bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of semiconductor surface states and metal–semiconductor interfaces
Authors: Louie Steven G. , Chelikowsky James R. , Cohen Marvin L.
Published: July 2002

The electronic structure of vacuum–semiconductor and metal–semiconductor interfaces has been studied using a method involving self-consistent pseudopotentials. Our model for an intimate metal–semiconductor interface consists of jellium in contact with a semiconductor described in the pseudopotential formalism. Local density of states and charge densities are used to analyze the electronic properties of the two types of interfaces. For the vacuum–semiconductor case, Si (111) surfaces and the (110) surfaces of III–V and II–VI zincblende semiconductors are investigated, with GaAs and ZnSe considered as zincblende prototypes. For metal–semiconductor interfaces, jellium of Al density in contact with (111) surfaces of Si and the (110) surfaces of GaAs, ZnSe, and ZnS are investigated. The calculated Schottky barrier heights are in good agreement with experimental results. It is shown that the variations in the experimental barrier height for different metals in contact with various semiconductors can be understood in terms of a simple model using the result of the pseudopotential calculations. The relation of our results to other theories and current experiments is discussed.
On the transmission probability for molecular gas flow through a tube
Authors: van Essen D. , Heerens W. Chr.
Published: July 2002

Starting with the fundamental theory of Knudsen about molecular gas flow through orifices and especially through circular cylindrical tubes, several investigators have derived formulas for the calculation of the transmission probability. Here the equations of Knudsen, Dushman, Clausing, De Marcus, Berman, and, very recently, Nawyn and Meyer are compared. For very accurate calculations the calculation method of Nawyn and Meyer must be used. If accuracies of 0.1% are adequate, the simple equations of Berman can be used.
Some thoughts on the existence of empty surface states and the effect of surface order on sorption
Authors: Mark Peter , So Edward , Bonn Matthew
Published: July 2002

Experimental and computational information on the existence of empty surface bands in the upper portion of the band gap on nonpolar compound semiconductor surfaces is reviewed and it is shown that agreement is poor. However, both sources of information have reached the point where meaningful agreement can be achieved, thus presenting one of the first cases of quantitative experimental and theoretical self-consistency of the electronic properties of semiconductor surfaces. The dominant influence of surface atomic order on sorption at free nonpolar surfaces of compound semiconductors is also reviewed and it is suggested that both experimenters and theorists must incorporate this order-dependence into their work before a fundamental understanding of sorption can be achieved.
Characteristics of ion-beam-sputtered thin films
Authors: Kane S. M. , Ahn K. Y.
Published: July 2002

Thin films of Au, Si, Al, NiFe, Ta, and W have been prepared by ion beam sputtering, and physical properties have been compared with films deposited by rf sputtering, and electron beam evaporation. In general, films of a given material appeared to be comparable for the properties measured, independent of deposition method. In the ion beam sputtering system, the effects of deposition rates are studied as a function of the accelerating voltage of the ion beam, and of the angle created by the target tilt and the path of the ion beam. Deposition rates generally leveled off at an accelerating voltage of 750 V; and for several materials, a peak in deposition rate was observed at a target-beam angle of 30°. The magnetic properties of ion-beam-sputtered NiFe films are compared before and after annealing in vacuum at 350°C for 30 minutes. Annealing produced a small drop in the resistivity of these films, with a corresponding increase in the magnetoresistive effect, Δρ/ρ. Electron microprobe measurements indicate no significant incorporation of ionizing gas in the films. Ion-beam-sputtered thin films appear to be as good as films deposited by the other techniques discussed, and have shown a distinct advantage for use in liftoff processes. Since ion beam deposition occurs in a field-free region, heating effects due to electron bombardment of the substrate are minimal. Photoresist images are not degraded, and subsequent lift-off is facilitated.
Reactive ion etching of silicon
Authors: Schwartz G. C. , Schaible P. M.
Published: July 2002

Reactive ion etching of silicon substrates in a plasma containing chlorinated species does not result in undercut of a permanent mask. When the silicon is very highly doped it behaves as a different material and undercut has been observed. This phenomenon will be discussed. For use in chlorinated plasmas, there is a choice of nonerodible masks that sputter slowly but will not be redeposited on the substrate surface. Both CCl4/Ar and Cl2/Ar plasmas will be described. The variation in etch rate of silicon with rf power, frequency, reactant concentration, reactant flow rate, gas presure, crystal orientation, and batch size will be presented. The possibilities of polymer formation and surface roughening will be discussed.
Steam-enhanced calcium segregation in dense alumina
Authors: Sinharoy S. , Levenson L. L. , Day D. E.
Published: July 2002

Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) have been used to study surface segregation of calcium in dense alumina. Thin (∠1 mm) disks ?99% alumina were exposed to saturated steam at 5.3×106 Pa or 750 psia (266 °C) for periods ranging from 1 to 48 days. SEM examination of the surface after exposure revealed growth of calcium- and silicon-rich structures at and near the surface. Auger depth profiles of the exposed specimens showed that the amount of calcium at the surface increased with increasing exposure time of up to 12 days. For longer exposures, the calcium-rich area began to extend from the surface into the bulk.
Schottky and Bardeen limits for Schottky barriers
Authors: Cohen Marvin L.
Published: July 2002

An estimate for the minimum (Bardeen limit) and maximum (Schottky limit) of the interface index, S, is calculated. It is shown that current measurements on high S materials are near or at the maximum value for S.
Chemical mechanisms of Schottky barrier formation
Authors: Brillson Leonard J.
Published: July 2002

Ultrahigh vacuum studies of reactive and unreactive metals on a wide range of semiconductors reveal new systematics of Schottky barrier formation. Surface work function, band bending, and chemical bonding measurements indicate several qualitatively different mechanisms of barrier formation, each determined by the degree of interface chemical reactivity. In general, the Schottky barrier formation can be characterized by a twofold process—local charge redistribution plus surface space charge transfer. The specific interface bonding determines the sign and magnitude of the local charge redistribution. Correlations between these interface phenomena and deviations from ideal barrier heights suggest a framework based on interface bonding for determining Schottky barrier heights.
Resolution and sensitivity dependence on the molecular weight distribution of a photoresist
Authors: Gong B. M. , Ye Y. D. , Gu M. Q. , Zhang Q. B.
Published: July 2002

It was found that the molecular weight distribution of a negative photoresist is closely related to the photosensitivity, resolution, and etching resistance. When the molecular weight distribution was narrowed down a fairly small range, corresponding to a dispersivity of 1.6 and an average degree of polymerization over 2100, photoresist of high resolution and good corrosion resistivity can be obtained. Thus, reagent grade polyvinyl alcohol, and its once and twice fractionated products were tested for their dispersivity and degree of polymerization, giving the figures: 2.0, 1750; 1.74, 1990; 2126, respectively. Examination of the properties of the corresponding polyvinyl cinnamates showed that the smaller the dispersivity and the greater the average degree of polymerization, the higher the resolution and resistivity were. Cinnamate, a negative resist, when produced from twice fractionated polyvinyl alcohol, could resolve lines 1.5 μm wide under UV exposure and 0.5 μm wide under electron beam exposure, at a resist thickness of 6000 Å. This would provide practical means for lithography in the submicron region. Positive photoresist could be made in a like manner with good results.
Chemical state of ion-implanted nitrogen in Fe18Cr8Ni steel
Authors: Singer I. L. , Murday J. S.
Published: July 2002

Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS), in conjunction with ion milling, were used to determine the chemical state of nitrogen implanted at relatively high doses (1016–1018/cm2) into type-304 steel. Nitrogen atoms were bonded in the nitrided state, at all doses, according to both AES and XPS. As the nitrogen concentration increased, the lineshape of the Cr MVV Auger spectra evolved from that of metallic Cr to Cr nitride; at highest doses, the Fe MVV lineshape could be identified as Fe nitride. Auger and XPS spectra of implanted 304 and thermally nitrided 304 were virtually identical at comparable nitrogen concentrations. The Cr(2p3/2) binding energies of both, however, were about 1 eV lower than found in Cr nitride but 0.5 eV higher than in metallic Cr.
Ambient effects on the out-diffusion of GaAs through thin gold films
Authors: Chang C.-A. , Chou N. J.
Published: July 2002

We have studied the ambient effects on the out-diffusion of GaAs through thin gold films. Auger spectroscopic measurement shows accumulation of both Ga and As on the gold surface upon annealing in N2. The accumulation, however, is reduced in the presence of CO. This suppressed out-diffusion by CO is in agreement with the surface potential model recently proposed.
A cleaning procedure for inertial fusion targets
Authors: Farnum E.H. , Gutacker A. R. , Mulford R.
Published: July 2002

We describe a method for cleaning microshells used for inertial fusion targets. Six stages of aqueous cleaning use Reagent grade water and commerical detergents in ultrasonically agitated baths. Further treatment with ultraviolet light and ozone gas (UV/ozone) is used to remove all traces of organic contaminant. Glass microshells cleaned with this technique show no surface contamination larger than 0.1μm as analyzed by scanning electron microscopy.
Analysis of low-energy electron diffraction intensities from ZnS(110)
Authors: Duke C. B. , Meyer R. J. , Paton A. , Kahn A. , Carelli J. , Yeh J. L.
Published: July 2002

Preliminary results for the atomic geometry of the (110) surface of ZnS are reported. The surface structure is determined by comparing dynamical calculations of elastic low-energy electron diffraction (ELEED) intensities with those measured at T = 300 K using a pulsed floodgun technique to suppress the charging of the insulating ZnS sample. In contrast to other tetrahedrally-coordinated, zincblende-structure compound semiconductors for which the (110) surfaces are known to be reconstructed, i.e., GaAs, InSb, InP, and ZnTe, the measured ELEED intensities from ZnS(110) are described acceptably by calculations based on the truncated bulk atomic geometry and minor variants thereof. Measures of the quality of this description of the ELEED intensity data are the x-ray reliability (R) factor, Rx = 0.222, and the Zanazzi–Jona R-factor, RZJ = 0.229. The minimum x-ray R-factor which we have achieved thus far, i.e., Rx = 0.219, is obtained for a puckered surface geometry in which the sulfur species relaxes vertically upward by 0.06 Å and the zinc species vertically upward by 0.02 Å. While we believe that the surface structure can be refined further, the quality of the description of the measured ELEED intensities by atomic geometries in the vicinity of the unreconstructed surfaces is surprisingly good relative to previously-studied zincblende-structure compound semiconductors.
Formation of surface states on the (111) surface of diamond
Authors: Pate B. B. , Stefan P.M. , Binns C. , Jupiter P. J. , Shek M. L. , Lindau I. , Spicer W. E.
Published: July 2002

Experimental studies of the diamond (111) surface are presented. While no intrinsic surface states have been found on the 1×1 surface, our new results in ultraviolet photoemission spectroscopy (15 eV?hν?25 eV) show evidence of a band of surface states on the 2×2/2×1 reconstructed surface. The band of surface states (with an integrated emission on the order of one filled state per surface atom) is centered at 2.5 eV below the Fermi level (1.1 eV below the valence-band maximum). The band width is 1.8 eV FWHM. In the case of the 2×2/2×1 surface structure, the downwards band-bending is greater by 0.75 eV and the electron affinity is greater by 1.4 eV over the respective values for the 1×1 surface. These new studies necessitate a quantitative revision of our band-bending model [J. Vac. Sci. Technol. 17, 1087 (1980)], although no change in basic concept is required. Preliminary photon-stimulated desorption (PSD) results over the photon energy range hν = 18–35 eV find hydrogen on the 1×1 surface. This surface hydrogen (possible in the form of hydroxide) may explain the absence of surface states on the diamond (111) 1×1 surface.
Growth of dislocation-free silicon films by molecular beam epitaxy (MBE)
Authors: Sugiura H. , Yamaguchi M.
Published: July 2002

The dependence of dislocation density on various growth conditions for Si MBE films is investigated. Dislocation density is determined by chemical etching and SEM observation. High temperature heat treatment generates dislocations in substrates, and film dislocation density increases with an increase in the substrate dislocation density. The dislocation generation in the substrate is prevented by using a substrate heater designed not to induce thermal stress. The film dislocation density decreases exponentially with an increase in the preheating time of the substrate. The density is proportional to exp (E/kBT) where T is growth temperature. The value of the activation energy is 3.5 eV. Dislocation-free films can be grown reproducibly at 860 °C on (100)-oriented substrates preheated to 1210 °C for 10–20 min.
Dry development of ion beam exposed PMMA resist
Authors: Adesida I. , Chinn J. D. , Rathbun L. , Wolf E. D.
Published: July 2002

The etching characteristics of polymethyl methacrylate (PMMA) exposed to 28Si+ ions and then dry developed by reactive ion etching in an oxygen plasma have been studied. It is shown that etch rates of resists exposed to doses greater than 1×1015 cm−2 (1.6×10−4 C/cm2) at 40 keV are much smaller than those of unexposed resists. A differential etch rate as high as 11 is demonstrated. This property of ion beam inhibited etching (IBIE) has been used to fabricate resist structures with submicrometer features using ’’see-through’’ thin silicon film masks. With further developments of high brightness ion sources, IBIE may be a useful technique to realize high resolution negative tone images in resists.
Low-temperature oxygen diffusion in alpha titanium characterized by Auger sputter profiling
Authors: Wittberg T. N. , Wolf J. Douglas , Keil R. Gerald , Wang Pu Sen
Published: July 2002

Anodic TiO2 films were grown on titanium foils under galvanostatic conditions at 20 mA/cm2 in an aqueous saturated solution of ammonium tetraborate. The samples were then heat treated at temperatures between 450 and 550 °C and changes in the profile of oxygen concentration as a function of depth were monitored using Auger electron spectroscopy (AES). The oxygen diffusivities were calculated from the Boltzmann–Matano method and compared with those obtained using an analytical solution to Fick’s second law. The activation energy for oxygen diffusion in this temperature range was then calculated to be 28 kcal/mol.
Heterojunction interface formation: Si on Ge, GaAs, and CdS
Authors: Katnani A. D. , Stoffel N. G. , Daniels R. R. , Zhao Te-Xiu , Margaritondo G.
Published: July 2002

We used photoemission spectroscopy with synchrotron radiation to investigate the formation and the microscopic properties of interfaces between semiconducting substrates and Si overlayers deposited by electron bombardment. The main goal of the experiment was to measure the valence band discontinuity in these systems and test the limits of the discontinuity transitivity rule predicted by all ‘‘linear’’ models. This rule could be tested by combining our present data on Si overlayers and previous data on Ge overlayers on Si, GaAs, and CdS. We found that the rule is valid for the above interfaces only within an accuracy of a few tenths of an electron volt. Our results are in agreement with recent evidence that the Harrison LCAO approach, one of the ‘‘linear’’ models, predicts the valence band discontinuities with an accuracy of the order of ∼0.25 eV.
Electron microprobe techniques for routine compositional analysis of (Hg,Cd)Te
Authors: Moore T. M.
Published: July 2002

Specialized data reduction methods have been applied to the quantitative x-ray analysis of (Hg,Cd)Te infrared detector materials in the scanning electron microscope. These methods provide a rapid and reliable characterization tool suitable for routine use in a process control environment. The technique is applicable to the entire composition range and provides submicron spatial resolution for compositional profiling of epitaxial layer cross sections.
Adsorption of water on rhenium studied by XPS
Authors: Schulze P. D. , Shaffer S. L. , Hance R. L. , Utley D. L.
Published: July 2002

The adsorption of D2O on clean polycrystalline rhenium has been studied by x-ray photoelectron spectroscopy. At 305 K, only dissociative adsorption occurs; at 135 K, D2O adsorbs molecularly with the formation of multilayer ice at high converges. The molecular D2O desorbs at 180 K with the formation of an OD complex.
Growth of CdTe films by close-spaced vapor transport
Authors: Anthony Thomas C. , Fahrenbruch Alan L. , Bube Richard H.
Published: July 2002

Based on a detailed investigation of close-spaced vapor transport (CSVT) growth of CdTe, a diffusion-limited transport model is proposed. Excellent quantitative agreement between the experimental data and the diffusion model supports its applicability. Surface temperatures used in the diffusion model are calculated from a one-dimensional thermal analysis of the CSVT system that includes conductive, radiative, and convective heat transfer. Comparison between growths in He and H2 reveals that H2 does not enhance the growth rate by acting as a chemical transport agent, although small deviations from diffusion-limited growth in H2 are attributed to H2Te formation. An equilibrium thermodynamic analysis of the H2, H2Te, Te2, Cd, and CdTe system supports this hypothesis.
Spectroscopic characterization of fluorinated silicon single crystal surfaces
Authors: Shinn N. D. , Morar J. F. , McFeely F. R.
Published: July 2002

Spectroscopic studies of Si(100) fluorinated by the dissociative chemisorption of XeF2 at 80 K are reported. LEED and XPS measurements suggest the formation of a disordered fluorosilyl layer after low fluorine doses. Separate EELS experiments identified a fluorine induced 800 cm−1 loss, suggesting that SiF is the major surface fluorosilyl species. Vibrational evidence for coadsorbed hydrogen, due to side reactions during fluorination, is also found. Recent synchrotron soft x-ray photoemission spectra support the SiF identification and also demonstrate dramatic crystallographic effects in comparative Si(111) studies.
Beam techniques for the analysis of poorly conducting materials
Authors: Werner H. W. , Warmoltz N.
Published: July 2002

Beam techniques for the surface analysis of solids [e.g., electron spectroscopy for chemical analysis (ESCA), Auger electron spectroscopy (AES), analytical electron microscopy (AEM), low energy ion scattering (LEIS), Rutherford backscattering (RBS), and secondary ion mass spectrometry (SIMS)], are based on the interaction of beams of photons, electrons, or ions, respectively, with a solid and the detection of the resulting information-carrying beams of photons, electrons, or ions. With these methods the chemical composition of the solid, across the sample surface and in depth (concentration profile) can be determined. A large number of artifacts may be introduced in any type of sample. In nonideal systems, such as glass and ceramic samples, additional difficulties can be introduced (A) by the poor electrical conductivity: charging of the sample occurs, leading to (a) deterioration and even a complete suppression of the spectra, (b) loss in lateral resolution in the imaging mode, and (c) migration of mobile ions in the sample and (B) by the extreme thin interfaces in polycrystalline materials which require the use of techniques with lateral resolution down to the 5 nm range. Mechanisms which lead to a deterioration of the spectra and procedures to overcome them will be discussed, followed by examples for the successful analysis of glasses and ceramics.
Barrier layers: Principles and applications in microelectronics
Authors: Wittmer Marc
Published: July 2002

The miniaturization of integrated circuit devices requires complex multilevel structures for the fabrication of their electrical contacts. Aluminum is the standard metal used in silicon device technology for low resistance interconnections. However, the low temperature reaction of aluminum with silicon makes aluminium contacts incompatible with shallow junction device processing. Therefore, a barrier layer is needed between the two. The basic function of the barrier layer is to prevent aluminum–silicon intermixing by reducing the mass transport. The realization of this property is discussed from a materials point of view. Various barrier materials are surveyed and compared to the requirements for microelectronics device applications.
The adsorption of oxygen on Pt(100)–(1×1) and hex surfaces at 123 K
Authors: Norton P. R. , Bindner P. E. , Griffiths K.
Published: July 2002

The interaction of O2 with Pt(100)–hex and (1×1) surfaces at 123 K was studied by thermal desorption spectroscopy, x-ray photoemission, and work function techniques. On the hex surface, oxygen is adsorbed entirely as the molecular state with a sticking coefficient of 0.13 and exhibits an activation energy for desorption ∼37 kJ mol−1. The saturation coverage is 0.24×1015 O2 molecules cm−2 and desorption does not lead to any dissociation. On the (1×1) surface, oxygen dissociates, even at 123 K. The sticking coefficient is 0.4, and the saturation coverage is 0.48×1015 O atoms cm−2. On warming to 240 K, a (2×1) LEED pattern is observed that is believed to originate from an oxygen overlayer on an unreconstructed substrate. At ∼360 K, the surface transforms irreversibly to the oxygen stabilized (3×1) reconstructed phase.
Molecular beam distributions from high rate sources
Authors: Jackson S. C. , Baron B. N. , Rocheleau R. E. , Russell T. W. F.
Published: July 2002

A mathematical model that describes the evaporated flux from high rate sources like those used in the deposition of semiconductor films were developed from mass balances with suitable constitutive equations describing the molecular beam distribution. The model was experimentally verified in laboratory scale equipment. Good agreement between the measured distributions and model results were obtained using a single fitting parameter. The fitting parameter is material specific and was correlated to the Knudsen number at a fixed location in the nozzle. A generalized procedure to evaluate this parameter and estimate the incident flux from a molecular beam source is outlined.
The development of photoemission spectroscopy and its application to the study of semiconductor interfaces: Observations on the interplay between basic and applied research
Authors: Spicer W. E.
Published: July 2002

A sketch is given of the development of photoemission electron spectroscopy (PES) with emphasis on the author’s own experience. Emphasis is placed: (1) on the period between 1958–1970; (2) on the various developments which were required for PES to emerge; and (3) on the strong interactions between applied/fundamental and knowledge/empirically based research. A more detailed discussion is given of the recent (1975–present) application of PES to study the interfaces of III–V semiconductors.
Structural damage produced in InP(100) surfaces by plasma-employing deposition techniques
Authors: Dautremont-Smith W. C. , Feldman L. C.
Published: July 2002

In previous work we demonstrated that rf diode sputter deposition of an oxide onto a clean InP(100) surface produced structural damage and/or interfacial mixing at any sputtering power. In contrast, plasma-enhanced chemical vapor deposition of SiO2 at 13.56 MHz at any plasma power was damage-free and gave an abrupt interface. Structurally damage-free means that the large majority of the surface and near-surface In and P atoms are not displaced from their lattice sites; this does not preclude the presence of electrical or optical modification. In this work, again using Rutherford backscattering under channeling conditions, we have investigated the interface between InP and a deposited dielectric or metal for various other types of plasma and sputter deposition. Dose and/or energy of species incident on the InP surface during the initial stage of deposition has been varied over the spectrum intermediate to the relative extremes of the two previously studied cases. The general conclusions are as follows. Plasma deposition is structurally damage-free, even at low frequency and at a low deposition rate to plasma power ratio, as for SiNx. Sputter deposition, however, is always damaging, even when there are no energetic negative ions incident on the InP substrate, or even when the InP substrate is remote from the plasma, as in ion beam sputter deposition. Use of a light sputtering gas (He) in place of Ar increases the magnitude of damage. Annealing at 450 °C can reduce but not remove the damage.
Confined oxygen glow discharge cleaning of silicon
Authors: Hoffman D. M. , Thomas J. H.
Published: July 2002

The reproducibility of the thickness of SiO2 films deposited on Si or Al surfaces is strongly affected by substrate cleanliness. Consistent correlation between film–substrate and film–quartz crystal data is not obtained when using wet chemical cleaning methods. Auger depth profiling has been used to characterize silicon dioxide films on silicon cleaned by a confined oxygen glow discharge in a commercial electron beam evaporator. Model studies were performed by x-ray photoelectron spectroscopy of in situ cleaning. Oxygen discharge cleaning is shown to be effective in removing carbonaceous material without damaging the silicon surface and without measurably increasing the silicon native-oxide thickness.
Surface analysis of gold containing polyimide films
Authors: Madeleine D. G. , Spillane S. A. , Taylor L. T.
Published: July 2002

Polyimide films with metallic surfaces were produced by thermal treatment of solutions containing HAuCl4⋅3H2O, hydrogen tetrachloroaurate trihydrate, and either Ciba–Geigy XU 218, poly[5(6)-amino-1-(4′ phenyl)-1,3,3 trimethylindane-3,3′,4,4′-benzophenonetetracarboxylimide] or BTDA-ODA, poly [ p, p′-oxybis (phenylene)-3,3′,4,4′-benzophenonetetracarboxylimide]. Scanning electron microscopy indicated that 0.1-μ particles covered a large portion of the air side surface; whereas, 10-μ particles covered the glass side surface of these films. Auger electron spectroscopy and x-ray photoelectron spectroscopy (XPS) indicated that the particles were gold in the metallic state. Model imide compounds were synthesized and used to evaluate the XPS spectra of both unmodified and gold containing films. Surface analysis data suggested that only a small amount of gold was located within the XPS sampling depth and that the observed gold particles resided underneath a polyimide overlayer. The discrete metal particles had an unexpected influence on the photoelectron spectra of the polyimide and the influence was dependent on particle size. A schematic model of the multilayered modified film surface was developed and the model further probed by electron flood gun and metal vapor deposition studies.
Electronic structure of oxygen vacancies on TiO2(110) and SnO2(110) surfaces
Authors: Munnix S. , Schmeits M.
Published: July 2002

The enhancement of the catalytic efficiency of semiconducting oxides by surface defects has led to intense experimental activity on their electronic and structural properties. By making use of the scattering theoretical method, we have investigated the electronic properties of point defects on the (110) face of two oxides of rutile structure: SnO2 and TiO2. A comparative study of various defect configurations is presented. For SnO2, which has a simple metal cation, we find no surface defect state inside the gap. The surface defect density of states presents essentially O-p derived resonances in the valence-band region and Sn-s derived resonances in the conduction-band region. For the transition metal oxide TiO2, we obtain similar features when O atoms are removed in the surface layer. However, in the case of a subsurface vacancy, we find a Ti-d derived surface defect state inside the gap. When allowance is made for relaxation of the atoms around the vacancy, this state is located at 0.7 eV below the conduction-band edge, in agreement with experimental results. The physical mechanism at the origin of this surface defect state is elucidated.
Electronically driven reconstruction of TaC(100)
Authors: Noonan J. R. , Davis H. L. , Gruzalski G. R.
Published: July 2002

The structure of a stoichiometric TaC(100) surface has been determined by an analysis of its low-energy electron diffraction I–V spectra. It was determined that the surface is reconstructed such that 〈011〉 rows of carbon atoms are displaced outward and 〈011〉 rows of Ta inward with respect to the truncated bulk atomic layer. The electrostatic forces on the surface atoms that are induced by the termination of the solid drive this rippled reconstruction.
Domain mixtures in the NiAl(111) surface
Authors: Noonan J. R. , Davis H. L.
Published: July 2002

The NiAl(111) surface structure has been determined using low-energy electron diffraction (LEED) I–V intensity analysis. From considerations of the stacking sequence of the (111) planes in the bulk, the surface would be expected to be terminated in one of two types of domains, i.e., in a layer of Al atoms or of Ni atoms. The results of the LEED analysis demonstrate that the NiAl(111) has ordered domains of both Al- and Ni-terminated layers. Importantly, the relaxations within the two types of domains are determined to be significantly different as well.
The emerging technologies of oil-free vacuum pumps
Authors: Hablanian M. H.
Published: July 2002

The demand for higher purity of vacuum environment has precipitated a number of attempts to develop pumps which do not contain any liquids, either as lubricants or as motive fluids. Although it is possible with careful design and operation to prevent hydrocarbon contamination from oil-sealed forepumps and diffusion pumps, the mere presence of oil or oillike substances in the pump provides the opportunity for backmigration, especially as a result of a malfunction. The trend toward the elimination of oil or other fluids began 35 years ago with development of ion-getter pumps, followed by turbomolecular pumps and cryopumps. In recent years the attention has shifted to the forevacuum pumps and a number of oil-free devices have been developed or are in the process of development. The paper concentrates primarily on this latter group which includes piston pumps, high-pressure-ratio blowers, scroll compressors, multistaged axial and centrifugal turbine pumps, special combination turbomolecular pumps, diaphragm pumps, etc. Some of these discharge directly into the atmosphere; some are arranged in series. It is practically feasible to produce pressure ratios near 100 000 in a self-contained single device discharging to atmosphere without using any conventional lubricants in the vacuum environment. Thus, a pressure near 1 Pa can be produced which is entirely sufficient for the initiation of high-vacuum pumping devices.
High-temperature stable TaSix –GaAs Schottky barrier
Authors: Kao C. H. , Huang F. S. , Huang S. L.
Published: July 2002

The effect of annealing TaSix –GaAs Schottky diodes at high temperature was investigated. Diodes with near-unity ideality factor were obtained for annealing temperature 500–800 °C. The I–V barrier height was found to remain stable at ∼0.7 eV, but the C–V barrier height shows a different result. For an annealing temperature above 700 °C for 20 min, the x-ray diffraction pattern gives the mixture phases of TaSi2 and Ta5 Si3 for Si-rich film. The Ta-rich film exhibits the single phase of Ta5 Si3 . Rapid thermal annealing at 900 °C for 10 s also produces good-quality films. Using Rutherford backscattering spectrum, we have observed Ga and As diffusion into the TaSix (x=0.6 and 2) layer for furnace annealing above 900 °C. But this outdiffusion disappears for rapid thermal annealing (RTA) at 900 °C for 10 s. The interface states of TaSix /GaAs were studied by deep-level transient spectroscopy and C–V analysis. The tantalum silicide films (x=0.6 and 2) exhibit high-temperature stability on GaAs for the RTA process.
Surface and trace analysis by high-resolution time-of-flight secondary ion mass spectrometry
Authors: Niehuis E. , Heller T. , Jürgens U. , Benninghoven A.
Published: July 2002

We have developed a high-performance time-of-flight secondary ion mass spectrometer that consists of an electron impact ion gun, a pulsed 90°-deflection producing mass separated subnanosecond ion pulses, a single-stage reflectron time-of-flight analyzer, a postacceleration detector, and an 800-MHz time-to-digital converter. This instrument offers a unique combination of high mass resolution, high transmission, and high-mass range, which opens new fields of application. A charge compensation system with a pulsed low-energy electron gun and a pulsed extraction for the secondary ions provides the analysis of insulating materials also. The fields of application range from trace analysis of elements and molecules in the uppermost monolayer of a solid, the characterization of polymer materials, study of surface reactions, and contaminations on all kind of materials (metals, ceramics, polymers, etc.), to the identification and structure analysis of biomolecules with sensitivities in the subpicomole range which is important for a wide variety of applications in life sciences. The performance of the instrument, regarding sensitivity, mass resolution, exact mass measurement, mass range, dynamic range, and abundance sensitivity, is discussed. Characteristic examples as trace element analysis on Si and GaAs wafers, and structural and surface investigations of polymers are presented.
Development of Ti Kα x radiation for electron spectroscopy for chemical analysis of polymer surfaces
Authors: Vargo Terrence G. , Gardella Joseph A.
Published: July 2002

It has been shown previously that implementing x-ray sources of different energies for electron spectroscopy for chemical analysis (ESCA) of various materials can be useful. In this study ESCA with Ti Kα x radiation as the excitation source is explored for the analysis of some common technical polymers. The investigation of these polymers is used to illustrate the usefulness of a Ti Kα x-ray source with respect to increasing the depth of ESCA analysis as well as its use in the analysis of atoms with high-binding-energy core level (1s) electrons, which are normally inaccessible to ESCA experiments using Mg Kα or Al Kα x radiation. Through the use of polymer standards a list of calculated sensitivity factors for C, N, O, F, Si, P, S, and Cl is derived from ESCA experiments utilizing Mg and Ti Kα x radiation. Also presented are corrected photoelectron binding energies, x-ray excited Auger kinetic energies, and modified Auger parameters (α′) which are accessible by Ti Kα radiation. Discussion of Ti Kα x-ray satellites is included with subsequent curve resolution results of core level photoemission from elements as well as polymer materials containing elements in different chemical states. Finally, the use of the new sensitivity factors is illustrated along with the increased depth profiling ability through the investigation of a segmented poly(ether–urethane) block copolymer system in which nitrogen containing ‘‘hard’’ segment domains are thought to segregate away from the surface into the bulk.
Surface chemistry of electrodes: Cu(111) in aqueous HCl
Authors: Stickney John L. , Ehlers Charles B.
Published: July 2002

Fundamental studies of the surface chemistry of copper electrodes have been performed. The composition and structure present at a six-sided Cu(111) electrode emersed (removed) from a 1mM HCl solution have been investigated with low-energy electron diffraction (LEED) and Auger spectroscopy. Experiments were performed in an UHV surface analysis instrument directly coupled to an antechamber containing a standard electrochemical cell. Electrode surface composition was examined as a function of emersion potential and the extent of oxidation and Cu deposition. A Cu(111) (√3×√3)R30°-split-Cl pattern was observed and was relatively unchanged under a variety of voltammetric programs and emersion potentials. The LEED pattern indicated a large surface unit cell. A proposed structure with a unit cell defined by phase boundaries and a local (√3×√3)R30° symmetry within the unit cell has been proposed. Electrochemical potential programs such as the dissolution and deposition of copper did not result either in the degradation of the LEED pattern or in significant changes in the Cl/Cu Auger ratio, indicating ordered stoichiometric deposition of copper. Rinsing of the copper chloride surface with water also resulted in no detectable changes in composition or structure.
Anisotropic surface mobility of aluminum on Si(111) during the initial stage of vapor deposition
Authors: Levenson L. L. , Usui H. , Yamada I. , Takagi T. , Swartzlander A. B.
Published: July 2002

Scanning Auger electron microprobe line scan analysis was used to determine the surface mobility of Al atoms on Si(111) substrates cut 4° off axis in the [11̄0] direction. These substrates are expected to have clusters of one atomic layer steps oriented normal to the [11̄0] direction. The Al films, ∼6 nm thick, were deposited from an ionized cluster beam source through a 50×1000 μm slot in a stainless-steel mask. The slot was oriented with its long axis in the [11̄2̄] direction, i.e., normal to the [11̄0]. The Auger line scans show that the surface mobility of Al is anisotropic. Furthermore, the anisotropy changes as the acceleration voltage increases from 0 to 3 kV and as the substrate temperature increases from 50 to 400° C. This anisotropy can be explained by the higher probability of Al atoms to be absorbed or reflected at steps in the ascending direction. In the descending direction, the passage of Al atoms over steps is facilitated by higher thermal energies provided by increased substrate temperature or by ion acceleration. However, increased evaporation of Al at high temperatures results in a decrease in diffusion distance above 200 °C.
The transition from αZr to αZrO2 growth in sputter-deposited films as a function of gas O2 content, rare-gas type, and cathode voltage
Authors: Kwok Chee-Kin , Aita Carolyn Rubin
Published: July 2002

In this study, a Zr target was sputtered in rf-excited, rare gas discharges (Kr, Ar, Ne) containing O–4% O2, operated at cathode voltages from −1.3 to −1.9 kV. In situ optical emission spectrometry was used to monitor three transitions of the neutral excited Zr atom in the discharge (λ3520, 3548, and 3601 Å). Films were deposited on fused SiO2 and their crystallography, optical behavior, and electrical resistivity were determined, postdeposition. The sequence of phases that evolved in the Zr–O system was αZr→transition phases→α+βZrO2→αZrO2. The sequence was shifted towards αZrO2 with increasing gas O2 content, changing rare gas from Kr→Ar→Ne, and decreasing cathode voltage. The results are discussed in terms of a two-step oxidation process involving both electrodes.
Time-of-flight secondary ion mass spectrometry of insulators with pulsed charge compensation by low-energy electrons
Authors: Hagenhoff B. , van Leyen D. , Niehuis E. , Benninghoven A.
Published: July 2002

A new charge compensation system for time-of-flight (TOF) secondary ion mass spectrometry is described. A pulsed low-energy electron source (10 eV) in combination with a pulsed extraction voltage of the TOF analyzer allows low-energy electrons to reach the target in the relatively long period of time between two excitation pulses. Low-energy electrons allow self-adjusting of the surface potential. Sample damages by these electrons are not detectable. Effects due to electron stimulated desorption are suppressed by pulsing the anode of the electron source. Compensation is possible for the accumulation of positive as well as negative spectra. With this experimental arrangement we investigated a variety of insulating materials (e.g., thick polymer films, glasses, ceramics). In all cases we found stable and reproducible spectra, even of insulators with extremely low conductivity.
Microfabrication of cantilever styli for the atomic force microscope
Authors: Albrecht T. R. , Akamine S. , Carver T. E. , Quate C. F.
Published: July 2002

Atomic force microscopy (AFM) is a newly developed high resolution microscopy technique which is capable of mapping forces near surfaces or, by means of these forces, the topography of the surface itself. In one mode of operation, AFM can resolve individual atoms on both conducting and insulating surfaces. A crucial component for the AFM is a flexible force-sensing cantilever stylus, whose properties should include, among other things: a sharp tip, a low force constant, and a high mechanical resonance frequency. These requirements can be met by reducing the size of the cantilever stylus through microfabrication techniques and employing novel methods to construct a sharp tip. Presented here are a number of microfabrication processes for constructing cantilever styli with properties ideal for the AFM. These fabrication processes include (1) a method for producing thin film SiO2 or Si3N4 cantilevers without tips, (2) a method for producing Si3N4 cantilevers with integrated pyramidal tips formed by using an etch pit on the (100) surface of Si as a mold, (3) a method for producing SiO2 cantilevers with conical tips formed by a combination of isotropic and anisotropic plasma etching of a small Si post, and (4) a method for producing SiO2 cantilevers with integrated tetrahedral tips formed by anisotropically etching a corner of a small Si post to a sharp point. Each of these processes uses a (100) Si wafer as a substrate and relies on conventional batch fabrication techniques. The quality (i.e., sharpness) of the tips produced by the above methods matches or exceeds that of conventional tips used in the AFM or scanning tunneling microscope (STM). Alternative methods for producing tips by evaporation of material through an orifice or by selective chemical vapor deposition of W metal into a pyramidal etch pit in Si have been demonstrated, but these methods have not yet been successfully used in cantilever assemblies.
Purity and thickness analysis of fluoropolymers by static secondary ion mass spectrometry
Authors: Newman J. G. , Viswanathan K. V.
Published: July 2002

An ongoing concern in the area of magnetic thin film storage media is that of proper lubrication of Winchester-type disks. Without proper lubrication, catastrophic head crashes can occur. While thin fluoropolymer lubrication technology has been known and used for several years, the method still contains inherent problems. The overall thickness of the lubricant may be wrong, contamination of the fluoropolymer may be present, diffusion of sublayers can occur, and inhomogeneities in the lubricant thickness are present. This paper describes the use of static secondary ion mass spectrometry (SIMS) for the analysis of fluoropolymer lubricants. Using extremely low primary ion current doses (<5×1012 ion/cm2), static SIMS produces a set of characteristic molecular fragment ions whose intensity is found to be linear with thickness up to approximately 30 Å. The method also provides information on the purity of the lubricant as well as on surface coverage inhomogeneities which may be present. The technique has been shown to work for a variety of different perfluoropolyether lubricants as well as for an organic surfactant film.
Accuracy of vacuum gauges
Authors: Jitschin Wolfgang
Published: July 2002

The performance characteristics of the following types of vacuum gauges are discussed: quartz Bourdon gauge (QBG), capacitance diaphragm gauge (CDG), thermal conductivity gauge (TCG), quartz friction gauge (QFG), spinning rotor gauge (SRG), cold cathode ionization gauge (CCG), hot filament ionization manometer (IM), quadrupole mass spectrometer (QMS) and other mass spectrometers. Depending on the pressure, the most stable gauges are the QBG, the CDG, the SRG, and the IM. In applications with modest demands on the measuring accuracy, it may be sufficient to employ an uncalibrated gauge and adopt an average gauge coefficient given by the manufacturer. In cases with higher demands on the accuracy, the calibration of individual gauges may be necessary. The highest demand arises if the gauges are used as transfer standards in intercomparisons of primary pressure standards. Here, the achievable accuracy is limited by the stability of the gauge with time. Gauges based on different operating principles and different constructions show different stabilities. Furthermore, the stability depends on the handling of the gauge. This paper compiles available data on the accuracy performance of the various gauges.
Magnetron sputtered TiAlON composite thin films. I. Structure and morphology
Authors: Luthier R. , Lévy F.
Published: July 2002

Composite TiAlON thin films were deposited by rf magnetron sputtering from TiN–Al2O3 ceramic targets. Samples deposited under conditions that minimize the adatom mobility are attractive as decorative coatings due to their lustrous, black color and their wear resistance. The critical relationships between the processing parameters and the resulting microstructural features were investigated. Coatings with thicknesses ranging from 0.05 to 5 μm were characterized by x-ray and electron diffraction, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Thus, both the small scale ‘‘nanostructure’’ and the larger scale ‘‘microstructure’’ are described. It is shown that the observed features fit in the evolutionary model of microstructures. For ‘‘standard’’ conditions, the nanostructure consisted of crystallized grains (up to 10 nm in size) of a mixed nitride (Ti,Al)NX embedded in an amorphous mixed oxide (Ti,Al)O2. These two phases formed nanocolumns with a base diameter of 10 nm, separated by voided regions for very thin films (50 nm). These small scale features merged into microcolumns with a lateral size of 0.1 μm in films thicker than 1 μm. The low mobility regime produced important intercolumnar porosity. Increased adatom mobility induced by low energy particle bombardment affected both structural levels. The lattice parameter of the (Ti,Al)NX grains relaxed to 0.417 nm, value in the reported range for single phase metastable (Ti0.5Al0.5)N coatings. The bombardment interrupted the columnar growth and yielded a denser material. If in addition, the deposition conditions increased the energy of ejected species (for low argon pressures), segregation of metallic aluminium was observed together with the (Ti,Al)N grains. The microstructure became totally featureless up to a thickness of 1.8 μm.