bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical ratio of hot electron current to thermal electron current in scanning hot electron microscopy
Authors: Zhang Bingyang , Furuya Kazuhito , Karasawa Shinya
Published: July 2002

Theoretical calculation of the ratio of hot electron current to thermal electron current in scanning hot electron microscopy (SHEM) is reported by modeling the sample-tip structure as a plane-sphere structure. The effects of hot electron energy, gap separation, and applied bias between the tip and the sample surface on the ratio are investigated. Comparison of the results of theoretical calculation and those obtained from SHEM experiment on a double-barrier resonant tunneling diode of an InGaAs/AlAs heterostructure emitter reveals agreement in the detected amplitude and the tendency of change of the hot electron current.
Phase separation as origin of the magnetic anomalies in La0.85Sr0.15CoO3
Authors: Mira J. , Rivas J. , Baio G. , Barucca G. , Caciuffo R. , Rinaldi D. , Fiorani D. , Señarı́s Rodrı́guez M. A.
Published: July 2002

The dependence of the ac-magnetic susceptibility of La0.85Sr0.15CoO3 on the annealing temperature used during synthesis is addressed. Such dependence has been previously attributed to compositional inhomogeneities. Nevertheless, the presence of distinct phases with different chemical compositions is excluded after explorations by several techniques. Instead it is proposed that an electronic phase separation takes place in the material, whose state is changed after charge-carrier redistributions due to the thermal history of the samples, analogously to parent manganese-based perovskites.
Gas phase ligand field photofragmentation spectroscopy
Authors: Puškar Ljiljana , Stace Anthony J.
Published: July 2002

Ligand field spectra have been recorded in the gas phase for the two complexes [Ag.(pyridine)4]2+ and [Cu.(pyridine)4]2+. Both spectra compare favorably with analogous condensed phase measurements; however, for [Ag.(pyridine)4]2+ the results differ in interpretation. The Ag(II) data are attributed to a ligand-to-metal charge transfer process, and the Cu(II) data (spectral region and extinction coefficient) match the characteristics of a d–d transition.
Effect of oxygen content on magnetization and magnetoresistance properties of CMR manganites
Authors: Trukhanov S. V. , Troyanchuk I. O. , Korshunov F. P. , Sirenko V. A. , Szymczak H. , Baerner K.
Published: July 2002

The influence of oxygen content on the magnetization and electrical resistivity of Ln0.5A0.5MnO3 (Ln=La, Pr, Nd; A=Ca, Ba) manganites with the perovskite structure is investigated. It is shown that the La0.5Ca0.5MnO3−γ compound undergoes a sequence of transitions from an antiferromagnetic (γ=0) to a spin-glass (γ=0.17) state and then to an inhomogeneous ferromagnetic (γ=0.3) state. A transition from an antiferromagnetic charge-ordered state to a ferromagnetic charge-disordered state in Nd0.5Ca0.5MnO3−γ is observed as the oxygen content is reduced to where γ=0.07. The Nd0.5Ba0.5MnO3−γ compound shows an increase of the Curie point from 110 K(γ=0) to 310 K(γ=0.3). In addition, a large magnetoresistance is revealed which develops below their Curie temperature despite the absence of Mn3+−Mn4+ pairs. A Zener double-exchange interaction is usually used in literature to explain the magnetic and electrical properties of hole-doped perovskite manganites. The data obtained support the mechanism of superexchange interactions between magnetic moments of the manganese ions via oxygen.
Characterization of nanopipes/dislocations in silicon carbide using ballistic electron emission microscopy
Authors: Reddy C. V. , Narayanamurti V.
Published: July 2002

We report on the characterization of the nanopipe/dislocation related defects in silicon carbide (SiC) wafers using a ballistic electron emission microscopy (BEEM) technique. While the rest of the techniques were just limited to providing structural information such as length and width, the dual mode of the BEEM technique is shown to provide physical insight into the nature of these defects in SiC. The presence of excess of carrier density at the dislocation site, and its impact on the device characteristics in terms of junction breakdown, is discussed within the scope of our experimental results.
Crystal lattice transition of behenic acid monolayer on pure water surface observed by polarization modulation infrared spectroscopy
Authors: Ren Yanzhi , Iimura Ken-ichi , Kato Teiji
Published: July 2002

The crystal lattice of CH3(CH2)20COOH (behenic acid) monolayer on pure water subphase was investigated in situ by polarization modulation infrared reflection absorption spectroscopy. Two kinds of centered rectangular unit cells were found: rectangular parallel and orthorhombic packing of the trans zigzag planar–(CH2)20–chain. In the former cell the chain was tilted on the water surface and the C–C–C plane at the rectangle center was parallel to that at the corner. In the latter cell the chain was normal to the water surface and the C–C–C plane at the rectangle center was perpendicular to that at the corner. A rectangular parallel-to-orthorhombic transition was discovered for two kinks from 0.260 to 0.220 nm2/molecule along the surface pressure-area isotherm at 283 K, evidenced by splitting of the CH2 scissoring mode. By holding the barrier at 0.240 nm2/molecule and cyclically varying the monolayer temperature, a partially reversible transition from rectangular parallel to orthorhombic unit cells was observed.
Enhanced ablation and photoacoustic excitation in near-threshold laser ablation of liquid-coated surfaces
Authors: Kim Dongsik , Lee Ho
Published: July 2002

The effect of an artificially deposited liquid film on a laser-ablated metal surface at near-threshold laser fluences is presented. Experiments utilizing a Q-switched Nd:YAG laser (wavelength=1064 nm, full width at half maximum=6 ns) reveal that deposition of a liquid film results in substantial reduction of ablation threshold and enhancement of ablation yield. In addition, enlarged photoacoustic emission is detected in the ablation involving the liquid film. The augmentation of ablation efficiency at high laser fluences is known to be induced by confinement of laser-generated plasma in the liquid layer. In contrast, the enhancement in the near-threshold regime, as in the present study, is primarily due to the photomechanical effect associate with the explosive liquid vaporization in the strongly superheated liquid layer. The effect of liquid-film thickness on near-threshold ablation are also examined for two different liquids, water and water/isopropanol mixture.
Reynolds number effects in the near-wall region of turbulent channel flows
Authors: Fischer M. , Jovanović J. , Durst F.
Published: July 2002

The continuity and momentum equations do not imply a Reynolds number dependence of turbulence data when wall variables are used for normalization. However, experimental and numerical results show a Reynolds number dependence of turbulence intensity very close to the wall. The cause of this is explained. It results from the behavior of a sink term in the dissipation rate equation which shows a Reynolds number dependence in the limit of two-component two-dimensional turbulence as it exists close to walls. Away from the near-wall region the Reynolds number dependence originates from the streamwise pressure gradient which enters into the equations for the turbulent kinetic energy and turbulent dissipation rate through the gradient production processes. The low-Reynolds number effects in turbulent channel flow were investigated experimentally using the laser Doppler anemometry (LDA) measuring technique. A new method was used to eliminate the influence of the limited spatial resolution of the LDA measuring control volume. Results are presented for the limiting behavior of the turbulent intensity near the wall and its variation with the Reynolds number. The present LDA measurements confirm the trend in the data of direct numerical simulations.
Decay of the elastic precursor in a filled glass
Authors: Bourne N. K. , Millett J. C. F.
Published: July 2002

Plate-impact experiments have been performed upon a lead-filled glass to understand its shock response. Stress measurements have been made using manganin stress gauges in both longitudinal and lateral orientations. It has been observed that there is a pronounced decay in Hugoniot elastic limit with distance into the target. In the experiments conducted, a steady value for the precursor amplitude has not been reached. Failure waves were seen to sweep the entire thickness of the glass specimens until stopped by release waves from the rear of the target.
Radio-frequency plasma jet generator of singlet delta oxygen with high yield
Authors: Schmiedberger Josef , Fujii Hiroo
Published: July 2002

O 2 (a  1 Δ g ) was generated in a flowing discharge of a radio-frequency (rf) hollow electrode. The radio frequency was 99.9 MHz and the rf power was 200 W. The discharge was done in the gas mixture O2:N2:NO=200:20:10 sccm and then it was chilled reactively by the mixture Ar:NO2=200:10 sccm. The O2(a 1Δg) relative yield of 32% was achieved at the pressure of 0.43 Torr. Usage of the mixtures O2:NO=200:100 sccm and Ar:NO2=100:100 sccm resulted in the O2(a 1Δg) yield of 25% at the pressure of 0.6–0.9 Torr. The effluent was mixed with molecular iodine in a far afterglow region and it was tested in an oxygen–iodine laser. The iodine flow rate was 0.3 mmol/min. A strong enhancement of atomic iodine spontaneous emission at the wavelength of 1315 nm was observed in the optical resonator.
Interfacial reactions in epitaxial Al/TiN(111) model diffusion barriers: Formation of an impervious self-limited wurtzite-structure AIN(0001) blocking layer
Authors: Chun J.-S. , Desjardins P. , Lavoie C. , Shin C.-S. , Cabral C. , Petrov I. , Greene J. E.
Published: July 2002

Single-crystal TiN(111) layers, 45 nm thick, were grown on MgO(111) by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges at Ts=700 °C. Epitaxial Al(111) overlayers, 160 nm thick, were then deposited at Ts=100 °C in Ar without breaking vacuum. Interfacial reactions and changes in bilayer microstructure due to annealing at 620 and 650 °C were investigated using x-ray diffraction and transmission electron microscopy (TEM). The interfacial regions of samples annealed at 620 °C consist of continuous ≃7-nm-thick epitaxial wurtzite-structure AlN(0001) layers containing a high density of stacking faults, with ≃22 nm thick tetragonal Al3Ti(112) overlayers. Surprisingly, samples annealed at the higher temperature are more stable against Al3Ti formation. TEM analyses of bilayers annealed at 650 °C (10 °C below the Al melting point!) reveal only the self-limited growth of an ≃3-nm-thick interfacial layer of perfect smooth epitaxial wurtzite-structure AlN(0001) which serves as an extremely effective deterrent for preventing further interlayer reactions.
Investigation of switching effects between the drains of an electron Y-branch switch
Authors: Worschech L. , Weidner B. , Reitzenstein S. , Forchel A.
Published: July 2002

By using high-resolution electron beam lithography and wet etching, Y-branched electron waveguides with lengths down to 70 nm have been fabricated on modulation-doped GaAs/AlGaAs heterostructures. Small positive bias applied between the source and the two drain electron reservoirs leads to enhanced switching of electrons into either of the two branches when a lateral external electric field is applied. The switching manifests itself by pronounced sawtooth oscillations in the conductance between source and one branch of an electron Y-branch switch as the gate voltage at the other branch is changed when the corresponding gate voltage is fixed.
Electron emission from single-walled carbon nanotubes with sharpened bundles
Authors: Ito Fuminori , Konuma Kazuo , Okamoto Akihiko
Published: July 2002

We investigated the influence of grinding as a pretreatment prior to the emitter film formation of single-walled carbon nanotubes (SWNTs) on the field emission and the geometric properties of SWNTs. Field emission from emitter films composed of ground SWNTs was found to be dramatically enhanced as well as to be homogeneous and stable, compared with that from emitter films composed of unground SWNTs. Transmission electron microscopy observations showed that several SWNTs protrude from the ends of ground SWNT bundles. The improved emission current is attributed to the large field enhancement at the sharpened ends of the SWNT bundles.
A supersymmetric type IIB Randall–Sundrum realization
Authors: Duff M. J. , Liu James T. , Stelle K. S.
Published: July 2002

We show that an earlier domain wall solution of type IIB supergravity provides a supersymmetric realization of the Randall–Sundrum brane-world, and give its ten-dimensional interpretation in terms of IIB 3-branes. We also explain how previous no-go theorems are circumvented. In particular, whereas D=5 supergravity scalars have AdS5 energy E0⩽4 and are unable to support a D=5 positive tension brane, our scalar has E0=8, and is the breathing mode of the S5 compactification. Another essential element of the construction is the implementation of a Z2 symmetry by patching together compactifications with opposite signs for their 5-form field strengths. This is thus a IIB analog of a previous D=5 3-brane realization of the Hořava–Witten orbifold. A mode-locking phenomenon avoids the appearance of negative energy zero-modes in spite of the necessity of a D=10 negative tension brane-source.
Relativistic Gaussian basis sets for molecular calculations: H–Xe
Authors: Koga Toshikatsu , Tatewaki Hiroshi , Matsuoka Osamu
Published: July 2002

Relativistic Gaussian basis sets suitable for molecular calculations are presented for the 54 atoms H through Xe. The basis sizes are rather compact and the same as the corresponding nonrelativistic basis sets reported by Koga et al. The exponent parameters of the Gaussian basis functions have been fully optimized separately for the l− and l+ symmetry species. The maximum truncation error in the total energies is 2.9 mhartree, and the virial deviation from −1 is less than 2.1×10−7. Test calculations are carried out on the Cu2 molecule.
Quasiphase-matched second-harmonic generation using periodically photoisomerized azo dye doped polymer film waveguide
Authors: Jung J. H. , Kato T. , Kinoshita T.
Published: July 2002

A process to form a periodically poled polymer film by photoisomerization is described. A poled film of polymethylmetaclyrate containing 5 wt % of 4-(4-nitrophenylazo)aniline (Disperse Orange 3) was exposed to an Ar+ laser beam using a photomask of a 100 μm period grating. Disorientation of the chromophore molecule resulted in decrease of d33 in the photoisomerized domain. Quasiphase-matched second-harmonic generation is demonstrated experimentally using a processed film. Phase-matching was confirmed by tuning the fundamental wavelength around 1200 nm. The effective propagation length due to a tightly focused fundamental wave and the influence of the absorption loss are discussed. The depoling process by photoisomerization is compared with photobleaching, optical poling, and electron beam treatment.
Optical characterization of GaAs pyramid microstructures formed by molecular beam epitaxial regrowth on pre-patterned substrates
Authors: Pritchard R. E. , Oulton R. F. , Stavrinou P. N. , Parry G. , Williams R. S. , Ashwin M. J. , Neave J. H. , Jones T. S.
Published: July 2002

Arrays of GaAs pyramids with square (001) bases of length 1–5 μm have been fabricated by molecular beam epitaxy regrowth on pre-patterned GaAs (001) substrates. The optical properties of the pyramid faces have been studied by microreflection and microtransmission imaging measurements with light (λ=900–1000 nm) incident through the pyramid base. Digitized charge coupled device images indicate that total internal reflection occurs at the {110} pyramid facets and that their reflectivities are greater than 80%, provided overgrowth of the facets does not occur. These properties suggest that such structures may be suitable as the top mirror in novel micron-scale vertical microcavity devices.
A Review of Recent Experiments and Calculations Relevant to the Kinetics of the HF Laser
Authors: Manke Gerald C. , Hager Gordon D.
Published: July 2002

A review of rate coefficients relevant to HF laser kinetics modeling is presented. The literature has been surveyed from the last published review in 1983 to the present. Updated HF Einstein emission coefficients are tabulated. Rate coefficients are categorized according to their role in the HF laser model: HF generation, reactive quenching, self-relaxation, V–V energy transfer, vibrational relaxation by atoms and molecules, F2 dissociation, and F atom recombination. In addition, a review of recent experiments and theoretical calculations relevant to the role of rotational nonequilibrium in HF lasers is presented. A list of recommended temperature dependent expressions for critical reaction rate coefficients is given.
A dynamic subfilter-scale model for plane parallel flows
Authors: Dubrulle B. , Laval J. P. , Nazarenko S. , Kevlahan N. K.-R.
Published: July 2002

We present a dynamic model of the subfiltered scales in plane parallel geometry using a generalized, stochastic rapid distortion theory (RDT). This new model provides expressions for the turbulent Reynolds subfilter-scale stresses via estimates of the subfilter velocities rather than velocity correlations. Subfilter-scale velocities are computed using an auxiliary equation which is derived from the Navier–Stokes equations using a simple model of the subfilter energy transfers. It takes the shape of a RDT equation for the subfilter velocities, with a stochastic forcing. An analytical test of our model is provided by assuming delta-correlation in time for the supergrid energy transfers. It leads to expressions for the Reynolds stresses as a function of the mean flow gradient in the plane parallel geometry and can be used to derive mean equilibrium profiles both in the near-wall and core regions. In the near-wall region we derive a general expression for the velocity profile which is linear in the viscous layer and logarithmic outside. This expression involves two physical parameters: the von Karman constant and the size of the viscous layer (which can be computed via a numerical implementation of our model). Fits of experimental profiles using our general formula provides reasonable values of these parameters (κ=0.4 to κ=0.45, the size of the viscous layer is about 15 wall units). In the core region, we find that the shape of the profile depends on the geometry of the flow; it ranges from algebraic in channel flow, to exponential in the bulk of boundary layers, and linear in plane Couette flow. This classification is consistent with Oberlack’s system, which is based on symmetry arguments. Fits of boundary layer flows or channel flows at different Reynolds number over the whole flow region are performed using our results, and are found to be in very good agreement with available data.
Trap-assisted tunneling at temperatures near 77 K in laser annealed Si n+-p junctions
Authors: Simeonov S. S. , Kafedjiiska E. , Szekeres A. , Ristoscu C. , Gyorgy E. , Mihailescu I. N.
Published: July 2002

The I–V characteristics of the n+-p Si junction diodes prepared by laser annealing of a p-type Si substrate, covered by a Si–P thin film, were measured in the 77–294 K temperature range. The I–V characteristics of these diodes were found to be independent of temperature in the range 77–120 K. It was established by C–V measurements that the acceptor concentration in the p side of these diodes is in the 1014–1015 cm−3 range. A trap-assisted tunneling mechanism was used to explain the temperature-independent conductivity of these laser annealed n+-p Si junction diodes. This tunneling mechanism is based on the recombination of tunneling holes in the n-side bulk adjacent to the space charge layer in the n+-p junction.
The effects of expansion strength on large-scale structures in compressible free shear layers
Authors: Smith K. M. , Dutton J. C.
Published: July 2002

Planar visualizations of two compressible free shear layers were performed immediately downstream of centered expansions of differing strengths in order to assess the influence of expansion strength on the embedded large-scale structures. The free shear layers studied here were formed through the separation of an approach flow, either a Mach 2.0 stream or a Mach 2.5 stream, from a planar backstep. In addition to side-view and end-view visualizations, spatial correlations (computed from large image ensembles) and laser Doppler velocimetry surveys of the free shear layers were also examined to discern relationships between the structure dynamics and the underlying pre- and postexpansion velocity fields. The instantaneous images clearly illustrate that ellipsoidal, highly coherent structures were present in both shear layers downstream of the expansion corner. The dissimilar expansion strengths did not appear to produce qualitatively different structures in the shear layers; however, as compared to the weaker expansion, the stronger expansion did result in an increase in the growth rate of the large-scale structures, apparently from an augmentation of the 〈u′v′〉∂U/∂y production term in the TKE equation. Furthermore, quantitative measurements of the mean structure geometry, as determined from the spatial correlation fields, revealed that a stronger expansion strength resulted in a larger aspect ratio of the mean structures (i.e., the structures were stretched preferentially in the streamwise and transverse directions as compared to the spanwise direction during the expansion process). Quadrant decompositions of the instantaneous velocity fluctuations within the approach boundary layers and within the free shear layers indicated a definite increase in structure organization across the expansion region, which is in contrast with studies of expanded supersonic boundary layers without separation. The instantaneous image data, spatial correlations, and velocity decompositions uniformly suggest that the separation process itself, and not the expansion strength, is the primary influence on initial eddy structure in the postexpansion free shear layer.
Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes
Authors: Nguyen N. D. , Germain M. , Schmeits M. , Schineller B. , Heuken M.
Published: July 2002

Thermal admittance spectroscopy measurements at temperatures ranging from room temperature to 90 K are performed on Schottky structures based on Mg-doped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The analysis of the experimental data is made by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structures. Numerical simulations are based on the solution of the basic semiconductor equations for the structure consisting of two Schottky diodes connected back to back by a conduction channel formed by the GaN layer. The description explicitly includes the Mg-related acceptor level, with its temperature- and position-dependent incomplete occupation state, leading to a dynamic exchange with the valence band. It fully reproduces the variations with temperature of the capacitance–frequency and conductance over frequency curves, allowing to give for all temperature ranges the origin of the various contributions to the junction capacitance and of the microscopic mechanisms responsible for the capacitance–frequency cutoff. Series resistance effects are shown to be dominant at temperatures above 230 K, whereas the Mg-related acceptor level governs the electrical behavior below 230 K. The existence of a second acceptor level with an activation energy of several tens of meV is revealed from the analysis of the characteristics at low temperature. An optimized fitting procedure based on the comparison of the electrical characteristics obtained from the numerical simulations to the experimental data allows one to determine the microscopic parameters describing the structure, among which the acceptor activation energies, thermal capture cross sections, concentrations, and the Schottky contact barrier heights are the most important ones. The obtained activation energy of the Mg-acceptor level of 210 meV is by a factor of 2 larger than that obtained from a classical Arrhenius plot, showing that a complete description of Mg-doped GaN junctions requires the correct treatment of the Mg level, acting as a dopant and as deep impurity, as well as the inclusion of series resistance effects.
Numerical modeling of fluctuation phenomena in semiconductor devices
Authors: Józwikowski Krzysztof
Published: July 2002

A method enabling numerical modeling of fluctuation phenomena in semiconductor devices is presented. The method is based on the assumption that fluctuations of generation–recombination processes and carrier mobility result in the fluctuations of carrier and ionized impurity concentrations. These, in turn, may be expressed by the fluctuations of the electrical potential and quasi-Fermi levels. Fluctuations of the electrical potential and quasi-Fermi-levels were calculated by solving the set of “transport equations for fluctuations” in which the fluctuations of generation–recombination processes (both thermal and optical) and fluctuations of mobility play roles of random source terms. The method enables the calculation of fluctuations of all physical quantities enclosed in a set of transport equations. The spatial distribution of the fluctuations of the electrical potential, electron concentration, and noise current density is shown. The noise spectrum in selected, cooled, long-wavelength HgCdTe photoresistors is calculated and the contribution of different noise sources is determined. Theoretical results are compared with experimental data.
Characterization of plasmas produced by laser–gas jet interaction
Authors: Malka V. , Faure J. , Amiranoff F.
Published: July 2002

An experiment has been performed with one of the six nanosecond beams of the Laboratoire pour l’Utilisation des Lasers Intenses laser facility in order to create long scale uniform plasmas over a wide range of electron density (1×1019–1.6×1020 cm−3) and electron temperature (0.5–1.3 keV). Electron density and temperature evolution have been measured using Thomson scattering. Numerical simulations obtained by using a simple model are presented. Scaling law related electron density and electron temperature have been established in agreement with experimental data.
Electrostatic force density for a scanned probe above a charged surface
Authors: Passian A. , Wig A. , Meriaudeau F. , Buncick M. , Thundat T. , Ferrell T. L.
Published: July 2002

The Coulomb interaction of a dielectric probe tip with a uniform field existing above a semi-infinite, homogeneous dielectric substrate is studied. The induced polarization surface charge density and the field distribution at the bounding surface of the dielectric medium with the geometry of half of a two sheeted hyperboloid of revolution located above the dielectric half space interfaced with a uniform surface charge density is calculated. The force density on the hyperboloidal probe medium is calculated as a function of the probe tip shape. The calculation is based on solving Laplace’s equation and employing a newly derived integral expansion for the vanishing dielectric limit of the potential. The involved numerical simulations comprise the evaluation of infinite double integrals involving conical functions.
Effect of pressure on the magnetic and transport properties of the ferrimagnetic semiconductor FeCr2S4
Authors: Tsurkan V. , Fita I. , Baran M. , Puzniak R. , Samusi D. , Szymczak R. , Szymczak H. , Klimm S. , Klemm M. , Horn S. , Tidecks R.
Published: July 2002

The influence of hydrostatic pressure on magnetization, electrical conductivity and magnetoresistance of FeCr2S4 single crystals has been studied. Significant pressure influence on the temperature of a spin-glass like magnetization anomaly, dTm/dp ≈ 2.7 K/kbar, on the Curie temperature, dTC/dp≈0.9 K/kbar, as well as on the resistivity and magnetoresistance, was found. The negative magnetoresistance maximum at the Curie temperature is shifted under pressure towards higher temperatures. Pressure induced changes of the low field magnetoresistance and of the conductivity anomalies at Tm are correlated well with the changes in the magnetization. In the low temperature region the appearance of low symmetry anisotropy has been observed. This effect is distinctly enhanced by the hydrostatic pressure. The low symmetry component of magnetic anisotropy was suggested to be due to the static Jahn–Teller effect.
Buried oxide and defects in oxygen implanted Si monitored by positron annihilation
Authors: Kruseman A. C. , van Veen A. , Schut H. , Mijnarends P. E. , Fujinami M.
Published: July 2002

One- and two-detector Doppler broadening measurements performed on low (∼1014 to 1015 O+/cm2) and high dose (∼1017 to 1018 O+/cm2) oxygen-irradiated Si using variable-energy slow positrons are analyzed in terms of S and W parameters. After annealing the low-dose samples at 800 °C, large VxOy complexes are formed at depths around 400 nm. These complexes produce a clear-cut signature when the ratio of S to that of defect-free bulk Si is plotted. Similar behavior is found for samples irradiated with 2 and 4×1017 O+/cm2 and annealed at 1000 °C. After irradiation with 1.7×1018 O+/cm2 and anneal at 1350 °C a 170 nm thick almost-bulk-quality Si surface layer is formed on top of a 430 nm thick buried oxide layer. This method of preparation is called separation by implantation of oxygen. S−W measurements show that the surface layer contains electrically inactive VxOy complexes not seen by electron microscopy. A method is presented to decompose the Doppler broadening line shape into contributions of the bulk, surface, and defect.
Surface modification of CdS nanocrystallites doped in SiO2 matrix
Authors: Wang H. , Zhu Y. , Ong P. P.
Published: July 2002

Nanometer-sized CdS semiconductor particles were prepared by pulsed laser deposition (PLD) and dispersed in SiO2 matrix. Surface modification of the CdS nanocrystallites deposited with the SiO2 material on a substrate caused the generation of CdO composite, which formed an encapsulating shell layer surrounding the inner CdS crystallite. Raman and photoluminescence (PL) analyses clearly identified and characterized this component. Annealing of the film caused growth in size of the CdO shell as established from the redshift of its PL band gap emission. This growth was found to keep in unison with the corresponding inner CdS crystalline growth, and a possible mechanism is suggested. Finally, through proper selection of the substrate temperature during PLD to remove crystalline stress, it was possible to obtain a complete optimum CdS profile without the accompanying CdO formation or characteristics.
Gyrokinetic study of the electron temperature gradient instability in plasmas with slightly hollow density profiles
Authors: Dong J. Q. , Sanuki H. , Itoh K.
Published: July 2002

The electron temperature gradient driven instabilities in plasmas with slightly hollow density profiles are studied. The gyrokinetic integral eigenvalue equation valid for a sheared slab configuration is employed. The Debye shielding effect on the modes is investigated. The effects of a sheared E×B flow on the modes are considered. Six modes with even or odd parities are found to be simultaneously unstable. The mixing length estimate for the transport is calculated. The flow shear suppression on the modes, as well as on the transport, is demonstrated. The correlations of the results with helical system Heliotron/Torsotron and tokamak experiments are discussed.
First operation of circular dichroism measurements with periodic photon-helicity switching by a variably polarizing undulator at BL23SU at SPring-8
Authors: Agui A. , Yoshigoe A. , Nakatani T. , Matsushita T. , Saitoh Y. , Yokoya A. , Tanaka H. , Miyahara Y. , Shimada T. , Takeuchi M. , Bizen T. , Sasaki S. , Takao M. , Aoyagi H. , Kudo T. P. , Satoh K. , Wu S. , Hiramatsu Y. , Ohkuma H.
Published: July 2002

This article presents the first operation of the magnetic circular dichroism (MCD) measurement system with periodic photon-helicity switching. The measurements were performed at the newly constructed soft x-ray beamline—BL23SU— at the third-generation synchrotron radiation facility, SPring-8. The monochromator control system was synchronized to the movement of the magnetic row (phase shift) of an APPLE-2 (Sasaki) type variably polarizing undulator. The periodic phase shift of the undulator provided the switching of helicity polarizing soft x rays up to 0.1 Hz. The closed-orbit distortion of the storage ring was controlled to avoid optical axis disturbances at this beamline as well as at other beamlines. The circular dichroism spectra with helicity switching by APPLE-2 show the possibility of high-sensitivity MCD measurements. This method promotes precise MCD measurements and can be a powerful technique to study magnetism as well as dichroism.
Effects of thickness on the electrical properties of metalorganic chemical vapor deposited Pb(Zr, Ti)O3 (25–100 nm) thin films on LaNiO3 buffered Si
Authors: Lin C. H. , Friddle P. A. , Ma C. H. , Daga A. , Chen Haydn
Published: July 2002

Pb(Zr, Ti)O 3 (PZT) thin films with (100) preferred orientation were prepared using metalorganic chemical vapor deposition on LaNiO3 (LNO) buffered platinized Si with thickness varying from 25–100 nm. The dependence of electrical properties of PZT films on thickness was studied using several techniques, including polarization–electric field (P–E), temperature variable current–voltage (I–V), and capacitance–voltage (C–V) measurements. Because of the formation of Schottky barriers at ferroelectric/electrode interfaces, built-in electric fields are present. A progressive increment in carrier concentration and interfacial built-in electric field versus reducing PZT film thickness was observed, which is believed to be a dominant factor controlling the measured dielectric/ferroelectric properties. The higher built-in electric field in thinner PZT films would pin the dipoles at the interfacial region and retard the response of dipoles to the external electric field.
Influence of a partially oxidized calcium cathode on the performance of polymeric light emitting diodes
Authors: Andersson G. G. , de Jong M. P. , Janssen F. J. J. , Sturm J. M. , van IJzendoorn L. J. , Denier van der Gon A. W. , de Voigt M. J. A. , Brongersma H. H.
Published: July 2002

We investigated the influence of the presence of oxygen during the deposition of the calcium cathode on the structure and on the performance of polymeric light emitting diodes (pLEDs). The oxygen background pressure during deposition of the calcium cathode of polymeric LEDs was varied. Subsequently, the oxygen depth distribution was measured and correlated with the performance of the pLEDs. The devices have been fabricated in a recently built ultraclean setup. The polymer layers of the pLEDs have been spincoated in a dry nitrogen atmosphere and transported directly into an ultrahigh vacuum chamber where the metal electrodes have been deposited by evaporation. We used indium–tin–oxide as anode, OC1C10 PPV as electroluminescent polymer, calcium as cathode, and aluminum as protecting layer. We achieved reproducibility of about 15% in current and brightness for devices fabricated in an oxygen atmosphere of ≪10−9 mbar. For further investigations the calcium deposition was carried out in an oxygen atmosphere from 10−8 to 10−5 mbar. We determined the amount of oxygen in the different layers of the current–voltage-light characterized pLEDs with elastic recoil detection analysis and correlated it with the characteristics of the devices. The external efficiency of the pLEDs decreases continuously with increasing oxygen pressure, the current shows a pronounced minimum. The brightness mostly decreases with increasing oxygen with an indication of a slight minimum. PLEDs with completely oxidized calcium are not operational. The first contact of the pLEDs with the dry glove box environment leads to an immediate reduction of current and brightness which is caused by the cooling of the devices by several degrees. Determining reproducible characteristics of pLEDs in the vacuum requires the measurement of their temperature.
Magneto-optical study of electron occupation and hole wave functions in stacked self-assembled InP quantum dots
Authors: Hayne M. , Maes J. , Moshchalkov V. V. , Manz Y. M. , Schmidt O. G. , Eberl K.
Published: July 2002

We have studied the magnetophotoluminescence of doubly stacked layers of self-assembled InP quantum dots in a GaInP matrix. 4.0±0.1 monolayers of InP were deposited in the lower layer of each sample, whereas in the upper layer 3.9, 3.4, and 3.0 monolayers were used. Low-temperature photoluminescence measurements in zero magnetic field are used to show that, in each case, only one layer of dots is occupied by an electron, and imply that when the amount of InP in both layers is the same, the dots in the upper layer are larger. High-field photoluminescence data reveal that the position and extent of the hole wave function are strongly dependent on the amount of InP in the stack.
Switching effect perpendicular to the plane of Pr0.5Ca0.5MnO3−y thin films
Authors: Sakai Joe , Kitagawa Atsushi , Imai Syozo
Published: July 2002

Au/Pr 0.5 Ca 0.5 MnO 3−y  (PCMO)/YBa2Cu3O7−z(YBCO) sandwich-type junctions were fabricated on MgO (001) substrates, and their resistivity was measured with a constant voltage applied between the top Au and bottom YBCO layers. In the low-temperature region, the resistivity was drastically suppressed for several orders with only a few volts applied due to field-induced switching effect of PCMO. When the voltage was removed from the junctions in a low-resistivity state, two types of behavior were observed: one which recovers to a high-resistivity state and another which does not.
Convertible transmission-reflection time-domain terahertz spectrometer
Authors: Khazan Max , Meissner Reinhold , Wilke Ingrid
Published: July 2002

The creation of reliable instrumentation for performing complex reflectance and transmittance measurements of dielectrics, metals, and superconductors in the frequency range from 60 GHz to 1.5 THz is reported. The system allows continuous variation of the THz radiation incidence angle from 25° to 80° without major realignment of the optics and provides the signal-to-noise ratio of 1000:1.
Formation and characterization of (553)B In0.15Ga0.85As/GaAs quantum wire structure
Authors: Yan Fa-Wang , Zhang Wen-Jun , Zhang Rong-Gui , Cui Li-Qi , Liang Chun-Guang , Liu Shi-Young
Published: July 2002

Strained In0.15Ga0.85As/GaAs quantum wires (QWRs) are naturally formed on a (553)B-oriented GaAs substrate by molecular beam epitaxy. The QWR structure with a corrugated upper interface and a flat lower interface is directly confirmed by atomic force microscopy. The average lateral period and step height of (553)B QWRs are revealed to be about 25 and 1.6 nm, respectively. Accordingly the density of (553)B QWRs is as high as 4.0×105 cm−1, which is several times larger than that of QWR arrays fabricated by the finest lithography. Moreover, the (553)B QWRs are intensively studied by photoluminescence (PL). In comparison with the reference (100) In0.15Ga0.85As/GaAs quantum wells, the PL peaks of the (553)B QWRs show slight redshifts and strong polarization dependence at 12 K, indicating a two-dimensional carrier confinement effect due to the corrugated structure. The narrow full width at half maximum of the PL peak at λ=861 nm is as small as 9.2 meV, which indicates the high uniformity and good optical quality of the (553)B QWRs.
Characteristics of two Coulomb blockade transistors separated by an island to which an oscillating potential is applied: Theory and experiment
Authors: Altebaeumer Thomas , Amakawa Shuhei , Ahmed Haroon
Published: July 2002

Two Coulomb blockade devices connected in series and separated by a main island to which a rf signal was applied, are characterized electrically. The main island is large enough so that Coulomb phenomena in it can be neglected. Although no source–drain voltage is applied, the rf signal causes a current. The polarity of this current depends on the dc gate voltages, which control the two Coulomb blockade devices. The experimental data correspond closely to the simulations. Furthermore, it is evident from theoretical considerations that the circuit can pump electron packets against an applied potential.
Resistive wall modes and nonuniform wall rotation
Authors: Taylor J. B. , Connor J. W. , Gimblett C. G. , Wilson H. R. , Hastie R. J.
Published: July 2002

The resistive wall mode (RWM) poses a threat to many plasma confinement devices. The continuous rotation of the wall relative to the plasma makes it appear perfectly conducting, because of the skin effect, but this is ineffective if the perturbation locks to the wall. This raises the question of whether a nonuniformly rotating wall is more effective. In this paper we discuss the effect of such nonuniform wall rotation, in both the toroidal and poloidal directions, on resonant and nonresonant RWMs. In the case of toroidal rotation it is shown that at large wall velocity both the resonant and nonresonant RWMs are stabilized, even though the nonresonant mode rotates with the maximum wall velocity. In the case of poloidal rotation RWMs do not lock to the wall and have a complicated behavior at intermediate velocities. However they are again stabilized by large wall velocity.
Colossal magnetoresistance in doped manganites: A consequence of percolation and phase separation
Authors: Pradhan A. K. , Feng Y. , Roul B. K. , Sahu D. R.
Published: July 2002

We report measurements of the resistivity and magnetization of high-quality melt-processed La0.67Ca0.33MnO3 (LCM) and Sr-doped LCM materials that exhibit a sharp and enhanced metal–insulator transition (TMI) with significant colossal magnetoresistance (CMR) and two competing magnetic phases in the vicinity of TMI. Our results can well be explained using percolation in combination with the phase-separation scenario proposed for CMR. We propose that the spin polarization through grain boundaries and the enhanced grain connectivity between the ferromagnetic domains facilitate better percolation through a junction and filament-like resistor network.
Full-dimensional quantum study of the vibrational predissociation of the I2⋯Ne2 cluster
Authors: Meier C. , Manthe U.
Published: July 2002

The dynamics of the vibrational predissociation of the I2Ne2(B,ν=21) cluster is studied using a full-dimensional time-dependent wave packet approach. The 6-D propagation is performed with the multiconfiguration time-dependent Hartree (MCTDH) method. The time dependence of the I2 vibrational state populations is monitored during the dissociation process. The final vibrational populations are compared to experimental findings of Levy et al.
Fast 1 kV metal-oxide-semiconductor field-effect transistor switch
Authors: Dedman C. J. , Roberts E. H. , Gibson S. T. , Lewis B. R.
Published: July 2002

A fast, high-voltage switch based on cheap and readily available components is described. This simple circuit can switch 1 kV to ground with a fall time of ∼2.5 ns, and has proved a cost-effective means of driving electrostatic gating and rereferencing devices in pulsed ion-beam experiments.
Mechanism of terahertz lasing in SiGe/Si quantum wells
Authors: Blom A. , Odnoblyudov M. A. , Cheng H. H. , Yassievich I. N. , Chao K. A.
Published: July 2002

Intense terahertz (THz) stimulated emission from boron-doped SiGe/Si quantum well structures with internal strain has been observed recently. We present a theoretical calculation which shows the formation of resonant states, and explains the origin of the observed temperature dependence of the dc conductivity under low bias voltage. Thus, the mechanism of THz lasing is population inversion of the resonant state with respect to the localized impurity states. This is the same mechanism of lasing as in uniaxially stressed p-Ge THz lasers.
Matrix-isolation investigation of the diatomic anion radicals of aluminum and gallium (Al2− and Ga2−): An electron spin resonance and ab initio theoretical study
Authors: Stowe Ashley C. , Kaup John G. , Knight Lon B. , Davis Jeffrey R. , McKinley Allan J.
Published: July 2002

The diatomic aluminum and gallium anion radicals (Al2− and Ga2−) have been studied by electron spin resonance (ESR) as isolated ions in neon matrices near 4 K. Apparently no previous examples of purely metallic anions have been studied by the rare gas matrix isolation trapping technique with any type of spectroscopic method. Pulsed laser ablation of the metals was the experimental generation method which has enabled the first direct characterization of the ground electronic states of these anions. The X 4Σ assignment agrees with previous interpretations of photoelectron spectroscopic results. The analysis of fine structure and nuclear hyperfine interactions has yielded a detailed description of the electronic structure in these seven-valence electron anion radicals, with comparisons to other isovalent X 4Σ radicals presented. Experimental values of the nuclear hyperfine interactions (A tensors) were compared with theoretical results obtained from various ab initio computational methods, including configuration interaction and density functional theory. Reasonable agreement between these experimental results and theoretical predictions was observed.
Fabrication of epitaxial CoSi2 nanowires
Authors: Kluth P. , Zhao Q. T. , Winnerl S. , Lenk S. , Mantl S.
Published: July 2002

We have developed a method for fabricating epitaxial CoSi2 nanowires using only conventional optical lithography and standard silicon processing steps. This method was successfully applied to ultrathin epitaxial CoSi2 layers grown on Si(100) and silicon-on-insulator substrates. A nitride mask induces a stress field near its edges into the CoSi2/Si heterostructure and leads to the separation of the CoSi2 layer in this region during a rapid thermal oxidation step. A subsequent etching step and a second oxidation generate highly homogenous silicide wires with dimensions down to 50 nm.
Nitrogen incorporation in ultrathin gate dielectrics: A comparison of He/N2O and He/N2 remote plasma processes
Authors: Khandelwal Amit , Smith Bradley C. , Lamb H. Henry
Published: July 2002

Ultrathin Si oxynitride films grown by low-temperature remote plasma processing were examined by on-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy to determine the concentration, spatial distribution, and chemical bonding of nitrogen. The films were grown at 300 °C on Si(100) substrates using two radio-frequency remote plasma processes: (i) He/N2O remote plasma-assisted oxidation (RPAO) and (ii) two-step remote plasma oxidation/nitridation. A 5 min He/N2O RPAO process produces a 2.5 nm oxynitride film incorporating approximately 1 monolayer of nitrogen at the Si–SiO2 interface. The interfacial nitrogen is bonded in a N–Si3 configuration, as in silicon nitride (Si3N4). By comparison, a 90 s He/N2 remote plasma exposure of a 1 nm oxide (grown by 10 s He/O2 RPAO) consumes substrate Si atoms creating a 1 nm subcutaneous Si3N4 layer. The nitrogen areal density obtained via the two-step process depends on the initial oxide thickness and the He/N2 remote plasma exposure time. Moreover, as the oxide thickness is increased (by increasing the He/O2 remote plasma exposure), the nitrogen distribution shifts away from the Si–SiO2 interface and into the oxide. More nitrogen with a tighter distribution is incorporated using He versus Ar dilution. Insight into the remote plasma chemistry was provided by optical emission spectroscopy. Strong N2 first positive and second positive emission bands were observed for He/N2O and He/N2 remote plasmas indicating the presence of N2 metastables and ground-state N atoms.