bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of nanopatterned permalloy electrodes for a lateral hybrid spin-valve structure
Authors: Last T. , Hacia S. , Wahle M. , Fischer S. F. , Kunze U.
Published: November 2004

Ferromagnetic electrodes of a lateral semiconductor-based spin-valve structure are designed to provide a maximum of spin-polarized injection current. A single-domain state in remanence is a prerequisite obtained by nanopatterning permalloy thin film electrodes. Three regimes of aspect ratios m are identified by room temperature magnetic force microscopy: (i) high-aspect ratios of m⩾20 provide the favored remanent single-domain magnetization states, (ii) medium-aspect ratios m∼3–m∼20 yield highly remanent states with closure domains, and (iii) low-aspect ratios of m⩽3 lead to multidomain structures. Lateral kinks, introduced to bridge the gap between microscale and macroscale, disturb the uniform magnetization of electrodes with high- and medium-aspect ratios. However, sloped regions (vertical flanks) help to maintain a uniformly magnetized state at the ferromagnet-semiconductor contact by domain wall pinning.
Analysis of nitrogen plasma generated by a pulsed plasma system near atmospheric pressure
Authors: Hayakawa R. , Yoshimura T. , Ashida A. , Fujimura N. , Kitahata H. , Yuasa M.
Published: November 2004

Stable discharging of pure nitrogen can be maintained even at atmospheric pressure when alternative pulsed voltage is applied between two parallel plate electrodes. We evaluated the nitrogen plasma generated by an alternative pulsed voltage system. The excited nitrogen species in a pure nitrogen plasma was evaluated using optical emission spectroscopy. In the discharging space, the largest peak detected corresponded to the N2 second positive system. Additionally, the emission peaks from the excited nitrogen atom were detected at a pressure of as high as 45Torr. In the downstream space, we detected emission peaks from the N2 Herman’s infrared system as well as the N2 second positive system. The N2 (AΣu+3) state is considered to be the origin of the N2 Herman’s infrared system. The emission intensities from the N2 second positive system and the N2 Herman’s infrared system increase with increasing nitrogen gas pressure, whereas the emission intensity from the N2+ first negative system decreases.
Growth of InGaAsP based asymmetric quantum well infrared photodetector using metalorganic vapor phase epitaxy
Authors: Li H. , Mei T. , Lantz K. P. , Karunasiri G.
Published: November 2004

A lattice-matched InyGa1−yAszP1−z∕InwGa1−wAsvP1−v∕InxGa1−xAs asymmetric step quantum well infrared photodetector grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) using N2 carrier with tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) is reported. The spectral responsivity of the detector has its peak at a wavelength of 10.7 μm with a peak responsivity of 0.19 A/W under 0.8 V bias at 25 K. A maximum peak detectivity of 1.9×109cmHz1∕2∕W was achieved under 0.6 V bias at 25 K. The measured activation energy using thermionic emission of carriers is found to be about 81 meV. This work demonstrates the fabrication of InP based quantum well infrared detectors using MOVPE with TBA and TBP sources with performance comparable to that achieved using molecular beam epitaxy.
Threshold voltage shift in organic field effect transistors by dipole monolayers on the gate insulator
Authors: Pernstich K. P. , Haas S. , Oberhoff D. , Goldmann C. , Gundlach D. J. , Batlogg B. , Rashid A. N. , Schitter G.
Published: November 2004

We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self-assembled monolayer (SAM). The observed shifts of the transfer characteristics range from −2to50V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4×1012∕cm2. In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1×1012∕cm2.
Structural, magnetic, and transport properties of full-Heusler alloy Co2(Cr1−xFex)Al thin films
Authors: Okamura S. , Goto R. , Sugimoto S. , Tezuka N. , Inomata K.
Published: November 2004

The structural, magnetic, and transport properties are investigated for full-Heusler alloy Co2(Cr1−xFex)Al (CCFA) thin films sputtered on thermally oxidized Si substrates at room temperature (RT). X-ray diffraction reveals that the films possess the B2 structure for x=0, decrease the atomic site ordering by substituting Fe for Cr(0.4≦x≦0.6), and form the A2 structure for x=1. Both the magnetic moment and the Curie temperature of the films increase with increasing Fe content (x), although the moment for x<1 is significantly smaller than that of the calculated value for the L21 structure. Magnetic tunnel junctions (MTJs) with CCFA films as either upper or bottom ferromagnetic layers are also fabricated by using metal masks. The maximum tunneling magnetoresistance (TMR) at RT for the MTJ was observed to be 19.1% for x=0.4 with the CCFA film as an upper ferromagnetic layer, despite the atomic disorder of the CCFA film. This is consistent with our previous TMR observation with the CCFA film as a bottom ferromagnetic layer, indicating that the CCFA film even with the B2 structure is a good candidate for future spintronic devices due to its disorder tolerance, high reproducibility, and large spin polarization.
Effect of a Ge interlayer on the high-temperature behavior of NiSi films
Authors: He Y. , Liu X. L. , Feng J. Y. , Wu Q. L.
Published: November 2004

The effect of a thin Ge interlayer on the formation of Ni silicides on (100)Si substrates has been investigated. X-ray diffraction shows a remarkable increase of the nucleation temperature of NiSi2 in the presence of the Ge interlayer. Four-probe measurements show that the sheet resistance of silicide formed in Ni∕Ge∕Si system remains stable up to 850 °C, while the sheet resistance of silicide formed in Ni∕Si system presents a significant increase at 750 °C. Scanning electron microscopy indicates that island formation is not observed in the NiSi film grown on Ge∕Si substrate annealing at 800 °C. The classical nucleation theory is employed to explain the increased temperature of the nucleation of NiSi2 in the Ni∕Ge-Si system.
Piezoelectric performances of lead-reduced (1−x)(Bi0.9La0.1)(Ga0.05Fe0.95)O3-x(Pb0.9Ba0.1)TiO3 crystalline solutions in the morphotropic phase boundary
Authors: Cheng Jinrong , Meng Zhongyan , Eric Cross L.
Published: November 2004

In this investigation, (1−x)(Bi0.9La0.1)(Ga0.05Fe0.95)O3-x(Pb0.9Ba0.1)TiO3(BLGF-PBT) crystalline solutions have been fabricated by using the solid-state reaction method. A ferroelectric rhombohedral/tetragonal morphotropic phase boundary (MPB) of (1−x)BLGF-xPBT was observed for x=0.4. In the vicinity of MPB, 0.6BLGF-0.4PBT revealed the maximum dielectric constant K and piezoelectric d33 constant of 1168 and 186pC∕N, respectively. The substitution of 10at.% Ba for Pb dramatically increased K and d33 of (1−x)BLGF-xPBT relative to (1−x)BLGF-xPT at the same x content. The Curie temperature Tc of (1−x)BLGF-xPBT was determined to be above 386°C through the compositions investigated. The phase diagram of (1−x)BLGF-xPBT revealed a V-shaped relationship between Tc and x content. The planar coupling coefficient Kp was measured to be 0.37 for 0.6BLGF-0.4PBT, which was stable with increasing the measurement temperature until 170°C. It is demonstrated that BLGF-PBT is a competitive alternative piezoelectric material, with the superior piezoelectricity and lead-reduced composition.
Modeling of anomalous shift and asymmetric hysteresis behavior of ferroelectric thin films
Authors: Wong C. K. , Shin F. G.
Published: November 2004

An analytical bilayer model has been developed to consider the effect of the existence of a dead layer (e.g., due to polarization degradation) at the film-electrode interface in an otherwise homogeneous ferroelectric thin film. By introducing asymmetric conductivity in the dead layer, the anomalous horizontal (along the field axis) shift behavior of hysteresis loops in ferroelectric thin films is successfully reproduced. Assuming that the ferroelectric P-E hysteresis loops of the layers are parallelogramlike, explicit expressions are derived for calculating the internal fields in the film, as well as the “apparent” D-E loop as measured from a Sawyer-Tower circuit. The general switching sequence for the ferroelectric phases will be considered. Using the ferroelectric-ferroelectric bilayer model, other anomalous phenomena, including vertical shift and deformed loop shape are also modeled.
Tensile failure of water due to shock wave interactions
Authors: Boteler J. M. , Sutherland G. T.
Published: November 2004

A series of low stress shock impact experiments were performed on water to examine the dynamic response under tension and establish a lower bound for water rupture or cavitation threshold. The experimental cell configuration permitted particle velocity measurements at the water-air free surface separated by a 5-μm-thick aluminized Mylar diaphragm. Water samples were triply distilled, de-ionized, and degassed prior to experiments. The average tensile strength for shock-induced cavitation in the water was found to be 8.7±0.2MPa. Experiments are compared with hydrocode simulations using a simple fracture criterion and published experimental data.
X-ray diffraction and extended x-ray absorption fine-structure characterization of nonspherical crystallographic grains in iron thin films
Authors: Jiménez-Villacorta F. , Muñoz-Martín A. , Prieto C.
Published: November 2004

The characterization of iron thin films grown at different substrate temperatures has been performed by x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS). The film growing of iron at low temperatures provides an excellent system to test the results obtained from both techniques because the crystallographic grains present a variation of size and shape as a function of the growing temperature. In both cases, the shape of the particle must be taken into account to calculate their size. The comparison gives a very good agreement when appropriate models are used, showing the reasons for possible differences between the results obtained from a more simple XRD and EXAFS analysis applied to columnar growth systems.
The role of C2 in nanocrystalline diamond growth
Authors: Rabeau J. R. , John P. , Wilson J. I.B. , Fan Y.
Published: November 2004

This paper presents findings from a study of nanocrystalline diamond (NCD) growth in a microwave plasma chemical vapor deposition reactor. NCD films were grown using Ar∕H2∕CH4 and He∕H2∕CH4 gas compositions. The resulting films were characterized using Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. Analysis revealed an estimated grain size of the order of 50nm, growth rates in the range 0.01–0.3μm∕h, and sp3- and sp2-bonded carbon content consistent with that expected for NCD. The C2 Swan band (dΠg3↔aΠu3) was probed using cavity ring-down spectroscopy to measure the absolute C2(a) number density in the plasma during diamond film growth. The number density in the Ar∕H2∕CH4 plasmas was in the range from 2to4×1012cm−3, but found to be present in quantities too low to measure in the He∕H2∕CH4 plasmas. Optical emission spectrometry was employed to determine the relative densities of the C2 excited state (d) in the plasma. The fact that similar NCD material was grown whether using Ar or He as the carrier gas suggests that C2 does not play a major role in the growth of nanocrystalline diamond.
Phonon spectra of terbium-doped lutetium orthophosphates
Authors: Moura M. R. , Ayala A. P. , Guedes I. , Loong C.-K. , Boatner L. A.
Published: November 2004

Raman spectroscopy has been applied to investigations of the effects of substitutionally replacing Lu by Tb in single crystals of the tetragonal, zircon structure host LuPO4. Room-temperature, polarized Raman spectra of Lu1−xTbxPO4 (x=0, 0.01, 0.1, and 0.12) have been studied. No appreciable changes in the frequencies of the phonons were observed, indicating that the force fields in the LuPO4 lattice are not strongly altered by Tb doping on the Lu site. A discussion of the fluorescence bands and a revised assignment of the LuPO4 vibrations are presented.
Nanomechanical properties studied by atomic force microscopy in combination with an inverse methodology
Authors: Chang Win-Jin , Fang Te-Hua
Published: November 2004

This study presents a method for calculating the applied force during the nanoindentation process using atomic force microscope (AFM). The determination of the applied force in the nanoindentation system is regarded as an inverse vibration problem. The conjugate gradient method is applied to treat the inverse problem using available displacement measurements. Initially, the nanoindentation force should be applied and then the nanomechanical properties of the thin film, including the elastic modulus and the hardness, can be obtained using the geometric relationships between the indenter, the applied force, and the penetration depth. Without using the inverse methodology the nanomechanical properties of ultrathin coatings or films are very difficult to be obtained with precision using the nanoindentation equipment or solely using AFM. This proposed method is useful for designing a surface measurement system for nanostructured materials.
InAs quantum dots on GaAs(1¯1¯2¯)B
Authors: Suzuki T. , Temko Y. , Xu M. C. , Jacobi K.
Published: November 2004

InAs quantum dots (QDs) were prepared by molecular beam epitaxy on GaAs(1¯1¯2¯)B substrates. Shape and size distribution of the QDs were investigated using in situ scanning tunneling microscopy as function of preparation temperature between 435 and 550°C. The wetting layer is not flat but undulated in submicrometer scale in a similar way as the bare substrate. The atomic structure of the wetting layer is the same as found for the flat base of InAs QDs grown on GaAs(1¯1¯3¯)B substrates. The shape of the QDs is given by {110}, (1¯1¯1¯)B, and {1¯4¯3¯}B bounding facets and a round vicinal (001¯) region. Unexpectedly, the number density increases and the size distribution sharpens, when the growth temperature is increased from 435to470°C, which is attributed to lattice defects incorporated into the QDs during growth at 435°C.
Surface kinetics and subplantation phenomena affecting the texture, morphology, stress, and growth evolution of titanium nitride films
Authors: Patsalas P. , Gravalidis C. , Logothetidis S.
Published: November 2004

We present a thorough study of the microstructure, texture, intrinsic stress, surface, and interface morphology of transition metal nitride (mainly TiN but also CrN) films grown on Si by reactive sputter deposition, with emphasis to the mechanisms of adatom migration on the surface and subplantation of energetic species. In order to study the effects of adatom mobility and the subplantation probability we vary the ion energy and growth temperature. For the experimental part of this work we used nondestructive, statistically reliable x-ray techniques (diffraction, reflectivity, scattering). The x-ray results are compared and correlated with supporting data of in situ spectroscopic ellipsometry as well as Monte Carlo simulations of the irradiation effects and surface diffusion of adatoms. We found that the texture and the surface and interface morphology are sensitive to the mechanism of dissipation of the impinging ions. If the energy is enough to overcome the subplantation threshold (∼50eV), then the films are highly compressed and exhibit ultrasmooth surfaces and rough interfaces. In this case, the texture of the films is not affected much by the ion energy, since the energy is dissipated in the bulk and contributes less to the surface mobility of adatoms. On the other hand, when the ion energy is below the subplantation threshold the texture of the films strongly depends on the ion energy and flux, the interfaces are atomically sharp and the surface morphology depends on the mobility and surface diffusion length of adatoms. However, in both cases these effects are dominant at the homogeneous growth. At the initial stages of nucleation and island growth the differences in the growth due to irradiation conditions are not pronounced and the thermodynamics of wetting of TiN on Si are prevailing factors.
Thermophysical analysis of thin films by lock-in thermography
Authors: Wolf A. , Pohl P. , Brendel R.
Published: November 2004

A method for the determination of the thermophysical properties of thin films is introduced. The procedure is based on the contact-free thermal analysis of free standing thin films by means of a lock-in thermography system. The thermophysical properties are deduced from the thermal diffusion length and the temperature amplitude of a thermal wave propagating in the sample excited by a laser heat source. An infrared camera images the wave and the thermal diffusion length Λa of the amplitude and the thermal diffusion length Λp of the phase are measured. Thermal losses influence both Λa and Λp. Their geometric mean ΛaΛp is, however, not effected by thermal losses. In turn 1∕Λa2−1∕Λp2 is determined by the thermal losses and does not depend on the lock-in frequency, as Λa and Λp do. These theoretical predictions are experimentally verified in this work. The measured values of Λa and Λp yield the in-plane thermal diffusivity and a damping factor. The latter quantifies the thermal losses to the ambient. The use of a vacuum chamber and a temperature calibration are not required. If, however, the camera is calibrated, the in-plane thermal conductivity and the volumetric heat capacity are obtained from the temperature amplitude of the thermal wave. The measurement accuracy is 10% for the thermal diffusivity, thermal conductivity, and volumetric specific heat. The thermophysical properties measured for thin films of copper, nickel silver, and polyimide agree with literature values.
Electric-field poling in Mg-doped LiNbO3
Authors: Mizuuchi K. , Morikawa A. , Sugita T. , Yamamoto K.
Published: November 2004

We report on electric-field poling of MgO:LiNbO3 (MgLN) using patterned electrodes. Investigation of electric properties of MgLN reveals that polarization switching causes reversible resistance change and that this phenomenon can explain the physical mechanism of random domain growth. Based on these results, we propose a domain-control method in MgLN single crystals by suppressing resistance reduction during the poling process. Using this method, short periodic structures consisting of submicron domain geometries have been achieved in X-, Y-, and Z-cut MgLN crystals. High performances of these periodic domain-inverted structures are also demonstrated by evaluating their potentials as second-harmonic generation devices.
Hydrogen content estimation of hydrogenated amorphous carbon by visible Raman spectroscopy
Authors: Adamopoulos G. , Robertson J. , Morrison N. A. , Godet C.
Published: November 2004

In the present study, we report the hydrogen content estimation of the hydrogenated amorphous carbon (a-C:H) films using visible Raman spectroscopy in a fast and nondestructive way. Hydrogenated diamondlike carbon films were deposited by the plasma enhanced chemical vapor deposition, plasma beam source, and integrated distributed electron cyclotron resonance techniques. Methane and acetylene were used as source gases resulting in different hydrogen content and sp2∕sp3 fraction. Ultraviolet-visible (UV-Vis) spectroscopic ellipsometry (1.5–5eV) as well as UV-Vis spectroscopy were provided with the optical band gap (Tauc gap). The sp2∕sp3 fraction and the hydrogen content were independently estimated by electron energy loss spectroscopy and elastic recoil detection analysis-Rutherford back scattering, respectively. The Raman spectra that were acquired in the visible region using the 488nm line shows the superposition of Raman features on a photoluminescence (PL) background. The direct relationship of the sp2 content and the optical band gap has been confirmed. The difference in the PL background for samples of the same optical band gap (sp2 content) and different hydrogen content was demonstrated and an empirical relationship between the visible Raman spectra PL background slope and the corresponding hydrogen content was extracted.
Minority-spin band parameters in a NiMnSb thin film determined by spectral conductivity
Authors: Grigorescu C. E. A. , Trodahl H. J. , Strickland N. M. , Bittar A. , Manea S. A. , Giapintzakis J. , Monnereau O. , Notonier R. , Kennedy V. J.
Published: November 2004

NiMnSb is expected to be a ferromagnetic half metal, an expectation that is based in part on band structure calculations. Here we report optical conductivity studies of the band structure for a film prepared by pulsed laser deposition onto a Si substrate held at a relatively low temperature as is required for some device applications—films which are susceptible to site disorder associated with the vacant site in this half-Heusler compound. We demonstrate that the direct interband transitions are essentially unshifted in comparison with bulk material, though they are somewhat broadened. Below the direct-transition absorption edge we report the presence of indirect spin-reversing transitions between the Fermi energy (Ef) and the extrema of the minority-spin valence and conduction bands, providing a measure of the band edge energies. Both of these edges appear closer to Ef than is seen in well-ordered bulk NiMnSb, with the conduction-band minimum showing weight at only 200cm−1 above Ef, close enough to have substantial occupation at ambient temperature.
Large-angle magnetization dynamics investigated by vector-resolved magnetization-induced optical second-harmonic generation
Authors: Gerrits Th. , Silva T. J. , Nibarger J. P. , Rasing Th.
Published: November 2004

We examine the relationship between nonlinear magnetic responses and the change in the Gilbert damping parameter α for patterned and unpatterned thin Permalloy films when subjected to pulsed magnetic fields. An improved magnetization-vector-resolved technique utilizing magnetization-induced optical second-harmonic generation was used to measure magnetization dynamics after pulsed-field excitation. The magnetization excitations were achieved with pulsed fields aligned parallel to the hard axis of thin permalloy (Ni80Fe20) films while a dc bias field is applied along the easy axis. At low bias fields, α was inversely related to the bias field, but there was no significant reduction in the absolute value of the magnetization, as might be expected if there was significant spin-wave generation during the damping process. We discuss the discrepancies between data obtained by ferromagnetic resonance, whereby spin-wave generation is prevalent, and pulsed-field studies, with the conclusion that fundamental differences between the two techniques for the excitation of the ferromagnetic spin system might explain the different proclivities toward spin-wave generation manifest in these two experimental methods.
Influence of regrowth conditions on the hole mobility in strained Ge heterostructures produced by hybrid epitaxy
Authors: Morris R. J. H. , Leadley D. R. , Hammond R. , Grasby T. J. , Whall T. E. , Parker E. H. C.
Published: November 2004

Strained Ge p-channel heterostructures have been produced using a hybrid-epitaxy method, which allows the advantages offered by different growth techniques to be exploited. Chemical vapor deposition of thick strain-tuned virtual substrates has been combined with growth of the active layers by solid-source molecular beam epitaxy. This paper discusses optimization of the regrowth conditions, to achieve a high hole mobility, and correlates structural characterization with electrical measurements. Initial ex situ chemical cleaning of the virtual substrate was found to be essential for successful regrowth. Structural analysis, using cross-sectional transmission electron microscopy and atomic force microscopy, showed that the regrowth temperature significantly affects the growth mode of the active layers and that planar growth was only achieved below 400°C. Samples with Ge channels from 8to30nm thick were analyzed with plan view transmission electron microscopy to study the formation of misfit dislocations and estimate the degree of relaxation—two effects detrimental to hole transport properties. For intermediate thickness layers, postgrowth annealing at 650°C was found to significantly improve the hole mobility, by eliminating point defects but not leading to substantial relaxation. As a result, the mobility was found to almost double at room temperature and increase fourfold at 10K. The level of diffusion and interface integrity, for both the as-grown and annealed structures, has been investigated using low energy secondary ion mass spectrometry.
Observation of silicon front surface topographs of an ultralarge-scale-integrated wafer by synchrotron x-ray plane wave
Authors: Suzuki Yoshifumi , Tsukasaki Yoshimitsu , Kajiwara Kentaro , Kawado Seiji , Iida Satoshi , Chikaura Yoshinori
Published: November 2004

Surface roughness and undulation of unpatterned silicon wafers are serious issues for ultralarge-scale-integrated circuit devices, even after fine mechanochemical polishing. It has never been clarified whether the undulations exist only on the surface or also exist inside the bulk crystal. We produced grazing incident diffraction topographs at three x-ray photon energies, with penetration depths estimated to be 3.85 nm, 4.78 nm, and 1.28 μm. All the topographs contained striation. We also obtained clear total reflection images using synchrotron x-ray plane waves, which also showed striation patterns at penetration depths from 3.85 nm to 1.28 μm. These results indicate that the origin of the patterns is not at the surface but is inside the Si wafer. The origin of striation patterns, observed in the topographs, was found not to be due to mechanochemical polishing processes but to crystal growth.
Confined optical modes in small photonic molecules with semiconductor nanocrystals
Authors: Rakovich Yu. P. , Gerlach M. , Bradley A. L. , Donegan J. F. , Connolly T. M. , Boland J. J. , Przyjalgowski M. A. , Ryder A. , Gaponik N. , Rogach A. L.
Published: November 2004

We report on the coherent coupling of whispering gallery modes (WGMs) in a photonic molecule formed from two melamine-formaldehyde spherical microcavities coated with a thin shell of light-emitting CdTe nanocrystals (NCs). Utilizing different excitation conditions, the splitting of the WGM resonances originating from bonding and antibonding branches of the photonic states is observed, and fine structure consisting of very sharp peaks resulting from lifting of the WGM degeneracy has been detected. Time-resolved measurements showed a slight increase in the spontaneous emission rate of NCs in a photonic molecule when compared to the spontaneous emission rate for NCs coating a single microsphere.
Self-consistency of Vuks equations for liquid-crystal refractive indices
Authors: Li Jun , Wu Shin-Tson
Published: November 2004

Vuks equations correlate the microscopic molecular polarizabilities to the macroscopic refractive indices of anisotropic crystalline materials. For anisotropic liquid crystals, the molecular polarizabilities are difficult to measure directly due to the short- and long-range interactions. We have converted Vuks equations into different forms. By measuring the refractive indices at different temperatures and wavelengths, the Vuks equations can be validated. Five liquid-crystal materials with refractive index ranging from ∼1.46 to ∼1.86 are used to validate the modified Vuks equations. The experimental results agree with the theory very well. Based on the Vuks equations, the molecular polarizabilities of 4-cyano-4-n-pentylbiphenyl are calculated.
Suppression of skin effect in metal∕ferromagnet superlattice conductors
Authors: Rejaei Behzad , Vroubel Marina
Published: November 2004

We propose and theoretically investigate a superlattice consisting of alternating normal-metal and ferromagnetic layers as a low-loss conductor for realization of planar-integrated radio-frequency devices. At sufficiently high frequencies, the negative permeability of the ferromagnetic films effectively compensates the positive permeability of the nonmagnetic metal layers, leading to an overall suppression of the skin effect. Simulations based on realistic material parameters demonstrate the effectiveness of this approach regardless of intrinsic relaxation losses in the magnetic films.
A diodelike conduction model for the postbreakdown current in metal–oxide–semiconductor structures
Authors: Miranda E.
Published: November 2004

The postbreakdown current in metal-oxide-semiconductor structures with ultrathin silicon dioxide layers is investigated within the framework of a diodelike conduction model with high series resistance. The diodelike behavior is ascribed to the contact effect between the semiconductor electrodes, and the series resistance is mainly attributed to the crowding of the current lines or funneling of the energy states at the constriction which represents the breakdown path. It is shown how this model is able to capture some distinctive features exhibited by the normalized differential conductance and how the model parameters can be extracted from the experimental characteristics without invoking the solution of the generalized diode equation.
High-field dielectric and piezoelectric performance of soft lead zirconate titanate piezoceramics under combined electromechanical loading
Authors: Zhou Dayu , Kamlah Marc
Published: November 2004

Piezoelectric actuators normally have complicated structures and work under severe loading conditions, e.g., high driving electric field and significant compressive preload. This study is focused on the experimental investigation of the electromechanical properties of a commercial soft lead zirconate titanate material under loading conditions simulating the in-service environment of high-strain actuators. The polarization and strain responses were first measured under a constant-stress preload. A significant enhancement of the dielectric and piezoelectric performance is observed within a small prestress range. At much higher preload levels, the predominant mechanical depolarization effect makes the material exhibits hardly any piezoeffect. In the other two series of tests, the specimen was subjected to cyclic mechanical load with different mean stresses and amplitudes. When the stress is applied in-phase with electrical loading, the polarization and strain outputs are found to monotonically decrease with an increase in stress amplitude, until mechanical loading completely impedes the piezoelectric response. An inverse effect occurs for the out-of-phase electromechanical loading tests, in which the polarization and strain outputs increase with increasing stress amplitude. In general, the enhanced polarization and strain responses are accompanied by an unfavorable increased hysteresis and, consequently, increased energy loss. An attempt has been made to explain the experimental findings by simultaneously taking into account the effects of dielectric response, elastic deformation, irreversible domain switching, and piezoeffects.
The origin of slow electron recombination processes in dye-sensitized solar cells with alumina barrier coatings
Authors: Fabregat-Santiago Francisco , García-Cañadas Jorge , Palomares Emilio , Clifford John N. , Haque Saif A. , Durrant James R. , Garcia-Belmonte Germà , Bisquert Juan
Published: November 2004

We investigate the effect of a thin alumina coating of nanocrystalline TiO2 films on recombination dynamics of dye-sensitized solar cells. Both coated and uncoated cells were measured by a combination of techniques: transient absorption spectroscopy, electrochemical impedance spectroscopy, and open-circuit voltage decay. It is found that the alumina barrier reduces the recombination of photoinjected electrons to both dye cations and the oxidized redox couple. It is proposed that this observed retardation can be attributed primarily to two effects: almost complete passivation of surface trap states in TiO2 that are able to inject electrons to acceptor species, and slowing down by a factor of 3–4 the rate of interfacial charge transfer from conduction-band states.
Hot-phonon temperature and lifetime in biased 4H-SiC
Authors: Matulionis A. , Liberis J. , Matulionienė I. , Cha H.-Y. , Eastman L. F. , Spencer M. G.
Published: November 2004

Experimental results on electron transport and microwave noise are used to investigate, in the electron temperature approximation, dissipation of hot-electron energy in a biased donor-doped 4H-SiC channel (n=2∙1017cm−3). The electron energy relaxation time is estimated to be 2.8±1ps in the electric-field range below 25kV∕cm at room temperature. Longitudinal-optical phonons are found responsible for electron energy dissipation when the supplied power ranges from 0.5to25nW per electron. In this range, accumulation of nonequilibrium (hot) longitudinal-optical phonons takes place. Equivalent hot-phonon temperature reaches 3000K near 30kV∕cm, hot-phonon lifetime is comparable to the electron energy relaxation time. Dependence of the lifetime on electric field is weak in the investigated range of electric fields below 50kV∕cm.
Propagation characteristics of surface-plasmon waveguides operating in the mid- and far infrared: Nonperturbative approach
Authors: Zietkowski W. , Załużny M.
Published: November 2004

The modal propagation characteristics of metal surface-plasmon waveguides (MSPWs) containing n-doped multiple quantum wells with normal and inverted subband occupation are studied theoretically. “Ordinary” (“enhanced”) waveguides modeled by simple three (four) media structures are considered. The appropriate dispersion equations are derived employing the transfer-matrix approach and solved numerically. Analytical solutions are also obtained employing the thin-layer approximation. Special attention is paid to the role of the coupling between the modes guided by passive MSPWs and the intersubband plasmon. The obtained results indicate that modification of the propagation characteristics induced by the above-mentioned coupling plays a very important role. We show that the commonly used perturbed approach based on the concept of the confinement factor has a restricted range of applicability, particularly in the case of the ordinary MSPWs.
Monodisperse nanoparticles via metal induced crystallization
Authors: Chandra A. , Clemens B. M.
Published: November 2004

This paper discusses the self-assembled formation of monodisperse gold-rich nanoparticles and associated crystalline silicon nanostructures. Multilayer films comprising of amorphous Au25Si75 and amorphous silicon were grown via dc magnetron sputtering and subsequently annealed under varying thermal conditions. The films were characterized by electron microscopy before and after the thermal anneal. Thermal decomposition of the multilayer films results in the metal induced crystallization of amorphous silicon, as well as the formation of uniform Au-rich nanoparticles. Further annealing did not alter the size or position of the nanoparticles, indicating that the particles are too small to induce further silicon crystallization. Through thermodynamic modeling, two mechanisms are shown to be viable means for nanoscaled size selection. The first mechanism entails crystallization of Au25Si75 followed by metal induced crystallization of amorphous silicon, while the second utilizes spinodal decomposition of Au25Si75 to select a single nanoparticle radius.
Growth and two-color holographic storage properties of Mn-doped lithium niobate crystals with varying Li∕Nb ratio
Authors: Liu Youwen , Kitamura Kenji , Ravi Ganesan , Takekawa Shunji , Nakamura Masaru , Hatano Hideki
Published: November 2004

Near-stoichiometric LiNbO3 crystals slightly doped with Mn from melts with different compositions are grown by the top seeded solution growth method. The growth parameters such as temperature field, pulling, and rotation rates are optimized. Two-color holograms are recorded in these crystals by the use of a near infrared laser of 778 nm for writing and an ultraviolet laser of 350 nm for gating. The light-induced absorption changes and two-color holographic characteristic performances are presented and compared. The results show that crystals with composition closer to stoichiometry exhibit better two-color holographic storage performance.
Observing sub-microsecond telegraph noise with the radio frequency single electron transistor
Authors: Buehler T. M. , Reilly D. J. , Starrett R. P. , Chan V. C. , Hamilton A. R. , Dzurak A. S. , Clark R. G.
Published: November 2004

Telegraph noise, which originates from the switching of charge between metastable trapping sites, becomes increasingly important as device sizes approach the nanoscale. For charge-based quantum computing, this noise may lead to decoherence and loss of readout fidelity. Here we use a radio frequency single electron transistor (rf-SET) to probe the telegraph noise present in a typical semiconductor-based quantum computer architecture. We frequently observe microsecond telegraph noise, which is a strong function of the local electrostatic potential defined by surface gate biases. We present a method for studying telegraph noise using the rf-SET and show results for a charge trap in which the capture and emission of a single electron is controlled by the bias applied to a surface gate.
Thin polycrystalline diamond for low-energy x-ray detection
Authors: Conte G. , Rossi M. C. , Salvatori S. , Ascarelli P. , Trucchi D.
Published: November 2004

The response of polycrystalline diamond detectors to low-energy x-ray beams is presented. Diamond metal∕semiconductor∕metal vertical structures have been realized to investigate the influence of the electric field in the dark and under x-ray irradiation. Nonlinear contributions to the current, compatible with field-assisted thermal ionization of trapped charges, have been evidenced, as well as the presence of defect-related electronic states acting as traps has been confirmed by subband-gap photoconductivity measurements. Moreover, correlations have been evidenced among irradiation time, variation of the current in the dark, and photoconductive response of the devices to the x-ray beam. An evaluation of the minority-carrier mobility-lifetime μτ product, at the end of the activation process, has led to a value of 9.2×10−7cm2∕V. Two orders of magnitude linear response to the beam intensity variations have been evaluated after removal of the bremsstrahlung radiation contribution on a 43-μm-thick device.
Electrical characteristics and interface structure of magnetic tunnel junctions with hafnium oxyfluoride barrier
Authors: Yu Y. Y. , Kim D. S. , Char K.
Published: November 2004

We have studied the effects of fluorine inclusion on the electrical transport characteristics and interface structure of the hafnium oxide barrier in a magnetic tunnel junction. The tunneling magnetoresistance (TMR) and resistance-area (RA) as a function of oxidation time show that the TMR ratio of the hafnium oxyfluoride barrier is higher (8.3%) than that of the hafnium oxide barrier (5.7%) at their optimum conditions, and the oxyfluoride barrier junctions maintain a high TMR ratio even when the RA product increases by three orders of magnitude. X-ray photoelectron spectroscopy analysis shows that the fluorine atoms in the oxyfluoride barrier play an important role in the formation of a barrier with uniform composition. We believe that the initial fluoride layer is causing the subsequent oxygen diffusion to slow down, resulting in the formation of a defect-free hafnium oxide layer. These results are consistent with what we have found for aluminum oxyfluoride barriers.
Effects of positron concentration, ion temperature, and plasma β value on linear and nonlinear two-dimensional magnetosonic waves in electron–positron–ion plasmas
Authors: Mushtaq A. , Shah H. A.
Published: November 2004

Magnetosonic waves are intensively studied due to their importance in space plasmas and also in fusion plasmas where they are used in particle acceleration and heating experiments. This work considers magnetosonic waves propagating obliquely at an angle θ to an external magnetic field in an electron–positron–ion plasma, using the effective one-fluid magnetohydrodynamic model. Two separate modes (fast and slow) for the waves are discussed in the linear approximation, and the Kadomstev–Petviashvilli soliton equation is derived by using reductive perturbation scheme for these modes in the nonlinear regime. It is observed that for both the modes the angle θ, positron concentration, ion temperature, and plasma β-value affect the propagation properties of solitary waves and behave differently from the simple electron–ion plasmas. Likewise, current density, electric field, and magnetic field for these waves are investigated, for their dependence on the above mentioned parameters.
Spectroscopy of GdAl3(BO3)4:Tm3+ crystal
Authors: Jia G. H. , Tu C. Y. , Li J. F. , Zhu Z. J. , You Z. Y. , Wang Y. , Wu B. C.
Published: November 2004

Tm 3 + : GdAl 3 ( BO 3 ) 4 crystals were grown successfully from K2Mo3O10-B2O3 solvent by using the top seeded solution growth method. The absorption spectra for both σ and π polarizations were presented. The Judd-Ofelt (J-O) intensity parameters were found to be Ω2=4.54×10−20cm2,Ω4=2.25×10−20cm2, and Ω6=3.39×10−20cm2. The spectroscopic parameters of this crystal such as the oscillator strengths, radiative transition probabilities, radiative lifetime as well as the branching ratios were calculated. Unpolarized and polarized emission spectra were obtained under the excitations of 468 and 797 nm, respectively. This crystal may be preferable for GaAlAs laser pumping.
Spectroscopic diagnostics and modeling of Ar∕H2∕CH4 microwave discharges used for nanocrystalline diamond deposition
Authors: Lombardi G. , Hassouni K. , Bénédic F. , Mohasseb F. , Röpcke J. , Gicquel A.
Published: November 2004

In this paper Ar∕H2∕CH4 microwave discharges used for nanocrystalline diamond chemical vapor deposition in a bell-jar cavity reactor were characterized by both experimental and modeling investigations. Discharges containing 1% CH4 and H2 percentages ranging between 2% and 7% were analyzed as a function of the input microwave power under a pressure of 200mbar. Emission spectroscopy and broadband absorption spectroscopy were carried out in the UV-visible spectral range in order to estimate the gas temperature and the C2 density within the plasma. Infrared tunable diode laser absorption spectroscopy was achieved in order to measure the mole fractions of carbon-containing species such as CH4, C2H2, and C2H6. A thermochemical model was developed and used in order to estimate the discharge composition, the gas temperature, and the average electron energy in the frame of a quasihomogeneous plasma assumption. Experiments and calculations yielded consistent results with respect to plasma temperature and composition. A relatively high gas temperature ranging between 3000 and 4000K is found for the investigated discharge conditions. The C2 density estimated from both experiments and modeling are quite high compared with what is generally reported in the literature for the same kind of plasma system. It ranges between 1013 and 1014cm−3 in the investigated power range. Infrared absorption measurements and model predictions indicate quite low densities of methane and acetylene, while the atomic carbon density calculated by the model ranges between 1013 and 1015cm−3. The methane and hydrogen introduced in the feed gas are subject to a strong dissociation, which results in a surprisingly high H-atom population with mole fraction ranging between 0.04 and 0.16. Result analysis shows that the power coupling efficiency would range between 70% and 90%, which may at least explain the relatively high values obtained, as compared with those reported in the literature for similar discharges, for gas temperature and C2 population. The high H-atom densities obtained in this work would indicate that growing nanocrystalline diamond films would experience a very high etching. Simulation results also confirm that sp species would play a key role in the surface chemistry that governs the diamond growth.
X-ray and neutron reflectivity study of solid-supported lipid membranes prepared by spin coating
Authors: Generosi Johanna , Castellano Carlo , Pozzi Daniela , Castellano Agostina Congiu , Felici Roberto , Natali Francesca , Fragneto Giovanna
Published: November 2004

We present a study of x-ray synchrotron radiation and neutron reflectivity on solid-supported lipid membranes prepared by spin coating. This technique has the advantage of allowing the control of the number of lipid layers by varying the deposition parameters. The experiments were performed on the cationic lipid 1,2-dioleoyl-3-trimethylammonium-propane chloride salt (DOTAP), the neutral lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the lipid mixture (DOTAP-DOPC), and the complex (DOTAP-DOPC∕DNA) deposited on wafers. Only single neutral lipids or lipid-peptide mixtures were deposited on solid substrate using the spin coating technique and characterized. Results on the structure of the deposited lipid layers indicate that DNA contributes to the order in the lipoplexes.
Effects of high-energy electron irradiation on heavy-metal fluoride glass
Authors: Jiang Nan , Qiu J. , Silcox J.
Published: November 2004

The effects of high-energy (100keV) electron irradiation have been observed in a heavy-metal fluoride glass using in situ time-resolved electron energy loss spectroscopy in a scanning transmission electron microscope. Formation of F2 has been observed. The irradiation-induced changes of configuration and composition are discussed based on the evolution of the near-edge fine structures of F K edge. We suggest that the high sensitivity to electron irradiation is due to the existence of nonbridging F in heavy-metal fluoride glasses so that is there is a tendency of the electron irradiation to eliminate nonbridging F in the irradiated region.
Elliptic instability in a Rankine vortex with axial flow
Authors: Lacaze Laurent , Birbaud Anne-Laure , Le Dizès Stéphane
Published: November 2004

The elliptic instability of a Rankine vortex with axial flow subject to a weak strain field perpendicular to its axis is analyzed by asymptotic methods in the limit of small strain rate. General unstable modes associated with resonant Kelvin modes of arbitrary azimuthal wavenumbers are considered. Both the effects of axial flow and viscosity are analyzed in details.
Silicon-based honeycomb photonic crystal structures with complete photonic band gap at 1.5μm wavelength
Authors: Ye Jia-Yu , Matsuo Shigeki , Mizeikis Vygantas , Misawa Hiroaki
Published: November 2004

Photonic crystal structures consisting of dielectric cylinders arranged in a honeycomb pattern were fabricated in silicon using electron-beam lithography and inductively-coupled plasma dry-etching. Lattice parameters of the fabricated structures (rod diameter ∼0.4μm, distance between the centers of neighboring rods ∼0.85μm, and rod height ∼5.2μm) allowed to achieve and confirm experimentally the complete photonic band gap in vicinity of the optical communications wavelengths around 1.5μm.