bluuebunny/crossref_metadata_2025 huggingface dataset:
A spatial detector array for measuring plasma turbulence with the heavy ion beam probe
Authors: Beckstead J. A. , Aceto S. C. , Crowley T. P. , Demers D. , McLaren P. E. , Ouroua A. , Schatz J. G. , Schoch P. M.
Published: July 2002

The heavy ion beam probe (HIBP) has become a valuable diagnostic for measuring the plasma space potential and electron density in high-temperature plasmas. However, due to the limitations placed on the design of the HIBP by the size and complexity of the electrostatic energy analyzer, only limited information can be extracted from the HIBP’s fluctuation measurements. By extending the range and number of sample volumes simultaneously recorded by the beam probe, a better understanding of the turbulence distribution, S(k,ω), can be achieved. This can be accomplished by installing an array of secondary particle detectors close to the plasma edge. InterScience, Inc. has developed a particle detector capable of measuring the small signal levels required by the HIBP in the high background environment associated with operating near the plasma edge. In addition, this detector has been incorporated into the design of a 20-element, spatial array for the 2 MeV HIBP on TEXT. A flexible design has been developed such that the array can make measurements that are nearly poloidally or radially aligned. With the use of spatial Fourier transform, this will allow the HIBP to better determine S(k,ω). Details of the design of the detector array and initial experimental results will be presented.
Soft x-ray and bolometric tomography in RFX (abstract)
Authors: Martin P. , Murari A. , Buffa A. , Marrelli L. , Baker W. , Gadani G. , Hemming O. , Manduchi G. , Parini A. , Taliercio C. , Hoffmann A. , Stasek G. , Wirth R.
Published: July 2002

The article presents the design and the first results of a new eight camera, soft x-ray (SXR) and bolometric tomography which has been installed in RFX, a large (R=2 m, a=0.46 m), high current reverse field pinch experiment operating in Padova. The main features of the diagnostic, which has been designed with standards compatible with the requirements of large fusion experiments, are: (a) it allows simultaneous reconstruction of both SXR and total radiation emissivity structures with high spatial and time resolution (bandwidth up to 200 kHz for SXR and 10 kHz for bolometry). Moreover, computer controlled remote actions which interchange up to five material filters in front of the detectors allow the exploration of different energy ranges; (b) it can be easily decoupled from the machine without breaking the main vacuum and can be operated during first wall baking up to 300 °C thanks to a cooling circuit; (c) the detection chain is designed to measure very low signals [in the order of 1 nA for SXR diodes and of 100 μV for bolometers]; (d) it is equipped with a VME based 1 MHz data acquisition system which allows on-line processing of a large amount of data. To accomplish these features the diagnostic consists of eight fully remotely controlled manipulators which can insert eight photocameras, carrying 78 top-rated Si SXR and 48 miniaturized metal film bolometric detectors in their operating positions. Initial results are presented.
Measurements of nonthermal confined alpha particles in Tokamak Fusion Test Reactor D–T plasmas (invited)
Authors: Stratton B. C. , Budny R. V. , Duong H. H. , Mansfield D. K. , Medley S. S. , Redi M. H. , Fonck R. J. , McKee G. R. , Ödblom A. , Wising F. , Fisher R. K. , McChesney J. M. , Parks P. B. , Petrov M. P. , Gorelenkov N. N.
Published: July 2002

Experience with the α charge exchange recombination spectroscopy and pellet charge exchange confined, nonthermal alpha particle diagnostics over the first two years of Tokamak test fusion reactor (TFTR) D–T operation is summarized. A brief summary of the concept, instrumentation, and analysis techniques for each diagnostic is given, followed by examples of alpha physics results. Issues important to further development of these diagnostic techniques for TFTR and ITER are discussed.
Pressure effect on yttrium doped La0.60Y0.07Ca0.33MnO3 compound
Authors: Arnold Z. , Kamenev K. , Ibarra M. R. , Algarabel P. A. , Marquina C. , Blasco J. , García J.
Published: July 2002

We report the effect of hydrostatic pressure up to 8 kbar on magnetoelastic, magnetic, and transport properties in the temperature range 10–300 K. An increase of Curie temperature Tc with pressure ΔTc/Δp=2.6 K/kbar was determined from ac susceptibility measurements. The anomalies in resistivity and magnetovolume effects are significantly suppressed by pressure but the temperatures of these anomalies increased with pressure with the same slope as Tc. These results indicate that impurity state of yttrium rather than lattice parameter reduction is responsible for the large increase in the magnetoresistance observed in the doped compound.
Electronic high-temperature radio frequency superconducting quantum interference device gradiometers for unshielded environment
Authors: Borgmann J. , David P. , Ockenfuss G. , Otto R. , Schubert J. , Zander W. , Braginski A. I.
Published: July 2002

This article will discuss improved electronically formed gradiometers based on high-temperature radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. For gradiometer balancing, a system of adjustable superconducting plates was developed. This technique was used to build first- and second-order, axial gradiometers with adjustable baselines, which operate at 77 K. Each magnetometer combines a washer rf-SQUID with bulk or a thin-film flux concentrator in flip chip geometry. In an unshielded environment, the magnetic field sensitivity in the white noise region is about 80 fT/√Hz for first-order and 150 fT/√Hz for second-order gradiometer. Common mode rejection could be balanced to better than 104 for uniform background fields and better than 200 for gradient fields.
A novel digital feedback scheme of shear-force control in near-field scanning optical microscopy
Authors: Kim Ki Hyun , Eah Sang-Kee , Lee Byoungho , Cho Chang Ho , Jhe Wonho
Published: July 2002

We have developed a novel digital feedback scheme of distance control via shear-force measurement in the near-field scanning optical microscope. Using simple comparators, full digital control is achieved. In this new scheme, the probe tip stops at a reference point as it approaches the sample and then draws back when the scanning piezoelectric tube executes lateral translation of the sample by one step. The resulting feedback motion is similar to tapping at each point, which removes the feedback oscillation and the hysteresis effect in the conventional analog feedback scheme. Because the control program monitors the tip position at each step and there is no settling time of the feedback, the scanning speed can be improved without oscillation and tip–sample collision.
An ultrahigh vacuum scanning Kerr microscope
Authors: Vaterlaus A. , Maier U. , Ramsperger U. , Hensch A. , Pescia D.
Published: July 2002

A new ultrahigh vacuum instrument allowing in situ Kerr microscopy and scanning tunneling microscopy is described. The Kerr microscope has a spatial resolution of about 1 μm. First experimental results are reported on the magnetism of a 5 μm wide stripe consisting of six atomic layers of Fe grown in situ by molecular beam epitaxy on a W(110) surface.
Simple device for making optical fiber tips for scanning near field optical microscopes
Authors: Xiao M. , Nieto J. , Siqueiros J. , Machorro R.
Published: July 2002

We present a simple heating-pulling setup for making optical fiber tips for scanning near field optical microscopes. The heating is realized with a resistant coil and the pulling is performed by a mechanical spring. There is no longer the need to use a laser, a multistage puller, or a computer. Fabrication can therefore be realized in situ with the fiber aligned on the microscope and this drastically reduces the cost.
Experimental investigation of the interaction of 100 TW laser with solid targets (abstract)
Authors: Zakharenkov Y. , Wharton K. , Offenberger A. , Moody J. , Stuart B. , Herman S. , Miller J. , Hammel B. , Perry M.
Published: July 2002

We report on the first plasma experiments with the 100 TW laser at LLNL. We have experimentally investigated the interaction of 1 μm, 40 J, 400 fs, 5×1019 W/cm2 laser pulses with multilayer solid targets. Various diagnostics were used to characterize this interaction. An x-ray pinhole camera monitored laser spot size and plasma location on target. Part of the laser beam was split off to probe the density scale length of the preformed plasma. An optical system for Schlieren photography was designed to record the images of plasma with electron density gradients as low as 1021 cm−4 with characteristic scale length of 10 μm. The range of electron densities was 1018–2020 cm−3. CR-39 plastic track detectors were employed to measure the fast ion blowoff from the plasma. Using a foil-filter technique the proton energy spectrum up to 6 MeV was measured. It was found that the high energy tail usually had a narrow (FWHM ∼10°) cone of expansion. Energy conversion to the fast ions (above 0.5 MeV) was estimated to be ∼0.3%–1% of laser energy. In addition, charge-coupled devices were used for detecting characteristic K α emission caused by fast (>1 MeV) electrons. (See article by K. Wharton for further discussion of the fast electron measurements.) Estimated fast electron yield ∼3%.
Selective digital integrator for modulated-polarization spectroscopy: An evaluation using (+)-3-methylcyclopentanone
Authors: Vrancic A. , Rupnik K. , McGlynn S. P.
Published: July 2002

A new device, a selective digital integrator (SDI), for the acquisition of modulated polarization spectroscopy (MPS) signals is described. Special attention is given to the accurate measurement of very small signals (ac component of interest <10−3×dc component) buried in noise. The polarization of light is altered with fast modulators (modulation frequency ∼50 kHz). A SDI connected to a single detector is used to separately and selectively measure the polarization-dependent intensities of electronic transitions. The SDI is a major improvement over the lock-in approach: it has on-the-fly time-resolved data acquisition and integration capability, and a wide gain-switching-free dynamic range (10 orders of magnitude). This design permits a variety of applications that require detection of modulated or pulsed signals. The advantages of the MPS-SDI method are demonstrated on the first Rydberg electronic transitions of (+)-3-methylcyclopentanone. The selectively measured MPS intensities are provided in 3D form (intensity versus both wavelength and light polarization). A discussion of the scattering problems that bedevil measurements at higher optical densities is included, and ways to correct for it are outlined.
Electron attachment mass spectrometry as a diagnostics for electronegative gases and plasmas
Authors: Stoffels E. , Stoffels W. W. , Tachibana K.
Published: July 2002

Electron attachment mass spectrometry (EAMS) has been developed to study mixtures of electronegative gases and plasmas. A quadrupole mass spectrometer (QMS) has been used to detect negative ions, formed from sampled species by attachment of low energy electrons. Varying the electron energy allows to collect the attachment cross section of the considered species. EAMS appears to be a very powerful technique to study the chemistry of electronegative gases. Unlike ionization mass spectrometry, where cross sections are low at the threshold and rather flat over a broad range of electron energies, attachment resonances are sharp and distinct. Also very limited fragmentation of the parent negative ion occurs, so a given molecule yields only a few different negative ions. This facilitates identification of components in a gas mixture. It is particularly advantageous for detection of large, fragile molecules, which break up after ionization, but can be easily transformed into large negative ions. Moreover, sensitive detection of active species is possible due to their relatively high attachment cross sections. A particularly important application of EAMS is the determination of an effective attachment cross section in a plasma. Recording this cross section allows to decide on the actual negative ion formation mechanism in the plasma environment, where active products of plasma conversion can significantly alter the negative ion production channels and consequently the whole balance of charged particles. Examples of EAMS applications to fluorocarbon gases and low-pressure radio-frequency plasmas are discussed. In a CF4 discharge conversion of the parent gas into species like C2F6 and C3F8 is easily visualized. The dominant mechanism of negative ion formation in the plasma is electron attachment to these minority species and not to the parent gas. Also larger polymers are readily formed in fluorocarbon plasmas. In a C2F6 discharge molecules with up to ten carbon atoms (the mass limit of our apparatus) have been detected using EAMS.
Study of proton transfer processes in solution using the laser induced pH-jump: A new experimental setup and an improved data analysis based on genetic algorithms
Authors: Viappiani Cristiano , Bonetti Germano , Carcelli Mauro , Ferrari Francesco , Sternieri Armando
Published: July 2002

We present an experimental setup for studying proton transfer reactions in solution and an innovative data analysis based on a genetic algorithm. We have used o-nitrobenzaldehyde to irreversibly release protons in an aqueous solution upon photolysis with an intense UV nanosecond laser pulse. Reactions of the photodetached protons with pH indicators have been followed by monitoring the changes in absorbance at 632.8 nm with a He–Ne laser and a Si avalanche photodiode. The setup has laser-limited time resolution and single shot sensitivity. The pH jump established with the laser shot lasts ∼30 ms, during which proton transfer reactions can be followed. The proposed methodology represents a simplification in the kinetic schemes with respect to the previous protocols based on the use of reversible caged protons.
Neon gas target for the production of radioactive fluorine beams
Authors: Decrock P. , Nolen J. A.
Published: July 2002

A neon gas target has been developed to produce radioactive fluorine. Small CF4 impurities were added to the neon gas and the recovery efficiency of 18F-labeled CF4 has been measured as a function of the impurity level. Extraction efficiencies up to 90% have been obtained, which makes this technique to produce and extract radioactive fluorine from a production target a powerful method to generate intense radioactive 17F and 18F beams, using the 20Ne(p,α)17F and 20Ne(d,α)18F reactions, respectively.
High precision electrical substitution radiometer based on superconducting-resistive-transition edge thermometry
Authors: Reintsema Carl D. , Koch Jonathan A. , Grossman Erich N.
Published: July 2002

We describe the design and measured electrical performance of the first electrical substitution radiometer based on superconducting thermometers. Prospective applications emphasize high precision measurement of low power, mid- and far-IR blackbody radiation. The minimum noise floor for a single measurement of substitution power was 2 pW for an integration time of 2.7 s. For an extended series of experiments covering a range of substitution power from 0.5 nW to 5 μW, the noise floor can be roughly approximated as 4 pW plus 7×10−6 times the measured power. Analysis of the results indicates the precision is limited by a combination of thermal parasitics and electronic noise contributions.
Compact ultrahigh vacuum sample exchange device with wide temperature range
Authors: Schmidt J. , Laitenberger P. , Palmer R. E.
Published: July 2002

We describe the design and performance of a compact ultrahigh vacuum sample holder, which allows (1) sample transfer, (2) liquid-nitrogen cooling, (3) heating by electron bombardment, and (4) temperature measurement in direct proximity to the sample. The device operates from close to liquid-nitrogen temperatures up to temperatures of 800–900 K in the steady-state regime with peak temperatures in excess of 1700 K and allows easy and reliable sample transfer.
Electron cyclotron resonance applicator of the Wilkinson microwave power divider type for large diameter ion sources
Authors: Fusellier C. , Wartski L. , Coste Ph. , Schwebel C. , Aubert J.
Published: July 2002

We have designed and tested a new model of coaxial 2.45 GHz electron cyclotron resonance (ECR) ion source based on the splitting of the microwave power traveling along a coaxial line into a set of periodically distributed antennas. Our motivation was the study of the scaling-up feasibility of a coaxially fed ion source in which it is desirable to circumvent the transverse electric and magnetic (TEM) mode conversion into azimuthally unsymmetrical higher-order modes. Another important characteristic of this source is the possible incorporation of permanent magnets into the individual antennas in order to create the ECR zones around the latter. The validity of these concepts was tested on a 5 cm in external diameter coaxial ECR ion source in which the single thick coaxial antenna was replaced by a set of quarter wavelength magnetized rod antennas in order to enhance the magnetic field created by the external splitted ring magnets. For an incident power of 100 W and an argon pressure of 1.2×10−4 mbar, a 500 eV singly charged ion beam with a current density of 1 mA/cm2 was delivered through an accel-decel set of grids. Implementation of a scaled-up 20 cm in diameter ion source is in progress.
Local mechanical spectroscopy with nanometer-scale lateral resolution
Authors: Oulevey F. , Gremaud G. , Sémoroz A. , Kulik A. J. , Burnham N. A. , Dupas E. , Gourdon D.
Published: July 2002

A new technique has been developed to probe the viscoelastic and anelastic properties of submicron phases of inhomogeneous materials. The measurement gives information related to the internal friction and to the variations of the dynamic modulus of nanometer-sized volumes. It is then the nanoscale equivalent to mechanical spectroscopy, a well-known macroscopic technique for materials studies, also sometimes called dynamic mechanical (thermal) analysis. The technique is based on a scanning force microscope, using the principle of scanning local-acceleration microscopy (SLAM), and allows the sample temperature to be changed. It is called variable-temperature SLAM, abbreviated T-SLAM. According to a recent proposition to systematize names of scanning probe microscope based methods, this technique should be included in the family of “mechanothermal analysis with scanning microscopy.” It is suited for studying defect dynamics in nanomaterials and composites by locating the dissipative mechanisms in submicron phases. The primary and secondary relaxations, as well as the viscoplasticity, were observed in bulk PVC. The wide range of phenomena demonstrate the versatility of the technique. A still unexplained increase of the stiffness with increasing temperature was observed just below the glass transition. All of these observations, although their interpretation in terms of physical events is still tentative, are in agreement with global studies. This technique also permits one to image the variations of the local elasticity or of the local damping at a fixed temperature. This enables the study of, for instance, the homogeneity of phase transitions in multiphased materials, or of the interface morphologies and properties. As an illustration, the homogeneity of the glass transition temperature of PVC in a 50/50 wt % PVC/PB polymer blend has been demonstrated. Due to the small size of the probed volume, T-SLAM gives information on the mechanical properties of the near-surface, which may differ from bulk properties.
High-frequency demodulation of multi-photon fluorescence in hyper-Rayleigh scattering
Authors: Olbrechts Geert , Strobbe Rik , Clays Koen , Persoons André
Published: July 2002

A novel technique for suppression of the multi-photon fluorescence contribution in second-order nonlinear optical hyper-Rayleigh scattering experiments is described. The technique takes advantages of the demodulation and the phase shift in the frequency domain of the time-delayed (multi-photon) fluorescence in the time domain. We demonstrate the effectiveness of demodulation at high modulation frequencies of the fundamental laser beam by determining the molecular second-order nonlinear polarizability for a reference molecule under fluorescent conditions. The value that was obtained for crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore compares very well with the values that were previously obtained. The possibility of complete suppression of all fluorescence, based on phase-sensitive measurements in quadrature with the fluorescence, is also discussed.
Bolt-on source of spin-polarized electrons for inverse photoemission
Authors: Schedin Fredrik , Warburton Ranald , Thornton Geoff
Published: July 2002

We have developed a portable spin-polarized electron gun which can be bolted on to an ultrahigh vacuum chamber. The gun has been successfully operated with an electron gun to target distance of about 150 mm. This allows accommodation of other surface science equipment in the same vacuum system. The spin-polarized electrons are obtained via photoemission from a negative electron affinity GaAs(001) surface with circularly polarized light. A transversely polarized beam is achieved with a 90° electrostatic deflector. A set of two three-element electrostatic tube lenses are employed to transport and to focus the electrons onto a target. The measured transmission through the electron optics is >70% for electron energies in the range 7–20 eV. This is achieved by using large diameter electron transport lenses. The energy resolution of the electron beam is measured to be better than 0.27 eV and the polarization is determined to be 25±5%.
High-resolution multiposition Thomson scattering for the TJ-II stellarator
Authors: Barth C. J. , Pijper F. J. , v. d. Meiden H. J. , Herranz J. , Pastor I.
Published: July 2002

In this article, the recent installed multiposition Thomson scattering (TS) of the TJ-II stellarator is presented. Light from a pulsed ruby laser is scattered by the free electrons of the plasma and led to a spectrometer for spectral analysis. The Doppler broadened spectrum of the scattered light is analyzed with a spectrometer in the Littrow configuration, detected with a GaAsP image intensifier and recorded with two intensified charged coupled device cameras. Values of the electron temperature (Te) in the range of 50 eV up to 4 keV can be measured at 160 spatial elements of 2.25 mm along a chord of 360 mm. First results show that the observational error for Te is close to the expected error of ∼10% at an electron density (ne) of 1×1019 m−3 using a laser energy of 8 J.
High resolution pulsed field ionization photoelectron spectroscopy using multibunch synchrotron radiation: Time-of-flight selection scheme
Authors: Jarvis G. K. , Song Y. , Ng C. Y.
Published: July 2002

We have developed an efficient electron time-of-flight (TOF) selection scheme for high resolution pulsed field ionization (PFI) photoelectron (PFI-PE) measurements using monochromatized multibunch undulator synchrotron radiation at the Advanced Light Source. By employing a simple electron TOF spectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. A near complete suppression of prompt electrons was achieved in PFI-PE measurements by gating the PFI-PE TOF peak, as indicated by monitoring background electron counts at the Ar(11s′) autoionizing Rydberg peak, which is adjacent to the Ar+(2P3/2) PFI-PE band. The rotational-resolved PFI-PE band for H2+ (X 2Σg+,v+=0) measured using this electron TOF selection scheme is nearly free from residues of nearby autoionizing features, which were observed in the previous measurement by employing an electron spectrometer equipped with a hemispherical energy analyzer. This comparison indicates that the TOF PFI-PE scheme is significantly more effective in suppressing the hot-electron background. In addition to attaining a high PFI-PE transmission, a major advantage of the electron TOF scheme is that it allows the use of a smaller pulsed electric field and thus results in a higher instrumental PFI-PE resolution. We have demonstrated instrumental resolutions of 1.0 cm−1 full width at half maximum (FWHM) and 1.9 cm−1 FWHM in the PFI-PE bands for Xe+(2P3/2) and Ar+(2P3/2) at 12.123 and 15.760 eV, respectively. These resolutions are more than a factor 2 better than those achieved in previous synchrotron based PFI-PE studies.
Temperature measurement of an atmospheric-pressure plasma torch
Authors: Kuo S. P. , Koretzky E. , Vidmar R. J.
Published: July 2002

A simple method to make a temperature measurement in an atmospheric-pressure plasma torch with a density of 1013 electrons/cm3 is developed. The method is based on thermal equilibrium and a detailed analysis of heat loss from a copper wire placed in a torch. The wire diameter, which regulates heat loss, is systematically reduced to increase the temperature of the wire segment in the torch. This process yields a critical wire diameter: Smaller diameters melt and larger diameters are not affected. For the critical diameter the wire temperature in the torch is approximated as the wire melting temperature. This temperature and an analysis of heat input from the torch and heat loss from the wire combine to provide a temperature measurement in the 1300–2200 K range. Using this technique, the temperature of a plasma torch was determined to be approximately 1760 K.
Random telegraph noise analysis in time domain
Authors: Yuzhelevski Y. , Yuzhelevski M. , Jung G.
Published: July 2002

A new procedure for analysis of random telegraph signals in time domain has been developed and applied to the analysis of voltage fluctuations in the current induced dissipative state in superconducting thin films. The procedure, based entirely on the difference in the statistical properties of discrete Marcovian telegraph fluctuations and Gaussian background noise, ascribes each point of the experimental time record to one of the telegraph states. The average statistical lifetimes and amplitudes of the telegraph signal are then determined in an iterative way by fitting the amplitude histogram of thus obtained record of the redistributed data to the two-Gaussian histogram of the original experimental signal. The procedure allows for analyzing “noisy” random telegraph signals with low ratio between the signal amplitude and the intensity of the background noise that cannot be analyzed by the classical approach. Separation of the time record into two subrecords relative to two telegraph states also enables in-depth analysis of the spectral properties of the background noise observed together with the telegraph fluctuations.
Surface roughness and defect morphology in electron cyclotron resonance hydrogen plasma cleaned (100) silicon at low temperatures
Authors: Hwang Ki-Hyun , Yoon Euijoon , Whang Ki-Woong , Lee Jeong Yong
Published: July 2002

Surface roughening of (100) Si at low temperatures during electron cyclotron resonance hydrogen plasma cleaning is studied in an ultrahigh vacuum environment. The effects of process parameters on surface roughness are quantitatively analyzed by atomic force microscopy besides reflection high energy electron diffraction. Crystalline defect morphology is studied by transmission electron microscopy to understand its role in surface roughness. Surface roughness is strongly related to the nucleation and growth of {111} platelet defects at the Si subsurface region and the preferential etching at positions where {111} platelet defects intersect the Si surface. Hydrogen ion flux and substrate temperature can be successfully controlled to tailor the {111} platelet defects, therefore, surface roughness.
Preparation of nitrogen containing carbon films using chemical vapor deposition enhanced by electron cyclotron resonance plasma
Authors: Inoue Tohru , Ohshio Shigeo , Saitoh Hidetoshi , Kamata Kiichiro
Published: July 2002

A chemical vapor deposition apparatus enhanced by electron cyclotron resonance plasma was employed to deposit nitrogen containing carbon films. In the apparatus, negative dc bias voltage was applied to the substrate for acceleration of positive ions toward the substrate. The deposition rate and nitrogen content of the film was found to be mainly dependent upon the deposition conditions. Although a large N2 flow rate and bias voltage contribute to inhibit film growth through surface sputtering of the substrate, an optimum [N2]/([CH4]+[N2]) flow rate of 0.67 and a bias voltage of 50 V promote nitrogen implantation into the growing films through possible nitrogen ion bombardment.
Electric field induced second harmonic generation spectroscopy on a metal-oxide-silicon structure
Authors: Godefroy P. , de Jong W. , van Hasselt C. W. , Devillers M. A. C. , Rasing Th.
Published: July 2002

Spectroscopic electric-field-induced second harmonic generation on a Si(111)–SiO2–Cr metal-oxide-silicon structure shows a bias-independent ‘‘interface’’ resonance at 3.25 eV and a ‘‘bulk’’ resonance at 3.43 eV which is strongly bias dependent. The symmetry forbidden bulk dipole contribution becomes observable, and even dominating, due to the bias-induced band-bending that breaks the bulk inversion symmetry. The origin of these resonances is discussed, as well as the prospects for using second harmonic generation as a probe of metal-oxide-silicon characteristics.
Magnetization reversal in the picosecond range measured with time-resolved magneto-optical Kerr effect
Authors: Guarisco D. , Burgermeister R. , Stamm C. , Meier F.
Published: July 2002

We have performed time-resolved magneto-optical Kerr effect measurements with picosecond resolution in a pump-probe experiment using a 30 ps pulsed dye laser (λ=577 nm). Two typical magneto-optical storage materials were investigated: a Gd13Tb12Fe75 amorphous film evaporated on a glass substrate and a Co/Pt multilayer on a Si substrate. Curie-point thermomagnetic writing has been performed in less than 2 ns with the Co/Pt multilayer, whereas with the amorphous film magnetization reversal could not be observed within 6 ns. These differences in switching times are likely the consequence of the different thermal conductivities of the substrates.
Oxynitridation of silicon by remote-plasma excited nitrogen and oxygen
Authors: Saito Yoji
Published: July 2002

Direct oxynitridation of silicon is performed by remote-plasma excited nitrogen and oxygen gaseous mixtures at 550 °C. Nitrogen atoms are mainly incorporated near the SiO2–Si interfaces with Auger electron spectroscopy measurements. We have controlled the peak density of nitrogen up to at least several atomic percents, varying the partial pressure of nitrogen and oxygen. The supply of active oxygen species is required for the growth of oxynitride films, but their excess supply reduces the density of nitrogen in the films. The proposed technique is a unique process to obtain high quality ultrathin dielectrics.
Peak-to-valley ratio of small resonant-tunneling diodes with various barrier-thickness asymmetries
Authors: Schmidt T. , Tewordt M. , Haug R. J. , Klitzing K. v. , Schönherr B. , Grambow P. , Förster A. , Lüth H.
Published: July 2002

Resonant tunneling through small double-barrier heterostructures is investigated in dependence of the charge accumulation in the quantum well and the device diameter. The study comprises a series of resonant-tunneling diodes with four different barrier-thickness ratios and diameters between 300 nm and 10 μm. Special emphasis lies on the peak-to-valley ratio of the resonant-tunneling current peaks, which drops drastically with decreasing device diameter for weak electron accumulation, while it is size independent in the strong-charging case.
Continuous wave operation of midinfrared (7.4–8.6 μm) quantum cascade lasers up to 110 K temperature
Authors: Sirtori Carlo , Faist Jérôme , Capasso Federico , Sivco Deborah L. , Hutchinson Albert L. , Chu S. N. George , Cho Alfred Y.
Published: July 2002

The continuous wave (cw) operation of quantum cascade lasers operating at wavelengths in the 7.4–8.6 μm range is reported. The GaInAs/AlInAs structures, grown by molecular beam epitaxy, are based on a vertical intersubband transition scheme and use a plasmon-enhanced waveguide geometry to reduce losses and increase the confinement factor. The single mode cw optical power from one facet is 2 mW up to a maximum temperature of 110 K. In pulsed operation, the highest temperature is 210 K and the threshold shows a weak temperature dependence typical of these class of lasers, with a T0=110 K.
Size effects in small oxide confined vertical-cavity surface-emitting lasers
Authors: Oh T.-H. , Huffaker D. L. , Deppe D. G.
Published: July 2002

Data are presented on small area vertical-cavity surface-emitting lasers with lateral sizes ranging from 5 to 1 μm that use both oxide confinement and a high-contrast distributed Bragg reflector as the upper mirror. Optical loss becomes appreciable for device sizes less than 3 μm in diameter, and is analyzed through measurements of threshold, differential output efficiency, and the far-field radiation patterns. The results suggest that loss of optical confinement occurs for thin apertures and small device sizes.
Quantitative study of interface roughness replication in multilayers using x-ray reflectivity and transmission electron microscopy
Authors: Chládek M. , Valvoda V. , Dorner C. , Holý C. , Grim J.
Published: July 2002

The recently appeared distorted wave Born approximation formalism is used for quantitative determination of interface roughness replication and lateral correlation length in periodical multilayered structures. The results obtained from x-ray diffraction are in very good agreement with analysis of cross-section transmission electron micrographs. We interpret transparently the obtained parameters and demonstrate the ability of the low-angle x-ray diffraction methodology for nondestructive and quantitative studying of interface roughness in magnetic multilayers.
Observation of compositional modulation in (111)A InGaAs quantum wells and the effect on optical properties
Authors: Chin Albert , Chen W. J.
Published: July 2002

We have studied the growth of InGaAs/GaAs multiple quantum wells (MQWs) on (111)A GaAs. Uniform thickness of (111)A quantum wells is observed by the cross-sectional transmission electron microscopy (TEM). Growth induced long-range In- and Ga-rich InxGa1−xAs/InyGa1−yAs superlattice in (111)A is also observed by cross-sectional TEM in the ternary InGaAs wells. In contrast, none of the above superstructure was observed by TEM on a side-by-side grown (100) oriented substrate. However, the photoluminescence (PL) linewidth is broadened by such compositional modulation. Low-temperature (15 K) photoluminescence showed a broad PL linewidth of 27.5 meV for In0.16Ga0.84As/GaAs MQWs grown on (111)A substrates at 520 °C. A decreased PL linewidth of 15.7 meV and a reduced compositional modulation in InGaAs wells can be achieved at a higher growth temperature of 560 °C.
New interpretation of the dominant recombination center in platinum doped silicon
Authors: Sachse J.-U. , Sveinbjörnsson E. Ö. , Jost W. , Weber J. , Lemke H.
Published: July 2002

The midgap level in platinum doped n-type silicon, which was proposed to be the dominant recombination center, is identified as a platinum-hydrogen complex. Hydrogenation of the samples is achieved by wet-chemical etching at room temperature. Defect profiles, determined by deep level transient spectroscopy, clearly associate the level with the concentration profile of atomic hydrogen.
Low-frequency noise of the leakage current in undoped low-pressure chemical vapor deposited polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A. , Brini J. , Kamarinos G.
Published: July 2002

The origin of the leakage current in low-pressure chemically vapor deposited polycrystalline silicon (polysilicon) thin-film transistors is investigated by low-frequency noise measurements. The leakage current depends on the structure of the polysilicon layer. When the grain size is relatively large (about 120 nm), the noise spectra show a pure 1/f behavior caused by carrier fluctuation within the space charge region of the drain junction. For smaller grain size (about 50 nm), the observed 1/f1.5 spectra are attributed to thermal noise of the bulk polysilicon film.
Spectroscopic ellipsometric characterization of undoped ZnTe films grown on GaAs
Authors: Kim Y. D. , Choi S. G. , Klein M. V. , Yoo S. D. , Aspnes D. E. , Xin S. H. , Furdyna J. K.
Published: July 2002

We report highly accurate dielectric function data for ZnTe. These data were made possible by the high quality of the heteroepitaxial material and the development of a chemical etching procedure for producing abrupt surfaces on ZnTe; they provided the first observation of the E0+Δ0 structure in ZnTe by spectroscopic ellipsometry and evidence for several contributions to the E2 structure. Accurate critical point energies were obtained by Fourier analysis.
Enhancement of the Hall mobility in undoped GaAs with low carrier concentration by light excitation
Authors: Kažukauskas V. , Kühnel G. , Siegel W.
Published: July 2002

We report the investigation of the steady-state Hall mobility behavior upon extrinsic light excitation in n-type liquid-encapsulated Czochralski GaAs crystals. The carrier concentration of the samples measured in the dark ranged from 108 to 3×1011 cm−3. The Hall mobility demonstrated a pronounced minimum in the concentration region 109–1011 cm−3. In samples of this region a significant increase of the Hall mobility from ⩾1460 to 6300–7800 cm2 V s could be induced by light, in some cases without an increase of the measured carrier concentration. Such behavior was explained by reduction of the mesoscopic nonuniformities related to the cellular structure of dislocations by the carriers generated from defect levels in the band gap.
Optical imaging of carrier dynamics in silicon with subwavelength resolution
Authors: La Rosa A. H. , Yakobson B. I. , Hallen H. D.
Published: July 2002

Characteristic rate variations of carrier processes are imaged using near-field scanning optical microscopy. We couple both a visible pump and an infrared probe light through a subwavelength aperture to investigate the interband recombination and intraband diffusion of excess carriers in oxidized silicon. Typical values of the locally measured life time constants agree well with those obtained by conventional space-averaged techniques. Moreover, the images locate defects, reveal variations, and can map the regions in which a recombination process is active.
Variation of GaN valence bands with biaxial stress and quantification of residual stress
Authors: Edwards N. V. , Yoo S. D. , Bremser M. D. , Weeks T. W. , Nam O. H. , Davis R. F. , Liu H. , Stall R. A. , Horton M. N. , Perkins N. R. , Kuech T. F. , Aspnes D. E.
Published: July 2002

Low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal-space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1meV with increased confidence.
High trapped fields in bulk YBa2Cu3O7−δ samples at temperatures around 50 K
Authors: Fuchs G. , Krabbes G. , Schätzle P. , Gruβ S. , Stoye P. , Staiger T. , Müller K.-H. , Fink J. , Schultz L.
Published: July 2002

Bulk melt textured YBa2Cu3O7−δ samples with single grains of about 24 mm diameter were obtained by use of SmBa2Cu3O7−x seed crystals. The maximum trapped field B0 in the gap between two samples was investigated as function of temperature. B0 increased from 1 T at 77 K to 8.5 T at 51.5 K, which is the highest trapped field achieved in nonirradiated samples. At low temperatures, cracking of the samples was observed under magnetic pressure. In this temperature range, the trapped field is limited by the mechanical strength of the samples, for which a value of 25 MPa was estimated.
Self-assembled Ge nanowires grown on Si(113)
Authors: Omi Hiroo , Ogino Toshio
Published: July 2002

We grew coherent Ge islands on Si(113) substrates by molecular beam epitaxy. Atomic force microscopy and reflection high energy electron diffraction were used to examine surface morphology as a function of Ge coverage and growth temperature. The as-grown coherent islands were shaped like wires and formed dense arrays over the entire surface. The islands bounded by {519} facets were elongated in the [332̄] direction and were linearly ordered across steps. The wire-shaped islands formed when Ge coverage was 5–8 monolayers and the growth temperature was 400–500 °C. Cross-sectional transmission electron microscope images confirm that the Ge islands are coherently grown on the Si substrates. The anisotropic shape of the Ge islands was due to an anisotropic strain relief mechanism on Si(113), which had been theoretically predicted. Our findings suggest that the coherent island formation of Ge on Si(113) may be a possible method to fabricate self-assembled Ge nanowires.
Preferential Co–Si bonding at the Co/SiGe(100) interface
Authors: Boyanov B. I. , Goeller P. T. , Sayers D. E. , Newmanich R. J.
Published: July 2002

The initial stages of the reaction of Co with Si0.79Ge0.21(100) were studied in situ with extended x-ray absorption fine structure spectroscopy and reflection high energy electron diffraction. The Si:Ge ratio in the first coordination shell of Co in sub-monolayer Co films was found to increase with film thickness and annealing temperature, indicating preferential formation of Co–Si bonds. The impact of the observed preference for Co–Si bonding on the morphology of epitaxial CoSi2/Si1−xGex heterostructures is discussed.
La
0.5
Sr
0.5
CoO
3
/Pb(Nb
0.04
Zr
0.28
Ti
0.68
)O
3
/La
0.5
Sr
0.5
CoO
3
thin film heterostructures on Si using TiN/Pt conducting barrier
Authors: Yang B. , Aggarwal S. , Dhote A. M. , Song T. K. , Ramesh R. , Lee J. S.
Published: July 2002

A high density ferroelectric memory process flow requires the integration of conducting barrier layers to connect the drain of the pass-gate transistor to the bottom electrode of the ferroelectric stack. We are studying the effect of crystallinity of the TiN/Pt barrier layer with Si wafers on the ferroelectric properties of La0.5Sr0.5CoO3/Pb(Nb0.04Zr0.28Ti0.68)O3/La0.5Sr0.5CoO3 (LSCO/PNZT/LSCO) capacitors. Structural studies indicate complete phase purity (i.e., fully perovskite) in both epitaxial and polycrystalline materials. The polycrystalline capacitors show lower remnant polarization and coercive voltages. However, the retention, fatigue, and imprint characteristics are similar, indicating minimal influence of crystalline quality on the ferroelectric properties.
Phase stability of guest/host photorefractive polymers studied by light scattering experiments
Authors: Hendrickx E. , Volodin B. L. , Steele D. D. , Maldonado J. L. , Wang J. F. , Kippelen B. , Peyghambarian N.
Published: July 2002

We report on light scattering measurements in guest/host photorefractive polymers doped with different polar dyes as a function of temperature and sample composition. Crystallization processes of the polar dye are found to follow a nucleation and growth mechanism. The structure and melting point of the polar dye, the storage temperature, and the amount of plasticizer are shown to exert a major influence on the shelf lifetime of the samples. New composites with high efficiency and long shelf lifetime are presented.
Measurement of the dynamic input impedance of a dc superconducting quantum interference device at audio frequencies
Authors: Falferi Paolo , Mezzena Renato , Vitale Stefano , Cerdonio Massimo
Published: July 2002

The coupling effects of a commercial dc superconducting quantum interference device (SQUID) to an electrical LC resonator which operates at audio frequencies (≈1 kHz) with quality factors Q≈106 are presented. The variations of the resonance frequency of the resonator as functions of the flux applied to the SQUID are due to the SQUID dynamic inductance in good agreement with the predictions of a model. The variations of the quality factor point to a feedback mechanism between the output of the SQUID and the input circuit.
Solid-state tunable cavity lasing in a poly(para-phenylene vinylene) derivative alternating block co-polymer
Authors: Kumar N. Deepak , Bhawalkar Jayant D. , Prasad Paras N. , Karasz Frank E. , Hu B.
Published: July 2002

There is a great deal of interest in developing π-conjugated polymeric semiconductor based diode lasers which can yield lasing under electrical excitation. Towards this goal, tunable cavity lasing was achieved in a poly(para-phenylene vinylene) derivative alternating block copolymer, BMPPV, dispersed in an active solid matrix of poly(9-vinylcarbazole), with which this block co-polymer also exhibits strong electroluminescence. The laser operated in the blue region of the spectrum over the range 480 to 510 nm, with an optical efficiency approaching 10%. Linewidth narrowing and a narrowing of the temporal profile are reported. Operating lifetime of the solid-state laser was in excess of 5000 pulses at the same pumping position on the sample. Lasing properties of the same block copolymer in the solution phase were also investigated and the results compared with the solid state laser performance.
Spin-polarized quasiparticle injection devices using Au/YBa2Cu3O7/LaAlO3/Nd0.7Sr0.3MnO3 heterostructures
Authors: Dong Z. W. , Ramesh R. , Venkatesan T. , Johnson Mark , Chen Z. Y. , Pai S. P. , Talyansky V. , Sharma R. P. , Shreekala R. , Lobb C. J. , Greene R. L.
Published: July 2002

Oxide heterostructures were used for studies of quasiparticle injection effects in high-Tc superconducting thin films. The effect of injection of spin polarized quasiparticles from a ferromagnetic gate layer was compared to that of unpolarized quasiparticles from a nonmagnetic metallic gate. Transport measurements of the superconducting layer showed strong suppression in the supercurrent by the injection of spin-polarized quasiparticles, and a current gain of as large as five was attained. This is 10 to 30 times larger than the gain of unpolarized injection devices. Such large effects could be useful in a variety of active high-Tc superconductor/colossal magnetoresistance heterostructure based devices.
Pd/Ge/Ti/Au ohmic contact to AlGaAs/InGaAs pseudomorphic high electron mobility transistor with an undoped cap layer
Authors: Kim Yi-Tae , Lee Jong-Lam , Mun Jae Kyoung , Kim Haecheon
Published: July 2002

The Pd/Ge/Ti/Au ohmic contact to AlGaAs/InGaAs pseudomorphic high electron mobility transistor was investigated with the etch depth of an undoped GaAs/AlGaAs cap layer. The contact resistivity decreases from 9.5×10−5 to 2.3×10−6 Ω cm2 when the contacts were formed on a n-Al0.23Ga0.77As layer by removing the undoped cap layer. X-ray diffraction results show that the good ohmic contact is due to the formation of Au2Al as well as β-AuGa. Both compounds play a role to create group-III vacancies, followed by the incorporation of Ge into group-III vacancies, namely, creation of free electron below the contact. This results in the considerable elimination of contact resistivity by lowering the effective tunneling barrier.
Maximum output power and maximum operating temperature of quantum well lasers
Authors: Makino T. , Evans J. D. , Mak G.
Published: July 2002

An efficient phenomenological model describing the temperature dependence of the output power of multiquantum well (MQW) lasers is presented. The model can predict all the general features of the experimental results. The effect of series resistance, thermal resistance, cavity length, leakage current, and facet reflectivity on the maximum achievable power are analyzed in a systematic manner. The calculated maximum operating temperature (Tm), defined as the heat sink temperature at which the saturated output power vanishes due to heating, is in reasonable agreement with reported experimental results.