bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions between silica xerogel and tantalum
Authors: Rogojevic Svetlana , Jain Anurag , Wang Feng , Gill William N. , Wayner Peter C. , Plawsky Joel L. , Lu Toh-Ming , Yang Guang-Rong , Lanford William A. , Kumar Atul , Bakhru Hassaram , Roy Alok Nandini
Published: July 2002

Thin films of tantalum, a potential diffusion barrier, were deposited on xerogel films. The structures were annealed at different temperatures and in different ambient environments and analyzed using Rutherford backscattering spectrometry (RBS), optical microscopy, and the Scotch™ tape test, in order to assess the extent of interaction with the xerogel film. When annealed in nitrogen with at most trace amounts of oxygen (equivalent to 10−7–10−6 Torr vacuum), RBS analysis for Ta does not show diffusion of this metal through the xerogel when annealed up to 450 °C. However, the presence of larger concentrations of oxygen in the annealing environment was found to cause oxidation of Ta thin films, and this leads to Ta film buckling and diffusion-like RBS spectra. Ta does not buckle nor diffuse into the xerogel film at temperatures up to 640 °C when the metal is capped with a Si3N4 layer. Analysis of the samples by RBS microbeam shows that the low energy Ta tail originates from only a portion of the sample surface, the shape of which resembles Ta buckles. RBS spectra collected from other parts of the samples show no Ta diffusion into xerogel. Adhesion of Ta to porous xerogel is reduced compared to SiO2, due to the reduced contact area at the interface between the materials.
Speed distribution of C2H6 molecular beam scattered through chattering collision on a LiF(001) surface
Authors: Tomii Takashi , Kondo Takahiro , Yagyu Shinjiro , Yamamoto Shigehiko
Published: July 2002

With a high resolution cross-correlation time-of-flight technique we have successfully eliminated the component of C2H6 molecules scattered after experiencing a temporary trapping on a LiF(001) surface at 300 K. The incident translational energies of C2H6 molecules are varied in the 260–700 meV range and the azimuthal directions of incidence are set at [100] and [110]. While the major peak in each of the time-of-flight spectra, which consists of rotationally excited C2H6 molecules upon single collision, can be well reproduced by a shifted Maxwell–Boltzmann speed distribution function, the temporarily trapped component is fitted to a single Maxwellian distribution with a characteristic temperature slightly higher than the target surface temperature. This temperature tends to increase from around 300 to 600 K with the translational energy of the incident C2H6 beam. The mean translational energy of the temporarily trapped component does not show any special angular dependencies in contrast to that of the directly scattered component which qualitatively follows the washboard model predictions. Temporarily trapped molecules are believed to be the ones which during the first few bounces have stored their translational energy in form of rotation, either cartwheel or helicopter mode, and tangential motion on a corrugated LiF(001) surface.
Microscopic composition difference related to oxidizing humidity near the ultrathin silicon oxide–Si(100) interface
Authors: Yamada Hiroshi
Published: July 2002

The numerical densities of Si and O atoms for 3.5–6.5-nm-thick silicon oxide films grown on Si(100) at oxidizing humidities of 0.5 ppb–500 ppm were determined by Rutherford backscattering spectrometry. Especially, the numerical density near the oxide–Si interface was rigorously investigated by comparing the results with the previous volumetric density evaluations using charged-particle activation analysis (CPAA). It was confirmed that excess Si atoms relative to the stoichiometric SiO2 composition exist near the interface and their number decreases with decreasing humidity. In addition, this humidity dependence was similar to that for the “slight discrepancy of the oxide thickness” (ΔTox) found by CPAA, which was strongly correlated with the humidity dependence of the volumetric density, device reliability, etc. Therefore, it is possible that all the humidity dependence has a common origin: Dehydration results in a reduction of the excess Si atoms near the interface, and thus produces a decrease in Si dangling bonds.
Evaluation of plasmas fed with hydrofluorocarbons–oxygen mixtures for SiO2 dry etching
Authors: Fracassi F. , d’Agostino R. , Illuzzi F.
Published: July 2002

The etching characteristics of SiO2 thin films were tested in plasmas fed with CH2F2, CHF2CHF2, and CF3CH2F in mixture with oxygen. The research was aimed to replace conventional perfluorocompounds with new compounds with lower impact on global warming. SiO2 etch rate and selectivity to Si as well as x-ray photoelectron spectroscopy surface composition of treated surfaces and anisotropy have been investigated.
Stable titanium silicide formation on field oxide after BF2 ion implantation
Authors: Mollat Martin , Demkov Alexander A. , Fejes Peter , Werho Dennis
Published: July 2002

The use of titanium silicide for low resistivity interconnects in a complementary metal–oxide–semiconductor process is investigated. After a source-drain processing, a wet oxide strip, and a 600 Å Ti deposition, a two-step anneal forms stable TiSi2 in the diffused regions and amorphous silicon gate. Extraneous regions or islands of TiSi2 were found to form on BF2 implanted thick field oxide, and were not present on B11, n-type (N+), or nonimplanted field areas. The growth and nucleation of TiSi in the presence of oxygen is discussed, and an oxygen solubility model is used to explain the nucleation of TiSi2 from a Ti5Si3 interlayer. Two models are presented to explain the availability of Si to form stable TiSi in field oxide regions. In the first, B is shown to promote the formation of oxygen vacancies resulting in a Si rich oxide, while the second involves oxide network strain from the incorporation of F in the oxide, facilitating Si segregation to the surface and subsequent availability of Si atoms.
Scanning field-emission force microscopy and spectroscopy of chemical-vapor-deposited carbon field-emission cathodes
Authors: Inoue Takahito , Ogletree D. Frank , Salmeron Miquel
Published: July 2002

A scanning-force microscope with an electrically conducting tip was used in both contact and noncontact scanning-polarization force microscopy modes to study the field-emission properties of diamond-like carbon chemical-vapor-deposited films in vacuum. Using the tip as an anode, the emission current and work function were measured with 100 nm lateral resolution. Emission was detected from individual micron-size grains. Large current fluctuations on a ms time scale were observed, correlated with large changes in surface potential, possibly due to charge trapping. There were no significant differences in work function between emitting and nonemitting regions, but the emitting regions showed low conductivity and large band gaps, while the nonemitting regions were either insulating or highly conductive. No asperities were observed at the film–vacuum interface. The current dependence on tip–sample separation suggests that emission occurs below, and not at, the film–vacuum interface.
Dependence of energy gaps with the stoichiometric deviation in a CuIn0.5Ga0.5Se2 ingot: A schematic band model
Authors: Dı́az R.
Published: July 2002

The optical properties of a CuIn0.5Ga0.5Se2 ingot with strong stoichiometric deviations have been analyzed. The value of the first energy gap has been obtained by fitting the experimental reflectance data to the function R=A0+A1/(hν)2−A2/(hν)3, whereas the values of three other gaps were obtained through the fit of the absorption coefficient, obtained by photomodulated spectral ellipsometry, to the function (α*hν)m=B*(hν−Eg) (m=2 or 2/3 for the direct or forbidden gap). The four transition energy values have been found to be in the 1.251–1.294, 1.837–1.996, 2.963–3.052, and 3.365–3.419 eV ranges, respectively. A correlation has been found between the first energy gap and the Ga sublattice occupation while the other energy gap values have been found to be associated with the Se position in the unit cell, as determined by the relative coordinate of the anion. On its turn, this position depends on the point defect concentration in the lattice. The energy gaps increase with the force constants of the Ga–Se bond. Each gap has been assigned to a transition in the band structure. Eg2 corresponds to the transition from the Γ5v(2) level at the valence band to Γ1c, the conduction-band minimum. For Eg3 and Eg4, the transition are from Γ4v(2) (the maximum valence band) to Γ3c and Γ2c, respectively. The shifts in the valence- and conduction-band levels have been found by analyzing the differences between energy gaps and have been associated with changes in the structural properties. The Eg2−Eg1 difference is affected by stoichiometric deviations. The Γ5v(2) level is closer to Γ4v(2) in samples with vacancy concentrations higher than in the stoichiometric ones.
Theoretical study of GaN molecular beam epitaxy growth using electron cyclotron resonance nitrogen plasma
Authors: Fu Wenning , Venkat Rama , Meyyappan M.
Published: July 2002

III–V nitrides (GaN, InN, and AlN) are intensely researched for optoelectronic applications spanning the entire visible spectrum. In spite of the realization of commercial devices and advances in processing of materials and devices, the understanding of the processing and epitaxial growth of these materials is incomplete. In this study, a rate equation based on physically sound surface processes to investigate the molecular beam epitaxial growth of GaN using an electron cyclotron resonance (ECR) plasma source is proposed. A surface riding layer of Ga and N plasma species is included in the model. The surface riding species are allowed to undergo several physical and chemical processes. Rates of all surface processes are assumed Arrhenius type. In the ECR plasma, the flux and reactivity of the active nitrogen are modeled based on plasma dynamics and used in our study. The necessary model parameters, which are unknown, were found by fitting results from simulation to experimental values. As the ECR power increases, the flux of active nitrogen and all other N species, which are by-products, increase almost linearly. Thus the Ga to active N flux ratio increases and hence Ga incorporation rate increases and saturates at a maximum rate. Results of growth rate versus temperature behavior are also presented and discussed based on physical mechanisms. Electron concentration obtained from bulk vacancy concentrations of Ga and N decreases linearly with ECR power, unlike the experimental observation of exponential decrease. The discrepancy is due to the fact that the electron concentration is strongly influenced by the incorporation of unintentional impurities from the plasma chamber such as Si, C, and O, which are not modeled in our study.
SCALPEL mark detection using Si/SiO2 and 100 keV backscattered electrons
Authors: Farrow R. C. , Mkrtchyan M. , Kizilyalli I. C. , Waskiewicz W. K. , Hopkins L. C. , Alakan A. , Gibson G. , Brown P. , Misra S. , Trimble L.
Published: July 2002

Scattering with angular limitation in projection electron-beam lithography (SCALPEL) marks for alignment and registration have been fabricated in SiO2 deposited in Si trenches using a process that is similar to that used for shallow trench isolation in complementary metal–oxide–semiconductor (CMOS) integrated circuits. The marks were detected using backscattered electrons in a SCALPEL exposure tool using 100 keV incident electrons. The signal-to-noise from the Si/SiO2 marks is comparable to that measured from Si/WSi2 marks fabricated in CMOS gate material. The Si/SiO2 marks fabricated from this process are a viable option for gate alignment to the thin oxide level and is extensible to circuits with critical dimensions less than 100 nm.
Sound velocity and Young’s modulus in plasma deposited amorphous hydrogenated carbon
Authors: Dillon R. O. , Ali Abbas , Ianno N. J. , Ahmad A. , Furtak T.
Published: July 2002

The first carbon-film sound velocities obtained with the piezoelectric method are given. The a-C:H films were deposited from methane using rf plasma chemical vapor deposition at different substrate biases and thus contain varying hydrogen concentrations. Measurements of density allowed the Young’s modulii of the films to be calculated. Both the sound velocity and the Young’s modulus reached a maximum as the substrate bias changed from −47 to −175 V. The film with the maximal properties occurred at a bias of −76 V and had a sound velocity of 16.4 km/s, a Young’s modulus of 589 GPa, an optical gap of 2.16 eV and a density of 2.19 g/cm3. Although the modulus is 52% that of the directionally averaged value of diamond, it produced a sound velocity 91% that of diamond due to the lower film density. The film densities were in the range of 1.81–2.43 g/cm3 with the densest films occurring at the highest bias magnitudes. Optical gap measurements were taken with a photospectrometer and yielded a gap that decreased from 3.86 to 1.40 eV with increasing bias magnitude and thus decreasing hydrogen content. These values indicated sp3 concentrations that increased with the gap from 43% to 94% and these were consistent with Raman spectroscopy results.
Dynamics and thermal stability of Cs superstructures on a Pt(111) surface
Authors: Kondo Takahiro , Kozakai Hiroyuki , Sasaki Takashi , Yamamoto Shigehiko
Published: July 2002

Utilizing thermal energy helium atom scattering we have made clear the dynamics from formation to destruction and the thermal stability of the Cs superstructures on a Pt(111) surface. Among three structures identified by low energy electron diffraction, both the (3×3) and the (3×3)R30° structures are unstable and are destroyed at around 350 K, and transform into the (2×2) structure. The (2×2) structure which is stable up to 450 K, exhibits large corrugation amplitudes of the potential energy surface. This suggests a strong bond between Cs and Pt atoms and the localization of electron charge distribution as a result of the charge transfer from Cs atoms to the substrate Pt(111) surface. It is also suggested that the work function minimum is closely related to this charge transfer of Cs atoms forming the (2×2) structure.
Ge-rich Si1−xGex nanocrystal formation by the oxidation of an as-deposited thin amorphous Si0.7Ge0.3 layer
Authors: Yoon Tae-Sik , Kim Ki-Bum
Published: July 2002

Ge-rich Si1−xGex nanocrystals are formed by the selective oxidation of Si during the dry oxidation processing of an amorphous Si0.7Ge0.3 layer. The oxidation kinetics of the alloy film in the temperature ranges from 600 to 800 °C are well explained by the classical model proposed by Deal and Grove with the activation energies of the linear rate and parabolic rate regime of about 1.35 and 1.02 eV, respectively. As a result of the selective oxidation process, Ge-rich Si1−xGex nanocrystals are formed with the size of 5.6±1.7 nm and with the spatial density of 3.6×1011/cm2 at 600 °C. With an increase of the oxidation temperature to 700 and 800 °C, the size of the nanocrystal is increased to about 20 nm. The variation of size of the nanocrystals as a function of temperature is explained considering the solid phase crystallization of amorphous film, oxidation rate, and grain growth.
Study of the SiO2/Si interface using spectroscopic ellipsometry and x-ray reflectometry
Authors: Itoh Hitoshi , Mitani Yuichirou , Satake Hideki
Published: July 2002

To analyze the structure of the SiO2/Si interface, we developed a method that uses spectroscopic ellipsometry and x-ray reflectometry simultaneously. We applied this method to the characterization of SiO2 grown in dry oxygen. We found that the thickness of the interface layer showed no significant dependence on the oxidation temperature and oxide film thickness. The mean value of the interface layer density was 2.31 g/cm3, clearly higher than that of the upper layer, which was 2.28 g/cm3. Furthermore, we found that the distribution of the interface layer density in the 6 in. wafer was similar to the distribution of the interface state density, Dit, which was obtained by the charge-pumping method using metal–oxide–semiconductor field-effect transistors with SiO2 formed in the same oxidation process. This fact strongly suggests that the density of the interface layer is correlated closely with the electrical characteristics of the SiO2/Si interface.
Carrier concentrations and deep trap concentrations in high temperature GaAs
Authors: Halder N. C. , Krishnan V. , Parshall D. , Totzke D. G.
Published: July 2002

The carrier concentration profiles and deep trap concentrations in molecular beam epitaxy-grown, Si-implanted, high temperature GaAs have been investigated. The samples were grown at 1023 K and the Schottky diodes were fabricated using Au contacts. The capacitance–voltage measurements were made in the temperature range 100–350 K at an interval of 2 K. The effective carrier concentration and the donor ionization energy both increased with the temperature. The energy of the carriers indicated a linear relationship, whereas the carrier concentration showed a two-step linear rise ending with a tendency to saturation at higher temperatures. The deep trap concentrations were determined from these measured values of the carrier concentration using (i) experimental data for the deep trap peaks obtained from field effect deep level transient spectroscopy measurement and (ii) the appropriate theory for the lambda effect.
Determination of Thermal Accommodation of Translational Energy of Vapors at a Glass Surface
Authors: Morrison J. A. , Tuzi Y.
Published: July 2002

A new method has been developed for the determination of translational thermal accommodation coefficients, αt, of vapors interacting with solid surfaces and has been used to measure αt for bromine, iodine, benzene, naphthalene, n-hexane, and ethylacetate on Pyrex glass in the temperature region 195 ° to 465 °K. The values obtained (0.55, 0.64, 0.37, 0.37, 0.47, and 0.46, respectively) have a probable accuracy of ±0.06 or better and are only slightly temperature-dependent.

The method, which can be easily adapted for the study of interactions with surfaces other than glass, requires the measurement of the mean velocity of molecules leaving the solid surface. This is accomplished by passing a selected beam of the molecules through the sensitive volume of an ion gauge. The only restriction on the method is that the pressure of the vapor at the temperature of the wall of the gauge should be very low (e.g., 10−12 Torr).
Use of Matching Cleavage Faces of Sodium Chloride in the Study of Thin-Film Growth
Authors: Stirland D. J. , Campbell D. S.
Published: July 2002

A method of examining matching faces of vacuum-cleaved gold-decorated rocksalt has been used to study nucleation behavior. It has been shown that there is no site for site correspondence between nuclei on the two surfaces even if the surfaces are given identical treatments and are not heated above 150 °C. Rearrangement of many of the surface steps occurs if the surfaces are heated above 150 °C. If one surface is exposed go air after cleavage, most of the surface features are completely changed.
Ion Storage in Three-Dimensional, Rotationally Symmetric, Quadrupole Fields. II. A Sensitive Mass Spectrometer
Authors: Dawson P. H. , Whetten N. R.
Published: July 2002

A mass spectrometer has been constructed in which ions are trapped in a three-dimensional, rotationally symmetric, quadrupole field. Ions of a specific charge to mass ratio are stored and then periodically drawn out of the trapping field into an electron multiplier. The ion trapping feature permits integration of the ion current over long periods of time. Electrodynamic ion trapping also enables a long effective path length to be attained in a small size device. Ions have been stored for several days using a low-resolution mode. Experimental data are presented concerning storage time, resolution, and sensitivity. Factors affecting the peak shape include storage time, resolution, and bias voltage between the end-caps. The theoretical basis for the trapping of charged particles in a three-dimensional, quadrupole field is discussed in a companion paper.
Controlled Preparation of Alloys by Simultaneous, Multitarget Sputtering
Authors: Segatto Peter R.
Published: July 2002

This work describes the preparation of alloy films by multitarget, triode sputtering. An apparatus designed to sputter two or three targets simultaneously onto a rotating substrate holder is described. The sputtering parameters necessary to vary and reproduce alloy composition are discussed. Changes in alloy composition are obtained by voltage variation of the several targets. Reproducible film compositions are assured by maintaining target voltage, geometry, and sputtering atmosphere constant. Zero or a constant partial pressure of hydrogen is required to obtain reproducible alloy properties. Composition, resistance, and thickness data are presented for the alloy systems Nb–Zr and Nb–Zr–Mo. Sheet resistance values for these films are reproducible to ±2%.
Calibration to High Precision in the Medium Vacuum Range with Stable Environments and Micromanometer
Authors: Ruthberg Stanley
Published: July 2002

Ultrastable vacuum environments are combined with a micrometer-point contact manometer to obtain a calibration uncertainty of 1.2×10−4 Torr + 6 parts in 105 of the reading for the range 1×10−4 Torr to several Torr. The stable pressures are generated with a dynamic vacuum system and a servo control loop which employs cascaded error signals from a capacitance-diaphragm gauge to obtain pressure stabilities of better than ±1 part in 105 at 1 Torr. Precise evaluation of systems and procedures is given. Operational factors that affect accuracy were studied, for example, sorption in the manometer fluid, pressure distribution in the test chamber, time constants of the micromanometer, and thermal response of the control loop.
Accurate Pressure Measurements in the Region 0.1–1000 mTorr; The Intercalibration of Two McLeod Gauges and a Capacitance Manometer
Authors: Bromberg J. Philip
Published: July 2002

The capacitance manometer appears to be a convenient and reliable pressure measuring device over the range 0.1–1000 mTorr. We find that deviations from linearity in this region do not exceed a few tenths of a percent. On the other hand, mercury streaming in a trapped McLeod gauge can introduce errors in the measured pressures which are dependent upon the pressure in a complicated manner.
Methane production and sorption capacities of flashed barium getters
Authors: Lange W. J.
Published: July 2002

Evaporated barium film getters are heavily relied upon by the TV tube industry to provide the required low−operating pressures and low−partial pressures of gases which are deleterious to cathode operation and life. The studies reported here, utilizing nitrogen doped getters, show that while they are effective for nitrogen, carbon monoxide, carbon dioxide, and oxygen, the getters themselves are a major source of methane which is not sorbed. The capacity for carbon monoxide, the customary test gas for such getters, is found to agree with manufacturers’ data. After saturation with carbon monoxide, comparable residual capacities remain for carbon dioxide and oxygen. The methane evolved during flashing of the getters is attributed to hydrogen and carbon impurities which are found in the unfired getter material by independent analyses.
Tubular hollow cathode sputtering onto substrates of complex shape
Authors: Thornton John A. , Hedgcoth Virgle L.
Published: July 2002

Hollow cathode sputtering, wherein substrates are mounted on the axis of an elongated tubular source, provides an attractive method for coating objects of complex shape. For a uniform current density and cosine emission, the sputtered flux at all points within the cathode where end effects are unimportant is equal to the cathode erosion flux.This paper describes experiments in which thick (∠5 mil) OFHC copper coatings were deposited at reasonably high rates (∠4000 Å/min) onto substrates incorporating deep channels as well as convex and concave corners. Argon working gas pressures were varied from 1 to 150 mTorr; average substrate bias current density from zero to 1.25 mA/cm2. Coating thickness distributions and microstructures were examined by preparing metallographic cross sections. Substrate heating and plasma measurements are also reported. Low pressure operation yielded best microstructures and most complete surface coverage. The method compares favorably with ion plating for many applications.
Photoelectron spectra of condensed gases on an inert substrate
Authors: Yu K. Y. , McMenamin J. C. , Spicer W. E.
Published: July 2002

There have been numerous attempts to use ultraviolet photoemission spectroscopy (UPS) to monitor the chemical states of adsorbed gas molecules on metal surfaces. To interpret the data correctly, one has to determine the effect of photoemission on the measured energy levels of the molecule. We have measured the UPS spectra of seven gases (C6H6, C5H5N, CH3OH, C2H5OH, H2CO, H2O, NH3) condensed on a LN2 cooled MoS2 substrate at hν = 21.2 eV. The inertness of the MoS2 substrate assures that no strong chemical bonding exists between the substrate and adsorbed molecules. For each gas, the spectrum of the condensed phase is similar to the corresponding spectrum of the gas phase except all the energy levels are shifted up by the same amount. This shift ranges from 1 to 1.65 eV for the gases studied. The energy shift is attributed to the dielectric screening of the hole produced during the optical excitation.
On the chemisorption of oxygen on platinum (111) surfaces
Authors: Joebstl Johann A.
Published: July 2002

Chemisorption of oxygen on clean platinum (111) surfaces at room temperature can be readily observed by the development of a (2×2) −O LEED pattern provided the carbon monoxide content of the residual gas is less than 0.5%. The (2×2) −O pattern disappears during reevacuation of the surface research system, indicating that a critical ratio of pO2/pCO is required for the formation of an adsorbed layer of oxygen. These results suggest that similar ’’clean−off’’ reactions occur in the platinum/oxygen chemisorption system as were observed recently by other researchers for the silver−oxygen and copper−oxygen systems. These results may also explain the failure to observe the chemisorption of oxygen on low index surfaces of platinum in vacuum systems which have been extensively used for studies of carbonaceous gases.
Nondestructive determination of energy-gap grading in thin films by optical transmission measurements
Authors: Hall W. F. , Tennant W. E. , Cape J. A. , Harris J. S.
Published: July 2002

In hererojunction devices where band-gap variation is achieved by control of alloy composition, grading of material properties may extend a micrometer or more from the metallurgical interface. For a narrow-gap film on a wide-gap substrate, this grading affects the optical absorption for frequencies above the fundamental edge in a particularly simple fashion. It is shown in the work reported here that the frequency variation of the optical absorption in this case can be used to determine the depth profile of the energy gap, provided only that the dependence of bulk optical properties on composition is known. A specific application to GaAs1−xSbx films on GaAs is reported in which the depth profile is obtained with a resolution of 0.1 μm.
Prepration of alloy films of constant composition by diffusion-limited evaporation from the liquid state
Authors: Ast D. G. , Krenitsky D. J.
Published: July 2002

A new type of evaporation source based on diffusion-controlled steady-state evaporation from the liquid state was constructed and operated with a Cu–Ag alloy. The result of a compositional analysis of the deposited films as a function of time agreed well with the theoretical prediction which was based on numerical solutions of the diffusion equation. The diffusion constant of Ag in liquid Cu at 1675 K was found to be 2.01×10−5 cm2/s which is within a factor of 2.4 of the value estimated from the self-diffusion of liquid Cu, corrected for the different mass of Ag. The compositional output of the new source is generally much more stable than the traditional open pot source and can, via the source geometry and operating temperature, be adjusted to give a constant compositional output for the life of the source.
Angular-resolved UPS studies of the chemisorption of ethylene and acetylene on nickel (100)
Authors: Horn K. , Bradshaw A. M. , Jacobi K.
Published: July 2002

In photoemission from surfaces, simple selection rules govern the relationship between normal exit emission and the symmetry of the initial state wave function. UPS measurements on CO/Ni(100) with unpolarized light have already shown that the dependence of normal emission intensity on photon angle of incidence can be used for the assignment of adsorbate molecular orbitals. In the present paper the method is extended to the (√2×√2) R45 structures formed by C2H2 and C2H4 on Ni(100) at 80 K. Heating the surfaces formed by the interaction at low temperature results in pronounced changes in the UPS spectrum, which in turn indicates changes in the chemical nature of the adlayers. An ordered surface structure is, however, retained until above room temperature.
Oxygen adsorption on polycrystalline Cu films studied by means of gravimetric uptake and work function change
Authors: Benndorf C. , Egert B. , Keller G. , Seidel H. , Thieme F.
Published: July 2002

Past investigations into the interactions of oxygen with copper have been made in which the superstructures caused by oxygen adsorption on the main crystallographic orientations were studied by low energy electron diffraction (LEED). Although the low sticking coefficient of oxygen on Cu is generally accepted, no systematically gravimetric measurements of the oxygen adsorption are available. We report our results from combined measurements on evaporated Cu films by means of gravimetric mass gain and work function changes up to an oxygen dosage of 7×105 L.
Application of matrix isolation spectroscopy to the measurement of sputtering yields
Authors: Steinbrüchel Christoph , Gruen Dieter M. , Dawson Jane
Published: July 2002

Matrix isolation spectroscopy has been used to measure sputtering yields of D+ and Ar+ on Au and Ti in the energy range 0.25–2.0 keV. The technique involves bombarding a target with ions produced by a sputter ion gun, collecting sputtered atoms in a cryogenic noble gas matrix, and determining the amount of metal collected by optical spectroscopy. The present preliminary results are compared with previous results obtained by more conventional experimental techniques.
Chemical characterization from carbon Auger spectra by application of pattern recognition and factor analysis
Authors: Gaarenstroom Stephen W.
Published: July 2002

The feasibility of using pattern recognition and factor analysis to extract chemical state information from Auger line shapes was explored. These data analysis techniques were applied to x-ray-excited carbon Auger spectra. It was found that Auger line shapes could be described by using the amplitudes of selected frequencies from the Fourier-transformed Auger spectrum as variables during the computations. Factors analysis was found to predict the length of the alkyl chain in a series of poly(alkyl methacrylate) spectra and also indicated a chemical state difference in carbonaceous residues on sheet steels exhibiting good corrosion resistance and poor corrosion resistance.
Surface impurity studies in the ISX-A tokamak
Authors: Emerson L. C. , Clausing R. E. , Heatherly L.
Published: July 2002

The Impurity Study Experiment (ISX-A) was a tokamak designed to study impurity sources, plasma wall interactions, impurity behavior within the plasma, and related phenomena. An Auger electron spectrometer, attached to the tokamak by means of a UHV sample transfer system, has been the primary tool used for surface studies of the first wall. Stainless steel samples positioned at the wall edge were used to follow the progressive ’’cleanup’’ of the torus wall. Cleaning consisted of low energy hydrogen discharge plasmas, and later titanium gettering. The principal contaminants initially were carbon, oxygen, and sulfur. As cleaning proceeded, the surface oxide layers were reduced to substoichiometric levels, and the carbon deposits appeared to be converted to metallic carbides. The sulfur was largely removed. A decrease in the effective nuclear charge of the plasma could be correlated with these surface changes. Discharge cleaning reduced Zeff from ∠4 to 1.6 over a period of several months while titanium gettering produced values approaching 1.0.
Critique of the empirical tight-binding method for semiconductor surfaces and interfaces
Authors: Pantelides Sokrates T. , Pollmann J.
Published: July 2002

The empirical tight-binding method has been used extensively over the last several years to study the electronic and atomic structure of semiconductor surfaces and interfaces. The purpose of this paper is to provide a critical review of both the method itself and the variety of techniques that have been used to implement the basic idea. The foundations of the method are examined, identifying its strengths, its limitations, the types of properties it can describe reliably, its range of applicability and its overall usefulness. Individual techniques e.g., slab techniques, transfer-matrix techniques, Koster–Slater techniques, etc.), which often introduce additional assumptions and approximations, are discussed and compared. Selected applications of the method, both to electronic- and atomic-structure problems, are described and discussed in order to illustrate the general conclusions.
Development of bakeable seals for large noncircular ports on the Tokamak Fusion Test Reactor
Authors: Fleming R. B. , Brocker R. W. , Mullaney D. H. , Knapp C. A.
Published: July 2002

The Tokamak Fusion Test Reactor (TFTR), being constructed at the Princeton Plasma Physics Laboratory, has a total of 60 large ports of rectangular and racetrack shape that must be equipped with demountable seals. The largest port is a rectangle about 0.74×0.89 m. Seals must be compatible with a 10−8 Torr base pressure, be bakeable to 250°C, and be capable of remote demounting and resealing. A literature search and initial experiments showed that conventional commercially available sealing approaches were not useful, partly because of the large sizes and the noncircular shapes. Successful tests were made with ’’Helicoflex’’ seals. These seals consisted of a helical Inconel wire, surrounded by a stainless steel jacket that is covered with a soft metal, and were compressed between flat flange surfaces. General results were: a silver covering performed better than aluminum; with a smooth flange surface, higher compression produced better results; and performance was improved by striations produced in flange surfaces along the direction of the seal axis.
Chemisorption of oxygen and aluminum on the GaAs (110) surface from a
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Authors: Barton John J. , Swarts Coenraad A. , Goddard William A. , McGill T. C.
Published: July 2002

We have applied quantum chemical methods to the structure of the clean GaAs (110) surface and the chemisorption of oxygen and aluminum. We find that results for small clusters give geometries for the clean surface in agreement with those observed experimentally. We propose that an intermediate stage of oxidation would exist in which the O atom binds to a surface As. Our studies indicate that the initial site for Al chemisorbed on GaAs (110) has the Al bonded to the surface Ga. Geometries and chemical shifts are reported for both O and Al on GaAs (110). The chemical shifts are in agreement with recent experimental results.
Microfabrication of hemispherical shells for laser fusion targets
Authors: Goldstein I. S. , Kalk F. , Trovato J.
Published: July 2002

Hemispherical shells for use in laser fusion targets have been fabricated by using reactive ion etching to produce a hemispherical original master pattern from a selected glass microsphere. Then micromolding techniques using silicone rubber and positive photoresist are employed to produce a large number of final molds which are coated with the shell material (parylene or copper). The flat background film is removed by a number of etching techniques and the final molds are dissolved to give free standing hemispherical shells. The shells were found to replicate the hemispherical section of the original microsphere within the measurement accuracy of about 3%. The advantages and current limitations of this technique are also discussed.
Electronic structure of defects at Si/SiO2 interfaces
Authors: Herman F. , Kasowski R. V.
Published: July 2002

After summarizing the essential features of an idealized model of the Si/SiO2 interface, we discuss first-principles EMTO calculations of the electronic structure of SiO2 supercells and Si/SiO2 superlattices containing periodic arrays of noninteracting localized defects. Among the defects investigated were Si and O interstitials and vacancies; interstitial O2, SiO, and H2O molecules; and substitutional Al-interstitial H complexes. As an application of these theoretical studies, we suggest an interpretation of the U-shaped continuum of localized interface states which is often derived from experimental measurements of suitably processed Si/SiO2 interfaces.
Angle-resolved surface Penning ionization: CO chemisorbed on Pd(110)
Authors: Wang S.-W.
Published: July 2002

First principle calculations are presented of angular distributions of electrons ejected from the CO-covered Pd(110) surface using metastable He* beams as soft probes (i.e., surface Penning ionization). Excellent agreement has been obtained in comparison with recent available experimental data. These theoretical results indicate for the first time the importance of intermolecular and intramolecular shadow effects and surface corrugations and show that CO stands perpendicularly to the surface with oxygen towards the vacuum. The great surface sensitivity of angle-resolved surface Penning ionization (ARSPI) to adlayer electronic and geometrical structures has opened a new area for surface applications.
Failure mechanism of TV tubes
Authors: Wu Chong-ruo , Zhu Yu-hua , Chen Zhang-qi
Published: July 2002

In an attempt to extend the life of TV tubes, several factors causing the failure of TV tubes have been studied. The residual gas composition in TV tubes operating for more than 5000 h, and the residual gas in many TV tubes after failure has been analyzed with an omegatron. With the aid of AES, SEM, and SIMS the coating and the base metal of the oxide-coated cathodes have also been studied. It has been found that the main source of gas release is the EID of the fluorescent screen. The rate of desorption decreases with time. The influences of the coating materials and technology on EID have been measured. The method of reducing EID is discussed. A barium film getter is able to sorb a large number of oxy-gases, but it also introduces some argon into the tubes. The getter containing the argon, the sorption mechanism of argon, and a way to lower the argon pressure have been studied in some detail.
Adsorption of O2 and CO on cleaved GaAs(110) at low temperatures
Authors: Frankel D. J. , Yukun Y. , Avci R. , Lapeyre G. J.
Published: July 2002

We have studied the low-temperature (∼50 K) adsorption of O2 and CO on GaAs (110) with UV photoemission. Spectra were taken using a He discharge lamp and a double pass cylindrical mirror analyzer (CMA) in the angle-integrated mode. Spectra showing peaks characteristic of gas-phase O2 and CO were observed after dosing GaAs (110) surfaces cleaved at a temperature T ≂50 K with <5 L of O2 or CO. The molecular CO peaks disappeared on warming the sample to 100 K leaving a spectrum characteristic of the clean surface. In the case of O2, the O2 1πg orbital was 5.4 eV below the valence band maximum (VBM) and 13.2 eV above the Ga 3d core, which implies a relaxation shift towards smaller binding energies of ∼1.4 eV for adsorbed O2 as compared to the gas phase. On warming the O2-dosed sample the molecular peaks disappeared by ∼60 K and by 300 K the spectrum was transformed to one characteristic of chemisorbed oxygen. Auger analysis indicated that oxygen concentrations of a few percent remained at 300 K after O2 dosing with only 10–40 L at 50 K. The 300 K spectra are attributed to atomic rather than molecular oxygen, which implies a transition from the physisorbed molecule to the chemisorbed atom.
Tritium storage/delivery and associated cleanup systems for TFTR
Authors: Gill J. T. , Anderson B. E. , Watkins R. A. , Pierce C. W.
Published: July 2002

The tritium storage and delivery system (TSDS) will provide the tritium for high power plasmas in TFTR. TSDS will store tritium as solid uranium tritide at room temperature. At ≳400 °C gas may be released from uranium and pumped through a metal bellows pump to a holding volume. Acceptably pure T2 will be passed along a ∼100-ft manifold into the storage volumes of three piezoelectric gas injection valves above the torus. Tests on several above components are described. Process components are contained within evacuable housings and three seismically qualified gloveboxes. Two of these boxes are serviced by catalytic oxidation/dessicant adsorption-based T2 cleanup systems, one of which is dedicated and the other also detritiating torus and neutral beam exhaust gases. TSDS is located in a sealable vault, it is air purifiable by a third such system. Emergency dehumidification of the torus test cell and its basement is also provided. TSDS and the cleanup systems will operate remotely under hardwired interlocks and operator–interactive computer control. Interlocks are safety-oriented and, e.g., prevent overheating of uranium, overpressurization of bellows pumps, and large glovebox pressure excursions. Tritium monitors are important to cleanup interlocks.
Process monitoring of tritium concentration
Authors: Ellefson R. E.
Published: July 2002

Tritium fueling and recycling processes for the fusion industry need measurements of tritium concentrations, at various points, for control decisions. A prototype process sampling and analysis system built to monitor tritium concentration in a thermal diffusion column (used to enrich tritium) is described. The system draws a 2-cm3 gas sample from a selected point and analyzes tritium concentration. Concentration is determined using a beta scintillation detector to measure the tritium partial pressure in ratio to the total sample pressure measured, using a 1 Torr capacitance manometer. The percent relative (% rel) calibration accuracy of the beta scintillation detector is 1.4% rel/°C plus the electrometer zero drift. Accuracies ≤6% rel for 1 mol % of tritium reducing to ≤1.5% rel at 90 mol% tritium are routinely produced in a typical air-conditioned laboratory. Temperature control of ±0.1 °C improves accuracies to one-fifth the level for ambient conditions. For high purity tritium (>90 mol%), a miniature quadrupole mass spectrometer with Faraday ion detection is required to assay tritium purity by differences (100% minus nontritium components). The detection limits of 0.2 mol% for DT and 0.5 mol% for HT and HD place ultimate limits on purity measurements. The sampling and analysis system is fully automated allowing arbitrary sampling points or a timed sampling sequence. A method is also presented for direct measurement of tritium partial pressure in exhaust lines of a fusion device.
Initial wall conditioning for the TMX-U fusion experiment
Authors: Allen S. L. , Clower C. , Drake R. P. , Hooper E. B. , Hunt A. L. , Munger R. , Bastasz R. J. , Bauer W. , Hsu W. L.
Published: July 2002

The introduction of impurities and hydrogen from the walls and liners of the TMX-U device can have a deleterious effect on the plasma. The area of these surfaces is large (∼107 cm2) and has a complicated shape, so cleaning is difficult. Techniques that have been used to condition these walls include (1) initial solvent cleaning; (2) baking; (3) glow discharge cleaning; (4) gettering; and (5) plasma clean up. Several diagnostic techniques, including UV and visible spectroscopy and residual gas analysis, have been used to evaluate the effectiveness of the conditioning procedures. A series of experiments was performed to optimize the glow discharge in the TMX-U geometry; during approximately 80 h of glow discharge cleaning, low-Z impurities were removed. Gettering along with pumping with liquid-nitrogen-cooled liners and cryopumps resulted in base pressures ∼10−8 Torr. This paper describes these procedures and their importance in the formation of initial plasmas in TMX-U.
Dynamic dielectric response to carrier–carrier interactions in narrow-gap semiconductors
Authors: Meyer J. R. , Bartoli F. J.
Published: July 2002

For the first time, the dynamic dielectric response of the screening medium within a narrow-gap semiconductor is properly taken into account in treating electron–hole scattering. The interaction potential is shown to have the strikingly simple form U(q,ω)=V(q)/ε (q,ω=q⋅vh), where V(q) is the bare Coulomb potential, vh is the hole velocity, and ε(q,ω) is the dielectric constant of the medium. The screening medium is assumed to consist of core electrons, lattice ions, free electrons, and free holes. The effects of dynamically treating the slowly responding lattice ions and free holes are discussed in detail. It is shown that previous approximations of employing the static and high frequency limits lead to significant error in the calculated electron–hole scattering mobility.
Test of structural models for Ag{110}1×2–O by LEED intensity analysis
Authors: Zanazzi E. , Maglietta M. , Bardi U. , Jona F. , Marcus P. M.
Published: July 2002

A quantitative LEED intensity analysis of the Ag{110}1×2–O structure is described. Thirteen structural models were tested with the conclusion that, while the model of oxygen atoms in the bridge sites within the ‘‘troughs’’ of the substrate surface is better than the others in approaching agreement with experiment, none of the models tested in this work is satisfactory. A set of experimental LEED intensity data is presented.
Outgassing characteristics of TiC-coated graphite
Authors: Sukenobu S. , Gomay Y.
Published: July 2002

Outgassing characteristics of POCO graphite coated with TiC by chemical vapor deposition are reported. Results obtained from fully TiC-coated samples showed that the coating layers are not completely gas tight. The air included in the porous part of the graphite was gradually released at an outgassing rate on the order of 10−10 Torr⋅l/s⋅cm2. After exposing a part of the graphite surface, the outgassing rate was reduced to the order of 10−12 Torr⋅l/s⋅cm2, after vacuum baking at 250 °C for 24 h.
Electron spectroscopy study of SiC
Authors: Bozso Ferenc , Muehlhoff Lucia , Trenary Michael , Choyke W. J. , Yates John T.
Published: July 2002

A silicon carbide single crystal has been studied using x-ray photoelectron spectroscopy and x-ray and electron excited Auger spectroscopy. A procedure for producing an atomically clean SiC crystal surface has been perfected. The SiC exhibits prominent bulk plasmon loss features associated with Si and C photoelectrons. This loss, at 22.5 eV, agrees well with optical data establishing the bulk plasmon energy.