bluuebunny/crossref_metadata_2025 huggingface dataset:
Pump-probe spectroscopy of band tail states in metalorganic chemical vapor deposition-grown InGaN
Authors: Schmidt T. J. , Cho Yong-Hoon , Gainer G. H. , Song J. J. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: July 2002

Nanosecond nondegenerate optical pump-probe experiments have been performed on InGaN thin films and InGaN/GaN multiple quantum wells. Bleaching of absorption of the localized band tail states was observed with increasing excitation density (Iexc) of the pump pulse. The dynamics of the bleaching was found to depend on the localization depth of the band tail states and on Iexc. With high Iexc, large blueshifts in the spontaneous emission luminescence peaks were also observed, the magnitude of which was again found to depend on the localization depth of the band tail states. Stimulated emission is observed from the samples with increasing Iexc and correlates with significant changes in the behavior of the absorption bleaching. The observed bleaching dynamics of the band tail states are well explained by considering the effective lifetime of the band tail states as measured by time-resolved photoluminescence experiments.
Controlled ink-jet printing and deposition of organic polymers and solid particles
Authors: Perçin Gökhan , Lundgren Thomas S. , Khuri-Yakub Butrus T.
Published: July 2002

In this letter, we present a technique for the deposition of inks, organic polymers and solid particles, using a fluid ejector. The ejector design is based on a flextensional transducer that excites axisymmetric resonant modes in a clamped circular membrane. It is constructed by bonding a thin piezoelectric annular ring to a thin, edge supported, circular membrane. Liquids or solid particles are placed behind one face of the membrane which has a small orifice (50–200 μm diam) at its center. By applying an ac signal across the piezoelectric element, continuous or drop-on-demand ejection of photoresist (Shipley Microposit S1400-21, S1400-27, S1805, and S1813), oil-based ink, water, or talcum powder [Mg3Si4O10(OH)2] has been achieved. Successful deposition of photoresist has been accomplished without spinning, and thus without waste. Patterning of 10 μm features, by baking, exposure, and developing, has revealed no defects in the deposition process. A boundary integral method was used to numerically simulate drop formation from the vibrating orifice. Simulations have been used to optimize ejection performance.
Nonferroelectric epitaxial Sr–Bi–Ta oxide thin film with a high dielectric constant
Authors: Hyun S. J. , Park B. H. , Bu S. D. , Jung J. H. , Noh T. W.
Published: July 2002

Two kinds of epitaxial Sr–Bi–Ta oxides were grown on La0.5Sr0.5CoO3(001)/LaAlO3(001) substrates by pulsed laser deposition. At a temperature higher than 650 °C, c-axis oriented SrBi2Ta2O9 films could be deposited epitaxially. At 550 °C, an epitaxial film with a cubic fluorite-like phase could be grown. Although the film with the cubic phase does not show ferroelectric properties, it has good electrical properties which are suitable for high capacitance applications.
Incongruent transfer related to surface segregation in pulsed-laser-deposited La–Ca–Mn–O films
Authors: Zhang Wei , Wang Xiaoru , Boyd Ian W.
Published: July 2002

A range of La–Ca–Mn–O films have been grown using pulsed-laser deposition on hot Si substrates. The composition of the films is found to be strongly dependent on substrate temperature (Ts) with the calcium and oxygen content being significantly reduced at high Ts (>700 °C). The kinetic energies of the various ions in the laser generated plume are found to be distributed in the 10–100 eV range, high enough to induce resputtering from deposited films. Surface segregation was also observed in the films grown at high Ts (>700 °C), resulting in a Ca-rich surface. Preferential resputtering of these surface Ca-rich segregated samples, driven by a mixture of high energy ion irradiation and thermally activated processes, is proposed to explain the observed phenomena.
Recombination processes in InxGa1−xN light-emitting diodes studied through optically detected magnetic resonance
Authors: Glaser E. R. , Kennedy T. A. , Carlos W. E. , Ruden P. P. , Nakamura S.
Published: July 2002

Optically detected magnetic resonance (ODMR) has been observed on photoluminescence from InGaN light-emitting diodes (LEDs) under low photoexcitation conditions. The samples have the usual p-i-n structure but without etching or metallizations. Distinct ODMR features from the recombining electron and hole are found with strength that indicates significant charge separation and long lifetimes (>100 ns). The electron and hole g tensors are determined for green and extra-blue LEDs. The recombination is assigned to electrons in the InGaN quantum well (QW) and holes either bound at Mg acceptors outside the well or localized at potential minima in the QW but spatially separated from the electrons.
Effects of mechanical stress on electromigration-driven transgranular void dynamics in passivated metallic thin films
Authors: Gungor M. Rauf , Maroudas Dimitrios , Gray Leonard J.
Published: July 2002

The combined effects of mechanical stress and surface electromigration on the dynamics of transgranular voids in passivated metallic thin films are analyzed based on self-consistent dynamical simulations. Depending on the strength of the electric and stress fields, void morphological instabilities can lead to film failure by propagation from the void surface of either faceted slits or finer-scale crack-like features. Most importantly, there exists a narrow range of applied stress for given strength of electric field over which slit formation can be inhibited completely.
Quantum wells due to ordering in GaInP
Authors: Hsu Y. , Stringfellow G. B. , Inglefield C. E. , DeLong M. C. , Taylor P. C. , Cho J. H. , Seong T.-Y.
Published: July 2002

CuPt ordering results in a reduction of the band-gap energy of GaInP. Thus, heterostructures and quantum wells can be produced by simply varying the order parameter, without changing the solid composition. Changes in the order parameter can be induced by changes in growth conditions. The disordered/ordered/disordered quantum wells described here are grown by changing the PH3 flow rate. Transmission electron microscopy results show that the quantum wells produced in this way are clearly defined, with abrupt interfaces. Low-temperature photoluminescence spectra show distinct peaks from quantum wells (QWs) of different widths. The QW photoluminescence peak energy increases with decreasing well width due to quantum size effects. The difference in band-gap energy between the ordered and disordered single layers is determined from photoluminescence excitation spectroscopy to be 0.06 eV.
Probing submicron nanomagnets by magneto-optics
Authors: Cowburn R. P. , Koltsov D. K. , Adeyeye A. O. , Welland M. E.
Published: July 2002

A hybrid magneto-optical magnetometer and optical microscope has been designed and constructed for probing the magnetic properties of submicron nanomagnets. 10-nm-thick square nanomagnets have been fabricated individually and in small arrays from Ni80Fe14Mo5 (“supermalloy”) by electron-beam lithography. Hysteresis loops with a good signal-to-noise ratio have been obtained from individual nanomagnets as small as 400 nm and from (5 μm)2 arrays of nanomagnets ranging in size from 500 to 75 nm.
Microwave characterization of Nd0.67Sr0.33MnO3−x thin films for magnetically tunable filters
Authors: Wosik J. , Xie L.-M. , Strikovski M. , Miller J. H. , Przyslupski P.
Published: July 2002

We report on the microwave properties of Nd0.67Sr0.33MnO3−x (NSMO) and NSMO/YBa2Cu3O7−x thin film heterostructures. The quality factor (Q) and center frequency of a 13 GHz shielded dielectric cavity resonator, with the film comprising one surface, were measured as functions of temperature and direct current magnetic field. The Q versus field data was theoretically simulated using the Landau–Lifschitz–Gilbert dynamic permeability equation thus demonstrating that the microwave losses are determined by the ferromagnetic properties of the films. Our results indicate that the field-dependent permeability μ(B) of NSMO films holds the potential to create magnetically tunable microwave devices.
Annealing effects and Te mixing in CdTe/CdS heterojunctions
Authors: Soo Y. L. , Huang S. , Kao Y. H. , Compaan A. D.
Published: July 2002

Angular dependence of x-ray fluorescence techniques have been employed to investigate the effects of thermal annealing at various temperatures ranging from 340 to 387 °C on a series of CdTe/CdS heterojunctions. Changes in depth profiles of Te and Cd atoms in the CdS layer as a result of heat treatment has been observed. A simple model is proposed for comparison of the ratio of Te Kα to Cd Kα fluorescence yield at different annealing temperatures. Our results suggest that the concentration of migrated Te atoms in the CdS layer increases with the annealing temperature, and the Te/Cd ratio in the CdS layer could even become greater than that in CdTe with annealing temperatures higher than 370 °C.
Effects of the surface Cu2−xSe phase on the growth and properties of CuInSe2 films
Authors: Niki S. , Fons P. J. , Yamada A. , Lacroix Y. , Shibata H. , Oyanagi H. , Nishitani M. , Negami T. , Wada T.
Published: July 2002

Drastic changes in average molecularities (m=Cu/In) from m≫1 to m=0.92–0.93 and in hole concentrations from p≫1019 cm−3 to as low as p=7.5×1016 cm−3 have been observed in molecular beam epitaxy grown CuInSe2 after selective etching of the Cu–Se phase by a KCN aqueous solution; high hole concentrations and Cu-excess compositions of the as-grown films were attributed to the Cu–Se phase. On the other hand, well-defined photoluminescence emissions were found characteristic of intrinsic CuInSe2. The presence of the Cu–Se phase made possible the growth of high-quality CuInSe2 epitaxial films at a temperature well below the melting point of any Cu–Se compound. Surface topology measurements showed that the surface of the as-grown films was not fully covered by Cu–Se grains, leaving holes with depths of 200–300 nm after KCN etching. The enhanced two-dimensional growth and the reduced defect concentration imply that a very thin Cu-excess surface layer controls the growth of CuInSe2 when grown under Cu-excess conditions.
Room-temperature photoluminescence, contactless electroreflectance, and x-ray characterization of a double-side delta-doped GaAlAs/InGaAs high electron mobility transistor structure
Authors: Huang Y. S. , Sun W. D. , Malikova L. , Pollak Fred H. , Ferguson I. , Hou H. , Feng Z. C. , Ryan T. , Fantner E. B.
Published: July 2002

Using room-temperature photoluminescence and contactless electroreflectance we have characterized a double-side delta-doped Ga0.8Al0.2As/In0.2Ga0.8As pseudomorphic high electron mobility transistor structure fabricated by metal-organic chemical vapor deposition. Signals have been observed from every region of the sample making it possible to evaluate In and Al compositions, channel width, and two-dimensional electron gas density as well as the properties of the GaAs/GaAlAs superlattice buffer layer. The optical determination of the In composition and channel width are in good agreement with an x-ray measurement.
GaN evaporation in molecular-beam epitaxy environment
Authors: Grandjean N. , Massies J. , Semond F. , Karpov S. Yu. , Talalaev R. A.
Published: July 2002

GaN(0001) thick layers were grown on c-plane sapphire substrates by molecular-beam epitaxy using NH3. The evaporation of such GaN layers in vacuum was studied as a function of substrate temperature. In situ laser reflectivity was used to quantitatively measure the decomposition rate of the GaN(0001) plane. It is nearly zero below 750 °C, increases rapidly above 800 °C, and reaches 1 μm/h at 850 °C. An activation energy of 3.6 eV is deduced for the thermal decomposition of GaN in vacuum. The evaporation rate as a function of the incident NH3 flux was also investigated for different substrate temperatures. A kinetic model is applied for the interpretation of the experimental results.
Four-wave mixing of strong picosecond optical pulses in passive semiconductor waveguides
Authors: Tang J. M. , Shore K. A.
Published: July 2002

Four-wave mixing (FWM) of strong picosecond optical pulses in passive semiconductor waveguides is investigated numerically, taking into account two-photon absorption (TPA), ultrafast nonlinear refraction, probe depletion, and cross-gain modulation, as well as interband and intraband carrier processes. Simulations show good agreement with published experimental measurements, and indicate that interband and intraband nonlinear mixing and TPA-induced pulse reshaping play significant roles in determining conjugate pulse behaviors for strong pulse energy cases. In particular, for increased pulse energy, an increase in FWM conversion efficiency is predicted, and an optimum pulse width is identified.
Electrical properties of covalently linked silicon/polypyrrole junctions
Authors: Vermeir Inge E. , Kim Namyong Y. , Laibinis Paul E.
Published: July 2002

Electrodeposited polypyrrole films were formed on chemically modified hydrogen-terminated silicon surfaces that expose tethered pyrrole units. Semiconductor/polypyrrole junctions on the native and modified substrates exhibit diode-like characteristics, with those on the latter substrate exhibiting higher current densities and better ideality factors. Impedance measurements revealed that the improved electrical properties of junctions on the modified substrates were not due to a change in barrier height but rather a consequence of incorporating sites on the silicon surface where the polymer and semiconductor have direct contact.
Prediction of room-temperature high-thermoelectric performance in
n
-type La(Ru1−xRhx)4Sb12
Authors: Fornari Marco , Singh David J.
Published: July 2002

First-principles calculations are used to investigate the band structure and the transport-related properties of unfilled and filled 4d skutterudite antimonides. The calculations show that, while RhSb3 and p-type La(Rh,Ru)4Sb12 are unfavorable for thermoelectric application, n-type La(Rh,Ru)4Sb12 is very likely a high figure-of-merit thermoelectric material in the important temperature range 150–300 K.
Transmitted signal detection of optical disks with a superresolution near-field structure
Authors: Nakano Takashi , Sato Akira , Fuji Hiroshi , Tominaga Junji , Atoda Nobufumi
Published: July 2002

We observed transmitted signals of phase change marks recorded in super-resolution near-field structure (super-RENS) disks under high speed disk rotation. The observed minimum mark size was 81 nm and the carrier to noise ratio was about 6 dB at a constant linear velocity of 4.8 m/s. The mark size was far beyond the diffraction limit of an optical pickup with a wavelength of 635 nm and a numerical aperture of 0.6. It was clear that near-field scattering actually occurred in super-RENS disks.
Proton irradiation effects on the intersubband transition in GaAs/AlGaAs multiple quantum wells with bulk or superlattice barriers
Authors: Manasreh M. O. , Ballet P. , Smathers J. B. , Salamo G. J. , Jagadish Chennupati
Published: July 2002

The optical absorption spectra, measured at either 77 or 300 K, of the intersubband transition in Si-doped GaAs/AlGaAs multiple quantum wells were studied before and after 1 MeV proton irradiation. The intersubband transition in samples with superlattice barriers was completely washed out after irradiation with doses as low as 1.0×1014 cm−2. The total integrated areas of the intersubband transitions were studied as a function of doses. It was observed that the intersubband transitions in samples with superlattice barriers degrade at a faster rate as compared to those transitions in samples with bulk barriers.
Time-resolved photoluminescence studies of free and donor-bound exciton in GaN grown by hydride vapor phase epitaxy
Authors: Bunea G. E. , Herzog W. D. , Ünlü M. S. , Goldberg B. B. , Molnar R. J.
Published: July 2002

Time-resolved photoluminescence (PL) spectroscopy was used to study the radiative recombination of free and donor-bound excitons in unintentionally doped GaN grown by hydride vapor phase epitaxy. Low temperature (4 K), time-integrated PL spectra identified the free exciton (A), the donor-bound exciton peak ∼6 meV below, and the acceptor-bound exciton ∼20 meV below the free exciton peak. A radiative recombination lifetime of 295 ps for the free exciton and 530 ps for donor-bound exciton were found at 4 K. The decay of the free exciton remained single exponential to room temperature, with an increase in lifetime to 530 ps, consistent with the thermal excitation of exciton states.
Gated negative-effective-mass ballistic terahertz generators
Authors: Korshak A. N. , Gribnikov Z. S. , Vagidov N. Z. , Mitin V. V.
Published: July 2002

We consider gate control of terahertz generation in planar ballistic diodes with a negative-effective-mass section in a dispersion relation of current carriers in a current-conducting channel. Such a generation in ballistic p+pp+ or n+nn+ diodes occurs as a result of plasma instability development and self-organization of a regular oscillation regime. Conditions of existence and oscillation frequencies are calculated. The gate can also serve as an oscillation-collecting electrode. We consider double-gate designs, side by side with conventional single-gate designs. The double-gate devices allow us to separate circuits for direct and high-frequency currents.
Negative ion enhancement in caesium-seeded hydrogen discharges—a volume or surface effect?
Authors: Steen P. G. , Graham W. G.
Published: July 2002

Measurements of plasma parameters, including H− ion densities, made in conjunction with wall temperature, visible and vacuum ultraviolet emission spectroscopy verify that there is little caesium in the plasma volume of the H− ion source. Surface work function measurements indicate that there is significant caesium coverage of the inner walls of the ion source. It is found that, as the work function of a test surface decreases due to caesium seeding, the H− ion fraction in the discharge volume increases. These observations combine to indicate that, in the present source, the H− ion enhancement mechanism is a surface dominated effect.
Arsenic clusters on the surface of vertically aligned InAs islands on GaAs substrate by annealing
Authors: Fan T. W. , Mo Q. W. , Lin F. , Wang Z. G. , Zhang W.
Published: July 2002

The formation of arsenic clusters in a system of vertically aligned InAs quantum islands on GaAs during thermal annealing under As overpressure has been investigated by transmission electron microscopy (TEM) and Raman scattering. Semicoherent arsenic clusters, identified by TEM examination, have been formed on the surface of the GaAs capping layer. The existence of arsenic precipitates is also confirmed by Raman spectra, showing new peaks from the annealed specimen at 256 and 199 cm−1. These peaks have been ascribed to A1g and Eg Raman active phonons of crystalline arsenic. The phenomenon can be understood by a model of strain-induced selected growth under As overpressure.
Hydrogen in the mechanically prepared nanostructured graphite
Authors: Orimo S. , Majer G. , Fukunaga T. , Züttel A. , Schlapbach L. , Fujii H.
Published: July 2002

Nanostructured graphite was prepared by mechanical milling under hydrogen atmosphere. Several samples obtained after different milling times were systematically examined to get fundamental information about the structures and hydrogen concentrations. After the expansion of the graphite interlayer, the long-range ordering of the interlayer disappears continuously with increasing milling time. The hydrogen concentration reaches up to 7.4 mass % (CH0.95) after milling for 80 h. Judging from the radial distribution function determined by the neutron diffraction measurement, there are two types of deuterium coordinations: deuterium atoms in the graphite interlayers and that with the CDx covalent bonds, respectively.
Interface properties of AlxGa1−xN/AlN heterostructures from optical waveguiding information
Authors: Dogheche E. , Belgacem B. , Remiens D. , Ruterana P. , Omnes F.
Published: July 2002

A complete optical characterization using the prism coupling technique is proposed for AlGaN/AlN heterostructures grown on sapphire substrates by metalorganic vapor-phase epitaxy. In this study, we have qualified the film behavior and the substrate-to-layer interface directly from the measured optical data. The experimental and theoretical approach used for this purpose is described in detail. The results have clearly shown essential changes in the refractive index profile at the interface, which may be related to structural defects, and indicate a good agreement with the trends observed by transmission electron microscopy analysis.
Strongly reduced bias dependence in spin–tunnel junctions obtained by ultraviolet light assisted oxidation
Authors: Boeve H. , Girgis E. , Schelten J. , De Boeck J. , Borghs G.
Published: July 2002

For future implementation of ferromagnetic tunnel junctions, we need a better understanding of the influence of the insulating barrier preparation method on the junction resistance, tunnel magnetoresistance (TMR), and its voltage bias dependence. In this letter, we focus on the bias dependence of junctions (Co–Al2O3–Ni80Fe20) prepared by ultraviolet light assisted in situ oxidation in an O2 ambient. For an initial Al thickness of 1.3 nm, the resistance times area product of the junctions is 60 kΩ μm2, while showing up to 20% TMR at 5 mV bias. The decrease of TMR with bias voltage up to 1 V is remarkably small leading to V1/2, for which half of the low-bias TMR remains, well over 0.6 V.
Transient electroluminescence under double voltage pulse: Charge accumulation in light-emitting devices based on alizarin violet
Authors: Das Sharmistha , Pal Amlan J.
Published: July 2002

Transient characteristics of organic light-emitting devices with alizarin violet as emitting material have been studied. A sequence of two voltage pulses separated by a time delay was applied. From the transient response of electroluminescence, the effects of accumulated and injected carriers have been separated out. During the first voltage pulse, the intrinsically accumulated charges generated a peak in luminance, which was absent during the latter pulses. The separation time between the two pulses has been varied to study the relaxation dynamics of the accumulated carriers. By varying the molar concentration of the emitting material, we have studied the parameters related to electron barrier modification and discussed the contribution of accumulated carriers in device operation.
Probing local leakage current and ferroelectricity of Pb(Zr0.53, Ti0.47)O3/YBa2Cu3O7−x heterostructure by a modified atomic force microscope
Authors: Xie Z. , Luo E. Z. , Xu J. B. , Wilson I. H. , Peng H. B. , Zhao L. H. , Zhao B. R.
Published: July 2002

In this letter we present an investigation on probing leakage current and ferroelectricity of Pb(Zr, Ti)O3(PZT)/YBa2Cu3Ox heterostructure using a modified atomic force microscopy. Inhomogeneities of leakage current and ferroelectric properties were concurrently mapped and quantitatively studied on a nanometer scale. It shows that microscopically the leakage current can be described by a model based on space-charge-limited current.
Intersubband absorption from InGaAlAs/InAlAs multiple quantum-well structures grown by molecular-beam epitaxy
Authors: Zhang D. H. , Zhang W. M. , Osotchan T. , Zhang P. H. , Yoon S. F. , Shi X.
Published: July 2002

The intersubband absorption from the InGaAlAs/InAlAs multiple quantum-well structures, lattice matched to InP, for the long-wavelength infrared detection has been investigated. It is found that the strong absorption resulting from the bound-to-bound transition in the quaternary material is observable and the wavelength of the absorption varies with the well width while the barrier width remains unchanged. The photoluminescence results indicate that the absorption originates from the transition of the electrons from the ground energy level to the first excited energy level in the conduction band of the well material. Our experimental results are also in good agreement with the theoretical estimation based on the simple finite barrier model.
Thermally stable, soft FeXN thin films
Authors: Liu Yi-Kuang , Kryder M. H.
Published: July 2002

Significant improvement of the thermal stability in FeXN (X=Al, Zr, Ta,…) thin films is reported. Sputtered at the reduced target–substrate spacing of 38 mm, 200-nm-thick thermally stable FeXN thin films are obtained. They have hard-axis coercivity ≈ 0.1–2.0 Oe, easy-axis coercivity ≈1.5–3.0 Oe, Hk≈8–16 Oe, and Bs≈19–20 kG. Results of transverse-field annealing experiments in a uniform field of 700 Oe show no significant change of magnetic properties at 150 °C for 3 h. At 150 °C for 24 h Hk decreases by 2–4 Oe. Their easy/hard axes do not rotate and the coercivity remains almost unchanged. This superior thermal stability of FeXN films is very promising for high-moment write heads.
Biexciton emission from high-quality ZnO films grown on epitaxial GaN by plasma-assisted molecular-beam epitaxy
Authors: Ko H. J. , Chen Y. F. , Yao T. , Miyajima K. , Yamamoto A. , Goto T.
Published: July 2002

We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for bulk ZnO. The biexciton binding energy is estimated to be 15 meV, in agreement with previous results. At the higher excitation regime, the emission line due to exciton–exciton scattering dominates the PL spectrum.
Luminescence of dye-doped polymer films induced by corona discharge
Authors: Suzuki Hironori , Zou De Chun , Izumi Takao , Yamamoto Hitoshi , Tao Xu Tang , Watanabe Toshiyuki , Usui Hiroaki , Miyata Seizo
Published: July 2002

A type of luminescence was observed from fluorescent dye-doped polymer films by corona discharge excitation. The polymer film was deposited on a transparent electrode, and direct current corona discharge was caused in argon atmosphere using this electrode as the anode. Emission of three elemental colors, i.e., red, green, and blue, were obtained from polyvinilcarbazole films doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, 3-(2′-Benzothiazolyl)7-diethylaminocoumarin, and 1,1,4,4-tetraphenyl-1,3-butadiene, respectively. The drive current required for the luminance of 10 cd/m2 was as low as 100 μA/cm2 and the maximum quantum efficiency was 9.95 cd/A, which is nearly four orders of magnitude higher than that of the common injection type electroluminescent cells with the same material. The origin of this high efficiency can be attributed to the efficient hole blocking by the gas phase and also by concurrent effects of different luminescence processes including electroluminescence by charge recombination, photoluminescence by ultraviolet excitation, and cathodoluminescence by electron impact excitation.
Laminar phase flow for an exponentially tapered Josephson oscillator
Authors: Benabdallah A. , Caputo J. G. , Scott A. C.
Published: July 2002

Exponential tapering and inhomogeneous current feed were recently proposed as means to improve the performance of a Josephson flux flow oscillator. Extensive numerical results backed up by analysis are presented here that support this claim and demonstrate that exponential tapering reduces the small current instability region and leads to a laminar flow regime where the voltage wave form is periodic giving the oscillator minimal spectral width. Tapering also leads to an increased output power. Since exponential tapering is not expected to increase the difficulty of fabricating a flux flow oscillator, we suggest that this feature should be incorporated in future designs.
Development of reference states for use in absolute free energy calculations of atomic clusters with application to 55-atom Lennard-Jones clusters in the solid and liquid states
Authors: Amon L. M. , Reinhardt W. P.
Published: July 2002

In this paper four reference states allowing computation of the absolute internal free energies of solid and liquid clusters are introduced and implemented. Three of these are introduced for the first time. Two of these references are useful for highly fluctional liquidlike clusters while the other two are appropriate for more rigid solidlike clusters. These reference states are combined with a finite time variational method to obtain upper and lower bounds to the absolute free energies of clusters of Lennard-Jones (LJ) atoms, LJ4 and LJ55, allowing the efficiency of each of the four reference states to be elucidated. The optimal references are then applied to obtain upper and lower bounds to the internal free energies (the absolute free energy in the cluster center of mass frame) of LJ55 over a series of fixed temperatures including the solid–liquid coexistence regime. The reversible scaling method, recently introduced by de Koning, Antonelli, and Yip, is then used to extend the results over a continuous range of temperatures. Estimation of the rotational free energy allows comparisons to free energies of LJ55 in the nonrotating center of mass frame as estimated by Doye and Wales.
Comment on “A potential dependent polarizability?” [J. Chem. Phys. 96, 5289 (1992)]
Authors: Donald Kelling J. , Mulder Willem H. , Szentpály László v.
Published: July 2002

We investigate the validity of employing a potential dependent polarizability in a model to determine atom in molecule (AIM) polarizabilities. It is found that the assumption of an electric potential dependent polarizability leads to a contravention of the principle of conservation of energy.
Effect of initial conformation, flow strength, and hydrodynamic interaction on polymer molecules in extensional flows
Authors: Agarwal U. S.
Published: July 2002

Brownian dynamics simulations are used to study the unraveling process of polymer molecules in dilute solutions under strong elongational flows. We follow chain extension, segmental alignment, and viscosity contribution behavior of individual, randomly coiled, freely jointed bead–rod chain model molecules. In the absence of hydrodynamic shielding, segmental orientation at an intermediate strain rate begins only when aided by overall chain extension. However, at a very high strain rate, rapid initial segmental orientation and lateral chain compression precedes overall chain extension, resulting in the formation of sharp folds in most chains. Fold formation during the extension process is characterized by a sudden decrease in the rate of overall chain extension, an intermediate plateau in birefringence, and a disproportionately low chain end-to-end distance. Hydrodynamic screening generally slows down the uncoiling process, sometimes enough to avoid the formation of folded conformations.
The study of the influence of Jahn-Teller coupling and low symmetry strain on the anomalous electron paramagnetic resonance spectrum of titanium(III) doped CsAl(SO4)2⋅12H2O
Authors: Tregenna-Piggott Philip L. W. , Noble Christopher J. , Pilbrow John R.
Published: July 2002

The controversial EPR spectrum of titanium(III) as an impurity in cesium aluminum sulphate alum has been re-examined using conventional CW- and a novel pulsed-EPR technique, which monitors the nutation frequency as a function of field. The CW-spectra display a high degree of structure, which is interpreted as arising from chemically distinct titanium(III) species. The two-dimensional nutation spectrum maps the g∥ vs g⊥ relation from just one crystal orientation, and to far greater precision than available from CW-EPR. This novel technique shows that the origin of the linewidths observed for some of the EPR lines is inhomogeneous broadening, the nature of which can be described adequately only in the two-dimensional nutation spectrum. Calculations of g∥ vs g⊥ have been undertaken by numerical diagonalization of the vibronic Hamiltonian. It is found that the relationship between g∥ and g⊥ can be modeled only by assuming that the titanium(III) ions are subject to both dynamic Jahn-Teller coupling and low symmetry strain. Furthermore, it is shown that the calculated g∥ vs g⊥ relation is strongly dependent upon the nature of the vibronic interaction assumed. An excellent reproduction of the experimental data is obtained, using parameters consistent with those employed to model the susceptibility data of the isostructural cesium titanium sulphate alum.
Dual Lanczos simulation of dynamic nuclear magnetic resonance spectra for systems with many spins or exchange sites
Authors: Dumont Randall S. , Hazendonk Paul , Bain Alex
Published: July 2002

A stable formulation of dual Lanczos tridiagonalization of non-Hermitian matrices, along with solution of tridiagonal systems of equations, is used to simulate liquid nuclear magnetic resonance (NMR) spectra for systems with chemical exchange. The method provides computer storage and performance advantages over our previously developed sparse-matrix methodology [Dumont, Jain, and Bain, J. Chem. Phys. 106, 5928 (1997)], in addition to the incorporation of full blocking of the system Liouvillian with respect to the conservation of z magnetization. Convergence with respect to number of Lanczos iterations is investigated in some detail in order to achieve optimal performance.
Large-eddy simulation of a turbulent piloted methane/air diffusion flame (Sandia flame D)
Authors: Pitsch H. , Steiner H.
Published: July 2002

The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame (Sandia flame D). The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel.
Dust: A new challenge in nuclear fusion research?
Authors: Winter J.
Published: July 2002

Small particles (dust) exist in magnetic confinement fusion devices. Their origin is due to plasma–surface interactions. Dust particles may contain significant amounts of hydrogen isotopes, 50% of which will be tritium in future devices. It is important to assess and understand the processes by which dust is formed and by which it interacts with the fusion device and its plasma. Dust may be a safety hazard due to its high chemical reactivity and due to the mobile tritium inventory. Dust may influence the plasma performance and the operation of fusion devices. The radioactive decay of tritium incorporated into carbonaceous dust may lead to its charging and to the formation of a nuclear induced plasma associated with levitation and transport of dust inside the vacuum vessel. Some important aspects of dust in fusion devices will be discussed, making use of information from the area of plasma–surface interactions in fusion devices and from the field of dusty plasma research.
Multi-wavelength lasers fabricated by an Al layer controlled quantum well intermixing technology
Authors: Teng J. H. , Chua S. J. , Huang Y. H. , Li G. , Zhang Z. H. , Helmy A. Saher , Marsh J. H.
Published: July 2002

We report that the shift in the band gap of Al0.3Ga0.7As/GaAs quantum well structures can be precisely controlled by an Al layer buried between a spin-on silica film and a wet-oxidized GaAs surface. The blueshift in wavelength of the Al0.3Ga0.7As/GaAs quantum well photoluminescence (PL) depends linearly on the thickness of the buried Al layer. By changing the Al layer thickness, the PL peak wavelength can be tuned from 7870 Å for the as-grown sample to 7300 and 7050 Å after 20 and 45 s rapid thermal annealing at 850 °C, respectively. Applying this technology, Al layers with different thickness, i.e., no Al, 200 and 300 Å thick, were applied to the oxidized GaAs surface in three adjacent regions with 200 μm spacing on a quantum well laser structure sample. Three wavelength lasers were successfully fabricated in a single chip by a one step rapid thermal annealing. All the lasers have similar threshold current and slope efficiency.
Current conduction mechanism of Si/Ti-based Ohmic contacts to n-GaN
Authors: Kim Dae-Woo , Baik Hong Koo
Published: July 2002

We have investigated the Ohmic contact formation mechanism of Si/Ti-based contacts to n-GaN. The Si/Ti contact system was selected because Ti silicides have a low work function and also Si has been used widely as an n-type dopant. Our experimental results show that the Ohmic behavior of Si/Ti-based contacts were attributed to the low barrier height of the Ti silicide/GaN interface. The contact resistance of Si/Ti-based Ohmic contacts decreased exponentially with the measuring temperature. It can be concluded that current flows over the low barrier height by thermionic emission.
Ab initio group model potentials including electron correlation effects
Authors: Hernández Norge Cruz , Sanz Javier Fdez.
Published: July 2002

A method for determination of ab initio group model potentials, with the intention of describing the effects of a whole molecule or a chemical group within a density functional theory framework, is reported. The one-electron part of the Kohn–Sham equations is modified by incorporation of a Coulomb operator, which accounts for the classical electron interaction arising from the group. Exchange and correlation effects are introduced by a suitable modification of the exchange-correlation functionals. The strong orthogonality condition, usually required by the theory of separability of many electron systems, is written in terms of first order reduced density matrices. In order to check the method a group model potential for H2O (environment) was obtained and employed in the calculation of HF⋯H2O and H2O⋯H2O complexes using several functionals. Equilibrium intergroup distances and binding energies are compared with all-electron calculations.
A nine-dimensional high order perturbative study of the vibration of silane and its isotopomers
Authors: Wang Xiao-Gang , Sibert Edwin L.
Published: July 2002

The vibrations of silane isotopomers with Td, C3v, and C2v symmetry are studied by means of high order canonical Van Vleck perturbation theory (CVPT). Transforming the quartic ab initio force field of Martin, Baldridge, and Lee [Mol. Phys. 95, 254 (1999)] into curvilinear normal coordinates, CVPT is used to calculate energies that agree well with experimental data. Both low energy stretch–bend combination bands and high energy stretch local mode bands of silane up to 12 000 cm−1 are well reproduced. The choice of polyad quantum number is discussed with respect to different molecules. Comparing sixth- to eighth-order level of theory, most of the levels agree to within 0.1 cm−1. Spectroscopic constants are given for all the major isotopomers. The construction of the full cubic and quartic resonance operators for symmetric top species are summarized. The Si–H and Si–D stretch modes of Td and C3v symmetry species are studied with a 4D stretch variational model using both the fitted three-parameter potential and the quartic ab initio stretch potential. Large energy level differences between these two calculations indicate that the fitted potential constants of the stretch variational model are different from those derived directly from the ab initio surface. By comparing with the nine-dimensional CVPT calculation, it is shown that the direct and indirect Fermi resonances are responsible for the inability of the four-dimensional model to fit some stretch levels.
A note on families of spin observables and a generalization of Wigner’s theorem in C2
Authors: Poschadel Norbert
Published: July 2002

We present an elementary geometric method to derive a full quantum mechanical description of the spin of a spin-1/2 particle from the only requirement that no two different orthonormal bases in R3 could be distinguished from each other by any spin measurement. Further, this method is used to obtain a generalization of Wigner’s theorem on symmetry operations for the special Hilbert space C2.
Effect of electrostatic interactions on the dynamics of semiflexible monodisperse DNA fragments
Authors: Liu Hui , Gapinski Jacek , Skibinska Lidia , Patkowski Adam , Pecora R.
Published: July 2002

The dynamics of three monodisperse linear duplex DNA fragments—a 2311 base pair restriction fragment and 1500 and 1100 base pair polymerase chain reaction fragments—in dilute solution are studied as functions of added salt (NaCl) concentration by dynamic light scattering-photon correlation spectroscopy. Translational diffusion coefficients and intramolecular relaxation times are extracted from the measured light scattering intensity time autocorrelation functions as the added salt concentration is reduced from 100 mM to approximately 0.1 mM. The relaxation times of the first intramolecular mode increase as the added salt concentration is lowered. The dependence of the translational diffusion coefficient D on the added salt concentration is not very large, although it exhibits a maximum for all three fragments. The maximum is interpreted as the consequence of two opposing effects—the stiffening of the molecule that produces an increase of the size (decrease of D) as the added salt concentration is lowered, and the coupling of the diffusion of the DNA through the electrostatic forces to the motion of the small and other polyions in the solution that results in an increase of its mobility (increase of D). The increase of the slowest intramolecular relaxation times as the salt concentration is lowered is interpreted in terms of a theory relating this time to the mean-squared radius of gyration of the molecule.
Correlation between N 1s core level x-ray photoelectron and x-ray absorption spectra of amorphous carbon nitride films
Authors: Quirós C. , Gómez-Garcı́a J. , Palomares F. J. , Soriano L. , Elizalde E. , Sanz J. M.
Published: July 2002

This work presents a comparative analysis of the N 1s core level spectra, as measured by x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS), of amorphous CNx films which gives evidence of the existing correlation between the different components that constitute the respective spectra. After annealing, the contribution of XPS at 399.3 eV and the components of XAS at 399.6 and 400.8 eV are clearly enhanced. They are assigned to sp2 with two neighbors and to sp states of nitrogen. In addition, the XPS component at 401.3 eV is related to the XAS feature at 402.0 eV and has been assigned to sp2 nitrogen bonded to three carbon neighbors.
Domain wall in ultrathin magnetic film: Internal structure and dynamics
Authors: Sobolev V. L. , Ivanova S. V. , Huang H. L.
Published: July 2002

Detailed micromagnetic study of internal structure and dynamics of domain wall in ultrathin magnetic film with thickness tf≪lex (lex is the exchange length) is carried out. It is revealed that deviations of stationary magnetization distribution inside the wall from the one of the Bloch domain wall are small and proportional to (tf/lex)Q−1. The limiting velocity of uniform domain wall motion coincides with the same for the Bloch wall (Walker’s critical velocity) with an accuracy of terms proportional to (tf/lex)2. It is also found that the same small parameter describes deviation of stationary distribution of magnetization in a vertical Bloch line and deviation of Bloch line velocity from the expressions found for films with tf>lex.
Investigation of the vibrational dynamics of the HCN/CNH isomers through high order canonical perturbation theory
Authors: Sugny Dominique , Joyeux Marc , Siber Edwin L.
Published: July 2002

Molecular vibrations of the molecule HCN/CNH are examined using a combination of a minimum energy path (MEP) Hamiltonian and high order canonical perturbation theory (CPT), as suggested in a recent work [D. Sugny and M. Joyeux, J. Chem. Phys. 112, 31 (2000)]. In addition, the quantum analog of the classical CPT is presented and results obtained therefrom are compared to the classical ones. The MEP Hamiltonian is shown to provide an accurate representation of the original potential energy surface and a convenient starting point for the CPT. The CPT results are subsequently used to elucidate the molecular dynamics: It appears that the isomerization dynamics of HCN/CNH is very trivial, because the three vibrational modes remain largely decoupled up to and above the isomerization threshold. Therefore, the study of the three-dimensional HCN/CNH system can be split into the study of several one-dimensional bending subsystems, one for each value of the numbers v1 and v3 of quanta in the CH and CN stretches. In particular, application of high order CPT to the most precise available ab initio surface provides simple expressions (quadratic polynomials) for the calculation of the heights of the isomerization barrier and of the CNH minimum above the HCN minimum for each value of v1 and v3.
FTIR studies of annealing processes and irradiation effects at 266 nm in ozone–amorphous ice mixtures
Authors: Chaabouni H. , Schriver-Mazzuoli L. , Schriver A.
Published: July 2002

Fourier transform infrared (FTIR) spectroscopy is used to study the vibrational spectrum of ozone trapped in amorphous ice (a situation observed on icy satellites in the solar system). Evaporation of ozone from ice is investigated from 30 to 150 K under a static pressure of 10−7 Torr. Condensed and chemisorbed ozone on the surface of micropores is released at a temperature between 40 and 80 K, and ozone in water lattice evaporates starting from 120 K. The release of ozone probes the gradual transformation of water ice. The photochemistry of ozone in excess ice is also investigated using 266 nm laser irradiation. At low temperature, condensation of H2O/O3 mixtures leads to ozone trapped in pores and cavities, and H2O2 is produced through the hydrogen-bonded complex between ozone and free OH bonds. At higher temperature, when a solid solution of ozone in water is observed, H2O2 is formed by the reaction of the excited oxygen atom O(1D) with the nearest water molecules. Kinetic studies suggest that recombination of the dioxygen molecule with ground-state atomic oxygen O(3P) is a minor channel.