bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of oxygen on diamond growth
Authors: Harris Stephen J. , Weiner Anita M.
Published: July 2002

I n situ mass spectral measurements of gas composition at the substrate surface were made during filament-assisted diamond growth. The input gases were various mixtures of CH4, O2, and H2 chosen in order to discern the effects of oxygen addition on diamond formation and growth. The gas phase chemistry was modeled as a one-dimensional flow reactor, and the measured and calculated species mole fractions were in good agreement. The model was then used to estimate mole fractions of several atomic and radical species which could not be measured. We find that addition of O2 has only a small effect on the radical mole fractions. However, O2 can reduce the effective initial hydrocarbon mole fraction, which is important because higher quality diamond is grown at a lower initial hydrocarbon mole fraction. Most important, perhaps is that O2 addition leads to the formation of sufficient gas phase OH to remove nondiamond (pyrolytic) carbon from the film. Thus, O2 addition allows diamond films to be grown under composition and temperature conditions which otherwise would produce largely nondiamond carbon.
Direct observation of ion transfer in contact charging between a metal and a polymer
Authors: Mizes H. A. , Conwell E. M. , Salamida D. P.
Published: July 2002

Triboelectric charging between metals and insulators is usually thought to involve electron transfer. Doping some polymers with a small amount of salt can significantly change their charging properties, even reversing the sign to which they charge upon contact with a given metal. We show by means of secondary-ion mass spectrometry that ions of the salt are transferred across the interface in contacts between a doped polymer and a metal. Specifically, we observe a transfer of bromine ions when polystyrene doped with a small amount of the salt cetylpyridinium bromide is contacted to an indium surface.
Origin of the silver doping effects on superconducting oxide ceramics
Authors: Matsumoto Yasumichi , Hombo Jukichi , Yamaguchi Yoshiaki , Nishida Minoru , Chiba Akira
Published: July 2002

The origin of silver doping effects on Y-Ba-Cu-O superconducting ceramics, such as a high-temperature shift of the point at zero resistivity in resistivity measurements and an increase in critical current density, was sought by transmission electron microscopy observations. Sintering was strongly promoted together with suppression of the formation of the liquid phase in the grain boundaries by silver. Consequently, the liquid phase, which is not a superconductor, was not present in almost all the grain boundaries of the silver-doped ceramics, while this phase was present in almost all the grain boundaries in pure ceramics.
Increased flux pinning in cation-deficient Bi2(Sr,Ca)3−xCu2O8+δ
Authors: Nomura Shunji , Chiang Yet-Ming
Published: July 2002

We show that flux pinning in Bi2(Sr,Ca)3−xCu2O8+δ is increased by intentional deviations in cation nonstoichiometry. Polycrystalline samples of alkaline earth-deficient composition (x=0.24–0.32) show a larger irreversibility in magnetization (ΔM), and a slower decay of ΔM with both increasing field and time than stoichiometric compositions. The difference between compositions grows with increasing field out to 5 T, and is greatest at T≤20 K but still significant at 40 K. The critical state model yields a difference in intragranular current density (Jcm) at 20 K and 2 T of ≳102 between stoichiometric and x=0.3 compositions. Flux pinning energies determined from relaxation of magnetization are several times greater for cation-deficient than for stoichiometric compositions. These improvements are only achieved upon slow cooling or low-temperature annealing, indicating that defect clustering or incipient precipitation is necessary to form more effective pinning defects.
Field-induced resonant tunneling between parallel two-dimensional electron systems
Authors: Eisenstein J. P. , Pfeiffer L. N. , West K. W.
Published: July 2002

Resonant tunneling between two high-mobility two-dimensional (2D) electron systems in a double quantum well structure has been induced by the action of an external Schottky gate field. Using one 2D electron gas as source and the other as drain, the tunnel conductance between them shows a strong resonance when the gate field aligns the ground subband edges of the two quantum wells.
Surface outgrowth problem in c-axis oriented Y-Ba-Cu-O superconducting thin films
Authors: Ramesh R. , Inam A. , Hwang D. M. , Sands T. D. , Chang C. C. , Hart D. L.
Published: July 2002

The origin of surface outgrowths in c-axis oriented superconducting Y-Ba-Cu-O thin films has been identified. We find that a,b-axis regions nucleate heterogeneously at second phase regions and rapidly grow outward, leading to the formation of lenticular-shaped outgrowths. The second phase has been identified by electron diffraction and high-resolution electron microscopy to be the Y1Ba3Cu2O7−x phase that forms as microscopic platelet-shaped precipitates on the a-b plane. The heterogeneous nucleation of such outgrowths is general and has been observed in both in situ sputtered and laser-deposited films. It is probably related to the ledge growth mechanism of the c-axis oriented Y-Ba-Cu-O thin film and to the precise overall composition of the film and needs to be studied in detail in order to produce films with smooth surfaces.
Induced nucleation of diamond powder
Authors: Frenklach M. , Howard W. , Huang D. , Yuan J. , Spear K. E. , Koba R.
Published: July 2002

The effects of heteroatom addition on the nucleation of solid carbon in a low-pressure plasma reactor were investigated. Silane or diborane were added to acetylene mixed in hydrogen or argon. Oxygen was added to some of the diborane containing gas mixtures. Silane containing mixtures resulted in powder comprised of weakly bonded amorphous hydrogenated carbon-silicon material. The addition of diborane resulted in substantial production of diamond particles, 5 to 450 nm in diameter, under the conditions that show no diamond formation without diborane present. The observed yield of the oxidation-resistant powder produced in boron-containing mixtures reached 1.3 mg/h with the linear growth rates of diamond particles on the order of 102–104 μm/h. Implication of these results to interstellar dust formation is discussed.
Characteristics of advanced YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox edge type junctions
Authors: Gao J. , Boguslavskij Yu. , Klopman B. B. G. , Terpstra D. , Gerritsma G. J. , Rogalla H.
Published: July 2002

This letter describes the scaling behavior and Josephson properties of improved YBCO/PBCO/YBCO edge-type junctions. The critical current, normal-state resistance, and IcRn product scale with barrier thickness and junction area. The coherence length of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of microwave power, as well as the current modulation with applied magnetic field, have been studied: well-developed Shapiro steps at 10 GHz have been observed, and the modulation of Ic(H) shows Fraunhofer-like behavior with 95% Ic suppression.
Determination of H concentration in LiNbO3 by photorefractive fixing
Authors: Müller R. , Arizmendi L. , Carrascosa M. , Cabrera J. M.
Published: July 2002

A new method to determine the proton concentration in LiNbO3 is presented. The method is based on the measurement of the diffraction efficiency of a photorefractive grating in two situations. It is first measured after recording at room temperature, and second after saturation of the fixing process at a given temperature (about 150 °C). From only these two experimental data, the value obtained for the proton concentration in our sample is H0=(4.4±0.7)×1018 cm−3. This value agrees, within the experimental error, with that obtained from the infrared absorption arising from the OH− stretching bond.
Detection of Si—H bonds in silicon oxide by x-ray photoelectron spectrum difference
Authors: Ogawa Hiroki , Hattori Takeo
Published: July 2002

Detection of Si—H bonds in silicon oxide from the measurement of x-ray photoelectron spectrum difference is confirmed by the measurement of infrared absorption spectrum difference in the case of silicon native oxides formed during wet chemical treatments of atomically flat hydrogen terminated Si(111) surfaces. The desorption of hydrogen from native oxide is also found from x-ray photoelectron spectrum difference.
Noise gain and operating temperature of quantum well infrared photodetectors
Authors: Liu H. C.
Published: July 2002

The difference between the noise gain associated with dark current and the photoconductive gain in quantum well infrared photodetectors is discussed in light of recent experiments. The theoretical model is based on a single key parameter: the electron trapping probability. An empirical expression for the trapping probability or, alternatively, the electron escape probability is proposed. Using the dark current, the gain, the trapping probability expressions, and the device operating temperature for achieving background limited infrared performance is discussed.
Cubic boron nitride formation on Si (100) substrates at room temperature by pulsed laser deposition
Authors: Friedmann T. A. , McCarty K. F. , Klaus E. J. , Boehme D. , Clift W. M. , Johnsen H. A. , Mills M. J. , Ottesen D. K.
Published: July 2002

We are studying the boron nitride system by using a pulsed excimer laser to ablate from hexagonal BN(hBN) targets to form BN films. We have deposited BN films on heated (600 °C) and room-temperature silicon (100) surface in an ambient background gas of N2. Fourier transform infrared (FTIR) reflection spectroscopy indicates that the films grown at high temperature have short-range sp2 (hexagonal-like) order, whereas films grown at room temperature are a mixture of sp3-bonded BN and sp2-bonded BN. Electron diffraction confirms the presence of cubic BN (cBN) material in the films grown at low temperature and the corresponding TEM lattice images show a grain size of ∼200 Å. The presence of cBN in the films correlates with laser energy density, with cubic material appearing around 2.4 mJ/cm2. Auger electron spectroscopy (AES) indicates that the films are nitrogen deficient.
InTlSb: An infrared detector material?
Authors: van Schilfgaarde Mark , Sher Arden , Chen An-Ban
Published: July 2002

In1−xTlxSb is proposed as promising infrared material. A number of optical and structural properties are studied within local density-functional theory. The alloy at x=0.09 is estimated to have a gap of 0.1 eV. Although TlSb is found to favor the CsCl structure, the zinc blende alloy is stable for low x values. A phase diagram is calculated to estimate the regions of stable phases and explore the conditions for growing narrow-gap In1−xTlxSb alloys.
Characteristics of plasma enhanced chemical vapor deposited tungsten nitride thin films
Authors: Lee Chang Woo , Kim Yong Tae , Min Suk-Ki
Published: July 2002

Low resistive tungsten nitride (W100−xNx) thin films have been deposited at 350–400 °C by plasma enhanced chemical vapor deposition. X-ray photoemission spectroscopy, Rutherford backscattering spectrometry, and x-ray diffraction show that the nitrogen composition in W100−xNx films can be easily controlled between 15 and 72 at. % corresponding to an increase of the NH3/WF6 partial pressure ratio and fcc structure W2N can be obtained. The resistivities of W100−xNx films are varied from 70 to 440 μΩ cm according to nitrogen composition.
Growth of an inverse tetragonal distorted SiGe layer on Si(001) by adding small amounts of carbon
Authors: Osten H. J. , Bugiel E. , Zaumseil P.
Published: July 2002

Si1−x−yGexCy layers have been grown on Si(001) substrates with molecular beam epitaxy and investigated with transmission electron microscopy and x-ray diffraction. We show that it is possible to adjust the strain in pseudomorphic SiGe layers by adding small amounts of carbon. A simple linear extrapolation between the different lattice constants opens the possibility to predict the SiGeC structure in dependence on the carbon content. It is possible to grown epitaxial SiGeC layers with up to 2% carbon. Larger carbon concentrations lead to a crystallographic degradation of the layers. We were able to grow the first pseudomorphic SiGeC layer on Si(001) that is under tensile stress. These layers exhibit a lattice plane spacing in growth direction smaller than that of silicon.
Ultrathin device quality oxide-nitride-oxide heterostructure formed by remote plasma enhanced chemical vapor deposition
Authors: Ma Y. , Yasuda T. , Lucovsky G.
Published: July 2002

In this letter, we report our study on an oxide-nitride-oxide (ONO) heterostructure as a dielectric material in a metal-insulator-semiconductor field-effect transistor with Si as a semiconductor. The electrical properties of the ONO dielectrics have been correlated with: (i) the process related effects; (ii) the accumulation of N atoms and its bonding with Si at the SiO2/Si interface; and (iii) the chemical bonding within the nitride layers. By combining the remote plasma enhanced chemical vapor deposition and the rapid thermal annealing process, the device quality ONO structure with an oxide equivalent thickness of 4.7 nm has been successfully manufactured.
Deposition angle-dependent morphology of laser deposited YBa2Cu3O7 thin films
Authors: Lee Soon-Gul , Hwang Doo-Sup , Park Yong Ki , Park Jong-Chul
Published: July 2002

Surface morphology of pulsed laser deposited YBa2Cu3O7 films have been studied as a function of the plume angle with respect to the substrate’s normal. All the films showed c-oriented epitaxial growth according to x-ray diffraction analyses, and Tc(R=0)≥88 K and Jc≥106 A/cm2 at 77 K. Surface morphology, types of particles, and their number density, changed drastically with the deposition angle. As the angle was changed from 0° to 90°, particle type was changed from larger droplets to much smaller outgrowths with a gradual number density change. While larger droplets had the same composition as the target in accordance with their direct flight origination, smaller outgrowths were Cu rich and were believed to be byproducts of the locally inhomogeneous reaction of the ablated species resulting from their highly anisotropic momenta on the surface. High angle films were also found to contain a-oriented grains.
Carrier lifetime saturation in InGaAs single quantum wells
Authors: Ongstad Andrew P. , Gallant David J. , Dente Gregory C.
Published: July 2002

The carrier recombination rate in strained InGaAs–GaAs single quantum well lasers of varying potential depth, as determined by the well/barrier band offsets, is investigated both theoretically and experimentally. At higher current densities (J≥300 A cm−2), the carrier lifetime saturates. The saturation lifetime in the shallow well is considerably longer (τsat∼4.2 ns) than in the deep quantum wells (QWs) (τsat∼0.9 ns). The recombination rate law of bulk material is inadequate to predict the recombination rates in the InGaAs QWs. Consequently, a local recombination model has been developed which accurately predicts the lifetime saturation behavior observed in the QWs. Overall, it appears that an adequate model of carrier recombination is dependent both on material composition factors (i.e., accurate recombination coefficients) and on the detailed electron-hole densities which are influenced by structural factors such as QW potential depth and QW width.
Perpendicular magnetic anisotropies of (Pt/Co/Pt)/X superlattices
Authors: MacLaren J. M. , Victora R.H.
Published: July 2002

We present ab initio electronic structure calculations for (Pt/Co/Pt)/X superlattices with X = Pd, Ag, or Cu. Large intrinsic perpendicular magnetic anisotropies, as compared to similar Co/X superlattices, are predicted. On average, the calculated anisotropies decrease in the order Pd ≳ Ag ≳ Cu. In particular, the anisotropy of (1Pt/1Co/1Pt)/1Pd, 3.2×108 ergs/cm3 (Co), is three times larger than that predicted for either 1Co/3Pt or 1Co/3Pd.
Temperature dispersion of refractive indices in crystalline and amorphous silicon
Authors: Ghosh Gorachand
Published: July 2002

The temperature dependence of refractive indices (dn/dT) for crystalline and amorphous silicon is analyzed critically by use of a recently introduced physically meaningful model to find refractive indices at any operating temperature and wavelength. The opposite sign of the measured thermo-optic coefficients of the amorphous silicon at 0.6328 and 0.752 μm is investigated and is explained as due to the inverted position of the isentropic band gap rather than the crystalline silicon.
Modifications required on a power-compensated differential scanning calorimeter to obtain heat of adsorption measurements
Authors: Vannice M. A. , Sen B. , Chou P.
Published: July 2002

A commercial, power-compensated, differential scanning calorimeter (DSC) has been modified and incorporated into a gas-handling system to improve its performance and to allow its use to accurately measure heats of adsorption on supported metal catalysts. The use of gas flow controllers, needle valves, and appropriate purge gas mixtures minimized base-line perturbations due to uneven flow rates to the sample and references cavities and to changes in thermal conductivity upon introduction of the adsorbate into the purge gas. However, errors in the energy trace were still observed when large differences existed between the thermal conductivities of the adsorbate and the purge gas. These errors can be especially pronounced with processes involving the adsorption or reaction of hydrogen (if Ar is the purge gas, for example), and the severity of the problem has not previously been recognized. However, the use of a purge gas mixture with a thermal conductivity near that of the adsorbate minimized this problem, and employing a DSC block temperature near that of the adsorption temperature further decreased this error. Utilizing He as the purge gas allowed accurate values to be obtained for the enthalpy of Pd hydride formation and for heats of adsorption of H2 on Pt surfaces. The difficulties with CO adsorption were much less severe because its thermal conductivity is much more similar to that of Ar, and Qad values for CO on Pt were more easily obtained. Finally, the error in the energy trace for H2 when Ar was used was proportional to the amount of H2 adsorbed; therefore a correction factor could be obtained which alternatively allowed Ar to be used as a purge gas and provided values in good agreement with those obtained under optimum conditions using He.
Infrared reflection absorption spectro-electrochemical cell for the i
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i
t
u study of redox-active species at variable temperature
Authors: Best Stephen P. , Clark Robin J. H. , McQueen Roderick C. S. , Cooney Ralph P.
Published: July 2002

The design of an improved infrared reflection–absorption cell which facilitates the study of redox-active chemical species is described. The modifications to existing cell designs permit (i) the sample to be cooled to temperatures limited by the characteristics of the solvent, (ii) the sample to be maintained under an inert atmosphere, (iii) the study of small (1–2-cm3) volumes of solution, and (iv) the entire optical path to be maintained under vacuum. These features of the cell design facilitate the study of small quantities of compounds which are unstable with respect to reaction with oxygen, moisture, and redox partners. The study of tris-maleonitriledithiolato complexes of chromium in formal oxidation states iv, v, and vi demonstrates the advantages of the cell.
Scanning differential polarization microscope: Its use to image linear and circular differential scattering
Authors: Mickols William , Maestre Marcos F.
Published: July 2002

A differential polarization microscope that couples the sensitivity of single-beam measurement of circular dichroism and circular differential scattering with the simultaneous measurement of linear dichroism and linear differential scattering has been developed. The microscope uses a scanning microscope stage and single-point illumination to give the very shallow depth of field found in confocal microscopy. This microscope can operate in the confocal mode as well as in the near confocal condition that can allow one to program the coherence and spatial resolution of the microscope. This microscope has been used to study the change in the structure of chromatin during the development of sperm in Drosophila.
Heavy-ion beam probe diagnostic systems (invited)
Authors: Schoch P. M. , Carnevali A. , Conner K. A. , Crowley T. P. , Forster J. C. , Hickok R. L. , Lewis J. F. , Schatz J. G. , Hallock G. A.
Published: July 2002

Heavy-ion beam probing generally consists of passing a beam of 1+ ions through a plasma imbedded in a magnetic field. Secondary ions with higher ionization levels are produced by ionizing collisions with the plasma electrons. Detection of the secondary ions with a small-aperture electrostatic energy analyzer allows continuous fluctuation measurements of the plasma density and space potential with both spatial and temporal resolution. Spatial resolution is the order of 0.1 cm3 and temporal resolution is presently electronics limited to ∼1 μs. The energy of the probing beam is determined primarily by the requirement that the secondary ion must escape from the plasma. Typical beam energies extend from 10 to 500 keV. The range of plasma densities that have been investigated is 1012 cm−3<ne<1014 cm−3. At the higher densities, beam attenuation becomes a serious problem. Higher beam energies provide better penetration of the magnetic field, and reduced beam attenuation. Heavy-ion beam probes were first used to measure a coherent density fluctuation on a hollow cathode arc in 1969, and soon afterward to measure the space potential. Since then beam probes have been used to measure the space potential and fluctuations in both density and space potential for plasmas with varying magnetic geometries. There is continuing development work to study the feasibility of using beam probes to measure magnetic fluctuations and magnetic field structure. Sensitivity for measuring density and potential fluctuations is best demonstrated by what is the most sophisticated beam probe to date: the 500-keV system on TEXT. For broadband measurements (50–250 kHz), the TEXT beam probe has demonstrated a sensitivity to space potential fluctuations of 2 V (rms), and resolution for ñe/ne of 10−3.

Recent measurements on both ISX-B and TEXT have demonstrated the capability of obtaining simultaneous ñ and φ̃ measurements at three separate locations in the plasma. For some locations the sample volumes are poloidally separated and S(kθ, w) can be estimated for both ñ and φ̃. This permits evaluation of the net electrostatic-fluctuation-induced particle flux. Some of the problems still being encountered with present beam probes are the nonideal behavior of the energy analyzer, cross talk between ñ and φ̃ for high-wave-number fluctuations, the effect of finite sample volume and sample volume spacing on the evaluation of k spectra, simultaneous measurement of two components of the k vector, probing of the complex 3-D magnetic fields, and extension of the measurements to higher-energy beam probe systems.
Device for measuring the force and separation between two surfaces down to molecular separations
Authors: Parker J. L. , Christenson H. K. , Ninham B. W.
Published: July 2002

We present an apparatus for measuring the force as a function of distance between surfaces at separations down to the order of molecular dimensions. The device is a simplified yet improved version of the surface force apparatus first developed by Israelachvili and Adams [J. Chem. Soc. Faraday Trans. 1, 74, 975 (1978)]. It gives the same measurement resolution and has the same possibilities of studying various phenomena in thin films such as friction, viscosity, adhesion, and phase transitions. It offers improved performance with regard to control of surface separation and increased versatility by virtue of variable chamber dimensions and geometry.
Twenty-four percent efficient silicon solar cells with double layer antireflection coatings and reduced resistance loss
Authors: Zhao J. , Wang A. , Altermatt P. , Green M. A.
Published: July 2002

Significant performance increase for silicon solar cells is reported. This has been achieved by a combination of several mechanisms. One is the reduction of recombination at cell surfaces using atomic hydrogen passivation of silicon/silicon dioxide interfaces. Joule resistive losses in the cell have been reduced by a process which allows different thickness for fine and coarse features in the top cell metallization. Finally, reflection losses have been reduced by the use of a double layer antireflection coating. For successful incorporation, this required the development of techniques for growing the surface passivating oxide very thin, without reducing its passivation qualities. The cells display a monochromatic light energy conversion efficiency of 46.3% for 1.04 μm wavelength light, also the highest ever for a silicon devices.
Vortex flowmeter designed with wall pressure measurement
Authors: Miau J. J. , Liu T. W.
Published: July 2002

An axisymmetric bluff body is proposed for a vortex flowmeter as the vortex shedder. It is found that vortex shedding frequencies measured can be nondimensionalized into a unified relation for area blockage ratios of circular disks up to 29.2% and Reynolds numbers in a range of 2.5×103–9.7×104. Vortex shedding frequency can be clearly sensed by a pressure transducer installed on the pipe wall in the neighborhood of the maximum pressure fluctuation being measured. This suggests a feasible design that the sensor of a flowmeter can be removed from the flow field.
Ultraviolet inverse photoemission spectrograph with parallel multichannel isochromat acquisition
Authors: Sancrotti M. , Braicovich L. , Chemelli C. , Ciccacci F. , Puppin E. , Trezzi G. , Vescovo E.
Published: July 2002

A simple and cost effective inverse photoemission spectrograph based on a spherical grating with very large optical acceptance ( f/2.8) is described. The system works in the 10–25 eV photon energy range. The dispersed photons are detected by a detector assembly based on a microchannelplate and consisting of twelve independent channels. Measurements are taken in the isochromat mode by sweeping the impinging electron energy, and the acquisition of the twelve channels is done in parallel. As an example of the apparatus performances data on polycrystalline Pt are presented.
A segmented concentric Faraday cup for measurement of time-dependent relativistic electron beam profiles
Authors: Peyser T. A. , Antoniades J. A. , Myers M. C. , Lampe M. , Pechacek R. E. , Murphy D. P. , Meger R. A.
Published: July 2002

A multi-element segmented concentric Faraday collector has been developed for measuring the time evolution of the beam half-current radius (a1/2) of an intense relativistic electron beam. Each collector segment measures the total current within its radius. The data analysis procedure fits the data from all five segments at a given time to a prescribed beam profile and calculates a1/2 from the parameters of the fitted curves. The effect of beam centroid offsets on the data analysis was investigated numerically. Beam centroid offsets as large as half the beam radius produce only a 10% error in the experimental measurement of a1/2. The use of a thin graphite overlayer followed by range-thick stainless steel reduces scattering from one collector element to the next. The instrument has been used extensively on the SuperIBEX relativistic electron beam accelerator for measurement of the half-current radius as a function of time. Radius variations in excess of 4:1 have been measured over the duration of the beam pulse for beams with 5-MeV energy, 20-kA peak current and 1-cm final half-current radii.
Experiments on the multiampere negative ion source in National Institute for Fusion Science
Authors: Ando A. , Takeiri Y. , Tsumori K. , Kaneko O. , Oka Y. , Akiyama R. , Kawamoto T. , Mineo K. , Kurata T. , Kuroda T.
Published: July 2002

The multiampere negative hydrogen ion source has been developed in National Institute for Fusion Science (NIFS). The ion source is a volume-production type multicusp one with an extraction area of 25×25 cm2. It is found that high density negative hydrogen ions of more than 1 × 1012 cm−3 are produced in the center of the arc chamber with a double magnetic filter configuration. A supply of a small amount of cesium vapor into the arc chamber has greatly enhanced the H− ion current and reduced the operation pressure. The H− ion current of 3.3 A has been extracted from a Cs-seeded plasma at the pressure of 0.9 Pa.
Hydrogen passivation of Mg acceptors in GaN grown by metalorganic chemical vapor deposition
Authors: Götz W. , Johnson N. M. , Walker J. , Bour D. P. , Amano H. , Akasaki I.
Published: July 2002

The effects of the deliberate hydrogenation of GaN were investigated for heteroepitaxial layers grown by metalorganic chemical vapor deposition. The GaN layers were either Mg-doped, p-type after thermal activation, or Si-doped, n type. Elemental depth profiles from secondary ion mass spectroscopy reveal a striking contrast after a deuteration at 600 °C: the deuterium concentration in Mg-doped GaN is ∼1019 cm−3 while there is no detectable deuterium incorporation in the n-type material. Variable temperature Hall effect measurements provide the most direct evidence to date for Mg–H complex formation with the decrease in the hole concentration upon hydrogenation accompanied by an increase in the hole Hall mobility.
A novel capacitance microscope
Authors: Lányi Š. , Török J. , Řehůřek P.
Published: July 2002

A capacitance microscope has been used for imaging conducting surfaces. It differs from earlier designs in three major respects: the principle of capacitance detection, the coaxial probe employed, and the operating frequency. The impedance of the probe with respect to a conducting backplane is sensed, which allows the instrument to resolve the components of the complex capacitance. The probe and input stage design reduces the parasitic capacitances to approximately 4×10−14 F. The lateral resolution achieved is approximately 10 nm.
Development of a low-pressure aerosol sampler
Authors: Whitby Evan , Hoshino Masakazu
Published: July 2002

A low-pressure aerosol sampler (LPAS) has been built to interface between low-pressure source chambers and aerosol/gas-analysis systems operating at atmospheric pressure. The LPAS is computer controlled and takes batch samples of aerosol at low pressure from a source chamber, repressurizes the sampled aerosol to atmospheric pressure, and then delivers the sampled aerosol at steady flow to analysis instruments operating at atmospheric pressure. The LPAS is able to (1) sample and deliver particles in the 20–1000 nm size range with a minimum simulated transfer efficiency of 50% at 300 Pa, (2) maintain a background aerosol concentration of 2 [♯/cm3] in the delivered aerosol flow (the background concentration results from internal particle generation), and (3) deliver a continuous flow of aerosol for arbitrary periods of time, thereby providing a virtual interface between low-pressure source chambers and atmospheric-pressure analysis instruments.
Performance of spherically focusing Ge(444) backscattering analyzers for inelastic x-ray scattering
Authors: Macrander A. T. , Kushnir V. I. , Blasdell R. C.
Published: July 2002

A spectrometer designed for use as an undulator source and having targeted resolutions of 0.01 eV in one mode of use and 0.2 eV in another will operate at the APS. We report here on analyzers that we have constructed for use on this spectrometer for 0.2 eV resolution. We have tested them at NSLS beamline X21 using focused wiggler radiation and at the Cornell high energy synchrotron source (CHESS) using radiation from the CHESS-ANL undulator. Analyzers were constructed by gluing and pressing 90-mm-diam, (111) oriented Ge wafers into concave glass forms having a radius near 1 m. An overall inelastic scattering resolution of 0.3 eV using the (444) reflection was demonstrated at CHESS. Recent results at X21 revealed a useful diameter of 74 mm at an 87° Bragg angle.
Measurement of sensitivity distributions of capacitance tomography sensors
Authors: Yang W. Q. , Conway W. F.
Published: July 2002

Currently most electrical capacitance tomography systems employ the linear back-projection algorithm to reconstruct cross sectional images, the processing of which requires sensitivity maps for all electrode pairs. Usually these sensitivity maps are obtained by finite element analysis with accuracy limited by available data on sensor geometry. Alternatively they can be more accurately obtained by physical measurements so that the imaging accuracy can be improved. A test rig has been built at UMIST for this purpose. It can be controlled manually or by a computer. The experimental result shows the feasibility.
High-pressure light scattering apparatus to study pressure-induced phase separation in polymer solutions
Authors: Xiong Yan , Kiran Erdogan
Published: July 2002

A new high-pressure time- and angle-resolved light scattering apparatus has been developed to study the kinetics of phase separation in polymer solutions and other fluid mixtures under pressure at near- and supercritical conditions. The system consists of a high-pressure polymer loading chamber, a solvent charge line, a variable-volume scattering cell (with a built-in movable piston connected to a pressure generator, and an expansion rod driven by an air-actuated diaphragm), and a recirculation pump which are all housed in a temperature-controlled oven. The system is operable at pressures up to 70 MPa, and temperatures up to 473 K. The scattering cell is a short path-length cell made of two flat sapphire windows that are separated by 250 μm. It is designed to permit measurements of transmitted and scattered light intensities over an angle range from 0° to 30°. A linear image sensor with 256 elements is used to monitor the time evolution of the scattered light intensities at different angles. With this sensor, the angle range from 2° to 13° is scanned at a sampling rate of 3.2 ms/scan. The pressure quenches are achieved by movement of the air-actuated movable expansion rod, or by the movement of the piston with the aid of the pressure generator to bring about either rapid (at rates approaching 2000 MPa/s) or slow pressure changes in the system. Quench depth is also adjustable, and very deep (70 MPa) or very shallow (as low as 0.1 MPa) pressure quenches are readily achievable. The temperature and the pressure of the solution in the scattering cell, and the transmitted and scattered light intensities at different angles are recorded in real time through a computerized data acquisition system before and during phase separation. The experimental system is especially suited to follow the kinetics of phase separation in polymer solutions and to assess the metastable and unstable regions where phase separation proceeds by the nucleation and growth, and the spinodal decomposition mechanisms, respectively. The capability of the instrument is demonstrated for polystyrene/methylcyclohexane solutions.
Design and performance of a programmable-temperature scanning tunneling microscope
Authors: Hoogeman M. S. , van Loon D. Glastra , Loos R. W. M. , Ficke H. G. , de Haas E. , van der Linden J. J. , Zeijlemaker H. , Kuipers L. , Chang M. F. , Klik M. A. J. , Frenken J. W. M.
Published: July 2002

In this article we introduce a novel scanning tunneling microscope (STM), which operates in a sample temperature range from 60 to at least 850 K. The most important new feature of this STM is that, while one selected part of the surface is kept within the microscope’s field of view, the sample temperature can be varied over a wide range of several hundreds of degrees during actual imaging. The extremely low drift of the scanner and sample was achieved by the combination of a thermal-drift compensated piezoelectric scanner design with a newly developed sample stage. The design of the sample stage defines a fixed center from which thermal expansions, in all three directions, are forced outwards. The performance of the microscope is demonstrated for several surfaces including Au(110), on which we follow one particular surface region over a temperature range of more than 270 K.
Production and acceleration of tracer encapsulated solid pellets for particle transport diagnostics
Authors: Khlopenkov K. V. , Sudo S.
Published: July 2002

A new method for producing a tracer-encapsulated solid pellet (TESPEL) has been developed for a local deposition of the tracer ions in the core plasma and an accurate measurement of the particle transport. The method allows manufacturing of TESPELs in the form of polystyrene shells containing lithium hydride inside as a tracer. The TESPEL acceleration has been successfully performed and photos of the pellets in flight confirmed the TESPEL integrity. For the pellets with diameter 300–400 μm and wall thickness 40–50 μm the pellet fragility becomes insignificant. Calculation of the TESPEL ablation rate has showed that the achieved pellet velocities and sizes are appropriate for the injection into a medium size plasma. It was proposed to fractionate the tracer contents in order to provide better localization of the deposited tracer ions in the plasma. The data obtained in these experiments have proved that injection of the TESPEL made from the plastic shells can be a promising tool for the particle transport diagnostics.
A far-ultraviolet contamination-irradiation facility for in situ reflectance measurements
Authors: Meier Steven R. , Tveekrem June L. , Keski-Kuha Ritva A. M.
Published: July 2002

In this article, a contamination-irradiation facility designed to measure contamination effects on far-ultraviolet optical surfaces is described. An innovative feature of the facility is the capability of depositing a contaminant, photopolymerizing the contaminant with far-ultraviolet light, and measuring the reflectance of the contaminated sample, all in situ. In addition to describing the facility, we present far-ultraviolet reflectance measurements for a contaminated mirror.
Variable path cryogenic acoustic interferometer
Authors: Kucera D. M. , Ketterson J. B.
Published: July 2002

We describe a variable path acoustic interferometer for use at cryogenic temperatures. Movement is enabled without mechanical coupling via two piezoelectric bimorphs wired and mounted in a manner that preserves the parallelism of two ultrasonic transducers that define the acoustic path. A certain degree of in situ alignment can also be accomplished. Path length sweeps from 0 to 180 μm have been made at cryogenic temperatures and preliminary sound velocity measurements in liquid He4 and gaseous He3 near 4 K are presented which agree well with past measurements.
High-pressure instrument for small- and wide-angle x-ray scattering. II. Time-resolved experiments
Authors: Steinhart M. , Kriechbaum M. , Pressl K. , Amenitsch H. , Laggner P. , Bernstorff S.
Published: July 2002

An instrument to facilitate small- and wide-angle x-ray scattering measurements of samples under elevated hydrostatic pressures or exposed to pressure jumps is described. Pressure from atmospheric up to 0.35 GPa is produced by a motor-driven, piston-type generator and transferred through a network containing pressurizing liquid to a sample cell. The cell, with the optical path length of 1.6 mm, has two Beryllium windows with a total transmission of 0.4 (for x rays of λ=0.154 nm) and low background scattering. Scattering can be observed at angles up to 30°. Samples can be solid or liquid with a minimal volume less than 30 μl and the irradiated volume up to 3 μl. Separation of the samples from the pressurizing medium is accomplished by Teflon pistons. The high-pressure network has two sections separated by a pneumatic valve. The inner section is connected permanently to the cell and the outer one to the pressure generator. For pressure jumps, the outer section is brought to a different pressure level than the inner one and the jumps are accomplished by opening fast the connection between both sections. At the same time a trigger signal is sent to the data acquisition electronics. All functions of the instrument are PC controlled. To illustrate the performance characteristics of the instrument, time-resolved small-angle x-ray scattering measurements of phase transitions in liquid crystalline phospholipid, dioleoylphosphatidylethanolamine are shown. Pressure-jump experiments with 5 ms time resolution as well as temperature scans at a constant elevated pressure are presented.
Construction and testing of an 11.4 GHz dielectric structure based traveling wave accelerator
Authors: Zou P. , Gai W. , Konecny R. , Sun X. , Wong T. , Kanareykin A.
Published: July 2002

We report on the design, numerical modeling, and experimental testing of a cylindrical dielectric loaded traveling wave structure for charged particle beam acceleration. This type of structure has similar accelerating properties to disk-loaded metal slow wave structures but with some distinct advantages in terms of simplicity of fabrication and suppression of parasitic wakefield effects. Efficient coupling of external rf power to the cylindrical dielectric waveguide is a technical challenge, particularly with structures of very high dielectric constant ε. We have designed and constructed an X-band structure loaded with a permittivity ε=20 dielectric to be powered by an external rf power source. We have attained high efficiency broadband rf coupling by using a combination of a tapered dielectric end section and a carefully adjusted coupling slot. Bench testing using a network analyzer has demonstrated a power coupling efficiency in excess of 95% with bandwidth of 30 MHz, thus providing a necessary basis for construction of an accelerator using this device. We have also simulated the parameters of this structure using a finite difference time domain electromagnetic solver. Within the limits of the approximations used, the results are in reasonable agreement with the bench measurements.
Epitaxial ferromagnetic MnAs thin films grown by molecular beam epitaxy on Si (001) substrates
Authors: Akeura K. , Tanaka M. , Ueki M. , Nishinaga T.
Published: July 2002

We have grown ferromagnetic MnAs thin films on Si(001) substrates by molecular beam epitaxy. Epitaxial monocrystalline MnAs films with the growth plane of (1̄101) were obtained when the Si surface was first exposed to an As4 flux and then Mn and As4 fluxes were codeposited. It was found that the very first monolayer of As on Si(001) plays an essential role to obtain epitaxial MnAs thin films. Magnetization measurements indicate that the easy axis of the MnAs thin films is in-plane, along the [1̄1̄20] of MnAs and the [110] of Si, normal to the substrate misorientation. The M-H curve of a 300-nm-thick film shows a hysteresis with a saturation magnetization Ms of 694 emu/cm3 and a coercive field Hc of 94 Oe, when the magnetic field is applied along the easy axis.
Oxidation of GaAs/AlGaAs heterostructures studied by atomic force microscopy in air
Authors: Reinhardt F. , Dwir B. , Kapon E.
Published: July 2002

Oxide growth on cleaved (011̄) cross sections of GaAs/AlGaAs heterostructures exposed to air was investigated using atomic force microscopy. Measurements of the height profile at constant force provide information on the oxide growth rate as a function of time and Al-mole fraction. We observe an initial field-aided oxidation mode that follows the Cabrera–Mott model for metal oxidation. After a critical time, oxidation proceeds more slowly, probably assisted by a thermal diffusion process. We used the dependence of oxide thickness on alloy composition for imaging the Al content in GaAs/AlGaAs heterostructures deposited on nonplanar substrates, with a lateral resolution of ≊1 nm.
Synthesis and characterization of heteroepitaxial diamond-structured Ge1−xCx (x=1.5–5.0%) alloys using chemical vapor deposition
Authors: Todd M. , Kouvetakis J. , Smith David J.
Published: July 2002

We report growth of high quality heteroepitaxial Ge1−xCx alloys on Si(100) with C concentrations ranging from 1.5 to 5 at.%. The materials were obtained by reactions of the novel methylgermanes CH3GeH3 and HC(GeH3)3 with GeH4 at 470 °C using ultrahigh-vacuum chemical-vapor deposition techniques. The crystallinity, composition, and microstructure of the films were characterized by Rutherford backscattering, carbon-resonance spectroscopy, and cross-sectional transmission electron microscopy. Ge1−xCx (x=0.015–0.030) materials were deposited using CH3GeH3 and displayed excellent crystallinity. Films with higher C content were deposited using HC(GeH3)3. These films were epitaxial and of high crystallinity but they also displayed interfacial defects which sometimes persisted through the entire layer. Electron diffraction data and carbon-ion channeling experiments indicated that carbon primarily occupied substitutional sites in the Ge diamond-cubic lattice. Secondary ion mass spectrometry revealed that the films were pure and highly homogeneous.
Electric breakdown in GaN p-n junctions
Authors: Dmitriev V. A. , Irvine K. G. , Carter C. H. , Kuznetsov N. I. , Kalinina E. V.
Published: July 2002

Electric breakdown in GaN p-n junctions was investigated. GaN p+-p-n+ structures were grown on 6H–SiC substrates by metalorganic chemical vapor deposition. Mg and Si were used as dopants. Mesa structures were fabricated by reactive ion etching. Capacitance–voltage measurements showed that the p-n junctions were linearly graded. The impurity gradient in the p-n junctions ranged from 2×1022 to 2×1023 cm−4. Reverse current–voltage characteristics of the p-n junctions were studied in the temperature range from 200 to 600 K. The diodes exhibited abrupt breakdown at a reverse voltage of 40–150 V. The breakdown had a microplasmic nature. The strength of the electric breakdown field in the p-n junctions depended on the impurity gradient and was measured to be from 1.5 to 3 MV/cm. It was found that the breakdown voltage increases with temperature. The temperature coefficient of the breakdown voltage was ∼2×10−2 V/K.
Influence of N2O oxidation of silicon on point defect injection kinetics in the high temperature regime
Authors: Tsamis C. , Kouvatsos D. N. , Tsoukalas D.
Published: July 2002

In this work we investigate the influence of N2O oxidation on the kinetics of point defects at high temperatures. The interstitials that are injected during the oxidation process are monitored by the growth of preexisting oxidation stacking faults. We show that at high temperatures (1050–1150 °C), the supersaturation of self-interstitials in the silicon substrate is enhanced when oxidation is performed in a N2O ambient compared to 100% dry oxidation. This behavior is attributed to the presence of nitrogen at the oxidizing interface. However, at lower temperatures this phenomenon is reversed and oxidation in N2O ambient leads to reduced supersaturation ratios.
Enhanced diffusion in nonstoichiometric quantum wells and the decay of supersaturated vacancy concentrations
Authors: Lahiri I. , Nolte D. D. , Melloch M. R. , Woodall J. M. , Walukiewicz W.
Published: July 2002

Enhanced superlattice disordering in nonstoichiometric AlAs/GaAs quantum wells exhibits weak temperature dependence because of the decay of the supersaturated concentration of group-III vacancies. We present a formalism for transient enhanced diffusion in nonstoichiometric materials with which we can extract migration enthalpies Hm by assuming that the vacancy decay is thermally activated with an enthalpy Ha. By analyzing the electroabsorption from the quantum-confined Stark effect for a set of isochronal and isothermal anneals, we extract a migration enthalpy Hm=(1.8±0.2) eV for group-III vacancies, as well as an activation enthalpy Ha=(0.7±0.2) eV for vacancy annihilation.