bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray absorption of As low-energy ion implanted into Si(100) grown by molecular-beam epitaxy
Authors: Tyliszczak T. , Hitchcock A. P. , Jackman T. E.
Published: July 2002

Information about the local environment of dopants in semiconductor materials is required to understand and help optimize the electrical activity in highly doped samples. The near-edge (XANES) and extended x-ray absorption fine structure (EXAFS) components of x-ray absorption spectra provide detailed information on local geometric and electronic structure about selected atoms. Electron yield and fluorescence detection of As K EXAFS and XANES, with sample rotation to eliminate substrate diffraction, have been used to study the As sites in Si(As) epilayers grown by molecular-beam epitaxy (MBE) and doped in situ by low-energy As implantation. Marked differences are found in both the near edge and EXAFS signals between samples prepared under low temperature (460 °C) growth/implantation conditions and those prepared under the preferred, higher temperature (700 °C) growth conditions. The EXAFS analysis indicates that As is located primarily in the substitutional site in the optimal (700 °C) sample but that a large fraction of the As is in a defect site, with possibly some incorporated as some type of As–As clusters in the samples grown at 460 °C.
Modulated reflectance and Raman scattering studies of metalorganic chemical vapor deposited homolayers of GaAs
Authors: Kallergi M. , Roughani B. , Aubel J. , Sundaram S. , Chu T. , Chu S. , Green R.
Published: July 2002

Homoepitaxial GaAs films grown by metalorganic chemical vapor deposition (MOCVD) using two different growth conditions were characterized by electrolyte electroreflectance and Raman scattering techniques. We report here the results for two sets of GaAs films deposited on n-type GaAs substrates using MOCVD and trimethyl-gallium. The first set was grown at low temperature by ArF excimer laser-induced MOCVD with the use of arsine, and the second set was grown at about 700 °C by conventional MOCVD with the use of elemental arsenic. Electrolyte electroreflectance studies were used to investigate the energy gaps, structural quality, and topographical carrier concentrations for these homolayers. Raman scattering studies were performed to establish that the crystal orientation was the same as that of the substrate, which was (100) orientation. The spectral results are correlated with the growth parameters for the homolayers of GaAs on GaAs.
Scanning tunneling microscopic immunoassay: A preliminary experiment
Authors: Masai Junji , Sorin Takaaki , Kondo Shunzo
Published: July 2002

Imaging of gold colloidal particles bound to immune complexes on conductive substrates was carried out with scanning tunneling microscope (STM) operated in air. STM images of the gold colloids were obtained on models of immunoassay sample of ‘‘gold colloidal particle/immune complex/substrate.’’ One model is composed of gold colloidal particles (average diameter of 15 nm) and an antigen–antibody complex [human immunoglobulin G(IgG) and rabbit anti-human IgG] on a gold substrate. The other the same kind of gold colloids and another kind of immune complex [Hepatitis B surface (HBs) antigen and anti-HBs] on a highly oriented pyrolytic graphite. It was possible to obtain the STM images of gold colloids bound to the immune complexes without heavy metal coatings. The STM images showed corrugations consistent with the coverages expected from the nominal size of the gold colloidals used. The successful measurement suggests that we can make use of STM for a novel immunoassay, ‘‘STM immunoassay,’’ for detecting a trace amount of macromolecules of biological importance.
Studies in modified polybismaleimide resins and its composite materials
Authors: Xiaoming Yang , Luxia Jiang , Meili Xie , Xingxian Cai
Published: July 2002

This paper reports studies on modified polybismaleimide resin and its composite materials. Using the differential scanning calorimetry (DSC) and torsion braid analysis (TBA) instruments, we studied its curing behavior while heated. Its thermal stability was determined by thermogravimetric (TG) and differential thermal analysis (DTA). Its apparent active energy in thermal decomposition was calculated using an Apple II microcomputer. The results showed that these resins had better dissolubility and moldability as well as higher thermal stability than nonmodified resin. Based on the modified polybismaleimide resins, molding plastics filled with ‘‘red-earth’’ or micapowder and the composite materials reinforced with glass fiber were manufactured, and the physicomechanical properties of these composites were tested. The results showed that these materials could be used as structural and insulating material in high temperatures. Finally, this paper reports the production of the vanes of direct-link-type vacuum pump made from the glass fiber reinforced composite material. The results were satisfactory in the practical application.
Scanning tunneling spectroscopy of metal surfaces
Authors: Kuk Y. , Silverman P. J.
Published: July 2002

Tunneling spectroscopy of Au(100)-(5×20) and Fe on Au(100) surfaces has been studied. Using a well-defined tunneling tip, I–S, I–V, and V/I*dI/dV–V spectra were obtained. The results confirm that the characteristics of the spectra resemble those of previously reported semiconductors. From I–S relations, it was found that the tunneling barrier decreases abruptly when the tunneling gap is <6 Å.
A versatile low-temperature scanning tunneling microscope
Authors: Renner Ch. , Niedermann Ph. , Kent A. D. , Fischer O/.
Published: July 2002

We present a new scanning tunneling microscope (STM) developed specifically for use at low temperatures within a standard helium cryostat. This compact STM (diameter 25 mm and length 61 mm) has been designed to be compatible with a superconducting magnet. It incorporates a novel approach system which operates in any orientation including the vertical one, without any mechanical connections to the STM. It is able to translate a sample by 30 Å steps in any orientation. The coarse and the fine approach are driven by a single piezoelectric tube via inertial slip-stick motion of the sample holder. During the coarse approach, the translator can be moved over 15 mm at 0.25 mm/s against gravity. We have operated this STM down to 1.6 K and we have obtained images with atomic resolution on highly oriented pyrolitic graphite (HOPG) and NbSe2 from 300 K down to 90 K. This partially thermal compensated instrument is able to monitor temperature dependent surface changes on atomic scale in real time.
Molecular dynamics calculations of nuclear stimulated desorption
Authors: Glikman E. , Kelson I. , Doan N. V.
Published: July 2002

Molecular dynamics calculations of nuclear stimulated desorption are carried out for a palladium crystal containing radioactive palladium atoms. The total desorption probability from various sites are computed, as well as the angular distribution of the desorbing atoms. The implications of the results to different experimental scenarios are discussed.
Sulfur deposition on Fe(100) from methanethiol decomposition
Authors: Lu J.-P. , Albert M. R. , Bernasek S. L.
Published: July 2002

Sulfur overlayers on the Fe(100) surface were deposited by decomposition of methanethiol (CH3SH), and characterized by Auger electron spectroscopy and low-energy electron diffraction. Thermal decomposition of CH3SH results in the formation of c(2×2) sulfur overlayers. In the presence of electron beam irradiation, high coverage (greater than half monolayer) sulfur overlayers with antiphase domain structures can be produced.
The reaction pathway for the growth of alumina on high surface area alumina and in ultrahigh vacuum by a reaction between trimethyl aluminum and water
Authors: Soto C. , Tysoe W. T.
Published: July 2002

It is demonstrated that it is possible to grow thin alumina films on an alumina substrate by the sequential adsorption of water and trimethyl aluminum (TMA) both on high-surface-area alumina and in ultrahigh vacuum (UHV) at room temperature. The presence of the resulting layer is demonstrated on high surface-area substrates using infrared spectroscopy and in UHV using electron energy-loss spectroscopy (EELS). The correspondence between the reaction on high surface-area alumina and in UHV allow the surface species to be identified using infrared spectroscopy on the high surface-area sample, and the kinetics to be measured in UHV. A film formation reaction mechanism is proposed that is consistent with the experimental results. It is suggested that TMA can react with hydroxylated alumina to form an adsorbed dimethyl aluminum species and evolve methane. This can further react with water to evolve further methane and leave adsorbed methyl groups. These react rather slowly with water resulting finally in the formation of a hydroxylated alumina surface.
The influence of target oxygen on the YBa2Cu3O6+δ direct-current magnetron sputtering process
Authors: Larsson G. , Selinder T. I. , Helmersson U. , Rudner S.
Published: July 2002

The oxygen partial pressure and the target potential have been monitored under a range of process conditions during single target direct-current (dc) magnetron sputtering of Y–Ba–Cu–O. The introduced sputtering gas consisted in most instances of pure argon and hence the oxygen present in the plasma originated mainly from the target. During the first hours of sputtering the oxygen partial pressure was of the same magnitude as the argon pressure (3.0 Pa) and the film composition was off stoichiometric. During the sputtering the oxygen pressure decreased, the target potential increased and the film composition became more stoichiometric. After 30–40 h of sputtering the target potential and the oxygen pressure stabilized and the film composition was equal to that of the stoichiometric target. If an oxygen flow exceeding a critical level was mixed into the sputtering gas the target potential and the deposition rate decreased swiftly. This was due to target oxidation. In some instances the stabilization after ‘‘presputtering’’ in pure argon was incomplete and oscillations in target voltage and oxygen partial pressure were observed. The fluctuations made it virtually impossible to obtain stoichiometric films. The oscillative behavior of the sputtering process is tentatively explained by a target temperature dependent oxygen diffusion.
Toughening and crack tip shielding in brittle materials by residually stressed thin films
Authors: Lambropoulos John C.
Published: July 2002

The apparent fracture toughness of brittle components can be greatly enhanced via surface-deposited thin films which are under residual, possibly nonuniform compression since the residual compression counteracts the effect of the applied loads at the tip of a surface crack. Examples are epitaxial thin films on crystalline components, thin films in a state of compression due to the film deposition process, or thin films of ion-exchange in glass. We review the micromechanics of the strengthening process by considering a surface microcrack under the combined action of the residual stress in the film (which shields the crack tip and may cause partial crack closure) and of the externally applied loads (thermal or mechanical). We examine crack tip shielding as the interaction of microscopic dilatant spots with the crack tip. This model predicts rising crack growth resistance curves for limited amounts of crack growth. We review and analyze experimental results on ion exchange in thin films of glass in small-, medium-, and large-size glass components for high average power solid state lasers. These measurements show that the apparent fracture strength increases by as much as five to six times.
A versatile low-pressure ozone source
Authors: Coleman E. , Siegrist T. , Mixon D. A. , Trevor P. L. , Trevor D. J.
Published: July 2002

A low-pressure source for pure ozone has been developed and successfully tested in the growth of oxide superconductor thin films. Ozone is selectively adsorbed on silica gel at a temperature of −77 °C from an O3/O2 gas mixture stream containing 8 wt% O3, produced by a commercial ozone generator. Since only O3 is trapped, no further purification steps are required. Ozone/gel loading factors below 10% (wt/wt) ensure the safe storage of O3 over prolonged periods. Pure O3 is extracted from the silica gel trap, in which the O3 vapor pressure is maintained at around 10 mTorr, by applying a dynamic vacuum. Films of Ba2YCu3O7−δ with Tc above 86 K have been grown in situ by molecular beam epitaxy techniques using this ozone source.
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u stress measurements on niobium nitride thin films produced by hollow cathode enhanced direct current reactive magnetron sputtering
Authors: Dawson-Elli David F. , Plantz David , Stone Donald S. , Nordman James E.
Published: July 2002

A method of measuring thin film coating stress in situ has been developed, based on measuring the deflection of a cantilevered beam substrate. Using a slender piece of a Si wafer as a substrate, a bending force per unit width of ∼75–150 dyne/cm is achieved, dependent on the thickness of the substrate. This method is used to compare direct current reactive magnetron sputtered films deposited with and without hollow cathode enhancement. Factorial experimental design was used to investigate the effects of total sputtering pressure, sputtering power and nitrogen flow on the measured stress. The films were also characterized by deposition rate, Auger electron spectroscopy, x-ray diffraction. Select films were characterized by nanoindention to obtain hardness and Young’s modulus.
Formation of aluminum nitride by intensified plasma ion nitriding
Authors: Meletis E. I. , Yan S.
Published: July 2002

Ion nitriding of pure Al with an intensified glow discharge was attempted in the present work. The results showed that formation of aluminum nitride is feasible under low pressure, enhanced plasma conditions. The nitride formed had a dense structure and an ultrafine nuclei size (less than 100 Å). The x-ray and electron diffraction results showed that the nitride phase has a face centered cubic structure with a lattice parameter of 4.38 Å. The Auger analysis results indicated that the composition of the compound was close to Al3N. It is suggested that the mechanism of aluminum nitride formation closely relates to the presence of Al2O3 at the surface that can be sputtered by the energetic glow discharge permitting nitrogen reaction with the aluminum substrate.
Instrumental considerations for acquiring quantitative Auger electron spectra
Authors: Smith M. A.
Published: July 2002

Acquisition of Auger electron spectra which can be used for quantitative measure of surface composition depends fundamentally on knowledge of instrumental parameters which are often neglected or only poorly characterized. In this paper instrumental factors such as magnetic shielding, electron multiplier response, and the spectrometer transmission function of a commercial hemispherical analyzer are treated. An example of severe magnetic field interference is given, and a remedy for it is demonstrated that should be of application on other systems. Procedures for measuring the multiplier response function and the transmission function are described, and an instrumental correction function is shown. A spectrum of copper, after proper correction is shown to agree well with a spectrum that has been proposed by others as a standard for comparison among laboratories.
Outgassing characteristics of an electropolished stainless-steel pipe with an operating extractor ionization gauge
Authors: Yoshimura Nagamitsu , Hirano Haruo , Ohara Kenji , Ando Ichiro
Published: July 2002

The outgassing of a Bayard–Alpert ionization gauge (BAG) with a tungsten filament was observed to increase with the pressure in an isolation test. This was caused by the temperature rise of the hot filament, resulting from the gradual pressure rise. The outgassing of an extractor ionization gauge with an iridium filament coated with thorium oxide was much lower than that of the BAG. The net outgassing rate of an electropolished (EP)-type 304 stainless-steel (SS304) pipe wall was measured by the isolation method. The outgassing rate of the pipe wall at 3.0×10−6 Pa, after an ‘‘in situ’’ bakeout (150 °C, 20 h), was measured as 1.1×10−11 Pa ℓ/s cm−2 by neglecting the outgassing and pumping effects of the operating EXG.
Silicon nitride formation from a silane–nitrogen electron cyclotron resonance plasma
Authors: Barbour J. C. , Stein H. J. , Popov O. A. , Yoder M. , Outten C. A.
Published: July 2002

Good quality, low temperature silicon nitride and oxynitride films were deposited downstream from an electron cyclotron resonance (ECR) plasma source using SiH4 and N2 gas mixtures. The Si/N ratio and H content in the deposited films were determined using Rutherford backscattering spectrometry and elastic recoil detection. The H concentration was minimum for films with compositions closest to that of stoichiometric Si3N4. The optimum conditions for producing a stoichiometric Si3N4 were a SiH4/N2 flow ratio between 0.1 and 0.2, and an electrically isolated sample far from the ECR source. Infrared absorption spectra showed that as the film composition changed from N rich to Si rich the dominant bonds associated with H changed from N–H to Si–H.
Outgassing rate of some austenitic stainless steels
Authors: Ishikawa Yuichi , Koguchi Yuko , Odaka Kenji
Published: July 2002

Outgassing rates were measured and compared for 304L, 316L, and 316LN stainless steels in fully degassed condition produced by repeated pumpdown and baking. In addition, thermal desorption characteristics and surface composition were measured. In particular, the effect of grade, vacuum remelting, and prebaking is discussed. The grade itself does not appear to affect the outgassing rate significantly, while vacuum remelting reduces hydrogen in the steel, thus giving lower outgassing rate than commercially available steel. However the effect of prebaking at 450 °C appears more pronounced on reduction of diffusive outgassing of hydrogen than that of vacuum remelting. Application of prebaking to the vacuum remelted steel produces the extremely low outgassing surface, as low as 1.3×10−10 Pa m3/sm2 by baking at 150 °C for 14 h.
The interaction of ion implantation with photoresist ashing: A statistical experimental design study
Authors: Stefani Jerry A. , Loewenstein Lee M. , Michael Christopher
Published: July 2002

We applied statistical experimental design concepts to the study of the interaction of ion implantation with remote-plasma photoresist ashing. A 28–4IV fractional-factorial screening experiment revealed that implant species and resist hard bake conditions were not significant factors for resist ash rate and uniformity. We obtained quadratic polynomial response surface models using a G-optimal experimental design as a function of the remaining four ash variables (temperature, pressure, oxygen, and hydrogen mass flow rates) and two implant factors (implant dose and energy). Ion implant dose had a significant effect on ash rate while implant energy was an important factor for ash rate uniformity.
Design considerations for high-flux collisionally opaque molecular beams
Authors: Gray David C. , Sawin Herbert H.
Published: July 2002

Flux distributions and fractional interceptions for high-flux straight-tube sources have been calculated in the collisionally opaque gas flow regime. Previous theoretical work in this regime has been assimilated to provide a useful design guide similar to that provided by Campbell and Valone [J. Vac. Sci. Technol. A 3, 408 (1985)] for transparent regime molecular-beam sources. Several of these large throughput straight-tube sources have been constructed in the laboratory for use in etching studies of semiconductor materials. Reasonable agreement has been found between the available theories and measurements of peak beam fluxes and angular dispersions. While angular dispersion is a function of aspect ratio only (tube diameter-to-length ratio) in the transparent regime theory discussed by Campbell and Valone, additional parameters are required in the opaque regime since gas-phase collisions effect the beam directionality. The dimensionless group η=a/λtip, the ratio of tube diameter to gas mean free path at the doser tip, is suggested as the figure of merit for characterizing angular dispersion in opaque regime tube flow.
Minority-carrier lifetime and photon recycling in n-GaAs
Authors: Ahrenkiel R. K. , Keyes B. M. , Lush G. B. , Melloch M. R. , Lundstrom M. S. , MacMillan H. F.
Published: July 2002

We are reporting here the hole or minority-carrier lifetime in n-type GaAs over a concentration range from 1 × 1017 to 2 × 1018 cm−3. The lifetimes were resolved by means of time-resolved photoluminescence using the time-resolved single photon counting technique. Diagnostic isotype double heterostructures were grown by metalorganic chemical vapor deposition. The isotype double heterostructures had confinement layers of composition Al0.3Ga0.7As. At a fixed majority-carrier concentration, active layer thicknesses were varied from ∼0.25 to 10.0 μm. Using the accepted B coefficient as 2 × 10−10 cm3/s, the photoluminescence lifetime is over an order of magnitude larger than the radiative lifetime. The data were interpreted in terms of photon recycling, and the experimental results agreed quite well with theory.
X-ray photoelectron spectroscopy study of the stability of Fomblin Z25 on the native oxide of aluminum
Authors: Herrera-Fierro Pilar , Pepper Stephen V. , Jones William R.
Published: July 2002

Thin films of Fomblin Z25, a perfluoropolyalkylether lubricant, were vapor deposited onto clean, oxidized aluminum, and sapphire surfaces, and their behavior at different temperatures was studied using x-ray photoelectron spectroscopy and temperature desorption spectroscopy (TDS). It was found that the interfacial fluid molecules decompose on the native oxide at room temperature, and continue to decompose at elevated temperatures, as previous studies had shown to occur on the clean metal. TDS indicated that different degradation mechanisms were operative for clean and oxidized aluminum. On sapphire substrates, no reaction was observed at room temperature. Our conclusion is that the native oxide of aluminum is neither passive nor protective towards Fomblin Z25. At higher temperatures (150 °C) degradation of the polymer on sapphire produced a debris layer at the interface with a chemical composition similar to the one formed on aluminum oxide. Rubbing a Fomblin film on a single crystal sapphire also induced the decomposition of the lubricant in contact with the interface and the formation of a debris layer.
Concerted x-ray photoelectron spectroscopy study of the character of select carbonaceous materials
Authors: Barr Tery L. , Yin Mengping
Published: July 2002

In the present study we describe an analysis of the interrelationship of several x-ray photoelectron spectroscopy (XPS) techniques recently employed to describe different types of carbon–carbon and carbon–hydrogen bonding. Several systems containing only these types of bonds (e.g., graphite and polyethylene) have also been subjected to ozonation and their degree and type of oxidation analyzed. The latter has led to a novel view of the resulting bonding and the XPS peak previously classified as O(2s).
Two-dimensional phase transitions associated with surface miscibility gaps
Authors: Wynblatt Paul , Liu Yinshi
Published: July 2002

This article provides a thermodynamic treatment of two-dimensional phase transitions associated with a surface miscibility gap, in the regular solution approximation. The treatment is an extension of an approach due to Helms. In the context of surface segregation, this type of phase change manifests itself as an abrupt change in surface composition at a transition temperature which depends, among others, on the bulk composition of the solution. It is also shown that the surface compositions corresponding to the end points of the abrupt change are identical with the limits of miscibility of a hypothetical two-dimensional closed system. In addition, the article summarizes the results of previous Monte Carlo simulation, and reports on some previous as well as some new experimental measurements of this transition in Cu–Ag alloys.
Nucleation and growth of elongated Si islands on Si(100)
Authors: Kleban P.
Published: July 2002

We consider the nucleation and growth of Si islands on Si(100). Recent scanning tunneling microscopy (STM) observations find that small islands formed at ≊200 °C or above are greatly elongated, with aspect ratios of ≊10–20. On annealing, the islands grow and become less anisotropic, with aspect ratios ≊2–3. We show that this interesting behavior can be explained by very general thermal equilibrium effects occuring at or near a phase transition. Our theory reconciles the STM observations with other independent research on this system: molecular-beam epitaxy results, other experimental observations, and recent theoretical calculations, based on elasticity theory, that predict a phase transition for the equilibrium Si(100) surface at ≊200 °C.
Surface haze after heat treatment in hydrogen prior to Si epitaxy
Authors: Orner C. H. , Lewis R. K. , Aumick S.
Published: July 2002

High levels of haze−variations in the background scattering of incident laser light−have occasionally been observed on P− silicon wafers after the hydrogen heat treatment prior to Si epitaxy. These haze patterns have been found to correlate with discrete surface defects consisting of micropits or mound structures. The micropits, which are visible using scanning force microscopy, typically measure less than a micron in diameter and tens of angstroms in depth. Mound structures of similar size have also been observed after the hydrogen heat treatment. Scanning electron microscopy characterizations of these mound defects has indicated that they contain nonconducting material. The discrete surface defects which produce the haze are attributed to preferential etching at anomalous precipitates formed at or near the surface in heat treatments during wafer and device processing prior to epitaxy. Using secondary ion mass spectroscopy, ion images were obtained at successive depths. In these images near surface precipitates were found which had high levels of oxygen and carbon. As the epitaxial process normally is never interrupted after the in situ hydrogen heat treatment, these defects can easily go undetected in manufacturing processes and could lead to leakage channels in device structures.
Coverage dependent interaction of N2O and O2 with Si(001)2×1 as monitored by the O KLL Auger intensity ratio
Authors: Keim Enrico G. , Wormeester Herbert
Published: July 2002

The adsorption behavior of O2 on Si(001)2×1 at 100 K sample temperature has been studied by measuring the intensity ratio of the KL1L1 and KL2,3L2,3 O Auger transitions α as a function of the fractional oxygen coverage in an attempt to solve a longstanding discussion whether this reaction also involves molecular chemisorption. Recent cluster calculations provided evidence that the bond distance between Si and O and between the two oxygen atoms in the O2 molecule is a key parameter affecting the magnitude of α. By referring to the room temperature adsorption of N2O on the same surface we have found evidence for the coadsorption of a metastable molecular oxygen species at advanced adsorption stages, a result which is essentially in agreement with previous experimental predictions and theoretical calculations.
Brownian motion of a particle levitated in vacuum
Authors: Hinkle L. D. , Kendall B. R. F.
Published: July 2002

A glass microbubble (diameter ∼15 μm, wall 0.65 μm) was levitated in high vacuum by an actively controlled electric field. While held vertically, the particle moved freely in the horizontal plane under the influence of a central restoring force. The Brownian motion of the particle was observed and quantified by successive measurements of the oscillation amplitude in a horizontal axis. Brownian motion was exhibited throughout the pressure range from 7×10−6 to 7×10−4 Torr. The statistical behavior of the oscillation amplitude is consistent with the theoretical prediction for Brownian motion in a low pressure gas. These measurements constitute the first direct observation of Brownian motion of a levitated particle in an environment below 1 Torr. In addition to the experimental verification of low pressure Brownian motion theory, this work serves as an important step in the development of a standardizing high vacuum gauge.
Limits on the use of tunneling to describe the Pd–Ge ohmic contact to GaAs
Authors: Herrera-Gómez A. , Meissner P. L. , Bravman J. C. , Spicer W. E.
Published: July 2002

Tunneling theory predicts a break in the slope of the contact resistivity (ρc) versus the inverse of the temperature at the transition from the thermionic to the field emission regime. Although temperature dependent data of the newly developed Pd–Ge ohmic contact to GaAs has often been described by this model, we found that the low barrier heights obtained from such data, are not consistent with the approximations of the method used. We also found that it was not possible to fit the experimental results using the complete thermionic-field expression for ρc. Numerical calculation based on the Wentzel–Kramers–Brillouin (WKB) approximation for a nonuniform doping profile gave also a negative result in reproducing the experimental curves. Then, either the WKB method is not precise enough, or another transport mechanism is playing an important role. We suggest that an alternative transport mechanism to explain the experimental data is the interface states assisted tunneling. We also demonstrate that the low experimental activation energies found cannot be explained without taking the temperature dependence of the Fermi level into account.
Influence of monoenergetic surface state occupation on Fermi level pinning of metal–GaAs interfaces
Authors: Darling Robert B.
Published: July 2002

Fermi level pinning at metal–GaAs interfaces is conventionally attributed to a high density of surface states. This article points out that not only the density and energy level of the surface states, but more importantly their electron occupancy determines the location and degree of pinning. For device applications, Fermi level pinning is most often based upon the Cowley–Sze model, which postulates a thin, electrically transparent interfacial layer, as would be expected for metalizations on GaAs which form arsenide compounds such as aluminum. However, this model is very difficult to apply to the more common surface spectroscopies (x-ray and ultraviolet photoemission spectroscopies) because of its assumption of a uniform distribution of surface states with energy. This article presents a modified model which is based upon any multiplicity of monoenergetic donorlike or acceptorlike surface states and which therefore can be better correlated to the measured surface density of states. This model allows the full electronic structure of the surface states to be included, and it determines the degree and location of Fermi level pinning as a function of this defect structure alone. This eliminates the need for any empirically determined surface neutral energy, while still encompassing both the ideal Schottky and Bardeen limits of the Cowley–Sze model. An important result is that a high density of surface states is not by itself sufficient to cause pinning. The electrostatic restrictions of Gauss’ law require a surface charge on the interfacial layer in order to support any band bending within the semiconductor. This surface charge can arise only from sufficient occupancy of the surface states, e.g., the ionized surface acceptor states associated with arsenic vacancies or antisite defects in n-type GaAs.
Ballistic transport-reaction prediction of film conformality in tetraethoxysilane O2 plasma enhanced deposition of silicon dioxide
Authors: Cale T. S. , Raupp G. B. , Gandy T. H.
Published: July 2002

EVOLVE, a low pressure deposition process simulator based on a fundamental model for free molecular transport and heterogeneous surface reactions in features, is used to predict the evolution of silicon dioxide film profiles during plasma enhanced chemical vapor deposition from mixtures of tetraethoxysilane (TEOS) and oxygen. The constitutive relationships required by EVOLVE are supplied by simple models for the plasma, the plasma sheath and the surface chemistry. The intrinsic kinetic model used in the simulations involves film growth by oxidative attack on adsorbed TEOS and/or TEOS fragments by both oxygen ions and oxygen atoms. Recombination of oxygen atoms on the surface of the growing film competes with TEOS oxidation by atoms. The plasma models are used to predict the fluxes of oxygen ions and oxygen atoms to the surface. The fluxes of all neutral species from the source and from all surfaces are assumed to obey cosine distribution functions. Oxygen ions are assumed to follow an exponential distribution; the standard deviation of the distribution is adjusted to match predicted film profiles with experimentally determined profiles. This combination of an almost directional component and an almost isotropic component allows the prediction of the experimental trends in deposition rate and film conformality with operating conditions. The constitutive models are used by EVOLVE to predict film profiles as a function of temperature, within their window of validity, for deposition in ideal rectangular trenches with an aspect ratio of 2. Film conformality decreases as temperature increases, even though deposition rate actually decreases. Film conformality decreases with increasing pressure and increases with increasing power. All of these predicted trends in conformality agree with our experimental results.
D–T and D2 retention in plastic shells
Authors: Collins G. W. , Sanchez J. J. , Fearon E. M.
Published: July 2002

We have measured the D2 and D–T gas pressure versus time inside multilayer polymer shells [polystyrene, poly(vinyl alcohol) (PVA), and plasma polymer CH1.3] from which we calculate an effective permeability for the PVA layer. From this data, we have developed a D–T pressurization procedure for current direct drive fusion targets that increases the D–T fuel retention time an order of magnitude over previous pressurization schemes. The D–T fuel retention time will be decreased from the shells original D2 retention time (∼45 h) by; (1) 50% immediately after D–T pressurization, (2) 60% after D–T pressurization and cryogenic storage for 1 day, and (3) 85% after D–T pressurization and cryogenic storage for 13 days. We have also found that heat treating the polymer shell increases the fuel retention time for D2 from 10% to 40% and that the D2 mass transfer through the composite polymer shell does not follow Fick’s law. Finally, we describe an efficient target pressurization procedure for D2 fuel.
SiO2/Si etching with CHF3 in a high-field magnetron
Authors: McNevin S. C. , Ciampa N. A. , Miner J.
Published: July 2002

This paper will describe SiO2/Si etching with CHF3 in a magnetron prototype with a field variable up to 1000 G. Calculations based on the Lorentz equations indicate that this high magnetic field may be necessary to fully confine the dark sheath electrons and thus to optimize the incident ion formation and corresponding etch rates. A response surface analysis of the CHF3 etching of SiO2 and Si dependence on magnetic field and pressure is reported, and this is correlated with the observed wafer dc bias. There is a close relation between the Si etching and the dc bias voltage: as the pressure and magnetic field increase, the Si etching increases while the wafer dc bias decreases. Both effects are attributed to electron confinement by the magnetic field: this increases the density of the electrons, ions, and radicals which are involved in Si etching and also lowers the impedance of the plasma which decreases the dc bias voltage. In contrast, the SiO2 etch rate is optimum at intermediate pressures and magnetic fields (∼10 mTorr, ∼500 G). This maximum in the oxide etch rate is qualitatively attributed to the competing effects of reactant limited etching at low pressures and polymer-deposition-limited etching at high pressures. At this maximum, the effects of changing pressure are minimized. This also minimizes the etch rate dependence on contact solid angle [reactive ion etch (RIE) lag] which results from variation in the effective pressure at the bottom of the contact. It is also shown that the etch uniformity depends strongly on the wafer clamp and depends very little on the magnitude of the magnetic field.
Hydrogen based reactive ion etching of zinc sulfide
Authors: Orloff Glennis J. , Elkind J. L. , Koch David
Published: July 2002

II–VI materials are important for optoelectronic devices. These devices are currently wet etched and due to the isotropic nature of wet etching, their etch geometries are limited. Reactive ion etching (RIE), a dry etch process, is a more desirable alternative, because it offers a means of producing small anisotropic structures. This study describes the use of a novel hydrogen based RIE chemistry that is shown to clearly etch anisotropic features in ZnS. Etch characteristics as a function of pressure, power, and temperature are presented for both patterned and unpatterned material. Three different types of ZnS films are studied: evaporated, metalorganic chemical vapor deposited (MOCVD), and sputtered. Among the three types of ZnS films investigated, evaporated, and MOCVD consistently etched faster than sputtered ZnS films under the same conditions. However, when patterned, all three ZnS films exhibited similar etch rates.
In situ substrate temperature measurement in high-T
c superconducting film deposition
Authors: Choi B. I. , Anderson Alfredo C. , Westerheim A. C. , Flik M. I.
Published: July 2002

An adaptively calibrated pyrometer (ACP) is employed to measure the substrate surface temperature during the YBa2Cu3O7−δ film growth process. The film-thickness-dependent emittance of the film-substrate composite is determined from thin-film optics for instantaneous calibration of the pyrometer. The optical constants of the LaAlO3 substrate and the YBa2Cu3O7−δ film are measured at high temperatures using a Fourier transform infrared spectrometer with a special sample holder that controls the sample temperature and oxygen pressure. The accuracy and precision of the ACP are demonstrated through both in situ and ex situ measurements. The present study facilitates exact and reproducible temperature control in high-Tc superconducting film growth processes.
New approach to quantitative surface photovoltage spectroscopy analysis
Authors: Kronik L. , Shapira Yoram
Published: July 2002

A new approach is presented for determining surface state parameters at semiconductor interfaces using time resolved surface photovoltage spectroscopy. The analysis provides precise determination of surface state energies, densities, occupancies, and cross sections for electrons and photons (including the wave-length dependence of the photonic cross sections). Unlike previous methods, this approach does not require prior knowledge of the absolute band bending, but rather determines it among the other parameters. The algorithm is extended for any number of discrete surface energy levels.
Scanning tunneling microscopy and atomic force microscopy studies of biomaterials at a liquid–solid interface
Authors: Lindsay S. M. , Lyubchenko Y. L. , Tao N. J. , Li Y. Q. , Oden P. I. , DeRose J. A. , Pan J.
Published: July 2002

We have compared the scanning tunneling microscopy (STM) and atomic force microscopy (AFM) using a clean gold surface under electrochemical potential control, finding that the STM usually yields higher resolution, although this may be a consequence of better production methods for STM tips. New methods for tethering DNA molecules to substrates have yielded many excellent AFM images of DNA, even under water. The highest resolution has been obtained with specially sharpened tips, but recent work suggests that this may not be necessary. Control of contamination, and tip–sample interactions (i.e., adhesion) are also important factors. We describe a scheme of magnetic control of AFM cantilevers which should overcome the mechanical instability that places a lower limit on contact force. This should permit active control of interaction forces with the instrument operated in water. We illustrate local force measurements made under water with examples of the measurements of the microelastic properties of bone and of the electrostatic interactions between pairs of colloidal particles. The STM has been used to image small DNA oligomers on a gold electrode in water at high resolution. STM images of DNA bases show that the purines form stacked aggregates with the same base-stacked structure as found in DNA. There is evidence of a connection between the macroscopically measured electrochemical properties of thymine and its STM contrast. Such electronic sensitivity might be useful for chemical identification of molecules.
Electron beam lithography over large scan fields
Authors: Bazán Greg , Bernstein Gary H.
Published: July 2002

We have developed an advanced, research-oriented electron beam lithography system using a three channel arbitrary wave form generator and a scanning electron microscope. The system is capable of exposing patterns with features less than 100 nm over scan fields greater than 1.0 mm, as well as features less than 20 nm in smaller scan fields. Other system features include capabilities for registration and for generating exposure matrices for evaluating new electron beam resists and optimizing exposure parameters. We report the details of the system and show results made possible by our system.
Thin film sensors: The role of defects and impurity sites in controlling sensor response and selectivity
Authors: Collins G. E. , Armstrong N. R. , Pankow J. W. , Oden C. , Brina R. , Arbour C. , Dodelet J.-P.
Published: July 2002

Thin film gas sensors are discussed which are produced from polycrystalline organic dyes, such as the phthalocyanines, perylenes, etc. or from metal oxides, such as SnO2, ZnO, etc. Both types of materials are typically used to detect gas phase analytes by virtue of the surface conductance changes which accompany chemisorption of the analyte. In the case of the thin film oxides this process is understood in terms of the change in surface charge density and band bending which accompanies chemisorption of electron donors or acceptors. Defects in the oxide structure, or intentionally added impurities in the near surface region, can enhance the charge density at the surface and provide specific chemisorption sites for various analytes, which are the main means of introducing selectivity to these sensors. In the case of the organic dye thin films, hopping processes, rather than movement of charge through valence and conduction bands, control the flow of charge. Significantly higher defect and impurity densities are anticipated, versus the metal oxide thin films, and movement of charge through trap sites accounts for a significant fraction of the charge transfered. Illumination of certain organic thin films can enhance the sensitivity of the sensor, by a factor of up to 104, depending upon the ratio of dark to photoconductivity in the solid. Addition of electron or hole traps in the near surface region of the thin film can introduce chemisorption sites which interact more selectively with the analyte, and further increase sensitivity. Examples of this approach are given, for the detection of NH3 and NO2, using selected photoconductive phthalocyanine thin films.
Production of a dense metal-ion stream for ion coating by a pulsed vacuum arc in a strong magnetic field
Authors: Nakagawa Yoshiro , Itami Masao
Published: July 2002

A dense metal-ion stream (∼1×1018/shot) was produced by a high-current, long-pulse vacuum arc (∼2 kA, ∼25 ms) in a strong magnetic field. The ion stream was transported 40 cm by the magnetic field to a substrate and then deposited on it. Fast film formation (∼200 Å/shot), such as TiN and TiC films, was achieved by this device using a Ti electrode discharge in N2 or CH4 gas.
Time-of-flight secondary ion mass spectrometry study of P2S5/(NH4)2S- and ultraviolet/ozone-treated GaAs
Authors: Bennett J. , Dagata J. A.
Published: July 2002

Time-of-flight secondary ion mass spectrometry has been applied to the characterization of P2S5/(NH4)2S-treated and ultraviolet (UV)/ozone-oxidized GaAs (100) surfaces. Emphasis is placed on investigating the structural and chemical characteristics of the UV/ozone-induced oxide. Negative ion mass spectra obtained from passivated and UV/ozone-treated GaAs surfaces showed the presence of an arsenic-oxide-rich top layer with traces of S. Shallow depth profiling through the topmost layers indicated the presence of a gallium-oxide-rich layer under the arsenic-oxide-rich layer in samples exposed to UV/ozone for 15 min or longer. The gallium-oxide-rich layer also contained traces of S. The growth of the gallium-oxide-rich region is directly related to the time spent in the UV/ozone oxidation step.
Extremely high vacuum system for dynamical surface analysis
Authors: Tsukui Katsuyuki , Endo Kazuhiko , Hasunuma Ryu , Osaka Toshiaki , Ohdomari Iwao , Yagi Nobuaki , Aihara Hajime
Published: July 2002

In order to investigate surface structures dynamically, it is necessary to reduce the influence of adsorbates on the surface. We have developed an extremely high vacuum (XHV) system that maintains the required pressure even during dynamical reflection high energy electron diffraction (RHEED) observations. The main analysis chamber was evacuated by a Ti sublimation pump and a 4.2 K class cryosorption pump, and the XHV condition (<1×10−12 Torr) could be kept for longer than 200 h without any additional operations. To reduce the outgassing from the components of the pump such as the casing wall and cryopanels, the cryosorption pump was modified to allow baking. Addition of a RHEED system has been done successfully without degrading the XHV condition by using a differential pumping system for a hot cathode electron gun. The total pressure has been kept below 4×10−12 Torr even when specimens were heated to 1000 K. By using this analysis system, we can observe solid surfaces dynamically by electron diffraction.
Thermal stability of two-dimensional atomic structures of Au–Ag adsorbates on Si(111) surfaces
Authors: Yuhara J. , Inoue M. , Morita K.
Published: July 2002

Isochronal annealing of Au–Ag binary adsorbates at the Si(111) surface at temperatures from 200 to 430 °C has been studied by means of low energy electron diffraction, Auger electron spectroscopy, and Rutherford backscattering spectroscopy techniques. It was found that a √3×√3−(Au,Ag) structure was formed by annealing for 15 min at 200 °C Ag atoms on the Si(111)–5×1-Au surface at Au coverages less than 0.60 ML, and that the √3×√3 structure was kept until Ag decreases from 0.70 ML down to 0.18 ML due to the annealing, while the initial Au coverage was kept. Furthermore, when the Ag atoms, which are more than 0.60 ML, on the Si(111)–√3×√3-Au surface were annealed at 200 °C, a new structure of 2√3×2√3-(Au,Ag) was observed. Then, the √21 ×√21R(±10.89 °)-(Au,Ag) structure was formed by prolonged isochronal annealing and was kept until Ag decreases down to 0.12 ML, while the initial Au coverage was kept. From analysis of the decay curves of Ag coverage, it was determined that the activation energies for Ag atoms on Si(111)–5×1-Au and Si(111)–√3×√3-Au surfaces to be dissolved into the bulk were 0.61 ± 0.15 and 0.52 ± 0.15 eV, respectively.