bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser-induced evolution of hypocycloidal formation of vortex filaments from nonlinear Rayleigh–Taylor instability in a thin layer of molten metal
Authors: Lugomer S. , Maksimović A.
Published: July 2002

A tiny vortex filament, the self-organization of which follows the contour of the nonlinear Rayleigh–Taylor (RT) instability was generated by the Gaussian laser pulse on a ns time scale. Vortex filament self-organization on the nonplanar target surface follows the asymmetric RT evolution consisting of the compressed half of the hypocycloide whose cusps have evolved into spikes, and of the other half whose cusps have evolved into loops. Since the loops cannot be formed, the filaments break into short sets of parallel rolls, oriented in radial direction. The asymmetric contour of the filament self-organization was reproduced by the two-dimensional multimode model that gives the analytical solution, based on the model of Ott. The asymmetry in the vortex filament self-organization is generated by the random wave number mode values, and by the specific phase relations between the modes.
Modeling and three-dimensional simulation of the neutral dynamics in an air discharge confined in a microcavity. I. Formation and free expansion of the pressure waves
Authors: Eichwald O. , Yousfi M. , Bayle P. , Jugroot M.
Published: July 2002

A three-dimensional numerical analysis of the neutral dynamics is performed in the case of a short-gap (0.5 mm) spark discharge in air confined in microcavities at atmospheric pressure (760 Torr) and ambient temperature (293 K). This work is undertaken in the framework of silicon microsystems bearing a micropump actuated by pressure waves which result from a discharge. The short-gap discharge characteristics are taken from experimental results namely 470 ns for the duration and 13.5 W for the maximum injected power. The neutral gas evolution is described by the classical transport equations and solved by a powerful numerical monotonic upstream-centered scheme for conversion laws. The gas–solid interaction occurring in thermal and hydrodynamic boundary layers is taken into account assuming that the microcavity temperature remains invariant (293 K). This article (part I) is devoted to the first evolution phase of the neutral dynamics whose the duration corresponds to the discharge time. Our results clearly show that the first phase can again be split into a neutral inertia phase (during which the thermal energy transferred is stored in the ionized channel) followed by a free expansion one where this thermal energy is dissipated in the microcavity volume. The latter phase is analyzed before the neutral heterogeneities reach the microcavity’s walls. We also discuss the specific gas behaviors of the gas nearby the electrode surfaces, following heat exchanges and viscous stress.
Laser ablation induced surface modification of NdBa2Cu3O7−δ single crystal target in pulsed laser deposition
Authors: Li Yijie , Yao Xin , Tanabe K.
Published: July 2002

The surface modification of both morphology and composition of a single crystal NdBa2Cu3O7−δ (NBCO) target induced by laser ablation has been studied by scanning electron microscopy with x-ray energy dispersive spectroscopy. After a long period of laser ablation, the sintered target displays a cone-shaped columnar surface morphology. However, the single crystal target exhibits a continuous and smooth undulating morphology with ridges and grooves. This cone-free surface morphology is thought to inhibit the ejection of droplets from the target surface and thus lead to NBCO films with excellent surface morphology. After a large number of laser shots per site on the single crystal target, both the surface composition and deposition rate remain unchanged. Our experimental results prove that the observed results for the sintered target are in agreement with the erosion model, while the laser ablation induced surface modification for the single crystal target is dominated by the melt–resolidification process.
Energy considerations during the growth of a molten filament in metal-to-metal amorphous-silicon antifuses
Authors: Vasudevan N. , Fair R. B. , Massoud H. Z.
Published: July 2002

A model for the growth of a conducting filament in metal-to-metal amorphous-silicon antifuses is presented. The transition from a high-resistance state to a low-resistance one is initiated by the formation of a localized hot spot. The growth of the filament occurs by melting the surrounding amorphous silicon. The latent heat required for filament growth is provided by the power dissipation in the melt. As the filament grows, power dissipation drops rapidly and the growth slows. For a given set of programming conditions and at a certain value of the melt radius, the power dissipation in the filament is no longer sufficient to provide the energy needed for the growth process. This condition leads to ending the filament growth. The thermal model presented here predicts several characteristics of the ON state, such as the dependence of the final filament radius rfil on the programming voltage Vpp and the series resistance Rser. It also predicts that the ON resistance is inversely related to the programming voltage Vpp. The model predictions agree with experimental results.
c-axis resistivity of Hg-1201 single crystals
Authors: Villard G. , Daignere A. , Maignan A. , Ruyter A.
Published: July 2002

The out-of-plane (c-axis) resistivity, ρc, of Hg-1201 crystals has been determined. The values obtained, i.e., ρc(300 K)∼0.3–0.4 Ω cm, are found to be similar to those of Tl-2201 and Hg-1223, and a quasi-T-independent ρc is observed in the normal state. The Tc value of 92 K, obtained from the ρc(T) curves, confirms the nearly optimized state of this crystal. The T dependence of the irreversibility line (IL), obtained from the ρc(T)H curves (H∥c-axis geometry), is found to be similar to that determined magnetically. The correlation between ρc and the IL location in the (B,T) plane emphasizes the strong influence of the separating mercury layers upon the c-axis transport properties of this cuprate.
The role of size effects on the crystallization of amorphous Ge in contact with Bi nanocrystals
Authors: Wilson N. , Petford-Long A. K. , Doole R. C. , Serna R. , Afonso C. N.
Published: July 2002

The kinetics of metal-induced crystallization of amorphous Ge in contact with Bi nanocrystals (NCs) have been studied by in situ transmission electron microscope annealing. Series of nanostructured films consisting of layers of Bi NCs in an amorphous Ge matrix have been grown by pulsed laser deposition. The a-Ge crystallization temperature depends strongly on both the size and shape of the NCs and the separation between the NCs in the film-normal direction. The size of the NCs controls the crystal nucleation process through the amount of metal surface in contact with the semiconductor, the shape of the NCs determines the initial Ge crystallization in the direction perpendicular to the film plane, and the separation between the NCs in the film-normal direction controls the overall pattern of the Ge crystal growth process.
Spectroellipsometry and electroreflectance of PbS
Authors: Kanazawa Hideyuki , Adachi Sadao
Published: July 2002

We have reanalyzed our published ε(E) data of PbS measured by spectroscopic ellipsometry (SE). The electroreflectance (ER) technique has been used to independently assess critical points in PbS. A large negative peak at ∼2.6 eV is found in the measured ER spectra which have no observable correlation with the SE data. Excellent agreement with the published experimental data is achieved by considering this critical point in the calculation.
Spectroscopic assessment of LiTaO3:Tm3+ as a potential diode-pumped laser near 1.9 μm
Authors: Sokólska I. , Ryba-Romanowski W. , Goła̧b S. , Baba M. , Łukasiewicz T.
Published: July 2002

The spectroscopic properties of LiTaO3:Tm3+ single crystal relevant for the Tm3+ laser transition near 1.9 μm are presented. The absorption cross section for the H63→H43 transition, the emission cross section for the F43→H63 transition and gain curves are determined for σ and π polarization. The broad absorption band at about 795 nm and high values of the above cross sections are promising features for obtaining a laser diode-pumped Tm3+ laser. The gain coefficient curves imply that the tunability range is up to 200 nm.
Alignment of two-valley resonance levels and I–V characteristics of GaAs/AlAs resonant tunneling diodes
Authors: Krishnamurthy Srinivasan , Chen A.-B. , Sher A.
Published: July 2002

In the resonant tunneling devices made of mixed gap materials such as AlAs/GaAs/AlAs, the quantum well for the Γ valley resided in the GaAs layer and those of the X valley in the AlAs layer. There are resonance energy levels in each of these valleys. Our calculation of I–V characteristics suggests a new possibility of a considerable increase in the peak-to-valley current ratio if one resonance level of the Γ valley aligns with that in the second AlAs X valley. For one choice of empirical tight-binding band structures of GaAs and AlAs, the resonant level alignment increases the peak-to-valley ratio by two orders of magnitude. We further show that the predicted resonance levels are exceedingly sensitive to the barrier thickness and the quality of underlying band structures.
Effects of heat treatment on the electronic structure of nanometric Si/C/N powders by x-ray photoelectron spectroscopy
Authors: Senemaud C. , Gheorghiu-de La Rocque A. , Dufour G. , Herlin N.
Published: July 2002

The electronic structure of ultrafine laser synthesized powders with controlled composition have been investigated by using x-ray photoelectron spectroscopy. The Si 2p, C 1s, and N 1s core levels as well as the valence band distributions have been analyzed. The experimental analysis is focused on intermediate composition samples with C/N equal to 0.58 and 0.67; in this range the C/N value remains approximately the same under heat treatments. In as-formed samples, the nanopowders exhibit a high degree of chemical disorder; mixed tetrahedra with both C and N atoms around each Si atom are likely to occur. After a 1500 °C treatment, the powders remain amorphous and the presence of N–C and C–C bonds is clearly shown. After a 1600 °C annealing the samples are crystallized; the measured core levels and valence spectra reveal the presence of C–Si and N–Si bonds in Si–C4 and Si–N4 groups. It is noteworthy that in this case the C 2s and N 2s valence band states appear as two well separated peaks in the valence band. Our results show that the Si/C/N nanopowders, in the composition range close to C/N=0.6, are not a simple mixture of carbide and nitride groups at an atomic scale, but exhibit a specific atomic structure including C–N bonds. The evolution of the atomic structure of these systems, as a function of heat treatments, depends noticeably on the initial composition of the nanopowder.
Reactions in the afterglow of time modulated inductive discharges of Xe and I2 mixtures
Authors: Barnes Paul N. , Kushner Mark J.
Published: July 2002

An investigation was conducted using absorption and emission spectroscopy of the afterglow from rf inductive discharges in Xe/I2 mixtures. RF power at 11.5 MHz was supplied to coils surrounding a cylindrical discharge cell. Total pressures were between 0.8 and 5.3 Torr. Measurements were made as a function of radius and time following the termination of the discharge. Results presented here indicate that in the afterglow the XeI* excimer is formed by highly excited I2 and ground state Xe. Evidence is also presented that ion–ion neutralization is a major precursor reaction for the formation of other excited species in the afterglow.
On the frequency dependence of the magnetic permeability of FeHfO thin films
Authors: Bloemen P. J. H. , Rulkens B.
Published: July 2002

The frequency dependence of the magnetic permeability as well as of the electrical impedance have been investigated for soft-magnetic granular FeHfO thin films. The impedance measurements indicate that capacitive effects resulting from the inhomogeneous structure of the layers are of no importance for the roll off of the permeability of the present films. The frequency behavior at various FeHfO thicknesses shows that eddy current effects start to play a role above thicknesses of 10 μm. Below this thickness ferromagnetic resonance dominates the roll off.
Effect of humidity on microwave dielectric losses of porous alumina
Authors: Mollá J. , González M. , Vila R. , Ibarra A.
Published: July 2002

The microwave dielectric loss in highly porous alumina is measured in dry and moist atmospheres. Data are compared with those for sapphire and a fully densified alumina grade. Results indicate that the combination of humidity and porosity gives rise to a very high dielectric loss which is drastically reduced by replacing the moist atmosphere by dry gas. Measurements over a wide frequency range from 1 mHz to 100 MHz indicate that the origin of the microwave loss is due to the high frequency tail of a low frequency process. This low frequency loss peak shifts to lower frequencies with decreasing humidity, explaining the observed dependence of the microwave loss on humidity.
Domain structure in polycrystalline MnZn ferrite imaged by magnetic force microscopy
Authors: Aarts J. , Shiekah I. Abu , van der Zaag P. J.
Published: July 2002

We investigated the occurrence of magnetic domains in MnZn-ferrite grains by magnetic force microscopy (MFM). At the surface of samples with different grain sizes in the range from 2 to 10 μm, small grains generally contain only one domain, while large grains contain two domains. The crossover between these two intragranular domain states occurs at a critical grain size Dcr of about 4 μm. This finding is in agreement with the conclusions from earlier neutron depolarization measurements on the same samples and reemphasizes the large discrepancy between theoretical calculations of Dcr for isolated grains and experimental values in the ferrites. Furthermore, we find that the magnetic field profiles are not those expected from isolated grains. Modeling of the MFM response shows that the magnetic environment of a grain plays an important role in its magnetic configuration.
Nonequilibrium distribution in quantum dots lasers and influence on laser spectral output
Authors: Jiang H. , Singh J.
Published: July 2002

Self-assembled quantum dot structures used for lasers have shown significant variation in the dot size distribution. In this article, we address the issues related to carrier occupation of these dots as a function of temperature in the absence and presence of lasing. The carrier distributions among different dots are derived in this paper through detailed balance. It is found that at low temperatures the carrier occupation is highly nonequilibrium but with increased temperature it tends towards an equilibrium distribution. Based on this distribution, the threshold current density versus temperature has been calculated. Multimode operation of lasers at different injection levels and temperatures is also examined. The theoretical results are compared with published experimental results.
Cyano-containing phenylene vinylene-based copolymer as blue luminescent and electron transport material in polymer light-emitting diodes
Authors: Chang En-Chung , Chen Show-An
Published: July 2002

The new soluble cyano-containing phenylene vinylene copolymer with well-defined conjugating length was synthesized and characterized by use of thermal analysis and spectroscopy (infrared, ultraviolet-visible, photoluminescence, cyclic voltammetry) measurement. This new polymer can be used as a blue electroluminescent material and as an electron transport/hole blocking layer in polymer light-emitting diodes using poly(p-phenylene vinylene) (PPV) as the active luminescent layer. The transfer efficiency of current density to brightness of PPV devices with the introduction of the new polymer as electron transporting layer increases by a factor of 25.
Magnetic structure of as-quenched silicate glasses containing iron oxides
Authors: Hayashi Miyuki , Susa Masahiro , Nagata Kazuhiro
Published: July 2002

The effect of valence states of iron on the magnetic structure of as-quenched silicate glasses containing iron oxides has been investigated. The glasses, all having the same compositions, are melted in various oxygen pressures to control the ratios of Fe3+ ions to the total number of iron ions and quenched at the same speed, and the magnetic properties are measured. There are only a few reports which focus particularly on the relation between valence states of iron and the magnetic properties of the glasses. Here, the dc magnetizations and ac susceptibilities of the glasses are explained based upon a proposed magnetic structure model in which both microcrystalline clusters and free iron ions in the glass matrix bear magnetism. (In this article free iron ions mean iron ions which are distributed in the glass matrix.) The number density of microcrystalline clusters is highest in a sample of x=0.73 (x: the ratio of Fe3+ ions to the total number of iron ions) and it becomes smaller as the value of x becomes smaller and larger than 0.73. The average magnetic interaction among free iron ions in the glass matrix is stronger in the samples having larger values of x.
Formation of needle shaped twin domains in cordierite: A computer simulation study
Authors: Blackburn John F. , Salje Ekhard K. H.
Published: July 2002

The formation of needle shaped domains in cordierite is simulated using a simplified two-dimensional computer model with hexagonal symmetry and nonlocal (strain-induced) atomic interactions. Domain walls form along elastically soft directions as expected and sometimes needle domains appear during annealing. During annealing needles are then consumed by the coarsening process or retract into the domains to which they are attached. Needles have rounded, parabolic tips symptomatic of cordierite’s comparatively low anisotropy energy. Needle splitting is not observed but a novel wetting mechanism by which auxiliary domains form at the needle tip is observed. These domains form such as to make a section of the needle tip a strain allowed domain wall and so reduce the bending energies of the domain walls in this region. At higher temperatures, needle domain walls become only poorly orientated along soft directions and needle domains become patchy with blunt tips and increased “wetting” at the needle tips.
Modeling of transitions in Mn2+ doped ZnS nanocrystals and predicting reduced lasing threshold current density and enhanced electro-optic effects in ZnCdSe–ZnMgSSe and InGaN–AlGaN pseudomorphic quantum dots
Authors: Jain F. , Huang W.
Published: July 2002

A three-dimensional coupled-well excitonic model is presented to explain the observed shift in the photoluminescence excitation spectrum, emission peak from Mn2+ ions, and five-to-six orders reduction of radiative lifetime in ZnS:Mn2+(35 Å) doped nanocrystals. For pseudomorphic cladded nanocrystals such as ZnCdSe–ZnMgSSe, a modified excitonic model predicts enhancements in the absorption coefficient (α∼160 000 cm−1) and electric field-dependent index of refraction change (Δn/n∼0.1–0.2), and a significant reduction of radiative lifetime τr∼14.5 fs. Optical gain and threshold current density (Jth) are computed for ZnCdSe–ZnMgSSe and InGaN–AlGaN quantum dot lasers. In the case of InGaN–AlGaN quantum dot lasers, the effect of dislocation-induced traps, enhancement of optical gain due to excitonic transitions, and dot size are considered in the computation of Jth. It is shown that InGaN dots with larger cross sections (∼200×250 Å, such as self-organized dots) and consisting of a trap density of 2.9×1017 cm−3 yield Jth of 4500 A/cm2 in comparison to 136 A/cm2 for dots of size 35×35×35 Å. This explains the observation of higher current density in InGaN quantum well lasers having self-organized dots.
The origin of the large perpendicular magnetic anisotropy in Co3Pt alloy thin films
Authors: Yamada Y. , Suzuki T. , Kanazawa H. , Österman J. C.
Published: July 2002

The origin of the large perpendicular magnetic anisotropy constant Ku obtained in Co3Pt alloy thin films is discussed in connection with a superlattice structure. The Ku and the order parameter S are influenced not only by substrate deposition temperature Ts but also deposition rate γ. For Ts=330 °C, Ku and S increase with lowering γ, whereas for Ts=400 °C, the opposite trend is found. A model, in which the origin of Ku results from two mechanisms: Magnetocrystalline anisotropy in Co3Pt disordered phase and interface-induced magnetic anisotropy in Co3Pt ordered phase, can account for the observed dependence of Ku on S.
Mechanism of time-dependent oxide breakdown in thin thermally grown SiO2 films
Authors: Kimura Mikihiro , Koyama Hiroshi
Published: July 2002

In the thermally grown silicon dioxide (SiO2) films, thermochemical-breakdown and hole-induced-breakdown models are theoretically formulated to explain the external electric-field dependence of time-dependent dielectric breakdown (TDDB) phenomenon. Long-term TDDB test results proved to support the thermochemical-breakdown model. The time-dependent oxide breakdown mechanism is further studied on the basis of quantum physical chemistry. The structural transformations of a-SiO2 up to breakdown are simulated by a semiempirical molecular orbital calculation method (PM3 method) using Si5O16H12 clusters. The structural transformations can be classified into: (a) amorphous-like SiO2 (a-SiO2), (b) hole-trapped SiO2 (hole trap), and (c) electrically broken down SiO2 (breakdown) structures. The atom configuration shows a shortened length between the nearest oxygen atoms due to hole trapping. This leads to time-dependent oxide breakdown, and the breakdown structure consists of a pair of oxygen-excess (Si–O–O–Si) and oxygen-vacancy (Si–Si) defects. The heat of formation and frontier orbital energies of structural transformations account well for the physical aspects of the TDDB phenomenon.
Analytical model of the relaxation of a collisionless ion matrix sheath
Authors: Riemann K.-U. , Daube Th.
Published: July 2002

The temporal evolution of a collisionless ion matrix sheath in front of an electrode biased to a pulsed high negative voltage is investigated analytically and numerically. In the relaxation process a matrix extraction phase and a subsequent sheath expansion phase can be distinguished. For the matrix extraction phase we present an analytical model that is based on special solutions of the ion fluid equations and that is free of artificial assumptions. The model results in an explicit formula for the ion current to the electrode. The results are compared with numerical solutions of the ion fluid equations and show excellent agreement. By a simple parameter ansatz the model is extended to describe the sheath expansion phase.
Observation and application of optical interference and diffraction effects in reflection from photochemically fabricated Gaussian interfaces
Authors: Koker Lynne , Kolasinski Kurt W.
Published: July 2002

A HeNe laser has been used to fabricate photochemically a photoluminescent porous Si thin film on top of crystalline Si. The porous Si film has Gaussian shaped upper and lower interfaces. When the reflection of a laser beam from this film during, or after, the photochemical process is observed, two distinct, concentric circular interference patterns are observed. A pattern of thick rings is superimposed upon a pattern of fine rings. The reflected beam is far more divergent than the incident beam. The formation of the outer rings is a coherent phenomenon. Analysis of the patterns indicates that reflection from the upper interface is not involved in the ring formation process but that optical interference and Fresnel diffraction of the light reflected from the bottom interface cause the pattern formation. It is shown that the radius of the pattern is linearly proportional to the optical path length through the film. Therefore, measurements of pattern sizes yield information about the depth and index of refraction of the porous Si film. This observation provides us with a novel, in situ technique for measuring the kinetics of formation of the photoluminescent silicon thin films and might be exploited for applications in chemical sensing.
Electrical characterization of ZnO ceramics by scanning tunneling spectroscopy and beam-induced current in the scanning tunneling microscope
Authors: Dı́az-Guerra C. , Piqueras J.
Published: July 2002

A correlative study of the electrically active grain boundary structure of ZnO polycrystals has been carried out using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined instrument. Current imaging tunneling spectroscopy (CITS) measurements reveal reduced surface band gaps, as compared with grain interiors, at the charged boundaries imaged by SEM-based remote electron beam induced current (REBIC). ZnO grain boundaries were also imaged in the STM-REBIC mode with a resolution of up to 20 nm. The contrast differences observed in the SEM-REBIC and STM-REBIC images are discussed in terms of the different experimental conditions used in both techniques.
Experimental and theoretical study of strain-induced AlGaAs/GaAs quantum dots using a self-organized GaSb island as a stressor
Authors: Wang T. , Forchel A.
Published: July 2002

Low-temperature photoluminescence (PL) measurement and strain distribution calculation have been performed on strain-induced AlGaAs/GaAs quantum dots (QD) using self-organized GaSb islands on a GaAs surface as stressors (the lattice mismatch between GaSb and GaAs is 7.8%). Since lattice mismatch plays a crucial role in fabricating this strain-induced QD, the article compared self-organized GaSb island stressor and self-organized InP island stressor. Both of these self-organized islands are formed on a GaAs surface (the lattice mismatch between InP and GaAs is 3.8%). This article indicates that it is not an effective way to increase the strain effect by choosing a stressor with higher lattice mismatch. It also shows that higher lattice mismatch results in smaller coherent self-organized islands. This weakens the strain effect. The strain effect can be enhanced by decreasing a top layer thickness under the stressor, but the enhanced surface state due to the strain also decreases the PL emission intensity greatly. Based on this study, this article puts forward a suggestion on fabricating this strain-induced QD. In addition, our strain calculation predicts that the strain effect could be enhanced by thickening the wetting layer, which may be realized by lowering the growth temperature of the stressor.
Temperature-independent dc conduction in BaTiO3-based ceramic capacitors
Authors: Zhou Y. X. , Yoshimura N.
Published: July 2002

In this work, measurements of temperature-dependent dc conduction current in BaTiO3-based multilayer ceramic capacitors have been made. It was found that there is tunnel-like conduction current near Curie temperature of BaTiO3 where the current showed a temperature-independent characteristic. For the study of this abnormal phenomenon, the structural nature of insulating solids of practical importance is emphasized. A new concept has been introduced to explain the positive temperature coefficient anomaly in BaTiO3-based ceramic dielectrics. It was presumed that a Schottky process dominates the conduction current at the Curie temperature. It was also assumed that the Schottky process is controlled by the phase transition of BaTiO3-based ceramics near the Curie temperature where the effective work function, which changes with the temperature, is inherently related to the density of the paraelectric cubic crystals. Based on the proposed model, it was found that the calculated results are accurately correlated with the experimental ones.
Oxide ridge formed around octahedral void defects on (001)-oriented Czochralski silicon
Authors: Itsumi Manabu , Maeda Masahiko , Ueki Takemi , Tazawa Satoshi
Published: July 2002

We found that thermal oxidation of (001)-oriented Czochralski silicon produces oxide ridges which surround the oxide pits above the grown-in defects (or octahedral void defects), whereas thermal oxidation of (111)-oriented Czochralski silicon produces oxide pits solely above the octahedral void defects. When the thermal oxide layers on (001)-oriented silicon were 400 nm, the heights of the oxide ridges were about 10–20 nm. Considering that the side walls of the octahedral void defects are (111) oriented, we assume that the oxidation rate of (111)-oriented silicon exceeding that of (001)-oriented silicon is closely related to the formation of oxide ridges.
Charge trapping and charge compensation during Auger electron spectroscopy on SiO2
Authors: Guo Hansheng , Maus-Friedrichs W. , Kempter V.
Published: July 2002

It is well known that radiolysis, induced by the electron bombardment, leads to different defects in SiO2, but little is known about the trapping behavior of these defects. In this article, the charging behavior and the depth profiles of the irradiation damage introduced by electron bombardment are studied with Auger electron spectroscopy (AES). For charge reduction an environmental AES using O2 (up to 8×10−8 Torr) and specimen heating are applied. In environmental AES, a strong charge reduction is observed, and the importance of the environment for charge compensation reveals a correlation with electron stimulated desorption. By heating the sample, a strong charge reduction takes place above 500 °C; complete charge compensation can be expected by heating the sample above 700 °C in an O2 environment of 5×10−8 Torr. We suggest that E′ centers, nonbridging oxygen hole centers, and peroxy radicals, are responsible for the efficient trapping of electrons. The observed discharging is believed to be related to the annealing of the defects.
Influence of a surface electric field on the anchoring characteristics of nematic phases at rubbed polyimides
Authors: Meister Roland , Jérôme Blandine
Published: July 2002

We report measurements of the anchoring energy of 5CB (4-cyano-4′-n-pentyl-biphenyl) at a commercial polyimide (AL1051) with a method using a large applied electric field on thick measurement cells. The experiments reveal a deviation from the expected behavior of the system and cannot be described by the usual Frank elastic bulk energy density plus anchoring surface energy within the thermodynamic limit. We, therefore, discuss the effect of a surface electric field coupled to director distortions by the flexoelectric effect. Numerical calculations show, indeed, that the experiments can be described reasonably by the model. Experiments for thin cells indicate that the Debye screening length does not depend on the cell thickness. We derive an easy way to get an estimate of the Debye screening length from the experiments.
Tantalum–ruthenium dioxide as a diffusion barrier between Pt bottom electrode and TiSi2 ohmic contact layer for high density capacitors
Authors: Yoon Dong-Soo , Baik Hong Koo , Lee Sung-Man , Lee Sang-In
Published: July 2002

The effects of RuO2 addition on the barrier properties of the Ta layer were investigated at the temperature range of 650–800 °C in air. For the Pt/Ta/TiSi2/poly-Si/SiO2/Si structure, its layer structure was completely collapsed after annealing at 650 °C, resulting in higher total resistance and Schottky characteristics. For the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si structure, however, it exhibited the lower total resistance and ohmic characteristics, and its layer structure was retained up to 800 °C. A Ta amorphous structure by the chemically strong Ta–O or Ta–Ru–O bonds and nonstoichiometric, nanocrystalline RuOx resulted in the formation of a conductive RuO2 phase in the Ta+RuO2 barrier film after an annealing, leading to the prevention of the interdiffusion of Pt, Si, and external oxygen through the diffusion barrier as well as surface oxidation up to 800 °C. Therefore, the thermal stability for the Pt/Ta+RuO2/TiSi2/poly-Si/SiO2/Si structure was higher than that for Pt/Ta/TiSi2/poly-Si/SiO2/Si structure.
Determination of contact angles: Consistency between experiment and theory
Authors: El Ghzaoui A.
Published: July 2002

The Lifshitz–van der Waals theory has been used to calculate the contact angles of dispersive liquids and solids: diiodomethane, α-bromonaphtalene, methylnaphtalene, benzene, and n-octane, liquids on polytetrafluoroethylene, polystyrene, polyisobutene, polyvinylchloride, and polyethylene. The theoretical calculation of the contact angles was based on the nonretarded Hamaker constants which have been calculated from the dielectric properties of the materials and application of the Lifshitz theory. These theoretical contact angles were compared with the experimental contact angles measured by the Wilhelmy plate method. Closely related values have been found for the theoretical and experimental contact angles.
Thermal properties of H-related complexes in electron-irradiated Si doped with H
Authors: Suezawa Masashi
Published: July 2002

The thermal properties, namely, the thermal stability and the activation energy, of H-point defect complexes in Si were investigated. Specimens were doped with H by annealing in H2 gas followed by quenching. Then, they were irradiated by 3 MV electrons at room temperature. Subsequently, they were annealed isochronally or isothermally. Optical absorption spectra of H-point defect complexes were measured at 7 K. Due to isochronal annealing, the 2122, 1838, and 817 cm−1 peaks disappeared below 200 °C. On the other hand, 2223 and 2166 cm−1 peaks formed at above 125 and 175 °C, respectively. From isothermal annealing experiments, the binding energies of H2* (1838 cm−1 peak) and I (I: a self-interstitial) H2 or V (V: a vacancy) H2 (1987 and 1990 cm−1 peaks) were determined to be about 1.5 and 2.0 eV, respectively. The generation of the 2223 cm−1 peak was due to reaction between H2 and the 2122 cm−1 defect.
Time domain half-space dyadic Green’s functions for eddy-current calculations
Authors: Bowler J. R.
Published: July 2002

The field due to an impulsive current dipole embedded in a half-space conductor adjoining a nonconducting half space is given by an exact solution of the quasistatic field equations. This solution has been used to construct a half-space dyadic Green’s function containing a term for an unbounded conductor plus terms representing the field reflected at the interface between conducting and nonconducting regions. The resulting kernel can be used in the formulation of time-dependent scattering problems to express the electric field in a conductor as an integral over an electric dipole distribution.
A quantitative analysis of single pulse ultraviolet dry laser cleaning
Authors: Halfpenny D. R. , Kane D. M.
Published: July 2002

Particles as small as 0.3 μm in diameter have been successfully removed from a glass surface using a single ultraviolet pulse from a frequency doubled copper vapor laser (255.3 nm). Quantitative analysis of the particle density before and after laser irradiation shows that laser cleaning occurs after a fluence threshold is reached. The cleaning efficiency after threshold follows a nonlinear trend with respect to fluence. A model is presented which reveals that the cleaning efficiency is a function of the irradiance distribution of the beam used. Results of modeling thermal expansion of the substrate and particles, and particle adhesion do not confirm a thermal expansion mechanism for laser cleaning in this study, in contrast with other recent reports.
Lack of proportionality of total electron yield and soft x-ray absorption coefficient
Authors: Henneken H. , Scholze F. , Ulm G.
Published: July 2002

For many applications, the total electron yield (TEY) η is assumed to be proportional to the product of the linear absorption coefficient and the photon energy. To test this model we measured the total electron yield of copper and gold at the (111) surface of single crystals in the photon energy range between 50 eV and 1500 eV with relative uncertainties lower than 3.6%. In addition, the data for the absorption coefficient were improved by measuring the transmittance of polyimide films covered either with thin gold or copper layers. The experiments were performed at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY I. For photon energies below 150 eV, the total electron yield is proportional to the absorption coefficient, provided the saturation effects as determined here, by measurements of the TEY as a function of the angle of incidence of the radiation, are taken into account. At higher photon energy, the ratio between the TEY and the product of absorption coefficient times the photon energy decreases continuously by about 30% down to 1500 eV. We present a new analytical model for the total electron yield describing the contributions of primary and secondary electrons. Input parameters such as the electron escape depth and the reflection of the soft x-rays has been experimentally determined. When these parameters are used, our model achieves good agreement with the measured TEY, when the only free parameter, i.e., the efficiency with which the absorbed photon energy is converted into secondary electrons, is adjusted.
Thermal expansion of Ti-substituted barium hexaferrite
Authors: Hernández-Gómez P. , de Francisco C. , Brabers V. A. M. , Dalderop J. H. J.
Published: July 2002

Thermal expansion measurements in the range of 20–500 °C were carried out on both poly- and single crystalline samples of the hexagonal magnetoplumbite ferrite with composition BaTiFe11O19. The continuous scanning of the thermal expansion reveals the existence of a λ-type anomaly near the Curie temperature. In contrast to the usual observed anisotropic behavior for magnetoplumbite hexaferrites, the averaged thermal expansion coefficient for the a direction is larger than for the c direction, i.e., 12.8×10−6 and 9.5×10−6 °C−1, respectively.
Temperature and stress dependences of the magnetostriction in ternary and quaternary Terfenol alloys
Authors: Restorff J. B. , Wun-Fogle M. , Clark A. E.
Published: July 2002

The magnetization and magnetostriction of a variety of Laves phase rods of TbxDyyHo1−x−yFe1.95 grown in the form of [112] oriented dendritic platelets were measured as a function of magnetic field H(−2200<H<2200 Oe), compressive stress (20, 30, and 40 MPa), and temperature T(−60<T<80 °C). Compared to Terfenol-D (Tb0.3Dy0.7Fe1.95), Ho containing alloys can have lower hysteresis with only slightly lower magnetostriction. Sample compositions were chosen both along a line of minimum near room-temperature anisotropy (Tb0.3Dy0.7Fe1.95–Tb0.20Dy0.22Ho0.58Fe1.95) and at Ho0.15 and Ho0.30 across the line.
Role of orbital polarization in calculations of the magnetic anisotropy
Authors: Schneider G. , Jansen H. J. F.
Published: July 2002

Magnetic anisotropy is a difficult topic to describe by electronic structure theory. The results of ab initio calculations for iron and nickel are disappointing, especially since these calculations require high numerical precision and are very time consuming. For iron the value of the energy is too small by a factor of 3, while for nickel the sign is wrong and the value is too small by a factor of 5. The local density approximation can be improved by adding corrections that mimic the inclusion of Hund’s second rule. This is equivalent to increasing the effective spin–orbit parameter. For iron a small increase in the effective spin–orbit parameter is sufficient to reconcile theory and experiment. For nickel, this is not possible.
Strict control of grain size in Co–Cr–Ta recording layer for longitudinal magnetic media by intermittently depositing Cr underlayer
Authors: Fukuda Tadahiro , Nakagawa Shigeki , Naoe Masahiko
Published: July 2002

The intermittent deposition technique using 10-nm-thick Cr unit layers was performed to improve structural and magnetic characteristics of the Co–Cr–Ta recording layer deposited on the Cr underlayers. The Cr layers were composed of 10-nm-thick Cr unit layers intermittently deposited at intervals of 3 min or with N2 flow for 10 s between the depositions of each unit layer, in order to prevent the growth of Cr crystallites. The intermittent deposition of the Cr underlayers decreased in crystallite size 〈D〉Cr(200) and increased the coercivity Hc of the Co–Cr–Ta recording layer. N2 flow degraded the crystalinity of the Cr underlayer and decreased Hc∥ of the recording layer.
Switching in spin-valve devices in response to subnanosecond longitudinal field pulses
Authors: Kaka Shehzaad , Russek Stephen
Published: July 2002

We have fabricated spin-valve devices in a high-speed test structure that allows subnanosecond pulsed field excitation and high-bandwidth observation of the magnetoresistance response. The switching response varies for low-amplitude field pulses and approaches a consistent fast switch of less than 1 ns for field pulses of higher amplitude. For several pulse widths and amplitudes, the device switches into metastable states. The threshold amplitude of the write-pulse was measured as a function of pulse duration for pulses as small as 250 ps in duration.
Measurements of exchange anisotropy in NiFe/NiO films with different techniques
Authors: Fermin J. R. , Lucena M. A. , Azevedo A. , de Aguiar F. M. , Rezende S. M.
Published: July 2002

One of the puzzles of the recent investigations on the exchange anisotropy in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is the fact that different techniques yield different values for the exchange field (HE) between the layers. We report an investigation on sputtered NiFe/NiO carried out with three different techniques, namely, magneto-optical Kerr effect magnetometry (MOKE), Brillouin light scattering (BLS), and ferromagnetic resonance (FMR). In an attempt to reconcile the measurements obtained with the various techniques, we interpret the data with a model that includes the formation of a planar domain wall in the AF layer, giving rise to a torque on the FM moment represented by an effective domain wall field (HW). We find out that while the same pair of values of HE and HW provide equally good fits to the reversible FMR and BLS measurements, different pairs are necessary to fit the irreversible magnetometry data.
Oscillatory cyclotron resonance effective mass near ν=1 and 2 in modulation-doped ZnSe/Zn1−x−yCdxMnySe
Authors: Ng H. K. , Leem Y. A. , Storr K.
Published: July 2002

The fabrication by molecular-beam epitaxy of modulation-doped magnetic semiconductor heterostructures results in the creation of novel “magnetic” two-dimensional electron gases (2DEGs) with unusual transport properties. We report on the measurements of cyclotron resonance (CR) and effective mass in magnetic 2DEGs formed in modulation-doped ZnSe/Zn1−x−yCdxMnySe single quantum wells. Far-infrared absorption measurements are carried out at a nominal temperature of 4.2 K and in fields up to 30 T on these magnetic samples, as well as on corresponding nonmagnetic samples (ZnSe/Zn1−xCdxSe single quantum wells). The samples have carrier densities ranging from 1 to 4×1011 cm−2 and mobilities as high as 42 000 cm2/V s for the nonmagnetic samples. For the magnetic samples, the magnetic field dependence of the CR frequency shows marked oscillatory deviations from linearity in the vicinity of integer filling factors ν=2 and ν=1. For instance, compared to the nonmagnetic samples, the amplitude of the oscillations in the effective mass of the magnetic samples changes by 15% at half-filling factor. This is significantly larger than similar oscillatory effective mass behavior observed in standard semiconductor 2DEGS (e.g., GaAs/GaAlAs), suggesting a qualitatively different physical origin for this phenomenon in these magnetic systems.
Influence of coercivity and relative orientation of magnetization on reversible permeability
Authors: Garshelis I. J.
Published: July 2002

Magnetic fields affect magnetization by combinations of vector rotation and displacement of domain walls. Rotation is opposed by the prevailing anisotropy while wall displacement is resisted by factors associated with material heterogeneity. The extent of each process depends not only on the relative strengths of their respective opposing influences but also on the field orientation relative to the moment distribution. Comparison of measured values of reversible susceptibility χ in samples magnetically processed to obtain different distributions of domain orientations and densities of domain walls, inform on the extent of each process. Analysis predicts that materials of moderate coercivity, dominated by cubic crystal anisotropy, should exhibit maximum χ when remanently magnetized transversely to a probing field and minimum χ when remanently magnetized parallel to the field. Experimental measurements support this model. Results show strong correlations with remanence ratios.
Evaluation of vacuum bonded GaAs/Si spin-valve transistors
Authors: Dessein K. , Boeve H. , Anil Kumar P. S. , De Boeck J. , Lodder J. C. , Delaey L. , Borghs G.
Published: July 2002

In this article a new type of spin-valve transistor, a hybrid GaAs/Si device, is presented. In this device the Si emitter is replaced by a GaAs emitter launcher structure. The integration of the GaAs with the Si was done by means of a room temperature vacuum bonding technique. By using a soft NiFe/Au/Co spin-valve structure as metal base, a 63% change in collector current is obtained at room temperature for a saturation field of 30 Oe. The corresponding in-plane magnetoresistance is only 1%.
Electronic scattering from Co/Cu interfaces: In situ measurement, comparison with microstructure, and failure of semiclassical free-electron models
Authors: Bailey William E. , Wang Shan X. , Tsymbal Evgueni Yu.
Published: July 2002

We show that novel thickness-dependent film conductance measurements, taken in situ during deposition of NiO/Co/Cu/Co spin valves and compared with thickness-dependent microstructural characterization, indicate a qualitative failure of widely accepted semiclassical free-electron models of giant magnetoresistance (GMR). Both in situ Auger electron spectroscopy covering experiments and ex situ x-ray diffraction measurements of peak intensity for Co/Cu/Co films indicate that the defect concentration does not vary noticeably as a function of thickness. The microstructural measurements suggest that the bulk scattering parameters ρ and λ should be considered to be constant within each layer, and that the surface scattering parameter p does not change between layers. Under these constraints, it becomes difficult to fit even qualitatively the highly asymmetric scattering behavior measured during the formation of Co/Cu vs Cu/Co interfaces. Calculations incorporating realistic band structure resolve the observed inconsistency between free-electron model calculations and experiment. The asymmetry in scattering is understood here to arise simply from the higher density of unfilled d states in Co compared with Cu.
Nature of the native deep localized defect recombination centers in the chalcopyrite and orthorhombic AgInS2
Authors: Krustok J. , Raudoja J. , Krunks M. , Mändar H. , Collan H.
Published: July 2002

We studied the deep photoluminescence (PL) emission in polycrystalline chalcopyrite and orthorhombic AgInS2. In both phases several PL bands were detected at 8 K. On the energy scale these deep PL bands are positioned according to a regular pattern. This is explained as being due to electron-hole recombination within very close deep donor-deep acceptor pairs, with different distances between donor and acceptor defects. The deep donor defect is an interstitial silver Agi and the native deep acceptor defect appears to be situated at the Ag or In place. The two different crystal modifications also cause slightly different distances between donor and acceptor defects in the AgInS2 lattice and, as a result of this, different spectral positions of the deep PL bands. It is shown that these deep localized donor–acceptor pairs can be reasonably efficient radiative recombination centers up to distances of 5.3 Å between the deep donor and the deep acceptor and, thus, up to six distinct deep PL bands are visible in AgInS2. The deep donor-deep acceptor pair model is confirmed also by the temperature quenching experiments and by the excitation power dependences.