bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms for the ozonolysis of ethene and propene: Reliability of quantum chemical predictions
Authors: Chan Wai-To , Hamilton I. P.
Published: January 2003

Reactions of ozone with ethene and propene leading to primary ozonide (concerted and stepwise ozonolysis) or epoxide and singlet molecular oxygen (partial ozonolysis) are studied theoretically. The mechanism of concerted ozonolysis proceeds via a single transition structure which is a partial diradical. The transition structures and intermediates in the stepwise ozonolysis and partial ozonolysis mechanisms are singlet diradicals. Spin-restricted and unrestricted density functional methods are employed to calculate the structures of the closed-shell and diradical species. Although the partial diradicals exhibit moderate to pronounced instability in their RDFT and RHF solutions, RDFT is required to locate the transition structure for concerted ozonolysis. Spin projected fourth-order Møller–Plesset theory (PMP4) was used to correct the DFT energies. The calculated pre-exponential factors and activation energies for the concerted ozonolysis of ethene and propene are in good agreement with experimental values. However, the PMP4//DFT procedure incorrectly predicts the stepwise mechanism as the favored channel. UCCSD(T) predicts the concerted mechanism as the favored channel but significantly overestimates the activation energies. RCCSD(T) is found to be more accurate than UCCSD(T) for the calculation of the concerted mechanism but is not applicable to the diradical intermediates. The major difficulty in accurate prediction of the rate constant data for these reactions is the wide range of spin contamination for the reference UHF wave functions and UDFT solutions across the potential energy surface. The possibility of the partial ozonolysis mechanism being the source of epoxide observed in some experiments is discussed.
Ion beam synthesis of diamond-like carbon thin films containing copper nanocrystals
Authors: Gerhards I. , Ronning C. , Hofsäss H. , Seibt M. , Gibhardt H.
Published: January 2003

Amorphous carbon thin films containing 0–50 at. % Cu have been grown by mass selected ion beam deposition in order to synthesize isolated Cu nanoparticles within a diamond-like matrix. Raman spectroscopy and x-ray photoelectron spectroscopy show that the sp3 content of the matrix decreases with increasing Cu content. Simultaneously, the mean particle size of the embedded Cu nanocrystals increases, as x-ray diffraction and transmission electron microscopy analysis reveal. There is apparently no dependence of the matrix structure on the Cu+ ion energy, while the Cu content is strongly influenced by this deposition parameter.
Magnetoelastic effects of (La1−x Gdx)0.7 Ca0.3 MnO3 (x=0, 0.05, 0.2, 0.3, and 0.4) compound
Authors: Gwag Jin Seog , Kim Ho-Il , Kim Jang Whan
Published: January 2003

Polycrystalline samples (La1−xGdx)0.7 Ca0.3 MnO3 with x=0, 0.05, 0.2, 0.3, and 0.4 were synthesized by a solid-phase reaction method and we measured their ac magnetic susceptibility, resistivity, linear thermal expansion coefficient, magnetization, and magnetostriction as a function of temperature by using strain gauge, ac susceptometer, and a vibrating sample magnetometer. An extra contribution to the expansion coefficient was observed at Tc as a consequence of the polaron effect as well as the paramagnetic–ferromagnetic and metal–insulator transition. Electrical, magnetic, and lattice structural transition temperatures correspond to 253±5 K and 224±5 with respect to x=0 and 0.05, respectively. These results prove the presence of strong electron–phonon and spin coupling. Under the external magnetic field, transition temperatures shift to higher ones and the peak on the temperature dependence of the expansion coefficient is suppressed. These phenomena could be related to colossal magnetoresistance.
Effect of Fe57 ion implantation on magnetotransport in epitaxial La0.67Ca0.33MnO3 thin films
Authors: Bathe Ravi , Patil S. I. , Adhi K. P. , Hannoyer B. , Marest G.
Published: January 2003

Epitaxial thin films of La0.67Ca0.33MnO3 were implanted with 30 and 100 keV Fe57 ions at different fluence values to achieve a uniform implant distribution. The resistivity of the film increases as fluence increases, but the peak resistivity temperature Tp does not change significantly for the as-implanted sample. Annealing of the implanted samples at 950 °C allows recovery of the structural properties of the films and leads to interesting properties: increase of the peak resistivity and of the colossal magnetoresistance values, and decrease of Tp. The decrease in Tp is of ∼130 K, and the increase in resistivity is almost a factor of 28 as compared to the as-implanted sample.
Magnetization switching in 70-nm-wide pseudo-spin-valve nanoelements
Authors: Zhu Xiaobin , Grütter P. , Hao Y. , Castaño F. J. , Haratani S. , Ross C. A. , Vögeli B. , Smith H. I.
Published: January 2003

The magnetic domain structures and magnetization reversal of patterned 70-nm-wide pseudo-spin-valve (PSV) elements were studied by magnetic force microscopy (MFM). Both magnetically soft and hard layers form single-domain states at remanence, and can be magnetized either parallel or antiparallel to each other. The switching field of each layer, and the coupling between the layers, are quantified using MFM. Individual elements show well-defined switching fields, while the ensemble has a large switching field distribution due to variability between the PSV elements.
Effects of annealing ambient on the formation of compensation defects in InP
Authors: Deng A. H. , Mascher P. , Zhao Y. W. , Lin L. Y.
Published: January 2003

Positron annihilation lifetime (PAL) and photoinduced current transient spectroscopies (PICTS) have been employed to study the formation of compensation defects in undoped InP under different annealing processes with pure phosphorus (PP) ambience and iron phosphide (IP) ambience, respectively. The different annealing ambiences convert the as-grown n-type undoped InP into two types of semi-insulating (SI) states. The positron average lifetimes of as-grown InP, PP SI-InP, and IP SI-InP are found to be 246, 251, and 243 ps, respectively, which are all longer than the bulk lifetime of 240 ps, indicating the existence of vacancy-type positron-trapping defects. For as-grown InP, VInH4 complexes are the dominant defects. They dissociate into VInHn(0⩽n⩽3) acceptor vacancies under PP ambience annealing, compensating the residual shallow donors and turning the material semi-insulating. In forming IP SI-InP, diffusion of iron into VIn complexes under IP ambience annealing produces the substitutional compensation defect FeIn, causing a shorter positron average lifetime. The PICTS measurements show that a group of vacancy-type defects has been suppressed by iron diffusion during the annealing process, which is in good agreement with the PAL results.
Nonradiative recombination centers and electrical aging of organic light-emitting diodes: Direct connection between accumulation of trapped charge and luminance loss
Authors: Kondakov D. Y. , Sandifer J. R. , Tang C. W. , Young R. H.
Published: January 2003

Organic light-emitting diodes (OLEDs) are attractive for display applications because of their high brightness, low driving voltage, and tunable color. Their operating lifetimes, hundreds or thousands of hours, are sufficient for only a limited range of applications. The luminance efficiency decreases gradually as the device is operated (electrically aged), for reasons that are poorly understood. A prototypical OLED has the structure anode|HTL|ETL|cathode, where the HTL and ETL are hole- and electron-transporting layers, and the recombination and emission occur at or near the HTL|ETL interface. We find that the decreasing luminance efficiency is linearly correlated with an accumulation of immobile positive charge at the HTL|ETL interface, and the magnitude of the charge is comparable to the total charge at that interface when an unaged device is operated. A natural explanation of the connection between the two phenomena is that electrical aging either generates hole traps (and trapped holes) or drives metal ions into the device, and that either species act as nonradiative recombination centers. To estimate the accumulating immobile charge and determine its location, we use a variant of a recently introduced capacitance versus voltage technique. In the prototypical OLEDs described here, the HTL is a ca. 1000 Å layer of NPB, and the ETL is a 300−1800 Å layer of Alq3. A device with an additional “emission layer” (EML) of an anthracene derivative between the HTL and ETL, in which the electroluminescence spectrum is characteristic of the EML, behaved similarly. We surmise that the phenomena reported here may be common to a wider variety of OLED structures and compositions.
On the validity of dielectric continuum models in application to solvation in molecular solvents
Authors: Milischuk Anatoli , Matyushov Dmitry V.
Published: January 2003

We report Monte Carlo simulations of solvation of a point dipole in dipolar–quadrupolar solvents of varying dipole moment and axial quadrupole. The simulations are carried out to test the prediction of dielectric solvation models of a monotonic increase of the absolute value of the solvation chemical potential |μp| with the solvent dielectric constant ε. Dielectric constants are obtained from pure liquid simulations carried out for each solvent used in solvation simulations. A raising dependence of |μp| on ε, in qualitative agreement with dielectric solvation models, is seen when the solvent dipole moment is varied at constant solvent quadrupole. An increase in the axial quadrupole at constant solvent dipole reduces the dielectric constant at the same time leading to higher |μp| values. The simulations and dielectric models thus give the opposite dependence on the solvent quadrupole for any solvent dipole. We also show that for solvation in dipolar–quadrupolar solvents the saturation limit |μp|→const at ε≫1 predicted by linear response dielectric continuum models actually occurs in the range of nonlinear solvation.
The dissociation energy of BrCl and the equilibrium constant for its formation reaction in the gas phase
Authors: Tellinghuisen Joel
Published: January 2003

For the reaction, Br2(g)+Cl2(g)⇄2BrCl(g), recently used values for the equilibrium constant K° range from 6 to 10. The spread is due to the use (explicit or implicit) of different values for the dissociation energy of BrCl. The most recent assessment of this quantity yielded De=18 247±5 cm−1, from which the equilibrium constant is 8.95±0.5. It should be possible to sharpen the determination of De and K° by observing the predissociation-induced breakoff of fluorescence from low J levels of v=7 in the B state of the Br 37Cl isotopomers.
Modification of the lattice thermal conductivity in semiconductor rectangular nanowires
Authors: Lü X. , Chu J. H. , Shen W. Z.
Published: January 2003

A model for lattice thermal conductivity in a semiconductor nanowire with a rectangular cross section is investigated. It is based on solving the equations of phonon radiative transfer taking into account interface scattering and modification of the acoustic phonon dispersion. From the numerical calculations, we predict a decrease by an order of magnitude of the lattice thermal conductivity of a 10-nm-thick and 20-nm-wide rectangular nanowire. The interface scattering and phonon confinement play important roles in the reduction of lattice thermal conductivity. The nanowire lattice thermal conductivity is found to decrease with increasing temperature at moderate and high temperatures. It is shown that the phonon confinement becomes more important than interface scattering in the highly specular scattering case. The effect of interface roughness scattering and phonon confinement on the lattice thermal conductance in a rectangular wire at low temperature is also examined. We calculate the contribution to the lattice thermal conductance due to the longitudinal mode for the catenoid contact. It is found that phonon confinement leads to a significant decrease of the lattice thermal conductance at sufficiently low temperatures (T<0.1 K).
Elastic modulus of amorphous boron suboxide thin films studied by theoretical and experimental methods
Authors: Music Denis , Kreissig Ulrich , Chirita Valeriu , Schneider Jochen M. , Helmersson Ulf
Published: January 2003

Boron suboxide BOx thin films have been deposited on Si (100) by reactive rf magnetron sputtering of B powders in an Ar/O2 atmosphere. Elastic recoil detection analysis and x-ray diffraction were used to study the influence of the O incorporation on the film composition and structure and relate them to mechanical properties, which were evaluated by nanoindentation. As x in BOx was increased from 0.08 to 0.18, the elastic modulus of the x-ray amorphous films decreased from 273 to 231 GPa, by 15%. This can be understood using classical molecular dynamics (MD) with a Buckingham-like interaction potential: The increase in the O concentration and corresponding formation of B–O bonds, shown to be longer than the B–B bonds, resulted in larger ionic contributions as well as a density reduction. This increased ionicity was responsible for the observed decrease in elastic modulus. As even more O was incorporated (x>0.18), the H concentration increased, exceeding 0.3 at. %. This may cause the formation of boric acid (H3BO3) as a result of the chemical reaction with H2O upon atmosphere exposure. The presence of van der Waals and hydrogen bonding, associated with H3BO3 formation, provides a reasonable explanation for the extensive decrease in elastic modulus from 231 to 15 GPa, by 94%. The parameterization for the Buckingham-like interaction potential, introduced in this study, can be used for MD simulations of amorphous BOx systems without H3BO3.
Interface properties and structural evolution of TiN/Si and TiN/GaN heterostructures
Authors: Patsalas P. , Logothetidis S.
Published: January 2003

Spectroscopic ellipsometry (SE) is employed to study the evolution of microstructure, stoichiometry, and electron-transport properties of titanium nitride (TiN) heterostructures grown on Si and gallium nitride (GaN) by reactive magnetron sputtering. In order to achieve subnanometer resolution for the SE analysis, we developed and validated the appropriate methods of interpreting the optical data. Thus, we used (a) effective medium theories describing the heterostructures in terms of their constituent materials (Si, GaN, TiN, over-stoichiometric TiNx, and voids), and (b) a combined Drude–Lorentz model describing the optical response of the conduction and valence electrons of TiN and TiNx. In the case of TiN/Si, the SE results indicate a pure Volmer–Weber type of growth with four distinct growth stages. A TiNx layer is formed before TiN due to the stress variations during growth. On the other hand, TiN/GaN exhibits a quasi two-dimensional growth character. In both cases, the TiN resistivity is very high for films thinner than the critical thickness for homogeneous growth. Thus, for practical applications in electronic devices the thickness of the TiN layer should be at least 4 and 14 nm for TiN/GaN and TiN/Si, respectively.
Switching as a dynamical process in electron diodes
Authors: Akimov P. V. , Schamel H. , Ender A. Ya. , Kuznetsov V. I.
Published: January 2003

The transient nature of electron diode dynamics, caused by a weak amplitude switching of the collector voltage, is investigated both analytically and numerically. Based on the Lagrangian flow description, an analytical solution is found, which depicts the numerical findings accurately. Discussing in detail the trajectories of the electrons, we recognize two groups of electrons, behaving differently depending on whether they are already present in the diode at the moment of switching (“old” electrons) or they are injected later (“fresh” electrons). Their mutual interaction via the electric field yields a rapid, nonlinear transition between the initial and the final steady state, the spatial–temporal behavior of which could be extracted by solving a retarded differential equation for the perturbed emitter electric field and by a proper transformation back to the Eulerian space.
Optical absorption in Co-doped SiO2-GeO2 glass rods and fibers
Authors: Morishita Yuichi , Tanaka Keiji
Published: January 2003

Optical absorption spectra in rods and fibers of Co-doped SiO2-GeO2 glass have been studied comparatively, and the result is extended to other transition-metal ions. In the glass with a high-Co concentration (≈2000 wt. ppm), absorption peaks due to Co2+ ions appear differently in the rod and the fiber. A deconvolution analysis shows that this difference arises from the ratio of CoO4 and CoO6 units, the latter being preferred in the rod. This observation can be understood in terms of fictive temperature and free energy. The thermodynamical idea is also applied to the coordination behavior of other transition-metal ions in oxides, which suggests a close correlation between glasses and crystals. Results for a low-Co-concentration glass are also discussed.
Crystal growth and optical properties of Cr4+:Li2TiGeO5
Authors: Sharonov M. Yu. , Bykov A. B. , Owen S. , Petricevic V. , Alfano R. R.
Published: January 2003

Crystal growth and optical properties of Cr4+ centers in Li2TiGeO5 single crystals were investigated. Polarized absorption spectra at low and room temperature indicate a lowering of symmetry of the local environment of the Cr4+ ion from Td to C2 group of symmetry. The crystal field parameter Dq and the Racah paprameter B were determined to be Dq=930 cm−1, B=510 cm−1, and Dq/B=1.8. Emission spectra show wideband fluorescence from 1100 to 1600 nm with the maximum at 1200 nm. The lifetime of the metastable level decreases from 62 μs at 77 K to 6 μs at 300 K, indicating strong nonradiative quenching and a quantum efficiency of less than 10%.
Spectroscopic properties and thermal stability of erbium-doped bismuth-based glass for optical amplifier
Authors: Yang Jianhu , Dai Shixun , Zhou Yuefen , Wen Lei , Hu Lili , Jiang Zhonghong
Published: January 2003

The spectroscopic properties and thermal stability of Er2O3 doped Bi2O3–B2O3–Na2O glasses are investigated experimentally. A close correlation is observed between the B2O3 content and the spectroscopic properties such as Judd–Ofelt intensity parameters Ωt (t=2,4,6), emission spectra, and lifetime of I13/24 level of Er3+. The value of Ω6 increases with an increase of B2O3 content, which is attributed to the decrease of the covalency of Er–O bond in terms of the glass microstructure and electronegativity theory. The emission spectra of I13/24→4I15/2 transition broadens as a consequence of the enhancement of the inhomogeneous distribution of Er3+ sites when increasing B2O3 content. The lifetime of I13/24 level of Er3+ in bismuth-based glass, compared with those in other glasses, is relative low due to the high-phonon energy of the B–O bond, the large refractive index of the host, and the existence of OH− in the glass. In addition, the glass stability is improved in which the substitution of B2O3 for Bi2O3 strengthens the network structure. The suitability of bismuth-based glass as a host for Er3+ doped for a broadband amplifier and its advantages over other glass hosts are also discussed.
Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism
Authors: Koch N. , Kahn A. , Ghijsen J. , Pireaux J.-J. , Schwartz J. , Johnson R. L. , Elschner A.
Published: January 2003

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.
Quantitative magnetization measurements on nanometer ferromagnetic cobalt wires using electron holography
Authors: Snoeck E. , Dunin-Borkowski R. E. , Dumestre F. , Renaud P. , Amiens C. , Chaudret B. , Zurcher P.
Published: January 2003

The magnetic remanent states of 4-nm-diameter single-crystalline Co nanowires are characterized using off-axis electron holography. Measurements are obtained from isolated wires, as well as from bundles of wires and “multibranch” wires joined by 15-nm-diameter nodes of Co. The fraction of magnetically active moments in a single 4-nm-diameter wire is measured to be 1.01±0.19, indicating that, to within experimental error, the wire is fully magnetized throughout its diameter.
Magnetism and magnetic entropy change of LaFe11.6Si1.4Cx (x=0−0.6) interstitial compounds
Authors: Chen Yuan-fu , Wang Fang , Shen Bao-gen , Wang Guang-jun , Sun Ji-rong
Published: January 2003

Chemically stable LaFe11.6Si1.4Cx (x=0−0.6) interstitial compounds were prepared. The Curie temperatures increases from 195 to 250 K with an increase in X from 0 to 0.6 due to obvious lattice expansion. The maximal magnetic entropy changes |ΔS| with a change in magnetic field of 0–5 T, are 24.8, 24.2, 18.8 and 12.1 J/kg K for x=0, 0.2, 0.4, and 0.6, respectively, notably exceeding that of Gd (|ΔS|∼9.8 J/kg K at TC=293 K). The magnetic transition varies from first order to second order with an increase in carbon concentration, which has a decisive influence on |ΔS|. The large |ΔS|, chemical stability, and low cost make LaFe11.4Si1.6Cx a promising candidate as a magnetic refrigerant in the corresponding temperature ranges.
Seam-free fabrication of submicrometer copper interconnects by iodine-catalyzed chemical vapor deposition
Authors: Pyo Sung Gyu , Kim Sibum , Wheeler D. , Moffat T. P. , Josell D.
Published: January 2003

Kinetic parameters from studies of deposition on planar deposits are used to predict superconformal filling of fine features during iodine-catalyzed chemical vapor deposition. The mechanism behind the superconformal filling is described and the metrology required to predict it is identified and quantified. The dominant effect is the change in coverage of adsorbed catalyst with the surface area during interface evolution. Experimental filling results are described and are shown to be consistent with the predictions. An associated effect on surface roughness of planar deposits is also described.
Characterization of porous indium tin oxide thin films using effective medium theory
Authors: Ederth J. , Niklasson G. A. , Hultåker A. , Heszler P. , Granqvist C. G. , van Doorn A. R. , Jongerius M. J. , Burgard D.
Published: January 2003

Effective medium theory was used to model optical properties in the 0.3 – 30 μm wavelength range for films comprised of nanoparticles of a transparent conducting oxide that are connected in a percolating network characterized by a filling factor f. The model is based on charge carrier density ne and resistivity ρ of the particles, and it enables analyses of these microscopic parameters upon posttreatment of the film. The theory was used to interpret data on spin coated layers consisting of nanoparticles of indium tin oxide (i.e., In2O3:Sn) with f close to the percolation limit. It showed that the as-deposited film contained nanoparticles with ne as large as ∼5×1020 cm−3 and ρ≈5×10−4 Ω cm. The model also provided important data on f, ne, and ρ after heat treatment of the film.
Birefringence study of the freezing mechanism of lanthanum-modified lead zirconate titanate relaxor ferroelectrics
Authors: Kim Dal-Young , Choi Jong-Jin , Kim Hyoun-Ee
Published: January 2003

The temperature dependence of the electric field-induced birefringence and electro-optic coefficient of Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 (PLZT) relaxor ferroelectrics were investigated. The remanent birefringence of the poled specimen disappeared around 268 K, thus indicating this to be the freezing temperature. On the other hand, a maximum value of the electro-optic coefficient was observed around 235 K, which is much lower than the freezing temperature. A brief discussion is given on the electro-optic coefficient behavior from the viewpoint of the freezing mechanism of PLZT relaxor ferroelectrics.
Influence of contamination on the dislocation-related deep level C1 line observed in deep-level-transient spectroscopy of n-type silicon: A comparison with the technique of electron-beam-induced current
Authors: Knobloch Klaus , Kittler Martin , Seifert Winfried
Published: January 2003

Misfit dislocations containing different amounts of contamination were analyzed by deep-level-transient spectroscopy (DLTS). The amount of dislocation contamination was determined from the temperature dependence of the dislocation contrast, c(T), measured by electron-beam-induced current (EBIC). The C1 line was not observed for clean 60° dislocations in n-type Si, but appeared upon contamination of dislocations by gold. The density of levels related to C1 measured by DLTS is in good agreement with the density of impurities determined from the EBIC c(T) behavior. The width of the distribution of energy levels that form the C1 line increases with the amount of contamination. It is rather sharp for dislocations contaminated with less than 105 impurities/cm dislocation length, and becomes broadened for dislocations with about 106 impurities/cm length (ΔE=50 meV). The experimental observations are explained by accommodation of impurities in both the core and in the strain field of the dislocations.
Effects of additive elements on improvement of the dielectric properties of Ta2O5 films formed by metalorganic decomposition
Authors: Salam K. M. A. , Fukuda Hisashi , Nomura Shigeru
Published: January 2003

Ta 2 O 5 -based composite thin films formed by metalorganic decomposition have been investigated with respect to their dielectric properties. The dielectric and insulating properties of composite (1−x)Ta2O5−xTiO2 and (1−x)Ta2O5−xWO3 thin films are found to be improved compared to those of pure Ta2O5 thin films. In particular, thin films with x=0.08 composition of additive TiO2 or WO3 to Ta2O5 exhibited superior dielectric and insulating properties. The maximum dielectric constant and charge storage density of composite films are about 20 and 53.6 fC/μm2, respectively, higher than those of pure Ta2O5 films (about 13 and 34.5 fC/μm2). The temperature coefficient of the dielectric constant of composite films dramatically decreases from 65 ppm/°C for pure Ta2O5 to less than 11 ppm/°C. The leakage current density of composite films is lower than 1×10−9 A/cm2 up to an applied electric field of 3 MV/cm. The dominant conduction is Poole–Frenkel conduction in the films according to the measurement temperature dependence of the I−V characteristics.
Modeling of disorder influenced Auger recombination in strained-layer type-II superlattices
Authors: Grein C. H. , Ehrenreich H.
Published: January 2003

The effects of disorder on Auger recombination lifetimes in strained-layer type-II superlattices are estimated. Disorder is modeled in two ways: (i) by completely neglecting momentum conservation in Auger transitions, and (ii) by including phonon scattering in the transitions. The purpose of (i) is to obtain establish bounds on the effects of disorder, and of (ii) to provide more realistic lifetime estimates for high-quality materials. In both cases, the disorder is assumed to be less severe than what would cause significant changes in the electronic band structure. The complete relaxation of momentum conservation results in Auger lifetimes decreasing by approximately one order of magnitude. However, electron–phonon effects have a relatively small effect because the interaction is weak. Hence, the benefits of band engineering in such superlattices remain substantial even in the presence of typical disorder.
Untrapped dynamics of molecules within an accelerating optical lattice
Authors: Dong Guangjiong , Lu Weiping , Barker P. F.
Published: January 2003

We investigate the dynamics of untrapped molecules within a far-off resonant accelerating optical lattice. Our analysis shows that untrapped molecules can be temporarily transported by the lattice, and those that are transported for the longest time reach a unique, well-defined critical velocity that depends on the mass to polarizability ratio of the molecular species. We show that this species-dependent critical velocity leads to a velocity dispersion for different species within a gas mixture. Our numerical simulations show that the velocity distribution of a multicomponent gas evolves to form well-separated peaks in velocity space for each species. We propose a time-of-flight analysis technique that transforms the velocity dispersion to a temporal separation of different species, even for small differences in the mass to polarizability ratio. Separation utilizing this concept is demonstrated for atmospheric species and isotopes of nitrogen. Finally, we present an extension of this concept for both temporal and angular dispersion.
Alignment of polyamic acid molecules containing azobenzene in the backbone structure: Effects of polarized ultraviolet light irradiation and subsequent thermal imidization
Authors: Sakamoto Kenji , Usami Kiyoaki , Kikegawa Manabu , Ushioda Sukekatsu
Published: January 2003

We have investigated the molecular orientation in the films of polyamic acid (PAA) with azobenzene units in the backbone structure. Anisotropic molecular orientation was induced by irradiation of linearly polarized ultraviolet light (LPUVL). The change in the molecular orientation caused by subsequent thermal imidization was also investigated. The orientation of the PAA and polyimide backbone structures was determined by measuring the polarized infrared absorption spectra of the films. When the PAA film was exposed to LPUVL of wavelength 365 to 400 nm at normal incidence, permanent orientational change of the PAA backbone structure occurred through repeated photoisomerization reactions of the azobenzene unit. The PAA backbone structure rotated toward the plane perpendicular to the polarization direction of LPUVL. In subsequent thermal imidization the molecular order increased significantly around the direction perpendicular to both the polarization direction of LPUVL and the surface normal. This enhancement of the molecular order was tentatively attributed to the crystallization of the film caused by thermal imidization.
Influence of dynamic annealing on the shape of channeling implantation profiles in Si and SiC
Authors: Posselt M. , Bischoff L. , Teichert J. , Ster A.
Published: January 2003

The influence of the dose rate and temperature on the dose dependence of the shape of Ge depth profiles obtained by channeling implantation into Si and SiC is investigated. A focused ion-beam system is employed which enables the application of two widely different dose rates (1011 and 1018 cm−2 s−1). Implantations into Si are performed at room temperature (RT) and 250 °C. SiC is implanted at RT, 225, 450, and 580 °C. The Ge depth distributions are measured by secondary ion mass spectrometry. The shape of the channeling implantation profiles is affected by the formation and evolution of complex defects formed during ion bombardment, since these defects cause significant dechanneling of the implanted particles. The competing influence of the dose rate and temperature on the shape of Ge depth profiles is explained in terms of intracascade defect relaxation. The time scale for the reduction of complex defects is estimated. At RT, in Si some defect relaxation occurs within the first 100 s after an ion impact. At temperatures of 225 and 250 °C, in SiC and Si, a considerable defect reduction is found within the first 10 μs as well as between 10 μs and 100 s after an ion impact. The complex defects in Si vanish entirely between 10 μs and 100 s, whereas in SiC some of them survive. At 450 and 580 °C, defects in SiC relax mainly within the first 10 μs after an ion impact. The defect reduction increases with growing implantation temperature. Different mechanisms which may be responsible for the dynamic annealing in Si and SiC are discussed. A phenomenological model is developed in order to treat the dose rate and temperature dependence of the defect-induced dechanneling within the framework of atomistic computer simulations of ion implantation. The simulated Ge depth profiles agree very well with the measured data.
Phase transitions and polarizations in epitaxial BaTiO3/SrTiO3 superlattices studied by second-harmonic generation
Authors: Jiang A. Q. , Scott J. F. , Lu Huibin , Chen Zhenghao
Published: January 2003

BaTiO 3 / SrTiO 3 superlattices are found to have a critical periodicity of 30 SrTiO3 unit cells, below which the room-temperature symmetry is mm2 orthorhombic. The orthorhombic-tetragonal phase-transition temperature is shifted upward from the bulk value (<3 K and normally not observed at finite temperature) of the quantum paraelectric pure strontium titanate to temperatures above ambient (599 K for 30/30-unit-cell superlattices). Antidipole patterns appear in superlattices with unit cells below 10/10. A model is presented that accounts quantitatively for the change in second-harmonic generation intensity with superlattice periodicity from 4 unit cells to 50.
Quasiparticle nonequilibrium dynamics in a superconducting Ta film
Authors: Li L. , Frunzio L. , Wilson C. M. , Prober D. E.
Published: January 2003

Nonequilibrium quasiparticle dynamics in Ta are studied using a superconducting Ta film with an Al tunnel junction connected at each end. The quasiparticle system is driven out of the equilibrium by absorption of an x-ray photon. Millions of quasiparticles, created by each photon, diffuse in the Ta film. When the quasiparticles reach the Al junctions they lose energy by emitting phonons and are trapped in the Al film. By measuring the tunneling current, the number of excess quasiparticles can be calculated. In Ta, the diffusion constant of 8.2±0.2 cm2/s and quasiparticle lifetime of 83±5 μs at 0.21 K are derived from fitting the measured current pulses, and are compared with theoretical predictions.
Quantum transport in a nanosize silicon-on-insulator metal-oxide-semiconductor field-effect transistor
Authors: Croitoru M. D. , Gladilin V. N. , Fomin V. M. , Devreese J. T. , Magnus W. , Schoenmaker W. , Sorée B.
Published: January 2003

An approach is developed for the determination of the current flowing through a nanosize silicon-on-insulator metal-oxide-semiconductor field-effect transistors. The quantum-mechanical features of the electron transport are extracted from the numerical solution of the quantum Liouville equation in the Wigner function representation. Accounting for electron scattering due to ionized impurities, acoustic phonons, and surface roughness at the Si/SiO2 interface, device characteristics are obtained as a function of a channel length. From the Wigner function distributions, the coexistence of the diffusive and the ballistic transport naturally emerges. It is shown that the scattering mechanisms tend to reduce the ballistic component of the transport. The ballistic component increases with decreasing the channel length.
Three-dimensional hydrogen microscopy using a high-energy proton probe
Authors: Dollinger G. , Reichart P. , Datzmann G. , Hauptner A. , Körner H.-J.
Published: January 2003

It is a challenge to measure two-dimensional or three-dimensional (3D) hydrogen profiles on a micrometer scale. Quantitative hydrogen analyses of micrometer resolution are demonstrated utilizing proton–proton scattering at a high-energy proton microprobe. It has more than an-order-of-magnitude better position resolution and in addition higher sensitivity than any other technique for 3D hydrogen analyses. This type of hydrogen imaging opens plenty room to characterize microstructured materials, and semiconductor devices or objects in microbiology. The first hydrogen image obtained with a 10 MeV proton microprobe shows the hydrogen distribution of the microcapillary system being present in the wing of a mayfly and demonstrates the potential of the method.
Organic optical bistable switch
Authors: Xue Jiangeng , Forrest Stephen R.
Published: January 2003

We demonstrate an organic optical bistable switch by integrating an efficient organic photodetector on top of a transparent electrophosphorescent organic light-emitting diode (TOLED). The bistability is achieved with an external field-effect transistor providing positive feedback. In the “LOW” state, the TOLED is off and the current in the photodetector is solely its dark current. In the “HIGH” state, the TOLED emits light that is directly coupled into the integrated photodetector through the transparent cathode. The photocurrent then is fed back to the TOLED, maintaining it in the HIGH state. The green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a 4,4′-N,N′-dicarbazole-biphenyl host was used as the luminescent material in the TOLED, while alternating thin layers of copper phthalocyanine and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole were used as the active region of the organic photodetector. The circuit has a 3 dB bandwidth of 25 kHz, and can be switched between HIGH and LOW using pulses as narrow as 60 ns. The bistable switch can be both electrically and optically reset, making it a candidate for image-retaining displays (e.g., electronic paper) and other photonic logic applications. The integrated organic device also has broad use as a linear circuit element in applications such as automatic brightness control.
Effects of crystallization on the electrical properties of ultrathin HfO2 dielectrics grown by atomic layer deposition
Authors: Kim Hyoungsub , McIntyre Paul C. , Saraswat Krishna C.
Published: January 2003

Microstructural evolution and resulting changes in electrical properties of atomic-layerdeposition-grown HfO2 on SiO2/Si substrates were studied as a function of annealing temperature in a N2 ambient. As deposited ∼30-Å-thick HfO2 on 15 and 25 Å thermal SiO2 were almost entirely amorphous, although a low density of crystalline seeds were observed and crystallization occurred from these nuclei during furnace anneals at temperatures >∼500 °C. The major crystalline phase thus formed was monoclinic, and some fraction of tetragonal phase was observed during crystallization according to transmission electron microscopy electron diffraction analysis. Complete crystallization occurred around 700 °C and, at higher temperatures, significant interfacial silicon dioxide growth was observed due to the presence of a small partial pressure of oxygen in the annealing ambient. No significant increase of leakage current in the trap-assisted tunneling conduction regime was observed during the intermediate and final stage of crystallization process.
Manipulation of vortices by magnetic domain walls
Authors: Goa P. E. , Hauglin H. , Olsen Å. A. F. , Shantsev D. , Johansen T. H.
Published: January 2003

In a type-II superconductor, the magnetic field penetrates in the form of thin filaments called vortices. The controlled behavior of these vortices may provide the basis for a new generation of nanodevices. We present here a series of experiments showing simultaneous manipulation and imaging of individual vortices in a NbSe2 single crystal. The magnetic field from a Bloch wall in a ferrite garnet film (FGF) is used to manipulate the vortices. High-resolution magneto-optical imaging enables real-time observation of the vortex positions using the Faraday effect in the same FGF. Depending on the thickness of the sample, the vortices are either swept away or merely bent with the Bloch wall.
Interlayer segregation in magnetic multilayers and its influence on exchange coupling
Authors: Yu G. H. , Li M. H. , Zhu F. W. , Jiang H. W. , Lai W. Y. , Chai C. L.
Published: January 2003

Experimental results show that the exchange coupling field (Hex) of NiFe/FeMn for Ta/NiFe/FeMn/Ta multilayers is higher than that for spin-valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. X-ray photoelectron spectroscopy shows that Cu atoms segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. While studying Ta/X(X=Bi,Pb,Ag,In)/NiFe/FeMn multilayers, we also find that X atoms segregate to the NiFe/FeMn interface, which results in a decrease of the Hex. However, a small amount of Bi, Pb, etc. deposited between Cu and pinned NiFe layer for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers can increase Hex.
Determination of spin distributions in ion-beam magnetic textured iron films by magnetic orientation Mössbauer spectroscopy
Authors: Müller G. A. , Gupta R. , Lieb K.-P. , Schaaf P.
Published: January 2003

A promising approach by which to gain information about the spin distributions in different sample layers using depth-sensitive angle-dependent conversion electron Mössbauer spectroscopy is presented in combination with magneto-optical Kerr effect, Rutherford backscattering spectrometry and x-ray diffraction investigations. Three 77 nm thick Fe films containing a 15 nm Fe57 marker layer at different depths were deposited on Si(100) substrates and irradiated under well defined mechanical stress with 200 keV Xe ions. The distribution of the spins in the sample plane was analyzed by Mössbauer spectroscopy and by tilting the sample toward the incident γ beam and observing the variation in angle-dependent relative intensity of the sextet lines. For the sample with the sensitive marker layer in the middle of the ferromagnetic film, all spins were found to be oriented exactly along the mechanical stress axis. This agrees with the angular properties of the macroscopic hysteresis curves measured by the magneto-optic Kerr effect.
Influence of the thickness and doping of the emission layer on the performance of organic light-emitting diodes with PiN structure
Authors: Huang Jingsong , Blochwitz-Nimoth Jan , Pfeiffer Martin , Leo Karl
Published: January 2003

We have studied the behavior of various intrinsic emission zones on the characteristics of organic light-emitting diodes with a p-doped hole-transport layer and an n-doped electron-transport layer based on our previous work [J. S. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, and S. Liu, Appl. Phys. Lett. 80, 139 (2002)]. This configuration is referred to as a PiN structure. Because the p- and n-doped regions occupy nearly 80% of the total thickness in our PiN device, the intrinsic region becomes a narrow layer between two doped regions. This intrinsic region includes the region where the radiative recombination occurs. Thus, the nature of this layer plays an important role in determining the actual device performance. Employing 8-tris-hydroxyquinoline aluminum as an emitter, we investigated the influence of the thickness of the emitter layer on the performance of the device. The optimum thickness of the emitter layer is found to be 20 nm. Combining the fluorescence dye doping method, we have optimized the PiN structure device. Two emitter systems have been used: Alq3 doped with two highly fluorescent laser dyes, Quinacridone or Coumarin 6, respectively. We have demonstrated the influence of the thickness and the doping of the emission zone on the characteristics of a doped emitter device with PiN structure, and obtained higher-efficiency PiN structure devices. The different properties of PiN devices corresponding to two different emitter dopants with different trapping effect are also discussed.
Observation of defect complexes containing Ga vacancies in GaAsN
Authors: Toivonen J. , Hakkarainen T. , Sopanen M. , Lipsanen H. , Oila J. , Saarinen K.
Published: January 2003

Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018 cm−3 with increasing N composition and decreases after annealing at 700 °C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers.
Nonvolatile memory element based on a ferroelectric polymer Langmuir–Blodgett film
Authors: Reece Timothy J. , Ducharme Stephen , Sorokin A. V. , Poulsen Matt
Published: January 2003

We report the operation of a potential nonvolatile bistable capacitor memory element consisting of a metal gate, a 170 nm thick ferroelectric Langmuir–Blodgett film of vinylidene fluoride (70%) with trifluoroethylene (30%) copolymer, and a 100 nm thick silicon-oxide insulating layer, all deposited on an n-type silicon semiconductor substrate. The device exhibited clear capacitance hysteresis as the gate voltage was cycled between ±25 V, with a capacitance dynamic range of 8:1 and threshold voltage shift of 2.8 V. The results are in good agreement with the model of Miller and McWhorter [J. Appl. Phys. 72, 5999 (1992)].
Isolated hexaphenyl nanofibers as optical waveguides
Authors: Balzer F. , Bordo V. G. , Simonsen A. C. , Rubahn H.-G.
Published: January 2003

Laser-supported, dipole-assisted self-assembly results in blue-light guiding nanostructures, namely single-crystalline nanofibers of hexaphenyl molecules. The nanofibers are up to 1 mm long, extremely well-aligned to each other and their cross sections can be tuned to span the range from nonguiding to guiding single optical modes at λ=425.5 nm. An analytical theory for such organic waveguides can reproduce quantitatively the experimentally observed behavior. From the measured damping of propagating, vibrationally dressed excitons the imaginary part of the dielectric function of isolated nanoscaled organic aggregates is determined.
Impact of microstructure on transport properties of nanometric epitaxial SrRuO3 films
Authors: Herranz G. , Martı́nez B. , Fontcuberta J. , Sánchez F. , Garcı́a-Cuenca M. V. , Ferrater C. , Varela M.
Published: January 2003

We report on structural and magnetotransport properties of SrRuO3 films—grown on SrTiO3—and its dependence on thickness t (4 nm⩽t⩽320 nm). At early stages of film growth, a self-ordered finger-like pattern of growth units is formed and gives rise to a prominent in-plane anisotropy of transport properties. We argue that this behavior originates from defective regions formed at grain-merging regions and we show that appropriate annealing allows eliminating these defects. In addition, there is a progressive shift towards a lower temperature of the characteristic feature associated with the onset of the ferromagnetism. Since the films are fully strained, we argue that both effects are not associated to homogeneous cell modifications, but reflects the microstructural disorder concomitant to the island growth observed at early stages of film growth, the resulting stress distribution and the polymorphism of SrRuO3.
Time response in tunneling to a pn junction
Authors: Young Park Jeong , Phaneuf R. J.
Published: January 2003

We report on the variation in the response time in scanning tunneling microscopy (STM) measurements across a Si pn junction, and its effect on STM images of such devices. The response time of the tip height subsequent to a stepwise change in the voltage at a fixed demanded current varies by almost two orders of magnitude across the junction, with the slowest response of nearly 80 ms corresponding to the tip–sample junction in strong inversion. Measurements at decreasing separation show an increase in response time up to a saturation point, consistent with screening due to the inversion charge. The slow response in regions where the tip–sample junction is in inversion explains observations of anomalously deep features in STM topography images and scan direction-dependent features in STM conductance maps.