bluuebunny/crossref_metadata_2025 huggingface dataset:
Introduction of defects into HfO2 gate dielectrics by metal-gate deposition studied using x-ray photoelectron spectroscopy and positron annihilation
Authors: Uedono A. , Naito T. , Otsuka T. , Shiraishi K. , Yamabe K. , Miyazaki S. , Watanabe H. , Umezawa N. , Chikyow T. , Akasaka Y. , Kamiyama S. , Nara Y. , Yamada K.
Published: October 2006

The impact of TiN deposition on thin HfO2 films formed on Si substrates was studied using x-ray photoelectron spectroscopy and a monoenergetic positron beam. For the predeposition sample, the positrons implanted into Si were found to diffuse toward the HfO2∕Si interface under the influence of the electric field, suggesting the presence of negative charges in HfO2. After TiN was deposited by chemical vapor deposition, no large change in the defect species was observed. After TiN was deposited by physical vapor deposition, however, positive charges were introduced at the TiN∕HfO2 interface, which were associated with the incorporation of nitride into HfO2 and/or the plasma damage at the surface of HfO2, and the resultant formation of positively charged oxygen vacancies.
Steady flow and evaporation of a volatile liquid in a wedge
Authors: Markos Mulugeta , Ajaev Vladimir S. , Homsy G. M.
Published: October 2006

We develop a lubrication-type model of a liquid flow in a wedge in the limit of small capillary numbers and negligible gravity. Liquid flows under the action of capillary pressure gradients and thermocapillary stresses, and evaporates due to heating from the solid walls on which a constant axial temperature gradient is imposed. Steady vapor-liquid interface shapes are found for different wedge angles and material properties of the liquid. In the limit of weak evaporation (e.g., in the adiabatic region of a heat pipe) and negligible Marangoni number, the flow rate is the same in all cross sections and can be controlled by changing the wedge angle. We find the wedge angle that results in the maximum value of the flow rate for a given contact angle. For finite evaporation rates, both the flow rate and the amount of liquid in each cross section along the wedge decrease until the point of dry-out is reached. The location of the dry-out point is studied as a function of evaporation conditions. Somewhat counterintuitively, we find that the dry-out point shifts toward the region of higher temperature as evaporation intensity is increased. The effect of thermocapillary stresses on the vapor-liquid interface shape is also investigated in the limit of negligible evaporation. Since thermocapillarity generally opposes the capillary flow, it leads to shorter wetted lengths. The implications of the results for design and optimization of micro heat pipes are discussed.
Effect of fluorine addition on transparent and conducting Al doped ZnO films
Authors: Kim Inho , Lee Kyeong-Seok , Lee Taek Seong , Jeong Jeung-hyun , Cheong Byeong-ki , Baik Young-Joon , Kim Won Mok
Published: October 2006

Al doped ZnO (AZO) films with varying fluorine content were prepared by radio frequency magnetron sputtering at a room temperature to investigate doping effects of fluorine on the structural, the optical, and the electrical properties. The small amount of fluorine addition to AZO films resulted in beneficial effect on the electrical conductivity by improving the direct current (dc) Hall mobility, and the minimum specific resistivity was as low as 5.9×10−4Ωcm. With increasing fluorine content in AZO films, the optical absorption loss in the visible range decreased regardless of carrier concentration in the films. X-ray diffraction and scanning electron micrograph analyses showed that the crystallinity of AZO films was deteriorated by addition of fluorine. Small amount of fluorine addition to AZO film resulted in decrease of absorption loss as well as increase in Hall mobility, and the beneficial effects of fluorine addition was deduced to be caused by killing in-grain point defects. From the comparison between the dc Hall mobility and the optical mobility, it was concluded that fluorine excessively added to AZO increased grain boundary scattering, which was explained using two phase model rather than charge trapping model.
Sub- and overcritical stable states of composite high-Tc superconductors with different E(J) dependences and their unavoidable overheating
Authors: Romanovskii V. R. , Awaji S. , Nishijima G. , Watanabe K.
Published: October 2006

To understand the underlying physical trends of the current instability in a composite high-Tc superconductor, the limiting margin of its current-carrying capacity is derived in dc magnetic fields in the framework of the macroscopic continuum approximation. A static zero-dimensional model was used to formulate the peculiarities of the nonisothermal electric field distribution in a composite in the fully penetrated current states. The power and exponential equations describing the E(J) dependences of a superconductor are used. The boundary of the allowable stable values of the electric field, current, and temperature are investigated using qualitative and quantitative models. Permissible stable values of the electric field and current, which might be lower (subcritical states) or higher (overcritical states) than those determined by the critical voltage criterion, are discussed. It is stated that the subcritical quenching electric states are more probable in the operating regimes, which are observed in the high magnetic field. The overcritical stable quantities of the electric field exist, for example, if the superconducting composite has a relatively small volume fraction of the superconductor in a composite. In the meantime, the stable current modes may be both subcritical and overcritical when the permissible value of the electric field is overcritical. As a consequence of these features, an unavoidable increase in temperature of the composite superconductor occurs before its transition to the normal state. The latter depends on a broad shape of the E(J) dependence of high-Tc superconductor and the current sharing between the superconducting core and the matrix. In the limiting case, a stable value of the composite temperature may equal the critical temperature of the superconductor. For such operating states, the criterion of the complete thermal stability condition is written taking into consideration the nonlinear character of the E(J) dependence. Simultaneously, an allowable change in temperature of the superconducting composite leads to the thermal degradation of its current-carrying capacity. It depends on the critical current density of the superconductor at bath temperature, amount of a superconductor, and cross section of a composite under fixed cooling conditions. In particular, it is shown that the currents corresponding to the instability onset do not increase proportionally with relevant increase of the superconductor’s amount. The estimates presented have general character and may be used to verify the operating states of low-Tc superconducting composite.
Surface photovoltage spectroscopy and photoluminescence study of vertically coupled self-assembled InAs∕GaAs quantum dot structures
Authors: Chan C. H. , Chen H. S. , Kao C. W. , Hsu H. P. , Huang Y. S. , Wang J. S.
Published: October 2006

Multilayer self-assembled InAs∕GaAs quantum dot (QD) structures with varying GaAs spacer layer (SL) thickness are systematically investigated using surface photovoltage spectroscopy (SPS) and photoluminescence (PL). The optical transitions are more clearly visible in the room-temperature SPS spectra and exhibit more features in comparison to PL. The enhanced electronic coupling effect for thinner SL results in the appearance of an additional excited state lying higher than the second excited quantum dot state. A peculiar feature denoted as QDX below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is lower than that of vertically coupled ones. The transition blueshifted with a decrease of the SL thickness indicates that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain. The intensity ratio between the ground state and QDX in the temperature dependent PL spectra is found to be associated with the processes of enhanced thermal carrier transfer between QDs and tunneling of carriers between QDs in adjacent layers for a stack due to a reduction in SL thickness. The results demonstrate considerable diagnostic value of SPS for the nondestructive characterization of vertically coupled self-assembled QD structures.
Gas phase preparation and analysis of semiconductor surfaces in a clustered reactor apparatus
Authors: Finstad Casey C. , Montaño-Miranda Gerardo , Thorsness Adam G. , Muscat Anthony J.
Published: October 2006

An integrated reactor system was built for studying gas phase surface preparation chemistries. The system integrates HF/vapor and UV photochemistry modules with an ultrahigh vacuum deposition reactor and a surface analysis chamber (x-ray photoelectron spectroscopy and Auger) for in situ surface preparation, deposition, and analysis. Each vacuum chamber is mounted on a separate, isolated branch from a main sample transfer tube. The system was designed for samples with variable shapes and thickness, but less than 64mm (212in.) in diameter. This design allows for rapid transfer times between chambers (<5min) and for the simultaneous processing and storage of up to four samples. Use of standard sample transfer and vacuum hardware components minimized initial equipment costs and system maintenance. The capabilities of the clustered reactor apparatus and the importance of surface termination were demonstrated by (1) the removal of a mixed oxide and fluorocarbon residue on silicon, leaving the surface completely terminated with Cl atoms, (2) the removal of copper oxide and copper metal from silicon, (3) the deposition of Ti preferentially on a nonannealed, aqueous-cleaned SiO2 surface relative to an annealed surface, and (4) the use of complementary surface analysis techniques to chemically identify hydrogen-bonded silanol groups on a silicon surface after HF/vapor etching. Gas phase cleaning and surface termination utilized a combination of HF/vapor (100Torr, 27°C for 200s) and UV∕Cl2 (10SCCM Cl2, 90°C for 15min) steps. The results demonstrate that integrated processing provides a means to clean thin layers of organic, oxide, and metal contaminants from semiconductor surfaces and to control the terminating atom or chemical group.
Carrier compensation and scattering mechanisms in Si-doped InAsyP1−y layers grown on InP substrates using intermediate InAsyP1−y step-graded buffers
Authors: Hudait M. K. , Lin Y. , Sinha P. M. , Lindemuth J. R. , Ringel S. A.
Published: October 2006

Electronic transport properties of strain-relaxed Si-doped InAsyP1−y layers with arsenic mole fractions between y=0.05 and y=0.50 were studied. All layers were grown on semi-insulating InP substrates by solid source molecular beam epitaxy using intermediate InAsyP1−y step-graded buffers to reduce dislocation density. Variable magnetic field (0–8.5T) Hall effect measurements in conjunction with quantitative mobility spectrum analysis in the temperature range of 25–300K were used to extract individual carrier mobilities, densities, and donor ionization energy as a function of temperature and alloy composition. The low field mobility is calculated by taking into account various scattering mechanisms, and these results are compared with the experimental results. At a constant electron carrier concentration of ∼2×1016cm−3, the 300K carrier mobility increases from 2856to5507cm2∕Vs with increasing arsenic mole fraction from 0.05 to 0.50. The experimental mobilities are in close agreement with the theoretical results using various scattering mechanisms. Both optical polar phonon scattering and ionized impurity scattering are important at 300K while at 100K, ionized impurity scattering is the limiting process. Alloy scattering is found to be only of second order importance. The Si donor ionization energy was determined to be ∼2–4meV for all alloy compositions.
Optical, structural, and electrical properties of Mg2NiH4 thin films in situ grown by activated reactive evaporation
Authors: Westerwaal R. J. , Slaman M. , Broedersz C. P. , Borsa D. M. , Dam B. , Griessen R. , Borgschulte A. , Lohstroh W. , Kooi B. , ten Brink G. , Tschersich K. G. , Fleischhauer H. P.
Published: October 2006

Mg 2 Ni H 4 thin films have been prepared by activated reactive evaporation in a molecular beam epitaxy system equipped with an atomic hydrogen source. The optical reflection spectra and the resistivity of the films are measured in situ during deposition. In situ grown Mg2NiH4 appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg2NiH4 phase is kinetically blocked. Hydrogen desorption only takes place when a Pd cap layer is added. The optical band gap of the in situ deposited Mg2NiH4 hydride, 1.75eV, is in good agreement with that of Mg2NiH4 which has been formed ex situ by hydrogenation of metallic Pd capped Mg2Ni films. The microstructure of these in situ grown films is characterized by a homogeneous layer with very small grain sizes. This microstructure suppresses the preferred hydride nucleation at the film/substrate interface which was found in as-grown Mg2Ni thin films that are hydrogenated after deposition.
Intense terahertz laser fields on a quantum dot with Rashba spin-orbit coupling
Authors: Jiang J. H. , Weng M. Q. , Wu M. W.
Published: October 2006

We investigate the effects of the intense terahertz laser field and the spin-orbit coupling on single spin in a quantum dot. The laser field and the spin-orbit coupling can strongly affect the electron density of states and can excite a magnetic moment. The direction of the magnetic moment depends on the symmetries of the system, and its amplitude can be tuned by the strength and frequency of the laser field as well as the spin-orbit coupling.
Nonlinear compressibility effects in fluid-structure interaction and their implications on the air-blast loading of structures
Authors: Kambouchev N. , Noels L. , Radovitzky R.
Published: October 2006

The impulse imparted by a blast wave to a freestanding solid plate is studied analytically and numerically focusing on the case in which nonlinear compressibility effects in the fluid are important, as is the case for explosions in air. The analysis furnishes, in effect, an extension of Taylor’s pioneering contribution to the understanding of the influence of fluid-structure interaction (FSI) on the blast loading of structures [The Scientific Papers of Sir Geoffrey Ingram Taylor, edited by G. K. Batchelor (Cambridge University Press, Cambridge, 1963), Vol. III, pp. 287–303] to the nonlinear range. The limiting cases of extremely heavy and extremely light plates are explored analytically for arbitrary blast intensity, from where it is concluded that a modified nondimensional parameter representing the mass of compressed fluid relative to the mass of the plate governs the FSI. The intermediate asymptotic FSI regime is studied using a numerical method based on a Lagrangian formulation of the Euler equations of compressible flow and conventional shock-capturing techniques. Based on the analytical and numerical results, an approximate formula describing the entire range of relevant FSI conditions is proposed. The main conclusion of this work is that nonlinear fluid compressibility further enhances the beneficial effects of FSI in reducing the impulse transmitted to the structure. More specifically, it is found that transmitted impulse reductions due to FSI when compared to those obtained ignoring FSI effects are more significant than in the acoustic limit. This result can be advantageously exploited in the design and optimization of structures with increased blast resistance.
The effect of pulse-modulated surface acoustic waves on acoustoelectric current quantization
Authors: Kataoka M. , Ford C. J. B. , Barnes C. H. W. , Anderson D. , Jones G. A. C. , Beere H. E. , Ritchie D. A. , Pepper M.
Published: October 2006

The effect of pulse modulation of surface acoustic waves (SAWs) on the acoustoelectric current quantization through a one-dimensional channel is investigated. We found that the pulse modulation is effective in eliminating the effects of cross-talk and problems associated with reflected SAWs that are the sources of uncontrollable potential fluctuations in the channel. The drawback of this method is that the short pulse length (<650ns) required to achieve cross-talk elimination degrades the quality of the quantized current plateaus. We calculate the expected current from calibrations of the transducer passband and acoustoelectric current as a function of transducer power and find similar behavior.
Absorbance spectra of aligned Ag nanorod arrays prepared by oblique angle deposition
Authors: Zhao Y.-P. , Chaney S. B. , Zhang Z.-Y.
Published: October 2006

The s- and p-polarization absorbance spectra of Ag nanorod arrays with different heights prepared by oblique angle deposition are studied in detail. For short nanorods, both s- and p-polarization spectra, are dominated by a similar and single resonant peak. For long nanorods, a sharp UV peak dominates the s-polarization spectrum, while a broad vis–near infrared peak dominates the p-polarization spectrum. Three different absorbance peaks located at 293, 360, and 420–720nm, respectively, have been observed and evolve differently for s- and p-polarization absorbances with the nanorod height. The change of the absorbance as a function of the nanorod height is a reflection of the growth dynamics of the nanorod. The discrete dipole approximation was used to calculate the extinction spectra of Ag nanorod with 80-nm-diameter and different lengths, and the calculated results are qualitatively consistent with the experimental results.
Effect of magnesium oxide buffer layer on performance of inverted top-emitting organic light-emitting diodes
Authors: Choi Ho Won , Kim Soo Young , Kim Woong-Kwon , Hong Kihyon , Lee Jong-Lam
Published: October 2006

The effect of magnesium oxide (MgO) buffer layer between cathode and emitting materials on performance of inverted top-emitting organic light-emitting diodes (ITOLEDs) was investigated. The operation voltage at the current density of 100mA∕cm2 decreased from 14.9to9.7V for ITOLEDs with 1nm thick MgO buffer layers. The maximum luminance value increased about 78% in ITOLEDs using MgO buffer layer, which is 1000cd∕m2 at the current density of 191mA∕cm2. Synchrotron radiation photoelectron spectroscopy results revealed that the atomic concentration of Al–O bond increased after deposition of MgO on Al, indicating the oxidation of Al surface. Secondary electron emission spectra showed that the work function increased about 0.8eV by inserting the insulating MgO buffer layer. Therefore, the enhancement of device performance results from the decrease of the energy barrier for electron injection based on the tunneling model.
Electrical characteristics of pentacene-based thin film transistor with conducting poly(3,4-ethylenedioxythiophene) electrodes
Authors: Kang H. S. , Lee J. W. , Kim M. K. , Joo J. , Ko J. M. , Lee J. Y.
Published: October 2006

This is a report on the fabrication and electrical characteristics of an all-organic-based thin film transistor that uses conducting poly(3,4-ethylenedioxythiophene) (PEDOT) as electrodes. The conducting PEDOT layers as source, drain, and gate electrodes were patterned by using photolithography. The poly(vinyl cinnamate) (PVCN) was spin coated and cross-linked as a gate insulator. The pentacene as an active layer was vapor deposited onto the PVCN layer. In order to compare the characteristics of the pentacene-based organic thin film transistor (OTFT) with PEDOT electrodes, we fabricated another pentacene-based OTFT using a Si-based pattern with Au electrodes. The electrical characteristics of the devices, such as charge carrier mobility (μ), threshold voltage (Vth), and on/off current ratio (Ion∕off), were measured from its current-voltage (I-V) characteristic curves. The μ, Vth, and Ion∕off of the pentacene-based OTFT with PEDOT electrodes were ∼2.3×10−3cm2∕Vs, 4V, and ∼100, respectively. We evaluated the activation energy (Ea) of the pentacene layer of the OTFT devices by analyzing the transfer characteristic curves measured in a temperature range from 10to300K based on the multitrap and release model. The Ea of the OTFT with PEDOT electrodes was measured to be ∼0.33eV, in the saturation region. This energy was larger than that of the OTFT with Au electrodes which was measured to be ∼0.13eV. However, the μ’s of both OTFTs were almost the same, in spite of the relatively lower electrical conductivity of the PEDOT and the larger Ea of the OTFT with the PEDOT electrodes. From the results of temperature dependence of current density based on the Schottky emission model, we analyze that the lower barrier height between the PEDOT electrode and the pentacene active layer resulted in easier charge injection from the PEDOT electrode into the active layer.
Switchable resonant x-ray Bragg scattering on a magnetic grating patterned by ion bombardment
Authors: Höink V. , Sacher M. D. , Schmalhorst J. , Reiss G. , Engel D. , Weis T. , Ehresmann A.
Published: October 2006

A local manipulation of the exchange bias coupling between antiferromagnetic and ferromagnetic layers by ion bombardment induced magnetic patterning enables a patterning of the magnetization direction of the ferromagnetic layer without considerable structural changes. We show that a magnetic line grating with alternating antiparallel orientations of the magnetization of the ferromagnetic layer acts as a magnetically switchable reflective grating for soft x-ray radiation. A common rotational direction of the magnetization at all boundaries between bombarded and not bombarded lines is shown by magnetic force microscopy measurements. Scattering at the edges of the bombarded lines might be responsible for the observed interference pattern.
Interactions of radially outgoing jets through gaps between periodically arranged bars in a circle
Authors: Takemoto Yukio , Mizushima Jiro
Published: October 2006

Mutual interactions of jets radially spouting from gaps between periodically arranged sector-shaped bars in a circle are investigated. Confluences of jets were confirmed by flow visualization and numerical simulation assuming a two-dimensional and incompressible flow field. The jets are independent of each other at small Reynolds numbers, but interact in a complex manner to yield various flow patterns such as confluences of several jets or oscillatory flows synchronized in the same phase between several jets. We found that the confluence of two or three steady jets occurs due to a pitchfork bifurcation and that the synchronized oscillation is a consequence of a Hopf bifurcation. Quasi-periodic oscillation in jets was also found and confirmed to originate from a Neimark-Sacker bifurcation; i.e., a Hopf bifurcation for maps. The critical Reynolds numbers for these bifurcations were determined numerically for various values of the pitch-to-nozzle-width ratio.
Particle transport and flow modification in planar temporally evolving mixing layers. II. Flow modification due to two-way coupling
Authors: Narayanan Chidambaram , Lakehal Djamel
Published: October 2006

Simulations of two-dimensional, two-way coupled particle-laden mixing layers were performed for particles with various Stokes numbers, at mass loadings between 0.1 and 0.5. This component complements Part I of the study, where particle and fluid transport was analyzed under one-way coupling. Under two-way coupling, the accumulation of particles in the periphery of the Kelvin-Helmholtz vortices results in the formation of intricate undulating patterns, and rupture and break-up of the vortices. At higher mass loadings the vortex structure is completely destroyed. The overall accumulation at the edges of the mixing layer is significantly reduced due to two-way coupling. The rate of evacuation of the vortex core was found to be much slower compared to one-way coupling. In a global sense, particles delay the development of the mixing layer in terms of saturation of the fundamental and the subharmonic modes. Significant generation of small-scale vorticity and higher energy in the small scales is observed at higher mass loadings. Particles are shown to increase the modal kinetic energy dissipation rate. The mean fluid kinetic energy balance shows that most of the kinetic energy exchange between the particle and fluid phases takes place at the edges of the mixing layer. As the mixing layer evolves, the kinetic energy exchange with the particle phase was shown to decrease in importance compared to the other terms in the mean kinetic energy balance; namely, the energy exchange between the mean and the modal fields and the modal transport term.
Improved optical properties of InAs quantum dots grown with an As2 source using molecular beam epitaxy
Authors: Sugaya Takeyoshi , Amano Takeru , Komori Kazuhiro
Published: October 2006

We demonstrate the effects of using an As2 source to fabricate self-organized InAs∕GaAs quantum dot (QD) structures. QDs grown with an As2 source have narrower photoluminescence (PL) linewidths and higher PL intensities than those grown with an As4 source at high growth rates. The density of QDs grown with an As2 source is smaller, and the dot size larger than those of QDs grown with an As4 source. The coalescence of QDs is reduced under an As2 source, resulting in improved optical properties. These results are thought to result from the difference in the surface migration of In atoms and the surface structures under As2 and As4 sources.
Rectifying characteristics of La1−xSrxMnO3∕TiO2 (x=0.2,0.15,0.04) heterostructures
Authors: Li Tong , Zhang Ming , Song Xuemei , Wang Bo , Yan Hui
Published: October 2006

La 1 − x Sr x Mn O 3  (LSMO)∕TiO2 heterostructures have been synthesized by the rf magnetron sputtering and exhibit excellent rectifying properties. The diffusion potentials increase with increasing Sr concentration and decrease with the increase of temperature, which is attributed to modulating the interfacial electronic structure of La1−xSrxMnO3∕TiO2 heterostructure while changing temperatures. It is worth noting that the metal-insulator (M-I) transition of LSMO can also be observed in heterostructures and the increased sheet resistance of heterostructures at low temperatures is related to the introduction of TiO2.
Polarization of gain and symmetry breaking by interband coupling in quantum well lasers
Authors: Boxberg Fredrik , Tereshonkov Roman , Tulkki Jukka
Published: October 2006

We have studied the influence of conduction band–valence band coupling on the polarization of gain in quantum well (QW) lasers. As a reference we used the eight-band k∙p description of the gain polarization. Our eight-band k∙p model accounts for the crystal orientation, lack of inversion symmetry, strain induced deformation potentials, and piezoelectricity. We have studied both strained and unstrained (001) and (111) QWs. The results are compared with the transition dipole model of the gain polarization [M. Asada et al., IEEE J. Quantum Electron. 20, 745 (1984)], which is based on a phenomenological generalization of Kane’s [J. Phys. Chem. Solids 1, 249 (1957)] linear k∙p model of bulk crystals. We found a quantitative difference between our multiband model and the transition dipole model of Asada et al. The difference is addressed to lack of orthogonality between the transition dipole and the electron wave vectors. The orthogonality is broken outside the Γ point by both the QW heterostructure geometry and the interband coupling. Results obtained by the complete eight-band model are also compared with restricted multiband models excluding the conduction band.
A detailed experimental and analytical study of the thermal expansion of dielectric thin films on Si by x-ray reflectivity
Authors: Phung T. M. , Johnson D. C. , Antonelli G. A.
Published: October 2006

We present a method of using x-ray reflectivity to measure the thermal expansion coefficient for submicron dielectric thin films. Technique criteria are discussed including importance of thickness accuracy, thickness boundaries, and detecting thickness changes due to irreversible phenomena. The thin film mechanics required to extract the thermal expansion parameter for a freestanding film as opposed to an attached film are discussed. Thermal expansion measurements on silicon carbide and silicon nitride thin films using this method agree with literature values obtained for bulk samples. The thermal expansion of several carbon-doped silicon oxide thin films was also measured.
Quantitative structural characterization of InAs∕GaSb superlattices
Authors: Liu Ge “Joseph” , Fruhberger Bernd , Schuller Ivan K. , Haugan Heather J. , Brown Gail J.
Published: October 2006

Molecular beam epitaxy grown InAs∕GaSb superlattices, containing InSb-like interfacial layers, were analyzed by a combination of x-ray diffraction (XRD) and structural refinement. The superlattice refinement from x rays (SUPREX) method determines with high accuracy the average thicknesses and d spacings of the individual InAs and GaSb layers in addition to standard structural parameters usually obtained by XRD, such as the modulation length (periodicity), average out-of-plane interplanar spacings, and total thickness. The combined SUPREX/XRD experiments show that the absence of certain odd order satellite features in the x-ray data is due to asymmetric and inhomogeneous lattice strain.
Effect of laser pulse duration in picosecond ultrasonics
Authors: Dehoux T. , Perton M. , Chigarev N. , Rossignol C. , Rampnoux J.-M. , Audoin B.
Published: October 2006

An optical grating has been introduced in a picosecond ultrasonics experiment, in order to vary continuously the duration of the laser beam pulse from 0.1to150ps. The evolution of the measured signal has been observed and analyzed through the comparison with a theoretical approach based on a two-temperature model. The latter allows matching the acoustic echoes together with the thermal background and the coincidence peak, for each pulse duration and at any time scale. The broadening of the acoustic echoes and the disappearing of its Brillouin component, along with the diminishing of the thermal coincidence peak, have been demonstrated when increasing the pulse duration. For a constant incident pulse energy, the efficiency of acoustic generation is optimum for the shortest pulses. Nevertheless, for longer pulses designed to obtain thermal conditions below the ablation threshold, acoustic generation could be enhanced.
Defects in Ga(In)NAs thin films grown by atomic H-assisted molecular beam epitaxy
Authors: Shimizu Yukiko , Mura Yusuke , Uedono Akira , Okada Yoshitaka
Published: October 2006

The vacancy-type defects in Ga1−yInyNxAs1−x dilute nitride films grown by atomic H-assisted molecular beam epitaxy (H-MBE) were investigated. The positron annihilation measurements showed that the densities of vacancy-type defects in GaNxAs1−x (x=0%–1.3%) films grown under an optimized atomic H flux were as low as that for a liquid encapsulated Czochralski (LEC) GaAs substrate. Further, the influence of vacancy-type defects on the crystal quality and optical properties were studied by x-ray diffraction and photoluminescence (PL) measurements. The integrated PL intensity at 77K drastically decreased as N composition was increased, but we found no clear correlation between the density or volume of vacancy defects and optical properties, and the S parameters were nearly constant at a value of ∼0.516 in all Ga1−yInyNxAs1−x films grown by our H-MBE technique.
Generalized theory and application of Stokes parameter measurements made with a single photoelastic modulator
Authors: Liu Yanwei , Jones Grenville A. , Peng Yong , Shen Tiehan H.
Published: October 2006

We report in this paper a generalized theory that describes the interaction between a monochromatic light beam and an optical system that includes one photoelastic modulator, one analyzer, and one photodetector. Based on the theory, a detailed four-step procedure is presented, which allows a precise measurement of the four Stokes parameters. An analysis of the systematic and random errors arising from the four-step measurements is also given as well as a calibration procedure that involves the use of a general retardation plate. As a practical application the procedure is used to analyze the magneto-optical properties of magnetic thin films grown on GaAs(001) substrates.
Homogeneity and Markovity of electronic dephasing in liquid solutions
Authors: Ka Being J. , Zhang Ming-Liang , Geva Eitan
Published: October 2006

The electronic dephasing dynamics of a solvated chromophore is formulated in terms of a non-Markovian master equation. Within this formulation, one describes the effect of the nuclear degrees of freedom on the electronic degrees of freedom in terms of a memory kernel function, which is explicitly dependent on the initial solvent configuration. In the case of homogeneous dynamics, this memory kernel becomes independent of the initial configuration. The Markovity of the dephasing process is also the most conveniently explored by comparing the results obtained via the non-Markovian master equation to these obtained via its Markovian counterpart. The homogeneous memory kernel is calculated for a two-state chromophore in liquid solution, and used to explore the sensitivity of photon echo signals to the heterogeneity and non-Markovity of the underlying solvation dynamics.
Atomic layer deposition of gadolinium scandate films with high dielectric constant and low leakage current
Authors: Kim Kyoung H. , Farmer Damon B. , Lehn Jean-Sebastien M. , Venkateswara Rao P. , Gordon Roy G.
Published: October 2006

Gd Sc O 3 films were deposited on hydrogenated silicon substrates by atomic layer deposition. The films were pure and amorphous, both as-deposited and after a 5min anneal at 950°C. Cross-sectional transmission electron microscopy revealed a sharp, smooth interface between GdScO3 and Si. Capacitance and leakage current measurements on metal oxide semiconductor capacitors made from atomic layer deposited WN∕GdScO3 stacks showed that the amorphous GdScO3 films have a high dielectric constant (∼22), low fixed charge density, and low interface trap density. A film with 1nm equivalent oxide thickness also demonstrated that the leakage current density is less than 2mA∕cm2 at 1V gate bias.
Effects of BaTiO3 on dielectric behavior of BaTiO3–Ni–polymethyl methacrylate composites
Authors: Choi Hyung-Woo , Heo Young-Woo , Lee Joon-Hyung , Kim Jeong-Joo , Lee Hee-Young , Park Eun-Tae , Chung Yul-Kyo
Published: October 2006

Effects of ferroelectric BaTiO3 particles were investigated on the dielectric behavior of BaTiO3–Ni–polymethyl methacrylate composites fabricated by a two-step mixing and hot-molding method. The percolation power law in the smearing region was employed to explain the experimental results. The composites exhibit the dependence of the dielectric constant, dielectric loss, and percolation threshold on the concentration and particle size of BaTiO3. The results suggest that BaTiO3 particles play an important role in increasing the percolation threshold and the smearing region as well as the dielectric constant of BaTiO3–Ni–polymethyl methacrylate composites.
Efficient beaming from photonic crystal waveguides via self-collimation effect
Authors: Tang Donghua , Chen Lixue , Ding Weiqiang
Published: October 2006

Emission behavior of a subwavelength photonic crystal (PC) W1 waveguide covered by a self-collimation PC to the exit surface is investigated using finite-difference time-domain method. When the thickness and the relative position of the covering PC is designed appropriately, highly efficient beaming emission is obtained. The authors analyze the numerical results and attribute the beaming effect to the interference of the multiple self-collimation beams excited by the waveguide. This simple structure may find its potential applications in integrated optical circuits.
Growth and control of microscale to nanoscale carbon nitride particles
Authors: Li H. Y. , Shi Y. C. , Feng P. X.
Published: October 2006

Microscale to nanoscale carbon nitride (CN) particles are prepared using plasma sputtering deposition techniques. The preferred orientation of nanoscale CN particle distributions is obtained. Particles are examined using scanning electron microscopy (SEM) and Raman scattering spectroscopy. SEM micrographs show that the CN particles are spherical with nearly the same diameters of 2.5μm prepared without setting bias voltage. The distribution of these particles is random. Setting bias voltage up to 5kV, plasma sputtering deposition yields several dispersed ring patterns of particle distributions where many small groups of nanoscale particles are observed. Each group of these particles is in a sunflower type of distribution, in which the biggest (85nm) particle at the center is surrounded by many small sizes (30nm) of CN particles. Disk type of the particles with a diameter of 10μm is also observed at different deposition conditions. Typical carbon bands and CN band in the Raman spectra of the samples are identified. The intensity of the bands obviously varies at the different deposition conditions.
Dielectric constant enhancement due to Si incorporation into HfO2
Authors: Tomida Kazuyuki , Kita Koji , Toriumi Akira
Published: October 2006

The authors investigated the dielectric constant change of Hf(1−x)SixO2 film as functions of Si concentration and annealing temperature. As a result, the dielectric constant of Hf(1−x)SixO2 was increased when doped with a small amount of Si after 800°C annealing. The authors revealed that the dielectric constant enhancement of Hf(1−x)SixO2 films is related to the phase transformation from the monoclinic to the tetragonal phase of HfO2. By using the Clausius-Mossotti relation, it is concluded that the dielectric constant enhancement through the structural phase transformation is derived from the molar volume shrinkage rather than the molar polarizability increase.
Optical control of the singlet-triplet transition in Rb2
Authors: González-Vázquez Jesús , Sola Ignacio R. , Santamaria Jesus , Malinovsky Vladimir S.
Published: October 2006

By controlling nonresonant dynamic Stark shifts it is possible to effectively decouple the intramolecular couplings of simple molecules. We have illustrated this effect in the 1Σu1→1Πu3 transition in Rb2. The laser scheme implies two important control knobs: the laser frequency, which must be chosen to avoid both single and multiphoton resonances and to select different electronic environments for the singlet and triplet states, and the pulse intensity, which must amplify the asymmetry in the dynamic polarizabilities that allows the decoupling, avoiding undesired strong-pulse effects. The mechanism of the scheme implies not only brute-force energy shifts but also light-induced reshaping of the potentials to avoid the undesired crossings. Quantitative aspects of the method are analyzed by using molecular models of increasing complexity for Rb2.
A note on the stabilization of bluff-body wakes by low density base bleed
Authors: Sevilla A. , Martínez-Bazán C.
Published: October 2006

Base blowing is a simple and well-known means of stabilizing both two-dimensional and axisymmetric bluff-body wakes at supercritical Reynolds numbers. In the present Brief Communication we consider a generalization of previous works by studying the effect of the bleed-to-freestream density ratio, S=ρb∕ρ∞, on the stability properties of the wake behind an axisymmetric, slender body with a blunt trailing edge. Since a lower density in the slow stream is known to inhibit absolute instability, here we restrict our attention to the case of light bleed fluid, S<1. This mechanism is shown to inhibit the wake for values of the bleed coefficient, defined as the bleed-to-freestream velocity ratio Cb=ub∕u∞, smaller than those obtained in the homogeneous case of S=1. Our approach consists of obtaining the basic, axisymmetric flow by integration of the full set of Navier-Stokes and species conservation equations in cylindrical coordinates, and calculating its linear, local, spatio-temporal stability downstream of the body base. The analysis predicts a critical bleed coefficient Cb*, which decreases with the density ratio according to the linear law Cb*≃0.01+0.05S for 0.1⩽S⩽1.
The Soret effect in dilute polymer solutions: Influence of chain length, chain stiffness, and solvent quality
Authors: Zhang Meimei , Müller-Plathe Florian
Published: October 2006

Thermal diffusion in dilute polymer solutions is studied by reverse nonequilibrium molecular dynamics. The polymers are represented by a generic bead-spring model. The influence of the solvent quality on the Soret coefficient is investigated. At constant temperature and monomer fraction, a better solvent quality causes a higher affinity for the polymer to the cold region. This may even go to thermal-diffusion-induced phase separation. The sign of the Soret coefficient changes in a symmetric nonideal binary Lennard-Jones solution when the solvent quality switches from good to poor. The known independence of the thermal diffusion coefficients of the molecular weight is reproduced for three groups of polymers with different chain stiffnesses. The thermal diffusion coefficients reach constant values at chain lengths of around two to three times the persistence length. Moreover, rigid polymers have higher Soret coefficients and thermal diffusion coefficients than more flexible polymers.
Numerical studies for the linear growth of resistive wall modes generated by plasma flows in a slab model
Authors: Cui Shaoyan , Wang Xiaogang , Liu Yue , Yu Bo
Published: October 2006

The resistive wall mode generated by plasma-wall relative rotations is studied numerically in a slab model with a compressible plasma flow parallel to the magnetic field. The linear growth of the mode is investigated with different parameters in numerical simulations. The critical plasma flow velocities for the instability are calculated as the wave number of the mode and other parameters vary. It is found that in the long wavelength regime, the critical velocity is in the range of the sound speed cs, as predicted in theory. In the short wavelength regime however, the critical velocity increases to a level of Alfvén velocity VA and a second stable region is found. This region eventually merges with the first stable region as the wave number increases and stabilizes the mode. The growth rate of the mode decreases with the wave number of the mode and the plasma viscosity. The critical wave number for the instability is also calculated as the plasma velocity changes.
Acoustic resonant spectroscopy for characterization of thin polymer films
Authors: Tohmyoh Hironori , Imaizumi Takuya , Saka Masumi
Published: October 2006

An acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity, and density of micron-scale polymer films is developed. The method, which is based on spectral analysis, observes the acoustic resonance between water, the film, and a tungsten plate with high acoustic impedance in the frequency range of 20–70MHz. The interface between the film being examined and the plate is vacuum sealed, enabling us to characterize the low-density polyethylene film with acoustic impedances as low as about 1.9MNm−3s and the poly(vinyl chloride) film as thin as about 8μm. The error in the film density measurements is found to be less than 1%, and the validity of the technique is verified.
Dynamic responses of a two-dimensional flapping foil motion
Authors: Lu Xi-Yun , Liao Qin
Published: October 2006

The investigation of a flapping foil, which is used as a basic mode of the flapping-based locomotion in insects, birds, and fish, is performed by solving the Navier-Stokes equations numerically. In this Brief Communication we provide insight into the understanding of dynamics of a flapping foil. A critical flapping Reynolds number based on the flapping frequency and amplitude, above which a forward flapping movement occurs, is predicted. The dynamics of the flapping foil are analyzed in two dynamic responses, i.e., an oscillatory movement and a steady movement, which depend on the density ratio between the foil and the surrounded fluid. The steady movement response is related to the forward flapping motion. The Strouhal number that governs a vortex shedding for the forward flapping foil is calculated and lies in the range where flying and swimming animals will be likely to tune for high propulsive efficiency.
Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface
Authors: Castillo J. F. , Aoiz F. J. , Bañares L.
Published: October 2006

An ab initio interpolated potential energy surface (PES) for the Cl+CH4 reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH4 and Cl+CD4 reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH4 and Cl+CD4 reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH4 molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH3 and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v′=0) and DCl(v′=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.
Tunable dielectric properties of lead barium zirconate niobate films
Authors: Kuo Ya-Ling , Wu Jenn-Ming
Published: October 2006

The effect of substitution of niobium for zirconium on tunable behavior of lead barium zirconate (PBZ) films was investigated. Lead barium zirconate niobate films were grown on Pt∕Ti∕SiO2∕Si substrates using chemical solution deposition method. The substitution of Nb for Zr enhances tunable properties of PBZ films. The dielectric tunabilities are excellent, all higher than 45% with a maximum=60%. The substitution of Nb for Zr raises values of figure of merit (FOM) of films. The maximum FOM takes place at 5mol% Nb with a value of 90, which is about three times that of the corresponding PBZ film.