bluuebunny/crossref_metadata_2025 huggingface dataset:
Versatile device for low temperature in situ generation of forces up to 25 kN: Application to hydrostatic pressure experiments
Authors: Salce B. , Thomasson J. , Demuer A. , Blanchard J. J. , Martinod J. M. , Devoille L. , Guillaume A.
Published: July 2002

We describe a versatile modular device built to generate forces up to 25 kN at cryogenic temperatures in order to achieve very fine in situ hydrostatic pressure tuning in the range 0–21 GPa. This device was designed to save time during measurements and protect the experimental setup by avoiding warming and cooling cycles to vary the pressure. The force is generated by a bellow operated with pressurized He4 and amplified mechanically. Diamond and sapphire anvil cells are used to perform electrical resistivity, magnetic susceptibility, and specific heat measurements under highly hydrostatic conditions by using helium as the transmitting medium. The pressure is determined by the ruby fluorescence technique. The performance of the device is illustrated by measurements of the superconducting transition of Pb and the magnetic transitions of CeRu2Ge2 in the range 0–10 GPa. This device is currently being adapted in a dilution fridge in order to be operated down to 50 mK.
Control of epitaxial growth of pulsed laser deposited LiNbO3 films and their electro-optic effects
Authors: Lee See-Hyung , Noh T. W. , Lee Jai-Hyung
Published: July 2002

Epitaxial LiNbO3 films with two orientations were grown on sapphire (112̄0) substrates using pulsed laser deposition. The film deposited at a low oxygen pressure (i.e., 10−4 Torr) was epitaxially grown along the a axis, and that at a oxygen pressure of 10−1 Torr was epitaxially grown along the c axis. Electro-optic properties of these films were quite different, demonstrating importance of controlling the epitaxial growth behaviors. The birefringence measurements of the LiNbO3 (112̄0) film showed a quadratic electro-optic behavior with an effective quadratic coefficient of 2.38×10−15 m2/V2. This behavior was analyzed in terms of the quadratic electro-optic tensor for LiNbO3.
Room temperature operation of a single electron transistor made by the scanning tunneling microscope nanooxidation process for the TiOx/Ti system
Authors: Matsumoto K. , Ishii M. , Segawa K. , Oka Y. , Vartanian B. J. , Harris J. S.
Published: July 2002

The single electron transistor (SET) is fabricated using the scanning tunneling microscope (STM) as a fabrication process, and the fabricated SET operates at room temperature. Using the STM tip as a cathode, the surface of the titanium metal can be oxidized, and the few tens of nanometer wide oxidized titanium line can be made. The small island region of the SET of ∼30×∼35 nm2 is formed by the oxidized titanium line. The Coulomb staircase of 150 mV period is observed in the current–voltage characteristics of the SET at room temperature.
Void formation on ultrathin thermal silicon oxide films on the Si(100) surface
Authors: Wei Yi , Wallace Robert M. , Seabaugh Alan C.
Published: July 2002

The formation of voids on the thermally grown (650 °C) ultrathin (∼1 nm) silicon oxide films on the Si(100) surface was investigated by using ultrahigh vacuum scanning tunneling microscopy. Voids form randomly on the ultrathin oxide film upon thermal annealing at 750 °C. In contrast to void formation observed on thicker (>5 nm) thermal silicon oxide films and that observed on ultrathin (∼1 nm) oxide films formed by room temperature O2 adsorption, the number of voids increases during annealing. We find that Si monomer creation and SiO production compete kinetically in the void formation process.
Nonequilibrium photon-induced hotspot: A new mechanism for photodetection in ultrathin metallic films
Authors: Kadin A. M. , Johnson M. W.
Published: July 2002

A new class of photodetectors is proposed, based on localized heating of an ultrathin metallic film due to absorption of individual photons. For a 1 eV photon, the transient temperature rise can be of order 10 K or greater in a nonequilibrium ‘‘hotspot’’ on the nm spatial scale and ps timescale. If current is flowing in a metallic film with a temperature-dependent resistance, such a hotspot can give rise to a voltage pulse. This can provide the basis for an ultrafast photodetector with spectral sensitivity, in contrast to a conventional bolometer. Prospects for practical realization of infrared photodetectors based on this mechanism are discussed.
Exciton region reflectance of homoepitaxial GaN layers
Authors: Korona K. P. , Wysmol/ek A. , Pakul/a K. , Ste̢pniewski R. , Baranowski J. M. , Grzegory I. , L/ucznik B. , Wróblewski M. , Porowski S.
Published: July 2002

Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; EA=3.4776 eV, EB=3.4827 eV, and EC=3.502 eV. The spin-orbit parameter Δso=19.7±1.5 meV and the crystal field parameter Δcr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).
Mechanism for ordering in SiGe films with reconstructed surface
Authors: Araki T. , Fujimura N , Ito T.
Published: July 2002

A model of the ordering mechanism in SiGe films is developed to explain the occurrence of two types of ordered structures. We investigate the stability of ordered structures by strain energy calculation. It is suggested that atomic diffusion, which is enhanced by strain in a film, influences the formation of ordered structures. The process of atomic exchange that forms the ordered structure at a reconstructed surface during growth is also discussed.
Photoinduced enhancement of the c-axis conductivity in oxygen-deficient YBa2Cu3Ox thin films
Authors: Markowitsch W. , Stockinger C. , Lang W. , Bierleutgeb K. , Pedarnig J. D. , Bäuerle D.
Published: July 2002

High quality thin films of oxygen-depleted YBa2Cu3Ox (x≈6.6) were prepared by pulsed-laser deposition on SrTiO3, substrates that were cut with tilt angles of 10° and 20° with respect to the [001] direction, resulting in a steplike growth of the layers. The resistance showed a semiconducting behavior along the projection of the c axis to the film surface, but a metallic behavior in the perpendicular direction, indicating that the former is dominated by the c-axis resistivity and the latter by the ab-plane resistivity of YBa2Cu3Ox. Long-term illumination of the samples with a 100 W halogen lamp resulted in a significant conductivity enhancement in both directions. The photoinduced change of the out-of-plane resistance vs temperature characteristics is comparable to the effect of large hydrostatic pressure, introducing structural changes similar to that of photoexcitation.
Improved field emission of electrons from ion irradiated carbon
Authors: Walter K. C. , Kung H. H. , Maggiore C. J.
Published: July 2002

Electron field emission from allotropes of carbon (graphite, diamondlike carbon, and diamond) have been reported many times in the literature. This work explores the use of ion irradiation for improving electron field emission from carbon fibers. Carbon fibers have been irradiated with H, C, Ar, and Xe ions. Field emission characteristics have been measured as a function of ion dose. A reversible reduction in the required field for a fixed current level has been observed. The critical dose, Dc, defines the dose corresponding to the lowest field necessary to emit a fixed current (5 μA). The critical dose appears to correlate with the nuclear energy loss (collisions with atoms) of the ion in the carbon fiber. Transmission electron microscopy and parallel electron energy loss spectroscopy analysis indicate an amorphous surface, and an increase in the sp3 content of the fiber surface to 20%–30%. A corresponding decrease in the work function is expected and may account for the improvement in electron emission.
Formation of diamond in carbon onions under MeV ion irradiation
Authors: Wesolowski P. , Lyutovich Y. , Banhart F. , Carstanjen H. D. , Kronmüller H.
Published: July 2002

Spherical carbon onions are generated by irradiating graphitic carbon soot with Ne+ ions of 3 MeV energy. Under continued irradiation, a transformation of their cores to cubic diamond crystals is observed. In comparison to earlier electron irradiation experiments, the yield of diamond is much higher. The output of the irradiation experiment is characterized by electron microscopy and electron energy loss spectroscopy. Knock-on displacements of carbon atoms by Ne+ ions are assumed to be responsible for a self-compression of the onions, leading to the nucleation of diamond in their cores. The increased diamond yield is explained by the higher displacement cross-section, the higher energy transfer, and the higher total beam current on the specimen.
Studies of density and surface roughness of ultrathin amorphous carbon films with regards to thickness with x-ray reflectometry and spectroscopic ellipsometry
Authors: Logothetidis S. , Stergioudis G.
Published: July 2002

Specular x-ray reflectivity (XRR) measurements were used to study the density and surface roughness of ultrathin hydrogen-free amorphous carbon films deposited by sputtering, of thickness varying from 25 to 325 Å. The film thickness and surface roughness obtained from XRR measurements are in good agreement with that found by spectroscopic ellipsometry (SE) and atomic force microscopy. The results for the film composition obtained from SE and XRR are supported by stress measurements. Films (especially those with thickness below 100 Å) deposited with positive substrate bias voltage were found to exhibit a reduction in density, sp3 C–C bonding, and internal compressive stresses and an increase in surface roughness by increasing film thickness.
Internal distortion in ZrO2–CeO2 solid solutions: Neutron and high-resolution synchrotron x-ray diffraction study
Authors: Yashima Masatomo , Sasaki Satoshi , Yamaguchi Yasuo , Kakihana Masato , Yoshimura Masahiro , Mori Takeharu
Published: July 2002

Neutron and high-resolution synchrotron x-ray diffraction measurements have indicated a tetragonal zirconia phase with both an axial ratio of unity and internal shear deformation: The oxygen displacements from ideal fluorite positions of 0.088 and 0.078 Å are observed for 65 and 70 mol % CeO2–ZrO2 samples, respectively, although both c/a values are 1.000±0.001. The c/a ratio decreases with increasing of CeO2 content and decreases discontinuously to unity around a 60 mol % CeO2 composition, while the oxygen displacement decreases continuously up to about 90 mol % CeO2.
Defect induced lowering of activation energies at step bands in Co/Cu(100)
Authors: Coyle S. T. , Scheinfein M. R. , Blue James L.
Published: July 2002

Complex topological features such as rectangular voids and step inclusions that were seen in secondary electron micrographs of Co films grown on Cu(100) at room temperature were reproduced in Monte Carlo simulations in the presence of step bands. Lowered activation energies at defects such as steps, kinks, and vacancies enhance step edge restructuring during growth and upon annealing. This results in features such as faceted step edges, rectangular pits, incorporation of Co into terraces, surface alloying, and surface segregation. Simulated growth structures are directly compared with those observed in an ultrahigh vacuum scanning transmission electron microscope.
Optical absorption, disorder, and the disorderless limit in amorphous semiconductors
Authors: O’Leary Stephen Karrer
Published: July 2002

We study the role that disorder plays in shaping the functional form of the optical absorption spectra of amorphous semiconductors. We find, for hydrogenated amorphous silicon, amorphous germanium, amorphous gallium arsenide, and amorphous indium phosphide, that, while the breadth of the absorption tail is a strong function of disorder, the mean energy gap is insensitive to the amount of disorder. As the disorder is decreased, the optical absorption spectra associated with these amorphous semiconductors approach well defined disorderless limits, the energy gaps associated with these limits being greater than the corresponding crystalline gaps. The physical implications of these results are discussed.
A nanotube-based field-emission flat panel display
Authors: Wang Q. H. , Setlur A. A. , Lauerhaas J. M. , Dai J. Y. , Seelig E. W. , Chang R. P. H.
Published: July 2002

A matrix addressable diode flat panel display has been fabricated using a carbon nanotube–epoxy composite as the electron emission source. Field-emission uniformity has been confirmed by measuring the I–V curves of pixels across the panel. This prototype display demonstrates well-lit pixels under ±150 V biasing signals. The “on” and “off” of the pixels are well controlled by the half voltage “off-pixel” method. Further improvement of this technology may lead to easy-to-make and inexpensive flat panel displays.
Twin peak distribution of electron emission profile and impact ionization of ambient molecules during laser ablation of silver target
Authors: Issac Riju C. , Gopinath Pramod , Varier Geetha K. , Nampoori V. P. N. , Vallabhan C. P. G.
Published: July 2002

Laser-induced plasma generated from a silver target under partial vacuum conditions using the fundamental output of nanosecond duration from a pulsed Nd:yttrium aluminum garnet laser is studied using a Langmuir probe. The time of flight measurements show a clear twin peak distribution in the temporal profile of electron emission. The first peak has almost the same duration as the laser pulse while the second lasts for several microseconds. The prompt electrons are energetic enough (≈60 eV) to ionize the ambient gas molecules or atoms. The use of prompt electron pulses as sources for electron impact excitation is demonstrated by taking nitrogen, carbon dioxide, and argon as ambient gases.
Back-illuminated GaN/AlGaN heterojunction photodiodes with high quantum efficiency and low noise
Authors: Yang Wei , Nohova Thomas , Krishnankutty Subash , Torreano Robert , McPherson Scott , Marsh Holly
Published: July 2002

Back-illuminated GaN/AlGaN ultraviolet (UV) heterojunction photodiodes with high quantum efficiencies are demonstrated. Photovoltaic (zero bias) responsivity of 0.2 A/W at 355 nm was achieved. The improved efficiencies primarily arise from the use of AlGaN/GaN heterojunction in which photons are absorbed within the p-n junction thus eliminates carrier losses due to surface recombination and diffusion processes in previously reported homojunction devices. Very high dark impedance and large visible rejection ratio were obtained. These results indicate high quality GaN/AlGaN interface and efficient photocarrier collection in the photodiode.
InGaN/GaN quantum wells studied by high pressure, variable temperature, and excitation power spectroscopy
Authors: Perlin Piotr , Kisielowski Christian , Iota Valentin , Weinstein B. A. , Mattos Laila , Shapiro Noad A. , Kruger Joachim , Weber Eicke R. , Yang Jinwei
Published: July 2002

The energies of photo- and electroluminescence transitions in InxGa1−xN quantum wells exhibit a characteristic “blueshift” with increasing pumping power. This effect has been attributed either to band-tail filling, or to screening of piezoelectric fields. We have studied the pressure and temperature behavior of radiative recombination in InxGa1−xN/GaN quantum wells with x=0.06, 0.10, and 0.15. We find that, although the recombination has primarily a band-to-band character, the excitation-power induced blueshift can be attributed uniquely to piezoelectric screening. Calculations of the piezoelectric field in pseudomorphic InxGa1−xN layers agree very well with the observed Stokes redshift of the photoluminescence. The observed pressure coefficients of the photoluminescence (25–37 meV/GPa) are surprisingly low, and, so far, their magnitude can only be partially explained.
Giant magnetoresistance near the magnetostructural transition in Gd5(Si1.8Ge2.2)
Authors: Morellon L. , Stankiewicz J. , Garcı́a-Landa B. , Algarabel P. A. , Ibarra M. R.
Published: July 2002

Zero-field electrical resistivity over the temperature range of 4–300 K and magnetoresistance in magnetic fields of up to 12 T have been measured in Gd5(Si1.8Ge2.2). This system undergoes a first-order magnetostructural transition at TC≅240 K, from a high-temperature paramagnetic to a low-temperature ferromagnetic phase, accompanied by a large drop in the resistivity. The application of an external magnetic field above TC can induce this transition, and a giant negative magnetoresistance effect (Δρ/ρ≅−20%) is observed associated with this first-order field-induced transition.
Depth-resolved imaging of nematic liquid crystals by third-harmonic microscopy
Authors: Yelin D. , Silberberg Y. , Barad Y. , Patel J. S.
Published: July 2002

Third-harmonic microscopy is shown to be a powerful tool for the study of liquid crystal structures. The third-harmonic probe enables the investigation of molecular order well inside a liquid crystal cell. The observation of phase transition in various nematic liquid crystal samples is presented as a demonstration for the power of this technique. For example, nematic and isotropic regions are shown to coexist across the depth of the cell. Thermal fluctuations are observed in the nematic regions close to the isotropic–nematic transition temperature.
Giant electric fields in unstrained GaN single quantum wells
Authors: Langer R. , Simon J. , Ortiz V. , Pelekanos N. T. , Barski A. , André R. , Godlewski M.
Published: July 2002

We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.
Selective-area and lateral overgrowth of chromium dioxide (CrO2) films by chemical vapor deposition
Authors: Gupta A. , Li X. W. , Guha S. , Xiao Gang
Published: July 2002

Selective-area growth has been utilized to deposit chromium dioxide (CrO2) films in specific regions of a prepatterned surface using atmospheric pressure chemical vapor deposition from CrO3 precursor. For films deposited on single crystal TiO2 substrates covered with patterned SiO2 thin film, epitaxial growth occurs selectively in the windows exposed to the substrate with no deposition in the regions masked by SiO2. The substrate provides a crystallographic template for the initial vertical growth of CrO2 in the patterned stripe windows. With increasing film thickness, lateral epitaxial overgrowth of CrO2 takes place over the masked regions, with the growth rate and facet formation behavior dependent on the orientation of the stripe openings. Selective growth of polycrystalline CrO2 has also been achieved on prepatterned titanium films deposited on oxidized silicon wafers. In this case, the titanium film is oxidized prior to deposition, with localized growth of CrO2 occurring only on top of the oxidized titanium and not on the SiO2.
Continuous wave simultaneous multi-self-frequency conversion in Nd3+-doped aperiodically poled bulk lithium niobate
Authors: Capmany J. , Bermúdez V. , Callejo D. , Garcı́a Solé J. , Diéguez E.
Published: July 2002

We report simultaneous participation of the fundamental laser wave in self-frequency doubling and self-sum-frequency mixing with pump radiation in Nd3+-doped bulk aperiodically poled lithium niobate structures. A green continuous wave visible laser output power of 1.5 mW generated by self-frequency doubling and 0.5 mW of blue generated by self-sum-frequency mixing have been obtained simultaneously from 30 mW of laser output power available in the fundamental. Stable and efficient infrared laser action at room temperature is obtained in both F3/24→I11/24 and F3/24→I13/24 laser channels of Nd3+ ion without oxide codoping, although some residual photorefractive damage for the visible outputs is observed.
Trapping levels in nanocrystalline porous silicon
Authors: Ciurea M. L. , Drãghici M. , Lazanu S. , Iancu V. , Nassiopoulou A. , Ioannou V. , Tsakiri V.
Published: July 2002

Trapping levels in fresh (one month) and naturally aged (one year) nanocrystalline porous silicon have been investigated using the optical charging spectroscopy method. Four significant maxima and/or shoulders were observed for fresh samples and five for aged ones. They have been attributed to five and six trapping levels, respectively. The trapping centers corresponding to the most shallow four levels are situated at or nearby the internal surface of the porous silicon films.
Growth oscillation decay rates for control of III–V molecular beam epitaxy near stoichiometry
Authors: Owen J. H. G. , Barvosa-Carter W. , Zinck J. J.
Published: July 2002

We have investigated the decay of reflection high-energy electron diffraction oscillations during growth of InAs (001), as a function of growth parameters, such as the V/III ratio. We have shown that the decay constants are sensitive to changes in growth morphology, as measured by scanning tunneling microscopy. Our experiments show that the values of these decay constants decrease at high V/III ratios, in agreement with previous work. Additionally, we have found that the values of the decay constants diverge as the transition between the (2×4) and (4×2) reconstructions is approached. We propose that the decay constants of the growth oscillations may be used as inputs for control of interface morphology.
Ordering and orientation of CoPt/SiO2 granular films with additive Ag
Authors: Chen Chen , Kitakami Osamu , Okamoto Satoshi , Shimada Yutaka
Published: July 2002

We have studied the chemical ordering of granular films (CoPt)1−xAgx/SiO2 with the concentration x=2–6 at. %. It was found that the additive Ag efficiently promotes the ordering process, reducing the ordering temperature by 100 °C compared with that of without Ag. The additive Ag also stimulates nearly perfect (001) orientation of the CoPt-ordered grains in the films with a thickness of less than 50 nm, resulting in a large perpendicular magnetic anisotropy.
In situ-grown carbon nanotube array with excellent field emission characteristics
Authors: Rao A. M. , Jacques D. , Haddon R. C. , Zhu W. , Bower C. , Jin S.
Published: July 2002

In situ catalytic thermal decomposition method was used for producing aligned multiwalled carbon nanotubes (MWNTs) in bulk quantities on stable and electrically conducting substrates. Very low turn-on electric fields of 0.75 V/μm and low threshold fields of ∼1.6 V/μm (for current density of 10 mA/cm2) were obtained from the MWNT arrays grown on TiN substrate. Furthermore, large emission current densities of 1–3 A/cm2 were obtained at reasonably low fields of less than ∼8 V/μm. These enhanced emission properties are tentatively attributed to the oriented and high-density nature of the emitting carbon nanotube structure and the high-conductivity, stable nature of the TiN substrate onto which the nanotubes are attached.
Electrostatic drift-like waves in a bounded dusty plasma
Authors: Mamun A. A. , Shukla P. K.
Published: July 2002

A number of different types of obliquely propagating dust-associated electrostatic drift-like waves, namely, the Shukla–Varma mode, the lower-hybrid mode, coupled electron–drift–dust–ion–acoustic waves, coupled ion–drift–dust–electron–acoustic waves, coupled dust–drift–dust–acoustic waves, and coupled dust–drift–dust–cyclotron waves, in a nonuniform bounded dusty magnetoplasma are theoretically investigated. The last two are studied by considering the dynamics of the negatively charged dust grains, whereas the other ones are studied by considering the presence of the static dust grains. The dispersion properties of these drift-like waves, which are found to be significantly modified by the combined effects of plasma density inhomogeneity, finite boundary of the dusty plasma, obliqueness of the propagating mode, and external magnetic field, are examined. The relevance of this investigation to low-temperature laboratory dusty magnetoplasmas is discussed.
In-plane and out-of-plane resistivities of vicinal Hg-1212 thin films
Authors: Yun S. H. , Pedarnig J. D. , Rössler R. , Bäuerle D. , Obradors X.
Published: July 2002

Vicinal Hg1−x Rex Ba2CaCu2O6+δ films have been grown on tilted (001)SrTiO3 substrates. The c axis of films is oriented in parallel to the c axis of the substrate for tilt angles up to 12°. The vicinal films show a step-like surface morphology with smooth terraces that are tilted with respect to the substrate surface. The in-plane and out-of-plane resistivities ρab and ρc reveal metallic behavior with a slight upturn near Tc for the latter. The resistivity anisotropy derived from these measurements is ρc/ρab∼650±150 at 300 K. This value is between that found for YBa2Cu3O7−δ and the Bi-based cuprates, and it increases with decreasing temperature.
Mass spectrometric characterization of BCl3/SF6 plasmas
Authors: Lee Y.-S. , Sia J. F. , Nordheden K. J.
Published: July 2002

Significant increases in the etch rates of both GaAs and GaN have been observed with the addition of SF6 to BCl3 plasmas. Mass spectrometric characterization of neutrals in these gas mixtures shows that increasing the SF6 percentage in the flow enhances the dissociation of BCl3, resulting in nearly 100% dissociation at 70% SF6. This increased dissociation is believed to be due to electron attachment heating. Both Cl and Cl2 mass intensities also maximize at 70% SF6. The detection of BClF and BCl2F suggest the possibility of reactions occurring between BCl and BCl2 radicals and fluorine, which may inhibit the recombination of Cl.
Particle sizing of magnetite-based magnetic fluid using atomic force microscopy: A comparative study with electron microscopy and birefringence
Authors: Lacava B. M. , Azevedo R. B. , Silva L. P. , Lacava Z. G. M. , Neto K. Skeff , Buske N. , Bakuzis A. F. , Morais P. C.
Published: July 2002

Atomic force microscopy (AFM), transmission electron microscopy (TEM), and static magnetic birefringence (SMB) were used to unfold the particle size polydispersity profile of a magnetite-based magnetic fluid sample. The data obtained from different techniques were curve fitted using the lognormal distribution function, from which the mean particle diameter (Dm) and the standard deviation (σ) were obtained. In comparison to the TEM data, the AFM data show a reduction of Dm (about 20%) and an increase of σ (about 15%). In contrast, close agreement between the TEM and SMB data was found.
Full dimensional quantum calculations of the CH4+H→CH3+H2 reaction rate
Authors: Huarte-Larrañaga Fermı́n , Manthe Uwe
Published: July 2002

Accurate full-dimensional quantum mechanical calculations are reported for the CH4+H→CH3+H2 reaction employing the Jordan–Gilbert potential energy surface. Benchmark results for the thermal rate constant and the cumulative reaction probability are presented and compared to classical transition state theory as well as reduced dimensionality quantum scattering calculations. The importance of quantum effects in this system is highlighted.
Microwave dielectric properties of strained (Ba0.4Sr0.6)TiO3 thin films
Authors: Kim W. J. , Wu H. D. , Chang W. , Qadri S. B. , Pond J. M. , Kirchoefer S. W. , Chrisey D. B. , Horwitz J. S.
Published: July 2002

Ferroelectric Ba0.4Sr0.6TiO3 (BST) thin films grown on (001) MgO by pulsed laser deposition show a strong correlation between their structure and their microwave dielectric properties. Epitaxially grown BST films are observed by x-ray diffraction to be tetragonally distorted. The oxygen deposition pressure affects the magnitude of the tetragonal distortion (the ratio of in-plane and surface normal lattice parameters, D=a/c) of the deposited BST films. D varied from 0.996 to 1.004 at oxygen deposition pressure of 10–800 mTorr. The dielectric properties of BST films measured at microwave frequencies (1–20 GHz) exhibit an oxygen deposition pressure dependent dielectric constant (ε=100–600), and quality factor Q (1/tan δ=10–60). The BST film grown at the oxygen deposition pressure of 200 mTorr exhibits the highest figure of merit [% tuning in ε×Q0V, where % tuning is 100×(ε0−εb)/ε0, and ε0 and εb are dielectric constant at 0 and 80 kV/cm]. This corresponds to the film with the lowest distortion (D=1.001). The observed microwave properties of the films are explained by a phenomenological thermodynamic theory based on the strain along in-plane direction of the films.
Diffusion barrier and electrical characteristics of a self-aligned MgO layer obtained from a Cu(Mg) alloy film
Authors: Lee W. H. , Cho H. L. , Cho B. S. , Kim J. Y. , Nam W. J. , Kim Y-S. , Jung W. G. , Kwon H. , Lee J. H. , Lee J. G. , Reucroft P. J. , Lee C. M. , Lee E. G.
Published: July 2002

Diffusion barrier characteristics and electrical properties of self-aligned MgO layers obtained from a Cu(Mg) alloy film have been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing a Cu(Mg) film in an oxygen ambient and prevented interdiffusion of Cu in SiO2 up to 700 °C. The thermal stability of a pure Cu/TiN/Si multilayer system has been significantly enhanced up to 800 °C by the MgO layers by forming a MgO/Cu/MgO/TiN/Si multilayer system. A combined structure of Si3N4(500 Å)/MgO(100 Å) increased the breakdown voltage up to 20 V from 15 V and reduced the leakage current density down to 3×10−9 A/cm2 from 1×10−8 A/cm2 compared to a pure copper system. Consequently, the deposition of Cu(Mg) alloy followed by annealing in an oxygen ambient gives rise to the formation of a self-aligned MgO layer with excellent diffusion barrier and electrical characteristics and the film can be applied as a gate electrode in thin-film transistor/liquid-crystal displays, resulting in a reduction of process steps.
Y-junction carbon nanotubes
Authors: Satishkumar B. C. , Thomas P. John , Govindaraj A. , Rao C. N. R.
Published: July 2002

Carbon nanotubes with junctions are considered to be of potential value in nanoelectronics. A simple pyrolysis procedure for producing Y-junction carbon nanotubes is described. The method involves the pyrolysis of the organometallic precursor, nickelocene, along with thiophene at 1273 K. Tunneling conductance measurements showed that at the Y junction, the I-V characteristics are asymmetric with respect to zero bias as in a junction diode.
Inherent structures and nonequilibrium dynamics of one-dimensional constrained kinetic models: A comparison study
Authors: Crisanti A. , Ritort F. , Rocco A. , Sellitto M.
Published: July 2002

We discuss the relevance of the Stillinger and Weber approach to the glass transition investigating the nonequilibrium behavior of models with nontrivial dynamics, but with simple equilibrium properties. We consider a family of 1D constrained kinetic models, which interpolates between the asymmetric chain introduced by Jäckle and Eisinger [Z. Phys. B 84, 115 (1991)] and the symmetric chain introduced by Fredrickson and Andersen [Phys. Rev. Lett 53, 1244 (1984)], and the 1D version of the Backgammon model [Phys. Rev. Lett. 75, 1190 (1995)]. We show that the configurational entropy obtained from the inherent structures is the same for all models irrespective of their different microscopic dynamics. We present a detailed study of the coarsening behavior of these models, including the relation between fluctuations and response. Our results suggest that any approach to the glass transition inspired by mean-field ideas and resting on the definition of a configurational entropy must rely on the absence of any growing characteristic coarsening pattern.
Alkaline metal-doped n-type semiconducting nanotubes as quantum dots
Authors: Kong Jing , Zhou Chongwu , Yenilmez Erhan , Dai Hongjie
Published: July 2002

A 0.4 μm long semiconducting single-walled carbon nanotube is doped into n type by potassium (K) vapor. Electrical measurements of the doped nanotube reveal single-electron charging at temperatures up to 160 K. The K-doped sample manifests as a single quantum dot or multiple quantum dots in series depending on the range of applied gate voltage. This is explained by an inhomogeneous doping profile along the nanotube length. Similarities between K-doped nanotubes and silicon-based quantum dots and the possibility of room-temperature nanotube single-electron transistors are discussed.
Strain profiles in epitaxial films from x-ray Bragg diffraction phases
Authors: Vartanyants I. , Ern C. , Donner W. , Dosch H. , Caliebe W.
Published: July 2002

We present a versatile and model-independent approach for the analysis of strain distributions in thin-film systems which is based on the x-ray phase retrieval by an iterative Gerchberg–Saxton algorithm. This scheme is used to determine the strain profile across a thin ordered Cu3Au(111) film grown epitaxially on Al2O3.
Single-electron transistor made of two crossing multiwalled carbon nanotubes and its noise properties
Authors: Ahlskog M. , Tarkiainen R. , Roschier L. , Hakonen P.
Published: July 2002

A three-terminal nanotube device was fabricated from two multiwalled nanotubes by pushing one on top of the other using an atomic-force microscope. The lower nanotube, with gold contacts at both ends, acted as the central island of a single-electron transistor while the upper one functioned as a gate electrode. Coulomb blockade oscillations were observed on the nanotube at sub-Kelvin temperatures. The voltage noise of the nanotube single-electron transistor (SET) was gain dependent as in conventional SETs. The charge sensitivity at 10 Hz was 6×10−4 e/Hz.
Magnetoresistance in manganite/alumina nanocrystalline composites
Authors: Hueso L. E. , Rivas J. , Rivadulla F. , López-Quintela M. A.
Published: July 2002

Magnetotransport properties of manganite/insulator composites are studied in this article. By merging the half metallic character of La2/3Ca1/3MnO3 and the mixed composition with alumina grains dispersed in the structure we have been able to increase three times intergranular magnetoresistance around the percolation threshold. The transport properties have been studied employing a theoretical model for ferromagnetic–insulator systems and a two channel equation in order to reproduce the behavior of resistivity in the whole temperature range. The percolation theory is introduced to try to understand and improve the fascinating properties of these mixed systems.
Observation of molecular layering in a confined water film and study of the layers viscoelastic properties
Authors: Antognozzi M. , Humphris A. D. L. , Miles M. J.
Published: July 2002

A transverse dynamic force microscope, more commonly known as shear force microscope, has been used to investigate confined water films under shear. A cylindrically tapered glass probe was mounted perpendicularly to the sample surface. Pure water was confined between the probe and a freshly cleaved mica surface and a sinusoidal shear strain was applied by setting the probe into transverse oscillation. Repeated measurements of the probe oscillation amplitude and relative phase lag, at different tip-sample separations, exhibited a clear step-like behavior. The periodicity, recorded over several curves, ranged between 2.4 and 2.9 Å, which is similar to the diameter of the water molecule. The in-phase (elastic) and the out-of-phase (viscous) stress response of the confined water film was evaluated (from the experimental data) by assuming a linear viscoelastic behavior. Finally, by modeling the water film with the Maxwell mechanical model, the values for the shear viscosity and shear rigidity were obtained.
Ionized physical vapor deposition of Cu on 300 mm wafers: A modeling study
Authors: Rauf Shahid , Ventzek Peter L. G. , Arunachalam Valli
Published: July 2002

A two-dimensional model has been used to understand the physics and process engineering issues associated with a conceptual 300 mm Cu internal-coil ionized physical vapor deposition reactor. It has been found that inductive coupling from the coil is the primary source of plasma production. Since the coil is in direct contact with the plasma, a significant fraction of the coil power is deposited in the gas capacitively as well. This results in sputtering of the Cu coil, which tends to improve Cu flux uniformity at the outer edges of the wafer. Since the Cu ionization threshold is much lower than Ar, Cu+ density is comparable to Ar+ density even though ground state Cu density is much smaller than Ar. Significant fraction of the neutral Cu flux to the wafer is in the metastable or athermal state. The effects of several actuators, reactor dimensions, and buffer gas on important plasma and process quantities have also been investigated. Electron density in the reactor and Cu ionization fraction increases with increasing total coil power because of enhanced ionization. Total coil power however does not affect the Cu density appreciably, except near the coil where enhanced coil sputtering increases the Cu density. Decrease in dc target voltage with increasing coil power decreases Cu+ loss to the target and results in an increase in total Cu flux to the wafer. Electron and Cu density in the reactor increase with increasing dc target power. This is due to enhancement in target sputtering and consequent ionization of the sputtered Cu. While this increases the total Cu flux to the wafer, ionization fraction is not affected much. It is demonstrated that uniformity of Cu flux to the wafer and ionization fraction can be controlled by means of the terminating capacitor at the coil. Decreasing the terminating capacitance increases the coil voltage, enhances coil sputtering and enhances Cu flux toward the outer edges of the wafer. This, however, decreases the amount of power that is transferred to the plasma inductively, reducing the ionization efficiency. Increasing the coil–wafer distance results in fewer sputtered Cu atoms being ionized as the target–coil distance becomes smaller than the mean free path for thermalization of hot sputtered Cu atoms. Also, one can control the ionization fraction of Cu flux to the wafer by replacing Ar by Ne or Xe, without significantly impacting the total Cu flux.
An apertureless near-field scanning optical microscope and its application to surface-enhanced Raman spectroscopy and multiphoton fluorescence imaging
Authors: Nieman Linda T. , Krampert Gerhard M. , Martinez Robert E.
Published: July 2002

We describe a home-built apertureless near-field scanning optical microscope and present preliminary results of its operation. Raman scattering from samples of polydiacetylene para-toluene sulphonate, and two-photon-induced fluorescence from crystallites of coumarin I dye are strongly enhanced in the presence of a sharp gold-coated atomic force microscope tip. We verify the dependence of the scattered intensity on the polarization of the incident beam relative to the tip axis. Finally, we show near-field fluorescence images taken in the presence of a strong far-field background whose spatial resolution is limited by the size of the tip.
State selective vibrational (2ν3) activation of the chemisorption of methane on Pt (111)
Authors: Higgins J. , Conjusteau A. , Scoles G. , Bernasek S. L.
Published: July 2002

The state-selective dissociative sticking coefficient on Pt(111) surfaces for CH4 in the 2ν3 J=1,2 ro-vibrational levels has been measured using thermal energy atomic scattering. Continuous wave laser excitation of a molecular beam of CH4 seeded in He with a 1.5 μm color center laser, tunable around 6000 cm−1 and coupled to the beam by means of a resonant build-up cavity, allows pumping of up to 11% of the molecules to the excited ro-vibrational state. The laser/molecular beam combination allows precise control over the translational energy as well as the vibrational state of the methane that impinges on the clean Pt(111) surface. The intensity of the specular reflection of the incident helium beam is used to quantitatively monitor the coverage of chemisorbed methane on the platinum surface as a function of time (exposure). The sticking coefficient (S0) of CH4 with 5.4 kJ/mol normal translational energy is found to increase from 6.2×10−6 to 1.8×10−4 upon 2ν3 excitation (the overtone of the asymmetric stretch of methane). This represents a ∼30 fold enhancement in reactivity of the methane with the Pt(111) surface upon vibrational excitation. We also measured the changes of S0 obtained by varying the nozzle temperature and methane concentration over a tenfold range of energy. We find that 72 kJ/mol of vibrational energy in the excited CH4 is approximately equivalent to at least 30 kJ/mol of normal translational energy. This corresponds to a utilization efficiency of the 2ν3 vibrational energy of greater than 40%. In the only other measurement of this kind published in the literature, [L. B. F. Juurlink, P. R. McCabe, R. R. Smith, C. L. DiCologero, and A. L. Utz, Phys. Rev Lett. 83, 868 (1999)] for the fundamental (v=1) excitation of ν3 of CH4 on Ni(100) surfaces, a comparable value for the vibrational energy utilization efficiency was found (59%). Further work is necessary to determine if this result is general and if and how it may change by changing the vibrational mode excited by the laser.
The oxidation of CO on RuO2(110) at room temperature
Authors: Fan C. Y. , Wang J. , Jacobi K. , Ertl G.
Published: July 2002

RuO 2 (110) surfaces were prepared by exposing Ru(0001) to 107 L of O2 at 700 K. Postexposure of O2 at 300 K resulted in an additional oxygen species (O-cus) adsorbed on coordinatively unsaturated Ru atoms (Ru-cus). The surface was then exposed to CO at 300 K and studied by thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). It is demonstrated that CO is oxidized at 300 K through reaction with both the O-cus as well as with surface O-atoms held in bridge positions (O-bridge). Although—at room temperature—CO adsorbs intermediately on the Ru-cus atoms, it is stable only at the Ru atoms underneath the O-bridge after the latter has been reacted off. At room temperature only surface oxygen takes part in the CO oxidation and the oxygen-depleted surface can be restored by O2 exposure, so that under steady-state flow conditions an oxygen-deficient surface will exist whose stoichiometry will be determined by the ratio of partial pressures.
Electronic structure studies of undoped and nitrogen-doped tetrahedral amorphous carbon using high-resolution electron energy-loss spectroscopy
Authors: Waidmann S. , Knupfer M. , Fink J. , Kleinsorge B. , Robertson J.
Published: July 2002

The electronic structure of undoped and nitrogen-doped tetrahedral amorphous carbon (ta-C) has been studied using high-resolution electron energy-loss spectroscopy in transmission to determine the local bonding of the carbon and nitrogen atoms. We found that the carbon and nitrogen atoms are mainly sp3-bonded for nitrogen contents up to 10 at. %. However, the data analysis is also consistent with some N atoms occupying trivalent σ or pyrrole configurations. The real (ε1) and imaginary parts (ε2) of the dielectric function were derived over a wide energy range carrying out a Kramers–Kronig analysis of the loss spectra. The ε2 spectra show two features due to the excitation of the π and σ valence states. The π feature is very small in ta-C deposited at 100 eV, consistent with its large sp3 content (∼85%). We have determined the ratios of sp2/sp3-hybridized atoms in the samples by both the C 1s edge and the optical sum rules.
Optical limiting properties of phthalocyanine-fullerene derivatives
Authors: Zhu Peiwang , Wang Peng , Qiu Wenfeng , Liu Yunqi , Ye Cheng , Fang Guangyu , Song Yinglin
Published: July 2002

The solubility of fullerene (C60) is improved by combining it with an alkyl group of substituted copper (II) phthalocyanine (CuPc) through Diels–Alder reaction. Optical limiting properties of this compound were investigated by using a double frequency Nd:YAG laser system at a wavelength of 532 nm with pulse duration of 45 ps. The results show that there are two separated conjugation systems in this kind of molecule. Both of them independently contribute to the reverse saturable absorption. Broadband optical limiting materials might be designed based on the combination of various optical limiting molecules through covalent bonds. Phase separation problems existing in the mixture of various materials can be avoided.
Elastic fields of quantum dots in subsurface layers
Authors: Romanov A. E. , Beltz G. E. , Fischer W. T. , Petroff P. M. , Speck J. S.
Published: July 2002

In this work, models based on conventional small-strain elasticity theory are developed to evaluate the stress fields in the vicinity of a quantum dot or an ordered array of quantum dots. The models are based on three different approaches for solving the elastic boundary value problem of a misfitting inclusion embedded in a semi-infinite space. The first method treats the quantum dot as a point source of dilatation. In the second approach we approximate the dot as a misfitting oblate spheroid, for which exact analytic solutions are available. Finally, the finite element method is used to study complex, but realistic, quantum dot configurations such as cuboids and truncated pyramids. We evaluate these three levels of approximation by comparing the hydrostatic stress component near a single dot and an ordered array of dots in the presence of a free surface, and find very good agreement except in the immediate vicinity of an individual quantum dot.
Light degradation and voltage drift in polymer light-emitting diodes
Authors: Silvestre G. C. M. , Johnson M. T. , Giraldo A. , Shannon J. M.
Published: July 2002

It is shown that the voltage drift and light degradation in polymer light-emitting diodes are related and can be explained by the formation of traps and the modification of the space charge in the bulk of the polymer. The energy released by nonradiative carrier recombination is believed to be the driving force for the generation of traps in poly(p-phenylene vinylene) conjugated polymers. A first-approximation model is derived for the voltage drift and the light decrease during operation, which is in good agreement with experimental observations for time and current density dependencies.
How to build a better pair potential for water
Authors: Guillot Bertrand , Guissani Yves
Published: July 2002

With the objective of improving the effective pair potentials for water, we develop a potential model that employs diffuse charges, in addition to the usual point charges, on the oxygen and hydrogen atoms, to account for charge penetration effects. The potential has better transferability from the liquid to gaseous phases since, unlike many existing models, it does not require an enhanced dipole moment. As a result it accurately reproduces the structural and thermodynamic properties of water over a wide range of conditions. Moreover, by allowing for electronic polarization when evaluating the total dipole moment of the simulated fluid, the model leads to the correct value of the dielectric constant for virtually any state point. At room temperature the calculation produces an average dipole moment of 3.09 D, in accord with recent theoretical and experimental evaluations. This supports the idea that induction effects in water are more important than previously expected.