bluuebunny/crossref_metadata_2025 huggingface dataset:
Fourier transform analysis of electromodulation spectra: Effects of the modulation amplitude
Authors: Alperovich V. L. , Jaroshevich A. S. , Scheibler H. E. , Terekhov A. S. , Tober Richard L.
Published: July 2002

Fourier transform analysis of Franz-Keldysh oscillations is applied to electroreflectance spectra of GaAs p–i–n diode structures in order to elucidate the role of the modulation amplitude in electric-field-modulated spectroscopies. The effects of modulation amplitude are quantitatively evaluated by considering the imaginary and real parts of the complex Fourier transform in addition to its modulus. The experimentally measured electroreflectance spectra and their Fourier transforms are in good agreement with model calculations.
Surface diffusion growth and stability mechanism of BN nanotubes produced by laser beam heating under superhigh pressures
Authors: Louchev Oleg A.
Published: July 2002

A growth mechanism for BN nanotubes appearing at superhigh pressures under laser beam heating [D. Golberg, Y. Bando, M. Eremets, K. Takemura, K. Kurashima, and H. Yusa, Appl. Phys. Lett. 69, 2045 (1996)] is proposed based upon consideration of the dependence of the energy of adsorption and surface diffusion on the surface curvature. According to this mechanism, the growth of BN nanotubes is due to surface diffusion along the external surface, which also ensures morphological stability of the open end during growth. It is suggested that the observed nanotube heights are limited by the corresponding diffusion length.
Positive charging of thermal SiO2/(100)Si interface by hydrogen annealing
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

Annealing of SiO2/(100)Si interfaces in hydrogen in the temperature range of 500–800 °C is found to introduce a considerable density of fixed positive charge. The charge is diamagnetic, and shows no correlation with any kind of dangling bond defects at the Si surface or in the oxide. The observed charged state is attributed to hydrogen bonding to a bridging oxygen atom at the interface (threefold coordinated oxygen center), which may account for the well-known oxidation-induced charge at the Si/SiO2 interfaces.
Probing quantum nanostructures with near-field optical microscopy and vice versa
Authors: Bryant Garnett W.
Published: July 2002

A theory for using near-field optical microscopy to probe quantum nanostructures is presented. Optical transition rates are determined for dots excited by the near field of a tip. Local-field corrections are neglected. Results for different tip/dot configurations and sizes show that near-field excitation can enhance light-hole transitions, excite selection-rule breaking transitions with rates comparable to allowed transitions, and map electron–hole pair wave functions. Conversely, dot response can be used to characterize tip near fields.
Polymer-stabilized diffraction gratings from cholesteric liquid crystals
Authors: Lee S. N. , Chien L. C. , Sprunt S.
Published: July 2002

We report the use of a low concentration polymer network to stabilize electric field-induced diffraction gratings in cholesteric liquid crystals. The stabilized gratings can be electrically switched between a zero-field “on” state, which exhibits approximately 75% diffraction efficiency, and a moderate field (typically 3 V/μm) “off” state. Grating spacings between 1.7 and 30 μm are obtained in a 10 μm thick cell by variation of the concentration of chiral dopant used to form the cholesteric liquid crystal.
Linear and quadratic electro-optic coefficients of self-organized In0.4Ga0.6As/GaAs quantum dots
Authors: Qasaimeh O. , Kamath K. , Bhattacharya P. , Phillips J.
Published: July 2002

The electro-optic properties of self-organized In0.4Ga0.6As/GaAs quantum dots have been studied experimentally. Single-mode ridge waveguide structures were grown by molecular beam epitaxy with self-organized In0.4Ga0.6As/GaAs quantum dots in the guiding region. The measured linear and quadratic electro-optic coefficients are 2.58×10−11 m/V and 6.25×10−17 m2/V2, respectively, which are much higher than those obtained for bulk GaAs or quantum well structures. The measured transmission characteristics indicate that low-voltage amplitude modulators can be realized with quantum dot active regions.
Tunneling and enhanced magnetoresistance in Nd2/3Sr1/3MnO3 thin films with microcracks
Authors: Satyalakshmi K. M. , Fisher B. , Patlagan L. , Koren G. , Sheriff E. , Prozorov R. , Yeshurun Y.
Published: July 2002

We found that microcracks in thin Nd2/3Sr1/3MnO3 films create a series of intrinsic break junctions which are ideal for investigating tunneling phenomena in this system. A comparison of films with and without cracks, which have similar ferromagnetic Curie temperature (Tc) of 140–150 K, shows that the cracked film has a lower insulator to metal transition temperature TM (97 K vs 140 K), three orders of magnitude higher resistivity at TM, and two times larger magnetoresistance at 1 T near TM. At T>TM we observed that ln ραT−1/4 in the uncracked film while in the cracked film a ln ραT−1/2 dependence was found. This indicates that the conductivity in the first case is due to variable range hopping in three dimension, while in the second case it is dominated by thermally activated tunneling across the insulating barriers (the microcracks).
Single-electron effects in heavily doped polycrystalline silicon nanowires
Authors: Irvine Andrew C. , Durrani Zahid A. K. , Ahmed Haroon , Biesemans Serge
Published: July 2002

We have observed single-electron charging effects in heavily doped polycrystalline silicon nanowires at 4.2 K. Wires of approximately 20 nm by 30 nm active cross section were defined by electron-beam lithography and thermal oxidation in standard polycrystalline silicon material. We have measured a Coulomb staircase and periodic current oscillations with gate bias, attributed to localized carrier confinement resulting from a statistical variation in the intergrain tunnel barriers. A sharp change in the current oscillation period is seen and we speculate that it is due to electrostatic screening of the gate bias by grain boundary defect states.
Influence of the water layer on the shear force damping in near-field microscopy
Authors: Davy S. , Spajer M. , Courjon D.
Published: July 2002

The influence of the water layer on the shear force damping is investigated in the case of a perfectly flat mica surface. In ambient conditions it is shown that the damping curve exhibits three particular regimes depending on the tip-sample distance. Moreover, the damping varies significantly over the first hour, pointing out the complexity of the distance control by shear force detection.
Suppression of penetration of aluminum into 8-hydroxyquinoline aluminum via a thin oxide barrier
Authors: Huang M. B. , McDonald K. , Keay J. C. , Wang Y. Q. , Rosenthal S. J. , Weller R. A. , Feldman L. C.
Published: July 2002

We report a direct observation of Al penetration into an organic film of 8-hydroxyquinoline aluminum (Alq) in a Alq/Al bilayer, a common interface occurring in many organic light-emitting diode structures. The Al penetration has a strong correlation with Alq photoluminescence quenching. We also demonstrate that the penetration and luminescence quenching can be effectively suppressed by ∼20 Å of an intervening layer of Al2O3 at the Alq/Al interface.
The contribution of vacancies to carbon out-diffusion in silicon
Authors: Scholz R. F. , Werner P. , Gösele U. , Tan T. Y.
Published: July 2002

Diffusion of carbon is mostly assumed to be governed by carbon interstitials via the kick-out mechanism. Carbon in-diffusion experiments are associated with thermal equilibrium concentrations of point defects, whereas in the case of carbon out-diffusion a remarkable undersaturation of Si self-interstitials may develop provided the carbon concentration is several orders of magnitude over its solubility value. New carbon out-diffusion experiments demonstrate that this model qualitatively describes the observed carbon diffusion profiles. However, we demonstrate that an accurate description of the experimental profiles is only possible if the Frank–Turnbull mechanism, involving vacancies, is additionally taken into account. Detailed investigations of carbon and boron profiles in the same sample can be used to determine the splitting of the known vacancy component of the silicon self-diffusion coefficient into the vacancy diffusion coefficient and the vacancy thermal equilibrium concentration at 900 °C.
GaInN/GaN multiple-quantum-well light-emitting diodes grown by molecular beam epitaxy
Authors: Grandjean N. , Massies J. , Dalmasso S. , Vennéguès P. , Siozade L. , Hirsch L.
Published: July 2002

GaInN and GaN were grown by molecular beam epitaxy on c-plane sapphire using NH3. 9 K photoluminescence performed on both GaInN thin layers and GaInN/GaN multiple-quantum wells (MQWs) exhibits narrow emission (∼50 meV linewidths). Transmission electron microscopy images show sharp GaInN/GaN interfaces and homogeneous GaInN layers. Strong indium surface segregation is also evidenced. Light-emitting diodes were fabricated from 5×GaInN (25 Å)/GaN (35 Å) MQW heterostructures. The 300 K electroluminescence yields blue light at 440 nm.
Growth and optical characterization of aluminum nitride thin films deposited on silicon by radio-frequency sputtering
Authors: Dogheche E. , Rémiens D. , Boudrioua A. , Loulergue J. C.
Published: July 2002

Highly textured hexagonal aluminum nitride (AlN) thin films were deposited on silicon substrates by radio-frequency magnetron sputtering at a substrate temperature below 400 °C and annealed in the temperature range of 400–450 °C by rapid thermal annealing. The optical and the electro-optical properties have been investigated using the prism-coupling technique. Both ordinary and extraordinary refractive indices (n0=2.0058 and ne=2.0374 at 632.8 nm) were respectively determined from the transverse electric and the transverse magnetic mode excitations. Furthermore, refractive index profiles analysis by using an improved inverse Wentzel–Kramer–Brillouin method reveals a step-like behavior of AlN thin films. The optical losses have been evaluated to be around 7 dB cm−1. The electro-optic coefficient r13 of 0.98 pm/V has been measured from the variation of the shift of guided-modes spectrum as a function of the applied electric field in the experiment.
Consistent time-of-flight mobility measurements and polymer light-emitting diode current–voltage characteristics
Authors: Campbell I. H. , Smith D. L. , Neef C. J. , Ferraris J. P.
Published: July 2002

We present time-of-flight mobility measurements and measured and calculated current–voltage (I–V) characteristics of structures fabricated using a soluble poly(p-phenylene vinylene) derivative. Time-of-flight measurements were used to determine the electric field dependent hole mobility. This mobility was then used, without adjustable parameters, to calculate the I–V characteristics of space-charge-limited, hole only devices. The measured and calculated I–V characteristics are in good agreement over five orders of magnitude in current. These results demonstrate that an electric field dependent mobility, without invoking trapping effects, provides an accurate description of hole transport in this polymer.
Ferroelectricity in thin perovskite films
Authors: Tybell T. , Ahn C. H. , Triscone J.-M.
Published: July 2002

We report on the investigation of ferroelectricity in thin tetragonal single-crystalline perovskite films of Pb(Zr0.2Ti0.8)O3 grown by off-axis rf magnetron sputtering. The local ferroelectric properties of atomically smooth films, with thicknesses ranging from a few unit cells to 800 Å, were measured using a combination of electric force microscopy and piezoelectric microscopy. The time dependence of the measured signals reveals a stable ferroelectric polarization in films down to thicknesses of 40 Å.
Control of the initial oxidation on Cu(001) surface by selection of translational energy of O2 molecules
Authors: Yata Masanori , Rouch Hervé
Published: July 2002

In the initial oxidation of Cu(001) by O2, the surface is oxidized in a layer-by-layer manner up to one monolayer, followed by Cu2O islanding. The layer-by-layer oxidation is promoted by the increasing translational energy of incident O2 and is insensitive to surface temperature. By contrast, the formation of Cu2O is promoted by the decreasing translational energy of incident O2 and by increasing surface temperature in the temperature range between room temperature and 650 K. Our results demonstrate controllable initial oxidation by the adjustment of the translational energy of O2, offering the prospect of using this approach to aid in the development of new fabrication of metal oxide and Si devices.
GaAs/AlGaAs-based microcylinder lasers emitting at 10 μm
Authors: Gianordoli S. , Hvozdara L. , Strasser G. , Schrenk W. , Unterrainer K. , Gornik E.
Published: July 2002

The realization of electrically pumped GaAs/AlGaAs quantum cascade microcylinder lasers is reported. Design and fabrication of special resonator shapes (microcylinder and ridge waveguide) are presented. Threshold characteristics and optical output of different resonators of the same quantum cascade laser material emitting at 10 μm are investigated. A low threshold current of 318 mA is obtained for a microcylinder resonator with circular cross section. The maximum working temperature of the microcylinder lasers is 165 K. Single mode emission is detected with a side mode suppression ratio better than 20 dB.
Hot-electron far-infrared intrasubband absorption and emission in quantum wells
Authors: Towe E. , Vorobjev L. E. , Danilov S. N. , Kochegarov Yu. V. , Firsov D. A. , Donetsky D. V.
Published: July 2002

This letter reports the experimental study of hot-electron absorption and emission of far-infrared radiation in GaAs/(Al,Ga)As quantum-well structures. The emission and absorption processes discussed here are associated with intrasubband transitions; these processes are assisted by nonequilibrium optical phonons, and by scattering at rough interfaces and at impurity sites. We show how the far-infrared emission spectra of the hot-electron gas can be used to extract the electron temperature Te.
Short-coherence photorefractive holography in multiple-quantum-well devices using light-emitting diodes
Authors: Tziraki M. , Jones R. , French P. M. W. , Nolte D. D. , Melloch M. R.
Published: July 2002

We demonstrate an application of light-emitting diodes to photorefractive holography with multiple-quantum-well devices. Holograms corresponding to three-dimensional images with a depth resolution of less than 10 μm were recorded, with image acquisition times as short as 5 ms.
A simple reflectance method for estimation of the Al mole fraction of bulk AlGaN and AlGaN/GaN heterostructures
Authors: Yu L. S. , Qiao D. , Lau S. S. , Redwing J. M.
Published: July 2002

In this study, we report a simple optical reflectance method to characterize the Al mole fraction in bulk AlGaN and AlGaN/GaN heterostructures. This method is based on the concept of Fabry–Perot oscillations of a probing beam when the photon energy of the beam falls below the band-gap energy of the semiconductor. The accuracy and merits of this method in comparison with the more elaborate photoluminescence method are discussed.
Enhancement of erbium photoluminescence by substitutional C alloying of Si
Authors: Markmann M. , Neufeld E. , Sticht A. , Brunner K. , Abstreiter G. , Buchal Ch.
Published: July 2002

Photoluminescence (PL) at 1.54 μm of erbium-doped Si1−yCy alloys grown by molecular beam epitaxy (MBE) has been analyzed depending on sample temperature, excitation density, and growth conditions. Erbium activation raises with increasing incorporation of substitutional carbon compared to interstitial carbon. For [Er]=4.5×1019 cm−3 and y=0.1% maximum PL output at 1.54 μm was achieved for growth temperatures at 430 °C. Additional annealing could further enhance PL intensity at 1.54 μm. For increasing sample temperature a decrease of PL intensity with two characteristic activation energies around 100 and 10–20 meV is observed, which results in quenching of PL intensity at lower temperatures for Si:Er:C layers compared to Si:Er:O layers. PL spectra show different fine structure for oxygen and carbon codoping by MBE or ion implantation, higher efficiency, and lower PL background for MBE-grown samples in contrast to ion-implanted layers.
High-frequency AlGaN/GaN polarization-induced high electron mobility transistors grown by plasma-assisted molecular-beam epitaxy
Authors: Murphy M. J. , Chu K. , Wu H. , Yeo W. , Schaff W. J. , Ambacher O. , Eastman L. F. , Eustis T. J. , Silcox J. , Dimitrov R. , Stutzmann M.
Published: July 2002

High-quality AlGaN/GaN heterostructures have been grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. Polarization effects are exploited to achieve a two-dimensional electron-gas sheet density of 8.8×1012 cm−2 and greater on intentionally undoped material with a measured room-temperature mobility as high as 1478 cm2/V s. Transistors were then fabricated from this material, yielding a unity current gain frequency of 50 GHz and a unity power gain frequency of 97 GHz. By increasing the buffer layer thickness, output powers of 1.88 W/mm at 4 GHz with an efficiency of 34% were achieved. These results prove that the polarization effects in the nitrides are as enormous as theory predicts. The key to the improved mobility and operation of the devices of the all-molecular-beam-epitaxy-grown material, the AlN nucleation layer, will be discussed.
Radiation losses of waveguide-based two-dimensional photonic crystals: Positive role of the substrate
Authors: Benisty H. , Labilloy D. , Weisbuch C. , Smith C. J. M. , Krauss T. F. , Cassagne D. , Béraud A. , Jouanin C.
Published: July 2002

Radiation losses occurring in photonic crystals etched into planar waveguides are analyzed using a first-order perturbation approximation. Assuming the incoherent scattering limit, the model indicates that losses diminish as the cladding index approaches the core index. A simple scheme is devised to include these losses into purely two-dimensional calculations by using an imaginary index. Such calculations are shown to agree with corresponding experimental transmission through near-infrared photonic crystals, reproducing the contrasting behavior of the “dielectric” and “air” band edges.
Chemical vapor deposition of Si nanowires nucleated by TiSi2 islands on Si
Authors: Kamins T. I. , Williams R. Stanley , Chen Y. , Chang Y.-L. , Chang Y. A.
Published: July 2002

Silicon “nanowires” can be formed by chemical vapor deposition of Si onto Si substrates on which nanometer-scale, Ti-containing islands have been grown. At the growth temperatures used, the Ti-containing islands remain solid and anchored to the substrate, while the Si nanowires grow out from the islands, which remain at their bases. The nanowire growth mechanism, therefore, differs from the usual vapor-liquid-solid process and provides a potential route for the formation of oriented Si nanostructures or semiconductor-metal-semiconductor structures compatible with Si integrated circuits.
Surface and bulk passivation of GaAs solar cell on Si substrate by H2+PH3 plasma
Authors: Wang G. , Ogawa T. , Umeno M. , Soga T. , Jimbo T.
Published: July 2002

A promising passivation method for GaAs solar cell grown on Si substrate (GaAs/Si solar cell) by phosphine-added hydrogen (PH3/H2) plasma exposure has been envisaged. The defect-hydrogenation and the surface-phosphidization effects of GaAs/Si solar cell are realized simultaneously by this single passivation process. Consequently, surface recombination states are reduced and the minority carrier lifetime is increased, resulting in a significant reduction in saturation current density (J0) of the GaAs/Si p–n junction. High open-circuit voltage (0.93 V) and fill factor (80.9%) are obtained for the PH3 plasma exposed GaAs/Si solar cells. As a result, the conversion efficiency is increased from 15.9% to 18.6%. This approach provides a simple and effective method to improve the photovoltaic properties of GaAs/Si solar cell.
Transient enhanced diffusion of implanted boron in 4H-silicon carbide
Authors: Janson M. S. , Linnarsson M. K. , Hallén A. , Svensson B. G. , Nordell N. , Bleichner H.
Published: July 2002

Experimental evidence is given for transient enhanced diffusion of boron (B) in ion-implanted silicon carbide (SiC). The implanted B is diffusing several μm into the samples when annealed at 1600 and 1700 °C for 10 min, but the in-diffused tails remain unaffected when the annealing times are increased to 30 min at the same temperatures. A lower limit of the effective B diffusivity at 1600 °C is determined to 7×10−12 cm2/s, which is 160 times larger than the equilibrium B diffusivity given in the literature.
Room-temperature operation of a memory-effect AlGaAs/GaAs heterojunction field-effect transistor with self-assembled InAs nanodots
Authors: Koike K. , Saitoh K. , Li S. , Sasa S. , Inoue M. , Yano M.
Published: July 2002

This letter describes the memory effect of an AlGaAs/GaAs heterojunction field-effect transistor that contains InAs nanodots in the barrier layer. The device experiences a shift of threshold gate voltage, as a function of the amount of the electrons trapped in the nanodots. These trapped electrons can be injected by applying a positive gate voltage and be erased by a visible light illumination at negative gate bias. Although the shift of the threshold gate voltage volatilizes with the time after the memory programing operation, a considerable part of the shift is retained even after 100 h at room temperature.
Correlation between surface step structure and phase separation in epitaxial GaInAsSb
Authors: Wang C. A.
Published: July 2002

A strong correlation between the surface step structure and phase separation in metastable GaInAsSb epitaxial layers grown by organometallic vapor-phase epitaxy is reported. The full width at half maximum (FWHM) of the 4 K photoluminescence (PL) peak energy is used as a semiquantitative measure of the degree of phase separation: FWHM values increase with phase separation. The step structure of GaInAsSb grown at 525 °C is vicinal, while it is step bunched for layers grown at 575 °C. The corresponding 4 K PL FWHM data indicate smaller FWHM values for layers grown at the lower temperature, and suggest a lower degree of phase separation. Extreme PL broadening is associated with an aperiodic surface structure. Furthermore, the PL FWHM values decrease when the growth rate increases from 1.2 to 5 μm/h. It is proposed that longer adatom lifetimes, which are associated with longer terrace lengths of a step-bunched surface compared to a vicinal one, allow more time for the adatoms to reach equilibrium and thus phase separate.
Dynamics of amplified spontaneous emission in InAs/GaAs quantum dots
Authors: Lingk C. , von Plessen G. , Feldmann J. , Stock K. , Arzberger M. , Böhm G. , Amann M.-C. , Abstreiter G.
Published: July 2002

We investigate the dynamics of amplified spontaneous emission (ASE) in self-assembled InAs/GaAs quantum dots, using pulsed optical excitation of an edge-emitting sample at room temperature. A material gain of 1.5×104 cm−1 is determined for 800 nm excitation with 1.5 μJ/cm2 pulses. Using photoluminescence up-conversion, we show that increases in both electron–hole pair density and photon density in the excited stripe cause a significant decrease in the decay time from 2.2 ns, corresponding to spontaneous emission, to about 0.9 ns, corresponding to stimulated emission or ASE. A carrier capture time of 10 ps limits the onset of the ASE process for short stripe lengths.
Gain in 1.3 μm materials: InGaNAs and InGaPAs semiconductor quantum-well lasers
Authors: Hader J. , Koch S. W. , Moloney J. V. , O’Reilly E. P.
Published: July 2002

The absorption and gain for an InGaNAs/GaAs quantum-well structure is calculated and compared to that of a more conventional InGaAs/InGaPAs structure, both lasing in the 1.3 μm range. Despite significant differences in the band structures, the gain value is comparable for high carrier densities in both structures and the transition energy at the gain maximum shows a similar blueshift with increasing carrier density. For low and intermediate carrier densities, the calculated gain in the InGaPAs system is significantly lower and the bandwidth smaller than in the InGaNAs system.
Stochastic mechanics and the Feynman integral
Authors: Pavon Michele
Published: July 2002

The Feynman integral is given a stochastic interpretation in the framework of Nelson’s stochastic mechanics employing a time-symmetric variant of Nelson’s kinematics recently developed by the author.
Laser induced fluorescence measurements of dissolved oxygen concentration fields near air bubble surfaces
Authors: Roy Sabita , Duke Steve R.
Published: July 2002

This article describes a laser-induced fluorescence (LIF) technique for measuring dissolved oxygen concentration gradients in water near the surface of an air bubble. Air bubbles are created at the tip of a needle in a rectangular bubble column filled with water that contains pyrenebutyric acid (PBA). The fluorescence of the PBA is induced by a planar pulse of nitrogen laser light. Oxygen transferring from the air bubble to the deoxygenated water quenches the fluorescence of the PBA. Images of the instantaneous and two-dimensional fluorescence field are obtained by a UV-intensified charge-coupled device (CCD) camera. Quenching of fluorescence intensity is determined at each pixel in the CCD image to measure dissolved oxygen concentration. Two-dimensional concentration fields are presented for a series of measurements of oxygen transfer from 1.6 mm bubbles suspended on the tip of a needle in a quiescent fluid. The images show the spatially varying concentration profiles, gradients, and boundary layer thicknesses at positions around the bubble surfaces. These direct and local measurements of concentration behavior within the mass transfer boundary layer show the potential of this LIF technique for the development of general and mechanistic models for oxygen transport across the air–water interface.
Stationary uniform plasmas by electron cyclotron heating in a linear multipole
Authors: Fukao Masayuki , Mizuno Tamotsu , Saito Yoshiharu , Matsuo Hironobu
Published: July 2002

A description is given of experimental results on stationary plasmas produced by 430 MHz electron cyclotron heating (ECH) in a linear multipole (hexadecapole). A uniform and stable plasma was produced with 200 W rf power in the magnetic-field-free central region over a large area (20 cm in diameter) and with an electron density of up to 1.5×1011 cm−3. This corresponds to more than 60 times the cutoff density for rf power at a frequency of 430 MHz. A silicon substrate was inserted into the midplane perpendicular to the axis. The depletion in electron density due to the insertion was small and radial uniformity was well maintained. The changes in field-free zone area and current density required to produce a stable confinement magnetic field with strong enough intensity for ECH application are described for the case when the number of current rods on a constant circumference is increased. It is concluded that in principle the uniform plasma area can be widened without limit by increasing the pole number while maintaining the same interval between adjacent rods.
Matrix readout scheme for superconducting tunnel junction arrays
Authors: Martin D. D. E. , Peacock A. , Verhoeve P. , Poelaert A. , Venn R.
Published: July 2002

We report on the design and testing of a new readout scheme for superconducting tunnel junction (STJ) arrays. By grouping the electrodes in rows and columns, this method drastically reduces the number of connections and electronic circuits required for reading out a large format array of pixelated detectors. Using charge sensitive amplifiers with junction field-effect transistors we verify that the energy resolution degrades primarily due to capacitance increase at the amplifier’s input node. However, since each detector is readout by two independent circuits, these two outputs can be combined to increase the signal-to-noise level. For a N×N array, we show that the equivalent noise charge at the input worsens by a factor much less than N as compared to single device readout. We also report on a proof-of-principle experiment carried out on a two-by-two array of STJs using optical photon excitation. These measurements show that stable biasing of STJs is possible in this configuration and that the line resolutions are consistent with our theoretical predictions.
Active length control of two phase locked CO2 lasers with a digital signal processor
Authors: Weingartner Wendelin , Schröder Kurt , Schuöcker Dieter
Published: July 2002

Two CO2 lasers are phase locked together by injection locking, where radiation between the two lasers is exchanged using a copper prism as a beam folding device in the resonators. Extraction of the output radiation is achieved by a common output coupler. As this method is only utilizing reflective optics (except for the output coupler), it is also practical as a technique for phase-locking lasers with powers in the multikilowatt range. Single frequency oscillation is only achieved if the lengths of both resonators are within a so-called locking range. This can only be fulfilled if the length of one resonator is actively controlled. The method of detection of the phase difference between the lasers and actively controlling the length of one resonator is presented as well as its implementation into a digital signal processor. Stable phase locked operation is achieved and proved through detection of twice the intensity in the far field in the phase-locked regime.
Electronic excitation and ionization spectra of cyclopentadiene: Revisit by the symmetry-adapted cluster–configuration interaction method
Authors: Wan Jian , Ehara Masahiro , Hada Masahiko , Nakatsuji Hiroshi
Published: July 2002

Electronic excitation and ionization spectra of cyclopentadiene (CP) were reinvestigated by the symmetry-adapted cluster (SAC) and SAC–configuration interaction (SAC-CI) method with an extended basis set and a wide active orbital space. To give a satisfactory interpretation of the general profile of the observed excitation spectrum, 40 low-lying excited singlet and triplet states (with excitation energies of up to 9.5 eV) were computed. The calculated excitation energies were greatly improved compared to those reported previously. All of the peaks in the experimental spectrum were reassigned theoretically with small deviations. The natures of the low-lying valence and Rydberg-excited states were discussed in detail, and the results were also compared with those of some other recent theoretical studies. The ionization energies calculated by the SAC-CI general-R method agree well with the experimental peaks in the photoelectron spectrum. A number of two-electron shake-up states were calculated below 23 eV.
Reaction dynamics of Mg(4 1S, 3 1D2) with H2: Harpoon-type mechanism for highly excited states
Authors: Liu Dean-Kuo , Lin King-Chuen , Chen Jye-Jong
Published: July 2002

Using a pump–probe technique, the reactions of Mg(4 1S0 and 3 1D2) with H2 have been measured to yield similar rotational distributions of MgH(v=0 and 1) as that obtained for the reaction of the Mg(3 1P1) state with H2. A series of measurements is conducted to clarify that the reactions are initiated directly by these higher states, rather than occurring from the lower 3 1P1 state following radiative and collisional relaxation. The reactivity of the Mg 4 1S0 state with H2 is found to be comparable to that of the 3 1P1 state, but about three times larger than that of the 3 1D2 state. The Mg(4 1S0, 3 1D2)–H2 reactions proceed via a harpoon-type process, and are closely associated with the Mg(3 1P1)–H2 reaction coordinate through evolution of a series of surface crossings. To support our suggestion that the harpoon mechanism is involved, the cross sections of collisional deactivation by H2 for various excited states are measured. The ratios of cross sections observed for the 3 1P1, 4 1S0, and 5 1S0 state, equal to 1:2.85:4.3, are consistent with the calculated prediction of 1:2.62:4.24. The calculated cross sections are based on a simple hard sphere model with effective radii evaluated differently. Here, the effective radii for the higher states are determined from the crossing of ionic and covalent curves, while the Mg(3 1P1)–H2 radius is estimated from the nonadiabatic crossing between the reactive 1 1B2 state and the ground state. Consistency between observation and prediction confirms that the harpoon mechanism proposed in this work is plausible.
Oscillatory pump–probe signals from delocalized wave packets
Authors: Ermoshin V. A. , Engel V. , Meier C.
Published: July 2002

We calculate pump–probe signals obtained from femtosecond excitation of I2 molecules. It is shown that, depending on the parameters of the pump laser-pulse, the signals can exhibit a regular oscillatory behavior although the vibrational wave packet which is produced in the pump process, is delocalized over the entire classically allowed region. The respective structures can be related to temporal variations of an enhanced probability density which, in turn, are related to the underlying classical motion. We further demonstrate that the use of time-resolved photoelectron spectroscopy allows for a characterization of the structures which appear in the time-dependent wave function.
Doppler-free UV-visible optical–optical double resonance polarization spectroscopy of the 2 1Σu+ double minimum state and the C 1Πu state of Li2
Authors: Kasahara Shunji , Kowalczyk Paweł , Kabir Md. Humayun , Baba Masaaki , Katô Hajime
Published: July 2002

Doppler-free UV-visible optical–optical double resonance polarization spectra of the 2 1Σu+←X 1Σg+ and C 1Πu←X 1Σg+ transitions in Li27 molecules were measured. In the case of the double minimum 2 1Σu+ state transitions to all vibrational levels (except the lowest two) localized in the inner potential well, to several levels in the outer well and some levels beyond the potential barrier were observed. Potential energy curves of the C 1Πu(v=0–10) state and the inner well of the 2 1Σu+ state are determined by the RKR method. A V(R) curve describing the whole potential of the 2 1Σu+ state (including both wells) is constructed using the Inverted Perturbation Approach (IPA). Tunneling splittings and energy shifts of the 2 1Σu+ levels were observed, and their magnitudes are well represented in a semiclassical model using the IPA potential. Perturbations between the C 1Πu(v,J) and 2 1Σu+(v′,J) levels were detected in the region above the potential barrier of the 2 1Σu+ state, where the rotational constant of the 2 1Σu+ state is much smaller than that of the C 1Πu state.
Orientation dependence of the ferroelectric and piezoelectric behavior of Ba(Ti1−xZrx)O3 single crystals
Authors: Yu Zhi , Guo Ruyan , Bhalla A. S.
Published: July 2002

The dielectric properties, ferroelectric polarization, and piezoelectric strain behavior of Ba(Ti1−xZrx)O3 (x=0.05 and 0.08) single crystals are studied for different crystal orientations. The study of dielectric properties suggests that the three phase transitions in BaTiO3 are pinched by substituting Ti with Zr but the anisotropic dielectric behavior is present in crystals of various orientations. The remnant polarization and strain behavior are also studied for different crystal directions. Higher strain and lower polarization are obtained along the pseudocubic 〈001〉 direction compared to those observed along the 〈110〉 and 〈111〉 directions in both orthorhombic (x=0.05) and rhombohedral (x=0.08) phase. The results for the 〈001〉 oriented orthorhombic crystals with x=0.05 indicate the highest strain level (∼0.25%) with almost hysteresis-free behavior and high piezoelectric coefficient (d33=500 pC/N).
Aggregation of water molecules: Atmospheric implications
Authors: Evans Glenn T. , Vaida Veronica
Published: July 2002

The equilibrium constants for water oligomers ranging from dimers to cyclic hexamers are determined using Wertheim’s theory of associating systems. In the present model for water, the pair potential has a spherical hard core, and tetrahedral hydrogen bonds which are represented by an energy parameter and an interaction volume. On the basis of the present theory, one predicts that in earth’s troposphere, water dimers and perhaps trimers may contribute to the absorption of solar radiation, but concentrations of higher oligomers are too low to influence the optical properties of the earth’s atmosphere.
Vibrational density of states of nanocrystalline iron and nickel
Authors: Bonetti Ennio , Pasquini Luca , Sampaolesi Euro , Deriu Antonio , Cicognani Giovanna
Published: July 2002

We present an experimental determination of the vibrational density of states in nanocrystalline Fe and Ni by inelastic neutron scattering. Nanocrystalline specimens with different grain size and microstrain have been prepared by ball milling and thermal annealing. The vibrational density of states has been extracted from neutron time-of-flight spectra obtained with the spectrometer IN6 at the Institute Laue–Langevin. In comparison with reference coarse-grained specimens measured in the same conditions the nanocrystalline specimens exhibit: (i) a modest increase in the population of low-frequency modes and (ii) a distinct broadening of the transverse and longitudinal phonon peaks. The former feature is related to the presence of interface modes and it is critically compared with other observations in pure nanocrystalline metals. The latter is discussed in terms of reduced phonon lifetime due to the nanometric size of the crystallites.
Two-step growth of high-quality GaN by hydride vapor-phase epitaxy
Authors: Tavernier P. R. , Etzkorn E. V. , Wang Y. , Clarke D. R.
Published: July 2002

The use of a low-temperature layer of GaN formed by hydride vapor-phase epitaxy (HVPE) as a template to grow high-quality HVPE films is demonstrated. Using layers formed by reacting GaCl and NH3 at 550 °C and annealed at a growth temperature of 1050 °C, thick films of GaN can be grown by HVPE with fewer than 108 dislocations per cm2. Dislocation densities measured by high-resolution x-ray diffraction, atomic-force microscopy step termination density and plan-view transmission electron miscroscopy reveal that ∼23 μm films have dislocation densities of ∼6×107 cm−2. Obtaining high-quality single-crystal character films was found to be dependent on several factors, most importantly, the rate of temperature increase to growth temperature and the layer thickness.
Effect of film thickness on the properties of indium tin oxide thin films
Authors: Kim H. , Horwitz J. S. , Kushto G. , Piqué A. , Kafafi Z. H. , Gilmore C. M. , Chrisey D. B.
Published: July 2002

Transparent conducting indium tin oxide (ITO) thin films (40–870 nm) were grown by pulsed laser deposition on amorphous substrates and the structural, electrical, and optical properties of these films were investigated. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2, at substrate temperature of 300 °C and 10 mTorr of oxygen pressure. For ITO films (30–400 nm thickness) deposited at 300 °C in 10 mTorr of oxygen, a resistivity of 1.8–2.5×10−4 Ω cm was observed and the average transmission in the visible range (400–700 nm) was about 85%–90%. The Hall mobility and carrier density for ITO films (40–870 nm thickness) were observed to be in the range of 24–27 cm2/V s and 8–13×1020 cm−3, respectively. The ITO films have been used as the anode contact in organic light emitting diodes and the effect of ITO film thickness on the device performance has been studied. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 60–100 nm. The device with the optimum thickness of ITO anode showed an external quantum efficiency of about 0.85% at 100 A/m2.
Open-system quantum dynamics for gas-surface scattering: Nonlinear dissipation and mapped Fourier grid methods
Authors: Nest Mathias , Saalfrank Peter
Published: July 2002

When a gas phase atom or molecule hits a nonrigid surface, both elastic and inelastic scattering and sticking can occur. We suggest treating the dynamics of these processes using open-system density matrix theory. For the “free→free” and “free→bound” events at hand, both fundamental and numerical problems arise. The fundamental problem is that the adsorbate “system” is anharmonic and the coupling between the system and the substrate “bath” has to be nonlinear at least in the system coordinates. Here we propose a new Lindblad-type open-system density matrix approach which accounts for system anharmonicity and nonlinearity of the system–bath coupling. The numerical problem is that for a dissipative scattering process large basis sets or grids are required, making the storage and direct propagation of a density matrix difficult. To overcome this problem we use a mapped Fourier method which reduces the grid size and hence the storage requirements significantly. We apply the new methods and techniques to a simple model resembling the simultaneous scattering and sticking of an O2 molecule at a metal surface.
Investigation of the sites of dark spots in organic light-emitting devices
Authors: Liew Yoon-Fei , Aziz Hany , Hu Nan-Xing , Chan Hardy Sze-On , Xu Gu , Popovic Zoran
Published: July 2002

Poor environmental stability has been a major concern for organic light-emitting devices. Exposure to ambient conditions leads to the formation of nonemissive areas (dark spots) that result in a decrease in device luminescence. Although a number of mechanisms for the formation of dark spots have been proposed, the causes underlying their initiation, and their nucleation sites are still far from being clear. In this study, optical microscopy is used to investigate the sites of dark spots of devices in which the original cathodes are peeled off and replaced by newer cathodes. Results confirm that the growth of dark spots occurs primarily due to cathode delamination. The growth of dark spots is also associated with changes in the organic layers, especially at the organic/cathode interface. Results also suggest that the nucleation of dark spots takes place at the organic/cathode interface and originates during the deposition of the cathode. On the other hand, both the anode and the hole transport layer do not appear to play a role in the formation of dark spots.
Switching stability of magnetic tunnel junctions with an artificial antiferromagnet
Authors: Schmalhorst J. , Brückl H. , Reiss G. , Kinder R. , Gieres G. , Wecker J.
Published: July 2002

Avoiding fatigue in the switching of magnetic tunnel junctions is crucial for their long-term use in nonvolatile magnetic memories. We compare the switching stability of two types of junctions with different soft layers: Fe or Ni81Fe19, both with Co dusting at the barrier interface. The magnetically hard electrode is a Co/Cu/Co artificial antiferromagnet. While the tunneling magnetoresistance (TMR) remains unchanged after 104 cycles in a 4 kA/m rotating field, it decreases by more than 45% due to uniaxial switching. Fringing fields of domain walls in the soft layer and an intrinsic instability of Co/Cu/Co are identified as the main reasons. Magnetization reversal by two perpendicular switching pulses avoids this magnetic degradation and maintains a full TMR signal.
Dynamics of Ar+CH4/Ni{111} collision-induced desorption
Authors: Sun Lipeng , de Sainte Claire Pascal , Meroueh Oussama , Hase William L.
Published: July 2002

Classical trajectory simulations were used to study Ar+CH4/Ni{111} collision-induced desorption and compared with experiment. To perform the simulations, analytic potentials were determined for Ar/CH4 and CH4/Ni{111}. An accurate form for the former potential was derived by carrying out a series of ab initio calculations at various levels of theory, while previously published ab initio calculations were used to develop the latter CH4/Ni{111} potential. Overall the simulation and experimental desorption cross sections are in excellent agreement, except at small incident angles θi (with respect to the surface normal) and low initial Ar translational energies, Ei, where the simulation cross sections are approximately a factor of 2 too large. Most of the desorption occurs by trajectories in which Ar first strikes CH4, but for both large θi and Ei, a small fraction of the desorption occurs by trajectories in which Ar first strikes the Ni surface. Excitation of the CH4 vibrational modes is negligible and CH4 rotation receives less than 10% of the available energy. Most of the available product energy is partitioned to CH4 translation and to the Ni surface and Ar atom. At low Ei, CH4 translation receives the majority of the available energy, with the effect greater for large θi. At high Ei, approximately 40% of the available energy goes to CH4 translation, independent of θi. The CH4 translational energy distribution is multimodal and its peaks may be associated with trajectories in which the Ar atom rebounds off or sticks to the Ni surface and collisions in which Ar strikes CH4 with small and large impact parameters.
Thermal relaxation of the free layer anisotropy in spin valves
Authors: Baril Lydia , Mauri Daniele , McCord Jeffery , Gider Savas , Lin Tsann
Published: July 2002

While the thermal stability of the pinned layer magnetization in spin valves (SVs) has been studied extensively, less attention has been paid to the stability of the free layer anisotropy. Control of this anisotropy is important since it contributes significantly to the sensitivity and magnetic stability of the device. The kinetics of anisotropy relaxation in SVs are studied through isothermal hard axis anneals at temperatures ranging from 120 °C to 200 °C. A general quantitative method is applied to extract the anisotropy in a complete SV in the presence of interlayer coupling and pinned layer rotation. For a free layer consisting of a Ni81Fe19/Co90Fe10 bilayer, it is found that the anisotropy relaxation is proportional to T ln(t/t1), i.e., linear in temperature T and logarithmic in time t. The characteristic onset time t1 is described by t1_0 exp(Ea/kT). These results are compared with previously reported data for single NiFe films.
Blocking of resonant tunneling by electron Bragg reflectors in GaAs/AlxGa1−xAs quantum well structures
Authors: Lee Seung-Chang , Malloy Kevin J.
Published: July 2002

We examine the blocking of electrons selectively incident on a GaAs/Al0.22Ga0.78As electron Bragg reflector (EBR) from a GaAs/Al0.3Ga0.7As double barrier resonant tunneling structure (DBRTS). An EBR has a minimum transmittance at a specified blocking energy level, EB. This energy level is varied with respect to a resonant tunneling energy, EINJ, of the DBRTS. We find that the blocking efficiency is decreased as EB moves away from EINJ. To satisfy the Bragg condition, any potential barrier in the EBR must be lower than EINJ. We present experimental evidence of blocking by EBRs and compare it with blocking by potential barriers higher than EINJ.