bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth-sector dependence of fine structure in the first-order Raman diamond line from large isolated chemical-vapor-deposited diamond crystals
Authors: Stuart S.-A. , Prawer S. , Weiser P. S.
Published: July 2002

The growth-sector dependence of fine structure in the first-order Raman diamond line is investigated for the first time in high resolution spectra taken from large isolated diamond single crystals grown using microwave plasma chemical vapor deposition on tungsten wire tips. The volumes of crystal beneath the (100) and (111) surfaces of these crystals were sampled using a high resolution Raman microprobe from which the line shape of the 1332 cm−1 diamond line was found to be distinctly different. A splitting of the diamond line into two components of up to 7 cm−1 was observed for (100) growth sectors. This splitting may be caused by the buildup of directional strain fields caused by the different fundamental growth processes occurring on the (100) and (111) growth surfaces. An additional third peak near 1326 cm−1 was only observed in (111) growth sectors and may be attributable to the presence of stacking faults.
Microscopic studies of semiconductor lasers utilizing a combination of transmission electron microscopy, electroluminescence imaging, and focused ion beam sputtering
Authors: Hull R. , Bahnck D. , Stevie F. A. , Koszi L. A. , Chu S. N. G.
Published: July 2002

The microstructure of semiconductor laser diodes is studied using a combination of focused ion beam sputtering, electroluminescence imaging, and cross-sectional transmission electron microscopy. Careful control of focused ion beam sputtering allows fabrication of high quality thin membranes for transmission electron microscope imaging, which can be located to submicron accuracy at a given position on the laser active stripe. By correlation with electroluminescence imaging, the membrane may then be positioned at an optically degraded region of the active stripe. In addition, imaging of the complete cross-sectional laser structure, from substrate to surface contact layers is possible. The applications of these techniques to studies of laser degradation mechanisms are demonstrated and discussed.
Near-field scanning optical microscopy imaging of individual threading dislocations on relaxed GexSi1−x films
Authors: Hsu J. W. P. , Fitzgerald E. A. , Xie Y. H. , Silverman P. J.
Published: July 2002

We have identified individual threading dislocations on relaxed, compositionally graded GexSi1−x structures through simultaneous imaging of topography and photoresponse. The threading dislocations show shallow depressions in surface morphology and a 5%–10% reduction in photoresponse compared with the defect-free regions. The average spatial extent of the reduced photoresponse is (0.66±0.16) μm, which is larger than the associated morphology. This study further demonstrates the application of near-field scanning optical microscopy to characterization of electrically active defects. The spatial resolution is ten times higher than conventional optical techniques.
Application of a position-sensitive detector to atom probe microanalysis
Authors: Cerezo A. , Godfrey T. J. , Smith G. D. W.
Published: July 2002

A position-sensitive detector system based on a wedge-and-strip anode has been used to build a short flight-path atom probe which identifies both the chemical nature and position of single atoms field evaporated from the surface of a field-ion specimen. The detector also allows digitized field-ion images to be obtained from the region being analyzed. The prototype instrument has a lateral resolution during analysis of substantially below 1 nm, and a depth resolution of one atomic layer. Initial applications of the instrument to the analysis of nanometer-scale precipitates in metallic alloys has shown the capability of reconstructing the three-dimensional microstructure and microchemistry of materials.
Large dynamic range, parallel detection system for electron diffraction and imaging
Authors: Spence J. C. H. , Zuo J. M.
Published: July 2002

A parallel detection system for electrons in the 10–800-kV energy range is described. The design is based on a 576×382 pixel charge-coupled device array operating at liquid-nitrogen temperature. This is coupled by fiber optics to a single-crystal YAG screen, which serves as the primary detection element. The performance of the system for quantitative electron diffraction and imaging studies is described. The system will record a maximum count of 16 000 per pixel with low noise at a frame rate of 2.5 frames per second.
Generation of diamond nuclei on amorphous SiO2 by alternating-current bias microwave plasma chemical vapor deposition
Authors: Mao M. Y. , Jin X. F. , Wang T. P. , Xie J. F. , Tan S. S. , Wang W. Y. , Zhang X. K. , Zhuang Z. C.
Published: July 2002

The nucleation of diamond on the amorphous SiO2 mirror surface has been achieved by means of adding an ac signal to the negative dc bias in the microwave plasma chemical vapor deposition. It is found in experiment that the nucleation of diamond happens only after the frequency of the ac signal exceeds a threshold. The results also show that the diamond nucleation density depends not only on the ac frequency but also on the magnitudes of the ac signal and the dc bias.
A gold/platinum thermocouple reference table
Authors: Gotoh M. , Hill K. D. , Murdock E. G.
Published: July 2002

The thermoelectric voltage of the gold/platinum thermocouple has been measured over the range 0–962 °C by comparison with calibrated platinum resistance thermometers. From 0 to 500 °C, the measurements were carried out in stirred liquid baths; from 660 to 964 °C, they were carried out in a pressure-controlled sodium-filled heat pipe furnace that provided an isothermal intercomparison environment. Measurements were also made in the metal freezing points of indium, tin, zinc, antimony, and silver, at the melting point of gallium, and at the liquidus point of the silver-copper eutectic. By fitting the measured thermoelectric voltages to a single eighth-degree polynomial in temperature by the method of least squares, a reference function is obtained for the Au/Pt thermocouple that provides emf as a function of temperature (ITS-90) to within ±10 mK from 0 to 962 °C. The Au/Pt thermocouple merits serious consideration for precise temperature measurements as its stability approaches that of the high-temperature platinum resistance thermometer.
Production of intense micrometer-sized x-ray beams with tapered glass monocapillaries
Authors: Thiel Daniel J. , Bilderback Donald H. , Lewis Aaron
Published: July 2002

Methods were developed to characterize tapered capillaries as x-ray concentrators capable of forming spots of intense x-ray radiation with micrometer diameters. These tapered capillaries, with somewhat controlled tapers, were produced using a gravity-based capillary puller. A device was constructed to microscopically inspect these capillaries along two orthogonal axes in order to accurately measure the tapering and bending. Both monochromatic and white hard x rays were concentrated with a variety of tapered capillaries, and the subsequent gains in intensity (flux/area) ranging from 14 to 35 are reported. Using these unique x-ray concentrators, a simple high-powered x-ray fluorescence microscope was constructed and tested. We also found that hard x-ray beams could be successfully steered by bending the capillary tip with radii as small as 5 m. In addition, preliminary ray-tracing results obtained from a two-dimensional ray-tracing program are described.
Large-aperture TV detector with a beryllium-windowed image intensifier for x-ray diffraction
Authors: Amemiya Y. , Ito K. , Yagi N. , Asano Y. , Wakabayashi K. , Ueki T. , Endo T.
Published: July 2002

A large-aperture (150 mm and 230 mm in diameter) x-ray TV-type detector has been developed for x-ray diffraction with synchrotron radiation. The detector consists of a beryllium-windowed x-ray image intensifier, an optical lens, a charge coupled device (CCD) image sensor, and data acquisition system. The spatial resolution is 270 μm(FWHM), and the dynamic range is 6000:1. The noise level is quantum limited. The nonuniformity of response and image distortion is corrected by software. When a TV-rate (NTSC-mode) CCD is used as an image sensor, time-resolved measurements with a rate of 30 frame/s can be achieved with its noise quantum limited.
Apparatus for measuring viscoelastic properties over ten decades: Refinements
Authors: Brodt M. , Cook L. S. , Lakes R. S.
Published: July 2002

This paper describes refinements to an instrument for determining the viscoelastic properties of a solid material isothermally, with a single apparatus, over 10 decades of time and frequency. Torque is applied electromagnetically to a specimen fixed at one end. Specimen deformation is determined via a laser beam reflected from the other end upon a split diode detector. Phase resolution is improved by the use of a lock-in amplifier at high frequency and by the use of Lissajous figures to measure phase, allowing the study of materials of moderate loss (0.008≤ tan δ≤0.2) in addition to materials with high loss (tan δ≊1). The rigidity of the instrument is increased by modifications in the specimen support geometry. The range of equivalent frequency for torsion is from less than 10−6 Hz to more than 104 Hz. Digital methods are incorporated in the creep measurements and in the phase measurements.
A tangentially viewing visible TV system for the DIII-D divertor
Authors: Fenstermacher M. E. , Meyer W. H. , Wood R. D. , Nilson D. G. , Ellis R. , Brooks N. H.
Published: July 2002

A video camera system has been installed on the DIII-D tokamak for two-dimensional spatial studies of line emission in the lower divertor region. The system views the divertor tangentially at approximately the height of the X point through an outer port. At the tangency plane, the entire divertor from the inner wall to outside the DIII-D bias ring is viewed with spatial resolution of ∼1 cm. The image contains information from ∼90 deg of toroidal angle. In a recent upgrade, remotely controllable filter changers were added which have produced images from nominally identical discharges using different spectral lines. Software was developed to calculate the response function matrix of the optical system using distributed computing techniques and assuming toroidal symmetry. Standard sparse matrix algorithms are then used to invert the three-dimensional images onto a poloidal plane. Spatial resolution of the inverted images is 2 cm; higher resolution simply increases the size of the response function matrix. Initial results from a series of experiments with multiple identical discharges show that the emission from C II and C III, which appears along the inner scrape-off layer above and below the X point during ELMing H mode, moves outward and becomes localized near the X point in radiative divertor operation induced by deuterium injection.
Energy resolution and high count rate performance of superconducting tunnel junction x-ray spectrometers
Authors: Frank M. , Hiller L. J. , le Grand J. B. , Mears C. A. , Labov S. E. , Lindeman M. A. , Netel H. , Chow D. , Barfknecht A.T.
Published: July 2002

We present experimental results obtained with a cryogenically cooled, high-resolution x-ray spectrometer based on a 141 μm×141 μm Nb-Al-Al2O3-Al-Nb superconducting tunnel junction (STJ) detector in a demonstration experiment. Using monochromatized synchrotron radiation we studied the energy resolution of this energy-dispersive spectrometer for soft x rays with energies between 70 and 700 eV and investigated its performance at count rates up to nearly 60 000 cps. At count rates of several 100 cps we achieved an energy resolution of 5.9 eV (FWHM) and an electronic noise of 4.5 eV for 277 eV x rays (the energy corresponding to C K). Increasing the count rate, the resolution 277 eV remained below 10 eV for count rates up to ∼10 000 cps and then degraded to 13 eV at 23 000 cps and 20 eV at 50 000 cps. These results were achieved using a commercially available spectroscopy amplifier with a baseline restorer. No pile-up rejection was applied in these measurements. Our results show that STJ detectors can operate at count rates approaching those of semiconductor detectors while still providing a significantly better energy resolution for soft x rays. Thus STJ detectors may prove very useful in microanalysis, synchrotron x-ray fluorescence (XRF) applications, and XRF analysis of light elements (K lines) and transition elements (L lines).
A high resolution long travel friction-drive micropositioner with programmable step size
Authors: Chang S. H. , Li S. S.
Published: July 2002

A piezoelectric-driven Scott–Russel linear micropositioner utilizing the stick-slip effect of friction to drive a slider is presented. Effects of sawtooth, impulse, and transcendental electrical wave forms on the device performance are studied via numerical simulation and experiment test. The experiment demonstrates that positioning step sizes of 0.05–120 μm can be achieved at low input voltages of 2–25 V and essentially with unlimited travel range.
Photoluminescence studies of self-assembled InSb, GaSb, and AlSb quantum dot heterostructures
Authors: Glaser E. R. , Bennett B. R. , Shanabrook B. V. , Magno R.
Published: July 2002

Photoluminescence (PL) spectroscopy has been performed on a set of self-assembled InSb, GaSb, and AlSb quantum dot (QD) heterostructures grown on GaAs. Strong emission bands with peak energies near 1.15 eV and linewidths of ∼80 meV are observed at 1.6 K from 3 monolayer (ML) InSb and GaSb QDs capped with GaAs. The PL from a capped 4 ML AlSb QD sample is weaker with peak energy at 1.26 eV. The PL bands from these Sb-based QD samples shift to lower energy by 20–50 meV with decreasing excitation power density. This behavior suggests a type II band lineup. Support for this assignment, with electrons in the GaAs and holes in the (In,Ga,Al)Sb QDs, is found from the observed shift of GaSb QD emission to higher energies when the GaAs barrier layers are replaced by Al0.1Ga0.9As.
Superlattices of PbZrO3 and PbTiO3 prepared by multi-ion-beam sputtering
Authors: Kanno I. , Hayashi S. , Takayama R. , Hirao T.
Published: July 2002

Artificial superlattices consisting of antiferroelectric PbZrO3 (PZO) and ferroelectric PbTiO3 (PTO) have been fabricated by a multi-ion-beam sputtering technique. The epitaxial PZO and PTO layers were sequentially grown on (100)Pt/MgO substrates at a low substrate temperature of 415 °C with a periodicity from 5 to 100 perovskite unit cells. X-ray diffraction studies revealed the superlattice structures with a-axis oriented PTO layers on a-axis oriented PZO layers. The dielectric and ferroelectric properties of the superlattice films were enhanced with increasing periodicity.
Ge segregation during the initial stages of Si1−xGex alloy growth
Authors: Jernigan Glenn G. , Thompson Phillip E. , Silvestre Conrad L.
Published: July 2002

Segregation of Ge in the ‘‘leading edge’’ of Si1−xGex alloys grown by molecular beam epitaxy is investigated using x-ray photoelectron spectroscopy (XPS). Alloys of 5%, 10%, 20%, and 40% Ge were grown in varying thickness (0–20 nm) at 500 °C to observe segregation during the initial stages of alloy growth. The length of the leading edge was found to decrease with increasing Ge concentration (4.8, 2.8, 2.4, and 2.0 nm, respectively). The amount of segregated Ge was found to increase with Ge concentration. A complete monolayer of Ge was found on the surface for all Ge concentrations and an increasing amount of Ge (20%, 55%, 80%, and 95%, respectively) was found in the second atomic layer.
X-ray detection using a superconducting transition-edge sensor microcalorimeter with electrothermal feedback
Authors: Irwin K. D. , Hilton G. C. , Wollman D. A. , Martinis John M.
Published: July 2002

We have developed a new type of x-ray detector based on a superconducting transition-edge thermometer operated near 100 mK. A superconducting quantum interference device is used to measure the current through the thermometer, and negative electrothermal feedback is used to improve the energy resolution and shorten the thermal time constant. We have used a detector mounted on a scanning electron microscope to measure the energy of titanium Kα (4.5 keV) fluorescence x rays with a resolution better than 14 eV full width at half-maximum. Using two other devices, we have measured an energy resolution for Joule heat pulses of 2.6 eV at 1 keV and 0.2 eV at 4 eV, the best reported for any calorimeter. An electrical noise equivalent power of 3×10−18 W/√Hz was also measured, suggesting the use of these detectors as infrared bolometers.
Monolithic polycapillary focusing optics and their applications in microbeam x-ray fluorescence
Authors: Gao N. , Ponomarev I. Yu. , Xiao Q. F. , Gibson W. M. , Carpenter D. A.
Published: July 2002

Monolithic polycapillary focusing optics can collect a large solid angle of x rays from a point source and form intense focused beams for microbeam x-ray fluorescence (MXRF) analysis. Such an optic was tested and characterized in a MXRF setup. For the Cu Kα line, the measured focal spot size of the optic was 43 μm full width at half-maximum (FWHM). With the 16 W microfocusing x-ray source (50 μm×10 μm), the average Cu Kα intensities over the focal spot was measured to be 8.9×104 photons/s/μm2. When compared to a straight single channel capillary optic with approximately the same output beam size, an intensity gain of 44 was obtained. The optic was applied to the MXRF setup to analyze trace elements in various samples and a minimum detection limit (MDL) of about 2 pg was achieved for the transition elements (V, Cr, Mn, and Fe). The optic was also used to map the distributions of trace elements in various samples.
Fermi-level pinning position at the Au–InAs interface determined using ballistic electron emission microscopy
Authors: Bhargava S. , Blank H.-R. , Narayanamurti V. , Kroemer H.
Published: July 2002

Ballistic electron emission microscopy (BEEM) has been used to determine the Fermi-level pinning position at the Au/InAs interface. Using BEEM’s three-terminal capabilities, collector current–voltage scans were taken on Au/InAs/AlSb samples. The extracted BEEM threshold values (1.22 eV) correspond to the highest energy band position in the conduction band at the InAs/AlSb interface. By subtracting the InAs/AlSb conduction-band offset (1.35 eV), an estimate of the Au Fermi-level position on InAs is obtained (0.13 eV).
Influence of SrTiO3 bicrystal microstructural defects on YBa2Cu3O7 grain-boundary Josephson junctions
Authors: McDaniel E. B. , Gausepohl S. C. , Li C.-T. , Lee Mark , Hsu J. W. P. , Rao R. A. , Eom C. B.
Published: July 2002

Using near-field scanning optical microscopy (NSOM), we observe an inhomogeneous distribution of submicron-sized structural defects at the fusion boundary of polished SrTiO3 bicrystal substrates. Both NSOM and scanning force microscopy show that these substrate defects cause the grain boundary of a YBa2Cu3O7 thin film grown on the bicrystal to wander up to a micron in the film. These structural defects are shown to correlate qualitatively with the electrical characteristics of grain-boundary Josephson junctions patterned on the YBa2Cu3O7 film.
Oxygen surface diffusion in three-dimensional Cu2O growth on Cu(001) thin films
Authors: Yang J. C. , Yeadon M. , Kolasa B. , Gibson J. M.
Published: July 2002

By studying the growth of Cu2O islands during the initial oxidation stage of Cu(001) with in situ transmission electron microscopy, it is found that the dominant mechanism for the growth of three-dimensional islands is surface diffusion of oxygen. However, there exists a non-negligible contribution to the metal oxide growth by another mechanism, probably direct impingement of the oxygen atoms on the oxide island. These results demonstrate the importance of surface conditions in oxidation.
Observation of vertical and lateral Ge segregation in thin undulating SiGe layers on Si by electron energy-loss spectroscopy
Authors: Walther T. , Humphreys C. J. , Cullis A. G.
Published: July 2002

The local Ge composition in undulating Si0.8Ge0.2 layers on Si has been studied in a scanning transmission electron microscope using electron energy-loss spectroscopy. We observe Ge enrichment of the SiGe layer near the free surface (vertical Ge segregation in the growth direction) as well as Ge depletion of the ripple troughs compared to the peaks (lateral segregation). These lateral compositional fluctuations are likely to retard the generation of misfit dislocations and might be relevant to the Stranski–Krastanov growth of strained epitaxial alloy layers as well as to the self-organized growth of quantum dot structures.
Defect structure in selectively grown GaN films with low threading dislocation density
Authors: Sakai Akira , Sunakawa Haruo , Usui Akira
Published: July 2002

We have characterized by transmission electron microscopy (TEM) defect structures in GaN films grown selectively in hydride vapor-phase epitaxy (HVPE). In this experiment, growth was achieved on SiO2-stripe-patterned GaN layers that had been grown by metalorganic vapor-phase epitaxy (MOVPE) on sapphire substrates. Cross-sectional TEM revealed unambiguously that most of the dislocations, which originated from threading dislocations vertically aligned in the MOVPE-grown layer, propagated laterally around the SiO2 mask in the HVPE-grown film before the film thickness amounted to about 5 μm. This change of the propagation direction prevented the dislocations from crossing the film to the surface region and thus principally led to a drastic reduction in the threading dislocation density in thicker films.
Optical and terahertz power limits in the low-temperature-grown GaAs photomixers
Authors: Verghese S. , McIntosh K. A. , Brown E. R.
Published: July 2002

Optical heterodyne conversion, or photomixing, occurs in an epitaxial low-temperature-grown GaAs layer with voltage-biased metal electrodes on which two laser beams are focused with their frequencies offset by a desired difference frequency. Difference-frequency power couples out of the photomixer through a log-spiral antenna at THz frequencies. Pumping such a device with the maximum optical power of ∼90 mW at 77 K led to a measured output power of 0.2 μW at 2.5 THz, approximately twice the maximum output power of a photomixer operated near 300 K. Photomixers that were operated above the maximum optical power were destroyed, often because of a thermally induced fracture in the GaAs substrate. The fracture seemed to occur at high pump power when the temperature of the photomixer active area was elevated by roughly 110 K, independent of the bath temperature.
Microstructure of epitaxial SrRuO3 thin films on (001) SrTiO3
Authors: Jiang J. C. , Pan X. Q. , Chen C. L.
Published: July 2002

Metallic oxide films of SrRuO3 deposited on (001) SrTiO3 by pulsed laser deposition have been investigated by transmission electron microscopy (TEM) techniques. These films have a single crystalline structure with an extremely smooth surface. A TEM study of cross-sectional samples shows that the film grew epitaxially on the (001) surface of the SrTiO3 substrate. The films grew along the [110] directions with an in-plane orientation relationship of either SrRuO3[1̄10]//SrTiO3 [100] and SrRuO3[001]//SrTiO3[010], or SrRuO3[11̄0]//SrTiO3[010] and SrRuO3[001]//SrTiO3 [100]. Domains with a rotation of 90° around SrRuO3[110] were observed in the dark-field image of plan-view samples.
Growth of InGaAs multi-quantum wells at 1.3 μm wavelength on GaAs compliant substrates
Authors: Zhu Z. H. , Zhou R. , Ejeckam F. E. , Zhang Z. , Zhang J. , Greenberg J. , Lo Y. H. , Hou H. Q. , Hammons B. E.
Published: July 2002

InGaAs multiple quantum wells at 1.3 μm wavelength have been grown on a twist-bonded GaAs compliant substrate. The GaAs compliant substrate contains a 30 Å GaAs thin layer bonded to a GaAs bulk substrate with a 22-degree angle. Nomarski phase contrast microscopy, transmission electron microscopy (TEM), and photoluminescence were used to characterize the heteroepitaxial layers. The smooth and crosshatch-free surface morphology, dislocation-free cross-sectional TEM, and strong luminescence intensity all provide convincing evidences for substantial improvement of the quality of heteroepitaxial material using the compliant substrate technique. Research is underway to apply the concept and technique of compliant substrate to Si and other materials.
Energy dissipation in tapping-mode atomic force microscopy
Authors: Cleveland J. P. , Anczykowski B. , Schmid A. E. , Elings V. B.
Published: July 2002

A method is presented to measure the energy dissipated by the tip–sample interaction in tapping-mode atomic force microscopy (AFM). The results show that if the amplitude of the cantilever is held constant, the sine of the phase angle of the driven vibration is then proportional to changes in the tip–sample energy dissipation. This means that images of the cantilever phase in tapping-mode AFM are closely related to maps of dissipation. The maximum dissipation observed for a 4 N/m cantilever with an initial amplitude of 25 nm tapping on a hard substrate at 74 kHz is about 0.3 pW.
Exciton localization and the Stokes’ shift in InGaN epilayers
Authors: Martin R. W. , Middleton P. G. , O’Donnell K. P. , Van der Stricht W.
Published: July 2002

We report a comparative study of the emission and absorption spectra of a range of commercial InGaN light-emitting diodes and high-quality epilayers. A working definition of the form of the absorption edge for alloys is proposed, which allows a unique definition of the Stokes’ shift. A linear dependence of the Stokes’ shift on the emission peak energy is then demonstrated for InGaN using experimental spectra of both diode and epilayer samples, supplemented by data from the literature. In addition, the broadening of the absorption edge is shown to increase as the emission peak energy decreases. These results are discussed in terms of the localization of excitons at highly indium-rich quantum dots within a phase-segregated alloy.
Dynamic electron scattering distinguishes dodecahedral and tetrahedral crystallographic sites in garnet structure
Authors: Lábár János L.
Published: July 2002

It is shown in this letter that, in contrast to the accepted belief in the literature, it is possible to determine if a minority component is located on the dodecahedral, octahedral, or tetrahedral sites in a garnet single crystal. This prediction of ours is based on dynamical Bloch-wave calculations and proved experimentally with x-ray measurements in a transmission electron microscope. The previous literature seemed to agree on the assumption that the dodecahedral and tetrahedral sites are indistinguishable from each other.
Synthesis of uniformly distributed carbon nanotubes on a large area of Si substrates by thermal chemical vapor deposition
Authors: Lee Cheol Jin , Kim Dae Woon , Lee Tae Jae , Choi Young Chul , Park Young Soo , Kim Won Seok , Lee Young Hee , Choi Won Bong , Lee Nae Sung , Kim Jong Min , Choi Yong Gak , Yu Soo Chang
Published: July 2002

We have synthesized carbon nanotubes by thermal chemical vapor deposition of C2H2 on transition metal-coated silicon substrates. Multiwalled carbon nanotubes are uniformly synthesized on a large area of the plain Si substrates, different from previously reported porous Si substrates. It is observed that surface modification of transition metals deposited on substrates by either etching with dipping in a HF solution and/or NH3 pretreatment is a crucial step for the nanotube growth prior to the reaction of C2H2 gas. We will demonstrate that the diameters of carbon nanotubes can be controlled by applying the different transition metals.
High-displacement spiral piezoelectric actuators
Authors: Mohammadi F. , Kholkin A. L. , Jadidian B. , Safari A.
Published: July 2002

A high-displacement piezoelectric actuator, employing spiral geometry of a curved piezoelectric strip is described. The monolithic actuators are fabricated using a layered manufacturing technique, fused deposition of ceramics, which is capable of prototyping electroceramic components with complex shapes. The spiral actuators (2–3 cm in diameter) consisted of 4–5 turns of a lead zirconate titanate ceramic strip with an effective length up to 28 cm. The width was varied from 0.9 to 1.75 mm with a height of 3 mm. When driven by the electric field applied across the width of the spiral wall, the tip of the actuator was found to displace in both radial and tangential directions. The tangential displacement of the tip was about 210 μm under the field of 5 kV/cm. Both the displacement and resonant frequency of the spirals could be tailored by changing the effective length and wall width. The blocking force of the actuator in tangential direction was about 1 N under the field of 5 kV/cm. These properties are advantageous for high-displacement low-force applications where bimorph or monomorph actuators are currently employed.
Reconstruction of three-dimensional chemistry and geometry using focused ion beam microscopy
Authors: Dunn D. N. , Hull R.
Published: July 2002

A technique to reconstruct high resolution three-dimensional structural images and chemical maps using focused ion beam microscopy is presented. A focused ion beam microscope is used to collect secondary electron images and secondary ion mass spectroscopy elemental maps as a function of depth. These images and elemental maps are then used to reconstruct volume image and chemical maps with 20 nm lateral and depth resolutions. Methods to improve lateral resolution and to reduce uncertainties due to differential sputtering are also discussed.
Effects of temperature oscillations on the growth of carbon nanotubes by chemical vapor deposition
Authors: Sun L. F. , Xie S. S. , Mao J. M. , Pan Z. W. , Chang B. H. , Zhou W. Y. , Wang G. , Qian L. X.
Published: July 2002

We have grown carbon nanotubes at oscillatory growth temperature by chemical vapor deposition. The high-resolution transmission electron microscopy (HRTEM) images show that they contain a number of incomplete carbon layers both on the outer and inner periphery and dislocations in the middle part of the carbon nanotubes walls. The defect structure of these nanotubes depends on the period of the T oscillation. In addition, HRTEM examinations of these carbon nanotubes from end-on reveal that some have polyhedral sections and contain dislocations. These features make them more active chemically and might have significant influence on the properties and applications of the carbon nanotubes.
Density functional theory analysis of the local chemical bonds in the periodic tantalum dichalcogenides TaX2 (X=S, Se, Te)
Authors: Doublet M.-L. , Remy S. , Lemoigno F.
Published: July 2002

The electronic structure of layered tantalum dichalcogenides 1T-TaX2 (X=S, Se, Te) have been studied both with the linear muffin tin orbitals-atomic sphere approximation (LMTO-ASA) and the Amsterdam density functional for band (ADF-band) programs. The first code (LMTO) provides band structures, density of states (DOS), and crystal orbitals Hamiltonian populations (COHP) while the second one allows accurate atomic charge calculations by means of a powerful electron density numerical integration. All those analyses were used to rationalize the electronic structures of the three 1T-TaX2 phases, in particular to enlighten the 13×13 structural modulations observed in TaS2 and TaSe2, and to put forward the influence of the local chemical Ta–Te bonds on the relative stability of the 1T-TaTe2 phase vs the distorted monoclinical one. The indirect overlap between the two bands responsible for the metallic properties of TaS2 and TaSe2 has been shown to significantly increase the tantalum d electron count compared to its formal value (d1) leading to a more realistic occupation of the threefolded t2g-like bands involved in the 13×13 instability. Owing to the low electronegative character of Te compared to S and Se, the direct overlap occurring at the Fermi level results in an electron transfer from local Ta–Te bonding states to local Ta–Te antibonding ones yielding a destabilization of the metal–chalcogen bonds.
Imaging charge trap distributions in GaN using environmental scanning electron microscopy
Authors: Toth M. , Kucheyev S. O. , Williams J. S. , Jagadish C. , Phillips M. R. , Li G.
Published: July 2002

We present direct experimental evidence for a field assisted component in images acquired using the gaseous secondary electron detector (GSED) employed in environmental scanning electron microscopes. Enhanced secondary electron (SE) emission was observed in GSED images of epitaxial GaN bombarded with MeV He ions. The increase in SE emission is attributed to an electric field generated by electrons trapped at defects produced by ion implantation. The presence of nonradiative recombination centers and of trapped charge in implanted GaN was established by cathodoluminescence spectroscopy and energy dispersive x-ray spectrometry. The field assisted SE component is distinguishable from the “normal” GSED signal by characteristic pressure and temperature dependencies. The presented results demonstrate the utility of the GSED for imaging charge trap distributions in semiconductors.
High-resolution transmission electron microscopy study of epitaxial oxide shell on nanoparticles of iron
Authors: Kwok Y. S. , Zhang X. X. , Qin B. , Fung K. K.
Published: July 2002

Remarkably oxidation-resistant nanoparticles of iron prepared at Tianjin University have been studied by electron diffraction and high-resolution transmission electron microscopy (HRTEM). HRTEM images clearly show the polyhedral epitaxy of γ-Fe2O3 layers on equivalent {100} facets of the {100} truncated {110} rhombic dodecahedral nanoparticles of iron. Oxide layers on {110} facets are formed by the lateral extension of the epitaxial {100} γ-Fe2O3 layers over adjacent {110} facets. The {110} oxide layers are bent and concave inward. The compressive γ-Fe2O3 layers which are about 4 nm thick give effective protection, according to the theory of Caberra–Mott, to the iron core from further oxidation.
Global model of inductively coupled Ar plasmas using two-temperature approximation
Authors: Kimura T. , Ohe K.
Published: July 2002

The electron energy distribution function (EEDF) is measured with a Langmuir probe in an inductively coupled rf (13.56 MHz) Ar discharge in the pressure range from 5 to 70 mTorr, by changing the power injected into the plasma up to 100 W. The EEDFs measured at a pressure of 5 mTorr formed a bi-Maxwellian structure, which is not prominent due to high electron density, in the energy region lower than the lowest excitation threshold energy. The EEDF structure in the energy region higher than the threshold has a significant depletion of high energy electrons. The EEDF measured at a pressure higher than 10 mTorr can be approximated using a two-temperature distribution, which consists of the higher temperature in a low-energy region below the lowest excitation threshold and the lower temperature in a high-energy region. A global model using the two-temperature distribution is proposed and compared with the experimental results. The model consists of the rate equations for neutrals and charged particles and an energy-balance equation for electrons together with the balance equation for high-energy electrons. Pressure dependences of the electron density and temperatures predicted in this global model agree well with the experimental results except in the pressure range lower than 10 mTorr.
Global simulations of ion turbulence with magnetic shear reversal
Authors: Garbet X. , Bourdelle C. , Hoang G. T. , Maget P. , Benkadda S. , Beyer P. , Figarella C. , Voitsekovitch I. , Agullo O. , Bian N.
Published: July 2002

This paper presents the results of three-dimensional fluid global simulations of electrostatic ion turbulence in tokamaks with reversed magnetic shear. It is found that a transport barrier appears at the location of magnetic shear reversal. This is due to a rarefaction of resonant surfaces in this region. For the same reason, the barrier is more pronounced when the minimum of the safety factor is a simple rational number. The barrier is broadened by velocity shear effects. It is also found that large-scale transport events hardly cross a transport barrier. Finally, a significant amount of toroidal rotation is generated by the turbulence. This rotation changes its sign at the position of magnetic shear reversal, as expected from a quasi-linear estimate of the Reynolds stress tensor.
Theoretical study of the structure of silver clusters
Authors: Fournier René
Published: July 2002

Neutral silver cluster isomers Agn (n=2 to 12) were studied by Kohn–Sham density functional theory. There is a strong even-odd oscillation in cluster stability due to spin subshell closing. Nearest-neighbor interatomic distances do not evolve continuously from the diatomic (2.53 Å) to the bulk (2.89 Å). After adding an empirical correction to the calculated values, we estimate that they are always near 2.68 Å for 3⩽n⩽6, and near 2.74 Å for 7⩽n⩽12. We find several low-energy isomers at all cluster sizes larger than seven atoms with one exception: Ag10 has a D2d twinned pentagonal bipyramid isomer predicted to be 0.20 eV more stable than any other isomer. The ellipsoidal jellium model predicts rather well the shapes of stable silver clusters. Other models (extended Hückel, empirical potential) fail to reproduce the energy ordering of cluster isomers. The structural attributes of low-energy silver cluster isomers Agn (n⩾7) are, in decreasing order of importance: a high mean coordination; a shape that conforms to the ellipsoidal jellium model; and uniformity in atomic coordinations.
Domain reversal and nonstoichiometry in lithium tantalate
Authors: Kim Sungwon , Gopalan Venkatraman , Kitamura K. , Furukawa Y.
Published: July 2002

Recent studies have shown that lithium nonstoichiometry has a tremendous influence on domain reversal characteristics in ferroelectric lithium tantalate. This work presents a systematic study of the domain reversal characteristics such as threshold coercive fields for domain reversal, domain stabilization times, “backswitching” phenomena, domain switching and wall pinning times, and sideways wall mobility in near-stoichiometric LiTaO3 with Li/(Li+Ta)∼0.498. These properties are contrasted with those of congruent LiTaO3 [Li/(Li+Ta)∼0.485]. A qualitative model is proposed based on nonstoichiometric dipolar defects to explain the dependence of threshold coercive field on defect density, and on repeated field cycling, the origin of domain backswitching, and domain stabilization times.
Dielectric and piezoelectric response of lead zirconate–lead titanate at high electric and mechanical loads in terms of non-180° domain wall motion
Authors: Chaplya Pavel M. , Carman Gregory P.
Published: July 2002

The effect of prestress on the nonlinear dielectric (polarization) and piezoelectric (strain) response of lead zirconate–lead titanate (PZT–5H) piezoelectric ceramic is studied. The response to bipolar (−2/+2 MV/m) and unipolar (0/+2 MV/m, −0.4/+2 MV/m) electric field under constant prestress (up to 175 MPa) is experimentally evaluated. In the bipolar regime, prestress mainly influences the first non-180° process. In the unipolar regime, the dielectric and piezoelectric response achieve maximum values near 50–60 MPa because the prestress increases the number of available non-180° domains. A detailed description of the effect of the prestress on electro–mechanical response is provided in terms of non-180° domain wall motion. Based on rule of mixtures formulation, an analytical model is developed to estimate the optimum prestress value for the unipolar electric loading condition. It is found that the dielectric and piezoelectric response of the material is proportional to the volume fraction of the non-180° domains and the difference in domain wall pressure created by mechanical and electrical loads.
Dependence of the photoreduction and oxidation behavior of indium oxide films on substrate temperature and film thickness
Authors: Bender M. , Katsarakis N. , Gagaoudakis E. , Hourdakis E. , Douloufakis E. , Cimalla V. , Kiriakidis G.
Published: July 2002

Indium oxide (InOx) films with a thickness of 10–1100 nm were deposited onto Corning 7059 glass and silica substrates at various substrate temperatures. An unusual decrease of the lateral grain size with increasing substrate temperature during deposition was found. The changes in the conductivity of the films after exposure to ultraviolet light in vacuum and subsequent oxidation in ozone atmosphere were analyzed and related to their structural and morphological properties. It is suggested that the photoreduction and oxidation treatments affect only a thin layer less than 10 nm at the surface of the film, while the minimum bulk conductivity is mainly determined by the structural and morphological properties.
Phase transition at low temperature in NaNbO3 ceramic
Authors: Lanfredi S. , Lente M. H. , Eiras J. A.
Published: July 2002

The electric permittivity of the sodium niobate ceramic was investigated in the temperature range from 15 up to 900 K, at frequencies from 100 Hz to 1 MHz. Three anomalies were found in the permittivity curve. A phase transition for NaNbO3 was assigned to anomaly at around 280 K, while other two ones have previously been reported. In addition, the dielectric permittivity dependence with the temperature and frequency, at temperatures lower than 400 K, showed two regions with typical characteristics of a relaxor ferroelectric materials.
Time scales of overdamped nonlinear Brownian motion in arbitrary potential profiles
Authors: Malakhov A. N.
Published: July 2002

This paper presents a new approach and new results concerning the problem of the rate of the nonlinear Brownian motion in a field of force. Within the scope of one-dimensional overdamped Brownian motion the exact values of time scales are obtained in detail for three types of arbitrary dimensionless potential profiles φ(x) which differ from one another by their limiting values at x=±∞. Three time scales, the relaxation time, the decay time of a metastable state and the life time of a nonstable state, are found via given potential profiles φ(x) by explicit quadrature formulae, as it takes place for the well-known mean first passage times. Specific examples are considered.
Dielectric breakdown in a simplified parallel model
Authors: Fowler Howland A. , Devaney Judith E. , Hagedorn John G. , Sullivan Francis E.
Published: July 2002

The growth of streamer trees in insulating fluids (a submicrosecond process that triggers high-voltage breakdown) has been simulated with a combination of parallel-coding tools. Large grids and arrays display well the multifractal, self-avoiding character of the streamer trees. Three physical cases have been approximated by different power-law weightings of the statistical growth filter: dense anode trees, in the uniform field; sparse cathode trees (a rarer experimental case); and ultrasparse anode trees (seen in some fluids of higher viscosity). The model is contained in a software package that is written in Fortran 90 with data parallel extensions for distributed execution. These extensions encapsulate an underlying, invisible message-passing environment, thus enabling the solution of memory-intensive problems on a group of limited-memory processors. Block partitioning creates processes of reasonable size, which operate in parallel like small copies of the original code. The user needs only to express his model in transparent array-directed commands; parallel interfacing between blocks is handled invisibly. Breakdown is performed in parallel in each of the local blocks. Results are presented for experiments run on eight and nine nodes of the IBM SP2, and four and eight nodes of the SGI Onyx and Origin, three examples of multiple-processor machines. Display is carried out in three dimensions. Timing of the growth can be shown by color banding or by frame animation of the results. The adequacy of the growth rules and size scaling are tested by comparing the simulations against snapshots from high-voltage discharge events.
Metastable phase formation in titanium-silicon thin films
Authors: Beyers Robert , Sinclair Robert
Published: July 2002

The formation of TiSi2 thin films on silicon substrates has been investigated with several transmission electron microscope techniques. For films formed either by reacting titanium with a silicon substrate or by sintering a codeposited (Ti+Si) mixture, electron diffraction patterns show that a metastable phase—TiSi2 (C49 or ZrSi2 structure)—forms prior to the equilibrium phase—TiSi2 (C54 structure). High-resolution images indicate that the metastable TiSi2-silicon interface is atomically sharp, with no ‘‘glassy membrane’’ layer present. The annealing temperature required to transform the metastable TiSi2 to the low resistivity, equilibrium TiSi2 increases as the thin-film impurity content increases. Previous studies of TiSi2 formation are discussed in light of these results.
Some considerations on the electric field induced in insulators by electron bombardment
Authors: Cazaux Jacques
Published: July 2002

Starting from a simple model of the distributions of charge created in an insulator by bombardment with electrons, the components of the electric field are evaluated by using Maxwell’s equations and image effects. The results are applied to the most common experimental situations: a semi-infinite sample (i) bounded by a vacuum or (ii) covered by a conducting film, and a sample in the form of a film (iii) unsupported or (iv) covering a conducting substrate. The results are compared to some experimental data concerning, for instance, electromigration and electron-stimulated desorption. In surface analysis the decay of the Auger signal from ions of opposite charges and the opposite behavior of ions of the same charge are explained. Similar effects observed in electron-probe microanalysis of glasses are also elucidated. The results concern scanning electron microscopy, transmission electron microscopy, and electron-beam lithography applied to biological objects, polymers, ceramics, minerals, glasses, and electronic devices. With slight modifications, the same model can be applied to cases of irradiation with ions or x rays. The evolution of the trapped charges with time is suggested, and the need to indicate the electric parameters (ε and γ) of the investigated samples is outlined.
Thermal expansion of the hexagonal (4H) polytype of SiC
Authors: Li Z. , Bradt R. C.
Published: July 2002

Thermal expansion of the hexagonal (4H) polytype of SiC was measured from 20 to 1000 °C by the x-ray diffraction technique. The principal axial coefficients of thermal expansion are expressed by the second-order polynomials: α11=3.21×10−6+3.56×10−9T−1.62×10−12T2, and  α33=3.09×10−6+2.63×10−9T−1.08×10−12T2 (°C−1). The α11 is larger than the α33 over the entire temperature range, yielding a thermal expansion anisotropy ‘‘A’’ of 0.04 at room temperature which increases to 0.11 at 1000 °C. The thermal expansion of the (4H) structure is compared with previously published results for the cubic (3C) and the hexagonal (6H) SiC polytypes.