bluuebunny/crossref_metadata_2025 huggingface dataset:
Sub-eV electron spectroscopy in ion-atom collisions
Authors: Pieksma Marc , van der Meiden H. J. , van Eck J. , Westerveld W. B. , Niehaus A.
Published: July 2002

A newly designed spectrometer is presented, which is eminently suited for the measurement of electron velocity distributions in the sub-eV electron energy region. The application of this spectrometer is demonstrated in an ion-atom collision experiment, using a time-of-flight technique. As an example, the low-energy electron spectrum of 4 keV ionizing H+−H2 collisions is shown.
A comparison of CaF2:Mn thermoluminescent dosimeter chips to aluminum and silicon x-ray calorimeters in the pulsed Hermes III environment
Authors: Fehl D. L. , Sujka B. R. , Vehar D. W. , Westfall R. L. , Lorence L. J. , Rice D. A. , Gilbert D. W.
Published: July 2002

Hermes III is a pulsed power, Bremsstrahlung simulator used for radiation-hardness testing of electronics components [19-MeV spectrum, 20-ns pulse duration, and typical doses (silicon) ≤100 krad (1 kGy)]. CaF2:Mn thermoluminescent dosimeter chips (TLDs) have been compared to a set of x-ray calorimeters in the Hermes III environment for doses between 10–75 krad. Similar to a design reported by Murray and Attix, this set of detectors included different dosimetric materials (silicon and aluminum) and two independent temperature sensors (thermistors and thermocouples). The electronic recording system was also updated. The average disagreement between TLDs and calorimeters was 1%–3%. Radiation transport calculations, however, suggest a possible bias of 4%–6% (source unknown). With the silicon calorimeter the ac bridge, which measured the resistance of thermistor temperature sensors, was extremely sensitive to EMP.
MLM polychromator monitoring soft-x-ray spectral line emissions of C, O, and Fe used in transport studies of the Phaedrus-T tokamak plasma during ohmic, rf-heated, and H-mode discharges
Authors: Regan S. P. , Finkenthal M. , May M. J. , Moos H. W.
Published: July 2002

The MLM (multilayer mirror) polychromator has two modes of operation. In the first mode, it simultaneously monitors the Ly α and β emissions of H-like O at 19.0 and 16.0 Å, respectively, the Ly α emission of C at 33.7 Å, and the singlet and triplet transitions of He-like C at 40.5 Å. In the second mode, it simultaneously monitors the Ly α and β emissions of H-like C at 33.7 and 28.5 Å, respectively, as well as the 15.6 and 93.9 Å emissions of F-like Fe. The MLM polychromator has a wavelength resolution that varies from 0.3 Å at 16.0 Å to 2 Å at 40.5 Å and 7 Å at 93.9 Å. It was mounted on the midplane of the Phaedrus-T tokamak with its line of sight along the major radius. On a shot by shot basis, the MLM polychromator, with a temporal resolution of 1 ms and a spatial resolution of 1 cm, scanned the plasma in the z direction ±15 cm from the toroidal magnetic axis. Vertical asymmetries in the soft x-ray spectral line brightness profiles were measured and found to depend on the direction of BT. The brightness profiles were inverted and modeled with a poloidally asymmetric Gaussian emissivity profile. The measured results were compared with the predicted emissivity profiles to determine the transport of carbon.
Core excitons in silicon and silicon oxides
Authors: Dorssen G. E. van , Roper M. D. , Padmore H. A. , Smith A. D. , Greaves G. N.
Published: July 2002

Using a newly developed reflectometer we have measured the reflectivity at the silicon K edge for different silicon compounds. Combining these measurements with silicon L edge reflectivity measurements carried out on a different reflectometer we determined the binding energy of core excitons at the L edge. The results show that it is possible to carry out reflectivity measurements with the resolution necessary for the determination of the exciton parameters. Comparison with literature values shows that a rigid Kramers–Kronig analysis is not needed when the angle of incidence is well below the critical angle of total external reflection.
X-ray standing wave measurements on III–V compound heterostructures
Authors: Lessmann A. , Brennan S. , Materlik B. , Schuster M. , Riechert H.
Published: July 2002

New applications of the x-ray standing wave method are described, in which not only Bragg reflections from the substrate, but also from a heteroepitaxial layer and from a superlattice satellite are utilized. The investigated samples are an InP/InGaAsP structure on InP(001) and a short-period AlAs/GaAs superlattice on GaAs(001). For efficient detection of the photoelectron yield without vacuum requirements, the sample was placed inside a gas cell, and the induced photocurrent was monitored. In addition to the electron yield, which probes only a thin surface layer, the depth-integrating x-ray fluorescence was analyzed. The results give new insights into the structure of heteroepitaxial systems and their buried interfaces, as well as information about electron escape depths and spatial coherence of x-ray wavefields.
Acceleration of neon pellets to high speeds for fusion applications
Authors: Combs S. K. , Love T. L. , Jernigan T. C. , Milora S. L. , Frattolillo A. , Migliori S.
Published: July 2002

The injection of impurity pellets into the plasmas of tokamak fusion reactors has been proposed as a technique to lessen the deleterious effects of plasma disruptions. Equipment and techniques that were previously developed for pneumatic hydrogen pellet injection systems and used for plasma fueling applications were employed for a limited experimental study with neon pellets. Isotopic hydrogen pellets doped with neon have previously been used for injection into fusion plasmas to study impurity particle transport, and pure neon pellets are applicable for disruption studies. Using a repeating pneumatic injector in the laboratory, it was found that the formation and acceleration of 2.7-mm-diam neon pellets were relatively straightforward; reliable operation was demonstrated with both a single- and a two-stage light gas gun, including velocities of ∼700 m/s with a single-stage injector and up to 1740 m/s with a two-stage injector. Based on the operating sequences and successful tests demonstrated in the laboratory experiments, a three-barrel repeating pneumatic injector installed on the DIII-D tokamak was equipped with the necessary components for neon operation and has been used in initial disruption experiments with 1.8-mm-diam neon pellets.
A new method of radial potential profile measurement utilizing magnetic field gradient near a mirror throat
Authors: Ishii K. , Katsuki Y. , Abe A. , Takahashi T. , Goto T. , Kikuno N. , Katanuma I. , Itakura A. , Yatsu K. , Tamano T.
Published: July 2002

A gold neutral beam is perpendicularly injected into a plasma column along a main chord near an inner mirror throat of a plug/barrier cell in the tandem mirror GAMMA 10, in order to directly measure time evolution of the electrostatic radial potential profile during one shot. The new method pays attention to the fact that trajectories of the ions, which are ionized at each point on the path of the neutral beam in the plasma column, are shifted due to the magnetic field gradient and the electrostatic potential. After traveling about a half period of Larmor motion, ions are detected by a plate type of ion detector. The radial profiles of the plasma potential are determined analyzing positions of the ion beam spots on the detector plane. The main advantage of this method is that radial potential profiles in a strong magnetic field region can be measured as a function of time during one shot without sweeping incident beam angles.
New end station for the study of gases, liquids, and solid films at the MAX laboratory
Authors: Svensson S. , Forsell J.-O. , Siegbahn H. , Ausmees A. , Bray G. , Södergren S. , Sundin S. , Osborne S. J. , Aksela S. , Nõmmiste E. , Jauhiainen J. , Jurvansuu M. , Karvonen J. , Barta P. , Salaneck W. R. , Evaldsson A. , Lögdlund M. , Fahlman A.
Published: July 2002

A new end station equipped with a very high-resolution SES-200 electron energy analyzer has been constructed for the study of gases and soft molecular materials. The analyzer is rotatable around the direction of the photon beam, allowing angular-dependent studies. The system has been built in a way allowing the investigation of free molecules and atoms, metal vapors, liquid interfaces as well as solid films. The first electron spectroscopy results for gases, liquids, and polymer films are presented.
A calibrated scanning tunneling microscope equipped with capacitive sensors
Authors: Holman A. E. , Laman C. D. , Scholte P. M. L. O. , Heerens W. Chr. , Tuinstra F.
Published: July 2002

Piezoelectric actuators that are used in scanning probe microscopes and in precision X–Y translation stages have undesirable properties. Piezoelectric effects like hysteresis, nonlinearity, and drift introduce geometric deformations in the scanning probe microscope images. Due to these deformations, the quantitative interpretation of the absolute size of surface features is difficult and often not accurate. A calibrated X–Y scanning stage has been developed that is combined with a scanning tunneling microscope. This stage is equipped with capacitive sensors that measure the X–Y position in real time. By incorporating the sensors in a feedback loop, it is possible to correct the piezoelectric voltages dynamically and reach the desired scanning positions with a minimal position error. Therefore, a surface can be scanned in a geometrically proper way, and stage drift and low frequency vibrations are corrected in real time.
Test of an 18-m-long suspended modecleaner cavity
Authors: Skeldon K. D. , Strain K. A. , Grant A. I. , Hough J.
Published: July 2002

Fluctuations in laser beam geometry such as lateral beam movement and beam width variation can produce perturbations in the output signal of a laser interferometer through various coupling mechanisms. In order to avoid this type of laser noise from degrading the sensitivity of interferometers designed to detect gravitational radiation, currently planned long-base-line detectors will employ resonant optical filters called modecleaners to suppress beam geometry noise. Here we describe a prototype modecleaner cavity constructed at Glasgow having an optical path length of 18 m and containing four suspended mirrors. We present results detailing various aspects of its performance.
A simple current-to-voltage interface for dielectric relaxation measurements in the range 10−3 to 107 Hz
Authors: Richert Ranko
Published: July 2002

A new circuit is presented for detecting low-level harmonic currents as needed in dielectric relaxation spectroscopy in the low- to medium-frequency range. For a standard operational amplifier-based current-to-voltage converter, a special feedback network is designed such that the output voltage is fairly insensitive to frequency over many decades if a sine generator is coupled to the input via a capacitor with a lossy dielectric. In combination with a gain-phase analyzer or lock-in amplifier this system measures ε*(ω) in the frequency range 10−3 to 107 Hz with high resolution needed for conclusive dielectric relaxation spectroscopy.
Experimental results of amplitude modulation reflectometry on the FTU tokamak
Authors: Zerbini M. , Amadeo P. , Buratti P.
Published: July 2002

The amplitude modulation (AM) reflectometer realized for the FTU tokamak has been successfully operated in a wide range of plasma conditions. The reflectometer has been calibrated using the signal reflected by the inner part of the vacuum chamber (back wall) in absence of the plasma. Density profiles have been obtained for steady-state discharges by inverting the AM phase measured as a function of the carrier frequency. The results are in good agreement with density profiles obtained from multichannel far infrared interferometry.
High-luminosity monochromatic x-ray backlighting using an incoherent plasma source to study extremely dense plasmas (invited)
Authors: Pikuz S. A. , Shelkovenko T. A. , Romanova V. M. , Hammer D. A. , Faenov A. Ya. , Dyakin V. A. , Pikuz T. A.
Published: July 2002

A new diagnostic method for dense plasmas, monochromatic x-ray backlighting, is described. In this method, shadow images of a bright, dense plasma can be obtained with high spatial resolution using monochromatic radiation from a separate plasma, permitting a major reduction in the required backlighting source power. The object plasma is imaged utilizing spherically bent mica crystals as x-ray optical elements. Experimental results, namely images of test objects obtained using x-ray radiation having different photon energies, are presented. Shadow images of exploding Al wire plasmas in the 1s2–1s2p line radiation of He-like Al XII are also shown. Spatial resolution as fine as 4 μm is demonstrated. The scheme described here is useful for backlighting extended high density plasmas, and could be a less costly alternative to using x-ray lasers for such purposes.
Time-dependent, x-ray spectral unfolds and brightness temperatures for intense Li+ ion beam-driven hohlraums
Authors: Fehl D. L. , Chandler G. A. , Biggs F. , Dukart R. J. , Moats A. R. , Leeper R. J.
Published: July 2002

X-ray-producing hohlraums are being studied as indirect drives for inertial confinement fusion targets. In a 1994 target series on the PBFAII accelerator, cylindrical hohlraum targets were heated by an intense Li+ ion beam and viewed by an array of 13 time-resolved, filtered x-ray detectors (XRDs). The unfold operator (UFO) code and its suite of auxiliary functions were used extensively in obtaining time-resolved x-ray spectra and radiation temperatures from this diagnostic. The UFO was also used to obtain fitted response functions from calibration data, to simulate data from blackbody x-ray spectra of interest, to determine the suitability of various unfolding parameters (e.g., energy domain, energy partition, smoothing conditions, and basis functions), to interpolate the XRD signal traces, and to unfold experimental data. The simulation capabilities of the code were useful in understanding an anomalous feature in the unfolded spectra at low photon energies (⩽100 eV). Uncertainties in the differential and energy-integrated unfolded spectra were estimated from uncertainties in the data. The time–history of the radiation temperature agreed well with independent calculations of the wall temperature in the hohlraum.
An improved linear retarder for time-resolved circular dichroism studies
Authors: Esquerra R. M. , Lewis J. W. , Kliger D. S.
Published: July 2002

Time-resolved natural and magnetic circular dichroism measurements on the nanosecond to second time scale have been used to investigate many chemical and biological processes. The key element that makes these measurements possible is a simple and inexpensive variable linear retarder (a strain plate) which produces well-characterized, elliptically polarized light throughout a broad spectral range (180–1500 nm). It essentially consists of a fused silica plate of high optical quality appropriately compressed from the edges to produce ∼1° of retardation. Early designs that consisted of a square plate compressed with wedges have worked satisfactorily in the past, but their use was complicated by inhomogeneity of strain across the probe beam (as large as 6 mm diameter for white light measurements with a flashlamp) and a temperature dependence of the strain. An improved strain plate that consists of a round fused silica plate compressed with springs is presented, and its performance is evaluated with respect to earlier designs. This improved strain plate not only simplifies time-resolved circular dichroism measurements but also improves the quality of the data and the kinetic resolution of intermediates.
A combined absorption–wavelength modulation technique for diode laser spectroscopy of van der Waals complexes in a pulsed planar jet
Authors: Pak I. , Hepp M. , Roth D. A. , Winnewisser G.
Published: July 2002

A tunable diode laser spectrometer modified for the investigation of molecular gases and van der Waals complexes in a supersonic jet is described. Simply constructed, reliable pulsed sources with 15 μm×(7–40 mm) slits were used at a repetition rate of the order of 100 Hz for the cooling of gases and for the formation of complexes. A double modulation technique based on a simultaneous modulation of the jet and a 10 kHz wavelength modulation of the diode laser enabled measurements of absorbances 1.3×10−5 (3×10−5 in relative absorption) at operating conditions corresponding to 1 s output time constant of the registration system. White type multireflection optics enlarged the absorption path of the laser radiation through the jet by 16 times. New spectra of the van der Waals complexes Ar–CH4 and Kr–CH4 in the 7 μm region are presented in order to show the characteristics of the spectrometer. The final resolution in the recorded spectra was better than 40 MHz. The rotational temperature of the complexes produced by the present jet source was 7–10 K, which could be compared to the 13 K excitation temperature measured for the two lowest rotational states of F symmetry of the methane monomer.
Quantitative mapping of species moving in solution by x-ray projection microscopy
Authors: Rondot S. , Cazaux J.
Published: July 2002

An x-ray projection microscope equipped with a digital camera allows a rapid and independent mapping of copper and iron ions in an aqueous sulphate solution during the corrosion/deposition process of a metallic iron electrode in a copper sulphate solution. The concentration distributions of the two species can be quantified from a pair of x-ray images acquired in 20 s with a resolution on the order of 10 μm. Four pairs of images have been acquired each minute of the corrosion deposition process. This technique makes it possible to follow the concentration distribution of the two species as a function of the space (x,y) and time coordinates with a detection limit in the 10 ppm range.
Two-color polarimeter for electron density measurement on large tokamaks
Authors: Kawano Yasunori , Nagashima Akira
Published: July 2002

A two-color laser polarimeter on large tokamaks such as ITER has been proposed for electron density measurement based on the midplane Faraday rotation. The two-color scheme in the polarimeter eliminates the Faraday rotation at vacuum windows from measured polarization angles. Two CO2 lasers which have different wavelengths of 10.6 and 9.27 μm are favorable as light sources of the two-color laser polarimeter for large tokamaks. A couple of photoelastatic modulators are used for stable polarization measurement even though the intensity and the propagation axis of an incident laser beam are changed.
Preliminary results of the 14 GHz SERSE superconducting electron cyclotron resonance ion source working in high-B mode
Authors: Ludwig P. , Bourg F. , Briand P. , Girard A. , Melin G. , Guillaume D. , Seyfert P. , La Grassa A. , Ciavola G. , Di Bartolo G. , Gammino S. , Cafici M. , Castro M. , Chines F. , Marletta S. , Passarello S.
Published: July 2002

After the first operation of the SERSE superconducting source assembled with reduced performance coils, the tests of the new SERSE magnetic system were carried out with excellent results. The specifications of the design field were reached and exceeded. The new coils were mounted and SERSE is now able to run at full performance magnetic field. The today’s status of the facility, electron cyclotron resonance source and new magnetic configuration, is described. The first tests as a multicharged ion source have just started, and a few preliminary results are given and commented.
Piezoelectric shear force detection: A geometry avoiding critical tip/tuning fork gluing
Authors: Salvi J. , Chevassus P. , Mouflard A. , Davy S. , Spajer M. , Courjon D. , Hjort K. , Rosengren L.
Published: July 2002

A technique for controlling the tip–sample distance in near-field optical microscopes is presented. It consists of mechanically exciting a fiber tip inserted without any adhesive between the two prongs of a high Q-piezoelectric tuning fork. The detection of the shear forces is classically achieved by measuring the decrease of the dithering amplitude when the tip approaches the surface. This simple setup greatly simplifies tip replacement and its resonance frequency tuning.
Digital linearization and cancellation of capacitive coupling for a scanning tunneling microscope
Authors: Paillard Bruno , Tang Ran , Rowntree Paul
Published: July 2002

This article describes the development and test of a digital control loop, to control the tip-to-specimen distance in a scanning tunneling microscope. This digital controller performs a frequency-independent linearization of the gap-to-current relationship, as well as the compensation of the undesirable capacitive coupling between the electrodes of the scan tube and the input of the current-to-voltage converter, two difficulties normally associated with analog controllers. In the described work, the control loop is implemented on an inexpensive fixed-point DSP, processing the signals at a 25 kHz sampling rate.
Proximity gettering of transition metals in separation by implanted oxygen structures
Authors: Skorupa W. , Hatzopoulos N. , Yankov R. A. , Danilin A. B.
Published: July 2002

The gettering behavior of Cu and Fe in ion beam synthesized silicon on insulator (SOI) material incorporating a buried oxide layer is investigated before and after the formation of deep gettering zones by either C or He implantation. Secondary ion mass spectroscopy (SIMS) analysis is employed to obtain information as to the C, O, Fe, and Cu depth distributions. It is shown that the proximity gettering approach using C and He renders the possibility of removing and stabilizing metal contaminants not only away from the near-surface region, but also remote from the buried oxide/substrate interface to which they normally segregate in the absence of efficient implantation induced gettering sinks. C implants are found to have better gettering efficiency as they getter both Cu and Fe whereas He implants getter Cu only. In addition, the C implant dose needed to achieve one and the same gettering effect is an order of magnitude lower than the He dose.
Boron incorporation in Si1−xGex films grown by ultrahigh vacuum chemical vapor deposition using Si2H6 and GeH4
Authors: Chen L. P. , Chou C. T. , Huang G. W. , Tsai W. C. , Chang C. Y.
Published: July 2002

0.1% B2H6 diluted in hydrogen is used as the p-type dopant gas in Si1−xGex grown by ultrahigh vacuum chemical vapor deposition (UHVCVD) using Si2H6 and GeH4. The boron concentration is evaluated by secondary ion mass spectrometry (SIMS). The boron concentration of Si1−xGex increases with the increase of the GeH4 flow rate, that is, Ge fraction, by keeping Si2H6 and B2H6 flow rates constant. The result may be due to the increase of the vacant surface sites which is caused by the increase of the hydrogen desorption rate when a higher Ge fraction epilayer is grown.
Theory of optical gain in ideal GaN heterostructure lasers
Authors: Meney A. T. , O’Reilly E. P.
Published: July 2002

We present gain calculations for ideal bulk GaN and 100 Å GaN quantum well laser structures. We show that due to the large electron and hole effective masses in GaN, the room-temperature material gain characteristics of a 100 Å quantum well are a little different from those of bulk GaN up to a gain level of 1000 cm−1, and that the transparency and threshold carrier density is approximately 2.5 times that in an equivalent GaAs structure, with the radiative current density being of order eight times larger. Comparing the unstrained zinc blende and wurtzite crystal structures, we predict improved gain characteristics in the wurtzite case. The introduction of compressive strain, e.g., through pseudomorphic growth between unstrained AlGaN barriers, will benefit both crystal structures, with wurtzite remaining better than zinc blende for mismatch up to about 1.5%. Finally, we note that the gain characteristics would be further improved if it were possible to grow tensile-strained zinc blende layers.
Design and performance of the JAERI superconducting linac for high-power free-electron laser
Authors: Sawamura Masaru , Nagai Ryoji , Kikuzawa Nobuhiro , Sugimoto Masayoshi , Nishimori Nobuyuki , Minehara Eisuke J.
Published: July 2002

A superconducting rf linac has been required to realize high-duty operation and low-electricity consumption for a high-average-power and high-efficiency free-electron laser (FEL). The JAERI FEL superconducting linac consists of a 250 kV electron gun, a subharmonic normal-conducting buncher (SHB) of 83.3 MHz, and two single-cell and two five-cell superconducting cavities of 499.8 MHz. The gun was typically operated around 200 kV to reduce space-charge effects. A combination of the SHB and the two single-cell cavities enabled a high-current beam of more than 10 A by utilizing a thermionic cathode and a grid pulser. The full width at half maximum bunch length and energy resolution were measured to be 22 ps and 1% or less, respectively. The measured results agreed with calculations utilizing the modified PARMELA code. This showed the validity of the linac design for the FEL. With this beam quality, high-average FEL power of 0.1 kW at a 24–28 μm wavelength was obtained in a quasi-cw mode.
An improved method for detecting hot spots in field emission cathode arrays
Authors: Wei Yi , Smith Bruce G. , Chalamala Babu R.
Published: July 2002

We present an improved liquid crystal microthermography technique for the detection of defects in large area field emission cathode arrays. Defects can cause electrical leakage paths, leading to localized heating at the defect location. The thermal gradients at these hot spots can be detected by liquid crystal microthermography, but, the direct placement of the liquid crystal is a two step process. First the liquid crystal is deposited by spin coating the device and the liquid crystal has to be removed from the device after the test. This process, apart from being time consuming, can lead to contamination of the test device. In this article, we present an improved liquid crystal microthermography technique for the detection of hot spots in field emitter arrays. The improvement is obtained by hermetically sealing the liquid crystal material inside a packaged assembly made from a glass support and a thin plastic membrane. We have used the new method for the detection of hot spots in large area field emission cathode arrays. This technique provides accurate detection of hot spot locations caused by leakage currents as low as 100 μA.
A method for calculating the average solid angle subtended by a circular disk from uniformly distributed points within a coaxial circular plane
Authors: Tryka Stanislaw
Published: July 2002

Methods for calculating the solid angles subtended at a point by closed contours of some given objects are required in many areas of optical and nuclear physics to estimate the fluxes or particle beams of radiation. It is therefore of interest to derive solid angle equations for various closed contours and the location of such contours with respect to a single point or some points. In this article two methods are described for calculating the solid angles for objects having the shape of a circular disk. In the first method the formula for calculating the solid angle at a point subtended by a circular disk was presented in a more general form than the expressions reported by S. Tryka [Opt. Commun. 137, 317 (1997)]. In the second method this formula was used to derive another formula for calculating the average solid angle subtended by a circular disk from uniformly distributed multiple points lying on the planar circular surface coaxial to the disk. Both of these formulas were represented by superpositions of simple elementary functions with the complete Legendre–Jacobi elliptic integrals. These superpositions were given as dependencies on the radii of the disk and of the circular surface and on the distance between the disk and the surface. The influences of these radii and distance on the solid angle value were illustrated graphically by some three-dimensional surface plots. Some representative results were calculated to eight decimal places and given in tabular form. In addition, a similar table of results computed for the solid angle at a point subtended by a circular disk was included. It was shown that the formulas obtained are simple and directly applicable in some high-language programs. As an example of such an application all plots and calculations in this article were performed using MATHEMATICA 2.2.3 software.
Gas flow meter using a positive temperature coefficient thermistor as the sensor
Authors: Chatterjee Suman , Sengupta Kamalendu , Maiti Himadri S.
Published: July 2002

Positive temperature coefficient (PTC) thermistor sensors have been used successfully for sensing and measurement of gas flow through a tube. Tuned detection technique using an ac signal is found to be a more efficient measuring circuit than the conventional dc Wheatstone bridge. Any particular sensor can be used to measure a wide range of flow rates using sensing heads of varying geometries. For a specific geometry, the plot of electrical voltage output against flow rate produces an “S” shaped curve which follows from the basic principles of heat transfer by both conduction and convection mechanisms.
Minority carrier lifetime scan map in crystalline silicon wafers
Authors: Gervais J. , Palais O. , Clerc L. , Martinuzzi S.
Published: July 2002

Improved resolution lifetime scan map of minority carriers in excess in monocrystalline and multicrystalline silicon wafers are obtained by means of the phase-shift technique at a practically constant injection level. This technique is contactless, based on reflected microwave power variations which occur when the sample is illuminated by a focused near infrared light. Surfaces are passivated by means of an aqueous iodine solution, whose passivating efficiency is remarkably constant. Scan maps are obtained with a lateral resolution of 50 μm thanks to a coaxial cable which directs 9,4 GHz microwaves onto the wafer and a fiber coupled laser diode which generates minority carriers in excess. Lifetime scan maps are found to be in acceptable agreement with minority carrier diffusion length scan maps obtained by the light beam induced current technique.
Determination of the dynamic response of a nitric oxide detector
Authors: Civerolo Kevin L. , Stehr Jeffrey W. , Dickerson Russell R.
Published: July 2002

A chopper wheel apparatus to characterize the time response of a nitric oxide detector is described. At low chopping rates, fast Fourier transform analysis of the output signal indicates that the instrument has a response time of about 0.45±0.14 s with respect to a periodic input signal. This detector has an “e-folding” response time of about 0.3±0.1 s to a simple step change in input signal, demonstrating reasonable agreement within experimental uncertainty between the two techniques.
A high pressure device for measurements of the speed of sound in liquids
Authors: Żak Andrzej , Dzida Marzena , Zorȩbski Michał , Ernst Stefan
Published: July 2002

A new measuring set for measurements of the speed of sound in liquids under high pressures (up to 300 MPa), designed and constructed in our laboratory, is described. It operates on the principle of the pulse-echo-overlap method. A single transmitting–receiving piezoelectric ceramic transducer operating at 4 MHz and an acoustic reflector were applied. Details concerning the designing and operation of all the elements of the acoustic path, assuring high accuracy of the speed measurements, are given.
Alignment of defect dipoles in polycrystalline ferroelectrics
Authors: Warren W. L. , Dimos D. , Pike G. E. , Vanheusden K. , Ramesh R.
Published: July 2002

Using electron paramagnetic resonance (EPR), we show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline BaTiO3 ceramics by subjecting the capacitors to a dc bias at elevated temperatures. The alignment is demonstrated to occur via orientation dependent EPR signals in the polycrystalline perovskite lattice. The alignment of the defect dipoles is found to strongly enhance the light sensitivity of the defects. This observation may have important implications for photorefractive applications.
In situ scanning tunneling microscopy observation of surface evolution in magnetically coupled Co/Cu multilayers
Authors: Minvielle Timothy J. , Wilson Robert J. , White Robert L.
Published: July 2002

A series of bilayers of the form Cu(100)/[Co(21 Å)/Cu(21 Å)]n were grown by both ion beam and DC magnetron sputtering techniques. Scanning tunneling microscopy images of the developing layers demonstrate a marked difference in the way in which roughness evolves through the films. The higher energy ion beam sputtered systems show a nonconformal roughness that is characterized by comparatively large lateral length scales. The less energetic magnetron-formed systems exhibit an island-upon-island growth that is conformal from layer to layer. Kerr effect measurements show that the former is ferromagnetically coupled and the latter is antiferromagnetically coupled. An explanation is presented that attributes the differences in roughness to the potential barriers at step edges. Adatom mobility and incident energy are shown to be the determining factors for this kind of conformal growth.
Low temperature scanning electron microscopy study of YBa2Cu3O7−δ multilayer dc SQUID magnetometers
Authors: Gerber R. , Koelle D. , Gross R. , Huebener R. P. , Ludwig F. , Dantsker E. , Kleiner R. , Clarke John
Published: July 2002

Low temperature scanning electron microscopy (LTSEM) has been used to analyze the superconducting properties of integrated magnetometers involving a dc superconducting quantum interference device. The study yielded spatially resolved information on the critical temperature Tc and critical current density Jc in the upper and lower superconducting layers of the YBa2Cu3O7−δ/SrTiO3/YBa2Cu3O7−δ structure. The Tc of the lower film was depressed by several kelvin, and the Jc of the upper film was lower where it crossed the edge of a lower YBa2Cu3O7−δ film covered with SrTiO3. A gradient in Tc was observed in the lower film near the edge of a via, arising presumably from oxygen diffusion during fabrication.
The growth characteristics of (001) oriented diamond layers on (111) diamond face via bias-assisted chemical vapor deposition
Authors: Zhang W. J. , Jiang X.
Published: July 2002

A new phenomenon of diamond growth was observed. (001) oriented diamond layers can be grown on a (111) diamond face in a bias-assisted chemical vapor deposition process. The growth characteristics were investigated by means of scanning electron microscopy and Raman spectroscopy. It was found that, due to the ion bombardment, the epitaxial growth of the (111) diamond face was interrupted and a secondary nucleation occurred. This ion-bombardment-induced secondary nucleation leads to the growth of a top layer with a nonparallel orientation relationship with the substrate.
Theoretical study of room temperature optical gain in GaN strained quantum wells
Authors: Chow W. W. , Wright A. F. , Nelson J. S.
Published: July 2002

The determination of gain properties in group III nitride quantum wells is complicated by the incomplete knowledge of band structure properties, and the need for a consistent treatment of many-body Coulomb effects. This letter describes an approach that involves a first-principles band structure calculation, the results of which are incorporated into a microscopic laser theory where many-body Coulomb effects are treated in a consistent manner. Using this approach, we investigate quantum well structures composed of alloys of GaN, AlN, and InN, in particular, GaN–AlInN, which has high confinement potentials in both strained and unstrained configurations.
Time-resolved optical waveguide spectroscopy: Application to a study of the switching process of a nematic liquid crystal under the influence of an electric field
Authors: Mitsuishi M. , Ito S. , Yamamoto M. , Knoll W.
Published: July 2002

We report a novel optical spectroscopy, time-resolved optical waveguide spectroscopy. This technique was developed in order to investigate the transient behavior of the dielectric constants of thin dielectric medium, e.g., changes of the refractive indices induced by an external trigger such as light, electric field, etc. Optical guided waves propagate in a thin dielectric medium whose optical properties, in particular film thickness and anisotropic dielectric tensor diagonals in molecular and laboratory coordinates, can be evaluated with high accuracy. We employed this technique to a nematic liquid crystal (5CB) as a thin dielectric medium, and molecular reorientation on the switching process was observed through the recording of transient waveguide patterns.
Chemical information in positron annihilation spectra
Authors: Myler U. , Goldberg R. D. , Knights A. P. , Lawther D. W. , Simpson P. J.
Published: July 2002

Positron annihilation spectra of arsenic- and gold-implanted silicon are compared with spectra from bulk samples of arsenic and gold. Spectra with strongly reduced background intensities were recorded using a two detector coincidence system with a variable-energy positron beam. It is shown that features in the high-momentum region of the spectra (∼514–520 keV) can be identified with particular elements and that this identification is independent of structure, i.e., whether the element forms the bulk or is an implanted impurity. Proportionality between the intensity of characteristic spectral features and the fraction of annihilating positrons is also demonstrated, using the native oxide on a silicon wafer as a test case.
Cross-sectional high resolution electron microscopy of Zn+ implanted and low-power pulsed-laser annealed GaAs
Authors: Vitali G. , Zollo G. , Pizzuto C. , Manno D. , Kalitzova M. , Rossi M.
Published: July 2002

High resolution transmission electron microscopy has been used to investigate the lattice damage distribution in Zn+ implanted and implanted plus low-power pulsed-laser annealed (LPPLA) GaAs. The damage distribution of implanted samples has been examined in detail showing the presence of a continuous amorphous layer under the surface and stacking fault tetrahedra nuclei at the inner a–c interface. A solid phase epitaxial regrowth of ion implanted GaAs has been induced by LPPLA technique. In the annealed samples, the crystalline recovering is characterized by a low density of residual extended defects lying in the fully recrystallized amorphous layer.
Stable photorefractive memory effect in sol-gel materials
Authors: Darracq B. , Canva M. , Chaput F. , Boilot J.-P. , Riehl D. , Lévy Y. , Brun A.
Published: July 2002

We report on the synthesis and the optical characterization of a photorefractive sol-gel material which contains nonlinear azo chromophore and carbazole charge transporting molecules. Both of these functional groups are covalently attached to the silica based backbone. Thin sol-gel films exhibit stable optically nonlinear properties: the electro-optic coefficient r13 measured at 633 nm is found to be 17 pm/V one day after corona poling and 15 pm/V after three weeks and over a period of several months. Photorefractivity is demonstrated by two beam coupling experiments without external applied electric field. Sol-gel films present a stable photorefractive memory effect with a net internal gain of 200 cm−1.
Influence of the erbium and oxygen content on the electroluminescence of epitaxially grown erbium-doped silicon diodes
Authors: Reittinger A. , Stimmer J. , Abstreiter G.
Published: July 2002

The electroluminescence behavior of erbium-oxygen-doped silicon light emitting diodes grown by molecular beam epitaxy was studied for a fixed oxygen to erbium concentration ratio of about six. The diodes were operated in reverse bias. An increase of the erbium and oxygen content leads to a stronger erbium intensity at 5 K up to an erbium concentration of 1.5×1020 cm−3, an enhanced decrease of the erbium intensity with higher temperatures, and a shift of the temperature quenching onset to lower temperatures. The strongest erbium electroluminescence emission in reverse bias in this set of samples was obtained from annealed Si:Er-diodes doped with concentrations of 5×1019, or 1.5×1020 cm−3 erbium and 3×1020 or 3×1021 cm−3 oxygen, respectively. Electroluminescence emission due to erbium was detected up to 440 K.
Many-body effects on optical gain in strained hexagonal and cubic GaN/AlGaN quantum well lasers
Authors: Park Seoung-Hwan , Ahn Doyeol
Published: July 2002

Theoretical results on the optical gain of strained hexagonal and cubic GaN/AlGaN quantum well (QW) lasers taking into account of many-body effects are presented. These results are also compared with those of the free-carrier theory without the many-body effects. The band structure is calculated by using the Hamiltonian based on the Rashba–Sheka–Pikus 6×6 Hamiltonian. It is found that the peak gain for cubic and hexagonal QW lasers is increased by about 45% and 37%, respectively, near the transparency carrier density due to the Coulomb enhancement. Also, the gain peaks with many-body effects are redshifted relative to those without many-body effects by ∼130 meV at higher carrier densities. These results indicate the importance of the many-body effects for both cubic and hexagonal GaN/AlGaN QW lasers.
Active mode locking of a p-Ge hot hole laser
Authors: Hovenier J. N. , Muravjov A. V. , Pavlov S. G. , Shastin V. N. , Strijbos R. C. , Wenckebach W. Th.
Published: July 2002

The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50–140 cm−1) was achieved due to active mode locking by electrical intracavity modulation of the gain.
Stable single-frequency operation of a high-power external cavity tapered diode laser at 780 nm
Authors: Goyal A. K. , Gavrilovic P. , Po H.
Published: July 2002

We report on grating-tuned tapered lasers that oscillate on a single external-cavity mode at powers as high as 720 mW. Frequency stabilization and side-mode discrimination are achieved through the use of an intracavity etalon. The emission frequency was stabilized against mode hops for a period of >2000 s and exhibited a short term jitter of ∼85 MHz over a period of 300 s. The single-frequency emission was tunable over a 17 nm range centered at 783 nm. Furthermore, the far-field emission remained near-diffraction limited throughout the measured current range (<2 A).
Origin of dislocation-related photoluminescence bands in very thin silicon–germanium layers grown on silicon substrates
Authors: Lee Hosun , Choi Suk-Ho , Seong T.-Y.
Published: July 2002

We measured the photoluminescence spectra of very thin and partially strained Si1−xGex (0.2⩽x⩽0.5) layers grown on silicon substrates with varying degrees of strain relaxation. We observed photoluminescence lines, so-called D lines, which arise from dislocations in the Si1−xGex/Si alloys. Surprisingly, we observed no D lines originating from the Si1−xGex layers. We identify the origin of the D lines as the penetrated dislocations in the Si substrates extending from the SiGe/Si interface with the assistance of transmission electron microscopy. We discuss possible mechanisms of the dominance of luminescence from the dislocations in the Si substrates over those in the SiGe layers.
Green electro- and photoluminescence from nanocrystalline Si film prepared by continuous wave Ar+ laser annealing of heavily phosphorus doped hydrogenated amorphous silicon film
Authors: Wang Mingxiang , Chen Kunji , He Lei , Li Wei , Xu Jun , Huang Xinfan
Published: July 2002

A thin layer of plasma enhanced chemical vapor deposited (PECVD) heavily phosphorus doped hydrogenated amorphous silicon (a-Si:H) film was annealed by cw Ar+ laser scanning. Different from conventionally prepared polycrystalline Si films, it was found that nanocrystalline Si (nc-Si) was formed in our laser annealed sample. Room-temperature green electroluminescence (EL) peaked at 530 nm was achieved from our nc-Si film. Photoluminescence (PL) from the same sample also shows the 530 nm green peak, in addition to the red peak located at 680 nm. The film had a rather high electrical conductivity of 10S/cm as well. The light emitting and highly conductive nc-Si film provides a new possibility to fabricate optoelectronic devices along with the well-developed laser annealing techniques of a-Si:H.
1.35 W of stable single-frequency emission from an external-cavity tapered oscillator utilizing fiber Bragg grating feedback
Authors: Goyal A. K. , Gavrilovic P. , Po H.
Published: July 2002

An external-cavity tapered diode laser utilizing fiber Bragg grating feedback was designed to oscillate on a single-longitudinal mode. Stable single-longitudinal mode operation was attained at powers as high as 1.35 W. The emission frequency tuned smoothly with current over the cavity’s free spectral range of 6 GHz. The power in the lasing mode was >30 dB above the background amplified spontaneous emission and the side-mode suppression ratio was >15 dB. No deterioration in the beam quality was measurable up to the highest output power. The coupling efficiency into a Gaussian resonator mode was >65% up to 1.35 W.
Kerr-rotation imaging in scanning near-field optical microscopy using a modified Sagnac interferometer
Authors: Petersen B. L. , Bauer A. , Meyer G. , Crecelius T. , Kaindl G.
Published: July 2002

A scanning near-field optical microscope is combined with a modified Sagnac interferometer to generate high-resolution magnetic contrast images of a magneto-optical disk (MOD) in reflection. The interferometer acts as a magnetic sensor to determine the Kerr rotation of reflected circularly polarized light. A separate shear-force sample topography is obtained simultaneously with the optical. This technique allows successful magnetic imaging of samples with complicated surface topographies, as demonstrated with the MOD. For these initial measurements, the resolution with nonmetal-coated fiber tips is 300 nm, and the magnetic phase sensitivity is 1 mrad.
Energetics of AlN thin films on the Al2O3(0001) surface
Authors: Di Felice R. , Northrup J. E.
Published: July 2002

We present an ab initio study of the energetics and atomic structure of films consisting of approximately 1 bilayer of AlN on the c-plane sapphire surface. We show that these films are unstable with respect to three-dimensional islands, and we attribute this instability to both strain and chemical mismatch between the oxide and the nitride. The relative stability of the AlN films depends on the chemical potentials of Al and N. Films having (0001) polarity are expected to form under Al-rich conditions. Films with (0001̄) polarity appear to form only for undersaturation conditions of bulk AlN in the initial stages of growth.
Very large continuous-wave-laser-induced optical absorption in porous silicon films: Evidence for thermal effects
Authors: Koyama Hideki , Fauchet Philippe M.
Published: July 2002

A very large optical absorption increase induced by continuous-wave laser irradiation is reported for free-standing porous silicon films oxidized at 800–950 °C. A maximum reversible optical-density change of 2.5 is obtained with a pump laser intensity of ∼20 W/cm2. The induced absorption is found to be strongly decreased when the sample is attached to materials with a higher thermal conductivity. The experimental results can be satisfactory fitted with an empirical formula for the thermally induced absorption increase in bulk crystalline Si. These results strongly suggest that the observed nonlinearity is originating from thermally induced band gap shrinking in Si micro/nanostructures.