bluuebunny/crossref_metadata_2025 huggingface dataset:
A sensitive double beam laser interferometer for studying high-frequency piezoelectric and electrostrictive strains
Authors: Pan W. Y. , Cross L. E.
Published: July 2002

A double beam laser interferometer is built up to study high-frequency piezoelectric and electrostrictive strains. The system is capable of resolving a displacement of 10−2 Å using lock-in detection and measuring the strain all the way to the piezoelectric resonance frequencies using a digital oscilloscope for detection. The interference of sample bending to the detected signal is effectively avoided.
Electroluminescence and photoluminescence of Ge-implanted Si/SiO2/Si structures
Authors: Shcheglov K. V. , Yang C. M. , Vahala K. J. , Atwater Harry A.
Published: July 2002

Electroluminescent devices were fabricated in SiO2 films containing Ge nanocrystals formed by ion implantation and precipitation during annealing at 900 °C, and the visible room-temperature electroluminescence and photoluminescence spectra were found to be broadly similar. The electroluminescent devices have an onset for emission in reverse bias of approximately −10 V, suggesting that the mechanism for carrier excitation may be an avalanche breakdown caused by injection of hot carriers into the oxide. The electroluminescent emission was stable for periods exceeding 6 h.
Step bunching in chemical vapor deposition of 6H– and 4H–SiC on vicinal SiC(0001) faces
Authors: Kimoto Tsunenobu , Itoh Akira , Matsunami Hiroyuki
Published: July 2002

Step bunching and terrace widening on 6H– and 4H–SiC epilayers on vicinal {0001} faces have been studied with atomic force microscopy (AFM) and transmission electron microscopy (TEM). Macroscopically, hill-and-valley structures are observed on off-oriented (0001)Si faces, whereas surfaces are rather flat on off-oriented (0001̄)C faces. High-resolution TEM analysis revealed that 3 bilayer-height steps are dominant on 6H–SiC and 4 bilayer-height steps on 4H–SiC. The distribution of step height and terrace width are also investigated.
Vortex phase diagram and J
c limiting factor in high T
c Bi2Sr2Ca2Cu3O10/Ag superconducting tapes
Authors: Li Qiang , Wiesmann H. J. , Suenaga M. , Motowidlo L. , Haldar P.
Published: July 2002

Based on our detailed measurements of current–voltage characteristics for a Bi2Sr2Ca2Cu3O10/Ag tape (Jc≂2×104 A/cm2 at 77 K and zero field) immersed in liquid neon (27 K) and in liquid oxygen (54–90 K) as a function of applied magnetic field up to 17 T, we extended the vortex phase diagram of superconducting Bi2Sr2Ca2Cu3O10 in the mixed state and show that Jc for this tape at T≥27 K in magnetic field (except at very low field) is fundamentally limited by the transition of the vortex-solid-to-liquid within grains rather than the dissipation from grain boundaries.
Soft x-ray detection with diamond photoconductive detectors
Authors: Kania D. R. , Pan L. , Kornblum H. , Bell P. , Landen O. N. , Pianetta P.
Published: July 2002

Photoconductive detectors (PCDs) fabricated from natural IIa diamonds have been used to measure the x-ray power emitted from laser-produced plasmas. The detector was operated without any absorbing filters which distort the x-ray power measurement. The 5.5 eV band gap of the detector material practically eliminates its sensitivity to scattered laser radiation thus permitting filterless operation. Excellent agreement was achieved between a diamond PCD and a multichannel photoemissive diode array in the measurement of radiated x-ray power and energy.
Bifunctional chromophore for photorefractive applications
Authors: Zhang Yue , Ghosal Saswati , Casstevens Martin K. , Burzynski Ryszard
Published: July 2002

A bifunctional chromophore has been developed which serves as a charge transporting agent as well as a second-order nonlinear optical compound. The chromophore has been incorporated into an inert polymer to form a photorefractive material when doped with a photocharge generation sensitizer. The dependence of the four-wave mixing diffraction efficiency, holographic grating writing rate, and two-beam coupling gain on the chromophore concentration has been studied. The results confirm the photorefractive character of the composite.
Simultaneous spectral and temporal resolution in a single photon counting technique
Authors: Ainbund M. R. , Buevich O. E. , Kamalov V. F. , Men’shikov G. A. , Toleutaev B. N.
Published: July 2002

We have demonstrated the performance of a position-sensitive and time-resolved photon counting system based on a microchannel plate photodetector with a delay-line anode, in combination with a picosecond laser source and two-channel fast timing technique. A spatial resolution of 220 μm and a temporal resolution of 500 ps were achieved. Examples of two-dimensional, both spectrally and temporally resolved molecular emission spectra are presented. The results obtained clearly show the method to be suitable for those spectroscopic applications that require an extremely high sensitivity and multichannel spectral analysis with picosecond time resolution.
Intercomparison of hydraulic pressure measurements to 28 MPa using a single-piston gauge in the controlled-clearance, reentrant, and simple configurations
Authors: Jain K. , Ehrlich C. , Houck J.
Published: July 2002

The use of controlled-clearance piston gauges as highly accurate primary pressure standards is well established. However, the operation of a controlled-clearance gauge is more complicated than that of the familiar simple or reentrant designs. An investigation has been performed on a modified design of a piston gauge for which the same piston/cylinder can be operated in either the controlled-clearance, simple, or reentrant modes, to determine the degree of degradation in accuracy of measured pressure using the gauge in either the simple or reentrant modes as a standard. How the particular characterization of the standard gauge affects the calibration parameters of a test gauge is also explored. The effective areas of the test gauge are within 4.5 parts per million (ppm) when the standard is used in the simple mode instead of the controlled-clearance mode, up to a pressure of 28 MPa, whereas in the case of the reentrant mode this deviation increases from 2.5 ppm at 7 MPa to 14 ppm at 26 MPa.
Analysis of the properties of an electrostatic triplet quadrupole lens used as an electron beam transport device
Authors: Marynowski M. , Franzen W. , El-Batanouny M.
Published: July 2002

The electron optical properties of an electrostatic triplet quadrupole lens used as an electron beam transport device were reexamined, both analytically and by computer simulation, in order to derive limits on the choice of the design parameters of the lens. Furthermore, computer simulation of electron trajectories through a single quadrupole lens, and comparison with experimental results obtained by other authors, have led to a graphical relation between the fringing field end correction and the aspect ratio of the lens.
Beam-centroid tracking instrument for ion thrusters
Authors: Pollard James E.
Published: July 2002

Thrust vector stability for an electrostatic ion engine can be measured with improved sensitivity and time resolution by the method described here. Four double-wire Langmuir probes, aligned in the form of a cross, are placed in the exhaust plume and are translated by a motorized positioning system to balance the currents collected along two orthogonal axes. The thrust vector position is thereby measured with an angular resolution of <0.01° and a response time of <5 s.
Charge measurement and control for the Gravity Probe B gyroscopes
Authors: Buchman Saps , Quinn Theodore , Keiser G. M. , Gill Dale , Sumner T. J.
Published: July 2002

We describe a technique based on photoemission for controlling the charge of the Gravity Probe B electrostatically suspended gyroscopes, and three methods for measuring this charge. Charging is caused by cosmic radiation in orbit and by enhanced field emission in ground testing. Errors induced by disturbing torques require the potential of the gyroscope to be smaller than 15 mV (15 pC) during the space experiment. The disturbing drift rate produced by measuring and controlling the charge in orbit is smaller than 10−13 deg/h, as compared with the 10−11 deg/h systematic drift rate of the gyroscope. The charge control technique is based on ultraviolet photoemission of electrons from both the gyroscope and a charge control electrode on the gyroscope housing. We demonstrate the effectiveness of this method in ground testing and therefore its suitability for the space experiment. Calculations indicate that heating by absorbed photons is, in the worst case, smaller than 1 nW and thus not a problem for the experiment. The principal charge measurement method is based on the determination of the control effort needed to balance a force modulation applied to the suspension electrodes. This technique is insensitive to pickup from the suspension system and to gyroscope miscentering, and is independent of the gyroscope acceleration. We demonstrate that the force modulation method is suitable for charge monitoring in orbit with an accuracy equal to or better than 4 mV.
Two new optical designs for soft x-ray spectrometers using variable-line-space gratings
Authors: Osborn K. D. , Callcott T. A.
Published: July 2002

Two new optical designs for soft x-ray spectrometers using variable-line-space (VLS) gratings are described, characterized, and compared. One design, using VLS rulings on a spherical substrate, is characterized both analytically and with ray tracing. The second, using a concave focusing mirror and a plane VLS grating, is ray traced. Performance is optimized using line spacing described by a third-order polynomial. For the single element design, resolving powers of approximately 1300 and above are achieved over the energy range from 300 to 1000 eV in a compact (1-m-long) instrument, with performance limited by the resolution of the electronic area detector to be used with the design.
Inelastic x-ray scattering spectroscopy: Medium resolution (invited) (abstract)
Authors: Schülke W.
Published: July 2002

A review of the application of medium-energy resolution inelastic x-ray scattering spectroscopy on problems of solid-state physics is presented, where medium-energy resolution means the range around 1 eV. The review starts with a description of the fundamental information about electron density–density correlation, which can be obtained by means of IXSS, and how this information can be related to electronic excitation (band structure) and dielectric screening (dielectric functions). Then the necessary experimental synchrotron based setup for IXSS is demonstrated at the HASYLAB inelastic-scattering beamline as an example, where the method of dispersion compensation is stressed. As the first solid-state problem, investigated with IXSS, the following special features of electron density–density correlations in s-p metals and semiconductors are presented: (1) lattice-induced fine structure in the dielectric response of Al metal, comparison with theoretical calculations, (2) lattice-induced additional collective excitation, demonstrated on Be, (3) plasmon-Fano-resonances, detected in the dielectric response of Si, and (4) first direct experimental evidence for volume plasmon bands in Si by means of standing-wave inelastic scattering. The investigation of electronic properties of layered structures by means of IXSS is demonstrated by means of experiments on graphite and graphite based alkali-metal intercalation compounds. Finally, it is stressed that IXSS also allows the measurement of core excitation spectra, so that edge structures in the 10–1000 eV range of low-Z elements can be investigated using deeply penetrating hard x-rays in the 10 keV range, thus making accessible the edge fine structure of low-Z components of solids in environments, which forbid the application of VUV or electrons. Application of IXSS core excitation is demonstrated both on core excitation spectra of the 1s edge of Li in LiC6 intercalation compounds and on core 1s-excitation spectrum of solid hcp helium single crystals, grown and in situ investigated in a Be 600 bars pressure cell at 5 K.
Investigation of the iron pre-edge x-ray absorption near-edge structure
Authors: Bajt S. , Hanson A. L.
Published: July 2002

The x-ray absorption near-edge structure (XANES) spectra of iron bearing minerals with iron in two valence states show a remarkable property that the energies of the pre-edge XANES peaks vary linearly with the net valence state of the iron. The K-edge XANES spectra of fayalite (Fe2SiO4), olivine [(Mg, Fe)2 SiO4], and hematite (Fe2O3) were investigated by measuring the fluorescence spectra with a high energy-resolution spectrometer while varying the incident x-ray energy. It was determined that the pre-edge XANES structure is the result of scattering through both real and virtual electronic state transitions.
High-resolution triple-crystal x-ray-diffraction experiments performed at the Australian National Beamline Facility in Japan (abstract)
Authors: Nikulin A. Yu. , Stevenson A. W. , Hashizume H. , Wilkins S. W. , Cookson D. , Foran G. , Garrett R. F.
Published: July 2002

The x-ray-diffraction results reported here are from the first high-resolution triple-crystal experiments to be performed at the Australian National Beamline Facility at the Photon Factory. The heart of the facility is a multipurpose two-axis high-resolution vacuum diffractometer (BIGDIFF) Z. Barnea et al., Rev. Sci. Instrum. 63, 1069 (1992) capable of use for high-resolution powder diffraction (using both conventional scintillation detectors and imaging plates), protein crystallography, reflectometry, as well as single-crystal diffractometry. The present experiments were conducted on BIGDIFF in triple-crystal diffraction mode with a monolithic channel-cut Si monochromator (supplied by Professor M. Hart), a single-crystal Si sample, and a four-reflection monolithic channel-cut Si analyzer crystal. The Si(111) sample is a part of a wafer which had been implanted with 100 keV B+ ions (doses 1×1015 and 5×1015 cm−2) through a one-dimensional 0.5 μm thick oxide strip pattern with a 5.83 μm period and 4 μm open region. The triple-crystal data were collected in the form of two-dimensional intensity maps in the vicinity of the 111 Bragg peak, varying the sample rotation (ω) and the analyzer/scintillation detector rotation (2θ). The first results were collected in air both with the as-described sample and after the oxide layer had been removed. Certain slice scans (one-dimensional sections of the two-dimensional intensity maps) were also collected with a vacuum of 1 Torr and reveal considerable improvement in signal to background.

The data will be compared with a recent similar study A. Yu. Nikulin et al., J. Appl. Cryst. 27, 338 (1994) performed on BL-14B at the Photon Factory. The new data collected in air indicate that lattice distortion may be mapped with a resolution of approximately 160 Å, to a depth of approximately 1.0 μm, providing valuable quantitative information on ion diffusion in such implanted materials. The slice scans collected in vacuum indicate that a depth resolution of 50 Å is certainly achievable using BIGDIFF. The data show the excellent potential of BIGDIFF for extremely good signal to noise and very high resolution in such experiments, and the advantages of working entirely in vacuum.
A high-intensity, pulsed supersonic carbon source with C(3P
j) kinetic energies of 0.08–0.7 eV for crossed beam experiments
Authors: Kaiser R. I. , Suits A. G.
Published: July 2002

An enhanced supersonic carbon source produces carbon atoms in their C(3Pj) electronic ground states via laser ablation of graphite at 266 nm. The 30 Hz (40±2) mJ output of a Nd-YAG laser is focused onto a rotating graphite rod with a 1000 mm focal length UV-grade fused silica plano-convex lens to a spot of (0.5±0.05) mm diameter. Ablated carbon atoms are subsequently seeded into helium or neon carrier gas yielding intensities up to 1013 C atoms cm−3 in the interaction region of a universal crossed beam apparatus. The greatly enhanced number density and duty cycle shift the limit of feasible crossed beam experiments down to rate constants as low as 10−11–10−12 cm3 s−1. Carbon beam velocities between 3300 and 1100 m s−1, with speed ratios ranging from 2.8 to 7.2, are continuously tunable on-line and in situ without changing carrier gases by varying the time delay between the laser pulse, the pulsed valve, and a chopper wheel located 40 mm after the laser ablation. Neither electronically excited carbon atoms nor ions could be detected within the error limits of a quadrupole-mass spectrometric detector. Carbon clusters are restricted to ∼10% C2 and C3 in helium, minimized by multiphoton dissociation, and eliminating the postablation nozzle region.
A spatially scanning vacuum ultraviolet and visible range spectrometer for spectroscopy of tokamak plasmas in ASDEX-Upgrade
Authors: Field A. R. , Fink J. , Dux R. , Fussmann G. , Wenzel U. , Schumacher U.
Published: July 2002

A spatially scanning, combined vacuum-ultraviolet (VUV) and visible range spectrometer system for the spectroscopy of tokamak plasmas in the ASDEX-Upgrade experiment is described. This system is designed to allow flexible observation of about 2/3 of the boundary plasma using VUV (30–200 nm) and visible range spectrometers viewing along a common line of sight which can be scanned during the plasma discharge by means of a rotatable mirror. From successive spectra recorded using intensified, multichannel photodiode detectors and the recorded position data, spatial profiles of the plasma emission can be reconstructed. Because radiation losses from the boundary plasma can largely be attributed to line emission in the VUV spectral region, this instrument finds application in quantitative studies of radiation loss processes as well as to studies of impurity production and transport. Simultaneous observation in the visible spectral range facilitates an in situ absolute calibration of the VUV instrument by means of the ‘‘branching-ratios’’ technique.
Absolute intensities in photoacoustic spectroscopy
Authors: Barnes J. A. , Gough T. E. , Murdock T. Q. , Stoer M.
Published: July 2002

A novel method for the calibration of spectral line intensities measured by photoacoustic spectroscopy is presented. This technique uses an electrically heated wire to mimic the acoustic signal generated by the laser absorption of a sample gas. A model was devised to describe the acoustic signal generated by both sources and to calibrate the cell. The transition dipole moment of the R(7) transition of the ν2+3ν3 vibrational overtone band of acetylene was measured and found to be in good agreement with the literature value.
Focusing properties of micron-sized immersion lenses
Authors: Schmid Heinz , Fink Hans-Werner , Schiller Christoph , van Rooy Theo L.
Published: July 2002

We report here on the focusing properties of a two-electrode electrostatic immersion lens of micron dimensions that is combined with the electron point source. Depending on the mode of operation of the lens, the resulting focused electron beam can have an energy as low as 15 eV if the lens is used in the decelerating mode, or an energy of typically 200 eV if the lens is used in the accelerating mode. We will also present results of using the focused beam in a TEM setup as well as in the scanning mode.
A mass- and velocity-broadband ion deflector for off-axis ion injection into a cyclotron resonance ion trap
Authors: Guan Shenheng , Marshall Alan G.
Published: July 2002

Off-axis ion injection into an FT-ICR ion trap is desirable for capturing ions from a continuously generated beam (e.g., electrospray). A conventional E×B (Wien) filter focuses ions of a single velocity (independent of mass). Here we show that by segmenting opposed flat electrodes into small sections, the electric field may be tailored to produce well-focused ion trajectories over a wide range of ion velocity and mass-to-charge ratio, m/z. In the limit of infinitely extended deflector electrodes, small m/z, and/or high B, ion trajectories vary as powers or roots of distance.
Dial indicators checking-up by laser interferometry
Authors: Sporea D. G. , Miron N.
Published: July 2002

Dial indicators are mechanical gauging devices widely used for measurements in high accuracy machining. Checking-up the measuring accuracy of these devices, by classical means, is a time-consuming task for both manufacturers and users of dial indicators. Laser-based equipment for a fast and accurate checking-up of all types of dial indicators, according to international standards, is reported. This article contains the description of the equipment’s operating principle, the construction of the specially designed angular transducer used for the dial indicator’s hand angular movement reading, and the Michelson interferometer. The length measuring accuracy is 0.15 μm, while the dial indicator’s hand angular detection accuracy is 0.25°.
Evaluation of a 4 MHz integrated circuit oscillator for the measurement of solute dipole moments in dilute solution
Authors: Liu ChangSheng , Kauffman John F.
Published: July 2002

A simple device for the measurement of solute dipole moments in dilute solutions of nonpolar solvents is described and evaluated. The device is based on an integrated circuit radio frequency oscillator and is demonstrated to be capable of measuring changes in solution dielectric constant of less than one part in 10 000. This allows the measurement of solute dipole moments using solutions in the 0.0001–0.005 molar range. Solute–solute interactions are shown to be minimized within this concentration range, and a simple means of extracting the solute dipole moment from concentration-dependent dielectric constant measurements is described. Dipole moments of para-(dimethylamino)benzonitrile, nitrobenzene, para-(nitro)anisole, and para-(nitro)aniline are measured, and are shown to compare favorably with previously reported values. The device is demonstrated to be capable of distinguishing solution dielectric constant changes associated with solute concentration changes as low as 3×10−5 M. It is therefore well suited to the measurement of dipole moments of polar compounds whose solubility in nonpolar solvent is low.
Fast microchannel plate detector for particles
Authors: Wurz Peter , Gubler Lukas
Published: July 2002

In this article we report on the timing capabilities of a new microchannel plate detector we designed and built. The detector assembly has an impedance-matched transition line (50 Ω line resistance) from anode to cable connector which is considerably smaller than other, commercially available solutions and at the same time has about four times the active area. The detector was tested with an alpha particle source and excellent time response was achieved. Using 10 μm pore size channel plates, a rise time of 300 ps and a pulse width of 520 ps are obtained. The details of the signal analysis are also given in the article.
Efficient production of a 48Ca beam from oxide material in an electron cyclotron resonance ion source using a low power miniature oven
Authors: Harkewicz R.
Published: July 2002

A technique is described which allows the highly efficient production of a 48Ca beam from oxide material with reduction by zirconium in an electron cyclotron resonance ion source at a temperature of 875 °C, easily attainable with a low power miniature oven. This method yields superior beam quality in terms of stability, intensity, and longevity compared to methods which have been used in the past. Details on the design and fabrication of the miniature oven are also provided.
Modification of the optical properties of liquid crystals using near-field scanning optical microscopy
Authors: Moyer Patrick J. , Walzer Karsten , Hietschold Michael
Published: July 2002

Near-field scanning optical microscopy (NSOM) has been utilized to study the behavior of liquid crystals at room temperature. The material, 4′-octyl-4-biphenylcarbonitrile (8cb) was studied at the air–liquid interface. The NSOM probe is shown to take an active role in modifying, and subsequently reading, the liquid crystal with about 65 nm resolution. The optical modification is proven to be independent of topographic modification. Results indicate potential technological uses in data storage and displays.
Experimental instrument for observing angle- and frequency-scanned attenuated total reflection spectra
Authors: Hayashi T. , Fukumoto H. , Okamoto T. , Haraguchi M. , Fukui M.
Published: July 2002

An attenuated total reflection (ATR) instrument for the observation of angle- and frequency-scanned ATR spectra has been constructed. Light characterized by various wave vectors and dispersed into a spectrum has been detected by a charge coupled device camera after reflection from a rotating mirror and a diffraction grating. Two elliptic mirrors and a prism-sample unit have been set between the rotating mirror and the diffraction grating for the purpose of controlling the light path. The images obtained have been analyzed by a digital image processing system so that angle-scanned ATR signals are obtained for various wavelengths (500–800 nm). The time required to acquire one image has been 0.7 s in the present system. The dependence of the dielectric constant of a Ag film on wavelength is given here as an example measured by our system.
An extension to laser-induced fluorescence measures multiple velocity components simultaneously
Authors: Ross T. J. , Newsham D. , Rynn N.
Published: July 2002

An extension to laser-induced fluorescence is presented. The new technique, called intermodulated optical tagging, can be used to measure multidimensional velocity distribution functions directly as well as to tag ions in two velocity and three spatial coordinates. The first application makes the technique a possible replacement for tomography, the second makes it useful for heretofore difficult transport measurements. Application to velocity-space transport perpendicular to the magnetic field is discussed for a quiescent plasma.
Analysis of noise in electrically compensated pyranometers
Authors: De Lima L. C. , Duarte M. A. V.
Published: July 2002

This article analyzes electronic self-generated noise, signal-to-noise ratio and sensitivity for electrically compensated pyranometers. The conclusion from this study is that the source of self-generated noise in this kind of instrument depends mainly on the electrical resistance ratio on its sensors. A sensitivity analysis also demonstrated that the voltage output of these pyranometers is most sensitive to variation in resistance value of the compensation sensor.
A pneumatically powered mechanical translator–rotator for the direct laser vaporization of solid materials
Authors: Stone Earle G. , Bach Stephan B. H.
Published: July 2002

A pneumatically powered mechanical translator–rotator system has been designed and constructed for use in the direct laser vaporization (DLV) of materials. This translator–rotator was initially developed for the reproducible DLV production of refractory metal atoms to be reacted with small molecules and characterized in matrix isolation experiments, but has applications wherever a reproducible DLV stream of a material is required, such as matrix assisted laser desorption ionization. Key features of the new translator–rotator design are the employment of an inexpensive air ratchet to provide power for the translator–rotator mechanism, the elimination of magnetic relays and electrical limit switches through the use of an all mechanical gear and slot mechanism, and a triple O-ring gland capable of maintaining high vacuum, 10−7 Torr, while the translator–rotator is in operation.
Feedback control experiments on 1.0 and 1.5 cycle ac operations in the STOR-M tokamak
Authors: Mitarai Osamu , Xiao Chijin , White Darren , McColl David , Zawalski Wade , Hirose Akira
Published: July 2002

Complete one cycle and 1.5 cycle ac operations are performed in the STOR-M tokamak with the plasma current of ∼20 kA using newly developed feedback control and Ohmic heating circuits. Bias voltage adjustment is installed in the plasma position circuit to optimize the plasma position in the second negative plasma current phase for multicycle ac operation. The key to successful, reproducible multicycle ac tokamak operations on STOR-M is to control both the total vertical field by the feedback control system and the plasma position by application of the bias voltage.
Universal crossed molecular beams apparatus with synchrotron photoionization mass spectrometric product detection
Authors: Yang X. , Lin J. , Lee Y. T. , Blank D. A. , Suits A. G. , Wodtke A. M.
Published: July 2002

Vacuum ultraviolet radiation was generated from an undulator at the Advanced Light Source Synchrotron facility and used for photoionization detection of reaction products in a new universal crossed molecular beams machine. A description of the machine and its performance is presented. Initial experiments on the photodissociation of methylamine (CH3 NH2), ozone (O3), oxalyl chloride [ (OCCl)2] as well as the reactive scattering of Cl with C3 H8 show many of the advantages of photoionization in comparison to electron impact ionization, which has been exclusively used in such instruments in the past. “Momentum matching” of reaction products is much more easily accomplished than in electron impact studies due to suppression of dissociative ionization. The tunability of the vacuum ultraviolet radiation can be used to suppress background from residual gases especially when it is desired to detect free radical reaction products. Even when the tunability cannot be used to suppress background, the fact that little heat is generated by the ionizing beam allows background to be substantially suppressed by cryogenic pumping. The energy resolution of the apparatus is comparable to instruments that have previously been designed with electron impact ionization which have more than twice as long a flight path. This new instrument provides outstanding performance for fundamental studies of chemical dynamics.
A new x-ray diffraction method for structural investigations of solid-liquid interfaces
Authors: Huisman Willem Jan , Peters J. F. , Derks J. W. , Ficke H. G. , Abernathy D. L. , van der Veen J. F.
Published: July 2002

A synchrotron x-ray diffraction method is presented for structural investigations of interfaces between low-Z substrates and heavier liquids. The method, similar to methods used in neutron scattering, is based on illuminating the interface through the solid substrate. The backgrounds arising from bulk scattering and the signal-to-background ratio are estimated and compared with experimental results. An ultrahigh vacuum (UHV) setup is described in which the atomic arrangement and roughness of clean interfaces can be studied in situ. Our first results illustrate the possibilities for both out-of-plane and in-plane diffraction studies. The specular reflectivity of the Ga/diamond(111)-2×1 interface was measured for perpendicular momentum transfers up to 2.2 Å−1. In an in-plane study of Ga/Si(111)-7×7 the in-plane structure factor of Ga liquid within a depth of ∼50 Å was compared to the structure factor of the bulk liquid.
A low-cost and flexible double-crystal monochromator for an x-ray beamline
Authors: Hwang C. S. , Lin F. Y. , Lee Chih-Hao , Yu Kuan-Li , Hsieh C. H. , Tseng P. K. , Lin J. T. , Pong W. F.
Published: July 2002

This work designs and implements an inexpensive and simple double-crystal monochromator, which yields a high performance at a bending magnet x-ray beamline. A primary feature of the proposed monochromator is that the offset between the incident and monochromatic exit of the x-ray beam can be adjusted to cover a wide energy range with a short X–Y traveling stroke. Such a short range of X–Y strokes can reduce the monochromator’s overall size and, also, increase the stability of motions. Meanwhile, in this monochromator, instead of using a complicated hardware linkage mechanism, a software program is developed to effectively control the X–Y stage for positioning the second crystal in a fixed exit configuration, alleviating mechanic motion errors to maintain the parallelism of the two crystals. The entire software control mechanical linkage offers a relatively inexpensive and simple assembly monochromator that is more flexible than the commercially available one.
Specific heat (Cp) of Apiezon N grease (1 to 108 K) and calorimetry: Cp of copper below 30 K
Authors: Swenson C. A.
Published: July 2002

Heat capacity (Cp) data for Apiezon N grease from 1 to 108 K are compared with previous results for this grease and for Apiezon T, with good agreement. Addenda results for a calorimeter in which Apiezon N grease is used for thermal contact between the tray and a sample suggest that a thin film of this grease has an appreciably larger Cp, below 4 K, than bulk grease. For high accuracy experiments using Apiezon N grease, care should be taken in the calculation of addenda Cp’s from bare tray data and the Cp for bulk grease. During these measurements, Cp data for a high purity electron beam melted copper sample were found to be in excellent agreement from 1 to 30 K (±0.2%) with previous measurements [Holste, Cetas, and Swenson, Rev. Sci. Instrum. 43, 670 (1972)] on a different sample using a quite different calorimeter.
Visible luminescence from semiconductor quantum dots in large ensembles
Authors: Leon R. , Fafard S. , Leonard D. , Merz J. L. , Petroff P. M.
Published: July 2002

Visible luminescence has been obtained from ensembles of defect-free, InxAl(1−x)As islands of ultrasmall dimensions embedded in AlyGa(1−y)As cladding layers. These structures were grown by molecular beam epitaxy and studied with low-temperature photoluminescence (PL) and transmission electron microscopy. Visible luminescence is produced using various compositions of InxAl(1−x)As/AlyGa(1−y)As. Quantum dot size distributions, planar densities, dot heights, and wetting layer thicknesses are presented and correlated with the PL spectra.
Laser diode pumped 106 mW blue upconversion fiber laser
Authors: Sanders S. , Waarts R. G. , Mehuys D. G. , Welch D. F.
Published: July 2002

A laser diode pumped Tm3+-doped ZBLAN fiber upconversion laser is demonstrated with blue output power levels up to 106 mW. Differential optical-to-optical conversion efficiencies up to 30% are measured with respect to pump power coupled into the upconversion fiber. A single spatial mode blue output beam is demonstrated, with an M2 value of 1.4.
Quasistatic and high frequency capacitance–voltage characterization of Ga2O3–GaAs structures fabricated by in situ molecular beam epitaxy
Authors: Passlack M. , Hong M. , Mannaerts J. P.
Published: July 2002

Interface properties of Ga2O3–GaAs structures fabricated using in situ multiple-chamber molecular beam epitaxy have been investigated. The oxide films were deposited on clean, atomically ordered (100) GaAs surfaces at ≂600 °C by electron-beam evaporation using a Gd3Ga5O12 single-crystal source. Metal–insulator–semiconductor structures have been fabricated in order to characterize the Ga2O3–GaAs interface by capacitance–voltage measurements in quasistatic mode and at frequencies between 100 Hz and 1 MHz. The formation of inversion layers in both n and p-type GaAs has been clearly established. Using the quasistatic/high frequency technique, the interface state density has been derived as a function of band gap energy and a midgap interface state density in the mid 1010 cm−2 eV−1 range has been inferred. Charge trapping in the oxide has been revealed as the dominant trapping mechanism.
The role of quantum-confined excitons vs defects in the visible luminescence of SiO2 films containing Ge nanocrystals
Authors: Min K. S. , Shcheglov K. V. , Yang C. M. , Atwater Harry A. , Brongersma M. L. , Polman A.
Published: July 2002

Synthesis of Ge nanocrystals in SiO2 films is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The films exhibit strong room-temperature visible photoluminescence. The measured photoluminescence peak energy and lifetimes show poor correlations with nanocrystal size compared to calculations involving radiative recombination of quantum-confined excitons in Ge quantum dots. In addition, the photoluminescence spectra and lifetime measurements show only a weak temperature dependence. These observations strongly suggest that the observed visible luminescence in our samples is not due to the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, observations of similar luminescence in Xe+-implanted samples and reversible PL quenching by hydrogen or deuterium suggest that radiative defect centers in the SiO2 matrix are responsible for the observed luminescence.
Accuracy of electron counting using a 7-junction electron pump
Authors: Keller Mark W. , Martinis John M. , Zimmerman Neil M. , Steinbach Andrew H.
Published: July 2002

We have operated a 7-junction electron pump as an electron counter with an error per pumped electron of 15 parts in 109 and an average hold time of 600 s. The accuracy and hold time are sufficient to enable a new fundamental standard of capacitance. We compare the measured accuracy of the pump as a function of pumping speed and temperature with theoretical predictions based on a model which includes stray capacitance.
Lattice-matching SiC substrates with GaN
Authors: Ren Shang Yuan , Dow John D.
Published: July 2002

Using arguments based on both electronic structure calculations and ionic radii, we propose that interfacial charges contribute to the ionic part of the lattice mismatch. As a result, the Si-terminated 6H–SiC(0001) surface is a superior substrate for GaN to the C-terminated surface. This viewpoint unifies (i) the Sasaki–Matsuoka proposal that substrate quality depends on electrical polarity and (ii) the conventional viewpoint that the best substrate has zero lattice mismatch with the film deposited on it.
Self-organized fabrication of planar GaAs nanowhisker arrays
Authors: Haraguchi K. , Hiruma K. , Katsuyama T. , Tominaga K. , Shirai M. , Shimada T.
Published: July 2002

GaAs lateral nanowhiskers are grown on the side wall of a ridge formed on a GaAs substrate. The growth positions of the lateral nanowhiskers are controlled by a technique based on electron beam lithography. Also, lateral nanowhiskers bridging between two parallel wall surfaces are grown. These methods are potentially applicable to the fabrication of planar-type quantum functional devices.
Lattice parameters of gallium nitride
Authors: Leszczynski M. , Teisseyre H. , Suski T. , Grzegory I. , Bockowski M. , Jun J. , Porowski S. , Pakula K. , Baranowski J. M. , Foxon C. T. , Cheng T. S.
Published: July 2002

Lattice parameters of gallium nitride were measured using high-resolution x-ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects.
Piezoelectric properties of c-axis oriented Pb(Zr,Ti)O3 thin films
Authors: Kanno I. , Fujii S. , Kamada T. , Takayama R.
Published: July 2002

Piezoelectric properties of the c-axis oriented Pb(Zr,Ti)O3 (PZT) thin films were investigated. The PZT films with a composition near the morphotropic phase boundary were epitaxially grown on (100)Pt-coated MgO substrates by rf-magnetron sputtering. The PZT films exhibited excellent ferroelectricity with a remanent polarization more than 50 μC/cm2. In order to examine intrinsic piezoelectric properties, cantilever structures were microfabricated with the PZT films. The piezoelectric coefficient d31 of PZT films, which were not subjected to poling treatments, was measured directly from the transverse expansion of the cantilever beams. The measurements revealed that the PZT films were naturally polarized and had a relatively large piezoelectric coefficient d31 of 100×10−12 m/V without poling.
Improvement of leakage currents of Pt/(Ba, Sr)TiO3/Pt capacitors
Authors: Joo Jae-Hyun , Seon Jeong-Min , Jeon Yoo-Chan , Oh Ki-Young , Roh Jae-Sung , Kim Jae-Jeong
Published: July 2002

Pt/(Ba, Sr)TiO 3 /Pt capacitors fabricated by sputtering technique showed abnormally higher leakage current when negative bias was applied to the top electrode. In this letter, two kinds of processes were attempted to reduce high leakage current of Pt/BST/Pt capacitors for dynamic random access memory devices: (1) postannealing under O2 atmosphere and (2) adding oxygen into sputtering gas of platinum top electrode. These processes were very effective to reduce the oxygen vacancy in the BST films which are mostly responsible for such a high leakage current. The higher reverse currents were significantly lowered by these processes, so that we could obtain symmetric current versus voltage curves of Pt/BST/Pt capacitors.
Temperature dependent electron-hole recombination in polymer light-emitting diodes
Authors: Blom P. W. M. , de Jong M. J. M. , Breedijk S.
Published: July 2002

The current density–voltage characteristics of poly(dialkoxy p-phenylene vinylene) based polymer are investigated as a function of temperature. Model calculations show that the differences between single and double carrier devices can be well understood by taking into account a bimolecular recombination process. It is found that the bimolecular recombination is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin type, and explains why the conversion efficiency (photon/carrier) of a polymer light-emitting diode is temperature independent.
Effects of photo-oxidation on conjugated polymer films
Authors: Hale G. D. , Oldenburg S. J. , Halas N. J.
Published: July 2002

We have used two-photon photoemission as a sensitive monitor of the effects of photo-oxidation on poly[2-methoxy,5-(2′-ethylhexoxy)-1,4-phenylenevinylene] films. We observe a downward shift in the triplet exciton energy as well as a dramatic narrowing of the triplet exciton photoemission linewidth as photo-oxidation progresses. Both of these effects are attributed to changes in the physical and electronic structure of the film due to carbonyl defect formation on the polymer chains. This technique provides a highly sensitive probe of photo-oxidation which may be useful in the optimization of conjugated-polymer-based devices and displays.
Electron-irradiation-induced deep level in n-type GaN
Authors: Fang Z.-Q. , Hemsky J. W. , Look D. C. , Mack M. P.
Published: July 2002

Deep-level transient spectroscopy measurements of n-type GaN epitaxial layers irradiated with 1-MeV electrons reveal an irradiation-induced electron trap at EC−0.18 eV. The production rate is approximately 0.2 cm−1, lower than the rate of 1 cm−1 found for the N vacancy by Hall-effect studies. The defect trap cannot be firmly identified at this time.
Piezoelectric measurements with atomic force microscopy
Authors: Christman J. A. , Woolcott R. R. , Kingon A. I. , Nemanich R. J.
Published: July 2002

An atomic force microscope (AFM) is used to measure the magnitude of the effective longitudinal piezoelectric constant (d33) of thin films. Measurements are performed with a conducting diamond AFM tip in contact with a top electrode. The interaction between the tip and electric field present is a potentially large source of error that is eliminated through the use of this configuration and the conducting diamond tips. Measurements yielded reasonable piezoelectric constants of X-cut single-crystal quartz, thin film ZnO, and nonpiezoelectric SiO2 thin films.
Intra- and interwell transitions in GaInN/GaN multiple quantum wells with built-in piezoelectric fields
Authors: Kollmer H. , Im Jin Seo , Heppel S. , Off J. , Scholz F. , Hangleiter A.
Published: July 2002

We have studied optical transitions in GaInN/GaN single and multiple quantum wells using time-resolved photoluminescence spectroscopy. Our results show that the energy positions of the dominant emission lines strongly depend both on the well width and on the number of wells. In the case of multiple quantum wells, time-resolved measurements clearly distinguish multiple emission lines. These observations are consistently explained by considering the large built-in piezoelectric field in strained GaInN quantum wells. The multiple emission lines are attributed to intra- and interwell transitions between nearest and next-nearest neighbors.
Imaging microwave electric fields using a near-field scanning microwave microscope
Authors: Dutta S. K. , Vlahacos C. P. , Steinhauer D. E. , Thanawalla Ashfaq S. , Feenstra B. J. , Wellstood F. C. , Anlage Steven M. , Newman Harvey S.
Published: July 2002

By scanning a fine open-ended coaxial probe above an operating microwave device, we image local electric fields generated by the device at microwave frequencies. The probe is sensitive to the electric flux normal to the face of its center conductor, allowing different components of the field to be imaged by orienting the probe appropriately. Using a simple model of the microscope, we are able to interpret the system’s output and determine the magnitude of the electric field at the probe tip. We show images of electric field components above a copper microstrip transmission line driven at 8 GHz, with a spatial resolution of approximately 200 μm.
Bright-blue electroluminescence from a silyl-substituted ter-(phenylene–vinylene) derivative
Authors: Gao Zhiqiang , Lee C. S. , Bello I. , Lee S. T. , Chen Ruey-Ming , Luh Tien-Yau , Shi J. , Tang C. W.
Published: July 2002

A bright-blue electroluminescent device has been fabricated by using an emissive dopant and an electron-transporting host. The dopant was a highly photoluminescent silyl-substituted ter-(phenylene–vinylene) derivative [1,4-bis[4-(2-trimethylsilylvinly) styryl]-2,5-dibutoxybenzene (BTSB)]. BTSB was doped into a trimer of N-arylbenzimidazoles (TPBI) which functioned as the host and electron transporter. N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4-4′-diamine (NPB) was used as the hole transporter. The device, with a structure of indium tin oxide\NPB\TPBI:10%BTSB\TPBI\Mg:Ag, shows a bright blue emission of 11000 cd/m2 at a current density of 360 mA/cm2. The current efficiency of this device is about 3.2 cd/A at 20 mA/cm2.
Epitaxially ideal oxide–semiconductor interfaces: Silicate adlayers on hexagonal (0001) and (0001̄) SiC surfaces
Authors: Bernhardt J. , Schardt J. , Starke U. , Heinz K.
Published: July 2002

The preparation of hexagonal {0001} 4H and 6H silicon carbide surfaces by hydrogen plasma or etching in hydrogen flow produces highly ordered monolayers of silicon dioxide. Their structure and epitaxial relationship to the SiC substrate were analyzed by quantitative low-energy electron diffraction and Auger electron spectroscopy. The bond angles and distances retrieved agree with those of bulk SiO2. Due to the saturation of all dangling bonds the semiconductor surface is passivated and preserves its perfect order also in air. The practically ideal oxide monolayers may serve as a seed for growing epitaxial oxides with low defect density and only few structural distortions at the interface to the SiC substrate.
An ultrafast spectroscopy study of stimulated emission in poly(9,9-dioctylfluorene) films and microcavities
Authors: Virgili T. , Lidzey D. G. , Bradley D. D. C. , Cerullo G. , Stagira S. , De Silvestri S.
Published: July 2002

We report measurements of stimulated emission in poly(9,9-dioctylfluorene). We find a gain coefficient of 1.2×104 cm−1 for 100 nm-thick films at an excitation energy density of 9.9 mJ/cm2. Using ultrafast pump–probe measurements, we have studied the effect of cavity optical confinement. A strong shift of the cavity resonance due to nonlinear refraction is observed.