bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic pulse compressor using saturable transformer to excite excimer lasers
Authors: Kurihara K. , Kobayashi S. , Satoh I. , Shibata K. , Shigeta M. , Masugata K. , Yatsui K.
Published: July 2002

To reduce the load on switching elements in the excitation circuit of excimer lasers, a new circuit has been developed to study the basic characteristics, where a saturable transformer is utilized in the magnetic pulse compressor. We have used a saturable transformer with a winding ratio of 1:4. The initial pulse of 8.5 kV, 2.0 kA, 3 μs was transferred to the output pulse of 34 kV, 8 kA, 109 ns, with an energy-transfer efficiency of 58%. Using a saturable inductor as a charging inductor, furthermore, we have succeeded in reduction of the prepulse of the output voltage and enhancement of the energy-transfer efficiency.
A laboratory-scale arc furnace for melting volatile metals under elevated inert gas pressures
Authors: Bradshaw A. R. , Fort D.
Published: July 2002

A laboratory-scale arc furnace encompassing a nonconsumable electrode and a cold hearth and operating at inert gas pressures of up to 20 bar is described. Using this equipment, weight losses due to volatilization when melting high vapor pressure metallic materials have been reduced to about 20% of those encountered with a conventional arc furnace operating at 1 bar. The arc furnace has been designed to be evacuable to high vacuum levels, as well as attaining high pressures, so that specimen purity can be retained by thoroughly evacuating and degassing the system prior to admitting inert gas and melting. Increases in gas pressure have been found to have a pronounced effect on the stability of the arc, especially at low arc currents, and to achieve minimum volatilization losses a balance is required between pressure and arc current for a particular sample.
Hydrogen passivation of donors and acceptors in SiC
Authors: Gendron F. , Porter L. M. , Porte C. , Bringuier E.
Published: July 2002

The effect of hydrogen on donors (N) and acceptors (Al, B) in 6H-SiC crystals has been evidenced by electron spin resonance and transport measurements. Typical passivation (i.e., complexing with H) levels of 75% have been obtained by annealing in a H2 atmosphere, and a corresponding decrease in free-carrier density has been observed.
A quasielastic light scattering detector for chromatographic analysis
Authors: Lago P. , Rovati L. , Cantù L. , Corti M.
Published: July 2002

Very sensitive laser light scattering equipment has been designed to be used in conjunction with chromatographic column separation. The system is prealigned and fully computer controlled. It gives the hydrodynamic radius of the eluting particles on line during separation. With a He-Ne laser source, the sensitivity is reached with four optical channels and four correlation functions of the scattered light measured in parallel with a special correlator.
Design and characterization of a versatile Faraday cup
Authors: Seamans J. F. , Kimura W. D.
Published: July 2002

The design and characterization of a Faraday cup utilizing modular components are presented. Design specifications were primarily tailored to satisfy the specific electron beam (e-beam) energy (∼375 keV), rise time (∼60 ns), and magnitude (30 A/cm2, peak) used in this work and permit convenient sampling of large e-beam areas up to 7 cm×7 cm. Characterization during evacuated conditions included Z-dependence measurements using beryllium, carbon, aluminum, and lead collector plates. Electron beam transmission measurements were made utilizing combinations of various metal screens and Kapton foils in both gas and evacuated conditions. Gas environments tested were air, krypton, and a Kr/Ar mixture. An attacher gas, SF6, was also added inside the Faraday cup. Results reveal decreasing current densities with increasing gas stopping power and increasing electron propagation distance in a gas. Employing a carbon collector plate and a 25-μm Kapton foil insulator, current densities measured through a 3.6-cm thick 760 Torr air slab are reduced ≤6% from the evacuated Kapton-free condition. Applying profile and full-aperture Faraday cup measurements, a consistent description of the e-beam is also presented.
An electrostatic levitator for high-temperature containerless materials processing in 1-g
Authors: Rhim Won-Kyu , Chung Sang K. , Barber Daniel , Man Kin F. , Gutt Gary , Rulison Aaron , Spjut R. Erik
Published: July 2002

This article discusses recent developments in high-temperature electrostatic levitation technology for containerless processing of metals and alloys. Presented is the first demonstration of an electrostatic levitation technology which can levitate metals and alloys (2–4 mm diam spheres) in vacuum and of superheating-undercooling-recalescence cycles which can be repeated while maintaining good positioning stability. The electrostatic levitator (ESL) has several important advantages over the electromagnetic levitator. Most important is the wide range of sample temperature which can be achieved without affecting levitation. This article also describes the general architecture of the levitator, electrode design, position control hardware and software, sample heating, charging, and preparation methods, and operational procedures. Particular emphasis is given to sample charging by photoelectric and thermionic emission. While this ESL is more oriented toward ground-based operation, an extension to microgravity applications is also addressed briefly. The system performance was demonstrated by showing multiple superheating-undercooling-recalescence cycles in a zirconium sample (Tm=2128 K). This levitator, when fully matured, will be a valuable tool both in Earth-based and space-based laboratories for the study of thermophysical properties of undercooled liquids, nucleation kinetics, the creation of metastable phases, and access to a wide range of materials with novel properties.
Space-resolving flat-field vacuum ultraviolet spectrograph for plasma diagnostics
Authors: Yamaguchi Naohiro , Katoh Jungo , Sato Yukiko , Aota Tatsuya , Mase Atsusi , Tamano Teruo
Published: July 2002

A spatial imaging vacuum ultraviolet (VUV) spectrograph has been constructed for simultaneous observation of spatial and spectral distributions of plasma radiation in the wavelength range 150–1050 Å. The spectrograph consists of an entrance slit of limited height which provides spatial resolution, an aberration-corrected concave grating with varied spacing grooves which gives a flat-field spectral output plane, and an image-intensified two-dimensional detector system. The basic characteristics of the spectrograph have been investigated by ray-tracing calculations. The expected performance has been confirmed through experiments using a dc glow discharge source on the reciprocal dispersion, the spatial resolution, or the incident angle dependence of spectral images. VUV spectra with spatial and time resolution have been obtained successfully in the GAMMA10 tandem mirror experiment.
Optical interference method to obtain thickness and refractive indices of a uniaxial medium
Authors: Carreño F. , Martínez-Antón J. C. , Bernabeu E.
Published: July 2002

Optical interference fringe measurements of the thickness of weakly absorbing media can be rapid, accurate, and nondestructive. When the refractive index n of the sample is known, it will give us the layer thickness d. If, however, n is unknown, at least two independent spectrophotometric measurements are needed to obtain both n and d. A statistically based scheme is proposed to analyze the interference pattern in order to determine the refractive index and the thickness of the sample. The absolute interference order is also determined with the proposed technique. The major approximation inherent in the method is that the layer must be weakly absorbing and nondispersive over the wavelength region of interest. The method is applied to determine the optical constants of a uniaxial medium with the optical axis parallel to the faces.
Aero-acoustic levitation: A method for containerless liquid-phase processing at high temperatures
Authors: Weber J. K. Richard , Hampton D. Scott , Merkley Dennis R. , Rey Charles A. , Zatarski Mark M. , Nordine Paul C.
Published: July 2002

A method for containerless liquid-phase processing was developed which has practical application in process and property research on virtually any material which is involatile at the melting point. It combines aerodynamic and acoustic forces to support and position the levitated material. The design provides forced convection control of the thermal boundary in the gas surrounding beam-heated specimens, which stabilizes the acoustic forces and allows acoustic positioning necessary to stabilize the aerodynamic levitation forces on molten materials. Beam heating and melting at very high temperatures was achieved. Experiments were conducted on specimens with diameters in the range 0.25–0.4 cm, of density up to 9 g/cm3, at temperatures up to 2700 K, and in oxygen, air, or argon atmospheres. Unique liquid-phase processing results included deep undercooling of aluminum oxide, glass formation at exceptionally small cooling rates, complete melting and undercooling of YBa2Cu3Ox superconductor materials, direct formation of the YBa2Cu3Ox from the liquid phase, and the vaporization of volatile constituents from a low-liquefaction point glass to form a refractory, high melting material. The application of rapid containerless batch processing operations to materials synthesis is discussed.
High spatial resolution interferometry and polarimetry in hot plasmas
Authors: Donné A. J. H.
Published: July 2002

Interferometry is a standard technique for measuring the line-integrated electron density of magnetically confined plasmas. It is based on the fact that an electromagnetic wave, on its passage through a plasma, experiences a phase change with respect to the vacuum situation. Moreover, the plane of polarization of the wave will in general rotate because the plasma is birefringent. The total (Faraday) rotation angle of the wave, which can be measured by polarimetric techniques, is proportional to the line-integrated value of the magnetic field component parallel to the beam. The optimum wavelength for interferometry/polarimetry, for diagnosing magnetically confined plasmas is in the far-infrared and infrared spectral region. To invert the line-integrated data of the electron density and magnetic field component to local values, without using too stringent assumptions on the plasma symmetry, it is necessary to probe the plasma along multiple chords. In many confinement devices this is done by using a limited number of discrete probing chords. In recent years much attention has been devoted to the development of multichannel interferometer/polarimeter systems with a much better coverage of the plasma, which could be achieved by expanding the probing beam either to a slab- or a fanlike beam, covering almost the complete minor cross section of the plasma. A much higher number of detectors can be implemented in this way, thus substantially improving the spatial resolution of the measurements. In this paper the present status of interferometer/polarimeter systems featuring slablike or fanlike probing beams will be reviewed. Results of detailed measurements that have been performed to study the effect of magnetohydrodynamic activity (in particular sawteeth), disruptions, additional heating, and pellet injection on the electron density distribution, will be used to illustrate the merits of interferometry/polarimetry with high spatial resolution.
Stokes polarimetry for x rays using multiple-beam diffraction (invited)
Authors: Shen Qun , Shastri S. , Finkelstein K. D.
Published: July 2002

We show that the Stokes–Poincaré polarization parameters for an x-ray beam can be completely determined using the method of multiple-beam Bragg diffraction. We first summarize the principles of this technique, and then show results from an experiment in which we applied the method to characterize an x-ray phase plate.
A scanning near-field optical microscope for the imaging of magnetic domains in reflection
Authors: Silva T. J. , Schultz S.
Published: July 2002

The design and implementation of a scanning near-field optical microscope expressly intended for the imaging of magnetic domains in reflection with subwavelength resolution is described. The microscope employs ∼40 nm silver particles optically excited at the plasmon resonance as the near-field probes. Special attention is given to the Newton ring interferometer used to regulate the separation between the near-field probe and the sample. The results of two tests of the microscope performance are presented. The first is a measurement of the dependence of the near-field magneto-optic Kerr effect on probe/sample spacing for a longitudinally magnetized metallic thin film. The effect decreases rapidly with increasing separation with a length scale comparable to the probe size. The second is a series of images of a domain in a perpendicularly magnetized multilayer metallic film at different probe/sample spacings. The images indicate a persistent effect with increasing separation but diminishing resolution. Fundamental differences between the near-field magneto-optic Kerr effect for perpendicularly and longitudinally magnetized samples is discussed. An estimate is made as to the fundamental limit of resolution with this particular form of near-field microscopy.
Amorphous-crystal transition of organic dye assemblies: Application to rewritable color recording media
Authors: Naito Katsuyuki
Published: July 2002

Media composed of a color former (leuco dye), a developer (phenol compound), and a reversible matrix (steroid) were colored in the crystalline states of the matrix and colorless in the amorphous states. Another medium composed of a color former and a developer serving for a reversible matrix (steroid substituted by a phenol group) was colorless in the crystalline state and colored in the amorphous state. Reversible color changes were possible by a heat treatment. Melting, glass transition, and crystallization temperatures were widely controlled by changing the materials.
Application of a digital phase comparator technique to interferometer data
Authors: Jiang Y. , Brower D. L. , Zeng L. , Howard J.
Published: July 2002

Substantial improvements have been made to increase the frequency bandwidth of the UCLA multichannel interferometer system on TEXT-Upgrade. Recently, a digital phase comparator technique was implemented that has several advantages over the conventional analog phase comparator. The probe and reference wave forms are directly digitized and the phase computed in software using a numerical demodulation algorithm. For the purpose of reducing the data load, aliasing of the laser difference frequency is employed to vary the time response according to experimental demands.
Quantitative method for precise, quick, and reliable alignment of collection object fields in the JT-60U Thomson scattering diagnostic
Authors: Yoshida H. , Naito O. , Matoba T. , Yamashita O. , Kitamura S. , Hatae T. , Nagashima A.
Published: July 2002

The design concept and operation performance of a novel alignment system for accurate electron density profile measurement is presented, which has been developed on the basis of the original method for adjusting quantitatively and actively an object field of collection fiber optics to a laser-beam axis. The quantitative expression needed for an appropriate alignment was able to be formulated using the beam width measured in situ by this alignment system and the four laser scattering intensity signals from special alignment fiber bundles. The effectiveness of this alignment system for precise and reliable measurement of electron density has been successfully demonstrated in the JT-60U Thomson scattering diagnostic through direct monitoring and quick restoration of alignment. The important and distinctive feature of this alignment system is to attain compatibility between a stable and accurate maintenance of alignment and an improvement of the signal-to-noise ratio of Thomson scattering to plasma light accompanying the reduced fiber bundle width.
Fiber interferometer-based variable temperature scanning force microscope
Authors: Euler R. , Memmert U. , Hartmann U.
Published: July 2002

A scanning force microscope designed for an operation at temperatures between 4.2 and 300 K is presented. The deflection of the microfabricated force sensing cantilever is detected via an optical fiber interferometer. For low temperature imaging the whole instrument is incorporated into a bath cryostat which is suitable for both liquid helium and liquid nitrogen cooling. The instrument is of highly symmetric design in order to avoid large inner misalignment of the interferometer due to thermal expansion/contraction during temperature changes. In addition to this thermally compensated design, the interferometer can be adjusted by piezo actuators in situ in three dimensions.
A synchrotron x-ray liquid surface spectrometer
Authors: Schlossman Mark L. , Synal Dennis , Guan Yongmin , Meron Mati , Shea-McCarthy Grace , Huang Zhengqing , Acero Anibal , Williams Scott M. , Rice Stuart A. , Viccaro P. James
Published: July 2002

The design of a synchrotron x-ray liquid surface spectrometer at beamline X19C at the National Synchrotron Light Source is described. This spectrometer is capable of performing the full range of x-ray surface scattering techniques. A few examples of measurements made using this spectrometer are presented, including studies of organic monolayers on the surface of water and of the structure of strongly fluctuating oil–microemulsion interfaces. The measurements discussed illustrate the accuracy, resolution, and capabilities of the spectrometer.
Origin of compositional variations in sputter-deposited TixW1−x diffusion barrier layers
Authors: Bergstrom D. B. , Tian F. , Petrov I. , Moser J. , Greene J. E.
Published: July 2002

Sputter-deposited Ti1−xWx diffusion barriers in microelectronic devices have been reported by many groups to be Ti deficient with respect to the target composition. In the present experiments, polycrystalline TixW1−x alloys were grown on oxidized Si(001) substrates at temperatures Ts between 100 and 600 °C by ultrahigh-vacuum magnetron cosputter deposition from pure W and Ti targets in 5 mTorr (0.65 Pa) Ar and Xe discharges. Films deposited in Ar were found by Rutherford backscattering and Auger electron spectroscopies to be increasingly Ti deficient with increases in the Ti sputtering rate and/or Ts at a constant W sputtering rate. TRIM calculations and Monte Carlo gas-transport simulations were used, in combination with the experimental results, to show that the Ti loss was due primarily to differential resputtering of the growing film by energetic Ar particles backscattered from the heavier W target. This effect is exacerbated at elevated film growth temperatures by Ti surface segregation in the alloy. The use of Xe, rather than Ar, as the sputtering gas greatly reduces both the flux and the average energy of backscattered particles incident at the substrate such that measurable Ti loss is no longer observed.
Measurements of dead time and characterization of ion counting systems for mass spectrometry
Authors: Fahey Albert J.
Published: July 2002

A method for characterization of electron multiplier-based pulse counting systems is presented. Specific attention is paid to measurement of the counting system dead time. The systems discussed here are typically used to measure the abundance of ions passing through a mass spectrometer. A method for the determination of dead time by measurement of at least two isotopic ratios of an element with at least three isotopes is given. This method differs from the “text book” method typically used to measure the dead time. The theoretical basis for the measurements is given and an example of measurements of Ti isotopes by secondary ion mass spectrometry (SIMS) is shown. Dead time measurements on SIMS instruments can be performed in a few hours of machine time. A brief description of the pulse detection and counting circuit and its operation is also given. Guidelines for the proper measurement of pulse shapes and the determination of reasonable settings for electron multiplier gain, preamplifier gain, and discriminator threshold are also discussed.
Absolute values of specific heat capacity and thermal conductivity of liquids from different modes of operation of a simple photopyroelectric setup
Authors: Caerels J. , Glorieux C. , Thoen J.
Published: July 2002

Photothermal techniques offer a nice tool for the determination of changes in thermal parameters of different types of samples. Using a very thin pyroelectric transducer, a very simple and versatile ac calorimeter can be built. The described inverse pyroelectric technique allows high-resolution ac-calorimetric measurements of the temperature dependence of the specific heat capacity. Since measuring frequencies up to a few hertz can be used, which are higher than in most alternative ac-calorimetric setups, noise can be reduced and measuring times shortened. With a small modification of the measurement cell, the determination of absolute specific heat capacity and thermal conductivity values of liquid samples was achieved. Comparison of the frequency response of the system with and without a solid thermal load in the neighborhood of the sample yields data with an accuracy of about 5% and 10% for specific heat capacity and thermal conductivity, respectively. If necessary, absolute values of specific heat capacity and thermal conductivity can also be obtained by making contact between a (solid) thermal load and the liquid. A pure ac-calorimetric operation is then no longer possible, thus restricting this mode of operation to homogeneous or to properly aligned anisotropic liquids.
Transfer of samples between separated ultrahigh vacuum instruments for semiconductor surface studies
Authors: Jiřı́ček P. , Cukr M. , Kolařı́k V. , Koc S.
Published: July 2002

A simple, portable ultrahigh vacuum (UHV) chamber is described to transport samples prepared in a molecular beam epitaxy machine to a photoelectron spectrometer for surface studies of III-V semiconductors. Special care is devoted to maintain UHV conditions during the sample transfer and to remove possible sources of contamination, especially by degassing the chamber joint exposed to air during the transport. Low energy electron diffraction patterns taken before and after transport, subsequent x-ray photoelectron spectroscopy tests of surface purity, as well as the observed dispersion curve of GaAs (001) c(4×4) superstructure confirm that method of transport does not cause significant (detectable) surface contamination.
Simultaneous measurement of q and Er profiles using the motional Stark effect in high-performance DIII-D plasmas (invited)
Authors: Rice B. W. , Nilson D. G. , Burrell K. H. , Lao L. L.
Published: July 2002

The motional Stark effect (MSE) diagnostic was developed to provide a measurement of the magnetic pitch angle or q profile in tokamaks. The technique relies upon polarization measurements of Stark broadened Dα emission to determine the pitch angle of the Lorentz vb×B electric field, where vb is the injected neutral beam particle velocity and B is the total magnetic field. However, in many advanced confinement regimes, large values of the plasma radial electric field, Er, are observed and can affect the interpretation of MSE measurements. Viewing fixed locations in the plasma from two different viewing angles allows one to separate the Er field from the vb×B field, thus providing simultaneous measurement of the Er and q profiles. To achieve this measurement, the DIII–D MSE diagnostic was recently upgraded from 16 to 35 channels with three independent viewing angles. The new instrument provides an Er resolution of 5–10 kV/m with a time response of 1 ms. Measurement results from the VH mode, reverse shear, and H mode plasmas are presented.
Two dimensional electronic flute
Authors: Dyakonov M. I. , Shur M. S.
Published: July 2002

Based on a complete similarity between the plasma waves in a field effect transistor and sound waves, we discuss a possibility of realizing an electronic terahertz oscillator utilizing the excitation of the plasma waves in the two dimensional resonance cavities by adjacent electron flow.
Instrumentation for real-time fluorescence lifetime imaging in endoscopy
Authors: Mizeret Jérôme , Stepinac Thomas , Hansroul Marc , Studzinski André , van den Bergh Hubert , Wagnières Georges
Published: July 2002

The fluorescence lifetime of living tissues is, in certain cases, related to their pathologic state and is therefore of interest for cancer detection. Measuring fluorescence lifetime in vivo during an endoscopic examination has thus been a challenging objective for several years. The present article deals with the development and first clinical trails of an instrumentation producing fluorescence lifetime images in real time. The acquisition of such fast phenomenon (nanosecond time scale) on an image has been made possible by using the homodyne detection approach, in which the excitation light and the detection gain are modulated in a phase-coherent way. Based on images acquired at different phase between the excitation and detection modulation, the fluorescence lifetime is calculated for each pixel of the image. Different configurations of excitation modulation characteristics (pulse train versus sine-wave amplitude modulation) have been investigated and compared using Fourier transforms. Interestingly, a pulsed excitation combined with a sine-wave modulation detection gives valuable results. The expected auto-fluorescence signal emitted by human tissues under subthermal light excitation irradiance has been estimated at the wavelengths of interest. The limited number of auto-fluorescence photons results in relatively high noise on the lifetime calculated. The typical standard deviation is about 125 ps for lifetimes of 2.5 ns with a 322-pixel image (spatial integration). An in vivo image in the bronchi illustrates the potentiality of the new instrumentation. The results of this preliminary study indicate that the healthy bronchial mucosa, excited in the blue or in the green, fluoresces with a lifetime of 2.5 ns.
Beam optics in inertial electrostatic confinement fusion
Authors: Ohnishi Masami , Hoshino Chikara , Yoshikawa Kiyoshi , Masuda Kai , Yamamoto Yasushi
Published: July 2002

We study the transport of ions and electrons near the cathode of the inertial electrostatic confinement fusion that is expected to be a portable neutron source. We carry out a PIC particle simulation in order to obtain the self-consistent electrostatic potential and the transparency of the cathode for the accelerated ions. The transparency is shown to be much less than a geometrical transparency and possesses a strong dependence on the energy of ions. The increase of the applied voltage results in larger neutron production due to increased fusion cross section and also increased ion current by the improved transparency.
Pulsed spectrometer for nuclear quadrupole resonance for remote detection of nitrogen in explosives
Authors: Anferov V. P. , Mozjoukhine G. V. , Fisher R.
Published: July 2002

We describe a pulsed spectrometer for detection of nuclear quadrupole resonance on the nuclei of nitrogen N-14 with fast Fourier transform. The use of a multipulse sequence, four channel system for data registration and processing permits detection of the nuclear quadrupole resonance (NQR) signal in the presence of strong interference and the piezo effect. Using this spectrometer we registered the NQR signal from an explosive sample of 150 g (92% RDX) at a distance of 22 cm, and the time of detection was 81 s.
Comparison of two methods of measuring the rapid temperature rises in split Hopkinson bar specimens
Authors: Walley S. M. , Proud W. G. , Rae P. J. , Field J. E.
Published: July 2002

The temperature rise produced in iron and copper specimens by high strain deformation in a compression split Hopkinson pressure bar was measured simultaneously using two independent techniques: (i) small thermocouples (0.2 mm junction size) and (ii) an infrared (IR) camera system based on mercury cadmium telluride (spectral response between 8 and 12 μm). The response time of the thermocouple system was limited by the time taken for heat to diffuse into the junction. In order to obtain useful data with the IR camera system, the emissivity of the specimen surface needed to be modified by depositing a layer of soot. Even so, the measured emissivity was low (0.4), suggesting that the soot layer was semitransparent to IR radiation. The thermocouples, however, yielded temperature measurements that were consistent with all the mechanical work performed on the specimens being converted to heat. The main positive result obtained with the IR camera system is that heating of the iron specimens was spatially nonuniform.
Pulsed corona and dielectric-barrier discharge processing of NO in N2
Authors: Penetrante B. M. , Hsiao M. C. , Merritt B. T. , Vogtlin G. E. , Wallman P. H. , Neiger M. , Wolf O. , Hammer T. , Broer S.
Published: July 2002

Experimental results on pulsed corona and dielectric-barrier discharge processing of very dilute concentrations of NO in N2 are presented. These NO reduction experiments measure the G value for electron-impact dissociation of N2 and are used to infer the effective electron mean energy in an N2 discharge plasma at atmospheric pressure. The data have been obtained from three different laboratories using widely differing electrode structures, voltage wave forms, power measurements, and chemical analyses. The NO reduction yields from the discharge reactors tested are all similar, corresponding to an electron mean energy of 4.0±0.5 eV.
Low interface trap density for remote plasma deposited SiO2 on n-type GaN
Authors: Casey H. C. , Fountain G. G. , Alley R. G. , Keller B. P. , DenBaars Steven P.
Published: July 2002

Metal-oxide-semiconductor capacitors were prepared with remote plasma-enhanced chemical vapor deposition of SiO2 at ∼300 °C on an n-type GaN epitaxial layer grown by atmospheric pressure metalorganic chemical-vapor deposition on a sapphire substrate. No hysteresis was observed in the high-frequency capacitance-voltage (C−V) measurements, and the measured C−V curve agreed with the C−V behavior calculated for an ideal oxide with the same flat-band voltage as the measured C–V curve. The absence of hysteresis and stretchout in the measured C–V curve and the increase of capacitance with incident ultraviolet light while in deep depletion suggest a low concentration of interface traps. These results demonstrate previous predictions of the absence of Fermi-level stabilization at the interface for the ionic crystal GaN.
Microstructure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts for n-GaN
Authors: Ruvimov S. , Liliental-Weber Z. , Washburn J. , Duxstad K. J. , Haller E. E. , Fan Z.-F. , Mohammad S. N. , Kim W. , Botchkarev A. E. , Morkoç H.
Published: July 2002

Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts on n-type GaN (∼1017 cm−3) epitaxial layers. The metals were deposited either by conventional electron-beam or thermal evaporation techniques, and then thermally annealed at 900 °C for 30 s in a N2 atmosphere. Before metal deposition, the GaN surface was treated by reactive ion etching. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112]TiN//[10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.
Fabrication of GaAs and InAs wires in nanochannel glass
Authors: Berry A. D. , Tonucci R. J. , Fatemi M.
Published: July 2002

A newly developed porous glass, nanochannel glass, was used to fabricate uniform, high-density GaAs and InAs micro- and nanowires with high aspect ratios. The fabrication process utilized reactions between organogallium and organoindium compounds with arsine to produce polycrystalline GaAs and InAs with crystallite sizes of approximately 50–130 Å when annealed at 400–500 °C. At the higher annealing temperatures, the InAs wires exhibited an increase in surface porosity and grain size, whereas the GaAs wires maintained a uniform, smooth texture.
Time-resolved pump-probe experiments with subwavelength lateral resolution
Authors: Vertikov A. , Kuball M. , Nurmikko A. V. , Maris H. J.
Published: July 2002

We demonstrate picosecond time-resolved pump-probe experiments with subwavelength resolution using a reflection near-field scanning optical microscope (NSOM). The sample was a laterally patterned gold nanostructure. Strain-induced reflectivity changes as low as ΔR/R≊10−4 were detected in the near-field.
Gallium diffusion and diffusivity in CuInSe2 epitaxial layers
Authors: Schroeder David J. , Berry Gene D. , Rockett Angus A.
Published: July 2002

Ga diffusion in single crystal Cu(In1−xGax)Se2 (CIGS) epitaxial layers on GaAs was measured as a function of the Cu/In ratio in the CIGS. No Ga was supplied intentionally during deposition but Ga was released by the substrates into the growing film. The concentration profile of the Ga was determined by secondary ion mass spectrometry and was fit by numerical solution of Fick’s second law including motion of the surface and the possibility of Ga desorption from the surface. All films studied had Cu/(In+Ga) ratios greater than 1.0, which was determined by the Ga outdiffusion from the GaAs. The Cu/In ratio was controlled by the deposition conditions. The Ga diffusivity was a minimum near a Cu/In ratio of 1.0 and increased rapidly for both higher and lower values of this ratio. The diffusivity ranged from a minimum of 2.7×10−13 cm2/s at Cu/In=0.94 to 5×10−11 cm2/s−12 cm2/s at Cu/In=1.41 and 7×10−12 cm2/s at Cu/In=0.43. It was concluded that Ga diffuses on metal vacancies on either the Cu or In sublattice.
Nature of native oxide on GaN surface and its reaction with Al
Authors: Prabhakaran K. , Andersson T. G. , Nozawa K.
Published: July 2002

In this letter, we describe the surface properties of GaN thin films grown on sapphire substrate by molecular beam epitaxy, as revealed by ultraviolet and x-ray photoelectron spectroscopic and Auger electron spectroscopic studies. The samples are seen to contain overlayer of native oxides, which are predominantly in the Ga2O3 form. Ammonia is shown to be a good etchant for these native oxides. Furthermore, we investigated the early stages of the reaction of monolayer Al with a GaN surface covered with native oxide. Aluminum reacts preferentially with the surface oxygen and leads to the formation of a mixture of oxides at the interface.
Epitaxial relationships between GaN and Al2O3(0001) substrates
Authors: Grandjean N. , Massies J. , Vennéguès P. , Laügt M. , Leroux M.
Published: July 2002

GaN thin layers (200 Å) were grown by gas-source molecular beam epitaxy on c-plane Al2O3 substrates. Transmission electron microscopy reveals that two different epitaxial relationships may occur. The well-known GaN orientation with the c axis perpendicular to the Al2O3 surface and [11̄00]GaN‖[112̄0]Al2O3 is observed when the substrate is nitridated prior to the growth. On the other hand, GaN layers deposited on bare Al2O3 surfaces exhibit a different crystallographic orientation: [112̄0]GaN‖[11̄00]Al2O3 and [11̄03]GaN‖[112̄0]Al2O3. This corresponds to a tilt of about 19° of the c axis with respect to the substrate surface.
Initial stage for heteroepitaxy of 3C–SiC on the Si(001) surface in dimethylgermane source molecular beam epitaxy
Authors: Hatayama Tomoaki , Tanaka Norihiro , Fuyuki Takashi , Matsunami Hiroyuki
Published: July 2002

By the use of dimethylgermane [(CH3)2GeH2 : DMGe], a Si clean surface can be carbonized reproducibly at as low as 650 °C in a gas source molecular beam epitaxy. The initial stage for heteroepitaxy in the 3C–SiC/Si system has been studied with time resolved in situ reflection high-energy electron diffraction (RHEED) analysis. A RHEED pattern from a carbonized layer indicates single-crystalline 3C–SiC without any 3C–SiC twin spots and Ge-related patterns. An activation energy of about 51.1 kcal/mol is obtained in the initial stage of 3C–SiC growth. The successive growth of 3C–SiC layers formed with DMGe is also discussed.
Composition dependence of the ultraviolet absorption edge in lithium niobate
Authors: Kovács L. , Ruschhaupt G. , Polgár K. , Corradi G. , Wöhlecke M.
Published: July 2002

A convenient and fast method to determine the composition of LiNbO3 single crystals consisting of a measurement of the position of the fundamental absorption edge in the near UV region is refined and extended to the stoichiometric range. Using unsophisticated apparatus and simple reflection correction, the proposed two-parameter calibration equation provides an absolute accuracy of 0.1 mol % with the relative accuracy reaching an unusual value of better than 0.01 mol % for near stoichiometric compositions.
Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to
n
-type GaN
Authors: Luther B. P. , Mohney S. E. , Jackson T. N. , Asif Khan M. , Chen Q. , Yang J. W.
Published: July 2002

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN.
Anisotropic epitaxial lateral growth in GaN selective area epitaxy
Authors: Kapolnek D. , Keller S. , Vetury R. , Underwood R. D. , Kozodoy P. , Den Baars S. P. , Mishra U. K.
Published: July 2002

Epitaxial lateral mask overgrowth which occurs during GaN selective epitaxy has been studied using linear mask features. The lateral growth varies between its maximum and minimum over a 30° angular span and exhibits hexagonal symmetry. Vertical growth follows an opposite trend, with lateral growth maxima, and vertical growth minima occurring for lines parallel to the GaN 〈10•0〉. Large variations in the lateral growth are also obtained through variations in the growth temperature and NH3 flow. Under proper growth conditions, lateral to vertical growth rate ratios of up to 4.1 can be achieved, resulting in significant lateral mask overgrowth and coalescence of features without excessive growth times.
Scanning thermal microscopy of a vertical-cavity surface-emitting laser
Authors: Luo K. , Herrick R. W. , Majumdar A. , Petroff P.
Published: July 2002

A scanning thermal microscope was used to measure the temperature distribution inside a vertical-cavity surface-emitting laser. The peak temperature occurred at the intersection of the optical axis and the active quantum wells, and increased with input power at a rate of 0.74 °C/mW. Comparison with model predictions showed that the n mirrors and the substrate produce higher heat generation rates, possibly due to Joule heating and/or the absorption of spontaneous emissions that are often neglected in models.
Planar aluminum-implanted 1400 V 4H silicon carbide p-n diodes with low on resistance
Authors: Peters D. , Schörner R. , Hölzlein K.-H. , Friedrichs P.
Published: July 2002

Planar p-n diodes with edge termination were fabricated by aluminum implantation on n-type 4H silicon carbide. These diodes exhibited an excellent blocking behavior up to 1400 V reverse voltage with stable avalanche breakdown at an electric field strength of 2.8 MV/cm. In addition, a nearly classical forward characteristic was observed with both recombination and diffusion current mechanism represented by ideality factors of 1.05 and 1.93, respectively. The turn-on voltage was 2.8 V. At a forward voltage drop of 6.2 V a current density of 4000 A/cm2 and a differential on resistance below 1 mΩ cm2 were achieved.
Low-threshold quasi-cw type-II quantum well lasers at wavelengths beyond 4 μm
Authors: Lin Chih-Hsiang , Pei S. S. , Le H. Q. , Meyer J. R. , Felix C. L.
Published: July 2002

Optically pumped InAs/InGaSb/InAs/AlSb type-II quantum well lasers emitting from 4.1 to 4.36 μm were operated up to 226 K with a characteristic temperature T0 of 30 K. The absorbed threshold pump intensity at 0.98 μm was 0.12 kW/cm2 at 100 K, and 3.25 kW/cm2 at 200 K with a pulse length of 5 μs and a repetition rate of 2 kHz. At 73 K, the peak output power was 250 mW per facet with a pulse length of 10 μs and a repetition rate of 10 kHz. A cw output power of 14.7 mW was observed at 74 K.
Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo
Authors: Bharathan Jayesh , Yang Yang
Published: July 2002

Inkjet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting (or conjugated) polymers are solution processable, IJP technology becomes an ideal method for printing polymer light-emitting diodes with high resolution. Unfortunately, the polymer film printed from an inkjet printer usually consists of pin-holes, and this intrinsic character makes it unsuitable for fabricating high quality polymer electronic devices, particularly for devices in the sandwich structure. In this letter, we submit a hybrid structure, which consists of an inkjet printed layer in conjunction with another uniform spin coated polymer layer, as an alternative to the regular inkjet printed structure. The uniform layer serves as a buffer layer to seal the pin-holes and the IJP layer is the layer consisting of the desired pattern, for example the red–green–blue dots for a multicolor display. To demonstrate, we applied this hybrid technology to fabricate efficient and large area polymer light-emitting logos. The use of this concept represents a whole new technology of fabricating polymer electronic devices with lateral patterning capability.
Effect of lattice mismatch strains on the structural and magnetic properties of barium ferrite films
Authors: Shinde S. R. , Ramesh R. , Lofland S. E. , Bhagat S. M. , Ogale S. B. , Sharma R. P. , Venkatesan T.
Published: July 2002

The effect of substrate-induced lattice strains on the structural and magnetic properties of epitaxial barium ferrite (BaFe12O19) thin films is studied. The 5000 Å thick film shows a very narrow ferromagnetic resonance (FMR) line with a width of about 140 Oe. After appropriate annealing, the linewidth measured at 58 GHz decreases to 37 Oe, which to the best of our knowledge is about two times smaller than the best previously reported value for epitaxial thin films. FMR studies show that the presence of strains broadens resonance absorption.
Dynamic nucleation of ice induced by a single stable cavitation bubble
Authors: Ohsaka K. , Trinh E. H.
Published: July 2002

Evidence is presented of the dynamic nucleation of ice induced by an isolated stable cavitation bubble in undercooled water. The evidence supports the notion of dynamic nucleation as a result of the freezing point shift due to high pressure pulses associated with a collapsing bubble. It is found that the cavitation bubble requires minimum undercooling, 5 K, to initiate nucleation on ice. This undercooling may be attributed to the temperature rise of the water due to the heat generated by compression of the bubble or the undercooling required for normal ice to nucleate on a high pressure phase.
Field emission from single-wall carbon nanotube films
Authors: Bonard Jean-Marc , Salvetat Jean-Paul , Stöckli Thomas , de Heer Walt A. , Forró László , Châtelain André
Published: July 2002

We report on the field emission properties of single-wall carbon nanotube films, with emphasis on current–versus–voltage (I–V) characteristics and current stability. The films are excellent field emitters, yielding current densities higher than 10 mA cm−2 with operating voltages that are far lower than for other film emitters, but show a significant degradation of their performances with time. The observed deviations from the Fowler-Nordheim behavior in the I–V characteristics point to the presence of a nonmetallic density of states at the tip of the nanotubes.
Decay of ultraviolet-induced optical absorption in Ge-doped SiO2 glass
Authors: Ohama Motoshi , Fujiwara Takumi , Ikushima Akira J.
Published: July 2002

We report quantitative measurements on the decay of optical absorption at various elevated temperatures in ultraviolet (UV)-irradiated SiO2 glasses doped with Ge. From the Arrhenius plots of decay time constants, the activation energy of the Ge E′ center in the UV-irradiated glasses has been determined to be 2.4±0.3 eV, while the reported activation energy of Ge E′ decay in UV-poled glasses is 0.4±0.1 eV. The present experimental results strongly suggest that the decay process of the Ge E′ center induced in UV-irradiated glasses is quite different from that in UV-poled glasses.
Large blue shifts induced by the quantum confined stark effect in asymmetric quantum wells
Authors: Gug R. K. , Hagston W. E.
Published: July 2002

The quantum-confined stark effect is calculated for an asymmetric AlxGa1−xAs/GaAs double quantum well system. It is shown that, with slight modifications to this system, the resultant blue shift in the one-particle energy separation of the electron and hole can be increased to values exceeding 30 meV. It is also shown that the value of this blue shift is very sensitive to the potential profile of the quantum well.
Absolute frequency stabilization of two diode-pumped Er–Yb:glass lasers to the acetylene P(15) line at 1534 nm
Authors: Svelto C. , Bava E. , Taccheo S. , Laporta P.
Published: July 2002

Two identical diode-pumped bulk Er–Yb:glass lasers, operating at 1534-nm wavelength, have been independently locked to the P(15) rovibrational line of the acetylene molecule by the FM sideband technique. Measurements of the beat note between the two lasers show a long-term frequency stability of 170 kHz over a 1-h period and a short-term laser linewidth below 50 kHz over 1 ms. The Allan standard deviation of the beat frequency was measured yielding values below 10−10 for integration times between 10 ms and 1 s, reaching the 4×10−11 level at 0.1 s.
Phase transitions in ferrimagnetic and ferroelectric ceramics by ac measurements
Authors: Peláiz-Barranco A. , Gutiérrez-Amador M. P. , Huanosta A. , Valenzuela R.
Published: July 2002

Ac conductivity measurements were carried out on polycrystalline samples of a ferrimagnetic spinel (Zn0.44Mn0.56Fe2O4) and a ferroelectric perovskite (Sr0.25Bi4Ti3.25O12.75), in the temperature range 20–160 and 20–660 °C, respectively, and in the frequency range 5 Hz–13 MHz. The impedance response in both cases could be resolved into two contributions, associated with the bulk (grains) and the grain boundaries. An analysis by means of the ac conductivity power law showed evidence of a critical temperature of 132 and 536 °C, for the ferrimagnetic and the ferroelectric samples, respectively, which corresponds to the Curie temperature for each type of material. These results are interpreted in terms of the disorder increase approaching the phase transition.
Argon ion damage in self-assembled quantum dots structures
Authors: Schoenfeld W. V. , Chen C.-H. , Petroff P. M. , Hu E. L.
Published: July 2002

The effects of radiation damage exposure on InGaAs quantum wells and InAs quantum dots are compared using luminescence spectroscopy techniques. A large increase in the radiation resistance of the InAs quantum dots is observed and attributed to exciton localization in the quantum dots and a point defect strain gettering effect.