bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrasoft x-ray imaging system for the National Spherical Torus Experiment
Authors: Stutman D. , Finkenthal M. , Soukhanovskii V. , May M. J. , Moos H. W. , Kaita R.
Published: July 2002

A spectrally resolved ultrasoft x-ray imaging system, consisting of arrays of high resolution (<2 Å) and throughput (⩾tens of kHz) miniature monochromators, and based on multilayer mirrors and absolute photodiodes, is being designed for the National Spherical Torus Experiment. Initially, three poloidal arrays of diodes filtered for C 1s-np emission will be implemented for fast tomographic imaging of the colder start-up plasmas. Later on, mirrors tuned to the C Lyα emission will be added in order to enable the arrays to “see” the periphery through the hot core and to study magnetohydrodynamic activity and impurity transport in this region. We also discuss possible core diagnostics, based on tomographic imaging of the Lyα emission from the plume of recombined, low Z impurity ions left by neutral beams or fueling pellets. The arrays can also be used for radiated power measurements and to map the distribution of high Z impurities injected for transport studies. The performance of the proposed system is illustrated with results from test channels on the CDX-U spherical torus at Princeton Plasma Physics Laboratory.
X-ray line diagnostics on the Tore Supra tokamak
Authors: Platz P. , Margerit Y. , Artaud J.-F. , Mattioli M. , Moulin B.
Published: July 2002

We describe a multi-crystal, high-resolution, fully remote-controlled, absolutely calibrated, Johann-type spectrometer for the 1–4 Å region. Spectra from the helium- and lithium-like charge states of impurity elements from Ar to Cu are analyzed by least-squares fitting of synthetic spectra. The intrinsic impurity elements Cr and Fe are used for a routine, time-resolved, hands-off diagnostic for electron and ion temperatures, and for toroidal rotation velocities. Impurity densities are obtained from absolute line intensities. Injection of Ar, Ti, Mn, and Ni, is used for transport studies. We have also obtained the precise wavelengths of transitions in neon-like and near-neon-like charge states of Pd and Ag, which are of much interest in atomic physics.
A toroidal focusing mirror based vacuum ultraviolet diagnostic for TJ-II
Authors: McCarthy K. J. , Zurro B. , Baciero A. , Ahero B. , Burgos C.
Published: July 2002

We describe a vacuum ultraviolet spectrometer having spatial, spectral, and temporal resolution capabilities that is being built as a diagnostic for the TJ-II flexible heliac device. The system is based around a renovated 0.5 m radius Rowland circle grazing incidence duochromator and will cover the wavelength range from 300 to 2000 Å with good spatial resolution. A toroidal mirror used at grazing incidence, mounted on a specially designed mounting positioned between the plasma and spectrometer, will permit ∼80% of the bean shaped plasma cross section to be scanned with spatial resolution on the order of 10 mm. In addition, filters and slits are incorporated to tailor the incident light beam. The duochromator was coupled directly to the TJ-II during the machine start up phase to check the good operation of the duochromator. These data have been fed back into the design program in order to optimize the instrument design.
Two fast VME transient recorder systems with 12 bits and multiple differential inputs
Authors: Correia C. , Combo A. , Correia M. , Simões J. B. , Coelho P. , Carvalho B. B. , Sousa J. , Varandas C. A. F.
Published: July 2002

This article describes two on-site developed, VME transient recorder systems, with 12 bit resolution, eight (four) differential input channels, sampling frequency up to 1.25 (20) MSPS and 512 (250 or 1000) kWords of memory per channel. Autonomous operation is guaranteed by standard static random access memory, under the control of a programmable logic array device for address generation. This device also manages the external trigger signal and provides the control signals for the analog-to-digital converter in articulation with a programmable timing element that determines the acquisition rate. Software drivers have been developed for the OS-9 operating system. Tests to quantify the channel crosstalk, the common mode noise rejection, and the spectral characterization are presented.
Experimental study of self-injection locking oscillator on copper vapor laser
Authors: Ohzu Akira , Kato Masaaki , Maruyama Yoichiro , Arisawa Takashi
Published: July 2002

As a novel method to obtain a low divergence laser beam, a self-injection locking oscillator system has been developed for a copper vapor laser. The system is made of one laser tube, and uses temporal superposition of two pulsed discharges, predischarge and main discharge. The laser pulse is produced in the main discharge by amplifying a seeded laser pulse produced by the predischarge. By optimizing the discharge conditions including an interval period between the two pulsed discharges, a high quality laser beam containing 80% of the whole laser beam energy in divergence as small as eight times diffraction limited has been obtained. The quality of the laser beam is comparable to that obtained with a conventional injection locking oscillator system consisting of two separate laser tubes.
Photo-induced dissociation of electrospray generated ions in an ion trap/time-of-flight mass spectrometer
Authors: Gabryelski Wojciech , Li Liang
Published: July 2002

Laser photo-induced dissociation (PID) is an attractive alternative to collision-induced dissociation (CID) in probing structural features of biomolecules, such as peptides, by mass spectrometry. We report a new experimental setup for PID studies of biomolecules. It involves the use of an ion trap/time-of-flight mass spectrometer for the detection of PID products. The intact molecular ions are produced by electrospray ionization and introduced into a quadrupole ion trap. The ions stored in the trap are then dissociated by using a 266 nm laser beam from a pulsed Nd:yttrium–aluminium–garnet laser. After a short delay, all the fragment ions are extracted out of the trap and mass analyzed by a linear time-of-flight (TOF) mass spectrometer. The applications of this instrument for the PID studies of several peptides are demonstrated. The PID spectra are compared to those obtained by CID in the same instrument. It is shown that the amount of photoenergy deposited for fragmentation can be controlled by laser absorption property of the ions, laser power, number of laser pulses, and ion trap buffer gas pressure. Energetically optimal PID process results in spectra providing structural information similar to that obtained from CID. An excessive amount of photoenergy deposited to the fragmenting ions favors the formation of deep fragmentation products at the cost of sequence related ions. It is demonstrated that in some cases the PID technique has the potential in probing structural features of peptides that cannot be fragmented in ion trap CID. Finally, it is shown that the combination of the ion trap with the TOF detector provides a unique capability for fast detection of ions formed via PID. The PID products generated after a time delay from 1.5 μs to several milliseconds following the dissociation laser pulse can be characterized.
Low-energy electron point source microscope with position-sensitive electron energy analyzer
Authors: Park Jeong-Young , Kim S. H. , Suh Y. D. , Park W. G. , Kuk Y.
Published: July 2002

A low-energy electron point source microscope equipped with a position-sensitive energy analyzer is constructed. A nanometer-sized feature can be zoomed in and its energy-loss spectrum can be measured with a retarding field-type energy analyzer mounted in front of the imaging screen. The geometric and the electronic structures of carbon nanotubes are measured with the present system. Interference between the scattered and the transmitted electron beams through the carbon nanotubes is observed using an atomically sharp field emitter. The electron energy-loss spectrum shows two prominent peaks at ∼7 and 16–17 eV, which are identified as the π plasmon and (π+σ) surface-plasmon peaks. This result is consistent with the measurements of high-energy electron energy-loss spectroscopy as well as the theoretical calculation.
A vacuum chamber with a radial rotating port
Authors: Lee S. G. , Bak J. G.
Published: July 2002

An engineering design of a vacuum chamber with a radial rotating port (VCRP) and the performance test results from the VCRP are presented.
Analysis of dopant concentration effects in praseodymium-based fluorescent fiber optic temperature sensors
Authors: Mitchell I. R. , Farrell P. M. , Baxter G. W. , Collins S. F. , Grattan K. T. V. , Sun T.
Published: July 2002

An analysis of concentration effects in praseodymium-doped glasses used as the sensor element of several fiber optic temperature sensors has been performed. Results show the dependence of a range of relevant parameters on concentration, and the determination of concentration-independent parameters used in the design of effective probe sensor elements with both doped and co-doped rare earth materials. Low concentrations of Pr3+, commensurate with an acceptable signal-to-noise ratio in the optical system, are preferable for the design of the most effective optical fiber fluorescence sensors based upon this species.
Proximity heater for elevated temperature in situ vacuum scanning tunneling microscopy of metal surfaces
Authors: Pearl T. P. , Sibener S. J.
Published: July 2002

The design and application of a radiant heater assembly for elevated temperature scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) is presented. The proximity heater is a noninvasive modification to an existing commercial room-temperature microscope and is capable of radiatively heating samples up to 650 K in situ. Imaging at higher temperatures should be readily accessible with other microscope construction designs. It is demonstrated that this heater is well suited for enabling an STM to capture surface morphological transformations such as the motion of atomic steps on metal surfaces at elevated temperature. Various design issues and solutions related to variable temperature UHV-STM are also discussed. We believe the approach described to be general in nature, offering a direct route to adapting UHV-STM designs for elevated temperature imaging.
Advances in hydride phase growth: Automatic high precision calorimeter-volumetric devices, for thermodynamic and kinetics analyses
Authors: Dantzer P. , Millet P.
Published: July 2002

A new automatic apparatus has been specifically developed for investigating phase transformations in hydrogen absorbing intermetallic compounds, providing accurate characterizations of the thermodynamic properties as well as of the dynamic aspects of the hydride phase growth over a wide range of pressures 0–4 MPa and temperatures 250–800 K. It consists essentially of a heat flow calorimeter coupled with high precision volumetric devices. The ensemble constitutes of a closed system in which high purity hydrogen gas within the system is transferred between hydrides reservoirs and reactors with high thermal transfer capacity. The excellent stability of the signal of the calorimeter, ±4 nV over a long period of time (>10 days), allows one direct measurement of the heat of H2 absorption or desorption during a scan of an hysteresis loop with an average accuracy of 1%. To maintain quasi-isothermal conditions during the transformation, a reliable control of the temperature inside the sample is insured by optimizing the hydrogen gas flow rate. Simultaneously the heat flux, pressure, temperature, composition data collected have been used to obtain kinetic parameters through two different and complementary techniques. The first one is based on an analysis of the measured heat flux evolved during the reaction which gives the true rate law at the sample level by deconvolution of the measured signal. It is shown that only overall information can be expected by this method. The results of the numerical treatment raise the problem of the location of the heater used for calibration of the calorimeter. The second technique takes into account the component volumes of the system. Here, the time variation of hydrogen mass flow (excitation of the system) and hydrogen pressure in the reactor (response of the system) is analyzed in the frequency domain which requires knowledge of the experimental transfer function of the volumetric equipment. In the complex plane, the shape of the transfer function appears as a signature of a proposed mechanism. ZrNi–H2 and LaNi5–H2 systems have been used to demonstrate the detailed analysis.
High-efficiency Stark-geometry photorefractive quantum wells with intrinsic cladding layers
Authors: Lahiri I. , Aguilar María , Nolte D. D. , Melloch M. R.
Published: July 2002

Output diffraction efficiencies approaching 40% and input diffraction efficiencies approaching 3% have been achieved in photorefractive p-i-n quantum well diodes operating in the longitudinal Stark geometry. The device structure consists of a low-temperature-grown multiple quantum well isolated from the doped contacts by an intrinsic standoff layer of Al0.5Ga0.5As. All charge trapping and screening occurs within the quantum wells without the need for trapping in specialized buffer layers used in previous designs. This new design operates at lower voltages (18 V) and lower fields (6 V/μm) than previously demonstrated.
Polarity determination of GaN films by ion channeling and convergent beam electron diffraction
Authors: Daudin B. , Rouvière J. L. , Arlery M.
Published: July 2002

By using ion channeling and convergent beam electron diffraction techniques, we have determined the absolute polarity of various GaN films grown by MOCVD on (0001) sapphire. We observe two main classes of GaN films, namely flat and rough pyramidal ones. We find that flat GaN films have a Ga polarity. Rough pyramidal samples contain many tiny columnar inversion domains (with Ga polarity) imbedded in a matrix exhibiting an N polarity.
Sputtered AlN encapsulant for high-temperature annealing of GaN
Authors: Zolper J. C. , Rieger D. J. , Baca A. G. , Pearton S. J. , Lee J. W. , Stall R. A.
Published: July 2002

Reactively sputtered AlN is shown by electrical characterization of Pt/Au Schottky diodes to be an effect encapsulant for GaN annealed at 1100 °C. Schottky diodes formed on GaN encapsulated with AlN during the anneal had low reverse leakage currents with breakdown voltages in excess of 40 V. In contrast, samples annealed without the AlN layer had 3–4 orders-of-magnitude higher reverse leakage currents. Atomic force microscopy images of as-grown and annealed samples also demonstrate an increase in surface roughness and a change in morphology of the uncapped samples following annealing. Auger electron spectroscopy supports the hypothesis that the AlN encapsulant is reducing N loss from the GaN substrate. N loss in the uncapped samples is expected to create an n+-region at the surface that accounts for the high reverse leakage current and improved Ohmic behavior for the uncapped samples. The use of AlN encapsulation will enable the realization of all ion implanted GaN metal semiconductor field effect transistors.
Correlation between the valence- and conduction-band-tail energies in hydrogenated amorphous silicon
Authors: Sherman S. , Wagner S. , Gottscho R. A.
Published: July 2002

We provide experimental evidence for a correlation between the characteristic energies of the exponential conduction- and valence-band tails of hydrogenated amorphous silicon (a-Si:H). We measured subgap optical-absorption spectra of isolated a-Si:H films in order to extract their valence-band-tail energies. Also, we modeled the current–voltage characteristics of thin-film transistors which had a-Si:H layers deposited identically to the isolated films, in order to extract their conduction-band-tail energies. When the quality of the a-Si:H was varied, the characteristic energies of the two tails scaled linearly with each other.
Ultra-low resistive ohmic contacts on n-GaN using Si implantation
Authors: Burm Jinwook , Chu Kenneth , Davis William A. , Schaff William J. , Eastman Lester F. , Eustis Tyler J.
Published: July 2002

Implanted ohmic contacts were made on molecular beam epitaxy grown GaN materials. Si was implanted at a doping density of about 4×1020  cm-3 to decrease the contact resistance of the contact, followed by an activation anneal at 1150 °C for 30 s. The overlay metal Ti/Au was evaporated. Four-probe measurements were performed on transmission line model patterns. The measured maximum contact resistance was 0.097 Ω mm and the apparent specific contact resistance was 3.6×10−8 Ω cm2.
Fabrication and electroluminescence of double-layered organic light-emitting diodes with the Al2O3/Al cathode
Authors: Li F. , Tang H. , Anderegg J. , Shinar J.
Published: July 2002

The effects of a controlled Al2O3 buffer layer on the behavior of highly efficient vacuum evaporated aqua regia-treated indium tin oxide (ITO)/triphenyl diamine (TPD)/8-tris-hydroxyquino-line aluminum Alq3/Al2O3/Al light-emitting diodes are described. It is found that, with a buffer layer of suitable thickness, both current injection and electroluminescence output are significantly enhanced. The enhancement is believed to be due to increased charge carrier density near the TPD/Alq3 interface that results from enhanced electron tunneling, and removal of exciton-quenching gap states that are intrinsic to the Alq3/Al interface.
Anisotropic magnetic coupling of permalloy micron dots forming a square lattice
Authors: Mathieu C. , Hartmann C. , Bauer M. , Buettner O. , Riedling S. , Roos B. , Demokritov S. O. , Hillebrands B. , Bartenlian B. , Chappert C. , Decanini D. , Rousseaux F. , Cambril E. , Müller A. , Hoffmann B. , Hartmann U.
Published: July 2002

Static magnetic and spin wave properties of square lattices of permalloy micron dots with thicknesses of 500 and 1000 Å and with varying dot separations have been investigated. A magnetic fourfold anisotropy was found for the lattice with dot diameters of 1 μm and a dot separation of 0.1 μm. The anisotropy is attributed to an anisotropic dipole–dipole interaction between magnetically unsaturated parts of the dots. The anisotropy strength (order of 105 erg/cm3) decreases with increasing in-plane applied magnetic field.
Ohmic contacts to n-GaN using PtIn2
Authors: Ingerly D. B. , Chang Y. A. , Perkins N. R. , Kuech T. F.
Published: July 2002

A new metallization scheme has been developed to form Ohmic contacts to n-GaN. Contacts were fabricated by sputtering the intermetallic compound, PtIn2 on metal–organic vapor phase epitaxy grown n-GaN (n∼5×1017 cm−3) with some of the contacts subjected to rapid thermal annealing. Contacts in the as-deposited state exhibited nearly Ohmic behavior with a specific contact resistance of 1.2×10−2 Ω cm2. Contacts subjected to rapid thermal annealing at 800 °C for 1 min exhibited linear current–voltage characteristics and had specific contact resistances less than 1×10−3 Ω cm2. Auger depth profiling and glancing angle x-ray diffraction were used to examine the interfacial reactions of the PtIn2/n-GaN contacts. Consistent with estimated phase diagram information, the results from Auger depth profiling and glancing angle x-ray diffraction indicated the formation of (InxGa1−x)N at the contact interface, which could be responsible for the Ohmic behavior of PtIn2 contacts.
Nonlinear optical effects in ferrofluids induced by temperature and concentration cross coupling
Authors: Du Tengda , Luo Weili
Published: July 2002

Both experiment and numerical calculation were performed to study the nonlinear optical (NLO) effects in ferrofluids consisting of magnetic particles suspended in an organic solvent. We show that, in contrast to all the mechanisms responsible for NLO phenomena known so far, the novel NLO effects in ferrofluids in the zero applied field can be explained by the cross coupling between the temperature of the system and the concentration of particles through thermophoresis, which in turn leads to the spatial distribution of the refractive index. This NLO effect is enhanced by applying a moderate magnetic field whose mechanism is unclear so far.
Near band-edge transition in aluminum nitride thin films grown by metal organic chemical vapor deposition
Authors: Tang Xiao , Hossain Fazla , Wongchotigul Kobchat , Spencer Michael G.
Published: July 2002

Cathodoluminescence measurements were performed for carbon doped and undoped aluminum nitride thin films in the temperature range from liquid helium to room temperature. The AlN films were grown on three different substrates: 6H–SiC, 4H–SiC, and sapphire. From these samples, a strong luminescence peak surrounded by two weaker peaks in the near band-edge region, near 6 eV, was observed. For AlN on sapphire, this near band-edge transition can be further resolved into three peaks at 6.11, 5.92, and 5.82 eV. These peaks are believed to be due to exciton recombination. The effects of substrate materials and carbon doping on the exciton peak were discussed. The temperature dependence of the peak position and line width of this transition was also studied. The temperature coefficient of the band-gap energy is estimated to be 0.51 meV/K.
Valence-band offsets and Schottky barrier heights of layered semiconductors explained by interface-induced gap states
Authors: Mönch Winfried
Published: July 2002

Many metal chalcogenides are layered semiconductors. They consist of chalcogen–metal–chalcogen layers that are themselves bound by van der Waals forces. Hence, heterostructures involving layered compounds are abrupt and strain-free. Experimental valence-band offsets of heterostructures between GaSe, InSe, SnS2, SnSe2, MoS2, MoTe2, WSe2, and CuInSe2 and between some of these compounds and ZnSe, CdS, and CdTe as well as barrier heights of Au contacts on GaSe, InSe, MoS2, MoTe2, WSe2, ZnSe, CdS, and CdTe are analyzed. The valence-band discontinuities of the heterostructures and the barrier heights of the Schottky contact compounds are consistently described by the continuum of interface-induced gap states as the primary mechanism that governs the band lineup at semiconductor interfaces.
Semiconducting optical properties of silver/silica mesoporous composite
Authors: Cai Weiping , Zhang Ye , Jia Junhui , Zhang Lide
Published: July 2002

The optical absorption properties were investigated for mesoporous silica loaded with different amounts (up to 5% in weight) of silver (Ag) nanoparticles (less than 4 nm in diameter) into its pores. In contrast to the previously reported glasses containing Ag particles, in this Ag particle-loaded silica no apparent surface plasmon resonance peak of Ag particle, other than an absorption edge, was observed. This composite exhibits the optical features of a semiconductor with direct band gap. The absorption edge shifts in the range from about 200 to about 700 nm with an increasing Ag loading amount up to 5% in weight, and hence, is controllable. Adsorption of oxygen in air on the surface of Ag particles within the pores, and scattering inside the porous structure were used to explain these results.
Depth profiling of vacancy clusters in MeV-implanted Si using Au labeling
Authors: Venezia V. C. , Eaglesham D. J. , Haynes T. E. , Agarwal Aditya , Jacobson D. C. , Gossmann H.-J. , Baumann F. H.
Published: July 2002

A technique for profiling the clustered-vacancy region produced by high-energy ion implantation into silicon is described and tested. This technique takes advantage of the fact that metal impurities, such as Au, are trapped in the region of excess vacancies produced by MeV Si implants into silicon. In this work, the clustered-vacancy regions produced by 1-, 2-, and 8-MeV Si implants into silicon have been labeled with Au diffused in from the front surface at 750 °C. The trapped Au was profiled with Rutherford backscattering spectrometry. The dynamics of the clustered-vacancy region were monitored for isochronal annealing at 750–1000 °C, and for isothermal annealing at 950 °C, for 10–600 s. Cross-sectional transmission electron microscopy analysis revealed that after the drive-in anneal, the Au in the region of vacancy clusters is in the form of precipitates. The results demonstrate that the Au-labeling technique offers a convenient and potentially quantitative tool for depth profiling vacancies in clusters.
Micromagnetic tailoring of periodic antidot permalloy arrays for high density storage
Authors: Torres L. , Lopez-Diaz L. , Iñiguez J.
Published: July 2002

A micromagnetic study on antidot permalloy arrays is carried out in order to delimit their possibilities for high density storage. The dependence of the recorded bit quality on antidot size, separation distance, and film thickness is reported as well as hysteresis loops and coercivities. The simulations show a limit for the antidot size around 80 nm leading to maximum areal storage densities of about 10 Gbits/in2. The results are interpreted in terms of demagnetizing, anisotropy, and exchange energies’ balance.
Growth of straight nanotubes with a cobalt–nickel catalyst by chemical vapor deposition
Authors: Sun L. F. , Mao J. M. , Pan Z. W. , Chang B. H. , Zhou W. Y. , Wang G. , Qian L. X. , Xie S. S.
Published: July 2002

In this letter, we report the catalytic synthesis of a large amount of straight carbon nanotubes using a transition-metal cobalt–nickel/zeolite catalyst. High-resolution transmission electron microscopy images show that they are well graphitized. Raman spectrum shows its peak at 1349 cm−1 (D band) is much weaker than that at 1582 cm−1 (G band). We believe that straight carbon nanotubes contain much less defects than curved nanotubes and might have potential applications in the future.
Mechanism for the reduction of interstitial supersaturations in MeV-implanted silicon
Authors: Venezia V. C. , Haynes T. E. , Agarwal Aditya , Pelaz L. , Gossmann H.-J. , Jacobson D. C. , Eaglesham D. J.
Published: July 2002

We demonstrate that the excess vacancies induced by a 1 MeV Si implant reduce the excess interstitials generated by a 40 keV Si implant during thermal annealing when these two implants are superimposed in silicon. It is shown that this previously observed reduction is dominated by vacancy annihilation and not by gettering to deeper interstitial-type extended defects. Interstitial supersaturations were measured using B doping superlattices (DSL) grown on a silicon-on-insulator (SOI) substrate. Implanting MeV and keV Si ions into the B DSL/SOI structure eliminated the B transient enhanced diffusion normally associated with the keV implant. The buried SiO2 layer in the SOI substrate isolates the deep interstitials-type extended defects of the MeV implant, thereby eliminating the possibility that these defects getter the interstitial excess induced by the keV Si implant.
CdTe(111)B grown on Si(111) by molecular beam epitaxy
Authors: Rujirawat S. , Xin Y. , Browning N. D. , Sivananthan S. , Smith David J. , Tsen S.-C. Y. , Chen Y. P. , Nathan V.
Published: July 2002

We demonstrate the growth of large-area, domain-free CdTe(111)B single crystals on As-passivated Si(111) substrates by molecular beam epitaxy using ZnTe buffer layers. The crystal quality of the CdTe(111)B/ZnTe(111)B/Si(111) films was examined by x-ray diffraction (56 arcs), etch-pit-density (2×105 cm−2) analysis, and transmission electron microscopy, and was found to be comparable to or better than the best CdTe(111)B films grown directly on vicinal Si(001). Surface reconstructions were observed by reflection high-energy electron diffraction at different stages. Diffraction intensity oscillations demonstrated the layer-by-layer growth mode of the CdTe surface. An interface model for these films is proposed.
Cavitation-induced nucleation of zirconium in low earth orbit
Authors: Hofmeister W. H. , Bayuzick R. J. , Hyers R. , Trapaga G.
Published: July 2002

Liquid-to-solid nucleation experiments on pure zirconium were performed onboard Spacelab. No evidence for fluid flow effects on nucleation was found for maximum flows from 5 to 43 cm s−1. Evidence for fluid flow effects on nucleation was found at flow velocities above 50 cm s−1. At this flow velocity and above, the dynamic pressure in the drop exceeds the static pressure, a condition that is known to provide a driving force for cavitation in fluids. Under these conditions undercooling was severely limited.
Tensile tests of ropes of very long aligned multiwall carbon nanotubes
Authors: Pan Z. W. , Xie S. S. , Lu L. , Chang B. H. , Sun L. F. , Zhou W. Y. , Wang G. , Zhang D. L.
Published: July 2002

We have directly measured the Young’s modulus and tensile strength of multiwall carbon nanotubes by pulling very long (∼2 mm) aligned nanotube ropes with a specially designed stress-strain puller. This puller can apply an axial force to the rope and simultaneously measure the corresponding rope elongation and the change in rope resistance. The average Young’s modulus and tensile strength obtained were 0.45±0.23 TPa and 1.72±0.64 GPa, respectively, which are lower than those calculated and measured previously. The factors that affect the mechanical strengths of nanotubes are discussed.
Excitons bound to nitrogen clusters in GaAsN
Authors: Francoeur S. , Nikishin S. A. , Jin C. , Qiu Y. , Temkin H.
Published: July 2002

We studied the photoluminescence from GaAsN/GaAs, with the nitrogen content of less than 0.5%. The low-temperature photoluminescence spectra are composed of several excitons bound to nitrogen complexes, each associated with different composition or configuration. These features were studied as a function of the excitation intensity, temperature, concentration, and growth conditions. The dependence of the binding energy of the dominant recombination center on the nitrogen concentration is interpreted in terms of a hierarchy of nitrogen complexes, from centers composed of at least two nitrogen atoms to more extended clusters. These excitonic transitions are very sensitive to growth parameters and can be used to study the statistical distribution of nitrogen in nominally uniform layers. We also show that the transition from nitrogen doping to alloy formation occurs for nitrogen concentrations above 0.25%.
Polarization-enhanced Mg doping of AlGaN/GaN superlattices
Authors: Kozodoy Peter , Smorchkova Yulia P. , Hansen Monica , Xing Huili , DenBaars Steven P. , Mishra Umesh K. , Saxler A. W. , Perrin R. , Mitchel W. C.
Published: July 2002

The hole-transport properties of Mg-doped AlGaN/GaN superlattices are carefully examined. Variable-temperature Hall-effect measurements indicate that the use of such superlattices enhances the average hole concentration at a temperature of 120 K by over five orders of magnitude compared to a bulk GaN film (the enhancement at room temperature is a factor of 9). An unusual modulation-doping scheme, which has been realized using molecular-beam epitaxy, has yielded high-hole-mobility superlattices and conclusively demonstrated the pivotal role of piezoelectric and spontaneous polarization in determining the band structure of the superlattices.
Sum-frequency generation with a blue diode laser for mercury spectroscopy at 254 nm
Authors: Alnis J. , Gustafsson U. , Somesfalean G. , Svanberg S.
Published: July 2002

Blue diode lasers emitting 5 mW continuous-wave power around 400 nm have recently become available. We report on the use of a blue diode laser together with a 30 mW red diode laser for sum-frequency generation around 254 nm. The ultraviolet power is estimated to be 0.9 nW, and 35 GHz mode-hop-free tuning range is achieved. This is enough to perform high-resolution ultraviolet spectroscopy of mercury isotopes. The possibility to use frequency modulation in the ultraviolet is demonstrated; however, at present the ultraviolet power is too low to give advantages over direct absorption monitoring. Mercury detection at atmospheric pressure is also considered which is of great interest for environmental monitoring.
Cerenkov generation of high-frequency confined acoustic phonons in quantum wells
Authors: Komirenko S. M. , Kim K. W. , Demidenko A. A. , Kochelap V. A. , Stroscio M. A.
Published: July 2002

We analyze the Cerenkov emission of high-frequency confined acoustic phonons by drifting electrons in a quantum well. We find that the electron drift can cause strong phonon amplification (generation). A general formula for the gain coefficient α is obtained as a function of the phonon frequency and the structure parameters. The gain coefficient increases sharply in the short-wave region. For the example of a Si/SiGe/Si device, it is shown that the amplification coefficients of the order of hundreds of cm−1 can be achieved in the subterahertz frequency range.
Quantification of excess vacancy defects from high-energy ion implantation in Si by Au labeling
Authors: Kalyanaraman R. , Haynes T. E. , Venezia V. C. , Jacobson D. C. , Gossmann H.-J. , Rafferty C. S.
Published: July 2002

It has been shown recently that Au labeling [V. C. Venezia, D. J. Eaglesham, T. E. Haynes, A. Agarwal, D. C. Jacobson, H.-J. Gossmann, and F. H. Baumann, Appl. Phys. Lett. 73, 2980 (1998)] can be used to profile vacancy-type defects located near half the projected range (12 Rp) in MeV-implanted Si. In this letter, we have determined the ratio of vacancies annihilated to Au atoms trapped (calibration factor “k”) for the Au-labeling technique. The calibration experiment consisted of three steps: (1) a 2 MeV Si+ implant into Si(100) followed by annealing at 815 °C to form stable excess vacancy defects; (2) controlled injection of interstitials in the 12 Rp region of the above implant via 600 keV Si+ ions followed by annealing to dissolve the {311} defects; and (3) Au labeling. The reduction in Au concentration in the near-surface region (0.1–1.6 μm) with increasing interstitial injection provides the most direct evidence so far that Au labeling detects the vacancy-type defects. By correlating this reduction in Au with the known number of interstitials injected, it was determined that k=1.2±0.2 vacancies per trapped Au atom.
Laser emission from quantum dots in microdisk structures
Authors: Michler P. , Kiraz A. , Zhang Lidong , Becher C. , Hu E. , Imamoglu A.
Published: July 2002

We report optically pumped continuous-wave lasing from self-assembled InAs and InGaAs quantum dots (QDs) embedded in high-quality-factor microdisk laser structures. The microdisk emission spectra show lasing on 1–5 well separated modes in the wavelength range between 900 and 990 nm. The estimated threshold pump densities are between 20 and 200 W/cm2. The lasing characteristics are discussed in terms of both inhomogeneously and homogeneously broadened QD transitions.
Electric field and plasma emission responses in a low pressure positive column discharge exposed to a low Mach number shock wave
Authors: Bletzinger P. , Ganguly B. N. , Garscadden A.
Published: July 2002

Low Mach number shock waves propagating through a low pressure, nonequilibrium positive column gas discharge have been observed to experience dispersion and velocity changes. It is shown that these effects depend on discharge polarity. Optical and electrical measurements are described which show further polarity-dependent effects in discharge light emission and changes in electrical properties. Using two types of probes, electrical measurements were made of both the global changes in discharge voltage and current and time resolved local electric field changes. The measured behaviors of discharge and shock wave point to very localized triple or quadruple layer electric sheaths connected with the propagating shock wave, which provide local enhanced ionization at the shock front which can sustain the discharge, at least during the short shock propagation time. The postulated density gradient driven large local recirculation current in the potential minimum near this sheath [H. S. Maciel and J. E. Allen, J. Plasma Phys. 42, 321 (1989)] at the shock front will result in large local Joule heating, causing the shock dispersion and shock speed increase, which have been observed in many experiments. Thus, the conclusion is that the effects are due to highly localized gas heating which is facilitated by the response of the positive column plasma to the acoustic shock.
Performance of a very high resolution soft x-ray beamline BL25SU with a twin-helical undulator at SPring-8
Authors: Saitoh Y. , Kimura H. , Suzuki Y. , Nakatani T. , Matsushita T. , Muro T. , Miyahara T. , Fujisawa M. , Soda K. , Ueda S. , Harada H. , Kotsugi M. , Sekiyama A. , Suga S.
Published: July 2002

We report on the excellent performance of a newly constructed soft x-ray helical undulator beamline BL25SU of SPring-8 for photon energies 500–1800 eV. The full beamline was designed to perform very high resolution soft x-ray spectroscopy of solids with using high brilliance, highly circularly polarized undulator radiation. The grazing incidence monochromator employs varied-line-spacing plane gratings which operate in convergent light from a spherical mirror and focuses monochromatic light onto the exit slit. A resolving power in excess of 15 000 was measured at 540 and 870 eV for a grating with a central groove density of 600 lines/mm from the photoemission spectra of Au. A resolving power of more than 20 000 is estimated near 870 eV for a grating with a central groove density of 1000 lines/mm. A photon flux of more than 1×1011 photons/s/100 mA/0.02% b.w. is supplied onto the sample between 500 and 1800 eV with very low amount of higher-order light. The low heat load from the twin-helical undulator gives high stability to all optical components, which is essential to obtain high resolution in a wide energy region. Three experimental stations are installed in tandem on this beamline for various high resolution spectroscopy measurements.
Interchange modes in a collisional plasma
Authors: Kesner J.
Published: July 2002

Gross plasma stability can derive from plasma compressibility in the bad curvature regions of closed field line systems. In this situation magnetohydrodynamic (MHD) theory predicts that the maximum pressure gradient that is stable is proportional to γ, the ratio of specific heats. This article will examine the accuracy of the MHD prediction for electrostatic interchange modes using kinetic theory. The maximum sustainable pressure gradient is found to be dependent on the ratio of the temperature and density gradients (η≡(n/T)(∇T/∇n)) as well as on the ion gyro-radius scale length. For η=2/3 the MHD stability condition is reproduced. When η deviates from 2/3 the mode changes character and the stability criterion becomes more stringent.
Influence of impurities on the optical damage of KTiOPO4 crystals
Authors: Zaldo C. , Carvajal J. , Solé R. , Dı́az F. , Bravo D. , Kling A.
Published: July 2002

Optical damage has been induced in doped c-cut KTiOPO4 (KTP) crystals by low intensity (I0⩽0.1 W/cm2) continuous-wave illumination with ultraviolet (λ=364 nm) light. The presence of two growth sectors with different sensibility to optical damage has been observed and it is related to their different impurity concentrations. Impurities shift the optical absorption edge of KTP to lower energy and make the induced damage more stable at room temperature. The damage saturation depends on the ultraviolet power density and on the impurity concentration. It has also been observed that some impurities (W and others) change their valence due to ultraviolet irradiation. A dynamic model considering the excitation of electrons from donors and their release from traps is proposed. The model allows us to describe the main features of the optical damage and erasure kinetics.
The meaning of the irreducible memory function in stochastic theories of dynamics with detailed balance
Authors: Pitts Steven J. , Andersen Hans C.
Published: July 2002

We investigate the relationship between the memory functions that arise in stochastic theories of fluctuations at equilibrium and those appropriate for an underlying microscopic (deterministic) description. We consider the class of stochastic theories that are Markovian with transition rates that satisfy the detailed balance condition. This class includes, for example, Smoluchowski dynamics, kinetic lattice gas models, and kinetic Ising models. When a time autocorrelation function is calculated using stochastic and deterministic descriptions, and the projection operator method of Mori is used, first and second order memory functions arise in both descriptions. We find a close and simple relationship between the first order memory functions of the two descriptions but not for the second order memory functions. Instead, the second order memory function of the microscopic description is simply related to the so-called irreducible memory function of the stochastic description. The latter was introduced for Smoluchowski dynamics by Cichocki and Hess and generalized by Kawasaki. This explains the empirical findings that for stochastic dynamics the irreducible memory function, rather than the second order memory function, has a more fundamental physical interpretation and is more useful for constructing mode coupling theories.
Some features of chaos and oscillations in plasma-filled diodes
Authors: Chen X. , Lindsay P. A. , Zhang J.
Published: July 2002

The physical background concerning the existence of four dynamical states (stable, oscillatory, chaotic and unstable) in a plasma-filled diode has been investigated in some detail, the results being presented with the help of charge density and other three-dimensional graphs. It has been established that the determining factor is a dynamic ions/electrons charge-balance in the interelectrode space.
High resolution transmission electron microscopy of elevated temperature Zn+ implanted and low-power pulsed laser annealed GaAs
Authors: Zollo G. , Pizzuto C. , Vitali G. , Kalitzova M. , Manno D.
Published: July 2002

Cross-sectional high resolution transmission electron microscopy was applied to 140 keV Zn+ implanted GaAs at elevated temperature (110±10 °C). Gaussian-like in-depth distributions of damage clusters, retaining some features of the original crystal lattice, were observed. The distribution maximum was found located between about 55 and 80 nm below the implanted surface. Low-power pulsed-laser annealing of the implanted samples induced both migration and clustering of radiation defects in the region extending from the surface and 80 nm depth, combined with nearly complete recrystallization of the material below this layer.
Kinetics of activation of group V impurities in Hg1−xCdxTe alloys
Authors: Schaake H. F.
Published: July 2002

A kinetic model is developed for the activation of group V impurities in Hg1xCdxTe alloys as acceptors. The model assumes that mercury interstitials are introduced at the surface and diffuse into the alloy. There, they displace group V impurities residing on the metal sublattice and place them on the tellurium sublattice, generating tellurium interstitials. These tellurium interstitials then diffuse back to the top surface, or to climbing dislocations. The rate-controlling process is the out-diffusion of the tellurium interstitials. A key finding is that the conversion rate is inversely proportional to the impurity concentration. The model is found to be in good agreement with published data for the activation of arsenic.
Composition effects on structural and optical infrared properties of CuIn0.5Ga0.5Se2
Authors: Dı́az R. , Martı́n T. , Merino J. M. , León M. , Martı́n de Vidales J. L. , Rueda F.
Published: July 2002

The dependence of structural parameters and force constants for Cu–Se, Ga–Se, and In–Se bonds on compositional deviations in CuIn0.5Ga0.5Se2 have been studied. The composition gradient along the ingot was obtained by a single fusion at 1150 °C of the components and subsequent slow cooling in a still ampoule placed in a vertical furnace. All along the sample, a single chalcopyrite phase is present and its composition along its length was found by energy dispersive analysis of x-ray measurements on slices. Unit cell parameters, anion displacement, and Cu occupation fraction in its sublattice were analyzed by x-ray powder diffraction and Rietveld refinement methods. The anion displacement found is a function of the Cu defect in its sublattice. The existence of associated defects, i.e., two Cu vacancies and one Ga in Cu site, [2V(Cu)+GaCu], is proposed to explain the Cu defect in its sublattice and the changes in lattice parameters. This leads to the existence of BIII vacancies (BIII=In+Ga), and interstitial Cu up to 8 at. % that also cause changes in the structural parameters. Infrared reflectance measurements led to the imaginary dielectric constant determination which, fitted to a Lorentz function, permitted to obtain atomic vibration modes. Using the model of Neumann for chalcopyrites, the values of force constants for Cu–Se, Ga–Se, and In–Se bonds were computed. These appear to increase when the occupation of each sublattice increases.
Electron spin resonance centers associated with oxygen precipitates in Czochralski silicon crystals
Authors: Koizuka M. , Yamada-Kaneta H.
Published: July 2002

We have previously concluded that the oxygen-precipitate-associated defects that we identified by the deep levels at Ev+0.30 eV and Ec−0.25 eV were the Pb centers generated in the interface between the oxygen precipitates and the surrounding silicon crystal lattice [M. Koizuka and H. Yamada-Kaneta, J. Appl. Phys. 84, 4255 (1998)]. In order to confirm this conclusion, we have made electron spin resonance (ESR) measurements on the Czochralski-grown silicon crystals containing the oxygen precipitates generated by the two-step anneals of 500 °C, 20 h+700 °C, 60 h. We have found the ESR lines whose g values well coincide with those of the Pb0 and Pb1 centers. Thus it has been clarified that the Pb0 and Pb1 centers are generated by the oxygen precipitation as well as by the thermal oxidation. The present successful application of the ESR method to the annealed silicon crystals suggests that it can be a useful tool to characterize the precipitation state of the oxygen in silicon.
The field-assisted emission effect on the acceptor energy of GaN:Mg in admittance spectroscopy measurements
Authors: Kim D. J.
Published: July 2002

A simulation program for admittance calculations in a semiconductor Schottky junction with a trap was developed using physical parameters including the Poole–Frenkel electric field-assisted emission effect. The simulation revealed limitations in the validity of the admittance measurement technique in determining thermal activation energies of semiconductors. The controversy in the observed thermal activation energy of the GaN:Mg structures between the Hall effect and admittance spectroscopy measurements is explained using the simulation results. The admittance measurement technique can only determine the real energies when the overall doping concentration of the shallow (NS) and deep (NT) levels is low and while NT⩽NS. While the condition of NT⩾NS causes a reduction in the apparent measured energies, the electric field built up at the crossover by NS or/and NT further reduces the energies by lowering the barrier potential for the trap emission. Particularly if NT≫NS, the electric field induced by the deep level itself induces the reduction in the barrier potential. This is the case for the rather highly doped GaN:Mg structures.
Comment on “Thermal nonequilibrium of the shock-compressed state of polymers realized by 1 GPa shock waves” [J. Appl. Phys. 84, 6592 (1998)]
Authors: Rosenberg Z.
Published: July 2002

In a recent article, Nagayama and Mori [J. Appl. Phys. 84, 6592 (1998)] arrived at several conclusions concerning the thermodynamic nature of shock loaded plexiglass in the low stress range. Their departure point is the kink in the shock velocity-particle velocity curve for this material from which they obtain kinks in the temperature versus shock stress and Gruneisen parameter versus compression curves. These results are in sharp contrast with our conclusions concerning these entities which were published in the early 80’s. Our experimental work relied on measuring the temperature of the shock loaded plexiglass using very thin copper thermistors embedded in the specimens. This technique and our main results are reviewed here.