bluuebunny/crossref_metadata_2025 huggingface dataset:
Degradation of GaN-based quantum well light-emitting diodes
Authors: Zhao L. X. , Thrush E. J. , Humphreys C. J. , Phillips W. A.
Published: January 2008

Electrical and optical properties of AlGaN∕InGaN-based near-ultraviolet (UV) multiquantum well light-emitting diodes have been studied during operation at high junction temperatures. Light output decreased exponentially by 40% in the first 30h after which there was no obvious change. The current-voltage characteristics were almost ideal before degradation, with an ideality factor of approximately 2. During degradation, an additional contribution to the current becomes apparent at voltages of below 2.5V, with a temperature-independent logarithmic slope of the current-voltage characteristic, suggesting an additional transport mechanism by tunneling through defects created during degradation. The evolution of the additional current component corresponded to the two different stages observed in the reduction of the light emission with stress time. Generation of defects with similar effects on forward and reverse bias currents could be correlated with the rapid degradation during the first 30h, but for the second stage, the change in the I-V characteristics did not correlate with the change in light emission. Electroluminescence spectra showed that the emission peaks shifted to slightly lower energies during degradation.
Crystallization kinetics of amorphous equiatomic NiTi thin films: Effect of film thickness
Authors: Wang X. , Rein M. , Vlassak J. J.
Published: January 2008

We have investigated the crystallization of amorphous equiatomic NiTi thin films sandwiched between two protective silicon nitride barrier films using optical, atomic force, and transmission electron microscopies. Crystallite nucleation occurs homogeneously inside the NiTi films because small composition shifts at the interfaces between NiTi and surrounding layers suppress heterogeneous nucleation at these interfaces. The crystallite growth rate is independent of film thickness for films thicker than 600nm. Below 600nm, the growth rate decreases rapidly and has an apparent activation energy that increases with decreasing film thickness. We suggest that diffusion of hydrogen from the film interfaces may be responsible for this unusual behavior.
Control of voice coil motor nanoscanners for an atomic force microscopy system using a loop shaping technique
Authors: Youm Woosub , Jung Jongkyu , Lee SungQ , Park Kyihwan
Published: January 2008

The voice coil motor nanoscanner has the advantages of large working range, easy control, and low cost compared to the conventional lead zirconate titanate driven nanoscanner. However, it has a small damping problem which causes mechanical vibration. The mechanical vibration reduces the accuracy as well as servobandwidth, which deteoriates the atomic force microscopy (AFM) image of the samples. In order to solve the vibration problem, the loop shaping technique [for vertical (z)] and input prefilter [for lateral (xy)] are applied. Experimental results of the proposed techniques are presented for vertical (z) and lateral (xy) scanner. Finally, the AFM images are provided to investigate its effect.
A novel high-throughput scanning microscope for label-free detection of protein and small-molecule chemical microarrays
Authors: Fei Y. Y. , Landry J. P. , Sun Y. S. , Zhu X. D. , Luo J. T. , Wang X. B. , Lam K. S
Published: January 2008

We describe a novel scanning optical microscope based on a polarization-modulated nulling ellipsometry. The new microscope employs a combination of scanning mirror and sample translation and thus enables high-throughput label-free detection of biomolecular microarrays with more than 10 000 protein or small-molecule targets. For illustration, we show the image of a 2760-spot protein microarray on a functionalized glass slide obtained with such a microscope. The new scanning microscope is also capable of determining, in parallel, the real-time binding kinetics of multiple molecular species under aqueous conditions.
Instabilities in the dynamics of neutrally buoyant particles
Authors: Sapsis Themistoklis , Haller George
Published: January 2008

The asymptotic dynamics of finite-size particles is governed by a slow manifold that is globally attracting for sufficiently small Stokes numbers. For neutrally buoyant particles (suspensions), the slow dynamics coincide with that of infinitesimally small particles, therefore the suspension dynamics should synchronize with Lagrangian particle motions. Paradoxically, recent studies observe a scattering of suspension dynamics along Lagrangian particle motions. Here we resolve this paradox by proving that despite its global attractivity, the slow manifold has domains that repel nearby passing trajectories. We derive an explicit analytic expression for these unstable domains; we also obtain a necessary condition for the global attractivity of the slow manifold. We illustrate our results on neutrally buoyant particle motion in a two-dimensional model of vortex shedding behind a cylinder in crossflow and on the three-dimensional steady Arnold–Beltrami–Childress flow.
Blue luminescent silicon nanocrystals prepared by nanosecond laser ablation and stabilized in electronically compatible spin on glasses
Authors: Švrček Vladimir , Sasaki Takeshi , Shimizu Yoshiki , Koshizaki Naoto
Published: January 2008

We investigated the blue luminescent silicon nanocrystal (Si-nc) preparation in liquid spin on glass (SOG) by nanosecond laser ablation of the Si target in SOG. The confinement of laser-generated plasma in liquids significantly enhanced the formation of Si-nc. Furthermore, the silicon-dioxide-based SOG inhibited Si-nc aggregation compared to the case of laser ablation in de-ionized water. The silicon-dioxide-based SOG during the solidification process accelerated Si-ncs surface oxidation and passivation. As a result, visible room temperature photoluminescence (PL) of stabilized Si-ncs in solid SOG was achieved with a maximum located at 2.9eV. This PL spectrum was very similar to that from the Si-nc colloidal solution obtained by laser ablation of the Si target and subsequent aging for six months in de-ionized water. The mathematical description of dynamical Si-nc formation processes within laser plasma confined by liquid SOG was applied to qualitatively describe the obtained results. A Si-nc formation scheme could be described as serial processes of rapid formation and growth of embryotic Si particles, consecutive quick oxidation in SOG, and growth termination by quenching. The quantum confinement size effect and surface∕defect states in the surrounding silicon oxide were responsible for efficient blue PL. The solidification of Si-nc in SOG solution enabled the formation of self-supporting films with well-defined Si-nc concentrations that could be simply varied by laser fluence. The controllability of blue luminescent intensity from a film by laser fluence during laser ablation is of significance for optoelectronic applications.
Electronic properties of the EC-0.6 eV electron trap in n-type GaN
Authors: Pernot Julien , Muret Pierre
Published: January 2008

The electronic properties of the EC-0.6 eV electron trap in n-type GaN are investigated by deep-level transient spectroscopy with the help of an experimental method relying on space-charge depth modulation [D. Pons, J. Appl. Phys. 55, 3644 (1984)]. The free energy and capture cross-section temperature dependence are determined between 250 and 330 K. The capture cross section is found at σn=8×10−16 cm2, corresponding to a neutral center with a degeneracy factor of gn≤4.
Core-shell photonic band gap structures fabricated using laser-assisted chemical vapor deposition
Authors: Wang H. , Lu Y. F.
Published: January 2008

Laser-assisted chemical vapor deposition (LCVD), in combination with three-dimensional (3D) self-assembly of colloidal silica particles, was used to fabricate 3D core-shell photonic band gap (PBG) structures. Self-assembled multilayer silica particles were formed on silicon substrates using the isothermal heating evaporation approach. A continuous-wave CO2 laser (10.6 μm wavelength) was used as the energy source in the LCVD to fabricate a silica-core–silicon-shell PBG structure. This technique is capable of fabricating structures with various PBGs by adjusting the silica particle size and Si-shell thickness using different LCVD parameters. This capability enables us to engineer positions and widths of PBGs by flexibly controlling the particle size and shell thicknesses. In the fabricated PBG structures, face-centered cubic structures consist of silica-core–silicon-shell “effective atoms.” A series of PBG structures with designed PBGs was obtained under different experimental conditions. Incidence-angle-resolved spectroscopic ellipsometry was used to identify specific PBGs. The refractive indices of the effective atoms with different Si-shell thicknesses were calculated using the Bruggeman composite model. The plain-wave expansion method was used to simulate the photonic dispersion diagrams, which supported the experimental results.
Roles of oxidizing species in a nonequilibrium atmospheric-pressure pulsed remote O2/N2 plasma glass cleaning process
Authors: Iwasaki Masahiro , Matsudaira Yuto , Takeda Keigo , Ito Masafumi , Miyamoto Eiji , Yara Takuya , Uehara Tsuyoshi , Hori Masaru
Published: January 2008

Atmospheric pressure plasma treatments have attracted attention for various application processes. The effect of O2 additions below 0.2% to N2 was investigated for the efficiency of removing organic contaminants on a glass surface using nonequilibrium atmospheric-pressure pulsed plasma. A remarkably high efficiency of cleaning was obtained by a plasma treatment with ca. 0.03% O2 additions to N2. The concentration of ozone (O3) and the ground-state oxygen radical [O(P32)] were measured using ultraviolet absorption spectroscopy and vacuum ultraviolet laser absorption spectroscopy, respectively. It was found that the key factors for surface cleaning were the scission of carbon bonds due to ultraviolet irradiation and subsequent oxidation due to O(P3), and that the surface cleaning proceeded in broad areas due to the photodissociation of O3.
Photoluminescence study of ZnCdO alloy
Authors: Mohanta A. , Thareja R. K.
Published: January 2008

The photoluminescence (PL) properties of ZnCdO alloy are investigated at various temperatures. Each PL profile contains four distinct peaks: P1, P2, P3, and P4. Peak P4 is due to free-excitonic recombination. The temperature mapping of the P3 peak position shows an S-shaped shift similar to the InGaN system due to the inhomogeneous distribution of Cd in ZnCdO alloy. The peak P2 is attributed to the phonon replica of the localized excitonic emission (P3). The linewidth of the photoluminescence profiles of the ZnCdO alloy is shown to depend on the concentration of Cd.
Highly efficient and tunable fluorescence of a nanofluorophore in silica/metal dual shells with plasmonic resonance
Authors: Chen Xue-Wen , He Sailing , Choy Wallace C. H.
Published: January 2008

A dual-shell structure consisting of a silica buffer shell and a metallic outer shell is studied and designed for highly efficient and tunable fluorescence. The enhanced Purcell effect due to the plasmonic resonance and the dependence of the resonance on the structural parameters are analyzed and discussed through a classical electromagnetic approach. With the present dual-shell structure a fivefold enhancement of the fluorescence efficiency can be achieved for a nanofluorophore comprised of multiple tetramethylrhodamine isothiocyanate dye molecules, as compared to the nanofluorophore within a single silica shell. Furthermore, the peak wavelength of the fluorescence spectrum can be tuned easily (due to the variation of the surface plasmon resonance) by over 50 nm when the refractive index of the ambient material varies from 1.3 to 1.6.
Coexistence of ferroelectricity and antiferroelectricity in epitaxial PbZrO3 films with different orientations
Authors: Pintilie Lucian , Boldyreva Ksenia , Alexe Marin , Hesse Dietrich
Published: January 2008

The temperature dependence of the ferroelectric hysteresis and capacitance in PbZrO3 epitaxial films with (120)O and (001)O orientations was investigated in the 4.2–400K temperature range. It was found that the films with (120)O orientation show a mixture of ferroelectric and antiferroelectric phases on the entire temperature range up to room temperature, with the ferroelectric phase more stable at low temperatures. Above room temperature the (120)O oriented films seem to behave only as an antiferroelectric material. By contrast, films with (001)O orientation show only ferroelectric behavior up to a temperature of about 60K when the single hysteresis loop splits into a double loop characteristic for antiferroelectrics. Above this temperature the (001)O oriented films show only antiferroelectric behavior up to 400K. The temperature dependence of capacitance and loss tangent clearly shows a maximum at around 16K in the case of the (001)O oriented film. This might be associated with a low temperature ferroelectric-antiferroelectric phase transition. However, this transition is not visible in the (120)O oriented films.
Effect of Tb doping on electric and magnetic behavior of BiFeO3 thin films
Authors: Wang Yao , Nan Ce-Wen
Published: January 2008

The effect of Tb doping on crystal structure and electric and magnetic behavior of BiFeO3 thin films prepared via a solution processing on (111)Pt∕Ti∕SiO2∕Si substrates was studied. The substitution of Bi ions by a small amount of Tb ions could stabilize the valence of Fe3+ in BiFeO3 thin films, and as indicated by transmission electron microscope images and largely reduced leakage current in comparison with pure BiFeO3 thin films, the quality of the films was improved by Tb doping. Electric and magnetic behavior of Tb-doped BiFeO3 films was investigated with employing La-doped BiFeO3 films as comparison. Large enhancement in both remnant and saturation polarization and the polarization switching were observed in Tb-doped BiFeO3 films, while the inhomogeneous spin-modulated magnetic structure of BiFeO3 was not modified through Tb substitution but by La substitution.
Gain, loss, and internal efficiency in interband cascade lasers emitting at λ=3.6–4.1μm
Authors: Bewley W. W. , Lindle J. R. , Canedy C. L. , Kim M. , Kim C. S. , Larrabee D. C. , Vurgaftman I. , Meyer J. R.
Published: January 2008

We employ a cavity-length study to determine the temperature variation of the internal loss and gain per unit current density in a ten-stage interband cascade laser that operated cw up to 269K with an emission wavelength of 4.05μm. The characteristic temperature for the gain per unit current density is 39K, which is slightly lower than T0 of the threshold current and is consistent with dominance by Auger recombination. The internal loss for the 150-μm-wide mesa devices increased from 11cm−1 at 78Kto28cm−1 at 275K.
Implications of changes in the injection mechanisms on the low temperature electroluminescence in InGaN∕GaN light emitting diodes
Authors: Pavesi M. , Manfredi M. , Rossi F. , Meneghini M. , Meneghesso G. , Zanoni E. , Zehnder U.
Published: January 2008

The presence of traps is sometimes favorable, and sometimes detrimental to the electrical transport and optical efficiency in III-nitride quantum heterostructures. This work presents the results of a joint analysis of electrical features and electroluminescence in InGaN∕GaN-based blue light emitting diodes; a detailed and exhaustive reading of the carrier injection mechanisms highlights the central role of trap centers near the active region. Some suggestions will be eventually advanced as to the design of devices with better emission performances.
Focused ion beam processing of organic crystal (TMTSF)2PF6. A combined conducting probe atomic force microscopy and secondary ion mass spectrometry study
Authors: Wang K. , Schneegans O. , Moradpour A. , Jomard F.
Published: January 2008

We study the behavior of the organic material (TMTSF)2PF6 (TMTSF: tetramethyl tetraselena fulvalene) under focused Ga+ ion beam impact using conducting probe atomic force microscopy (CP-AFM) and secondary ion mass spectrometry. The (TMTSF)2PF6 crystals are exposed to a 25 keV focused ion beam with the sample native surface both perpendicular and slightly tilted to the incident beam axis, where drastically different etching effects are observed. Moreover, very large ion implantation depth, as compared to conventional substrates, is obtained when the sample surface is set perpendicular to the beam axis, while the implantation depth is much smaller at the tilted angle. This phenomenon is discussed in relation with the sample structure anisotropy. Finally, the transport property modifications of the impact areas are investigated by CP-AFM and discussed in relation with both the crystalline structure and the polarizability of the matrix.
Structure and optical properties of pulsed sputter deposited CrxOy∕Cr∕Cr2O3 solar selective coatings
Authors: Barshilia Harish C. , Selvakumar N. , Rajam K. S. , Biswas A.
Published: January 2008

Spectrally selective CrxOy∕Cr∕Cr2O3 multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O2 and Ar plasmas to deposit a CrxOy (bottomlayer)∕Cr∕Cr2O3 (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899–0.912) and low emittance (0.05–0.06). The x-ray diffraction data in thin film mode showed that the CrxOy∕Cr∕Cr2O3 coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A1g and Eg modes, characteristic of Cr2O3. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr3+ and no phases of CrO2 and CrO3 were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr2O3 layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the CrxOy∕Cr∕Cr2O3 coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity (α∕ε) of 0.895∕0.06 even after heat treatment in air up to 300°C for 2h. At higher temperatures, the solar selectivity decreased significantly (e.g., α∕ε=0.855∕0.24 at 350°C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr3+ vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500°C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325°C) and vacuum (up to 600°C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250h at 250°C with a solar selectivity of 0.898∕0.11.
Elasticity, band-gap bowing, and polarization of AlxGa1−xN alloys
Authors: Duan Yifeng , Li Jingbo , Li Shu-Shen , Xia Jian-Bai
Published: January 2008

Elastic constants, the bulk modulus, Young’s modulus, band-gap bowing coefficients, spontaneous and piezoelectric polarizations, and piezoelectric coefficients of hexagonal AlxGa1−xN ternary alloys are calculated using first-principles methods. The fully relaxed structures and the structures subjected to homogeneous biaxial and uniaxial tension are investigated. We show that the biaxial tension in the plane perpendicular to the c axis and the uniaxial tension along the c axis all reduce the bulk modulus, whereas they reduce and enhance Young’s modulus, respectively. We find that the biaxial and uniaxial tension can enhance the bowing coefficients. We also find that the biaxial tension can enhance the total polarization, while the uniaxial tension will suppress the total polarization.
Hydrogen effusion from tritiated amorphous silicon
Authors: Kherani N. P. , Liu B. , Virk K. , Kosteski T. , Gaspari F. , Shmayda W. T. , Zukotynski S. , Chen K. P.
Published: January 2008

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300°C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04eV. The second process, slow tritium outgassing with a time constant of 200–300h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60years.
The use of combination of nonlinear optical materials to control terahertz pulse generation and detection
Authors: Nazarov M. M. , Makarova S. A. , Shkurinov A. P. , Okhotnikov O. G.
Published: January 2008

We suggest composite nonlinear optical media based on the pair of ZnTe and GaP crystals for terahertz pulse detection or generation. The influence of laser wavelength, pulse duration, and media thickness on the terahertz pulse conversion are studied. The optimization of the composite structure performed numerically allowed for essential improvement of the phase-matching condition observed in the experiment.
Long-term stabilized two-beam combination laser amplifier with stimulated Brillouin scattering mirrors
Authors: Kong Hong Jin , Yoon Jin Woo , Shin Jae Sung , Beak Du Hyun
Published: January 2008

The beam combination method is the promising technique for constructing a very high energy laser with a high repetition rate over 10Hz such as a real fusion driver. In our previous works, the phase control technique essential for realizing this system was proposed and demonstrated experimentally. However, these previous works were done without amplifiers. In this work, we employed amplifiers to test the real beam combination system and obtained a well stabilized phase controlling with λ∕51 fluctuation by standard deviation during 5000 laser shots (500s) at 204mJ total output energy.
Electronic structure of GeO2-passivated interfaces of (100)Ge with Al2O3 and HfO2
Authors: Afanas’ev V. V. , Stesmans A. , Delabie A. , Bellenger F. , Houssa M. , Meuris M.
Published: January 2008

Analysis of internal photoemission and photoconductivity in Ge/thermal germanium oxide/high-dielectric constant oxide (HfO2,Al2O3) structures reveals that the bandgap of the germanium oxide interlayer is significantly lower (4.3±0.2eV) than that of stiochiometric GeO2 (5.4–5.9eV). As a result, the conduction and valence band offsets at the interface appear to be insufficient to block electron and hole injection leading to significant charge trapping in the GeOx∕high-κ oxide stack. Formation of a hydroxyl-rich Ge oxide phase is suggested to be responsible for the modification of the oxide properties.
Effect of magnetization direction on irreversible magnet demagnetization in brushless dc motor
Authors: Kim Tae Heoung
Published: January 2008

The magnetization direction of a permanent magnet is an important variable in the design process of a brushless dc motor. This paper describes the effect of the magnetization direction on the various characteristics. Especially, we concentrate on the irreversible demagnetization characteristic of a permanent magnet. Two kinds of magnetization direction (radial and parallel direction) and finite element method are used to analyze the effect. From the simulation and experimental results, we suggest that the radial magnetization direction has more advantage in terms of the irreversible magnet demagnetization.
Magnetic and electric properties and phase diagram of single crystalline TmAlB4
Authors: Sung N. H. , Kim J. Y. , Han S. H. , Cho B. K.
Published: January 2008

The magnetic and electric properties of TmAlB4 single crystals were investigated. It was found that temperature-dependent magnetization showed a large anisotropy with the c axis being easy magnetic direction. The anisotropy is likely due to crystalline electric field effect. Isothermal magnetization at T=2K represented six metamagnetic transitions at HC1=9.6kOe, HC2=12kOe, HC3=17.1kOe, HC4=21.4kOe, HC5=24.1kOe, and HC6=26kOe. Field-dependent resistivity at T=2K with the applied field along the c axis and current perpendicular to the c axis exhibited a noticeable sharp change at the fields of metamagnetic transitions, confirming the six transitions. Based on the isothermal magnetization and field-dependent resistivity at several temperatures, a field-temperature phase diagram of TmAlB4 was constructed, which contains at least seven magnetic phases.
Electronic trap characterization of the Sc2O3∕La2O3 high-κ gate stack by scanning tunneling microscopy
Authors: Ong Y. C. , Ang D. S. , Pey K. L. , Wang Z. R. , O’Shea S. J. , Tung C. H. , Kawanago T. , Kakushima K. , Iwai H.
Published: January 2008

The tunneling current versus voltage characteristic of the Sc2O3∕La2O3∕SiOx high-κ gate stack is examined using scanning tunneling microscopy in ultrahigh vacuum. Different measurement bias polarities allow information on the location (i.e., in the high-κ or interfacial SiOx layer) of the electronic traps to be extracted. Two types of localized leakage sites may be distinguished. Lowering of the electron barrier height and trap-assisted tunneling are proposed as the two leakage mechanisms.
Half-metallic chromium dioxide (CrO2) nanostructures and field-dependent magnetic domain evolution
Authors: Zou Xiaojing , Xiao Gang
Published: January 2008

Both polycrystalline and epitaxial chromium dioxide structures in nanoscale have been fabricated using selective-area growth technique. Multiple grains were formed in the polycrystalline CrO2 dots larger than 100nm, however, below which only a single grain was allowed to exist. The lateral growth rate of epitaxial CrO2 nanocrystals was found to be highly anisotropic, which is affected by both the orientation and the deposited thickness. Field-dependent magnetic force microscopy was used to study the domain structures of single crystal CrO2 nanowires. The magnetization states were found to be closely related to the wire width. A stripelike domain structure with alternating magnetization parallel to the magnetic easy axis was observed within the CrO2 nanowires aligned along the [010] directions, indicating the existence of a strong uniaxial magnetocrystalline anisotropy.
Superfluid density of superconductor-ferromagnet bilayers
Authors: Lemberger Thomas R. , Hetel Iulian , Hauser Adam J. , Yang F. Y.
Published: January 2008

We report the first measurements of the effective superfluid density nS(T)∝λ−2(T) of superconductor-ferromagnet (SC/FM) bilayers, where λ is the effective magnetic field penetration depth. Thin Nb∕Ni bilayers were sputtered in ultrahigh vacuum in quick succession onto oxidized Si substrates. Nb layers are 102Å thick for all samples, while Ni thicknesses vary from 0to100Å. TC determined from λ−2(T) decreases rapidly as Ni thickness dNi increases from zero to 15Å, then it has a shallow minimum at dNi≈25Å. λ−2(0) behaves similarly, but has a minimum several times deeper. In fact, λ−2(0) continues to increase with increasing Ni thickness long after TC has stopped changing. We argue that this indicates a substantial superfluid density inside the ferromagnetic Ni films.
Measuring the circular motion of small objects using laser stroboscopic images
Authors: Wang Hairong , Fu Y. , Du R.
Published: January 2008

Measuring the circular motion of a small object, including its displacement, speed, and acceleration, is a challenging task. This paper presents a new method for measuring repetitive and/or nonrepetitive, constant speed and/or variable speed circular motion using laser stroboscopic images. Under stroboscopic illumination, each image taken by an ordinary camera records multioutlines of an object in motion; hence, processing the stroboscopic image will be able to extract the motion information. We built an experiment apparatus consisting of a laser as the light source, a stereomicroscope to magnify the image, and a normal complementary metal oxide semiconductor camera to record the image. As the object is in motion, the stroboscopic illumination generates a speckle pattern on the object that can be recorded by the camera and analyzed by a computer. Experimental results indicate that the stroboscopic imaging is stable under various conditions. Moreover, the characteristics of the motion, including the displacement, the velocity, and the acceleration can be calculated based on the width of speckle marks, the illumination intensity, the duty cycle, and the sampling frequency. Compared with the popular high-speed camera method, the presented method may achieve the same measuring accuracy, but with much reduced cost and complexity.
Highly efficient tip-enhanced Raman spectroscopy and microscopy of strained silicon
Authors: Tarun Alvarado , Hayazawa Norihiko , Motohashi Masashi , Kawata Satoshi
Published: January 2008

We present a versatile tip-enhanced Raman spectroscopy (TERS) system that permits efficient illumination and detection of optical properties in the visible range to obtain high signal-to-noise Raman signals from surfaces and interfaces of materials using an edge filter. The cutoff wavelength of the edge filter is tuned by changing the angle of incident beam to deliver high incident power and effectively collect scattered near-field signals for nanoscopic investigation in depolarized TERS configuration. The dynamic design of the instrument provides a unique combination of features that allows us to perform reflection or transmission mode TERS to cover both opaque and transparent samples. A detailed description of improvements of TERS was carried out on a thin strained silicon surface layer. The utilization of an edge filter for shorter collection time, specialized tip for higher field enhancement and for elimination of Raman signal from the tip, shorter wavelength, sample orientation relative to probing polarization, and depolarized configuration for higher contrast Raman signal is discussed.
Monte Carlo analysis of noise spectra in self-switching nanodiodes
Authors: Iñiguez–de-la-Torre I. , Mateos J. , Pardo D. , González T.
Published: January 2008

By means of a semiclassical two-dimensional Monte Carlo technique, we analyze current noise spectra of InAlAs∕InGaAs-based submicron self-switching diodes. Shot noise (at low bias) and diffusion noise in the series resistance (at high bias) are found to dominate the current noise at low frequency (in the plateau beyond the 1∕f range). Two peaks of different origins, which may limit the device performance, are found in the noise spectra at higher frequencies. The dependence of the amplitude and frequency of these peaks on the topology of the diodes is analyzed and discussed. Design indications to improve the noise performance of the devices are provided.
Dependence of electron peak current on hollow cathode dimensions and seed electron energy in a pseudospark discharge
Authors: Cetiner S. O. , Stoltz P. , Messmer P. , Cambier J.-L.
Published: January 2008

The prebreakdown and breakdown phases of a pseudospark discharge are investigated using the two-dimensional kinetic plasma simulation code OOPIC™ PRO. Trends in the peak electron current at the anode are presented as function of the hollow cathode dimensions and mean seed injection velocities at the cavity back wall. The plasma generation process by ionizing collisions is examined, showing the effect on supplying the electrons that determine the density of the beam. The mean seed velocities used here are varied between the velocity corresponding to the energy of peak ionization cross section, 15 times this value and no mean velocity (i.e., electrons injected with a temperature of 2.5eV). The reliance of the discharge characteristics on the penetrating electric field is shown to decrease as the mean seed injection velocity increases because of its ability to generate a surplus plasma independent of the virtual anode. As a result, the peak current increases with the hollow cathode dimensions for the largest average injection velocity, while for the smallest value it increases with the area of penetration of the electric field in the hollow cathode interior. Additionally, for a given geometry an increase in the peak current with the surplus plasma generated is observed. For the largest seed injection velocity used a dependence of the magnitude of the peak current on the ratio of the hole thickness and hollow cathode depth to the hole height is demonstrated. This means similar trends of the peak current are generated when the geometry is resized. Although the present study uses argon only, the variation in the discharge dependencies with the seed injection energy relative to the ionization threshold is expected to apply independently of the gas type. Secondary electrons due to electron and ion impact are shown to be important only for the largest impact areas and discharge development times of the study.
Electron plasma wave propagation in external-electrode fluorescent lamps
Authors: Cho Guangsup , Kim Jung-Hyun , Jeong Jong-Mun , Hong Byoung-Hee , Koo Je-Huan , Choi Eun-Ha , Verboncoeur John P. , Uhm Han Sup
Published: January 2008

The optical propagation observed along the positive column of external electrode fluorescent lamps is shown to be an electron plasma wave propagating with the electron thermal speed of (kTe∕m)1∕2. When the luminance of the lamp is 10000–20000cd∕m2, the electron plasma temperature and the plasma density in the positive column are determined to be kTe∼1.26–2.12eV and no∼(1.28–1.69)×1017m−3, respectively.
Microstructure and dislocation of epitaxial InN films revealed by high resolution x-ray diffraction
Authors: Liu B. , Zhang R. , Xie Z. L. , Lu H. , Liu Q. J. , Zhang Z. , Li Y. , Xiu X. Q. , Chen P. , Han P. , Gu S. L. , Shi Y. , Zheng Y. D. , Schaff W. J.
Published: January 2008

This article reports on the study of microstructure and dislocation of InN films using high resolution x-ray diffraction grown on sapphire (0001) both by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). The mosaic tilt, twist, and correlation lengths of InN films are determined by using symmetrical and asymmetrical reflections as well as reciprocal space mapping. Deducing from these results, MBE-grown InN film exhibits the edge-type dislocations of 4.0×109 cm−2, which is about ten times higher than the density of screw-type dislocations. In MOCVD-grown InN sample, the edge-type dislocations density is as high as 2.1×1010 cm−2, and the screw-type dislocations density is 1.3×109 cm−2. They indicate that edge type is the predominant dislocation type in the InN films. By comparing the reported transmission electron microscopy results, the accuracy of evaluation for the dislocation density using the mosaic model is proved.
Active waveguide in Nd3+:MgO:LiNbO3 crystal produced by low-dose carbon ion implantation
Authors: Chen Feng , Tan Yang , Jaque Daniel , Wang Lei , Wang Xue-Lin , Wang Ke-Ming
Published: January 2008

We report on Nd3+:MgO:LiNbO3 active planar waveguides produced by 3MeV carbon ion implantation at dose of 7.5×1014cm−2. The extraordinary refractive index of the sample surface experiences positive alternations after the implantation and subsequent thermal annealing treatment, constructing enhanced-well confined waveguide structures. The propagation loss of the waveguide is 1–2dB∕cm, which means acceptable quality for further guide-wave applications. The microluminescence spectra of the waveguide show fairly good potentials for laser action at 929 and 1085nm.
Enhanced exchange bias effects in a nanopatterned system consisting of two perpendicularly coupled ferromagnets
Authors: Bollero A. , Dieny B. , Sort J. , Buchanan K. S. , Landis S. , Nogués J.
Published: January 2008

The effect of patterning on the shift of the hysteresis loop HE and coercivity HC in a system composed of two perpendicularly exchange-coupled ferromagnets (NiFe sputtered onto a [Pt∕Co] multilayer) is investigated in long stripes and square dots. Setting the exchange bias coupling along the stripes results in a threefold increase of HE compared to the continuous films. HC increases dramatically when the coupling is set perpendicular to the stripes and also in the dots. Magnetic force microscopy studies and micromagnetic simulations suggest that differences in the number and orientation of the magnetic domains can account for the observed effects.
Concept of a nanowire array magnetoresistance device
Authors: Shvets I. V. , Wu H. C. , Usov V. , Cuccureddu F. , Arora S. K. , Murphy S.
Published: January 2008

We describe a concept of the magnetoresistive medium comprising of a planar array of intermingled nanowires with pinned and unpinned magnetic moments. We propose a bottom up method of forming the medium and demonstrate the feasibility of the proposed approach. We present the results of the simulations of the magnetic moments in the nanowires under an external magnetic field to illustrate the concept.
Band gaps of lower-order Lamb wave in thin plate with one-dimensional phononic crystal layer: Effect of substrate
Authors: Gao Jian , Zou Xin-Ye , Cheng Jian-Chun , Li Baowen
Published: January 2008

We study the substrate effect on the band gaps of lower-order Lamb waves propagating in thin plate with one-dimensional phononic crystal layer coated on uniform substrate. The transmitted power spectra are calculated by using the finite element method (FEM). The results show that when the substrate is hard, the influences on band gap are significant and the band gaps disappear rapidly as the substrate becomes thicker. However, when the substrate is soft, the depth of band gaps becomes larger as the thickness of the substrate increases. A virtual plane wave expansion method is developed to calculate the dispersion curves of Lamb wave. The locations and widths of band gaps on the dispersion curves are in good agreement with the results from the transmitted power spectra by FEM.
Growth and physical property of epitaxial Co70Fe30 thin film on Si substrate via TiN buffer
Authors: Ji C.-X. , Lu Feng , Chang Y. Austin , Yang J. Joshua , Rzchowski M. S.
Published: January 2008

Epitaxial Co70Fe30 films with the bcc structure were grown on a Si(001) substrate with TiN as a buffer by sputtering technique. The x-ray diffraction results confirmed the epitaxial nature of the films and the crystallographic relationship was determined as Co70Fe30(002)⟨110⟩∕∕TiN(002)⟨100⟩∕∕Si(004)⟨100⟩. The surface morphology characterized by atomic force microscopy on our films revealed that smooth surfaces could be obtained at growth temperatures below 350°C. The strain state of 60nm epitaxial Co70Fe30 films was studied as a function of growth temperature. Magnetization hysteresis loops of the films grown at 300°C were measured using superconducting quantum interface device magnetometer.
Realization of spin-wave logic gates
Authors: Schneider T. , Serga A. A. , Leven B. , Hillebrands B. , Stamps R. L. , Kostylev M. P.
Published: January 2008

We demonstrate the functionality of spin-wave logic exclusive-not-OR and not-AND gates based on a Mach-Zehnder-type interferometer which has arms implemented as sections of ferrite film spin-wave waveguides. Logical input signals are applied to the gates by varying either the phase or the amplitude of the spin waves in the interferometer arms. This phase or amplitude variation is produced by Oersted fields of dc current pulses through conductors placed on the surface of the magnetic films.
Origins of the damping in perpendicular media: Three component ferromagnetic resonance linewidth in Co–Cr–Pt alloy films
Authors: Mo Nan , Hohlfeld Julius , ul Islam Misbah , Brown C. Scott , Girt Erol , Krivosik Pavol , Tong Wei , Rebei Adnan , Patton Carl E.
Published: January 2008

The 9.41GHz ferromagnetic resonance field and linewidth have been measured as a function of the angle (θH) between the external magnetic field and film normal for a series of 17.5nm thick Co–Cr–Pt alloy films. The linewidths ranged from hundreds of Oersted to kiloersted, with different values at θH=0 and θH=90° and additional minima and maxima for θH-values from 16° to 64°. The profiles can be fitted with a combination of inhomogeneity line broadening, grain boundary two magnon scattering, and magnon-electron (m-e) scattering processes, with a notably small Gilbert damping α-value of 0.004 for the m-e term.
Two-dimensional spin diffusion in multiterminal lateral spin valves
Authors: Saha D. , Basu D. , Holub M. , Bhattacharya P.
Published: January 2008

The effects of two-dimensional spin diffusion on spin extraction in lateral semiconductor spin valves have been investigated experimentally and theoretically. A ferromagnetic collector terminal of variable size is placed between the ferromagnetic electron spin injector and detector of a conventional lateral spin valve for spin extraction. It is observed that transverse spin diffusion beneath the collector terminal plays an important role along with the conventional longitudinal spin diffusion in describing the overall transport of spin carriers. Two-dimensional spin diffusion reduces the perturbation of the channel electrochemical potentials and improves spin extraction.
Single-walled carbon nanotubes for high-energy optical pulse formation
Authors: Song Yong-Won , Yamashita Shinji , Maruyama Shigeo
Published: January 2008

We passively generate picosecond 6.5nJ optical pulses directly from a single-stage oscillator using carbon nanotube mode locker. The mode locker immunized to the high optical power induced damage employs the evanescent field interaction of propagating light with the nanotubes. The mode locker endures the intracavity power of up to 27.7dBm. For the enhanced interaction, vertically aligned nanotubes are synthesized and applied. The output pulses are monitored for 200h to find that there is no significant degradation of average power and spectral width.
Shaping tight-focusing patterns of linearly polarized beams through elliptic apertures
Authors: Ando Taro , Ohtake Yoshiyuki , Inoue Takashi , Itoh Haruyasu , Matsumoto Naoya , Fukuchi Norihiro
Published: January 2008

We propose elliptic deformation of apertures for shaping focusing patterns of tightly focused linearly polarized beams. Numerical integration of the vectorial Rayleigh-Sommerfeld diffraction formula predicts the formation of symmetric focusing patterns by shrinking the aperture shape in the direction perpendicular to the polarization of the beams. The elliptic deformation is also applied to a focused linearly polarized beam through an annular aperture to demonstrate the formation of smaller symmetric focal spot than that through a simple oval aperture.
Controlled thermal emission of polarized infrared waves from arrayed plasmon nanocavities
Authors: Ikeda K. , Miyazaki H. T. , Kasaya T. , Yamamoto K. , Inoue Y. , Fujimura K. , Kanakugi T. , Okada M. , Hatade K. , Kitagawa S.
Published: January 2008

We have demonstrated thermal emission of linearly polarized and narrow-band midinfrared waves from subwavelength gratings of narrow and deep rectangular cavities engraved on a Au surface. 100-nm-wide and 1000-nm-deep, high-aspect trenches were accurately manufactured by inversion from master molds. Organ pipe resonance of surface plasmons in the cavities exhibits a Lorentzian emission peak centered at 2.5–5.5μm. The maximum emittance reaches 0.90 and the peak width Δλ∕λ is as narrow as 0.13–0.23. This simple emitter is expected to play a key role in the infrared sensing technologies for analyzing our environment.
Room-temperature intracavity difference-frequency generation in butt-joint diode lasers
Authors: Zvonkov B. N. , Biryukov A. A. , Ershov A. V. , Nekorkin S. M. , Aleshkin V. Ya. , Gavrilenko V. I. , Dubinov A. A. , Maremyanin K. V. , Morozov S. V. , Belyanin A. A. , Kocharovsky V. V. , Kocharovsky Vl. V.
Published: January 2008

We observe room-temperature intracavity difference-frequency generation in the mid-infrared range in a butt-joint GaAs∕InGaAs∕InGaP quantum-well laser diode which supports lasing at two closely spaced wavelengths in the near-infrared range around 1μm. We employ a special asymmetric waveguide design and a low-doped substrate that minimize midinfrared losses and phase mismatch for the difference-frequency generation process.
Thermal annealing effects on intersubband transitions in (CdS∕ZnSe)∕BeTe quantum wells
Authors: Li B. S. , Akimoto R. , Shen A.
Published: January 2008

The authors report the study of thermal annealing effects on intersubband transition (ISB-T) properties of (CdS∕ZnSe)∕BeTe quantum wells (QWs). With the increase of annealing temperature, the ISB absorption wavelength shifts to lower energy and absorption intensity gradually decreases. The dependence of linewidths on the annealing temperature is more complicated and shows opposite trends for the QWs with different well thicknesses. Photoinduced ISB-T measurements indicate that the decrease of ISB absorption intensity results from the loss of free carriers in the well layers. The change of structural properties obtained from x-ray diffraction measurements were used to explain the observed change of ISB absorption characteristics.
Luminescence enhancement by Si ring resonator structures on silicon on insulator
Authors: Lin Shiyun , Kobayashi Yosuke , Ishikawa Yasuhiko , Wada Kazumi
Published: January 2008

Si ring resonators on silicon on insulator show at least 50 times stronger photoluminescence (PL) with sharp peaks than the surrounding Si slab region at room temperature. The frequencies of the PL peaks are well explained by the resonant frequencies of the rings and the intensities are by the ratio of the quality factors of the PL peaks and the modal volumes of resonances. This suggests that the Purcell effect should be responsible for the enhancement.
Fatigue mechanism of the ferroelectric perovskite thin films
Authors: Yang Feng , Tang M. H. , Zhou Y. C. , Liu Fen , Ma Y. , Zheng X. J. , Tang J. X. , Xu H. Y. , Zhao W. F. , Sun Z. H. , He J.
Published: January 2008

Incorporating the vacancy electromigration theory into the switching-induced charge-injection mechanism into the local phase decomposition model has led to an analytical model for the dielectric fatigue behavior and the remnant polarization in perovskite structured ferroelectric thin films. The model has allowed us to reproduce the fatigue behavior in various ferroelectric thin films measured under different voltages, temperatures, and frequencies. We concluded the essential reason for electrical fatigue in ferroelectrics is the local phase separation induced directly or indirectly by other fatigue mechanisms proposed in previous papers.