bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermixing in quantum-dot ensembles with sharp adjustable shells
Authors: Fafard S. , Allen C. Nı̀.
Published: July 2002

State-filling spectroscopy is used to study the effects of alloy intermixing in quantum-dot (QD) ensembles having well-defined electronic shells. Rapid thermal annealing is performed on samples of self-assembled QDs grown with different intersublevel energy spacings. For InAs/GaAs QDs, the intersublevel is tuned between ∼90 and 25 meV. The intense and sharp shell structures observed in photoluminescence indicate unambiguously that the QDs retained their zero-dimensional density of states after the diffusion of the potential, which also causes strong blueshifts (up to ∼200 meV) and a pronounced narrowing of the inhomogeneously broadened emission (down to ∼12 meV).
An oxide-diluted magnetic semiconductor: Mn-doped ZnO
Authors: Fukumura T. , Jin Zhengwu , Ohtomo A. , Koinuma H. , Kawasaki M.
Published: July 2002

Epitaxial thin films of an oxide-diluted magnetic semiconductor, Mn-doped ZnO, were fabricated by pulsed-laser deposition technique. Solubility of Mn into ZnO exceeds thermal equilibrium limit as a result of nonequilibrium film growth process. As Mn content is increased, the lattice constants of both a and c axes of wurtzite Zn1−xMnxO films (x<0.35) increase and the band gap expands although considerable in-gap absorption develops. Itinerant electrons over 1019 cm−3 can be doped into the Zn1−xMnxO films by Al doping, in contrast to low carrier density in the other II–VI diluted magnetic semiconductors. The temperature dependence of the resistivity is almost metallic and considerable magnetoresistance is observed at low temperatures.
Femtosecond Z-scan measurement of GaN
Authors: Huang Yong-Liang , Sun Chi-Kuang , Liang Jian-Chin , Keller S. , Mack M. P. , Mishra U. K. , DenBaars S. P.
Published: July 2002

Nonlinear refractive index n2 of GaN was measured for the below-band-gap ultraviolet (UV) wavelength region using Z-scan techniques with femtosecond UV pulses. A large nonlinear refractive index of −2.9±1.2×10−12 cm2/W was obtained at a wavelength of 368 nm. The distribution of n2 versus wavelength was found to be consistent with a model described by the quadratic Stark effect, which is the dominant factor contributed to the nonlinear refractive index near the band gap. Our experiments show that GaN is an excellent nonlinear material with large negative n2 in the UV region.
Quality of silica capping layer and its influence on quantum-well intermixing
Authors: Fu L. , Deenapanray P. N. K. , Tan H. H. , Jagadish C. , Dao L. V. , Gal M.
Published: July 2002

The quality of spin-on silica films prebaked at different temperatures has been studied using Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and P-etch [HF(40%):HNO3(70%):H2O=3:2:60] measurements. Low-temperature photoluminescence (PL) was performed on GaAs/AlGaAs quantum-well (QW) structures encapsulated by the same films. For all the prebaked films, not only the Si–O–Si peaks, but also OH-related peaks were detected in the IR spectra. After annealing at 950 °C for 60 s, almost all OH-related peaks disappeared. Spectroscopic ellipsometry modeling and P-etch measurements showed that the porosity of high-temperature (>300 °C) baked samples was similar, and was significantly higher than the low-temperature (210 °C) baked sample. The same trend was observed in the PL energy shifts from the GaAs/AlGaAs QWs, indicating a direct correlation between the film quality and quantum-well intermixing.
Fabrication of a single-electron transistor by current-controlled local oxidation of a two-dimensional electron system
Authors: Keyser U. F. , Schumacher H. W. , Zeitler U. , Haug R. J. , Eberl K.
Published: July 2002

The surface layers of a GaAs/AlGaAs heterostructure are locally oxidized using an atomic force microscope. The local anodic oxidation depletes the underlying two-dimensional electron gas leading to the formation of tunneling barriers. The height of the barriers is determined by measuring the thermally activated current. By varying the oxidation current, the barrier heights can be tuned between a few meV and more than 100 meV. Using these barriers as tunneling elements, a side gated single-electron transistor is fabricated.
Epitaxial growth of AlN and Al0.5Ga0.5N layers on aluminum nitride substrates
Authors: Schowalter L. J. , Shusterman Y. , Wang R. , Bhat I. , Arunmozhi G. , Slack G. A.
Published: July 2002

High quality epitaxial AlN and AlxGa1−xN layers have been grown by organo-metallic vapor-phase epitaxy on single crystal a-face AlN substrates. Here we report the characterization of these layers using Rutherford backscattering/ion channeling spectroscopy, atomic force microscopy, double crystal x-ray diffraction, and preliminary electrical results. Ion channeling along the [112̄0] axis gave a minimum yield of 1.5% for an AlN layer and 2.2% for an Al0.5Ga0.5N, indicating excellent crystal quality. A resistivity of 20 Ω cm and a mobility of 20 cm2/V s was measured in a Si-doped, 1-μm-thick Al0.5Ga0.5N grown epitaxially on the AlN substrates.
Er
3+
ions–Si
nanocrystals interactions and their effects on the luminescence properties
Authors: Franzò Giorgia , Pacifici Domenico , Vinciguerra Vincenzo , Priolo Francesco , Iacona Fabio
Published: July 2002

A detailed investigation on the interaction mechanisms between Er ions and Si nanocrystals (nc) is reported. Silicon nc were produced by high-temperature annealing of substoichiometric SiOx thin films grown by plasma-enhanced chemical vapor deposition. Subsequently, some of the samples were implanted by Er. These samples show intense room-temperature luminescence at both 1.54 and 0.98 μm. High-resolution luminescence spectra of Er-implanted Si nc suggest that the emitting Er ions are located in the SiO2 or at the Si nc/SiO2 interface. The pump-power dependence and the time decay of the 1.54 μm emission in Si nc with different Er contents have evidenced the presence of several nonradiative decay processes due to Er–Er and Er–Si nc interactions. Moreover, the number of Er ions per Si nc is shown to be a quite critical parameter in determining the final properties of the overall system.
Highly polarized blue electroluminescence from homogeneously aligned films of poly(9,9-dioctylfluorene)
Authors: Whitehead K. S. , Grell M. , Bradley D. D. C. , Jandke M. , Strohriegl P.
Published: July 2002

Homogeneous alignment of poly(9,9-dioctylfluorene) films on thin layers of rubbed precursor-route poly(p-phenylenevinylene) allows the construction of light-emitting diodes that emit highly polarized blue light (λem=458 nm). The rubbed poly(p-phenylenevinylene) acts as an effective hole-injecting alignment layer. Annealing of poly(9,9-dioctylfluorene) in its nematic phase followed by rapid quenching orients the polymer as a glassy monodomain on the alignment layer and gives devices with a polarization ratio of 25:1 and a luminance of up to 250 cd/m2.
Nanowire formation in a polymeric film
Authors: Bhattacharrya S. , Saha S. K. , Chakravorty D.
Published: July 2002

Silver nanowires of diameter ∼2 nm and length ∼2 mm have been grown by an electrodeposition method within a polyvinyl alcohol film subjected to a two-stage treatment, viz., in ammonium persulphate and pyrrole solutions, respectively. A staircase current–voltage characteristic has been measured in this composite. Also, the material shows a dielectric constant ∼104. Both these effects are observed when the applied electric field is in a direction perpendicular to that of the electrodeposition process.
AlAs/GaAs quantum cascade lasers based on large direct conduction band discontinuity
Authors: Becker Cyrille , Sirtori Carlo , Page Hideaki , Glastre Geneviève , Ortiz Valentin , Marcadet Xavier , Stellmacher Max , Nagle Julien
Published: July 2002

The design and operation of quantum cascade (QC) lasers using AlAs/GaAs coupled quantum wells are reported. In this material system, the conduction band offset at the Γ point (∼1 eV) is much higher than in previously reported QC lasers. The use of high band discontinuity allows us to increase the energy separation among the subbands, thus suppressing thermally activated processes which limit device performance at high temperature. The measured thermal characteristics of these promising devices are strongly improved from previously reported GaAs-based QC lasers: The temperature dependence of the threshold current density is described by a very large T0 (320 K) and the laser slope efficiency does not vary for increasing heat sink temperatures. The maximum operating temperature is 230 K, limited by negative differential resistance effects that occur when the applied bias reaches 8 V.
The radio frequency magnetic field effect on electron heating in a low frequency inductively coupled plasma
Authors: Chung ChinWook , Seo Sang-Hun , Chang Hong-Young
Published: July 2002

Radio frequency power with a low frequency of 4 MHz is delivered to a solenoidal inductively coupled plasma at a low pressure of 1 mTorr. The electron energy distribution functions (EEDFs) are measured by a rf compensated Langmuir probe at different rf powers. As the rf power increases, a Maxwellian EEDF evolves into a bi-Maxwellian EEDF with a low energy peak. This means that the electron heating in the plasma greatly changes. This EEDF transition can be understood by considering the rf magnetic field effect, which is strong at low frequency.
Ab initio study of the electric-field-gradient-induced birefringence of a polar molecule: CO
Authors: Rizzo Antonio , Coriani Sonia , Halkier Asger , Hättig Christof
Published: July 2002

An ab initio coupled cluster singles and doubles (CCSD) investigation of the electric-field- gradient-induced birefringence (EFGB) of a polar molecule, CO, is presented. The so-called “effective quadrupole center” (EQC), the origin to which the quadrupole moment deduced from EFGB experiments refers for dipolar molecules and which experimentalists cannot determine directly, is computed within two different semiclassical theories of the EFGB. The temperature independent frequency dependent contribution to the birefringence is determined as well. The molecular electric quadrupole moment with the origin at the EQC is determined within the two theories via an accurate value of the quadrupole moment referring to the center of mass, obtained using well-established hierarchies of basis sets and wave function models, and the calculated EQCs. The final theoretical values result as the basis set limit at the CCSD(T) level completed with estimates for relativistic effects, correlation contributions beyond CCSD(T), rovibrational effects, and residual errors in the EQC. The comparison of the theoretical results with the corresponding experimentally deduced values sheds some light on the differences between the two semiclassical approaches to the EFGB.
Scattering on triply periodic minimal surfaces—the effect of the topology, Debye–Waller, and molecular form factors
Authors: Garstecki Piotr , Hołyst Robert
Published: July 2002

We compute scattering patterns for four triply periodic surfaces (TPS). Three minimal—Schwarz P (Im3̄m), Schwarz D—diamond (Pn3̄m), Schoen G—gyroid (Ia3̄d), and one nodal S1 (Ia3̄d). Simple approximations are adopted to examine the influence of the molecular form factor, and the Debye–Waller factor on the scattering pattern. We find that the Debye–Waller factor has a much smaller influence on the scattering intensities of TPS than on the intensities of the lamellar structure consisting of parallel surfaces. This is caused by an almost spherelike distribution of normal vectors for TPS. We give a simple formula that allows a comparison of the experimental scattering data with the data for the P, D, G mathematical surfaces. Finally, the spectra of the two surfaces G and S1 of the same space group symmetry and different topologies are compared. It is found that in the case of the more complex S1 structure the intensities of the first two peaks are very small.
Conjugate flows for a three-layer fluid
Authors: Lamb Kevin G.
Published: July 2002

A method for computing conjugate flows for a non-Boussinesq, three-layer fluid with arbitrary constant currents is developed. The general solution for a two-layer fluid is obtained as a special case. Symmetric stratifications at rest, with the upper and lower layer depths equal to h and identical density jumps across each interface, are considered in detail using the Boussinesq approximation. Mode-1 conjugate flows exist for hhc2>hc1 two additional asymmetric solutions exist. Comparisons with solutions for continuous stratifications with thin pycnoclines are made. Mode-2 solutions are more sensitive to the width of the pycnocline than are mode-1 solutions. Comparisons between three-layer non-Boussinesq and Boussinesq solutions are also made. For a total density variation of 4% of the mean value the two solutions are similar. For larger density variations the mode-2 solutions can be significantly different.
Instabilities of self-gravitating dusty clouds in magnetized plasmas
Authors: Mamun A. A. , Shukla P. K.
Published: July 2002

A rigorous theoretical investigation has been made of electromagnetic modes and associated filamentational instabilities in a nonuniform, partially ionized, self-gravitating, dusty magnetoplasma, taking into account the effects of the neutral fluid dynamics, inhomogeneities in the external magnetic field and the plasma density, the drag forces, the ionization and recombination, etc. It is found that there exist two new types of filamentational instabilities; one type is due to the combined effects of collisions of dust particles or ions with stationary neutrals and the self-gravitational force, and the other type is due to the combined effects of the dusty plasma and the neutral fluid dynamics. Furthermore, the effects of the external magnetic field and its inhomogeneity, the neutral fluid temperature, collisions of dust particles with neutrals, and the ionization play the stabilizing role, whereas the effects of the neutral fluid mass density, the plasma number density inhomogeneity, collisions of ions with neutrals, and the recombination play the destabilizing role. The newly found instabilities may be responsible for the formation of stars in galaxies via fragmentation or filamentation of interstellar dense molecular clouds.
Alternating current conduction behavior of excimer laser ablated SrBi2Nb2O9 thin films
Authors: Bhattacharyya S. , Bharadwaja S. S. N. , Krupanidhi S. B.
Published: July 2002

Bi-layered Aurivillius compounds prove to be efficient candidates of nonvolatile memories. SrBi2Nb2O9 thin films were deposited by excimer laser ablation at low substrate temperature (400 °C) followed by an ex situ annealing at 750 °C. The polarization hysteresis behavior was confirmed by variation of polarization with the external applied electric field and also verified with capacitance versus voltage characteristics. The measured values of spontaneous and remnant polarizations were, respectively, 9 and 6 μC/cm2 with a coercive field of 90 kV/cm. The measured dielectric constant and dissipation factors at 100 kHz were 220 and 0.02, respectively. The frequency analysis of dielectric and ac conduction properties showed a distribution of relaxation times due to the presence of multiple grain boundaries in the films. The values of activation energies from the dissipation factor and grain interior resistance were found to be 0.9 and 1.3 eV, respectively. The deviation in these values was attributed to the energetic conditions of the grain boundaries and bulk grains. The macroscopic relaxation phenomenon is controlled by the higher resistive component in a film, such as grain boundaries at lower temperatures, which was highlighted in the present article in close relation to interior grain relaxation and conduction properties.
Vibrationally excited states and fragmentation geometries of NeN and ArN clusters, N=3–6, using hyperspherical coordinates
Authors: Blume D. , Greene Chris H.
Published: July 2002

We calculate the ground state and a class of zero orbital angular momentum (L=0) vibrationally excited state energies for NeN and ArN clusters using an adiabatic hyperspherical representation to solve the nuclear Schrödinger equation. The Schrödinger equation in the hyperangular coordinates is solved for a sequence of fixed hyperradii by diffusion Monte Carlo techniques, which determines the lowest effective potential curve. We monitor structural properties such as the pair and angle distribution as a function of the hyperspherical radius. These structural studies allow us to identify configurational changes as the N atom cluster fragments into an (N−1)-atom cluster plus an atom. We also determine separately the ground state of the full 3N-dimensional nuclear Schrödinger equation for the ground state, and compare the resulting structural properties with those calculated in the adiabatic hyperspherical approximation.
Internal kink stability of large aspect ratio high-β tokamaks
Authors: Hurricane O. A. , Chandran B. D. G. , Cowley S. C.
Published: July 2002

Theoretically, high-β (β∼1) tokamaks offer a large fusion power efficiency advantage over low-β devices. However, if high-β tokamaks are inherently unstable then such devices will never be realized. In particular, kink modes are thought to be the most serious obstacle to high-β operations. High-β tokamaks are characterized by a very large Shafranov shift with a thin “boundary layer” on the outboard side of the device and a large “core” region of vertical flux surfaces comprising most of the central volume. In this paper, the energy principle is used to compute the magnetohydrodynamic internal kink stability of such devices in the large aspect ratio limit with a low toroidal mode number. A class of internal kink mode similar to the usual low-β kink is present; the stability against these modes is computed. A set of parameters describing a kink stable high-β equilibrium is given. Stability is shown to be dependent on the shape of the plasma boundary.
Statistical Lorentzian geometry and the closeness of Lorentzian manifolds
Authors: Bombelli Luca
Published: July 2002

I introduce a family of closeness functions between causal Lorentzian geometries of finite volume and arbitrary underlying topology. When points are randomly scattered in a Lorentzian manifold, with uniform density according to the volume element, some information on the topology and metric is encoded in the partial order that the causal structure induces among those points; one can then define closeness between Lorentzian geometries by comparing the sets of probabilities they give for obtaining the same posets. If the density of points is finite, one gets a pseudo-distance, which only compares the manifolds down to a finite volume scale, as illustrated here by a fully worked out example of two two-dimensional manifolds of different topology; if the density is allowed to become infinite, a true distance can be defined on the space of all Lorentzian geometries. The introductory and concluding sections include some remarks on the motivation for this definition and its applications to quantum gravity.
Reflective polarizer based on a stacked double-layer subwavelength metal grating structure fabricated using nanoimprint lithography
Authors: Yu Zhaoning , Deshpande Paru , Wu Wei , Wang Jian , Chou Stephen Y.
Published: July 2002

A reflective polarizer consisting of two layers of 190 nm period metal gratings was fabricated using nanoimprint lithography. Measurements with a He–Ne laser (wavelength=632.8 nm) showed that at normal incidence, this polarizer reflects light polarized perpendicular to the grating lines (transverse magnetic polarization) with a reflectance of 54%, but strongly absorbs parallel-polarized light (transverse electric polarization) with a reflectance of only 0.25%. The enhanced polarization extinction ratio of over 200 at this wavelength is possibly related to the resonance between the two layers of metal gratings. This polarizer is thin, compact, and is suited for integrated optical systems.
Instantaneous spherical electron focusing in a Penning trap
Authors: Zhuang Ge , Liu Wandong , Yu Changxuan , Zheng Jian , Fu Chengjiang , Xie Jinlin , Zhao Kai , Bai Bo , Liang Xiaoping , Zhao Shujun , Chi Ji
Published: July 2002

In the electron beam Penning trap, electron focusing at the trap center by means of injecting low-canonical-angular-momentum electrons has been identified. It is found that the focusing occurs instantaneously under the condition that the injection current is larger than a threshold and the effective electric potential well is spherical. A self-consistent model is proposed to describe the setup process of the dense core. According to the model the instantaneous core density is evaluated to be about one order greater than the corresponding Brillouin density.
Chemical mapping and formation of V-defects in InGaN multiple quantum wells
Authors: Sharma Nikhil , Thomas Paul , Tricker David , Humphreys Colin
Published: July 2002

InGaN multiple-quantum-well structures grown by metal–organic chemical-vapor deposition on GaN and capped by p-type GaN are found to contain inverted pyramids of indium-free GaN. High-resolution structural and chemical analyses of these “V-defects” by a number of complementary transmission electron microscopy techniques show that the InGaN quantum wells end abruptly at the V-defect interfaces, which lie on {10–11} planes. Each V-defect has at its center a threading edge dislocation, indicating that the defects are initiated at edge dislocation cores in the presence of indium. The lower temperatures of InGaN/GaN quantum-well growth (790 °C/950 °C) assist the formation of V-pits, which are subsequently filled in during the growth at higher temperature (1045 °C) of the p-type capping layer.
Active control of edge localized modes by radio frequency waves
Authors: Avinash K. , Diamond P. H.
Published: July 2002

The effect of radio frequency wave driven torque on edge localized mode (ELM) activity is studied. It is shown that the radio frequency driven torque causes transition of type I giant ELMs to the benign grassy ELM behavior. The efficiency of this process scales directly with minor radius and inversely with the plasma density. It is argued that the technique of active ELM control will be efficient in the present for moderate-sized machines and in the future for large machines.
Direct iterative solution of the generalized Bloch equation. III. Application to H2-cluster models
Authors: Meißner Holger , Paldus Josef
Published: July 2002

A state-selective multiconfigurational single-reference method that was outlined in the preceding paper of this series (H. Meißner and J. Paldus, J. Chem. Phys. 113, 2594 (2000); preceding paper), and is based on a quadratic iterative algorithm enabling the direct solution of the generalized Bloch equation, is applied to several model systems consisting of interacting hydrogen molecules, nowadays referred to as the H4, S4, and H8 models. These exactly solvable models are often used to test the efficacy of post-Hartree–Fock methods in their ability to recover both the dynamic and nondynamic correlation energies, since they enable a continuous variation of the degree of quasidegeneracy from the degenerate to nondegenerate limit by varying a single geometrical parameter, while simulating the dissociation of one or more single bonds. Various approximation schemes that were outlined in Part II, as well as their combinations, are tested and their performance evaluated. The size-extensivity deviations of those approximations that do not rely on the exponential cluster ansatz for the wave operator are also examined using larger hydrogen molecule clusters. It is shown that the so-called BQ4 approximation performs extremely well in all cases and even outperforms the externally corrected, reduced multireference (RMR) CCSD in the quasidegenerate region of geometries.
Pyramidal defects in metalorganic vapor phase epitaxial Mg doped GaN
Authors: Vennéguès P. , Benaissa M. , Beaumont B. , Feltin E. , De Mierry P. , Dalmasso S. , Leroux M. , Gibart P.
Published: July 2002

A transmission electron microscopy study of structural defects induced by the introduction of Mg during the growth of metalorganic vapor phase epitaxy GaN is presented. These defects are assumed to be pyramidal inversion domains with an hexagonal base and {112̄3} inclined facets. The tip of the pyramids is always pointing toward the [0001̄] direction, i.e., in a Ga-terminated film, toward the substrate and in a N-terminated film, toward the surface. A chemical quantitative analysis shows that these pyramidal defects are Mg rich. They are present in all the studied films, independent of the doping level.
Extension of drift magnetic island theory beyond the common profile function approximation
Authors: Mikhailovskii A. B. , Konovalov S. V. , Tsypin V. S. , Nascimento I. C. , Galvão R. M. O.
Published: July 2002

The approach of several profile functions for studying drift magnetic islands is developed. This approach yields more favorable predictions for the role of the polarization current than the less adequate approach of a common profile function adopted in preceding studies. It is shown that the polarization current is stabilizing not only for subdrift islands propagating in the ion drift direction but also for islands propagating in the electron drift direction with a sufficiently low rotation frequency.
Energy gain calculations in Penning fusion systems using a bounce-averaged Fokker–Planck model
Authors: Chacón L. , Miley G. H. , Barnes D. C. , Knoll D. A.
Published: July 2002

In spherical Penning fusion devices, a spherical cloud of electrons, confined in a Penning-like trap, creates the ion-confining electrostatic well. Fusion energy gains for these systems have been calculated in optimistic conditions (i.e., spherically uniform electrostatic well, no collisional ion-electron interactions, single ion species) using a bounce-averaged Fokker–Planck (BAFP) model. Results show that steady-state distributions in which the Maxwellian ion population is dominant correspond to lowest ion recirculation powers (and hence highest fusion energy gains). It is also shown that realistic parabolic-like wells result in better energy gains than square wells, particularly at large well depths (>100 kV). Operating regimes with fusion power to ion input power ratios (Q-value) >100 have been identified. The effect of electron losses on the Q-value has been addressed heuristically using a semianalytic model, indicating that large Q-values are still possible provided that electron particle losses are kept small and well depths are large.
Photodissociation and recombination of F2 molecule in Ar54 cluster: Nonadiabatic molecular dynamics simulations
Authors: Niv M. Y. , Bargheer M. , Gerber R. B.
Published: July 2002

Photodissociation and recombination of an F2 molecule embedded in an Ar cluster is investigated. The electronic states involved are described by the valence bond approach for the F(2P)+F(2P) interaction, with spin–orbit coupling included and the anisotropic interactions between F and Ar atoms described by the diatomics-in-molecules (DIM) approach. The potential energy surfaces for 36 electronic states and the nonadiabatic couplings between them are constructed in this basis. The surface hopping method is used for dynamical simulations. The main results are: (i) Spin nonconserving transitions play a crucial role both in the dissociation and in the recombination dynamics. (ii) The ratio between the population of the triplet states and the population of the singlet states reaches the statistical equilibrium value of 3:1 60 fs after the photoexcitation, but the population of specific singlet and triplet states remains nonstatistical for at least 1.5 ps. (iii) Recombination on the only bound excited state (3Πu) becomes significant within 100 fs and builds up to 40% of the trajectories within 1 ps after excitation of the cluster with 4.6 eV. This is in accord with recent experiments on ClF/Ar solid, where strong emission from this state was found. (iv) 3% of recombination on the ground Σg1 state is found as well. (v) For excitation energy of 4.6 eV, the dissociation can be direct or delayed. In delayed dissociation the F photofragments hit the Ar cage more than once before escaping the cage. (vi) For excitation energy of 6.53 eV the yield of dissociation was found to be 100%, and the dissociation is direct only.
Magnetosonic modes with dust mass distributions
Authors: Jacobs Gerald , Verheest Frank , Hellberg M. A. , Pillay S. R.
Published: July 2002

Previously it was shown that perpendicularly propagating magnetosonic waves in a self-gravitating dusty plasma can significantly increase the Jeans length characteristic of gravitational collapse. Whereas that work was based on the presence of a single dust species, an arbitrary number of charged dust species is now considered and a modified Jeans criterion is obtained, subject to neglect of the dust thermal speeds with respect to the magnetosonic speed. As an illustration, the case of two dust masses is considered and used to justify that assumption. It is found that, in general, the criterion derived for a single (average) dust species provides an adequate approximation to the physics.
Structure of laser-crystallized Ge2Sb2+xTe5 sputtered thin films for use in optical memory
Authors: Yamada Noboru , Matsunaga Toshiyuki
Published: July 2002

The structure of laser-crystallized thin films of Ge2Sb2+xTe5 (0.0<x⩽1.0) formed by the sputtering method were identified by x-ray diffraction studies to be composed of two phases: one phase is chiefly NaCl type crystal with a lattice constant of about 6 Å and a composition corresponding to Ge2Sb2Te5; the other phase comprises a small amount of an amorphous component such as Sb metal. Results of the Rietveld and the whole-powder-pattern fitting analyses show good agreement when assuming that (i) the 4(a) site is wholly occupied by only Te, (ii) the 4(b) site is randomly occupied by Ge or Sb atoms, and (iii) a little less than 20% of the 4(b) site is always vacant independent of the x value. The above results and the fact that halo noise rises with x increasing from 0.0 to 1.0 indicate a more precise model of crystal structure as follows. That is, Sb atoms added beyond the stoichiometric ratio, Ge2Sb2Te5, never fill up the vacancies of the 4(b) site in the NaCl type structure; the excess Sb atoms will remain in the amorphous state and concentrate, for example, at the grain boundary. The authors conclude that the amounts of the amorphous component produced through the crystallization process predominantly determine the crystallization rates and the critical temperatures of Ge–Sb–Te amorphous films, reportedly that they show a gentle and continuous dependence on the compositional deviation from the GeTe–Sb2Te3 pseudobinary line.
Collision-dominated to collisionless electron-free space-charge sheath in a plasma with variable ion temperature
Authors: Benilov M. S.
Published: July 2002

A theory of the near-cathode space-charge sheath in the case when the near-cathode voltage is high enough and the presence of electrons in the sheath is unessential is developed on the basis of a fluid description of the ion motion with account of ion-atom collisions and of variable ion temperature. The model spans the range of conditions from a collision-free to collision-dominated space-charge sheath. Detailed analytical and numerical results are presented for two models of ion-atom interaction, the model of rigid spheres (constant mean free path) and the model of Maxwell molecules (constant frequency of momentum transfer). It is found, in particular, that the assumption of cold ions provides a good accuracy in the problem considered. An analytical solution has been obtained under this assumption for the model of Maxwell molecules.
Coupled moment expansion model for the dynamics in a beam–plasma system
Authors: Backhaus E. Yu. , Wurtele J. S.
Published: July 2002

A new technique is developed to study beam dynamics in an overdense plasma. This technique is an alternative to, and extension of, the well-known envelope equation formalism, and it can be used in systems with nonlinear forces. It is based on a systematic moment expansion of the Vlasov equation. In contrast to the envelope equation, which is derived assuming constant rms beam emittance, this coupled moment expansion (CME) model allows the emittance to vary through coupling to higher order moments. The CME model is implemented in slab geometry in the absence of return currents. The evolution of the low order moments of the distribution function are followed with ∼100 ordinary differential equations. The CME simulation yields rms beam sizes,velocity spreads, and emittances that are in good agreement with particle-in-cell simulations for a wide range of system parameters. New analytical results for the second-order moments in the phase-mixed state are obtained. Unlike previous results, these connect the final values of the second-order moments with the initial beam mismatch. These analytical estimates are in good agreement with the CME model and particle-in-cell simulations.
Isoelectronic As doping effects on the optical characteristics of GaN films grown by metalorganic chemical-vapor deposition
Authors: Huang H. Y. , Lin W. C. , Lee W. H. , Shu C. K. , Liao K. C. , Chen W. K. , Lee M. C. , Chen W. H. , Lee Y. Y.
Published: July 2002

We have studied the As doping effects on the optical characteristics of GaN films by time-integrated photoluminescence and time-resolved photoluminescence. When As is incorporated into the film, the localized defect levels and donor–acceptor pair transition become less resolved. The recombination lifetime of neutral-donor-bound exciton (I2) transition in undoped GaN increases with temperature as T1.5. However, the I2 recombination lifetime in As-doped GaN first decreases exponentially from 98 to 41 ps between 12 and 75 K, then increases gradually to 72 ps at 250 K. Such a difference is related to the isoelectronic As impurities in GaN, which generate nearby shallow levels that dominate the recombination process.
Quantum molecular dynamics and spectral simulation of a boron impurity in solid para-hydrogen
Authors: Krumrine Jennifer R. , Jang Soonmin , Alexander Millard H. , Voth Gregory A.
Published: July 2002

Using path-integral molecular dynamics, we investigate the equilibrium properties of a boron impurity trapped in solid para-hydrogen. Because of its singly filled 2p orbital, the B atom interacts anisotropically with the pH2 molecules in the matrix. To assess the effect of this electronic anisotropy, we compare with similar simulations in which an orientation-averaged B–H2 potential is used. We investigate three matrices: (a) a single B atom site substituted for a pH2 molecule, (b) a similar site-substituted matrix with a nearest-neighbor vacancy, and (c) a B atom site substituted not in the bulk but near the pH2 surface. It is found that small distortions of the lattice occur to permit an energetically favorable orientation of the 2p orbital, even in the absence of a vacancy. When the B impurity is located near the surface, the spherically-averaged potential provides a noticeably different description from the case of the anisotropic potential. The 3s←2p absorption spectra of the B chromophore is also predicted by means of a semiclassical Franck–Condon technique using path integrals to sample the quantum lattice configurations. These spectral simulations provide additional insight into the interpretation of experimental observations of trapped B in a solid pH2 matrix.
Study of the electrostatic force between a conducting tip in proximity with a metallic surface: Theory and experiment
Authors: Patil Shivprasad , Kulkarni Arun V. , Dharmadhikari C. V.
Published: July 2002

A formula has been derived for the electrostatic force between the conducting tip and a planer metallic surface by exploiting the fact that the tip–sample geometry can be described by confocal hyperboloids of revolution. The prolate spheroidal coordinate system was found to be most convenient for this purpose. The general behavior of force curves obtained in the attractive regime using a conducting cantilever and an optical beam deflection system is in reasonably good agreement with the theory over a wide range of distances. The results are important in the context of design, development, and understanding of scanning probe microscopes involving voltage bias between the probe and sample.
Bragg scattering from periodically microstructured light emitting diodes
Authors: Lupton John M. , Matterson Benjamin J. , Samuel Ifor D. W. , Jory Michael J. , Barnes William L.
Published: July 2002

We present a simple method of generating a periodic wavelength scale structure in the optically active layer of a light emitting diode. This is achieved by solution deposition of a light emitting polymer on top of a corrugated substrate. The periodic structure allows waveguide modes normally trapped both in the substrate and in the thin polymer film to be Bragg scattered out of the structure, thus leading to a doubling of efficiency. This scattering process gives rise to a polarization of the emission spectrum as well as angular dispersion effects.
The semiclassical propagator for spin coherent states
Authors: Stone Michael , Park Kee-Su , Garg Anupam
Published: July 2002

We use a continuous-time path integral to obtain the semiclassical propagator for minimal-spread spin coherent states. We pay particular attention to the “extra phase” discovered by Solari and Kochetov, and show that this correction is related to an anomaly in the fluctuation determinant. We show that, once this extra factor is included, the semiclassical propagator has the correct short time behavior to O(T2), and demonstrate its consistency under dissection of the path.
Dimensional crossover of diffusion-limited reaction in a quasi-one-dimensional lattice
Authors: Lee Jae Woo
Published: July 2002

We have studied a dimensional crossover of a diffusion-limited reaction A+B→0, with and without a drift in a quasi-one-dimensional lattice W×L where the length of the lattice L is large and W is the width of the lattice. The density follows a scaling function such as C(t)∼W−xf(t/tc), where f(z)∼z−α,z≪1 with α=0.59(1) regardless of the drift and f(z)∼z−β,z≫1 with β=0.254(8) without the drift and β=0.31(2) with the drift. We found the scaling exponent x=0.87(1) for the isotropic diffusion and x=1.05(1) for the maximum drift. We observed that the crossover time had a power law like tc∼Wy with y=1/2(β−α).
Two-center nonexchange integrals over Slater orbitals
Authors: Barnett Michael P.
Published: July 2002

We have used computer algebra (CA) to construct general formulas for the two-center overlap, resonance, Coulomb and hybrid integrals over Slater orbitals (STOs). Individual formulas then were produced for the Coulomb integrals containing all combinations of K, L and M shell orbitals. The numerical evaluation of these formulas is very rapid and allows unrestricted accuracy. Tests give complete agreement with the output of numerical programs written by other workers. Computer algebra eliminates terms of equal magnitude and opposite sign that can cause catastrophic loss of accuracy in a purely numerical computation. Strong parallelization is possible in the construction and evaluation of the formulas. Their availability opens up the prospect of carrying symbolic computation into the eigenvalue finding stage of quantum chemistry. The present calculations also highlight several needs and opportunities in the field of computer algebra.
Destabilizing effect of thermal conductivity on the Rayleigh–Taylor instability in a plasma
Authors: Ryutov D. D.
Published: July 2002

It is shown that the Rayleigh–Taylor instability in a compressible medium is not necessarily stabilized by thermal conductivity. It is pointed out that one can use the destabilization effect for in situ measurements of thermal conductivity in high-energy-density experiments. The other consequence is generation of small-scale turbulence in supernovae, giving rise to significant turbulent viscosity.
High-voltage mesa-structure GaN Schottky rectifiers processed by dry and wet etching
Authors: Zhu T. G. , Lambert D. J. H. , Shelton B. S. , Wong M. M. , Chowdhury U. , Dupuis R. D.
Published: July 2002

We have fabricated and investigated high-voltage GaN vertical Schottky-barrier rectifiers grown by metalorganic chemical vapor deposition. A mesageometry Schottky-barrier rectifier having a 5-μm-thick i region, and processed using reactive-ion etching, exhibited a reverse breakdown voltage of −450 V (at 10 mA/cm2) and an on-resistance of 23 mΩ cm2. For comparison, we have also applied wet chemical etching for the fabrication of mesageometry Schottky-barrier rectifiers. The 2-μm-thick i-region GaN mesa-Schottky rectifiers showed a breakdown voltage of −310 and −280 V for wet-etched and dry-etched devices, respectively, and an on-resistance of 8.2 and 6.4 mΩ cm2, respectively. These results indicate that the performance of the wet-etched rectifiers is comparable to or better than that of comparable dry-etched devices.
Evolution of structural and electronic properties of highly mismatched InSb films
Authors: Weng X. , Goldman R. S. , Partin D. L. , Heremans J. P.
Published: July 2002

We have investigated the evolution of structural and electronic properties of highly mismatched InSb films, with thicknesses ranging from 0.1 to 1.5 μm. Atomic force microscopy, cross-sectional transmission electron microscopy, and high-resolution x-ray diffraction show that the 0.1 μm films are nearly fully relaxed and consist of partially coalesced islands, which apparently contain threading dislocations at their boundaries. As the film thickness increases beyond 0.2 μm, the island coalescence is complete and the residual strain is reduced. Although the epilayers have relaxed equally in the 〈110〉 in-plane directions, the epilayer rotation about an in-plane axis (epilayer tilt) is not equal in both 〈110〉 in-plane directions. Interestingly, the island-like surface features tend to be preferentially elongated along the axis of epilayer tilt. Furthermore, epilayer tilt which increases the substrate offcut (reverse tilt) is evident in the [110] direction. High-resolution transmission electron microscopy indicates that both pure-edge and 60° misfit dislocations contribute to the relaxation of strain. In addition, as the film thickness increases, the threading dislocation density decreases, while the corresponding room-temperature electron mobility increases. The other structural features, including the residual strain, and the surface and interface roughness, do not appear to impact the electron mobility in these InSb films. Together, these results suggest that free-carrier scattering from the threading dislocations is the primary room-temperature mobility-limiting mechanism in highly mismatched InSb films. Finally, we show quantitatively that free-carrier scattering from the lattice dilation associated with threading dislocations, rather than scattering from a depletion potential surrounding the dislocations, is the dominant factor limiting the electron mobility.
Ewald sums for Yukawa potentials
Authors: Salin Gwenaël , Caillol Jean-Michel
Published: July 2002

The numerical simulation of systems involving Yukawa interaction y(r)=exp(−αr)/r (e.g., colloids, dusty plasmas,…) needs some caution in the case where the potential cannot be neglected on the boundaries of the cubic simulation cell [i.e., when y(L/2)−L side of the cube — is larger than the absolute uncertainties wanted for the thermal average of the energy]. In that case the usual minimum image convention fails and it is necessary to introduce Ewald sums similar to those used for Coulomb systems. In this study we derive the expression of the Ewald sums associated to Yukawa interactions and discuss the numerical errors induced by their truncation.
Improved fabrication approach for carbon nanotube probe devices
Authors: Stevens R. , Nguyen C. , Cassell A. , Delzeit L. , Meyyappan M. , Han Jie
Published: July 2002

An improved process is developed for simple and efficient fabrication of carbon nanotube probe devices. This process requires only two steps to make nanotube probes. First a nanotube cartridge is created using chemical vapor deposition, then the nanotubes are transferred from the cartridge to a device using an electric field. Multiwall nanotube probes are made into different device geometries in this approach. Their applications are illustrated by atomic force microscopy imaging of the surface of a terrestrial rock granule selected to simulate the morphology and consistency of a grain of Mars dust and nanolithography on a silicon substrate.
Thermal stability of Cu/Ta/GaAs multilayers
Authors: Chen Chang-You , Chang Li , Chang Edward Y. , Chen Szu-Houng , Chang Der-Fu
Published: July 2002

Copper metallization for GaAs was evaluated by using Cu/Ta/GaAs multilayers for its thermal stability. A thin Ta layer of 40 nm was sputtered on the GaAs substrate as the diffusion barrier before copper film metallization. As judged from sheet resistance, x-ray diffraction, Auger electron spectroscopy and transmission electron microscopy, the Cu/Ta films with GaAs were very stable up to 500 °C without migration into GaAs. After 550 °C annealing, the interfacial mixing of Ta with GaAs substrate occurred, resulting in the formation of TaAs2. At 600 °C annealing, the reaction GaAs with Ta and Cu formed TaAs, TaAs2, and Cu3Ga, resulting from Cu migration and interfacial instability.
Doping of 6H–SiC by selective diffusion of boron
Authors: Soloviev S. I. , Gao Y. , Sudarshan T. S.
Published: July 2002

Experimental evidence of selective boron doping of 6H–SiC via diffusion is given. Selective diffusion has been realized at 1800–2100 °C using graphite film as a protecting mask. Cathodoluminescence measurements as well as an anodic oxidation technique have been employed to identify the local doped regions. In addition, a diffused planar p–n diode based on the local p-type emitter region was fabricated. The ideality factor extracted from current–voltage characteristics was 1.97, which indicates that the Shockley–Hall–Read recombination is the dominant mechanism in the conduction region. The value of breakdown voltage for this diode measured at room temperature was a little greater than 800 V, and a leakage current density of 5.7×10−6 A/cm2 at 800 V was achieved.
Sequential ionization of C60 with femtosecond laser pulses
Authors: Campbell E. E. B. , Hoffmann K. , Rottke H. , Hertel I. V.
Published: July 2002

Multiply charged C60q+, q<6, is observed in the multiphoton ionization of C60 with 790 nm, 25 fs pulses. Determination of the laser intensity dependence of the ion intensities indicates that the multiple ionization occurs predominantly sequentially. Coupling of the electronic excitation energy to vibrational degrees of freedom occurs in competition with further ionization.
Proton transfer and tautomerization in 7-hydroxyquinoline⋅(NH3)n clusters: Structure and energetics at the self-consistent field level
Authors: Coussan Stéphane , Meuwly Markus , Leutwyler Samuel
Published: July 2002

The structures and energetics of 23 different 7-hydroxyquinoline⋅(NH3)n clusters with n=1–6 are investigated using ab initio calculations at the SCF/6-31G** level. Clusters based on both the enol and keto tautomers of 7-hydroxyquinoline (7-HQ) are investigated, as well as ion-pair (zwitterionic) clusters resulting from proton transfer from 7-HQ to the (NH3)n cluster. In all cases, the solvent cluster forms hydrogen bonds at both the -O-H and N positions of 7-HQ. For n=2–4, ammonia-chain clusters are found, for n⩾4, additional hydrogen-bond topologies appear such as bifurcated chains, cycles and mixed chain/cycles. Although the bare keto 7-HQ is calculated to be 17 kcal/mol less stable than the enol form, the energy difference decreases continuously to 11 kcal/mol for 7-HQ⋅(NH3)6 with increasing n. For one of the enol clusters involving six NH3 molecules, proton transfer can occur from either the -O-H or N-H group to the cluster, yielding a locally stable zwitterion cluster 7-HQ−⋅(NH3)6H+. For the keto tautomer, locally stable ion-pair clusters were found for n=2, 5, and 6, but these are calculated to be considerably less stable than the neutral enol forms.