bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of thermal transport in low stress silicon nitride films
Authors: Holmes W. , Gildemeister J. M. , Richards P. L. , Kotsubo V.
Published: July 2002

We have measured the thermal conductance, G, of ≈1 μm thick low stress silicon nitride membranes over the temperature range, 0.06<T<6 K, as a function of surface condition. For T>4 K, G is independent of surface condition indicating that the thermal transport is determined by bulk scattering. For T<4 K, scattering from membrane surfaces becomes significant. Membranes which have submicron sized Ag particles glued to the surface or are micromachined into narrow strips have a G that is reduced by a factor as large as 5 compared with that of clean, solid membranes with the same ratio of cross section to length.
Spontaneous and reverse-bias induced recovery behavior in organic electroluminescent diodes
Authors: Zou Dechun , Yahiro Masayuki , Tsutsui Tetsuo
Published: July 2002

Double-layer organic electroluminescent diodes composed of a spin-coated polyvinylcarvbazole layer and a vacuum-sublimed tris-(8-hydroxyquinoline)aluminum layer were prepared. The diodes were driven at constant voltage and were also kept under short-circuit or reverse-bias conditions. Observations of luminance-current density-voltage relations at constant voltage driving were repeated. The decrease of both luminance and current density during constant voltage driving was observed. Both spontaneous and reverse-bias assisted recoveries of device performances were observed, and these degradation and recovery phenomena were discussed in terms of the movement of ionic impurities in organic layers.
Giant magnetoresistance enhancement in inhomogeneous semiconductors
Authors: Thio Tineke , Solin S. A.
Published: July 2002

We show that the low-field magnetoresistance (MR) of a semiconductor with inhomogeneities which are more conducting than the surrounding matrix is enhanced significantly compared to that of the homogeneous material. The enhancement results from a magnetic field induced geometric effect in which at high field the current is deflected around the conducting inhomogeneity. A model based on previous work by Wolfe et al. has been augmented to include the physical MR. The augmented model is used to account quantitatively for a 50-fold anomalous enhancement of the giant magnetoresistance (GMR) observed at low field in (near) zero-band-gap thin-film Hg1−xCdxTe (x≈0.1). The practical implications of the GMR boost are noted.
Infrared study of light-induced reactivation of neutralized dopants in hydrogenated n-type GaAs doped with silicon
Authors: Loridant-Bernard D. , Mezière S. , Constant M. , Dupuy N. , Sombret B. , Chevallier J.
Published: July 2002

Effects of light-induced reactivation of neutralized dopants in hydrogenated n-type silicon-doped GaAs epilayers have been investigated using infrared spectroscopy and electrical measurements as well. Photoexcitation of the passivated material has been performed over a wide wavelength domain (250 nm–1.06 μm), and the samples have been analyzed by using extensively both specular reflection and transmission infrared techniques. In all cases, the observed structural modifications have been correlated to the electrical properties evaluated by measuring sheet resistance of the material. The obtained experimental data permit a simplified analysis of the dissociation of dopant-hydrogen complexes versus the photon energy of the incident light.
Effects of precipitate distribution on electromigration in Al–Cu thin-film interconnects
Authors: Han J. H. , Shin M. C. , Kang S. H. , Morris J. W.
Published: July 2002

This letter reports that electromigration lifetimes of Al–2Cu (wt. %) thin-film conducting lines increase by more than three times when the lines are optimally aged to facilitate finely dispersed Al2Cu precipitates in the interior of the grains. In contrast to other studies which did not report a beneficial aging effect for Al–Cu films, the present work substantiates the fact that proper control of Al2Cu precipitates improves resistance to electromigration failure. However, the benefit of aging the Al-2Cu lines investigated here was less pronounced and confined to a more narrow heat-treatment “window” than that previously found for the Al–2Cu–1Si lines.
Incorporation of indium during molecular beam epitaxy of InGaN
Authors: Böttcher T. , Einfeldt S. , Kirchner V. , Figge S. , Heinke H. , Hommel D. , Selke H. , Ryder P. L.
Published: July 2002

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.
Long wavelength superlattice quantum cascade lasers at λ≃17 μm
Authors: Tredicucci Alessandro , Gmachl Claire , Capasso Federico , Sivco Deborah L. , Hutchinson Albert L. , Cho Alfred Y.
Published: July 2002

We report the realization of a semiconductor injection laser based on intraband transitions with emission wavelengths extending beyond the atmospheric windows. The structure uses the quantum cascade scheme with “chirped” superlattices as active material. Laser action in pulsed operation is achieved at λ≃17 μm up to 150 K, with peak output powers of ∼12 mW at cryogenic temperatures.
Intermixing in strained InGaAs/GaAs quantum-well infrared photodetectors
Authors: Lee Alex S. W. , Li E. Herbert , Karunasiri Gamani
Published: July 2002

The effect of interdiffusion on strained InGaAs/GaAs quantum-well infrared photodetectors is investigated. Photoluminescence measurements of the interband transition indicate that there is minimal deterioration of the annealed heterostructures, as it is also evident from both the transverse electric and transverse magnetic infrared intersubband optical transitions. The absorption peak wavelength is redshifted from the as-grown 10.2 μm to 10.5 and 11.2 μm for 5 and 10 s annealing, respectively, at 850 °C without appreciable degradation in absorption strength. The peak responsivity of the as-grown and annealed spectra is of comparable amplitude, whereas the annealed spectra become narrower in shape. The dark current of the annealed devices is about an order of magnitude higher than the as-grown one at 77 K.
Measurement of drift mobility in AlGaN/GaN heterostructure field-effect transistor
Authors: Dang X. Z. , Asbeck P. M. , Yu E. T. , Sullivan G. J. , Chen M. Y. , McDermott B. T. , Boutros K. S. , Redwing J. M.
Published: July 2002

Low-field mobilities for electrons in the channel of an Al0.15Ga0.85N/GaN heterostructure field-effect transistor are derived from direct current transistor characteristics. The dependencies of mobility on gate bias, sheet carrier concentration, and temperature are obtained. For negative gate bias voltages, mobility is found to increase monotonically with increasing sheet carrier concentration, which we interpret as a consequence of increased screening of carrier scattering. For positive gate bias voltages, mobility is found to decrease with increasing gate bias due to the onset of parallel conduction in the AlGaN barrier layer. The mobility varies approximately as T−α with α≈1.6–1.8 for temperature ranging from 200 to 400 K, indicating that phonon scattering is dominant in the two-dimensional electron gas in this temperature range.
Boron nitride nanotubes, webs, and coexisting amorphous phase formed by the plasma jet method
Authors: Shimizu Yoshiki , Moriyoshi Yusuke , Tanaka Hideki , Komatsu Shojiro
Published: July 2002

Boron nitride (BN) nanotubes were prepared by a high-temperature plasma jet method. While the BN nanotubes were found in quantity on the surface of a BN sintered disk, amorphous BN webs were found dominant in the water-cooled collector. These structures were studied on the basis of transmission electron microscopy observations. One of them was composed of 21 layers grown coaxially with a regular spacing of 0.34 nm and their tip ends were terminated with parallel bases of the coaxial cylindrical shapes. The diameter of the innermost tube was 0.6 nm and this dimension is in fairly good agreement with the diameters of the smallest BN fullerenes.
Enhanced magnetoresistance in sintered granular manganite/insulator systems
Authors: Petrov D. K. , Krusin-Elbaum L. , Sun J. Z. , Feild C. , Duncombe P. R.
Published: July 2002

We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.
Effects of growth temperature on GaN nucleation layers
Authors: Yi M. S. , Lee H. H. , Kim D. J. , Park S. J. , Noh D. Y. , Kim C. C. , Je J. H.
Published: July 2002

The effects of growth temperature on the microscopic structure of GaN nucleation layers were studied in a synchrotron x-ray scattering experiment. As the growth temperature increases from 467 to 655 °C, the stacking of GaN changes from random stacking to a mixture of cubic and hexagonal stacking. With increasing the growth temperature, the order in the atomic layer positions in the out-of-plane direction increases and the mosaic distribution becomes narrow. The optimal photoluminescence spectrum was obtained on the GaN epilayer deposited on the nucleation layer grown at 505 °C.
Evolutionary phase diagrams for plasma-enhanced chemical vapor deposition of silicon thin films from hydrogen-diluted silane
Authors: Koh Joohyun , Ferlauto A. S. , Rovira P. I. , Wronski C. R. , Collins R. W.
Published: July 2002

Real-time optical studies have been applied to develop phase diagrams that characterize plasma-enhanced chemical vapor deposition (PECVD) of silicon thin films at low temperature (200 °C). The deposition phase diagrams describe regimes over which predominantly amorphous and microcrystalline Si phases are obtained as a function of the accumulated thickness and the hydrogen-to-silane gas flow ratio R=[H2]/[SiH4] in the PECVD process. The diagrams for different substrates provide insights into optimization of amorphous Si materials and solar cells.
Al composition dependence of breakdown voltage in AlxGa1−xN Schottky rectifiers
Authors: Zhang A. P. , Dang G. , Ren F. , Han J. , Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Redwing J. M. , Cao X. A. , Pearton S. J.
Published: July 2002

Planar geometry, lateral Schottky rectifiers were fabricated on high resistivity AlxGa1−xN (x=0–0.25) epitaxial layers grown on sapphire substrates. The reverse breakdown voltages of unpassivated devices increased with Al composition, varying from 2.3 kV for GaN to 4.3 kV for Al0.25Ga0.75N. The reverse current–voltage (I–V) characteristics showed classical Shockley–Read–Hall recombination as the dominant mechanism, with I∝V0.5. The reverse current density in all diodes was in the range 5–10×10−6 A cm−2 at 2 kV. The use of p+ guard rings was effective in preventing premature edge breakdown and with optimum ring width increased VB from 2.3 to 3.1 kV in GaN diodes.
Enhancement of ion extraction from a cold plasma with radio-frequency plasma heating
Authors: Ohzu Akira , Suzuki Yoji , Maruyama Yoichiro , Arisawa Takashi
Published: July 2002

Ion extraction from a cold plasma produced by laser multiphoton ionization has been enhanced by using radio-frequency (140 MHz) dielectric plasma heating. When 10 W in heating power is applied to a plasma of ion density around 3×1010 cm−3, it is observed that the peak ion flux extracted with an electrostatic voltage of 20 V increases by a factor of more than 2. The time for extracting all the ions becomes short by a factor of 3 without any production of radio-frequency discharged plasma. It is considered that these results are due to the increase in the electron temperature of the plasma. This method will be very useful for the process of ion extraction in atomic vapor laser isotope separation.
Low-threshold index-guided 1.5 μm long-wavelength vertical-cavity surface-emitting laser with high efficiency
Authors: Ortsiefer M. , Shau R. , Böhm G. , Köhler F. , Amann M.-C.
Published: July 2002

A significantly improved InP-based vertical-cavity surface-emitting laser with record device performance is demonstrated. Utilizing a twofold epitaxial growth process, self-adjusted lateral current confinement and index guiding are accomplished by means of a buried InGa(Al)As tunnel junction. Front and back mirrors are realized by 35 epitaxial InGaAlAs/InAlAs layer pairs and a 1.5 MgF2/a-Si layer pair, respectively. At room temperature and under continuous wave condition, lasers with small aperture diameters of only 13 μm exhibit record output powers of 1.6 mW with quantum efficiencies around 25%. For these devices, threshold current and voltage are as low as 4 mA and 1.2 V, respectively, because of low series resistances around 70 Ω.
Performance degradation of small silicon devices caused by long-range Coulomb interactions
Authors: Fischetti M. V. , Laux S. E.
Published: July 2002

In small silicon devices, conduction electrons in the channel are subject to long-range Coulomb interactions with electrons in the heavily doped drain, source, and gate regions. We show that, for devices with channel lengths shorter than about 40 nm and oxides thinner than 2.5 nm, these interactions cause a reduction of the electron velocity. We present results obtained using both semiclassical two-dimensional self-consistent Monte Carlo/Poisson simulations and a quantum-mechanical model based on electron scattering from gate–oxide interface plasmons.
Metallization scheme for highly low-resistance, transparent, and thermally stable Ohmic contacts to p-GaN
Authors: Jang Ja-Soon , Park Seong-Ju , Seong Tae-Yeon
Published: July 2002

We report on a promising metallization scheme for high-quality Ohmic contacts to surface-treated p-GaN:Mg (2–3×1017 cm−3). It is shown that the as-deposited Pt/Ru contact produces a specific contact resistance of 7.8(±2.2)×10−4 Ω cm2. However, annealing of the contact at 600 °C for 2 min results in a resistance of 2.2(±2.0)×10−6 Ω cm2. It is also shown that the light transmittance of the annealed contact is 87.3% at 470 nm. Furthermore, the surface of the contact annealed at 600 °C for 30 min is found to be very smooth with a rms roughness of 0.8 nm. These results strongly indicate that the Pt/Ru can be a suitable scheme for the fabrication of high-performance laser diodes or other devices.
Blue–violet photoluminescence from large-scale highly aligned boron carbonitride nanofibers
Authors: Bai X. D. , Wang E. G. , Yu J. , Yang Hui
Published: July 2002

We report on the strong blue–violet photoluminescence (PL) at room temperature from the large-scale highly aligned boron carbonitride (BCN) nanofibers synthesized by bias-assisted hot filament chemical vapor deposition. The photoluminescence peak wavelength shifts in the range of 470–390 nm by changing the chemical composition of the BCN nanofibers, which shows an interesting blue and violet-light-emitting material with adjustable optical properties. The mechanism for the shift of the PL peaks at room temperature is also discussed.
Investigation of deep levels in rapid thermally annealed SiO2-capped n-GaAs grown by metal-organic chemical vapor deposition
Authors: Deenapanray P. N. K. , Tan H. H. , Jagadish C. , Auret F. D.
Published: July 2002

Defects created in rapid thermally annealed (RTA) SiO2-capped epitaxial GaAs layers grown by metal-organic chemical vapor deposition have been investigated by deep level transient spectroscopy. RTA introduced four electron traps S1 (Ec−0.23 eV), S2 (Ec−0.46 eV), S3 (Ec−0.72 eV), and S4 (Ec−0.74 eV). S1 may be the so-called EL9 defect. We propose that S2 is a defect complex involving the Ga vacancy and Si dopant atoms, VGa–SiGa, and associate it with the EL5. S2 is introduced almost uniformly within the first 0.8 μm below the surface with an activation energy of 4.4 eV. S4 is most probably one of the EL2 family. The concentration of S4 decreased exponentially below the surface with a characteristic decay length ∼0.2 μm. The activation energy for the introduction of S4 is 2.5 eV.
Interfacial stability of an indium tin oxide thin film deposited on Si and Si0.85Ge0.15
Authors: Ow-Yang Cleva W. , Shigesato Yuzo , Paine David C.
Published: July 2002

The stability of the interface formed by depositing indium tin oxide (ITO) thin films on Si and Si0.85Ge0.15 substrates was investigated. Cross-sectional transmission electron microscopy combined with Fourier-transform infrared spectroscopy, energy dispersive x-ray analysis, x-ray diffraction, and capacitance–voltage measurements were used to characterize the interface immediately after rf magnetron sputter deposition as a function of annealing time in ultrahigh purity (UHP) N2 at 785 °C for 10–80 min. The In–Si–O2 ternary phase equilibrium diagram was calculated to predict the possible product layer sequences. A 2-nm-thick interfacial amorphous silicon oxide, associated with ion implantation intermixing, is present in the ITO/Si as-deposited sample, while a 3–7-nm-thick amorphous oxide interlayer is observed in the ITO/Si0.85Ge0.15 sample. During annealing in UHP N2, the interlayer oxide growth rate follows the initial stage of conventional oxidation. In the ITO/Si system, experimental observations revealed a preference for the displacement reaction limited by the diffusivity of Si through the SiO2 layer, i.e., Si/SiO2/In/In2O3. This solid-state oxidation process has the potential for in situ fabrication of ITO metallized SiO2 gates in Si thin film transistor applications. On the other hand, the more complicated quaternary In–Si–Ge–O2 system reveals two distinct reaction layer morphologies, suggesting that the presence of Ge strongly influences the stability of the interfacial thermodynamics and kinetics.
The baroclinic secondary instability of the two-dimensional shear layer
Authors: Reinaud Jean , Joly Laurent , Chassaing Patrick
Published: July 2002

The focus of this study is on the numerical investigation of two-dimensional, isovolume, high Reynolds and Froude numbers, variable-density mixing layers. Lagrangian simulations, of both the temporal and the spatial models, are performed. They reveal the breaking-up of the strained vorticity and density-gradient braids, connecting two neighboring primary structures. The secondary instability arises where the vorticity has been intensified by the baroclinic torque. A simplified model of the braid of the variable-density mixing layer, consisting of a strained vorticity and density-gradient filament, is analyzed. It is concluded that the physical mechanism responsible for the secondary instability is the forcing of the vorticity field by the baroclinic torque, itself sensitive to perturbations. This mechanism suggests a rapid route to turbulence for the variable-density mixing layer.
Electrical depth profile of p-type GaAs/Ga(As, N)/GaAs heterostructures determined by capacitance–voltage measurements
Authors: Krispin P. , Spruytte S. G. , Harris J. S. , Ploog K. H.
Published: July 2002

Capacitance–voltage measurements on metal-semiconductor contacts are used to examine depth-resolved electrical characteristics of GaAs/Ga(As, N)/GaAs heterostructures. The experimental depth profiles of the carrier concentration are compared with calculations based on self-consistent solutions of the Poisson equation. As-grown Ga(As, N) layers are p type, and hole concentrations of about 3×1016 cm−3 are observed for undoped Ga(As, N) layers with a GaN mole fraction of 3% and thicknesses below 80 nm. This hole concentration is stable during rapid thermal annealing. For a GaN mole fraction of about 3%, the valence band offset between GaAs and Ga(As, N) is found to be +(11±2) meV. The heterointerfaces are of type I. The dominant carrier depletion in as-grown heterostructures is due to donor-like defect levels, which are accumulated at the GaAs-on-Ga(As, N) interface. The amount of these interfacial defects rises remarkably in thicker Ga(As, N) layers, but can be completely removed by rapid thermal annealing after growth. By release spectroscopy, further hole traps with definite level energies are distinguished at the Ga(As, N)-on-GaAs interface, which are probably due to the specific GaAs growth conditions.
Magneto-optical disk properties enhanced by a nonmagnetic mask layer
Authors: Kim J. H. , Buechel D. , Nakano T. , Tominaga J. , Atoda N. , Fuji H. , Yamakawa Y.
Published: July 2002

Magneto-optical (MO) recording signals were enhanced by a nonmagnetic mask layer combining a silver oxide (AgOx) thin film and near-field coupled super-resolution. The signal intensity of 300 nm marks recorded by a 680 nm laser and a lens numerical aperture of 0.55 with light-intensity modulation was amplified 100 times in comparison with conventional MO media. Resolution of less than 200 nm was achieved by near-field coupling between a light-scattering center generated in the AgOx film and light polarization of MO marks. To identify near-field enhancement, we simulated electrical field components, finding that near-field coupling between the light-scattering center and a MO layer enhances MO signals about 20 times due to the surface plasmon effect.
Microstructure of epitaxial La0.7Ca0.3MnO3 thin films grown on LaAlO3 and SrTiO3
Authors: Lu C. J. , Wang Z. L. , Kwon C. , Jia Q. X.
Published: July 2002

Epitaxial La0.7Ca0.3MnO3 (LCMO) thin films of a thickness ∼170 nm were grown on (001) LaAlO3 (LAO) and (001) SrTiO3 (STO) substrates by pulsed laser deposition. Transmission electron microscopy and associated techniques have been applied to investigate the microstructures introduced by lattice mismatch that are responsible for the observed differences in properties between these two films. Numerous secondary phase rods were observed in both films. For the LCMO/LAO film, Ca-deficient secondary-phase rods originated in the film after a thickness of about 25 nm and were found to be responsible for relieving in-plane compressive stress during the island growth. In the case of STO substrate, however, almost all of secondary-phase rods initiated at the film–substrate interface. The lattice mismatch between LCMO and STO is relaxed into regions of good coherent fit separated by such secondary phases, possibly resulting from interfacial reaction. The two types of substrates lead to the formation of two different crystallographic domain structures in the LCMO films. The film on LAO exhibits an almost pure [110] out-of-plane texture with 90° domains in plane. In contrast, the film grown on STO consists of mixed domains of [001] and [110] orientations and is dominated by [001] texture.
Complete set of deep traps in semi-insulating GaAs
Authors: Pavlović M. , Desnica U. V. , Gladić J.
Published: July 2002

Reevaluation and recalculation of thermally stimulated current (TSC) data from semi-insulating (SI) GaAs, published by many different authors over a period of three decades were done by means of the new analytical method, simultaneous multiple peak analysis (SIMPA). The SIMPA procedure clearly resolved contributions from various overlapping TSC peaks and enabled the precise determination of signatures (activation energy, Ea and capture cross section, σ) of all observed deep traps. The analyzed TSC spectra refer to SI GaAs samples that have been grown/treated in quite different ways (various growth techniques, growth under As or Ga rich conditions, different annealing procedures, irradiation with neutrons, γ rays, etc.). Although the SIMPA procedure was applied to apparently quite different TSC spectra, in all cases excellent fits were achieved, with the unique set (or subset from it) of eleven different deep traps, the only difference being in relative and absolute concentrations of traps. Despite a broad variety of samples analyzed in this article, the set of deep traps obtained is the same as the one being previously seen in the narrow range of SI GaAs samples. This finding suggests that this set of traps is a finite and complete set of all defects with deep levels in SI GaAs. It was also concluded that these defects are primarily complexes containing simple native defects.
Particle segregation in suspensions subject to high-gradient ac electric fields
Authors: Dussaud Anne D. , Khusid Boris , Acrivos Andreas
Published: July 2002

An experimental and theoretical study is described dealing with the dielectrophoretic motion of individual particles in a static as well as in a flowing suspension subject to high-gradient ac electric fields. The experiments were performed on very dilute suspensions of neutrally buoyant hollow ceramic spheres in a specially designed device in which the electric-field lines and the dielectrophoretic force were along the plane perpendicular to the streamlines of the main flow. Upon application of a high-gradient field (∼several kV/mm) to a quiescent suspension, the particles were found to move away from the electrodes and then to concentrate above the grounded electrodes, forming a distinct boundary between the clean fluid and the remaining suspension. This same field, when applied to a flowing suspension, caused the particles to concentrate within thin stripes parallel to the flow above the grounded electrodes and to travel with the suspending fluid within these stripes. The theoretical model for the particle motion included only the dielectrophoretic force and the viscous drag, and required no fitting parameters because the particle polarizability was calculated independently by measuring the concentration dependence of the complex permittivity of the suspension in a spatially uniform electric field of low strength (∼several V/mm). The computed particle motions and pattern formations were found to be in a good agreement with the experimental data. These results demonstrate that the expression for the dielectrophoretic force which employs the value of the particle polarization measured in fields of low strength can be used for describing the particle motions in fields of high strength. This approach enables one to model a broad range of electro-hydrodynamic phenomena in suspensions irrespective of whether or not they are perfectly insulating or perfectly conducting.
Atomic-layer-deposited silicon-nitride/SiO2 stacked gate dielectrics for highly reliable p-metal–oxide–semiconductor field-effect transistors
Authors: Nakajima Anri , Yoshimoto Takashi , Kidera Toshiro , Obata Katsunori , Yokoyama Shin , Sunami Hideo , Hirose Masataka
Published: July 2002

An extremely thin (∼0.4 nm) silicon-nitride layer has been deposited on thermally grown SiO2 by an atomic-layer-deposition (ALD) technique. The boron penetration through the stacked gate dielectrics has dramatically been suppressed, and the reliability has been significantly improved, as confirmed by capacitance–voltage, gate-current–gate-voltage, and time-dependent dielectricbreakdown characteristics. The ALD technique allows us to fabricate an extremely thin, very uniform silicon-nitride layer with atomic-scale control.
Optical properties of a photonic crystal of hollow spherical shells
Authors: Rengarajan Rajesh , Jiang Peng , Colvin Vicki , Mittleman Daniel
Published: July 2002

We report on optical transmission studies of three-dimensionally ordered photonic crystals of close-packed spherical shells. These samples are fabricated using a double-template method, which allows for extensive control over shell thickness. The transmission spectra exhibit an optical stop band, whose spectral position and width depend on the thickness of the shell and on the overlap between adjacent spheres, in a manner consistent with numerical simulations. These parameters can be controlled over a wide range, thus permitting systematic studies of the optical properties, and providing a valuable method for engineering the characteristics of the optical stop band in colloidal photonic media.
Low-order scaling local correlation methods II: Splitting the Coulomb operator in linear scaling local second-order Møller–Plesset perturbation theory
Authors: Hetzer Georg , Schütz Martin , Stoll Hermann , Werner Hans-Joachim
Published: July 2002

A novel multipole approximation for the linear scaling local second-order Møller–Plesset perturbation theory (MP2) method is presented, which is based on a splitting of the Coulomb operator into two terms. The first one contains the singularity and is rapidly decaying with increasing distance. It is treated by a conventional two-electron transformation, where the rapid decay leads to significant savings. The second term is long range, but nonsingular and can therefore be approximated by a multipole expansion. Reliability, accuracy, and efficiency of this method are demonstrated by an extensive benchmark study. It is shown that the goal to further improve the efficiency of the existing linear scaling local MP2 algorithm has been achieved. Moreover, the new method is a promising starting point for future developments, such as coupling of MP2 with density functional theory.
Evolution and properties of nanodiamond films deposited by direct current glow discharge
Authors: Heiman A. , Gouzman I. , Christiansen S. H. , Strunk H. P. , Comtet G. , Hellner L. , Dujardin G. , Edrei R. , Hoffman A.
Published: July 2002

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 vol % methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950 °C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the morphology of the films was examined showing that the formation of the nanodiamond phase is initially accompanied with an increase in surface roughness which decreases with film growth. By high-resolution transmission electron microscopy it was determined that the continous nanodiamond films are composed of diamond nanocrystallites, 3–5 nm in diameter.
Effect of temperature on longitudinal optical phonon-assisted exciton luminescence in heteroepitaxial GaN layer
Authors: Xu S. J. , Liu W. , Li M. F.
Published: July 2002

First-order and second-order longitudinal optical (LO) phonon-assisted luminescence lines of neutral donor-bound excitons and free excitons in heteroepitaxial GaN film have been investigated in temperature range from 3.5 to 50 K. The energy spacing between the exciton resonant lines and their corresponding LO phonon replicas is found to be strong temperature dependent. Permogorov’s theory on LO phonon-assisted luminescence of free excitons could be employed to explain the main experiments. It is also found that the interaction strength of the free exciton with LO phonon is stronger than that of the bound exciton with LO phonon.
Gallium oxide as an insulating barrier for spin-dependent tunneling junctions
Authors: Li Zisen , de Groot Cornelis , Moodera Jagadeesh H.
Published: July 2002

Spin-dependent tunneling has been shown to occur through Ga2O3 as the insulating tunnel barrier. Magnetic tunnel junctions of the type Co/Ga2O3/Ni80Fe20 were prepared by oxidizing thin metal layer of Ga in oxygen plasma and characterized. The highest junction magnetoresistance observed was 18.2% at room temperature, increasing to 27.6% at 77 K. The average barrier height was estimated to be about 2 eV, as opposed to over 3 eV for junctions with Al2O3 barrier of comparable quality. Otherwise these junctions behave similar to those with Al2O3. This shows the feasibility of obtaining lower resistance junctions with Ga2O3 as the barrier for magnetic storage applications.
Long-range Coulomb interactions in small Si devices. Part II. Effective electron mobility in thin-oxide structures
Authors: Fischetti M. V.
Published: July 2002

In metal–oxide–semiconductor structures with polycrystalline Si gates, electrons in the inverted channel of the substrate scatter with electrons in the gate via long-range Coulomb interactions. For thin oxides, these interactions can cause a significant transfer of momentum from the channel to the gate, thus reducing the effective mobility of the two-dimensional electron gas in the substrate. We present calculations of the dispersion of the interface plasmons in poly-Si/SiO2/Si structures, comparing the results obtained in the long-wavelength limit to those obtained using the random-phase approximation. Employing the former model, we compute the effect of plasmon scattering on the effective electron mobility in Si inversion layers. We find a significant reduction of the mobility for oxides thinner than about 3 nm.
Aluminum doping of poly(vinylidene fluoride with trifluoroethylene) copolymer
Authors: Xu B. , Borca C. N. , Ducharme S. , Sorokin A. V. , Dowben P. A. , Fridkin V. M. , Palto S. P. , Petukhova N. N. , Yudin S. G.
Published: July 2002

The interface between aluminum and crystalline copolymer thin film of vinylidene fluoride (70%) with trifluoroethylene (30%) [PVDF-TrFE] has been studied. The ratio of carbon 1s and aluminum 2p core level photoemission peak intensities changes little with increasing emission angle after deposition of 5 Å aluminum on the surface of PVDF-TrFE. This indicates that the distribution of aluminum atoms in the copolymer film is quite uniform in the near surface region and that the interface between PVDF-TrFE and aluminum is not abrupt. There is evidence for changes in photohole screening with aluminum doping within the polymer film.
The plasma-sheath transition with a constant mean free path model and the applicability of the Bohm criterion
Authors: Franklin R. N. , Snell J.
Published: July 2002

The fluid model for positive ions is used together with the assumption of a constant ion mean free path to examine the structure of the plasma-wall sheath at low and medium pressures in plasmas where ionization is by electron impact and the ion temperature is set to zero. It is shown that in the near collisionless regime there is the same structure as was found with the constant collision frequency for momentum transfer model with the Bohm speed playing the same role and similar scaling of the transitional layer. At higher pressures there is again no transitional layer, only plasma joining smoothly to a collisional sheath, thus the Bohm criterion has no significance, but the scaling is with a different exponent than for the mobility case. No fundamental differences are found between the two models. An expression is derived to describe the boundary between the parameter region where the Bohm criterion applies and that where it does not.
Spectroscopic determination of the melting energy of a gold nanorod
Authors: Link S. , El-Sayed M. A.
Published: July 2002

Gold nanorods in colloidal solution can be melted into spherical nanoparticles by excitation with intense femtosecond laser pulses of the proper energy. The threshold of the laser pulse energy for the complete melting of the nanorods with a mean aspect ratio of 4.1 in solution is determined by observing the change in the absorption intensity of the longitudinal absorption band (measure of the rod concentration) at 800 nm with increasing number of laser pulses of known energy. The number of laser pulses needed to reduce the band intensity (rod concentration) by 1/e of its initial value is determined as the laser energy per pulse increases. For pulses of lower energy than threshold, it is found that the number of pulses required to melt the gold nanorods present in solution increases significantly with decreasing laser pulse energy. Above threshold, this number is constant since the additional absorbed laser energy will only further heat the particles to temperatures above their melting point. The gold concentration in the colloidal solution is measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), from which the gold nanorod concentration is determined from the known shape and size distribution obtained from transmission electron microscopy (TEM) results. A simple analysis using the determined threshold energy and the nanorod concentration showed that it takes an average of ∼ 60 femtojoule (fJ) to melt a single gold nanorod. Experiments using 820 nm as well as 410 nm femtosecond laser pulses yield similar values, indicating that the laser induced shape transformation of the nanorods is independent of the irradiation wavelength and that this process is therefore photothermal in origin.
Using the finite-difference time-domain pulse propagation method to simulate time-resolved THz experiments
Authors: Beard Matthew C. , Schmuttenmaer Charles A.
Published: July 2002

The finite-difference time-domain (FDTD) method has been applied to time-resolved THz spectroscopy (TRTS) experiments. Time-resolved THz spectroscopy utilizes an optical pump pulse to excite the sample, followed by a far-infrared (FIR) probe pulse with frequency components that span from 10 to 100 cm−1. The subpicosecond evolution of the FIR spectrum is obtained as a function of time after the visible photoexcitation event. Significant challenges arise in interpreting these experimental results due to the very different frequencies of the pump and probe pulses. Therefore, it is essential to simulate the experiment. The method described entails numerically propagating both the THz probe pulse and the visible pump pulse simultaneously, keeping track of the transiently induced polarization from absorption of the visible pulse. Group velocity mismatch between the visible and THz pulse and a transiently changing response function are completely accounted for in the calculation. Furthermore, a spatially varying polarization can be included to account for a nonuniform excited region of the sample under investigation. The response function of the material is described as a multimode Brownian oscillator that can describe dispersive media in a very general sense. In particular, the overdamped, underdamped, and critically damped cases are all included, as well as special cases such as a Debye or Drude response. As a specific example, we present results of modeling a TRTS experiment of photoexcitation of a dye in solution, namely, 2,11,20,29-tetra-tert-butyl-2,3-napthalocyanine, dissolved in toluene. We carry out a nonlinear least squares fit of a parameterized model to the measured data to show that the FDTD–TRTS method is able to accurately reproduce the features observed in the measured data set.
Shock response of the elastomer, polychloroprene
Authors: Millett J. C. F. , Bourne N. K.
Published: July 2002

The high-strain rate response of polymers is becoming increasingly important to understand, given their widespread use in many industries. In particular, they are used as bonding agents in the production of materials such as polymer bonded explosives. In this case, their properties become especially important to understand since the safety of the energetic composition depends upon their response. There are three broad classes of polymer microstructure; elastomers, thermoplastics, and thermosets. The shock response of one of the first of these three classes is investigated in this work. Work is described in which the Hugoniot of the material is investigated using plate impact experiments, measuring the stress at varying positions to determine the shock and the stress-particle velocity state.
Magnetic noise measurements using cross-correlated Hall sensor arrays
Authors: Jung G. , Ocio M. , Paltiel Y. , Shtrikman H. , Zeldov E.
Published: July 2002

An experimental technique for measuring magnetic fluctuations by means of a double-layer Hall sensor array is described. The technique relies on cross-correlating Hall signals from two independent sensors positioned one above the other in two separate two-dimensional-electron-gas layers of a GaAs/AlGaAs heterostructure. The effectiveness of the technique is demonstrated by a reduction of the magnitude of the background noise floor of the correlated sensors with respect to the noise level of the best single sensor.
Photoluminescence and ultraviolet laser emission from nanocrystalline ZnO thin films
Authors: Mitra Anirban , Thareja R. K.
Published: July 2002

We report on photoluminescence and ultraviolet laser emission from ZnO pellets and thin films of ZnO. Laser emission from disordered polycrystalline thin films and pellets was observed in all directions. ZnO films were deposited on glass substrate at room temperature in various ambient gas pressures of oxygen using the pulsed laser deposition technique. The dependence of laser emission on the size of nanocrystallites observed at different pressures of ambient gas is discussed. Photoluminescence spectra depend on the stoichiometry and the microstructure of the film.
Dielectric spectroscopy for bioanalysis: From 40 Hz to 26.5 GHz in a microfabricated wave guide
Authors: Facer G. R. , Notterman D. A. , Sohn L. L.
Published: July 2002

We report developing coplanar waveguide devices which can perform dielectric spectroscopy on biological samples within a microfluidic channel or well. Since coupling to the fluid sample is capacitive, no surface functionalization or chemical sample preparation are required. Data on cell suspensions and solutions of proteins and nucleic acids spanning the frequency range from 40 Hz to 26.5 GHz are presented. Low-frequency data are well explained using a simple dispersion model. At microwave frequencies, the devices yield reproducible and distinguishable spectral responses for hemoglobin solution and live E. coli.
Instabilities and structures in optical and microwave breakdown processes
Authors: Gildenburg V. B. , Vvedenskii N. V.
Published: July 2002

The results of investigations on the nonlinear dynamics of the field and plasma in the gas breakdown processes produced by a high-intensity laser and microwave pulses are presented. The ionization nonlinearity dominates under the conditions considered and results in a number of important effects of a new type, which are absent for other nonlinear mechanisms. The scope of effects focused on here is associated with the phenomena of mutual concentration (joint localization) of the field and field-created plasma. They, in particular, manifest themselves as fast small-scale instabilities at the gas breakdown stage, the generation of high local peaks in the field amplitude, electron density and temperature, and the creation of long-lived states with a high level of Langmuir turbulence.
On the modes and loss mechanisms of a high Q mechanical oscillator
Authors: Liu Xiao , Morse S. F. , Vignola J. F. , Photiadis D. M. , Sarkissian A. , Marcus M. H. , Houston B. H.
Published: July 2002

We have performed laser-Doppler vibrometry measurements of the vibration of a double-paddle oscillator. Seven modes with principally out-of-plane motion have been identified. Their resonance frequencies and mode shapes are in excellent agreement with three-dimensional finite element simulations. We have found that the second antisymmetric torsional mode has exceptionally good vibration isolation of its mode shape. This explains its extremely small low temperature internal friction below 10 K (2×10−8). By correlating the internal friction of each mode with features of their mode shapes, a criterion has been established to develop high Q oscillators.
Dilute magnetic semiconductors in spin-polarized electronics (invited)
Authors: Schmidt Georg , Molenkamp Laurens W.
Published: July 2002

Dilute magnetic semiconductors have proven to be very useful in building an all-semiconductor platform for spintronics—so far they provide the only viable route to establish spin-polarized current injection into a nonmagnetic semiconductor. The reasons for this become apparent from a simple spin-channel model, which predicts that spin injection into a semiconductor can, within linear response, only readily be achieved from a ferromagnetic injector that has: (i) a resistivity that is comparable to the semiconductor and (ii) preferably is 100% spin polarized. Both of these criteria can be met in magnetic semiconductors, but (so far) are hard to achieve using other materials. Experimentally, we demonstrate how dilute magnetic II–VI semiconductors can be used to inject a strongly (up to 90%) spin-polarized current into a light emitting diode. In addition, we discuss the implications of the spin-channel model for the observation of giant magnetoresistance-like effects in the magnetoresistance of an all-semiconductor device.
Influence of substrate structure on the growth of CdTe thin films
Authors: Yan Y. , Dhere R. G. , Jones K. M. , Al-Jassim M. M.
Published: July 2002

We report on a high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy study of the microstructure of CdTe thin films epitaxially grown on single-crystal hexagonal CdS and cubic CdTe substrates. We find that the different structures of the substrates do not make a great structural difference on the grown CdTe films; i.e., on both substrates, the grown CdTe films have a cubic structure and high density of planar defects near the interface regions. At the CdTe/CdS interface, interdiffusion occurs, forming CdTe1−xSx and CdS1−xTex alloys. These alloys lead to significantly reduced mismatch at the interface.
Measurement of solution viscosity by atomic force microscopy
Authors: Ahmed Nabil , Nino Diego F. , Moy Vincent T.
Published: July 2002

We report on studies aimed at employing the atomic force microscope (AFM) to measure the viscosity of aqueous solutions. At ambient temperature, the AFM cantilever undergoes thermal fluctuations that are highly sensitive to the local environment. Here, we present measurements of the cantilever’s resonant frequency in aqueous solutions of glycerol, sucrose, ethanol, sodium chloride, polyethylene glycol, and bovine plasma albumin. The measurements revealed that variations in the resonant frequency of the cantilever in the different solutions are largely dependent on the viscosity of the medium. An application of this technique is to monitor the progression of a chemical reaction where a change in viscosity is expected to occur. An example is demonstrated through monitoring of the hydrolysis of double stranded deoxyribonucleic acid by DNase I.
Ultrafast dynamics of InAs/GaAs quantum-dot microdisk lasers
Authors: Luo K. J. , Xu J. Y. , Cao H. , Ma Y. , Chang S. H. , Ho S. T. , Solomon G. S.
Published: July 2002

The dynamical response of InAs/GaAs quantum-dot microdisk lasers has been experimentally investigated using femtosecond optical pumping. Because surface recombination and carrier diffusion are suppressed in the quantum dots, the response speed of a quantum-dot microdisk laser is much faster than that of a quantum-well microdisk laser. A turn-on time as short as 7.8 ps has been achieved in a quantum-dot microdisk laser at 5 K. The temperature dependence of the dynamical response of the quantum-dot microdisk lasers has also been studied over a wide temperature range. At the same pumping level, the turn-on time of the laser decreases as the temperature increases from 5 to 120 K. Such behavior may be due to a faster carrier relaxation process at higher temperature.