bluuebunny/crossref_metadata_2025 huggingface dataset:
Relation between microstructure and stress in titanium nitride films grown by plasma immersion ion implantation
Authors: Lim S. H. N. , McCulloch D. G. , Bilek M. M. M. , McKenzie D. R.
Published: March 2003

Polycrystalline and microcrystalline materials grown as thin films often exhibit a preferred crystallographic orientation. The mechanism by which this preferred orientation develops has been the subject of some debate. In this article we present the results of electron microscopy and diffraction experiments examining preferred orientation in titanium–nitride (TiN) films deposited by physical vapor deposition with bombardment energies up to 18 keV using plasma immersion ion implantation. We show that as the bombardment energy increases the intrinsic stress is reduced. For example, at 3 kV more than a 50% reduction in stress is achieved. The results are consistent with a model in which the preferred orientation is the result of minimizing the energy, which includes both surface energy and bulk strain energy terms.
Magnetic properties, Mössbauer and aftereffect studies of Pr10Fe90−xBx (x=5.88–11.5) nanocomposites
Authors: Chang H. W. , Chang W. C. , Ho J. C. , Unver M. , Hamdeh H. H.
Published: March 2003

Magnetic properties, phase evolution and aftereffect of melt-spun Pr10Fe90−xBx (x=5.88–11.5) nanocomposite ribbons have been studied. Summarizing the results of thermal magnetic analysis and Mössbauer spectroscopy, two phases, namely Pr2Fe14B and α-Fe, were found for ribbons with x=5.88 and 7.5, while additional two metastable phases, Pr2Fe23B3 and Fe3B, prevailed for x=9. For x=10 and 11.5, only three of these phases: Pr2Fe14B, Pr2Fe23B3, and Fe3B, but not α-Fe were present. The optimal magnetic properties of Br=10.1 kG, Hci=6.4 kOe, and (BH)max=16.1 MGOe were achieved in the Pr10Fe84.12B5.88 ribbon, while Hci 8.71 kOe and (BH)max 14.7 MGOe were obtained in Pr10Fe80B10. Furthermore, Pr10Fe78.5B11.5 having the optimal Hci (9.33 kOe) exhibited minimum values in activation volume V=5.03×10−19 cm3, because it possessed the least volume fraction of the magnetically soft phases. Finally, confirmation of the absence of spin reorientation in Pr2Fe14B down to 5 K assures its advantage over Nd2Fe14B for low-temperature applications.
Investigation of bottom-contact organic field effect transistors by two-dimensional device modeling
Authors: Li T. , Ruden P. P. , Campbell I. H. , Smith D. L.
Published: March 2003

We present device simulations for p-channel organic field effect transistors. The current conservation equation and Poisson’s equation are solved self consistently in two dimensions in the drift-diffusion approximation. We focus on modeling transistor structures consisting of a p+ Si gate electrode, a silicon dioxide gate insulator, and a polymer layer as the active (channel) material. The source and drain contacts are taken to be deposited directly on the gate insulator (bottom contact structure). We focus particularly on calculations of the surface potential for which experimental data have recently been published [L. Burgi, H. Sirringhaus, and R. H. Friend, Appl. Phys. Lett. 80, 2913 (2002)]. We find that the surface potential on the polymer layer closely matches the channel potential as argued by Burgi et al. Furthermore, we show that the experimentally observed drops in the surface potential above the source and drain contacts can be explained as a consequence of polymer material defects near the contacts and, significantly, that these results cannot be explained solely by large contact Schottky energy barriers. We also present results for the transistor output characteristics and examine the effects associated with changes in charge carrier injection for different source and drain contact materials and different models for the injection process. These results elucidate the device physics of ideal organic field effect transistors and model the behavior of current experimental devices including the most likely parasitic effects.
Direct measurement of the etching rates on Si (111) and silicon dioxide surfaces in 40% ammonium fluoride aqueous solution via atomic force microscopy
Authors: Ouyang Jian Hua , Zhao Xin Sheng , Li Ting , Zhang Da Cheng
Published: March 2003

The etching process on micropatterned Si (111) and silicon dioxide surfaces in 40% ammonium fluoride aqueous solution has been studied with atomic force microscopy. The etching rates of silicon and silicon dioxide are obtained from air-saturated and oxygen-free solutions. From the measurements at different temperatures (20–40 °C), the apparent activation energies are deduced. It is found that the etching rates are substantially different in silicon and silicon dioxide and that the dissolved oxygen in the solution facilitates the etching of silicon but obstructs it for silicon dioxide. It is also demonstrated that the thickness of the silicon dioxide film on the silicon substrate can be determined accurately from the jump of the etching rate at the SiO2/Si interface.
On the diffusion of lattice matched InGaAs/InP microstructures
Authors: Bollet F. , Gillin W. P. , Hopkinson M. , Gwilliam R.
Published: March 2003

Photoluminescence and high-resolution x-ray diffraction (HRXRD) studies of the diffusion in lattice matched InGaAs/InP quantum wells show that at high temperatures intermixing can be modeled by Fick’s law with an identical diffusion rate for both the group III and group V sublattices. This results in materials that remain lattice matched for all compositions created by the diffusion. At lower temperatures, the photoluminescence shows that the diffusion process changes and HRXRD shows that strained layers are produced within the structure. This may be due to the presence of the miscibility gap within the InGaAsP phase diagram.
Optical properties of Bi3.25La0.75Ti3O12 thin films using spectroscopic ellipsometry
Authors: Hu Zhigao , Wang Genshui , Huang Zhiming , Chu Junhao
Published: March 2003

Using spectroscopic ellipsometry, the optical properties of the Bi3.25La0.75Ti3O12 (BLT) thin films with different film thicknesses on platinized silicon substrates prepared by chemical solution methods have been investigated in the 400–1700 nm wavelength range. By fitting the measured ellipsometric parameter (Ψ and Δ) data with a multilayer model system and the classical dispersion relation for the BLT thin films, the optical constants and thicknesses of the thin films have been obtained. Meanwhile, some degree of inhomogeneity (in the form of low density layers or surface roughness) has been found in the BLT thin films except for the thinnest film, which was homogeneous throughout its thickness. The multilayer model system became very complicated with increasing thickness. It indicated that the microstructure of the BLT thin films varied with increasing thickness. The refractive index n of the BLT thin films increases with increasing thickness, and on the contrary, the extinction coefficient k decreases with increasing thickness. The dispersion of the refractive index can be well explained by a single-term Sellmeier relation.
Extinction ratio improvement of an electroabsorption modulator by using Be as a p-type dopant with small diffusion coefficient
Authors: Miyazaki Yasunori , Hanamaki Yoshihiko , Tada Hitoshi , Takagi Kazuhisa , Takemi Masayoshi , Aoyagi Toshitaka , Mihashi Yutaka , Mitsui Yasuo
Published: March 2003

Beryllium (Be) has been applied as an acceptor in the p-type cladding layer and the contact layer of an InGaAsP-based electroabsorption modulator (EAM) fabricated by metalorganic chemical vapor deposition (MOCVD). The concentration of diffused dopants in the absorption layer of the EAM was significantly reduced by employing Be instead of zinc (Zn) which has been conventionally used as an acceptor for MOCVD-grown III–V semiconductors. Numerical simulation of the diffusion of Be and Zn in the EAM device structure showed that the diffusion of Be was dominated by interstitials with small diffusion coefficient, while the diffusion of Zn was dominated by a fast diffusion mechanism of a divacancy-related complex which was not observed in the Be-doped EAM. The measured extinction ratio of the Be-doped EAM was higher by approximately 40% than that of the Zn-doped counterpart. The improvement of the extinction ratio is due to improved uniformity of the electric field applied to the absorption layer of the EAM.
Electronic and vibrational population transfer in diatomic molecules as a function of chirp for different pulse bandwidths
Authors: Chang Bo Y. , Kim Bongsoo , Solá Ignacio R.
Published: March 2003

We study the dynamics of two-photon nonresonant electronic excitation of diatomic molecules driven by chirped pulses. While the majority of the experimental results address the role of the chirp for fixed pulse bandwidth, we analyze the possibility of selective excitation for fixed time, as a function of the pulse bandwidth, depending on the sign of the chirp. With strong picosecond pulses and positive chirp it is shown that the dynamics always prepare the molecule in the ground vibrational level of the excited electronic state. The robustness of the dynamics inherits the properties of an effective Landau–Zener crossing. For negative chirp the final state is very sensitive to the specific pulse bandwidth. The dynamics of the system follow a complex convoluted behavior, and the final state alternates between low vibrational levels of the excited electronic state and excited vibrational levels of the ground potential, which become increasingly more excited with increasing bandwidth. The final electronic populations follow a double-period oscillatory behavior. We present a model based on sequential independent crossings which correlates the long-oscillation period with changes in the final vibrational state selected. We show that the short-oscillation period is related with nonadiabatic effects that give rise to fast dynamic Rabi flipping between the electronic states, providing only information of the field–molecule effective coupling. Although the short-oscillation period partially masks the expected results of the final populations, we show that it is still possible to retrieve information from the long-oscillation period regarding the frequencies of the electronic potentials. In order to do so, or in order to control the outcome of the dynamics, it is necessary to perform experiments scanning very different pulse bandwidths, and we propose a possible experimental implementation. All the numerical results of the paper are calculated for a model of the Na2 dimer.
Thermodynamics of crystal nucleation in multicomponent droplets: Adsorption, dissociation, and surface-stimulated nucleation
Authors: Djikaev Y. S. , Tabazadeh A. , Reiss H.
Published: March 2003

We extend our previous work on crystallization of unary liquids [Djikaev et al., J. Phys. Chem. A 106, 10247 (2002)] to multicomponent systems and develop the Gibbsian thermodynamics of the crystallization of multicomponent liquid solutions (bulk as well as droplets). Our treatment takes into account the possibility of formation of a solid phase from a nonstoichiometric liquid solution (noncongruent solidification). Using the capillarity approximation and taking into account both dissociation and surface adsorption effects, we derive the reversible work of formation of a crystal nucleus in homogeneous and pseudoheterogeneous modes (the density difference between crystal and liquid phases is also taken into account). In the former mode, the nucleus forms homogeneously within the (supercooled) bulk liquid solution, while in the latter mode, it forms “pseudoheterogeneously” at the liquid–vapor interface. Comparison of the works of formation in the two modes provides an inequality which must hold in order for pseudoheterogeneous crystal nucleation to be thermodynamically favored over the homogeneous process. As for unary systems, this inequality is identical to the condition of partial wetting of at least one crystal facet by its melt, i.e., the effects of adsorption and dissociation do not explicitly alter the thermodynamic condition for surface-stimulated crystallization in multicomponent systems. These effects do play an important role in determining the crystal nucleation mode, because they contribute to the liquid–vapor and liquid–solid surface tensions that enter the condition of partial wetting. The influence of these effects on the freezing behavior of droplets is expected to depend on the droplet size.
An ab initio study of self-trapped excitons in α-quartz
Authors: Van Ginhoven Renée M. , Jónsson Hannes , Peterson Kirk A. , Dupuis Michel , Corrales L. René
Published: March 2003

The structure and properties of self-trapped excitons (STE), were investigated using density functional theory (DFT) and wave function-based [UHF, UMP2, CAS-SCF, and CCSD(T)] electronic structure methods. The DFT results were compared to those obtained using the different wave function-based electronic structure methods that treat the electron correlation and exchange with varying degrees of accuracy. The calculations were carried out on cluster configurations extracted from supercell DFT calculations of the STE in α-quartz. Two luminescent STEs were found, as well as a nonradiative state at a crossing of the singlet and triplet surfaces. One of the luminescent STEs is the same as that previously found by Fisher, Hayes, and Stoneham [J. Phys.: Condens. Matter 2, 6707 (1990)]. It was furthermore determined that the PW91 functional underestimates the energy of the triplet state, and that this error is greater with greater delocalization of the excess spin density of the state.
Silicon epitaxial growth on the Si(001)2×1 surface from silane using dynamic Monte Carlo simulations
Authors: Satake Koji , Graves David B.
Published: March 2003

Dynamic Monte Carlo (DMC) simulations are carried out on silicon (001)2×1 surface under 100% silane gas chemical vapor deposition condition as a function of surface temperature (600–800° C) and pressure (6 and 12 mTorr). The reactant on the surface from gas-phase is assumed to be the silane molecule. The rates and probabilities of surface reactions are determined a priori by recent ab initio calculation results in the literature. The DMC method can reveal not only the short-time microscopic mechanism but also predict the macroscopic phenomena such as deposition. The calculated growth rate and Arrhenius activation energy of growth depending on temperature show good agreement with experimental results. The results suggest that the low activation energy regime above 700 °C is associated with a process controlled by silane dissociative adsorption. In contrast, the higher activation energy regime below 700 °C is supposed to be governed by hydrogen desorption. The periodic change of surface structure that is similar to reflection high-energy electron diffraction intensity oscillation in the process of molecular beam epitaxy is observed. The periodic behavior results from the repeat of a series of hydrogen desorption, silane adsorption, surface incorporation, and adatom diffusion on the growing surface.
Low frequency noise of GaAs Schottky diodes with embedded InAs quantum layer and self-assembled quantum dots
Authors: Hastas N. A. , Dimitriadis C. A. , Dozsa L. , Gombia E. , Amighetti S. , Frigeri P.
Published: March 2003

The electrical properties of InAs quantum layer (QL) and self-assembled quantum-dots (QDs), embedded in GaAs, are investigated by low-frequency noise measurements using Au/n-GaAs Schottky diodes as test devices. The measurements are carried out in the forward conduction regime with forward current IF as a parameter. Current–voltage and capacitance–voltage measurements indicate that GaAs and GaAs/InAs-QL Schottky diodes are nearly ideal, even though defects are present in the space–charge region of GaAs/InAs-QD Schottky diodes. In GaAs and GaAs/InAs-QL Schottky diodes, the power spectral density of the current fluctuations, S1, shows 1/f behavior and is proportional to IF2, which is explained by modulation of the barrier height due to trapping and detrapping phenomena. In GaAs/InAs-QD Schottky diodes, S1 shows 1/fγ (with γ≈0.6) behavior and is proportional to IF2 in the low current region and proportional to IF2.5 in the high current region. These noise data are explained by the generation of band tail states with exponential energy distribution in the GaAs layers due to QD formation.
Optical waveguides in lead zinc niobate–lead titanate by He ion implantation
Authors: Levy M. , Moran P. D.
Published: March 2003

Optical waveguides have been formed in bulk single crystals of the relaxor ferroelectric (0.955)[Pb(Zn1/3Nb2/3)O3]−(0.045)[PbTiO3] (0.955PZN–0.045PT) by He ion implantation. Unusually large contrast in the peak refractive index in the buried cladding layer (|Δn/n|=18%) and tetragonal distortion in the waveguiding region (Δa⊥/a⊥=2%) are observed after implantation. These large magnitudes are attributed to the proximity of 0.955PZN–0.045PT to a compositional morphotropic phase boundary in PZN–PT materials system. Rapid thermal annealing slightly reduces the magnitude of the peak refractive index contrast, but significantly changes its depth. A commensurate reduction in the tetragonal distortion and local strain distribution in the waveguiding region to values close to those observed before implantation is measured by triple crystal high resolution x-ray diffraction. These data demonstrate a viable method by which to form optical waveguides in PZN–PT.
Cathodoluminescence studies of threading dislocations in InGaN/GaN as a function of electron irradiation dose
Authors: Henley S. J. , Cherns D.
Published: March 2003

Cathodoluminescence (CL) studies have been carried out on a 30 nm GaN/1.5 nmIn0.28Ga0.72N/(0001) GaN single quantum well (SQW) structure in a field emission scanning electron microscope at voltages <5 kV and temperatures down to 8 K. A direct comparison of CL maps with transmission electron microscope studies carried out on back-thinned samples showed that the QW emission was reduced close to defects identified as “V-shaped” pits and around threading edge-type dislocations. Extended area CL spectra showed progressive blueshifting in the QW emission from around 460 to about 445 nm as the electron dose increased. This blueshift, which could be reversed by exposure to UV light, was accompanied by degradation in the spatial resolution of QW CL maps consistent with an increase in the carrier diffusion length from 200 to 330 nm. It is proposed that these effects can be explained by filling of the trap states in the vicinity of the quantum well by free carriers generated during irradiation.
Theoretical analysis of optical and magneto-optical properties of one-dimensional magnetophotonic crystals
Authors: Kato Hideki , Matsushita Takeshi , Takayama Akio , Egawa Motoji , Nishimura Kazuhiro , Inoue Mitsuteru
Published: March 2003

We present a procedure for analyzing the properties of multilayer structures with magnetic film. The magneto-optical effect in multilayer-structure magnetic films [e.g., bismuth-substituted yttrium iron garnet (YIG)] and dielectric films (e.g., SiO2 and Ta2O5) was investigated theoretically and experimentally to examine the correlation between the magneto-optical properties and the localization effect of light. The film structure of interest, which has symmetric periodicity, is the (Ta2O5/SiO2)n/Bi:YIG/(SiO2/Ta2O5)n structure, where n is the repetition number. We fabricated experimentally a one-dimensional magnetophotonic crystal film of (Ta2O5/SiO2)n/Bi:YIG/(SiO2/Ta2O5)n (n=2,6) structures. The theoretical calculation and the measured value for the Faraday rotation were in close agreement. Furthermore, the Faraday rotation of the multilayer structure with n=6 is 150 times larger than that of a Bi:YIG single-layer film. This is of interest from the point of view of actual magneto-optical device (e.g., optical isolator) fabrication.
Measurement of the band offsets of SiO2 on clean n- and p-type GaN(0001)
Authors: Cook T. E. , Fulton C. C. , Mecouch W. J. , Tracy K. M. , Davis R. F. , Hurt E. H. , Lucovsky G. , Nemanich R. J.
Published: March 2003

The band alignment at the SiO2-GaN interface is important for passivation of high voltage devices and for gate insulator applications. X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy have been used to observe the interface electronic states as SiO2 was deposited on clean GaN(0001) surfaces. The substrates, grown by metallorganic chemical vapor deposition, were n- (1×1017) and p-type (2×1018) GaN on 6H-SiC(0001) with an AlN(0001) buffer layer. The GaN surfaces were atomically cleaned via an 860 °C anneal in an NH3 atmosphere. For the clean surfaces, n-type GaN showed upward band bending of 0.3±0.1 eV, while p-type GaN showed downward band bending of 1.3±0.1 eV. The electron affinity for n- and p-type GaN was measured to be 2.9±0.1 and 3.2±0.1 eV, respectively. To avoid oxidizing the GaN, layers of Si were deposited on the clean GaN surface via ultrahigh vacuum e-beam deposition, and the Si was oxidized at 300 °C by a remote O2 plasma. The substrates were annealed at 650 °C for densification of the SiO2 films. Surface analysis techniques were performed after each step in the process, and yielded a valence band offset of 2.0±0.2 eV and a conduction band offset of 3.6±0.2 eV for the GaN-SiO2 interface for both p- and n-type samples. Interface dipoles of 1.8 and 1.5 eV were deduced for the GaN-SiO2 interface for the n- and p-type surfaces, respectively.
Humidity sensing by nanocomposites of silver in silicate glass ceramics
Authors: Pal B. N. , Kundu T. K. , Banerjee S. , Chakravorty D.
Published: March 2003

Silver nanoparticles of diameters in the range 3.4 to 13.2 nm were grown within a silicate glass ceramics containing barium titanate phase. The glass ceramics were filled with silver particles by subjecting the former to a Na+–Ag+ ion exchange process followed by a reduction treatment in hydrogen. Silver particles were formed at the interfaces of the silicate glass and the barium titanate phases, respectively. The silver particle sizes could be varied by controlling the fractal structure of the crystalline phase by prior heat treatment. Electrical resistivity measurements were carried out on cold-pressed specimens of nanocomposite powders prepared as just stated. A five order of magnitude resistivity change was recorded in the case of nanocomposite specimen with a silver particle diameter of 10.1 nm in the relative humidity range of 25% to 85%. The resistivity of the nanocomposites was found to be controlled by a variable range hopping conduction. It is believed that the silver nanoparticles provide sites where physisorption of water molecules takes place which increases the number of localized states near the Fermi level.
Analysis of Pb centers at the Si(111)/SiO2 interface following rapid thermal annealing
Authors: Hurley P. K. , Stesmans A. , Afanas’ev V. V. , O’Sullivan B. J. , O’Callaghan E.
Published: March 2003

In this work, an experimental study of defects at the Si(111)/SiO2 interface following rapid thermal annealing (RTA) in a nitrogen ambient at 1040 °C is presented. From a combined analysis using electron spin resonance and quasistatic capacitance–voltage characterization, the dominant defects observed at the Si(111)/SiO2 interface following an inert ambient RTA process are identified unequivocally as the Pb signal (interfacial Si3≡Si⋅) for the oxidized Si(111) orientation. Furthermore, the Pb density inferred from electron spin resonance (7.8±1)×1012 cm−2, is in good agreement with the electrically active interface state density (6.7±1.7)×1012 cm−2 determined from analysis of the quasistatic capacitance–voltage response.
Particle size dependence and model for iron-catalyzed growth of carbon nanotubes by thermal chemical vapor deposition
Authors: Nerushev O. A. , Dittmar S. , Morjan R.-E. , Rohmund F. , Campbell E. E. B.
Published: March 2003

The catalytic particle size dependence of chemical vapor deposition growth of multiwall carbon nanotubes was systematically investigated using two different molecules, C2H2 and C60, as carbon feedstock gases. In the particle size range between 25 and 500 nm, the use of C2H2 leads exclusively to growth of carbon nanotubes. The nanotube diameters increase with increasing catalytic particle sizes but do not scale 1:1. In contrast, nanotube formation from C60 is observed only if the particle sizes are sufficiently small with an optimum between 20 and 30 nm. For catalyst samples with considerably larger diameters, C60 is transformed into a nontubular deposit. A growth model is given that explains the different behavior.
Temperature dependence of the exchange parameter and domain-wall properties
Authors: Vértesy G. , Tomáš I.
Published: March 2003

The A exchange parameter and its temperature dependence were determined for epitaxially grown magnetic thin garnet films. The method is based on the measurement of saturation magnetization, uniaxial anisotropy field, and zero-field stripe domain period. The temperature dependence of domain-wall energy density and domain-wall width was also determined. A correlation was found between domain-wall pinning field and domain-wall parameters, which can be useful for the experimental verification of theories that describe the coercivity on the basis of domain-wall parameters.
Optical activity of electrorheological fluids under external electric field
Authors: Zhao X. P. , Zhao Q. , Gao X. M.
Published: March 2003

The optical activity of electrorheological (ER) fluids under an external dc electric field was investigated. According to the nonhomogeneous structure model of ER fluids, we propose an expression of rotation angle from Maxwell’s equations and Fresnel reflection. The optical activity of several kinds of ER fluids was measured by varying an applied electric field, the angle θ, and weight concentrations. The experiments show that the rotation angle increases with the concentration and electric field, and it first increases, passes through a maximum at an optimum θ, then monotonically decreases with θ. The experimental results are in qualitative agreement with our theoretical simulations. It is thought that the structure symmetry breaking of ER fluids under an external electric field may be the origin of the optical activity.
InGaAs/InAlAs avalanche photodiode with undepleted absorber
Authors: Li Ning , Sidhu Rubin , Li Xiaowei , Ma Feng , Zheng Xiaoguang , Wang Shuling , Karve Gauri , Demiguel Stephane , Holmes Archie L. , Campbell Joe C.
Published: March 2003

We report an avalanche photodiode with an undepleted p-type InGaAs absorption region and a thin InAlAs multiplication layer. The motivation for utilizing an undepleted absorption layer, which is similar to that in the unitraveling carrier photodiode, is to reduce the dark current. A dark current below 1 nA at a gain of 10 and a gain–bandwidth product of 160 GHz are demonstrated.
Proton-transport catalysis, proton abstraction, and proton exchange in HF+HOC+ and H2O+HOC+ and analogous deuterated reactions
Authors: Collins Michael A. , Radom Leo
Published: March 2003

Classical simulations of the reactions of HF and H2O with HOC+ have been carried out on interpolated ab initio potential energy surfaces. Rearrangement (X+HOC+→OCH++X), abstraction (X+HOC+→XH++OC), (X=HF or H2O), exchange (e.g., DY+HOC+→HY+DOC+), and exchange-rearrangement (e.g., DY+HOC+→OCD++YH) (Y=F or HO or DO) reactions are observed. However, the abstraction reaction is dominant for both the HF+HOC+ and H2O+HOC+ systems.
Conformational stability of allylbenzene: A combined study by dispersed fluorescence spectroscopy and quantum chemistry calculation
Authors: Panja Sujit Sankar , Chakraborty Tapas
Published: March 2003

Two conformational isomers of allylbenzene are identified in a supersonic free jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. With the aid of the predictions of ab initio quantum chemistry calculations at the MP2 level for a series of extended basis sets [6-311+G(d,p), 6-311++G(d,p), and cc-pVTZ], the major species of the electronic spectrum is shown to be an eclipsed conformer in which the allyl group is oriented perpendicular to the plane of the benzene ring and a terminal hydrogen atom of the ethylene moiety is poised nearly above the aromatic π electrons. The minor species is identified as an internal rotational isomer that is obtained by rotating the ethylene group about the Cα–Cβ bond by 120° from the eclipsed configuration. This predicted order of conformational preference is reversed for calculations at relatively low levels of theory: MP2/6-31G(d,p), HF/6-311++G(d,p), HF/6-31G(d,p), and B3LYP/6-31G(d,p). The relative intensities of the vibronically induced nontotally symmetric and totally symmetric transitions are significantly different in the electronic spectra of the two conformers.
Electric dipole polarizabilities of Nb2–27
Authors: Knickelbein Mark B.
Published: March 2003

The static electric dipole polarizabilities of Nb2–Nb27 have been measured via a molecular beam deflection method. The clusters display per-atom polarizabilities that vary between 5.4±0.3 Å3 per atom (Nb10) to 37.7±0.3 Å3 (Nb13). The factor of ∼7 variation observed in the per-atom polarizabilities is the largest observed for any metal cluster system investigated thus far and implies large size-dependent variations in their electronic and/or geometric structures.
Investigation of high saturation magnetization of low rare-earth content α-Fe/Nd2Fe14B bulk exchange-spring magnets
Authors: Ono H. , Tayu T. , Waki N. , Sugiyama T. , Shimada M. , Kanou M. , Fujiki A. , Yamamoto H. , Takasugi K. , Tani M.
Published: March 2003

An investigation was made of the magnetic properties of Nd–Fe–Co–V bulk exchange-spring magnets fabricated by spark plasma sintering and consisting of a hard Nd2Fe14B phase and a soft α-Fe phase and having a low rare-earth content of Nd=2–7 at. %. The high saturation magnetization of the bulk exchange-spring magnets was confirmed experimentally, and the measured data agreed well with the saturation magnetization values estimated from their respective rare-earth content. The saturation magnetization values were nearly uniform and were not dependent on changes in the grain size. Additionally, the specific resistance of the samples nearly coincided with the values interpolated from the specific resistances of an ordinary sintered pure Fe bulk compact and a sintered Nd–Fe–B magnet. These results suggest that the α-Fe/Nd2Fe14B bulk exchange-spring magnets did not contain any appreciable grain boundary phase between the fine nanoscale-size grains or tertiary phase.
Low-frequency Raman scattering study of tert-butyl alcohol–water and tetrahydrofuran–water binary mixtures
Authors: Fukasawa Toshiko , Amo Yuko , Tominaga Yasunori
Published: March 2003

Low-frequency Raman scattering spectra have been measured for tert-butyl alcohol (TBA)–water and tetrahydrofuran (THF)–water binary mixtures as a function of concentration. We confirmed the isosbestic point in the reduced Raman spectra χ″(ν) of the TBA–water and THF–water mixtures. The χ″(ν) spectra could be decomposed into a linear combination of the neat TBA spectrum and the pure water spectrum in the frequency range from 80 to 400 cm−1. On the other hand, below 80 cm−1, the χ″(ν) spectra could not be decomposed into a linear combination. The superposed spectra deviated systematically from the experimental spectra as a function of solute concentration. This result indicated that cooperative dynamics was present in TBA–water and THF–water mixtures although the intermolecular vibrations showed independent character.
Perturbed hard fluid theoretical analysis of the effects of solvation on the thermodynamics of a hemiketal formation reaction
Authors: Gift Alan D. , Ben-Amotz Dor
Published: March 2003

The effects of temperature and pressure on the equilibrium reaction of acetone and methanol to form a hemiketal (2-methoxy-2-propanol), when dissolved in either tetrahydorfuran or diethyl ether, are analyzed using the perturbed hard fluid (PHF) model. Experimental global reaction thermodynamic results (ΔG,ΔH,ΔS,ΔV,ΔU,ΔA) are obtained from pressure- and temperature-dependent calibrated Raman peak area measurements. The PHF theoretical analysis separates the effects of repulsive and attractive solute–solvent interactions by combining hard sphere fluid and van der Waals mean field approximations. The results offer a critical test of these approximations and elucidate the way in which the balance of repulsive and attractive interactions contribute to each reaction thermodynamic function.
Aging continuous time random walks
Authors: Barkai Eli , Cheng Yuan-Chung
Published: March 2003

We investigate biased and nonbiased aging continuous time random walks (ACTRW), using fractal renewal theory. For example, a biased ACTRW process describes a Montroll–Weiss CTRW process which starts at time −ta and then at time t=0 a bias is added to the random walk (i.e., an external field is switched on). Statistical behaviors of the displacement of the random walker r=r(t)−r(0) in the time interval (0,t) are obtained, after aging the random walk in the time interval (−ta,0). In ACTRW formalism, the Green function P(r,ta,t) depends on the age of the random walk ta and the forward time t. We derive a generalized Montroll–Weiss equation, which yields an exact expression for the Fourier double Laplace transform of the ACTRW Green function. Asymptotic long times ta and t behaviors of the Green function are shown to be related to the arc–sine distribution and Lévy stable laws. In the limit of t≫ta, we recover the standard nonequilibrium CTRW behaviors, while the important regimes t≪ta and t≃ta exhibit interesting aging effects. Convergence of the ACTRW results towards the CTRW behavior, becomes extremely slow when the diffusion exponent becomes small. In the context of biased ACTRW, we investigate an aging Einstein relation. We briefly discuss aging in Scher–Montroll type of transport in disordered materials.
On independence of the solvation of interaction sites of a water molecule
Authors: Předota Milan , Ben-Naim Arieh , Nezbeda Ivo
Published: March 2003

To support simplifying assumptions used in analytic theories of aqueous systems, we have used computer simulations to examine correlations in the bonding of the individual sites of a water molecule using two qualitatively different extended primitive models, EPM4 and EPM5. We have studied these correlations not only for the fully interacting water molecule (considered as a solute) but also for a series of other solutes made from the water molecule by turning off some of its interaction sites. We have found that for the EPM5 solvent the local density of water molecules bound to a specific site is independent of the state of the other sites being turned on or off; for the EPM4 solvent such an independence does not hold exactly, but the correlations have been found to be very small. These facts fully justify previously used speculative approximations for the calculation of the solvation Helmholtz free energy of a water molecule, and also lend support to the first-order thermodynamic perturbation theory of Wertheim.
Formation of droplets on active centers in supersaturated vapors
Authors: Kožı́šek Zdeněk , Demo Pavel
Published: March 2003

Kinetics of nucleation on active centers in supersaturated vapors is studied by a numerical solution of kinetic equations, including depletion of active centers during the phase transition process. The basic characteristics of the nucleation process (the size distribution of nuclei, nucleation rate and the total number of nuclei) are determined at various conditions. Nuclei are formed on all active centers at a sufficiently long time in coincidence with a widely used Avrami model. An increase of the total number of nuclei with time is steeper in the Avrami model as compared with our numerical solution of kinetic equations. The nucleation rate does not reach its stationary limit and becomes negligibly small with increasing time.
Stability of particles adsorbed at liquid/fluid interfaces: Shape effects induced by line tension
Authors: Faraudo Jordi , Bresme Fernando
Published: March 2003

We analyze the stability of particles at liquid–fluid interfaces as a function of their shape and orientation with respect to the interface plane. To do this we apply the generalized Young’s equation, including the line and surface tensions, to the specific shapes of prolate and oblate particles. We show that particles with aspect ratio α larger than a critical value (depending on the surface and line tensions) cannot be adsorbed at the interface. The stability of nonspherical particles at interfaces exhibits a rather complex dependence with line tension and orientation. The interplay between these variables results in a destabilization of prolate particles. In general the stability decreases in the order, oblate>sphere>prolate. It is argued that in experiments aiming to investigate the effect and magnitude of the line tension, the optimum experimental choice corresponds to very elongated particles.
Nonlinear surface impedance of Tl2Ba2CaCu2O8 thin films as a function of temperature, frequency, and magnetic field
Authors: Gaganidze E. , Heidinger R. , Halbritter J. , Shevchun A. , Trunin M. , Schneidewind H.
Published: March 2003

YBa 2 Cu 3 O 7 epitaxial film quality has reached a stage where rf applications at T⩽80 K are carried through. Higher-operating temperatures or better power handling are very much desired. Tl2Ba2CaCu2O8 (TBCCO) is an excellent candidate with its higher-transition temperature Tc≈100–105 K. We study the rf field dependence of surface impedance Zs of epitaxial TBCCO films using dielectric resonators with TE011-modes at 8.5 and 15.3 GHz at 4.2 K<T<100 K and at 0.01 mT⩽Hrf⩽10 mT in the frequency and time domain. High-quality films show surface resistances at low fields Rs(77 K) superior to YBCO and promising power handling with a nonlinearity onset above turn-up field μ0H*⩾3 mT. Defective TBCCO films show anomalous nonlinear behavior δRs(Hrf)∝√Hrf for μ0Hrf⩽μ0H*∼0.3–1 mT, extremely pronounced at T<40 K, indicating the presence of very weak links yielding flux flow, followed at higher fields μ0Hrf≳0.5 mT by hysteresis losses with a linear field dependence δRs(Hrf)∝Hrf. Better quality TBCCO films show δRs(Hrf)∝Hrf2 up to fields of about 5–10 mT. The δRs(Hrf)∝Hrfn (n=1,2) dependencies together with ratios r(T)=δXs/δRs∼1–10 hint to hysteresis losses of Josephson fluxons enhanced by slits or holes in the films.
Thermoelectric properties of Al1−xInxN and Al1−y−zGayInzN prepared by radio-frequency sputtering: Toward a thermoelectric power device
Authors: Yamaguchi Shigeo , Iwamura Yasuo , Yamamoto Atsushi
Published: March 2003

With the aim of fabricating a thermoelectric power device using III-nitride semiconductors, we recently studied thermoelectric properties of Al1−xInxN and Al1−y−zGayInzN prepared by rf sputtering. For Al0.35In0.65N with 0.63-μm thickness, the maximum power factor was 3.63×10−4 W/mK2. For Al0.26Ga0.44In0.30N, the maximum power factor was 5.58×10−5 W/mK2.
Effect of composition on the band gap of strained InxGa1−xN alloys
Authors: McCluskey M. D. , Van de Walle C. G. , Romano L. T. , Krusor B. S. , Johnson N. M.
Published: March 2003

The band gap of pseudomorphically strained InxGa1−xN alloys has been measured using optical absorption spectroscopy. X-ray diffraction measurements indicated that the in-plane lattice parameter of the InxGa1−xN film equaled that of the underlying GaN layer. For strained InxGa1−xN, it was determined that the band gap shift versus composition is given by dEg/dx=−4.1 eV for x<0.12. Our results contradict some recent reports that InxGa1−xN has a relatively small bowing parameter. Possible reasons for the discrepancies are discussed.
Thermal stability of InP-based high electron mobility transistor epitaxial wafers
Authors: Sugiyama Hiroki , Watanabe Kazuo , Yokoyama Haruki , Kobayashi Takashi
Published: March 2003

Systematic annealing of the InP-based high electron mobility transistor (HEMT) structure was performed. The thermal stability of the sheet carrier concentration (Ns) and its relation to fluorine contamination were investigated by using van der Pauw–Hall effect measurements, secondary ion mass spectroscopy (SIMS), and photoluminescence (PL) measurement. The Ns decreased after annealing at 450 °C in nitrogen ambient, which was due to fluorine contamination of the carrier-supply layer (CSL). The PL spectra from the InGaAs channel reflected the Ns reduction. In addition, the PL spectra from the Si delta-doped InAlAs CSL drastically changed in line shape as the fluorine concentration increased. Such changes of PL and Ns were not observed in the samples after annealing at 350 °C, where the CSL was scarcely contaminated by fluorine. The series of PL characterization and Hall effect and SIMS measurements can effectively analyze how fluorine contamination affects the Ns in the InP-based HEMT structures.
H/D isotope effects on formation and photodissociation of HKrCl in solid Kr
Authors: Khriachtchev Leonid , Saarelainen Mia , Pettersson Mika , Räsänen Markku
Published: March 2003

The HCl (DCl)/Kr matrixes were irradiated at 193 nm, which produced matrix-isolated H (D) and Cl atoms as permanent photolysis products. The annealing-induced formation of HKrCl and DKrCl was used as a measure of atomic hydrogen (H and D) mobility at various annealing temperatures (from 24 to 30 K). The temperature dependencies of the HKrCl and DKrCl formation rates yield ∼64 and 68 meV for the corresponding activation energies estimating the isotope effect on atomic hydrogen mobility in solid Kr (D slower than H). The difference in mobility of H and D atoms allowed us to demonstrate a reaction between D atoms and HKrCl molecules, and the suggested kinetic model is in good agreement with experiment. In addition, the H/D isotope effects on the solid-state photodissociation of HCl and HKrCl are studied and discussed.
Thermal stress characteristics of Cu/oxide and Cu/low-k submicron interconnect structures
Authors: Rhee Seung-Hyun , Du Yong , Ho Paul S.
Published: March 2003

Thermal stress characteristics of single damascene Cu lines passivated with tetraethyl orthosilicate oxide and methyl silsesquioxane low-k dielectrics were investigated by x-ray diffraction method and finite element analysis. Lines with different aspect ratios were studied, and the effect of line geometry was evaluated. The stress characteristics of low-k passivated Cu lines indicate that the diffusion barrier plays an important role in controlling the stress behavior in damascene structure. The effect of material properties, process conditions, and interconnect structure were examined and the implications in Cu/low-k interconnect reliability are discussed.
Enhancement of the current in a superconductor strip by means of curved superconducting shields
Authors: Genenko Y. A. , Rauh H.
Published: March 2003

The distribution of the sheet current in a superconductor strip located between superconducting shields of various geometries is studied on the basis of exact analytic forms. Whereas the current becomes increasingly uniform when flat shields approach the strip, strong inhomogeneities are found to develop for curved shields, with concomitant enhancements by several times of the maximum total current predicted in the marginal Meissner state; a trait which could be favorably exploited regarding superconductor electronics applications.
Escape and reentry of a Brownian particle through a hole in a cavity
Authors: Berezhkovskii A. M. , Barzykin A. V.
Published: March 2003

We show that the kinetics of diffusive escape from a cavity through a small hole can be described, to a sufficient degree of accuracy, by a formal kinetic scheme for reversible dissociation of a spherical binding site with appropriately defined effective association and dissociation rate constants. At short times, the population of the cavity decays exponentially, while at long times a crossover to the inverse power law behavior is observed. These simple predictions are in excellent agreement with the results of Brownian dynamics simulations.
Site-averaging in the integral equation theory of interaction site models of macromolecular fluids: An exact approach
Authors: Krakoviack V.
Published: March 2003

A simple “trick” is proposed, which allows us to perform exactly the site-averaging procedure required when developing integral equation theories of interaction site models of macromolecular fluids. It shows that no approximation is involved when the number of Ornstein–Zernike equations coupling the site–site correlation functions is reduced to one. Its potential practical interest for future theoretical developments is illustrated with a rederivation of the so-called molecular closures.
Electronic and optical properties of polyfluorene and fluorene-based copolymers: A quantum-chemical characterization
Authors: Cornil J. , Gueli I. , Dkhissi A. , Sancho-Garcia J. C. , Hennebicq E. , Calbert J. P. , Lemaur V. , Beljonne D. , Brédas J. L.
Published: March 2003

We report a detailed quantum-chemical characterization of the electronic and optical properties of polyfluorene chains and compare them to those in copolymers containing alternating fluorene and benzothiadiazole or ethylenedioxythiophene units. The introduction of the comonomer can strongly modify the excitonic properties as well as the efficiency of charge- and energy-transfer processes. The choice of the comonomer is thus critical in targeting specific optical properties while maintaining good transport properties.
Proton-irradiation-induced intermixing of InGaAs quantum dots
Authors: Lever P. , Tan H. H. , Jagadish C. , Reece P. , Gal M.
Published: March 2003

Proton irradiation was used to create interdiffusion in In0.5Ga0.5As quantum dots (QDs), grown by low-pressure metalorganic chemical vapor deposition. After 25-keV proton irradiation, the QD samples were annealed at two temperatures (700 or 750 °C) for 30 s. It was found that much lower annealing temperatures were needed to recover the photoluminescence signals than in the quantum-well case. Large blueshifts (120 meV) and narrowing of the photoluminescence spectra were seen. Various doses (5×1013–1×1015 cm−2) and implant temperatures (20–200 °C) were used to study the interdiffusion processes in these samples. In QD samples, much lower doses were required to achieve similar energy shifts than reported in quantum-well samples.
Device for the independent verification of subresonant mechanical damping measurements
Authors: Brunner W. M. , Getting I. C. , Spetzler H. A.
Published: March 2003

An existing subresonant damping spectrometer for making mechanical damping measurements in ∼0.1 m scale samples at low (0.001–100 Hz) frequencies has been characterized for accuracy and precision. In this study, we have designed and constructed a viscoelastic structure with a calculated dynamic response to determine the accuracy of the spectrometer. We present an analytical derivation of the viscoelastic behavior of the structure, and calculate damping as a function of frequency. We also present results from measurements of the structure using the damping spectrometer. Both theory and experiment show modified Debye relaxation peaks that coincide in magnitude (Q−1=δ∼0.05) and center frequency (0.1 Hz). Above 1 Hz, theoretical and experimental values diverge. We interpret this divergence as a broadened phase shift associated with a resonance in the spectrometer near 100 Hz. We conclude that damping measurements below 1 Hz using this spectrometer are valid, implying that absolute values of mechanical damping can be legitimately compared. The standard deviation of measurements within this range is ⩽0.003. This conclusion suggests the development of a viscoelastic standard for uniaxial-mode mechanical damping measurements under quasistatic conditions to facilitate the development of new spectrometers.