bluuebunny/crossref_metadata_2025 huggingface dataset:
Preliminary results of a Te/Si heterostructure
Authors: Shih I. , Despins C. , Champness C. H. , Gundjian A. A.
Published: July 2002

Polycrystalline tellurium films have been vacuum deposited on (111) n-type Si substrates. The surface morphology of the deposited films was found to be strongly dependent on the substrate temperature. When the substrate temperature was increased from room value to 300 °C, the grain size of the Te crystallites increased to about 5×10 μm2 with the long axis lying approximately parallel to the surface of the substrate. Preliminary studies on the electrical properties of the Te–Si heterostructure show rectifying characteristics. When cooled to liquid nitrogen temperature, the structure shows a photovoltaic effect under illumination in the wavelength range between 1 and 4 μm.
Gas flow measurements of a novel geometry for neutral beam neutralizers
Authors: Pirkle D. R. , Conrad J. R.
Published: July 2002

The conductance and line density of a novel geometry (pumped geometry) for neutral beam neutralizers were measured and compared with a conventional (passive) neutralizer geometry in the transition flow regime. A passive neutralizer geometry is merely a duct attached to the ion source. For the pumped geometry, the top and bottom surfaces of the duct are replaced by a venetian blind geometry which opens into ballast vacuum pumping volumes. A one-half scale model with pumped neutralizer geometry was built and compared with a passive neutralizer geometry of comparable dimensions. A gas source was maintained at fixed pressure and the conductance and line density of the two geometries were measured. These measurements were repeated for several values of gas source pressure, covering the range of pressures typically used in neutral beam ion sources. With the vanes on the pumped neutralizer geometry opened to 55 deg and the gas source pressure ‘‘low,’’ i.e., in the range of pressures typical for ‘‘bucket-type’’ magnetic field ion sources, the amount of gas flowing from the exit of the pumped geometry was about 3 times less than the amount flowing from the passive geometry. As the source gas pressure was increased, the ratio of gas flow from the exit of the passive geometry to that of the pumped geometry increased also. At gas source pressures typical of magnetic field free ion sources, up to 5 times less gas flowed from the pumped geometry. On the other hand, the neutral gas line density of the pumped geometry was only 1.6 times less than that of the passive geometry, independent of the source gas pressure.
An evaporation system for the preparation of ternary compounds
Authors: Webb R. J. , Goldman A. M.
Published: July 2002

A computer-controlled high vacuum evaporation system has been constructed for the fabrication of ternary compounds such as the Chevrel phase materials. The latter are formed on a substrate held at high temperatures with the constituent elements being deposited from some combination of electron beam sources, resistively heated Langmuir sources, and Knudsen sources. Unique features of the system include the techniques employed to handle sulfur in a high vacuum environment, and the monitoring and control system used to ensure the formation of stoichiometric compounds with a high degree of compositional uniformity.
The effects of hydrogenation on the properties of ion beam sputter deposited amorphous carbon
Authors: Jansen F. , Machonkin M. , Kaplan S. , Hark S.
Published: July 2002

The incorporation of hydrogen in ion beam sputtered carbon profoundly affects the physical properties of the amorphous film. Optical absorption, electrical conductivity, and ESR measurements suggest that incorporated hydrogen efficiently passifies the dangling bonds and saturates the graphitic bonding to some extent. By single ion beam sputter deposition, amorphous carbon films containing up to 35 at. % of hydrogen were obtained. This material acts as a narrow band gap (1.2 eV) semiconductor with a relatively low density of states but has an abundance of unsaturated sp2 bonding. In fact, 13C-NMR measurements on this material show that the sp2/sp3 bonding ratio is about 1.5. High density and extreme hardness are properties of ion beam deposited carbon films which contain little or no hydrogen. However, the physical properties of such amorphous carbon which, due to its unique mechanical properties is sometimes referred to as amorphous diamond, are largely dominated by the unsaturated graphitic bonding.
A new tool for studying epitaxy and interfaces: The XPS searchlight effect
Authors: Egelhoff W. F.
Published: July 2002

Very recently, a phenomenon long known in angle-resolved x-ray photoelectron spectroscopy (XPS) has been reinterpreted. The new interpretation is that XPS peak intensities are enhanced at angles corresponding to axes connecting the photoemitting atom to its immediate neighboring atoms. These enhanced intensities thus identify the bond axes present near the surface. In this paper, examples are presented of the great power of this ‘‘XPS searchlight’’ effect as a new tool for studying epitaxy and interfaces.
The deuterium kinetic isotope effect in the decomposition of methanol on Ni(110)
Authors: Richter Lee J. , Ho W.
Published: July 2002

Utilizing time-resolved electron energy loss spectroscopy (TREELS) the decompositions of methoxy and methoxy-d3 on Ni(110) have been studied for coverages near 0.16 monolayer. The kinetic parameters for the decomposition of CH3O are: εa(kcal mol−1) =17.0±1.6, log ν(s−1)=12.3±1.4 and for the decomposition of CD3O are: εa(kcal mol−1) =18.8±1.2, log ν(s−1)=13.1±1.0. The value of the isotopic rate ratio kH/kD at 270 K is 5.1±0.8. This is interpreted within a simple activated complex theory as indicative of C–H bending character in the reaction coordinate.
Surface and interface properties of Hg1−xMnxSe
Authors: Franciosi A. , Reifenberger R. , Furdyna J. K.
Published: July 2002

Synchrotron radiation photoemission studies of Hg1−xMnxSe single crystals for x≤0.15 show the cleaved semiconductor surfaces to be stable in ultrahigh vacuum with no loss of Hg or change in surface stoichiometry oberved with time. Comparison with results for HgSe samples points out a well defined Mn-derived emission feature centered 4.1 eV below the Fermi level EF with a full width at half maximum (FWHM) of 1.1 eV. Analysis of the photoemission cross section demonstrates that the corresponding states have dominant Mn 3d character and are therefore responsible for the semiconductor magnetotransport and magneto-optical properties. Preliminary studies of Hg1−xMnxSe–Al interface formation show that the semiconductor surface is depleted of both Hg and Mn upon metal deposition, while an Al–Se reacted phase is established as the true microscopic interface at the Hg1−xMnxSe–Al junction.
Kinetics and mechanism of HgCdTe oxidation by ac impedance
Authors: Talasek R. T. , Ohlson M. J. , Syllaios A. J.
Published: July 2002

The kinetics and mechanism of anodic oxidation of (Hg,Cd)Te in 0.1 N KOH in 90% ethylene glycol/ 10% water were explored by ac impedance. The oxidation rate was found to be independent of KOH concentration in the absence of mass transport effects. The mechanism was found to be of a general ECE (electron transfer, chemical, electron transfer) type, with a switch from mass transport to charge transfer control caused by potential dependent migration of hydroxide ions to the surface of the electrode.
Surface chemistry of phosphorus-containing molecules: I. Interaction of PH3 with Rh(100) and the effect of preadsorbed phosphorus
Authors: Hegde R. I. , Tobin J. , White J. M.
Published: July 2002

The interaction of PH3 with Rh(100) has been investigated using temperature-programmed desorption spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, and work function change measurements. A mobile precursor state is involved in the adsorption kinetics at 100 K. The overlayer is saturated after an exposure of 3 L, at which point the work function has decreased by 1.2 eV, indicating adsorption through the phosphorus atoms. Heating the crystal above 100 K desorbs molecular PH3 and H2, and leaves adsorbed phosphorus atoms on the surface. The PH3 thermal desorption spectra show a coverage-dependent adsorption energy associated with repulsive lateral interactions between the adsorbed molecules. There are two thermal desorption states of hydrogen, one of which is characteristic of hydrogen on clean Rh(100). Preadsorbed phosphorus atoms partially passivate the surface, preventing PH3 decomposition until higher temperatures are reached. Preadsorbed phosphorus also reduces the capacity of the surface for H2 adsorption, but does not alter the activation energy for H2 desorption.
Simplified variably shaped beam for electron beam lithography
Authors: Nakasuji M. , Kuniyoshi K. , Takigawa T. , Wada H.
Published: July 2002

It is shown analytically that a shaped beam has 1.8 times higher beam current available than a Gaussian beam for a single pixel exposure system, when spherical aberration is dominant. A stable, small size, and low cost electron optical column is proposed. Critical illumination is adopted and beam size is varied by two shaping apertures, which are close to each other, and four deflectors which are positioned symmetrically with respect to the shaping apertures. The critical illumination condition is maintained by a condenser lens which is positioned between the LaB6 electron gun and shaping aperture. By tilting 45° between the beam side and raster scan direction, pattern edge roughness, including the 45° oblique pattern, may be zero at optimum dose and process conditions.
Calibration stability of some hot-cathode ion gauges
Authors: Warshawsky I.
Published: July 2002

Data are presented on the stability of calibration of some hot-cathode ion gauges: a conventional triode used in the range of 0.0002–0.2 Pa, and Bayard-Alpert-type gauges used in the range of 0.0002–0.02 Pa. Numerical magnitudes are given of the systematic corrections that must be applied to the gauge indication in order to achieve the highest accuracy of pressure indication. The relative sensitivities of these gauges to Ar, N2, Ne, and He are also given.
Effect of gas species and partial pressure on chromium thin film adhesion
Authors: Torkington R. S. , Vaughan J. G.
Published: July 2002

Vacuum evaporated chromium thin films on glass substrates were evaluated for adhesion by the scratch test as a function of partial pressures of Ar, H2, CH4, O2, CO, and CO2 deliberately introduced during chromium deposition. Partial pressures examined ranged from 0.67 to 13.3 mPa. Initial adhesion results suggest a reactive evaporation process of chromium with O2, CO, and CO2 backfills and are in agreement with the theory of an oxide layer formation causing increased initial adhesion. Gas/chromium atom impingement ratios corresponding to noticeably improved adhesion are reported. Time dependent adhesion results reveal a distinction in adhesion behavior between films deposited in O2, CO, and CO2 backfills and those deposited in Ar, CH4, and H2 backfills. Possible mechanisms for this behavior are discussed.
Plasma deposited hydrogenated carbon on GaAs and InP
Authors: Warner J. D. , Pouch J. J. , Alterovitz S. A. , Liu D. C. , Lanford W. A.
Published: July 2002

The properties of ‘‘diamondlike’’ carbon films grown by glow discharge 30 kHz plasma using methane are reported. The C ls XPS line shape of films showed localized hybrid carbon bonds ranging from as low as 40% to as high as 97% percent of the total carbon bonds. Infrared spectroscopy and N15 nuclear reaction profiling data indicated 35% to 42% hydrogen, depending inversely on deposition temperature. The deposition rate of films on Si falls off exponentially with substrate temperature, and nucleation does not occur above 200 °C on GaAs and InP. Optical data of the films showed band gap values of 2.0 to 2.4 eV increasing monotonically with methane flow rate.
P2O5–SiO2 film deposition by reactive sputtering in phosphorus vapor
Authors: Serikawa T. , Okamoto A.
Published: July 2002

P2O5–SiO2 (PSG) films were deposited from a nondoped silicon target by reactive sputtering in an oxygen–argon mixture added to phosphorus vapor. The phosphorus vapor was generated by heating a crucible containing solid red phosphorus in the sputtering system. PSG films with phosphorus concentration of up to 3×1021 cm−3 were successfully deposited at low substrate temperature. Phosphorus concentration in the films could be controlled by not only the crucible temperature but also the sputtering gas pressure.
Structural properties of evaporated silicon films
Authors: Esquivias I. , Sanz-Maudes J. , Sangrador J. , Rodríguez T.
Published: July 2002

Amorphous and polycrystalline silicon films have been deposited by vacuum evaporation onto different types of substrates, held at temperatures varying between 300 and 800 °C. Film structure has been characterized by x-ray diffraction and scanning electron microscopy. The transition temperature between the amorphous and crystalline state is found to be around 400 °C. Polycrystalline films grow preferentially in the 〈110〉 direction, with a strong increment of the (220) peak near 800 °C. Two different microstructures have been observed: (a) A microcolumnar growth in amorphous and polycrystalline films deposited onto titanium-coated stainless steel, fused silica or monocrystalline silicon. The diameter of the columns ranges between 0.1 and 0.3 μm and does not depend on the substrate temperature. However, the structure is found to be more ordered as substrate temperature increases. (b) A nodular type growth is found in films deposited onto aluminium-coated stainless steel. The film is composed of tapered grains (0.5–10 μm size). Each grain is composed of a bundle of smaller columns, quite similar to the former microstructure. The origin of these structures can be explained by using the structural zone models: the microcolumnar growth is produced by the limited self-diffusion of adatoms, while the nodular growth is a consequence of the existence of preferential nucleation centers and a self-shadowing process.
Hydrazine reduction of transition metal oxides: I
n s
i
t
u characterization using x-ray photoelectron spectroscopy
Authors: Littrell D. M. , Tatarchuk B. J.
Published: July 2002

Among the first row transition metal oxides from V2O5 to ZnO, CuO was found to be the most easily reduced by hydrazine. CuO surfaces exposed to hydrazine at ∼10−6 Torr could be reduced to the metallic state at room temperature. Reduction kinetics, continuously measured at the surface using in situ x-ray photoelectron spectroscopy (XPS) at hydrazine pressures of 6×10−3 Torr and temperatures from 295 to 381 K, indicated that the reaction was first order with respect to CuO. The kinetics of this reaction, as monitored by the rate of change of the shakeup feature which accompanies the CuO spectrum at 943 eV, proceeded with a measured activation energy of 35±4 kJ/mol and a preexponential factor of 103.3±0.7/s. In situ XPS observations of reduced copper surfaces at temperatures of 295 K indicated that two types of adsorption occurred: (i) an irreversible component which could not be removed by extended vacuum pumping and provided a N:Cu atomic ratio of 0.28 and (ii) a reversible component which increased the measured N:Cu ratio to ∼0.60 at hydrazine pressures up to 0.5 Torr. Gas phase and adsorbed species were readily discernible based on the values of their observed N 1s photoemission peaks at 406 and 400 eV, respectively. While the pressure of hydrazine over metallic copper could significantly increase the measured N:Cu ratio, the N ls spectrum could not be used to distinguish between the various adsorbed components which yielded only a single peak spectrum at a binding energy of ∼400 eV. This value is in agreement with that found by previous investigators (M.H. Matloob and M. W. Roberts, J. Chem. Soc. Faraday Trans. 1 1977, 1393) who suggested that hydrazine was molecularly adsorbed at these conditions.
The interaction of CO, NO, and O2 with sodium-promoted Rh(100) surfaces
Authors: Höchst H. , Colavita E.
Published: July 2002

The promotor effect of Na on the dissociative adsorption of CO, NO, and O2 at 300 K on Rh(100) has been studied by work function measurements and photoemission spectroscopy using synchrotron radiation in the photon energy range hν=15–35 eV. Preadsorption of Na lowers the work function up to −4.2 eV. Below a critical Na concentration, the work function increases again by ∼0.9 eV by coadsorption of CO, NO, and O2. Compared to clean Rh(100), the saturation exposure of the dissociative adsorption of NO and O2 is about ten times greater for Na/Rh(100). The promotional effect of Na is less pronounced for the coadsorption of CO, which shows only fractional dissociation at 300 K.
State-selective ionization detection of neutral NO following electron-stimulated desorption
Authors: Burns A. R.
Published: July 2002

A tunable UV laser is used to selectively ionize neutral NO molecules that are in specific rotational and vibrational states following electron-stimulated desorption (ESD) from NO-covered Ni and Pt surfaces. An analysis of the results supports previous studies that reported a shallow valence-level mechanism for ESD neutrals.
The effect of alkali ion migration on the adhesion of sputtered chromium metallizations to glass
Authors: Baird R. J. , Haeberle R. J.
Published: July 2002

Alkali ion migration has been identified as a mechanism for the loss of adhesion of magnetron sputtered, chromium metallizations to glass. The onset of this mechanism of adhesion loss has been correlated with the power dissipated during the magnetron sputter deposition. It is proposed that the combination of substrate heating and electric fields produced during magnetron sputter deposition leads to alkali–ion migration to the chromium–glass interface, which affects the film adhesion. The concentration of alkali at the interface was measured by x-ray photoelectron spectroscopy. It was found that for 7740 Pyrex glass, the concentration of sodium at the interface was greatly increased over that at the substrate surface prior to sputter deposition. As a further test of this mechanism of adhesion loss, the adhesion of chromium films to glass was modified by the application of static electric fields to heated sputter coated specimens. Again, sodium was found to accumulate at the interface of poorly adhering films.
Space technology today
Authors: Cohen Aaron
Published: July 2002

Technologies developed in support of our civil space program have progressed considerably over the 25-year life of the National Aeronautics and Space Administration. Many of these advances are the result of a continuing increase in knowledge relative to the environment of space and the utilization of these previously designated hostile elements to gain design application advantages. Much more significant utilization of the environmental elements is forecast as space research and commercial goals and objectives begin to gain momentum. Challenges in the design of our Shuttle System and the next major goal, the Space Station, are significant and will result in greater utilization of the microgravity and vacuum of space as a benefit to research and operational goals.
Models for the oxidation of silicon
Authors: Lewis E. A. , Irene E. A.
Published: July 2002

Since the 1960’s many adaptations to the linear parabolic model for silicon oxidation have been proposed. For the purposes of process engineering, curve fitting procedures which are sometimes devoid of physical content but numerically precise are employed. However, a truly unified physical model which is in quantitative accord with all known facts is still lacking. In this review we will discuss the ‘‘facts’’ and newer models.
A quick temperature cycling sample holder for ultrahigh vacuum surface studies
Authors: O’Grady W. E. , Melo A. V. , Hoffman R. W. , Chottiner G. S.
Published: July 2002

An ultra−high vacuum compatible sample holder has been designed with copper gasketed flanges that allows quick cycling between low and high temperatures (77 K−1300 K). (AIP)
Adsorption of atomic oxygen on the Si(110)5×1 surface via interaction with N2O
Authors: Keim E. G. , van Silfhout A. , Wolterbeek L.
Published: July 2002

This paper describes the interaction of a clean Si(110) surface, showing a prominent 5×1 superstructure, with N2O at 300 K in UHV. Differential reflectometry in the photon energy range 1.5–4.5 eV, Auger electron spectroscopy, and low-energy electron diffraction have been used to monitor this solid–gas reaction. Chemisorption of atomic oxygen (as released by decomposition of N2O occurs via an immobile precursor state in the initial adsorption stage in which the N2O molecule is weakly bound at the surface. Every stage of the reaction involves oxygen attachment to the dangling bonds, the initial sticking probability being (6.0±0.1)×10−6. This reaction terminates at 0.42±0.06 ML oxygen coverage, thereby leaving part of the dangling bonds unreacted. This phenomenon can be explained from the nature of the dangling bonds at the Si(110) surface: these hybrids point in off-normal directions thus creating a barrier against effective decomposition of the N2O molecule at higher oxygen coverages. Additional exposure to O2 results in a passivating oxygen adlayer at 0.71±0.11 ML coverage. The optical spectrum indicates that the Si(110)5×1 surface probably has several types of (dangling bond) surface states.
Potassium promotion of carbon reactivity: 104-fold increase of oxidation rate in O2
Authors: Sjövall P. , Hellsing B. , Keck K.-E. , Kasemo B.
Published: July 2002

We have investigated, experimentally and theoretically, the oxidation rate of UHV evaporated carbon films with added surface deposits of potassium. The oxidation rate in O2 increases faster than linearly with increasing K coverage. A monolayer of K increases the rate by a factor ∼104, in comparison with pure carbon. A theoretical calculation assumes that O2 dissociation is the rate limiting step. Based on a charge transfer mechanism, this calculation reproduces the order of magnitude of the rate increase.
Dislocations in HgCdTe/CdTe and HgCdTe/CdZnTe heterojunctions
Authors: Yoshikawa M. , Maruyama K. , Saito T. , Maekawa T. , Takigawa H.
Published: July 2002

Behavior of misfit and threading dislocations in HgCdTe/CdTe and HgCdTe/CdZnTe heterojunctions has been investigated by the dislocation-pit etching technique. It is confirmed that by lattice matching the linear pit density caused by misfit dislocations on (1̄10) cleaved plane was decreased to 500 cm−1, which is about 1% of that obtained using CdTe substrates. It has been found that the density of the threading dislocations is not decreased by lattice matching, and nearly coincides with the dislocation density of the substrate.
Analysis of junction depths and lattice point defect interdiffusion coefficients in Hg0.8Cd0.2Te
Authors: Harman Theodore C.
Published: July 2002

A model for the interdiffusion of lattice defects in Hg0.8Cd0.2Te is proposed and applied to experimental results. It is assumed that the electrical properties of state-of-the-art nominally undoped Hg0.8Cd0.2Te are determined by native defects. The model has been used to derive an equation for the junction depth obtained when a Te-rich sample is diffused under Hg-saturation conditions at low temperatures. Calculations using the equation are found to agree with experimental data for junction depth versus excess Te concentration, diffusion time, and temperature.
Surface electronic behavior of face-centered-cubic iron on copper
Authors: Fernando Gayanath W. , Lee Y. C. , Montano Pedro A. , Cooper Bernard R. , Moog E. R. , Naik H. M. , Bader S. D.
Published: July 2002

We have studied the electronic structure of fcc iron grown epitaxially on (100) copper with ultraviolet photoemission (UPS), surface magneto-optic Kerr effect (SMOKE), low-energy electron diffraction (LEED), and self-consistent film linearized muffin-tin-orbital (FLMTO) calculations. This study identifies and explains novel characteristic UPS features of fcc iron and a substantial change in the work function compared to bcc iron. The SMOKE measurements show that fcc iron grown on copper has two metastable states lying close in energy. One of these is not ferromagnetic and occurs at room temperature, while the other is ferromagnetic and occurs at 190 °C. As shown by LEED, the ferromagnetic state has an expanded surface interplanar lattice; and UPS shows exchange splittings in that state.
A new type of vacuum pump for corrosive gases and vapors
Authors: Cole Michael
Published: July 2002

A vacuum pump is described based on a novel gas pumping principle. A rotating drum is partially filled with a liquid, which is made to flow round the outside of the drum, as in a liquid ring pump, leaving the center clear. A probe is held in the rotating liquid and is shaped to create a low-pressure region on its surface where there is a hole which connects, through the static hub of the drum, to the vessel to be evacuated. The center of the drum is connected, through suitably placed holes in the central hub, to atmosphere. Gas is drawn from the vessel thought the hole in the probe into the circulating liquid. Centripetal force acts on the gas bubbles in the liquid ring to propel them towards the center where they burst at the gas/liquid interface. The gas then escapes to atmosphere through the outlet hole system. Small experimental pumps have achieved vacua better than 0.1 mbar and speeds of 10 m3/h. Higher vacua and faster pumping speeds are thought to be attainable. There are no rubbing components inside the pump and there is, therefore, no need for the working liquid to be a lubricant. Also, choice of materials in contact with the possibly aggressive gases is not restricted by the need for rubbing compatibility. A corrosion-resistant laboratory pump is described with a 3 m3/h pumping speed and a 0.5 mbar ultimate vacuum.
Large ultrahigh vacuum joints
Authors: Wikberg T. , Poncet A.
Published: July 2002

The antiproton accumulator at CERN comprises a 160-m-long UHV system with apertures up to 1100 mm. The design parameters of this machine called for an average pressure <10−10 Torr employing bakeout temperatures up to 350 °C. Prior to its construction several sealing techniques were examined with a view to their space requirements, ease of mounting, reliability, and cost. These studies included different types of demountable, as well as in situ, welded joints. Particular constraints were the limited space available for the joints and the lack of freedom with respect to axial rotation of the components to be joined. The solution adopted consists essentially of demountable joints of the Wheeler type (Varian trademark), but with some special improvements. During six years about 100 such joints have been successfully in use at CERN on various machines operating at pressures down to 2×10−12 Torr.
A systematic approach for selecting pumping technology: Ultrahigh vacuum application
Authors: Shapira Yoram , Arbel Ami
Published: July 2002

An analytic model of the selection of vacuum pumping technology is presented, which identifies selection issues directly contributing to or hindering the pumping system performance. These two groups of issues, entitled benefit and cost, respectively, are analyzed through a decision-making methodology called the analytic hierarchy process. Application of this methodology to the benefit and cost issues results in a priority structure leading to a systematic selection of the best choice out of a given set of candidate systems. Results of this evaluation are given for a test case of ultrahigh vacuum.
Heteroepitaxy of Ge on (100) Si substrates
Authors: Baribeau J. M. , Jackman T. E. , Maigné P. , Houghton D. C. , Denhoff M. W.
Published: July 2002

The epitaxial growth of GaAs on Si is a subject of growing interest because of the possibility of integrating high-speed and optical devices in Si-based technology. The large lattice mismatch between Si and GaAs makes the growth of high-quality epilayers very difficult. However, Ge and GaAs have similar lattice parameters and thermal expansion coefficients and thus a promising way to improve epitaxial growth of GaAs on Si is to introduce an intermediate Ge buffer layer. This technique will only be effective when the structural defects formed at the Si–Ge interface can be prevented from propagating into GaAs. A series of Ge epilayers have been grown on (100) Si by molecular-beam epitaxy and several methods to reduce dislocation propagation have been examined. These include postgrowth annealing, graded layering, growth interruption, and/or superlattice buffers. The efficiency of these methods was assessed by transmission electron microscopy, Rutherford backscattering/channeling, and double-crystal x-ray diffraction. It was found that the threading dislocation density tends to decrease with increasing thickness. The graded layering technique gave poor results. The introduction of a superlattice buffer reduced the threading dislocation density; however, the best overall quality material was obtained by depositing pure Ge on Si at 600 °C and annealing 30 min at 700 °C.
Properties of nitrogen implanted nickel foils
Authors: Ehni P. D. , Unertl W. N.
Published: July 2002

Surface nitride layers about 90 nm thick have been formed on high-purity, polycrystalline Ni foils by ion implantation under ultrahigh-vacuum conditions. Fluences were 5.5–8.0×1017 N+2/ cm2 at 80 keV. The substrate was free of impurities as judged by Auger electron spectrosopy (AES). AES depth profiles were used to determine the depth distribution in as-implanted and annealed samples. As-implanted N has a two-peaked profile with maxima at the surface and at about 45 nm. Annealing at T <250 °C increases the surface N concentration, decreases the concentration between the maxima, decreases the 45-nm maxima, and broadens it toward larger depths. Small amounts of C on the surface at the start of implantation significantly alter the N depth distribution. These effects are consistent with terms of recent model calculations.
Computational and experimental studies on the implosion processes of laser fusion targets
Authors: Ido S. , Takabe H. , Azechi H. , Izawa Y. , Yamanaka C.
Published: July 2002

This paper describes the study of implosion processes of glass microballoon targets irradiated directly and uniformly by the GEKKO XII glass laser system. In this direct driven mode using a high-aspect-ratio target, high neutron yields were achieved. Comparison between experiment and the computer simulation using one-dimensional hydrodynamic codes shows good agreement. Progress of direct driven implosion shows the possibility of achieving breakeven by using the 100 kJ class laser of 0.35 μm wavelength.
X-ray microscopy of inertial fusion targets using a laser-produced plasma as an x-ray source
Authors: Kim Hyo-gun , Wittman Mark D.
Published: July 2002

With the advent of an ablation layer coating technology for mechanically nonsupported inertial fusion targets, the conventional optical interferometric technique for the characterization of these targets is no longer feasible since the batch process of this bounce coating technique makes it difficult to account for individual targets. A soft x-ray contact microradiography method, which was developed for this purpose, is limited in resolution by the 0.5-μm grain size of the photographic medium used. Utilization of an x-ray sensitive photoresist extends the resolution to as low as 50 Å. The use of a laser-generated x-ray source for x-ray microscopy appears to require much less energy than published photoresist sensitivity data. A single-shot exposure of 100 J, 0.527-μm wavelength, and 1-ns laser shot suffices for image formation. The resulting x-ray micrographs show a higher spatial resolution than the images obtained by previous radiography techniques. The resolution of 0.2 μm in the present study is due to the finite size of the x-ray source.
Microstructural aspects of vacuum annealing-induced solute depletion in sputter-deposited 304 stainless-steel films
Authors: Fabis P. M.
Published: July 2002

Type 304 stainless-steel (SS) films were dc-planar-magnetron sputter deposited onto substrates maintained at 77 K. The as-deposited ferromagnetic films of bcc symmetry possessed equiaxed and columnar grain morphologies in the size range of 10–20 nm. These nanocrystalline films were found to be compositionally homogeneous and to be identical in composition to the sputtering target. Interest in protective coating application for these films stimulated a thermal stability investigation of their microstructure. Following vacuum annealing at 1146 K at 1.33×10−3 Pa for 15 min, analytical electron microscopy was performed on cross-sectional specimens revealing (1) a significant Cr depletion from 18.1 wt. % at 775 nm to 4.4 wt. % at 20-nm depth from the film surface, (2) a slight Ni depletion from 8.9 to 8.1 wt. % over the depth (from surface) increment of 75 to 20 nm, and (3) a near-surface bcc–fcc phase transformation and associated grain size increases. Theoretical weight loss and surface Cr concentration calculations did not take into account the microstructural effects on the solute depletion profile and, therefore, were not in good agreement with the experimental analyses conducted under identical exposure conditions. Stereological analyses revealed that the nanocrystalline microstructure contained approximately 1017 interfaces/cm3 which comprised over 106 cm2/cm3 (≊100 m2/cm3) of interfacial material. The grain size–grain morphology relationship was quantified and results indicated it to be complex; very small multifaceted grains comprising a film which possessed a large volume fraction of interfacial material. Calculations of grain matrix (lattice) and interfacial (grain boundary) Cr flux have shown the interfacial component to be significantly greater than the flux emerging from grain matrix regions. The disordered characteristics of the interfacial material relaxed the requirements for transport along, and evaporation from, these interfacial regions thereby significantly enhancing the Cr loss from these films.
Numerical modeling of vacuum systems using electronic circuit analysis tools
Authors: Wilson Scott R.
Published: July 2002

The analysis of large and complex high-vacuum systems is a tedious process, especially when distributed loads and pumps are involved. It has been suggested that by using the appropriate transformations, a vacuum system can be modeled as an electrical network. Well-developed numerical techniques exist for the computational characterization of electrical networks, often implemented in the form of a general purpose circuit simulation code. In this paper, a method is presented for the modeling of high-vacuum systems using a circuit analysis code. The method includes the capability to model outgassing, and to obtain the response of a system to transient and time-varying loads. An example is given of the analysis of a vacuum system with distributed pumps and distributed loads using the commonly available spice ii program. The use of this technique is expected to significantly reduce the effort required to analyze such systems as accelerators, storage rings, and process lines.
Process monitoring of a-C:H plasma deposition
Authors: Wild Ch. , Wagner J. , Koidl P.
Published: July 2002

The deposition process of amorphous hydrogenated carbon (a-C:H) in a 13.56 MHz hydrocarbon glow discharge system is investigated. Mass spectrometry was used to measure the relative abundance of the different ionic species. While there is a strong dependence of the mass spectra on the type of hydrocarbon gas (e.g., benzene, hexane, methane), the film properties are independent of the precursor. Spatially resolved spectroscopy of the optical emission was applied to study the interaction between energetic ions and substrate surface. Strong emission from small fragments, such as C, C2, and CH in the vicinity of the negatively self-biased cathode shows the importance of impact-induced fragmentation and explains the independence of film properties of the hydrocarbon process gas. With an electrically heated Langmuir probe we measured the potential, the density and the electron temperature of the plasma. In situ determination of the properties of the growing film, such as refractive index, thickness, absorption, and optical gap, was achieved by recording the reflectivity of the substrate during deposition.
Morphological and structural features of Cu seed cones
Authors: Morishita S. , Fujimoto Y. , Okuyama F.
Published: July 2002

Morphologies and structures of surface cones formed on polycrystalline Cu targets seeded with Mo were surveyed by means of high-resolution electron microscopy. The sputtering of the target with a focused Ar+-ion beam generated Cu cones having diversified configurations and crystalline states, obviously reflecting an intricate mechanism of cone evolution. Some of the cones observed were characterized by an amorphouslike structure, commonly grown at the apex areas, and Mo was never detected from coned areas. These findings strongly suggest that the role of Mo seeds was to nucleate cones, and that the direct supply of sputtered Cu atoms to the cone nucleation sites interplayed with the ion-etching process so as to develop the nucleated cones.
Pumping mechanism for N2 gas in a triode ion pump with A1100 aluminum cathode
Authors: Liu Y. C. , Lin C. C. , Lee S. F. , Chen J. R. , Su C. S.
Published: July 2002

The pumping speeds of a triode ion pump with an A1100 aluminum cathode were measured for N2, O2, H2, and Ar gases. In addition, results from a pumping lifetime test, a gettering test, x-ray diffraction analysis, and Auger electron spectroscopy surface analysis were studied to understand the pumping mechanism for N2 gas in the pump. It was shown that the interaction between aluminum and nitrogen with sufficient energy might occur in the ion pump. This kind of interaction would probably be the major pumping mechanism for nitrogen in an ion pump with an aluminum cathode.
An efficient algorithm for the simulation of hyperthermal energy ion scattering
Authors: Goodstein David M. , Langer Stephen A. , Cooper B. H.
Published: July 2002

The use of ion scattering spectroscopy to study surface properties often requires sophisticated computer simulations of the scattering process. Their feasibility is constrained by the tremendous number of scattering events that contribute to a spectrum. This paper presents a method for minimizing the number of trajectories that must be computed to achieve a given energy and angular resolution in an ion scattering simulation. The method is independent of how ion trajectories are calculated, and can therefore be used with existing simulation algorithms. The method is compared with a Monte Carlo algorithm for a simulation of hyperthermal K+ scattering from Cu(100).
Atomic and electronic structure of tetrahedrally coordinated compound semiconductor interfaces
Authors: Duke C. B.
Published: July 2002

The most extensively specified atomic interface geometries for tetrahedrally coordinated compound semiconductors are those of the nonpolar (110) cleavage surfaces of zinc-blende structure semiconductors, specifically AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, and CdTe. These geometries have been determined experimentally by a variety of techniques, low-energy electron diffraction intensity analysis and ion scattering, in particular. They have been predicted by several quantum chemical methods, including extended tight-binding, pseudopotential, and ab initio models. All of the structures are known to be qualitatively identical: a bond-length-conserving rotation in the top layer followed by a relaxation of this layer toward the substrate and possibly small counter rotations in deeper layers. The atomic rotation in the top layer is driven by a rehybridization of the surface chemical bonds to redistribute the charge in the dangling bonds of the unrelaxed surface into the remaining surface and back bonds. The magnitudes of the surface atomic relaxations scale linearly with the bulk lattice constants. Atomic geometries also are available for a few overlayer structures on these surfaces, i.e., p(1×1) structures of Al on GaAs and of Sb on GaAs and InP. An area of current research is the extension of such studies to the cleavage faces, (101̄0) and (112̄0), of wurtzite-structure compound semiconductors. Theoretical predictions are available for ZnO, ZnS, ZnSe, CdS, and CdSe.

Experimental structure analyses have been reported only for the (101̄0) surfaces of ZnO and CdSe. The relaxed geometries of these surfaces are analogous to those of zinc-blende (110), and are driven by the same mechanism of dangling-bond charge redistribution. Another active research area is the study of the polar surfaces of zinc-blende structure materials. These surfaces exhibit complex reconstructions which depend both on the surface stoichiometry and on the sample preparation procedure. Structure analyses have been reported for GaAs(111)-(2×2), InSb(111)-(2×2), GaP(111)-(2×2), GaAs(1̄1̄1̄)-(2×2), InP(1̄1̄1̄)-(1×1), and GaAs (100)-(2×4), as well as for GaAs(311) surfaces. These reconstructions also are driven by rehybridization of the surface chemical bonds, but the precise nature of this rehybridization differs from one structure to another. The electronic structures of all three classes of interface have been studied by a combination of photoemission spectroscopy and quantum chemical calculations. The interface states are dominated by the surface-relaxation-induced rehybridization and/or consequences of interface chemical reactions. In those cases for which the atomic geometries are known independently, the qualitative features of the observed interface-state eigenvalue spectra are well reproduced by the model calculations, although the models disagree both among themselves and with the measurements on several details.
The deposition rate dependence of plasma polymerization on the radius of curvature of the substrate
Authors: Lintz H. K. , Murrell S. R. , Crawley R. L. , Daukas J. C.
Published: July 2002

In the plasma polymerization deposition of CHx onto microspherical inertial fusion targets we have found that the coating rate varies with substrate diameter. Spherical shells ranging in diameter from 100 to 1000 μ were coated with several microns of plasma polymerized trans-2-butene under identical conditions and their coating thicknesses measured interferometrically. We found that the coating rate variation with shell radius could be described by an equation having the form dr/dt=A/(R+r)+B, where dr/dt is the coating rate, R the initial sphere radius, r the coating thickness, and A and B parameters related to the plasma conditions. We also found that there exists a functional relationship between the parameters A and B which is dependent on the rf power density. A model has been developed to show how this observed rate dependence relates to the buildup of an electrical charge on the nonconducting substrate surface. The electric field which arises is dependent on the radius of curvature of the substrate and in turn affects the contribution to the film growth by the ionized species impinging upon the surface. Neutral species arriving at the surface are not affected and thus lead to the B term in the empirical form of the rate dependence.
Voltage-dependent scanning tunneling microscopy imaging of semiconductor surfaces
Authors: Stroscio Joseph A. , Feenstra R. M. , Newns D. M. , Fein A. P.
Published: July 2002

We present voltage-dependent scanning tunneling microscopy (STM) data from various semiconductor surfaces. Different techniques for acquiring the data are demonstrated, and the interpretation of spectroscopic data is discussed. For the Si(111)2×1, GaAs(110), and O/GaAs(110) surfaces, we find that the voltage dependence of constant-current contours provides the most direct method for understanding the electronic and geometric structure of the surface. For the case of oxygen on n-type GaAs, we observe a reversal in the constant-current contours when the polarity of the bias voltage is reversed, which is interpreted in terms of band bending due to a negatively charged adsorbate. Theoretical calculations for a screened Coulomb potential are shown to agree with the observed reversal in the STM contours of the adsorbed oxygen.
Direct writing with the scanning tunneling microscope
Authors: Ehrichs E. E. , Silver R. M. , de Lozanne A. L.
Published: July 2002

A scanning tunneling microscope has been used to write metallic features directly on metallic and semiconducting surfaces without further process steps. Organometallic gases were used to obtain features as small as 20 nm.
Imaging deoxyribose nucleic acid molecules on a metal surface under water by scanning tunneling microscopy
Authors: Lindsay S. M. , Barris B.
Published: July 2002

We have investigated the mechanism of a scanning tunneling microscope operated in air and water by measuring current–distance (It–s) characteristics and making spatial maps of dIt/ds, for Pt and W tips and gold substrates. Although the It–s curves are characteristic of tunneling, their interpretation in terms of work functions would require values between a few tenths of and a few thousandths of the vacuum values. These values do not depend on the metals or the tunneling medium. We conclude that the tip and substrate are always in contact via an insulating adsorbate layer. We prepare flat, clean gold surfaces onto which we can form stable aggregates of DNA molecules under a buffer solution in a small electrochemical cell by a cycle of plating followed by stripping. The aggregates are imaged both as depressions in a constant-current topology map, and dramatic changes on a dIt/ds map. 20-Å corrugations are clearly visible in dense aggregates of high molecular weight DNA which appear to pack with liquid–crystalline order. On occasion the path of an individual molecule can be followed.
Design of a scanning tunneling microscope for biological applications
Authors: Guckenberger R. , Kösslinger C. , Baumeister W.
Published: July 2002

A scanning tunneling microscope (STM) designed for biological application and operating at ambient pressure is described. It is combined with a light microscope, monitoring the specimen through a transparent support, which is used to search for specimen areas of interest. Glass or mica coated with a thin conductive layer (e.g., carbon) are used as a specimen support. The STM itself, is essentially a Besocke-type STM; modifications include a convenient and reliable mechanism for sample approach to the tip (z adjustment).
The oxygen Auger line in ZnO and MgO
Authors: Weisz S. Z. , Resto O. , Goldstein Y. , Yaron G. , Many A.
Published: July 2002

High-resolution measurements reveal a double-peak structure in the oxygen KLL main line of ZnO which is absent in MgO. We suggest that part of this fine structure is due to contributions from interatomic transitions and, as such, corresponds to two maxima in the valence-band density of states of ZnO.
Ellipsometric studies of sputtered Au–TiNx thin films
Authors: Luthier R. , Lévy F.
Published: July 2002

Sputtered Au–TiNx thin films were deposited from two composite powder targets of Au and TiN with 4.4 and 8.5 mol % TiN, respectively. The composition of the sputtering atmosphere Ar–N2 was varied with the N2 partial pressure ranging from 0 to 2×10−1 Pa. The influence of the reactive gas on the optical properties of the sputtered films has been investigated by means of differential reflectivity measurements and polarization modulation ellipsometry, in the photon energy range 1.5–5 eV. The observed properties are qualitatively interpreted on the basis of a composite microstructure: a dilute Au:Ti alloy acts as the host phase containing a small fraction of segregated TiNx grains, of typical dimension lower than 2.5 nm. The concentration of the matrix alloy (Au:Ti) was deduced from the measured dielectric function on the basis of the virtual-bound-state model, which applies to transition-metal impurities in noble-metal hosts. The influence of the dispersed refractory phase has then been examined within a scheme of mean-field theory.