bluuebunny/crossref_metadata_2025 huggingface dataset:
ac susceptibility of cobalt in mercury magnetic fluids
Authors: Abu-Aljarayesh I. , Bayrakdar A. , Yusuf N. A. , Abu-Safia H.
Published: July 2002

The temperature dependence of the frequency-dependent magnetic susceptibility χ′ has been measured for tin-coated cobalt particles dispersed in mercury, in the temperature range 80–290 K and for a driving frequency of 10 kHz. A rounded maximum in χ′ was observed at certain temperature below the melting point of the mercury Tm. For T≳Tm, a rapid increase of χ′ with temperature is observed. This maximum disappeared for the heat-treated sample. The results show that χ′(T) follows a Curie–Weiss behavior for TB<T<Tm, with positive ordering temperatures for the three samples. The results were analyzed within the context of Neel theory of superparamagnetism.
Structural properties of ZnSe films grown by migration enhanced epitaxy
Authors: Gaines J. M. , Petruzzello J. , Greenberg B.
Published: July 2002

We describe the structural properties of ZnSe grown on (001) GaAs by migration enhanced epitaxy, and show that the heterointerface plays a significant role in determining the structural properties of the ZnSe films. Films were grown with thicknesses ranging from 900 to 10 000 Å. Transmission electron microscopy and high-resolution x-ray diffraction measurements show that the resulting structure is highly dependent on how growth is initiated. Nearly perfect films are obtained, for thicknesses up to about 2500 Å [considerably thicker than the critical thickness for molecular beam epitaxy (MBE)-grown films], when growth begins with an initial exposure of the GaAs substrate to Zn. However, if growth begins with an initial high-temperature Se exposure, then stacking fault densities are greatly increased, and more rapid relaxation occurs. Comparison to MBE-grown films shows greater defect densities and a faster rate of relaxation of the misfit strain for the MBE-grown films.
Superconductivity in transparent Sn-doped In2O3 films
Authors: Mori Natsuki
Published: July 2002

Superconducting thin films of Sn-doped In2O3 with high visible transmittances were prepared by electron-beam evaporation on polyester and glass substrates, followed by post-deposition annealing. Effects of annealing on electrical and optical properties were examined and optimum conditions under which superconductivity appears were determined. With increasing annealing temperature or annealing time, films change their electrical conduction from semiconducting to metallic behavior, and the superconducting state could be obtained in the transition region between the two phases. Post-oxidation at a low temperature (∼140 °C) is important to produce superconducting films, which have transition temperatures Tc of 2–4 K for resistivities of the order of 10−3 Ω cm and carrier densities in the range of 1021 cm−3. Superconducting films with much higher transmittance (∼80% at the wavelength 550 nm) were obtained on polyester substrates, whereas glass substrates were used to produce films with higher Tc’s and lower resistivities. The temperature variations of the resistivity near Tc and of the perpendicular critical field can be reconciled with those expected for a dirty superconducting thin film. It is found that the Tc is well correlated with the resistivity and transmittance, and that the superconducting state appears in restricted ranges of these parameters. Chemical bonding and valence-band structures in films were measured by means of x-ray photoelectron spectroscopy. The present results are compared with those for other superconducting oxides including high-Tc cuprate systems. The appearance of superconductivity in our samples is discussed on the basis of the electrical, optical, and chemical properties.
Remanent magnetic measurements on perpendicular recording materials with compensation for demagnetizing fields
Authors: Samwel E. O. , Bissell P. R. , Lodder J. C.
Published: July 2002

Existing techniques for characterization of longitudinal recording media using remanence measurements are extended to perpendicular media, in particular to Alumite, and correction for demagnetizing fields is taken into account. It is found that these techniques have limited value because of the sensitivity of the analysis to the correction factor used. Measurement of the recoil lines is investigated as an alternative method of probing the reversal processes.
Raman monitoring of semiconductor growth
Authors: Wagner V. , Drews D. , Esser N. , Zahn D. R. T. , Geurts J. , Richter W.
Published: July 2002

In this paper it is demonstrated that Raman spectroscopy can be used for in situ online monitoring of semiconductor layer growth. For this purpose an ultra-high-vacuum chamber was designed to suit molecular beam epitaxial growth as well as simultaneous optical measurements utilizing a multichannel detection Raman system. As a result Raman spectra can be taken while growth progresses. As shown by the example of InSb growth, they provide information on the interface chemistry, crystal quality of the growing layer, and the growth mode as well.
Quantized acoustic phonon modes in quantum wires and quantum dots
Authors: Stroscio Michael A. , Kim K. W. , Yu SeGi , Ballato Arthur
Published: July 2002

Acoustic phonon modes in isotropic cubic media are derived for a number of quantum-wire and quantum-dot geometries of significant interest in nanoelectronics and optoelectronics. In each case, the mode amplitude is determined by requiring that the mode energy be given by that of the properly quantized phonon. For the case of cylindrical quantum wires and quantum dots with rectangular faces, the Hamiltonians for the deformation potential interactions are derived. These quantized acoustic modes and the associated deformation potential Hamiltonians provide a basis for modeling carrier-acoustic-phonon interactions in a variety of mesoscopic devices. Our new results supplement previous treatments of related piezoelectric effects in cylindrical quantum wires.
Scanning tunneling microscope contrast of perylene-3,4,9,10-tetracarboxylic-dianhydride on graphite and its application to the study of epitaxy
Authors: Hoshino Akitaka , Isoda Seiji , Kurata Hiroki , Kobayashi Takashi
Published: July 2002

Epitaxial films of perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) on graphite (0001) were investigated by scanning tunneling microscopy. Molecular image contrast of PTCDA was found to depend strongly upon the molecular orientation and the position on graphite. In particular, the periodic discrepancy between PTCDA and graphite lattice points results in a modulation of contrast, which can be used to determine the epitaxial relation of PTCDA relative to the substrate accurately. By analyzing this modulation of contrast, we determined two kinds of epitaxial orientation of PTCDA. These orientations have no exact commensurate relation with graphite, but every lattice point of PTCDA lies on a lattice line parallel to the a axis (or b axis) of graphite. This specific feature contributes to decreasing the interfacial energy. The contrast mechanism of adsorbed molecules is also discussed.
A closed-form inversion-type polysilicon thin-film transistor dc/ac model considering the kink effect
Authors: Chen S. S. , Shone F. C. , Kuo J. B.
Published: July 2002

This paper reports a closed-form inversion-type polysilicon thin-film transistor dc/ac model considering kink effect for circuit simulation. Using a quasi-two-dimensional approach and an impact ionization model, the kink effect has been explained in terms of dc/ac models as verified by the experimental data. Based on the analytical model, a smaller grain leads to a higher avalanche multiplication factor as a result of a larger average trapped charge density.
Strain determination and microstructural characterization of 50 keV Sn-ion-implanted Si(001)
Authors: Sardela M. R. , Turan R. , Willander M. , Hansson G. V. , Hultman L.
Published: July 2002

Si(001) structures, implanted with Sn at energy of 50 keV and with doses in the range 2–9×1015 cm−2, were investigated by multicrystal x-ray diffraction, reciprocal space mapping (RSM), high-resolution transmission electron microscopy, and secondary-ion-mass spectrometry (SIMS). For Sn doses up to 3.30×1015 cm−2, annealing at 600 °C for 30 min under dry N2 atmosphere resulted in recrystallization by solid-phase epitaxy (SPE) to a layer thickness of more than 50 nm. These SPE-grown layers were shown to be free of extended defects and Sn redistribution was negligible. As measured by x-ray diffraction, the Sn-induced strain in Si increased with the implant dose. From RSM measurements, this strain was shown to be tetragonal with negligible in-plane relaxation. Mosaicity and defect-related effects were shown to be negligible. Instead, limited thickness effects and strain variation due to the implantation profile appeared to be the major sources of the observed broadening in the diffraction peaks. The lattice expansion coefficient for Sn in Si was estimated from the measurements to be 2.5×10−24 cm3/atom. For Sn doses above 3.3×1015 cm−2, a reduction in the Sn-induced strain in Si was observed despite the fact that Sn concentrations were higher. In this high-dose regime, the SPE growth under the same annealing conditions was limited to ∼10 nm. The remainder of the structure showed a succession of layers dominated by twinned Si(001), polycrystalline Si, nanocrystalline Si:Sn, and an untransformed amorphous top layer. In addition, Sn redistribution was detected in the SIMS measurements at levels much higher than expected from trace-diffusivity values at the employed annealing conditions. The observed SPE retardation was related to the high concentrations of Sn in these structures.
Low frequency noise in polysilicon-emitter bipolar junction transistors
Authors: Deen M. Jamal , Ilowski John , Yang Ping
Published: July 2002

This paper describes experimental results on low frequency noise in several types of polysilicon-emitter NPN bipolar junction transistors. The experimental data were modelled using a combination of 1/f noise, generation-recombination noise (g-r), and shot noise, and good agreement between model calculations and experimental measurements were obtained. Observed differences in the experimental low frequency noise spectra of devices with similar geometry and under similar biasing conditions could be explained by the differences in the generation-recombination (g-r) noise contributions. Experiments were performed on devices with emitter areas varying from 1.6 to 144 μm2, and it was found that the magnitude of the flicker noise contribution KF varied inversely with emitter area AE or emitter perimeter PE. The fact that KF∼A−1E or KF∼P−1E is explained by the observation that AE∼PE within fabrication errors/tolerances. Using a set of 3.2 μm2 BJTs with pronounced g-r noise, input current noise was measured at different temperatures and from a deconvolution of the noise spectra, a trap at ∼400 meV and with a capture cross section of ∼4×10−17 cm2 was identified. Finally, the effect of different base implant and rapid thermal annealing conditions, polysilicon emitter thicknesses, and surface etching conditions on the noise magnitude were studied, and these results are also reported.
Theory for the multiloop dc superconducting quantum interference device magnetometer and experimental verification
Authors: Drung D. , Knappe S. , Koch H.
Published: July 2002

A theoretical analysis of the multiloop dc superconducting quantum interference device (SQUID) magnetometer fabricated from low-Tc (transition temperature) or high-Tc materials is presented. Using simple analytic formulas, the essential parameters of a multiloop magnetometer can be estimated: the effective area A, the effective SQUID inductance L, the transfer function VΦ, and the flux density noise √SB. The theoretical predictions are compared with experimental results of seven different low-Tc versions and good agreement is found. Based on the analytical description, a high-Tc magnetometer design with a 7 mm pickup coil and 16 parallel loops giving a sufficiently small SQUID inductance L≂145 pH is presented. At T=77 K a voltage swing 2δV≂8 μV and a white noise √SB≂8 fT/√Hz are predicted assuming a critical current I0=20 μA and a normal resistance R=2 Ω per junction and a damping resistance Rd=R across the SQUID inductance.
Growth of nodular defects during film deposition
Authors: Dubost Laurent , Rhallabi Ahmed , Perrin Jérôme , Schmitt Jacques
Published: July 2002

Defects in films created by dust or particles are usually called nodules. A simple geometric model is often used to explain their shape, but their size is not well described. Here we analyze how the growth and dimensions of the nodule depend on the surface reaction probabilities of gaseous precursors which determine the conformality of film deposition. The shape of nodules obtained from calibrated particles is investigated. Hydrogenated amorphous silicon deposited by plasma-enhanced chemical-vapor deposition and chromium layers deposited by sputtering have been used to study the effects of conformality. We demonstrate a satisfactory model based on a modification of a simple geometric growth approach by comparisons of calculations with scanning electron microscopy measurements. Furthermore, a 2D analytical simulation using two precursors and their respective loss and sticking probabilities has been carried out. The computed nodule diameters are in excellent agreement with the measured ones. These observations lead us to infer the presence of an interface between the nodule and the film in specific conditions.
Fracture of synthetic diamond
Authors: Drory M. D. , Dauskardt R. H. , Kant A. , Ritchie R. O.
Published: July 2002

The fracture behavior of synthetic diamond has been investigated using indentation methods and by the tensile testing of pre-notched fracture-mechanics type samples. Specifically, the fracture toughness of free-standing diamond plates, grown by chemically-vapor deposited (CVD) methods, was measured using Vickers indentations and by the use of disk-shaped compact-tension specimens; the latter method provides an evaluation of the through-thickness fracture properties, whereas the indentation method was performed on the nucleation surface of the sample. Measured fracture toughness (Kc) values were found to be approximately 5–6 MPa√m by both methods, indicating that the fracture resistance of CVD diamond does not vary appreciably with grain size (within the certainty of the testing procedures). Complications, however, arose with the fracture-mechanics testing regarding crack initiation from a relatively blunt notch; further work is needed to develop pre-cracking methods to permit more reliable fracture toughness testing of diamond.
Characterization of anisotropic stress around Si trenches by polarized Raman spectroscopy
Authors: Yoshikawa M. , Maegawa M. , Katagiri G. , Ishida H.
Published: July 2002

Polarized Raman spectra around trenches formed on (100) silicon wafers have been measured and it has been found that the peak frequency shift varies with the polarization configuration, suggesting that anisotropic stresses occur around the trenches. The different stress components have been calculated by the use of the polarization Raman technique and it was found that the stress distribution of each component approximately agrees with that of each component simulated by a finite element method. Polarized Raman spectroscopy is a powerful technique for the estimation of an anisotropic stress of an electronic silicon device in situ.
Numerical model of an ac plasma display panel cell in neon-xenon mixtures
Authors: Meunier J. , Belenguer Ph. , Boeuf J. P.
Published: July 2002

We present a self-consistent 1D model of the discharge initiated in an ac plasma display panel cell. The model is based on a two-moments fluid description of electron and ion transport, coupled with Poisson’s equation, and with a set of kinetic equations characterizing the evolution of the population of excited states leading to UV emission in neon-xenon mixtures. Results are presented in a 90% neon, 10% xenon gas mixture, for a gap length of 100 μm and a gas pressure of 560 Torr at ambient temperature. Under the conditions above, and for typical sustaining voltages, the duration of the discharge current pulse predicted by the model is on the order of 10 ns while the UV emission lasts for about 5 μs. The UV production efficiency in the discharge is about 10% for a Ne-Xe (90-10) mixture. Results for other neon-xenon mixtures are also discussed. The model also shows that a non-negligible part of the UV production occurs in the transient ‘‘plasma column’’ and not only in the sheath region. Voltage transfer curves and margin obtained with this model are presented and compared with available experimental measurements.
On the structural properties of a-Si1−xCx:H thin films
Authors: Mastelaro V. , Flank A. M. , Fantini M. C. A. , Bittencourt D. R. S. , Carreño M. N. P. , Pereyra I.
Published: July 2002

The structural and chemical properties of hydrogenated amorphous silicon carbide (a-Si1−xCx:H) thin films, deposited by plasma-enhanced chemical-vapor deposition, were determined by extended x-ray absorption fine structure (EXAFS), x-ray-absorption near-edge spectroscopy (XANES), small-angle x-ray scattering, Fourier transform infrared (FTIR) spectroscopy, Auger electron spectroscopy, and visible spectrometry. The EXAFS and XANES results show the crucial influence of the ‘‘starving’’ plasma deposition conditions on the structural properties of wide-gap a-Si1−xCx:H films and are consistent with the FTIR and optical-absorption data. The first-neighbors distance for alloys with smaller carbon content or deposited at higher silane flow are very close to the mean Si–Si distance obtained for a-Si:H. On the other hand, the EXAFS spectra of films with higher carbon content (x≳50 at. %) and deposited under ‘‘starving’’ plasma regime show Si–C distances similar to crystalline SiC (c-SiC). The presence of a typical c-SiC resonance in the XANES spectra of the same samples is evidence that the material has a chemical order close to that of c-SiC.
GaAs nanocrystals formed by sequential ion implantation
Authors: White C. W. , Budai J. D. , Zhu J. G. , Withrow S. P. , Zuhr R. A. , Hembree D. M. , Henderson D. O. , Ueda A. , Tung Y. S. , Mu R. , Magruder R. H.
Published: July 2002

Sequential ion implantation of As and Ga into SiO2 and α-Al2O3 followed by thermal annealing has been used to form zinc-blende GaAs nanocrystals in these two matrices. In SiO2, the nanocrystals are nearly spherical and randomly oriented, with diameters less than 15 nm. In Al2O3, the nanocrystals are three dimensionally aligned with respect to the crystal lattice. Infrared reflectance measurements show evidence for surface phonon modes in the GaAs nanocrystals in these matrices.
High temperature magnetic properties of 49%Co–2%V–Fe alloy
Authors: Li Lin
Published: July 2002

Direct current hysteresis loops of 49%Co–2%V–Fe alloy have been measured at temperatures up to 800 °C. Two phase structure is evident in the B–H loops between 580 and 730 °C. This alloy cannot be reliably operated above 580 °C. The second magnetic phase is believed to result from the nucleation and growth of disordered α phase in the ordered α′ phase matrix. Magnetic properties (measured as B at H=2.5 and 10 Oe) degrade with time during aging at 450 °C, but approach stable values after about 1000 h, which provides the possibility for long-term high temperature (e.g., 450 °C) operation if the degraded properties are still acceptable for the application. The coercive field remains unchanged while the permeability decreases irreversibly during aging. The irreversible magnetic property changes that occur during aging at 450 °C are most likely caused by an increase in the long range order parameter.
Modal analysis of optical guides with two-dimensional photonic band-gap boundaries
Authors: Benisty H.
Published: July 2002

Guided modes propagating in dielectric material surrounded by periodic two-dimensional photonic band-gap material are theoretically studied using the plane-wave expansion and a supercell. Two opposite boundary designs heavily affecting optical feedback at zone edge (Bragg reflection) are investigated. Limits to the validity range of simple bi-mode coupling schemes in these periodic guides are emphasized and coupling coefficients presented in adequate cases. Mode symmetries are studied and related to dispersion and frequency-width relationship. Finally, the smallest guide thickness allowing for a midgap mode is found to be much smaller than the analogous one-dimensional case, raising prospects for enhanced light-matter interaction.
The application of the cathodic arc to plasma assisted chemical vapor deposition of carbon
Authors: Yin Y. , McKenzie D. R. , Zou J. , Das A.
Published: July 2002

This article reports on the properties of hydrogenated carbon films deposited using a cathodic arc decomposition process with a graphite cathode used in a plasma assisted chemical vapor deposition mode. In this application of the cathodic arc, acetylene is broken down into radicals to form hydrogenated amorphous carbon films. Some results are also reported using hydrogen gas. The density in terms of plasmon energy or sp3 fraction of the films has been found to decrease with increasing acetylene or hydrogen flow rate, approaching the value for glow discharge deposited polymer-like hydrogenated amorphous carbon. When substrate bias is used, the hydrogen content is reduced and graphite microcrystals appear, together with cauliflower-like growths. In the range between 100 and 500 V, negative substrate bias increases the density of the films. Overall, the hydrogen content decreases with the density of the films.
Conduction mechanism and origin of stress-induced leakage current in thin silicon dioxide films
Authors: Kimura Mikihiro , Ohmi Tadahiro
Published: July 2002

The conduction mechanism and origin of the electrical stress-induced leakage current (SILC) in thin silicon dioxide (SiO2) films thermally grown on silicon substrate were clarified from various electrical properties. The properties examined consisted of the I-V characteristics, the oxide trap charge buildup, the generation of the Si/SiO2 interface states, and the generation of the neutral oxide traps. The electrical properties were obtained from films of different oxide thicknesses fabricated by different oxidation processes. The conduction mechanism of SILC was investigated from the viewpoint of oxide thickness dependence, using 92- and 56-Å-thick oxide films. From the oxide-thickness-dependent studies it was found that the SILC phenomenon was not correlated with the oxide trap charge buildup and interface state generation, but rather closely correlated with neutral electron trap generation. The conduction mechanism for nonequilibrium SILC was theoretically deduced from one-dimensional ballistic triangular barrier tunneling that occurred only during the filling process. The tunneling was directed from a leakage spot at the electron-injecting cathode to neutral electron trap sites uniformly generated within the oxide at a trap level (≊1.17 eV from the cathode conduction band and ≊2.0 eV from the SiO2 conduction band) lower than the SiO2 barrier height during only the filling process. The origin of the SILC was also investigated from the viewpoint of oxidation process dependence, using both wet and dry oxides of 86 and 50 Å thicknesses. The oxidation-process-dependent studies revealed that the SILC associated with a wet oxide after the stress application was less than that of a stressed dry oxide. The oxide trap charge buildup and the interface state generation associated with a wet oxide after the stress application was, however, greater than that of a stressed dry oxide. This result suggested that the SILC originated not from water-related chemical reactions, but from the distortion of the thermally grown SiO2 bond structure during electrical stressing. The SILC of both wet and dry oxides after the application of stress were well fitted by Fowler-Nordheim lines, confirming that the leakage conduction mechanism is independent of the oxidation process.
Multi-mode noise analysis of cantilevers for scanning probe microscopy
Authors: Salapaka M. V. , Bergh H. S. , Lai J. , Majumdar A. , McFarland E.
Published: July 2002

A multi-mode analysis of micro-cantilever dynamics is presented. We derive the power spectral density of the cantilever displacement due to a thermal noise source and predict the cantilevers’s fundamental resonant frequency and higher harmonics. The first mode in the multi-mode model is equivalent to the traditional single-mode model. Experimental results obtained with a silicon nitride cantilever at 300 K are in excellent qualitative agreement with the multi-mode model. The multi-mode model may be used to obtain accurate values of the cantilever properties such as the elastic modulus, effective mass, thickness and moment of inertia.
Radiation resistance of fluorophosphate glasses for high performance optical fiber in the ultraviolet region
Authors: Zou Xuelu , Toratani Hisayoshi
Published: July 2002

Transmission loss characteristics of a fluorophosphate fiber was investigated in 200–800 nm wavelength region. The transmission loss was found to be primarily dominated by extrinsic absorption due to transition metal impurities. The total scattering is the next most important loss factor which is in fact Rayleigh in character exhibiting a λ−4 dependence, and the ultraviolet absorption tail is the least significant loss factor. Radiation resistance characteristics of the bulk glasses corresponding to the core and cladding have also been investigated. Upon exposure to ultraviolet radiation from a 1 kW Hg-Xe lamp, the radiation-induced defect centers leading to additional absorption bands in the ultraviolet region result mainly from the photoionization processes of Fe2++hν=Fe3++e− and 2Cl−+hν=Cl2−+e− for the Cl− doped glasses, and the former one is also responsible for the glasses without doping of Cl−. The electron capture centers in the glasses are the [PO4] and/or [P2O7] groups. If the irradiation is carried out with a KrF excimer laser having high-energy density, the formation of radiation-induced defect centers is due not only to the photoionizable multivalent ions, but also to the photoionization of phosphate groups in the glasses. The latter process gives the purple color to the irradiated glasses.
Theory of thin-skin eddy-current interaction with surface cracks
Authors: Harfield N. , Bowler J. R.
Published: July 2002

Eddy-current non-destructive evaluation is commonly performed at relatively high frequencies at which the skin depths are significantly smaller than the dimensions of a typical crack. A thin-skin analysis of eddy currents is presented in which the electromagnetic fields on the crack faces are described in terms of a potential which obeys a two-dimensional Laplace equation. Solutions of this equation for defects in both magnetic and non-magnetic materials are determined by applying thin-skin boundary conditions at the crack perimeter. The impedance change of an eddy-current coil due to the defect is then calculated by numerical evaluation of one-dimensional integrals over the line of the crack mouth, the impedance integrals having been derived with the aid of a reciprocity relationship. Theoretical predictions are compared with experimental data for long, uniformly deep slots in aluminium and mild steel and good agreement between theory and experiment is obtained.
Effect of magnetic field annealing on the giant magnetoimpedance in FeCuMoSiB ribbons
Authors: Ku Wanjun , Ge Fuding , Zhu Jing
Published: July 2002

The magnetic permeability and giant magnetoimpedance effect of Fe73.5Cu1Mo3Si13.5B9 alloy ribbons in different annealed states have been measured as functions of the external magnetic field and the ac driving current frequency. It is found that the giant magnetoimpedance effect in the magnetic-field annealed state is larger than that in the nonfield annealed state. In the field annealed state, the easy magnetization direction is along the driving current as well as the external magnetic field. This leads to significant change of the permeability at high frequencies, and consequently the magnetoimpedance is enhanced in this state.
The precipitation of Fe at the Si–SiO2 interface
Authors: Wong-Leung J. , Eaglesham D. J. , Sapjeta J. , Jacobson D. C. , Poate J. M. , Williams J. S.
Published: July 2002

We report on the precipitation kinetics of Fe contamination at Si–SiO2 interfaces during dry oxidation at 900 °C. Transmission electron microscopy (TEM) and atomic force microscopy are used to monitor the interface roughness and precipitate density for oxides formed in the presence of both bulk and surface contamination of the Si. We show that the precipitating phase is β-FeSi2, the equilibrium Fe–Si phase (rather than α-FeSi2 or an iron silicate or iron oxide). The interface precipitate density is proportional to the surface Fe, and differs in float-zone and Czochralski Si. These precipitates correlate with the presence of sharp asperities at the interface. Detectable Fe precipitation is measured in TEM for contamination levels down to 1×1013 cm−2. In contrast, bulk contamination to above the solid solubility does not generate observable precipitation at the oxide interface.
Dynamic response times of electrorheological fluids in steady shear
Authors: Wang Zuowei , Lin Zhifang , Fang Haiping , Tao Ruibao
Published: July 2002

Transient responses of electrorheological fluids to square-wave electric fields in steady shear are investigated by computational simulation method. The structure responses of the fluids to the field with high frequency are found to be very similar to that to the field with very low frequency or the sudden applied direct current field. The stress rise processes are also similar in both cases and can be described by an exponential expression. The characteristic time τ of the stress response is found to decrease with the increase of the shear rate γ̇ and the area fraction of the particles φ2. The relation between them can be roughly expressed as τ∝γ̇−3/4φ2−3/2. The simulation results are compared with experimental measurements. The aggregation kinetics of the particles in steady shear is also discussed according to these results.
High-pressure melting curve of platinum
Authors: Kavner Abby , Jeanloz Raymond
Published: July 2002

The melting curve of platinum, determined to 70 GPa by spectroradiometry and visual observation through the laser-heated diamond cell, is described by Tm(P)=2057+27.2×P−0.1497×P2 K, where Tm is the melting temperature in K and P is pressure in GPa. This expression is valid to a precision of ±97 K in the pressure range 10 to 70 GPa.
Lattice model calculation of the strain energy density and other properties of crystalline LiCoO2
Authors: Hart F. X. , Bates J. B.
Published: July 2002

The strain energy densities for various crystalline planes of LiCoO2 were calculated from the stiffness tensors obtained from lattice model calculations using the program GULP. In addition to Coulomb and Buckingham potentials, it was necessary to include shell models for the oxygen and cobalt ions in order to obtain acceptable agreement between the observed and calculated structural parameters and high frequency dielectric constant. The strain energy densities u due to differential thermal expansion were calculated using the theoretical stiffness tensors and estimated values for the thermal expansion coefficients of LiCoO2. For a temperature change of 675 °C, these ranged from 0.5 to 1.3×108 erg/cm3 or 5 to 13 J/m2 for 1-μm-thick films on alumina substrates. In particular, the energies for the (003), (101), and (104) planes were ordered as u(003)≫u(104)>u(101). This suggests that the strong (101) preferred orientation of LiCoO2 films (⩾1 μm thick) is due to the tendency to minimize volume strain energy that arises from differential thermal expansion between the film and the substrate. Additional properties obtained from the GULP calculations include the free energy, heat capacity, and the k=0 vibrational modes.
Pressure-induced phase transformations in diamond
Authors: Gogotsi Yury G. , Kailer Andreas , Nickel Klaus G.
Published: July 2002

The stability of diamond under pressure and the structure of hypothetical high-pressure phases have been a controversial issue for a long time. “Will diamond transform under megabar pressures?” asked Yin and Cohen in the title of their paper [Phys. Rev. Lett. 50, 2006 (1983)] which attempted to predict an answer to this question 15 years ago. Before and after that, many other scientists tried to find the answer doing both modeling and experiments. However, the cubic structure of diamond seems to be experimentally stable up to the highest static pressures that the modern high-pressure technology can achieve. We addressed the problem by decreasing the contact area of pressurization instead of increasing the total load. Experimentally this can be easily done in indentation tests using a sharp diamond indenter. In addition to hydrostatic stresses, such a test creates shear stresses as well. Here deformations may be realized, which are either impossible or would require much higher pressures when utilizing only hydrostatic stresses. By coupling the indentation loading with micro-Raman spectroscopy, we were able to drive and monitor phase transformations in diamond. A very similar phenomenon can be observed by scratching a diamond with another diamond. Thus, phase transformations in diamond may in fact be a very common feature of wear.
Spectroscopic and structural characterization of electrochemically grown ZnO quantum dots
Authors: Mahamuni Shailaja , Borgohain Kavita , Bendre B. S. , Leppert Valerie J. , Risbud Subhash H.
Published: July 2002

We report a novel method for the synthesis of stable, OH free zinc oxide quantum dots, using an electrochemical route. The optical properties of these quantum dots were studied at room temperature, by taking the optical absorption and luminescence spectra. The band gap luminescence is predominant in ZnO quantum dots synthesized by the present technique, while the green defect induced luminescence, typical of ZnO, is strongly quenched. The role of defects in photoluminescence emission is discussed.
Particle size dependence of the volume plasmon energy in cadmium sulphide quantum dots by electron energy loss spectroscopy
Authors: Nakashima P. N. H. , Tsuzuki T. , Johnson A. W. S.
Published: July 2002

The shift of the volume plasmon energy with particle size in cadmium sulphide nanoparticles was examined by energy filtered transmission electron microscopy. This technique allowed the plasmon energy of individual, well-separated particles to be measured. The measured plasmon energy was found to increase with decreasing particle size, in general agreement with the theoretical approximation for plasmon shifting due to the quantum size effect. The nanoparticle diameters ranged between 1.5 and 3.8 nm and they were prepared by mechanochemical processing techniques and then electrophoretically deposited on the transmission electron microscope specimen grids.
Effect of the electrode layer on the polydomain structure of epitaxial PbZr0.2Ti0.8O3 thin films
Authors: Alpay S. P. , Nagarajan V. , Bendersky L. A. , Vaudin M. D. , Aggarwal S. , Ramesh R. , Roytburd A. L.
Published: July 2002

PbZr 0.2 Ti 0.8 O 3 (PZT) thin films with and without La0.5Sr0.5CoO3(LSCO) electrodes were grown epitaxially on (001) SrTiO3 at 650 °C by pulsed laser deposition. The domain structure of the 400 nm thick PZT films with different electrode layer configurations was investigated by x-ray diffraction and transmission electron microscopy. The c-domain fractions of the PZT films with no electrode layer, with a 50 nm electrode layer between the film and the substrate, and with 50 nm electrode layers on top and bottom of the PZT film were found to be equal. Theoretical estimation of the c-domain fraction based on the minimization of the energy of internal stresses in films is in good agreement with experimental results. This means that depolarizing fields do not affect the polydomain structure of the film. Calculations of the in-plane strains based on the lattice parameters of the LSCO layer in the above configurations led to the conclusion that the bottom electrode layer is coherently strained to match the substrate.
Influence of misfit stress on the magnetoresistive properties of La0.7Ca0.3MnO3−δ thin films
Authors: Gommert E. , Cerva H. , Wecker J. , Samwer K.
Published: July 2002

For this study La0.7Ca0.3MnO3−δ (LCMO) films of various thicknesses were deposited on SrTiO3(100), LaAlO3(100), and MgO(100) substrates. The substrate–film lattice mismatch causes changes in the microstructure of the films and can therefore influence their magnetoresistive behavior. The structure of the films was characterized by x-ray diffraction and high-resolution transmission electron microscopy. Magnetic and electric properties were determined using standard techniques. Samples grown on SrTiO3 show a strongly distorted growth. A large number of defects are incorporated in the initial LCMO layers. Beginning at a thickness of approximately 500 nm films on these substrates show strong deviations in electric behavior and develop macroscopic cracks. In the case of LaAlO3 substrates the films show only a few lattice defects. The substrate–film lattice mismatch is compensated by large stresses in the LCMO films. The best samples with the least amount of defects are grown on MgO substrates even though the substrate–film mismatch is largest in this case. The misfit is completely compensated in a narrow region near the substrate interface. From there on the LCMO grows with only few lattice defects and little strain. These films exhibit electric and magnetic properties similar to bulk materials. The results demonstrate that the colossal magnetoresistance properties of thin films are significantly influenced by strain caused by the substrate. They behave different than bulk samples under hydrostatic pressure. These aspects must be considered when interpreting the properties of such films and comparing the results of individual experiments.
Nematic polar anchoring strength measured by electric field techniques
Authors: Nastishin Yu. A. , Polak R. D. , Shiyanovskii S. V. , Bodnar V. H. , Lavrentovich O. D.
Published: July 2002

We analyze the high-electric-field technique designed by Yokoyama and van Sprang [J. Appl. Phys. 57, 4520 (1985)] to determine the polar anchoring coefficient W of a nematic liquid crystal-solid substrate. The technique implies simultaneous measurement of the optical phase retardation and capacitance as functions of the applied voltage well above the threshold of the Frederiks transition. We develop a generalized model that allows for the determination of W for tilted director orientation. Furthermore, the model results in a new high-field technique, (referred to as the RV technique), based on the measurement of retardation versus applied voltage. W is determined from a simple linear fit over a well-specified voltage window. No capacitance measurements are needed to determine W when the dielectric constants of the liquid crystal are known. We analyze the validity of the Yokoyama–van Sprang (YvS) and RV techniques and show that experimental data in real cells often do not follow the theoretical curves. The reason is that the director distribution is inhomogeneous in the plane of the bounding plates, while the theory assumes that the director is not distorted in this plane. This discrepancy can greatly modify the fitted value of 1/W, and even change its sign, thus making the determination of W meaningless. We suggest a protocol that allows one to check if the cell can be used to measure W by the YvS or RV techniques. The protocol establishes new criteria that were absent in the original YvS procedure. The results are compared with other data on W, obtained by a threshold-field technique for the same nematic-substrate pair.
Simulation of resonant tunneling structures: Origin of the I–V hysteresis and plateau-like structure
Authors: Zhao Peiji , Cui H. L. , Woolard D. , Jensen K. L. , Buot F. A.
Published: July 2002

Hysteresis and plateau-like behavior of the I–V curves of a double-barrier resonant tunneling structure are simulated in the negative differential resistance region. Our simulation results show that the creation of an emitter quantum well after the current passes its maximum value is the key point in understanding the origin of the I–V plateau-like structure. It is demonstrated that the plateau-like behavior of the I–V curves is produced by the coupling between the energy level in the emitter quantum well and that in the main quantum well. The hysteresis is a manifestation of the above-mentioned energy level coupling, the accumulation and distribution of electrons in the emitter, and the coupling between the energy level in the quantum well and the conduction band edge or the three-dimensional continuum states in the emitter. The effects of the structural parameters on the bistability of the I–V curves of resonant tunneling devices are discussed. The creation and disappearance mechanism of the emitter quantum well is presented. The effects of device temperature on the hysteresis and plateau-like behavior of the I–V curves are obtained. These results provide the physical basis for utilizing the plateau-like structure of I–V curves in designing resonant tunneling devices.
Strain-impedance properties of a CoSiB/Cu/CoSiB layered film
Authors: Yamadera Hideya , Nishibe Yuji
Published: July 2002

A type of strain sensor element utilizing the magnetostrictive effect and the magneto-impedance effect was presented and the strain-impedance properties of the element were evaluated. The strain-impedance (SI) element constructed from a CoSiB/Cu/CoSiB layered film was prepared on a Corning No. 0313 glass substrate by magnetron sputtering under a magnetic field. The element was composed of an inner copper conductive layer and outer magnetostrictive layers which covered the conductive layer. The Co73Si12B15 negative magnetostriction films (λ=−6×10−6) were adopted as magnetostrictive layers and magnetic anisotropy was induced perpendicular to the applied strain direction. Impedance Z drastically changed with applied compressive strain ε up to −2×10−3 and exhibited a good reversibility and reproducibility. The impedance change ratios ΔZ/Z0=(Zmax−Z[ε=0])/Z[ε=0] of the element were 40% at 15 MHz and 24% at 1 MHz. The maximum gauge factor Gmax defined as the maximum fractional change in impedance to strain (ΔZ/Z)/ε was 960–1910 in low frequency ranges from 1 to 15 MHz. These values were much larger than those of the single layer SI element constructed from only a CoSiB film and ten times larger than those of conventional semiconductor strain gauges. Therefore, the layered SI element will be very useful for the application of sensing small amounts of strain.
Time resolved study of the B 3Σ−u and B″ 3Πu states of diatomic sulfur in a supersonic free jet
Authors: Matsumi Y. , Suzuki Takanori , Munakata Toshiaki , Kasuya Takahiro
Published: July 2002

Fluorescence decay rates were measured for individual rovibronic spin levels of the B 3Σ−u and B″3Πu states of diatomic sulfur (S2) in a supersonic free jet. The radiative lifetimes of the B 3Σ−u and B″3Πu states were found to be strongly dependent on the spin and vibrational quantum numbers, and fall in the range of 34.2 to 49.3 and 108 to 3888 ns, respectively. The marked variation of the lifetimes was interpreted satisfactorily in terms of the homogeneous interactions between the B 3Σ−u and B″3Πu states. A quantitative deperturbation treatment of both the spectroscopic and lifetime data leads to the evaluation of interaction strengths between the B 3Σ−u and B″ 3Πu states.
Compositional variations in the near surface layers, an atom-probe study of cosegregation of sulfur in Pt–Rh and Pt–Ir alloys
Authors: Ahmad M. , Tsong T. T.
Published: July 2002

Equilibrium composition depth profiles with true single atomic layer depth resolution have been obtained for the (001) plane of a Pt–Ir and five Pt–Rh alloys using the atom-probe field ion microscope. Pt segregates to the top two layers in Pt–Ir and Rh segregates to the top layer in Pt–Rh alloys after annealing at 700 °C for 5 min. In addition, an overlayer of sulfur is found to cosegregate to the surface of these alloys even though the bulk impurity content is less than 100 ppm. The coverage of sulfur overlayer for Pt–Rh alloys varies between 0.22 to 0.52 of a monolayer, and it increases with increasing surface Rh concentration. The depth dependence of the composition depth profiles has been found to be monotonic for the Pt–Ir alloy and nonmonotonic, or possibly oscillatory, for the Pt–Rh alloys. Pending the uncertainty of the effect of the sulfur overlayer, Pt segregation in Pt–Ir is consistent with most theoretical models, and Rh segregation in Pt–Rh is consistent with the bond-breaking model and the atomic model, but is inconsistent with a surface energy model and an electronic model. Rh segregation may also be an effect of chemisorption enhanced surface segregation. While the experimental result on the top surface layer can be explained by various theoretical models, the compositional variation in the near surface layers may shed some light on the atomic interactions in the surface region. For the Pt–Ir alloy, we have also observed a diffusion kinetic effect in a composition depth profile.
Trapping of low energy electrons at preexisting, cold water clusters
Authors: Knapp M. , Echt O. , Kreisle D. , Recknagel E.
Published: July 2002

Long-lived water cluster anions [(H2O)−n, n≥11, and (D2O)−n, n≥12] can be formed by electron attachment to a beam of preformed, neutral clusters; collisional stabilization is not required. Attachment occurs resonantly at or very close to 0 eV incident electron energy; the ion yield is unexpectedly large.
Vibrational predissociation spectroscopy of (HF)3
Authors: Michael Daniel W. , Lisy James M.
Published: July 2002

The vibrational predissociation spectra of (HF)3, (HF)2DF, and HF(DF)2 in the HF stretching region, and (DF)3 in the DF stretching region have been measured using a molecular beam apparatus equipped with a mass spectrometer and a LiNbO3 optical parametric oscillator. The structure of (HF)3 has been found to be cyclic with C3h symmetry. No evidence for a noncyclic complex was found. Following vibrational excitation of the HF stretch, there was direct evidence for the trimer vibrational predissociation to the dimer plus monomer and indirect evidence for complete predissociation to three monomers. The total translational energy in the dimer plus monomer channel was 30 cm−1. The inability to resolve rotational features in the vibrational band, combined with negligible broadening of the rotational band contour bracketed the upper state lifetime between 2.5 and 21 ps. The largest ion fragment of the trimer produced by electron impact ionization was the protonated monomer.
Cluster isomerization induced by electron attachment
Authors: Scharf Dafna , Jortner Joshua , Landman Uzi
Published: July 2002

In this paper we explore the mechanisms of cluster isomerization induced by electron attachment to small, neutral Na4Cl4 clusters, with the localization of an excess electron inducing structural configurational changes within the cluster. The constant temperature quantum path integral molecular dynamics method was applied to obtain information of the structure and energetics of the cluster as well as the binding energy and the charge distribution of the excess electron for Na4Cl−4 clusters, over the temperature range of 50–1200 K. The attachment of an excess electron to the ionic cluster induced two types of configurational modifications, which can be traced to the role of the excess electron as a pseudonegative ion and to the partial neutralization of a single cation by the excess electron. Consequently, the induction of isomerization at moderately low temperatures, in conjunction with the appearance of new nuclear configurations of the negative cluster, which have no counterpart in the neutral parent cluster, are exhibited.