bluuebunny/crossref_metadata_2025 huggingface dataset:
Production of standardized air bubbles: Application to embolism studies
Authors: Palanchon Peggy , Klein Jan , de Jong Nico
Published: March 2003

Gaseous emboli may arise and enter into the circulation system during various clinical procedures. In order to better understand their immediate and long-term consequences, clinical investigations following the insertion of air bubbles into the body as well as new detection techniques need to be carried on. To this end, a device has been developed to generate a uniform stream of microbubbles with diameters ranging between 20 and 300 μm. This device comprises a glass micropipette connected to an air pressure source. The micropipette tip crosses a variable liquid flow and the bubbles produced are carried away by the flow. These bubbles have a very narrow size and density distribution: 90% of the bubbles lie within ±6% of the mean radius and the number of bubbles does not vary more than 10%. The size and density of the produced bubbles can be controlled by adjusting three independent parameters: the liquid flow, the gas pressure level, and the micropipette shape. For a given micropipette, increasing the liquid flow or decreasing the gas pressure level leads to a reduction of bubble size while the number of bubbles produced increases. As an example, doubling of the liquid flow results in a variation in bubble size up to 40%. This technique offers the advantage of generating uniform bubbles of known size and number depending on the settings selected. It appears to be a valuable tool for embolism studies such as the development of ultrasonic methods for detection of gaseous emboli.
Design of a compact synchrotron for medical applications
Authors: Al Harbi Nader , Lee S. Y.
Published: March 2003

An optimal design of a low energy (300 MeV) proton synchrotron for medical applications is addressed. The machine has the following properties: (1) The transition energy is higher than the targets final proton beam energy of 300 MeV; (2) the betatron tunes are chosen such that the machine is free of systematic resonances; (3) the machine can accommodate both slow and fast extraction systems; and (4) the machine can provide rapid cycling operations depending on the rf cavity voltage. Applications of this low energy synchrotron are discussed.
Separatrix shape measurement on field-reversed configuration plasmas
Authors: Gota Hiroshi , Akiyama Takahiko , Fujimoto Kayoko , Ohkuma Yasunori , Takahashi Tsutomu , Nogi Yasuyuki
Published: March 2003

In order to determine the separatrix shapes of field-reversed configuration plasmas with high accuracy, an iterative method that compares measured magnetic fluxes with the solution of the Grad–Shafranov equation is discussed in detail. Several suggestions for successfully treating the iterative method are given using numerical simulation and a mock-up experiment where conductors with three kinds of shape are inserted into the coil instead of the plasma. The iterative method is also applied to the field-reversed configuration plasma, and it is found that the separatrix shape has distinct ends and the axial location of the X point can be determined.
OPL-3D: A novel, portable optical digitizer for fast acquisition of free-form surfaces
Authors: Sansoni Giovanna , Patrioli Alessandro , Docchio Franco
Published: March 2003

The paper presents OPL-3D, a novel, powerful three-dimensional optical digitizer based on structured, incoherent light illumination, developed in our laboratory for noncontact acquisition and digitization of free forms in space. The system exploits active stereovision using time multiplexing based on a combination of Gray code and phase-shifting techniques. The system has been specifically designed for applications to reverse engineering and rapid prototyping of free-form shapes, as well as for applications to measurement and quality control. Suitable estimation of the measurement parameters, as well as specifically developed two-dimensional image analysis, allows us to retrieve very dense point clouds in a few seconds. Special care has been devoted to the implementation of easy-to-use fast calibration procedures, and to enhancement of the flexibility of the system to the measuring problem as well as its portability. A kernel for elaboration of the views and for their alignment is provided. OPL-3D exhibits low-measurement uncertainty (120 μm) over large measurement areas (450 mm×340 mm), linearly scalable in the case of smaller areas. The output formats of the data files are fully compatible with the formats commonly used by elaboration environments dedicated to the production of polygonal models and to computer-aided design models of the shapes. The performance of OPL-3D has been tested in a number of applications, ranging from industry to biomedicine and virtual reality.
A camera based position control of a suspended optical bench used in a gravitational wave detector
Authors: Beauville F. , Buskulic D. , Flaminio R. , Marion F. , Masserot A. , Massonnet L. , Mours B. , Ramonet J. , Tournefier E. , Verkindt D. , Veziant O. , Yvert M.
Published: March 2003

The VIRGO gravitational wave detector uses several optical benches suspended inside a large vacuum system. One of these benches is placed at the interferometer output port and it includes all of the optics required to collect and filter the interferometer dark fringe beam. The position of this bench is controlled with respect to ground by means of a charge coupled device camera and 400 light-emitting diodes placed outside the vacuum tank. The read-out sensitivity is a few 10−8 m/√Hz for the translations degrees of freedom and 10−7 rad/√Hz for the rotational degrees of freedom. Displacements as large as ±10 mm and rotations at the level of ±5 mrad can be measured. The feedback is based on a digital architecture and it has been fully tested at the VIRGO site. It is able to control all the six degrees of freedom of the bench with a precision better than 1 μm for the translations and 0.5 μrad for all the rotations.
Application of time-sliced ion velocity imaging to crossed molecular beam experiments
Authors: Lin Jim J. , Zhou Jingang , Shiu Weicheng , Liu Kopin
Published: March 2003

A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining conventional two-dimension velocity mapping, the third velocity component was mapped linearly to the ion time of flight. A weak extraction field was used to spread the ion turnaround time to several hundred nanoseconds, which permits good resolution for selection of the longitudinal velocity. A fast gated (⩾5 ns) intensified charge coupled device camera was used to record time-sliced ion images. Calibration of the apparatus was done by measuring O+ images from the multiphoton dissociation/ionization of O2. The resolution in velocity achieved was about 1% (Δv/v) in slicing through the center of a Newton sphere. The overall performance was examined by observing product ion images from the F+CD4→DF+CD3 reaction. To detect CD3+ with kinetic energy release of about 1 eV, 50 ns time slicing provides sufficient velocity resolution, such that resolution of the image is mainly limited by the spread in velocity of the two molecular beams. These ion optics can focus on a large volume of ion cloud, which is crucial in crossed molecular beam experiments. Direct 3D imaging also simplifies data analysis. This direct 3D ion imaging method provides a powerful tool with which to study systems with no cylindrical symmetry.
Reversible phase transition and structure memory effect of metastable phase in electron-irradiated poly(vinylidene-fluoride-trifluoroethyline) copolymers
Authors: Li W. P. , Tang Y. W. , Guo S. S. , Wang D. H. , Yang G. , Wang R. H. , Zhao X. Z.
Published: March 2003

We report the results of x-ray diffraction studies on phase transition behavior of irradiated poly(vinylidene-fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer. It is found that the transformations between the polar ferroelectric phase, the metastable paraelectric phase, and normal paraelectric phase induced by thermal and electric field are reversible. The reversible phase transition of the metastable paraelectric phase is very important to explain the appearance of giant strain in the irradiated P(VDF-TrFE) copolymer. The macroscopic strain caused by these reversible transitions was estimated.
Cu
(
In
1−x
Ga
x
)
Se
2
growth studies by in situ spectroscopic light scattering
Authors: Scheer R. , Neisser A. , Sakurai K. , Fons P. , Niki S.
Published: March 2003

The growth of polycrystalline Cu(In1−xGax)Se2 thin films using a three-stage deposition process has been investigated by in situ diffuse spectroscopic light scattering in the wavelength range of 400–800 nm. Differences in the time dynamics of the intensity at different wavelengths can be explained by the development of surface roughness at different length scales. Of specific interest is the development of roughness around the stoichiometric points of film formation. It is shown that on-line monitoring of the spectroscopic light scattering can be used for process control and for adjustment of the final film roughness.
Function principle of a relaxation oscillator based on a bistable quantum Hall device
Authors: Nachtwei G. , Kalugin N. G. , Sağol B. E. , Stellmach Ch. , Hein G.
Published: March 2003

We present a simple relaxation oscillator based on a quantum Hall device with Corbino geometry near the breakdown of the quantum Hall effect. In the hysteresis region of the breakdown, the quantum Hall device exhibits bistable behavior. If a resistance is connected in series and a capacitor in parallel to the quantum Hall device, the bistable switching leads to subsequent charging and discharging of the capacitor, detectable as relaxation oscillations. We explain the observed oscillations by solving Kirchhoff’s equations and obtain a good quantitative description of the experiment. From this, we deduce some dynamical parameters of the Corbino device and discuss the performance limits of the oscillator.
Shape and growth of InAs quantum dots on GaAs(113)A
Authors: Temko Y. , Suzuki T. , Jacobi K.
Published: March 2003

The shape of InAs quantum dots (QDs), grown by molecular-beam epitaxy on the GaAs(113)A surface, is studied by in situ scanning tunneling microscopy. At an early growth stage, the QDs adopt a shape given by {110}, (111)A, and {2 5 11}A bounding facets and an unresolved rounded {001} region. At a later growth stage, the QDs become elongated along [332̄], with a reduction of the (111)A facet size and a flattening of the rounded region. This is explained by facet growth kinetics. The broad size distribution indicates growth limitation. The symmetry of the substrate is retained in the QDs, proving epitaxial growth.
Electrical characterization of electrochemically grown single copper nanowires
Authors: Toimil Molares M. E. , Höhberger E. M. , Schaeflein Ch. , Blick R. H. , Neumann R. , Trautmann C.
Published: March 2003

Single- and poly-crystalline copper wires with diameters down to 30 nm are grown in etched ion-track membranes. Individual nanowires are isolated and contacted by means of optical lithography. Electronic transport properties and oxidation processes are investigated. Depending on the oxidation state, the wire resistance varies between a few hundred ohms and several megaohms, enabling its usage as metallic or semiconducting structural elements for devices on the nanometer scale.
Comparison of oxygen-chain models for late thermal double donors in silicon
Authors: Lee Y. J. , von Boehm J. , Pesola M. , Nieminen R. M.
Published: March 2003

The electronic and atomic structures of the oxygen chains assigned to late thermal double donors (TDDs) in silicon are studied using accurate total-energy calculations. We find that the ring-type O-chain model is best suited for TDDs and better than the di-Y-lid-type O-chain model. The ring-type O chains have slightly alternating C2v–C1h symmetry consistent with the recent high-field electron paramagnetic resonance experiments. The spin densities of the double-donor states are located outside the region of the O atoms, which makes the hyperfine interaction of an unpaired donor electron with the O17 nuclear spins very weak.
Cathodoluminescence and Hall-effect measurements in sulfur-doped chemical-vapor-deposited diamond
Authors: Nakazawa Kazushi , Tachiki Minoru , Kawarada Hiroshi , Kawamura Aki , Horiuchi Kenji , Ishikura Takefumi
Published: March 2003

Dominant n-type conductivity in sulfur-doped chemical-vapor-deposited diamond is observed by Hall-effect measurement. The activation energy is estimated at 0.5–0.75 eV above 600 K. Below 600 K, the carrier concentration deviates from the activation energy, and Hall mobility decreases in comparison with that above 600 K. It is considered that hopping conduction takes place. By cathodoluminescence measurement, free-exciton recombination radiation is observed in spite of a very high sulfur doping level of 2.5% during deposition, where boron is not detected by secondary ion mass spectroscopy. Therefore, the n-type conductivity of sulfur-doped diamond is caused by a sulfur-related mechanism.
Temperature- and injection-dependent lifetime spectroscopy for the characterization of defect centers in semiconductors
Authors: Schmidt Jan
Published: March 2003

A highly sensitive, contactless defect characterization technique, temperature- and injection-dependent lifetime spectroscopy (TIDLS), is introduced. In this method, the injection-level-dependent carrier lifetime curves of a semiconductor are measured at various temperatures by means of an inductively coupled resonant circuit. In order to illustrate certain features of the TIDLS, the lifetime-limiting defect center in aluminum-doped Czochralski silicon is analyzed. It is shown that, compared to previous lifetime spectroscopy techniques, TIDLS not only determines the defect energy level with improved accuracy, but is also capable of determining the temperature dependence of the electron and hole capture cross sections. In addition, the method permits the separation between shallow and deep-level centers.
Atomistic simulation of strain-induced amorphization
Authors: Lund Alan C. , Schuh Christopher A.
Published: March 2003

The process of solid-state amorphization through extensive plastic straining (i.e., mechanical alloying) is studied through molecular simulation of binary Cu–Zr alloys. Without such confounding factors as impurity pick-up, or such thermal effects as melting or diffusion, we find that amorphization can be driven solely by accumulation of crystal defects and interfacial roughening between the component phases. The tendency for glass formation is also seen to depend on composition, and the results are in line with extensive prior experimental work.
Layer-guided shear acoustic plate mode sensor
Authors: Newton M. I. , McHale G. , Martin F.
Published: March 2003

Experimental data are presented for an acoustic wave sensor based on a layer-guided shear acoustic plate mode excited on a thin quartz substrate. The effect of coating the front and back faces with polymer waveguiding layers is shown to convert the plate modes into layer-guided plate modes in a manner analogous to Love waves and to produce a similar enhancement of mass sensitivity. These layer-guided plate mode devices offer the possibility of liquid-phase sensing with transducers situated on the face of the substrate isolated from the liquid, while offering a mass sensitivity comparable to a Love wave device.
Determination of the charge carrier compensation mechanism in Te-doped GaAs by scanning tunneling microscopy
Authors: Gebauer J. , Weber E. R. , Jäger N. D. , Urban K. , Ebert Ph.
Published: March 2003

We identified the charge carrier compensation mechanism in Te-doped GaAs with atomically resolved scanning tunneling microscopy. Three types of defects were found: tellurium donors (TeAs), Ga vacancies (VGa), and Ga vacancy–donor complexes (VGa–TeAs). We show quantitatively that the compensation in Te-doped bulk GaAs is exclusively caused by vacancy–donor complexes in contrast to Si-doped GaAs. This is explained with the Fermi-level effect as the universal mechanism leading to Ga vacancy formation in n-doped GaAs, and a Coulomb interaction leading to the formation of the complexes. The quantification of the carrier compensation yields a −3e charge state of VGa in bulk GaAs.
Low-light divergence in photovoltaic parameter fluctuations
Authors: Shvydka Diana , Karpov V. G. , Compaan A. D.
Published: March 2003

We study statistics of the major photovoltaic (PV) parameters, such as open-circuit voltage, short-circuit current, etc., versus light intensity on a set of nominally identical thin-film CdTe/CdS solar cells. A crossover light intensity is found, below which the relative fluctuations of the PV parameters diverge inversely proportional to the square root of the light intensity. We propose a model in which the observed fluctuations are due to lateral nonuniformities in the device structure. The crossover is attributed to the lateral nonuniformity screening length exceeding the device size. From the practical standpoint, our study introduces a simple uniformity diagnostic technique.
Efficient 3–5-μm negative luminescence from HgCdTe/Si photodiodes
Authors: Lindle J. R. , Bewley W. W. , Vurgaftman I. , Meyer J. R. , Varesi J. B. , Johnson S. M.
Published: March 2003

The negative luminescence of a midwave-infrared HgCdTe photodiode (cutoff wavelength=5.3 μm at 295 K) was investigated using a modulated, self-referencing, optical technique. Internal efficiencies were measured to be 88% throughout the 3–5-μm spectral region, nearly independent of temperature in the 240–300-K range. This corresponds to an apparent temperature reduction of greater than 50 K at room temperature and greater than 35 K at 240 K when the reverse bias is applied. The reverse saturation current density of 1.3 A/cm2 is lower than any reported previously for a negative luminescence device with cutoff wavelength beyond 4 μm.
Superconducting MgB2 thin films on silicon carbide substrates by hybrid physical–chemical vapor deposition
Authors: Zeng X. H. , Pogrebnyakov A. V. , Zhu M. H. , Jones J. E. , Xi X. X. , Xu S. Y. , Wertz E. , Li Qi , Redwing J. M. , Lettieri J. , Vaithyanathan V. , Schlom D. G. , Liu Zi-Kui , Trithaveesak O. , Schubert J.
Published: March 2003

We have used two polytypes of silicon carbide single crystals, 4H-SiC and 6H-SiC, as the substrates for MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD). The c-cut surface of both polytypes has a hexagonal lattice that matches closely with that of MgB2. Thermodynamic calculations indicate that SiC is chemically stable under the in situ deposition conditions for MgB2 using HPCVD. The MgB2 films on both polytypes show high-quality epitaxy with a Rutherford backscattering channeling yield of 12%. They have Tc above 40 K, low resistivities, high residual resistivity ratios, and high critical current densities. The results demonstrate that SiC is an ideal substrate for MgB2 thin films.
High-electric-field photocurrent in thin-film ZnS formed by pulsed-laser deposition
Authors: Yano S. , Schroeder R. , Sakai H. , Ullrich B.
Published: March 2003

Photocurrent spectra of thin-film ZnS on glass fabricated by pulsed-laser deposition have been studied employing electric fields up to 20 kV/cm. The spectra show a shift towards lower energy at and beyond 10 kV/cm. By modeling the film absorption with the density of states and the Urbach rule, it is shown that, without the involvement of the Franz–Keldysh effect and excitonic transitions, the slope of the Urbach tail depends on the electric field, owing to impurity ionization.
Valence-band structure of self-assembled InAs quantum dots studied by capacitance spectroscopy
Authors: Bock C. , Schmidt K. H. , Kunze U. , Malzer S. , Döhler G. H.
Published: March 2003

Hole transport into self-assembled InAs quantum dots (QDs) embedded in a GaAs/AlAs matrix was studied by capacitance spectroscopy. From the differential capacitance, a Coulomb blockade energy of EC0h≈22 meV for holes in the ground state was extracted. When the front barrier between the dot layer and the Schottky contact is precisely reduced by selective wet chemical etching, the QD ground state signal shifts to lower gate voltages according to a simple leverage law. From the linear fit of the voltage shift versus the front barrier thickness the hole binding energy of E0h≈194 meV was determined.
Dynamics of causal beam refraction in negative refractive index materials
Authors: Cummer Steven A.
Published: March 2003

A finite difference simulation of a causally excited electromagnetic Gaussian beam incident on an interface between free space and a physically realizable negative refractive index material shows that negative refraction of finite beams does occur. A discontinuity in the phase normal direction is established very quickly when the beam front reaches the interface of positive and negative index materials. Once the beam enters the negative index material, the causal wave-front propates with the group velocity consistent with theory. During the transient portion of the beam development, the beam undergoes small direction changes, which can be explained as a consequence of higher frequencies present during the beam turn-on. After this transient, the theoretically predicted sinusoidal steady state is reached.
Significant enhancement of terahertz radiation from InSb by use of a compact fiber laser and an external magnetic field
Authors: Takahashi Hiroshi , Suzuki Yuji , Sakai Masahiro , Ono Shingo , Sarukura Nobuhiko , Sugiura Toshiharu , Hirosumi Tomoya , Yoshida Makoto
Published: March 2003

We investigated the magnetic-field dependence of terahertz (THz) radiation power from InSb. Significant enhancement of THz-radiation power is observed by using a compact fiber laser that delivered 100 fs optical pulses at a center wavelength of 1560 nm. Additionally, applying external magnetic fields dramatically enhanced the THz-radiation power. THz-radiation power reaches a maximum value at around 1.2 T, and its enhancement factor exceeds 100. From an applications viewpoint, this is a significant finding for practical light source design, since it is easily achieved by using a compact fiber laser and a conventional magnet.
Random networks of carbon nanotubes as an electronic material
Authors: Snow E. S. , Novak J. P. , Campbell P. M. , Park D.
Published: March 2003

We report on the transport properties of random networks of single-wall carbon nanotubes fabricated into thin-film transistors. At low nanotube densities (∼1 μm−2) the networks are electrically continuous and behave like a p-type semiconductor with a field-effect mobility of ∼10 cm2/V s and a transistor on-to-off ratio ∼105. At higher densities (∼10 μm−2) the field-effect mobility can exceed 100 cm2/V s; however, in this case the network behaves like a narrow band gap semiconductor with a high off-state current. The fact that useful device properties are achieved without precision assembly of the nanotubes suggests the random carbon nanotube networks may be a viable material for thin-film transistor applications.
Origin of high-temperature ferromagnetism in (Ga,Mn)N layers grown on 4H–SiC(0001) by reactive molecular-beam epitaxy
Authors: Dhar S. , Brandt O. , Trampert A. , Däweritz L. , Friedland K. J. , Ploog K. H. , Keller J. , Beschoten B. , Güntherodt G.
Published: March 2003

We report on the growth, structural as well as magnetic characterization of (Ga,Mn)N epitaxial layers grown directly on 4H–SiC(0001) by reactive molecular-beam epitaxy. We focus on two layers grown under identical conditions except for the Mn/Ga flux ratio. Structural characterization reveals that the sample with the lower Mn content is a uniform alloy, while in the layer with the higher Mn content, Mn-rich clusters are found to be embedded in the (Ga,Mn)N alloy matrix. Although the magnetic behavior of both the samples is similar at low temperatures, showing antiferromagnetic characteristics with a spin-glass transition, the sample with higher Mn content additionally exhibits ferromagnetic properties at and above room temperature. This ferromagnetism most likely originates from the Mn-rich clusters in this sample.
Atomistic modeling of amorphization and recrystallization in silicon
Authors: Pelaz Lourdes , Marqués Luis A. , Aboy Marı́a , Barbolla Juan , Gilmer George H.
Published: March 2003

We propose an atomistic model to describe the evolution of the damage generated by irradiation in Si, going from isolated point defects to the formation of continuous amorphous layers. The elementary units used to reproduce the defective zones are Si interstitials, vacancies and the bond defect, which is a local distortion of the Si lattice without any excess or deficit of atoms. More complex defect structures can be formed as these elementary units cluster. The amorphous pockets are treated as agglomerates of bond defects characterized by their local coordination. The model is able to reproduce the abrupt regime in the crystal-amorphous transition in Si and the epitaxial recrystallization upon annealing as observed in the experiments. The model extends the atomistic kinetic Monte Carlo simulation technique to high implant doses, adequately describing the amorphization and regrowth in Si.
Josephson device for simultaneous time and energy detection
Authors: Esposito E. , Ejrnaes M. , Pagano S. , Perez de Lara D. , Cristiano R.
Published: March 2003

The development of a detection device for simultaneous measurement of energy and impact time, to be used in time-of-flight mass spectrometry, is reported. In this device, two superconducting tunnel junctions are coupled through a passive network. The first junction operates in the quasiparticle regime in order to measure the energy of a molecule impact and to act as a proportional detector. The second one operates in the Josephson regime in order to act as a fast discriminator for the impact time of a molecule impinging on the detector junction. In this way, a very accurate time determination can be achieved limited only by the intrinsic detector response, thus improving the spectrometer mass resolution. To demonstrate the feasibility of this detection scheme in mass spectrometry, calibration measurements have been carried out using a Fe55 x-ray source to simulate the molecule impact. The experimental results successfully demonstrated simultaneous detection of energy and arrival time in coincidence with photon impacts, with a time resolution limited, in practice, only by the electronics used.
Ge-fraction-dependent metal-induced lateral crystallization of amorphous-Si1−xGex (0≦x≦1) on SiO2
Authors: Kanno Hiroshi , Tsunoda Isao , Kenjo Atsushi , Sadoh Taizoh , Miyao Masanobu
Published: March 2003

Metal-induced low-temperature (≦550 °C) crystallization of amorphous-Si1−xGex (0≦x≦1) on SiO2 has been investigated. In the case of low Ge fraction (0≦x≦0.2), Ge-doping enhanced plane growth was observed. This achieved strain-free poly-Si0.8Ge0.2 with large grains (18 μm). On the other hand, dendrite growth became dominant in the case of intermediate Ge fractions (0.4≦x≦0.6). By optimizing the growth conditions (x: 0.4, annealing: 450 °C, 20 h), very sharp needle-like crystal regions (width: 0.05 μm, length: 10 μm) were obtained. These polycrystalline SiGe films on SiO2 should be used for the system-in-display, three-dimensional ultralarge scale integrated circuits, and novel one-dimensional wires.
Color variation of ZnGa2O4 phosphor by reduction-oxidation processes
Authors: Kim J. S. , Kang H. I. , Kim W. N. , Kim J. I. , Choi J. C. , Park H. L. , Kim G. C. , Kim T. W. , Hwang Y. H. , Mho S. I. , Jung M.-C. , Han M.
Published: March 2003

The color of the emission of zinc gallate (ZnGa2O4) oscillates between ultraviolet and blue by hydrogen ambient reduction and air ambient oxidation heat treatments. The photoluminescence spectra and electron paramagnetic resonance signals show that ultraviolet emission of reduced ZnGa2O4 always accompanies 680 nm emission originating from single oxygen vacancies (VO*). The increasing difference in binding energy between Ga3+ and O2− in reduced ZnGa2O4 indicates that the configuration of octahedral sites is distorted due to VO* generation and it becomes more ionic which shifts the emission band from 430 to 360 nm. The x-ray diffraction patterns and Raman scattering spectra show that β-Ga2O3 from ZnGa2O4 is formed in both reduction and oxidation processes which suggests the vaporization of Zn ions. We propose a model in which the origin of 360 nm emission is the Ga–O transition at distorted octahedral sites with VO* in ZnGa2O4, whereas 430 nm emission originates from the Ga–O transition of regular octahedral sites without VO* in ZnGa2O4.
Electron tunneling from channel to gate
Authors: Price Peter J.
Published: March 2003

A general treatment based on the Gamow formulation of tunneling escape through an enclosing barrier is developed for analysis of the escape of electrons from the conducting channel of a field effect transistor through the barrier layer into the gate. The result gives the tunneling rate calculated from computed wave functions of the quantum levels, without requiring these to correspond to quasiclassical motion in the channel region, but rather for the general case including the ground state.
Valence band alignment with a small spike at the CuI/CuInS2 interface
Authors: Konovalov I. , Szargan R.
Published: March 2003

From the point of view of the “doping pinning rule,” application of p-type buffer layer materials for CuInS2 solar cells may lead to record levels of cell efficiency due to an optimal band offset at the interface. Under simplified simulation conditions, an increase in efficiency of up to about 18% was predicted. Evaluation of the valence band offset between CuI and single crystalline CuInS2 by photoelectron spectroscopy displays a spike height of (0.1±0.2) eV, optimal for solar cell applications. The positions of the valence band edge were determined by assuming a parabolic distribution of the density of states.
Investigation of trap processes in polycrystalline silicon thin film transistors by ac measurement
Authors: Yan Feng , Migliorato Piero , Shimoda Tatsuya
Published: March 2003

We report a method for characterizing trap generation–recombination processes in polycrystalline silicon thin film transistors. A small ac voltage was superimposed on a dc gate voltage and the ac current was measured at the source. A theoretical model was developed, whereby n-channel thin film transistors were analyzed. A resonant peak in the imaginary part of the ac current and a corresponding step in the real part were found at frequency of 147 Hz. Our data indicate that the ac response is dominated by a single trap level. By combining the ac current measurement with a low frequency capacitance measurement, we have determined the trap energy, capture cross section, and trap density to be, respectively, 0.91 eV above the valence band, 3.1×10−21 cm2, and 5.7×1015 cm−3.
Doping diagnosis by evaluation of the surface Fermi level using scanning Maxwell-stress microscopy
Authors: Matsukawa Takashi , Kanemaru Seigo , Masahara Meishoku , Nagao Masayoshi , Tanoue Hisao , Itoh Junji
Published: March 2003

A microscopic approach for evaluation of the Fermi level on the silicon surface under various doping conditions is reported. We have utilized scanning Maxwell-stress microscopy (SMM) for measurement of the surface potential. The surface Fermi level evaluated by SMM was almost identical to the bulk Fermi level determined by doping when the surface states are passivated by thermal oxide. The SMM measurement on the pn junction under an electrically shorted condition exhibited contrast in potential which is consistent with that of the bulk Fermi level. The results for the open circuit condition exhibited an inconsistent potential profile, which is due to the photovoltaic effect from light irradiation of the sample.
Observation of interface defects in thermally oxidized SiC using positron annihilation
Authors: Dekker James , Saarinen Kimmo , Ólafsson Halldór , Sveinbjörnsson Einar Ö.
Published: March 2003

Positron annihilation has been applied to study thermally oxidized 4H- and 6H-SiC. The SiC/SiO2 interface is found to contain a high density of open-volume defects. The positron trapping at the interface defects correlates with the charge of the interface determined by capacitance–voltage experiments. For oxides grown on n-SiC substrates, the positron annihilation characteristics at these defects are nearly indistinguishable from those of a silicon/oxide interface, with no discernable contribution from C-related bonds or carbon clusters. These results indicate that those defects at the SiC/oxide interface, which are visible to positrons, are similar to those at the Si/oxide interface. The positron annihilation characteristics suggest that these defects are vacancies surrounded by oxygen atoms.
Current control for magnetized plasma in direct-current plasma-immersion ion implantation
Authors: Tang Deli , Chu Paul K.
Published: March 2003

A method to control the ion current in direct-current plasma-immersion ion implantation (PIII) is reported for low-pressure magnetized inductively coupled plasma. The ion current can be conveniently adjusted by applying bias voltage to the conducting grid that separates plasma formation and implantation (ion acceleration) zones without the need to alter the rf input power, gas flux, or other operating conditions. The ion current that diminishes with an increase in grid bias in magnetized plasmas can be varied from 48 to 1 mA by increasing the grid voltage from 0 to 70 V at −50 kV sample bias and 0.5 mTorr hydrogen pressure. High implantation voltage and monoenergetic immersion implantation can now be achieved by controlling the ion current without varying the macroscopic plasma parameters. The experimental results and interpretation of the effects are presented in this letter. This technique is very attractive for PIII of planar samples that require on-the-fly adjustment of the implantation current at high implantation voltage but low substrate temperature. In some applications such as hydrogen PIII-ion cut, it may obviate the need for complicated sample cooling devices that must work at high voltage.
Variation of domain formation in a 15 nm NiFe layer exchange coupled with NiO layers of different thicknesses
Authors: Liu Z. Y. , Adenwalla S.
Published: March 2003

The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a gradual decrease. This variation of domain wall formation in the NiFe layer with the NiO thickness possibly reflects the variation of the domain structure in the NiO layer through interfacial exchange coupling.
Effect of Be++O+ coimplantation on Be acceptors in GaN
Authors: Nakano Yoshitaka , Kachi Tetsu , Jimbo Takashi
Published: March 2003

P-type regions were produced in undoped GaN films by Be+ and Be++O+ implantation and subsequent annealing at temperatures between 1000 and 1050 °C. From thermal admittance spectroscopic measurements, the activation energy of the Be acceptor level was found to decrease from ∼240 to ∼163 meV by the implantation of additional O atoms, which is in reasonable agreement with the improvement in p-type doping characteristics determined by room-temperature Hall-effect measurements. These results indicate that Be++O+ coimplantation reduces the depth of the Be acceptor level based on a site-competition effect. Therefore, these acceptor levels are most probably attributable to Be atoms at interstitial and Ga-lattice sites.
Magnetism in cobalt-doped Cu2O thin films without and with Al, V, or Zn codopants
Authors: Kale S. N. , Ogale S. B. , Shinde S. R. , Sahasrabuddhe M. , Kulkarni V. N. , Greene R. L. , Venkatesan T.
Published: March 2003

Thin films of 5% Co doped Cu2O were grown on single-crystal (001) MgO substrates by pulsed-laser deposition, without and with 0.5% codoping with Al, V, or Zn. Structural, electrical, and magnetic properties were studied. The films showed phase-pure character under the chosen optimum growth conditions. Spin-glass-like behavior was observed in Co-doped films without codoping. A clear ferromagnetic signal at room temperature was found only in the case of Co:Cu2O films codoped with Al.
Damping as a function of pulsed field amplitude and bias field in thin film Permalloy
Authors: Nibarger J. P. , Lopusnik R. , Silva T. J.
Published: March 2003

We have measured the step response in thin film Permalloy as a function of both a hard-axis pulsed field amplitude and an easy-axis longitudinal magnetic bias field using a pulsed inductive microwave magnetometer. The bias field ranged from 0 to 8000 A/m (0 to 100 Oe) and the pulsed field varied from 0.32 to 320 A/m (0.004 to 4 Oe). The rotation angle of the equilibrium magnetization direction varied from 0.002° to 40° for this range of field values. Data were analyzed to extract the Gilbert damping parameter, α. The damping parameter decreased monotonically with an increase in longitudinal bias field. However, there is no observed dependence of α on the pulse amplitude, indicating that the damping is independent of the angle of rotation. We conclude that there is no significant nonlinear generation of spin waves that affects the damping in the case of free induction decay for the range of field pulses employed.
Strain-induced band gap shrinkage in Ge grown on Si substrate
Authors: Ishikawa Yasuhiko , Wada Kazumi , Cannon Douglas D. , Liu Jifeng , Luan Hsin-Chiao , Kimerling Lionel C.
Published: March 2003

Band gap shrinkage induced by tensile strain is shown for Ge directly grown on Si substrate. In Ge-on-Si pin diodes, photons having energy lower than the direct band gap of bulk Ge were efficiently detected. According to photoreflectance measurement, this property is due to band gap shrinkage. The origin of the shrinkage is not the Franz–Keldysh effect but rather tensile strain. It is discussed that the generation of such a tensile strain can be ascribed to the difference of thermal expansion between Ge and Si. Advantages of this tensile Ge for application to photodiode are also discussed.
Second-order generation of point defects in gamma-irradiated float-zone silicon, an explanation for “type inversion”
Authors: Pintilie I. , Fretwurst E. , Lindström G. , Stahl J.
Published: March 2003

Radiation-induced defects in silicon diodes were investigated after exposure to high doses of Co60-gamma irradiation using the thermally stimulated current method. We have found that, for high irradiation doses, a second-order defect can be detected. This defect is largely suppressed in oxygen-enriched material while it is the main cause for the space charge sign inversion effect observed in standard float-zone material.
Optical properties of single-crystalline ZnO nanowires on m-sapphire
Authors: Ng Hou Tee , Chen Bin , Li Jun , Han Jie , Meyyappan M. , Wu J. , Li S. X. , Haller E. E.
Published: March 2003

ZnO nanowires have been synthesized using a catalyst-assisted heteroepitaxial carbothermal reduction approach on a m-sapphire substrate. Intricate and uniform arrays have been obtained with each nanowire forming an angle ∼30° with the substrate normal. Photoluminescence studies at room temperature for wavelengths between 335 and 620 nm reveal a strong single exciton peak at ∼380 nm (3.26 eV) with accompanying deep-level blueshifted emission peaks at ∼486, 490, and 510 nm. UV resonant Raman spectroscopy has been used to characterize the nanowires at room temperature with multiphonon scattering exhibiting phonon quantum confinement.
High critical-current density in robust MgB2/Mg nanocomposites
Authors: Li Qiang , Gu G. D. , Zhu Y.
Published: March 2003

A robust and high critical-current density (Jc>1 MA/cm2 at 5 K and self-field) superconducting composite based on MgB2 has been developed. The composite consists of 75% MgB2 matrix and 25% dispersed Mg on the scale of tens of nanometers. Transmission electron microscope investigation revealed that the composites are extremely dense, and consist of nanosized MgB2 grains connected by the clean strongly coupled grain boundaries. The nanodispersed Mg can blunt crack tips and provides regions for plastic flow. Both magnetization and magneto-optical studies demonstrated that the composite is free of any weak links. Furthermore, the addition of Mg in the matrix substantially reduces the flux jumps commonly observed in the monolithic MgB2 superconductors.
Detection of electrical spin injection by light-emitting diodes in top- and side-emission configurations
Authors: Fiederling R. , Grabs P. , Ossau W. , Schmidt G. , Molenkamp L. W.
Published: March 2003

Detection of the degree of circular polarization of the electroluminescence of a light-emitting diode (LED) fitted with a spin injecting contact (a spin-LED) allows the direct determination of the spin polarization of the injected carriers. Here, we compare the detection efficiency of (Al,Ga)As spin-LEDs fitted with a (Zn,Be,Mn)Se spin injector in top- and side-emission configurations. In contrast to top emission, we cannot detect the electrical spin injection in side emission by analyzing the degree of circular polarization of the electroluminescence. To reduce resonant optical pumping of quantum-well excitons in side emission, we have analyzed structures with mesa sizes as small as 1 μm.
Density functional theory analysis of infrared modes in carbon-incorporated SiO2
Authors: Petersen Max , Schulberg Michelle T. , Gochberg Lawrence A.
Published: March 2003

Low dielectric constant (low k) materials become increasingly important for microelectronics as interconnect delays limit circuit performance. One of the most promising low k candidates is carbon-incorporated SiO2 (SiOC:H). Despite the technological relevance of these materials, little is known about their atomistic structure. In order to address this issue, this letter compares structures obtained from the density functional theory to experimental data. This is done by analyzing both the calculated vibrational modes and experimental Fourier-transform infrared spectra. The resulting agreement is excellent. This can allow for future modeling of mechanical and electronic properties of low k dielectrics.
High-efficiency polymer light-emitting diodes with stable saturated red emission based on blends of dioctylfluorene-benzothiadiazole-dithienylbenzothiadiazole terpolymers and poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene]
Authors: Niu Yu-Hua , Hou Qiong , Cao Yong
Published: March 2003

High-efficiency light-emitting diodes with stable saturated red emission are realized based on blends of poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEHPPV) and terpolymers of dioctylfluorene, benzothiadiazole, and dithienylbenzothiadiazole (DBT). External quantum efficiency of more than 3.2 emitted photons per hundred electrons injected (ph/el %) is realized for a device with the Commission Internationale de I’Eclairage coordinates x=0.65, y=0.35. Such high device performance results from effective hole injection induced by the addition of MEHPPV, efficient energy transfer from the [0-0] ground-state emission of MEHPPV to the DBT segments together with the direct contribution of the [0-1] phonon coupling emission of MEHPPV.
Strain manipulation in BaTiO3/SrTiO3 artificial lattice toward high dielectric constant and its nonlinearity
Authors: Kim Leejun , Jung Donggeun , Kim Juho , Kim Young Sung , Lee Jaichan
Published: March 2003

Enhancement of dielectric properties has been demonstrated in BaTiO3 (BTO)/SrTiO3 (STO) strained artificial lattice. Large variation of lattice distortion in the consisting BTO and STO layers was achieved by varying a stacking sequence of BTO and STO layers. From this strain manipulation, it is found that BTO and STO lattices have a dielectric constant sensitively influenced by the lattice distortion and, more importantly, maximum dielectric constant at a certain degree of lattice distortion. An appropriate degree of lattice distortions of the consisting layers is needed to obtain the large dielectric constant and its nonlinearity of the artificial lattice. The artificial lattice exhibited the large dielectric constant (1230) and extremely large nonlinearity (94%) at the periodicity of BTO2 unit cell/STO2 unit cell. These results suggest that the strain is a macroscopically important factor to influence the dielectric properties and can be manipulated via oxide artificial lattice to obtain large dielectric constant and its nonlinearity.
Stress-induced suppression of piezoelectric properties in PbTiO3:La thin films via scanning force microscopy
Authors: Kholkin A. L. , Shvartsman V. V. , Emelyanov A. Yu. , Poyato R. , Calzada M. L. , Pardo L.
Published: March 2003

Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity of the SFM tip.
Thermochemical description of dielectric breakdown in high dielectric constant materials
Authors: McPherson J. , Kim J-Y. , Shanware A. , Mogul H.
Published: March 2003

A thermochemical/molecular model is developed for breakdown in high dielectric constant materials and the model suggests that a fundamental relationship exists between dielectric breakdown strength (Ebd) and dielectric constant (k). The model indicates that Ebd should show an approximate (k)−1/2 dependence over a wide range of high dielectric constant materials. The model also predicts that the field-acceleration parameter (γ), from time-dependent dielectric breakdown (TDDB) testing, should increase with dielectric constant. TDDB and Ebd data are presented for model support. The thermochemical model suggests that the very high local electric field (Lorentz-relation/Mossotti-field) in high-k dielectrics tends to distort/weaken the polar molecular bonds making them more susceptible to bond breakage by standard Boltzmann processes and/or by hole capture and thus lowers the breakdown strength.
Application of convergent beam electron diffraction to two-dimensional strain mapping in silicon devices
Authors: Armigliato A. , Balboni R. , Carnevale G. P. , Pavia G. , Piccolo D. , Frabboni S. , Benedetti A. , Cullis A. G.
Published: March 2003

A method of obtaining quantitative two-dimensional (2D) maps of strain by the convergent beam electron diffraction technique in a transmission electron microscope is described. It is based on the automatic acquisition of a series of diffraction patterns generated from digital rastering the electron spot in a matrix of points within a selected area of the sample. These patterns are stored in a database and the corresponding strain tensor at each point is calculated, thus yielding a 2D strain map. An example of application of this method to cross-sectioned cells fabricated for the 0.15 μm technology of flash memories is reported.