bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ observation of surface deformation of polymer films by atomic force microscopy
Authors: Nishino Takashi , Nozawa Akiko , Kotera Masaru , Nakamae Katsuhiko
Published: July 2002

The tensile X–Y stage, providing a load cell and a stretching device, has been constructed to observe the surface deformation of polymer film in situ by using an atomic force microscope (AFM). From the three-dimensional AFM images, the streak-like bumps were observed on a polyethylene terephalate (PET) film surface. By monitoring the change in the distance between them by the tensile load, the strain was evaluated in the direction both parallel and perpendicular to the tensile direction. The microscopic stress–strain relationship by AFM coincided with the macroscopic one, which indicates so-called affine deformation of PET film. Young’s modulus was obtained as 2.3 GPa for PET from the initial slope of the stress–strain curve by AFM. The apparent Poisson ratio of the PET film surface could be also evaluated.
Analysis of light scattering centers in crystals using modified light scattering tomography
Authors: Sakai K.
Published: July 2002

Light scattering measurements using an infrared laser light and a bias light have been carried out for the first time to investigate defect energy levels in indium-doped GaAs crystals. The principle of this method is based on the analysis of scattered light intensity variation induced by the bias light. A drastic quenching effect is observed at some wavelength of the bias light. It was found that there are two kinds of light scattering centers in as-grown indium-doped GaAs crystals.
Fast mixing device inside a nuclear magnetic resonance magnet: A tool for observing early steps in protein folding
Authors: Hamang M. , Sanson A. , Liagre L. , Forge V. , Berthault P.
Published: July 2002

We describe a device designed to perform a fast mixing of two solutions inside the magnet of a nuclear magnetic resonance (NMR) spectrometer. The apparatus is totally nonmagnetic; it operates via a system of pneumatic valves, which drives a syringe to inject the first solution into the NMR tube that already contains the other solution. An important point is that before and after the injection, the solution in the NMR tube is free of any perturbing injecting device. The injection is controlled by the spectrometer software in the NMR pulse sequence. Simple experiments have been performed to assess the viability of the device, and evaluate the dead time after injection, as well as the loss of homogeneity and the mixing capabilities. The device described can mix up to 200 μl of solution with the solution already present in the NMR tube in less than 50 ms, with a negligible loss of homogeneity during injection. The device was specifically designed for protein folding studies but can find a general purpose for chemical reactions involving small amounts of compounds.
Pulse radiolysis of liquid water using picosecond electron pulses produced by a table-top terawatt laser system
Authors: Saleh Ned , Flippo Kirk , Nemoto Koshichi , Umstadter Donald , Crowell Robert A. , Jonah Charles D. , Trifunac Alexander D.
Published: July 2002

A laser based electron generator is shown, for the first time, to produce sufficient charge to conduct time resolved investigations of radiation induced chemical events. Electron pulses generated by focussing terawatt laser pulses into a supersonic helium gas jet are used to ionize liquid water. The decay of the hydrated electrons produced by the ionizing electron pulses is monitored with 0.3 μs time resolution. Hydrated electron concentrations as high as 22 μM were generated. The results show that terawatt lasers offer both an alternative to linear accelerators and a means to achieve subpicosecond time resolution for pulse radiolysis studies.
Crystal adjustment by means of blocking pattern
Authors: Ellmer H. , Aichinger R. , Winkler E. , Semrad D. , Mergel V. , Jagutzki O.
Published: July 2002

A facility is described to record the blocking pattern of a single crystal produced by 400 keV protons. By this we aim at a reliable, fast, and careful procedure to align single crystals to the incoming ion beam. The two-dimensional position sensitive detector (2D-PSD) system from Roentdek GmbH Frankfurt/M, Germany, is based on three microchannel plates arranged in a Z stack. The essential shortcoming of this detector is its low sensitivity to high energy (>200 keV) projectiles. To reproduce the pattern formed by 400 keV protons on the 2D-PSD we had to decrease their energy to a few tens of kiloelectron-volts by a stopping foil of 3.5 μm Mylar®. This foil has the additional advantage of absorbing slow projectiles and sputtered atoms emerging from the target. On their way to the detector, these slow particles, in case they are charged, would be strongly influenced by electric fields and would thus obscure the true pattern. At present, our facility allows us to routinely align crystals for channeling measurements accurately, fast and, for most applications, with an acceptable amount of radiation damage.
Detection efficiency of a channel electron multiplier for low energy incident noble gas ions
Authors: Tassotto M. , Watson P. R.
Published: July 2002

The absolute detection efficiency of a continuous channel electron multiplier (DeTech 407-EIC) has been measured for the noble gas ions He+, Ar+, and Xe+, with energies up to 4.9 keV. For all ions, the detection efficiency initially rises linearly as a function of velocity. Our results and a reanalysis of literature data show that the threshold velocity below which a multiplier does not respond to impinging ions cannot be regarded as independent of the ion’s atomic number, Z, as previously reported. Instead, the threshold velocity is proportional to Z−c. The experimentally obtained efficiency-versus-velocity curves for our detector have been analyzed in the region of linear increase. The resulting expression for the detector efficiency, ε, is a function of the ion’s atomic number and velocity, v, and can be written as ε(v,Z)=a(v−b/Zc)Zn, with a, b, c, and n being constants.
Apparatus for deformation tests of solids in liquid 3He
Authors: Hashimoto T. , Katakura S. , Edagawa K. , Takeuchi S. , Suzuki T.
Published: July 2002

An apparatus for deformation of solids in liquid 3He is constructed. Either tensile deformation or compression of a specimen can be performed by exchanging the assemblies in the 3He pot which has a capacity of about 30 cm3. The pulling rod for transmitting load from the tensile testing machine to the specimen runs inside the outlet tube of 3He, being isolated from 4He bath and almost free from mechanical friction. To measure the change in flow stress with the supernormal transition of superconducting metals, a superconducting magnet is mounted outside of the vacuum chamber which separates the 3He pot and the 4He bath. Under an applied load for plastic deformation the system is stably operative down to 0.6 K, while the lowest temperature achieved is 0.5 K. Some results on Ta and NaCl are presented.
Low-field microwave magnetoabsorption in manganites
Authors: Tyagi S. D. , Lofland S. E. , Dominguez M. , Bhagat S. M. , Kwon C. , Robson M. C. , Ramesh R. , Venkatesan T.
Published: July 2002

We report the magnetic field dependence of microwave absorption at low fields (200 Oe), in thin film and bulk samples of La1−xBaxMnO3−z. We demonstrate, using a very simple cavity configuration, that the magnetic field sensitivity of the microwave response of the giant magnetoresistance materials offers possibilities of using such materials in devices operating at low fields. Low-field dc magnetization and ac resistivity measurements are also presented.
Mechanism for anneal-induced interfacial charging in SiO2 thin films on Si
Authors: Warren W. L. , Vanheusden K. , Schwank J. R. , Fleetwood D. M. , Winokur P. S. , Devine R. A. B.
Published: July 2002

H-induced positive charging is observed at both the top Si/SiO2 and bottom SiO2/Si interfaces in Si/SiO2/Si structures. The response of the H-induced positive charge to thermal annealing and electron injection is very different from that of simple oxygen vacancy hole traps in SiO2. To explain this H-induced positive charging, we propose a scheme in which H reacts to form positively charged over-coordinated oxygen centers in close proximity to both top and bottom interfaces. The annealing-induced entity may also provide a natural explanation for the origin of the fixed oxide charge that forms during oxidation of Si.
Time-resolved photoluminescence studies of GaN epilayers grown by gas source molecular beam epitaxy on an AlN buffer layer on (111) Si
Authors: Godlewski M. , Bergman J. P. , Monemar B. , Rossner U. , Barski A.
Published: July 2002

Optical properties of GaN epilayers grown by gas source molecular beam epitaxy on a (111) Si substrate with an AlN buffer layer are described. The mechanism of energy transfer between different excitonic emissions is proposed based on the results of time-resolved photoluminescence (PL) and PL kinetics measurements. It is suggested that tunneling of excitons between bound and free excitons is responsible for the observed PL decay transients and their temperature dependence.
Pulsed laser recording of gratings in SiO-Cu quantum dot thin films
Authors: Kaganovskii Yu. , Rosenbluh M.
Published: July 2002

Interference patterns are written by two intersecting pulsed laser beams in thin films composed of Cu quantum dots in SiO glass. The rapid local optical heating of the metal clusters forming the quantum dots in the glass film results in the writing of patterns in the thin film. The high cluster temperatures attained drive the selective growth of the larger quantum dots and also result in a stress induced peeling of the film from the substrate. Estimates of the kinetics of the selective quantum dot growth agrees well with experimental observations.
Monte Carlo calculations of hot-carrier noise under degenerate conditions
Authors: Tadyszak P. , Danneville F. , Cappy A. , Reggiani L. , Varani L. , Rota L.
Published: July 2002

We present a Monte Carlo investigation of noise and velocity fluctuations in Si at 300 K under full degenerate conditions. The presence of the Pauli principle is found to strongly modify the shape of the correlation functions: at low electric fields, the time decay decreases due to the shortening of momentum relaxation time and the variance evidences nonparabolicity effects. At increasing electric fields, the correlation function exhibits a negative part that can be associated with ballistic carriers crossing the velocity space from negative to positive values of the Fermi velocity without scattering. A microscopic analysis in terms of diagonal and off-diagonal contributions to the velocity correlation confirms this interpretation.
Ion assisted growth of diamond
Authors: Sattel S. , Robertson J. , Scheib M. , Ehrhardt H.
Published: July 2002

Diamond crystallites up to 40 nm in size have been grown from a highly ionized plasma beam of acetylene for ion energies close to 100 eV per C atom and substrate temperatures above 450 °C. This shows that diamond can be grown by physical vapor deposition from an ion-rich plasma as well as by chemical vapor deposition from a radical-rich plasma. The formation mechanism is argued to be one of nucleation and growth rather than a stress induced transformation from graphite.
Efficient carrier blocking by an attractive potential in strained Si1−xGex/Si single quantum well
Authors: Kishimoto Y. , Shiraki Y. , Fukatsu S.
Published: July 2002

Efficient carrier blocking by a single ‘‘attractive’’ potential is demonstrated using a strained Si0.85Ge0.15/Si quantum well (QW). The carrier blocking becomes pronounced at increased temperatures as most of the carriers at the QW bound state are thermalized with the barrier band edge. The carrier blocking efficiency at 100 K is almost of the order of unity as compared to insufficient carrier blocking due to a large ‘‘repulsive’’ potential associated with SiO2. The carrier blocking arises from efficient hole reemission from the QW that reflects a highly efficient carrier capture to the strained Si1−xGex/Si QWs from Si barriers. It is further shown that a single QW is the optimized geometry for efficient carrier blocking and that multiple well potentials lead only to a reduced blocking efficiency.
Carbon–hydrogen complexes in vapor phase epitaxial GaN
Authors: Yi Gyu-Chul , Wessels Bruce W.
Published: July 2002

Carbon–hydrogen complexes in undoped and Mg-doped epitaxial GaN films have been investigated using Fourier transform infrared spectroscopy. Infrared absorption peaks in the Mg-doped material were observed at 2853, 2923, and 2956 cm−1. The absorption peaks are attributed to the symmetric and asymmetric vibrational stretching modes of C–H in CHn (n=1–3) defect complexes. The carbon–hydrogen complexes were unintentionally incorporated during the Mg-doped GaN film growth. The absorbances of the vibrational modes increased for heavily Mg-doped GaN. Upon annealing at 700 °C for 30 min under nitrogen environment, the complexes decomposed. The origin of the carbon–hydrogen complexes is discussed.
Guided-wave acousto-optic Bragg diffraction in indium gallium arsenide phosphide waveguides
Authors: Tsai C. S. , Sun B. , Kar-Roy A.
Published: July 2002

Theoretical and experimental studies on guided-wave acousto-optic (AO) Bragg diffraction in a composite indium gallium arsenide phosphide (InGaAsP) planar waveguide were carried out. In this letter, the experimental results of the excitation and propagation of a 〈110〉-directed surface acoustic wave (SAW) at the center frequency of 167 MHz, and the AO Bragg diffraction at a 1.31 micron wavelength in In0.22Ga0.78As0.47P0.53 waveguide are reported. An AO Bragg diffraction efficiency as high as 14% was achieved at an acoustic drive power of 8.5 mW with an interaction length (transducer aperture) of only 1.0 mm. Two orders of magnitude improvement in the Bragg diffraction efficiency versus rf drive power should be achievable by the optimization of the SAW transducer and the optical waveguide parameters.
Atomic force microscope tip-induced local oxidation of silicon: kinetics, mechanism, and nanofabrication
Authors: Avouris Phaedon , Hertel Tobias , Martel Richard
Published: July 2002

Atomic force microscope induced local oxidation of silicon is a process with a strong potential for use in proximal probe nanofabrication. Here we examine its kinetics and mechanism and how such factors as the strength of the electric field, ambient humidity, and thickness of the oxide affect its rate and resolution. Detection of electrochemical currents proves the anodization character of the process. Initial very fast oxidation rates are shown to slow down dramatically as a result of a self-limiting behavior resulting from the build up of stress and a reduction of the electric field strength. The lateral resolution is determined by the defocusing of the electric field in a condensed water film whose extent is a function of ambient humidity.
Ballistic Hall micromagnetometry
Authors: Geim A. K. , Dubonos S. V. , Lok J. G. S. , Grigorieva I. V. , Maan J. C. , Hansen L. Theil , Lindelof P. E.
Published: July 2002

We report a magnetization measurement technique which allows quantitative studies of thermodynamic properties of individual submicron superconducting and ferromagnetic particles.
Self-diffraction from free surface relief gratings in a photorefractive Bi12TiO20 crystal
Authors: Stepanov S. , Korneev N. , Gerwens A. , Buse K.
Published: July 2002

Experimental results on self-diffraction of periodically phase-modulated laser beams from a relief grating formed on a free surface of a cubic photorefractive Bi12TiO20 crystal due to the piezoelectric effect are reported. Periodic intensity modulations of the reflected beams up to 10−2 due to energy exchange at this unshifted phase grating are measured. The modulation depends linearly on the fringe spacing and on the amplitude of an externally applied electric dc field. The observed self-diffraction process is independent of the light polarization.
Asymmetric microtrenching during inductively coupled plasma oxide etching in the presence of a weak magnetic field
Authors: Schaepkens Marc , Oehrlein Gottlieb S.
Published: July 2002

When fabricating microscopic features in SiO2 layers using low pressure, high-density fluorocarbon plasmas, microtrenching has commonly been observed. Microtrenching has been explained either as due to ion scattering from sloped sidewalls or negative charging of the sidewalls by electrons, and the influence of the associated electric field on ion trajectories. In this work, we show that a weak magnetic field produces a significant asymmetry in microtrenching. Our results demonstrate unambiguously that electron-based sidewall charging is to a significant extent responsible for microtrenching, and, more generally, that differential charging is an important effect in microstructure fabrication using high-density plasmas.
Red shift of plasmon resonance frequency due to the interacting Ag nanoparticles embedded in single crystal SiO2 by implantation
Authors: Liu Zhengxin , Wang Honghong , Li Hao , Wang Xuemei
Published: July 2002

A metal nanoparticle system has been prepared by 200 Kev Ag+ ion implantation into perfect single crystal SiO2 at room temperature to dose: 6.7×1016/cm2. The system presents quasidual-layer structure: the shallower implanted layer containing noninteracting small Ag nanoparticles and the deeper layer containing interacting large nanoparticles, in which great red shift, about 1 eV, comparing with the plasmon resonance frequency of the noninteracting nanoparticle, can be clearly observed. The red shift is attributed to the multipoles interaction among the high density nanoparticles at external electric field. Moreover, the magnitude of red shift increases with implanted dose.
Observation of small interfacial strains in YBa2Cu3Ox sub-micron-thick films grown on SrTiO3 substrates
Authors: Lin W. J. , Hatton P. D. , Baudenbacher F. , Santiso J.
Published: July 2002

We have observed x-ray intensity oscillation fringes around low index Bragg reflections in a YBa2Cu3Ox (YBCO) thin film. By employing a combination of crystal truncation rod interference fringe measurements and x-ray reflectivity using synchrotron radiation, we have been able to probe the structures of highly oriented [001] YBCO grown on [001] SrTiO3 substrates. The results demonstrate the presence of a thin disordered surface layer, the excellent coherence between the YBCO surface and the film-substrate interface, the presence of a small (3.2×10−4) interfacial strain existing in the YBCO film. This strain exists close to the film-substrate interface and extends approximately 300 Å into the micron-thick film. Our results demonstrate that high quality detailed information can be obtained, nondestructively, from thin film superconductors of thicknesses typically used for microwave and other applications.
Termination, surface structure and morphology of the molecular beam epitaxially grown HgTe(001) surface
Authors: Oehling S. , Ehinger M. , Gerhard T. , Becker C. R. , Landwehr G. , Schneider M. , Eich D. , Neureiter H. , Fink R. , Sokolowski M. , Umbach E.
Published: July 2002

The surface structure of molecular beam epitaxially grown HgTe(001) has been studied under in situ ultrahigh vacuum conditions. The as-grown samples were investigated by means of x-ray photoelectron spectroscopy (XPS), spot profile analysis of low-energy electron diffraction, reflection of high-energy electron diffraction and scanning tunneling microscopy (STM). They exhibited a c(2×2) surface reconstruction with an additional weak (2×1) component in both diffraction experiments. The surface was shown by XPS to be terminated with Hg atoms. In addition, by means of STM experiments, we have been able to resolve the atomic structure of the reconstructed surface and to detect domain boundaries along the [11̄0] direction whose local symmetry is twofold and, therefore, the probable cause of the weak (2×1) reconstruction.
Tomographic reconstruction of an integrated circuit interconnect
Authors: Levine Zachary H. , Kalukin Andrew R. , Frigo Sean P. , McNulty Ian , Kuhn Markus
Published: July 2002

An Al–W-silica integrated circuit interconnect sample was thinned to several μm and scanned across a 200 nm focal spot of a Fresnel zone plate operating at photon energy of 1573 eV. The experiment was performed on beamline 2-ID-B of the Advanced Photon Source, a third-generation synchrotron facility. Thirteen scanned projections of the sample were acquired over the angular range ±69.2°. At least 301×301 pixels were acquired at each angle with a step size of 77×57 nm. A three-dimensional image with an approximate uncertainty of 400 nm was reconstructed from projection data using a standard algorithm. The two layers of the integrated circuit and the presence of the focused ion beam markers on the surface of the sample are clearly shown in the reconstruction.
Spatially resolved photoluminescence in partially ordered GaInP2
Authors: Smith S. , Cheong H. M. , Fluegel B. D. , Geisz J. F. , Olson J. M. , Kazmerski L. L. , Mascarenhas A.
Published: July 2002

Scanning confocal microscopy combined with high-resolution spectroscopy is used to probe the spatial variations in the low-temperature (5.0 K) photoluminescence (PL) of partially ordered GaInP2 with a spatial resolution of 0.7 μm. We observe large regions (1–2 μm) wherein the excitonic PL is suppressed up to a factor of four (“defect-rich” regions) when compared to unaffected areas. These defect-rich regions show a commensurate enhancement in the lower energy below gap emission. The spatial extent of this effect is inconsistent with the picture that the low-energy emission originates solely at the antiphase boundaries of the ordered domains and therefore must originate from other defects within the ordered domain as well.
Field enhancement effect of small Ag particles on the fluorescence from Eu3+-doped SiO2 glass
Authors: Hayakawa Tomokatsu , Selvan S. Tamil , Nogami Masayuki
Published: July 2002

We have investigated the effects of nanometer-sized silver particles on the optical properties of Eu3+ ions in SiO2 glass. Glass samples were prepared by the sol-gel method. The mean particle size and volume fraction of silver particles, which were estimated from UV-VIS absorption spectra using Mie-Drude theory modified by electron mean free path model, rapidly increased with the reduction time of Ag+ up to ∼3 min at 900 °C. The fluorescence from Eu3+ ions for the excitation by N2 laser was greatly enhanced in the presence of silver particles of 4.3 nm size. Our experimental results suggest that the origin of enhanced fluorescence is from local field enhancement around Eu3+ ions, owing to the surface plasmon resonance of small silver particles.
Evidence that arsenic is incorporated as As4 molecules in the molecular beam epitaxial growth of Hg1−xCdxTe:As
Authors: Garland J. W. , Grein C. H. , Yang B. , Wijewarnasuriya P. S. , Aqariden F. , Sivananthan S.
Published: July 2002

Molecular arsenic, As4, is commonly used as the source for in situ p-type doping of Hg1−xCdxTe grown by molecular beam epitaxy. As incorporated, the arsenic is strongly self-compensated, requiring annealing for its p-type electrical activation. Here, a quasithermodynamic model is used to interpret the dependence of the arsenic concentration, cAs, as measured by secondary ion mass spectroscopy, on the incident As4 and Hg fluxes during growth. The results strongly suggest that the As4 is absorbed in its molecular form rather than being dissociated on the growth surface, as has previously been assumed. This clearly is relevant to the self-compensation of the arsenic in as-grown Hg1−xCdxTe.
Periodic and depth-graded Cu/Si multilayer films for hard x-ray optics
Authors: Windt David L.
Published: July 2002

We describe the growth and characterization of periodic and depth-graded Cu/Si multilayer structures for use as coatings in hard x-ray optics. Films have been grown by dc magnetron sputtering, and were characterized by grazing-incidence x-ray (8 keV) reflectance analysis. The x-ray reflectance of both periodic and depth-graded structures was found to be high, and stable at room temperature over a period of at least several months. The interface widths determined from fits to the x-ray reflectance data were found to lie in the range 0.23–0.3 nm. X-ray diffraction revealed no crystalline phases present in any of the films, only broad peaks indicating either an amorphous Cu or possibly copper–silicide phase. As a result of the small measured interface widths and the low x-ray absorption of Cu just below, and also far above the Cu K edge (∼9 keV), highly efficient depth-graded Cu/Si multilayer coatings can now be used for broadband, grazing-incidence x-ray mirrors, for x-ray energies greater than at least 100 keV.
Polarization modulation in optoelectronic generation and detection of terahertz beams
Authors: Chen Q. , Zhang X.-C.
Published: July 2002

Compared with the conventional laser amplitude modulation with a lock-in amplifier, the optical polarization modulation in optoelectronic generation and detection of a free-space terahertz (THz) radiation provides up to twofold increase of the dynamic range. The total laser power for the optoelectronic generation of THz beams can be fully utilized.
Growth and luminescence properties of self-organized ZnSe quantum dots
Authors: Tawara Takehiko , Tanaka Satoru , Kumano Hidekazu , Suemune Ikuo
Published: July 2002

Self-organized ZnSe quantum dots (QDs) were grown on (100) ZnS/GaAs surfaces to study the relation of the size dispersion and luminescence. The exact dot sizes were obtained by measurements of atomic force microscope with its tip calibration and transmission electron microscope. The average dot size was 2.0 nm high and 11 nm in its diameter and the density was 1×1010 cm−2. Transition energies of ZnSe QDs were calculated using these measured dot sizes. These calculated peaks were in reasonable agreement with measured photoluminescence (PL) peaks. It was also revealed that the broadening of the PL spectra from ZnSe QDs were consistently explained by the dot size distribution.
Interface and tunneling barrier heights of NbN/AlN/NbN tunnel junctions
Authors: Wang Zhen , Terai Hirotaka , Kawakami Akira , Uzawa Yoshinori
Published: July 2002

The tunneling barrier height of NbN/AlN/NbN tunnel junctions was measured by investigating the barrier thickness dependence of the current density, and the junction interface was studied by cross-sectional transmission electron microscopy (TEM). We found that the current density of the junctions has two distinct types of dependency on the AlN barrier thickness, corresponding to two average barrier heights in different regions for the current density. The TEM observations showed that the junctions had a very smooth and clear electrode–barrier interface, and the crystal structures of the counterelectrode NbN films were strongly dependent on the thickness of AlN barriers. The average barrier height was estimated to be 2.35 eV in the low-Jc region, Jc<5 kA/cm2, and to be 0.88 eV in the high-Jc region, Jc>5 kA/cm2.
Scaling effect on statistical behavior of switching parameters of ferroelectric capacitors
Authors: Gruverman Alexei
Published: July 2002

Scanning force microscopy (SFM) has been used to study nanoscale variations in switching parameters in layered perovskite films of SrBi2Ta2O9 and to investigate the effect of capacitor scaling on the standard deviation of the capacitors’ integral polarization signal. Ferroelectric poling and SFM piezoresponse imaging were performed in a number of regions on the film surface sized 2×2, 1×1, 0.5×0.5, and 0.3×0.3 μm2 with subsequent statistical analysis of the obtained data. It has been found that variations of the polarization signal can be approximated by the normal distribution function. The standard deviation increases with the decrease in the capacitor size, suggesting a stronger effect of grain misalignment in smaller capacitors. The obtained results imply that reliable high-density ferroelectric memories cannot be realized unless a certain capacitor size/grain size ratio is maintained.
Transconductance oscillations in metal–oxide–semiconductor field-effect transistors with thin silicon-on-insulator originated by quantized energy levels
Authors: Takahashi Tetsuyasu , Miura-Mattausch Mitiko , Omura Yasuhisa
Published: July 2002

Transconductance oscillations have been observed in silicon-on-insulator metal–oxide–semiconductor field-effect transistors with the separation by implanted oxygen technology at 39 K. Here, we demonstrate that the origin of the oscillations is attributed to a terrace structure of the buried oxide surface. The terrace results in different thicknesses of the active Si layer. Quantized energy levels at the thin Si layer are higher than those at the thicker one. These different energy levels act as barriers for carriers moving in the channel. The observed transconductance oscillations are well reproduced by fitting the effective terrace periodicity.
Reactive-ion-etched gallium nitride: Metastable defects and yellow luminescence
Authors: Brown S. A. , Reeves R. J. , Haase C. S. , Cheung R. , Kirchner C. , Kamp M.
Published: July 2002

Gallium nitride has been reactive-ion etched with SF6 and argon plasmas. The Ar-etched samples show a striking transition from a dominant blue luminescence band to a dominant yellow luminescence band after less than 5 min of low power illumination. The observation of metastable defects which are associated with both the yellow and blue bands has important consequences for our understanding of defect-related luminescence in gallium nitride.
Electrostriction in gallium nitride
Authors: Guy I. L. , Muensit S. , Goldys E. M.
Published: July 2002

Electromechanical effects in the compound semiconductor gallium nitride have been measured. The electromechanical response is found to include a significant contribution from electrostriction. The measured value of the electrostrictive coefficient M33 in a polycrystalline sample of GaN is (1.2±0.1)×10−18 m2 V−2. This finding may have significance for devices using strained layers of this material.
Single-domain spectroscopy of self-assembled photonic crystals
Authors: Vlasov Yu. A. , Deutsch M. , Norris D. J.
Published: July 2002

We show how optical microscopy can be used to study the optical properties of a single crystalline domain in a self-assembled photonic crystal. By measuring spatially resolved reflection and emission spectra from a synthetic opal, inhomogeneities due to averaging over inherent disorder can be avoided. From “defect-free” reflection and emission spectra, the intrinsic photonic band structure can be extracted and inhibition of spontaneous emission can be verified.
Single-mode surface-plasmon laser
Authors: Tredicucci Alessandro , Gmachl Claire , Capasso Federico , Hutchinson Albert L. , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

Surface-plasmon modes confined at the interface between a metal and a semiconductor are exploited in place of conventional dielectric waveguides for the realization of a λ∼17 μm semiconductor laser. The device is based on the quantum cascade concept and outperforms with its 38 mW of peak output power and 240 K of maximum operating temperature any previous semiconductor laser of comparable wavelength. Pure single-wavelength emission with a tuning rate of ∼1 nm/K is achieved using Bragg reflection from a two-metal grating that modulates the skin depth of the surface plasmons.
Interface structure of hydride vapor phase epitaxial GaN grown with high-temperature reactively sputtered AlN buffer
Authors: Valcheva E. , Paskova T. , Tungasmita S. , Persson P. O. Å. , Birch J. , Svedberg E. B. , Hultman L. , Monemar B.
Published: July 2002

Thick hydride vapor phase epitaxy GaN layers have been grown on a-plane sapphire using high-temperature ion-assisted reactively sputtered AlN as a buffer layer. Transmission electron microscopy and atomic force microscopy were carried out to study the formation of the two interfaces sapphire/AlN and AlN/GaN, and their influence on the microstructure of both the buffer layer and the main GaN layer. It was demonstrated that the high-temperature reactively sputtered buffer layer provides a good alternative for hydride vapor phase epitaxy growth of GaN layers. In particular, the buffer promotes a specific interface ordering mechanism different from that observed on low-temperature buffers.
Green upconversion luminescence in Er3+:BaTiO3 films
Authors: Zhang H. X. , Kam C. H. , Zhou Y. , Han X. Q. , Buddhudu S. , Xiang Q. , Lam Y. L. , Chan Y. C.
Published: July 2002

Green upconversion emissions at 548 and 528 nm have been obtained from sol-gel derived nanocrystalline Er3+:BaTiO3 films under 980 nm excitation. The green emissions are attributed to the ground-state (4I15/2) -directed transitions from H11/22 (528 nm) and S3/22 (548 nm) of Er3+ ions. Analysis of the temporal evolution of the emission at 548 nm shows that both excited state absorption of individual ion and energy transfer between the two neighboring ions contribute to the upconversion emissions in Er3+ (3 mol %):BaTiO3 film. The lifetime of the dominant emission at 548 nm is about 90 μs.
Artifact-free near-field optical imaging by apertureless microscopy
Authors: Labardi M. , Patanè S. , Allegrini M.
Published: July 2002

A method for optical near field discrimination, leading to drastic artifact reduction in superresolved imaging by scanning interference apertureless microscopy is presented. The method relies on second harmonic detection of the modulated optical signal scattered by a vibrating silicon tip. An edge resolution of 15 nm, or 7 nm Rayleigh-type resolution, with optical contrast as high as 50%, has been obtained on aluminum projection pattern samples in the constant gap width mode. Our method has been determined not to be affected by topographical artifacts by constant height mode scans.
The properties of ion clouds in a Paul trap: A statistical model
Authors: Hou Jidong , Wang Yiqiu , Yang Donghai
Published: July 2002

We present a statistical model to calculate the spatial and energy properties of an ion cloud in a Paul trap. In this model, we consider the collision between the ions and the molecules of background gases as the reason to cause the quasistable state of an ion cloud. The space-charge potential in an ion cloud is taken into account by a self-consistent method. From the real parameters of a Paul trap the properties of an ion cloud can be computed. The comparison between the calculated and experimental results shows good agreement. The influences of background gases and space charge on the properties of ion clouds are studied. Our calculation shows that laser cooling does not work when the number of confined ions is larger than 106.
Continuum modeling of multi-mode vibronic excitations in near-edge x-ray absorption fine structure spectra
Authors: Privalov Timofei , Plashkevych Oleksandr , Gel’mukhanov Faris , Ågren Hans
Published: July 2002

The shape of electro-vibrational bands in near-edge x-ray absorption fine structure (NEXAFS) spectra is often made up by a few strong lines near a phononless resonance. In addition—as predicted here—there can be extended tails due to the large density of vibrational states in the short-wavelength region of the x-ray spectrum where the spacings between vibrational levels are smaller than the lifetime broadening. We propose a continuum approach for simulating vibronic bands in x-ray absorption spectra for systems with a high density of states. This approach is found to provide an accurate description of the spectral shape of the multi-mode vibrational bands in the NEXAFS spectra of polyenes having even a relatively small number of vibrational modes. Ab initio simulations using the discrete linear coupling model confirm the common trends obtained by the continuum approach.
New open-loop actuating method of piezoelectric actuators for removing hysteresis and creep
Authors: Jung Hewon , Shim Jong Youp , Gweon DaeGab
Published: July 2002

Many researchers have studied the actuation of piezoelectric actuators using a model of the hysteresis of piezoelectric transducer (PZT). However, these hysteresis modeling methods to remove the nonlinearity in PZT did not include the consideration of PZT’s creep effect which is a very important factor in many open-loop application of piezoelectric actuator. In order to compensate for the creep effect of PZT as well as the hysteresis of displacement, the notion of “voltage creep” is proposed in this article. And, with the notion, a new actuation method of piezoelectric actuators for removing the hysteresis and creep will be proposed and the actuation result will be presented.
Manipulation of adsorbates with electric fields
Authors: Saalfrank Peter
Published: July 2002

Various strategies, which lead to the manipulation of molecules adsorbed at surfaces by means of external electric fields are proposed. First, explicitly time-dependent laser radiation is considered. For the example system NH3–Cu(111), a two-dimensional adiabatic model and nuclear wave packet propagation are adopted to study the desorption of NH3 by “vibrational ladder climbing” and “above threshold desorption,” both driven by a series of ultrashort infrared pulses. Another strategy makes use of static fields and leads to “field desorption,” for which isotope effects and higher-order contributions to the molecule–field interaction are addressed in some detail. We also study the reverse process to desorption, adsorption. Starting with (i) the elastic and inelastic scattering of “randomly oriented” (molecular beam) ammonia molecules off the surface, we then consider (ii) the scattering of static-field aligned molecules and, finally, (iii) the laser-induced adsorption (LIA) of gas-phase molecules by stimulated emission of photons.
Hole transport in polyphenylenevinylene-ether under bulk photoexcitation and sensitized injection
Authors: Im C. , Bässler H. , Rost H. , Hörhold H. H.
Published: July 2002

Employing the time-of-flight (TOF) technique, the hole mobility in films of polyphenylenevinylene-ether has been measured as a function of electric field and temperature. Charge carriers were generated upon either photoexcitation into the S1←S0 transition of the polymer or via sensitized injection from a thin rhodamine 6G layer. The results will be interpreted in terms of the concept of hopping among polymer segments featuring a Gaussian distribution of energies, its variance being 91 meV. While TOF signals generated via sensitized injection are similar to those in molecularly doped polymers bearing out a transition from nondispersive to dispersive transport, intrinsically generated TOF signal features a cusp at higher temperature and almost no dispersion. The latter phenomenon turns out to be a consequence of charge generation at the tail of the density distribution of hopping states.
Electro- and magneto-rheology of nematic liquid crystals: Experiment and nonequilibrium molecular dynamics computer simulation
Authors: Eich A. , Wolf B. A. , Bennett L. , Hess S.
Published: July 2002

The viscosity of the nematic liquid crystal PCH-5 [4-(trans-4′-pentylcyclohexyl)-benzonitrile], measured in the presence of an electric field parallel to the gradient of the velocity, shows a complex dependence both on magnitude E of the electric field and on the shear rate γ̇. When plotted versus E2/γ̇, all data points fall unto a master curve. In nonequilibrium molecular dynamics computer simulations, performed for a Gay–Berne model fluid, the corresponding influence of a magnetic field on the viscosity was calculated. In both cases, the shape of the master curve for the viscosity can be computed by considering the competition between the torques exerted by the velocity gradient and by the orienting field. It involves the Leslie viscosity coefficients γ1 and γ2, the Miesowicz and Helfrich viscosities η1,η2, and η12. Thus it should be possible to extract these coefficients from the data. This is straightforward in the magnetic case since the molecules in the fluid “see” the externally applied field. The internal electric field, however, differs from the applied field E. When this effect is taken into account, using the known experimental values for dielectric coefficients ε‖ and ε⊥, the electro-rheological master curve agrees very well with the experimental data. Values for the viscosity coefficients are obtained and presented.
Soliton interpretation of relation between driving force and velocity of interface motion in martensitic transformation
Authors: Zhao Yu , Zhang Jihua , Hsu (Xu Zuyao) T. Y.
Published: July 2002

The relationship between the driving force for a martensitic transformation, ΔG, and the motion velocity of the martensite/parent interface, v, is derived for the Hamiltonian of a one-dimensional atomic chain with a nonlinear periodic potential model suggested by Peyrard and Remoissenet [Phys. Rev. B 26, 2886 (1982)] as ΔG=(2ma/π2)ω[C/1−v2/C2](4.026 92−2.931 92r−0.762 24r2), where m is the atomic mass, a the spacing of the one-dimensional lattice, r the shape parameter indicating the periodic lattice potential and C and ω are the characteristic velocity and frequency of the system, respectively. This result shows that the velocity of interface motion increases as the driving force increases. This equation is valid for different materials, regardless of whether the softening for martensitic transformation exists.
Interfaces, wetting, and capillary nematization of a hard-rod fluid: Theory for the Zwanzig model
Authors: van Roij René , Dijkstra Marjolein , Evans Robert
Published: July 2002

We investigate interfacial and capillary phenomena in a simple model for a fluid of hard rods, viz. the Zwanzig model, in which the orientations of rectangular blocks are restricted to three orthogonal directions. The theory, which is based on an Onsager-like free energy functional, predicts local biaxial ordering at the “free” interface between the coexisting isotropic and nematic phases. For an isotropic bulk fluid in contact with a single planar hard wall, we find a continuous surface phase transition from uniaxial to biaxial local symmetry, followed by complete wetting of the wall–isotropic fluid interface by a nematic film with director parallel to the wall, as the reservoir density approaches its value at bulk coexistence. For a fluid confined by two parallel hard walls we determine a first-order capillary nematization transition at large wall separation, which terminates in a capillary critical point when the wall separation is about twice the length of the rods. This transition is the analog of the capillary condensation observed for simple fluids confined by attractive walls but is purely entropy driven here.
Theoretical study of the ground and excited states of silicon clusters: Si8Hx
Authors: Nakajima Keizo , Yoshizawa Kazunari , Yamabe Tokio
Published: July 2002

The electronic structures of small silicon clusters in the ground and excited states have been studied based on ab initio configuration interaction by the single-substitutions approximation method. We consider Si8Hx clusters of chain, branch, ladder, and cubic types as model compounds, due to their variety of molecular structures. Calculated Stokes shifts and oscillator strengths successfully reproduce the experimental electronic spectrum. These phenomena are well explained by the detailed analysis of the orbital patterns and energy level changes related to the excitation. The optical properties of the cubic Si cluster are considerably different from those of the other Si8 clusters. It shows zero oscillator strength, i.e., “forbidden transition,” from the first to 20th excited states because of its high symmetry.
Simulation of the optical effects of particles residing at the bottom of KrF positive photoresist films coated on Si substrates
Authors: Kamoshida Mototaka , Inui Hirotomo , Ohta Toshiyuki , Kasama Kunihiko
Published: July 2002

The reflection effects of Si, SiO2, and Al particles on developed cross-sectional photoresist patterns are simulated by using a two-dimensional waveguide model. The particles were assumed to reside at the bottom of chemically amplified positive photoresist film coated on the Si substrates. The reflection effects were investigated in both cases of particles on the flat surface and on the side slope of the stepped pattern of the substrates. The smallest sizes of particles capable of causing pattern shorts are discussed through a comparison between this work and the authors’ previous work, where the particles reside at the top of photoresist film. In conclusion, the smallest sizes of particles residing at the bottom of the resist film are larger than those of particles residing at the top of the resist film. In addition, because the reflecting light exposes the bottom of the positive resist, after development it apparently helps to realize 0.2 μm critical dimension. Thus the smallest size of the Al particle is larger than that of the Si particle. The reflection from the side of the steps on the Si wafer surface influences the cross-sectional developed pattern of the photoresist film.
Work function study of rhenium oxidation using an ultra high vacuum scanning Kelvin probe
Authors: Baikie I. D. , Petermann U. , Speakman A. , Lägel B. , Dirscherl K. M. , Estrup P. J.
Published: July 2002

We have undertaken a study of high work function (φ) surfaces as part of an ongoing project searching for efficient target materials for use in hyperthermal surface ionization (HSI), a new mass spectroscopy ionization technique. HSI relies on high φ surfaces for the production of positive ions. Rhenium is particularly interesting in this respect as oxidation substantially increases φ to approximately 7 eV. Using a novel ultrahigh vacuum scanning Kelvin probe and Auger electron spectroscopy we have followed the oxidation kinetics of clean, polycrystalline rhenium at temperatures in the range (300–800) K and examined the effects of oxidation via high resolution φ topographies. Our results indicate a Δφ increase of 1050 meV at 300 K rising to 1950 meV at 800 K. We observe two reaction stages in the 300 K data, with a transition at 150 L, characterized by different rates of oxide growth. Sputter-cleaned surfaces exhibit significant surface roughness even after annealing, which dramatically influences the second oxidation stage.
The effect of forcing on the spatial structure and spectra of chaotically advected passive scalars
Authors: Neufeld Zoltán , Haynes Peter H. , Picard Guillemette
Published: July 2002

The stationary distribution of passive tracers chaotically advected by a two-dimensional large-scale flow is investigated. The value of the tracer is conserved following each fluid element except when the element enters certain localized regions. The tracer value is then instantenously reset to a value associated with the region entered. This resetting acts as a forcing for the tracer field. This problem is mathematically equivalent to advection in open flows and results in a fractal tracer structure. The spectral exponent of the tracer field is different from that for a passive tracer with the usual additive forcing (the so-called Batchelor spectrum) and is related to the fractal dimension of the set of points that have never visited the forcing regions. We illustrate this behavior by considering a time-periodic flow whose effect is equivalent to a simple two-dimensional area-preserving map. We also show that similar structure in the tracer field is found when the flow is aperiodic in time.