bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromigration drift velocity in Cu interconnects modeled with the level set method
Authors: Nathan M. , Glickman E. , Khenner M. , Averbuch A. , Israeli M.
Published: July 2002

Electromigration (EM) drift velocity (DV) experiments in polycrystalline pure Cu lines are simulated numerically with the level set method. The simulation is based on a grain boundary (GB) grooving model, incorporating an electric field. The model is distinguished by two key requirements imposed at the triple point where two surfaces and a GB meet: that of GB and surface flux coupling (flux continuity), and that of permanent equilibrium between surface and GB tensions. Surface diffusion exists only at the advancing cathode edge, and is driven both by local curvature gradients and by the local field. Using independent, literature diffusivity values, the simulation yields both the DV prefactor and the EM activation energy in an Arrhenius-type expression. An excellent match is obtained with experimental DV values in the T range of 573–723 K. Some implications regarding the material transport mechanism are discussed.
Charge diffusion noise in monocrystalline PbS nanoparticle films
Authors: Otten F. , Kish L. B. , Granqvist C.-G. , Vandamme L. K. J. , Vajtai R. , Kruis F. E. , Fissan H.
Published: July 2002

Charge transport in monocrystalline lead sulfide (PbS) nanoparticle films is investigated by current noise measurements. Monocrystalline and single-sized PbS nanoparticles are synthesized via the gas phase and deposited electrostatically onto semiconducting (GaAs) and on isolating (SiNx) substrates with planar electrode contacts. The particles form inhomogeneous films. Low-frequency current noise of 1-ML-thick films are measured at various voltages, exhibiting diffusion noise characteristics, which indicates a random-walk (diffusion) phenomenon of electrons between the particles.
Ultrafast carrier dynamics around nanoscale Schottky contacts studied by femtosecond far- and near-field optics
Authors: Achermann M. , Siegner U. , Wernersson L.-E. , Keller U.
Published: July 2002

Far- and near-field femtosecond pump–probe spectroscopy has been used to study ultrafast carrier dynamics in matrices of nanometer-scale tungsten disks embedded in GaAs. These studies reveal the dynamics of carrier transfer from a semiconductor into embedded metal clusters in the presence of Schottky contacts and built-in electric fields. The carrier transfer involves transport towards and trapping into the tungsten disks. We find picosecond time constants at higher carrier densities when the built-in field is screened, allowing for efficient carrier transport. Near-field measurements reveal the spatial variation of carrier dynamics. The spatially averaged dynamics can be controlled by the tungsten disk spacing.
Effects of dry processing on the optical properties of GaN
Authors: Cheung R. , Reeves R. J. , Brown S. A. , van der Drift E. , Kamp M.
Published: July 2002

Effects of dry processing on the band-edge, blue, and yellow luminescence behavior of GaN surfaces exposed to SF6 and Ar plasmas have been investigated. Our results indicate that a higher concentration of donor-related defects are introduced on the top 100 nm GaN surface after Ar compared to SF6 plasma treatment. The higher defect concentration can be correlated to the yellow luminescence. In particular, the sample exposed to Ar plasma exhibits a factor of 20 increase in yellow luminescence intensity compared to the unetched sample. This effect is attributed to the introduction of defects—metastable states on the GaN surface after ion bombardment. Using a laser beam in the ultraviolet region, we have been able to write to and read from regions on the Ar-bombarded GaN surface. We demonstrate that the presence of reactive ion etch-induced metastable defects is central to the observed enhanced optical memory effect. The metastable states are identified as nitrogen deficient surfaces limited to the top few monolayers, as well as unidentified defect propagation down to 100 nm from the etched surface.
Midinfrared photoluminescence of InAsSb quantum dots grown by liquid phase epitaxy
Authors: Krier A. , Huang X. L. , Hammiche A.
Published: July 2002

Photoluminescence in the 2–5 μm spectral region is reported from InAs1−xSbx quantum dots grown from the liquid phase at 580 °C on an InAs (100) substrate. Atomic force microscopy shows that coalesced quantum dots and then isolated quantum dots are formed with increasing Sb composition (x=0.2–0.3) and strain. The 4 K photoluminescence of the isolated and coalesced quantum dots was observed to peak in the midinfrared at 289 and 316 meV, (4.29 and 3.92 μm), respectively. These peaks are due to type II transitions and begin to quench at temperatures above 100 K as holes become thermally activated out of the quantum dot confinement potential.
C-MoS
2
and C-WS2 nanocomposites
Authors: Hsu W. K. , Zhu Y. Q. , Kroto H. W. , Walton D. R. M. , Kamalakaran R. , Terrones M.
Published: July 2002

Composite C-MoS2 and C-WS2 nanostructures were produced by pyrolysing H2S/N2 over a mixture of MoO3 (or WO3) and carbon particles/nanotubes. MoS2 particles embedded in the nanotubes were also observed occasionally.
Influence of output waveguide on mode quality factor in semiconductor microlasers with an equilateral triangle resonator
Authors: Huang Yong-Zhen , Guo Wei-Hua , Wang Qi-Ming
Published: July 2002

Semiconductor microlasers with an equilateral triangle resonator (ETR) and an output waveguide are proposed and analyzed by the finite-difference time-domain technique and the Padé approximation. The numerical results show that microlasers with an output waveguide still have a high-quality factor (Q factor) and are suitable to realize directional emission. For the ETR with a 0.46-μm-width opening in one of the vertices connected to the output waveguide, we have the Q factor of 1.5×103 and 2.5×102 for the TM fundamental mode at the wavelength of 1.55 μm, as the side length of the ETR is 5 and 3 μm. The simulated intensity distributions are presented for the fundamental mode in the ETR with a side length of 3 μm and an opening of 0.23 μm.
Separation of cascaded and direct fifth-order Raman signals using phase-sensitive intrinsic heterodyne detection
Authors: Golonzka O. , Demirdöven N. , Khalil M. , Tokmakoff A.
Published: July 2002

Cascaded third-order Raman signals can be separated from direct two-dimensional fifth-order Raman signals using heterodyne detection with a local oscillator that is cycled from in-phase to out-of-phase with the direct signal. The difference of these scans gives the direct signal, while the sum gives the in-quadrature cascaded signal.
Stabilization of iron-rich NdFe13−xBex compounds
Authors: Shaheen S. A. , Zhang C. , Mohanty R. C. , Murugaiah A. , Raman A. , Grenier C. G. , Ferrell R. E.
Published: July 2002

We have synthesized a new Fe-rich material, NdFe13−xBex (x=2–5) that crystallizes in the NaZn13 type cubic structure. Structural and magnetic properties of this material have been investigated. Stabilization of the single phase NaZn13 type cubic structure is limited to x=2–4. The magnetic properties of this system are quite complex, as anomalously low magnetic moments are observed in this system, and no saturation is achieved in M versus μ0H measurements in fields up to 5 T. The magnetization decreases as the content of Fe increases; such behavior is typical for the Invar alloys.
Optical characterization of the director profile in a ferroelectric liquid crystal cell with homeotropic alignment
Authors: Hodder B. , Yang F. , Sambles J. R.
Published: July 2002

The optic tensor configurations (director profiles) in surface stabilized ferroelectric liquid crystal cells with homeotropic alignment are considerably simpler than in cells with the more conventional homogeneous alignment and, with the exception of thin surface regions, are essentially that of an unconstrained or “free” FLC sample. Previously the half-leaky guided mode (HLGM) technique, in which angle dependent reflectivity only is measured, has been used to characterize the director profile in such cells. The extended HLGM technique reported in this paper enables detection of both angle dependent reflectivity and transmissivity data. Combined analysis of these features significantly enhances the sensitivity of the original HLGM technique enabling the director profile in cells with homeotropic alignment to be explored in even greater detail.
Spin-polarized current switching of a Co thin film nanomagnet
Authors: Albert F. J. , Katine J. A. , Buhrman R. A. , Ralph D. C.
Published: July 2002

A thin film Co nanomagnet in the shape of an elongated hexagon has been incorporated in a vertical device structure consisting of the nanomagnet and a thin Cu spacer layer formed on top of a thick Co film. The spin-polarized current flowing between the nanomagnet and the Co film is used to abruptly switch the magnetic alignment of the nanomagnet relative to that of the thick Co layer by the transfer of spin angular momentum from the conduction electrons to the nanomagnet moment. The shape anisotropy in the nanomagnet promotes the single domain behavior required for nonvolatile memory applications.
Sensitivity of a micromechanical displacement detector based on the radio-frequency single-electron transistor
Authors: Blencowe Miles P. , Wybourne Martin N.
Published: July 2002

We investigate the tunneling shot noise limits on the sensitivity of a micromechanical displacement detector based on a metal junction, radio-frequency single-electron transistor (rf SET). In contrast with the charge sensitivity of the rf-SET electrometer, the displacement sensitivity improves with increasing gate voltage bias and, with a suitably optimized rf SET, displacement sensitivities of 10−6 Å/Hz may be possible.
Excitonic properties of MgS/ZnSe quantum wells
Authors: Urbaszek B. , Balocchi A. , Bradford C. , Morhain C. , O’Donnell C. B. , Prior K. A. , Cavenett B. C.
Published: July 2002

Magnetic field and temperature dependent measurements are used to study the excitonic properties of high quality ZnSe quantum wells in MgS barriers grown by molecular beam epitaxy. The small inhomogeneous broadening of the samples allows the observation of higher excited exciton states. Due to the large difference in band gap between ZnSe and MgS a value of 43.9 meV was measured for the exciton binding energy which is the largest reported for this material system. The full width at half maximum of the heavy hole transitions is measured as a function of temperature and the broadening of the excitonic transitions in narrow quantum wells is reduced compared to the ZnSe bulk value due to the expected reduction in the LO-phonon scattering.
Near-field aperture fabricated by solid–solid diffusion
Authors: Suzuki Yoshimasa , Fuji Hiroshi , Tominaga Junji , Nakano Takashi , Atoda Nobufumi
Published: July 2002

It is well known that some metals (such as Ag) can diffuse into chalcogenides (such as Te) to produce alloys at room temperature. Using this solid–solid diffusion, we propose a technique to fabricate a near-field probe. Some basic experiments were carried out by this technique, and we obtained images on both the atomic-force microscope and scanning near-field optical microscope at the same time. Furthermore, it was found that the shading of Te is superior to that of other metals such as Ag and Al.
Optical parametric amplification in composite polymer/ion exchanged planar waveguide
Authors: Khalil M. Alshikh , Vitrant G. , Raimond P. , Chollet P. A. , Kajzar F.
Published: July 2002

A simple hybrid polymer/ion exchanged planar optical waveguiding structure has been fabricated which allows us to realize phase matching between different guided modes without any corrugation or modulation of the structure. A nonlinear polymer is deposited on top of glass substrate with a waveguide realized by the ion-exchange technique. Efficient optical parametric amplification is demonstrated for a pump at 532 nm and a signal at 814 nm. A 1 dB internal optical gain is achieved after propagation on 5 mm, with a pump power of 15 kW/mm.
Accelerated and reproducible oxidation of strain-compensated short-period superlattice structures for incorporation in InP based devices
Authors: Koley B. , King O. , Johnson F. G. , Saini S. S. , Dagenais M.
Published: July 2002

An accelerated oxidation process has been demonstrated in InAs/AlAs-based strain-compensated ultrashort-period superlattices grown on an InP substrate. It has been observed that the uniformity as well as the rate of the oxidation process in the strain-compensated short-period superlattice depends on the composition of the surrounding semiconductor layers. A suitable layer structure has been designed to obtain accelerated and reproducible oxidation rate in InP based optoelectronic devices.
Spectroscopy of competing mechanisms generating stimulated emission in gallium nitride
Authors: Herzog W. D. , Bunea G. E. , Ünlü M. S. , Goldberg B. B. , Molnar R. J.
Published: July 2002

Two competing recombination mechanisms of stimulated emission in the vicinity of 145 K have been directly observed in the temperature dependence of the optical emission spectra for high-quality, unintentionally doped gallium nitride. Our analysis of the spectra indicates that exciton-exciton scattering is responsible for stimulated emission below 145 K, while at higher temperatures an electron-hole plasma becomes the dominant mechanism.
Scanning probe energy loss spectroscopy: Angular resolved measurements on silicon and graphite surfaces
Authors: Eves B. J. , Festy F. , Svensson K. , Palmer R. E.
Published: July 2002

We report angle resolved “scanning probe energy loss spectroscopy” measurements from Si(111)-7×7 and graphite surfaces. Electrons incident on the surface after field emission from a scanning tunneling microscope tip are backscattered and detected with an energy and angle resolved hemispherical analyzer. We find that the reflected signal is sharply peaked in the direction parallel to the surface plane. Characteristic energy loss peaks corresponding to bulk and surface plasmon modes of the different surfaces are observed.
Low-loss transmission through tightly bent standard telecommunication fibers
Authors: Donlagić Denis , Culshaw Brian
Published: July 2002

This letter presents a principle of low-loss light transmission through tightly bent standard telecommunication optical fibers. The design is based on selective launching and filtering of the fundamental mode in standard graded index multimode fiber. The proposed system has demonstrated significantly better macrobend tolerance than commercially available bend resistant fibers.
Organic template directed growth of one- and two-dimensional GeX2/template superstructures (X=S, Se)
Authors: Chen L. , Klar P. J. , Heimbrodt W. , Oberender N. , Kempe D. , Fröba M.
Published: July 2002

We have obtained GeX2 nanostructures (X = S, Se) via a template-directed synthesis using organic templates and inorganic precursors. Depending on the choice of organic template, the GeS2/template superstructures are either lamellar or have a two-dimensional superstructure of hexagonal symmetry. All GeSe2/template superstructures are lamellar. The superstructure periods are typically below 5 nm. The microscopic structure of the IV–VI2 walls is revealed by Raman spectroscopy. In the GeSe2-based superstructures, the semiconductor layers consist of an amorphous network of GeSe4 tetrahedrons. In the GeS2-based nanostructures, the walls are a network of adamantan-like Ge4S10 cage units.
Broad-area optical characterization of well-width homogeneity in GaN/AlxGa1−xN multiple quantum wells grown on sapphire wafers
Authors: Pomarico A. , Lomascolo M. , Passaseo A. , Cingolani R. , Berti M. , Napolitani E. , Natali M. , Sinha S. K. , Drigo A. V.
Published: July 2002

We have performed spatially resolved photoluminescence spectroscopy on the entire 2 in. sapphire wafers containing GaN/AlxGa1−xN multiple quantum wells grown by metalorganic chemical vapor deposition. We have observed an energy shift of about 50 meV in the ground level emission energy between the center and the peripheric regions of the samples. We show that such a variation in the emission energy is due to a well narrowing of about 3 ML from the center to the outer regions of the wafers, which induces a large Stark shift through the built-in field.
Application of the thermal wave resonator to the measurement of the thermal diffusivity of gas mixtures
Authors: Lima J. A. P. , Marı́n E. , da Silva M. G. , Sthel M. S. , Cardoso S. L. , Vargas H. , Miranda L. C. M.
Published: July 2002

A gas analyzer device based on thermal wave interference in a cavity is presented. The thermal diffusivity of CO2:air mixtures as a function of the relative concentration is measured. It is demonstrated that different concentrations of CO2 in air can be detected with accuracy using the described experimental device. The results presented here open the possibility to perform routine measurements of thermal diffusivity of binary gas mixtures and using this parameter to monitor the relative gas concentration.
Diode-pumped Nd:YAG laser intensity noise suppression using a current shunt
Authors: Abbott R. S. , King P. J.
Published: July 2002

A current shunt actuator has been used to stabilize the intensity of a 10 W cw Nd3+:YAG laser. The current shunt developed exhibited a better actuator response than the pump diode current adjust actuator provided with the laser. Using the current shunt actuator, the relative intensity noise was suppressed from ∼10−51 / Hz to below ∼5 × 10−71 / Hz.
Distributed Bragg reflectors for visible range applications based on (Zn,Cd,Mg)Se lattice matched to InP
Authors: Guo S. P. , Maksimov O. , Tamargo M. C. , Peiris F. C. , Furdyna J. K.
Published: July 2002

Lattice-matched (Zn,Cd,Mg)Se epilayers were grown by molecular-beam epitaxy on InP substrates. X-ray diffraction and photoluminescence measurements show the high crystalline quality of the epilayers. Using a prism coupler technique, the index of refraction of the epilayers was investigated at four discrete wavelengths. With these results, (Zn,Cd,Mg)Se-based distributed Bragg reflector structures have been grown, covering nearly the entire visible spectral range. A maximum reflectivity of 95% has been achieved for structures having 16 periods.
Electronic properties of the diamond films with nitrogen impurities: An x-ray absorption and photoemission spectroscopy study
Authors: Chang Y. D. , Chiu A. P. , Pong W. F. , Tsai M.-H. , Chang Y. K. , Chen Y. Y. , Chiou J. W. , Jan C. J. , Tseng P. K. , Wu R. T. , Chung S. C. , Tsang K. L. , Lin I. N. , Cheng H. F.
Published: July 2002

X-ray absorption near-edge structure (XANES) measurements have been performed for nitrogen (N) containing diamond films with three different N concentrations at the C K-edge using the sample drain current mode. The C K-edge XANES spectra of these diamond films resemble that of the pure diamond regardless of the N concentration, which suggests that the overall bonding configuration of the C atom is unaltered. N impurities are found to reduce the intensities of both the sp2- and sp3-bond derived resonance features in the XANES spectra. The valence-band photoelectron spectra indicate that N atoms cause the broadening of the valence band σ- and π-bond features and the enhancement and reduction of the σ- and π-bond features, respectively.
In-plane anisotropic lasing characteristics of (110)-oriented GaInAsP quantum-well lasers
Authors: Oe Kunishige , Bhat Raj , Ueki Mineo
Published: July 2002

(110)-oriented GaInAsP quantum-well (QW) lasers have been fabricated to investigate growth direction effects of the QW structure on laser performance. Large in-plane anisotropic threshold current densities in the lasers were observed between the [001] and [11̄0] cavity directions of the (110)-oriented QW structure lasers. This large anisotropy is able to be ascribed to a stronger oscillator strength for [11̄0]-polarized light. Fairly low threshold current densities of less than 0.6 kA/cm2 were obtained for the lasers with cavities along [001] direction in spite of the lower reflectivity of the reactive ion etching etched mirror surface.
Photon scanning tunneling optical microscopy with a three-dimensional multiheight imaging mode
Authors: Balistreri M. L. M. , Korterik J. P. , Kuipers L. , van Hulst N. F.
Published: July 2002

A photon scanning tunneling microscope with a three-dimensional multiheight mode has been developed for the mapping of optical field distributions in integrated optical waveguide structures. The optical field is measured at different heights above the waveguide surface. The multiheight measurements also contain the optical information gathered with the commonly used constant gap measurements in addition to the topography of the waveguide surface. With the multiheight method, the decay length of the evanescent field is readily determined as function of the in-plane coordinate. Moreover, the evanescent light can be distinguished from scattered light.
Thermally stimulated current method applied on diodes with high concentration of deep trapping levels
Authors: Pintilie I. , Pintilie L. , Moll M. , Fretwurst E. , Lindstroem G.
Published: July 2002

We propose an improved method of thermally stimulated currents (TSC) spectra analysis in the case of diodes having a concentration of traps higher than that of doping impurities. Beside the calculation of trap concentrations from TSC peaks analysis, the method allows us to evaluate the density and the type of the very deep trapping level which, due to the contribution of leakage current, can not be detected in a real TSC experiment. The proposed method is applied to a p+-n Silicon diode irradiated with 1.82×1013neutrons/cm2.
Properties of latent interface-trap buildup in irradiated metal–oxide–semiconductor transistors determined by switched bias isothermal annealing experiments
Authors: Jaksic Aleksandar B. , Pejovic Momcilo M. , Ristic Goran S.
Published: July 2002

Isothermal annealing experiments with switched gate bias have been performed to determine the properties of the latent interface-trap buildup during postirradiation annealing of metal–oxide–semiconductor transistors. It has been found that a bias-independent process occurs until the start of the latent interface-trap buildup. During the buildup itself, oxide-trap charge is not permanently neutralized, but is temporarily compensated.
Atomic scale protection using fullerene encapsulation
Authors: Cotier B. N. , Upward M. D. , Jones F. H. , Moriarty P. , Beton P. H.
Published: July 2002

The use of fullerene multilayers as a removable protective coating on a clean, reconstructed semiconductor surface has been investigated using an ultrahigh vacuum scanning tunneling microscope (UHV-STM). We have found that the Ag/Si(111)-√3×√3R30° surface, which is normally stable only under an UHV environment, can survive exposure to ambient conditions beneath a protective layer of C60. The C60 capping layer may be removed by annealing after reinsertion into UHV. For optimum protection a thickness >5 monolayers is required. The protective effect lasts for times of order several minutes.
Nano-oxidation of H-terminated p-Si(100): Influence of the humidity on growth and surface properties of oxide islands
Authors: Jungblut H. , Wille D. , Lewerenz H. J.
Published: July 2002

Nanometer-scale oxide islands were formed on p-Si(100) by use of an atomic-force microscope in ambient air. The islands were investigated using contact and friction mode. Volumes were determined from cross-sectional measurements before and after HF etching. A pronounced influence of the relative humidity (hr) of the ambient air on the size and on the friction behavior of the islands was found. A distinct peak of the size distribution was observed at hr≈85%. The friction images showed a clear contrast inversion at this humidity value. At lower humidities, the friction between tip and oxide was larger than on H-terminated silicon, at higher humidities, it was lower. A condensation mechanism occurring at the meniscus between the tip and H-terminated silicon surface is suggested, which explains the results.
The effect of damping on autoresonant (nonstationary) excitation
Authors: Fajans J. , Gilson E. , Friedland L.
Published: July 2002

When a nonlinear oscillator with an amplitude dependent frequency is driven by a swept frequency drive, the oscillator’s amplitude will, in some circumstances, automatically adjust itself so that the oscillator’s nonlinear frequency closely matches the drive frequency. This phenomenon is called autoresonance, and allows the amplitude of the oscillator to be controlled simply by sweeping the drive frequency. Previous studies of autoresonance were in undamped systems; the effect of damping on autoresonance is considered here. In particular, the question of a threshold for entering autoresonance in a dissipative system is investigated. The resulting theory accurately describes the behavior of experiments on the diocotron mode in pure-electron plasmas.
Ultrahigh vacuum cantilever magnetometry with standard size single crystal substrates
Authors: Höpfl Th. , Sander D. , Höche H. , Kirschner J.
Published: July 2002

A cantilever magnetometer is described that measures the magnetic moment of ferromagnetic films with submonolayer sensitivity. The magnetometer is incorporated into an ultrahigh vacuum chamber for sample preparation and in situ magnetometry. Standard size single crystals of 5 mm diameter and 2 mm thickness can be used, which are mounted on thin sheet metal. This composite sampleholder works as a cantilever when the bending is induced by the torque exerted by an external magnetic field on a monolayer ferromagnetic film deposited onto the single crystal substrate. We demonstrate the submonolayer sensitivity on Fe monolayers on Cu(100) in the thickness range from 2 to 68 monolayers. The sample holder is designed for internal calibration by passing a current through it and exploiting the well-known current induced magnetic moment.
Ion-assisted deposition of amorphous GaN: Raman and optical properties
Authors: Bittar A. , Trodahl H. J. , Kemp N. T. , Markwitz A.
Published: July 2002

We report the preparation of amorphous GaN by ion-assisted deposition and studies of the Raman and optical response of the resulting films. The films are transparent across the visible and show an edge whose energy and structure are in close agreement with crystalline material, suggesting a low density of gap states and homopolar bonds. The Raman spectrum is similar to a broadened vibrational density of modes calculated for wurtzite GaN, with a Raman cross section which varies among the vibrational bands.
Ultrathin epitaxial Fe films on vicinal GaAs(001): A study by spin-resolved photoelectron spectroscopy
Authors: Zhang T. , Spangenberg M. , Greig D. , Takahashi N. , Shen T.-H. , Matthew J. A. D. , Cornelius S. , Rendall M. , Seddon E. A.
Published: July 2002

Thin epitaxial Fe films have been grown on vicinal GaAs(001) substrates and their remanent magnetic properties and the degree of substrate atom diffusion investigated using synchrotron-based photoelectron spectroscopy. The vicinal Fe films, though exhibiting greater As diffusion than their singular homologues, displayed better film quality both from the structural and the magnetic points of view. The spin-resolved valence spectra of the vicinal films resemble those for crystalline bulk Fe at lower film thicknesses than for singular films.
Excitation of a microdischarge with a reverse-biased pn junction
Authors: Wagner C. J. , Park S.-J. , Eden J. G.
Published: July 2002

Excitation of cylindrical microdischarges, 300–360 μm in diameter, by a reverse-biased, Si pn junction has been demonstrated. Devices fabricated from commercial diodes have been operated with Ne gas pressures in the 200–700 Torr range and dc voltages as low as 120 V. For a Ne gas pressure of 700 Torr, the wavelength-integrated (300–800 nm) output power—emitted into a solid angle of ∼6×10−2 sr—of a 360-μm-diam device is 48±1 μW for an operating current and voltage of 5.7±0.1 mA and 134 V, respectively. This hybrid solid state/gas device represents the demonstration of the generation of a gas discharge by a pn junction and lends itself to the fabrication of large arrays.
Density functional theory studies of the adsorption of ethylene and oxygen on Pt(111) and Pt3Sn(111)
Authors: Watwe Ramchandra M. , Cortright Randy D. , Mavrikakis Manos , Nørskov Jens K. , Dumesic James A.
Published: July 2002

Density functional theory, employing periodic slab calculations, was used to investigate the interactions of ethylene and oxygen with Pt(111) and Pt3Sn(111). The predicted energetics and structures of adsorbed species on Pt(111) are in good agreement with experimental data. The binding energies of π-bonded ethylene, di-σ-bonded ethylene, and ethylidyne species are weaker on Pt3Sn(111) than on Pt(111) by 21, 31, and 50 kJ/mol, respectively. Hence, the electronic effect of Sn on the adsorption of ethylene depends on the type of adsorption site, with adsorption on three-fold site weakened more than adsorption on two-fold and one-fold sites. Oxygen atoms bond as strongly on Pt3Sn(111) as on Pt(111), and these atoms prefer to adsorb near Sn atoms on the surface. The addition of Sn to Pt(111) leads to a surface heterogeneity, wherein ethylidyne species prefer to adsorb away from Sn atoms and oxygen atoms prefer to adsorb near Sn atoms. Implications of this surface heterogeneity on hydrocarbon reaction selectivity on Pt-based catalysts are discussed.
Time-resolved spectroscopy of strained GaN/AlN/6H–SiC heterostructures grown by metalorganic chemical vapor deposition
Authors: Pozina G. , Edwards N. V. , Bergman J. P. , Paskova T. , Monemar B. , Bremser M. D. , Davis R. F.
Published: July 2002

Temperature-dependent time-resolved photoluminescence measurements were performed on GaN film/AlN buffer/6H–SiC substrate heterostructures grown by metalorganic chemical vapor deposition. The overlying GaN layers were under tension, as estimated from the free A exciton (FEA) position. The recombination lifetimes were determined for the FEA and for the neutral-donor-bound exciton (D0X). We observed that the recombination lifetime for the FEA has the same value of 40–50 ps in all the layers, whereas the recombination time for the D0X varies for different samples. We observed that the recombination lifetimes for D0X have a clear dependence on the position of FEA, i.e., the recombination lifetime increases with decreasing strain in the layers. We discuss the results in term of the hole states involved in the donor-bound exciton recombination.
Active feedback stabilization of the resistive wall mode on the DIII-D device
Authors: Okabayashi M. , Bialek J. , Chance M. S. , Chu M. S. , Fredrickson E. D. , Garofalo A. M. , Gryaznevich M. , Hatcher R. E. , Jensen T. H. , Johnson L. C. , La Haye R. J. , Lazarus E. A. , Makowski M. A. , Manickam J. , Navratil G. A. , Scoville J. T. , Strait E. J. , Turnbull A. D. , Walker M. L. ,   DIII-D Team
Published: July 2002

A proof of principle magnetic feedback stabilization experiment has been carried out to suppress the resistive wall mode (RWM), a branch of the ideal magnetohydrodynamic (MHD) kink mode under the influence of a stabilizing resistive wall, on the DIII-D tokamak device [Plasma Phys. Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159; Phys. Plasmas 1, 1415 (1994)]. The RWM was successfully suppressed and the high beta duration above the no-wall limit was extended to more than 50 times the resistive wall flux diffusion time. It was observed that the mode structure was well preserved during the time of the feedback application. Several lumped parameter formulations were used to study the feedback process. The observed feedback characteristics are in good qualitative agreement with the analysis. These results provide encouragement to future efforts towards optimizing the RWM feedback methodology in parallel to what has been successfully developed for the n=0 vertical positional control. Newly developed MHD codes have been extremely useful in guiding the experiments and in providing possible paths for the next step.
Intense laser interactions with sprays of submicron droplets
Authors: Gumbrell E. T. , Comley A. J. , Hutchinson M. H. R. , Smith R. A.
Published: July 2002

Picosecond laser interaction experiments conducted at peak intensities of 1.5×1017 W cm−2 using a new target medium consisting of a dense spray of 0.5 micron radius ethanol droplets indicate a strong laser–plasma coupling. The laser absorption exceeds that seen in solid targets of greater Z, and remains high over more than four orders of magnitude of intensity. Invariance in the laser absorption with wavelength and polarization is also reported. Together with x-ray spectroscopy studies, absorption measurements have been used to implement nonlocal thermodynamic equilibrium (NLTE) plasma simulations in order to isolate the important features of the droplet heating and explosion dynamics. These simulations show that the interplay of laser heating and energy transport processes is significantly different from those seen in continuous solid target interactions and that a substantial fast electron fraction must be inferred.
Photoluminescence and electrical characteristics of the two-dimensional electron gas in Si delta-doped GaN layers
Authors: Teng C. W. , Aboelfotoh M. O. , Davis R. F. , Muth J. F. , Kolbas R. M.
Published: July 2002

We have studied the electrical and photoluminescence (PL) properties of a Si delta-doped GaN layer grown by metalorganic chemical vapor deposition. The Hall mobility and electron sheet concentration are 726 cm2/V s and 1.9×1012 cm−2, respectively, at 2 K. A PL peak located at 78 meV below the band gap of GaN is observed at 77 K. This PL peak is attributed to the radiative recombination between electrons in the two-dimensional quantum states and photoexcited holes in GaN, which is consistent with simulation results using a one-dimensional Poisson and Schrödinger equation solver. The peak disappears at temperatures higher than 77 K and is not observed in uniformly doped GaN layers.
Electrostatic probe diagnostics of a planar-type radio-frequency inductively coupled oxygen plasma
Authors: Seo D. C. , Chung T. H. , Yoon H. J. , Kim G. H.
Published: July 2002

An inductively coupled oxygen radio-frequency (13.56 MHz) discharge is investigated based on modeling and experiment. Experimental measurement is done at a range of gas pressure of 1–30 mTorr, and rf power of 100–1000 W. We measure most of the important plasma parameters such as the densities of charged species, electron temperature, plasma potential, and electron energy distribution function. The measured values are compared with the results of the spatially averaged global model. We observe a generally good agreement between the modeling and the experiment. The scaling features, the transition of the operating region, and the radial distributions of charged species are also discussed.
Role of the kinetic and potential sputtering in the regeneration of the soot
Authors: Ahmad Shoaib , Akhtar M. N.
Published: July 2002

We have used the photoemission spectroscopy of the graphite hollow cathode sooting discharge to identify the roles played by the kinetic and potential sputtering, respectively. Our indicators are the relative densities of the sputtered carbon C, the Ne metastable atoms, and the ionized components (Ne+) of the regenerative sooting plasma. We find that the metastable atoms are the main agents for the regeneration of the soot.
Adiabatic pumping of two-dimensional electrons in a ratchet-type lateral superlattice
Authors: Höhberger E. M. , Lorke A. , Wegscheider W. , Bichler M.
Published: July 2002

A pair of interdigitated gates, interlacing off center, is used to generate a widely tunable one-dimensional lateral superlattice in the two-dimensional electron gas at a GaAs/AlGaAs heterointerface. Magnetotransport measurements with statically biased gates demonstrate that the conduction-band modulation is strongly influenced by higher harmonics, thus allowing the realization of both symmetric and ratchet-like periodic potential modulations. Temporally phase-shifted ac biasing of the gates results in adiabatic pumping of electrons. The generated dc current increases linearly with the pumping frequency and can be reversed in situ by inverting the phase shift.
Surface reaction of nitrogen with liquid group III metals
Authors: Romanowski Z. , Krukowski S. , Grzegory I. , Porowski S.
Published: July 2002

Surface reactions of atomic and molecular nitrogen (N2) with liquid group III metals: Al, Ga, and In has been investigated by quantum mechanical calculations in density functional theory (DFT) formulation, using cluster representation of metal surface. It has been shown that the N2 molecule dissociates during adsorption on the surfaces of liquid group III metals. The N2 dissociation energy barriers are equal to 3.0 eV, 3.4 eV, and 3.6 eV for Al, Ga, and In, respectively. They are much smaller that the dissociation energy of free N2 molecule, equal to 9.76 eV. It has been also determined that the adsorption of N2 on surface of liquid Al is an exothermic and on Ga and In is an endothermic process. These results are consistent with experimentally observed combustion of liquid Al in high pressure of nitrogen and the absence of combustion of both Ga and In. The process of dissolution of N atoms adsorbed on liquid Al surface has been also analyzed. The energy barriers for the direct jump of the N adatom from the surface position into the liquid Al interior is equal to 1.3 eV. This suggests that the dissolution of N in liquid Al proceeds not by direct jumps of N adatoms into the liquid interior but by Brownian motion of clusters consisting of these adatoms and neighboring Al atoms. The results of the calculations indicate that nitrogen solution in liquid group III metals consists of single N atoms strongly attached to the surrounding Me atoms.
Electronic transitions with quantum trajectories
Authors: Wyatt Robert E. , Lopreore Courtney L. , Parlant Gérard
Published: July 2002

The quantum trajectory method (QTM) is extended to the dynamics of electronic nonadiabiatic collisions. Equations of motion are first derived for the probability density, velocity, and action function for wave packets moving on each of the coupled electronic potential surfaces. These discretized equations are solved in the Lagrangian (moving with the fluid) picture to give the trajectory dynamics of fluid elements evolving on each potential surface. This trajectory method is fully quantum mechanical and does not involve “trajectory surface hopping.” The method is applied to nonadiabiatic collision models involving two coupled electronic states. The quantum trajectory results are in excellent agreement with solutions computed (using space-fixed grid methods) directly from the time-dependent Schrödinger equation.
Spectral diffusion of individual pentacene, terrylene, and dibenzanthanthrene molecules in n-tetradecane
Authors: Durand Yannig , Bloeß Andreas , Köhler Jürgen , Groenen Edgar J. J. , Schmidt Jan
Published: July 2002

The spectral stability of three guest molecules embedded in a quickly frozen n-tetradecane matrix has been investigated on a single-molecular level at liquid-helium temperature. In total, about 2500 spectral trajectories of 476 terrylene molecules, 328 dibenzanthanthrene molecules, and 252 pentacene molecules were recorded. Both line broadening and spectral jumps are analyzed and the latter are found to be mainly light induced. The spectral changes in essence reflect the dynamics of the host matrix and the differences between the guest molecules may be reduced to differences in their pumping cycles.
Improved particle control for high integrated plasma performance in Japan Atomic Energy Research Institute Tokamak-60 Upgrade
Authors: Takenaga H. ,   JT-60 Team
Published: July 2002

Pellet fueling and impurity transport have been investigated in high poloidal-beta (βp) and reversed shear (RS) plasmas. It is observed that pellet injection from the high-field side at the top is more effective for density increase in the neutral-beam-heated plasma compared with pellet injection from the low-field-side midplane. This observation is consistent with the radial displacement theoretically predicted based on the E×B drift model, where deeper deposition is suggested for the high-field-side pellet injection. Using the high-field-side top pellets, the accessible density region of the high βp plasmas is extended to 70% of the Greenwald density with a confinement enhancement factor over the ITER89P scaling of 1.94, a normalized beta of 2.2, and a bootstrap current fraction of 60%. The impurity diffusivity at the internal transport barrier is only higher by a factor of 2–4 than the neoclassical prediction in the RS plasma. In contrast, the impurity diffusivity is two orders of magnitude higher than the neoclassical prediction and smaller by a factor of 5 than the theoretical prediction based on turbulence model in the high βp plasma. The helium-exhaust capability is enhanced and Zeff is reduced by increasing divertor recycling in the RS plasma, although confinement degrades with increasing divertor recycling.
Dielectric exchange-force effect on the rupture force of adsorbed bilayers of self-assembled surfactant films
Authors: Teschke O. , Ceotto G. , de Souza E. F.
Published: July 2002

We measured and formulated dielectric exchange forces between adsorbed layers of self-assembled surfactant films and atomic-force microscope tips in water. The dielectric exchange-force model is in agreement with the observation that the surfactant-layer rupture forces (tip-applied force necessary to obtain tip/substrate contact) are smaller in the thickest layers, where the compactness of the adsorbed film results in the smallest values of the dielectric permittivity. Within experimental accuracy, a dielectric permittivity value of ∼4 for bilayers and of ∼36 for monolayers is found.
Origin and control of the lead-enriched near-surface region of (Pb, La)TiO3
Authors: Vasco E. , Böhme O. , Román E. , Zaldo C.
Published: July 2002

The chemical composition, structure and extension of the Pb-enriched nonferroelectric near-surface region of stoichiometric (Pb, La)TiO3 ferroelectric films was characterized by x-ray diffraction, Rutherford backscattering spectroscopy and x-ray photoemission spectroscopy. Its origin as a result of the Pb excess segregation from the growing (Pb, La)TiO3 bulk and subsequent oxidation was revealed. A postdeposition procedure to control its composition and thereby its properties was provided. Thus, a decrease of the near-surface Pb excess of about 75% was achieved.
Physics and design interplay phenomena in an actively cooled tokamak: Tore Supra
Authors: Chatelier M. ,   Tore Supra Team
Published: July 2002

In order to achieve long pulse high performance plasma discharges, actively cooled plasma facing components (PFCs) have been installed since the beginning of Tore Supra in 1988. So far, up to 280 MJ of energy could be removed within a single discharge of 2 mn duration. Active cooling creates a specific environment, like, e.g., constant low wall temperature (<200 °C) or the presence of water pipes close to the plasma boundary, as compared to inertially cooled devices. Physical processes, specific of the actively cooled environment occur, which may sometimes induce irreversible damage that would not even have been observed on conventional PFCs. After a short description of the development status of the actively cooled PFCs program at Tore Supra, the paper concentrates on those aspects where physics and design aspects are mixed, which deserve thorough analysis for the conception of a next step fusion device environment.
Alignment of grain boundary in a Si film crystallized by a linearly polarized laser beam on a glass substrate
Authors: Horita Susumu , Nakata Y. , Shimoyama A.
Published: July 2002

We performed crystallization of an amorphous Si film deposited on a Pyrex glass substrate using a Nd:YAG pulse-laser beam with linear polarization. It was found that, in the crystallized film, the grain boundaries were aligned with a period of about 550 nm or the wavelength of the laser beam. Meanwhile, in the Si film crystallized by circularly polarized beam that passed through the λ/4 plate, the grain boundaries were randomly generated. This means that linear polarization of the laser beam is essential to align grain boundaries periodically or to produce a periodic temperature distribution in the irradiated Si film.
Metastability of defects in p-GaAs grown from a Ga-rich melt
Authors: Fang Z.-Q. , Kim J. W. , Yu P. W.
Published: July 2002

We report the observation of metastability for defects in Si-doped p-GaAs grown from a Ga-rich melt. A configuration change occurs suddenly at a critical temperature (near 120 K) during cooldown scans, giving rise to discontinuous deep-level transient spectra. A similar phenomenon was observed on electron-irradiated float-zone-refined Si and explained in terms of entropy variations in the configurational space. We believe that the metastability found in the p-GaAs sample is also due to a barrierless transformation of defect configuration, driven by variations in both entropy and the charge state of GaAs.
Adjustable ultraviolet-sensitive detectors based on amorphous silicon
Authors: Topič Marko , Stiebig Helmut , Krause Mathias , Wagner Heribert
Published: July 2002

Thin-film detectors made of hydrogenated amorphous silicon (a-Si:H) and amorphous silicon carbide (a-SiC:H) with adjustable sensitivity in the ultraviolet (UV) spectrum were developed. Thin PIN diodes deposited on glass substrates in N–I–P layer sequence with a total thickness of down to 33 nm and a semitransparent Ag front contact were fabricated. The optimized diodes with a 10 nm Ag contact exhibit spectral response values above 80 mA/W in the wavelength range from 295 to 395 nm with a maximum of 91 mA/W at 320 nm. For longer wavelengths, the spectral response drops by 50% at 450 nm. Increasing the thickness of the Ag front contact leads to a narrowing of the spectral response at around 320 nm, which allows the adjustment from a broad UV to a selective UV–B-sensitive detector.
Changes in the interface capacitance for fatigued lead–zirconate–titanate capacitors
Authors: Meng X. J. , Sun J. L. , Yu J. , Bo L. X. , Jiang C. P. , Sun Q. , Guo S. L. , Chu J. H.
Published: July 2002

From the capacitance–voltage dependence of Pt/PbZr0.5Ti0.5O3/Pt capacitors, we mathematically separated the capacitance into two parts corresponding to two regions of the samples, the uniform electric-field region and the nonuniform region. They are correlated with the bulk ferroelectric film region and the Pt/PbZr0.5Ti0.5O3 Schottky barrier interface region, respectively. The calculations based on the in-series capacitor model show a slight decrease of dielectric permitivity for the fatigued bulk films. By assuming a much smaller dielectric permitivity of the interface region than that of bulk films, it was found that the interface capacitance decreased remarkably compared with that of the bulk ferroelectric film after fatigue. This decrease was attributed to the lowering of ferroelectricity in the interface layer, which suggests that the fatigue is mainly an interface state controlled process. The asymmetricity in the interface capacitance–voltage curve is attributed to the different defect concentration levels in the top and bottom interface regions.
Fluorescence x-ray absorption fine structure study on local structures around Fe atoms heavily doped in GaN by low-temperature molecular-beam epitaxy
Authors: Ofuchi H. , Oshima M. , Tabuchi M. , Takeda Y. , Akinaga H. , Németh S̆. , De Boeck J. , Borghs G.
Published: July 2002

A local structural transition in heavily Fe-doped GaN films related to the magnetic properties has been revealed by fluorescence x-ray absorption fine structure (XAFS) analysis. The structural transition is explained (or considered to be induced) by the change in the degree of hybridization between Fe 3d and N 2p states, which can be evaluated by x-ray absorption near edge structure spectra. The XAFS analysis indicates that the present diluted magnetic semiconductor based on GaN can be fabricated by electron cyclotron resonance microwave plasma-assisted low-temperature molecular-beam epitaxy.