bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleation and propagation of plastic collapse bands in aluminum honeycomb
Authors: Mohr Dirk , Doyoyo Mulalo
Published: August 2003

Uniaxial compression experiments on aluminum honeycomb are performed to investigate localization of deformation in cellular materials. Physical experiments, combined with numerical simulation of the honeycomb microstructure, feature the indepth analysis of the nucleation and propagation of plastic collapse bands. The onset of inelasticity is determined by the von Kármán collapse load for the honeycomb microstructure. The plastic collapse mechanism yields localization of deformation in the form of collapse bands. At the same time, microstructural imperfections are generated in the vicinity of those bands. As a result, three microstructural configurations characterize the honeycomb specimen: Uncrushed material with deformation-induced imperfections, crushed material containing folded cell walls, and an active interface between the uncrushed and crushed regions. Globally, the active interface emerges as a flat propagating crushing front that travels down the specimen. It appears that the behavior of the interface governs the macroscopic response of the honeycomb. Consequently, a one-dimensional interface-based constitutive law is developed along with an additional field equation that monitors the imperfection intensity. The results provide a basis for the mechanical modeling of materials that evolve statistically inhomogeneous microstructures during deformation.
Modeling of temperature field and solidified surface profile during gas–metal arc fillet welding
Authors: Kim C.-H. , Zhang W. , DebRoy T.
Published: August 2003

The temperature profiles, weld pool shape and size, and the nature of the solidified weld pool reinforcement surface during gas–metal arc (GMA) welding of fillet joints were studied using a three-dimensional numerical heat transfer model. The model solves the energy conservation equation using a boundary fitted coordinate system. The weld pool surface profile was calculated by minimizing the total surface energy. Apart from the direct transport of heat from the welding arc, additional heat from the metal droplets was modeled considering a volumetric heat source. The calculated shape and size of the fusion zone, finger penetration characteristic of the GMA welds, and the solidified free surface profile were in fair agreement with the experimental results for various welding conditions. In particular, the computed values of important geometric parameters of fillet welds, i.e., the leg length, the penetration depth, and the actual throat, agreed well with those measured experimentally for various heat inputs. The weld thermal cycles and the cooling rates were also in good agreement with independent experimental data.
The structure of silicon doped intermediate size carbon clusters
Authors: González Noya Eva , Menon Madhu
Published: August 2003

The lowest energy configurations of silicon doped carbon clusters of intermediate size (CnSim, n+m=11,12, m=1,2,3) are investigated using generalized tight binding molecular dynamics scheme and ab initio calculations. Our results favor low dimensional structures over three-dimensional arrangements for these clusters. This trend is in agreement with photolysis experiments that suggest linear chains to be more stable isomers.
Dielectric relaxor properties of K0.5Bi0.5TiO3 ferroelectrics prepared by sol–gel method
Authors: Li Z. F. , Wang C. L. , Zhong W. L. , Li J. C. , Zhao M. L.
Published: August 2003

Lead-free potassium–bismuth titanate, K0.5Bi0.5TiO3 (KBT), ferroelectric ceramics were fabricated from the natural sintering of powders prepared by the thermal decomposition of adequate precursor solutions. Their crystal structure was determined by x-ray diffraction, and the temperature dependence of dielectric constants were measured. The results show that KBT may be a kind of order–disorder relaxor ferroelectric with a first-order phase transition below the temperature of dielectric constant maximum.
Pyroelectric properties of ferroelectric ceramic/ferroelectric polymer 0–3 composites
Authors: Yang Fengxia , Zhang Duanming , Yu Boming , Zheng Keyu , Li Zhihua
Published: August 2003

A general analytic expression is derived for pyroelectric coefficient p of a 0–3 composite composed of ferroelectric ceramic particles dispersed in a ferroelectric polymer by employing the Lapalace equation in the static case where three factors are considered including the interaction between ceramic particles, the contributions of the polymer, and the poling ratios of the two constituents. Good agreement is obtained between theoretical and experimental results when the volume fraction v2 of the ceramic particles is below 0.5. The limitation for the particle volume fraction v2>0.5 is also discussed according to a simple model.
Factors influencing the capacitance–voltage characteristics measured by the scanning capacitance microscope
Authors: Buh G. H. , Kopanski J. J. , Marchiando J. F. , Birdwell A. G. , Kuk Y.
Published: August 2003

A scanning capacitance microscope (SCM) can measure the local capacitance–voltage (C–V) characteristics of a metal-oxide-semiconductor structure formed by the SCM probe tip and a doped semiconductor sample. A common realization of the SCM depends on a parallel atomic force microscope, which includes a laser focused on the end of the cantilever to monitor the position of the probe tip. In this configuration, it is found that the stray light from the laser can dramatically affect the measured C–V curve. The difference between the SCM C–V curves measured in this high stray light condition and those measured in the true dark condition are shown and discussed. Also discussed is the distortion of the measured C–V curves caused by the SCM method of measuring the differential capacitance using a capacitance-modulating ac voltage and a lock-in amplifier. After reducing and accounting for these effects, the SCM C–V curves show markedly different behavior from that of conventional one-dimensional C–V curves. The measured C–V curves are stretched out in voltage and have a larger |dC/dV| signal in the depletion and the inversion regions, as compared to the conventional one-dimensional C–V curve. The measured C–V curves are compared with the results of three-dimensional calculations for different values of the probe-tip size.
Time-resolved cathodoluminescence and photocurrent study of the yellow band in Si-doped GaN
Authors: Dı́az-Guerra C. , Piqueras J. , Castaldini A. , Cavallini A. , Polenta L.
Published: August 2003

Time-resolved cathodoluminescence (TRCL) and photocurrent (PC) spectroscopies have been applied to the study of the yellow band of Si-doped GaN. Measurements carried out combining both techniques unambiguously reveal the complex nature of this broad emission and confirm that different deep defect levels are involved in the observed luminescence. Five emission bands centered at 1.89, 2.03, 2.16, 2.29, and 2.38 eV were found by steady state and time-resolved CL investigations, while PC spectra showed four transitions at about 2.01, 2.14, 2.28, and 2.43 eV. The behavior of the deep-level emissions intensity as a function of the excitation pulse width as well as their decay times were investigated by TRCL. A decay time of 245 μs was measured for the 2.29 eV emission band, while longer decay times of 315 and 340 μs were found, respectively, for the 2.16 and 2.38 eV bands, in agreement with TRCL spectra. The appearance of the 2.03, 2.16, 2.29 eV and 2.38–2.43 eV peaks both in PC and CL spectra suggests that these bands are related to deep acceptor to band transitions, as supported by the single exponential character of the corresponding decay transients.
Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics
Authors: Méndez-Ramos J. , Lavı́n V. , Martı́n I. R. , Rodrı́guez-Mendoza U. R. , Rodrı́guez V. D. , Lozano-Gorrı́n A. D. , Núñez P.
Published: August 2003

Optical properties of Eu3+ ions in oxyfluoride glasses and glass ceramics doped with two different concentrations, 0.1 and 1 mol %, have been analyzed and compared with previous results for higher concentrated samples, 2.5 mol %. The Eu3+ ions in the 0.1 mol % doped glass ceramics are diluted into like crystalline environments with higher symmetry and lower coupled phonon energy than in the precursor glasses; meanwhile in the 1 mol % doped glass ceramics the presence of EuF3 clusters has been observed in addition to diluted ions. Fluorescence line narrowing measurements indicate the presence of two main fluoride site distributions for the diluted Eu3+ ions in both glass ceramics.
Comment on “Electromagnetic resonances in individual and coupled split-ring resonators” [J. Appl. Phys. 92, 2929 (2002)]
Authors: Marqués R. , Medina F. , Rafii-El-Idrissi R.
Published: August 2003

It is pointed out that the recently reported resonances appearing in split-ring resonators are explained by the recently reported theory about the magnetoelectric coupling (or bianisotropy) in the tensor polarizability of these particles.
Polyelectrolyte chain dimensions and concentration fluctuations near phase boundaries
Authors: Prabhu V. M. , Muthukumar M. , Wignall G. D. , Melnichenko Y. B.
Published: August 2003

We have measured the temperature (T) dependence of the correlation length (ξ) for concentration fluctuations in aqueous solutions of sodium–poly(styrene sulfonate) with a fixed level of added barium chloride salt. Apparent critical behavior is observed upon lowering the temperature to precipitation phase boundaries that complements our earlier work on salt-dependent behavior. We interpret experimental deviations from ξ−2 versus T−1 as crossover from the mean field to the Ising universality class. We also measured the radius of gyration (Rg) of labeled chains and ξ for semidilute polyelectrolyte solutions at low ionic strengths. We recovered the familiar result of ξ scaling with polymer concentration (Cp) and degree of polymerization (N), such that ξ=(73±9) N0Cp−0.48±0.03 [Å], and using SANS high concentration labeling Rg=(400±28)Cp−0.24±0.01 [Å] (for N=577) and Rg=(2.8±2.1)N0.6±0.1 [Å] (for Cp=206 gL−1), respectively. The indices recovered are in agreement with theoretical predictions for low ionic strength semidilute solutions. Such experiments offer insight into relatively unexplored phase behavior in charged macromolecular solutions.
Complex dynamics in a nonlinear chemical system switching between two stable stationary states
Authors: Wang Jichang
Published: August 2003

In this paper we investigated the dynamic behavior of a nonself-oscillatory chemical system switched periodically between two nearby stable stationary states in a concentration phase space. Despite the absence of self-oscillating dynamics, a variety of nonlinear phenomena including periodic oscillations of large amplitudes and chaos has been uncovered when the switching frequency is varied systematically. Our analysis suggests that the onset of large amplitude oscillations in the studied system is due to an unusual response of the system to an inhibitory perturbation. In addition, this study further demonstrates that the waveform of a square-wave switching can be equally important as other control parameters in determining the dynamics of a forced system.
The polymerization of actin: Thermodynamics near the polymerization line
Authors: Niranjan Priya S. , Yim Peter B. , Forbes Jeffrey G. , Greer Sandra C. , Dudowicz Jacek , Freed Karl F. , Douglas Jack F.
Published: August 2003

Studies of the dependence of actin polymerization on thermodynamic parameters are important for understanding processes in living systems, where actin polymerization and depolymerization are crucial to cell structure and movement. We report measurements of the extent of polymerization, Φ, of rabbit muscle actin as a function of temperature [T=(0–35) °C], initial G-actin concentration [[G0]=(1–3) mg/ml], and initiating salt concentration [[KCl]=(5–15) mmol/l with bound Ca2+], in H2O and D2O buffers and in the presence of adenosine triphosphate (ATP). A preliminary account of the data and analysis for H2O buffers has appeared previously [P. S. Niranjan, J. G. Forbes, S. C. Greer, J. Dudowicz, K. F. Freed, and J. F. Douglas, J. Chem. Phys. 114, 10573 (2001)]. We describe the details of the studies for H2O buffers, together with new data and analysis for D2O buffers. The measurements show a maximum in Φ(T) for H2O buffers and D2O buffers. For H2O buffers, Tp decreases as either [G0] or [KCl] increases. For D2O buffers, Tp decreases as [KCl] increases, but Tp is not monotonic in [G0]. The measurements are interpreted in terms of a Flory–Huggins-type lattice model that includes the essential steps: monomer activation, dimerization of activated species, and propagation of trimers to higher order polymers. The competition between monomer activation and chain propagation leads to the observed nonmonotonic variation of Φ(T). The actin polymerization in D2O buffer differs considerably from that in the H2O buffer and underscores the significant deuterium effect on hydrophobic interactions and hydrogen bonding in the polymerization process.
Transverse nuclear spin relaxation induced by director fluctuations in a nematic liquid crystal polymer. Evaluation of the anisotropic elastic constants
Authors: Frezzato Diego , Moro Giorgio J. , Tittelbach Markus , Kothe Gerd
Published: August 2003

Transverse deuteron spin relaxation measurements, employing Carr–Purcell–Meiboom–Gill (CP) sequences, have been used to determine the anisotropic elastic constants of a thermotropic main chain/side chain liquid crystal polymer (LCP) in the nematic phase. The observed relaxation rates, R2CP(ω), exhibit a square root dependence on the inverse pulse frequency, ω, i.e., R2CP(ω)∝ω−1/2, over more than one order of magnitude in ω in the kHz regime. This is precisely the dispersion law expected for nematic director fluctuations. Analysis of the experimental dispersion profile is performed using a slow-motional model for director fluctuations, in which five independent Leslie viscosities and three Frank elastic constants are considered. Using additional information from a step-rotation rheo-nuclear magnetic resonance (NMR) experiment, the analysis provides absolute values for the splay, bend, and twist elastic constant of the studied LCP. It is the first time that such data are available for this class of polymers. The splay elastic constant of K1∼8×10−8 N exceeds that of monomers by four orders of magnitude, in substantial agreement with theoretical predictions. The values for the bend and twist elastic constant of K2∼K3≃5×10−10 N are by a factor of 100 larger than those of low molecular weight liquid crystals. The results show that transverse NMR relaxation measurements involving CP sequences represent a powerful tool for the study of the anisotropic viscoelastic properties of LCPs.
Structural and electronic properties of amorphous and polycrystalline In2Se3 films
Authors: Chaiken A. , Nauka K. , Gibson G. A. , Lee Heon , Yang C. C. , Wu J. , Ager J. W. , Yu K. M. , Walukiewicz W.
Published: August 2003

Structural and electronic properties of amorphous and single-phase polycrystalline films of γ- and κ-In2Se3 have been measured. The effect of deposition conditions on the film phase was studied extensively. The stable γ phase nucleates homogeneously in the film bulk and has a high resistivity, while the metastable κ phase nucleates at the film surface and has a moderate resistivity. The microstructures of polycrystalline hot-deposited and postannealed, cold-deposited γ films are quite different but their electronic properties are similar. The increase in the resistivity of amorphous In2Se3 films upon annealing is interpreted in terms of the replacement of In–In bonds with In–Se bonds during crystallization. Great care must be taken in the preparation of In2Se3 films for electrical measurements as the presence of excess chalcogen or surface oxidation may greatly affect the film properties.
Stability of polythiophene-based transistors and circuits
Authors: Ficker J. , Ullmann A. , Fix W. , Rost H. , Clemens W.
Published: August 2003

Detailed investigations on shelf life and operation lifetime of polymer field-effect transistors (PFETs) and circuits are reported. All examined devices consisted entirely of polymer materials except the electrodes. Regioregular poly(3-alkylthiophene) was used as a semiconductor. Unencapsulated devices were produced, stored, and measured under ambient conditions. The performance of PFETs was maintained for more than 12 months after production. Even under extreme conditions of 85 °C and 85% relative humidity, a stable shelf life of more than 1400 h was measured. Transistors exceeded a continuous operation time of 1000 h. Operation lifetimes showed that the degradation did not follow the Arrhenius lifetime-temperature relationship. Similar results were found for ring oscillators.
Thermal strain in indium phosphide on silicon obtained by epitaxial lateral overgrowth
Authors: Sun Y. T. , Baskar K. , Lourdudoss S.
Published: August 2003

High-resolution x-ray diffraction reciprocal lattice mapping and low-temperature photoluminescence (PL) were used to study the thermal strain in InP layers grown on Si (001) substrate by hydride vapor-phase epitaxial lateral overgrowth (ELO) technique. Good agreement is found between the PL and x-ray measurements. We show that strain in the grown ELO InP/Si layers is affected by the aspect ratio (width to height ratio) of the ELO InP layer. Almost strain-free InP layer with high crystallographic quality is obtained on Si substrate, which is similar to that of a homoepitaxial InP layer.
Electric transport mechanism in intrinsic and p-doped microcrystalline silicon thin films
Authors: Concari S. B. , Buitrago R. H. , Gutiérrez M. T. , Gandı́a J. J.
Published: August 2003

Intrinsic microcrystalline silicon thin films, as well as p-type doped with boron prepared by very high-frequency plasma enhanced chemical vapor deposition, have been studied. Raman spectroscopy, atomic-force microscopy, lateral dark conductivity, and ultraviolet-visible transmittance were used to characterize each sample. Conductivity of all samples, as a function of the inverse of temperature, showed a thermally activated behavior of electric carriers with temperature-independent activation energy in all of the temperature ranges studied. Following the method proposed by Godet [C. Godet, J. Non-Cryst. Solids 299, 333 (2002)], assuming an exponential density of states for this group of different films, variable range hopping between defects near the Fermi level was established as a predominant electronic transport mechanism. Using classical equations of percolation theory, as well as the correlation found by Godet, the density of states near the Fermi level was calculated and found to give values that are consistent with the results of other independent experiments.
Multiplication and excess noise in AlxGa1−xAs/GaAs multilayer avalanche photodiodes
Authors: Chia C. K , Ng B. K. , David J. P. R. , Rees G. J. , Tozer R. C. , Hopkinson M. , Airey R. J. , Robson P. N.
Published: August 2003

Impact ionization in 1–2.5 μm thick AlxGa1−xAs/GaAs multiple quantum well (MQW) avalanche photodiodes with various barrier and well widths has been investigated in detail. The measured avalanche multiplication suggests that in all cases no enhancement of the ionization coefficients over that of equivalent GaAs is possible. Excess noise measurements on these structures corroborate the multiplication results and show that no enhancement of the ionization coefficient ratio can be obtained in AlxGa1−xAs/GaAs MQWs by virtue of the heterojunction interfaces, irrespective of the number of repeating units. These results suggest that the excess energy gained by carriers crossing the AlxGa1−xAs/GaAs heterojunction interface is negligible and that the lower ionization coefficients of the AlxGa1−xAs results in behavior that is similar to that of a pseudoalloy of the wells and barriers.
Experimental analysis and theoretical model for anomalously high ideality factors (n≫2.0) in AlGaN/GaN p-n junction diodes
Authors: Shah Jay M. , Li Y.-L. , Gessmann Th. , Schubert E. F.
Published: August 2003

Diode ideality factors much higher than the expected values of 1.0 to 2.0 have been reported in GaN-based p-n junctions. It is shown that moderately doped unipolar heterojunctions as well as metal-semiconductor junctions, in particular the metal contact to p-type GaN, can increase the ideality factor to values greater than 2.0. A relation is derived for the effective ideality factor by taking into account all junctions of the diode structure. Diodes fabricated from a bulk GaN p-n junction and a p-n junction structure with a p-type AlGaN/GaN superlattice display ideality factors of 6.9 and 4.0, respectively. These results are consistent with the theoretical model and the fact that p-type AlGaN/GaN superlattices facilitate the formation of low-resistance ohmic contacts.
Magnetic properties and exchange coupling of nanocomposite (Nd,Y)2Fe14B/α-Fe
Authors: Zhang M. , Ren W. J. , Zhang Z. D. , Sun X. K. , Liu W. , Geng D. Y. , Zhao X. G. , Grössinger R. , Triyono D.
Published: August 2003

Magnetic properties and exchange couplings of nanocomposite (Nd,Y)2Fe14B/α-Fe magnets prepared by mechanical milling have been investigated at low temperatures. With decreasing temperature, due to Y substitution for Nd, an extension of the exchange coupling length in nanocomposite magnets occurs in a wider temperature range. This can be attributed a decreasing anisotropy of the hard-magnetic phase so that more soft-magnetic grains participate the exchange coupling. Close to 135 K, due to the spin reorientation, the hard magnetic phase is not more uniaxail, which leads to a drastic decrease of the magnetic properties of the Nd2Fe14B-based nanocomposite magnets.
Macroporous-silicon-based three-dimensional photonic crystal with a large complete band gap
Authors: Hillebrand R. , Senz St. , Hergert W. , Gösele U.
Published: August 2003

A photonic crystal (PC) structure is described revealing a complete three-dimensional (3D) photonic band gap of about 25% if realized as a silicon/air structure. It is based on two systems of parallel circular pores being orthogonal to each other. The gap size depends on the degree of mutual penetration of the pore systems. A possible fabrication route is based on macroporous silicon (lattice constant a=0.5 μm), into which orthogonal pores are drilled, e.g., by focused-ion-beam etching. This yields a 3D photonic crystal with a complete band gap in the near infrared. The dispersion behavior of the PC is theoretically analyzed (band structure, density of states), varying the pore radii. We discuss the influence of pore shape variations and topological modifications on the size of the gap.
Cathodoluminescence study of carrier diffusion in AlGaN
Authors: Barjon J. , Brault J. , Daudin B. , Jalabert D. , Sieber B.
Published: August 2003

This article presents a method to estimate the carrier diffusion length in the optical waveguide region of a separate confinement heterostructure (SCH) semiconductor laser. The electron incident energy dependence of waveguide and active region cathodoluminescence yields are compared with a Monte Carlo simulation of ionization processes. This procedure is applied to a SCH consisting of GaN quantum dots inserted in an Al0.27Ga0.73N/AlN optical waveguide grown by plasma assisted molecular-beam epitaxy on 6H–SiC. A diffusion length of 7.5 nm is estimated in AlGaN material at room temperature.
Strong interplay between giant and anisotropic magnetoresistance observed in face-centered-cubic-Co/Au multilayers between 5 and 280 K
Authors: Christides C.
Published: August 2003

(111) oriented [Co(1 nm)/Au(2.5 nm)]30 multilayers exhibit a strong thermal variation of the magnetoresistive (MR) signal between 5 and 280 K, that consists of field-direction dependent giant (GMR) and anisotropic magnetoresistance (AMR) contributions. Both, isothermal magnetization and the MR loops reveal the appearance of a canted magnetic state that accompanies the enhancement of the AMR contribution at lower temperatures. In addition, the experimental results reveal that the GMR and AMR contributions not only depend on the relative direction between the magnetic field and the current but also depend strongly on the relative direction of the field and the tilting angle of the mean easy-magnetization axis. The large differences observed in the magnetization and the MR curves of the as-made and the annealed films reveal the important role played by the field dependence of the magnetic domain correlations in the spin conductance of face-centered-cubic-Co/Au multilayers.
Femtosecond electron diffraction for direct measurement of ultrafast atomic motions
Authors: Cao J. , Hao Z. , Park H. , Tao C. , Kau D. , Blaszczyk L.
Published: August 2003

We have developed a femtosecond electron diffraction system capable of directly measuring the complete transient structures with atomic level detail and on 400-fs time scale in solid materials. Additionally, a diffraction image with significant signal-to-noise ratio to reveal the long-range order can be obtained with a single electron pulse of 700 fs in duration. A direct observation of ultrafast lattice expansion following the irradiation of femtosecond pulsed laser of Ag film has been demonstrated.
Role of the fast Ar atoms, Ar+ ions, and metastable Ar atoms in a hollow cathode glow discharge: Study by a hybrid model
Authors: Baguer N. , Bogaerts A. , Gijbels R.
Published: August 2003

The role of the fast Ar atoms, Ar+ ions, and metastable Ar atoms in a cylindrical hollow cathode discharge (HCD) is studied based on a self-consistent model. The model comprises submodels based on the principle of Monte Carlo and fluid simulations. With Monte Carlo models the movement of the fast electrons, fast Ar atoms, and fast Ar+ ions as particles is described, while with the fluid models, the slow electrons, ions, and metastable atoms are treated as a continuum. Typical results are, among others, the fast atom, fast ion, and fast electron excitation and ionization rates, the electron, ion, and metastable atom densities and fluxes, the energy distribution function of the fast atoms, fast ions, and fast electrons, and the electric field and potential distribution. Also the relative importance of different processes determining the metastable density in an Ar HCD is analyzed, as well as the influence of the fast atoms and fast ions on the discharge properties.
Ga-doping effects on electrical and luminescent properties of ZnO:(La,Eu)OF red phosphor thin films
Authors: Fujihara Shinobu , Suzuki Akira , Kimura Toshio
Published: August 2003

Red thin-film phosphors were fabricated based on a ZnO:(La,Eu)OF nanocomposite structure where (La,Eu)OF nanoparticles were dispersed in a ZnO film matrix. The films were deposited on glass substrates by a sol-gel method at a low temperature of 600 °C using trifluoroacetic acid as a fluorine source. Doping gallium into the films suppressed the grain growth of ZnO, increased the optical band gap, and decreased electrical resistivity, which indicates that Ga3+ was selectively incorporated into the ZnO lattice. Eu3+ was practically doped in the LaOF lattice. As a result, strong red emissions due to the D05→F27 transition of Eu3+ were observed in both photo- (PL) and cathodoluminescence (CL) measurements. The efficiency of ultraviolet light excitation at 274 nm was promoted by the charge transfer from O2− to Eu3+ in PL. Ga doping was found to increase the CL intensity of the film, which was attributed to suppression of charge accumulation on the films.
Electric fatigue in Pb(Nb,Zr,Sn,Ti)O3 thin films grown by a sol–gel process
Authors: Zhai Jiwei , Chen Haydn
Published: August 2003

Antiferroelectric Pb(Nb,Zr,Sn,Ti)O3 (PNZST) thin films were deposited via a sol–gel process on LaNiO3-buffered Pt/Ti/SiO2/Si substrates. The highly (100)-oriented LaNiO3 buffer layer facilitated the formation of high-quality PNZST films with a strong (100) preferred orientation. These films showed improved electric fatigue properties than those grown on Pt/Ti/SiO2/Si substrates. With increasing cycling field, the remanent polarization increases but the saturated polarization decreases. Fatigue properties of PNZST antiferroelectric thin films might be closely related to the nonuniform strain buildup due to switching that tends to stabilize the ferroelectric phase.
Protein printing with an atomic force sensing nanofountainpen
Authors: Taha Hesham , Marks Robert S. , Gheber Levi A. , Rousso Ittay , Newman John , Sukenik Chaim , Lewis Aaron
Published: August 2003

We demonstrate the direct printing of proteins on a surface using a cantilevered nanopipette as the probe of a scanned probe microscope. Protein features as small as ∼200 nm were directly delivered through the ∼100 nm aperture of the nanopipette by simply contacting the probe with any surface. This allows for the direct connection of this methodology to standard separation techniques so that multiple proteins can be printed through one nanopipette at different locations in ambient conditions.
Intrinsic Josephson junctions in c-axis-oriented La1.85Sr0.15CuO4 thin films
Authors: Mizugaki Y. , Uematsu Y. , Kim S.-J. , Chen J. , Nakajima K. , Yamashita T. , Sato H. , Naito M.
Published: August 2003

We investigated current–voltage characteristics (IVCs) of intrinsic Josephson junctions (IJJs) for c-axis oriented La1.85Sr0.15CuO4 thin films. We fabricated IJJs with two kinds of structures. One was a mesa structure with a junction area of 22–194 μm2 fabricated by using conventional photolithography and Ar-ion milling, and the other was a microbridge structure with a junction area of 1.1–3.6 μm2 fabricated by using a focused-ion-beam technique. The mesa-type IJJs exhibited resistively-shunted-junction-like IVCs with no hysteresis. The temperature dependence of their critical current followed the Ambegaokar–Baratoff relation. IVCs of the microbridge-type IJJs, on the other hand, exhibited voltage jump and clear hysteresis. The different behaviors between the mesa-type and the microbridge-type IJJs were explained by the different numbers of grain boundaries involved in the IJJs.
Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots
Authors: Kako S. , Miyamura M. , Tachibana K. , Hoshino K. , Arakawa Y.
Published: August 2003

Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots is reported. Two samples having different average size of quantum dots (QDs) have been investigated at the temperature of 3.5 K. The measurement has revealed that larger-QD sample shows longer photoluminescence (PL) decay time and smaller emission energy than smaller one. The dependence of radiative decay time of the samples on emission energy smoothly connects with each other reflecting the size distribution. The radiative decay time strongly increases by almost three orders magnitude, reaching microseconds, upon increasing the size of QDs. The increase of PL decay time with increasing the size of QDs is attributed to the reduction of oscillator-strength due to the strong built-in electric field in the GaN/AlN heterostructures.
Study on interfacial dislocations of Si substrate/epitaxial layer by self-interstitial decoration technique
Authors: Shao Lin , Wang Xuemei , Rusakova Irene , Chen Hui , Liu Jiarui , Thompson Phillip E. , Chu Wei-Kan
Published: August 2003

Trapping of migrating Si interstitials at substrate/epitaxial interfaces during high-energy Si ion bombardment has been observed. It shows that the interface of Si/Si layer, grown by molecular-beam epitaxy, is a strong sink for self-interstitials during MeV bombardment at room temperature. We reported the finding and applied it as a decoration technique to study evolution of interfacial dislocations. After the thermal annealing of Si/Si layers at a temperature ranging from 450 to 600 °C, samples were bombarded with MeV Si ions at room temperature. Trapped Si interstitials at the interface were quantitatively measured by Rutherford backscattering spectrometry. The integration of trapped interstitials, which indicates an activation energy for the interfacial defect release of 0.65 eV, suggests a weak binding of stored Si atoms in the interfacial dislocations.
Wet chemical cleaning of InP surfaces investigated by in situ and ex situ infrared spectroscopy
Authors: Pluchery O. , Chabal Y. J. , Opila R. L.
Published: August 2003

Wet chemical cleaning is central to semiconductor device processing. For InP-based optoelectronic devices, controlling wet processing is challenging because of the high reactivity of InP surfaces in ambient air. In situ techniques are therefore critical to monitor and understand wet chemical etching and oxidation. We have combined in situ and ex situ Fourier transform infrared absorption spectroscopy, in direct transmission and multiple internal reflection configurations, with ex situ x-ray photoelectron spectroscopy to investigate the wet-chemical removal of “epi-ready” oxides on InP(100). Even dilute solutions of HCl, HF, or H2SO4 are found to completely remove the epi-ready oxides. For both HCl and HF, the InP surface is left unpassivated, exhibiting rapid recontamination and reoxidation upon air exposure. In contrast, H2SO4 aggressively etches the surface, and forms a thick and fragile oxide cap that can be washed away during a subsequent water rinse. This systematic in situ vibrational study of InP oxide chemistry provides a basis for a mechanistic understanding of wet chemical etching and oxidation.
Low-threshold lasing of InGaN vertical-cavity surface-emitting lasers with dielectric distributed Bragg reflectors
Authors: Tawara Takehiko , Gotoh Hideki , Akasaka Tetsuya , Kobayashi Naoki , Saitoh Tadashi
Published: August 2003

Lasing action is achieved in InGaN vertical-cavity surface-emitting lasers (VCSELs) with dielectric distributed Bragg reflectors (DBRs). We fabricated III-nitride VCSELs by removing a SiC substrate from a III-nitride cavity with a dry etching technique and then wafer bonding the cavity and SiO2/ZrO2 DBRs. These VCSELs have a high quality factor of 460 and a spontaneous emission factor of 10−2. We observed lasing at a wavelength of 401 nm at room temperature with optical pumping. This lasing action was demonstrated at a low threshold of 5.1 mJ/cm2 by using a high-quality crystalline cavity and quantum-well layers without surface roughening or cracking.
Synthesis and characterization of composition-spread (Sr,Ca)2CuO3 thin films with high third-order optical nonlinearity
Authors: Ohtani M. , Fukumura T. , Sakurada H. , Nishimura J. , Kawasaki M. , Makino T. , Yamamoto K. , Segawa Y.
Published: August 2003

The optical properties of composition-spread (SrxCa1−x)2CuO3(0⩽x⩽1) films showing high third-order optical nonlinearity are systematically investigated. Desired orientations and high crystallinity are obtained for the entire composition x by employing a composition-spread [(Sr2TiO4)x(LaSrAlO4)1−x] buffer layer and a temperature-gradient method in order to compensate the change in lattice mismatch and growth temperature depending on x. Linear absorption spectra and third-harmonic intensity with an exciting wavelength of 1.56 μm are measured as a function of x. As the Cu–O bond length increases with x, the charge-transfer gap decreases and the third-harmonic intensity increases, and both the charge-transfer gap and third-harmonic intensity change almost linearly with x between Ca2CuO3 and Sr2CuO3.
Ga-filled single-crystalline MgO nanotube: Wide-temperature range nanothermometer
Authors: Li Y. B. , Bando Y. , Golberg D. , Liu Z. W.
Published: August 2003

A highly effective one-step approach was developed to synthesize single-crystalline MgO nanotubes and in situ fill nanotubes with Ga. The axes of nanotubes are in the [100] direction of cubic MgO. The prepared nanotube exhibits a square-like cross section both for its interior and exterior. The liquid metal-assisted route is suggested to be a general way to prepare oxide nanotubes. Linear thermal expansion behavior recorded for liquid gallium column confined in the MgO nanotube makes possible creation of a wide-temperature range nanothermometer with superior mechanical properties and environmental structural stability.
Effect of Al doping in GaN films grown by metalorganic chemical vapor deposition
Authors: Lee Jae-Hoon , Hahm Sung-Ho , Lee Jung-Hee , Bae Sung-Bum , Lee Kyu-Seok , Cho Yong-Hoon , Lee Jong-Lam
Published: August 2003

The effect of Al doping in GaN films grown by metalorganic chemical vapor deposition was investigated using photoluminescence (PL), time-resolved PL, Hall measurements, and reciprocal space map. The electron mobility measured at 300 (150) K by a Hall measurement significantly increased from 170 (185) cm2/V s in the undoped sample to 524 (744) cm2/V s in the Al-doped sample grown with a molar flow rate ratio Al/(Al+Ga) of 0.056. When increasing the incorporation of Al in GaN, the band edge photoluminescence emission intensity was enhanced by about one order of magnitude compared to the undoped GaN. In addition, an increase in the decay lifetime of the GaN band edge emission was observed with the Al-doped GaN. In conclusion, the incorporation of only a small amount of Al in GaN was found to significantly reduce the point-defect-related electron scattering center associated with the compensating acceptors (Ga vacancies or their complexes) and nonradiative recombination centers, thereby improving the electrical and optical properties of GaN films.
Rectification properties of layered boron nitride films on silicon
Authors: Nose K. , Tachibana K. , Yoshida T.
Published: August 2003

Cubic boron nitride (c-BN)/turbostratic boron nitride (t-BN) layered films were deposited on n-type Si substrates, and their rectification properties were investigated. Rectification in a typical n-type/p-type diode was observed in the current–voltage characteristics of c-BN film with a thin t-BN initial layer. However, the rectification polarity was inverted in the double-layered film with thick t-BN, where conduction was found to be caused by Schottky and Frenkel–Poole emission conduction mechanisms, depending on the range of bias applied. In the case of a thick t-BN single-layered film, the Frenkel–Poole emission conduction mechanism governed the conduction.
Propagating spin wave spectroscopy in a permalloy film: A quantitative analysis
Authors: Bailleul Matthieu , Olligs Dominik , Fermon Claude
Published: August 2003

We report on the microwave response of micrometer-wide antennas inductively coupled to a 30 nm thick permalloy film. We show that the self-inductance of a single antenna measures the coupling between the exciting current and the spin wave modes of the film. On the other hand, the signal transmitted to a second, distant antenna is used to observe the free relaxation law of a magnetostatic wave packet. All measurements, performed over a frequency range of 1–20 GHz, are quantitatively consistent with magnetostatic wave theory combined with Gilbert formulation of the damping.
Very large giant magnetoresistance of spin valves with specularly reflective oxide layers
Authors: Hong Jongill , Noma Kenji , Kanda Eiichi , Kanai Hitoshi
Published: August 2003

We developed a spin valve with oxide specular layers that shows a giant magnetoresistance value of 20%. The spin valve also showed an exchange bias field of over 1000 Oe. The giant magnetoresistance was mainly due to an increase in the sheet resistance change and resulted from additional specular reflection at the interface of the free layer. The method we used was to modify specularly reflective oxide layers of the free and the capping layers.
Nanoscale mechanical behavior of individual semiconducting nanobelts
Authors: Mao Scott X. , Zhao Minhua , Wang Zhong Lin
Published: August 2003

Nanobelts are a group of materials that have a rectangle-like cross section with typical widths of several hundred nanometers, width-to-thickness ratios of 5–10, and lengths of hundreds of micrometers. In this letter, nanoindentations were made in individual ZnO and SnO2 nanobelts by a cube corner diamond indenter. It is shown that the effect of indentation size is still obvious for indentation depths less than 50 nm. It is also demonstrated that nanomachining of nanobelts is possible using an atomic force microscope tip.
Temperature-mediated switching of magnetoresistance in Co-contacted multiwall carbon nanotubes
Authors: Chakraborty S. , Walsh K. M. , Alphenaar B. W. , Liu Lei , Tsukagoshi K.
Published: August 2003

We present temperature-dependent measurements of the magnetoresistance in ferromagnetically contacted multiwall nanotubes. At low temperature, the resistance increases sharply near zero-field due to misalignment of the contact magnetizations. As temperature increases, the resistance peak transforms into a resistance dip, with a peak-to-valley ratio of similar magnitude, but opposite sign. The resistance switch has a distinct temperature dependence compared with the background magnetoresistance, suggesting that the two have different origins. We propose that a ferromagnetic transition near the contact interfaces reverses the polarity of the injected spin, and changes the sign of the resistance switch.
Organic bistable molecular memory using photochromic diarylethene
Authors: Tsujioka Tsuyoshi , Kondo Hayato
Published: August 2003

A principle of organic memory device using a bistable photochromic molecule is presented that allows extremely high bit densities and very low power consumption. This device is based on an isomerization reaction of photochromic diarylethene molecule via its excited state by an electric carrier injection, not by photon absorption. Experimental data show that the reversible writing and nondestructive reading of information by the carrier injection is feasible. The advantages and properties of such an organic semiconductor memory using a bistable molecule are discussed.
Nanoscale radio-frequency thermometry
Authors: Schmidt D. R. , Yung C. S. , Cleland A. N.
Published: August 2003

We experimentally demonstrate the high bandwidth readout of a thermometer based on a superconductor–insulator–normal metal (SIN) tunnel junction, embedded in a rf resonant circuit. Our implementation enables basic studies of the thermodynamics of mesoscopic nanostructures. It can also be applied to the development of fast calorimeters, as well as ultrasensitive bolometers for the detection of far-infrared radiation. We discuss the operational details of this device, and estimate the ultimate temperature sensitivity and measurement bandwidth.
Effect of high temperature and interface treatments on photoluminescence from InGaN/GaN multiple quantum wells with green light emissions
Authors: Liu W. , Chua S. J. , Zhang X. H. , Zhang J.
Published: August 2003

InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.
Polarization-dependent reflectivity from dielectric nanowires
Authors: Du Y. , Han Song , Jin Wu , Zhou C. , Levi A. F. J.
Published: August 2003

The presence of GaN nanowires grown primarily normal to the surface of a sapphire substrate has a dramatic influence on the polarization dependence of laser light reflectivity at λ=1550-nm wavelength. Even at 12% substrate surface coverage, there is a factor of 2 enhancement in polarization dependence of reflectivity relative to bulk sapphire at values of incident angle greater than φ=72°.
Optical and structural anisotropy of InP/GaInP quantum dots for laser applications
Authors: Manz Y. M. , Christ A. , Schmidt O. G. , Riedl T. , Hangleiter A.
Published: August 2003

Self-assembled InP quantum dots, embedded in Ga0.52In0.48P and grown by solid source molecular-beam epitaxy, exhibit strong structural and optical anisotropy. Photoluminescence measurements reveal that the quantum dots are elongated in [11̄0] crystal direction and the optical transitions of both the dots and the surrounding GaInP material dominate for light polarized along this direction, whereas embedded compressively strained GaxIn1−xP quantum wells behave isotropically. The comparison of the optical gain of a strained GaxIn1−xP quantum well laser and a threefold stacked quantum-dot laser in [110] and [11̄0] directions (edge emission) emphasizes this difference. The gain of the quantum-well laser shows no directional dependence. The quantum-dot laser reveals significantly larger gain for light propagating perpendicular to the dot elongation. Thus, particular care has to be taken to align the cavities of InP/GaInP quantum-dot lasers in [110] direction.
Thermal oxidation of (0001) 4H-SiC at high temperatures in ozone-admixed oxygen gas ambient
Authors: Kosugi Ryoji , Fukuda Kenji , Arai Kazuo
Published: August 2003

The method of oxidation by atomic oxygen has been developed for gate oxide formation in SiC metal–oxide–semiconductor (MOS) devices. Ozone (O3)–admixed oxygen (O2) gas is introduced into the cold-wall oxidation furnace, where atomic oxygen in a ground state is formed by thermal decomposition of O3 molecules at elevated sample temperatures. The growth rate of oxide in the O3-admixed gas shows a maximum at around 666.4 Pa and 950–1200 °C, whereas the rate in pure O2 gas is negligible below 6664.5 Pa. Interface trap density (Dit) of the MOS capacitors fabricated using atomic oxygen strongly depends on the oxidization temperature; oxidation at 1200 °C results in significant reduction of Dit in comparison with that at 950 °C.
Investigation of boron diffusion in 6H-SiC
Authors: Gao Y. , Soloviev S. I. , Sudarshan T. S.
Published: August 2003

p -type doping of 6H-SiC was implemented by diffusion of boron at temperatures higher than 1900 °C. The doping profiles were clearly divided into steep (zone I) and long-tail (zone II) regions. The boron diffusions in both regions are well fitted by erfc functions but with different diffusion coefficients, which are an activation energy (EA) of 6.1 eV and a prefactor (D0) of 3.2 cm2/s for zone I and 4.6 eV and 0.1 cm2/s for zone II, respectively. Further, it has been confirmed that the boron acceptors in zone I are primarily located at shallow energy levels (∼300 meV) and the ones in zone II are located at deep energy levels (∼700 meV).
Fabrication and photoresponse of a pn-heterojunction diode composed of transparent oxide semiconductors, p-NiO and n-ZnO
Authors: Ohta Hiromichi , Hirano Masahiro , Nakahara Ken , Maruta Hideaki , Tanabe Tetsuhiro , Kamiya Masao , Kamiya Toshio , Hosono Hideo
Published: August 2003

Transparent trilayered oxide films of ZnO/NiO/indium tin oxide were heteroepitaxially grown on a YSZ (111) substrate by pulsed-laser deposition combined with a solid-phase-epitaxy technique, and were processed to fabricate a p-NiO/n-ZnO diode. The diode exhibited a clear rectifying I–V characteristic with an ideality factor of ∼2 and a forward threshold voltage of ∼1 V. Although the photoresponsivity was fairly weak at the zero-bias voltage, it was enhanced up to ∼0.3 A W−1 through the application of a reverse bias of −6 V under the irradiation of 360 nm light, a value comparable to that of commercial devices.