bluuebunny/crossref_metadata_2025 huggingface dataset:
Comprehensive Assessment of Semi-Active Vibration Suppression Including Energy Analysis
Authors: Makihara Kanjuro , Onoda Junjiro , Minesugi Kenji
Published: January 2007

This paper presents an extensive investigation on the LR-switching method (also called the energy-recycling semi-active method). Compared with the energy-dissipative R-switching method, the LR-switching method has been shown to have significantly better vibration suppression performance. However, certain essential issues affecting a system employing the LR-switching method remained to be dealt with. In particular, we had to clarify its vibration suppression mechanism from the viewpoint of mechanical and electrical energy exchange. Second, the robustness of the method against model errors and control time delays had to be verified. The experiments and numerical simulations that we conducted on a 10-bay truss structure demonstrate that the LR-switching method outperforms other suppression methods under sinusoidal and random excitations, which are more common in real systems and more difficult to deal with than transient vibrations. This paper provides fundamental insights on the LR-switching method and gives the method a guarantee for actual applications.
Braking Impact of Normal Dither Signals
Authors: Badertscher Jeff , Cunefare Kenneth A. , Ferri Aldo A.
Published: January 2007

Dither control is a method of introducing high-frequency control efforts into a system to suppress a lower-frequency disturbance. One application of dither control is the suppression of automotive brake squeal. Brake squeal is a problem that has plagued the automotive industry for years. Placing a piezoceramic stack actuator in the piston of a floating caliper brake creates an experimental normal dither system. Many theoretical models indicate a reduction in the braking torque due to the normal dither signal. Using a Hertzian contact stiffness model, the loss in friction is due to lowering the average normal force. There are also theories that the dither signal eliminates the “stick-slip” oscillation causing an effective decrease in the friction force. Yet another theory indicates that the effective contact area is reduced, lowering the mean coefficient of friction. A particular approach considering a single-degree-of-freedom friction oscillator predicts a maximum friction reduction of 10%, occurring at the primary resonance of the system. This paper will concentrate on validating this claim by experimentally determining braking torque reduction for a variety of dither control signals. Several dither control frequencies were chosen at system resonances, while others were chosen at frequencies most likely to provide control of the system. These frequencies were chosen based on previous squeal suppression research. The results indicate that dither control frequencies at system resonances have a greater impact on the braking system’s performance. In general, dither control reduces braking torque by no more than 2%.
Anisotropic Polarized Emission of a Doped Silicon Lamellar Grating
Authors: Marquier F. , Laroche M. , Carminati R. , Greffet J.-J.
Published: January 2007

Thermal emission of a doped silicon grating has been studied in the plane perpendicular to the grooves. We show how the excitation of surface plasmons produce a resonant emission weakly depending on the polarization and azimuthal angle. We analyze in detail the polarization and angular dependence of the emission out of the plane perpendicular to the grooves. Two kinds of thermal sources, directional and quasi-isotropic, are studied. They have been designed in a previous paper. We also compute the total hemispherical emissivity of these gratings. In addition we show that in applications such as radiative cooling, these sources are less efficient than other structures.
Advances in Laser Cooling of Solids
Authors: Ruan X. L. , Kaviany M.
Published: January 2007

We review the progress on laser cooling of solids. Laser cooling of ion-doped solids and semiconductors is based on the anti-Stokes fluorescence, where the emitted photons have a mean energy higher than that of the absorbed photons. The thermodynamic analysis shows that this cooling process does not violate the second law, and that the achieved efficiency is much lower than the theoretical limit. Laser cooling has experienced rapid progress in rare-earth-ion doped solids in the last decade, with the temperature difference increasing from 0.3to92K. Further improvements can be explored from the perspectives of materials and structures. Also, theories need to be developed, to provide guidance for searching enhanced cooling performance. Theoretical predictions show that semiconductors may be cooled more than ion-doped solids, but no success in bulk cooling has been achieved yet after a few attempts (due to the fluorescence trapping and nonradiative recombination). Possible solutions are discussed, and net cooling is expected to be realized in the near future.
Modeling of Elastic, Thermal, and Strength/Failure Analysis of Two-Dimensional Woven Composites—A Review
Authors: Onal Levent , Adanur Sabit
Published: January 2007

The usage of textile structures as a reinforcement for polymer composites became essential in many industrial applications in, for example, the marine and aerospace industries because of their favorable stiffness and strength to weight ratio. Determination of elastic properties and failure behavior of textile reinforced composites is vital for industrial design and engineering applications. This paper aims to present a review of numerical and analytical models for elastic, thermal, and strength/failure analysis of 2D reinforced woven composites. Major modeling techniques and approaches are presented. A state of the art review of woven fabric composites is presented starting from earlier one-dimensional models to recent three-dimensional models. The intention is not to give a detailed analysis of the mathematical approaches to the models discussed, but rather to inform researchers about the main ideas of previous works. This review article cites 122 references.
Effects of Wall Calcifications in Patient-Specific Wall Stress Analyses of Abdominal Aortic Aneurysms
Authors: Speelman Lambert , Bohra Ajay , Bosboom E. Marielle H. , Schurink Geert Willem H. , van de Vosse Frans N. , Makaroun Michel S. , Vorp David A.
Published: January 2007

It is generally acknowledged that rupture of an abdominal aortic aneurysm (AAA) occurs when the stress acting on the wall over the cardiac cycle exceeds the strength of the wall. Peak wall stress computations appear to give a more accurate rupture risk assessment than AAA diameter, which is currently used for a diagnose. Despite the numerous studies utilizing patient-specific wall stress modeling of AAAs, none investigated the effect of wall calcifications on wall stress. The objective of this study was to evaluate the influence of calcifications on patient-specific finite element stress computations. In addition, we assessed whether the effect of calcifications could be predicted directly from the CT-scans by relating the effect to the amount of calcification present in the AAA wall. For 6 AAAs, the location and extent of calcification was identified from CT-scans. A finite element model was created for each AAA and the areas of calcification were defined node-wise in the mesh of the model. Comparisons are made between maximum principal stress distributions, computed without calcifications and with calcifications with varying material properties. Peak stresses are determined from the stress results and related to a calcification index (CI), a quantification of the amount of calcification in the AAA wall. At calcification sites, local stresses increased, leading to a peak stress increase of 22% in the most severe case. Our results displayed a weak correlation between the CI and the increase in peak stress. Additionally, the results showed a marked influence of the calcification elastic modulus on computed stresses. Inclusion of calcifications in finite element analysis of AAAs resulted in a marked alteration of the stress distributions and should therefore be included in rupture risk assessment. The results also suggest that the location and shape of the calcified regions—not only the relative amount—are considerations that influence the effect on AAA wall stress. The dependency of the effect of the wall stress on the calcification elastic modulus points out the importance of determination of the material properties of calcified AAA wall.
Obtaining Subwavelength Optical Spots Using Nanoscale Ridge Apertures
Authors: Jin E. X. , Xu X.
Published: January 2007

Concentrating light into a nanometer domain is needed for optically based materials processing at the nanoscale. Conventional nanometer-sized apertures suffer from low light transmission, therefore poor near-field radiation. It has been suggested that ridge apertures in various shapes can provide enhanced transmission while maintaining the subwavelength optical resolution. In this work, the near-field radiation from an H-shaped ridge nanoaperture fabricated in an aluminum thin film is experimentally characterized using near-field scanning optical microscopy. With the incident light polarized along the direction across the gap in the H aperture, the H aperture is capable of providing an optical spot of about 106nm by 80nm in full-width half-maximum size, which is comparable to its gap size and substantially smaller than those obtained from the square and rectangular apertures of the same opening area. Finite different time domain simulations are used to explain the experimental results. Variations between the spot sizes obtained from a 3×3 array of H apertures are about 4–6%. The consistency and reliability of the near-field radiation from the H apertures show their potential as an efficient near-field light source for materials processing at the nanoscale.
Surface Plasmon Scattering by Gold Nanoparticles and Two-Dimensional Agglomerates
Authors: Venkata Pradeep Garudadri , Aslan Mustafa M. , Mengüç M. Pinar , Videen Gorden
Published: January 2007

There has long been an interest in nanosized metallic particles for numerous novel applications, from the productions of colored glass in medieval times to the molecular-level sensors of today. These particles are known to display considerably different, and size-dependent, optical properties than those of their bulk counterparts. Yet it is very difficult to determine the size and structure of these particles in situ, such as monitoring the actual self-assembly process, because of their small size. In this paper, we present a methodology to predict the patterns of nanosized particles and agglomerates subjected to surface plasmon waves. For this characterization, the scattering patterns of different types of particles and agglomerates on or near the surface are needed. A combination of the T-matrix method, image theory, and a double interaction model are considered. The incident and scattered fields are expanded by employing spherical harmonic functions. The surface effects are incorporated using the Fresnel equations, in the incident-field expansion coefficients, and by including particle-surface interaction fields. The premise of the method is that the T-matrix is independent of incident and scattered fields and hence can be used effectively for cases involving incident surface waves. By obtaining the T-matrix for clusters or agglomerates of metallic particles, the scattering matrix elements (M11, M12, M33, and M34) of agglomerated structures on the surface are calculated using an additional T-matrix operation. The effect of size, shape, and orientation of gold nanosized particles on their scattering patterns are explored both in the visible spectrum and at resonance wavelengths. The results show that the normalized scattering matrix elements at certain observation angles and incident wavelengths provide significant information to monitor the structural change of gold nanosized particles on a gold substrate.
Radiative Properties
of Patterned Wafers With Nanoscale Linewidth
Authors: Chen Y.-B. , Zhang Z.
M. , Timans P.
J.
Published: January 2007

Temperature nonuniformity is a critical problem in rapid thermal processing (RTP) of wafers because it leads to uneven diffusion of implanted dopants and introduces thermal stress. One cause of the problem is nonuniform absorption of thermal radiation, especially in patterned wafers, where the optical properties vary across the wafer surface. Recent developments in RTP have led to the use of millisecond-duration heating cycle, which is too short for thermal diffusion to even out the temperature distribution. The feature size is already below 100nm and is smaller than the wavelength (200-1000nm) of the flash-lamp radiation. Little is known to the spectral distribution of the absorbed energy for different patterning structures. This paper presents a parametric study of the radiative properties of patterned wafers with the smallest feature dimension down to 30nm, considering the effects of temperature, wavelength, polarization, and angle of incidence. The rigorous coupled wave analysis is employed to obtain numerical solutions of the Maxwell equations and to assess the applicability of the method of homogenization based on effective medium formulations.
Infrared Microscopy Thermal Characterization of Opposing Carbon Nanotube Arrays
Authors: Hu X. Jack , Panzer Matthew A. , Goodson Kenneth E.
Published: January 2007

Carbon nanotubes (CNTs) have received much recent research interest for thermal management applications due to their extremely high thermal conductivity. An advanced thermal interface structure made of two opposing, partially overlapped CNT arrays is designed for thermally connecting two contact surfaces. The performance of this interface structure is thermally characterized using diffraction-limited infrared microscopy. Significant temperature discontinuities are found at the CNT-CNT contact region, which indicates a large thermal resistance between CNTs. Due to this intertube resistance, the thermal performance of the CNT-based interface structure is far below expectation (with a thermal resistance value about 3.8×10−4Km2∕W).
On Higher Order Point-Line and the Associated Rigid Body Motions
Authors: Zhang Yi , Ting Kwun-Lon
Published: January 2007

This paper presents a study on the higher-order motion of point-lines embedded on rigid bodies. The mathematic treatment of the paper is based on dual quaternion algebra and differential geometry of line trajectories, which facilitate a concise and unified description of the material in this paper. Due to the unified treatment, the results are directly applicable to line motion as well. The transformation of a point-line between positions is expressed as a unit dual quaternion referred to as the point-line displacement operator depicting a pure translation along the point-line followed by a screw displacement about their common normal. The derivatives of the point-line displacement operator characterize the point-line motion to various orders with a set of characteristic numbers. A set of associated rigid body motions is obtained by applying an instantaneous rotation about the point-line. It shows that the ISA trihedrons of the associated rigid motions can be simply depicted with a set of ∞2 cylindroids. It also presents for a rigid body motion, the locus of lines and point-lines with common rotation or translation characteristics about the line axes. Lines embedded in a rigid body with uniform screw motion are presented. For a general rigid body motion, one may find lines generating up to the third order uniform screw motion about these lines.
Numerical and Experimental Study of the Loaded Transmission Error of a Spiral Bevel Gear
Authors: de Vaujany Jean-Pierre , Guingand Michèle , Remond Didier , Icard Yvan
Published: January 2007

The design of spiral bevel gears in aeronautical gear boxes requires very precise and realistic numerical simulations. One important criteria is the loaded transmission error (LTE) that gear designers attempt to reduce at the nominal torque. This paper presents a numerical tool that simulates the loaded meshing of spiral bevel gears and experimental tests carried out on a real helicopter gear box. Tooth profile is defined by the Gleason cutting process and tooth bending effects and contact deformations are both taken into account. The bending effect computation uses a three-dimensional finite element model, while the contact deformations are obtained by using Boussinesq’s theory. Experimental measurements of the LTE were performed using magnetic and optical encoders rigidly connected with the pinion and gear shafts, giving access to the records of the instantaneous angular positions. The numerical simulations fit quite well the experimental results.
Load Distribution in Spiral Bevel Gears
Authors: Simon Vilmos
Published: January 2007

A new approach for the computerized simulation of load distribution in mismatched spiral bevel gears with point contact is presented. The loaded tooth contact is treated in a special way: it is assumed that the point contact under load spreads over a surface along the “potential” contact line (Simon, 2006, Mech. and Machine Theory, in press), which line is made up of the points of the mating tooth surfaces in which the separations of these surfaces are minimal, instead of assuming the usually applied elliptical contact area. The bending and shearing deflections of gear teeth, the local contact deformations of mating surfaces, gear body bending and torsion, the deflections of supporting shafts, and the manufacturing and alignment errors of mating members are included. The tooth deflections of the pinion and gear teeth are calculated by the finite element method. As the equations governing the load sharing among the engaged tooth pairs and load distribution along the tooth face are nonlinear, an approximate and iterative technique is used to solve this system of equations. The method is implemented by a computer program. By using this program the load and tooth contact pressure distributions, the angular displacements of the driven gear and the stresses in the pinion and gear teeth are calculated. The influence of design data and transmitted torque on load distribution parameters and fillet stresses is investigated and discussed.
Finite Element Modeling of Self-Loosening of Bolted Joints
Authors: Zhang Ming , Jiang Yanyao , Lee Chu-Hwa
Published: January 2007

A three-dimensional finite element (FE) model with the consideration of the helix angle of the threads was developed to simulate the second stage self-loosening of a bolted joint. The second stage self-loosening refers to the gradual reduction in clamping force due to the back-off of the nut. The simulations were conducted for two plates jointed by a bolt and a nut and the joint was subjected to transverse or shear loading. An M12×1.75 bolt was used. The application of the preload was simulated by using an orthogonal temperature expansion method. FE simulations were conducted for several loading conditions with different preloads and relative displacements between the two clamped plates. It was found that due to the application of the cyclic transverse load, microslip occurred between the contacting surfaces of the engaged threads of the bolt and the nut. In addition, a cyclic bending moment was introduced on the bolted joint. The cyclic bending moment resulted in an oscillation of the contact pressure on the contacting surfaces of the engaged threads. The microslip between the engaged threads and the variation of the contact pressure were identified to be the major mechanisms responsible for the self-loosening of a bolted joint. Simplified finite element models were developed that confirmed the mechanisms discovered. The major self-loosening behavior of a bolted joint can be properly reproduced with the FE model developed. The results obtained agree quantitatively with the experimental observations.
Recommended practice for process sampling for partial pressure analysis
Authors: Blessing James E. , Ellefson Robert E. , Raby Bruce A. , Brucker Gerardo A. , Waits Robert K.
Published: January 2007

This Recommended Practice describes and recommends various procedures and types of apparatus for obtaining representative samples of process gases from >10−2Pa (10−4Torr) for partial pressure analysis using a mass spectrometer. The document was prepared by a subcommittee of the Recommended Practices Committee of the American Vacuum Society. The subcommittee was comprised of vacuum users and manufacturers of mass spectrometer partial pressure analyzers who have practical experience in the sampling of process gas atmospheres.
Epitaxial (SrTiO3∕NiO)n∕MgO multiferroic heterostructure
Authors: Kirby S. D. , Polking M. , van Dover R. B.
Published: January 2007

High quality epitaxial (SrTiO3∕NiO)n∕MgO thin film multiferroic heterostructures have been fabricated using reactive off-axis sputtering. Crystal quality is verified using x-ray diffraction and ion channeling. These heterostructures comprise of dielectric and antiferromagnetic layers that exhibit dielectric resonance and antiferromagnetic resonance, respectively, at terahertz frequencies. In order to achieve epitaxy despite the large lattice mismatch between SrTiO3 and NiO or MgO, high processing temperature is required. The high temperature has led to a small amount of interdiffusion between any SrTiO3∕MgO interfaces. Less interdiffusion is evident for the NiO∕MgO interface as seen through Rutherford backscattering. NiO and SrTiO3 have been shown to be compatible and nonreacting at temperatures as high as 1500°C through the use of bulk ceramic techniques.
High-power pulsed sputtering using a magnetron with enhanced plasma confinement
Authors: Vlček Jaroslav , Kudláček Pavel , Burcalová Kristýna , Musil Jindřich
Published: January 2007

High-power pulsed dc magnetron discharges for ionized high-rate sputtering of metallic films were systematically investigated. The depositions were performed using two unbalanced circular magnetrons of different types with a directly water-cooled planar copper target of 100mm in diameter. The repetition frequency was 1kHz at a fixed 20% duty cycle and an argon pressure of 0.5Pa. Time evolutions of the discharge characteristics were measured to provide information on absorption of energy in the discharge plasma and on transfer of arising ions to the substrate at a target power density in a pulse up to 950W∕cm2. Time-averaged mass spectroscopy was performed at the substrate position to characterize ion energy distributions and composition of total ion fluxes onto the substrate. The deposition rate of the copper films formed on a floating substrate at the distance of 100mm from the target was 2.2μm∕min at an average target power density over a pulse period of 96W∕cm2. Very effective ionization of sputtered copper atoms resulted in a strong predominance of copper ions (up to 92%) in total ion fluxes onto the substrate. Trends in measured values of the deposition rate per average target power density and the ionized fraction of sputtered copper atoms in the flux onto the substrate (up to 56%) were explained on the basis of model predictions.
Inertial interlock bonding: A new approach to sputtering target construction
Authors: Wickersham C. E. , Workman David
Published: January 2007

Inertial interlock bonding (IIB) was developed to provide a reliable low cost automated method for joining dissimilar metal plates at low bulk bonding temperatures for the production of sputtering targets. Joining dissimilar metals at elevated temperatures creates residual stress from the differential thermal expansion of the bimetallic plates. The IIB method of joining dissimilar metal plates alleviates the need to increase the bulk temperature of the metal plates during bonding and thereby significantly reduces the residual stress. In IIB, the heat generated by inertial friction between two dissimilar metal plates is used to simultaneously (1) soften an intercalated ring that is forged into a matching T groove and (2) melt a thin solder layer that wets the other plate creating a reliable solder bond. While finite element modeling of the process indicates that the surface temperatures during bonding approach 1000°C, the bulk temperature for the assembly remains below 70°C. In this way, the differential thermal residual stress remains low, while a reliable bimetallic joint is formed. Results for joining tantalum to copper alloy plates for sputtering target applications are provided.
Phosphor coatings to enhance Si photovoltaic cell performance
Authors: Chung Philip , Chung Hsiu-hsin , Holloway Paul H.
Published: January 2007

Polycrystalline Si photovoltaic cells were coated with photoluminescent phosphors to increase the power conversion efficiency by down-conversion of high energy ultraviolet photons to less energetic visible photons, for which the Si solar cell conversion efficiency is greater. Phosphor coatings consisting of Y2O3:Eu3+, Y2O2S:Eu3+, 3,9-perylenedicarboxylic acid bis(2-methylpropyl) ester, and tetraphenylporphyrin dispersed in either polyvinyl alcohol or polymethylmethacrylate were tested. Cells coated with the Y2O3:Eu3+ or Y2O2S:Eu3+ showed an increased power conversion efficiency by up to a factor of 14 under UV illumination. The increased power conversion efficiency was shown to depend on the quantum efficiency of the down-conversion phosphor, the overlap between the photoluminescence excitation and the illumination spectra, and the overlap of the emission spectrum and the solar cell spectral response function.
Penetration of fluorine into the silicon lattice during exposure to F atoms, F2, and XeF2: Implications for spontaneous etching reactions
Authors: Winters Harold F. , Graves D. B. , Humbird D. , Tougaard Sven
Published: January 2007

The purpose of this article is to present evidence about the quantity and distribution of fluorine in silicon after and during spontaneous etching with F atoms, F2, and XeF2. X-ray photoelectron spectroscopy spectra were analyzed using the method developed by Sven Tougaard designated in this paper as “ST.” It is found that fluorine penetrates deeply into the silicon lattice during the etching reaction. It is shown that the surface concentration of fluorine (first 10Å) is relatively independent of whether F atoms of XeF2 were used and is also relatively independent of doping level. In contrast, the amount of fluorine in the silicon lattice does depend upon these parameters. It is suggested that the fluorine in the lattice exists primarily as negative ions. Fluorine on the surface (top 10–20Å) is probably SiFx (x=1–3). The authors data are consistent with the data and interpretation of Lo et al. [J. Vac. Sci. Technol. A 11, 2054 (1993)] who found the surface concentration of F at saturation to be ∼1.7×1015F∕cm2 (1.7 ML, 1.1×1015F/ML). They found that SiF3 was the dominant species on the surface at saturation. The concentration of F− ions in the lattice is so large [∼1020to4×1021∕cm3] that they probably neutralize holes and donor sites and also are likely to dope the system with deep lying acceptor states which lead to an increased concentration of holes at the valence band maximum. A p-n junction may be created. In addition, the total quantity of fluorine in the lattice is estimated. These new values are derived from previously published work using a more reliable calibration. They are about 2.5 times higher than the original estimate. It will also be shown that the etch properties are influenced by the negative ions in the silicon lattice. The ions adjust their concentration and depth distribution to fit the etch conditions. The time constant for this adjustment is undetermined but is many minutes at room temperature and moderate fluxes [see Fig. 15 of Ref. 3(a)].
Development of a method for investigating carbon removal processes during photoassisted film growth using organometallic precursors: Application to platinum
Authors: Cahill John J. , Panayotov Valentin G. , Cowen Kenneth A. , Harris Ernest , Koplitz Lynn V. , Birdwhistell Kurt , Koplitz Brent
Published: January 2007

A method is used to investigate carbon removal pathways during metal film growth using organometallic precursors. The approach combines a time-of-flight mass spectrometer with a growth chamber from which substrates can be removed during real-time film growth. Consequently, evolving mass spectral signatures can be correlated with changing film properties. Although more general as a technique, results are presented for the photoassisted growth of Pt from CH3CpPt(CH3)3 in a D2 atmosphere. Here, a marked increase in deuterium/hydrogen exchange is clearly correlated with an increase in the Pt:C ratio for the metal films, as determined by x-ray photoelectron spectroscopy. However, results for growth with CH3CH2CpPt(CH3)3 as well as (CH3)3CCpPt(CH3)3 suggest that while extensive D∕H exchange can be a feature of the growth process, it is not a prerequisite for producing films with relatively high Pt:C ratios.
Corrosion behavior of Zr modified CrN coatings using metal vapor vacuum arc ion implantation
Authors: Purushotham K. P. , Ward L. P. , Brack N. , Pigram P. J. , Evans P. , Noorman H. , Manory R. R.
Published: January 2007

In recent years, attention has focused on the use of alternative metal nitride coatings as replacements for TiN for not only improved wear resistance and surface hardness but also for increased corrosion resistance in selected environments. While these coatings display excellent wear resistance and surface hardness, like many nitride coatings, their corrosion behavior is determined to a large extent by the presence of defects such as pinholes within the coating. Improved corrosion resistance is expected through minimizing the porosity/number of pinholes within the coating, through postdeposition surface modification. The aim of this study was to modify the surface of CrN coatings using metal vapor vacuum arc ion implantation. CrN coatings were deposited on AISI 316 stainless steel and AISI 1020 mild steel substrates using physical vapor deposition technology, followed by implantation of Zr ions into the coating at doses varying from 6×1016to2×1017ions∕cm2. The corrosion behavior was assessed in saline environments using linear polarization techniques and the corroded surface of the coatings was characterized using x-ray photoelectron spectroscopy and scanning electron microscopy. The results of the study showed that implantation of Zr ions into CrN resulted in a lowering of the corrosion current density, suggesting improved corrosion resistance. This was though to be associated with two factors. Firstly, partial closure of the pinholes as a result of the implantation process and secondly, the formation of ZrN, CrZrN, and various oxynitrides/oxides at the surface.
Modeling of the chemically assisted ion beam etching process: Application to the GaAs etching by Cl2∕Ar+
Authors: Elmonser L. , Rhallabi A. , Gaillard M. , Landesman J. P. , Talneau Anne , Pommereau F. , Bouadma N.
Published: January 2007

The development of two-dimensional chemically assisted ion beam etching model of GaAs by Cl2∕Ar+ allowed the authors to analyze the role of some critical parameters of etch process on the morphology of trench and mesa structures. In a fact, the simulation results show that the etch rate variation with substrate temperature Ts reveals three regimes: For Ts<350K, the etch rate of GaAs is mainly controlled by ion assisted etching component. For 350<Ts<TM, where TM corresponds to the maximum of etch rate, a high variation of etch rate is observed. In this range of temperature, the etch rate is controlled by the chemical component and mainly by the exponential rise of desorption coefficient of GaClx,λ3, with Ts. For Ts>TM a fast decrease of the etch rate is observed because of the high decrease of the coverage rate of Cl, ξ, on the gallium sites with the substrate temperature. On the other hand, the increase of the Cl2 flow rate allows to improve the anisotropy and contributes to the elimination of both the microtrenching and the transfer of the facets from the mask into the substrate.
Characterization of neutral beam source based on pulsed inductively coupled discharge: Time evolution of ion fluxes entering neutralizer
Authors: Abolmasov Sergey N. , Ozaki Takuya , Samukawa Seiji
Published: January 2007

Low-energy neutral beam sources are very promising candidates for realization of next generation ultralarge-scale integrated devices. The use of pulsed inductively coupled plasma and surface (wall) neutralizer appears to be an efficient way of producing high-flux low-energy neutral beams. Measurement of the time evolution of ion fluxes entering the neutralizer plays an essential role in understanding and control of these neutral beam systems. Here the authors present a simple method for measuring the temporal dynamics of ion fluxes in neutral beam source described elsewhere [S. Samukawa et al., J. Vac. Sci. Technol. A 20, 1566 (2002)]. The method is based on the use of a low aspect ratio orifice in the center of neutralizer, magnetic filter, and Faraday cup. At some conditions, it allows (1) to measure the magnitudes of positive and negative wall ion fluxes in pulsed plasmas with an extremely high temporal resolution (better than 1μs) and (2) to examine the difference in surface neutralization between positive and negative ions. The measurements show that neutralization of hyperthermal ions is mainly controlled by geometry of plasma sheath adjacent to the surface neutralizer; however, negative ions are neutralized more easily than positive ones. The experimental results for SF6 (ion-ion) and Ar plasmas in combination with dc/rf bias are reported.
Initial nucleation and growth of atomic layer deposited HfO2 gate dielectric layers on Si surfaces with the various surface conditions using in situ medium energy ion scattering analysis
Authors: Chung K. B. , Whang C. N. , Chang H. S. , Moon D. W. , Cho M.-H.
Published: January 2007

The initial nucleation and growth of atomic layer deposited HfO2 films under various surface conditions were investigated by in situ medium energy ion scattering analysis. The influences of an O–H terminated surface on the initial growth stage were investigated in detail using the atomic density of Hf that reacted on the surface. The measured growth rate of HfO2 per cycle was applied to a mathematical model based on classical chemical kinetics. A parabolic initial growth with an extremely low rate at the initial stage of growth was observed for the film with a hydrogen-terminated surface. However, linear growth, with a value of 1.41×1014Hfatoms∕cm2cycle, was maintained for films grown on an O–H terminated surface. The ∼1∕6 steric hindrance factor extracted from a phenomenological model was related to the size of the tetrahedral HfCl4 molecule and the possible attachment sites. Moreover, the surface roughness and electrical properties of the atomic layer deposited HfO2 films show a strong dependence on the initial nucleation and growth on the different surface conditions.
Module to guide the expert use of x-ray photoelectron spectroscopy by corrosion scientists
Authors: Castle J. E.
Published: January 2007

This contribution, to the potential development of data systems having some degree of “expert” character for use in x-ray photoelectron spectroscopy (XPS), illustrates the manner in which models of “Rules” might be developed by the user community. The field of corrosion science is taken as an example of one community of researchers who make regular use of XPS for well defined needs. These “needs” are redefined as a series of Goals that have to be reached in order to characterize the surface in terms of layer sequences and the enrichment of given elements within them. Rules are written to allow a structured approach to achieve each Goal. A feature of this set of Rules is that they are designed expressly to allow automated interpretation of the survey scan. This approach is facilitated by the use of a recommendation that the survey spectrum be acquired as a series of accumulated scans instead of the usual approach of making a single scan through the spectrum. Repeat scans enable the information extracted by the operation of the Rules to be processed and displayed for information during the period that is normally used for the survey scan. It is intended that this information will inform the setting up of any subsequent high resolution scans and their interactive interpretation. It will also inform any future operations such as ion etching or angle-resolved measurements. In some cases, the information made available may be all that is required by the user and in this case the “expert module” approach becomes particularly cost effective. The operation of the rules is illustrated throughout by an examination of data obtained for passivated stainless steel, giving a data set of measurements, typical of those made by corrosion scientists, that can be compared with the literature values obtained by more conventional XPS interpretation.
Power-law scaling during shadowing growth of nanocolumns by oblique angle deposition
Authors: Tang F. , Karabacak T. , Li L. , Pelliccione M. , Wang G.-C. , Lu T.-M.
Published: January 2007

The authors have investigated the power-law behaviors of various morphological parameters during the shadowing growth of ruthenium (Ru) nanocolumns by an oblique angle sputter deposition technique with substrate rotation. Particularly, wavelength and column number density were measured at different column heights (h). The exponents associated with the wavelength (pλ) and column number density (pn), correlated by the geometrical relationship pλ≈−12pn, were measured by atomic force microscopy to be ∼0.5 and ∼−1.0, respectively. Using a one-dimensional facet growth model based upon the principle of evolutionary selection under oblique angle deposition, they showed that the exponents associated with the column number density and wavelength can be predicted. The authors also illustrated that the exponent value associated with column number density originates from the competitive growth among columns that have different vertical growth rates. The simulated exponent values are independent of the shape of the facet, which indicates the universality of these power-law exponents.
Numerical and experimental comparative study of metal-organic chemical vapor deposition of ZnO
Authors: Liu S. M. , Gu S. L. , Zhu S. M. , Ye J. D. , Liu W. , Zhang R. , Zheng Y. D.
Published: January 2007

In this article a numerical approach has been developed to simulate the ZnO growth carried in a vertical reactor by metal-organic chemical vapor deposition (MOCVD). The coupled transport equations are solved numerically by employing the control-volume-based finite difference method. A three-dimensional model is put forward to analyze the deposition process. Simulations show that the deposition rate decreases with the substrate temperatures ranging between 500 and 700°C. Substrate rotation and large density of the inject nozzles are found to favor significantly improvements on the film uniformity. Computational results show the same general features with the experiment data, which shows that the developed numerical technique is suitable for the simulation and optimization of the MOCVD reactor and could shed some light on the practical growth of ZnO for optoelectric device applications.
In situ spectroscopic ellipsometry analyses of hafnium diboride thin films deposited by single-source chemical vapor deposition
Authors: Yang Yu , Jayaraman Sreenivas , Sperling Brent , Kim Do Young , Girolami Gregory S. , Abelson John R.
Published: January 2007

In situ spectroscopic ellipsometry was used to analyze hafnium diboride thin films deposited by chemical vapor deposition from the single-source precursor Hf(BH4)4. By modeling the film optical constants with a Drude-Lorentz model, the film thickness, surface roughness, and electrical resistivity were measured in situ. The calculated resistivity for amorphous films deposited at low temperature ranged from 340to760μΩcm. These values are within 25% of those measured ex situ with a four-point probe, indicating the validity of the optical model. By modeling the real-time data in terms of film thickness and surface roughness, the film nucleation and growth morphology were determined as a function of substrate type, substrate temperature, and precursor pressure. The data show that at low precursor pressures (∼10−6Torr) and at low substrate temperatures (<300°C), the onset of growth is delayed on both Si and SiO2 surfaces due to the difficulty of nucleation. A higher substrate temperature or precursor pressure reduces this delay. At low temperatures the film morphology is a sensitive function of the precursor pressure because site-blocking effects change the reaction probability; the authors show that the morphology of newly grown film can be reversibly transformed from dense smooth to rough columnar by decreasing the precursor pressure.
Fabrication of layered self-standing diamond film by dc arc plasma jet chemical vapor deposition
Authors: Chen G. C. , Dai F. W. , Li B. , Lan H. , Askari J. , Tang W. Z. , Lu F. X.
Published: January 2007

Layered self-standing diamond films, consisting of an upper layer, buffer layer, and a lower layer, were fabricated by fluctuating the ratio of methane to hydrogen in high power dc arc plasma jet chemical vapor deposition. There were micrometer-sized columnar diamond crystalline grains in both upper layer and lower layer. The size of the columnar diamond crystalline grains was bigger in the upper layer than that in the lower layer. The orientation of the upper layer was (110), while it was (111) for the lower layer. Raman results showed that no sp3 peak shift was found in the upper layer, but it was found and blueshifted in the lower layer. This indicated that the internal stress within the film body could be tailored by this layered structure. The buffer layer with nanometer-sized diamond grains formed by secondary nucleation was necessary in order to form the layered film. Growth rate was over 10μm∕h in layered self-standing diamond film fabrication.
Divalent Metal Nanoparticles
Authors: DeVries Gretchen A. , Brunnbauer Markus , Hu Ying , Jackson Alicia M. , Long Brenda , Neltner Brian T. , Uzun Oktay , Wunsch Benjamin H. , Stellacci Francesco
Published: January 2007

Nanoparticles can be used as the building blocks for materials such as supracrystals or ionic liquids. However, they lack the ability to bond along specific directions as atoms and molecules do. We report a simple method to place target molecules specifically at two diametrically opposed positions in the molecular coating of metal nanoparticles. The approach is based on the functionalization of the polar singularities that must form when a curved surface is coated with ordered monolayers, such as a phase-separated mixture of ligands. The molecules placed at these polar defects have been used as chemical handles to form nanoparticle chains that in turn can generate self-standing films.
Unexpected Stability of Al
4
H
6
: A Borane Analog?
Authors: Li X. , Grubisic A. , Stokes S. T. , Cordes J. , Ganteför G. F. , Bowen K. H. , Kiran B. , Willis M. , Jena P. , Burgert R. , Schnöckel H.
Published: January 2007

Whereas boron has many hydrides, aluminum has been thought to exhibit relatively few. A combined anion photoelectron and density functional theory computational study of the Al
4
H
–
6
anion and its corresponding neutral, Al
4
H
6
, showed that Al
4
H
6
can be understood in terms of the Wade-Mingos rules for electron counting, suggesting that it may be a borane analog. The data support an Al
4
H
6
structure with a distorted tetrahedral aluminum atom framework, four terminal Al-H bonds, and two sets of counter-positioned Al-H-Al bridging bonds. The large gap between the highest occupied and the lowest unoccupied molecular orbitals found for Al
4
H
6
, together with its exceptionally high heat of combustion, further suggests that Al
4
H
6
may be an important energetic material if it can be prepared in bulk.
Comment on "The Geometry of Musical Chords"
Authors: Headlam Dave , Brown Matthew
Published: January 2007

Tymoczko (Reports, 7 July 2006, p. 72) proposed that the familiar sonorities of Western tonal music cluster around the center of a multidimensional orbifold. However, this is not true for all tonal progressions. When prototypical three-voice cadential progressions by Bach converge on the tonic, the chords migrate from the center to the edge of the orbifold.
Brilliant Whiteness in Ultrathin Beetle Scales
Authors: Vukusic Pete , Hallam Benny , Noyes Joe
Published: January 2007

The colored appearances of animals are controlled by pigmentation, highly periodic ultrastructure, or a combination of both. Whiteness, however, is less common and is generated by neither of these, because it requires scattering processes appropriate for all visible wavelengths. We report whiteness resulting from a three-dimensional photonic solid in the scales of Cyphochilus spp. beetles. Their scales are characterized by their exceptional whiteness, their perceived brightness, and their optical brilliance, but they are only 5 micrometers thick. This thickness is at least two orders of magnitude thinner than common synthetic systems designed for equivalent-quality whiteness.
Global-Scale Similarities in Nitrogen Release Patterns During Long-Term Decomposition
Authors: Parton William , Silver Whendee L. , Burke Ingrid C. , Grassens Leo , Harmon Mark E. , Currie William S. , King Jennifer Y. , Adair E. Carol , Brandt Leslie A. , Hart Stephen C. , Fasth Becky
Published: January 2007

Litter decomposition provides the primary source of mineral nitrogen (N) for biological activity in most terrestrial ecosystems. A 10-year decomposition experiment in 21 sites from seven biomes found that net N release from leaf litter is dominantly driven by the initial tissue N concentration and mass remaining regardless of climate, edaphic conditions, or biota. Arid grasslands exposed to high ultraviolet radiation were an exception, where net N release was insensitive to initial N. Roots released N linearly with decomposition and exhibited little net N immobilization. We suggest that fundamental constraints on decomposer physiologies lead to predictable global-scale patterns in net N release during decomposition.
Balanced Inhibition and Excitation Drive Spike Activity in Spinal Half-Centers
Authors: Berg Rune W. , Alaburda Aidas , Hounsgaard Jørn
Published: January 2007

Many limb movements are composed of alternating flexions and extensions. However, the underlying spinal network mechanisms remain poorly defined. Here, we show that the intensity of synaptic excitation and inhibition in limb motoneurons varies in phase rather than out of phase during rhythmic scratchlike network activity in the turtle. Inhibition and excitation peak with the total neuron conductance during the depolarizing waves of scratch episodes. Furthermore, spike activity is driven by depolarizing synaptic transients rather than pacemaker properties. These findings show that balanced excitation and inhibition and irregular firing are fundamental motifs in certain spinal network functions.
Breaking the Embargo
Authors: Horton Richard
Published: January 2007

Embargoed Science
.
By Vincent Kiernan
. University of Illinois Press, Urbana, 2006. 190 pp. $30. ISBN 9780252030970.

The author argues that the practice of embargoing research publications stifles competitive journalism and favors uncritical reporting of science and medical news.