropic phase. The rotational viscosities of the two lower concentration samples increased only moderately after UV curing (3 mW/cm2, 25 min). The 1.5 wt % sample gave under the same condition a nematic with a fairly rigid anisotropic network that did not realign in magnetic field up to 20 kG. We estimate that the mesh size of the network was in the order of 1 μm, significantly smaller than the magnetic coherence length of the solvent that is in the order of 5 μm. The diamagnetic anisotropies of the aligned samples were not affected significantly by the polymer. We studied electro-optical properties on thin films. The films were strongly light scattering but they could be switched to transparent state. We conclude that the polymer forms a network of loosely connected fibrils that interacts strongly with the director field.
Anisotropic properties of high-temperature polyimide thin films: Dielectric and thermal-expansion behaviors
Authors: Ree M. , Chen K.-J. , Kirby D. P. , Katzenellenbogen N. , Grischkowsky D.
Published: July 2002

Multilayer poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide films of 172 μm total thickness (11.4 μm per layer) were prepared from the poly(amic acid) precursor solution through repetition of a spin-coat/softbake/cure process. Wide-angle x-ray diffraction results indicate that the polyimide molecules in the multilayer films are highly ordered along the chain axes as well as in the lateral direction and furthermore are highly oriented in the film plane as observed in a single-layer film of 11.4 μm thickness. The multilayer films showed the same dynamic mechanical properties and glass transition behavior (Tg = 330 °C) as a single-layer film. For the multilayer films both the in-plane dielectric constant (ε′XY) and out-of-plane thermal-expansion coefficient (αZ) were measured using time-domain spectroscopy and conventional thermal mechanical analysis, respectively. The ε′XY at room temperature was 3.69 (±0.08) over a frequency range of 0.35–2.50 THz. A similar ε′XY is predicted at frequencies of ≤0.35 THz. In contrast to the ε′XY, a relatively lower out-of-plane dielectric constant (ε′Z) was observed: ε′Z = 2.96–3.03 (±0.02) at 1 MHz, depending on moisture content in the film. The dielectric loss ε″Z at 1 MHz was 0.011–0.014 (±0.001), depending on moisture content. The measured αZ was 74 ppm/°C over the temperature range of 25–150 °C, which was much higher than αXY = 2.6–5 ppm/°C. Consequently, large anisotropic ε′ and α have been observed in the in plane and out of plane of the thermally imidized BPDA-PDA films. The anisotropic ε′ and α were caused by high in-plane orientation of the polyimide molecules highly ordered along the chain axes in the films.
Analysis of diamond growth in subatmospheric dc plasma-gun reactors
Authors: Coltrin Michael E. , Dandy David S.
Published: July 2002

The growth of diamond in a subatmospheric dc-arc plasma-jet reactor has been studied theoretically. Full transport equations for this geometry, including gas-phase and surface chemistry, have been solved numerically. The surface-reaction mechanism includes pathways for the incorporation of CH3, C2H2, and C from the gas phase, as well as growth of graphite. The surface mechanism includes full reversibility for all reactions, based on estimates of the thermochemistry. Results are presented for degrees of dissociation of H2 in the plasma gun ranging from 2.6% to 90%, and inlet levels of CH4 spanning 0.1–5.0 mol %. It is seen that CH3 is the predominant growth species when there is little H2 dissociation within the plasma gun, but C becomes the dominant species at higher dissociation levels. The third growth species, C2H2, does not play a role in diamond growth under these conditions when there is less than 1% CH4 in the feed; but, at higher CH4 levels both C and CH3 addition rates drop to 50 times greater than C2H2.
Investigation of laser sputtering of iron at low fluence using resonance ionization mass spectrometry
Authors: Gibert T. , Dubreuil B. , Barthe M. F. , Debrun J. L.
Published: July 2002

Emission of neutral and ionized Fe atoms induced by N2 laser irradiation (λ=337 nm) of iron targets was investigated for laser fluences ranging from the sputtering threshold (a few tens mJ/cm2) up to 320 mJ/cm2. The unique sensitivity of resonance ionization mass spectrometry permitted to identify the laser-sputtered neutral particles, to study their velocity distribution and excitation state, and to measure the laser-sputtering yield at a very low emission level (100 Fe atoms removed per laser shot). In this soft ablation mode, it appears that the sputtered atoms carry direct information on the primary mechanisms involved in the laser sputtering of an iron surface. The present study corroborates a thermal process in which the laser beam acts as a pulsed heat source. Energy characteristics of the emitted neutral atoms reflect rather well the thermal state of the surface, the atom temperature increasing from the ambient at threshold to melting and boiling temperatures for growing laser energies.
Scaling laws for diamond chemical-vapor deposition. I. Diamond surface chemistry
Authors: Goodwin D. G.
Published: July 2002

A simplified model of the gas-surface chemistry occurring during chemical-vapor deposition of diamond thin films is presented. The model results in simple scaling relations, useful for process scale-up and optimization, for growth rate and defect density in terms of the local chemical environment at the substrate. A simple two-parameter expression for growth rate is obtained, which with suitable parameter choices reproduces the results of more detailed mechanisms and experiment over two orders of magnitude in growth rate. The defect formation model suggests that the achievable growth rate at specified defect density scales approximately quadratically with the atomic hydrogen concentration at the substrate.
Scanning force microscope springs optimized for optical-beam deflection and with tips made by controlled fracture
Authors: Gustafsson M. G. L. , Clarke John
Published: July 2002

Springs for scanning force microscopy (SFM) are described which incorporate two separate advances: Their shape significantly reduces the effect of photon shot noise in the optical beam deflection method, and they are endowed with very sharp tips through a unique process based on controlled fracture of micromachined structures. With this readout method, the noise in the measured tip position is limited by photon shot noise, which is proportional to the ratio of the effective length of the spring to the length of the mirror. Cantilever springs optimized for this parameter are described, with a measured white noise level of 29 fm/√Hz at 210 μW of laser power; torsion springs for which the mirror length exceeds the effective length yielded a theoretical noise level of 7.2 fm/√Hz at this power. Cantilever springs optimized for this parameter are described, with a measured white noise level of 29 fm/√Hz at 210 μW of laser power; torsion springs for which the mirror length exceeds the effective length yielded a theoretical noise level of 7.2 fm/√Hz at this power. The latter is lower than the noise of an ideal fiber interferometer at the same power, demonstrating that the optical-beam-deflection method can be made competitive with interferometric methods for noise-critical SFM applications. This novel tip making technique generates cube-corner-shaped tips, with a radius which is unresolvable in scanning electron microscope images, implying that it is less than 10 nm. These tips are intended for samples whose features are in the size range 0.5–20 nm, where the tip curvature is the resolution limiting factor. To demonstrate their performance in this regime, the tips were used to image single collagen molecules.
Soft laser sputtering of InP(100) surface
Authors: Dubreuil B. , Gibert T.
Published: July 2002

Laser sputtering of InP(100) surface with 337 nm photons was investigated for fluences ranging from the threshold for particle emission up to about 370 mJ/cm2. Sputtered atoms and molecules are detected during their flight using resonant laser post-ionization and mass spectrometry. From the shot number and the energy dependencies of the sputtering yield, it is shown that two sputtering regimes exist. For low fluence values (<190 mJ/cm2), the sputtering results mainly from absorption and excitation of defect sites. Conversely, at higher fluences, interband transitions in the whole absorption volume lead after relaxation to a process similar to thermal evaporation. This thermal-like process induces the preferential emission of phosphorus in the form of atoms and molecules and the quite different velocities of phosphorus and indium populations which in absence of collisions separate during their flight. The limit between the two regimes might correspond to the point where melting of the surface occurs.
FMR studies of magnetic properties of Co and Fe thin films on Al2O3 and MgO substrates
Authors: Goryunov Yu. V. , Khaliullin G. G. , Garifullin I. A. , Tagirov L. R. , Schreiber F. , Bödeker P. , Bröhl K. , Morawe Ch. , Mühge Th. , Zabel H.
Published: July 2002

The effect of substrates on the magnetic properties has been studied for Co and Fe films both on Al2O3 (112̄0) and MgO (001) substrates by using ferromagnetic resonance techniques. For Fe(001)/MgO(001) samples the thickness dependence of the magnetocrystalline constant and of the effective magnetization values have been determined from the in-plane angular variation of the resonance field H0. Different reasons for the thickness dependencies of these parameters are discussed. For Co(111)/Al2O3(112̄0) the angular variation of H0 exhibits an uniaxial anisotropy, for which several causes are discussed. For Co(112̄0)/MgO(100) a four-fold in-plane anisotropy was observed which is due to the twinned structure of these samples.
Average widths of grain boundaries in nanophase alloys synthesized by mechanical attrition
Authors: Fultz B. , Kuwano H. , Ouyang H.
Published: July 2002

Many binary ferrous alloys were synthesized by mechanical attrition in a high-energy ball mill. X-ray diffractometry and transmission electron microscopy were used to measure grain sizes, which were as small as a few nanometers in several alloys. The nanocrystalline alloys showed new features in their Mössbauer spectra, which we associated with 57Fe atoms at and near grain boundaries. The experimental data on the fraction of 57Fe atoms at and near grain boundaries were correlated to the measured grain sizes to obtain an average width of the grain boundaries. The average grain-boundary widths of the fcc alloys Fe–Mn and Ni–Fe were approximately 0.5 nm, but the average widths of grain boundaries in the bcc alloys Cr–Fe, Mo–Fe, and Fe–Ti were somewhat larger than 1 nm.
Nondegenerate continuum model for polymer light-emitting diodes
Authors: Davids P. S. , Saxena A. , Smith D. L.
Published: July 2002

We present a device model to describe polymer light-emitting diodes (PLEDs) under bias conditions for which strong electrical injection does not occur (i.e., reverse, zero, and weak forward bias). The model is useful to interpret: capacitance–voltage measurements, which probe the charged trap density in the PLEDs; electroabsorption measurements on PLEDs, which probe the built-in electric field in the device; and internal photoemission measurements, which probe the effective Schottky barriers at the contacts of the PLED. The device model is based on the low-density nondegenerate continuum model for the electronic structure of polymers. Polarons and bipolarons are the principal charged excitations in this model. Polarons are singly charged excitations which play the primary role in charge injection and in experiments such as internal photoemission which probe single particle interface properties. Bipolarons are doubly charged excitations which can play an important role in establishing Schottky barriers at metal/polymer interfaces. In the device model, the region of the polymer near each contact is assumed to be in quasiequilibrium with that contact. The charge density as a function of position is found from the electrostatic potential and equilibrium statistics. Poisson’s equation is integrated to determine the electrostatic potential. We find that a large charge density is transferred into the polymer if the chemical potential of a contact is higher than the negative bipolaron formation energy per particle or lower than the positive bipolaron formation energy per particle. The transferred charge pins the Fermi level and establishes the effective Schottky barrier. If the contact chemical potential is between the formation energy per particle of the two types of charged bipolarons, there is little charge transfer into the polymer and the Fermi level is not pinned. The electric field in the device is found for different contacts and bias conditions. Capacitance as a function of voltage is calculated for various trap binding energies and densities. The calculated results are used to interpret recent measurements on PLEDs.
Plasma-assisted decomposition of methanol and trichloroethylene in atmospheric pressure air streams by electrical discharge processing
Authors: Hsiao M. C. , Merritt B. T. , Penetrante B. M. , Vogtlin G. E. , Wallman P. H.
Published: July 2002

Experiments are presented on the plasma-assisted decomposition of dilute concentrations of methanol and trichloroethylene in atmospheric pressure air streams by electrical discharge processing. This investigation used two types of discharge reactors, a dielectric-barrier and a pulsed corona discharge reactor, to study the effects of gas temperature and electrical energy input on the decomposition chemistry and byproduct formation. Our experimental data on both methanol and trichloroethylene show that, under identical gas conditions, the type of electrical discharge reactor does not affect the energy requirements for decomposition or byproduct formation. Our experiments on methanol show that discharge processing converts methanol to COx with an energy yield that increases with temperature. In contrast to the results from methanol, COx is only a minor product in the decomposition of trichloroethylene. In addition, higher temperatures decrease the energy yield for trichloroethylene. This effect may be due to increased competition from decomposition of the byproducts dichloroacetyl chloride and phosgene. In all cases plasma processing using an electrical discharge device produces CO preferentially over CO2.
Reaction chemistry and optimization of plasma remediation of NxOy from gas streams
Authors: Gentile Ann C. , Kushner Mark J.
Published: July 2002

Increasing environmental awareness and regulatory pressure have motivated investigations into energy efficient methods to remove oxides of nitrogen (NxOy) from gas streams resulting from the combustion of fossil fuels. Plasma remediation of NxOy is potentially an efficient removal technique due to the relative ease of generating reactants by electron-impact processes. Previous works have investigated the use of electron-beam, corona, and dielectric barrier discharge (DBD) generated plasmas for this purpose. In those works, reduction (N+NO→N2+O) and oxidation (NO2+OH→HNO3) reactions were identified as major removal channels. A computational study of the plasma remediation of NxOy from humid air using repetitively pulsed DBDs is reported. The dominant reaction pathways are discussed and scaling laws are proposed to optimize the energy efficiency of removal. Three reaction periods are identified: the current pulse (during which electron-impact processes generate radicals), the postpulse remediative period (during which NxOy is removed), and the interpulse period (during which the densities of various nitrogen oxides are reapportioned with little net removal). The lifetimes of reactants (OH and O3 in particular) determine the length of these periods and hence the optimum repetition frequency. Optimum repetition rates are typically less than hundreds of Hz. It is also found that a larger number of current pulses producing less energy deposition per pulse results in a higher removal efficiency due to reduced competition from radical-radical reactions which deplete the reactants. The production of unwanted species (e.g., O3 and N2O) can be minimized by reducing or terminating power deposition when the densities NO and NO2 have been reduced to ppm levels. The energy efficiency of remediation generally increases with increasing water content by removing NOx through the oxidation channel, although at the price of producing an acidic end product.
Two-dimensional distributions of C2, CH, and OH in a diamond depositing oxyacetylene flame measured by laser induced fluorescence
Authors: Klein-Douwel R. J. H. , Spaanjaars J. J. L. , ter Meulen J. J.
Published: July 2002

Two-dimensional laser induced fluorescence measurements are applied to the chemical vapor deposition of diamond by an oxyacetylene flame. The fluorescence distributions of C2, CH, and OH are measured for various deposition conditions. The influence of the temperature of the molybdenum substrate and the distance between the substrate and the flame front on the two-dimensional distributions and on the quality and morphology of the deposited diamond is investigated. A relation is observed between the C2 distribution in the flame and the local growth rate and quality of the diamond layer, therefore C2 is thought to be an important species for diamond growth. The exact role of CH as a growth species is less clear; OH seems to be of minor importance. All measurements are performed during diamond deposition.
Magnetocaloric properties of doped lanthanum manganite films
Authors: Morelli Donald T. , Mance Andrew M. , Mantese Joseph V. , Micheli Adolph L.
Published: July 2002

LaMnO3 films doped with Ca, Ba, or Sr have been fabricated using the metalorganic decomposition technique. These films exhibit paramagnetic-to-ferromagnetic phase transitions at 250, 300, and 350 K, respectively. By measuring the film magnetization as a function of field and temperature we have determined the entropy change associated with these transitions. The large magnetization of these materials results in a total entropy change a factor of five less than that of gadolinium, the prototypical high-temperature magnetocaloric material. Improvements in film morphology and composition may provide a further increase in the magnetization and total entropy change in these materials.
Microstructure and magnetostriction for (Dy0.65Tb0.25Pr0.1) (Fe1−xAlx)1.8 alloys
Authors: Wang B. W. , Wu C. H. , Deng W. , Tang S. L. , Jin X. M. , Chuang Y. C. , Li J. Y.
Published: July 2002

The microstructure, Curie temperature and magnetostriction of R(Fe1−xAlx)1.8 alloys (R=Dy0.65Tb0.25Pr0.1, x≤0.3) have been investigated using optical microscopy, x-ray diffraction, transmission electron microscopy, electron probe microanalysis, positron annihilation technique, vibrating sample magnetometer, and standard strain gauge technique. It was found that the alloys are essentially single phase up to x=0.30. The defects in R(Fe1−xAlx)1.8 decrease with increasing Al content when x≤0.1. The magnetostriction of the alloys decreases with increasing Al content in high applied magnetic fields, but it exhibits a peak when x=0.05 in low applied magnetic fields (H≤160 k A/m).
Morphological evolution of diamonds in combustion synthesis
Authors: Zhang Bi , Chen Sihua
Published: July 2002

This research investigates the morphological evolution of diamond films in combustion synthesis as a basis for understanding the nucleation and growth mechanisms of the diamond phase. Various diamond films were synthesized from an oxyacetylene flame on a molybdenum substrate under atmospheric conditions. To identify their morphological characteristics, the films were studied by scanning electron microscopy and were characterized by μ-Raman spectroscopy. These investigations suggest that initial diamond growth resembles a ball-like structure consisting of a mixture of graphite and diamond crystallites. As growth progresses, varying the gas ratio and substrate temperature conditions can cause the diamond crystallites to evolve from the cauliflower-like, to octahedral, and cubic structures. The nucleation and growth mechanisms of the diamond phase are discussed at the atomic, lattice, and macroscopic levels. The research concludes that morphological evolution varies with the rate of deposition and etching on both sp3 and non-sp3 bonded carbons.
Effective conductivity of composites containing aligned spheroidal inclusions of finite conductivity
Authors: Lu Shih-Yuan , Lin Hway-Chi
Published: July 2002

The effective conductivity of composites containing aligned spheroids of finite conductivity is determined with the pair interaction rigorously taken into account. The pair interaction is evaluated by solving a boundary-value problem involving two aligned spheroids with a boundary collocation scheme. Our results are in the form of virial expansion in the inclusion volume fraction f, truncated at the O(f2) term. Effective conductivities obtained from this study agree well with Willis’s [J. Mech. Phys. Solids 25, 185 (1977)] bounds and Kim and Torquato’s [J. Appl. Phys. 74, 1844 (1993)] first-passage-time simulation results. An expression for the effective conductivity tensor based on an equivalent inclusion method originated by Hasselman and Johnson [J. Compos. Mater. 21, 508 (1987)] is also derived, from which a criterion for testing the virial expansion validity is constructed. The relevant series ratio is found to be a good measure of intensity of the system thermal interaction. The smaller the value of the series ratio, the better agreement of the virial expansion with the simulation results. Explicit expressions for the second-order virial expansion for the parallel and perpendicular components of the effective conductivity tensor are tabulated for inclusion aspect ratios 1, 10/9, 2, 5, 10 and phase conductivity ratios σ=0, 0.1, 2, 10, 100, 10 000, and ∞.
Theory of spin-dependent tunneling and transport in magnetic nanostructures (invited)
Authors: Maekawa S. , Inoue J. , Itoh H.
Published: July 2002

We study the linear response theory of the electric transport in magnetic nanostructures. The conductance is expressed by using the wave functions of electrons on the Fermi level in the mean field theory at zero temperature. The theory is extended to the system with single resistive layer in which there exists electron–electron interaction and the conductance is derived in the single-site approximation. The theoretical results may be applied to a variety of magnetic nanostructures such as magnetic multilayers and ferromagnetic tunneling junctions.
Visible photoluminescence from Si clusters in γ-irradiated amorphous SiO2
Authors: Nishikawa Hiroyuki , Watanabe Eiki , Ito Daisuke , Sakurai Yuryo , Nagasawa Kaya , Ohki Yoshimichi
Published: July 2002

Visible photoluminescence (PL) bands around 2 eV were studied in 60Co γ-irradiated (dose<1 MGy) oxygen-deficient-type amorphous SiO2 (a-SiO2) excited by 2–4 eV photons. In addition to the well-known 1.9 eV PL band due to nonbridging oxygen hole centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron-spin-resonance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of the Eδ′ center, a paramagnetic state of a cluster of silicons. After much higher γ irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si-implanted a-SiO2, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters in a-SiO2 by the high-dose γ irradiation.
Properties of single crystalline semiconducting CoSb3
Authors: Caillat T. , Borshchevsky A. , Fleurial J.-P.
Published: July 2002

A study of the thermoelectric properties of the skutterudite compound CoSb3 was carried out on single crystals grown by the Bridgman gradient freeze technique. p- and n-type samples were obtained over a wide range of carrier concentration. Undoped As-grown crystals show p-type conductivity while n-type samples were obtained by addition of Te or Pd. Samples were characterized by x-ray diffractometry, electron microprobe analysis, and density measurements. The physical properties of CoSb3 such as linear thermal expansion coefficient, sound velocity, and Debye temperature were also determined and are presented. Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall effect measurements were performed between room temperature and about 900 K. Exceptionally high Hall mobilities were obtained on p-type samples with a maximum room-temperature Hall mobility of 3300 cm2 V−1 s−1 at a carrier concentration of 1×1017 cm−3. The results of the transport property measurements are discussed and are in agreement with some recent predictions based on band structure calculations. The potential of CoSb3 for thermoelectric applications is evaluated.
Microstructure dependence of electrical properties of (Ba0.5Sr0.5)TiO3 thin films deposited on Pt/SiO2/Si
Authors: Lee Won-Jae , Kim Ho-Gi , Yoon Soon-Gil
Published: July 2002

Stoichiometric (Ba0.5Sr0.5)TiO3 (BST) thin films with various thicknesses have been prepared on Pt/SiO2/Si substrates using conventional rf magnetron sputtering method with a ceramic target containing excess BaO and SrO. With increasing film thickness, the grain size of the BST films gradually increased. It was observed that the dielectric constant increased from 348 to 758 when grain size increased from 32 to 82 nm in BST films deposited at 600 °C. The decrease of dielectric constant in thinner BST films was attributed to the initial low dielectric constant layer and small grain size. In the current–voltage curves for BST films, higher leakage current densities and narrower flat regions (hopping conduction region) in the low bias field were obtained with increasing grain size.
Characterization of nanocrystalline Ni33Zr67 alloy
Authors: Liu X. D. , Umemoto M. , Deng W. , Xiong L. Y. , Ping D. H. , Lu K.
Published: July 2002

Nanocrystalline samples with grains in the range of 12–94 nm were synthesized via thermal crystallization of the amorphous alloy. High-resolution electron microscope observations show that the nanocrystalline Ni33Zr67 sample is composed of many “orientation regions.’’ Each orientation region consists of a high density of well-defined microtwin domains. Positron annihilation spectroscopy reveals that two types of defects are present in the Ni33Zr67 samples. With the increase of annealing temperature up to 973 K, the two types of defects show remarkable change while microhardness changes slightly. The contributions of the microtwin domain and the nature of constituent phases to microhardness in the present alloy system are discussed.
Two-stage switching behavior of polymer stabilized cholesteric textures
Authors: Dierking I. , Kosbar L. L. , Afzali-Ardakani A. , Lowe A. C. , Held G. A.
Published: July 2002

The electro-optical behavior of polymer stabilized cholesteric texture cells has been investigated for three different polymers. The switching process was studied with respect to the electric field dependence of the diffuse reflectivity, diffuse transmittance, and the dynamics of the reorientation process. For certain polymer concentrations, a two-stage reorientation process was observed. This behavior is consistent with the cholesteric liquid crystal being divided between two distinct environments. In the first, the liquid crystal is strongly dominated by the polymer network, while in the second a bulklike behavior, comparable to the unstabilized cholesteric material, is observed. Scanning electron micrographs of the polymer networks further support this model. Measurements of the diffuse scattering indicate that the polymer influenced regions contribute largely to the observed back scattering, whereas the bulklike material contributes primarily to forward scattering.
Domain wall displacements in amorphous films and multilayers studied with a magnetic force microscope
Authors: Pokhil Taras G.
Published: July 2002

The magnetic force microscope (MFM) was used to study the displacement of domain walls (DW) in amorphous TbFe alloy films and Co/Pd multilayer films with high spatial resolution. The reversible bending of domain wall segments pinned to defects and irreversible, jumplike displacement of domain wall segments were imaged with the MFM in an applied magnetic field. The maximum reversible displacement of domain walls was 50–100 nm and the length of the segments which reversibly curved in the field was about 150 nm. Measurement of the change in radius of curvature of a DW segment in response to an applied field allowed estimation of the DW energy density and self-demagnetizing field of the film acting on the DW. The DW energy density for the TbFe films was about 1 erg/cm2. It was shown that the self-demagnetizing field acting on a domain wall depends on the domain structure surrounding the studied DW segment. For instance, for a film with saturation magnetization 100 G and thickness 80 nm, which exhibited a mazelike domain structure, the demagnetizing field varied from 100 G in the center of a mazelike domain to 400 G near the edge of a domain. The irreversible displacement of a DW was not a continuous process. The 200–400 nm long DW segments exhibited jumplike motion over distances of 100–150 nm.
Micromagnetic studies of switching speed in longitudinal and perpendicular polycrystalline thin film recording media
Authors: Peng Qingzhi , Bertram H. Neal
Published: July 2002

Micromagnetic modeling is used to study the gyromagnetic switching process in longitudinal and perpendicular thin film media. Medium magnetization switching time under an applied uniform reverse field is studied by varying the damping constant α. The remanent coercivity dependence on the time duration of applied pulse field is studied for media with different inter-granular exchange coupling. At a constant recording density of 635 kfci, square wave recording is simulated with different bit cell time (0.4–4.0 ns) by varying the relative head medium linear speed from 10 to 100 m/s. The gyromagnetic switching limit on longitudinal and perpendicular media is examined at data rates from 500 MHz to 2.5 GHz with α=0.02 and 0.05. Incomplete magnetization reversal occurs for perpendicular media at a lower recording data rate than for longitudinal media.
Magnetic properties of exchange-coupled α-Fe/Nd–Fe–B multilayer thin-film magnets
Authors: Shindo M. , Ishizone M. , Sakuma A. , Kato H. , Miyazaki T.
Published: July 2002

The multilayer thin films having the form of Ti(30 nm)/Fe(dFe)/[Nd–Fe–B(dNd–Fe–B)/Fe(dFe)] ×5/Ti(30 nm)/glass were fabricated on glass substrates by means of radio frequency (rf) sputtering, with dFe varied from 0 to 50 nm and dNd–Fe–B from 0 to 100 nm. Magnetization measurements have revealed that the minor loops of the films are reversible in a certain range of demagnetization fields except for the films with dFe∼0 nm or dNd–Fe–B<10 nm. Micromagnetic calculation of magnetization curves has been performed, which has well reproduced the observed hysteresis loops including spring-back behavior and the dependence of the coercive field HcJ on dFe and dNd–Fe–B. From the comparison between experiment and calculation, we can infer that the interlayer exchange-coupling strength is about 10% of the intralayer couplings, which are found to be almost independent of dFe and dNd–Fe–B.
Spin dynamics in magnets: Quantum effects and numerical simulations (invited)
Authors: Antropov V. P. , Tretyakov S. V. , Harmon B. N.
Published: July 2002

We briefly review the key ideas and important physical approximations which are used to derive quantum dynamical equations of motion for the spin degrees of freedom in magnetic materials. This approach to spin dynamics uses first principles methods to obtain time and temperature dependent magnetic properties. A new stochastic approach to simulate time and temperature effects in extended spin systems is presented. Future applications and extensions are briefly covered.