bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-frequency planar Doppler velocimetry (2ν-PDV)
Authors: Charrett Tom O. H. , Ford Helen D. , Nobes David S. , Tatam Ralph P.
Published: October 2004

A planar Doppler velocimetry (PDV) system has been designed which is able to generate two beams from a single source separated in frequency by 690 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser and a set of acousto-optic modulators used to affect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other in a region of zero absorption. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapor cell onto the camera. The reference beam that lies in a zero absorption region is unaffected by passage through the cell, and provides a reference image. The signal beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object and is currently achieving a velocity resolution of about +/−2 ms−1, limited by the quality of the light sheet generated from the multimode fiber. Automatic superposition of the signal and reference images is achieved, and polarization errors caused by the beam splitter in the conventional system are eliminated. Measurements have also been made on an axisymmetric air jet, seeded with a commercial smoke generator, which has maximum velocities of ∼100 ms−1. A comparison with data obtained simultaneously, using a conventional two camera PDV arrangement has been made and the difference between the measurements found to be within a few m/s.
Radiofrequency sheath fields above a metal-dielectric interface
Authors: Barnat E. V. , Hebner G. A.
Published: October 2004

Two-dimensional maps of the sheath electric fields formed around a metal-dielectric interface were measured in a radio frequency (rf) argon plasma using laser-induced fluorescence-dip spectroscopy. Experimentally determined Stark shifts of the argon Rydberg 13d[3∕2]1 state were used to quantify the electric fields in the sheath as functions of the rf cycle, voltage, and pressure. Both the structure of the sheath fields and the discharge characteristics in the region above the electrode depend on the discharge conditions and the configuration of the surface. Dissimilar materials placed adjacent to each other result in electric fields with a component parallel to the electrode surface.
Assembly of FePt L1 nanoparticles grown on MgO(110) with self-organized groove structure
Authors: Okamoto S. , Kitakami O. , Miyazaki T. , Shimada Y. , Takahashi Y. K. , Hono K.
Published: October 2004

We have fabricated an FePt nanoparticle assembly on a MgO(110) with a self-organized structure. Prior to FePt deposition, the MgO(110) substrate was annealed at 1273K in air to develop self-organized grooves running along the [11¯0] direction with average periodicity of 14.5nm. Subsequently a very thin FePt layer with nominal thickness of 1.5nm was sputtered onto the MgO substrate at 973K, resulting in FePt L10 nanoparticles with 2–4nm in diameter aligned along the grooves. Each FePt nanoparticle was a L10 single crystal with its c-axis tilting 45° or −45° from the substrate normal that corresponds to MgO [100] or [010]. The magnetic properties of the FePt nanoparticle assembly well reflected such c-axis distribution, and their switching behavior almost followed the Stoner-Wohlfarth model.
The influence of Cr doping on the charge-ordering state in bilayered LaSr2Mn2O7
Authors: Zhang R. L. , Zhao B. C. , Song W. H. , Ma Y. Q. , Yang J. , Sheng Z. G. , Dai J. M. , Sun Y. P.
Published: October 2004

The effect of Cr doping on the charge-ordered (CO) state of the bilayered manganite LaSr2Mn2O is investigated by electronic transport and magnetic property measurements. With Cr doping, the metal-insulator transition disappears and the temperature dependence of resistivity ρ(T) exhibits semiconducting behavior. Both the antiferromagnetic transition temperature TN and the CO temperature TCO shift to lower temperature and the cluster glass state appears with increasing Cr doping level. The increase of resistivity and the slight change in the magnetization with increasing Cr content may be related to the combined effect of the partial destruction of the Mn-O-Mn interaction network within the ferromagnetic (FM) layers of an A-type antiferromagnet and the appearance of Cr3+-O-Mn3+ FM interaction. Additionally, the introduction of the random Coulomb potential caused by the substitution of Cr3+ for Mn3+ is also a source of increased resistivity. Both, the different effects of Mn-site doping on the CO state between the perovskite and layered manganites and the different effects of Co doping and Cr doping in the Mn site in layered manganites on the CO state are discussed.
An effective thermal conductivity measurement system
Authors: Madrid F. , Jordà X. , Vellvehi M. , Guraya C. , Coleto J. , Rebollo J.
Published: October 2004

In the technical literature, there is a lack of reliable thermal parameters and, often, it is necessary to do in situ measurements for every particular material. An effective thermal conductivity measurement system has been designed and implemented to provide reliable and accurate values for that thermal parameter. The thermal conductivity of a given material is deduced from thermal resistance differential measurements of two samples. All parts of the implemented system as well as practical and theoretical solutions are described, including a power controller circuit exclusively conceived for this application. Experimental considerations to reduce the measurement error are exposed, as well as some results obtained for three different materials.
Time resolved laser-induced fluorescence of electrosprayed ions confined in a linear quadrupole trap
Authors: Friedrich Jochen , Fu Jinmei , Hendrickson Christopher L. , Marshall Alan G. , Wang Yi-Sheng
Published: October 2004

We have designed and constructed a linear quadrupole ion trap for the measurement of laser-induced fluorescence (LIF) of mass selected gas-phase ions produced by electrospray ionization. The instrument consists of a simple electrospray source, radiofrequency octopole guide, a dc quadrupole bender, a quadrupole mass filter, the linear quadrupole trap (which is equipped with optics for LIF collection and a channeltron ion detector), and several multielement focusing lenses. With this instrument, the LIF decay lifetime of gas-phase Rhodamine 640 radical cations is determined for the first time.
The role of electron heat flux in guide-field magnetic reconnection
Authors: Hesse Michael , Kuznetsova Masha , Birn Joachim
Published: October 2004

A combination of analytical theory and particle-in-cell simulations are employed in order to investigate the electron dynamics near and at the site of guide field magnetic reconnection. A detailed analysis of the contributions to the reconnection electric field shows that both bulk inertia and pressure-based quasiviscous processes are important for the electrons. Analytic scaling demonstrates that conventional approximations for the electron pressure tensor behavior in the dissipation region fail, and that heat flux contributions need to be accounted for. Based on the evolution equation of the heat flux three tensor, which is derived in this paper, an approximate form of the relevant heat flux contributions to the pressure tensor is developed, which reproduces the numerical modeling result reasonably well. Based on this approximation, it is possible to develop a scaling of the electron current layer in the central dissipation region. It is shown that the pressure tensor contributions become important at the scale length defined by the electron Larmor radius in the guide magnetic field.
Effect of selected atomic layer deposition parameters on the structure and dielectric properties of hafnium oxide films
Authors: Kukli Kaupo , Aarik Jaan , Ritala Mikko , Uustare Teet , Sajavaara Timo , Lu Jun , Sundqvist Jonas , Aidla Aleks , Pung Lembit , Hårsta Anders , Leskelä Markku
Published: October 2004

Hf O 2 films were atomic layer deposited from HfCl4 and H2O on Si(100) in the temperature range of 226–750°C. The films consisted of dominantly the monoclinic polymorph. Elastic recoil detection analysis revealed high residual chlorine and hydrogen contents (2–5at.%) in the films grown below 300–350°C. The content of residual hydrogen and chlorine monotonously decreased with increasing growth temperature. The effective permittivity insignificantly depended on the growth temperature and water partial pressure. Capacitance-voltage curves exhibited marked hysteresis especially in the films grown at 400–450°C, and demonstrated enhanced distortions likely due to the increased trap densities in the films grown at 700–750°C. Changes in water pressure led to some changes in the extent of crystallization, but did not induce any clear changes in the capacitance of the dielectric layer.
Parametric investigation of laser nanoimprinting of hemispherical cavity arrays
Authors: Li L. P. , Lu Y. F. , Doerr D. W. , Alexander D. R. , Chen X. Y.
Published: October 2004

A monolayer of self-assembled silica particles can be imprinted into a silicon substrate by laser irradiation (KrF excimer laser, λ=248nm). Periodical hemispherical cavities can be therefore created on the substrate surface. The influences of various particle sizes and laser fluence were investigated. In addition, preheating of the substrate significantly improves the performance. One-dimensional thermal calculation was employed to understand the thermal effect in this process. Three-dimensional optical simulation provided an accurate insight into the light intensity enhancement. Raman spectroscopy was used to examine the stress induced by the laser imprinting process resided in the cavity structures.
Hole effective mass in strained Si1−xCx alloys
Authors: Lin C. Y. , Chang S. T. , Liu C. W.
Published: October 2004

The directional, density-of-states, and carrier-concentration effective masses of light, heavy, and split-off holes have been calculated for strained Si1−xCx alloys on Si (001) substrate. The results for the directional effective mass show that the effect of strain makes the constant energy surface of light holes near the band edge more symmetric than that in pure silicon. The effect of strain on the heavy and split-off hole bands is rather regular; up to 7% of carbon concentration the strain effect monotonically reduces the density-of-states effective mass for the two bands at energy values within energy interval of 0.4eV below the valence band edge. This reduction is obtained for the carrier-concentration effective mass at temperatures from 0to600K. The strain effect on the light hole band is less trivial; at nonzero carbon concentrations the strain effect influences the density-of-states and the carrier-concentration effective mass in a similar way as it does to the heavy and split-off bands but irregular behavior shows up in the energy interval of 0.02eV below the valence band edge and at the temperature range from 0to140K. At 7% of carbon doping the total density-of-states effective mass for holes at 77 and 300K are almost the same, namely, the values are 0.39 and 0.40 in units of free electron mass, respectively.
Analytical fit of the I-V characteristic for cylindrical and spherical probes in electronegative plasmas
Authors: Crespo R. Morales , Palop J. I. Fernández , Hernández M. A. , Borrego del Pino S. , Ballesteros J.
Published: October 2004

This paper developes a theoretical model for the sheath that surrounds cylindrical and spherical Langmuir probes immersed in an electronegative plasma. The model allows the electric potential profile surrounding the negatively polarized probe, the densities of the different charged particles within the sheath, and the I-V characteristic curves to be determined for several negative ion density and temperature values. These curves are fitted analytically, permitting expressions to be obtained for the potential profile, the floating potential, and the sheath edge.
Resonant tunneling through defects in an insulator: Modeling and solar cell applications
Authors: Jiang Chu-Wei , Green Martin A. , Cho Eun-Chel , Conibeer Gavin
Published: October 2004

In this paper, a model for electron tunneling through defects in an insulator is presented. The three-dimensional results for the electron transmission coefficient can be obtained by characterizing the tunneling process in terms of a defect density and capture cross section. Fitting the model parameters by comparison with the results of a full three-dimensional tunneling-through-defect simulation, this model can be used to calculate and predict the electron transmission for various spatial distributions of defects without performing the complex three-dimensional calculations. Energy selective contacts using the resonant tunneling for carrier extraction have been proposed as a means to achieve a higher efficiency in future generations of photovoltaic devices. Resonant tunneling through defects in an insulator, where the defects may be atoms or quantum dots, may provide a possible implementation for such energy selective contacts. With the present model, the influences of the tunneling effective mass, insulator thickness, and defect distribution on the electron transmission coefficient have been investigated. The results suggest that the introduced defects should lie in the middle of a reasonably thick insulator to improve the carrier energy selectivity.
Rhenium ohmic contacts on 6H-SiC
Authors: McDaniel G. Y. , Fenstermaker S. T. , Lampert W. V. , Holloway P. H.
Published: October 2004

Rhenium (Re) thin-film contacts (100-nm thick) were deposited on carbon-rich, nominally stoichiometric, and silicon-rich 6H–SiC surfaces, which were moderately doped with nitrogen (1.28×1018cm−3). Morphology (Dektak), phase formation (x-ray diffraction), chemistry (Auger electron spectroscopy), and electrical properties (I–V) were characterized for the as-deposited and annealed (120min, 1000°C, vacuum <1×10−6Torr) contacts. As-deposited films were nonohmic. Films grown on carbon-rich surfaces were nonspecular, granular, and often delaminated during characterization. At room temperature in air, the Re films on stoichiometric SiC remained optically specular reflecting for 3h, but then became hazy from oxidation. The Re films on silicon-rich surfaces, stored in air at room temperature, resisted ex situ oxidation for approximately 24h. The annealed samples remained specular without visible signs of oxidation. The annealing resulted in a reduction in surface roughness for all the films regardless of substrate chemistry. The phase separation between carbon and rhenium was observed based on the formation of interfacial Re clusters and a ∼10-nm graphite surface layer after annealing. Auger data showed that Si layers (5–10nm) deposited to create Si-rich surfaces were partially consumed to form rhenium silicide during annealing, and the sharp Re∕Si∕SiC interface became more diffused with Re detected ∼50nm deeper into the structure. The annealing of Re films on moderately doped (1.28×1018cm−3) SiC resulted in ohmic contacts with an average specific contact resistance of 7.0×10−5Ωcm2 for stoichiometric and 1.6×10−5Ωcm2 for silicon-rich samples. The annealed contacts on carbon-rich surfaces remained rectifying.
Production of ultrahigh-current-density ion beams by short-pulse skin-layer laser–plasma interaction
Authors: Badziak J. , Głowacz S. , Jabłoński S. , Parys P. , Wołowski J. , Hora H.
Published: October 2004

We report experimental evidence, supported by a simple theory and numerical calculations, that the skin-layer subrelativistic interaction of a short (⩽1ps) low-energy (<1J) laser pulse with a thin preplasma layer in front of a solid target can produce a collimated fast ion flux of extremely high ion current density (⩾1010A∕cm2 close to the target), comparable to those predicted for ballistically focused ion beams from relativistic laser–plasma interactions.
Ultrafast measurements of vibrational relaxation in the conjugated polymer poly(9,9-dioctylfluorene)
Authors: Wells Jon-Paul R. , Lidzey David G. , Phillips P. Jonathan , Carder Damian A. , Fox A. Mark
Published: October 2004

We report the resonant pump–probe measurements used to study the dynamics of molecular vibrations in the conjugated polymer poly(9,9-dioctlyfluorene) (PFO). Free-electron-laser excited, pump–probe measurements on a drop-cast polymer film yield the lifetime of a series of different infrared active, high frequency vibrational modes at 5K. A general trend of decreasing lifetime with increasing frequency of the vibrational mode seems consistent with an enhanced density of accepting states for high frequency modes. Our measurements provide an insight into the dissipation of energy in conjugated polymers, and have implications for exciton generation and dissociation mechanisms in organic optoelectronic devices.
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Orientated lead salt midinfrared lasers
Authors: Shi Z. , Lv X. , Zhao F. , Majumdar A. , Ray D. , Singh R. , Yan X. J.
Published: October 2004

A Lead salt midinfrared quantum-well (QW) laser on [110] orientation is proposed. Theoretical simulations of [110] QW edge-emitting lasers show a 70° temperature increase in continuous-wave operation compared to the conventional [100]-orientated lasers. This is because the gain on [100]-orientated QW structure is significantly increased. Among [100], [111], and [110] orientations, [110]-orientated QW structure offers the highest gain. PbSe∕PbSrSe QW structures were successfully grown on [110]-orientated BaF2 substrate by molecular-beam-epitaxy (MBE). Photoluminescence intensity of [110]-orientated samples was twice as high as that on [111]-orientated BaF2 substrates from the same MBE run.
The dynamic characteristics of sulfur sensitization centers in T-grain AgBrI microcrystals
Authors: Fu Guang-Sheng , Yang Shao-Peng , Li Xiao-Wei , Liu Rong-Juan , Tian Xiao-Dong , Han Li
Published: October 2004

The microwave absorption dielectric-spectrum technique was used to study the decay kinetics of the free photoelectrons and shallow-trapped electrons in the sulfur-sensitized T-grain AgBrI crystals prepared under different sensitization times. It was found that, from the dependence of the kinetic behavior of photoelectrons in the sensitized T-grain AgBrI emulsions, the optimal sensitization time was 45min. With an increase of the sensitization time, the trap effect of the sulfur sensitization centers varies from the hole trap to shallow electron trap, and to deep electron trap, corresponding to the sulfur sensitization centers in the forms of a monomer, dimmer, and cluster of Ag2S, respectively. The range of the shallow electron trap depth traps was determined to be 0.26–0.27eV.
Study of photoelectron spectroscopy from extremely uniform Si nanoislands on Si(111) 7×7 substrate
Authors: Negishi R. , Suzuki M. , Shigeta Y.
Published: October 2004

The electronic and structural properties of self-assembled Si nanoislands on a Si(111) 7×7 dimer-adatom-stacking fault substrate are investigated by photoelectron spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Uniform Si nanoislands are formed on the Si(111) 7×7 substrate by control of the growth conditions. For the nanoislands fabricated on the substrate, the photoelectron spectrum shows a significant peak shift of ≈0.1eV, which is caused by a surface state related to a dangling bond at the nanoisland.
Resonantly pumped optical pumping injection cavity lasers
Authors: McAlpine T. C. , Greene K. R. , Santilli M. R. , Olafsen L. J. , Bewley W. W. , Felix C. L. , Vurgaftman I. , Meyer J. R. , Lee H. , Martinelli R. U.
Published: October 2004

An optical parametric oscillator is tuned to the resonance wavelength of the etalon in an optical pumping injection cavity (OPIC) laser with a type-II “W” active region, thereby minimizing the threshold pump intensity and maximizing the output slope efficiency. Previous OPIC experiments employed fixed-wavelength sources with only limited tuning available by adjusting the incident angle. Low threshold pump intensities of 330W∕cm2 at 100K and 14kW∕cm2 at 300K (where the output wavelength is 3200nm) were achieved. The energy conversion efficiency is found to decrease by over a factor of 100 when the pump wavelength is tuned from the resonance condition (1822nm at 300K) to only slightly off resonance (e.g., 1808nm).
Reducing the critical switching current in nanoscale spin valves
Authors: Manschot Jan , Brataas Arne , Bauer Gerrit E. W.
Published: October 2004

The current induced magnetization reversal in nanoscale spin valves is a potential alternative to magnetic field switching in magnetic memories. We show that the critical switching current can be decreased by an order of magnitude by strategically distributing the resistances in the magnetically active region of the spin valve. In addition, we simulate full switching curves and predict a new precessional state.
Ferromagnetism and coupling between charge carriers and magnetization at room temperature in Ge∕MnAs multilayers
Authors: Lee J. J. , Cui Y. , Song J. H. , Freeman A. J. , Ketterson J. B. , Cho S. L.
Published: October 2004

Ge ∕ MnAs multilayers are grown on (001) GaAs substrates by molecular-beam epitaxy. All samples investigated showed strong anomalous Hall effects at room temperature with p-type conductivity and temperature-dependent hysteresis loops in the magnetization. Ge∕MnAs multilayers also revealed an in-plane magnetic easy axis and a vanishingly small in-plane magnetic anisotropy. These results are in sharp contrast to MnAs∕GaAs digital alloys, where the reported Curie temperatures are at or below 50K, and demonstrate the potential of germanium-based spintronic devices.
All jet-printed polymer thin-film transistor active-matrix backplanes
Authors: Arias A. C. , Ready S. E. , Lujan R. , Wong W. S. , Paul K. E. , Salleo A. , Chabinyc M. L. , Apte R. , Street Robert A. , Wu Y. , Liu P. , Ong B.
Published: October 2004

Thin-film transistor (TFT) backplanes fabricated by using jet printing as the only patterning method are reported. Additive and subtractive printing processes are combined to make 128×128 pixel active matrix arrays with 340μm pixel size. The semiconductor used, a regioregular polythiophene, poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene]; (PQT-12) is deposited by inkjet printing and exhibits average TFT mobility of 0.06cm2∕Vs, on/off ratios of 106, and minimal bias stress. The printed TFTs have high yield with a narrow performance distribution. The pixel design benefits from the registration accuracy of jet printing and it is shown that the electrical performance is suitable for addressing capacitive media displays.
Performance of single-junction and dual-junction InGaP∕GaAs solar cells under low concentration ratios
Authors: Khan Aurangzeb , Yamaguchi Masafumi , Takamoto Tatsuya
Published: October 2004

A study of the performance of single-junction InGaP∕GaAs and dual-junction InGaP∕GaAs tandem cells under low concentration ratios (up to 15 suns), before and after 1 MeV electron irradiation is presented. Analysis of the tunnel junction parameters under different concentrated light illuminations reveals that the peak current (JP) and valley current (JV) densities should be greater than the short-circuit current density (Jsc) for better performance. The tunnel junction behavior against light intensity improved after irradiation. This led to the suggestion that the peak current density (JP) and valley current density (JV) of the tunnel junction were enhanced after irradiation or the peak current was shifted to higher concentration. The recovery of the radiation damage under concentrated light illumination conditions suggests that the performance of the InGaP∕GaAs tandem solar cell can be enhanced even under low concentration ratios.
Diameter dependence of the electronic structure of ZnO nanorods determined by x-ray absorption spectroscopy and scanning photoelectron microscopy
Authors: Chiou J. W. , Kumar K. P. Krishna , Jan J. C. , Tsai H. M. , Bao C. W. , Pong W. F. , Chien F. Z. , Tsai M.-H. , Hong I.-H. , Klauser R. , Lee J. F. , Wu J. J. , Liu S. C.
Published: October 2004

O K -, ZnL3, and K-edges x-ray absorption near-edge structure (XANES) spectra and scanning photoelectron microscopy (SPEM) spectra were obtained for ZnO nanorods with various diameters. The analysis of the XANES spectra revealed increased numbers of O2p and Zn4p unoccupied states with the downsizing of the nanorods, which reflects the enhancement of surface states when the diameter is decreased. Valence-band photoemission spectra show a significant narrowing of the valence band for the 45nm diameter nanorod. The Zn3d intensities in the Zn3d SPEM spectra are drastically diminished for all nanorods as compared to the ZnO reference film, which can be interpreted as a reduction in density of itinerant final states or in transition probability.
Reversal mechanisms and metastable states in magnetic nanoelements
Authors: Liu X. , Chapman J. N. , McVitie S. , Wilkinson C. D. W.
Published: October 2004

Transmission electron microscopy and micromagnetic modeling have been used to study magnetization processes in small elongated magnetic elements with different end shapes. Their response to fields applied both parallel and perpendicular to the element long axes has been investigated with particular emphasis on the latter. A rich variety of magnetic states have been revealed and it is shown that very similarly shaped elements can respond quite differently; moreover the same element need not respond in the same way when subjected to repeated field cycles. The importance of thermal excitation and the resulting stability at remanence of the various metastable states observed is discussed.
Physical mechanisms for anisotropic plasma etching of cesium iodide
Authors: Yang Xiaoji , Hopwood Jeffrey A.
Published: October 2004

The physical mechanisms for the interaction between a reactive plasma and a cesium iodide surface are investigated. Under conditions of ion bombardment and elevated substrate temperature, CsI is found to sputter etch slowly (15nm∕min). If atomic fluorine, fluorocarbon radicals, of SFx radicals are present in the discharge, however, CsI is reactively etched at substantially higher rates (up to 200nm∕min). The roles of plasma radicals and energetic ion bombardment are investigated by first exposing the surface to plasma radicals and then bombarding the surface with argon ions. The optical emission from Cs and I atoms is found to correlate with the etch rate of CsI and is used as an in situ monitor of radical-enhanced etching. Small surface exposures to CFx, SFx, and F radicals are shown to enhance the etch rate of CsI. If the exposure of the CsI surface is increased, however, these same radical species act as etch inhibitors. A simple model for reactive etching of CsI is proposed, and this model is shown to compare reasonably well with experimental etch rates.
In situ fabrication of single-crystal Fe nanomagnet arrays
Authors: Choi J. H. , Kim T.-H. , Seo J. , Kuk Y. , Suh M. S.
Published: October 2004

We produced single-crystalline Fe nanodot arrays grown in situ on a W(110) substrate in an ultrahigh vacuum system. An alumina shadow mask with perfectly ordered holes was used for Fe deposition. Polycrystalline Fe nanodots could be crystallized to single-domain nanodots by thermal annealing. After annealing, Fe wets tungsten substrate with one monolayer, but Fe islands neither coalesce nor form an alloy with the substrate.
Charge state and time resolved plasma composition of a pulsed zirconium arc in a nitrogen environment
Authors: Rosén Johanna , Anders André , Hultman Lars , Schneider Jochen M.
Published: October 2004

The species and ion charge state evolution of a pulsed cathodic arc plasma was investigated at different pressures. A zirconium cathode was operated in a nitrogen environment, and the plasma composition was analyzed by time-of-flight charge-to-mass spectrometry. Large plasma chemistry changes were detected with respect to time and pressure. The 250μs plasma pulse can be divided in two characteristic phases: a transient phase before 150μs and a steady state phase for all later times. The measured changes in plasma chemistry in the transient phase at p∼10−5Torr are explained by charge transfer collisions, while the increasing N+ fraction in the p>10−5Torr range most likely originates from erosion of the nitrided cathode surface. In the steady-state phase, a pressure-induced change from higher to lower charge states was observed, which was mainly due to scattering of self-sputtered metal followed by metal ion-atom charge exchange collisions. These results are of importance for understanding the evolution of thin film composition and microstructure during reactive plasma deposition.
Effect of free-carrier absorption on the threshold current density of GaAs∕(Al,Ga)As quantum-cascade lasers
Authors: Giehler M. , Kostial H. , Hey R. , Grahn H. T.
Published: October 2004

Ga As ∕ Al 0.33 Ga 0.67 As quantum-cascade lasers with plasmon-assisted waveguides exhibit a decreasing threshold current density jth with increasing wave number ν0 of the laser line, which changes as a function of the injector doping density. We have developed an analytical approach based on the effective dielectric tensor component for the p-polarized light emitted from a quantum-cascade laser, which explains the observed dependence of jth(ν0) in terms of losses due to free-carrier absorption predominantly in the doped waveguides αWG(ν0). A contribution to the losses by free-carrier absorption in the quantum-cascade structure itself and subsequently to jth can be neglected except for very high injector doping densities. The calculated values for αWG(ν0) are in good agreement with the experimental data. Our approach quantitatively predicts the observed decrease of jth from 17to7kAcm−2 with increasing ν0 between 900 and 1100cm−1. In addition to achieving a direct physical insight into the influence of free-carrier absorption on the laser performance, the proposed analytical approach provides a simple tool for the determination of the waveguide losses for any quantum-cascade laser without adopting a numerical solver.
Improvement in spatial resolution of plasma-enhanced quantum-well intermixing by stress-inducing dielectric mask
Authors: Djie H. S. , Mei T. , Arokiaraj J.
Published: October 2004

We report the use of a stress-inducing dielectric mask to improve the spatial resolution of the proximity quantum-well intermixing process. Photoluminescence and Raman spectroscopy were used to study the band gap modification and the spatial resolution using Ar plasma in the InGaAs∕InGaAsP laser structure. A spatial resolution of 2.4μm has been achieved with the presence of an SixNy annealing cap as a stress-inducing mask. The simple technique provides a promising approach of lateral band gap tuning with a high spatial resolution for high-density photonic integrated circuits.
Microstructure and ferroelectric properties of sol-gel derived Bi3.15Nd0.85Ti3O12 thin films on Pt∕Ti∕SiO2∕Si(100)
Authors: Li J. H. , Qiao Y. , Liu X. L. , Nie C. J. , Lu C. J. , Xu Z. X. , Wang S. M. , Zhang N. X. , Xie D. , Yu H. C. , Li J. Q.
Published: October 2004

Bi 3.15 Nd 0.85 Ti 3 O 12 (BNdT) thin films were deposited on Pt∕Ti∕SiO2∕Si substrates using a sol-gel process. The film annealed at 750°C is composed of grains of 50–100nm in diameter. The fine grains show nearly random orientations. “Micropores” were frequently observed at the junctions of the grains and they are mostly amorphous, while sometimes containing very fine crystalline particles with sizes of only a few nanometers. The BNdT film capacitors with a Pt top electrode showed excellent ferroelectric properties. The remanent polarization and the coercive field were in the range of 41–43μC∕cm2 and 70–84kV∕cm, respectively. The BNdT capacitors did not show any significant fatigue up to 5×109 switching cycles at a frequency of 1MHz.
Transition in growth mode by competing strain relaxation mechanisms: Surfactant mediated epitaxy of SiGe alloys on Si
Authors: Kammler M. , Hoegen M. Horn-von
Published: October 2004

Surfactant mediated epitaxy of Si(1−x)Gex alloys on Si(111) can, besides technological importance, clarify the influence of the lattice mismatch during surfactant mediated heteroepitaxial growth. For low Ge concentration we find an immediate layer-by-layer growth, whereas at high Ge concentration a roughening transition followed by relaxation of the lattice mismatch in a periodic dislocation network is preferred. This behavior can be explained by comparing the dislocation nucleation rate on a smooth surface with the island nucleation rate on a pseudomorphic film.
Unified explanation for suppressed electron ionization coefficient and its weak temperature dependence in InGaP
Authors: Neo Wah-Peng , Wang Hong
Published: October 2004

Experimental evidence of electron multiplication, extracted from InGaP–GaAs–InGaP double heterojunction bipolar transistors, reveals a suppressed electron impact ionization coefficient, αn, with weak temperature dependence in InGaP compared to those measured from binary InP and GaP, which could not be predicted by the widely used Okuto–Crowell relation assuming that the optical phonon scattering is the only dominant scattering process during impact ionization. We show that discrepancies between the experimental data and the Okuto–Crowell relation can be consistently ascribed to the involvement of alloy scattering. This provides an improved insight into understanding the presence of alloy scattering on carrier impact ionization behavior in InGaP.
Pure Bi4Ti3O12 thin films with improved ferroelectric properties
Authors: Chiou Tai-Yi , Kuo Dong-Hau
Published: October 2004

Pure Bi4Ti3O12 thin films with outstanding ferroelectric properties are reported. These films have been prepared by spin coating on substrates with a chemical solution. A (117)-preferred growth occurs at the annealing temperature of 600°C. These films have a low coercive field of ∼35kV∕cm, a remanent polarization >20μC∕cm2, a nonvolatile polarization >37μC∕cm2 after 1010 sweep cycles, and a leakage current density ∼10−5A∕cm2 at the coercive field. The excellent ferroelectric properties are attributed to the chemical preparation, which corresponds to chemical bonding and defects, dissolution of the chemical precursors, and precipitation of the spin-coating solution.
Surface acoustic wave-driven planar light-emitting device
Authors: Cecchini Marco , De Simoni Giorgio , Piazza Vincenzo , Beltram Fabio , Beere H. E. , Ritchie D. A.
Published: October 2004

Electroluminescence emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers for SAW generation were integrated onto pLEDs fabricated following the scheme which we have recently developed [Cecchini et al., Appl. Phys. Lett. 82, 636 (2003)]. Current-voltage, light-voltage, and photoluminescence characteristics are presented at cryogenic temperatures. We argue that this scheme represents a valuable building block for advanced optoelectronic architectures.
Metamaterial exhibiting left-handed properties over multiple frequency bands
Authors: Chen Hongsheng , Ran Lixin , Huangfu Jiangtao , Zhang Xianmin , Chen Kangsheng , Grzegorczyk Tomasz M. , Kong Jin Au
Published: October 2004

We present experimental measurements at microwave frequencies of a double band left-handed metamaterial. The sample of metamaterial is composed of an extended version of the S-shaped resonators that exhibit simultaneously a negative permittivity and a negative permeability response at comparable frequencies. The experimental results clearly show that there exist two frequency bands where the refraction index is negative. The double left-handed passbands are due to the multiple capacitances and inductances induced in the structure, which can be further tuned to realize a metamaterial with multiple (more than two) left-handed frequency bands.
Temperature dependence of breakdown voltage in AlxGa1−xAs
Authors: Groves C. , Harrison C. N. , David J. P. R. , Rees G. J.
Published: October 2004

Avalanche breakdown voltage has been measured in a series of AlxGa1−xAs(0<x<0.8)p+in+ and n+ip+ diodes at temperatures between 20 and 500K. The temperature sensitivity falls with decreasing avalanche region width for all alloy compositions, with a minimum sensitivity at x=0.6, for which we offer a physical explanation. While Monte Carlo simulations show that alloy scattering reduces the temperature dependence of breakdown voltage, the effect is too small to account for the observations.
Weak anti-localization of the two-dimensional electron gas in modulation-doped AlxGa1−xN∕GaN heterostructures with two subbands occupation
Authors: Lu J. , Shen B. , Tang N. , Chen D. J. , Zhao H. , Liu D. W. , Zhang R. , Shi Y. , Zheng Y. D. , Qiu Z. J. , Gui Y. S. , Zhu B. , Yao W. , Chu J. H. , Hoshino K. , Arakawa Y.
Published: October 2004

Weak anti-localization of the two-dimensional electron gas (2DEG) in a modulation-doped Al0.22Ga0.78N∕GaN single heterostructure has been investigated through magnetoresistance measurements at low temperatures. The elastic scattering time τe, dephasing time τϕ and spin-orbit scattering time τso at various temperatures are obtained. When the second subband in the triangular quantum well at the heterointerface is occupied by the 2DEG, the weak anti-localization is observed clearly, which is thought to be due to the strong spin-orbit effect induced by the intersubband scattering. The spin-orbit effect and the intersubband scattering become stronger with increasing temperature.
Horizontal growth and in situ assembly of oriented zinc oxide nanowires
Authors: Nikoobakht Babak , Michaels Chris A. , Stranick Stephan J. , Vaudin Mark D.
Published: October 2004

The positioning and directed assembly of semiconductor nanowires (NWs) is of considerable current interest for “bottom-up” approaches to the engineering of intricate structures from nanoscale building blocks. We report a horizontal growth mode for ZnO NWs on the (112¯0) sapphire surface in which NWs grow in the [11¯00]sap direction. This growth mode strictly depends on the size and spacing of the Au nanodroplet catalysts and competes with the vertical growth of the NWs. An approach is presented which promotes the horizontal growth, in situ alignment, and predictable positioning of ZnO NWs. This strategy allows for the large scale assembly of NWs, width control, and production of quantum wires.
Stable styrylamine-doped blue organic electroluminescent device based on 2-methyl-9,10-di(2-naphthyl)anthracene
Authors: Lee Meng-Ting , Chen Hsian-Hung , Liao Chi-Hung , Tsai Chih-Hung , Chen Chin H.
Published: October 2004

We have developed a highly efficient and stable blue organic electroluminescent (EL) device based on a blue fluorescent styrylamine dopant, p-bis(p-N,N-diphenyl-aminostyryl)benzene, in a morphologically stable high band-gap host material, 2-methyl-9,10-di(2-naphthyl)anthracene, which achieved an EL efficiency of 9.7cd∕A and 5.5lm∕W at 20mA∕cm2 and 5.7 V, with Commission Internationale d’Eclairage coordinates of (x=0.16,y=0.32). The blue-doped device achieved a half-decay lifetime (t1∕2) of 46 000 h at an initial brightness of 100cd∕m2.
Electronic properties of ferroelectric BaTiO3∕MgO capacitors on GaAs
Authors: Murphy T. E. , Chen D. , Phillips J. D.
Published: October 2004

Thin films of MgO and BaTiO3 were deposited on (001) GaAs substrates using molecular beam epitaxy and pulsed laser deposition, respectively. X-ray diffraction scans indicate crystalline MgO and BaTiO3 thin films with preferential c-axis orientation. Capacitors fabricated from the BaTiO3∕MgO∕GaAs structures demonstrate ferroelectric hysteresis behavior with a remenant polarization of 0.4μC∕cm2. Small-signal capacitance measurements indicate an effective dielectric constant for the BaTiO3∕MgO structure of 45.5ε0. A hysteresis behavior indicative of ferroelectric switching is observed in the capacitance–voltage characteristics with a memory window of approximately 2 V. The observation of ferroelectric behavior in these materials are promising for future multifunctional devices on GaAs.
Probing subpicosecond dynamics using pulsed laser combined scanning tunneling microscopy
Authors: Takeuchi Osamu , Aoyama Masahiro , Oshima Ryuji , Okada Yoshitaka , Oigawa Haruhiro , Sano Nobuyuki , Shigekawa Hidemi , Morita Ryuji , Yamashita Mikio
Published: October 2004

Time-resolved tunneling current measurement in the subpicosecond range was realized by ultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pair method. A low-temperature-grown GaNxAs1−x(x=0.36%) sample exhibited two ultrafast transient processes in the time-resolved tunnel current signal, whose lifetimes were determined to be 0.653±0.025 and 55.1±5.0ps. These values are of the same order of magnitude as those measured in the conventional pump–probe reflectivity measurement.
Quantum antidot as a controllable spin injector and spin filter
Authors: Zozoulenko I. V. , Evaldsson M.
Published: October 2004

We propose a device based on an antidot embedded in a narrow quantum wire in the edge-state regime, that can be used to inject and/or control spin-polarized current. The operational principle of the device is based on the effect of resonant backscattering from one edge state into another through localized quasibound states, combined with the effect of Zeeman splitting of the quasibound states in sufficiently high magnetic field. We outline the device geometry, present detailed quantum-mechanical transport calculations, and suggest a possible scheme to test the device performance and functionality.
Time-resolved photoluminescence studies of bound excitons in CuInS2 crystals
Authors: Wakita Kazuki , Nishi Kazuhito , Ohta Yoshihiko , Onishi Takashi
Published: October 2004

The time-resolved photoluminescence of bound excitons in bulky single crystalline CuInS2 grown by the traveling heater method is investigated. It is found that radiative decay of the bound exciton at 1.530 eV is exponential with two characteristic decay times while that of the bound excitons at 1.525 and 1.520 eV is well-represented by a single exponent at low temperatures. The radiative lifetimes of the bound excitons at 1.530, 1.525, and 1.520 eV are obtained to be 500 ps, 2.1 ns, and 3.5 ns, respectively. According to our estimates, a neutral charge is to be assigned to the defect centers associated with the observed bound excitons.
High luminescent efficiency of InGaN multiple quantum wells grown on InGaN underlying layers
Authors: Akasaka Tetsuya , Gotoh Hideki , Saito Tadashi , Makimoto Toshiki
Published: October 2004

InGaN multiple quantum wells were grown on InGaN underlying layers 50 nm thick by metalorganic vapor phase epitaxy. Photoluminescence (PL) measurements were performed by selective excitation of the quantum wells under a weak excitation condition. The PL intensity was almost constant at temperatures ranging from 17 to 150 K. Assuming that the internal quantum efficiency (ηint) equals unity at 17 K, we obtained ηint as high as 0.71 even at room temperature. The reason for the high ηint is the reduction of nonradiative recombination centers by the incorporation of indium atoms into the underlying layer.
Fabrication temperature effect of the field emission from closed and open tip carbon nanotube arrays fabricated on anodic aluminum oxide films
Authors: Kang Dong Won , Suh Jung Sang
Published: October 2004

We have studied the field emission from highly ordered closed and open tip carbon nanotubes (CNTs) fabricated, in the range of 550–750°C, on porous anodic aluminum oxide films. The field-emission efficiency of the closed and open tip CNTs showed the opposite behavior as the fabrication temperature increased. The former decreased, whereas the latter increased with an increase in the fabrication temperature. From the HRTEM analysis, as the fabrication temperature increased, the crystallinity of the inner walls was improved, whereas that of the outer walls deteriorated due to the formation of a composite of carbon and alumina. This could be a direct cause of the opposite field-emission behavior of the closed and open tip CNTs with an increasing fabrication temperature, because for the open tips, the field emission could take place from all the walls including the inner walls but for the closed tips, only from the outer walls. This is supported by the observation that the field emission of the closed tip CNTs fabricated at a high temperature was greatly improved by milling the surface of the tips, slightly, with ion bombardment. Our results suggest a way to fabricate a high efficiency CNT field-emitter array.
Irradiator to study damage induced to large nonvolatile molecules by low-energy electrons
Authors: Zheng Yi , Cloutier Pierre , Wagner J. Richard , Sanche Léon
Published: October 2004

We report on the design and performance of an irradiator to study the fragmentation of large nonvolatile molecules induced by low-energy electron impact under ultrahigh vacuum. The apparatus consists essentially of a new type of electron gun which can bombard molecules spin-coated on the inside surface of a cylinder. With this configuration, it is possible to irradiate a relatively large area (26 cm2) of a solid molecular film and thus, produce a sufficient amount of degraded material for subsequent analysis, outside vacuum, of the compounds remaining on the inner surface of the cylinder. The electron energy is tunable from 3 to 130±0.5 eV and the current density adjustable up to 2.6±0.5×1012 electron s−1 cm−2, respectively. Choosing thymidine as a model system for deoxyribose nucleic acid damage, we show that nonvolatile fragments produced by 5–100 eV electron irradiation can be characterized by high-pressure liquid chromatography/ultraviolet detector and gas chromatography/mass spectroscopy.
Improved algorithm for the suppression of interference fringe in absorption spectroscopy
Authors: Faggin Marc F. , Hines Melissa A.
Published: October 2004

An improved computational method for the suppression of interference fringe in absorption spectroscopy has been developed. The interference fringe is isolated using a combination of digital filter techniques in the energy, or cm−1, domain and Fourier analysis in the wavelength, or cm, domain. The isolated fringe spectrum is then cleanly subtracted from the original spectrum. This process does not distort spectral features or the spectral base line. Applications of this technique to surface infrared spectroscopy are presented.
High-resolution transmission electron microscopy studies of planar defects in the magnetic superconductor RuSr2EuCu2O8
Authors: Vasiliev A. L. , Aindow M. , Han Z. H. , Budnick J. I. , Hines W. A. , Klamut P. W. , Maxwell M. , Dabrowski B.
Published: October 2004

The defect structure in powders of the 1212-type phase RuSr2EuCu2O8 was investigated using high-resolution transmission electron microscopy. Both 90° domain boundaries and planar “faults” on {100} were observed. By comparing images of the faults with simulations for various plausible atomic models, it was shown that they are crystallographic shear planes with a displacement vector of R=(1∕6)⟨332⟩. Such defects correspond to a local deficiency in Ru, Cu, and O, and give a local discontinuity in the Ru-O charge reservoir planes. As such, these defects may be responsible for the sensitivity of the superconducting and magnetic properties to processing conditions.