bluuebunny/crossref_metadata_2025 huggingface dataset:
High voltage operation (>80 V) of GaN bipolar junction transistors with low leakage
Authors: Limb J. B. , Xing H. , Moran B. , McCarthy L. , DenBaars S. P. , Mishra U. K.
Published: July 2002

We have demonstrated the high voltage operation of n-p-n GaN bipolar junction transistors using regrown emitters. Devices were grown by metalorganic chemical vapor deposition on sapphire substrates. The n-type emitter was grown selectively on a base-collector p-n junction diode using a dielectric mask. A thin base (1000 Å) was used to increase the current gain over our previous result with a regrown emitter [J. B. Limb, L. McCarthy, P. Kozodoy, H. Xing, J. Ibbetson, Y. Smorchkova, S. P. DenBaars, and U. K. Mishra, Electron. Lett. 35, 19 (1999)]. The base contacts were better than expected despite the use of a thin base. Common emitter operation showing a voltage operation of over 80 V with negligible leakage has been demonstrated. Room temperature current gain was ∼3 corresponding to a current transfer ratio of ∼0.75. This results in a calculated minority carrier lifetime of about 80 pS in the base.
Well-width dependence of the quantum efficiencies of GaN/AlxGa1−xN multiple quantum wells
Authors: Zeng K. C. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

A set of GaN/AlxGa1−xN(x≈0.2) multiple quantum wells (MQWs) with well widths, Lw, varying from 6 to 48 Å has been grown by metalorganic chemical vapor deposition under the optimal GaN-like growth conditions. Picosecond time-resolved photoluminescence spectroscopy has been employed to probe the well-width dependence of the quantum efficiencies (QE) of these MQWs. Our results have shown that these GaN/AlGaN MQW structures exhibit negligibly small piezoelectric effects and hence enhanced QE. Furthermore, GaN/AlxGa1−xN MQWs with Lw between 12 and 42 Å were observed to provide the highest QE, which can be attributed to the reduced nonradiative recombination rate as well as the improved quantum-well quality. The decreased QE in GaN/AlxGa1−xN MQWs with Lw<12 Å is due to the enhanced carrier leakage to the underlying GaN epilayers, while the decreased QE in MQWs with Lw>42 Å is associated with an increased nonradiative recombination rate as Lw approaching the critical thickness of MQWs. The implications of our results on device applications are also discussed.
Transport properties of Bi nanowire arrays
Authors: Lin Yu-Ming , Cronin Stephen B. , Ying Jackie Y. , Dresselhaus M. S. , Heremans Joseph P.
Published: July 2002

To explain various temperature-dependent resistivity measurements [R(T)] on bismuth (Bi) nanowires as a function of wire diameter down to 7 nm, a semiclassical transport model is developed, which explicitly considers anisotropic and nonparabolic carriers in cylindrical wires, and the relative importance of various scattering processes. R(T) of 40 nm Bi nanowires with various Te dopant concentrations is measured and interpreted within this theoretical framework.
Dynamics of a gravitationally loaded chain of elastic beads
Authors: Manciu Marian , Tehan Victoria N. , Sen Surajit
Published: July 2002

Elastic beads repel in a highly nonlinear fashion, as described by Hertz law, when they are compressed against one another. Vertical stacking results in significant compressions of beads at finite distances from the surface of the stack due to gravity. Analytic studies that have been reported in the literature assume acoustic excitations upon weak perturbation [J. Hong et al., Phys. Rev. Lett. 82, 3058 (1999)] and soliton-like excitations upon strong perturbation [V. Nesterenko, J. Appl. Mech. Tech. Phys. 5, 733 (1983); S. Sen and M. Manciu, Physica A 268, 644 (1999)]. The present study probes the position, velocity and acceleration and selected two-point temporal correlations and their power spectra for individual beads for cases in which the system has been (i) weakly, (ii) strongly, and (iii) moderately perturbed at the surface in the sense specified in the text. Our studies reveal the existence of distinctly different dynamical behavior of the tagged beads, in contrast to conventional acoustic response, as the strength of the perturbation is varied at fixed gravitational loading. We also comment on the effects of polydispersity on system dynamics and probe the relaxation of isolated light and heavy beads in the chain.
Effect of excess Bi2O3 on the ferroelectric properties of SrBi2Ta2O9 ceramics
Authors: Lee Jung-Kun , Park Byungwoo , Hong Kug-Sun
Published: July 2002

In this study, the effect of bismuth content on phase-transition behavior, ferroelectric property, and crystal structure of strontium–bismuth–tantalate (SBT) ceramics was explored with the aid of εr(T), ferroelectric hysteresis loop, x-ray diffraction (XRD), and Raman spectroscopy. Both the phase-transition behavior and ferroelectric properties such as spontaneous polarization (Ps) showed a dependence on Bi content. The ferroelectric Curie temperature Tc, was found to decrease with increasing the Bi content and Ps was maximized when 2–3 mol % excess Bi2O3 was added. It was found that Raman spectroscopy and XRD can explain the Bi-content dependence of SBT ceramics. The frequency and the width of the mode below 60 cm−1 in Raman spectra revealed that a site exchange of Sr2+ and Bi3+ ions occurred. The formation of antisite defects was confirmed by the change in the intensity ratio of I(008)/I(105) in the XRD patterns. The calculation of structure factors showed it was also related to antisite defects (SrBi,BiSr).
Diffusion and transport phenomena in a two-component singly charged magnetized and anisotropic plasma stream
Authors: Mohanty J. N. , Baral K. C.
Published: July 2002

The presence of a beam in an electron–ion plasma embedded in magnetic field lines entails streaming or mass motion of plasma species. It causes a kind of energy dissipation (v0⋅p±) in the plasma, where v0 represents (v0≪c) streaming velocity of the species in the same frame and system, transverse to the direction of the applied uniform magnetic field. A modified Chapman–Enskog collisional analysis is employed in the calculation of the flux across the magnetic field, assuming a small ratio of gyroradius to the density inhomogeneity scale length and particle mean-free path. Explicit formulas concerning diffusion and modified transport coefficients are analytically derived. The E×B drift velocity is found to be independent of streaming velocity (v0), and the electrical resistivity (η⊥) decreases slowly with increasing v0, whereas the thermoelectric coefficient (λ) registers marked enhancement; however, the thermal conductivity (K) decreases slowly with the rise in streaming parameters. In the limit V0→O, early nonstreaming results are recovered. It is also found that η⊥, λ and K all decrease with the increase in thermal energy. In addition, the results are of relevance to solar flares, sunspots, etc., and the influence of streaming in fusion temperature (kT→1 keV) range is equally emphasized.
Thermal condensation modes in weakly ionized hydrogen plasmas
Authors: Birk Guido T.
Published: July 2002

The dispersion relations for condensation modes in weakly ionized two-fluid hydrogen plasmas in ionization equilibrium with magnetic fields are derived. The excitement of unstable modes is determined by different instability criteria and the modes evolve on different time scales for the neutral gas and the ionized fluid component, respectively. Equilibrium magnetic fields do not necessarily hinder the condensation of the ionized component.
Combinatorial explosion in model gene networks
Authors: Edwards R. , Glass L.
Published: July 2002

The explosive growth in knowledge of the genome of humans and other organisms leaves open the question of how the functioning of genes in interacting networks is coordinated for orderly activity. One approach to this problem is to study mathematical properties of abstract network models that capture the logical structures of gene networks. The principal issue is to understand how particular patterns of activity can result from particular network structures, and what types of behavior are possible. We study idealized models in which the logical structure of the network is explicitly represented by Boolean functions that can be represented by directed graphs on n-cubes, but which are continuous in time and described by differential equations, rather than being updated synchronously via a discrete clock. The equations are piecewise linear, which allows significant analysis and facilitates rapid integration along trajectories. We first give a combinatorial solution to the question of how many distinct logical structures exist for n-dimensional networks, showing that the number increases very rapidly with n. We then outline analytic methods that can be used to establish the existence, stability and periods of periodic orbits corresponding to particular cycles on the n-cube. We use these methods to confirm the existence of limit cycles discovered in a sample of a million randomly generated structures of networks of 4 genes. Even with only 4 genes, at least several hundred different patterns of stable periodic behavior are possible, many of them surprisingly complex. We discuss ways of further classifying these periodic behaviors, showing that small mutations (reversal of one or a few edges on the n-cube) need not destroy the stability of a limit cycle. Although these networks are very simple as models of gene networks, their mathematical transparency reveals relationships between structure and behavior, they suggest that the possibilities for orderly dynamics in such networks are extremely rich and they offer novel ways to think about how mutations can alter dynamics.
Alfvén vortices and chaos in electron–positron dusty magnetoplasma with equilibrium flows
Authors: Mirza Arshad Majid
Published: July 2002

A set of coupled nonlinear differential equations which govern the dynamics of low-frequency short-wavelength electromagnetic waves in a nonuniform magnetized electron–positron (pair) plasma with dust has been derived. In the linear limit, a local dispersion relation has been derived and analyzed. It is shown that in the absence of density gradients and equilibrium sheared plasma flow, can make the dust shear Alfvén waves unstable. In the nonlinear case, the temporal behavior of nonlinear dissipative system can be expressed in the form of well known Lorenz and Stenflo type equations that admit chaotic trajectories. On the other hand, a quasi-stationary solution of the mode coupling equations can be represented in the form of dipolar and vortex-chain solutions. The results of our present investigation should be helpful for understanding plasma transport and wave phenomena in the pulsars magnetosphere.
Profile control in BF3 plasma doping
Authors: Kwok Dixon T. K. , Chu Paul K. , Chan Chung
Published: July 2002

Plasma doping (PD) is an alternative technique to form shallow junctions in deep-submicrometer microelectronic devices. Previous studies have demonstrated that PD produces shallow junctions with better efficiency than those by conventional low energy beam-line doping (BD). In addition, even though cross-sectional transmission electron microscopy reveals that the surface layer is amorphized after high dose BF3 PD or BD implantation, PD samples show less residual defects after rapid thermal annealing. For ultrashallow junctions, doping profiles with a high dopant concentration near the surface are required for the formation of low resistant contacts. In this article, we demonstrate the use of nonideal voltage pulse shape in achieving advantageous doping profiles that are difficult to obtain via BD. By performing particle-in-cell (PIC) simulation, we derive the ion energy distributions for different sample voltage pulse shapes for BF3 PD. Comparison of the PD boron depth profiles simulated by PIC and an assumed Gaussian implant profile to the BD boron depth profiles simulated by TRIM shows a low energy component that does not exist in BD samples. The rise and fall time of the sample voltage pulse contributes to the overall energy distribution since a long rise or fall time increases the low energy component. We postulate that these low energy ions may also change the nature of the amorphized layer and are one of the reasons for the reduction of residual defects after rapid thermal annealing. The preferred sample voltage pulse for plasma doping is suggested to be a short one with a relatively long rise and fall time. This is something that is very difficult to achieve by beam-line ion implantation.
Structural and optical properties of self-assembled InAs/GaAs quantum dots covered by InxGa1−xAs (0⩽x⩽0.3)
Authors: Liu H. Y. , Wang X. D. , Wu J. , Xu B. , Wei Y. Q. , Jiang W. H. , Ding D. , Ye X. L. , Lin F. , Zhang J. F. , Liang J. B. , Wang Z. G.
Published: July 2002

Optical and structural investigations of InAs quantum dots (QDs) covered by InxGa1−xAs (0⩽x⩽0.3) overgrowth layer have been systematically reported. The decrease of strain in the growth direction of InAs quantum dots covered by InGaAs layer instead of GaAs is demonstrated by transmission electron microscopy experiments. In addition, the atomic force microscopy measurement shows that the surface of InAs islands with 3-nm-thick In0.2Ga0.8As becomes flatter. However, the InGaAs islands nucleate on the top of quantum dots during the process of InAs islands covered with In0.3Ga0.7As. The significant redshift of the photoluminescence peak energy and reduction of photoluminescence linewidth of InAs quantum dots covered by InGaAs are observed. The energy gap change of InAs QDs covered by InGaAs could be explained in terms of reducing strain, suppressing compositional mixing, and increasing island height.
Radio frequency power deposition in a high-density helicon discharge with helical antenna coupling
Authors: Enk Th. , Krämer M.
Published: July 2002

A high-density helicon discharge (ne⩽1020 m−3) produced through a new m=1 helical antenna is investigated. Various diagnostics are applied to measure the discharge parameters and the radio frequency (rf) quantities like the plasma resistance and the rf field distribution. Special attention is paid to the axial asymmetry of the discharge, which is characteristic for helicon devices with helical antennas. The axial profiles of the rf wave fields, as well as the energy deposition profiles, reveal that the rf power is mainly transferred and absorbed via the m=+1 helicon mode traveling in the positive magnetic-field direction. The experimental findings are compared with numerical results obtained from a fully electromagnetic model, which takes into account the rf current distribution of the launching antenna, as well as the finite size of the plasma column. The antenna–plasma coupling, as well as the total rf power deposited in the plasma, can be explained satisfactorily if the measured profiles are taken in the computations. In particular, the axial asymmetry of the helicon discharge can be understood in terms of the radial inhomogeneity of the plasma column. Furthermore, the calculations show that the small-scale Trivelpiece–Gould waves may be excited near the plasma edge. These waves would carry a considerable fraction of the absorbed rf power and may thus be important for the rf power coupling.
Amplification of spontaneous magnetic fields due to inverse bremsstrahlung absorption of high frequency radiation
Authors: Ferrante G. , Zarcone M. , Uryupin S. A.
Published: July 2002

The theory of the linear stage of the Weibel instability growth due to anisotropic heating of electrons undergoing inverse bremsstrahlung absorption of high frequency electromagnetic radiation is reported. The possibility is shown of a significant amplification of spontaneous magnetic fields for both the cases of circularly and linearly polarized high frequency radiation. Numerical calculations and comparisons are reported for a wide range of the process parameters, covering the cases of weak and strong high-frequency radiation fields. Among others, if the linearly polarized field is strong, the magnetic field amplification for typical conditions is found to be less significant as compared to the case of strong circularly polarized field. The reverse result is found for the case of weak linearly and circularly polarized fields.
Doping of covalently bound fullerene monolayers: Ag clusters on C60/Si(111)
Authors: Taylor M. D. R. , Moriarty P. , Cotier B. N. , Butcher M. J. , Beton P. H. , Dhanak V. R.
Published: July 2002

The deposition of silver on a C60 monolayer chemisorbed on Si(111) leads to the formation of nanoscale clusters which, as determined from valence band photoemission measurements, donate minimal charge to the adsorbed fullerene molecules. The low doping level of the C60 monolayer leads to a slow rate of Ag cluster neutralization following the emission of a photoelectron, resulting in a significant shift of the cluster Fermi level and valence band features to higher binding energy.
Plasma current in tokamak conditions: An application of the guiding field line model
Authors: Petrov Yu. , Huang T. S.
Published: July 2002

A new kinetic representation for the plasma current is examined for tokamak conditions. Within this representation, the periodic bounce motion is extracted from the guiding center motion of particles. The drift orbit of a guiding center is viewed as a closed loop stretched along the guiding field line—a field line that moves at the bounce-averaged drift rate. Because all quantities, including the distribution function, are defined not on the basis of the particle’s guiding center, but rather on the basis of the guiding field line, an additional magnetization term appears, which corresponds to the wobbling deviation of the guiding center from the guiding field line. Thus, the current is decomposed into three terms: The flow of orbits as a whole, the gyro-magnetization, and bounce–wobble magnetization. The great advantage of the new model is that the zeroth-order distribution function, a nonshifted Maxwellian distribution, yields a current that satisfies the fluid equations.
Nonperturbative theory of multiphonon anharmonic transitions
Authors: Hizhnyakov V. , Kaasik H.
Published: July 2002

A method of time-dependent self-consistent phonons is applied for a nonperturbative description of the relaxation of a local mode caused by multiphonon anharmonic transitions between its vibrational levels. Theory predicts a sharp change of the relaxation law at the critical vibrational level. In the vicinity of this level the rates of the transitions attain a very high value, comparable to the dephasing rate of phonons. In the case of weak interactions, the results agree with perturbation theory.
1.54 μm Er3+ photoluminescent and waveguiding properties of erbium-doped silicon-rich silicon oxide
Authors: Han Hak-Seung , Seo Se-Young , Shin Jung H. , Kim Dong-Su
Published: July 2002

1.54 μm Er3+ photoluminescent and waveguiding properties of erbium-doped silicon-rich silicon oxide (SRSO) are investigated. Optimum Er3+ luminescence was obtained after an anneal of at least 5 min at 950 °C, and at least 1 at. % excess silicon in SRSO was necessary for the excitation of erbium to be dominated by carriers. The refractive index and the bulk waveguide loss of erbium-doped SRSO film with 0.1 at. % erbium and 1 at. % excess silicon after the optimal anneal treatment was 1.4817 and 4.0 dB/cm, respectively. Fabrication of an erbium-doped SRSO strip waveguide using the standard Si processing techniques and the guiding of internal 1.54 μm Er3+ emission by such a strip waveguide are demonstrated.
Influence of technological parameters on the behavior of the hole effective mass in SiGe structures
Authors: Rodrı́guez Salvador , Gámiz F. , Palma A. , Cartujo P. , Carceller J. E.
Published: July 2002

We present a study of some features of a two-dimensional hole gas confined in a Si/Si1−xGex/Si/SiO2 structure when the external applied gate bias is varied. From the framework of the effective-mass theory, and applying the semiaxial approximation to separate the 6×6 Luttinger Hamiltonian into two 3×3 matrices, we calculated the hole density profile and the band structure. This enabled us to evaluate, in an iterative process, the Poisson and Schrödinger equations until convergence was achieved, obtaining the above-mentioned results for a discretional angle in the kx–ky plane, and thus including the warping. We identified in this way the influence of the different technological parameters which determine the behavior of the device, in particular, through the determination of a unique effective mass. We conclude that the utility of this kind of device lies more in the possibility of modifying the band structure due to the strain introduced than in the fact of being able to confine carriers in the Si1−xGex channel. Accurate calculation of the band structure is, therefore, needed and although simpler simulations can qualitatively model some features of the device, a complete study as described in this article must be carried out in order to obtain better insight into the physics of the system.
Response to “Comment on ‘Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling’ ” [J. Appl. Phys. 88, 2172 (2000)]
Authors: Pollak Fred H.
Published: July 2002

This communication is a response to the Comment on “Optical properties of CdTeS: Experiment and modeling” by A. B. Djurišić and E. H. Li. The shortcomings of a number of schemes to model the optical constants of diamond- and zincblende-type semiconductors, which do not include continuum exciton effects at either the E0, E0+Δ0 or E1, E1+Δ1 critical points (CPs), are discussed. The Holden model, which has been used to fit the CdTeS data, does include these effects, making it possible to extract important parameters, such as the binding energy of the exciton associated with the E1, E1+Δ1 CPs, not deduced by the other approaches.
Effect of Bi substitution at the Sr site on the ferroelectric properties of dense strontium bismuth tantalate ceramics
Authors: Noguchi Yuji , Miyayama Masaru , Kudo Tetsuichi
Published: July 2002

Ferroelectric properties for dense bulk ceramics of Sr-deficient and Bi-excess strontium bismuth tantalate, Sr1−xBi2+xTa2O9, were investigated, and their crystal structures were analyzed by the powder x-ray diffraction Rietveld method. A successful pattern fitting was obtained under the assumption that Bi was substituted at the Sr site for samples with x⩽0.16, and the lattice in the a-b plane was reduced linearly with an increase in x. Furthermore, dielectric measurements showed that the Curie temperature, TC, rose with x and the sample with x=0.27 had a TC of 430 °C, which is much higher than that of the stoichiometric sample (x=0: TC=300 °C). These results strongly support the Bi substitution at the Sr site. The polarization hysteresis measurements revealed that a small amount of Bi substitution of 5% results in a marked increase in remanent polarization, however that further Bi substitution is not effective for improving the ferroelectric properties.
In situ electrical characterization of dielectric thin films directly exposed to plasma vacuum-ultraviolet radiation
Authors: Cismaru C. , Shohet J. L.
Published: July 2002

In this article we report a method for in situ electrical characterization of dielectric thin films under direct exposure to plasma in an electron-cyclotron-resonance etcher. This method is based on the development of a special test structure that allows for the measurement of the influence of plasma vacuum-ultraviolet (VUV) radiation on the electrical conductivity of thin dielectric layers. Results show that the measured conductivity of SiO2 layers temporarily increases during exposure to argon and oxygen plasmas, with controlled VUV emission. Based on the measurements made through this method, a model of the VUV-induced conductivity of SiO2 is developed. These measurements are very important for plasma processing of semiconductor devices, because the temporary increase in the conductivity of these layers upon exposure to processing plasmas can decrease the plasma-induced charging of these dielectric layers depending on the intensity of the plasma VUV emission. This can have an impact on the properties and reliability of processed devices.
Spatially resolved heat conduction in polar perhydrotriphenylene inclusion compounds studied by means of thermal waves
Authors: Wübbenhorst M. , van Turnhout J. , Quintel A. , Hulliger J.
Published: July 2002

Scanning pyroelectric microscopy was applied to study the local thermal diffusivity in polar perhydrotriphenylene (PHTP) inclusion compounds. A thermal wave technique with special heat flow and pyroelectric detection principles was used to measure the longitudinal (K∥) and transversal (K⊥) thermal diffusivity of needle shaped crystals. As expected, we have found a higher thermal conductivity along the channel direction: K∥>K⊥. K∥ also showed a variation along the needle axis, the K∥ values near the capping faces being higher than in the center region. Based on these observations we suggest a relation between the longitudinal thermal conductivity and the local density of orientational defects of the dipolar guest molecules which reaches a maximum in the area around the seed. These findings are in agreement with previous pyroelectric studies, revealing a macrodomain structure and a spatially dependent pyroelectric activity for two different PHTP inclusion compounds.
Hall mobilities in B-doped strained Si1−xGex and Si1−x−yGexCy layers grown by ultrahigh vacuum chemical vapor deposition
Authors: Kar G. S. , Dhar A. , Ray S. K. , John S. , Banerjee S. K.
Published: July 2002

Hall mobilities in a temperature range of 80–300 K have been measured in fully strained Si1−xGex and partially strain-compensated p-type Si1−x−yGexCy alloy layers grown on Si (100) by ultrahigh vacuum chemical vapor deposition. The effect of the addition of C on strain compensation of Si1−xGex films has been studied by high-resolution x-ray diffraction analysis. The Hall hole mobility is found to increase with decreasing compensative strain or effective Ge content in the layer throughout the studied temperature range. The effect of a Si-cap layer on the hole mobility of Si1−x−yGexCy film has been investigated.
Oxide and hydrogen capped ultrasmall blue luminescent Si nanoparticles
Authors: Belomoin Gennadiy , Therrien Joel , Nayfeh Munir
Published: July 2002

We dispersed electrochemical etched silicon into a colloid of ultrasmall ultrabright Si nanoparticles. Direct imaging using transmission electron microscopy shows particles of ∼1 nm in diameter, and infrared and electron photospectroscopy show that they are passivated with hydrogen. Under 350 nm excitation, the luminescence is dominated by an extremely strong blue band at 390 nm. We replace hydrogen by a high-quality ultrathin surface oxide cap by self-limiting oxidation in H2O2. Upon capping, the excitation efficiency drops, but only by a factor of 2, to an efficiency still two-fold larger than that of fluorescein. Although of slightly lower brightness, capped Si particles have superior biocompatability, an important property for biosensing applications.
Optical constants and ellipsometric thickness determination of strained Si1−xGex:C layers on Si (100) and related heterostructures
Authors: Zollner Stefan , Hildreth Jill , Liu Ran , Zaumseil P. , Weidner M. , Tillack B.
Published: July 2002

The complex dielectric functions ε(ω) from 0.75 to 6.6 eV of pseudomorphically strained Si1−xGex (0<x<0.275) and Si1−x−yGexCy (x≈0.21, 0<y<0.013) alloys grown on Si (001) were determined using spectroscopic ellipsometry. Our rotating-analyzer instrument uses a computer-controlled MgF2 Berek waveplate as a compensator to improve the accuracy of the ellipsometric angles, particularly below 3 eV. By performing a least-squares analysis of the raw data, taken at three angles of incidence, we obtain the thicknesses of the alloy and the native oxide cap as well as ε(ω) for the alloy, which is parametrized using a semiempirical oscillator model. Differences between our data and those in the literature are due to differences in strain conditions and/or the improved accuracy of our instrument employing a compensator, which allows the determination of the native oxide thickness from measurements at long wavelengths. We apply our dielectric functions to analyze a variety of group-IV heterostructures and find good agreement with high-resolution x-ray diffractometry. The optical constants reported here are important, because they allow thickness and composition measurements using automated inline spectroscopic ellipsometers and reflectometers.
Self-affine silver films and surface-enhanced Raman scattering: Linking spectroscopy to morphology
Authors: Douketis C. , Haslett T. L. , Wang Z. , Moskovits M. , Iannotta S.
Published: July 2002

The relationship between the self-affine structure of cold-deposited films and the surface-enhanced Raman (SERS) intensity of benzene adsorbed on the films is examined. Based on variable temperature STM studies the structure of cold-deposited silver films is shown to be self-affine with a fractal dimension ∼2.6, more or less independent of temperature for T less than ∼270 K. The fractal structure is shown to collapse to a more or less compact structure when the films are annealed to ∼280 K. SERS activity ceases at a somewhat lower temperature (∼250 K) for all the films examined. SERS enhancements rise by factors as great as 8 as a function of annealing temperature over and above their values at the lowest deposition temperatures used (24 K). The rise reaches a maximum at a temperature that depends both on the deposition temperature of the films and the excitation wavelength. (Such an annealing effect on the SERS intensity has been known for many years.) We suggest that the observations are consistent with recently developed theories of the optical properties of fractal clusters so long as one includes arguments regarding the optimal dimensions of the monomers comprising those clusters whose mean value change during annealing. (Fractality, however, is not an essential condition for observing intense SERS.) The disappearance of the SERS signal before the self-affine structure of the film collapses is, therefore, attributed to the monomer size having significantly passed its optimal size before the temperature at which the collapse of the fractal nature of the film is reached. We argue that this collapse is mediated by surface diffusion. The narrow temperature range over which this occurs is intriguing and worthy of a proper theoretical investigation.
A nearly diffraction limited surface emitting conjugated polymer laser utilizing a two-dimensional photonic band structure
Authors: Riechel S. , Kallinger C. , Lemmer U. , Feldmann J. , Gombert A. , Wittwer V. , Scherf U.
Published: July 2002

We have fabricated a mechanically flexible conjugated polymer laser utilizing distributed feedback due to a two-dimensional photonic band structure. An ultraviolet-embossing process is used for nanopatterning a plastic substrate. On top we spin-coat a ladder-type poly(p-phenylene) as the active laser medium. Upon optical pumping, we observe a low threshold and nearly diffraction limited monomode laser emission perpendicular to the surface. Our results are explained within a Laue formulation for the feedback mechanism in the two-dimensional organic photonic crystal.
Clepsydrae, from Galilei to Torricelli
Authors: Clanet C.
Published: July 2002

The whole article deals with free fall: the free fall of a solid particle carefully studied by Galileo Galilei and the free fall of a fluid particle along a stream line introduced by Evangelista Torricelli. Both limits are brought together in the problem of the vertical emptying of a cylindrical tube of diameter D0 through a hole of diameter d, and one can move continuously from one limit to the other varying d, from d=D0, to d≪D0. The limit, d=D0, corresponds to Galilei’s problem of the solid free fall, in which the velocity of the upper interface, |Vi|, increases as it comes closer to the hole following the law |Vi|=2g|Z0−Zi|, where g is the acceleration due to gravity, Zi is the liquid height above the hole (defined by Z=0), and Z0 is the initial location of the interface. The opposite limit, d≪D0, corresponds to Torricelli’s problem, in which the velocity of the interface slows down as it comes closer to the hole, following the law |Vi|=(d/D0)22g|Zi|. Theoretically, the problem reduces to the integration of the differential equation: dvi2/dzi−[(D0/d)4−1](vi2/zi)+1=0, where length and velocity have respectively been reduced by Z0 and 2gZ0, and the z-axis oriented from the hole z=0, towards the initial interface location z=1. With α≡(D0/d)4−2, the above equation leads to the solution vi=−zi/α(1−ziα), when α≠0, and vi=−zi ln 1/zi when α=0. Galilei’s and Torricelli’s regimes correspond respectively to the limit α=−1 and α≫1. These solutions are compared to experimental measurements conducted over a large range of geometrical and physical parameters. In a second stage, the model, developed for nonconstant cross sectional area, is compared to the experimental results obtained in conical Clepsydrae.
Structural evolution during chemical vapor deposition of diamond thin films
Authors: Morell G. , Cancel L. M. , Figueroa O. L. , González J. A. , Weiner B. R.
Published: July 2002

In situ phase-modulated ellipsometry was employed to monitor the nucleation and growth processes of diamond thin films fabricated by chemical vapor deposition. The effective extinction coefficient (k) at 1.96 eV was used as a basis for dividing the deposition process into intervals. The film growth was aborted at various k values yielding diamond film samples that represent snapshots of the growth process at different stages. Ex situ characterization of the films was performed using Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The results indicate that the diamond film deposition process consists of various stages in which the crystalline quality, the net compressive stress, and the relative amount of non-sp3 carbon follow different trends. A correlation between the effective k value measured in situ and the film microstructure characterized ex situ was established which enables the monitoring of the diamond film growth process in real time.
Particle number fluctuations in a membrane channel
Authors: Bezrukov Sergey M. , Berezhkovskii Alexander M. , Pustovoit Mark A. , Szabo Attila
Published: July 2002

Channel-facilitated transport of metabolites across biological membranes results in excess noise in the current carried by small ions. This noise originates from fluctuations of the number of metabolite molecules in the channel due to their diffusion. We have carried out a theoretical study of particle number fluctuations in a cylindrical pore. First, we obtain the power spectral density of these fluctuations as a function of pore length and radius, as well as the diffusion constants of the particle in the pore and in the bulk, in the absence of particle–pore interactions. We then perform three-dimensional Brownian dynamics simulations that show excellent agreement with the analytical result. Finally, we demonstrate that explicit expressions for the low-frequency limit of the spectral density can be found even when the particle interacts with the pore.
Theoretical study of the electronic states of platinum trimer (Pt3)
Authors: Majumdar D. , Dai Dingguo , Balasubramanian K.
Published: July 2002

Geometries and energy separations of various low-lying spin-states of the platinum trimer (Pt3) have been investigated. The complete active space multiconfiguration self-consistent field (CASMCSCF) method followed by large scale multiconfiguration singles plus doubles configuration interaction (MRSDCI) that included up to 4.26 million configuration spin functions were used to compute several electronic states. A relativistic configuration interaction (RCI) technique was employed to compute the spin-orbit effects in different electronic states. Although an equilateral triangular A11 state was found to be global minimum in the absence of spin-orbit effects, this state was found to be nearly degenerate with the B13(A1) spin-orbit state when spin-orbit coupling was included. The A11 and B13(A1) states were found to be heavily mixed by spin-orbit coupling. We also compared our MRSDCI results with the density functional as well as Møller–Plesset second order perturbation calculations. The dissociation and atomization energies have been computed and compared with experiment.
Fabrication of semiconductor nanostructures by nanoindentation of photoresist layers using atomic force microscopy
Authors: Wiesauer K. , Springholz G.
Published: July 2002

Fabrication of semiconductor nanostructures by atomic force microscopy (AFM) is investigated. In our processing scheme, ultrathin photoresist layers are mechanically modified by an AFM tip with a subsequent pattern transfer by reactive ion or wet chemical etching. In combination with conventional optical lithography, this method allows the selective modification of micrometer scale patterns. Several parameters influence the size and shape of the nanostructures as well as the reliability of the process: the shape and the material of the tip, the thickness and hardness of the resist layer, the force applied to the tip, and the corrections of the scanner nonlinearities. Pattern transfer by reactive ion etching enforces a compromise between etch depth and resolution: Thinner resist layers allow the fabrication of smaller structures but inhibit a deep pattern transfer due to the limited resistance of the photoresist to sputtering. Application of our mask-based lithographic process for the fabrication of Si, SiGe, and GaAs nanostructures is demonstrated.
Rapid construction of a phase diagram of doped Mott insulators with a composition-spread approach
Authors: Fukumura T. , Ohtani M. , Kawasaki M. , Okimoto Y. , Kageyama T. , Koida T. , Hasegawa T. , Tokura Y. , Koinuma H.
Published: July 2002

We propose a method of rapid construction of a structural–magnetic–electronic phase diagram of doped Mott insulators. The composition-spread method is utilized for fabricating a film whose doping concentration varies from 0 to 1 continuously. The concurrent x-ray diffractometer that measures x-ray diffraction spectra of all the composition simultaneously, the scanning superconducting quantum interference device microscope, and the infrared optical spectroscopy are employed for characterizing the film. A demonstration is given for a colossal magnetoresistive material, La1−xSrxMnO3.
Practical limits of high-voltage thyristors on wide band-gap materials
Authors: Trivedi Malay , Shenai Krishna
Published: July 2002

This article presents a detailed simulation study of the highest achievable voltage ratings on bi-directional semiconductor controlled rectifiers designed using wide band-gap materials. It is shown that the voltage ratings of 4H–SiC and GaN are limited by small carrier lifetimes, as a result of material defects or crystal structure. Symmetric thyristors rated up to 8 kV are possible with commercially available 4H–SiC wafers that have carrier lifetimes longer than 300 ns. With further improvements in wafer quality, devices with voltage rating higher than 15 kV may be achievable. The performance of 8 kV 4H–SiC thyristors is compared with that of 8 kV Si thyristors. It is shown that the on-state voltage of 4H–SiC thyristors has very little dependence on temperature up to 300 °C unlike Si thyristors, with a positive temperature coefficient. Finally, simulations suggest that symmetric GaN thyristors may be impractical primarily because of the low carrier lifetimes resulting from direct band structure.
Modeling of gate-induced drain leakage current in n-type metal–oxide–semiconductor field effect transistor
Authors: Touhami A. , Bouhdada A.
Published: July 2002

Gate-induced drain leakage current (Igidl) in n-type metal–oxide–semiconductor field effect transistor is distinguished from other leakage processes by a weak dependence on temperature and a strong variation with transverse electric field and junction doping. The source of this current is the band-to-band tunneling effect in the gate-drain overlap region. We propose a model of Igidl current, which takes into account the spatial distribution of the transverse electric field and the doping profile in the drain region. This model also takes into account the donor or acceptor type of defects located in the oxide above the gate-drain overlap region. These defects can be simulated by a Gaussian distribution.
Glass transition and layering effects in confined water: A computer simulation study
Authors: Gallo P. , Rovere M. , Spohr E.
Published: July 2002

Single particle dynamics of water confined in a nanopore is studied through computer molecular dynamics. The pore is modeled to represent the average properties of a pore of Vycor glass. Dynamics is analyzed at different hydration levels and upon supercooling. At all hydration levels and all temperatures investigated a layering effect is observed due to the strong hydrophilicity of the substrate. The time density correlators show, already at ambient temperature, strong deviations from the Debye and the stretched exponential behavior. Both on decreasing hydration level and upon supercooling we find features that can be related to the cage effect typical of a supercooled liquid undergoing a kinetic glass transition. Nonetheless the behavior predicted by mode coupling theory can be observed only by carrying out a proper shell analysis of the density correlators. Water molecules within the first two layers from the substrate are in a glassy state already at ambient temperature (bound water). The remaining subset of molecules (free water) undergoes a kinetic glass transition; the relaxation of the density correlators agree with the main predictions of the theory. From our data we can predict the temperature of structural arrest of free water.
I. Three-center versus four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: Bimodal rotational distribution of HCl (v⩽7) detected with time-resolved Fourier-transform spectroscopy
Authors: Lin Shiaw-Ruey , Lin Shih-Che , Lee Yu-Chang , Chou Yung-Ching , Chen I-Chia , Lee Yuan-Pern
Published: July 2002

Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000–3310 cm−1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to v=7 is observed, with J>32 (limited by overlap at the band head) for v=1–3. All vibrational levels show bimodal rotational distribution with one component corresponding to ∼500 K and another corresponding to ∼9500 K for v⩽4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center (α, α) elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition states during bond breaking predict substantial rotational excitation for three-center elimination of HCl but little rotational excitation for four-center (α, β) elimination; observed internal energy of the low-J component is consistent with that predicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9×1011 s−1 for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberger-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to v⩽6 for high-J and low-J components. Hence we conclude that observed high-J and low-J components correspond to HCl (v, J) produced from three-center and four-center elimination channels, respectively.
Bifurcation phenomena in a Taylor–Couette flow with asymmetric boundary conditions
Authors: Mullin T. , Blohm C.
Published: July 2002

We present the results of an experimental and numerical study of bifurcation phenomena in the flow between a rotating inner cylinder and a stationary outer one, the so-called Taylor–Couette problem. Novel asymmetric boundary conditions have been used where one end plate is rotated with the inner cylinder while the other is held fixed. The steady cellular flows consist of one or three vortices in the aspect ratio range considered. We have investigated the selection procedure for the preferred steady states experimentally and numerically and good quantitative agreement is found between the two sets of results. The sequence involves a fold in the solution surface which produces a cusp in the bifurcation set. Hopf bifurcations to three-dimensional oscillatory flow are also present and so the full bifurcation structure is revealed using the combined numerical and experimental approach in a complimentary manner.
Thermodynamics of trapped interacting bosons in one dimension
Authors: Gu Shi-Jian , Li You-Quan , Ying Zu-Jian
Published: July 2002

On the basis of Bethe ansatz solution of bosons with δ-function interaction in a one-dimensional potential well, the thermodynamics equilibrium of the system in finite temperature is studied by using the strategy of Yang and Yang. The thermodynamics quantities, such as specific heat, etc., are obtained for the cases of the strong coupling and weak coupling limits respectively.
Electronic excitation spectrum of thiophene studied by symmetry-adapted cluster configuration interaction method
Authors: Wan Jian , Hada Masahiko , Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

Electronic excitation spectrum of thiophene was investigated by the symmetry-adapted cluster (SAC)/SAC configuration interaction method. Seventy singlet and four lowest triplet electronic states of thiophene were computed to give a detailed satisfactory theoretical interpretation of the vacuum ultraviolet (VUV) spectrum and the electron energy loss spectrum of thiophene. The present calculations gave the 2 1A1 valence state at 5.41 eV and the 1 1B2 valence state at 5.72 eV with oscillator strengths 0.0911 and 0.1131, respectively, and the 5 1A1 valence state at 7.32 eV and the 4 1B2 valence state at 7.40 eV with oscillator strengths 0.3614 and 0.1204, respectively. These valence-excited states were assigned to the two strong absorption bands of the VUV spectrum centered around 5.5 and 7.05 eV, respectively. A number of Rydberg transitions were obtained and assigned to the 6.0, 6.6, and 7.5–8.7 eV, etc. energy regions. The similarities and differences in the electronic excitations between thiophene and other five-membered ring compounds were discussed. The accuracy and assignment of the present results are compared with those of the recent theoretical studies by CASPT2 and multireference double configuration interaction methods.
The equivalence of the class of Rivlin–Sawyers equations and a class of stochastic models for polymer stress
Authors: Feigl Kathleen , Öttinger Hans Christian
Published: July 2002

This paper establishes the precise relationship between the macroscopic class of factorized Rivlin–Sawyers equations and a class of microscopic-based stochastic models. The former is a well-established and popular class of rheological models for polymeric fluids, while the latter is a more recently introduced class of rheological models which combines aspects of network and reptation theory with aspects of continuum mechanic models. It is shown that the two models are equivalent in a defined sense under certain unrestrictive assumptions. The first part of the proof gives the functional relationship between the linear viscoelastic memory function of the Rivlin–Sawyers model and the probability density for creation times of random variables in the stochastic model. The main part of the proof establishes the relationship between the strain descriptions in each model by showing that the difference in corresponding strain expressions can be made arbitrarily small using the appropriate weighted norm from spectral approximation theory.