bluuebunny/crossref_metadata_2025 huggingface dataset:
Deformation behavior and plastic instabilities of ultrafine-grained titanium
Authors: Jia D. , Wang Y. M. , Ramesh K. T. , Ma E. , Zhu Y. T. , Valiev R. Z.
Published: July 2002

Ultrafine-grained (UFG) Ti samples have been prepared using equal channel angular pressing followed by cold rolling and annealing. The deformation behavior of these materials, including strain hardening, strain rate dependence of flow stress, deformation/failure mode, and tensile necking instability, have been systematically characterized. The findings are compared with those for conventional coarse-grained Ti and used to explain the limited tensile ductility observed so far for UFG or nanocrystalline metals.
Theory of emission state of tris(8-quinolinolato)aluminum and its related compounds
Authors: Sugimoto Manabu , Sakaki Shigeyoshi , Sakanoue Kei , Newton Marshall D.
Published: July 2002

Fluorescence of fac-AlQ3 (Q=8-quinolinolato), mer-AlQ3, mer-Al(mQ)3 (mQ=4-methyl-8-quinolinolato), and BeQ2 were investigated with electronic structure calculations. The molecular structure of the first singlet excited state (the emission state) was optimized with the ab initio “configuration interaction with single excitations” (CIS) method. Ab initio CIS and semiempirical “Zerner’s intermediate neglect of differential overlap” (ZINDO) methods were used to calculate the emission energies (ΔE) and also the corresponding absorption energies. Although the ab initio CIS method overestimated the experimental value of ΔE by 1.09–1.16 eV, the ZINDO method reproduced it to a reasonable accuracy (within 0.26 eV). The optimized excited-state structure has an interesting feature in that one of the equivalent ligands distorts appreciably, while the thers keep their ground-state structures. As a result the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) are localized on the distorted ligand. Since the excited state is characterized as the HOMO–LUMO transition, the emission from AlQ3 (and its analogues) directly reflects that of the ligand. In order to analyze this intrigung excited-state structure, exciton transfer theory was applied, where the exciton coupling between ligands and the structural relaxation of the ligand upon excitation were taken into account. By examining these two factors for BeQ2, it is shown that the exciton localization results from weak exciton coupling and/or large structural relaxation energy.
Lattice location of phosphorus in n-type homoepitaxial diamond films grown by chemical-vapor deposition
Authors: Hasegawa Masataka , Teraji Tokuyuki , Koizumi Satoshi
Published: July 2002

The lattice location of phosphorus dopant atoms in n-type homoepitaxial diamond {111} films grown by chemical-vapor deposition has been investigated by Rutherford backscattering spectrometry and particle-induced x-ray emission under ion-channeling conditions. It is found that phosphorus dopant atoms occupy the substitutional sites almost completely in the host diamond lattice. The substitutional fraction of phosphorus was more than 0.9 for 〈011〉 and 〈111〉 directions. Present observation implies that the deep ground-state energy level of phosphorus in diamond, which is at 0.6 eV below the bottom of the conduction band, is attributed to the relaxation of surrounding carbon atoms.
A density functional theory study of the adsorption of sulfur, mercapto, and methylthiolate on Au(111)
Authors: Gottschalck J. , Hammer B.
Published: July 2002

We have studied the adsorption of sulfur (S=), mercapto (HS–), and methylthiolate (CH3S–) on Au(111). The three adsorbates are found to form strong bonds to the gold surface through the S-end. While the sulfur atom adsorbs in the high coordinated fcc site, the mercapto and methylthiolate molecules adsorb in bridge or bridgelike sites. The bending of the sulfur–hydrogen and sulfur–methyl bonds away from the surface normal plays a key role in this sites preference. We show that the back bonded sulfur species prefer the bridge sites because of lower steric repulsion at these sites between the gold surface and the hydrogen and methyl groups, respectively.
Observation of spontaneous neoclassical tearing modes
Authors: Fredrickson E. D.
Published: July 2002

In this paper data are presented from the Tokamak Fusion Test Reactor (TFTR) [D. Meade and the TFTR Group, in Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9–14] which challenge the commonly held belief that extrinsic magnetohydrodynamic (MHD) events such as sawteeth or edge localized modes are required to provide the seed islands that trigger neoclassical tearing modes (NTMs). While sawteeth are reported to provide the trigger for most of the NTMs on DIII-D [J. Luxon and DIII-D Group, in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. I, p 159] and ASDEX-U [O. Gruber, H.-S. Bosch, S. Günter et al., Nucl. Fusion 39, 1321 (1999)], the majority of NTMs seen in TFTR occur in plasmas without sawteeth, that is, which are above the β threshold for sawtooth stabilization. Examples of NTMs appearing in the absence of any detectable extrinsic MHD activity will be shown. Conversely, large n=1 modes in plasmas above the NTM β threshold generally do not trigger NTMs. An alternative mechanism for generating seed islands will be discussed.
Zero permittivity materials: Band gaps at the visible
Authors: Garcia N. , Ponizovskaya E. V. , Xiao John Q.
Published: July 2002

We theoretically discuss the possibility of having materials with zero effective permittivity that would create band gaps in a wide range of frequencies up to the visible. The physical realization of these materials is also discussed in terms of embedding metallic nanoparticles and nanowires in a dielectric medium. In the limit of long wavelengths, these composites will behave like a homogeneous medium with zero permittivity that will completely reflect electromagnetic waves. We present transmittivity calculations by using finite-difference time domain for periodic structures that proves the concept and shows the validity of the long wavelength approximation. The striking result is that the cutoff frequency ωc is determined by the lattice parameter of the composite. By properly choosing the lattice constant of the composite and permittivity of metal and dielectric constituents, we can have full band gaps at any frequency range but especially in the visible.
Origin of large magnetostriction in FeGa alloys
Authors: Wu Ruqian
Published: July 2002

Using the highly precise full potential linearized augmented plane wave method, the phase stability, magnetism, and magnetostriction of Fe3Ga alloys were investigated. The magnetostrictive coefficients strongly depend on the atomic arrangement. The B2-like structure, although it is unstable in the small unit cell chosen here, appears to play a crucial rule for the large positive magnetostriction in the FeGa alloys as observed recently. Electronic origin of enhancement in magnetostriction is discussed in terms of density of states and band structures.
Eigenfrequencies of vortex state excitations in magnetic submicron-size disks
Authors: Guslienko K. Yu. , Ivanov B. A. , Novosad V. , Otani Y. , Shima H. , Fukamichi K.
Published: July 2002

We have theoretically and numerically studied the dynamic properties of the vortex magnetic state in soft submicron ferromagnetic dots with variable thickness and diameter. To describe the vortex translation mode eigenfrequencies, we applied the equation of motion for the vortex collective coordinates. We calculated the vortex restoring force with an explicit account of the magnetostatic interaction on the bases of the “rigid” vortex and two-vortices “side charges free” models. The latter model well explains the results of our micromagnetic numerical calculations. The translation mode eigenfrequency is inversely proportional to the vortex static initial susceptibility and lies in GHz range for submicron in-plane dot sizes.
Charge collection and absorption-limited sensitivity of x-ray photoconductors: Applications to a-Se and HgI2
Authors: Kabir M. Zahangir , Kasap S. O.
Published: July 2002

A generalized expression for charge carrier transport and absorption-limited sensitivity of x-ray photoconductors is derived by analytically solving the continuity equation for both holes and electrons considering the drift of electrons and holes in the presence of deep traps. The normalized sensitivity equation is applied to stabilized a-Se and HgI2. In the latter case, the sensitivity model is fitted to published data to determine the electron and hole ranges (6.4×10−6 cm2/V and 7×10−8 cm2/V, respectively) in screen-printed polycrystalline HgI2.
Control of neoclassical tearing modes in DIII–D
Authors: La Haye R. J. , Günter S. , Humphreys D. A. , Lohr J. , Luce T. C. , Maraschek M. E. , Petty C. C. , Prater R. , Scoville J. T. , Strait E. J.
Published: July 2002

The development of techniques for neoclassical tearing mode (NTM) suppression or avoidance is crucial for successful high beta/high confinement tokamaks. Neoclassical tearing modes are islands destabilized and maintained by a helically perturbed bootstrap current and represent a significant limit to performance at higher poloidal beta. The confinement-degrading islands can be reduced or completely suppressed by precisely replacing the “missing” bootstrap current in the island O-point or by interfering with the fundamental helical harmonic of the pressure. Implementation of such techniques is being studied in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] in the presence of periodic q=1 sawtooth instabilities, a reactor relevant regime. Radially localized off-axis electron cyclotron current drive (ECCD) must be precisely located on the island. In DIII-D the plasma control system is put into a “search and suppress” mode to make either small rigid radial position shifts of the entire plasma (and thus the island) or small changes in the toroidal field (and, thus, the ECCD location) to find and lock onto the optimum position for complete island suppression by ECCD. This is based on real-time measurements of an m/n=3/2 mode amplitude dBθ/dt. The experiment represents the first use of active feedback control to provide continuous, precise positioning. An alternative to ECCD makes use of the six toroidal section “C-Coil” on DIII-D to provide a large nonresonant static m=1, n=3 helical field to interfere with the fundamental harmonic of an m/n=3/2 NTM. While experiments show success in inhibiting the NTM if a large enough n=3 field is applied before the island onset, there is a considerable plasma rotation decrease due to n=3 “ripple.”
Insight into why the Langmuir–Hinshelwood mechanism is generally preferred
Authors: Baxter R. J. , Hu P.
Published: July 2002

In heterogeneous catalysis, the two main reaction mechanisms which have been proposed are the Langmuir–Hinshelwood and the Eley–Rideal. For the vast majority of surface catalytic reactions, it has been accepted that the Langmuir–Hinshelwood mechanism is preferred. In this study, we investigate catalytic CO oxidation on Pt(111). It is found that reaction barriers for Langmuir–Hinshelwood mechanisms actually tend to be higher than those for Eley–Rideal ones. An explanation is presented as to why it is still more probable for the reaction to proceed via the Langmuir–Hinshelwood mechanism, despite its higher reaction barrier.
Response of silicon carbide to high velocity impact
Authors: Holmquist Timothy J. , Johnson Gordon R.
Published: July 2002

This article presents an analysis of the response of silicon carbide to high velocity impact. This includes a wide range of loading conditions that produce large strains, high strain rates, and high pressures. Experimental data from the literature are used to determine constants for the Johnson–Holmquist constitutive model for brittle materials (JH-1). It is possible to directly determine the strength and pressure response of the intact material from test data in the literature. After the ceramic has failed, however, there are not adequate experimental data to directly determine the response of the failed material. Instead, the response is inferred from a comparison of computational results to ballistic penetration test results. After the constants have been obtained for the JH-1 model, a wide range of computational results are compared to experimental data in the literature. Generally, the computational results are in good agreement with the experimental results. Included are computational results that model interface defeat, which occurs when a high velocity projectile impacts a ceramic target and then dwells on the surface of the ceramic with no significant penetration.
Spectral profile of the two-photon absorption coefficients in CaF2 and BaF2
Authors: Tsujibayashi Toru , Toyoda Koichi , Sakuragi Shiro , Kamada Masao , Itoh Minoru
Published: July 2002

Two-photon absorption spectra due to the valence excitons in CaF2 and BaF2 have been measured using the second harmonic of the output of a XeCl-excimer-laser pumped dye laser as a light source. The band gap energies of CaF2 and BaF2 are obtained as 11.8 and 10.6 eV at 298 K, respectively. The two-photon absorption coefficient β in the spectral region below the 2P exciton is expressed by an exponential function of the photon energy in both crystals. The values of β at the 2P exciton peak are about 2×10−9 cm/W for CaF2 and 3×10−9 cm/W for BaF2.
Manipulation of the equilibrium between diamond growth and renucleation to form a nanodiamond/amorphous carbon composite
Authors: Zhou X. T. , Li Q. , Meng F. Y. , Bello I. , Lee C. S. , Lee S. T. , Lifshitz Y.
Published: July 2002

Composite films of ∼10 nm nanodiamond particles embedded in an amorphous carbon matrix were formed using a double bias assisted hot filament chemical vapor deposition system with a feeding gas mixture of 1% CH4:99% H2. The structure was obtained via the equilibrium of a multistage process including: (1) bias enhanced nucleation of diamond in an amorphous carbon matrix, (2) growth of both amorphous carbon and diamond, (3) suppression of the diamond growth by the surrounding amorphous carbon matrix, and (4) bias enhanced renucleation of diamond on the new amorphous carbon boundaries. The work adds insight to the diamond nucleation and growth processes.
Theory of photopyroelectric spectroscopy of solids
Authors: Mandelis Andreas , Zver Martin M.
Published: July 2002

Light absorption by a solid material and conversion of part, or all, of the optical energy into heat due to nonradiative deexcitation processes within the solid can give rise to an electrical signal in a pyroelectric thin film in contact with the sample. This effect forms the basis of a new spectroscopic technique for the study of condensed phase matter. A one-dimensional theory is presented, which describes the dependence of the pyroelectric signal on the optical, thermal, and geometric parameters of the solid/pyroelectric system. Specifically, the theory examines the conditions under which the photopyroelectric signal exhibits a linear dependence on the optical absorption coefficient of the solid. Thus a theoretical basis for the technique of photopyroelectric spectroscopy is established. Qualitative comparisons between predictions of the theory and preliminary experimental observations are used to test the applicability of the theory to experimental configurations of practical interest.
Geometrical model of conductive and dielectric properties of partially saturated rocks
Authors: Feng Shechao , Sen P. N.
Published: July 2002

Dielectric constant and conductivity of three-component porous media are calculated using a geometrical model. Applications are made to experiments on clay-free sedimentary rocks partially saturated with hydrocarbon and water, and air and water. A good agreement with experimental data at high frequency (GHz) is obtained using only a spherical grain shape. The low-frequency (MHz) data cannot be explained by the spherical grain alone, but a bimodal distribution of spherical and platey grains gives a fair agreement. The low-frequency dielectric constant is very sensitive to the grain-shape distribution, but at high frequency is not.
Radiant refrigeration by semiconductor diodes
Authors: Berdahl Paul
Published: July 2002

Dousmanis et al. [Phys. Rev. 133, A316 (1964)] demonstrated that GaAs light-emitting diodes could produce a cooling effect if the quantum efficiency (ratio of photon flux to junction current) is very close to unity (e.g., >0.97). Here, it is pointed out that for narrow-bandgap semiconductors, the quantum efficiency need not be so high to produce cooling. Also, for narrow-bandgap semiconductors there is an additional cooling mode in which the reverse-biased diode cools its radiant environment by absorbing infrared radiation. Maximum cooling rates per unit of junction area are on the order of n2σT4, where n is the index of refraction, σ is the Stefan–Boltzmann constant, and T is the temperature. For small cooling rates the efficiency for cooling can approach the limit imposed by the second law of thermodynamics.
Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass
Authors: Arai Kazuo , Namikawa Hiroshi , Kumata Ken , Honda Tatsutoku , Ishii Yoshiro , Handa Takashi
Published: July 2002

To utilize the excellent properties of silica (SiO2) glass for a glass laser host, neodymium-aluminum (Nd–Al) and neodymium-phosphorous (Nd–P) co-doped SiO2 glasses were studied. They were prepared by plasma-torch chemical vapor deposition (CVD). It was found that a doping level less than ten times the number of Nd for the Al co-dopant and less than about fifteen times for the P co-dopant was enough to remove undesirable fluorescence properties of Nd-doped SiO2 glasses and make them suitable for laser application. The clustering Nd ions disperse well in a glass matrix and lasing fluorescence increases. The effects of the Al dopant on the density and Raman spectra were also studied to obtain structural information. On the basis of glass science and solution chemistry, the marked effects of both dopants were explained by the following model. Nd ions can be well incorporated into a SiO2 glass network through co-dopant oxide forming a solvation shell around the Nd ions. This model leads to an expansible method for coordination control around active ions in SiO2 glass. A preliminary experiment on laser oscillation using Nd–Al co-doped SiO2 glass was also carried out.
A rotary magnetic refrigerator for superfluid helium production
Authors: Hakuraku Y. , Ogata H.
Published: July 2002

A new rotary-magnetic refrigerator designed to obtain superfluid helium temperatures by executing a magnetic Carnot cycle is developed. A rotor containing 12 magnetic refrigerants (gadolinium-gallium-garnet) is immersed in liquid helium at 4.2 K and rotated at constant speed in a steady magnetic field distribution. Performance tests demonstrate that the new rotary refrigerator is capable of obtaining a temperature of 1.48 K. The maximum useful cooling power obtained at 1.8 K is 1.81 W which corresponds to a refrigeration efficiency of 34%.
High-speed photography of surface geometry effects in liquid/solid impact
Authors: Dear J. P. , Field J. E.
Published: July 2002

Recent theoretical studies of liquid/solid impact, in particular the geometric wave theory of Lesser and Field, have emphasized the importance of the detailed geometry in the contact area. In parallel with the theoretical work, we have developed a two-dimensional technique using gels for impact and shock studies. A combination of high-speed photography and schlieren optics allows the shocks in the liquid and solid, if it is transparent, to be visualized, as well as important processes, such as jetting, to be recorded. This paper describes the gel technique and gives results for a range of surface geometries for collision velocities of a few hundred meters per second. The relevance to damage initiation in liquid/solid impact problems, such as rain erosion, steam turbine blade erosion, and cavitation, are discussed.
Logarithmic detrapping response for holes injected into SiO2 and the influence of thermal activation and electric fields
Authors: Lakshmanna V. , Vengurlekar A. S.
Published: July 2002

Relaxation of trapped holes that are introduced into silicon dioxide from silicon by the avalanche injection method is studied under various conditions of thermal activation and external electric fields. It is found that the flat band voltage recovery in time follows a universal behavior in that the response at high temperatures is a time scaled extension of the response at low temperatures. Similar universality exists in the detrapping response at different external bias fields. The recovery characteristics show a logarithmic time dependence in the time regime studied (up to 6000 s). We find that the recovery is thermally activated with the activation energy varying from 0.5 eV for a field of 2 MV/cm to 1.0 eV for a field of −1 MV/cm. There is little discharge in 3000 s at room temperature for negative fields beyond −4 MV/cm. The results suggest that the recovery is due to tunneling of electrons in the silicon conduction band into the oxide either to compensate or to remove the charge of trapped holes.
Optical and structural properties of GaAs grown on (100) Si by molecular-beam epitaxy
Authors: Stolz W. , Guimaraes F. E. G. , Ploog K.
Published: July 2002

GaAs epitaxial layers (0.5 μm<d<4 μm) were grown by molecular-beam epitaxy (MBE) on (100) Si substrates (2° off in 〈011〉 direction) by using the two-step growth mode. The structural properties of the grown films (lattice constants, misorientation, crystal quality) have been measured by high-resolution double-crystal x-ray diffraction. The GaAs layers are under biaxial tensile strain at temperatures below the growth temperature because of the difference in thermal expansion coefficients of GaAs and Si. With the use of temperature-dependent photoluminescence and excitation spectroscopy, the band edges as well as the dominant near-band-edge recombination processes have been determined. Excitonic resonances are clearly observed in the excitation spectra. The strain splitting between the heavy- and light-hole valence bands amounts to 13 meV at 4.5 K. The dominant low-temperature luminescence line at 1.489 eV is assigned to exciton-related recombination processes. In addition to the band-to-carbon acceptor (e,A0C) recombination at 1.470 eV, a defect-related (d,A0C) recombination process is identified at 1.411 eV for a sample temperature of 4.5 K. The defect concentration is higher near the GaAs to Si interface region and decreases with increasing GaAs layer thickness. With increasing temperature (80–300 K) the valence-band splitting diminishes and reaches a value of 9 meV at room temperature, in good agreement with the strain values determined by x-ray diffraction.
Structure and composition of amorphous Ge1−xSnx thin films
Authors: Chambouleyron I. , Marques F. C. , de Souza J. P. , Baumvol I. J. R.
Published: July 2002

The composition and bonding configuration of amorphous germanium-tin (a-Ge1−xSnx) thin films are reported (0≤x<0.3). Mössbauer spectroscopy analyses show that under the reported deposition conditions all tin atoms enter the a-Ge network in a perfect substitutional way, i.e., in a covalent tetrahedral configuration. The absence of defect structures in the tin sites is discussed and compared with results on films prepared under different conditions.
A model for the discharge kinetics and plasma chemistry during plasma enhanced chemical vapor deposition of amorphous silicon
Authors: Kushner Mark J.
Published: July 2002

A model for the plasma enhanced chemical vapor deposition of amorphous hydrogenated silicon (a-Si:H) in rf and dc discharges is presented. The model deals primarily with the plasma chemistry of discharges sustained in gas mixtures containing silane (SiH4). The plasma chemistry model uses as input the electron impact rate coefficients generated in a separate simulation for the electron kinetics and therefore makes no a priori assumptions as to the manner of power deposition. Radical densities and contributions to film growth are discussed as a function of gas mixture, electrode separation, and locale of power deposition, and comparisons are made to experiment. A compendium of reactions and rate constants for silane neutral and ion chemistry is also presented.
Magnetization reversal in nucleation controlled magnets. II. Effect of grain size and size distribution on intrinsic coercivity of Fe-Nd-B magnets
Authors: Ramesh R. , Thomas G. , Ma B. M.
Published: July 2002

Results of experiments to study the effect of grain size and grain-size distribution on the intrinsic coercivity and the hysteresis loop of sintered Fe-Nd-B magnets are presented. It is shown that the intrinsic coercivity decreases as the average grain size of the magnet is increased. It is also shown that the intrinsic coercivity decreases linearly with the logarithm of the square of the grain size. This is consistent with the predictions made based upon the statistical model developed in Part I. An increase in the sintering temperature leads to an increase in the average grain size, which consequently leads to a narrower hysteresis loop and lower intrinsic coercivity compared to magnets sintered at a lower temperature. It is also shown that a heterogeneous grain-size distribution, such as a bimodal distribution, causes kinks to appear in the second quadrant of the hysteresis loop. By examining magnets with different fractions of large grains, the prediction that the magnitude of the kinks increases with the volume fraction of the large grains, has been verified experimentally.
Thermal conductivity of a boron phosphide single-crystal wafer up to high temperature
Authors: Kumashiro Y. , Mitsuhashi T. , Okaya S. , Muta F. , Koshiro T. , Takahashi Y. , Mirabayashi M.
Published: July 2002

The thermal diffusivity of a well-characterized boron phosphide (BP) single-crystal wafer made by a chemical vapor deposition process was measured by a unique ring-flash light method. Thermal conductivity calculated from the products of thermal diffusivity, specific heat capacity, and density of the wafer yields a high thermal conductivity of 4.0 W cm−1 deg−1 at room temperature, and shows a pronounced temperature dependence due to phonon scattering. Boron phosphide is a promising material for heat-sink substrates for semiconductor devices.
The role of hydrogen in vapor deposition of diamond
Authors: Frenklach Michael
Published: July 2002

The effect of hydrogen addition on growth of diamond films under the conditions of chemical vapor deposition was investigated computationally. A detailed chemical kinetic mechanism was composed to describe the evolution of reaction species in pyrolysis of hydrogen- and argon-diluted methane mixtures with imposed temperature profiles, simulating the gas-phase conditions of diamond film growth in an idealized hot-filament reactor. The reaction mechanism was comprised of two basic parts: decomposition of methane, and formation and growth of polycyclic aromatic hydrocarbons; it contained a total of 120 elementary reactions and 45 chemical species. The reaction rate coefficients included temperature and pressure dependencies. The computations were performed for a variety of initial conditions, elucidating the effects of critical parameters on the product composition in the regime of diamond deposition. Analysis of the computational results indicated that the key role of the hydrogen addition in the diamond deposition process is to suppress the formation of aromatic species by H2 in the gas phase and thereby to prevent the formation and growth of nondiamond, graphitic phases on the deposition surface.
Metastable phase formation in the Zr-Al binary system induced by mechanical alloying
Authors: Fecht H. J. , Han G. , Fu Z. , Johnson W. L.
Published: July 2002

We have studied the sequence of phase transformations induced in the Zr-Al binary system by mechanical alloying of mixed Zr and Al powders. The structure of these materials has been studied by transmission electron microscopy and by x-ray diffraction measurements. Three different metastable phases have been found experimentally with variation of the initial composition xAl: (1) a nanocrystalline supersaturated solid solution of α-Zr for xAl≤0.15, (2) an amorphous phase for 0.15<xAl≤0.4, and (3) a metastable face-centered-cubic phase for xAl=0.5 with a grain size of 4 nm. The crystallization reaction of the amorphous phase was monitored by differential scanning calorimetry, and the kinetics of the reaction have been examined as well. A possible explanation based on thermodynamic arguments is given for the defect-driven vitrification of the crystalline Zr phase.
Relationship between crystallization process and magnetic properties of Fe-(Cu-Nb)-Si-B amorphous alloys
Authors: Noh T. H. , Lee M. B. , Kim H. J. , Kang I. K.
Published: July 2002

The effect of Cu and/or Nb addition on crystallization processes and soft magnetic properties for Fe-Si-B alloys has been investigated. For Fe-Si-B, Fe-Cu-Si-B, and Fe-Nb-Si-B alloys annealed at various temperatures, the effective permeabilities were very low and decreased considerably as they crystallized, while for Fe-Cu-Nb-Si-B alloy annealed at a temperature above Tx , a very good soft magnetic property was obtained. From DTA analysis of Fe-Cu-Nb-Si-B alloys, it became clear that the combined addition of Cu and Nb expanded largely the temperature range over 100 °C where a single phase of α-Fe solid solution can exist. Above 580 °C, the second crystalline phase such as Fe2 B and some unidentified phases other than the α-Fe phase precipitated, and soft magnetic properties deteriorated rapidly. It is concluded that the combined addition of Cu and Nb makes the α-Fe single phase stable in a broad temperature range, which is considered to be the necessary condition for the superior soft magnetic properties.
Migration and immobilization of hydrogen and helium in gold and tungsten at low temperatures
Authors: Soltan A. S. , Vassen R. , Jung P.
Published: July 2002

Hydrogen, deuterium, and helium were implanted at 5 K with energies from 0.25 to 3 keV into thin films of 80–320 nm of gold and tungsten. The annealing of the implantation-induced resistivity was measured during isochronally heating of the specimens up to 400 K. The onset of resistivity annealing after implantation to low concentrations was used as indication that the implanted atoms become mobile. The corresponding temperatures were ≤5 K for helium in both metals, ≊18 K for H in Au, ≊50 K for D in Au, and 80 K for H and D in W. The increasing suppression of annealing with increasing concentration of implanted atoms shows that the incomplete annealing above the onset of mobility is mainly due to clustering of implanted atoms and only to a lesser extent due to trapping by impurities.
Soft magnetic properties of nanocrystalline bcc Fe-Zr-B and Fe-M-B-Cu (M=transition metal) alloys with high saturation magnetization (invited)
Authors: Suzuki Kiyonori , Makino Akihiro , Inoue Akihisa , Masumoto Tsuyoshi
Published: July 2002

A mostly single bcc phase with nanoscale grain sizes of 10 to 20 nm was found to form by annealing amorphous Fe-Zr-B, Fe-Hf-B, and Fe-M-B-Cu(M=Ti, Zr, Hf, Nb, and Ta) alloys for 3.6 ks in the range of 723 to 923 K. The high permeability (μe) above 10 000 at 1 kHz combined with high saturation magnetization (Bs) above 1.5 T was obtained for the bcc alloys. The highest μe and Bs values reach 14 000 and 1.7 T for Fe91Zr7B2, 20 000 and 1.55 T for Fe87Zr7B5Cu1, and 48 000 and 1.52 T for Fe86Zr7B6Cu1. Magnetostriction (λs) decreases significantly by the phase transition from amorphous to bcc phase and is measured to be 1 × 10−6 for the bcc Fe86Zr7B6Cu1 alloy. The small λs as well as the small grain size is concluded to be the reason for the good soft magnetic properties. The lattice parameter of this bcc phase is 0.2870 nm being larger than that of pure α-Fe. The small λs seems to be achieved by the dissolution of solute elements above an equilibrium solubility limit. The bcc Fe86Zr7B6Cu1 alloy also shows the low core loss of 0.066 W/kg at 1 T and 50 Hz, which is considerably smaller than that of amorphous Fe78Si9B13 and bcc Fe-3.5mass%Si alloys in practical uses as core materials in transformer.
Dynamic high-pressure properties of AlN ceramic as determined by flyer plate impact
Authors: Rosenberg Z. , Brar N. S. , Bless S. J.
Published: July 2002

The dynamic properties of hot-pressed aluminum nitride ceramics were determined in a series of plate impact experiments using longitudinal and transverse in-material manganin gauges. The Hugoniot curve for hot pressed specimens was determined in the range of 0–190 kbar with a value of 94±2 kbar for the Hugoniot elastic limit (HEL). Using both gauge orientations, the stress deviator in the specimens was determined as the difference between longitudinal and transverse stresses. It was found that the stress deviator remains relatively constant above the HEL, and is about 10% higher than the value at the HEL point. The inferred Hugoniot converges to the extrapolation of the hydrostat. Since the convergence is not due to loss of strength, it may be due to a phase change in the AlN from low-pressure (wurtzite) structure to high-pressure (rocksalt) structure.
Films and junctions of cadmium zinc telluride
Authors: Chu T. L. , Chu S. S. , Ferekides C. , Britt J.
Published: July 2002

Cadmium telluride (CdTe) and zinc telluride (ZnTe), direct gap semiconductors with room-temperature band gap energies of 1.45 and 2.25 eV, respectively, form a continuous series of solid solutions (Cd1−xZnxTe). The band gap energy of Cd1−xZnxTe can be tailored in the 1.45–2.25 eV range. Cd1−xZnxTe with band gap energy of 1.65–1.75 eV is suitable as the upper member of a two-cell tandem structure for the photovoltaic conversion of solar energy. In this work, polycrystalline films of Cd1−xZnxTe have been deposited on glass, CdS/SnO2:F/glass, and Cd1−xZnxS/SnO2:F/glass substrates at 400 °C by the reaction of dimethylcadmium (DMCd), diethlyzinc (DEZn), and diisopropyltellurium (DIPTe) in a hydrogen atmosphere. The composition of Cd1−xZnxTe films determined by wavelength dispersive spectroscopy and x-ray diffraction has been correlated with the band gap energy deduced from the junction photovoltage spectroscopy and optical transmission. The structural and electrical properties of Cd0.7Zn0.3Te (band gap energy 1.70 eV) films have been evaluated. Thin film Cd0.7Zn0.3Te/CdS and Cd0.7Zn0.3Te/Cd0.7Zn0.3S heterojunctions have been prepared and characterized.
Determination of complex dielectric functions of ion implanted and implanted-annealed amorphous silicon by spectroscopic ellipsometry
Authors: Fried M. , Lohner T. , Aarnink W. A. M. , Hanekamp L. J. , van Silfhout A.
Published: July 2002

Measuring with a spectroscopic ellipsometer (SE) in the 1.8–4.5 eV photon energy region we determined the complex dielectric function (ε = ε1 + iε2) of different kinds of amorphous silicon prepared by self-implantation and thermal relaxation (500 °C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted a-Si (i-a-Si) differs from that of relaxed (annealed) a-Si (r-a-Si). Moreover, its ε differs from the ε of evaporated a-Si (e-a-Si) found in the handbooks as ε for a-Si. If we use this ε to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the Davis–Mott plot based on the relation: (ε2E2)1/3 ∼ (E− Eg). The results are: 0.85 eV (i-a-Si), 1.12 eV (e-a-Si), 1.30 eV (r-a-Si). We attribute the optical change to annihilation of point defects.
Impact of illumination level and oxide parameters on Shockley–Read–Hall recombination at the Si-SiO2 interface
Authors: Aberle Armin G. , Glunz Stefan , Warta Wilhelm
Published: July 2002

The experimentally observed dependence of effective surface recombination velocity Seff at the Si-SiO2 interface on light-induced minority carrier excess concentration is compared with theoretical predictions of an ‘‘extended Shockley–Read–Hall (SRH) formalism.’’ The calculations of SRH-recombination rates at the Si-SiO2 interface are based on the theory of a surface space charge layer under nonequilibrium conditions and take into account the impact of illumination level, gate metal work function, fixed oxide charge density, and the energy dependence of capture cross sections σn, σp and interface state density Dit. Applying this theory to p-type silicon surfaces covered by high quality thermal oxides, the experimentally observed strong increase of Seff with decreasing minority carrier excess concentration could quantitatively be attributed to the combined effect of the σn/σp ratio of about 1000 at midgap and the presence of a positive fixed oxide charge density Qf of about 1×1011 charges/cm2. Due to the favorable work function of aluminum, surface recombination velocities below 1 cm/s can be obtained at Al-covered Si-SiO2 interfaces for minority carrier densities above 1013 cm−3.
Electrical and structural properties of low resistivity tin-doped indium oxide films
Authors: Shigesato Yuzo , Takaki Satoru , Haranoh Takeshi
Published: July 2002

Tin-doped indium oxide (ITO) films with the resistivity less than 1.35×10−4 Ω cm were formed by low voltage dc magnetron sputtering (LVMS) and highly dense plasma-assisted electron beam (EB) evaporation using the arc plasma generator (HDPE). The structural properties of these films were investigated using x-ray diffraction, scanning electron microscope, and electron spectroscopy for chemical analysis, in comparison with the films formed by conventional magnetron sputtering and EB evaporation, in order to clarify the key factors for low resistivity. With decreasing plasma impedance and sputtering voltages from 540 to 380 V, the resistivity of the films deposited at Ts=400 °C decreased from 1.92 to 1.34×10−4 Ω cm, due mostly to increase in the carrier density. This LVMS film showed higher crystallinity because of lower damages of high-energy particles during the deposition, which might increase the number of electrically active species. For HDPE, the film with resistance of 1.23×10−4 Ω cm was deposited at Ts=280 °C, which showed more flat surface morphology and less surface segregation of tin than the conventional EB films.
Thermal wave imaging with phase sensitive modulated thermography
Authors: Busse G. , Wu D. , Karpen W.
Published: July 2002

Thermography and thermal wave techniques can be combined to provide in a short-time low-frequency phase angle images where nonthermal features can be suppressed. The principle is optical thermal wave generation simultaneously on the whole sample surface and sequential monitoring of all pixels using both thermographic techniques and lock-in data analysis. Due to parallel stationary excitation one can use low modulation frequencies allowing for a depth range that is of relevance for applications.
Elastic constants of single-crystal transition-metal nitride films measured by line-focus acoustic microscopy
Authors: Kim J. O. , Achenbach J. D. , Mirkarimi P. B. , Shinn M. , Barnett S. A.
Published: July 2002

The elastic constants of single-crystal NbN, VN, and TiN films were determined from surface acoustic wave (SAW) dispersion curves obtained by the use of an acoustic microscope with a line-focus beam. Measurements were carried out for single-crystal nitride films grown on the (001) plane of single-crystal cubic-symmetric MgO substrates. The phase velocities measured as functions of the angle of propagation display the expected anisotropy. Dispersion curves of SAWs propagating along the symmetry axes were obtained by measuring the wave velocities for various film thicknesses and frequencies. Using a modified simplex method, an inversion of the SAW dispersion data yielded the elastic constants of cubic symmetry, namely c11, c12, and c44. The Rayleigh surface wave velocities calculated from the determined elastic constants and known mass densities agree well with a result measured by Brillouin scattering spectroscopy reported elsewhere.
Quasielastic release in shock-compressed solids
Authors: Johnson J. N. , Hixson R. S. , Gray G. T. , Morris C. E.
Published: July 2002

Shock- and release-wave measurements are reported for 6061-T6 aluminum [J. R. Asay and L. C. Chhabildas, in Shock Waves and High-Strain-Rate Phenomena in Metals, edited by M. A. Meyers and L. E. Murr (Plenum, New York, 1981), pp. 417–431], oxygen-free-electronic copper, and a Si-bronze alloy. Significant departure from ideal elastic-plastic response is observed in all three materials. Experimentally determined release-wave profiles show evidence for the onset of reverse plastic flow immediately upon release from the shocked state. This phenomenon is analyzed in terms of internal stresses acting on straight dislocation pileups and pinned dislocation loops created by the shock-compression process. Following shock compression and prior to release, the internal stresses are opposed by the applied shear stress; that is, they exactly balance each other and no plastic flow occurs. As the applied stress is reduced in the unloading wave, reverse plastic flow occurs immediately due to internal reverse stresses acting on these pileups and pinned loops. This effect reduces the longitudinal modulus, and hence, the release-wave speed in what we normally think of as the ‘‘elastic-wave’’ regime. Interpretations of quasielastic release-wave data and calculations are expressed in terms of micromechanical concepts.
Simple expressions for elastic constants c11, c12, and c44 and internal displacements of semiconductors
Authors: Muramatsu Shinji , Kitamura Michihide
Published: July 2002

Simple expressions are given for elastic stiffness constants c11, c12, and c44 and Kleinman’s internal displacement parameter ζ. They all are represented by the bulk modulus and covalency alone, and predict fairly well the experimental results. These expressions are useful for getting a simple estimate of ζ as well as c11, c12, and c44.
Electromagnetic particle simulation of electron cyclotron resonance microwave discharge
Authors: Koh Wook Hee , Choi Nak Heon , Choi Duk In , Oh Yong Ho
Published: July 2002

We present a numerical model to study the electron cyclotron resonance (ECR) microwave discharge using a one-dimensional electromagnetic particle-in-cell Monte Carlo collision method [C. K. Birdsall, IEEE Trans. Plasma Sci. 19, 65 (1991)]. In our model, the electromagnetic wave is polarized circularly and propagates along an external static magnetic field and elastic, excitational, and ionizing electron-neutral collisions and elastic and charge exchange ion-neutral collisions are included. The discharge for helium gas is simulated and the simulation results explain well the physical properties of the ECR discharge which include the energy absorption of electrons through ECR coupling, the propagation of microwave, the transports of the charged particles, and the effect of divergent external magnetic field.
Localized Kerr study of the magnetic properties of an ultrathin epitaxial Co wedge grown on Pt(111)
Authors: McGee N. W. E. , Johnson M. T. , de Vries J. J. , aan de Stegge J.
Published: July 2002

We have used the magneto-optical Kerr effect to study the thickness dependence of the perpendicular magnetic anisotropy, coercivity, nucleation field and remanence in an ultrathin Co wedge (0–9 monolayers) grown epitaxially on Pt(111) and capped by a thin Pt overlayer. In addition, the spin-reversal mechanism in the Co layers has been investigated using Kerr domain imaging. The sample showed perpendicular magnetization up to an extrapolated Co thickness of 15 monolayers (ML), with derived volume and interface anisotropy constants of −0.77 MJ/m3 and 1.15 mJ/m2, respectively. The Kerr rotation and Kerr ellipticity demonstrated linear dependences on the Co thickness (tCo), with sizeable extrapolated offsets at tCo=0, attributed to polarization of the Pt by the Co. The coercivity (Hc) and nucleation field both rose to peak values at tCo=1.5 ML, with a subsequent monotonic fall-off for higher Co thicknesses. In the case of Hc, this fall-off did not demonstrate the tCo−5/2 dependence shown by a similar Pd(111)/Co/Pd sample investigated earlier.
Diffusion barrier properties of TiW between Si and Cu
Authors: Wang Shi-Qing , Suthar Sailesh , Hoeflich Christine , Burrow Brad J.
Published: July 2002

100-nm-thick TiW (30 at. % Ti) films were used as diffusion barriers between silicon substrates and thin Cu films. Sheet resistance measurements, Rutherford backscattering spectrometry, Auger electron spectroscopy, transmission electron microscopy, scanning electron microscopy, and x-ray diffractometry indicated the absence of interdiffusion and structural change for unpatterned Si/TiW/Cu samples up to 775 °C if there was no exposure to air between TiW and Cu deposition and 850 °C if there was an exposure, respectively. Leakage current measurements showed no deterioration of diode junctions up to 725 °C for TiW without air exposure and 775 °C for air-exposed TiW.
Stress evolution due to electromigration in confined metal lines
Authors: Korhonen M. A. , Bo/rgesen P. , Tu K. N. , Li Che-Yu
Published: July 2002

Electromigration is an important concern in very large scale integrated circuits. In narrow, confined metal interconnects used at the chip level, the electromigration flux is resisted by the evolution of mechanical stresses in the interconnects. Solutions for the differential equation governing the evolution of back stresses are presented for several representative cases, and the solutions are discussed in the light of experimental as well as theoretical developments from the literature.
Damage-free selective etching of Si native oxides using NH3/NF3 and SF6/H2O down-flow etching
Authors: Nishino H. , Hayasaka N. , Okano H.
Published: July 2002

Damage-free selective etching of Si native oxides against Si has been achieved by NH3/NF3 and SF6/H2O down-flow etching. In the NH3/NF3 etching, the wafer was covered with a film, and after its removal by heating above 100 °C, only SiO2 was found to be etched with an extremely high selectivity with respect to Si. Selective etching of Si oxides has also been obtained for SF6/H2O microwave discharge. In this case, a film of liquid solution containing HF and H2SOx is considered to form on the wafer surface. The selective etching of SiO2 takes place by the dissolved HF just as in the wet etching by an HF solution. The mechanisms of these selective reactions are discussed in detail based on the covalency of Si and SiO2 bondings.