bluuebunny/crossref_metadata_2025 huggingface dataset:
A quantum reactive scattering study of the spin-forbidden CH(X 2Π)+N2(X 1Σg+)→HCN(X 1Σ+)+N(4S) reaction
Authors: Wada Akira , Takayanagi Toshiyuki
Published: July 2002

The dynamics of the spin-forbidden CH(X 2Π)+N2(X 1Σg+)→HCN(X 1Σ+)+N(4S) reaction has been studied theoretically using the reduced dimensionality quantum scattering method. Three degrees of freedom have been considered in the dynamics calculations by treating CH as a united atom. The problem is thus reduced to the usual atom–diatom scattering calculation. Three-dimensional potential energy surfaces for both the doublet and quartet states were constructed using ab initio electronic structure calculations while the spin–orbit coupling element was taken from previous work. Time-independent quantum reactive scattering calculations have been performed using the hyperspherical close-coupling method. The calculated cumulative reaction probabilities show that the reaction dynamics is exclusively resonance-dominated. The thermal rate constants calculated using the reduced dimensionality cumulative reaction probabilities with the energy shifting and J-shifting approximations were found to be much smaller than experimental measurements and previous reduced-dimensionality results of Seideman [J. Chem. Phys. 101, 3662 (1994)] by a factor of more than two orders of magnitude. In order to understand this serious disagreement, we have carried out the scattering calculations with the use of modified potential energy surfaces and spin–orbit couplings but found that the calculated rate constants were still much smaller than experimental data. The present computational study strongly suggests that further experimental studies including direct detection of N(4S) and/or any other mechanism for the “prompt-NO” formation will be necessary.
X-ray scattering study of the structure of water around myoglobin for several levels of hydration
Authors: Dorbez-Sridi R. , Cortès R. , Mayer E. , Pin S.
Published: July 2002

Water has been shown to play a crucial role in the stability and catalytic function of protein. Our purpose is to understand the modification of bulk liquid behavior when water is around myoglobin, in particular, the perturbation of liquid water structure by hydrophobic and hydrophilic sites of the protein. For this, the structure of water adsorbed at two different surfaces of a hydrophobic medium such as activated charcoal and a hydrophilic one such as polyHEMA (a synthetic hydrogel) was studied using both x-ray and neutron scattering techniques in earlier works. Following this study, the structure of water around myoglobin is investigated by x-ray scattering at room temperature and down to 77 K. The structure factors and the corresponding radial distribution functions are determined for several levels of hydration. At high water content (3 g water/g protein) the properties of water are very similar to those of bulk water. In partially hydrated samples (<0.4 g water/g protein), the molecular structure of water determined by water–water radial distribution function exhibits significant change, especially in the range of 3.3<r<3.9 Å. These changes correspond to a distortion of the hydrogen-bonded network at the level of the first and second neighbors, and water does not crystallize when the temperature is lowered.
Differential cross sections for O(3P)+alkane reactions by direct imaging
Authors: Liu Xianghong , Gross Richard L. , Suits Arthur G.
Published: July 2002

We report a crossed molecular beam study of the reaction of O(3P) with cyclohexane with velocity map imaging of the cyclohexyl radical product using single photon ionization. The results represent the first direct measurement of the differential cross sections for ground state oxygen atom reactions with alkanes. The experimental approach has the advantage that the collision energy is not only well defined but also continuously tunable in a reasonable range. The results show that at 5.8 kcal/mol collision energy the products are mainly backward scattered, clearly confirming the anticipated rebound reaction mechanism. More surprising is the large fraction, nearly 60%, of the available energy partitioned into the internal degrees of freedom of the products.
Passive-scalar diffusion in a fluid membrane
Authors: Fujitani Youhei
Published: July 2002

A lipid-bilayer membrane in the fluid phase when agitated by ambient fluids not only undergoes a shape fluctuation but also a two-dimensional disordered flow. Thus, besides moving individually, lipid molecules in a two-dimensional fluid element can move collectively in the lateral direction. This collective motion, which has been often overlooked in studying diffusion in a fluid membrane, can shift a lipid molecule as well as its Brownian motion. Calculating the self-diffusion constant of a passive scalar—a particle convected neither affecting the flow nor making the Brownian motion—in a fluid membrane fluctuating around a plane in aqueous environments, we show that the collective motion cannot be neglected in explaining observed values of the self-diffusion constant of a lipid molecule.
Modeling of nonlinear vibrational relaxation of large molecules in shock waves with a nonlinear, temperature-varying master equation
Authors: Davis Michael J. , Kiefer John H.
Published: July 2002

We model recent experiments on the vibrational relaxation of oxirane in a shock tube. A master equation is developed which includes self-collisions of the oxirane, leading to a nonlinear master equation. This master equation is also applied to a more limited study of vibrational relaxation for cyclopropane in a shock tube. The time variation of the temperature dependence of the bath is also included in the calculations. Good agreement between the modeling and experiments are obtained through a fit to the energy transfer parameters. These fits demonstrate that self-collisions are dominant in promoting the relaxation even for mixtures of Kr and oxirane where the oxirane is 2% and 4% dilute. This dominance comes from two sources: (1) much larger energy transfer per collision for oxirane–oxirane collisions and (2) resonant energy transfer effects. For cyclopropane, some of the good fits show smaller energy transfer characteristics for self-collisions than buffer gas collisions. Even in these cases self-collisions are an important part of the energy transfer process through resonant energy transfer effects.
Charge retention in metal–oxide–semiconductor capacitors on SiC used as nonvolatile-memory elements
Authors: Dimitrijev Sima , Cheong Kuan Yew , Han Jisheng , Harrison H. Barry
Published: July 2002

In this letter, the possibility of using metal–oxide–semiconductor capacitors on SiC as nonvolatile random-access memory elements has been experimentally investigated. Because of the wide energy gap and the very low minority-carrier generation rate in SiC, it should be possible to achieve very long retention times. The investigations show that charge leakage through the gate oxide may prevent the use of SiC metal–oxide–semiconductor (MOS) capacitors as memory elements. Importantly, the experiments demonstrate that both the charge leakage and carrier-generation rate are low in the case of nitrided SiO2–SiC interfaces. The retention time extrapolated to room temperature is in the order of 109 years for the case of MOS capacitors on 4H–SiC, which is approximately equal to the theoretical limit.
Excitation of torsional modes of proteins via collisional energy transfer: A quantum dynamical approach
Authors: Clary David C. , Meijer Anthony J. H. M.
Published: July 2002

Quantum dynamical calculations have been carried out on the excitation of the torsional vibrations of a protein by collision with a solvent molecule. This energy transfer process represents the first step in the unfolding of the protein. The method developed for this purpose is the torsional close coupling, infinite order sudden approximation. Both time-independent and time dependent methods are used to solve the scattering problem and individual excitation of all the torsional modes of the protein is treated. The method is applied to the excitation of the HIV protein gp41 colliding with a water molecule. This protein has 1101 atoms, 56 amino acids, and 452 torsional modes. A major mode-selective effect is found in the computations: it is much easier to excite backbone torsions than sidechain torsions in the protein. In addition, resonances arise in the collisional process and these complexes involve temporary trapping of the water molecule inside the pockets of the protein.
A study of diode perveance for high-power relativistic electron beams
Authors: Uhm Han S. , Choi Eun H. , Choi Myung C.
Published: July 2002

The space-charge-limited current in a diode, including the relativistic effects of beam electrons on it, is investigated in connection with development of an analytical model of the perveance for high-power relativistic electron beams. A simple scaling law for perveance in a high-power diode is obtained and compared with experimental data, which agree reasonably well with theoretical predictions.
Chain conformations and correlations in thin polymer films: A Monte Carlo study
Authors: Müller M.
Published: July 2002

Using Monte Carlo simulations of a coarse grained lattice model, we study conformations and concentration fluctuations of polymers confined into thin films. We vary the chain length from N=32 to 512. As we reduce the film thickness to the unperturbed radius of gyration Rg, the chain extension parallel to the surface increase only by about 5% for the largest chain length. Confinement has, however, a much more pronounced effect on intra- and intermolecular correlations: the surfaces make the chain fold back into the volume it occupies and squeeze out neighboring chains. Decreasing the film thickness, we reduce the interdigitation of chains. This gives rise to deviations from the ideal Gaussian chain structure for wave vectors qRg>1, which are qualitatively similar to corrections observed in semidilute (bulk) solutions. Fitting the structure factor with a Debye function in this Kratky regime, we systematically overestimate the chain extension by up to 15%. Additional (minor) deviations from the incompressible random phase approximation at large wave vectors are found. As we reduce the film thickness the correlation function of the centers of mass develops a very strong correlation hole and resembles in thin films that of a two-dimensional fluid of soft particles.
Characterization of vapor-phase-grown ZnSe nanoparticles
Authors: Sarigiannis Demetrius , Peck John D. , Kioseoglou George , Petrou Athos , Mountziaris T. J.
Published: July 2002

ZnSe nanoparticles were synthesized by reacting vapors of (CH3)2Zn:N(C2H5)3 and H2Se, diluted in H2, in an opposed flow reactor operating at room temperature and 120 Torr. The particles were collected as random aggregates on silicon substrates and transmission electron microscopy (TEM) grids placed downstream of the reaction zone. Particle characterization using TEM and electron diffraction revealed the presence of polycrystalline nanoparticles with diameter of about 40 nm that were apparently formed from coagulation of smaller nanocrystals with characteristic size of about 4 nm. Raman spectra of the nanoparticles revealed an asymmetrically broadened feature associated with the LO phonon of ZnSe, indicating the presence of smaller single-crystalline grains.
High frequency components of current fluctuations in semiconductor tunneling barriers
Authors: Oriols Xavier , Martı́n Ferran , Suñé Jordi
Published: July 2002

The power spectral density of current noise in phase-coherent semiconductor tunneling scenarios is studied in terms of Bohm trajectories associated to time-dependent wave packets. In particular, the influence of the particles reflected by the barrier on the noise spectrum is analyzed. An enhancement of the power spectral density of the current fluctuations is predicted for very high frequencies. The experimental measurement of this high frequency effect is discussed as a possible test of Bohm trajectories.
Musical variations from a chaotic mapping
Authors: Dabby Diana S.
Published: July 2002

A chaotic mapping provides a technique for generating musical variations of an original work. This technique, based on the sensitivity of chaotic trajectories to initial conditions, produces changes in the pitch sequence of a piece. A sequence of musical pitches {pi}, i.e., any piece ranging from Bach (or earlier) to contemporary music, is paired with the x-components {xi} of a Lorenz chaotic trajectory. Each pi is marked on the x axis at the point designated by its xi. In this way, the x axis becomes a pitch axis configured according to the notes of the original composition. Then, a second chaotic trajectory, whose initial condition differs from the first, is launched. Its x-components trigger pitches on the pitch axis (via the mapping) that vary in sequence from the original work, thus creating a variation. There are virtually an unlimited number of variations possible, many appealing to expert and nonexpert alike.
Abelian sandpiles
Authors: Creutz Michael
Published: July 2002

A class of models for self-organized critical phenomena possesses an isomorphism between the recursive states under addition and the Abelian operator algebra on them. Several exact results follow, including the existence of a unique identity state, which when added to any configuration C in the recursive set relaxes back to that configuration. In this relaxation process, the number of topplings at any lattice site is independent of the configuration C.
Efficient x-ray production from ultrafast gas-puff Z pinches
Authors: Spielman R. B. , Hanson D. L. , Palmer M. A. , Matzen M. K. , Hussey T. W. , Peek J. M.
Published: July 2002

The Proto-II accelerator has been used to implode krypton and xenon annular gas puffs. A significant fraction of the machine electrical energy was converted first to plasma kinetic energy and then to x rays when the plasma pinched on axis. Quantitative measurements using time-resolved bolometers have shown as much as 10% of the total radiation yield near 1 keV in Xe and 2 keV in Kr. We have compared this radiation yield to the predictions from one-dimensional magnetohydrodynamic code calculations. The implosions were also observed with both time-integrated pinhole cameras and spectrographs. No hard x-ray (E>10 keV) output was observed.
Comparison of one-dimensional simulations of magnetically insulated gaps with analytic models
Authors: Lockner T. R. , Poukey J. W.
Published: July 2002

A comparison is made between several one-dimensional (1-D) analytic theories of magnetically insulated gaps and 1-D particle simulations. The electron sheath current flowing in the E×B drift direction is the most model-sensitive parameter. This parameter is significant in experiments where there is a symmetry discontinuity in the electron flow. It is found that the theory of Bergeron is in the best agreement with the simulations.
Transport of bromine out from a graphite molecular beam source: A case of bulk diffusion
Authors: Holmlid Leif , Sigurdsson Arnar
Published: July 2002

The Br and Br2 molecular beam fluxes from graphite sources have been studied mass spectrometrically at 300–1200 K. The results show that the transport out from the sources goes via bulk diffusion in the vicinity of the normal effusion opening. The pressure dependence of the signals shows that the Br and Br2 fluxes are not in chemical equilibrium. Dissociation is also higher than expected at equilibrium. The diffusing species is Br2 in most cases. This efficient diffusion process is in accord with the well-known facile intercalation of graphite with bromine. The method used may be developed to study further molecular details of transport processes. Sources using controlled diffusion and increased dissociation may also be used as high intensity molecular beam sources.
Formation of ion beam mixed silicides on Si (100) at elevated substrate temperatures
Authors: Fathy D. , Holland O. W. , Narayan J.
Published: July 2002

High-resolution transmission electron microscopy and Rutherford backscattering spectroscopy have been used to study the structure and composition of intermixed regions between thin films of Ni deposited on single crystals of (100) silicon. A comparison has been made between thin films of silicide which have been either thermally grown or ion-beam reacted at different substrate temperatures. It is shown that not only can high temperature, silicide phases be produced at lower temperatures by ion-beam mixing, but also the structure of the Si/silicide interface and the morphology of the silicide depend on whether the mixing was induced thermally or by ion irradiation. It was found that depending on the temperature of the substrate, Ni3Si2, NiSi, NiSi2, were observed to form. Ion mixing at 500 °C led to the formation of a unique morphological structure. At this temperature, two distinct silicide layers formed (NiSi and NiSi2) with the silicon-rich phase at the surface away from the Si/silicide interface. Premixing by ion irradiation before thermal treatment of the samples was found to influence the detailed microstructure of the reacted region. The premixing produced less oxidation of nickel during thermal treatment compared to that observed on samples which were reacted solely by thermal processing; it also resulted in a more uniform, planar interface.
Probabilistic-topological representation for crosslink, entanglement, or contact networks in (bio)polymer systems
Authors: Klonowski Wlodzimierz
Published: July 2002

Structure-property relationships for (bio)polymer systems, especially for crosslinked network systems, are discussed and calculated. Physical properties of the system, e.g., modulus of elasticity, gel fraction, etc., are expressed as certain functionals of the system’s actual structure. A new topological gelation criterion for the sol-gel transition point is proposed. For the first time the gel fraction for a system with closed cycles is calculated, using a new method of equivalent crosslinks. In contrast to many existing theories, the present theory may easily be applied to crosslinking systems far from thermodynamic equilibrium and to nonrandom systems. It is suggested that a new kind of auto-oscillatory phenomenon, namely an oscillating sol-gel transition, is to be found in open crosslinked systems, e.g., in actomyosin solutions maintained far from equilibrium by a constant influx of ATP. Such phenomena may be of great importance in living cells. The theory is easily adaptable to a broad spectrum of systems to which probabilistic-topological notions similar to the ones introduced here may be applied, e.g., to entanglement networks for systems of flexible chains or to contact networks for systems of stiff molecules.
Factors affecting the spatial distribution of the principal midgap donor in semi-insulating gallium arsenide wafers
Authors: Blakemore J. S. , Dobrilla P.
Published: July 2002

The spatial distribution of an extrinsic variable across a wafer from a melt-grown crystal can provide clues concerning processes during and following solidification. This has encouraged several recent wafer mapping studies in semi-insulating GaAs: for dislocations, for the midgap native donor known as EL2, and for other flaw-related properties. This paper provides maps for the optically measurable neutral EL2 concentration in wafers from Czochralski-grown GaAs crystals. These figures illustrate comments about the factors believed to influence the magnitude and spatial distribution of that midgap flaw. Those factors include the melt stoichiometry and convection, any post-freezing stress, dislocation climb as plastic relief from such stress, and defect reactions in hot solid GaAs. A full accounting for these factors has been hampered by the continuing lack of a generally accepted atomic-scale model for EL2.
Fundamental stability limits for the diode-laser-pumped rubidium atomic frequency standard
Authors: Camparo J. C. , Frueholz R. P.
Published: July 2002

Recently, there has been considerable interest in the use of single-mode diode lasers in atomic frequency standards. In the present paper theoretical calculations are performed in order to quantify the expected performance improvement upon incorporation of diode lasers in rubidium gas cell atomic frequency standards. We assume that clock signal shot noise, the diode laser’s quantum noise, and diode laser frequency locking noise all contribute to the atomic frequency standard’s stability. Our results indicate that white-noise Allan variances of ∼6×10−15/(τ)1/2 are possible if enhanced cavity Q diode lasers are employed, whereas for presently available commercial diode lasers we predict white-noise Allan variances of ∼3×10−14/(τ)1/2. These variances represent a 2–3 orders of magnitude improvement in frequency stability over that currently obtained with rubidium gas cell atomic clocks.
Measurement of high Rydberg 1P spectra of atomic helium and its application to glow-discharge diagnostics
Authors: Ganguly B. N.
Published: July 2002

The high Rydberg spectra of atomic helium np 1P states were measured up to n=46 in the negative glow region of a discharge, with mJ=±1 polarization, by optogalvanic spectroscopy. The net electric field values in the negative glow region, in the transition region between the negative glow and the positive column, and also on the radial axis of the positive column were determined from series-termination measurements. The spatial variations of the ion density in the negative glow region were also obtained from the net-field measurements.
Charged defect states at silicon grain boundaries
Authors: Stützler F. J. , Queisser H. J.
Published: July 2002

Grain boundaries (GB’s) can induce electrostatic potential barriers due to trapping of majority carriers into defect states at the interface. The as-grown electrical activity of GB’s in silicon is related with the density of decorated dislocations in the interface plane. Heat treatments actuate as-grown electrically active and inactive boundaries. The measured transverse conductance is reduced due to the presence of a potential barrier. The density of these generated states is determined by duration and temperature of the treatment. We explain this actuation by impurity-related formation of trap states at the interface. The annealing-temperature dependence of the density of trap states is independent of the atomistic structure of the interface. We observe strong impurity gettering of GB’s. Hydrogen incorporation in plasma discharge can repassivate as-grown electrically active and the heat-actuated GB’s. The majority-carrier transport properties of silicon GB’s are thus determined by impurities instead of the atomistic interface structure.
Comparison of measured and calculated SF6 breakdown in rf electric fields
Authors: Thompson Brian E. , Sawin Herbert H.
Published: July 2002

Measured breakdown characteristics of SF6 at 4–14 MHz are compared with the breakdown predicted with an electron-balance model. This balance includes terms for the formation of electrons from ionization of neutral molecules and electron losses by attachment to neutral molecules, electric-field-driven fluxes, and diffusion. With model parameters obtained from existing literature on electron properties in dc electric fields, the model predicts radio frequency (rf) breakdown voltages in agreement with the experimental values. The electron balance model is also expressed in a nondimensional form to aid in generalizing rf breakdown to other gases. The electron concentration profiles formed when including the electric-field-driven fluxes are compared with the sinusoidal distribution achieved for diffusion-controlled breakdown. The success of this model suggests that glow discharges can be modeled in a similar fashion.
Optical properties of silicon related insulators
Authors: Ravindra N. M. , Narayan J.
Published: July 2002

We report a study of the optical properties of silicon related insulators. Essentially, the following materials have been considered in this study; noncrystalline Si3N4, SiO2, SiO, and crystalline SiO2. Utilizing the available data of the refractive index and the extinction coefficient in the literature, the related optical properties such as (i) the real (ε1) and imaginary (ε2) parts of the complex-dielectric constant, (ii) absorption coefficient (α), and (iii) electronic polarizability (αe) are calculated. The optical gaps EG of these materials have been estimated employing the Tauc rule and the Cody alternative. Extrapolation of the band-structural and spectroscopic models of Penn [Phys. Rev. 128, 2093 (1962)] and Wemple and Didomenico [Phys. Rev. B 3, 1338 (1971)] are then made to evaluate some band-structural parameters of these materials. Interpretations of the energy corresponding to the peak in ε2 are then sought in terms of the Penn gap Ep. Whenever relevant, comparisons and correlations of the optical properties are also made with some dielectric properties.
Effects of implantation impurities and substrate crystallinity on the formation of NiSi2 on silicon at 200–280 °C
Authors: Chen L. J. , Doland C. M. , Wu I. W. , Chu J. J. , Lu S. W.
Published: July 2002

The influences of implantation impurities, such as BF2, B, F, As and P, as well as silicon substrate crystallinity on the formation of NiSi2 at 200–280 °C in nickel thin films on ion-implanted silicon, have been investigated by transmission electron microscopy. In implantation-amorphous samples no NiSi2 formation was detected at 200–280 °C. The presence of BF2, B, and F atoms was found to promote the epitaxial growth of NiSi2 in nickel thin films on crystalline silicon at low temperatures. Little or no effect on the formation of NiSi2 was found for As+- and P+-implanted samples. The results indicated that the influences of the implantation impurities are not likely to be chemical in origin. The presence of electrically active impurities in crystalline silicon alone was not sufficient to induce the formation of epitaxial NiSi2 at low temperatures. On the other hand, good correlation was found between the atomic size factor and resulting stress and NiSi2 epitaxy at low temperatures.
An intrinsic stress scaling law for polycrystalline thin films prepared by ion beam sputtering
Authors: Windischmann H.
Published: July 2002

The intrinsic stresses of Al, Ti, Fe, Ta, Mo, W, Ge, Si, AlN, TiN, and Si3N4 films prepared by ion beam sputtering were investigated at low Td/Tm values. The intrinsic stress is compressive and its origin is explained in terms of the ion peening model. Knock-on linear cascade theory of forward sputtering is applied to derive a simple scaling law with the film’s physical properties. The results show that the stress is directly proportional to the elastic energy/mole, given by the quantity Q=EM/(1−ν)D, where E is Young’s modulus, M the atomic mass, D the density, and ν Poisson’s ratio. Stress data taken from the literature for a variety of materials deposited by low-pressure magnetron sputtering, and rf and ion beam sputtering also fit the correlation with Q. Furthermore, the model predicts a square-root dependence on the incident ion energy, suggesting that the stress is momentum rather than energy driven.
Some design considerations on using modulated intense annular electron beams for particle acceleration
Authors: Lau Y. Y.
Published: July 2002

The space-charge wave and the stability of an intense annular electron beam propagating in a metallic pipe are examined. It is shown that the beam can propagate stably in the parameter regime of some proposed experiments where such a beam is used to generate an electromagnetic wake for particle acceleration. Modulation of an intense beam with a large diameter appears promising at high frequencies.
Study of the rapid thermal nitridation and silicidation of Ti using elastic recoil detection. II. Ti on SiO2
Authors: Krooshof G. J. P. , Habraken F. H. P. M. , van der Weg W. F. , Van den hove L. , Maex K. , De Keersmaecker R. F.
Published: July 2002

This paper describes a study of the rapid thermal processing of Ti thin films on SiO2 in Ar and N2 in the temperature range 400–1100 °C. The resulting layer structures were analyzed by means of Rutherford backscattering spectrometry and elastic recoil detection. The latter technique yielded quantitative information with a low detection limit of the depth distribution of H, O, and N. The underlying SiO2 film starts to react with the Ti at 500 °C. Oxygen originating from the reduction of SiO2 is dissolved in the Ti layer. As long as the solubility limit of O in Ti is not reached, nitridation of the Ti in the surface region causes O to be snow-plowed out of the top surface layers. The reaction of Ti with SiO2 is characterized by an activation energy of 0.7 eV. For the case of rapid thermal processing in N2, the evolution of the layer structure is governed by a competition between nitridation and the above-mentioned oxidation of Ti. In between the Ti oxide and the remaining SiO2, a silicide of probable composition Ti5Si3 is formed.
Thermal stability of five sintered rare-earth–cobalt magnet types
Authors: Li D. , Mildrum H. F. , Strnat K. J.
Published: July 2002

Five commercially available magnet types—SmCo5, high- and low-coercivity Sm2(Co,Fe,Cu,Zr)17, (Sm,Ce)(Co,Cu,Mn)7, and Ce(Co,Cu,Fe)5—were systematically studied to obtain a comprehensive overview of the following magnetic properties of interest to the user: reversible and irreversible losses during thermal cycling between 20 °C and 250 °C; open-circuit remanent flux loss during aging at 150 °C and 250 °C up to 2100 h, at permeances of p=−2.5 and −0.5. Room-temperature demagnetization curves before and after aging are compared to distinguish recoverable (magnetic) from irrecoverable (structural) aging effects. The Sm0.5Ce0.5(Co,Cu,Mn)7 magnets had the best long-term stability under all test conditions, followed by the high-Hc 2-17 and SmCo5. The low-Hc 2-17 exhibited the greatest absolute flux loss under all conditions, but it stabilized fairly well after 10 h, and the loss was recoverable by remagnetizing. In contrast, for Ce(Co,Cu,Fe)5 the losses continued up to 2100 h without a sign of flux stabilization.
Deep-level transient spectroscopy studies of near-surface hole and electron traps in Zn-doped InP using high barrier Yb/p-InP Schottky diodes
Authors: Singh A. , Anderson W. A.
Published: July 2002

High barrier Yb/p-InP Schottky diodes were used to study hole and electron traps in Zn-doped p-InP single crystals by deep-level transient spectroscopy (DLTS). DLTS spectra were recorded at different quiescent bias voltages in the temperature range between 80 and 450 K. From the DLTS measurements at reverse bias of 3 V, three hole traps, H1 (EV +0.56 eV), H2 (EV +0.22 eV), H3 (EV +0.12 eV), and one electron trap, E1 (EC −0.40 eV) were observed. The concentration of the traps ranged between 1012 and 1015 cm−3 with a capture cross section between 10−19 and 10−17 cm2. The DLTS spectra, at a quiescent forward bias of 0.2 V, revealed only one interfacial hole trap (EV +0.25 eV) which was different from the hole trap H2 . The measurements at different reverse bias values on the most prominent hole trap indicated an anomalous field dependence of the DLTS signal which suggested that the concentration of the trap level decreased and its activation energy as well as capture cross section increased with depth from the metal–semiconductor junction.
Thin-film CdSe: Photoluminescence and electronic measurements
Authors: Jäger-Waldau R. , Stücheli N. , Braun M. , Lux Steiner M. , Bucher E. , Tenne R. , Flaisher H. , Kerfin W. , Braun R. , Koschel W.
Published: July 2002

The electronic properties of thin CdSe films prepared through physical vapor transport were investigated using photoluminescence (PL) and electronic measurements. The films were studied at each of the main preparation steps, i.e., evaporation, annealing, etching, and finally photoetching. At 3 K two distinct donor-acceptor (DA) transitions at 1.75 and 1.70 eV were found in the photoluminescence spectra in addition to deep states at about 1.55 eV at 20 K. These DA transitions which are produced mainly during the evaporation might be associated with group VII and with alkali metal impurities. After each preparation step the DA transitions change their intensities. It is shown that photoetching of the films leads to a removal of the deep centers, while the 1.75 eV transition is blue shifted. In contrast with single-crystal CdSe the intensity of the PL increases after photoetching. The results of the PL are consistent with the electronic measurements. They are explained in terms of a previously published model.
Energy deposition in N and N+ by high-energy electron beams
Authors: Taylor R. D. , Slinker S. P. , Ali A. W.
Published: July 2002

A discrete, time-dependent energy deposition model is used to study high-energy electron-beam (100 eV–10 MeV) deposition in N and N+. Both time-dependent and steady-state secondary electron distributions are computed. The loss function, mean energies per electron-ion pair production (W), production efficiencies, and distribution functions are presented for a wide range of energies. The latest experimental and theoretical cross sections are used in the model which predicts that W is approximately 31 eV for N and 72 eV for N+ over a wide range of beam energies. The sensitivity of these results to assumed background ionization fractions is also investigated.
Resonant tunneling through one-, two-, and three-dimensionally confined quantum wells
Authors: Liu H. C. , Aers G. C.
Published: July 2002

Resonant tunneling through one-, two-, and three-dimensionally confined quantum wells from a three-dimensional electron source (emitter) is investigated theoretically. The three cases correspond to (1) the usual double-barrier structure, (2) a quantum-well ‘‘wire,’’ and (3) a quantum-well ‘‘box’’ or ‘‘dot.’’ Resonant tunneling currents exhibit different features for the three structures which can be easily understood in terms of the energy and momentum conservation involved in each case. A transfer Hamiltonian approach is used to predict current-voltage characteristics for square and round quantum wires and dots.
Refractive fringe diagnostics of sparks in air
Authors: Bacon M. , Campbell R. N. , Cunningham P. F. , Michaelis M. M.
Published: July 2002

The refractive fringe diagnostic is applied to sparks in air. Shot-to-shot density profiles are deduced at various times in the history of the shock which are consistent with published data. A qualitative description of the fringe formation is presented.
Evaluation methods and evaporation conditions for low-resistivity contacts on high T
c superconductors
Authors: Liu Y. P. , Warner K. , Chan C. , Chen K. , Markiewicz R. , Moore R. L.
Published: July 2002

Low-resistivity contacts to bulk high Tc superconductors have been prepared by resistive evaporation with values of ρs(Ω cm2) as low as 10−10 (77 K) for Ag/YBaCuO, 10−9 (77 K) for Ag/TlPbBaCaCuO, and 10−7 (60 K) for Ag/BiCaSrCuO. ρs has been found to be improved by increasing the length of a preevaporation step. This effect has been further investigated by secondary ion-mass spectrometry analysis on several Ag/Si samples. Both three-terminal and four-terminal methods have been used to determine ρs; the three-terminal method shows less dependency on the sample/contact geometry for measurements taken above Tc.
Relaxation phenomena of image sensors made from a-Si:H
Authors: Hoheisel M. , Brutscher N. , Wieczorek H.
Published: July 2002

Image sensors made from amorphous silicon ( a-Si:H ) are under development. Their elements consist of back-to-back Schottky diodes. For practical operation, long-term stability is of great importance. We investigated dark conductivity and photoconductivity, capacitance-voltage characteristics, and response behavior after switching off illumination. Even after light soaking for many hours, no change in photocurrent occurred, whereas dark current, capacitance, and response time increased. These changes are metastable and can be reversed by annealing above 200 °C. Contrary to the Staebler–Wronski effect, [Appl. Phys. Lett. 31, 292 (1977)], the dark-current increase disappears at room temperature after several hours. We investigated the time dependence of this relaxation and calculated the energetic depth of the states involved. The contact between a-Si:H and indium-tin-oxide is described as a Schottky–Bardeen-metal-insulator-semiconductor junction. Its properties are strongly dependent on interface states, in particular on the position of the neutrality energy of the interface states with respect to the Fermi energy. We show that besides the well-known Staebler–Wronski effect, a new degradation process is observed. We suggest a model where holes are trapped in interface states about 1.0–1.4 eV above the valence band. Their thermal emission governs the relaxation behavior of the dark current.
The excess carrier lifetime in p-type HgCdTe measured by photoconductive decay
Authors: Fastow R. , Nemirovsky Y.
Published: July 2002

The excess carrier lifetime has been measured in p-type Hg1−xCdxTe, with x=0.225 and p∼1×1016/cm3 , using the technique of photoconductive decay. Measurements as a function of temperature were carried out on bulk vacancy-doped crystals grown by three different techniques, as obtained from four independent sources. The lifetimes at 77 K were found to vary between 23 and 100 ns, and were correlated with the inverse carrier concentration. We present simple theoretical arguments which show that the lifetime can, in certain cases, be limited by recombination at the 15-meV acceptor level. By considering this recombination mechanism, as well as recombination at deeper levels, we were able to model all our data and derive the dependence of lifetime on the doping level.
The characterization of high-frequency ultrasonic fields using a polarimetric optical fiber sensor
Authors: Chan H. L. W. , Chiang K. S. , Price D. C. , Gardner J. L.
Published: July 2002

A polarimetric ultrasonic sensor consisting of a polarization-maintaining fiber carrying light from a laser is described, and its application to the characterization of high-frequency (1–10 MHz) ultrasonic fields is demonstrated. The principle of the sensor is based on the detection of ultrasonically induced birefringence in the fiber. The advantage of using a polarization-maintaining fiber instead of a single-mode fiber is discussed. The response of the sensor to both continuous and pulsed ultrasound at frequencies ranging from 1 to 10 MHz and at different power levels has been investigated, and a tomographic technique was used to compute transducer beam profiles with high spatial resolution (∼0.1 mm). Profiles measured in this way are compared with those measured using a polyvinylidene fluoride (PVDF) hydrophone, and are shown to have superior spatial resolution. The sensor output is observed to be a linear function of the square root of the total acoustic power, which is measured with a radiation force balance. The advantages of this sensor over available PVDF hydrophones are its ability to withstand very high ultrasonic power, and its excellent spatial resolution for beam profile measurements. It is most accurately employed in situations where the acoustic beam is both narrow and plane wave like as is the case in transducer focal regions. This is just the situation in which conventional ‘‘point’’ hydrophones, with their limited spatial resolution, are least accurate. This sensor should therefore find applications in the characterization of medical ultrasonic transducers.
Role of boron in the multistable carbon-related defect in silicon
Authors: Su Z. , Wald P. G. , Farmer J. W.
Published: July 2002

The conversion of the carbon-interstitial–phosphorous-substitutional (Ci-Ps) pairs from configuration M4 to M1 is shown to have a strong dependence on the method of sample fabrication. This conversion can be observed only in samples that have been contaminated by boron during sample fabrication. The temperature at which the conversion to M1 occurs is closely related to the level of boron contamination. In addition, it is shown that the M1 peak is not a single defect level. M1 consists of at least two overlapping levels that have similar properties but which are clearly related to different configurations. It is proposed that the conversion of M4 to M1 involves long-range migration of boron interstitials and that the M1 configurations involve a boron interstitial trapped at an isolated Ci-Ps pair.
Does continuous trail of damage appear at the change in the electronic stopping power damage rate?
Authors: Toulemonde M. , Enault N. , Fan Jin Yun , Studer F.
Published: July 2002

Y3Fe5O12 samples have been irradiated by Kr ions with incident energies between 9 and 37 MeV/A and Xe ions between 9 and 25 MeV/A. In this way a range of electronic stopping power between 7 and 28 MeV/μm has been explored. The irradiated samples have been etched chemically. A homogeneous track etching radius is observed after xenon irradiation but not after krypton irradiation. This result indicates that at an electronic stopping power value (dE/dx) of 17±3 MeV/μm a change in the damage morphology occurs: above this value the latent track becomes continuous cylinder. This value of stopping power is markedly higher than the dE/dx (8±1 MeV/μm) at which we have previously observed a change in the latent track damage rate and in the damage morphology from spherical to cylindrical shape of the defects.
Electromechanical coupling in thickness-multimode piezoelectric resonators and transducers
Authors: Strashilov V. L. , Boyanov B. I.
Published: July 2002

A theoretical analysis of the electromechanical coupling properties of piezoelectric transducers and resonators in the general thickness-multimode case is presented. A new concept for material characterization using three mode thickness resonator geometries is discussed. The insertion loss function of the piezoelectric transducer is derived and numerically analyzed for arbitrary polarizations of the excited modes.
An Auger and electron energy-loss study of reactions at the Ti-SiO2 interface
Authors: Wallart X. , Zeng H. S. , Nys J. P. , Dalmai G. , Friedel P.
Published: July 2002

In this paper, we use Auger electron spectroscopy and electron energy-loss spectroscopy in order to investigate the room-temperature formation of the Ti/SiO2 interface and its reactivity upon annealing. We perform anneals in the temperature range 200–900 °C for which the phenomena involved are discussed with respect to the Si oxide and Ti film thicknesses.
Dependence of the polar Kerr angle and coercivity on repeat distance in sequentially prepared Tb/FeCo films
Authors: Shin Sung-Chul
Published: July 2002

Multilayer thin films of Tb and FeCo were prepared by sequential thermal evaporation of Tb and Fe90Co10 alloy on glass substrates at ambient temperature in a vacuum of about 1 × 10−6 Torr. The repeat distance of the films was varied from 3.5 to 175 Å while maintaining the composition constant for all samples. Low-angle x-ray diffraction and Auger electron spectroscopy showed that a multilayer structure was achieved for films fabricated with a repeat distance Λ≥31.5 Å. The polar Kerr rotation angle and coercivity of the samples were obtained by measuring the polar Kerr hysteresis loop using a 632.8-nm HeNe laser. It was observed that for the samples having a multilayer structure, the remnant Kerr angle and coercivity monotonically decreased with increasing repeat distance, and eventually disappeared for Λ≥70 Å. For the samples showing a nonmultilayer structure, the remnant Kerr angle was independent of repeat distance. However, the coercivity increased with repeat distance until a broad maximum was obtained around Λ≂14–21 Å.
Kinetics of formation and dissociation of a dominant native defect (EL2) in GaAs
Authors: Morrow Richard A.
Published: July 2002

It is shown that a simple kinetic model can account for existing data both on the formation of the native defect EL2 in the temperature range 644–800 °C in GaAs samples from which EL2 was eliminated by a 1200 °C anneal/quench and on the disappearance of EL2 during anneals in the temperature range 1000–1200 °C. Our analysis suggests that EL2 consists of VGa bound to an unidentified ‘‘kernel’’ which, if not actually stable at temperatures up to 1200 °C, forms relatively rapidly at the lower temperatures and dictates the final concentration of EL2 in the sample. The change in enthalpy involved in the capture or release of VGa by the kernel is estimated to be 5.6 eV.
Phonon attenuation and velocity measurements in transparent materials by picosecond acoustic interferometry
Authors: Lin H. -N. , Stoner R. J. , Maris H. J. , Tauc J.
Published: July 2002

A detailed analysis of the method of picosecond acoustic interferometry to study attenuation and velocity of longitudinal acoustic phonons in transparent materials in the Brillouin frequency range with picosecond laser pulses is presented. Experimental results for fused quartz from 90 to 300 K show good agreement with previous Brillouin scattering data. Measurements on a borosilicate glass (Corning 7059) and sapphire have also been made. This method makes these measurements possible under conditions where conventional approaches are not applicable.
Analysis of thin-film silicon-on-insulator structures formed by low-energy oxygen ion implantation
Authors: Li Y. , Kilner J. A. , Robinson A. K. , Hemment P. L. F. , Marsh C. D.
Published: July 2002

The characteristics of the formation and growth of buried oxide layers, formed by oxygen implantation into silicon at lower energies (50–140-keV 16O+), have been studied using secondary-ion mass spectrometry. Some results have been checked and compared with the results obtained by Rutherford backscattering and cross-sectional transmission electron microscopy. The critical doses, required to form a continuous buried stoichiometric oxide layer during implantation (ΦIc) and after annealing (ΦAc) have been estimated from experimental results. The thicknesses of the silicon overlayer (TASi) and buried silicon dioxide layer (TASiO2) for the annealed wafers have also been estimated. A set of semi-empirical formulas for ΦIc, ΦAc, TASi, and TASiO2 has been introduced. These formulas can be used to quickly calculate the critical doses and the layer thickness values.
In situ observation of the strain of Co and Ni deposited on Pd (111) by reflection high energy electron diffraction
Authors: Yamazaki T. , Suzuki Y. , Katayama T. , Taninaka M. , Nakagawa K. , Itoh A.
Published: July 2002

Co/Pd(111) and Ni/Pd(111) bilayer films were grown epitaxially on single crystal MgO (111) substrates by ultrahigh vacuum deposition. In situ observation of the strain of Co and Ni deposited on Pd (111) plane was performed by reflection high energy electron diffraction (RHEED) during fabrication. From RHEED patterns it was confirmed that the lattice constant of a Co film of 2 Å in thickness deposited on Pd was about 3.7 Å, which was different from that of Pd (3.89 Å). The strain (εCo) of the Co layer was rapidly released during further Co deposition. The strain εCo of the third Co monatomic layer on Pd is nearly equal to zero. The large strain of the Co layer is localized at about one or two monatomic layers of the interface between Co and Pd. On the other hand, a Ni film deposited on Pd was not strained from the beginning of the deposition. The relationship between the perpendicular magnetic anisotropy of Co/Pd and Ni/Pd compositionally modulated multilayer films and the strain of Co and Ni is discussed.