bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron heating in low-pressure rf glow discharges
Authors: Surendra M. , Graves D. B. , Morey I. J.
Published: July 2002

Particle-in-cell simulations of radio frequency glow discharges between parallel-plate electrodes were performed to test the role of secondary emission at applied frequencies above the ion plasma frequency. With the secondary electron emission coefficient set to zero, the high-energy tail of the electron energy distribution function in the center of the glow is modulated at the applied frequency, and these energetic electrons show substantial anisotropy. In addition, a pulse of ionization propagates across the glow, correpsonding to the directed velocity of the high-energy electrons in the center of the glow. These high-energy electrons originate at the plasma-sheath boundary and appear to be the result of sheath heating.
Balanced phase matching in segmented KTiOPO4 waveguides
Authors: Bierlein J. D. , Laubacher D. B. , Brown J. B. , van der Poel C. J.
Published: July 2002

A new phase matching technique is described which is based on phase mismatch balancing in crystal or waveguide segments that are short compared to their respective coherence lengths. The technique is demonstrated experimentally in a mixed bulk crystal/waveguide structure in KTiOPO4, resulting in a second-harmonic generation conversion efficiency of 15%/W/cm2 at 1.064 μm, close to the theoretical maximum. We show that this technique can significantly broaden processing latitude for fabricating practical nonlinear optical waveguide devices.
Photon avalanche upconversion laser at 644 nm
Authors: Koch M. E. , Kueny A. W. , Case W. E.
Published: July 2002

We have obtained efficient laser action from Pr-doped LaCl3 using a novel pumping method. Pump energy is coupled into the crystal through Pr ions in an excited state rather than the ground state. Excited-state absorption is made possible by a regenerative feedback process (the photon avalanche) utilizing cross relaxation to convert the energy of a single ion in a high-lying state into energy shared by multiple lower lying state ions.
Mechanism for efficient blue second-harmonic generation in periodically segmented waveguides
Authors: Khurgin J. , Colak S. , Stolzenberger R. , Bhargava R. N.
Published: July 2002

We present a theoretical investigation of the mechanisms responsible for the recently reported efficient type I blue second-harmonic generation in periodically segmented KTiOPO4:Rb,Tl waveguides. In these guides, grating-assisted phase matching of second-harmonic light between 390 and 480 nm has been achieved. 5-mm-long guides give output powers on the order of 1 mW at around 425 nm with fundamental powers in the 50–100 mW range. We show that such high efficiencies can be expected from strongly perturbed guides through terms representing the modulation of phase mismatch and mode size in addition to the nonlinear susceptibility and refractive index modulations.
Pressure dependence of ionization efficiency in sputtering magnetrons
Authors: Sheridan T. E. , Goeckner M. J. , Goree J.
Published: July 2002

Using a Monte Carlo simulation, we show how electron confinement allows sputtering magnetrons to operate at lower neutral pressures than similar unmagnetized devices. We find that at both high and low pressures, the ionization efficiency in a magnetron is constant, and it varies by only 40% between the two regimes. In contrast, the efficiency of an unmagnetized discharge varies linearly with pressure, becoming very small at low pressures.
Enhanced hardness in lattice-matched single-crystal TiN/V0.6Nb0.4N superlattices
Authors: Mirkarimi P. B. , Hultman L. , Barnett S. A.
Published: July 2002

Single-crystal TiN/V0.6Nb0.4N superlattice films were deposited on MgO(100) substrates using ultrahigh vacuum reactive sputtering from two magnetron sources with a computer-controlled shutter. X-ray diffractometer measurements showed first- and second-order superlattice peaks while electron diffraction showed up to seventh-order superlattice reflections. Direct observations using cross-sectional transmission electron microscopy showed that the superlattices had flat, abrupt interfaces, but contained threading dislocations. Microhardness values for the superlattices ranged from to 3100 to 4100 kg/mm2, depending on the superlattice period (λ), compared with 1500 for an alloy with equal average composition. The hardness values for these lattice-matched superlattices agreed with those previously reported for TiN/VN superlattices with a lattice mismatch of 2.4%, although the hardness peak was less pronounced. The present results indicate that coherency strains do not play the major role in enhancing the hardness of these nitride superlattices.
Initial stage of epitaxial growth at the high temperature of GaAs and AlGaAs on Si by metalorganic chemical vapor deposition
Authors: Soga T. , George T. , Jimbo T. , Umeno M. , Weber E. R.
Published: July 2002

GaAs and AlGaAs grown directly on Si were characterized by transmission electron microscopy. Both GaAs and AlGaAs grow three-dimensionally on Si at 750 °C. The spacing between GaAs islands is large, while the AlGaAs islands appear to be contiguous for a nominal thickness of 22.5 nm. There is a high density of dislocations, stacking faults, and microtwins in the thin GaAs layer, but drastic reduction of such defects was observed in the planar AlGaAs nucleation layer.
Oxide thickness effect and surface roughening in the desorption of the oxide from GaAs
Authors: Van Buuren T. , Weilmeier M. K. , Athwal I. , Colbow K. M. , Mackenzie J. A. , Tiedje T. , Wong P. C. , Mitchell K. A. R.
Published: July 2002

The temperature for thermal desorption of the gallium oxide from GaAs is shown to increase linearly with oxide thickness. In addition, we show by diffuse light scattering that highly polished GaAs substrates roughen during the oxide desorption. These results are interpreted in terms of a model in which the oxide evaporates inhomogeneously.
Generating near-earth-core pressures with type-IIa diamonds
Authors: Ruoff Arthur L. , Luo Huan , Vanderborgh Craig , Vohra Yogesh K.
Published: July 2002

In the past year static pressures of 378 and 416 GPa, greater than the pressure at the center of the earth (361 GPa), were achieved and carefully measured for the first time (x-ray diffraction on W and Mo, respectively) using extremely high-quality type-Ia diamonds (containing nitrogen platelets). In this letter, we show that type-IIa diamonds (containing no nitrogen) can reach similar pressures (338 GPa by x-ray diffraction on Pt) thus showing that the strength depends primarily on crystalline perfection (and not on nitrogen platelets). Nearly perfect type-IIa diamonds are rare.
Vacuum field emission from a Si-TaSi2 semiconductor-metal eutectic composite
Authors: Kirkpatrick D. A. , Bergeron G. L. , Czarnaski M. A. , Hickman J. J. , Levinson M. , Nguyen Q. V. , Ditchek B. M.
Published: July 2002

We report on measurements of vacuum field emission from ungated field emission cathode arrays fabricated from Si-TaSi2 eutectic composite wafers. The Si-TaSi2 material is an ideal candidate for large area field emission array cathodes due to the large density of TaSi2 fibers incorporated into the Si matrix, the high melting point of the TaSi2 material, the ease with which single-crystal large diameter (2.5 cm) material can be fabricated, and the promise of integrability of the field emission array with conventional Si technology through the use of epitaxial Si layers grown on the cathode backplane.
Generalization of Fowler–Nordheim field emission theory for nonplanar metal emitters
Authors: He Jun , Cutler P. H. , Miskovsky N. M.
Published: July 2002

Field emitter tips can now be fabricated with radii of curvature of the order of nm or even the size of a single atom. To include these geometric effects, we have calculated the field emission tunneling currents for hyperboloidal and conical free-electron tip models using geometry-dependent image interactions and bias fields. The numerical results can be fitted by an equation of the form J=AV2 exp(−B/V−C/V2), where A, B, and C are constants depending on material and geometric parameters. The calculated results, plotted as log J/V2 vs 1/V, do not exhibit the straight line behavior predicted by the Fowler–Nordheim model for field emission from a planar surface. Furthermore, the calculated current densities are dramatically enhanced for both the hyperboloidal (rt=10 nm) and conical (cone half-angle=70°) emitter models. In addition, field emission energy distributions for both models are significantly different from that of the Fowler–Nordheim planar model.
Propagation loss measurements in silicon-on-insulator optical waveguides formed by the bond-and-etchback process
Authors: Evans A. F. , Hall D. G. , Maszara W. P.
Published: July 2002

We report the results of loss measurements for guided waves in silicon-on-insulator (SOI) optical waveguides formed by the bond-and-etchback process. Losses as low as 2.6 (2.0) dB/cm were found for the TE0 (TM0) mode for wavelength λ=1.319 μm. Grating coupling into these waveguides is demonstrated and discussed.
Large (14.5 dB) reduction of intensity noise from a semiconductor laser by amplitude-phase decorrelation
Authors: Newkirk Michael A. , Vahala Kerry J.
Published: July 2002

By the method of amplitude-phase decorrelation, intensity noise from a distributed feedback semiconductor laser is passively reduced up to 14.5 dB below its intrinsic level. At high laser power, reduction to 1.3 dB above the photon shot noise level is observed. The results agree with a simple model incorporating a power-independent source of phase noise in the laser rate equations.
Field-induced surface modification on the atomic scale by scanning tunneling microscopy
Authors: Huang Jin-Lin , Sung Yung-Eun , Lieber Charles M.
Published: July 2002

Scanning tunneling microscopy has been used to study the modification of tin diselenide (SnSe2) and molybdenum disulfide (MoS2) surfaces in ultrahigh vacuum. We have shown that there are positive bias voltage pulse thresholds that must be exceeded to remove material from the surfaces of SnSe2 and MoS2. The voltage threshold for modification of SnSe2(+1.4 V) is significantly smaller in magnitude than the threshold for modification of MoS2(+3.5 V). These threshold results and tip-sample distance dependence data suggest that modification occurs by field evaporation. Additionally, near threshold pulses create stable atomic sizes defects that can be erased by high voltage scanning.
Optical monitoring of nucleation and growth of diamond films
Authors: Smolin A. A. , Ralchenko V. G. , Pimenov S. M. , Kononenko T. V. , Loubnin E. N.
Published: July 2002

Early stages of diamond film deposition on molybdenum substrates using dc arc discharge in CH4/H2 gas mixtures were studied by in situ measurements of optical reflectivity of growing film. Ultrafine diamond grit of ≊200 Å size was used for seeding to increase nucleation density up to 2×109 cm−2 and to produce smooth thin films. Evolution of He-Ne laser beam reflection at 0.63 μm wavelength is described in terms of Mie scattering by nonabsorbing dielectric spheres in the case of nucleated film and of light interference in the system of continuous diamond film on a metal substrate. During the deposition process the growth rate passes through a minimum at the moment when a minimum roughness is supposed to be achieved.
Photoelectric quantum yield of nanometer metal particles
Authors: Schleicher B. , Burtscher H. , Siegmann H. C.
Published: July 2002

The absolute photoelectric quantum yield of nanometer metal particles (Ni, Pd, Cu, Ag, Au) is measured from threshold up to photon energies hν of 10 eV. The particles are produced and measured in ultrapure helium at atmospheric pressure. Yield Y and photothreshold φ are compared to bulk data. The shape of Y(hν) is similar for particle and bulk with all investigated materials except Ag. All metals have in common that the particle yield is larger by a factor ≊100 compared to the bulk yield. Apart from electrostatic corrections due to the image and Coulomb potential, the photoelectric threshold is identical for the bulk and the particle.
Deposition of amorphous BaTiO3 optical films at low temperature
Authors: Liu W.-T. , Lakshmikumar S. T. , Knorr D. B. , Lu T.-M. , van der Leeden Ir. Gerard A.
Published: July 2002

A reactive partially ionized beam technique has been used to deposit amorphous BaTiO3 films at room temperature. The procedure employing independently controlled evaporation and ionization of Ba and TiO has been optimized to produce highly transparent films. Films deposited on quartz or glass show 100% optical transmission over the visible wavelengths with respect to the substrate. The refractive index can be varied over a range of 1.9–2.0 by varying the Ba/Ti ratio without effecting the optical transparency. Films with refractive indices up to 2.4 can be deposited but the optical transmission decreases. The best optical properties are obtained for films with near stoichiometric Ba/Ti ratio. Annealing at temperatures less than 500 °C leads to densification of the films with a 2%–3% decrease in film thickness. The anneal increases the refractive index marginally. The optical band gap of the films was calculated from the experimental transmission plots. The Eg value of 4.6±0.1 eV for stoichiometric films decreases to 4.3 eV for nonstoichiometric films.
Optical detection of misfit dislocation-induced deep levels at InGaAs/GaAs heterojunctions
Authors: Raisanen A. , Brillson L. J. , Goldman R. S. , Kavanagh K. L. , Wieder H. H.
Published: July 2002

Using variable-depth luminescence excitation probes, we have observed discrete emission at characteristic energies from deep electronic states associated with misfit dislocations at InGaAs/GaAs interfaces. These states are localized near the buried heterointerface within the InGaAs layer and exhibit only minor variations in energy with composition and strain. The dislocation-induced spectral features appear only in strain-relaxed InGaAs films and are uncorrelated with additional features due to native bulk defects.
Lateral structuring of silicon thin films by interference crystallization
Authors: Heintze M. , Santos P. V. , Nebel C. E. , Stutzmann M.
Published: July 2002

Laterally structured microcrystalline silicon in the submicron range has been produced from amorphous silicon thin films by transient holography using a high-energy pulse laser. The energy density along the lines of the transient optical grid is sufficient to induce crystallization at the intensity maxima. Large area laterally structured microcrystalline silicon has been produced by selectively etching the amorphous phase with simultaneous growth of μc-Si:H in a hydrogen-silane plasma.
Formation of epitaxial and textured platinum films on ceramics-(100) MgO single crystals by pulsed laser deposition
Authors: Narayan J. , Tiwari P. , Jagannadham K. , Holland O. W.
Published: July 2002

We have investigated the formation of textured and epitaxial metallic films on (100) MgO single crystals substrates (lattice constant a=4.21 Å) as a function of deposition temperature during pulsed laser ablation. Platinum (a=3.92 Å) films on MgO with lattice misfit of 7.4% were found to grow epitaxially in the temperature range 500–700 °C. Three-dimensional x-ray diffraction results (theta, phi, and chi scans) show 〈100〉 epitaxy with the alignment of all three cube axes. Rutherford backscattering and channeling measurements on a film deposited at 700 °C showed a minimum yield of 2.2%, which is very close to the defect-free single crystal value. In the temperature range 200–500 °C both 〈100〉 and 〈111〉 textures were observed. The 〈111〉 oriented (normal to the surface) films were random in the plane of the substrate, whereas 〈100〉 crystallites were epitaxial. Below 200 °C, only 〈111〉 crystallites were observed. The 〈111〉 texture of platinum films is also observed when grown on amorphous substrates such as SiO2. The experimental results are normalized with theoretical simulations addressing the minimization of film energy.
Low threshold, wafer fused long wavelength vertical cavity lasers
Authors: Dudley J. J. , Babić D. I. , Mirin R. , Yang L. , Miller B. I. , Ram R. J. , Reynolds T. , Hu E. L. , Bowers J. E.
Published: July 2002

We demonstrate electrically injected InGaAsP (1.3 μm) vertical cavity lasers (VCLs) fabricated on GaAs substrates and employing GaAs/AlAs mirrors. The technique of wafer fusion allows for integration of GaAs/AlAs mirrors with InP double heterostructures without degradation of device performance, despite a 3.7% lattice mismatch between the wafers. The wafer fused VCLs have the lowest threshold current (9 mA) and lowest threshold current density (9.5 kA/cm2) and the highest characteristic temperature (T0=67 K) reported to date of any room-temperature long wavelength VCL.
Sensitivity analysis for Love mode acoustic gravimetric sensors
Authors: Wang Z. , Cheeke J. D. N. , Jen C. K.
Published: July 2002

An approximate mass sensitivity formula for Love mode sensors is presented. The optimal thickness of the guiding layer, at which the sensitivity, Sfm is maximum for a given frequency or wavelength is obtained. The (Sfm)max can be expressed as −k0/ρ1λ1, where ρ1 is the density, λ1 is the wavelength of the surface transverse wave of the substrate, and k0 is a coefficient determined by the ratio of shear wave velocity and the ratio of density of the substrate to those of the overlayer. The (Sfm)max of the Love mode device can be 20 times higher than that of the shear horizontal SH–surface acoustic wave device fabricated on the same substrate and operating at the same frequency.
Reliability and degradation of organic light emitting devices
Authors: Burrows P. E. , Bulovic V. , Forrest S. R. , Sapochak L. S. , McCarty D. M. , Thompson M. E.
Published: July 2002

We present a simple encapsulation technique for organic light emitting devices (OLEDs). By studying the degradation of a population of OLEDs, we show that the lifetime of encapsulated devices is increased by more than two orders of magnitude over that of unencapsulated devices. In both cases, degradation is primarily due to the formation of nonemissive regions, or dark spot defects. By studying the structure and evolution of the dark spots, we infer that the growth of electrode defects limits device lifetime. Hermetic packaging of OLEDs is essential if they are to be used in commercially viable flat panel displays.
Interface roughening in surfactant deposition
Authors: Hasegawa Shigehiko , Ryland Robert G. , Williams Ellen D.
Published: July 2002

We report on a scanning tunneling microscopy study demonstrating silicon mass transport and the resulting roughening of the surface during Sb deposition on Si(111). It is found that small amounts of Sb, which cause the Sb-induced d-(7×7) reconstruction, lead to Si island formation due to the replacement of Si atoms by Sb. The succeeding adsorption of Sb causes formation of pits in addition to the islands, in parallel with the structural transition to a (2×1) or (√3×√3) reconstruction. The extent of the roughening is directly related to the changes of surface Si atom density induced by Sb adsorption.
Dielectric strength of thermal oxides on 6H-SiC and 4H-SiC
Authors: Friedrichs P. , Burte E. P. , Schörner R.
Published: July 2002

This work reports on the dielectric strength of oxide layers formed by thermal oxidation of silicon carbide (SiC). SiC epilayers grown homoepitaxially on the silicon face of 6H-SiC and 4H-SiC substrates were oxidized in dry or wet ambient at 1100 °C. The dielectric strength was investigated using metal–oxide-semiconductor capacitors and was found to be tightly bound to 10 MV/cm for oxide thicknesses around 65 nm and independent of the SiC polytype and substrate doping. Considering the current-voltage characteristics in the prebreakdown region, dry oxides exhibit superior quality. Fowler–Nordheim tunneling was identified as the limiting current mechanism in the dry oxides. The corresponding barrier heights between the two SiC polytypes and thermal silicon dioxide were determined.
High-brightness blue and green light-emitting diodes
Authors: Eason D. B. , Yu Z. , Hughes W. C. , Roland W. H. , Boney C. , Cook J. W. , Schetzina J. F. , Cantwell G. , Harsch W. C.
Published: July 2002

We report high-brightness blue and green light-emitting diodes (LEDs) based on II–VI heterostructures grown by molecular beam epitaxy on ZnSe substrates. The devices consist of a 2–3 μm thick layer of n-type ZnSe:Cl, a ∼0.1 μm thick active region of Zn0.9Cd0.1Se (blue) or ZnTe0.1Se0.9 (green), and a 1.0 μm thick p-type ZnSe:N layer. The blue LEDs produce 327 μW (10 mA, 3.2 V), with the light output sharply peaked at 489 nm, and exhibit an external quantum efficiency of 1.3%. The green LEDs produce 1.3 mW (10 mA, 3.2 V) peaked at 512 nm, corresponding to an external quantum efficiency of 5.3%. In terms of photometric units, the luminous performance (luminous efficiency) of the devices is 1.6 lm/W (blue) and 17 lm/W (green), respectively, when operated at 10 mA.
Optically induced surface gratings on azoaromatic polymer films
Authors: Rochon P. , Batalla E. , Natansohn A.
Published: July 2002

The surface of an azoaromatic polymer film is optically altered to produce local highly efficient diffraction gratings. The gratings obtained are stable but can be erased by heating the polymer above its glass transition temperature and no permanent damage of the film is observed. Multiple gratings can be simultaneously written and gratings can be overwritten. Atomic force microscopy was used to investigate the gratings produced on the surfaces. Possible mechanisms responsible for the surface alteration are discussed.
Delta function convolution method (DFCM) for fluorescence decay experiments
Authors: Zuker M. , Szabo A. G. , Bramall L. , Krajcarski D. T. , Selinger B.
Published: July 2002

A rigorous and convenient method of correcting for the wavelength variation of the instrument response function in time correlated photon counting fluorescence decay measurements is described. The method involves convolution of a modified functional form F̃s of the physical model with a reference data set measured under identical conditions as the measurement of the sample. The method is completely general in that an appropriate functional form may be found for any physical model of the excited state decay process. The modified function includes a term which is a Dirac delta function and terms which give the correct decay times and preexponential values in which one is interested. None of the data is altered in any way, permitting correct statistical analysis of the fitting. The method is readily adaptable to standard deconvolution procedures. The paper describes the theory and application of the method together with fluorescence decay results obtained from measurements of a number of different samples including diphenylhexatriene, myoglobin, hemoglobin, 4′, 6-diamidine–2-phenylindole (DAPI), and lysine–trytophan–lysine.
Simplified pipe gun
Authors: So/rensen H. , Nordskov A. , Sass B. , Visler T.
Published: July 2002

A simplified version of a deuterium pellet gun based on the pipe gun principle is described. The pipe gun is made from a continuous tube of stainless steel and gas is fed in from the muzzle end only. It is indicated that the pellet length is determined by the temperature gradient along the barrel right outside the freezing cell. Velocities of around 1000 m/s with a scatter of ±2% are obtained with a propellant gas pressure of 40 bar.
Observations on the use of a thermal conductivity cell to measure the para hydrogen concentration in a mixture of para and ortho hydrogen gas
Authors: Bradshaw T. W. , Norris J. O. W.
Published: July 2002

This paper describes the use of a thermal conductivity cell to measure the para hydrogen concentration in a mixture of para and ortho hydrogen gas. The device was calibrated over its entire range and the output compared with the theoretically expected form. The output of this device had been previously assumed to be linear, this assumption can lead to systematic errors of up to 5.6% in the determination of the para hydrogen concentration. A simple calibration procedure is described.
Vertical-viewing electron cyclotron emission diagnostic for the DIII-D tokamak
Authors: James Reed , Janz S. , Ellis R. , Boyd D. , Lohr John
Published: July 2002

The vertical-viewing electron cyclotron emission diagnostic on DIII-D will be used to assess the nonthermal electron distributions resulting from electron cyclotron heating and electron cyclotron current drive experiments. Electron cyclotron emission along a vertical chord is collected using an ellipsoidal focusing mirror and retroreflector (the latter to minimize wall reflections). The emission is then transported ∼20 m using a quasioptical transport system composed of eight lenses and three mirrors, and detected between the 2nd and the 10th harmonics by a fast-scanning (40-Hz) Michelson interferometer. The entire system has been aligned using a Gaussian beam simulator and absolutely calibrated in situ using a cold liquid-nitrogen bath. Details of the design, installation, and calibration will be discussed.
A differential calorimetric technique for heat capacity and thermal conductivity measurements of liquids
Authors: Bertolini D. , Cassettari M. , Salvetti G. , Tombari E. , Veronesi S.
Published: July 2002

Details are reported for a differential calorimetric technique to measure both the heat capacity and the thermal conductivity of liquid samples. The accuracy of the measured heat capacity is better than 0.4%, while sensitivity is about 4×10−4 J/°C. The accuracy of the measured thermal conductivity kL is about 1% if the difference between the kL value of the reference and the sample liquid is within 40%. Measurements were performed on: (a) water+t-butanol, (b) water+n-butanol, and (c) methanol+t-butanol mixtures at 25 °C. The experimental results for solute molar fraction x less than 0.2 are compared with data in the literature in the case of water+t-butanol mixtures. The heat capacity and thermal conductivity data versus x are probably the first ones published for mixtures (b) and (c).
Growth and characterization of pseudomorphic single crystal zinc blende MnS
Authors: Skromme B. J. , Zhang Y. , Smith David J. , Sivananthan S.
Published: July 2002

We report the growth and characterization of crystalline, single-phase zinc blende MnS (i.e., β-MnS). The material was grown on ZnSe buffer layers on (100) GaAs substrates, using solid-source molecular beam epitaxy and a novel valved S cracker with deposition at low (∼110 °C) substrate temperatures. Characterization by reflection high energy electron diffraction, high resolution transmission electron microscopy, and low temperature photoluminescence confirms the pseudomorphic nature of the growth. Beyond a certain critical thickness, the layers either become amorphous or convert into the equilibrium rock salt polymorph (α-MnS), depending on the growth temperature.
Graded-composition buffer layers using digital AlGaAsSb alloys
Authors: Fritz I. J. , Dawson L. R. , Olsen J. A. , Howard A. J.
Published: July 2002

We describe step-graded digital-alloy buffers using alternate layers of Al0.5Ga0.5As and Al0.5Ga0.5As0.65Sb0.35 grown on GaAs substrates by molecular beam epitaxy. The buffers consist of three sets of superlattices with AlGaAs/AlGaAsSb layer thicknesses of 7.7/2.3 nm, 5.4/4.6 nm, and 3.1/6.9 nm, respectively, terminating in a lattice constant equal to that of bulk In0.32Ga0.68As. Transmission electron micrographs show that most of the misfit-generated dislocations lie near the steps in pseudoalloy composition, and atomic force micrographs indicate a rms surface roughness of 3.6 nm. A 20.5-period lattice-matched InGaAs/InAlAs reflector stack grown on such a buffer has a peak reflectivity of 98% near 1.3 μm. These buffers provide potentially useful substrates for optoelectronic device applications near 1.3 μm using strained InGaAs active regions.
Colossal magnetoresistance of 1 000 000-fold magnitude achieved in the antiferromagnetic phase of La1−xCaxMnO3
Authors: Gong Guo-Qiang , Canedy Chadwick , Xiao Gang , Sun Jonathan Z. , Gupta Arunava , Gallagher William J.
Published: July 2002

Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3 system. At 125 K, the CMR of the La0.5Ca0.5MnO3 reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated.
Impact of Pb doping on the optical and electronic properties of ZnO powders
Authors: Vanheusden K. , Warren W. L. , Voigt J. A. , Seager C. H. , Tallant D. R.
Published: July 2002

Electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy have been combined to characterize Pb-doped ZnO ceramic powders. We observe a decrease in the 2.26 eV emission peak and a concomitant smearing of the band edges, narrowing the effective gap of the grains to ≊2 eV with increasing lead content. Both phenomena are at least in part attributed to the formation of a separate PbO-like phase, likely residing at the grain boundaries. The free-carrier concentration in the grains was also observed to decrease with increasing Pb content. Our EPR results suggest that this may be due to electron transfer from oxygen vacancy donors to substitutional Pb centers, acting as electron traps.
Scanning tunneling microscope data acquisition and control in visual basic
Authors: Porter T. L.
Published: July 2002

A general purpose data acquisition and control system for scanning tunneling microscopy (STM) using Visual Basic is presented. This Windows hosted Visual Basic environment is highly desirable for use in STM image manipulation, storage, and printing, but in its standard form is not suitable for most data acquisition and display applications. Many of the inherent limitations in the Visual Basic language have been overcome by the use of direct calls to the Windows Application Program Interface. In this paper, we describe a general Visual Basic STM user interface and control system, and the extensions to the language using the Windows API needed to implement this system.
Characterization of plasma beam deposited amorphous hydrogenated silicon
Authors: Severens R. J. , Brussaard G. J. H. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

Fourier transform infrared spectrometry, visual transmission spectroscopy, and in situ ellipsometry have been performed on plasma beam deposited (PBD) amorphous hydrogenated silicon layers. From these measurements refractive index at infrared wavelengths and at 632.8 nm, the optical band gap and the hydrogen content of the layers have been determined. The hydrogen concentration of the layers varies between ∼9 and 25 at. %. It was found that the refractive index decreases more with hydrogen concentration in the layer than predicted by theoretical calculations assuming tetrahedral structures. The band gap of the material remains constant at ∼1.72 eV for the range of hydrogen contents measured. The resonance frequency of the SiH stretching mode (around 2000 cm−1) increases with increased hydrogen content. This is additional evidence to support the assumption that clustered SiH (SiH on voids) does not have its stretching mode near the 2100 cm−1 SiH2 peak. From the results presented it is concluded that PBD layers show behavior similar to plasma enhanced chemical vapor deposition layers with respect to the hydrogen content in the layers.
Vertical viewing of electron-cyclotron emissions for diagnosing fast-electron dynamics in TEXT-U
Authors: Roberts D. R. , Steimle R. F. , Giruzzi G. , Cima G. , Watts C.
Published: July 2002

A system for vertically observing the second-harmonic electron-cyclotron emissions from the TEXT-U tokamak in conjunction with fundamental electron-cyclotron heating is described. Using external collection optics and heterodyne receivers, measurements of the distribution function can be performed over two-thirds of the plasma radius and on transport time scales. Observations are typically of electrons with energy E<100 keV and either steep or shallow pitch angle, depending upon the toroidal viewing angle relative to the plasma current.
Generation of a fast atomic-oxygen beam from O− ions by resonant cavity radiation
Authors: Stephen T. M. , Van Zyl B. , Amme R. C.
Published: July 2002

An apparatus has been developed for producing a beam of ground-electronic-state oxygen atoms with energies variable from 4 to 1000 eV with a 1.5 eV FWHM energy distribution. The technique involves extraction of negative ions from a low-voltage gas-discharge source, mass selection of the extracted O− with a Wien-type velocity filter, O− acceleration or deceleration and focusing by electrostatic ion optics, and electron detachment from O− by intracavity laser radiation. A 25 W argon-ion-laser cavity has been extended to include the ion-beam vacuum chamber so that the intracavity radiation intersects the O− ion trajectories normally. Depending on the laser configuration in use, ion-neutralization efficiencies between 5% and 25% have been achieved at 5 eV O− energy. Thus, 5 eV O-atom fluxes of ∼1011 atoms/s (∼1012 atoms/cm2 s) have been achieved for O− currents of ∼10−7 A. The advantages and limitations of the technique are discussed, and preliminary measurements of the secondary-negative-charge production from low-energy O-atom impact on copper and stainless-steel surfaces are presented.
Novel in situ cleavage technique for cross-sectional scanning tunneling microscopy sample preparation
Authors: Kim Y.-C. , Nowakowski M. J. , Seidman D. N.
Published: July 2002

A novel in situ sample cleavage technique has been developed for fabricating specimens for cross-sectional scanning tunneling microscopy applications. This technique can be easily adapted to any ultrahigh vacuum scanning tunneling microscope that has coarse motion and tip ex- change capabilities. A 90° bent diamond tip attached to a tip holder is used to make micron long scratches on GaAs(001) surfaces along a 〈110〉 direction. The sample is then fractured and the cross-sectional surface is scanned in the conventional way. Atomic resolution images of {110}-type GaAs surfaces are readily and reproducibly obtained.
Point projection radiography with the flexible x-ray imager
Authors: Budil K. S. , Perry T. S. , Alvarez S. A. , Hargrove D. , Mazuch J. R. , Nikitin A. , Bell P. M.
Published: July 2002

Radiography techniques utilizing large-area x-ray sources (typically ⩽7 keV) and pinhole-imaging gated x-ray diagnostics have long been used at the Nova laser facility. However, for targets requiring higher-energy x-ray backlighters (>9 keV), low conversion efficiencies and pinhole losses combine to make this scheme unworkable. The technique of point projection radiography has been improved upon to make imaging at high x-ray energies feasible. In this scheme a “point” source of x-rays, usually a small diameter (⩽25 μm) fiber, is illuminated with a single, 100-ps pulse from the Nova laser. A gated x-ray imager with a 500-ps electronic gate width is used to record the projected image. [K. S. Budil et al., Rev. Sci. Instrum. 67, 485 (1996).] The experimental challenges this technique presents and experimental results will be discussed.
An ultra-low-noise, low-frequency, six degrees of freedom active vibration isolator
Authors: Newell D. B. , Richman S. J. , Nelson P. G. , Stebbins R. T. , Bender P. L. , Faller J. E. , Mason J.
Published: July 2002

To extend the operating frequency band of earth-based interferometric gravitational wave (GW) detectors down to 1 Hz, an unconventional system is required that provides approximately a factor of one million vibration isolation for the horizontal and more for the vertical, starting at 1 Hz. To this end, a three-stage ultra-low-noise, high-performance active and passive isolation system is being designed and constructed in order to demonstrate that a high degree of reduction can be achieved for vibrations at the support points of a GW detector’s final pendulum system. The first stage of this system has been fully characterized and is the main subject of this article. It is an equilateral triangular platform, 1.1 m on a side, with a total mass of 460 kg, including the vacuum system that will contain the other two stages. Active isolation is achieved by six servocontrol loops, using signals from low-frequency displacement sensors to feed back to noncontacting force transducers. The first stage has been actively isolated from vibrations in all six degrees of freedom by at least a factor of 100 at 1 Hz and above. The system noise limitations at 2 Hz and above are roughly 2×10−11 and 5×10−11 m/Hz for vertical and horizontal translations, respectively.
Kilovolt x-ray scattering from a plasma
Authors: Woolsey Nigel C. , Riley David , Nardi Eran
Published: July 2002

We describe a diagnostic suitable for the investigation of strong coupling effects in a plasma. The diagnostic is based on x-ray scattering of kilovolt x rays from a plasma and the determination of total x-ray scattering cross sections. The first experimental results of the total scattering cross sections from a strongly coupled plasma measured with kilovolt x rays are presented. The scattering plasma is formed by radiatively heating an Al foil with soft x rays created by Au laser-conversion foil; the plasma is probed with Ti XXII 1s2–1s2p line x rays emitted from a second laser produced plasma. A detailed description of the scattering technique is presented and the potential of x-ray scattering as a diagnostic of strongly coupled plasmas is explored.
Low cost PC based scanning Kelvin probe
Authors: Baikie I. D. , Estrup P. J.
Published: July 2002

We have developed a novel, low cost, scanning Kelvin probe (SKP) system that can measure work function (wf) and surface potential (sp) topographies to within 1 meV energy resolution. The control and measurement subcomponents are PC based and incorporate a flexible user interface, permitting software control of major parameters and allowing easy user implementation via automatic setup and scanning procedures. We review the mode of operation and design features of the SKP including the digital oscillator, the compact ambient voice-coil head-stage, and signal processing techniques. This system offers unique tip-to-sample spacing control (to within 40 nm) which provides a method of simultaneously imaging sample height topographies and is essential to avoid spurious or “apparent” wf changes due to scanning-induced spacing changes. We illustrate SKP operation in generating high resolution wf/sp profiles of metal interfaces (as a tip characterization procedure) and operational electronic devices. The SKP potentially has a very wide range of applications ranging from semiconductor quality control thin film and surface analyses to corrosion and biopotential imaging.
Moiré interferometry of short wavelength Rayleigh–Taylor growth
Authors: Matsuoka M. , Azechi H. , Nakai M. , Izumi N. , Shigemori K. , Nagaya T. , Miyanaga N.
Published: July 2002

One of the most critical quantities to be measured for better understanding of the ablative Rayleigh–Taylor (RT) instability is the cutoff wavelength below which the RT instability is stabilized. Unfortunately, this wavelength is expected to be below or close to spatial resolutions of most x-ray imagers used in the RT experiments. We propose and test a new technique using moiré interferometry to convert short wavelength perturbations to longer wavelength perturbations. The RT target is backlit by x rays and imaged just like in conventional experiments, but a grid mask is inserted between the target and the x-ray imager. The grid mask is one dimensional grid with a period slightly different from the wavelength of the RT target. It is set so that the grid and the RT perturbation groove are parallel to each other. With this operation, the long wavelength moiré interference is measured as the beat with the RT target. As an initial test, we use a grid mask with 10 μm period coupled to RT targets with 8 and 12 μm perturbation wavelengths. The corresponding moiré interference wavelengths are 40 and 60 μm, respectively.
Active shielding to reduce low frequency disturbances in direct current near biomagnetic measurements
Authors: Platzek D. , Nowak H. , Giessler F. , Röther J. , Eiselt M.
Published: July 2002

Measurements of dc near biomagnetic fields are disturbed by low frequency noise that is not reduced sufficiently by most of the magnetically shielded rooms or gradiometers. For this reason an active shielding system has been developed at the Biomagnetic Center of the University of Jena. This work describes the principle of the active shielding system and demonstrates its properties concerning the attenuation of disturbing fields, frequency range, and some applications in biomedical measurements. We achieved a reduction of external low frequency magnetic fields by more than 50 dB and an attenuation of the field gradient by about 25 dB. This active shielding enables measurements of near dc biomagnetic fields in investigations of periinfarct depolarizations after ischemic stroke and spreading depression in migraine patients.
Study of the capability of energy dispersive small angle x-ray scattering with synchrotron radiation
Authors: Yu Kuan-Li , Lee Chih-Hao , Hwang Ching-Shiang , Tseng Huan-Chi , Tseng Poh-Kun , Lin Tsang-Lang , Chang Szu-Li , Sheu Rong-Jiun , Chen Sow-Hsin
Published: July 2002

Energy dispersive small angle x-ray scattering is a unique method that takes advantage of a continuous spectrum of a synchrotron radiation source. In this study, the capability of this scattering measurement is explored. Using a detector mask with three acceptance rings of different ring radius and a sample to detector distance of 440 mm with the usable x-ray energy ranging from 4 to 20 keV, we can cover a q range from 0.01 to 0.4 Å−1. However, sample transmission and detector response function limit the application range at low energy. Slit scattering is the dominant source of background. The signal to background ratio for a standard cross-linked polyethylene sample can be more than 100. For a time-resolved experiment, data acquisition rate is limited by the throughput of the detector.
Nanotip array photoimprint lithography
Authors: Dam Thuc H. , Pantano Paul
Published: July 2002

Nanotip arrays have been fabricated on the distal faces of coherent fiber-optic bundles. A nanotip array comprised ∼6000 individual optical fibers that were etched chemically. Individual tips were ∼4 μm long with radii of curvatures as small as 15 nm. Nanotip arrays served as a template for a novel polymeric patterning process called photoimprint lithography. This lithographic method generated an array of polysiloxane microwells on glass surfaces. Individual wells had ∼1 μm diameters and were dispersed regularly ∼4 μm apart (center-to-center). Nanotip arrays were also used as templates for an imprint patterning process. This lithographic method generated an array of polystyrene microwells on glass surfaces dispersed regularly ∼4 μm apart with ∼1 μm diameters and ∼4 μm well depths. Both lithographic methodologies provide a simple, technically-expedient method to pattern surfaces with arrays of picoliter-volume wells suitable for microanalytical device utilization.
Morphology of Si(100) surfaces exposed to a remote H plasma
Authors: Montgomery J. S. , Schneider T. P. , Carter R. J. , Barnak J. P. , Chen Y. L. , Hauser J. R. , Nemanich R. J.
Published: July 2002

This study addresses the formation of roughness and near surface defects on Si(100) surfaces that are exposed to a remotely excited H plasma. The remote H plasma processing can be employed for in situ wafer cleaning. Atomic force microscopy, transmission electron microscopy, and residual gas analysis are used to measure the surface roughness, the near surface defects, and the etching, respectively. For remote H plasma exposures at substrate temperatures ≤300 °C, etching is observed along with a significant increase in the surface roughness and the formation of platelet defects in the near surface region. As the substrate temperature is increased to above 450 °C, etching is significantly reduced and no subsurface defects or increases in surface roughness are observed.
Ultrafast polarization-discriminating Mach-Zehnder all-optical switch
Authors: Tajima K. , Nakamura S. , Sugimoto Y.
Published: July 2002

We propose a polarization-discriminating Mach-Zehnder (PDMZ) all-optical switch. The switching speed of this PDMZ all-optical switch is not limited by the slow relaxation time of highly efficient incoherent nonlinearities. We demonstrate a squarelike modulation characteristic, which is necessary in most switching applications, at a switching speed (on-off time) of 40 ps. We also demonstrate ultrafast switching at a detector limited speed of ∼8 ps.
Voltage shifts and imprint in ferroelectric capacitors
Authors: Warren W. L. , Dimos D. , Pike G. E. , Tuttle B. A. , Raymond M. V. , Ramesh R. , Evans J. T.
Published: July 2002

Voltage offsets in the polarization-voltage characteristics of Pb(Zr,Ti)O3 capacitors can be induced by either thermal or optical processes. The thermally (optically) induced voltage shift occurs by heating (illuminating) the sample under remanence or a saturating bias. Generally speaking, the thermally induced voltage shifts are greater than those obtained optically; this is attributed to the role of oxygen vacancy-related defect dipoles throughout the film. We find that the inclusion of a dopant element that occupies a portion of the Ti(Zr) sites and has an oxidation state greater than +4 reduces the thermally induced voltage shifts observed in the capacitors. This may result because these particular dopants reduce the oxygen vacancy density and, hence, the defect-dipole contribution to the voltage shift.
InAs quantum boxes: Highly efficient radiative traps for light emitting devices on Si
Authors: Gérard J. M. , Cabrol O. , Sermage B.
Published: July 2002

We have compared by photoluminescence (PL) the radiative quantum efficiencies η of an array of InAs/GaAs quantum boxes (QBs) obtained by self-organized growth and of a single high quality InGaAs quantum well (QW). On GaAs substrates, η is essentially the same for both structures. A growth on a commercial GaAs-on-Si substrate entails drastic quenching of the integrated PL intensity and shortening of the carrier lifetime τ for the InGaAs QW, whereas both τ and η are not modified for the QB array. The efficient carrier capture by InAs QBs, combined with the localized nature of QB excitons hinders in this case the carrier diffusion toward dislocations. These superior properties of QBs on Si, which are observed over a wide range of excitation powers and for temperatures up to 300 K, opens a novel route toward efficient and reliable light emitters on Si.
Observation of the quantum confined ground state in InP quantum dots at 300 K
Authors: Giessen H. , Fluegel B. , Mohs G. , Peyghambarian N. , Sprague J. R. , Micic O. I. , Nozik A. J.
Published: July 2002

Colloidal suspensions of InP quantum dots (diameters 25 and 35 Å) show steplike absorption spectra which are blue-shifted by about 1 eV with respect to bulk material. Time resolved femtosecond spectroscopy at room temperature identifies the first quantum confined state. The nonlinear bleaching signal rises within 300 fs and persists longer than 200 ps.