bluuebunny/crossref_metadata_2025 huggingface dataset:
Power transfer and mode transitions in low-frequency inductively coupled plasmas
Authors: Ostrikov K. N. , Xu S. , Yu M. Y.
Published: July 2002

Operation and mode jumps in low-frequency (500 kHz) radio-frequency inductively coupled plasmas are investigated. The discharge is driven by a flat inductive coil which can excite the electrostatic (E) and electromagnetic (H) discharge modes. The power transfer efficiency and mode transition behavior are studied. It is found that the power reflection coefficient as a function of the input power is minimal in the vicinity of the mode transitions and exhibits hysteresis, which is also observed when the operating gas pressure is varied.
Microheterogeneity and relaxation in 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 relaxor single crystals
Authors: Jiang F. M. , Kojima S.
Published: July 2002

Micro-Brillouin scattering has been applied to the observation of the microheterogeneity and the fast relaxation of 0.65 Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 single crystals in various microareas with the size of 1–2 μm. Cubic–tetragonal at TC–T and tetragonal–rhombohedral around TT–R transitions show different types of acoustic anomalies. It was found that local symmetry varies from one microarea to others below TC–T. Intense quasielastic scattering was observed below 250 °C; it is correlated with thermally activated fast relaxations of polar microregions. The results show that microheterogeneity and related dynamics of polar microregions are origins of relaxor phase transitions and are responsible for macroscopic properties like hypersonic damping.
Piezoelectric-induced quantum-confined Stark effect in self-assembled InAs quantum dots grown on (N11) GaAs substrates
Authors: Sanguinetti S. , Gurioli M. , Grilli E. , Guzzi M. , Henini M.
Published: July 2002

We investigate the optical properties of InAs self-assembled quantum dots grown on (N11)A/B GaAs substrates, by means of cw photoluminescence under different excitation power densities. We observe a sizeable blue-shift of photoluminescence band induced by increasing the photogenerated carrier density. The shift depends on the substrate orientation and exhibits a strong asymmetric dependence on the substrate termination. We attribute the photoluminescence blue-shift to a reverse quantum confined Stark shift of ground state transition energies in the quantum dots. This effect arises from the photogenerated charge screening of the built-in piezoelectric field present in such strained structures grown on high index planes.
Metastable vibrationally excited HF (v=1) in helium nanodroplets
Authors: Nauta K. , Miller R. E.
Published: July 2002

High-resolution infrared laser spectroscopy is used to study hydrogen fluoride solvated in helium nanodroplets. The results clearly show that the vibrationally excited HF (v=1) does not relax on the time scale of the experiments (0.5 ms) and that the large linewidth of the R(0) transition (0.43 cm−1) results from rotational relaxation. A large dc electric field is applied to induce a Q(0) transition, providing an accurate value for the rotational constant of HF in solution (19.47 cm−1), only 2% smaller than in the gas phase.
Approach to fabricating Co nanowire arrays with perpendicular anisotropy: Application of a magnetic field during deposition
Authors: Ge Shihui , Li Chao , Ma Xiao , Li Wei , Xi Li , Li C. X.
Published: July 2002

Cobalt (Co) nanowire arrays were electrodeposited into the pores of polycarbonate membranes. A magnetic field parallel or perpendicular to the membrane plane was applied during deposition to control the wire growth. X-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer were employed to investigate the structure as well as the magnetic properties of the nanowire arrays. The results show that the magnetic field applied during deposition strongly influences the growth of Co nanowires, inducing variations in their crystalline structure and magnetic properties. The sample deposited with the field perpendicular to the membrane plane exhibits a perpendicular magnetic anisotropy with greatly enhanced coercivity and squareness as a result of the preferred growth of Co grains with the c axis perpendicular to the film plane. In contrast, the deposition in a parallel magnetic field forces Co grains to grow with the c axis parallel to the film plane, resulting in in-plane anisotropy.
Growth of highly transparent nanocrystalline diamond films and a spectroscopic study of the growth
Authors: Chen L. C. , Kichambare P. D. , Chen K. H. , Wu J.-J. , Yang J. R. , Lin S. T.
Published: July 2002

A series of nanocrystalline diamond films with grain size ranged from 4 nm to a few hundreds of nanometers were grown by microwave plasma enhanced chemical vapor deposition. Effects of the substrate pretreatment and the methane fraction in the source gas on the microstructure, surface roughness, and optical transmittance of the resultant films were studied. Specifically, comparison was made between two different sizes, 4 nm and 0.1 μm, of the diamond powder used for substrate pretreatment. Interestingly, the films grown on substrates scratched with coarser powder (0.1 μm) can be smoother and more transparent than those on substrates scratched with finer powder (4 nm), despite of the similarity in the grain size of these two types of films prepared at high methane fractions. It is also demonstrated that the major factor that controls the optical transparency is the surface roughness irrespective of the grain size as long as the sp2-bonded carbon in the film is avoided. In situ optical emission spectroscopy was employed to monitor the plasma chemistry, from which possible growth species for the films were discussed.
Evolution of the surface morphology of Fe grown on GaAs (100), (311)A, and (331)A substrates by molecular beam epitaxy
Authors: Schönherr Hans-Peter , Nötzel Richard , Ma Wenquan , Ploog Klaus H.
Published: July 2002

We study the growth of Fe by molecular beam epitaxy on GaAs (100), (311)A, and (331)A substrates in dependence on the termination (reconstruction) of the GaAs surface and Fe growth temperature. Crystal quality and surface morphology of the 20- and 160-nm-thick Fe layers are characterized by double-crystal x-ray diffraction and atomic force microscopy. On GaAs (100) substrates we obtain very smooth Fe layers for As-rich surface reconstructions at a growth temperature of 50 °C. Less As-rich surface reconstructions produce macroscopic defects whose density increases on more Ga-rich surface reconstructions. On GaAs (311)A and (331)A substrates smooth layers with good crystal quality are obtained at 0 °C. The high density of macroscopic defects in these Fe layers is again eliminated on As-saturated surfaces. The evolution of the Fe surface morphology on the micron-length scale and the successful elimination of macroscopic defects on As-saturated GaAs substrates is highly relevant for application of these layers, in particular, their integration with the unique lateral semiconductor nanostructures formed on high-index GaAs (311)A and (331)A substrates.
Polarization retention in SrBi2Ta2O9 thin films investigated at nanoscale
Authors: Gruverman A. , Tanaka M.
Published: July 2002

The nanoscale mechanism of retention behavior in SrBi2Ta2O9 (SBT) thin films has been investigated by means of piezoresponse scanning force microscopy. It has been found that SBT films with Pt electrodes (SBT/Pt) exhibit a strong dependence of retention characteristics upon domain polarity: positive domains with the polarization vector pointing to the bottom electrode exhibit excellent stability in sharp contrast to the negative domains which show poor retention. The backswitching of the negative domains proceeds via sidewise motion of the domain walls and shows a log-time dependence. The strong effect of the poling voltage parameters (duration and amplitude) and the previous switching pattern on domain retention behavior is reported. On the other hand, in SBT films with IrO2 electrodes (SBT/IrO2), the written domains of both polarities show no sign of relaxation to the original state irrespective of the parameters of the poling voltage. The difference in retention behavior of opposite domains in SBT/Pt films is explained by the presence of the polarization-independent built-in bias at the SBT/Pt interface pointing to the bottom electrode. This built-in bias triggers the backswitching of a negative domain by generating positive nuclei at the interface. The forward growth of the nuclei is further governed by the field of the space charge which screens the original polarization state. The symmetric retention behavior and the stability of domain in SBT films with IrO2 electrodes are attributed to the inhibition of the nucleation process at the SBT/IrO2 interface.
Calculation of mean distances between the randomly distributed particles in the model of points and hard spheres (the method of Voronoi polyhedra)
Authors: Lavrik N. L. , Voloshin V. P.
Published: July 2002

The method of Voronoi polyhedra is used to introduce the concept of nth neighbor. For hard spheres a theoretical dependence of the mean distance between the nth neighbors on the coefficient of filling was obtained. A simple formula was obtained to estimate the mean distance between the nth neighbors.
Magnetohydrodynamic calculation for free surface velocity
Authors: Fujisaki Keisuke , Yamada Takahiro , Satoh Shouji
Published: July 2002

The estimate of free surface velocity at the billet mold in magnetohydrodynamic calculation is important for steel surface quality. It is considered that the free surface velocity in the electromagnetic stirring operation is transferred through the wall, a boundary of the molten steel. Then the wall friction phenomena between the wall and the molten steel are modeled so as a wall friction, force is a function of wall velocity. The calculated velocity distribution in consideration of the wall friction in which the friction coefficient is estimated as 0.03 is in good agreement with the measured value.
Phase behavior of monomeric mixtures and polymer solutions with soft interaction potentials
Authors: Wijmans C. M. , Smit B. , Groot R. D.
Published: July 2002

We present Gibbs ensemble Monte Carlo simulations of monomer–solvent and polymer–solvent mixtures with soft interaction potentials, that are used in dissipative particle dynamics simulations. From the simulated phase behavior of the monomer–solvent mixtures one can derive an effective Flory–Huggins χ-parameter as a function of the particle interaction potential. We show that this χ-parameter agrees very well with the free energy difference between a monomer surrounded by solvent particles, and a solvent particle surrounded by solvent particles. We develop a new “identity change” Monte Carlo move to equilibrate the polymer–solvent mixtures. In this move a polymer chain from one box is exchanged with an equal number of solvent particles from the other box. At realistic densities this new move offers a large computational advantage over the convential insertion method for a polymer chain using a configurational bias Monte Carlo algorithm. The new algorithm is demonstrated for polymer–solvent mixtures with a chain length of up to 150 segments. Significant differences are found between the simulated polymer–solvent phase behavior and results predicted by mean-field theory. Finally, we fit a master–equation to the simulated binodal curves at different chain lengths. This function is used to make a quantitative comparison between the simulations and experimental data for the phase equilibrium of the polystyrene–methylcyclohexane system.
Time-dependent properties of liquid water isotopes adsorbed in carbon nanotubes
Authors: Martı́ J. , Gordillo M. C.
Published: July 2002

Dynamics of liquid water and its isotopes when adsorbed inside carbon nanotubes of different radii is studied by means of molecular dynamics simulations. Water molecules have been modeled with a flexible simple point charged (SPC) potential while carbon forces were accounted for with Lennard–Jones-type potentials. Diffusive behavior and the librational, rotational, intra- and intermolecular motions of the constrained molecules have been investigated by means of the spectral densities computed from atomic velocity autocorrelation functions. The results show in all cases significant new vibrational bands and frequency shifts absent in bulk water.
Slip boundary condition on an idealized porous wall
Authors: Jeong Jae-Tack
Published: July 2002

Slip boundary condition on a porous wall is investigated by considering a viscous flow near an idealized porous wall based on the Stokes’ approximation. The idealized porous wall is composed of a large number of parallel and equidistant thin semi-infinite plates. The flow is assumed to be a simple shear flow far from the plates and stagnant deep inside the channels of the plates. The slip velocity on the idealized porous wall is calculated by solving a Wiener–Hopf equation. From the streamline patterns shown, it is found that Moffatt’s infinite sequence of viscous eddies developed between the two adjacent plates. Pressure and shear stress distributions on the plates are shown and local flow near the edge of the plate is discussed. Laminar shear flow at the farfield unidirectional along the edges of the plates is also considered.
First principles investigation of scaling trends of zirconium silicate interface band offsets
Authors: Kawamoto Atsushi , Cho Kyeongjae , Griffin Peter , Dutton Robert
Published: July 2002

First principles density functional theory calculations are carried out to investigate the scaling trends of band offsets at model silicon/zirconium silicate interfaces. Owing to the d character of zirconium silicate conduction bands, the band gap and band offset are shown to decrease as the zirconium concentration is increased. Since the valence band character of silicates remains unchanged relative to SiO2, the conduction band offset alone decreases, leading to increasingly asymmetric band offsets at higher zirconium concentrations. The use of charge transfer dipoles at the interface is investigated as a possible remedy to restore the band offset symmetry by shifting the silicate bands relative to the silicon bands.
Voltage required to detach an adhered particle by Coulomb interaction for micromanipulation
Authors: Takahashi Kunio , Kajihara Hideaki , Urago Masataka , Saito Shigeki , Mochimaru Yoshihiro , Onzawa Tadao
Published: July 2002

An adhered particle can be detached by Coulomb interaction. The voltage required for detachment for micromanipulation is theoretically analyzed by employment of a boundary element method. The system consists of a manipulating probe, a spherical particle, and a substrate plate, all of these objects being conductive. The manipulator and the substrate are cylindrical, and axial symmetry is assumed. Although a numerical method is used to solve the equations, all parameters are normalized. The effect of the shape parameters on the Coulomb force is systematically calculated. The force is independent of system size and depends on the relative shape of the system. The force is proportional to the applied voltage raised to the second power. The force generated by the Coulomb interaction is compared with the adhesion force deduced from the Johnson–Kendall–Roberts theory, and the voltage required for detachment is clearly expressed. The possibilities and limitations of micromanipulation using both the adhesion phenomenon and Coulomb interaction are theoretically discussed.
Determination of the pyroelectric coefficient in strained InGaAs/GaAs quantum wells grown on (111)B GaAs substrates
Authors: Cho Soohaeng , Majerfeld A. , Sanz-Hervás A. , Sánchez J. J. , Sánchez-Rojas J. L. , Izpura I.
Published: July 2002

We report an experimental determination of the pyroelectric coefficient for strained InGaAs layers in a pseudomorphic piezoelectric In0.17Ga0.83As/GaAs multiquantum well in a p-i-n diode configuration which was grown on a (111)B GaAs substrate by molecular-beam epitaxy. By analyzing the Franz–Keldysh oscillations in the photoreflectance spectra over the temperature range 11–300 K, we obtained the temperature dependence of the piezoelectric field, from which the temperature variation of the piezoelectric constant e14 was deduced. A linear dependence of e14 with temperature was observed. Therefore, the strain-induced component of the pyroelectric coefficient of (9.3±0.3)×10−7 C/m2 K was determined for this material system.
The dynamics of ethylene adsorption on Pt(111) into di-σ and π-bonded states
Authors: Carlsson A. F. , Madix R. J.
Published: July 2002

The dynamics of ethylene adsorption on Pt(111) into both the di-σ- and π-bonded states were investigated at 95 and 40 K, respectively, using supersonic molecular beam techniques. The angular dependence of ethylene adsorption into both states is similar to the angular dependence for ethane adsorption, which has a much weaker bond to the surface in its final state. In contrast to ethane, high adsorption probabilities for ethylene prevail to high incident kinetic energies, suggesting that the strong interaction of ethylene with the surface influences adsorption. The initial adsorption probability of ethylene is approximately independent of surface temperature between 40 and 450 K, suggesting that there is no reversible, thermalized intrinsic precursor to adsorption. At 40 K, the adsorption probability increases with coverage (in the π-bonded state). However, at 95 K, the adsorption probability of ethylene remains constant with increasing self-coverage (in the di-σ- bonded state) for trajectories incident with low parallel momentum, but decreases with coverage at high parallel momentum. High parallel momentum may contribute to an increased overall scattering probability from a “hot” extrinsic precursor, resulting in a decreased net adsorption probability at higher ethylene coverage in the rigidly bound di-σ state.
Switchable fiber coupling using variable-focal-length microlenses
Authors: Smith P. J. , Taylor C. M. , McCabe E. M. , Selviah D. R. , Day S. E. , Commander L. G.
Published: July 2002

An array of microlenses, with focal lengths that can be electrically varied, can control the coupling of light into a single mode fiber using an applied voltage. We have examined this technology with a view to the design of a confocal microlens/image-fiber endoscope. Maximum coupling to the fiber was found to occur over a voltage range of ∼1 V and over an axial range of 1 cm. Contrast ratios between minimum and maximum coupling of up to 30 dB were demonstrated.
Photoluminescence excitation spectra of CuInS2 crystals
Authors: Wakita Kazuki , Fujita Futoshi , Yamamoto Nobuyuki
Published: July 2002

The photoluminescence excitation (PLE) spectra of CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT) have been examined. THM crystals show significant structure with many peaks and dips in the PLE spectra, whereas IT crystals exhibits two clear peaks at two free exciton energies in the spectrum. The peak and dip structures of PLE spectra at the free exciton energy depend on the monitoring PL peak, the kind of crystals, and temperatures. A proposed relaxation process of excited electron–hole pairs explains peak and dip structures of PLE spectra at the free exciton energies.
The formation of satellite droplets by unstable binary drop collisions
Authors: Brenn G. , Valkovska D. , Danov K. D.
Published: July 2002

Experimental investigations on the process of satellite droplet formation by unstable binary drop collisions are presented. The experiments are carried out using two monodisperse streams of drops of equal size. A systematic variation of the parameters influencing the collisions leads to an extended version of the stability nomogram which involves the numbers of satellite droplets formed by stretching separation after off-center collisions. The time scales for the formation of liquid filaments and their breakup into the satellites are measured and, in the case that a single satellite is formed, the satellite size is measured by means of a phase-Doppler anemometer. Furthermore, a theoretical model for the breakup of cylindrical liquid filaments in head-on and off-center collisions is presented. The model is based on a linear stability analysis of the filament formed after the collision. The critical wavelength associated with the largest deformation energy is calculated and identified with the disturbance which eventually breaks the filament and determines the number of satellites formed. Comparisons with experiments by Ashgriz and Poo [J. Fluid Mech. 221, 183 (1990)] for the head-on and near-head-on cases yield agreement of the numbers of satellites formed.
Molecular dynamics and microstructure development during cold crystallization in poly(ether-ether-ketone) as revealed by real time dielectric and x-ray methods
Authors: Nogales A. , Ezquerra T. A. , Denchev Z. , Šics I. , Baltá Calleja F. J. , Hsiao Benjamin S.
Published: July 2002

The isothermal crystallization process of poly(ether-ether-ketone) from the glass has been studied in real time by dielectric spectroscopy and x-ray scattering experiments. The combination of these two techniques revealed a complete picture of the crystallization processes from the point of view of both amorphous and crystalline phases. Analysis of results shows that the sample morphology consists of lamellar stacks, separated by rather broad amorphous regions. The lamellar stacks are highly crystalline (∼70%), as obtained from both dielectric and x-ray scattering measurements, and the amorphous phase within the stacks is constrained up to a level where no segmental relaxation is possible. The remaining amorphous phase, after completion of the primary crystallization process, still has a certain mobility, but it is significantly slower than the initial amorphous mobility. Dielectric data and x-ray results are found to be highly congruent.
Nonlinear analysis of oscillatory flow in the annulus of an elastic tube: Application to catheterized artery
Authors: Sarkar A. , Jayaraman G.
Published: July 2002

The changed flow pattern of pulsatile blood flow in an annulus with elastic outer wall has been studied through a mathematical model. The main objective is to apply the model to study the combined effect of introduction of the catheter and elastic properties of the arterial wall on the pulsatile nature of the blood flow. The diameter variation of the wall is considered small for the perturbation analysis to be valid. The steady streaming effect brings into focus the existence of a nonzero mean pressure gradient in addition to the one predicted by the linear theory—a fact overlooked by previous authors. Thus, our results are intended to provide a correction to the “mean pressure gradient–flow rate relationship” usually calculated by neglecting the nonlinear inertia terms. This correction depends on the amplitude of the diameter variation, flow rate wave forms, and the phase difference between them. The calculations based on the geometry and the flow conditions representing a real physiological situation as closely as possible suggest that mean pressure gradient changes with catheter size for any frequency parameter. The results obtained for arbritrary frequency parameter and for small steady streaming Reynolds number, Rst, show that the geometry of the wall plays an important role in the dynamics of the flow even for small catheter radius. The interaction of the amplitude of catheter oscillation and the amplitude of the wall movement is first manifested through the induced mean pressure gradient and induced mean velocity. Further, the results are sensitive to the elastic nature of the wall reflected by the phase difference between the diameter variation and the flow rate. Interesting streamline patterns depict distinct boundary layer characteristics both at artery wall and catheter wall. Depending upon the material properties, a thin catheter experiencing small oscillations due to the flow conditions is likely to have a similar influence to a thicker catheter which remains fairly stationary inside the artery. Finally, the effect of catheterization on various physiologically important flow rate characteristics—mean velocity, mean pressure gradient, wall shear stress—is studied for a range of different catheter sizes and frequency parameters.
Multilevel nanoimprint lithography with submicron alignment over 4 in. Si wafers
Authors: Zhang Wei , Chou Stephen Y.
Published: July 2002

We demonstrate that multilevel nanoimprint lithography (NIL) with submicron alignment over an entire 4 in. Si wafer can be achieved. Average alignment accuracy of 1 μm with a standard deviation 0.4 μm in both X and Y directions was obtained in ten consecutive tests of multilevel NIL. The multilevel alignment was achieved by aligning the wafer and the mask with an aligner, fixing them with a holder, and imprinting in an imprint machine. The issues that are critical to the alignment accuracy, such as relative movement during the press, relative thermal expansion, wafer bending, and resist, are discussed. The alignment accuracy currently achieved on the system is limited by the aligning accuracy of the aligner, instead of the process of multilevel NIL.
Observations of Al segregation around dislocations in AlGaN
Authors: Chang L. , Lai S. K. , Chen F. R. , Kai J. J.
Published: July 2002

Transmission electron microscopy has been used to observe Al segregation around the threading dislocations in Al0.1Ga0.9N and Al0.3Ga0.7N grown by metalorganic chemical vapor deposition on 6H–SiC. Dislocation lines were found to have up to 70% more Al concentration than those regions free of dislocations in the matrix. The Al-depleted regions around the dislocations are shown to be within a few nanometers from the dislocation lines. The results also show that more Al segregate to edge dislocations than to screw ones.
A comparison of the properties of 2,2,2-trifluoroethanol and 2,2,2-trifluoroethanol/water mixtures using different force fields
Authors: Chitra Rajappa , Smith Paul E.
Published: July 2002

It is necessary to evaluate the quality of a force field by comparison with known experimental properties before it can be used with confidence in a simulation. Here, such a study is reported for pure 2,2,2-trifluoroethanol (TFE) solutions and for TFE/water mixtures at two different compositions. Six literature force fields were examined using molecular dynamics simulations. It is found that none of the currently available force fields describe all the properties of pure liquid TFE. The models of van Buren and Berendsen, Duffy and Jorgensen, and Fioroni et al. produce properties that are in reasonable agreement with experiment. The ability to adequately describe properties of TFE/water mixtures varied with the property of interest. The heat of mixing is too unfavorable for all the models. The aggregation properties of the mixtures were well described by the model of Duffy and Jorgensen at the lower composition (30% by volume of TFE), while the Fioroni et al. model performed better at higher TFE concentrations. The activity coefficient derivative from the Fioroni et al. model was in very good agreement with experiment for all compositions studied here. Consequently, the choice of the most appropriate force field will depend on the major properties of interest.
Elastic constants from direct correlation functions in nematic liquid crystals: A computer simulation study
Authors: Phuong Nguyen Hoang , Germano Guido , Schmid Friederike
Published: July 2002

Density functional theories such as the Poniewierski–Stecki theory relate the elastic properties of nematic liquid crystals with their local liquid structure, i.e., with the direct correlation function (DCF) of the particles. We propose a way to determine the DCF in the nematic state from simulations without any approximations, taking into account the dependence of pair correlations on the orientation of the director explicitly. Using this scheme, we evaluate the Frank elastic constants K11, K22, and K33 in a system of soft ellipsoids. The values are in good agreement with those obtained directly from an analysis of order fluctuations. Our method thus establishes a reliable way to calculate elastic constants from pair distributions in computer simulations.
Ferroelectric properties of KNbO3/KTaO3 superlattices by atomic-level simulation
Authors: Sepliarsky M. , Phillpot S. R. , Wolf D. , Stachiotti M. G. , Migoni R. L.
Published: July 2002

We use atomic-level simulation methods to determine the zero-temperature structure and properties of coherent KNbO3/KTaO3 superlattices. We find that the in-plane behavior is essentially bulk-like with an abrupt jump in the polarization at the interfaces. By contrast, the polarization in the modulation direction is continuous through the interfaces with the interior of the KTaO3 layers remaining polarized for modulation lengths of up to 160 unit cells. The zero-frequency dielectric constant in the modulation direction has a similar magnitude to that of KNbO3. An investigation of the switching behavior shows that for modulation lengths greater than 24 unit cells, each KNbO3 layer behaves essentially independently. For modulation lengths of less than 12 unit cells, the KNbO3 layers interact so strongly with each other that the superlattice essentially behaves as a single artificial structure.
Determining the locus for photocarrier recombination in dye-sensitized solar cells
Authors: Zhu Kai , Schiff E. A. , Park N.-G. , van de Lagemaat J. , Frank A. J.
Published: July 2002

We present intensity-modulated photocurrent and infrared transmittance measurements on dye-sensitized solar cells based on a mesoporous titania (TiO2) matrix immersed in an iodine-based electrolyte. Under short-circuit conditions, we show that an elementary analysis accurately relates the two measurements. Under open-circuit conditions, infrared transmittance, and photovoltage measurements yield information on the characteristic depth at which electrons recombine with ions (the “locus of recombination”). For one particular series of samples recombination occurred near the substrate supporting the titania film, as opposed to homogeneously throughout the film.
On the self-consistent modeling of a traveling wave sustained nitrogen discharge
Authors: Guerra V. , Tatarova E. , Dias F. M. , Ferreira C. M.
Published: July 2002

We present a self-consistent formulation to study low-pressure traveling wave (azimuthally symmetric surface transverse magnetic mode) driven discharges in nitrogen. The theoretical model is based on a self-consistent treatment of the electron and heavy particle kinetics, wave electrodynamics, gas thermal balance, and plasma–wall interactions. The solution provides the axial variation (as a result of nonlinear wave power dissipation along the wave path) of all discharge quantities and properties of interest, such as the electron energy distribution function and its moments, population densities of all relevant excited and charged species [N2(X 1Σg+,ν),N2(A 3Σu+,a′ 1Σu−,B 3Πg,C 3Πu,a 1Πg,w 1Δu), N2+, N4+, e], gas temperature, degree of dissociation [N(4S)]/N, mean absorbed power per electron, and wave attenuation. A detailed analysis of the energy exchange channels among the degrees of freedom of the heavy particles is presented. Particular attention is paid to the axial variation of the gas and wall temperatures, which affect in a complex way the discharge operation. For the high electron densities and reduced electric fields achieved at 2.45 GHz, it is shown that the contribution of exothermic reactions involving excited molecules in metastable states to the total gas heating can be significant. The role of the triplet N2(A 3Σu+) metastable state as an energy “reservoir” that pumps translational modes of gas particles is pointed out. A strong correlation between the degree of dissociation, the concentration of metastable N2(A 3Σu+), N(2D,2P) particles, and surface kinetics is shown to exist. Spatially resolved measurements of the gas and wall temperatures, electron density, and wave propagation characteristics provide a validation of the model’s predictions.
Continuous wave electron paramagnetic resonance investigation of the hyperfine structure of O2−17 adsorbed on the MgO surface
Authors: Chiesa Mario , Giamello Elio , Paganini M. Cristina , Sojka Zbigniew , Murphy Damien M.
Published: July 2002

The adsorption of molecular oxygen (enriched with O17) onto high surface area MgO has been studied by electron paramagnetic resonance (EPR) spectroscopy. The oxide surface was pretreated in such a way so that surface trapped electron FS+(H) centers are produced. Subsequent dioxygen adsorption results in an electron transfer reaction from FS+(H) centers to O2, producing a surface stabilized superoxide (O2−) anion. The resulting EPR spectrum of the paramagnetic anion is complicated by the simultaneous presence of a high number of “normal” hyperfine lines along the principal axes and also by several off-axis extra features which have complicated previous interpretations of the Ayy and Azz components. By adopting a suitable adsorption procedure which suppresses the superoxide speciation, using a highly crystalline MgO material and controlling the isotopomer composition through appropriate O17 enrichments, the resolution of the EPR spectrum has been dramatically improved. Analysis of the H1 superhyperfine structure (|AH|/βeg=[3.9,2.2,1.3]G), resulting from a dipolar interaction between the adsorbed O2− anion and a neighboring OH group, and positions of the extra absorption lines in the spectrum, have provided us with auxiliary sources of information to determine for the first time the complete O17 hyperfine tensor (AO/βeg=[−76.36,7.18,8.24] G). The tensor has been analyzed in detail using a localized spin model. The spin density is shared among the 2pπx(0.495), 2pxy(−0.024) and 2s(0.011) orbitals. The total spin density on O2− indicates that a complete surface electron transfer from the FS+(H) center to dioxygen occurs upon adsorption, in line with recent ab initio calculations.
Photoelectron spectroscopy of naphthalene cluster anions
Authors: Song Jae Kyu , Han Sang Yun , Chu Inho , Kim Jeong Hyun , Kim Seong Keun , Lyapustina Svetlana A. , Xu Shoujun , Nilles J. Michael , Bowen Kit H.
Published: July 2002

Mass spectrometric and anion photoelectron spectroscopic studies of homogeneous naphthalene cluster anions, (Nph)n=2–7−, were conducted to characterize the nature of their anionic cores. The smallest stable species in this series was found to be the naphthalene dimer anion. The vertical detachment energies of naphthalene clusters, n=2–7, were determined and found to increase smoothly with cluster size. By extrapolation, the vertical detachment energy of the isolated naphthalene molecule was found to be −0.18 eV, in agreement with its adiabatic electron affinity value from literature. The strong similarity between the spectral profiles of (Nph)2− and (Nph)1−(H2O)1 implied that (Nph)2− possesses a solvated monomeric anion core. All of the naphthalene cluster anions studied here were interpreted as having monomer anion cores.
Fluorescence intensity and color purity improvement in nanosized YBO3:Eu
Authors: Wei Zhenggui , Sun Lingdong , Liao Chunsheng , Yan Chunhua , Huang Shihua
Published: July 2002

In order to improve the color purity of YBO3:Eu, which is practically used as a plasma phosphor, the lowering of the structural symmetry is proposed and subsequently realized in nanosized YBO3:Eu so as to increase the contribution of D05–F27 transition of the luminescent centers, Eu3+ ions. Spherical YBO3:Eu nanocrystals were fabricated via thermal decomposition of complex precursors and could be crystallized at lower temperature than required by the bulks. The particle size could be controlled by adjusting the annealing temperature and time. Unlike the bulk YBO3:Eu synthesized by solid-state reaction, the quenching concentration increased with the decrease of the particle size. Furthermore, the fluorescence intensity and color purity were significantly improved in nanocrystals. Possible mechanisms responsible for these improvements were proposed.
Optical emission spectroscopy of pulsed hexalfuoropropylene oxide and tetrafluoroethylene plasmas
Authors: Cruden Brett A. , Gleason Karen K. , Sawin Herbert H.
Published: July 2002

The optical emission spectra of tetrafluoroethylene (TFE) and hexafluoropropylene oxide (HFPO) have been examined for chemistry and electron distribution transients as a function of plasma pulsing, under depositing conditions (1 torr). The major identifiable peaks in these plasmas are attributable to CF2 and CF. Other expected species, including CF3, atomic F and atomic C are not observed. HFPO plasmas show significant emission from CO and O. Comparison of CF2 emission transients to absolute absorption measurements show that the continuum emission surrounding CF2 is not attributable to CF2+ emission, as has been previously proposed, but is likely rovibrational overlap. Argon emission in TFE shows a relatively slow transient toward steady state (∼20 ms), implying slower chemistry transients, such as attachment, control the electron transient in pulsed TFE plasmas. In HFPO plasmas, CO and O actinometry are applied. CO emission trends suggest that CO is actually produced during the plasma off times, and is here attributed to etching reactions in the film. The rotational structure of CO is also examined and fit to estimate plasma rotational temperature, giving temperature transients on the order of ∼20 ms.
Photoemission and ab initio theoretical study of interface and film formation during epitaxial growth and annealing of praseodymium oxide on Si(001)
Authors: Fissel A. , Da̧browski J. , Osten H. J.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) was performed to study the formation process of Pr2O3/Si(001) interfaces and films during epitaxial growth and postgrowth annealing. A significant shift in the Pr and O core-level binding energy was found accompanied by an analogous shift in the Pr2O3 valence band maximum. This shift depends on the oxide layer thickness and interface structure, as indicated by ab initio pseudopotential calculations. It is caused by interface dipoles in the Si/Pr2O3 interface region due to the accumulation of oxygen. Besides providing a insight into the growth mechanism and interface properties of high-K dielectrics on Si, our results also demonstrate the usefulness of in vacuoXPS for investigating epitaxial growth processes.
Gallium distribution in the clathrates Sr8Ga16Ge30 and Sr4Eu4Ga16Ge30 by resonant diffraction
Authors: Zhang Yuegang , Lee Peter L. , Nolas George. S. , Wilkinson Angus P.
Published: July 2002

The distribution of gallium/germanium in the type-I clathrates Sr8Ga16Ge30 and Sr4Eu4Ga16Ge30 over the three crystallographically distinct framework sites has been determined by resonant diffraction. The analyses indicate a strong preference for the occupation of the 6c site by gallium. This is consistent with theoretical predictions, but contrary to the results of a previous neutron diffraction study. The gallium distribution, and, hence, the thermoelectric properties of these materials, may be amenable to manipulation by heat treatment.
Barrier height determination of SiC Schottky diodes by capacitance and current–voltage measurements
Authors: Raynaud C. , Isoird K. , Lazar M. , Johnson C. M. , Wright N.
Published: July 2002

Extractions of barrier heights of 6H and 4H-SiC Schottky diodes have been performed on structures with various gate metallization, using both capacitance–voltage (C–V) and current–voltage (I–V) measurements. The sum of the two barriers extracted by C–V measurements on both n-type and p-type materials is found to be higher than the band gap energy EG, whereas the one extracted by I–V is less than EG. However, above room temperature, temperature variations of barrier heights are in agreement with the variations of EG. We have also computed theoretical I–V characteristics using a two-barrier height model. By taking account of temperature variations of a large number of parameters, e.g., the carrier mobility, free carrier concentration, and barrier height, we have achieved a good fit with experimental data. The model is shown to be valid for n-type Schottky diodes over a wide range of temperatures (from 100 to 500 K).
Observation of s-d exchange force between domain walls and electric current in very thin Permalloy films
Authors: Freitas P. P. , Berger L.
Published: July 2002

Large dc current pulses, ≂2 μs long, are sent through 30–40-nm-thick Ni87Fe13 films containing Néel walls. Wall displacements are seen for current densities ≥1.2×107 A/cm2. Displacements reverse when current sense is reversed. Walls always move in direction of charge carriers in this electronlike material. Our results agree with a theory of s-d exchange interaction between walls and 4s conduction electrons. Hydromagnetic ‘‘domain-drag’’ forces are too small in such very thin films to explain our data.
Permanent magnet properties of sintered Nd-Fe-B between −40 and +200 °C
Authors: Li D. , Mildrum H. F. , Strnat K. J.
Published: July 2002

Easy and hard-axis magnetization curves, and second quadrant demagnetization lines were measured on two early commercial Nd-Fe-B based pilot products at temperatures between −40 and +200 °C. Open-circuit flux losses during short-term heating were determined. The high-Hc grade 30-H shows generally better temperature stability of magnetic properties than the lower-coercivity grade 35. The (negative) temperature coefficients of Br, BHc, and (BH)max at 100 °C of Nd-Fe-B are approximately twice those of SmCo5, that of MHc is three times larger. After thermal demagnetization by cycling to the Curie temperature all losses are fully recoverable by remagnetizing. The required recharging field strength of previously field-demagnetized magnets can be reduced by the simultaneous application of field and heat.
Frequency-scanned deep-level transient spectroscopy
Authors: Henry P. M. , Meese J. M. , Farmer J. W. , Lamp C. D.
Published: July 2002

The output signal in a deep-level transient spectroscopy experiment is a function of both the rate-window settings and sample temperature. Usually, the rate window is held fixed and the temperature scanned to produce the deep-level spectrum. We will demonstrate that a deep-level spectrum can also be obtained by fixing the temperature and scanning the rate window.
Hydrogen bonding configurations in silicon nitride films prepared by plasma-enhanced deposition
Authors: Maeda Masahiko , Nakamura Hiroaki
Published: July 2002

Hydrogen chemical bonding configurations in amorphous silicon nitride films prepared by plasma-enhanced chemical vapor deposition are determined for the first time. This is accomplished by measuring the difference spectra between the absorption spectrum of an annealed or a hydrogen implanted film and that of a reference film, mostly as-grown film. Three- and two-bonding configurations for Si-hydrogen and N-hydrogen bonds are separated and assigned to Si-H(2120 cm−1), Si-H2 and/or (Si-H2)n chains (2180 cm−1), Si-H3(2255 cm−1), N-H(3320 cm−1), and N-H2(3280 and 3345 cm−1), respectively. Si-H2 bonds are the most thermally stable and N-H2 bonds are more thermally stable than N-H bonds. It is found that N-H bonds form the hydrogen bonding in the films.
Epitaxial growth of LaF3 on GaAs(111)
Authors: Sinharoy S. , Hoffman R. A. , Rohatgi A. , Farrow R. F. C. , Rieger J. H.
Published: July 2002

We report the first epitaxial growth of lanthanum trifluoride on the (111) surface of gallium arsenide. Smooth, crack-free, and high-crystalline quality films of thickness up to 200 nm were grown at 500 °C on a GaAs(111) surface that was cleaned by ion heat treatment and post-anneal. The film surfaces were examined in situ by low-energy electron diffraction (LEED) and ex situ by reflection high-energy electron diffraction (RHEED) and Nomarski optical microscopy. Capacitance-voltage (C-V) and current-voltage (I-V) measurements showed a breakdown strength of 2×106 V/cm and no significant flat-band shift due to insulator charges.
Comparative oxidation studies of SiC(0001̄) and SiC(0001) surfaces
Authors: Muehlhoff L. , Bozack M. J. , Choyke W. J. , Yates John T.
Published: July 2002

A comparison of the kinetics of oxidation at 995 and 1345 K of the SiC(0001̄) and SiC(0001) crystal faces is made. The oxidation rate on the SiC(0001̄) (C-rich face) is higher at both temperatures. SiO2 is formed. At 1345 K, the initial oxidation process is retarded by excess surface carbon. When the excess carbon is volatilized by either CO or CO2 formation, the oxidation rate is higher at 1345 K than at 995 K.
Magnetization process in rapidly solidified neodymium-iron-boron permanent magnet materials
Authors: Pinkerton F. E. , Van Wingerden D. J.
Published: July 2002

Initial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd-Fe-B permanent magnet materials: melt-spun ribbons, hot-pressed magnets, and die upset magnets. In all three materials, the results are consistent with domain-wall pinning at Nd2Fe14B grain boundaries as the coercivity mechanism. Thermally demagnetized ribbons are comprised of single domain Nd2Fe14B grains, and both initial magnetization and demagnetization occur by depinning and wall motion. Domain walls move easily in die upset magnets until they become strongly pinned at grain edges. Complete magnetization requires an applied field greater than the coercive field. Hot-pressed magnets show a mixture of ribbon and die upset behavior.
Photoluminescence excitation measurements on GaAs:Er grown by molecular-beam epitaxy
Authors: Ennen H. , Wagner J. , Müller H. D. , Smith R. S.
Published: July 2002

Using a tunable color center laser, photoluminescence excitation measurements were performed on GaAs:Er grown by molecular-beam epitaxy. These measurements show that only one type of Er3+ center is responsible for the sharply structured emission band at 1.54 μm. The multiplicity of the zero-phonon lines indicates that this Er3+ center has lower than cubic symmetry and that the luminescence arises from the intracenter transition 4I13/2→4I15/2 of Er3+ (4f 11).
Optical and electronic properties of vanadium in gallium arsenide
Authors: Hennel A. M. , Brandt C. D. , Ko K. Y. , Lagowski J. , Gatos H. C.
Published: July 2002

The effects of vanadium doping on the electrical and optical properties of GaAs were systematically studied in melt-grown crystals prepared by the liquid-encapsulated Czochralski and horizontal Bridgman techniques and in epitaxial crystals prepared by liquid-phase electroepitaxy. By employing deep-level transient spectroscopy, Hall-effect measurements and the V2+(3d3) and V3+(3d2) intracenter optical-absorption spectra, one vanadium-related level was identified in all crystals, i.e., the substitutional-vanadium acceptor level (V3+/V2+) at 0.15±0.01 eV below the bottom of the conduction band. From the absorption measurements we conclude that the vanadium (V4+/V3+) donor level must be located within the valence band. Because of its energy position, the above level cannot account for the reported semi-insulating properties of V-doped GaAs. We observed no midgap levels resulting from vanadium-impurity (defect) complexes. The high resistivity reported for certain V-doped GaAs crystals must result from indirect effects of vanadium, such as the gettering of shallow-level impurities.
Application of deep level transient spectroscopy to metal-oxide-semiconductor relaxation transients
Authors: Pearce N. O. , Hamilton B. , Peaker A. R. , Craven R. A.
Published: July 2002

A method for determining minority carrier generation lifetimes and hold times in metal-oxide-semiconductor devices is presented. The technique overcomes some of the uncertainties inherent in the commonly used Zerbst lifetime measurement and can be used on very small devices. In the method a deep level transient spectroscopy system is used to separate generation currents of different activation energies, and to provide values for these energies, so indicating the source of the generation at a particular temperature. In the range where generation in the depletion region is dominant, the minority-carrier generation lifetime can be derived. In all cases the data can be directly related to the ‘‘hold’’ time of a metal-oxide-semiconductor capacitor.