bluuebunny/crossref_metadata_2025 huggingface dataset:
Equilibrium flow structures and scaling of implosion trajectories in wire array Z pinches
Authors: Chittenden J. P. , Lebedev S. V. , Oliver B. V. , Yu E. P. , Cuneo M. E.
Published: February 2004

The hypothesis that wire array Z-pinch radiation sources can be represented as an ablating mass source embedded within a Lorentz force field is examined and the effects that this has upon the trajectory and spatial structure of the ensuing implosion are studied. Two-dimensional (2D) resistive magnetohydrodynamic (MHD) simulations of the ablating core regions and of the array cross-section indicate that the core ablation rate is determined by force balance at the ablation surface. This implies a weak dependence of the ablation velocity (the ratio of the magnitude of the Lorentz force to the mass ablation rate) on the array parameters (current, radius, mass, etc.). In the case of a constant ablation rate, the radial profiles in the flow region between the wires and the axis are found to converge to a set of time independent equilibria. These profiles represent a unique solution to the ideal MHD equations for super-Alfvénic flow in cylindrical geometry. Comparisons of simulated implosion trajectories with experimental optical streak photography data are used as a code validation exercise and show important deviations from the scenario of invariant ablation velocity. The importance of the number of wires in the array in determining the ablation rate and thus the trajectory and structure of the implosion is highlighted. The effects upon the inferred implosion symmetry and the x-ray pulse shape and peak power are discussed.
A perturbative solution of the drift kinetic equation yields pinch type convective terms in the particle and energy fluxes for strong electrostatic turbulence
Authors: Baker D. R.
Published: February 2004

Approximate linearized solutions to the drift kinetic equation (DKE) can provide physical insights into turbulent transport processes in a tokamak plasma. These types of solutions can provide a useful supplement to the results of numerical solutions to either the gyrokinetic equation or to nonlinear fluid equations. Here the DKE is solved in an iterative fashion with an attempt to include the tokamak geometry in a realistic way. The gradients in the DKE are expressed in tokamak geometry, not just by the plane wave approximation. The ballooning type spatial dependence of the electrostatic turbulence is assumed to have a given poloidal dependence. There is no attempt to solve for the radial, poloidal or toroidal dependence of the turbulence. This approximate solution shows that the nonadiabatic part of the perturbed electron distribution function yields particle fluxes which have pinch-like terms proportional to the electron temperature gradient and the safety factor gradient. The terms proportional to the safety factor gradient are inward, but the terms proportional to the temperature gradient can be either inward or outward.
Electrowetting-induced morphological transitions of fluid microstructures
Authors: Klingner Anke , Mugele Frieder
Published: February 2004

We studied the morphology of liquid droplets on hydrophobic substrates with hydrophilic stripes. Instead of using chemical surface patterning we made use of the electrowetting effect (with striped electrodes). This offers the unique opportunity to externally control the contact angle by varying the applied voltage. Upon increasing the voltage, droplets become elongated along the direction of the stripe. Beyond a critical volume-dependent voltage, they undergo a first order morphological transition to an elongated cylindrical shape with the contact line pinned along the edge of the stripe. This observation both confirms recent theoretical and numerical work by [M. Brinkmann and R. Lipowsky, J. Appl. Phys. 92, 4296 (2002)] and it demonstrates the flexibility of electrowetting for wetting studies with heterogeneous substrates. Finally, we outline possible applications of morphological transitions for microfluidic devices.
Understanding degradation and breakdown of SiO2 gate dielectric with “negative Hubbard U” dangling bonds
Authors: Karp James , Gitlin Daniel , Jeong Steve , Moyzhes Boris
Published: February 2004

Degradation and time dependent breakdown of SiO2 gate oxides are discussed based on the Anderson–Mott theory of amorphous solids with dangling bonds as diamagnetic “negative Hubbard U” centers. Negative-U dangling bonds in the oxide are either positive D+ centers or two-electron negative D− centers. Due to a large difference in mobility between electrons and holes, hopping current in SiO2 is mainly electron current on D+ centers. Degradation of isolation properties and time dependent breakdown of SiO2 gate oxide under voltage stress are due to the conversion of D− into D+ centers caused by the hole-hopping current in SiO2. The reaction of conversion is stress polarity dependent. Thermal conductivity of Si is approximately 100 times higher than thermal conductivity of SiO2. Heat dissipation and accumulation of D+ centers inside the oxide are important in understanding the time dependent breakdown of the oxide.
Thermal transport properties of magnetic refrigerants La(FexSi1−x)13 and their hydrides, and Gd5Si2Ge2 and MnAs
Authors: Fujieda S. , Hasegawa Y. , Fujita A. , Fukamichi K.
Published: February 2004

La(Fe x Si 1−x ) 13 and their hydrides exhibit large magnetocaloric effects due to the itinerant-electron metamagnetic transitions in a wide temperature range covering room temperature. Thermal conductivity and diffusivity of La(Fe0.88Si0.12)13 and La(Fe0.88Si0.12)13H1.0 have been investigated, together with those of other candidates for magnetic refrigerants working in the vicinity of room temperature such as Gd, Gd5Si2Ge2 and MnAs. The thermal conductivity in the vicinity of room temperature for La(Fe0.88Si0.12)13H1.0 is larger than that for Gd5Si2Ge2 and MnAs, and almost identical to that for Gd. Furthermore, the thermal diffusivity in the vicinity of room temperature for La(Fe0.88Si0.12)13H1.0 is as large as that for Gd and Gd5Si2Ge2, and larger than that for MnAs. Consequently, La(FexSi1−x)13 and their hydrides are promising as the magnetic refrigerants from the standpoint of thermal transport properties.
Annealing-induced-type conversion of GaInNAs
Authors: Kurtz Sarah , Geisz J. F. , Friedman D. J. , Metzger W. K. , King R. R. , Karam N. H.
Published: February 2004

When grown by metalorganic chemical vapor deposition (MOCVD), nominally undoped GaInNAs is commonly observed to have an acceptor concentration of ∼1017 cm−3. However, after annealing in the MOCVD reactor at a temperature of 650 °C, p-type GaInNAs sometimes converts to n type with an electron concentration of ∼1017 cm−3. This n-type material has a slightly higher electron mobility (∼400–450 cm2/V s) than has usually been reported for 1 eV GaInNAs. Secondary ion mass spectroscopy shows significant hydrogen and some carbon contamination of these layers. The type conversion is correlated with both the nitrogen and hydrogen concentration and is relatively insensitive to the choice of growth precursors (trimethylgallium versus triethylgallium, or unsymmetric dimethylhydrazine versus nitrogen trifluoride). The data are consistent with theoretical predictions that the donors arise from a N–H complex. Annealing in the absence of hydrogen reduces the background acceptor concentration, but does not produce an electron concentration as high as 1017 cm−3.
Instantaneous line heat source near a plane interface
Authors: Shendeleva Margarita L.
Published: February 2004

A model comprising an instantaneous line heat source situated parallel to the interface between two semi-infinite heat-conductive media in perfect thermal contact is studied theoretically. The Green functions to this problem have been derived recently using the Cagniard–de Hoop method. Here we apply another method of derivation based on the R. Bellman, R. E. Marshak, and G. M. Wing [Philos. Mag. 40, 297 (1949)] approach developed for an instantaneous point source. The Green functions derived by this method have a great advantage in numerical computation in comparison with the Green functions obtained by the Cagniard–de Hoop method. The results of calculations of temperature fields and heat fluxes in space and their interpretation with the use of wave diagrams are presented.
Excitonic transitions in β-FeSi2 epitaxial films and single crystals
Authors: Birdwell A. G. , Shaffner T. J. , Chandler-Horowitz D. , Buh G. H. , Rebien M. , Henrion W. , Stauß P. , Behr G. , Malikova L. , Pollak F. H. , Littler C. L. , Glosser R. , Collins S.
Published: February 2004

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of β-FeSi2 at low temperatures (T⩽180 K). A model based on the results of low-temperature absorption [M. Rebien et al., Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12–180 K, we extract a free exciton binding energy of (0.009±0.002) eV and a direct energy gap at T=0 K of (0.934±0.002) eV.
Cantilever noise in off-cantilever-resonance force-detected nuclear magnetic resonance
Authors: Harrell Lee E. , Thurber Kent R. , Smith Doran D.
Published: February 2004

Early work in force-detected nuclear magnetic resonance (FD-NMR) and magnetic resonance force microscopy was restricted to nuclei with a relatively large gyromagnetic ratio γ. Increasingly, as researchers look to apply FD-NMR to practical problems, observing isotopes with a small γ is becoming necessary. The small γ of these isotopes places severe restrictions on the radio frequency field strength necessary to flip the sample spins at practical cantilever frequencies by adiabatic rapid passage. These restrictions led us to investigate the feasibility of observing FD-NMR by flipping sample spins at a rate well below the cantilever frequency. In this article we show that there is no increase in thermomechanical force noise in off-cantilever-resonance FD-NMR relative to on-cantilever-resonance work. Further, we show that working off-cantilever resonance can reduce artifacts and decrease data acquisition time. The major disadvantage to working off-cantilever resonance—reduced cantilever response—increases the importance of low noise detection of cantilever oscillation.
Pulsed laser annealing of Sn-implanted Si single crystal
Authors: Klinger D. , Auleytner J. , Żymierska D. , Kozankiewicz B. , Nowicki L. , Stonert A.
Published: February 2004

A very disturbed near-surface layer was formed by Sn-ion implantation in a Si single crystal. For crystal lattice reconstruction of this layer and for change of the dopant distribution pulsed laser annealing was applied. In order to determine the optimal value of the energy density of laser pulse numerical calculations were performed. The experiments carried out by means of reflection high-energy electron diffraction and by Rutherford backscattering have shown that laser annealing caused the formation of the epitaxial Si1−xSnx layer. A good crystal quality of this layer, comparable with the quality of the single crystal Si matrix, as well as a good substitutional location of the Sn dopant atoms were obtained for the laser beam energy density value equal to the calculated optimal one.
Effect of current density on electron beam induced charging in sapphire and yttria-stabilized zirconia
Authors: Thome T. , Braga D. , Blaise G.
Published: February 2004

This article presents a study performed with a dedicated scanning electron microscope on the electrical property evolution of surfaces of (0001)-oriented sapphire (Al2O3) and (100)-oriented yttria-stabilized zirconia (YSZ) single crystals, during a 1.1 keV electron irradiation at room temperature. The type of charges trapped on the irradiated areas and the charging kinetics are determined by measuring the total secondary electron emission yield σ during the injection process, by means of two complementary detectors. At low current density (<7×106 pA cm−2) where positive charging is observed in both materials, charges trapped in Al2O3 are stable, whereas they are unstable in YSZ. This leads to two different charging kinetics. As charging is progressing in Al2O3, σ varies from its initial intrinsic value 7.5 down to a steady value σ=1 which corresponds to the self-regulated regime. Under the same conditions, σ varies in YSZ from 2.35 down to a steady value above 1 (σ=1.1 in the experiment presented). At high current density (above 7×106 and 6×109 pA cm−2, respectively, for Al2O3 and YSZ), the regulation of the charge regime is controlled by the formation of a negative charge layer due to the reduction of the secondary electron emission by the elastic interaction of incident electrons with secondaries. The difference in the charging kinetics of the two materials is attributed to the difference in conductivities. The higher conductivity of YSZ is responsible for the slower charging kinetics in YSZ, the less pronounced current density effect, and the vanishing of positive charges when irradiation stops.
Effects of Br and Cl on organometallic vapor phase epitaxial growth and ordering in GaInP
Authors: Howard A. D. , Rieth L. W. , Chapman D. C. , Wixom R. R. , Stringfellow G. B. , Kim B. J. , Seong T. Y.
Published: February 2004

CuPt ordering in GaInP has significant effects on the electrical and optical properties. In fact, band gap reductions as large as 160 meV are potentially useful for devices. Thus, control of ordering is important. This has led to the investigation of surfactants that affect the surface processes during growth with little incorporation into the solid. A potentially interesting class of surfactant elements are those from group VII, that are known not to be incorporated during growth, but to change the dynamics of surface processes. This article describes the results of the addition of Cl and Br during the organometallic vapor phase epitaxial growth of GaInP. The addition of Br in small concentrations is observed to systematically decrease the amount of CuPt ordering observed directly via transmission electron diffraction and photoluminescence measurements. A ratio of Br/III of 0.042 is found to virtually eliminate ordering. The reduction in order parameter is not found to correspond to a decrease in the 405 nm peak measured during in situ surface photoabsorption measurements. This indicates that a change in surface reconstruction is not responsible for the decrease in order parameter. However, both Cl and Br are found to significantly roughen the surface morphology. Atomic force microscopy shows that facets are formed that increase in height and angle to the (001) growth surface as the surfactant concentration increases. This is postulated to be the origin of the reduction in order parameter, since previous studies have indicated that an intentional misorientation of the substrate in either the A or B direction leads to a decrease in order parameter.
Magnetization for lower temperature, selective diamond and carbon nanotube formation: A milestone in carbon physicochemical condensation
Authors: Little Reginald B. , Goddard Robert
Published: February 2004

Diamonds and carbon nanotubes (CNTs) have extraordinary properties with the potential for vast technological and scientific advancements. However, the syntheses of these super materials have required extreme conditions. Recent synthetic developments surrounding catalytic chemical vapor deposition (CCVD) have contributed to more suitable, practical and economical preparations, but more progress is needed for better selectivity, purity, and mass production of CNTs and diamonds. Such synthetic advancements require a deeper understanding of the mechanisms of formation on the atomic scale. A recent comprehensive mechanism of Little suggests the importance of high-spin electronic states and the rehybridization mechanics of carbon atoms and metal atoms (for catalytic synthesis) during the nucleation and growth of CNTs and diamonds. The significance of these predictions is demonstrated in this work by using an intense, static magnetic field of several teslas to influence carbon fixation events during carbon CCVD. Single crystalline diamonds are discovered to nucleate and grow under the influence of the static magnetic field (19.3 T) under catalytic CVD conditions that normally result in carbon nanotubes. Furthermore, this technique results in a bottom-up approach for creating diamond nucleation sites on the basis of a so-called chemical preabrasion of the silicon substrate with the potential advantage of the control of seeding nucleation-site density and nonrandom patterning for larger single crystal diamond syntheses. This technique also provides a basis for diamond–CNT composite super materials. Moreover, the observed influence of high magnetic field on diamond formation provides implications concerning natural diamond genesis in the earth’s mantle and core in comparison to celestial diamond formation. Furthermore, these findings suggest uses of polarized and nonpolarized neutron irradiation for static and dynamic magnetic interactions, leading to diamond and CNT productivity, respectively.
Improvement in hard magnetic properties of FePt films by N addition
Authors: Wang H. Y. , Mao W. H. , Ma X. K. , Zhang H. Y. , Chen Y. B. , He Y. J. , Jiang E. Y.
Published: February 2004

The structure and magnetic properties of (Fe56Pt44)100−xNx alloy films with x=0–15 at. % prepared by rf magnetron sputtering were investigated. Compared to an Fe56Pt44 binary film, (Fe56Pt44)100−xNx alloy films showed great enhancement in hard magnetic properties. A coercivity Hc of 1027 kA/m, a remanence Mr of 1.24 T, and a maximum energy product (BH)max of 260 kJ/m3 were obtained for an (Fe56Pt44)93N7 film annealed at 600 °C for 10 min, whereas the Fe56Pt44 film annealed at the same conditions gave Hc=682 kA/m, Mr=0.87 T, and (BH)max=132 kJ/m3. Structural analysis revealed that N addition influenced the phase structure and microstructure of FePt films. N atoms incorporated in the disordered FePt phase in as-deposited films, they released rapidly out of the FePt phase by vacuum annealing. It is suggested that the rapid release of N atoms from the FePt phase promotes the transformation to the ordered phase, results in a higher coercivity of the film. The high remanence of the (Fe56Pt44)93N7 film is attributed to enhanced exchange coupling of the soft iron nitride phase and the hard FePt phase.
High-efficiency AlGaInP light-emitting diodes for solid-state lighting applications
Authors: Gessmann Th. , Schubert E. F.
Published: February 2004

AlGaInP lattice matched to GaAs is suited for light-emitting diodes (LEDs) operating in the red, orange, yellow, and yellow–green wavelength range. Such long-wavelength visible-spectrum devices will play an important role in solid-state lighting applications. This review discusses the major classes of AlGaInP device structures, including absorbing-substrate (AS) LEDs, absorbing substrate LEDs enhanced by distributed-Bragg-reflectors (DBRs), transparent substrate (TS) LEDs, thin-film (TF) LEDs, and LEDs using omnidirectional reflectors (ODRs). Some of these device structures have well-known deficiencies: A significant fraction of light is absorbed in the GaAs substrate in AS-LEDs; DBRs are essentially transparent at oblique angles of incidence leading to substantial optical losses. More recent developments such as TS-LEDs and TF-LEDs avoid these drawbacks. High-reflectivity, electrically conductive ODRs were recently developed that considerably outperform conventional distributed Bragg reflectors. LEDs using such conductive ODRs can replace DBRs in AlGaInP LEDs and are potential candidates for low-cost high-efficiency LEDs suitable for high-power solid-state lighting applications.
High-speed optical switching based on diffusive conduction in an optical waveguide with surface-normal optical control
Authors: Sabnis V. A. , Demir H. V. , Yairi M. B. , Harris J. S. , Miller D. A. B.
Published: February 2004

We report a surface-normal optically controlled waveguide switch made of a p-i-n diode that utilizes optically induced voltage screening to turn on and diffusive conduction to turn off. We examine theoretically diffusive conduction in these switches and show that this mechanism offers the possibility of creating optical switches operating at several tens of GHz. As a proof-of-concept demonstration, we present experimental results of wavelength-converting, optical switching at frequencies up to 2.5 GHz in a multiple-quantum-well, GaAs-based waveguide device using milliwatt-level switching powers. Optimized arrays of these devices can be used for high-speed, multichannel wavelength conversion and optical regeneration in wavelength-division-multiplexed systems.
Measurement of current profile dynamics in the Madison Symmetric Torus
Authors: Terry S. D. , Brower D. L. , Ding W. X. , Anderson J. K. , Biewer T. M. , Chapman B. E. , Craig D. , Forest C. B. , O’Connell R. , Prager S. C. , Sarff J. S.
Published: February 2004

The current profile and core magnetic field fluctuation amplitudes in a reversed-field pinch are measured by using a high-resolution polarimetry–interferometry system. This paper presents data showing a redistribution of the current during a sawtooth crash. Also, the core magnetic field fluctuation amplitude is observed to increase at a sawtooth crash consistent with the idea of nonlinearly driven dynamo current. In addition, the parallel current density increases in the outer region of the plasma during auxiliary pulsed parallel current drive. This was expected, as the external application of an edge parallel electric field is designed to flatten the current profile providing an equilibrium closer to the minimum energy Taylor state. However, the current density also increases in the core, relative to standard plasmas. This increase can be explained by a reduction of the dynamo (anti)current drive in the core that should accompany the measured reduction of magnetic fluctuations and by a drop in resistivity caused by the increased confinement of fast electrons.
Splitting and mating properties of dispersion curves of wave modes in a cold magnetoplasma-filled cylindrical conducing waveguide
Authors: Ding Z. F. , Chen L. W. , Wang Y. N.
Published: February 2004

Under varieties of typical conditions, the splitting and mating properties of dispersion curves of wave modes in the ω–kz plane are systematically analyzed for wave modes, such as waveguide EH, HE and cyclotron HEc modes with opposite polarity. The obtained results show that such properties depend on the location of an EH mode with respect to two neighboring HE modes (waveguide or cyclotron). Anisotropic modes HE−ma waves are solitary in terms of their particular mating properties.
Study of kinetic shear Alfvén instability in tokamak plasmas
Authors: Dong J. Q. , Chen L. , Zonca F. , Jian G. D.
Published: February 2004

Kinetic shear Alfvén modes in tokamak plasmas (with or without temperature gradient) are numerically investigated in the full gyrokinetic limit. It is shown that, in the presence of ion temperature gradient (ITG), the threshold value of plasma pressure gradient is well below that for ideal magnetohydrodynamic ballooning instability. It is also demonstrated in a more general sense that, without ITG, the former is identical with the latter. The electromagnetic instability is also found to exist in the second stable region of the ideal modes when a finite ITG is present. The results are compared with previous similar analyses for kinetic collisionless ballooning modes. Possible correlation of the instability with tokamak experiments is discussed.
Prepulse controlled splitting of relativistically self-guided channel and suppression of Raman forward scattering instability
Authors: Chien T.-Y. , Chen W.-T. , Chen Y. H. , Lee C.-H. , Lin J.-Y. , Wang J. , Chen S.-Y.
Published: February 2004

The effects of laser prepulse on splitting of a relativistically self-guided channel and suppression of Raman forward scattering instability in the propagation of an intense ultrashort laser pulse in an underdense plasma are studied. They are resolved by using probing interferometry, shadowgraphy, and spectrometry. By changing the prepulse intensity, the propagation of the laser beam can be controlled to show self-guiding or channel splitting. The effect of prepulse on Raman forward scattering instability shows that the instability is significantly reduced if the gas target is preionized, identifying ionization wakefield as the seeding mechanism for this process.
Modulational instability of two colliding whistlers
Authors: Shukla P. K. , Mamun A. A.
Published: February 2004

It is shown that the dynamics of two colliding whistlers is governed by a pair of coupled nonlinear Schrödinger equations. The latter exhibit a modulational instability of constant amplitude whistlers that are nonlinearly interacting through quasistationary density perturbations created by the combined action of the ponderomotive forces of two whistlers. It has been found that the growth rate of the modulational instability increases with the intensity of any of the colliding whistlers, but decreases with the wave number per unit length of the modulated wave packet.
Low-temperature-grown GaAs coplanar waveguide single-photon/two photon absorption autocorrelator
Authors: Montoya Juan , Hu Qing
Published: February 2004

Previously, we have described a low temperature grown GaAs device that uses single-photon absorption to perform a carrier lifetime limited optical autocorrelation of picosecond optical pulses. In this article, we describe how this same device could be used to perform an autocorrelation of femtosecond optical pulses by utilizing two-photon absorption (TPA). Furthermore, we propose how to model and minimize the photocurrent’s dependence on the single-photon absorption (SPA) response of the midlevel traps. We find that the SPA response produces a distortion on the TPA autocorrelation signal at low intensities. At large peak intensities (Ipeak≈3G W cm−2), however, we find that the SPA distortion becomes nearly two orders of magnitude smaller than the TPA signal, and decreases further with increasing intensity. In our discussion, we also describe some of the tradeoffs between using a photoconductor with a large two-photon absorption coefficient and midlevel states as a TPA autocorrelator.
Beyond the diffraction limit: Super-resolving pupils
Authors: Ranfagni A. , Mugnai D. , Ruggeri R.
Published: February 2004

In an attempt to transfer the results obtained with super-gain antennas to optical systems in order to increase the resolving power, an original way for overcoming the diffraction limit was proposed in 1952. In this work, we report on the results of microwave experiments which demonstrate the correctness of these predictions. Their interpretation in relation to the uncertainty principle is then discussed, and a possible connection with the “weak measurement” theory is then suggested.
Characterization of magnetoelectric laminate composites operated in longitudinal-transverse and transverse–transverse modes
Authors: Dong Shuxiang , Li Jie-Fang , Viehland D.
Published: February 2004

Magnetostrictive and piezoelectric laminate composites of terfenol-D and Pb(Zr1−xTix)O3 have been studied. The magnetoelectric (ME) coefficients have been characterized for the different operational modes: (i) a longitudinally magnetized and transversely polarized longitudinal transverse (LT) mode, and (ii) a transversely magnetized and transversely polarized transverse–transverse (TT) mode. The results demonstrate that the (LT) magnetoelectric mode has dramatically higher ME coefficients than the TT one. The LT magnetoelectric coefficient is up to 5–7 times larger than the TT one, when operated in low magnetic bias ranges.
Heteroepitaxial evolution of AlN on GaN Grown by metal-organic chemical vapor deposition
Authors: Gherasimova M. , Cui G. , Ren Z. , Su J. , Wang X.-L. , Han J. , Higashimine K. , Otsuka N.
Published: February 2004

We have investigated the morphological evolution during heteroepitaxial growth of AlN on GaN by metal-organic chemical vapor deposition at different V/III ratios. Two-dimensional layer–by–layer and step flow growth modes, combined with strain-induced cracking, are observed at low and intermediate V/III ratios, while nitrogen-rich conditions yield three-dimensional domain-like growth due to limited Al adatom diffusion. Samples grown at the metal-rich conditions exhibit a crosshatch pattern of surface undulations possibly related to the presence of misfit dislocations that form at the early stages of nucleation. Our observations show that the local stoichiometry at the vapor-solid interface strongly influences the adatom kinetics during the growth, thereby affecting the nature of strain relaxation and growth mode.
Application of radiative transport theory to light extraction from organic light emitting diodes
Authors: Shiang J. J. , Duggal Anil R.
Published: February 2004

One limitation on organic light emitting diode (OLED) performance is the optical extraction efficiency ηex, which is defined as the ratio of light generated within the device to light emitted into the ambient. Typical estimates for ηex, in OLEDs range between 0.17 and 0.5. We develop a simple radiative transport model that quantifies the effect of volumetric light scattering on light output in OLEDs in terms of a small set of readily measured parameters. The methodology is sufficiently general to parametrize and describe many of the light extraction schemes found in the literature. A set of model calculations is presented using typical OLED parameters; these calculations show that the introduction of light scattering sites within the otherwise transparent substrate can increase light extraction efficiencies to values between 0.55 and 1.
Experimental demonstration of increased organic light emitting device output via volumetric light scattering
Authors: Shiang J. J. , Faircloth T. J. , Duggal Anil R.
Published: February 2004

A set of experimental measurements of scattering films and organic light emitting devices (OLEDs) is presented. We measure the reflectance, transmission, and emission characteristics of scattering media and OLED devices separately, and use this data as input into a simple radiative transfer model to predict the effect of light scattering on OLED light output. We find quantitative agreement between the radiative transfer model predictions and experimental results. We find that the introduction of volumetric scattering mechanisms increases the output of OLEDs by as much as 40%, which corresponds to over 70% of the light within a typical glass substrate being coupled to air.
Effect of dislocations on minority carrier lifetime in HgCdTe
Authors: Gopal Vishnu , Gupta Sudha
Published: February 2004

This article shows that the temperature dependence of minority carrier lifetime in mercury cadmium telluride samples of differing dislocation densities can be accounted well by including the dislocation contribution in addition to the contribution of well known radiative, auger, and Shockley–Read recombination mechanisms. The dislocation contribution to the minority carrier lifetime has been calculated on the basis of a recently proposed model [V. Gopal and S. Gupta, IEEE-ED 50, 1220 (2003)] by treating dislocation as a discontinuity of the lattice.
Radiative and nonradiative processes in strain-free AlxGa1−xN films studied by time-resolved photoluminescence and positron annihilation techniques
Authors: Onuma Takeyoshi , Chichibu Shigefusa F. , Uedono Akira , Sota Takayuki , Cantu Pablo , Katona Thomas M. , Keading John F. , Keller Stacia , Mishra Umesh K. , Nakamura Shuji , DenBaars Steven P.
Published: February 2004

Radiative and nonradiative processes in nearly strain-free AlxGa1−xN alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately −0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for x⩽0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies (VIII) and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to x=0.7. Consequently, certain trapping mechanisms associated with VIII where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency (ηint) with increasing x for x⩽0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the VIII-related trap density.
Growth model for thin oxides and oxide optimization
Authors: Ludsteck A. , Schulze J. , Eisele I. , Dietl W. , Nenyei Z.
Published: February 2004

The growth of thin silicon oxides cannot be completely described by the Deal-Grove-Model [B. E. Deal, A. S. Grove, J. Appl. Phys. 36, 3770 (1965)]. Analysis of 2.3 nm rapid thermal processing (RTP) silicon oxides SiO2 showed that it is necessary to consider not only the reaction of oxygen O2 and silicon Si at the Si/SiO2 interface but also the transport of silicon to the surface. According to process conditions the latter can cause SiO2 growth as well as desorption. Using well defined process parameters (temperature and gas ambient) in a RTP system the process can thus be switched between oxide growth and removal. The object of this article is to present a model which explains the above mentioned results by taking into account both reactions at the Si/SiO2 interface as well as at the SiO2/gas interface. On the basis of this model the oxide quality can be optimized with respect to interface quality and tunneling currents, and it is possible to develop an in-situ cleaning process in pure argon prior to oxidation.
Two-dimensional simulation of a miniaturized inductively coupled plasma reactor
Authors: Nam Sang Ki , Economou Demetre J.
Published: February 2004

A two-dimensional self-consistent simulation of a miniaturized inductively coupled plasma (mICP) reactor was developed. The coupled equations for plasma power deposition, electron temperature, and charged and neutral species densities, were solved to obtain the spatial distribution of an argon discharge. The effect of control parameters, such as power and pressure, on the evolution of plasma density and electron temperature was investigated. Strong ion density gradients were observed which can make spatially resolved Langmuir probe measurements particularly challenging. Simulation results were in reasonable agreement with available experimental data. The neutral gas temperature was predicted to be close to the wall temperature, due to the small length scale of the mICP, allowing for efficient heat transfer.
Optical properties of poly 2-methoxy-5-hexyloxy phenylenevinylene and related copolymers
Authors: Kang T. D. , Lee K. J. , Choi S. H. , Lee Hosun , Kim J. H. , Choi D. H.
Published: February 2004

We have measured the dielectric function of poly 2-methoxy-5-hexyloxy phenylenevinylene and the alternating copolymers with N-hexylphenothiazine grown on glass using variable angle spectroscopic ellipsometry and Raman spectroscopy at room temperature. Using the parametric optical constant model, we estimated the dielectric function of each polymer and also determined the band gap parameters, such as threshold energy, broadening, and amplitude. We also determined the band gap parameters by performing lineshape curve-fitting on the second derivative of the fit dielectric function using standard critical point model, and compared to those of the parametric optical constant model. We discussed the shift of band gap parameters and phonon wave numbers in terms of the chemical structures.
Nonlinear magnetohydrodynamics in the Dag confinement configuration
Authors: Strauss H. R.
Published: February 2004

The Dag magnetic fusion confinement configuration is a spheromak-like toroidal device. It consists of central vertical current channel, and an outer toroidal chamber with a toroidal current. It has a special magnetic topology. Whether this has consequences for plasma confinement is a motive for this study. A restricted class of computations, using the Multi-level 3D code [Park et al., Phys. Plasmas 6, 1796 (1999)], indicate stability for β<15%. For higher β, the simulations exhibit turbulent magnetic behavior similar to spheromaks and reverse field pinches. A reverse field pinch-like variant should be capable of a less turbulent start up and higher β than the spheromak-like Dag.
Analysis of low-frequency waves in inhomogeneous and bounded plasmas
Authors: Vranjes J. , Poedts S.
Published: February 2004

Electrostatic perturbations with low frequencies (less than or comparable to the ion cyclotron frequency) are analyzed in bounded and inhomogeneous plasmas. The effects of an arbitrary size density gradient are studied and the corresponding equation for the spatially dependent wave amplitude is solved analytically. The general solution of the eigenvalue problem reveals a global mode that is subject to given boundary conditions while the appropriate eigenvalues determine the eigenfrequencies. Exact values of frequencies and wave numbers are presented for waves propagating in a plasma with a Gaussian density distribution.
Sustainment of elongated field reversed configurations with localized rotating magnetic field current drive
Authors: Guo H. Y. , Hoffman A. L.
Published: February 2004

The effects of rotating magnetic field (RMF) antenna length on the sustainment of RMF driven field reversed configurations (FRC) have been investigated in the Translation, Confinement, and Sustainment (TCS) facility. FRCs could be sustained with axial lengths well beyond the RMF antenna, presumably due to induced axial flow. The FRC length, ls, in TCS was weakly dependent on the antenna length, la, with ls∼2la being achieved for the shortest antenna employed. Reducing antenna length led to more stable operation, with the FRCs less prone to rotational instabilities. Shorter antenna lengths also resulted in increases in the plasma temperature. At larger ratios of ls/la the RMF had to supply a greater torque per unit length, and it did this by increasing its penetration into the FRC, brought about automatically by a slight decrease in plasma density. Optimal overall current drive efficiency occurred at values of la/2rs of about 1.3, where rs is the separatrix radius, and values of ls/la of about 1.5. In contrast, operation with the RMF antenna covering the full TCS confinement chamber, la/2rs∼3.5, led to non-steady behavior with the FRC length decreasing during the discharge.
Optical observations of plasma formation and wire core expansion of Au, Ag, and Cu wires with 0–1 kA per wire
Authors: Hu Min , Kusse Bruce R.
Published: February 2004

Explosions of Au, Ag, and Cu wires (1 cm long, 13–25 μm diameter, with/without insulating coatings) driven by a current pulser (4.5 kA peak, rise time 350 ns) were studied. Diagnostics included three channels of Schlieren imaging and two channels of shearing interferometry using the second harmonic of a Nd:YAG laser (λ=532 nm, 5 ns pulse width). The formation of a plasma shell around the neutral wire core was observed relatively late in time compared to the voltage collapse time. The corresponding plasma density was measured. From the refractive index of the wire core, some properties of the wire core material will also be discussed. The expansion rates of exploding wires were measured by interferometry and compared to the previously reported results. Two-wire experiments were also performed. The merging of the plasma at an increased plasma density was observed between two wires. The effects of the plasma shell on the wire expansion are discussed.
Generalization of the Child–Langmuir law for nonzero injection velocities in a planar diode
Authors: Puri R. R. , Biswas Debabrata , Kumar Raghwendra
Published: February 2004

The Child–Langmuir law relates the voltage applied across a planar diode to the saturation value JCL of current density that can be transmitted through it in case the injection velocity of electrons is zero. The Child–Langmuir current density JCL is, at the same time: (i) the maximum current density that can be transmitted through a planar diode, (ii) the current density below which the flow is steady and unidirectional in the long time limit, and (iii) the average transmitted current density for any value of injected current density above JCL. Existing generalizations of Child–Langmuir law to nonzero velocities of injection are based on the characteristics (i) and (ii) of JCL. This paper generalizes the law to nonzero velocities of injection based on the characteristic (iii) by deriving an analytical expression for the saturation value of current density. The analytical expression for the saturation current density is found to be well supported by numerical computations. A reason behind preferring the saturation property of the Child–Langmuir current density as the basis for its generalization is the importance of that property in numerical simulations of high current diode devices.
Comment on “Formation of nanocrystallites governed by the initial stress in the ultrathin hydrogenated amorphous silicon films” [J. Appl. Phys. 90, 1067 (2001)]
Authors: Sperling Brent A. , Abelson John R.
Published: February 2004

A recent article suggested that “paracrystalline” silicon could be detected in very thin hydrogenated amorphous silicon films by using spectroscopic ellipsometry. We show that the important features of the ellipsometry data can be reproduced using a simple optical model of amorphous Si:H, with no assumptions about unusual structures or thickness-dependent optical properties.
Response to comment on “Formation of nanocrystallites governed by the initial stress in the ultrathin hydrogenated amorphous silicon films” [J. Appl. Phys. 90, 1067 (2001)]
Authors: Hazra Sukti , Sakata Isao , Yamanaka Mitsuyuki , Suzuki Eiichi
Published: February 2004

In this reply, we address the issues raised regarding our article [J. Appl. Phys. 90, 1067 (2001)] and show that spectroscopic ellipsometry is a tool to identify the paracrystallinity of the ultathin hydrogenated amorphous silicon (a-Si:H) films.
Fabrication of low loss two-dimensional InP photonic crystals by inductively coupled plasma etching
Authors: Pommereau F. , Legouézigou L. , Hubert S. , Sainson S. , Chandouineau J.-P. , Fabre S. , Duan G.-H. , Lombardet B. , Ferrini R. , Houdré R.
Published: February 2004

Low-loss bidimensional photonic crystals have been fabricated by using an optimized process, consisting of electron beam lithography of a poly(methyl methacrylate) resist, reactive ion etching of an intermediate dielectric layer, and inductively coupled plasma etching of InP-based heterostructures. A depth to diameter aspect ratio as high as 14 has been obtained for hole diameter varying from 260 to 620 nm. As a result of the high aspect ratio and of the almost cylindrical hole shape, very low radiation losses have been obtained. For example, a transmission coefficient in the air band as high as 0.8 has been achieved for eight rows ΓM structure, demonstrating a very low value of the radiation losses.
Beam shifting experiment for the characterization of left-handed properties
Authors: Ran Lixin , Huangfu Jiangtao , Chen Hongsheng , Zhang Xianmin , Chen Kangsheng , Grzegorczyk Tomasz M. , Kong Jin Au
Published: February 2004

In this article, we report experimental results on the shift of a beam that propagates through a slab of metamaterial, known to exhibit left-handed properties. A similar experiment with a Teflon sample of similar size is also carried out for comparison. The results show the existence of a frequency band over which the shift experienced by the beam exceeds the theoretical limit above which the material is left handed. This demonstrates the effectiveness of using a beam shift experiment to characterize the left-handed properties of metamaterial.
Effect of oxygen adsorption on the structure and spin-reorientation transition of Fe films on Cu(1 1 25)
Authors: Ma Xucun , Barthel J. , Klaua M.
Published: February 2004

The morphology, structure, and magnetism of Fe films on an oxygen-precovered, stepped Cu(1 1 25) surface are investigated by scanning tunneling microscopy, low-energy electron diffraction, and magneto-optical Kerr effect analysis. After exposure of Cu(1 1 25) to about 600 L of oxygen at 493 K, a well-ordered (2√×√)R45° superstructure is formed. The O-induced structure has a zig-zag morphology, showing a high density of facets and is very stable during subsequent Fe deposition. Fcc Fe can be stabilized up to 20 monolayer (ML) thickness, while the magnetization of the films reorients from perpendicular to in-plane at 15 ML. From 24 ML, the linear extrapolation line of the thickness dependence of the Kerr intensity of the transformed bcc Fe films does not go along with the pure fully magnetized bcc Fe films. We conclude that oxygen adsorption strongly affects the structure and spin-reorientation transition of Fe films.
Relating the physical structure and optical properties of conjugated polymers using neutron reflectivity in combination with photoluminescence spectroscopy
Authors: Mitchell William J. , Burn Paul L. , Thomas Robert K. , Fragneto Giovanna , Markham Jonathan P. J. , Samuel Ifor D. W.
Published: February 2004

Understanding the effect of physical structure and the role of interfaces is critical for gaining insight into the optoelectronic properties of conjugated polymers and their behavior in semiconductor devices such as organic light-emitting diodes and photovoltaic cells. We have developed an in situ neutron reflection measurement that allows the direct relationship between film photoluminescence and structure to be studied. In addition, we have found that by judicious deuteration of the conjugated polymers, the polymer/indium tin oxide (ITO) interface can be probed. Critically for both poly[2-(2-d17-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] and poly[9,9′-(2-d17-ethylhexyl)-2,6-fluorene] of thickness of order 140–150 nm on ITO, we found that a thermally stable low-density layer of 20 Å thickness was present between the polymer film and the ITO. The presence of the low-contact layer means that measurements involving these two families of polymers directly deposited onto ITO may need re-evaluating, and suggests why poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) may be so beneficial for polymer light-emitting diodes.