bluuebunny/crossref_metadata_2025 huggingface dataset:
Selective growth of Ge on Si(100) through vias of SiO2 nanotemplate using solid source molecular beam epitaxy
Authors: Li Qiming , Han Sang M. , Brueck Steven R. J. , Hersee Stephen , Jiang Ying-Bing , Xu Huifang
Published: December 2003

We demonstrate that Ge can be selectively grown on Si(100) through openings in a SiO2 nanotemplate by solid source molecular beam epitaxy. The selectivity relies on the thermal instability of GeO and SiO near 650 °C. Ge islands grow in the template windows and coalesce on top of the template, forming an epitaxial lateral overgrowth (ELO) layer. Cross-sectional transmission electron microscopy images show that the Ge seeds and the ELO layer are free of threading dislocations. Only stacking faults are generated but terminate within 70 nm of the Ge–Si interface, while twins along {111} planes are observed in the ELO layer. The threading-dislocation-free Ge seeds and ELO layer are attributed to epitaxial necking as well as Ge–Si intermixing at the interface.
Magnetic-field-induced strain of Fe–Ni–Ga in single-variant state
Authors: Morito H. , Fujita A. , Fukamichi K. , Kainuma R. , Ishida K. , Oikawa K.
Published: December 2003

The magnetic-field-induced strain (MFIS) and the magnetocrystalline anisotropy in Fe19.3Ni54.2Ga26.5 ferromagnetic shape memory alloy have been investigated in a single-variant state. From the magnetization curves, the magnetocrystalline anisotropy constant K in the single crystal Fe19.3Ni54.2Ga26.5 β′ martensite phase is estimated to be 1.8×106 erg/cm3 at 5 K. In the single-variant martensite phase, the reversible MFIS of 0.02% is observed, and the value of K is reduced with increasing temperature. On the other hand, the magnitude of MFIS increases up to 100 K, and then decreases with increasing temperature. Finally, no MFIS is observed above 150 K. From these data, the condition of K for the MFIS can be confirmed at low temperatures.
Electrostatic engineering of nanotube transistors for improved performance
Authors: Heinze S. , Tersoff J. , Avouris Ph.
Published: December 2003

With decreasing device dimensions, the performance of carbon nanotube field-effect transistors (CNFETs) is limited by high OFF currents except at low drain voltages. Introducing an asymmetry between source and drain electrostatics can improve the performance, reducing OFF currents and extending the usable range of drain voltage. The improvement is most dramatic for ambipolar Schottky-barrier CNFETs. Moreover, this approach allows a single device to exhibit equally good performance as an n- or p-type transistor, by changing only the sign of the drain voltage. Even for CNFETs having ohmic contacts, an asymmetric design can greatly improve the performance for small-bandgap nanotubes.
Electronically active layers and interfaces in polycrystalline devices: Cross-section mapping of CdS/CdTe solar cells
Authors: Visoly-Fisher Iris , Cohen Sidney R. , Cahen David , Ferekides Christos S.
Published: December 2003

Electronic mapping of cross sections of a polycrystalline device, the n-CdS/p-CdTe solar cell, show that the photovoltaic and metallurgical junctions coincide to within experimental resolution (50 nm), which rules out both type conversion of CdS and buried homojunctions. Compositional analysis of the CdS supports this. Mapping was done using scanning capacitance, complemented by scanning Kelvin probe microscopy. Our results explain why a high-resistance transparent conducting oxide layer is needed as contact to the CdS for successful device operation. They define limits on inputs for modeling performance of these devices.
Caterpillar structures in single-wire Z-pinch experiments
Authors: Johnston M. D. , Lau Y. Y. , Gilgenbach R. M. , Strickler T. S. , Jones M. C. , Cuneo M. E. , Mehlhorn T. A.
Published: December 2003

A series of experiments have been performed on single-wire Z pinches (1–2 kA, 20 kV, pulse length 500 ns; Al, Ag, W, or Cu wire of diameter 7.5–50 μm, length 2.5 cm). Excimer laser absorption photographs show expansion of metallic plasmas on a time scale of order 100 ns. The edge of this plasma plume begins to develop structures resembling a caterpillar only after the current pulse reaches its peak value. The growth of these caterpillar structures is shown to be consistent with the Rayleigh–Taylor instability of the decelerating plasma plume front at the later stage of the current pulse.
Photothermal detection of local thermal effects during selective nanophotothermolysis
Authors: Zharov Vladimir P. , Galitovsky Valentin , Viegas Mark
Published: December 2003

Photothermal (PT) technique was applied to the detection of laser-induced local thermal effects around absorbing nanoparticles into cells. The experiments demonstrated the capability of this technique for studying the threshold and the dynamic of thermal events around even a single particle in a broad temperature range, including monitoring of microbubble dynamic. PT technique shows promise for optimizing cancer-cell killing by incorporation of gold nanoparticles into the cells (selective “nanophotothermolysis”), as verified by electron microscopy and conventional viability tests.
High-k lithium phosphorous oxynitride thin films
Authors: Fu Zheng-Wen , Liu Wen-Yuan , Li Chi-Lin , Qin Qi-Zong , Yao Yin , Lu Fang
Published: December 2003

Lithium phosphorous oxynitride (Lipon) thin films have been fabricated onto n-Si substrate at room temperature by nitrogen plasma-assisted deposition of electron-beam reactive evaporated Li3PO4. The capacitance–voltage (C–V) and I–V characteristics of Al/Lipon/Si capacitors were measured. The accumulation, depletion, and inversion phenomena in the C–V curves of the as-deposited Lipon thin film could be clearly observed. The isothermal transient ionic current of Al/Lipon/Al as a function of time during voltage stepping from 0 to 3 V exhibits a large current response due to dipole orientation. The dielectric constant of Lipon thin films is found to be 16.6, and the leakage current density at an applied electric field of 5 kV/cm is about 6.0×10−7 A/cm2. These results suggest that lithium phosphorous oxynitride thin films are high-k materials. The incorporation of N into amorphous of Li3PO4 could significantly increase the dielectric constant of Lipon thin films.
Ultrahigh-vacuum scanning tunneling microscopy and spectroscopy of single-walled carbon nanotubes on hydrogen-passivated Si(100) surfaces
Authors: Albrecht P. M. , Lyding J. W.
Published: December 2003

Single-walled carbon nanotubes (SWNTs) have been studied on a Si(100)-2×1:H surface using an ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). Dry deposition of SWNTs in situ establishes the pristine interface necessary to elucidate fundamental physical and electronic interactions between SWNTs and silicon. We have achieved simultaneous atomic resolution STM images of isolated SWNTs and the local H-passivated Si(100) substrate. Scanning tunneling spectroscopy served to characterize both semiconducting and metallic SWNTs. In each case, electronic features unique to the nanotube can be identified within the substrate band gap. In contrast to previous UHV STM studies of SWNTs on Au(111), our investigation is motivated by the technological relevance of the Si(100) substrate and the potential for nanofabrication of hybrid SWNT-Si electronic devices on the Si(100)-2×1:H platform.
Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields
Authors: Hase Muneaki , Kitajima Masahiro , Nakashima Shin-ichi , Mizoguchi Kohji
Published: December 2003

We propose an experimental technique to generate large amplitude coherent phonons with irradiation of THz-rate pump pulses and to study the dynamics of phase transition in GeTe ferroelectrics. When a single pump pulse irradiates the sample at various pump power densities, the frequency of the soft phonon decreases sublinearly and saturates at higher pump powers. By contrast, when the THz-rate pump pulse sequence irradiates the sample at matched time intervals to forcibly drive oscillation, a large redshift of the phonon frequency is observed without any saturation effects. After excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped and leads to a near critical damping condition. This condition indicates that the lattice is driven to a precursor state of the phase transition.
Screening and collective effects on resonant charge-exchange processes in nonideal plasmas
Authors: Song Mi-Young , Jung Young-Dae
Published: December 2003

The resonant charge-exchange process by ion–ion collisions is investigated in nonideal plasmas. The interaction Hamiltonian in nonideal plasmas is given by an pseudopotential model that takes into account the plasma screening effect as well as the nonideal collective effect. Screened atomic states of the target and projectile ions in nonideal plasmas are obtained by the variational principle and perturbational analysis. The semiclassical straight-line trajectory method is used to describe the behavior of the projectile ion. It is found that the resonant charge-exchange probability in nonideal plasmas described by the pseudopotential is smaller than that in ideal plasmas described by the Debye–Hückel potential. The maximum position of the resonant charge-exchange probability, i.e., the position which takes place the resonant change-exchange process, is found to recede from the target ion with an increase of the nonideal effect.
Sharp ferroelectric phase transition in strained single-crystalline SrRuO3/Ba0.7Sr0.3TiO3/SrRuO3 capacitors
Authors: Dittmann R. , Plonka R. , Vasco E. , Pertsev N. A. , He J. Q. , Jia C. L. , Hoffmann-Eifert S. , Waser R.
Published: December 2003

Single-crystalline all-perovskite SrRuO3/Ba0.7Sr0.3TiO3/SrRuO3 thin-film capacitors epitaxially grown on SrTiO3 exhibit a sharp paraelectric-to-ferroelectric phase transition at 350 K with a maximum permittivity of about 6660. This value is comparable to that of bulk ceramics and exceeds by several times the highest values reported for Ba0.7Sr0.3TiO3 thin film capacitors. The observed thickness dependence of the dielectric response is analyzed with the aid of a thermodynamic theory. It is shown that a weak decrease of the permittivity with the Ba0.7Sr0.3TiO3 thickness decreasing from 200 to 10 nm can be explained solely by the thickness-dependent strain relaxation in epitaxial films without assuming the presence of low-permittivity layers at the film/electrode interfaces.
Effect of system size on particle-phase stress and microstructure formation
Authors: Lasinski M. E. , Curtis J. S. , Pekny J. F.
Published: December 2003

In this paper, we investigate the effect of particle number, or system size, on three-dimensional (3D) particle dynamic simulation results. Specifically, we simulate conditions with varying e (coefficient of restitution) and φ (solids volume fraction), containing particle numbers ranging from 250 to 300 000. Various algorithmic improvements are implemented in the simulation to efficiently handle these large numbers of particles. We observe, for the first time for 3D simulations, particle-phase microstructure formation at high coefficients of restitution. Furthermore, we show the onset of the particle-phase microstructure formation at various threshold system sizes, depending on the value of e and φ, and relate it to an increase in particle-phase stress. Visual observations are used to conduct a preliminary investigation of the nature of the microstructure formation. Multiple well-defined bands are observed for simulations of at least 100 000 particles.
On the inverse Magnus effect in free molecular flow
Authors: Weidman Patrick D. , Herczynski Andrzej
Published: December 2003

A Newton-inspired particle interaction model is introduced to compute the sideways force on spinning projectiles translating through a rarefied gas. The simple model reproduces the inverse Magnus force on a sphere reported by Borg, Söderholm and Essén [Phys. Fluids 15, 736 (2003)] using probability theory. Further analyses given for cylinders and parallelepipeds of rectangular and regular polygon section point to a universal law for this class of geometric shapes: when the inverse Magnus force is steady, it is proportional to one-half the mass M of gas displaced by the body.
Tunable optical fiber devices based on broadband long-period gratings and pumped microfluidics
Authors: Acharya Bharat R. , Krupenkin Tom , Ramachandran Siddharth , Wang Z. , Huang C. C. , Rogers John A.
Published: December 2003

This letter describes classes of tunable microfluidic fiber (μFF) devices that use specially designed long-period gratings in which the phase matching condition is satisfied over a wide spectral range. Dynamic tuning is achieved by electrowetting-based pumping of microfluidic plugs back and forth over the gratings. As specific examples, we demonstrate dynamically tunable broadband attenuators and filters with adjustable profiles by using fluids with different refractive indices. These devices have attractive features that include in-fiber design and polarization-independent behavior together with low-power, nonmechanical, fully reversible, and latchable tuning.
Fluorescence modulation of long DNA molecules adsorbed onto a microelectrode surface
Authors: Ueda Masanori , Takai Madoka , Taniguchi Kazutake , Takamura Yuzuru , Horiike Yasuhiro , Baba Yoshinobu
Published: December 2003

Using microfabricated electrodes, we demonstrate fluorescence modulation of deoxyribonucleic acid (DNA) stained with dye through the application of cyclic electric fields. The fluorescence of DNA adsorbed onto an electrode was quenched under the potentials greater than about 1 V, and was recovered by reducing the potentials to below about 0.6 V. The quenching was observed even for a single DNA molecule adsorbed on an electrode. Therefore, the fluorescence modulation of DNA by external electric fields can be expected as one of selective detection methods for adsorbed DNA onto electrodes.
Laterally patterned band structure in micromachined semiconductors
Authors: Stievater T. H. , Rabinovich W. S. , Boos J. B. , Katzer D. S. , Biermann M. L.
Published: December 2003

We demonstrate that micromachining lattice-matched InGaAs quantum wells grown on (001) InP with strained barriers produces precise laterally patterned modifications to the semiconductor band structure. The light-hole and heavy-hole excitonic transitions are mixed and differentially shifted by the micromachining, inducing a surface-normal optical anisotropy characterized by a peak birefringence of Δn=0.028. The measured optical properties agree with calculations based on finite-element models of the strain combined with an eight-band k⋅p model that includes deformation potentials. This technique may find applications in fields such as surface-normal polarization modulators, quasi-phase matching, and optically-acitve piezoelectric materials.
Compositional analysis of ultrathin silicon oxynitride gate dielectrics by quantitative electron energy loss spectroscopy
Authors: Stegmann Heiko , Zschech Ehrenfried
Published: December 2003

Silicon oxynitrides provide dielectric materials with improved properties for further enhanced metal–oxide–semiconductor field effect transistors. Spatially resolved quantitative analysis of the elemental composition across the gate dielectric is required to optimize this ultrathin layer. So far, this has not been possible for cross-sectioned transistors. We have obtained spatially resolved atomic ratios of N, O and Si from such samples. Electron energy-loss spectra were recorded in a scanning transmission electron microscope and processed quantitatively by fitting a set of reference spectra to them. Deposition parameters and electrical transistor properties can thus be correlated with the N and O depth distributions in the oxynitride gate dielectric of processed transistors.
Room-temperature ultraviolet light-emitting zinc oxide micropatterns prepared by low-temperature electrodeposition and photoresist
Authors: Izaki Masanobu , Watase Seiji , Takahashi Hisaya
Published: December 2003

High-quality (0001)-oriented zinc oxide (ZnO) micropatterns, 5-μm-wide microlines and 5-μm-diam microdot arrays, have been prepared on (111) Au-coated (100) Si wafer substrates by electrodeposition at 333 K in zinc nitrate aqueous solution and industrially employed positive type photoresist techniques. The ZnO micropatterns emitted ultraviolet light at photon energy of 3.25 eV due to recombination of bound excitons and visible light at 2.37 and 2.80 eV at room temperature.
AlGaAs/GaAs quantum wires with high photoluminescence thermal stability
Authors: Liu X.-Q. , Wang X.-L. , Ogura M. , Guillet T. , Voliotis V. , Grousson R.
Published: December 2003

We report a 5 nm thick V-shaped AlGaAs/GaAs single quantum wire (QWR) that showed high photoluminescence (PL) thermal stability as a result of our recent progress in fabrication techniques. The integrated PL intensity of the QWR sample was quenched only by a factor of about 2.5 when the temperature was increased from 5 to 300 K. This sample also showed higher PL thermal stability over the whole temperature range than a 5 nm thick single quantum well reference sample grown under similar conditions.
Simulations of threshold voltage instabilities in HfySiOx and SiO2/HfySiOx-based field-effect transistors
Authors: Houssa M. , Bizzari C. , Autran J. L.
Published: December 2003

Threshold voltage shifts in metal-oxide-semiconductor field-effect transistors with HfySiOx gate layers and SiO2/HfySiOx gate stacks have been simulated, taking into account the generation of Si trivalent dangling bonds at the Si/dielectric interface, resulting from the injection of electrons through the structure. While the tunneling current flowing through devices with HfySiOx single layers is predicted to be lower compared to devices with SiO2/HfySiOx gate stacks (with equivalent electrical thickness), it is found that the transistor lifetime, based on threshold voltage shifts, is improved in SiO2/HfySiOx gate stacks. This finding is attributed to the beneficial presence of the SiO2 interfacial layer, which allows the relaxation of strain at the Si/dielectric interface.
Formation of self-assembled InAs quantum dots on (110) GaAs substrates
Authors: Wasserman D. , Lyon S. A. , Hadjipanayi M. , Maciel A. , Ryan J. F.
Published: December 2003

The formation of indium arsenide self-assembled quantum dots (QDs) has been observed on (110) GaAs substrates. The quantum dots were grown using solid source molecular-beam epitaxy. Both (110) and (100) GaAs wafers were placed in the growth chamber and grown on simultaneously. QDs were found to form on the (110) GaAs substrate when grown upon a thin AlAs layer. Buried layers of InAs were studied using photoluminescence spectroscopy, while surface layers were studied by atomic force microscopy. While the luminescence spectra for simultaneously grown (110) and (100) QDs are similar in structure, the changes in each sample’s spectrum as a function of both temperature and excitation intensity suggest quantum dot density is over an order of magnitude lower on (110) samples than it is on the (100) samples. This difference is conditionally confirmed by atomic force microscopy studies.
Epitaxial ferromagnetic Mn5Ge3 on Ge(111)
Authors: Zeng Changgan , Erwin S. C. , Feldman L. C. , Li A. P. , Jin R. , Song Y. , Thompson J. R. , Weitering H. H.
Published: December 2003

Ferromagnetic Mn5Ge3 thin films were grown on Ge(111) with solid-phase epitaxy. The epitaxial relationship between the alloy film and substrate is Mn5Ge3(001)//Ge(111) with [100]Mn5Ge3//[11̄0]Ge. The alloy films exhibit metallic conductivity and strong ferromagnetism up to the Curie temperature, TC=296 K. These epitaxial alloy films are promising candidates for germanium-based spintronics.
Model for isothermal crystallization kinetics with metastable phase formation
Authors: Laine B. , Trapaga G. , Prokhorov E. , Morales-Sánchez E. , González-Hernández J.
Published: December 2003

In this letter we propose a simple computer stochastic model to represent and gain some insight into the kinetics of transformation in materials with metastable phase formation. This model explains deviations from the well-known Johnson–Mehl–Avrami–Kolmogorov kinetics theory and predicts the three slopes in Avrami’s plot. The model predictions were compared with experimental results obtained from thin-film samples of Ge2Sb2Te5 during isothermal phase transformations. The model includes the two-stage transformation and is capable to reproduce the experimental transformation curves.
Measuring the refractive indices of conjugated polymer films with Bragg grating outcouplers
Authors: Ziebarth Jonathan M. , McGehee Michael D.
Published: December 2003

Photoluminescence trapped inside a polymer film can be scattered out with a Bragg grating. The refractive index in the plane of the film and in the normal direction can be determined as a function of wavelength by measuring the scattering angles of the TE- and TM-waveguide modes. For the polymer poly[2-methoxy-5-(3′,7′-dimethyloctloxy)-paraphenylenevinylene], which is commonly used in light-emitting diodes, the refractive index in the plane of the film drops from 2.0 to 1.8 as the wavelength increases from 600 to 750 nm, while the refractive index in the normal direction is relatively constant at 1.57.
Far-infrared optical constants of CO2 near the critical point measured by terahertz spectroscopy
Authors: Mounaix P. , Moustakim M. , Le Boiteux S. , Delville J. P. , Wunenburger R. , Sarger L.
Published: December 2003

The optical index constants of CO2 at critical density are measured by terahertz (THz) time-domain spectroscopy (TDS–THz) in the frequency range 0.1–1.5 THz. Since supercritical CO2 is usually presented as a “universal” solvent, we systematically study the refractive index variations in the vicinity of its critical point along the critical isochore by scanning temperature from 295 to 308 K to explore the liquid, the gas, and the supercritical states. Density changes with temperature result in refractive index variations that are found to fit the Lorenz–Lorentz relationship in the THz frequency range.
Core structure and properties of partial dislocations in silicon carbide p-i-n diodes
Authors: Ha S. , Benamara M. , Skowronski M. , Lendenmann H.
Published: December 2003

The electroluminescence, mobility, and core nature of partial dislocations bounding stacking faults in 4H silicon carbide p-i-n diodes were investigated using optical emission microscopy and transmission electron microscopy (TEM). The stacking faults developed and expanded in the blocking layer during high current forward biasing. Their bounding partial dislocations showed two distinct characteristics. Bright luminescent segments were mobile while dark invisible ones were stationary during biasing. TEM analysis of their Burgers vectors indicated that the mobile segments were silicon-core 30° partial dislocations while the immobile segments were carbon-core 30° ones.
Effect of nitrogen addition on the morphology and structure of boron-doped nanostructured diamond films
Authors: Liang Qi , Catledge Shane A. , Vohra Yogesh K.
Published: December 2003

A chemical vapor deposition hydrogen/methane/nitrogen feedgas mixture with unconventional high methane (15% CH4 by volume) normally used to grow ultrahard and smooth nanostructured diamond films on Ti–6Al–4V alloy substrates was modified to include diborane (10% B2H6 in hydrogen) for boron doping. The flow rates of N2 were varied to investigate its effect on plasma chemistry, film morphology, and mechanical properties. As expected, boron is readily incorporated into the diamond film and results in a change in the lattice parameter. Nitrogen, on the other hand, competes with boron in the plasma and acts to prevent boron incorporation into the diamond structure. Glancing angle x-ray diffraction indicates a decrease of diamond lattice parameter with increasing N2/CH4 flow rate ratio. A critical N2/CH4 ratio of 0.4 was found to result in a film with a minimum in grain size and surface roughness, along with high boron incorporation (∼4×1020 cm−3). As shown from this critical limit, experimental conditions can be optimized to produce ultrahard, boron doped nanostructured diamond films with the potential for high temperature applications.
Single-grain thin-film transistor using Ni-mediated crystallization of amorphous silicon with a silicon nitride cap layer
Authors: Kim Jung Chul , Choi Jong Hyun , Kim Seung Soo , Kim Kyu Man , Jang Jin
Published: December 2003

Amorphous silicon was crystallized by metal-induced crystallization through a cap with a Ni area density of 2×1014 atoms/cm2 on the cap. The crystallized polycrystalline silicon (poly-Si) shows hexagonal-shaped or disk-shaped grains with average grain size of ∼40 μm. The p-type channel thin-film transistor (TFT) on a single-grain poly-Si exhibited a field-effect mobility of 114 cm2/V s, a threshold voltage of −4.7 V and a subthreshold slope of 0.5 V/dec. The gate bias-stressed changes in the TFT performance was found to be greatly reduced compared to the laser-annealed poly-Si TFT. The surface roughness of the poly-Si is 2.35 nm smaller than that of conventional excimer laser-annealed poly-Si (13.1 nm), which appears to be related to better device performance and improved stability.
Deformation twins in nanocrystalline Al
Authors: Liao X. Z. , Zhou F. , Lavernia E. J. , He D. W. , Zhu Y. T.
Published: December 2003

Due to its high stacking fault energy, no deformation twin has ever been observed in coarse-grained Al. Recent molecular dynamic (MD) simulations predicted the formation of deformation twins in nanocrystalline (nc) Al. Here, we report transmission-electron-microscopic observations of two types of twins in nc Al processed by cryogenic ball milling. They were formed via mechanisms suggested by the MD simulations. We also observed curved twin boundaries caused by partial dislocations. These results indicate that deformation mechanisms not accessible to coarse-grained Al are active in nc Al. They could be responsible for some unique mechanical properties of nc materials.
Design and ferroelectric properties of polar-axis-oriented polycrystalline Bi4−xPrxTi3O12 thick films on Ir/Si substrates
Authors: Matsuda Hirofumi , Ito Sachiko , Iijima Takashi
Published: December 2003

A route for polar-axis-oriented films with Bi4Ti3O12 (BIT)-type structure was presented. Bi4−xPrxTi3O12 (x=0.0, 0.3, 0.5, 0.7) films were grown on Ir/Si substrates from chemical solutions and formation of IrO2 from Ir layers fostered the nucleation of grains with a and b axes mixed orientation by lattice matching and pseudo-orthogonal c axes was aligned in-plane. Furthermore, by setting the heat treatment temperature for grain growth above the Curie temperature TC, the residual strain between the film and Si introduced lateral stress on cooling and aligned the ferroelectric polar axis separately along the film normal leaving the nonpolar axis in-plane. The polar-axis-oriented film exhibited superb ferroelectric properties with remanent and saturation polarizations of 2Pr=92  and Psat=50 μC/cm2 (x=0.3).
Low-resistance and highly-reflective Zn–Ni solid solution/Ag ohmic contacts for flip-chip light-emitting diodes
Authors: Song June-O , Leem Dong-Seok , Kwak J. S. , Nam O. H. , Park Y. , Seong Tae-Yeon
Published: December 2003

We have investigated a Zn–Ni solid solution/Ag scheme for use in producing high-quality ohmic contacts for flip-chip light-emitting diodes (LEDs). The as-deposited contact shows nonlinear I–V characteristics. However, oxidizing the contacts at temperatures of 350–550 °C for 1 min in air ambient results in linear I–V behaviors, yielding specific contact resistances of 10−4–10−5 Ω cm2. In addition, LEDs are fabricated with the oxidized Zn–Ni solid solution/Ag contacts and Ag single contacts. The typical I–V characteristics of the LEDs with the annealed Zn–Ni solid solution (2.5 nm)/Ag (200 nm) p-type contact layers reveal a forward-bias voltage of 3.25 V at an injection current of 20 mA, which is much better than that of the LEDs with the Ag (200 nm) contact layers.
Stochastic modeling of progressive breakdown in ultrathin SiO2 films
Authors: Miranda E. , Cester A. , Paccagnella A.
Published: December 2003

The breakdown dynamics of ultrathin SiO2 films in metal–oxide–semiconductor structures has been investigated. We show that the progressive increase of the leakage current that flows through the oxide when subjected to constant electrical stress can be modeled by the stochastic logistic differential equation. This approach relies on a time scale separation in which a deterministic term provides the S-shaped growth trajectory while a second term of the equation deals with the noisy behavior. Because of the inherent mean reverting property of the simulation process, the proposed model is also able to cover cases in which sudden upward and downward changes of the system’s conductance are registered.
Ge
1−x
Sn
x
alloys pseudomorphically grown on Ge(001)
Authors: Ladrón de Guevara H. Pérez , Rodrı́guez A. G. , Navarro-Contreras H. , Vidal M. A.
Published: December 2003

Ge 1−x Sn x alloys were grown on Ge(001) substrates in a conventional rf sputtering system. We determined the in-plane and in-growth lattice parameters, as well as the alloy bulk lattice parameter of the alloys for different Sn concentrations by high resolution x-ray diffraction. The Sn concentration was determined assuming Vegard’s law for the alloy lattice parameter. At low concentrations, we observed that Ge1−xSnx layers have pseudomorphic characteristics for layer thickness from 320 to 680 nm. These characteristics of Ge1−xSnx layers agree with the People and Bean critical thickness model. This structural study opens the possibility of growing dislocation-free Ge1−xSnx alloys below the critical thickness.
Electrical carrier-injection and transport characteristics of photochromic diarylethene films
Authors: Tsujioka Tsuyoshi , Masuda Kazuya
Published: December 2003

Electrical carrier-injection and transport characteristics of photochromic diarylethene thin films were investigated. Injected and transported carriers (injected current) to the diarylethene film were increased with the existence ratio of the closed-ring form, which was one isomerization state of the diarylethene molecule. A dramatic increase was observed at a 12-percent existence ratio of the closed-ring molecules. A lowering effect of the potential barrier, which is caused by a Poole–Frenkel-like effect with a short distance interaction potential for hole transportation by increasing the existence ratio of the closed-ring molecules is proposed as the mechanism behind this phenomenon.
Carbon nanotubes as elements to focus electron beams by Fresnel diffraction
Authors: Krüger A. , Ozawa M. , Banhart F.
Published: December 2003

Fresnel diffraction of electrons at curved edges can be used to focus electrons onto a spot when the concave side of the edge is transparent. It is shown that short multi-wall carbon nanotubes with diameters of the inner hollow core of a few nanometers act as electron focusers when they are aligned parallel to the beam in an electron microscope. The electron beam is focused onto the projected center of the tube, where an amplification of the electron intensity by several times can be attained.
Effect of carbon nanotube doping on critical current density of MgB2 superconductor
Authors: Dou S. X. , Yeoh W. K. , Horvat J. , Ionescu M.
Published: December 2003

The effect of doping MgB2 with carbon nanotubes on transition temperature, lattice parameters, critical current density and flux pinning was studied for MgB2−xCx with x=0, 0.05, 0.1, 0.2, and 0.3. The carbon substitution for B was found to enhance Jc in magnetic fields but depress Tc. The depression of Tc, which is caused by the carbon substitution for B, increases with an increasing doping level, sintering temperature, and duration. By controlling the extent of the substitution and addition of carbon nanotubes we can achieve the optimal improvement on critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in Tc. Under these conditions, Jc was enhanced by two orders of magnitude at 8 T and 5 K and 7 T and 10 K. Jc was more than 10 000 A/cm2 at 20 K and 4 T and 5 K and 8.5 T, respectively.
Positronium time-of-flight measurements of porous low-k films
Authors: Yu R. S. , Ohdaira T. , Suzuki R. , Ito K. , Hirata K. , Sato K. , Kobayashi Y. , Xu Jun
Published: December 2003

We demonstrate that positronium (Ps) time-of-flight (TOF) spectroscopy provides a powerful technique for probing the structure of porous low-dielectric constant (low-k) films. The emission of ortho-positronium (o-Ps) from the surface of a series of nano-porous silsesquioxane films was studied with a newly developed TOF spectrometer. Clear Ps emission peaks were observed from the two high porosity films (F38 and KI31), indicating their higher open porosity in comparison with other films. We also observed that the energy of o-Ps emission from F38 is much higher than that from KI31. It was concluded that the open pores in the former film are less tortuous, and o-Ps loses significantly less energy emerging from its surface than from that of latter film.
Decrease in Al acceptor density in Al-doped 4H-SiC by irradiation with 4.6 MeV electrons
Authors: Matsuura Hideharu , Aso Koichi , Kagamihara Sou , Iwata Hirofumi , Ishida Takuya , Nishikawa Kazuhiro
Published: December 2003

From the temperature dependence of the hole concentration p(T) in a lightly Al-doped 4H-SiC epilayer, an Al acceptor with ∼200 meV and an unknown defect with ∼370 meV are found. By irradiation with 4.6 MeV electrons, the Al acceptor density is reduced, while the unknown defect density is almost unchanged. This indicates that the substitutional Al atoms in 4H-SiC are displaced by irradiation or that the bonds between the substitutional Al atom and the nearest neighbor atom are broken by irradiation. Moreover, the reduction in p(T) with irradiation arises from the decrease of the Al acceptors, not from the formation of hole traps or donor-like defects.
Activation kinetics of implanted Si+ in GaN and application to fabricating lateral Schottky diodes
Authors: Irokawa Y. , Kim Jihyun , Ren F. , Baik K. H. , Gila B. P. , Abernathy C. R. , Pearton S. J. , Pan C.-C. , Chen G.-T. , Chyi J.-I.
Published: December 2003

The electrical activation characteristics of implanted Si+ in GaN were investigated as a function of annealing temperature (1000 °C–1200 °C). The maximum activation percentage for an ion dose of 2.5×1014 cm−2 was ∼30% with an apparent activation energy for electrical activation of 1.65±0.15 eV obtained from the temperature dependence of the saturated carrier density. Lateral Schottky diodes fabricated on this material showed forward ideality factors of ∼2 and a temperature coefficient of −0.15 V K−1 for their reverse breakdown voltage. These results show the feasibility of creating n+ tubs in p-GaN such as are needed as sources of minority carriers to achieve inversion in GaN metal-oxide-semiconductor field-effect transistors.
Characterization of nanoplasmonic structures by locally excited photoluminescence
Authors: Bouhelier Alexandre , Beversluis Michael R. , Novotny Lukas
Published: December 2003

A method is presented for the characterization of locally enhanced fields at laser-irradiated metal nanostructures. Excitation with 120 fs laser pulses gives rise to photoluminescence mediated by two-photon absorption. A metal tip used to locally scatter the photoluminescence renders a map of regions with high field strengths. Near-field photoluminescence images of particle clusters reveal the dipole nature of the electromagnetic field surrounding the particles. Spectra acquired with and without the presence of the tip show no significant shift of the surface plasmon resonance of the particle clusters, confirming that the tip acts as a passive probe.
Sculpting of three-dimensional nano-optical structures in silicon
Authors: Koonath Prakash , Kishima Koichiro , Indukuri Tejaswi , Jalali Bahram
Published: December 2003

Separation by IMplantation of OXygen (SIMOX) based process has been developed to sculpt three-dimensionally integrated nano-optical waveguiding structures in silicon. An approach, based on the implantation of oxygen ions into a silicon substrate, patterned with thermal oxide, has been adopted to synthesize low loss buried rib waveguides in a single implantation step of thickness 286 nm and widths varying from 2 μm to 12 μm. These waveguides show propagation losses in the range of 3–4 dB/cm. The capability of the process to sculpt three-dimensional (3-D) structures has also been demonstrated by defining rib waveguides on the top silicon layer.
Shear force imaging of soft samples in liquid using a diving bell concept
Authors: Koopman M. , de Bakker B. I. , Garcia-Parajo M. F. , van Hulst N. F.
Published: December 2003

We present a reliable and easy-to-use system, with a perfect analogy to a diving bell, to perform tuning fork based shear force microscopy on soft cells in liquid. Using the diving bell concept the tuning fork vibrates in air, while the tip is immersed in solution. In this way Q factors of 200 and higher in liquid are routinely obtained. The force feedback is reliable and stable over hours requiring a minimum adjustment of the set-point during imaging. With this system, tip–sample interaction forces are kept below 350 pN, enabling us to image soft dendritic cells in a buffer solution.
Effect of varying material anisotropy on critical current anisotropy in vicinal YBa2Cu3O7−δ thin films
Authors: Durrell J. H. , Evetts J. E. , Rössler R. , Delamare M. P. , Pedarnig J. D. , Bäuerle D.
Published: December 2003

The high Tc cuprate superconductors are noted for their anisotropic layered structure, certain of these materials indeed tend toward the limit of a Lawrence–Doniach superconductor. However, YBa2Cu3O7−δ has a smaller anisotropy than would be expected from its interlayer spacing. This is due to the cuprate chains in the structure. To investigate the influence of the chain oxygen on transport properties critical current versus applied field angle measurements were performed on fully oxygenated and deoxygenated YBa2Cu3O7−δ thin films and optimally oxygenated Y0.75Ca0.25Ba2Cu3O7−δ thin films. The films were grown on 10° miscut SrTiO3 substrates to enable the intrinsic vortex channeling effect to be observed. The form of the vortex channeling minimum observed in field angle dependent critical current studies on the films was seen to depend on film oxygenation. The vortex channeling effect is dependent on a angular dependent crossover to a string-pancake flux line lattice. The results obtained appear to be consistent with the prediction of [G. Blatter, M. V. Feigelman, V. B. Geshkenbein, A. I. Larkin, and V. M. Vinokur, Rev. Mod. Phys. 66, 1125 (1994)] that increased superconducting anisotropy leads to the kinked string-pancake lattice existing over a smaller angular range.
Inverted hybrid organic light-emitting device with polyethylene dioxythiophene-polystyrene sulfonate as an anode buffer layer
Authors: Dobbertin T. , Werner O. , Meyer J. , Kammoun A. , Schneider D. , Riedl T. , Becker E. , Johannes H.-H. , Kowalsky W.
Published: December 2003

In this study we demonstrate inverted organic light-emitting diodes (OLEDs) utilizing highly conductive polyethylene dioxythiophene–polystyrene sulfonate as a buffer layer to a radio-frequency sputter-deposited indium–tin–oxide as the anode. In comparison to an entirely small-molecule-based reference, a reduction in operation voltage of 8.4 V at superior efficiencies of 4.2 cd/A and 1 lm/W obtained at 1.000 cd/m2 was achieved. The inverted cell configuration is desirable for next-generation active-matrix OLED displays.
Structure of Nd-doped glasses measured by isotopic substitution in neutron diffraction
Authors: Benmore C. J. , Weber J. K. R. , Siewenie J. E. , Hiera K. J.
Published: December 2003

The structural environment of Nd ions in glasses can have a significant effect on their performance in optical devices. Isotopic substitution in neutron diffraction has been used to probe the structure of Nd2O3 doped SiO2 and La2O3+Y2O3+Al2O3 glasses. In the silicate glass, Nd is found to be ∼sixfold coordinated by oxygen atoms at a distance of 2.28(2) Å with additional Nd–O/Si correlations at 3.55(3), 4.78(3), 5.89(3) Å, and higher distances. In the rare-earth aluminate glass a peak at 2.49(2) Å and a shoulder at ∼3 Å indicate the presence of 8 or higher coordinate Nd polyhedra by comparison with bond valence theory.
Induction of combinatory characteristics by relaxor modification of Pb(Zr0.5Ti0.5)O3
Authors: Priya Shashank , Uchino Kenji , Ryu Jungho , Ahn Cheol-Woo , Nahm S.
Published: December 2003

This letter reports a Pb(Zr0.5Ti0.5)O3 (PZT) based composition, which provides high piezoelectric and electromechanical properties. The general form of the composition is Pb(Zn1/3Nb2/3)x(B1/3I′B2/3II′)y(Zr0.5Ti0.5)1−x−yO3+0.5 wt % MnO2 (x+y=0.2). Four compositions corresponding to the earlier formulation were studied. For the composition corresponding to y=0, the radial mode coupling factor (kp) of 0.58, longitudinal mode piezoelectric constant (d33) of 277 pC/N, dielectric constant (ε33/ε0) of 946, and mechanical quality factor (Qm) of 1402 are obtained. X-ray diffraction, dielectric, and polarization-electric field measurements were done to investigate the origin for this high performance. It was found that high soft properties are obtained due to the shifting of PZT composition towards morphotropic phase boundary and high hard properties are obtained due to the generation of dipolar defects which pin the polarization.
Green emission from c-axis oriented AlN nanorods doped with Tb
Authors: Liu Q. L. , Tanaka T. , Hu J. Q. , Xu F. F. , Sekiguchi T.
Published: December 2003

Green emission has been obtained from c-axis oriented aluminum nitride (AlN) nanorods doped with terbium (Tb) on silicon (111). The nanorods were prepared by reactive radio-frequency magnetron sputtering in argon and nitrogen atmosphere using a target of Al and TbN mixtures, and characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The sharp characteristic emission lines observed in the photoluminescence and cathodoluminescence (CL) spectra at room temperature correspond to the Tb3+ intra-4fn shell transitions. The monochromatic (wavelength=554 nm) CL images with the high spatial resolution directly show the green emission from the well-aligned nanorods.
Alfalfa nodulation by Sinorhizobium fredii does not require sulfated Nod-factors
Authors: Noreen Sadaf , Schlaman Helmi R. M. , Bellogín Ramón A. , Buendía-Clavería Ana M. , Espuny MaríaRosario , Harteveld Marga , Medina Carlos , Ollero F. Javier , Olsthoorn MaurienM.A. , Soria-Diaz M. Eugenia , Spaink Herman P. , Temprano Francisco , Thomas-Oates Jane , Vinardell José M. , Yang Su Sheng , Zhang Haiyu , Ruiz-Sainz José E.
Published: December 2003

Rhizobium strain 042B(s) is able to nodulate both soybean and alfalfa cultivars. We have demonstrated, by mass spectrometry, that the nodulation (Nod) factors produced by this strain are characteristic of those produced by Sinorhizobium fredii, which typically nodulates soybean; they have 3–5 N-acetylglucosamine (GlcNAc) residues, a mono-unsaturated or saturated C16, C18 or C20 fatty-acyl chain, and a (methyl)fucosyl residue on C6 of the reducing-terminal GlcNAc. In order to study Rhizobium strain 042B(s) and its nodulation behaviour further, we introduced an insertion mutation in the noeL gene, which is responsible for the presence of the (methyl)fucose residue on the reducing terminal GlcNAc of the Nod-factors, yielding mutant strain SVQ523. A plasmid (pHM500) carrying nodH, nodP and nodQ, the genes involved in sulfation of Nod-factors on C6 of the reducing-terminal GlcNAc, was introduced into SVQ523, generating SVQ523.pHM500. As expected, strain SVQ523 produces unfucosylated Nod-factors, while SVQ523.pHM500 produces both unfucosylated and unfucosylated sulfated Nod-factors. Plant tests showed that soybean nodulation was reduced if the inoculant (SVQ523.pHM500) produced sulfated Nod-factors. In the Asiatic alfalfa cultivar Baoding, SVQ523 (absence of a substitution at C6) failed to nodulate, but both 042B(s) (fucosyl at C6) and SVQ523.pHM500 (sulfate at C6) formed nodules. In contrast, SVQ523 showed enhanced nodulation capacity with the western alfalfa cultivars ORCA and ARC. These results indicate that Nod-factor sulfation is not a requisite for S. fredii to nodulate alfalfa.
Copper-induced inhibition of photosynthesis: limiting steps of in vivo copper chlorophyll formation in Scenedesmus quadricauda
Authors: Küpper Hendrik , Šetlík Ivan , Šetliková Eva , Ferimazova Naila , Spiller Martin , Küpper Frithjof C.
Published: December 2003

The in vivo substitution of Mg2+ in chlorophyll by heavy metals is an important damage mechanism in heavy metal-stressed plants that leads to an inhibition of photosynthesis. In photosynthetic organisms with LHC II antennae, the in vivo substitution of Mg2+ by Cu2+ occurs particularly readily under low irradiance with a dark phase — a phenomenon referred to as 'shade reaction'. In the present study the limiting steps of the shade reaction were investigated with synchronised cultures of the chlorococcal green alga Scenedesmus quadricauda (Turp.) Bréb. The rate of copper chlorophyll formation during shade reaction was shown to be controlled by several factors; firstly, in some phases of the cell cycle, especially at the end of the light period, Mg2+ in chlorophyll was not accessible to substitution. This pattern is likely to be caused by cell cycle-dependent changes in photosynthesis and thylakoid ultrastructure, which were published earlier and are reconsidered in the discussion of the present work. Secondly, prolonged culture in a medium containing 3 μM Cu2+ reversibly increased the resistance of the strain to Cu2+. Culturing without added Cu2+ lowered the threshold concentrations of various deleterious effects more than 10-fold within 8 months of de-adaptation. Adaptation to high Cu2+ levels is discussed in the context of studies of the regulation of metal transporter proteins. In addition, it was also observed that toxic Cu2+ levels impaired photosynthesis sooner than cell division.
Species diversity and spatial variation in fish assemblage structure of streams in connected tropical catchments in northern Australia with reference to the occurrence of translocated and exotic species
Authors: Russell D. J. , Ryan T. J. , McDougall A. J. , Kistle S. E. , Aland G.
Published: December 2003

The freshwater piscifauna of the watercourses of the Barron and Mitchell systems, two northern Australian catchments with adjacent headwaters, was surveyed. Fifty-eight species of fish from 26 families, including several marine vagrants, were sampled from the freshwater reaches of the Barron River, compared to 28�species from 15 families from the Mitchell River. Species diversity and richness was higher in the coastal section of the Barron catchment owing to the influence of marine vagrants and species that were estuarine dependent for part of their life cycle. Detrended correspondence analysis of fish abundance data showed that sites in the coastal Barron catchment and the Mitchell catchment were widely separated in ordination space. Historically, barriers to fish movement determined species composition in both catchments, but there have been major changes in species composition as a result of extensive translocations of native fish into the Barron catchment over the last 50 years. Five species of exotic fish, including the cichlids Tilapia mariae and Oreochromis mossambicus, were established in the Barron system and one species in the Mitchell catchment. Cherax quadricarinatus, a crustacean endemic to western drainages, has been translocated into the Barron catchment and has since become widely established. This has implications for the interbasin transfer of irrigation water between the Barron and the Mitchell catchments potentially providing a mechanism for fish, including the exotic species O. mossambicus, to spread across northern Australia.