bluuebunny/crossref_metadata_2025 huggingface dataset:
Model calculation of the femtosecond carrier dynamics in Al0.48Ga0.52As
Authors: Rego Luís G. C. , Andrade Leandro H. F. , Cruz Carlos H. B.
Published: July 2002

We present a model calculation capable of investigating the dynamics of photoexcited carriers in the Al0.48Ga0.52As indirect gap semiconductor. Nearly resonant excitation at the Γ point produces low excess energy carriers, so that we use Boltzmann like equations and assume thermalized carrier distributions for each of the conduction valleys. Some aspects of the carrier dynamics are discussed and pump and probe measurements are compared to the calculated saturation bleaching, evidencing a very good agreement between theory and experiment. We obtain a value of 3.5 eV/Å for the DΓX deformation potential.
Measurements and analysis of Hall effect of a two dimensional electron gas in the close proximity of a superconducting YBa2Cu3O7−x film
Authors: Tseng M. Z. , Jiang W. N. , Hu E. L.
Published: July 2002

A direct integration of YBa2Cu3O7−x and a two dimensional electron gas Hall probe was made possible through the use of a MgO buffer layer. We demonstrate the use of this structure for the measurements of the magnetization hysteresis of a superconducting YBa2Cu3O7−x thin film, and we make an estimate of the sensitivity and resolution that can be achieved with this probe structure. The close proximity of the YBa2Cu3O7−x to the two dimensional electron gas (∼1700 Å) allows sensitive measurements of interactions between the two; more importantly, closer superconductor-semiconductor spacing can be achieved without severe compromise of the component material quality.
Spectral dependence of the absorption coefficient in III-V multiple quantum well structures from transmitted light intensity measurements at a single wavelength
Authors: Abdelkader Hatem H. , Lengyel Gabriel , Valliath George
Published: July 2002

We report the development of a useful and a rather simple experimental technique that can be used to determine the wavelength dependence of the absorption coefficient as a function of electric field in III-V compound based multiple quantum well (MQW) devices. In this technique, the absorption coefficient at some wavelength and applied electric field is extracted from measurement of the transmitted light intensity through the MQW device at a fixed wavelength as a function of the applied electric field. Results show good agreement with direct measurements of the absorption coefficient.
Propagation velocities of laser-produced plasmas from copper wire targets and water droplets
Authors: Song Kyo-Dong , Alexander Dennis R.
Published: July 2002

Experiments were performed to determine the plasma propagation velocities resulting from KrF laser irradiation of copper wire targets (75 μm diameter) and water droplets (75 μm diameter) at irradiance levels ranging from 25 to 150 GW/cm2. Plasma propagation velocities were measured using a streak camera system oriented orthogonally to the high-energy laser propagation axis. Plasma velocities were studied as a function of position in the focused beam. Results show that both the shape of the plasma formation and material removal from the copper wire are different and depend on whether the targets are focused or slightly defocused (≊0.5 mm movement in the beam axis). Plasma formation and its position relative to the target is an important factor in determining the practical focal point during high-energy laser interaction with materials. At irradiance of 100 GW/cm2, the air plasma has two weak-velocity components which propagate toward and away from the incident laser while a strong-velocity component propagates away from the laser beam as a detonation wave. Comparison of the measured breakdown velocities (in the range of 2.22–2.27 ×105 m/s) for air and the value calculated by the nonlinear breakdown wave theory at irradiance of 100 GW/cm2 showed a quantitative agreement within approximately 50 % while the linear theory and Gaussian pulse theory failed. The detonation wave velocities of plasma generated from water droplets and copper wire targets for different focused cases were measured and analyzed theoretically. The propagation velocities of laser-induced plasma for liquid droplets obtained by previous research are compared with current work.
Spectral characterization of a Sn soft x-ray plasma source
Authors: Cerjan C.
Published: July 2002

The radiative emission from a soft x-ray laser-produced plasma source is examined in detail for the particular case of a Sn solid target under moderate illumination conditions: 1010–1012 W/cm2. The prominent spectral features which contribute to the relatively large conversion efficiency in the range 124–155 Å are identified using extensive multi-configuration Dirac–Fock calculations. These data are used to generate synthetic spectra with plasma characteristics derived from a two-dimensional simulation of the ablating, radiating plasma. The comparison to the experimental results is generally favorable.
Magnetic properties of shock-compacted high-coercivity magnets with a nanometer-sized microstructure
Authors: Oda Hideaki , Hirai Hisako , Kondo Ken-ichi , Sato Takashi
Published: July 2002

High-coercivity magnets with a nanometer-sized microstructure were fabricated by the shock-compaction technique using an acicular iron-alloy powder as a starting material. The magnetic properties of these magnets depended on the initial magnetic property of the acicular powder even after shock compaction and were considered to be attributed to the shape anisotropy of acicular single-domain fine particles. The preferred orientation of the acicular particles by which the powder was precompacted in the external magnetic field affected final magnetic properties of the shock-compacted magnets. The best value of the maximum energy product (BHmax) obtained in this study was 23 kJ/m3 (2.9 MG Oe), which was comparable to that of the elongated single-domain magnet.
Radius tailoring of an electron beam using a fast rise-time focusing coil: Experiment and simulation
Authors: Weidman D. J. , Nguyen K. T. , Rhee M. J. , Schneider R. F. , Stark R. A.
Published: July 2002

An experiment demonstrating the production of a ‘‘radius-tailored beam,’’ which has a larger head and smaller tail for more stable propagation through gas, using a fast rise-time focusing coil is described. The results and their analyses for the radius-tailored case, the untailored case, and the ‘‘reverse-tailored’’ case are presented. A two-dimensional particle simulation of the experiment was performed for each case using the parameters of the experiment for the inputs and it produces results consistent with the experiment.
Infrared optical absorption of iridium silicide films on silicon
Authors: Chi M. , Mooney J.
Published: July 2002

Optical constants and absorption data of thin iridium silicide films on silicon substrates are presented for incident light between 4 and 11 μm. The data were obtained from reflection and transmission measurements made on samples with IrSi thicknesses from 8 to 350 Å. The optical constants n and k are those that best fit the reflection and transmission data over the wavelength and thickness ranges.
Structural-modification mechanism for polyimide-doped poly(tetrafluoroethylene) at subthreshold fluences using 248 nm radiation
Authors: Davis C. R. , Snyder R. W. , Egitto F. D. , D’Couto G. C. , Babu S. V.
Published: July 2002

Single-photon excimer laser ablation of neat poly(tetrafluoroethylene) (PTFE) is not observed at emissions in the ‘‘quartz’’ UV, i.e., from about 190–380 nm. However, it has been successfully demonstrated that, when the fluoropolymer is doped with small quantities of polyimide (PI), ablation in the quartz UV, e.g., at 248 and 308 nm and pulse widths of about 25 ns, is readily achieved. When PI-PTFE blends are exposed to subthreshold fluences, considerable changes in surface topography occur although clearly defined structures, e.g., pits, are not formed. Using photoacoustic infrared spectroscopy to evaluate surface and bulk chemical changes to blends exposed to subthreshold excimer laser fluences, <100 mJ/cm2, it is shown that PI (1) is distributed throughout the bulk and resides at the surface and (2) is selectively absorbing the high-energy photons and as a result being preferentially removed from the surface.
Influence of temperature gradients on partial pressures in a low-pressure chemical-vapor-deposition reactor
Authors: Oosterlaken T. G. M. , Leusink G. J. , Janssen G. C. A. M. , Radelaar S. , Kuijlaars K. J. , Kleijn C. R. , van den Akker H. E. A.
Published: July 2002

Measurements and calculations of the influence of temperature gradients on the partial pressures of the gas species in a cold-wall chemical-vapor-deposition reactor are presented. The experiments were performed at low pressures (300–500 Pa total pressure) and gas mixtures consisting of hydrogen, nitrogen, and tetrafluoromethane. The partial pressures were determined by Raman spectroscopy. The Soret effect (or thermal diffusion) has a large influence on the partial pressures of heavy gases in the vicinity of the heated wafer. In some cases a decrease in partial pressure of 20% compared to the inlet partial pressures was observed. Numerical calculations were performed to predict the behavior of the gas mixture. For mixtures under investigation the gas temperatures as well as the changes in partial pressures due to the Soret effect were predicted correctly.
Chemical structure and reactivity of a silicon single crystal surface fluorinated by xenon fluoride
Authors: Takahagi T. , Ishitani A. , Wakao S.
Published: July 2002

A reconstructed Si single crystal surface was fluorinated using xenon fluoride. All dangling bonds on the 2×1 reconstructed surface of Si(100) were terminated by fluorine atoms, and Si—F bonds were generated. However, dimer bonds remained after the treatment. This is different from the situation where dimer bonds are easily terminated by hydrogen atoms in a hydrogenation treatment. A bulky Si—F group is considered to sterically hinder fluorine termination of a dimer bond. Also, only an adatom bond seems to be terminated by a fluorine atom in the fluorination treatment of a Si(111) 7×7 reconstructed surface. When the fluorinated Si(100) surface was exposed to the atmosphere, dimer bonds were observed to be oxidized immediately. Also following the reaction, the backbond of the Si—F group is oxidized and the S—F bond itself was hydrolyzed. The fluorinated surface, which is chemically active, differs from the hydrogen-passivated surface.
600 GHz resonant mode in a parallel array of Josephson tunnel junctions connected by superconducting microstrip lines
Authors: Kaplunenko V. K. , Larsen Britt H. , Mygind J. , Pedersen N. F.
Published: July 2002

The high frequency properties of the one-dimensional transmission line consisting of a parallel array of resistively shunted Josephson tunnel junctions have been studied in the limit of relatively low damping where this nonlinear system exhibits new and interesting phenomena. Here we report on experimental and numerical investigations of a resonant step observed at a voltage corresponding to 600 GHz in the dc current-voltage characteristic of a parallel array of 20 identical small NbAl2O3Nb Josephson junctions interconnected by short sections of superconducting microstrip line. The junctions are mutually phase locked due to collective interaction with the line sections excited close to the half wavelength resonance. The phase locking range can be adjusted by means of an external dc magnetic field and the step size varies periodically with the magnetic field. The largest step corresponds to a superconducting phase difference of π between neighboring junctions.
A laterally tunable quantum dot transistor
Authors: Chang H. , Grundbacher R. , Jovanovic D. , Leburton J.-P. , Adesida I.
Published: July 2002

A laterally tunable quantum dot transistor has been fabricated on a modulation-doped AlGaAs/GaAs heterostructure. The transistor consists of a singly gated quantum wire in which a quantum dot region is locally formed beneath the gate under the influence of an applied potential. Quantum confinement in this device is realized through structural and electrostatic means. The discrete energy levels in the quantum dot are tunable using the single gate control and the transport path is through a 1D-0D-1D constriction in the device. Reproducible resonant conductance peaks resulting from resonant transmission of electrons through zero-dimensional states from and to adjacent quantum wires are observed at 4.2 K.
Study of the oxygen-sensitive behavior of yellow lead oxide by defect chemistry
Authors: Ma Ying-Ren
Published: July 2002

The dependence of electrical conductivity on ambient oxygen partial pressure of yellow lead oxide ceramics is investigated. A conduction type transition is observed for pure PbO as the oxygen partial pressure varies through a certain value that corresponds to the minimum conductivity measured, and the conductivity is found to vary with −1/4 and +1/4 power of the oxygen partial pressure in the n region and the p region, respectively. It is also found that the conduction transition point can be moved to lower or higher oxygen partial pressures with acceptor or donor impurities doped into PbO. A point defect model of doubly charged oxygen vacancies with concentration affected by impurities is presented for PbO in the investigation and it is demonstrated that the theory is in good agreement with the experimental results.
Sequential tunneling in GaAs/AlGaAs multiquantum well structures grown by molecular beam epitaxy
Authors: Han Z. Y. , Yoon S. F. , Radhakrishnan K.
Published: July 2002

The sequential tunneling of electrons through Schottky diodes with GaAs/AlGaAs multiple quantum wells (MQWs) is reported. Two series of tunneling structures were observed in low-temperature current-voltage (I-V) characteristics in 200-Å-period MQWs. The oscillations in the conductance (G) were found to be persistent from 5 K up to 200 K. S-shape features in the current-voltage characteristics similar to those observed from double-barrier resonant tunneling structures were observed at low temperature in the MQW structures. The electron tunneling process through the MQW structures is discussed in terms of the creation and propagation of high-field domains as a function of the applied bias.
Electrical characterization of electron beam evaporated indium tin oxide/indium phosphide junctions
Authors: Manivannan P. , Subrahmanyam A.
Published: July 2002

The electrical properties of Indium tin oxide(ITO)/p-indium phosphide (InP) junctions prepared at different temperatures by reactive electron beam evaporation technique have been studied. A maximum of 10.0% photo conversion efficiency under 100 mW cm−2 illumination (without front metal grid and antireflection coating) has been observed. Analyses of the results indicate an interfacial oxide layer consisting of indium oxide and indium orthophosphate and the ITO/p-InP junction correspond to the semiconductor-insulator-semiconductor SIS model. An attempt has been made to understand the nature of the interfacial layer.
Characterization of complexities in Czochralski crystal growth by nonlinear forecasting
Authors: Miyano Takaya , Morita Hiroshi , Shintani Akira , Kanda Tadashi , Hourai Masataka
Published: July 2002

How the flow of the silicon melt affects Czochralski crystal growth in terms of nonlinear forecasts about the time series of striations of as-grown crystals and the melt temperature fluctuations beneath the crystal is investigated. The melt exhibits nonstationary self-affine random motion added to regular motion. The random contribution becomes dominant toward the center of the crucible. Such dynamical properties of the melt are found to be embedded into the striations.
Surface modification of YBa2Cu3Oy thin films with a scanning tunneling microscope
Authors: Chen Sufen , Wang L. M. , Jian W. B. , Wang S. Y. , Yang H. C. , Horng H. E.
Published: July 2002

We report a systematic modification of thin YBa2Cu3Oy films with a scanning tunneling microscope. The samples include YBa2Cu3Oy films on MgO(001) and YBa2Cu3Oy/PrBa2Cu3Oy bilayer films on SrTiO3 (001). The bias voltage of the tip was kept negative and varied from −600 to −1200 mV. The tip was operated in a constant tunneling current mode with tunneling current 0.3–0.7 nA and scanning rate 1 Hz. Modification of the surface begins with nucleation of holes, or at the edge of a dislocation or protrusion on the surface, and is achieved by successively scanning a fixed area. We show the image obtained from an atomic force microscope which can help us look into the mechanism of the modification.
Effect of oxygen aggregation processes on the recombining activity of 60° dislocations in Czochralski grown silicon
Authors: Acerboni S. , Pizzini S. , Binetti S. , Acciarri M. , Pichaud B.
Published: July 2002

The interaction of dislocations with light elements like oxygen and carbon presents a variety of aspects which are of basic interest for the understanding of gettering processes, as well as for a deeper knowledge of the electrical and optical properties of dislocations. We report the results of a systematic investigation of the influence of the dislocations on the segregation of oxygen and of the effect of oxygen segregation on their electrical activities. The experiments were carried out on p-type Czochralski silicon, in the 700–1100 °C temperature range. It has been shown that not only a direct oxygen-dislocation interaction occurs, but also that a competition between homogeneous and heterogeneous segregation processes occurs, which influences the overall electrical properties of the samples as well.
Effect of conjugated bonds on the charging of insulating polymers
Authors: Gong H. , Chooi K. M. , Ong C. K.
Published: July 2002

Different densities of conjugated bonds, which affect the polarization states necessary in forming polarons, are generated by stretching polymers at various rates. An investigation of the charging of the stretched high-impact polystyrene and high-density polyethylene by using a scanning electron microscope technique is made. The experimental results reveal that polymer charging is related to electron trapping as polarons at the polarized sites of the polymeric chains, confirming the recent interpretation of charging behavior in insulating materials.
Surface acoustic wave: Acoustogyric effect
Authors: Walikainen D. , Levy M.
Published: July 2002

An asymmetry is observed in the hysteretic surface acoustic wave (SAW) attenuation curve that is obtained when a magnetic field is applied in the plane of a magnetoelastic film which is deposited on the surface of the substrate carrying the SAW. We propose that a SAW acoustogyric effect may account for this asymmetry.
Characteristics of pseudomorphic AlGaAs/InxGa1−xAs (0≤x≤0.25) doped-channel field-effect transistors
Authors: Yang Ming-Ta , Chan Yi-Jen , Chen Chun-Hung , Chyi Jen-Inn , Lin Ray-Ming , Shieh Jia-Lin
Published: July 2002

The pseudomorphic properties of doped-channel field-effect transistors have been thoroughly investigated based on AlGaAs/InxGa1−xAs (0≤x≤0.25) heterostructures with different In contents. Through various analytical schemes and device characterization, we observed that by introducing a 150 Å strained In0.15Ga0.85As channel we can enhance device performance; however, this strained channel is not stable after high-temperature heat treatment. By further increasing the In content up to x=0.25, the device performance started to degrade, which is associated with strain relaxation in this highly strained channel.
Role of structural relaxations in the fracture of vitreous silica
Authors: Michalske Terry A. , Smith William L. , Houston Jack E.
Published: July 2002

The double cleavage drilled compression fracture mechanics specimen was used to obtain crack velocity data for vitreous silica in vacuum conditions at temperatures of 100 and 300 K. We observed stable crack propagation over a velocity range between 10−9 and 10−2 m/s. The measured fracture rate dependence with applied stress was modeled using a global energy balance approach that incorporated energy losses due to the anelastic response of the material in the stress field of the propagating crack. Measured mechanical loss data for vitreous silica were incorporated into an anelastic fracture model to produce qualitative agreement with the measured fracture behavior. These results indicate that the anelastic response of silicate glasses can contribute significantly to the measured fracture energy. This interpretation of silicate glass fracture suggests a new approach to design glass compositions for improved mechanical performance and provides a more complete picture of the atomic scale processes that control the macroscopic fracture energy of amorphous materials.
Reconstruction controlled dopant incorporation and ‘‘coercion’’ effects in molecular-beam epitaxial germanium grown on gallium arsenide
Authors: Wood C. E. C. , Tabatabaei S. A. , Mimomye L. V. , Grober L. M.
Published: July 2002

Incorporation of arsenic in molecular-beam epitaxial germanium depends upon the specific surface reconstruction. When both gallium and arsenic are incident on the surface during growth, incorporation of both species follow each other in magnitude and temperature dependence. This similarity of incorporation behavior is explained by ‘‘coercion’’ effects related to chemisorbed arsenic-induced surface charge separation.
Büttiker–Landauer traversal times in the radial direction of cylindrical single and double barriers
Authors: Ping Er-Xuan
Published: July 2002

Büttiker–Landauer traversal times in the radial direction of single and double barriers with cylindrical geometry are studied by modulating the incident wave and the barriers, respectively. The traversal times obtained by modulating the incident wave oscillate as a function of the incident wave energy. The oscillation is negligibly small in the traversal times obtained by modulating the barriers. The coherence of particle waves (Hankel functions) in cylindrical geometry is responsible for the oscillation.
Raman studies of plasmon modes in a drifting two-dimensional electron gas
Authors: Ó Súilleabháin L. C. , Hughes H. P. , Churchill A. C. , Ritchie D. A. , Grimshaw M. P. , Jones G. A. C.
Published: July 2002

We present the first Raman-scattering studies of the behavior of the intrasubband plasmon mode of a two-dimensional electron gas which is undergoing lateral drift in an applied electric field. The data clearly show the expected Doppler shifts of the modes traveling up- and downstream, together with the expected dependence on the wave vector, but at higher drift velocities, the expected linear shift is distorted because of electron heating effects.
Radio-frequency bias effects on SiO2 films deposited by distributed electron cyclotron resonance plasma enhanced chemical vapor deposition
Authors: Jiang N. , Agius B. , Hugon M. C. , Olivier J. , Puech M.
Published: July 2002

The effects of radio-frequency (rf) bias on the chemical, physical and electrical properties of SiO2 films deposited by distributed electron cyclotron resonance plasma enhanced chemical vapor deposition have been investigated. The rf bias power densities used here are relatively low (≤0.3 W/cm2) compared to that used by other researchers, but their influences on deposition processes and on film properties are apparent: the deposition rate decreases with increasing rf bias; the in situ relaxation process of the oxide and the observed surface polish effects have been found to be caused by ion bombardment. The most important effects of the rf bias are on the electrical properties of the deposited films. In particular, a SiO2 film without net fixed charges may be deposited with a low rf bias. It is also shown that the increase in interface trap density with rf bias power is not caused by ion bombardment but to the electron injection from the Si substrate towards the Si/SiO2 interface.
Energetics of dislocation dipoles in capped epitaxially strained layers
Authors: Atkinson A. , Jain S. C.
Published: July 2002

Most device structures based on strained epitaxial layers are capped by a second, unstrained layer to increase the mechanical stability of the structure. In order to calculate the energies of these structures it is necessary to synthesize the total energy from the energies of the line defects they contain (interfacial dislocations and dislocation dipoles). The self energies and interaction energies of dislocations and dipoles are calculated and their behavior examined as a function of their spacing and the thicknesses of the strained and capping layers. The results confirm the observations that capped strained layers are more stable than uncapped ones (of the same strained layer thickness) and that capping layers do not need to be thicker than approximately three times the strained layer thickness. An expression is deduced for the total energy of finite, nonuniform arrays of dipoles in capped layers and, by analogy with a similar earlier expression for dislocations in uncapped layers, it is concluded that the effect of a nonuniformity in the dipole spacing will be to increase the energy of the system compared with that of a uniform array having the same average spacing. The results in this paper can be used to assess the stability of devices and their rate of degradation by strain relaxation.
Capability of plasma-enhanced chemical-vapor-deposited SiO2 films as diffusion barriers on InGaAs
Authors: Ambrée P. , Wandel K.
Published: July 2002

Results on the diffusion behavior of InGaAs host atoms as well as Zn and Cd atoms in plasma-enhanced chemical-vapor-deposited SiO2 layers are reported. The group-III elements In and Ga diffuse very fast in SiO2 at annealing temperatures from 400 to 600 °C. A mobilization of these species by OH groups is proposed. An As diffusion can only be observed in the case of semiconductor samples etched in a H2SO4 solution before SiO2 deposition. An As-rich interface layer produced by this etchant is assumed to act as exhausting source for mobile As-O complexes. The InGaAs host atoms could be detected up to a concentration level of about 1018 cm−3 by secondary-ion-mass spectroscopy in the SiO2 films after annealing. Their effective diffusion coefficients were estimated to be in the order of 10−15–10−13 cm2/s in the investigated temperature range. Activation energies of about 0.6 eV (In), 0.3 eV (Ga), and 0.8 eV (As) could be determined. The In/Ga out-diffusion was accompanied by the occurrence of two new peaks in the photoluminescence spectra of the InGaAs layers. Binding energies of about 11 and 18 meV were obtained, respectively. Possible crystal defects as well as results of numerical simulations on base of simple diffusion models are discussed. The capability of plasma-enhanced chemical-vapor-deposited SiO2 layers as a diffusion mask during Zn as well as Cd acceptor diffusion at 500 and 600 °C was proved.
Barrier property dependence of band nonparabolicity effects in a quantum well
Authors: Kim B. W. , Charlson E. M.
Published: July 2002

The barrier material property dependence of band nonparabolicity effects on the discrete energy levels of a GaAs/AlxGa1−xAs quantum-well structure is reported. Results from the modified resonant tunneling method show clear barrier property dependence. The model includes the position-dependent nonparabolicity parameters from which energy-dependent effective mass is calculated. This model can be applied to any shape potential barrier. Both a rectangular quantum well and a parabolic quantum well were analyzed and these results are compared to those from previously reported calculations.
Heat-treatment effects on porous silicon
Authors: Sabet-Dariani R. , Haneman D.
Published: July 2002

High-resolution scanning electron microscopy of porous silicon (PS) subjected to various annealing treatments in vacuum has revealed a spheroidal particlelike structure that only becomes clearly apparent after treatment at about 250 °C. This is consistent with recent suggestions that any quantum effects arise from particles. It is deduced that the particles have a coating that is reduced or removed on heating. The concomitant effects on the current-voltage curves of PS, caused by annealing and also by prolonged air exposure, have been measured, and are consistent with the interpretation.
Increasing the yield of Ar(3P2,0) and Ne(3P2,0) effusing from a glow discharge by using a longitudinal magnetic field
Authors: Barrios A. , Ramos G. B. , Hardy K. A. , Sheldon J. W.
Published: July 2002

It is demonstrated that a longitudinal magnetic field applied to a low-pressure, low-voltage, hot cathode glow discharge will substantially increase the intensity of metastable atoms effusing from a central anode slit. For an argon discharge there is a fourfold increase in the effusing Ar(3P2,0) atom intensity and for neon there is a 14-fold increase in Ne(3P2,0) intensity.
Background formation in the low-energy region in Auger electron spectroscopy
Authors: Ding Z.-J. , Shimizu R. , Goto K.
Published: July 2002

Monte Carlo simulations of cascade processes of secondary electron generation in Si, Cu, and Au have been performed to study the energy distribution of backscattered electrons in the low-energy region. Calculation results show that the characteristic hump appearing in the energy distribution above Si-LVV Auger peak in the EN(E) spectrum can be well described by those electrons ejected from the L shell followed by the cascade process. The shape of EN(E) spectra in the low-energy region is dominated by the directly produced secondary electrons and, hence, strongly correlated with the excitation spectrum Im[−1/ε(ω)] for electron generation.
Undoped semi-insulating GaAs epitaxial layers and their characterization
Authors: Imaizumi T. , Okazaki H. , Yamamoto H. , Oda O.
Published: July 2002

Wafer annealing was applied to undoped conductive GaAs epitaxial layers grown by the chloride chemical vapor deposition method in order to realize semi-insulating GaAs epitaxial layers. It was found that, by wafer annealing at temperatures higher than 950 °C, semi-insulating epitaxial layers with a resistivity higher than 107 Ω cm and a mobility higher than 5000 cm2/V s can be obtained. The material quality has been evaluated by Hall measurement, isothermal capacitance transient spectroscopy, deep level transient spectroscopy, scanning photoluminescence, AB etching, ion implantation, and activation efficiency measurement. It was concluded that the semi-insulating behavior of undoped GaAs epitaxial layers is due to the increase of the EL2 concentration to the level of 5×1015 cm−3 realized by wafer annealing. The present material does not show any cell structures which are inherent to bulk GaAs materials. It was found to be of the best quality ever reported from the viewpoint of various material characterizations.
The ac-Josephson effect above the gap frequency
Authors: de Lange G. , Kuipers J. J. , Klapwijk T. M. , Panhuyzen R. A. , van de Stadt H. , de Graauw M. W. M.
Published: July 2002

The rf-power dependence of the ac-Josephson steps is measured at 720 GHz, using small area Nb tunnel junctions. This frequency is well above the gap frequency of Nb. The junction is placed in a waveguide, and connected to a superconducting stripline, which effectively tunes out the junction capacitance and facilitates the coupling of the radiation to the junction. We observe three Josephson steps, and the first step crosses the zero current axis over a considerable range of rf-power. This indicates the possible application of THz Josephson steps in voltage standards. The data are compared to the theory and we find clear evidence for the predicted intrinsic roll-off of the Josephson current amplitude above the gap frequency.
Band-gap renormalization effects on 980 nm strained-layer InGaAs/AlGaAs quantum-well lasers
Authors: Ahn Doyeol , Choi Sun C.
Published: July 2002

We calculate the optical gain and threshold current density of strained-layer InGaAs/AlGaAs quantum-well lasers with the band-gap renormalization effects taken into account. It is found that the band-gap renormalization shifts the lasing wavelength as much as 30 nm which is much larger than the allowed bandwidths, ±6 nm, of 980 nm Er-doped optical fiber amplifiers. We also try to optimize the graded-index separate-confinement-heterostructure optical waveguide layer in order to maximize the optical confinement factor and to reduce its fabrication dependence. The threshold current densities are calculated for the optimized structures for which the gain peak positions are predicted to be within the allowed bandwidths of 980 nm.
Instability of an expanding charged-particle beam
Authors: Book David L.
Published: July 2002

It is shown that a uniform collisionless charged-particle beam expanding under the influence of electrostatic and internal pressure forces is linearly unstable. The instability, which tends to cause spreading and change the radial profile of the beam, implies a limit on the achievable focusing even when the lens system corrects for expansion.
Fourier extrapolation of magnetic fields measured by electron-beam tomography
Authors: Shinada Hiroyuki , Suzuki Yoshio
Published: July 2002

A method for calculating the magnetic field on a plane near a magnetic recording head is presented. In this method, the magnetic field is measured at a plane farther from the head. The method involves taking the Fourier transform of the measured field, enhancing the large wave-number region of the spectrum, and effecting the reverse Fourier transform of the modified spectrum. It is estimated that a field 0.1 μm away from the head can be calculated from a field measured 0.3 μm from it, with a maximum error of less than 0.35%. A field 0.5 μm from a thin-film head calculated from the measured field at 0.7 μm has been compared with actual measurements of the field at 0.5 μm and found to agree within 8%.
Damping capacity of layered materials
Authors: Brandner E. D. , Munier Alain , Zhu Jian-Ming , Averill Bruce A. , Shivaram Bellave S.
Published: July 2002

Extensive measurements of the internal friction in a variety of layered materials such as graphite, metal dichalcogenides (MoS2, TiS2, NbSe2, TaS2, TaSe2, and TaTe2), and montmorillonite clays are reported. The internal friction of these layered materials exhibits features similar to those observed in graphite. In particular, a high temperature internal friction plateau develops at a temperature that is characteristic of the material. The physical mechanism for this plateau is discussed.
Influence of hydrogen on the magnetic properties of Terfenol-D
Authors: Ruiz de Angulo L. , Abell J. S. , Harris I. R.
Published: July 2002

The reactions with hydrogen of the C15-type Laves phase Tb0.27Dy0.73Fe2 have been studied by differential thermal analysis and the influence of this hydrogenation upon the magnetic properties was observed with a Curie–Faraday balance and a vibrating sample magnetometer. It has been found that a slightly rare earth rich composition allows the hydrogenation to take place at room temperature. Two endothermic peaks observed on heating under 1.2 bar of hydrogen are attributed to hydrogen desorption, due possibly to the form of the pressure isotherms. At a pressure of 1.2 bar, the amorphization and disproportionation peaks are combined as a single peak, but a higher pressures these two phenomena separate into two peaks. After the first absorption/desorption cycle, the hydrogenation occurs at temperatures as low as −75 °C, and is highly exothermic and very rapid in nature. The magnetic susceptibility of the compound suffers a dramatic drop when it is hydrided, and the magnetization at 1100 kA m−1 falls from 77.5 J T−1 kg−1 to 9.2 J T−1 kg−1. When heated in hydrogen, a peak in susceptibility was observed at about the same temperatures as those of the amorphization/disproportionation reactions. The Curie point of the amorphous hydrided material was found to be around 68 °C, with a magnetization at room temperature and 1100 kA m−1 of 46 J T−1 kg−1. The fully disproportionated material had a magnetization of 82 J T− kg−1 at 1100 kA m−1, which was related to the proportion of α-Fe formed during the disproportionation reaction.
Evaluation of the far-infrared dispersion properties of γ-irradiated polymer films
Authors: El-Tonsy M. M. , Shaban A. M.
Published: July 2002

The optical dispersion of polaren-R (polyethylene base) films was determined from the transmission spectrum in the far-infrared region of the spectrum. The refractive indices of polaren-R films, with different macromolecular structures, were calculated from the observed interference fringes in transmission due to the internal multi-reflections. The refractive indices were calculated in the wavelength range 14–34 μm. The macromolecular structure of polaren-R films was varied by γ-ray irradiation with different doses. The variations in the polymer structure were detected by the measurement of density variations using a vibrational technique at resonance conditions. A new physical quantity describing the degree of anisotropy of a polymer has been suggested. This quantity is the magnitude of the shift in the polymer optical dispersion from the normal dispersion of the isotropic media. The authors also suggest a characteristic frequency for the lattice vibration which was found to be dependent on the lattice structure.
Backscattering and electron microscopy study of mega-electron volt gold implantation into silicon
Authors: Alford T. L. , Theodore N. David
Published: July 2002

Rutherford backscattering spectrometry and cross-section transmission electron microscopy have been used to study implantation of MeV Au+ ions into silicon. Measured range (Rp) and straggle (ΔRp) values for MeV Au+ implanted silicon are found to be consistently larger than values predicted by trim simulations. The magnitude of the discrepancies are such that the differences cannot be attributed to implantation effects alone. We conclude that the trim computer program does not accurately predict Rp and ΔRp values for MeV Au+ implantation into crystalline Si. Experimental results show that for low-current low-energy implants a single Gaussian Au profile is achieved. Low-power implants produce a single band of damage consisting of simple point defects. High-current high-energy implants lead to the creation of more complex defect structures such as dislocation networks; these arise as a result of dynamic beam recrystallization. Multiple layers of precipitation are observed in silicon implanted with MeV Au+ ions in those samples where dynamic recrystallization occurred. Precipitation occurs as a result of the local Au concentration exceeding the solid-solubility during beam-induced recrystallization. Different mechanisms operate in conjunction to cause anomalous Au motion which results in formation of multiple precipitate layers. A first mechanism has the implanted Au segregating into a densely defected region; when the concentration exceeds the local solid solubility Au precipitates out of the matrix. A second mechanism has motion of Au along dislocations in a network; the diffusing Au reaches a dislocation-node where it exceeds the local threshold for precipitation and the Au therefore precipitates. Enhanced Au diffusion is dependent upon the magnitude of dynamic recrystallization occurring during the implantation.
Two types of additional maxima in magnetization curves of layered superconductors (abstract)
Authors: Bugoslavsky Yu. V. , Minakov A. A.
Published: July 2002

We report on magnetic measurements of LaSrCuO and YBaCuO single crystals in order to clear up their anomalous behavior and to attribute it to the crystals’ anisotropic properties. Magnetization curves were obtained by means of a vibrating sample magnetometer at temperatures from 4.2 to 90 K. The magnetic field up to 8 kOe was applied at various angles with respect to the crystal c axis. Our main findings are that there are two physically different types of additional maxima, as ruled out from the evolution of magnetization curves with varying the tilt angle of the external field. One type of the maxima is attributed to anisotropic flux penetration into a sample and flux-line-lattice rotation with increasing the field. We provide an experimental justification of this explanation. The second type of maxima is related to an anomalous increase of the critical current in magnetic field which is often referred to as the ‘‘fishtail’’ phenomenon. It was found that the existence of the fishtail is closely related to the anisotropy of flux pinning. On this basis a qualitative model was developed to describe this phenomenon. The model takes into account vortex interaction with stretched pinning sites and the vortex bending due to stray fields in the sample. The formation of staircase flux lines is also assumed, as specific for layered superconductors. The qualitative explanation of the fishtail is as follows. At low fields the flux lines are strongly curved. When increasing the field, they straighten and fit the linear pinning sites better, which gives rise to more effective flux line fixing. The proposed model allows to describe the angular and temperature dependencies of the additional maximum position.
Magnetic field dependence of T
c of EuB6 (abstract)
Authors: Lacerda A. , Graf T. , Sarrao J. L. , Hundley M. F. , Mandrus D. , Thompson J. D. , Fisk Z.
Published: July 2002

We report measurements of resistivity and transverse magnetoresistance (B⊥I) on a single crystalline sample of EuB6 in magnetic fields to 18 T in a temperature range from 4 to 100 K. The ferromagnetic transition (Tc) determined by resistivity versus temperature at constant field varies from 10 to 25 K at zero and 4 T, respectively. The transition is completely suppressed at 8 T. A very large and negative tranverse magnetoresistance is observed in the entire temperature range investigated: −5% at 4 K and −20% at 100 K at 18 T. Finally, the effect of high applied magnetic field on the transport properties is investigated in the temperature range below Tc.
Simultaneous surface and bulk magnetic properties investigations by using simultaneous gamma, x-rays and conversion electron Mössbauer spectroscopy: Method and experimental results
Authors: Kamzin A. S. , Grigor’ev L. A. , Ioffe A. F.
Published: July 2002

The surface and bulk phase transitions in a microscopic antiferromagnetic Fe3BO6 crystal was investigated by means of new method: Simultaneous gamma rays, x-rays, and electron Mössbauer spectroscopy (A. S. Kamzin, V. P. Rusakov, and L. A. Grigor’ev, in Proceedings of the International Conference ‘‘Physics of transition metals,’’ USSR-1988, Part 2, p. 271; A. S. Kamzin, and L. A. Grigor’ev, Pisma Zh. Tekh. Fiz. 16, 38 (1990) [Sov. Tech. Fiz. Lett. 16, 417 (1990)]), which makes it possible to study simultaneously the surface and volume properties of a bulk crystal.
New possibilities offered by high resolution Fourier transform spectroscopy in studying magnetic phase transitions
Authors: Popova M. N.
Published: July 2002

High resolution optical absorption spectra of an intrinsic R3+ ion or Er3+ probe were measured in cuprates R2Cu2O5 and R2BaCuO5 related to high Tc superconductors of the 123 type and in low-dimensional nickelates R2BaNiO5. We have registered for the first time phase transitions, some of them not detectable by other methods, in R2BaCuO5 with R=Y, Lu, Nd, Dy, Yb, and Lu, in R2BaNiO5, with R=Nd, Sm, Eu, Dy, Ho, Tm, and Lu. Coexistence of two different magnetic phases in Y2BaCuO5 and Lu2BaCuO5 in a broad temperature range was demonstrated. Fine splitting of the low temperature magnetic phase transition, possibly caused by the interaction of critical fluctuations, has been observed in Dy2BaCuO5. Low-dimensional correlations at temperatures higher than the temperature of three-dimensional (3-D) magnetic ordering have been analyzed in quasi-2D compounds R2Cu2O5 and quasi-1-D R2BaCuO5. We developed the method of the Er3+ probe in a series of isostructural f–d compounds to select the right magnetic structure of an ordered d subsystem from several structures that fitted neutron scattering data equally well.
How does mean-field theory work in magnetic multilayer systems?
Authors: Hu Xiao , Kawazoe Yoshiyuki
Published: July 2002

Phase transition and critical phenomena in magnetic multilayer systems are studied in terms of Ginzburg–Landau mean-field theory. Detailed calculations are carried out for a system consisting of a layer of finite thickness coupled to a semi-infinite bulk magnet. Correlation functions are derived and the shift in critical point with the overlayer thickness and coupling between the neighboring layer is evaluated. A new critical exponent ν̃=1 is derived for the divergence of the effective extrapolation length at the interface with temperature and/or layer thickness.
Magnetic phase transitions in CsEr(MoO4)2 and KDy(MoO4)2-chain-layered Ising compounds (abstract)
Authors: Khats’ko E. N. , Cherny A. S.
Published: July 2002

The crystals investigated belong to the family of the orthorhombic layered rare-earth double molybdates. These crystals have aroused interest because of the diversity of structural and magnetic phase transitions, which are due to the crystalline structure and the electron energy spectrum of rare-earth ions. These ions are distinguished for their Ising behavior. The magnetization and susceptibility measurements were performed at temperatures between 0.5 and 300 K in magnetic fields up to 0.4 T along three principal magnetic axes. The 3D magnetic ordering is found to occur at Tc=0.84 K for CsEr(MoO4)2 and Tc=1.1 K for KDy(MoO4)2. The magnetic properties are strongly anisotropic both above and below Tc. In the magnetic ordered state spin-orientation phase transitions were found in both of these compounds. For CsEr(MoO4)2 a typical metamagnetic transition was observed along one magnetic field direction (b). There is a more intricate situation in the case of KDy(MoO4)2. The spin-orientation phase transition has a rather complex character: it is anitferromagnetic along the a axis, and ferromagnetic along the b and c axes. The magnetic structures of these compounds were determined. In the case of CsEr(MoO4)2 a collinear two-sublattice structure of antiferromagnetically ordered ferromagnetic chains is realized. For KDy(MoO4)2, the magnetic structure is defined as a complicated noncollinear four sublattice structure with magnetic moments rotated 35° in the ac plane and 10° in the bc plane.
Schematic frictional model for interacting vortices in an isotropic superconducting plate (abstract)
Authors: Kouvel J. S. , Park S. J.
Published: July 2002

In the simple model proposed, repulsive intervortex forces are balanced by containing forces produced by the external field (H) and by frictional forces representing the effects of pinning on displaced vortices. For the field-cooled (FC) state, whose vortex density is presumably uniform, the empirical fact that the average flux density (B̄) in nearly equal to H yields an operational inverse-square dependence of the intervortex force on the intervortex spacing. For both the FC and zero-field-cooled (ZFC) states, expressions are derived for B̄ vs H (including the remanences at H=0) and for the profiles of B across the sample thickness. Calculations of these properties are compared with experiment and with the macroscopically related critical-state model, revealing again that the pinning forces are strongly dependent on H. The frictional interacting-vortex model is also used in deriving the critical current as a transport property of the FC and ZFC states.
Ruderman–Kittel–Kasuya–Yosida polarizations in inhomogeneous media
Authors: Baltensperger W. , Helman J. S.
Published: July 2002

An analytical treatment of the Ruderman–Kittel–Kasuya–Yosida polarization in inhomogeneous media is possible for some simple models. This is exemplified with the half-spaces and slabs in one and three dimensions, and with an especially chosen potential barrier in one dimension.