bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoluminescence spectra of some ternary and quaternary chalcopyrite semiconductors
Authors: Abou-Elfotouh F. , Dunlavy D. J. , Kazmerski L. L. , Albin D. , Bachman K. J. , Menner R.
Published: July 2002

Photoluminescence (PL) emission spectra of single crystals and thin films of CuGaxIn1−xSe2 compounds have been investigated at various measuring temperatures, and compared with the emission from CuGaSe2 and CuInSe2. The observed PL spectra consisted of two groups of emission lines: the near-band-gap group (A) and the lower energy group (B). It was found that the type of emission obtained is determined by the amount of stoichiometry and molecularity deviations. The PL data also showed a correspondence between the defect related transitions from the CuGaxIn1−xSe2 solid solution and CuGaSe2 for x<0.5. Based on the intrinsic defect states in the two components CuInSe2 and CuGaSe2, the predominant defect states transitions in the solid solution are defined for the m<1 and ΔS>0. Interpretation of the PL spectra of Se-deficient compounds with m>1 are rather complicated, and much work remains to be done before the defect chemistry of CuGaxIn1−xSe2 could be fully understood.
Scanning tunneling microscopy and optical spectrum studies of light-emitting tunnel junctions
Authors: Pappas D. , Sparks P. D. , Hopster H. , Rutledge J. E.
Published: July 2002

We have measured the optical spectra of light-emitting tunnel junctions with two different roughnesses and measured the power spectrum of the roughness with scanning tunneling microscopy (STM). Our experiments show that most of the information on the roughness relevant to light emission can be inferred from STM measurements. We discuss the aspects of the STM data that are unique to the determination of long-wavelength roughness relevant to optical studies.
Imaging graphite in air by scanning tunneling microscopy: Role of the tip
Authors: Colton R. J. , Baker S. M. , Driscoll R. J. , Youngquist M. G. , Baldeschwieler J. D. , Kaiser W. J.
Published: July 2002

We have been successful in obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current–voltage (I–V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. Our observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.
Local structure investigation of the ion sputtered oxide substrates
Authors: Chaug Y. S. , Kim Y. H. , Chou N. J.
Published: July 2002

Effect of substrate pretreatment by ion sputtering on adhesion has been studied for the case of thin Ti films deposited on two different oxide substrates, namely, fused silica and sapphire. Peel test measurements showed that the peel strength of Ti on the oxide substrates was enhanced as the ion sputter time was increased at a constant ion flux. In situ x-ray photoelectron spectroscopy was used to investigate the chemical interaction of Ti with the sputtered substrate. Intermediate oxidation state of titanium, i.e., a TiOx-like (1<x<1.5) state, was formed at the interaction sites on both treated and untreated substrate surfaces. Comparison of the x-ray induced O(KVV) Auger spectra evolved at different Ti coverages on these substrates suggested that ion sputtering created additional interaction sites on the substrate surface. Surface extended electron energy-loss fine structure above the oxygen K edge was used to investigate the local structure of the ion sputtered substrates. The bond length determined by this technique for the sputtered surface is consistent with the scenario that ion sputtering creates defects in the top monolayers of the substrate, which provide the increased interaction sites.
A study of organic/inorganic interactions in syncrude fines by x-ray photoelectron spectroscopy and core electron energy-loss spectroscopy
Authors: Zajac G. W. , Fleisch T. H. , Lee F. S. C.
Published: July 2002

Surface analysis including x-ray photoelectron spectroscopy (XPS), core electron energy-loss (CEEL), and temperature programmed oxidation–XPS have been used to characterize the fines from a tar sand oil obtained by extraction of a western tar sand, a shale oil obtained by retort of a western shale, and a coal liquid produced by direct liquefaction of a sub-bituminous coal. Our results identify the modes of interaction involved between different organic and mineral types. In tar sand fines, the major mineral component is clay. The dominant organic interaction with the clay phase is with N-rich asphaltenes. In shale, the major mineral component is Ca and Ca/Mg carbonates while clay represents a minor component. The major bonding between the organic and mineral phases is a cation–carboxylate interaction. However, the clay continues to bind up a substantial fraction of the N-rich organics although it represents a minor phase. In coal liquid, the fines consist of a heterogeneous mix of organic and mineral phases. The mineral particles, in particular Fe, are strongly associated with organics. CEEL analysis of the organics shows highly aromatic structure. Comparing organics on fines with coal feed indicates hydrogen incorporation during coal liquefaction.
Analysis of frequency responses for different rotor configurations of small turbomolecular pumps
Authors: Casaro F.
Published: July 2002

An experimental evaluation was made of the dynamic behavior and design of small turbomolecular pump rotors. In particular, the natural flexural frequencies of the rotor were considered as an indication of the potential capability of the pump to reliably reach a high rotational speed (which means, for a small pump, the achievement of high performance). The first natural flexural frequency should be sufficiently higher than the nominal speed, both to avoid passing through this frequency during the acceleration/deceleration of the rotor and to make the balancing easier. Some rotor geometries were examined and compared from the above point of view, highlighting their positive and negative aspects for the specific application of the turbomolecular pumps and leading to the determination of optimization criteria. For rotors which differ in materials and shape (but adopted in pumps having similar overall dimensions), the measured first natural frequencies were ranging from 1000 to 2000 Hz. Moreover, slight geometrical changes on a rotor caused a first natural frequency variation as high as 40%.
Interfacial reactions at copper surfaces coated with polyimide films prepared from poly(amide–acid) precursors
Authors: Burrell M. C. , Codella P. J. , Fontana J. A. , Chera J. J. , McConnell M. D.
Published: July 2002

Thin films of polyimide were prepared by spin coating the poly(amide–acid) precursor onto copper and aluminum substrates, followed by the usual heat treatment to promote imidization (curing) of the film. Films prepared on aluminum substrates were completely cured during the heat treatment, as shown by x-ray photoelectron spectroscopy and infrared measurements. On copper substrates, the thinnest films (2000 Å or less) showed considerable intermixing of copper ions in the polymer layer. This prevented the films from curing completely during heat treatment. The formation of a copper carboxylate at the acid site in the polyimide precursor is postulated.
Selective etching of silicon nitride using remote plasmas of CF4 and SF6
Authors: Loewenstein Lee M.
Published: July 2002

Silicon nitride is etched using a remote plasma discharge of CF4 or SF6, diluted in a He or N2 carrier gas. By itself, this process chemistry does not etch silicon nitride selectively, a desirable property of a useful process; however, by including a source of abstractable hydrogen, e.g., H2 or CH4, in the process mix, the Si3N4 :Si and Si3N4 :SiO2 etch rate ratios can be considerably increased. The hydrogen source serves to consume atomic fluorine, enhancing the preferential deposition of a fluorocarbon layer on Si and SiO2 (in the case of CF4 ). This fluorocarbon film impedes Si and SiO2 etching by atomic F. For SF6 , some form of etch-impeding surface layer also may be formed.
Stimulated desorption of neutral molecules as a real-time probe of surface kinetics: Methanol adsorption and decomposition on Ni(110)
Authors: Vajo J. J. , Campbell J. H. , Becker C. H.
Published: July 2002

The adsorption and time-resolved decomposition of methanol on Ni(110) has been studied by stimulated desorption using a pulsed Ar+ beam at 2 keV, followed by laser ionization at 193 nm and reflecting time-of-flight (TOF) mass spectrometry of the desorbed neutral species. Total ion doses are kept well below damage thresholds. Because ion (or electron) stimulated desorption is a nonthermal process, surface chemical changes can be directly monitored even for species not subject to thermal desorption. Since a complete induced-desorption TOF mass spectrum is collected for each laser pulse, real-time measurements are possible. At 170 K methanol adsorption occurs with an initial probability of 0.95 to 1.0 which remains constant until ∼0.8 of saturation. For an initial coverage of 0.2 of saturation, decomposition of molecularly adsorbed methanol during temperature programmed reaction occurs in two steps: deprotonation, which occurs between approximately 140 and 240 K, and methoxy decomposition to adsorbed carbon monoxide and hydrogen, which occurs between 240 and 270 K.
Photoemission study of the interaction of SO2 and H2S with titanium and vanadium oxides
Authors: Smith Kevin E. , Henrich Victor E.
Published: July 2002

Fundamental studies of the interaction of sulfur-containing molecules with titanium and vanadium oxide surfaces are of great importance given the widespread study and use of these materials as catalyst supports and the notorious nature of sulfur as a catalyst poison. We report here a detailed photoemission investigation of the interaction of SO2 with well-characterized single-crystal surfaces of TiO2, Ti2O3, and V2O3, and of H2S with well-characterized surfaces of TiO2. It is found that sulfur has a remarkable impact on the oxidation properties of these oxides. A complete oxidation of suboxides of Ti by exposure to SO2 has been observed using ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy; this is not possible by exposure to O2. TiS2 and TiO2 are formed at the Ti2O3(101̄2) surface following SO2 exposure. In contrast, the suboxide of V studied, V2O3, could not be significantly oxidized by exposure to SO2. Weak molecular and dissociative adsorption occurred, with the reaction saturating on a variety of different V2O3 surfaces for SO2 exposures <5 L. The reaction H2S with reduced TiO2(110) surfaces was also found to be weak, with an initial dissociative adsorption at exposures <10 L, followed by molecular adsorption. Exposure of the reduced surface to H2S passivates the surface, inhibiting O2 uptake.
Elastic electron fine-structure investigation of oxygen interaction with the Si(111) surface
Authors: Lin B. Y. , Kahn A.
Published: July 2002

The elastic electron fine-structure (EEFS) technique is applied to the study of the interaction of oxygen with the Si(111) surface and to the formation of the Si–SiOx interface. The SiOx is in the range of 0–5 monolayers. The Si–Si bond peak in the radius distribution function has a broad shoulder which suggests the existence of a distribution of the Si–Si bond length with a spread of up to 10% with respect to the bulk bond length. When the oxide thickness exceeds ∼2 monolayers, the presence of O–O bonds is also detected. After high exposure at high temperature, the O–Si bond length appears to have shifted toward a lower value by ∼0.1 Å. This experiment demonstrates the applicability of the EEFS technique in the submonolayer and monolayer region. The advantages and disadvantages of EEFS over other short-range-order probing techniques are discussed.
Ferromagnetic order at surfaces of ultrathin Fe(100)/Ag(100) and Tb/Fe(100)/Ag(100) films determined by electron capture spectroscopy
Authors: Rau C. , Xing G.
Published: July 2002

The temperature and magnetic field dependence of the electron-spin polarization (ESP) at atomically clean and flat surfaces of ultrathin (1–10 monolayers) bcc Fe(100)/Ag(100) and Tb/Fe(100)/Ag(100) films is studied as function of film thickness. For the investigation of the ESP, we use electron capture spectroscopy which enables us to study long-ranged ferromagnetic order at the topmost layer of a surface. The geometrical and chemical structure of the films is well characterized using Auger electron spectroscopy and low- and high-energy electron diffraction. Atomic flatness is studied using scanning tunneling microscopy. Nonzero ESP is found at the surface of all films. We find ambiguously that in-plane remanent ferromagnetic order exists at uncoated surfaces of 1 to 10 monolayer thick bcc Fe(100)/Ag(100) films for temperatures ranging between 173 and 303 K. It is further found that the Curie temperature of the Fe films depends on film thickness. Evidence is found for in-plane magnetic anisotropies which strongly depend on film thickness. In striking contrast to uncoated Fe films, large temperature-dependent perpendicular (out-of-plane) magnetic anisotropies exist at surfaces of Tb-coated Fe(100)/Ag(100) films.
Structure of the Si(111)/CaF2 interface
Authors: Tromp R. M. , Reuter M. C. , LeGoues F. K. , Krakow W.
Published: July 2002

The structure of the Si(111)–CaF2 interface has been determined with medium-energy ion scattering and high-resolution transmission electron microscopy. The interface structure is closely related to the bulk crystal structure of CaSi2. The interface is deficient in F, resulting in Si–Ca bonding and a charge–neutral interface.
Characterization of a fast atom source for secondary ion mass spectrometry
Authors: Corcoran S. , Parikh N. , Griffis D. P. , Linton R. W.
Published: July 2002

A fast atom bombardment (FAB) source has been installed on the Cameca IMS-3f, a stigmatic imaging secondary ion mass spectrometer (SIMS) [F. Degreve and J. M. Lang, Surf. Interface Anal. 7, 117 (1985)]. The FAB source is used routinely in the ion microscopy of poorly conducting materials, and is currently being evaluated for use in the study of polymer films. The FAB source consists of a collision cell placed along the optical axis of the primary ion column approximately 30 cm from the sample. During operation the pressure is raised inside the cell by adjusting the flow rate of the collision gas into the cell by an external leak valve. The primary ions are then neutralized through a charge exchange mechanism as they pass through the cell. Characterization of the primary beam dose rate (atoms/cm2 s) and energy distribution is important if the FAB source is to be used to generate reliable SIMS data, e.g., in applications such as organic polymer films where beam damage is an important consideration. This paper reports on an analytical methodology used to obtain information on the characteristics of the FAB source installed in this laboratory. A series of experiments were performed using the FAB source for on line implantation of Si wafers. Rutherford backscattering spectroscopy was performed on the implanted regions to determine the total dose to the sample over a given amount of time. SIMS was used to depth profile the implanted atoms by monitoring the (CsAr)+ secondary ions, revealing information on the energy distribution of the implanted neutral atoms. Presputtering with Cs+ prior to Ar atom implantation of the Si wafer was used to minimize surface ion yield transients in subsequent SIMS depth profiles with Cs+ primary ions. Results are presented for both Ar and Xe atom beams.
Laser-enhanced oxidation of nickel
Authors: Mesarwi A. , Ignatiev A.
Published: July 2002

Photoenhanced oxidation of nickel in air at 350 °C under laser irradiation with wavelengths 308 nm≤λ≤1064 nm has been studied. Significant oxidation enhancement over infrared oven heating has been observed under irradiation with short-wavelength lasers. A 690% increase in oxidation rate has been observed under irradiation with 308-nm wavelength laser. The enhanced oxidation decreased with increasing wavelength up to a threshold wavelength of λ=720 nm. The mechanism involved is believed to be the photoexcitation of adsorbed molecular oxygen at the nickel oxide surface leading to enhanced generation of the active atomic oxygen.
Surface characterization of the copper–epoxy adhesion interface from production printed circuit boards
Authors: Miller C. W. , Laberge P. C.
Published: July 2002

In the construction of multilayered printed circuit boards, the interface adhesion between copper foils and glass cloth reinforced FR4 epoxy is critical to the life performance of the product. This paper describes the surface characterization of the copper and epoxy interfaces obtained from production run printed circuit boards and control samples produced on production equipment. The interface surfaces, obtained by mechanically separating the copper foil from the copper–epoxy laminate, were characterized by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning electron microscopy, and energy dispersive spectroscopy. The results indicate that for copper foils, with the adhesion promoting ‘‘brown oxide’’ surface produced by oxidation with sodium chlorite, the separation occurs primarily within the epoxy matrix. This epoxy layer is very thin and on the order of the sampling depth of XPS and AES (approximately 50–75 Å). Analysis of the changes in the copper 2p3/2,1/2 photoelectron line shape indicates the copper is substantially reduced to the 1+ state after lamination relative to the initial fully oxidized 2+ state of the foil surface prior to lamination. Higher than expected nitrogen levels are also detected on the interface.
From atoms to integrated circuit chips, blood cells, and bacteria with the atomic force microscope
Authors: Gould S. A. C. , Drake B. , Prater C. B. , Weisenhorn A. L. , Manne S. , Hansma H. G. , Hansma P. K. , Massie J. , Longmire M. , Elings V. , Northern B. Dixon , Mukergee B. , Peterson C. M. , Stoeckenius W. , Albrecht T. R. , Quate C. F.
Published: July 2002

The atomic force microscope (AFM) can now bridge the gap from imaging objects that can be seen with an optical microscope to imaging atoms: a range in magnification of 104. High magnification images of germanium show single atoms separated by 0.4 nm while low magnification images of entire cells and portions of an integrated circuit chip provide lateral and vertical information over a range of 25 μm.
Reduction of particle contamination by controlled venting and pumping of vacuum loadlocks
Authors: Strasser G. , Bader H. P. , Bader M. E.
Published: July 2002

The control of particle and chemical contamination is one of the main challenges on the way to ever higher integration in the production of advanced semiconductor devices. Inadequate pumping and venting of vacuum equipment loadlocks has been identified as a major cause for particle contamination. To overcome the problem, ‘‘soft pumping’’ and ‘‘soft venting’’ procedures are in use. A quantitative assessment of the term ‘‘soft’’ has as yet been missing. We have used standard particle monitoring equipment like a Climet 6300 airborne particle counter and a Tencor 5000 surface particle counter to quantify the contamination level after different venting and pumping procedures. In contrast to a widespread belief we found that the resuspension of sub-micrometer particles does not start at the transition point from laminar to turbulent flow conditions but above. Most particles are generated at low pressure in the initial phase of venting. To account for the experimental effects observed, a theoretical model was developed. Using optimized pumping and venting procedures, airborne particle levels below class 10 were obtained in equipment loadlocks. A surface particle density increase below 10−3 cm−2 for particles larger than 0.3 μm was measured after proper venting and pumping for both vertical and horizontal oriented wafers.
Schottky barrier height and thermal stability of the NiAl/n-Ge/GaAs(001) interface
Authors: Chambers S. A. , Loebs V. A.
Published: July 2002

We have successfully grown epitaxial NiAl on single-crystal Ge interlayers, which were in turn grown on GaAs(001) substrates. The goal has been to use the single crystallinity of the NiAl overlayer with its characteristically low density of grain boundaries as a diffusion barrier to prevent outdiffusion of n-type dopant atoms from the Ge interlayer. Heavily As-doped Ge has proven effective in lowering the Schottky barrier height of GaAs substrates to 0.2–0.5 eV. The epitaxial overlayers and interfaces have been characterized in situ by means of x-ray photoemission, x-ray photoelectron diffraction, and low-energy electron diffraction. We have found that even at growth temperatures as low as 210 °C, outdiffusion of As from the Ge layer into the growing NiAl epifilm occurs, causing the barrier height to increase from ∼0.4 eV to ∼0.6 eV. Furthermore, limited Al atom indiffusion and Al–Ga cation exchange occur during the initial phase of NiAl growth, as does limited disruption of the Ge epilayer surface. Band alignment at the Ge/GaAs heterojunction is also perturbed as a result of the cross-diffusion that occurs, as evidenced by an ∼0.15 eV increase in the valence band offset. Annealing at 350 °C causes additional As loss from the Ge interlayer, a further barrier height increase, additional penetration of Al atoms to the Ge/GaAs interface, and enhanced Al–Ga cation exchange. Thus, it appears that such a system is of little value as a high-thermal-stability, low-barrier-height metal contact to GaAs. Interface degradation is dominated by interdiffusion and chemistry.
Changes in hydrophobic properties of glass surfaces by ion implantation
Authors: Ohwaki Takeshi , Taga Yasunori
Published: July 2002

Ion implantation was used to modify the hydrophobic properties of glass surfaces. The hydrophobicity of the glass surface was evaluated by contact angle measurements for water droplets as a function of ion implantation conditions. The results showed, irrespective of ion species, that the contact angle was influenced by the energy and dose of primary ions and atmospheric conditions. A maximum contact angle of 80° was obtained after ion implantation with an energy of 50–100 keV and a dose of 1015–1017 ions/cm2, though the contact angle before ion implantation was around 10°–20° for soda-lime glass. Characterization of hydrophobic glass surfaces thus prepared has been carried out by x-ray photoelectron spectroscopy (XPS). XPS analysis revealed the formation of carbon-related bonds such as C–H and C–H2 at the glass surface, which could be explained in terms of the interaction between primary ions and adsorbed species on the glass surface. In conclusion, hydrophobic glass surfaces were obtained by the formation of carbon-related bonds as a result of ion implantation.
Auger and x-ray photoelectron study of surface heterogeneity in ASTM F-75 alloy prepared for biomedical implantation
Authors: Moore R. L. , Grobe G. L. , Gardella J. A.
Published: July 2002

Previous studies, which evaluated the tissue bonding and in vivo stability of biomedical devices, paid little attention to the surface chemistry and properties of the biomaterials prior to their implantation. Better materials characterization before placement of biomedical devices is necessary for the understanding of the chemical and physical properties, which determine biocompatibility. Biological adhesion, in particular, is strongly affected by surface chemistry changes, which are in turn drastically affected by differing preparations. The particular focus of this investigation is on the differences between passivated (acid-treated) and nonpassivated (or untreated) ASTM F-75 alloy. Both types of surfaces were examined utilizing Auger electron spectroscopy and x-ray photoelectron spectroscopy, with moderate emphasis placed on the detection and identification of nickel species.
Quantitative secondary ion mass spectrometry dopant profiling in silicided metal–oxide semiconductor field effect transistors
Authors: Smith Howard E. , Beagle John L.
Published: July 2002

The shallow metal–oxide semiconductor field effect transistor junctions required for very large scale integrated microcircuits are typically 50–250 nm deep, including a 50–100 nm overlying metal silicide layer. Dopant distributions must be quantified through both the silicide and underlying silicon in order to measure junction depths, dopant evaporation, and dopant segregation. Accurate measurement of these distributions by secondary ion mass spectroscopy (SIMS) sputter profiling is problematic, due to concentration and depth scale variations of up to 500% and 20%, respectively in the silicide, relative to the silicon. These variations are caused by the effect of changing composition upon sputtered ion yields and sputtering rates. In this paper, yield variations in the silicide layer are quantified and archived as correction factors. Dopant profiles are corrected using a point-by-point silicide yield correction procedure. This procedure eliminates most concentration scale error. The magnitude of the remaining error depends on the day-to-day reproducibility of the silicide correction factors. The error is quantified for the example of a boron distribution in a silicided sample. The one-sigma uncertainty of the dopant concentration in the silicide layer is no better than ±17% when archived yield factors are used without the analysis of external standards.
Rate equation models of epitaxial growth on stepped surfaces
Authors: Resh J. , Strozier J. , Bensaoula A. , Ignatiev A.
Published: July 2002

We review the rate equation method of modeling epitaxial growth on a flat surface, where the predominant growth mode is via nucleation and growth of adatom islands. We apply a similar method to investigate the step-edge propagation growth mode on stepped (vicinal) surfaces. Unlike the intuitive picture of step-edge propagation whereby the surface morphology is invariant in a moving reference frame, results show oscillations in the step-edge positions about mean values (dependent on initial step-edge distribution). This could have important consequences in the growth of vertical superlattices. We have combined the two previous models to describe a more general case of growth, on stepped surfaces, where both nucleation through adatom islands and step-edge growth occur. Calculated reflection high-energy electron diffraction oscillations from this model will be compared with experimental results.
From chemisorption to mechanism on surfaces: An exploration of the pyrolysis of triisobutylaluminum in the chemical vapor deposition of aluminum thin films
Authors: Chan A. W. Edith , Hoffmann Roald
Published: July 2002

The chemisorption of H, CH3, C2H5, and C2H4 is examined on Al(111) and Al(100) surfaces by the extended Hückel method, using a tight-binding formalism. A local chemical viewpoint is sought through fragment analyses, decompositions of the density of states and overlap populations. Various adsorption sites have been studied. On both surfaces, the first three species prefer to bind in the on-top site by a σ-type interaction with the coordinated Al atom. Ethylene, on the other hand, favors the twofold bridging site by a di-σ bonded interaction with its highest occupied molecular orbital and lowest unoccupied molecular orbital. In addition, the mobility of all these species on surfaces is investigated. Finally, the mechanism for the thermal decompositions of the triisobutylaluminum is studied. A β-hydride elimination rate-determining step is established, in which the activation barrier of the reaction in Al(111) is lower than Al(100). This result is in agreement with the experimental findings.
Formation of compound semiconductors by electrochemical atomic layer epitaxy
Authors: Suggs D. Wayne , Villegas Ignacio , Gregory Brian W. , Stickney John L.
Published: July 2002

A method for the electrochemical formation of epitaxial deposits of compound semiconductors is being developed. It is referred to as electrochemical atomic layer epitaxy (ECALE). The method is the electrochemical analog of atomic layer epitaxy (ALE), where ALE is a method used to form compounds by alternately depositing atomic layers of the constituent elements. Atomic layers are formed in ECALE by using underpotential deposition (UPD). UPD is a phenomena where an atomic layer of an element deposits at a potential prior to that needed to deposit the bulk element, due to the increased stability afforded by reaction with a second element present at the substrate surface. This paper describes the structure of the first monolayer of Te formed on an Au(100) surface and the structure of a monolayer of CdTe, subsequently formed by deposition of an atomic layer of Cd. Deposits have been formed and analyzed in an ultrahigh vacuum (UHV) surface analysis instrument directly coupled to an electrochemical cell. Low-energy electron diffraction (LEED) and Auger electron spectroscopy have been used to follow the structures and compositions of deposits after various steps in the ECALE cycle. As well, some initial studies of the atomic arrangements have been performed using scanning tunneling microscopy. Reductive UPD of Te on the Au(100) plane resulted in a series of well ordered Te structures at increasing coverages: (2×2), (2×√37), (2×4), and (√2×√5)R45°. Oxidative UPD of Te also resulted in a well-ordered structure, a (2×√10) at 1/3 coverage of Te. Subsequent reductive UPD of Cd on this surface resulted in a well ordered c(2×2)-CdTe structure.
Potassium Auger emission by K+ ion bombardment of Ag surface
Authors: Terzić I. , Tošić M. M. , Rakočević Z.
Published: July 2002

The electron energy spectra induced by bombardment of Ag surface with 0.6–3.0 keV energy K+ ions were measured. A peak in the energy range 14.5–15.0 eV was found in the case when the target has been previously bombarded by K+ ions. This phenomenon was ascribed to the deactivation of M23 vacancies of K atom formed in symmetrical collisions of projectiles with K atoms implanted or adsorbed at the surface. The K atoms presence at Ag surface during K+ ion bombardment is independently determined by the low-energy ion scattering spectroscopy.
Uniformity of targets erosion and magnetic film thickness distribution in the target-facing-type sputtering method
Authors: Fan Qihua
Published: July 2002

In this article we report the uniformity of the targets erosion and the high rate deposited Fe film thickness distribution in the target-facing-type sputtering method. Both the columnar and annular magnet could confine the plasma in the space between the two facing targets effectively and resulted in high rate sputtering of magnetic targets. But the targets were much more uniformly etched when using annular magnet than that when using columnar magnet. A concept of uniform thickness distribution line was first put forward. According to this concept, the rectangle opening shield covers were used to replace the annular ones and the film thickness uniformity was greatly improved in the direction parallel to the targets surface. With the distance between two facing targets increasing from 70 to 100 mm, the uniformity of film thickness in the direction perpendicular to the targets surface improved.
Temperature programmed desorption mass spectrometry of organic matrix compounds
Authors: Beck Donald D. , Somers Nancy
Published: July 2002

The desorption of organic matrix compounds from a stainless steel sample probe in vacuum has been studied using temperature programmed desorption (TPD) to learn about their useful lifetimes and stabilities at temperatures and vacuum environments typically used in secondary ion mass spectrometry (SIMS) or fast atom bombardment mass spectrometry (FABMS) of organic samples. Glycerol, thioglycerol, triethanolamine, and 3-nitrobenzyl alcohol were chosen for study since they are commonly used as matrix compounds in FABMS. For all matrix compounds studied, molecular species desorb within a relatively narrow temperature range which is unique for each matrix, but consistent with the known liquid–gas phase transition temperature of each compound. For one compound (glycerol), additional species desorb which are attributed to decomposition of the compound during TPD. These results suggest that for each matrix compound used in SIMS or FABMS an optimum useful temperature range can be defined. The ultimate performance of each matrix, however, may also be affected by the thermal stability, which varies for the compounds studied in this work.
Design of selective transmissive/reflective coatings for windows
Authors: Koss V. A. , Belkind A. , Vossen J. L. , Wolfe J.
Published: July 2002

The optical design of selective transmissive/reflective coatings for windows presents unique requirements. Requirements may include specifications of color, luminous as well as solar reflectance and transmittance. Computational design is greatly simplified if the optical requirements are formulated in terms of integral characteristics over various parts of the spectrum. A comprehensive program was developed that allows one to use various integral characteristics in the design formulation. Material data can be presented as a set of optical constants, by dispersive equations and/or various effective medium theories. The effect of manufacturing uncertainties in layer thicknesses on optical performance of the designed coating can be analyzed by the program. Optical constants of new materials and/or parameters of effective medium theories and dispersive equations can be evaluated by the same program using any combination of photometric and ellipsometric data. The program application is illustrated on a few examples of window coatings.
Schottky barrier formation at nonreactive interfaces: Ga/GaAs(100) and Pb/GaAs(100)
Authors: Chen W. , Mao D. , Santos M. , Shayegan M. , Kahn A. , Mangat P. S. , Soukiassian P. , Florez L. T. , Harbison J. P.
Published: July 2002

The chemistry, overlayer morphology, and band bending occurring upon room temperature deposition of Ga and Pb on GaAs(100) are studied via high resolution photoemission spectroscopy, low energy electron diffraction and contact potential difference measurement. Both interfaces are abrupt and unreacted. At submonolayer coverage, Ga and Pb grow two dimensionally on GaAs(100). An ordered (1×2) superstructure is obtained for Pb(1/2 ML)/GaAs(100) by low temperature annealing of the interface (∼300 °C). Clustering is dominant at high coverage. With Ga, the Fermi level (EF) converges toward a position 0.68 eV above the valence band maximum (VBM), in agreement with the position obtained for the GaAs(110) surface. The room temperature deposition of Pb leads to EF stabilization at 0.57 eV above VBM, independent of the initial surface stoichiometry (Ga or As rich). The evolution of the band bending as a function of metal coverage emphasizes in both cases the importance of adatom-induced states at low coverages and the transition to final EF position upon metallization, in support of models based on metal-induced gap states.
Interface sharpness during the initial stages of growth of thin, short-period III–V superlattices
Authors: Pellegrino J. G. , Qadri S. B. , Cotell C. M. , Amirtharaj P. M. , Nguyen N. V. , Comas J.
Published: July 2002

Superlattices composed of III–V heterostructures have established applications in high-speed electronic and optoelectronic devices. As layer thicknesses are reduced, the role of heterostructure interface sharpness becomes more critical to ensuring high quality two-dimensional growth. In this work, short-period (less than 1 nm) superlattices with active layer thicknesses of 31 nm were investigated to assess interface roughness in the initial stages of growth. X-ray diffraction was used to evaluate interface roughness and to calculate superlattice periodicity. Results suggest that surface roughening by islanding may be promoted by GaAs buffer layers that are 10–100 nm thick. Smoother interfaces were obtained in samples with buffer layers 250 nm and greater.
Electron cyclotron resonance reactive ion etching of fine features in HgxCd1−xTe using CH4/H2 plasmas
Authors: Eddy C. R. , Dobisz E. A. , Meyer J. R. , Hoffman C. A.
Published: July 2002

To date, most dry etching of mercury cadmium telluride (MCT) has involved fabrication of conventional devices, whose minimum feature size is several tens of micrometers. Recent theoretical work has predicted novel effects in nanometer-scale devices making the ability to fabricate nanometer-scale features in narrow band-gap semiconductors such as MCT desirable. As MCT has a very low threshold for damage, realization of either conventional or nanometer size devices requires reduced, controllable ion energies and improved understanding of the etch processes involved. This work presents the results of parametric studies of the etching of MCT using an electron cyclotron resonance (ECR) microwave plasma source and CH4/H2 gas mixtures. The current work is focused on the application of ECR etching for the nanofabrication of MCT. The parameters investigated include % CH4 in H2 (10 to 25); ion energy (0 to 100 eV); pressure (0.10 to 1.0 mTorr), and substrate position with respect to the ECR condition. Optimized process conditions are total gas pressure of 0.50 mTorr, 18% CH4 in H2, ≊−100 V ion energy, and a 32 cm ECR-sample separation. These conditions have produced etch rates of up to 1500 Å/min and etch features with sidewall angles of ≥88°. Feature sizes from 40 nm to 2.0 μm have been investigated.
Effect of preparation conditions on the morphology and electrochromic properties of amorphous tungsten oxide films
Authors: Witham H. S. , Chindaudom P. , An I. , Collins R. W. , Messier R. , Vedam K.
Published: July 2002

Electrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of electrochromic coloration and spectroscopic ellipsometry experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with previous reports finding that amorphous tungsten oxide thin films are dominated by a columnar morphology and an intercolumnar void network that strongly influence the electrochromic properties. In addition real-time spectroscopic ellipsometry (SE) experiments on the tungsten oxide electrochromic coloration process have been performed for the first time. In studies of very thin (∼15 nm) films by real-time SE, the formation of tungsten bronze exhibits reaction-limited kinetic behavior, and thus we are able to extract accurate dielectric function spectra for the film in different stages of the coloration process. These spectra can be understood in terms of a two-phase physical mixture of tungsten oxide and tungsten bronze components with relative volume fractions determined in a regression analysis of the spectra.
Fourier transform infrared reflection absorption spectroscopy studies of adsorbates and surface reactions: Bridging the pressure gap between surface science and catalysis
Authors: Hoffmann F. M. , Weisel M. D.
Published: July 2002

Recent development of in situ Fourier transform–infrared reflection absorption spectroscopy has provided us with a sensitive surface probe to study catalatic reactions on single crystal surfaces at elevated pressure. This method has been utilized in combination with (ex situ) photoemission and other probes to investigate the CO hydrogenation reaction over a potassium promoted Ru(001) surface. Potassium carbonate and formate are identified as reaction intermediates. Isotope transient measurements demonstrate the activity of formate under equilibrium conditions. The comparison with the ultrahigh vacuum (UHV) model adsorbates allows us to establish that both formate and carbonate are bound to potassium and not to the ruthenium substrate. This suggests direct participation of the potassium in the synthesis of hydrocarbon species in this reaction.
Analysis of offgassed water: Calibration and techniques
Authors: Basford James A.
Published: July 2002

Analysis of water in vacuum systems presents large difficulties. Generation of known partial pressures is necessary to calibrate mass spectrometer response for quantitative analysis. For this purpose, two designs of leaks and their calibrations will be described. Two problems of offgas analyses dictated the development of special techniques. One study, examining comparative surface desorption, required rapid sample insertion and data acquisition. The other study, the monitoring of slowly changing offgas rates, required high precision over extended periods. Very constant, elevated temperatures of the entire vacuum system are required to keep the water partial pressure a function of current sample offgassing. Exposure of mass spectrometers to water can cause instabilities to their sensitivities to all gases. Methods of decreasing this effect through ionizer modification and of acquiring good data through special calibration procedures will be described.
Properties of aluminum overlayers on chemically modified TiO2(110) surfaces
Authors: Dake L. S. , Lad R. J.
Published: July 2002

The chemical interactions of ultra-thin aluminum films with clean and chemically modified stoichiometric TiO2 (110) surfaces have been studied by several electron spectroscopies (ultraviolet photoelectron, x-ray photoelectron, and Auger electron) and low-energy electron diffraction. The chemical properties of the Al overlayers on clean TiO2 surfaces were compared to those on surfaces pre-dosed with submonolayer amounts of carbon or potassium. The thermal stability of each system was also examined. On the clean surface, the first monolayer of aluminum completely wets the surface and becomes oxidized, while the Ti 4+ cations in the near surface region are reduced. Submonolayer amounts of potassium interact with the stoichiometric TiO2 surface in a similar manner and form a potassium oxide overlayer. Aluminum deposited onto a potassium-dosed TiO2 (110) surface competes with K for oxygen and reduces more Ti 4+ cations in the interface region. Carbon species adsorbed on the TiO2 (110) surface by electron beam decomposition of ethylene prior to the aluminum deposition interact only weakly with the oxide surface, but retard the oxidation rate of aluminum and decrease the temperature stability of the aluminum oxide overlayer.
Instability of the spatial electron current distribution in hot cathode ionization gauges as a source of sensitivity changes
Authors: Jousten K. , Röhl P.
Published: July 2002

When ionization gauges are being used as secondary standards in the high and ultrahigh vacuum, a stable sensitivity is essential. Our hypothesis is that the sensitivity stability may be affected by the path length changes of the electrons within the gauge. We have measured possible spatial changes of the electron current distribution onto the anode by subdividing the anode into nine separate, isolated partial electrodes. We found that up to about 10% of all emitted electrons change their target electrode on the anode after regular operation with the gauge, e.g., degassing, both for gauges with ring cathodes as well as for those with hairpin cathodes. These spatial changes of electron trajectories can explain the observed instabilities of the ionization gauge sensitivity.
Structural and chemical characterization of vapor-deposited polythiophene films
Authors: Dillingham T. R. , Cornelison D. M. , Townsend S. W.
Published: July 2002

The chemical structure, including changes in charge distribution upon doping, for vapor-deposited polythiophene films has been investigated using x-ray photoelectron spectroscopy (XPS). Two different methods of doping, coevaporation with FeCl3 and exposure to iodine vapor, are contrasted in this study and several chemical states are observed for the ‘‘dopants’’ associated with these complexes. In general, the FeCl3 codeposited complexes have considerably higher conductivities than the iodine exposed films (10–25 S/cm as compared to 0.01 S/cm). Furthermore, besides being a more effective doping mechanism, the FeCl3 codeposited complexes are much more stable upon exposure to atmosphere and also in the ultrahigh vacuum environment of the XPS system. Atomic force microscopy images of the same films show clear morphological differences between the as-deposited, the FeCl3 codeposited, and the iodine doped films. The as-deposited film shows a fibrillar-type structure while the FeCl3 codeposited film is observed to coalesce into larger bundles. For the iodine exposed film, the fibrillar structure is present but small bundles with dimensions of several hundred Å have formed throughout the network.
Rarefied gas flow through a long tube at any temperature ratio
Authors: Sharipov Felix
Published: July 2002

The mass flow rate of a rarefied gas through a long capillary caused by small pressure and temperature gradients has been calculated based on the s-model for the diffuse specular gas-surface interaction in the range of the rarefaction parameter from 0.005 to 50. A simple method of calculation of the thermomolecular pressure effect and the thermal creep caused by a large temperature ratio have been elaborated. Numerical calculations of both phenomena for the temperature ratio T2/T1=3.8—usually realized in experiments—have been carried out. It has been determined that the nonlinear thermomolecular pressure effect does not depend on the temperature distribution along the capillary. The nonlinear thermal creep has been calculated for two different temperature distributions along the capillary. A dependence of the creep on the temperature distribution has been found. It has been shown that the application of the linear theory based on the average rarefaction parameter to the gas flow at a large temperature ratio gives a significant error.
Ion-dust streaming instability in processing plasmas
Authors: Rosenberg M.
Published: July 2002

An ion-dust streaming instability is investigated for conditions representative of plasma sheath interface regions where dust grains are observed to be present in processing plasmas. Kinetic theory is used, and the effects of collisions of charged particles with neutrals is retained. It is found that an instability with growth rate of the order of the dust plasma frequency may be driven by the drift of ions relative to charged dust. Possible implication for plasma etching is briefly discussed.
Room temperature Coulomb blockade and Coulomb staircase from self-assembled nanostructures
Authors: Andres R. P. , Datta S. , Dorogi M. , Gomez J. , Henderson J. I. , Janes D. B. , Kolagunta V. R. , Kubiak C. P. , Mahoney W. , Osifchin R. F. , Reifenberger R. , Samanta M. P. , Tian W.
Published: July 2002

The self-assembly of well-characterized, nanometer-size Au clusters into ordered monolayer arrays spanning several microns has been achieved. Techniques to insert molecular wires to link adjacent clusters in the self-assembled array have also been developed. ‘‘Unit cell’’ nanostructures formed from individual Au clusters supported on a self-assembled monolayer film of the double-ended thiol molecule p-xylene-α,α′- dithiol show evidence for reproducible single electron effects at room temperature when studied by scanning tunneling microscopy. From these measurements, estimates for the electrical resistance of a single molecule can be obtained. The experimental values for this resistance are in reasonable agreement with theoretical calculations using the Landauer approach.