bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of self-organized surface nanostructures and anisotropic electron transport in nonpolar ZnO (10−10) homoepitaxy
Authors: Matsui Hiroaki , Tabata Hitoshi
Published: August 2006

Self-organized surface nanostructures were formed during laser molecular beam epitaxy of nonpolar ZnO (10−10). The growth mechanism of the nanostructures was related to a Schwoebel barrier effect generated during ZnO homoepitaxy. Small islands elongated along the [001] direction were generated on two-dimensional (2D) growing surfaces above the critical thickness. With increasing film thickness, the anisotropic islands developed homogeneous nanostripe arrays with stripe lengths above 5μm and connected by some branches. Highly anisotropic surface morphology markedly influenced electron transport of ZnO films and Mg0.12Zn0.88O∕ZnO multiquantum wells (MQWs) with conductivity parallel to the nanostripe arrays by more than one order of magnitude larger than that observed perpendicular to the nanostripe arrays. We propose that the origin of anisotropic conductivity of ZnO films resulted mainly from anisotropically distributed scattering centers of electrons related to grain boundaries between nanostripe arrays. Furthermore, the surface nanostructures consisting of nanostripe arrays gave rise to the anisotropic surface roughness at the heterointerface between Mg0.12Zn0.88O and ZnO layers and contributed to the anisotropy of 2D electron transport of the MQWs at low temperatures.
A spectroscopic comparison of femtosecond-laser-modified fused silica using kilohertz and megahertz laser systems
Authors: Reichman Wilbur J. , Krol Denise M. , Shah Lawrence , Yoshino Fumiyo , Arai Alan , Eaton Shane M. , Herman Peter R.
Published: August 2006

Waveguides were written in fused silica using both a femtosecond fiber laser with a 1MHz pulse repetition rate and a femtosecond amplified Ti:sapphire laser with a 1kHz repetition rate. Confocal Raman and fluorescence microscopies were used to study structural changes in the waveguides written with both systems. A broad fluorescence band, centered at 650nm, associated with nonbridging oxygen hole center (NBOHC) defects was observed after waveguide fabrication with the megahertz laser. With the kilohertz laser system these defects were only observed for pulse energies above 1μJ. Far fewer NBOHC defects were formed with the megahertz laser than with kilohertz writing, possibly due to thermal annealing driven by heat accumulation effects at 1MHz. When the kilohertz laser was used with pulse energies below 1μJ, the predominant fluorescence was centered at 550nm, a band assigned to the presence of silicon clusters (Eδ′). We also observed an increase in the intensity of the 605cm−1 Raman peak relative to the total Raman intensity, corresponding to an increase in the concentration of three-membered rings in the lines fabricated with both laser systems.
Numerical analysis of single photon detection avalanche photodiodes operated in the Geiger mode
Authors: Sugihara K. , Yagyu E. , Tokuda Y.
Published: August 2006

For a wide range of the thicknesses of the charge and the multiplication layers, detection efficiency and dark count probability are numerically investigated for GaInAs∕InP single photon detection avalanche photodiodes (APD’s) which are operated in the Geiger mode. Breakdown probability and dark currents are calculated to evaluate detection efficiency and dark count probability. The result shows that dark count probability can be significantly reduced by increasing the thickness of the charge layer, whereas detection efficiency is expected to decline steeply at some thickness of the charge layer. Moreover, increasing the thickness of the multiplication layer does not continue to reduce dark count probability, which increases when the multiplication layer is thicker than a critical thickness. Finally, we show a design guideline of single photon detection APD’s with higher detection efficiency and lower dark count probability.
Influence of defects in the output power of c- and a-axis growth Nd:YVO4 single laser crystals
Authors: Shawley Charles R. , Wang Cai-Lin , Lynn Kelvin G.
Published: August 2006

The differences in the output powers of a- and c-axis growth neodymium-doped yttrium orthovanadate (Nd:YVO4) crystals grown by the Czochralski method were investigated. It was found that the differences in the output powers are due to crystal defects, such as veils and grain boundaries, which lead to changes in the thermal and optical properties of Nd:YVO4 and reduce the quality of the laser beam. To relieve internal stresses introduced by the crystal growth process, boules form grain boundaries and other thermodynamically favorable defects in the boule. Because c-axis grown Nd:YVO4 crystals do not contain a slip system, no grain boundaries or ghost veils can form. As a result, c-axis grown boules are highly vulnerable to thermal and stress fractures and require careful growth parameter selection to overcome these defects. However, a-axis grown crystal contain a slip system and can form thermodynamically stable defects including grain boundaries, scatter centers, and ghost veils. In this paper, data are presented showing that properly grown c-axis growth crystal, having no grain boundaries or veils, is more suited for high output power applications than a-axis grown one.
All-optical diodes based on photonic crystal molecules consisting of nonlinear defect pairs
Authors: Zhou Hui , Zhou Kai-Feng , Hu Wei , Guo Qi , Lan Sheng , Lin Xu-Sheng , Venu Gopal Achanta
Published: August 2006

We investigate the unidirectional transmission behavior of photonic crystal (PC) molecules consisting of nonlinear defect pairs by use of the coupled mode theory and the finite-difference time-domain technique. As compared with the all-optical diodes based on single asymmetrically confined PC defects (or PC atoms), a significant enhancement in the transmission contrast is achieved. A qualitative comparison of the dynamical response to external excitation is carried out for nonlinear PC atoms and molecules and the physical origins responsible for the enhancement of transmission contrast are clarified. In addition, the tolerance of the resulting optical diodes against the fabrication error is discussed. Furthermore, it is revealed that the figure of merit, which is defined as the product of the threshold transmission and the transmission contrast, can be greatly improved by dropping the use of air defects. Some abnormal and intriguing transmission behaviors are observed in the PC molecules without using air defects.
Simulation of the dielectric resonator for radio frequency superconducting quantum interference devices
Authors: He D. F. , Itozaki H.
Published: August 2006

We simulate the dielectric SrTiO3 (STO) resonator for radio frequency superconducting quantum interference devices (rf SQUIDs) using electromagnetic analysis software. The simulated values of the resonant frequencies, the unloaded quality factor, the loaded quality factor under 50Ω matching, and the coupling with the SQUID are in good agreement with the experimental results. The simulation also shows that the strength of the electromagnetic field outside the resonator decreases exponentially with the distance from the resonator, which proves that the field is confined to a small volume near the resonator. Due to the strong temperature dependence of the permittivity of STO, the resonant frequency changes with the temperature fluctuations of liquid nitrogen, which may cause the low frequency noise of rf SQUIDs with dielectric resonators.
Nonlinear contributions to the dielectric permittivity and converse piezoelectric coefficient in piezoelectric ceramics
Authors: Eitel R. E. , Shrout T. R. , Randall C. A.
Published: August 2006

A measurement and analysis technique has been developed to quantify the complex contributions to the nonlinear dielectric and converse piezoelectric response in ferroelectric ceramics. The electric field dependent response was quantified in two important bulk ceramic piezoelectric systems, Pb(Zr1−x,Tix)O3 and (1−x)BiScO3–xPbTiO3 (BS-PT), at low frequency and room temperature. The nonlinear response was shown to obey the Rayleigh law in the specific field and frequency ranges reported. Analysis of BS-PT perovskites with different crystal structures revealed a ferroelectric phase dependence on the magnitude of the nonlinear contribution to the dielectric and piezoelectric responses. The magnitudes of the nonlinear coefficients, which quantify the irreversible extrinsic response, were found to be smallest in the ferroelectric tetragonal phase, larger in the rhombohedral composition, and highest at the morphotropic phase boundary. Finally, this work provides strong evidence that the same loss mechanism dominates the nonlinear portion of both the dielectric and piezoelectric responses in piezoelectric ceramics. Analysis of these contributions supports an earlier theoretical model inferring that non-180° domain wall motion in bulk ferroelectric ceramics is a major factor in the piezoelectric and dielectric responses. These results link the observed Rayleigh law behavior to the mechanistic domain wall model commonly employed to describe the extrinsic response of ferroelectric ceramics.
Weak ferromagnetism and exchange biasing in cobalt oxide nanoparticle systems
Authors: Tomou A. , Gournis D. , Panagiotopoulos I. , Huang Y. , Hadjipanayis G. C. , Kooi B. J.
Published: August 2006

Cobalt oxide nanoparticle systems have been prepared by wet chemical processing involving the encapsulation of the nanoparticles by an organic ligand shell (oleic acid and oleylamine). CoO nanoparticles were easily prepared by this method, while the synthesis of the CoPt∕CoO nanocomposites was achieved using a two step polyol process. CoPt nanoparticles were first obtained by simultaneous reduction of cobalt acetate and platinum acetylacetonate and then used as seeds for the growth upon them of cobalt oxide using a second polyol process. The antiferromagnetic CoO nanoparticles, when field cooled to temperatures below 200K, show displacement of the magnetization curves (along the magnetization axis) characteristic of weak ferromagnetism phenomena that are attributed to the uncompensated surface magnetic moments. The transition temperature of the particles is considerably lower than the Néel temperature of CoO and it is followed by an upswing at low temperatures, which is attributed to spins that are loosely coupled to the antiferromagnetic core. In the CoPt∕CoO nanocomposites, magnetic measurements show the appearance of increased coercivity with respect to the as-prepared CoPt particles and unidirectional anisotropy (loop shift of Heb=1125Oe) at temperatures below 20K, as a result of exchange coupling between CoO and CoPt.
Above-room-temperature 3–12μm Si emitting arrays
Authors: Malyutenko V. K. , Bolgov S. S. , Malyutenko O. Yu.
Published: August 2006

The design and parameters are presented for 16-element infrared (IR, 3–12μm) emitting arrays based on Si p+in+ structures and above-room-temperature (300K<T<480K) operated by the contact double injection. The device principle is in modulation of thermal emission beyond the fundamental absorption edge by controlling free charge carrier concentration in a base. Nearly 1.0mW output power and 420K apparent temperature of IR radiation could be achieved with ∼0.8% external power efficiency. This represents the longer wavelengths, higher operating temperatures, and output power from Si spontaneous emitters ever reported.
Micro-strip-line-based sensing chips for characterization of polar liquids in terahertz regime
Authors: Ohkubo T. , Onuma M. , Kitagawa J. , Kadoya Y.
Published: August 2006

We have developed embedded thin-film micro-strip-line-based sensor chips working in terahertz regime. In the chip, terahertz waves are generated and detected with femtosecond optical pulses accessed from the back side of the chips. Moreover, the spectroscopic sensitivity can be freely adjusted by changing the thickness of a polyimide cover layer. They make it easy to measure polar liquids. The measurement of water is demonstrated with spectral range from 30GHzto1–1.5THz, where the upper limit depends on the thickness of the cover layer.
Direct measurement of penetration length in ultrathin and/or mesoscopic superconducting structures
Authors: Hao L. , Macfarlane J. C. , Gallop J. C. , Lam S. K. H.
Published: August 2006

As the dimensions of thin superconducting structures become comparable with or less than the penetration length of magnetic flux into the structures, it becomes increasingly necessary to devise experimental tests of available theoretical models. One approach which we shall describe enables penetration lengths to be derived from the measurement of the effective area of planar, thin-film structures with linear dimensions in the range of 1–100μm. The effective area is defined by measurement of the inductive coupling of the structures to dc or low-frequency magnetic fields. The structures described consist of two parts: (1) an ultrathin annular superconducting film with transition temperature Tca (“washer”) and (2) surrounding the washer is a superconducting ring with transition temperature Tcs. Because the films are prepared in such a way that Tca<Tcs, the ring-washer combination acts as a dc superconducting quantum interference device up to and beyond Tca, enabling the effective area of the washer to be measured over a wide temperature range. Results for the temperature dependence of the Pearl penetration length Λ(T), derived directly from measurements of the effective area, are compared both with theory and with other experimental data. Whereas alternative methods may be restricted to narrow-band, high-frequency fields and require sample dimensions of order of 10mm or greater, the method is inherently broadband and is applicable to dimensions ⩾1μm.
Measurement of negative permittivity and permeability from experimental transmission and reflection with effects of cell misalignment
Authors: Wang Dongxing , Huangfu Jiangtao , Ran Lixin , Chen Hongsheng , Grzegorczyk Tomasz M. , Kong Jin Au
Published: August 2006

The measurement of the negative effective permittivity and permeability of a material is a direct way to verify its left-handed properties. In this paper, a transmission experiment for a left-handed material (LHM) sample is performed in a parallel plate waveguide. The effective frequency dispersive permittivity and permeability are retrieved from the experimental scattering data and found to be simultaneously negative in a specific frequency range. The simulation results also show that the misalignments of the metallic inclusions of the LHM introduce some dips to the retrieved curves, which prevent us from obtaining ideal curves following the Lorentz or Drude model.
Rotational resonance of magnetoinductive waves: Basic concept and application to nuclear magnetic resonance
Authors: Solymar L. , Zhuromskyy O. , Sydoruk O. , Shamonina E. , Young I. R. , Syms R. R. A.
Published: August 2006

Magnetoinductive waves propagating along a set of resonant metamaterial elements are studied under the condition when the wave travels round a closed circular path and the total phase shift is an integral multiple of 2π. The resonant frequency of the circulating wave is shown to be related to the resonant frequency of the element via the known dispersion relationship. The currents in the elements are determined with the aid of the impedance matrix when the excitation is by a rotating magnetic dipole located at the center of the structure. It is shown that the power taken out from one element in the loop may approach N times that from a single element, where N is the number of elements, provided the quality factor of the individual elements is sufficiently high and suitable modifications are made to nearby elements. Potential applications to magnetic resonance spectroscopy are discussed.
Exciton fine structure and biexciton binding energy in single self-assembled InAs∕AlAs quantum dots
Authors: Sarkar D. , van der Meulen H. P. , Calleja J. M. , Becker J. M. , Haug R. J. , Pierz K.
Published: August 2006

The exciton and biexciton emissions of a series of single quantum dots of InAs in an AlAs matrix have been studied. These emissions consist of linear cross polarized doublets showing large values of both the biexciton binding energy and the fine-structure splitting. At increasing exciton emission energy, corresponding to decreasing dot size, the biexciton binding energy of 9meV decreases down to zero, reflecting a possible crossover to an antibinding regime. Simultaneously the fine-structure splitting diminishes from a value of 0.3meV down to zero, at the same energy, suggesting a common origin for the two effects.
Effect of growth rate on the composition fluctuation of InGaAsN∕GaAs single quantum wells
Authors: Chen J. F. , Hsiao R. S. , Hsieh P. C. , Wang J. S. , Chi J. Y.
Published: August 2006

Effect of growth rate on the composition fluctuation is investigated in In0.34Ga0.66As0.98N0.02∕GaAs single quantum wells (QWs) by photoluminescence (PL), transmission electron microscopy, and admittance spectroscopy. In an InGaAsN layer grown at a normal growth rate, the PL spectra show a low-energy bump at the tail of an InGaAsN emission, suggesting the presence of composition fluctuation. Lowering the growth rate degrades the composition fluctuation by segregating into bimodal phases of an InGaAsN and InGaAs-rich phase. Further lowering the growth rate leads to a three-dimensional growth and enhances the InGaAs-rich phase. The carrier distribution for the InGaAsN layer grown at the normal rate shows a carrier bump at the tail of a strong accumulation peak, suggesting the presence of an electron state below the QW ground state. The admittance spectroscopy shows that the activation energy (32meV) of this electron state is comparable to the energy separation (30meV) between the InGaAsN emission and the low-energy bump, and thus it is possible that the composition fluctuation actually induces an electron state closely below the QW ground state.
Thermal characterization of dielectric and phase change materials for the optical recording applications
Authors: Yang Y. , Li C.-T. , Sadeghipour S. M. , Dieker H. , Wuttig M. , Asheghi M.
Published: August 2006

Advances in the phase change optical recording technology strongly depend on the optical and thermal optimizations of the metal/ZnS–SiO2/phase change multilayer structure, which requires accurate modeling and thermal characterization of the phase change media structure. In the present work, the thermal conductivities of the amorphous and crystalline Ge4Sb1Te5 phase change and ZnS–SiO2 dielectric layers of thicknesses in the range of 50–300nm have been measured using the transient thermoreflectance technique. The data are between factors of 2–4 different from the previously measured values for thin film and bulk samples. The thermal boundary resistance at a metal/ZnS–SiO2 interface is found to be around 7×10−8m2W−1. This might have serious implications for the future phase change recording application which attempts to achieve the high writing speeds by decreasing the thickness of a ZnS–SiO2 dielectric layer.
Magnetomechanical damping capacity of Tb0.36Dy0.64(Fe1−xTx)2 (T=Co,Mn) alloys
Authors: Ma Tianyu , Jiang Chengbao , Xiao Fu , Yu Liming , Xu Huibin , Pei Yongmao , Fang Daining
Published: August 2006

The magnetomechanical damping capacity of the giant magnetostrictive Tb0.36Dy0.64(Fe1−xTx)2 (x=0, 0.05, 0.10; T=Mn or Co) alloys has been calculated from quasistatic strain-stress hysteresis loops at room temperature. Stress ranges from 0 to 10, 20, 30, and 50MPa were used in a series of applied fields between 0 and 3250Oe. The largest damping capacities were obtained at zero field with values of 1.14 for Tb0.36Dy0.64Fe2, 1.62 for Tb0.36Dy0.64(Fe0.95Mn0.05)2, and 1.84 for Tb0.36Dy0.64(Fe0.90Co0.10)2, respectively. In low fields, the damping capacity ΔW∕W decreases drastically with the applied stress and magnetic field. However, in higher fields, ΔW∕W decreases slightly with the magnetic field but only increases slightly with the applied stress. The results show that both Mn and Co as substitutes for Fe are beneficial for obtaining higher damping capacity.
Growth and characterization of red-green-blue cathodoluminescent ceramic films
Authors: Gozzi Daniele , Latini Alessandro , Salviati Giancarlo , Armani Nicola
Published: August 2006

Gd 2 O 3 and Y2O3 films, respectively, doped with Eu3+, Tb3+, and Tm3+ have been grown by the electron beam physical vapor codeposition technique on optically polished quartz substrates. The film samples have been doped at different concentrations by the corresponding rare-earth oxides. The concentration range explored is from 0.9% to 9.8% on 18 samples. For each film sample an extended characterization has been performed by thin film-x-ray diffraction, scanning electron microscopy, energy dispersion spectroscopy, cathodoluminescence spectroscopy, and color coordinate analysis. Y2O3 films display the most intense red-green-blue (RGB) emission and their film morphology and structure are more compact and crystalline with respect to Gd2O3 monoclinic films. Eu3+ and Tb3+ doped Y2O3 films grow oriented along the (222) direction. The ratios between the intensities of the electric dipole and magnetic dipole transitions have been also evaluated. The blue emission of Tm3+ doped Gd2O3 is lacking in the dopant concentration range from 1.6% to 7.6%, whereas it is present in Tm3+ doped Y2O3 films, at approximately the same dopant concentration range (1.9%–9.8%). Commission Internationale de l’Eclairage plot of the color coordinates of all the RGB film samples has been reported together with the RGB phosphor standard used in cathodic ray tube TV screens.
Large second-order optical nonlinearity in 30BaO–15TiO2–55GeO2 surface crystallized glass with strong orientation
Authors: Masai Hirokazu , Fujiwara Takumi , Benino Yasuhiko , Komatsu Takayuki
Published: August 2006

The correlation between the orientation degree of surface crystalline phase and the second-order optical susceptibility, d coefficient for second-harmonic generation, in 30BaO–15TiO2–55GeO2 (BTG55) crystallized glass was investigated using x-ray diffraction measurements and a Maker fringe technique. The d coefficient in BTG55 crystallized glass was confirmed to depend on the orientation degree of the surface crystalline phase. An oriented fresnoite (Ba2TiGe2O8) phase was observed in transparent BTG55 crystallized glass which had been heat treated at 720°C for 3h. The orientation degree along the polar c axis in the fresnoite phase increased with increasing removal thickness by polishing from the surface up to several microns. A d coefficient in transparent BTG55 crystallized glass of 24±3pm∕V has been obtained, which is the largest value in crystallized glass ever reported.
Generation of electrical short pulse using Schottky line periodically loaded with electronic switches
Authors: Narahara Koichi , Otsuji Taiichi , Sano Eiichi
Published: August 2006

A method of the generation of an electrical short pulse, which uses the Schottky line periodically loaded with electronic switches as a key device, is proposed. As is well known, the Schottky line, which means a transmission line periodically loaded with Schottky diodes, simulates the Toda lattice. When a pulse with the longer temporal duration than the inverse of the Bragg frequency of the line is inputted, it is split to be several solitons. Moreover, these solitons have in general shorter temporal duration than the input pulse. We consider the case in which an electronic switch (the switch is open for voltages greater than some fixed threshold, and closed otherwise) is put in parallel with each Schottky diode. Once the input pulse crosses the threshold voltage of the shunt switches, this multiple solitons are all attenuated at the voltages below the threshold by the finite conductance. However, it is found that the larger solitons are less attenuated than the smaller ones. Thus, it is possible to obtain only the largest soliton among the multiple ones, when we obtain the output after the appropriate transmission of the pulse on the proposed nonlinear transmission line. In this paper, we describe the principle of the operation of the proposed method and quantify how well the method succeeds in the generation of short pulses through both the perturbative characterization and the numerical integration of the transmission equation of the line.
Photoionization cross section of hydrogenic impurities in cylindrical quantum wires: Finite well model
Authors: Ham Heon , Spector Harold N.
Published: August 2006

We have used a variational wave function for hydrogenic impurities in cylindrical quantum wires to calculate the dependence of the photoionization cross section of such impurities on the photon energy using the finite confining potential well model. The results are presented for various wire radii and for different locations of the impurity in the wire. We compare the results we find here to those found previously using the infinite confining potential well model.
Anisotropic plasma screening effects on bremsstrahlung spectrum in magnetized anisotropic plasmas
Authors: Chang Dong-Man , Jung Young-Dae
Published: August 2006

The anisotropic plasma screening effects on the low-energy electron-ion Coulomb bremsstrahlung process are investigated in magnetized anisotropic plasmas in the presence of an external magnetic field. The impact-parameter analysis is applied to the motion of the projectile electron in order to investigate the variation of the bremsstrahlung cross section as a function of the impact parameter, Debye length, radiation photon energy, and external magnetic field. It is found that the magnetic field effect significantly enhances the bremsstrahlung radiation cross section. The magnetic field effects on the bremsstranlung radiation cross section are found to be increased with increasing the impact parameter. It is also found that the magnetic field effects decrease with increasing the radiation photon energy.
Stable discretization of the Boltzmann equation based on spherical harmonics, box integration, and a maximum entropy dissipation principle
Authors: Jungemann C. , Pham A. T. , Meinerzhagen B. , Ringhofer C. , Bollhöfer M.
Published: August 2006

The Boltzmann equation for transport in semiconductors is projected onto spherical harmonics in such a way that the resultant balance equations for the coefficients of the distribution function times the generalized density of states can be discretized over energy and real spaces by box integration. This ensures exact current continuity for the discrete equations. Spurious oscillations of the distribution function are suppressed by stabilization based on a maximum entropy dissipation principle avoiding the H transformation. The derived formulation can be used on arbitrary grids as long as box integration is possible. The approach works not only with analytical bands but also with full band structures in the case of holes. Results are presented for holes in bulk silicon based on a full band structure and electrons in a Si NPN bipolar junction transistor. The convergence of the spherical harmonics expansion is shown for a device, and it is found that the quasiballistic transport in nanoscale devices requires an expansion of considerably higher order than the usual first one. The stability of the discretization is demonstrated for a range of grid spacings in the real space and bias points which produce huge gradients in the electron density and electric field. It is shown that the resultant large linear system of equations can be solved in a memory efficient way by the numerically robust package ILUPACK.
Self-assembled and etched cones on laser ablated polymer surfaces
Authors: Murthy N. S. , Prabhu R. D. , Martin J. J. , Zhou L. , Headrick R. L.
Published: August 2006

At least two different routes lead to conical structures on laser ablated polymer surfaces. These were investigated by studying laser ablation on the surfaces of different classes of polymers. Cones appeared readily in strongly absorbing polymers such as poly(ethylene terephthalate) (PET) and polyimide (PI), but only within narrow laser parameters in nylon 6, and rarely in poly(chlorotrifluoroethylene), the last two being weak ultraviolet (UV) absorbers. Self-assembled, close-packed cones occurred in PET, in which heat generated due to absorbed laser energy creates a thin, chemically stable, viscoelastic, highly compliant layer (above the glass transition temperature). Surface structure in such polymers evolves from nodules through donuts into ripples and finally to cones as the energy deposited per unit area on the surface (total fluence) is increased using a combination of single pulse fluence and number of pulses. A phase transition from a ripple phase to a cone phase is thought to occur as the thickness of the viscoelastic surface layer increases above a critical value. Cones began to appear from almost the beginning of the irradiation process at random locations in PI, a polymer whose surface irreversibly turns into a hard solid upon exposure to either or both UV and heat. It is proposed that the radiation hardened spots serve as nuclei, a cone “grows” out of this as the material surrounding this nuclei is ablated. The initial sparse occurrence of cones in PI-like polymers, and the increase in their number density with total fluence until the surface is densely packed with cones can be explained by a nucleation and growth model.
Effects of different potential barriers on the structural and optical properties of GaN∕AlxGa1−xN∕GaN coupled multiquantum wells
Authors: Park Y. S. , Park C. M. , Hwang B. R. , Im Hyunsik , Kang T. W. , Kim Chang Soo , Noh Sam Kyu
Published: August 2006

Coupled multiquantum well structures, GaN∕AlxGa1−xN∕GaN bounded by AlxGa1−xN barriers with varying Al content x, are characterized by using high-resolution x-ray diffraction and photoluminescence measurements. It is clearly demonstrated that the structural and optical properties strongly depend on the Al content x. Photoluminescence peaks continue to be redshifted with respect to the bulk GaN band gap with increasing x up to 0.5, but, in turn, become blueshifted beyond this value. This behavior is understood in terms of the different screening effects of the piezoelectric field induced by a strain with increasing Al content. It is found that the strain starts to be relatively relaxed around x=0.5, leading to the reduction in the screening of the piezoelectric field.
High-pressure tailored compression: Controlled thermodynamic paths
Authors: Nguyen Jeffrey H. , Orlikowski Daniel , Streitz Frederick H. , Moriarty John A. , Holmes Neil C.
Published: August 2006

We have recently carried out exploratory dynamic experiments where the samples were subjected to prescribed thermodynamic paths. In typical dynamic compression experiments, the samples are thermodynamically limited to the principal Hugoniot or quasi-isentrope. With recent developments in a functionally graded material impactor, we can prescribe and shape the applied pressure profile with similarly shaped, nonmonotonic impedance profile in the impactor. Previously inaccessible thermodynamic states beyond the quasi-isentropes and Hugoniot can now be reached in dynamic experiments with these impactors. In the light gas gun experiments on copper reported here, we recorded the particle velocities of the Cu–LiF interfaces and have employed hydrodynamic simulations to relate them to the thermodynamic phase diagram. Peak pressures for these experiments are on the order of megabars, and the time scales range from nanoseconds to several microseconds. The strain rates of these quasi-isentropic experiments are approximately 104–106s−1 in samples with thicknesses of up to 5mm. Though developed at a light gas gun facility, such shaped pressure profiles are also feasible using laser ablation or magnetically driven compression techniques and allow for previously unexplored directions to be taken in high pressure physics.
Doubling of conductance steps in Si∕SiO2 quantum point contact
Authors: Boxberg Fredrik , Häyrynen Teppo , Tulkki Jukka
Published: August 2006

We have calculated the effect of the oxidation-induced strain on the ballistic conductance in a Si∕SiO2 quantum point contact. The strain-induced deformation potential was calculated semiempirically using a viscoelastic continuum model. The charge carriers are confined to the corners of the waveguide by both the strain-induced deformation potential and the Si∕SiO2 band edge discontinuity. As a consequence nearly degenerate symmetric and antisymmetric transverse states are formed for the Si [001] minima. This additional degeneracy within the Landauer-Büttiker formalism leads to doubling of conductance steps for electrons in the [001] minima which govern the conductance near the cutoff energy. Due to the additional strain-induced confinement, the effective channel width of the quantum point contact is smaller and therefore the conductance steps are sharper.
Single crystalline aluminum nitride films fabricated by nitriding α-Al2O3
Authors: Fukuyama Hiroyuki , Kusunoki Shin-ya , Hakomori Akira , Hiraga Kenji
Published: August 2006

Single crystalline aluminum nitride (AlN) is a key material for deep-ultraviolet light emitting devices. In the present study, high-quality single crystalline AlN films have been fabricated by nitriding α-Al2O3 with a precise control of driving force of nitridation reaction. This process provides the ⟨0001⟩-axis oriented AlN film as large as 50.8mm in diameter formed on (0001) and (112¯0) planes of α-Al2O3 substrates. The crystalline qualities of the films have the values of full width at half maximum of rocking curves using (0002) and (101¯0) planes of AlN for tilt and twist components as follows; Tilt=165arcsec and twist=540arcsec in the case of nitriding (0001) plane of α-Al2O3 and tilt=127arcsec and twist=237arcsec in the case of nitriding (112¯0) plane of α-Al2O3.
Effect of exciplex formation on organic light emitting diodes based on rare-earth complex
Authors: Wang D. Y. , Li W. L. , Chu B. , Liang C. J. , Hong Z. R. , Li M. T. , Wei H. Z. , Xin Q. , Niu J. H. , Xu J. B.
Published: August 2006

An exciplex can be formed due to the charge transfer between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. By introducing a mixing layer composed of [N,N′-diphenyl-N,N′bis (3-methylphenyl)-1,1′-diphenyl-4,4′-diamine] (TPD) and europium(dibenzoylmethanato)3(bathophenanthroline) [Eu(DBM)3bath] and a graded interface, elimination of light emission from the exciplex and significant luminescence enhancement of trivalent europium ions (Eu3+) in organic light emitting devices have been achieved. The elimination mechanism of exciplex emission based on the concept that an exciplex can be formed between LUMO of the acceptor (Eu complex) and HOMO of donor (TPD) was investigated. To comprehensively understand the mechanism, devices consisting of a Eu(DBM)3bath as the emitting material and the devices using other rare-earth (RE) complex [RE(DBM)3bath] as the emitting material were fabricated with the same device configuration. As a reference, four spin-coated films with the blend composed of TPD and the gadolinium complex [Gd(DBM)3bath] were also fabricated. The electroluminescence (EL) spectra from the devices and photoluminescence spectra from the spin-coating films were fully investigated. The results show that the exciplex was formed by the charge transfer from the donor TPD to the acceptor RE complex, the exciplex state that acted as a transient excited state can be controlled by altering the molecular ratio in the mixing films. The relation of the exciplex formation based on EL devices with the RE complex versus the variety of the RE ions is also discussed by manipulating the energy level of the excited state.
Photochromism and polaronic photocharge localization in diluted KTa1−xNbxO3
Authors: Gubaev A. I. , Kapphan S. E. , Jastrabik L. , Trepakov V. A. , Syrnikov P. P.
Published: August 2006

Ultraviolet (UV)-light-induced optical absorption in the near infrared (NIR) region was observed in diluted KTa1−xNbxO3 single crystals (x=0,0.004,0.007,0.012,0.07) at low temperatures. Illumination by wideband light (3.10–4.13eV, 300–400nm) is accompanied by the appearance of a broad NIR absorption band with the position of the maxima varying in the 0.69–0.8eV (1.54–1.79μ, T=1.3K) region for different Nb concentrations. This UV-light-induced absorption is absent in nominally pure KTaO3, as well as in all Nb diluted specimens at elevated temperatures. The centers responsible for the photochromic NIR absorption bands are tied to interband optical transitions of pair Nb4+ electronic polarons. The photochromic experimental data, supplemented by luminescence studies in the visible range, evidence the strong localization of the photocharge carriers by pair Nb4+ polarons at low temperatures. It is suggested that namely the strong localization of the photocarriers plays a crucial role in photoinduced gigantic dielectric effects and possible phase transitions, which have been recognized recently in incipient ferroelectrics at low temperatures.
Glass-forming ability and fragility parameter of amorphous Fe67Co9.5Nd3Dy0.5B20
Authors: Biswas K. , Venkataraman S. , Zhang W. Y. , Ram S. , Eckert J.
Published: August 2006

The glass-forming ability of amorphous Fe67Co9.5Nd3Dy0.5B20 has been analyzed in terms of the width of the supercooled liquid region, the reduced glass transition temperature, the recently proposed parameter γ, the Hruby parameter [Czech. J. Phys., Sect. B 22, 1187 (1972)] Kgl, and the stability parameter S. The lack of a high glass-forming ability for this alloy cannot be predicted by these parameters. The glass-forming ability is also discussed with respect to the fragility parameter calculated from the dependence of the glass transition on the heating rate described by the Vogel-Fulcher-Tamman relation. The value of the fragility parameter indicates that this alloy is closer to the fragile side than the strong side in the frame of the classical scheme of glass-forming liquids, revealing that the fragility parameter can be reliably used to predict the glass-forming ability for this alloy rather than other well-known empirical parameters.
Comparison of quantum well intermixing in GaAs structures using a low temperature grown epitaxial layer or a SiO2 cap
Authors: Lee A. S. W. , Hulko O. , Thompson D. A. , Robinson B. J. , Simmons J. G.
Published: August 2006

Studies of quantum well intermixing (QWI) have been performed on Al-free GaAs based structures in which InGaAs quantum wells (QWs) have either GaAs barriers or InGaAsP quaternary barriers such that the barrier-QW compositional change consists solely of a group III change (GaAs barrier) or a group V change (quaternary barrier). These structures permit identification of the sublattice upon which intermixing occurs when the point defects responsible for the QWI are created by annealing in the presence of a (conventional) dielectric (SiO2) cap layer versus an InGaP cap layer grown at low temperature (LT-InGaP). QWI occurs on the group III sublattice via vacancy diffusion in both the LT-InGaP and SiO2 capped samples with identical group V compositions in the QW and barrier layers. For the samples with identical group III compositions for the QW and barriers, QWI is only observed with the LT-InGaP capping and occurs via group V interstitial diffusion and P–As exchange in the QW.
Crystallographic and morphological textures in laser surface modified alumina ceramic
Authors: Harimkar Sandip P. , Dahotre Narendra B.
Published: August 2006

Laser surface modification is an advanced technique for improving the surface performance of alumina ceramics in refractory and abrasive machining applications. Surface performance is expected to be greatly influenced by the crystallographic and morphological textures of surface grains generated during rapid solidification associated with laser processing. In this study, an investigation of the evolution of crystallographic and morphological textures during laser surface modifications of alumina ceramic was carried out using a 4kW Nd:YAG laser with fluences in the range of 458–726J∕cm2. In these regimes of laser surface processing, the formation of equilibrium α-alumina was found to be assisted by catalytic sites provided by the substrate. Microstructure evolution was explored in terms of the development of crystallographic and morphological (size and shape) textures of surface grains as a function of laser processing parameters. The interdependence of crystallographic and morphological textures of the surface grains is discussed within the framework of faceted growth model suggesting that the formation of crystal shapes is governed by the relative velocities of certain crystallographic facets. Also, the effect of thermal aspects of laser processing on the morphology of the surface grains is discussed from the viewpoint of existing solidification theories.
Mechanisms of Ag as a surfactant in giant magnetoresistance multilayer growth and thermal stability
Authors: An Yukai , Zhang Hongdi , Dai Bo , Mai Zhenhong , Cai Jianwang , Wu Zhonghua
Published: August 2006

The mechanisms played by Ag as a surfactant in giant magnetoresistance multilayers were investigated using interface sensitive x-ray anomalous scattering techniques. Analysis on [Cu∕Ni70Co30]20 and [Cu∕Ag∕Ni70Co30]20 multilayers revealed that 6Å thick NiCu and 6Å thick CuNi3Co intermixing regions are formed at the Ni70Co30-on-Cu interfaces of undoped and Ag-doped multilayers, respectively. The Cu-on-Ni70Co30 interfaces in both multilayers are sharp. Annealing causes severe diffusion across both types of interfaces in the undoped multilayer. But the interfaces in the Ag-doped multilayer do not change significantly upon annealing, except that Ag atoms diffuse into the whole Ni70Co30 layer and some parts of the Cu layer. The results suggest that addition of Ag during the deposition suppresses interfacial intermixing. X-ray diffuse scattering profiles show that the interfacial lateral correlation length of the Ag-doped multilayer is longer than that of the undoped multilayer and does not change significantly after annealing, suggesting that the addition of Ag gives rise to smoother interfaces and results in a good thermal stability.
In situ spectroscopic ellipsometry study on the growth of ultrathin TiN films by plasma-assisted atomic layer deposition
Authors: Langereis E. , Heil S. B. S. , van de Sanden M. C. M. , Kessels W. M. M.
Published: August 2006

The growth of ultrathin TiN films by plasma-assisted atomic layer deposition (PA-ALD) was studied by in situ spectroscopic ellipsometry (SE). In between the growth cycles consisting of TiCl4 precursor dosing and H2–N2 plasma exposure, ellipsometry data were acquired in the photon energy range of 0.75–5.0eV. The dielectric function of the TiN films was modeled by a Drude-Lorentz oscillator parametrization, and the film thickness and the TiN material properties, such as conduction electron density, electron mean free path, electrical resistivity, and mass density, were determined. Ex situ analysis was used to validate the results obtained by in situ SE. From the in situ spectroscopic ellipsometry data several aspects related to thin film growth by ALD were addressed. A decrease in film resistivity with deposition temperature between 100 and 400°C was attributed to the increase in electron mean free path due to a lower level of impurities incorporated into the films at higher temperatures. A change in resistivity and electron mean free path was observed as a function of film thickness (2–65nm) and was related to an increase in electron-sidewall scattering for decreasing film thickness. The TiN film nucleation was studied on thermal oxide covered c-Si substrates. A difference in nucleation delay was observed on these substrates and was related to the varying surface hydroxyl density. For PA-ALD on H-terminated c-Si substrates, the formation of an interfacial SiNx film was observed, which facilitated the TiN film nucleation.
Formation of extraordinarily uniform periodic structures on metals induced by femtosecond laser pulses
Authors: Wang Jincheng , Guo Chunlei
Published: August 2006

In this paper, we report an observation of extraordinarily uniform periodic structures on metals following p-polarized femtosecond laser pulses at various obliquely incident angles. The highly uniform structures are oriented parallel to the laser polarization. Angular dependence of the period of these structures is studied, and the results indicate that these structures are formed by the interference between the incident light and excited surface plasmons. Under identical experimental conditions, these highly uniform structures show distinctly different levels of morphological clearness on different metals, and this difference can be explained by considering the electron-phonon coupling strength in different metals.
Peculiarities in the electronic band structures of Cr∕Cu multilayered nanostructures and Cr1−xCux metastable alloy films: Ab initio linearized-augmented plane-wave and experimental optical studies
Authors: Kudryavtsev Y. V. , Uvarov V. N. , Oksenenko V. A. , Lee Y. P. , Kim J. B. , Gontarz R.
Published: August 2006

A set of Cr1−xCux alloy films with 0<x<1 and Cr∕Cu multilayered films (MLFs) has been prepared by dc sputtering onto glass substrates kept at 293K. The x-ray diffraction study reveals the formation of continuous solid solutions in bcc structures for the Cr1−xCux alloy films with x<0.3 and in fcc structures for x>0.3. The structural instability of Cr1−xCux alloy films near x≈0.3 is discussed in terms of band structures of Cr–Cu alloys with bcc and fcc types of structure, and related to the dramatic changes of the density of states at the Fermi level. The optical properties of Cr1−xCux alloy films can be classified into two groups, representing the film structures. The fcc-bcc structural transformation results in the appearance of interband absorption at 2eV in the optical-conductivity spectra of alloy films, which originates from Cr. The observed changes in the experimental optical properties of Cr1−xCux alloy films are explained in terms of band structures of Cr–Cu alloys. The comparison between experimental and modeled optical properties of Cr∕Cu MLF enables us to conclude that the interfacial alloy regions of about 1.6–2.2nm in thickness are spontaneously formed during the Cr∕Cu MLF fabrication. In addition, it was understood that the main features of electronic band structures for pure Cr and Cu sublayers of a few nanometers in Cr∕Cu MLF resemble greatly those of the bulk Cr and Cu.