bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoluminescence and electroluminescence from copper doped zinc sulphide nanocrystals/polymer composite
Authors: Que Wenxiu , Zhou Y. , Lam Y. L. , Chan Y. C. , Kam C. H. , Liu B. , Gan L. M. , Chew C. H. , Xu G. Q. , Chua S. J. , Xu S. J. , Mendis F. V. C.
Published: July 2002

Cu-doped ZnS nanocrystals were prepared in an inverse microemulsion at room temperature as well as under a hydrothermal condition. X-ray diffraction analysis showed that the diameter of the Cu-doped ZnS nanocrystals particles was about 9 nm. These particles showed a strong photoluminescence intensity and a broad emission band from 490 to 530 nm. The half-width of emission was about 60 nm. Cu-doped ZnS nanocrystals/polymethylmethacrylate composite as a light-emitting layer was used to fabricate a single layer structure electroluminescent device which had low turn on voltage (less than 5 V). The green light of electroluminescence was observed at room temperature. The electroluminescence and photoluminescence spectra were nearly identical at room temperature.
Influence of GaAs capping on the optical properties of InGaAs/GaAs surface quantum dots with 1.5 μm emission
Authors: Saito Hideaki , Nishi Kenichi , Sugou Shigeo
Published: July 2002

Uncapped InGaAs quantum dots (surface quantum dots) on a GaAs substrate emit photoluminescence at a long wavelength of 1.53 μm at room temperature. When the surface dots are covered by a GaAs cap layer, the emission energy of the dots increases by 287 meV. This large energy shift is mainly caused by inducing compressive stress from the cap layer. In segregation on the surface led to greater photoluminescence intensity in the surface quantum dots even at room temperature due to the suppression of nonradiative surface recombination.
Load transfer in carbon nanotube epoxy composites
Authors: Schadler L. S. , Giannaris S. C. , Ajayan P. M.
Published: July 2002

The mechanical behavior of multiwalled carbon nanotube/epoxy composites was studied in both tension and compression. It was found that the compression modulus is higher than the tensile modulus, indicating that load transfer to the nanotubes in the composite is much higher in compression. In addition, it was found that the Raman peak position, indicating the strain in the carbon bonds under loading, shifts significantly under compression but not in tension. It is proposed that during load transfer to multiwalled nanotubes, only the outer layers are stressed in tension whereas all the layers respond in compression.
Germanium “quantum dots” embedded in silicon: Quantitative study of self-alignment and coarsening
Authors: Kienzle O. , Ernst F. , Rühle M. , Schmidt O. G. , Eberl K.
Published: July 2002

We report on experiments aiming to produce Ge quantum dots embedded in Si. Employing cross-sectional transmission electron microscopy, we have studied the misfit stress-induced self-alignment of islands belonging to consecutive Stranski–Krastanov layers of Ge buried in Si by molecular beam epitaxy. Quantitative evaluation of the micrographs has revealed the critical Si interlayer thickness below which the island positions in successive Ge layers begin to correlate. Moreover, we have quantitatively analyzed the influence of the Si interlayer thickness on the coarsening of the Ge islands from one buried Ge layer to the next.
Low-resistance Pt/Ni/Au ohmic contacts to p-type GaN
Authors: Jang Ja-Soon , Chang In-Sik , Kim Han-Ki , Seong Tae-Yeon , Lee Seonghoon , Park Seong-Ju
Published: July 2002

We report on a Pt (20 nm) Ni (30 nm)/Au (80 nm) metallization scheme for low-resistance ohmic contacts to the moderately doped p-type GaN:Mg (3×1017 cm−3). Both as-deposited and annealed Pt/Ni/Au contacts on p-GaN exhibit linear current–voltage characteristics, showing that a high-quality ohmic contact is formed. The Pt/Ni/Au scheme shows the specific contact resistance of 5.1×10−4 Ω cm2 when annealed at 350 °C for 1 min in a flowing N2 atmosphere.
Observation of the impulse-like negative-differential resistance of superlatticed resonant-tunneling transistor
Authors: Cheng Shiou-Ying , Liu Wen-Chau , Chang Wen-Lung , Pan Hsi-Jen , Wang Wei-Chou , Chen Jing-Yuh , Feng Shun-Ching , Yu Kuo-Hui
Published: July 2002

An AlInAs/GaInAs superlatticed resonant-tunneling transistor (SRTT) is fabricated and demonstrated. A five-period AlInAs/GaInAs superlattice is used to provide resonant tunneling (RT) and confinement of minority carriers. Experimentally, an interesting impulse-like multiple negative-differential-resistance (MNDR) phenomenon and transistor action are obtained for the studied SRTT device at room temperature. It is believed that the N-shaped MNDR results mainly from RT within the five-period AlInAs/GaInAs superlattice.
p-type doping of Zn(Mg)BeSe epitaxial layers
Authors: Tournié E. , Faurie J.-P.
Published: July 2002

We have investigated the nitrogen-plasma p-type doping properties of both the ZnBeSe-ternary and the ZnMgBeSe-quaternary wide-bandgap alloys grown by molecular-beam epitaxy on (001) GaAs substrates. We show that net acceptor concentrations in the 1017 cm−3 range can be obtained for a bandgap as high as 3.2 eV in the ternary ZnBeSe alloy. In contrast, the bandgap of ZnMgBeSe quaternary alloys lattice matched onto GaAs should not exceed 2.97 eV to reach comparable doping levels. Our results reveal the detrimental influence of both ZnS and MgSe compounds on the p-type doping of wide-bandgap II–VI alloys.
Loop-wire medium for investigating plasmons at microwave frequencies
Authors: Smith D. R. , Vier D. C. , Padilla Willie , Nemat-Nasser Syrus C. , Schultz S.
Published: July 2002

We present numerical simulations and microwave measurements on a loop-wire structure that acts as an effective medium exhibiting a well-defined bulk plasma frequency in the microwave regime, with an effective negative dielectric function below this plasma frequency. The dependence of this plasmonic response on the self-inductance of the constituent wire elements is made explicit. A finite structure, approximately spherical, composed of this inductive medium is studied, and reveals subwavelength surface plasmon resonances below the bulk plasma frequency.
Holotomography: Quantitative phase tomography with micrometer resolution using hard synchrotron radiation x rays
Authors: Cloetens P. , Ludwig W. , Baruchel J. , Van Dyck D. , Van Landuyt J. , Guigay J. P. , Schlenker M.
Published: July 2002

Because the refractive index for hard x rays is slightly different from unity, the optical phase of a beam is affected by transmission through an object. Phase images can be obtained with extreme instrumental simplicity by simple propagation provided the beam is coherent. But, unlike absorption, the phase is not simply related to image brightness. A holographic reconstruction procedure combining images taken at different distances from the specimen was developed. It results in quantitative phase mapping and, through association with three-dimensional reconstruction, in holotomography, the complete three-dimensional mapping of the density in a sample. This tool in the characterization of materials at the micrometer scale is uniquely suited to samples with low absorption contrast and radiation-sensitive systems.
On the effect of hydrogen incorporation in strontium titanate layers grown by high vacuum magnetron sputtering
Authors: Schneider Jochen M. , Hjörvarsson Björgvin , Wang Xin , Hultman Lars
Published: July 2002

Hydrogen incorporation in strontium titanate thin films grown in a high vacuum environment by rf magnetron sputtering on MgO(001) substrates was investigated by nuclear resonance analysis. The amount of hydrogen in the films is strongly affected by the growth temperature. Films deposited at or below 200 °C have a close-to-amorphous microstructure, are oxygen rich and exhibit hydrogen incorporation up to 0.08 (atomic ratio). As the growth temperature is increased to ⩾250 °C, crystalline growth of stoichiometric SrTiO3 is obtained, with a hydrogen content of 0.006. The results provide evidence that hydrogen incorporation is decisive for the composition as well as the microstructure evolution of the layers.
Quantum dots in photonic dots
Authors: Artemyev M. V. , Woggon U.
Published: July 2002

A micrometer-sized spherical microcavity, the photonic dot, is made from semiconductor nanocrystals, the quantum dots. The coupling of electronic and photonic states is demonstrated for a single photonic dot by the observation of whispering gallery modes in the spectrum of spontaneous emission of the embedded CdSe quantum dots.
Waveguide optical isolator based on Mach–Zehnder interferometer
Authors: Fujita J. , Levy M. , Osgood R. M. , Wilkens L. , Dötsch H.
Published: July 2002

A waveguide optical isolator based on nonreciprocal interference is demonstrated. Ridge waveguides are fabricated in a Mach–Zehnder configuration on a single film of bismuth-, lutetium-, neodymium-iron garnet. With this design, no polarizers are required to achieve extinction in the backward propagation direction. This isolator exhibits a 19 dB extinction ratio at λ=1.54 μm. A flat wavelength dependence, to within 2 dB, has been observed in the range between 1.49 and 1.57 μm.
Infrared spectroscopy of ZnSiN2 single-crystalline films on r-sapphire
Authors: Mintairov Alexander , Merz James , Osinsky Andrei , Fuflyigin Vladimir , Zhu L. D.
Published: July 2002

This letter presents a study of optical phonon modes of single-crystalline orthorhombic ZnSiN2 semiconductor epitaxially deposited on r-sapphire. An epitaxial relationship for ZnSiN2 film was found from x-ray diffraction to be (0k0)ZnSiN2‖(10 1̄2)Al2O3 and [100]ZnSiN2‖(12̄10)Al2O3. Six B1 optical modes were revealed in 400–1000 cm−1 range in s-polarized infrared reflectance spectra. This is consistent with the analysis of the phonon symmetry and selection rules presented. The frequencies of the transversal and longitudinal components, phonon damping, and oscillator strengths of the B1 phonons as well as high frequency dielectric constant ε∞xx of the orthorhombic ZnSiN2 were determined.
Porous silicon microcavities as optical chemical sensors
Authors: Mulloni V. , Pavesi L.
Published: July 2002

The optical properties of porous silicon microcavities are strongly dependent on the environment. For highly luminescent samples, both the luminescence intensity and the peak position are affected by organic substances, giving the possibility to obtain dual-parameter optical sensors. While the peak position depends on the organic compound refractive index, luminescence intensity depends on its low-frequency dielectric constant. This allows the discrimination between different organic substances. This sensor is particularly interesting for solvents with low dielectric constant, where the response of electrical sensors is very weak.
Plasma patterning of carbon nanotubes
Authors: Chen Qidao , Dai Liming
Published: July 2002

We have prepared micropatterns of carbon nanotubes on a wide range of substrates either by patterned growth of aligned nanotubes on surfaces prepatterned with plasma polymers (e.g., n-hexane plasma polymer) or through region-specific adsorption of certain chemically modified carbon nanotubes (e.g., –COOH substituted nanotubes) onto surfaces prepatterned with various plasma-generated functionalities (e.g., –NH2). Micropatterns of carbon nanotubes prepared in both cases have resolutions on a micrometer scale, suitable for fabrication of various electronic and photonic devices.
High visible rejection AlGaN photodetectors on Si(111) substrates
Authors: Pau J. L. , Monroy E. , Naranjo F. B. , Muñoz E. , Calle F. , Sánchez-Garcı́a M. A. , Calleja E.
Published: July 2002

We report on the fabrication and characterization of Schottky barrier photodetectors based on Si-doped Al0.35Ga0.65N layers grown on Si(111) substrates, for solar UV-band monitoring (λ<320 nm). The epilayers have been obtained by plasma-assisted molecular-beam epitaxy, showing a full width at half maximum of 15 arcmin in x-ray diffraction measurements. A very high visible rejection (>104) and a responsivity of 5 mA/W at 257 nm are reached. The detector time response is limited by the resistance×capacitance product, with a minimum time constant of 20 ns in the zero-load-resistance limit. After photodiode voltage breakdown, the effect on the detector response is discussed.
Thermal stability of IrMn and MnFe exchange-biased magnetic tunnel junctions
Authors: Samant Mahesh G. , Lüning Jan , Stöhr Joachim , Parkin Stuart S. P.
Published: July 2002

The thermal stability of exchange-biased magnetic tunnel junctions is explored using near-edge x-ray absorption fine-structure spectroscopy. Structures with Ir–Mn antiferromagnetic exchange bias layers are much more thermally stable than similar structures with Fe–Mn exchange bias layers. In both cases, diffusion of Mn from the antiferromagnetic layer through thin exchange-biased ferromagnetic layers to the tunnel barrier is observed at elevated temperatures. This observation explains the diminished magnetoresistance of these structures on annealing even though the resistance of the tunnel junctions is hardly changed.
Optical properties of self-assembled thin-film of poly(p-phenylene vinylene)s and its application to light-emitting devices with microring geometry
Authors: Sonoda T. , Fujisawa T. , Fujii A. , Yoshino K.
Published: July 2002

Periodic multilayer structures of poly(p-phenylene vinylene)s have been fabricated by a self-assembly method on flat surfaces and round surfaces of optical fibers. Alternating multilayers consisting of poly{1,4-[2-(5-carboxypentyloxy)-5-methoxyphenylene]vinylene} (CPMOPPV) and poly(p-phenylene vinylene) (PPV) were adsorbed onto the positively charged substrates. The optical properties of the periodic multilayer structures of CPMOPPV/PPV have been studied. Periodic multilayers with microring geometry have also been fabricated around the quartz fibers. Their optical properties have been studied, and yellow electroluminescence from a light-emitting device with microring geometry has been observed.
Statistical analysis in the negative-U model of donors in AlxGa1−xAs:Si
Authors: Rziga-Ouaja F. , Mejri H. , Triki A. , Selmi A. , Rebey A.
Published: July 2002

Hall measurements were performed on molecular beam epitaxy grown AlxGa1−xAs:Si in the temperature range 77–300 K. The DX center has been detected through the observation of persistent photoconductivity at low temperature. Two statistics have been developed, using the negative-U model of Chadi and Chang, to analyze the temperature dependence of Hall electron densities. The first statistic is derived assuming that the conduction electrons originate exclusively from the DX center. The second statistic, however, supposes the existence of shallow donors and acceptors in addition to Si–DX centers. The concentrations of these centers are treated as fitting parameters. We have investigated, on the other hand, the pinning of the Fermi level EF and the trend of the probability fDX− of Si to be in the DX− state versus temperature using the previous statistics. We will attempt to explain all these results.
Quantum state-resolved reactive scattering of F+CH4→HF(v,J)+CH3: Nascent HF(v,J) product state distributions
Authors: Harper Warren W. , Nizkorodov Sergey A. , Nesbitt David J.
Published: July 2002

State-to-state reactive scattering of F+CH4→HF(v,J)+CH3 is studied using crossed supersonic jets and high-resolution (Δν≈0.0001 cm−1) IR laser direct absorption techniques. Rovibrational state-resolved HF column-integrated absorption profiles are obtained under single collision conditions and converted to populations via appropriate density-to-flux transformation. Nascent rovibrational distributions in each HF(v,J) state are reported. Summed over all product rotational levels, the nascent vibrational quantum state populations for HF(v) [(v=3) 0.106(3); (v=2) 0.667(14); (v=1) 0.189(27); (v=0) 0.038(78); 2σ error bars] are in agreement with previous flow cell studies by Setser, Heydtmann, and co-workers [Chem. Phys. 94, 109 (1985)]. At the rotational state level, however, the current studies indicate nascent distributions for HF(v,J) that are significantly hotter than previously reported, ostensibly due to reduced collisional relaxation effects under supersonic jet conditions. Final HF rotational states from F+CH4 are observed near the maximum energetically accessible J values in both the v=2 and v=3 vibrational manifolds, which provides experimental support for a bent F–H–C transition state structure.
Tight binding molecular dynamics studies of boron assisted nanotube growth
Authors: Hernández E. , Ordejón P. , Boustani I. , Rubio A. , Alonso J. A.
Published: July 2002

In this paper we report a theoretical study of the effects of the presence of boron in growing carbon nanotubes. We employ a well established tight binding model to describe the interactions responsible for the energetics of these systems, combined with the molecular dynamics simulation technique and structural relaxation calculations. We find, in agreement with the previous theoretical/experimental work of Blase et al. [Phys. Rev. Lett. 83, 5078 (1999)], that boron favors (n,0) (zig-zag) tubular structures over (n,n) (arm-chair) ones by stabilizing the zig-zag edge. Furthermore, it is shown that boron has the effect of delaying the tube closure process, a fact which could explain the improved aspect ratio experimentally observed in nanotubes synthesized in the presence of boron. Our dynamical simulations lead us to propose a mechanism through which this extension of the closure time can be explained.
Surface acoustic wave depth profiling of elastically inhomogeneous materials
Authors: Glorieux Christ , Gao Weimin , Kruger Silvio Elton , Van de Rostyne Kris , Lauriks Walter , Thoen Jan
Published: July 2002

The potential of Rayleigh wave spectroscopy for the in-depth reconstruction of elastic properties of multilayers for materials with a continuous profile of elastic properties is explored. Two models to calculate the surface acoustic wave (SAW) dispersion spectrum from the profile of the elastic parameters are elaborated and compared. It is found that the relevant elastic parameters for Rayleigh wave dispersion in multilayers are the “effective” Rayleigh velocities, i.e., the Rayleigh velocities calculated for virtually semi-infinite layers. For the solution of the inverse problem, a neural network and a singular value decomposition model are proposed and tested on simulated SAW spectra. The reconstruction techniques are applied to reconstruct the elastic depth profile of shot-peened steel samples from laser-generated and laser-detected SAW data.
Review of temperature measurement
Authors: Childs P. R. N. , Greenwood J. R. , Long C. A.
Published: July 2002

A variety of techniques are available enabling both invasive measurement, where the monitoring device is installed in the medium of interest, and noninvasive measurement where the monitoring system observes the medium of interest remotely. In this article we review the general techniques available, as well as specific instruments for particular applications. The issues of measurement criteria including accuracy, thermal disturbance and calibration are described. Based on the relative merits of different techniques, a guide for their selection is provided.
Orientational photorefractive effect in nematic liquid crystal with externally applied fields
Authors: Zhang Guoquan , Montemezzani Germano , Günter Peter
Published: July 2002

Light-induced space-charge fields and the related orientational photorefractive effects in nematic liquid crystal cells are studied both theoretically and experimentally. Analytical solutions for the refractive index change distribution are obtained under the single-elastic-constant approximation with various configurations of externally applied electric and magnetic fields and of the nematic liquid crystal director. The effects of the rigid boundary condition are investigated in detail. The grating spacing for which the refractive index change is maximized depends on the strength of external electric and magnetic fields. The responses of parallel cells differ from those of homeotropic cells, both upon the dependencies on the grating spacing and on the external fields. Under the same conditions, the orientational photorefractive effects are stronger in parallel cells than in homeotropic ones. It is also shown that the investigated effects will be greatly enhanced near the Freédericksz transition. Experimental verifications of the photorefractive response in parallel aligned cells confirm the necessity to consider the boundary conditions explicitly.
Optical properties of high-quality CuInSe2 single crystals
Authors: Mudriy A. V. , Bodnar I. V. , Viktorov I. A. , Gremenok V. F. , Yakushev M. V. , Tomlinson R. D. , Hill A. E. , Pilkington R. D.
Published: July 2002

CuInSe 2 single crystals have been studied employing photoluminescence (PL), optical reflection (OR), optical absorption (OA) and wavelength derivative reflection (WDR) techniques at temperatures from 4.2 to 300 K. Exciton-related peaks were observed in the near-band-edge region of the PL spectra: several narrow lines, with full width at half maximum (FWHM) of about 0.3 meV, and two wider peaks (FWHM about 0.7 meV) at 1.0414 (A) and 1.0449 eV (B). The A and B peaks were also observed in the OR and OA spectra and identified as A and B free excitonic states. The narrow lines were attributed to bound exciton recombination on intrinsic defects. A third exciton resonance (C) was observed in the WDR spectra at 1.2779 eV. The crystal-field and spin-orbit splittings were derived to be 5.3 and 234.7 meV, respectively.
Gyrocenter-gauge kinetic theory
Authors: Qin H. , Tang W. M. , Lee W. W.
Published: July 2002

Gyrocenter-gauge kinetic theory is developed as an extension of the existing gyrokinetic theories. In essence, the formalism introduced here is a kinetic description of magnetized plasmas in the gyrocenter coordinates which is fully equivalent to the Vlasov–Maxwell system in the particle coordinates. In particular, provided the gyroradius is smaller than the scale-length of the magnetic field, it can treat high-frequency range as well as the usual low-frequency range normally associated with gyrokinetic approaches. A significant advantage of this formalism is that it enables the direct particle-in-cell simulations of compressional Alfvén waves for magnetohydrodynamic (MHD) applications and of rf (radio frequency) waves relevant to plasma heating in space and laboratory plasmas. The gyrocenter-gauge kinetic susceptibility for arbitrary wavelength and arbitrary frequency electromagnetic perturbations in a homogeneous magnetized plasma is shown to recover exactly the classical result obtained by integrating the Vlasov–Maxwell system in the particle coordinates. This demonstrates that all the waves supported by the Vlasov–Maxwell system can be studied using the gyrocenter-gauge kinetic model in the gyrocenter coordinates. This theoretical approach is so named to distinguish it from the existing gyrokinetic theory, which has been successfully developed and applied to many important low-frequency and long parallel wavelength problems, where the conventional meaning of “gyrokinetic” has been standardized. Besides the usual gyrokinetic distribution function, the gyrocenter-gauge kinetic theory emphasizes as well the gyrocenter-gauge distribution function, which sometimes contains all the physics of the problems being studied, and whose importance has not been realized previously. The gyrocenter-gauge distribution function enters Maxwell’s equations through the pull-back transformation of the gyrocenter transformation, which depends on the perturbed fields. The efficacy of the gyrocenter-gauge kinetic approach is largely due to the fact that it directly decouples particle’s gyromotion from its gyrocenter motion in the gyrocenter coordinates. As in the case of kinetic theories using guiding center coordinates, obtaining solutions for this kinetic system involves only following particles along their gyrocenter orbits. However, an added advantage here is that unlike the guiding center formalism, the gyrocenter coordinates used in this theory involves both the equilibrium and the perturbed components of the electromagnetic field. In terms of solving the kinetic system using particle simulation methods, the gyrocenter-gauge kinetic approach enables the reduction of computational complexity without the loss of important physical content.
Composition and chemical bonding of pulsed laser deposited carbon nitride thin films
Authors: Riedo E. , Comin F. , Chevrier J. , Bonnot A. M.
Published: July 2002

We studied composition, structure, and growth parameters of amorphous diamond-like carbon (DLC) and carbon nitride (CNx) films deposited by pulsed laser deposition in vacuum and in nitrogen atmosphere. The composition (0⩽N/C⩽0.4), the structural and the electronic properties of the deposited carbon and carbon nitride films were investigated for different laser fluences (1–12 J/cm2). Electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and micro-Raman spectroscopy indicated an increase in sp3-bonded carbon sites in the DLC films and an increase in N-sp3 C bonded sites in the CNx films with increasing deposition laser fluence. Raman spectroscopy also showed the presence of a small amount of C≡N bonds in the CNx films. Furthermore, we observed that keeping the nitrogen pressure constant (P=100 mTorr) the increase in the deposition laser fluence is reflected by an increase in the nitrogen content in the films. All the results have been discussed in the framework of different theoretical models.
Near-field infrared imaging with a microfabricated solid immersion lens
Authors: Fletcher D. A. , Crozier K. B. , Quate C. F. , Kino G. S. , Goodson K. E. , Simanovskii D. , Palanker D. V.
Published: July 2002

We report imaging in the infrared with a microfabricated solid immersion lens. The integrated 15-μm-diameter lens and cantilever are fabricated from single-crystal silicon and scanned in contact with a sample to obtain an image. We demonstrate a focused spot size of λ/5 and an effective numerical aperture of 2.5 with λ=9.3 μm light. The total power transmitted through the lens is a factor of 103 greater than through a metal aperture giving the same spatial resolution. Two 1.0 μm holes in a metal film separated by 3.0 μm are imaged with the solid immersion lens in transmission and shown to be resolved.
A molecular beam/surface spectroscopy apparatus for the study of reactions on complex model catalysts
Authors: Libuda J. , Meusel I. , Hartmann J. , Freund H.-J.
Published: July 2002

We describe a newly developed ultrahigh vacuum (UHV) experiment which combines molecular beam techniques and in situ surface spectroscopy. It has been specifically designed to study the reaction kinetics and dynamics on complex model catalysts. The UHV system contains: (a) a preparation compartment providing the experimental techniques which are required to prepare and characterize single-crystal based model catalysts such as ordered oxide surfaces or oxide supported metal particles; and (b) the actual scattering chamber, where up to three molecular beams can be crossed on the sample surface. Two beams are produced by newly developed differentially pumped sources based on multichannel arrays. The latter are capable of providing high intensity and purity beams and can be modulated by means of a vacuum-motor driven and computer-controlled chopper. The third beam is generated in a continuous or pulsed supersonic expansion and is modulated via a variable duty-cycle chopper. Angular and time-resolved measurements of desorbing and scattered molecules are performed with a rotatable doubly differentially pumped quadrupole mass spectrometer with a liquid-nitrogen cooled ionizer housing. Time-resolved but angle-integrated measurements are realized with a second nondifferentially pumped quadrupole mass spectrometer. In situ measurements of adsorbed species under reaction conditions are performed by means of an adapted vacuum Fourier transform infrared spectrometer. The spectrometer provides the possibility of time-resolved measurements and can be synchronized with any of the beam sources. This contribution provides a general overview of the system and a description of all new components and their interplay. We also present test data for all components employing simple adsorption/desorption and reaction systems.
Two-dimensional particle simulation of a sustained discharge in an alternating current plasma display panel
Authors: Ikeda Y. , Suzuki K. , Fukumoto H. , Verboncoeur J. P. , Christenson P. J. , Birdsall C. K. , Shibata M. , Ishigaki M.
Published: July 2002

A plasma discharge due to a sustaining pulse of an alternating current plasma display panel was analyzed using a two-dimensional particle-in-cell code, and basic characteristics of the plasma discharge were calculated. The characteristics of the plasma discharge due to a sustaining pulse are as follows. (i) A large amount of space charge remained after drawing the discharge current. This excess space charge did not contribute to wall charge formation or ultraviolet radiation. (ii) The electron energy distribution function was evaluated and could be well fitted by the Druyvesteyn distribution in the high-energy region. The Druyvesteyn distribution was a consequence of the cross section for electron-Ne elastic scattering. (iii) The calculated ultraviolet radiation efficiency η of the plasma discharge due to a sustaining pulse was between 5.51% and 30.7%. Examination of the sensitivity of the efficiency to the electron temperature showed that reducing the electron temperature was a key to improving the efficiency. (iv) A detailed understanding of the conditions for a stable plasma discharge, memory margin, firing voltage, and electrode voltage of the sustaining pulse were obtained from the voltage transfer curve. The voltage transfer curve could be used to optimize the discharge cell design.
Photoluminescence of InAs quantum dots grown on GaAs surface
Authors: Wang J. Z. , Yang Z. , Yang C. L. , Wang Z. G.
Published: July 2002

InAs quantum dots (QDs) grown on GaAs surface are investigated. The observed abnormal photoluminescence (PL) properties, including extremely sharp high-energy peaks, almost temperature-independent linewidth, and fast thermal quenching, are discussed in terms of the strong quantum confinement effects due to the absence of a cap layer and the lack of carrier redistribution channel caused by the small number of QDs capable of contributing to PL and the high-density surface defects.
Enhanced reflectivity contrast in confocal solid immersion lens microscopy
Authors: Karrai Khaled , Lorenz Xaver , Novotny Lukas
Published: July 2002

The reflected image of a diffraction limited focused spot is investigated using confocal solid immersion microscopy. We find that the spot’s image shows aberrations when reflected off objects with optical indexes lower than that of the solid immersion lens (SIL) material. We demonstrate that such aberrations are only apparent and that the actual size of the spot at the SIL/object interface remains diffraction limited. The aberrations are due to lateral waves at the SIL surface. These von Schmidt waves originate from the total internal reflected components of a diverging spherical wave front. We make use of this image aberration in conjunction with the spatial filtering inherent to confocal microscopy in order to dramatically enhance the optical contrast of objects with low optical indexes.
Low-resistance ohmic contacts on p-type GaN using Ni/Pd/Au metallization
Authors: Chu Chen-Fu , Yu C. C. , Wang Y. K. , Tsai J. Y. , Lai F. I. , Wang S. C.
Published: July 2002

In this letter, a low-resistance ohmic contact on p-type GaN using an alloy of Ni/Pd/Au is reported. The Mg doped p-type GaN samples were grown by metalorganic chemical vapor deposition with a carrier concentration of 4.1×1017 cm−3. The as-grown Mg doped samples were deposited with Ni (20 nm)/Pd (20 nm)/Au (100 nm) and then annealed in air, nitrogen, and oxygen ambient conditions at different annealing temperatures ranging from 350 to 650 °C. Linear I–V ohmic characteristics were observed with specific resistance as low as 1.0×10−4 Ω cm2 for the samples annealed in oxygen atmosphere. Similar contact metal composition was also deposited on Be-implanted p-type GaN samples with a carrier density of 8.1×1019 cm−3. Without further annealing process, the samples show good ohmic contact with a lowest specific resistance of 4.5×10−6 Ω cm2.
Anisotropic spectra in two-dimensional turbulence on the surface of a rotating sphere
Authors: Huang Huei-Ping , Galperin Boris , Sukoriansky Semion
Published: July 2002

The anisotropic characteristics of small-scale forced 2D turbulence on the surface of a rotating sphere are investigated. In the absence of rotation, the Kolmogorov k−5/3 spectrum is recovered with the Kolmogorov constant CK≈6, close to previous estimates in plane geometry. Under strong rotation, in long-term simulations without a large-scale drag, a −5 slope emerges in the vicinity of the zonal axis (kx→0), while a −5/3 slope prevails in other sectors far away from the zonal axis in the wave number plane. This picture is consistent with the new flow regime recently simulated by Chekhlov et al. [Physica D 98, 321–334 (1995)] and Smith and Waleffe [Phys. Fluids 11, 1608–1622 (1999)] on the beta plane. The concentration of energy in the zonal components and breaking of isotropy are caused by the strongly anisotropic spectral energy transfer and the stabilization of zonal mean flow by the meridional gradient of the planetary vorticity. The sharp tilt-up of the spectrum along the zonal axis was qualitatively understood through the scale-dependent stability property of the zonal flow. Under planetary rotation, the capacity for the zonal jets to hold energy and remain stable sharply increases with an increase of the meridional scale of the jets. In our simulations that were virtually inviscid at the large scales, the energy spectrum along the zonal axis tilts up all the way to the largest possible scale, indicating an apparent up-scale energy “cascade” along the zonal axis. This apparent up-scale cascade corresponds to a process of continuous mergers of zonal jets that does not cease until reaching the largest scale. This picture is consistent with the inviscid scenario for jet merging discussed by Manfroi and Young [J. Atmos. Sci. 56, 784–800 (1999)]. It contrasts the viscous scenario (for flows under the influence of a constant bottom drag) simulated in several previous studies, in which a distinct and finite jet scale emerges asymptotically.
Optical losses in porous silicon waveguides in the near-infrared: Effects of scattering
Authors: Ferrand P. , Romestain R.
Published: July 2002

Benefitting from the long path inside planar waveguides, we have investigated the optical losses of porous silicon, in the continuous 0.8–1.6 μm (0.77–1.55 eV) range. The obtained values, typically a few cm−1, are 1 order of magnitude larger than “pure” absorption losses measured previously. The other main sources of loss, including scattering on both interface roughness and nanocrystallites, are invoked. Calculations give the same order of magnitude as measurements. We also detected scattered light close to the direct beam.
Reaction of the ethynyl radical, C2H, with methylacetylene, CH3CCH, under single collision conditions: Implications for astrochemistry
Authors: Stahl Frank , von Ragué Schleyer Paul , Bettinger H. F. , Kaiser R. I. , Lee Y. T. , Schaefer H. F.
Published: July 2002

The reaction between the ethynyl radical, C2H (X 2Σ+), and methylacetylene (X 1A1′), which yields ethynylallene, pentadiyne, and butadiyne, has been studied at the density functional (B3LYP/6-311+G**) and coupled cluster (coupled-cluster single double perturbative triple/cc-pVTZ) levels of theory. These results agree with data from crossed molecular beam experiments where ethynylallene (10) and pentadiyne (13) have been observed. The C2H(1) radical initially attacks the π system of methylacetylene (2) without an entrance barrier to form Z-1-ethynylpropen-2-yl (3) or Z-2-ethynylpropen-1-yl (4) in highly exothermic reactions. Geometric considerations as well as the computed enthalpies suggest Z-1-ethynylpropen-2-yl (3) to be the dominant initial intermediate. Assuming single collision conditions as found in cold molecular clouds in the interstellar medium and distinct planetary atmospheres, numerous rearrangements may ensue the initial reaction step before ejection of a hydrogen atom or a methyl group releases the accumulated reaction energy.
Investigating material and functional properties of static random access memories using cantilevered glass multiple-wire force-sensing thermal probes
Authors: Dekhter Rimma , Khachatryan Edward , Kokotov Yuri , Lewis Aaron , Kokotov Sophia , Fish Galina , Shambrot Yefim , Lieberman Klony
Published: July 2002

A double-wire cantilevered glass probe has been produced for scanned probe microthermal, resistivity, and topographic measurements. The structure has many potentially unique properties for scanned probe microscopy and other nanotechnological measurements. In this letter, a double Pt wire probe was fused at the tip and applied to thermal resistive measurements. The probe operation is based on the linear dependence of Pt resistance on temperature. Most microscopic structures are composed of a variety of materials. In the present study, the features of a static random access memory chip are investigated. Such memory chips are composed of materials such as dielectrics, metals, and semiconductors. We demonstrate that these samples, which are prepared using a chemical–mechanical polishing procedure and have essentially no surface topography, can be inspected using the thermal conductivity, resistivity, and topographic sensitivity of these probes.
Raman-spectroscopic evaluation of the long-range order in Ba(B1/3′B2/3″)O3 ceramics
Authors: Moreira Roberto L. , Matinaga Franklin M. , Dias Anderson
Published: July 2002

Ordered Ba(B1/3′B2/3″)O3 ceramics are currently used as dielectric resonators for microwave and millimeter wave technologies. The degree of ordering is generally determined by x-ray techniques. In this work, we demonstrate that Raman spectroscopy can be used to evaluate the degree of long-range order of these materials. We also show how varying the degree of order of samples allows partial assignment of the optical vibrational modes.
Enhanced depth resolution in terahertz imaging using phase-shift interferometry
Authors: Johnson Jon L. , Dorney Timothy D. , Mittleman Daniel M.
Published: July 2002

We describe an imaging technique for few-cycle optical pulses. An object to be imaged is placed at the focus of a lens in one arm of a Michaelson interferometer. This introduces a phase shift of approximately π between the two arms of the interferometer, via the Gouy phase shift. The resulting destructive interference provides a nearly background-free measurement, and a dramatic enhancement in depth resolution. We demonstrate this using single-cycle pulses of terahertz radiation, and show that it is possible to resolve features thinner than 2% of the coherence length of the radiation. This technique could have important applications in low-coherence optical tomographic measurements.
Spin wave quantization in laterally confined magnetic structures (invited)
Authors: Jorzick J. , Krämer C. , Demokritov S. O. , Hillebrands B. , Bartenlian B. , Chappert C. , Decanini D. , Rousseaux F. , Cambril E. , So/ndergard E. , Bailleul M. , Fermon C. , Slavin A. N.
Published: July 2002

An overview of the current status of the study of spin wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the most important issues; the quantization of localized spin waves due to the in-plane confinement of spin waves in elements, dipolar coupling between the quantized modes, and the localization of the modes within rectangular elements due to an inhomogeneous demagnetizing field.
Photoluminescence and structural studies on extended defect evolution during high-temperature processing of ion-implanted epitaxial silicon
Authors: Giri P. K. , Coffa S. , Raineri V. , Privitera V. , Galvagno G. , La Ferla A. , Rimini E.
Published: July 2002

Low-temperature photoluminescence (PL) spectroscopy, in conjunction with transmission electron microscopy (TEM) and optical microscopy (OM) have been carried out to investigate the origin of radiative recombination from various extended defects that evolve during high-temperature processing of ion-implanted epitaxial silicon. From PL studies on N2-annealed samples, we provide spectroscopic evidence of precipitation of the implanted impurities well below the solid-solubility limit. This result is being supported by observations from secondary ion mass spectrometry and spreading resistance profiling of the implanted ions. Cross sectional TEM analyses on these samples reveal 〈111〉-oriented precipitates located in a region containing a high dislocation density. Postimplantation annealing in oxygen ambient results in the formation of dislocations and oxidation-induced stacking faults (OISF). A systematic analysis of PL spectra on different-implanted and preannealed samples, in conjunction with TEM and OM analyses, reveals that the conventionally observed dislocation-related D1 and D2 lines in the PL spectrum is not a characteristic of the OISF, but of the dislocations only. It is shown that the OISF acts as a nonradiative channel for luminescence in silicon. Various other sources of nonradiative channels in silicon are also presented and the efficacy of photoluminescence technique in the characterization of process-induced defects in silicon is discussed.
Static and dynamic scattering from ternary polymer blends: Bicontinuous microemulsions, Lifshitz lines, and amphiphilicity
Authors: Morkved Terry L. , Stepanek Petr , Krishnan Kasiraman , Bates Frank S. , Lodge Timothy P.
Published: July 2002

Ternary polymer blends, comprising two homopolymers and the corresponding diblock copolymer, have been examined by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Two chemical systems have been employed: one consisting of polyethylethylene, polydimethylsiloxane, and poly(ethylethylene-b-dimethylsiloxane), and another containing polyethylenepropylene, polyethyleneoxide, and poly(ethylenepropylene-b-ethylene oxide). The molecular weights and compositions were chosen to emphasize the region of the phase prism dominated by the bicontinuous microemulsion (BμE) phase; the homopolymer molecular weights and volume fractions were approximately equal. The SANS intensity was compared quantitatively with the Teubner–Strey structure factor, and interpreted via the amphiphilicity factor fa. The transition from a fully disordered mixture at higher temperatures to a well-developed BμE upon cooling did not correlate well with either the disorder line (fa=+1) or the total monomer Lifshitz line (fa=0). However, DLS provided a clear signature of this transition, via a distinct maximum in the temperature dependence of the dynamic correlation length. We hypothesize that this maximum is closely correlated to the homopolymer/homopolymer Lifshitz line. The structure of the interfaces in the BμE was further examined in terms of the curvature and the copolymer coverage, as functions of copolymer concentration and temperature.
Preparation and magnetic properties of the double-perovskite A2FeMoO6 (A=Ca, Sr, Ba) polycrystals with nanometer-scale particles
Authors: Song W. H. , Dai J. M. , Ye S. L. , Wang K. Y. , Du J. J. , Sun Y. P.
Published: July 2002

Polycrystalline double perovskites A2FeMoO6 (A=Ca, Sr, Ba) with nanometer-scale grain size have been synthesized using a sol–gel method. The grain size of the samples is controlled within nanometer scale by sintering at different temperatures. The phase purity and the crystal structure of the samples are analyzed by x-ray powder diffraction measurements. The electrotransport and magnetic properties are also measured in this article. In comparison with the large grain samples with micrometer scale, the nanometer-scale grain samples have different magnetic properties, such as lower magnetic transition temperatures and larger magnetoresistance, which can be explained in terms of size effect.
Rupture of thin viscous films by van der Waals forces: Evolution and self-similarity
Authors: Vaynblat Dimitri , Lister John R. , Witelski Thomas P.
Published: July 2002

The van der Waals driven rupture of a freely suspended thin viscous sheet is examined using a long-wavelength model. Dimensional analysis shows the possibility of first-type similarity solutions in which the dominant physical balance is between inertia, extensional viscous stresses and the van der Waals disjoining pressure, while surface tension is negligible. For both line rupture and point rupture the film thickness decreases like (t*−t)1/3 and the lateral length scale like (t*−t)1/2, where t*−t is the time remaining until rupture. In each geometry these scalings are confirmed by numerical simulations of the time-dependent behavior, and a discrete family of similarity solutions is found. The “lowest-order” mode in the family is the one selected by the time-dependent dynamics.
Electron collisions with octafluorocyclobutane, c-C4F8
Authors: Winstead Carl , McKoy Vincent
Published: July 2002

We present calculated cross sections for elastic and inelastic collisions of low-energy electrons with octafluorocyclobutane, c-C4F8. The integral elastic cross section displays a rich resonance structure, which we analyze in terms of temporary trapping in virtual valence orbitals. The differential elastic cross sections compare well with recent measurements at energies where the approximations used in the calculations are expected to be valid. Integral and differential cross sections for electron-impact excitation of the lowest singlet and triplet excited states were obtained. We relate the small magnitude of the inelastic integral cross sections and the unusual form of the inelastic differential cross sections to the symmetries of the electronic states involved in the transition.
Analysis of the recombination probability of geminate triplet radical ion pairs at single encounter by means of magnetic field effects
Authors: Lavrik N. L.
Published: July 2002

The recombination probability of geminate triplet radical ion pairs at single encounter α has been determined by means of the proposed method of experimental data processing based on the Noyes theory regarding the Onsager concept. The method is based on measuring magnitude of magnetic field effect in the yield of the geminate recombination products of triplet pairs depending on medium viscosity. The method was used to determine α for the recombination of geminate triplet radical ion pair {pyrene−+N,N-diethylaniline+}3 in methanol. It amounts to 10−4, which is four orders of magnitude smaller than that obtained by the standard kinetic method. This is the basis upon which to conclude that even with the account of Onsager correction for polar media the use of Noyes theory for α determination is incorrect.
Nonlinear Schrödinger-type equations from multiscale reduction of PDEs. II. Necessary conditions of integrability for real PDEs
Authors: Calogero F. , Degasperis A. , Xiaoda Ji
Published: July 2002

The multiscale method has been applied in J. Math. Phys. 41, 6399 (2000) to evolution PDEs to obtain various nonlinear Schrödinger-type equations. Here these results are exploited to derive necessary conditions of integrability for nonlinear, real, evolution PDEs in 1+1 dimensions. These conditions are given by explicit formulas, and several examples are detailed.