bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of epitaxial CoSi2 by a Cr or Mo interlayer: Comparison with a Ti interlayer
Authors: Detavernier C. , Van Meirhaeghe R. L. , Cardon F. , Maex K. , Bender H. , Brijs B. , Vandervorst W.
Published: July 2002

The influence of Cr and Mo on phase formation and preferential orientation of CoSi2 is reported. Three different regimes are distinguished, depending on the thickness of the interlayer. For a thin interlayer or a capping layer, CoSi forms first, as in the standard Co/Si reaction. The remaining Cr or Mo can be considered as a contaminant that is present in the CoSi layer, causing a delay in CoSi2 nucleation and inducing preferential (220) and (400) nucleation. For interlayers with intermediate thickness, epitaxially (400) oriented CoSi2 is formed. For a thick interlayer, a polycrystalline layer of CrSi2 (or MoSi2) is formed first, followed by CoSi formation on top of the CrSi2. At higher temperatures, the CoSi layer is transformed into a polycrystalline, continuous layer of CoSi2 on top of the CrSi2 or MoSi2 layer, while some grains of CoSi2 are formed underneath the CrSi2. A similar behavior for Ti interlayers is observed, although a much thicker Ti layer is needed before the third regime is reached.
Accurate comparative measurement of oxygen content variations in YBa2Cu3O7−δ films due to postdeposition annealing
Authors: Zhai H. Y. , Zhang Z. H. , Chu W. K.
Published: July 2002

Non-Rutherford elastic resonance scattering, O16(α,α)16O at 3.045 MeV, has been used to measure the change in relative oxygen content, δ of dc-sputtered YBa2Cu3O7−δ (YBCO) films due to postdeposition annealing to a precision of 1%. Experimental results show that only 3.4% of the total amount of oxygen in YBa2Cu3O7−δ is absorbed in the postdeposition annealing process, and this agrees with our x-ray diffraction data. The presence of oxygen during annealing, which leads to the transition to the superconducting orthorhombic structure, may also be understood as providing a more favorable environment for structure reformation, rather than only adding more oxygen into the YBCO film.
Modern theory of Fermi acceleration: A new challenge to plasma physics
Authors: Malkov M. A. , Diamond P. H.
Published: July 2002

One of the main features of astrophysical shocks is their ability to accelerate particles to extremely high energies. The leading acceleration mechanism, the diffusive shock acceleration, is reviewed. It is demonstrated that its efficiency critically depends on the injection of thermal plasma into acceleration which takes place at the subshock of the collisionless shock structure that, in turn, can be significantly smoothed by energetic particles. Furthermore, their inhomogeneous distribution provides free energy for magnetohydrodynamic (MHD) turbulence regulating the subshock strength and injection rate. Moreover, the MHD turbulence confines particles to the shock front controlling their maximum energy and bootstrapping acceleration. Therefore, the study of the MHD turbulence in a compressive plasma flow near a shock is a key to the understanding of the entire process. The calculation of the injection rate became part of the collisionless shock theory. It is argued that the further progress in diffusive shock acceleration theory is impossible without a significant advance in these two areas of plasma physics.
Conductive electron heat flow along magnetic field lines
Authors: Held E. D. , Callen J. D. , Hegna C. C. , Sovinec C. R.
Published: July 2002

In this work, a unified closure for the conductive electron heat flux along magnetic field lines is derived and examined. Both free-streaming and collisional pitch-angle scattering of electrons are present in the drift kinetic equation which is solved using an expansion in pitch-angle eigenfunctions (Legendre polynomials). The closure takes the form of a generic integral operator involving the electron temperature variation along a magnetic field line and the electron speed. Derived for arbitrary collisionality, the heat flux closure may be written in forms resembling previous collisional and collisionless expressions. Electrons with two to three times the thermal speed are shown to carry heat for all collisionalities and thermal electrons make an important contribution to the heat flow in regimes of moderate to low collisionality. As a practical application, the flow of electron heat along a chaotic magnetic field is calculated in order to highlight the nonlocal nature of the closure which allows for heat to flow against local temperature gradients.
Catalytic synthesis and photoluminescence of β-Ga2O3 nanowires
Authors: Liang C. H. , Meng G. W. , Wang G. Z. , Wang Y. W. , Zhang L. D. , Zhang S. Y.
Published: July 2002

Monoclinic gallium oxide (β-Ga2O3) nanowires were synthesized by heat treating a composite material of GaAs and pre-evaporated Au at 1240 °C in dry oxygen atmosphere. The catalytic Au metal generated liquid nanoclusters that serve as reactive sites confining and directing the growth of β-Ga2O3 nanowires during the vapor-liquid-solid growth process. The β-Ga2O3 nanowires have diameters ranging from 20 to 50 nm and lengths of several micrometers. Photoluminescence measurement under excitation at 250 nm shows that the bulk β-Ga2O3 nanowires have a stable blue emission at 475 nm and an ultraviolet emission at 330 nm, which may be related to the defects such as the oxygen vacancy and the gallium–oxygen vacancy pair.
Evolution of the electron energy distribution and plasma parameters in a pulsed magnetron discharge
Authors: Gudmundsson J. T. , Alami J. , Helmersson U.
Published: July 2002

We demonstrate the creation of high-density plasma in a pulsed magnetron discharge. A 2.4 MW pulse, 100 μs wide, with a repetition frequency of 50 Hz is applied to a planar magnetron discharge to study the temporal behavior of the plasma parameters: the electron energy distribution function, the electron density, and the average electron energy. The electron density in the vicinity of the substrate, 20 cm below the cathode target, peaks at 8×1017 m−3, 127 μs after initiating the pulse. Towards the end of the pulse two energy groups of electrons are present with a corresponding peak in average electron energy. With the disapperance of the high-energy electron group, the electron density peaks, and the electron energy distribution appears to be Maxwellian like. Following the electron density peak, the plasma becomes more Druyvesteyn like with a higher average electron energy.
Investigation of surface blistering of hydrogen implanted crystals
Authors: Bedell Stephen W. , Lanford William A.
Published: July 2002

The time required to observe the onset of blistering was measured for 100 keV H implanted Si, SiC, and Ge crystals as a function of dose and annealing temperature. The calculated blistering activation energy (EA) for Si was found to decrease rapidly with increasing H dose. In Ge, EA decreased only slightly, while EA did not vary with dose in SiC. Hydrogen profiling using the H1(N15,αγ)C12 technique was used to study the evolution of the H distribution in these crystals. It was observed that implanted H concentrates upon annealing in Si and SiC, but not in Ge. By measuring the H profiles on the surfaces of bonded and transfered Si layers it was concluded that fracture occurred at the H peak and 50% of the implanted H was liberated during fracture. Ion beam channeling using 2 MeV He4 was used to study the implant damage in these materials. The dechanneling levels in the channeling spectra were attributed to the presence of lattice distortions near the implant peak. It was determined that the extent of lattice distortion was greatest in Ge and least in SiC. An observed decrease in the dechanneling level in as-implanted Si above 9×1016 H/cm2 was attributed to strain relaxation during implantation. Amorphization in SiC dramatically decreased the growth rate of the microcracks, implying that amorphous materials may be difficult to use in conjunction with Smart-Cut™ technology.
Finite Reynolds number effects in the Bretherton problem
Authors: Heil Matthias
Published: July 2002

In this paper we investigate the effect of fluid inertia in the classical Bretherton problem in which a semi-infinite air finger displaces viscous fluid in a two-dimensional channel. The governing free-surface Navier–Stokes equations are discretized by a finite element method and the system’s behavior is studied for capillary and Reynolds numbers in the ranges 0.05<Ca<5 and 0<Re<280, respectively. It is shown that at a finite Reynolds number, a sequence of closed vortices develops in the recirculating flow region ahead of the bubble tip. The largest of these vortices is located near the bubble tip where it significantly affects the pressure distribution and leads to a noticeable increase in the overall pressure rise across the bubble tip. The underlying physical mechanisms are discussed in detail.
A first principles study of methanol decomposition on Pd(111): Mechanisms for O–H bond scission and C–O bond scission
Authors: Zhang C. J. , Hu P.
Published: July 2002

There is some dispute as to whether methanol decomposition occurs by O–H bond scission or C–O bond scission. By carrying out density functional theory calculations, we investigate both scenario of the reaction pathways of methanol decomposition on a Pd(111) surface. It is shown that the O–H bond scission pathway is much more energetically favorable than the C–O bond scission pathway. The high reaction barrier in the latter case is found to be due to the poor bonding abilities of CH3 and OH with the surface at the reaction sites.
Raman scattering from LO phonon-plasmon coupled modes and Hall-effect in n-type silicon carbide 4H–SiC
Authors: Chafai M. , Jaouhari A. , Torres A. , Antón R. , Martı́n E. , Jiménez J. , Mitchel W. C.
Published: July 2002

The transport properties, particularly free carrier density and mobility, of SiC are usually determined by the Hall effect. Raman spectroscopy has been shown to yield transport parameters similar to the Hall effect. The analysis of the longitudinal optical plasmon coupled (LOPC) modes in doped SiC provides such information. In the case of damped plasmons in n-type 4H–SiC, changes in the carrier concentration result in a frequency shift of the LOPC mode, which appears close to the A1(LO) phonon mode. The validity of this approach for different free carrier concentrations (plasma frequencies) and mobilities (plasmon damping constants) is analyzed. The theoretical results obtained show that reliable estimation of the free carrier density can be obtained from the frequency shift of the LOPC mode for low carrier concentrations. At such a carrier concentration a reliable correlation between the Raman shift of the LOPC mode and the plasma frequency is established, without noticeable influence of the plasmon damping constant. However, when the free electron concentration increases, a non-negligible influence of the plasmon damping constant on the Raman frequency shift of the LPOC mode is observed. Therefore reliable transport data can only be obtained by a full line-shape analysis. These results are confirmed by the experimental results obtained on n-type 4H–SiC bulk samples with free electron concentration between 1×1017 and 5×1018 cm−3. © 2001 American Institute of Physics.
Structure and charge transfer dynamics of the (Ar–N2)+ molecular cluster
Authors: Candori R. , Cavalli S. , Pirani F. , Volpi A. , Cappelletti D. , Tosi P. , Bassi D.
Published: July 2002

In this paper we have investigated the interaction potential and the charge transfer processes at low collision energies in the (Ar–N2)+ system. The angular dependence of the lowest doublet potential energy surfaces (PES), correlating with Ar+(2Pj)–N2 and Ar–N2+(2Σ,2Π), has been given in terms of spherical harmonics, while the dependence on the intermolecular distance has been represented by proper radial coefficients. Such coefficients, which account for van der Waals, induction, charge transfer, and electrostatic contributions, have been predicted by empirical correlation formulas. The PES so obtained have been employed to calculate cross sections for the charge transfer process Ar++N2→Ar+N2+ at low collision energy (E⩽2 eV). A good agreement between calculated and experimental cross sections is obtained by assuming that the duration of the nonadiabatic transition has to match the time required for the molecular rearrangement into the final vibrational state. As a consequence the efficient formation of product ions into specific vibrational states is limited to well defined ranges of impact parameters. This treatment leads to a unified description of the major experimental findings.
Electric-field-directed growth of aligned single-walled carbon nanotubes
Authors: Zhang Yuegang , Chang Aileen , Cao Jien , Wang Qian , Kim Woong , Li Yiming , Morris Nathan , Yenilmez Erhan , Kong Jing , Dai Hongjie
Published: July 2002

Electric-field-directed growth of single-walled carbon nanotubes by chemical-vapor deposition is demonstrated. The field-alignment effect originates from the high polarizability of single-walled nanotubes. Large induced dipole moments lead to large aligning torques and forces on the nanotube, and prevent randomization of nanotube orientation by thermal fluctuations and gas flows. The results shall open up possibilities in directed growth of ordered molecular-wire architectures and networks on surfaces.
Effects of partial substitution of Ni by Pd on the thermoelectric properties of ZrNiSn-based half-Heusler compounds
Authors: Shen Q. , Chen L. , Goto T. , Hirai T. , Yang J. , Meisner G. P. , Uher C.
Published: July 2002

We report on the effects of partial substitution of nickel by palladium on the thermoelectric properties of ZrNiSn-based half-Heusler compounds. It is shown that the substitution of palladium for nickel results in a significant, beneficial reduction of the thermal conductivity. The Seebeck coefficient also decreases, but only by a small amount. In the Hf0.5Zr0.5Ni0.8Pd0.2Sn0.99Sb0.01 compound, a power factor of 22.1 μW K−2 cm−1 and a thermal conductivity as low as 4.5 W/m K are measured at room temperature. The dimensionless figure of merit ZT increases with increasing temperature and reaches a maximum value of 0.7 at about 800 K.
Ion and neutral energy flux distributions to the cathode in glow discharges in Ar/Ne and Xe/Ne mixtures
Authors: Capdeville H. , Pédoussat C. , Pitchford L. C.
Published: July 2002

The work presented in the article is a study of the heavy particle (ion and neutral) energy flux distributions to the cathode in conditions typical of discharges used for luminous signs for advertising (“neon” signs). The purpose of this work is to evaluate the effect of the gas mixture on the sputtering of the cathode. We have combined two models for this study: a hybrid model of the electrical properties of the cathode region of a glow discharge and a Monte Carlo simulation of the heavy particle trajectories. Using known sputtering yields for Ne, Ar, and Xe on iron cathodes, we estimate the sputtered atom flux for mixtures of Ar/Ne and Xe/Ne as a function of the percent neon in the mixture.
Use of thermodynamic integration to calculate the hydration free energies of n-alkanes
Authors: Wescott J. T. , Fisher L. R. , Hanna S.
Published: July 2002

Explicit atom molecular dynamics simulations were used in conjunction with the thermodynamic integration method to calculate hydration free energies for short n-alkane molecules, up to C5H12. The OPLS all-atom parameter set [Kaminski et al., J. Phys. Chem. 98, 13077 (1994)] was used to represent the n-alkanes, together with the TIP3P water model [Jorgensen et al., J. Chem. Phys. 79, 926 (1983)]. The approach of Beutler et al. [Chem. Phys. Lett. 222, 529 (1994)] was used to avoid singularities in nonbonded interaction potentials that can otherwise be problematical with this technique. Electrostatic interactions were treated using a cutoff radius of 0.9 nm, and a functional form that was shifted and scaled smoothly to zero. The values obtained for the solvation free energies were of similar accuracy to those from previously published simulations, but were systematically about 2 kJ mol−1 higher than experimental values. However, the calculated free energies of transformation for the reaction CnH2n+1(aq)→Cn+1H2n+4(aq), show a considerably improved agreement over previous values, and reproduce well the experimental trend versus n. The merits of the thermodynamic integration technique are discussed in relation to the popular thermodynamic perturbation method.
Electrical transport properties in LiMn2O4, Li0.95Mn2O4, and LiMn1.95B0.05O4 (B=Al or Ga) around room temperature
Authors: Iguchi E. , Tokuda Y. , Nakatsugawa H. , Munakata F.
Published: July 2002

In order to identify the carrier responsible for the electrical transport at room temperature in LiMn2O4 from the viewpoint of practical applications as a cathode material, the bulk conductivity measurements by complex-plane impedance analyses have been carried out on LiMn2O4, Li0.95Mn2O4, and LiMn1.95B0.05O4 (B=Al3+ or Ga3+) together with the measurements of four-probe dc conductivities and dielectric relaxation processes, because these are two candidates for the carrier, a Li ion or a nonadiabatic small polaron of an eg electron on Mn3+. The comparison of the ionic conductivity estimated numerically from the parameters obtained experimentally for the Li-diffusion in LiMn2O4 with the bulk conductivity indicates that the Li-diffusion seems difficult to play the primary role in the electrical conduction. Instead, a hopping-process of nonadiabatic small polarons of eg electrons is likely to dominate predominantly the electrical transport properties. The dielectric relaxation process, and the activation energies and the pre-exponential factors of the bulk conductivities in Li0.95Mn2O4 and LiMn1.95B0.05O4 are explained self-consistently in terms of the polaronic conduction.
Transport properties of Mn δ-doped GaAs and the effect of selective doping
Authors: Nazmul Ahsan M. , Sugahara S. , Tanaka M.
Published: July 2002

We have grown Mn δ-doped GaAs layers on GaAs(001) substrates by molecular beam epitaxy. Secondary ion mass spectroscopy and transmission electron microscopy revealed that Mn dopants were abruptly confined. The doping profiles still retained abruptness even at elevated growth temperature up to 400 °C. Mn δ-doped GaAs samples showed high resistivity at low temperature and did not show a ferromagnetic behavior. However, in a selectively doped heterostructure (Mn δ-doped GaAs / Be-doped AlGaAs), where holes were supplied from the Be-doped AlGaAs layer, a ferromagnetic order was observed with the ferromagnetic transition temperature as high as 70 K.
Size effect in mesoscopic epitaxial ferroelectric structures: Increase of piezoelectric response with decreasing feature size
Authors: Bühlmann S. , Dwir B. , Baborowski J. , Muralt P.
Published: July 2002

An epitaxial 200 nm thick film of Pb(Zr0.40Ti0.60)O3 (PZT) has been deposited by reactive rf magnetron sputtering on conductive Nb-doped SrTiO3 (100) (STO). The patterning process involved electron-beam lithography of polymethylmethacrylate, fabrication of a 75 nm thick Cr hard mask layer by means of a lift-off process, and dry etching of PZT. The smallest PZT features obtained were 100 nm in lateral dimensions. Piezoelectric sensitive scanning force microscopy in the contact mode revealed a strong increase of the piezoelectric response for feature sizes with lateral dimensions below 300 nm. It is proposed that this behavior is mainly due to vanishing a domains.
Structure, microstructure, and magneto-optical properties of laser deposited Bi3Fe5O12/Gd3Ga5O12(111) films
Authors: Kahl S. , Khartsev S. I. , Grishin A. M. , Kawano K. , Kong G. , Chakalov R. A. , Abell J. S.
Published: July 2002

Bi 3 Fe 5 O 12 films were deposited onto single crystal Gd3Ga5O12(111) substrates by pulsed laser deposition. X-ray diffraction patterns and cross sectional transmission electron micrographs show epitaxial film growth and a flat interface between the film and substrate. However, there are small inclusions at the interface and a network of cracks is observed by optical microscope. The refractive index is 3.4 at 800 nm wavelength. The optical transmission is low below 540 nm but increases to 90% for a 1.8 μm thick film at 850 nm. The magneto-optical Faraday rotation is high, with a peak value of −24 deg/μm at 560 nm. The angle of Faraday ellipticity reaches a maximum value of 15 deg/μm at 545 nm.
Study of cracking mechanism in GaN/α-Al2O3 structure
Authors: Itoh Nobuo , Rhee Jung Chul , Kawabata Toshiharu , Koike Susumu
Published: July 2002

Imperfection (cracking, stress) in (0001)GaN/(0001)α-Al2O3 grown by using the metalorganic chemical vapor deposition (MOCVD) or chemical vapor deposition (CVD) method is investigated. First, the epitaxial relation is confirmed by using the oscillating-crystal method. Subsequently, imperfections, especially cracks, are observed in detail by optical microscopy and x-ray diffraction topography. Cracking occurs at the layer thickness larger than 13 μm in the crystals having epitaxial layers of good quality, while it does not appear even at the thickness of 30 μm in crystals including inferior layers. Finally, stress in the layer and in the substrate are determined by measuring the warp of the wafer. Compressive stress in the good epitaxial layer is 1.6×109 dyne cm−2, and in this case, cracking occurs at the maximum tensile stress in the substrate less than 3×108 dyne cm−2. However, cracking does not occur even at the compressive stress of 3×109 dyne cm−2 in the inferior layer and at the tensile stress of 1×109 dyne cm−2 in the substrate. From these facts, a possible mechanism of cracking is proposed. During the epitaxial growth, microcracks occur in the substrate at the interface owing to the lattice mismatching. During the cooling, macrocracks which can be observed are grown from the already existing microcracks owing to the thermal stress. In the latter course, the cracking in the substrate must be simultaneously propagated to the epitaxial layer.
Cathodoluminescence scanning electron microscopy of semiconductors
Authors: Yacobi B. G. , Holt D. B.
Published: July 2002

This paper reviews applications of cathodoluminescence scanning electron microscopy in the assessment of optical and electronic properties of semiconductors. The assessment includes, for example, information on band structure and impurity levels derived from spectroscopic cathodoluminescence, analysis of dopant concentrations at a level which is in some cases several orders of magnitude better than x-ray microanalysis, and mapping of carrier lifetimes and defects. Recent advances in both the various cathodoluminescence techniques and the processes leading to electron-beam-induced luminescence in semiconductors are reviewed. Possible future trends are also discussed.
Magnetic material R,Fe,Mo,(Co) with ThMn12 structure
Authors: Müller A.
Published: July 2002

Preparation and magnetic properties of ThMn12-type phases R,Fe,Mo,(Co), R=Y, Ce, Nd, Sm, Gd, Dy, Er are described. The ternary material, without Co, exhibits saturation magnetization up to 1 T and Curie temperature up to 215 °C, the quaternary material, with Co, up to 1.2 T, and 295 °C, respectively.
Nanometer-size fiber composite synthesis by laser-induced reactions
Authors: Chow Gan-Moog , Klemens Paul G. , Strutt Peter R.
Published: July 2002

A new approach in ultrafine composite synthesis involves rapid condensation of metallic and nonmetallic species, produced by laser-induced evaporation. A heated tungsten filament is simultaneously employed for codeposition of W via a chemical transport mechanism. This process occurs in a reducing environment of hydrogen gas, where evaporated species were produced by a laser-induced plume. Composite layers were formed on a Ni alloy substrate surface, at a rate of about 1 μm/s. The matrix of the composite films was either Al or W, and the dispersed phase was amorphous silica fibers. The diameter of the fibers was between 25 and 120 nm, depending on the laser-beam materials-interaction time. Various analytical techniques have been employed to characterize the as-synthesized layers. Experimental evidence does not support the vapor-liquid-solid model for fiber growth. An alternative fiber growth mechanism is proposed: ultrafine silica particles (10–25 nm diam) form by rapid condensation from the laser plume, and by virtue of a strong dipole moment coagulate to form clearly defined linear arrays which eventually transform into fibers.
Solid-state thin-film memistor for electronic neural networks
Authors: Thakoor S. , Moopenn A. , Daud T. , Thakoor A. P.
Published: July 2002

We report on a tungsten-oxide-based, nonvolatile, electrically reprogrammable, variable resistance device as an analog synaptic memory connection for electronic neural networks. A voltage controlled, reversible injection of H+ ions in electrochromic thin films of WO3 is utilized to modulate its resistance. A hygroscopic thin film of Cr2 O3 is the source of H+ ions. The resistance of the device can be tailored and stabilized over a wide dynamic range (∼four orders of magnitude), and the programming speed is modulated by the control voltage. The suitability of such a device in terms of its response speed, reversibility, stability, and cyclability for its use in electronic neural networks is discussed.
Nucleation and growth during reactions in multilayer Al/Ni films: The early stage of Al3Ni formation
Authors: Ma E. , Thompson C. V. , Clevenger L. A.
Published: July 2002

Solid-state interdiffusion reactions at Al/Ni interfaces in multilayer films have been studied using differential scanning calorimetry, cross-sectional transmission electron microscopy/microanalysis, and thin-film x-ray diffraction. Multilayer films with various modulation periods and an overall atomic concentration ratio of three Al to one Ni were prepared by alternate electron-beam evaporation in high- and ultrahigh-vacuum systems. We show calorimetric, microstructural, and compositional evidence that interdiffusion of Al and Ni leading to solid solutions precedes the formation of intermetallic crystalline compounds. Isothermal calorimetry indicates that Al3Ni subsequently nucleates in the interdiffused region at preferred sites. Calorimetric analyses also suggest that nucleation sites quickly saturate in the early stage of Al3Ni formation and that the nucleation site density strongly depends on the grain sizes of the deposited films. After coalescence into a continuous layer at the interface, Al3Ni thickens through a diffusion-limited process, in agreement with previous reports. A kinetic model is developed which yields calculated calorimetric traces in good agreement with experimental data. Our results suggest the importance of prenucleation interdiffusion, in addition to nucleation, in the selection of the first phase during thin-film reactions.
Empirical expressions for the alloy composition and temperature dependence of the band gap and intrinsic carrier density in GaxIn1−xAs
Authors: Paul Sajal , Roy J. B. , Basu P. K.
Published: July 2002

The band gap and the intrinsic carrier concentration in a semiconductor are important material parameters needed in the interpretation of various experimental and theoretical data. In the present work, empirical expressions for both the parameters as a function of alloy composition x and temperature are proposed for GaxIn1−xAs. The calculated results for band gap are in close agreement with the available data, while the same for intrinsic concentration give fair agreement with the data at the two ends of the alloy composition.
Effects of argon presputtering on the formation of aluminum contacts on polycrystalline diamond
Authors: Tachibana T. , Glass J. T.
Published: July 2002

The results of in vacuo x-ray photoelectron spectroscopy and Auger electron spectroscopy of Al-diamond interfaces for Al overlayer thicknesses up to 10 Å are presented. Postdeposition annealing effects up to 430 °C are also discussed. Ex situ current-voltage (I-V) measurements were also made on thick (∼1500 Å) Al contacts on diamonds. The as-grown diamond surface, on which Al was a rectifying contact, did not chemically interact with Al, even after annealing. An Ar+-sputtered diamond surface, on the other hand, did react with Al to form Al—C bonds upon annealing at temperatures as low as 430 °C. Al on the sputtered surface resulted in an ohmic contact. The distortion of the diamond network and formation of vacancies and unsatisfied bonds via Ar+ sputtering of the diamond surface change the I-V characteristics of the Al contact and also facilitate the solid state interdiffusion of Al and C as well as interface reactions at elevated temperatures.
Direct determination of grain-boundary and dislocation self-diffusion coefficients in silver from experiments in type-C kinetics
Authors: Sommer J. , Herzig Chr.
Published: July 2002

The use of an implanted, mass-separated carrier-free 105Ag radiotracer improves the sensitivity for measuring grain-boundary (GB) penetration profiles. In this way the GB self-diffusion coefficient DGB in silver was directly determined from experiments in type-C kinetics conditions. When multiplying the DGB values with a GB width δ=0.5 nm the results are in agreement with GB diffusivities P=δDGB measured in the same Ag material in the usual type-B kinetics experiments which were performed between 911 and 540 K and extrapolated to the low-temperature regime of type-C kinetics experiments (455–353 K). From the near-surface part of the type-C diffusion profiles the dislocation diffusion coefficient Dd was directly determined. These Dd values are compared with experiments of Turnbull and Hoffman [Acta Metall. 2, 419 (1954)] who applied the dislocation pipe model for a low-angle-tilt GB in silver. The good agreement confirms a dislocation core diameter of about 0.5 nm.
Magnetic domains and microstructural defects in Terfenol-D
Authors: Al-Jiboory M. , Lord D. G. , Bi Y. J. , Abell J. S. , Hwang A. M. H. , Teter J. P.
Published: July 2002

The ternary alloy Tb0.3Dy0.7Fe2 (Terfenol-D) is of significant technological interest as it possesses the largest known magnetostriction to anisotropy ratio near room temperature. Results of microstructural and magnetic domain observations by Lorentz microscopy and reflection x-ray topography are presented from both stoichiometric twinned single crystals prepared by a free-standing zoning technique, and pseudo single crystals prepared by the Czochralski method. Direct evidence of wall interactions with dislocations, twin boundaries and precipitates are presented which demonstrate the complex interaction between the magnetic and strain energies within this material.
Multistage plasma initiation process by pulsed CO2 laser irradiation of a Ti sample in an ambient gas (He, Ar, or N2)
Authors: Hermann J. , Boulmer-Leborgne C. , Mihailescu I. N. , Dubreuil B.
Published: July 2002

New experimental results are reported on plasma initiation in front of a titanium sample irradiated by ir (λ=10.6 μm) laser pulses in an ambient gas (He, Ar, and N2) at pressures ranging from several Torr up to the atmosphere. The plasma is studied by space- and time-resolved emission spectroscopy, while sample vaporization is probed by laser-induced fluorescence spectroscopy. Threshold laser intensities leading to the formation of a plasma in the vapor and in the ambient gases are determined. Experimental results support the model of a vaporization mechanism for the plasma initiation (vaporization-initiated plasma breakdown). The plasma initiation is described by simple numerical criteria based on a two-stage process. Theoretical predictions are found to be in a reasonable agreement with the experiment. This study provides also a clear explanation of the influence of the ambient gas on the laser beam-metal surface energy transfer. Laser irradiation always causes an important vaporization when performed in He, while in the case of Ar or N2, the interaction is reduced in heating and vaporization of some surface defects and impurities.
Spectroscopy of defects in germanium-doped silica glass
Authors: Gallagher Matt , Osterberg Ulf
Published: July 2002

We present data concerning the dynamics of photoexcitation of defect centers in germanium-doped silica glass optical fibers and fiber preforms. It is shown that two-photon absorption of mode-locked and Q-switched light at 527 nm results in partial ionization of the germanium oxygen deficiency center (GODC) 400-nm luminescence band. This bleaching induces loss in the 320–600-nm wavelength range, which we argue is due to the introduction of Ge(1) centers and charges trapped at structural defects in the glass. The excitation and relaxation of dynamics of the GODC are analyzed by examining the photoluminescence and absorption spectra, and two models are proposed to explain the observed behavior.
Generation of shock waves by laser-induced plasma in confined geometry
Authors: Devaux D. , Fabbro R. , Tollier L. , Bartnicki E.
Published: July 2002

Confined plasmas induced by neodynium glass laser at 1.06 μm and pulse width of 3 and 30 ns are studied. The metallic target is covered with a dielectric layer, glass or water, transparent to the laser radiation. Experimental measurements of the pressure induced by the plasma have been performed. For a certain range of laser power density these measurements agree particularly well with an analytical model. At high power densities (10 GW/cm2), the dielectric breakdown appears to be the main limiting process of the confining method. It is observed that this breakdown induces a saturation of the pressure. It is shown that the use of a short-rise-time laser pulse is the only way to reduce the effects of the breakdown and to obtain much higher-pressure shock waves. This is due to the dependence of the dielectric breakdown threshold on the laser pulse rise time.
Parametric study of the in situ growth of BaKBiO thin films by laser ablation
Authors: Lin Wen-Tai , Pan Shyi-Ming , Chen Yung-Fu
Published: July 2002

In situ growth of Ba1−xKxBiO3 (BKBO) films by laser ablation has been studied as a function of gas pressure and substrate temperature in O2, N2O, and N2O/O2 (1:1), respectively. The superconducting BKBO films can be grown at 410–500 °C in 15–60 mTorr of O2, at 390–510 °C in 20–2000 mTorr of N2O, and at 390–500 °C in 15–700 mTorr of N2O/O2 (1:1), respectively. Oxygen partial pressure plays an important role in controlling the doping of potassium into the BKBO phase during in situ deposition and hence the growth of the superconducting films. The optimum deposition pressures are 40 mTorr of O2, 30–50 mTorr of N2O, and 20–50 mTorr of N2O/O2 (1:1), under which not only the temperature range available for growing the superconducting films is wider but also the Tc’s of the as-grown films are higher. N2O used as an oxidation gas during growth is efficient in suppressing the crack formation in the BKBO films on cooling. The target composition also plays an important role in film growth. The optimal target composition studied is Ba:K:Bi=0.6:0.8:1.0.
Magnetic field and current distribution in metallized capacitors
Authors: Joubert Ch. , Béroual A. , Rojat G.
Published: July 2002

We attempt to establish the mathematical expression of the current and the magnetic field in a metallized capacitor. The expression of the impedance of this capacitor is also presented. The distribution of the current is discussed through the variation of the capacitor impedance and compared to experimental ones. There is good agreement between experimental and theoretical results. The analytical expressions of the magnetic field and the current also show a dependence on parameter δ2 akin to the skin depth in conductors. δ2 also depends on frequency. When δ2 is smaller than the outer radius of the capacitor, the current distribution becomes inhomogeneous.
Heat transfer and material removal in pulsed excimer-laser-induced ablation: Pulsewidth dependence
Authors: D’Couto G. C. , Babu S. V.
Published: July 2002

The pulsewidth-dependent ablation of polyimide, poly(methylmethacrylate), poly (etheretherketone), poly(ethylene terepthalate), and poly(ethersulphone) exposed to 248 and 308 nm, nanosecond UV laser pulses was modeled assuming one-dimensional heat transfer and a published model [G. H. Pettit and R. Sauerbrey, Appl. Phys. A 56, 51 (1993)] for photon absorption. The polymers were assumed to degrade/ablate after reaching a threshold temperature determined either from published temperature calculations of the ablating surface or the ceiling temperature. Since heat transfer calculations suggest that this temperature is reached before the end of the laser pulse, it was assumed that the degraded/ablated material continues to attenuate the incoming laser energy for the remaining duration of the laser pulse. Since the fluence-dependent absorption coefficient of this degraded material is unknown, it was obtained by fitting the experimental pulsewidth dependent ablation rate data of Schmidt, Ihlemann, and Wolff-Rottke (unpublished). The resulting values are consistent with mass spectrometric analysis of the ablation products and with the absence or occurrence of incubation. The threshold fluences and ablation rates predicted by this model are in good agreement with reported literature values; however, the use of a well-defined threshold temperature in the model leads to a different limiting etch rate dependence on the fluence at the threshold.
Strong ultraviolet induced absorption and absorption gratings in BaTiO3
Authors: Ross Graeme , Montemezzani Germano , Bernasconi Pietro , Zgonik Marko , Günter Peter
Published: July 2002

Efficient optical pumping of impurity levels in BaTiO3 is achieved using low intensity ultraviolet (UV) exposure. The resulting large photoinduced absorption in the visible is assessed in the context of interband charge transfer. An induced absorption of up to 12 cm−1 is observed at 633 nm. The effect, which takes place close to the UV-exposed crystal surface, occurs on a millisecond time scale.
Laser removal of particles from magnetic head sliders
Authors: Lu Y. F. , Song W. D. , Hong M. H. , Teo B. S. , Chong T. C. , Low T. S.
Published: July 2002

A quantitative investigation of laser-induced removal of particles from magnetic head slider surfaces has been carried out. The damage thresholds of magnetic head sliders for laser fluence and pulse number were found to be about 150 mJ/cm2 and 5000 pulses at 100 mJ/cm2, respectively. For laser fluence or pulse number above the damage threshold, laser irradiation onto magnetic head slider surfaces can cause microcracks around the pole tips. It is found that laser cleaning efficiency increases with increasing laser fluence and pulse number, but does not depend on repetition rate up to 30 Hz. Laser cleaning efficiency of removing particles from magnetic head slider surfaces can reach about 90% for Al particles and 100% for Sn particles, respectively, under appropriate conditions without causing damage. The mechanisms of laser cleaning of particles from magnetic head slider are laser-induced surface vibration, particle vibration, particle thermal expansion, and ablation with high laser fluence, which produce forces strong enough to detach particles from slider surfaces. Based on the above cleaning mechanisms, the dependence of laser cleaning efficiency on laser parameters can be explained.
Nanoindentation studies of single-crystal (001)-, (011)-, and (111)-oriented TiN layers on MgO
Authors: Ljungcrantz H. , Odén M. , Hultman L. , Greene J. E. , Sundgren J.-E.
Published: July 2002

The mechanical properties of (001)-, (011)-, and (111)-oriented MgO wafers and 1-μm-thick TiN overlayers, grown simultaneously by dc magnetron sputter deposition at 700 °C in a mixed N2 and Ar discharge, were investigated using nanoindentation. A combination of x-ray-diffraction (XRD) pole figures, high-resolution XRD analyses, and Auger electron spectroscopy was used to show that all TiN films were single crystals with N/Ti ratios of 1.0±0.05. The nanoindentation measurements were carried out using a three-sided pyramidal Berkovich diamond indentor tip operated at loads ranging from 0.4 to 40 mN. All three orientations of MgO substrates, as-received, exhibited identical hardness values as determined using the Oliver and Pharr method. After a 1 h anneal at 800 °C, corresponding to the thermal treatment received prior to film growth, the measured hardness of MgO(001) was 9.0±0.3 GPa. All TiN films displayed a completely elastic response at low loads. Measured hardness values, which decreased with increasing loads, increased in the order (011)<(001)<(111). After a 30 s postdeposition anneal at 1000 °C, however, hardness was found to be independent of load except at displacements >100 nm where substrate effects were apparent. TiN(001) and (111) films had hardnesses of 20±0.8 and 21±1 GPa, respectively, while data obtained from (011) layers exhibited large scatter due to surface roughness effects. Young’s moduli for annealed samples, calculated from the elastic unloading curves, were found to be 307±15 GPa for MgO (001) and 445±38 and 449±28 GPa for TiN (001) and TiN (111), respectively.
Power losses in magnetic laminations with hysteresis: Finite element modeling and experimental validation
Authors: Basso V. , Bertotti G. , Bottauscio O. , Fiorillo F. , Pasquale M. , Chiampi M. , Repetto M.
Published: July 2002

Dynamic hysteresis loop shapes and magnetic power losses are studied in nonoriented Fe-Si laminations exhibiting significant excess losses. Measurements are carried out under controlled sinusoidal induction in the frequency range from 1 Hz to 1.6 kHz, at various peak inductions from 0.25 to 1.5 T. Excess losses are found to obey a f3/2 law up to frequencies of 200–400 Hz, depending on peak induction. Beyond this limit, definite deviations are observed, due to eddy current shielding. Detailed information on the flux and field distribution in this high frequency regime is obtained by finite element solutions of Maxwell equations employing the dynamic Preisach model to describe quasi-static hysteresis and dynamic wall processes. The agreement between theoretical predictions and measurements is discussed.
High-coercivity, c-axis oriented Nd2Fe14B films grown by molecular beam epitaxy
Authors: Keavney D. J. , Fullerton Eric E. , Pearson J. E. , Bader S. D.
Published: July 2002

Thin films of Nd2Fe14B with a c-axis orientation and bulklike magnetic properties have been grown via molecular beam epitaxy with thicknesses as low as 300 Å. The films are grown on single-crystal MgO(100) wafers overcoated with epitaxial Mo(100) buffer layers. The 2–14–1 phase is crystallized either by sequential deposition or codeposition of Fe, Nd, and B from pure elemental evaporation sources onto 600–700 °C substrates. The structure of each film was characterized in situ with reflection high-energy electron diffraction and ex situ with x-ray diffraction. For the sequentially deposited films, the in-plane saturation field is 60–70 kOe at 300 K, consistent with the bulk anisotropy field of 73 kOe. The spin–reorientation transition at 135 K can also be clearly seen in the in-plane and out-of-plane magnetization versus temperature data. The out-of-plane coercivities typically range from 15 to 20 kOe at 20 K, and from 3 to 8 kOe at 300 K. Codeposition results in a multiphase structure, with Nd2Fe14B show the minority phase. The multiphase structure results in reduced perpendicular anisotropy.
Conductance quantization above 30 K in GaAlAs shallow-etched quantum point contacts smoothly joined to the background 2DEG
Authors: Kristensen A. , Jensen J. Bo , Zaffalon M. , So/rensen C. B. , Reimann S. M. , Lindelof P. E. , Michel M. , Forchel A.
Published: July 2002

The electrical characteristics of shallow etched GaAs/GaAlAs quantum point contacts (QPCs) of various shapes have been studied as a function of temperature above 0.3 K. Quantized conductance was observed up to 36 K, and from the temperature dependence of the conductance staircase we find energy separations between the lowest one-dimensional subbands up to 20 meV. This value exceeds the highest values so far reported for laterel QPC constrictions in GaAs/GaAlAs heterostructures. In addition, very well behaved quantized conductance plateaus were observed at the lowest temperatures.
Thermoelectric power, specific heat, and giant magnetoresistance of La0.85MnO3
Authors: Suryanarayanan R. , Berthon J. , Zelenay I. , Martinez B. , Obradors X. , Uma S. , Gmelin E.
Published: July 2002

Magnetization, thermoelectric power (TEP), specific heat (Cp), and magnetoresistance (MR) of the as prepared (AP) and oxidized (OX) polycrystalline La0.85MnO3 are reported. Both the samples have rhombohedral symmetry. The magnetization of the OX sample showed a sharper rise as a function of temperature with a Tc∼270 K and a well defined peak of Cp close to Tc. The TEP of the AP sample was positive between 10<T<300 K with a maximum located near Tc, but the OX sample showed a negative value between 240 and 300 K. A maximum of 75% in MR was observed for the OX sample in a field of 5 T at 270 K. The data were analyzed in light of similar data obtained on divalent (Sr,Ca) substituted LaMnO3 and the existing models.
Magnetoquenched superconducting valve
Authors: Clinton T. W. , Johnson Mark
Published: July 2002

A superconducting switch has been developed in a simple bilayer cross strip geometry using the magnetic fringe field of a ferromagnetic film to control the critical current in an underlying superconducting bridge. The magnetization of the ferromagnet is rotated in the plane of the film to vary the magnitude of the fringe field locally applied to the superconductor from negligible to substantial values. In the latter case, the magnetization is oriented such that the magnetic poles are along the edges of the cross strip directly above the superconductor. The large fringe field near the poles suppresses superconductivity over a length of order microns, giving rise to superconducting weak link behavior. A large modulation of the critical current is observed. The effect is demonstrated in the low Tc superconductors Pb (Tc=7.3 K) and Sn (Tc=3.9 K). Fabrication of the device involves minimal processing. Applications as a high speed switch, amplifier, nonvolatile storage cell, and controllable weak link are possible.
Microstructural analysis of strip cast Nd–Fe–B alloys for high (BH)max magnets
Authors: Bernardi J. , Fidler J. , Sagawa M. , Hirose Y.
Published: July 2002

High energy density magnets >400 kJ/m3 are increasingly used in many applications. Conventional casting techniques for sintered magnets reveal the formation of a high quantity of α-Fe and large Nd-rich regions. New techniques, like strip casting, produce homogeneous and fine scaled microstructures and are already used for producing high (BH)max magnets. The fast cooling rate during strip casting suppresses the formation of α-Fe dendrites and of large Nd-rich pockets. Directional solidification causes a formation of columnar grains containing a typical arrangement of hard magnetic Nd2Fe14B regions and Nd-rich regions. The Nd regions occur as intragranular platelets as well as intergranular phases. Intragranular lamellae show a periodicity which corresponds to a eutectoidal solidification according to the composition of the liquid and are directed parallel to the temperature gradient during solidification. The lamellae show an average width of 150 nm, a spacing of 3 μm, and a length up to the size of the hard magnetic grains. The fine separation of the hard magnetic and Nd phases is advantageous for the milling of the alloy after hydrogen decripitation and improves sinterability of magnets. Although the microstructure of strip cast alloys is much finer than that of ordinary cast alloys, the alignment of the powder is not deteriorated and Br is not reduced due to a sufficient large interlamellar spacing between the Nd-rich platelets that enables the formation of single crystal powder particles after milling.