bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral narrowing in a rhodamine-doped layered TiO2/surfactant thin film
Authors: Stathatos Elias , Lianos Panagiotis , Couris Stelios
Published: July 2002

Spectral narrowing has been observed in the emission of a rhodamine-6G-doped layered TiO2/surfactant thin film by both visible and near-ultraviolet laser excitation. Narrowing threshold was extensively decreased by combining waveguide and scatter gain and by exploiting the self-organizing capacity of the surfactant molecules and the unusual optical matching between the TiO2 nanoparticle layer and the organic surfactant layer.
Alternating current loss in coplanar arrays of superconducting strips with bidirectional currents
Authors: Mawatari Yasunori , Yamasaki Hirofumi
Published: July 2002

Magnetic field, current density, and hysteretic alternating current loss power P in coplanar arrays of superconducting strip lines are analyzed on the bases of the critical state model. For a simplified model of a film-type fault current limiter, we consider a strip array in which multiple strip lines are periodically arranged in one plane and are carrying bidirectional currents. We investigate the effect of spacing between the strip lines on P. The P of the strip array is higher than that of a single strip, because the magnetic field in a strip is enhanced by currents in neighboring strips.
X-ray standing waves effects for a multilayer mirror modulated by surface acoustic waves
Authors: Roshchupkin D. V. , Tucoulou R. , Brunel M.
Published: July 2002

The x-ray standing wave technique has been used to characterize a multilayer mirror modulated by surface acoustic waves. Upon sinusoidal modulation of a multilayer by surface acoustic waves, fields of x-ray standing waves are found to form around the Bragg conditions and at the angular positions of the rocking curve diffraction satellites. It is shown that the angular position of the diffraction peaks in the rocking curve and the amplitude of x-ray standing waves are determined by the wavelength and the amplitude of the surface acoustic waves, respectively.
Vibrational properties of tetrahedral amorphous carbon from first principles
Authors: Mauri Francesco , Dal Corso Andrea
Published: July 2002

We study the vibrational properties of hydrogenated tetrahedral amorphous carbon from first principles. Our results reproduce the C density of states (DOS) measured by electron energy loss. We decompose the theoretical DOS in terms of vibrational modes of microscopic units. These partial DOS have large overlaps in frequency. Only the region between 1350 and 1600 cm−1 is covered by a single type of stretching (sp2–sp2), whose DOS reproduces the experimental visible Raman spectra. We find a linear correlation between CH stretch frequency and CH bond length, and we show that the spreading of CH distances due to the constraints imposed by the C network is an important source of broadening of the CH stretching spectrum.
Optically-induced growth of fiber patterns into a photopolymerizable resin
Authors: Shoji Satoru , Kawata Satoshi
Published: July 2002

We present evidence of optically-induced growth of fiber patterns into a photopolymerizable resin. Optical growth of a single or multiple fibers is achieved by focusing an ultraviolet (UV)-laser light into the photopolymerizable resin used. The fiber growth is due to an effect in which photopolymerization of the resin upon light irradiation produces an increase of the resin refractive index, the change of which, in turn, confines the light propagation into waveguide-type fiber structures. We have also observed that two optically grown independent fibers can merge to form a single fiber under specific conditions. We have studied the dependence of this optical growth of fiber structures phenomena on all the experimental parameters, including the numerical aperture (NA) of the lens used to focus the light, the light power, and the exposure time.
Diffraction from oxide confinement apertures in vertical-cavity lasers
Authors: Roos P. A. , Carlsten J. L. , Kilper D. C. , Lear K. L.
Published: July 2002

Direct measurement of scattered fields from oxide confinement apertures in vertical-cavity lasers is presented. Diffraction fringes associated with each transverse lasing mode are detected in the far field from devices with varying oxide aperture dimensions and with quantum efficiencies as high as 48%. The diffracted pattern symmetries match the rectangular symmetry of the oxide apertures present in the devices and fringe locations are compared to Fraunhofer theory. The fraction of power diffracted from the lasing mode remains roughly constant as a function of relative pump rate, but is shown to depend on both transverse mode order and oxide aperture size.
Measurement of mechanical resonance and losses in nanometer scale silicon wires
Authors: Carr Dustin W. , Evoy S. , Sekaric L. , Craighead H. G. , Parpia J. M.
Published: July 2002

We present data on nanofabricated suspended silicon wires driven at resonance. The wires are electrostatically driven and detected optically. We have observed wires with widths as small as 45 nm and resonant frequencies as high as 380 MHz. We see a strong dependence of the resonant quality factor on the surface to volume ratio.
Realization of an axially aligned Au-ion source of atomic size
Authors: Purcell S. T. , Binh Vu Thien
Published: July 2002

Nanotips are high-brightness electron sources with a principal characteristic source-size of one atom. We explore here nanotips as a type of atom-size source of Au ions. Conditions were found for a reproducible fabrication of a single Au nanotip on the principal axis of a W-base tip that emitted a beam of Au exclusively from its apex. The measured source characteristics were (i) the atomic-size source; (ii) currents: 10−14–1012 A; (iii) stability: 5 h; (iv) beam opening angle: 3.2°; and (v) energy dispersion <1 eV. First focusing experiments showed crossover and that the entire emitted beam was focused.
Excess noise characteristics of amorphous silicon staircase photodiode films
Authors: Sawada Kazuaki , Akiyama Masahiro , Ishida Makoto
Published: July 2002

An a-Si:H (hydrogenated amorphous silicon)/a-SiC:H (hydrogenated amorphous silicon carbide) staircase photodiode with linear graded-gap multiplication regions is useful for photoconversion. Noise characteristics of the a-Si:H/a-SiC:H staircase photodiode films with one and three linear graded-gap region were studied. The measured shot-noise characteristics of the photodiode film, which was operated in a photocurrent multiplication region, corresponded to the ideal value which was calculated as free-excess noise. It was found that in the graded-gap region, the a-Si:H/a-SiC:H staircase photodiode film does not generate extra noise during the photocurrent multiplication. These results indicate that the staircase photodiode films are a promising device for the high-sensitive imaging sensor.
Origin of yield problems of single electron devices based on evaporated granular films
Authors: Müller H.-O. , Boero M. , Vincent J. K. , Inkson J. C. , Mizuta H. , Mulheran P. A.
Published: July 2002

An evaporated nanometer scale granular film provides a simple system for studying Coulomb blockade effects. This technique has often been used during the last few decades. However with respect to potential devices, specific problems continue to obstruct broader application. It is virtually impossible to observe Coulomb blockade in one–dimensional structures, and even for wide two–dimensional systems the yield is frustratingly low. We study these problems using a comprehensive theoretical framework that enables us to model both the growth aspects, and the electrical characteristics. In particular, we study how the morphology of the islands influences their electrical properties. Explanations for the observed behavior are put forward.
Amplification of Raman scattering by stimulated emission in a stretched free-standing poly(p-phenylenevinylene) film
Authors: Shin Seungwoo , Shin Jeongmi , Kim Hongki , Ro Jung Hoon , Choi Heayoung , Lee Kwanghee , Cha Myoungsik
Published: July 2002

Photoexcitation of a stretched poly(p-phenylenevinylene) thick film results in a light emission with a very narrow linewidth when the excitation intensity exceeds a threshold. By measuring the excitation and the Raman spectra, we found that the narrow emission is an amplification of Raman scattering during waveguide propagation.
High-quality coalescence of laterally overgrown GaN stripes on GaN/sapphire seed layers
Authors: Fini P. , Zhao L. , Moran B. , Hansen M. , Marchand H. , Ibbetson J. P. , DenBaars S. P. , Mishra U. K. , Speck J. S.
Published: July 2002

We have characterized GaN stripes grown by lateral epitaxial overgrowth on large-area (2 in.) SiO2/GaN/Al2O3 wafers by low-pressure metalorganic chemical vapor deposition before and after coalescence. Using scanning electron microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscope (AFM), it is shown that by first obtaining “wings” (laterally overgrown material) with low tilt relative to the “seed” (underlying) GaN, very few extended defects are formed when wings from neighboring stripes coalesce. After wings with a tilt of ∼0.1° are coalesced and an additional ∼10 μm of GaN is grown, it is found with XRD that peak splitting due to tilt is no longer detectable. TEM and AFM results show that few dislocations (with a linear density <4×103 cm−1) are formed at coalescence fronts.
Interaction of Cu and Cu3Ge thin films with Si1−xGex alloys
Authors: Aboelfotoh M. O. , Borek M. A. , Narayan J.
Published: July 2002

The interaction of Cu and Cu3Ge thin films with Si1−xGex (x=0.5) alloy layers epitaxially grown on Si(100) has been studied in the temperature range of 250–400 °C. In this temperature range, Cu reacts with the alloy to form a Cu3(Si1−xGex) ternary phase with an ordered body-centered-cubic crystal structure. The Cu3(Si1−xGex) phase exhibits high-room-temperature (∼150 μΩ cm) and nonmetallic resistivity. However, this ternary phase is not observed and the diffusion of Cu into the alloy is suppressed when Cu is replaced by low resistivity (typically less than 10 μΩ cm at room temperature) ε1-Cu3Ge phase. In contrast to the results reported for films of ε1-Cu3Ge formed on Si(100), the outdiffusion of Si into the ε1-Cu3Ge films is found to be suppressed when the films are formed on Si0.5Ge0.5 layers, indicating the increased stability of ε1-Cu3Ge on Si1−xGex alloys compared to pure silicon.
Mg/GaAs(001): A highly anisotropic reaction morphology
Authors: Robey S. W.
Published: July 2002

The reaction of Mg thin films with a Ga-stabilized 4×2 GaAs(001) surface was observed to produce elongated, wire-like structures with widths on the order of 50 nm and lengths up to 1000 nm. These structures are composed of an epitaxial, cubic phase and develop asymmetrically, with the elongated direction along GaAs[110]. In situ electron diffraction, Auger spectroscopy, and ex situ atomic force microscopy were used to investigate this interesting reaction morphology. A potential formation mechanism is proposed in which migration of Ga or Ga-rich “droplets” across the GaAs surface leads to the formation of a ridge of Mg3As2 that is aligned preferentially along [110] due to strain and/or chemical anisotropy.
Evanescent wave Doppler velocimetry for a wall’s near field
Authors: Yamada Jun
Published: July 2002

This study proposes a Doppler velocimetry utilizing evanescent waves that are induced by a laser beam. This velocimetry allows us to measure the flow velocity within the near field less than 1 μm from a dielectric surface. When evanescent waves propagate in opposite directions on a dielectric surface, the waves interfere with each other and produce a standing wave within the near field. When a tracer particle goes through the standing wave, the particle scatters the wave. The flow velocity within the near field can be determined by measuring the blinking frequency similar to conventional laser Doppler velocimetry. The validity of this velocimetry is empirically examined by applying it to a slip velocity measurement of a solid on a dielectric surface.
Ideal performance of cascade and noncascade intersubband and interband long-wavelength semiconductor lasers
Authors: Flatté Michael E. , Olesberg J. T. , Grein C. H.
Published: July 2002

The ideal performance of cascade and noncascade intersubband and interband laser active regions is directly compared in a cavity-insensitive way. For devices not limited by series resistance or series voltage (such as can in principle be accomplished by cascading) the relevant figure of merit is the net material gain per unit volumetric power dissipation density in the active region. This figure of merit is evaluated at 77 and 300 K for a variety of structures relying on interband and intersubband transitions, each of which may constitute the active region of a cascade or noncascade device. A design for an 11 μm laser active region is proposed whose ideal performance exceeds that of current intersubband lasers.
Double-chirped semiconductor mirror for dispersion compensation in femtosecond lasers
Authors: Paschotta R. , Spühler G. J. , Sutter D. H. , Matuschek N. , Keller U. , Moser M. , Hövel R. , Scheuer V. , Angelow G. , Tschudi T.
Published: July 2002

A double-chirped mirror structure with broadband negative dispersion was realized with semiconductor technology. The necessary high precision of the fabrication was achieved by using special calibration structures. A single reflection on the obtained low-loss mirror produces sufficient negative dispersion for dispersion compensation in a femtosecond laser cavity. In this way we demonstrate 200 fs pulses from a compact Nd:glass laser without any additional dispersion compensation.
Oxidized AlxGa1−xAs heterostructure planar waveguides
Authors: Luo Y. , Hall D. C. , Kou L. , Steingart L. , Jackson J. H. , Blum O. , Hou H.
Published: July 2002

Waveguiding by total internal reflection is demonstrated within AlxGa1−xAs semiconductor heterostructures which have been fully oxidized in water vapor at ∼490 °C. Refractive index, mode propagation constant, propagation loss (⩽3 cm−1) at λ0=1.3 and 1.55 μm, secondary ion mass spectrometry depth profile, and Fourier transform infrared transmission spectra measurements are presented to characterize a multimode single-heterostructure oxide waveguide. An index contrast of Δn=0.06 is observed between oxidized x=0.4 and x=0.8 AlxGa1−xAs oxide layers. Absorption loss at 1.55 μm is observed due to OH groups. Near-field images are presented showing waveguiding in a single-mode oxide double heterostructure.
Mechanochemical reaction and formation of an amorphous nitride phase during ball milling of Si in NH3
Authors: Li Z. L. , Williams J. S. , Llewellyn D. J. , Giersig M.
Published: July 2002

The formation of an amorphous SixNyH phase is obtained by a solid–gas mechanochemical reaction induced by ball milling of Si in NH3 gas. We suggest that mechanical damage together with absorbed N and H play an important role in the formation and stability of the amorphous SixNyH phase during milling. Transmission electron microscopy, x-ray diffraction, thermal analysis, and composition analysis have been used to characterize the samples after ball milling and subsequent annealing. Crystallization of the amorphous phase to α-Si3N4 is observed at temperatures above 800 °C.
Optical properties of CdSe nanocrystals in a polymer matrix
Authors: Firth Andrea V. , Cole-Hamilton D. J. , Allen J. W.
Published: July 2002

Cadmium selenide nanocrystals have been made in a transparent polymer matrix by synthesis at room temperature. Particle size is controlled by reaction time or reagent concentration. Larger nanocrystals (≳3 nm) show near-band-edge photoluminescence. Smaller ones have a broad emission extending across the visible region. Atmospheric moisture causes the emission intensity to decrease but subsequent exposure to light reverses this and gives a material which is no longer quenched by moisture. The broad emission and stability against atmospheric moisture leads to possible device applications of a “white” phosphor.
Direct gap in ordered silicon carbon alloys
Authors: Krishnamurthy Srinivasan , Berding M. A. , Sher A. , van Schilfgaarde Mark , Chen A.-B.
Published: July 2002

We show that alloying silicon with a few percent of carbon can render the band gap direct with strong optical absorption, provided the carbon atoms are ordered. The addition of carbon introduces a significant s character into the conduction band minimum, resulting in a large dipole matrix element. First-principles calculations of the optical absorption in ordered in CxSi1−x alloys for x=1/54 and 1/32 show a near band edge absorption coefficient about half that of GaAs.
Room-temperature coercivity enhancement in mechanically alloyed antiferromagnetic-ferromagnetic powders
Authors: Sort J. , Nogués J. , Amils X. , Suriñach S. , Muñoz J. S. , Baró M. D.
Published: July 2002

The coercivity, HC, and squareness of Co powders have been enhanced at room temperature by mechanically alloying them with antiferromagnetic powders with Néel temperature, TN, above room temperature. The enhancement is maximum after field annealing above TN. The existence of loop shifts and the dependence of HC on the annealing and measuring temperatures indicate that exchange bias effects are responsible for this behavior.
Hot electron spectroscopy of carrier relaxation within indirect AlAs tunnel barriers
Authors: Sivaraya S. , Bending S. J. , Henini M.
Published: July 2002

Hot electron spectroscopy has been performed on electrons which have tunneled through indirect AlAs potential barriers in a tunneling hot electron transfer amplifier. We find that only about 1% of the electrons are collected close to their injection energy in stark contrast to an otherwise identical structure with an Al0.5Ga0.5As barrier where this fraction was 30%. Measurements under hydrostatic pressure show clear evidence for the real-space transfer of electrons from the emitter electrode into an X-point barrier subband adjacent to the emitter Fermi band. A detailed analysis of hot electron spectra reveals that the transferred electrons undergo strong inelastic scattering within the AlAs barrier and relax down through the ladder of X-point subbands there before being reemitted into the base layer.
Photovoltaic effect of cubic GaN/GaAs(100)
Authors: Zhao D. G. , Jiang D. S. , Yang Hui , Zheng L. X. , Xu D. P. , Li J. B. , Wang Q. M.
Published: July 2002

We have studied the photovoltaic effect in cubic GaN on GaAs at room temperature. The photovoltaic spectra of cubic GaN epitaxial film were concealed by the photovoltaic effect from the GaAs substrate unless additional illumination of a 632.8 nm He–Ne laser beam was used to remove the interference of the GaAs absorption in the measurement. On the basis of the near-band-edge photovoltaic spectra of cubic GaN, we obtained the minority carrier diffusion lengths of about 0.32 and 0.14 μm for two undoped n-type cubic GaN samples with background concentrations of 1014 and 1018 cm−3, respectively.
Energy relaxation by photoexcited carriers in the InAs/GaAs quantum-dot system: Bolometric detection of strong acoustic-phonon emission
Authors: Hawker P. , Kent A. J. , Henini M.
Published: July 2002

We have used bolometric detection to observe directly the phonons emitted by photoexcited carriers in the InAs/GaAs self-organized quantum-dot system. We find that about 74% of the energy lost by carriers in the InAs dots and wetting layer is via emission of low-frequency acoustic phonons and argue that this is facilitated by Auger scattering.
Impurity states are the origin of yellow-band emission in GaN structures produced by epitaxial lateral overgrowth
Authors: Li X. , Bohn P. W. , Coleman J. J.
Published: July 2002

GaN grown by selective area epitaxy and subsequent lateral overgrowth exhibits sharply peaked anisotropic structures in the form of hexagonal pyramids and ridges. Spatially resolved optical emission from these structures, using both cathodoluminescence and collection-mode near-field scanning optical microscopy, of radiation centered near 550 nm, the so-called yellow band, indicates that the emission arises predominantly from the apex regions of the pyramids and ridges. In contrast, transmission electron microscopy shows that the apex region is nearly dislocation free and that dislocations cluster at the vertical growth core region. The spatial separation of the dislocations and optical emission indicates that the yellow-band emission has no direct relationship to dislocations. The observation of yellow-band emission strongly localized in the apical regions of both types of structures and the tendency of impurity species to concentrate in these areas argues that it arises instead from impurity states, the most likely candidate of which is a complex formed between a gallium vacancy, VGa, and Si or O.
Prepulse-enhanced narrow bandwidth soft x-ray emission from a low debris, subnanosecond, laser plasma source
Authors: Dunne P. , O’Sullivan G. , O’Reilly D.
Published: July 2002

Soft x-ray emission from 170 ps laser-produced plasmas formed on cerium-doped borosilicate glasses has been recorded in the 7–17 nm region using a 2 m grazing incidence vacuum spectrograph. Broadband spectra have been recorded on photographic plates, while intensity comparisons have been made using an absolutely calibrated, extreme ultraviolet sensitive photodiode. The use of a laser prepulse to prime the target has been seen to enhance the emission with the maximum flux produced at an interpulse delay of 5.1 ns. The peak conversion efficiency is found to be 4.8%±1.5% into 3% bandwidth, centered at 8.8 nm. In addition, the level of debris emitted by the target is greatly reduced by comparison with solid metallic targets.
Sheet resistance modeling of the Ti/SiO2 system upon high temperature annealing
Authors: Zeng Yuxiao , Chen Linghui , Alford T. L.
Published: July 2002

The reaction of Ti/SiO2 structure at elevated temperatures has been studied by measuring the sheet resistance as a function of annealing temperature for different initial Ti thicknesses. A sheet resistance model has been developed to account for the effects of the thickness and resistivity variations of the reaction products [Ti(O) and Ti5Si3]. The simulated sheet resistance values varying with the Ti5Si3 thickness are in good agreement with the general trend as obtained from the experimental measurements for different initial Ti thicknesses. The approach used in modeling can be applied to other thin film structures with similar reaction mechanism to simulate the overall sheet resistance variation.
Electro-optical examination of the band structure of ordered InGaAs
Authors: Spieler J. , Kippenberg T. , Krauß J. , Kiesel P. , Döhler G. H. , Velling P. , Prost W. , Tegude F. J.
Published: July 2002

Using electroabsorption measurements, we have studied the effects of atomic superlattice ordering on the electronic band structure of InGaAs for different growth parameters. We have observed ordering-induced polarization anisotropy, valence-band splitting and band gap reduction strongest for 550 °C growth and 2°[111]B tilted substrates. Back-folded conduction-band states show an ordering dependent energy shift. The position of the split-off valence-band, however, is almost unaffected. An extension to extremely low growth temperatures exhibits ordering also for 450 °C growth. Atomic force microscopy measurements reveal a temperature-dependent change of InGaAs surface from step-like to island formation.
Response to “Comment on ‘Emission of prompt electrons during excimer laser ablation of aluminum targets’ ” [Appl. Phys. Lett. 76, 248 (2000)]
Authors: Amoruso S. , Armenante M. , Bruzzese R. , Spinelli N. , Velotta R. , Wang X.
Published: July 2002

In a comment on our recent letter, Hong and Lu questioned that the sharp peak in the time-of-flight (TOF) signal observed during laser ablation of the Al target in vacuum was due to photoelectrons produced by laser target interaction. In particular, they claimed that very similar signals have been observed during KrF excimer laser ablation of Cu targets in ambient air at a laser fluence of 9.2 J cm−2 and with a probe placed at 0.75 and 1.8 mm apart from the target surface. In a recent article, such signals have been ascribed to plasma induced electric fields at early stages of the laser ablation process, when the emitted electrons and positive ions are in fast dynamics. We wish to point out that the analogy between the two electric signals is only apparent. In fact, the experimental approach by Lu and Hong is based on the detection of electromagnetic signals due to the emitting dipole formed by electrons and ions in the plume. This is realized by a tiny metal probe put nearby the target (<1 cm). Both the r−2 dependence of the signal amplitude (r being the probe distance) and the independence of the peak position on r ensure that the signal they observed was due to electromagnetic radiation.
High luminescent efficiency in light-emitting polymers due to effective exciton confinement
Authors: Sun R. G. , Wang Y. Z. , Wang D. K. , Zheng Q. B. , Kyllo E. M. , Gustafson T. L. , Epstein A. J.
Published: July 2002

Highly efficient light-emitting polymers have become possible by molecular engineering. Photoluminescence (PL) quantum yield above 90% in the solid state is reported for the alternating block copolymer of distyrylbenzene. We conclude that the alternate arrangement of conjugated and nonconjugated segments with surrounding side groups for chromophores effectively confine the excitons for radiative emission. The effectiveness of the exciton confinement is confirmed through the temperature independence of the PL quantum yield. The time-resolved PL decay measurement supports this model through the independence of the PL yield on temperature and emission wavelength. The synthesized copolymers have been employed for the fabrication of electroluminescent (EL) devices, demonstrating high external EL efficiency with low operation threshold.
Temperature dependence of band gap energies of GaAsN alloys
Authors: Uesugi Katsuhiro , Suemune Ikuo , Hasegawa Tatsuo , Akutagawa Tomoyuki , Nakamura Takayoshi
Published: July 2002

The temperature dependence of band gap energies of GaAsN alloys was studied with absorption measurements. As the N concentration in GaAsN increased, the temperature dependence of the band gap energy was clearly reduced in comparison with that of GaAs. The redshift of the absorption edge in GaAsN for the temperature increase from 25 to 297 K was reduced to 60% of that of GaAs for the N concentration larger than ∼1%. The differential temperature coefficient of the energy gap at room temperature was also reduced to 70% of that of GaAs. The main factor for this reduced temperature dependence in GaAsN was attributed to the transition from band-like states to nitrogen-related localized states with detailed studies of the temperature-induced shift of the absorption edge.
Improved characteristics of InGaN multiple-quantum-well light-emitting diode by GaN/AlGaN distributed Bragg reflector grown on sapphire
Authors: Nakada N. , Nakaji M. , Ishikawa H. , Egawa T. , Umeno M. , Jimbo T.
Published: July 2002

An InGaN multiple-quantum-well light-emitting diode (LED) containing a GaN/AlGaN distributed Bragg reflector has been grown on a sapphire substrate by metalorganic chemical vapor deposition. Comparing with the conventional LED, the output power has been improved from 79 to 120 μW under 20 mA direct current biasing condition and the external quantum efficiency has been also improved from 0.16% to 0.23% under 10 mA dc current.
Temperature measurements of a thermal wave at static high pressures
Authors: Pangilinan G. I. , Ladouceur H. D. , Russell T. P.
Published: July 2002

Experiments probing the dynamics of a thermal wave in a material compressed at static high pressures are presented. The method involves launching a thermal wave in a sample compressed in an anvil cell and monitoring the temporal profile of temperature at a fixed point downstream. A pulsed laser incident on a metal foil provides the thermal wave, while fluorescence from a small ruby sphere yields the time-resolved temperature. Results from compressed NaCl show that temperature rises quickly and decays much more slowly, consistent with the expected heat flow. This temporal profile evaluated with a numerical solution of the heat-conduction equation or an analytical approximation provides values for thermal conductivity, thermal diffusivity, and heat capacity at pressures to 1.7 GPa.
Reversible phase transition between amorphous and crystalline in Zr41.2Ti13.8Cu12.5Ni10Be22.5 under high pressure at room temperature
Authors: Sun Liling , Wang W. K. , He D. W. , Wang W. H. , Wu Q. , Zhang X. Y. , Bao Z. X. , Zhao Q.
Published: July 2002

A reversible phase transition between amorphous and crystalline in bulk metallic glass (BMG) Zr41.2Ti13.8Cu12.5Ni10Be22.5 has been investigated under high pressure at room temperature. The BMG displayed a structure memory under high pressure as detected by in situ synchrotron radiation of x-ray diffraction and resistance measurement in a diamond anvil cell. Direct experimental observations found that the crystallization of the BMG occurred at 24 GPa on uploading and the crystalline phase reverted back to the amorphous state during downloading. This unusual phenomenon was discussed thermodynamically.
X-ray photoelectron spectroscopy characterization of AlGaN surfaces exposed to air and treated in NH4OH solution
Authors: Hashizume Tamotsu , Ootomo Shin-ya , Nakasaki Ryuusuke , Oyama Susumu , Kihara Michio
Published: July 2002

Chemical properties of Al0.17Ga0.83N surfaces exposed to air and treated in an NH4OH solution were systematically investigated by x-ray photoelectron spectroscopy (XPS). An air-exposed sample showed highly nonstoichiometric surface which included a large amount of Ga and Al oxides. The angle-resolved XPS analysis revealed that the natural oxide layer possessed a complicated composition distribution in depth and that the Al-oxide component was dominant on the topmost layer. A drastic reduction of such Al-oxide component as well as Ga-related oxide was achieved after the treatment in an NH4OH solution at 50 °C for 10 min, resulting in a constant in-depth composition distribution. The NH4OH-based treatment was found to enhance the intensity of the E2 (high) Raman peak and to reduce the root-mean-square value of surface roughness.
Lattice strains and composition of self-organized Ge dots grown on Si(001)
Authors: Jiang Z. M. , Jiang X. M. , Jiang W. R. , Jia Q. J. , Zheng W. L. , Qian D. C.
Published: July 2002

X-ray diffraction measurements at different grazing angles for self-organized Ge dots grown on Si(001) are carried out by using synchrotron radiation as a light source. The lattice parameters parallel and perpendicular to the surface are determined from the grazing angle and ordinary x-ray diffraction spectra. A 1.2% lattice constant expansion parallel to the interface and a 3.1% lattice expansion along the growth direction, as compared with the Si lattice, are found within the Ge dots. Based on the Poisson equation and the Vegard law, the Ge dot should be a partially strain relaxed SiGe alloy with the Ge content of 55%. The composition change in Ge dots is suggested to be caused by the atomic intermixing during the islanding growth. In the small grazing angle x-ray diffraction spectrum, a peak located at the higher angle side of Si(220) is observed. The origin of this peak is attributed to the near surface compressive strain in the peripheral substrate regions surrounding the Ge dots. This compressive strain is induced by the formation of Ge dots and leads to a −0.8% lattice constant change parallel to the interface.
Formation of carbon nitride with sp3-bonded carbon in CNx/ZrN superlattice coatings
Authors: Wu M. L. , Guruz M. U. , Dravid V. P. , Chung Y. W. , Anders S. , Freire F. L. , Mariotto G.
Published: July 2002

A carbon nitride phase with primarily sp3-bonded carbon is stabilized in CNx/ZrN superlattice coatings. Rutherford backscattering spectrometry measurements give a N/C atomic ratio ∼1.3 for the CNx layers. High-resolution transmission electron microscopy and Z-contrast imaging from the superlattice show a well-defined layer structure. Raman and near-edge x-ray absorption fine structure spectroscopy results reveal the formation of sp3-bonded carbon atoms in short-period CNx/ZrN superlattices.
Significant reduction of leakage current in the TiO2/Si structure by the insertion of the CeO2 intermediate layer
Authors: Bae Geunhag , Song Youngil , Jung Donggeun , Roh Yonghan
Published: July 2002

The insertion of the CeO2 intermediate layer between the TiO2 layer and the Si substrate reduced the leakage current significantly after rapid thermal annealing (RTA) in O2 ambient for 3 min. After RTA, the TiO2/CeO2/Si structure showed a significantly lower leakage current than the TiO2/Si structure. At a proper voltage of −2 V, which was defined as the difference between the applied gate bias and the flatband voltage, the leakage current density of the TiO2/CeO2/Si structure was ∼10−8 A/cm2, while the leakage current density of the TiO2/Si sample was ∼10−2 A/cm2. The formation of the intermixed structure of TiO2 and CeO2 at the TiO2/CeO2 interface by RTA was thought to contribute to the reduction of the leakage current.
Periodic ab initio calculations of orthoboric acid
Authors: Zapol Peter , Curtiss Larry A. , Erdemir Ali
Published: July 2002

Periodic ab initio calculations for orthoboric acid were performed. Geometries of a monolayer and a three-dimensional molecular crystal were optimized. Intermolecular interactions within the layer are found to be about three times stronger than the interaction between layers. Layer buckling is shown to be caused by interlayer interactions. Deformation density maps are given and changes in the density distribution induced by intermolecular interactions are discussed.
Polarization locked vector solitons and axis instability in optical fiber
Authors: Cundiff Steven T. , Collings Brandon C. , Bergman Keren
Published: July 2002

We experimentally observe polarization-locked vector solitons in optical fiber. Polarization locked-vector solitons use nonlinearity to preserve their polarization state despite the presence of birefringence. To achieve conditions where the delicate balance between nonlinearity and birefringence can survive, we studied the polarization evolution of the pulses circulating in a laser constructed entirely of optical fiber. We observe two distinct states with fixed polarization. This first state occurs for very small values birefringence and is elliptically polarized. We measure the relative phase between orthogonal components along the two principal axes to be ±π/2. The relative amplitude varies linearly with the magnitude of the birefringence. This state is a polarization locked vector soliton. The second, linearly polarized, state occurs for larger values of birefringence. The second state is due to the fast axis instability. We provide complete characterization of these states, and present a physical explanation of both of these states and the stability of the polarization locked vector solitons.
Speed-dependent line profile: A test of a unified model from the Doppler to the collisional regime for molecule–molecule collisions
Authors: Chaussard F. , Saint-Loup R. , Berger H. , Joubert P. , Bruet X. , Bonamy J. , Robert D.
Published: July 2002

A speed-dependent line profile combining soft and hard fully correlated Dicke-narrowing collisions was recently successfully tested on Ar-broadened H2 spectra in a wide density and temperature range. A further test for mixtures of H2 in nitrogen molecules (instead of Ar atoms) is presented. This test is also based on high resolution Raman investigation of the isotropic Q(1) line of H2 from low to high density at various temperatures. The same consistency of the speed-dependent line profile as for H2–Ar is obtained for H2–N2 through a remarkable agreement with all the data by using a unique set of four parameters (the collisional width and shift, the kinetic frequency, and a characteristic velocity memory parameter). The present study is a preliminary step for the hydrogen CARS thermometry in H2–air flames at high pressure.
Treatment of the semiclassical Boltzmann equation for magnetic multilayers
Authors: Pereira L. G. , Duvail J. L. , Lottis D. K.
Published: July 2002

We present an analytical treatment of the Camley–Barnaś theory of the giant magnetoresistance (GMR) in magnetic layered structures and obtain an exact and general expression for the resistivity. We used this expression to evaluate the resistivity and GMR numerically, comparing the results with experimental observations.
Room temperature enhancement and inhibition of spontaneous emission in semiconductor microcavities
Authors: Bourdon G. , Robert I. , Adams R. , Nelep K. , Sagnes I. , Moison J. M. , Abram I.
Published: July 2002

We have fabricated planar semiconductor microcavities with metallic mirrors in which we have observed both enhancement and inhibition of spontaneous emission, at room temperature. Our results are quantitatively accounted for by modeling the band-to-band emission as arising from point dipoles. Observation of these cavity quantum electrodynamics effects in room temperature semiconductor structures opens the way to optoelectronic devices with controlled spontaneous emission.
Vibrational energy relaxation of azulene in the S2 state. I. Solvent species dependence
Authors: Yamaguchi T. , Kimura Y. , Hirota N.
Published: July 2002

We have measured the time-resolved fluorescence spectra of azulene in the S2 state in compressed gases and in liquids. We have found that the band shape of the fluorescence changes significantly in the earlier time scale after the photoexcitation when large excess energy (about 6500 cm−1) is given. The change of the band shape is similar both in the compressed gases and in the liquids, although the time scales of the change are quite different. We have measured the excitation energy dependence of the fluorescence band shape of the isolated molecule separately, and shown that the time dependence of the fluorescence band shape in gases and liquids corresponds to the vibrational energy relaxation in the S2 state. Comparing with the excitation energy dependence of the fluorescence band shape of the isolated molecule, we have succeeded in determining the transient vibrational excess energy. The vibrational energy relaxation rates in the S2 state are 1–2 times faster than those in the ground state both in compressed gases and in liquids.
Electric dipole polarizability of one excess-electron alkali–halide cluster
Authors: Rayane Driss , Antoine Rodolphe , Dugourd Philippe , Broyer Michael
Published: July 2002

In this communication, we present the first measurement of the electric dipole polarizability of alkali–halide clusters with one excess electron. The polarizability is strongly size dependent. Very large values are observed for certain sizes. Ionization potential measurement is also presented for comparison.
Reactions of iron clusters with oxygen and ethylene: Observation of particularly stable species
Authors: Huisken F. , Kohn B. , Alexandrescu R. , Morjan I.
Published: July 2002

Iron clusters have been produced by CO2-laser-induced decomposition of iron pentacarbonyl in a flow reactor. The absorption of CO2 laser photons was achieved by using SF6 as a sensitizer. By adding an oxidizing gas, N2O, or a hydrocarbon, C2H4, molecules which are also dissociated in the laser field, the iron clusters may react with several radicals. The as-synthesized species are extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where they are analyzed with a time-of-flight mass spectrometer. In the experiment with N2O, we observe a magic peak at m=856 amu which can be readily assigned to the particularly stable Fe13O8 cluster. If C2H4 is added to the reactant gas, the mass spectrum reveals a magic peak at mass m=884 amu. Using deuterated ethylene, the magic peak shifts by 12 amu to larger masses, indicating that the magic cluster contains 12 hydrogen atoms. With the given restrictions, we readily derive the molecular formula Fe13C12H12. Chemical stability and symmetry considerations suggest that the detailed chemical formula of the magic cluster is Fe13(C2H2)6 and that its structure corresponds to a Fe13 icosahedron with six HC=CH or C=CH2 groups bound to six pairs of the 12 iron surface atoms.
Dynamical properties of chemical systems near Hopf bifurcation points
Authors: Ipsen M. , Schreiber I.
Published: July 2002

In this paper, we numerically investigate local properties of dynamical systems close to a Hopf bifurcation instability. We focus on chemical systems and present an approach based on the theory of normal forms for determining numerical estimates of the limit cycle that branches off at the Hopf bifurcation point. For several numerically ill-conditioned examples taken from chemical kinetics, we compare our results with those obtained by using traditional approaches where an approximation of the limit cycle is restricted to the center subspace spanned by critical eigenvectors, and show that inclusion of higher-order terms in the normal form expansion of the limit cycle provides a significant improvement of the limit cycle estimates. This result also provides an accurate initial estimate for subsequent numerical continuation of the limit cycle.
Diode-free magnetic random access memory using spin-dependent tunneling effect
Authors: Wang Frank Z.
Published: July 2002

A diode-free magnetic random access memory comprises two sets of conductive lines, an array of magnetic tunnel junctions at each intersection, and a peripheral circuitry. Such a simplified diode-free architecture described in this letter overcomes the diode-area constraint in the prior art and achieves a significant breakthrough in storage density.
Spin-dependent tunneling junctions with AlN and AlON barriers
Authors: Sharma Manish , Nickel Janice H. , Anthony Thomas C. , Wang Shan X.
Published: July 2002

We report on ferromagnetic spin-dependent tunneling (SDT) junctions with NiFe/AlN/NiFe and NiFe/AlON/NiFe structures. Good barriers were formed by plasma nitridation and oxy-nitridation of Al films. Tunneling magnetoresistance ratios (TMR) up to 18% were observed at room temperature. The devices exhibit lower resistance-area products than those seen in reference junctions with Al2O3 barriers. The degradation in TMR at higher bias voltages is found to be less than that found in standard alumina junctions. AlN and AlON could thus be alternate materials for the tunnel barrier in SDT junctions.
Line shape parameters for HF in a bath of argon as a test of classical path models
Authors: Hartmann J.-M. , Boulet C.
Published: July 2002

The classical path method proposed by Neilsen and Gordon [J. Chem. Phys. 58, 4131 (1973)] is tested by comparison with extensive fully quantal calculations and recent measurements of cross-sections for HF-Ar. Widths and shifts are found to be in satisfactory agreement both with the close-coupling and experimental results. Comparisons are also made with more “downgraded” theoretical approaches based on the peaking approximation and the neglect of the time ordering. Finally, the Neilsen and Gordon approach is used in order to demonstrate that vibration-rotation coupling significantly contributes to the rotational dependence of both widths and shifts cross-sections.
Interaction between soot particles and NOx during dielectric barrier discharge plasma remediation of simulated diesel exhaust
Authors: Dorai Rajesh , Hassouni Khaled , Kushner Mark J.
Published: July 2002

Plasma remediation is being investigated as a means to remove NOx from combustion effluent and from diesel exhausts in particular. Soot particles are inevitably present in actual exhausts and may, through heterogeneous chemistry, affect the remediation process. In this article, a computational investigation of the effect of soot on the plasma chemistry of NOx removal in a simulated diesel exhaust processed in a dielectric barrier discharge reactor is presented using a zero-dimensional global-kinetics simulation. A surface chemistry model is employed to describe soot oxidation by O and OH radicals, and soot-NOx interactions. The NOx chemistry may be substantially affected by the reactions at the soot surface. In particular, for soot particles having densities of 108 cm−3 and diameters of 100 nm, significant increases of NO are obtained when taking into account NO2→NO conversion on the soot surface. Heterogeneous reaction of NO2 also results in an increase in the gas-phase OH density which results in the increased formation of HNO2, thereby adding to the NOx remediation. NO2→NO heterogeneous conversion also results in an increase of CO and a subsequent reduction of the soot mass. The mechanism for soot-NOx interactions depends on the deactivation of active hydrocarbon radicals on the soot.
Dependence of composition fluctuation on indium content in InGaN/GaN multiple quantum wells
Authors: Lin Yen-Sheng , Ma Kung-Jeng , Hsu C. , Feng Shih-Wei , Cheng Yung-Chen , Liao Chi-Chih , Yang C. C. , Chou Chang-Cheng , Lee Chia-Ming , Chyi Jen-Inn
Published: July 2002

The information on the variations of indium composition, aggregation size, and quantum-well width is crucially important for understanding the optical properties and, hence, fabricating efficient light-emitting devices. Our results showed that spinodal decomposition could occur in InGaN/GaN multiple quantum wells with indium content in the range of 15%–25% (grown with metal–organic chemical-vapor deposition). A lower nominal indium content led to a better confinement of indium-rich clusters within InGaN quantum wells. The InGaN/GaN interfaces became more diffusive, and indium-rich aggregates extended into GaN barriers with increasing indium content. It was also observed that indium-rich precipitates with diameter ranging from 5 to 12 nm preferred aggregating near V-shaped defects.
F
+
and F centers in α-Al2O3 by electron-induced x-ray emission spectroscopy and cathodoluminescence
Authors: Jonnard P. , Bonnelle C. , Blaise G. , Rémond G. , Roques-Carmes C.
Published: July 2002

Study of an α-Al2O3 single crystal by electron-induced x-ray emission spectroscopy and cathodoluminescence is reported. The relative intensities of optical emissions due to F+ and F centers have been determined as a function of the parameters of the electron beam and the annealing of the sample. It is shown that the F+ centers, i.e., the oxygen vacancies with one trapped electron, are predominant when the density of the incident electron beam increases. Similar variation is observed when the electron energy varies from 1 to 4 keV. From the comparison between x-ray and optical spectra, the F+ centers are determined to be stable defects in the bulk of the sample.
Notes on the boundary conditions for fluid-dynamic equations on the interface of a gas and its condensed phase
Authors: Sone Yoshio , Takata Shigeru , Golse François
Published: July 2002

The boundary conditions for the fluid-dynamic equations on an interface of a gas and its condensed phase derived by an asymptotic analysis of the Boltzmann equation are considered. Some general relations satisfied by the conditions and their application showing a fundamental property of a flow accompanying evaporation and condensation are presented.
Midinfrared photoconductivity of Ge/Si self-assembled quantum dots
Authors: Rappaport N. , Finkman E. , Brunhes T. , Boucaud P. , Sauvage S. , Yam N. , Le Thanh V. , Bouchier D.
Published: July 2002

We have investigated the midinfrared photoconductivity of Ge/Si self-assembled quantum dots. The self-assembled quantum dots were grown by ultra-high-vacuum chemical vapor deposition on Si(001). The photoresponse of the p-type device exhibits resonances in the midinfrared around 10 μm wavelength. The resonance of the photocurrent shifts to lower energy as the applied bias increases. The photocurrent is weakly dependent on the incoming polarization of the infrared light. The photocurrent is analyzed in terms of bound-to-bound and bound-to-continuum transitions in the valence band. The photocurrent peaks are correlated to the photoluminescence of the device.