bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonantly coupled surface plasmon polaritons in the grooves of very deep highly blazed zero-order metallic gratings at microwave frequencies
Authors: Went H. E. , Sambles J. R.
Published: July 2002

Sharp features are observed in the wavelength-dependent reflectivity of very deep highly blazed zero-order metallic gratings at microwave frequencies. These features are attributed to the resonant excitation of self-coupled surface plasmon polaritons in the cavities of such samples. The slat structures may, in a simplistic model, be treated as a class of Fabry–Pérot cavity resonator.
Physical mechanisms governing pattern fidelity in microscale offset printing
Authors: Darhuber Anton A. , Troian Sandra M. , Wagner Sigurd
Published: July 2002

We have studied the offset printing of liquid polymers curable by exposure to ultraviolet light onto flat and unpatterned silicon and glass substrates. The interplay of capillary, viscous, and adhesion forces dominates the dynamics of ink transfer at small feature sizes and low capillary number. For smooth and nonporous substrates, pattern fidelity can be compromised because the ink contact lines are free to migrate across the substrate during plate separation. Using a combination of experiments and equilibrium simulations, we have identified the physical mechanisms controlling ink transfer and pattern fidelity. In considering the resolution limit of this technique, it appears that the dynamics of ink flow and redistribution during transfer do not explicitly depend on the absolute feature size, but only on the aspect ratio of film thickness to feature size. Direct printing holds promise as a high-throughput fabrication method for large area electronics.
Effect of hydrogenation on Al-related photoluminescence in 6H–SiC
Authors: Koshka Yaroslav , Mazzola Michael S.
Published: July 2002

Interaction of hydrogen with Al acceptors in SiC is investigated using low-temperature photoluminescence (PL) spectroscopy. Hydrogenation is performed in hydrogen plasma using a standard inductively coupled plasma etching system. Appearance of H-related PL peaks after hydrogenation is accompanied with a significant reduction in relative intensity of Al bound exciton (Al–BE) PL. A gradual quenching of the remaining Al–BE photoluminescence is observed in hydrogenated samples under excitation with above band gap light, resulting in a complete disappearance of Al–BE PL emission. High-temperature annealing completely restores the shape of the PL spectrum to its prehydrogenation form.
Surface photovoltage spectroscopy characterization of a GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structure
Authors: Cheng Y. T. , Huang Y. S. , Lin D. Y. , Tiong K. K. , Pollak Fred H. , Evans K. R.
Published: July 2002

Using room-temperature surface photovoltage spectroscopy (SPS), we have characterized a GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (pHEMT) structure. Signals have been observed from every region of the sample. From a line shape fit to the normalized first derivative of the surface photovoltage signal with respect to photon energy, the two-dimensional electron gas density (Ns) was obtained and found to be in good agreement with Hall measurement. The Al composition and the properties of the GaAs/GaAlAs superlattice buffer layer also were obtained from the SPS spectrum. The results demonstrate the considerable potential of SPS for the contactless and nondestructive characterization of pHEMT structures at room temperature.
Radiation response of n-type base InP solar cells
Authors: Walters Robert J. , Messenger S. R. , Summers G. P. , Romero M. J. , Al-Jassim M. M. , Araújo D. , Garcia R.
Published: July 2002

The effects of particle irradiation on the electrical properties of high efficiency p/n InP solar cells have been studied using a variety of techniques including current–voltage and spectral quantum efficiency measurements (QE), electron beam induced currents (EBIC), and deep level transient spectroscopy. A detailed analysis of the radiation response of the solar cell photovoltaic response is presented, and the primary damage mechanisms are identified. Data measured after irradiation by protons of various energies are correlated in terms of displacement damage dose to produce a characteristic degradation curve for the p/n InP technology. This characteristic curve is compared to that of the n/p InP technology to provide an assessment of the relative radiation hardness of the p/n devices. Radiation-induced decreases in the minority carrier diffusion length in both the p-type emitter and n-type base at low damage levels have been extracted from the QE and EBIC measurements, and damage coefficients have been determined. At high damage levels, EBIC profiles suggest that the primary device degradation mechanism is an increase in bulk resistivity due to electron trapping in the base. However, capacitance–voltage measurements did not indicate any change in the junction capacitance. A model to account for these effects based on radiation-induced defect kinetics is presented.
Laser performance of pyrromethene 567 dye in solid polymeric matrices with different cross-linking degrees
Authors: Costela A. , Garcı́a-Moreno I. , Gómez C. , Garcia O. , Sastre R.
Published: July 2002

We report on the laser action of pyrromethene 567 (PM567) incorporated into copolymers of methyl methacrylate (MMA) with different methacrylic and acrylic cross-linking monomers: ethyleneglycol dimethacrylate, trimethylolpropane trimethacrylate, tetraethyleneglycol diacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate (PETRA). The vol/vol proportion of the different co-monomers in each copolymer formulation was systematically varied, and the effect of each composition on the lasing properties of PM567 was evaluated. The laser samples were transversely pumped at 534 nm with 5.5 mJ/pulse from a frequency doubled Q-switched Nd:KGW laser. Lasing efficiencies of up 26% and good stability with a drop of the initial laser output of 30% after 100 000 pump pulses at 5 Hz in P(MMA:PETRA 95:5) were demonstrated.
Back-channel-type scanning charge pumping method for characterization of interface traps in silicon-on-insulator wafer
Authors: Yoshida Haruhiko , Sasakura Hiroshi , Yabuuchi Tomoyuki , Takami Toshinori , Uchihashi Takayuki , Kishino Seigo
Published: July 2002

A scanning charge pumping method using a back channel is proposed for the characterization of interface traps in a silicon-on-insulator (SOI) wafer. In this method, a contactless gate electrode is used instead of the permanent gate electrode of normal metal–oxide–semiconductor transistors, allowing the interface trap density of SOI wafers to be mapped. A preliminary study is performed using a sample device with many permanent gate electrodes fabricated on an oxidized SOI wafer. The results demonstrate that the back-channel-type scanning charge pumping method is effective in characterizing interface trap density and is potentially applicable to SOI wafer inspection.
Polymeric micromechanical components with tunable stiffness
Authors: Manias E. , Chen J. , Fang N. , Zhang X.
Published: July 2002

We present a microstereolithographic technique that enables the manufacturing of polymeric components for microelectromechanical systems. Model microstructures were fabricated in the form of end-supported microbeams (10 μm in diameter), in order to characterize the mechanical properties of the produced structures at the micron scale. The flexural modulus of these microbeams was measured by atomic force microscopy, using cantilevers with attached metal spheres, and employed in a three-point bending geometry. Postfabrication treatment of the microstructures allows for the tailoring of their stiffness.
Amorphous-iron disilicide: A promising semiconductor
Authors: Milosavljević M. , Shao G. , Bibić N. , McKinty C. N. , Jeynes C. , Homewood K. P.
Published: July 2002

We report here the synthesis and the measurements of the microstructural and optical properties of a promising semiconductor, amorphous-iron disilicide. The material was obtained by ion-beam mixing of Fe layers on Si, with Ar8+ ions, at 300 °C. Optical absorption measurements indicate a semiconductor with a direct band gap of 0.88 eV. The significance of this discovery is that it demonstrates the existence of such a material. It should be possible to synthesize by other techniques and could be applied in large-area electronics.
Vortex matter in superconductors
Authors: Pan V. M. , Pan A. V.
Published: July 2002

The behavior of the ensemble of vortices in the Shubnikov phase in biaxially oriented films of the high-temperature superconductor YBa2Cu3O7−δ (YBCO) in an applied magnetic field is investigated for different orientations of the field. The techniques used are the recording of the current–voltage characteristics in the transport current and of resonance curves and damping of a mechanical oscillator during the passage of a transport current. It is shown that the behavior of the vortex ensemble in YBCO films, unlike the case of single crystals, is determined by the interaction of the vortices with linear defects—edge dislocations, which are formed during the pseudomorphic epitaxial growth and are the dominant type of defect of the crystal lattice, with a density reaching 1015 lines/m2. The effective pinning of the vortices and the high critical current density (Jc⩾3×1010 A/m2 at 77 K) in YBCO films are due precisely to the high density of linear defects. New phase states of the vortex matter in YBCO films are found and are investigated in quasistatics and dynamics; they are due to the interaction of the vortices with crystal defects, to the onset of various types of disordering of the vortex lattice, and to the complex depinning process. A proposed H–T phase diagram of the vortex matter for YBCO films is proposed.
Fluorine incorporation into gate stacks of advanced silicon memory technologies: Simulation, depth distribution, and reliability
Authors: Krüger D. , Dabrowski J. , Gaworzewski P. , Kurps R. , Pomplun K.
Published: July 2002

Fluorine segregation influences the intrinsic reliability of thin gate oxides in poly-Si/SiO2/Si structures. We analyze diffusion and segregation kinetics of F in gate stacks with 5 nm gate oxides using secondary ion mass spectroscopy. Well defined doses of F were introduced by ion implantation. We find that F segregation at interfaces to the gate oxide is diffusion limited with an effective activation energy of 1.4 eV. For F concentrations lower than 5×1015 cm−2, F segregation is beneficial improving, for example, the tails of the Weibull plots and slightly increasing the breakdown field. For F doses higher than 1×1016 cm−2, detrimental consequences were found, degrading the charge to breakdown values by about a factor of 5 after long-term thermal treatments. From ab initio density functional theory pseudopotential calculations for atomic structures and total energies of segregated F, we conclude that the segregation to the interface is driven mainly by the energy difference between Si–O and Si–Si bonds in the interfacial region, by oxygen assisted strain relaxation around Si atoms attacked by F and, possibly, by kinetic bottlenecks in diffusion of network oxygen through SiO2.
Phase-locking Josephson junctions arrays
Authors: Cirillo M. , Rotoli G. , Mueller F. , Niemeyer J. , Poepel R.
Published: July 2002

We demonstrate that a large area Josephson junction oscillating in the fluxon oscillator mode can be synchronized to other large junctions and simultaneously pump, by emitted radiation, a small area junction. We study the synchronization of the oscillations of the long junctions as a function of relevant experimental parameters such as bias current and stripline coupling characteristics. Experimental results obtained on coupled series arrays of Josephson junctions designed on the basis of our calculations are presented. We have fabricated coupled arrays containing each up to 1500 junctions in order to estimate the usefulness of our calculations for voltage standard devices.
Charge carrier response time in sputtered a-C/n-Si heterojunctions
Authors: Konofaos N. , Angelis C. T. , Evangelou E. K. , Dimitriadis C. A. , Logothetidis S.
Published: July 2002

Amorphous carbon (a-C)/n-Si heterojunctions were developed by rf magnetron sputtering from a carbon target on Si(100) n-type substrates kept at room temperature. Subsequent metallization by the deposition of sputtered TiN on top of the carbon films resulted in the creation of effective heterojunction devices as shown by electrical characterization. The electrical performance of the devices was further investigated by admittance spectroscopy, allowing the calculation of the charge carrier response time which was found to be of the order of 10−6 s at room temperature, the lower value reported so far when compared to similar values reported for chemically vapor deposited films. These results showed that the devices were suitable for use in fast electronics working in hard environments.
Formation and decay of nonbridging oxygen hole centers in SiO2 glasses induced by F2 laser irradiation: In situ observation using a pump and probe technique
Authors: Kajihara Koichi , Skuja Linards , Hirano Masahiro , Hosono Hideo
Published: July 2002

Formation and decay of nonbridging oxygen hole centers (NBOHC, an oxygen dangling bond) in SiO2 glasses by F2 excimer laser (7.9 eV) irradiation were in situ analyzed by monitoring 1.9 eV photoluminescence of NBOHC using a pump and probe technique. In wet SiO2, the SiO–H bond was efficiently photolyzed by F2 laser photons to form NBOHC with a quantum yield of ∼0.2. However, the recombination with dissociated hydrogenous species suppressed the buildup of NBOHC. In dry SiO2, in contrast, NBOHC formation by dissociation of strained Si–O–Si bonds was inefficient but NBOHC accumulated with the number of F2 pulses due to a negligibly slow reverse recombination reaction.
Spin-dependent current transmission across a ferromagnet–insulator– two-dimensional electron gas junction
Authors: Hammar P. R. , Johnson Mark
Published: July 2002

The importance of using a low-transmission tunnel barrier for electrical spin injection from a ferromagnetic metal to a semiconductor is experimentally tested. A set of ferromagnet–insulator–two-dimensional electron-gas device stuctures is fabricated. Spin-dependent transport is measured in both diode and potentiometric geometries. Results are compared for devices with junction resistances that vary by an order of magnitude. The spin polarization of the junction current is about 40% for high-resistance barriers, with little temperature dependence over the range 4 K< T<295 K.
Refractive indices of sapphire under elastic, uniaxial strain compression along the a axis
Authors: Jones S. C. , Vaughan B. A. M. , Gupta Y. M.
Published: July 2002

Sapphire crystals were shocked to 190 kbar along the a axis to characterize their use as optical windows, for velocity interferometry measurements, up to their Hugoniot elastic limit. When partially polarized light is incident on the samples, birefringence in the material is manifested as a beat frequency in the probe light that is returned from the specimens. Proper procedures for interpreting the velocity interferometry data for various polarization conditions were developed. The refractive indices at 514.5 nm wavelength decreased linearly with the density. The data were analyzed to yield three photoelastic coefficients: p12, p31, and p41. Calibration is developed for any polarization state of the probe light. Particle velocity wave forms are consistent with elastic behavior up to 170 kbar shock stress, and evidence of deviation from elastic behavior is present at 190 kbar impact stress. High precision shock velocity measurements are reported to 170 kbar stress along the a axis.
Controlled formation of nanoparticles utilizing laser irradiation in a flame and their characteristics
Authors: Lee Donggeun , Yang Sangsun , Choi Mansoo
Published: July 2002

Controlled synthesis of nanoparticles has been carried out using laser beam irradiation on aggregates forming in a flame. This coalescence enhanced method transforms titania aggregates into much smaller unagglomerate spherical anatase nanoparticles by reducing the characteristic time of coalescence of nanoparticles. These transformed nanoparticles exhibit not only better thermal stability, but also unusually small grain growth during a simple pressureless sintering, which ultimately lead to a full dense bulk ceramic with 60 nm grains. Such small grain size in full dense ceramics has not been achieved before without employing special sintering techniques. This would be attributed not only to the successful control of size and morphology of nanoparticles, but also to the synthesis of oxygen vacancy free particles, which was confirmed from Raman spectroscopy.
Highly electrostrictive poly(vinylidene fluoride–trifluoroethylene) networks
Authors: Casalini R. , Roland C. M.
Published: July 2002

We report greatly improved electromechanical performance from a poly(vinylidene fluoride–trifluoroethylene) copolymer by chemical cross linking. Curing with an organic peroxide in combination with a free-radical trap, followed by crystallization, yields a ferroelectric network exhibiting a high electrostrictive response. The electric-field-induced strains were measured at low frequency using an air gap capacitance method and by a noninterferometric optical technique. At electric fields as low as 9 MV/m, longitudinal strains of 12% are obtained. This is more than two orders of magnitude better than conventional vinylidene fluoride materials, and more than an order of magnitude larger than the best results reported to date on any fluoropolymer. Since the mechanical modulus of the network is high (∼0.5 GPa), this work demonstrates the potential for extending the range of utility for polymer-based electromechanical materials.
A molecular dynamics simulation study of the phase behavior of an ensemble of rigid bead-necklace molecules
Authors: Tian Pu , Bedrov Dmitry , Smith Grant D. , Glaser Matthew
Published: July 2002

We have performed molecular dynamics simulation investigations of the phase behavior of an ensemble of rigid molecules interacting via a soft-core repulsive potential. The system consisted of 600 bead-necklace molecules each composed of 11 interaction centers (beads). The system manifested two liquid crystalline phases, a nematic phase and a smectic A phase. Initial points on the isotropic–nematic and nematic–smectic A (P,T) phase coexistence curves were established through rigorous calculation of chemical potentials. The Gibbs–Duhem integration method was subsequently employed to trace the isotropic–nematic and nematic–smectic A coexistence curves over a wide range of pressure and temperature. This simple model was found to capture qualitatively many of the features of the phase behavior of real thermotropic liquid crystals. The isotropic–nematic transition was found to be weakly first order. The enthalpy, entropy, and density of transition for the isotropic-nematic increased with increasing temperature, while the temperature (pressure) range over which the nematic phase is stable increased with increasing pressure (temperature). The nematic–smectic A transition was also found to be weakly first order and was accompanied an increase in the orientational order parameter and a decrease in the thermal fluctuations of the orientational order parameter, indicating coupling between positional order and thermal fluctuations of orientational order. However, contrary to mean-field predictions and the observed behavior for some real thermotropic liquid crystalline materials, the enthalpy of the nematic–smectic A transition was found to increase with increasing temperature/pressure.
Effect of Cr concentration gradient on the intergranular exchange in CoCr(Ta,Pt) thin films
Authors: Valcu Bogdan , Bertram H. Neal
Published: July 2002

Intergranular exchange in CoCr-alloy polycrystalline thin films utilized for longitudinal recording media is analyzed in terms of details of their chemical composition. Two neighboring interacting grains are modeled by a linear “atomic” chain extending from the center of one grain to the center of another. The composition at each spin site determines the interspin exchange interaction. Thermal fluctuations are included via mean field theory and they yield spacial variations of the saturation magnetization and the crystal anisotropy that differ from the bulk properties. Equilibrium spin configurations for the linear chain are obtained for different Cr concentrations at the grain boundary and for different temperatures. Chromium segregation at the boundary is confirmed to be the mechanism for grain decoupling. An effective exchange parameter, assuming uniformly magnetized grains like those in large-scale micromagnetic simulations, is determined. This parameter decreases significantly with increasing temperature and increases slightly with relative anisotropy axis orientation.
Observation of crystallographic wing tilt in cantilever epitaxy of GaN on silicon carbide and silicon (111) substrates
Authors: Katona T. M. , Craven M. D. , Fini P. T. , Speck J. S. , DenBaars S. P.
Published: July 2002

We have characterized the growth of GaN on both SiC and Si (111) using the cantilever epitaxy technique. Cantilever epitaxy employs growth from periodic, parallel mesas which are formed by etching the substrate [C.I.H. Ashby et al. Appl. Phys. Lett. 77, 3233 (2000)]. GaN/AlN/Si and GaN/AlN/SiC layers were grown via metalorganic chemical vapor deposition on substrates with reactive ion etched trenches. This single step approach allows GaN to laterally overgrow the substrate trenches yielding low defect density material over the trench, while avoiding use of a mask which is commonly employed in lateral epitaxial overgrowth. Scanning electron microscopy, atomic force microscopy, and x-ray diffraction were used to characterize the resulting material. Significant threading dislocation reduction was observed in the “wing” regions (the material directly over the trenches). Contrary to previous reports, crystallographic wing tilt ranging from 0.22° to 0.77° was also observed, despite the absence of a wing/mask interface using this technique.
Role of anisotropy in noncontacting thermoelectric materials characterization
Authors: Nayfeh Adnan H. , Carreon Hector , Nagy Peter B.
Published: July 2002

Inclusions and other types of imperfections in metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents that appear when the specimen is subjected to directional heating and cooling. The detectability of small imperfections is ultimately limited by the intrinsic thermoelectric anisotropy and inhomogeneity of the material to be inspected. This article presents an analytical method for calculating the magnetic field produced by thermoelectric currents in anisotropic materials under two-dimensional directional heating and cooling. Experimental results from a textured Ti–6Al–4V titanium-alloy plate are shown to be in very good agreement with the predictions of this model. The described analytical method can be used to optimize thermoelectric inspection procedures and to evaluate the macroscopic texture of metals from their characteristic magnetic signatures.
Atomic-scale processes involved in long-term changes in the density of states distribution at the Si/SiO2 interface
Authors: Lenahan P. M. , Mishima T. D. , Fogarty T. N. , Wilkins R.
Published: July 2002

We utilize very sensitive magnetic resonance measurements to observe changes in the densities of interface trap centers hundreds of hours after irradiation. Our observations provide direct atomic-scale evidence for slow changes in Si/SiO2 interface-state density distributions which appear after the devices have been damaged. Our observations also explain (at least in part) why different groups report somewhat different shapes for the density of interface states in the silicon band gap.
Structural and chemical characterization of 4.0 nm thick oxynitride films
Authors: Banerjee S. , Gibaud A. , Chateigner D. , Ferrari S. , Fanciulli M.
Published: July 2002

We report x-ray reflectivity and secondary ion mass spectrometry (SIMS) analysis of several silicon oxynitride films of 4.0 nm thick as a function of nitrogen concentration at the interface between the oxide and the Si substrate. The x-ray reflectivity data have been analyzed using a model-dependent matrix method, and the results were compared with the model-independent method based on the distorted wave Born approximation and Fourier inversion refinement technique based on the Born approximation. Limitation of each of these techniques is also discussed. The x-ray reflectivity analysis of the films reveals the existence of high electron density at the region where nitrogen accumulation has been observed. Nitrogen accumulation has been observed using dual-beam time-of-flight-SIMS. The results of x-ray reflectivity have been compared with the results of SIMS.
Mid-infrared photonic-crystal distributed-feedback laser with enhanced spectral purity and beam quality
Authors: Bewley W. W. , Felix C. L. , Vurgaftman I. , Bartolo R. E. , Lindle J. R. , Meyer J. R. , Lee H. , Martinelli R. U.
Published: July 2002

We report a photonic-crystal distributed-feedback (PCDFB) laser with an antimonide type-II “W” active region. Optical lithography and dry etching were used to pattern the two-dimensional rectangular lattice with a second-order grating tilted by 20° relative to the facet normal. For pulsed optical pumping, the emission line centered on λ=4.6–4.7 μm is considerably narrower (7–10 nm) than for Fabry-Pérot and angled-grating DFB (α-DFB) lasers fabricated from the same wafer. The PCDFB beam quality is also substantially enhanced, e.g., by a factor of 5 compared with the α-DFB at a pump-stripe width of 200 μm.
Simple efficient coordinate markers for investigating synthetic nanofibers
Authors: Kim Gyu-Tae , Waizmann Ulrike , Roth Siegmar
Published: July 2002

A system of simple coordinate markers has been developed to facilitate the identification of individual synthetic nanofibers adsorbed on a substrate. The markers are deposited by electron beam lithography and allow to locate any spot on an atomic force microscope image and to deposit new structures close to this spot, such as lithographic contacts to nanotubes. As a demonstration, current–voltage characteristics of a junction between a metallic carbon nanotube and an n-type V2O5 nanofiber were recorded.
Nonlinear optical figures of merit of processible composite of poly(2-methoxy,5-(2′-(ethyl)hexyloxy)-p-phenylene vinylene) and poly(methyl methacrylate)
Authors: Lin Y. , Zhang J. , Brzozowski L. , Sargent E. H. , Kumacheva E.
Published: July 2002

We report ultrafast nonlinear optical figures of merit for a highly processible guest–host blend of poly(2-methoxy,5-(2′-(ethyl)hexyloxy)-p-phenylene vinylene) with poly(methyl methacrylate). Our experiments employ 120 fs pulses at 840 nm and are designed to eliminate slow thermal nonlinearity and focus exclusively on ultrafast electronic nonlinearity. We report a two-photon absorption coefficient β of 1.5±0.2 cm/GW, a nonlinear refraction coefficient n2 of −(2.1±0.2)×10−13 cm2/W and a two-photon figure of merit T of 0.6. The blend hybridizes the desirable features of nonlinearity and processibility of its two constituents to provide (1) ultrafast response one to two orders of magnitude faster than achievable in electronic switching devices; (2) a two-photon figure of merit compatible with harnessing this nonlinearity in a practical optical device geometry; and (3) a materials system and processing methodology compatible with spin-coating and photopatterning in an ambient environment.
Nonlinear optical probing of nanocrystalline orientation in epitaxial ferroelectric thin films
Authors: Woltersdorf G. , Seifert G. , Graener H. , Pignolet A , Hesse D.
Published: July 2002

Epitaxially grown ferroelectric BaBi4Ti4O15 thin films were studied by optical second-harmonic (SH) generation using a femtosecond titanium:sapphire laser at 800 nm wavelength. By varying both the incidence and the azimuthal angle and registering the second-harmonic intensity, a significant correlation was found between the azimuthal dependence of the measured SH signal and the nanoscopic texture of the samples, which was determined by electron microscopy and x-ray diffraction. In particular, two different types of grains with different crystallographic orientations generate different SH intensity maxima with fourfold azimuthal symmetry, distinguishable from an isotropic background due to a c-axis oriented main layer. This correlation suggests the use of the SH technique for a purely optical characterization of the film orientation and thickness.
Tunneling current through a quantum dot array
Authors: Kuo David M.-T. , Guo G. Y. , Chang Yia-Chung
Published: July 2002

The tunneling current through a quantum dot array (QDA) is studied theoretically. Strong electron correlation effect is taken into account in the QDA in which the quantum dots provide a strong three-dimensional confinement effect. A mixed Hubbard and Anderson model is used to simulate the system. It is found that Coulomb charging splits the band resulting from interdot coupling into two subbands. The tunneling current is thus influenced significantly by both Coulomb charging and interdot coupling.
Influence of gas transport on the oxidation rate of aluminum arsenide
Authors: Cich M. J. , Zhao R. , Anderson Erik H. , Weber E. R.
Published: July 2002

The effect of gas transport on the lateral oxidation kinetics of aluminum arsenide has been studied by limiting the gas transport to the gas–oxide interface using closely spaced mesas. At 440 °C the gas transport factor is found to be 8.5±1.3 times the reaction rate coefficient, resulting in a measurable decrease in oxidation rate for mesas closer than 200 nm. This confirms the usual assumption that the initial oxidation rate is limited by the reaction kinetics at the oxidizing interface and not the gas transport for normal oxidation conditions
Picosecond large angle reorientation of the magnetization in Ni81Fe19 circular thin-film elements
Authors: Wu J. , Schmool D. S. , Hughes N. D. , Moore J. R. , Hicken R. J.
Published: July 2002

Large angle picosecond reorientation of the magnetization has been studied in circular Ni81Fe19 thin-film elements of 30 μm diameter and 500 Å thickness by means of an optical pump–probe technique. The sample was pumped by an optically triggered magnetic field pulse and probed by a time resolved magneto-optical Kerr effect measurement. The temporal profile of the pulsed field and the in-plane uniaxial anisotropy of the element were first determined from measurements made in large static fields where the magnetization exhibited small amplitude ferromagnetic resonance oscillations. Measurements of large amplitude oscillations were then made in a smaller static field that was still larger than the in-plane uniaxial anisotropy field and sufficient to saturate the sample. Using the measured temporal profile of the pulsed field, the Landau–Lifshitz–Gilbert equation was used to model the motion of the magnetization as a coherent rotation process. The same values of the anisotropy and damping constants provided an adequate simulation of both the high and low field data. The magnetization was found to move through an angle of up to about 30° on subnanosecond time scales. The dependence of the reorientation upon the direction of the static applied field and observed deviations from the coherent precession model are discussed.
Direct hysteresis measurements of single nanosized ferroelectric capacitors contacted with an atomic force microscope
Authors: Tiedke S. , Schmitz T. , Prume K. , Roelofs A. , Schneller T. , Kall U. , Waser R. , Ganpule C. S. , Nagarajan V. , Stanishevsky A. , Ramesh R.
Published: July 2002

Direct hysteresis measurements on single submicron structure sizes were performed on epitaxial ferroelectric Pb(Zr,Ti)O3 thin films grown on SrTiO3 with La0.5Sr0.5CoO3 (LSCO) electrodes. The samples were fabricated by focused-ion-beam milling resulting in pad sizes down to 200 nm×200 nm. The influence of parasitic capacitance of the measurement setup was eliminated by applying an enhanced compensation procedure. No size effects were observed in capacitors milled down to 400 nm×400 nm. Thus, a published increase of Pmax1 for decreasing pad size can be explained by the parasitic influence of the setup. Finally, the inaccuracy of increasing coercive voltage due to the coating of the cantilever of the atomic force microscope is discussed.
Electron acceleration based on self-trapping by plasma wake fields
Authors: Suk H.
Published: July 2002

A method for self-injection of background plasma electrons in a plasma wake field is introduced. In this scheme, a plasma wake wave, which can be generated by an intense laser pulse or an electron beam pulse propagating in a plasma, is passed through a sharply downward plasma density transition. When the plasma wake wave passes the density transition, some background plasma electrons are trapped by the ultrastrong plasma wake field that is several orders of magnitude stronger than the accelerating electric field in conventional rf-based accelerators. In this paper, the basic mechanism of the trapping is introduced and two-dimensional particle-in-cell simulation results including trapped-beam quality studies are presented. In addition, the experimental program to investigate the self-injection-based acceleration at the Center for Advanced Accelerators of KERI is introduced.
Model for doping-induced contrast in photoelectron emission microscopy
Authors: Ballarotto V. W. , Siegrist K. , Phaneuf R. J. , Williams E. D.
Published: July 2002

We present a model that describes doping-induced contrast in photoelectron emission microscopy by including the effect of surface state distributions and doping-induced band gap reduction. To quantify the contrast, the photoyield from the valence band for near-threshold photoemission is calculated as a function of p-type doping concentration in Si(001). Various surface state distributions appropriate for a native-oxide covered Si device are investigated in order to determine the effect on doping-induced contrast. The lower limit on the number of surface states necessary for doping-induced contrast to occur is approximately 5×1013 cm−3. An interesting result is that neither the position nor the energy distribution of the surface donor states affects the contrast, which corresponds to approximately a factor of 2 change in intensity for each decade change in doping density. However, the overall intensity increases with any one of: increased surface state density, narrowing of surface state distribution, or increased energy of surface states with respect to the valence band. The band bending profile generated by the model predicts that doping-induced contrast will be affected by varying the incident photon energy. Experimentally, we verify this prediction by imaging with photon energies between 4.5 and 5.2 eV.
Changes in the density of ultrathin silicon oxide films related to excess Si atoms near the oxide–Si(100) interface
Authors: Yamada Hiroshi
Published: July 2002

To clarify the considerable changes in the (volumetric) density ρox of 3.5–8.0-nm-thick silicon oxide films on Si(100) oxidized at 800–950 °C, which were confirmed by our previous work, ρoxs and (areal) number densities of the 700–750 °C oxidation films were investigated. The ρoxs and number densities of Si and O atoms were determined by charged-particle activation analysis and Rutherford backscattering spectrometry, respectively. It was confirmed again that excess Si atoms relative to the stoichiometric SiO2 composition exist near the oxide–Si substrate interface and the number densities of them NSi(excess)s were changed with oxidation temperature. The ρoxs were also changed with oxidation temperature but the changes in ρox were diametrically contrasted with those in NSi(excess). For the 2.0–8.0-nm-thick films oxidized at 700–950 °C, the ρox characteristics exhibited a maximum at 850 °C but the NSi(excess) ones exhibited a minimum at 850 °C. It is, therefore, believed that this ρox change is governed mainly by microscopic changes in the atomic arrangement structure due to the NSi(excess) difference.
Design of lightweight multilayer partitions based on sonic crystals
Authors: Uris Antonio , Rubio Constanza , Estelles Hermelando , Sanchez-Perez Juan V. , Martinez-Sala Rosa , Llinares Jaime
Published: July 2002

The sound transmission coefficient of different multilayer partitions commonly encountered in buildings has been measured as a function of frequency. Most of the samples studied showed an increase in the sound transmission coefficient over a specific frequency, called the critical frequency, depending on the layer material. However, for partitions built with the same materials, but built with a periodic arrangement of layers, this behavior has not been observed. This kind of periodic multilayer partition can be considered as a sonic crystal, because the stopband corresponding to a one-dimensional sonic crystal with a constant lattice equal to the modulation of the partition is in the same range as the critical frequency of the panel.
Single-mode operation of coupled-cavity lasers based on two-dimensional photonic crystals
Authors: Happ Thomas D. , Markard Alexander , Kamp Martin , Forchel Alfred , Anand Srinivasan
Published: July 2002

We report the fabrication of short-cavity lasers with highly reflective two-dimensional photonic crystal mirrors on an InGaAsP/InP laser structure emitting at 1.57 μm. An intracavity photonic crystal mirror creates two coupled cavities, which provide additional longitudinal mode selection for stable single-mode operation with side-mode suppression ratios exceeding 35 dB. The shortest lasers with l=100 μm overall length have a threshold current of 13 mA and provide more than 4 mW power under continuous wave operation. Longer devices with l=200 μm deliver up to 9 mW. A maximum modulation bandwidth of 7.9 GHz was determined by relative intensity noise measurements.
Comment on “Evidence for edge gradients as control parameters of the spontaneous high-mode transition” [Phys. Plasmas 8, 2722 (2001)]
Authors: Shoucri Magdi
Published: July 2002

Using an Eulerian Vlasov code, we show in the present comment that finite ion orbits at a plasma edge conjugate with steep gradients to produce a charge separation and an electric field in the region where the gradients are steep, which can provide an important “control parameter” for the L–H transition.
Polymer PBT/n-GaN metal–insulator–semiconductor structure
Authors: Tu L. W. , Tsao P. H. , Lee K. H. , Lo Ikai , Bai S. J. , Wu C. C. , Hsieh K. Y. , Sheu J. K.
Published: July 2002

Organic poly(p-phenylenebenzobisthiazole), PBT, is spin coated on n-GaN epilayer to serve as a gate insulating layer. The GaN is grown on c sapphire by the metalorganic-chemical-vapor-deposition method. A metal-insulator-semiconductor structure is fabricated with Al ohmic contact to the n-GaN, and Au/Al gate metal on top of the PBT. High-frequency capacitance measurements are performed. A doping concentration of 8.2×1016 cm−3 is obtained from the capacitance in the depletion region as compared with a Hall value of 7.8×1016 cm−3. Another technique of max–min capacitance method is also used in the calculation of the doping concentration. The threshold voltage is calculated as −5.1 V, and negative charges with an effective charge number density of 9.3×1011 cm−2 shift the capacitance curve towards the positive voltage side. The hysteresis phenomenon is observed and analyzed. The current–voltage curve shows a low, constant current value up to 20 V.
Investigation of tearing modes behavior in predisruption phase
Authors: Farshi E. , Brevnov N. , Gott Y.
Published: July 2002

The m=2 tearing mode in predisruptive activity with the visible radiation spectrum and the charge exchange neutral have been investigated in the Damavand Tokamak [Amrollahi et al., Plasma Phys. Rep. 23, 561 (1997)]. The oscillations amplitude of the mode has been increased about 500 microseconds before disruption. After that instead of complete ceasing the oscillations, which is observed usually in locked modes, only a reduction in the amplitude of oscillations and decreasing in the frequency have been observed. The experiments show when the amplitude of oscillations increases, the radiation power increases also, but with reduction of amplitude, the radiation power at the plasma center does not have much changes whereas the radiation power at the plasma edge begins to reduce. Therefore, a change in the current density profile has been suggested. When the temperature recovers at the edge, another cycle starts that the result will be as the bursts of large-amplitude m=2 fluctuations, and prolongation of predisruption phase.
Observation of supersonic shock wave mitigation by a plasma aero-spike
Authors: Bivolaru Daniel , Kuo S. P.
Published: July 2002

A wind-tunnel model in the shape of a 30° half-angle truncated-cone is designed to generate a strong bow shock behind a weak conical (oblique) shock wave attached to the tip of a protruding central-electrode, in a non-ionized supersonic flow. Plasma is generated between two shocks by an on-board discharge. Its effect on shock waves is explored. The results show that the plasma spike has drastically modified the original complicated shock structure to a simple structure having only a single conical shock attached to the tip of the model, similar to the one generated by a perfect cone.
A review of the first ionization potential effect on elemental abundances in the solar corona and in flares
Authors: Feldman U. , Widing K. G.
Published: July 2002

The elemental composition of solar upper atmosphere plasmas was studied from spectra obtained by instruments aboard all major solar observatories that were launched into space in the last three decades. The studies show that the first ionization potential (FIP) of the elements has a profound effect on their abundance in the various solar upper atmosphere regions. In this paper we review properties of the FIP effect as they relate to coronal holes, quiet regions, active regions and flares and we show that although the derived compositions vary considerably with the type of plasma observed regularities in their behavior are emerging. The observed behavior may help identify the energy deposition mechanism that fuels the corona and solar flares.
Analysis of ion temperature gradient modes in high β plasmas with sheared slab configuration model
Authors: Gao Zhe , Dong J. Q. , Liu G. J. , Ying C. T.
Published: July 2002

A set of integral equations is developed to study drift instabilities in any β (plasma pressure/magnetic pressure) plasmas with the sheared slab magnetic configuration model. Both components of the perturbed vector potential, Ã∥ and Ã⊥, are considered in the equations, as well as the perturbation of the electrostatic potential φ̃. The magnetic gradient drift effects are taken into account. The ion temperature gradient modes are analyzed and found to be unstable in the high β regime. The stability of the high β modes is very sensitive to the mode frequency. The lower frequency modes are more difficult to be stabilized since the β effects cannot effectively change the frequency and then the particle-wave interaction in the lower frequency regime. The magnetic shear is shown to have strong stabilizing effects on the high β modes.
Strain partitioning in coherent compliant heterostructures
Authors: Zubia D. , Hersee S. D. , Khraishi T.
Published: July 2002

A self-consistent set of equations describing strain partitioning in planar bilayers is developed using a typical definition of strain, the assumption of a coherent interface and a mechanical equilibrium criterion. This approach eliminates the need for the concepts of lattice mismatch and compatibility of deformation, leading to a general solution for the strains and in-plane lattice parameter in bilayer structures. Using the strain equations, the strain energies in the system are calculated as a function of the epilayer to substrate thickness ratio. It was found that for a given substrate thickness, the epilayer strain energy contains a maximum at a layer thickness ratio of ∼1. The peak epilayer strain energy is only ∼25% of the maximum possible in the system. A criterion based on energy considerations is proposed for determining whether to use the epilayer or substrate dislocation formation energy when calculating the epilayer critical thickness. This criterion is applied to the GexSi1−x/Si(100) material system and is manifested by a kink in the critical thickness versus substrate thickness curves. The kink is interpreted as a boundary identifying whether a threading dislocation will most favorably be injected into the substrate or epilayer when the critical thickness is exceeded.
Liquid meniscus condensation in dip-pen nanolithography
Authors: Jang Joonkyung , Schatz George C. , Ratner Mark A.
Published: July 2002

The condensation of a liquid meniscus between a curved tip and a completely wetting substrate is theoretically studied in the context of dip-pen nanolithography (DPN). Utilizing the grand canonical Monte Carlo simulation of a two-dimensional lattice gas model, we studied the onset and broadening of menisci by tips with a range of curvatures. The tip–liquid interaction is characterized in terms of the wettability of the tip, and both wetting and drying tips are considered to mimic the various (hydrophilic and hydrophobic) ink molecule–water interactions possible in DPN. We study the microscopic details of the meniscus formation and examine the thermodynamic stability of the meniscus by focusing on the fluctuation in its width. After its initial formation, a meniscus grows continuously with increasing saturation (relative humidity), but the meniscus is typically wider than 10 molecular diameters until the saturation is sufficiently high that the entire interfacial region frills with liquid. For large tip–substrate distances, meniscus formation only occurs in the high saturation limit where conventional capillary condensation occurs. A general trend is that a sharp (small radius of curvature), dry tip results in a smaller meniscus width which further shrinks upon shortening the tip–substrate distance and/or raising temperature. At very short tip–substrate distances, the nascent meniscus is unstable and its width is independent of the tip curvature and wettability. The minimum width corresponds to a physical dimension of about 2.5 nm.
Ga flux dependence of Er-doped GaN luminescent thin films
Authors: Lee D. S. , Steckl A. J.
Published: July 2002

Er-doped GaN thin films have been grown on (111) Si substrates with various Ga fluxes in a radio frequency plasma molecular beam epitaxy system. Visible photoluminescence (PL) and electroluminescence (EL) emission at 537/558 nm and infrared (IR) PL emission at 1.5 μm from GaN:Er films exhibited strong dependence on the Ga flux. Both visible and IR PL and visible EL increase with the Ga flux up to the stoichiometric growth condition, as determined by growth rate saturation. Beyond this condition, all luminescence levels abruptly dropped to the detection limit with increasing Ga flux. The Er concentration, measured by secondary ion mass spectroscopy and Rutherford backscattering, decreases with increasing Ga flux under N-rich growth conditions and remains constant above the stoichiometric growth condition. X-ray diffraction indicated that the crystalline quality of the GaN:Er film was improved with increasing Ga flux up to stoichiometric growth condition and then saturated. Er ions in the films grown under N-rich conditions appear much more optically active than those in the films grown under Ga-rich conditions.