bluuebunny/crossref_metadata_2025 huggingface dataset:
Increasing the exchange-bias field of Ni0.5Co0.5O films by microstructural control
Authors: Martien Dinesh , Takano Kentaro , Berkowitz A. E. , Smith David J.
Published: July 2002

The exchange-bias field produced by antiferromagnetic monoxide films has been shown to scale with the density of uncompensated interfacial spins. The uncompensated spin density is inversely proportional to the interfacial grain diameter. Accordingly, we have prepared Ni0.5Co0.5O films in which the average interfacial grain diameter is small, but the grains are elongated, thus avoiding thermally activated fluctuations of the spin system associated with small grain volumes. The exchange-bias field remained inversely proportional to the interfacial grain diameter, and was increased to twice its previous maximum value without introducing significant thermal instability.
Transverse strain responses in the electrostrictive poly(vinylidene fluoride–trifluorethylene) copolymer
Authors: Cheng Z.-Y. , Xu T.-B. , Bharti Vivek , Wang Shexi , Zhang Q. M.
Published: July 2002

A large transverse electrostrictive strain S1 with a high elastic strain energy density is observed along the drawing direction in the stretched poly(vinylidene fluoride–trifluorethylene) copolymer film under a proper electron irradiation treatment condition, which results in a ratio of |S1/S3| larger than 1 where S3 is the longitudinal strain. In contrast, unstretched films generate a relatively small transverse strain with a small ratio of |S1/S3| (∼0.25). Hence, in this copolymer system, the ratio of |S1/S3| can be varied over a broad range. In addition, a large volume strain, which amounts to about half of the longitudinal strain, is observed in the films investigated.
Enhanced photoluminescence in epitaxial ZnGa2O4:Mn thin-film phosphors using pulsed-laser deposition
Authors: Lee Yong Eui , Norton David P. , Budai John D.
Published: July 2002

The growth and properties of ZnGa2O4:Mn thin-film phosphors on single crystal substrates using pulsed-laser deposition were investigated. Epitaxial film properties were compared to polycrystalline films deposited on glass substrates. Green photoluminescence was observed for as-deposited films with no postannealing required. Enhanced luminescent intensity in the epitaxial films was observed as compared to randomly oriented polycrystalline films, suggesting that grain boundaries and grain alignment strongly influence the luminescent properties. The ratio of Zn/Ga in the films also affected photoluminescence properties, with strong green emission observed in Zn-deficient films.
Photoluminescence and Raman studies in thin-film materials: Transition from amorphous to microcrystalline silicon
Authors: Yue Guozhen , Lorentzen J. D. , Lin Jing , Han Daxing , Wang Qi
Published: July 2002

We measured photoluminescence (PL) and Raman spectra for films deposited by hot-wire chemical vapor deposition using various hydrogen to silane ratios. We observed: (a) a PL peak energy increase from 1.25 to 1.4 eV as the material approaches the a- to μc-Si transition region; (b) a dual-PL peak at 1.3 and 1.0 eV for the film with a H dilution ratio of 3; and (c) as the H ratio increases, the 1.3 eV PL fades away and the low energy PL dominates. Meanwhile, a redshift of the peak position, a decrease of the intensity, and a narrower bandwidth for the low energy PL are also observed. The low energy PL is explained by band-tail radiative transitions from two types of grain boundaries.
Room-temperature continuous-wave operation of InGaN multiple-quantum-well laser diodes with an asymmetric waveguide structure
Authors: Kneissl Michael , Bour David P. , Van de Walle Chris G. , Romano Linda T. , Northrup John E. , Wood Rose M. , Teepe Mark , Johnson Noble M.
Published: July 2002

Room-temperature continuous-wave (cw) operation is demonstrated with InGaN multiple-quantum-well laser diodes containing an asymmetric waveguide structure. Pulsed threshold current densities as low as 5.2 kA/cm2 have been obtained for ridge-waveguide laser diodes grown on sapphire substrates by metal-organic chemical vapor deposition. For improved thermal management, the sapphire substrate was thinned and the devices were mounted p side up onto a copper heatsink. Under cw conditions at 20 °C, threshold current densities were 8.3 kA/cm2 with threshold voltages of 6.3 V. The emission wavelength was 401 nm with output powers greater than 3 mW per facet. Under cw conditions, laser oscillation was observed up to 25 °C. The room-temperature cw operation lifetimes, for a constant current, exceeded one hour.
Parallel nanodevice fabrication using a combination of shadow mask and scanning probe methods
Authors: Lüthi Roli , Schlittler Reto R. , Brugger Jürgen , Vettiger Peter , Welland Mark E. , Gimzewski James K.
Published: July 2002

We describe a resistless proximal probe-based lithography technique, which enables the direct patterning of complex and submicron-sized structures of various materials. The method is based on a combination of scanning probe microscopy and the shadow masking technique, whereby structures are locally deposited through pinhole-like apertures situated in the proximity of a cantilever tip. Predefined excursions of the sample lead to the direct fabrication of arbitrary structures on the surface. Complex patterns such as rings and intersecting lines with linewidths well below 0.1 μm are presented.
Electroluminescence of polymer/J-aggregate composites
Authors: Mal’tsev Eugene I. , Lypenko Dmitry A. , Shapiro Boris I. , Brusentseva Maria A. , Milburn George H. W. , Wright Jeffrey , Hendriksen Andre , Berendyaev Vladimir I. , Kotov Boris V. , Vannikov Anatoly V.
Published: July 2002

Efficient electroluminescence was revealed in single-layer light-emitting diodes based on electron-hole conducting polymers containing the nanocrystalline phase of cyanine dyes (J-aggregates). These species exhibit a very narrow emission band with a maximum in the red to infrared spectrum range. The J-aggregates play an active role in the charge carrier transport of the composites.
Realization of high tunability barium strontium titanate thin films by rf magnetron sputtering
Authors: Padmini P. , Taylor T. R. , Lefevre M. J. , Nagra A. S. , York R. A. , Speck J. S.
Published: July 2002

Ferroelectric thin films are currently being used to develop tunable microwave circuits based on the electric-field dependence of the dielectric constant. (Ba0.5Sr0.5)TiO3 (BST) films prepared by sputtering on Pt/TiO2/SiO2/Si substrates are found to exhibit a capacitance change (tunability) of nearly 4:1. Higher tunability has been attributed to the (100) texturing of the BST films and is a result of the biaxial tensile stress imposed by Si on BST making the polar axis oriented in plane. Electrical characterization shows that the dielectric permittivity increases with increase in film thickness (up to ∼200 nm).
Enhanced magnetic coercivities in Fe nanowires
Authors: Grobert N. , Hsu W. K. , Zhu Y. Q. , Hare J. P. , Kroto H. W. , Walton D. R. M. , Terrones M. , Terrones H. , Redlich Ph. , Rühle M. , Escudero R. , Morales F.
Published: July 2002

We describe a way of generating films (<2 mm2; <40 μm thick) of aligned Fe-filled carbon nanotubes. These Fe nanowires are usually composed of single Fe crystals, and have dimensions from 5–40 nm outer diameter and <10 μm in length. The carbon tubes, which coat the wires, have external diameters of ∼20–70 nm and are <40 μm in length. High-resolution electron energy loss spectroscopy, x-ray powder diffraction, and elemental mapping of the tubular structures reveal only characteristic metallic signals and the effective absence of oxygen (or any other nonmetallic element) within the wires. The material exhibits coercivities in the 430–1070 Oe range, i.e., greater than those reported for Ni and Co nanowires.
Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers
Authors: Ashrafi A. B. M. Almamun , Ueta Akio , Avramescu Adrian , Kumano Hidekazu , Suemune Ikuo , Ok Young-Woo , Seong Tae-Yeon
Published: July 2002

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.
Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon
Authors: Yoo J. H. , Jeong S. H. , Mao X. L. , Greif R. , Russo R. E.
Published: July 2002

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.
Large zinc cation occupancy of octahedral sites in mechanically activated zinc ferrite powders
Authors: Oliver S. A. , Harris V. G. , Hamdeh H. H. , Ho J. C.
Published: July 2002

The cation site occupancy of a mechanically activated nanocrystalline zinc ferrite powder was determined as (Zn0.552+Fe0.183+)tet[Zr0.452+Fe1.823+]octO4 through analysis of extended x-ray absorption fine structure measurements, showing a large redistribution of cations between sites compared to normal zinc ferrite samples. The overpopulation of cations in the octahedral sites was attributed to the ascendance in importance of the ionic radii over the crystal energy and bonding coordination in determining which interstitial sites are occupied in this structurally disordered powder. Slight changes are observed in the local atomic environment about the zinc cations, but not the iron cations, with respect to the spinel structure. The presence of Fe3+ on both sites is consistent with the measured room temperature magnetic properties.
Atomic force microscopy using an integrated comb-shape electrostatic actuator for high-speed feedback motion
Authors: Akiyama T. , Staufer U. , de Rooij N. F.
Published: July 2002

A microfabricated cantilever with integrated comb-shape electrostatic actuator is proposed to yield a high-speed feedback motion in atomic force microscopy with optical deflection detection. The actuator has a linear response to the driving voltage. The dynamic range of tip amounts to 1 μm with an actuation efficiency of 11.4 nm/V. Using this cantilever, an imaging bandwidth of ∼80 kHz was obtained. High-speed constant force imaging with a tip velocity of 1.22 mm/s is demonstrated. The tip-sample force is verified to be constant by using a cantilever with an integrated piezoresistor as the scanned sample.
Ferroelectricity and electronic defect characteristics of c-oriented Sr0.25Ba0.75Nb2O6 thin films deposited on Si substrates
Authors: Yang Y. S. , Ryu M. K. , Joo H. J. , Lee S. H. , Lee S. J. , Kang K. Y. , Jang M. S.
Published: July 2002

Sr 0.25 Ba 0.75 Nb 2 O 6 (SBN) thin films postannealed at 750 °C for 30 min were epitaxially grown in the [00l] direction on p-type (100) Si substrates by the rf magnetron sputtering method. The capacitance–voltage (C–V) characteristics of Au/SBN/Si capacitors, which depend on postannealing conditions, were measured. The C–V curve of the crystalline SBN film had a hysteresis curve with a clockwise rotation. The memory window and surface charge density calculated from the hysteresis loop are 2.1 V and 85 nC/cm2, respectively. From the data of a deep level transient spectroscopy, the activation energies of major traps in the crystalline SBN film obtained were about Ev+0.26 eV and Ev+0.28 eV, by using the Arrhenius plot.
Carrier transport and electron field-emission properties of a nonaligned carbon nanotube thick film mixed with conductive epoxy
Authors: Lee Yun-Hi , Kim Dong-Ho , Hoon-Kim  , Ju Byeong-Kwon
Published: July 2002

We have studied the conduction characteristics of multiwalled carbon nanotubes (MWNTs), which were screen printed in a thick film form for field-emission displays. Resistivity and magnetoresistance were measured as a function of temperature T in the range of 1.7–390 K and magnetic field, respectively. The resistivity of the MWNTs for temperatures of 10–390 K indicates that the system is intrinsically metallic and the resistivity–temperature characteristics are well described by the Mott’s T−1/4 law in temperatures above 10 K, suggesting that the density of states at the Fermi level is constant in the range of 10–100 K. We found that the main contribution to the conductivity comes from carriers that hop directly between localized states via variable-range hopping. The temperature dependence above 10 K is in good agreement with that of an individual multiwalled carbon nanotube. However, below 10 K the resistivity is well fit to Efros T−1/2 law, confirming the presence of a Coulomb gap for the system. With the decrease of temperature below 10 K the charge carriers in the system are localized by strong disorder, bringing a nearly insulating state. The thick-film form for large-area display resulted in a highly bright light as well as a very low turn-on field just like individual multiwalled nanotubes at room temperature. Also, the electron field-emission characteristics followed typical Fowler–Nordheim conduction under high electric field.
Nonlinear properties of small amplitude dust ion acoustic solitary waves
Authors: Ghosh Samiran , Sarkar S. , Khan Manoranjan , Gupta M. R.
Published: July 2002

In this paper some nonlinear characteristics of small amplitude dust ion acoustic solitary wave in three component dusty plasma consisting of electrons, ions, and dust grains have been studied. Simultaneously, the charge fluctuation dynamics of the dust grains under the assumption that the dust charging time scale is much smaller than the dust hydrodynamic time scale has been considered here. The ion dust collision has also been incorporated. It has been seen that a damped Korteweg–de Vries (KdV) equation governs the nonlinear dust ion acoustic wave. The damping arises due to ion dust collision, under the assumption that the ion hydrodynamical time scale is much smaller than that of the ion dust collision. Numerical investigations reveal that the dust ion acoustic wave admits only a positive potential, i.e., compressive soliton.
Effect of signal modulated optical illumination on the Schrödinger wave function in a quantum well in a modulation doped field effect transistor and related device characteristics
Authors: Singh P. K. , Pal B. B.
Published: July 2002

The effect of signal modulated optical radiation on the Schrödinger wave function has been studied in a quantum well in a n-AlGaAs/GaAs modulation doped field effect transistor (MODFET) and related device characteristics. Partial depletion of the active region of the modulation doped field effect transistor has been considered. At the heterojunction interface two different models for the quantum well has been assumed: (1) a triangular potential well and (2) a modified triangular potential well of finite depth. From the knowledge of frequency dependent Schrödinger wave function sheet concentration of the two-dimensional electron gas, the drain-source current of the MODFET and the transfer characteristics as a function of signal frequency have been evaluated. The frequency dependent current–voltage characteristics are compared with the published theoretical results since no experimental data are available on similar studies.
Preparation of n-type conductive transparent thin films of AgInO2:Sn with delafossite-type structure by pulsed laser deposition
Authors: Ibuki Shuntaro , Yanagi Hiroshi , Ueda Kazushige , Kawazoe Hiroshi , Hosono Hideo
Published: July 2002

Sn-doped AgInO2 thin films were prepared on α-Al2O3(0001) single-crystal substrates by pulsed laser deposition. The films prepared under optimized conditions have high optical transmittance up to the near-ultraviolet region and high electrical conductivity. The optical band gap was estimated to be ∼4.1 eV, and electrical conductivity was 7.3×101S cm−1 at 300 K. The carrier concentration and Hall mobility at 300 K were 3.3×1020 cm−3 and 1.4 cm2 V−1s−1, respectively.
Double critical behavior and micellar size effect in the multicomponent surfactant solution
Authors: Isojima T. , Fujii S. , Kubota K. , Hamano K.
Published: July 2002

Multicomponent surfactant system, sodium dodecyl sulfate and butanol in an aqueous NaCl solution, which exhibits a closed-loop type reentrant phase separation, was investigated by light scattering experiments focusing on the double critical behaviors and the finite micellar size effect on the dynamic critical behavior. The loop size decreased with the decrease of NaCl concentration, and the double critical point appeared. Approaching the double critical point, the critical exponents ν for the long-range correlation length ξ and γ for the isothermal osmotic compressibility χT determined by use of the spinodal divergency theory showed a crossover behavior from Fisher’s renormalized Ising model values to the doubling of them. ξ and χT formed master curves as a function of εUL=|(TU−T)(TL−T)|/TUTL, and had the effective critical exponents ν=0.73 and γ=1.41 ascertaining the validity of Fisher’s renormalized Ising model. Here, TU and TL are the upper and lower critical solution temperatures, respectively. Dynamic critical behaviors were well explained in a unified manner over the hydrodynamic and critical regions by the modified dynamical droplet model taking the finite micellar size into account, which assumes that the local fluctuations activated thermally behave as the physical clusters with a fractal dimension of df=2.49 and a polydispersity exponent of τ=2.21. The evaluated monomer unit sizes are in good agreement with the micellar sizes obtained independently.
Probe diagnostics of supermagnetron plasma with applications of continuous and pulse-modulated rf electric fields
Authors: Kinoshita Haruhisa , Fukushima Fumihiko , Kando Masashi , Nakagawa Yukito , Tsukada Tsutomu
Published: July 2002

Parameters of supermagnetron plasma produced by rf (radio frequency) pulse, with 49.6 ms on and 0.4 ms off, were investigated by probe characteristics measured at 10–100 μs after the rf pulse in order to avoid disturbance in the measurement. The plasma parameters obtained by a continuous rf electric field and the phase differences of the rf electric field to both parallel electrodes were compared with those of rf pulse plasma. A double probe was mainly used in Ar as a standard gas, and a single probe was used in He gas for rf electric field free measurements using pulse-modulated rf plasma. He gas was used for extended time measurements because of a low probe contamination as a result of sputtering from two electrodes. A high electron density of an order of 1011 cm−3 was observed at low gas pressure (20 mTorr) for Ar plasma, and a low electron temperature of 1.1–1.2 eV was observed for He rf plasma. As for an rf phase difference dependence between two rf powers supplied to two parallel electrodes, maximum and minimum electron densities were observed at phase differences of 180° and 0°, respectively.
Void-containing materials with tailored Poisson’s ratio
Authors: Goussev Olga A. , Richner Peter , Rozman Michael G. , Gusev Andrei A.
Published: July 2002

Assuming square, hexagonal, and random packed arrays of nonoverlapping identical parallel cylindrical voids dispersed in an aluminum matrix, we have calculated numerically the concentration dependence of the transverse Poisson’s ratios. It was shown that the transverse Poisson’s ratio of the hexagonal and random packed arrays approached 1 upon increasing the concentration of voids while the ratio of the square packed array along the principal continuation directions approached 0. Experimental measurements were carried out on rectangular aluminum bricks with identical cylindrical holes drilled in square and hexagonal packed arrays. Experimental results were in good agreement with numerical predictions. We then demonstrated, based on the numerical and experimental results, that by varying the spatial arrangement of the holes and their volume fraction, one can design and manufacture voided materials with a tailored Poisson’s ratio between 0 and 1. In practice, those with a high Poisson’s ratio, i.e., close to 1, can be used to amplify the lateral responses of the structures while those with a low one, i.e., close to 0, can largely attenuate the lateral responses and can therefore be used in situations where stringent lateral stability is needed.
Fourier grid Hamiltonian multiconfigurational self-consistent-field: A method to calculate multidimensional hydrogen vibrational wavefunctions
Authors: Webb Simon P. , Hammes-Schiffer Sharon
Published: July 2002

The Fourier Grid Hamiltonian Multiconfigurational Self-Consistent-Field (FGH-MCSCF) method for calculating vibrational wavefunctions is presented. This method is designed to calculate multidimensional hydrogen nuclear wavefunctions for use in mixed quantum/classical molecular dynamics simulations of hydrogen transfer reactions. The FGH-MCSCF approach combines a MCSCF variational method, which describes the vibrational wavefunctions as linear combinations of configurations that are products of one-dimensional wavefunctions, with a Fourier grid method that represents the one-dimensional wavefunctions directly on a grid. In this method a full configuration interaction calculation is carried out in a truncated one-dimensional wavefunction space [analogous to complete active space self-consistent-field (CASSCF) in electronic structure theory]. A state-averaged approach is implemented to obtain a set of orthogonal multidimensional vibrational wavefunctions. The advantages of the FGH-MCSCF method are that it eliminates the costly calculation of multidimensional integrals, treats the entire range of the hydrogen coordinates without bias, avoids the expensive diagonalization of large matrices, and accurately describes ground and excited state hydrogen vibrational wavefunctions. This paper presents the derivation of the FGH-MCSCF method, as well as a series of test calculations on systems comparing its performance with exact diagonalization schemes.
Symmetry-adaptive ferroelectric mesostates in oriented Pb(BI1/3BII2/3)O3–PbTiO3 crystals
Authors: Viehland Dwight
Published: July 2002

An intermediate orthorhombic ferroelectric phase has been found in oriented crystals of (0.92)Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 (PZN-PT 92/8). Investigations have been performed by electrically induced polarization and strain methods, and reciprocal phase space mapping. The lattice parameters of this intermediate ferroelectric state have been shown to be equal to those predicted by the adaptive martensite theory. A hierarchy of symmetries is believed to exist on the mesoscale which are due to symmetry reductions by domain averaging.
Residual stress in deuterium implanted nominal copper coatings
Authors: Inal M. Y. , Alam M. , Peascoe R. A. , Watkins T. R.
Published: July 2002

The effects of deuterium (D) implantation on the residual stresses in Cu and CuAl2 phases present in nominal Cu coatings (containing Al) deposited on Al-alloy (Al-6061) substrates were measured using an x-ray diffraction technique. The coatings were deposited by radio frequency magnetron sputtering of a pure Cu target under identical conditions and Al was incorporated in the coatings during growth by diffusion from the substrate. Deuterium was implanted in the coatings at energies of 40 or 40+120 keV with fluences of 1×1021, 2×1021, or 3×1021 D+/m2. Pole figures of the Cu phase in the coatings prior to and after implantation indicated no effect of implantation on the fibrous texture. Triaxial stress analysis indicated the surface normal stress component to be negligible in Cu and slightly tensile in CuAl2 under all conditions. Furthermore, under all conditions, the in-plane residual stresses in both phases were found to be compressive and nearly isotropic. The magnitude of the isotropic compressive stress was always higher in CuAl2 as compared to Cu. The compressive residual stresses in the Cu phase changed only mildly with increasing coating weight, ion energy, and fluence. However, in the CuAl2 phase the compressive residual stresses changed markedly with increasing ion energy (initial decrease followed by leveling off) and increasing ion fluence (initial decrease followed by an increase), but remained unaffected by increasing coating weight.
Radiometric analysis of the light coupled by optimally cut plastic optical fiber amplitude modulating reflectance displacement sensors
Authors: Hadjiloucas S. , Irvine J. J. , Bowen J. W.
Published: July 2002

A radiometric analysis of the light coupled by optical fiber amplitude modulating extrinsic-type reflectance displacement sensors is presented. Uncut fiber sensors show the largest range but a smaller responsivity. Single cut fiber sensors exhibit an improvement in responsivity at the expense of range. A further increase in responsivity as well as a reduction in the operational range is obtained when the double cut sensor configuration is implemented. The double cut configuration is particularly suitable in applications where feedback action is applied to the moving reflector surface.
Filamentless operation of a high-power electron bombardment furnace used for refractory metals atom beam generation
Authors: Bhatia M. S. , Dongare A. S. , Mago V. K. , Lal B.
Published: July 2002

Filamentless operation of a 10 kW electron bombardment vacuum furnace in the temperature range 2000–3000 K is described. This novel mode of operation was useful in obtaining low magnetic field ambient around the furnace. This enabled us to carry out sensitive measurement of atomic properties of refractory metal atoms with the help of well-collimated neutral atomic beams. In this furnace, the necessary electron emission was derived from the innermost heat shield, which was coated with a low work-function metal and placed closer to the boat to attain high enough temperatures. The destabilizing effect of high degree of electrothermal feedback was effectively countered by using a nonlinear ballast consisting of a bank of tungsten lamps. The ionized fraction in the atomic beam was removed with the help of biased grids. This furnace has provided uninterrupted service to many users over the last few years.
Mechanisms for Si dopant migration in molecular beam epitaxy AlxGa1−xAs
Authors: Mills A. P. , Pfeiffer L. N. , West K. W. , Magee C. W.
Published: July 2002

Because of well-known surface segregation effects in molecular beam epitaxy growth, Si dopant atoms deposited as thin layers in AlxGa1−xAs typically become distributed over many atomic layers. We have measured the Si depth distributions in (100) and (311)A samples grown at temperatures between 420 and 655 °C, with Al fraction x=0, 0.1, and 0.32. The surface migration decay length Λ for a Si atom on a growing (100) surface is strongly temperature dependent but nearly independent of x, with Λ≈8 nm at 655 °C. The x=0(100) measurements show evidence for a minimum value Λ≈0.6 nm at low temperatures and a maximum value Λ≈8.5 nm at high temperatures. The data are in accord with a thermally activated surface segregation process with activation energy (1.8±0.4) eV acting in parallel with a temperature independent surface segregation mechanism. The (311)A surface shows Λ=(3.3±0.1) nm virtually independent of temperature for x=0. The Si decay length for the (311)A surface strongly increases with x, and for x=0.32 there is no significant difference in Λ for the (100) and (311)A surfaces.
Oxidation induced precipitation in Al implanted epitaxial silicon
Authors: La Ferla A. , Galvagno G. , Giri P. K. , Franzò G. , Rimini E. , Raineri V. , Gasparotto A. , Cali D.
Published: July 2002

The behavior of Al implanted in silicon has been investigated during thermal oxidation. It has been found that precipitation of Al into Al–O-defect complexes depends on the implant energy, i.e., on the distance of the dopant from the surface. It occurs at 650 keV, but it does not at 2.0 MeV or higher energies. This phenomenon has been explained taking into account the diffusivity of self-interstitials introduced during oxidation, the oxygen present in the Si, the Al concentration, and the annealing out of defects.
Band discontinuities at epitaxial SrTiO3/Si(001) heterojunctions
Authors: Chambers S. A. , Liang Y. , Yu Z. , Droopad R. , Ramdani J. , Eisenbeiser K.
Published: July 2002

We have used photoemission methods to directly measure the valence and conduction band offsets at SrTiO3/Si(001) interfaces, as prepared by molecular-beam epitaxy. Within experimental error, the measured values are the same for growth on n- and p-Si, with the entire band discontinuity occurring at the valence band edge. In addition, band bending is much larger at the p-Si heterojunction than at the n-type heterojunction. Previously published threshold voltage behavior for these interfaces can now be understood in light of the present results.
Characterization of paramagnetic defect centers in three polytypes of dry heat treated, oxidized SiC
Authors: Macfarlane P. J. , Zvanut M. E.
Published: July 2002

This work describes the characterization of defect centers in 3C–SiC, 4H–SiC, and 6H–SiC. The different SiC crystal structures are examined with electron paramagnetic resonance after thermal oxidation, and after dry (<1 ppm H2O) N2 or O2 heat treatment. The centers are described by g values that range from 2.0025 to 2.0029, which are typical of C dangling bonds. Because the centers are activated in ambients that eliminate H2O and are passivated in ambients that contain H2O, it is suggested that the centers are C dangling bonds created during the dry heat treatment when hydrogen or a hydrogenous species releases from C bonds. The activation characteristics for the centers is the same for both 6H and 3C polytypes; however, centers in the 6H–SiC samples are passivated at lower temperatures than the centers in the 3C–SiC samples. The passivation behavior is attributed to differences in the hydrogen diffusion rates in these materials rather than significant differences in the chemistry of the centers. Etching studies conducted with hydrofluoric acid indicate that the centers are not located in the SiO2, but are located in the SiC at a distance of, at most, 200 nm from the SiO2/SiC interface.
Runaway electron production in DIII-D killer pellet experiments, calculated with the CQL3D/KPRAD model
Authors: Harvey R. W. , Chan V. S. , Chiu S. C. , Evans T. E. , Rosenbluth M. N. , Whyte D. G.
Published: July 2002

Runaway electrons are calculated to be produced during the rapid plasma cooling resulting from “killer pellet” injection experiments, in general agreement with observations in the DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] tokamak. The time-dependent dynamics of the kinetic runaway distributions are obtained with the CQL3D [R. W. Harvey and M. G. McCoy, “The CQL3D Code,” in Proceedings of the IAEA Technical Committee Meeting on Numerical Modeling, Montreal, 1992 (International Atomic Energy Agency, Vienna, 1992), p. 489] collisional Fokker–Planck code, including the effect of small and large angle collisions and stochastic magnetic field transport losses. The background density, temperature, and Zeff are evolved according to the KPRAD [D. G. Whyte and T. E. Evans et al., in Proceedings of the 24th European Conference on Controlled Fusion and Plasma Physics, Berchtesgaden, Germany (European Physical Society, Petit-Lancy, 1997), Vol. 21A, p. 1137] deposition and radiation model of pellet–plasma interactions. Three distinct runway mechanisms are apparent: (1) prompt “hot-tail runaways” due to the residual hot electron tail remaining from the pre-cooling phase, (2) “knock-on” runaways produced by large-angle Coulomb collisions on existing high energy electrons, and (3) Dreicer “drizzle” runaway electrons due to diffusion of electrons up to the critical velocity for electron runaway. For electron densities below ≈1×1015 cm−3, the hot-tail runaways dominate the early time evolution, and provide the seed population for late time knock-on runaway avalanche. For small enough stochastic magnetic field transport losses, the knock-on production of electrons balances the losses at late times. For losses due to radial magnetic field perturbations in excess of ≈0.1% of the background field, i.e., δBr/B⩾0.001, the losses prevent late-time electron runaway.
Defects and interfaces in an epitaxial ZnO/LiTaO3 heterostructure
Authors: Lim S.-H. , Shindo D.
Published: July 2002

ZnO films were grown on a (011̄2) r-plane LiTaO3 substrate by electron cyclotron resonance-assisted molecular-beam epitaxy. The detailed structures of the interface and defects are investigated by high-resolution electron microscopy and image simulation. The epitaxial relationship was found to be [0001]ZnO∥[01̄11]LiTaO3 and (112̄0)ZnO∥(011̄2)LiTaO3. This epitaxial relationship corresponds to the c axis of ZnO parallel to the piezoelectric r-plane LiTaO3, which results in the enhanced electromechanical coupling factor. The interfaces were very smooth and structurally semicoherent with a comparative regular array of misfit dislocations at the interface accommodating a lattice mismatch of 9.49% when the incident electron beam is parallel to the [0001]ZnO∥[01̄11]LiTaO3 direction. A high efficiency of transfer of acoustic energy across the interface is expected for surface acoustic wave devices with such an interface. The dominant defects commonly observed in ZnO films were found to be the type-I1 intrinsic stacking fault. The formation of stacking faults was shown to be growth kinetics on particular crystallographic planes during the initial stage of film growth. The effects of these defects and interfaces on electrical and optical properties for device applications are discussed.
Comparison of surface spin wave modes at Fe/MnF2 and Fe/Mn interfaces
Authors: Siu I. L. , Islam M. N. , Walker J. C.
Published: July 2002

We have studied the thermal demagnetization in semi-infinite ferromagnets in Fe/MnF2 and Fe/Mn bilayers using Mössbauer spectroscopy. We find that the hyperfine field at the Fe/MnF2 interface follows a quasilinear temperature dependence, which reverts to a T3/2 dependence further into the bulk. The region in which linear temperature dependence was observed also showed significantly higher spin canting than in the film’s bulk layers. The interface in the Fe/Mn system immediately showed a T3/2 dependence which persisted deeper into the bulk. We attribute the linear temperature behavior to surface spin wave modes created by a perpendicular surface anisotropy at the interface. This behavior diminishes farther away from the interface, until the hyperfine field goes like T3/2 as expected for bulk, 3D spin waves. We conclude that the perpendicular surface anisotropy is much stronger at the Fe/MnF2 than the Fe/Mn interface.
Nonlinear evolution of double tearing modes
Authors: Ishii Y. , Azumi M. , Kurita G. , Tuda T.
Published: July 2002

The linear and nonlinear behaviors of the tearing mode is systematically studied for nonmonotonic q-profiles on the basis of the reduced magnetohydrodynamics (MHD) equations in cylindrical geometry, and some new features about the double tearing mode are revealed. The linear eigenmode scales as a resistive internal mode for a small distance between two rational surfaces with the same q-value, Δrs, and as the tearing one for large Δrs. New nonlinear phenomena appear in the tearing mode regime and for shorter Δrs. The linear eigenfunction shows sharply localized fluid motion at both resonant surfaces and small but global convective motion between the resonant surfaces; consequently the mode goes through a Rutherford-type regime. When the islands have grown enough, the mode shows explosive growth. This results from the nonlinear coupling among the higher harmonics, so that the inner and outer magnetic islands interact with each other leading to an internal disruption.
Photoacoustic cell for spectroscopy at low temperatures
Authors: Störkel U. , Grill W.
Published: July 2002

Based on the photoacoustic effect we have developed a cell for application in optical spectroscopy at low temperatures (between 1 and 2 K). A superconducting bolometer serves as a detector for temperature waves (so-called second sound) in liquid helium. These waves are generated after transformation of the absorbed light energy into heat due to nonradiative decay in the sample. The cell consists of an elliptical mirror with two focal points, in which the sample and the detector are placed. In this article we show some characteristic properties of the cell. In addition to that we present an absorption spectrum of a doped inorganic glass as a first example of possible applications of the photoacoustic method.
Plasma heating and ultrafast semiconductor laser modulation through a terahertz heating field
Authors: Li Jianzhong , Ning C. Z.
Published: July 2002

Electron-hole plasma heating and ultrafast modulation in a semiconductor laser under a terahertz electrical field are investigated using a set of hydrodynamic equations derived from the semiconductor Bloch equations. The self-consistent treatment of lasing and heating processes leads to the prediction of a strong saturation and degradation of modulation depth even at moderate terahertz field intensity. This saturation places a severe limit to bandwidth achievable with such scheme in ultrafast modulation. Strategies for increasing modulation depth are discussed.
Influence of nickel substrate grain structure on YBa2Cu3O7−x supercurrent connectivity in deformation-textured coated conductors
Authors: Feldmann D. M. , Reeves J. L. , Polyanskii A. A. , Kozlowski G. , Biggers R. R. , Nekkanti R. M. , Maartense I. , Tomsic M. , Barnes P. , Oberly C. E. , Peterson T. L. , Babcock S. E. , Larbalestier D. C.
Published: July 2002

Coupled magneto-optical imaging and local misorientation angle mapping have been used to demonstrate the percolative nature of supercurrent flow in YBa2Cu3O7−x(YBCO) coated conductors grown on deformation-textured Ni substrates. Barriers to current flow occur at many YBCO grain boundaries (GBs) which have propagated through the buffer layers from the underlying Ni substrate, and all Ni GBs with misorientation angles >4° initiate percolative current flow. This type of current barrier is characteristic of the conductor form and has been found to exist in samples with Jc(0 T,77 K) values >2 MA/cm2. Sharpening of the local substrate texture or improving in low-angle GB properties should lead to higher Jc values.
Development of a high-speed, reciprocating electrostatic probe system for Hall thruster interrogation
Authors: Haas James M. , Gallimore Alec D. , McFall Keith , Spanjers Greg
Published: July 2002

The use of electrostatic probes to measure local plasma parameters inside the discharge chamber of a Hall thruster presents significant difficulties. The high-temperature, dense plasma, and Hall current in the accelerating channel heat the probe rapidly causing ablation of probe material, which perturbs thruster operation and reduces probe lifetime. Results are presented which show the extent of perturbation to discharge current, cathode potential, and thrust for the case where probe material is ablated. A simple thermal model of probe material heating is developed and ablation times for a typical probe configuration are presented. Using the results of the thermal model, a high-speed axial reciprocating probe (HARP) system was developed to enable probe survival and reduce thruster perturbations during interrogation of the discharge chamber of a Hall thruster. Results using the HARP system are presented showing a significant reduction in thruster perturbation. The results also indicate that a mechanism other than material ablation is contributing to perturbation of the thruster. Based on emissive probe data, the tungsten conductor appears to provide a low impedance path between magnetic field lines, enhancing electron transport to the anode.
Optical gain of strained GaAsSb/GaAs quantum-well lasers: A self-consistent approach
Authors: Liu Guobin , Chuang Shun-Lien , Park Seoung-Hwan
Published: July 2002

We present a self-consistent model for the band structure and optical gain spectrum of GaAs1−xSbx/GaAs quantum-well (QW) lasers with carrier population. Experimental data indicate that this material system has a type-II QW configuration. By fitting the experimental photoluminescence data from various groups using our proposed empirical model, which assumes that 90% of the band-gap bowing parameter (1.2 eV) appears in the valence band, we find that the unstrained valence band edge discontinuity ratio Qv0 is close to 0.9 for an arbitrary Sb mole composition x of GaAs1−xSbx/GaAs QWs and the QW structure becomes type-II when the compressive strain induced band-edge shifts are taken into account. We show that for type-II QWs the self-consistent solution, which solves the Schrödinger equation and Poisson equation simultaneously, is necessary. Due to the free-carrier screening effect in the self-consistent potential, the electrons are attracted toward the barrier–well interfaces by the holes in the wells, and, therefore, sufficient gain can be achieved for lasing action with type-II QWs. Our theoretical results are compared with experimental data of laser operation wavelength and modal gain, with reasonable agreement. Our model will be important for modeling of type-II QW lasers and useful for design of GaAsSb/GaAs QW vertical-cavity surface-emitting lasers.
The AM(1) automata related to crystals of symmetric tensors
Authors: Hatayama G. , Hikami K. , Inoue R. , Kuniba A. , Takagi T. , Tokihiro T.
Published: July 2002

A soliton cellular automaton associated with crystals of symmetric tensor representations of the quantum affine algebra Uq′(AM(1)) is introduced. It is a crystal theoretic formulation of the generalized box–ball system in which capacities of boxes and carriers are arbitrary and inhomogeneous. Scattering matrices of two solitons coincide with the combinatorial R matrices of Uq′(AM−1(1)). A piecewise linear evolution equation of the automaton is identified with an ultradiscrete limit of the nonautonomous discrete Kadomtsev–Petviashivili equation. A class of N soliton solutions is obtained through the ultradiscretization of soliton solutions of the latter.
Spectroscopy of single metallic nanoparticles using total internal reflection microscopy
Authors: Sönnichsen C. , Geier S. , Hecker N. E. , von Plessen G. , Feldmann J. , Ditlbacher H. , Lamprecht B. , Krenn J. R. , Aussenegg F. R. , Chan V. Z-H. , Spatz J. P. , Möller M.
Published: July 2002

We have developed a simple, fast, and flexible technique to measure optical scattering spectra of individual metallic nanoparticles. The particles are placed in an evanescent field produced by total internal reflection of light from a halogen lamp in a glass prism. The light scattered by individual particles is collected using a conventional microscope and is spectrally analyzed by a nitrogen-cooled charge-coupled-device array coupled to a spectrometer. This technique is employed to measure the effect of particle diameter on the dephasing time of the particle plasmon resonance in gold nanoparticles. We also demonstrate the use of this technique for measurements in liquids, which is important for the potential application of particle plasmons in chemical or biological nanosensors.
Discharge phenomena of an atmospheric pressure radio-frequency capacitive plasma source
Authors: Park Jaeyoung , Henins I. , Herrmann H. W. , Selwyn G. S. , Hicks R. F.
Published: July 2002

Discharge phenomena of a nonthermal atmospheric pressure plasma source have been studied. An atmospheric pressure plasma jet (APPJ) operates using rf power and produces a stable homogeneous discharge at atmospheric pressure. After breakdown, the APPJ operation is divided into two regimes, a “normal” operating mode when the discharge is stable and homogeneous, and a “failure” mode when the discharge converts into a filamentary arc. Current and voltage (I–V) characteristics and spatially resolved emission intensity profiles have been measured during the normal operating mode. These measurements show that the APPJ produces an alpha (α) mode rf capacitive discharge. Based upon a dimensional analysis using the observed I–V characteristics, a rough estimate is made for plasma density of 3×1011 cm−3 and an electron temperature of 2 eV. In addition, the gas temperature of 120 °C has been spectroscopically measured inside the discharge. These plasma parameters indicate that the APPJ shows promise for various materials applications as it can produce substantial amounts of reactive species and avoid thermal damages, while having the advantage of atmospheric pressure operation.
Spectroscopy of the anharmonic cantilever oscillations in tapping-mode atomic-force microscopy
Authors: Stark Martin , Stark Robert W. , Heckl Wolfgang M. , Guckenberger Reinhard
Published: July 2002

By spectroscopic analysis of the cantilever oscillation in tapping-mode atomic-force microscopy (TM–AFM), we demonstrate that the transition from an oscillatory state dominated by a net attractive force to the state dominated by repulsive interaction is accompanied by the enhanced generation of higher harmonics. The higher harmonics are a consequence of the nonlinear interaction and are amplified to significant amplitudes by the eigenmodes of the cantilever. The results show that in a quantitative description of TM–AFM higher eigenmode excitation must be considered to account for internal energy dissipation.
Zero-field magnetic resonance of the photo-excited triplet state of pentacene at room temperature
Authors: Yang Tran-Chin , Sloop David J. , Weissman S. I. , Lin Tien-Sung
Published: July 2002

The pulsed EPR free induction decay (FID) signals of the photo-excited pentacene triplet state are reported for three mixed crystals at room temperature: pentacene-h14 in p-terphenyl, pentacene-h14 in benzoic acid, and pentacene-d14 in p-terphenyl. The recorded FID signals have relatively long decay times of about four microseconds, presumably due to the reduced hyperfine interactions in the zero magnetic field. The time domain FID signals transform to spectral components typically narrower than 500 kHz, allowing us to determine the pentacene triplet zero field splitting parameters to better accuracy than previously reported. Further, a new experimental technique using the high speed magnetic field jumping capability enables us to examine the anisotropic hyperfine and quadrupole interactions.