bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of unstable mixing region in wurtzite In1−x−yGaxAlyN
Authors: Matsuoka Takashi
Published: July 2002

The wurtzite structure In1−x−yGaxAlyN quaternary system with a wide band gap, which is useful for light emitters in the wavelength region shorter than green, is studied with respect to the unstable region in mixing. This unstable region in mixing is calculated from the free energy of mixing using the strictly regular solution model. The interaction parameter used in this calculation is obtained using the delta-lattice-parameter method. From this calculation, the ternary alloys of InAlN, InGaN, and GaAlN are, respectively, predicted to always, sometimes, and hardly ever have a unstable mixing region at the temperature lower than 3000 °C.
Transient analysis of ballistic transport in stublike quantum waveguides
Authors: Burgnies L. , Vanbésien O. , Lippens D.
Published: July 2002

Switching properties of a T-shaped quantum waveguide are studied by solving the two-dimensional time-dependent Schrödinger equation under continuous injection. Assuming a ballistic electron transport, the conductance transient alternately overshoots and undershoots the steady-state value and shows a self-sustaining harmonic oscillation in the long-time limit. Analysis by Fourier transform resolves the various spectral contributions in the terahertz range and it is found that the quantum system keeps a memory of its initial state at short times. Pronounced mixing effects are shown with an electron wave function shared between the bound and metastables states of the initial and final eigenspectra.
Engineering individual grain boundaries using a zone-confining process in chemical vapor deposited Cu films
Authors: Rauf I. A. , Gadgil P. N. , Egerton R. F. , Boyd J. D. , Sayer M.
Published: July 2002

Materials scientists have struggled for a while to find a process which can be used to engineer individual grain boundaries separating two adjacent grains. Considerable effort has also been devoted to finding a technique that can control the orientation of individual grains. A temperature gradient applied during the chemical vapor deposition of thin copper films provides an effect such that chains of grains oriented in the same direction are produced at chosen positions in the film. As a result of an interaction between defect migration and crystal growth, interconnected grains, oriented in the 〈110〉 direction and separated by twin grain boundaries, form chains along contours of equal temperature. When the temperature gradient is small, low-angle tilt boundaries, low-energy coincidence-site boundaries, and twin boundaries are observed. For a sharp temperature gradient, all boundaries are coherent twin boundaries.
A model for photoemission from prepoled ferroelectric ceramics
Authors: Benedek G. , Boscolo I.
Published: July 2002

The emission of electrons from a prepoled ferroelectric material when shined with a laser light pulse cannot be interpreted as a normal photoemission process, nor due to polarization switching. Another picture is proposed for the emission: in a prepoled ferroelectric disk a very narrow layer of electrons is set at the surface. The high-power laser pulse is absorbed by the electrons and an Auger process occurs. Hence, part of the hot electrons acquire enough energy to tunnel and/or to overcome the potential barrier and exit the crystal.
A metal/oxide tunneling transistor
Authors: Snow E. S. , Campbell P. M. , Rendell R. W. , Buot F. A. , Park D. , Marrian C. R. K. , Magno R.
Published: July 2002

We have fabricated a nanometer-scale transistor that operates by using a gate field to modulate the transmission of electrons through a lateral metal/oxide tunnel barrier. Our initial devices have a 30-nm-wide lateral Nb/NbOx tunnel junction on top of a planar Al2O3/Al buried gate. We observe effective modulation of the source–drain current with gate bias at room temperature with negligible gate leakage current. We identify the materials issues that currently limit the device performance, and we offer direction for future device improvements.
In-plane strain and strain relaxation in laterally patterned periodic arrays of Si/SiGe quantum wires and dot arrays
Authors: Darowski N. , Pietsch U. , Zhuang Y. , Zerlauth S. , Bauer G. , Lübbert D. , Baumbach T.
Published: July 2002

The depth dependent strain relaxation in photolithographically defined and reactive ion etched Si/SiGe quantum wire and dot arrays is determined by high resolution grazing incidence x-ray diffraction. The laterally periodic structures were aligned along two orthogonal [110] and [1̄10] directions on the (001) surface. By recording reciprocal space maps around the (220) and (2̄20) reciprocal lattice points, the shape and in-plane strain could be determined independently of each other. Using triple axis diffractometry and changing the effective penetration depth of the x-ray radiation between 5 and 300 nm the strain relaxation in the wires and dots could be determined depth resolved.
Observation of photoluminescence from Al1−xInxN heteroepitaxial films grown by metalorganic vapor phase epitaxy
Authors: Yamaguchi Shigeo , Kariya Michihiko , Nitta Shugo , Takeuchi Tetsuya , Wetzel Christian , Amano Hiroshi , Akasaki Isamu
Published: July 2002

We have observed photoluminescence of Al1−xInxN films. The films were grown on GaN by atmospheric pressure metalorganic vapor phase epitaxy. GaN was grown on a c-plane sapphire substrate with a low-temperature deposited AlN buffer layer. Photoluminescence, absorption, and x-ray diffraction measurements have shown that each spectral peak shifts with alloy composition x and that Al1−xInxN heteroepitaxial films are not macroscopically in phase separation and are constituted in the wurzite structure.
Measurement of AlInAsSb/GaInAsSb heterojunction band offset by photoluminescence spectroscopy
Authors: Chang J. R. , Su Y. K. , Lin C. L. , Wu K. M. , Huang W. C. , Lu Y. T. , Jaw D. H. , Li W. L. , Chen S. M.
Published: July 2002

We have grown unstrained Al0.66In0.34As0.85Sb0.15/Ga0.64In0.36As0.84Sb0.16 multiple-quantum-well (MQW) structures on InP substrates by metalorganic vapor phase epitaxy. Low-temperature photoluminescence was performed for these MQW structures. By comparing the luminescence peak energies with the theoretical calculations, we estimated the conduction-band offset ratio to be 0.75±0.10 for the Al0.66In0.34As0.85Sb0.15/Ga0.64In0.36As0.84Sb0.16 heterostructure.
Magnetic properties of cast Nd60−xFe20Al10Co10Cux alloys
Authors: Fan G. J. , Löser W. , Roth S. , Eckert J. , Schultz L.
Published: July 2002

The effect of Cu on the magnetic properties of slowly cooled Nd60−xFe20Al10Co10Cux (0⩽x⩽10) bulk amorphous alloys has been studied by room-temperature magnetic measurements. Ferromagnetic bulk amorphous Nd60Fe20Al10Co10 cylinders of 3 mm diameter and 50 mm in length with a coercivity of 298 kA m−1 were obtained by copper mold casting. The addition of 1 at. % Cu into the Nd60Fe20Al10Co10 alloy causes an increase in coercivity to 321 kA m−1. Further addition of Cu by replacing neodymium, however, results in an abrupt decrease in coercivity to 41 kA m−1 for alloys with x=10 due to the precipitation of a Fe-rich magnetically soft Nd(Fe, Co, Al)2 phase. The improvement of the hard magnetic properties by the addition of 1 at. % Cu for the cast Nd60Fe20Al10Co10 based bulk amorphous alloys may be attributed to an enhanced clustering of Nd–(Fe, Co) pairs in the amorphous alloys.
AlGaInP-based microcavity light-emitting diodes: Controlled on-wafer detuning and measurement of the internal quantum efficiency
Authors: Royo P. , Stanley R. P. , Houdré R. , Ilegems M. , Moser M. , Hövel R. , Schweizer H. P. , Gulden K. H.
Published: July 2002

We present results on visible red top-emission microcavity light-emitting diodes grown by metal organic chemical vapor deposition. The emission characteristics dependence with respect to the detuning between the quantum well emission and the cavity mode was experimentally investigated. The detuning was varied during growth by 60 nm across a 2 in. wafer radius according to a parabolic and reproducible dependence with respect to the position on the wafer. Numerical simulation reproduced very well the experimental results using the internal quantum efficiency ηint as a fitting parameter, resulting in a value of ηint=56%+/−5% at a current density of 20 A/cm2.
The role of particle energy and pulsed particle flux in physical vapor deposition and pulsed–laser deposition
Authors: Mayr Stefan G. , Moske Michael , Samwer Konrad , Taylor Maggie E. , Atwater Harry A.
Published: July 2002

Surface morphology evolution of thin films generated by physical and pulsed-laser deposition depending on the incident particle energy and the pulse rate is investigated using a continuum growth model. The model includes curvature-induced surface diffusion, the Schwoebel barrier and surface atom displacement as main surface processes. The numerical solution of the model is in very good agreement with the results of kinetic Monte Carlo simulations, which also serve to estimate the continuum growth parameters, and with experimental results on thin Si films. The increase of the incident particle energy, starting from thermal energy, fundamentally influences the surface topography, changing from self-affine to self-organized morphology.
Dynamic study and experimental “two-step process” of substrate step preparation for high-Tc Josephson junctions
Authors: Zhai H. Y. , Zhang L. , Chu W. K. , Morishita T. , Tanaka S. , Xu F. Z. , Yang Q. S.
Published: July 2002

We report theoretical and experimental studies of the dynamics of substrate step preparation for high-Tc Josephson junctions. A maximum step edge angle of 70.8° has been calculated for SrTiO3 (STO) substrates with a Nb mask. This calculated angle agrees well with our experimental result of 66°. Step-edge angles can be predicted for different purposes using this method. We also utilized a “two-step process” to improve the surface morphology of the stepped substrate, and step-edge Josephson junctions were fabricated with good uniformity.
Two-pulse echo experiments in the spectral diffusion regime
Authors: Berg Mark A. , Rector K. D. , Fayer M. D.
Published: July 2002

The two-pulse echo sequence is examined for the case in which the frequency modulation time τm of the transition is intermediate between the well known limiting cases of very fast modulation (motional narrowing) and very slow or static modulation (inhomogeneous broadening). Within this spectral diffusion regime, the interpretation of the echo decay differs markedly from standard treatments. If the frequency-frequency correlation function initially decays as 1−tβ, the echo decay time TE is proportional to τmβ/(β+2). These results reduce to those of Yan and Mukamel [J. Chem. Phys. 94, 179 (1991)] for β=1. Drawing on a viscoelastic model, the theoretical results are compared to viscosity and temperature dependent vibrational echo experiments on myoglobin–CO. A τm1/3 dependence is observed, as is predicted for an exponential decay of the frequency–frequency correlation function.
Pseudorotation in tetrahydrofuran⋯HF heterodimer
Authors: Alonso J. L. , López J. C. , Blanco S. , Lesarri A. , Lorenzo F. J.
Published: July 2002

Seven isotopomers of the hydrogen-bonded heterodimer tetrahydrofuran⋯HF have been investigated in the frequency range 6–18 GHz using molecular-beam Fourier transform microwave spectroscopy. The rotational spectra of C4H8O⋯HF and C4H8O⋯DF exhibit small tunneling splittings, which were not observed for C4D8O⋯HF and the four single 13C isotopomers. These observations analyzed in terms of symmetry considerations allow us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit of the complex and not to HF inversion. The spectroscopic parameters of the complex have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to the twisted ring form, with HF lying on the plane bisector to the COC ring angle.
Effect of O:Er concentration ratio on the structural, electrical, and optical properties of Si:Er:O layers grown by molecular beam epitaxy
Authors: Scalese S. , Franzò G. , Mirabella S. , Re M. , Terrasi A. , Priolo F. , Rimini E. , Spinella C. , Carnera A.
Published: July 2002

The structural, electrical, and optical properties of crystalline Si codoped with Er and O by molecular beam epitaxy (MBE) have been investigated in detail. Si:Er:O layers (∼250 nm thick) have been grown by MBE, realizing uniform dopant concentrations in the range 8×1018–1.5×1020 cm−3 for Er and up to 5×1020 cm−3 for O. The O:Er ratio was varied between 0 and ∼20. Samples have been subsequently annealed at 900 °C for 1 h. We observed that clear constraints to the Er and O contents exist in order to incorporate them in a good quality single crystal. We also found that the O:Er ratio represents the main parameter in determining the properties of this system. For instance, Er is observed to behave as a donor in MBE grown samples and the donor concentration increases with the O:Er ratio until a saturation regime is achieved for a ratio higher than 6–8. All the samples emit light at 1.54 μm and similar behavior is also found for the optical activation of the Er ions. The thermal process usually increases the number of light emitting Er ions which is also increased by increasing the O:Er ratio; however, for O:Er ratios higher than 6–8, no further activation is measured. In contrast, the most intense room temperature photoluminescence (PL) peak is obtained in samples having an O:Er ratio ∼2, for which PL temperature quenching is strongly reduced. Indeed, the coupling of these observations with structural measurements allows us to clearly identify the best conditions for an MBE grown Si:Er:O sample. These phenomena are investigated in detail and discussed.
Rydberg states of the allyl radical observed by two-photon resonant ionization spectroscopy
Authors: Wu Jen-Chieh , Li Runhua , Chang Jia-lin , Chen Yit-Tsong
Published: July 2002

The vibronic spectrum of the allyl radical (CH2CHCH2) at 6–8 eV has been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The allyl radicals were produced in the nozzle of a supersonic jet expansion by the pyrolysis of allyl iodide. Five new Rydberg states, a 3d and four ns states (n=4,6–8), are observed for the first time. Prominent vibrational progressions with the gross spacing of ∼430 cm−1 are observed in the s Rydberg series. The totally symmetric ∠CCC-bending vibration in the Rydberg states is responsible for the observed vibrational progressions. Facilitated with the ab initio calculation of Franck–Condon factors, other observed vibrational states in the s Rydberg series have been identified. The term values for the ns Rydberg series converge to an adiabatic ionization energy of 65 638±18 cm−1 with a quantum defect of 0.966±0.005.
Control-matrix approach to stellarator design and control
Authors: Mynick H. E. , Pomphrey N.
Published: July 2002

The full space Z≡{Zj=1,…,Nz} of independent variables defining a stellarator configuration is large. To find attractive design points in this space, or to understand operational flexibility about a given design point, one needs insight into the topography in Z-space of the physics figures of merit Pi which characterize the machine performance, and means of determining those directions in Z-space which give one independent control over the Pi, as well as those which affect none of them, and so are available for design flexibility. The control matrix (CM) approach described here provides a mathematical means of obtaining these. In this work, the CM approach is described and used in studying some candidate Quasi-Axisymmetric (QA) stellarator configurations the National Compact Stellarator Experiment design group has been considering. In the process of the analysis, a first exploration of the topography of the configuration space in the vicinity of these candidate systems has been performed, whose character is discussed.
Finger instability in radiatively driven dusty plasmas
Authors: Schröer Andreas , Shchekinov Yuri
Published: July 2002

In this work the dynamics of a three-fluid plasma consisting of electrons, ions and charged dust particles are considered. The plasma is illuminated by a radiation field, which, through absorbtion, accelerates the dust particles. By direct and Coulomb collisions the gained momentum is partially transmitted to electrons and ions. A steady state can be achieved by an external gravitational field acting against the radiation. After presenting the steady-state flow, linear perturbations of dynamical variables with the wave vector perpendicular to the radiation flux are considered. A linear analysis is performed by reducing the full three-fluid description to a suitable one-fluid system of equations, assuming the plasma to be quasineutral and omitting plasma species separation. It is demonstrated that when the drag force between ions and dust particles is a decreasing function of their relative velocity, the steady state is unstable against the formation of parallel layered streams with high and low relative flow velocities, i.e., multiple current sheets. The temporal evolution of this instability is then investigated by means of numerical simulations in the framework of a complete three-fluid model.
The diocotron spectrum of a toroidal non-neutral plasma
Authors: Bhattacharyya S. N.
Published: July 2002

The equilibrium and stability of a toroidal non-neutral plasma of low density has been studied numerically. The equilibrium is computed using the variational moment method, while linear stability is computed using a Fourier representation in the poloidal coordinate and a finite difference approximation in the radial coordinate. The computation has been carried out for various configurations to obtain frequencies of stable modes and the growth rate of instabilities.
How do shape anisotropy and spatial orientation of the constituents affect the permittivity of dielectric heterostructures?
Authors: Brosseau C. , Beroual A. , Boudida A.
Published: July 2002

The study of dielectric heterostructures has been advancing at a rapid pace. Much of the interest in these materials stems from the fact that their physical properties can be systematically tuned by variation of the size and shape of the constituents. Here we report on extensive computer simulations of the effective permittivity of dielectric periodic (deterministic) heterostructures, having monosized hard core inclusions of anisotropic shape (rod, ellipsoid) embedded in an otherwise homogeneous and isotropic matrix. The real and imaginary parts of the permittivity, in the quasistatic limit, are rigorously evaluated with the use of the PHI3D field calculation package and the resolution of boundary integral equations. In this article, we show that the effective permittivity has critical properties near a conduction threshold. The conduction threshold concentration can be significantly modified by the size, shape, and spatial arrangement of the constituents. More specifically, it obeys a square law dependence as a function of the aspect ratio, i.e., the ratio of the smaller dimension to the larger dimension in both the rodlike and ellipsoidal inclusions. The data exhibit a scaling behavior and can all be collapsed onto a single master curve, indicative of a remarkable universality in the conductivity property. The critical exponents which determine how the real and imaginary parts of the effective permittivity scale with the distance from the conduction threshold are determined. Our results are compared with the scaling prediction of the standard percolation theory for infinite three-dimensional random lattices of insulator–normal metal composite systems. We also observed that the conduction transition is shifted towards higher concentrations as the angle between the symmetry axis and the direction of the applied electric field increases. Increasing the contrast ratio, between the permittivity and the conductivity of the background medium and the inclusions, results in dramatic changes of the complex effective permittivity, depending on the geometry of the inclusions. The scale-dependent properties and the mechanism which govern criticality are related to the actual area of contacts between the inclusions.
In situ single wavelength ellipsometry studies of high rate hydrogenated amorphous silicon growth using a remote expanding thermal plasma
Authors: Smets A. H. M. , Schram D. C. , van de Sanden M. C. M.
Published: July 2002

An in situ single wavelength HeNe rotating ellipsometry study of high rate (∼80 Å s−1) hydrogenated amorphous silicon (a-Si:H) deposition using an expanding thermal plasma is presented. An optical growth model is used to simulate the measured ellispometric trajectories similar to models used for low rate a-Si:H growth in the literature. The in situ growth at high growth rates was studied as function of the substrate temperature. The refractive index n (at 632.8 nm) increases with increasing temperature corresponding to an increase in the density of the films. The in situ extinction coefficient k (at 632.8 nm) increases with increasing substrate temperature due to a smaller optical band gap and due to an increase in indirect absorption. It is shown that the ellipsometry setup in combination with optical modeling enables us to monitor the surface roughness evolution during deposition and to obtain the dynamic scaling exponent β for postinitial growth.
Release isentropes of overdriven plastic-bonded explosive PBX-9501
Authors: Hixson R. S. , Shaw M. S. , Fritz J. N. , Vorthman J. E. , Anderson W. W.
Published: July 2002

Continuous release isentropes for the plastic-bonded explosive PBX-9501 are obtained from velocity interferometer system for any reflector measurements at a high-explosive/LiF interface. Forward calculations from a tabular representation of the isentropes to the measured u(t) data at the interface are iterated to yield isentropes that give agreement with the data. Curves for the isentropes and for the isentropic gamma, γS=−(∂lnP/∂lnV)S are presented. Because isentropes from different overdriven states differ, a crude estimate of the Grüneisen parameter is obtained. An overall representation of the data is achieved with this Grüneisen parameter and a single isentrope through the Chapman–Jouguet state.
Poloidal rotation effects on a simulated resistive kink mode
Authors: Kissick M. W. , Leboeuf J.-N. , Cowley S. C. , Dawson J. M.
Published: July 2002

Through the principal use of the reduced magnetohydrodynamic version of the finite aspect ratio code [L. A. Charlton et al., J. Comput. Phys. 86, 270 (1990)], an m/n=1/1 resistive kink mode was poloidally rotated with the accompanying rotational shear. It was observed that the growth rate of this unstable mode can either decrease or increase as the applied equilibrium rotation is increased to near poloidal sonic speeds. Shear in the poloidal rotation profile is stabilizing, but only if the destabilizing effects of bulk rotation can be overcome. Therefore, the mode’s stability was sensitive to the location of the rotation’s peak relative to the eigenmode’s spatial extent. The destabilizing effects of bulk rotation are apparently a rotationally enhanced beta, and the stabilizing effects appear to be caused by exceeding a critical rotational shear spatially averaged over the eigenmode structure.
Calculation of refractive indices and third-harmonic generation susceptibilities of liquid benzene and water: Comparison of continuum and discrete local-field theories
Authors: Reis H. , Papadopoulos M. G. , Theodorou D. N.
Published: July 2002

The refractive indices (linear response) and the nonlinear susceptibility for third harmonic generation (THG) at λ=1064 nm of liquid water and benzene are predicted, using ab initio molecular (hyper)polarizabilities and local-field corrections calculated according to the Lorentz model, the generalized reaction-field model of Onsager, and a discrete local-field theory applied to liquid structures obtained by classical molecular simulation methods. The ratio of the calculated THG susceptibilities for the two liquids is compared with experimental results, in order to avoid ambiguities concerning the factors used to calibrate the THG experiments. All models are able to reproduce the experimental refractive indices of both liquids accurately, but are less successful for the THG susceptibility. The discrete local-field corrections for the THG susceptibility are considerably larger than those of the continuum model in the form of the more accurate extended reaction-field approach.
Interface of directly bonded GaAs and InP
Authors: Jin-Phillipp N. Y. , Sigle W. , Black A. , Babic D. , Bowers J. E. , Hu E. L. , Rühle M.
Published: July 2002

The structure and composition of the interface of directly bonded GaAs and InP (001) wafers has been studied with various techniques in electron microscopy. For each interface three different dislocation networks have been identified and analyzed. They have been confirmed to accommodate the lattice mismatch, the tilt misfit between the two wafers, and the thermal misfit, respectively. Interdiffusion of both group-III and group-V elements takes place. Indium diffusion is enhanced by the Zn dopant and its segregation. The “cavities” at the interface, reported in the literature, have been found to be associated with indium depletion. In the case of inadequate surface preparation prior to bonding an amorphous layer of native oxide(s) forms at the interface. The actual bonding temperature Tb at which the atomic bonds construct locally across the two surfaces of the wafers is lower than the annealing temperature employed in the present experiments. It is therefore suggested that a better interface may be achieved by improving the preparation of the surfaces of the wafers with appropriate chemicals and by bonding the wafers at a lower temperature.
Electron–electron coalescence and counterbalance functions for atoms
Authors: Koga Toshikatsu
Published: July 2002

For many-electron atoms, spherically averaged electron–electron coalescence h0(R) and counterbalance d0(u) functions are studied which, respectively, represent the probability densities that any electron pairs with zero interelectronic distance are located at a radius R from the nucleus and that any electron pairs with zero center-of-mass radius have a relative distance u. For the exact and Hartree–Fock (HF) wave functions, cusp relations h0′(0)/h0(0)=−4Z and d0′(0)/d0(0)=−2Z are derived theoretically, where the prime denotes the first derivative and Z is nuclear charge. At the Hartree–Fock limit level, both functions h0(R) and d0(u) are found to be monotonically decreasing with a single maximum at R=0 or u=0 for all the 102 atoms He through Lr. The long-range asymptotic behavior of the coalescence and counterbalance functions is governed in general by the orbital energy of the highest occupied atomic orbital.
Self-capacitance of a Thomas–Fermi nanosphere
Authors: Krčmar Maja , Saslow Wayne M. , Zangwill Andrew
Published: July 2002

We calculate the self-capacitance and charging energy of a spherical nanoparticle in the Thomas–Fermi approximation. The result is CTF=C0[1−p−1 tanh p]/[1−(1−ε−1)p−1 tanh p], with CTF⩾C0. Here C0=4πε0R is the classical geometrical value, p=R/l is the ratio of the particle radius R to the Thomas–Fermi screening length l, and ε is the material dielectric constant. The addition of surface localized states drives C toward C0. These results should be relevant to tunneling spectroscopy studies of giant carbon onions and “large” semiconductor nanocrystals that do not require a full quantum treatment.
Integration of GaN with Si using a AuGe-mediated wafer bonding technique
Authors: Funato Mitsuru , Fujita Shizuo , Fujita Shigeo
Published: July 2002

This letter describes integration of GaN with Si using a AuGe alloy as a bonding material. GaN is first grown on GaAs and then GaN/GaAs/AuGe/Si and GaAs/GaN/AuGe/Si structures are fabricated by wafer bonding. For the latter structure, the GaAs substrate is removed by mechanical and chemical etching. From the current–voltage measurements of both structures, it is found that the bonded interfaces do not obstruct the carrier transport. Furthermore, the optical reflection measurements reveal that AuGe works well as a mirror, which is a suitable characteristic for the integration of GaN light-emitting devices with Si.
Growth of epitaxial and polycrystalline thin films of the electron doped system La1−xCexMnO3 through pulsed laser deposition
Authors: Mitra C. , Raychaudhuri P. , John J. , Dhar S. K. , Nigam A. K. , Pinto R.
Published: July 2002

The polycrystalline La1−xCexMnO3 manganites do not exist in single phase in bulk under the preparation conditions so far employed, but their polycrystalline and epitaxial films deposited by the pulsed laser deposition (PLD) technique form readily in single phase. The cerium oxide (CeO2) remains partially unreacted when the bulk sample is prepared through the solid state reaction route. The resistivity of the bulk La0.7Ce0.3MnO3 sample shows a broad metal insulator transition (MIT) clearly resolved into two peaks, suggesting the presence of a second (impurity) phase, which is identified as unreacted CeO2 by the intensity analysis of the x-ray diffraction (XRD) data. However, when prepared as thin films by PLD, La0.7Ce0.3MnO3 forms in single phase, as corroborated by the uniqueness and sharpness of the MIT peak and also by the XRD patterns of the polycrystalline films. We also performed a detailed study of the epitaxial films by a high-resolution XRD system with a four-circle goniometer and did not find any impurity phase. The magnetization data shows a very sharp transition followed by a sharp MIT in resistivity at the same temperature in the epitaxial thin film. These results suggest that PLD can be used as a useful technique to synthesize unconventional compounds, which do not form easily in bulk.
Energy assignments for the I15/24 and S3/24 multiplets of the Er3+ ion in (Er0.05Y0.95)2BaZnO5
Authors: Cruz G. K. , Carvalho R. A. , Basso H. C.
Published: July 2002

The absorption and emission spectra between the I15/24 and S3/24 multiplets of the Er3+ ion in (Er0.05Y0.95)2BaZnO5 are recorded. From these spectra we have demonstrated the occupancy of two nonequivalent positions by the Er3+ ion in the sample. Time decay measurement, emission versus temperature analysis, and selective site excitation were made to confirm our conclusion. From emission versus temperature for S3/24→I15/24 transitions and from the selective site excitation, we were able to select the group of lines that belong to each site. In this article, we present a table with the energy-level values for the ground I15/24 and the excited S3/24 multiplets of the Er3+ ion in each site.
A mass spectrometry study of n-octane: Electron impact ionization and ion-molecule reactions
Authors: Jiao C. Q. , DeJoseph C. A. , Garscadden A.
Published: July 2002

Electron impact ionization of n-octane over an energy range of 10–70 eV and the subsequent ion-molecule reactions with the parent molecule have been studied using Fourier-transform mass spectrometry. Molecular ion and fragment ions C1+–C6+ are produced from the electron impact with a total ionization cross section of 1.4±0.2×10−15 cm2 between 60 and 70 eV. C3H7+ is the most abundant ion at most of the ionizing energies with the exception for E⩽16 eV where C6H13+ and C6H12+ are the most abundant. Among the fragment ions only C4H7+ and smaller ions react readily with the parent molecule, primarily producing C5H11+ and C4H9+, with rate coefficients of 0.32–2.4×10−9 cm3 s−1. Essentially all of the ions, including the molecular ion and the large fragment ions, undergo decomposition upon collision with neutral molecules after they are kinetically excited to an energy range of 1–5 eV, forming a variety of small hydrocarbon ions. Many of the decomposition product ions in turn are capable of further reacting with n-octane. Isotope reagents have been utilized in experiments to probe the type of the ion-molecule reactions studied.
Translational–orientational coupling during passage of methane through the bottleneck in zeolite A
Authors: Chitra R. , Anil Kumar A. V. , Yashonath S.
Published: July 2002

Diffusion of a five-site model of methane within porous zeolite A has been investigated by molecular dynamics simulation using potential available in the literature. Equilibrium and dynamical quantities especially those relating to the orientational degrees of freedom of methane have been computed. Methane exhibits interesting orientational preference during intercage diffusion through the eight-membered window which is the rate determining step for diffusion. A predominance of (2+2) orientation in place of (1+3) orientation is seen suggesting a strong translational–orientational coupling. This suggests that orientational motion might strongly influence the translational diffusivity of methane.
Observation of light propagation in two-dimensional photonic crystal-based bent optical waveguides
Authors: Yamada S. , Koyama T. , Katayama Y. , Ikeda N. , Sugimoto Y. , Asakawa K. , Kawai N. , Inoue K.
Published: July 2002

An observation of light propagation in a two-dimensional photonic crystal-based (2D–PC) optical waveguide is reported. Bent waveguides with a bending angle of 60° embedded in a 2D–PC triangular lattice with air columns were fabricated on a molecular-beam epitaxially grown AlGaAs/GaAs structure. The light-propagation characteristics were examined by observing scattered light from PC regions with incident light in the 850–950 nm range under an optical microscope using a charge coupled device camera. The incident light was strongly guided for wavelengths corresponding to the photonic band gap.
Polarization selectivity in fifth-order electronically nonresonant Raman scattering from CS2
Authors: Kaufman Laura J. , Blank David A. , Fleming Graham R.
Published: July 2002

This paper presents several experimentally collected tensor elements of the direct fifth-order Raman signal of room temperature CS2. All results were collected so as to reduce contamination from third-order cascade signals, and it is shown that while these cascade signals are of varying importance in different tensor elements, phase matching considerations are sufficient to make the direct fifth-order signal the dominant signal in every tensor element collected. We show, as predicted, that the different tensor elements appear to weight particular Liouville pathways of the direct fifth-order signal, and thus particular dynamics of the studied system, differently. The magic angle signals are shown to contain information about the intermolecular vibrational echo and to put a limit on the time scale of rephasing of the room temperature liquid. The Ryzzzzy tensor element, in conjunction with the Ryyzzzz tensor element, is shown to be useful in studying the pure dephasing and population dynamics of the liquid. From these data it appears that population dynamics occur on a much longer time scale than pure dephasing dynamics, even though this is not obvious for the intermolecular motions of a room temperature liquid that are studied here. Further, intensities and behaviors of different tensor elements of the experimental data are compared with the only detailed theory currently available that explicitly includes polarization dependence.
Microwave frequency ferroelectric domain imaging of deuterated triglycine sulfate crystals
Authors: Steinhauer D. E. , Anlage Steven M.
Published: July 2002

We have used a near-field scanning microwave microscope to image domain structure and quantitatively measure dielectric permittivity and nonlinearity in ferroelectric crystals at 8.1 GHz with a spatial resolution of 1 μm. We imaged ferroelectric domains in periodically poled LiNbO3, BaTiO3, and deuterated triglycine sulfate (DTGS) with a signal-to-noise ratio of 7. Measurement of the permittivity and nonlinearity of DTGS in the temperature range 300–400 K shows a peak at the Curie temperature, TC≈340 K, as well as reasonable agreement with thermodynamic theory. In addition, the domain growth relaxation time shows a minimum near TC. We observe coarsening of ferroelectric domains in DTGS after a temperature quench from 360 to 330 K and evaluate the structure factor.
Preparation and Faraday effect of EuS microcrystal-embedded oxide thin films
Authors: Tanaka Katsuhisa , Tatehata Naoki , Fujita Koji , Hirao Kazuyuki
Published: July 2002

EuS microcrystal-embedded SiO2, Al2O3, and TiO2 thin films have been prepared by using rf sputtering method. X-ray diffraction analysis indicates that EuS microcrystal is precipitated as a single phase in Al2O3 and TiO2 films while precipitation of EuS is not detected in SiO2 film. Faraday effect attributable to the 4f7–4f65d transition of Eu2+ in EuS microcrystalline phase is observed in Al2O3 and TiO2 films. In particular, Faraday rotation angle observed for EuS-embedded TiO2 film is large; for instance, the magnitude of Verdet constant for as-deposited TiO2 film prepared without heating of substrate during the sputtering is 0.15 deg/cm Oe at wavelength of 700 nm. This value is larger by two orders of magnitude than those of Eu2+- or Tb3+-containing oxide glasses which show the largest Faraday rotation angle among rare-earth-containing glasses, and is comparable to the value for EuSe single crystal.
Optical transition in infrared photodetector based on V-groove Al0.5Ga0.5As/GaAs multiple quantum wire
Authors: Fu Y. , Willander M. , Liu X.-Q. , Lu W. , Shen S. C. , Tan H. H. , Jagadish C. , Zou J. , Cockayne D. J. H.
Published: July 2002

Photoconductors based on V-grooved Al0.5Ga0.5As/GaAs multiple quantum wires (QWR) were fabricated. The geometric structure of the QWR was carefully characterized by transmission electron microscopy and spatially resolved microphotoluminescence measurements. Infrared response at 9.2 μm is observed from the photocurrent spectrum measured at 80 K. It is attributed as the intersubband transition in the quantum wire region. Due to the effective quantum confinement from the two (111)-surfaces forming the V groove, the overlapping between the ground state in the QWR and the one in the vertical quantum well is very small. This explains the weak photocurrent signal from the QWR photodetector. Theoretical design for a better wave function overlapping and optical coupling is outlined from the analysis of two-dimensional spatial distributions of the wave functions.
Zero-field mobility, exact mean dwell times, and disorder-induced steps in a Gaussian energy distribution
Authors: Soos Z. G. , Sin J. M.
Published: July 2002

Hopping transport in molecularly doped polymers (MDPs) is usually modeled in terms of energetic disorder corresponding to a Gaussian distribution of states, g(ε,σ), whose width σ is taken from experiment. Time-of-flight profiles indicate normal transport and finite mean dwell time 〈τ(T,E)〉. At zero field, thermal equilibrium and detailed balance are shown to yield 〈τ(T)〉 exactly for hopping rates that are products of energetic and geometrical factors. Hopping in g(ε,σ) for standard rates such as Miller-Abrahams, Marcus, or symmetric leads to ln〈τ(T)〉 that goes strictly as T−2 only for symmetric rates. Disorder-induced steps dominate the zero-field mobility μ(T). Monte Carlo simulations with Marcus rates show that extra steps depend on the overall disorder. Dilution and orientation increase the slope of ln μ(T) vs T−2. We interpret extra steps using an auxiliary lattice with spacing a′ chosen to give the same μ(T). Evidence for disorder-induced steps comes from previous studies with variable donor concentration in the same polymer, from much larger mobility changes than the concentration ratio, and from modeling of tritolylamine:polystyrene systems. Exact dwell times for hopping in g(ε,σ) show that current estimates of σ are upper bounds that neglect geometrical disorder. They provide direct tests for the convergence of simulations, either in terms of 〈τ(T)〉 or the distribution of visited sites in an infinite random walk.
Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VII. Localized plasma formation on NaCl single crystal surfaces
Authors: Kawaguchi Y. , Dawes M. L. , Langford S. C. , Dickinson J. T.
Published: July 2002

Wide band gap insulators containing defects exposed to nanosecond pulses of UV laser radiation at fluences close to the damage threshold often display highly localized flashes of light. In this work, we show that flashes observed during irradiation of cleaved, single crystal NaCl at relatively low fluences are due to localized plume fluorescence. By comparing time-resolved optical images of this fluorescence with subsequent scanning electron microscope images of surface topography, we show that these flashes are often associated with micron-dimension surface and near-surface damage, typically associated with cleavage steps. With continued laser irradiation, plume fluorescence at previously damaged regions usually grows stronger from pulse to pulse. In some cases, weak plume fluorescence disappears after one laser pulse, and may or may not reappear with continued irradiation. We interpret these results in terms of localized laser absorption by deformation-induced defects generated during cleavage. Deliberately deformed material, produced by indentation, is damaged at considerably lower laser fluences, consistent with this interpretation. We suggest that mobile excitations produced by laser absorption preferentially decay along dislocation cores, which strongly localizes laser-induced thermal stresses and damage.
Photoinduced phase-matched second-harmonic generation in azodye-doped polymer films
Authors: Si Jinhai , Qiu Jianrong , Kitaoka Kenji , Hirao Kazuyuki
Published: July 2002

Phase-matched second-harmonic generation (SHG) in azodye-doped polymethylmethacrylate films was demonstrated using the nonresonant optical poling technique. During the seeding process, samples were irradiated simultaneously by coherent superposition of the fundamental and the second-harmonic light of a femtosecond laser. Optical poling experiments were performed at three wavelengths of 1500, 1320, and 1300 nm, respectively. The quadratic dependence of SHG on the film thickness showed that a χ(2) grating that satisfied the phase-matching condition for SHG was optically induced in the polymer films. The SHG conversion efficiency of a 105 μm thick film was estimated to be about 2%. A relaxation retardation effect of the photoinduced χ(2) was also observed in the thick films.
Optical functions of low-k materials for interlayer dielectrics
Authors: Postava K. , Yamaguchi T.
Published: July 2002

The optical functions of low dielectric constant (low-k) materials have been determined using a high-precision four-zone null spectroscopic ellipsometer in the spectral range from 1.5 to 5.4 eV (230–840 nm wavelength region). The ellipsometric data were fitted simultaneously with near-normal incidence reflectivity spectra (ranging from 0.5 to 6.5 eV). A general method of simultaneous treatment of ellipsometric and reflectivity data is demonstrated on representative materials used in the semiconductor industry for interlayer dielectrics: (1) SiLK—organic dielectric resin from the Dow Chemical Company, (2) Nanoglass—nanoporous silica from the Honeywell Electronic Materials Company, and (3) tetra-ethyl-ortho-silicate (TEOS) (SiO2)—the standard dielectric material. The low-k materials (SiLK and Nanoglass) were prepared by a standard spin-coating process, while the SiO2 layer was prepared by thermal decomposition from TEOS onto single-crystal silicon wafers.
Ultrafast time-resolved photoluminescence from novel metal–dendrimer nanocomposites
Authors: Varnavski O. , Ispasoiu R. G. , Balogh L. , Tomalia D. , Goodson T.
Published: July 2002

We report the first results of ultra-fast enhanced light emission from gold– and silver–dendrimer nanocomposites. There is a fast (70 fs) fluorescence decay component associated with the metal nanocomposites. Anisotropy measurements show that this fast component is depolarized. The enhanced emission is suggestively due to local field enhancement in the elongated metal–dendrimer nanoparticles.
Studies on the behavior of mixed-metal oxides: Adsorption of CO and NO on MgO(100), NixMg1−xO(100), and CrxMg1−xO(100)
Authors: Rodriguez J. A. , Jirsak T. , Pérez M. , González L. , Maiti A.
Published: July 2002

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption mass spectroscopy (TDS), and first-principles density functional (DF) generalized-gradient-corrected calculations were used to study the adsorption of CO and NO on MgO(100), Ni0.06Mg0.94O(100), and Cr0.07Mg0.93O(100) surfaces. UPS spectra and DF calculations show clear differences in the electronic properties of these oxides. After doping MgO with nickel, states with Ni 3d character appear ∼1.5 eV above the occupied {O 2p+Mg 3s} band. A similar phenomenon is found after adding Cr, but now the dopant levels are ∼3 eV above the {O 2p+Mg 3s} band. In CO- and NO-TDS experiments, the reactivity of the oxide surfaces increases in the sequence: MgO(100)<Ni0.06Mg0.94O(100)<Cr0.07Mg0.93O(100). Cr-bonded molecules exhibit adsorption energies as large as 15 (CO) and 20 kcal/mol (NO). For CO and NO on MgO(100), the mixing between the frontier orbitals of the adsorbate and the bands of the surface is poor, and the low adsorption energy is mainly due to weak MgO↔CO or MgO↔NO electrostatic interactions. On the other hand, the Cr 3d levels in Cr0.07Mg0.93O(100) are energetically well positioned for responding to the presence of adsorbates, leading to substantial binding of CO and NO. DF results for a series of TM0.06Mg0.94O(100) systems (TM=Zn, Ni, Fe, or Cr) show a correlation between their electronic and chemical properties: the less stable the occupied levels of a mixed-metal oxide, the higher its chemical reactivity. An important parameter to consider when designing a mixed-metal oxide catalyst is the final energy position of the occupied states provided by the second metal or dopant agent.
Structural study of nanometric electrodeposited Co films using Co59 NMR
Authors: Cerisier M. , Attenborough K. , Jedryka E. , Wojcik M. , Nadolski S. , Van Haesendonck C. , Celis J. P.
Published: July 2002

A Co59 NMR experiment has been used to investigate the structure of two series of Co films with thicknesses varying from 5 to 400 nm which were electrodeposited on Cu at the electrolyte pH value of 2.1 and 3.7, respectively. It was shown that the overall structure of studied Co films consists of a very good quality fcc phase and a heavily faulted hcp phase in about equal proportions. The exceptions are very thin Co layers (below 20 nm) where the hcp structure was stabilized at pH 3.7 and overpotential of 0.9 V. This effect is attributed to the formation of hexagonal cobalt hydroxide in the early stage of deposition, which acts as a buffer layer stabilizing Co hcp structure.
Micromechanical detectors for local field measurements based on ferromagnetic resonance (invited)
Authors: Jander Albrecht , Moreland John , Kabos Pavel
Published: July 2002

Ferromagnetic resonance (FMR) measurements were performed on micrometer-size thin-film samples deposited onto a micromechanical cantilever detector. The FMR response is coupled to cantilever motion in one of three ways: (1) By measuring the change in torque on the sample in a uniform field; the FMR precession reduces the static magnetic moment of the sample with a resultant change in torque. (2) By measuring the damping torque acting on the FMR precession. (3) By measuring the energy absorbed in FMR using a bimaterial cantilever as a calorimeter sensor. Our instrument is capable of measuring the FMR response in permalloy samples as small as 2×10−11 cm3 in ambient conditions with a signal-to-noise ratio of 100. In addition we demonstrate that this system can be used as a quantitative scanning probe magnetic field microscope. Using the magnetic field sensitivity of the FMR response in a small ferromagnetic particle, we have achieved 50 A/m field resolution on 20 μm length scales. Both dc fields and microwave fields were imaged.