bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical emission from neon/oxygen rf sputtering glow discharges
Authors: Aita C. R. , Marhic M. E.
Published: July 2002

The results of an optical emission study of neon and oxygen species which exist in Ne/O2 rf diode sputtering glow discharges are presented. Comparing emission intensities in pure Ne and Ne/O2 mixtures at constant total gas pressure, it was found that the addition of O2 to the sputtering gas quenches Ne i emission and increases emission from O i and O+2 species to a greater extent than that which would be expected from the relative changes in neon and oxygen concentrations, respectively. Although this effect could be caused by a change in the electron density and electron energy distribution function, the correlation between Ne i, O i, and O+2 emission suggests that metastable neon atoms may interact directly with ground state diatomic oxygen molecules in two competing ways: (1) by Penning ionization which produces O+2, and (2) by a quasiresonant transfer of excitation which leads to the production of O. The results are compared to those previously obtained for Ar/O2 discharges using identical sputtering conditions, where the dominant interaction between the rare gas and O2 leads entirely to the production of monatomic oxygen.
A multiple sample transfer device for use with surface analytical systems
Authors: Nelson G. C. , Shapnek L. R.
Published: July 2002

This note describes a sample transfer system that has been designed with a modified sample carousel for use with a Physical Electronics model 15−600 specimen stage. This system allows all of the twelve positions on the sample carousel to be used. It has been built using commercially available parts at a nominal cost. (AIP)
High resolution electron energy loss spectroscopy of an anisotropic insulator surface: A test for the dielectric theory
Authors: Liehr M. , Thiry P. A. , Pireaux J. J. , Caudano R.
Published: July 2002

Electron energy loss measurements were performed on the α-Al2O3(0001) surface at temperatures between 120 and 350 K. Spectra of the Fuchs–Kliewer phonons were recorded with high energy resolution (26 cm−1). Numerical fits revealed that the spectra are quantitatively described by the macroscopic dielectric theory for anisotropic crystals. Comparisons are made with infrared data.
Performance of titanium nitride diffusion barriers in aluminum–titanium metallization schemes for integrated circuits
Authors: Suni I. , Blomberg M. , Saarilahti J.
Published: July 2002

Three different contact schemes Al–Si, Al/Ti–Si, and Al/TiN/Ti–Si have been studied for their electrical characteristics and thermal stability on shallow n+p-junctions. Contact resistance and leakage current measurements indicate that the Al/Ti structure remains stable up to 450 °C and the Al/TiN/Ti structure up to 500 °C for 15 min, whereas Al–Si contacts degrade already at 400 °C. The stability of the Al/Ti contacts is limited by the thin film reaction between Al and Ti while the failure of the Al/TiN/Ti structure is attributed to local defects such as pinholes in the TiN barrier.
Electron-induced damage of ThF4 thin films in the presence of XeF2
Authors: Loudiana M. A. , Dickinson J. T. , Ashley E. J.
Published: July 2002

During extensive bombardment with a 1-keV, 50-μA electron beam, a ThF4 thin film lost the equivalent of all of the fluorine in 27 monolayers as measured with a quartz crystal microbalance. The loss of surface fluorine was monitored with AES. This process is so efficient that initially almost every electron removes a fluorine atom. When the fluorine depleted surface was exposed to XeF2 it rapidly gained back 1 equivalent monolayer of fluorine. Simultaneous exposure of a ThF4 film to XeF2 and 1-keV electrons showed a loss of 13 equivalent monolayers of fluorine. In the presence of XeF2 and low-energy electrons ThF4 can be made to rapidly absorb large quantities of fluorine. The influence of electron bombardment on fluorine desorption, sorption, and diffusion will be discussed.
Metallizing system for coating an astronomical telescope mirror
Authors: Roth F. Miguel , Graper Edward B.
Published: July 2002

A vacuum metalizing system developed for coating a 2.12 m astronomical telescope mirror is described. The 3 m diameter chamber, installed at 2800 m altitude at the Mexican National Astronomical Observatory at San Pedro Mártir, Baja California, was developed with uniquely economical equipment and simplicity of design to facilitate ease of operation, safe handling of the mirror, and reliability in this remote location. In order to meet strict performance specifications within a very limited budget, nontraditional equipment suppliers and fabricators were used. The project design and administration philosophy are described.
Effective hydrogen gas temperature in a cryopumped beam line vessel
Authors: Kim Jinchoon , Fasolo John A.
Published: July 2002

Thermal accommodation of hydrogen gas molecules to cold surfaces modifies the effective gas temperature in cryogenically pumped vacuum vessels. In Doublet III neutral beam lines, gas pumping is achieved by liquid-helium-cooled panels which are surrounded by liquid-nitrogen-cooled thermal shields. By measuring the gas pressure during the injection of a known amount of gas while cooling the thermal shields by liquid nitrogen, we have determined the effective gas temperature according to T/T0=(Pi/Pio)2, where Pi is the pressure monitored by an ionization gauge and the subscript o refers the case of room temperature. The effective gas temperature was determined to be about 150 K when only the thermal shields were cooled down to the liquid nitrogen temperature. Implications of the reduced gas temperature on pressure measurement, pumping speed measurement, and neutral beam reionization loss calculation are discussed.
Throwing power and shadowing effect in planar magnetron sputtering process
Authors: Stowell W. , Foster J. , Berner W. , Wan C. , Chambers D. , Hanes H.
Published: July 2002

In magnetically enhanced sputtering, coating thickness varies over the surface of a substrate, which can be a planar, spherical, parabolic, or any irregular shape. The throwing power and shadowing effect were determined for specific geometries of coatings deposited with differences in (a) sputtering pressure, (b) target-to-substrate distance, and (c) substrate bias. The geometry of the substrate was formed by flat planels of 25×75-mm glass slides or metal plates, oriented at different angles with respect to the cathode surface with a varying amount of overlapping. Coatings deposited at P=0.27 Pa have more throwing power and less shadowing effect than those deposited at P=2.7 Pa. A substrate bias of 400 V definitely improves coating uniformity. A target-substrate distance of 102 mm causes slightly more uniform coating than a distance of 38 mm.
Characterization of laboratory plasmas with probes
Authors: Manos D. M.
Published: July 2002

This paper presents a broad overview of various types of probes employed to study plasmas produced in laboratory environments. Methods reviewed include single, double, and triple Langmuir probes, biased calorimeters, E×B, RFA, momentum discriminating, and sample exposure probes.
Structural studies of hydrogen-bombarded silicon using ellipsometry and transmission electron microscopy
Authors: Collins R. W. , Yacobi B. G. , Jones K. M. , Tsuo Y. S.
Published: July 2002

Hydrogen-bombardment-induced structural changes in single-crystal silicon were studied using ellipsometry and transmission electron microscopy techniques. Hydrogen ion energies ranged from 400 to 1900 eV and the total dose was about 5×1019 ions/cm2. Various degrees of damage and phase mixtures in the layers were identified. It was concluded that ellipsometry can be used effectively as a nondestructive characterizational tool for analyzing bombardment-induced microstructural changes in this material.
Deposition of zirconium boride thin films by direct current triode sputtering
Authors: Chakrabarti U. K. , Barz H. , Dautremont-Smith W. C. , Lee J. W. , Kometani T. Y.
Published: July 2002

Specular, crack-free thin films of the refractory conductor zirconium boride have been deposited for possible applications in combined contact/diffusion barrier metallization schemes. Films were deposited by dc triode sputtering, which allowed the independent study of the effects of sputtering pressure, target voltage, and current on the film properties. The mole ratio of boron in the films increased (composition tending to ZrB2) and the resistivity decreased with increasing deposition rates which at a fixed target voltage and sputtering pressure increased almost linearly with target current. Decrease in sputtering pressure, with only a minor change in deposition rate, dramatically decreased resistivity and caused stress in the films to change from tensile to compressive. X-ray photoelectron spectroscopy correlated reduced oxygen content to reduced resistivity. Triode sputtering permitted deposition of films at 2 mTorr with a resistivity of 162 μΩ cm which is the lowest reported value for as-deposited films.
Schottky barriers on MgxZn1−xTe alloys
Authors: El Akkad F. , Felimban A. , Sallam F.
Published: July 2002

Indium surface barriers on MgxZn1−xTe alloys have been investigated. For the composition x=0 (ZnTe), capacitance–voltage and photoelectric measurements yielded a barrier height φB of 1.84+0.02 eV. A gradual decrease of φB with increasing Mg content has been observed through photoelectric measurements. The results are interpreted on the basis of the simple Schottky theory. Electron affinity values in the range 3.68>χs>2.79 eV have been deduced for MgxZn1−xTe in the composition range 0<x<0.28.
Near edge x-ray absorption fine-structure studies of molecules and molecular chains bonded to surfaces
Authors: Stöhr J. , Outka D. A.
Published: July 2002

A review and future outlook are given of the capabilities of the near edge x-ray absorption fine-structure (NEXAFS) technique for the study of simple and complex molecules bonded to surfaces. NEXAFS resonances arise from K-shell excitations to final states which are highly localized on the core excited atom. The localized nature of the measured excitations allows the determination of the orientation, hybridization, and bond lengths of specific functional groups (e.g., C–C, C–C, or C–O) in molecules ranging from diatomic to complex polyatomic species. Breakdown of this localized excitation picture is found when the molecular orbitals of two or more molecular units or functional groups interact. As examples we discuss interactions between π and/or σ states in aromatic hydrocarbon rings and conjugated π orbitals in complex acids which lead to a splitting of NEXAFS resonances. NEXAFS spectra of Langmuir–Blodgett chains reveal a pronounced polarization dependence which allows the determination of the macroscopic chain orientation on the surface.
Synchrotron radiation photoemission studies of CO chemisorption on Pt/Ta(110) and Ni/Ta(110)
Authors: Ruckman M. W. , Strongin Myron , Pan Xiaohe
Published: July 2002

The effect of overlayer structure and electronic density near the Fermi level on CO chemisorption is studied for Pt or Ni deposited on Ta(110) using angle-resolved synchrotron radiation photoemission spectroscopy. Pt films form two-dimensional pseudomorphic islands at submonolayer coverage, but undergo a structural change to a close-packed Pt(111)-like structure at ∼0.7 monolayer coverage. Absence of CO induced photoemission features at monolayer coverage suggests that the CO–Pt bond has been weakened and this prevents the retention of CO on a close-packed Pt monolayer at room temperature. Close-packed Pt films three or more layers thick readily chemisorb CO. At liquid-nitrogen temperature, CO is retained on Pt/Ta(110), and photoemission features, such as the CO 4σ shake-up satellite seen for weakly bonded CO on noble metals, appear for the CO phase chemisorbed on a Pt monolayer. This photoemission feature is absent for CO chemisorbed on thicker Pt films. Photoemission examination of the CO–metal bonding geometry suggests that the CO molecules lie close to the surface normal for both Pt monolayers and the thicker Pt films. From the photoemission data, we conclude that the CO–metal bond strength is reduced for CO–Pt/Ta(110). Comparable studies of CO on Ni/Ta(110) were not possible because the three-dimensional clustering of Ni produced a more complicated surface which had a number of inequivalent CO bonding sites.
The effects of radio frequency waves on the sputtering of ion cyclotron resonance heating antenna Faraday shields
Authors: Caughman J. B. O. , Ruzic D. N. , Hoffman D. J.
Published: July 2002

The sputtering of the graphite covered Faraday shield of an ion cyclotron resonance heating loop antenna has been studied. The antenna operated at 50 MHz and was exposed to a weakly magnetized plasma produced by the High Particle Flux Facility at Oak Ridge National Laboratory. The flux at the antenna was on the order of 3×1017 cm−2 s−1. A dc bias voltage applied to the antenna allowed the effective energy of the impinging hydrogen ions to be varied up to 65 eV. Coupled rf power varied from 0 to 3 kW. Sputtering depths were directly measured by surface topography. Sputtering enhancement due to rf was between a factor of 2 to 4, depending on the position along the antenna. Antenna voltage profiles were calculated and used to correlate measured erosion coefficients with ion energies. Various sputtering theories and possible chemical sputtering effects are discussed.
A vacuum-to-air interface for the advanced test accelerator
Authors: Cruz G. E. , Erikson E. D. , Schwab T. L.
Published: July 2002

The operation of some future accelerators will require that a pulsed particle beam exit a region which is at a pressure of 10−5 Torr or less into a region which is at atmospheric pressure. The separation between atmosphere and vacuum could be achieved with small apertures or differential pumping stages. In this paper we report some of the results of tests carried out on a component which could be used as the first stage of a differentially pumped system. The component employs an expendable foil and foil advancement mechanism which serve as both an orifice and a fast-acting vacuum valve. In operation, the particle beam penetrates the thin foil, thereby creating an aperture of minimum geometry between the atmosphere and the vacuum system. The beam duration is on the order of 70 to 100 ns. During the balance of the (1-s) cycle, the foil is advanced to seal the opening and recover the loss in vacuum. A series of differentially pumped stages will be used to further separate the two pressure regimes. The design goal for this first stage was to achieve a pressure reduction of 760:1. Our test results show that for a 1-mm-diam orifice, undergoing multiple cycles, the system pressure rise is approximately 1.7 Torr/cycle and for a 10.0-mm-diam orifice, the pressure rise is approximately 2.9 Torr/cycle. The gas throughput for the 10-mm orifice was approximately 9700 Torr l/s. Future objectives include the further reduction of the system pressure rise in order to maintain 1.0 Torr or less during the 1-s cycle time.
Applications of turbomolecular pumps in manufacturing plants
Authors: Deters L. , Goetz D. , Schwarz W.
Published: July 2002

Vacuum pumps used in production processes have to fulfill various requirements such as high throughput, low ultimate pressure, and hydrocarbon-free vacuum. In certain applications a constant pumping speed in the pressure range between 10−2 and 10−3 mbar is absolutely necessary in order to avoid pressure fluctuations. Turbomolecular pumps on coating machines for the production of optical films, thin layers for the optical or magnetic data storage, hard and wear-resistant coatings, and transparent and electrically conductive layers on discrete substrates or webs are discussed. Turbomolecular pumps offer various advantages compared to other pumps. The main features are the capability of providing a hydrocarbon-free vacuum and a high stable throughput at relatively high pressures.
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u spectroscopic ellipsometry during molecular-beam epitaxy of cadmium mercury telluride
Authors: Demay Y. , Arnoult D. , Gailliard J. P. , Medina P.
Published: July 2002

To improve the control of the growth, a II–VI molecular-beam epitaxy chamber has been equipped with an in situ spectroscopic ellipsometer. Using this characterization technique we checked the interdiffusion between CdTe and HgTe. A thin CdTe layer was deposited on a HgTe layer. The initial abrupt interface (less than 1 nm) evolved during successive annealings. Spectroscopic measurements were performed at each step. A modeling permits the evaluation of the thickness of a graded composition layer resulting from the interdiffusion at 260 °C.
Effects of argon pressure and substrate heating on the chromium underlayer used for high-density longitudinal CoNiCr media
Authors: Ravipati Durga P. , Haines William G. , Dockendorf James L.
Published: July 2002

Sputtered CoNiCr on Cr is one of the preferred material systems for high-density longitudinal recording media. dc magnetron sputtered Cr films, on Al substrates, were investigated for the effects of argon pressure and substrate temperature on the microstructure. High-resolution scanning electron micrographs reveal that the films made at lower argon pressures have acicular-shaped grains changing to faceted grain structures at higher argon pressures. The effect of substrate heating prior to deposition was to change the structure from acicular shape to cubic shape. X-ray diffraction work reveals a bcc(110) orientation for films deposited at lower argon pressures changing to a mixed phase at higher argon pressures. Increased substrate temperature caused a mixture of (110) and (200) orientation.
Energy and momentum transport by sputtered and reflected streams in a glow discharge
Authors: Gras-Marti Alberto , Vallés-Abarca José A. , Bensaoula Abdelhak
Published: July 2002

Extending established concepts from the field of atomic collisions in solids we evaluate, for planar geometry, the spatial profile of the energy deposited in the gas phase by the streams of particles emitted at the cathode. Both sputtered and reflected energetic streams are considered. It is estimated, also, the pressure profile due to the transport of momentum both by the emitted streams, and by the elastic collision cascades generated by them. The analysis of the transport of momentum and matter is useful for a quantitative understanding of the dynamic rarefaction which is measurable in the vicinity of the cathode in a glow discharge.
An easily constructed, inexpensive cold stage for use in ultrahigh vacuum
Authors: Nelson G. , Ohlhausen T. , Hardegree E. , Schulze P.
Published: July 2002

A liquid-nitrogen-cooled sample holder for use in ultrahigh vacuum systems is described. The holder is well suited for either resistive or electron-beam heating, and requires less coolant than some other designs. The holder can be constructed from commercially available parts for about $350, using commonly available tools.
Matrix calculation of pressures in high-vacuum systems
Authors: Hirano Haruo , Kondo Yukihito , Yoshimura Nagamitsu
Published: July 2002

A matrix analysis method has been applied to two high-vacuum systems: an outgassing pipe and an electron microscope. These vacuum systems were analyzed as linear vacuum circuits with pressure sources, current sources, and conductances by using a digital computer. The pressures at six points along an outgassing pipe were calculated for two different models. A 5% difference of pressures was noted between the models. Pressures throughout an electron microscope high-vacuum system were also successfully calculated by a computer. The pressure varied over three orders of magnitude depending on position.
The role of oxygen and fluorine in the electron emission of some kinds of cathodes
Authors: Lin Li-wei
Published: July 2002

Experiments have proved that lowering of the work function of a cathode by adsorbing both oxygen and electropositive metallic elements on its surface may be more effective than by adsorbing simply the metallic element. However, the role played by oxygen is not yet completely clear. In this paper we first discuss briefly the situation of the thermionic cathode. Then the role of oxygen or fluorine in the activation of the negative electron affinity GaAs photocathode is discussed. Emphasis will be placed on the atomic core potential field of the surface atoms of the cathode, the bonding state of the surface adsorption layer (Cs or O), and the transfer of electron charges in the surface atoms. The conclusion is drawn that an appropriate quantity of oxygen adsorbed on the cathode surface would be essential to result in a better dispenser cathode and also high quantum yield negative electron affinity photocathodes, while fluorine would play an important part in the electron emission stability.
Aluminum deposition by ultraviolet laser photofragmentation of trimethylaluminum on Al: Identification of photoproducts and desorption dynamics
Authors: Orlowski T. E. , Mantell D. A.
Published: July 2002

Important new mechanistic details regarding aluminum deposition by excimer laser photodecomposition of trimethylaluminum adsorbed on aluminum covered SiO2/Si surfaces have been obtained using a time-of-flight quadrupole mass spectrometer. CH3 radicals and Al–(CH3)n(n=1,2,3) species desorb efficiently at 193 nm leaving the surface with up to 0.22 eV of translational energy. No evidence is found for desorption of Al from the surface indicating that Al is more tightly bound than methyl-aluminum fragments. A threshold behavior in the laser intensity dependence of the time-of-flight transient intensity for Al(CH3)2 and AlCH3 fragments suggests a dissociation mechanism involving stepwise removal of methyl fragments. Desorbed fragment translational energy depends upon surface coverage indicating a distribution of surface binding sites. Finally, no evidence of ethane or methane desorption from the sample surface is found implying that radical recombination and hydrogen abstraction are primarily secondary gas phase reactions which are not surface initiated.
Temperature dependence of intrinsic stress in Fe, Si, and AlN prepared by ion beam sputtering
Authors: Windischmann H.
Published: July 2002

The intrinsic stress in ion beam sputtered Fe, Si, and AlN films was investigated as a function of deposition temperature. At low deposition temperature (300 K) the intrinsic stress is high and compressive for all materials investigated. With increasing deposition temperature the stress initially decreases gradually followed by a rapid drop for Td >Tm /3. The initial slope of the stress variation with temperature appears to be material dependent, being steepest for iron and shallowest for AlN with silicon falling in between. Unlike AlN and Si, Fe converts from compressive to tensile stress at Td /Tm =0.25, where Tm is the absolute melting temperature. The inverse temperature dependence of the intrinsic stress is explained in terms of the grain growth and annealing of a strained microstructure models. Applying the forward-sputtering model [which relates the intrinsic stress to the elastic energy/mole, EM/(1−ν)D, where E is Young’s modulus of elasticity, D the mass density, M the atomic mass, and ν Poisson’s ratio] it is shown that the temperature dependence of the material properties do not contribute significantly to the intrinsic stress variation with temperature.
Water on Ni(110): Resolution of monolayer, bilayer, and ice layers by thermal desorption, Δφ, and associated absolute coverages
Authors: Griffiths K. , Memmert U. , Callen B. W. , Norton P. R.
Published: July 2002

The various adsorbed states of D2O on Ni(110) have been resolved by thermal desorption and simultaneous Δφ measurements, permitting determination of the effective dipole moment in each state. The states detected by Madey and co-workers are all confirmed but an additional state at the end of the ice-desorption region is clearly resolved by Δφ methods. The three higher temperature desorption states (T>ice layer desorption temperature) can be selectively saturated by water exposure at 180 K. The absolute coverage of water in these states was then determined by nuclear reaction analysis to be 5.3×1014 water molecules cm−2. This coverage is significantly lower than the coverage proposed by Madey and co-workers for the bilayer structure (1.1×1015 mol cm −2). The state at the end of the ice desorption region can also accommodate 5–6×1014 mol cm−2 and may result from water bonded mostly to other water molecules, with weak residual bonding to Ni.
Kinetics of CO adsorption on epitaxial (111)Cu on (111)Pd thin films
Authors: Oral B. , Kothari R. , Vook R. W.
Published: July 2002

CO adsorption has been studied on (111)Cu/Pd thin-film surfaces grown epitaxially on mica in UHV of base pressure 5×10−11 Torr. Auger electron spectroscopy investigations of the growth of Cu on (111)Pd films showed that layer growth occurred. The Kelvin probe, work function method was used to monitor the CO adsorption at 298 K as a function of Cu overlayer thickness. It was found that very thin Cu overlayers had a drastic effect on saturation CO coverage: one monolayer of copper reduced the saturation CO coverage by ∼95%. For the pure (111)Pd thin-film surface, the data showed that the rate of CO adsorption changes when the CO fractional coverage approaches ∼0.4. This result is most likely due to the previously reported change in CO superlattice structure that occurs with increasing coverage. The kinetic adsorption data for various bilayers were interpreted in terms of a first-order Kisliuk mobile precursor model.
Surface studies of electroless nickel/copper plated polycarbonate subjected to Batelle class II flowing mixed gas environment
Authors: Isern-Flecha Ileana
Published: July 2002

Polymeric structural components used in the electronic industry require electromagnetic interference shielding which can be provided by copper, overplated by nickel for corrosion protection. This paper discusses what effects field environments have on electroless nickel over copper plated polycarbonate components when subjected to degradation by accelerated corrosion testing, specifically to the class II Battelle flowing mixed gas environment. Surface chemical changes have been studied ex situ using scanning Auger microscopy and x-ray photoelectron spectroscopy. Changes include the formation of copper sulfide, cuprous oxide, and nickel oxide. Corrosion of copper can be attributed to imperfections (i.e., thin spots, mud cracks, and pinholes) in the nickel film and/or its migration through the nickel layer when subjected to the accelerated corrosion test conditions.
Specifying and evaluating vacuum system purchases
Authors: O’Hanlon John F. , Bridwell Merl
Published: July 2002

This paper reviews a procedure for purchasing large industrial vacuum systems. It describes the elements of a request for quote, reviews a procedure for evaluating the responses to the quotation, and discusses the process of selecting the supplier, and following through with the installation. It concludes by examining some difficulties which can arise in the process.
Uniform solid deuterium–tritium fuel layers resulting from radioactively induced sublimation
Authors: Hoffer J. K. , Foreman L. R.
Published: July 2002

Recently, it has been demonstrated that radioactive self-heating in a solid can induce preferential sublimation from thicker, warmer layers to thinner, cooler ones, thus tending to make the layers become uniform in thickness. Following experiments in pure T2 , we have investigated solid deuterium–tritium (DT) in optical cells with isothermal boundaries. Backlighting permits direct observations with a telescope and a video camera. As the temperature is lowered, we follow the stages of condensation, freezing, and subsequent layer equilibration or isotropization. Measured rate constants for freshly prepared DT samples agree well with the theoretically determined value of 26.6 min. Due to the impedance presented by 3 He in the vapor space, the rate constants for solid layers made from aged DT increase by nearly 12 min/day. Equilibrated shells of solid DT exhibit slow optical degradation, presumably due to 3He accumulation at grain boundaries. The optical effects can be reduced by annealing near the triple point.
Optical second-harmonic generation study of barium deposition on Si(001)
Authors: Hollering R. W. J. , Dijkkamp D. , Lindelauf H. W. L. , van der Heide P. A. M. , Krijn M. P. C. M.
Published: July 2002

Experimental results are presented on the deposition of barium on clean Si(001) studied by optical second-harmonic generation and work function measurements. Barium deposition up to one monolayer coverage enhances the signals by nearly a factor of 10. Continued deposition of Ba leaves the signal arising from the in-plane polarization component unaffected, indicating that this signal arises from the Si–Ba interface or the Ba–vacuum interface. In contrast, the second-harmonic signals related to the normal polarization components are increased by another order of magnitude around ∼4 ML Ba coverage.
Platinum/iridium tips with controlled geometry for scanning tunneling microscopy
Authors: Musselman Inga Holl , Russell Phillip E.
Published: July 2002

An electrochemical etching procedure using a saturated CaCl2/H2O/concentrated HCl (60%/36%/4% by volume) solution has been developed to fabricate platinum/iridium tips with controlled geometry for scanning tunneling microscopy (STM). These tips, which have a high aspect ratio (5°–10° cone half angle) and a small radius of curvature (∼500 Å), are particularly useful for the imaging and metrology of precision-engineered surfaces. Initial attempts to use the tips showed them to be unreliable for STM imaging. Auger electron spectroscopy indicated the presence of a carbon contamination layer. By eliminating CO2 from the etching environment, the contamination was reduced and the reliability of the resulting tips improved dramatically. The imaging versatility of these tips is demonstrated for sputter-deposited gold on silicon, for a gold-coated polymethylmethacrylate lithographic test pattern, and for highly oriented pyrolytic graphite.
Quantitative surface analysis at atomic resolution, atom-probe field ion microscopy
Authors: Tsong Tien T.
Published: July 2002

There are at least three different aspects of a high resolution microscopy of the surface, namely the atomic imaging aspect, the chemical analysis aspect, and the energy spectroscopy aspect. The atom-probe field ion microscopy (FIM) is a combination of a field ion microscope and a single ion detection sensitivity time-of-flight spectrometer. With the field ion microscope, the atomic structure of the surface and the bulk of a solid, and the behavior of single atoms and small atomic clusters on the surface can be investigated. In chemical analysis, surface layers and interface layers of a solid can be analyzed atom by atom and atomic layer by atomic layer. Thus compositional variations in surface and impurity segregations, in grain boundary segregations, in precipitations, in oxidation and corrosion, and in compound layer formation etc., can be studied quantitatively with subnanometer spatial resolution. In energy spectroscopy, the binding energy of surface atoms from well defined adsorption sites, the formation of cluster ions in pulsed-laser stimulated field desorption, resonance tunneling phenomena, and the orientation and isotope effects in field dissociation of compound ions by atomic tunneling etc., can be studied with a mass and energy resolution of one to two parts in 100,000. Some of our recent studies in these areas are briefly described.
Scanning tunneling microscope observations of MgO fracture surfaces
Authors: Langford S. C. , Zhenyi Ma , Jensen L. C. , Dickinson J. T.
Published: July 2002

Scanning tunnel microscope (STM) observations of surfaces of single crystal MgO fractured in three-point bend and coated with thin gold films are reported. Macroscopically, the fracture geometry employed yields a smooth, approximately (027) surface along the tensile side of the sample, and a heavily stepped (001) surface along the compressive side. STM observations of the tensile side show smooth regions approximately (027) in orientation with relatively little relief. These smooth regions are separated by rough regions which often could only be partially imaged with the present instrument. The compressive side showed much more varied structures, with many (001) features as well as features approximately (720) in orientation. Fractal dimension measurements were made on topographs of tensile and compressive side features. Observations of triangular features on the compressive side of one MgO sample show evidence of interaction between the growing crack and dislocation structures. The significance of these microscopic measurements to energy dissipation processes during crack growth in MgO are discussed.
Coadsorption of bismuth with electrocatalytic molecules: A study of formic acid oxidation on Pt(100)
Authors: Kizhakevariam N. , Stuve E. M.
Published: July 2002

Bismuth is an effective promoter in formic acid electrocatalysis on Pt(100), yet this system is problematic and the reaction mechanisms not well understood. We investigate this from a surface science perspective by studying Bi adsorption and coadsorption with CO, O2, H2O, and HCOOH on Pt(100) with low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). The ‘‘hex’’ reconstruction of clean Pt(100) is gradually and completely lifted by increasing Bi coverage up to 0.25 monolayer (ML); saturation coverage is 0.5 ML with a c(2×2) LEED pattern. Low coverages of Bi (below 0.25 ML) exhibit moderate attractive interactions, whereas repulsive interactions occur for higher coverages. The heat of adsorption for Bi in the limit of zero coverage is 380 kJ/mol. Bismuth is an effective site blocker for CO and O2 with each adatom blocking two CO molecules and two O atoms. Strangely, Bi is not hydrated by coadsorbed H2O and a Pt(100) surface with only 0.2 ML Bi is hydrophobic. Both clean and Bi-covered Pt(100) are inert to HCOOH; only molecular adsorption occurs below 220 K. Preadsorbed oxygen dramatically enhances HCOOH reaction, however, with complete oxidation to CO2 and H2O occurring at 300 K via the formate intermediate. The implications of these results towards electrocatalysis are discussed.
Band bending at Al, In, Ag, and Pt interfaces with CdTe and ZnTe (110)
Authors: Wahi A. K. , Miyano K. , Carey G. P. , Chiang T. T. , Lindau I. , Spicer W. E.
Published: July 2002

Band bending behavior and interfacial chemistry for Al, In, Ag, and Pt overlayers on vacuum-cleaved p-CdTe and p-ZnTe (110) have been studied using ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). These metals provide a range of metal–substrate reactivities: Al reacts strongly with Te, Ag moderately, and In minimally, with no evidence seen for In reaction on ZnTe. Pt exhibits strong alloying behavior with both Cd and Zn. All four metals are found to yield Schottky barriers on CdTe and ZnTe, with a narrow range of final Fermi level positions, Efi=Ef−EVBM, observed on CdTe, from 0.9 to 1.05±0.1 eV, and on ZnTe from 0.65 to 1.0±0.1 eV. The prediction of the MIGS model that a difference in barrier height exists for two semiconductors dependent upon their band lineup (valence band offset) is examined and found to agree with experiment for Ag, Pt, and Al, but not for In. For the highly reactive Al, no evidence for the overlayer metallicity required for metal-induced gap states (MIGS) to operate is seen on CdTe or ZnTe until after band bending has stabilized. Reaction and intermixing for Al, Ag, and Pt overlayers on CdTe and ZnTe indicate these interfaces are not ideal. The possible role of defects at these four metal/CdTe and metal/ZnTe interfaces is considered, and provides a consistent explanation for the final Fermi level positions observed.
Electrical and structural characterization of Mo/Si contact to n-GaAs
Authors: Kulkarni A. K. , Patkar M. P.
Published: July 2002

Mo/Si contacts to n-GaAs are fabricated by deposition of three alternate layers of Mo and Si using e-beam deposition process. The electrical and structural characteristics are studied as a function of annealing temperatures (320 to 800 °C for five min) to study silicide formation and its effect on the electrical properties of Schottky diodes. The best ideality factor is 1.38 for a contact annealed at 470 °C. The least value of saturation current density based on thermionic-field emission model is 1.23×10−7 A/cm2 for a contact annealed at 320 °C. This results in a barrier height of 0.76 V. The electrical properties degrade when the contacts are annealed at 800 °C for 5 min. The capacitance–voltage characteristics do not follow the depletion model because of incomplete silicide formation. This incomplete silicide formation is observed by Auger analysis which indicates that in layered contacts Mo does not diffuse into silicon but Si does diffuse into Mo to form a silicide.
Structural characterization of molecular-beam epitaxially grown Zn1−xMnxSe and Zn1−xMnxSe/ZnSe superlattices
Authors: Qadri S. B. , Samarth N. , Furdyna J. K.
Published: July 2002

Epitaxially grown Zn1−xMnxSe and its superlattices with ZnSe have been characterized by x-ray diffraction techniques. Rocking curves of the (004) and (115) reflections were measured using parallel configuration. Values of unit cell parameters perpendicular to the substrate surface were obtained from the (004) and (006) reflections. The lattice parameter parallel to the substrate surface was obtained from the (444) and (115) reflections. The superlattices of (Zn,Mn)Se and ZnSe exhibit considerable tetragonal distortion and have significantly fewer dislocations than (Zn,Mn)Se films for the same Mn composition.
Pressure and temperature influence on CdTe thin-film deposit by close-spaced vapor transport technique
Authors: Sosa Víctor , Castro Román , Peña J. L.
Published: July 2002

CdTe thin films were grown by the close-spaced vapor transport (CSVT) technique, and the sublimation rate had a transition as ambient pressure was changed. The transition is discussed in terms of diffusion and free sublimation processes. The interpretation of these results is given based on an argument on the mean free path of gas molecules. A transition in the activation energy is reported.
Simple modeling techniques for analysis of laser beam induced current images
Authors: Hennessy John , McDonald Paul
Published: July 2002

Spatially resolved laser beam induced current measurements yield large volumes of data. We present here a data reduction technique based on simple emulation of the laser beam induced current (LBIC) response expected from a distribution of charge in a medium. A model for the current expected when scanning over a buried charged sheet allows some LBIC data to be reduced to a geometric model for a cracked sample. The results obtained are sufficiently encouraging to indicate that such modeling methods can reduce the often complex LBIC images obtained from HgCdTe material to a digest of geometric and electrical properties.
Structure and property correlation of plasma source ion implanted layers in Monel K-500 alloy
Authors: Madapura M. , Dodd R. A. , Conrad J. R. , Plantz D. , Worzala F. J.
Published: July 2002

Monel series of nickel base alloys (Ni–Cu type) are attractive for components like bearings and shafts operating in high magnetizing environments because of their nonmagnetic characteristic coupled with superior corrosion and oxidation resistance at extreme service temperatures. Unfortunately, the current bearing technology is based on the use of two classes of materials, 5000 series alloy steels and 400 series stainless steels, both of which are magnetic. Thus, the successful operation of a magnetic device requires a new class of bearing materials. Some of the nonferrous materials, such as monels, have not demonstrated superior wear characteristics as bearing materials. Plasma source ion implantation (PSII), a non-line-of-sight technology for implanting preferably gaseous species, is a new and innovative approach to surface modification of materials. Recently, it was reported that nitrogen implantation of the solution annealed and age hardened Monel K-500 alloy by the PSII process resulted in a significant improvement in the adhesive/sliding wear resistance with no perceptible increase in the low-load microhardness. The Auger analysis of the implanted layer showed a peak nitrogen concentration of about 10 at. %. The principal objective of the present investigation was to understand the unusual nature of the implanted layer and to arrive at plausible mechanisms for wear improvement in this material. Monel K-500 alloy samples were nitrogen implanted by the PSII process. The modified layers were investigated by nanohardness testing, transmission electron microscopy, and pin on disk wear and friction measurements. The results of these studies show (1) 20%–60% increase in hardness; (2) significant reduction in the friction coefficient under unlubricated condition; and (3) a more complex microstructure, due to nitrogen implantation.
Influences of noble gases (Ne, Ar, and Kr) on magnetic properties of ion-beam-sputtered Fe/SiO2 multilayer films
Authors: Senda Masakatsu , Nagai Yasuhiro
Published: July 2002

This paper investigates the sputter gas (Ne, Ar, and Kr) influences on magnetic properties of Fe/SiO2 multilayer film. The magnetostriction changes due to Fe layer thickness are strongly dependent on sputter gas species. This phenomenon cannot be explained only by Fe crystal orientation change. X-ray photoelectron spectroscopy analysis indicates the magnetostriction change is also related to the Fe–Si-O mixed state formed between Fe and SiO2 layers, and to the thickness of the region. Ne improves the soft-magnetic properties of Fe/SiO2 film, but Kr deteriorates the properties. This result is thought to be caused by the difference in magnetic anisotropy dispersion due to Fe crystal grain size. Saturation magnetization of 1.72 T, hard-axis coercivity of 0.3 Oe, and relative permeability (at 5 MHz) of more than 5000 are achieved under nearly zero magnetostriction in Ne80Ar20 ion-beam-sputtered Fe/SiO2 (10/2.5 nm) film, with no post annealing.
Scanning tunneling microscopy of silicon surfaces in air: Observation of atomic images
Authors: Nakagawa Y. , Ishitani A. , Takahagi T. , Kuroda H. , Tokumoto H. , Ono M. , Kajimura K.
Published: July 2002

Atomic images were successfully observed on hydrogen terminated Si (100) and (111) surfaces in air by scanning tunneling microscopy. The surfaces were prepared by ultraviolet irradiation in an oxygen atmosphere followed by dipping in 1% hydrofluoric acid. For the (100) surface, the atomic images exhibit various structures such as rows along the [010] and [001] directions, an apparent (1/(2)1/2×1/(2)1/2)R45° ‘‘sublattice,’’ and a c(2×1) superlattice in addition to the square lattice corresponding to the Si(100)-(1×1) structure. For the (111) surface, a triangular lattice pattern which agrees with the Si(111)-(1×1) structure was observed. Such atomic images appeared in restricted areas. It is possible that slight oxidation or some adsorption enables or prevents the observation of atomic images.
The surface electronic structure and scanning tunneling microscopy of WTe2
Authors: Tang S. L. , Kasowski R. V. , Suna A.
Published: July 2002

We have investigated the surface electronic structure of the van der Waals surfaces of tungsten ditelluride (WTe2) with first principles calculations of the spatial distribution of the surface valence charge densities and compared the results to images obtained with scanning tunneling microscopy (STM). The energy-dependent calculations show that the valence charge density distributions above the Te surface could be derived from the surface Te layer, as we previously calculated, but the charge density distribution has a distortion that makes it appear to have a symmetry close to the paired, zig-zag and buckled rows of the W layer. These results dramatically illustrate that the surface valence charge density distribution does not necessarily follow the surface atomic positions even on an ideal, unreconstructed surface. An alternative interpretation of the STM images of this surface will be proposed in light of these new findings in the surface electronic structure.
Properties of pin hole orifices as transfer leak artifacts in the 10−11 to 10−6 mol/s range
Authors: Thornberg Steven M.
Published: July 2002

A simple leak artifact has been developed using a single orifice only a few microns in diameter. The new single orifices have been calibrated over several orders of magnitude in leak rate by varying the gas pressure applied to one side of the element. The short term stability of the leak rate is limited primarily by the stability of the applied pressures. Long term stability appears to be limited by the cleanliness of the orifice and calibration system. The typical calibration curves obtained will be presented.
A Monte Carlo study of the angular dependence of the depth distribution function of signal electrons in electron spectroscopies
Authors: Werner W. S. M. , Gries W. H. , Störi H.
Published: July 2002

The emission angle dependence of the depth distribution function of signal electrons in electron spectroscopies has been studied by means of a Monte Carlo simulation in which elastic scattering plays a significant role. Calculations were made for Si, InP, and In substrates at different emission angles (0°, and 40° to 80° off normal). The calculated depth distribution of signal electrons showed significant deviations from exponential attenuation at the more oblique emission angles. In consequence, the ‘‘attenuation length’’ of signal electrons loses its status of a material constant. A new empirical depth distribution function is proposed which consists of the sum of two exponential terms. This function describes the calculated Monte Carlo depth distribution as a function of the emission angle. The first term of this depth distribution function represents the contribution of those electrons which follow parallel straight-line paths, whereas the second term accounts for a randomization of electron directions due to elastic scattering. Common to both terms is a parameter which we propose to call the attenuation parameter. A relationship is established between the attenuation parameter and the ‘‘effective attenuation length,’’ as proposed earlier [W. H. Gries and W. Werner, in Proceedings of the European Conference on Applied Surface and Interface Analysis, ECASIA, Antibes, France, 1989 (unpublished)]. The attenuation parameter is independent of the angle of emission, in contrast to the effective attenuation length. Evidence was found and is discussed, which indicates that the results apply equally to other materials. In special situations, the thickness dependence of the depth distribution function in the case of an overlayer situation is accounted for by using the new depth distribution function.
Surface mechanisms in aluminum chemical vapor deposition
Authors: Mantell David A.
Published: July 2002

Triisobutylaluminum (TIBA) can be used to selectively grow patterned aluminum films in chemical vapor deposition. This is possible because film nucleation occurs much more rapidly on a metal or semiconductor surface than on an oxide surface. In this study we examine the adsorption and decomposition of TIBA on a silicon oxide surface. Two of the three isobutyl groups in the TIBA are removed easily upon adsorption on a silicon oxide surface, but the third isobutyl group will not decompose thermally. Ultraviolet laser irradiation has previously been shown to be effective at aiding film growth by decomposing adsorbed organometallics and creating adsorbed metal particles that act as nucleation sites for thermal growth. We show here that x rays are also effective at decomposing the isobutyl groups through some surface mediated process such as generation of hot electrons, but only when the surface is heated. Chemical changes in the surface are linked to this process.