bluuebunny/crossref_metadata_2025 huggingface dataset:
High energy (MeV) ion-irradiated π-conjugated polyaniline: Transition from insulating state to carbonized conducting state
Authors: Park S. K. , Lee S. Y. , Lee C. S. , Kim H. M. , Joo J. , Beag Y. W. , Koh S. K.
Published: August 2004

High energy (MeV)C2+,F2+, and Cl2+ ions were irradiated onto π-conjugated polyaniline emeraldine base (PAN-EB) samples. The energy of an ion beam was controlled to a range of 3–4.5MeV, with the ion dosage varying from 1×1012 to 1×1016ions∕cm2. The highest dc conductivity (σdc) at room temperature was measured to be ∼60S∕cm for 4.5MeV Cl2+ ion-irradiated PAN-EB samples with a dose of 1×1016ions∕cm2. We observed the transition of high energy ion-irradiated PAN-EB samples from insulating state to conducting state as a function of ion dosage based on σdc and its temperature dependence. The characteristic peaks of the Raman spectrum of the PAN-EB samples were reduced, while the D-peak (disordered peak) and the G peak (graphitic peak) appeared as the ion dose increased. From the analysis of the D and G peaks of the Raman spectra of the systems compared to multiwalled carbon nanotubes, ion-irradiated graphites, and annealed carbon films, the number of the clusters of hexagon rings with conducting sp2-bonded carbons increased with ion dosage. We also observed the increase in the size of the nanocrystalline graphitic domain of the systems with increasing ion dosage. The intensity of normalized electron paramagnelic resonance signal also increased in correlation with ion dose. The results of this study demonstrate that π-conjugated pristine PAN-EB systems changed from insulating state to carbonized conducting state through high energy ion irradiation with high ion dosage.
Interaction between the electromagnetic fields and the plasma in a microwave plasma reactor
Authors: Hagelaar G. J. M. , Hassouni K. , Gicquel A.
Published: August 2004

We investigate the interaction between electromagnetic (EM) fields and a hydrogen plasma in a resonance-type microwave plasma reactor, by combining an elementary theoretical analysis and a self-consistent two-dimensional numerical model. We show that the EM fields in the reactor are strongly modified by the presence of plasma; due to absorption the standing wave patterns of the field intensity are less pronounced. In turn, the electric field controls the electron temperature and the plasma formation. For typical conditions (field frequency 2.45GHz, gas pressure 10–200mbar, gas temperature 2000–3000K) it is not just the electric field that controls the plasma, but rather the reduced electric field, i.e., the ratio of electric field strength to gas particle density. We show that the dependence of the reduced field on the gas temperature has a reinforcing effect on plasma formation and may cause jumps in the spatial plasma configuration as a function of power or pressure. We also show that the plasma size is limited by the skin effect.
Surface-optical phonon assisted transitions in quantum dots
Authors: Chen Chen , Dutta Mitra , Stroscio Michael A.
Published: August 2004

The Fröhlich interaction between electrons and surface-optical phonons is studied by using the dielectric continuum model. First, the eigenfrequencies of the surface phonons are calculated. A result for the normalized displacement of the phonon is obtained and a more accurate electron-surface phonon Fröhlich interaction Hamiltonian is calculated. The surface-optical-phonon-assisted scattering rate of an electron in free-standing GaN quantum dots is calculated with natural linewidth broadening. Finally, illustrative examples of surface-optical-phonon-assisted scattering rates are calculated to clarify the potential importance of SO-phonon-assisted transitions in potential applications of quantum dots.
Annealing behavior of the carbon vacancy in electron-irradiated 4H-SiC
Authors: Zolnai Z. , Son N. T. , Hallin C. , Janzén E.
Published: August 2004

Electron paramagnetic resonance (EPR) was used to study the annealing behavior of the positively charged carbon vacancy (EI5 center) in electron-irradiated 4H-SiC. At ∼1000°C the EPR signal of the defect starts decreasing gradually. Clear ligand hyperfine structure is still observed after annealing at 1350°C, while the central line can be detected after a 1600°C anneal. A similar annealing behavior was also observed for the EI6 center suggesting that this defect may be also the positively charged carbon vacancy but at the hexagonal lattice site.
Pinning lattice: Effect of rhenium doping on the microstructural evolution from Tl-2212 to Hg-1212 films during cation exchange
Authors: Zhao H. , Wu J. Z.
Published: August 2004

In a cation exchange process developed recently by some of us, epitaxial HgBa2CaCu2O6 films can be obtained by diffusing volatile Tl cations out of, and simultaneously diffusing Hg cations into, the crystalline lattice of epitaxial Tl2Ba2CaCu2O8 (Tl-2212) precursor films. When a large number of Tl cations diffuse out from the same local area of the precursor lattice simultaneously, it causes lattice collapse locally and leads to formation of pores of micrometer dimension. To eliminate such large-scale lattice collapse, “lattice pins” were introduced on the original Tl-2212 lattice by partially replacing volatile Tl cations with nonvolatile Re ones. Since the Re cations remain on the lattice during the Tl-Hg cation exchange, they pin the lattice around them. HgBa2CaCu2O6 films obtained from these Re-doped Tl-2212 precursor films have much improved microstructures with the pore dimension reduced by an order of magnitude.
Optical properties of nanocrystalline silicon within silica gel monoliths
Authors: Posada Y. , Miguel L. San , Fonseca L. F. , Resto O. , Weisz S. Z. , Kim C.-H. , Shinar J.
Published: August 2004

Described herein is the incorporation of nanocrystalline silicon nc-Si from porous silicon (PSi) in a silica matrix fabricated by the sol-gel technique that yields highly photoluminescent (PL) and optically transparent monoliths with uniformly distributed nc-Si inclusions or nanoclusters. The sample monoliths were prepared with PSi-derived nanoclusters (PSi-n) with average diameters of 14–45nm. Concentrated samples of PSi-n-exhibited blueshifted orange emission bands with maximum peaks between 600 and 750nm with PL emission intensities ten times stronger than those of the original PSi, while diluted samples exhibited UV to blue (350–450nm) emission bands. The PL quantum yield of the typical PSi-n monoliths was 44% higher than the native PSi. Light absorption measurements showed a linear response to laser powers before the saturation threshold at 80mW. PL bleaching following 3h of constant laser power exposure resulted in 90% reduction of the maximum initial PL. Mechanical and thermal stability properties of nc-Si were greatly improved within the silica matrix, demonstrating that PSi-n monoliths’ are more manageable materials that enable the fabrication of samples with high densities of nc-Si for semiconducting and optoelectronic purposes. No special chemical passivation of the nc-Si surfaces was used in the preparation of the PSi-n monoliths. A strong relation between the optical properties of this nanophase material and the size distribution and concentration of nc-Si in the sample is demonstrated.
Inspection of an end quenched 0.15%–0.2% C, 0.6%–0.9% Mn steel jominy bar with photothermal radiometric techniques
Authors: Liu Yue , Baddour Natalie , Mandelis Andreas , Wang Chinhua
Published: August 2004

The effect of the cooling rate on hardness and thermal conductivity in a metallurgical Jominy bar made of 0.15%–0.2% C, 0.6%–0.9% Mn (AISI 1018) steel, by means of a water end-quenched heat treatment process without diffusion-controlled case depth, is studied with photothermal radiometry (PTR). It is concluded that our two PTR techniques, common-mode rejection demodulation and conventional 50% duty-cycle square-wave frequency scan, are sensitive to low hardness values and gradients, unlike the high values all previous photothermal studies have dealt with to-date. Both PTR methods have yielded an anticorrelation between thermal conductivity and microhardness in this case as in previous cases with heat-treated and diffusion-controlled case depth profiles. It is shown that the cooling rate strongly affects both hardness and thermal conductivity in the Jominy-bar heat-treating process.
Multielement magnetic probe using commercial chip inductors
Authors: Romero-Talamás C. A. , Bellan P. M. , Hsu S. C.
Published: August 2004

A 60-element magnetic probe array has been constructed using miniature commercial chip inductors. The array consists of twenty clusters of three coils each mounted on a linear fixture. The coils are oriented in orthogonal directions to yield three-dimensional information. The array has been used to investigate magnetic properties of spheromaks.
New phase and amplitude high resolution pulse shaper
Authors: Monmayrant Antoine , Chatel Béatrice
Published: August 2004

We present the first realization of a femtosecond pulse shaper in phase and amplitude using two liquid crystal devices with 640 pixels, leading to a wide temporal window around 25 ps at 800 nm. Several examples illustrate the high resolution of such device. The use of a folded zero dispersion line is also successfully demonstrated.
Investigation of dielectric anomalies at cryogenic temperatures in (1−x)[Pb(Mg1∕3Nb2∕3)O3]–xPbTiO3 system
Authors: Lente M. H. , Zanin A. L. , Andreeta E. R. M. , Santos I. A. , Garcia D. , Eiras J. A.
Published: August 2004

Complex electrical permittivity measurements in (1−x)[Pb(Mg1∕3Nb2∕3)O3]–xPbTiO3 ceramics for 0.10⩽x⩽0.40 were performed in the frequency and temperature range from 1to100kHz and from 15to600K, respectively. Unexpected dielectric anomalies, which were frequency dependent, were found at cryogenic temperatures. It was verified that this dielectric behavior is always observed, independent of whether the composition presents a “normal” ferroelectric or a relaxor characteristic. The results were analyzed within the framework of two current models found in the literature. It was concluded that none of the proposed mechanisms uniquely satisfies the experimental results in the composition range investigated here.
Diffusion of O2 in C60 crystal by measuring the decay of electrical conductivity
Authors: Yang Chih-Ming , Liao Jian-Liang , Chiu Kuan-Cheng
Published: August 2004

The time dependence of the decay of dark electrical conductivity σ(t) from C60 crystals under air exposure was investigated at different temperatures. The decay of σ(t) can be fitted via a stretched exponential function, indicating that the diffusion of O2 in C60 crystal follows a hierarchically constrained dynamic. The diffusivity of molecular oxygen in C60 crystal is estimated using the least-square fitted time constant, and an activation energy EA=1.5eV corresponding to the diffusion process is deduced from the Arrhenius plot. The mechanism of the diffusion process is then discussed.
Current, charge, and capacitance during scanning probe oxidation of silicon. I. Maximum charge density and lateral diffusion
Authors: Dagata J. A. , Perez-Murano F. , Martin C. , Kuramochi H. , Yokoyama H.
Published: August 2004

A comprehensive analysis of the electrical current passing through the tip-substrate junction during oxidation of silicon by scanning probe microscopy (SPM) is presented. This analysis of experimental results under dc-bias conditions resolves the role of electronic and ionic contributions, especially for the initial stages of the reaction, determines the effective contact area of the tip-substrate junction, and unifies the roles of space charge and meniscus formation. In Part I of this work, we demonstrate that SPM oxidation is governed by a maximum charge density generated by electronic species within the junction at the onset of the oxidation process. Excess charge is channeled into lateral diffusion, keeping the charge density within the reaction zone constant and reducing the aspect ratio of the resulting oxide features. A uniform charge density implies that SPM oxides contain a fixed defect concentration, in accordance with the space-charge model. The effective (electrical) thickness of SPM oxides determined by these defects is investigated by Fowler-Nordheim analysis. We conclude that most of the electrical current involved in high voltage SPM oxidation of Si does not actually induce surface oxide growth, and that lateral diffusion and small aspect ratios are unavoidable aspects of contact-mode conditions.
Current, charge, and capacitance during scanning probe oxidation of silicon. II. Electrostatic and meniscus forces acting on cantilever bending
Authors: Dagata J. A. , Perez-Murano F. , Martin C. , Kuramochi H. , Yokoyama H.
Published: August 2004

A comprehensive analysis of the electrical current passing through the tip-substrate junction during oxidation of silicon by scanning probe microscopy (SPM) is presented. This analysis identifies the electronic and ionic contributions to the total current, especially at the initial stages of the reaction, determines the effective contact area of the tip-substrate junction, and unifies the roles of space charge and meniscus formation. In this work, we concentrate on noncontact SPM oxidation. We analyze simultaneous force-distance and current-distance curves to demonstrate that total current flow during noncontact oxidation is significantly less for noncontact mode than for contact oxidation, although the resulting oxide volume is nearly identical. Ionization of water layers and mobile charge reorganization prior to and following meniscus formation is also shown to alter the tip-substrate capacitance and, therefore, the bending of the SPM cantilever.
Dynamics of femtosecond laser-induced melting and amorphization of indium phosphide
Authors: Bonse J. , Wiggins S. M. , Solis J.
Published: August 2004

Laser-induced melting and resolidification of single-crystalline indium phosphide (InP) upon irradiation with 150fs laser pulses at 800nm has been investigated by means of real-time-reflectivity measurements with subnanosecond time resolution. Melting of the surface is observed to occur very rapidly on a time scale shorter than our experimental resolution while the lifetime of the liquid phase is several tens of nanoseconds. As a result of the subsequent rapid solidification process, a thin layer of amorphous material with a thickness of several tens of nanometers is formed on the surface. The formation of this amorphous layer has been observed for every fluence above the melting and below the ablation threshold. The evolution of the reflectivity has been modeled for several different solidification scenarios and compared to the experimental results. This comparison shows that solidification proceeds interfacially from the solid interface towards the surface. A lower limit for the critical solid-liquid interface velocity for amorphization in this compound semiconductor has been estimated to be in the range of 1 –4m∕s.
Effects of degree of three-dimensional order and Fe impurities on photoluminescence of boron nitride
Authors: Yao B. , Liu Li , Su W. H. , Shen Z. X. , Liu L. , Sun W. X. , Ding J.
Published: August 2004

Amorphous boron nitride (a-BN) was produced by ball milling of hexagonal BN (h-BN). Turbostratic BN (t-BN) and mesographite BN (m-BN) with different degrees of three-dimensional order (DTDO) were subsequently prepared by annealing the a-BN at different temperatures. The photoluminescence (PL) of the BN samples with different structures and DTDOs was investigated in the visible region from 460 to 800nm. The h-, a-, and t-BN samples, which have completely ordered, disordered and two-dimensionally ordered structures, respectively, emitted weak continuums. The m-BN samples with three-dimensionally ordered structures gave discrete PL bands, which we attributed to the presence of NB antisites or a complex formed by an Fe atom and a B vacancy in the BN layer. The intensity and energy of the PL bands changed greatly with DTDO and could be tuned by varying the annealing temperature. The strongest PL bands at 593 and 612nm were observed in one of the m-BN samples produced at an annealing temperature of 1100°C, and their intensity was about 1000 times the intensity of h-BN. Unintentional Fe contamination did not affect the PL nature of the m-BN samples but increased the PL intensities of some bands greatly.
Analysis of fluctuations in semiconductor devices through self-consistent Poisson-Schrödinger computations
Authors: Andrei Petru , Mayergoyz Isaak
Published: August 2004

The effects of random doping and random oxide thickness fluctuations in metal-oxide-semiconductor field-effect transistors are analyzed by using self-consistent Poisson-Schrödinger computations. The Poisson and Schrödinger equations are solved by using the Newton iteration technique in which the Jacobian matrix is computed through first-order perturbation theory in quantum mechanics. A very fast technique based on linearization of the transport equations is presented for the computation of threshold voltage fluctuations. This technique is computationally much more efficient than the traditional Monte Carlo approach and it yields information on the sensitivity of threshold voltage fluctuations to the locations of doping and oxide thickness fluctuations. Hence, it can be used in the design of fluctuation resistant structures of semiconductor devices. Sample simulation results obtained by using this linearization technique are reported and compared with those obtained by using the Monte Carlo technique.
Influences of the spacer layer growth temperature on multilayer InAs∕GaAs quantum dot structures
Authors: Liu H. Y. , Sellers I. R. , Gutiérrez M. , Groom K. M. , Soong W. M. , Hopkinson M. , David J. P. R. , Beanland R. , Badcock T. J. , Mowbray D. J. , Skolnick M. S.
Published: August 2004

The growth temperature of spacer layers (SPLs) is investigated as a means to obtain identical layers for multilayer quantum dot (QD) structures. A 5-layer 1.3-μm InAs∕GaAs QD structure with 50-nm GaAs SPLs served as a model system. It is found that the growth temperature of the GaAs SPLs has pronounced effects on both the structural and optical properties of the InAs QDs. For GaAs SPLs grown at a low temperature of 510°C, dislocations are observed in the second and subsequent layers, a result of significant surface roughness in the underlying spacer layer. However by increasing the growth temperature to 580°C for the final 35nm of the 50-nm GaAs SPLs, a much smoother surface is achieved, allowing the fabrication of essentially identical, defect free QD layers. The suppression of defect formation enhances both the room-temperature photoluminescence efficiency and the performance of 1.3-μm multilayer InAs∕GaAs QD lasers. An extremely low continue-wave room-temperature threshold current density of 39A∕cm2 is achieved for an as-cleaved 5-layer device with emission at 1.306μm and ground state operation up to 100°C.
Investigation of shock-induced reaction behavior of as-blended and ball-milled Ni+Ti powder mixtures using time-resolved stress measurements
Authors: Xu Xiao , Thadhani Naresh N.
Published: August 2004

The shock-compression response of as-blended and ball-milled elemental Ni and Ti powder mixtures is investigated in this study. An 80-mm diameter single-stage gas-gun was employed to perform time-resolved measurements using piezoelectric stress gauges to monitor the stress wave profiles at the front (input) and back (propagated) surfaces of the ∼50% dense Ni+Ti powder mixture compacts. Shock-compression experiments performed on as-blended Ni+Ti powder mixtures in the range of 522m∕sto1046m∕s impact velocities, showed characteristics of powder densification at measured input stress of 1.12GPa, and shock-induced chemical reaction indicated by volume expansion and wave speed increase at measured input stress of 3.2GPa. Ball-milled powder mixtures also showed similar evidence of shock-induced chemical reaction at stresses exceeding 3GPa, with the degree of expansion depending on the energy release associated with the reaction in the powder mixtures ball-milled for various times. The measured high-pressure expanded state was observed to correspond to that calculated using a thermodynamic “ballotechnic” model for shock-induced formation of reaction products in as-blended and ball-milled powder mixtures. The results of instrumented experiments provide clear evidence of shock-induced chemical reactions occurring in Ni+Ti powder mixtures (following densification at stresses exceeding the crush strength) as evidenced by increased shock velocity and generation of an expanded state of products resulting from the associated heat of reaction.
Various magnetic domain structures in a Ni–Mn–Ga martensite exhibiting magnetic shape memory effect
Authors: Ge Y. , Heczko O. , Söderberg O. , Lindroos V. K.
Published: August 2004

Magnetic domain structures of the Ni–Mn–Ga martensite were observed by means of type I and type II magnetic contrast in scanning electron microscope. The different configuration of magnetic domain patterns coupled together with the twin structures were studied in multivariant, two-variant, and single-variant martensite. The martensitic band contains broad stripelike magnetic domains following the easy axis of magnetization, i.e., the crystallographic c axis. These stripe domains are connected by 90° domain walls creating a staircaselike structure in the adjoining bands. It is found that the internal twins, substructures of the martensite twin domains, are distorted into a zig–zag shape in order to accommodate the main band magnetization. Furthermore, the dagger-shaped stripe domains occur only when the internal twins are present. When the sample exhibits the single-variant state, the internal twins disappear totally and the stripe magnetic domains spread over the whole specimen. The configuration observed here for the magnetic microstructure together with the crystallographic microstructure can help in understanding the magnetic shape memory effect.
Soft breakdown enhanced hysteresis effects in ultrathin oxide silicon-on-insulator metal-oxide-semiconductor field effect transistors
Authors: Chen M. C. , Ku S. H. , Chan C. T. , Wang Tahui
Published: August 2004

The impact of oxide soft breakdown location on threshold voltage hysteresis in partially depleted silicon-on-insulator metal-oxide-semiconductor field effect transistors with an ultrathin oxide (1.6nm) is investigated. Two breakdown enhanced hysteresis modes are identified. In a drain-edge breakdown device, excess holes result from band-to-band tunneling flow to the floating body, thus causing threshold voltage variation in drain bias switching. In contrast, in a channel breakdown device, enhanced threshold hysteresis is observed during gate bias switching because of increased valence band electron tunneling. Our findings reveal that soft breakdown enhanced hysteresis effect can be a serious reliability issue in silicon-on-insulator devices with floating body configuration.
Comment on “Infrared spectra and second-harmonic generation in barium strontium titanate and lead zirconate titanate thin films: ‘Polaron’ artifacts” [J. Appl. Phys. 94, 3333 (2003)]
Authors: Niklasson Gunnar A. , Berggren Lars , Azens Andris
Published: August 2004

Scott et al. [Scott et al., J. Appl. Phys. 94, 3333 (2003)] have questioned our measurements of the optical absorption of amorphous tungsten oxide, and have suggested that the polaron absorption might be an artifact due to interference effects. In this comment we point out that we obtained the absorption coefficient from measurements of both the transmittance and the reflectance of thin films deposited on transparent substrates. By this method the “true” absorption is easily obtained.
Response to “Comment on ‘Infrared and second harmonic generation in barium strontium titanate and lead zirconate titanate thin films: ‘Polaron' artifacts’ ” [J. Appl. Phys. 96, 2409 (2004)]
Authors: Scott J. F.
Published: August 2004

Several authors have recently interpreted infrared features in the 0.3–0.6eV region of the spectra of perovskite-family oxides as polarons or as plasmons; our view is that without measurements on different film thicknesses and∕or quantitative fits to linewidths and lineshapes, these features may be optical artifacts.
Effects of oxidation by O2 plasma on formation of Ni∕Au ohmic contact to p-GaN
Authors: Chen Z. Z. , Qin Z. X. , Tong Y. Z. , Hu X. D. , Yu T. J. , Yang Z. J. , Yu L. S. , Zhang G. Y. , Zheng W. L. , Jia Q. J. , Jiang X. M.
Published: August 2004

Oxidation of Ni∕Au (5nm∕10nm) contact to p-GaN layer was performed by O2 plasma in a reactive ion etching system. The structural characteristics of the Ni∕Au p-GaN for different oxidation time were investigated by x-ray diffraction (XRD) measurements, using an intense synchrotron x-ray source. The XRD measurements indicated that the grains of nickel oxide polycrystalline in the contact were grown continually when the oxidation time increased in 10min. However, Au showed amorphouslike and the intensities of Bragg diffraction peaks were hardly changed when oxidation time increased to 10min. The nickel oxide formed by O2 plasma without sequent thermal annealing did not reduce the specific contact resistance (ρc) to p-GaN, but it took an important role in lowering ρc followed by thermal annealing in N2 at 500°C for 10min. Optical transmission spectra confirmed that the nickel was easy to be oxidized and few interdiffusions occurred at the metal interface in O2 plasma ambient. Finally, the mechanism of oxidation on the formation of low ρc ohmic contact was also discussed.
Fiber laser relaxation oscillation noise suppression through the use of self-biased intracavity loss modulator
Authors: Zhu G. , Wang Q. , Dong H. , Dutta N. K.
Published: August 2004

We demonstrate a technique for suppressing the intensity noise of erbium doped fiber lasers. We show that by introducing negative feedback into the system through the use of a self-biased intracavity loss modulator, the relaxation oscillation noise of the fiber laser could be suppressed by as much as 20dB. To explain the observed noise reduction phenomenon, a simple theory based on the linearized rate equation has been developed and is supported by experimental measurements. We also derive a formula for estimating the maximum achievable intensity noise suppression ratio.
Two-dimensional space-time-resolved emission spectroscopy on atmospheric pressure glows in helium with impurities
Authors: Anderson C. , Hur M. , Zhang P. , Mangolini L. , Kortshagen U.
Published: August 2004

The time dependence of the spectral emission of an atmospheric pressure glow discharge in helium with impurities has been studied with a two-dimensional spatial resolution. Emissions from HeI(3s3S→2p3P:λ=706.5nm) and N2(C3Πu→B3Πg:λ=337.1nm) are used to qualitatively map the distributions of high- and low-energy electrons, respectively. The emission from N2+(B2∑u+→X2∑g+:λ=391.4nm) provides qualitative information about the relative spatial distribution of He* metastable atoms within the discharge gap. The results demonstrate a radial spreading of the discharge during the current pulse as well as the formation of a ringlike, radially propagating cathode layer late in the current pulse.
Atomic cracks and (23×2×3)-R30° reconstruction at 6H-SiC(0001) surface
Authors: Amy F. , Soukiassian P. , Brylinski C.
Published: August 2004

We investigate the Si-rich 3×3 to Si-terminated 3×3 phase transition of the 6H-SiC(0001) surface by atom-resolved scanning tunneling microscopy. We find a 23×23-R30° reconstruction, coexisting with few 3×3 domains. While a high-quality 3×3 surface preparation is achieved with a very low defect density (<2% of atomic defects), the (23×2×3)-R30° reconstruction instead exhibits long atomic cracks suggesting important stress relief during the phase transition. A structural model containing 13Si atoms per unit cell distributed in three layers above the Si terminated bulk SiC substrate is proposed.
Paramagnetic defects of silicon nanowires
Authors: Baumer A. , Stutzmann M. , Brandt M. S. , Au F. C.K. , Lee S. T.
Published: August 2004

The paramagnetic defects in and on Si nanowires (SiNWs) obtained by oxide-assisted growth were studied by conventional electron spin resonance spectroscopy. For the as-grown nanowires, three different defects were found: Dangling bonds or Pb-centers with g=2.0065, located at the interface of the crystalline core to the surrounding oxide, E′-centers with g=2.0005 and EX-centers with g=2.00252, located in the oxide. For the EX-centers, the characteristic hyperfine lines separated by 16.4G were detected. The as-grown SiNWs showed a spin density of about 1018cm−3. H termination of the nanowires via hydrofluoric acid decreases the spin density drastically to 3×1016cm−3. The optical absorption spectra of SiNWs determined by photothermal deflection spectroscopy are comparable to those of microcrystalline silicon and show a similar decrease of defect density upon H termination.
Dynamics of inertia dominated binary drop collisions
Authors: Roisman Ilia V.
Published: August 2004

A head-on impact of two equal drops is studied theoretically. The initial drops deformation, the radial expansion of the obtained liquid mass, and the subsequent axial stretching are considered. The deformation of the drop in the radial direction is governed by the motion of a rim bounding an expanding lamella, whereas the axial deformation of the drop is governed by the motion of two globules formed at the ends of a stretching liquid jet. The equations of motion of the rim and of the globules are obtained from the mass and the momentum balance. The theory accounts for the inertial effects, surface tension and the viscous stresses. The theoretical predictions of the drop diameter and its length are compared with experiments. The agreement is rather good in spite of the fact that no adjustable parameters are used.
Infrared dielectric function and phonon modes of Mg-rich cubic MgxZn1−xO(x⩾0.67) thin films on sapphire (0001)
Authors: Bundesmann C. , Schubert M. , Rahm A. , Spemann D. , Hochmuth H. , Lorenz M. , Grundmann M.
Published: August 2004

Infrared dielectric function spectra and phonon modes of single-phase rocksalt-type MgxZn1−xO thin films with 0.67⩽x⩽1 prepared by pulsed-laser deposition on c-plane sapphire substrates were obtained from infrared spectroscopic ellipsometry (360cm−1 to 1500cm−1). A one-mode behavior is found. Phonon mode frequencies, the high-frequency limit of the dielectric function, and phonon mode broadening parameters reflect a considerable and systematic dependence on the Mg content x. X-ray diffraction measurements revealed the single-phase growth and a decreasing lattice constant with increasing x.
Charge transfer from TiO2 into adsorbed benzene diazonium compounds
Authors: Merson A. , Dittrich Th. , Zidon Y. , Rappich J. , Shapira Yoram
Published: August 2004

Electron transfer from sol–gel–prepared TiO2 into adsorbed benzene diazonium compounds has been investigated using cyclic voltammetry, x-ray photoelectron spectroscopy, contact potential difference, and surface photovoltage spectroscopy. The results show that the potential of maximum electron transfer depends strongly on the dipole moment of the benzene compound. Two reactive surface sites at which electron transfer occurs have been identified.
All-electrical switching and control mechanism for actomyosin-powered nanoactuators
Authors: Mihajlović Goran , Brunet Nicolas M. , Trbović Jelena , Xiong Peng , von Molnár Stephan , Chase P. Bryant
Published: August 2004

A fast all-electrical activation and control mechanism for biomolecular motor-powered nanoactuators has been developed. Rapid and reversible on–off control of actomyosin biomolecular motors was experimentally demonstrated using in vitro motility assays. The results show that the motility of the actin filaments can be cycled repeatedly by electrically controlled thermal activation in the temperature range from 10°C to 50°C without functional loss. The fast response of the filaments upon rapid temperature switching suggests that thermal activation provides an effective method for turning actomyosin-powered nanoactuators on and off.
Laser-assisted nanoscale deposition of diamond-like carbon films on tungsten tips
Authors: Shi J. , Lu Y. F. , Cherukuri R. S. , Mendu K. K. , Doerr D. W. , Alexander D. R. , Li L. P. , Chen X. Y.
Published: August 2004

Diamond-like carbon (DLC) films were deposited on tungsten tips under KrF excimer laser irradiation in benzene solution. The deposition process was found to be highly dependent on tip sharpness. Tips with larger curvature radii and smaller aspect ratios could not be coated with DLC films under the same condition as that for sharp tips. Raman spectra showed that more sp3 tetrahedral structures were present in the DLC films on a tip with a smaller curvature radius. Simulation results showed that the tip sharpness dependent local optical enhancement played an important role in the DLC deposition process. An optical field gradient from apex to tip body was also found in the simulation. We suggest that there are two modes in the process of DLC deposition on nanotips under different laser fluences, i.e., local apex DLC deposition under low laser fluences and phase-graded DLC deposition under high laser fluences.
Band offset at the CuGaSe2∕In2S3 heterointerface
Authors: Schulmeyer T. , Klein A. , Kniese R. , Powalla M.
Published: August 2004

We have investigated the electronic properties of the CuGaSe2∕In2S3 heterointerface by photoelectron spectroscopy. In2S3 was evaporated by physical vapor deposition onto contamination free polycrystalline CuGaSe2 surface prepared by the selenium decapping process. A valence band offset ΔEVB=0.78±0.1 has been determined.
Amorphous structures induced in monocrystalline silicon by mechanical loading
Authors: Zarudi I. , Zou J. , McBride W. , Zhang L. C.
Published: August 2004

Different amorphous structures have been induced in monocrystalline silicon by high pressure in indentation and polishing. Through the use of high-resolution transmission electron microscopy and nanodiffraction, it was found that the structures of amorphous silicon formed at slow and fast loading/unloading rates are dissimilar and inherit the nearest-neighbor distance of the crystal in which they are formed. The results are in good agreement with recent theoretical predictions.
Observation of a 0.7eV electron trap in dilute GaAsN layers grown by liquid phase epitaxy
Authors: Dhar S. , Halder N. , Kumar J. , Arora B. M.
Published: August 2004

The growth of GaAsN layers by liquid phase epitaxy, using polycrystalline GaN as the source of nitrogen, is reported. The presence of nitrogen in the grown layer is indicated by a nitrogen-related shoulder in the Fourier transform absorption spectrum and a resultant band-gap reduction of 90meV is measured by optical transmission and photocurrent techniques. Data from photocurrent and photocapacitance measurements show the presence of a 0.7eV electron trap in the material which originates due to nitrogen. Compared with earlier published data on GaAsN, grown by other techniques, the trap is tentatively related to (N–N) defects at As sites.
Resonant response of a field-effect transistor to an alternate current signal
Authors: Kushwaha Manvir S.
Published: August 2004

A theoretical investigation is made of the response of a field-effect transistor (FET) to incoming electromagnetic radiation in the presence of a perpendicular magnetic field within the framework of hydrodynamics. The treatment is valid for a nondegenerate electron gas in which the mean free path for electron–electron scattering λee is much smaller than the device length L and the mean free path due to collisions with impurities and/or phonons λcoll. These requirements, written as λee⪡L⪡λcoll, are fulfilled for magnetic fields weak enough to prevent Landau quantization. It is our general observation that the shorter device lengths, weaker magnetic fields, and lower temperatures (or higher electron mobility) are most favorable to achieve a greater resonant response of the device to an ac signal. Such resonant response makes FET a promising device for new types of sources, detectors, mixers, and multipliers in the GHz and THz frequency range.
Y-branched Bi nanowires with metal–semiconductor junction behavior
Authors: Tian Yongtao , Meng Guowen , Biswas Sujit K. , Ajayan Pulickel M. , Sun Shuhui , Zhang Lide
Published: August 2004

Y-branched Bi nanowires (NWs) embedded in anodic aluminum oxide templates were synthesized by electrochemical deposition. Transmission electron microscope observations revealed that the “stem” and the “branches” of the Y-branched Bi NWs are about 80 and 50nm in diameter, respectively. Selected area electron diffraction studies showed that both the stem and the branches are single crystalline. Current–voltage measurement revealed that the parallel Y-branched Bi NWs have characteristics of conventional metal–semiconductor junctions. Our approach to produce one-dimensional metal–semiconductor junctions using Y-branched NWs consisting of only one kind of semimetal and without any external doping can be exploited to create metal–semiconductor junctions of other semimetals, which may find various applications in nanodevices.
Long-wavelength laser based on self-assembled InAs quantum dots in InAlGaAs on InP (001)
Authors: Kim Jin Soo , Lee Jin Hong , Hong Sung Ui , Han Won Seok , Kwack Ho-Sang , Lee Chul Wook , Oh Dae Kon
Published: August 2004

Seven stacks of self-assembled InAs quantum dots (QDs) separated by 28nm thick InAlGaAs barriers were grown on InP (001) substrate by a solid-source molecular-beam epitaxy and were investigated by cross-sectional transmission electron microscopy and photoluminescence spectroscopy. Gain guided broad-area lasers with a stripe width of 75μm were fabricated by using the seven-stacked InAs QD layers with the InAlGaAs–InAlAs material system on InP (001). The lasing operation from InAs QDs was observed up to 260K and the characteristic temperature of the uncoated QD laser calculated from the temperature dependence of threshold current density was 377K for temperatures up to 200K, and 138K above 200K. The drastic decrease in the characteristic temperature above 200K was mainly related to the thermal behavior of carriers in QDs, and possibly the thermal coupling of the QDs to the wetting layer and the waveguide region.
Spectroscopic ellipsometric analysis of interfaces: Comparison of alloy and effective-medium-approximation approaches to a CdMgTe multilayer system
Authors: Ghong T. H. , Kim T. J. , Kim Y. D. , Aspnes D. E.
Published: August 2004

We discuss the accuracy and detectability of interface layers in the analysis of ellipsometric spectra in the CdxMg1−xTe system. Using parametric-alloy and effective-medium-approximation (EMA) representations to simulate interfaces in a single-quantum-well structure, we show that EMA analysis overestimates thicknesses of alloy interfaces by more than a factor of 3. While detailed results will clearly depend on the nature of the epitaxial materials involved, the results suggest that analyses of interfaces by the EMA should be done with caution.
Experimental demonstration of all-optical 1×2 quantum routing
Authors: Ham B. S.
Published: August 2004

Using a quantum switching technique based on a two-photon coherence swapping, we experimentally demonstrate an all-optical 1×2 quantum routing, where the observed switching time of ∼5μs is two orders of magnitude faster than the carrier lifetime or the population relaxation time. Owing to the nonabsorption resonance and its enhanced nonlinearity, the observed switching efficiency (intensity ratio of switch-out signal to switch-in signal) is as high as 10%. Such an efficient and fast optical routing method has the potential to hyperterahertz all-optical quantum routers, where semiconductor quantum wells or quantum dots are chosen as the optical medium with a spin dephasing time in the range of picoseconds.
Deposition of Ti2AlC and Ti3AlC2 epitaxial films by magnetron sputtering
Authors: Wilhelmsson O. , Palmquist J.-P. , Nyberg T. , Jansson U.
Published: August 2004

Thin films of the Mn+1AXn-phases Ti2AlC and Ti3AlC2 have been deposited by dc magnetron sputtering. In agreement with the Ti–Si–C system, the MAX-phase nucleation is strongly temperature dependent. At 900°C epitaxial films of Ti2AlC and Ti3AlC2 were grown, but at 700°C only a cubic (Ti,Al)C phase was formed. In addition, a perovskite carbide, Ti3AlC was grown at 800°C. A bulk resistivity of 0.51μΩm, 0.44μΩm, and 1.4μΩm was measured for the Ti3AlC2, Ti2AlC, and Ti3AlC films deposited at 900°C, respectively. By nanoindentation the hardness and Young’s module was determined for an epitaxial Ti3AlC2 film to 20GPa and 260GPa, respectively.
Planar THz quasioptics
Authors: Dai Jianming , Coleman S. , Grischkowsky D.
Published: August 2004

Two-dimensional (2D) quasioptical elements are demonstrated as guiding structures in the THz frequency regime. A planar polyethylene lens is incorporated in a parallel metal plate structure and is shown to focus guided THz radiation to a spot comparable to that expected for a three-dimensional (3D) optical element in free space. A 2D double-slit aperture is also demonstrated to diffract the incoming THz beam as expected for 3D. The spatial resolution of the measurement techniques employed is illustrated by the sharpness of the measured diffraction pattern.
Vertically-coupled micro-resonators realized usingthree-dimensional sculpting in silicon
Authors: Koonath Prakash , Indukuri Tejaswi , Jalali Bahram
Published: August 2004

A modified separation by implantation of oxygen process has been developed to sculpt vertically coupled microdisk resonators in silicon. The approach involves the implantation of oxygen ions into a silicon substrate, patterned with thermal oxide, to define waveguides on the bottom silicon layer, and photolithography and reactive ion etching to define the microdisk resonators on the top silicon layer. The top and the bottom silicon layers are separated by the oxide layer that was formed after the oxygen implantation. Fabricated microdisk resonators show resonances with a Q value of 10 300 and a free spectral range of 5.4nm.
Experimental evidence for index modulation by carrier depletionin SiGe∕Si multiple quantum well structures
Authors: Lupu A. , Marris D. , Pascal D. , Cercus J.-L. , Cordat A. , Le Thanh V. , Laval S.
Published: August 2004

Experimental results for the refractive index variation obtained by hole depletion in SiGe∕Si multiple quantum wells inserted in a reverse-biased p-i-n junction are reported. The electronic contribution to the index variation is unambiguously separated from the thermal one. Measured refractive index changes around 4.2×10−5V−1 are in quite good agreement with modeling.
Tapasin enhances MHC class I peptide presentation according to peptide half-life
Authors: Howarth Mark , Williams Anthony , Tolstrup Anne B. , Elliott Tim
Published: July 2004

Understanding how peptides are selected for presentation by MHC class I is crucial to vaccination strategies based on cytotoxic T lymphocyte priming. We have studied this selection of the MHC class I peptide repertoire in terms of the presentation of a series of individual peptides with a wide range of binding to MHC class I. This series was expressed as minigenes, and the presentation of each peptide variant was determined with the same MHC class I peptide-specific antibody. In wild-type cells, the hierarchy of presentation followed peptide half-life. This hierarchy broke down in cells lacking tapasin but not in cells lacking calreticulin or in cells lacking transporter associated with antigen processing-associated ERp57. We demonstrate a key role for tapasin in shaping the MHC class I peptide repertoire, as enhancement of presentation in the presence of tapasin correlated with peptide half-life.
Peroxynitrite inhibition of Coxsackievirus infection by prevention of viral RNA entry
Authors: Padalko Elizaveta , Ohnishi Tomokazu , Matsushita Kenji , Sun Henry , Fox-Talbot Karen , Bao Clare , Baldwin William M. , Lowenstein Charles J.
Published: July 2004

Although peroxynitrite is harmful to the host, the beneficial effects of peroxynitrite are less well understood. We explored the role of peroxynitrite in the host immune response to Coxsackievirus infection. Peroxynitrite inhibits viral replicationin vitro, in part by inhibiting viral RNA entry into the host cell. Nitrotyrosine, a marker for peroxynitrite production, is colocalized with viral antigens in the hearts of infected mice but not control mice. Nitrotyrosine coprecipitates with the viral polypeptide VP1 as well. Guanidinoethyl disulfide, a scavenger of peroxynitrite, blocks peroxynitrite inhibition of viral replicationin vitroand permits an increase in viral replicationin vivo. These data suggest that peroxynitrite is an endogenous effector of the immune response to viruses.
Slow wave sleep in crayfish
Authors: Ramón Fidel , Hernández-Falcón Jesús , Nguyen Bao , Bullock Theodore H.
Published: July 2004

Clear evidence of sleep in invertebrates is still meager. Defined as a distinct state of reduced activity, arousability, attention, and initiative, it is well established in mammals, birds, reptiles, and teleosts. It is commonly defined by additional electroencephalographic criteria that are only well established in mammals and to some extent in birds. Sleep states similar to those in mammals, except for electrical criteria, seem to occur in some invertebrates, based on behavior and some physiological observations. Currently the most compelling evidence for sleep in invertebrates (evidence that meets most standard criteria for sleep) has been obtained in the fruit fly
Drosophila melanogaster
. However, in mammals, sleep is also characterized by a brain state different from that at rest but awake. The electrophysiological slow wave criterion for this state is not seen in
Drosophila
or in honey bees. Here, we show that, in crayfish, a behavioral state with elevated threshold for vibratory stimulation is accompanied by a distinctive form of slow wave electrical activity of the brain, quite different from that during waking rest. Therefore, crayfish can attain a sleep state comparable to that of mammals.
Kinetic profiles of p300 occupancy
in vivo
predict common features of promoter structure and coactivator recruitment
Authors: Smith James L. , Freebern Wendy J. , Collins Irene , De Siervi Adriana , Montano Idalia , Haggerty Cynthia M. , McNutt Markey C. , Butscher Wayne G. , Dzekunova Inna , Petersen David W. , Kawasaki Ernest , Merchant Juanita L. , Gardner Kevin
Published: July 2004

Understanding the language encrypted in the gene regulatory regions of the human genome is a challenging goal for the genomic era. Although customary extrapolations from steady-state mRNA levels have been effective, deciphering these regulatory codes will require additional empirical data sets that more closely reflect the dynamic progression of molecular events responsible for inducible transcription. We describe an approach using chromatin immunoprecipitation to profile the kinetic occupancy of the transcriptional coactivator and histone acetyltransferase p300 at numerous mitogen-induced genes in activated T cells. Comparison of these profiles reveals a class of promoters that share common patterns of inducible expression, p300 recruitment, dependence on selective p300 domains, and sensitivity to histone deacetylase inhibitors. Remarkably, this class also shares an evolutionarily conserved promoter composition and structure that accurately predicts additional human genes with similar functional attributes. This “reverse genomic” approach will have broad application for the genome-wide classification of promoter structure and function.