bluuebunny/crossref_metadata_2025 huggingface dataset:
A versatile data acquisition system for low-energy electron diffraction
Authors: Anderegg J. W. , Thiel P. A.
Published: July 2002

We have developed a system for rapid data acquisition from low-energy electron diffraction (LEED) patterns. The essential components of the system are a sensitive silicon intensified target video camera, a commercial video interface, and a PDP 11/23 microcomputer. The system is designed to acquire up to ten diffraction spot intensities and profiles (total) in a short time relative to the time scales of typical surface reactions. The data acquisition system is used in conjunction with standard Varian■ four-grid LEED optics in an ultrahigh vacuum chamber. The abilities of the system with respect to versatility, speed, linearity, and signal-to-noise are demonstrated, and data from a study of oxygen chemisorption on Pd(100) are presented.
Mass spectrometric studies of the electrical breakdown of thin polymer films
Authors: Kendall B. R. F. , Rohrer V. S. , Bojan V. J.
Published: July 2002

Little is known of the composition of the neutral particle fluxes released during the electrical breakdown of insulating films in vacuum. Analysis of the released particles is unusually difficult because of the transient nature of the event, the unpredictability of its exact position and timing, and the very high rate at which information is generated. A special type of time-of-flight mass spectrometer triggered from the breakdown event has been developed to study this problem. The first materials investigated have been thin polymer films similar to those used on the exteriors of spacecraft. A slowly increasing potential gradient is applied from a high-impedance source through a movable contact until breakdown occurs. The observed neutral-particle bursts can be correlated with actual discharge sites left behind on the sample. For Teflon FEP and PFA films the material released consists mainly of fluorocarbon fragments, some of them having masses greater than 350 amu, while the material released from Kapton, Tefzel, and Mylar films consists mainly of light molecules with masses at or below 44 amu. Kapton samples which had been exposed to an oxygen ion beam to simulate conditions in low earth orbit gave generally similar results except for reduced neutral emission in the predischarge phase.
Electron stimulated desorption and its relation to molecular structure at surfaces
Authors: Madey Theodore E.
Published: July 2002

Recent examples of the use of electron stimulated desorption ion angular distribution (ESDIAD) in determining the structures of molecules on surfaces are discussed. These include the adsorption and decomposition of NH3 on Fe(100), and the stabilization of H2O on Ag(110) by impurity O and Br atoms. ESDIAD evidence for ‘‘lying down’’ CO on several surfaces is presented, and the influence of beam damage in producing new surface structures for H2O on Ni(110) is discussed. Calculations of the influence of the surface image potential and reneutralization effects on ion trajectories are described briefly.
Structure-property relations for surface processes at the metal/solution interface
Authors: Ross Philip N.
Published: July 2002

The recent development of special apparatus coupling electroanalytical cells directly to UHV surface analytical chambers has made possible definitive determination of structure-property relations for processes at metal electrode surfaces. Low-energy electron diffraction analyses of emersed platinum electrodes have provided direct evidence for place-exchange processes accompanying anodic oxidation, and combined with recent x-ray photoelectron spectroscopy and thermal desorption spectroscopy results these studies have refined our understanding of aqueous oxidation phenomena. The use of UHV annealed well-ordered single-crystal surfaces has produced some surprising results. The process of submonolayer metal ion deposition (so-called underpotential deposition) exhibits unexpected sensitivity to long-range order in the substrate, indicating that lateral bonding forces are of the same magnitude as the perpendicular bonding force. On a well-ordered Pt(111) surface, unexpected new surface processes have been observed that are unique to this surface, i.e., are not seen on either (100) or (110) surfaces, nor on polycrystalline surfaces. It appears that the new processes are associated with atomically flat regions of the (111) surface with a critical region size that is quite large, at least 2 nm. It is suggested that cooperative interactions in the double layer form a critical ensemble that react to form the new surface species. However, much further study utilizing in situ spectroscopy will be required to understand this new surface process in Pt(111).
Alkali overlayers on aluminum, alumina, and aluminum carbide
Authors: Paul J.
Published: July 2002

Adsorption data for potassium and sodium atoms on the surfaces of aluminum metal, aluminum oxide, and aluminum carbide are presented. Low coverage adsorption of K(Na) on Al(100) is characterized by a 4.6(3.2) D surface dipole and desorption around 620(590) K. Increasing the alkali coverage lowers the desorption temperature as well as the effective surface dipole per alkali atom. Clustering on aluminum metal is activated by annealing to around room temperature. The adsorption of K and Na monomers on dehydrogenated Al2O3 is characterized by desorption around 350 K and, for sodium, by cluster formation even at submonolayer coverage. Finally, potassium but not sodium atoms bind strongly to Al4C3 (Id>700 K). This explains a previous observation of potassium at a promoted aluminum surface after exposure to carbon monoxide and annealing to 700 K.1,2
Coverage dependence in the electron-stimulated desorption of neutral NO from Pt(111)
Authors: Burns A. R. , Jennison D. R. , Stechel E. B.
Published: July 2002

We report a strong coverage dependence in the yield of a neutral molecule desorbed by low-energy excitations produced by electron impact. For NO/Pt(111), there are two distinct desorption channels revealed by state-specific translational energy distributions, measured by time-of-flight laser resonance ionization. The dominant low-energy channel (Etrans =∼0.05 eV) has been previously assigned to a simple 5σ excitation. The high-energy channel (Etrans =∼0.35 eV) involves complex (multielectron) excitations. Here we present the coverage dependence of the specific yield for the two channels. Strikingly different behavior is seen in that a strong inverse dependence occurs for low-energy NO but relative independence of coverage is seen for high-energy NO. A model is developed which describes the coverage dependence in terms of varying localization (or self-trapping) probabilities as a function of nearest-neighbor site occupancy. Comparison to experiment using a random vacancy model is rather good, with deviations from randomness consistent with higher coordination than random at intermediate coverages.
Gallium arsenide thin films by low-temperature photochemical processes
Authors: Nishizawa J. , Kurabayashi T. , Abe H. , Sakurai N.
Published: July 2002

Molecular layer epitaxy (MLE) is a new crystal-growth method which is able to produce thin films of single crystal in the accuracy of atomic order at relatively low temperature. In MLE, adsorption and surface reaction processes are controlled by alternate injection of reactant gases. MLE of GaAs was demonstrated in trimethylgallium–AsH3 (TMG–AsH3) and triethylgallium–AsH3(TEG–AsH3) systems on various faces of GaAs substrate; monomolecular layer growth was realized for (100) face in TMG–AsH3 system. The film thickness, electric properties, and surface morphology were studied as a function of substrate surface crystalline orientations. By using mass spectroscopy, adsorption and surface reaction processes of MLE by TMG–AsH3 systems were studied. The formation and migration of Ga complex adsorbate which was produced from TMG, and the surface reaction of adsorbate with AsH3 were clarified. Influence of photoirradiation on the growing film has been investigated by use of excimer lasers. UV light irradiation improved the surface morphology and, in certain instances, also lowered impurity concentrations of the film. From the results of mass spectroscopic measurements and photoirradiation effects, it is supposed that surface migration species is an adsorbate like Ga (CHn)x, x=0 at the temperature higher than 520 °C and x=1 at the temperature lower than 500 °C, in MLE.
A surface study of the oxidation of polycrystalline tin
Authors: Asbury Douglas A. , Hoflund Gar B.
Published: July 2002

The oxidation of a polycrystalline tin foil was examined using Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), ion scattering spectroscopy (ISS), and electron-stimulated desorption (ESD). An oxidation state which does not correspond to SnO or SnO2 has been identified using XPS and retarding mode AES at low oxygen exposures (≤3000 L). It has been suggested in previous studies that oxygen penetrates beneath the outermost layers during the early stages of oxidation of certain metals such as tin. This suggestion has been verified in this study using ISS and ESD. Very small amounts of oxygen are detected with ISS on a sputter-cleaned Sn surface, and this O signal does not increase with oxygen exposure even though XPS and AES show a significant oxygen uptake. Additionally, no O+signal is detected with ESD whereas large ESD O+ signals are obtained from surfaces of bulk tin oxides. The oxygen concentration is uniform across the sample surface at these exposures as determined by SAM implying that grain boundaries do not provide preferential adsorption sites.
The compensation effect in thermal desorption of adsorbate systems with lateral interactions
Authors: Niemantsverdriet J. W. , Wandelt K.
Published: July 2002

Within the Polanyi–Wigner model of thermal desorption, lateral interactions are revealed by a coverage-dependent activation energy of desorption E(θ) and a preexponential factor ν(θ). Several experiments have shown that ν and E often satisfy the ‘‘compensation effect’’ ln ν(θ)=E(θ)/RTc+const. In this contribution we introduce this compensation effect into the rate expression for thermal desorption and present calculated thermal desorption spectra assuming pairwise lateral interactions and first- or second-order desorption kinetics. The results emphasize that simplified thermal desorption spectroscopy analysis procedures based on peak maximum temperatures and peak widths only will almost always lead to incorrect results.
Scanning tunneling microscopy topography of electron-beam evaporated niobium thin films in Nb–NbOx–Pb tunnel junctions on quartz substrates
Authors: Walmsley D. G. , Turner R. J. , Hagan H. P. , Garcia N. , Baro A. , Vazquez L. , Paganini R.
Published: July 2002

The topography of electron-gun evaporated niobium films incorporated into Nb–NbOx–Pb crossed film tunnel junctions has been examined in a scanning tunneling microscope. It is found that films deposited on a quartz substrate at 20 °C are rough with irregular strong surface ridges which suggest a crystallite structure on a scale of 100 to 500 Å. Films deposited on quartz at 600±50 °C are smooth (to within 30 Å) over distances >1 μ. The films on cold substrates give high current leakage characteristics in thin-film tunnel sandwiches; those on heated substrates have much lower leakage. It is suggested that the leakage in rough film structures may be understood on the basis of a recent theory of tunneling by Garcia, Flores, and Guinea. Attempts to determine the topography of evaporated films of Al, In, and Pb exposed to ambient atmosphere were unsuccessful: the natural oxide is thought to frustrate the measurement in these materials.
Stress and microstructure in tungsten sputtered thin films
Authors: Haghiri-Gosnet A. M. , Ladan F. R. , Mayeux C. , Launois H. , Joncour M. C.
Published: July 2002

Tungsten films, 0.05–1 μm thick, were deposited on silicon wafers by rf magnetron sputtering. Induced film stresses were investigated as a function of substrate temperature and background pressure in connection with microstructural observations and limited compositional analysis. Homologous substrate temperatures Ts/Tm, where Tm is the melting point of tungsten (3683 K), ranged from 0.08 to 0.15. Argon pressures investigated ranged from 1 to 7 Pa. This corresponds to the low-temperature part of Thornton’s microstructure model, within zones I and T. Two regions were distinguished for increasing values of the homologous temperature and/or decreasing values of argon pressure: (i) The first region, observed for low substrate temperature and high argon pressure, was a voided zone I microstructure with small grain size (200–400 Å). Film stresses increased from zero towards tensile values as voids disappeared. (ii) The second region, observed for higher values of Ts/Tm and/or low pAr, was a denser zone T structure, with a bimodal grain distribution (<3000 Å). After reaching a maximum tensile value σ=2×103 MPa, corresponding to the transition from zone I to zone T microstructure, the stress decreased abruptly to a constant value σ=−2.7×103 MPa. All stress values were found to be quite independent of thickness in the range from 0.05 to 1 μm. Even though β metastable phase is usually observed for zone I deposits, a quasipure α film was obtained at low temperature and high pressure (within zone I) under especially clean conditions. Independent of phase composition (α or β phase) the voided zone I was observed to be highly reactive in air. The shift with time toward compressive stress for the zone I structure was attributed to oxygen absorption in the pores. For 3-month-old films, a high oxygen content of ∼20% was measured by nuclear reaction analysis. In the denser zone T films without voids, no change in film stresses was observed over time, and oxygen content was negligible (<1%) after 3 months.
The influence of crystallographic order upon the electronic structure of thin mercury overlayers
Authors: Dowben P. A. , Onellion M. , Varma Shikha , Kime Y. J. , Erskine J. L.
Published: July 2002

Thin (1–7 monolayer) well-ordered, disordered, and liquid Hg overlayers on Ag(100) have been investigated with angle resolved photoemission and low-energy electron diffraction. The well-ordered overlayers exhibit new electronic states as a result of the strained, cubic, overlayer crystallographic structure. The new electronic states depend upon the long-range crystallographic order of the overlayer, not simply the local crystallographic symmetry. The photoemission cross section and branching ratio of the 5d mercury states depends on the overlayer long-range crystallographic order and cannot be explained by photoelectron diffraction.
A molecular-beam study of the dissociative chemisorption of O2 on Ir(110)-(1×2)
Authors: Mullins C. B. , Wang Y. , Weinberg W. H.
Published: July 2002

The zero-coverage probability of dissociative chemisorption of O2 on Ir(110)-(1×2) has been measured using molecular-beam techniques for a wide range of incident kinetic energies, incident angles, and surface temperatures. The data indicate that a trapping-mediated mechanism is responsible for dissociative chemisorption at low energies, whereas at high energies a direct mechanism accounts for dissociative adsorption. Total energy scaling approximately describes the dissociative dynamics on the very corrugated Ir(110)-(1×2) surface.
Diffusion measurements on interactive systems
Authors: Tringides M. C.
Published: July 2002

The nonlinearity of the diffusion process in interactive systems suggests a classification of the methods that measure diffusion coefficients into equilibrium and nonequilibrium ones. Diffusion coefficients obtained by different methods, in general, do not agree with each other. This is shown explicitly with the use of Monte Carlo simulations by comparing results obtained with an equilibrium method (fluctuation method) and a nonequilibrium one (laser-induced-desorption method). However, the same microscopic information about the interactions between the particles can be obtained by any method with proper modeling.
Ion beam effects on the composition and structure of glass surfaces
Authors: Brow R. K.
Published: July 2002

Changes in the surface composition and structure of a number of alkali silicate glasses, bombarded by 1–5 keV argon ions, have been characterized by x-ray photoelectron spectroscopy. Quantitative analyses of peak area changes indicate that the glass surfaces are dealkalized by the ion beam, and decomposition of the respective O 1s spectra reveal that the surfaces simultaneously repolymerize.
Ethylene π species on bare and cesiated Pt(111) surfaces
Authors: Hugenschmidt M. B. , Dolle P. , Jupille J. , Cassuto A.
Published: July 2002

The adsorption of ethylene on bare and cesiated Pt(111) surfaces has been investigated by means of thermal desorption spectroscopy and ultraviolet photoemission spectroscopy, at temperatures ranging between 37 and 800 K. On the bare Pt(111) surface, a new π-bonded C2H4 species has been observed at T<52 K. It is believed to be nearly undistorted with respect to the gas phase molecule. At T>52 K, it transforms into di-σ C2H4 and then into ethylidyne (∼300 K). Conversely, cesium preadsoption gives rise to reversibly adsorbed π-bonded ethylene, meanwhile it inhibits the usual reaction path (di-σ species leading to ethylidyne). Furthermore, valence-level binding energy shifts induced by cesium adatoms have been shown to be consistent with the theoretical picture of the electrostatic potential surrounding adsorbed alkali elements.
Physical and chemical properties of the anodic oxide/HgCdTe interface
Authors: Stahle C. M. , Helms C. R. , Schaake H. F. , Strong R. L. , Simmons A. , Pallix J. B. , Becker C. H.
Published: July 2002

In this paper we show that anodic oxide/Hg0.8Cd0.2Te interface is a two-layered structure with a thin CdTeO3 layer next to the Hg0.8Cd0.2Te substrate and a second thicker layer of CdTeO3, TeO2, HgTeO3, and HgTe particles. For a 700-Å-thick anodic oxide, the thin CdTeO3 layer was 30–50 Å thick and the second interface layer was ∼120–150 Å thick. The large positive fixed charge measured on metal-insulator semiconductor devices which have been fabricated with the anodic oxide may be caused by a large concentration of positively charged oxygen vacancies associated with the HgTe particles in the second interface layer. The low interface trap densities are probably due to the thin CdTeO3 layer which forms a good interface with HgCdTe.
Determination of electrode/substrate surface temperatures through the solid–liquid transition of metals/eutectics in capacitively coupled radio frequency-plasma chemical vapor deposition reactors
Authors: Shinohara M. , Tashiro H. , Saio K. , Arai K. , Takayama K.
Published: July 2002

In a capacitively coupled radio frequency-plasma chemical vapor deposition reactor, we took pure metals, In, Sn, Bi, and Pb with the melting points ranging from 157 to 327 °C, and the eutectic Al (45 wt. %)/Ge alloy with the eutectic point of 423 °C as the fixed point materials to determine surface temperatures of the electrode and a piece of stainless steel (SLS) plate placed on it. The surface temperatures thus obtained were compared with temperatures detected with a thermocouple mounted within the electrode. An analysis of the obtained results based on a simplified heat transfer model revealed the thermal contact resistance at the specimen-SLS plate interface to be as small as about one sixth that at an interface between two pieces of SLS plate. Criteria for the choice of eutectics as the thermometric fixed-point materials are discussed and some possible candidates usable for this purpose are listed.
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u simultaneous radio frequency discharge power measurements
Authors: Godyak V. A. , Piejak R. B.
Published: July 2002

The power dissipated in a radio frequency (rf) discharge has been measured using three different techniques simultaneously. In one technique, the power dissipated in the matching network and in the discharge itself have been determined from rf voltage and in-line power measurements. Two other techniques based on voltage and current measurements have been used to calculate discharge power directly. Measurements based on all three techniques have been compared and are in good agreement. Typical results of these measurements are given for rf electroded discharges at 13.56 MHz in argon gas at pressures of 0.01 and 1.0 Torr.
Measurement of the local density of states on a metal surface: Scanning tunneling spectroscopic imaging of Au(111)
Authors: Everson M. P. , Jaklevic R. C. , Shen Weidian
Published: July 2002

We describe a method for measurement of the local density of states on metals with the scanning tunneling microscope and apply it to the study of the sp-gap surface state on Au(111). Local surface state peak intensity changes are found to correlate with surface features. Spectroscopic images of the surfaces are produced. The surface state intensity is substantially reduced at step edges as compared to the value for large terraces. A change in the intensity by a factor of 2 over the 23×(3)1/2 reconstruction unit cell is also observed. These effects are attributed to a spatial variation of the surface state intensity with the local potential. Upward shifts of the surface state energy have also been observed on narrow terraces, however more detailed measurements are necessary to understand this effect.
Equilibrium surface hydrogen coverage during silicon epitaxy using SiH4
Authors: Liehr M. , Greenlief C. M. , Offenberg M. , Kasi S. R.
Published: July 2002

Epitaxial silicon has been grown on Si(100) wafers using SiH4 in a rapid thermal chemical vapor deposition reactor in the temperature regime from 450–700 °C. Gas analysis during growth and thermal desorption spectra (TDS) after growth were measured with a differentially pumped mass spectrometer. We have attempted to estimate the surface population of hydrogen during epitaxial growth by ‘‘freezing out’’ the surface hydrogen with a rapid cool down and pump down followed by a temperature programmed desorption taken in the reactor. SiH is found as the majority species in equilibrium during growth, with the surface population decreasing from one monolayer around 550–600 °C, at a pressure of three mTorr SiH4. Molecular hydrogen does not interfere with silane adsorption in this pressure regime.
Auger and x-ray absorption studies of solid molecular oxygen
Authors: Chen Jie , Lin C. L. , Qiu S. L. , Strongin Myron , denBoer M. L.
Published: July 2002

We have studied the x-ray absorption spectrum (XAS) and the Auger electron spectrum (AES) of condensed molecular oxygen in the vicinity of the O 1s (K) core level. The XAS spectrum, which is similar to that previously reported for gas phase O2, has sharp peaks at about 530.8 and 540.3 eV, which we attribute to excitations from the 1s level into π* and σ* states, respectively. We assign a third peak at 543.3 eV to Rydberg transitions. Compared with gas phase O2, the σ* and Rydberg states are shifted to higher energies, an effect that we attribute to solid state effects in the condensed layer. The AES spectra of the ionized molecule and excited molecule are both shifted relative to the spectra of gas phase O2, but by different amounts, 10 and 2 eV, respectively, which may be due to different final state screening effects. A tentative assignment of the features in the AES spectra is also made.
A new experimental method for determining secondary ion yields from surfaces bombarded by complex heterogeneous ions
Authors: Blain M. G. , Della-Negra S. , Joret H. , Le Beyec Y. , Schweikert E. A.
Published: July 2002

We report on a new experimental method for determining the relative negative secondary ion (SI) yields for a number of different molecular and cluster primary ions. The method is based on the time-of-flight (TOF) analysis of primary ions which are desorbed from a source target by fission fragments of 252Cf. By using secondary electrons, emitted from a sample target upon primary ion impact, and an event-by-event coincidence counting technique, secondary ion TOF spectra correlated to several different primary ions may be obtained simultaneously. We have used this technique to measure the relative SI yields from organic (e.g., phenylalanine) as well as metallic (e.g., gold) surfaces using (CsI)nCs+ (n=0, 1, 2) and the monomer and dimer ions of coronene, C24H12, and phenylalanine, C6H5CH2CH(NH2)COOH, as primary ions. For both the organic and metallic surfaces, strong enhancements in the SI yields are observed when a more complex projectile is used.
Effects of annealing and surface preparation on the properties of polycrystalline CdZnTe films grown by molecular beam epitaxy
Authors: Ringel S. A. , Sudharsanan R. , Rohatgi A. , Owens M. S. , Gillis H. P.
Published: July 2002

The effects of annealing and chemical etching on the chemical, compositional, and electrical properties of polycrystalline molecular-beam epitaxy (MBE)-grown Cd1−xZnxTe thin films were investigated in detail for the first time to identify and eliminate some of the undesirable process-induced side effects resulting from the CdZnTe/CdS solar cell fabrication procedure. Depth-resolved x-ray photoelectron and Auger electron spectroscopies on processed surfaces along with current-voltage and capacitance-voltage measurements on In/Cd1−xZnxTe Schottky diodes were used to characterize the treated Cd1−xZnxTe surface. Films were annealed in air at 200–400 °C and etched in Br:CH3OH and K2Cr2O7:H2SO4 solutions, similar to processes used to achieve high efficiency CdTe/CdS solar cells. Air annealing was found to enhance the uniformity of the p-type conductivity within the film, yielding a doping value of ∼3×1015 cm−3. However, the air anneal resulted in a highly Zn-rich surface compared to hydrogen and argon anneals. Furthermore, the air anneal resulted in the oxidation of ∼80% of the Zn at the surface which was found to extend ∼0.1 μm into the bulk. This is in contrast to Cd and Te oxides which were detected only at the surface.

A model is proposed to explain the Zn pileup near the surface. According to this model, the oxygen diffuses into the film to oxidize Zn resulting in a concentration gradient and outdiffusion of Zn. Subsequent etching of the annealed films by the standard Br2:CH3OH etch used for CdTe processing removed the surface Te and Cd oxides but failed to remove the Zn-rich surface region and Zn–O and did not result in a p+ Te-rich surface which is required to facilitate ohmic contacts. In contrast, etching of the annealed Cd1−xZnxTe surfaces with a concentrated K2Cr2O7:H2SO4 solution removed all oxides and left a surface completely depleted in Cd and Zn. Subsequently fabricated In/Cd1−xZnxTe Schottky barrier diodes on etched surfaces confirmed a significant reduction in effective barrier height, from ∼1.2 eV for the Br:CH3OH surface to ∼0.65 eV for the K2Cr2O7:H2SO4 etched surface due to a Te-rich surface layer with a p-type doping concentration of ∼3×1017 cm−3. It is expected that these results will provide guidelines to increase present-day Cd1−xZnxTe cell performance by reducing the resistance of ‘‘ohmic’’ contacts.
Fabrication of cylindrical, microcellular foam-filled targets for laser-driven experiments
Authors: Falconer John W. , Golnazarians Wigen , Baker Michael J. , Sutton Douglas W.
Published: July 2002

Small, open ended, parylene cylinders of 150 μm diameter, 250 μm length and 10 μm wall thickness are manufactured and filled with solutions of acrylate monomers which are polymerised in situ with uv light to produce gels. These gels are then precipitated in a non-solvent such as methanol and dried in a critical point drying apparatus to produce foam-filled parylene cylinders for use in laser-driven plasma experiments. The foams have densities in the range of 20 to 200 mg cm−3 and cell sizes of up to 1 μm, fill the cylinders completely, do not shrink during the drying process and do not require subsequent machining.
Angular dependence of the secondary electron emission crystal current: Effects of surface modification
Authors: Peeters F. , Puckrin E. R. , Slavin A. J.
Published: July 2002

The variation of the angle between the incident electron beam and sample surface contributes in two ways to the secondary electron emission from single crystals. These contributions are a monotonically varying background due to the changing number of secondaries produced near the surface, and an oscillatory component previously explained as a bulk effect related to Kikuchi patterns. This work shows that for emission from a Au(111) surface, the background can be described well by a simple model based on the semiempirical theory of Kanaya and Kawakatsu. It also provides additional support for the explanation of the oscillations as a bulk effect by showing that the oscillation amplitude remains essentially unchanged either by surface sputtering or by the deposition of a thin lead layer. The presence of these oscillations requires some care in the use of the secondary electron crystal current as a measure of the thickness of thin metal films.
A simple molecular beam system for surface reactivity studies
Authors: Bowker Mike , Pudney P. D. A. , Barnes C. J.
Published: July 2002

A simple design for a thermal molecular beam system is described, which can be added as a bolt-on to a conventional vacuum system. It provides a beam of well-defined spatial distribution, which can be heated or cooled, and used for a variety of applications. These include the accurate and rapid determination of sticking probabilities and their coverage dependence, the gas temperature dependence of sticking, and reactive scattering measurements. The use of the beam is exemplified for the dissociative sticking of oxygen on Cu(110) and the reaction of a beam of methanol with a predeposited patch of oxygen. The initial O2 sticking probability is determined to be 0.21 (±0.01) independent of substrate temperature (300–600 K). Methanol is dehydrogenated by preadsorbed O atoms, but by different mechanisms at high and low substrate temperatures.
Scanning tunneling microscopy study of the structure of sulfur [2(3)1/2×2(3)1/2] R 30° overlayer on rhenium (0001)
Authors: Ogletree D. F. , Ocal C. , Marchon B. , Somorjai G. A. , Salmeron M. , Beebe T. , Siekhaus W.
Published: July 2002

The structure of the (2(3)1/2×2(3)1/2) R30° sulfur overlayer chemisorbed on rhenium(0001) has been studied with scanning tunneling microscopy (STM). The ordered sulfur overlayer was prepared in ultrahigh vacuum and then transferred through air to an STM operating at a vacuum of 10−7 Torr. The (2(3)1/2×2(3)1/2) R30° sulfur overlayer passivates the rhenium substrate in air. STM images show the atomic structure of the overlayer unit cell to be a hexagonal ring of six sulfur atoms. Defect structures in the two-dimensional lattice of sulfur hexagons, overlayer domain boundaries and substrate dislocations were also observed.
The effects of low-energy ion impacts on graphite observed by scanning tunneling microscopy
Authors: Shedd G. M. , Russell P. E.
Published: July 2002

A focused ion beam has been used to expose graphite substrates to gallium ions with kinetic energies ranging from 10 eV to 2.5 keV. The effects of the ion impacts on the substrates were then observed by scanning tunneling microscopy. For landing energies of 250 eV and above, essentially every ion impact produces an amorphous, cluster-like feature of 1–5 nm in diameter that protrudes from the substrate by about one-tenth of the feature diameter. As ion energy is decreased below 250 eV the probability that an ion impact will cause a protruding features decreases, until, for landing energies of 10 eV and below, it becomes negligible.
Preparation of low resistivity Cu–1 at. %Cr thin films by magnetron sputtering
Authors: Cabral C. , Harper J. M. E. , Holloway K. , Smith D. A. , Schad R. G.
Published: July 2002

Copper is an attractive metal for very large scale integrated interconnections because of its low bulk resistivity of 1.7 μΩ cm. Alloying of the copper may be required to improve the mechanical stability, adhesion, electromigration lifetime, and resistance to corrosion. However, the addition of chromium to copper increases its bulk resistivity at an initial rate of 4 μΩ cm per atomic percent at 0 °C. We have investigated the effects of sputtering parameters and post-deposition annealing on the resistivity of Cu–1 at. %Cr thin films deposited by magnetron sputtering on thermally oxidized silicon wafers. Resistivities as low as 2.3 μΩ cm have been obtained. The resistivities of as-deposited films decreased with increasing thickness, from 10 to 2.3 μΩ cm, corresponding to film thicknesses of 100 to 1200 nm, respectively. The thicker films were heated in situ by increasing the number of passes the substrate makes in front of the target in the side-scan sputtering system. Ex situ resistivity versus temperature analysis, measured in purified He up to 600 °C on the deposited films, reveals that there are two drops in resistivity; at about 200 °C and 400 °C. Transmission electron microscopy shows the onset of grain growth at 125 °C–200 °C and the development of a bimodal grain size distribution at 400 °C–500 °C. The final resistivities of the films ranged from 2.3 to 4.5 μΩ cm. In general, the final resistivity of the in situ heated Cu–1 at. %Cr films is lower than that of the ex situ heated films. This suggests that Cr surface diffusion during heated deposition is more effective in lowering the resistivity than Cr bulk diffusion during post-deposition annealing. This model is supported by Auger electron spectroscopy, which reveals surface segregation of Cr during annealing. Heating during deposition allows low resistivity (<2.5 μΩ cm) films to be achieved at a low substrate temperature (200 °C–250 °C), whereas post-deposition annealing requires about 600 °C to achieve the same values.
Optical emission investigation of the plasma enhanced chemical vapor deposition of silicon oxide films
Authors: Banerjee A. , DebRoy T.
Published: July 2002

During plasma enhanced chemical vapor deposition of silicon oxide films, plasma diagnostics is important for process control and reproducibility of film quality. In this study the film growth rate has been investigated with special emphasis on plasma properties to seek a better understanding of the deposition process. Light emissions from the plasma were monitored using an optical multichannel analyzer for various deposition conditions. The electron temperature and density were determined from the spectral data. With increasing silane concentration, the number density of the electrons, the concentrations of active species and the deposition rate increased. The growth rate also increased with nitrous oxide concentration. However, the emission intensities did not show a concomitant increase because of the radiation quenching effect. Analysis of the results emphasizes the crucial importance of electron density to generate sufficient concentration of active species responsible for determining the film growth rate. Among the peaks monitored, the intensity of a hydrogen peak correlated well with the deposition rate. The hydrogen peak intensity can be used for reliable, on-line, non-contact, instantaneous monitoring of the film growth rate.
Synthesis and characterization of SiO2 films deposited using tetraethylorthosilicate/ozone at low processing pressures (10−1 to 10−3)
Authors: Stonnington K. D. , Hsieh K. Y. , King L. L. H. , Bachman K. J. , Kingon A. I.
Published: July 2002

High quality, low growth temperature SiO2 thin films are necessary for the fabrication of submicron devices. A new deposition technique for SiO2 films utilizing chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS) in an ultrahigh vacuum chamber at low processing pressures (10−1 to 10−3 Torr) has been investigated. Ozone is introduced to provide a source of active oxygen. The oxide was characterized by ellipsometry, infrared absorption spectrometry, and etch rate. Films for comparison were grown using TEOS/oxygen and TEOS only. Results indicate that the active oxygen substantially reduces the deposition temperature. It was also found that the growth rate is strongly affected by the processing pressure at these very low pressures. The effect of important parameters (substrate temperature, TEOS/ozone ratio, oxidizing agent, and processing pressure) on the resultant oxide growth rate and properties are addressed. Our findings are compared to results reported in the literature for plasma-enhanced CVD (PECVD) of SiO2 films grown in a similar pressure range and atmospheric CVD of SiO2 using an ozone ambient.
Magnitude and effects of adsorbent temperature nonuniformities in temperature-programmed desorption
Authors: Carter Robert N. , Anton A. Brad
Published: July 2002

In a typical apparatus for temperature-programmed desorption (TPD) measurements, heat is not added uniformly to the entire adsorbent volume, but rather is generated in support wires and conducts in at the contact points, forming a temperature gradient in the adsorbent during heating. Based on a simplified, one-dimensional treatment, we determine the approximate magnitude of the temperature profile and investigate its deleterious effects in TPD simulations for first-order kinetics. Analysis of the coupled equations governing surface temperature and coverage leads to a dimensionless group of parameters involving the heating rate, the desorption rate parameters, and the size and physical properties of the adsorbent that determines the magnitudes of systematic errors introduced by the temperature nonuniformity. The simulations allow a general expression to be developed for the maximum heating rate in TPD that will give estimates of E and ln(ν) reliable to within a few percent. The predictions of the model are verified by comparison to TPD data for CO/Ni(110).
Inverse photoemission study of the defective TiO2(110) surface
Authors: See A. K. , Bartynski R. A.
Published: July 2002

We present inverse photoemission spectroscopy (IPE) data from the nearly perfect and defective TiO2(110) surfaces. In the spectra from the nearly perfect surface, features appearing ∼1 and ∼4 eV above the Fermi level are identified as the primarily Ti-derived 2t2g levels and 3eg levels. This result is consistent with earlier electronic structure calculations and x-ray absorption studies by other groups. Spectra from the defective TiO2(110) surface exhibit a new feature ∼3 eV above the Fermi level. This is in agreement with calculations based on defective TiO2(110) surface but has never been observed experimentally before. In contrast to ultraviolet photoemission, IPE spectra from the nearly perfect and ion bombarded TiO2(110) surfaces differ considerably at very low ion doses. IPE spectra from Ti-deposited TiO2(110) surfaces are extremely similar to those from the ion bombarded surface indicating that the spectral changes typical of the defective surface are characteristic of a reduced TiOx phase.
X-ray photoelectron spectroscopic study of the interactions of fluorine ions with gallium arsenide
Authors: Williston L. R. , Bello I. , Lau W. M.
Published: July 2002

The surface of gallium arsenide has been bombarded with low energy fluorine ions in order to study their role in modifying the surface during a CHF3 reactive ion etch of oxide overlayers on GaAs. Compositional changes caused by the bombardment of GaAs with ions ranging in energy from 10 to 300 eV have been investigated using x-ray photoelectron spectroscopy. Depth profiling information was obtained using angle-resolved photoemission measurements. Annealing in vacuum of 1 × 10−6 Pa was investigated as a means of returning the bombarded surface to its original condition. It was found that F+ ion bombardment incorporated fluorine into the surface of GaAs and depleted it of arsenic by forming gallium fluoride and apparently releasing volatile arsenic fluoride. Monitoring the Ga 3d peak revealed that three gallium fluoride phases corresponding to GaF, GaF2, and GaF3 were formed. The extent of fluorine incorporation and arsenic depletion were dependent upon incident F+ ion energy, reaching a maximum at low incident ion energies around 50 eV. At higher ion energies the amounts of GaF2 and GaF3 decreased more than that of GaF. Post-bombardment annealing in vacuum was successful in removing the residual fluoride phases at temperatures below 305 °C. GaF3 was the first component to desorb and GaF the last. The resulting GaAs surface, however, was still gallium rich, likely due to the presence of some residual gallium oxide.
Ultrathin Fe films on TiO2(110): Growth and reactivity
Authors: Pan Jian-Mei , Madey Theodore E.
Published: July 2002

As part of a systematic study of metals on oxides, we report here a study of the growth of Fe films on TiO2(110) surfaces using low-energy ion scattering and x-ray photoelectron spectroscopy (XPS). Our studies reveal that Fe grows initially in an island growth mode, and completely covers the TiO2 substrate for thicknesses of a few monolayers. XPS shows evidence for a strong interaction between Fe films and the oxide substrate, i.e., oxidation of Fe overlayers and reduction of the oxide substrate, at submonolayer thickness. Comparing the wetting ability of Fe with that of Cu, a less reactive metal, and with that of Cr, a more reactive metal, we observe a relation between the wetting ability of the metal films and the reactivity of metal with oxygen. Complete coverage of the TiO2 substrate can be obtained for a 1 monolayer Fe film following oxidation of the film. Studies of Fe film growth on sputtered TiO2 surfaces indicate that surface roughness also plays an important role in the morphology of the metal films.
Aluminum nitride thin film sensor for force, acceleration, and acoustic emission sensing
Authors: Zheng L. , Ramalingam S. , Shi T. , Peterson R. L.
Published: July 2002

To meet the real-time process monitoring and control needs in machining automation, aluminum nitride thin film sensors have been implemented on WC–Co cutting inserts by rf sputtering. These piezoelectric sensors detect force, acceleration, and acoustic emission signals. Method of constructing, characterizing and evaluating AlN piezoelectric sensors are presented here. It is shown that c-axis oriented AlN films can be deposited on WC–Co substrates to sense force, acceleration and acoustic emission signals. As the transducer characteristics vary with rf sputtering conditions, parametric studies have been carried out to determine ‘‘best processing’’ conditions to obtain the needed transducers sensitivities and process reproducibility. Results of these studies are also presented here. It is shown, in addition, that AlN films deposited WC–Co can provide useful signals even though the cutting tool substrate may experience temperature rise during use in metal machining.
Development of foam shell with plastic ablator for cryogenic laser fusion target
Authors: Takagi M. , Ishihara M. , Norimatsu T. , Yamanaka T. , Izawa Y. , Nakai S.
Published: July 2002

A low density, low Z foam shell overcoated with a plastic ablation layer was developed for use as a cryogenic laser fusion target in which main liquid or solid fuel is sustained by the uniformly thick foam layer. The high quality ablator with thickness of 1–10 μm, uniformity of ≳95%, and surface smoothness of <0.2 μm was coated on the foam shell by the interfacial polycondensation method using hydroxyethylcellulose or poly(p-vinyl phenol) (PVP) and isophthaloyl chloride. The tensile strength of the crosslinked PVP at low temperature and its permeability to deuterium at room temperature are comparable with those for polystyrene. In this reaction, we found that the existence of surfactant in the emulsion was essential to form an ablation layer which adhered to the foam layer.
Multiple steady states in electron cyclotron resonance plasma reactors
Authors: Aydil Eray S. , Gregus Jeffrey A. , Gottscho Richard A.
Published: July 2002

Although electron cyclotron resonance (ECR) plasma reactors are being used in the microelectronics industry for etching and deposition of thin films they are prone to instabilities that can wreak havoc with manufacturing processes. Plasma conditions are often unstable because of nonlinear coupling between wave propagation, power absorption, neutral density, and charge density profiles. We report on hysteresis, multiple steady states and abrupt transitions in an ECR plasma reactor that can alter the plasma properties drastically. Substrate platen floating potential and Langmuir probe measurements are used to identify several abrupt transitions and regions in the operating parameter space where the plasma can exist in either one of two different states under identical operating conditions (microwave power, pressure, flow rate, magnetic field, rf bias etc.) Depending on how the plasma is started either branch can be obtained. Abrupt transitions and hysteresis in plasma properties are observed both with and without rf bias under typical conditions used in plasma processes. Such transitions can cause drastic deviations in plasma parameters—density, current, uniformity—and may result in degraded process characteristics—uniformity, rate, selectivity, linewidth control. Statistical techniques used to design and develop processes cannot properly account for discontinuous changes in the state of the plasma and can produce brittle processes that can suddenly lead to catastrophic failure. Care must be taken to map out the regions of bistability for the purpose of avoiding them or controlling the plasma in these regions. While the mechanism responsible for the multiple steady states and abrupt transitions reported here is not well understood, it is clear that these phenomena depend critically on the neutral density.
Thickness-dependent stress in sputtered carbon films
Authors: Puchert M. K. , Timbrell P. Y. , Lamb R. N. , McKenzie D. R.
Published: July 2002

The intrinsic compressive stress in thin carbon films up to 350 nm in thickness, deposited on Si(100) substrates via dc magnetron sputtering, has been measured using ex situ optical interferometry. A plateau value in compressive stress (∼1.5×109 Pa) is observed for carbon films thicker than 85 nm, but an anomalously large stress (up to 50% increase) occurs for film thicknesses between 18 and 85 nm. The carbon film stress data are interpreted in terms of a simple structural model in which the average film density influences the macroscopic stress. Ultrathin carbon films (<10 nm) in the initial stages of growth were further investigated by atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM images of ∼1-nm-thick carbon films and uncoated substrates reveal topographic features with average heights of 4 and 1.5 Å, respectively, but no obvious island film growth structures. XPS measurements, in which the native SiO2 peak is used as a marker of exposed substrate area, suggest that films of ≤2 nm thickness are not continuous. The reported enhanced film stresses are therefore associated with a continuous film structure.
Pulsed desorption kinetics using micromachined microhotplate arrays
Authors: Cavicchi Richard E. , Poirier Gregory E. , Suehle John S. , Gaitan Michael , Semancik Steve , Burgess Donald R. F.
Published: July 2002

We demonstrate a technique for obtaining desorption kinetics parameters by applying temperature pulses to small samples. In these experiments we use an array of micromachined hotplates, each with a temperature range from 300 to 1300 K and a thermal response time of 5 ms. Array elements are 100 μm×200 μm and have a mass of 0.4 μg. A metal film is deposited on the microhotplate array surface. After exposing the sample to a gas species of interest, a sequence of temperature pulses of controlled duration is applied to the array elements. The desorption signal from each pulse is recorded using a mass spectrometer. For each temperature, the desorption time constant is determined. A simple Arrhenius plot of desorption time constant versus inverse temperature yields directly the activation energy for desorption and the rate constant prefactor or frequency factor. The results are independent of the pumping speed of the vacuum system. The technique is demonstrated for adsorbed CO on platinum using temperature pulses that range over nearly four orders of magnitude in duration.