bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman study on electrochemical lithium insertion into multiwalled carbon nanotubes
Authors: Kim Yoong Ahm , Kojima Masashito , Muramatsu Hiroyuki , Shimamoto Daisuke , Hayashi Takuya , Endo Morinobu , Terrones Mauricio , Dresselhaus Mildred S.
Published: April 2008

We have carried out in situ Raman studies during the electrochemical insertion of lithium ions (Li+) into pristine and thermally treated multiwalled carbon nanotubes (MWNTs). We found an improved structural integrity as well as the removal of defects in the thermally treated tubes. The different Li+ insertion behaviors above 0.5 V in as‐grown and thermally treated tubes could be explained by the presence of defects on the outer surface of the tubes. No change of Raman spectra from 2.8 to 0.8 V is characterized by the coverage of Li+ on the outer surface of tubes, whereas the upshift of G band and the absence of a separated G band below 0.75 V indicate the formation of diluted graphite intercalation below the stage‐2 phase (LiC12). Copyright © 2008 John Wiley & Sons, Ltd.
Thermophysical properties of compressor oils for refrigerating plant
Authors: Semenyuk Yury V. , Sechenyh Vitaly V. , Zhelezny Vitaly P. , Skripov Pavel V.
Published: April 2008

In the present study, an accurate experimental information for the widely used commercial refrigerating compressor oils ONF 46, ACD 100 FY, Azmol, FR 32, XC 15, XC 40 and WF 15A is presented. Experimental data on the capillary constant, surface tension, density, refractive index and kinematic viscosity at the temperature range from 293 to 353 K for seven commercial compressor oils are reported. In addition, the experimental data on the isobaric heat capacity at temperature range from 288 to 393 K, the vapor pressure at liquid‐saturated curve and average molecular weight for four commercial compressor oils are firstly presented in the paper. The data obtained are fitted by few‐constant correlations. Based on results obtained, the values of pseudocritical temperature for the compressor oils have been determined. Copyright © 2008 John Wiley & Sons, Ltd.
Stationary phases for hydrophilic interaction chromatography, their characterization and implementation into multidimensional chromatography concepts
Authors: Jandera Pavel
Published: April 2008

Hydrophilic interaction chromatography (HILIC) is becoming increasingly popular for separation of polar samples on polar columns in aqueous‐organic mobile phases rich in organic solvents (usually ACN). Silica gel with decreased surface concentration of silanol groups, or with chemically bonded amino‐, amido‐, cyano‐, carbamate‐, diol‐, polyol‐, or zwitterionic sulfobetaine ligands are used as the stationary phases for HILIC separations, in addition to the original poly(2‐sulphoethyl aspartamide) strong cation‐exchange HILIC material. The type of the stationary and the composition of the mobile phase play important roles in the mixed‐mode HILIC retention mechanism and can be flexibly tuned to suit specific separation problems. Because of excellent mobile phase compatibility and complementary selectivity to RP chromatography, HILIC is ideally suited for highly orthogonal 2‐D LC‐LC separations of complex samples containing polar compounds, such as peptides, proteins, oligosaccharides, drugs, metabolites and natural compounds. This review attempts to present an overview of the HILIC separation systems, possibilities for their characterization and emerging HILIC applications in 2‐D off‐line and on‐line LC‐LC separations of various samples, in combination with RP and other separation modes.
Misalignment effects on steady‐state and dynamic behaviour of compliant journal bearings lubricated with couple stress fluids
Authors: Boucherit Hamid , Lahmar Mustapha , Bou‐Said Benyebka
Published: April 2008

This work concerns the steady‐state and dynamic analysis of misaligned compliant journal bearings considering the effects of couple stresses arising from the lubricant blended with polymer additives. Based on the Stokes micro‐continuum theory, a modified form of the Reynolds equation is derived. The displacement field at the fluid film–bearing liner interface due to pressure forces is determined using the elastic thin liner model. The effects of the misalignment and the couple stress parameters on static and dynamic performances such as pressure distribution, load‐carrying capacity, power loss, side leakage flow, misalignment moment, critical mass and whirl frequency are presented and discussed. Copyright © 2008 John Wiley & Sons, Ltd.
A parameterisation of impact laws in perfect multi‐contact collisions
Authors: Aeberhard U. , Glocker Ch.
Published: April 2008

Impacts in rigid multibody systems cause instantaneous jumps in the generalised velocities. Naturally, the velocity after impact depends on the chosen impact law. An impact law should fulfil kinematic, kinetic as well as energy restrictions. In this paper, we study the domain of possible post‐impact velocities for arbitrary impact laws. For single‐contact collisions, this domain is at most one‐dimensional but the domain becomes higher dimensional in the multi‐contact case. The domain of possible post‐impact velocities is a compact convex subset of the tangent space to the configuration manifold. Using a complete canonical parameterisation, the post‐impact velocity of all impact laws can be addressed. For instance, the impact law corresponding to maximal dissipation as well as Newtons (extended) impact law are examples of incomplete canonical parameterisations. These impact laws are not complete as non‐local impact effects are not addressed. Here, we try to find a complete canonical parameterisation which covers non‐local impact effects. Moreover, the relationship between symmetries and conservation laws in this context will be elucidated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Differential topology methods for shape description
Authors: Biasotti Silvia , Giorgi Daniela , Patané Giuseppe
Published: April 2008

Differential topology, and specifically Morse theory, provides a suitable setting for formalizing and solving several problems related to shape analysis. In this field, we discuss how a shape can be analyzed according to the properties of a real function defined on it (e.g., harmonic fields or laplacian eigenfunctions), and how these properties can be stored in compact and informative descriptors. We refer to Reeb graphs, that encode the configuration of level sets and critical points of the function
Asymptotics for the potential in a periodic network with localized defects
Authors: Bonnetier Eric , Sista Sivaji Ganesh
Published: April 2008

We consider a network of conductors built on the lattice h Z2 and study how the potential is affected by a localized interior defect. We derive an asymptotic expansion of the difference between the potential of the perturbed network and that of an ideal, defect‐free network. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A stall‐delay model for rotating blades
Authors: Dumitrescu Horia , Cardos Vladimir
Published: April 2008

The paper is aimed at describing a fundamental phenomenon: the effect of rotation on the inboard blade boundary layer on a wind turbine. The three‐dimensional incompressible steady momentum integral boundary‐layer equations are used to predict the boundary‐layer growth and limiting streamline angles on the blade surface for both attached and separating flow. The chordwise skin friction coefficient is used to identify boundary layer separation and shear layer reattachment locations. The nature of flow near the axis of rotation is discussed and the physical mechanism associated with 3‐D and rotational effects is identified. A semi‐empirical correction law for the lift coefficient based on 2‐D airfoil data is established. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Flexibility and leadership advantages in a model with uncertain demand
Authors: Ferreira Fernanda A. , Ferreira Flávio , Pinto Alberto A.
Published: April 2008

We consider a differentiated Stackelberg model with demand uncertainty only for the first mover. We study the advantages of flexibility over leadership as the degree of the differentiation of the goods changes
Numerical simulation of non‐viscous liquid pinch off using a coupled level set boundary integral method
Authors: Garzon Maria , Sethian James A. , Gray Len
Published: April 2008

The pinch off of an inviscid fluid column is described using a potential flow model with capillary forces. The interface velocity is obtained via a Galerkin boundary integral method for the 3D axisymmetric Laplace equation, whereas the interface location and the velocity potential on the free boundary are both approximated using level set techniques on a fixed domain. The algorithm is validated computing the Raleigh‐Taylor instability for liquid columns which provides an analytical solution for short times. The simulations show the time evolution of the fluid tube and the algorithm is capable of handling pinch‐off and after pinch‐off events. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Validation for turbulent mixing simulations
Authors: Glimm James , Jin Hyeonseong
Published: April 2008

We compare simulation studies of acceleration driven mixing (the Rayleigh‐Taylor instability) to laboratory experiments. Good agreement is obtained for variables describing the large scale aspects of the mixing process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimal control of constrained time lag systems: Necessary conditions and numerical treatment
Authors: Göllmann Laurenz , Kern Daniela , Maurer Helmut
Published: April 2008

We consider retarded optimal control problems with constant delays in state and control variables under mixed controlstate inequality constraints. First order necessary optimality conditions in the form of Pontryagin's minimum principle are presented and discussed as well as numerical methods based upon discretization techniques and nonlinear programming. The minimum principle for the considered problem class leads to a boundary value problem which is retarded in the state dynamics and advanced in the costate dynamics. It can be shown that the Lagrange multipliers associated with the programming problem provide a consistent discretization of the advanced adjoint equation for the delayed control problem. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Higher‐order energy consistent time integrators for nonlinear thermoviscoelastodynamics
Authors: Groß Michael , Betsch Peter
Published: April 2008

An advantage of the temporal fe method is that higher‐order accurate time integrators can be constructed easily. A further important advantage is the inherent energy consistency if applied to equations of motion. The temporal fe method is therefore used to construct higher‐order energy‐momentum conserving time integrators for nonlinear elastodynamics (see Ref. [1]). Considering finite motions of a flexible solid body with internal dissipation, an energy consistent time integration is also of great advantage (see the references [2, 3]). In this paper, we show that an energy consistent time integration is also advantageous for dynamics with dissipation arising from conduction of heat as well as from a viscous material. The energy consistency is preserved by using a new enhanced hybrid Galerkin (ehG) method. The obtained numerical schemes satisfy the energy balance exactly, independent of their accuracy and the used time step size. This guarantees numerical stability. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effective transport properties for flashing ratchets using homogenization theory
Authors: Latorre Juan C. , Kramer Peter R. , Pavliotis Grigorios A.
Published: April 2008

We study effective transport properties of Brownian motor models of molecular motors. The effective drift and diffusivity can be calculated by solving cell problems, given explicitly by homogenization theory. We briefly describe how this approach is equivalent to theWang‐Peskin‐Elston (WPE) [3] numerical algorithm for calculating effective transport properties of flashing ratchets. For an on‐off flashing ratchet we examine the optimization of the Peclet number as a function of the free parameters of the system. We also present a numerical method for solving the cell equations for a flashing ratchet with Gaussian multiplicative noise. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A high order finite volume method for the 2D shallow water equation
Authors: Messerschmidt Bettina
Published: April 2008

In this paper, we give a brief introduction into two numerical flux functions in the context of Finite Volume Schemes on unstructured grids of theoretically arbitrary high order, namely the well‐ known ADER‐ and the new STE‐ Scheme. We will emphasize in particular the differences with regard to computing time. Further we give an example comparing the results of the schemes applied for the first time to the 2D Shallow Water Equation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Scaling dynamics for solvable coagulation equations with dust and gel
Authors: Menon Govind , Pego Robert L.
Published: April 2008

We study limiting behavior of rescaled size distributions that evolve by Smoluchowski's rate equations for coagulation, with rate kernel K=2, x+y or xy. We find that the dynamics naturally extend to probability distributions on the half‐line with zero and infinity appended, representing populations of clusters of zero and infinite size. The “scaling attractor” (set of subsequential limits) is compact and has a Levy‐Khintchine‐type representation that linearizes the dynamics and allows one to establish several signatures of chaos. In particular, for any given solution trajectory, there is a dense family of initial distributions (with the same initial tail) that yield scaling trajectories that shadow the given one for all time. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Continuous solutions of some boundary layer problem
Authors: Dreglea Aliona I. , Sidorov Nikolay A.
Published: April 2008

The continuous solutions for BVP of third order nonlinear differential equations appears in [1] mathematical model of the melt spinning process. The existence theorem is proved for such BVP. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the structure of phase spaces in Synergetic Inter‐Representation Network
Authors: Sugimoto Takeshi
Published: April 2008

Exchanging ideas and concepts among people can be handled with by so‐called Synergetic Inter‐Representation Network, which is described by n ‐dimensional ordinary differential equations with cubic nonlinearity. The novel find is the fact that the basic equation set is transformed to a set of n ‐dimensional Lotka‐Volterra equations. This discovery allows us to scrutinize the existence and stability of the fixed points: (1) The origin is an unstable fixed point; (2) The single representation survival, for example (0, 0, …, λj /C, …, 0), is a stable fixed point; (3) The coexistence of several representations exists as a saddle point. If all of these exist, the number of the saddle points is 2n – n – 1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coupling of various methods for the investigation of the morphology of blends of natural rubber and polybutadiene
Authors: Portal J. , Carrot C. , Majeste J.C. , Cocard S. , Pelissier V. , Baran K. , Anselme‐Bertrand I.
Published: April 2008

The aim of this work is to develop a set of experimental methods to investigate the morphology of blends of two immiscible elastomers, namely natural rubber (NR) and polybutadiene (PB). Selective extraction, dynamic mechanical spectroscopy in the melt, and transmission electron microscopy (TEM) were used to get information on the structure of unfilled and uncured blends. 1,2‐dichloroethane and 4‐methyl‐2‐pentanone (MIBK) were employed to dissolve selectively NR and PB in specific conditions to assess the continuity of the phases. In oscillatory shear flow at low frequencies and low volume fractions of the minor phase, the storage modulus shows an excess of elasticity that increases with the dispersed phase content. At higher concentration of the minor phase, the elasticity decreases in relation to the morphology. Samples were also ultra cryo‐microtomed and then analyzed by TEM. The superposition of the three methods highlights the existence of a phase‐inversion point for these systems. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers
Recycled and virgin LDPE as rheology modifiers of lithium lubricating greases: A comparative study
Authors: Martín‐Alfonso J.E. , Valencia C. , Sánchez M.C. , Franco J.M. , Gallegos C.
Published: April 2008

In this work, a new application for recycled low‐density polyethylene (LDPE), as rheology modifier of standard lithium lubricating grease formulations, was studied. The effectiveness of this additive was compared with that achieved with a virgin LDPE. With this aim, both types of polymers were added to the formulation during the manufacturing process of greases, following the same standard protocol, to reinforce the role of the thickening agent, the lithium 12‐hidroxystearate. The effect that both lithium soap and LDPE concentrations exert on the rheology of lubricating grease formulations and its relationship with grease microstructure were discussed. Lubricating greases were rheologically characterized through small‐amplitude oscillatory shear and viscous flow measurements. In addition to these, scanning electron microscopy observations and mechanical stability tests were also carried out. In all cases, an increase in soap concentration yields higher values of apparent viscosity and linear viscoelasticity functions. On the other hand, the values of the rheological functions obtained for recycled LDPE‐based lubricating greases are, in general, higher than those obtained for virgin LDPE‐based grease formulations. However, the structural skeleton developed in greases containing recycled LDPE demonstrates less resistance to severe working conditions, showing lower mechanical stability than virgin LDPE‐based grease formulations. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers
On the Capability of the Generalized Gamma Function to Represent Spray Drop‐Size Distribution
Authors: Lecompte Matthieu , Dumouchel Christophe
Published: April 2008

The three‐parameter, Generalized Gamma function solution of a recent MEF formulation used to derive liquid spray drop‐size distribution, is applied to sprays resulting from three different atomization processes. The objectives of these applications are to determine the sign of the parameters for which this function reports a more reliable fit and to further understand the parameter stability problem reported elsewhere. It is found that the lack of stability of the parameters is related to a characteristic feature of the mathematical function and appears for a series of spray drop‐size distributions with constant shape. For each situation analyzed in the present study, the Generalized Gamma function provides a very good fit with parameters that are either constant or correlated to the working conditions. As far as the sign of the parameters is concerned, the results show that the best formulation is a function of the spray and that it is impossible to know, a priori, which parameter sign will report the best fit. Finally, for one situation, it is found that the Generalized Gamma function allows extrapolation of drop sizes outside the measured values. All of the results converge to conclude that the three‐parameter Generalized Gamma function, which is identical to the well‐known Nukiyama‐Tanasawa distribution, accumulates valuable attributes to represent liquid spray drop‐size distributions.
Growth and magnetization study of transition metal doped GaN nanostructures
Authors: Gupta Shalini , Kang Hun , Kane Matthew H. , Park Eun‐Hyun , Ferguson Ian T.
Published: April 2008

This work presents the MOCVD growth and characterization of optically active GaN nanostructures which have been doped with the transition metals manganese and iron for potential spintronic applications. Introduction of both these transition metals in GaN nanostructures enhanced the nucleation of the nanostructures resulting in reduced lateral dimensions and increased nanostructure density. Both Ga1–xMnxN and Ga1–xFexN nanostructures showed hysteresis behaviour at 5 K. Further VSM measurements on Ga1–xFexN nanostructures at 300 K showed a hysteresis curve with a reduced coercive filed and displayed superparamagnetic behaviour. These magnetically active nanostructures are promising and provide an incentive to further study them with the aim of eventually utilizing them in spintronic applications and improving device efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Laser threshold and optical gain of blue optically pumped InGaN/GaN multiple quantum wells (MQW) grown on Si
Authors: Lutsenko E. V. , Danilchyk A. V. , Tarasuk N. P. , Andryieuski A. V. , Pavlovskii V. N. , Gurskii A. L. , Yablonskii G. P. , Kalisch H. , Jansen R. H. , Dikme Y. , Schineller B. , Heuken M.
Published: April 2008

Optical and laser properties of a series of MQW heterostructures with varying geometry grown on silicon with Al predeposition were investigated. Photoluminescence (PL) band positions covered a spectral range of 430 – 460 nm under Iexc=1 MW/cm2 and 445‐505 nm under Iexc=0.15 W/cm2. Laser action was achieved under transversal optical pumping at room temperature using only cleaved lateral facets of the samples as laser mirrors. The laser threshold rose from 137 kW/cm2 to 300 kW/cm2 with laser wavelength increase from 440 nm to 465 nm. Numerical simulation of the laser conditions shows that the minimal threshold is realized on the fifth order mode. However, the calculated value of material optical gain of InGaN at the laser threshold increases only from 750 cm–1 to 1020 cm–1, mainly due to absorption rise in the substrate with increasing wavelength. Correlation was observed between PL characteristics and laser threshold. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of surface passivation films on AlGaN/GaN HEMT with MIS gate structure
Authors: Yagi S. , Shimizu M. , Ide T. , Yano Y. , Akutsu N.
Published: April 2008

We investigated the effect of ebeam‐deposited SiNx, SiO2, and TiO2 passivation films on the AlGaN/GaN HEMT structure. The AlGaN/GaN heterojunction structure was grown on a sapphire substrate by metal organic chemical vapor deposition. The sheet resistance of the AlGaN/GaN HEMT structure was 420‐510 Ω/□. TiO2, SiNx and SiO2 passivation films were employed as the insulator in MIS gate structure. All passivation films were deposited by electron beam evaporation at 200 °C. The thickness of SiNx, SiO2 and TiO2 passivation films were 15, 10 and 15 nm, respectively. The relative dielectric constants of the deposited SiNx, SiO2 and TiO2 were measured to be 4, 7 and 80, respectively by fabricating the capacitance structures. The sheet resistance was changed after the deposition of the insulator films. We found to be 512, 628 and 992 Ω/□ for the SiNx, SiO2 and TiO2 films, respectively. We considered that the surface passivation using the TiO2 and SiO2 films reduces the carrier density in the AlGaN/GaN channel. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Recombination dynamics of localized excitons in AlGaN‐based quantum wells
Authors: Murotani Hideaki , Saito Takuya , Kato Nobuo , Yamada Yoichi , Taguchi Tsunemasa
Published: April 2008

The effect of internal electric field on exciton localization in AlGaN‐based quantum wells (QWs) has been studied by means of time‐resolved photoluminescence (PL) spectroscopy. The observed time‐dependent PL was analyzed using rate equations, which enabled us to obtain the PL lifetime and the localization time of excitons for three QWs with different well‐layer thickness. The localization time became longer with increasing well‐layer thickness. This result indicated that a quantum confined Stark effect caused the reduction in transition probability of excitons from extended to localized states. In addition, the localization time decreased with increasing excitation density and the three QWs indicated almost the same localization time at higher excitation density. Since the alloy composition was the same for the three QWs, the degree of localization due to alloy disorder did not change so much. Therefore, the screening of the internal electric field resulted in the same localization time. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
340 nm‐band high‐power InAlGaN quantum well ultraviolet light‐emitting diode using p‐type InAlGaN layers
Authors: Fujikawa Sachie , Takano Takayoshi , Kondo Yukihiro , Hirayama Hideki
Published: April 2008

In order to achieve high‐efficiency ultraviolet (UV) light‐emitting diodes (LEDs), p‐type layers with sufficient hole concentration is quite important to suppress the electron overflow. We introduced Mg‐doped InAlGaN for p‐type layers of the UV‐LEDs for the purpose of obtaining higher hole concentration, and achieved high power CW operations of the UV‐LEDs. 340 nm‐band InAlGaN‐based multi‐quantum‐well (MQWs) UV‐LEDs with Mg‐doped InAlGaN electron‐blocking layer (EBL) and Mg‐doped InAlGaN layers were grown on sapphire substrates by metal‐organic chemical vapor deposition (MOCVD). The output power of UV‐LED with p‐InAlGaN layers was approximately 4.7 times larger than that with p‐AlGaN layers. Maximum output power and the external quantum efficiency (EQE) of the LED with p‐InAlGaN layers were 8.4 mW and 0.9%, respectively, at emission wavelength of 346 nm under room temperature (RT) CW operation. From these results, p‐InAlGaN is shown to be useful for the realization of high‐efficiency UV‐LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimization of GaN MOS capacitors and FETs
Authors: Huang W. , Khan T. , Chow T. P.
Published: April 2008

In this paper, three different gate oxide processes were explored and compared. All MOS capacitors were fabricated on n‐type GaN epitaxial layers grown by MOCVD on sapphire substrates. C‐V measurements were performed on the three samples and their characteristics were compared. The very small interface‐state density makes OX3 a very good process choice for GaN MOSFETs and the trend of decreasing interface‐state density deeper into the bandgap allows the demonstration of GaN inversion‐mode MOSFET.

Both accumulation and inversion‐mode MOSFETs were fabricated and characterized using OX3 process conditions. Field‐effect mobilities were extracted on both circular and linear devices with different channel lengths. Mobilities in two perpendicular directions in (0001)‐plane show similar values. Current collapse testing was also performed on our GaN MOSFETs and no current collapse was observed. In addition, an NMOS inverter stage, utilizing an enhancement‐mode MOSFET and a depletion‐mode MOSFET has also been demonstrated and characterized. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Surface treatment on the growth surface of semi‐insulating GaN bulk substrate for III‐nitride heterostructure field‐effect transistors
Authors: Ryou J.‐H. , Liu J. P. , Zhang Y. , Horne C. A. , Lee W. , Shen S.‐C. , Dupuis R. D.
Published: April 2008

Al0.25Ga0.75N/GaN heterostructure field‐effect transistor (HFET) structures are grown on semi‐insulating GaN substrates by metalorganic chemical vapor deposition with Fe‐doped GaN buffer layer and various surface treatments prior to the epitaxial growth of the structure. The surface treatments are intended to remove suspected surface‐charge‐containing layer and they include wet chemical etching, in‐situ thermal etching, plasma dry etching, and photo‐enhanced chemical etching. Photo‐enhanced chemical (PEC) etching was effective in achieving microscopically smooth surface and in eliminating surface charge layer on the surface of semi‐insulating GaN substrates. RMS surface roughness of PEC etched surface of the semi‐insulating GaN substrate is 0.1∼0.2 nm for 1×1 μm2 scan. C ‐V measurement shows well‐defined 2‐dimensional‐gas‐related charge without any growth‐interface‐related charge for a HFET grown on a semi‐insulating GaN substrate with optimized photo‐enhanced chemical etching of the surface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Luminescence properties of AlxGa1–xN(0.4 < x < 0.5)/AlyGa1–yN (0.6 < y ≤ 1) quantum structures grown by gas source molecular beam epitaxy
Authors: Nikishin S. , Borisov B. , Kuryatkov V. , Song D. , Holtz M. , Garrett G. A. , Sarney W. L. , Sampath A. V. , Shen H. , Wraback M.
Published: April 2008

We report structural and optical properties of AlxGa1–xN (0.4 < x < 0.5)/AlyGa1–yN (0.6 < y ≤ 1) quantum structures grown by gas source molecular beam epitaxy with ammonia on (0001) sapphire substrates. The structures are designed for light emission at ∼ 280 nm. The AlxGa1–xN (0.4 < x < 0.5) well material was grown under two dimensional (2D), three dimensional (3D), and (2D+3D) conditions by the varying group‐III/ammonia ratio. The formation of nanoscale islands, or quantum dots (QDs), in the wells grown in 3D and (2D+3D) modes was observed using transmission electron microscopy. Optical properties are investigated using room temperature cathodoluminescence and time‐resolved photoluminescence. Systematic studies allow us to obtain well growth conditions to produce ∼ 60 fold intensity enhancement over purely two‐dimensional structures. Under these conditions, corresponding to deposition ∼ 10 monolayers of well material, we obtain emission at ∼ 280 nm with narrowest line width and longest photoluminescence decay time. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Closed‐loop MBE growth of droplet‐free GaN with very metal rich conditions using Metal Modulated Epitaxy with Mg and In
Authors: Burnham S. D. , Henderson W. , Doolittle W. A.
Published: April 2008

Improvements to the Metal Modulated Epitaxy (MME) technique are achieved through computer controlled shutter transitions based on feedback from RHEED transients (S. D. Burnham and W. A. Doolittle, J. Vac. Sci. Technol. B 24, 2100 (2006) [1]), thus creating a closed‐loop control system for nitride MBE, the first of its kind. A high‐sensitivity 22 bit camera is used to determine when RHEED transients have subsided, upon which a shutter transition is initiated allowing the efficient buildup and depletion of the metal bilayer, which improves surface morphology and growth rate compared to the standard MME technique. RMS surface roughness was reduced by 41% by using this “Smart Shuttering” improvement to MME. A substantially higher peak concentration of Mg, approaching 2% atomic concentration, was achieved using the MME technique. As expected, a negligible amount of In was incorporated into the very Ga‐rich films. Using the new closed loop control system developed for MBE, the surface morphology was improved for GaN films while highly repeatable holes concentrations as high as 4.7×1018 cm–3 with 1.07 cm2/V‐sec mobility was obtained. This approach offers great promise for improved reproducibility and improved material quality. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Integration of a matrix addressable blue/green LED array with multicore imaging fiber for spatiotemporal excitation in endoscopic biomedical applications
Authors: Xu H. , Davitt K. M. , Dong W. , Song Y.‐K. , Patterson W. R. , Nurmikko A. V. , Aizenman C. D.
Published: April 2008

We have developed a micro‐scale endoscopic optical image projection device, integrating an InGaN MQW blue/green two‐dimensional (2D) LED array to a multicore imaging fiber. A scalable matrix addressing scheme was used for the electrical access to individual elements in the densely packed 2D‐LED array. A prototype 10×10 element array was fabricated by specialized device process flow, using deep reactive ion etching and polyimide etch‐back processes for device isolation and planarization. Individual elements of the array have been operated under cw current injection up to 3 kA/cm2, suggesting uncompromised performance and robustness of the planarized device structure. The device was butt‐coupled to a 30,000 pixel multicore image fiber to generate a spatiotemporal pattern of light at the output end of the fiber. As application demonstration, selected illumination patterns have been projected to the eye of Xenopus laevis tadpoles in studies of their developing visual system, as well as optically excitable neural cells in the brain. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Low‐temperature grown compositionally graded InGaN films
Authors: Miller N. , Jones R. E. , Yu K. M. , Ager J. W. , Liliental‐Weber Z. , Haller E. E. , Walukiewicz W. , Williamson T. L. , Hoffbauer M. A.
Published: April 2008

A new thin film growth technique known as energetic neutral atomic‐beam lithography/epitaxy (ENABLE) provides a large energetic N atom flux and eliminates the need for high substrate temperatures as compared to molecular beam epitaxy, making isothermal growth over the entire InGaN alloy composition range possible without phase separation. 500–800 nm thick compositionally graded InGaN films were grown by ENABLE at ∼450 °C with the following structure types: (1) with the Ga‐rich material on the surface and (2) with the Inrich material on the surface. Rutherford backscattering spectrometry, transmission electron microscopy, X‐ray diffraction, absorption spectroscopy, photoluminescence, and Hall effect measurements were used to assess the thickness, composition, crystalline quality, and optical and electrical properties of the films. The results establish the new ENABLE method as uniquely capable of growing compositionally graded InGaN films and, in the future, InN/GaN heterostructures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High‐resolution Rutherford backscattering spectrometry for InAlGaN/AlGaN single quantum well structure
Authors: Sakuta H. , Fukui T. , Miyachi T. , Kurai S. , Taguchi T.
Published: April 2008

InAlGaN/AlGaN single quantum well (SQW) structure has been studied by means of high‐resolution Rutherford backscattering spectrometry (HRBS). The HRBS spectrum of a SQW structure was clearly observed. The composition of each layer was estimated to be In0.02Al0.17Ga0.81N/Al0.28Ga0.72N from the HRBS measurement. The channeling measurement shows the high degree of the crystalline perfection that can be achieved in such epitaxial layers from the minimum yield of 3.2% in the direction of the <11–23> axis. The channeling measurement also shows atomic disorder differences between InAlGaN epitaxial layer and AlGaN epitaxial layer. Furthermore Al, Ga and In minimum yields in the InAlGaN layer were estimated to be χmin(Ga) = 4.9%, χmin(Al) = 9.4% and χmin(In) = 6.6%, respectively. Larger normalized yield of Al than those of Ga and In in the InAlGaN layer indicates that Al atomic disorder becomes larger to compensate the compressive stress of Ga and In. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
GaN HEMT thermal behavior and implications for reliability testing and analysis
Authors: Green Daniel S. , Vembu Bharath , Hepper David , Gibb Shawn R. , Jin Daniel , Vetury Rama , Shealy Jeffrey B. , Beechem L. Thomas , Graham Samuel
Published: April 2008

GaN HEMT reliability evaluation in a typical Arrhenius manner requires establishing peak junction temperature for a particular stress condition. Several new techniques have yielded promising results toward establishing peak temperature for these devices in combination with detailed physical modeling, particularly micro‐Raman imaging. This paper compares results from finite element modeling to measurements by infrared imaging and micro‐Raman imaging. The limitations of IR imaging were confirmed similar to earlier reports. Two techniques for establishing temperature from micro‐Raman measurements were used to reveal excellent correlation to the model, and also provide insight into the relationship between temperature and structural change in the device. Temperature modeling data is reported for base plate temperature from 85°C to 250°C for practical GaN HEMT devices. Implications of the measurements for GaN HEMT reliability stress testing and analysis will be discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Formation of ohmic contacts to ultra‐thin channel AlN/GaN HEMTs
Authors: Zimmermann Tom , Deen David , Cao Yu , Jena Debdeep , Xing Huili Grace
Published: April 2008

AlN/GaN‐based high electron mobility transistors with ultra‐thin AlN barriers of 2.3 ‐ 5 nm are attractive candidates for very high speed applications owing to the aggressive scalability such structures afford. We report the first study on formation of ohmic contacts to these high quality ultra‐thin channel heterostructures (ns > 1x1013 cm–2 and μ > 900 cm2/Vs) with systematically varying barrier thicknesses. While the conventional ohmic contacts to AlGaN/GaN structures generally require high temperature annealing, these ohmic contacts were found to behave ohmic or near ohmic as‐deposited. Annealing (400‐860 0C) improves the contact resistance to a range of 0.8 ‐ 2 ohm‐mm but the annealing conditions strongly depend on the AlN thickness as well as the heterostructure quality (μ). All alloyed contacts show smooth morphology, making them suitable for e‐beam lithographically defined gate patterning. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A novel GaN‐based multiparameter sensor system for biochemical analysis
Authors: Lübbers B. , Kittler G. , Ort P. , Linkohr S. , Wegener D. , Baur B. , Gebinoga M. , Weise F. , Eickhoff M. , Maroldt S. , Schober A. , Ambacher O.
Published: April 2008

GaN and its ternary alloys offer several advantages for the fabrication of biochemical sensors systems due to their chemical stability and optical transparency. We employ ion‐sensitive field effect transistors (ISFET) based on a surface sensitive AlGaN/GaN heterostructure. Combined with a picoliter dosing system, an integrated micro reference electrode and an optical spectroscopy system a novel multi‐parameter analysis system for aqueous solutions in the micro‐ to nanoliter range was constructed. Application examples are presented to illustrate the application potential of the sensor system e.g. in the high‐throughput screening of cancer drugs. Simultaneous opto‐electrochemical monitoring of enzyme reaction kinetics by pH and absorption measurements in 700 nl and parallel automated measurements with functionalized enzyme‐modified FETs (EnFET) and a reference ISFET in 3 µl are demonstrated. The experimental data obtained agree well with published values for the reaction kinetic and with reference measurements performed with a state‐of‐the‐art pH‐meter and spectrophotometer which require 1000‐fold larger volumes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
InGaN‐based LEDs grown by plasma‐assisted MBE on (0001) sapphire with GaN QDs in the nucleation layer
Authors: Thomidis C. , Nikiforov A. Yu. , Xu Tao , Moustakas Theodore D.
Published: April 2008

Blue‐violet LEDs based on InGaN MQWs were grown by RF‐plasma MBE on (0001) sapphire with GaN QDs incorporated in the nucleation layer as a mechanism of dislocation filtering. Devices with dimensions 800 μm x 800 μm were fabricated and evaluated at a bare die configuration by measuring their EL spectra and power output. At low injection current, the EL spectra peak at 440 nm. At high injection current a second peak at 385 nm becomes the dominant one and it is attributed to tunneling of injected electrons through the electron blocking layer. The dependence of the power output as a function of the injection current was studied systematically in order to evaluate the effects of heating. Power outputs up to 9 mW were measured at injection current of 1 A. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical properties of single and multi‐layer InGaN quantum dots
Authors: Sebald K. , Lohmeyer H. , Herlufsen S. , Kalden J. , Gutowski J. , Tessarek C. , Yamaguchi T. , Hommel D.
Published: April 2008

We discuss the optical properties of single InGaN QDs grown by MOVPE. The samples were structured into mesas by focused‐ion‐beam etching and investigated by micro‐photoluminescence measurements. The QDs are characterized by a high temperature stability of their emission up to 150 K and strong localization of the carriers within the quantum dots, indicated by the observation of antibinding biexcitonic complexes. The influences of piezoelectric fields and spectral diffusion effects were investigated. Furthermore, the polarization of individual QD emission lines was analyzed giving an insight into their geometrical shape. The quantum dots were used for the realization of quantum dot stacks. The photoluminescence of these samples can be traced up to room temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Synthesis of rare‐earth activated AlN and GaN powders via a three‐step conversion process
Authors: Tao Jonathan H. , Perea‐Lopez Nestor , McKittrick Joanna , Talbot Jan B. , Klinedinst Keith , Raukas Madis , Laski Joseph , Mishra Kailash C. , Hirata Gustavo
Published: April 2008

Using a three‐step solution method, we have successfully synthesized rare‐earth (RE) activated AlN, GaN, and pure GaAlN powders. Aluminium and/or gallium nitrates are first dissolved in water. For AlN:RE, rare‐earth nitrates converted from their corresponding oxides of Dy3+ and Tm3+ are then added to the precursor nitrate solution. The mixture is subsequently converted into Al(OH)3:RE by combining with aqueous NH4OH at room temperature. The product is then mixed with aqueous NH4F to form (NH4)3AlF6:RE. The conversion process is complete when the hexafluoride compound is flushed with anhydrous ammonia to produce the final nitride product AlN:Dy3+,Tm3+. GaN, GaN:Tb3+ and GaAlN are synthesized similarly. Single phase, pure GaN and GaAlN formation has been confirmed. Dysprosium oxide has been observed in AlN:Dy3+, Tm3+. Oxygen concentration of approximately 4 at% has been measured in GaN powders, and is slightly higher in AlN powders. Spectroscopic measurements show clearly observable emission from Dy3+ and Tm3+ co‐activated AlN samples. In activated GaN samples, Tb3+ emission has been observed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The force field picture of molecular shape response
Authors: Kryachko Eugene S.
Published: April 2008

The concept of molecular response that is likely related to an enormous variety of phenomena of chemistry and biochemistry, such as supramolecular catalysts, enzymes reactions, folding, the concept of recognition, sensoring, and drug design, is approached in the present work from both graph theory invoked to rationalize the bonding patterns and from the molecular force field applied to quantify this response. Regarding the need of some supplementary criteria to characterize the chemical bond, we partially focus on the existence of the central C–C bond ofsyn‐1,6:8,13‐biscarbonyl [14]annulene and report its three lower‐energy conformers. Some class of triatomic molecules HXY is computationally studied to illustrate the proposed approach to molecular response. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
Hydraulic habitat composition and diversity in rural and urban stream reaches of the North Carolina Piedmont (USA)
Authors: Shoffner Deborah , Royall Dan
Published: April 2008

The differences between urban and rural streams regarding hydrological process, channel morphodynamics and ecosystem functioning have been highlighted by a number of studies in recent decades. The need to understand lotic ecosystem functioning in these environments at scales relevant to individual organisms has led to research focusing on hydraulic composition and structure over small areas of channel bed. In this study we map and analyze the hydraulic biotope composition of two urban and two rural stream reaches in the North Carolina (USA) Piedmont to determine if urban flow regimes and attendant channel modification processes might translate into important differences in hydraulic environment, and if so, what those differences are. Hydraulic biotope assemblages were found to vary only moderately in diversity per unit stream length between sites, but were distinctly different in composition. One important control on the differences between rural and urban streams was found to be the localized incision of urban channels into bedrock and saprolite. Resistant rock outcrop in the beds of urban streams creates rapid and riffle biotopes and long stretches of upstream pool habitat by impoundment. Urban reaches were found to be more homogeneous than rural reaches in hydraulic composition and were dominated by pools. Rural reaches, characterized by copious sandy alluvium in the bed, were dominated by runs or glides. Quantitative differentiation of biotopes based on four hydraulic indices generally yields coherent associations, although these may vary in content. Comparisons between hydraulic and biotic diversities suggest relationships favoring biotic functional group aggregation over species richness‐based indices of diversity. Because the majority of published hydraulic diversity analyses are based on coarse‐bed streams, further study of hydraulic diversity in streams with finer substrate is likely to be beneficial. Copyright © 2008 John Wiley & Sons, Ltd.
Macroinvertebrate community response to inter‐annual and regional river flow regime dynamics
Authors: Monk Wendy A. , Wood Paul J. , Hannah David M. , Wilson Douglas A.
Published: April 2008

Spatio‐temporal variability in river flow is a fundamental control on instream habitat structure and riverine ecosystem biodiversity and integrity. However, long‐term riverine ecological time‐series to test hypotheses about hydrology–ecology interactions in a broader temporal context are rare, and studies spanning multiple rivers are often limited in their temporal coverage to less than five years. To address this research gap, a unique spatio‐temporal hydroecological analysis was conducted of long‐term instream ecological responses (1990–2000) to river flow regime variability at 83 sites across England and Wales. The results demonstrate clear hydroecological associations at the national scale (all data). In addition, significant differences in ecological response are recorded between three ‘regions’ identified (RM1–3*) associated with characteristics of the flow regime. The effect of two major supra‐seasonal droughts (1990–1992 and 1996–1997) on inter‐annual (IA) variability of the LIFE scores is evident with both events showing a gradual decline before and recovery of LIFE scores after the low flow period. The instream community response to high magnitude flow regimes (1994 and 1995) is also apparent, although these associations are less striking. The results demonstrate classification of rivers into flow regime regions offers a way to help unravel complex hydroecological associations. The approach adopted herein could easily be adapted for other geographical locations, where datasets are available. Such work is imperative to understand flow regime–ecology interactions in a longer term, wider spatial context and so assess future hydroecological responses to climate change and anthropogenic modification of riverine ecosystems. Copyright © 2008 John Wiley & Sons, Ltd.
Erosion properties of cohesive sediments in the Colorado River in Grand Canyon
Authors: Akahori Ryosuke , Schmeeckle Mark W. , Topping David J. , Melis Theodore S.
Published: April 2008

Cohesive sediment deposits characterized by a high fraction of mud (silt plus clay) significantly affect the morphology and ecosystem of rivers. Potentially cohesive sediment samples were collected from deposits in the Colorado River in Marble and Grand Canyons. The erosion velocities of these samples were measured in a laboratory flume under varying boundary shear stresses. The non‐dimensional boundary shear stress at which erosion commenced showed a systematic deviation from that of non‐cohesive sediments at mud fractions greater than 0.2. An empirical relation for the boundary shear stress threshold of erosion as a function of mud fraction was proposed. The mass erosion rate was modelled using the Ariathurai–Partheniades equation. The erosion rate parameter of this equation was found to be a strong function of mud fraction. Under similar boundary shear stress and sediment supply conditions in the Colorado River, cohesive lateral eddy deposits formed of mud fractions in excess of 0.2 will erode less rapidly than non‐cohesive deposits. Copyright © 2008 John Wiley & Sons, Ltd.