bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteady and Three-Dimensional Simulation of Blood Flow in the Human
Aortic Arch
Authors: Shahcheraghi N. , Dwyer H. A. , Cheer A. Y. , Barakat A. I. , Rutaganira T.
Published: October 2002

A three-dimensional and pulsatile blood flow in a human aortic arch and its three major branches has been studied numerically for a peak Reynolds number of 2500 and a frequency (or Womersley) parameter of 10. The simulation geometry was derived from the three-dimensional reconstruction of a series of two-dimensional slices obtained in vivo using CAT scan imaging on a human aorta. The numerical simulations were obtained using a projection method, and a finite-volume formulation of the Navier-Stokes equations was used on a system of overset grids. Our results demonstrate that the primary flow velocity is skewed towards the inner aortic wall in the ascending aorta, but this skewness shifts to the outer wall in the descending thoracic aorta. Within the arch branches, the flow velocities were skewed to the distal walls with flow reversal along the proximal walls. Extensive secondary flow motion was observed in the aorta, and the structure of these secondary flows was influenced considerably by the presence of the branches. Within the aorta, wall shear stresses were highly dynamic, but were generally high along the outer wall in the vicinity of the branches and low along the inner wall, particularly in the descending thoracic aorta. Within the branches, the shear stresses were considerably higher along the distal walls than along the proximal walls. Wall pressure was low along the inner aortic wall and high around the branches and along the outer wall in the ascending thoracic aorta. Comparison of our numerical results with the localization of early atherosclerotic lesions broadly suggests preferential development of these lesions in regions of extrema (either maxima or minima) in wall shear stress and pressure.
Making Process Visible: A Grammatical Approach to Managing Design Processes
Authors: Chung Moon Jung , Kwon Patrick , Pentland Brian T.
Published: October 2002

This paper presents a novel framework for managing design processes using a formal grammar as the theoretical foundation to represent, manipulate and execute design processes. The grammatical approach allows designers to represent a complex activity concisely with a small number of higher-level tasks and to explore alternative processes within a space of feasible alternatives. These capabilities allow the engineers to “visualize” the design process so that they can fully understand the alternative methods before making any design decisions. The framework, called MIDAS, includes separate layers for process specification and execution. Using the process specification layer, designers can capture the overall design process and each designer can understand his or her task with respect to the whole design process. In the process execution layer, design tasks are executed according to the information in specification layer so that designers can be informed of the current design status, alternative design methods, and their impacts in a whole design process. The framework has the potential to improve design productivity by accessing, reusing, and revising previous processes for a similar design. We use a gearbox design process to demonstrate the framework.
The Tornado Flow Configuration—An Effective Method for Screening of a Solar Reactor Window
Authors: Kogan Abraham , Kogan Meir
Published: October 2002

The working fluid in solar receivers, utilized for effecting chemical reactions, is usually flown through a sealed enclosure provided with a quartz window. When one of the reactants or products of reaction is a powder, care must be taken to prevent contact of the incandescent powder particles with the window, in order to obviate its destruction by overheating. Attempts made in the past to screen the window against particle deposition by a “curtain” of an auxiliary gas stream showed that very substantial flow rates of auxiliary gas (30—80% of the main stream flow rate) were necessary for perfect window screening. The heat absorbed by the auxiliary gas stream represented a major loss of energy. In an effort to reduce the auxiliary stream flow rate to a minimum, a certain flow pattern akin to the natural tornado phenomenon has recently been developed in our laboratory. It enabled effective reactor window screening by an auxiliary gas flow rate less than 5% of the main gas flow rate. The tornado effect is discussed and demonstrated by a smoke flow visualization technique.
Simulation of an Air-Cooled Solar-Assisted Absorption Air Conditioning System
Authors: Alva Luis H. , Gonza´lez Jorge E.
Published: October 2002

This paper investigates the technical feasibility of using a compact, air-cooled, solar-assisted, absorption air conditioning system in Puerto Rico and similar regions. Computer simulations were conducted to evaluate the system’s performance when subjected to dynamic cooling loads. Within the computer model, heat and mass balances are conducted on each component of the system, including the solar collectors, thermal storage tank, the air-cooled condenser, and the air-cooled absorber. Guidance on component design and insight into the effects of such operating factors as ambient air temperature were gained from exercizing the simulation model. Comparisons are made with an absorption air conditioning system that uses a cooling tower instead of air-cooled components. The particular absorption system of study is one that uses lithium bromide and water as the absorbent and refrigerant, respectively. The heat input to the absorption system generator is provided by an array of flat plate collectors that are coupled to a thermal storage tank. Systems having nominal cooling capacities of 10.5, 14, and 17.5 kW were considered. Useful information about the number of collectors needed, storage tank volume, and efficiency of the overall system is presented.
Comparison of the Flame Characteristics of Turbulent Circular and Elliptic Jets in a Crossflow
Authors: Gollahalli S. R. , Pardiwalla D.
Published: October 2002

This study was directed to understand the coupling effects of the noncircular geometry of the burner and a crossflow on the combustion of gas jets. This paper compares the characteristics of turbulent propane jet flames from circular (diameter=0.45 cm) and elliptic (major axis/minor axis=3) burners of equivalent exit area in a crossflow. The elliptic burner was oriented with its major axis or minor axis aligned with the crossflow. Experiments were conducted in a wind tunnel provided with optical and probe access and capable of wind speeds up to 12.5 m/s. The burners were fabricated with metal tubes. Instrumentation included a Pt-Pt/13% Rh thermocouple, a quartz-probe gas sampling system, chemiluminescent and nondispersive infrared analyzers, a video-recorder, and a computer data acquisition system. The measurements consisted of the upper and lower limits of jet velocity for a stable flame, flame configuration, and visible length. Flame structure data including temperature profiles and concentration profiles of CO2,O2, CO, and NO were obtained in a two-zone flame configuration (at jet to crossflow momentum flux ratio=0.11), where a planar recirculation exists in the wake of the burner tube followed by an axisymmetric tail. The relative emission indicators of CO and NO were estimated from the composition data. Results show that the upper and lower limits of the fuel jet velocity increase with the crossflow velocity for all burners, and the rate of increase is highest for the elliptic burner with its minor axis aligned with the crossflow. That burner configuration also produces the longest flame. The relative emission indicators show that the CO production is lower and NO production is higher with elliptic burners than with circular burners in crossflow.
Entropy Production During Laser Picosecond Heating of Copper
Authors: Yilbas Bekir Sami
Published: October 2002

Nonequilibrium energy transport taking place in the surface region of the metallic substrate due to laser short-pulse heating results in entropy production in electron and lattice systems. The entropy analysis gives insight into the irreversible processes taking place in this region during the laser short-pulse heating process. In the present study, entropy production during laser shortpulse heating of copper is considered. Equations governing the nonequilibrium energy transport are derived using an electron kinetic theory approach. The entropy equations due to electron and lattice systems and coupling of these systems are formulated. The governing equations of energy transport and entropy production are solved numerically. Two pulse shapes, namely step input intensity and exponential intensity, are employed in the analysis. It is found that entropy production due to coupling process attains higher values than those produced due to electron and lattice systems. The effect of pulse shape on the entropy production inside the substrate material is significant.
Variable Water Flow Pumping for Central Chilled Water Systems
Authors: Liu M.
Published: October 2002

A new variable water (VW) flow system for chilled water system application is presented. Design, operational, and control issues are discussed. Analytical models of pump power and evaporative temperature are developed. The VW systems consume considerably less pump and compressor energy than the primary secondary (PS) systems under partial load conditions for most existing chillers. The VW system consumes less compressor energy since it results in higher refrigerant temperature in the evaporator. The VW systems also have the decoupling capability of the primary and secondary pumping systems.
Competing Effects of Buckling and Anchorage Strength on Optimal Wheat
Stalk Geometry
Authors: Farquhar Tony , Zhou Jiang , Wood William H.
Published: October 2002

We seek the ideal wheat stalk, which minimizes the structural mass required to support a fixed grain load in the presence of gravity and wind. The optimization search is restricted to stepped cylindrical stems of known moduli and density but unknown dimension. Stem buckling and root anchorage strength are assumed to place restrictions on the permissible stalk resonant frequency in the presence of a specified wind forcing frequency. These effects are described mathematically, and the penalty parameter method is used to find stem mass minima for various stalk heights. In general, there are two alternative solution branches. The lower solution is the global minimum but it is probably impractical for field crops exposed to natural wind. The upper minimum is more conservative and therefore requires more stem mass. Due to the competing requirements of buckling versus anchorage strength, the parameter study shows that optimal wheat stem geometry has a nonlinear dependence on the intensity of gravity and the frequency spectra of the wind.
Predicting Globe Control Valve Performance—Part I: CFD Modeling
Authors: Davis James A. , Stewart Mike
Published: October 2002

Computational Fluid Dynamics (CFD) tools are evaluated for use in industrial design applications by predicting primary control valve performance characteristics. The performance parameter of primary interest to the manufacturer is the flow coefficient, Cv. Valves having relative valve capacity factors between 2.5 and 13 were modeled. The control valve Cv was experimentally measured and numerically predicted. Both equal percentage and linear characteristic valves were represented in the study. The numerical (simulation) study presented in Part 1 showed that the valve Cv and the inherent valve characteristic could be accurately predicted using axisymmetric flow models over most of the plug travel. In addition, the study demonstrates the usefulness of simplified CFD analysis for relatively complex 3-D flows.
Numerical Simulation of Droplet Flows and Evaluation of Interfacial Area
Authors: Watanabe T. , Ebihara K.
Published: October 2002

Droplet flows with coalescence and breakup are simulated numerically using the lattice Boltzmann method. It is shown that the rising velocities are in good agreement with those obtained by the force balance and the empirical correlation. The breakup of droplets after coalescence is simulated well in terms of the critical Weber number. A numerical method to evaluate the interfacial area and the volume fraction in two-phase flows is proposed. It is shown that the interfacial area corresponds to the shape, the number and the size of droplets, and the proposed method is effective for numerical evaluation of interfacial area even if the interface changes dynamically.
Neighboring Optimal Feedback Law for Higher-Order Dynamic Systems
Authors: Veeraklaew Tawiwat , Agrawal Sunil K.
Published: October 2002

In recent years, using tools from linear and nonlinear systems theory, it has been shown that classes of dynamic systems in first-order forms can be alternatively written in higher-order forms, i.e., as sets of higher-order differential equations. Input-state linearization is one of the most popular tools to achieve such a representation. The equations of motion of mechanical systems naturally have a second-order form, arising from the application of Newton’s laws. In the last five years, effective computational tools have been developed by the authors to compute optimal trajectories of such systems, while exploiting the inherent structure of the dynamic equations. In this paper, we address the question of computing the neighboring optimal for systems in higher-order forms. It must be pointed out that the classical solution of the neighboring optimal problem is well known only for systems in the first-order form. The main contributions of this paper are: (i) derivation of the optimal feedback law for higher-order linear quadratic terminal controller using extended Hamilton-Jacobi equations; (ii) application of the feedback law to compute the neighboring optimal solution.
Potentialities of Ultrasonics for Evaluating Residual Stresses: Influence of Microstructure
Authors: Walaszek H. , Lieurade H. P. , Peyrac C. , Hoblos J. , Rivenez J.
Published: October 2002

The good control of residual stress level in mechanical components is an important factor, particularly for a good fatigue strength of these components. This paper presents advances obtained at the technical center for mechanical engineering industries (CETIM) in the field of development of an ultrasonic method for stress measurements. This method is potentially advantageous because it is nondestructive, has good portability, and is easy to use. In the paper are discussed the results obtained with ultrasonics on steel welded plate, and a comparison is made with stress measurement obtained by incremental hole-drilling method, and X-ray diffraction. These results are also validated by thermal relaxation of the plates. The paper discusses also the microstructure influence on ultrasonic measurements and methods for adjusting the ultrasonic measurements to improve the agreement with results obtained from other techniques. In conclusion is emphasized the interest for studying the ability of the ultrasonic residual stress measurement method in different industrial cases.
Plane Thermal Stress Analysis of an Orthotropic Cylinder Subjected to an Arbitrary, Transient, Asymmetric Temperature Distribution
Authors: Yee K.-C. , Moon T. J.
Published: October 2002

A closed-form, analytical solution is presented for the transient, plane thermal stress analysis of a linearly elastic, homogeneously orthotropic hollow cylinder subjected to an arbitrary temperature distribution. The thermoelastic solution, obtained by a stress function approach, can be used as the basis for the corresponding thermoviscoelastic solution for thermorheologically simple viscoelastic materials by invoking the viscoelastic Correspondence Principle. This solution can also be directly extended to the class of weakly inhomogeneously orthotropic cylinders using perturbation methods. The transient asymmetric temperature field is characterized by Fourier-Bessel eigenfunction expansions. The analytically derived stress function satisfies a linear, fourth-order inhomogeneous partial differential equation and the Cesaro integral conditions, which assure the existence of a single-valued displacement field. The corresponding thermal stresses are then computed by the stress-stress function relations. A key feature of the analytical solution is that the hoop, radial, and shear stresses, due to the transient arbitrary temperature distribution, are expressed explicitly in terms of the scalar temperature field. A polymer composite example is presented to validate the current method and to qualitatively illustrate the distribution of thermal stresses due to an asymmetric temperature distribution. Numerical results are presented for the thermally driven hoop, radial and (interlaminar) shear stresses in a hollow, hoop-wound glass/epoxy cylinder. This analysis demonstrates that potentially debilitating interlaminar shear stresses can develop in laminated composites when subjected to an even modest transient asymmetric temperature distribution. Their magnitudes depend on the severity of the spatial and temporal thermal gradients in the circumferential direction. While still relatively low compared to the hoop stress, the shear stress may cause thermal failure due to the typically low interlaminar shear strengths of laminated composite materials.
Guided Wave Resonance Tuning for Pipe Inspection
Authors: Barshinger James , Rose Joseph L. , Avioli, Michael J.
Published: October 2002

Tremendous interest has surfaced recently on the use of guided waves in pipe inspection in the oil, chemical, and power generating industries. Relatively long lengths of piping can be inspected for corrosion and cracking from a single probe position. This saves a great deal of time and money compared to using more standard point-by-point normal beam inspection procedures. Pipes can be inspected without removing insulation or tar coatings by controlling the guided wave modes and frequencies used to carry out the study. This paper will review the history and state of the art of the guided wave techniques in piping. Benefits and limitations of the various methods will be pointed out along with a vision of future directions in the area of pipe inspection.
Nondestructive Monitoring of Stress Relaxation in Welded Steel Plates
Authors: Chance Brent H. , Bray Don E.
Published: October 2002

This study investigates transverse stress relaxation in welded steel plates. Two different methods of stress measurement were used; strain gages and critically refracted longitudinal LCR waves. The material investigated was ASTM 1008/1010 steel. T-shaped slots were cut from each edge of each specimen in order to provide a uniaxial tension stress field in the area of investigation. After the plates were stress-relieved by annealing, the transverse slots were then welded and allowed to cool. Strain gages were applied to the bridge area joining the middle of the plate and oriented parallel to the long side of the plate. Initial strain gage and ultrasonic measurements were then performed. At different time intervals, cutting through a sample plate’s welds relieved the stresses. Strain gage and ultrasonic measurements were then repeated. Both the strain gage and LCR methods indicate that there is a measurable stress relaxation in the plates, and that it occurs in a time-dependent and predictable manner.
Micro-Mechanics of Creep-Fatigue Damage in PB-SN Solder Due to Thermal Cycling—Part II: Mechanistic Insights and Cyclic Durability Predictions From Monotonic Data
Authors: Sharma Pradeep , Dasgupta Abhijit
Published: October 2002

This paper illustrates the micromechanics approach to cyclic creep-fatigue damage of Pb-Sn, developed by the authors with the help of a simple case study. It is demonstrated that durability predictions can be made based solely on monotonic test data. Various parametric studies of practical interest are conducted to obtain mechanistic insights into various aspects of damage in solders e.g., effect of hydrostatic stresses, grain size, ramp rate, etc.
The Effects of Surface Roughness on the Mean Velocity Profile in a Turbulent Boundary Layer
Authors: Bergstrom Donald J. , Kotey Nathan A. , Tachie Mark F.
Published: October 2002

Experimental measurements of the mean velocity profile in a canonical turbulent boundary layer are obtained for four different surface roughness conditions, as well as a smooth wall, at moderate Reynolds numbers in a wind tunnel. The mean streamwise velocity component is fitted to a correlation which allows both the strength of the wake, Π, and friction velocity, Uτ, to vary. The results show that the type of surface roughness affects the mean defect profile in the outer region of the turbulent boundary layer, as well as determining the value of the skin friction. The defect profiles normalized by the friction velocity were approximately independent of Reynolds number, while those normalized using the free stream velocity were not. The fact that the outer flow is significantly affected by the specific roughness characteristics at the wall implies that rough wall boundary layers are more complex than the wall similarity hypothesis would allow.
Performance Study of a Bi-Directional Thermodiode Designed for Energy-Efficient Buildings
Authors: Chun Wongee , Chen Kuan , Kim Hyung Taek
Published: October 2002

A new, bi-directional thermodiode designed for energy-efficient buildings was constructed and tested. Experimental results are presented and discussed for solar-heating applications. The thermodiode system consisted of a number of rectangular loops filled with water. The tilting angle of the loops can be altered to reverse the direction of natural convection within the loops for bi-directional operations. The horizontal segments of the loops were attached to metallic panels facing indoors or outdoors. The amount of thermal radiation incident on the outdoor-facing surfaces can be adjusted by rotating the panels or by installing a removable shading device in front of the surfaces. Results of the indoor tests for winter use of the diode showed an onset time between 7 to 20 min for natural convection to be induced throughout the loops in the thermodiode. Before the throughflow started, the fluid in the heated copper tubes reached its maximum temperature. A sudden drop and rebound in this temperature was observed immediately after the onset of throughflow. After that, temperatures at different locations on the thermodiode rose at approximately the same rate until a steady state was reached. During the cool-down phase, the temperatures decreased at the same rate without humps, indicating only conduction took place in the rectangular loops when the thermodiode was reverse-biased. A simple analytical model was developed to estimate the temperature variations and heat transfer rates in the diode system. The diode under forward-biased condition increases the heat transfer rate by nearly 100 times for an incident radiation of 600 W/m2.
Probabilistic gel formation theory in negative tone chemically amplified resists used in optical and electron beam lithography
Authors: Patsis G. P. , Glezos N.
Published: October 2002

In this article a probabilistic model able to describe gel formation in negative tone chemically amplified resists (CARs) is developed. This model is a continuation and development of previous work on polymer cross-linking and on gelation of negative tone nonchemically amplified resists. The relation between gel fraction and exposure dose in negative tone CARs is obtained. A new parameter called inhibition activity with a vital role in the quantification of the cage effect (i.e., acid species trapping) is analyzed. The description of cage effect evolution with exposure dose, postexposure temperature, and time in CARs is accomplished through this parameter. The theory is tested against two negative tone CARs. The first one is EPR, an experimental CAR with known chemistry and the other is SAL-601, a commercial CAR from Shipley.
Control of shape of silicon needles fabricated by highly selective anisotropic dry etching
Authors: Kanechika Masakazu , Sugimoto Noriaki , Mitsushima Yasuichi
Published: October 2002

The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing effect by the silicon needle with a high aspect ratio.
Electrical contrast observations and voltage measurements by Kelvin probe force gradient microscopy
Authors: Girard P. , Ramonda M. , Saluel D.
Published: October 2002

Kelvin probe force gradient microscopy is proposed to image and measure local dc voltage variations using the double pass method. The various voltages between sensor and sample induce electrical force gradients that change the resonance of the sensor. Images of the various phase shifts show contrasts, which, as we demonstrate, can be interpreted in terms of local changes in voltage and capacitive coupling. The interest of this method for observation and local voltage measurements is demonstrated and explained.
Study of the acid-diffusion effect on line edge roughness using the edge roughness evaluation method
Authors: Yoshizawa Masaki , Moriya Shigeru
Published: October 2002

The acid-diffusion effect on line edge roughness (LER) in a negative-type chemically amplified resist of electron beam lithography was evaluated experimentally. The edge roughness evaluation method was utilized to quantify the acid-diffusion length. Adjusting the prebaking temperature controlled the acid-diffusion coefficient in the range from 8.1 nm2/s at 70 °C to 0.8 nm2/s at 130 °C. Acid diffusion reduced the LER of a 100 nm isolated line from 6.9 to 4.0 nm while increasing the diffusion length from 15 to 44 nm, but a too-long diffusion length degraded line and space patterns. These results suggest that acid diffusion has two opposing effects on LER: one is the smoothing effect reducing LER and the other is the degrading image contrast effect increasing LER. It is concluded that LER occurs as a consequence of the amplification of the so-called uncertainty band by the slope of the quasibeam profile. The bandwidth signifies the effect of molecular structure, the spatial dispersion of photochemical events caused by shot noise, the smoothing effect of acid diffusion, and development uncertainty at the pattern edge. The magnitude of the amplification is determined by the slope of the quasibeam profile that comprises aerial-image quality, acid-diffusion length, and resist contrast.
Scanning tunneling potentiometry of semiconductor junctions
Authors: Dong Yang , Feenstra R. M. , Hey R. , Ploog K. H.
Published: October 2002

A method for performing scanning tunneling potentiometry of semiconductor heterojunctions is described. The method yields a direct measure of the electrostatic potential distribution across the interface, with microscopic resolution. The measurement is accomplished by scanning the probe tip at constant sample–tip separation across the junction, and adjusting the sample–tip voltage to maintain a constant tunnel current. An example is given of potentiometry across a GaAs pn junction.
Scanning capacitance spectroscopy of an AlxGa1−xN/GaN heterostructure field-effect transistor structure: Analysis of probe tip effects
Authors: Schaadt D. M. , Yu E. T.
Published: October 2002

We have performed an analysis of the influence of the probe tip geometry in scanning capacitance microscopy and spectroscopy measurements on an AlxGa1−xN/GaN heterostructure field-effect transistor (HFET) structure. The extremely small probe tip size (typical apex radius ∼10–30 nm) makes a detailed analysis essential in comparisons of dC/dV spectra with standard, large-area C/V measurements. Using three-dimensional simulation software, we have calculated dC/dV spectra for various tip geometries and have found that the nanoscale tip structure influences dC/dV spectra strongly, while the macroscopic shape has a much smaller influence on the dC/dV spectra. Thus, caution must be exercised in comparison of individual dC/dV spectra obtained over periods during which the probe tip geometry might have changed. We have analyzed these effects in detail and assessed their influence on the extraction of parameters such as threshold voltage, layer thickness, doping concentrations, and information about trap states in nitride HFETs and other electronic device structures.
Surface and interface properties of In0.8Ga0.2As metal–insulator–semiconductor structures
Authors: Wieder H. H. , Sari Huseyin
Published: October 2002

More than two decades ago it was shown that the surface barrier of InxGa1−xAs alloys, positive, in depletion, and negative, in accumulation passes through zero at, or near, x=0.8. Consequently, the energy bands at the surfaces of In0.8Ga0.2As might be the same as in the bulk; i.e., at flatband. Electrical, galvanomagnetic, and surface photovoltage measurements made on transistor-like, gated, eight-arm, In0.8Ga0.2As metal–insulator–semiconductor structures confirm that the equilibrium surface Fermi level is, indeed, at flatband. Although the density of surface states is of the order 1012/cm2 applied gate voltages can displace, quasistatically, the Fermi level, from above the conduction band edge to the vicinity of the valence band edge. An interpretation of the data applied to this as well as to all of the other InxGa1−xAs alloys is based on Zunger’s “vacuum pining rule” and Walukiewicz’s Fermi level stabilization energy. The energies of the states which determine their surface barriers are not referred to their conduction and valence band edges. Instead, they are amphoteric charged defects located on a composition-independent reference level, located ∼4.95 eV below the vacuum level.
Properties of (In,Ga)N/GaN quantum wells grown by plasma-assisted molecular beam epitaxy
Authors: Brandt O. , Waltereit P. , Dhar S. , Jahn U. , Sun Y. J. , Trampert A. , Ploog K. H. , Tagliente M. A. , Tapfer L.
Published: October 2002

We investigate the synthesis of (In,Ga)N/GaN multiple quantum wells by plasma-assisted molecular beam epitaxy (MBE). For metal-stable growth, unexpectedly strong In surface segregation is revealed. The In depth profiles obtained by secondary ion-mass spectrometry exhibit a top-hat In distribution and are thus indicative of a zeroth order segregation mechanism instead of a first order process as observed for other materials systems. As additionally evidenced by transmission electron microscopy, the segregation of In during metal-stable growth results in quantum wells with smooth interfaces but significantly larger width than intended. The resulting blueshift of the transition energy may be the reason for the frequent conclusion that the theoretical polarization fields of Bernardini et al. [Phys. Rev. B 56, R10024 (1997)] are too large for (In,Ga)N. Being in possession of the (at least approximately) correct structural parameters, we find the theoretical fields to be in very satisfactory agreement with those deduced from experimental data. For a thorough understanding of the spontaneous emission from these structures both electrostatic fields and compositional fluctuations have to be taken into account. Both the transition energies and radiative decay times in photoluminescence are shown to be in agreement with the quantum-confined Stark effect in these structures. Using cathodoluminescence spectroscopy, we investigate the dependence of transition energy and quantum efficiency on both temperature and excitation density. At low temperatures, recombination is governed by localized states, whereas for high temperatures extended states dominate. This finding is in agreement with the temperature dependence of the radiative decay time which is utilized as a probe of the dimensionality of the system. A quantitative analysis of these experiments via coupled rate equations shows that the localization depth in these MBE-grown (In,Ga)N/GaN quantum wells is around 20–30 meV. This shallow localization is found to significantly enhance the internal quantum efficiency up to a temperature of about 100 K.
Controlling the threshold voltage of a metal–oxide–semiconductor field effect transistor by molecular protonation of the Si:SiO2 interface
Authors: Yang Jinman , de la Garza L. , Thornton T. J. , Kozicki M. , Gust D.
Published: October 2002

We present results from a hybrid molecular/metal–oxide–semiconductor field effect transistor (MOSFET) structure that is sensitive to the presence of a molecular monolayer on its surface. The device is fabricated from a silicon-on-insulator substrate, and unlike a conventional MOSFET a substrate voltage is used to invert the buried Si:SiO2 interface. This allows the top surface of the silicon to be free of any insulating layers, apart from a thin native oxide that forms on exposure to air. The buried inversion layer is less than 40 nm away from the exposed surface, and the threshold voltage of the device is strongly influenced by the surface potential. Measurements of the drain current as a function of substrate voltage can be accurately reproduced from numerical simulation by treating the charge at the native oxide interface as a fitting parameter. The shift in threshold voltage after molecular attachment can be accounted for by a simple increase in the (positive) fixed oxide charge density, all of the other parameters being kept constant. This suggests that the shift in threshold voltage occurring after attachment of the molecular monolayer results from protonation of the native oxide. We explain this result in terms of the higher acidity of the molecular group compared to that of the native oxide.
Core level photoemission and scanning tunneling microscopy study of the interaction of pentacene with the Si(100) surface
Authors: Hughes Greg , Roche Jason , Carty Darren , Cafolla Tony , Smith Kevin E.
Published: October 2002

The chemical bonding interactions of molecular pentacene with the Si(100) surface were investigated by high resolution core level photoemission spectroscopy and by scanning tunneling microscopy (STM). Thin films of pentacene were deposited from a thermal evaporator onto the atomically clean Si(100) surface in ultrahigh vacuum. Analysis of the Si 2p core level spectra reveal evidence of a strong chemical interaction between the molecule and the surface. Three chemically shifted components at kinetic energies—0.27, −0.65, and −1.1 eV with respect the bulk peak—are required to consistently fit the Si 2p core level. The −0.27 eV chemically shifted component resulting from the bonding interaction suggests the formation of Si–C bonds between the pentacene and the silicon surface. The other two components are attributed to different adsorption sites on the surface. Annealing the pentacene covered surface in the 100–200 °C temperature range results in the desorption of molecular layers which had been deposited on top of this chemically reacted layer, leaving a monolayer coverage intact. Valence band spectra of the annealed surface indicate that the bonding interaction occurs between the π orbitals of the pentacene and the silicon surface. By acquiring valence band spectra of thicker layers with s and p polarized light it was possible to conclude that the pentacene molecules absorb molecularly on top of the reacted layer with the plane of the molecule lying parallel to the Si(100) surface. The STM studies of submonolayer coverages of pentacene on the clean Si surface allow three distinct molecular orientations to be identified. Adsorption taken place both on top of, and between, the dimer rows with the molecule aligned parallel to the dimer row axis. Molecular adsorption perpendicular to the dimer rows is also observed.
Mechanism of cleaning Si(100) surface using Sr or SrO for the growth of crystalline SrTiO3 films
Authors: Wei Yi , Hu Xiaoming , Liang Yong , Jordan D. C. , Craigo Brad , Droopad Ravi , Yu Z. , Demkov Alex , Edwards John L. , Ooms William J.
Published: October 2002

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.
Electrical characteristics of polycrystalline silicon thin film transistors using the Cu-field aided lateral crystallization process
Authors: Kwon Se-Youl , Park Kyoung-Wan , Lee Jae-Bok , Choi Duck-Kyun
Published: October 2002

Polycrystalline silicon thin film transistors (poly-Si TFTs) were fabricated on glass substrate by using a concept of Cu-field aided lateral crystallization (Cu-FALC). The crystallization of a-Si was significantly enhanced when an electric field of 30 V/cm was applied to selectively Cu-deposited amorphous silicon (a-Si) film during thermal annealing at 500 °C for 3 h. These FALC TFTs using Cu exhibited a low off-state leakage current of 6.2 ×10−12 A at Vg=−10 V and a maximum on/off current ratio of 3.1×105. The field-effect mobility and the threshold voltage of the fabricated poly-Si TFT were about 22.0 cm2/V s and 3.9 V, respectively. Therefore, the possibility of high-performance and low-temperature (<500 °C) poly-Si TFTs was demonstrated by using Cu-FALC technology.
Ballistic electron emission microscopy studies of ZnSe–BeTe heterojunctions
Authors: Chahboun A. , Fink V. , Fleischauer M. , Kavanagh K. L. , Lu R. P. , Hansen L. , Becker C. R. , Molenkamp L. W.
Published: October 2002

Ballistic electron and hole emission microscopies, have been used to probe Au/ZnSe/GaAs and Au/ZnSe/BeTe/GaAs (001) heterostructures as a function of the ZnSe/BeTe interface stoichiometry. Ballistic electron and hole transport into n- and p-type ZnSe epilayers, respectively, were observed with spectra that conformed well to theoretical behavior. The measured average voltage thresholds +1.25 (p type) and −1.41 V (n type) were in good agreement with previously observed values for the Au/ZnSe Schottky barrier heights and consistent with the value of the ZnSe direct band gap (2.67 eV). However, the ballistic transport into the BeTe conduction bands required to measure the ZnSe/BeTe conduction band offset was not confirmed. Time-dependent and spatial variations of the voltage thresholds were observed for p- and n-type diodes, with and without BeTe epilayers, and were presumed to be linked to switching Fermi level pinning positions from varying Au/ZnSe interface states densities.
Influence of in situ ultrasound treatment during ion implantation on amorphization and junction formation in silicon
Authors: Krüger D. , Romanyuk B. , Melnik V. , Olikh Ya. , Kurps R.
Published: October 2002

We report the first study of the effect of in situ ultrasound treatment during ion implantation on amorphization and transient enhanced diffusion (TED) in silicon. Rutherford backscattering spectroscopy, ion channeling, and Raman spectroscopy measurements show that amorphization of Si during Ar ion implantation is increased by ultrasound treatment, especially at ultrasound frequencies around 2 MHz. By secondary ion mass spectroscopy we investigate the influence of ultrasound treatment on TED of B atoms. Our data show that TED is reduced due to the influence of ultrasound treatment. The results are discussed in terms of the interaction of ultrasound waves with point defects and the ultrasound stimulated enhanced diffusion of interstitials.
Studies on the plasma localization of a magnetic neutral loop discharge using normalized radio frequency electric field
Authors: Sung Youl-Moon , Honda Chikahisa
Published: October 2002

In order to establish the conditions required for the formation of a neutral loop discharge (NLD) plasma, the distributions of excited atoms that are closely related to that of the ionization rates were measured under various discharge conditions. Also, computer-based simulations in three dimensions were used to examine the experimental results obtained. The measurements of the distributions of excited atoms were performed using laser-induced fluorescence and optical emission spectroscopy(s). The results of the measurements were consistent with the theoretically predicted dependencies of electron heating on the magnetic field strength, electric field, frequency and NL radius. In addition, it was found that the plasma localization increased with the normalized rf electric field (F0), which can be used to characterize the localization degree of the plasma production region.
Microstructural interpretation of Ni ohmic contact on n-type 4H–SiC
Authors: Han Sang Youn , Shin Jong-Yoon , Lee Byung-Teak , Lee Jong-Lam
Published: October 2002

Using cross-sectional transmission electron microscopy (TEM), microstructural changes in Ni contacts on n-type 4H–SiC as a function of annealing temperature were investigated. From these results, the correlation between the microstructural change and electrical properties was interpreted. After annealing at 800 °C, which yielded rectifying behavior, the silicide phases were formed, composed of Ni2Si and Ni31Si12. From the results shown in microbeam diffraction patterns, Ni31Si12 remains at the surface and Ni2Si is dominant in the contact, indicating that Ni2Si started to grow at the interface through the outdiffusion of Si atoms. When the sample was annealed at 950 °C, ohmic behavior was shown, and the layer structure was changed to a C-rich layer/Ni2Si/NiSi/n-type SiC. The NiSi phase was observed. These results imply that the composition of Si in nickel silicide at the interface with SiC increased with the increase of annealing temperature. The observation of the graphite phase at the surface indicates that the C atoms diffused out to the surface at 950 °C. This leads to the formation of carbon vacancies, acting as donors for electrons. These suggest that the production of carbon vacancies plays a major role in the formation of ohmic contact through the reduction of the effective Schottky barrier height for the transport of electrons.
Atomic resolution imaging of a single-crystal Cu (100) surface by scanning tunneling microscopy in ultrahigh vacuum at room temperature
Authors: Zou Z. Q. , Dong Z. C. , Trifonov A. S. , Nejo H.
Published: October 2002

Atomically ordered array of Cu atoms on a clean single-crystal Cu (100) surface has been observed on both occupied and empty sample states by scanning tunneling microscopy in ultrahigh vacuum at room temperature. An average corrugation amplitude of ∼0.4 Å is found in the empty state while it is only ∼0.06 Å in the occupied state. Atomic spacing is measured and the results are in good agreement with the known copper interatomic distance of 2.6 Å.
Steps on the (001) SrTiO3 surface
Authors: Zhang Xiaodong , Demkov Alexander A.
Published: October 2002

We have carried out local atomic orbital basis first-principles total-energy calculations of (001) SrTiO3 (STO) surfaces with a half unit cell high step. The thermodynamic analysis shows that an SrO-terminated surface (A-site layer) has the lower surface energy under most chemical environments. We introduce a method to estimate the step energy for a SrTiO3 surface with a step based on thermodynamic considerations. We show that an STO surface with a half unit cell high step prefers an oxygen terminated step edge under oxygen rich conditions and an Sr- or TiO-terminated step edge under oxygen deficient conditions. However, under the majority of chemical environments the edge termination is mixed, and it is the stoichiometry that drives the terrace termination.
Aliasing effects in digital images of line‐pair phantoms
Authors: Albert Michael , Beideck Daniel J. , Bakic Predrag R. , Maidment Andrew D. A.
Published: October 2002

Line‐pair phantoms are commonly used for evaluating screen‐film systems. When imaged digitally, aliasing effects give rise to additional periodic patterns. This paper examines one such effect that medical physicists are likely to encounter, and which can be used as an indicator of super‐resolution.
Optimization of Ga‐67 imaging for detection and estimation tasks: Dependence of imaging performance on spectral acquisition parameters
Authors: El Fakhri Georges , Moore Stephen C. , Kijewski Marie Foley
Published: October 2002

We have compared the use of two (93 and 185 keV) and three (93, 185, and 300 keV) photopeaks for Ga‐67 tumor imaging and optimized the placement of each energy window. Methods: The bases for optimization and evaluation were ideal and Bayesian signal‐to‐noise ratios (SNR) for the detection of spheres embedded in a realistic anthropomorphic digital torso phantom and ideal SNR for the estimation of their size and activity concentration. Seven spheres of radii ranging from 1 to 3 cm, located at several sites in the torso, were simulated using a realistic Monte Carlo program. We also calculated the ideal SNR for the detection from simple phantom acquisitions. Results: For detection and estimation tasks, the optimum windows were identical for all sphere sizes and locations. For the 93 keV photopeak, the optimal window was 84–102 keV for the detection and 87–102 keV for estimation; these windows are narrower than the 20% window often used in the clinic (83–101 keV). For the 185 keV photopeak, the optimal window was 170–220 keV for the detection and 170–215 keV for estimation; these are substantially different than the 15% window used in our clinic (171–199 keV). For the 300 keV photopeak, the optimal window for detection was 270–320 keV, and for estimation, 280–320 keV. Using the three optimized, rather than only the two lower‐energy, windows yielded a 9% increase in the SNR for the detection of the 3 cm diam sphere (a 12% increase for a 2 cm diam sphere) and a 7% increase in the SNR for estimation of its size. For the acquired phantom data, detection also increased by 9%–12% when using three, rather than two, energy windows.