bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluctuation measurements in tokamaks with microwave imaging reflectometry
Authors: Mazzucato E. , Munsat T. , Park H. , Deng B. H. , Domier C. W. , Luhmann N. C. , Donné A. J. H. , van de Pol M. J.
Published: July 2002

To study the mechanism of anomalous transport in tokamaks requires the use of sophisticated diagnostic tools for the measurement of short-scale turbulent fluctuations. In this article, we describe an attempt at developing a technique capable of providing a comprehensive description of plasma fluctuations with k⊥ρi<1, such as those driven by the ion temperature gradient mode in tokamaks. The proposed method is based on microwave reflectometry, and stems from a series of numerical calculations showing that the spatial structure of fluctuations near the cutoff could be obtained from the phase of reflected waves when these are collected with a wide aperture optical system forming an image of the cutoff onto an array of phase sensitive detectors. Preliminary measurements with a prototype apparatus on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [U. Samm, Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (IEEE, Piscataway, NJ, 1995), p. 470] confirm the validity of these conclusions. Technical issues in the application of the proposed technique to tokamaks are discussed in this article, and the conceptual design of an imaging reflectometer for the visualization of turbulent fluctuations in the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] is described.
Emission gain narrowing from single crystals of a thiophene/phenylene co-oligomer
Authors: Nagawa Michifumi , Hibino Ryota , Hotta Shu , Yanagi Hisao , Ichikawa Musubu , Koyama Toshiki , Taniguchi Yoshio
Published: July 2002

Emission gain narrowing has been observed for single crystals of a thiophene/phenylene co-oligomer. The hexagon flake crystals were placed on a quartz substrate with the crystals’ face in close contact with the substrate plane. These crystals were irradiated with a N2 laser with a 337.1 nm wavelength at a repetition rate 10 Hz that tuned its intensity to 100–1150 μJ/cm2. The emission gain narrowing takes place at 21490 (465.4 nm) and 20220 cm−1 (494.5 nm) with increased intensities, with their half width at half maxima reaching ∼50 cm−1. On the basis of the nonlinear relationship between the emission peak intensities and the laser light intensity, the gain narrowing has been attributed to the amplified spontaneous emission.
Ferromagnetic resonance in GaMnAs
Authors: Sasaki Y. , Liu X. , Furdyna J. K. , Palczewska M. , Szczytko J. , Twardowski A.
Published: July 2002

We report a systematic study of ferromagnetic resonance (FMR) on thin films of the ferromagnetic semiconductor GaMnAs grown by low temperature molecular beam epitaxy. Pronounced shifts of the FMR line are observed towards fields above and below the g=2.00 resonance position for the magnetic field orientations perpendicular and parallel to the plane of the layer. At higher temperatures these shifts decrease rapidly as the Curie temperature is approached, converging on the g=2.00 resonance position that is characteristic of the Mn++ ion, in agreement with the effects of demagnetization on FMR in thin film geometry. At low temperatures, however, all specimens show a striking deviation from the simple demagnetization-induced picture. We ascribe this to anisotropy fields, which increase in importance as the temperature decreases.
Quantitative photothermal characterization of ion-implanted layers in Si
Authors: Salnick Alex , Opsal Jon
Published: July 2002

Quantitative analysis of ion-implanted layers in Si using the damage-based theoretical modeling and experimental results obtained with the photomodulated reflectance (PMR) technique are described. Our theoretical approach combines the conventional quantum mechanics based calculations of the ion-induced damage depth profiles in semiconductors with the corresponding scaling of the thermal and carrier plasma parameters followed by the calculation of the photothermal response from a multilayered sample. The theoretical limit of the photothermal signal sensitivity to the implantation dose in the absence of optical and carrier plasma-wave interference effects is estimated. Simulations of the photothermal amplitude and phase dose dependencies allow us to follow the dynamics of the thermal- and carrier plasma waves in an ion-implanted semiconductor. The validity of the proposed damage-based modeling approach to the problem of quantitative analysis of surface-modified semiconductors is analyzed. It is shown that the results of the photothermal damage-based modeling are in a very good agreement with experimentally observed PMR signal implantation dose behavior for B+-implanted Si across the entire range of practically important implantation doses: 109–1015 cm−2.
Electron paramagnetic resonance in boron carbide
Authors: Kakazey M. G. , Gonzalez-Rodriguez J. G. , Vlasova M. V. , Shanina B. D.
Published: July 2002

Electron paramagnetic resonance (EPR) signals from powdered samples of boron carbide B4C are recorded at g factor 2.0028±0.0002. The dependence on temperature and thermal treatment of the samples is studied. We demonstrated that native defects of boron carbide and conduction electrons are responsible for EPR absorption. The temperature dependence of the EPR line width ΔB(T) is accounted for via the Korringa mechanism. By fitting experimental and theoretical curves for ΔB(T), distances of donor levels from the edge of the conduction band ΔEd are obtained. The process of sample preparation strongly effects the value of ΔEd. We assume a radical formation of the type B3C with local concentration Nloc∼1019–2×1020 cm−3 is responsible for the EPR signal under these conditions.
Electro-optical characteristics and switching behavior of a twisted nematic liquid crystal device based upon in-plane switching
Authors: Oka Shinichirou , Kimura Munehiro , Akahane Tadashi
Published: July 2002

A driving mechanism for a twisted nematic liquid crystal device that could possibly improve the viewing-angle characteristics and the cell gap error margin is proposed. It is important that the surface azimuthal anchoring strength of the liquid crystal cell differs at the upper and lower substrates, unlike a conventional twisted nematic (TN) mode. Electro-optical characteristics were investigated with an electric field applied in the plane of the substrates. It is numerically demonstrated and predicted that the proposed driving mechanism can have both the desirable features of in-plane switching mode (such as an excellent viewing angle) and those of a conventional TN type device (such as a small color shift and wide cell gap margin).
Nested sums, expansion of transcendental functions, and multiscale multiloop integrals
Authors: Moch Sven , Uwer Peter , Weinzierl Stefan
Published: July 2002

Expansion of higher transcendental functions in a small parameter are needed in many areas of science. For certain classes of functions this can be achieved by algebraic means. These algebraic tools are based on nested sums and can be formulated as algorithms suitable for an implementation on a computer. Examples such as expansions of generalized hypergeometric functions or Appell functions are discussed. As a further application, we give the general solution of a two-loop integral, the so-called C-topology, in terms of multiple nested sums. In addition, we discuss some important properties of nested sums, in particular we show that they satisfy a Hopf algebra.
Long wavelength (15 and 23 μm) GaAs/AlGaAs quantum cascade lasers
Authors: Ulrich Jochen , Kreuter Johann , Schrenk Werner , Strasser Gottfried , Unterrainer Karl
Published: July 2002

Quantum cascade lasers based on GaAs/AlGaAs chirped superlattice active regions have been achieved at wavelengths of 15 and 23 μm. In pulsed mode they operate up to temperatures of 220 and 100 K, respectively. While the 15 μm-laser employs a standard waveguide with n+-doped cladding, the 23 μm-laser was made with a metallic surface plasmon waveguide. The threshold current densities at cryogenic temperatures of 2.2 kA/cm2 (15 μm) and 10.2 kA/cm2 (23 μm) reflect the differences in intersubband lifetimes and waveguide losses close to the reststrahlenband.
Optimal chaos control through reinforcement learning
Authors: Gadaleta Sabino , Dangelmayr Gerhard
Published: July 2002

A general purpose chaos control algorithm based on reinforcement learning is introduced and applied to the stabilization of unstable periodic orbits in various chaotic systems and to the targeting problem. The algorithm does not require any information about the dynamical system nor about the location of periodic orbits. Numerical tests demonstrate good and fast performance under noisy and nonstationary conditions.
Complex mixed-mode periodic and chaotic oscillations in a simple three-variable model of nonlinear system
Authors: Kawczyński Andrzej L. , Khavrus Vyacheslav O. , Strizhak Peter E.
Published: July 2002

A detailed study of a generic model exhibiting new type of mixed-mode oscillations is presented. Period doubling and various period adding sequences of bifurcations are observed. New type of a family of 1D (one-dimensional) return maps is found. The maps are discontinuous at three points and consist of four branches. They are not invertible. The model describes in a qualitative way mixed-mode oscillations with two types of small amplitude oscillations at local maxima and local minima of large amplitude oscillations, which have been observed recently in the Belousov–Zhabotinsky system.
Lattice defects within grain volumes that affect the electrical quality of cast polycrystalline silicon solar-cell materials
Authors: Yoo K. C. , Johnson S. M. , Regnault W. F.
Published: July 2002

Lattice defect structures within large grains of cast polycrystalline silicon have been investigated using optical microscopy, x-ray topography, and electron-beam-induced conductivity (EBIC) measurements. Optical observations through a series of wafers of an ingot indicate that kinks in high-angle grain boundaries develop by forming straight grain boundaries rather than curved boundaries. It is also observed that a junction of three grain forms a kink in the grain boundary. X-ray topographic observations of lattice defects show that cumulative dislocations are generated at these kinks when the material is subjected to thermal stresses at sufficiently elevated temperatures during solidification. Subsequently the dislocations propagate into the grain volume and form subgrain boundaries. In some cases, they pile up in the coherent {111} twin boundary interfaces. Observed EBIC constrast has shown that both dislocations and subgrain boundaries are electrically active and serve to decrease the effective minority carrier diffusion length.
Optically enhanced defect reactions in semi-insulating bulk GaAs
Authors: Jiménez J. , González M. A. , Hernández P. , de Saja J. A. , Bonnafé J.
Published: July 2002

We investigate in this paper a photomemory effect in the photocurrent of semi-insulating GaAs when it is illuminated with photons ranging from 1 to 1.35 eV. We observe a strong enhancement of the photocurrent after long excitation with those photons, which is thermally quenched between 123 and 135 K. We explain this phenomenon by a defect reaction, dealing with the association of two or more complex defects, as a result of the electrostatic interaction between the original defects when their charge state is changed by means of the illumination. The formation of this complex drives the sample into a high-sensitivity state, which we have called ‘‘on-state.’’ The dissociation of the photogenerated complex defect, due to thermal emission of electrons to the conduction band, restores the crystal into its normal photosensitivity state, which we have called ‘‘off-state.’’
Comparison between one-dimensional and two-dimensional models for Josephson junctions of overlap type
Authors: Eilbeck J. C. , Lomdahl P. S. , Olsen O. H. , Samuelsen M. R.
Published: July 2002

A two-dimensional model of Josephson junction of overlap type is presented. The energy input is provided through induced magnetic fields modeled by a set of boundary conditions. In the limit of a very narrow junction, this model reduces to the one-dimensional model. Further, an equation derived for the critical current leads in this limit to the critical current obtained from the one-dimensional model. Comparisons between stationary fluxon velocities obtained from the two models by means of numerical computations show that the difference is negligible. This supports the experimental observation that measurements are insensitive to the width of the junction.
Behavior of oxygen in the crystal formation and heat treatment of silicon heavily doped with antimony
Authors: Nozaki Tadashi , Itoh Yoshiko , Masui Tsumoru , Abe Takao
Published: July 2002

Using charged particle activation analysis, oxygen concentration in silicon crystals heavily doped with antimony has been shown to be considerably lower than in undoped crystals grown under otherwise the same conditions. The difficulty of oxygen precipitation in these doped crystals is therefore due to the lowered oxygen concentration and also to the difference in the dopant effect on precipitate formation.
Effect of the statistical shift on the anomalous conductivities of n-type hydrogenated amorphous silicon
Authors: Yoon Byung-Gook , Lee Choochon , Jang Jin
Published: July 2002

We calculated the shift of Fermi energy EF with temperature, using a model density of states for hydrogenated amorphous silicon (a-Si:H) similar to that of deep level transient spectroscopy results, by a numerical method. The conductivity σ was calculated from the calculated EF as a function of temperature. It was found that some features of anomalous transport phenomena of n-type a-Si:H such as kinks or the continuous bending of log σ vs 1/T curves and the Meyer–Neldel-type preexponential factors can be explained, at least in part, by the statistical shift alone.
Extrinsic photoconductive characteristics of semi-insulating GaAs crystals
Authors: Mita Yoh
Published: July 2002

Extrinsic photoconductive characteristics of undoped, semi-insulating GaAs have been investigated, especially the characteristic enhancement effect at low temperatures. It has been shown that two quasistable photoconductive states exist, in which photoconductivity differs by orders of magnitude and that the two states are partly interchangeable by irradiating with an appropriate wavelength light. Spectra and temperature dependencies were investigated to elucidate the enhancement mechanism. Discussions are made on the basis of metastable states in As-rich defects.
Optical self-regulation during laser-induced oxidation of copper
Authors: Baufay L. , Houle F. A. , Wilson R. J.
Published: July 2002

The oxidation of copper induced by a cw Ar+ laser beam is investigated using time-resolved reflectance measurements at λ=632.8 and 514.5 nm, together with supporting studies of film properties by scanning Auger and electron microscopies. The optical measurements and film composition analyses indicate that the oxide layer is mainly Cu2O. Since this material is partially transparent to both the probe and oxidizing laser beams, interference of each beam within the film results. This gives rise to strong reflectance variations which can be used to gain information about the oxidation reaction via theoretical simulations of the process. A model which explicitly treats the intimate relationship between temperature, oxide growth, and optical absorption is proposed. Free of adjustable parameters, the theory uses only optical, thermal, and furnace oxidation data from the literature, and gives calculated curves in good agreement with experiment. The results of this study have important consequences for investigations of the kinetics of laser-induced reactions whenever they are driven by absorption of light by a system whose optical properties depend on the reaction, i.e., are optically self-regulating.
Evaluation of the interface state energy distribution from Schottky I-V characteristics
Authors: Horváth Zs. J.
Published: July 2002

The effect of the presence of an interfacial layer and interface states on the I-V characteristics of Schottky contacts is analyzed. A simple evaluation of the interface state energy distribution and the interfacial layer thickness to its relative dielectric constant ratio (relative interfacial layer thickness) is presented for two different models concerning the quasi-Fermi level position in Schottky contacts. The first experimental results evaluated from the I-V characteristics of n-type (100) GaAs-Cr/Au mesa Schottky contacts, are in agreement with those published earlier.
Bleaching and recovery characteristics of optical absorption bands in semi-insulating GaAs crystals
Authors: Mita Yoh
Published: July 2002

Optical absorption and related characteristics in undoped, semi-insulating GaAs crystals have been investigated, especially at the bleaching and thermal recovery stages. It has been shown that a broad absorption band at a longer wavelength grows as bleaching proceeds, in a reciprocal relation with the near-infrared absorption band. It has been found that the bleached near-infrared absorption shows rather abrupt recovery at a temperature around 130 K and that the recovery profile, i.e., the temperature derivative of the recovering absorption, is dependent upon the crystal composition and thermal prehistory. Hence, the present method, recovery characteristics of the bleached absorption method, may have applicability for elucidating defect properties as well as crystal characterization. Simultaneous measurements of extrinsic photoconductivity combined with related experimental results have lead to a conclusion that the extrinsic photoconductivity in the enhanced state is related to the metastable state of the deep lying defects. These experimental results are discussed systematically on the basis of the defect transition to and recovery from the metastable state.
Magnetic anisotropy of IBS Ni films
Authors: Sato K. , Kondo H. , Mizoguchi T.
Published: July 2002

The magnetic anisotropy in Ni films deposited on Pyrex glass substrates by different sputtering methods, i.e., ion beam sputtering (IBS), triode rf sputtering, and diode rf sputtering, was studied in relation to the internal stress in the films. Strong in-plane easy magnetic anisotropy (Ku=−4.9×105 erg/cm3) was observed in IBS Ni films. The average in-plane stress in the IBS film was estimated from the radius of convex curvature of the disk substrate as large as −1.2×1010 dyn/cm2 (a negative sign implies the compressive stress), which is due to the atomic peening effect by kinetic incident atoms during deposition. The stress-induced magnetic anisotropy is calculated to be about −1.2×106 erg/cm3, which is overwhelming to the observed magnetic anisotropy in the IBS Ni film. Positive Ku and tensile internal stress, one or two order of magnitude smaller in absolute values than that in IBS film, were observed in Ni films prepared by the other sputtering methods.
As+ and Ga+ implantation and the formation of buried GaAs layers in silicon
Authors: Madakson Peter , Ganin Eti , Karasinski J.
Published: July 2002

A buried layer of GaAs was formed in single-crystal silicon by dual implantation of extremely high doses of As+ plus Ga+ at 200 keV, followed by furnace annealing. The layer consists of polycrystalline grains with random orientation. Rapid thermal annealing, in the presence of oxygen, does not result in the formation of GaAs. Instead, Ga and As migrate to the surface to form an oxidized layer, which is separated from the underlying silicon by a thin layer of SiO2. Analysis of the samples with single implants of Ga+ or As+ indicates the oxides formed to be Ga2O3 and As2O2. Samples implanted with As+ alone have essentially dislocation loops after annealing, while those implanted with only Ga+ have mostly microtwins and precipitates. Up to 88% Ga and 62% As from the single implants and 31% As and Ga from the dual implants are lost during annealing. This is probably due to the migration of the implanted species to the surface and the subsequent formation of volatile oxides. However, such outward migration does not result in redistribution or broadening of the implanted species.
Interparticle correlations in Fe3O4 ferrofluids, studied by the small-angle neutron scattering technique
Authors: Rosman R. , Janssen J. J. M. , Rekveldt M. Th.
Published: July 2002

Aggregation in sterically stabilized Fe3O4 ferrofluids has been studied with the small-angle neutron scattering technique. The influence on the aggregation of the particle volume fraction, varying between 0.02 and 7 vol % Fe3O4, and the influence of an applied magnetic field is examined. Contrast variation enabled the scattering of particles and surfactants to be measured independently. The particle size distribution derived from the scattering is in good agreement with the field dependence of the magnetization and the optical anisotropy. From the anisotropy in the neutron scattering patterns of fluids subjected to a magnetic field of 0.10 T perpendicular to the neutron beam, it is derived that at all volume fractions below 1 vol % Fe3O4 most of the particles are located in small chainlike aggregates oriented parallel to the field direction. Larger, three-dimensional aggregates arise at higher volume fractions, both in the applied field and in zero field. The interparticle distance within an aggregate corresponds to surface-to-surface contact of the surfactant coatings.
Photothermal reflectance investigation of processed silicon. I. Room-temperature study of the induced damage and of the annealing kinetics of defects in ion-implanted wafers
Authors: Christofides Constantinos , Vitkin I. Alex , Mandelis Andreas
Published: July 2002

Silicon films damaged by ion implantation have been examined using the photothermal reflectance technique in the thermal wave regime (low-modulation frequencies, large laser beam spot sizes). Data are presented on the sensitivity of this method to the implant dose and to the effects of thermal annealing. It has been shown that the technique provides information about the state of the implanted layer, and is a sensitive probe for monitoring the annihilation of the induced damage as a function of the annealing temperature. A model for the kinetics of damage annihilation has been presented to estimate the activation energy of the local annealing recovery mechanism, found to be 0.15 eV. The presence of negative annealing has been detected at about 500 °C, an advantage over the essentially insensitive Hall mobility method.
Annealing behaviors of dislocation loops near the projected range in high-dose As+- implanted (001)Si
Authors: Hsu S. N. , Chen L. J. , Wu S. C.
Published: July 2002

Annealing behaviors of dislocation loops formed near the projected range (Rp loops) in 150 keV, 5×1015/cm2 As+- implanted (001)Si have been studied by both planview and cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry. Factors influencing the formation and growth of the Rp loops, such as annealing temperature and time, as well as single- and two-step annealings have been investigated. The Rp loops were observed in samples annealed at 550–900 °C for 0.5 h. The loops were also found in samples annealed at 800 °C for a time as short as 1 s and as long as 8 h. However, they were not seen in samples annealed at 470 °C for 4 h, 500 °C for 2 h, or 550 °C for 15 min. The annealing behaviors of Rp loops in single- and two-step annealed samples and in samples with an oxide capping layer were found to be consistent with the suggestion that their formation is related to the agglomeration of self-interstitials mediated by the presence of a high concentration of electrically inactive arsenic atoms.
Helium implantation effects in hard hydrogenated carbon layers
Authors: Boutard D. , Gudowska I. , Scherzer B. M. U. , Möller W.
Published: July 2002

Hard amorphous films of a-C:D were deposited by a rf glow discharge in CD4. The implantation of 40-keV 3He+ ions in these films was studied as a function of the helium fluence at temperatures between 100 K and room temperature. The trapped amounts of helium and the deuterium losses in the layer were measured in situ using nuclear reaction analysis with 0.5-MeV D+ and 1-MeV 3He+ beams, respectively, and subsequent ex-situ elastic-recoil detection analysis. A transient helium retention appears in the carbonized layers, occurring only at temperatures below 200 K. Above a critical fluence which depends on temperature and ion flux, outdiffusion of He is observed which is ascribed to the formation of diffusion channels by radiation damage. The deuterium depletion induced by helium bombardment below 200 K sets on in correlation with the helium outdiffusion, but is otherwise independent of the helium trapping.
Photoluminescence study of native defects in the filled tetrahedral semiconductor LiZnP
Authors: Kuriyama K. , Mineo N. , Takahashi Yukimi
Published: July 2002

The radiative recombination related to a native defect in a direct wide-gap semiconductor LiZnP (Eg = 2.13 eV at 77 K) was studied at 77 K using photoluminescence (PL) technique. One type of crystal yields PL emission that consists of two peaks: One is a peak at 615 nm (2.02 eV) associated with a phosphorus vacancy (lying at 110 meV below the conduction band)-valence band transition and the other a broad peak at 848 nm (1.46 eV) associated with a phosphorus vacancy (VP)-acceptor complex. Another type of crystal exhibits only a broad emission around 830 nm. Both types of PL emissions are attributed to the difference in concentration between free VP defects and VP-acceptor complexes in as-grown crystals. The origin of the broad emissions observed in both types is likely to be essentially identical.
Onset of thermal convection in an anisotropic and inhomogeneous porous layer underlying a fluid layer
Authors: Chen Falin , Hsu Long H.
Published: July 2002

The onset of thermal convection in a single-component fluid layer bounded above by a rigid wall and saturating an underlying porous medium whose permeability and thermal diffusivity may be anisotropic and inhomogeneous is considered. A linear stability analysis is used to systematically implement a parametric study that covers a wide range of depth ratio ζ (ratio of fluid layer depth to saturated porous layer depth), the permeability ratio ξ (ratio of horizontal to vertical permeability), thermal diffusivity ratio ξT (ratio of horizontal to vertical thermal diffusivity), and inhomogeneities of permeability and thermal diffusivity of the porous medium in the vertical direction. For ζ≤0.1, the critical Rayleigh number Rcm was found to be essentially an explicit function of ξT/ξ and the corresponding critical wave number acm was found to be a function of (ξ×ξT)−1/4; since the porous layer dominates the two-layer system by convection, the anisotropic and inhomogeneous effects are significant. For ζ≥0.2, however, no explicit function for either Rcm or acm is obtained and the anisotropic and inhomogeneous effects are not significant because the onset of convection is largely confined to the fluid layer. Generally, the vertically increasing permeability leads to a less stable state with smaller critical wavelength; and the vertically increasing thermal diffusivity leads to a more stable state with larger critical wavelength. 
Magnetic susceptibility of Pr2CuO4 monocrystals and polycrystals
Authors: Földeáki Maria , Ledbetter Hassel , Hidaka Yoshikazu
Published: July 2002

We measured the field dependence and temperature dependence of Pr2CuO4 monocrystals and polycrystals and found indications of field-dependent Cu+2-Pr+3 interactions. One can interpret the non-Curie-Weiss behavior by considering crystal-field effects.
Flux density dependence of magneto-acoustic emission in pipeline steel
Authors: Dhar Ajay , Atherton D. L.
Published: July 2002

Magneto-acoustic emission (MAE) in pipeline steel was investigated to study its dependence on the amplitude of the excitation flux density, ‖B‖, in the sample. At low magnetizations, the MAE rms voltage exhibited a nearly linear dependence on ‖B‖ in the sample, while at higher magnetizations it tended to saturate. The ‖B‖ dependence of MAE activity was analyzed in terms of the pulse-height distribution of the MAE wave forms. It was observed that at low ‖B‖, the MAE activity is due to domain wall motion while at high ‖B‖ the genesis seems to be also from rotation of the domain magnetization vectors. The importance of analyzing MAE activity in terms of the pulse-height distribution of the MAE wave forms is emphasized.
Tunable, short pulse hard x-rays from a compact laser synchrotron source
Authors: Sprangle P. , Ting A. , Esarey E. , Fisher A.
Published: July 2002

A compact laser synchrotron source (LSS) is proposed as a means of generating tunable, narrow bandwidth, ultra-short pulses of hard x rays. The LSS is based on the Thomson backscattering of intense laser radiation from a counterstreaming electron beam. Advances in both compact ultra-intense solid-state lasers and high brightness electron accelerators make the LSS an attractive compact source of high brightness pulsed x rays, particularly at photon energies beyond ∼30 keV. The x-ray wavelength is λ[Å]=650 λ0[μm]/Eb2[MeV], where λ0 is the laser wavelength and Eb is the electron beam energy. For Eb=72 MeV and λ0=1 μm, x rays at λ=0.12 Å (100 keV) are generated. The spectral flux, brightness, bandwidth, and pulse structure are analyzed. In the absence of filtering, the spectral bandwidth in the LSS is typically ≲1% and is limited by electron beam emittance and energy spread. Two configurations of the LSS are discussed, one providing high peak power and the other moderate average power x rays. Using present day technology, the LSS can generate picosecond pulses of x rays consisting of ≳109 photons/pulse with a peak brightness of ≳1020 photons/s mm2 mrad2 (0.1% BW) and photon energies ranging from 50 to 1200 keV.
Magnetic order and critical behavior at surfaces of ultrathin Fe(100)p(1×1) films on Pd(100) substrates
Authors: Rau C. , Mahavadi P. , Lu M.
Published: July 2002

The magnetic order and critical behavior at well-defined surfaces of ultrathin (1.9–2.1) monolayer Fe(100)p(1×1) films, deposited at room temperature on atomically flat Pd(100) substrate crystals, is investigated using electron-capture spectroscopy (ECS). Ferromagnetic order sets in at a surface Curie temperature TCs=613.1 K. At room temperature, the long-ranged electron-spin polarization (ESP) amounts to 34% and is oriented in-plane for well-annealed films. Near TCs, the ESP follows the power law (TCs−T)β, with a critical exponent β=0.125±0.01 evaluated near TCs. This result is in good agreement with the exact value β=1/8 of the two-dimensional Ising model with short-ranged forces. For temperatures far from TCs, the ESP follows precisely the temperature dependence of the magnetization as predicted by Yang’s exact solution of the two-dimensional Ising model including long-ranged forces.
Annealing behavior of MeV implanted carbon in silicon
Authors: Isomae Seiichi , Ishiba Tsutomu , Ando Toshio , Tamura Masao
Published: July 2002

The annealing behavior of carbon implanted at 1.3 MeV with a dose of 5×1019 ions/m2 into Czochralski-grown silicon wafers is investigated using an x-ray double-crystal method, transmission electron microscopy, secondary-ion-mass spectroscopy, and infrared-absorption spectroscopy. For comparison, the behavior of boron implanted at 2.0 MeV with 5×1019 ions/m2 is also investigated. X-ray rocking curve analysis shows that carbon produces a larger lattice strain than boron. For the samples annealed at 1273 K, x-ray data indicate that the carbon atoms, unlike the boron atoms, do not occupy substitutional sites. In addition, the present experimental data suggest that the interaction of interstitial carbon atoms with silicon self-interstitials produced by ion implantation suppresses the generation of dislocations and brings about the reduction of lattice strain in the implanted region.
Microstructural studies of epitaxial CoSi2 layers on silicon produced by ion beam synthesis and rapid thermal annealing
Authors: Meekison C. D. , Booker G. R. , Reeson K. J. , Spraggs R. S. , Gwilliam R. M. , Sealy B. J.
Published: July 2002

Structures of cobalt disilicide layers fabricated by ion beam synthesis on (001) silicon wafers have been studied by cross-sectional transmission electron microscopy. Implantation at 350 °C with doses of 5 and 7×1017 cm−2 of 200 keV Co+ ions was used, followed by rapid thermal annealing. For the as-implanted wafer with the lower dose, a CoSi2 layer in a parallel (A-type) epitaxial orientation was formed, and below this there were CoSi2 precipitates, some in twinned (B-type) orientations, and {113} defects. With the higher dose, polycrystalline CoSi was also present at the surface and there was substantial surface roughening. For the annealed wafers, as the annealing temperature increased from 700 to 1100 °C, the CoSi2 layer progressively increased in thickness, and the CoSi at the surface of the CoSi2 layer was eliminated. In the silicon beneath the silicide layer, the CoSi2 precipitates were greatly reduced in number and the {113} defects were eliminated.
Photothermal measurements near a 90° edge. II. Characterization of subsurface rectangular voids and close planar cracks
Authors: Sánchez-Lavega A. , Salazar A.
Published: July 2002

We investigate the scattering of spherical thermal waves by planar subsurface structures located close to the corner of an opaque and thermally thick sample. Two kinds of planar features have been studied theoretically and experimentally: isolating rectangular holes and weakly scattering thermal barriers. We have modeled by means of the Green’s function method the temperature of the heated surface for each case, incorporating the Gaussian shape of the exciting beam and the strength (thermal resistance) of the barrier. The resulting thermal field is the convolution of that produced by the scattering by the subsurface structure and by the 90° edge. Good qualitative agreement is found between the theoretical surface temperature and the photothermal signal measured with a mirage device on fabricated structures on a niquel sample. We discuss how the scattering processes influence the spatial forms of the normal (φn) and transverse (φt) mirage deviations, and how they are also modified when probe and pump beams are centered and fixed for imaging inspection. These results can be employed not only to detect and separate the effect of the corner from that of the feature, but also to characterize the subsurface structure.
Die-upset Pr-Fe-B-type magnets from melt-spun ribbons
Authors: Fuerst C. D. , Brewer E. G. , Mishra R. K. , Zhu Yimei , Welch D. O.
Published: July 2002

In order to optimize the magnetic properties of die-upset Pr-Fe-B magnets, we have systematically altered alloy compositions, including partial substitution of cobalt and neodymium, and low-level additions of gallium. In general, the remanences of Pr-Fe-B die-upset magnets were lower by ∼1 kG, and coercivities were higher by ∼4 kOe, than analogous Nd-Fe-B magnets. The microstructure of the Pr-Fe-B die-upset magnets was similar to Nd-Fe-B magnets. Both systems are two phase, consisting of oriented flat platelet-shaped grains of 2-14-1 separated by a rare earth-rich intergranular phase. The intergranular phase is thicker for the Pr-Fe-B magnets, resulting in enhanced magnetic domain wall pinning and increased coercivity. Even higher coercivities were obtained with small amounts of cobalt substitution (≤5 at. % of the transition metal) and by very low-level additions of gallium (≤0.5 at. %). The largest remanence (Br=13.7 kG) for a Pr-based 75% die-upset magnet was obtained with the alloy (R13TM81B6)0.9975Ga0.25, where R=(Pr0.8Nd0.2) and TM=(Fe0.95Co0.05).
Carrier tunneling and device characteristics in polymer light-emitting diodes
Authors: Parker I. D.
Published: July 2002

In this paper it is demonstrated that the characteristics of light-emitting diodes based upon MEH-PPV [more fully known as poly(2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene- vinylene)] are determined by tunneling of both the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating voltage of the device as well as its efficiency. A model is developed that clearly explains the device characteristics of a wide range of diodes based upon MEH-PPV. The turn-on voltage for an ideal device is shown to be equal to the band gap, i.e., 2.1 eV for MEH-PPV, and is slightly lower at 1.8 eV for an indium-tin oxide/MEH-PPV/Ca device. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I–V characteristics, while the larger barrier determines the device efficiency. In indium-tin-oxide/MEH-PPV/Ca devices, the barrier to hole injection is 0.2 eV and the barrier to electron injection is only 0.1 eV. This combination of electrodes is close to optimal for MEH-PPV, but lowering the hole barrier can still lead to a doubling of the device efficiency.
Transparent and conductive impurity-doped GaN thin films prepared by an electron cyclotron resonance plasma metalorganic chemical vapor deposition method
Authors: Sato Hirotoshi , Minami Tadatsugu , Yamada Eiji , Ishii Makoto , Takata Shinzo
Published: July 2002

Transparent and conductive doped GaN thin films have been prepared on glass substrates by the metalorganic chemical vapor deposition method utilizing electron cyclotron resonance plasma. Highly transparent oxygen-, silicon-, and titanium-doped GaN thin films with resistivities of 1.8×10−2, 3.8×10−2, and 8.7×10−3 Ω cm, respectively, were prepared at 350 °C using triethylgallium and ammonia (NH3) as source gases. Carrier concentrations above 2×1020 cm−3 were obtained for all three types of GaN thin films doped with each impurity. These GaN thin films exhibited excellent thermal and chemical stability in various gas environments up to 350 °C and chemical stability in either acid or basic solutions depending on the dopant used.
Pyroelectric and electro-optical effects in the SmC* phase of a polysiloxane liquid crystal
Authors: Kocot A. , Wrzalik R. , Vij J. K. , Zentel R.
Published: July 2002

The pyroelectric coefficient, the spontaneous polarization Ps, the optical tilt angle θ, and the switching time of a polysiloxane liquid crystalline polymer have been measured. The pyroelectric coefficient is found to be one order of magnitude greater and the response broader than for a low molar liquid crystal reported in the literature and is comparable to that of the solid ferroelectrics. Ps vs θ is found to be nonlinear, and the ratio of Ps/θ shows anomalies close to the SmC*-SmA transition temperature. The measurements of θ and the optical response time are used to calculate the switching time which spans from 1 to 100 ms, depending on the temperature. The optical response time in the SmA phase close to the SmC*-SmA transition temperature, together with a finite value of the pyroelectric coefficient, provide evidence of a SmC* phase induced by an external field. The rotational viscosity as a function of temperature shows critical behavior at the SmC*-SmA phase transition temperature.
Effect of boron doping on the structural properties of polycrystalline silicon films grown at reduced pressures
Authors: Lin Horng-Chih , Lin Hsiao-Yi , Chang Chun-Yen , Jung Tz-Gwei , Wang P. J. , Deng Ray-Chern , Lin Jandel
Published: July 2002

The deposition and properties of in situ boron-doped polycrystalline silicon (poly-Si) films grown at 550 °C were investigated using an ultrahigh vacuum chemical vapor deposition system. It is observed that, if the doping level is high enough, the boron incorporation would significantly reduce the deposition rate, impede the grain growth, and degrade the crystallinity of the poly-Si films. We also found that the preferential adsorption of boron atoms on the SiO2 surface at the initial stage of deposition shortened the incubation time of deposition. The property of trapping states at the grain boundary is also examined, and a density of about 4.7×1012 cm−2 is obtained for poly-Si films with a doping level less than 2.2×1019 cm−3.
Analysis of the thermal and acoustic properties of ion-implanted diamondlike carbon films using the transient reflecting grating technique
Authors: Shen Qing , Harata Akira , Sawada Tsuguo
Published: July 2002

The transient reflecting grating (TRG) technique was applied to evaluate both nitrogen-ion-implanted and nonimplanted thin-film coatings (thickness: 40 nm) of diamondlike carbon (DLC) on metallic multilayered substrates. Different TRG responses corresponding to the different nitrogen ion doses were observed. Hypersonic surface wave velocities as well as the thermal diffusivities were obtained with a variety of grating spacings. Effective elastic moduli (Young’s moduli) for the DLC coatings were given by using a theoretical treatment with a model. Nitrogen ion implantation hardened the DLC coating.
Correlation of YBa2Cu3O7 step-edge junction characteristics with microstructure
Authors: Herrmann K. , Kunkel G. , Siegel M. , Schubert J. , Zander W. , Braginski A. I. , Jia C. L. , Kabius B. , Urban K.
Published: July 2002

Current-voltage characteristics, Josephson radiation spectra, and critical current versus magnetic-field dependences were measured in epitaxial, c-axis YBa2Cu3O7 step-edge Josephson junctions (SEJs) on SrTiO3 and LaAlO3 substrates with various step angles α. The results were correlated with microstructural data to determine the origin of the observed weak-link behavior. It was shown that on steps with α≳45° the SEJ is a series connection of two weak links unambiguously correlated with the occurrence of two 90° tilt grain boundaries. On steep steps, α≥70°, the boundary at the upper step edge has, on average, the (103) symmetry, while the lower one is predominantly of the basal-plane-faced (010)(001) type. Correspondingly, one link is weaker than the other, with the weaker link originating on the (010)(001) boundary. However, others have shown that analogous grain boundaries in planar (103) and biepitaxial a-axis/c-axis films do not exhibit a strong magnetic-field dependence of critical current, which is characteristic of a weak link. Hence, it is proposed that the weak-link behavior of boundaries on step edges originates from their defect structure.
Electrical conduction and deep levels in polycrystalline diamond films
Authors: Gonon P. , Deneuville A. , Fontaine F. , Gheeraert E.
Published: July 2002

We have studied the dark conductivity (field, temperature, and frequency dependence), and the photoconductivity in undoped polycrystalline diamond films. Detailed analysis reveals that either of two alternative models can be invoked to explain all the observed features of the dark conductivity. The first model is a Hill-type hopping conduction involving the presence of discrete acceptor states located at 0.91 eV above the valence band with a density around 1017 cm−3. The second model involves the presence of a band-tail of acceptor states extending about 1 eV above the valence band. In this case, variable range hopping conduction dominates at low fields with a density of states at the Fermi level around 5×1015 cm−3 eV−1, while space charge limited currents dominate at high fields. The states controlling the dark conductivity give rise to photoconduction with a threshold around 0.85 eV and a peak at 1.1 eV. The shape of the photoconductivity spectrum suggests that lattice relaxation (with a Franck-Condon shift around 0.08 eV) occurs at these states. Peaks in the photoconductivity at 1.4 eV and at 1.9 eV give evidence for the presence of deeper states in these films.
Particle clustering and dielectric enhancement in percolating metal–insulator composites
Authors: Doyle William T.
Published: July 2002

An effective cluster model has been developed [Phys. Rev. B 42, 9319 (1990)] that treats a disordered suspension of monodisperse metal spheres as a mixture of isolated spheres and close-packed spherical clusters of spheres using the Clausius–Mossotti or Maxwell equations. The effective cluster model is adapted to such suspensions with a random intermingled cluster topology using Bruggemann’s symmetrical equation. Model susceptibilities for the two cluster topologies are contrasted with one another and compared with experiments. Guillien’s permittivity measurements [Ann. Phys. (Paris) Ser. 11 16, 205 (1941)] and Turner’s conductivity measurements [Chem. Eng. Sci. 31, 487 (1976)] exemplify nonpercolating island topology suspensions. The permittivity measurements of Grannan, Garland, and Tanner [Phys. Rev. Lett. 46, 375 (1981)] exemplify percolating random topology clusters. The models for both cluster topologies are in excellent agreement with experiment over the entire accessible range of volume loading.
Observation of diamond nanocrystals in carbon films deposited during ion-assisted microwave plasma nucleation pretreatments
Authors: McGinnis Sean P. , Kelly Michael A. , Hagström Stig B. , Alvis Roger L.
Published: July 2002

Diamond nanocrystals were observed in carbon films deposited by ion-assisted microwave plasma pretreatments for diamond thin films. Both selected-area electron diffraction and x-ray diffraction confirm the presence of diamond in the bias-deposited carbon films despite micro-Raman spectra and scanning electron micrographs which do not provide conclusive evidence of this carbon phase. These analyses indicate that carbon deposited by the ion-assisted nucleation process consists of a high density of nanocrystalline diamond nuclei in a nondiamond carbon matrix. Subsequent growth of these nuclei using unbiased microwave plasma chemical-vapor deposition leads to the formation of relatively smooth and thin continuous polycrystalline diamond films with small grain sizes.
Low frequency (1/f ) noise model for the base current in polysilicon emitter bipolar junction transistors
Authors: Mounib A. , Ghibaudo G. , Balestra F. , Pogany D. , Chantre A. , Chroboczek J.
Published: July 2002

A new model for the low frequency (LF) noise in the base current of polysilicon emitter bipolar junction transistors (BJTs) is proposed. This model is based on the carrier number fluctuation approach and satisfactorily accounts for the base current noise characteristics. The base current fluctuations are ascribed to surface and volume noise sources. The surface noise arises from the low frequency fluctuations of the surface generation–recombination base current component due to the dynamic trapping–detrapping of carriers into/from slow states located in the spacer oxide at the periphery of the emitter/base junction. The volume noise results from the intrinsic fluctuations of the diffusion base current due to the carrier number fluctuations in the emitter or at the emitter–base junction. This LF noise model has been applied to BJTs subjected to hot carrier stress after reverse biasing of the base–emitter junction. The evolution of the base current noise characteristics after stress clearly demonstrates that the hot carrier injection leads to a substantial increase of the surface base current noise component whereas the volume noise component as well as the collector current noise is nearly insensitive to the degradation-induced defects. The application of the model provides an evaluation of the slow and fast trap densities generated as a function of the stress level.