bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-voltage 0.1 μm organic transistors and complementary inverter circuits fabricated with a low-cost form of near-field photolithography
Authors: Rogers John A. , Dodabalapur Ananth , Bao Zhenan , Katz Howard E.
Published: July 2002

This letter describes the combined use of a form of near-field photolithography that relies on a conformable phase masks with microcontact printing and shadow masking for low-cost fabrication of organic transistors and simple complementary inverter circuits with critical dimensions of ∼0.1 μm. The good performance of the devices and their low-voltage operation make them and the fabrication procedures potentially attractive for many applications.
Broadband phase-matched difference frequency mixing of femtosecond pulses in GaSe: Experiment and theory
Authors: Kaindl R. A. , Eickemeyer F. , Woerner M. , Elsaesser T.
Published: July 2002

Phase-matched difference frequency mixing within the broad spectrum of single 13 fs pulses from a cavity-dumped, mode-locked Ti:sapphire laser in a GaSe crystal allows the generation of femtosecond mid-infrared pulses, continuously tunable in the wavelength range from 7 to 20 μm. Pulse durations down to 95 fs at 9.3 μm are directly measured. Model calculations including the full dispersion of GaSe show that the chirp of the pump pulses allows to change both the pulse duration and conversion efficiency obtained in the nonlinear process, in agreement with the experiment.
Evidence of multiple states of ordered structures and a phase transition in magnetic fluid films under perpendicular magnetic fields
Authors: Hong Chin-Yih , Horng Herng-Er , Kuo F. C. , Yang S. Y. , Yang H. C. , Wu J. M.
Published: July 2002

When a magnetic fluid thin film is subjected to a perpendicular magnetic field H, the particles in the film agglomerate and form particle columns. With the increasing H, the columns evolve from a disordered column phase to the first-level hexagonal structural pattern and finally reach the second-level hexagonal structural pattern through a phase transition. During the transition phase, each column split into two columns. The split of a column may be attributed to the further alignment of the magnetic particles in each column under higher H’s. Hence the dipolar repulsive force in each column becomes dominant and causes the split of a column. The formation of the second-level hexagonal structure reveals that the minimum energy of the equilibrium system of magnetic fluid thin films under perpendicular magnetic fields has multiple local minima when H varies.
SiO
2
/
6H-SiC
(0001)3×3
initial interface formation by Si overlayer oxidation
Authors: Amy F. , Soukiassian P. , Hwu Y.-K. , Brylinski C.
Published: July 2002

We investigate the initial oxidation and SiO2/6H-SiC interface formation by core level photoemission spectroscopy using synchrotron radiation. The results indicate that the direct oxidation of the 6H–SiC(0001)3×3 surface leads to SiO2 formation at low temperatures (500 °C) with a nonabrupt interface having significant amounts of mixed (Si–O–C) and intermediate (Si3+,Si2+,Si+) oxidation products. In contrast, C-free and a much more abrupt SiO2/6H-SiC(0001) interface formation is achieved when predeposited Si overlayer is thermally oxidized at low oxygen exposures and low temperatures (500 °C).
Resputtering during the growth of pulsed-laser-deposited metallic films in vacuum and in an ambient gas
Authors: Fähler S. , Sturm K. , Krebs H.-U.
Published: July 2002

To determine the effective sputter yield during pulsed-laser deposition a method by measuring the deposition rate on tilted substrates is proposed. Under vacuum conditions, sputter yields of up to 0.17 and 0.55 were found at a laser fluence of 4.5 J/cm2 for Fe and Ag, respectively. These strong resputtering effects are induced by the large fraction of energetic ions occurring during deposition. With decreasing laser fluence or increasing Ar gas pressure, the sputter yields are reduced due to a decrease of the kinetic energy of the ions. For the deposition of stoichiometric films, an optimum Ar partial pressure of about 0.04 mbar exists, where the deposition rate is highest and the sputter yield is reduced.
Anomalous features in the optical properties of Al1−xInxN on GaN grown by metal organic vapor phase epitaxy
Authors: Yamaguchi Shigeo , Kariya Michihiko , Nitta Shugo , Takeuchi Tetsuya , Wetzel Christian , Amano Hiroshi , Akasaki Isamu
Published: July 2002

We have studied the optical properties of Al1−xInxN thin films grown on GaN by metal organic vapor phase epitaxy. X-ray diffraction analysis of ω and ω-2θ scans showed that both the compositional fluctuation and the degree of crystalline mosaicity increase with increasing x. While the energy positions of both the absorption edge and photoluminescence peak shift to a lower-energy region with increasing x, the linewidth of the photoluminescence spectra and the value of the absorption edge tail decrease. The Stokes shift also decreases with increasing x, following which both energy positions become 1.66 eV, which is smaller than the band gap of InN (1.9 eV). These anomalous features of the optical properties of Al1−xInxN might be affected by the absorption in the infrared region caused by the high electron concentration.
Vacancy formation in (Pb,La)(Zr,Ti)O3 capacitors with oxygen deficiency and the effect on voltage offset
Authors: Friessnegg T. , Aggarwal S. , Ramesh R. , Nielsen B. , Poindexter E. H. , Keeble D. J.
Published: July 2002

Vacancy-related defect profiles have been measured for La0.5Sr0.5CoO3/(Pb0.9La0.1)(Zr0.2Ti0.8)O3/La0.5Sr0.5CoO3 ferroelectric capacitors using a variable-energy positron beam. By varying the layer thickness and the postgrowth processing in a reducing ambient, a capacitor showing oxygen deficiency dominantly in the top electrode and one with deficiency in both electrodes were produced. The capacitor with an asymmetric defect profile showed a voltage offset polarization–voltage hysteresis loop, that with a symmetric distribution of vacancy-related defects showed no offset. These results are discussed in the context of current models for imprint.
Highly efficient blue-green emission from organic light-emitting diodes using dibenzochrysene derivatives
Authors: Tokito Shizuo , Noda Koji , Fujikawa Hisayoshi , Taga Yasunori , Kimura Makoto , Shimada Kou , Sawaki Yasuhiko
Published: July 2002

We have fabricated highly efficient organic light-emitting diodes (OLEDs) using new hole-transporting emissive materials based on dibenzochrysene. Hole drift mobilities of the dibenzochrysene derivatives were measured in the vacuum-deposited films and found to be 5×10−4–2×10−3 cm2/V s (at 5×105 V/cm). The OLEDs consist of an emitting layer of the dibenzochrysene derivative and an electron-transport layer of tris(8-quinolinolato) aluminum. Emission colors of the OLEDs were blue-green and their spectra were consistent with the photoluminescence spectra with a peak wavelength of 490 nm. High external quantum efficiency of 2% was obtained at a luminance of 300 cd/m2, and good durability in a continuous operation at room temperature and high temperatures was achieved.
Superlattice-like stacking fault and phase separation of InxGa1−xN grown on sapphire substrate by metalorganic chemical vapor deposition
Authors: Cho H. K. , Lee J. Y. , Kim K. S. , Yang G. M.
Published: July 2002

In x Ga 1−x N alloys were directly grown on sapphire substrate with a GaN nucleation layer. The degree of phase separation in the InGaN layer on sapphire substrate is maximized at higher growth temperature than for an InGaN layer grown on a thick GaN layer. For high indium composition, a superlattice-like stacking fault in the InxGa1−xN grown on sapphire substrate was detected by the selected area diffraction pattern and high-resolution transmission electron microscopy. The superlattice-like arrangement of stacking faults leads to the formation of split spots and the distance of split spots corresponds to the distance between stacking faults.
Ferroelectric domain inversion by electron beam on LiNbO3 and Ti:LiNbO3
Authors: Restoin C. , Darraud-Taupiac C. , Decossas J. L. , Vareille J. C. , Hauden J. , Martinez A.
Published: July 2002

Periodically poled titanium in-diffused lithium niobate (Ti:LiNbO3) seems to be an attractive candidate for applications in the field of integrated optics for wavelength division multiplexed networks it could allow building of polarization-independent devices. In this study, we present the fabrication of reversed ferroelectric domain gratings by direct electron beam writing on both clear LiNbO3 and Ti:LiNbO3. First, a preliminary study was carried out on clear lithium niobate in order to determine the irradiation parameters (voltage, probe current, scanning speed) needed to create inverted ferroelectric domains, and then the same irradiation parameters were used for Ti:LiNbO3. The irradiated patterns were observed after chemical etching using a scanning electron microscope and optical microscopy. HeNe laser diffraction through the samples shows that the irradiation induces a variation of the linear optical refractive index in Ti:LiNbO3 but not in LiNbO3. Moreover, no topographical modification was observed before chemical etching by atomic force microscopy on either sample.
A vortex-based model for the subgrid flux of a passive scalar
Authors: Pullin D. I.
Published: July 2002

A model for the flux of a passive scalar by the subgrid motions in the large-eddy simulation of turbulent flow is proposed within the framework of the stretched-vortex subgrid stress model. The model is based on an analytical solution for the winding of a scalar field by an elemental subgrid vortex. This gives a tensor gradient-diffusion expression for the local flux of the scalar with subgrid turbulent diffusivity which depends upon the subgrid energy, the local cell size, and the vortex orientation in space. The scalar-flux subgrid model is tested by comparison of the results of 323 large-eddy simulation of passive-scalar transport by forced isotropic turbulence in the presence of a mean scalar gradient, with the direct-numerical simulation results of Overholt and Pope [Phys. Fluids 8, 2128 (1996)]. The present large-eddy simulation results predict that at large Taylor–Reynolds numbers, the ratio of the scalar variance to the squared product of the scalar gradient with the dissipation length of the turbulence, is asymptotic to a nearly constant value c′2/(α1 Lε)2≈0.36.
Epitaxial growth of Pt and oxide multilayers on MgO by laser ablation
Authors: Morcrette M. , Gutierrez-Llorente A. , Seiler W. , Perrière J. , Laurent A. , Barboux P.
Published: July 2002

We have studied the epitaxial growth of Pt films on MgO substrates by laser ablation. Depending upon oxygen pressure during deposition, different Pt film textures were observed. Pure (001) films are grown under oxygen (0.1 mbar), while (111) films are formed under high vacuum (<10−6 mbar). By the complementary use of Rutherford backscattering spectrometry in channeling geometry and x-ray diffraction, the crystalline quality was found nearly perfect. Finally multilayered structures with LiCoO2 or LiMn2O4 layer on Pt film on MgO substrates were epitaxially grown by laser ablation. Such oxide films could present interesting properties as insertion compounds in lithium batteries, and we demonstrate that LiMn2O4 could be used for the accurate determination of lithium concentration in solution.
Persistence of zero velocity fronts in reaction diffusion systems
Authors: Kramer Lorenz , Gottwald Georg , Krinsky Valentin I. , Pumir Alain , Barelko Viktor V.
Published: July 2002

Steady, nonpropagating, fronts in reaction diffusion systems usually exist only for special sets of control parameters. When varying one control parameter, the front velocity may become zero only at isolated values (where the Maxwell condition is satisfied, for potential systems). The experimental observation of fronts with a zero velocity over a finite interval of parameters, e.g., in catalytic experiments [Barelko et al., Chem. Eng. Sci., 33, 805 (1978)], therefore, seems paradoxical. We show that the velocity dependence on the control parameter may be such that velocity is very small over a finite interval, and much larger outside. This happens in a class of reaction diffusion systems with two components, with the extra assumptions that (i) the two diffusion coefficients are very different, and that (ii) the slowly diffusing variables has two stable states over a control parameter range. The ratio of the two velocity scales vanishes when the smallest diffusion coefficient goes to zero. A complete study of the effect is carried out in a model of catalytic reaction.
Third-order derivatives of the dipole moment function for the ozone molecule
Authors: Sulakshina O. N. , Borkov Yu. G. , Tyuterev Vl. G. , Barbe A.
Published: July 2002

The third-order contributions to the dipole moment function of ozone has been evaluated from available experimental values of the transition moment parameters for the second overtone and ternary combination vibration–rotation bands. The calculations are based on the formalism of effective dipole moment operators. The purely vibrational part of the transformed transition moment operators for three-quanta bands is presented in the form suitable for an iterative programming. It allows one to determine the values and to make the optimal choice of signs of the third derivatives of the dipole moment function using the transition moment parameters deduced from experimental spectra. The estimation of the errors have been done by an error propagation of uncertainties in anharmonicity parameters of the potential function and previously determined first- and secondorder dipole moment derivatives.
Energy levels and optical properties of neodymium-doped barium fluorapatite
Authors: Stefanos Sennay M. , Bonner Carl E. , Meegoda Chandana , Rodriguez Waldo J. , Loutts George B.
Published: July 2002

Energy levels of the 4f3 electronic configuration of Nd3+ in barium fluorapatite, Ba5(PO4)3F(B-FAP) have been determined from polarized absorption and fluorescence spectra using crystals at 8 K. Experimental energy-level assignments were made initially by comparing the crystal spectra energy levels with those obtained from those previously reported for Nd3+ in strontium fluorapatite and fluorapatite. The initial crystal-field parameters were calculated by using lattice summation techniques. The crystal-field parameters were varied to obtain a best fit between experimental and theoretical energies and the final values give a root-mean-square deviation of 7.1 cm−1. The odd-fold crystal-field components are used to calculate the emission intensities and lifetimes of the Nd3+ ions in B-FAP. These calculations yield results in good agreement with the experimental measurements of the absorption and emission cross sections and lifetimes.
Experimental determination of the transmission factor for the Omicron EA125 electron analyzer
Authors: Ruffieux P. , Schwaller P. , Gröning O. , Schlapbach L. , Gröning P. , Herd Q. C. , Funnemann D. , Westermann J.
Published: July 2002

In this article a study of the transmission factor of the Omicron EA125 analyzer equipped with the universal lens is presented. The procedure is based on a model by Cross and Castle [J. Electron Spectrosc. Relat. Phenom. 22, 53 (1981)] and is applicable to every spectrometer which can be operated in the constant analyzer energy (CAE) and in the constant retarding ratio measuring mode. The advantage of the method is its independence on the sample and on the inelastic mean free path of the electrons. We find that the transmission factor for the CAE mode is proportional to Ekin−1 for most measuring setups. This dependence is predicted by theory for an ideal analyzer. Deviations from this behavior are observed if the retarding ratio for a given kinetic energy is too small. The limit value of the retarding ratio for ideal behavior, i.e., an Ekin−1 transmission factor, depends on the analyzer entrance slit aperture which has been selected.
Improved drive voltages of organic electroluminescent devices with an efficient p-type aromatic diamine hole-injection layer
Authors: Ganzorig Chimed , Fujihira Masamichi
Published: July 2002

An SbCl5-doped N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) thin film was studied as a hole-injection layer in low-molecular-weight organic electroluminescent (EL) devices. EL characteristics of devices with a TPD hole-injection layer doped with other oxidizing reagents, such as iodine, FeCl3, and tris(4-bromophenyl)aminium hexachloroantimonate were compared with that of SbCl5-doped TPD. The device with SbCl5-doped TPD on a cleaned indium–tin–oxide (ITO) substrate exhibited the best performance of all the devices studied. The improvement in device performance was attributed to an increase in work function of ITO due to acid formation as a result of hydrolysis of SbCl5 and by thinning the tunneling barrier for hole injection due to formation of the space charge region in highly doped TPD with SbCl5.
Linear eddy simulations of Reynolds number and Schmidt number effects on turbulent scalar mixing
Authors: Chakravarthy V. K. , Menon S.
Published: July 2002

Effects of molecular diffusion on turbulent scalar mixing are studied using the linear eddy model. Unlike the energy spectrum which is determined only by the viscous and eddy time scales, the scalar spectra also depend on the diffusion time scales. The linear eddy model, being one-dimensional, offers an inexpensive way of capturing these time scale dependencies. Some of the observations made using experiments and direct simulations are verified using the model. Simulations reported here indicate that certain features of scalar mixing continue to depend on the Reynolds number (Reλ) and the Schmidt number (Sc) in the range of parameter space (32⩽Reλ⩽775 and 0.001⩽Sc⩽700) that is unattainable using the current direct simulation capabilities. However, some of the models developed for differential diffusion using direct simulations are shown to be accurate even when the difference in the Schmidt numbers of the scalars and (or) the Reynolds numbers are very high.
Correlation between leakage current density and threading dislocation density in SiGe p-i-n diodes grown on relaxed graded buffer layers
Authors: Giovane Laura M. , Luan Hsin-Chiao , Agarwal Anuradha M. , Kimerling Lionel C.
Published: July 2002

A correlation between bulk leakage current density and threading dislocation density in silicon–germanium mesa-isolated diodes fabricated on relaxed graded buffer layers is presented. Si0.75Ge0.25 p-i-n diodes were grown on SiGe graded buffers with different grading rates. Graded buffers with different grading rates yielded “virtual substrates” with varying densities of threading dislocations. Bulk leakage current densities were differentiated from surface leakage currents by using p-i-n diodes with different areas. We demonstrate that the increase in bulk leakage current density in SiGe p-i-n diodes can be modeled by generation processes assisted by deep levels related to threading dislocations.
Microstructural and photoluminescence studies of germanium nanocrystals in amorphous silicon oxide films
Authors: Choi W. K. , Ho Y. W. , Ng S. P. , Ng V.
Published: July 2002

Ge nanocrystal growth in amorphous silicon oxide film was studied using the transmission electron microscopy and x-ray photoelectron spectroscopy techniques. The as-sputtered sample contained mainly GeO2 and Ge suboxides. GeO2 and/or suboxides dissociate when rapid thermal annealed and provide Ge for nanocrystal formation. Nanocrystals of similar size (∼60 Å in diameter) were obtained when annealed at 800 °C. Nanocrystals with diameters of 200–280 Å consisting of multiple twin structures near the Si–SiO2 interface were observed when annealed at 1000 °C. The twin structure was attributed to the enhanced diffusion of Ge at 1000 °C and the short annealing time. Our photoluminescence (PL) results show that the best PL response was obtained with samples that exhibit uniform nanocrystal size.
Negative ion density fronts
Authors: Kaganovich Igor
Published: July 2002

Negative ions tend to stratify in electronegative plasmas with hot electrons (electron temperature Te much larger than ion temperature Ti, Te≫Ti). The boundary separating a plasma containing negative ions, and a plasma without negative ions, is usually thin so that the negative ion density falls rapidly to zero—forming a negative ion density front. Theoretical, experimental, and numerical results giving the spatio-temporal evolution of negative ion density fronts during plasma ignition, the steady state, and extinction (afterglow) are reviewed. During plasma ignition, negative ion fronts are the result of the break of smooth plasma density profiles during nonlinear convection. In a steady-state plasma, the fronts are boundary layers with steepening of ion density profiles due to nonlinear convection also. But during plasma extinction, the ion fronts are of a completely different nature. Negative ions diffuse freely in the plasma core (no convection), whereas the negative ion front propagates towards the chamber walls with a nearly constant velocity. The concept of fronts turns out to be very effective in the analysis of plasma density profile evolution in strongly nonisothermal plasmas.
Epitaxial growth of dielectric Ba0.6Sr0.4TiO3 thin film on MgO for room temperature microwave phase shifters
Authors: Chen C. L. , Shen J. , Chen S. Y. , Luo G. P. , Chu C. W. , Miranda F. A. , Van Keuls F. W. , Jiang J. C. , Meletis E. I. , Chang H. Y.
Published: July 2002

Dielectric Ba0.6Sr0.4TiO3 thin films were epitaxially grown on (001) MgO by using pulsed laser ablation. Microstructure studies from x-ray diffraction and electron microscopy suggest that the as-grown films are c-axis oriented with an interface relationship of 〈100〉BSTO//〈100〉MgO. A room temperature coupled microwave phase shifter has been developed with a phase shift near 250° at 23.675 GHz under an electrical field of 40 V/μm and a figure of merit of ∼53°/dB. The performance of the microwave phase shifter based on the epitaxial Ba0.6Sr0.4TiO3 thin films on (001) MgO is close to that needed for practical applications in wireless communications.
Full-band simulation of indirect phonon assisted tunneling in a silicon tunnel diode with delta-doped contacts
Authors: Rivas Cristian , Lake Roger , Klimeck Gerhard , Frensley William R. , Fischetti Massimo V. , Thompson Phillip E. , Rommel Sean L. , Berger Paul R.
Published: July 2002

Full-band simulations of indirect, phonon assisted, interband tunneling are used to calculate the current–voltage response of a low-temperature molecular-beam-epitaxy-grown silicon tunnel diode with delta-doped contacts. Electron confinement in the contacts results in weak structure in the current–voltage characteristic. The structure is lost when finite lifetime effects are included. The approach uses the nonequilibrium Green function formalism in a second-neighbor sp3s* planar orbital basis.
Study of copper silicide retardation effects on copper diffusion in silicon
Authors: Lee C. S. , Gong H. , Liu R. , Wee A. T. S. , Cha C. L. , See A. , Chan L.
Published: July 2002

A B-buried layer with a dose of 1×1014 atoms/cm2 was introduced into p-doped Si at a depth of 2.2 μm to enhance copper diffusion via its inherent gettering effect. Copper was then introduced into silicon either via a low-energy implantation followed by a thermal anneal, or through the thermal drive in of physical vapor deposited (PVD) copper film. Secondary ion mass spectrometry depth profiling of both annealed samples later indicated that while substantial amounts of copper was gettered by the B layer in the former sample, no copper was gettered by the B-buried layer in the latter sample. Further analysis with an x-ray diffraction technique showed that copper silicide, Cu3Si was formed in the latter sample. It is thus surmised that the formation of this silicide layer impeded the diffusion of copper towards the B-buried layer. This work investigates the cause of CuSix formation and the underlying reasons for the lower mobility of Cu in PVD Cu film samples.
Physicochemical Properties of Higher Nonaromatic Hydrocarbons: A Literature Study
Authors: Verbruggen Eric M. J. , Hermens Joop L. M. , Tolls Johannes
Published: July 2002

Few data are available on the behavior of higher aliphatic hydrocarbons in the environment and especially in relation to hydrophobicity. Sorption to soil and bioconcentration by aquatic organisms are examples of processes that can be estimated from hydrophobicity parameters, such as aqueous solubility and octanol-water partition coefficient. A literature study was carried out on these physicochemical properties of aliphatic hydrocarbons containing ten or more carbon atoms. In general, very few experimental data were found, probably as a consequence of the fact that these parameters are very difficult to determine for these compounds. Reliable data on hydrophobicity parameters, for example, were not found for n-alkanes higher than tridecane.
Slow β process in simple organic glass formers studied by one- and two-dimensional H2 nuclear magnetic resonance. I
Authors: Vogel M. , Rössler E.
Published: July 2002

We study the Johari–Goldstein β process of organic glass formers by one- (1D) and two-dimensional (2D) H2 nuclear magnetic resonance (NMR). In particular, we compare systems with pronounced secondary relaxation in dielectric spectroscopy, namely toluene-d5 and polybutadiene-d6 (PB), with compounds which do not exhibit a distinct β peak, i.e., glycerol-d5 and polystyrene-d3 (PS). Choosing large interpulse delays in the applied echo pulse sequences we increase the sensitivity on small angle rotational jumps. This way, we are able to probe clearly the β process of toluene and PB in the line shape of 1D 2H NMR spectra and in the orientational correlation functions of 2D 2H NMR in time domain which is not possible when using the conventional techniques. Below the glass transition temperature Tg, the secondary relaxation of both glass formers is caused by a highly restricted reorientation of essentially all molecules. Comparing our results with simulations we estimate that the reorientation of most toluene molecules and PB monomeric units is characterized by an amplitude χ<10°. This amplitude is approximately unchanged below Tg, but strongly increases above the glass transition. Closer investigating the 1D 2H NMR line shape for large interpulse delays we moreover demonstrate that the reorientation involved in the β process takes place step-by-step via many elementary rotational jumps. On the other hand, for glycerol and PS, hardly any effects are observed in 1D and 2D 2H NMR experiments below Tg when applying comparable experimental parameters. We conclude that reorientations with an amplitude χ>1° do not occur on a time scale of μs−ms for the majority of molecules in glassy glycerol and PS.
Quantitative evaluation of electron beam writing in passivated gold nanoclusters
Authors: Bedson T. R. , Palmer R. E. , Jenkins T. E. , Hayton D. J. , Wilcoxon J. P.
Published: July 2002

We report a quantitative investigation of direct electron beam writing in monolayer films of passivated gold nanoclusters. In this process, the passivating organic ligands are (partially) removed to create gold-based nanostructures. We report the fabrication of lines with width as narrow as 26 nm, while measurements of the linewidth as a function of dose allow us to obtain a quantitative measure of the sensitivity, for comparison with established negative tone resists.
Strain effects on As and Sb segregates immersed in annealed GaAs and GaSb by Raman spectroscopy
Authors: Campos C. E. M. , Pizani P. S.
Published: July 2002

Raman scattering was used to obtain information about residual strains on As and Sb segregates immersed in GaAs and GaSb annealed at high temperatures. Eg and A1g modes of As and Sb in annealed GaAs and GaSb Raman spectra showed that the segregates are in the crystalline phase. Tensile stresses on As segregates were evident, while the Sb segregates were found to be practically relaxed. The temperature dependence of the stresses on segregates were determined. The different annealing temperatures used in this work were found to not cause substantial changes in stresses on segregates.
Enhancement of the molecular nitrogen dissociation levels by argon dilution in surface-wave-sustained plasmas
Authors: Tabbal M. , Kazopoulo M. , Christidis T. , Isber S.
Published: July 2002

In this work, the nitrogen molecular dissociation level in Ar/N2 surface-wave plasma is evaluated as a function of plasma parameters such as Ar percentage in the gas mixture, power absorbed in the plasma, and total pressure in order to design an efficient N-atom source that can be used for various applications such as thin-film deposition and materials surface modification. This plasma is operated at 40.68 MHz and the nitrogen dissociation rate is determined, in the remote plasma, by analyzing the optical emission of the first positive molecular nitrogen band. For all operating conditions, the dissociation rate ([N]/[N2]) of N2 molecules was enhanced, as the percentage of Ar in the mixture increased from 0 to ∼95%, and dissociation rates higher than 2.5% were measured. This gain in the dissociation rate became more pronounced when the plasma power and total pressure increased from 40 to 120 W and from 4 to 7.5 Torr, respectively. These results are discussed in terms of the kinetics of the electrons, nitrogen atoms, and molecules and confirm theoretical kinetic models presented in the literature.
A simple model for baroplastic behavior in block copolymer melts
Authors: Ruzette A.-V. G. , Banerjee P. , Mayes A. M. , Russell T. P.
Published: July 2002

A simple model for the free energy of mixing of compressible polymer blends is derived, based on the regular solution model. Its ability to predict phase behavior for weakly interacting polymer pairs using only the pure component properties of mass density, solubility parameter, and thermal expansion coefficient is illustrated for mixtures of polystyrene and poly(n-alkyl methacrylates) with n⩽12. The model yields a clear explanation for the strong pressure effects observed in some of these systems, including the first reported baroplastic elastomer.
Compact and fast response ultrasonic detection device based on two-wave mixing in a gallium arsenide photorefractive crystal
Authors: Campagne Benjamin , Blouin Alain , Pujol Lionel , Monchalin Jean-Pierre
Published: July 2002

An improved phase demodulator for the detection of ultrasound based on two-wave mixing (TWM) in the diffusion regime in a semi-insulating photorefractive GaAs crystal is presented. A new optical layout is proposed in which the total laser power is injected into the crystal to reduce the grating buildup time. The device is then less sensitive to ambient vibrations or motion of the inspected part. Another new feature of the device is a balanced receiver based on large area InGaAs detectors. The measured rejection ratio of this balanced receiver is 50 dB. However, in this new optical configuration the signal and pump beam paths cannot be made equal which results in some sensitivity to high frequency laser phase noise. The performance of the device operated with a cw and a pulsed laser source is described. A comparison with the conventional confocal Fabry–Perot in the transmission mode is also presented. As expected, the GaAs-based TWM photorefractive system is less sensitive than the Fabry–Perot at high frequencies but becomes more sensitive below some frequency value. For the 30 W pumping level used into the crystal and a 1 m long Fabry–Perot with 85% reflectivity mirrors, this value was found to be about 1.5 MHz. This feature is known to be useful in the cases where low frequencies have to be detected, higher frequencies being attenuated by the material. This is the case of coarse microstructure materials or viscous or mushy materials. This photorefractive configuration is also attractive like all photorefractive systems for compactness and the absence of any need for active stabilization. To obtain a sensitivity equal or better than that of the confocal Fabry–Perot in the transmission mode, this new setup can be used with an InP:Fe or a CdTe:V photorefractive crystal under an applied field.
Photoluminescence of nanostructured PbTiO3 processed by high-energy mechanical milling
Authors: Leite E. R. , Santos L. P. S. , Carreño N. L. V. , Longo E. , Paskocimas C. A. , Varela J. A. , Lanciotti F. , Campos C. E. M. , Pizani P. S.
Published: July 2002

This letter reports on a process to prepare nanostructured PbTiO3 (PT) at room temperature with photoluminescence (PL) emission in the visible range. This process is based on the high-energy mechanical milling of ultrafine PbTiO3 powder. The results suggest that high-energy mechanical milling modifies the particle’s structure, resulting in localized states in an interfacial region between the crystalline PT and the amorphous PT. These localized states are believed to be responsible for the PL obtained with short milling times. When long milling times are employed, the amorphous phase that is formed causes PL behavior. An alternative method to process nanostructured wide-band-gap semiconductors with active optical properties such as PL is described in this letter.
Polaron hopping conduction and thermoelectric power in LaMnO3+δ
Authors: Pal Sudipta , Banerjee Aritra , Rozenberg E. , Chaudhuri B. K.
Published: July 2002

Two different phases of LaMnO3+δ [one showing a metal–insulator transition (MIT), referred to as LaMn–C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn–S followed a nonadiabatic hopping conduction mechanism while LaMn–C and the Pb doped samples viz. La1−xPbxMnO3 (x=0.05–0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(EF), Debye temperature (θD), electron–phonon interaction constant (γP), etc. of LaMn–S showed appreciable differences from those of LaMn–C and La1−xPbxMnO3, which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons.
Photodissociation and photoisomerization pathways of the HNCN free radical
Authors: Bise Ryan T. , Hoops Alexandra A. , Neumark Daniel M.
Published: July 2002

The photodissociation spectroscopy and dynamics of the HNCN free radical have been investigated by fast beam photofragment translational spectroscopy. Predissociative transitions for both the B̃ 2A′←X̃ 2A″ band and a higher-energy band system assigned to the C̃ 2A″←X̃ 2A″ band were observed. Photofragment mass distributions indicate that N2 loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N2 fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN2 intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN2 and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D0 and heat of formation ΔfH0(HNCN) were determined to be 2.80±0.03 eV and 3.35±0.03 eV, respectively.
Probing nuclear quadrupole interactions in the rotationally resolved S1←S electronic spectrum of 2-chloronaphthalene
Authors: Plusquellic David F. , Davis Scott R. , Jahanmir Farid
Published: July 2002

Rotationally resolved S1←S0 fluorescence excitation spectra of 2-chloronaphthalene (2ClN) are measured using a newly constructed ultraviolet (UV) laser/molecular beam spectrometer. More than 1000 well-resolved rotational lines are recorded at a resolution of ∼3 MHz for the two overlapping band origins of the Cl35 and Cl37 isotopomers (Δνorigin∼3 GHz) over a 3.5 cm−1 spectral region at a rotational temperature of 10 K. Both spectra are hybrid bands, having 18% a-/82% b-type character in accordance with La1←1A1 excitations observed for other 2-substituted naphthalenes. Additionally, Cl nuclear quadrupole splittings are observed as broadened asymmetric line shapes with linewidths from 5 to 20 MHz. Least-squares fits of these line shapes coupled with high-precision Fourier-transform microwave measurements of the ground state have provided accurate S1 quadrupole coupling constants. Specified relative to the principal quadrupole tensor orientation of S0, the S0 and S1 values of eQqzz are −71.198(5) MHz and −67.8(30) MHz, and the asymmetry parameters, η=(eQqyy−eQqxx)/eQqzz, are 0.076 38(5) and 0.13(5), respectively. The experimental line shapes are fit to Voigt functions, yielding 4.9(3) MHz homogeneous (lifetime) and 3.0(2) MHz inhomogeneous (Doppler) contributions. All of the observed structure are well reproduced by more than 20 000 transitions predicted for the nuclear-quadrupole-split rotational spectra of the two hybrid-band origins. Furthermore, the homogeneous linewidths show no dependence on either the upper state energy or angular momentum quantum numbers. The absence of additional level structure at the resolution of 1 MHz or more, from heavy-atom-enhanced intersystem crossing mechanisms, is discussed within the framework of radiationless transition theory. The quadrupole tensor components are used in simple theoretical models to investigate the orbital character of the S1 state and its radiationless decay mechanisms.
Absorption of gas-phase atomic hydrogen by Si(100): Effect of surface atomic structures
Authors: Maeng Jae Yeol , Kim Sehun , Jo S. K. , Fitts W. P. , White J. M.
Published: July 2002

The atomic-scale surface structural evolution of Si(100) exposed to gas-phase thermal hydrogen atoms, H(g), has been investigated by scanning tunneling microscopy and temperature-programed desorption mass spectrometry. For the substrate temperature (Ts) between 420 and 530 K, dihydride species in 3×1:H domains were selectively etched upon extensive exposures to H(g). As a result, etch pits grew laterally along Si surface dimer rows. The presence of these pits correlates with the absorption of H(g) into the bulk of Si(100), confirming our earlier suggestion that atomic-scale surface roughening caused by etching is a prerequisite for H(g) absorption.
Electromigration effect upon the Sn–0.7 wt% Cu/Ni and Sn–3.5 wt% Ag/Ni interfacial reactions
Authors: Chen Chih-ming , Chen Sinn-wen
Published: July 2002

This study investigates the effect of electromigration upon the interfacial reactions between the promising lead-free solders, Sn–Cu and Sn–Ag, with Ni substrate. Sandwich-type reaction couples, Sn–0.7 wt% Cu/Ni/Sn–0.7 wt% Cu and Sn–3.5 wt% Ag/Ni/Sn–3.5 wt% Ag, were reacted at 160, 180, and 200 °C for various lengths of time with and without the passage of electric currents. Without passage of electric currents through the couples, only one intermetallic compound Ni3Sn4 with ∼7 at. % Cu solubility was found at both interfaces of the Sn–0.7 wt% Cu/Ni couples. With the passage of an electric current of 500 A/cm2 density, the Cu6Sn5 phase was formed at the solder/Ni interface besides the Ni3Sn4 phase. Similar to those without the passage of electric currents, only the Ni3Sn4 phase was found at the Ni/solder interface. Directions of movement of electrons, Sn, and Cu atoms are the same at the solder/Ni interface, and the growth rates of the intermetallic layers were enhanced. At the Ni/solder interface, the electrons flow in the opposite direction of the Sn and Cu movement, and the growth rates of the intermetallic layers were retarded. Only the Ni3Sn4 phase was formed from the Sn–3.5 wt% Ag/Ni interfacial reaction with and without the passage of electric currents. Similar to the Sn–0.7 wt% Cu/Ni system, the movement of electrons enhances or retards the growth rates of the intermetallic layers at the solder/Ni and Ni/solder interfaces, respectively. Calculation results show the apparent effective charge za* decreases in magnitude with raising temperatures, which indicates the electromigration effect becomes insignificant at higher temperatures.
On the ground state structure of monolayers on the (100) face of fcc crystals
Authors: Patrykiejew A. , Sokolowski S. , Binder K.
Published: July 2002

While the (classical) ground state structure of an atomic monolayer adsorbed at a smooth substrate with negligible corrugation potential is trivial, namely, a triangular lattice with a spacing given by the minimum of the interatomic (Lennard-Jones) potential, nontrivial ground state structures occur on corrugated substrate surfaces. Assuming as a substrate surface the (100) plane of a face-centered cubic crystal, the possible (classical) ground state structures of the monolayer are studied by Monte Carlo simulation, varying both the density of the adsorbed film and the strength of the potential due to the surface. Different incommensurate structures are found, which can be interpreted as (two-dimensional) patterns of domain walls separating commensurate regions. Typically the heavy walls are oriented along the face diagonals of the square substrate lattice. This nonuniform order also is reflected in the distribution of distances of the adsorbed atoms from the substrate surface plane, which is not a delta function but may even display a bimodal character.
Origin of low dielectric constant of carbon-incorporated silicon oxide film deposited by plasma enhanced chemical vapor deposition
Authors: Kim Jin Yong , Hwang Moo Sung , Kim Yoon-Hae , Kim Hyeong Joon , Lee Young
Published: July 2002

Dielectric properties of carbon-incorporated silicon oxide (SiOC) films deposited by plasma enhanced chemical vapor deposition (PECVD), using a bis-trimethylsilylmethane precursor, were compared with the dielectric properties of silicon oxide thermally grown in a furnace (thermal oxide) and also with oxide deposited by PECVD using a tetraethoxysilane (TEOS) precursor (PE-TEOS oxide). The electronic contribution to the dielectric constant of the three oxide films was calculated from their refractive indices measured at 632.8 nm by ellipsometry. Ionic contributions were computed from their IR reflection spectra, measured at 650–4000 cm−1, by using the Kramers–Kronig relation. The dipolar contribution was qualitatively analyzed from temperature dependence of the polarizability, on a per unit volume basis. The dielectric constant of the SiOC films, which was measured at 1 MHz, decreased from 4.2 to 2.3 as the carbon content increased from 0 to 19.6 at. %. Although there was a significant reduction of the dielectric constant, the electronic contribution was only slightly changed from 2.10 to 1.90, whereas the ionic polarization noticeably changed from 1.86 to 0.25. SiOC films showed considerable dipolar contribution when compared to thermal oxide and PE-TEOS oxide films, but the dipolar contribution in the SiOC films became negligible as the carbon content was increased. The variation of each contribution with the carbon content shows that carbon incorporation leads to a decrease in the electronic and ionic contributions. The reduction of the ionic contribution was the predominant factor leading to a decrease in the overall dielectric constant.
Control of the colossal magnetoresistance by strain effect in Nd0.5Ca0.5MnO3 thin films
Authors: Rauwel Buzin E. , Prellier W. , Simon Ch. , Mercone S. , Mercey B. , Raveau B. , Sebek J. , Hejtmanek J.
Published: July 2002

Thin films of Nd0.5Ca0.5MnO3 manganites with colossal magnetoresistance (CMR) properties have been synthesized by the pulsed-laser deposition technique on (100)-(SrTiO3). The lattice parameters of these manganites and correlatively their CMR properties can be controlled by the substrate temperature TS. The maximum CMR effect at 50 K, calculated as the ratio ρ(H=0T)/ρ(H=7T) is 1011 for a deposition temperature of TS=680 °C. Structural studies show that the Nd0.5Ca0.5MnO3 film is single phase, [010]-oriented and has a pseudocubic symmetry of the perovskite subcell with a=3.77 Å at room temperature. We suggest that correlation between lattice parameters, CMR, and substrate temperature TS result mainly from substrate-induced strains which can weaken the charge-ordered state at low temperature.
Some results concerning the potential energy of interfaces with nonuniformly distributed surfactant
Authors: Schwartz L. W. , Roy R. V.
Published: July 2002

A functional is presented for the stored energy content, or potential energy, of a nonuniform distribution of surfactant on a liquid interface. This is the energy available to drive fluid motion using surface-tension-gradient forces. The functional is shown to be non-negative. An energy evolution equation is derived for the motion of a thin liquid film, whose shape evolves under the combined influence of surface tension, viscous, and surfactant effects. Numerical calculations show the time history of these global energy components. Results are relevant to the flow behavior of wetting agents, soap films, foams, and emulsions with possible applications in the industrial and biological worlds.
Structure and stability of fluorine-substituted benzene-argon complexes: The decisive role of exchange-repulsion and dispersion interactions
Authors: Tarakeshwar P. , Kim Kwang S. , Kraka Elfi , Cremer Dieter
Published: July 2002

The van der Waals complexes benzene-argon (BAr), fluorobenzene-argon (FAr), p-difluorobenzene-argon (DAr) are investigated at the second-order Møller–Plesset (MP2) level of theory using the 6-31+G(d), cc-pVDZ, aug-cc-pVTZ, and [7s4p2d1f/4s3p1d/3s1p] basis sets. Geometries, binding energies, harmonic vibrational frequencies, and density distribution are calculated where basis set superposition errors are corrected with the counterpoise method. Binding energies turn out to be almost identical (MP2/[7s4p2d1f/4s3p1d/3s1p]: 408, 409, 408 cm−1) for BAr, FAr, and DAr. Vibrationally corrected binding energies (357, 351, 364 cm−1) agree well with experimental values (340, 344, and 339 cm−1). Symmetry adapted perturbation theory (SAPT) is used to decompose binding energies and to examine the influence of attractive and repulsive components. Fluorine substituents lead to a contraction of the π density of the benzene ring, thus reducing the destabilizing exchange-repulsion and exchange-induction effects. At the same time, both the polarizing power and the polarizability of the π-density of the benzene derivative decreases thus reducing stabilizing induction and dispersion interactions. Stabilizing and destabilizing interactions largely cancel each other out to give comparable binding energies. The equilibrium geometry of the Ar complex is also a result of the decisive influence of exchange-repulsion and dispersive interactions.
Scanning probe electromagnetic tweezers
Authors: Barbic Mladen , Mock Jack J. , Gray Andrew P. , Schultz S.
Published: July 2002

We present a micromanipulation technique that utilizes integrated microcoils and magnetic microtips for localized positioning of micron-sized magnetic objects. Forces of 10 pN, and submicron positioning control are demonstrated on the 2.8 μm diameter superparamagnetic beads. The technique also implements an optical illumination scheme that provides a clear viewing of the magnetically trapped objects without including the scattering background from the magnetic manipulator tip. This simple instrument provides a noninvasive, low cost alternative to the optical trapping techniques normally used in micromanipulation. Among the possible advantages are the negligible heating of the manipulated sample, effective decoupling of the manipulation component of the experiment from the optical studies of the systems of interest, and the ability to perform studies in a variety of fluids.