bluuebunny/crossref_metadata_2025 huggingface dataset:
Variable magnetic bottle for precision geonium experiments
Authors: Van Dyck R. S. , Moore F. L. , Farnham D. L. , Schwinberg P. B.
Published: July 2002

A novel technique has been developed which allows a quadratic magnetic field to be continuously varied (via a superconducting flux transformer) from the outside of an ultrahigh-vacuum container placed within a superconducting solenoid. Enclosed within each of these vacuum vessels is a high-precision (compensated) Penning trap which is used in a variety of geonium experiments at the University of Washington. By varying the dc current in an outer (normal) primary solenoid, the secondary current flowing within a shorted superconducting loop is varied accordingly. The superconducting loop need only be placed in a cylindrically symmetric position within the Penning trap in order to generate the B2 term, and with a clever choice of geometry, the zeroth-order term can be totally eliminated. As an additional highlight, the uniform field from the main solenoid is drift stabilized to the same degree as the basic cancellation and further, an auxiliary trim coil can be used to fine trim the zeroth-order null.
Direct detection of low-frequency NMR using a dc SQUID
Authors: Friedman L. J. , Wennberg A. K. M. , Ytterboe S. N. , Bozler H. M.
Published: July 2002

A system has been developed to directly observe NMR free induction signals at very low frequencies. We use a nonresonant pickup arrangement with a commercial dc SQUID to monitor the precessing magnetization in a 0.5–50-kHz range of Larmor frequencies. A relocking interval for the SQUID’s flux locked loop presently sets our pulse recovery around 20–30 μs throughout this frequency range. The noise for this system is less than 1.5 times that recorded for the SQUID with shorted input terminals.
Response function of modulated grid Faraday cup plasma instruments
Authors: Barnett Alan , Olbert Stanislaw
Published: July 2002

Modulated grid Faraday cup plasma analyzers are a very useful tool for making in situ measurements of space plasmas. One of their great attributes is that their simplicity permits their angular response function to be calculated theoretically. In this paper we derive an expression for this response function by computing the trajectories of the charged particles inside the cup. We use the Voyager plasma science (PLS) experiment as a specific example. Two approximations to the ‘‘rigorous’’ response function useful for data analysis are discussed. The theoretical formulas were tested by multisensor analysis of solar wind data. The tests indicate that the formulas represent the true cup response function for all angles of incidence with a maximum error of only a few percent.
In situ fabricated Ga2O3–GaAs structures with low interface recombination velocity
Authors: Passlack M. , Hong M. , Schubert E. F. , Kwo J. R. , Mannaerts J. P. , Chu S. N. G. , Moriya N. , Thiel F. A.
Published: July 2002

Amorphous Ga2O3 films have been deposited in situ on (100) GaAs layers grown by molecular beam epitaxy in ultrahigh vacuum. The Ga2O3–GaAs interface is stable during photoexcitation and the photoluminescence (PL) intensity, measured at 514.5 nm excitation wavelength, is enhanced drastically by a factor of 420 as compared to a corresponding bare GaAs surface. The Ga2O3–GaAs interface recombination velocity derived from a modified dead layer model is below 104 cm/s. Furthermore, the PL intensity of Ga2O3–GaAs structures approaches that of a very low interface state density (2×109 eV−1 cm−2) AlGaAs–GaAs reference structure.
Use of thorium as a target in electron-spin analyzers
Authors: McClelland J. J. , Scheinfein M. R. , Pierce D. T.
Published: July 2002

Measurements of the effective Sherman function have been carried out for 10–100-keV spin-polarized electrons scattering from a thick thorium target in a retarding Mott analyzer. At 20 and 100 keV the dependence on the maximum energy loss accepted by the detector has been measured. Comparison is made with scattering from a 1250-Å gold film. Thorium is seen to have a Seff up to 30% higher than gold. This higher Seff can not only improve the figure of merit of a spin detector, but also lessen its sensitivity to instrumental asymmetries. Comparison is also made with preliminary theoretical results. Good agreement between theory and experiment is seen in the thorium Sherman function relative to that of gold.
Langmuir trough dedicated for x-ray study of monolayer systems
Authors: Yun W. B. , Bloch J. M.
Published: July 2002

Recently, several x-ray techniques have been demonstrated to have unique capabilities of making microscopic measurement on a monolayer supported on a liquid subphase. Substantial information about the phase transitions and chemical reactions of the monolayer system previously unavailable has been obtained. In this paper a Langmuir trough designed to perform these x-ray experiments is described. Design considerations specific for the x-ray applications are discussed, and the hardware implementation is described. The trough has been used successfully in our monolayer study using surface extended x-ray absorption fine structure (SEXAFS) and near total external fluorescence (NTEF) techniques.
Computer control of the tunnel barrier width for the scanning tunneling microscope
Authors: Piner R. , Reifenberger R.
Published: July 2002

The design of a computer system which directly controls tunnel barrier width in a scanning tunneling microscope (STM) is described. This capability allows STM scans to be performed at a speed which is automatically determined by the roughness of the surface under study. The computer system is inexpensive and designed with readily available components for the VME bus.
Broadband reflectometry for the density profile and fluctuation measurements in the JT-60 tokamak
Authors: Fukuda T. , Nagashima K. , Konoshima S. , Haraguchi K. , Takahashi T. , Nagashima A. , Matoba T.
Published: July 2002

A broadband reflectometric system in O-mode operation has been developed for the density profile determination, MHD and turbulent fluctuation measurement, and evaluation of the particle diffusion coefficient in the JT-60 tokamak. For the profile determination, full Ka-Q band frequencies of BWOs have been swept in 750 μs, in order to minimize the effect of doppler shift due to the fluctuation of reflection layers. The heterodyne fixed-frequency reflectometer has unveiled its effectiveness to probe MHD activities, diagnosing the m=1 tearing mode oscillations. It has also enabled the observation of the dramatic suppression of edge plasma density fluctuations at the L- to H-mode transition in the LHCD limiter plasma. Furthermore, the propagation delay of density pulses has been observed by different fixed-frequency channels during a series of sawteeth, from which the particle diffusion coefficient was evaluated, with a newly proposed method applicable to fully diffusive plasmas.
Improved microtips for scanning probe microscopy
Authors: Lemke H. , Göddenhenrich T. , Bochem H. P. , Hartmann U. , Heiden C.
Published: July 2002

Improved electrochemical techniques for the reproducible fabrication of sharp metallic tips are presented. Radii of curvature down to 10 nm make the tips particularly suitable for scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Additionally, simple methods are developed for preparing AFM cantilevers. A new type of spherical probe suitable for long-range scanning force microscopy has been fabricated. The probes consist of nearly perfect spheres with adjustable radii between about 50 and several 100 nm deposited at the very tip of tiny probe holders. Both probe and probe holder may consist of any metal. First experimental investigations confirm that the spherical probes are particularly suitable for van der Waals and magnetic force microscopy.
Thermal stress-induced, high-strain fragmentation of buried SiGe layers grown on Si
Authors: Fatemi M. , Thompson P. E. , Twigg M. E.
Published: July 2002

Buried Si1−xGex layers grown on Si at elevated temperatures of 700 to 800 °C generally exhibit x-ray rocking curves which are significantly broader than those predicted for perfect crystals, suggesting that the layers are strain-relieved. However, calculations using these rocking curves show the materials to be either nearly- or fully-strained. The source of x-ray broadening accompanied by high strain is found to be an abrupt, thermally-induced fragmentation of the layer into small, slightly misoriented regions during the cool-down, such that the as-grown strain remains unchanged. The fragments are typically rectangles a few micrometers wide, with well-defined boundaries along [110]-type directions.
Stoichiometry control of atomic beam fluxes by precipitated impurity phase detection in growth of (Pr,Ce)2CuO4 and (La,Sr)2CuO4 films
Authors: Naito Michio , Sato Hisashi
Published: July 2002

This letter describes a method to sense slight off-stoichiometry (less than 2%–3%) in the cation ratio [Cu/(Pr+Ce) or Cu/(La+Sr)] of atomic beam fluxes by detecting precipitated impurity phases on the surface using reflection high-energy electron diffraction during growth of T′-(Pr,Ce)2CuO4 and T-(La,Sr)2CuO4 films. The method is based on the facts that off-stoichiometry of fluxes inevitably produces precipitates of certain impurity phases on the surface in film growth of these oxides, and that the species of the impurity phases are solely determined by the type of off-stoichiometry; copper rich or lanthanoid rich. Furthermore, it has been found that it is possible to recover a precipitate-free surface, from a surface with precipitates produced by lanthanoid-rich deposition, by later flux readjustment, but it is difficult from a surface with precipitates produced by copper-rich deposition.
High resolution stimulated Brillouin gain spectrometer
Authors: Grubbs W. Tandy , MacPhail Richard A.
Published: July 2002

We describe a stimulated Brillouin gain (SBG) spectrometer based on low power continuous-wave frequency-stabilized lasers. The high resolution and broad tuning range of this spectrometer are demonstrated through a SBG spectrum of glycerol in the glassy state (T=146 K). The narrow Brillouin linewidth (17 MHz half-width at half-maximum) and large Brillouin shift (17.31 GHz) in this spectrum illustrate the extremely high equivalent ‘‘finesse’’ of the instrument, a finesse that would be difficult to achieve through interferometric techniques. The accuracy and precision with which the Brillouin shift can be measured are demonstrated using SBG spectra of liquid methylene chloride as an example. Our Brillouin shift values obtained over the temperature range 178–300 K are in excellent agreement with values measured previously by others using spontaneous and stimulated Brillouin scattering.
The Quadrumafios electron cyclotron resonance ion source: Presentation and analysis of the resultsa)
Authors: Girard A. , Briand P. , Gaudart G. , Klein J. P. , Bourg F. , Debernardi J. , Mathonnet J. M. , Melin G. , Su Y.
Published: July 2002

The Quadrumafios electron cyclotron resonance ion source (ECRIS) has been especially designed to permit physical studies of the plasma; this paper describes the source itself (which has been operated at 10 GHz in a first step), its preliminary performances, and the different diagnostics involved, which mainly concern the electron population (ECE, x rays, diamagnetism, microwave interferometer, and electron analyzer). The results are presented and discussed: There is of course a close relationship between the parameters of the plasma and the performances of the source; this point will be discussed in the article.
Frankfurt EBIS development: Fundamental research and new applications
Authors: Kleinod M. , Becker R. , Kester O. , Lakatos A. , Thomae H. , Zipfel B. , Klein H.
Published: July 2002

Fundamental questions of electron beam ion sources (EBIS) are studied using three different setups. Cryogenic classical EBIS: The limit of ion–ion cooling in EBIS devices operating near the space-charge neutralization limit has been studied experimentally. To investigate different degrees of compensation of the electron beam by highly charged ions, a hemispherical analyzer has been constructed including a novel deceleration optic for high resolution. Using its energy definition we tested different modes of ion transportation in our multipassage magnetic spectrometer, which can be operated chromatically as well as extremely dispersively: up to 200 passages could be obtained. EBIS without magnetic field: First results on ion production in the short trap at the very dense crossover of an electron beam (XEBIS) confined only inertially without the use of magnetic fields are presented. Normal conducting short EBIS: Using a normal conducting field of 0.8 T of 25-cm length an EBIS is under construction to study extremely short ion extraction of approximately 4 μs for single turn injection into a medical synchrotron for cancer therapy (MEDEBIS).
Compact thermal ion detector for space and laboratory applications
Authors: Enloe C. L. , Lloyd J. , Meassick S. , Chan C. , McGarity J. O. , Huber A. , Hartnett P.
Published: July 2002

We have developed a new compact thermal ion sensor suitable for spaceflight or laboratory applications. The device is highly sensitive and has a wide dynamic range due to its use of a microchannel plate charge multiplier. It allows spatial variations in velocity and anisotropic temperatures to be measured in a flowing plasma stream using a segmented anode. It has a high rejection to solar UV due to the trajectories of the particles within the device. The device has been flight tested as part of the Charging Hazards and Wake Studies flight experiment and sample data from the flight are presented. We discuss interpretation of the output of the device including extracting the parallel and perpendicular temperatures and the flow energy of a plasma stream.
Lateral photoeffect in large area one-dimensional thin-film position-sensitive detectors based in a-Si:H P-I-N devices
Authors: Martins R. , Fortunato E.
Published: July 2002

The aim of this work is to provide the basis for the interpretation, under steady state conditions, of the lateral photoeffect in p-i-n a-Si:H one-dimensional thin-film position-sensitive detectors (1D TFPSD) and the determination of its linear spatial detection limits, function of the device, and light spot source characteristics. This leads to the development of a model, based on the application of the Poisson, continuity, and current density equations in the p-i-n junction, where two thin resistive layers, as equipotentials, are considered on both sides of the doped layers. The experimental data recorded in 1D TFPSD devices with different performances are compared with the predicted curves and the obtained correlations discussed.
High-resolution x-ray spectrometer based on a cylindrically bent crystal in nondispersive geometry
Authors: Hämäläinen K. , Krisch M. , Kao C.-C. , Caliebe W. , Hastings J. B.
Published: July 2002

The results from a new crystal spectrometer for high-resolution inelastic scattering and resonant Raman scattering spectroscopy are presented. It is based on a cylindrically bent silicon crystal in standard Rowland circle geometry where the energy dispersion is obtained nonconventionally by placing a position sensitive detector perpendicular to the scattering plane. The advantage of this geometry is that the energy dispersion is only about one order of magnitude larger than the intrinsic energy resolution of the instrument, allowing the optimization of the collection efficiency within a narrower energy range of interest while preserving the practical realization of a cylindrically bent crystal. A total energy resolution of 0.7 eV at 8 keV with an energy dispersion of only a few eV was reached. Potential applications of this new geometry as well as comparison with other focusing geometries are also discussed.
Calorimetric study of the energetics and kinetics of interdiffusion in Cu/Cu6Sn5 thin-film diffusion couples
Authors: Dreyer K. F. , Neils W. K. , Chromik R. R. , Grosman D. , Cotts E. J.
Published: July 2002

Differential scanning calorimetry was used to characterize the energetics and kinetics of interdiffusion in solder/metal diffusion couples. The heat of formation of Cu3Sn from Cu6Sn5 and Cu thin films was found to be ΔHr=−4.3±0.3 kJ/mol, similar to the results of previous measurements on bulk samples. We have seen that the nucleation of Cu3Sn begins at temperatures near 360 K, but that the nucleation and initial growth of Cu3Sn is not a well-defined Arrhenius process in these diffusion couples. Later portions of our differential scanning calorimetry scans were identified with diffusion-limited growth of Cu3Sn. From these calorimetry data we have estimated the averaged interdiffusion coefficient, D̃(cm2/s)=D0 exp (−E/kbT), where kb is Boltzmann’s constant and D0=3.2×10−2 cm2/s and E=0.87 eV/atom.
PLS bending magnet x-ray beamline 3C2
Authors: Park B.-J. , Rah S.-Y. , Park Y.-J. , Lee K.-B.
Published: July 2002

The design of the 3C2 beamline at the Pohang Light Source (PLS) is described. The beamline is designed for x-ray scattering studies with x-ray photons over an energy range of 4–12 keV. A cylindrical Pt-coated silicon mirror intercepts a photon beam up to 3 mrad at a distance of 16 m from a source point and focuses the beam sagittally at the sample position with a magnification of 1:1. For the vertical focusing it will be dynamically bent by the mirror holder. A double-crystal monochromator of two Si(111) crystals with a fixed-exit beam geometry provides monochromatic photons and is located in front of the experimental hutch to reduce the motion of the beam position. The hutch is equipped with a four-circle diffractometer and various photon detection electronics for x-ray scattering experiments.
A flexible implementation of scanning probe microscopy utilizing a multifunction system linked to a PC-Pentium controller
Authors: Barchesi C. , Cricenti A. , Generosi R. , Giammichele C. , Luce M. , Rinaldi M.
Published: July 2002

A flexible electronic setup on a PC platform and the software implementation in Windows Microsoft environment, for a multipurpose head for scanning probe microscopy (SPM), has been developed. The integrated, multiapplication data acquisition system is linked to a PC-Pentium controller, through a digital I/O board, and consists of: (i) an asynchronous acquisition for real time removal of following error from SPM images; (ii) a three-axes, computer controlled micropositioning stage; (iii) software for electronic control, data acquisition, and graphics elaboration performed through subroutines of Visual Basic (Visual Basic Programming System Professional edition for Windows is a registered trademark of Microsoft Corporation, USA.), and PV-WAVE personal edition. (PV-WAVE Personal edition for Windows is a registered trademark of Visual Numerics, USA.)
Royal Military College of Science light gas gun facility
Authors: Fellows N. A. , Barton P. C.
Published: July 2002

Research into the high velocity impact of materials has necessitated the setting up of a new facility at the Royal Military College of Science. A light-gas gun and flash x-ray system were acquired to meet the research objectives. In this article, the light-gas gun and auxiliary equipment are described. Modifications were made to the facility to ensure the safety of personnel during a gas gun and flash x-ray firing. An analysis was carried out to select operating parameters within the maximum safe loading condition of the gas gun. The work met the safety and operational objectives, and suggestions for future work are made.
High-luminosity, high-resolution, x-ray spectroscopy of laser-produced plasma by vertical-geometry Johann spectrometer
Authors: Renner O. , Missalla T. , Sondhauss P. , Krouský E. , Förster E. , Chenais-Popovics C. , Rancu O.
Published: July 2002

Successful applications of the vertical-geometry Johann spectrometer (VJS) in advanced plasma spectroscopy are reported. Different experimental configurations are discussed, and a complete quantitative analysis of the spectrometer function including the transfer of the spectral lines is presented. The method for reconstruction of the spectra emitted from extended, quasilinear sources is described; the precision attainable and possible sources of errors are discussed. Due to the combination of high collection efficiency, and spectral and one-dimensional spatial resolution, the instrument is particularly suitable for high-precision measurements of the spectral line profiles and positions in nonhomogeneous plasmas. The examples of experimental results, which are superior to those obtained in earlier measurements, demonstrate the VJS performance and suggest a broad field of possible applications.
Spectroscopic ellipsometry of RuO2 films prepared by metalorganic chemical vapor deposition
Authors: Hones Peter , Gerfin Tobias , Grätzel Michael
Published: July 2002

Conducting thin films on RuO2 were deposited on glass by metalorganic chemical vapor deposition using tris-trifluoroacetylacetonate-ruthenium (III) [Ru(tfa)3] as precursor. Smooth, specular, and well adherent films were grown at temperatures as low as 400 °C, if the reaction gas contained water. RuO2 was the only phase detected by x-ray diffraction. The films were investigated by spectroscopic ellipsometry in the energy range of 1.5 to 5.0 eV. Thereby, the real (ε1) and imaginary (ε2) parts of the dielectric function were determined at room temperature. The optical constants could be described by three harmonic oscillators in combination with a Drude term for free-charge carriers.
Monochromatic focusing of subpicosecond x-ray pulses in the keV range
Authors: Missalla T. , Uschmann I. , Förster E. , Jenke G. , von der Linde D.
Published: July 2002

An effective x-ray optical method to focus keV x-ray pulses shorter than one picosecond by using spherically or toroidally bent crystals is presented. The spectral, spatial, and time-dependent properties of focusing by two-dimensional bent crystals are calculated by considering geometrical effects, physical limitation in high performance crystal optics, and reflectivities obtained by x-ray diffraction theory. These properties are compared with first experimental results of focusing x rays from a plasma created by a laser pulse with 4.5 mJ energy and 100 fs pulse length. The x-ray signals, simultaneously obtained from a von Hámos spectrometer and two-dimensional bent crystals are compared and found in good agreement with theoretical data. The possibilities and aspects of laser pump x-ray probe experiments using this type of x-ray optics system and currently available laser systems are discussed.
An enriched K40 source for fermionic atom studies
Authors: DeMarco B. , Rohner H. , Jin D. S.
Published: July 2002

We have developed a novel, enriched potassium source for use in trapping and cooling of the fermionic isotope K40. The source is clean, easily controllable, and consistent with high vacuum applications. With this enriched source, we are able to collect 108 K40 atoms in a vapor cell magneto-optical trap.
Theory of laser gain in group-III nitrides
Authors: Chow W. W. , Knorr A. , Koch S. W.
Published: July 2002

A many-body calculation of the nonlinear optical response of bulk group-III nitrides is presented. For the example of GaN it is shown that the Coulomb effects contribute significantly to the magnitude and spectral extension, as well as the temperature and carrier density dependences of the optical gain and absorption.
Electro-optical characteristics and switching behavior of the in-plane switching mode
Authors: Oh-e Masahito , Kondo Katsumi
Published: July 2002

Electro-optical characteristics related to the threshold behavior of liquid crystals when using the in-plane switching (IPS) mode were investigated with interdigital electrodes. In order to analyze the switching behavior of liquid crystals, an equation, which expresses the threshold transition, was derived using the continuum elastic theory. It was made clear that it was the electric field and not the voltage that drives the liquid crystals in the IPS mode. Significantly, an inversely proportional relationship between the threshold voltage and the gap between the substrates was found to hold. Furthermore, the electro-optical characteristics were recognized to change with the variation of the gap between the substrates. This behavior is due to the independence of electric field on liquid crystal layer normal.
Two-photon pumped cavity lasing in novel dye doped bulk matrix rods
Authors: He Guang S. , Zhao Chan F. , Bhawalkar Jayant D. , Prasad Paras N.
Published: July 2002

Trans-4-[p-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridi that possesses a much greater two-photon absorption cross section and much stronger upconversion fluorescence emission than common organic dyes (such as rhodamine), when excited with near infrared laser radiation. Utilizing ASPT doped bulk polymer rods, two-photon pumped frequency upconverted cavity lasing has been accomplished using a Q-switched Nd:YAG laser as the pump source. The wavelength and pulse duration were ∼600 nm and 3–6 ns, respectively, for the cavity lasing; whereas the corresponding values for pump pulses were 1.06 μm and ∼10 ns, respectively. For a 7 mm long sample rod with a dopant concentration d0=8×10−3 M/L, the conversion efficiency from the absorbed pump energy to the cavity lasing output was ∼3.5% at a pump energy level of 1.3 mJ. The lasing lifetime, in terms of pulse numbers, was more than 4×104 pulses at 2 Hz repetition rate and room temperature.
Resonant Raman scattering in hexagonal GaN
Authors: Behr D. , Wagner J. , Schneider J. , Amano H. , Akasaki I.
Published: July 2002

We performed resonant Raman scattering in hexagonal GaN using discrete laser lines in the violet and UV spectral range for optical excitation. To tune the energetic position of the fundamental gap E0 of GaN relative to the exciting photon energy the sample temperature was varied between 77 and 870 K. Analyzing both Stokes and anti-Stokes Raman spectra, the resonance profiles for Fröhlich-induced one-E1(LO) and two-E1(LO) phonon scattering could be deduced, covering the energy range from 0.5 eV below the E0 gap up to the gap energy. The strength of deformation-potential scattering by the A1(TO) mode was used as an internal reference. For excitation slightly above the E0 gap energy E1(LO) multiphonon scattering up to the fourth order was observed, which reflects the stronger polarity of the Ga-N bond as compared to conventional III-V semiconductors.
Tunneling through grain boundaries of YBa2Cu3O7 step-edge junctions
Authors: Yi H. R. , Winkler D. , Claeson T.
Published: July 2002

Tunneling experiments were done on YBa2Cu3O7 step-edge junctions. The temperature dependence of the superconducting gap parameter, Δ(T), was measured. From the peak-to-peak distance in the conductance curve, Δ was estimated to vary from 24 to 32 meV at 4.2 K. The conductance at high-voltage bias does not increase with increasing voltage. The tunneling results, together with the well-behaved magnetic field dependence of the critical Josephson current, hysteresis, and Fiske-type resonances in current-voltage curves, indicate a dielectric behavior of the grain boundary junction, i.e., a tunnel junction.
Broadening of interband resonances in thin AlAs barriers embedded in GaAs
Authors: Weimar U. , Wagner J. , Gaymann A. , Köhler K.
Published: July 2002

AlAs barriers embedded in GaAs were studied by spectroscopic ellipsometry and resonant Raman scattering. Heterostructures with AlAs barrier widths ranging from 2 to 30 nm were grown by molecular-beam epitaxy at growth temperatures between 410 and 660 °C. For layer widths below 10 nm the E1 and E1+Δ1 critical point resonance in the dielectric function of the AlAs was found to broaden and to be smeared out completely for a width of 2 nm. Resonant Raman scattering by the AlAs LO phonon reveals for layer widths ≤10 nm a considerable broadening of also the E0 interband transition in the AlAs. The magnitude of the critical point broadening and redistribution of oscillator strength, however, was found to be independent of the growth temperature and thus of the cation intermixing observed by Raman spectroscopy for growth temperatures ≥600 °C. Therefore, the observed critical point broadening is not caused by the formation of graded composition (AlGa)As barriers. Instead, the broadening of interband resonances is attributed to a spread of the carrier wave functions into the surrounding GaAs, which are not confined within the AlAs barrier for neither the E0 nor the E1 and E1+Δ1 interband transitions.
Pulsed laser deposition of highly conductive iridium oxide thin films
Authors: El Khakani M. A. , Chaker M. , Gat E.
Published: July 2002

Highly conductive IrO2 thin films have been deposited on Si (100) substrates by means of pulsed laser ablation of iridium metal target in an oxygen ambient pressure of 200 mTorr. IrO2 films grown at substrate temperatures in the 400–550 °C range are polycrystalline with a preponderant (101) IrO2 reflection and exhibit a dense granular morphology. Their room-temperature resistivities are very comparable to that of bulk single-crystal IrO2. IrO2 thin films with a resistivity of (39±4) μΩ cm are obtained at a substrate temperature as low as 400 °C. The dependence of IrO2 films properties on the nature and/or the preparation of their underlying substrates is pointed out.
Cryogenic scanning probe characterization of semiconductor nanostructures
Authors: Eriksson M. A. , Beck R. G. , Topinka M. , Katine J. A. , Westervelt R. M. , Campman K. L. , Gossard A. C.
Published: July 2002

We demonstrate the use of a scanned probe microscope (SPM) at 4 Kelvin to study electron transport through a ballistic point contact in the two-dimensional electron gas inside a GaAs/AlGaAs heterostructure. The electron gas density profile is locally perturbed by the charged SPM tip providing information about the electron flow through the point contact. As the tip is scanned, one obtains a spatial image of the ballistic electron flux as well as the topographic profile of the structure. Calculations indicate the spatial resolution is comparable to the electron gas depth.
An analysis of the anisotropic and topographic gratings in a side-chain liquid crystalline azobenzene polyester
Authors: Holme N. C. R. , Nikolova L. , Ramanujam P. S. , Hvilsted S.
Published: July 2002

We have examined in detail the formation of surface relief structures in azobenzene polyesters formed by polarization holography with orthogonally circularly polarized laser beams. We show that it is possible to separate the contribution to the diffraction efficiency into an anisotropic part and a surface relief part by examining the polarization content of the first order diffracted beam. By studying the dynamics of the growth of the grating, we show that the gratings due to anisotropy and surface relief appear at the same time. Atomic force microscopic investigations of the film after irradiation reveal a strongly polarization dependent surface relief pattern.
Excitation and deexcitation of Er3+ in crystalline silicon
Authors: Kik P. G. , de Dood M. J. A. , Kikoin K. , Polman A.
Published: July 2002

Temperature dependent measurements of the 1.54 μm photoluminescence of Er implanted N codoped crystalline Si are made. Upon increasing the temperature from 12 to 150 K, the intensity quenches by more than a factor thousand, while the lifetime quenches from 420 to 3 μs. The quenching processes are described by an impurity Auger energy transfer model that includes bound exciton dissociation and a nonradiative energy backtransfer process. Electron and hole trap levels are determined. Direct evidence for a backtransfer process follows from spectral response measurements on an Er-implanted Si solar cell.
Polarization-induced trapped charge in ferroelectrics
Authors: Warren W. L. , Pike G. E. , Tuttle B. A. , Dimos D.
Published: July 2002

We have examined the nature of the defects in BaTiO3 capacitors that have been subjected to an electric field at elevated temperatures (an accelerated aging process) using thermally stimulated current (TSC) and electron paramagnetic resonance measurements. The accelerated aging stress causes the ferroelectric capacitor to exhibit a reduction in its insulating resistance and a voltage offset in its polarization-voltage hysteresis loop. Following the accelerated aging treatment, the trapped charge estimates obtained from the TSC and the hysteresis measurements (as ascertained from the voltage offset) differ significantly and are attributed to either local charge compensation or uniform charge trapping within the dielectric. We also show that some of the trapped charge in ferroelectric materials is directly associated with the net polarization.
Enhanced Sb segregation in surfactant-mediated-heteroepitaxy: High-mobility, low-doped Ge on Si
Authors: Reinking D. , Kammler M. , Horn-von Hoegen M. , Hofmann K. R.
Published: July 2002

Surfactant-mediated epitaxy (SME) allows the growth of smooth, continuous, relaxed, and principally defect free Ge films directly on Si(111); however, the very high surfactant doping level in the range of the solid solubility limit made them unacceptable for most device applications. By using high temperature SME we have reduced the Sb surfactant background doping level by more than three orders of magnitude. This is attributed to an enhanced surfactant segregation without kinetic limitations. The low Sb incorporation has been determined by an electrical characterization: An electron concentration of 1.1×1016 cm−3 and a very high electron Hall mobility of 3100 cm2/V s at 300 K (12 300 cm2/V s at 77 K) suggest an interesting potential of SME grown Ge films for future device applications.
Parallel epitaxy of TiN(100) thin films on Si(100) produced by pulsed reactive crossed-beam laser ablation
Authors: Timm R. , Willmott P. R. , Huber J. R.
Published: July 2002

TiN(001)[100]‖Si(001)[100] parallel epitaxy of thin films grown by pulsed reactive crossed-beam laser ablation (KrF, 248 nm) is investigated in situ by reflection high-energy electron diffraction and ex situ by x-ray diffraction, full-hemispherical X-ray photoelectron diffraction and low-energy electron diffraction. TiN films are grown on atomically flat, initially two domain 2×1 reconstructed Si(001) surfaces at 100⩽T⩽800° C. Parallel epitaxy is found to prevail at T⩾400 °C with growth rates in the 10−2 monolayer/pulse−1 range and a repetition rate of 2 Hz. The substrate and film morphologies are investigated using atomic force microscopy.
Nitridation of silicon oxide layers by nitrogen plasma generated by low energy electron impact
Authors: Kobayashi H. , Mizokuro T. , Nakato Y. , Yoneda K. , Todokoro Y.
Published: July 2002

Low temperature nitridation of silicon oxide layers by nitrogen plasma generated by electron impact is investigated using x-ray photoelectron spectroscopy (XPS) and synchrotron radiation ultraviolet photoelectron spectroscopy and it is found that a large amount of nitrogen can be incorporated in the layers. The valence band structure of the oxide surface nitrided at 25 °C is similar to that of Si3N4, while that nitrided at 700 °C resembles the mixture of silicon oxide and silicon oxynitride. Measurements of XPS depth profiles show that the nitrogen concentration is high near the surface and the oxide/Si interface.
H-complexed oxygen vacancy in SiO2: Energy level of a negatively charged state
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

The defects generated in SiO2 during irradiation with energetic (10 eV) photons were found to trap electrons at a level 3.1 eV below the oxide conduction band. The electron spin resonance data and the behavior upon hydrogen passivation indicate that the optically active state may be ascribed to a H-complexed oxygen vacancy in SiO2. The observed injection of electrons to these traps from Si advances the revealed defects as the possible origin of the degradation-induced electrical conduction of thin SiO2 layers.
Gain studies of (Cd, Zn)Se quantum islands in a ZnSe matrix
Authors: Strassburg M. , Kutzer V. , Pohl U. W. , Hoffmann A. , Broser I. , Ledentsov N. N. , Bimberg D. , Rosenauer A. , Fischer U. , Gerthsen D. , Krestnikov I. L. , Maximov M. V. , Kop’ev P. S. , Alferov Zh.I.
Published: July 2002

By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.
Fiske and flux-flow modes of the intrinsic Josephson junctions in Bi2Sr2CaCu2Oy mesas
Authors: Irie A. , Hirai Y. , Oya G.
Published: July 2002

We report the observation of the highest velocity mode of Josephson vortex motion in mesas consisting of 5–15 intrinsic Josephson junctions with an area of 37×30 μm2 on Bi2Sr2CaCu2Oy single crystals. By applying the dc magnetic field parallel to CuO2 planes in the mesas, resonant steps and flux flow branches have been observed on the current–voltage characteristics along the c-axis of the mesas. The characteristic velocities of the intrinsic Josephson junctions are estimated to be (1.5–1.8)×106 m/s, which corresponds to the highest velocity mode, from the voltages of Fiske steps and the maximum voltages of the flux-flow branches.
Assisted tunneling in ferromagnetic junctions and half-metallic oxides
Authors: Bratkovsky A. M.
Published: July 2002

Different mechanisms of spin-dependent tunneling are analyzed with respect to their role in tunnel magnetoresistance (TMR). Microscopic calculation within a realistic model shows that direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. The larger observed values of the TMR might be a result of tunneling involving surface polarized states. It is found that tunneling via resonant defect states in the barrier radically decreases the TMR by order of magnitude. One-magnon emission is shown to reduce the TMR, whereas phonons increase the effect. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR. The model presented here is applied qualitatively to half metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000%. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2.
Density of states of the tetragonal-phase germanium nanocrystals using x-ray photoelectron spectroscopy
Authors: Sato S. , Nozaki S. , Morisaki H.
Published: July 2002

The valence-band x-ray photoelectron spectroscopy (XPS) spectra of the Ge nanocrystals deposited by the cluster-beam evaporation technique were studied, and suggest the presence of the tetragonal crystal structure (ST-12) of Ge in the Ge nanocrystals. Although the ST-12 structure of Ge is known to be metastable and is transformed to the diamond structure, which is the crystal structure of bulk Ge, at relatively low temperatures, the XPS and absorption spectra of the Ge nanocrystals annealed at various temperatures show that the obtained ST-12 structure is thermally stable. The critical temperature for the phase transformation of the Ge nanocrystals is found to be higher than 700 °C. These results are consistent with our earlier study using Raman spectroscopy.
Coherent control of acoustic phonons in semiconductor superlattices
Authors: Bartels Albrecht , Dekorsy Thomas , Kurz Heinrich , Köhler Klaus
Published: July 2002

Coherent acoustic phonons are generated in GaAs/AlAs superlattices by excitation with femtosecond laser pulses. Several modes of the acoustic phonon spectrum are observed, in agreement with the effect of zone folding in the mini-Brillouin zone of the superlattice. By applying successive pump pulses we are able to silence the first back-folded mode near q=0, while selectively enhancing the coherent amplitude of higher order backfolded modes. This increase in the spectroscopic sensitivity opens the way to detailed time resolved studies of higher order acoustic modes in superlattices.
Electro-optic beam deflector using epitaxial Pb(Zr,Ti)O3 waveguides on Nb-doped SrTiO3
Authors: Nashimoto K. , Nakamura S. , Moriyama H. , Watanabe M. , Osakabe E.
Published: July 2002

Pb(Zr,Ti)O 3 (PZT) thin-film optical waveguides were grown on Nb-doped SrTiO3 substrates by solid-phase epitaxy to fabricate an electrode/waveguide/conductor structure. The propagation loss was relatively large for a PZT waveguide on a lightly Nb-doped SrTiO3 as compared with the propagation loss of 4 dB/cm for the structure using undoped SrTiO3. An electro-optic beam deflection device was fabricated by preparing a prism electrode on the surface of the PZT waveguide on a lightly Nb-doped SrTiO3 substrate. Efficient deflection of the coupled laser beam in the PZT waveguide as large as 11 mrad was observed by applying 35 V between the prism electrode and the substrate.
Green emission from Er-doped GaN grown by molecular beam epitaxy on Si substrates
Authors: Birkhahn R. , Steckl A. J.
Published: July 2002

Visible light emission has been obtained from Er-doped α-GaN thin films grown on Si(111). The GaN was grown by molecular beam epitaxy using solid sources (for Ga and Er) and a plasma gas source for N2. Photoexcitation with a He–Cd laser resulted in strong green emission from two narrow green lines at 537 and 558 nm identified as Er transitions from the H11/22 and S3/24 levels to the I15/24 ground state. X-ray diffraction shows the GaN:Er to be a wurtzitic single crystal film. The growth temperature is seen to have a strong effect on the GaN:Er surface morphology.
Fine structure in the secondary electron emission peak for diamond crystal with (100) negative electron affinity surface
Authors: Asnin V. M. , Krainsky I. L.
Published: July 2002

A fine structure was discovered in the low-energy peak of the secondary electron emission spectra of the diamond surface with negative electron affinity. We studied this structure for the (100) surface of the natural type-IIb diamond crystal. We have found that the low-energy peak consists of a total of four maxima. The relative energy positions of three of them could be related to the electron energy minima near the bottom of the conduction band. The fourth peak, having the lowest energy, was attributed to the breakup of the bulk exciton at the surface during the process of secondary electron emission.
Spin-glass behavior in LiMn2O4 spinel
Authors: Jang Young-Il , Chou F. C. , Chiang Yet-Ming
Published: July 2002

LiMn 2 O 4 spinel exhibits spin-glass behavior below 25 K. A cusp in the ac susceptibility appears at 25 K, below which the susceptibility increases with decreasing frequency. The dc magnetization shows irreversibility, and relaxation of remanent magnetization is seen on the macroscopic time scale. The randomness and frustration necessary for spin-glass behavior are explained in a manner compatible with the cation and charge ordering present in this material.
Two color blinking of single strain-induced GaAs quantum dots
Authors: Bertram D. , Hanna M. C. , Nozik A. J.
Published: July 2002

In this letter we report on a temporal instability in the ground and excited state luminescence of a single strain-induced quantum dot. Using a microscopic photoluminescence technique, we record spectra from a single strain-induced quantum dot in the GaAs/(AlGa)As material system. On a time scale of seconds the luminescence shows an increase and decrease in intensity with an increase of the ground state luminescence correlating with a decrease in the excited state luminescence intensity and vice versa. We term the observed effect two color blinking.
Low temperature processed 0.7SrBi2Ta2O9–0.3Bi3TaTiO9 thin films fabricated on multilayer electrode-barrier structure for high-density ferroelectric memories
Authors: Ryu S. O. , Joshi P. C. , Desu S. B.
Published: July 2002

Thin films of solid-solution material 0.7SrBi2Ta2O9–0.3Bi3TaTiO9 (0.7SBT–0.3BTT) were fabricated on n+-polycrystalline (n+-poly) Si substrates by a metalorganic solution deposition technique at a low processing temperature of 650 °C using a Pt–Rh/Pt–Rh–Ox electrode-barrier structure. The Pt–Rh/Pt–Rh–Ox structure was deposited using an in situ reactive radio frequency sputtering process. The electrodes had a smooth and fine-grained microstructure and were excellent diffusion barriers between the 0.7SBT–0.3BTT thin film and Si substrate. The ferroelectric (0.7SBT–0.3BTT) test capacitors using these electrode-barrier grown directly on Si showed good ferroelectric hysteresis properties, measured through n+-poly Si substrate, with 2Pr and Ec values of 11.5 μC/cm2 and 80 kV/cm, respectively, at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics (<10% decay) under bipolar stressing up to 1011 switching cycles and the leakage current density was lower 10−7 A/cm2 at an applied electric field of 200 kV/cm. The good ferroelectric properties of 0.7SBT–0.3BTT solid-solution thin films at a low processing temperature of 650 °C and excellent electrode-diffusion barrier properties of a Pt–Rh/Pt–Rh–Ox structure are encouraging for the realization of high-density nonvolatile ferroelectric random access memories on silicon substrates.
Single quantum dot states measured by optical modulation spectroscopy
Authors: Bonadeo N. H. , Lenihan A. S. , Chen Gang , Guest J. R. , Steel D. G. , Gammon D. , Katzer D. S. , Park D.
Published: July 2002

Using optical modulation spectroscopy, we report the direct observation of absorption lines from excitons localized in GaAs single quantum dot potentials. The data provide a measurement of the linewidth, resonance energy, and oscillator strength of the transitions, and show that states which decay primarily by nonradiative processes can be directly probed using this technique. The experiments establish this technique for the characterization of single quantum dot transitions, thereby complementing luminescence studies.
Characterization of hydrogen-terminated Si(111) surfaces by sum-frequency surface vibrational spectroscopy
Authors: Mao M. Y. , Miranda P. B. , Kim D. S. , Shen Y. R.
Published: July 2002

Sum-frequency surface vibrational spectroscopy was used to characterize H-terminated Si(111) surfaces prepared by different methods. It was found that flash heating of a sample to 1500 K followed by H2 dosing at 900 K yields a surface as good in quality as a wet chemically treated surface. Both surfaces are far superior than that prepared by atomic H adsorption.
Design and realization of an all d-wave dc π-superconducting quantum interference device
Authors: Schulz R. R. , Chesca B. , Goetz B. , Schneider C. W. , Schmehl A. , Bielefeldt H. , Hilgenkamp H. , Mannhart J. , Tsuei C. C.
Published: July 2002

The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.