bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic CdSe-based quantum dots grown on Mn-passivated ZnSe
Authors: Titova L. V. , Furdyna J. K. , Dobrowolska M. , Lee S. , Topuria T. , Moeck P. , Browning N. D.
Published: July 2002

In this letter we describe the properties of self-assembled CdSe quantum dots (QDs) grown on Mn-passivated ZnSe buffers. We show that the Mn deposited on the ZnSe surface during the passivation process acts as a nucleating seed for self-assembled QD formation. For moderate amounts of Mn deposition, the dots grown in this way show a significant improvement in size uniformity compared to CdSe dots grown on ZnSe without Mn passivation. Using photoluminescence, we also show that the dots exhibit large Zeeman splitting, indicating that this growth method is suitable for fabricating magnetic QDs that exhibit strong spin polarization effects.
Strong band edge luminescence from InN films grown on Si substrates by electron cyclotron resonance-assisted molecular beam epitaxy
Authors: Yodo Tokuo , Yona Hiroaki , Ando Hironori , Nosei Daiki , Harada Yoshiyuki
Published: July 2002

We observed strong band edge luminescence at 8.5–200 K from 200–880 nm thick InN films grown on 10 nm thick InN buffer layers on Si(001) and Si(111) substrates by electron cyclotron resonance-assisted molecular beam epitaxy. The InN film on the Si(001) substrate exhibited strong band edge photoluminescence (PL) emission at 1.814 eV at 8.5 K, tentatively assigned as donor to acceptor pair [DAP (α-InN)] emission from wurtzite-InN (α-InN) crystal grains, while those on Si(111) showed other stronger band edge PL emissions at 1.880, 2.081 and 2.156 eV, tentatively assigned as donor bound exciton [D0X(α-InN)] from α-InN grains, DAP (β-InN) and D0X (β-InN) emissions from zinc blende-InN (β-InN) grains, respectively.
Lateral copolymer domain patterning through thermal gradients
Authors: Pickett Galen T.
Published: July 2002

Lateral patterns in thin films of microsegregated copolymers can be controlled through the application of a static thermal gradient. Patterns are promoted lateral to the imposed gradient. Calculations in a lattice self-consistent field method are presented and compare favorably with predictions of a simple scaling theory.
Extending the polarizable continuum model to effective ab initio pair potentials in multicomponent solutions: A test on calcium–water and calcium–ammonia potentials
Authors: Floris F. M. , Martı́nez José M. , Tomasi J.
Published: July 2002

The use of the polarizable continuum model to develop ab initio effective pair potentials is extended to multicomponent solutions. The methodology takes into account nonadditivity effects on pair interactions computing wave functions perturbed by the solvent. Ca2+–water and Ca2+–ammonia potentials suitable for aqueous ammonia solutions are presented. These effective ab initio pair potentials present smaller binding energies with respect to strictly ab initio two-body potentials. The reduction is higher in Ca2+–ammonia (28%) than in Ca2+–water (22%) and brings to a small gap the difference between the binding energies of the two ligands with Ca2+ when solvent effects are considered. As a first test, metal-ligand clusters of different size and composition have been studied. The comparison with restricted Hartree–Fock ab initio calculations shows good agreement for the largest clusters considered. Results confirm that the presented methodology, based on the polarizable continuum model, describes in a proper way the interactions in the condensed phase, where the ion completes its coordination sphere. The cluster results also show that ammonia can displace water in the first ion coordination with a tendency to change the coordination number from 8 to 9 when the ion is fully surrounded by the former, the ninth ammonia molecule being positioned in an intermediate situation between the first and the second coordination shells.
Gyrokinetic particle-in-cell simulation of Alfvénic ion-temperature-gradient modes in tokamak plasma
Authors: Zhao Gang , Chen Liu
Published: July 2002

A linearized gyrokinetic particle-in-cell code is developed to study the excitations of electromagnetic instabilities due to finite ion temperature gradients in tokamak plasmas. The code employs the δf scheme to solve the gyrokinetic equations for ion dynamics; while electrons are modeled as massless fluid. The instability threshold condition of the Alfvénic ion-temperature-gradient (ITG) mode as well as the finite β stablization of electrostatic ITG mode are studied. Here β=8πp/B2 is the ratio between the plasma and magnetic pressure. In the β→0 limit, our code recovers the electrostatic results. As β increases, the destablization of kinetic ballooning mode due to finite ion temperature gradient is observed. The simulation results also reveal two possible critical values of ηi, where ηi=d ln Ti/d ln n is the ratio between the density scale length to the temperature scale length; one of the critical values is for the predominantly electrostatic ITG mode and one for the Alfvénic ITG mode, where ηi,cAITG<ηi,cESITG. The transition domain between the Alfvénic ITG mode and the ITG mode, meanwhile, is found to depend on β and ηi.
Nanofabrication with holographic optical tweezers
Authors: Korda Pamela , Spalding Gabriel C. , Dufresne Eric R. , Grier David G.
Published: July 2002

We describe optical trapping techniques based on holographic optical tweezer arrays useful for assembling colloidal particles into arbitrary arrangements for photonics, optoelectronic, and sensor applications.
Plastic relaxation of solid GeSi solutions grown by molecular-beam epitaxy on the low temperature Si(100) buffer layer
Authors: Bolkhovityanov Yu. B. , Gutakovskii A. K. , Mashanov V. I. , Pchelyakov O. P. , Revenko M. A. , Sokolov L. V.
Published: July 2002

The role of a low temperature Si buffer layer (LT-Si) in the process of plastic relaxation of molecular-beam epitaxy grown GeSi/Si(001) is studied. Probable sources and mechanisms of generation of misfit dislocations (MD) are discussed. Transmission electron microscopic and x-ray diffraction techniques are used for studying 100 nm GexSi1−x films with LT-Si and those free of such a buffer layer. The MD density is found to be much lower in the former than in the latter, and the level of the film plastic relaxation is not higher than 20% in both as-grown and annealed films with LT-Si. As the thickness of the solid solution layer reaches 300-400 nm, the plastic relaxation of the films increases to almost 100%. Therefore, the determining role of the MD multiplication is supposed. We assume the double role of the LT-Si buffer layer. First, the diffusion flux of vacancies from the LT-Si layer to the GeSi/Si interface may cause erosion of the interface and, as a result, a decrease in the rate of MD generation at the early stages of epitaxy. Second, generation of intrinsic defect clusters in the LT-Si, which are potential sources of MDs, occurs in the field of mechanical stresses of the growing pseudomorphic layer. This process is thought to be the key feature of the plastic relaxation of GeSi/LT-Si/Si(100) films which promotes MD self-organization.
Electronic quenching of O(1D) by collisions with O2: A theoretical study in a collinear case
Authors: Miura Nobuaki , Hashimoto Kenro , Takahashi Kenshi , Taniguchi Nori , Matsumi Yutaka
Published: July 2002

Potential energy curves of triplet states for collinear O–O2 were calculated by ab initio CASSCF and MRSDCI methods. We found a pseudocrossing between 1 3Σ− (6 3A″) and 2 3Σ− (7 3A″) states at long O–O2 separation. The electronic quenching reaction, O(1D)+O2(X 3Σg−)→O(3P)+O2(b 1Σg+), is dominated by the nonadiabatic transition via the pseudocrossing. The collision energy dependence of the quenching reaction probability, which is evaluated by Zhu and Nakamura’s formula, is found to be in good agreement with experiment.
Electron attachment to SF5CF3 (296–563 K) and calculations of the neutral and anion thermochemistry
Authors: Miller Thomas M. , Arnold Susan T. , Viggiano A. A. , Knighton W. B.
Published: July 2002

Møller–Plesset (MP) perturbation theory and density functional theory (DFT) were used to examine the structure and bonding of trifluoromethyl sulfurpentafluoride, SF5CF3, and the corresponding anion, SF5CF3−. The structural parameters, charge analysis, and energetics are all consistent with the anion having ion-dipole character (i.e., SF5−–CF3). Results from G2(MP2) theory yield a neutral D2980(SF5–CF3)=301 kJ mol−1 (3.12 eV), anion D2980(SF5−–CF3)=21.5 kJ mol−1 (0.22 eV), EA(SF5CF3)=119 kJ mol−1 (1.24 eV), ΔfH2980(SF5CF3)=−1639 kJ mol−1, and ΔfH298(SF5CF3−)=−1750 kJ mol−1. The calculated value for the standard enthalpy of formation for SF5CF3 differs from the previous estimate by 78 kJ mol−1. DFT was found to perform poorly for quantities related to the neutral SF5–CF3 bond. Calculations were also carried out for SF5, SF5−, CF3, and CF3− fragments, and both DFT and G2(MP2) methods performed well for these open-shell species. Rate constants for electron attachment to SF5CF3 were measured over the temperature range 296–563 K in 133 Pa helium gas using a flowing afterglow Langmuir probe apparatus. The 296 K rate constant is (8.6±2.2)×10−8 cm3 s−1, which agrees within uncertainties with the estimate reported by Kennedy and Mayhew [Int. J. Mass Spectrom. 206, i–iv (2001)]. The only ionic product of attachment is SF5−. The temperature dependence of the electron attachment rate constant implies an activation energy of 25 meV. Implications of this work for estimating the atmospheric lifetime of SF5CF3 are discussed.
Thermodynamic stability of high-K dielectric metal oxides ZrO2 and HfO2 in contact with Si and SiO2
Authors: Gutowski Maciej , Jaffe John E. , Liu Chun-Li , Stoker Matt , Hegde Rama I. , Rai Raghaw S. , Tobin Philip J.
Published: July 2002

We present theoretical and experimental results regarding the thermodynamic stability of the high-k dielectrics ZrO2 and HfO2 in contact with Si and SiO2. The HfO2/Si interface is found to be stable with respect to formation of silicides whereas the ZrO2/Si interface is not. The metal–oxide/SiO2 interface is marginally unstable with respect to formation of silicates. Cross-sectional transmission electron micrographs expose formation of nodules, identified as silicides, across the polycrystalline silicon/ZrO2/Si interfaces but not for the interfaces with HfO2. For both ZrO2 and HfO2, the x-ray photoemission spectra illustrate formation of silicate-like compounds in the MO2/SiO2 interface.
Dissociative chemisorption of methylsilane on the Si(100) surface
Authors: Silvestrelli Pier Luigi , Sbraccia Carlo , Ancilotto Francesco
Published: July 2002

The chemisorption of methylsilane on the Si(100) surface is studied from first principles. Methylsilane is found to chemisorb dissociatively; during SiC film synthesis using methylsilane the dissociation of the molecule can take place either before or during interaction with the Si(100) surface. In the latter case we suggest a possible dissociation reaction pathway involving the scission of a Si–H bond, while preserving instead the Si–C bond. We investigate the geometry, the energetics, and the vibrational properties of a number of possible configurations following dissociative chemisorption and compare our results with recent experimental data. Finally, the results of a molecular dynamics simulation, aimed at qualitatively describing the process of heating methylsilane on Si(100), are reported.
Effect of binding on particle number fluctuations in a membrane channel
Authors: Berezhkovskii Alexander M. , Pustovoit Mark A. , Bezrukov Sergey M.
Published: July 2002

Transport of solutes through membrane channels produces additional noise in the channel ion current because the number of solute molecules in the channel fluctuates. We obtain a general expression for the power spectral density of these fluctuations in a cylindrical channel in the presence of a binding site of arbitrary strength. The expression shows how the spectral density transforms from that in the case of no-binding to the Lorentzian spectral density corresponding to the strong-binding limit. Brownian dynamics simulations confirm our analytical results.
Analysis of subband structures and optical properties of periodic strained quantum wires by a finite element method
Authors: Ishigaki T. , Ogawa M. , Morita M. , Miyoshi T.
Published: July 2002

We present theoretical investigations of subband structures and optical absorption properties of periodic quantum wires. The calculation is based on the multiband effective mass theory, where effects such as the valence-band mixing effects, the dependence of crystallographic orientation, and the strain effect using a total energy minimization technique are duly taken into account. We use the finite element method to solve the eigenvalue problem with periodic boundary conditions for the quasitwo-dimensional quantum confinement effect. We study strain effects on subband structures and optical properties, and dependence of the optical properties on the crystallographic orientation.
A Melnikov function for the break-up of closed streamlines in steady Navier–Stokes flows
Authors: Yannacopoulos A. N. , Rowlands G. , King G. P.
Published: July 2002

A Melnikov approach is proposed to study the existence/nonexistence of closed streamlines and asymptotic orbits in solutions of the steady Navier–Stokes equations. In certain limiting cases the method gives important insight into the physical processes that cause the break-up of closed and heteroclinic streamlines. The approach sheds light on the important issue of transport in three-dimensional Navier–Stokes flows. Specifically we show that the criterion for the existence of closed streamlines is that the line integral of the viscous force, induced from the perturbation of the flow along the unperturbed closed streamline, must vanish. We apply the approach to wavy Taylor vortex flow and show that the space averaged Melnikov function is related to the effective axial diffusivity.
Numerical studies of a three-dimensional flow in suddenly contracted channels
Authors: Chiang T. P. , Sheu Tony W. H. , Hwang Robert R.
Published: July 2002

This paper aims to provide deeper insight into the suddenly contracted channel flow. The channels under investigation have a width of 18D, where D represents the channel height upstream of the step plane, and two contraction ratios: C=2 and 4. In the present three-dimensional finite volume analysis, the advective fluxes are discretized using an upwind scheme that provides a third-order accurate solution in uniform grid cases. For efficient calculation of field variables, working equations are solved separately based on the semi-implicit velocity-pressure coupling procedures. We exploit a theoretically appealing topological theory to determine lines of separation and reattachment, from which the pitchfork bifurcation flow can be identified in the contraction channel. To further elucidate the vortical details of the channel flow, we trace massless particles seeded in the flow, showing the spanwise spiraling flow motion. Emphasis is placed on flow phenomena in the upstream salient corner eddy and downstream tip corner eddy.
Soft x-ray imaging and spectroscopy of single nanocrystals
Authors: Rockenberger Jörg , Nolting Frithjof , Lüning Jan , Hu Jiangtao , Alivisatos A. Paul
Published: July 2002

Resonant photoemission electron microscopy (PEEM) at the Fe L3,2 absorption edge was utilized to image single γ-Fe2O3 nanocrystals of 10 nm average diameter (∼20 000 Fe atoms) and to record soft x-ray absorption spectra of individual particles. Within the spectral resolution of the experiment, no damage to the individual nanoparticles occurs during repeated, prolonged exposure to the intense x-ray beam. Furthermore, no differences in the position or shape of the soft x-ray absorption spectrum of a single nanocrystal and the ensemble are observed within the experimental resolution. PEEM contrast images and soft x-ray absorption spectra, however, show strong intensity variations between different particles reflecting the size distribution of the sample. This proof-of-principle experiment successfully demonstrates the applicability of x-ray spectromicroscopy to the study of nanoscale systems on a hitherto unachieved length scale.
Schrödinger equation for current carrying states
Authors: Kosov D. S.
Published: July 2002

Schrödinger equation with given, a priori known current is formulated. A nonzero current density is maintained in the quantum system via a subsidiary condition imposed by vector, local Lagrange multiplier. Constrained minimization of the total energy on the manifold of an arbitrary current density topology results into a nonlinear self-consistent Schrödinger equation. The applications to electronic transport in two-terminal molecular devices are developed and new macroscopic definition of a molecular current–voltage characteristic is proposed. The Landauer formula for the conductance of an ideal one-dimensional lead is obtained within the approach. The method is examined by modeling of current carrying states of one-dimensional harmonic oscillator.
Infrared analogs of heteronuclear nuclear magnetic resonance coherence transfer experiments in peptides
Authors: Scheurer C. , Mukamel S.
Published: July 2002

Using short pulses whose bandwidth covers only one amide band and that are resonant with either the amide I or the amide II band, we propose two-color multiple pulse infrared experiments that are analogs of heteronuclear nuclear magnetic resonance (NMR) experiments for probing the structure of peptides and proteins. These pulse sequences yield simpler spectra of nonoverlapping vibrational bands with fewer overlapping peaks than one-color techniques, and open new ways to more sophisticated control over coherence transfer pathways. Differences between NMR and IR techniques stemming from the different initial conditions and the pulse field strengths are taken into account. We find that in several cases, π pulses commonly used in NMR to simplify the spectra can be avoided in vibrational spectroscopy due to the low temperature initial condition (i.e., vibrational energies are large compared to kT).
Effects of high potential barrier on InAs quantum dots and wetting layer
Authors: Kim Jin Soo , Yu Phil Won , Leem Jae-Young , Jeon Minhyon , Noh Sam Kyu , Lee Joo In , Kim Gu Hyun , Kang Se-Kyung , Kim Jong Su , Kim Song Gang
Published: July 2002

Effects of a thin AlAs layer (1 nm) with different position on InAs quantum dots (QDs) and wetting layer have been investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR). The PL peak position of InAs QDs directly grown on the thin AlAs is blueshifted from that of InAs QDs grown on the GaAs layer by 171 meV mainly due to the high potential barrier and reduced dot size shown in the TEM image. As the additional GaAs layer (1 and 2 nm) is inserted on top of the AlAs layer, the PL peak position is systematically shifted toward longer wavelength with increase in the thickness. Temperature dependent PL of QD samples shows that a thin AlAs layer significantly influences the thermal activation energy. The wetting layer related peak in PR spectra is changed to lower energy with increase in the thickness of an additional GaAs layer, which is mainly caused by the reduction in the effects of the AlAs layer.
Nanoparticle-templated carbon nanotube ring nucleus formation
Authors: Hester James R. , Louchev Oleg A.
Published: July 2002

A mechanism for nanotube ring nucleus formation via the interaction of a carbon nanosheet with a nanoparticle is proposed based on consideration of both short-range covalent and long-range interactions. In nanosheet–nanoparticle interaction, the potential barrier for nanosheet folding into a nanoring segment caused by covalent bonds distortions can be significantly decreased by long-range interaction with the nanoparticle. This effect, alone or in combination with the kinetic energy of collision, is shown to be able to fold the nanosheet around the nanoparticle and lead to the eventual formation of a nanotube ring nucleus.
Advanced computations in plasma physics
Authors: Tang W. M.
Published: July 2002

Scientific simulation in tandem with theory and experiment is an essential tool for understanding complex plasma behavior. In this paper we review recent progress and future directions for advanced simulations in magnetically confined plasmas with illustrative examples chosen from magnetic confinement research areas such as microturbulence, magnetohydrodynamics, magnetic reconnection, and others. Significant recent progress has been made in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics, giving increasingly good agreement between experimental observations and computational modeling. This was made possible by innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning widely disparate temporal and spatial scales together with access to powerful new computational resources. In particular, the fusion energy science community has made excellent progress in developing advanced codes for which computer run-time and problem size scale well with the number of processors on massively parallel machines (MPP’s). A good example is the effective usage of the full power of multi-teraflop (multi-trillion floating point computations per second) MPP’s to produce three-dimensional, general geometry, nonlinear particle simulations which have accelerated progress in understanding the nature of turbulence self-regulation by zonal flows. It should be emphasized that these calculations, which typically utilized billions of particles for thousands of time-steps, would not have been possible without access to powerful present generation MPP computers and the associated diagnostic and visualization capabilities. In general, results from advanced simulations provide great encouragement for being able to include increasingly realistic dynamics to enable deeper physics insights into plasmas in both natural and laboratory environments. The associated scientific excitement should serve to stimulate improved cross-cutting collaborations with other fields and also to help attract bright young talent to plasma science.
Influence of Ca concentration on the electric, morphological, and structural properties of (Pb,Ca)TiO3 thin films
Authors: Pontes F. M. , Pontes D. S. L. , Leite E. R. , Longo E. , Santos E. M. S. , Mergulhão S. , Chiquito A. , Pizani P. S. , Lanciotti F. , Boschi T. M. , Varela J. A.
Published: July 2002

Pb 1−x Ca x TiO 3  (0.10⩽x⩽0.40) thin films on Pt/Ti/SiO2/Si(100) substrates were prepared by the soft solution process and their characteristics were investigated as a function of the calcium content (x). The structural modifications in the films were studied using x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate a decrease in lattice tetragonality with the increasing content of calcium in these films. Raman spectra exhibited characteristic features of pure PbTiO3 thin films. Variations in the phonon mode wave numbers, especially those of lower wave numbers, of Pb1−xCaxTiO3 thin films as a function of the composition corroborate the decrease in tetragonality caused by the calcium doping. As the Ca content (x) increases from 0.10 to 0.40, the dielectric constant at room temperature abnormally increased at 1 kHz from 148 to 430. Also calcium substitution decreased the remanent polarization and coercive field from 28.0 to 5.3 μC/cm2 and 124 to 58 kV/cm, respectively. These properties can be explained in terms of variations of phase transition (ferroelectric-paraelectric), resulting from the substitution the lead site of PbTiO3for the nonvolatile calcium.
Room-temperature single-electron charging phenomena in large-area nanocrystal memory obtained by low-energy ion beam synthesis
Authors: Kapetanakis E. , Normand P. , Tsoukalas D. , Beltsios K.
Published: July 2002

We investigated the dependence of implantation dose on the charge storage characteristics of large-area n-channel metal–oxide–semiconductor field-effect transistors with 1-keV Si+-implanted gate oxides. Gate bias and time-dependent source–drain current measurements are reported. Devices implanted with 1×1016 cm−2 Si dose exhibit a continuous (trap-like) charge storage process under both static and dynamic conditions. In contrast, for 2×1016 cm−2 implanted devices, electrons are stored in Si nanocrystals in discrete units at low gate voltages, as revealed by a periodic staircase plateau of the source–drain current with a low gate voltage sweep rate, and the step-like decrease of the time-dependent monitoring of the channel current. These observations of room-temperature single-electron storage effects support the pursuit of large-area devices operating on the basis of Coulomb blockade phenomena.
Generation of micrometer-scale discharge in a supercritical fluid environment
Authors: Ito Tsuyohito , Terashima Kazuo
Published: July 2002

The generation of micrometer-scale discharge in very high-pressure CO2 up to a supercritical fluid (SCF) environment using micrometer-gap (1, 2 μm) electrodes has been performed. A peculiar characteristic of breakdown voltage for CO2 is reported here. The results reveal an inflection near 2.5 MPa and a trough near the critical point. Near the critical point, the discharge can be generated with almost one fifth of the voltage required for a conventional gas, as estimated from Paschen’s law, which is probably caused by unstable clusters in SCF. This result reveals that an ionized state or plasma state in a SCF environment can be generated with very low voltage.
White electroluminescence from hydrogenated amorphous-SiNx thin films
Authors: Pei Zingway , Chang Y. R. , Hwang H. L.
Published: July 2002

White electroluminescence (EL) was observed from hydrogenated amorphous-SiNx-based light-emitting device. Silicon nitride thin films were deposited on the indium-tin-oxide (ITO)-coated glass substrate by plasma enhanced chemical vapor deposition method with a mixture of Ar-diluted 5% SiH4 and pure N2 gases, in the ratio 2 to 1. Measured x value of the film is 0.56, and the corresponding photoluminescence of a-SiN0.56:H thin film exhibited a red-infrared spectrum, centered at 630 nm. The layer structure of the EL device is ITO/a-SiN0.56:H (80 nm)/Al, with light emitting from the ITO layer, recognizable by the naked eye in the dark, under the 14 V forward bias conditions. White EL spectra from ∼400 to 750 nm, with a central peak at 560 nm, were observed in the hydrogenated amorphous silicon nitride EL device. A carrier transport mechanism was suggested, and the EL was attributed to the recombination of carriers through the luminescent states.
Low-resistant and high-transparent Ru/Ni ohmic contact on p-type GaN
Authors: Jang Ho Won , Urbanek W. , Yoo M. C. , Lee Jong-Lam
Published: July 2002

We report a low-resistant and high-transparent Ru-based ohmic contact on p-type GaN through oxidation annealing under O2 atmosphere. A low resistivity of 4.5×10−5 Ω cm2 and high light transmittance (84.6%) were simultaneously obtained from oxidized Ru (50 Å)/Ni (50 Å) contact. The RuO2 formed on p-type GaN played a role in reducing the effective Schottky barrier height for injection of holes and NiO layer produced on the RuO2 acted as a diffusion barrier for the outdiffusion of released Ga and N atoms, resulting in the reduction of the contact resistivity as well as the enhancement of light transmission.
Time domain optical susceptibility of intrinsic GaAs
Authors: Thomas M. E. , Miragliotta J. A. , Joseph R. I.
Published: July 2002

Intrinsic GaAs optical constant values are well known as functions of frequency (10 000–65 000 cm−1 or 1.24–8.06 eV) and temperature (22–754 K). Room-temperature far-infrared optical constant data also exist as a function of frequency, and are representable by a classical oscillator model. In this article, the frequency-domain, temperature-dependent intrinsic dielectric function of GaAs has been Fourier transformed to obtain an analytical, closed-form representation of the time-domain susceptibility. Results from these expressions are consistent with the temporal characteristics of electronic transitions impeded by elastic scattering, which are in the femtosecond regime. The closed form nature of these expressions makes them well suited for finite difference time domain simulations of waveguides, optoelectronic devices, and microwave devices.
Length of magnetic field lines in turbulent plasmas
Authors: Núñez Manuel
Published: July 2002

An estimation of the length of any magnetic field line in a two-dimensional periodic magnetohydrodynamic problem is provided. This is done by using some classical function theory results on the analytic extension of the vector potential. The essential parameter, the maximum of this extension, may be analyzed in the case of turbulent plasmas by admitting the Iroshnikov–Kraichnan statistics, establishing in this way a relation between the length of any magnetic field line and the energy dissipation scale.
Practical formulations of the electron capture rate in quantum wells by phonon emission at low carrier density
Authors: Vallone Marco
Published: July 2002

In this article the Green’s functions formalism is exploited to describe the coupled phonon-plasmon system in the plasmon pole approximation for the longitudinal dielectric constant, for two-dimensional quantum wells. Electron capture time in quantum wells is obtained in a closed form, separating contributions coming from different two-dimensional plasmon modes. Moreover, useful criteria are pointed out, in order to decide under which conditions a simpler description in the static limit of the plasmon pole approximation may be enough accurate. In the limit of low carrier density, an analytical expression for the capture rate by LO-phonon emission has been given, in the two-dimensional static limit of the longitudinal dielectric constant. By means of this analytic description, effects of screening, Fermi filling, and electrons momentum distribution at finite temperature on capture time may be separately investigated in detail.
Coulomb scattering model for ultrathin silicon-on-insulator inversion layers
Authors: Gamiz F. , Jiménez-Molinos F. , Roldán J. B. , Cartujo-Cassinello P.
Published: July 2002

A Coulomb scattering model for ultrathin silicon-on-insulator inversion layers has been developed. This model simultaneously takes into account (i) screening of charged centers by mobile carriers, (ii) the distribution of charged centers inside the structure, (iii) the actual electron distribution, (iv) the charged center correlation, and (v) the effect of image charges. We have used this model in a Monte Carlo simulator for single-gate silicon-on-insulator inversion layers and have calculated electron mobility curves in these devices taking into account phonon, surface roughness and Coulomb scattering for different values of the silicon slab thickness sandwiched between the two oxide layers.
Quantifying the drift velocity of carrier ensembles in time-dependent electric fields
Authors: Bieler M. , Hübner J. , Oestreich M. , Koch M. , Hein G. , Pierz K. , Siegner U.
Published: July 2002

If a spatially nonuniform carrier distribution is optically excited in an electrically biased semiconductor, complicated carrier motion can be expected due to temporal and spatial variations of the screened electric field. We present an experimental method that allows one to quantify the drift velocity of carrier ensembles versus time in time-dependent electric fields. The method is based on the analysis of spatially resolved photoluminescence images at different times, recorded with a streak camera. With this technique, we have studied the details of carrier sweep out in photoconductive switches.
Quantitative x-ray photoelectron spectroscopy study of Al/AlOx bilayers
Authors: Batlle Xavier , Hattink Bart Jan , Labarta Amı́lcar , Åkerman Johan J. , Escudero Roberto , Schuller Ivan K.
Published: July 2002

An x-ray photoelectron spectroscopy (XPS) analysis of Nb/Al wedge bilayers, oxidized by both plasma and natural oxidation, is reported. The main goal is to show that the oxidation state—i.e., O:(oxidize)Al ratio—, structure and thickness of the surface oxide layer, as well as the thickness of the metallic Al leftover, as functions of the oxidation procedure, can be quantitatively evaluated from the XPS spectra. This is relevant to the detailed characterization of the insulating barriers in (magnetic) tunnel junctions.
Characterization on microstructures of tungsten/barrier metals (TiN,WNx)/silicon multilayer films
Authors: Yang Jun-Mo , Choi Il-Sang , Kim Yong Soo , Park Ju-Chul , Lee Sang-Moo , Hong Tae-Eun , Park Yoon-Beak , Lee Soun-Young , Kim Yil Wook
Published: July 2002

Microstructures of W/barrier metals (TiN,WNx)/Si multilayer films followed by heat treatment were precisely investigated by x-ray diffraction and high-resolution transmission electron microscopy. The analysis results showed that in the W/TiN/Si stacked film having the W (110) and (200) preferred orientations, the TiN film itself plays a role as a barrier for the reaction of W and Si, whereas in the case of the WNx barrier having the W (110) one, the amorphous SixNy layer with a thickness of a few nanometers works effectively as a barrier, which was formed during the deposition and denudation process of the WNx film. Furthermore, the nanometer-scaled interfacial reaction in the multilayer films was clearly investigated by x-ray photoelectron spectroscopy coupled with chemical etching and an energy-filtered elemental mapping technique. Based on the results, effects of barrier metals on the W preferred orientation and the interfacial reaction were crystallographically and thermodynamically discussed.
Evaluation of the (Ba,Sr)RuO3 and (Ba,Sr)RuO3/Ru bilayer as an oxygen diffusion barrier
Authors: Park Jeong-Hee , Hong Duck-Hwa , Kim Young-Bae , Choi Duck-Kyun
Published: July 2002

The (Ba,Sr)RuO3 (BSR) oxide electrode which can enhance electrical properties of (Ba,Sr)TiO3 (BST) dielectric film due to structural and chemical matches with BST, was evaluated as an oxygen diffusion barrier. It was possible to restrain the oxidation of TiN layer under BSR to TiO2 by sequential depositions of amorphous BSR and crystalline BSR, in which the amorphous BSR eventually crystallized into crystalline BSR during the deposition of crystalline BSR. When two-step BSR layers on TiN, however, were annealed in oxygen ambient at 700 °C, oxygen atoms diffused and oxidized TiN layer to TiO2. On the other hand, oxygen could be effectively blocked by the BSR/Ru bilayer. In this system, the Ru sublayer plays a role as an oxygen getter and the bilayer tends to block oxygen diffusion. The blocking effect was more obvious when the thickness of BSR in the bilayer increased. The BST/bilayer showed higher dielectric constant due to the suppression of formation of a low dielectric layer between BST and the bilayer. Although the BST/bilayer showed a slightly higher leakage current density, it was possible to reduce the leakage current density to 10−8 order A/cm2 at 1 V by increasing the BSR thickness in the bilayer.
Photovoltage spectroscopy of InAs/GaAs quantum dot structures
Authors: Toušková J. , Samochin E. , Toušek J. , Oswald J. , Hulicius E. , Pangrác J. , Melichar K. , Šimeček T.
Published: July 2002

In addition to widely used photoluminescence spectroscopy photovoltaic measurement of quantum dot structures can give complementary information about electron and hole transitions. Structures with self-organized InAs quantum dots in GaAs matrix were grown by the Stranski–Krastanov mechanism using the low pressure metalorganic vapor phase epitaxy technique. Two types of samples were studied, with single and multiple quantum dot layers. We have shown that surface photovoltage spectroscopy can be used for the study of single, as well as multiple quantum dot layer structures.
Silicon fiber formed on silicon without using a gas process
Authors: Takami Tomohide , Kusunoki Isao
Published: July 2002

We demonstrate the formation of self-organized silicon fibers on a Si(111) surface without the use of a gas process. The fibers are formed as follows. (1) Silicon particles are placed on a silicon surface. (2) Monolayer-equivalent gold atoms are deposited on the silicon surface in vacuum. (3) The gold-deposited silicon surface is annealed at 1000 °C for several hours.
Blueshifts in the ultraviolet absorption spectra of amphoteric SnO2−x nanocrystalline particles
Authors: Tsunekawa Shin , Kang Junyong , Asami Katsuhiko , Kasuya Atsuo
Published: July 2002

Blueshifts in the ultraviolet absorption spectra of amphoteric SnO2−x nanocrystalline particles differ greatly in acidic and alkaline sols. X-ray diffraction measurements show that the average lattice strain of the nanocrystalline particles in aqueous ammonia sols is about twice as large as that in oxalic acid sols. The size dependence of the blueshift without lattice strain is proposed to be about three times as large as that in an alkaline sol with large strain. X-ray photoelectron spectroscopic measurements reveal that the valence of Sn in acidic and alkaline sols is not 4 but less than 3. It is strongly suggested that the blueshift depends not only on the crystalline size but also most importantly on the lattice strain and second on the valence state of the Sn ions.
Diffuse phase transition and relaxor behavior in (PbLa)TiO3 thin films
Authors: Bhaskar S. , Majumder S. B. , Katiyar R. S.
Published: July 2002

Room-temperature micro-Raman scattering and temperature-dependent dielectric measurements were carried out on sol-gel-derived ferroelectric Pb1−xLaxTi1−x/4O3 (PLT x=0.05–0.30) thin films. Results indicate that the crystal structure and the electrical properties of PLT films were strongly influenced by the La contents. The dielectric properties of PLT thin films were studied in the temperature range of 80–700 K and frequencies in the range of 1 kHz–1 MHz. Results show that PLT thin films undergo normal-to-relaxor ferroelectric transformation with 30 at. % La contents. The observed relaxor behavior is established in terms of diffuse phase transition characteristics and Vögel–Fulcher relationship.
Finding finite-time invariant manifolds in two-dimensional velocity fields
Authors: Haller G.
Published: July 2002

For two-dimensional velocity fields defined on finite time intervals, we derive an analytic condition that can be used to determine numerically the location of uniformly hyperbolic trajectories. The conditions of our main theorem will be satisfied for typical velocity fields in fluid dynamics where the deformation rate of coherent structures is slower than individual particle speeds. We also propose and test a simple numerical algorithm that isolates uniformly finite-time hyperbolic sets in such velocity fields. Uniformly hyperbolic sets serve as the key building blocks of Lagrangian mixing geometry in applications.
The electronic characteristics of n-type CdIn2Te4
Authors: Ou S. S. , Eshraghi S. A. , Stafsudd O. M. , Gentile A. L.
Published: July 2002

The elecstronic properties of n-type CdIn2Te4 crystals were investigated. The mobility and carrier concentration were measured using Hall techniques. I-V and C-V measurements were performed on Al/n-CdIn2Te4 Schottky barrier devices. The I-V data are interpreted in terms of space-charge-limited currents. The dominant electron trap level has been determined to be 0.21 eV below the conduction band. The barrier height was found to be approximately 0.65 eV.
Synchronizable Q-switched, mode-locked, and cavity-dumped ruby laser for plasma diagnostics
Authors: Houtman H. , Meyer J.
Published: July 2002

We report on the design and operation of an optimized version of a Q-switched, mode-locked, and cavity-dumped ruby-laser oscillator. The modulator window is much narrower than that assumed in conventional active mode-lock theory, and is shown to yield much shorter pulses than the latter in cases where the number of round trips is restricted. To allow a high-power pulse (≊1 GW) to evolve in the oscillator, and to allow simple synchronization to a (∼100 ns fixed delay) CO2 laser, a limit of 23 round trips was chosen, but similar limits may be imposed by lasers having short-gain duration as in an excimer laser. Details are given on the single spark gap switching element and Pockels cells, with an analysis of their expected switching speeds, in order to establish the effectiveness of the modulator, as compared to conventional sinusoidally driven active mode lockers. Single pulses of 50–70 mJ are reliably cavity-dumped after only 100-ns delay (23 round trips) with pulse length adjustable from 50–100 ps with ±5-ps stability. Relative timing between the main (CO2) and probe (ruby) pulses allows a measurement accuracy of ±50 ps to be attained.
Piezoresistance response of longitudinally and laterally oriented ytterbium foils subjected to impact and quasi-static loading
Authors: Gupta Satish C. , Gupta Y. M.
Published: July 2002

Resistance change measurements in ytterbium foils, encapsulated in a polymethylmethacrylate matrix and subjected to impact loading (to 25 kbars) and quasi-static triaxial loading (to 4 kbars), are presented. Two gauge orientations were examined. Residual resistance measurements were obtained for impact experiments. Both sets of experiments confirm the overall validity of the theoretical analysis described in Ref. 5. In particular, the crossover in resistance change-stress curves, predicted at low stresses, was measured. The present results show that gauge resistance measurements cannot always be uniquely related to a particular matrix stress component. Restricting the use of ytterbium foils to 20 kbars is inferred from the experimental results. The residual resistance data depend on both foil and matrix response. Finally, analysis of the resistance measurements, along the two orientations in the impact experiments, shows that polymethylmethacrylate retains significant material strength to 25 kbars.
Au acceptor levels in Si under pressure
Authors: Li Ming-fu , Chen Jian-xin , Yao Yu-shu , Bai Guang
Published: July 2002

The hydrostatic pressure coefficient of Au acceptor levels ET in Si was measured by transient capacitance method. Under the pressure range of 0–8 kbar, the pressure coefficient ∂(Ec−ET)/∂P=−1.9 meV/kbar. The electron capture cross section of Au acceptor centers is independent of pressure within experimental accuracy. For defect levels with defect potential of Td symmetry, the uniaxial stress coefficient ∂(Ēc−ĒT)/∂F is isotropic and equal to one-third of corresponding hydrostatic pressure coefficient. By comparing the present result of hydrostatic pressure coefficient with the uniaxial stress coefficient reported by X. C. Yau, G. G. Qin, S. R. Zeng, and M. H. Yuan [Acta Phys. Sin. 33, 377 (1984)], one concludes that the defect potential is far from Td symmetry. Therefore, the Au acceptor levels are unlikely to have been originated by simple gold substitutional or interstitial configuration in Si.
Durable metal carbide layers on steels formed by ion implantation at high temperatures
Authors: Singer I. L. , Bolster R. N. , Sprague J. A. , Kim K. , Ramalingam S. , Jeffries R. A. , Ramseyer G. O.
Published: July 2002

High-power beams (4–10 W/cm2) of Ti ions have been used to heat Fe and steel substrates to 600–800 °C during high fluence (5×1017/cm2) implantation. Auger sputter depth profiles find a stoichiometric TiC surface layer, about 100 nm deep, graded continuously into both Fe and steel substrates. Secondary ion mass spectrometry of Fe and steels implanted in 13CO atmospheres indicate that the carbon originates from the bulk in carbon steels but from the atmosphere in Fe foils. Transmission electron microscopy reveals a continuous layer of fine-grained (50–100 nm) TiC crystallites in a preferred Baker–Nutting orientation relationship with respect to underlying Fe grains. Abrasive-wear measurements performed with diamond paste (1–5 μm) show the TiC layer on hardened M2 steel is 3–10 times more wear resistant than the substrate. Sliding-wear studies find an extremely durable layer that reduces friction by up to 60%, and increases by 50% the contact-stress threshold of M2 tool steel to boundary lubrication. The metallurgical processes responsible for the TiC layer will be discussed, and the advantages of this high-temperature treatment will be presented.
Inelastic light scattering from heavily doped and highly compensated GaAs:Si
Authors: Kamijoh T. , Hashimoto A. , Takano H. , Sakuta M.
Published: July 2002

The first-order Raman scattering from heavily doped and highly compensated GaAs:Si has been measured with a constant impurity concentration of 1.1 × 1019 cm−3. The damped q=0 L− and L+ branches due to the compensation-limited low mobilities have been observed. The impurity-induced forbidden mode which had been found in uncompensated p-GaAs:Zn has not appeared in the measured spectra. The line-broadening of plasmon-coupled LO phonon, L(q), and line and peak shift of TO phonon line are attributed to the impurity-induced Fröhlich interaction and the self-energy effect, respectively.
Raman study of phosphorous-implanted and pulsed laser-annealed GaAs
Authors: Ashokan R. , Jain K. P. , Mavi H. S. , Balkanski M.
Published: July 2002

Raman spectroscopy has been used to study structural disorder in phosphorous ion-implanted GaAs. Line-shape asymmetry of the one-phonon mode and the oscillator strength of the two-phonon combination spectra are used to characterize the disorder. This analysis permits a rough identification of the threshold for amorphization. Localized vibrational mode of phosphorous impurities in ion-implanted and pulsed laser-annealed (PLA) GaAs is reported and its intensity variation with fluence of the implant is studied. The softening of the LO-phonon mode in ion-implanted, PLA GaAs is explained as an inherent impurity effect. The presence of a partially annealed deep layer is investigated by observing a disorder activated mode in the Raman spectrum and by an enhancement in the intensity of the forbidden TO phonon. The occurrence of defects on the surface after PLA is investigated by the wavelength dependence of the LO-phonon softening.
Relationship between infrared absorption and resistivity in liquid-encapsulated Czochralski semi-insulating GaAs crystals
Authors: Katsumata Tooru , Okada Hideo , Obokata Takeshi , Fukuda Tsuguo
Published: July 2002

The relationship between the infrared absorption coefficient and the resistivity was examined experimentally in undoped and In-doped liquid-encapsulated Czochralski grown semi-insulating GaAs. The resistivity increases from ∼106 to 5×108 Ω cm with an increase in the infrared absorption coefficient at 1 μm from 0.6 to 1.0 cm−1 in Ga-rich GaAs, and decreases from about 5×108 to 5×106 Ω cm with an increase in the infrared absorption coefficient from 1.0 to 3.0 cm−1 in As-rich GaAs. This fact provides a physical basis for the use of infrared transmission topography as a nondestructive, rapid characterization technique for the evaluation of the two-dimensional uniformity in resistivity of semi-insulating GaAs.
Nonlinear, elastic, piezoelectric, electrostrictive, and dielectric constants of lithium niobate
Authors: Cho Yasuo , Yamanouchi Kazuhiko
Published: July 2002

All eight electrostrictive, all three third-order dielectric, all thirteen third-order piezoelectric, and all fourteen third-order elastic constants of lithium niobate have been determined. Measurements for the electrostrictive constants have been made by measuring the electric capacity in selected directions when the directed static stresses were applied to the crystal. The third-order dielectric constants have been calculated from these measured electrostrictive constants and the measured capacitance as functions of the applied static electric field. The third-order piezoelectric and elastic constants have both been determined from the above-mentioned two kinds of measured constants and the measured velocities for small-amplitude ultrasonic waves as functions of several applied stresses and static electric fields.