bluuebunny/crossref_metadata_2025 huggingface dataset:
Discrepancies between photocurrent and absorption spectroscopies in intersubband photoionization from GaAs/AlGaAs multiquantum wells
Authors: Rosencher E. , Martinet E. , Luc F. , Bois Ph. , Böckenhoff E.
Published: July 2002

Intersubband transitions between a bound state and extended states in GaAs/AlGaAs multiquantum wells are studied by simultaneous absorption and photocurrent spectroscopy under different electric field conditions. It is found that both types of spectra exhibit different line shapes, with the photocurrent maximum occurring at lower photon energy than in absorption spectra. Moreover, there is a blue shift of absorption peak and a red shift of photocurrent peak with increasing electric fields. These results suggest that a sequential mechanism is involved in the photocurrent collection from the quantum well. The blue shift is then well fitted by a quadratic Stark effect and the red shift by a barrier lowering mechanism.
Gibbs free energy difference between the undercooled liquid and the β phase of a Ti-Cr alloy
Authors: Ohsaka K. , Trinh E. H. , Holzer J. C. , Johnson W. L.
Published: July 2002

The heat of fusion and the specific heats of the solid and liquid have been experimentally determined for a Ti60Cr40 alloy. The data is used to evaluate the Gibbs free energy difference, ΔG, between the liquid and the β phase as a function of temperature to verify a reported spontaneous vitrification (SV) of the β phase in Ti-Cr alloys. The results show that SV of an undistorted β phase in the Ti60Cr40 alloy at 873 K is not feasible because ΔG is positive at the temperature. However, ΔG may become negative with additional excess free energy to the β phase in the form of defects.
Dependence of superconducting properties of thin films of YBa2Cu3O7−x on structural parameters of the films and substrates
Authors: Schieber M. , Ariel Y.
Published: July 2002

High-temperature superconducting (HTS) properties such as critical current Jc (77 K) of up to 4×106 A/cm2, microwave surface resistance, Rs (77 K and 50.8 GHz) as low as 15 mΩ and Tc(R=0) of up to 91 K have been measured for YBa2Cu3O7−x, YBCO, layers prepared by pulsed laser deposition on MgO and (11̄02) Al2O3 buffered with yttrium stabilized zirconia (YSZ). The following x-ray structural parameters have been measured: percentage of residual (200) or (103) lines, value of c-axis parameter, percentage of (111) unwanted lines in (100) YSZ buffer layers, ratio of 〈100〉 to 〈110〉 orientation in the basal plane as determined by double crystal diffraction phi-scan and full width half maximum (FWHM) of (005) rocking curve. It was found that the most critical structural parameter is the substrate quality and 100% 〈100〉 basal plane orientation of the individual grains.
Quarter-micrometer GaAs Schottky barrier diode with high video responsivity at 118 μm
Authors: Peatman W. C. B. , Wood P. A. D. , Porterfield D. , Crowe T. W. , Rooks M. J.
Published: July 2002

A quarter-micrometer diameter Schottky barrier mixer diode has been fabricated on n+ GaAs using electron beam lithography and reactive ion etching (RIE). The anodes were formed using a Pt/Au electroplate technique. The diode zero-bias capacitance of 0.25 fF and series resistance of about 25 Ω, measured at dc, correspond to a ‘‘figure-of-merit’’ cuttoff frequency of about 25 THz. The video responsivity at 118 μm (2540 GHz) was as high as 200 V/W, over three times higher than the best previously reported. The design, fabrication, and evaluation of this diode is described.
Time-dependent Hall effect analysis method used for investigation of the DX center in AlGaAs:Si
Authors: Brunthaler G. , Stöger G. , Aumayr A. , Köhler K.
Published: July 2002

A new time-dependent Hall effect analysis method is demonstrated on the DX center in AlGaAs:Si. Due to the time dependence of the carrier concentration and the mobility in the metastable temperature region of the DX center, errors are usually made in a standard estimation of the Hall effect values. By using an interpolation method for the individual measured voltages, we are able to omit these errors and also to explain them by a linear error approximation. We present mobility data and show that correlation effects between DX centers are important.
Catalytic growth of carbon microtubules with fullerene structure
Authors: José-Yacamán M. , Miki-Yoshida M. , Rendón L. , Santiesteban J. G.
Published: July 2002

A method to grow carbon microtubules with fullerene structure (buckytubes) has been identified. The method consists of the catalytic decomposition of acetylene over iron particles at 700 °C. Carbon microtubules of up to 50 μm in length are synthesized by this method. Electron diffraction and high resolution electron microscopy studies demonstrate that the structure of these microtubules corresponds to the helical structure recently reported by S. Iijima, Nature 354, 56(1991), prepared using an arc-discharge evaporation method.
Electrical characteristics of plasma loaded, radio frequency driven, capacitively coupled electrodes
Authors: Forster John
Published: July 2002

A simple, nonlinear circuit model of a plasma sheath was used to study impedance and harmonic generation at a radio frequency (rf) driven electrode immersed in a plasma. The impedance can be estimated from linear expressions of the conduction and displacement currents; i.e., nonlinear effects on impedance are small. Harmonic generation occurs from nonlinearities in the sheath. The harmonic level at the electrode is influenced both by the plasma parameters and the electrical circuitry attached to the electrode. Thus any use of the harmonic levels for a plasma diagnostic must take into account the stray and actual components attached to the electrode.
Temperature control during chemical vapor deposition of polycrystalline silicon with substrate heating by microwaves
Authors: Buchta R. , Zhang S.-L. , Sigurd D. , Lindgren K.
Published: July 2002

Contactless and accurate measurement of the temperature is an important issue in rapid thermal processing. In this letter we report on the variation of the emissivity of the surface during chemical vapor deposition of polycrystalline silicon onto oxidized silicon wafers. The observed periodic change in the emissivity of the polycrystalline silicon/silicon dioxide/silicon structure is due to interference phenomena in the growing layer. In these experiments the silicon wafers were heated by direct absorption of microwaves in a single wafer reactor. It is shown that with this method of heating, the change of emissivity does not influence the actual temperature of the substrate.
Two-dimensional Shubnikov–de Haas oscillations in modulation-doped CdTe/CdMnTe quantum-well structures
Authors: Scholl S. , Schäfer H. , Waag A. , Hommel D. , von Schierstedt K. , Kuhn-Heinrich B. , Landwehr G.
Published: July 2002

For the first time clear evidence for two-dimensional Shubnikov–de Haas oscillations in modulation-doped CdTe/CdMnTe quantum-well structures is reported. The structures were grown by molecular-beam epitaxy using ZnBr2 as a novel source material for the n-type doping of II-VI epitaxial layers. From an analysis of the Shubnikov–de Haas oscillations a carrier density of 9×1011 cm−2 and an effective mass of 0.1 m0 could be deduced. Due to band filling the Fermi energy in the subbands is shifted above the conduction-band edge. This can be detected as a Stokes shift of absorption compared to photoluminescence recombination. From the Fermi energy shift the carrier concentration can be estimated, which agrees well with values determined by Hall-effect measurements.
Atomic probe microscopy of 3C SiC films grown on 6H SiC substrates
Authors: Steckl A. J. , Roth M. D. , Powell J. A. , Larkin D. J.
Published: July 2002

The surface of 3C SiC films grown on 6H SiC substrates has been studied by atomic probe microscopy in air. Atomic-scale images of the 3C SiC surface have been obtained by scanning tunneling microscopy (STM). STM has confirmed the 〈111〉 orientation of the cubic 3C layer grown on the {0001} surface of the hexagonal 6H substrate. The nearest-neighbor atomic spacing for the 3C layer has been measured to be 3.29±0.2 Å, which is within 7% of the bulk value. Shallow terraces in the 3C layer have been observed by STM to separate regions of very smooth growth in the vicinity of the 3C nucleation point from considerably rougher 3C surface regions. These terraces are oriented at right angles to the growth direction. Atomic force microscopy has been used to study etch pits present on the 6H substrate due to high temperature HCl cleaning prior to chemical vapor deposition growth of the 3C layer. The etch pits have hexagonal symmetry and vary in depth from 50 nm to 1 μm.
Magnetic microscopy using a liquid nitrogen cooled YBa2Cu3O7 superconducting quantum interference device
Authors: Black R. C. , Mathai A. , Wellstood F. C. , Dantsker E. , Miklich A. H. , Nemeth D. T. , Kingston J. J. , Clarke J.
Published: July 2002

We have developed a magnetic flux microscope which uses a liquid nitrogen cooled thin-film YBa2Cu3O7 dc superconducting quantum interference device (SQUID) to produce two-dimensional images of magnetic fields. The instrument operates as a scanning probe microscope with a spatial resolution of about 80 μm and a field of view of about 100 mm2. The equivalent magnetic field noise in the SQUID ranges from 80 pT Hz−1/2 at 1 Hz to 20 pT Hz−1/2 at 1 kHz, yielding a field resolution of about 200 pT in our images. We have used the microscope to image magnetic domains in ferromagnetic samples, small currents flowing in fine wires, and trapped flux and diamagnetic susceptibility in superconducting thin films.
Hydrogen desorption kinetics from the growing Si(100) surface during silane gas-source molecular beam epitaxy
Authors: Kim K.-J. , Suemitsu M. , Miyamoto N.
Published: July 2002

Hydrogen desorption from Si(100) surfaces during silane gas-source molecular beam epitaxy was investigated by temperature-dependent measurements on the growth rate (GR) and the surface hydrogen coverage (θ) during growth. By use of a prediction from a balance between adsorption and desorption of surface hydrogens that GR/θn should follow the Arrhenius relation, the reaction order n and the activation energy were obtained. It is most likely that the desorption proceeds via a first order reaction with the activation energy of 2.0 eV. This energy value is a reasonable one for the hydrogen desorption process and is compared to 1.29 eV for the growth rate itself.
Washer size and shape dependencies of current steps due to standing waves in superconducting quantum interference devices (SQUIDS)
Authors: Takeda Eriko , Nishino Toshikazu , Takagi Kazumasa
Published: July 2002

To find out the quantitative relationship between the voltages of current steps caused by standing waves and the size of the SQUID washer, experimental and numerical studies have been conducted. The wavelength of the standing wave depends on both the width and the hole size of the washer. A model treating a multiturn input coil as a plate explains the measured systematic change in the step voltages. For shortening the wavelength, a new figure of SQUID with an incision in the washer is proposed. The figure is effective for increasing the output voltage of the SQUID.
Surface acoustic wave and Bleustein–Gulyaev wave generation in KTiOPO4 crystals
Authors: Chu David K. T. , Bierlein John D.
Published: July 2002

Surface acoustic waves (SAW) are observed in a z-cut KTP plate. Measured frequency response for x-propagation SAW waves indicates a SAW velocity of about 3940 m/s which agrees with theoretical calculations. The calculated electromechanical coupling coefficient (𝓀SAW2) is about 1%. X-propagation Bleustein–Gulyaev waves (BG) have also been observed in a y-cut KTP plate. Center frequency of a delay line with uniform interdigitated transducers indicates that BG waves possess a velocity about 4256 m/s which is also in good agreement with theory. Using known bulk parameters, the calculated electromechanical coupling coefficient for this BG wave (𝓀BG2) is ≊4%.
Photon counting using a large area avalanche photodiode cooled to 100 K
Authors: Woodard Nathan G. , Hufstedler Eric G. , Lafyatis Gregory P.
Published: July 2002

We have detected single 650 nm photons with quantum efficiencies greater than 60% using a large area silicon avalanche photodiode. We cool a 5 mm diam commercially available device to 100 K and operate in the gain-mode. For most applications—from the near-infrared to the ultraviolet—this device is the most sensitive photon counting detector that has ever been demonstrated. In typical photon counting applications, this detector should prove to be between two and one hundred times more sensitive than the best currently available devices.
Buried-oxide ridge-waveguide InAlAs-InP-InGaAsP (λ∼1.3 μm) quantum well heterostructure laser diodes
Authors: Krames M. R. , Holonyak N. , Epler J. E. , Schweizer H. P.
Published: July 2002

Native oxide technology is used to fabricate long wavelength (λ∼1.3 μm) InAlAs-InP-InGaAsP quantum well heterostructure laser diodes with a buried oxide undercutting and constricting the ridge-waveguide active region. The buried native oxide of InAlAs constricts the current and reduces edge and surface losses. Data are presented showing threshold currents as low as ∼140 mA for ∼13-μm-wide stripes (L∼750 μm), with maximum continuous wave output powers as high as ∼225 mW/facet and external differential quantum efficiencies up to 67% (300 K, uncoated facets).
Photoreflectance characterization of an InAlAs/InGaAs heterostructure bipolar transistor
Authors: Hsu K. T. , Chen Y. H. , Chen K. L. , Chen H. P. , Lin H. H. , Jan G. J.
Published: July 2002

We have measured the photoreflectance spectrum at 300 K from a lattice-matched InAlAs/InGaAs heterostructure bipolar transistor grown by molecular beam epitaxy. The energy features of photoreflectance spectra have been identified and the built-in dc electric fields and associated doping profiles have been evaluated in the n-InAlAs emitter from the observed Franz–Keldysh oscillations. The undoped InGaAs spacer between emitter and base was added on to change the built-in electric field. The results showed that the energy features above the InGaAs band gap are the transitions from the valence band to the quantized state of the conduction band. The quantum well of the conduction band is in the interface of the InAlAs and InGaAs heterojunction. The interface charge densities in the spacer channel are determined to be 3.54×1011 cm−2 and 4.22×1011 cm−2, corresponding to the samples with spacer thicknesses of 300 and 500 Å, respectively. A triangular potential profile model was used to calculate the microstructure in the potential well and electron energy transition. The theoretical and experimental results were compared and good agreements were also found.
Characteristics of chemically assisted ion beam etching of gallium nitride
Authors: Adesida I. , Ping A. T. , Youtsey C. , Dow T. , Khan M. Asif , Olson D. T. , Kuznia J. N.
Published: July 2002

Chemically assisted ion beam etching (CAIBE) characteristics of gallium nitride (GaN) have been investigated using a 500-eV Ar ion beam directed onto a sample in a Cl2 ambient. Enhanced etch rates were obtained for samples etched in the presence of Cl2 over those etched only by Ar ion milling at a substrate temperature of 20 °C. The CAIBE etch rates were further enhanced at higher substrate temperatures whereas etch rates for Ar ion milling were not influenced by substrate temperature. Etch rates as high as 210 nm/min are reported. The etch rates reported here are the highest so far reported for GaN. Anisotropic etch profiles and smooth etched surfaces in GaN have been achieved with CAIBE.
Influence of barrier height distribution on the parameters of Schottky diodes
Authors: Dobročka Edmund , Osvald Jozef
Published: July 2002

I-V curves of Schottky diodes are simulated for a Gaussian type of the Schottky barrier height (SBH) distribution using the model of noninteracting parallel diodes. The mean value and the standard deviation of the distribution are supposed to be constant, i.e., not dependent on the voltage and the temperature. The influence of the distribution parameters and the temperature on the apparent barrier height and the ideality factor is analyzed. It is shown that the ideality factor increases and the apparent barrier height decreases with increasing standard deviation and decreasing temperature. The simulation also provides a rough estimate for the standard deviation. Values of ∼0.09 V can result in ideality factors up to 1.2. The importance of the effect of series resistance in the approach of noninteracting diodes is emphasized.
Silicon wafer bonding studied by infrared absorption spectroscopy
Authors: Feijoó Diego , Chabal Y. J. , Christman S. B.
Published: July 2002

The interface of hydrophilic and hydrophobic Si wafers joined at room temperature is studied using multiple internal transmission infrared absorption spectroscopy as a function of 30-min annealing steps in N2 atmosphere up to 1100 °C. For hydrophilic wafers, water molecules are observed up to 300 °C, silanol groups up to 900 °C, and hydrogen up to 1000 °C. The absence of water spectral features above 300 °C implies that, if present, water molecules are mostly oriented in a plane parallel to the interface. For the hydrophobic wafers, the temperature dependence of the Si–H stretch spectra shows that the majority of H does not participate in the bonding, pointing to the role of impurities or defects.
Crystalline structure of AlGaN epitaxy on sapphire using AlN buffer layers
Authors: Ponce F. A. , Major J. S. , Plano W. E. , Welch D. F.
Published: July 2002

The crystalline structure near the substrate interface has been studied for AlGaN films grown on (0001) sapphire substrates by metalorganic chemical vapor deposition, using AlN buffer layers. Transmission electron lattice images show that the sapphire/AlN interface is coherent, with misfit dislocations separated by 2.0 nm, corresponding to relaxed bulk lattice parameters. The interface between the buffer layer and the AlGaN film is discussed. The defect structure of the epilayer near the substrate interface consists mostly of dislocations and stacking faults lying on basal planes.
Metallic glass formation in highly undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 during containerless electrostatic levitation processing
Authors: Kim Y. J. , Busch R. , Johnson W. L. , Rulison A. J. , Rhim W. K.
Published: July 2002

Various sample sizes of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 with masses up to 80 mg were undercooled below Tg (the glass transition temperature) while electrostatically levitated. The final solidification product of the sample was determined by x-ray diffraction to have an amorphous phase. Differential scanning calorimetry was used to confirm the absence of crystallinity in the processes sample. The amorphous phase could be formed only after heating the samples above the melting temperature for extended periods of time in order to break down and dissolve oxides or other contaminants which would otherwise initiate heterogeneous nucleation of crystals. Noncontact pyrometry was used to monitor the sample temperature throughout processing. The critical cooling rate required to avoid crystallization during solidification of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy fell between 0.9 and 1.2 K/s.
Polarized Raman spectroscopy of chemically vapor deposited diamond films
Authors: Prawer S. , Nugent K. W. , Weiser P. S.
Published: July 2002

Polarized micro-Raman spectra of chemically vapor deposited diamond films are presented. It is shown that important parameters often extracted from the Raman spectra such as the ratio of the diamond to nondiamond component of the films and the estimation of the level of residual stress depend on the orientation of the diamond crystallites with respect to the polarization of the incident laser beam. The dependence originates from the fact that the Raman scattering from the nondiamond components in the films is almost completely depolarized while the scattering from the diamond components is strongly polarized. The results demonstrate the importance of taking polarization into account when attempting to use Raman spectroscopy in even a semiquantitative fashion for the assessment of the purity, perfection, and stress in chemical vapor deposition diamond films.
Fabrication of visibly photoluminescent Si microstructures by focused ion beam implantation and wet etching
Authors: Xu J. , Steckl A. J.
Published: July 2002

A technique is reported for the fabrication of optically active Si microstructures embedded in a crystalline Si (c-Si) substrate. The process combines Si microstructure fabrication by localized high dose Ga+ (1016/cm2) focused ion beam (FIB) implantation at 30 kV into n-type (100) Si followed by anisotropic etching in KOH:H2O (1:5 by volume). Self-selective porous Si (PoSi) formation of the microstructures is obtained by stain etching in HF:HNO3:H2O (1:3:5 by volume). Upon UV 365 nm or Ar+ 488 nm excitation, selective visible room-temperature photoluminescence (PL) was observed from the Si microstructures only. The PL, peaked at ∼670 nm with a full width at half-magnitude (FWHM) of ∼130 nm, is similar to that of PoSi obtained from c-Si substrate.
Electron-nuclear double resonance of the zinc vacancy in ZnGeP2
Authors: Halliburton L. E. , Edwards G. J. , Scripsick M. P. , Rakowsky M. H. , Schunemann P. G. , Pollak T. M.
Published: July 2002

Electron-nuclear double resonance (ENDOR) has been used to identify the singly ionized zinc vacancy (VZn− center) in ZnGeP2. This S=1/2; defect is the dominant paramagnetic acceptor in the material, and it is associated with the absorption from 0.7 to 2.5 μm that limits the use of ZnGeP2 in optical parametric oscillators. The unpaired spin of the VZn− center is shared nearly equally by two phosphorus nuclei adjacent to the vacancy with little overlap of the wave function onto the other two phosphorus neighbors. Angular dependence of the ENDOR spectrum shows that the two primary 31P nuclei have nearly axial hyperfine matrices with unique axes pointing approximately toward the center of the vacancy. The internuclear axis for these two phosphorus makes an angle of 37.8° with the basal plane.
Edge magnetoplasmons in single two-dimensional electron disks at microwave frequencies: Determination of the lateral depletion length
Authors: Dahl C. , Manus S. , Kotthaus J. P. , Nickel H. , Schlapp W.
Published: July 2002

We present a new method to determine the lateral depletion length (LDL) at the boundary of a two-dimensional electron system (2DES). This approach exploits the fact that in high magnetic fields certain plasma excitations of the confined 2DES (edge-magnetoplasmons, EMPs) are sensitive to details of the density profile near the edge. Comparing the measured magnetic-field dispersion of EMPs in a circular mesa with existing classical theories, the LDL is found to be between 0.3 and 0.6 μm for an electron density of 3.0×1011 cm−2.
Influence of platinum interlayers on the electrical properties of RuO2/Pb(Zr0.53Ti0.47)O3/RuO2 capacitor heterostructures
Authors: Al-Shareef H. N. , Bellur K. R. , Kingon A. I. , Auciello O.
Published: July 2002

Lead zirconate titanate, Pb(ZrxTi1−x)O3 or PZT, thin films grown on RuO2 electrodes by the sol-gel process have excellent resistance to polarization fatigue, but they generally have two drawbacks. The films have high leakage currents and large property variation. In this letter we show that the use of a thin Pt layer (100 Å) deposited on the bottom RuO2 electrode to fabricate RuO2/PZT/Pt/RuO2/(100)MgO capacitors has two important effects. It reduces capacitor leakage by two to four orders of magnitude and it significantly reduces the large property variation. In addition, these capacitors retain their excellent resistance to polarization fatigue which is characteristic of the RuO2/PZT/RuO2 heterostructure.
Epitaxial lift-off of ZnSe based II–VI structures
Authors: Brys C. , Vermaerke F. , Demeester P. , Van Daele P. , Rakennus K. , Salokatve A. , Uusimaa P. , Pessa M. , Bradley A. L. , Doran J. P. , O’Gorman J. , Hegarty J.
Published: July 2002

The epitaxial lift-off technique is applied to II–VI based structures. Epilayers of 255 nm thickness containing quantum wells are lifted off their substrates and redeposited onto polyimide coated GaAs. The technique has also been applied to II–VI samples onto which dielectric films had been deposited. Photoluminescence measurements show that the material quality has not been degraded during the processing. The success of this technique with II–VI’s opens up many possibilities for the integration of these materials with metals and dielectrics in vertical structure devices.
Simple capacitor charging power supply for a 100-pps laser pulser
Authors: Bhadani P. K. , Thattey S. S. , Bhawalkar D. D.
Published: July 2002

A very simple, compact, and inexpensive capacitor charging power supply for a 100-pps laser pulser is described. Unlike dc resonant charging, this power supply does not use a high-voltage filter capacitor, charging choke, and diode.
Automation of an alternating field microelectrophoresis apparatus
Authors: Hakim B. H. , Delatour E. , Hanss M.
Published: July 2002

The automation of a microelectrophoresis apparatus is described. It relies on digital techniques for the electric field generation, the conductance measurement, the cuvette positioning, and the temperature rise estimation deduced from the conductance. A current source is used with programmable shape, frequency, duty cycle, and duration. All the measurement steps are successively controlled by the microprocessor. As described herein, the particles motion is recorded with a camera. The apparatus is built from independent (software and hardware) modules and other detecting devices could easily be adapted.
Two-color Z-scan technique with enhanced sensitivity
Authors: Ma H. , de Araújo Cid B.
Published: July 2002

A procedure to improve the sensitivity of the two-color Z-scan technique is presented. The sensitivity of the technique as a function of the pump and the probe beam confocal parameters and their relative transverse dimensions is evaluated. The results show that, in the case of small nonlinear phase shift, the sensitivity of the measurements at the probe beam center may be a factor of 4 greater by the appropriate choice of the beams parameters. Results for arbitrary phase shift values and for detection at or out of the probe beam center are also reported.
Infrared absorption spectrometer for liquid semiconductors
Authors: Bell F. G. , Cutler M.
Published: July 2002

An infrared absorption spectrometer for use with high-temperature liquids is described. The system has been used for liquid semiconductors at temperatures up to 550 °C having absorption edges as low as 0.3 eV. The optical cell consists of a thin (100–600 μm) gasket of graphite foil sandwiched between two optical flat windows and equipped with a fluid reservoir and heating block. This design allows the use of any optical windows chemically compatible with the sample. We have used several infrared windows to measure the optical absorption edge in selenium, selenium–tellurium, and selenium–thallium alloys down to 0.2 eV. This represents an improvement in the previous range of measurement which has been limited by the 0.5-eV cutoff of fused quartz cells.
Device for longitudinally compressing single pulp fibers
Authors: Sachs I. B.
Published: July 2002

Paper and paperboard products are often subjected to edgewise compression loads in use. The basic component of paper and paperboard is the individual pulp fiber. Observance of external changes on the fiber wall should provide information as to how and why pulp fibers fail under compressive load. A fiber compression device was designed for use in the specimen chamber of a scanning electron microscope. This device permits observation of changes in the cell walls of individual pulp fibers while the fiber undergoes longitudinal compressive loading.
New high-precision bridge for low-resistance thermometry
Authors: Bruschi L. , Storti R. , Torzo G.
Published: July 2002

We describe a simple four-terminal ac bridge designed to be used with sensors of low resistance, such as platinum thermometer at low temperature.
Windowless position-sensitive x-ray detector for a Rowland circle crystal spectrometer
Authors: Duval B. P. , Bateman J. E. , Peacock N. J.
Published: July 2002

The output from an x-ray detector using a Chevron pair of microchannel plates has been linearly encoded using an artificial delay line technique. The detector, operating in the wavelength region 12–30 Å, has been mounted on the focusing plane of a Johann crystal spectrometer, radius 25 cm, and used to survey the spectrum from the Divertor Injection Tokamak Experiment (DITE) tokamak. The 20×20 mm2 detector is encoded by a remote, CAMAC based, computer acquisition system which at an electronic count rate of 1×105 Hz shows negligible degradation of the spectral profiles. Events encoded along the 20-mm dimension are directly histogrammed into a CAMAC memory which is capable of recording 128 time frames each with 256 spectral bins with 12 bits/bin. In nonoptimized tests, a FWHM of 60 μm is observed, and a count rate up to 2×104 Hz/mm has been achieved. The high-resolution capability and the shallow detection depth of the microchannel plate both complement the low f number of the spectrometer. Results are reported for the DITE tokamak.
Development of a magneto-optic current sensor for high, pulsed currents
Authors: Lassing H. S. , Oomens A. A. M. , Woltjer R. , van der Laan P. C. T. , Wolzak G. G.
Published: July 2002

A description is given of a current-measurement system based on the Faraday rotation in a one-turn fiber coil. It is shown that an ultralow birefringence ‘‘spun’’ single-mode fiber is extremely suitable in a magneto-optic current sensor. The properties of this fiber were investigated. Measurements of high, pulsed currents (Imax∼1 MA; f=25 kHz), with good accuracy (1%), and in agreement with a conventional method, are presented. Because of the low birefringence, this good accuracy was also reached in the low-current region (ϑ≤π/4 rad). In order to be able to compute the value of large currents unambiguously from the Faraday rotation, a setup with a beam-splitter prism and a second detector was constructed. Some measurements were carried out with this setup.
Cryogenic motorized waveguide switch
Authors: Kaminsky P. G. , Rupp L. W. , Yurke B.
Published: July 2002

Simple modifications to a commercially available motorized waveguide switch are described which allow the operation of the switch at both room temperature and cryogenic temperatures.
State-machine digital pulse generator
Authors: Sidky E. Y. , Wachter E. A. , Farrar T. C.
Published: July 2002

An architecturally simple yet versatile memory-state-based digital pulse generator (or state machine) has been developed for control of instrumentation with PC-compatible microcomputers. The pulse-programming device uses parallel memory allocation to allow simultaneous output-state and address change capabilities. The pulse sequence is loaded into parallel sections of resident memory as a program consisting of output states, state durations, and partial pointers to succeeding states. The complete pointers are generated by concatenation of these address bits with the outputs of duration and loop counters, making complex jumps and loops possible. Minimum event duration and timing resolution is limited only by memory access time; fast or slow memory can be selected based on individual needs. The memory used in the device described has access times of 120 ns, allowing clock rates as high as 8 MHz. The cost of this device is under $200.
Piezoelectric probe for the detection of shock-induced spray and spall
Authors: Speight C. S. , Harper L. , Smeeton V. S.
Published: July 2002

A piezoelectric probe has been developed to study the formation and evolution of spray and spall in explosively driven materials. The probe senses the rate of change of momentum of the impacting material and thereby provides an estimate of the density and time-of-arrival of the various features evolving from the shocked surface. The principle of operation is explained and examples of probe construction and performance are given.
A simple solar-blind photodiode for light near 200 nm useful for angle-locking nonlinear optical crystals
Authors: Rettner C. T. , Bethune D. S.
Published: July 2002

We describe a simple solar-blind split-cathode photodiode for use in controlling the orientation of a nonlinear optical cyrstal such as β-barium borate (BBO) to maintain optimum conversion of pump light to tunable ultraviolet radiation in the 200-nm region. This device avoids the need for any wavelength filtering and is rugged, inexpensive and easy to build.
MASTIF: Mass analysis of secondaries by time-of-flight technique. A new approach to secondary ion mass spectrometry
Authors: Gruntman M. A.
Published: July 2002

A new approach to surface studies by time-of-flight secondary ion mass spectrometry is proposed. The time-of-flight technique is implemented in an unconventional way utilizing a continuous-probing beam. The ‘‘birth’’ moment of the secondary ion is fixed by the detection of secondary electrons emitted when a probing-beam particle strikes the surface. Multichannel mass identification of secondary ions is thus possible using a low-intensity continuous-probing beam, which permits one to study fragile and extremely thin (down to few monolayers) objects, sensitive to damage by intense probing beam and high doses. The technique has been applied to thin (30-Å) self-supporting carbon foil, semiconducting material, and the sensitive surface of a microchannel plate.
On the local structure of optically active Er centers in Si
Authors: Przybylinska H. , Hendorfer G. , Bruckner M. , Palmetshofer L. , Jantsch W.
Published: July 2002

We report high resolution (<0.05 cm−1) photoluminescence (PL) spectra of erbium implanted float-zone (FZ) and Czochralski grown (CZ) silicon. We show that the PL spectrum of cubic Er centers observed in CZ-Si annealed at 900°C is the dominant emission in FZ-Si for the same annealing conditions. We assign it to isolated, interstitial erbium. We observe also two other kinds of optically active Er centers with lower than cubic site symmetry: (i) O-related (found only in CZ Si) and (ii) those related to radiation defects. We conclude that coimplantation with light elements does not lead to the formation of Er-codopant complexes, but rather to Er forming complexes with implantation induced lattice defects.
Integrated materials science facility on the SRS
Authors: Derbyshire G. , Dobson B. , Greaves G. N. , Harris N. , Mackle P. , Moore P. R. , Roberts K. J. , Allinson N. , Nicoll J. , Doyle S. , Oldman R. J.
Published: July 2002

The development of a new integrated materials science facility (station 9.3) on the wiggler line of the Daresbury Synchrotron Radiation Source is outlined. The facility combines instrumentation for data acquisition using both spectroscopic (XANES and EXAFS) and diffraction techniques and is optimized towards industrial applications such as catalysis and electrochemistry using in situ techniques.
Ion temperature measurements on TMX-U using a gridless electrostatic analyzer
Authors: Falabella S. , Molvik A. W.
Published: July 2002

A small retarding-potential analyzer was used on the Tandem Mirror Experiment-Upgrade (TMX-U) to investigate the radial profiles of ion temperature, density, and plasma potential during ion cyclotron resonance heating (ICRH). The analyzer uses the magnetic field of TMX-U, rather than biased grids, to prevent electrons from reaching the biased ion collector. The probe, the Radial Energy Analyzer (REA), has been inserted into the central-cell plasma at temperatures of 200 eV and densities of 3×1012 cm−3 without damage to the probe or major degradation of the plasma. This analyzer measured an increase in ion temperature from 20 to 150 eV with 60 kW of ICRH. The REA measurements agree with other diagnostics on TMX-U. A one-dimensional code was written to investigate the effect of space charge on the flow of ions inside the probe tip. The effect was found to be minimal, in agreement with experimental data.
Low pass filter for soft x-ray monochromators
Authors: Terminello L. J. , McLean A. B. , Santoni A. , Spiller E. , Himpsel F. J.
Published: July 2002

A low pass filter is described that suppresses higher-order diffraction light from vacuum ultraviolet and soft x-ray monochromators. It consists of a triple silicon reflector with the angle of incidence matched to the desired photon energy cutoff. The performance is calculated and tested experimentally using a toroidal grating monochromator.
A novel sample-positioning system for microgravity application
Authors: Sridharan G.
Published: July 2002

Containerless processing of materials is achieved by the use of a variety of sample levitation techniques. However, suspension of a nonmetallic specimen in a high-vacuum, high-temperature environment is not possible with the current methods. The paper describes how to generate mild forces for enabling sample positioning in microgravity experiments, by a new scheme based on pulsed lasers.
High resolution time-of-flight positron emission tomograph
Authors: Ishii Keizo , Orihara Hikonojo , Matsuzawa Taiju , Binkley David M. , Nutt Ronald
Published: July 2002

A time-of-flight positron emission tomograph (TOF-PET) was recently developed for the purpose of clinical medical study and brain research. It provides high-quality positron images with a spatial resolution of 8 mm full width at half-maximum (FWHM) which is obtained by the use of time-of-flight (TOF) location techniques and small 10×18-mm BaF2 detectors to detect the positron-electron annihilation γ-rays. The time-of-flight resolving time was 623 ps (FWHM) in the system, which has improved the S/N ratio for positron images. This new TOF-PET has been designed to digitally record the arriving time of γ-rays at the detectors using the system clock. With this method, the architecture of hard and software for taking TOF sinograms and reconstructing TOF images is simplified compared to that of other TOF-PET systems. The details of this system are described here.
The thermometric properties of MODEL RS-11 Chinese carbon resistors at low temperatures and in magnetic fields
Authors: Chen Pu-fen , Feng Xue-me , Besley Laurence Michael
Published: July 2002

The resistance-temperature relationship and self-heating characteristics of encapsulated RS-11 carbon resistors from the People’s Republic of China demonstrate their suitability as thermometers for the temperature range 1–30 K. A simple interpolation method for them is accurate to 10 mK while polynomial fitting will give 1-mK precision. At 20 K their stability over 30 thermal cycles is typically of the order of 0.1 K, superior to commercially available cryogenic carbon resistance thermometers although inferior to carbon-glass thermometers. These resistors are useful in magnetic fields. At 4.2 K at field intensities up to 10 T, their magnetic field-dependent temperature error is no greater than 5%, and they show no orientation effect.
Variances of parameters obtained from nonlinear curve fits
Authors: Anton A. Brad
Published: July 2002

An analytical expression is derived that quantifies the variances of parameters obtained from curve fits in terms of the variances of the individual data. In contrast to solutions of this problem offered previously, this solution accounts rigorously for nonlinear dependence of the fitting function on the fit parameters.
Analysis of stray capacitance in the Kelvin method
Authors: Baikie I. D. , Venderbosch E. , Meyer J. A. , Estrup P. J. Z.
Published: July 2002

We present a theoretical analysis of the Kelvin probe circuit taking into account both the parallel capacity induced by the connecting cables and fringing fields. We demonstrate a simple explicit solution for low modulation index ε and suggest an optimized detection method for ε close to unity. We extend the analysis to include stray capacitance terms for both the static- and vibrating-plate earthed (spe, vpe) configurations and examine the variation in apparent contact potential difference Vapp as a function of the Kelvin probe mean spacing. This analysis is primarily intended for UHV applications where shielding problems, due either to connecting cables within the system or nonideal system configurations, e.g., imposed by sample mounting constraints, are nontrivial. Using a specially developed computer-steered Kelvin probe and shield potential Vs coupled to a data acquisition system (DAS) we have tested the above model. We find Vapp to be linear with Vs and varies quadratically with mean spacing in both spe and vpe configurations. In the latter case the effect was much smaller; however, it must be taken into account for sub-meV operation.
Plasma diagnostic x-ray tomography system
Authors: Eshelman C. D. , Tseng H. K. , Dolan T. J. , Prelas M. A.
Published: July 2002

A radiation-hardened 60-channel x-ray tomography system has been developed to determine the two-dimensional distribution of x-ray emissivity from magnetically confined plasmas. In order to maximize their field of view, the diode arrays are mounted in re-entrant tubes inside the plasma chamber diagnostic ports. Metal foil vacuum windows serve as x-ray filters and permit the diodes and cables to be at atmospheric pressure. Preamplifiers are mounted at the outside end of the re-entrant tubes. The diode arrays and preamplifiers are protected from the harsh radiation environment by lead shielding. Image reconstruction is done using the harmonic expansion method or the linear algebraic method. For plasmas with some cylindrical symmetry the harmonic expansion method is superior, but for small discrete objects the Cartesian algebraic method is better. Preliminary data from the Missouri Magnetic Mirror plasma with electron cyclotron resonance heating show evidence of a hot electron ring.
Photoconduction in porous TiO2 sensitized by PbS quantum dots
Authors: Hoyer P. , Könenkamp R.
Published: July 2002

Photoconduction in heterogeneous thin films consisting of a porous nanocrystalline TiO2 matrix and internally adsorbed quantum size PbS clusters is reported. The TiO2 serves to establish electrical contact to the PbS clusters, and the PbS cluster size is chosen so that photogenerated excess electrons are directly injected from the PbS to the TiO2. The films exhibit strong photoconductance in the visible region, indicating photoelectric sensitization of the TiO2 by the PbS clusters. Comparison of the absorption and photocurrent spectra shows that only PbS clusters of sizes smaller than ∼25 Å contribute to the sensitization. For larger clusters the band alignment at the TiO2/PbS interface appears to be unfavorable for carrier transfer. Further improvement in the photoresponse will have to involve optimization of the transport properties in the nanocrystalline TiO2.
A new triple correlation technique for measuring ultrashort laser pulses
Authors: Paulter Nicholas G. , Majumdar Arun K.
Published: July 2002

A new triple correlation technique for measuring the complete intensity profile of ultrashort optical pulses is described. The triple correlation preserves the phase information of the input pulse so that a reconstruction of the triply correlated signal will provide a unique reconstruction of the input. The new technique described here uses two second-order, nonlinear optical interactions for the generation of a triply correlated signal. A derivation of the measured triple-correlation signal and the pulse reconstruction is given. The effects of noise on the measured signal are also examined.
The systematic uncertainty in the calibration of the Langavant calorimeter
Authors: Balbás M. , Díaz J. I. , Gascón F. , Varadé A.
Published: July 2002

A mathematical study is made of the systematic uncertainty in the Langavant calorimeter calibration, determining the most influent variables being able to optimize the method. The influence of calibration constants on the determination of the heat of hydration of a cement is analyzed. Accuracy modification of several measuring instruments is suggested and imprecisions correction in the UNE 80-118-86 Spanish Standard and in the NF P 15-436-88 French Standard are proposed.
Design, construction, and evaluation of an integrating transient recorder for data acquisition in capillary gas chromatography/time-of-flight mass spectrometry
Authors: Holland J. F. , Newcombe B. , Tecklenburg R. E. , Davenport M. , Allison J. , Watson J. T. , Enke C. G.
Published: July 2002

An integrating transient recorder (ITR) has been designed, constructed, and evaluated to accomplish time-array detection in gas chromatography time-of-flight mass spectrometry (GC-TOFMS) applications. The ITR consists of a 200-MHz flash analog-to-digital converter, 16 high-speed 100K emitter-couple logic (ECL) summing boards, three parallel processors for real-time data reduction, instrument control and routing functions, and a 300-Mbyte mass storage device. The ITR is capable of recording 80 μs bursts of transient information with a time resolution of 5 ns. For each transient, up to 16 384 sequential time-resolved channels may be recorded. An operator-selectable number of sequential transients may be summed in a locked time registry creating a summed scan file while maintaining the integrity of the transient time resolution. The information from each transient is read, summed, and stored in one of two summing registers (16×1024×24 bits). While incoming information is being stored in one summing register, the information in the other summing register is processed and read out to disk, thus permitting high-speed data collection continuously for long periods of time. The information from successive transients is summed in order to improve signal-to-noise, dynamic range, and sensitivity, and produces scan files at a rate sufficient to maintain all of the chromatographic information. GC-MS data collected at 1, 20, and 50 spectra per second are presented for a nine-component aliphatic/aromatic mixture. Although the ITR was specifically designed for GC-TOFMS studies, the overall design concepts of the ITR are universal and apply to any situation where information from two or more phenomena occur at the output of a single detector and occur over vastly different time domains.
Piezoelectric copolymer hydrophones for ultrasonic field characterization
Authors: Chan H. L. W. , Ramelan A. H. , Guy I. L. , Price D. C.
Published: July 2002

Hydrophones to be used in the characterization of medical ultrasonic transducers have been fabricated using a new polyvinylidene fluoride/trifluoroethylene (VF2/VF3) copolymer. The copolymer has an advantage over VF2 in that it does not require prestretching before poling. Thin copolymer films can be cast from solution and then poled using the corona discharge method. As there is a need for small-diameter hydrophones to provide good spatial resolution in measuring highly focused ultrasonic beams, hydrophones with diameter as small as 0.1 mm have been made. Both needle-type and line hydrophones have been tested and their performance reported. In the case of line hydrophones, the output signal is proportional to the line integral of the acoustic pressure and a computer tomographic technique has been used to reconstruct the beam profiles.
High-field ion sources and applicationsa)
Authors: Mühle R.
Published: July 2002

High electric fields are used for ion generation in gas field ion sources and liquid metal ion sources. With the preparation of stable microprotrusions on refractory metals, stably emitting gas field ion sources for hydrogen and helium ions with angular intensities of 35 and 20 μA sr−1, respectively, are available with a high chromatic angular intensity. Liquid metal ion sources can stably deliver a variety of ions with a source life in some cases over more than 1000 h. They are an essential element for pattern generation in the nanometer region and other focused ion beam applications.