bluuebunny/crossref_metadata_2025 huggingface dataset:
Friction and wear characteristics of ceramic nanocomposite coatings: Titanium carbide/amorphous hydrocarbon
Authors: Cao D. M. , Feng B. , Meng W. J. , Rehn L. E. , Baldo P. M. , Khonsari M. M.
Published: July 2002

Friction and wear characteristics of titanium-containing amorphous hydrocarbon (Ti–C:H) coatings were measured during unlubricated sliding against WC–Co. These Ti–C:H coatings consist of nanocrystalline TiC clusters embedded in an amorphous hydrocarbon (a-C:H) matrix, i.e., they are TiC/a-C:H nanocomposites. The elastic modulus and hardness of the coatings exhibit smooth variations with increasing Ti composition. In contrast, a relatively abrupt transition occurs in the friction coefficient and wear rate of the coatings over a relatively narrow (20–30 at. %) Ti composition range. Our results reveal bimodal friction and wear behaviors for the TiC/a-C:H nanocomposites, a-C:H like at Ti compositions below 20%, and TiC like at Ti compositions above 30%. The two different wear mechanisms that operate as the volume fraction of nanocrystalline TiC clusters changes are discussed.
Superconducting MgB2 thin films by pulsed laser deposition
Authors: Shinde S. R. , Ogale S. B. , Greene R. L. , Venkatesan T. , Canfield P. C. , Bud’ko S. L. , Lapertot G. , Petrovic C.
Published: July 2002

Growth of MgB2 thin films by pulsed laser deposition is examined under ex situ and in situ processing conditions. For the ex situ process, boron films grown by pulsed laser deposition were annealed at 900 °C with excess Mg. For the in situ process, different approaches involving ablation from a stoichiometric target under different growth conditions, as well as multilayer deposition involving interposed Mg layers were examined and analyzed. Magnetic measurements on ex situ processed films show Tc of ∼39 K, while the current best in situ films show a susceptibility transition at ∼22 K.
Autoparametric optical drive for micromechanical oscillators
Authors: Zalalutdinov M. , Zehnder A. , Olkhovets A. , Turner S. , Sekaric L. , Ilic B. , Czaplewski D. , Parpia J. M. , Craighead H. G.
Published: July 2002

Self-generated vibration of a disk-shaped, single-crystal silicon micromechanical oscillator was observed when the power of a continuous wave laser, focused on the periphery of the disk exceeded a threshold of a few hundred μW. With the laser power set to just below the self-generation threshold, the quality factor for driven oscillations increases by an order of magnitude from Q=10 000 to Qenh=110 000. Laser heating-induced thermal stress modulates the effective spring constant via the motion of the disk within the interference pattern of incident and reflected laser beams and provides a mechanism for parametric amplification and self-excitation. Light sources of different wavelengths facilitate both amplification and damping.
Concentration measurements in molecular gas mixtures with a two-pump pulse femtosecond polarization spectroscopy technique
Authors: Hertz E. , Chaux R. , Faucher O. , Lavorel B.
Published: July 2002

Recently, we have demonstrated the ability of the Raman-induced polarization spectroscopy (RIPS) technique to accurately determine concentration or polarizability anisotropy ratio in low-pressure binary molecular mixtures [E. Hertz, B. Lavorel, O. Faucher, and R. Chaux, J. Chem. Phys. 113, 6629 (2000)]. It has been also pointed out that macroscopic interference, occurring when two revivals associated to different molecules time overlap, can be used to achieve measurements with picosecond time resolution. The applicability of the technique is intrinsically limited to a concentration range where the signals of both molecules are of the same magnitude. In this paper, a two-pump pulse sequence with different intensities is used to overcome this limitation. The relative molecular responses are weighted by the relative laser pump intensities to give comparable signals. Furthermore, by tuning the time delay between the two-pump pulses, macroscopic interference can be produced regardless of the accidental coincidences between the two molecular temporal responses. The study is performed in a CO2–N2O gas mixture and the concentration is measured with and without macroscopic interference. Applications of the method in the field of noninvasive diagnostics of combustion media are envisaged.
Observation of an anomalously large blueshift of the photoluminescence peak and evidence of band-gap renormalization in InP/InAs/InP quantum wires
Authors: Mu Xiaodong , Zotova Ioulia B. , Ding Yujie J. , Yang Haeyeon , Salamo Gregory J.
Published: July 2002

We have investigated polarization-dependent photoluminescence in InP/InAs/InP quantum wires directly formed on the top of InP substrates. With excitation laser intensity we have observed an anomalously large blueshift of the photoluminescence peak using a cw laser with extremely low intensities. We have also observed evidence of band-gap renormalization. In addition, we have measured two-photon luminescence spectra and confirmed their dependence on photoluminescence polarization.
Strain and composition distribution in uncapped SiGe islands from x-ray diffraction
Authors: Stangl J. , Daniel A. , Holý V. , Roch T. , Bauer G. , Kegel I. , Metzger T. H. , Wiebach Th. , Schmidt O. G. , Eberl K.
Published: July 2002

We have investigated the strain and composition distribution in uncapped SiGe islands grown on Si (001) by x-ray diffraction. In order to be sensitive to the dot layer on the sample surface, and at the same time being able to measure in-plane strain and strain in growth direction, we utilized a scattering geometry at grazing incidence angles, but with high exit angles. The measured intensity distribution is compared to simulations based on the strain distribution calculated by a finite element method. Although pure Ge has been deposited during island growth by molecular beam epitaxy, the Ge composition varies from 0.5 at the island base to 1.0 at the top of the islands. Even at this top, the elastic relaxation reaches only about 50%.
Optical and electronic properties of TiCxNy films
Authors: Fuentes G. G. , Elizalde E. , Sanz J. M.
Published: July 2002

The optical and electronic properties of TiCxNy(x+y∼1,0<y<1) thin films have been investigated by spectroscopic ellipsometry in the 1.5–4.5 eV energy range and by valence band x-ray photoemission spectroscopy as a function of the composition. The dielectric functions measured in the energy range of intraband transitions are analyzed in terms of a Drude-like approximation. Both the free plasma energy and the damping constant are observed to depend on the nitrogen content of the samples, suggesting a certain tunability of the optical and electronic properties of these films. Analysis of the valence band reveals that the C 2p band shifts toward higher binding energies upon an increase of the nitrogen content, in good agreement with the shift observed in the minimum of the optical reflectivity associated with the threshold of the interband transitions. The enhancement of the metallic character of the films as the nitrogen content increases is also evidenced by x-ray photoemission spectroscopy as a continuous intensity growth of the conduction band at the Fermi level.
Intense and stable blue-light emission of Pb(ZrxTi1−x)O3
Authors: Sun Chang Q , Jin Da , Zhou Ji , Li S. , Tay B. K. , Lau S. P. , Sun X. W. , Huang H. T. , Hing P.
Published: July 2002

A single band of photoluminescence (PL) has been detected at room temperature from the Pb(ZrxTi1−x)O3 (x=0–1.0) surface. The intense and stable PL band covers 475±50 nm (or 2.67±0.25 eV) wavelengths and the corresponding photoexcitation (PE) band is in the range of 305±45 nm. It is found that changing the x value causes a negligible shift of the PL and PE peaks, and the light emission is subject to the Pb presence. The minimal lifetime of the excited photons was found to be 0.03 ms at x=0.5 and the maximum lifetime of 0.60 ms can be achieved by either decreasing or increasing the x values.
Large room-temperature magnetoresistance and phase separation in La1−xNaxMnO3 with 0.1⩽x⩽0.3
Authors: Ye S. L. , Song W. H. , Dai J. M. , Wang K. Y. , Wang S. G. , Du J. J. , Sun Y. P. , Fang J. , Chen J. L. , Gao B. J.
Published: July 2002

The structural, magnetic, and electronic properties of the polycrystalline La1−xNaxMnO3 (x=0.10, 0.15, 0.20, and 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in a rhombohedrally distorted structure with space group R3̄C. The magnetic measurement shows that Curie temperature TC of the studied samples is near or above room temperature. The temperature dependence of resistivity shows that all samples undergo a sharp transition accompanying a paramagnetic to ferromagnetic with the decrease of temperature, however, for x⩾0.15 samples, double transition peaks with a single ferromagnetic transition is observed. In the meanwhile, a large room-temperature magnetoresistance with low applied magnetic field is observed. The co-existing ferromagnetic metallic phases and ferromagnetic insulating (FMI) phases induced by the electronic inhomogeneity as well as the additional FMI phases caused by the presence of vacancies at the A sites, are presented to account for the transport properties and large magnetoresistance in these compounds.
Melting point of MgO
Authors: Ronchi Claudio , Sheindlin Mikhail
Published: July 2002

Despite large commercial production of MgO-based ceramics for a wide gamut of applications, the melting point of magnesia remained uncertain for almost one century. This article shows that a number of problems must be solved to attain experimental conditions where the solid–liquid phase transition of magnesia can be unambiguously detected, and the temperature be measured with sufficient accuracy. The method adopted in the reported work is based on controlled laser pulse heating. The solidification point was measured by the thermal arrest occurring during cooldown from the melt. The measurement of temperature, a most delicate problem for pyrometry applications in semitransparent materials, was obtained by using combined brightness and spectral pyrometers. The experimental and analytical methods are described in some detail. The resulting melting point of MgO is 3250±20 K, which is approximately 150 K higher than the value currently recommended.
Infrared fluorescence of Xe2 molecules in electron/proton beam excited pure Xe gas and in an Ar/Xe gas mixture
Authors: Borghesani A. F. , Bressi G. , Carugno G. , Conti E. , Iannuzzi D.
Published: July 2002

We report experimental results of proton- and electron-beam-induced near-infrared fluorescence in high-pressure Xe gas and in a 90% Ar–10% Xe gaseous mixture at room temperature. The investigated wavelength band spans the range 0.7⩽λ⩽1.8 μm. In the previously unexplored range for λ⩾1.05 μm we have detected a broad continuum near-infrared fluorescence centered at λ≈1.3 μm. The continuum shifts towards longer wavelengths as pressure is increased up to 1.5 MPa. The shift is quantitatively different in pure Xe or in the mixture, but qualitatively similar. We believe that this continuum is produced by a bound-free Xe2 excimer transition in a way similar to the well-known first and second vacuum ultraviolet continua of noble gas excimers. The pressure-dependent shift can then be explained by the interaction of the outer electron in the excimer with the atoms of the host gas.
Experimental comparison between optical spectroscopy and laser-ultrasound generation in polymer-matrix composites
Authors: Dubois Marc , Lorraine Peter W. , Filkins Robert J. , Drake Thomas E.
Published: July 2002

Laser ultrasound is a technique based on lasers to generate and detect ultrasound. The generation mechanism involves several parameters among which one of the most important is the optical penetration depth. This letter presents a comparison between the amplitude of ultrasonic waves generated by an optical parametric oscillator and optical penetration depth spectra measured by photoacoustic spectroscopy in the 3.0 to 3.5 μm wavelength range for three different composite samples. The laser-ultrasound amplitude spectra closely track the photoacoustic spectra. The results presented in this letter experimentally demonstrate why the 3.0–3.5 μm wavelength range generates more efficiently generates ultrasonic waves in the ultrasonic frequency range of interest for the inspection of polymer-matrix composites than the 10.6 μm wavelength of the CO2 laser.
Temperature-dependent conduction of W-containing composite diamond films
Authors: Terranova M. L. , Sessa V. , Piccirillo S. , Orlanducci S. , Manno D. , Micocci G. , Serra A. , Tepore A. , Rossi M.
Published: July 2002

We report on the synthesis and electrical characterizations of composite W-containing diamond films. These layers have been produced by a hybrid chemical-vapor-deposition/powder flowing technique that allows control of the dispersion of the foreign W phase inside a diamond matrix. The electrical behavior of such material in the 25–500 K temperature range is found characterized by a nonlinear response, typical of metal/insulator composite materials, with conductivity reaching the maximum value of 95 Ω−1 cm−1 at 148 K. Structural and morphological investigations performed by reflection high-energy electron diffraction, scanning electron microscopy, and dispersive x-ray spectrometry indicate that the presence of a percolating network of metallic grains did not perturb the quality of the diamond lattice.
Improved phase diagram of polyelectrolyte solutions
Authors: Nishida K. , Kaji K. , Kanaya T.
Published: July 2002

A previously reported phase diagram of polyelectrolyte solutions as functions of degree of polymerization and the concentration is improved. The new phase diagram is also intended for salt-free solutions of polyelectrolytes with the constitutional repeating unit -C-CX-, X being a charged group. The main improved point is the crossover concentrations between the dilute and semidilute regimes in the low concentration range of less than 10−2 mol/l, which is based on the recent most reliable experimental data on the persistence length of flexible polyion chains. This phase diagram as well introduces both the concentrated regime, which has been not only theoretically predicted but also experimentally confirmed, and the experimentally newly found swollen regime. Furthermore, it is proposed that the dilute regime is separated into four regions, gaslike, poor order, liquidlike order and crystal. The degrees of orderliness in these regions have been assumed to depend on the ratio δ/ξcent of the amplitude of thermal fluctuations to the mean intermolecular distance. Typically, δ/ξcent=1/5 as the critical Lindemann ratio of melting is assigned for the crossover between the liquidlike order and crystal regions. Interestingly, the most probable region for the formation of a 3-D lattice is located within the experimental approach.
Characterization of phosphorus-doped and boron-doped diamond-like carbon emitter arrays
Authors: Tsai Chia-Lun , Chen Chia-Fu , Lin Chien-Liang
Published: July 2002

We synthesized phosphorus-doped and boron-doped emitters by using trimethylphosphite P(OCH3)3 and trimethylborate B(OCH3)3 as doping sources in a microwave plasma chemical vapor deposition system. Based on our experimental results from scanning electron microscopy and Raman spectra, there is much difference among undoped, phosphorus-doped, and boron-doped diamondlike material. In addition, doping both phosphorus and boron can enhance electric properties by reducing the turn-on voltage and can increase the emission current density. The turn-on voltages of undoped, boron-doped, and phosphorus-doped emitters in triode-type field emitter arrays are 15, 8, and 5 V, respectively. The emission currents of boron-doped and phosphorus-doped emitters are about 20 and 80 times larger than the undoped.
Schottky barrier height and surface state density of Ni/Au contacts to (NH4)2Sx-treated n-type GaN
Authors: Lee Ching-Ting , Lin Yow-Jon , Liu Day-Shan
Published: July 2002

By using capacitance–voltage and photoluminescence measurements, we have investigated the Schottky barrier height and surface state density of Ni/Au contacts to n-type GaN with, and without, (NH4)2Sx treatment. The Schottky barrier height of 1.099 eV is very close to the Schottky limit of 1.10 eV for Au/Ni/n-type GaN treated with (NH4)2Sx. This result indicates that there is no severe Fermi level pinning induced by surface states. The reduction of the surface state density for the (NH4)2Sx-treated n-type GaN is attributed to the decrease of dangling bonds and occupation of nitrogen-related vacancies due to the formation of Ga–S bonds.
Superconducting magnesium diboride films on Si with Tc0∼24 K grown via vacuum annealing from stoichiometric precursors
Authors: Zhai H. Y. , Christen H. M. , Zhang L. , Cantoni C. , Paranthaman M. , Sales B. C. , Christen D. K. , Lowndes D. H.
Published: July 2002

Superconducting magnesium diboride films with Tc0∼24 K and sharp transition ∼1 K were prepared on Si by pulsed-laser deposition from stoichiometric MgB2 target. Contrary to previous reports, anneals at 630 °C and a background of 2×10−4 Ar/4%H2 were performed without the requirement of Mg vapor or Mg cap layer. This integration of superconducting MgB2 film on Si may thus prove enabling in superconductor-semiconductor device applications. Images of surface morphology and cross-section profiles by scanning electron microscopy show that the films have a uniform surface morphology and thickness. Energy-dispersive spectroscopy study reveals these films were contaminated with oxygen, originating either from the growth environment or from sample exposure to air. The oxygen contamination may account for the low Tc for those in situ annealed films, while the use of Si as a substrate does not result in a decrease in Tc as compared to other substrates.
Hydrogenation of S to H2S on Pt(111): A first-principles study
Authors: Michaelides A. , Hu P.
Published: July 2002

Density-functional theory has been used to investigate the chemisorption of S, SH, and H2S as well as the coadsorption of S and H and SH and H on Pt(111). In addition reaction pathways and energy profiles for the conversion of adsorbed S and H into gas-phase H2S have been determined. It has been found that S, SH, and H2S bind preferentially at face-centered-cubic (fcc), bridge, and top sites, respectively. Both the S+H and SH+H reactions have high barriers (∼1 eV) and high exothermicities (∼1 eV). This reveals that adsorbed H2S and SH are highly unstable adsorbates on Pt(111) and that adsorbed S (and H) is the most stable SHX (X=0,1,2) intermediate on Pt(111)
Schlieren imaging of laser-generated ultrasound
Authors: Don-Liyanage Dilhan K. L. , Emmony David C.
Published: July 2002

The interaction of a pulse from a Q-switched laser with a metal in water is the basis for the laser peening process where the metallurgical properties are modified. An investigation of the process, using schlieren imaging, shows that high-frequency ultrasound is generated when the metal is in the form of a thin plate. The period of the ultrasonic waves in the water is given by the plate thickness and the velocity of longitudinal sound waves in the metal. Ultrasonic wave trains with a frequency of 7.5 MHz up to 10 μs long are shown.
Nanocluster crystals of lacunary polyoxometalates as structure-design-flexible, inorganic nonlinear materials
Authors: Murakami Hidetoshi , Kozeki Toshimasa , Suzuki Yuji , Ono Shingo , Ohtake Hideyuki , Sarukura Nobuhiko , Ishikawa Eri , Yamase Toshihiro
Published: July 2002

Lacunary polyoxometalates, large inorganic, structure-design-flexible, nanocluster crystals are found to have higher optical nonlinearity than KH2PO4 by the powder second-harmonic-generation method. Moreover, the capability of generating ultraviolet radiation down to around 300 nm is found. The basic criteria to design the high nonlinearity are also discovered by the reduction of the molecular symmetry.
The stability margin for stable weightless liquid bridges
Authors: Slobozhanin Lev A. , Alexander J. Iwan D. , Patel Viral D.
Published: July 2002

The stability of weightless axisymmetric liquid bridge equilibrium configurations to “large” disturbances is examined by calculating the stability margin. For bridges held between coaxial equidimensional circular disks (radius R0) separated by a distance H, the stability to infinitesimal perturbations (linear stability) has been thoroughly investigated and the stability region is constructed in the (Λ,V) plane. Here, the slenderness Λ (=H/2R0) and the relative volume V (ratio of the actual liquid volume to that of a cylinder with radius R0 and height H) are the parameters that define the system. To assess stability with respect to finite amplitude disturbances we use a potential energy analysis based on the concepts of a potential energy well and the equilibrium stability margin introduced by Myshkis [USSR Comput. Math. Math. Phys. 5, 193 (1965); Math. Notes Acad. Sci. USSR 33, 131 (1983); Introduction to the Dynamics of a Body Containing a Liquid Under Zero-Gravity Conditions (Vychisl. Tsentr Akad. Nauk SSSR, Moscow, 1968)]. The stability margin represents the height of a local potential energy barrier adjacent to the well of a given stable equilibrium. Wherever a linearly stable equilibrium is nonunique equilibrium, the stability margin corresponds to the smallest among the heights of saddle points on the potential energy surface that are adjacent to the well. The saddle point that determines the stability margin is the point of emergence from the well and leads to the energy wells corresponding to other equilibria or to infinity. Unless the total energy of perturbations exceeds the stability margin for a given stable equilibrium, the liquid bridge will return to that equilibrium state. In this work we determined the stability margin in part of the stability region where axisymmetric bridges that are already unstable to small axisymmetric perturbations coexist with stable ones. The domains of existence of a variety of unstable axisymmetric bridges are constructed using previous results concerning the bifurcation structure. This enabled us to construct contours of the dimensionless stability margin within the linear stability region not only in the vicinity of the stability boundary, but also far from it. The stability margins for bridges with fixed values of the slenderness, as well as for cylindrical and catenoidal bridges, are also calculated.
Longitudinal and transverse structure functions in sheared and unsheared wind-tunnel turbulence
Authors: Shen X. , Warhaft Z.
Published: July 2002

The scaling exponents of the longitudinal structure function, 〈Δuxn〉 and two different transverse structure functions: the longitudinal velocity, u, measured in the transverse, y direction (the direction of the mean shear), 〈Δuyn〉, and the transverse velocity v, measured in the flow direction, x, 〈Δvxn〉, up to n=8, are studied in an approximately uniform shear flow [Shen and Warhaft, Phys. Fluids 12, 2976 (2000)], as well as in decaying (shearless) grid turbulence. Results, for low Reynolds numbers (Rλ∼100–200) and high Reynolds numbers (Rλ∼800–1000) are compared. It is shown that at high Rλ, the scaling exponents of 〈Δuyn〉, ζu(y)(n), are the same as those of the longitudinal structure function exponents, ζu(x)(n), for both even and odd values of n. The observation that ζu(y)(n)∼ζu(x)(n) (n odd) is in violation of the postulate of local isotropy (PLI) and implies that the anomalous scaling (departures from the Kolmogorov 1941 scaling) is fully captured by the anisotropy since these odd moments are zero in isotropic turbulence. In the low Rλ shear flow ζu(y)(n)<ζu(x)(n) suggesting that here this transverse component has not yet reached its high Rλ limit. In the absence of shear, ζu(y)(n)∼ζu(x)(n) at high Rλ (similar to the shear case but with the odd moments of 〈Δuyn〉 equal to zero). Similarly, at low Rλ, ζu(y)(n)<ζu(x)(n). On the other hand, the scaling exponent of the v structure function (〈Δvxn〉), ζv(x)(n), was lower than that of ζu(x)(n), even at high Rλ. The results are related to numerical, and other laboratory studies.
Structure, surface composition, and electronic properties of CuInS2 and CuIn(1−x)AlxS2
Authors: Kamoun N. , Jebbari N. , Belgacem S. , Bennaceur R. , Bonnet J. , Touhari F. , Lassabatere L.
Published: July 2002

CuInS 2 and CuIn(1−x)AlxS2 thin films have been deposited on different substrates using the spray pyrolysis technique at 320 °C. X-ray diffraction, atomic force microscopy, and scanning electron microscopy were used to characterize the structure of the films; the surface compositions of the films were studied by Auger electrons spectroscopy and energy dispersive spectrometry (EDS), the work function and the photovoltage by the Kelvin method. Using these techniques, we have specified the effect of the nature of the substrate, its surface morphology, and the introduction of small amounts of Al in the layer on the properties of the films. The best crystallized composition of CuInS2 was obtained for deposits on pyrex. This was improved by the introduction of Al atoms. The work function differences (φmaterial−φprobe) for CuInS2 and CuIn(1−x)AlxS2 deposited on pyrex were equal to −350 meV and to −400 meV, respectively. Putting Al atoms in the film increases φm (by about 50 meV) and induces the formation of a negative surface barrier. The best composition was obtained for CuInS2 deposited on SnO2 and annealed. Auger studies shows that the concentration of S and C elements increased when the samples were annealed under a vacuum, whereas the concentration of Cu, In, and O decreased. In the case of CuInS2 grown on pyrex, the introduction of Al increases the O, S, and Cl concentrations and reduces Cu and C concentrations. Analysis of the film compositions by EDS gives the following concentrations [Cu]=24.270%, [In]=24.487%, [S]=46.670%, and [Cl]=4.573%.
Alignment of self-assembled magnetic nanostructures: Co dot chains and stripes on grooved Ru(0001)
Authors: Yu Chengtao , Li Dongqi , Pearson J. , Bader S. D.
Published: July 2002

We have deposited 0–60 nm Co with a wedge-shaped thickness gradient onto a grooved Ru(0001) substrate by means of molecular-beam epitaxy at 350 °C. The resultant epitaxial Co island (dot) structure was characterized ex situ using atomic- and magnetic-force microscopy and the magneto-optic Kerr effect. The dots self-assemble with ∼70–500 nm diameter, ∼1–7 nm height, and align into linear chains along the grooves with spacings in the chains to decrease from ∼1 μm until they form continuous stripes, depending on dosage and groove structure. The dots have magnetic single domains with in-plane easy axes.
Zeeman quantum-beat spectroscopy of NO2: Eigenstate-resolved Landé gF factors near dissociation threshold
Authors: Xin Ju , Reid Scott A.
Published: July 2002

The sign and magnitude of Landé gF factors for single NO2 rovibronic (J=3/2) eigenstates in the 15 cm−1 region below dissociation threshold (D0=25 128.57 cm−1) were investigated using Zeeman quantum-beat spectroscopy. The derived Landé gF factors exhibit pronounced fluctuations about an average much smaller than expected in the absence of rovibronic perturbations, which destroy the goodness of the N and K quantum numbers and the J=N+S coupling scheme. The F=J+I coupling scheme was found to be valid near D0 to within the uncertainty of our measurements, and the average Landé gF factors near dissociation threshold are in good agreement with those calculated under the assumption of complete rovibronic mixing. Our findings do not provide evidence for the participation of repulsive quartet states near dissociation threshold.
Role of annealing conditions and surface treatment on ohmic contacts to p-GaN and p-Al0.1Ga0.9N/GaN superlattices
Authors: Zhang A. P. , Luo B. , Johnson J. W. , Ren F. , Han J. , Pearton S. J.
Published: July 2002

The effects of the initial Mg reactivation annealing temperature, surface cleaning treatments, and contact annealing conditions on the specific contact resistance of Ni/Au ohmic contacts on p-GaN are reported. The lowest contact resistances were obtained for 900 °C activation annealing and cleaning steps that reduced the native oxide thickness (i.e., KOH rinsing). Removal of this interfacial oxide reduced the barrier for hole transport, providing a contact resistance of 9×10−4 Ω cm2 for Ni/Au metallization annealed at 500 °C. The use of a ten period p-Al0.1Ga0.9N(Mg)/GaN(Mg) superlattice with individual layer thickness 50 Å led to a specific contact resistance of 9×10−5 Ω cm2 under the same conditions.
Particle dynamics during nanoparticle synthesis by laser ablation in a background gas
Authors: Nakata Yoshiki , Muramoto Junichi , Okada Tatsuo , Maeda Mitsuo
Published: July 2002

Particle dynamics during Si nanoparticle synthesis in a laser-ablation plume in different background gases were investigated by laser-spectroscopic imaging techniques. Two-dimensional laser induced fluorescence and ultraviolet Rayleigh scattering techniques were used to visualize the spatial distribution of the Si atoms and nanoparticles grown, respectively. We have developed a visualization technique called re-decomposition laser-induced fluorescence to observe small nanoparticles (hereafter called clusters) which are difficult to observe by the conventional imaging techniques. In this article, the whole process of nanoparticle synthesis in different background gases of He, Ne, Ar, N2 and O2 was investigated by these techniques. In He, Ne, Ar and N2 background gases at 10 Torr, the clustering of the Si atoms started 200, 250, 300 and 800 μs after ablation, respectively. The growth rate of the clusters in He background gas was much larger than that in the other gases. The spatial distributions of the Si nanoparticles were mushroom like in He, N2 and O2, and column like in Ne and Ar. It is thought that the difference in distribution was caused by differences in the flow characteristics of the background gases, which would imply that the viscosity of the background gas is one of the main governing parameters.
Complete suppression of the transient enhanced diffusion of B implanted in preamorphized Si by interstitial trapping in a spatially separated C-rich layer
Authors: Napolitani E. , Coati A. , De Salvador D. , Carnera A. , Mirabella S. , Scalese S. , Priolo F.
Published: July 2002

A method for completely suppressing the transient enhanced diffusion (TED) of boron implanted in preamorphized silicon is demonstrated. Boron is implanted in a molecular beam epitaxy (MBE) grown silicon sample that has been previously amorphized by silicon implantation. The sample is then annealed in order to epitaxially regrow the amorphous layer and electrically activate the dopant. The backflow of silicon interstitials released by the preamorphization end-of-range (EOR) damage is completely trapped by a carbon-rich silicon layer interposed by MBE between the damage and the implanted boron. No appreciable TED is observed in the samples up to complete dissolution of the EOR damage, and complete electrical activation is obtained. The method might be considered for the realization of ultrashallow junctions for the far future complementary metal–oxide–semiconductor technology nodes.
Self-assembly of surfactants in a supercritical solvent from lattice Monte Carlo simulations
Authors: Lı́sal Martin , Hall Carol K. , Gubbins Keith E. , Panagiotopoulos Athanassios Z.
Published: July 2002

We modify Larson’s lattice model [J. Chem. Phys. 83, 2411 (1985)] and use it to study self-assembly of surfactants in a supercritical solvent by large-scale Monte Carlo simulations. Carbon dioxide and perfluoroalkylpoly(ethylene oxide) serve as prototypes for the solvent and surfactant, respectively. Larson-model type parameters for carbon dioxide and perfluoroalkylpoly(ethylene oxide) are obtained using experimental values of critical parameters and solubility along with a modified Berthelot combining rule. We perform canonical Monte Carlo simulations at a supercritical temperature, varying the number of surfactant head and tail segments, the solvent density and the surfactant concentration. Various properties such as the critical micelle concentration, the aggregate size distribution, and the size and shape of the micelles are evaluated and pseudophase diagrams are constructed. We further investigate the ability of the surfactant solutions to dissolve more solute than solutions without surfactants by calculating the partition coefficient. Water serves as a prototype for the solute and Larson-model type parameters for water are obtained in the same way as for carbon dioxide and perfluoroalkylpoly(ethylene oxide).
Direct detection of H atoms in the catalytic chemical vapor deposition of the SiH4/H2 system
Authors: Umemoto Hironobu , Ohara Kentaro , Morita Daisuke , Nozaki Yoshitaka , Masuda Atsushi , Matsumura Hideki
Published: July 2002

The absolute densities of H atoms produced in catalytic chemical vapor deposition (Cat-CVD or hot-wire CVD) processes were determined by employing two-photon laser-induced fluorescence and vacuum ultraviolet absorption techniques. The H-atom density in the gas phase increases exponentially with increases in the catalyzer temperature in the presence of pure H2. When the catalyzer temperature was 2200 K, the absolute density in the presence of 5.6 Pa of H2 (150 sccm in flow rate) was as high as 1.5×1014 cm−3 at a point 10 cm from the catalyzer. This density is one or two orders of magnitude higher than those observed in typical plasma-enhanced chemical vapor-deposition processes. The H-atom density decreases sharply with the addition of SiH4. When 0.1 Pa of SiH4 was added, the steady-state density decreased to 7×1012 cm−3. This sharp decrease can primarily be ascribed to the loss processes on chamber walls.
Measurement of the sheet resistance of resistive films on thin substrates from 120 to 175 GHz using dielectric waveguides
Authors: Collier R. J. , Hasko D. G.
Published: July 2002

A method is reported for the measurement of the sheet resistance, at microwave frequencies, of conducting films supported on thin dielectric substrates. The sheet resistance is found from measurements of the millimeter-wave power transmission through the film using a millimeter-wave source and power meter coupled through dielectric waveguides. The accuracy of this technique does not depend on the precise placement of the waveguide terminations with respect to the substrate, in contrast to methods using metallic waveguides or coils. This method is used to characterize the sheet resistance of semiconductor samples in the frequency range 120–175 GHz and the results are compared to the dc values obtained by conventional techniques. Sheet resistance values can be easily measured by this method in the range from 1 to 1000 Ω.
Analysis of residual stress in diamond films by x-ray diffraction and micro-Raman spectroscopy
Authors: Ferreira N. G. , Abramof E. , Leite N. F. , Corat E. J. , Trava-Airoldi V. J.
Published: July 2002

We investigate the residual stress in diamond films grown on (001) silicon substrates as a function of film thickness. The diamond films were deposited at 1070 K by the conventional hot filament technique using a gas mixture of methane (1.0% vol) and hydrogen (99.0% vol). The film thickness, obtained from cross section scanning electron micrographs, varied from 3.0 to 42 μm as the growth time increased from 1 to 10 h. These images evidenced that the columnar growth is already established for films thicker than 10 μm. Top view micrographs revealed predominantly faceted pyramidal grains for the films at all growth stages. The grain size, obtained from these images, was found to vary linearly with film thickness. Using a high resolution x-ray diffractometer, the residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ values ranging from −60° to +60°, and applying the sin2 ψ method. For the micro-Raman spectroscopy, we used the summation method, which consists in recording and adding a large number of spectra in different places of a selected area of the sample. All Raman spectra were fitted with Lorentzian lines to separate the contribution of the pure diamond and the other nondiamond (graphite) phases. This spectral analysis performed in each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which increases from 70% to 90% as the thickness goes from 3 to 42 μm. The type and magnitude of the residual stress obtained from x-ray and micro-Raman measurements agreed well for films thicker than 10 μm. For films thinner than this value, an opposite behavior between both results was observed. We attributed this discrepancy to the domain size characteristic of each technique.
Epitaxial growth of Cu(In, Ga)Se2 on GaAs(110)
Authors: Liao D. , Rockett A.
Published: July 2002

Epitaxial Cu(In, Ga)Se2 (CIGS) films were grown on (110)-oriented GaAs substrates using a hybrid sputtering and evaporation process. The morphological and structural properties were determined by scanning electron microscopy, atomic force microscopy, x-ray diffraction, and electron backscatter diffraction. Pronounced faceting was observed on the surfaces of the films and Ga diffusion was observed at higher growth temperatures from the substrates into the films. The (220)/(204) surface of CIGS was found to be unstable under the growth conditions. The resulting surface consists entirely of {112} type facets with no observable (220)/(204)-oriented surfaces. The epitaxial temperature for the (220)/(204) layers is considerably lower than that on any other surface tested and is attributed to the reduced diffusion distance required for adatoms to reach growth sites. The surface is proposed to grow by rapid nucleation and gradual growth of Se terminated steps across Se terminated surface terraces. This causes the Se-terminated terraces to be rough and leaves the metal-terminated terraces, on which nucleation is slow, relatively smooth.
Current-induced fluorescence quenching in organic light-emitting diodes
Authors: Young Ralph H. , Tang Ching W. , Marchetti Alfred P.
Published: July 2002

The electroluminescence quantum efficiency of organic light-emitting diodes with a doped Alq [tris(8-quinolinolato)aluminum] emitting layer is found to decrease markedly with increasing current density. This phenomenon was investigated using multilayer device structures permitting bipolar or unipolar carrier transport, and luminescence measurements with simultaneous optical and electrical excitation. The loss of electroluminescence quantum efficiency is attributed to the quenching of the singlet-excited state of the dopant by a cationic species.
Three-dimensional simulation of an inductively coupled plasma reactor
Authors: Panagopoulos Theodoros , Kim Doosik , Midha Vikas , Economou Demetre J.
Published: July 2002

A three-dimensional finite element fluid model and a corresponding simulation tool have been developed for studying azimuthal asymmetries and their effect on etch uniformity in inductively coupled plasma (ICP) reactors. For silicon etching with chlorine in an ICP reactor with a planar coil, four different cases were examined: (a) uniform power deposition without a focus ring, (b) uniform power deposition with a focus ring, (c) nonuniform power deposition without a focus ring, and (d) nonuniform power deposition with a focus ring. When etching is ion driven, the power deposition profile is most important for etch uniformity, because azimuthal nonuniformities in the ion production rate can persist even down to the wafer level. For uniform power deposition, the effect of asymmetric pumping becomes more important. A focus ring can play an important role in alleviating azimuthal nonuniformities, especially in the nonuniform power deposition cases. Gas inlets pointing parallel to the wafer plane introduce only local disturbances in the species profiles.
An effective centroid Hamiltonian and its associated centroid dynamics for indistinguishable particles in a harmonic trap
Authors: Blinov Nicholas , Roy Pierre-Nicholas
Published: July 2002

We show that incorporating the effects of Bose–Einstein or Fermi–Dirac quantum statistics within the centroid molecular dynamics formalism leads to additional correlations in the system due to exchange effects. In the case of Bose–Einstein statistics they appear as an additional attraction between physical particles while an additional repulsion is observed for Fermi–Dirac statistics. We show that we can account for these correlations through the effective centroid Hamiltonian. Within the approach based on the phase space centroid density, this Hamiltonian depends on centroid momenta in a nonclassical way. We illustrate the above findings using a simple model of two bosons and fermions in a harmonic potential. The average of a centroid variable along centroid trajectories based on such an effective Hamiltonian can be used to study the equilibrium properties of quantum systems. Is is also shown that the dynamics of the centroid variables derived from the quantum mechanical dynamics of the corresponding physical observables does not depend on exchange effects for a harmonic system.
Magnetocapacitance effect of spin tunneling junctions
Authors: Kaiju H. , Fujita S. , Morozumi T. , Shiiki K.
Published: July 2002

Magnetocapacitance effect of coercive differential spin tunneling junctions Co(100 Å)/Al2O3(∼20 Å)/Co(500 Å) fabricated onto a glass substrate by ion-beam mask sputtering was investigated. The impedance was measured by a four-probe method at room temperature in the frequency range from 120 Hz to 1 MHz. It is found that the effective capacitance changes with the application of an external magnetic field. At high frequencies, the magnetocapacitance ratio is as large as the dc magnetoresistance ratio. However, at low frequencies, capacitance changes cannot be observed because the measurement sensitivity is too low. The magnetocapacitance effect was hence found to be a promising tool for high frequency magnetic sensing.
Enhanced magnetic and metal–insulator transition in Ru-doped layered manganites
Authors: Sahu Ranjan K. , Manoharan S. Sundar , Mohammad Qureshi , Rao Manju L. , Nigam A. K.
Published: July 2002

Double exchange mediated magnetism and transport properties are exemplified in two-dimensional layered manganites, La1.2Ca1.8Mn2O7. We demonstrate here the role of ruthenium in showing increased metal–insulator transition temperature, TP and Curie temperature Tc, by about 15 K when Ru is substituted up to 10% (x=0.2) at the Mn site. We discuss these unique features based on the existence of a redox ionic pair involving Mn3+/Mn4+↔Ru4+/Ru5+ up to x∼0.2. Beyond the critical level (x∼0.2), Ru exists mostly in Ru4+ oxidation state leading to a weak double exchange interaction. We document here, an enhanced Tc and TP in the layered manganites despite the disorder induced at the B site.
Electrical characterization of vapor-phase-grown single-crystal ZnO
Authors: Auret F. D. , Goodman S. A. , Legodi M. J. , Meyer W. E. , Look D. C.
Published: July 2002

Gold Schottky-barrier diodes (SBDs) were fabricated on vapor-phase-grown single-crystal ZnO. Deep-level transient spectroscopy, using these SBDs, revealed the presence of four electron traps, the major two having levels at 0.12 eV and 0.57 below the conduction band. Comparison with temperature-dependent Hall measurements suggests that the 0.12 eV level has a temperature activated capture cross section with a capture barrier of about 0.06 eV and that it may significantly contribute to the free-carrier density. Based on the concentrations of defects other than this shallow donor, we conclude that the quality of the vapor-phase-grown ZnO studied here supercedes that of other single-crystal ZnO reported up to now.
Rare-earth doped polymers for planar optical amplifiers
Authors: Slooff L. H. , van Blaaderen A. , Polman A. , Hebbink G. A. , Klink S. I. , Van Veggel F. C. J. M. , Reinhoudt D. N. , Hofstraat J. W.
Published: July 2002

Optical waveguide amplifiers based on polymer materials offer a low-cost alternative for inorganic waveguide amplifiers. Due to the fact that their refractive index is similar to that of standard optical fibers, they can be easily coupled to existing fibers with low coupling losses. Doping the polymer with rare-earth ions that yield optical gain is not straightforward, as the rare-earth salts are poorly soluble in the polymer matrix. This review article focuses on two different approaches to dope a polymer waveguide with rare-earth ions. The first approach is based on organic cage-like complexes that encapsulate the rare-earth ion and are designed to provide coordination sites to bind the rare-earth ion and to shield it from the surrounding matrix. These complexes also offer the possibility of attaching a highly absorbing antenna group, which increases the pump efficiency significantly. The second approach to fabricate rare-earth doped polymer waveguides is obtained by combining the excellent properties of SiO2 as a host for rare-earth ions with the easy processing of polymers. This is done by doping polymers with Er-doped silica colloidal spheres.
A high-density multichannel surface electromyography system for the characterization of single motor units
Authors: Blok J. H. , van Dijk J. P. , Drost G. , Zwarts M. J. , Stegeman D. F.
Published: July 2002

An electromyography (EMG) system is presented that noninvasively records the electrical activity of a muscle with 126 densely spaced skin-surface electrodes. The electrodes are arranged in a two-dimensional array and integrated in a single container for ease of application. Signals are recorded “monopolarly”, with a reference electrode placed at a distance from the array. With this recording configuration, the surface EMG (sEMG) potential distribution can be described not only as a function of time, but also topographically. The availability of topographical information opens up a range of applications. Some of these have been described previously. However, the system presented is unique in that it allows exploration of all clinical and scientific possibilities of topographical sEMG. In its design, special attention was paid to user-friendliness and flexibility. With high-density multichannel sEMG, both the properties of a whole muscle and those of single motor units, the functional units of a muscle, can be studied. The latter belong to a realm that was long considered accessible only with needle-EMG, a conventional, invasive diagnostic technique. It is demonstrated that the additional topographical information can be used to characterize motor units in a way that is partially superior to needle EMG.
Physically motivated density functionals with improved performances: The modified Perdew–Burke–Ernzerhof model
Authors: Adamo Carlo , Barone Vincenzo
Published: July 2002

In this paper we propose a modification of the exchange functional introduced by Perdew, Burke, and Ernzerhof, which significantly enlarges the original field of applications. This modification is obtained by a series expansion of the functional, which introduces one additional parameter, but retains all the asymptotic and scaling properties of the original model. The results obtained for structural, thermodynamic, kinetic, and spectroscopic properties are satisfactory and not far from those delivered by the most reliable functionals including heavy parametrization. The way in which the functional is derived and the lack of empirical parameters fitted to specific properties makes the new exchange functional widely applicable, for both quantum chemistry and for condensed-matter physics.