bluuebunny/crossref_metadata_2025 huggingface dataset:
Epitaxial growth of iron iodide films on Fe(110)
Authors: Mueller D. R. , Rhodin T. N. , Dowben P. A.
Published: July 2002

The interaction of iodine vapor on an Fe(110) single crystal surface at room temperature has been investigated primarily through the use of LEED and UPS. Both a series of chemisorbed overlayers and an epitaxial iodide layer are observed. Iodide formation proceeds through an island growth mechanism with the iodide basal plane parallel to the Fe(110) surface. Evidence is presented that the defects introduced on the surface during argon ion sputtering may be important as nucleation sites for iodide growth. The results are compared with the results of previous studies.
Synchrotron photoemission evidence for ‘‘lying-down’’ CO on Cr(110)
Authors: Shinn Neal D.
Published: July 2002

Synchrotron ultraviolet photoemission spectroscopy (UPS) has been used to identify two sequentially populated molecular CO binding modes on Cr(110) at 90 K. These are distinguished by both intensity and electron-binding-energy differences in the CO-derived valence-band UPS features. These results support the previously proposed models in which the first binding mode (α1CO) is ‘‘lying down’’ on the surface, with both the carbon and oxygen coordinated to chromium atoms, and the second binding mode (α2CO) is terminally bonded and oriented roughly along the [110] surface normal.
Energy transfer processes in glow discharges
Authors: Abril I. , Gras-Marti A. , Valles-Abarca J. A.
Published: July 2002

The characterization of plasma environments requires the evaluation of elastic and inelastic energy loss processes undergone by the species present in the plasma. For instance, the slowing down of sputtered fluxes due to elastic collisions with the background gas determines the distributions of the thermalized and energetic components of deposited fluxes. The inelastic processes undergone by the species in the sheath determine the energy and angular distributions of the sputtering fluxes that reach the cathode in a glow discharge, and also influence the spatial dependence of the electric field in the sheath. We have modeled several processes of interest in glow discharge systems. Estimates of the quantities mentioned above will be commented upon and, whenever possible, compared with other theories and with relevant experimental data.
Ion implantation damage in Hg0.8Cd0.2Te
Authors: Schaake H. F.
Published: July 2002

Damage produced by ion implantation of B+ and As+ through an anodic oxide into Hg0.8Cd0.2Te has been examined by transmission electron microscopy. Implant doses of 1013, 1014, and 1015 cm−2 were studied. At 100 keV the damage consisted primarily of interstitial dislocation loops. At 200 keV an array of dislocations was also formed. For B+ implant doses 1014 cm−2 and below, no damage was detected, indicating the existence of a threshold dose for defect formation. For all As+ implants, no defects were found, indicating that the ions had been stopped in the anodic oxide.
Electrostatic spline lenses
Authors: Szilagyi M.
Published: July 2002

A systematic investigation of electrostatic lenses is proposed on the basis of the cubic spline model. The model is a convenient tool for varying the basic parameters of the potential distributions of different classes of electrostatic lenses. The actual electrode system is reconstructed from the distribution with the best figure of merit by a straightforward procedure. The simple spline class consisting of two intervals is investigated in detail. A superior quality six-electrode lens is proposed as an example.
Low-range flowmeters for use with vacuum and leak standards
Authors: McCulloh K. E. , Tilford C. R. , Ehrlich C. D. , Long F. G.
Published: July 2002

Vacuum pressure standards of the orifice-flow type require known gas flows of 10−6 mol/s (10−2 atm cm3/s at 0 °C) and less. Known gas flows can also be used to calibrate ‘‘standard’’ leaks by comparing the pressures generated when flows from the leak and the flowmeter are alternately passed through a constant conductance. Two constant-pressure, piston displacement flowmeters developed at the National Bureau of Standards are described that can generate flows between 10−6 and 10−10 mol/s with an estimated uncertainty of 0.8% to 2%. Comparisons of the flowmeters with alternate calibration techniques, and repeated low-range leak and vacuum gauge calibrations, have been used to confirm the estimated uncertainty and random errors of the flowmeter.
Simple, inexpensive delay circuit for protection against vacuum loss from power interruptions
Authors: Polta J. A. , Thiel P. A.
Published: July 2002

We describe a simple, inexpensive electrical circuit which protects a turbomolecular pumped chamber against vacuum loss in the event of a power failure. The circuit is timed to ensure proper sequencing of events, and also to distinguish between a short and long power failure.
The effects of Na on the interaction of CO with Ta(110)
Authors: Shek M. L. , Pan Xiaohe , Strongin Myron
Published: July 2002

The interaction of CO with Ta(110) at 80 K proceeds by dissociative chemisorption followed by molecular adsorption. The molecular chemisorption bond strength is weak, as judged by the observation in photoemission spectra of a ‘‘shake-up’’ feature at 2.1 to 2.2 eV below the ‘‘4σ’’ orbital. The weakly bound molecular phase continues to exist in the presence of Na predoses, apparently without appreciable adsorption bond strengthening. This molecular phase is suppressed in favor of dissociation only after predoses of ∼1.0 and ∼1.29 ML Na, when a small to moderate enhancement in CO dissociation occurs relative to the pure surface at 80 K.
K–CO on transition metals: A local ionic interaction
Authors: Schultz Peter A. , Patterson Charles H. , Messmer Richard P.
Published: July 2002

We present a model for the bonding of CO adsorbed on alkali-promoted surfaces that accounts for differences in experimental observations between the singly and coadsorbed systems. Theoretical evidence indicates that the interaction between the alkali metal and the CO on the surface is local and dominantly an ionic one, namely, the alkali atom transfers its electron to the CO, resulting in a large Coulombic interaction. This conclusion is supported by the results of generalized valence-bond calculations and qualitatively accounts for the unusual behavior in thermal desorption, electron energy-loss spectroscopy, Auger, and core and valence photoemission in the coadsorbed system.
Fabrication of polyvinyl alcohol coated polystyrene shells
Authors: Burnham Alan K. , Grens John Z. , Lilley Eben M.
Published: July 2002

Glass shells have been used traditionally as the deuterium–tritium fuel container for direct-drive laser fusion experiments because of their convenience and availability, but lower-Z fuel containers have superior implosion characteristics and diagnostic possibilities. Unfortunately, polymers such as polystyrene (PS) that produce shells easily have very high permeabilities so require cryogenics to retain fuel, and impermeable polymers such as polyvinyl alcohol (PVA) are difficult to make into high-quality shells. We have developed improved methods of making PS shells with diameters from 0.2 to 0.7 mm and coating them with a 3-μm layer of PVA to obtain the advantages of both. Both the PS shells and the PVA coating are made in drop towers using gas-stripped nozzles. Details of the procedures and product quality are discussed.
Mechanism of two-dimensional AgAu alloy formation on Ru(001)
Authors: Markert K. , Dolle P. , Niemantsverdriet J. W. , Wandelt K.
Published: July 2002

Using the photoemission of adsorbed xenon technique we demonstrate that the two-dimensional alloy formation between ordered Ag islands on Ru(001) and Au atoms deposited on top of these islands proceeds via a two-step mechanism: The Au atoms do not penetrate the Ag layer directly, but first migrate from the islands onto the Ru substrate, from where they then ‘‘attack’’ the Ag islands to form a two-dimensional AgAu alloy.
Removal of surface relaxation of Cu(110) by hydrogen adsorption
Authors: Baddorf A. P. , Lyo I.-W. , Plummer E. W. , Davis H. L.
Published: July 2002

It is now established that the termination of a metal crystal by a vacuum causes oscillatory relaxation of the interplanar distances near the surface, even when there is no lateral reconstruction. Since the details of the driving forces for this oscillatory relaxation are not fully understood at the present time, we have undertaken low-energy electron diffraction (LEED)–IV experiments and calculations to study the structure of(1×1) Cu(110) as a function of atomic hydrogen concentration adsorbed at 90 K. At this temperature high-resolution electron energy-loss spectroscopy reveals that the hydrogen lies in inhomogeneous sites within the (110) troughs. At most coverages LEED indicates that the hydrogen is disordered. Hydrogen induced changes in the work function and in angle-resolved ultraviolet photoemission spectroscopy spectra are small, indicating a weak interaction between the hydrogen and copper substrate. This one-dimensional lattice gas causes a continuous shift in the Cu(110) interplanar distances, resulting eventually in a bulk lattice spacing between the top two layers. Subsurface lattice spacings change at a slower rate. We conclude that the simple model of an electrostatic driving force created by a lateral charge redistribution [see M. W. Finnis and V. Heine, J. Phys. F 4, L37 (1974)] can explain the nature of the experimental results. The adsorption of hydrogen reduces the charge-density corrugation of the surface, and eliminates the charge redistribution and related surface relaxation of the copper substrate.
Detection of neutral atoms sputtered from ion-bombarded single-crystal surfaces Rh{111} and p(2×2) O/Rh{111}: Ejection mechanism and surface structure determinations from energy- and angle-resolved measurements
Authors: Singh J. , Reimann C. T. , Baxter J. P. , Schick G. A. , Kobrin P. H. , Garrison B. J. , Winograd N.
Published: July 2002

A temporally sensitive ionization scheme is used in conjunction with a position-sensitive detector to measure simultaneously energy- and angle-resolved distributions of sputtered neutral atoms. We report results for 5-keVAr+ ion-bombarded Rh{111} single-crystal surfaces, both clean and with a p(2×2) overlayer of oxygen atoms. The angular distributions and their variation with ejection kinetic energy are shown to give information about simple collision sequences that produce directionally preferential atom ejection. The changes that occur in the ejection distributions upon O atom adsorption suggest that O atoms occupy the ‘‘expected’’ sites, the sites that would be occupied by Rh atoms in a new monolayer.
Magnetic field gradient effects on ion energy for electron cyclotron resonance microwave plasma stream
Authors: Matsuoka Morito , Ono Ken’ichi
Published: July 2002

The effects of mirror field application on ion energy and its dispersion for electron cyclotron resonance (ECR) microwave plasma stream are discussed. Low-energy ions with minimal dispersion are extracted from ECR microwave plasma in the mirror field. Mirror field effects on the ion energy profile are caused by a decrease in plasma microparameter distribution, including plasma potential, electron temperature, and plasma density. A highly dense ion stream is extracted from ECR plasma with the mirror field.
Metal–polymer chemistry. I. Charge-transfer-related modifications of polyimide (pyromellitic dianhydride-4,4′-oxydianiline)
Authors: Clabes J. G. , Goldberg M. J. , Viehbeck A. , Kovac C. A.
Published: July 2002

Metal interactions with polyimides, studied so far by x-ray photoelectron spectroscopy (XPS), have shown surprising similarities. Dramatic changes are observed in the core levels associated with the carbonyl groups of the dianhydride portion of the polymer, in many cases already at submonolayer coverage, suggesting a high degree of selectivity. The nature of this reaction has been investigated by XPS and infrared spectroscopy comparing Cr/pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) with reduced polyimide films and electrochemical and Cs+ intercalation experiments. The results demonstrate conclusively that charge transfer occurs from metal atoms into the PMDA portion of the polymer at the initial stage of metal–polymer interaction. Hence subsequent metal–polymer compound formation involving bond breaking and displacement of O, N, and C from the polymer chain is seen as a logical consequence of the polymer preactivation by reduction.
Promotion of dissociative chemisorption with vibrational and translational energy
Authors: Rettner C. T. , Pfnür H. E. , Stein H. , Auerbach D. J.
Published: July 2002

Molecular-beam techniques have been employed to probe the effects of translational and vibrational energy in promoting dissociative chemisorption. We review work on the three systems that we have examined to date: N2/Fe(111), CH4/W(110), and N2/W(110). These two forms of energy are found to be roughly equivalent in their ability to promote dissociation, both being highly efficient. Specifically, results for the CH4/W(110) system are found to be consistent with a model in which all vibrational modes are equally effective and vibrational energy and translational energy are approximately equivalent in promoting this highly activated process. In the N2/Fe(111) case, we find that for kinetic energies in the range 0.3–0.6 eV that vibrational energy is on average only about one-half as effective as translational energy. This is consistent with a proposed precursor mechanism, where access to the precursor state is blocked by a potential barrier that is more easily overcome with translational than with vibrational energy. Finally, for N2 on W(110), we again find a rough equivalence between these two forms of energy, but here results are clouded by the unknown effect of high levels of rotational energy present in the effusive beams employed for that study.
X-ray photoemission spectroscopy characterization of silicon surfaces after CF4/H2 magnetron ion etching: Comparisons to reactive ion etching
Authors: Oehrlein Gottlieb S. , Bright Arthur A. , Robey Steve W.
Published: July 2002

Silicon surfaces subjected to CF4/H2 magnetron ion etching (MIE) have been characterized by x-ray photoemission spectroscopy. Low-power-density (0.274 W/cm2) 25-mTorr MIE plasmas cause fluorocarbon films on Si which are characterized by C–CFx, CF, CF2, and CF3 groups. Graphitic carbon and silicon carbon type bonding can also be observed at the film/silicon interface. Thicker films are formed in hydrogen-rich CF4/H2 etching gas mixtures. Carbonaceous films with significant chemical differences are produced under high-power/low-pressure MIE operating conditions (1 W/cm2 at 4 mTorr). These films are characterized primarily by C–C/C–H type carbon and a very low intensity of highly fluorinated carbon groups. The carbon/fluorine ratio is 0.8 for the films formed in a pure CF4 magnetron discharge at a power density of 0.27 W/cm2, similar to that of films formed in a conventional reactive ion etch plasma, whereas it is 2.2 for the films formed at a power density of 1 W/cm2. The carbonaceous film formed during high-power MIE does not have a dominant role in determining the silicon etching behavior in contrast to fluorocarbon films formed under low-power MIE conditions which suppress the Si etch rate. This explains the lack of SiO2/Si etch selectivity observed under high-power MIE conditions using CF4/H2 in contrast to low-power magnetron ion etching where SiO2/Si etch selectivity is achieved. The formation of carbon-rich film in high-power MIE can qualitatively be explained by a large flux of highly dissociated CF4 derived plasma species to the silicon surface under high-power magnetron ion etching conditions and a Si substrate temperature difference of more than 100 °C between high-power MIE (>200 °C) and low-power MIE (near 100 °C or less) processing.
A scanning tunneling microscopy study of the reaction of Si(001)-(2×1) with NH3
Authors: Hamers R. J. , Avouris Ph. , Bozso F.
Published: July 2002

We have performed a study of a semiconductor surface chemical reaction at the atomic level using a scanning tunneling microscope (STM). The ‘‘topographic’’ structure and electronic density of states are measured before and after reacting Si(001)-(2×1) with NH3. While both clean and NH3-dosed surfaces exhibit a (2×1) symmetry, STM images reveal changes in the spatial distribution of occupied electronic states which allow us to distinguish reacted and unreacted Si(001) dimers. Using tunneling spectroscopy, the occupied and unoccupied surface states of the clean and reacted surface are identified. The results are interpreted in terms of a Si(001)-(2×1)H monohydride resulting from dissociative adsorption of NH3.
Atomic structure and bonding of Si(111)-(√3×√3)Al
Authors: Hamers R. J. , Demuth J. E.
Published: July 2002

The topographic structure and density of occupied and unoccupied states of the Si(111)-( 7/8 × 7/8 )Al surface are studied using scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy. Tunneling spectroscopy results are compared with theoretical band structure calculations and previous experimental studies. The results confirm a threefold adatom geometry for 7/8 Al/Si(111) and prefer assignment to the T4 site over the H3. The STM images are interpreted in terms of tunneling through localized electronic states at the surface.
Scanning tunneling microscopy of β-SiC and YBa2Cu3O7−x ceramic surfaces
Authors: Zheng N. J. , Knipping U. , Tsong I. S. T. , Petuskey W. T. , Barry J. C.
Published: July 2002

We have constructed a compact scanning tunneling microscope (STM) on a 6-in.-diam conflat flange for use in air and in ultrahigh vacuum. Topographies on a nanometer scale were imaged on a cubic β-SiC(100) surface and a single-phase polycrystalline YBa2Cu3O7−x surface with the STM at room temperature. In the case of the YBa2Cu3O7−x surface, we have succeeded in obtaining images showing the layered-perovskite structure on an atomic scale.
Laser-frequency mixing using the scanning tunneling microscope
Authors: Arnold L. , Krieger W. , Walther H.
Published: July 2002

Frequency mixing of infrared laser radiation in the tunneling junction of a scanning tunneling microscope (STM) was investigated. The dependence of the mixing signal on the tunneling current, the bias voltage, the power and polarization of the laser radiation, and on the difference frequency was studied using a W–Au STM junction. From the frequency dependence of the signals observed up to 90 MHz, the difference frequency generation is attributed to a thermal process. In addition, the generation of harmonics of the beat signal and mixing with a radio frequency signal is reported. The mixing experiments demonstrate a considerable extension of the frequency range accessible with the STM.
A high-performance scanning tunneling microscope
Authors: Cox M. P. , Griffin P. R.
Published: July 2002

The design and construction of a new ultrahigh vacuum scanning tunneling microscope are described. The microscope stage incorporates a number of novel features including a coarse x–y sample motion of several millimeters while still routinely producing atomic resolution images. A set of electronics elements and a datasystem which have been developed to control the microscope are also discussed. The performance of the microscope is illustrated by presenting images of evaporated gold and highly oriented pyrolytic graphite surfaces.
Hydrogen adsorption on some a-Ge1−xCx:H films prepared by radio-frequency sputtering
Authors: Shinar Ruth
Published: July 2002

Auger and electron energy-loss spectroscopy (EELS) were used to study the interaction of hydrogen atoms with clean, Ar+ sputter-etched amorphous germanium (a-Ge), hydrogenated amorphous germanium (a-Ge:H), and some hydrogenated amorphous germanium carbide (a-Ge1−xCx:H) films prepared by rf sputtering. The EEL spectra indicate preferential sputtering of hydrogen bonded to germanium by ion etching in a-Ge:H as well as in a-Ge0.9C0.1:H and the creation of new surface Ge–H bonds following exposure to atomic hydrogen. No such information could be obtained from EELS studies for films containing 25 at. % or more carbon. However, Auger line-shape measurements of Ge and C in a-Ge1−xCx:H, before and after exposure to dissociated hydrogen, yield the same conclusions for x≤0.5. There is no evidence of preferential removal of hydrogen bonded to carbon by Ar+ etching or of formation of new C–H surface bonds in the alloy films by hydrogen adsorption. The preferential removal of hydrogen bonded to Ge but not to C may explain the relatively high density of bulk C–H bonds in rf sputter-deposited a-Ge1−xCx:H films.
Magnetic field effects on cylindrical magnetron reactive ion etching of Si/SiO2 in CF4 and CF4/H2 plasmas
Authors: Yeom Geun Young , Kushner Mark J.
Published: July 2002

Etching of Si and SiO2 has been investigated as a function of magnetic field strength in a radio-frequency cylindrical magnetron discharge using CF4 and CF4/H2 gas mixtures. Si and SiO2 etch rates increased with increasing magnetic field strength by a factor as large as 5 at a given power density. Futher increases in magnetic field strength, though, decreased etch rates in both CF4 and CF4/H2 plasmas. The density of CF, CF2, and F radicals, inferred from optical emission intensities, increased continuously as a function of magnetic field strength. Ion densities, measured by electrical probes, increased by a factor of ≊15 as the magnetic field strength was increased from 0 to 250 G in a CF4 discharge (0.45 W/cm2). However, the self-bias voltage on the powered electrode decreased rapidly with increasing magnetic field, from ∼1 kV at 0 G to almost 0 V at 250 G. The maximum etch rates occurred at an intermediate magnetic field strength where the self-bias voltage was between 25 and 50 V, which corresponds to conditions which maximize both the effectiveness of ion bombardment and ion density. With these conditions, the polymer thickness formed on the Si was a minimum and C/F ratio on the surface was a maximum. Highly anisotropic etching was also obtained near these conditions.
Coverage dependence of photoemission core levels of alkali-metal overlayers
Authors: Sham T. K. , Shek M.-L. , Xu G.-Q. , Hrbek J.
Published: July 2002

Photoemission from alkali-metal core levels (Li 1s, Na 2s, K 3p, Cs 5p, and 4d) has been studied for different alkali-metal coverages on Ru(001) using synchrotron radiation from the Vacuum Ultraviolet (VUV) ring at the National Synchrotron Light Source (NSLS). The alkali atoms interacting directly with the Ru substrate show coverage-dependent binding-energy shifts towards the Fermi level, with the largest shift for Li (1.1 eV) and the smallest ones for K and Cs. At coverages > 1 monolayer (ML), the core-level shift between the second (surface) and the first (interface) layer is observed in all alkali overlayers. At ‘‘thin’’ multilayer coverages (∼3 ML), all alkali overlayers exhibit three sets of core-level photoemission bands which are assigned to the interface, the immediate ‘‘bulk,’’ and the surface in increasing binding energy. At coverages >5 ML (‘‘thick’’ multilayer) all alkali metals exhibit bulklike properties and surface–atom core-level binding-energy shifts are observed in all cases.
Laser Raman and laser ablation Fourier transform mass spectroscopy studies of chemically etched polyethylene
Authors: Siperko L. M. , Creasy W. R. , Brenna J. T.
Published: July 2002

Raman studies show that the chemical etching of low-density polyethylene (LDPE) films by chromium trioxide in sulfuric acid at 87 °C produces carbonyl groups (acid functionalities) within the film. A complexed chromium sulfato species, alkenes, and aldehyde functionalities are present on the etched surface. Etching by chromium trioxide in water under similar conditions produces bulk alkene and –C–O– groups (alcohols, peroxides) but no carbonyls. A chromium trioxide species is present on the surface, as are alkene and alkoxy groups. A chromium oxide species detected within the sulfuric acid etched films is not present in water etched films. The depth profile of chromium species within the chromium trioxide/sulfuric acid etched films detected by laser ablation/mass spectrometry shows the maximum concentration of this species lies several micrometers beneath the LDPE film surface, but for the chromium trioxide/water etch, chromium is present only on the surface of the film.
Use of a partial local density of states calculation to characterize the Auger electron Si-L2,3 V
V transitions of thin oxide layers
Authors: Sasse A. G. B. M. , Wormeester H. , van der Hoef M. A. , Keim E. G. , van Silfhout A.
Published: July 2002

The line shape of the Si-L2,3 VV Auger spectrum is to a first approximation equal to the sum of the convolution products of the partial local density of states (pLDOS), each weighted by the two electron Auger matrix elements and the escape depth. Semiempirical quantum chemical cluster calculations have been used to calculate the pLDOS of SiOx (x=0, 0.5, 1, 1.5, 2) from which the line shape in the derivative mode [dN(E)/dE] could be obtained by using the angular momentum selection rules of Feibelman et al., neglecting the structure in the radial two electron Auger matrix elements. Within this approximation we were able to interpret peaks in the measured Si-L2,3 VV Auger line shape of the initial oxidized silicon surfaces (100, 111, 110) and the fully oxidized Si(100) in the derivative mode in terms of local chemical bonding.
The Versailles Project on Advanced Materials and Standards (VAMAS) project on ion-implanted reference materials for surface analysis: September 1988
Authors: Gries W. H.
Published: July 2002

The unconventional approach followed for the Versailles Project on Advanced Materials and Standards (VAMAS) project on ion-implanted reference materials for surface analytical characterization of semiconductor materials is sketched and its current status is reviewed.
An evaluation of the capabilities of photon and electron spectroscopies for nondestructive quantitative measurements on analyte depth profiles by the angle-resolved signal ratio method
Authors: Gries W. H.
Published: July 2002

The angle-resolved signal ratio (AR/SR) technique of nondestructive measurement on shallow depth profiles of analytes in flat and smooth-surfaced samples has been developed earlier for use in quantitative x-ray fluorescence spectroscopy (XFS). The principles of the method have since been successfully applied also in electron beam microanalysis and x-ray photoelectron spectroscopy. The application in Auger electron spectroscopy is under study. The AR/SR technique can provide quantitative information on the type of depth profile, the centroid depth, the cumulative amount of analyte, and sometimes also on symmetrical profile broadening (by diffusion). The last three items of information are investigated for their dependence on measuring geometry as well as excitation function and/or attenuation coefficients of exciting and excited radiation. General conclusions are drawn from sample calculations for selected cases investigated by AR/SR/XFS.
Nucleation of new solid phases from chemical interactions at an interface
Authors: d’Heurle F. M.
Published: July 2002

Starting from the classical theory of nucleation as applied to such transitions as liquid–solid or liquid–gas in the vicinity of an equilibrium temperature it is shown that the same theory applies to the chemical formation of a new solid phase at the interface between two interacting phases, one of which is a solid. Proceeding from the oxidation of metals, where at constant pressure there exists a finite temperature where the metal, its oxide, and oxygen are in equilibrium, one considers next eutectoid reactions which also occur in the vicinity of an equilibrium point. The common feature of these nucleation-controlled reactions is that the driving free energy change is small. It is then finally shown that this is the condition necessary for nucleation to be rate controlling, even in the absence of any equilibrium point.
Room-temperature study of cobalt metal growth on Ge(111)
Authors: Smith G. A. , Luo L. , Hashimoto Shin , Gibson W. M. , Lewis N.
Published: July 2002

The room-temperature growth of Co on Ge(111) has been studied by MeV ion channeling and Auger electron spectroscopy in an ultrahigh vacuum environment. It has been found that even at room temperature, Co reacts with the Ge substrate and effectively displaces ∼3 monolayers of Ge atoms from their original lattice positions forming a possible interfacial layer of CoGe. Further deposition of Co resulted in a continuous and uniform metal overlayer. These room-temperature results are very similar to those found for Co on Si(111). After annealing a sample of 35 Å of Co on Ge at 600 °C for 1 h, ion channeling and transmission electron microscopy data showed the annealed film to consist of small crystalline islands with a stoichiometry of Co5Ge7. The similarities and differences between Co on Ge and Co on Si are also discussed.
The interaction of ultraviolet excimer laser light with sodium trisilicate
Authors: Eschbach P. A. , Dickinson J. T. , Langford S. C. , Pederson L. R.
Published: July 2002

We describe the results of irradiating sodium trisilicate glass with 248-nm excimer laser light at fluences from 1 to 5 J/cm2. A threshold for the onset of etching occurs at 3 J/cm2. We investigate the changes in surface topography as a function of laser fluence. We also identify the ionic and neutral species contained in the emission plume. A clear correlation is observed between the etching threshold and (i) onset of emission of fast excited neutrals, as well as (ii) the appearance of atomic Na D resonance radiation. The high velocities (10–15 km/s) of the excited neutrals and ions (H+, Na+, and Si+) are attributed to laser/plume interactions. The character and origin of lower velocity neutral species (atomic O, Si and molecular NaO) are presented. The possible role of surface fracture in the onset of etching is also discussed.
Scanning tunneling microscopy imaging of uncoated biological material
Authors: Jericho M. H. , Blackford B. L. , Dahn D. C. , Frame C. , Maclean D.
Published: July 2002

The scanning tunneling microscopy (STM) imaging of biological structures such as protein or cell membranes without metal coating has had limited success. A major difficulty is the mechanical interaction between needle and sample, which often results in a sideways displacement of, or damage to, the biological material during imaging. We recently reported a new scanning mode for the STM (referred to as hopping) in which the needle is periodically withdrawn from the surface. The buildup of lateral stress is thereby prevented and material that is weakly bonded to a substrate can be imaged. Using the new STM scanning technique we have obtained images of uncoated, large fragments of the cell wall sheath of the archaebacterium Methanospirillum hungatei (Mh). The sheath thickness was found to be 80 Å±10% which agrees with previous STM results on metal coated complete Mh sheaths. High resolution images taken on top of the sheaths show parallel rows, or corrugations, having a dominant lateral period of 60 Å±10% and heights of ≲4 Å. The minimum expected period of 30 Å was also present but the two-dimensional crystalline structure expected from diffraction studies was not seen in the STM images. We also report the first STM images of bare pepsin molecules and find the overall dimensions (∼70×40×25 Å) to be in good agreement with those expected from x-ray diffraction studies. The STM images also show reproducible structure of 10–20 Å lateral size on the surface of the pepsin molecules which has a resemblance to the x-ray crystallographically determined shape. For comparison, we also present normally scanned STM images of carbon coated pepsin molecules which show a complicated surface structure strongly dependent on the tunneling tip bias.
Gas-phase evolution and convective heat transfer in closed-cell vapor transport of organic thin films in microgravity and unit gravity
Authors: Debe M. K. , Poirier R. J. , Cook E. L. , Miller L. R. , Spiering M. S. , Floeder S. P.
Published: July 2002

The completed first phase of the analyses of a series of physical vapor transport experiments conducted both in low earth orbit on the Space Shuttle Orbiter and in the laboratory is presented. The experiments are the first to use physical vapor transport as a process to grow oriented organic thin films on epitaxially active substrates sealed within demountable ampoules. It was found that the films grown in the microgravity environment were smoother, optically more homogeneous and denser than their unit gravity grown counterparts, and contained predominantly a new polymorph of copper phthalocyanine (CuPc). In this report the gas-phase evolution within the ampoules and resulting convective effects on the substrate temperature have been characterized for four microgravity processed cells and over twenty ground control experiments. The nitrogen-equivalent total pressures and partial pressures of nine common gases are measured within the processed ampoules and the relationships determined empirically among the gas pressures, compositions, and the orientation of the ampoule relative to the earth’s gravitational field.

Significant gas evolution amounting to several times the original backfilled buffer gas is found in all the ampoules, and attributed to hydrogen evolution from stainless-steel components and the thermal decomposition of as little as 0.06% of the starting source material. The amounts and compositions are found to be very consistent among the different experiments, leading to the conclusion that this is not a significant variable in the comparisons of microstructure differences observed between the microgravity grown and laboratory grown films. It is observed that the substrate temperature is sensitive not only to the ampoule gas pressure and composition, but also to the ampoule orientation and its external gas environment during the thermal processing. It is concluded that enhanced convective heat transfer to the substrate occurs not only with the hotter end of the ampoule below the cooler end but also when the hotter end is above the cooler end under unit gravity conditions.

Finally, estimates of the thermophysical parameters are calculated for a representative ampoule gas composition, including the mixture kinetic viscosity, thermal conductivity, heat capacity, binary diffusivity of the CuPc, and the effective vapor pressure of the CuPc above the substrate.
Real-time i
n s
i
t
u studies of metal electrodeposition with scanning tunneling microscopy
Authors: Robinson R. S.
Published: July 2002

Scanning tunnelng microscopy was used to observe the deposition of silver from electrolytic solution under controlled-current electrochemical conditions onto a smooth graphite substrate. The silver was deposited at 20 μA cm−2 and the growth of the deposits which initially formed was monitored in situ with scanning tunneling microscope (STM) over a scan area of 460×460 nm. The deposits showed various two- and three-dimensional morphologies and growth rates after nucleation. The apparent diffusion of deposit during the STM scan may be ascribed to interactions between the deposit and the scanning tip. The influence of the tip must be carefully considered when interpreting the images, particularly with inhomogeneous systems where the binding strength between the materials being studied may be overcome by interactive forces between the tip and sample.
Force microscope with capacitive displacement detection
Authors: Göddenhenrich T. , Lemke H. , Hartmann U. , Heiden C.
Published: July 2002

We developed a force microscope using a capacitively controlled lever displacement. Both mechanical construction and electronics are simple and lead to a very compact device. Different measurement modes of the microscope are described. In the constant-capacitance mode the observed capacitance between tip and sample is in the order of femtofarad. In the force mode, various tip-sample interactions are investigated under ambient air. The obtained force resolution is comparable to that of laser-controlled atomic force microscopy. As an application, the magnetic force signal between the tip and the local surface–microfield configuration of a bit structure in a magnetic recording medium was imaged.
Tunneling tips imaged by scanning tunneling microscopy
Authors: Möller R. , Esslinger A. , Rauscher M.
Published: July 2002

Electrochemically etched tips commonly used for scanning tunneling microscopy (STM) have been investigated by STM itself. By appropriate positioning, the topography of the apex of the tips could be investigated, leading to a typical value for the tip radius of 300 nm. Different techniques have been attempted to untangle the convoluted STM image. By rotating one tip relative to the other, or by exchanging one tip and leaving the other in place, some distinct features can be attributed to the one or the other tip. Furthermore, the profile of the tip could be investigated by scanning over the edge of a cleaved crystal. Beside the fundamental interest in the tip structure itself, this configuration offers the unique feature of unambiguous relocation of the scanned area when the tip investigated in that experiment has been removed and reinstalled to the microscope.
Evaluation of the stopping depth of nonradiative recombination centers in Al0.5Ga0.5As by Ar+ ion beam sputtering by photoluminescence measurements
Authors: Akita K. , Taneya M. , Sugimoto Y. , Hidaka H. , Tajima M.
Published: July 2002

The stopping depth of nonradiative recombination centers in Al0.5 Ga0.5 As by Ar+ ion beam sputtering was evaluated by photoluminescence (PL) measurements using Al0.5 Ga0.5 As/GaAs single heterostructures. Al0.5 Ga0.5 As layers were sputtered by an Ar+ ion beam with energies from 0.5 to 3 keV at temperatures from 0 to 300 °C. The PL intensity of the underlying GaAs was measured to detect the damage (nonradiative recombination center). The thickness of the residual Al0.5 Ga0.5 As layer at which the PL intensity of the GaAs began to fall was taken as the stopping depth of nonradiative recombination centers. This depth increased as Ar+ ion energy increased and sputtering temperature decreased. The depth of damage was roughly proportional to the Ar+ ion energy E at about 25 °C for E less than 3 keV.
Electron cyclotron resonance plasmas excited by rectangular and circular microwave modes
Authors: Popov Oleg A.
Published: July 2002

Microwave plasmas were excited and studied in cylindrical chambers with inner diameters of 5–15 cm in the presence of the static axial magnetic fields of 800–1200 G. Rectangular (TE10) and circular (TM01) modes of electromagnetic waves at a frequency of 2.45 GHz were introduced into the plasma chamber via a dielectric vacuum window. It was found that a transition to the high level of microwave power absorption occurred when the plasma density reached a value of Ncr =7.4×1010 cm−3 and magnetic field was about 875 G. Hysteresis in the dependences of reflected microwave power, Pr, versus incident microwave power, Pi, and gas pressure were observed. Two ‘‘resonant’’ magnetic fields which provided the highest plasma densities in electron cyclotron resonance (ECR) plasma were found: 875 and 920–930 G. Both column- and donut-type high density (Ne ∼1012 cm−3) ECR plasmas were achieved in 15 cm diam chamber utilizing rectangular TE10 and circular TM01 modes.
Toward understanding photoemission in K+CO coadsorption systems
Authors: Schultz Peter A.
Published: July 2002

Alkali-induced changes in electronic energy levels, observed in photoemission from CO on transition-metal surfaces, are examined theoretically and shown to be consistent with the formation of two-dimensional ionic islands in the coadsorbed system. A point-charge model is applied to the K+CO system to calculate the levels. The analysis reveals that significant screening of the CO levels, by the extra electron in the K-modified anion, is masked by an equally large electrostatic stabilization of the electrons on the CO site by the Madelung potential (∼8 eV) of the K+–CO− layer. The model explains the splitting of the 1π-derived level, by ∼1 eV, without requiring any symmetry lowering of the molecular geometry. The results are in reasonable agreement with experimental observations and compare favorably with more sophisticated calculations.
Kinetics of physical adsorption of ethane on Ir(110)-(1×2): Molecular beam reflectivity measurements and Monte Carlo simulations
Authors: Kang H. C. , Mullins C. B. , Weinberg W. H.
Published: July 2002

Experimental results, obtained using a reflectivity method, for the probability of physical adsorption of ethane on the Ir(110)-(1×2) surface are presented. We analyze these results using Monte Carlo simulations and show that physical adsorption can occur either directly or through a precursor state in which an ethane molecule is trapped in a second layer above a first layer of physically adsorbed ethane. From the Monte Carlo simulations, we are able to establish that the energy barrier for desorption of an ethane molecule from the precursor state is ∼4.5 kcal/mol. We also find that the energy barrier for diffusion of an ethane molecule on top of a monolayer of physically adsorbed ethane on the Ir(110) surface is ∼3.7 kcal/mol.
Outgassing characteristics and microstructure of an electropolished stainless steel surface
Authors: Yoshimura Nagamitsu , Sato Tomoshige , Adachi Sachiko , Kanazawa Toshiyuki
Published: July 2002

Outgassing characteristics of an electropolished stainless steel (304) pipe wall were investigated by an isolation method. The free outgassing rate after an in situ bakeout (150 °C, 20 h) was estimated as low as 1.6×10−12 Pa ℓ s−1cm−2. After the in situ bakeout, H2 molecules were steadily evolved from the pipe wall, whereas most of CO, C, CH4, and CO2 molecules were emitted from the operating mass spectrometer and Bayard–Alpert gauge with incandescent filaments. Auger depth profile analysis revealed that the oxide layer of an electropolished surface was cleaner, thinner, and finer in microstructure than that of a buff-polished surface. This is the reason why an electropolished surface showed a very low outgassing rate.
Material dependence of electron inelastic mean free paths at low energies
Authors: Tanuma S. , Powell C. J. , Penn D. R.
Published: July 2002

We present and discuss electron inelastic mean free path (IMFP) data for aluminum and gold in the 50–200 eV range. These elements serve as examples of IMFP calculations that have been made for 50–2000 eV electrons in 31 materials (27 elements and 4 compounds). Substantial differences are found in the shapes of the IMFP versus energy curves for Al and Au and these can be understood in terms of the different inelastic scattering mechanisms in the two metals. The minimum IMFP value occurs at 40 eV in aluminum and at 120 eV in gold, a result which is consistent with the trends expected from free-electron IMFP calculations. This result differs, however, from that expected from the Seah and Dench attenuation length formula which shows essentially no material dependence at low energies. We have extended a general formula derived earlier to describe the calculated IMFPs over the 200–2000 eV energy range to give the IMFP dependences on material and energy from 50 to 2000 eV.
The influence of radio frequency sputter variables on the composition of YBa2Cu3O7 films
Authors: Klein J. D. , Yen A.
Published: July 2002

The compositions and thicknesses of Y–Ba–Cu–O films radio frequency (rf) sputtered from a nonstoichiometric oxide target were determined as a function of deposition conditions. Variations in chamber pressure, rf power, and substrate position were observed to shift the as-deposited cation composition along a single line in the Y–Ba–Cu ternary composition diagram. Deposition rates of up to 24 nm/min were achieved with good cation stoichiometry. Films deposited on YSZ substrates at the optimized sputter conditions produced a zero resistance transition temperature of 88.7 K after a post deposition helium/oxygen heat treatment. Scanning electron microscopy (SEM) and x-ray examination of the films revealed a strongly oriented large-grain polycrystalline microstructure.
Field ion-scanning tunneling microscopy of alkali metal adsorption on the Si(100) surface
Authors: Hashizume T. , Hasegawa Y. , Kamiya I. , Ide T. , Sumita I. , Hyodo S. , Sakurai T. , Tochihara H. , Kubota M. , Murata Y.
Published: July 2002

We have constructed a scanning tunneling microscope (STM) equipped with a field ion microscope (FIM), by which one can monitor and shape the STM probe tip on an atomic scale in situ in the STM chamber. Taking advantage of a well-defined tip prepared by a FIM, we have greatly improved the stability and reproducibility of the performance of the STM. Li and K adsorption on the Si(001) 2×1 surface has been investigated by field ion-scanning tunneling microscopy (FI-STM). The STM images have shown that at the initial stage of adsorption, Li (K) atoms (1) adsorb on top of one of the dimer-forming Si surface atoms and (2) stabilize the asymmetric (buckled) dimerization, and (3) form linear chains, perpendicular to the substrate 2×1 dimer rows. Our observations suggest that alkali metal adsorption on the Si(001) 2×1 surface may be significantly different from the conclusions of earlier reports.
Electron cyclotron resonance plasma chemical vapor deposition of large area uniform silicon nitride films
Authors: Shapoval S. Y. , Petrashov V. T. , Popov O. A. , Yoder M. D. , Maciel P. D. , Lok C. K. C.
Published: July 2002

Electron cyclotron resonance plasma (f=2.45 GHz, microwave power P=200–800 W) generated in a radially uniform magnetic field (B=875–1000 G) was used to produce a large area (15–20 cm diam) uniform plasma stream at 20–30 cm from the source output. Low temperature (70–300 °C) silicon nitride films with a thickness of 800–3000 Å were deposited on 5–20 cm diameter wafers with deposition rates of 100–350 Å/min. Film thickness uniformity was ±1% for 7.6–10.0 cm diam wafers, ±3% for 15 cm diam wafers, and ±9% for 20.0 cm diam wafers. It was found that the film deposition rate Wg increased linearly with the silane flow rate, while Wg increased slower than power 1/2 with the nitrogen flow rate. The film refractive index was 1.9–2.0 at a silane/nitrogen flow rate ratio of 0.40–0.6. The effects of plasma density and its profile on the film growth rate and uniformity are discussed.
Cryopump vacuum recovery after pumping Ar and H2
Authors: Longsworth R. C. , Webber R. J.
Published: July 2002

For many vacuum processes the rate of recovery to a specific base pressure after gas injection is used as a criterion for determining the cleanliness of the vacuum system. When a cryopump is used, a long recovery time does not necessarily mean that undesirable gas species are present, because the recovery time of the cryopump itself increases as it becomes loaded with gases. This paper reports on experimental measurements of recovery time for APD-8 Cryopumps after flowing Ar and H2 at different rates for extended periods of time. Tests were run to measure the effect of using a new supercarbon versus standard charcoal and to see the effect of the cryotrapping of H2 by Ar without the use of charcoal. The recovery time after injecting Ar correlates with the time for the surface temperature of the solid Ar to return to base temperature. The recovery time for H2 is a function of the amount of H2 that has been pumped, cryopanel temperature, type of charcoal, gas flow rate, H2 concentration, and sequence of gas injection.