bluuebunny/crossref_metadata_2025 huggingface dataset:
Suppression of implantation-induced damage in 6H–SiC by simultaneous excimer laser irradiation during ion implantation
Authors: Hishida Yuji , Watanabe Masanori , Sekine Kohei , Sugino Kenichi , Kudo Jun
Published: July 2002

The implantation-induced damage suppression effect on 6H–SiC by simultaneous excimer laser irradiation during ion implantation (SLII) was demonstrated. A 308-nm XeCl excimer laser was used as the light source. Secondary ion mass spectroscopy indicated that SLII causes N atom diffusion in SiC. The damage suppression effect was evaluated by Rutherford backscattering channeling measurement. SLII proved to be effective for the suppression of implantation-induced damage in SiC. In the vicinity of the surface, SiC with almost no implantation-induced damage was obtained without postimplantation annealing.
Formation of single and double self-organized InAs quantum dot by selective area metal-organic vapor phase epitaxy
Authors: Hahn Cheol-Koo , Motohisa Junichi , Fukui Takashi
Published: July 2002

Position- and number-controlled InAs self-organized quantum dots (SOQDs) were formed by selective-area-metal-organic chemical vapor phase epitaxy on the partially SiNx patterned GaAs (001) substrate. The mask layer was patterned along the [110] direction, and somewhat wider rectangular openings were also attached to the line. As a result of GaAs SA growth, a pyramidal shaped structure was formed on the rectangular region of the pattern. The top area of the pyramidal structure is a very narrow hexagonal-shaped (001) facet which is surrounded by two-{111}B and four-{124} facets. The SOQD was preferentially formed on the top (001) facet because the growth rate on the (001) facet is far much higher than on the surrounding sidewalls. It is found that the number of SOQDs formed is strongly dependent on the width of the top (001)-facet so that control of single, double, and multiple SOQD(s) is possible.
Electron emission from Pb-based ferroelectrics, antiferroelectrics, and paraelectrics by pulse electric field
Authors: Kim Yong Tae , Yoon Ki Hyun , Kim Tae Heui , Park Kyung Bong
Published: July 2002

Pulse electric-field-induced electron emissions from Pb-based ferroelectrics, antiferroelectrics, and paraelectrics were investigated. The electron emission charge increased exponentially with applied pulse field regardless of the dielectric phases. Electrons were emitted at the falling point as well as at the rising point of an applied pulse field when the antiferroelectrics and paraelectrics were used. The variation of emission charge from antiferroelectrics and paraelectrics was similar to that from ferroelectrics, as the emission charge increased exponentially with the polarization change [Ps-(−Pr)].
Visualization of photoexcited free carriers by scanning near-field millimeter-wave microscopy
Authors: Nozokido Tatsuo , Bae Jongsuck , Mizuno Koji
Published: July 2002

Visualization of transition phenomena of photoexcited free carriers by scanning near-field millimeter-wave microscopy has been demonstrated. A scanning millimeter-wave microscope using a metal slit-type probe and an image reconstruction algorithm based on computerized tomographic imaging has been used in the experiment to achieve two-dimensional time-resolved imaging. Experiments performed at 60 GHz (λ=5 mm) under room temperature conditions show that generation, extinction, and diffusion processes of photoexcited free carriers generated in the silicon layer of a silicon on quartz substrate can be imaged with a time division of one nanosecond and a spatial resolution of 110 μm (∼λ/45).
In-plane magnetophotoluminescence studies of modulation-doped GaAs/AlGaAs coupled double quantum wells
Authors: Kim Yongmin , Perry C. H. , Simmons J. A. , Klem J. F.
Published: July 2002

In-plane magnetic-field photoluminescence spectra from a series of n-type modulation-doped GaAs/Al0.3Ga0.7As coupled double quantum wells show distinctive doublet structures related to the tunnel-split ground sublevel states. The magnetic-field behavior of the upper transition from the antisymmetric state strongly depends on sample mobility. In a lower mobility sample, the transition energy displays an 𝒩-type kink with field (namely, a maximum followed by a minimum), whereas higher mobility samples have a linear dependence. The former is attributed to a coupling mechanism due to homogeneous broadening of the electron and hole states. The results are in good agreement with recent theoretical calculations.
Evidence for ferroelectric border traps near the SrBi2Ta2O9/Si interface through capacitance–voltage measurement
Authors: Li W. P. , Zhang R. , Shen J. , Liu Y. M. , Shen B. , Chen P. , Zhou Y. G. , Li J. , Yuan X. L. , Chen Z. Z. , Shi Y. , Liu Z. G. , Zheng Y. D.
Published: July 2002

A metal–ferroelectric–semiconductor (MFS) structure has been developed by depositing SrBi2Ta2O9 (SBT) films directly on n-type (100) Si by pulsed laser deposition. In the MFS structure, evidence for ferroelectric border traps in the SBT film has been obtained by high-frequency capacitance–voltage (C–V) measurement. When the ramp rate of voltage is higher than 200 mV/s, typical ferroelectric C–V hysteresis loops with the counterclockwise direction are obtained in C–V plots. When the ramp rate is lower than 80 mV/s, the ferroelectric hysteresis loops are replaced by the trap-induced ones with the clockwise direction. This pronounced change results from the fact that more and more border traps in SBT can communicate with the underlying Si. The border-trap density at the ramp rate of 10 mV/s is as high as 1.8×1012 cm−2. Moreover, the width of the hysteresis loops changes linearly with the logarithmic decrease in ramp rate, which is consistent with the ferroelectric border traps communicating with Si by tunneling or a thermally activated process.
Depolarization by high aperture focusing
Authors: Bahlmann K. , Hell S. W.
Published: July 2002

We introduce a method employing ferroelectric monomolecular layers, by which it is possible to measure the light field polarization in the focus of a lens. This method allowed us to noninvasively establish the perpendicularly oriented focal field that is anticipated at high apertures. For a numerical aperture 1.4 oil immersion lens illuminated with linearly polarized plane waves, the integral of the modulus square of the perpendicular component amounts to (1.51±0.2) % of that of the initial polarization. It is proven that depolarization decreases with decreasing aperture angle. Whereas for regular imaging conditions depolarization is largely negligible, it plays a significant role in microscopy of highest resolution, microspectroscopy, and single molecule studies.
Nonlinear interactions in a rotating disk flow: From a Volterra model to the Ginzburg–Landau equation
Authors: Floriani E. , Dudok de Wit T. , Le Gal P.
Published: July 2002

The physical system under consideration is the flow above a rotating disk and its cross-flow instability, which is a typical route to turbulence in three-dimensional boundary layers. Our aim is to study the nonlinear properties of the wavefield through a Volterra series equation. The kernels of the Volterra expansion, which contain relevant physical information about the system, are estimated by fitting two-point measurements via a nonlinear parametric model. We then consider describing the wavefield with the complex Ginzburg–Landau equation, and derive analytical relations which express the coefficients of the Ginzburg–Landau equation in terms of the kernels of the Volterra expansion. These relations must hold for a large class of weakly nonlinear systems, in fluid as well as in plasma physics.
On the role of the pore filling medium in photoluminescence from photochemically etched porous silicon
Authors: Kolasinski Kurt W. , Barnard John C. , Ganguly Santanu , Koker Lynne , Wellner Anja , Aindow Mark , Palmer Richard E. , Field Christian N. , Hamley Paul A. , Poliakoff Martyn
Published: July 2002

Porous silicon thin films created under laser illumination in fluoride solutions without biasing have been studied by a variety of techniques to investigate the film structure and photoluminescence (PL). The use of ultrathin silicon wafers allows us to perform plan view transmission electron microscopy studies without recourse to thinning procedures that might adversely affect the film structure. Supercritically dried samples are compared to air dried samples and clearly demonstrate the deleterious effects of air drying on film structure. PL studies were performed (a) while the sample is submerged in aqueous HF, (b) in Ar after rinsing in ethanol, and (c) in air after rinsing in ethanol. The wavelength of light used to fabricate the film is found to correlate strongly with the peak PL wavelength when measured in solution. Little correlation is found in Ar or in air. Exposure to air can change the PL spectrum dramatically on a time scale of just seconds. We demonstrate that samples can exhibit essentially identical PL spectra in one medium but have spectra that differ from one another when the samples are placed in a different medium. The PL results indicate that band-to-band recombination cannot explain photon emission under all circumstances, and that surface states must also be involved in radiative processes under those conditions in which the bands are sufficiently separated to allow for the appearance of gap states.
High accuracy non-Born–Oppenheimer calculations for the isotopomers of the hydrogen molecule with explicitly correlated Gaussian functions
Authors: Kinghorn Donald B. , Adamowicz Ludwik
Published: July 2002

The first rigorous, variational results for the nonadiabatic (i.e., non-Born–Oppenheimer) ground states of the six isotopomers of the hydrogen molecule are reported. Ground-state energies in Hartrees are: H2[−1.164 025 023 2] [this result was reported by us earlier in Phys. Rev. Lett. 83, 2541 (1999)], D2[−1.167 168 78], T2[−1.168 535 65], HD[−1.165 471 906], HT[−1.166 002 033], and DT[−1.167 819 642]. Expectation values for the kinetic and potential energies, the internuclear distance and the square of the internuclear distance, the virial coefficient, and the square of the energy gradient norm for the optimized wave functions are also reported. The calculations were performed with a direct nonadiabatic variational approach using a new diatomic correlated Gaussian basis set exponentially dependent on interparticle distances and including pre-exponential powers of the internuclear distance.
Impedance field and noise of submicrometer n+nn+ diodes: Analytical approach
Authors: Bulashenko O. M. , Gaubert P. , Varani L. , Vaissiere J. C. , Nougier J. P.
Published: July 2002

A theoretical model for the noise properties of n+nn+ diodes in the drift-diffusion framework is presented. In contrast with previous approaches, our model incorporates both the drift and diffusive parts of the current under inhomogeneous and hot-carrier conditions. Closed analytical expressions describing the transport and noise characteristics of submicrometer n+nn+ diodes, in which the diode base (n part) and the contacts (n+ parts) are coupled in a self-consistent way, are obtained.
Electronic transport in thin film electroluminescence of SrS:Ce
Authors: Xu Chunxiang , Cui Yiping , Xu Xurong
Published: July 2002

Thin film electroluminescent (TFEL) devices based on the scheme of SiO2/SrS:Ce/SiO2/SiO were fabricated, and blue–green emission was observed in the devices. The electronic transport process is strongly affected by the mixing interaction between the conduction band of SrS and the excited state of Ce3+. The photoluminescent excitation and the wave forms of time-dependent electroluminescent emission show that recombinant luminescence is the main process in SrS:Ce TFEL. The experimental results reveal that the excitation mechanism is related to the sulfur vacancies.
Quantum solvation and molecular rotations in superfluid helium clusters
Authors: Kwon Yongkyung , Huang Patrick , Patel Mehul V. , Blume Dörte , Whaley K. Birgitta
Published: July 2002

Spectroscopic experiments on molecules embedded in free clusters of liquid helium reveal a number of unusual features deriving from the unique quantum behavior of this nanoscale matrix environment. The apparent free rotation of small molecules in bosonic He4 clusters is one of the experimentally most well documented of these features. In this Focus article, we set this phenomenon in the context of experimental and theoretical advances in this field over the last ten years, and describe the microscopic insight which it has provided into the nature and dynamic consequences of quantum solvation in a superfluid. We provide a comprehensive theoretical analysis which is based on a unification of conclusions drawn from diffusion and path integral Monte Carlo calculations. These microscopic quantum calculations elucidate the origin of the empirical free rotor spectrum, and its relation to the boson character and superfluid nature of the quantum nanosolvent. The free rotor behavior of the molecular rotation is preserved because of inefficient angular momentum coupling between the dopant and its quantum liquid surroundings. This is consistent with the superfluid character of the droplet, and has significant implications for the hydrodynamic response of the local quantum fluid environment of the embedded molecule. The molecule–helium interaction appears to induce a local nonsuperfluid density component in the first quantum solvation shell. This can adiabatically follow the molecular rotation, resulting in a reduction of the rotational constant. The dynamic nature of this adiabatically following density, its relation to the magnitude of the gas-phase molecular rotational constant and to the anisotropy of the interaction potential, are characterized with several examples. The role of the local superfluid density is analyzed within a continuum hydrodynamic model which is subject to microscopic quantum constraints. The result is a consistent theoretical framework which unites a zero temperature description based on analysis of cluster rotational energy levels, with a quantum two-fluid description based on finite temperature analysis of local quantum solvation structure in the superfluid.
Relaxation model of coherent island formation in heteroepitaxial thin films
Authors: Budiman R. Arief , Ruda Harry E.
Published: July 2002

A model for coherent island formation in heteroepitaxial thin films is presented by focusing on the interplay between surface and strain relaxation energies. The resulting free energy is mapped onto the Landau free energy, and yields the local surface slope as the order parameter for island formation. Isotropic islands are found to exhibit second-order transitions. We argue that our model is appropriate for describing the nucleation and stability of island formation. The spinodal curve indicates that there exists an unstable regime where spinodal-like islands emerge as low-slope islands, even in a highly mismatched heteroepitaxial system.
Time evolution of ion energy distributions and optical emission in pulsed inductively coupled radio frequency plasmas
Authors: Misakian Martin , Benck Eric , Wang Yicheng
Published: July 2002

This article reports the results of time-resolved measurements of ion energy distributions (IEDs), relative ion densities, as well as optical emissions and electrical characteristics in pulsed inductively coupled plasmas for the simple gas mixture of oxygen and argon (50%Ar:50%O2). The peak radio frequency power, frequency, repetition rate, and duty cycle were 200 W, 13.56 MHz, 500 Hz, and 85%, respectively. Examination of the time evolution of the data over the pulse cycle indicates that when the plasma is energized, it begins in the dim (E) mode undergoing at first slow changes in certain plasma parameters. After about 1.2 ms, a sudden transition to the bright (H) mode occurs. The characteristics of the IEDs are consistent with an average plasma potential profile that has (a) a relatively large collisionless sheath and narrow presheath during the dim mode and (b) a very narrow sheath and greatly extended presheath during the bright mode; the ion mean free path influences the peak position of the IED during the bright mode. For most of the pulse cycle, the relative abundances of the ions Ar+, O2+, and O+ maintain the relation %O2+⩾%Ar+>%O+.
Fresnel corrections to measurements of ambipolar diffusion length
Authors: Nicholson J. P.
Published: July 2002

The steady state photocarrier grating method is re-examined to take account of Fresnel reflection losses at the surface. With the present conventional experimental method, the key experimental quantity β is the ratio of two lock-in amplifier readings of the conductance of the specimen with and without photocarrier grating fringes, respectively. It is shown that because of the change in optical polarization between the two readings a multiplicative factor on the measured β is necessary. Both experimental and computed results show that this can have an effect on the derived ambipolar diffusion length and surface recombination velocity.
Electrical properties of MgO insulating layers in spin-dependent tunneling junctions using Fe3O4
Authors: Kiyomura T. , Maruo Y. , Gomi M.
Published: July 2002

We investigated the growth conditions and electrical properties of MgO epitaxial thin films, which have potential applications as insulating layers for spin-dependent tunneling devices where Fe3O4 serves as one of the magnetic electrodes. Our investigation showed that epitaxial MgO films with high crystalline quality can be successfully grown at temperatures as low as 473 K in oxygen pressures less than 1×10−5 Torr. This is a very important result because it indicates that the oxidation of the underlying Fe3O4 electrode is not a factor in fabrication of spin-dependent tunneling devices. We also examined the electron tunneling properties of Au/MgO/Fe3O4 junction with an ultrathin MgO layer prepared under the conditions described above and found excellent electron tunneling properties, as will be discussed. Barrier height and thickness estimated by curve fitting current density–voltage curves using the Simmons equation yielded barrier height and thicknesses of 0.9 eV and 2.5 nm, respectively. These values were consistent with those estimated by taking into account the reduction of the barrier height due to image forces. These results indicate that the MgO insulating layers grown under the restricted conditions have satisfactory electrical qualities required for spin tunneling devices.
Nature of the Schottky-type barrier of highly dense SnO2 systems displaying nonohmic behavior
Authors: Bueno P. R. , de Cassia-Santos M. R. , Leite E. R. , Longo E. , Bisquert J. , Garcia-Belmonte G. , Fabregat-Santiago F.
Published: July 2002

The electrical characteristics of highly dense SnO2 ceramic varistors are believed to be caused by the existence of potential barriers at the grain boundary. A complex plane analysis technique (to eliminate the influence of trapping activity associated with the conductance term observed via depression angle of a semicircular relaxation in the complex capacitance plane), allied with an approached Mott–Schottky model, are used to demonstrate that the potential barriers at the grain boundary are Schottky-type barriers in SnO2 varistors such as those observed in the traditional ZnO varistor.
Stress-free two-way thermoelastic shape memory and field-enhanced strain in Ni52Mn24Ga24 single crystals
Authors: Wang W. H. , Wu G. H. , Chen J. L. , Yu C. H. , Gao S. X. , Zhan W. S. , Wang Z. , Gao Z. Y. , Zheng Y. F. , Zhao L. C.
Published: July 2002

Stress-free and two-way thermoelastic shape memory, with 1.2% strain and 6 K temperature hysteresis, has been found in single crystalline Ni52Mn24Ga24. The deformation can be enhanced more than three times, up to 4.0% shrinkage with a bias field 1.2 T applied along the measurement direction, or changed to 1.5% expansion by the 1.2 T applied perpendicular to the measurement direction. For achieving a large deformation, the magnetic field exhibits a more evident contribution than an external stress on this material. These characteristics can be attributed to the low level of internal stress and the preferential orientation of the martensitic variants.
First-principles electronic structure studies for the cluster modeled Si/Si(111) chemisorption system
Authors: Wang Sanwu , Radny M. W. , Smith P. V.
Published: July 2002

Ab initio density functional theory (DFT) methods have been employed to investigate the electronic structure of atomic clusters representing the T4 and H3 adatom bonding configurations of the Si(111)3×3R30°-Si reconstructed surface. The nature of the bonding between the Si adatom and its nearest neighbor atoms on the Si(111) surface is discussed by analyzing the density of states (DOS), projected DOS, charge distributions and the molecular orbital overlap population (MOOP). The results provide an explanation for the difference in stability between the T4 and H3 configurations.
Pulsed laser ablation of aluminum in the presence of nitrogen: Formation of aluminum nitride
Authors: Sharma A. K. , Thareja R. K.
Published: July 2002

We report on the pulsed laser ablation of aluminum in the presence of nitrogen gas using a 1.06 μm wavelength of Nd:YAG laser. A prominent band of aluminum nitride corresponding to the (0-0) band of the system belonging to a π3−π3 transition was observed at 507.8 nm. An attempt is made to identify the ionized states of aluminum and nitrogen contributing to formation of the AlN band. AlN films were deposited at room temperature and characterized using x-ray diffraction. A direct correlation between the laser ablated aluminum plasma and the deposited AlN film is reported.
Evaluating the continuous-wave performance of AlGaInP-based red (667 nm) vertical-cavity surface-emitting lasers using low-temperature and high-pressure techniques
Authors: Sweeney Stephen J. , Knowles Gareth , Sale Terry E.
Published: July 2002

By measuring visible AlGaAs/AlGaInP vertical-cavity surface-emitting lasers (VCSELs) and similar edge-emitting lasers (EELs) as a function of temperature and pressure, we study the contributions of electron leakage and gain-cavity detuning on the threshold current, Ith, of VCSELs. In the EELs, leakage accounts for ∼20%Ith at room temperature, rising to ∼70%Ith at 80 °C. Similarly, leakage accounts for almost all of the increase in the VCSEL Ith above −100 °C and limits the cw output power. At low temperature or high pressure, however, the VCSEL Ith increases sharply due to misalignment of the gain peak to the high-energy side of the cavity mode. Under normal operating conditions, carrier leakage has the greatest effect on the VCSELs with gain-cavity detuning only becoming important at very low temperatures.
The degree and nature of radiation damage in zircon observed by Si29 nuclear magnetic resonance
Authors: Farnan I. , Salje E. K. H.
Published: July 2002

A quantitative analysis of Si29 nuclear magnetic resonance spectra of radiation damaged, natural zircons showed that the local structure in crystalline and amorphous regions depend explicitly on radiation dose. Nonpercolating amorphous islands of high density “glass” within the crystalline matrix show a low interconnectivity of SiO4 tetrahedra. This structural state is quite different from that of the high dose, percolating regions of low density glass with more polymerised tetrahedra. A continuous nonlinear dose dependence between the high and low density glass states is reported. A continuous evolution of the local structure of the crystalline phase up to the percolation point is also reported. No phase separation into binary oxides was observed. The total number of permanently displaced atoms per α-recoil event is ∼3800 atoms for low radiation doses and decreases to ∼2000 atoms for 10×1018 α events/g. No indication of partitioning of paramagnetic impurities between crystalline and amorphous regions was found for these natural zircons. The amorphous fractions of the metamict zircons were determined as a function of their accumulated radiation dose. These values coincide closely with those recently determined by x-ray diffraction studies. They are much greater than previously assumed based on density measurements. The dose dependence is consistent with the concept of direct impact amorphization in the atomic cascade following an α-recoil event.
Quadrupolar-driven recoupling of homonuclear dipolar interactions in the nuclear magnetic resonance of rotating solids
Authors: Edén Mattias , Frydman Lucio
Published: July 2002

We discuss the recoupling of homonuclear dipolar interactions between quadrupolar nuclei under magic-angle spinning conditions, caused by the first-order quadrupolar interaction. This recoupling leads to NMR linewidths displaying a nonmonotonic dependence on the spinning frequency, meaning that broader lines may result as the spinning rate is increased. The effect depends on geometrical parameters of the spin system, which makes it suitable for distance measurements and for obtaining the relative orientations of dipolar and quadrupolar tensors. We propose a theoretical model of the dipolar recoupling based on average Hamiltonian theory, and find it in good agreement with numerically exact simulations and experiments on I=3/2 systems.
The physics of the International Thermonuclear Experimental Reactor FEAT
Authors: Campbell D. J.
Published: July 2002

The International Thermonuclear Experimental Reactor FEAT design for a long-pulse tokamak burning plasma experiment (R=6.2 m, a=2 m, B=5.3 T, I=15 MA) is intended to achieve extended burn in inductively driven deuterium–tritium plasmas with the ratio of fusion power to auxiliary heating power, Q, of at least 10 and a nominal fusion power output of ∼500 MW. It also aims to demonstrate steady-state plasma operation using noninductive current drive with a Q of at least 5. Particular features of the design are: a significant operating window for Q=10 inductive operation; long inductive pulses (several hundred seconds burn); a capability for studying steady-state scenarios, specifically in cases where α-particles make a significant contribution to the plasma pressure; disruption physics processes which are comparable to those expected at the reactor scale; and an α-particle density and heating power which permit the key issues of α-particle confinement and α-particle driven magnetohydrodynamic instabilities to be investigated under conditions appropriate to a reactor.
The coalescence of two equal-sized drops in a two-dimensional linear flow
Authors: Yang Hong , Park C. Charles , Hu Y. Thomas , Leal L. Gary
Published: July 2002

A four-roll mill was used to experimentally investigate the coalescence of two equal-sized drops in general linear flows. The experimental system consisted of polybutadiene drops suspended in polydimethylsiloxane. Under the experimental conditions studied, the bulk-phase rheological properties of both fluids are Newtonian. We studied both head-on collisions for a purely extensional “hyperbolic” flow that always lead to coalescence, and collisions with a finite offset from the inflow axis for several different flow types produced in the four-roll mill. The experimental results have been compared with approximate theoretical predictions of coalescence, based on an asymptotic theory for small capillary number, where the drops are spherical apart from a small planar deformation at the frontal surfaces between the two drops. In head-on collisions, it was found experimentally that the product of the film drainage time and strain rate is independent of capillary number (Ca) and drop radius at very low Ca. This scaling behavior agrees qualitatively with theoretical predictions from the approximate small Ca theory. At higher values of Ca, this dimensionless drainage time varies as Ca3/2. This scaling is also consistent with theoretical predictions. For collisions with a nonzero offset from the head-on configuration, coalescence occurs only for capillary numbers below a critical value, Cac. Measurements were made of Cac as a function of the drop size and the flow type, for various values of the offset. The critical capillary number for coalescence was found to decrease with increasing offset, in qualitative agreement with predictions from the theoretical model. However, these Cac versus offset results do not agree quantitatively with the theoretical predictions. In the model the minimum film thickness occurs when the two-drop pair has rotated to the angle at which the external flow just begins to pull them apart. However, for configurations with small but nonzero offsets, it is found experimentally that coalescence occurs earlier in the collision process. Thus, the actual time available for film drainage is shorter, and the critical capillary number is smaller than what is predicted by the model. At the same time, for larger offsets, it is shown experimentally that the collision and initiation of film drainage is delayed relative to what is predicted, and thus there are offset values where the model predicts that coalescence is possible, whereas, in fact, no coalescence is observed.
Annealing effects of tantalum films on Si and SiO2/Si substrates in various vacuums
Authors: Liu Ling , Wang Yue , Gong Hao
Published: July 2002

The annealing effects of 550 nm thick β-Ta films sputtered on Si and SiO2 substrates have been investigated under various vacuum conditions. Phase transformation from the tetragonal β-Ta into body-centered-cubic α-Ta of much higher conductivity occurred at annealing temperatures lower than 500 °C and 80% of β-Ta transformed into α-Ta after annealing at 600 °C for Ta on a Si substrate. For Ta on a SiO2 substrate, no phase transformation was observed at 500 °C annealing, and only 20% of β-Ta transformed into α-Ta at 600 °C. Oxygen diffusion into the Ta film at the interface of Ta/SiO2 could hinder β-Ta to α-Ta transformation. Both Ta on Si and Ta on SiO2 samples have smooth surfaces after annealing in 2×10−5 Torr. After annealing in a vacuum lower than 2×10−4 Torr, surface oxidation of the Ta thin films was detected. The increase of oxygen content in the Ta films caused higher compressive stress, and resulted in the film peeling in a serpentine pattern during annealing at 500 °C in 2×10−2 Torr for Ta on a SiO2 substrate. The Ta films cracked and detached from the SiO2 substrate after being annealed at 750 °C in 2×10−2 Torr. In contrast, no crack was found in Ta on Si, probably because of the relief of film stress due to more β-Ta being transformed into α-Ta during annealing. The residual oxygen and moisture in low vacuum may build up stress in Ta thin films during thermal processes, which can cause major reliability problems in electronic and other applications.
Stability of colloidal clusters in shear flow near a wall: Stokesian dynamics simulation studies
Authors: Jones R. B.
Published: July 2002

An earlier algorithm for Stokesian dynamics simulation of colloid particles in a fluid bounded by a hard wall is extended to the case when a linear shear flow is applied. The algorithm includes many-body hydrodynamic interactions arising from the shear flow with lubrication corrections for pairs of close particles. The extended algorithm is used to simulate small clusters of particles which may interact with each other and with the bounding wall through model potentials with repulsive cores and attractive tails. The pair problem is studied first, in bounded and unbounded fluid, both with and without pair and wall interactions. The critical shear rate necessary to break a bound pair is determined for a range of initial configurations. Whereas in unbounded fluid a bound pair rotates with the vorticity of the shear flow, near a wall the hydrodynamic forces introduce a new breakup mechanism in which the pair tilts through a finite angle relative to the wall before separating. This tilt mechanism requires significantly stronger shear fields than are needed to separate a pair in unbounded fluid. Linear arrays of particles as well as examples of 2- and 3-D bound clusters are studied next to show how the wall and the tilt mechanism modify the shear-induced breakup. With a wall potential included, significant changes are seen in the final distribution of particles resulting from cluster breakup.
A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. III. 1,2-butadiene, H2CCCH(CH3) (X 1A′)—a non-Rice–Ramsperger–Kassel–Marcus system?
Authors: Balucani N. , Lee H. Y. , Mebel A. M. , Lee Y. T. , Kaiser R. I.
Published: July 2002

Crossed molecular beam experiments were conducted to investigate the reaction of ground state carbon atoms, C(3Pj), with 1,2-butadiene, H2CCCH(CH3) (X 1A′), at three collision energies of 20.4 kJ mol−1, 37.9 kJ mol−1, and 48.6 kJ mol−1. Ab initio calculations together with our experimental data reveal that the reaction is initiated by a barrier-less addition of the carbon atom to the π system of the 1,2-butadiene molecule. Dominated by large impact parameters, C(3Pj) attacks preferentially the C2–C3 double bond to form i1 (mechanism 1); to a minor extent, small impact parameters lead to an addition of atomic carbon to the C1–C2 bond yielding i2 (mechanism 2). Both cyclic intermediates i1 and i2 ring open to triplet methylbutatriene complexes i3′ (H2CC*CCH(CH3)) and i3″, (H2CCC*CH(CH3)); C* denotes the attacked carbon atom. i3′ is suggested to decay nonstatistically prior to a complete energy randomization via atomic hydrogen loss forming 1- and 4-methylbutatrienyl CH3CCCCH2 (X 2A″) and HCCCCH(CH3) (X 2A″), respectively. The energy randomization in i3″ is likely to be complete. This isomer decomposes via H atom loss to 3-vinylpropargyl, H2CCCC2H3(X 2A″), as well as 1- and 4-methylbutatrienyl radicals. In high-density environments such as the inner regions of circumstellar envelopes of carbon stars and combustion flames, these linear C5H5 isomers might undergo collision induced isomerization to cyclic structures like the cyclopentadienyl radical. This isomer is strongly believed to be a key intermediate involved in the production of polycyclic aromatic hydrocarbon molecules and soot formation. These characteristics make the reactions of atomic carbon with C4H6 isomers compelling candidates to form C5H5 isomers in the outflow of AGB stars and oxygen-deficient hydrocarbon flames.
Heterogeneous solvation: An ab initio approach
Authors: Jørgensen Solvejg , Ratner Mark A. , Mikkelsen Kurt V.
Published: July 2002

A multiconfigurational self-consistent reaction field (MCSCRF) method is presented for solvation of a molecule on a metal surface. We assume that the molecular system is enclosed in a half-spherical cavity and adsorbed on the surface of a perfect conductor. The half-spherical cavity is embedded in a linear, homogeneous, isotropic dielectric medium. A multiconfigurational self-consistent reaction field linear response method is presented for calculating frequency dependent polarizabilities as well as electronic excitation energies and transition moments of a solvated molecule on a metal surface.
Long-range energy transfer of singlet and triplet excitations in dye-doped tris(phenylquinoxaline)
Authors: Blumstengel S. , Meinardi F. , Tubino R. , Gurioli M. , Jandke M. , Strohriegl P.
Published: July 2002

In order to obtain efficient organic light-emitting diodes (OLED) it is necessary to funnel both singlet and triplet excitons generated by electroexcitation to the emitting dye molecule. Energy transfer plays thereby a decisive role. We have studied mechanisms and kinetics of the energy transfer process involving singlet and triplet excitations in a donor–acceptor system suitable for use as active layer in OLEDs. Fluorescent and phosphorescent tris(phenylquinoxaline) served as donor and a red emitting platinum porphyrin as acceptor molecule. The results of our investigations show that efficient energy transfer from the singlet and triplet excited state of the donor to the acceptor molecules takes place by combined long-range dipole–dipole interaction and diffusion. Due to the very different oscillator strengths of radiative transitions from the singlet and triplet excited state to the singlet ground state the rate of energy transfer of triplet excitons is several orders of magnitude slower than that of singlet excitons. Since, however, the lifetime of both states differ by the same order of magnitude the transfer efficiency is comparable.
Path integral formulation of centroid dynamics for systems obeying Bose–Einstein statistics
Authors: Blinov Nicholas V. , Roy Pierre-Nicholas , Voth Gregory A.
Published: July 2002

This paper presents a formal foundation for the recent extension [J. Chem. Phys. 110, 3647 (1999)] of the centroid molecular dynamics (CMD) method to systems obeying Bose–Einstein statistics. It is shown that the introduction of centroid phase space coordinates corresponding to individual physical particles allows one to obtain (exact) canonical averages within the framework of the bosonic CMD method. It is also shown that formally exact expressions for quantum mechanical Kubo transformed correlation functions can be written in terms of individual particle centroids and that a CMD approximation can be introduced. Calculations for a bosonic trimer are used as an illustration of the new concepts introduced in this work.
Dynamics of the second-order nonlinearity in thermally poled silica glass
Authors: Faccio D. , Pruneri V. , Kazansky P. G.
Published: July 2002

We study the temporal evolution of both the second-order nonlinear coefficient and of the nonlinear thickness in thermally poled silica-glass slides by using a high-resolution all-optical technique. A time delay in the nonlinearity formation is observed, followed by an increase to a maximum, and a final decrease. The thickness is shown to increase at a rate that differs significantly from that reported for the corresponding ionic charge fronts. Our measurements also show strong dependencies on sample thickness and these can be attributed to different electric fields in the depletion region.
Observation of thermally activated conduction at a GaN–sapphire interface
Authors: Mavroidis C. , Harris J. J. , Kappers M. J. , Sharma N. , Humphreys C. J. , Thrush E. J.
Published: July 2002

Temperature-dependent differential Hall measurements have been performed on an undoped GaN epitaxial layer grown by metalorganic chemical vapor deposition on a sapphire substrate. The resultant depth profile shows that, for this sample, the epitaxial material is mostly insulating, and that the observed thermally activated conduction arises from an interface region ⩽0.65 μm thick. Comparison with cross-sectional transmission electron microscope micrographs suggests that this conducting region is correlated to the highly defective three-dimensional growth region, while the two-dimensional growth mode beyond this thickness corresponds to the insulating portion with a lower dislocation density. Such behavior is consistent with the presence of an impurity band in a heavily doped interface region formed by oxygen outdiffusion from the substrate.
Ion acoustic waves in one- and two-negative ion species plasmas
Authors: Ichiki Ryuta , Shindo Masako , Yoshimura Shinji , Watanabe Tsuguhiro , Kawai Yoshinobu
Published: July 2002

Ion acoustic waves in multi-ion plasmas including two negative ion species are investigated both numerically and experimentally. Numerically, the kinetic dispersion relation in two-negative ion plasmas is investigated. There are three modes of the ion acoustic waves in two-negative ion plasmas. In an Ar+–F−–SF6− plasma, only one of the three modes is dominant, regardless of the values of the electron and the ion temperatures. In a Xe+–F−–SF6− plasma, on the other hand, two modes can be important for a certain range of the electron–ion temperature ratio. The results also imply the possibility of the coexistence of the fast mode and the slow mode in one-negative ion plasmas. Experimentally, ion acoustic waves are observed in an Ar+–F−–SF6− plasma and are found to show a mode transition that agrees with the theoretical prediction for one of the three ion acoustic modes.
High-temperature electron transport properties in AlGaN/GaN heterostructures
Authors: Maeda Narihiko , Tsubaki Kotaro , Saitoh Tadashi , Kobayashi Naoki
Published: July 2002

Electron transport properties in the Al0.15Ga0.85N/GaN heterostructure field effect transistors (HFETs) have been examined from room temperature up to 400 °C. The temperature dependencies of the two-dimensional electron gas (2DEG) mobility have been systematically measured for the samples with different 2DEG densities. The 2DEG mobility has been shown to decrease with increasing the temperature, with the lower decrease ratio at higher temperatures, and moreover, shown to be less dependent on the 2DEG density at higher temperatures. These features well agree with those of the longitudinal optical phonon-limited mobility theoretically predicted, although the effect of alloy and interface scattering should further be examined and analyzed. The observed 2DEG mobilities at 400 °C were as high as from 100 to 120 cm2/V s, directly providing the evidence for suitability of the HFET of this material system for high-temperature applications. Moreover, Si-doped Al0.15Ga0.85N single layer has been shown to exhibit a relatively high bulk mobility of 50 cm2/V s at 400 °C, suggesting that AlGaN is attractive as the channel material when higher-voltage and higher-temperature device operation is required.
Continuous room-temperature operation of electrically pumped quantum-dot microcylinder lasers
Authors: Arzberger M. , Böhm G. , Amann M.-C. , Abstreiter G.
Published: July 2002

We report continuous-wave (cw) room-temperature operation of electrically pumped AlGaAs/GaAs microcylinder lasers using self-assembled InAs/GaAs quantum dots (QDs) as the active material. The threshold currents are lower than those of similar devices with an InGaAs quantum well fabricated for comparison. This is discussed in terms of nonradiative surface recombination at the sidewall of the resonator which has less influence in the case of the QD devices since the lateral carrier confinement suppresses lateral carrier diffusion.
New splitting formulations for lattice summations
Authors: Batcho Paul F. , Schlick Tamar
Published: July 2002

We present a new formulation for the efficient evaluation of pairwise interactions for large nonperiodic or spatially periodic infinite lattices. Our optimally designed splitting formulation generalizes the Ewald method and its Gaussian core function. In particular, we show that a polynomial multiplication to the Gaussian core function can be used to formulate desired mathematical or physical characteristics into a lattice summation method. Two optimization statements are examined. The first incorporates a pairwise interaction splitting into the lattice sum, where the direct (real) and reciprocal space terms also isolate the near-field and far-field pairwise particle interactions, respectively. The second optimization defines a splitting with a rapidly convergent reciprocal space term that allows enhanced decay rates in the real-space term relative to the traditional Ewald method. These approaches require modest adaptation to the Ewald formulation and are expected to enhance performance of particle-mesh methods for large-scale systems. A motivation for future applications is large-scale biomolecular dynamics simulations using particle-mesh Ewald methods and multiple time step integration.
Möbius structure of the spectral space of Schrödinger operators with point interaction
Authors: Tsutsui Izumi , Fülöp Tamás , Cheon Taksu
Published: July 2002

The Schrödinger operator with point interaction in one dimension has a U(2) family of self-adjoint extensions. We study the spectrum of the operator and show that (i) the spectrum is uniquely determined by the eigenvalues of the matrix U∈U(2) that characterizes the extension, and that (ii) the space of distinct spectra is given by the orbifold T2/Z2 which is a Möbius strip with boundary. We employ a parametrization of U(2) that admits a direct physical interpretation and furnishes a coherent framework to realize the spectral duality and anholonomy recently found. This allows us to find that (iii) physically distinct point interactions form a three-parameter quotient space of the U(2) family.
Oxidation states of titanium in bulk barium titanates and in (100) fiber-textured (BaxSr1−x)Ti1+yO3+z thin films
Authors: Stemmer S. , Höche T. , Keding R. , Rüssel C. , Schneider R. , Browning N. D. , Streiffer S. K. , Kleebe H.-J.
Published: July 2002

Chemical shifts of titanium L edges and oxygen K edges in electron energy-loss spectroscopy in transmission electron microscopy were used to detect valence state reduction of Ti in bulk barium titanates, used as reference materials, and in (BaxSr1−x)Ti1+yO3+z (BST) thin films grown with excess Ti. A hollandite-type Ba titanate, containing Ti with an average valence state of approximately 3.7 in octahedral coordination, showed large chemical shifts relative to rutile TiO2 and BaTiO3, containing only Ti4+. In BST, chemical shifts relative to BaTiO3 were measured from grain interiors of columnar films with different amounts of excess Ti. We found that shifts, corresponding to an average valence state of Ti smaller than nominal 4+, increase with increasing amounts of excess Ti in the films. The results show that at least some amount of excess Ti is accommodated in these films by a defect mechanism that requires a reduction of the average valence state of Ti.
Characterization of a cryogenic, high-pressure gas jet operated in the droplet regime
Authors: Parra E. , McNaught S. J. , Milchberg H. M.
Published: July 2002

We report on the characterization of a cryogenically cooled high-pressure pulsed valve for laser–droplet interaction experiments. The gas jet can produce liquid droplets of a variety of gases over a wide range of temperature (100–300±0.25 K). Measurements of total mass flow for argon and krypton are presented as well as droplet size distributions obtained via dark-field shadowgraphy. Using light scattering, the temporal behavior of the valve is also investigated and the droplet flow speeds measured. The results are explained in terms of the phase diagrams for these gases and the dynamic operation of the valve. This characterization allows for properly interpreting yields of extreme ultraviolet emission from laser-irradiated droplet plasmas.