bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferromagnetic properties of epitaxial manganite films on SrTiO3∕Si heterostructures
Authors: Pradhan A. K. , Dadson J. B. , Hunter D. , Zhang K. , Mohanty S. , Jackson E. M. , Lasley-Hunter B. , Lord K. , Williams T. M. , Rakhimov R. R. , Zhang J. , Sellmyer D. J. , Inaba K. , Hasegawa T. , Mathews S. , Joseph B. , Sekhar B. R. , Roy U. N. , Cui Y. , Burger A.
Published: August 2006

We report on the magnetic properties of epitaxial La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 films on Si (100) and Si (111) substrates using SrTiO3 template layer, which demonstrate magnetic and electrical properties at and above room temperature. The magnetization data show magnetic transition and magnetic hysteresis at and above room room temperature. The films show well-defined magnetic domains. The ferromagnetic resonance studies show anisotropic effects related to ferromagnetic properties of films. The smaller grain size of about 20nm in manganite films on SrTiO3∕Si may be one of the reasons to minimize the strain effect through strain relaxation at the interface between SrTiO3 and manganites through the formation of three-dimensonal islands.
Periodic solution and chaotic strange attractor for shunting inhibitory cellular neural networks with impulses
Authors: Gui Zhanji , Ge Weigao
Published: August 2006

By using the continuation theorem of coincidence degree theory and constructing suitable Lyapunov functions, we study the existence, uniqueness, and global exponential stability of periodic solution for shunting inhibitory cellular neural networks with impulses, dxij∕dt=−aijxij−∑Ckl∊Nr(i,j)Cijklfij[xkl(t)]xij+Lij(t),t>0,t≠tk; Δxij(tk)=xij(tk+)−xij(tk−)=Ik[xij(tk)], k=1,2,….} Furthermore, the numerical simulation shows that our system can occur in many forms of complexities, including periodic oscillation and chaotic strange attractor. To the best of our knowledge, these results have been obtained for the first time. Some researchers have introduced impulses into their models, but analogous results have never been found.
Eu-activated fluorochlorozirconate glass-ceramic scintillators
Authors: Johnson J. A. , Schweizer S. , Henke B. , Chen G. , Woodford J. , Newman P. J. , MacFarlane D. R.
Published: August 2006

Rare-earth-doped fluorochlorozirconate (FCZ) glass-ceramic materials have been developed as scintillators and their properties investigated as a function of dopant level. The paper presents the relative scintillation efficiency in comparison to single-crystal cadmium tungstate, the scintillation intensity as a function of x-ray intensity and x-ray energy, and the spatial resolution (modulation transfer function). Images obtained with the FCZ glass-ceramic scintillator and with cadmium tungstate are also presented. Comparison shows that the image quality obtained using the glass ceramic is close to that from cadmium tungstate. Therefore, the glass-ceramic scintillator could be used as an alternative material for image formation resulting from scintillation. Other inorganic scintillators such as single crystals or polycrystalline films have limitations in resolution or size, but the transparent glass-ceramic can be scaled to any shape or size with excellent resolution.
Sources of optical absorption between 5.7 and 5.9eV in silica implanted with Si or O
Authors: Magruder R. H. , Stesmans A. , Clémer K. , Weeks R. A. , Weller R. A.
Published: August 2006

To determine if the only source of optical absorption between 5.8 and 5.9eV is the E′ center (absorbing at 5.85eV) two separate suites of type III silica samples were implanted, one with Si and one with O. Several ion energies were used for implantation to produce layers 600 and 570nm thick in the Si case and in the O case, respectively. Concentrations of implanted ions in the layers ranged from >0.02to<2.1at.%, with uncertainties in the concentrations less than ±5%. Optical absorption measurements were made from 2.0to6.5eV and electron spin resonance (ESR) measurements were performed at ∼20.3 and 33GHz at temperatures in the 77–100K range. Several ESR active defects were observed including the E′γ center (O vacancy), the nonbridging oxygen hole center, the peroxy radical, a possibly oxygen related center, labeled OS, and a component around g=2.0026. In the O-implantation case, by comparing the increasing optical absorption at 5.85eV with the observed decrease in the ESR E′ densities with increasing O concentration, we conclude that an oxygen related band is created and its intensity increases with increasing O concentration. In the silicon case, the optical absorption coefficient and the E′γ density as a function of Si implantation dose have a correlation coefficient that is ∼1, within reasonable errors. We conclude that in the O-implantation case the absorption between 5.8 and 5.9eV is in part due to an O related defect state in addition to the E′γ center while in the Si case it is mainly the E′ center that is responsible for the 5.85eV absorption band.
Intermixing of InGaAs quantum dots grown by cycled monolayer deposition
Authors: Djie H. S. , Wang D.-N. , Ooi B. S. , Hwang J. C. M. , Fang X.-M. , Wu Y. , Fastenau J. M. , Liu W. K.
Published: August 2006

We investigate the thermal induced intermixing and the diffusion kinetics of InGaAs quantum dots grown by cycled monolayer deposition subjected to the overgrowth and ex situ annealing. The group-III intermixing, that obeys the Fickian law, reaches a steady state after ex situ annealing up to 850°C. An identical activation energy (Ea=1.5±0.3eV) obtained with and without subjecting to overgrowth implies that the intermixing is primarily governed by the dynamic annealing of intrinsic defects during the epitaxial overgrowth. The intrinsic intermixing is attributed to the instability of the interface morphology driven by the atomic migration during self-formation of quantum dot.
Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors
Authors: Scully Shawn R. , McGehee Michael D.
Published: August 2006

Exciton diffusion is of great importance to the future design of high efficiency organic photovoltaics. Exciton diffusion studies require accurate experimental techniques. This paper addresses two important complications that can arise in exciton diffusion length measurements made by analyzing luminescence from thin films on quenching substrates: namely, the effects of optical interference and of energy transfer to the quencher. When there is modest contrast in the refractive indices of the quencher and organic material, as is the case for titania or C60 and most organic materials, interference effects can overwhelm the measurement, thereby making it impossible to accurately determine the diffusion length of excitons in the organic material. We show that this problem can be fully eliminated by using thin (<5nm) quencher films. The second complication that can occur is energy transfer to the quenching layer. We model the effect this has when fullerenes are used as quenchers. If energy transfer was ignored, one would falsely measure exciton diffusion lengths that are much greater than, and in some cases more than double, the actual diffusion length. Using titania as a quencher we eliminate the possibility of energy transfer, and by using thin titania films we eliminate the effects of interference and accurately measure a diffusion length of 6±1nm for the commonly used polymer poly[2-methoxy,5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene.
Circuit elements at optical frequencies from first principles: A synthesis of electronic structure and circuit theories
Authors: Ramprasad R. , Tang C.
Published: August 2006

A first principles electronic structure based method is presented to determine the equivalent circuit representations of nanostructured physical systems at optical frequencies, via a mapping of the effective permittivity calculated for a lattice of physical nano-elements using density functional theory to that calculated for a lattice of impedances using circuit theory. Specifically, it is shown that silicon nanowires and carbon nanotubes can be represented as series combinations of inductance, capacitance and resistance. It is anticipated that the generality of this approach will allow for an alternate description of physical systems at optical frequencies, and in the realization of novel opto- and nanoelectronic devices, including negative refractive index materials.
Stress reduction in epitaxial GaN films on Si using cubic SiC as intermediate layers
Authors: Komiyama Jun , Abe Yoshihisa , Suzuki Shunichi , Nakanishi Hideo
Published: August 2006

Stress in the epitaxial films of GaN on Si is reduced by using SiC as intermediate layers. The crystalline films of cubic SiC (0–1μm), thin AlN (50nm), and GaN (1–3μm) were prepared on 3in. (1 1 1) Si substrates—stacked in the order of GaN∕AlN∕SiC∕Si—by metalorganic vapor-phase epitaxy. It is revealed by Raman spectroscopy that the tensile stress in GaN is reduced to half (reduction of about 300MPa) for GaN on Si with SiC intermediate layers compared with GaN on Si without SiC intermediate layers. Because of stress reduction, crack-free GaN on Si with a thickness of 2μm was obtained by using SiC intermediate layers. Cracking was minimized even on thicker GaN on Si (3μm thick) with SiC intermediate layers. The SiC intermediate layers are promising for the realization of nitride based electronic devices on Si.
A dielectric mode in electroclinic liquid crystals
Authors: Thakur A. K. , Choudhary A. , Kaur S. , Bawa S. S. , Biradar A. M.
Published: August 2006

The dielectric properties of electroclinic and ferroelectric liquid crystal materials have been investigated in the frequency range of 100Hz–10MHz. A dielectric mode has been predicted in electroclinic liquid crystals near the transition temperature of Sm-C*–Sm-A phase. It has been observed that the investigated material has nonlayer shrinkage near the transition temperature of Sm-C*–Sm-A phase and shows anomalous behavior of dielectric spectra, tilt, and texture which is entirely different from the behavior of ferroelectric liquid crystals (FLCs). The dielectric relaxation frequency and tilt angle are almost constant with respect to temperature near the transition temperature under high bias field, which is characteristically different from FLCs. The high dielectric permittivity near Tc owing to the presence of mode contribution leads to the fact that there is an intralayer phason variation few degrees before transition. The mode has been named random mode due to its origin from randomization of tilt near Tc. Tilt randomization has been considered as an order parameter because in both the phases molecules are tilted but it is disordered in Sm-A phase making its intralayer polarization zero.
Growth of CuCl thin films by magnetron sputtering for ultraviolet optoelectronic applications
Authors: Natarajan Gomathi , Daniels S. , Cameron D. C. , O’Reilly L. , Mitra A. , McNally P. J. , Lucas O. F. , Rajendra Kumar R. T. , Reid Ian , Bradley A. L.
Published: August 2006

Copper (I) chloride (CuCl) is a potential candidate for ultraviolet (UV) optoelectronics due to its close lattice match with Si (mismatch less than 0.4%) and a high UV excitonic emission at room temperature. CuCl thin films were deposited using radio frequency magnetron sputtering technique. The influence of target to substrate distance (dts) and sputtering pressure on the composition, microstructure, and UV emission properties of the films were analyzed. The films deposited with shorter target to substrate spacing (dts=3cm) were found to be nonstoichiometric, and the film stoichiometry improves when the substrate is moved away from the target (dts=4.5 and 6cm). A further increase in the spacing results in poor crystalline quality. The grain interface area increases when the sputtering pressure is increased from 1.1×10−3to1×10−2mbar at dts=6cm. Room temperature cathodoluminescence spectrum shows an intense and sharp UV exciton (Z3) emission at ∼385nm with a full width at half maximum of 16nm for the films deposited at the optimum dts of 6cm and a pressure of 1.1×10−3mbar. A broad deep level emission in the green region (∼515nm) is also observed. The relative intensity of the UV to green emission peaks decreased when the sputtering pressure was increased, consistent with an increase in grain boundary area. The variation in the stoichiometry and the crystallinity are attributed to the change in the intensity and energy of the flux of materials from the target due to the interaction with the background gas molecules.
Thermal evolution of hydrogen related defects in hydrogen implanted Czochralski silicon investigated by Raman spectroscopy and atomic force microscopy
Authors: Düngen W. , Job R. , Ma Y. , Huang Y. L. , Mueller T. , Fahrner W. R. , Keller L. O. , Horstmann J. T. , Fiedler H.
Published: August 2006

Micro-Raman spectroscopy and atomic force microscopy investigations have been applied on hydrogen implanted p-type Czochralski silicon samples to investigate the hydrogen related defects and their evolution after subsequent annealing. The thermal evolution of interstitial-hydrogen and vacancy-hydrogen complexes and hydrogen terminated silicon dangling bonds has been analyzed. Furthermore, the two Raman lines of molecular hydrogen attributed to free hydrogen molecules in blisters and molecular hydrogen trapped in multivacancies have been observed and analyzed. A mechanism based on the exchange of the hydrogen atoms between different hydrogen related defects during the annealing is proposed for the blister formation process, which is relevant for the hydrogen induced exfoliation for silicon-on-insulator fabrication.
Lorentz microscopy studies of domain wall trap structures
Authors: Brownlie C. , McVitie S. , Chapman J. N. , Wilkinson C. D. W.
Published: August 2006

Domain wall traps of varying geometry have been studied using Lorentz microscopy in a transmission electron microscope. Electron beam lithography and lift-off were used to fabricate the elements whose shape allowed the formation of a head-to-head domain structure in the central section. Previous micromagnetic simulations have shown that different head-to-head configurations are possible depending on the width and thickness of the strip. In the majority of our in situ magnetizing experiments a vortex domain wall configuration was nucleated. This could be moved reproducibly between the ends of the element under fields of a few tens of oersted.
Relationship between electrical impedance of a transducer and its electroacoustic behavior: Measurement without primary source
Authors: Loyau Vincent , Feuillard Guy
Published: August 2006

Forward and backward waves in a reflective medium that loads a piezoelectric transducer under electrical impedance test are usually considered as a perturbation for the measurement. However, substantial information about the transducer and the medium sample properties can be obtained from such electrical impedance measurements. This information is equivalent to that obtained from a pulse/echo experiment and it is shown that echo wave forms are easily deduced from such measurements. Furthermore, in the case of an electrical impedance deduced from a noise measurement, it is shown that echo wave forms can be obtained from a passive method without primary external source.
Characteristics of ZrO2 gate dielectrics on O2- and N2O-plasma treated partially strain-compensated Si0.69Ge0.3C0.01 layers
Authors: Mahapatra R. , Maikap S. , Lee Je-Hun , Ray S. K.
Published: August 2006

The characteristics of ZrO2 gate dielectric along with the interfacial layer on O2- and N2O-plasma treated partially strain-compensated Si0.69Ge0.3C0.01∕Si heterostructures have been investigated using spectroscopic and electrical measurements. Time-of-flight secondary ion mass spectroscopy and x-ray photoelectron spectroscopy analyses show the formation of an oxygen or nitrogen rich Zr-germanosilicate interfacial layer between the deposited ZrO2 and SiGeC films. The electrical and charge trapping properties under a constant current stressing have been studied using a metal-oxide-semiconductor structure. The N2O-plasma treated SiGeC film has a higher effective dielectric constant (k∼14) than that of the O2-plasma treated (k∼12) films. The equivalent areal densities of charge defects, Neq (cm−2), are found to be ∼1.8×1012 and ∼6×1011cm−2 for O2- and N2O-plasma treated films, respectively. Considerably less trapped charges in the N2O-treated gate dielectric stack under constant current stressing make it highly attractive for SiGeC based scaled metal-oxide-semiconductor device applications.
Photooxidation of plasma polymerized polydimethylsiloxane film by 172nm vacuum ultraviolet light irradiation in dilute oxygen
Authors: Ichikawa S.
Published: August 2006

Plasma polymerized polydimethylsiloxane films irradiated under different partial pressures of oxygen with a 172nm vacuum ultraviolet light were investigated in order to clarify the roles of molecular oxygen and photons in photooxidation. The thickness, densities, surface roughness, elemental compositions, and molecular structures of the irradiated and unirradiated films were examined by using glazing incidence x-ray reflectivity, Rutherford backscattering, infrared, and x-ray absorption (XAS) spectroscopies. Photooxidation is hardly promoted by irradiation in a high vacuum of 1×10−4Pa, though photodesorption of the methyl group and formation of Si–H bonds were observed. Silica films thicker than 140nm were formed at room temperature by irradiating them in low pressure oxygen gases. The degree of oxidation was smaller for the oxygen pressure of 10kPa than for 83Pa. Si K-edge XAS was performed to clarify the change of coordination environment of silicon by photooxidation in dilute oxygen flow containing less than 5ppm of molecular oxygen.
Improved stability of the induced second-order nonlinearity in soft glass by thermal poling
Authors: Moura A. L. , de Araujo M. T. , Vermelho M. V. D. , Aitchison J. S.
Published: August 2006

Stable and intense second-order nonlinearity in soda lime glass is investigated tailoring the induced electric current. This procedure allows the determination of the relative contributions of the dipole orientation as well as the ionic contributions to the poling process. The experiments are developed in the light of multiple-carrier models controlling the output power supply applied current to tailor the frozen-in induced electric field — Edc. This method permits the induction of the stable nonlinearity for applied electric fields above ∼5kV∕cm and temperatures ∼250°C. It is also possible to reach higher temperatures than the ones used in normal poling procedures avoiding the electric current breakdown. The controlled Edc formation enables it to participate in essential chemical reactions that determine the intensity and stability of the nonlinearity. The induced d33 of ∼0.41pm∕V measured 20 days after poling reduced only ∼50% during the next seven months.
Electric-field-induced microstructural transformation of carbon nanotubes
Authors: Bao Qiaoliang , Zhang Han , Pan Chunxu
Published: August 2006

In the present work, microstructural transformation of carbon nanotubes (CNTs) from the “herringbone” into highly crystalline structure in an electric field was experimentally observed by using a high-resolution transmission electron microscope and the growth mechanism was modeled and discussed by using a finite element method. It is found that the CNT microstructures can be changed by an electric field through the influence of the electrostatic force on the carbon surface and bulk diffusion on/in a deformed catalyst particle. Preliminary experiment revealed that an isomeric “graphite-nongraphite-graphite-nongraphite⋯” CNTs could be synthesized when a pulsed electric field is applied, which are expected to exhibit special properties and promising applications.
The effect of anion substitution and nonstoichiometry on the fast ion conductor Cu6PSe5I
Authors: Beeken R. B. , Lintereur A. T.
Published: August 2006

Cu 6 P Se 5 I is a fast ion conductor exhibiting an order-of-magnitude change in electrical conductivity at its 265K superionic-covalent phase transition. Replacing 5% of the selenide anions in this compound with sulfide results in a substantial enhancement of conductivity as well as a decrease in the phase transition temperature. Replacing 5% of the iodide anions with bromide produces a similar enhancement of conductivity, but has no effect upon the phase transition temperature. Introducing 2% halide vacancies into the Cu6PSe5I lattice via nonstoichiometric sample preparation has little effect upon the superionic conductivity, but significantly enhances conductivity and reduces activation energy in the covalent phase. Finally, the introduction of 1% vacancies into the chalcogenide sites via nonstoichiometric sample preparation shifts the superionic-covalent phase transition to higher temperatures.
Interface magnetism and magnetic structure of GdN∕Fe multilayers studied by x-ray magnetic circular dichroism
Authors: Leuenberger F. , Parge A. , Felsch W. , Neisius T. , Mathon O.
Published: August 2006

We have used the element specificity of x-ray magnetic circular dichroism to separate the contributions of the component layers to the magnetization and local magnetic structure of artificial nanoscale ferrimagnetic multilayers GdN∕Fe. The Fe layers, by strong antiferromagnetic interlayer exchange coupling, not only magnetize a narrow interface region in paramagnetic GdN but induce long-range magnetic order in the volume of the GdN layers at temperatures considerably above the Curie temperature of the bare layers (TCGdN≈60K), in support of a theoretical mean-field prediction. We propose that the effect may be related to the special electronic band structure of GdN, which shows a transition from narrow-gap semiconducting to metallic conduction at TCGdN. In an elevated external magnetic field the GdN- and Fe-sublayer magnetic moments adopt a canted configuration. In this state the local magnetization in each GdN layer is highly nonuniform at low temperature. The interior turns its moment into the field direction almost abruptly from one atomic layer to the next in a single block. This indicates weak magnetic coupling between the interfacial and volume magnetizations in GdN. The results are at variance with the much studied “model” multilayer system Gd∕Fe, where magnetic order in the Gd-layer volume appears intrinsically only significantly below the Curie temperature of bulk Gd. Furthermore, magnetization reversal in the Gd layers in a magnetic field occurs gradually on a considerably larger length scale, only if they are sufficiently thick.
Electronic and optical properties of CaCO3 porous nanoparticles
Authors: de Sousa J. S. , Freire V. N. , Albuquerque E. L.
Published: August 2006

The electronic and optical properties of porous CaCO3 nanoparticles are investigated. The pore size are statistically generated and the overall behavior is averaged over several samples. We observed that (i) the energy of the confined electronic states obeys the power law En∝nb (b<1), (ii) for a given state, the confined states are randomly localized for different samples, and (iii) excited states are randomly localized in different sites of the same particle. Concerning their optical properties, interband (intraband) transitions are very efficient (inefficient). Due to the small energy difference between adjacent states, absorption and luminescence spectra are expected to be very strong.
Magnetization of α′ iron nitride produced through the fcc→bct martensitic transformation in high magnetic field
Authors: Koyano T. , Nomiyama T. , Kanoh N. , Numata H. , Ohba T. , Kita E. , Ohtsuka H.
Published: August 2006

The γ iron nitride (nitrogen austenite) was subjected to high magnetic field process in order to drive the fcc→bct martensitic transformation. Molar fraction of martensite monotonically increased with increasing the magnetic field and reached 94% at 35T. With a combination of magnetization and Fe57 Mössbauer spectroscopy data, magnetization of bulk processed α′ phase with 9.6at.% N is determined to be 229emu∕g, the same as that for dc sputtered thin films.
Femtosecond electron pulse propagation for ultrafast electron diffraction
Authors: Reed Bryan W.
Published: August 2006

Ultrafast electron diffraction (UED) relies on short, intense pulses of electrons, which because of Coulombic repulsion will expand and change shape as they propagate. While such pulse expansion has been studied in other contexts, efforts to model this effect for typical UED parameters have only arisen fairly recently. These efforts have yielded accurate predictions with very simple models, but have left a number of unexplained results (such as the development of a linear self-similar profile with sharply defined end points). The present work develops a series of models that gradually incorporate more physical principles, allowing a clear determination of which processes control which aspects of the pulse propagation. This will include a complete analytical solution of the one-dimensional problem (including a fundamental limitation on temporal resolution), followed by the gradual inclusion of two-dimensional and inhomogeneous effects. Even very simple models tend to capture the relevant on-axis behavior to within 10% or better. This degree of success can be traced to the manner in which the pulse transitions from one dimensional to two dimensional. We also present methods for determining the most appropriate model for a given situation and suggest paths toward future modeling improvements as the field evolves.
Chaotic mixing and transport in a meandering jet flow
Authors: Prants S. V. , Budyansky M. V. , Uleysky M. Yu. , Zaslavsky G. M.
Published: August 2006

Mixing and transport of passive particles are studied in a simple kinematic model of a meandering jet flow motivated by the problem of lateral mixing and transport in the Gulf Stream. We briefly discuss a model stream function, Hamiltonian advection equations, stationary points, and bifurcations. The phase portrait of the chosen model flow in the moving reference frame consists of a central eastward jet, chains of northern and southern circulations, and peripheral westward currents. Under a periodic perturbation of the meander’s amplitude, the topology of the phase space is complicated by the presence of chaotic layers and chains of oscillatory and ballistic islands with sticky boundaries immersed into a stochastic sea. Typical chaotic trajectories of advected particles are shown to demonstrate a complicated behavior with long flights in both the directions of motion intermittent with trapping in the circulation cells being stuck to the boundaries of vortex cores and resonant islands. Transport is asymmetric in the sense that mixing between the circulations and the peripheral currents is, in general, different from mixing between the circulations and the jet. The transport properties are characterized by probability distribution functions (PDFs) of durations and lengths of flights. Both the PDFs exhibit at their tails power-law decay with different values of exponents.
Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3
Authors: Mazumder R. , Ghosh S. , Mondal P. , Bhattacharya Dipten , Dasgupta S. , Das N. , Sen A. , Tyagi A. K. , Sivakumar M. , Takami T. , Ikuta H.
Published: August 2006

We report results of a comprehensive study of the phase transition at TN (∼643K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300K under a magnetic field of 100–10000Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN, is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO3 with divergence between zero-field cooled and field cooled magnetizations below ∼100K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO3 one also observes finite coercivity at lower temperature, which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi∕Fe ions and/or oxygen nonstoichiometry seems to be giving rise to broad features in thermal, magnetic as well as electrical responses.
Conduction model of SnO2 thin films based on conductance and Hall effect measurements
Authors: Oprea A. , Moretton E. , Bârsan N. , Becker W. J. , Wöllenstein J. , Weimar U.
Published: August 2006

Thin and porous SnO2 films (70nm thick with grain size between 10 and 30nm) have been prepared by e-beam evaporation onto alumina substrate provided with platinum electrodes. The Ohmic character of the contacts was preserved in all measurement conditions utilized for investigations. The dependence of electrical conduction on the composition of the ambient atmosphere has been studied by means of Hall and four point conductance measurements. The experiments were performed in different gas atmospheres containing N2, O2, and CO and at different operation temperatures (between room temperature and 420°C). A relatively low effective mobility (5–30cm2V−1s−1) and a high charge carrier effective concentration (1018–1019cm−3) were deduced when using the single crystals recipe, as required by the established models for granular materials. The analysis of these experimental data showed the inadequacy of the geometrical models and effective medium theories to correctly extract the electrokinetic parameters from conductance and Hall measurements in the case of gas sensitive layers and to predict their temperature and gas composition dependences. The conventional approach fails because it considers the samples at different temperatures as one physical system while, in fact, the surface chemistry in oxygen atmosphere leads to new trap generation, which is equivalent to the doping level modification. The use of a nonconventional approach, taking into account the film interaction with the ambient through quasichemical equations, and associated mass action laws together with the surface scattering influence on the carrier mobility allowed for the understanding of the involved mechanisms and good fits for the experimental data.
Time dependent evolution of the carrier mobility in poly(3-hexylthiophene) based field effect transistors
Authors: Devine Roderick A. B.
Published: August 2006

Measurements have been made of the carrier mobility, threshold voltage, and contact resistance in poly(3-hexylthiophene) based field effect transistors. Initially the measured values were 0.0008cm2V−1s−1, +8V, and 107Ω, respectively. Within 190min these values increased to 0.0022cm2V−1s−1, +11.5V, and 3×107Ω. Subsequently the mobility and threshold voltage declined while the contact resistance increased monotonically reaching 108Ω after 480min. Reversibility of these effects is demonstrated for the threshold voltage but an irreversible decline is observed for the mobility.
Dielectric behavior of Bi3.25La0.75Ti3O12 ferroelectric film
Authors: Zhong N. , Shiosaki T.
Published: August 2006

Dielectric and ac conductivity measurements, at frequencies from 100Hzto1MHz, were carried out on lanthanum doped bismuth titanium (Bi3.25La0.75Ti3O12) ferroelectric films, in the temperature range of 25–600°C. In addition to a dielectric peak at 483°C attributed to the paraferroelectric phase transition, three dielectric relaxations were found at ∼250, ∼400, and ∼500°C, respectively. The dielectric relaxation ∼400°C can be eliminated by using both a higher sintering temperature and oxygen postannealing, and it can be induced again by a subsequent nitrogen postannealing process. The activity energy of the dielectric relaxation of this mode is estimated to be 1.89eV, according to Arrhenius law. The dielectric relaxation ∼500°C is not influenced by sintering temperature but is influenced by the postannealing process. The intensity of this anomaly decreases after oxygen postannealing and then increases after nitrogen postannealing. The relaxation behavior of this dielectric relaxation is not in accordance with Arrhenius law but is in accordance with Vögel-Fulcher law, and the activity energy is calculated as 0.31eV. Possible mechanisms for the observed dielectric relaxations ∼250, ∼400, and ∼500°C are proposed.
Photoluminescence in disordered Sm-doped PbTiO3: Experimental and theoretical approach
Authors: Lima R. C. , Espinosa J. W. M. , Gurgel M. F. C. , Paris E. C. , Leite E. R. , Joya M. R. , Pizani P. S. , Varela J. A. , Longo E.
Published: August 2006

Sm-doped PbTiO3 powder was synthesized by the polymeric precursor method, and was heat treated at different temperatures. The x-ray diffraction, photoluminescence, and UV-visible were used as a probe for the structural order degree short-, intermediate-, and long-range orders. Sm3+ ions were used as markers of these order-disorder transformations in the PbTiO3 system. From the Rietveld refinement of the Sm-doped PbTiO3 x-ray diffraction data, structural models were obtained and analyzed by periodic ab initio quantum mechanical calculations using the CRYSTAL 98 package within the framework of density functional theory at the B3LYP level. This program can yield important information regarding the structural and electronic properties of crystalline and disordered structures. The experimental and theoretical results indicate the presence of the localized states in the band gap, due to the symmetry break, which is responsible for visible photoluminescence at room temperature in the disordered structure.
Thermal stability and crystallization kinetics of mechanically alloyed TiC∕Ti-based metallic glass matrix composite
Authors: Zhang Lai-Chang , Xu Jian , Eckert Jürgen
Published: August 2006

A Ti-based metallic glass matrix composite with 10vol% TiC is synthesized by mechanical alloying. The thermal stability and crystallization kinetics of the metallic glass and composite powders are investigated by differential scanning calorimetry in the mode of isochronal heating and isothermal annealing. The isothermal transformation kinetics is analyzed by the Kolmogorov-Johnson-Mehl-Avrami equation. The values of the Avrami exponent calculated for low crystallization volume fractions imply that the crystallization of both types of powders is governed by diffusion-controlled three-dimensional growth. The mean activation energy of crystallization for the composite is slightly lower than that of the Ti-based metallic glass. The addition of 10vol% TiC particles into a Ti-based metallic glass matrix may slightly affect the crystallization kinetics of the glassy matrix.
Aging and detergent washing effects of the surface of (001) and (110) GaAs passivated with hexadecanethiol
Authors: Moumanis Kh. , Ding X. , Dubowski J. J. , Frost E. H.
Published: August 2006

We report on aging and detergent washing effects in GaAs wafers passivated with hexadecanethiol (HDT) [HS(CH2)15CH3]. The evolution of the photoluminescence (PL) signal from GaAs was measured at room temperature as a function of time, up to 1000h, for thiolated (001) GaAs that was solvent cleaned and etched using standard procedures, and for thiolated (110) GaAs that was obtained by cleaving in air. For 2h of thiolation time, the decay of the PL signal from (001) GaAs and (110) GaAs could be fitted, respectively, with double and single exponential functions. This indicates that at least two different types of non-adiative recombination centers (NRRC) form on the (001) surface. The overall density of surface states formed on (110) GaAs is significantly lower than those on (001) GaAs. A single exponential decay of the PL signal has been observed for 18h thiolated samples. The strong increase of the PL signal after detergent washing of (001) GaAs suggests that this treatment is effective in removal of some of the NRRC. The 19-fold enhancement of the PL signal, in comparison to that of the oxidized sample, has been obtained using the sample that, following the etching and detergent washing procedures, was thiolated with HDT.
Optical retardation of in-plane switched polymer dispersed liquid crystals
Authors: Drevenšek-Olenik I. , Čopič M. , Sousa M. E. , Crawford G. P.
Published: August 2006

We investigated the field-induced optical birefringence of a photopolymerized polymer dispersed liquid crystal (PDLC) with nearly a 50% volume ratio of phase-separated liquid crystal (LC) droplets. Measurements were performed in the in-plane switching configuration, which was attained by the use of interdigitated electrodes. The observed dependence of the sample transmittance on the external field magnitude is independent of the polarization state of the incident beam. The material transforms a linearly polarized wave into an elliptically polarized wave. Phase retardation between the two eigenpolarizations is proportional to the order parameter of the LC domains. The observed phenomena are attributed to the dominant role of the droplet proximity effects in the optical properties of the material. Dynamic response of the retardation takes place in a broad range of time scales, which are associated with relaxation of different degrees of freedom of the system. The observed dynamic features indicate an interconnected structure of the LC domains in the LC rich PDLC structures.
Planar laser imaging and modeling of matrix-assisted pulsed-laser evaporation direct write in the bubble regime
Authors: Lewis Brent R. , Kinzel Edward C. , Laurendeau Normand M. , Lucht Robert P. , Xu Xianfan
Published: August 2006

A combination of planar laser imaging and theoretical modeling has been used to examine matrix-assisted pulsed-laser evaporation direct write (MAPLE-DW) in the bubble regime. MAPLE-DW is a method for patterning substrates via laser-initiated forward transfer of an organic fluid containing metallic particles and coated on a transparent support. For our conditions, best deposition of a silver-based, thick-film ink was found to occur when laser-initiated vaporization forces the ink outward as a bubble. Planar laser imaging was used to monitor bubble growth as a function of time for three different ink films with nominal thicknesses of 12, 25, and 50μm and two laser beam diameters of 30 and 60μm. From these measurements, correlations were developed for predicting the maximum height and velocity of bubbles via three known process variables: laser energy, ink thickness, and beam diameter. Further insight on the physics of the MAPLE-DW process was obtained by developing a theoretical model for bubble growth based on a simple force balance associating vapor-pocket pressure and viscous forces. Primary parameters specifying the subsequent differential equation were related to the above process variables. Numerical solutions to the differential equation were used to predict successfully bubble growth versus time for the conditions analyzed in the imaging experiments.
Electron mobility in nanocrystalline silicon devices
Authors: Stieler Daniel , Dalal Vikram L. , Muthukrishnan Kamal , Noack Max , Schares Eric
Published: August 2006

Electron mobility in the growth direction was measured using space charge limited current techniques in device-type nin structure nanocrystalline Si:H and nanocrystalline Ge:H structures. The films were grown on stainless steel foil using either hot wire or remote plasma enhanced chemical vapor deposition techniques. Grain size and crystallinity were measured using x ray and Raman spectroscopy. The size of grains in films was adjusted by changing the deposition conditions. It was found that large ⟨220⟩ grain sizes (∼56nm) could be obtained using the hot wire deposition technique, and the conductivity mobility at room temperature was measured to be 5.4cm2∕Vs in films with such large grains. The plasma-grown films had smaller grains and smaller mobilities. The mobility was found to increase with increasing grain size and with increasing temperature.
Transition between N- and Z-shaped current-voltage characteristics in semiconductor multiple-quantum-well structures
Authors: Pupysheva O. V. , Dmitriev A. V. , Farajian A. A. , Mizuseki H. , Kawazoe Y.
Published: August 2006

We study theoretically the vertical electron transport in semiconductor multiple-quantum-well structures, where sequential tunneling between neighboring wells takes place. The nonuniformity of electric field along the growth axis and charge redistribution among the quantum wells, as well as between the inner wells and contacts, are taken into account. A simple and efficient model of charged contact layers is proposed. The calculated I-V curves exhibit regions of conventional N-shaped negative differential conductivity and Z-shaped portions of intrinsic bistability, both arising due to the tunneling resonances. A general explanation of their formation mechanism is given, which is valid for any form of interwell transitions of resonant nature. The conditions of N- and Z-shaped curve observation and controllable transition between them are discussed.
Unusual distribution of the diffraction patterns from liquid surface waves
Authors: Dong Jun , Miao Runcai , Qi Jianxia , Li Fangju
Published: August 2006

Light diffraction from liquid surface acoustic wave at low frequency was experimentally realized. In the experiment, steady and visible diffraction patterns were obtained, and obvious asymmetry of patterns was observed. The approximate conditions of the surface wave diffraction and the damped wave model were theoretically analyzed, and the corresponding analytic expression of the displacement of the nth-order diffraction component to the center of the patterns was derived. This explained the asymmetrical distribution well. The theoretical and experimental results are in good agreement.
Size dependence of energetic properties in nanowire-based energetic materials
Authors: Menon L. , Aurongzeb D. , Patibandla S. , Bhargava Ram K. , Richter C. , Sacco A
Published: August 2006

We prepared nanowire-array-based thin film energetic nanocomposites based on Al–Fe2O3. The ignition properties as a function of wire dimensions and interwire spacing have been investigated. We show significant variations in ignition behavior, which we relate to the kinetic and heat transfer dynamics of the various configurations studied. Our results indicate the possibility for nanoscale control of reaction parameters such as flame temperature and burn rate in such composites for optimized configurations (optimum wire size, interwire spacing, film thickness, etc.).
Ferroelectric properties of PbxSr1−xTiO3 and its compositionally graded thin films grown on the highly oriented LaNiO3 buffered Pt∕Ti∕SiO2∕Si substrates
Authors: Zhai Jiwei , Yao Xi , Xu Zhengkui , Chen Haydn
Published: August 2006

Thin films of ferroelectric PbxSr1−xTiO3 (PST) with x=0.3–0.7 and graded composition were fabricated on LaNiO3 buffered Pt∕Ti∕SiO2∕Si substrates by a sol-gel deposition method. The thin films crystallized into a single perovskite structure and exhibited highly (100) preferred orientation after postdeposition annealing at 650°C. The grain size of PST thin films systematically decreased with the increase of Sr content. Dielectric and ferroelectric properties were investigated as a function of temperature, frequency, and dc applied field. Pb0.6Sr0.4TiO3 films showed a dominant voltage dependence of dielectric constant with a high tunability in a temperature range of 25–230°C. The compositionally graded PST thin films with x=0.3–0.6 also showed the high tunability. The graded thin films exhibited a diffused phase transition accompanied by a diffused peak in the temperature variations of dielectric constants. This kind of thin films has a potential in a fabrication of a temperature stable tunable device.
Mesoscopic domain average engineering simulations of ferroelectric perovskites compared with multidomain group theoretical predictions
Authors: Keith J. Brandon , Hatch Dorian M.
Published: August 2006

Engineered, multidomain ferroelectrics are of current interest for mechatronic and optical applications. Previously [D. M. Hatch et al., J. Appl. Phys. 94, 5220 (2003)], we presented a set of tables/symmetry rules showing all possible domain average engineered multidomain structures arising from [100]-, [110]-, and [111]-ordering ferroelectric transitions from a Pm3¯m perovskite structure. In this work we test those predictions against three-dimensional mesoscopic fast quench simulations under various electric/stress field combinations and find excellent agreement with multidomain group theory. These simulations use a Langevin strain-displacement/polarization evolution strategy and are important to developing processing conditions for materials with precise domain structures.
Ni
∕
Au
contact to silicon quantum dot light-emitting diodes for the enhancement of carrier injection and light extraction efficiency
Authors: Kim Baek-Hyun , Cho Chang-Hee , Park Seong-Ju , Park Nae-Man , Sung Gun Yong
Published: August 2006

The effect of Ni∕Au metal contact on the carrier injection and the electroluminescence of silicon quantum dot light-emitting diodes (LEDs) was investigated. An LED with an annealed Ni∕Au contact at 400°C in air showed a lower threshold voltage compared to that of an as-deposited Ni∕Au contact by forming a nickel silicide, which has a lower work function than Ni at the interface between metal layers and silicon nitride. The optical output power of the LED with the annealed Ni∕Au contact was also increased due to a highly transparent NiO layer and a lowly resistant Au layer.
Quantifying oxygen diffusion in ZnO nanobelt
Authors: Liu Jin , Gao Puxian , Mai Wenjie , Lao Changshi , Wang Zhong L. , Tummala Rao
Published: August 2006

A method is presented for quantifying oxygen diffusion behavior in a nanodevice fabricated using individual ZnO nanowire/nanobelt. A nanodevice was built using a single nanobelt. Defects are introduced into nanobelt during specific nanofabrication procedure. Then, after the device being exposed to atmosphere for several days, oxygen in air diffused into the nanobelt and significantly changed the conductivity of the device. By comprising the experimentally measured conductivity and that of simulated result, the diffusion coefficient of oxygen in ZnO nanowires/nanobelts has been derived.
Fe adatoms along Bi nanolines on H∕Si(001)
Authors: Orellana W. , Miwa R. H.
Published: August 2006

The stability and electronic and magnetic properties of Fe atoms adsorbed on the self-assembled Bi-line nanostructure on the H∕Si(001) surface are addressed by spin-density functional calculations. Our results show that Fe adatoms are much more stable on sites closer to the Bi nanolines suggesting that they form one-dimensional atomic arrays. The most stable structure occurs on a missing dimer line beside the Bi dimers, which corresponds to an array with distances between Fe adatoms of about 8Å. In this array the irons are coupled antiferromagnetically with spin magnetic moment of about 1.5μB per Fe atom, whereas the coupling exchange interactions is found to be of about 14.4meV. We also estimate a large magnetic anisotropy energy of 3meV∕at. originated on the structural anisotropy of the Fe-adatom site. In addition, the electronic band structure of the Fe array at the most stable structure shows a magnetic half-metal behavior.