bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-correlated single photon counting imager for simultaneous magnetic resonance and near-infrared mammography
Authors: Ntziachristos Vasilis , Ma XuHui , Chance Britton
Published: July 2002

We describe a near infrared (NIR) imager for mammography, designed to work simultaneously with a magnetic resonance (MR) scanner. The imager employs two pulsing laser diodes, with average power of 25 μW, at 780 and 830 nm. The two wavelengths are time multiplexed into 24 source fibers. The detection part consists of eight parallel time-correlated photon-counting channels with overall counting capacity of 106 photons/s. We use long optical fibers to avoid interference with the magnetic field. Specially designed coupling plates, for breast soft compression, bear both the MR radio-frequency coils and the optical source and detector fibers. Capillaries containing water and copper sulfate mark the position of the plates on the MR images for accurate coregistration of NIR and MR images. Instrument compatibility has been successfully tested with volunteers in the MR scanner. The use of gallium arsenide photomultiplier tubes has allowed penetration depths of 10 cm in the human breast. Imaging algorithms, based on the analytical modeling of photon propagation in inhomogeneous media, have been applied successfully to image 0.8-mm-diam absorbing and scattering cylindrical perturbations in transmittance geometry of breast-like phantoms.
Monitoring of an atomic force microscope cantilever with a compact disk pickup
Authors: Quercioli F. , Tiribilli B. , Ascoli C. , Baschieri P. , Frediani C.
Published: July 2002

In the present study we test a compact disk pickup as the cantilever position sensor in an atomic force microscope (AFM). The pickup is placed on top of the optical microscope used for the visual inspection and alignment of the specimen. The AFM is also equipped with its own cantilever movement sensor system. Both the built-in and the new detection devices are simultaneously active for comparison purposes. Two different measurements are performed in sequence on the same sample each using one sensor at a time as the error signal source for the AFM feedback loop. The pickup has demonstrated good sensitivity as well as excellent performance in terms of compactness, reliability, and cost.
A combined time-of-flight and electrostatic analyzer for low-energy ion scattering
Authors: Denier van der Gon A. W. , Reijme M. A. , Rumphorst R. F. , Maas A. J. H. , Brongersma H. H.
Published: July 2002

A new instrument for low-energy ion scattering (LEIS) is presented which employs a combination of electrostatic and time-of-flight analysis. In this instrument, electrostatic analysis is used to determine the kinetic energy of the ions, and flight-time analysis is used to select the mass of the ions. The combination allows us to discriminate the signals resulting from particles with a different mass than the primary ions, resulting in a very efficient suppression of signals caused by sputtered particles. The suppression of signals from sputtered particles enables more accurate determination of LEIS signals, especially for light elements. This technique is especially suited for the study of polymers and oxides, but can also be valuable for determination of low concentration of heavier elements. In this article the design of the instrument is presented and the method is demonstrated by some examples of LEIS spectra.
A pulsed Laval nozzle apparatus with laser ionization mass spectroscopy for direct measurements of rate coefficients at low temperatures with condensable gases
Authors: Lee Seonkyung , Hoobler Ray J. , Leone Stephen R.
Published: July 2002

A pulsed Laval nozzle, low Mach number supersonic expansion kinetics apparatus has been constructed to study neutral–neutral kinetics by a rather general laser photolysis initiation and laser photoionization detection of the product species. This new apparatus permits laboratory studies of low temperature rate coefficients (e.g., 70–170 K) on condensable gases that have insufficient vapor pressures at low temperatures for conventional methods of kinetic measurements. The design considerations, the uniformity of the reaction zone over 10–20 cm, and the skimmer sampling of the pulsed Laval expansion are examined. The direct measurement of a rate coefficient at 90 K is also demonstrated using this new apparatus.
An electro-optically controlled liquid crystal diffraction grating
Authors: Chen J. , Bos P. J. , Vithana H. , Johnson D. L.
Published: July 2002

A structure for an electro-optically controlled liquid crystal diffraction grating is proposed, which can dramatically simplify the fabrication process of liquid crystal optical gratings. The structure consists of two alternating stripes. Each stripe is a hybrid liquid crystal cell with adjacent stripes oriented perpendicularly. This kind of electro-optically controlled diffraction grating in principle gives 100% diffraction efficiency and no polarization direction dependence. The detailed fabrication process is presented.
Growth and doping of GaAsSb via metalorganic chemical vapor deposition for InP heterojunction bipolar transistors
Authors: McDermott B. T. , Gertner E. R. , Pittman S. , Seabury C. W. , Chang M. F.
Published: July 2002

GaAsSb is a low band gap, lattice matched to InP, alternative to GaInAs. Growth and doping using diethyltellurium and carbon tetrachloride were investigated. Hole concentrations up to 1.3×1020 cm−3 have been achieved in as-grown carbon-doped GaAsSb [i.e., no postgrowth annealing was necessary for dopant activation, a key requirement for n-p-n heterojunction bipolar transistor (HBT) structures]. This is a sevenfold improvement over the best carbon-doped InGaAs reported by metalorganic chemical vapor deposition. Hall measurements indicate that GaAsSb’s hole mobility is 55%–60% of GaInAs’s, for a given carrier concentration. InP HBTs with carbon-doped GaAsSb base are demonstrated.
Laser-induced thermoelastic excitation of Scholte waves
Authors: Desmet C. , Gusev V. , Lauriks W. , Glorieux C. , Thoen J.
Published: July 2002

The results of experiments on an all-optical generation and detection of Scholte waves are reported. The Scholte wave was excited thermoelastically by pulsed laser action on mercury/plexiglas or mercury/fused silica interfaces and was subsequently detected by the technique of light beam deflection. Both the shape of the Scholte pulse in the time domain and its measured velocity of propagation are in line with theoretical predictions.
Fine droplets of liquid crystals in a transparent polymer and their response to an electric field
Authors: Matsumoto Shiro , Houlbert Marthe , Hayashi Takayoshi , Kubodera Ken-ichi
Published: July 2002

Fine droplets of nematic liquid crystal (LC) with a diameter less than 100 nm were obtained in ultraviolet curing polymers. The transparency of the material was kept high in the infrared regions because there was little scattering in comparison to materials containing much larger droplets. The fine droplets were found to respond to an electric field, causing a change in birefringence. This change was about 0.0004 at the applied voltage of 5.5 V/μm. The special features of low transmission loss and easy film formation will enable one to apply LCs to optical devices of the wave guide type.
Terahertz radiation from superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical pulses
Authors: Hangyo M. , Tomozawa S. , Murakami Y. , Tonouchi M. , Tani M. , Wang Z. , Sakai K. , Nakashima S.
Published: July 2002

Ultrashort electromagnetic waves (600 fs width) from superconducting YBCO thin films have been observed by irradiating current-biased samples with femtosecond optical laser pulses (80 fs width). The Fourier component of the pulse extends up to ∼2 THz. The characteristics of the radiation are studied and the radiation mechanism is ascribed to the ultrafast supercurrent modulation by the laser pulses, which induce the nonequilibrium superconductivity.
Limitations to n-type doping in diamond: The phosphorus-vacancy complex
Authors: Jones R. , Lowther J. E. , Goss J.
Published: July 2002

In spite of large concentrations of phosphorus being incorporated into diamond, the material often remains insulating. It is argued that this occurs through the formation of phosphorus-vacancy complexes which are deep acceptors and compensate any donor. The complex is analyzed using a first-principles cluster method. In the ionized state, the defect is diamagnetic and cannot give rise to any internal optical transitions although broad band donor–acceptor transitions are expected—and observed—in material codoped with nitrogen.
Growth of high quality Al0.48In0.52As/Ga0.47In 0.53As heterostructures using strain relaxed AlxGayIn1−x−yAs buffer layers on GaAs
Authors: Haupt M. , Köhler K. , Ganser P. , Emminger S. , Müller S. , Rothemund W.
Published: July 2002

Morphological and physical properties of Al0.48In0.52As/Ga 0.47In0.53As heterostructures grown by molecular beam epitaxy, lattice relaxed on GaAs substrates and lattice matched on InP substrates, are presented. Both a quaternary linear and step graded lattice relaxed buffer concept is implemented to accomodate the lattice mismatch between the GaAs substrate and the Al0.48In0.52As/Ga 0.47In0.53As layer sequence. The surface morphology and the transport properties of Al0.48In0.52As/Ga0.47In0.53As high electron mobility transistor structures were studied by atomic force microscopy and Hall effect measurements, respectively. Optical properties were investigated by low temperature photoluminescence experiments on quantum well structures. The linear graded buffer approach was found to result in superior heterostructure properties due to the two dimensional growth mode during the whole growth process resulting in the typical cross hatched surface morphology. In contrast, the use of step graded buffer layers resulted in three dimensional layer growth and inferior layer quality. However, by increasing the number of steps, i.e., reducing the change in the lattice constant for each step and thus approaching a linear grading, two dimensional growth is recovered.
Multiple combustion induced by ball milling
Authors: Takacs L.
Published: July 2002

An unusual multiple combustion effect has been observed during the mechanochemical reaction of zirconium and sulphur powders. Ball milling induces combustion in this highly exothermic system, but the combustion is quenched repeatedly, after consuming only a fraction of the reactants. The process concludes with a final, more energetic combustion, releasing about 70% of the reaction heat. The evaporation of sulfur at the reaction site may explain this behavior.
“Plug and play” systems for quantum cryptography
Authors: Muller A. , Herzog T. , Huttner B. , Tittel W. , Zbinden H. , Gisin N.
Published: July 2002

We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution. The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing. The use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber. First experimental results show a fringe visibility of 0.9984 for a 23-km-long interferometer, based on installed telecom fibers.
B diffusion and clustering in ion implanted Si: The role of B cluster precursors
Authors: Pelaz L. , Jaraiz M. , Gilmer G. H. , Gossmann H.-J. , Rafferty C. S. , Eaglesham D. J. , Poate J. M.
Published: July 2002

A comprehensive model for B implantation, diffusion and clustering is presented. The model, implemented in a Monte Carlo atomistic simulator, successfully explains and predicts the behavior of B under a wide variety of implantation and annealing conditions by invoking the formation of immobile precursors of B clusters, prior to the onset of transient enhanced diffusion. The model also includes the usual mechanisms of Si self-interstitial diffusion and B kick-out. The immobile B cluster precursors, such as BI2 (a B atom with two Si self-interstitials) form during implantation or in the very early stages of the annealing, when the Si interstitial supersaturation is very high. They then act as nucleation centers for the formation of B-rich clusters during annealing. The B-rich clusters constitute the electrically inactive B component, so that the clustering process greatly affects both junction depth and doping level in high-dose implants.
Fiber Sagnac interferometer temperature sensor
Authors: Starodumov A. N. , Zenteno L. A. , Monzon D. , De La Rosa E.
Published: July 2002

A modified Sagnac interferometer-based fiber temperature sensor is proposed. Polarization independent operation and high temperature sensitivity of this class of sensors make them cost effective instruments for temperature measurements. A comparison of the proposed sensor with Bragg grating and long-period grating fiber sensors is derived. A temperature-induced spectral displacement of 0.99 nm/K is demonstrated for an internal stress birefringent fiber-based Sagnac interferometer.
Reverse saturable absorption and optical limiting in C60 solution in the near-infrared
Authors: Mishra S. R. , Rawat H. S. , Mehendale S. C.
Published: July 2002

We report observation of reverse saturable absorption in C60 toluene solution for 10 ns duration laser pulses in the wavelength region 710–740 nm, for which the photon energy is less than the energy of the lowest excited singlet state. The observed nonlinear absorption appears to be due to excitation of molecules to the more absorbing lowest triplet state by direct absorption from the ground state. Our results imply that a C60 based optical limiter can operate over a broad spectral range extending into the near-infrared.
Cholesteric gratings with field-controlled period
Authors: Subacius D. , Shiyanovskii S. V. , Bos Ph. , Lavrentovich O. D.
Published: July 2002

Diffractive gratings with period varied by an electric field are developed using a cholesteric liquid crystal confined between two transparent electrodes with unidirectionally treated alignment layers. In the initial state (zero field), the cholesteric layers are parallel to the cell planes. The electric field creates an in-plane modulated structure of variable period. Diffraction regimes of both Raman-Nath and Bragg types are demonstrated.
Scanning capacitance spectroscopy: An analytical technique for pn-junction delineation in Si devices
Authors: Edwards Hal , McGlothlin Rudye , San Martin Richard , U Elisa , Gribelyuk Michael , Mahaffy Rachel , Ken Shih C. , List R. Scott , Ukraintsev Vladimir A.
Published: July 2002

Scanning capacitance spectroscopy (SCS), a variant of scanning capacitance microscopy (SCM), is presented. By cycling the applied dc bias voltage between the tip and sample on successive scan lines, several points of the high-frequency capacitance–voltage characteristic C(V) of the metal–oxide–semiconductor capacitor formed by the tip and oxidized Si surface are sampled throughout an entire image. By numerically integrating dC/dV, spatially resolved C(V) curves are obtained. Physical interpretation of the C(V) curves is simpler than for a dC/dV image as in a single-voltage SCM image, so that the pn junction may be unambiguously localized inside a narrow and well-defined region. We show SCS data of a transistor in which the pn junction is delineated with a spatial resolution of ±30 nm. This observation is consistent with the conclusion that SCS can delineate the pn junction to a precision comparable to the Si depletion width, in other words, the actual size of the electrical pn junction. A physical model to explain the observed SCS data near the pn junction is presented.
Very high power InGaAsP/InP distributed feedback lasers at 1550 nm wavelength
Authors: Chen T. R. , Hsin W. , Bar-Chaim N.
Published: July 2002

A continuous wave (cw) optical power of 108 mW was coupled into a polarization maintaining fiber from a high power distributed feed back (DFB) laser at 1550 nm wavelength. The laser also displays excellent high temperature performance, low noise, and narrow linewidth, which makes it very attractive for broadband supertrunking applications.
Photoluminescence of carbon in situ doped GaN grown by halide vapor phase epitaxy
Authors: Zhang R. , Kuech T. F.
Published: July 2002

Carbon was in situ doped into GaN during halide vapor phase epitaxy and photoluminescence properties of the C-doped GaN film were investigated. It has been found that incorporation of carbon into GaN produces a significant yellow luminescence around 2.2 eV. The peak position of the yellow band blueshifts linearly and the intensity of that band monotonically decreases with measurement temperature, with systematic changes in the linewidth. These results suggest that multiple donor–acceptor recombination channels are involved in the yellow luminescence.
Observation of optically-active metastable defects in undoped GaN epilayers
Authors: Xu S. J. , Li G. , Chua S. J. , Wang X. C. , Wang W.
Published: July 2002

Optically-active defects of undoped GaN epilayers grown on sapphire by metalorganic chemical vapor epitaxy was investigated with photoluminescence. A new metastable defect emitting blue light was found, besides the well-known yellow luminescence centers. With excitation by the 325 nm He–Cd laser, this metastable defect, at low temperature, exhibits the luminescence fatigue effect with the decay time determined to be about 6 min. When the temperature is increased to room temperature, it recovers its optically-active state. The yellow band emission increases in intensity as the blue band emission decreases in intensity. Analysis shows that this metastable center is a hole trap, and Ga vacancy is its most probable candidate.
Enhanced stability of deuterium in silicon
Authors: Biswas R. , Li Y.-P. , Pan B. C.
Published: July 2002

Tight-binding molecular dynamics has been used to simulate vibrationally excited Si–H and Si–D modes. Simulations find that vibrationally excited Si–D bending modes decay much more rapidly than Si–H bending modes, resulting in SiD bonds having much higher stability than SiH bonds. This provides a viable mechanism for reduced degradation in deuterated metal–oxide–semiconductor transistors and deuterated amorphous silicon devices.
Field-dependent absorption in superlattices: Comparison of theory and experiment
Authors: Thränhardt A. , Kolbe H. J. , Hader J. , Meier T. , Weiser G. , Koch S. W.
Published: July 2002

Absorption and differential electroabsorption spectra of a GaInAs/InP superlattice in the regime of Wannier–Stark localization are compared with spectra derived from the Semiconductors Bloch Equations. The absorption is strongly underestimated for higher energies by the effective mass approximation but good agreement is achieved by using an energy dependent in-plane effective mass. No fitting parameters are used except for a phenomenological broadening. The lineshape of the field-modulated spectra is dominated by the strong field dependence of Wannier–Stark transitions and depends on the amplitude of the modulating field which should be kept small for best results.
A wideband electronically tunable microwave notch filter in yttrium iron garnet–gallium arsenide material structure
Authors: Tsai Chen S. , Su Jun
Published: July 2002

A wideband electronically tunable microwave notch (band-stop) filter has been constructed in an yttrium iron garnet (YIG)/gallium arsenide (GaAs) material structure. An incident microwave propagating along the microstrip transmission line in the GaAs substrate is coupled into and to excite the magnetostatic surface waves in the YIG layer, which is laid upon the microstrip transmission line. Maximum coupling and thus the peak absorption of the output microwave power occur at the ferromagnetic resonance frequency in the YIG film as determined by a bias magnetic field. A tuning range as large as 2.5–23.0 GHz in the peak absorption frequency with the corresponding magnetic field tuning range of 290–7300 Oe has been accomplished. Peak absorption of 15–38 dB in the microwave output power has also been measured.
High-sensitivity piezoresistive cantilevers under 1000 Å thick
Authors: Harley J. A. , Kenny T. W.
Published: July 2002

Ultrathin, high-sensitivity piezoresistive cantilevers were constructed using vapor-phase epitaxy to grow the conducting layer. A fourfold reduction in thickness was achieved over the thinnest implanted piezoresistive cantilevers, allowing improved force or displacement sensitivity and increased bandwidth. In cantilevers 890 Å thick, the dopant is well confined to the surface, and the sensitivity is 70% of the theoretical maximum. A cantilever fabricated for high force resolution has a minimum detectable force of 8.6 fN/Hz in air. Additionally, the 1/f noise is shown to follow the relation proposed by Hooge [Phys. Lett A 29, 139 (1969)], increasing in inverse proportion to the number of carriers.
Influence of molybdenum on the formation of C54 TiSi2: Template phenomenon versus grain-size effect
Authors: Zhang S.-L. , d’Heurle F. M.
Published: July 2002

Experimental results are presented for the formation of TiSi2 in the presence of an ultrathin Mo layer deposited either at the interface between Ti and Si or on top of Ti/Si. The formation of C54 TiSi2 is clearly shown to be enhanced with a surface Mo layer, although the effect is less pronounced as compared to the use of an interposed Mo layer. The results can be accounted for with a template mechanism where the formation of C40 (Mo, Ti)Si2 is crucial for the epitaxial growth of C54 TiSi2 atop. Possible grain-size effects on the formation of C54 TiSi2 are also discussed.
Avalanche noise measurement in thin Si p+-i-n+ diodes
Authors: Tan C. H. , Clark J. C. , David J. P. R. , Rees G. J. , Plimmer S. A. , Tozer R. C. , Herbert D. C. , Robbins D. J. , Leong W. Y. , Newey J.
Published: July 2002

The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been investigated. In these thin diodes the excess noise is found to fall below that predicted by conventional local noise theory. Modeling of the multiplication and excess noise using a recurrence method, which includes the dead space for carrier ionization, gives good agreement with experiment. This suggests that the dead space can reduce the excess noise in submicron Si diodes.
Ultrafast Raman-induced Kerr-effect of water: Single molecule versus collective motions
Authors: Winkler Kathrin , Lindner Jörg , Bürsing Helge , Vöhringer Peter
Published: July 2002

The ultrafast optical Kerr-response of water and heavy water has been measured at 1 bar in the temperature range between 273 and 373 K. The nuclear Kerr response of the liquid exhibits a pronounced double exponential decay on longer time scales after dephasing of impulsively perturbed acoustic modes is completed. The time constant, τ2, characterizing the slowly decaying exponential component of the Kerr-response function is in quantitative agreement with rotational diffusion time constants of the water molecules obtained form nuclear magnetic resonance (NMR) spin-lattice relaxation rates. A detailed comparison with THz time domain spectroscopy demonstrates that the reorientational dynamics responsible for the long time tail of the Kerr response are due to single molecule as opposed to collective effects. Furthermore, a good agreement between the single molecule rotational diffusion and the Stokes–Einstein–Debye equation is found in the temperature range of thermodynamic stability of the liquid. The time constant, τ1, characterizing the fast exponential component of the Kerr-response of water is found to be in qualitative agreement with central Lorentzian linewidths obtained from frequency-domain, depolarized Raman scattering experiments. The temperature dependence of τ2 does not follow an Arrhenius-type behavior, which was previously taken as evidence for thermally activated crossing of a librational barrier with concomitant hydrogen-bond breakage. Instead, the temperature dependence of the fast relaxation time constant can be represented adequately by the Speedy–Angell relation which has been shown to accurately describe a number of transport parameters and thermodynamic properties of water.
Correlating the NMR self-diffusion and relaxation measurements with ionic conductivity in polymer electrolytes composed of cross-linked poly(ethylene oxide-propylene oxide) doped with LiN(SO2CF3)2
Authors: Hayamizu Kikuko , Aihara Yuichi , Price William S.
Published: July 2002

A solvent-free solid-polymer electrolyte based on a cross-linked poly(ethylene oxide-propylene oxide) random copolymer doped with LiN(SO2CF3)2, was studied using multinuclear NMR and ionic conductivity. The NMR spin-lattice relaxation times, T1, of the bulk polymer (1H), the lithium ion (7Li) and the anion (19F) were analyzed using single exponential analysis above the glass transition temperatures. Since the temperature dependent H1 and Li7 NMR T1 values had minima, the reorientational correlation times were obtained for the segmental motion of the CH2CH2O/CH2CH(CH3)O moiety of the bulk polymer and the hopping motion of the lithium ions correlated with the segmental motion. The spin–spin relaxation of the anion signals appeared single exponential with respect to time, whereas that of the polymer and the lithium echo signals were at least bi-exponential. Since both the spin-lattice and spin–spin relaxation of the anion indicated a single component, the self-diffusion coefficients, D, were measured using F19 pulsed-gradient spin-echo (PGSE) NMR measurements. Although the PGSE attenuation data appeared single exponential at each value of the separation between the gradient pulses, Δ, the measured D values had a Δ-dependence. Phenomenologically, the anion diffuses quicker in a shorter range and the activation energy of the shorter-time diffusion is smaller than that of the longer-time diffusion. The apparent self-diffusion coefficients became smaller for longer Δ and approached a constant when Δ was longer than 0.05 s. The mean square displacements of the anion were inconsistent with standard diffusion models including “anomalous diffusion” as found for a neutral particle diffusing in a fractal network [i.e., 〈r2(Δ)〉∝Δκ with κ<1(κ≡2/dw where dw is the random walk fractal dimension)]. The apparent diffusion coefficients of the lithium ions at Δ=0.02 s are almost independent of temperature and smaller than the corresponding diffusion coefficients of the anion. Since the activation energy of the anion determined for Δ longer than 0.05 s correlates well with those obtained from the ionic conductivity, the ion conduction in the solid-polymer medium is driven mainly by fast transfer of the anions.
Analysis of discretization in the direct simulation Monte Carlo
Authors: Hadjiconstantinou Nicolas G.
Published: July 2002

We propose a continuous-time formulation of the direct simulation Monte Carlo that allows the evaluation of the transport coefficient dependence on the time step through the use of the Green–Kubo theory. Our results indicate that the error exhibits quadratic dependence on the time step, and that for time steps of the order of one mean free time the error is of the order of 5%. Our predictions for the transport coefficients are in good agreement with numerical experiments. The calculation of the cell size dependence, first obtained by Alexander et al. [Phys. Fluids 10, 1540 (1998)], is reviewed and a correction is pointed out.
Simulation of physical vapor deposition into trenches and vias: Validation and comparison with experiment
Authors: O’Sullivan Peter L. , Baumann Frieder H. , Gilmer George H.
Published: July 2002

We have performed two-dimensional (2D) and three-dimensional (3D) (axisymmetric) numerical simulations of physical vapor deposition into high aspect ratio trenches and vias used for modern very large-scale integration interconnects. The topographic evolution is modeled using (continuum) level set methods. The level set approach is a powerful computational technique for accurately tracking moving interfaces or boundaries, where the advancing front is embedded as the zero level set (isosurface) of a higher dimensional mathematical function. We have validated both codes against analytic formulas for step coverage. First, we study the 2D case of long rectangular trenches including 3D out-of-plane target flux. The 3D flux can be obtained from molecular dynamics computations, and hence our approach represents a hybrid atomistic/continuum model. Second, we report results of axisymmetric 3D simulations of high aspect ratio vias, which we compare with experimental data for Ti/TiN barrier layers. We find that the simulations (using a cosine angular distribution for the flux from the target) overpredict bottom coverage in some cases by approximately 20%–30% for both collimated and uncollimated deposition, but in other cases provide a reasonably accurate comparison with experiment.
Transient and asymptotic growth of two-dimensional perturbations in viscous compressible shear flow
Authors: Farrell B. F. , Ioannou P. J.
Published: July 2002

A comprehensive assessment is made of transient and asymptotic two-dimensional perturbation growth in compressible shear flow using unbounded constant shear and the Couette problem as examples. The unbounded shear flow example captures the essential dynamics of the rapid transient growth processes at high Mach numbers, while excitation by nonmodal mechanisms of nearly neutral modes supported by boundaries in the Couette problem is found to be important in sustaining high perturbation amplitude at long times. The optimal growth of two-dimensional perturbations in viscous high Mach number flows in both unbounded shear flow and the Couette problem is shown to greatly exceed the optimal growth obtained in incompressible flows at the same Reynolds number.
Conductive layer near the GaN/sapphire interface and its effect on electron transport in unintentionally doped n-type GaN epilayers
Authors: Cheong M. G. , Kim K. S. , Oh C. S. , Namgung N. W. , Yang G. M. , Hong C.-H. , Lim K. Y. , Suh E.-K. , Nahm K. S. , Lee H. J. , Lim D. H. , Yoshikawa A.
Published: July 2002

Temperature-dependent Hall effect measurements on unintentionally doped n-type GaN epilayers show that, above room temperature, the Hall-mobility values of different samples vary parallel with each other with temperature. We demonstrate that this anomaly is mainly due to a conductive layer near the GaN/sapphire interface for thin samples with low carrier density. Through trapping electrons, threading edge dislocations (TEDs) debilitate the epilayer contribution in a two-layer mixed conduction model involving the epilayer and the near-interface layer. The trapping may, in part, explain low mobility and anomalous transport in pure GaN layers. Scattering by TEDs is important only at low temperatures.
Generating positrons with femtosecond-laser pulses
Authors: Gahn C. , Tsakiris G. D. , Pretzler G. , Witte K. J. , Delfin C. , Wahlström C.-G. , Habs D.
Published: July 2002

Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2×108 Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy.
The Sinai billiard, square torus, and field chaos
Authors: Liboff Richard L. , Liu Jack
Published: July 2002

An experiment is reported in which the Sinai quantum billiard and square-torus quantum billiard are compared for field chaos. In this mode of chaos, electromagnetic fields in a waveguide are analogous to the wave function. It is found that power loss in the square-torus guide exceeds that in the Sinai-billiard guide by approximately 3.5 dB, thereby illustrating larger field chaos for the square-torus quantum billiard than for the Sinai quantum billiard. Solutions of the Helmholtz equation are derived for the rectangular coaxial guide that illustrate that transverse electric or transverse magnetic modes exist in the guide provided the ratio of edge lengths of the outer rectangle to parallel edge lengths of the inner rectangle is rational. Eigenfunctions partition into four sets depending on even or odd reflection properties about Cartesian axis on which the concentric rectangles are oriented. These eigenfunctions are uniquely determined by four coaxial parameters and two eigen numbers. Justification of experimental findings is based on the argument that the rationals comprise a set of measure zero with respect to the irrationals. Consequently, from an observational point of view, these modes do not exist, which is in accord with the reported experiment.
Development of plasma chemical vaporization machining
Authors: Mori Yuzo , Yamauchi Kazuto , Yamamura Kazuya , Sano Yasuhisa
Published: July 2002

Conventional machining processes, such as turning, grinding, or lapping are still applied for many materials including functional ones. But those processes are accompanied with the formation of a deformed layer, so that machined surfaces cannot perform their original functions. In order to avoid such points, plasma chemical vaporization machining (CVM) has been developed. Plasma CVM is a chemical machining method using neutral radicals, which are generated by the atmospheric pressure plasma. By using a rotary electrode for generation of plasma, a high density of neutral radicals was formed, and we succeeded in obtaining high removal rate of several microns to several hundred microns per minute for various functional materials such as fused silica, single crystal silicon, molybdenum, tungsten, silicon carbide, and diamond. Especially, a high removal rate equal to lapping in the mechanical machining of fused silica and silicon was realized. 1.4 nm (p–v) was obtained as a surface roughness in the case of machining a silicon wafer. The defect density of a silicon wafer surface polished by various machining method was evaluated by the surface photo voltage spectroscopy. As a result, the defect density of the surface machined by plasma CVM was under 1/100 in comparison with the surface machined by mechanical polishing and argon ion sputtering, and very low defect density which was equivalent to the chemical etched surface was realized. A numerically controlled CVM machine for x-ray mirror fabrication is detailed in the accompanying article in this issue.
Single particle dynamics in fluid and solid hydrogen sulphide: An inelastic neutron scattering study
Authors: Andreani C. , Degiorgi E. , Senesi R. , Cilloco F. , Colognesi D. , Mayers J. , Nardone M. , Pace E.
Published: July 2002

Inelastic neutron scattering experiments were performed at intermediate and high momentum transfer, up to 88.2 Å−1, to study the temperature dependence of single hydrogen mean kinetic energy in polycrystalline and liquid hydrogen sulphide (H2S), in the temperature range 16–206 K. Values of the hydrogen mean kinetic energy were extracted, within the impulse approximation, by fitting to the high momentum transfer data a model response function, obtained from a momentum distribution which is the orientational average of a multivariate Gaussian function. The extracted kinetic energies are compared with a harmonic model for the vibrational and roto-translational dynamics. The model makes use of the hydrogen-projected density of states worked out from intermediate momentum transfer data, as well as of optical frequencies determined from Raman and infrared (IR) spectroscopy. A fairly good agreement is obtained in the whole temperature range, while noticeably lower values for the kinetic energy are found if a single atom momentum distribution of isotropic Gaussian shape is assumed in the model response function.
Very-low-operating-voltage organic light-emitting diodes using a p-doped amorphous hole injection layer
Authors: Zhou X. , Pfeiffer M. , Blochwitz J. , Werner A. , Nollau A. , Fritz T. , Leo K.
Published: July 2002

We demonstrate the use of a p-doped amorphous starburst amine, 4, 4′, 4″-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m2 even for a comparatively large (110 nm) total hole transport layer thickness.
A comparative study of near-wall turbulence in high and low Reynolds number boundary layers
Authors: Metzger M. M. , Klewicki J. C.
Published: July 2002

The present study explores the effects of Reynolds number, over three orders of magnitude, in the viscous wall region of a turbulent boundary layer. Complementary experiments were conducted both in the boundary layer wind tunnel at the University of Utah and in the atmospheric surface layer which flows over the salt flats of the Great Salt Lake Desert in western Utah. The Reynolds numbers, based on momentum deficit thickness, of the two flows were Rθ=2×103 and Rθ≈5×106, respectively. High-resolution velocity measurements were obtained from a five-element vertical rake of hot-wires spanning the buffer region. In both the low and high Rθ flows, the length of the hot-wires measured less than 6 viscous units. To facilitate reliable comparisons, both the laboratory and field experiments employed the same instrumentation and procedures. Data indicate that, even in the immediate vicinity of the surface, strong influences from low-frequency motions at high Rθ produce noticeable Reynolds number differences in the streamwise velocity and velocity gradient statistics. In particular, the peak value in the root mean square streamwise velocity profile, when normalized by viscous scales, was found to exhibit a logarithmic dependence on Reynolds number. The mean streamwise velocity profile, on the other hand, appears to be essentially independent of Reynolds number. Spectra and spatial correlation data suggest that low-frequency motions at high Reynolds number engender intensified local convection velocities which affect the structure of both the velocity and velocity gradient fields. Implications for turbulent production mechanisms and coherent motions in the buffer layer are discussed.
Low-resistance Ti/Al/Ti/Au multilayer ohmic contact to n-GaN
Authors: Wang Dong-Feng , Shiwei Feng , Lu C. , Motayed Abhishek , Jah Muzar , Mohammad S. Noor , Jones Kenneth A. , Salamanca-Riba L.
Published: July 2002

A metallization scheme has been developed for obtaining low ohmic contacts to n-GaN with a low contact resistance. The metal contact is a Ti/Al/Ti/Au composite with layers that are respectively 30, 100, 30, and 30 nm thick. Contacts with a specific contact resistivity ρs, as low as 6.0×10−7 Ω cm2 for a doping level of 1.40×1020 cm−3 were obtained after annealing the sample for 30 s at 750 °C in a rapid thermal annealer. The Ti placed on top of the traditional Ti/Al contact appears to have the advantage of tying up the excess Al; therefore it does not form a mottled contact. Some of the additional Ti–Al intermetallic alloys that are formed also have beneficial effects. The Ti–Au layer forms a robust upper portion of the composite, which enables the contacts to have high-temperature applications.
Dynamic behaviors of dust particles in the plasma–sheath boundary
Authors: Takamura S. , Misawa T. , Ohno N. , Nunomura S. , Sawai M. , Asano K. , Kaw P. K.
Published: July 2002

A variety of dynamic behaviors in dusty plasmas is expected under the experimental condition of weak friction with gas molecules. The device “KAGEROU” provides such an environment for dynamic collective phenomena. Self-excited dust oscillations in Coulomb crystals have been observed at low values of plasma density and gas pressure. An instability mechanism was identified to be delayed charging in an inhomogeneous equilibrium dust charge in the sheath. The theoretical growth rate was formulated in relation to the destabilization of a transverse dust lattice wave (T-DLW), which was found to be very sensitive to the presence of a small amount of hot electrons which produces a substantial positive equilibrium charge gradient ∇Qd-eq around the equilibrium position of dust particles in the plasma–sheath boundary. The first experimental observation of a correlated self-excited vertical oscillations in a one-dimensional dust chain indicates a destabilization of T-DLW. The experimental condition is very consistent with the parameter area which predicts numerically an instability of T-DLW.
Two-photon absorption and optical power limiting of bifluorene molecule
Authors: Morel Yannick , Irimia Adriana , Najechalski Pawel , Kervella Yann , Stephan Olivier , Baldeck Patrice L. , Andraud Chantal
Published: July 2002

We have studied the nonlinear absorption spectra and optical limiting properties of 2,2′-(9,9-dihexyl) bifluorene. Measurements were done in chloroform solution, in the visible range (450–650 nm), and for nanosecond time-duration pulses. The two-photon absorption (TPA) spectrum, obtained by the up-conversion fluorescence method, shows a resonance at λ=534 nm with a cross-section σTPA=60 10−50 cm4⋅s/photon-molecule. Semiempirical quantum chemistry calculation on fluorene and bifluorene suggests an enhancement of the bifluorene TPA due to coupling effects between monomers. In nonlinear transmission measurements, two-photon absorption is reinforced by the excited-state absorption that occurs during the pulse duration. At resonance, the three-photon absorption coefficient is α3=14 000 cm3/GW2 for a bifluorene concentration of 600 g/L. This strong nonlinear absorption leads to an efficient optical power limiting in the green and blue parts of the spectrum. The maximum transmitted energy is lower than 10 μJ for an input energy of up to 200 μJ in a F/5 optical geometry.
Laser induced crystallization of amorphous Ge2Sb2Te5 films
Authors: Weidenhof V. , Friedrich I. , Ziegler S. , Wuttig M.
Published: July 2002

The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by atomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier, and the Avrami coefficient. On the minute time scale, thin amorphous films were isothermally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to be (2.0±0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature dependent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron size were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized. For the formation of crystalline spots in an as deposited amorphous film a minimum time of (100±10) ns was found, which is identified as the minimum incubation time for the formation of critical nuclei. In contrast, the complete crystallization of melt-quenched amorphous bits in a crystalline matrix was possible in 10 ns. This is attributed to the presence of quenched-in nuclei inside the amorphous bits. The combination of optical measurements with atomic force microscopy reveals the formation and growth of crystalline bits and shows that the crystal growth in vertical direction strongly affects the reflectivity changes.
Wormlike chains in the large-d limit
Authors: Hansen Per Lyngs , Podgornik Rudi
Published: July 2002

We study the properties of an isolated, self-interacting wormlike polymer chain on the basis of a nonperturbative 1/d-expansion, where d denotes the dimension of embedding space. In the absence of an external force, we characterize the dimension R of the chain in embedding space via R∼Lν, where L is the internal size. (A) Long-range, repulsive segmental interactions decaying as 1/rα may control chain conformations that are either rodlike, ν=1(1<α<2), “wrinkled,” 1/2<ν<1(2<α<4), or random-walk-like, ν=1/2(α>4). (B) For short-range, screened, repulsive interactions, the crossover between rodlike and random-walk-like behavior is controlled by the persistence length whose interaction part we compute focusing on a Debye–Hückel interaction of strength V0, with inverse screening length κ0. The induced persistence length varies as V0βκ0−γ, with, as expected, (β,γ)=(1,2) when the chain is intrinsically stiff, and, surprisingly, with either (β,γ)=(1/6,7/6) or (β,γ)=(1,7) when the chain is intrinsically very flexible. The chances of experimentally observing the novel regimes may be limited. For a chain subject to an external stretching force f, we determine the force-extension relation ζ=ζ(f )=ζ0+δζ(f ), where ζ denotes the chain extension, ζ0 is the spontaneous extension. (A) If the interaction potential is either screened, or if the decay of a long-range interaction potential is fast, i.e., if α>4, the chain spontaneously generates an “effective tension” and responds linearly to weak forces with elastic constants “renormalized” by interactions. By contrast, “tension-free” chains, with either ν=1, where δζ∼f1/2, or with ν=2/α, where δζ∼f1/3, respond to the weakest force nonlinearly. (B) Near full extension the chain always responds nonlinearly. When the potential is screened, or if α>4, we find the 1/f corrections typical of wormlike chains.
Direct numerical simulation of decaying compressible turbulence and shocklet statistics
Authors: Samtaney Ravi , Pullin D. I. , Kosović Branko
Published: July 2002

We present results from 1283 and 2563 direct numerical simulations (DNS) of decaying compressible, isotropic turbulence at fluctuation Mach numbers of Mt∼0.1–0.5 and at Taylor Reynolds numbers Reλ=O(50–100). The presence or absence of fluctuations of thermodynamic quantities as well as velocity divergence in the initial conditions are found to have a negligible effect on the decay of turbulent kinetic energy. The decay of the turbulent kinetic energy shows no significant effect of Mt and power laws fitted to the timewise decay exhibit exponents n=1.3–1.7 that are similar to those found for decaying incompressible turbulence. The main new phenomenon produced by compressibility is the appearance of random shocklets which form during the main part of the decay. An algorithm is developed to extract and quantify the shocklet statistics from the DNS fields. A model for the probability density function (PDF) of the shocklet strength Mn−1 (Mn is the normal shock Mach number) is derived based on combining weak-shock theory with a model of the PDF of longitudinal velocity differences in the turbulence. This shows reasonable agreement with PDFs obtained from the shocklet extraction algorithm. The model predicts that at moderate Mt the most probable shocklet strength is proportional to Mt/Reλ1/2 and also that the PDF for the shock thicknesses has an inverse cubic tail. The shock thickness statistics are found to scale on the Kolmogorov length rather than the mean free path in the gas.
Resonance frequency and Q factor mapping by ultrasonic atomic force microscopy
Authors: Yamanaka Kazushi , Maruyama Yoshiki , Tsuji Toshihiro , Nakamoto Keiichi
Published: July 2002

We developed an improved ultrasonic atomic force microscopy (UAFM) for mapping resonance frequency and Q factor of a cantilever where the tip is in linear contact with the sample. Since the vibration amplitude at resonance is linearly proportional to the Q factor, the resonance frequency and Q factor are measured in the resonance tracking mode by scanning the sample in the constant force mode. This method enables much faster mapping of the resonance frequency and Q factor than the previous one using a network analyzer. In this letter, we describe the principle and instrumentation of the UAFM and show images of carbon-fiber-reinforced plastic composites.