bluuebunny/crossref_metadata_2025 huggingface dataset:
Thickness effects in ultrathin film chemical vapor deposition polymers
Authors: Senkevich Jay J.
Published: July 2002

An increasing scientific and technological interest exists to understand the structure and properties of ultrathin films and the associated solid/solid interface. In particular ultrathin polymer films are largely impacted by the presence of the substrate due to the cooperative motion needed for crystallization and the glass transition. The study here investigates the optical anisotropy of poly(p-xylylene) (PPXN) ultrathin films as-deposited as a function of thickness and then subsequently annealed; and poly(chloro-p-xylylene) (PPXC) ultrathin films as a function of film thickness and deposition temperature. Of the various methods available to deposit ultrathin polymer films, chemical vapor deposition (CVD) is of particular interest since it can deposit ultrapure highly conformal pinhole free films on the thickness level of nanometers. The most common class of CVD polymers are the parylenes that are polymerized by self-initiation through pyrolysis of the dimeric cyclophane precursor. The two parylene polymers studied here, PPXN and PPXC, are shown to be increasingly positively birefringent in the as-deposited condition as their thickness is decreased below ∼100 nm. Unlike PPXN whose Tg is ∼13 °C, PPXC whose Tg is ∼36 °C exhibits a thickness effect below ∼100 nm much like PPXN for films deposited above its Tg but not below its Tg. A possible explanation is given. Further, PPXNs birefringence as a function of its film thickness is investigated as a function of postdeposition anneals both below and above its crystallographic phase transformation from monoclinic to hexagonal at ∼220 °C.
Electrochemical fluorine source for ultrahigh vacuum dosing
Authors: Nakayama Koji S. , Sakurai T. , Weaver J. H.
Published: July 2002

A solid state electrochemical source was built that allows fluorine dosing at pressures below 8×10−11 Torr. The cell consists of a CaF2 crystal that is clamped between two Au plates that function as the cathode and anode. An applied voltage at elevated temperature produces an ionic current due to fluorine migration to the anode where it desorbs. This cell allows submonolayer halogen exposures, and it is ideal for studies of halogen etching because it minimizes degradation of the vacuum system and avoids toxic chemicals.
Microcalorimetry applications of a surface micromachined bolometer-type thermal probe
Authors: Li Mo-Huang , Gianchandani Yogesh B.
Published: July 2002

This article describes a surface micromachined scanning thermal probe that uses polyimide as the structural material and an embedded thin film metal resistor as the sensing element. The typical dimensions of a fabricated probe are 350 μm in length, 50 μm in width, and 3–10 μm in thickness. The resistor and the scanning tip are formed by sputter-deposited films of nickel and tungsten, which provide temperature coefficient of resistance of 2963 ppm/K. The probe is used to map surface and subsurface spatial variations in the thermal conductivity of a test sample. It is also used as a spatially localized microcalorimeter to measure the glass transition temperature of photoresists: the values obtained for Shipley 1813 and UV6 are 118±1 °C and 137±1 °C, respectively. These are in close agreement with results obtained by other methods that utilize larger samples.
Elastic and Inelastic Diffraction of Low-Energy Electrons from Al (100): Double-Diffraction Model
Authors: Duke C. B. , Howsmon A. J. , Laramore G. E.
Published: July 2002

The isotropic-scatterer, inelastic-collision model is used to analyze experimental elastic intensity profiles characteristic of electrons in the energy range 10≤E≤120 eV scattered from Al(100). Using the double-diffraction approximation a satisfactory description of these profiles is achieved for a variety of incident beam angles, θ≤25°. The temperature dependence of these profiles due to both thermal expansion and lattice vibrations is examined. Using the inelastic-collision-model description of the elastic scattering, the RPA plasmon dispersion relations and lifetimes, and the sharp-junction, “uniform” electron fluid electron-plasmon interaction vertices, the inelastic energy and angular profiles for the two-step excitation of bulk and surface plasmons are predicted using a double-diffraction analysis of this process. Observation of these profiles and their correlation with such an analysis constitutes a critical test of existing models of inelastic diffraction.
Monte Carlo Analysis of the Behavior of Divergent Conical Effusion Orifices
Authors: Ward John W. , Fraser Malcolm V. , Bivins Robert L.
Published: July 2002

Computer simulation of real experiment by Monte Carlo techniques has been extended to include the study of particulate fluxes in divergent conical Knudsen cell orifices. The effects of specular reflection and surface diffusion have been considered, in terms of the cone angle and length-to-radius ratio of the orifice. Large statistical sample sizes resulted in precise steady-state values for transmission coefficients, spatial distributions of effusate, and evaporation flux gradients. The results of other studies have been analyzed and interpreted in light of the Monte Carlo data.
Temperature Rise during Film Deposition by rf and dc Sputtering
Authors: Lau S. S. , Mills R. H. , Muth D. G.
Published: July 2002

The temperature rise during film deposition by rf sputtering of Ta, Au, and Pd films and dc sputtering of Ta films has been investigated. The film temperature was measured for various cathode to anode distances, as a function of applied power used for sputtering and sputtering time. It was found that by sputtering from water-cooled cathodes at power levels between 0.1 and 2 kW, the contribution due to radiation from the plasma and the cathode is negligible. To a first approximation, heating is caused in varying amounts by the kinetic energy of the sputtered atoms, the latent heat of condensation of the sputtered atoms, and electron bombardment. The film temperature also depends on the optical emissivity of the deposited materials causing the temperature of films with higher emissivities such as Pd to remain significantly lower for comparable sputter times and power. For dc sputtering from an uncooled cathode, it was estimated that radiation from the cathode contributes significantly to the heating of the sputtered films.
An ultrasensitive hydrogen detector and its applications
Authors: Das K. B.
Published: July 2002

An ultrasensitive hydrogen detector is described which has a detection sensitivity of a few parts per billion in the gas stream. The detector's performance, range, and accuracy were evaluated by analyzing titanium standards from NBS containing known amounts of hydrogen at the following levels: 32±5, 98±5, and 215±6 ppm. The fractional random error in the mean value of these standards at 95% confidence level was found to be ±5.01%, ±1.67%, and ±1.08%, respectively. The detector has been used on surface contamination, corrosion, hydrogen concentration gradient, and hydrogen embrittlement studies. When two such detectors are connected in a Y configuration, the system becomes a vacuum gauge calibrator which does not involve a flowmeter. The principle of operation and highlights of the above studies are presented along with the sensitivity factor for a Bayard–Alpert-type ionization gauge for hydrogen gas.
Performance of multicusp plasma ion source for focused ion beam applications
Authors: Scipioni L. , Stewart D. , Ferranti D. , Saxonis A.
Published: July 2002

The multicusp plasma ion source has found many uses such as in ion implanters, ion projection lithography, and injectors for particle accelerators. This source is favorable for such applications because of its low ion temperature, small energy spread, and good stability. These properties also offer the tantalizing possibility of utilizing such a source to create a noncontaminating focused ion beam (FIB) system for semiconductor fabrication line use and advanced photomask repair applications. We have made measurements of the brightness of this ion source and have done some preliminary applications work with a Kr+-enabled FIB system for mask repair and ion milling. A specially designed multicusp plasma ion source has been used to measure focused beam spot size versus current relationships for several inert gas ion species. From these data the axial brightness of the ion source was ascertained. For the extraction of Kr+ from the source we find an angular intensity of 12 μA/sr and a virtual source size of 16.9 μm, giving an approximate value for the image-side brightness (at 30 keV) of 1650 A/cm2 sr. We find good sputtering rates for the Kr+ beam, and the Aerial image measurement system measurements show essentially 100% transmission of 193 nm light through photomask quartz that had been implanted with a significant dose of krypton.
Polysubstituted derivatives of triphenylene as high resolution electron beam resists for nanolithography
Authors: Robinson A. P. G. , Palmer R. E. , Tada T. , Kanayama T. , Allen M. T. , Preece J. A. , Harris K. D. M.
Published: July 2002

We have explored the application of polysubstituted derivatives of triphenylene as high resolution, high etch durability electron beam resists. Room temperature spin coating was used to produce films of the derivatives on silicon substrates. Exposure to a 20 keV electron beam was found to alter the dissolution rate of these derivatives in various organic solvents. Doses of between ∼3×10−4 and ∼2.5×10−3 C/cm2 substantially increased the solubility of the derivative hexapentyloxytriphenylene in polar solvents (positive tone behavior). Doses greater than ∼2.5×10−3 C/cm2 led to a decrease in solubility in both polar and nonpolar solvents (negative tone behavior). Other derivatives also demonstrated a reduction in their dissolution rate for doses between ∼1.5×10−3 and ∼6.5×10−3 C/cm2. The etch durabilities of the positive and negative tone patterns were found to be, respectively, ∼25% less and ∼70% greater than that of a conventional novolac based negative tone resist. Line and space patterns were defined in one of the resists with a resolution of ∼14 nm and structures with an aspect ratio of ∼50–1 were etched into silicon.
Scanning Auger microscopy studies of an ancient bronze
Authors: Paparazzo E. , Lea A. S. , Baer D. R. , Northover J. P.
Published: July 2002

Scanning Auger microscopy (SAM) has been used to study the surface and interface microchemistry of a sheet bronze belt from the Urartian kingdom in NE Syria of the early first millennium B.C. We find that the patina contains no copper species at all (decuprification), whereas carbonaceous species, Ca-silicates and N-bearing species are detected, the last being tentatively identified as organic (primarily amine-like) residues deriving from the soil. A textured grain, which we qualify as a second phase of bronze originated by an imperfect alloying of the two major metals (i.e., consisting of Cu-rich and Sn-rich domains) is observed on the metallic side lying beneath the patina. SAM imaging with a submicron spatial resolution highlights the presence of SnO2 oxide inside what appears to be the hollow veins of the grain, whereas a Cu2O-like oxide is confined exclusively to the flat regions of the grain. We explain these results by noting that the hollow veins, offering a higher exposure to external fluids, are likely to have promoted preferential formation of the more stable tin oxide over copper oxide. In another region of the metal side we studied the chemistry of grain boundaries and their surrounding areas. We find that S species lie exclusively inside the grain boundaries, whereas Sn and Zn species accumulate just outside the boundary channels, and this lateral chemical inhomogeneity is highlighted with a ∼200 nm spatial resolution. Lateral segregation of Cu and Sn domains is imaged in another region with a spatial resolution of ∼15 nm. This result marks the best spatial resolution any analytical method has yet achieved in highlighting chemical heterogeneities of ancient bronzes. Although archaeomaterials lie outside the mainstream applications of Auger techniques, this study provides convincing evidence that SAM can greatly advance our understanding of these materials, as it provides clues relating to corrosion and patination phenomena, as well as manufacturing processes that are not easily reachable by other microscopies or microanalytical techniques.
Epitaxial growth and electric characteristics of SrMoO3 thin films
Authors: Wang H. H. , Yang G. Z. , Cui D. F. , Lu H. B. , Zhao T. , Chen F. , Zhou Y. L. , Chen Z. H. , Lan Y. C. , Ding Y. , Chen L. , Chen X. L. , Liang J. K.
Published: July 2002

Using computer-controlled pulsed laser deposition, high quality SrMoO3 thin films have been epitaxially grown on SrTiO3 (001) substrates. A growth interruption technique was employed for better epitaxy, and layer-by-layer growth was obtained by the intensity oscillations of reflection high-energy electron diffraction. The films were of epitaxial crystallinity as characterized by high-resolution transmission electron microscopy. The typical root mean square surface roughness was 2.2 Å. The SrMoO3 thin films showed metal-like conducting behavior with resistivity on the order of 60–120 μΩ cm between 4 K and room temperature. An x-ray photoelectron spectroscopy study shows that the valence band spectrum of the film has the typical character of a metal, and Mo 4dt2g electrons are responsible for the conduction of the films.
In situ X-ray Photoelectron, Ultraviolet Photoelectron, and Auger Electron Spectroscopy Spectra from First-Row Transition-Metal Nitrides: ScN, TiN, VN, and CrN
Authors: Gall D. , Haasch R. T. , Finnegan N. , Lee T.-Y. , Shin C.-S. , Sammann E. , Greene J. E. , Petrov I.
Published: July 2002

X-ray photoelectron (XPS), ultraviolet photoelectron (UPS), and Auger electron spectroscopy (AES) spectra are presented from epitaxial, single-crystal transition-metal (TM) nitride (ScN, TiN, VN, and CrN) layers, that were grown in situ in an ultrahigh-vacuum (UHV) magnetron sputter deposition system attached to the analysis chambers. The samples are near-stoichiometric with N/Me ratios determined by Rutherford backscattering spectroscopy (RBS), and contain no bulk or surface impurities detectable by XPS, AES, or RBS. The spectra therefore represent reliable reference data. We also present XPS and AES data from sputter-etched samples in order to quantitatively determine the effect of preferential sputtering of nitrogen in TM nitrides. The sputter etching was performed under conditions typical for sputter cleaning of air-exposed samples until a steady state N/Me ratio is reached.
Ion energy and angular distribution at the radio frequency biased electrode in an inductively coupled plasma apparatus
Authors: Mizutani Naoki , Hayashi Toshio
Published: July 2002

The incident energy and angle of ions that bombarded the rf biased electrode in an etching apparatus using inductively coupled Ar–O2 plasma, were analyzed for the pressure of 4–20 mTorr. For the energy analysis at the rf driven electrode, the analyzer was set to rf floating, that is, the electric potential reference of the analyzer was equal to the potential of the rf biased electrode. The ion energy distribution (IED) was measured by the retarding method, and the ion angular distribution (IAD) was measured by annular ion collector rings. The measured IED and IAD were qualitatively explained by a simple calculation model that included charge exchange and elastic collision in the sheath. From a comparison between the measured and calculated results, the ion transverse temperature at the ion sheath edge was estimated to be about 1000 K.
Clocking of molecular quantum-dot cellular automata
Authors: Hennessy Kevin , Lent Craig S.
Published: July 2002

Quantum-dot cellular automata (QCA) may provide a novel way to bypass the transistor paradigm to form ultrasmall computing elements. In the QCA paradigm information is represented in the charge configuration of a QCA cell, which maps naturally to a binary model. Molecular QCA implementations are being explored where the quantum dots in the cell take the form of redox centers in a molecule. Clocking has proved important in QCA cells synthesized so far. Here we examine a method to clock molecular QCA cells which are assembled at an interface. The clocking signals in this scheme originate from wires buried below the QCA surface. We present a simplified model of this clocking method and examine its behavior as a molecular shift register.
Etching of polysilicon in inductively coupled Cl2 and HBr discharges. II. Simulation of profile evolution using cellular representation of feature composition and Monte Carlo computation of flux and surface kinetics
Authors: Mahorowala Arpan P. , Sawin Herbert H.
Published: July 2002

A Monte Carlo simulator was developed to simulate the profile evolution during the Cl etching of patterned polysilicon in high density plasma etchers. The simulator used Monte Carlo techniques to compute the transport and surface kinetics combined with a cellular representation of the feature. The Monte Carlo algorithm permitted the incorporation of all the dominant physical and chemical mechanisms of the etching process such as angle-dependent ion-enhanced etching, physical sputtering, ion scattering, surface recombination, plasma deposition, sidewall passivation, and line-of-sight redeposition without encountering numerical difficulties. The technique allows for the modification of surface kinetics rates based on the surface composition. Simultaneous composition-dependent etching and deposition could be handled easily. A modification of the cellular representation of the feature (similar to a string-and-node algorithm) was developed to determine neutral species interactions with the surface enabling accurate simulation of deposition processes. A surface normal calculation algorithm involving least-squares fitting of the surface was developed to handle specular ion scattering. A generalizable structure allowing the incorporation of all chemical and physical processes was developed to handle the compositional, energy, and angular dependent surface processes. Test cases were run to quantify the accuracy of the combined Monte Carlo and cellular algorithms.
Thin film coatings in solar−thermal power systems
Authors: Peterson R. E. , Ramsey J. W.
Published: July 2002

The applications and requirements for thin film coatings in solar−thermal power systems are reviewed. The substantial impact of selective absorber coatings and antireflection coatings on both flat plate and concentrating type solar collectors is covered. The results of durability life tests on a high−temperature stable, vacuum−evaporated absorber coating consisting of layers of Al2O3−Mo−Al2O3 are reported. This coating was unaffected by 500 h at 930°C. Other tests included thermal cycling, thermal shock, ultraviolet irradiation, and solar wind simulation. An electroplated solar absorber coating for low−temperature applications (<200°C) has been developed which has a solar absorption of 0.96 and an infrared emittance of 0.07 at 100°C. A chemically etched antireflection coating for glass has been investigated. Solar transmission of 0.97 is possible with this low−cost technique.
Angle-resolved photoemission studies of surface states on (110) GaAs
Authors: Knapp J. A. , Lapeyre G. J.
Published: July 2002

Synchrotron radiation was used to study the angle-resolved uv photoemission spectra (UPS) of the (110) surface of GaAs. The wide photon energy continuum (hν<30 eV) coupled with 40 angular resolution permitted the identification of two occupied surface states, centered at ∠−1 and ∠−7 eV measured from the valence band (VB) maximum. These states have not previously been observed in photoemission experiments due to their degeneracy with the bulk VB emission. The relatively new technique of angle-resolved UPS enables one to obtain dispersion information about occupied states and thus lends itself to discriminating between surface and bulk transitions. Since the parallel component of the electron’s momentum is conserved upon leaving the sample, portions of the occupied surface band structure could be identified and mapped out. These were found to be in qualitative agreement with the dispersions predicted by Joannopoulos and Cohen. Sensitivity of the emission to surface contamination served as a test of their surface character.
Surface structures of compound semiconductors
Authors: Duke C. B.
Published: July 2002

A survey is given of surface-structure determinations for the low index faces of compound semiconductors via analyses of elastic low-energy electron diffraction intensities. The (100) face of (cubic-NaCl structure) MgO is thought to exhibit a surface structure that differs little from that of a truncated bulk crystal. Small deviations from the truncated bulk geometry have been reported for the layered transition-metal dichalcogenides MoS2 and NbSe2. The cleavage faces of tetrahedrally coordinated compound semiconductors [i.e., zincblende (110), wurtzite (101̄0), and wurtzite (112̄0)] exhibit the same space-group symmetry as the bulk crystal. The positions of the species in the uppermost atomic layer may be altered substantially from their bulk values, however, with the anion shifted outward, the cation inward, and the whole layer relaxed toward the bulk crystal. For the materials examined to date [i.e.,GaAs(110), ZnSe (110), ZnO(101̄0) and ZnO(112̄0)] the shifts in atomic position vary in magnitude but can exert profound effects on the electronic structure and chemical behavior of the associated surfaces.
Hydrogen and deuterium permeation in copper alloys, copper–gold brazing alloys, gold, and the i
n s
i
t
u growth of stable oxide permeation barriers
Authors: Begeal David R.
Published: July 2002

The deuterium permeation through several copper alloys has been measured over a temperature range of 550 to 830 K using the membrane technique. In some cases, the hydrogen permeability was also measured. The results were divided into three categories: common alloys, gold alloys, and stable oxide forming alloys. Common alloys which showed typical bulk metallic diffusion with litle change in the permeation activation energy as compared to copper (77 kJ/mol for D2) were: (additions are in weight percent) 5% Sn, 2.3% U, 0.15% Zr, 4% Sn+4% Pb+4% Zn, 3% Si, and 7% Al+2% Fe. Compared to copper, the D2 permeability at 573 K was reduced by factors of 2.0, 2.7, 4.5, 5.3, 5.9, and 7.0, respectively. A series of gold–copper alloys including pure gold, 80% Au, 50% Au, 49% Au, and 35% Au also showed typical bulk metallic diffusion with a trend of decreasing permeability (increasing activation energies for permeation) with increasing gold content. There were also pronounced inflections or shifts in the permeability at ∠370°C, or about the order–disorder transition for Cu3Au and CuAu, for the 80% and 50% alloys. Two alloys did not exhibit bulk metallic permeation behavior and the permeabiltiy was in fact controlled by surface oxide layers. It was found that a layer of beryllium oxide could be formed on Cu+2% Be and a layer of aluminum oxide could be formed on Cu+7% Al+2% Si. As compared to 0.25 mm-thick copper, the deuterium permeability at 500°C was reduced by a factor of ∠250 for Cu–Be and ∠1000 for Cu–Al–Si. The activation energies for deuterium permeation were 98 kJ/mol and 132 kJ/mol, respectively. The mechanism for the oxide growth is the high-temperature hydrogen reduction of nearby less stable oxides, simultaneous with oxidation of the active metal, Be or Al, by trace amounts of water in the hydrogen. Ion microprobe mass analysis identified the oxide layers as containing beryllium or aluminum but not containing copper. The trend of activation energies found lends support to a theoretical model that relates the activation energy for diffusion to the shear modulus of the host oxide and the molecular radius of the diffusing molecule.
X-ray lithography by synchrotron radiation of INS–ES
Authors: Aritome H. , Nishimura T. , Kotani H. , Matsui S. , Nakagawa O. , Namba S.
Published: July 2002

Synchrotron radiation from the electron-synchrotron INS-ES at the University of Tokyo has been used for x-ray lithography. Several mask patterns, such as the bubble pattern and the holographic grating pattern, are duplicated. Masks used are 0.4 μm-thick gold on 3-μm silicon. At an electron energy of 1.1 GeV the direct-total beam of synchrotron radiation can be used for mask replication. By using the grating-mask pattern with 692-nm period made by the holographic method, the grating pattern with a large height-to-width ratio is obtained. This result clearly demonstrates the high collimation of the x-ray beam from the synchrotron.
Silver contact on GaAs (001) and InP (001)
Authors: Massies J. , Devoldére P. , Linh N. T.
Published: July 2002

We report here the first results of our study of metal/III–V semiconductor contacts :Ag–GaAs (001) and Ag–InP (001). Clean GaAs (001) and InP (001) surfaces, under various conditions of stoichiometry, are obtained by ion etching followed by annealing and arsenic or phosphorous adsorption and carefully characterized by AES, LEED, and ELS. Using these techniques some correlations between surface stoichiometry, surface structures, and electronic surface states have been found. On these surfaces the metallization has been carried out by an MBE-like technique. The results obtained show that the Schottky barrier height changes according to the conditions of surface preparation. To explain these results, different hypotheses are discussed.
Thermal transpiration: A comparison of experiment and theory
Authors: Storvick T. S. , Park H. S. , Loyalka S. K.
Published: July 2002

The experimental measurements of the absolute thermal transpiration differential pressure generated in a capillary tube by helium, argon, carbon dioxide, and air are compared to the theoretical differential pressure calculated by Loyalka. The thermal creep flow calculated using the BGK model and Maxwell’s diffuse-specular reflection operator at the wall is quantitatively correct for these gases.
Surface reactions and interdiffusion
Authors: Bachrach R. Z. , Bauer R. S.
Published: July 2002

Local interface chemistry is important to understand before proceeding to unravel the electronic properties of interfaces. Characterization of the intrinsic aspects of reactions and interdiffusion is also important to understand possible device size limitations. Various types of local reactions are possible and we have investigated these with surface sensitive photoemission and photoyield spectroscopy using synchrotron radiation excitation. We show for both compound semiconductor and for oxide surfaces that reactions can occur which locally determine the chemical phases present. The heat of these reactions can drive the interdiffusion process. We present results for the important systems Al–GaAs, Al-oxidized GaAs and Al–SiO2. We compare these results with our previous results for Ge and Ga on GaAs as well as those obtained by others. We demonstrate that the I–V behavior of Ga on p-GaAs is ohmic and attribute this non-schottky character to As diffusion and vacancy production at the interface. Diffusion can clearly be identified as distinct from island formation by analyzing the core photoelectron line shape; Ge grown on GaAs(110) at differing substrate temperatures provides an illustrative example. Interfacial reactions are observed in real time for Al2O3 formation by Al reduction of oxidized Si.
Adsorption of type III and V elements on GaAs (110)
Authors: van Laar J. , Huijser A. , van Rooy T. L.
Published: July 2002

Angular-resolved photoemission and low energy electron loss spectroscopy (LELS) measurements are reported on GaAs(110) surfaces with adsorbed In, N, P, and As. It is shown that the photoemission from the ’’As dangling bond’’ surface state vanished after In adsorption, whereas the surface photoemission is unaffected by deposition of N, P, or As. This result is confirmed by tight-binding-type surface band structure calculations for models with a relaxed GaAs surface on which adsorbed In is assumed to be bound to the surface As atoms and N, P, or As to the Ga. However, some other observations like the surface Fermi level position and the behavior of the Ga 3d exciton peak in the LELS spectrum cannot be understood from this model.
Gas adsorption on cleaved GaAs(110) surfaces studied by surface photovoltage spectroscopy
Authors: Liehr M. , Lüth H.
Published: July 2002

Surface photovoltage (SPV) spectroscopy has been used to investigate electronic surface state distributions in the forbidden band of ultrahigh vacuum (UHV) cleaved clean and gas covered (O2, H2, H2O, H2S) GaAs(110) surfaces. On clean p-type surfaces a correlation is found between the cleavage quality and the absolute height of the SPV for h/ω≳1.5 eV. This effect is related to surface states near midgap which are due to crystallographic defects. A similar surface state distribution is also observed after O2 adsorption. Exposure to atomic hydrogen (H) causes surface states at about 0.1 eV below the conduction band or near 0.1 eV above the valence band edge. The results of H2O adsorption are interpreted in terms of empty surface states derived from a molecular H2O level; H2S adsorbs in a two-stage process causing two different types of extrinsic surface states within the forbidden band.
Obtaining numerical values for the elliptic emission functions used in calculating electron emission from surfaces
Authors: Craig Miller H.
Published: July 2002

The four elliptic emission functions v(y), t(y), s(y) and ϑ(y) [or fmg(y)] are used in calculating electron emission from surfaces under the influence of temperature and external electric fields. Critical evaluations are made of the published tables of these functions. Several existing approximation equations for the emission functions are given together with some additional polynomial approximations. The estimated accuracies of these approximation equations range from five to six significant figures for the most complex approximations to ±1% or so for the simplest.
Schottky barrier behavior in polycrystal GaAs
Authors: Cohen Marshall J. , Paul M. D. , Miller D. L. , Waldrop J. R. , Harris J. S.
Published: July 2002

The results of a study of the effects of grain boundaries on the electronic properties of Schottky barriers formed in polycrystal GaAs are reported. The samples investigated were fabricated on n-type (Sn doped) GaAs grown by MBE on large grain polycrystal n+-GaAs substrates. Al layers were deposited in situ in the MBE chamber immediately following the growth of the epilayers and 10 mil Schottky dots were subsequently isolated photolithographically. The diodes characterized fell into three categories: (i) diodes which were located entirely within a single grain and exhibited ideal, n=1 behavior, (ii) diodes which straddled a grain boundary but whose electrical characteristics were indistinguishable from the single crystal diodes, and (iii) diodes which straddled a grain boundary and whose forward current contained an excess leakage component. The data are explained in terms of a model in which the Fermi level is pinned by localized bandgap states due to dangling bonds at the grain boundary and the resulting band bending causes a double depletion region in the bulk material.
X-ray photoelectron spectroscopy study of the surface adsorption of lead naphthenate
Authors: Bertrand P. A. , Fleischauer P. D.
Published: July 2002

The adsorption of lead naphthenate on the surfaces of metals, oxides, and sulfides was studied by means of x-ray photoelectron spectroscopy (XPS). Lead naphthenate physically adsorbs on all surfaces. It chemisorbs on surfaces with basic sites, such as oxides, sulfides, and metals treated in basic solutions before adsorption. The two states have distinct XPS spectra: the Pb(4f7/2) binding energy for the physically adsorbed state is 139.1 eV, whereas that for the chemisorbed state is 136.6 eV. Conditions of surface preparation that result in the formation of each state are described.
Silicide interface stoichiometry
Authors: Freeouf J. L.
Published: July 2002

Recent studies of the initial stages of silicide formation on silicon have observed a chemical shift of silicide-like valence orbitals. The sign of the shift suggests a silicon-rich ’’silicide’’ close to the silicon substrate smoothly grading to the bulk silicide stoichiometry as the probed region is moved away from the substrate. Such results suggest a re-examination of the boundary values appropriate to a simple Schottky picture of the metal-semiconductor interface. It seems clear that the appropriate metal workfunction is that of the metallic region closest to the silicon-i.e., the region most deviant from the bulk silicide. The absence of reliable experimental values for either stoichiometry or the workfunciton of this region forces us to model both. Assuming the silicon electron affinity and doping to be unaffected by the interface, we conclude that such a silicon excess would reduce the dependence of barrier height upon bulk metal (silicide) workfunction. Further, we conclude that if this metallic layer has a stoichiometry very silicon-rich (∠(metal)Si4) the barrier height of most silicides would be fully explained by this effect and the Schottky picture of the interface.
Correlation of GaAs surface chemistry and interface Fermi-level position: A single defect model interpretation
Authors: Grant R. W. , Waldrop J. R. , Kowalczyk S. P. , Kraut E. A.
Published: July 2002

XPS measurements of core-level binding energies have been used to determine the interface Fermi-level position in (100) and (110) n-and p-type GaAs samples as a function of surface treatment. A variation in interface Fermi-level position of up to 0.7 eV has been observed. The difference in interface Fermi-level position for n- and p-type samples subjected to identical surface treatments remains nearly constant at ≊0.3 eV. A single defect model (SDM) is suggested which appears to provide the simplest explanation for the observed interface-potential variations.
Oxygen impurity effects at metal/silicide interfaces: Formation of silicon oxide and suboxides in the Ni/Si system
Authors: Grunthaner P. J. , Grunthaner F. J. , Scott D. M. , Nicolet M-A. , Mayer J. W.
Published: July 2002

The effect of oxygen impurities on the Ni/Ni2Si interface has been investigated via ion implantation using x-ray photoelectron spectroscopy (XPS), 4He+ backscattering, and 16O(d,α)14N nuclear reactions. Oxygen dosages corresponding to peak concentrations of 1, 2, and 3 atomic percent were implanted into Ni films evaporated on Si (100) substrates. The oxygen, nickel, and silicon core lines were monitored as a function of time during in situ growth of the Ni silicide to determine the chemical nature of the diffusion barrier known to form in the presence of oxygen impurities. It is shown that neither Ni oxide or mixed compounds such as Ni2SiO4 are involved in the barrier formation. The data demonstrate that as the advancing Ni/Ni2Si interface encounters oxygen in the Ni film, silicon suboxides (Si2O3, Si2O, and SiO) are formed. As more oxygen is encountered, Si takes on a full coordination of oxygen, forming SiO2. When a sufficient layer of SiO2 has formed, Ni metal is no longer able to diffuse through to the Si/Ni2Si interface to continue the solid phase reaction. It has been determined under UHV annealing conditions that the amount of oxygen necessary to stop the Ni diffusion is 2.2×1016 O/cm2. These experiments also provide a novel approach for synthesizing Si oxides and suboxides in a metallic matrix for examining relaxation effects in XPS as well as providing model compounds for Si/SiO2 interfacial studies.
A simple method for fabricating submicron lines
Authors: Speidell J. L.
Published: July 2002

A process for fabricating submicron lines using photolithography and vacuum evaporation is described. In this process, a photoresist layer is deposited and treated to obtain an undercut profile using chlorobenzene. The submicron line is then deposited by direct evaporation onto the photoresist edge. Lines as narrow as 0.06 μm and aspect ratios ranging from 5:1 to 30:1 have been demonstrated. The process offers several advantages such as simple processing, dimensional control, and a broad materials range.
Alloyed ohmic contacts to GaAs
Authors: Braslau N.
Published: July 2002

The alloyed AuGe-based contact is widely used to make ohmic connections to GaAs. It has been presumed that the regrown alloyed region is heavily doped so that carrier transport is by tunneling. The electrical and metallurgical properties of this heterogeneous system have been extensively studied and have been shown to be spatially nonuniform. Details of fabrication technique, analysis, and theoretical interpretation of its behavior will be discussed. It is suggested that the observed (doping)−1 dependence of the contact resistance is due to spreading resistance domination of current paths through submicron regions of the contact area where heavy doping occurs.
Theoretical investigation of hydrogen chemisorption on Ga-containing III–V compounds
Authors: Manghi F. , Bertoni C. M. , Calandra C. , Molinari E.
Published: July 2002

A fully self-consistent pseudopotential calculation of the electronic properties of atomic hydrogen chemisorbed on GaAs and GaP(110) surfaces is reported. Different chemisorption geometries and substrate coverages are considered. The results are compared with the experimental information to select a structural model.
Faceting of a Ge(111) surface and its vicinals during Ag adsorption
Authors: Suliga E. , Henzler M.
Published: July 2002

The stability of a Ge(111) surface and its vicinals were studied by LEED for clean and Ag covered conditions. Before deposition the surfaces were macroscopically smooth. After deposition of only a few tenths of a monolayer of Ag and heating to 400 °C, the surface consisted of narrow (544) facet planes and wide (111) terraces. At coverage above 0.7 the (544) facets disappeared and (211) facets started growing. The facets had dimensions in the range of 10 μ, so that they could be observed with an optical microscope. Furthermore, a rotation of ledges into directions with low indices was observed. The results show that the equilibrium shape of a surface is drastically changed by a coverage of a fraction of a monolayer due to the formation of new minima of the surface free energy at peculiar high index vicinal surfaces.
Alkali-promoted gas adsorption and surface reactions on metals
Authors: Bonzel H. P.
Published: July 2002

Alkali metals adsorbed on transition metal surfaces do not only change the electronic properties of these metals but alter also their chemical reactivity. This effect is well known in heterogeneous catalysis as the promotion of metallic catalysts by alkali additives. The more fundamental physics of this effect is being studied by applying surface spectroscopies to coadsorbed layers of alkali and gas molecules (e.g., O2, N2, CO, NO) on well defined transition metal surfaces. These studies show that the adsorbed alkali causes the transfer of additional electronic charge into certain molecular orbitals of the adsorbed molecules. This electron transfer may weaken or strengthen the metal–adsorbate or intramolecular bonds. In some cases the dissociation of the adsorbed molecule occurs. Heterogeneous surface reactions on these alkali promoted samples show significant changes in both the surface composition of the catalyst and the gaseous product spectrum. Examples for this effect are given in connection with a study of CO hydrogenation on K-promoted Fe foils.
The etching of Cu(100) with Cl2
Authors: Winters Harold F.
Published: July 2002

Modulated beam mass spectrometry has been used to investigate the etch products, surface chemistry and product desorption mechanisms for interaction of Cl2 with Cu(100). The dominant etch product is Cu3Cl3 for T<∼580 °C and CuCl for T>650 °C. Exposure of Cu(100) to Cl2 at temperatures below 150 °C leads to the growth of a copper chloride layer whose thickness increases linearly with time for exposures greater than approximately 1000 langmuirs. The rate of growth of the chloride layer decreases with increasing temperature. Temperature programmed desorption experiments indicate that the chloride desorbs as Cu3Cl3 with zero order kinetics and an activation energy for desorption of about 33 kcal/mol. The zero order desorption kinetics suggest that the quantity of chlorine on the immediate surface remains constant until the chlorine in the bulk is depleted. The activation energy for desorption of 33 kcal/mol is within experimental error equal to the heat of sublimation of Cu3Cl3 from pure copper chloride. This result suggests that etching is simply a ‘‘compound growth’’-evaporation reaction. Ion bombardment is shown to suppress the etch rate which is probably a consequence of ion-induced dissociation of Cu3Cl3 and possibly a decreased concentration of chlorine in the surface region.
Surface structure determination using alkali and noble gas ion scattering
Authors: Niehus H.
Published: July 2002

The potential and application of low-energy ion backscattering for surface structure determination is discussed. Low-energy impact collision ion scattering spectroscopy (ICISS) supplies a real-space image of the geometry of surfaces. ICISS with noble gas ions and ion detection only is hampered by the large neutralization effect. Substantial improvement is obtained by the use of primary alkali ions (ALICISS) or the detection of scattered noble gas neutral atoms (NICISS). A missing-row-type reconstruction has been found for Pt(110)-(1×2), Au(110)-(1×2), and the oxygen induced reconstruction of O/Ni(110)-(2×1) and O/Cu(110)-(2×1). The hydrogen induced reconstruction of H/Pd(110)-(1×2), on the other hand, is clearly represented by a pairing row atomic arrangement with an alternating 0.4-Å lateral displacement of close packed 〈110〉 Pd rows.
Interaction of boron with Mo(100) and its effect on surface chemistry
Authors: Fryberger T. B. , Grant J. L. , Stair P. C.
Published: July 2002

The interaction of boron with Mo(100) has been investigated for the first time by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. At 300 K, the B(1s) binding energy increases approximately linearly with boron atomic coverage from 186.8 eV at 0.2 monolayers (ML) to 187.6 eV at 1.1 ML (saturation). This suggests the growth of a uniform layer with increasing adatom–adatom interactions. Annealing studies showed that at 1073 K and above, two distinct phases are formed: (i) a low-coverage phase [0.2 ML, B(1s) at 186.8 eV], and a B-rich, surface ‘‘boride’’ phase [MoB2, B(1s) at 188.2 eV]. Adsorption studies showed that molecular CO adsorption is suppressed, but that ethene, propene, and trifluoropropene adsorption and dissociation are not inhibited when boron is present. These results are compared to those on O- and C-covered Mo(100) surfaces.
Characterization of phosphate crystal nucleation and growth on cold-rolled steel
Authors: Gaarenstroom Stephen W. , Ottaviani Robert A.
Published: July 2002

The formation of a dense, phosphate crystal coating on a steel surface is of prime importance in paint technology to prevent corrosion and to provide a proper substrate for paint adhesion. Crystal nucleation and growth, from an immersion phosphating bath, was examined in detail by infrared spectroscopy, scanning electron microscopy, x-ray diffraction, and electron spectroscopy. The application of high-spatial-resolution Auger electron spectroscopy, which probes individual crystals and substrate regions between crystals, was especially useful and yielded new information on the growth of phosphate coatings. In the system studied, the 15-nm-thick native oxide layer is removed in the first 5 s of immersion as the iron surface dissolves in the bath. Phosphate crystals are identified as nearly all phosphophyllite [Zn2Fe(PO4)2⋅4H2O], but with surfaces enriched in Ni and depleted in Zn. No precursor species form prior to the phosphophyllite growth, in contrast to observations on other phosphating systems. The combined application of our instrumental characterization techniques, together with our experimental protocol, is a powerful method for examining the initial steps in the formation of many surface coating materials.
Raman scattering characterization of carbon bonding in diamond and diamondlike thin films
Authors: Nemanich R. J. , Glass J. T. , Lucovsky G. , Shroder R. E.
Published: July 2002

The atomic bonding configurations of carbon bonding in diamond and diamondlike thin films are explored using Raman scattering. The general aspects of Raman scattering from composites are presented. Effects are discussed due to crystalline or amorphous structures, large versus microcrystalline domains, and strong optical absorption and transparent regions. The Raman scattering from diamondlike films shows several features which are attributed to microcrystalline graphitelike structures which all originate from the same region in the sample. In contrast, the spectra of diamond films show features attributed to different components of a composite film. Components identified are crystalline diamond, and disordered and microcrystalline graphitic structures. The presence of precursor microcrystalline or amorphous diamond structures is also suggested.
Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite
Authors: Tiedje T. , Varon J. , Deckman H. , Stokes J.
Published: July 2002

We have used electrical noise measurement and transmission electron microscopy to study the condition of metal tips used to obtain contrast current mode scanning tunneling microscope (STM) images of pyrolytic graphite in air. Electrochemically and mechanically sharpened tips of W, W0.97Re0.03, Pt, and Pt0.8Ir0.2, which gave giant corrugations in graphite images, were found to have various forms of carbonaceous contamination on the tip. Contamination in the form of liquid hydrocarbon, graphite clumps, and carbon fibers were observed. Tunneling current noise spectra from these tips exhibited a characteristic 1/f behavior from 10 Hz to 10 kHz. Clean atomic-resolution tips formed by sputtering thin layers of metal (Ni, Au, Pt) on prefabricated tips were found to exhibit lower noise in the tunneling current and ≤1-Å amplitude corrugations. Our interpretation of the present experiment, while similar to earlier explanations that associate the giant corrugations with contamination, differs in that we attribute the large corrugations to deformation of relatively soft carbonaceous material attached to the tip, rather than to elastic deformation of the graphite itself. In addition, we have found that several of the structural artifacts observed in atomic-resolution STM images of graphite can be explained by a moiré effect caused by a double tip.
Laser-induced fluorescence measurement of plasma ion temperatures: Corrections for power saturation
Authors: Goeckner M. J. , Goree J.
Published: July 2002

When using high-power pulsed tunable dye lasers to measure plasma ion temperatures, it is important to attenuate the laser intensity. The temperature is found from the Doppler broadening of a spectral line. This may be obscured by saturation broadening, an instrumental effect encountered when too much laser intensity is used. Three useful experimental methods for determining the optimum pulsed laser intensity are found from a semiclassical atomic physics model. As an example, an Ar II transition pumped by a 1-GHz bandwidth laser is examined. The fluorescence linewidth of room-temperature ions broadens from 1.50 to 2.87 GHz when the homogeneous laser intensity is increased from 50 kW/m2 to 5 MW/m2.