bluuebunny/crossref_metadata_2025 huggingface dataset:
Homogeneity and the inertial range in driven two-dimensional turbulence
Authors: Rivera Michael , Wu Xiao-Lun
Published: September 2002

To be compared with current theory, turbulent velocity fluctuations should be homogeneous throughout the measurement volume as well as have a significant inertial range. For two-dimensional (2D) systems, these requirements are difficult to achieve. The difficulty with homogeneity stems from the fact that the outer scale, rout, grows with increasing Reynolds number or with decreasing external friction. If the outer scale begins to exceed the system size, rsys, homogeneity is sacrificed throughout the system. Current 2D systems have rsys about a decade removed from the injection scale, rinj. Thus to maintain homogeneity, rout must be kept small. A consequence of small rout is that there is little to no inertial range in such systems. This is because the inertial range extends between rinj and the integral scale rint∝(rinjrout)1/2, which is smaller than rout. To have a significant inertial range as well as maintain homogeneity throughout that range, 2D systems must be made larger in size than current systems. This paper demonstrates these facts in a particular experimental setup, an electromagnetically driven soap film. However, the results are expected to be applicable to other 2D turbulence systems.
Single-crystal Si formed on amorphous substrate at low temperature by nanopatterning and nickel-induced lateral crystallization
Authors: Gu Jian , Chou Stephen Y. , Yao Nan , Zandbergen Henny , Farrer Jeffrey K.
Published: September 2002

Single-crystal silicon has been achieved by patterning amorphous silicon film on silicon dioxide substrate into nanoscale lines and nickel-induced lateral crystallization. Line width affects the single-crystal silicon formation significantly. Narrow line widths, 30 nm or less, resulted in little lateral crystallization; while for line widths above 250 nm, multiple grains started to form. In-situ transmission electron microscope observation has been used to study the crystallization process. Lithography-constrained single seeding is proposed to explain the single-crystal formation.
Three-dimensional pattern transfer and nanolithography: modified soft molding
Authors: Kim Y. S. , Park Joonhyung , Lee Hong H.
Published: September 2002

One-step transfer of molded three-dimensional polymer structures into underlying substrate is reported. The one-step transfer is made possible by a molding technique presented here in the form of modified soft molding. Formation of a desired three-dimensional structure in a polymer film by this method, followed by one-step reactive ion etching, is utilized for the transfer. The technique is also shown to be effective in transferring sub-100-nm features.
Phase diagrams of heteronuclear dimers adsorbed on a square lattice
Authors: Rżysko W. , Borówko M.
Published: September 2002

Monte Carlo simulation in the grand canonical ensemble has been performed to study phase transitions in monolayer films formed by heteronuclear dimers. The phase diagrams have been estimated for some model systems. The results reveal a great variety of vapor–liquid coexistence behaviors depending on the relative strengths of interactions between molecule segments. The structural transitions have been found.
Secondary electron emission from magnesium oxide on multiwalled carbon nanotubes
Authors: Kim Won Seok , Yi Whikun , Yu SeGi , Heo Jungna , Jeong Taewon , Lee Jeonghee , Lee Chang Soo , Kim J. M. , Jeong Hee Jin , Shin Young Min , Lee Young Hee
Published: September 2002

We have investigated effects of electric fields on the yield of secondary electron emission (SEE) from the primary electron bombardment on magnesium oxide (MgO) covering vertically grown multiwalled carbon nanotubes (MWCNTs). We observe that the yield of SEE increases up to at least 22 000 at a special condition. The strong local field generated by the sharp tip of vertically grown MWCNTs accelerates secondary electrons generated by primary electrons. This eventually gives rise to so called Townsend avalanche effect, generating huge number of secondary electrons in a MgO film. Emission mechanism for such a high SEE will be further discussed with energy spectrum analysis.
Microstructure of surface layers in Cu(In,Ga)Se2 thin films
Authors: Yan Y. , Jones K. M. , Abushama J. , Young M. , Asher S. , Al-Jassim M. M. , Noufi R.
Published: September 2002

In most Cu(In,Ga)Se2 thin films used for solar cells, there usually exist interfaces lying about 0.1 to 0.2 μm below surfaces. We report on convergent-beam electron diffraction and energy-dispersive x-ray spectroscopy study of the microstructure and chemical composition of the surface region in Cu(In,Ga)Se2 thin films. We find that the surface region and the bulk are structurally similar, with no ordered defect chalcopyrite structure observed. However, their composition is slightly different, indicating that they can have different point defect physics. Our results suggest that the subinterfaces and the bulk absorber may form homojunctions.
Extraction of positive and negative ions from electron-beam-generated plasmas
Authors: Walton S. G. , Leonhardt D. , Fernsler R. F. , Meger R. A.
Published: September 2002

The results of investigations using pulsed, electron-beam-produced plasmas in Ar/SF6 mixtures are reported. Time-resolved, positive, and negative ion fluxes were measured at a biased electrode located adjacent to the plasma. The measurements indicate that plasmas form with large negative ion densities due to electron attachment to SF6 at the low electron temperatures associated with electron-beam-generated plasmas.
Time-dependent density-functional theory investigation of the formation of the charge transfer excited state for a series of aromatic donor–acceptor systems. Part I
Authors: Jamorski Jödicke Christine , Lüthi Hans Peter
Published: September 2002

Singlet excitation energies for a series of acceptor para-substituted N,N-dimethyl-anilines that are dual (4DMAB-CN, 3M4MAB-CN, MHD) and nondual (4AB-CN, 3M4AB-CN, 4MAB-CN, 3M4DMAB-CN, HHD, and MMD) fluorescent have been performed using the TDDFT method. The aim of this study is to investigate the influence of changing donor groups as well as the addition of methyl groups to the benzene moiety, on the fluorescence behavior of these molecules. Calculations of excitation energies have been performed with both B3LYP and MPW1PW91 functionals using a 6-311*(2p,d) (Bg) basis set. For all systems, ground-state geometries were optimized using density-functional theory with the Becke three parameter Lee–Yang–Parr functional combined with a 6-31G(d) (Sm) basis set. In addition, 4AB-CN, 4DMAB-CN, and MMD ground-state geometry has also been optimized using the MPW1PW91 functional with the Sm basis set. For all molecules, the potential energy surface (PES) has been investigated following the twisting intramolecular charge transfer (TICT) model proposed in the literature as a possible mechanism to explain the fluorescence behavior. Both 4AB-CN and HHD molecules have been computed to be nondual fluorescent in full agreement with experimental spectra. The single band observed in the gas-phase fluorescence spectra of 3M4DMAB-CN, and MMD is clearly understood by the form of the PES of the charge transfer excited state that presents a minimum for the perpendicular structure. The qualitative picture of the PES along the twisting angle is in full agreement with experimental observations. The dual fluorescence of 4DMAB-CN and 3M4MAB-CN is explained, within the TICT model, by a double mechanism proposed by Serrano et al. [J. Am. Chem. Soc. 117, 3189 (1995)], that involves the presence of two low-lying states close enough in energy. The nondual fluorescence of 4MAB-CN is explained by the height of the energy barrier (larger than for 4DMAB-CN and 3M4MAB-CN). Finally, the dual fluorescence of the MHD molecule can be fully understood by a double mechanism within the TICT model. 3M4AB-CN is computed nondual fluorescent like 4AB-CN and HHD, but no experimental data has been reported in the literature so far. Our calculations give new evidence in favor of the TICT model as an explanation for the occurrence of dual fluorescence.
Time-dependent density-functional theory investigation of the formation of the charge transfer excited state for a series of aromatic donor–acceptor systems. Part II
Authors: Jamorski Jödicke Christine , Lüthi Hans Peter
Published: September 2002

TDDFT singlet excitation energies calculations have been performed for a series of donor–acceptor para-substituted N,N-dimethyl-anilines (4DMAB-CHO, 4DMAB-COMe, 3DMAB-CHO, 3DMAB-COMe, 4DMAB-COOH, 4DMAB-COOMe, 4DMAB-CONH2, 4DMAB-CON(CH3)2, and DMAPY) using both B3LYP and MPW1PW91 functionals with a 6-311*(2p,d) basis set. The aim of this study is to investigate the influence of the variation of the acceptor group on the fluorescence behavior of these molecules. This work is a complementary investigation to part I [J. Chem. Phys. 117, 4146 (2002), preceding paper], where variation of the donor group has been studied. Ground-state geometries were optimized using density-functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For all molecules, the potential energy surface has been investigated following the twisting intramolecular charge transfer (TICT) model proposed in the literature as a possible mechanism to explain the fluorescence behavior. Computed vertical absorption energies using the MWP1PW91 and B3LYP functionals compare within 0.02 and −0.09 eV in average with gas-phase experimental data (4DMAB-COMe and 4DMAB-COOMe). Almost all para compounds are predicted to be dual fluorescent within the TICT model, in agreement with experiments. The present calculations predict 4DMAB-CONH2 to be dual fluorescent in nonpolar solvent in disagreement with experimental results. For meta compounds, the literature is sparse and no fluorescence spectra have been reported for these systems. Our results indicate that these molecules should exhibit only one band in the fluorescence spectra. Our calculations reinforce the validity of the TICT model as a possible mechanism to explain the fluorescence activity of these donor–acceptor systems.
Vortex dynamics and mass entrainment in a rectangular channel with a suddenly expanded and contracted part
Authors: Sau Amalendu
Published: September 2002

We report results of three-dimensional simulation of the spatially developing incompressible flow through a rectangular channel with a suddenly expanded and contracted part. The dynamics and evolution of the streamwise vortices through the expansion and contraction regions, and their influence on mass entrainment has been presented to extend our knowledge base to such flows. In addition to the observed axis-switching mechanism of the jet, profound differences as found in the flow structure on two symmetric planes, and the relatively unknown and interesting three-dimensional free-slip separation patterns that formed away from the symmetry planes, particularly in presence of tabs, are included in the presentation. Rectangular tabs, placed on the plane of expansion, generated streamwise vortices which controlled the lateral spreading of the jet and the mass entrainment through their streamwise dynamics. A complete and extensive analysis of the pressure distribution over the channel has been presented, which helps to identify the major source of streamwise vortices, and clearly explains the role played by the pressure variation over the channel in the generation of the streamwise vortices. A relation concerning relative positions of the developed local high and low pressure regions inside the channel to the streamwise dynamics of the vortices in the setup has been established which can predict the presence of all the streamwise vortices in the flow field, and characterize their nature of dynamics without any ambiguity. The results of the present simulation reveal flow features closely consistent with the vortex dynamics and the axis switching mechanism suggested by previous experimental observations. In an interesting contrast with the two-dimensional flow in a symmetric channel with a suddenly expanded and contracted part which exhibited the symmetry breaking flow bifurcations, the corresponding three-dimensional flow investigated here remained symmetric and surprisingly without much sign of recirculation on the minor axis plane.
Bias-assisted photoelectrochemical oxidation of n-GaN in H2O
Authors: Seo J. W. , Oh C. S. , Jeong H. S. , Yang J. W. , Lim K. Y. , Yoon C. J. , Lee H. J.
Published: September 2002

Growth of gallium oxide on n-GaN was realized in H2O by bias-assisted photoelectrochemical (PEC) oxidation using Al as a counterelectrode instead of a Pt commonly used in the PEC process. Although the growth of the oxide was not observed at below 2 V, the initial oxide growth rate of 8.7 nm/min was shown at a bias of 15 V and ultraviolet light intensity of 300 mW/cm2. However, the growth rate lowered and oxide thickness was saturated to 340 nm. The saturated oxide thickness and initial growth rate were increased with the applied bias. The homogeneous oxide growth and near stoichiometric composition of Ga2O3 were observed in Auger electron spectroscopy analysis results.
Investigation of the energy band structure of orthorhombic BaSi2 by optical and electrical measurements and theoretical calculations
Authors: Nakamura Tomoyuki , Suemasu Takashi , Takakura Ken-ichiro , Hasegawa Fumio , Wakahara Akihiro , Imai Motoharu
Published: September 2002

Optical and electrical properties of polycrystalline orthorhombic BaSi2 prepared by arc melting in Ar atmosphere were investigated. The optical absorption spectra measured at room temperature showed that indirect and direct absorption edges were 1.15 and 1.25 eV, respectively. The activation energy estimated from temperature dependence of the resistivity was 1.10 eV. These results agreed well with a calculated band structure of the orthorhombic BaSi2 by first principles calculation using density functional theory.
Ultrathin CNx overcoats for 1 Tb/in.2 hard disk drive systems
Authors: Li D. J. , Guruz Murat U. , Bhatia C. Singh , Chung Yip-Wah
Published: September 2002

Carbon nitride films were grown on silicon and hard disk substrates using pulsed dc magnetron sputtering in a single-cathode deposition system. Substrates were mounted on a specially designed holder that allowed 45° tilt angle and substrate rotation about the surface normal up to 20 rpm. The influence of substrate bias, substrate tilt, and rotation on film growth and properties was studied. Films with the lowest rms surface roughness and corrosion performance were obtained at −100 V substrate bias with substrate tilt and rotation. Atomic force microscope scans over 10×10 μm2 sampling areas showed that 50 nm thick CNx films prepared under these conditions have roughness almost four times lower than those prepared without substrate tilt and rotation. We observed a twofold reduction in corrosion damage for hard disk substrates with 1 nm thick CNx overcoats deposited with substrate tilt and rotation. This improved performance is likely a result of more efficient and uniform momentum transfer parallel to the surface during deposition in this configuration.
“Self-leveling” uncooled microcantilever thermal detector
Authors: Corbeil J. L. , Lavrik N. V. , Rajic S. , Datskos P. G.
Published: September 2002

Bimaterial microcantilevers can be used as highly sensitive uncooled infrared (IR) detectors. Since the transduction efficiency of microcantilevers depends on their deflection, it is important to minimize cantilever deflections caused by factors other than IR radiation (e.g., intrinsic mechanical stresses and ambient temperature fluctuations). In this letter we report on a design of a microcantilever IR detector that is immune to ambient temperature changes and other sources of interfering mechanical stresses. We modeled and experimentally measured responses of such devices to IR radiation as well as to ambient temperature changes. Both modeling and experimental results indicate that the implemented innovative designs can combine excellent IR sensitivity with negligible sensitivity to the ambient temperature changes.
Reduction of the power dissipation in silicon carbide Schottky rectifiers by a dual-metal planar structure
Authors: Roccaforte F. , La Via F. , Di Franco S. , Raineri V.
Published: September 2002

A dual-metal-planar rectifier on 6H–SiC was fabricated using Ti and Ni2Si as Schottky metals. The forward current voltage (I–V) characteristic of the dual-metal devices was comparable with that of Ti diodes. On the other hand, under reverse bias, almost the same leakage current of the Ni2Si rectifiers was achieved, i.e., a factor 1000 lower than that of Ti diodes. The fabricated diodes allowed to obtain a power dissipation of 0.37 W/cm2, significantly reduced with respect to the Ti and Ni2Si diodes dissipation. Moreover, the breakdown voltage was the same as in the planar Ni2Si diode, thus indicating that the planar structure is very efficient in avoiding electric field crowding at the titanium stripes edges.
Bonding configurations and optical band gap for nitrogenated amorphous silicon carbide films prepared by pulsed laser ablation
Authors: Trusso S. , Barreca F. , Neri F.
Published: September 2002

SiC x N y thin films have been deposited by ablating a sintered silicon carbide target in a controlled nitrogen atmosphere. The structural and the optical properties of the films were investigated by x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. The nitrogen content was found to be dependent on the nitrogen partial pressure and did not exceed 7.5%. XPS measurements of the N 1s, C 1s, and Si 2p photoelectron peaks pointed out the nitrogen preferential bonding to silicon atoms with a concurrent decrease of the Si–C and Si–Si bond density. A slight increase of sp3 hybridized carbon bonds has been also observed. The optical band gap Eg values, obtained from fitting the ellipsometric data with an appropriate multiple layers model, were found to increase up to 2.4 eV starting from a value of 1.6 eV for a non-nitrogenated sample. This finding was related to progressive substitution of the weaker Si–Si bonds by the stronger Si–C and Si–N ones which, presumably, involves the removal of electronic states lying at the band edges.
Study of oxygen vacancies in SrTiO3 by positron annihilation
Authors: Uedono Akira , Shimayama Kazuo , Kiyohara Masahiro , Chen Zhi Quan , Yamabe Kikuo
Published: September 2002

Oxygen vacancies introduced by homoepitaxial growth of thin films on strontium titanate (SrTiO3) substrates were studied by means of positron annihilation. The SrTiO3 films were grown by molecular-beam epitaxy without using an oxidant. The Doppler broadening spectra of the annihilation radiation were measured as a function of incident positron energy for the SrTiO3/SrTiO3 samples fabricated by various growth conditions. The line shape parameter S, corresponding to the annihilation of positrons in the substrate, was found to be increased by the growth of the film. This increase was attributed to the diffusion of oxygen from the substrate into the film, and the resultant introduction of oxygen vacancies in the substrate. Two different types of defects in the substrate were identified: one is oxygen multivacancies, such as oxygen divacancies, and the other is their complexes with Sr vacancies. The concentration of oxygen vacancies in the subsurface region increased as the substrate temperature during the growth decreased (⩽320 °C). This fact was attributed to the decrease in the diffusion length of oxygen at low temperatures, and the resultant accumulation of oxygen vacancies in the subsurface region.
Mott–Schottky behavior of strongly pinned double Schottky barriers and characterization of ceramic varistors
Authors: Fernández-Hevia D. , de Frutos J. , Caballero A. C. , Fernández J. F.
Published: September 2002

This work studies the capacitive behavior of an intergranular double Schottky barrier, which describes nonlinear charge transport in polycrystalline semiconductors. It is found that: (i) a widely applied version of the Mott–Schottky equation can be inadequate, and can lead to significant errors; (ii) a property called strong barrier pinning (SBP), underlies most attempts to obtain physical parameters from C–V measurements; and (iii) under SBP, known results from one-sided Schottky barriers can be used to analyze C–V response, showing that correct physical parameters are obtained at low frequency and that high frequency measurements are not advantageous. A new characterization method is introduced, which allows high voltage devices to be directly measured, and yields comparative information about average donor density and barrier height. Besides its technological applicability, the method simplifies the study of scale effects. Experimental results support the theoretical considerations and the proposed characterization method. The usefulness of the method is illustrated by studying the effects of current–pulse degradation on the physical parameters of high voltage varistors.
Parallel transport in an entangled ring
Authors: Wootters William K.
Published: September 2002

This article defines a notion of parallel transport in a lattice of quantum particles, such that the transformation associated with each link of the lattice is determined by the quantum state of the two particles joined by that link. We focus particularly on a one-dimensional lattice—a ring—of entangled rebits, which are binary quantum objects confined to a real state space. We consider states of the ring that maximize the correlation between nearest neighbors, and show that some correlation must be sacrificed in order to have nontrivial parallel transport around the ring. An analogy is made with lattice gauge theory, in which nontrivial parallel transport around closed loops is associated with a reduction in the probability of the field configuration. We discuss the possibility of extending our result to qubits and to higher dimensional lattices.
Structural and optical properties of InGaN/GaN layers close to the critical layer thickness
Authors: Pereira S. , Correia M. R. , Pereira E. , Trager-Cowan C. , Sweeney F. , O’Donnell K. P. , Alves E. , Franco N. , Sequeira A. D.
Published: September 2002

In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1−xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1−xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at ∼2.50 and ∼2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectroscopy. DSR leads directly to the emergence of the second, lower-energy, peak. Based on this experimental evidence, it is argued that the appearance of luminescence doublets in InGaN is not evidence of “quantum dotlike In-rich” or “phase separated” regions, as commonly proposed.
Carbon nanotubes as masks against ion irradiation: An insight from atomistic simulations
Authors: Krasheninnikov A. V. , Nordlund K. , Keinonen J.
Published: September 2002

Recent experiments show that carbon nanotubes can be used as masks against ion irradiation to make metallic nanowires of a few nanometers in width. In order to ascertain the limitations of this technique, we use molecular dynamics and simulate ion irradiation of multiwalled nanotubes. We derive an equation which for a given nanowire material allows one to estimate the theoretical limit on the minimum width of the wire which can be made using this technique.
Why do (2×4) GaAs and InAs (001) surfaces exposed to phosphorus have so different behavior? Elastic strain arguments
Authors: Wallart X. , Priester C. , Deresmes D. , Gehin T. , Mollot F.
Published: September 2002

When exposed to phosphorus, the (2×4) GaAs surface tends rapidly to roughen whereas it is not the case for the (2×4) InAs one, even after long exposure times. X-ray photoemission measurements show that the incorporated phosphorus amount is similar in both cases. The elastic energy difference between these two materials with incorporated phosphorus, calculated within the valence force field approximation, appears to be strong enough to explain this phenomenon. The role of surface reconstruction, mainly dimerization, on phosphorus incorporation is also emphasized.
Combining implicit solvation models with hybrid quantum mechanical/molecular mechanical methods: A critical test with glycine
Authors: Cui Qiang
Published: September 2002

A combined approach to study reactions in solution in which the solute and a number of solvent molecules are described with a hybrid quantum mechanical/molecular mechanical (QM/MM) method, and the bulk solvent is represented by a polarizable continuum model (PCM) has been implemented. In this way, both short-range effects of the first-solvation shell and long-range electrostatics due to the bulk solvent can be taken into account. By carefully choosing the size of the solute–solvent cluster and the QM/MM partition, the current QM/MM/PCM approach can offer both computational efficiency and accuracy. The approach has been illustrated by two simple systems: water-dimer and glycine in water. The results demonstrated that the current approach offers a satisfactory description of solvation effects on the geometry and energetics of neutral and charged hydrogen-bonding systems. The method correctly produced the relative stability of the zwitterionic and neutral forms of glycine in solution, which was found to be a subtle issue in previous studies. The approach can be extended to study reactions in biomolecules in which part of the system is treated with QM/MM, and the bulk solvent plus part of the protein or nucleic acids are described with either a continuum or approximate microscopic representation.
Silver as a sensitizer for erbium
Authors: Strohhöfer C. , Polman A.
Published: September 2002

The photoluminescence of Er3+in borosilicate glass is strongly enhanced by the presence of silver. Samples prepared by a combination of erbium ion implantation and Na+↔Ag+ ion exchange show an increase of the Er3+excitation efficiency of up to a factor 70 when excited at 488 nm. Excitation of Er3+ is possible over a broad wavelength range in the near ultraviolet and visible. Our data suggest that absorption of light occurs at a silver ion/atom pair or similar defect, followed by energy transfer to Er3+. We can exclude that silver nanocrystals are part of the dominant excitation mechanism, neither via local field enhancement effects due to their surface plasmon resonance nor via absorption and subsequent energy transfer to Er3+.
Maximization of the open circuit voltage for hydrogenated amorphous silicon n–i–p solar cells by incorporation of protocrystalline silicon p-type layers
Authors: Koval R. J. , Chen Chi , Ferreira G. M. , Ferlauto A. S. , Pearce J. M. , Rovira P. I. , Wronski C. R. , Collins R. W.
Published: September 2002

In studies of hydrogenated amorphous silicon (a-Si:H) n–i–p solar cells fabricated by rf plasma-enhanced chemical vapor deposition (PECVD), we have found that the maximum open circuit voltage (Voc) is obtained by incorporating p-type doped Si:H layers that are protocrystalline in nature. Specifically, these optimum p layers are prepared by PECVD in the a-Si:H growth regime using the maximum hydrogen-to-silane flow ratio possible without crossing the thickness-dependent transition into the mixed-phase (amorphous+microcrystalline) growth regime for the ∼200 Å p-layer thickness. The strong dependence of the p-layer phase and solar cell Voc on the underlying i-layer phase also confirms the protocrystalline nature of the optimum Si:H p layer.
Direct measurement of the absolute value of the interaction force between the fiber probe and the sample in a scanning near-field optical microscope
Authors: Lapshin D. A. , Letokhov V. S. , Shubeita G. T. , Sekatskii S. K. , Dietler G.
Published: September 2002

The absolute values of the force exerted by the fiber probe of a scanning near-field optical microscope onto the surface were measured using an atomic force microscope in ambient conditions. We demonstrate that a usually neglected static attraction force is dominant at small dither amplitudes and is of the order of 200 nN. The tapping component of the force, often referred to as shear force, is of the order of 1 nN at these conditions for both the tuning fork-based and optical in resonance detection schemes. Other peculiarities of the shear force interaction are also discussed.
Comparison of the birefringence in an azobenzene-side-chain copolymer induced by pulsed and continuous-wave irradiation
Authors: Cimrová Věra , Neher Dieter , Hildebrandt Ralf , Hegelich Manuel , von der Lieth Arend , Marowsky Gerd , Hagen Rainer , Kostromine Sergej , Bieringer Thomas
Published: September 2002

A comparative study of photoinduced birefringence in azobenzene-side-chain copolymers under continuous-wave (cw) and pulsed irradiation is presented. Absolute refractive index changes were determined by the attenuated total reflection spectroscopy. We clearly demonstrated that pulsed, similarly to cw, irradiation induces a three-dimensional reorientation of azobenzene chromophores. For the same light dose, pulsed irradiation using ns pulses of appropriate pulse energy led to higher values of birefringence, which can be attributed to a cooperative motion of azobenzene side chains and thermal effects induced by the laser pulses.
Sound generation by the leapfrogging between two coaxial vortex rings
Authors: Inoue Osamu
Published: September 2002

The sound generated by the leapfrogging between two coaxial vortex rings is investigated by direct solution of the axisymmetric, unsteady, compressible Navier–Stokes equations. The results show that sound pressure waves are generated in two processes; slip-through process and vortex-core deformation process which appears after the merging of the two vortex rings. In the slip-through process, the generation of pressure waves is related to the change of direction of the vortex ring motion induced by the mutual interaction of the two vortex rings. The successive generation of pressure waves in the core deformation process is due to the periodic core deformation of the merged vortex ring and may be associated with the orientation of the vortex cores.
Hafnium oxide gate stack prepared by in situ rapid thermal chemical vapor deposition process for advanced gate dielectrics
Authors: Lee S. J. , Jeon T. S. , Kwong D. L. , Clark R.
Published: September 2002

A high-quality HfO2 gate stack with equivalent oxide thickness (EOT) of 7.8 Å and a leakage current of Jg=0.5 mA/cm2 @ Vg=−1.0 V has been achieved by an in situ rapid thermal chemical vapor deposition process. It is found that both NH3-based interface layer and N2 postdeposition annealing are very effective in reducing EOT and leakage, and at the same time, improving film qualities. These HfO2 gate stacks show negligible frequency dependence, small hysteresis in capacitance–voltage (C–V) and weak temperature dependence of the leakage current. They also show negligible charge trapping at high voltage stress.
Ultrafast switch-off of an electrically pumped quantum-dot laser
Authors: Platonov A. V. , Lingk C. , Feldmann J. , Arzberger M. , Böhm G. , Amann M.-C. , Abstreiter G.
Published: September 2002

We report on the subpicosecond switch-off of the emission of an electrically pumped quantum-dot (QD) laser with perturbation by a 100 fs optical pulse. The observed effect is in contrast to known ones for quantum-well laser diodes. This is explained by unique properties of the QDs, namely by the fast modal gain saturation and localization-enhanced intraband absorption. The QD laser response can be described in terms of hot carrier absorption in the barrier states via intraband optical transitions leading to the transient suppression of the laser mode and, hence, to the switch-off.
Crystallographic orientation of Cr in longitudinal recording media and its relation to magnetic anisotropy
Authors: Ajan Antony , Okamoto Iwao
Published: September 2002

A specific growth of Cr layer grains is found to exist when grown on the mechanically textured NiP–Al substrates used for longitudinal recording. High resolution transmission electron microscopy analysis of a large number of individual Cr grains indicate a Cr[110] preferential growth along the textured direction (groove or circumferential direction). This particular orientation of the Cr underlayer is found to be the cause of an in-plane magnetic anisotropy of the Co based magnetic layer. The temperature dependence of this in-plane magnetic anisotropy study indicated the importance of the specific crystallographic orientations of both the underlayer and the magnetic layer.
Photoluminescence of strained and relaxed multilayered Ge islands on Si(001)
Authors: Palange E. , Di Gaspare L. , Notargiacomo A. , Capellini G. , Evangelisti F.
Published: September 2002

In this letter, we report on the optical properties of multistacked Ge quantum dots grown on Si(001). We demonstrate that the two emission bands observed in the photoluminescence spectra are related to the presence of strained and relaxed islands. Within both types of islands, the experimental findings support the hypothesis of the presence of quantum confining regions whose dimensions are governed by the strain relaxation process.
Foreign and native coordination effects in core-level spectra of mixed Be-Mg clusters
Authors: Dobrodey N. V. , Streltsov A. I. , Cederbaum L. S. , Villani C. , Tarantelli F.
Published: September 2002

The core-ionization spectra of mixed Be-Mg clusters are computed by accurate ab initio methods. They are found to exhibit a variety of strong and unexpected effects originating from the presence of neighboring atoms of the same (native) or the other (foreign) element. The spectra reveal unusually low-lying intense satellites and the band shapes are sensitive to the local environment of the core-ionized atom. Foreign and native screening processes compete with each other resulting in specific spectral features which uniquely characterize the clusters.
Highly nondegenerate femtosecond four-wave mixing in tapered microstructure fiber
Authors: Abedin Kazi S. , Gopinath Juliet T. , Ippen Erich P. , Kerbage Charles E. , Windeler Robert S. , Eggleton Benjamin J.
Published: September 2002

We demonstrate efficient, highly nondegenerate four-wave mixing of femtosecond pulses, with a frequency shift of ∼6000 cm−1, in an 18 cm tapered microstructure fiber. Using a pump at 810 nm and a signal at 1540 nm, light is generated at wavelengths between 535 nm and 570 nm with 10% efficiency. Due to the walk-off between pump and signal pulses in the fiber, the interaction length in the tapered fiber is only 1.4 cm. Ten percent efficiency is achieved in this short length because of the enhanced nonlinearity of the tapered fiber and its unique dispersion characteristics.
Nanoscale anodization of an amorphous silicon surface with an atomic force microscope
Authors: Umezu Ikurou , Yoshida Takashi , Matsumoto Kimihisa , Sugimura Akira , Inada Mitsuru
Published: September 2002

Nanoscale anodization was performed on the surface of amorphous silicon thin films by means of an atomic force microscope. The anodization mechanism was different from that previously reported on metal thin films. We found that the anodization was a function of defect density and current through the sample. The optical properties of the anodized area were measured by means of micro-photoluminescence, in which photoluminescence intensity decreases with oxidation. We concluded that both defect reaction and creation processes are important during the nanoscale anodization of amorphous material.
Triple-optical autocorrelation for direct optical pulse-shape measurement
Authors: Liu Tzu-Ming , Huang Yin-Chieh , Chern Gia-Wei , Lin Kung-Hsuan , Lee Chih-Jie , Hung Yu-Chueh , Sun Chi-Kuang
Published: September 2002

Triple optical autocorrelation of femtosecond optical pulses was realized simply with third-harmonic-generation technique. This optical technique provides complete knowledge of transient pulse intensity variation directly in time domain. Only analytic calculation is needed to obtain the pulse-shape from data without direction-of-time ambiguity. Combined with a spectral measurement and the Gerchberg–Saxton algorithm, except for pulses with complete temporal and spectral symmetry that will cause a twofold ambiguity, exact phase variation in time can also be retrieved through an iterative calculation with an O(n) complexity.
Hydrogen-induced percolation in discontinuous films
Authors: Dankert Olof , Pundt Astrid
Published: September 2002

Hydrogen absorption in a metal leads to significant lattice expansion. This effect can be applied for discontinuous films to make them exceed the percolation limit, turning the film from nonconducting to conducting. Furthermore, in a percolating system, the conductivity should depend strongly on the external hydrogen pressure. This letter discusses the idea of a hydrogen-induced percolation mechanism. The process is illustrated by measurements on agglomerated palladium films, showing a maximum decrease of the resistivity by 50% at an exposure of the films to 5×104 to 9×104 Pa. The agglomerated films are prepared by Joule heating, using heat dissipation in the film itself.
Spectroscopic ellipsometry study of optical transitions in Zn1−xCoxO alloys
Authors: Kim Kwang Joo , Park Young Ran
Published: September 2002

Zn 1−x Co x O  (x⩽0.22) films were prepared on (0001)-oriented Al2O3 substrates by rf magnetron sputtering. The alloys show wurtzite crystal structure with the c-axis lattice constant increasing with increasing x. The optical properties of the samples were measured by spectroscopic ellipsometry at room temperature in the 1.5–5 eV photon energy region. As x increases, the optical band gap absorption edge (E0) of the alloys shows a redshift from that of pure ZnO, reaching 350 meV for x=0.22. The excitonic character of the E0 edge is gradually reduced as x increases and is replaced by the three-dimensional critical-point shape. Optical absorption structures are also observed below the E0 edge near 2 eV and interpreted as due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ ions substituting Zn2+ ions.
Electromechanical engraving and writing on bulk metallic glasses
Authors: de Oliveira M. F. , Botta F. W. J. , Kiminami C. S. , Inoue A. , Yavari A. R.
Published: September 2002

Using the intrinsic materials properties of bulk metallic glasses (BMG), namely electrical resistivities two orders of magnitude higher than good conductors and a Newtonian viscous-flow regime of deformability, a new electromechanical process has been developed for engraving on BMGs. Viscous flow in the supercooled liquid region between the glass transition temperature Tg and the crystallization temperature Tx of the bulk metallic glass allows rapid electromechanical engraving at low applied stresses. Miniaturization of the process is expected to allow submicron engraving in the future.
Heat-treatment-induced ferroelectric fatigue of Pt/Sr1−xBi2+yTa2O9/Pt thin-film capacitors
Authors: Shi-Zhao Jin , Lim Ji Eun , Cho Moon Joo , Hwang Cheol Seong , Kim Seung-Hyun
Published: September 2002

The variations in the level of ferroelectric fatigue of Sr1−xBi2+yTa2O9 (SBT, 220-nm-thick) thin-film capacitors with Pt electrodes as a function of the heat-treatment temperature were investigated. The ferroelectric SBT thin films were spin coated on 200-nm-thick Pt/TiO2/SiO2/Si and crystallized by furnace annealing at 800 °C. The post-heat-treatment temperature of the Pt/SrBi2Ta2O9/Pt capacitors was varied from 800 to 950 °C. The different post-heat-treatment temperatures barely affected the remanent polarization (Pr) vs. the applied voltage (Va) characteristics (saturation Pr of ∼10 μC/cm2 at a Va of 5 V). However, the samples annealed at 900 and 950 °C showed serious ferroelectric fatigue after ∼108–109 switching cycles whereas the sample annealed at 800 °C showed fatigue-free behavior up to 1010 cycles. This behavior appeared to have a close relationship with the loss of oxygen from the SBT layer during high temperature annealing.
Lateral oxidation kinetics of AlxGa1−xAs layer by capacitance technique
Authors: Das N. C. , Gollsneider B. , Newman P. , Chang W.
Published: September 2002

Capacitance voltage (C–V) technique is used to determine the kinetics of lateral oxidation of AlxGa1−xAs layer in a circular mesa structure. The oxide layer width determined by the C–V technique agrees with optical measurements. Additionally, the C–V technique has the advantage of determining different types of defects in the oxide, which are important for projecting the reliability of vertical cavity surface emitting laser devices. The wet oxidation between 400 and 435 °C shows negative charges in the oxide.
Copper diffusion characteristics in single-crystal and polycrystalline TaN
Authors: Wang H. , Tiwari Ashutosh , Zhang X. , Kvit A. , Narayan J.
Published: September 2002

We have investigated the diffusivity of copper in single-crystal (NaCl-structured) and polycrystalline TaN thin films grown by pulsed-laser deposition. Polycrystalline TaN films were grown directly on Si(100), while single-crystal films were grown with TiN buffer layers. Both poly- and single-crystal films with Cu overlayers were annealed at 500, 600, 650, and 700 °C in vacuum to study the copper diffusion characteristics. The diffusion of copper into TaN was studied using scanning transmission electron microscopy (STEM) Z contrast, where the contrast is proportional to Z2 (atomic number), and TEM. The diffusion distances (2Dτ) are found to be about 5 nm at 650 °C for 30 min annealing. The diffusivity of Cu into single-crystal TaN follows the relation D=(160±9.5)exp[−(3.27±0.1)eV/kB T] cm2 s−1 in the temperature range of 600–700 °C. We observe that Cu diffusion in polycrystalline TaN thin films is nonuniform with enhanced diffusivities along the grain boundary.
Highly ordered arrays of organic–inorganic nanophotonic composites
Authors: Levitsky I. A. , Liang J. , Xu J. M.
Published: September 2002

Highly ordered organic–inorganic nanocomposite arrays formed by filling anodized aluminum oxide nanopores (50 nm size) with Rhodamine 123 (Rh123) dye has been studied by visible absorptive, fluorescent spectroscopy, and atomic force/scanning electron microscopy techniques in this work. The absorbance and fluorescence spectra are found to drastically differ from that of conventional Rh123 films deposited onto flat substrates. The molecular assemblies in the nanopore array are mostly monomer-like, the fluorescence yield is many times higher than the conventional film counterpart and increases after exposure to intense laser illumination, and the fluorescence polarization dependence indicates a preferred molecular orientation along the pore axis. Such nanocomposite arrays open up opportunities in engineering molecular assemblies and in creating functional materials of superior properties for future nanophotonics.
Voltage-controlled wavelength conversion by terahertz electro-optic modulation in double quantum wells
Authors: Su M. Y. , Carter S. G. , Sherwin M. S. , Huntington A. , Coldren L. A.
Published: September 2002

An undoped double quantum well (DQW) was driven with a terahertz (THz) electric field of frequency ωTHz polarized in the growth direction, while simultaneously illuminated with a near-infrared (NIR) laser at frequency ωNIR. The intensity of NIR upconverted sidebands ωsideband=ωNIR+ωTHz was maximized when a dc voltage applied in the growth direction tuned the excitonic states into resonance with both the THz and NIR fields. There was no detectable upconversion far from resonance. The results demonstrate the possibility of using gated DQW devices for all-optical wavelength shifting between optical communication channels separated by up to a few THz.
Silicon clustering in Si–GaAs δ-doped layers and superlattices
Authors: Boscherini F. , Ferretti N. , Bonanni B. , Orani D. , Rubini S. , Piccin M. , Franciosi A.
Published: September 2002

To probe the development of Si–GaAs superlattices, we performed Si K-edge x-ray absorption measurements on periodic Si δ-doped structures in GaAs, for equivalent Si thicknesses of 0.02, 0.2, 0.5, 1, 2, and 4 monolayers (ML). We detected the presence of both Si–Si and Si–As (or Si–Ga) bonds and followed the variation of the coordination numbers as a function of the equivalent thickness of the Si layers. The Si–Si coordination number was found to gradually increase with increasing thickness, but Si–Si bonds were always detected, even at equivalent Si coverages of 0.02 ML. This indicates that the preferred growth conditions for Si–GaAs superlattices lead to widespread Si clustering and self-compensation, and suggests that lateral growth of such clusters to achieve coalescence may be the main mechanism for Si quantum well development.
Restrictive domain motion in polarization switching of lithium niobate
Authors: Peng L.-H. , Shih Y.-J. , Zhang Y.-C.
Published: September 2002

We investigate the electrostatic control of domain nucleation and motion for periodical polarization (Ps) switching on Z-cut congruent-grown lithium niobate (LiNbO3). A tenfold reduction in the tangential field (Ex) can be resolved as the perturbed, positive charge density intervening otherwise the single-domain crystal exceeds 10−3Ps. This concept is realized in a two-step procedure consisting of a high-temperature treatment followed by pulsed field poling. Bulk periodic polarization switching is then realized underneath the oxidized electrode with the lateral 180° domain motion confined to the edge of the charged domain boundary. The forward domain growth can extend through the 500-μm-thick substrate and reach an aspect ratio as large as 150. These observations are attributed to the constraint on domain nucleation and the suppression of Ex by the positive charge originating from the divergence of Ps at the inverted domain boundary near the subsurface regime.
Simple method to diagnose the performance of electroabsorption modulators on InP using optical low-coherence reflectometry
Authors: Rao E. V. K. , Gottesman Y. , Provost J. G.
Published: September 2002

This letter describes a judicious implication of optical low-coherence reflectometry (OLCR) measurements to evaluate the principal characteristics of an electroabsorption modulator with passive sections on either ends of its active layer fabricated on an InP substrate. We show here that careful monitoring of the reflections at the transition regions (passive to active and vice versa), and also transmission of a broadband OLCR (∼55 nm wide and centered at ∼1.56 μm) probe light as a function of applied reverse bias, permit a nearly complete assessment of the device. For example, in addition to assessing the quality of the transition regions, such a study further leads to the determination of the optimal operation wavelength and also the modulation depth of the device.