bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth, atomic structure, and oxidation of chromium overlayers on W(110)
Authors: Shinn Neal D. , Berlowitz Paul J.
Published: July 2002

Novel structural and chemical properties of Cr monolayer, bilayer, and multilayer films on W(110) have been identified using temperature programmed desorption, low-energy electron diffraction, and Auger electron spectroscopy. A pseudomorphic (1×1) Cr monolayer forms at T>100 K which is stable up to desorption at ∼1290 K despite a ∼9% lattice mismatch. The second monolayer forms a metastable (2×2) structure in the 500 K <T<800 K range, for which a quad-cluster model is proposed. An (11×11) coincidence lattice is observed for multilayer films. Oxidation of the Cr overlayers leads to both an interfacial oxide and Cr2O3-like phases. Cr2O3/W(110) incommensurate epitaxy is observed for thick Cr films.
Step structure and dimer row correlations in vicinal Si(100)
Authors: Aumann C. E. , Savage D. E. , Kariotis R. , Lagally M. G.
Published: July 2002

The surface structures of 7°, 3.5°, and 2.5° vicinal Si(100) samples tilted about [011̄] have been studied with low-energy electron diffraction. Double-atomic-height steps can be formed on these surfaces after sufficient heat treatment. Angular profile analysis of the integral-order diffracted beams is used to determine step height, average terrace size, terrace size distribution, and degree of step edge roughness. Angular profile analysis of the fractional-order diffracted beams is used to determine the relative fraction of single-atomic-height steps and to measure the degree of correlation in the (2×1) dimer row shifts. The latter is seen as a splitting in the (2×1) beam that is one-half the splitting distance in the (00) beam. This splitting is observed to be reversible with temperature, indicating a phase transition in the configuration of (2×1) dimer row shifts.
Effects of sputtering through small apertures on the refractive index of niobium pentoxide films
Authors: Hickernell Fred S.
Published: July 2002

Niobium pentoxide (Nb2O5) can be reactively rf sputtered to produce high-index low-loss amorphous films for use as integrated optical (IO) components. Tight tolerances are required on refractive index, loss, scatter, and geometric form. Of particular interest to IO devices are thin-film Luneburg lenses which have been successfully employed on lithium niobate (LiNbO3) based acousto-optic spectrum analyzers. To develop a particular lens configuration it is necessary to sputter through a circular aperture mask in contact with the LiNbO3. Experimental evidence has shown that there is a significant drop in refractive index under fixed deposition conditions as the ratio of aperture diameter to mask thickness A/t is decreased. For an A/t ratio of 7.0 the index is 2.272 while an A/t ratio of 1.0 results in an index of 2.224. Increasing the power to the Nb target and bias sputtering have been successfully applied to raise the index. An increase in power from 70 to 400 W raised lens indices from 2.220 to 2.280. Bias sputtering similarly increases lens indices but distorted the lens profile. The mechanisms influencing refractive index change in amorphous Nb2O5 films sputtered under varying deposition conditions are discussed.
Quantitative analysis of Cu implanted Co/Ni alloys by Auger electron spectroscopy/depth profiling using factor analysis
Authors: Wandass J. H. , Turner N. H.
Published: July 2002

Principal component analysis (PCA) methods were used to analyze Auger electron spectroscopy (AES) depth profile information from a Cu ion implanted 50–50 Co–Ni alloy. PCA has advantages (effective use of entire spectral region, analysis of complex spectral mixtures) over other methods of quantitative analysis by AES. The base alloy was ion implanted with Cu to an estimated peak atomic concentration of 5% at a depth of 50 nm. Previous PCA results on this sample using the LMM AES lines showed additional components possibly due to energy scale variations, alloying, and sputter effects. In this study, the MVV AES transitions were used and provided a severe test for PCA, since all the transitions are within a 15-eV window, and a large secondary electron background is present. If background correction was not used, the minority Cu component was not detected. With background corrected data and the use of three standards, the results indicated that there were only three components, however the calculated Cu atomic concentration was 1.2%, possibly indicating preferential sputtering. In addition, Ni surface enrichment was suggested. Exclusion of the Cu standard during PCA indicated a third component existed. The use of interexperimental standards for PCA is not recommended since it can suggest the existence of additional components. Electron beam current changes during profiling, modulation voltage settings, rf interference with system controller units, and ion induced Auger signals from A1 masking materials contributed to the errors in the experiment.
Cesium ion transport across a solid electrolyte–porous tungsten interface
Authors: Kim S. I. , Seidl M.
Published: July 2002

The electrified interface between a cesium–mordenite solid electrolyte and a porous tungsten electrode has been studied theoretically and experimentally. This interface double layer plays a major role in a novel cesium ion source of high current density. A self-sustained theory of the solid electrolyte–metal interface is presented. The coverage of cesium on the surface of the porous tungsten electrode (i.e., inner layer) is calculated to be 0.3–0.35 in the temperature range 800–1100 °C. The concentration profile in the diffusion layer is obtained numerically in equilibrium and steady state. The Cs+ ion emission current–voltage curves have been measured experimentally as a function of temperature and the biasing current. Cs+ ion emission current of up to 10 mA/cm2 has been extracted for 500 h at 1100 °C in steady-state operation. The emission current can be controlled by the biasing current.
Determining depth profiles from angle dependent x-ray photoelectron spectroscopy: The effects of analyzer lens aperture size and geometry
Authors: Tyler Bonnie J. , Castner David G. , Ratner Buddy D.
Published: July 2002

Lens apertures that reduce the solid angle of photoelectrons directed to the detector can be used to better resolve the variations in x-ray photoelectron spectroscopy (XPS) signals which occur with electron takeoff angle. The effects of aperture size and geometry on angle-dependent XPS data, and on the depth profiles calculated from the data, have been investigated. Both theoretical calculations and angle-dependent XPS experiments have been used in this study to determine the effects of apertures. Circular and slit geometry apertures varying in size from a 6° solid collection angle to a 30° solid collection angle were studied. Samples consisting of a substrate covered by a thin, uniform overlayer were examined. At low photoelectron takeoff angles, the differences between the apertures were insignificant. At high photoelectron takeoff angles, decreasing the size of the aperture improved surface sensitivity. Decreasing the width of the aperture also sharpened the transitions between layers in a generated depth profile. Even data collected through a 30° solid angle circle, however, can be used to determine useful information about the concentration depth profile of a sample. The effects of the aperture size on the estimated depth profile are similar to the effects of random noise, so improvements in angular resolution must be balanced against the signal-to-noise ratio. Slit geometry apertures offer a good compromise between increased resolution and a high signal-to-noise ratio.
Electron stimulated desorption of oxygen from, and subsequent type conversion of, thin-film p-CuInSe2
Authors: Matson R. J. , Kazmerski L. L. , Noufi R. , Cahen David
Published: July 2002

There are seemingly conflicting results from two sets of investigations in the literature concerning the electron stimulated desorption of oxygen incorporated into thin-film p-CuInSe2 This has a direct bearing on the model for the role of oxygen in the important postdeposition oxygen treatment of CdS/CuInSe2 solar cells, proposed in one of the published reports. By comparing the specific beam parameters of the scanning electron microscope (SEM)/ electron beam induced current (EBIC) versus scanning Auger microprobe (SAM) based studies and calculating charge density for the two cases, the SAM experiments are seen to support, rather than contradict, the results of the SEM/EBIC experiments. The evidence tying the addition and deletion of oxygen to changes in the electrical properties of the CuInSe2 are then discussed.
Possible evidence of single electron tunneling with the scanning tunneling microscope at room temperature
Authors: Valdeś J. , Choleva E.
Published: July 2002

Contradictory previous results concerning I–V curves for metal–insulator–metal tip to sample tunnel junctions are pointed out. In this paper we present experimental data acquired in ambient conditions, tested with different measuring methods, showing stable I–V curves, nonlinear at least for tunneling voltages higher as 100 mV. We also confirm the scanning tunneling microscopy (STM) behavior as a diode by mixing two microwave signals of different frequencies in the same way as a normal diode connected instead of the tip–sample junction does. The shape of the I–V curves that we obtained for low applied voltages in ambient conditions resemble those reported by other authors at helium temperature as giving evidence of a single electron tunneling (SET) current between tip and surface. Adopting the SET model, the voltage difference between shifted hypothetical asymptotes would imply here a dynamical capacitance C between tip and sample of the order of 10−19 F, which is a value low enough as to fulfill with the condition required for SET effects to become evident at room temperature. Finally we report some of the difficulties in a direct detection of the SET frequency, if any, in order to realize a dc quantum standard by using the STM set up at room temperature.
Prevention of oil vapor backstreaming in vacuum systems by gas purge method
Authors: Tsutsumi Y. , Ueda S. , Ikegawa M. , Kobayashi J.
Published: July 2002

Mechanical pumps have been used as roughing pumps in many vacuum systems because of the ease of their operation. However, it is a well known fact that heavy oil vapor backstreaming occurs when mechanical pumps are operated at pressures close to their ultimate pressure. This backstreaming oil vapor contaminates the vacuum system. Recently, oil contamination has become a serious problem in the semiconductor manufacturing process. In this paper, the results from experiments to prevent oil vapor backstreaming in a pipe that is 9 cm in diam. and 220 cm in length in the transition flow region, using gas purge is examined. Backstreaming oil is measured quantitatively with and without gas purge using a quadrupole mass analyzer and a differential pumping system. The results show that even in the transition flow region, oil vapor backstreaming is prevented by a small flow rate of purge gas.
Improvement in the method of searching for the fractal nature of rough thin-film top surfaces
Authors: Varnier F. , Llabaria A. , Rasigni G.
Published: July 2002

It was previously shown how the fractal nature of thin-film top surfaces can be obtained from the surface relief reconstructed by microdensitometer analysis of an electron micrograph of a shadowed replica of the surface. Here, we present some improvement in the determination of the fractal dimension Dl which characterizes a planar section of the thin film, as well as a different way of conducting such a determination. Results connected with several rough thin films are given and it is found that many different ways of characterizing the fractal nature of a thin-film top surface are possible from the reconstructed relief of the surface to be studied.
Scanning tunneling microscopy of biomolecules
Authors: Miles M. J. , McMaster T. , Carr H. J. , Tatham A. S. , Shewry P. R. , Field J. M. , Belton P. S. , Jeenes D. , Hanley B. , Whittam M. , Cairns P. , Morris V. J. , Lambert N.
Published: July 2002

We have used STM to study a range of biological molecules. The necessity of immobilizing the molecules on the surface was recognized and achieved by the formation of ordered layers or by the addition of chemical groups which strongly adsorbed to the graphite substrate. Clear images of the helical structure of the synthetic polypeptide poly γ-benzyl-L-glutamate were obtained. Images of protein molecules isolated from wheat were found to be consistent with the rare β-spiral conformation. The binding of the mutagen ethidium bromide to DNA was also studied and images of the cyclic glucan β-cyclodextrin, tosylated to improve adsorption of the molecule to graphite, were also obtained. Although the contrast mechanism is unclear in some of these examples, these results suggest that STM can be applied successfully to a wide range of biomolecules.
Molecular dynamics and quasidynamics simulations of the annealing of bulk and near-surface interstitials formed in molecular-beam epitaxial Si due to low-energy particle bombardment during deposition
Authors: Kitabatake M. , Fons P. , Greene J. E.
Published: July 2002

Molecular dynamics and quasidynamics simulations, utilizing the Tersoff many-body potential, were used to investigate the relaxation, diffusion, and annihilation of split and hexagonal interstitials resulting from 10 eV Si irradiation of (2×1)-terminated Si(001). The interstitials were created in layers two through six. Stable atomic configurations and total potential energies were derived as a function of site symmetry and layer depth. The interstitial Si atoms were allowed to diffuse and total potential energy changes calculated. Lattice configurations along each path, as well as the starting configurations, were relaxed and minimum energy diffusion paths derived. The results showed that the minimum energy paths were toward the surface and generally involved tetrahedral sites. Calculated interstital migration activation energies were always less than 1.4 eV and were much lower in the near-surface region than in the bulk. Thus, the defects could easily be annealed out over time periods corresponding to less than that required for monolayer deposition under typical molecular-beam epitaxial film growth conditions.
X-ray photoelectron spectroscopy study of the atomic structure change of amorphous carbon films annealed in vacuum
Authors: Wu X. M. , Fang C. S. Ares , Yang G. R. , Hill I. , Lu T.-M.
Published: July 2002

Amorphous carbon films deposited at near room temperature on Si substrates by a direct current plasma chemical vapor deposition technique using CH4 gas have been annealed in vacuum. X-ray photoelectron spectroscopy measurements show that, as the annealing temperature increases from room temperature to 300 °C, a significant shift (0.5 eV) of the C 1s electron binding energy is observed. The observed shift may be the indication of an atomic structure change of the amorphous films upon the thermal annealing. On the other hand, a large shift in the sp2 bond peaks of the Raman spectra only occurs at annealing temperatures higher than 300 °C, which may be the indication of a structural change of the amorphous carbon films by the annealing. A two stage stress release mechanism is suggested: One for the atomic structure change and one for the film structure change.
Radiation damages in TRISTAN vacuum systems
Authors: Momose T. , Ishimaru H.
Published: July 2002

The TRISTAN e+e− collider, the all aluminum (Al) alloy vacuum system, has been operated for 4 years at a beam energy between 27.5 and 32 GeV, with a characteristic energy of the synchrotron radiation between 187 and 295 keV. The radiation in the TRISTAN tunnel was measured using thermoluminescence dosimeters. Radiation damage in the aluminum vacuum system is mainly from HNO3 produced from NOx. N2 and SO2 in atmosphere are oxidized, combined with H2O, and turned to HNO3 and H2SO4 in TRISTAN tunnel. Freon also turned to HCl and HF. Corroded materials on aluminum surface is amorphous Al(NO3)3⋅9H2O. To decrease corrosion due to the radiation, one or two of the components in the atmosphere must be removed. This can be realized by pumping with a rotary pump (RP) or a RP together with a turbomolecular pump, or supplying N2 or He with low dew point, to protect beryllium (Be) windows for internal targets and beam injection and extraction, and race track type bellows. SiO2 coating superior to almite coating is applied to protect bellows from radiation damage. Most organic materials are decomposed with the radiation. Halogens in the materials are decomposed and turned to acids combining with H2O. Connectors of distributed ion pumps and cables of beam position monitors were damaged by fluorine gas, resulting from the decomposition of Teflon insulators in cables and connectors. Polyimide and polystyrene are durable under radiation of about 1010 rad. They however turned fragile and caused insulation failure in connectors. Rubbers and oils also received radiation damage. Organic materials must be exchanged to inorganic or metallic materials to avoid radiation damage. Mineral insulation cable is developed and is under test.
On-axis sputter deposition of superconducting YBa2Cu3O7-δ on Si(100)
Authors: Stamper A. K. , Greve D. W. , Schlesinger T. E.
Published: July 2002

Superconducting thin films of YBa2Cu3O7-δ have been deposited using radio-frequency diode sputtering from a single composite target. These films were deposited on silicon substrates at substrate temperatures up to 600 °C using a 90% argon and 10% oxygen sputtering gas mixture. Yttria-stabilized ZrO2 buffer layers were employed both for electrical isolation and to minimize the reaction between the silicon and the superconductor. The composition and electrical properties of these films, which were deposited on axis from 5 cm targets, were approximately constant over most of the 5 cm substrates and were reproducible. The effect of the sputtering gas pressure, target composition, and substrate temperature on the film composition have been studied. The films deposited on in situ heated substrates had Tczero=87 K with 10% to 90% transition widths of less than 5 K and were c-axis oriented. Films deposited on unheated substrates required processing at higher temperatures, displayed slightly lower resistive transitions (84 K), and were randomly oriented.
Interaction of evaporated copper with vapor-deposited thin polyimide films
Authors: Strunskus T. , Hahn C. , Frankel D. , Grunze M.
Published: July 2002

The interaction of evaporated copper deposits with vapor-deposited, vacuum-cured thin films of pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide was studied using x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection absorption spectroscopy (FTIRAS). XPS and FTIRAS measurements indicate a preferential interaction of the copper with the imide part of the molecule, some interaction with the ODA ether oxygen and only a very weak interaction with the phenyl rings. The spectral changes observed show a dependence on the rate of the copper deposition.
Stability of the SnO2(111) surface
Authors: Blanchard David L. , Baer D. R.
Published: July 2002

The (111) face of an Sb doped SnO2 crystal was studied. The surface was ion bombarded, cleaning and preferentially removing oxygen, and then reoxidized by annealing in vacuum and O2. Low energy electron diffraction (LEED) was used to characterize the surface structure, and Auger electron spectroscopy and x-ray photoemission spectroscopy were used to monitor the surface composition. Only one ordered LEED pattern was observed. A structure corresponding to this pattern is proposed that maximizes the coordination of tin cations at the surface. The surface composition (O/Sn) evolves with annealing temperature in a manner qualitatively similar to that observed for the previously studied (101) and (110) surfaces. The first ordered LEED pattern appears near 600 K, close to that seen for the other surfaces. Apparently the (111) and (101), which show only one ordered LEED pattern each, are more stable than the (110), which exhibits several. Reasons for this greater stability are proposed, based on the relative oxygen deficiency at the surface.
Observed ion mixing of Au with GaAs(001) during low-energy ion scattering
Authors: Charatan Robert M. , Williams R. Stanley
Published: July 2002

Thin Au films on GaAs(100) are extremely sensitive to bombardment by low energy (5 keV) He ions that are often used as the analysis projectiles in ion scattering spectroscopy. Fluences as low as 1015 ions cm−2 cause enhanced diffusion of the Au into the GaAs substrate.
Grid metallization and antireflection coating optimization for concentrator and one-sun photovoltaic solar cells
Authors: Gessert T. A. , Coutts T. J.
Published: July 2002

In photovoltaic (PV) solar cell research, often the mechanisms of junction formation are studied intensively, while the design of the grid and the antireflection coating (ARC) are somewhat neglected. This is usually because, at the research stage, fundamental information about the junction parameters of novel device materials is more essential than the need to produce high-efficiency devices. Additionally, the equipment and expertise necessary to enable state-of-the-art processing is relatively specialized and often not available to the PV researcher. Nevertheless, during the past few years, many PV research technologies have begun to incorporate optimally designed ARCs and very high-quality metallization, involving processes more typically associated with the micoelectronics industry. Partly because of this, significant improvements in conversion efficiencies have been reported in several key solar cell material systems. In this paper, the models and techniques necessary to design and optimize metal contacts and ARCs for PV solar cells are reviewed and discussed. Additionally, similarities and differences between grid metallizations of cells used under optical concentration and one sun are identified. Through these discussions, the advantages of several recently developed metallization and ARC techniques are reviewed, including very-fine linewidth, buried-contact metallizations, metallizations typically used in the thin-film solar cell industry, and short-circuit current contour modeling for ARC optimization.
Relaxation of the ZnTe and CuCl (110) surfaces
Authors: Tsai M.-H. , Hu W.-M. , Dow John D. , Sankey O. F.
Published: July 2002

Ab initio molecular dynamics simulations of the ZnTe and CuCl (110) surfaces are employed to study surface atomic relaxation. We believe that these are the first such computations for heteropolar semiconductors, and for their surfaces in particular. The molecular dynamics follows the Sankey–Niklewski method, and electrostatic interactions are incorporated using Ewald’s scheme for Gaussian atomic charge distributions. Hence the electrons are treated in the local-density approximation, forces are computed using the Hellmann–Feynman method, and atoms move to equilibrium according to Newton’s laws. Using ‘‘dynamical quenching,’’ we allow the ‘‘ideal’’ surfaces to relax according to these laws of physics and then address a controversy concerning whether Coulomb forces can play a significant role in determining the (110) zinc blende surface relaxation: Coulomb effects are not negligible for ZnTe (110) and are as dominant as covalent effects for CuCl (110). They reduce the (almost rigid) bond rotation angle ω1 due to movement of the anions up out of the (110) surface by about 6° for ZnTe, and cause the CuCl anion rotation angle to be nearly zero.
Bismuth postdosing thermal desorption mass spectroscopy: Performance and limitations
Authors: Hugenschmidt M. B. , Domagala M. E. , Campbell C. T.
Published: July 2002

The applications of bismuth postdosing thermal desorption mass spectroscopy (BPTDS) to the analysis of molecular and nonmolecular adsorbates on Pt(111) is briefly reviewed. In this method, about one monolayer of bismuth adatoms is vapor deposited onto a surface at 100 K containing some previously prepared adlayer. The surface is then heated while monitoring for gas phase species mass spectrometrically. All molecular adsorbates studied to date, which includes methyl iodide and hydrocarbons alone or in mixtures, are found to desorb intact and nearly quantitatively in BPTDS. Nonmolecular adsorbates such as Ha, CH3,a, and c-C6H9,a are found to give high yields of the singly hydrogenated gas phase product at ∼200 K in BPTDS when coadsorbed with Ha. Coadsorption with Da can be used to distinguish such reactions from simple desorption. The thermodynamic driving forces which cause these results are discussed, and it is argued that BPTDS should be generally applicable to many molecular and some nonmolecular adsorbates on transition metal surfaces.
Low-energy positron diffraction from GaAs(110)
Authors: Lessor D. L. , Duke C. B. , Chen X. M. , Brandes G. R. , Canter K. F. , Ford W. K.
Published: July 2002

Intensities of 16 beams of near normal incidence positrons have been measured at T=120 K and analyzed using a multiple scattering model of the low-energy positron diffraction (LEPD) process. Excellent correspondence between the measured and calculated intensities is obtained for a reconstruction that is primarily a bond-length-conserving rotation of the top layer, with As relaxed outward and Ga inward with a tilt angle ω1 = 28.6 ± 3°, confirming the results of previous structure analyses for this surface. The quality of the description of the measured intensities, as measured by the x-ray R factor, is significantly better for LEPD than for low-energy electron diffraction. This result is attributed to the repulsive character of the positron-ion core potential and a resulting more surface sensitive diffraction process for LEPD.
Pairing of curved atomic steps at the extrema of periodic gratings on Si(001)
Authors: Umbach C. C. , Keeffe M. E. , Blakely J. M.
Published: July 2002

Scanning tunneling microscopy has been used to study the shape of curved steps at the extrema of periodic step arrays produced from gratings on Si(001). These steps are created by the overlap of the step density associated with the gratings with a background step density in the perpendicular direction; the contour of such a step crosses from one side of an extremum to the other. A pairing of the crossing steps is explained as arising from the energetics of the steps and the reconstructed surface. Steps on gratings along both [110] and [010] show a lengthening of the low energy A-type portions of crossing step contours and a shortening of the high energy B-type portions; this results in the steps approaching to within 200 to 400 Å of one another. This pairing can be attributed to a balancing of the decrease in step energy as low-energy portions are formed with an increase in repulsive energy as the steps approach each other. The observed step shapes are consistent with model calculations based on the currently accepted values of step energies.
Particulate temperature in radio frequency glow discharges
Authors: Daugherty J. E. , Graves D. B.
Published: July 2002

A spectroscopic technique is described for measuring the temperature of phosphorescent particles suspended in a low pressure rf plasma. The particles are found to be slightly hotter than the neutral gas temperature. This result is consistent with models of particle heating in a low pressure discharge. Also, we report on an entrainment device for introducing the phosphorescent particles (or other types of particles) into a discharge.
Superlattices of YBa2Cu3O7−δ/PrBa2Cu3O7−δ grown by the pulsed organometallic beam epitaxy method
Authors: Duray S. J. , Buchholz D. B. , Zhang H. , Song S. N. , Schulz D. L. , Dravid V. P. , Marks T. J. , Ketterson J. B. , Chang R. P. H.
Published: July 2002

We report on the growth of YBa2Cu3O7−δ/PrBa2Cu3O7−δ superlattices by pulsed organometallic beam epitaxy technique. X-ray diffraction and analytical high resolution transmission electron microscopy measurements show high-quality superlattice crystalline structures are being grown. Electrical measurements of these layered oxides have also shown very promising results. This work demonstrates the capabilities of pulsed organometallic chemical vapor deposition as an alternative method for growing high temperature superconducting oxides in a controlled manner.
Monte Carlo simulation of surface kinetics during plasma enhanced chemical vapor deposition of SiO2 using oxygen/tetraethoxysilane chemistry
Authors: Stout Phillip J. , Kushner Mark J.
Published: July 2002

Tetraethoxysilane/oxygen (TEOS/O2) chemistries are now used for low plasma enhanced chemical vapor deposition (PECVD) of SiO2 when high conformality and low temperature are required. The surface processes leading to film growth, carbon elimination, and conformality are not well characterized. We have developed a model to investigate the surface kinetics of the PECVD of SiO2 films using TEOS/O2 chemistry. The model includes precursor adsorption and desorption, densification of the film, void formation, pyrolysis, plasma surface interactions, and geometric shadowing. Results for growth rate, fraction of carbon retained in the film, film conformality, and film roughness are discussed.
Effect of substrate temperature on the orientations of radio frequency sputtered Ba2Si2TiO8 thin films
Authors: Li Yi , Youdelis William V. , Chao Benjamin S. , Yamauchi Hisao
Published: July 2002

The effect of substrate temperature on orientations of rf sputtered Ba2Si2TiO8 (BST) thin films grown on Si (100) and (111) substrates was investigated using x-ray diffraction, optical microscopy, and scanning electron microscopy. The texture of the films deposited on Si (100) substrates varies from an amorphous structure at 750 °C, to a randomly oriented polycrystalline structure, then to (001), (111), (320), and (410) orientations as the substrate temperature increases to 950 °C. The sequence of textures coincides with the decrease in the planar atomic density of the films and the increase in the BST film/substrate interfacial energy for the corresponding orientations. The films deposited on Si(111) substrates show a similar temperature dependency of textures to that observed on films deposited on Si (100) substrates, but the optimum temperature for a particular orientation shifts to higher temperature than that for deposition on Si (100) substrate. The capillarity theory of nucleation is used to explain the growth mechanism of the BST films deposited on Si substrates.
Gas incorporation during ion-assisted deposition in bias sputtering
Authors: Window B. , Harding G. L.
Published: July 2002

An important factor in ion-assisted deposition with ion guns or bias sputtering is the incorporation of gas atoms, particularly inert gas atoms, in the growing film. The incorporation of argon or xenon in sputtered films of chromium, molybdenum, and tungsten, produced using dc unbalanced magnetrons to provide the fluxes of depositing metal atoms and the ions has been studied as functions of bias voltage, ion-to-atom ratio, and in some cases temperature. The burial rate increased approximately as the square of the energy of the ions, and depended on the ion-to-atom ratio in a nonlinear way. The incorporation of energetic heavy ions in light materials was particularly pronounced. The temperature dependences of incorporation for Cr/Ar, Cr/Xe, and Cu/Ar are consistent with the inert gas atom residing substitutionally in the lattice.
Core–valence Auger spectra of LiAl Zintl intermetallic compound
Authors: Jang G. E. , Curelaru I. M. , Kim H. G.
Published: July 2002

The core–valence–valence Auger spectra were measured for a nearly stoichiometric polycrystalline LiAl intermetallic compound. These spectra were compared with the self-convolution of the density-of-state curves obtained from theoretical band structure calculations. Major discrepancies between the experimental data and this simple one-particle model prompt a more elaborate treatment to include many-body and core-hold relaxation effects. Significant accumulation of lithium was observed at the surface of the ‘‘as-fractured’’ sample. Ion bombardment resulted in lithium removal from the damaged surface layer, as opposed to electron bombardment which caused surface segregation of lithium originating from the bulk.
Adverse effects on plasma polymerized films due to previous oxygen etching in an ‘‘electrodeless’’ reactor
Authors: Wymore Troy , Nichols Michael F.
Published: July 2002

Amorphous hydrogenated carbon (a-C:H) films prepared by plasma polymerization can contain chemical species due not only to the process gas species, but species from prior processes in the deposition chamber. Our studies showed that standard O2 etching/ cleaning processes done prior to plasma film formation will affect future film deposition rate measurements and carbonyl group formation. Additionally, substrate etching by polymer forming gas species is shown through the 956 cm−1 negative peak in the IR spectrum.
Pump down of evacuated glazing
Authors: Clugston D. A. , Collins R. E.
Published: July 2002

Evacuated glazing consists of two sheets of glass separated by a small gap, hermetically sealed around the edges, and containing a thermally insulating vacuum. This article describes techniques to determine the time necessary to evacuate such a device through a small pump out tube. The time constant for pressure reduction is determined by numerical modeling. An approximate analytic method is also developed which gives results in good agreement with the numerical data. The numerical results are validated by experimental measurements. It is shown that samples of area about 1 m2 and internal gap ∼0.2 mm can be evacuated in a few minutes through a small pump out tube. It is therefore likely that the evacuation time in the production process of evacuated glazing will be dominated by the heating and cooling cycles associated with outgassing of the internal surfaces, rather than by the pumping process itself.
Mass spectroscopic study of the cracking of trimethylgallium in a heated doser
Authors: Lin R. , Cadwell L. , Masel R. I.
Published: July 2002

Trimethylgallium (TMGa) is a common source gas in GaAs film production by metalorganic chemical vapor deposition and atomic layer epitaxy. Recent studies have shown that TMGa is also a useful intermediate for chemical beam epitaxy (CBE). However, Suto et al. found that improved films are produced when the TMGa is ‘‘cracked’’ before it impinges on the substrate. The objective of this article is to examine the products of the cracking process. TMGa was passed through a heated tantalum or stainless steel tube at low pressure, and the products of the decomposition were measured with a mass spectrometer. It was found that TMGa is unreactive in a stainless steel and/or tantalum tube at 250 K. However, by 300 K there is significant attenuation of the parent TMGa peak in both tubes. The dimethylgallium (DMGa) disappears between 600 and 700 K, while the monomethylgallium (MMGa) contribution disappears between 650 and 800 K. The gallium signal is lost between 700 and 800 K. Calibration runs were performed to determine if the cracking pattern of TMGa changes with temperature. It was found that the cracking pattern of TMGa is essentially constant from 200 to 500 K. As a result, the changes in mass spectrometer signal observed in our work must be due to changes in the composition of the gas, and not due to changes in the cracking pattern of the various species in the mass spectrometer. Therefore, the conclusion of our study is that TMGa decomposes at room temperature on a stainless steel or tantalum tube, to yield DMGa. The DMGa decomposes between 500 and 700 K to yield MMGa. The MMGa decomposes between 700 and 800 K to yield gallium. The gallium signal is lost above 800 K, probably due to deposition of gallium inside the cracker. These results show that under UHV conditions, TMGa is much more reactive than previous investigators had supposed. In particular, the results indicate that special precautions must be taken to get TMGa into a vacuum system. If one puts TMGa through a standard doser at temperatures slightly above room temperature one will get significant DMGa even after days of passivation.
Two-dimensional alloying of Au and Ag overlayer on Ru(001): Photoemission and thermal desorption spectroscopy studies
Authors: Bzowski A. , Kuhn M. , Sham T. K. , Tan K. H.
Published: July 2002

Photoemission spectroscopy with synchrotron radiation has been used to study the valence-band behavior as well as the Au 4f core-level binding energy shifts of coadsorbed Au and Ag overlayers on Ru(001) at one-monolayer coverage each as monitored with thermal desorption measurements. The results were compared with bulk Au–Ag results obtained previously to determine if and under what conditions a two-dimensional alloy is formed. We found that alloying occurred to some extent at room temperature and a stable alloy was formed at elevated temperatures (800 K) when Ag was deposited first, while there was no alloying when Au was deposited first on the Ru substrate. It appears that there is a significant difference in the electronic structure of the overlayers depending on the order of the deposition. These results and their implications are discussed.
Structural study of monolayers of Sb on Ge(111) with different surface reconstructions*
Authors: Kendelewicz T. , Woicik J. C. , Miyano K. E. , Yoshikawa S. A. , Pianetta P. , Spicer W. E.
Published: July 2002

X-ray standing waves (XSW) and low energy electron diffraction (LEED) have been applied to the structural study of a monolayer of Sb on cleaved Ge(111). Initial Ge(111) surfaces with 2×1, c(2×8), and 1×1 LEED reconstructions have been obtained by preannealing the 2×1 room temperature cleavage surfaces. Although in all three cases, the Sb deposition followed by a 600 °C anneal on these different initial reconstructions results in a 2×1 or 2×1/2×2 overlayer LEED geometry, the XSW data indicate large differences in the local Sb bonding structure between them. For the initial 2×1 surface, Sb forms Seiwatz chains. Preannealing of the surface causes a decrease in the Sb coherent fraction and an increase of the substrate overlayer distance which is inconsistent with a single site absorption geometry and indicates large disorder of the 1×1 surface prior to the Sb deposition.
Vacuum system design of the International Thermonuclear Experimental Reactor pellet fueling system
Authors: Langley R. A. , Gouge M. J. , Santeler D. J.
Published: July 2002

The International Thermonuclear Experimental Reactor (ITER) will use an advanced, high-velocity pellet injection system to fuel ignited plasmas. For rampup to ignition, a moderate-velocity (1–1.5 km/s) single-stage pneumatic injector and a high-velocity (1.5–5 km/s) two-stage pneumatic injector using pellets encased in sabots are envisioned. For the steady-state burn phase a continuous, single-stage pneumatic injector and a centrifugal injector are proposed. The purpose of this study is to simulate the ITER pellet injection line vacuum pumping system to determine the pump requirements. This study analyzed the injector vacuum system using commercially available vacuum pumps compatible with tritium operation. The vacuum system design program, VSD-II, was used to determine the gas flow through the system components for various pumping arrangements and component sizes and geometries. The VSD-II computer program allows changes to be made easily in the input so that results from different configurations are readily obtained and compared. Results are presented and issues in the design are discussed as well as limitations in the existing pump data.
Detection of small pressure pulses in an ion pumped ultrahigh vacuum system
Authors: Rupschus G. , Niepraschk R. , Jousten K. , Kühne M.
Published: July 2002

Pressure stability in the ultrahigh vacuum (UHV) system of a gravitational wave detector is a crucial point for its sensitivity limit. We carried out an investigation in order to detect pressure fluctuations in an UHV system pumped with an ion pump. The expected characteristics of pressure pulses caused by desorption were simulated with Monte Carlo methods. Test equipment was developed with a sensitivity limit for relative pressure changes of 10−3 at a base pressure of 10−6 Pa in the range up to 1 kHz, and of 10−4 below 40 Hz. Pressure fluctuations in the 10−4 range of working pressure were found in the frequency range <40 Hz and larger pressure pulses in the percent range with a frequency of about 1 in 4 h.
Vacuum outgassing of artificial dielectric ceramics
Authors: Nguyen-Tuong Viet
Published: July 2002

A special aluminum nitride (AlN)–glassy carbon artificial dielectric ceramic for microwave absorption at low temperature has been developed at the Continuous Electron Beam Accelerator Facility to manufacture the higher order mode (HOM) loads used in the superconducting cavities of the machine. As the HOM loads share the same ultrahigh vacuum as the superconducting cavities, very tight vacuum requirements are imposed on the HOM load’s material. Vacuum outgassing rates have been measured and compared for AlN–15% glassy carbon artificial ceramic in fully degassed condition produced by heating to high temperature in a vacuum furnace. In addition, the effect of exposure to air, nitrogen gas, and isopropanol is discussed. A typical outgassing rate at room temperature is 2.5×10−11 Torr ℓ/s/cm2, 24 h after initial pump down. Baking 24 h at 150 °C was sufficient to attain an outgassing rate of less than 4×10−12 Torr ℓ/s/cm2. However, when the ceramic has a lower bulk density or a higher apparent porosity, the outgassing rates can be two orders of magnitude higher.
Throughput-type pumps and ultrahigh vacuum
Authors: Hablanian M. H.
Published: July 2002

The development of ultrahigh-vacuum techniques had a profound effect on vacuum chamber design. It stimulated equally important improvements in pump design and in the understanding of some basic aspects of pump performance as well as their limitations. Design improvements also helped to clarify certain elusive concepts, for example, the meaning of the term ‘‘ultimate pressure’’ of a pump. Regarding partial pressures, the ordinary dual-stage sliding-vane mechanical pumps have been demonstrated to have full pumping speed at pressures as low as 10−10 Torr for gases nearly absent at the discharge. Modern vapor-jet pumps (diffusion pumps) appeared in the 1960’s with improvements in all performance aspects. Using high-performance pumping fluids, pressures near 1×10−9 Torr were achieved with only ambient temperature baffles, and near 1×10−11 Torr with liquid nitrogen cryobaffles. Even the extreme high-vacuum range was obtained with vapor-jet pumps assisted by gaseous helium- or liquid helium-cooled traps. However, the fear of accidental backstreaming events shifted the users to turbomolecular pumps which, after nearly 50 years of stagnation, finally became a major pumping choice. Turbopump improvements are continuing to this day driven by the need for cleaner pumping methods and rapid achievement of lower pressures. The trend is to optimized staging arrangements to increase the limiting pressure ratios (particularly for hydrogen), the introduction of light-weight, ceramic, grease-lubricated bearings, magnetic bearings, and at least ten times higher permissible discharge pressures which allow entirely oil-free backing pumps, the use of smaller backing pumps, and the achievement of an ultrahigh pressure range even in the presence of light gases such as hydrogen.
Ultrahigh vacuum/chemical vapor deposition epitaxy of silicon and germanium–silicon heterostructures
Authors: Greve D. W. , Misra R. , Strong R. , Schlesinger T. E.
Published: July 2002

Germanium–silicon epitaxial growth by chemical vapor deposition (CVD) has proven to be suitable for growth of many heterojunction devices. We report here on recent work with one CVD technique (UHV/CVD, or ultrahigh vacuum chemical vapor deposition) which is capable of multiwafer deposition of advanced device structures. First, the physics and chemistry of the growth process are outlined and the factors which influence layer uniformity and heterojunction abruptness are discussed. We then present recent results from the characterization of doped multiple quantum well structures suitable for far-infrared detectors.
Chemical states of the GaAs/Si interface
Authors: Zhou Huijian , Ho Wei
Published: July 2002

Auger line-shape analysis is employed in the study of the chemical states of the GaAs/Si interface. A fast Fourier transform algorithm is applied to quantitatively analyze the concentration of a particular element in an interrupted structure, such as an interface and a surface. The data are corrected for spectrometer distortions and electron-energy-loss processes. The correction schemes are presented. The changes of the Auger radial matrix elements for core–valence–valence, core–core–valence transitions (CVV, CCV) are analyzed in different depth profiles of the GaAs/Si interface. It has been found that Si bonds predominantly to As instead of Ga. Some p electrons are transferred from the valence band of Si to As, and a new interface state emerges at 2 eV above the top of the valence band of GaAs. But for Ga, there are no observable changes, suggesting that Ga has no bonding with Si in the GaAs/Si interface.
Si 1s x-ray absorption spectra of epitaxial Si–Ge atomic layer superlattice and alloy films
Authors: Hitchcock A. P. , Tyliszczak T. , Rocco M. L. M. , Francis J. T. , Urquhart S. G. , Feng X. H. , Lu Z. H. , Baribeau J.-M. , Jackman T. E.
Published: July 2002

The Si 1s (K-shell) x-ray absorption spectra of several [(Si)m(Ge)n]p atomic layer superlattices (ALS) and a range of SixGe1−x alloy thin films grown epitaxially on both Si(100) and Ge(100) have been investigated using plane-polarized synchrotron radiation. The near-edge spectral features of ALS and alloy samples with similar (average) chemical composition are remarkably similar. The spectra of both the strained ALS and alloy samples contain features at the Si 1s threshold which exhibit a small but characteristic polarization dependence. The polarization dependence is reduced or absent in strain-relaxed materials such as annealed ALS, annealed alloys, or thick alloy films. The polarization-dependent components of the signal are attributed to anisotropic states associated with strain-induced tetragonal distortions. The sense of the polarization is inverted between samples grown on Si and those grown on Ge, consistent with the expected inversion in the spatial orientation of the strain field. An explanation is proposed for the dependence of the magnitudes of the Si 1s polarization effect on the composition of alloys and ALS.