bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and thermal stability of pseudomorphic Ge1−yCy/Ge superlattices on Ge(001)
Authors: Duschl R. , Schmidt O. G. , Winter W. , Eberl K. , Dashiell M. W. , Kolodzey J. , Jin-Phillipp N. Y. , Phillipp F.
Published: July 2002

High quality Ge/Ge1−yCy superlattices with nominal carbon contents of 1.2% and 2.1% were grown by molecular beam epitaxy on Ge(001). In transmission electron microscopy the layers are planar and perfectly pseudomorphic without any extended defects observable. The infrared absorption line at 529 cm−1 is attributed to the local vibrational mode of substitutional carbon in germanium. However, in contrast to Si1−yCy alloys where almost 100% of the C is substitutional under optimized growth conditions, x-ray diffraction measurements indicate that the efficiency of carbon incorporation onto substitutional sites is only about 30% for low temperature growth at TS=200 °C. It reduces further for higher growth temperatures to only about 10% at TS=300 °C. Post-growth annealing experiments indicate thermal stability up to 450 °C. Annealing at higher temperature results in a reduction of substitutional carbon content. As in the case of Si1−yCy alloys the built-in strain is relaxed by C diffusion and not by nucleation of misfit dislocation.
Mechanics of rollable and foldable film-on-foil electronics
Authors: Suo Z. , Ma E. Y. , Gleskova H. , Wagner S.
Published: July 2002

The mechanics of film-on-foil devices is presented in the context of thin-film transistors on steel and plastic foils. Provided the substrates are thin, such transistors function well after the foils are rolled to small radii of curvature. When a substrate with a lower elastic modulus is used, smaller radii of curvature can be achieved. Furthermore, when the transistors are placed in the neutral surface by sandwiching between a substrate and an encapsulation layer, even smaller radii of curvature can be attained. Transistor failure clearly shows when externally forced and thermally induced strains add to, or subtract from, each other.
High-transparency Ni/Au ohmic contact to p-type GaN
Authors: Sheu J. K. , Su Y. K. , Chi G. C. , Koh P. L. , Jou M. J. , Chang C. M. , Liu C. C. , Hung W. C.
Published: July 2002

In this study, a very thin Ni/Au bilayer metal film was prepared by electron beam evaporation and thermal alloying to form ohmic contact on p-type GaN film. After thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film exhibited ohmic behavior. The Ni/Au contacts showed a specific contact resistance of 1.7×10−2 Ω cm2 at an alloying temperature of 450 °C. In addition, the light transmittance of the Ni/Au (2 nm/6 nm) bilayer on p-type GaN was measured to be around 85% at 470 nm. These results suggest that a suitable metallization technology for the fabrication of light emitting devices can be achieved.
InAs/InGaAs quantum dot structures on GaAs substrates emitting at 1.3 μm
Authors: Ustinov V. M. , Maleev N. A. , Zhukov A. E. , Kovsh A. R. , Egorov A. Yu. , Lunev A. V. , Volovik B. V. , Krestnikov I. L. , Musikhin Yu. G. , Bert N. A. , Kop’ev P. S. , Alferov Zh. I. , Ledentsov N. N. , Bimberg D.
Published: July 2002

InAs self-organized quantum dots inserted in InGaAs quantum well have been grown on GaAs substrates by molecular beam epitaxy. The lateral size of the InAs islands has been found to be approximately 1.5 times larger as compared to the InAs/GaAs case, whereas the island heights and surface densities were close in both cases. The quantum dot emission wavelength can be controllably changed from 1.1 to 1.3 μm by varying the composition of the InGaAs quantum well matrix. Photoluminescence at 1.33 μm from vertical optical microcavities containing the InAs/InGaAs quantum dot array was demonstrated.
Influence of oxygen content on dielectric and electromechanical properties of Pb(Mg1/3Nb2/3)O3 thin films
Authors: Catalan G. , Corbett M. H. , Bowman R. M. , Gregg J. M.
Published: July 2002

Pulsed laser deposition was used to grow Pb(Mg1/3Nb2/3)O3 (PMN) thin film planar capacitor structures. X-ray diffraction and plan-view transmission electron microscopy were used to verify PMN crystallography. The dc leakage current and ac capacitance and dielectric loss were measured as a function of temperature and frequency. Finally, crystallographic strain as a function of applied dc field was monitored in situ by x-ray diffraction. The electromechanical strain response was found to depend on the deposition conditions for each capacitor. Tensile strains of ∼0.2% and compressive strains of ∼0.35%, both parallel to the applied field, were measured for capacitors of different oxygen contents and thicknesses. Tensile strains achieved are higher than previously reported for PMN thin films or polycrystalline ceramics. We propose that the compressive strains are not an intrinsic property of the PMN. Instead they are induced by the combined effect of joule heating of the capacitor structure, caused by leakage currents, and thermal expansion mismatch between the substrate and films.
Generation of high-frequency coherent acoustic phonons in a weakly coupled superlattice
Authors: Glavin B. A. , Kochelap V. A. , Linnik T. L.
Published: July 2002

In this work we analyze conditions of generation of high frequency coherent acoustic phonons in a biased semiconductor superlattice. We show that the electron perpendicular transport provides the electron population inversion for interwell phonon-assisted transitions and generation of phonons propagating along the superlattice axis. The generation rate is large and exceeds the phonon losses in perfect superlattices. Under such conditions, the phonon subsystem is unstable with respect to generation of the coherent phonon modes. Influence of the phonon spectra modification in the superlattice on the generation rate is examined. Effects restricting the intensity of the phonon generation are also pointed out.
Patterning of highly oriented pyrolytic graphite by oxygen plasma etching
Authors: Lu Xuekun , Huang Hui , Nemchuk Nikolay , Ruoff Rodney S.
Published: July 2002

Patterning of highly oriented pyrolytic graphite (HOPG) was demonstrated by oxygen plasma etching of lithographically patterned substrates. Periodic arrays of islands, or holes of several microns on an edge, were obtained on freshly cleaved HOPG surfaces which had been prepared with SiO2 mask stops and then oxygen plasma etched. The etching process is described, including a study of etch rate as a function of rf power, and morphology was characterized with scanning electron microscopy.
Self-assembling ferroelectric Na0.5K0.5NbO3 thin films by pulsed-laser deposition
Authors: Cho Choong-Rae , Grishin Alex
Published: July 2002

Highly [100]-axis oriented single-phase Na0.5K0.5NbO3 (NKN) thin films have been grown on polycrystalline Pt80Ir20 (Pt) and SiO2 (native oxide)/Si (111) substrates using KrF excimer laser ablation of a stoichiometric ceramic target. X-ray diffraction θ–2θ scan and rocking curve data are evidence of the strong effect of film self-assembling along the [100] direction regardless of the substrate texture. Furthermore, multiple-cell structuring along the polar axis has been observed in NKN films grown onto the Pt substrate. Ferroelectric measurements yield remnant polarization Pr of 10 μC/cm2 and spontaneous polarization Ps of 17.5 μC/cm2 at 80 kV/cm. The electrical resistivity of the Na0.5K0.5NbO3 film was in the order of 1010 Ω cm at 10 kV/cm. Dielectric permittivity ε′ and dissipation factor tan δ have been found to vary 480–440 and 0.028–0.024, respectively, in the frequency range 0.4–100 kHz.
Void nucleation on intentionally added defects in Al interconnects
Authors: Doan J. C. , Lee S.-H. , Bravman J. C. , Flinn P. A. , Marieb T. N.
Published: July 2002

Void nucleation in passivated aluminum interconnects was studied using high voltage scanning electron microscopy. To test theories about stress-induced and electromigration void nucleation, Ar ions were implanted into Al specimens. The Ar atoms precipitated and formed bubbles that served as nucleation sites with high surface energy. In the implanted samples, voids formed away from the interconnect sidewalls, in contrast to voids in ordinary passivated Al interconnects. The evolution of the void volume was also affected by the reduction in the nucleation barrier. These results strongly support the theory of void nucleation on interface flaws in Al interconnects.
Light extraction from optically pumped light-emitting diode by thin-slab photonic crystals
Authors: Boroditsky M. , Krauss T. F. , Coccioli R. , Vrijen R. , Bhat R. , Yablonovitch E.
Published: July 2002

We describe a promising thin-slab light-emitting diode (LED) design, which uses a highly efficient coherent external scattering of trapped light by a two-dimensional (2D) photonic crystal. The light generation region was an unpatterned heterostructure surrounded by the light extraction region, a thin film patterned as a 2D photonic crystal. A six-fold photoluminescence enhancement was observed compared to an unpatterned thin film LED. That corresponded to 70% external quantum efficiency.
Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off
Authors: Wong W. S. , Sands T. , Cheung N. W. , Kneissl M. , Bour D. P. , Mei P. , Romano L. T. , Johnson N. M.
Published: July 2002

Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.
Growth of InGaN/GaN multiple-quantum-well blue light-emitting diodes on silicon by metalorganic vapor phase epitaxy
Authors: Tran Chuong A. , Osinski A. , Karlicek R. F. , Berishev I.
Published: July 2002

We report the growth of InGaN/GaN multiple-quantum-well blue light-emitting diode (LED) structures on Si(111) using metalorganic vapor phase epitaxy. By using growth conditions optimized for sapphire substrates, a full width at half maximum (FWHM) (102) x-ray rocking curve of less than 600 arcsec and a room-temperature photoluminescence peak at 465 nm with a FWHM of 35 nm was obtained. Simple LEDs emitting bright electroluminescence between 450 and 480 nm with turn-on voltages at 5 V were demonstrated.
Formation of single-wall carbon nanotubes by laser ablation of fullerenes at low temperature
Authors: Zhang Y. , Iijima S.
Published: July 2002

Laser ablation products from fullerene materials have been studied by transmission electron microscopy and Raman spectroscopy. Using nickel and cobalt as a catalyst, single-wall carbon nanotubes were produced at an ambient temperature of 400 °C. The results were compared with those using graphite as starting materials. It is suggested that the formation of single-wall carbon nanotubes is controlled by both the availability of proper precursors and the activity of the metal catalyst.
Experimental and theoretical studies of phonons in hexagonal InN
Authors: Davydov V. Yu. , Emtsev V. V. , Goncharuk I. N. , Smirnov A. N. , Petrikov V. D. , Mamutin V. V. , Vekshin V. A. , Ivanov S. V. , Smirnov M. B. , Inushima T.
Published: July 2002

The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and (11̄02) sapphire substrates. All six Raman-active optical phonons were observed and assigned: E2(low) at 87 cm−1, E2(high) at 488 cm−1, A1(TO) at 447 cm−1, E1(TO) at 476 cm−1, A1(LO) at 586 cm−1, and E1(LO) at 593 cm−1. The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be ε⊥0/ε∥0=0.91. The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.
Epitaxial ferroelectric (Pb, La)(Zr, Ti)O3 thin films on stainless steel by excimer laser liftoff
Authors: Tsakalakos L. , Sands T.
Published: July 2002

Epitaxial (Pb0.90La0.07)(Zr0.5Ti0.5)O3 (PLZT) films, 1.4 μm in thickness, have been transferred from their (100) MgO growth substrates to stainless-steel receptor substrates. The films on their MgO growth substrates were first attached to 25 μm stainless-steel shims using a transient-liquid-phase Pd–In bonding process, followed by irradiation through the MgO substrate with a single 248 nm excimer laser pulse. The 38 ns pulse at 400 mJ/cm2 fluence melted a thin layer of PLZT at the interface, expelling the MgO and leaving the PLZT film bonded to the stainless-steel receptor substrate. X-ray diffraction analysis of the PLZT films showed that the crystallographic structure of the film is maintained during laser liftoff. Scanning electron microscopy observations of the original interface reveal the presence of a thin quenched-liquid surface layer, as predicted by a thermal model of the process. Electrical testing of the films before and after laser liftoff demonstrated that the ferroelectric properties are retained by the transferred film.
Analysis of the turn-off dynamics in polymer light-emitting diodes
Authors: Pinner D. J. , Friend R. H. , Tessler N.
Published: July 2002

We present experimental techniques to analyze the electroluminescence (EL) of polymer light-emitting diodes following the removal of an applied voltage pulse. We explain the fast modulation of the EL intensity at turn-off in terms of the sudden reduction of the Langevin recombination rate, and extract the time evolution the device’s internal electric field at the recombination zone during the application of a voltage pulse. The results are compared to, and found to be consistent with, those of simple numerical modeling. The subsequent long-lived EL tail is analyzed to give the time evolution of the carrier distributions at the recombination zone once the voltage pulse has been removed.
Optically detected magnetic-resonance mapping on the yellow luminescence in GaN
Authors: Koschnick F. K. , Michael K. , Spaeth J.-M. , Beaumont B. , Gibart P.
Published: July 2002

A mapping investigation was performed with photoluminescence-detected electron paramagnetic resonance (PL-EPR) via the yellow luminescence on nominally undoped, metal–organic vapor-phase epitaxy-grown GaN on sapphire. From the results, it is concluded that the PL-EPR signals observed in these samples represent different recombination channels which contribute to the yellow luminescence. Therefore, our results do not support the model of Glaser et al. [Phys. Rev. B 51, 13326 (1995)] for the mechanism of the yellow luminescence proposed recently.
Defect annihilation in AlN thin films by ultrahigh temperature processing
Authors: Fan Z. Y. , Rong G. , Newman N. , Smith David J.
Published: July 2002

Postgrowth thermal processing in the range of 1200–1400 °C is shown to improve markedly the quality of thin (∼200 nm) AlN films grown by molecular beam epitaxy on SiC substrates. Comparison of both on-axis (0002) and off-axis (101̄2) x-ray diffraction peaks documents this improvement. Cross-sectional transmission electron micrographs confirm the reduction in dislocations and grain boundaries, while plan-view micrographs demonstrate that threading defect densities can be reduced to ∼3×108/cm2 after annealing. The thermal treatment is particularly effective because of the unusually large temperature window between the onset of a near-zero reactant sticking coefficient at ∼1200 °C and AlN thermal decomposition at ∼1400 °C. The Al sticking coefficient and the AlN decomposition rate are also reported.
Epitaxial Pb(Mg1/3Nb2/3)O3 thin films synthesized by metal-organic chemical vapor deposition
Authors: Bai G. R. , Streiffer S. K. , Baumann P. K. , Auciello O. , Ghosh K. , Stemmer S. , Munkholm A. , Thompson Carol , Rao R. A. , Eom C. B.
Published: July 2002

Metal-organic chemical vapor deposition was used to prepare Pb(Mg1/3Nb2/3)O3 (PMN) thin films on (001) SrTiO3 and SrRuO3/SrTiO3 substrates, using solid Mg β-diketonate as the Mg precursor. Parameters including the precursor ratio in the vapor phase, growth temperature, growth rate, and reaction pressure in the reactor chamber were varied in order to determine suitable growth conditions for producing phase-pure, epitaxial PMN films. A cube-on-cube orientation relationship between the thin film and the SrTiO3 substrate was found, with a (001) rocking curve width of 0.1°, and in-plane rocking-curve width of 0.8°. The root-mean-square surface roughness of a 200-nm-thick film on SrTiO3 was 2 to 3 nm as measured by scanning probe microscopy. The zero-bias dielectric constant and loss measured at room temperature and 10 kHz for a 200-nm-thick film on SrRuO3/SrTiO3 were approximately 1100 and 2%, respectively. The remnant polarization for this film was 16 μC/cm2.
Observation of bright monochromatic x rays generated by relativistic electrons passing through a multilayer mirror
Authors: Kaplin V. V. , Uglov S. R. , Zabaev V. N. , Piestrup M. A. , Gary C. K. , Nasonov N. N. , Fuller M. K.
Published: July 2002

We have observed the emission of 15 keV x rays produced by 500 MeV electrons passing through a x-ray multilayer mirror. The mirror consisted of 300 pairs of W and B4C layers with layer spacing of 12.36 Å and supported by a 100 μm Si substrate. The x rays were emitted at the Bragg angle θγ=33.15 mrad with respect to the mirror surface and at the angle θD=66.3 mrad with respect to the electron-beam direction. The spatial distribution and the spectral angular dependence of the x rays were measured and shown to be larger than the parametric x rays emitted from the Si substrate. The value of the differential photon efficiency was estimated to be about 0.22 photons/electron/str.
IV–VI compound midinfrared high-reflectivity mirrors and vertical-cavity surface-emitting lasers grown by molecular-beam epitaxy
Authors: Shi Z. , Xu G. , McCann P. J. , Fang X. M. , Dai N. , Felix C. L. , Bewley W. W. , Vurgaftman I. , Meyer J. R.
Published: July 2002

Midinfrared broadband high-reflectivity Pb1−xSrxSe/BaF2 distributed Bragg reflectors and vertical-cavity surface-emitting lasers (VCSELs) with PbSe as the active material were grown by molecular-beam epitaxy. Because of an extremely high index contrast, mirrors with only three quarter-wave layer pairs had reflectivities exceeding 99%. For pulsed optical pumping, a lead salt VCSEL emitting at the cavity wavelength of 4.5–4.6 μm operated nearly to room temperature (289 K).
Use of scanned laser annealing to control the bamboo grain length of Cu interconnects
Authors: Hau-Riege C. S. , Thompson C. V.
Published: July 2002

Microstructural evolution induced by scanned laser annealing (SLA) of Cu interconnects was found to produce unique large-grained “bamboo” grain structures, with bamboo grain lengths up to ten times the linewidth. These bamboo grain lengths are shown to depend on the scan rate and laser power. By comparing results from experiments on different structures with grain growth simulations, the bamboo grain length induced by SLA is shown to be a strong function of the thermal profile, where steeper thermal profiles yield longer bamboo grains.
Local and global dynamics of atactic polypropylene melts by multiple field 13C nuclear magnetic resonance
Authors: Qiu XiaoHua , Moe N. E. , Ediger M. D. , Fetters Lewis J.
Published: July 2002

13C NMR T1, NOE, and T1ρ were measured for an atactic polypropylene (aPP) melt at 5, 25, 75, and 125 MHz 13C Larmor frequencies from 43 °C to 260 °C. The data were fit very well with a model for C–H vector reorientation composed of very fast librational relaxation, intermediate segmental relaxation, and a normal mode contribution described by Rouse dynamics modified for entanglement effects. The longest relaxation time calculated from this fit agrees reasonably well with the value obtained from viscoelastic measurements on similar systems. The amplitude of the normal mode contribution for aPP is much smaller than that found for polyethylene in a previous study. The temperature dependence of the segmental dynamics is very similar to that of the viscosity indicating that conformational transitions are the fundamental motions for flow for aPP. The width of the distribution of relaxation times of segmental dynamics remains unchanged at about 1.3 decades from Tg+45 to Tg+250 K.
Band structure nonlocal pseudopotential calculation of the III-nitride wurtzite phase materials system. Part I. Binary compounds GaN, AlN, and InN
Authors: Goano Michele , Bellotti Enrico , Ghillino Enrico , Ghione Giovanni , Brennan Kevin F.
Published: July 2002

This work presents nonlocal pseudopotential calculations based on realistic, effective atomic potentials of the wurtzite phase of GaN, InN, and AlN. A formulation formulation for the model effective atomic potentials has been introduced. For each of the constitutive atoms in these materials, the form of the effective potentials is optimized through an iterative scheme in which the band structures are recursively calculated and selected features are compared to experimental and/or ab initio results. The optimized forms of the effective atomic potentials are used to calculate the band structures of the binary compounds, GaN, InN, and AlN. The calculated band structures are in excellent overall agreement with the experimental/ab initio values, i.e., the energy gaps at high-symmetry points, valence-band ordering, and effective masses for electrons match to within 3%, with a few values within 5%. The values of the energy separation, effective masses, and nonparabolicity coefficients for several secondary valleys are tabulated as well in order to facilitate analytical Monte Carlo transport simulations.
A parametric limiting absorption problem with degeneration
Authors: Eidus D.
Published: July 2002

We are concerned with acoustic waves propagation in inhomogeneous media in the case where density ρ(x), being independent of r=|x| and depending on a parameter τ, can vanish on some domains of R3 for some isolated value τ=τ0. We investigate the behavior of solutions of wave equation as τ→τ0, prove the limiting absorption principle for τ=τ0, and apply the obtained results to a wave fronts propagation problem.
High electron mobility in AlGaN/GaN heterostructures grown on bulk GaN substrates
Authors: Frayssinet E. , Knap W. , Lorenzini P. , Grandjean N. , Massies J. , Skierbiszewski C. , Suski T. , Grzegory I. , Porowski S. , Simin G. , Hu X. , Khan M. Asif , Shur M. S. , Gaska R. , Maude D.
Published: July 2002

Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities.
Electron emission from ferroelectrics
Authors: Rosenman G. , Shur D. , Krasik Ya. E. , Dunaevsky A.
Published: July 2002

Electron emission from ferroelectrics (FEE) is an unconventional electron emission effect. Methods of FEE excitation are quite different compared to classic electron emission from solids. Two kinds of FEE have been observed, “weak” and “strong.” “Weak” electron emission (current density 10−12–10−7 A/cm2) occurs from polar surfaces of ferroelectric materials in the ferroelectric phase only. A source of the electric field for “weak” FEE excitation is an uncompensated charge, generated by a deviation of macroscopic spontaneous polarization from its equilibrium state under a pyroelectric effect, piezoelectric effect, or polarization switching. The FEE is a tunneling emission current which screens uncompensated polarization charges. It is shown that the FEE is an effective tool for direct domain imaging and studies of electronic properties of ferroelectrics. “Strong” FEE, which is 10–12 orders of magnitude higher than “weak” FEE, achieves 100 A/cm2 and is plasma-assisted electron emission. Two modes of the surface flashover plasma formation followed by strong electron emission have been studied. The plasma of ferroelectric origin has been observed only in the ferroelectric phase and it is induced by polarization switching or a field-enforced phase transition, such as antiferroelectric–ferroelectric or relaxor–ferroelectric. The second mode of plasma is conventional surface flashover which may be initiated by a high voltage application in any phase from any dielectric, including ferroelectrics. In this review paper we consider numerous experimental results, as well as mechanisms of both types of electron emission from ferroelectrics. The main stress is placed on the material aspect in order to clarify the influence of ferroelectricity (ferroelectric phase transitions, polarization switching, etc.) on electron emission. Another aspect which is broadly discussed is the potential applications of these unconventional FEE emitters in various devices for development of high density FEE cathodes for microwave devices, as well as FEE converters of IR irradiation into visible light, x-ray imaging, FEE flat panel displays, etc.
Transparent p-type semiconductor: LaCuOS layered oxysulfide
Authors: Ueda K. , Inoue S. , Hirose S. , Kawazoe H. , Hosono H.
Published: July 2002

La 1−x Sr x CuOS  (x=0, 0.05) thin films prepared by radio-frequency sputtering were found to have high optical transmission (⩾70%) at the visible and near-infrared wavelengths and an energy gap of about 3.1 eV. The dc electrical conductivities of x=0 and 0.05 thin films at room temperature were 1.2×10−2 and 2.6×10−1 S cm−1, respectively. The Seebeck coefficients of these samples were positive, indicating that p-type electrical conduction is dominant in these materials. A sharp photoluminescence peak, probably originating from an interband transition, was observed at the optical absorption edge. The present study demonstrates that LaCuOS is a promising transparent p-type semiconductor for optoelectronic applications. Moreover, our material design, based on chemical modulation of the valence band, was successfully extended to oxysulfide systems.
Kähler potential of moduli space of Calabi–Yau d-fold embedded in CPd+1
Authors: Sugiyama Katsuyuki
Published: July 2002

We study a Kähler potential K of a one parameter family of a Calabi–Yau d-fold embedded in CPd+1. By comparing results of the topological B-model and the data of the conformal field theory (CFT) calculation at a Gepner point, the K is determined unambiguously. It has a moduli parameter ψ that describes a deformation of the CFT by a marginal operator. Also, a metric, a curvature and Hermitian two-point functions in the neighborhood of the Gepner point are analyzed. We use a recipe of the tt* fusion and develop a method to determine the K from the point of view of the topological sigma model. It is not restricted to this specific family of the d-fold and can be applied to other Calabi–Yau cases.
A homogeneous space–time model with singularities
Authors: Bromberg Shirley , Medina Alberto
Published: July 2002

We show the existence of a left invariant pseudo-Riemannian metric on the Oscillator Group of dimension four, such that this group becomes a space–time with singularities in the sense of Hawking and Penrose. As an application we exhibit new incomplete, nonhomeomorphic compact Lorentzian manifolds.
Kinetics of ordering for a ternary alloy in the pair approximation
Authors: Ni Jun , Gu Binglin , Ashino Toshihiro , Iwata Shuichi
Published: July 2002

The kinetics of ordering in the bcc ternary alloy is investigated by means of the micromaster equation method in the pair approximation. The time evolution of the long-range order and short-range order parameters are calculated. A transient ordered state results in distinct features, such as stepwise variation, overshooting effects, and peaks in the evolution to different equilibrium phases. It is found that the development of short-range order in the ordering stage of the transient ordered state is different from that in the equilibrium phase. Short-range order relaxes gradually in the ordering stage of the transient ordered state in contrast to the fast change of short-range order in the ordering stage of the equilibrium phase.
Kinetic oscillations in the catalytic CO oxidation on Pt(100) with adsorbed impurities
Authors: Chávez Francisco , Vicente Luis , Perera Aurélien
Published: July 2002

The effect of inert sites (random impurities) in the global oscillations in the oxidation of CO on Pt(100) is studied analytically via a stability analysis of the underlying mean field model. The analysis shows that the introduction of a fraction θd of inert sites can reduce considerably the extent of the oscillatory region on the bifurcation diagram. The study of a mean field model explains some of the basic features that arise upon increasing the density of impurities on the catalytic surface, and it allows the analytical calculation of the bifurcation diagram. The analytical solution of the model reproduces well cellular automata simulations. In particular, we show the existence of a critical concentration of inert sites at which the oscillatory region vanishes and oscillations are no longer possible, due to the inability of the lattice to reach the minimum local concentration of CO required to trigger the surface reconstruction mechanism. We have evaluated this maximum defect concentration in the mean field approximation to be exactly θd*=1−θCO+, where θCO+ is the minimum CO concentration required to trigger the (hex) to (1×1) Pt substrate transition. For the set of parameters chosen, we find here θd*=0.6, which is in perfect agreement with our earlier simulation findings.
Effect of composition and temperature on field-induced properties in the lead strontium zirconate titanate system
Authors: Yu Yongjian , Singh Raj N.
Published: July 2002

The electric field-induced antiferroelectric (AFE) to ferroelectric (FE) phase transition in lead strontium zirconate titanate (PSZT) ceramics was studied by means of dielectric, polarization, and strain hysteresis measurements. PSZT compositions with varying strontium and Zr/Ti ratio, located in the ferroelectric, antiferroelectric phase regions, and near the AFE/FE phase boundary were prepared. Dielectric properties were measured as a function of temperature for different compositions. The electric field required for AFE–FE phase transition and hysteresis were affected by the temperature and composition. The entropy change during phase transition and the field-induced strain were also measured and discussed.
Giant and time-dependent magnetocaloric effect in high-spin molecular magnets
Authors: Torres F. , Hernández J. M. , Bohigas X. , Tejada J.
Published: July 2002

We have measured and calculated the magnetocaloric effect in macroscopic samples of oriented high-spin molecular clusters like Mn12 and Fe8 as a function of the temperature and both the intensity and the sweeping rate of the applied magnetic field. We have observed a high magnetic entropy variation around the blocking temperature of the magnetic moment of molecules and calculated the shift of the entropy variation and cooling temperature, with the sweeping rate of the magnetic field.
Nanoscale selective-area epitaxial growth of Si using an ultrathin SiO2/Si3Ni4 mask patterned by an atomic force microscope
Authors: Yasuda T. , Yamasaki S. , Gwo S.
Published: July 2002

We report selective epitaxial growth of Si using an ultrathin bilayer mask. The key feature of this process is direct writing of nanoscale patterns by means of local anodic oxidation of a Si3N4 layer using an atomic force microscope operated in air. Windows for selective growth are defined by wet etching of the locally oxidized regions. High growth selectivity upon chemical vapor deposition of Si is accomplished by employing the bilayer mask structure which is formed by oxidizing the Si3N4 surface and then selectively desorbing SiO2 in the windows. High-quality homoepitaxial growth is verified by transmission electron microscopy. We also report a simple plasma-treatment technique which solves the problem of retarded SiO2 desorption in the nanoscale windows.
Sequence design in lattice models by graph theoretical methods
Authors: Sanjeev B. S. , Patra S. M. , Vishveshwara S.
Published: July 2002

A general strategy has been developed based on graph theoretical methods, for finding amino acid sequences that take up a desired conformation as the native state. This problem of inverse design has been addressed by assigning topological indices for the monomer sites (vertices) of the polymer on a 3×3×3 cubic lattice. This is a simple design strategy, which takes into account only the topology of the target protein and identifies the best sequence for a given composition. The procedure allows the design of a good sequence for a target native state by assigning weights for the vertices on a lattice site in a given conformation. It is seen across a variety of conformations that the predicted sequences perform well both in sequence and in conformation space, in identifying the target conformation as native state for a fixed composition of amino acids. Although the method is tested in the framework of the HP model [K. F. Lau and K. A. Dill, Macromolecules 22, 3986 (1989)] it can be used in any context if proper potential functions are available, since the procedure derives unique weights for all the sites (vertices, nodes) of the polymer chain of a chosen conformation (graph).
Effects of Cs deposition on the field-emission properties of single-walled carbon-nanotube bundles
Authors: Wadhawan A. , Stallcup R. E. , Perez J. M.
Published: July 2002

We report the effects of Cs deposition on the field-emission (FE) properties of single-walled carbon-nanotube bundles. We observe that Cs deposition decreases the turn-on field for FE by a factor of 2.1–2.8 and increases the FE current by six orders of magnitude. After Cs deposition, the FE current versus voltage (I–V) curves show non-Fowler–Nordheim behavior at large currents, consistent with tunneling from adsorbate states. At lower currents, the ratio of the slope of the FE I–V curves before and after Cs deposition is approximately 2.1. Exposure to N2 does not decrease the FE current, while exposure to O2 decreases the FE current.
Use of a multiwavelength pyrometer in several elevated temperature aerospace applications
Authors: Ng Daniel , Fralick Gustave
Published: July 2002

A multiwavelength pyrometer was developed for applications unique to aerospace environments. It was shown to be a useful and versatile technique for measuring temperature, even when the emissivity is unknown. It has also been used to measure the surface temperatures of ceramic zircornia thermal barrier coatings and alumina. The close agreement between pyrometer and thin film thermocouple temperatures provided an independent check. Other applications of the multiwavelength pyrometer are simultaneous surface and bulk temperature measurements of a transparent material, and combustion gas temperature measurement using a special probe interfaced to the multiwavelength pyrometer via an optical fiber. The multiwavelength pyrometer determined temperature by transforming the radiation spectrum in a broad wavelength region to produce a straight line (in a certain spectral region), whose intercept in the vertical axis gives the temperature. Implicit in a two-color pyrometer is the assumption of wavelength independent emissivity. Though the two data points of a two-color pyrometer similarly processed would result immediately in a similar straight line to give the unknown temperature, the two-color pyrometer lacks the greater data redundancy of the multiwavelength pyrometer, which enables it to do so with improved accuracy. It also confirms that emissivity is indeed wavelength independent, as evidenced by a multitude of the data lying on a simple straight line. The multiwavelength pyrometer was also used to study the optical transmission properties of a nanostructured material from which a quadratic exponential functional frequency dependence of its spectral transmission was determined. Finally, by operating the multiwavelength pyrometer in a very wide field of view mode, the surface temperature distribution of a large hot surface was obtained through measurement of just a single radiation spectrum.
Computer-generated holographic optical tweezer arrays
Authors: Dufresne Eric R. , Spalding Gabriel C. , Dearing Matthew T. , Sheets Steven A. , Grier David G.
Published: July 2002

Holographic techniques significantly extend the capabilities of laser tweezing, making possible extended trapping patterns for manipulating large numbers of particles and volumes of soft matter. We describe practical methods for creating arbitrary configurations of optical tweezers using computer-generated diffractive optical elements. While the discussion focuses on ways to create planar arrays of identical tweezers, the approach can be generalized to three-dimensional arrangements of heterogeneous tweezers and extended trapping patterns.
Optically induced pulse delay in a solid-state Raman amplifier
Authors: Lee Kijoon , Lawandy Nabil M.
Published: July 2002

The pump-induced group velocity reduction of subnanosecond pulses is calculated and measured in a Ba(NO3)2 solid-state Raman amplifier. 1.197 μm probe pulses with 90 ps duration were generated using a Raman-shifted mode-locked and Q-switched Nd:YAG laser, and propagated through a Ba(NO3)2 crystal synchronously pumped by 7-ns-long 1.06 μm pulses. The time delay of the pulse peak was measured with varying pump intensity and was compared with theoretical calculation up to the point where amplified spontaneous Raman emission becomes dominant. The maximum time delay was found to be 105 ps.
Near-unity below-band-gap absorption by microstructured silicon
Authors: Wu C. , Crouch C. H. , Zhao L. , Carey J. E. , Younkin R. , Levinson J. A. , Mazur E. , Farrell R. M. , Gothoskar P. , Karger A.
Published: July 2002

We increased the absorptance of light by silicon to approximately 90% from the near ultraviolet (0.25 μm) to the near infrared (2.5 μm) by surface microstructuring using laser-chemical etching. The remarkable absorptance most likely comes from a high density of impurities and structural defects in the silicon lattice, enhanced by surface texturing. Microstructured avalanche photodiodes show significant enhancement of below-band-gap photocurrent generation at 1.06 and 1.31 μm, indicating promise for use in infrared photodetectors.
Temporally stable coherent states for infinite well and Pöschl–Teller potentials
Authors: Antoine J.-P. , Gazeau J.-P. , Monceau P. , Klauder J. R. , Penson K. A.
Published: July 2002

This article is a direct illustration of a construction of coherent states which has been recently proposed by two of us (JPG and JK). We have chosen the example of a particle trapped in an infinite square-well and also in Pöschl–Teller potentials of the trigonometric type. In the construction of the corresponding coherent states, we take advantage of the simplicity of the solutions, which ultimately stems from the fact they share a common SU(1,1) symmetry à la Barut-Girardello. Many properties of these states are then studied, both from mathematical and from physical points of view.
Proton transport in polarizable water
Authors: Walbran S. , Kornyshev A. A.
Published: July 2002

Proton mobility in water determines the conductive properties of water-based proton conductors. We address the problem of proton mobility in pure water using a new, simple, Newtonian molecular dynamics water model which is applicable to proton-rich environments (e.g., polymer electrolyte membranes). This model has degrees of freedom that are “inertial” and “inertialess” relative to the proton. The solvated proton is treated using a local empirical valence bond Hamiltonian, which allows for the efficient simulation of full charge, energy-conserving dynamics in single and multiple-proton systems. The solvated proton displays the Grotthus-type proton transfer mechanism, giving significantly enhanced transport in comparison with the classical diffusion of an H3O+ ion. The model yields an activation energy of 0.11 eV, in excellent agreement with experiment. The results are consistent with the observation that nonpolarizable water models, conditioned to reproduce correct values of the static dielectric constant, are predestined to give too large activation energies of proton mobility due to the overweighted spectrum of the slower nuclear modes.
Lattice parameter in GaNAs epilayers on GaAs: Deviation from Vegard’s law
Authors: Li Wei , Pessa Markus , Likonen Jari
Published: July 2002

The N content and lattice parameter of GaNxAs1−xepilayers on GaAs (0<x<0.03) were determined by secondary ion mass spectroscopy and x-ray diffraction measurements, respectively. A significant deviation of the lattice parameter variation in GaNxAs1−x from Vegard’s law between GaAs and cubic GaN was observed, which leads to overestimation of the nitrogen content by up to 30% for x⩽2.5%. The physical origin of this negative deviation is discussed.
Matrix theory compactification on noncommutative T4/Z2
Authors: Kim Eunsang , Kim Hoil , Lee Chang-Yeong
Published: July 2002

In this article, we construct gauge bundles on a noncommutative toroidal orbifold Tθ4/Z2. First, we explicitly construct a bundle with constant curvature connections on a noncommutative Tθ4 following Rieffel’s method. Then, applying the appropriate quotient conditions for its Z2 orbifold, we find a Connes–Douglas–Schwarz type solution of matrix theory compactified on Tθ4/Z2. When we consider two copies of a bundle on Tθ4 invariant under the Z2 action, the resulting Higgs branch moduli space of equivariant constant curvature connections becomes an ordinary toroidal orbifold T4/Z2.
Switchable mirrors based on nickel–magnesium films
Authors: Richardson T. J. , Slack J. L. , Armitage R. D. , Kostecki R. , Farangis B. , Rubin M. D.
Published: July 2002

An electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin, magnesium-rich Ni–Mg films prepared on glass substrates by co-sputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on cathodic polarization in alkaline electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extraction, and to protect the metal surface against oxidation.
Anomalous colossal magnetoresistance effect in the Zn-doped Fe1−xZnxCr2S4 polycrystals with the spinel structure
Authors: Wang Shouguo , Sun Yuping , Song Wenhai , Ye Shuangli , Dai Jianming , Li Kebin , Fang Jun , Cao Xiaowen , Zhang Yuheng
Published: July 2002

The electrical resistivity, magnetization, and magnetoresistance effect have been studied on a series of Zn-doped polycrystalline samples Fe1−xZnxCr2S4 (0⩽x⩽1) with a spinel structure over a wide range of composition and temperature. It is found that the magnetism and magnetoresistance ratio are anomalous, which do not change monotonically with substituting nonmagnetic Zn2+ ions in place of Fe2+ ions. Considering the effect of the applied magnetic field and substitution of Fe2+ by Zn2+ ions on the magnetism and small polarons transport property, the anomalous magnetism and magnetoresistance effect can be explained.
Infrared properties of room-temperature-deposited ZrO2
Authors: Koltunski L. , Devine R. A. B.
Published: July 2002

ZrO 2 films have been deposited at room temperature using Zr(OC4H9)4 and O2 source gases in a low-pressure electron-cyclotron-resonance-excited plasma reactor. The deposited films were, in general, mixed (amorphous and polycrystalline; monoclinic or tetragonal), the crystallinity depending upon the substrate polarization during deposition and the postdeposition annealing. Infrared vibrational modes of the films have been characterized. In as-deposited films, the dielectric constants were ∼12–18, dependent upon sample bias during deposition, and the refractive indices were ∼1.85–1.92.
Scaling on hysteresis dispersion in ferroelectric systems
Authors: Liu J.-M. , Chan H. L. W. , Choy C. L. , Zhu Y. Y. , Zhu S. N. , Liu Z. G. , Ming N. B.
Published: July 2002

The hysteresis area as a function of frequency of the time-varying external electric field—hysteresis dispersion—for ferroelectric Pb(Zr0.52Ti0.48)O3 is measured, and the Monte-Carlo simulation on the hysteresis dispersion for a model ferroelectric lattice is performed too. We demonstrate the scaling behavior of the single-peaked hysteresis dispersion for the two ferroelectric systems, predicting a unique effective characteristic time for the domain reversal. This characteristic time shows an inversely linear dependence on the field amplitude as long as the amplitude is high enough that the reversible domain rotation response is negligible.
Autophobic dewetting of homopolymer on a brush and entropic attraction between opposing brushes in a homopolymer matrix
Authors: Matsen M. W. , Gardiner J. M.
Published: July 2002

The wetting behavior of homopolymer on a chemically identical polymer brush is mathematically equivalent to the interaction between a pair of opposing brushes in a matrix of parent homopolymer. We examine both systems using self-consistent field theory (SCFT) with a new highly efficient and accurate algorithm. Our calculations provide compelling evidence that the global minimum in the free energy curve remains at a finite film thickness, implying that the brush/homopolymer interfacial tension, γb/h, is always positive favoring dewetting, or equivalently that an attraction always exists between opposing brushes. Nevertheless, we identify a region at low homopolymer molecular weights where γb/h is negligible, in which case complete wetting is highly metastable and the attraction between opposing brushes is extraordinarily weak. Furthermore, we demonstrate that SCFT is in good quantitative agreement with experiment. In contrast, we find that earlier predictions based on strong-stretching theory are terribly inaccurate.
The complete Raman spectrum of nanometric SnO2 particles
Authors: Diéguez A. , Romano-Rodrı́guez A. , Vilà A. , Morante J. R.
Published: July 2002

The complete Raman spectrum of SnO2 nanoparticles in presented and analyzed. In addition to the “classical” modes observed in the rutile structure, two other regions shown Raman activity for nanoparticles. The Raman bands in the low-frequency region are attributed to acoustic modes associated with the vibration of the individual nanoparticle as a whole. The high-frequency region is activated by surface disorder. A detailed analysis of these regions and the changes in the normal modes of SnO2 are presented as a function nanoparticle size.