bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of adsorbed gases and temperature on the photovoltage spectrum of GaAs
Authors: Dahlberg S. C.
Published: July 2002

The retarding potential electron beam technique has been used to study the photovoltage spectrum of Ar-bombarded GaAs (100) surfaces. Clean surfaces and those with absorbed O2, NO, NH3, H2S, SO2, and CO2 were investigated. The spectral region above the band-gap energy shows structure which changes in position and intensity in ways characteristic of the adsorbed gas. Similar structure is seen on the GaAs (100) surface prior to any Ar bombardment. Heating and cooling the sample do not result in appreciable changes to the photovoltage spectra, but heating does cause a transient loss of photovoltaic activity. Several possibilities for the origin of the photovoltage structure are discussed, including surface states and electron–phonon interaction.
Insulator flashover mechanism in vacuum insulated cryocables
Authors: Graneau Peter , Montgomery D. Bruce
Published: July 2002

This paper describes research on vacuum insulated cryocables for underground power transmission. It deals particularly with the flashover mechanism observed on solid dielectric conductor supports in vacuum. The insulators were protected by ion shields. The work revealed the important role played by gas and vapor absorbed on dielectric surfaces bridging the high voltage gap. Photographic evidence proved that an early link in the chain of breakdown events is a gas discharge (short mean free path) in the absorbed layer. Gas layer breakdown also offers explanations for the beneficial effects of ion shields with regard to breakdown voltage and protection against arc damage. With the help of high voltage capacitors it has been established that ion shielded, inorganic insulators will survive the discharge of all electrostatically stored energy in a long cable. This is an important requirement for high voltage conditioning. The paper contains the first published photograph of a Lichtenberg discharge in vacuum.
Photoemision and LEED study of indium phosphide with a determination of minority carrier diffusion length
Authors: Chang Keun-Ho , Meijer Paul H. E.
Published: July 2002

Photoemission measurements were taken in indium phosphide from the band gap to 1.8 eV photon energies. The surface activation procedure using cesium and oxygen is described. The condition of the surface was monitored for its crystallinity by a low-energy-electron-diffraction (LEED) camera, and for its purity by Auger electron spectroscopy. The work functions were measured by the electron beam retarding potential method to determine the consequence of the activation processes. The result of the LEED analyses and the work-function measurements indicate that the activation takes place on the surface in a patchy form. The results of the photoemission data are analyzed in light of the three-step diffusion model photoemission theory. From the data of the photoemission, the bulk diffusion length is determined to be 530 nm and the surface escape probability to be less than 0.025. Correlating the results of the photoemission experiments with LEED and work-function measurements, it is found that the surface of indium phosphide is not stable when it is activated, i.e., the photoemission slowly changed. An estimate of the thermalization length is derived and the value is used to describe the discrepancy between the diffusion model theory and the experimental results at photon energies higher than 1.5 eV.
Ionicity effects on compound semiconductor (110) surfaces
Authors: Bauer R. S.
Published: July 2002

Properties of clean and controllably oxidized surfaces of in situ cleaved GaAs, CdTe, ZnTe, ZnSe, and ZnS were probed by synchrotron radiation-induced photoelectron spectroscopy. We concentrate on variations in submonolayer oxygen adsorption due to changing semiconductor ionicity. A roughly exponential dependence of O2 sticking coefficient on electronegativity difference correlates well with estimates based on other techniques when the molecular state of the adsorbate is considered. Monitoring the O2 interaction by the semiconductor substrate core-level chemical shift, changes in surface bonding with ionicity are shown by the cation behavior. We also point out that the predominant angular momentum of the intrinsic empty surface states is an important characteristic. Using the dipole selection rules governing photoemission partial-yield transitions, significant anion s-like empty surface state density exists on all (110) surfaces studied. Increasing ionicity appears mainly to change the atomic character of cation-derived empty surface states from p- to s-like. Core-level transitions to these surface states are strongly influenced by final-state effects. Our self-consistent measurements of p-core exciton-binding energies show a large increase in surface final-state effects with increasing ionicity, while bulk conduction-band-edge excitons become weaker. The varying bonding requirements and possibly the associated surface-atom positions provide a unifying concept for understanding these ionicity effects.
Contamination in vacuum-evaporated iron films by conversion electron Mössbauer spectroscopy
Authors: Goodwin J. G. , Parravano G.
Published: July 2002

Iron films, about 800 Å thick, were prepared by evaporation under low pressure conditions [2×10−7 Torr (2.7×10−5 Pa)] and were examined by conversion electron Mössbauer spectroscopy to determine the amount and nature of contaminants. After deposition the films exhibited a spectrum suprisingly similar to that of Fe–C martensite. The quantity of carbon present in the film could be estimated from the spectrum of the film after precipitation of Fe3C at 500°C and was found to be on the order of 8 at.%. Besides carbon, no other contaminant was detected. The implications of these findings on the interpretation of studies of surface and subsurface reactions on vacuum-evaporated Fe films are discussed.
Effect of experiments on the density distribution in a molecular shield
Authors: Hueser J. E. , Park S. K. , Brock F. J.
Published: July 2002

A steady-state, collisionless flow analysis has been made of the density distribution within a hemisphere–disk system due to independent, uniformly distributed internal gas sources. The model may be used to estimate the density within a molecular shield, deployed from the Shuttle Orbiter, which contains internal experiments having a prescribed gas source. Contour plots of the density distribution within the system are presented for disk-to-hemisphere radius ratios of 0.1, 0.3, 0.5, and 0.7, and for disk-to-hemisphere surface emission flux density ratios of 0.01, 1, and 100. The hemisphere–disk system is compared to the empty hemisphere and it is found that if the disk emission flux density is the same as the hemisphere and the disk radius is not greater than one-third of the hemisphere radius, the increase in density at the center of the hemisphere–disk system is less than 50%.
Deposition techniques for dielectric films on semiconductor devices
Authors: Amick James A. , Schnable George L. , Vossen John L.
Published: July 2002

Techniques for forming dielectric layers for fabricating semiconductor components are reviewed, including (1) low-pressure techniques: evaporation, sputtering, plasma deposition, and low-pressure CVD, (2) techniques operating at one atmosphere total pressure: thermal oxidation, chemical vapor deposition, anodization, electrophoresis, spin on, spray on, silk screening, and (3) a number of miscellaneous techniques: roller coating, offset printing, centrifugation–sedimentation, and transfer. The advantages and limitations of the methods are presented and typical applications are given. Novel applications of the technology to other-than-silicon semiconductor devices are outlined, and future trends in the technology are indicated.
Use of a nitrogen–argon plasma to improve adherence of sputtered titanium carbide coatings on steel
Authors: Brainard William A. , Wheeler Donald R.
Published: July 2002

Friction and wear experiments on 440-C steel surfaces that had been rf sputtered with titanium carbide when a small percentage of nitrogen was added to the plasma were conducted. X-ray photoelectron spectroscopy and x-ray diffraction were used to analyze the resultant coatings. Results indicate that a small partial pressure of nitrogen (∠0.5%) markedly improves the adherence, friction, and wear properties when compared with coatings applied on sputter-etched, oxidized surfaces or in the presence of a small oxygen partial pressure. The improvements are related to the formation of an interface containing a mixture of the nitrides of titanium and iron which are harder than their corresponding oxides.
Sensitivity to various gases of Granville–Phillips vacuum gauge, series 275, convectron
Authors: Chapman R. , Hobson J. P.
Published: July 2002

Comparison of pressure readings in the ’’rough vacuum’’ region of he Granville–Phillips vacuum gauge with two other gauges reveals large discrepancies for helium, methane, argon, and krypton measurements. (AIP)
Surface composition of some austenitic stainless steels after different surface treatments
Authors: Boschi Andrea , Ferro Carlo , Luzzi G. , Papagno L.
Published: July 2002

Three kinds of stainless steel have been analyzed: AISI 304 ordinary plane sheets, AISI 316 L cold-pressed corrugated sheets, and AISI 316 LN rolled-up and machined plates (the last two materials employed for the vacuum vessel of the Frascati Tokamak FT). Their surfaces were subjected to two different chemical cleaning treatments: one of them based on a detergent, alkaline bath and the other on a brightening, acid bath. Both of these treatments were followed by baking in vacuo to 350°C or more. As a common result outgassing rates below 10−12 Torr l s−1 cm−2 at room temperature were in any case obtained. Nevertheless, the residual content of nonmetallic impurities (mainly oxygen and carbon) found by the depth profile AES analysis significantly varied from sample to sample, depending on the different kind of material (and according to the different chemical process undergone. The adopted brightening bath proved very effective in relieving all the treated surfaces from every kind of impurities, including oxides (oxygen was reduced to the minimum possible amount). The alkaline bath proved more effective in removing carbon, which in any case was reduced to a very small amount. Both the AISI 316 materials used for the vacuum vessel of the FT device showed, after either of the mentioned treatments, fewer nonmetallic impurities than the most common AISI 304 sheets.
Chemisorption site geometry and interface electronic structure of Ga and Al on GaAs(110)
Authors: Chadi D. J. , Bachrach R. Z.
Published: July 2002

Valence-band photoemission spectra taken as a function of overlayer coverage during the formation of Ga–GaAs(110) and Al–GaAs(110) interfaces are analyzed. Possible chemisorption site geometries of Ga on GaAs(110) are studied through total energy calculations and the surface electronic structure for Ga (or Al) in an optimal twofold configuration as well as for the ideal onefold coordinated position is obtained. The calculations show that the interface formation results in a change in the relaxation of the GaAs surface from its vacuum interface configuration. This interfacial relaxation at the initial stage of metal–semiconductor interface formation pulls unoccupied surface states into the gap and pins the Fermi level.
Surface EXAFS investigation of oxygen chemisorption on GaAs(110)
Authors: Stöhr J. , Bauer R. S. , McMenamin J. C. , Johansson L. I. , Brennan S.
Published: July 2002

Surface extended x-ray absorption fine structure measurements (EXAFS) are reported for the initial oxidation stage of the GaAs(110) surface which is characterized by a +2.9 eV binding energy shift of the As 3d photoemission peak. The surface EXAFS measurements employed a variation of the previously used secondary-electron-collection technique which significantly increased the surface sensitivity. We find oxygen to be chemisorbed in atomic form. The importance of using phaseshifts derived from model systems rather than using theoretically calculated phaseshifts for distance determinations from low Z atoms is pointed out.
Initial steps of interface formation: Surface states and thermodynamics
Authors: Göpel W.
Published: July 2002

A general framework about the influence of reactive particles on surface electronic structure and barrier formation at low coverages (ϑ?10−2) is deduced from experimental results obtained under thermodynamic equilibrium conditions on a suitable compound semiconductor ’’model surface.’’ Adsorption isotherms enable a rough classification of weak and strong electronic interactions into physisorption, chemisorption, and surface reaction steps, which depend on temperatures and partial pressures. (1) Covalent bonding of particles on ionic compound semiconductor surfaces, characterized by small partial charges formally attributed to the adsorption complex, may lead to extremely large dipole effects even at very low coverages [e.g.,dipole moments ≳ 15 Debye may formally be attributed to the adsorption complex ZnO (101̄0)/CO2,chem at ϑ<10−2]. Correspondingly, drastic changes are found in surface atom relaxation. Sticking coefficients are close to unity. (2) Ionic bonding, characterized by pronounced band bending, Sticking coefficients are extremely low for ’’acceptor type’’ adsorption.(3) Point defects at compound semiconductor surfaces are, for entropy reasons, thermodynamically stable at high temperatures. We present for the first time quantitative results on equilibrium concentrations of surface point defects [oxygen vacancies at ZnO(101̄0)], which cause surface Fermi level pinning at extremely low coverages (ϑ<10−3).
Abstract: Observation of semiconductor–semimetal transition in InAs–GaSb superlattices
Authors: Chang L. L. , Sai–Halasz G. A. , Kawai N. J. , Esaki L.
Published: July 2002

Semiconductor to semimetal transitions are reported in superlattices prepared by molecular beam epitaxy. Electrons and holes are created in spatially separated regions without impurity introduction. (AIP)
Surface defects and thermodynamics of chemisorbed layers
Authors: Lagally M. G. , Lu T. -M. , Welkie D. G.
Published: July 2002

Chemisorbed overlayers on single crystal surfaces can in many cases be considered as lattice gases. The general features of temperature–coverage phase diagrams resulting from attractive or repulsive adatom interactions in such systems are discussed in analogy with binary solid solutions. The effect of surface and overlayer defects on the determination of phase boundaries is considered. The use of low-energy electron diffraction (LEED) to study surface defects and overlayer phase diagrams is briefly summarized.
Charge transfer to oxygen chemisorbed on cleaved GaAs(110) surfaces
Authors: Mönch W. , Enninghorst R.
Published: July 2002

The change of the work function of cleaved GaAs(110) surfaces due to the chemisorption of oxygen was measured at room temperature by using the Kelvin method. With increasing exposure to oxygen the contact potential first increases and then decreases. Using the dependence of the band bending on the oxygen doses as determined by Spicer and coworkers the change of the ionization energy was calculated. The experimental data show that the ionization energy linearly increases by ?0.4 eV up to a coverage of ϑ?0.6 and then decreases; i.e., the chemisorbed oxgen is charged negatively. The initial slope of the ΔI-vs-ϑ curve amounts to 0.65 eV per monolayer and gives μn/εs=0.04 eo Å. This result is explained in a simple model of localized bonds between surface arsenic and chemisorbed oxygen and then results in a charge transfer of approximately –0.28 eo to the chemisorbed oxygen.
Operation modes and its optical measurements of plasma stream transport system
Authors: Tsuchimoto T.
Published: July 2002

The plasma stream transport method is a method where material transport is caused to occur in a plasma stream contained in a magnetic flux pipe. As the plasma stream contains only ions and electrons, the optical emission spectral lines from the plasma stream are considered to be representative of the material constituents of the plasma stream. A prototype plasma stream transport system was constructed and operated in three configurations or modes. The emission pattern was observed to depend on the operating mode and the measuring position along the plasma stream. Transport rates were estimated for the three operating modes and evaluated in terms of the optical emission measurements. The spectral lines at the vicinity of the substrate were found to correspond to the constituent of the deposition film.
Nonresonant charge transfer in ion–surface scattering
Authors: Bhattcharya R. S.
Published: July 2002

The role of nonresonant charge exchange mechanisms in the formation of the charge state of ions backscattered from polycrystalline surfaces is further explored.(AIP)
Vacuum applications of metal foams
Authors: Kendall B. R. F.
Published: July 2002

Possible applications of metal foam materials in vacuum systems are described. Copper foam, for example, is highly porous, easily shaped and strong, easily brazed or soldered, and has good thermal conductivity. The molecular-flow conductance of metal foams is important in vacuum applications.
The compact torus
Authors: Furth H. P.
Published: July 2002

The objective of the compact torus approach is to provide toroidal magnetic-field configurations that are based primarily on plasma currents and can be freed from closely surrounding mechanical structures. Some familiar examples are the current-carrying plasma rings of reversed-field theta pinches and relativistic-electron ’’smoke ring’’ experiments. The spheromak concept adds an internal toroidal magnetic field component, in order to enhance MHD stability. In recent experiments, three different approaches have been used to generate spheromak plasmas: (1) the reversed-field theta pinch; (2) the coaxial plasma gun; (3) a new quasi-static method, based on the initial formation of a toroidal plasma sleeve around a mechanical ring that generates poloidal and toroidal fluxes, followed by field-line reconnection to form a detached spheromak plasma. The theoretical and experimental MHD stability results for the spheromak configuration are found to have common features.
Ionization of atomic clusters sputtered from TiO2 and SrTiO3 surfaces
Authors: Šroubek Z. , Zavadil J. , Žďánský K.
Published: July 2002

Measurement of positive and negative atomic cluster ions produced by 1.2 keV Ar+ sputtering of TiO2 and SrTiO3 surfaces is reported. The generalized LTE formalism is developed and changes of ion currents caused by changes of oxygen surface deficiency and by Cs deposition are analyzed within the framework of the described model. The analysis provides evidence that the studied positive and negative ions are formed by the same mechanism. The effective temperature and other LTE parameters are determined and their physical meaning discussed.
Unified theory of point–defect electronic states, core excitons, and intrinsic electronic states at semiconductor surfaces
Authors: Allen R. E. , Dow J D.
Published: July 2002

A simple unified theory has been developed for predicting three different types of electronic states at semiconductor surfaces: (i) point–defect states, (ii) core excitons, and (iii) intrinsic surface states. The predictions are in agreement with experiment for each class of states. The theory indicates that Fermi level pinning at III–V semiconductor interfaces is usually caused by surface antisite defects.
Atomic and electronic structure of InP–metal interfaces: A prototypical III–V compound semiconductor
Authors: Brillson L. J. , Brucker C. F. , Katnani A. D. , Stoffel N. G. , Margaritondo G.
Published: July 2002

Soft x-ray photoemission spectroscopy and electrical measurements of UHV-cleaved InP–metal interfaces reveal that the interface abruptness on an atomic scale, the stoichiometry of semiconductor diffusion into the metal overlayer, and consequently the Schottky barrier height depend on the strength and nature of interface chemical bonding. For InP and other III–V compound semiconductors, these atomic and electronic features can be controlled extrinsically by reactive metal interlayers. Thus a chemical framework systematically accounts for the electrically-active sites created during Schottky barrier formation at III–V compound semiconductor–metal interfaces.
Gauge calibration system based on piston manometer
Authors: Warshawsky I.
Published: July 2002

An unbaked calibration system is described that permits absolute calibration with a piston manometer in the range 0.0002 to 6 Pa, with a probable error of 5 μPa+0.8%, or in the range 0.00008 to 0.02 Pa, with a probable error of 2 μPa+1%. Procedures and techniques that permit this performance are detailed. For hot-cathode ion gauges, the magnitudes of systematic corrections for envelope temperature and grid current are also indicated.
Graphic data editor for Cambridge e beam
Authors: Zimmermann H. M.
Published: July 2002

Circuit research with e-beam exposure and new resist tests require a much greater data modification freedom than is possible by hand or cycling changes back through a CAD group. Developed for a Cambridge e-beam system is a graphics editor that runs directly on the e-beam’s computer and CRT display. The editor interactively changes rectangles, triangles, or trapezoids into any three or four sided polygon shape. Additions and deletions to the data file are easily done with a crosshair cursor of the display. Macrocommands create groups of data elements in any of the six data display levels and may be matrixed or positioned as desired. Existing data group structures may be modified as the program automatically redefines nested definitions up to ten levels deep. Particularly useful for the e-beam is scan field boundary detection with the scan field size variable up to 4 mm. Also, a switchable reference scale may be used to examine data or input commands in microns or the Cambridge’s internal data units. Further development will include proximity corrections selective to the area displayed to save the time required to do extensive calculations on noncritical areas.
Variably shaped electron beam lithography system, EB55: II Electron optics
Authors: Saitou Norio , Ozasa Susumu , Komoda Tsutomu , Tatsuno Goichi , Uno Yasumichi
Published: July 2002

A variably shaped electron beam exposure system (EB55) with a high exposure rate is developed for direct beam lithography of 0.5 μm patterns. The electron beam column consists of seven magnetic lenses. The projection lens is specially designed to minimize deflection aberations and beam landing angle deviation on the wafer. The column design facilitates a shortened distance between the object and its image plane. Accelerating voltage is 30 kV due to decreased Coulomb interaction and proximity effect. In a shaped beam system, the electron gun should have a wide emission angle, high brightness, and large crossover size. A square rod type LaB6 cathode is developed for this purpose. The shaped beam is vector scanned by electrostatic deflection plates and electromagnetic coils. The scanning area is 2.6 mm square and the maximum beam size is 5.10 μm square. The size change unit is 0.02 μm. A beam current density higher than 10 A/cm2 is obtained with a small beam covergence angle. A shape edge slope smaller than 0.2 μm is obtained. Experimental and theoretical results of the column’s optical properties are presented. Edge slope dependence on variation of the shaped beam size is also discussed. The system has been successfully used to write submicron patterns on silicon wafers.
Considerations on space charge effect for high current variable shaped beam forming
Authors: Idesawa M. , Soma T. , Goto E. , Sasaki T.
Published: July 2002

Variable shaped beam schemes are very promising for advanced VLSI pattern generation. In such a device, beam blurring caused by the space charge effect becomes greater as the beam current increases and the focal length of the electron lens is varied as the beam current changes. In order to keep the beam blurring within a permissible amount, refocusings are required whenever beam sectional areas are changed dramatically. However, it is often difficult to complete refocusing in negligible short time in comparison with the shot time of an element pattern and total exposure time is influenced considerably. Investigating the results of computer simulations and fundamental experiments, it is found that beam blurring caused by defocusing is almost proportional to the deviation of the beam current from that at the optimum focusing point and is kept under the permissible value in certain intervals of beam current. Considering this characteristic, the defocusing problem can be improved significantly by rearranging the exposure sequence of pattern elements so that the frequency of dramatic shape change becomes minimum.
Ion projection microlithography for submicron device fabrication
Authors: Stengl G. , Kaitna R. , Löschner H. , Rieder R. , Wolf P. , Sacher R.
Published: July 2002

Ion projection microlithography is a technique employing a high-resolution demagnifying ion-optical imaging system in connection with a precision step-and-repeat stage. Design patterns are contained in a self-supporting metal foil of 10× (or 20×) chip size, and are imaged in parallel, chip by chip. A tensile stress is induced in the mask foil such that the mask remains flat and undistorted, when heated by exposure to the ion beam. Different design layers are aligned to better than ±0.1 μm by a system that detects ions backscattered from registration marks.
Dosimetry for lithographic applications
Authors: Shaw J.M. , Hatzakis M.
Published: July 2002

Because of the demand for miniaturization of devices, lithographic tools are becoming more expensive and sophisticated with future emphasis being placed on x rays, e beams, and ultraviolets in the range of 2200–3200 Å. It is increasingly important to be able to easily monitor exposure dose and uniformity from tool to tool in an industrial environment. This paper investigates the use of commercially available dosimeter films for lithography applications. These films consist of radiachromic dye derivatives in various polymers. Upon exposure to ultraviolet light (<3400 Å) and other ionizing radiation, these films undergo photoionization, and change from a colorless to a deeply colored state as a direct function of the radiation exposure received. Therefore, the optical density is a means of measuring the amount of incident radiation. For e-beam dosimetry, a measurable optical density change is observed between 0.1 and 20 μC/cm2 with saturation occurring at ∠25 μC/cm2. Maximum penetration range of the 25-kV beam was determined to be 20–25 μ. For x-ray application, the film is similar to e-beam, with a linear response up to 100 mJ/cm2 using the Al–Kα line. For mid UV at 3130 Å and deep UV at 2200 Å, the dosimeter is very sensitive with a 5% change in transmission occurring for every 1 mJ/cm2 dose variation.
Quantum size resonance in semiconducting thin films
Authors: Arora V. K.
Published: July 2002

A quantum theory for the line shape of photon-assisted size resonance (SR) between quantized levels of an electron in a semiconducting thin film is presented. In the ultrathin limit (UTL), the peak value is shown to increase with the thickness of the film while the linewidth decreases with the thickness of the film for isotropic scattering.
Effect of glow discharge treatment of substrates on parylene-substrate adhesion
Authors: Sharma Ashok K. , Yasuda H.
Published: July 2002

A detailed investigation of the effect of argon, oxygen, and methane glow discharge (plasma) treatment of substrates on the adhesion characteristics of parylene polymer to a variety of substrates is reported. It is found that methane glow discharge treatment, which results in the deposition of a glow discharge polymer of methane on the substrate surface, is most effective in improving the bonding strength of the parylene polymer, while the oxygen glow discharge treatment which produces a hydrophilic substrate surface with no or insignificant polymer deposition is detrimental to parylene adhesion. The effect of argon glow discharge treatment may vary from true cleaning of the substrate surface to deposition of a thin layer of a polymeric coating produced by the simultaneous glow discharge polymerization of the ablated organic materials existing in the reactor and may or may not improve parylene adhesion. These results are supported by the Auger electron spectroscopy and contact angle measurements of the glow discharge-treated surfaces. Model studies using substrates of different surface free energies indicate that water-resistant adhesion of parylene polymer increases with the hydrophobicity of the substrate material and is in harmony with the above findings. From the viewpoint of adhesion the most advantageous and dependable effect is obtained with the methane plasma treatment. The treatment results not only in the formation of a more compatible, hydrophobic surface but also produces radical sites for covalent bonding with the parylene.
Ion-bombardment-induced whisker formation on graphite
Authors: Floro J. A. , Rossnagel S. M. , Robinson R. S.
Published: July 2002

The bombardment of surfaces with energetic ions has led to the development of various types of surface topography. In one special case, that of graphite, ion bombardment is responsible for the formation of whiskerlike structures. Whiskers formed under ion bombardment exhibit an initial rapid growth process followed by a slower, diffusion-fed process. The rates of lengthening and thickening during the diffusion-fed process appear to be constant in time. Whisker length increases nonlinearly with beam energy; population density decreases as beam energy increases. Whiskers grow parallel to the ion beam. Their alignment is apparently dominated by the destruction of nonparallel whiskers rather than electric field effects. Simultaneous impurity seeding inhibits whisker formation and can totally suppress formation if sufficient flux is present. A growth model involving stress relief at recrystallization front junctures qualitatively fits many experimental observations.
The atomic geometries of GaP(110) and ZnS(110) revisited: A structural ambiguity and its resolution
Authors: Duke C. B. , Paton A. , Kahn A.
Published: July 2002

The atomic geometries of GaP(110) and ZnS(110) are reexamined using our R-factor minimization procedure, developed for GaAs(110) and previously applied to GaSb(110), ZnTe(110), InAs(110), and AlP(110), to analyze experimental elastic low-energy electron diffraction intensities. Unlike most of the earlier cases, both GaP(110) and ZnS(110) exhibit two distinct minimum-Rx structures which cannot be distinguished by analysis of the shapes of the intensity profiles alone. One region of best-fit structures exhibits top-layer displacements normal to the surface characterized by a small bond-length-conserving, top-layer rotation (ω∼2–3°), a small relaxation of the top layer away from the surface, and a 10% expansion of the top-layer bond length. The other region of best-fit structures is the conventional one: nearly bond-length-conserving rotations of ω=26–28° in the top layer and a small (∼0.1 Å) contraction of the uppermost layer spacing. This ambiguity may be removed, however, by consideration of the integrated beam intensities. The conventional region of structural parameters provides a decisively better description of the relative magnitudes of the integrated beam intensities and hence is the preferred structure.
Infrared study of the chemisorption of hydrogen and water on vicinal Si(100) 2×1 surfaces
Authors: Chabal Y. J.
Published: July 2002

The influence of steps on the formation of hydrides and the decomposition of water has been investigated with high resolution infrared spectroscopy using a multiple internal reflection technique to increase sensitivity. On vicinal surfaces for which single domains are formed on the terraces, polarized radiation is used to separate vibrations of adsorbates on the terrace from those at steps. We find that H adsorbed at steps is less stable than the monohydride phase but more stable than the dihydride phase. Further water is found to dissociate preferentially on the terrace dimers and not at steps.
2
9Si nuclear magnetic resonance study of plasma-polymerized hexamethyldisiloxane
Authors: Assink R. A. , Hays A. K. , Bild R. W. , Hawkins B. L.
Published: July 2002

The structure of plasma-polymerized hexamethyldisiloxane (PP-HMDSO) was studied by 29Si nuclear magnetic resonance (NMR) using magic angle spinning and cross-polarization techniques. Spectra of the PP-HMDSO were recorded before and after it had been subjected to successive heat treatments in air at 100, 200, and 300 °C. The spectra exhibit major peaks at +11 ppm, −18 ppm, −55 to −64 ppm, and −99 to −106 ppm with respect to tetramethylsilane. These shifts correspond to silicon bonded to one, two, three, and four oxygens, respectively. The broadness of the tri- and tetrafunctional silicon peaks is attributed to SiOH or SiOCH3 functional groups in addition to the normal SiOSi moieties. Minor peaks at −5 and −33 ppm are attributed to mono- and difunctional silicons in which one of the methyl groups has been replaced by a hydrogen. As the material is heat treated there is a tendency for the PP-HMDSO to form additional silicon–oxygen bonds. Spectra simulations show that the oxygen content increases from 16 to 30 wt. % during the course of the heat treatment. The silicon to oxygen weight percent ratios measured by NMR spectra simulation and neutron activation analysis are comparable and decrease monotonically as the heat treatment progresses.
Coating requirements in gas turbine engines
Authors: Meetham G. W.
Published: July 2002

The complex thermal, chemical, and mechanical environment of the gas turbine engine makes it extremely difficult for one material to meet all of the different requirements imposed on the various engine components. Coatings are thus required to provide corrosion, resistance, wear resistance, insulation, and abradability. In the turbine section of the engine, in particular, the overall operating conditions are becoming progressively more hostile in terms of temperature and mechanical environment as engine efficiency and performance are improved. In certain types of operation the corrosive environment is emphasized. Thus, coatings must provide increasing protection, the need for which is often aggravated by the limitations which considerations such as strength requirements may place on the inherent environmental resistance of the base material. Perceived future coating needs are discussed. It is probably only in the last decade that coating requirements, in terms of application and behavior, have been recognized as an essential aspect of the design process. Significant future work remains to be done and this aspect is also discussed.
Transition metal containing plasma polymers
Authors: Morosoff N. , Haque R. , Clymer S. D. , Crumbliss A. L.
Published: July 2002

The plasma polymerization of the C2 hydrocarbons in the presence of Fe(CO)5 vapor can lead to the formation of soluble cluster complexes (I) and/or high oxidation state iron both in the form of the oxide (II) and associated with the carboxylate or β-diketonate ion (III). Analysis of the electron spectroscopy for chemical analysis (ESCA) spectra obtained yields information regarding the nature of I on the one hand, and regarding the prevalence of II relative to III, on the other. The relative yield of I with respect to (II+III) can be controlled, at will, by variation of W/F. This permits control of the coating’s morphology (continuity) as shown by the electrodeposition of Prussian blue on smooth surfaces coated with plasma polymer containing I+II+III.
Contact resistivity and secondary ion mass spectroscopy–Auger analysis of Au/Cr contacts on p-GaAs
Authors: Castanedo R. , Asomoza R. , Jiménez G. , Romero S. , Peña J. L.
Published: July 2002

Alloyed contacts to p-type GaAs(100) have been formed using Au/Cr bilayers. A minimum in the specific contact resistivity as a function of annealing temperature and time has been observed at around 430 °C and 1 min, respectively. Values of ρc as low as 1×10−5 Ω cm2 have been obtained for these conditions in Zn doped samples with p∼2.9×1018 atoms/cm3. In this work, in addition to the contact resistivity results, we report analysis performed by SIMS and Auger spectroscopies on samples prepared under the conditions mentioned above. The measurements show that the interface Cr/GaAs is very reactive, even at room temperature. We have observed interdiffusion of the different atomic species. However, the electrical behavior of the contacts having the minimum contact resistivity is very stable as a function of time as shown by an aging test.
Microwave electron cyclotron resonance plasma for chemical vapor deposition and etching
Authors: Keqiang Chen , Erli Zhang , Jinfa Wu , Hansheng Zhen , Zuoyao Guan , Bangwei Zhou
Published: July 2002

A highly activated microwave plasma is easily obtained based on the electron cyclotron resonance at relatively low gas pressures in the range from 10−3 to 10−1 Pa. The influence of the microwave power and gas pressures on the plasma parameters has been studied. Si3N4 film has been deposited on the silicon substrate utilizing the microwave electron cyclotron resonance (ECR) plasma without substrate heating. The film is examined by FT-IR, ellipsometry, and Auger electron spectroscopy. Hydrogen is not detected in the film. Reactive plasma etching is successful in the microwave ECR plasma. The dependence of the etch rate on the ratio of O2/CF4 in the gas and the bias voltage on the specimen has been investigated.
Catalytic oxidation of niobium by rare earths
Authors: Latta E.-E. , Ronay Maria
Published: July 2002

We have previously proposed a model for the catalytic formation of Nb2O5 from niobium covered by some monolayers of cerium, involving a change in the valence state of the cerium oxides on the surface. This model is further evaluated by studying the oxidation of Nb covered by some monolayers of erbium, praseodymium, or terbium under ultrahigh vacuum conditions using x-ray photoelectron spectroscopy. The oxidation took place at room temperature in the 5×10−8 1 Torr pressure range. The experiments showed that Er, which has no other valence state than three, is not a catalyst, while Pr and Tb, which can be both tri- and tetravalent, are catalysts, but only at such high pressures of oxygen, where the tetravalent oxides form. Due to its very low dissociation pressure, CeO2 forms already at 5×10−8 Torr oxygen pressure, therefore, the catalytic action of cerium does not depend on pressure in this range.
Adsorption of metal cations from aqueous solution onto tin oxide thin films
Authors: Tarlov Michael J. , Evans John F.
Published: July 2002

Secondary ion mass spectrometry, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, x-ray diffraction, transmission electron microscopy, and 15N hydrogen profiling have been used to characterize rf sputter-deposited tin oxide thin films (1500–2500 Å) before and after immersion in aqueous solutions to determine the extent of film hydration and ion exchange activity. The results support a model in which the tin oxide film is comprised of microcrystalline, colloidlike particles or grains (200–300 Å), the ion exchange sites being located on the hydroxylated grain surfaces. The uptake of Na+ in the bulk film increases sharply at a pH=3–5 corresponding to literature values for the isoelectric point of SnO2.
Surface phase transitions
Authors: Thiry P. , Jezequel G. , Petroff Y.
Published: July 2002

Angle resolved photoemission is a powerful tool to study the surface and bulk electronic structure of solids. In this paper we describe the use of this technique to learn about phase transitions at surfaces at low or high temperatures. The first example that we discuss is surface melting of metals. Predictions that surface melting should occur much below bulk melting have been made, and recently Frenken and Van der Veen have reported the observation of a solid–liquid transition at the (110) surface of lead of 40 K below the bulk melting point. Angle-resolved photoemission with synchrotron radiation is a very good tool to study these phase transitions because it allows us to probe only the first layers. Experiments on Pb(100) and Al(100) will be reported. No evidence of surface melting has been observed. We will also describe a surface metal–insulator transition observed at very low temperatures in Ce hydrides.
An axial-emission magnetron suppressor gauge
Authors: Chen J. Z. , Suen C. D. , Kuo Y. H.
Published: July 2002

Upon the basis of the axial-emission magnetron gauge (AEMG) a new axial-emission magnetron suppressor gauge (AEMSG) has been developed by adding a small photocurrent suppressor in front of the ion collector of AEMG. Due to the suppression of the residual photocurrent to the ion collector, the lowest measurable pressure (Px) of the AEMSG is reduced to ≲10−14 Pa (≲10−16 Torr) at I0=10−6 A, Us=500 V, and H=300 Oe. In comparison with other designs of hot-cathode ionization gauges, the AEMSG has some other advantages besides lower Px. It has a high sensitivity, its electrode construction is much simpler, so it is easier to degas, and its filament works at a lower temperature; these factors make the AEMSG more suitable for the measurement of extreme high vacuum. Furthermore, the upper limit of the AEMSG can be extended to 1.33×10−2 Pa (1×10−4 Torr) when the magnet is taken off and the emission current is increased to 1×10−3 A.
Preparation of CoCr thin films on polymer substrates by ion beam sputtering
Authors: Suzuki Y. , Takiguchi K. , Yoshitake M. , Yotsuya T. , Ogawa S.
Published: July 2002

CoCr thin films have been deposited on polymer substrates by ion beam sputtering. The magnetic and crystallographic properties have been investigated in the films deposited under a range of preparation parameters such as ion acceleration voltage, substrate temperature, and substrate bias voltage. The typical half-value width of the x-ray rocking curve Δθ50 is 10°. Transmission electron microscopy revealed that the films on the polyimide substrate under high substrate voltage showed columnar structure, but that without the voltage there were coarse grains and no clear columnar structure. The relations among coercive force (Hc⊥), saturation magnetization (Ms⊥), Δθ50, and Co content are discussed.
Copper carboxylate formation in the thermal oxidative degradation of atactic polypropylene on copper oxide films
Authors: Webb John D. , Czanderna A. W. , Pitts J. R.
Published: July 2002

The thermal oxidation of atactic polypropylene on CuO0.67 surfaces in air was studied using Fourier transform infrared reflection–absorption spectroscopy (FT–IR–RA). Degradative losses of primary, secondary, and tertiary alkyl hydrogens were observed. Films of CuO0.67 (55 nm thick) were prepared by the thermal oxidation at 122 °C of a copper film on 50 nm of gold, which was supported by a polished fused silica substrate. Thin films (70–200 nm) of purified atactic polypropylene were cast onto the copper oxide from xylene solution at 90 °C and air dried at 22 °C. The samples were heated in situ in the FT–IR–RA apparatus in synthetic or in laboratory air and then mounted for the IR–RA measurements from 600 to 5000 cm−1. At 60 and 73 °C, carboxylic acids are the primary degradation products, while at 85 and 100 °C, copper carboxylate formation predominates, and CuO0.67 is decomposed. The distinct change in the oxidative mechanisms between 73 and 85 °C apparently is related to an irreversible thermal transition in the atactic polypropylene films, which may favor carboxylate production by increasing the permeability of the films to oxygen and water vapor.
The interaction of oxygen with a NiCu(100) single-crystal surface
Authors: Shek M. L.
Published: July 2002

We have investigated the interplay between the near-surface composition of a Ni–7.5 at. % Cu(100) single-crystal surface and its oxygen uptake. The clean annealed surface consists of a normalized Cu fraction of ∼0.53 (∼0.47–∼0.57) within the top 4.2–5.7 Å, as judged by Auger electron spectroscopy. Compared with a Ni(100)-like surface, the oxygen uptake is slower and a coverage larger by ∼0.1 monolayer (ML) is required for the onset of oxide nucleation. At ∼600 K on the annealed alloy surface, oxygen exposures can lead to a decrease of ∼0.1 in the normalized Cu fraction, as well as an initially enhanced uptake relative to 300 K exposures. A c(2×2) structure is observed by low-energy electron diffraction. At ∼600 K, however, the oxygen signal seems saturated by ∼120 L or ∼0.50–∼0.66 ML.
Experimental and theoretical investigation of chemisorbed Ga on Si(111)
Authors: Thundat T. , Mohapatra S. M. , Dev B. N. , Gibson W. M. , Das T. P.
Published: July 2002

Study of the lattice location of Ga adsorbed from weakly acidic methanol solution on chemically cleaned Si(111) surfaces has been carried out by an x-ray standing wave (XSW) technique using a conventional Mo Kα x-ray source. XSW is a very sensitive and precise technique to determine impurity atom locations normal to the diffraction planes. In addition to measurements of adatom locations normal to the crystal surface plane, the atom locations relative to planes inclined to the surface were measured. Self-consistent field Hartree–Fock cluster investigations were carried out for Ga on Si(111) surfaces for different high-symmetry adsorption sites such as substitutional, atop, threefold open, and threefold eclipsed to determine the minimum energy positions. The effect of lattice relaxation was studied for the eclipsed site alone. Analyzing the XSW results together with the bond lengths and binding energies derived from cluster calculations indicates that Ga coadsorbs at the atop and threefold relaxed eclipse sites. Also it is shown that the ultraviolet photoemission spectra for these two sites obtained from cluster calculation explain satisfactorily the observed spectrum for Ga adsorbed on Si(111).
Ion-beam-induced desorption of Ar+n ion clusters
Authors: Christiansen J. W. , Tsong I. S. T. , Lin S. H.
Published: July 2002

The mass distribution of Ar+n ion clusters desorbed from Ar solid at 20 K was measured for 1≤n≤38. The desorption was induced by bombardment of the solid by three different primary ions, Ne+, Ar+, and Kr+, at 6 keV. The mass distribution is not influenced greatly by the bombarding species. This agrees with the description of the experimental data by Rice, Ramsperger, Kassel, and Marcus (RRKM) theory incorporating the charge-transfer process. Certain ‘‘magic number’’ cluster ions were also observed in the Ar+n mass distribution.
Surface potential effects in low-energy current image diffraction patterns observed on the Al(001) surface
Authors: Jette A. N. , Bargeron C. B. , Nall B. H.
Published: July 2002

Fine structure observed in high-resolution low-energy electron diffraction (LEED) measurements near the energy threshold for emergence of new beams has been attributed to surface barrier effects. Recently, the surface barrier has been suggested as the source of the fine structure observed in current image diffraction (CID) patterns obtained by rastering the primary beam across an Al(001) crystal surface at a constant energy. This suggestion was based on kinematic arguments which correlated the emergence angle for a new electron beam with the observed structure in the CID pattern. In this work, the angular dependence of the elastic component of the total crystal reflectivity is calculated at constant energy. The calculations are based on full dynamical theories such as used in LEED but incorporating different surface barrier models to account for the saturating image potential. The results are compared with the experimental CID results.