bluuebunny/crossref_metadata_2025 huggingface dataset:
A reflectometer for fluctuation and correlation studies on the Joint European Torus tokamak
Authors: Conway G. D. , Vayakis G. , Fessey J. A. , Bartlett D. V.
Published: July 2002

An integrated system of correlation reflectometers have been installed on the Joint European Torus tokamak. The diagnostic consists of three X-mode, heterodyne, dual-antenna (bistatic), correlation reflectometers. Two of the reflectometers are dual channel systems at fixed frequencies of 75 GHz (edge measurements) and 105 GHz (core measurements) with a selectable poloidal (25 mm) or toroidal (40 mm) antenna separation. The third reflectometer is a two frequency system for radial measurements with one channel fixed at 92 GHz and the other swept between 92 and 96 GHz. All three reflectometers operate simultaneously through the same antenna cluster. The diagnostic can provide unique information on the spatial and temporal characteristics of plasma fluctuations. The capabilities of the system are illustrated with a selection of results.
Design of a scanning Josephson junction microscope for submicron-resolution magnetic imaging
Authors: Plourde B. L. T. , Van Harlingen D. J.
Published: July 2002

We describe a magnetic field scanning instrument designed to extend the spatial resolution of scanning superconducting quantum interference device microscopy into the submicron regime. This instrument, the scanning Josephson junction microscope, scans a single Josephson junction across the surface of a sample, detecting the local magnetic field by the modulation of the junction critical current. By using a submicron junction and a scanning tunneling microscope feedback system to maintain close proximity to the surface, magnetic field sensitivity of 10 μG with a spatial resolution of 0.3 μm should be attainable, opening up new opportunities for imaging vortex configurations and core structure in superconductors and magnetic domains in magnetic materials.
The drift source: A negative ion source module for direct current multiampere ion beams
Authors: Simonin A. , Delogu G. , Desgranges C. , Fumelli M.
Published: July 2002

The concept of a modular large size ion source is under investigation in our laboratory for the development of the very intense (tens of amperes) negative ion (D−) beams needed for neutral beam injection in thermonuclear fusion research. The basic idea is to develop a compact small ion source producing the required ion flux (20–30 mA/cm2) over a total surface of about 200 cm2 and designed in such a way that it can easily be juxtaposed with other identical modules. A large negative ion source of any size (up to several m2) and shape could be realized as a set of several modules. The anticipated advantage of this concept is the minimization of the risk inherent in a large extrapolation in size (e.g., ≈1.0 m2 for an ignition device to test engineering concepts) of the present ion sources. In this context, we have developed and tested a source module, called the DRIFT source, whose main properties are presented in this article. The particular magnetic field configuration of this module ensures, in a simplified way, a very good plasma confinement allowing operation of the source at very low filling pressures. Up to now, a D− current of 1 A (20 mA/cm2) 50 keV energy, 1 s pulse length, was obtained with cesium vapor seeding at 0.15 Pa source pressure with an arc power of 2.5 kW/ℓ (12 kW).
Effects of doping in polymer light-emitting diodes
Authors: Romero D. B. , Schaer M. , Zuppiroli L. , Cesar B. , François B.
Published: July 2002

We observed a dramatic improvement in the performance of polymer light-emitting diodes (LEDs) upon light doping of the organic layer. The LEDs betrayed symmetrical electrical and light-emission characteristics. Their turn-on voltage is lower and their external quantum and power conversion efficiencies are higher by nearly an order of magnitude when compared with devices that utilized a nominally undoped organic layer. We attributed these results to the modification of the tunneling barrier in metal–polymer–metal junctions due to the presence of an induced polarization electric field associated with the ionized dopant counterions and charged polymer chains.
Measuring the surface tension of a liquid–gas interface by automatic stalagmometer
Authors: Molina C. , Victoria L. , Arenas A.
Published: July 2002

We present a variation of the stalagmometer method for automatically determining the surface tension of a liquid–gas interface using a pressure sensor to measure the pressure variation per drop. The presented method does not depend on a knowledge of the density of the problem liquid and obtains values with a measurement error in the range of 1%–2%. Its low cost and simplicity mean that the technique can be used in the teaching and instrumentation laboratory in the same way as other methods.
Nitrogen-filled tube as a sensor of ionizing radiation
Authors: Pejović Momčilo M. , Ristić Goran S.
Published: July 2002

The influence of secondary electrons released by gamma radiation from the cathode of a nitrogen-filled tube, on probability of electrical breakdown and memory curve, has been investigated. The results show that the nitrogen-filled tube is sensitive to gamma radiation only at longer afterglow periods in the case of overvoltage values grater than 50%. For shorter afterglow periods, the lower values of overvoltages can be used to separate the contribution of gamma rays.
New design of the variable angle infrared spectroscopic ellipsometer using double Fourier transforms
Authors: Xia Guo-Qiang , Zhang Rong-Jun , Chen Yu-Li , Zhao Hai-Bing , Wang Song-You , Zhou Shi-Ming , Zheng Yu-Xiang , Yang Yue-Mei , Chen Liang-Yao , Chu Jun-Hao , Wang Zhi-Ming
Published: July 2002

A new type of incidence-angle-variable infrared spectroscopic ellipsometer working in the 2100–10 000 wave number range has been designed and constructed. For the system, the analyzer and polarizer were driven by two microstepping motors having hollow shafts and rotating synchronously with a speed ratio of 2:1, i.e., A=2P. The incidence angle can be varied from 30° to 90° with an accuracy of 0.01°. The doubled Fourier transforms as both functions of the wave number and the azimuthal angles of the polarizer and analyzer were carried out and integrated in the system. Two sets of ellipsometric parameters measured in the experiment have been used to test the data self-consistency of the system. In data reduction, the error arising from the slight anisotropy of the stray light was corrected. In application, the complex dielectric function of the Au film was measured with a data accuracy better that 1% in the entire spectral range.
The microstructural stability of Al(Cu) lines during electromigration
Authors: Shaw T. M. , Hu C-K. , Lee K. Y. , Rosenberg R.
Published: July 2002

We have used transmission electron microscopy to examine in detail how microstructural changes take place in aluminum/copper lines adjacent to tungsten contacts during electromigration testing. Observations made on aluminum-4% copper lines show that significant changes in the grain structure surrounding precipitates occur during testing even under conditions where grain growth is not normally observed in the lines. Grain coarsening and precipitate growth produced by the applied current result in the formation of new diffusion blocking structures in the lines which are likely to significantly influence their time to failure.
Magnetic properties of rapidly quenched high remanence Zr added Sm–Fe–N isotropic powders
Authors: Yoneyama T. , Yamamoto T. , Hidaka T.
Published: July 2002

The magnetic properties and microstructures of rapidly quenched Sm–Fe–N powders with low Sm contents and Zr and Co addition have been studied. High remanence values of 9.4 kG with high HcJ values of 7–10 kOe were obtained for isotropic powders. The x-ray diffraction analysis and transmission electron microscopy (TEM) observation showed these alloy powders to consist of a nanoscale mixture with a SmFe7Nx phase with the TbCu7-type structure and α-Fe in the size range of 20–30 nm. It is presumed that the high remanence is due to the small crystal size and exchange coupling at interphase boundaries.
Lattice location of rare-earth ions in LiNbO3
Authors: Lorenzo A. , Jaffrezic H. , Roux B. , Boulon G. , García-Solé J.
Published: July 2002

The lattice location of three rare-earth ions (Pr3+, Ho3+, Yb3+) in LiNbO3 is investigated using Rutherford backscattering spectrometry/channeling techniques. All of them are found to occupy the Li+ octahedron but shifted from the Li+ regular position by different amounts (−0.45 Å for Pr3+, −0.38 Å for Ho3+, and −0.3 Å for Yb3+) as previously reported for other rare-earth ions. The amount of displacement is directly related to the variation of the ionic radius of the rare-earth ion, oppositely to transition metal ions which are not shifted. A simple model based on electronic configuration is given to account for this behavior.
1/f resistance noise in the large magnetoresistance manganites
Authors: Alers G. B. , Ramirez A. P. , Jin S.
Published: July 2002

Resistance noise has been measured in thin films of La0.6Y0.07Ca0.33MnO3 at the ferromagnetic metal to paramagnetic-insulator transition where a large magnetoresistance is observed. Fluctuations in resistance were found to have a 1/f power law form throughout the transition where f is frequency. This intrinsic 1/f noise is much larger than what is typical for metallic films and is consistent with the fluctuation-dissipation theorem and magnetization fluctuations in the ferromagnetic phase. The implications of this large low frequency noise for practical magnetoresistive devices will be addressed.
Absolute density measurements of ammonia synthetized in N2–H2 mixture discharges
Authors: Amorim J. , Baravian G. , Sultan G.
Published: July 2002

Measurements of absolute density of ammonia in nitrogen–hydrogen mixture dc glow discharges were performed by employing photofragment translational spectroscopy associated with laser induced fluorescence (LIF) of the H ground state atoms. The dissociation energy of ammonia using the photolysis at 205 nm was determined to be (4.34±0.07) eV. The H atom LIF measurements were calibrated by vacuum ultraviolet absorption spectroscopy on the Lyman-α (121 nm) line. The ammonia synthesized in the discharge has a maximum density of about 8×1012 cm−3 for a 75% H2–25% N2 mixture, a discharge current of 50 mA and a pressure of 2 Torr.
Tunable optical filters using a LiNbO3 torsional actuator with a Fabry–Perot etalon
Authors: Nakamura Kiyoshi , Kurosawa Yoko , Ishikawa Kazuo
Published: July 2002

An electrically tunable optical filter using a LiNbO3 torsional actuator with a Fabry–Perot solid etalon is proposed for use in wavelength division multiplexed (WDM) optical communication systems. The Fabry–Perot solid etalon acts as an optical filter, while a LiNbO3 plate of particular orientation with a ferroelectric inversion layer acts as a torsional actuator with no hysteresis. This enables the actuator to precisely rotate the Fabry–Perot etalon and thereby tune the center wavelength of the filter by application of a voltage. Such a tunable optical filter for the 1310 nm wavelength region fabricated using a 0.1 mm thick etalon is demonstrated to have a tunable range of 3 nm and a 3 dB bandwidth (FWHM) of 0.16∼0.26 nm for an applied voltage of +500 to −500 V.
Vibrational spectroscopy of arsenic-hydrogen complexes in ZnSe
Authors: McCluskey M. D. , Haller E. E. , Zach F. X. , Bourret-Courchesne E. D.
Published: July 2002

Using infrared absorption spectroscopy, we have observed hydrogen local vibrational modes (LVMs) in arsenic-doped ZnSe grown by metalorganic chemical vapor deposition (MOCVD). When hydrogen is used as a carrier gas, we find an absorption peak at 2165.6 cm−1, which we attribute to a bond-stretching mode of a hydrogen attached to an arsenic acceptor in a bond-centered orientation. With deuterium substituting the hydrogen carrier gas, we find an additional peak at 1557.1 cm−1 with 1/3 the intensity of the hydrogen peak. The isotopic frequency ratio is r=vH/vD=1.3908, similar to values found in many hydrogen/deuterium-related complexes. The larger area of the hydrogen peak indicates that most of the hydrogen comes from decomposition reactions involving the metalorganic molecules. The temperature dependent shift of the ZnSe:As,H mode is proportional to the lattice thermal energy U(T), a consequence of the anharmonic coupling between the LVMs and the lattice phonons.
Properties of laser ablated porous silicon
Authors: Savin D. P. , Roizin Ya. O. , Demchenko D. A. , Mugeński E. , Sokólska I.
Published: July 2002

Luminescent films of porous silicon were obtained by laser ablation of crystalline silicon. The spectral and temporal characteristics of light emission from laser ablated porous silicon (LAPS) films critically depend on the chemical composition of the atmosphere in which they were fabricated. Luminescent characteristics, spatial structure, and chemical composition of LAPS were compared with the properties of porous silicon obtained by a standard electrochemical method on the same silicon substrate.
Measurement of non-Maxwellian electron energy distributions in an inductively coupled plasma
Authors: Hori T. , Bowden M. D. , Uchino K. , Muraoka K.
Published: July 2002

The electron energy distribution function (EEDF) in an argon planar inductively coupled plasma was measured by the method of laser Thomson scattering. In a low-pressure discharge, a clear departure from a Maxwellian distribution function was observed; an unambiguous measurement of a non-Maxwellian distribution in a low-density glow discharge. The non-Maxwellian EEDF became Maxwellian in a higher pressure discharge. These observations were attributed to the lack of randomizing collisions in the low-pressure discharge and the increase in the number of these collisions at higher pressure.
Growth temperature dependence of long-range alloy order and magnetic properties of epitaxial FexPt1−x (x≂0.5) films
Authors: Farrow R. F. C. , Weller D. , Marks R. F. , Toney M. F. , Hom S. , Harp G. R. , Cebollada A.
Published: July 2002

The structural and magnetic properties of 1000-Å-thick epitaxial FexPt1−x(001) alloy films, grown by molecular beam epitaxy with x≂0.5, have been studied as a function of growth temperature. X-ray diffraction analysis showed that the long-range order parameter increased from near zero for films grown at 100 °C to a maximum of 0.93 in films grown at 500 °C. Over this range the magnetic easy axis changed from in-plane to perpendicular and the polar Kerr rotation increased strongly. Spontaneous long-range ordering in these films has an activation energy ∼0.2 eV, consistent with an energy barrier for surface and step down diffusion.
Shallow donors in GaN studied by electronic Raman scattering in resonance with yellow luminescence transitions
Authors: Ramsteiner M. , Menniger J. , Brandt O. , Yang H. , Ploog K. H.
Published: July 2002

Electronic excitations in nominally undoped GaN have been investigated by Raman scattering. Peaks in the range between 18.5 and 30 meV have been assigned to internal shallow donor transitions in cubic and hexagonal GaN, respectively. The photon energy dependence of the scattering efficiencies in the cubic phase is explained by Raman scattering in resonance with the so-called ‘‘yellow’’ luminescence transitions. This interpretation supports models for the notorious yellow luminescence in which shallow donors are involved. These shallow donors most likely do not originate from native point defects.
Stable Ti-based quasicrystal offers prospect for improved hydrogen storage
Authors: Stroud R. M. , Viano A. M. , Gibbons P. C. , Kelton K. F. , Misture S. T.
Published: July 2002

The desorption of hydrogen from a novel material, a Ti45Zr38Ni17-H quasicrystal, was observed using high-temperature powder x-ray diffraction, demonstrating the potential utility of Ti-based quasicrystals in place of crystalline or amorphous hydrides for hydrogen storage applications. The maximum observed change in hydrogen concentration was from 61 at. %, corresponding to a hydrogen-to-metal ratio (H/M) of 1.54, at 91 °C to less than 2.5 at. % (H/M=0.025) at 620 °C. The onset temperature of desorption is below 350 °C. Surface oxidation was found to promote the formation of crystalline hydride phases. Highly oxidized samples transformed to a mixture of the C14 Laves and C15 Laves crystalline hydrides, and the Ti2Ni phase. When the oxidation was less severe, a reversible transformation between the quasicrystal and crystalline hydride phases was clearly observed, demonstrating the stability of the Ti45Zr38Ni17 quasicrystal at very low hydrogen concentrations, and temperatures as high as 661 °C. This is the first evidence for a stable Ti-based quasicrystal and for reversible hydrogen storage in a quasicrystalline phase.
Linewidth of submillimeter wave flux-flow oscillators
Authors: Koshelets Valery P. , Shitov Sergey V. , Shchukin Alexey V. , Filippenko Lyudmila V. , Mygind Jesper
Published: July 2002

A reliable technique for wide band measurements of the spectral linewidth of superconducting oscillators integrated on-chip with superconductor-insulator-superconductor (SIS) detectors has been developed. The spectral linewidth of flux-flow oscillators (FFO) based on the unidirectional and viscous flow of magnetic vortices in a long overdamped Josephson tunnel junction was measured in the frequency range 250–580 GHz, and a linewidth as low as 200 kHz was obtained at 450 GHz. Also stable frequency locking of a FFO to very high (≤60th) harmonics of an external microwave reference source has been demonstrated. The proposed technique may improve the sensitivity, frequency resolution, and stability of the fully superconducting integrated submillimeter wave receiver.
Modal dispersion phase matching over 7 mm length in overdamped polymeric channel waveguides
Authors: Jäger Matthias , Stegeman George I. , Flipse Marinus C. , Diemeer Mart , Möhlmann Guus
Published: July 2002

Phase-matched second harmonic generation over 7 mm is demonstrated using the modal dispersion of a composite 4-dimethylamino-4′-nitrostilbene/polyetherimde channel waveguide. For a 1-mm-long sample a record figure of merit for polymers of η=P2ω/(PωL2)=14%/W cm2 was measured at 1550 nm. This is comparable to the value reported for LiNbO3 at that wavelength. The efficiency in longer samples is limited by fundamental and harmonic waveguide losses.
Evidence for thin-film protection of visible photocathodes
Authors: Breskin A. , Buzulutskov A. , Chechik R. , Prager M. , Shefer E.
Published: July 2002

NaI and CsI protective coatings on visible cesium-antimony photocathodes have been studied. With NaI protective films the photocathodes are shown to withstand exposure to considerable doses of oxygen and dry air. This opens the way to their handling and operation in gas media.
High-speed and high-sensitivity silicon-on-insulator metal-semiconductor-metal photodetector with trench structure
Authors: Ho Jacob Y. L. , Wong K. S.
Published: July 2002

The frequency response and quantum efficiency (QE) of silicon-on-insulator (SOI) metal-semiconductor-metal photodetectors in the near-infrared (∼800 nm) are greatly enhanced with a simple reactive ion etching to form electrodes inside the interdigitated trenches. Detectors with 1.25 μm trench spacing were fabricated on a SOI substrate with a 6-μm-thick silicon top layer. The unique device structure isolates carriers generated deep inside the semiconductor substrate and at the same time provides a highly uniform electric field throughout the active region of the detector, resulting in an instrumentation limited response time of 23 ps at 5 V bias and a −3 dB bandwidth of 2.3 GHz as measured at 790 nm. The dc responsivity is 0.12 A/W, corresponding to an external QE of 18.7% and an internal QE of 88.5%. The large bandwidth and good responsivity at the wavelength of interest, combined with their low operating voltages, make these detectors attractive for use in short-distance optical communication systems.
Acoustic excitation of the circular Bragg–Fresnel lens in backscattering geometry
Authors: Souvorov A. , Snigireva I. , Snigirev A. , Aristova E. , Hartman Ya.
Published: July 2002

An increment of the x-ray flux in crystal Bragg–Fresnel lens (BFL) focus in backscattering geometry obtained by means of acoustic excitation of the BFL crystal substrate has been investigated. The dependence of the x ray’s total reflected power versus ultrasound parameters has been studied in a low frequency range (10–50 MHz). The proposed technique allows an increase in the flux in a BFL focus by a factor of 2 which almost achieves the kinematic limit.
Ellipsometric characterization of amorphous and polycrystalline silicon films deposited using a single wafer reactor
Authors: Borghesi A. , Giardini M. E. , Marazzi M. , Sassella A. , De Santi G.
Published: July 2002

The optical functions of amorphous and polycrystalline silicon thin films deposited on single oxidized silicon substrates by chemical vapor deposition in a wide range of deposition temperatures have been determined using spectroscopic ellipsometry. The data analysis is performed by direct inversion of the experimental spectra, therefore, obtaining results independent of any film modeling. The optical results indicate that the film structure changes as the deposition temperature increases from amorphous to polycrystalline with different grain size and distribution.
Nonlinear scanning laser microscopy by third harmonic generation
Authors: Barad Y. , Eisenberg H. , Horowitz M. , Silberberg Y.
Published: July 2002

Third harmonic generation near the focal point of a tightly focused beam is used to probe microscopical structures of transparent samples. It is shown that this method can resolve interfaces and inhomogeneities with axial resolution comparable to the confocal length of the beam. Using 120 fs pulses at 1.5 μm, we were able to resolve interfaces with a resolution of 1.2 μm. Two-dimensional cross-sectional images have also been produced.
Novel cost-effective bifacial silicon solar cells with 19.4% front and 18.1% rear efficiency
Authors: Hübner Andreas , Aberle Armin G. , Hezel Rudolf
Published: July 2002

Bifacial solar cells are a promising possibility to lower the costs of solar electricity compared to conventional monofacial solar cells. These devices utilize the sunlight more efficiently since they are able to convert light incident on both sides of the cell. In this letter, a novel, cost-effective bifacial silicon solar cell structure is presented. On float-zone silicon, the cells exhibit conversion efficiencies of 19.4% and 16.5% under 1-sun front and rear illumination, respectively. Furthermore, a “symmetrical” version of the cells is presented demonstrating front and rear efficiencies of 18.4% and 18.1%. In spite of the simplicity of the cell fabrication process, these are the highest independently confirmed efficiencies ever reported for bifacial silicon solar cells.
Hole effective masses in relaxed Si1−xCx and Si1−yGey alloys
Authors: Lin C. Y. , Liu C. W.
Published: July 2002

We report hole effective mass calculations of Si1−xCx and Si1−yGey alloys. All calculations are based on a 16×16 Hamiltonian matrix constructed from the linear combination of atomic orbital approximation with spin-orbit interaction taken into consideration. The 1 meV constant energy surfaces below the valence band edge are used to determine the nominal hole effective masses. The effective masses of light hole and heavy hole of Si1−yGey alloys vary as linear functions of Ge content and increase linearly as the hole energy increases from 1 to 15 meV. The heavy hole effective masses of Si1−xCx alloys, however, exhibit a totally different trend. The effective mass of Si1−xCx remains relatively unchanged from x=0.0 to x=0.9, and increases abruptly by a factor of two from x=0.9 to x=1.0. The nonparabolicity increases as the C content rises up to x=0.9, and nearly disappears when turning into pure diamond. The interaction between the split-off hole band and the heavy hole band is proposed for the anomalous behavior of the heavy hole effective masses of SiC alloys.
Differential measurement of the coupling constant between laser eigenstates
Authors: Brunel M. , Vallet M. , Le Floch A. , Bretenaker F.
Published: July 2002

The possibility to measure the coupling constant C between eigenstates in a laser is demonstrated. Spatial separation of orthogonally polarized modes is shown to provide a simple differential tool to measure their coupling. The difference in the behaviors of linear and helicoidal eigenstates enlightens the influence of the nature of the eigenstates on their interaction in the active medium. The obtained experimental values are C=0.16 for linear and C=0.4 for helicoidal eigenstates in a longitudinally monomode Nd:YAG laser. The method is applicable to most kinds of laser.
Design and performance of singular electric field terahertz photoconducting antennas
Authors: Cai Y. , Brener I. , Lopata J. , Wynn J. , Pfeiffer L. , Federici J.
Published: July 2002

We present new designs of more efficient terahertz (THz) radiation emitters and detectors enhanced by electric field singularities using sharp and laterally offset electrodes. We compare the performances of the terahertz emission and different polarization properties resulting from these structures. An average THz radiation power of 3 μW is achieved under 20 mW excitation, calibrated by free space electro-optic sampling. We also study the gap size dependence of the THz radiation, and find an absence of a positive electrode effect in the small gap limit.
Highly anisotropic photoenhanced wet etching of n-type GaN
Authors: Youtsey C. , Adesida I. , Bulman G.
Published: July 2002

A room-temperature photoelectrochemical etching process for n-type GaN films using a 0.04 M KOH solution and Hg arc lamp illumination is described. The process provides highly anisotropic etch profiles and high etch rates (>300 nm/min) at moderate light intensities (50 mW/cm2 @365 nm). The etch rate and photocurrent are characterized as a function of light intensity for stirred and unstirred solutions, and the etch process is found to be diffusion limited for light intensities greater than 20 mW/cm2 @365 nm. A reaction mechanism for the etch process is proposed.
Study of dynamic space charge in SrS:Ce thin-film electroluminescent devices by tunable laser excitation
Authors: Peter Manuela , Kashio Yukinori , Nishimura Satoru , Ohmi Koutoku , Tanaka Shosaku , Kobayashi Hiroshi
Published: July 2002

The origin of the dynamic space charge in SrS:Ce ac-driven thin-film electroluminescent devices is studied using photoinduced current measurements. Field assisted photoionization of Ce3+ centers and of intrinsic electron traps is observed. The results show that about 50% of the dynamic space charge in SrS:Ce originates from cerium ionization. The remaining 50% results from the ionization of crystal imperfections.
Sonic and shock waves in gas-discharge plasma
Authors: Mishin G. I.
Published: July 2002

To interpret the observed anomalous properties of weakly ionized gas-discharge plasma, the modified van der Waals’ equation of state for real gases was used. Use of this equation provides a possible explanation for the anomalous phenomena. The analysis allows the deduction that, in addition to the kinetic energy, there exists in the plasma a reservoir of potential energy from particle interaction. Estimates indicate the feasibility and potential expediency of applying this feature of gas-discharge plasma to practical problems in gas dynamics.
Space-charge-enhanced electromechanical response in thin-film polyurethane elastomers
Authors: Su J. , Zhang Q. M. , Ting R. Y.
Published: July 2002

We show that the large electrical-field-induced strain response observed in certain polyurethane elastomers is a thin-film effect. Based on the frequency dispersion of the strain response in samples with different thicknesses and the thermally stimulated discharge current on samples with different processing conditions, we suggest that the charge injection, an interface effect, which results in a nonuniform space-charge distribution and, hence, a nonuniform electric-field distribution across the sample thickness, is responsible for the enhanced electromechanical response in thin polyurethane samples.
Slater model applied to polarization graded ferroelectrics
Authors: Mantese Joseph V. , Schubring Norman W. , Micheli Adolph L. , Catalan Antonio B. , Mohammed Majed S. , Naik Ratna , Auner Greg W.
Published: July 2002

Ferroelectric thin films of BaxSr1−xTiO3 with compositional gradients normal to the growth surface have been formed by the successive deposition and annealing of films having step-variable Ba to Sr ratios. By suitably tailoring the magnitude and sense of the gradient in Ba to Sr ratio, directional potentials can be built into the structures yielding a new, but controllable, hysteresis phenomenon. Slater’s empirical model for ferroelectric materials has been extended to also describe thin films with polarization gradients normal to the growth surface, i.e., graded ferroelectric devices. This model accounts for several aspects of these structures, including: the broadness of the permittivity plots with temperature, the formation of a spontaneous potential upon oscillatory field excitation, offsets in the hysteresis graphs along the displacement axis with directions which are gradient dependent, and the electric field dependence of that offset.
Hot electron impact excitation cross-section of Er3+ and electroluminescence from erbium-implanted silicon metal-oxide-semiconductor tunnel diodes
Authors: Wang S. , Eckau A. , Neufeld E. , Carius R. , Buchal Ch.
Published: July 2002

We have demonstrated the 1.5 μm electroluminescence from implanted Er ions inside the SiO2 insulator of a silicon metal-oxide-semiconductor structure under forward bias. The Er ions are excited by the direct impact from electrons tunneling through the oxide at electric fields larger than 6 MV/cm. Under these conditions, we measured an excitation cross-section of 6±2×10−15 cm2 and a lifetime of the excited I1313/2 level of 1.5 ms.
Self-organized growth of ZnTe nanoscale islands on (001)GaAs
Authors: Longo M. , Lovergine N. , Mancini A. M. , Passaseo A. , Leo G. , Mazzer M. , Berti M. , Drigo A. V.
Published: July 2002

The Stransky–Krastanow metalorganic vapor phase epitaxy growth of self-organized ZnTe islands on homoepitaxial (001)GaAs is demonstrated. The −7.4% lattice mismatch of the ZnTe/GaAs heterostructure leads to a strain-driven distribution of nanoscale ZnTe islands on top of a two-dimensionally (2D) grown wetting layer. Atomic force microscopy and Rutherford backscattering spectrometry are used to determine the island dimensions and the thickness of the wetting layer. The density of the islands, their average diameter, and aspect ratio turn out to be about 520 μm−2, 13.6 nm, and 0.20, respectively, for a 1.2 ML thick 2D layer. Furthermore, the average aspect ratio of the islands decreases by increasing the thickness of the wetting layer, as expected by the progressive extinction of the strain-driven island nucleation.
Effect of fluorine on chemical and electrical properties of room temperature oxide films prepared by plasma enhanced chemical vapor deposition
Authors: Kim Kihong , Song Juho , Kwon Daehyuk , Lee G. S.
Published: July 2002

The effect of fluorine on SiO2 films was investigated by comparing chemical and electrical properties of fluorinated silicon oxide (SiOF) films with those of SiO2 films. The SiOF films were prepared at room temperature by plasma enhanced chemical vapor deposition incorporating CF4 as the fluorine source into the deposition process of the SiO2 films using Si2H6 and N2O. The relative dielectric constant of the as-deposited SiO2 films was reduced from 5.95 to 4.43 by the incorporation of fluorine and with postmetallization anneal. The breakdown measurements on the SiOF films showed no early failures at a field strength of ⩽3 MV/cm, resulting in an average breakdown field strength of 7.11 MV/cm.
Low voltage cathodoluminescence properties of blue emitting SrGa2S4:Ce3+ and ZnS:Ag,Cl phosphors
Authors: Zhang F.-L. , Yang S. , Stoffers C. , Penczek J. , Yocom P. N. , Zaremba D. , Wagner B. K. , Summers C. J.
Published: July 2002

The low voltage properties of ZnS:Ag and SrGa2S4:Ce3+ have been investigated for applications in field emission displays. It was observed that although ZnS:Ag,Cl has slightly better chromaticity than SrGa2S4:Ce3+, the high luminous efficiency, fast decay time, and better saturation behavior of the thiogallate potentially make it a very important blue phosphor.
Nonreciprocal TE-mode phase shift by domain walls in magnetooptic rib waveguides
Authors: Popkov A. F. , Fehndrich M. , Lohmeyer M. , Dötsch H.
Published: July 2002

The nonreciprocal wave number shift of a TE-like mode propagating along a normally magnetized gyrotropic rib waveguide supporting a domain lattice is predicted theoretically using perturbation theory. The electromagnetic fields are calculated by the effective index method and by a more rigorous numerical method. Preliminary estimations for Bi-substituted iron garnets show that the maximum nonreciprocal phase shift of a TE-like mode may be comparable to that of a TM-like mode in a tangentially magnetized waveguide.
Application of solid immersion lens to high-spatial resolution photoluminescence imaging of GaAs quantum wells at low temperatures
Authors: Yoshita Masahiro , Sasaki Takeaki , Baba Motoyoshi , Akiyama Hidefumi
Published: July 2002

A solid immersion lens (SIL) was applied to microscopic photoluminescence (PL) imaging at low temperatures. The spatial resolution corresponding to a numerical aperture of 1.0 was achieved and confirmed to be independent of temperature from 5 to 300 K. We performed PL imaging with the SIL for a GaAs quantum well and obtained stable PL images with high-spatial resolution at a wide temperature range from 5 to 200 K. From the temperature dependence of the PL images with high-spatial resolution, we can directly reveal the diffusion of photocarriers in the sample, which indicates the usefulness of the SIL technique.
Thermal stability of W and WSix contacts on p-GaN
Authors: Cao X. A. , Pearton S. J. , Ren F. , Lothian J. R.
Published: July 2002

The annealing temperature (400–1100 °C) and measurement temperature (25–300 °C) dependencies of current–voltage characteristics of W and WSi0.45 contacts on p-GaN have been compared to the more common Ni/Au metallization. At 25 °C, slightly rectifying characteristics were obtained for all three types of contact, but at 300 °C specific contact resistances in the 10−2 Ω cm2 range were obtained for WSi0.45 and Ni/Au. This is due to an increase in Mg acceptor ionization efficiency (from 10% at 25 °C to 57% at 300 °C) and more efficient thermionic hole emission across the metal-GaN interface. Both WSi0.45 and W contacts retained featureless surface morphology for annealing at >900 °C, whereas Ni/Au showed substantial islanding at ⩽700 °C.
Effect of hydrogenation on room-temperature 1.54 μm Er3+ photoluminescent properties of erbium-doped silicon-rich silicon oxide
Authors: Shin Jung H. , Seo Se-young , Lee Seok-Ju
Published: July 2002

The effect of hydrogenation on the room-temperature 1.54 μm Er3+ photoluminescent properties of erbium-doped silicon-rich silicon oxide thin films is investigated. Two samples with 7 and 1 at. % excess silicon and 0.4 at. % erbium were prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition of SiH4 and O2 with cosputtering of erbium and subsequent rapid thermal anneal at 900 °C. Hydrogenation by exposure to D plasma doubles the 1.54 μm Er3+ luminescence intensity from the high excess silicon content sample but halves that from the low excess silicon content sample. The lifetimes and excitation power dependence of Er+ luminescence show that hydrogenation primarily affects the active erbium fraction, increasing it in case of the high excess silicon sample but decreasing it in case of the low excess silicon content sample. With proper treatments, Er3+ luminescence lifetime of over 7 ms is obtained.
Two species model for deposition and erosion of carbon-nitrogen films
Authors: Aarão Reis F. D. A. , Franceschini D. F.
Published: July 2002

We propose a model of random deposition and erosion for carbon–nitrogen film growth. Particles C and N represent carbon and nitrogen atoms and are released with probabilities p and 1−p, respectively. Particle C aggregates upon contact with the surface. Particle N annihilates with a top C (N) with a probability of q (1). The line p=q/2 separates the regimes of growth and erosion. As p decreases for fixed q the deposition rate decreases, while the bulk concentration of N and the surface roughness increase. It describes qualitatively the growth of plasma deposited carbon–nitrogen films, and is in good quantitative agreement with results in acetylene–nitrogen atmospheres.
High-speed, low-noise metal–semiconductor–metal ultraviolet photodetectors based on GaN
Authors: Walker D. , Monroy E. , Kung P. , Wu J. , Hamilton M. , Sanchez F. J. , Diaz J. , Razeghi M.
Published: July 2002

We present the fabrication and characterization of nonintentionally doped GaN and GaN:Mg Schottky metal–semiconductor–metal (MSM) photodetectors, grown on sapphire by metalorganic chemical vapor deposition. Low-leakage, Schottky contacts were made with Pt/Au. The devices are visible blind, with an ultraviolet/green contrast of about five orders of magnitude. The response times of the MSM devices were <10 ns and about 200 ns for GaN and GaN:Mg, respectively. The noise power spectral density remains below the background level of the system (10−24 A2/Hz) up to 5 V, for the undoped GaN MSM detector.
Interfacial differences between SiO2 grown on 6H-SiC and on Si(100)
Authors: Jernigan G. G. , Stahlbush R. E. , Das M. K. , Cooper J. A. , Lipkin L. A.
Published: July 2002

Oxides grown on p-type 6H-SiC and on Si(100) were studied using x-ray photoelectron spectroscopy and sputter depth profiling to determine what differences exist between the two systems. The oxide on SiC is found to be stoichiometric SiO2, but the oxide is structurally different from the oxide grown on Si(100). We propose that strain introduced during processing accounts for the structural differences. We also found that Si atoms at the SiO2/SiC interface are chemically different from Si atoms in the bulk of SiC and a number of possible explanations for this are given.
Self-limitation of AlGaN/GaN quantum well energy by built-in polarization field
Authors: Grandjean N. , Massies J. , Leroux M.
Published: July 2002

Wurtzite AlGaN/GaN quantum well (QW) structures were grown by molecular beam epitaxy on c-plane sapphire substrates and the QW transition energies were measured by low temperature photoluminescence. Both the well widths and the Al mole fraction in the AlxGa1−xN(0<x<0.3) 100-Å-thick barriers have been varied in order to assess the built-in electric field present in the quantum heterostructures. It is found that the electric field increases linearly with the Al composition. The magnitude of this electric field is as high as 1 MV/cm for an Al mole fraction of 0.27. The main consequence is that whatever the investigated Al composition range, the well thickness must be lower than 30 Å in order to get a transition energy greater than the band gap of GaN.
Electrochemical deposition of nickel nanowire arrays in single-crystal mica films
Authors: Sun L. , Searson P. C. , Chien C. L.
Published: July 2002

Nanopores with lateral dimensions as small as 30 nm have been fabricated by nuclear track etching in 5 μm thick, single-crystal muscovite mica wafers. The nanopores have a diamond shape with their axes aligned with the crystal axes of mica as a result of anisotropic etching. Nickel nanowire arrays have been fabricated by electrodeposition into the nanopores. The magnetic properties of the nanowire arrays are compared with those fabricated in polymer membranes.
Nanometer-scale studies of vertical organization and evolution of stacked self-assembled InAs/GaAs quantum dots
Authors: Lita B. , Goldman R. S. , Phillips J. D. , Bhattacharya P. K.
Published: July 2002

We have investigated the vertical organization and evolution of 1-, 5-, 10-, and 20-layer stacks of molecular beam epitaxially grown self-assembled InAs/GaAs quantum dots using high resolution and large-scale cross-sectional scanning tunneling microscopy. We report results regarding the evolution of the dot sizes and shapes, and the assembly of vertically organized columns of stacked dots. As the number of dot layers within a stack is increased, the average spacing between vertically organized columns decreases, and the corresponding dots become more uniform in size. The data also suggest that the coalescence of neighboring stacks of dots has not occurred and therefore coalescence is not the mechanism leading to the observed uniform distribution of dot sizes and column spacings.
Bias-enhanced nucleation and growth of the aligned carbon nanotubes with open ends under microwave plasma synthesis
Authors: Tsai S. H. , Chao C. W. , Lee C. L. , Shih H. C.
Published: July 2002

Aligned carbon nanotubes with open ends have been fabricated on silicon wafer in one step using a microwave plasma enhanced chemical vapor deposition system with a mixture of methane and hydrogen as precursors. High concentration hydrogen plasma and high negative bias voltage to the substrate induce anisotropic etching of carbon nanotubes and can effectively reduce the randomly oriented carbon nanotubes. The mechanism of aligned carbon nanotubes with open ends is proposed in this letter.
High mobility two-dimensional electron gas in AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy
Authors: Elsass C. R. , Smorchkova I. P. , Heying B. , Haus E. , Fini P. , Maranowski K. , Ibbetson J. P. , Keller S. , Petroff P. M. , DenBaars S. P. , Mishra U. K. , Speck J. S.
Published: July 2002

High quality AlGaN/GaN heterostructures have been grown by radio-frequency plasma-assisted molecular beam epitaxy on n-type GaN templates grown on sapphire by metal organic chemical vapor deposition. The unintentionally doped Al0.12Ga0.88N/GaN heterostructure exhibits a 77 K Hall mobility of 14 500 cm2/Vs and a 12 K mobility of 20 000 cm2/Vs (ns=5.0×1012 cm−2). A room temperature mobility of 1860 cm2/Vs (ns=4.8×1012 cm−2) was calculated for the two-dimensional electron gas channel using a two layer model from the measured mobility for the whole structure (template plus heterostructure). Magnetoresistance measurements at 4.2 K showed well-resolved Shubnikov–de Haas oscillations, which began at 2.6 T.
β-SiC nanorods synthesized by hot filament chemical vapor deposition
Authors: Zhou X. T. , Wang N. , Lai H. L. , Peng H. Y. , Bello I. , Wong N. B. , Lee C. S. , Lee S. T.
Published: July 2002

A one-step procedure has been developed to grow β-SiC nanorods from a solid carbon and silicon source on a Si substrate by hot filament chemical vapor deposition. This process is catalyzed by metallic particles which come from impurities in the solid source which is a plate made by pressing a mixture graphite and silicon powders at 150 °C. Hydrogen was introduced into the reaction chamber to react with the solid plate to produce hydrocarbon and hydrosilicon radicals which presumably reacted to form SiC nanorods. The nanorods consisted of a crystalline β-SiC core with an amorphous silicon oxide shell layer and grew along the [100] direction. The nanorods were 10–30 nm in diameter and less than 1 μm in length.
Punctuated island growth: An approach to examination and control of quantum dot density, size, and shape evolution
Authors: Mukhametzhanov I. , Wei Z. , Heitz R. , Madhukar A.
Published: July 2002

The later stages of the evolution of epitaxical island quantum dots are examined systematically for InAs depositions on GaAs(001) following the conventional continuous deposition mode and an approach introduced here called punctuated island growth (PIG). The comparative study provides clear structural and optical evidence for a change in InAs island shape at a self-limiting lateral size, first reached for depositions ∼2 ML. The PIG approach has also allowed realization of the narrowest reported inhomogeneous linewidth of 23 meV for low temperature photoluminescence from a single layer of binary InAs/GaAs quantum dots.
AlGaInP/AuBe/glass light-emitting diodes fabricated by wafer bonding technology
Authors: Horng R. H. , Wuu D. S. , Wei S. C. , Huang M. F. , Chang K. H. , Liu P. H. , Lin K. C.
Published: July 2002

An AlGaInP/AuBe/glass light-emitting diode (LED) was fabricated by a wafer bonding technique. The AlGaInP LED was grown on a temporary GaAs substrate by metalorganic vapor phase epitaxy. By bonding the AuBe/glass substrate on top of epitaxial layers, the temporary GaAs substrate was removed. The luminance of this wafer-bonded device is about 3050 cd/m2 (600 nm wavelength) at an operating current of 20 mA. It is about three times brighter than a conventional device with an absorbing GaAs substrate. This could be due to the fact that the AuBe/glass substrate serves as a reflective mirror, improving the light extraction efficiency.
Incorporation of N into Si/SiO2 interfaces: Molecular orbital calculations to evaluate interface strain and heat of reaction
Authors: Ushio Jiro , Maruizumi Takuya , Miyao Masanobu
Published: July 2002

The determining factor for the accumulation of N at a Si/SiO2 interface during oxynitridation of the interface was investigated using a quantum-chemical method. Both mechanical and chemical factors (the interface strains and the heats of reaction of the oxynitridation) were considered. Though a slight relaxation of interface strain occurs when the interface has a certain type of oxygen-vacancy defect, we found the N incorporation does not relax the interface strain in most cases. The exothermicity and endothermicity of the oxynitridation reaction in the Si and SiO2 films, respectively, are the primary cause of the accumulation of N at the interface.
Variable profile capillary discharge for improved phase matching in a laser wakefield accelerator
Authors: Kaganovich D. , Sasorov P. , Cohen C. , Zigler A.
Published: July 2002

We have demonstrated the possibility to generate long plasma channels with a variable plasma density profile in a longitudinal direction. Such plasma channels can be used to control the velocity of the guided high-intensity laser beams in order to extend the acceleration distance. The measured plasma density profiles at the capillary exit were found in excellent agreement with the numerical simulations.
Control of leakage conduction of high-fatigue-endurance (Pb, La)(Zr, Ti)O3 film ferroelectric capacitors with Pt/SrRuO3 electrodes
Authors: Stolichnov I. , Tagantsev A. , Setter N. , Cross J. S. , Tsukada M.
Published: July 2002

Leakage conduction of (Pb, La)(Zr, Ti)O3 (PLZT) films grown on Pt bottom electrode, with Pt and Pt/SrRuO3 (Pt/SRO) top electrodes is studied. It is found that the conduction behavior of the ferroelectric capacitors strongly varies depending on the degree of interdiffusion from the electrode into the ferroelectric material. If the diffusion is limited, the conduction properties of the Pt/SRO/PLZT/Pt capacitor are similar to that of the conventional Pt/PLZT/Pt system. For the opposite case of excessive interdiffusion, the former exhibits much higher conduction than the latter. We found that the diffusion of the top electrode material can be controlled in two ways—either by varying Pb excess in the PLZT film or by controlling the SRO layer thickness. Schottky barrier measurements show that in the case of limited diffusion the potential barrier values of SRO/PLZT and Pt/PLZT interfaces are nearly equal. Based on our results we conclude that: (a) the Schottky barrier values of Pt/PLZT and SRO/PLZT contacts are similar and mainly controlled by Fermi level pinning at the interface rather than by work functions of electrode materials; (b) the leakage conduction of Pt/SRO/PLZT/Pt capacitors can be efficiently suppressed without losing good polarization fatigue performance.