bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxidation-enhanced interdiffusion in Si1−xGex∕Si1−yGey superlattices
Authors: Ozguven Nevran , McIntyre Paul C.
Published: February 2007

The effects of Si surface oxidation on Si–Ge interdiffusion in epitaxial heterostructures are presented. Single crystal Si1−xGex∕Si1−yGey superlattices, with a nominal Ge composition of 8.5at.%, were grown by low-pressure chemical vapor deposition onto Si (001) substrates. An epitaxial Si cap, which was partially consumed during postdeposition annealings in dry O2, was grown onto these superlattices. We observed an enhancement of Si–Ge interdiffusion kinetics for the case of oxidation annealings when compared to inert atmosphere annealings. X-ray multilayer scattering measurements were used to quantify the effects of nonequilibrium point defect concentrations created during Si oxidation on the interdiffusivity of Si and Ge over the temperature range of 770–870°C. The activation enthalpy and preexponential constant for the interdiffusivity at this Si–Ge composition, which were determined by analysis of the x-ray multilayer satellite intensity decay after annealing, were found to be 3.99±0.2eV and 1.32cm2∕s, respectively. The extent of the observed interdiffusivity enhancement caused by Si surface oxidation is significantly less than that reported for Si self-diffusion. This suggests a smaller interstitial-mediated component for Si–Ge interdiffusion than for Si self-diffusion.
Giant magnetoresistance and unusual hysteresis behavior in La0.67Ca0.33MnO3∕xCuO (x=20%) composite
Authors: Miao J. H. , Yuan S. L. , Xiao X. , Ren G. M. , Yu G. Q. , Wang Y. Q. , Yin S. Y.
Published: February 2007

La 0.67 Ca 0.33 Mn O 3 ( LCMO ) ∕ x Cu O (x=0 and 20mole%) composites were fabricated and investigated in detail for their electrical and magnetic transport properties. Compared with pure LCMO, the magnetoresistance (MR) effect for x=20% sample is substantially enhanced especially at the vicinity of TIM, where TIM is the transition temperature from insulator to metallic state. Application of even a low magnetic field of H=0.5T leads to a MR value as large as ∼98%. It is interesting to note that at the same temperature region where substantial MR effect is obtained, considerable thermal and magnetic hystereses in resistivity (ρ) appear. The measurement of magnetization (M) versus magnetic field (H) also exhibits an unusual magnetic hysteresis. Due to the absence of these unusual hysteresis behaviors in pure LCMO, it is discussed that the abnormal experimental observations are attributed to the spin disorder especially at grain boundaries induced by the interaction product layer between LCMO and CuO.
Nanosecond-laser plasma-assisted ultradeep microdrilling of optically opaque and transparent solids
Authors: Paul Stanley , Kudryashov Sergey I. , Lyon Kevin , Allen Susan D.
Published: February 2007

A mechanism of ultradeep (up to tens of microns per pulse, submillimeter total hole depths) plasma-assisted ablative drilling of optically opaque and transparent materials by high-power nanosecond lasers has been proposed and verified experimentally using optical transmission and contact photoacoustic techniques to measure average drilling rates per laser shot versus laser intensity at constant focusing conditions. The plots of average drilling rates versus laser intensity exhibit slopes which are in good agreement with those predicted by the proposed model and also with other experimental studies. The proposed ultradeep drilling mechanism consists of a number of stages, including ultradeep “nonthermal” energy delivery into bulk solids by the short-wavelength radiation of the hot ablative plasma, bulk heating and melting, accompanied by subsurface boiling in the melt pool, and resulting melt expulsion from the target.
Hafnium titanate as a high permittivity gate insulator: Electrical and physical characteristics and thermodynamic stability
Authors: Li Min , Zhang Zhihong , Campbell Stephen A. , Li Hong-Jyh , Peterson Jeff J.
Published: February 2007

High permittivity hafnium titanate thin films deposited by chemical vapor deposition have been studied systematically as gate insulators. The electrical and physical characteristics of the films were analyzed using a variety of techniques. It was observed that the films with higher Ti content are more resistant to crystallization when subjected to high temperature annealing. The capacitance-voltage curves for devices with hafnium titanate displayed relatively low (∼50mV) hysteresis. When the concentration of Hf is comparable to the concentration of Ti, both the interfacial layer equivalent oxide thickness and permittivity of HfxTi1−xO2 increase linearly with increasing Ti content. The sample with higher Hf content showed weaker temperature dependence of the current. In metal-oxide-semiconductor field-effect transistor devices with hafnium titanate films, normal transistor characteristics were observed. The devices exhibited electron mobility degradation. The thermodynamic stability of stoichiometric films in contact with Si was also studied.
Photocurrent bistability in a GaAs∕AlxGa1−xAs superlattice under resonant-coupling conditions of Wannier-Stark localization states
Authors: Hasegawa Takayuki , Nakayama Masaaki
Published: February 2007

We have investigated the effects of the resonant coupling between the Wannier-Stark localization states on photocurrent properties in a GaAs (6.8nm)∕Al0.1Ga0.9As (4.0nm) superlattice embedded in a p-i-n structure. The resonant-coupling profiles were confirmed with electroreflectance spectroscopy and analyzed by calculating the Wannier-Stark localization states as a function of electric field. It is found that the photocurrent-voltage characteristics exhibit peak structures, which results in negative differential resistance, owing to the changes of the optical-transition probabilities under the first- and second-nearest-neighbor resonant-coupling conditions between the first and second quantized electron states in the Wannier-Stark localization. Utilizing the negative differential resistance properties under the resonant-coupling conditions, we have demonstrated photocurrent-bistability operations of a self-electro-optic effect device.
Growth behavior of oxide nanostructures by electrical and thermal conductivities of substrate in atomic force microscope nano-oxidation
Authors: Lee Sunwoo , Pyo Eol , Kim Jeong Oh , Noh Jaegeun , Lee Haiwon , Ahn Jinho
Published: February 2007

We report the growth behavior of oxide nanostructures according to physical properties such as work function, electrical and thermal conductivities, and roughness for high resolution nanostructure fabrication. Among these factors, threshold voltages, in particular, which induced the formation of a water meniscus and driving voltage, which drive oxyanions for oxidation, decreased as the mobility of electrons increased by the increasing electrical conductivity. Oxide growth increased as the diffusion of OH radical increased by the increasing conductivity of thermal energy. The high electrical and thermal conductivities imply that the reaction of the OH radical and surface was more easily activated over a wide reaction region (in the parallel direction of substrate) by the conductivity of the generated thermal energy at a low driving voltage. On the basis of these conductivity effects, the Cr film, which is the most sensitive to electron transport and conductivities, had hill-shaped nanostructures and could be applied as a candidate for high-speed atomic force microscope lithography at the lowest driving voltage. In addition, Ta and Ti, which are less sensitive, can be used to fabricate nanostructures with a high aspect ratio (spike shape).
Conductors, semiconductors, and insulators irradiated with short-wavelength free-electron laser
Authors: Krzywinski J. , Sobierajski R , Jurek M. , Nietubyc R. , Pelka J. B. , Juha L. , Bittner M. , Létal V. , Vorlíček V. , Andrejczuk A. , Feldhaus J. , Keitel B. , Saldin E. L. , Schneidmiller E. A. , Treusch R. , Yurkov M. V.
Published: February 2007

The results of a study of irreversible changes induced at surfaces of metals, semiconductors, and insulators by extreme ultraviolet (λ<100nm) ultrashort pulses provided by TESLA Test Facility Free-Electron Laser, Phase 1 (TTF1 FEL) are reported and discussed. The laser was tuned at 86, 89, and 98nm during the experiments reported here. Energy spectra of ions ejected from the irradiated surfaces are also reported. Special attention is paid to the difference in the ablation behavior of (semi)conductors and insulators that we have observed. The difference is dramatic, while the absorption coefficients are similar for all materials at the TTF1 FEL wavelength.
Determination of stress-optical and thermal-optical coefficients of Nb2O5 thin film material
Authors: Chen Tei-Chen , Chu Ching-Jiung , Ho Chang-Hsien , Wu Chung-Chen , Lee Cheng-Chung
Published: February 2007

The stress-optical and thermal-optical coefficients of Nb2O5 in thin film narrow bandpass filters (NBPFs) are determined by using the experimental data of temperature-dependent shift of center wavelength. The five-cavity subnanometer NBPFs under consideration are made by the same Nb2O5 and SiO2 film materials but deposited on two different substrates (WMS-02 and F-7) through ion-assisted deposition process. The shift of center wavelength in the bandpass filters due to temperature rise is theoretically formulated and related to a variety of factors including the change of refractive index and film thickness due to temperature rise as well as thermal stress. By using the experimental data of center wavelength shift, the unknown stress-optical and thermal-optical coefficients of Nb2O5 thin film material are evaluated. These coefficients are equal to −95.1×10−12Pa−1 and 1.43×10−5°C−1, respectively, at 1550nm wavelength.
Polarization memory in a system of CdSe nanorods
Authors: Kravtsova Y. , Krull U. , Musikhin S. F. , Levina L. , Ruda H. E. , Shik A.
Published: February 2007

Polarization characteristics and spectra of photoluminescence in CdSe nanorods suspended in liquid were investigated. Experiments demonstrated the presence of the polarization memory related to the mismatch of nanorods and solvent dielectric constants, as well as to anisotropy of optical matrix elements. Luminescent spectra contained two peaks corresponding to interband transitions between the ground and excited size-quantized states and having essentially different values of polarization memory, presumably due to different anisotropy of corresponding matrix elements. The amplitude of polarization memory decreased with the increase in concentration of nanorods confirming the hypothesis of their mutual influence.
Nanostructure evolution in hydrogenated amorphous silicon during hydrogen effusion and crystallization
Authors: Young David L. , Stradins Paul , Xu Yueqin , Gedvilas Lynn M. , Iwaniczko Eugene , Yan Yanfa , Branz Howard M. , Wang. Qi , Williamson Don L.
Published: February 2007

The authors report a study by small-angle x-ray scattering (SAXS) and tilting SAXS of nanovoids in amorphous silicon films undergoing solid phase crystallization (SPC) by annealing. SPC causes nanovoids in hot-wire chemical vapor deposited (HWCVD) amorphous Si films to decrease in total void fraction but increase in individual void volume. During annealing, the voids also change shape from prolate spheroids to more spherical geometries. Preexisting, large H clusters in as-deposited HWCVD films may provide favorable sites for void expansion driven by H2 pressure, while energy minimization and strain relaxation drive geometric changes.
Chemical routes to Ge∕Si(100) structures for low temperature Si-based semiconductor applications
Authors: Wistey M. A. , Fang Y.-Y. , Tolle J. , Chizmeshya A. V. G. , Kouvetakis J.
Published: February 2007

The authors describe very low temperature (350–420°C) growth of atomically smooth Ge films (0.2–0.4nm roughness) directly on Si(100) via gas-source molecular beam epitaxy. A carefully tuned admixture of (GeH3)2CH2, possessing unique pseudosurfactant properties, and conventional Ge2H6 provides unprecedented control of film microstructure, morphology, and composition. Formation of edge dislocations at the interface ensures growth of virtually relaxed monocrystalline Ge films (∼40–1000nm thick) with a threading dislocation density less than 105cm−2 as determined by etch pit measurements. Secondary ion mass spectroscopy showed no measurable carbon incorporation indicating that C desorbs as CH4, consistent with calculated chemisorption energies.
Cryogenic pellet launcher adapted for controlling of tokamak plasma edge instabilities
Authors: Lang P. T. , Cierpka P. , Harhausen J. , Neuhauser J. , Wittmann C. ,   ASDEX Upgrade Team, Gál K. , Kálvin S. , Kocsis G. , Sárközi J. , Szepesi T. , Dorner C. , Kauke G.
Published: February 2007

One of the main challenges posed recently on pellet launcher systems in fusion-oriented plasma physics is the control of the plasma edge region. Strong energy bursts ejected from the plasma due to edge localized modes (ELMs) can form a severe threat for in-vessel components but can be mitigated by sufficiently frequent triggering of the underlying instabilities using hydrogen isotope pellet injection. However, pellet injection systems developed mainly for the task of ELM control, keeping the unwanted pellet fueling minimized, are still missing. Here, we report on a novel system developed under the premise of its suitability for control and mitigation of plasma edge instabilities. The system is based on the blower gun principle and is capable of combining high repetition rates up to 143 Hz with low pellet velocities. Thus, the flexibility of the accessible injection geometry can be maximized and the pellet size kept low. As a result the new system allows for an enhancement in the tokamak operation as well as for more sophisticated experiments investigating the underlying physics of the plasma edge instabilities. This article reports on the design of the new system, its main operational characteristics as determined in extensive test bed runs, and also its first test at the tokamak experiment ASDEX Upgrade.
Ground state structures and photoelectron spectroscopy of [Com(coronene)]− complexes
Authors: Kandalam Anil K. , Kiran Boggavarapu , Jena Puru , Li Xiang , Grubisic Andrej , Bowen Kit H.
Published: February 2007

A synergistic approach involving theory and experiment has been used to study the structure and properties of neutral and negatively charged cobalt-coronene [Com(coronene)] complexes. The calculations are based on density functional theory with generalized gradient approximation for exchange and correlation potential, while the experiments are carried out using photoelectron spectroscopy of mass selected anions. The authors show that the geometries of neutral and anionic Co(coronene) and Co2(coronene) are different from those of the corresponding iron-coronene complexes and that both the Co atom and the dimer prefer to occupy η2-bridge binding sites. However, the magnetic coupling between the Co atoms remains ferromagnetic as it is between iron atoms supported on a coronene molecule. The accuracy of the theoretical results is established by comparing the calculated vertical detachment energies, and adiabatic electron affinities with their experimental data.
Ground state energy of the low density Hubbard model: An upper bound
Authors: Giuliani Alessandro
Published: February 2007

We derive an upper bound on the ground state energy of the three-dimensional (3D) repulsive Hubbard model on the cubic lattice agreeing in the low density limit with the known asymptotic expression of the ground state energy of the dilute Fermi gas in the continuum. As a corollary, we prove an old conjecture on the low density behavior of the 3D Hubbard model, i.e., that the total spin of the ground state vanishes as the density goes to zero.
Spectral gap and decay of correlations in U(1)-symmetric lattice systems in dimensions D<2
Authors: Koma Tohru
Published: February 2007

We consider many-body systems with a global U(1) symmetry on a class of lattices with the (fractal) dimension D<2 and their zero temperature correlations whose observables behave as a vector under the U(1) rotation. For a wide class of the models, we prove that if there exists a spectral gap above the ground state, then the correlation functions have a stretched exponentially decaying upper bound. This is an extension of the McBryan-Spencer method at finite temperatures to zero temperature. The class includes quantum spin and electron models on the lattices, and our method also allows finite or infinite (quasi)degeneracy of the ground state. The resulting bounds rule out the possibility of the corresponding magnetic and electric long-range orders.
Demonstration of an amorphous carbon tunnel diode
Authors: Bhattacharyya Somnath , Silva S. Ravi P.
Published: February 2007

Negative differential conductance in metal/amorphous nitrogenated carbon (a-CNx)∕Si structures is demonstrated at room temperature. These metal-insulator-semiconductor tunnel diodes are fabricated by optimizing the tunnel barrier at the a-CNx∕Si junction through the control of the band gap and nitrogen doping level in carbon where this a-C layer acts as a semi-insulator. A small electron tunneling effective mass of about 0.06 times the free electron mass, a coherence length of ∼10nm in these thin a-CNx layers and a low interface trap density suggest fast device operation similar to classical tunnel diodes.
Dynamic characterization of nanoelectromechanical oscillators by atomic force microscopy
Authors: Ilic B. , Krylov S. , Bellan L. M. , Craighead H. G.
Published: February 2007

Dynamic detection of vibrational characteristics of nanoelectromechanical systems (NEMS) was investigated through direct coupling with a micromechanical probe. The nanomechanical structures were harmonically driven using piezoelectric transducers and the resulting out-of-plane excitations were monitored with a conventional atomic force microscope (AFM) probe. Intermittent contact imaging data show quantitative linear classical resonance behavior. Additionally, noncontact AFM interrogation revealed the initiation of interaction between the two oscillators, providing a qualitative description of the resonant response. The vibrational spectra measured through optical excitation and detection were in good agreement with the coupled NEMS-AFM system measurement results. The dynamic response of the coupled system was modeled through a combination of long range van der Waals and contact forces using the Derjaguin–Muller–Toporov model. These results collectively demonstrate that this is a viable method for detecting the dynamic behavior of nanoscale suspended mechanical structures.
High-resolution simulations and modeling of reshocked single-mode Richtmyer-Meshkov instability: Comparison to experimental data and to amplitude growth model predictions
Authors: Latini Marco , Schilling Oleg , Don Wai Sun
Published: February 2007

The reshocked single-mode Richtmyer-Meshkov instability is simulated in two spatial dimensions using the fifth- and ninth-order weighted essentially nonoscillatory shock-capturing method with uniform spatial resolution of 256 points per initial perturbation wavelength. The initial conditions and computational domain are modeled after the single-mode, Mach 1.21 air(acetone)/SF6 shock tube experiment of Collins and Jacobs [J. Fluid Mech. 464, 113 (2002)]. The simulation densities are shown to be in very good agreement with the corrected experimental planar laser-induced fluorescence images at selected times before reshock of the evolving interface. Analytical, semianalytical, and phenomenological linear and nonlinear, impulsive, perturbation, and potential flow models for single-mode Richtmyer-Meshkov unstable perturbation growth are summarized. The simulation amplitudes are shown to be in very good agreement with the experimental data and with the predictions of linear amplitude growth models for small times, and with those of nonlinear amplitude growth models at later times up to the time at which the driver-based expansion in the experiment (but not present in the simulations or models) expands the layer before reshock. The qualitative and quantitative differences between the fifth- and ninth-order simulation results are discussed. Using a local and global quantitative metric, the prediction of the Zhang and Sohn [Phys. Fluids 9, 1106 (1997)] nonlinear Padé model is shown to be in best overall agreement with the simulation amplitudes before reshock. The sensitivity of the amplitude growth model predictions to the initial growth rate from linear instability theory, the post-shock Atwood number and amplitude, and the velocity jump due to the passage of the shock through the interface is also investigated numerically.
Decay and return of internal solitary waves with rotation
Authors: Helfrich Karl R.
Published: February 2007

The effect of rotation on the propagation of internal solitary waves is examined. Wave evolution is followed using a new rotating extension of a fully nonlinear, weakly nonhydrostatic theory for waves in a two-layer system. When a solitary wave solution of the nonrotating equations is used as the initial condition, the wave initially decays by radiation of longer inertia-gravity waves. The radiated inertia-gravity wave always steepens, leading to the formation a secondary solitary-like wave. This decay and reemergence process then repeats. Eventually, a nearly localized wave packet emerges. It consists of a long-wave envelope and shorter, faster solitary-like waves that propagate through the envelope. The radiation from this mature state is very weak, leading to a robust, long-lived structure that may contain as much as 50% of the energy in the initial solitary wave. Interacting packets may either pass through one another, or merge to form a longer packet. The packets appear to be modulated, fully nonlinear versions of the steadily translating quasi-cnoidal waves.
Nature of defects for bismuth layered thin films grown on Pt electrodes
Authors: Simões A. Z. , Cavalcante L. S. , Longo E. , Varela J. A. , Riccardi C. S. , Mizaikoff B.
Published: February 2007

The authors investigated the influence of defects on the piezoelectric and dielectric properties of Bi4Ti3O12 (BIT), SrBi4Ti4O15 (SBTi) and CaBi4Ti4O15 (CBTi144) thin films by x-ray photoemission spectroscopy measurements. In the SBTi film, Sr which is a nonpolarizable ion restricting the movement of Ti4+ ions and thus leads to a low piezoresponse. Meanwhile, the oxygen environment is quite different in the BIT and CBTi144 films exhibiting excellent piezoelectric properties. The piezoelectric coefficient and the dielectric behavior were larger for a-b axis oriented than for c axis-oriented films due to the defects created during the films crystallization.
Control of lasing in fully chaotic open microcavities by tailoring the shape factor
Authors: Fang W. , Cao H. , Solomon G. S.
Published: February 2007

The authors demonstrate experimentally that lasing in a semiconductor microstadium can be optimized by controlling its shape. Under spatially uniform optical pumping, the first lasing mode in a GaAs microstadium with large major-to-minor-axis ratio usually corresponds to a high-quality scar mode consisting of several unstable periodic orbits. Interference of waves propagating along the constituent orbits may minimize light leakage at particular major-to-minor-axis ratio. By making stadium of the optimum shape, the authors are able to maximize the mode quality factor and align the mode frequency to the peak of the gain spectrum, thus minimizing the lasing threshold.
Shock-wave induced tension and spall in a zirconium-based bulk amorphous alloy
Authors: Turneaure Stefan J. , Dwivedi S. K. , Gupta Y. M.
Published: February 2007

Dynamic tensile response and fracture of a Zr-based bulk amorphous alloy (BAA) were examined by subjecting samples to uniaxial tensile strain in plate-impact experiments. Following elastic compressive loading to peak stresses ranging between 3.9 and 6.1 GPa, wave interactions produced tensile loading resulting in spallation in the BAA samples. Rear-surface velocity histories, obtained using laser interferometry, provided a real-time measure of the tensile response including spallation. The initial tensile loading was elastic (loading rates approximately 8×105 s−1) and the data were analyzed to obtain a nonlinear, tensile stress-strain relation. Tensile fracture or spall, observed in all experiments, was initiated at a tensile stress of 3.8±0.3 GPa; this initiation value was independent of the impact stress and is significantly higher than that observed for crystalline metals. A phenomenological tensile fracture model was incorporated into one-dimensional wave propagation simulations to gain insight into the BAA tensile response and damage. Good agreement was obtained between the numerical simulations and the experimental measurements. With increasing impact stress, the BAA samples exhibited a change from ductile to brittle tensile response.
Photovoltaic characteristics of amorphous silicon solar cells using boron doped tetrahedral amorphous carbon films as p-type window materials
Authors: Han Jiecai , Tan Manlin , Zhu Jiaqi , Meng Songhe , Wang Binsheng , Mu Shaojun , Cao Dawei
Published: February 2007

Boron doped tetrahedral amorphous carbon (ta-C:B) was prepared by filtered cathodic vacuum arc deposition. A band gap of 2.0eV and a conductivity of 1.42×10−7S∕cm were obtained at the doping ratio of 2.13at.%. A device structure was deduced from the conventional amorphous silicon (a-Si:H) solar cell using the ta-C:B window layer. Photovoltaic parameters of the cells were studied by varying the boron content in the ta-C:B films. A roughly 10% relative improvement of conversion efficiency was observed compared to the normal a-Si:H solar cell. The improved cell performance results from the enhancement of short wavelength response.
Charge-state-resolved ion energy distribution functions of cathodic vacuum arcs: A study involving the plasma potential and biased plasmas
Authors: Anders André , Oks Efim
Published: February 2007

Charge-state-resolved ion energy distribution functions were measured for pulsed cathodic arcs taking the sheath into account that formed between the plasma and the entrance of a combined energy and mass spectrometer. An electron emitting probe was employed to independently determine the plasma potential. All results were obtained by averaging over several individual measurements because the instantaneous energy distributions and the plasma potential show large amplitude fluctuations due to the explosive nature of the arc plasma generation. It was found that the ion energy distribution functions in the plasma were independent of the ion charge state. This is in contrast to findings with continuously operating, direct-current arcs that employ a magnetic field at the cathode to steer the cathode spot motion. The different findings indicate the important role of the magnetic steering field for the plasma properties of direct-current arcs. The results are further supported by experiments with “biased plasmas” obtained by shifting the potential of the anode. Finally, it was shown that the ion energy distributions were broader and shifted to higher energy at the beginning of each arc pulse. The characteristic time for relaxation to steady state distributions is about 100μs.
Effect of ceramic dispersion on thermoelectric properties of nano-ZrO2∕CoSb3 composites
Authors: He Zeming , Stiewe Christian , Platzek Dieter , Karpinski Gabriele , Müller Eckhard , Li Shanghua , Toprak Muhammet , Muhammed Mamoun
Published: February 2007

In the present work, nano-ZrO2∕CoSb3 composites were fabricated by milling ZrO2 and CoSb3 powders and hot pressing at different sintering temperatures. For the prepared compacts, the phase purity, microstructure, and temperature-dependent thermoelectric properties were characterized. The effect of nano-ZrO2 dispersion on composite electrical conductivity and thermal conductivity is strictly clarified by comparing the transport properties of the nondispersed and dispersed CoSb3 at identical porosity, so that the effect of porosity on thermoelectric parameters could be eliminated. The effect of the insulating inclusion itself on transport properties is also considered and eliminated using effective media theories. It is clearly verified that charge carrier scattering and phonon scattering occur simultaneously to lower the electrical conductivity and the thermal conductivity of CoSb3 due to the introduction of nano-ZrO2 inclusions. The investigated composites show higher electrical conductivity due to existence of metallic Sb and lower thermal conductivity because of nanodispersion. At the ranges of high measuring temperature (673–723 K) and low porosity (6%–9%), the ratio of electrical conductivity to thermal conductivity of the dispersed CoSb3 is higher than that of nondispersed CoSb3, and the dimensionless figure of merit (ZT) of the composite could probably be improved at these ranges with the enhanced ratio of electrical conductivity to thermal conductivity and Seebeck coefficient, which is assumed to be increased by a potential barrier scattering.
Detailed study of the plasma-activated catalytic generation of ammonia in N2-H2 plasmas
Authors: van Helden J. H. , Wagemans W. , Yagci G. , Zijlmans R. A. B. , Schram D. C. , Engeln R. , Lombardi G. , Stancu G. D. , Röpcke J.
Published: February 2007

We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N2 and H2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N2 and H2, are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals at the surface and incoming N and H containing radicals. Furthermore, our results indicate that the ammonia production is independent of wall material. The high fluxes of N and H radicals in our experiments result in a passivated surface, and the actual chemistry, leading to the formation of ammonia, takes place in an additional layer on top of this passivated surface.
Structural and electro-optic properties of Ba0.7Sr0.3TiO3 thin films grown on various substrates using pulsed laser deposition
Authors: Wang D. Y. , Wang J. , Chan H. L. W. , Choy C. L.
Published: February 2007

Epitaxial Ba0.7Sr0.3TiO3 (BST) thin films were deposited on various single crystal substrates, including LaAlO3 (LAO) (001), (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) (001), and SrTiO3 (STO) (001), using pulsed laser deposition in order to study their structural and electro-optic properties. All the films exhibit a good crystalline quality and a pure perovskite phase with a distorted lattice structure. The in-plane temperature dependence of relative permittivity of the films was measured on interdigital electrodes. The films grown on LAO and LSAT exhibited an obvious room-temperature ferroelectric state, while the film grown on STO showed a broad phase transition peak near room temperature. Correspondingly, large linear electro-optic effects were observed in the BST films grown on LAO and LSAT in a transverse geometry at a wavelength of 632.8nm using a modified Sénarmont method. The linear electro-optic coefficient rcof the BST films grown on LAO and LSAT was found to be 82.7×10−12 and 125.0×10−12m∕V, respectively. Nevertheless, a predominantly quadratic and slightly asymmetric electro-optic behavior is observed for the film grown on STO with the quadratic electro-optic coefficient Rc of 12.9×10−18m2∕V2.
Effect of Al and Cu doping on the crystallization properties of the phase change materials SbTe and GeSb
Authors: Raoux Simone , Salinga Martin , Jordan-Sweet Jean L. , Kellock Andrew
Published: February 2007

For the application of phase change materials in solid state memory devices it is very desirable to modify the crystallization properties such as the crystallization temperature Tx by doping in a predictable fashion. We have applied a model for the calculation of the glass transition temperature Tg of phase change materials as a function of material composition to predict the effect of Cu and Al doping for the phase change materials SbTe and GeSb. The model predicts an increase in Tg for Al and Cu doping of SbTe and Al doping of GeSb (for all Sb:Te and Ge:Sb ratios) while it predicts a decrease of Tg for Cu doping of GeSb. We confirmed experimentally that Al and Cu doping of Sb:Te=72:28 and Al doping of Ge:Sb=14:86 increase their Tx while Cu doping decreases Tx of Ge:Sb=14:86. The effect of a given dopant on Tg predicted by the model is shown to be a good indicator for the effect of this dopant on Tx.
Planar stacking effect on elastic stability of hexagonal boron nitride
Authors: Qi Yue , Hector Louis G.
Published: February 2007

The elastic stability of five hexagonal boron nitride (h-BN) stacking sequences is investigated with density functional theory. All components of the elasticity tensor are computed and used to evaluate the Born stability criteria. Phonon spectra are computed for one elastically stable and one elastically unstable h-BN structure and the normal modes associated with instability are identified. Charge density difference contour plots provide a qualitative connection between elastic stability and charge transfer.
Double-contact spin-torque nano-oscillator with optimized spin-wave coupling: Micromagnetic modeling
Authors: Choi Sangkook , Kim Sang-Koog , Demidov Vladislav E. , Demokritov Sergej O.
Published: February 2007

The author numerically conducted micromagnetic modeling to optimize the shapes of patterned magnetic thin-film elements applicable to double-contact gigahertz-frequency-range nano-oscillators. Spin waves emitted from one of two foci in the optimized shapes of nanoscale elements can be collected in phase at the other focal point via coherent and constructive interference of all the spin waves propagating directly between the two foci and reflected at the different boundary positions of the elements. This work provides promising ways to design the shapes of patterned magnetic thin films with in-plane and out-of-plane magnetizations applicable to phase-locked gigahertz-frequency-range nanooscillators with optimized spin-wave coupling.
High density hexagonal and rhombic shaped nanostructures in a fcc aluminum alloy induced by severe plastic deformation at room temperature
Authors: Liu M. P. , Roven H. J.
Published: February 2007

High density hexagonal and rhombic shaped nanostructures with an average size of 4nm have been observed in a commercial Al–Mg–Si alloy processed by equal channel angular pressing at room temperature. High-resolution transmission electron microscopy investigation of the nanostructures suggests that these planar defects are preferentially present along {111} and (001) planes. This surprising observation of the nanostructures inside ultrafine grain is probably caused by reactions between partial dislocations and stacking faults on different 1∕2⟨110⟩ {111} and 1∕2⟨110⟩ {001} slip systems in face-centered-cubic metals.
Effect of ZnSe partial capping on the ripening dynamics of CdSe quantum dots
Authors: Lai Y. J. , Yang C. S. , Chen W. K. , Lee M. C. , Chang W. H. , Chou W. C. , Wang J. S. , Huang W. J. , Jeng Erik S.
Published: February 2007

The ripening dynamics of CdSe quantum dots (QDs) partially capped with ZnSe layer are investigated. Atomic force microscopy (AFM) images show that the ripening of QDs is dramatically accelerated by depositing a ZnSe partial capping layer. The driving force of ripening enhancement is attributed to the increasing strain energy with capping thickness. For a ZnSe partial capping layer of below 3 ML, photoluminescence exhibits a clear redshift with increasing ZnSe monolayers. It is attributed to the size of the CdSe QD increases with ZnSe partial capping, in a manner that is consistent with the results of the AFM study.
Experimental demonstration of directional acoustic radiation based on two-dimensional phononic crystal band edge states
Authors: Ke Manzhu , Liu Zhengyou , Pang Pei , Qiu Chunyin , Zhao Degang , Peng Shasha , Shi Jing , Wen Weijia
Published: February 2007

The authors have experimentally studied the radiation of a point acoustic source placed inside a two-dimensional phononic crystal of square lattice. They show that a highly directional radiation with a half-power angular width of 6° can be achieved when operating at the band edge frequency for the phononic crystal. Such combination of a point source and a phononic crystal may serve as highly directional acoustic source in applications.
Nanoscale switch elements from self-assembled monolayers on silver
Authors: Beebe Jeremy M. , Kushmerick James G.
Published: February 2007

Au/molecule/Ag junctions are shown to behave as voltage-controlled two-state switches. In the open state, the current-voltage behavior is consistent with a metal-molecule-metal tunnel junction. At a negative bias threshold, silver filaments bridge the gap between the two electrodes, resulting in direct metal-metal contact and an increase in current of several orders of magnitude. Under positive bias, the filaments dissolve, returning the switch to an open state. Switching rates of up to ≈10kHz have been observed. Because the only required components are silver and a self-assembled monolayer, this switch element can be incorporated into a wide array of device architectures.
Ordered arrays of quantum wires through hole patterning: ab initio and empirical electronic structure calculations
Authors: Rurali R. , Suñé J. , Cartoixà X.
Published: February 2007

We propose an approach to the fabrication of one-dimensional nanostructures, based on the design of a pattern of channels onto a semiconductor surface. The feasibility of this approach is demonstrated by means of ab initio and empirical electronic structure calculations. When the channel diameter is sufficiently larger than the interstitial space, the resulting pillars constitute an ordered array of electronically independent, though mechanically connected nanowires. In the opposite regime a tunable metamaterial results. The proposed method provides a path to the realization of uniform quantum wires—both in size and doping characteristics—while easing electrical contacting.
Spectroscopic observation of chemical change during molecular electronics experiments
Authors: Robinson David B. , Funamura Joshua R. , Talin A. Alec , Anderson Richard J.
Published: February 2007

A molecular electronics test platform is presented that combines spectroscopic and electrical characterizations of test molecules. Two-dimensional arrays of 12nm gold particles are formed between the gaps of addressable interdigitated electrodes on an oxidized silicon substrate. Molecules of interest are introduced onto the self-assembled Au nanoparticles by surface exchange in an ethanol solution. Conductivity changes of four to six orders of magnitude are observed. Molecular exchange is verified by the surface-enhanced Raman spectroscopy on the same test specimen. This technique also reveals real-time, irreversible voltage-induced chemical change during testing.
Selective trace gas detection of complex molecules with a continuous wave optical parametric oscillator using a planar jet expansion
Authors: Ngai Anthony K. Y. , Persijn Stefan T. , Harren Frans J. M. , Verbraak Harald , Linnartz Harold
Published: February 2007

The authors present a trace gas detection method for complex molecules using continuous cavity ring-down spectroscopy in combination with a continuous wave optical parametric oscillator (tunability wavelength: 2.8–4.8μm; power: 1W) sampling a supersonic planar jet expansion (nozzle dimension: 3cm×80μm). The improved molecular selectivity allows simultaneous detection of larger numbers of complex molecules. With a total optical path length of 180m in the planar jet, a detection limit for methanol in an air expansion was determined at 70ppbv, corresponding to a minimal detectable absorption of 2.2×10−8cm−1 (over 90s).
Measuring the hole chemical potential in ferromagnetic Ga1−xMnxAs∕GaAs heterostructures by photoexcited resonant tunneling
Authors: Thomas O. , Makarovsky O. , Patanè A. , Eaves L. , Campion R. P. , Edmonds K. W. , Foxon C. T. , Gallagher B. L.
Published: February 2007

The authors investigate the optical and electrical properties of a p-i-n GaAs∕AlAs resonant tunneling diode in which the p-type layer is the ferromagnetic alloy semiconductor Ga1−xMnxAs (x=3%). The high density of Mn acceptors affects significantly the electrostatic potential profile of the heterostructure and inhibits hole tunneling from Ga1−xMnxAs.The authors use photoconductivity to probe this potential and measure the hole chemical potential in Ga1−xMnxAs relative to the band edges of the adjacent undoped GaAs layers.
Observation of whispering gallery modes in nanocrystalline diamond microdisks
Authors: Wang C. F. , Choi Y-S. , Lee J. C. , Hu E. L. , Yang J. , Butler J. E.
Published: February 2007

Nanocrystalline diamond microdisks have been fabricated and characterized. The process conditions were chosen to ensure smooth and vertical sidewalls. Focused ion beam milling was used to create ultrasmooth sidewalls. Whispering gallery modes were observed near the nitrogen-vacancy center emission wavelength (637nm) by photoluminescence and near ∼1550nm by evanescent fiber coupling. The cavity quality factors (Q) are about 100 in both experiments. The Q’s for these disks were calculated to be as high as 105 by three-dimensional finite-difference time-domain simulations. The authors believe the Q’s to be limited by absorption and scattering within the nanocrystalline cavity material.
Photoreflectance and reflectance investigation of deuterium-irradiated GaAsN
Authors: Geddo M. , Ciabattoni T. , Guizzetti G. , Galli M. , Patrini M. , Polimeni A. , Trotta R. , Capizzi M. , Bais G. , Piccin M. , Rubini S. , Martelli F. , Franciosi A.
Published: February 2007

The effect of deuterium irradiation on the optical and strain properties of GaAsN∕GaAs heterostructures was investigated by photoreflectance and reflectance techniques. The strain occurring in as-grown and deuterated GaAsN layers is monitored and measured by means of photoreflectance spectroscopy, highlighting the strain inversion after irradiation. By combining static and modulated reflectance results, evidence is given that the deuterium-induced recovery of the GaAs band gap as well as the strain inversion in GaAsN layers are accompanied by a 0.4%–0.8% reduction of the refractive index in the 1.31 and 1.55μm spectral windows of interest for fiber optic communications. These results anticipate a single step process to an in-plane confinement of carriers and photons.
Imaging and density mapping of tetrahydrofuran clathrate hydrates by phase-contrast x-ray computed tomography
Authors: Takeya Satoshi , Honda Kazumasa , Kawamura Taro , Yamamoto Yoshitaka , Yoneyama Akio , Hirai Yasuharu , Hyodo Kazuyuki , Takeda Tohoru
Published: February 2007

In situ observations of tetrahydrofuran (THF) hydrates in THF solution using a phase-contrast x-ray imaging system at 273K are reported. Density analysis measurements using phase-contrast x-ray imaging suggest that the inhomogeneous growth of THF hydrate crystals occurs due to differences in the cage occupancy of guest molecules, caused by the diffusion of solute. Phase-contrast x-ray imaging developed in this study can also be applied to the time-resolved imaging and density analysis of the growth and dissociation processes for a variety of hydrates besides THF hydrates.
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noise and percolation in carbon nanotube random networks
Authors: Soliveres S. , Gyani J. , Delseny C. , Hoffmann A. , Pascal F.
Published: February 2007

The 1∕f noise in carbon nanotube random network films has been experimentally and theoretically investigated. The authors have established that the percolation process is the primary physical mechanism influencing the noise level in such films. Using percolation theory, the authors give an expression for the 1∕f noise scaling that describes how noise is affected by design variables of nanotube macrostructures. As expected for percolation networks, the noise amplitude strongly depends on the film homogeneity. This shows that noise analysis could be used as a tool to evaluate the quality of films.
Electrical spin injection into p-doped quantum dots through a tunnel barrier
Authors: Lombez L. , Renucci P. , Braun P. F. , Carrère H. , Marie X. , Amand T. , Urbaszek B. , Gauffier J. L. , Gallo P. , Camps T. , Arnoult A. , Fontaine C. , Deranlot C. , Mattana R. , Jaffrès H. , George J.-M. , Binh P. H.
Published: February 2007

The authors have demonstrated by electroluminescence the injection of spin polarized electrons through Co∕Al2O3∕GaAs tunnel barrier into p-doped InAs∕GaAs quantum dots embedded in a p-i-n GaAs light emitting diode. The spin relaxation processes in the p-doped quantum dots are characterized independently by optical measurements (time and polarization-resolved photoluminescence). The measured electroluminescence circular polarization is about 15% at low temperature in a 2T magnetic field, proving an efficient electrical spin injection yield in the quantum dots. Moreover, this electroluminescence circular polarization is stable up to 70K.
Phase-field modeling of microvoid evolution under elastic-plastic deformation
Authors: Hu S. Y. , Baskes M. I. , Stan M.
Published: February 2007

Plastic deformation reduces the local stress concentration and changes the interface coherency in a coherent microstructure. Therefore, it may affect the morphology and evolution kinetics of a microstructure. In this work, a very efficient method has been proposed to obtain the elastic-plastic solution in an elastically inhomogeneous solid. A phase-field model integrating this method has been developed to study the microvoid evolution with vacancy diffusion under elastic-plastic deformation. It is able to demonstrate the formation of slip bands around voids and to predict the effect of plastic deformations and the elastic interaction between vacancy diffusion and deformation on the void evolution and stress-strain curves.
Surface phase separation in nanosized charge-ordered manganites
Authors: Dong S. , Gao F. , Wang Z. Q. , Liu J.-M. , Ren Z. F.
Published: February 2007

Recent experiments showed that the robust charge ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted.
Enhanced light outcoupling in a thin film by texturing meshed surfaces
Authors: Cheng Yu-Hung , Wu Jia-Lin , Cheng Chien-Hong , Syao Kao-Chih , Lee Ming-Chang M.
Published: February 2007

The authors demonstrate a method of texturing a meshed surface on a poly(dimethyl siloxane) (PDMS) film for improving light extraction. This meshed surface is fabricated through a casting process by using a self-organized porous film as a template. Experimental results show that the light outcoupling efficiency increases on the meshed surface of a freestanding PDMS film with large incident angles. The external quantum efficiency of an organic light-emitting diode with the textured PDMS film was also demonstrated to have an enhancement of 46%.