bluuebunny/crossref_metadata_2025 huggingface dataset:
Review of high-power microwave source research
Authors: Gold Steven H. , Nusinovich Gregory S.
Published: July 2002

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: (1) plasma resonance heating and current drive; (2) rf acceleration of charged particles; (3) radar and communications systems; and (4) high-peak power sources for weapons-effect simulation and exploratory development.
Development of a multichord beam-attenuation probe of hydrogen and helium for plasma diagnostics
Authors: Kodera F. , Kojima M. , Yoshikawa T. , Goto S.
Published: July 2002

In order to estimate the density and the electron temperature profiles of a medium temperature plasma, a multichord beam probing system has been developed. The ion density can be estimated by hydrogen neutrals attenuation via charge exchange. The electron temperature could be inferred from the electron impact ionization attenuation of a helium atom beam under some assumption. Our beam system includes a large bucket ion source which can simultaneously emit both hydrogen and helium ions, a neutralization drift tube, a beam energy and momentum analyzer corresponding to six chords and a data acquisition system. The completed device is applied for the measurement of a field-reversed configuration plasma which has a typical electron temperature of 50 eV and a line density of 2.0×1015 cm−2.
Prospects for fluorescence based imaging/visualization of hydrodynamic systems on the National Ignition Facility
Authors: Suter L. J. , Landen O. L. , Koch J. I.
Published: July 2002

The next generation of large, high power lasers, such as the National Ignition Facility in the U.S., Laser Mega Joule in France or Helen Successor in the United Kingdom offers the prospect of x-ray fluorescence based diagnosis of hydrodynamic experiments. We discuss two promising techniques for pumping this fluorescence. One is to use a large fraction of these facilities’ high power to efficiently make multi-kilovolt x rays which, in turn, causes dopants placed in experimental packages to fluoresce. The second technique is to use the sizable multi-kilovolt photon background that we expect to be present in many hohlraum based experiments, while the driving laser is on, to pump x-ray fluorescence. Here, the fluorescing medium could be a dopant in an experimental package or, possibly, a relatively thick slab of material in the hohlraum wall which could serve as a backlighter.
Some problems of the material choice for the first mirrors of plasma diagnostics in a fusion reactor
Authors: Voitsenya V. S. , Konovalov V. G. , Shtan’ A. F. , Solodovchenko S. I. , Becker M. F. , Bardamid A. F. , Yakimov K. I. , Gritsyna V. T. , Orlinskij D. V.
Published: July 2002

We present the results of simulation experiments on the effect of the fusion reactor environment on the optical properties of the first mirrors for spectroscopy and laser diagnostics. We found the greatest effect on the degradation of mirror optical properties was due to charge exchange atoms. These atoms can affect mirror quality in two ways: by sputtering and by redeposition of material sputtered from other inner components of the vacuum chamber. The degradation rates of mirrors made of different metals and subjected to long-term bombardment by ions from hydrogen or deuterium plasmas are compared. From analysis of all data, we concluded that special experiments will have to be conducted in order to make a correct choice of a first mirror material. These tests should include the following: (1) study the role of swelling on mirror surface modification; (2) find the multimillion shot laser-induced-damage threshold of metal mirrors; and (3) develop and test methods to protect the first mirrors from redeposition. In addition, a model should be developed for the charge exchange atom flux and energy distribution that represents real data from operating tokamaks.
Serviceability of using the carbon–graphite materials for inner elements of microwave diagnostic for large fusion devices
Authors: Berezhnyj V. L. , Fomin I. P. , Ocheretenko V. L. , Skibenko A. I. , Voitsenya V. S.
Published: July 2002

As an alternative to metal-made inner elements of MM and subMM diagnostics (antennae, reflectors, waveguides) we propose the elements fabricated of carbon–graphite materials (CGM). A comparative study of antennae and reflectors made of CGM and metal for 30–150 GHz in MM and for subMM ranges was carried out under the influence of pulsed plasma streams and heating up to 1000 °C. The effect of neutron irradiation on reflectivity at 337 μm was studied as well. The obtained data demonstrated that CGM can be used as material for fabricating the above mentioned inner diagnostic components in those large-scale fusion devices where similar materials are in use for the first wall protection. The Fabry–Perot type resonator with spherical CGM-made mirrors showed rather good characteristics, and can be used for measuring plasma properties in a divertor flow.
A compact and high repetitive photodiode array detector for the JT-60U Thomson scattering diagnostic
Authors: Yoshida H. , Naito O. , Sakuma T. , Kitamura S. , Hatae T. , Nagashima A.
Published: July 2002

A compact and two dimensional detector has been developed for the JT-60U Thomson scattering system, that consists mainly of a proximity focused image intensifier, a 20×12 spatially and spectrally arrayed silicon photodiode, and a data processing unit enabling high repetitive measurement at 1 kHz. To secure a wide spectral bandwidth for an effective detection of the Thomson scattering spectrum, we have introduced a spectrometer and lens systems with enhanced transmission in a longer wavelength, which can compensate for the decreasing characteristic of quantum efficiency of a transmission-mode photocathode used in the image intensifier. The spectral divide of the photodiode array is designed to give its full performance in a wide electron temperature (Te) range of 0.2–15 keV with 12 spectral channels. The application of this newly developed detector to the JT-60U core plasma measurement is also presented.
The new infrared imaging system on Alcator C-Mod
Authors: Maqueda R. J. , Wurden G. A. , Terry J. L. , Stillerman J. A.
Published: July 2002

A new infrared imaging system has been installed on Alcator C-Mod. This system uses an Amber Radiance 1 IR video camera (filtered to the 4.2–4.4 μm band) to view a 30 cm×30 cm region of the lower divertor from above by means of a re-entrant 5-m long ZnSe based periscope. Capture of the standard 30 Hz video frames (8-bit) and camera control are performed remotely over fiber optic links by a Windows 95 PC, using a MuTech MV-1000 video grabber board. Plans are under way to directly capture the 60 Hz, 12-bit, 256×256 pixel images using a digital video camera interface with a fiber optic link from EDT (Beaverton, Oregon). Preliminary results show that during nondisruptive discharges no substantial surface temperature increase is observed on the upper sections of the divertor, with the exception of “hot spots,” although occasionally, increased heating in toroidal bands is seen. Bands can also be observed after disruptions that result in a downward movement of the plasma.
Split-electrode thin film transducer for acoustic applications
Authors: Strashilov V. L.
Published: July 2002

A high-frequency transducer for generation of shear acoustic waves in arbitrary substrates has been considered. It exploits thickness-shear resonance in thin piezoelectric films, excited by a lateral electric field through a specific electrode configuration. The resonant equation for the center-of-band frequencies has been derived and possible applications including efficient excitation of surface acoustic waves have been discussed.
Polarized Xe129 optical pumping/spin exchange and delivery system for magnetic resonance spectroscopy and imaging studies
Authors: Rosen M. S. , Chupp T. E. , Coulter K. P. , Welsh R. C. , Swanson S. D.
Published: July 2002

We describe the design and construction of a laser-polarized Xe129 production and delivery system that is used in our in vitro and in vivo magnetic resonance imaging (MRI) experiments. The entire apparatus including lasers and optics, rapidly actuated valves, heating and cooling, and transport tubing lies in the high magnetic field environment of a 2 T MRI magnet. With approximately 7.5% Xe129 polarization, 157 cc atm of xenon gas is produced and stored as xenon ice every 5 min. Large quantities of polarized Xe129 can be obtained by cycling this process. The xenon is subsequently delivered in a controlled fashion to a sample or subject. With this device we have established the feasibility of using laser-polarized Xe129 as a magnetic tracer in MRI. This reliable, effective, and relatively simple production method for large volumes of Xe129 can be applied to other areas of research involving the use of laser-polarized noble gases.
Effects of filament positions on the arc discharge characteristics of a negative hydrogen ion source for neutral beam injector
Authors: Asano Shiro , Tsumori Katsuyoshi , Okuyama Toshihisa , Suzuki Yasuo , Osakabe Masaki , Oka Yoshihide , Takeiri Yasuhiko , Kaneko Osamu
Published: July 2002

The influence of filament positions on the arc discharge characteristics of a negative hydrogen ion source for a neutral beam injector were studied by simulations and experiments. We have made a simulation code named “PRIMELOC2,” which calculates the orbits of primary electrons emitted from cathode filaments, including collisions with H2 gas molecules filled in the source, to obtain the spatial distribution of ionization points. We applied this code to a large negative hydrogen ion source having an external magnetic filter for LHD-NBI No. 1 (neutral beam injector 1 for a large helical device). As a result of the calculations, we have found that the moving area of emitted primary electrons differs markedly according to the filament positions. Calculation results agreed well with the experimental results of high-power arc discharge and the spatial distribution of plasma parameters obtained by Langmuir probe measurements. By applying PRIMELOC2 to filament–arc ion sources, we can choose appropriate areas in arc chambers for filament positioning.
A novel electron spin-polarization detector with very large analyzing power
Authors: Bertacco Riccardo , Onofrio Davide , Ciccacci Franco
Published: July 2002

The low energy electron reflectivity from a magnetic surface depends on the relative orientation between the electron spin and the sample magnetization. This effect has been exploited for realizing a new electron spin polarization detector. As a scattering surface we have used a well ordered Fe(001)-p(1×1)O surface, which is stable against surface contamination and gives rise to very large spin dependent effects. We describe in detail the preparation in vacuum of such a target surface and show that it can be transferred in a separate system, without performance losses. The analyzing power S of the detector is given by the relative variation of its response when the target magnetization is reversed. We find extremely large S values for electrons reflected at 3–6 eV kinetic energy, with maximum above 45%, i.e., roughly 3 times better than other polarimeters presently used.
High temperature ultrasonic sensor for the simultaneous measurement of viscosity and temperature of melts
Authors: Balasubramaniam Krishnan , Shah Vimal V. , Costley R. Daniel , Boudreaux Gary , Singh Jagdish P.
Published: July 2002

An ultrasonic sensor that simultaneously measures temperature and viscosity of molten materials at very high temperature is described. This sensor has applications as a process monitor in melters. The sensor is based on ultrasonic shear reflectance at the solid–melt interface. A delay line probe is constructed using refractory materials. A change in the time of flight within the delay line is used to measure the temperature. The results obtained from this sensor on various calibration glass samples demonstrate a measurement range of 100–20 000 P for the viscosity and 25–1500 °C for the temperature.
Viscosity measurements of viscous liquids using magnetoelastic thick-film sensors
Authors: Loiselle Keith T. , Grimes Craig A.
Published: July 2002

A ribbon-like magnetoelastic thick-film sensor can be considered the magnetic analog of an acoustic bell. In response to an externally applied magnetic field impulse the magnetoelastic sensor magnetically rings, in a bell-like fashion, emitting magnetic flux with a characteristic resonant frequency that is determined from a fast Fourier transform of the transient response captured using a pickup coil. The resonant frequency of the sensor changes in response to mass loading and, when immersed in liquid, interfacial shear forces associated with viscosity. We report on the use of magnetoelastic sensors to measure the viscosity and density of highly viscous liquids. When characterizing highly viscous liquids, to avoid over damping of the sensor due to the liquid mass load the sensor is suspended from a boat-like structure with one end of the sensor immersed a fixed distance into the liquid. Using this method liquid viscosities ranging from 1.3 to 12 P are measured.
Quantitative characterization of an x-ray source in an x-ray photoelectron spectroscopy system
Authors: Pepper Stephen V. , Wheeler Donald R.
Published: July 2002

The x-ray spectrum from a dual anode x-ray source in an x-ray photoelectron spectroscopy system was calculated by the NIST Desk Top Spectrum Analyzer used for synthesizing spectra in energy dispersive x-ray analyzers commonly found in scanning electron microscopes. The energy flux was obtained by integrating over the x-ray energy spectrum. The energy flux from the source was measured by a silicon photodiode. Very good quantitative agreement was found between the flux obtained from the calculated spectra and the flux measured by the photodiode, indicating that the calculated spectra are, in fact, physically valid for this source. The calculated spectra were used to obtain the energy deposition as a function of depth below the irradiated surface of polytetrafluoro- ethylene, a material used in spacecraft and exposed to the space radiation environment.
A simple microbeam profiling technique for x-ray optics
Authors: Lankosz M. , Sieber J.
Published: July 2002

Enhancement of the quality of images or maps obtained using micro x-ray fluorescence spectrometry requires knowledge of the flux distribution profile of the primary x-ray beam. A simple procedure based on a wire scan was developed for the determination of an x-ray microbeam profile. This technique is simple and can be used for a given excitation/detection geometry without any change in spectrometer configuration. However, the result of such a scan is a convolution of the beam profile with the shape of the wire. To restore the actual distribution of photon flux density, a nonlinear least-squares procedure was adapted to model the profile using a parametric function to describe the flux distribution. Among the algorithms tested, a sigmoid function was found to be the best for describing the beam profile. The results of calculations were verified by measurements using a charge coupled device camera. Reasonably good agreement was obtained between the two different techniques. The results of this work confirmed that a wire scan combined with a mathematical restoration procedure is useful for describing the actual profile of an x-ray microbeam created using a pinhole collimator. An advantage is that the microbeam profile is calculated at the location of the measured samples. The results indicated that the χ2 value could be used to select the optimum function to describe the microbeam profile. The procedure can also be used to evaluate the quality of a pinhole collimator. We plan to use measured microbeam profiles in combination with mathematical procedures to improve the spatial resolution of linear scans of patterned samples.
Ultrahigh pressure cell for materials synthesis
Authors: Hale Michael A. , Clausi Dominic , Willson C. Grant , Dallas Tim , Hashemi Javad , Wilson James , James Daryl , Holtz Mark , Kuhrts Kurtis , Combs Bret
Published: July 2002

We describe here an apparatus for generating ultrahigh pressures and temperatures for materials synthesis. The system uses a spherically shaped explosive charge to radially compress various materials contained within the sample cavity. The results show the system is completely reproducible and, because of the geometry of the loading system, the workpiece that holds the precursor material is in excellent postshock condition. Samples are subjected to a pressure pulse of approximately 230 GPa for a period of 4 μs. Using this system, C60 has been compressed using a variety of quenching materials to attempt to generate diamond. Two organic precursors have also been tested to determine if the high stress conditions are suitable to synthesize β-carbon nitride. In this article, the design of the system, the numerical modeling using Dyna East Finite Element Lagrangian and CTH hydrocodes, and the results of the synthesis experiments will be presented.
Alignment visualization using electroconvection of planar nematics
Authors: Bock T. M. , Bläsing J. , Frette V. , Rehberg I.
Published: July 2002

In electroconvection experiments with planarly aligned nematic liquid crystals the director orientation is, conventionally, fixed through a mechanical treatment (rubbing) of the polymer-coated electrodes. Without rubbing, on the other hand, the flow direction during sample filling imposes the director orientation. We report atomic force microscopy and x-ray measurements that show an anisotropy in the polymer surface structure on several scales as a result of the rubbing. In particular we observe a fish-bone structure on a 10 nm scale. We visualize the orientation of the director both during and after filling the system using the electroconvection pattern. This is a convenient tool for exploring new director configurations. We confirm for the observed surface structure that when flow and surface designate different orientations, the mechanical surface treatment dominates. We have been able to obtain regions with radial director orientation of millimeter size. Such an alignment renders possible new types of electroconvection experiments.
In situ x-ray topography of silicon carbide during crystal growth by sublimation method
Authors: Yamaguchi H. , Nishizawa S. , Kato T. , Oyanagi N. , Bahng W. , Yoshida S. , Arai K. , Machitani Y. , Kikuchi T.
Published: July 2002

We have developed an instrument for real-time observation of silicon carbide (SiC) crystal during the sublimation growth process by x-ray topography. It is constructed by combining an x-ray goniometer and a crystal-growth chamber. The vertical goniometer consists of three circles, an α circle for the x-ray source, a β circle for the detector, and a φ circle for the azimuthal rotation of the sample. The growing crystal boule is set at the center of the goniometer glued on a crucible lid. Transmission topographs are taken using an asymmetric (101̄1) reflection with the scattering angle 2θB=β−α from the (0001)-oriented boule. A rotating-anode 18 kW generator with a molybdenum target is employed as the x-ray source. Topographs are observed by a direct imaging system using a charge-coupled device camera. Incident and scattered x-ray beams pass through beryllium windows mounted on the bottom and top flanges of the crystal-growth chamber, respectively. The crucibles are also designed for in situ measurements so that the x-ray beam path is separated from the source materials. The in situ topographs demonstrated the movement of micropipes and other defects during the crystal growth.
The Mt. Fuji submillimeter-wave telescope
Authors: Sekimoto Yutaro , Yamamoto Satoshi , Oka Tomoharu , Ikeda Masafumi , Maezawa Hiroyuki , Ito Tetsuya , Saito Gaku , Iwata Mitsuhiro , Kamegai Kazuhisa , Sakai Takeshi , Tatematsu Ken’ichi , Arikawa Yuji , Aso Yoshiyuki , Noguchi Takashi , Miyazawa Keisuke , Shi Sheng-Cai , Saito Shuji , Ozeki Hiroyuki , Fujiwara Hideo , Inatani Junji , Ohishi Masatoshi , Noda Kazufusa , Togashi Yuichi
Published: July 2002

We have developed a 1.2 m submillimeter-wave telescope at the summit of Mt. Fuji to survey emission lines of the neutral carbon atom (CI) toward the Milky Way. A superconductor-insulator-superconductor mixer receiver on the Nasmyth focus is used to observe the 492 GHz band in SSB and the 345 GHz band in DSB simultaneously. The receiver noise temperature is 300 K in SSB and 200 K in DSB for 492 and 345 GHz, respectively. The intermediate frequency frequency is 1.8–2.5 GHz. An acousto-optical spectrometer which has the total bandwidth of 0.9 GHz and 1024 channel outputs has also been developed. The telescope was installed at the summit of Mt. Fuji (alt. 3725 m) in July 1998. It has been remotely operated via a satellite communication system from Tokyo or Nobeyama. Atmospheric opacity at Mt. Fuji was 0.4–1.0 at 492 GHz during 30% of the time and 0.07–0.5 at 345 GHz during 60% of the time from November 1998 to February 1999. The system noise temperature was 1000–3000 K in SSB at 492 GHz and 500–2000 K in DSB at 345 GHz. We observed the CI (3P1−3P0: 492 GHz) and CO (J=3−2: 345 GHz) emission lines from nearby molecular clouds with the beam size of 2.̓2 and 3.̓1, respectively. We describe the telescope system and report the performance obtained in the 1998 winter.
Thermal expansion coefficient of 3C–SiC
Authors: Talwar D. N. , Sherbondy Joseph C.
Published: July 2002

Theoretical results for the temperature dependence of the thermal expansion coefficient for 3C–SiC are reported using a phenomenological lattice dynamical theory in the quasiharmonic approximation. The linear thermal expansion coefficient α of 3C–SiC exhibits a variation with temperature much like that of the specific heat and, unlike other tetrahedrally coordinated materials, it does not attain a negative value at lower temperatures.
In situ x-ray diffraction study of the antiferroelectric–ferroelectric phase transition in PLSnZT
Authors: Blue C. T. , Hicks J. C. , Park S.-E. , Yoshikawa S. , Cross L. E.
Published: July 2002

In situ x-ray diffraction studies were performed on the PLSnZT antiferroelectric–ferroelectric phase switching ceramic and polycrystalline powder. The crystallography of both the antiferroelectic and electric-field-induced ferroelectric phases were determined for the approximate composition of Pb0.98La0.02(Zr0.55Sn0.33Ti0.12)0.995O3. A preferred antiferroelectric domain structure has been identified and possible mechanisms responsible for the domain structure are presented. A single tetragonal phase has been identified for the room temperature zero-field antiferroelectric material with unit cell dimensions a=4.107 Å and c=4.081 Å. An electric field-induced structure developing at the antiferroelectric–ferroelectric switching field has also been observed and determined to be of rhombohedral symmetry with a=4.105 Å and θ=89.85° indicating a volume increase of 0.49% for the tetragonal–rhombohedral transition.
Crystalline quality and phase stability of hexagonal GdSi1.7 layers formed by channeled ion-beam synthesis
Authors: Wu M. F. , Vantomme A. , Pattyn H. , Langouche G. , Bender H.
Published: July 2002

Previous reports show that, among all rare-earth silicides, GdSi1.7 is the most difficult one to grow epitaxially with a good crystalline quality on a Si substrate. However, this letter shows that by using channeled implantation, a continuous GdSi1.7 layer with good crystalline quality (χmin=10%) can be formed by implantation of 90 keV Gd ions in Si(111). Besides, the hexagonal phase of the GdSi1.7 layer is stable up to a temperature of 850 °C for 30 min, which is much more stable than previously reported. After annealing at temperatures ≥900 °C for 30 min, the hexagonal GdSi1.7 phase transforms into the orthorhombic GdSi2 phase. Rutherford backscattering/channeling, transmission electron microscopy, and x-ray diffraction are used in this study.
Light scattering in high-dislocation-density GaN
Authors: Liau Z. L. , Aggarwal R. L. , Maki P. A. , Molnar R. J. , Walpole J. N. , Williamson R. C. , Melngailis I.
Published: July 2002

Light scattering by edge dislocations and the resulting loss coefficient have been modeled for GaN layers. Phase-front deformation caused by the refractive-index variation in the dislocation’s strain field has been considered and the resulting scattering loss calculated. We show that the high dislocation densities observed in recent GaN layers can result in significant large loss coefficients. The present work also offers some insights for improved lasers.
Thermal resistance of thin diamond films deposited at low temperatures
Authors: Verhoeven H. , Rei\S H. , Fü\Ser H.-J. , Zachai R.
Published: July 2002

The thermal resistance for conduction normal to diamond layers only a few microns thick, deposited at temperatures of about 500 °C on silicon using microwave-plasma-assisted chemical vapor deposition (MPACVD) and electron cyclotron resonance enhanced MPACVD, is measured using a specific laser-heating method. A detailed analysis of the results obtained using this method allows the determination of the effective diamond–silicon boundary resistance and the resistance within the diamond layer independently of one another. The analysis shows that the effective boundary resistance is comparable to or even larger than the resistance within the thin diamond layers, and suggests that it depends on the grain dimensions at the interface. For grain sizes of 10 nm, an average effective boundary resistance of 2.4×10−7 m2 K/W is determined, whereas at 1.2×10−8 m2 K/W it is significantly lower for grains about an order of magnitude larger. The resistance within the diamond shows a similar qualitative behavior with respect to grain size.
Noise characteristics of thin multiplication region GaAs avalanche photodiodes
Authors: Hu C. , Anselm K. A. , Streetman B. G. , Campbell J. C.
Published: July 2002

It is well known that the gain-bandwidth product of an avalanche photodiode can be increased by utilizing a thin multiplication region. Previously, measurements of the excess noise factor of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption and multiplication regions indicated that this approach could also be employed to reduce the multiplication noise. This letter presents a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses. It is demonstrated that there is a definite ‘‘size effect’’ for multiplication regions less than approximately 0.5 μm. A good fit to the experimental data has been achieved using a discrete, nonlocalized model for the impact ionization process.
Hydrogen plasma passivation of InP: Real time ellipsometry monitoring and ex situ photoluminescence measurements
Authors: Bruno G. , Losurdo M. , Capezzuto P. , Capozzi V. , Trovato T. , Perna G. , Lorusso G. F.
Published: July 2002

The remote hydrogen plasma passivation of semi-insulating InP substrates has been investigated by in situ spectroscopic ellipsometry (SE), to characterize the changes of InP morphology, and ex situ photoluminescence (PL) to reveal the passivation effect of nonradiative centers operated by H atoms. The concurrence of the InP native oxide removal without surface damage (phosphorus ablation) and the passivation by H-atoms results in a remarkable enhancement of the PL intensity.
γ-radiation resistance of aluminum-coated all-silica optical fibers fabricated using different types of silica in the core
Authors: Griscom D. L. , Golant K. M. , Tomashuk A. L. , Pavlov D. V. , Tarabrin Yu. A.
Published: July 2002

Induced loss spectra in four Al-coated large-core optical fibers are measured in the range 380–1000 nm during and after 60Co γ irradiation. These fibers had F-doped-silica claddings and synthetic silica core materials produced by different technologies. Measurements were carried out independently in Washington and Moscow using differing techniques but yielding very similar results. The highest radiation resistance is exhibited by two fibers with extremely low OH and Cl contents in their cores, one with a core material containing 0.5 mass % of fluorine, fabricated specially for this study, and one based on KS-4V silica, an original technology developed in Russia. The latter fiber demonstrates a rather flat induced loss spectrum in the visible region at a level of 2–2.5 dB/m for γ-ray doses of ∼10 MGy(Si).
Quantum boxes as active probes for photonic microstructures: The pillar microcavity case
Authors: Gérard J. M. , Barrier D. , Marzin J. Y. , Kuszelewicz R. , Manin L. , Costard E. , Thierry-Mieg V. , Rivera T.
Published: July 2002

A GaAs/AlAs planar cavity containing a collection of InAs quantum boxes in its core region has been grown in a single step by molecular beam epitaxy, and processed by electron-beam lithography and reactive ion etching into pillar microresonators. The optical study by photoluminescence of these localized light emitters allows a systematic and precise determination of the energies of the first confined photon modes of such microstructures, in good agreement with theoretical estimates. More generally, such probes facilitate the experimental study of the modes of complex photonic microstructures and of the spontaneous emission alteration they entail on a quasimonochromatic light emitter.
Observation of leaky slab modes in an air-bridged semiconductor waveguide with a two-dimensional photonic lattice
Authors: Kanskar M. , Paddon P. , Pacradouni V. , Morin R. , Busch A. , Young Jeff F. , Johnson S. R. , MacKenzie Jim , Tiedje T.
Published: July 2002

An air-bridged, 120-nm-thick semiconductor slab with a two-dimensional (2D) square array of through holes on a 480 nm pitch (Λ) was fabricated using selective wet etching techniques. The second order photonic resonances of the structure were studied by comparing broadband optical scattering data with numerical solutions of Maxwell’s equations. Features observed in these spectra over a 1200 cm−1 range, near 9500 cm−1, indicate that the 2D texture splits the energy degeneracy of slab modes with propagation constants {±2π/Λ,0} and {0,±2π/Λ} by as much as 14%.
A possible nanometer-scale computing device based on an adding cellular automaton
Authors: Benjamin Simon C. , Johnson Neil F.
Published: July 2002

We present a simple one-dimensional cellular automaton (CA) which has the property that an initial state composed of two binary numbers evolves quickly into a final state which is their sum. We call this CA the adding cellular automaton (ACA). The ACA requires only 2N two-state cells in order to add any two N−1 bit binary numbers. The ACA could be directly realized as a wireless nanometer-scale computing device. A possible implementation using coupled quantum dots is outlined.
Direct thermal conductance measurements on suspended monocrystalline nanostructures
Authors: Tighe T. S. , Worlock J. M. , Roukes M. L.
Published: July 2002

We describe and demonstrate a new class of devices that enable direct thermal conductance measurements on monocrystalline nanostructures. These are possible through our newly developed techniques for three-dimensional, successive surface nanomachining of GaAs-based heterostructures. Our methods allow the patterning of complex devices comprising electrically insulating, mesoscopic thermal conductors with separate, thermal transducers in situ. Intimate thermal contact between these elements is provided by their epitaxial registry. Low-temperature thermal conductance measurements indicate that phonon boundary scattering in these initial nanometer is scale structures is partially specular. These devices offer promise for ultrasensitive bolometry and calorimetry.
Chemical vapor deposition of poly (p-phenylene vinylene) based light emitting diodes with low turn-on voltages
Authors: Vaeth Kathleen M. , Jensen Klavs F.
Published: July 2002

We report fabrication of single layer poly (p-phenylene vinylene) (PPV) light emitting diodes with low turn-on voltages by chemical vapor deposition of dihalo p-xylenes. Devices made from dibromo p-xylene exhibit turn-on fields of 2×106 V/cm, which is in good agreement with solution processed PPV devices. Devices made from dichloro p-xylene, however, have significantly lower turn-on fields of 6.5×105 V/cm. The electroluminescent output of bilayer PPV/tris (8-hydroxyquinoline) aluminum devices made from the two monomers suggests that the chlorine based polymer may have improved electron transport characteristics compared to solution processed polymer, which is tentatively attributed to residual chlorine from the thermal conversion process.
Role of defect distributions and mobility on ferroelectric phase transformations in lead zirconate titanate
Authors: Tan Qi , Xu Z. , Li Jie-Fang , Viehland Dwight
Published: July 2002

Studies of the influence of defect distributions and mobility on ferroelectric phase transformations in lead zirconate titanate have been performed by transmission electron microscopy, high resolution electron microscopy, polarization switching measurements, and dielectric spectroscopy. Investigations focused on La3+ and K1+ modified lead zirconate titanates. Defects which could be randomly quenched-in from temperatures significantly above that of the ferroelectric phase transformation resulted in relaxor ferroelectric behavior characterized by polar nanodomains. However, defects which remained mobile until temperatures significantly below that of the phase transformation resulted in a normal ferroelectric state which was characterized by domain pinning. Mobile impurities/vacancies are revealed to order into chain fragments or clusters which diffuse to domain boundaries resulting in a pinning of the boundaries on cooling.
Lattice strain relaxation study in the Ga1−xAlxSb/GaSb system by high resolution x-ray diffraction
Authors: Bocchi C. , Bosacchi A. , Franchi S. , Gennari S. , Magnanini R. , Drigo A. V.
Published: July 2002

Lattice strain relaxation has been studied in the Ga1−xAlxSb/GaSb system by analyzing layers with thicknesses ranging between 0.1 and 6 μm and with Al concentration x=0.4. The samples have been grown by molecular beam epitaxy at 550 °C on (001) oriented undoped GaSb substrates. The heterostructures were investigated by a high resolution x-ray diffraction method. The experimental critical thickness for the strain relaxation has been found to be tc≈ 310 nm. The measured residual strain εres shows a t−0.47 dependence on the layer thickness t. This result is in agreement with the theoretical predictions based on the energy balance model, and cannot be described in the frame of the equilibrium theories which yield εres∝t−1. The values of εres have been corrected by the so-called thermal misfit (ΔT) for epitaxial systems with different thermal expansion coefficients. For the Ga0.6Al0.4Sb/GaSb epitaxial system grown at TG=550 °C, the value ΔT=−3.28×10−4 has been calculated from the literature data.
Quenching with rapid decompression—a new method for rapid solidification
Authors: He D. W. , Zhang F. X. , Zhang M. , Liu R. P. , Qin Z. C. , Xu Y. F. , Wang W. K.
Published: July 2002

We report that a rapid drop of pressure applied to a melt whose crystallization point is a decreasing function of the pressure may, if the temperature, the initial pressure, and the final pressure are appropriately chosen, be equivalent to a rapid thermal quenching. In particular, it may lead to the formation of metastable crystalline or glassy phases bulk materials. The method might be useful to simulate adiabatic decompressions subsequent to shocks such as meteorite impacts. The relation between the undercooling rate (νc) obtained by rapid decompression and the rate of pressure release (νp) is given by νc=kνp, where k is the drop rate of the melting point with pressure. This method is evaluated for the Al–Ge system. The solidification products are either stable or metastable crystalline compounds, depending on the rate at which the pressure is released. Our experimental results indicate that quenching with rapid decompression is an effective method to generate high undercooling rates exceeding 105 Ks−1.
Phase formation and ferroelectricity of sol-gel derived (Pb, La)TiO3 thin films
Authors: Lee Su Jae , Kang Kwang Yong , Han Seok Kil , Jang Min Su , Chae Byung Gyu , Yang Yong Suk , Kim Seong Hyun
Published: July 2002

Ferroelectric lead lanthanum titanate (PLT) thin films with various lanthanum concentration were fabricated by sol-gel spin-on process onto Pt/Ti/SiO2/Si substrates. We investigated the crystal structure, microstructure, and dielectric and ferroelectric properties of PLT films according to lanthanum doping concentration. The films annealed at 600 °C for 30 min have the single perovskite phase having only tetragonal or pseudocubic structure. The film microstructures were changed according to the doping concentration of La. The PLT films showed the typical polarization-electric field hysteresis loops and capacitance-voltage characteristics representing the ferroelectric switching property. The coercive field and remanent polarization decreased due to the phase transformation from ferroelectric to paraelectric phase with increasing La concentration. The effective dielectric constant and the leakage current density increased according to the increase of La content.
High-temperature stimulated emission in optically pumped InGaN/GaN multiquantum wells
Authors: Bidnyk S. , Schmidt T. J. , Cho Y. H. , Gainer G. H. , Song J. J. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: July 2002

Stimulated emission (SE) in optically pumped InGaN/GaN multiquantum well (MQW) structures grown by metalorganic chemical vapor deposition was experimentally studied in the temperature range of 175–575 K. The GaN barriers were intentionally doped with a different Si concentration ranging from 1×1017 to 3×1019 cm−3 and the effects of Si doping of GaN barriers on the optical properties of InGaN/GaN MQWs were investigated. The SE threshold was measured as a function of temperature and compared with bulk GaN. We observed that the SE threshold had a low value and a weak temperature dependence: for example, ∼25 kW/cm2 at 175 K, ∼55 kW/cm2 at 300 K, and ∼300 kW/cm2 at 575 K for one of the samples. Low SE thresholds are attributed to the high-quantum efficiency of the MQWs, possibly associated with the large localization of excitons. The characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for development of laser diodes that can operate well above room temperature.
Anisotropic modulated structures upon annealing of epitaxial AuNi ultrathin films on Au(001)
Authors: Dressler C. , Abadias G. , Bayle-Guillemaud P. , Marty A. , Schuster I. , Thibault J. , Gilles B.
Published: July 2002

This letter presents experimental evidence of an anisotropic compositional modulation upon annealing in (Ni/Au)n multilayers and Au–Ni alloy ultrathin films embedded in Au layers. These materials were grown by molecular beam epitaxy, on a (001)Au buffer layer. The influence of temperature, local composition, and strain has been investigated by high-resolution electron microscopy and x-ray diffraction.
Lattice relaxation in ZnS epilayers grown on GaP
Authors: Nam Sungun , Rhee Jongkwang , Yu Young-Moon , Lee Chong-kook , O Byungsung , Lee Ki-Seon , Choi Yong Dae
Published: July 2002

The lattice relaxation of ZnS epilayers grown on GaP substrates by hot-wall epitaxy was investigated. The dependence of lattice constants and full widths at half-maximum of the double crystal rocking curves upon layer thickness was observed. The critical thickness for ZnS/GaP is found to be about 350 Å. The epilayers thinner than the critical thickness have almost the same lattice constants. The strain due to the lattice mismatch is almost relaxed in epilayers thicker than 2.5 μm.
Single-molecule spectroscopy with 27 fs pulses: Time-resolved experiments and direct imaging of orientational distributions
Authors: Bopp M. A. , Jia Y. , Haran G. , Morlino E. A. , Hochstrasser R. M.
Published: July 2002

Confocal microscopy of single molecules bound on a silica surface is performed with precompressed 27 fs laser pulses. Interferometric autocorrelation using a single molecule is demonstrated. It is also shown that orientational distributions can be directly obtained from one- and two-photon images produced with circularly polarized light.
Room temperature operation of 8–12 μm InSbBi infrared photodetectors on GaAs substrates
Authors: Lee J. J. , Kim J. D. , Razeghi M.
Published: July 2002

We report the room temperature operation of 8–12 μm InSbBi long-wavelength infrared photodetectors. The InSbBi/InSb heterostructures were grown on semi-insulating GaAs (001) substrates by low pressure metalorganic chemical vapor deposition. The voltage responsivity at 10.6 μm was about 1.9 mV/W at room temperature and the corresponding Johnson noise limited detectivity was estimated to be about 1.2×106 cmHz1/2/W. The carrier lifetime derived from the voltage dependent responsivity measurements was about 0.7 ns.
Chemical kinetics of mobile-proton generation and annihilation in SiO2 thin films
Authors: Vanheusden K. , Warren W. L. , Fleetwood D. M. , Schwank J. R. , Shaneyfelt M. R. , Draper B. L. , Winokur P. S. , Devine R. A. B. , Archer L. B. , Brown G. A. , Wallace R. M.
Published: July 2002

The chemical kinetics of mobile-proton reactions in the SiO2 film of Si/SiO2/Si structures were analyzed as a function of forming-gas anneal parameters in the 300–600 °C temperature range. Our data show that the initial buildup of mobile protons is limited by the rate of lateral hydrogen diffusion into the SiO2 films. The final density of mobile protons is determined by the cooling rate which terminates the annealing process and, in the case of subsequent anneals, by the temperature of the final anneal. To explain the observations, we propose a dynamical equilibrium model which assumes a reversible interfacial reaction with a temperature-dependent balance.
Iron solubility in highly boron-doped silicon
Authors: McHugo S. A. , McDonald R. J. , Smith A. R. , Hurley D. L. , Weber E. R.
Published: July 2002

We have directly measured the solubility of iron in high and low boron-doped silicon using instrumental neutron activation analysis. Iron solubilities were measured at 800, 900, 1000, and 1100 °C in silicon doped with either 1.5×1019 or 6.5×1014 boron atoms/cm3. We have measured a greater iron solubility in high boron-doped silicon as compared to low boron-doped silicon, however, the degree of enhancement is lower than anticipated at temperatures >800 °C. The decreased enhancement is explained by a shift in the iron donor energy level towards the valence band at elevated temperatures. Based on this data, we have calculated the position of the iron donor level in the silicon band gap at elevated temperatures. We incorporate the iron energy level shift in calculations of iron solubility in silicon over a wide range of temperatures and boron-doping levels, providing a means to accurately predict iron segregation between high and low boron-doped silicon.
Heteroepitaxial growth of BeSe on vicinal Si(001) surfaces
Authors: Chauvet C. , Venneguès P. , Brunet P. , Tournié E. , Faurie J. P.
Published: July 2002

The growth of BeSe on vicinal Si(001) substrates has been investigated by molecular beam epitaxy. Reflection high energy electron diffraction was used to study the initial growth mode and the surface structure. Efforts have been done at the early steps of the growth in order to optimize the interface quality. Transmission electron microscopy revealed a BeSe layer relaxed with misfit dislocations and stacking faults that are mainly confined near the heterointerface. These results are promising in view of the growth of Zn0.55Be0.45Se alloy that is lattice matched to silicon. There will be many potential applications of this alloy—in the case of a direct band gap—in the frame of Si-based optoelectronic devices.
Direct patterning of surface quantum wells with an atomic force microscope
Authors: Cortes Rosa J. , Wendel M. , Lorenz H. , Kotthaus J. P. , Thomas M. , Kroemer H.
Published: July 2002

We employ an atomic force microscope to directly pattern the electron system of InAs–AlSb surface quantum wells. Sharp and sturdy electron beam deposited tips are developed to withstand the comparatively high (≈μN) forces in the direct patterning process. By direct patterning the InAs surface quantum well we modulate the electron system without any mask. We are therefore able to directly transfer the excellent lithographic resolution of atomic force microscopy to an electron system. The magnetoresistance of such fabricated antidot arrays is discussed.
Carrier pocket engineering to design superior thermoelectric materials using GaAs/AlAs superlattices
Authors: Koga T. , Sun X. , Cronin S. B. , Dresselhaus M. S.
Published: July 2002

A large enhancement in the thermoelectric figure of merit for the whole superlattice, Z3DT, is predicted for short-period GaAs/AlAs superlattices relative to bulk GaAs. Various superlattice parameters (superlattice growth direction, superlattice period, and layer thicknesses) are explored to optimize Z3DT, including quantum well states formed from carrier pockets at various high symmetry points in the Brillouin zone. The highest room-temperature Z3DT obtained in the present calculation is 0.41 at the optimum carrier concentration for either (001)- or (111)-oriented GaAs (20 Å)/AlAs (20 Å) superlattices, which is about 50 times greater than the corresponding ZT for bulk GaAs.
Stress evolution during metalorganic chemical vapor deposition of GaN
Authors: Hearne S. , Chason E. , Han J. , Floro J. A. , Figiel J. , Hunter J. , Amano H. , Tsong I. S. T.
Published: July 2002

The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.
Schottky barrier heights of tantalum oxide, barium strontium titanate, lead titanate, and strontium bismuth tantalate
Authors: Robertson J. , Chen C. W.
Published: July 2002

The Schottky barrier heights of various metals on the high permitivity oxides tantalum pentoxide, barium strontium titanate, lead zirconate titanate, and strontium bismuth tantalate have been calculated as a function of the metal work function. It is found that these oxides have a dimensionless Schottky barrier pinning factor S of 0.28–0.4 and not close to 1 because S is controlled by Ti–O-type bonds not Sr–O-type bonds, as assumed in earlier work. The band offsets on silicon are asymmetric with a much smaller offset at the conduction band, so that Ta2O5 and barium strontium titanate are relatively poor barriers to electrons on Si.