bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural properties of hydrogenated carbon-nitride films produced by ion-beam-assisted evaporation of the molecular precursor C4N6H4
Authors: Hammer P. , Lacerda R. G. , Valente G. M. S. , Droppa R. , dos Santos M. C. , Alvarez F. , Zanatta A. R.
Published: July 2002

Hydrogenated carbon-nitride films (CNx:H) were deposited by evaporation of aza-adenine (8-aza-6-aminopurine, C4N6H4) and irradiated during film growth by a nitrogen ion beam with energies of 25, 50, and 100 eV. The relationship between the deposition conditions and the chemical bonding structure was investigated by x-ray photoelectron and infrared spectroscopies. The mass density of the films was estimated from the C 1s plasmon energy and the optical properties were examined by ultraviolet–visible spectroscopy. The infrared spectra indicate that in the case of evaporated films, the molecular structure of aza-adenine is preserved without substantial nitrogen loss (N/C≈1.3). With increasing energy of the assisting nitrogen ion beam, the structure is subsequently transformed into an amorphous phase. This transition is accompanied by an increasing nitrogen loss down to N/C≈0.8, a decrease of the optical band gap, and an increase of the film density. The known molecular structure of aza-adenine permits the reconstruction of the C 1s and N 1s core-level spectra with deconvoluted spectral components representing the different bonding environments of C and N atoms present in the molecule. The spectral components are identified by comparing the experimental data with theoretical calculations of the corresponding binding energies. By analysis of the spectral changes induced by the nitrogen ion beam with increasing energies, it is possible to study the microstructural modifications of the material.
300 K operation of a GaAs-based quantum-cascade laser at λ≈9 μm
Authors: Page H. , Becker C. , Robertson A. , Glastre G. , Ortiz V. , Sirtori C.
Published: July 2002

The room-temperature (300 K), pulsed mode operation of a GaAs-based quantum-cascade laser is presented. This has been achieved by the use of a GaAs/Al0.45Ga0.55As heterostructure which offers the maximum Γ–Γ band offset (390 meV) for this material system without inducing the presence of indirect barrier states. Thus, better electron confinement is achieved, countering the loss of injection efficiency with temperature. These devices show ∼100 K increase in operating temperature with respect to equivalent designs using an GaAs/Al0.33Ga0.67As heterostructure. We also measure 600 mW peak power at 233 K a temperature readily accessible by Peltier coolers.
Trench formation around and between self-assembled Ge islands on Si
Authors: Denker Ulrich , Schmidt Oliver G. , Jin-Philipp Neng-Yun , Eberl Karl
Published: July 2002

Investigations on near-surface diffusion during the formation of self-assembled Ge islands on Si(001) are presented. We measure the detailed shape of trenches around islands that are formed due to short range, strain enhanced diffusion. It is found that these trenches have anisotropic shape which we explain in terms of the intrinsic anisotropy of the elastic properties for the Si crystal. At high growth temperatures, long-range depletion of the substrate and trench formation between neighboring islands due to strong in-diffusion of Si into the nominally pure Ge islands is observed. A simple diffusion model which predicts trench depths as a function of island distance fits well to our experimentally observed data. Calculated diffusion lengths from this model are comparable to the average island distance on the surface.
Nanostructure of thin amorphous hydrogenated carbon films studied by positron annihilation and photoluminescence
Authors: Wang C. L. , Kobayashi Y. , Katoh R. , Suzuki R. , Ohdaira T.
Published: July 2002

We deposited polymer-like a-C:H films using the plasma-enhanced chemical vapor deposition technique and characterized film microstructure by variable-energy positron lifetime spectroscopy, photoluminescence (PL), and UV-visible absorption spectroscopy. It was confirmed that PL occurs from a chromophore in a sp2 cluster as a result of fast recombination of a photoexcited electron-hole pair. Positron annihilation lifetime spectroscopy showed that positronium (Ps) formation takes place via electron-positron recombination in the sp3 matrix. The lifetime of ortho-positronium (o-Ps) in our a-C:H films was similar to that in polyethylene, indicating their polymer-like nature. The relative PL efficiency increased by about an order of magnitude with increasing film band gap from 1.3 to 3.4 eV, which can be related to the decreasing concentration of nonradiative centers. On the other hand, Ps formation was much less influenced by the band gap and nonradiative centers. Comparison of this result with that for polyethylene mixed with carbon-black nanoparticles, where a considerable reduction in Ps formation was observed, showed that nonradiative centers were of a different nature from the defects on the carbon nanoparticle surface. This work demonstrated the usefulness of positron lifetime spectroscopy combined with optical measurements to study the nanostructure of a-C:H.
Linear instability of Poiseuille–Couette–Ekman flows: Local results for flows between differentially rotating disks with throughflow
Authors: Hoffmann N. P. , Busse F. H.
Published: July 2002

The instabilities of incompressible flows between two parallel plates in relative motion in a system rotating about an axis perpendicular to the plates are analyzed in the case when a pressure gradient perpendicular to the plate motion is imposed. This system represents the local approximation to flows between differentially rotating disks with imposed throughflow. Relationships of the instabilities to those of the Ekman layer and of the Ekman–Couette layer are discussed.
Quantitative atom-resolved force gradient imaging using noncontact atomic force microscopy
Authors: Oral Ahmet , Grimble Ralph A. , Özer H. Özgür , Hoffmann Peter M. , Pethica John B.
Published: July 2002

Quantitative force gradient images are obtained using a sub-angström amplitude, off-resonance lever oscillation method during scanning tunneling microscopy imaging. We report the direct observation of short-range bonds, and the measured short-range force interaction agrees well in magnitude and length scale with theoretical predictions for single bonds. Atomic resolution is shown to be associated with the presence of a prominent short-range contribution to the total force interaction. It is shown that the background longer-range interaction, whose relative magnitude depends on the tip structure, has a significant effect on the contrast observed at the atomic scale.
Characterization of thin polymer films using terahertz time-domain interferometry
Authors: Krishnamurthy S. , Reiten M. T. , Harmon S. A. , Cheville R. A.
Published: July 2002

An interferometer for broadband single-cycle THz pulses is developed based on the Michelson configuration. Total internal reflection of THz pulses in high-resistivity silicon prisms provides a nearly 180° phase shift of one arm relative to the other to achieve destructive interference. We show that due to automatic compensation for laser fluctuations by the interferometer, it is possible to measure the index and absorption of thin-film samples with more accuracy than is achievable with standard THz time-domain spectroscopy. We demonstrate characterization of the complex index of refraction of 2 μm thick Mylar (polyester) films. By measuring the signal amplitude directly in the time domain, the interferometer can be used for rapid measurements of film thickness with a resolution of better than 1 μm.
Thomson scattering measurement of gold plasmas produced with 0.351 μm laser light
Authors: Bai Bo , Zheng Jian , Liu Wandong , Yu C. X. , Jiang Xiaohua , Yuan Xiaodong , Li Wenhong , Zheng Z. J.
Published: July 2002

Detailed measurements of gold plasmas produced with 0.351 μm laser light are reported. The temporal and spatial variations of Z̄Te are obtained in the case that pump intensities are 6.5×1013 and 1.4×1014 W/cm2. The experimental results are qualitatively consistent with two nonlocalized absorption models.
Determination of the electric dipole moment and excited state Fermi contact parameter of the GeCH radical
Authors: Smith Tony C. , Clouthier Dennis J. , Steimle Timothy C.
Published: July 2002

The germanium methylidyne (GeCH) radical has been studied at high resolution by laser-induced fluorescence spectroscopy in a skimmed molecular beam. The radical was produced in a pulsed discharge jet source using methyltrichlorogermane as the precursor. Single rotational lines of the Ã 2Σ+–X̃ 2Π3/2 component of the 000 band at 14 070 cm−1 were recorded with linewidths of 35 MHz, showing resolved proton magnetic hyperfine splittings. The observed hyperfine splittings of 12 rotational lines were satisfactorily modeled by varying a single upper state Fermi contact parameter, giving bF=163(2) MHz. Optical Stark measurements were performed on the GeCH74 P11(3/2)F′=1 hyperfine transition and the permanent electric dipole moments determined to be μ(X̃ 2Π)=0.122(2) D and μ(Ã 2Σ+)=1.29(2) D. Our ab initio predictions of the dipole moments and excited state Fermi contact parameter are in fair agreement with experiment. The magnitudes of the determined parameters are discussed based on the molecular orbitals and compared to similar measurements on SiCH.
Magnetic patterning of chemically-ordered CrPt3 films
Authors: Hellwig Olav , Weller Dieter , Kellock A. J. , Baglin J. E. E. , Fullerton Eric E.
Published: July 2002

We report magnetic patterning of ferrimagnetic CrPt3 films by locally suppressing the magnetic order using ion-beam irradiation. Chemically-ordered CrPt3 films grown on MgO(110) single-crystal and amorphous SiNx substrates are ferrimagnetic and exhibit strain-induced uniaxial magnetic anisotropy. X-ray diffraction confirms the formation of the L12 phase and magneto-optical Kerr effect spectroscopy reveals the theoretically expected dependence of Kerr rotation and ellipticity on photon energy. Irradiation of the films with 700 keV N+ ions at doses ≳6×1016 ions/cm2 transforms the chemically-ordered CrPt3 ferrimagnetic alloy into the chemically-disordered nonmagnetic fcc phase. As a result, ordered CrPt3 layers can be patterned into magnetic and nonmagnetic regions and are candidate material for ion-beam-patterned magnetic recording media.
Electrical properties of SrTiO3 thin films on Si deposited by magnetron sputtering at low temperature
Authors: Wang Zhongchun , Kugler Veronika , Helmersson Ulf , Konofaos N. , Evangelou E. K. , Nakao Setsuo , Jin Ping
Published: July 2002

Deposition of SrTiO3 (STO) thin films by radio-frequency magnetron sputtering in an ultrahigh vacuum system at a low substrate temperature (∼200 °C) was performed in order to produce high-quality STO/p-Si (100) interfaces and STO insulator layers with dielectric constants of high magnitude. The STO films were identified as polycrystalline by x-ray diffraction, and were approximated with a layered structure according to the best fitting results of raw data from both Rutherford backscattering spectroscopy and variable angle spectroscopic ellipsometry. Room-temperature current–voltage and capacitance–voltage (C–V) measurements on Al/STO/p-Si diodes clearly revealed metal–insulator–semiconductor behavior, and the STO/p-Si interface state densities were of the order of 1011 eV−1 cm−2. The dielectric constant of the STO film was 65, and the dielectric loss factor varied between 0.05 and 0.55 for a frequency range of 1 kHz–10 MHz. For a 387 nm thick STO film, the dielectric breakdown field was 0.31 MV cm−1, and the charge storage capacity was 2.1 μC cm−2. These results indicate that STO films are suitable for applications as insulator layers in dynamic random access memories or as cladding layers in electroluminescent devices.
Particle-in-cell simulations of electron flow in the post-hole convolute of the Z accelerator
Authors: Pointon T. D. , Stygar W. A. , Spielman R. B. , Ives H. C. , Struve K. W.
Published: July 2002

The three-dimensional, particle-in-cell code QUICKSILVER [J. P. Quintenz et al., Lasers Part. Beams 12, 283 (1994)] is now being used to simulate the inner region of the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The simulations model electron flow and anode losses in the double post-hole convolute, which couples four radial, magnetically insulated transmission lines (MITLs) in parallel to a single MITL that drives a Z-pinch load. To efficiently handle the large range in the magnetic field, 0<B<200 T, the particle pusher is modified to subcycle the electron advance relative to the field solver. Results from a series of simulations using a constant-impedance load are presented. The locations of electron losses to the anode in the convolute are in qualitative agreement with damage to the Z hardware. The electron energy deposited in these anode regions rapidly heats the surface to temperatures above 400 °C—the threshold at which anode plasma formation is expected.
Photoelectron spectroscopy of chlorine dioxide and its negative ion: A quantum dynamical study
Authors: Mahapatra Susanta , Krishnan Gireesh M.
Published: July 2002

The photoelectron spectra of ClO2 and its negative ion are investigated theoretically by a time-dependent wave-packet method. The near equilibrium MRCI potential energy surfaces of Peterson and Werner [J. Chem. Phys. 99, 302 (1993)] are employed in the nuclear dynamical simulations. The theoretical findings are in good agreement with the experimental results. In the experimental recording, excitations along the symmetric stretching and bending vibrational modes of ClO2 were observed. The excitation along the asymmetric stretching vibrational mode is absent in the experimental results. Considering these observations, and utilizing the available electronic structure results, we in our dynamical study focused on the C2v nuclear arrangements of the system. The relevant intial wave function to describe the photoelectron transition is prepared in both ways by the Hamiltonian matrix diagonalization using the ab initio potential energy surface of the ground electronic state, as well as in terms of the dimensionless normal coordinates of the electronic ground state of ClO2. The stick vibronic spectra are calculated by solving the time-independent Schrödinger equation employing a basis set expansion approach and the Lanczos algorithm. The resulting vibrational eigenvalues are compared with the experimental results and are discussed. The inclusion of the asymmetric stretching vibration and the possible role of the nonadiabatic couplings in the nuclear dynamics are also emphasized.
Quantum-dot infrared photodetector with lateral carrier transport
Authors: Chu L. , Zrenner A. , Bichler M. , Abstreiter G.
Published: July 2002

In this letter, we present a normal-incident quantum-dot infrared photodetector. The detection principle is based on intersubband transition between the p states and the wetting-layer subband in the conduction band of self-assembled In(Ga)As/GaAs quantum dots. Carrier transport takes place in a channel next to the quantum-dot layers. The photoresponse is peaked at λ=6.65 μm (186 meV) and reaches a maximum value of over 11 A/W at T=30 K with a wavelength resolution of 12%. Detector response time τ is determined to be about 0.8 ms. Temperature and frequency dependence of the detector structure are discussed.
High-quality Ge films on Si substrates using Sb surfactant-mediated graded SiGe buffers
Authors: Liu J. L. , Tong S. , Luo Y. H. , Wan J. , Wang K. L.
Published: July 2002

High-quality Ge films were grown on Si substrates by solid-source molecular beam epitaxy using SiGe graded layer and Sb surfactant-mediation technique. Transmission electron microscopy measurements show that samples grown using this method have a lower threading dislocation density than those grown by other typical methods, such as grading at high temperature (700 °C) only, grading at intermediate temperature (510 °C) only, and the use of low temperature Si buffer. A relaxed Ge film on a 4-μm-thick graded buffer was grown and shown to have a threading dislocation density of 5.4×105 cm−2 and surface roughness of 35 Å. Ge p–i–n diodes were fabricated and tested. Under a reverse bias of 1 V, the p–i–n Ge mesa photodiodes exhibit a very low dark current density of 0.15 mA/cm2.
Tunnel-magnetoresistance system with an amorphous detection layer
Authors: Käufler A. , Luo Y. , Samwer K. , Gieres G. , Vieth M. , Wecker J.
Published: July 2002

We have studied magnetic tunnel junctions using an amorphous CoFeNiSiB soft magnetic layer. At room temperature a tunnel-magnetoresistance ratio of 12% was achieved with a switching field of the amorphous layer of 12 Oe. The effect could be enhanced up to 22% by annealing. Investigations on single amorphous layers show a thermal stability up to 350 °C annealing temperature even for very thin layers. Due to the special preparation technique, an in-plane anisotropy is induced resulting from an oblique-incidence effect which presumably affects the short-range order of the amorphous phase.
Universal field-emission model for carbon nanotubes on a metal tip
Authors: Zhong D. Y. , Zhang G. Y. , Liu S. , Sakurai T. , Wang E. G.
Published: July 2002

Electron-field-emission properties have been investigated systematically for carbon nanotubes (CNTs) fabricated on a metal tip. With a vacuum gap of 0.7 mm, the threshold field is as low as 0.7 V/μm and the current density approaches 10 mA/cm2 at an electronic field of 1.0 V/μm. The emission current is quite stable with very low fluctuation. The emission behavior is in excellent agreement with Fowler–Nordheim theory and no current saturation is found even with an emission current reaching 1 A/cm2. A universal relationship 1/β=d2/d+1/β0 between the field amplification factor β and the vacuum gap d is developed within a two-region field-emission model. This relationship provides the basis for a microscopic understanding of CNT emitters and is applicable to other systems as well.
Large size dependence of exciton-longitudinal-optical-phonon coupling in nitride-based quantum wells and quantum boxes
Authors: Kalliakos S. , Zhang X. B. , Taliercio T. , Lefebvre P. , Gil B. , Grandjean N. , Damilano B. , Massies J.
Published: July 2002

We present an experimental and theoretical study of the size dependence of the coupling between electron–hole pairs and longitudinal-optical phonons in Ga1−xInxN/GaN-based quantum wells and quantum boxes. We found that the Huang–Rhys factor S, which determines the distribution of luminescence intensities between the phonon replicas and the zero-phonon peak, increases significantly when the vertical size of the boxes or the thickness of quantum well increases. We assign this variation to (1) the strong electric field present along the growth axis of the system, due to spontaneous and piezoelectric polarizations in these wurtzite materials, and (2) the localization on separate sites of electrons and holes in the plane of the wells or boxes, due to potential fluctuations in the ternary alloy. Indeed, envelope-function calculations for free or localized excitons, with electron–hole distance only controlled by Coulomb interaction, do not account quantitatively for the measured behavior of the S factor. In fact, the latter is rather similar to what is obtained for donor–acceptor pairs, with a statistical distribution of distances between localization centers for electrons and holes.
A diagnosis of linear eddy-viscosity in turbulence modeling
Authors: Su W.-D. , Zhao H. , Cai Q.-D. , Xiong A.-K. , Wu J.-Z.
Published: July 2002

A general diagnosis is introduced to examine whether and under what condition any Reynolds-averaged Navier–Stokes or large-eddy simulation using models based on linear eddy-viscosity νt is mathematically compatible with the incompressible turbulent flows to be modeled. It is shown that νt is governed by two independent second-order linear partial differential equations with the spatial derivatives of mean or filtered velocity as coefficients. In particular, for two-dimensional mean or filtered flows the νt equation is globally hyperbolic except in some degenerated cases. Numerical examples show that when the hyperbolic equation is incompatible with the boundary condition for νt, the characteristics of the same family may self-intersect to form shocks with finite jumps of νt, caused by the strong dissipation gradient near strong shear layers and vortices. Since in any modeled flow the νt field is globally continuous, once the “νt-shocks” occur the modeling must distort the real flow by weakening or even eliminating some vortices. It is further argued that constitutive-type models should contain the mean or filtered vorticity tensor and hence be nonlinear.
Fractional dynamics approach to diffusion-assisted reactions in disordered media
Authors: Sung Jaeyoung , Barkai Eli , Silbey Robert J. , Lee Sangyoub
Published: July 2002

We present a theory for describing nonclassical dynamics of reactions occurring in disordered media based on the fractional diffusion equation. An exact expression is derived for the Green’s function required to calculate the survival probabilities of reactants. A novel temperature-dependent kinetic phase transition is found: The exponent γ in the asymptotic power-law decay (∝t−γ) of the geminate survival probability increases with temperature T below a critical temperature T*, but decreases with T above T*. The present theory explains in a unified manner the observed features of ligand-protein recombination reactions for a wide range of temperature and time scales.
A study on the drop formation of continuous liquid jet by an electrical method
Authors: Mutoh Masayuki
Published: July 2002

Acceptable satellite conditions and allowable switching regions of the charging control signal in continuous inkjet printing are studied. The charging control signal can be switched within the phase interval corresponding to the satellite interaction time defined by Pimbley and Lee except for the forbidden region of the satellite droplet in rear-merging satellites, and within the phase interval except for the forbidden region defined by Hertz and Samuelsson in forward-merging and nonsatellites. An electrical method for the investigation of the drop formation using a phase search pulse is presented. It is confirmed that the phases of drop formations and the forbidden regions predicted by the electrical measurement well coincide with the result of optical observation.
Tunability of electron spin coherence in III–V quantum wells
Authors: Lau Wayne H. , Flatté Michael E.
Published: July 2002

We have calculated both T1 and T2 for (110)-oriented GaAs/AlGaAs quantum wells near room temperature. The altered symmetry of (110)-oriented quantum wells leads to an increase in calculated spin coherence times (T1) compared to those of similar (100)-oriented quantum wells, exceeding 1 ns at room temperature. We have also studied the electron spin coherence times as a function of applied electric field in (100)- and (110)-oriented GaAs/AlGaAs quantum wells. T1 is considerably more responsive to the growth-direction electric field in (110)-oriented quantum wells than in (100)-oriented quantum wells, whereas the response of T2 is similar for both growth directions.
Reduction method for low-frequency noise of GaAs junction field-effect transistor at a cryogenic temperature
Authors: Fujiwara M. , Sasaki M. , Akiba M.
Published: July 2002

A GaAs junction field-effect transistor (JFET) is a promising candidate for low-frequency, low-noise, and low-power cryogenic electronics to read out high-impedance photodetectors. We report on the spectral noise characteristics of a SONY n-type GaAs JFET, operating at the depression mode, at a cryogenic temperature of 4.2 K. If the GaAs JFET is turned on at 4.2 K, a random telegraph signal (RTS) is found to be the dominant noise source at low frequencies. However, the switching rate of RTS can be drastically reduced if the GaAs JFET is heated up to 55 K and cooled down again to 4.2 K while keeping the same drain current flow. We refer to this phenomenon as the thermal cure (TC). With TC, low-frequency noise can be reduced to below 1 μV/Hz1/2 at 1 Hz. The critical temperature for TC is found to be ∼35 K for our GaAs JFET.
Lateral color integration on rare-earth-doped GaN electroluminescent thin films
Authors: Lee D. S. , Steckl A. J.
Published: July 2002

Lateral color integration has been obtained using GaN thin films doped with Er and Eu. These rare-earth doped GaN (GaN:RE) films were grown on Si (111) substrates by molecular beam epitaxy. Independent red and green emissions have been obtained from side-by-side Er and Eu electroluminescent devices. Photoluminescence and electroluminescence operation show green emissions at 537 and 558 nm from Er-doped GaN and red emission at 621 nm from Eu-doped GaN. Two patterning fabrication techniques have been investigated to obtain lateral integration: (a) use of shadow masks during 400 °C growth of GaN:RE films; (b) photoresist liftoff in conjunction with <100 °C GaN:RE growth. Devices fabricated by the shadow mask method were bright enough to be detected under the ambient light at a bias of 30 V. The GaN:RE films were clear and their surfaces were smooth with nanoscale GaN grains. The root mean square surface roughness was measured to be 5–10 nm.
Thin-film electroluminescent devices grown on plastic substrates using an amorphous AlN:Tb3+ phosphor
Authors: Richardson H. H. , Van Patten P. G. , Richardson D. R. , Kordesch M. E.
Published: July 2002

An alternating current thin-film electroluminescent device has been constructed on a flexible polymer substrate using an amorphous AlN:Tb3+ film as the phosphor. When the device is operated at 170 Vac and at a frequency of 1 kHz, a stripe of green light can be seen emanating from the 0.6 mm×14 mm active area. The electroluminescence spectrum from the device is very similar to the cathodoluminescence and photoluminescence spectra from the amorphous AlN:Tb3+ phosphor. Luminance for this phosphor on plastic substrates is about 1 cd/m2 under optimal conditions. Time-resolved photoluminescence measurements were used to gain insight into the nature of the Tb3+ excited states. The relatively slow (τ=850 μs), single exponential decay of the excited state suggests that quenching via excited state energy migration to trap sites in the semiconductor is negligible.
Ab initio studies of ClOx reactions. I. Kinetics and mechanism for the OH+ClO reaction
Authors: Zhu R. S. , Xu Z. F. , Lin M. C.
Published: July 2002

The reaction of OH with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces predicted by the G2M method are presented. The reaction was shown to take place primarily over the singlet surface by two main channels producing HO2+Cl and HCl+O2(1Δ), with the former being dominant. The predicted total rate constant, kt=5.27×10−9 T1.03 exp (−40/T) cm3 molecule−1 s−1, and product branching ratios in the temperature range 200–500 K at P<200 atm agree satisfactorily with experimental values. The computed branching ratios, k2/(k1+k2)=0.073 for HCl+1O2 and 0.045–0.048 for DCl+1O2 in the temperature range 200–500 K based on the recent experimental heat of formation for HO2 (4.0±0.8 kcal/mol) compare closely with the experimental values, 0.07±0.03 and 0.05±0.02, respectively. At higher temperatures (1000–2500 K), the branching ratios increase slightly to 0.084–0.137 and 0.061–0.111 for the OH and OD reactions, respectively. The rate constant for HO2+Cl and HCl+O2 production from OH+ClO in the temperature range, 500−2500 K, can be given by k1=3.4×10−13 T0.3 exp (725/T) and k2=5.85×10−19 T1.67 exp (1926/T) cm3 molecule−1 s−1, respectively.
Low frequency noise of reverse biased rectifier diodes in the avalanche breakdown regime
Authors: Marinov Ognian , Deen M. Jamal , Loukanov V. , Velikov V.
Published: July 2002

A detailed investigation of the low frequency noise in rectifier diodes in the breakdown regime showed that the noise is a nonmonotonic function of the reverse current. A dynamic “competition” between impact ionization and microplasma switching explains the nonmonotonic, repetitive, and correlated variations in the breakdown slope (change in voltage per unit change in current—dV/dI), the noise level, and the noise waveform. These variations are due to the bistable microplasma fluctuation. The theories for avalanche charge multiplication and microplasma fluctuation should be combined with the microplasma switching theory to develop a physical model for the avalanche breakdown.
Device for the determination of spring constants of atomic force microscope cantilevers and micromachined springs
Authors: Jericho S. K. , Jericho M. H.
Published: July 2002

We describe a simple device for the calibration of atomic force microscope cantilevers and micromachined springs. The device measures the deflection of the cantilevers in response to known forces that are derived from calibrated macroscopic cantilever springs. We have used the device to determine spring constants from several N/m to tenths of μN/m. On V-shaped and single beam Si3N4 and Si cantilevers, spring constants could be determined to better than 10% with most of the uncertainty coming from uncertainties in the position of the load point.
Composition fluctuation in InGaN quantum wells made from molecular beam or metalorganic vapor phase epitaxial layers
Authors: Ruterana P. , Kret S. , Vivet A. , Maciejewski G. , Dluzewski P.
Published: July 2002

Using strain analysis on high resolution electron microscopy images and finite element modeling of InGaN quantum wells (QWs), it is shown that the In composition changes inside the layers can be accurately determined. The analyzed samples were nominally grown with 15%–17% In composition by molecular beam or metalorganic vapor phase epitaxy. Inside these QWs, the In composition is not homogeneous. Finite element modeling strongly suggests that the measured strain corresponds most probably to InN clusters whose size depends on the growth method.
High speed data link between digital superconductor chips
Authors: Herr Quentin P. , Smith Andrew D. , Wire Michael S.
Published: July 2002

Superconductor digital devices using single-flux-quantum (SFQ) data encoding offer higher speed at lower power than any other integrated circuit technology. For this very reason, interconnect is challenging. We report SFQ data transfer between chips flip chip mounted on a passive microstrip carrier. The flip-chip structure achieves bandwidths greater than 250 GHz. Unlike previous designs, our signal lines are terminated at both ends; this is accomplished using a driver that produces a double-flux-quantum signal. We measured the circuit for pseudorandom data in the range of 10–60 Gb/s. Bit error rates are measured down to 1E−10 and extrapolate to negligible values. The signal power on the microstrip is only 30 nW at 60 Gb ps.
Decoupling of the dc conductivity and (α-) structural relaxation time in a fragile glass-forming liquid under high pressure
Authors: Psurek Tatiana , Hensel-Bielowka Stella , Ziolo Jerzy , Paluch Marian
Published: July 2002

Two relaxation processes, involving dc conductivity and the glass (α) structural relaxation, respectively, were measured in the low-molecular-weight glass-forming liquid, bisphenol- A-propoxylate(1 PO/phenol)diglycidylether, by dielectric spectroscopy, as a function of temperature and pressure. We focused on the correlation between dc conductivity and the α-relaxation time, proving that the fractional Debye–Stokes–Einstein relation is valid for both isobaric and isothermal data. The value of the exponent was independent of both temperature and pressure, and related to the ratio of the apparent activation volume of the relaxation processes being considered. The shape of the dielectric loss α peak did not change with either temperature or pressure; i.e., frequency–temperature–pressure superposition was valid in the vicinity of the glass transition.
Raman characterization of boron carbonitride nanotubes
Authors: Zhi C. Y. , Bai X. D. , Wang E. G.
Published: July 2002

A systematic Raman study of boron carbonitride (BCN) nanotubes, synthesized by bias-assisted hot filament chemical vapor deposition, is reported. Raman spectra up to the fourth order are observed from the BCN nanotubes. Comparing with pure carbon nanotubes, the Raman bands in BCN nanotubes are broadened and the relative intensity of the D mode with respect to the G mode varies with increasing B and N atomic concentrations. The underlying mechanism has been studied on the basis of the microstructures obtained by high-resolution transmission electron microscopy.
Effect of ionic mobility on the enhanced dielectric and electro-optic susceptibility of suspensions: Theory and experiments
Authors: Arroyo F. J. , Delgado A. V. , Carrique F. , Jiménez M. L. , Bellini T. , Mantegazza F.
Published: July 2002

It is a well-known fact that the presence of charged dispersed solid particles in an electrolyte solution considerably modifies the dielectric permittivity and conductivity of the system as compared to that of the pure dispersing medium. The enhanced conductivity of the electrical double layer, and its polarization under the action of the external field are responsible for that fact. A related phenomenon, which is also a manifestation of large induced dipole moments, is the enhanced electric birefringence (Kerr effect), which measures the electric torque on charged nonspherical colloids. Measurements of the Kerr constant are significant because a direct relationship exists between electrically induced birefringence and the particle’s electric polarizability. In this work we analyze, from the experimental and theoretical points of view, the effects of coion and counterion mobility on the enhancement of both dielectric and Kerr constants: we show that, quite unexpectedly, the diffusion coefficient of coions has a large effect on both dielectric response and electric birefringence of the suspensions. To our knowledge, this effect had never been described before. Experimental data have been obtained on suspensions of various polymer particles, in different concentrations of NaCl and Na-salicylate: since the particles are anionic, this choice enables to assess the effects of the mobility of coions. We find that both the dielectric response and the Kerr effect are smaller (beyond experimental errors) in the presence of salicylate solutions. Experimental results and physical reasons for this behavior are discussed, and it is concluded that the classical theory of the low-frequency dielectric dispersion of colloidal systems provides a quantitative explanation for the coion effect on the dielectric constant. In the case of the Kerr effect, only qualitative arguments can be given in the low-frequency regime. In contrast, the high-frequency behavior is better justified in terms of a Maxwell–Wagner model.
Spin splitting of the electron ground states of InAs quantum dots
Authors: Medeiros-Ribeiro G. , Pinheiro M. V. B. , Pimentel V. L. , Marega E.
Published: July 2002

Here, we present results on the spin splitting of electrons in the ground state of InAs self-assembled quantum dots (QDs). The spin splitting is assessed by capacitance spectroscopy, which allows direct measurement of the electronic g-factor modulus. By applying the magnetic field parallel to the [001], [110], and [11̄0] crystallographic directions, we found that for magnetic fields below 5 T the g factor depends on the orientation, reflecting the QD anisotropy. For higher fields, the g factor does not exhibit the same degree of anisotropy, indicating a compression of the wave function inside the QD along the [001] direction, and consequently, a dependence on the magnitude of the applied field.
Transverse coupling of chemical waves
Authors: Gáspár Vilmos , Maselko Jerzy , Showalter Kenneth
Published: July 2002

The transverse coupling of chemical waves is investigated using a model scheme for excitable media. Chemical waves supported on the surfaces of a semipermeable membrane couple via diffusion through the membrane, resulting in new types of spatiotemporal behavior. The model studies show that spontaneous wave sources may develop from interacting planar waves, giving rise to a complex sequence of patterns accessible only by perturbation. Coupled circular waves result in the spontaneous formation of spiral waves, which subsequently develop patterns in distinct domains with characteristic features. The long time entrainment behavior of coupled spiral waves reveals regions of 1:2 phase locking.
Quantum features of Brownian motors and stochastic resonance
Authors: Reimann Peter , Hänggi Peter
Published: July 2002

We investigate quantum Brownian motion sustained transport in both, adiabatically rocked ratchet systems and quantum stochastic resonance (QSR). Above a characteristic crossover temperature T0 tunneling events are rare; yet they can considerably enhance the quantum-noise-driven particle current and the amplification of signal output in comparison to their classical counterparts. Below T0 tunneling prevails, thus yielding characteristic novel quantum transport phenomena. For example, upon approaching T=0 the quantum current in Brownian motors exhibits a tunneling-induced reversal, and tends to a finite limit, while the classical result approaches zero without such a change of sign. As a consequence, similar current inversions generated by quantum effects follow upon variation of the particle mass or of its friction coefficient. Likewise, in this latter regime of very low temperatures the tunneling dynamics becomes increasingly coherent, thus suppressing the semiclassically predicted QSR. Moreover, nonadiabatic driving may cause driving-induced coherences and quantized resonant transitions with no classical analog.
Elastic wave scattering by a surface-breaking or subsurface planar crack. II. Three-dimensional geometry
Authors: Visscher William M.
Published: July 2002

The theory of the dynamical behavior of surface cracks which has previously been presented is here applied to the computation of the scattering of elastic waves in 3D geometry from planar cracks of a variety of shapes perpendicular to the surface of an elastic half-space. Effects of crack size, shape, burial depth, and incoming wave frequency are illustrated. Appendices include discussions of reciprocity, simplifications in the long-wavelength limit, the determination of model parameters from surface-breaking criteria, results for in-plane scattering from surface cracks in a 2D geometry, and scattering from buried and from partly closed 2D cracks.
Characteristics of TiO2 films deposited by a reactive ionized cluster beam
Authors: Fukushima K. , Yamada I. , Takagi T.
Published: July 2002

Titanium oxides films of 150–400 nm thickness were deposited on glass and Si substrates by reactive ionized cluster beam deposition. Characteristics of films deposited at different deposition conditions of oxygen partial pressure, ionization current, and acceleration voltage have been examined. The films prepared at an oxygen pressure of 1–2×10−4 Torr showed stoichiometry of TiO2. The film is a mixture of anatase and rutile structures. By increasing the ionization current from 0 to 400 mA, the refractive index of the film could be increased from 2.0 to 2.6. By deposition at a high ionization current, the film showed rutile structure, whereas the anatase structure could be formed at a low ionization current. The optical absorption of the films is low when the films are deposited at a higher acceleration voltage.
A study of phosphorus in silicate glass with 31P nuclear magnetic resonance spectroscopy
Authors: Douglass D. C. , Duncan T. M. , Walker K. L. , Csencsits R.
Published: July 2002

In this study, 31P nuclear magnetic resonance spectra of phosphorus-doped silica have been analyzed to determine the concentration, chemical bonding and spatial distribution of phosphorus. The silica samples were prepared as performs for optical waveguide fibers. The two samples examined contained 1.12±0.02 and 0.06±0.02 wt. % phosphorus. Comparison with the profile of the refractive index shows that a nominal 1.0 wt. % phosphorus increases the refractive index 0.055% relative to pure fused silica. A single, well-defined 31P chemical shielding powder pattern is observed which implies that one species of phosphorus is dominant. Based on the 31P isotropic chemical shift and the magnitude and orientation of the shielding anisotropy, the phosphorus species is interpreted to be P(O) (O−)3. That is, the phosphorus is pentavalent and forms one terminal P=O bond and three bridging P-O− bonds. The distribution of phosphorus-phosphorus internuclear distances, determined from the 31P spin-echo data, indicates that phosphorus is randomly substituted into the silica network: there is no evidence of phosphate clusters.
Conditions for self-sustained pulsation and bistability in semiconductor lasers
Authors: Ueno Masayasu , Lang Roy
Published: July 2002

Conditions have been analyzed for the occurrence for self-sustained pulsations and bistabilities in semiconductor lasers having a saturable absorber region. It is shown that there are three crucial parameters, namely ratios of differential gain and carrier lifetime between amplifying region and absorbing region, and ratio of absorbing magnitude in absorbing region to the cavity loss. Ranges of these three parameters are determined in which bistabilities and self-sustained pulsations take place.
The structure of plasma-deposited silicon nitride films determined by infrared spectroscopy
Authors: Knolle W. R. , Osenbach J. W.
Published: July 2002

Plasma-deposited silicon nitride, a-SiN:H, has been deposited at low ammonia-to-silane gas ratios. The nitrogen-to-silicon ratio in the film is proportional to the NH3/SiH4 flow ratio in the reactor. The Si-H peak in the infrared spectrum of the a-SiN:H shifts to lower frequency as the N/Si of the film decreases. We use the random bonding model to calculate the average electronegativity that a Si-H bond experiences for a particular N/Si ratio in the film. The measured Si-H frequency correlates with the calculated electronegativities and agrees with a similar correlation of Si-H obtained for molecules. For N/Si less than 0.27 we observe an additional peak at 650 cm−1 that also appears in the spectrum of plasma-deposited amorphous Si and is the Si-H wagging vibration. The random bonding model predicts amorphous Si to be the predominant constituent for N/Si less than 0.27, in agreement with the infrared data.
Substrate chemical etching prior to molecular-beam epitaxy: An x-ray photoelectron spectroscopy study of GaAs {001} surfaces etched by the H2SO4-H2O2-H2O solution
Authors: Massies J. , Contour J. P.
Published: July 2002

X-ray photoelectron spectroscopy has been performed in order to investigate the effects of the chemical etching of GaAs {001} surfaces by the H2SO4/H2O2/H2O solution used following the procedure currently practiced in the molecular-beam-epitaxy technique. It is demonstrated that, in contrast to what is generally believed, rinsing in running deionized water after etching does not produce any passivating oxide film on the surface. The surface-oxidized phases are only due to the sample manipulation in air after etching. This oxidation process is enhanced by the sample heating for indium soldering on the sample holder. It is shown that the surface-oxidized phases can be avoided by handling the sample under an inert atmosphere. Results of thermal desorption of the surface-oxidized phases are also given.
Diffusion coefficient of holes in silicon by Monte Carlo simulation
Authors: Reggiani Lino , Brunetti Rossella , Normantas Edmundas
Published: July 2002

A theoretical investigation of the diffusivity of holes in Si as a function of temperature, field strength, and field direction is reported. Calculations have been performed with the Monte Carlo procedure. The theoretical analysis explains the main features exhibited by the experimental data available from the literature. Minor discrepancies between theory and experiment are discussed in terms of microscopic mechanisms and/or reliability of the different experimental techniques used.
Penetration depths of low-energy hydrogen-ion implantation on ZnO surfaces
Authors: Yaron G. , Levy J. , Goldstein Y. , Many A.
Published: July 2002

The penetration depths of protons in the (0001̄) face of ZnO produced by 100- and 400-eV hydrogen-ion bombardment are studied by three methods: calibrated argon-ion sputtering, calibrated etching, and space-charge capacitance measurements in the ZnO/electrolyte system. For the 100-eV implantation, the latter method provides unequivocal support to our previous conclusion that the protons, acting as fully ionized donors, penetrate only to a depth of 10–20 Å below the surface. The narrow space-charge layer so produced, having surface electron densities of up to 2 × 1014 cm−2, constitutes a quantized, two-dimensional electron gas system. In the 400-eV implanted surface, the proton penetration depth is considerably larger. In this case an approximate depth profile could be derived from the combined measurements. It consists of a Gaussian distribution, peaked about 40 Å below the surface with a standard deviation of some 80 Å. However, some 10% of the implanted protons are found to penetrate much deeper, being distributed up to 1000 Å or more below the surface. For 100-eV implanted surfaces, similarly large penetration depths were observed, but the percentage of the deep lying protons is less than 1%. Such huge penetration depths arise most probably from a channeling mechanism.
Impurity defect interactions in GaAs
Authors: Stievenard D. , Bourgoin J. C.
Published: July 2002

Bulk Czochralski (CZ) and vapor phase epitaxy (VPE) layers of n GaAs have been irradiated with 1-MeV electrons, with doses ranging from 2×1015 to 1017 cm−2 and temperatures ranging from 240 to 330 °C. In CZ material, up to seven defects are detected. In VPE layers, five defects are observed. The variation of their introduction rates have been determined versus the temperature of irradiation. This study allows us to show that these traps are complex defects, due to the interaction of arsenic interstitial (IAs) with the impurities contained in the material. The fact that we detected the introduction of traps having the same electrical characteristics as the well-known EL5 and EL2 traps found usually in unirradiated materials provides information on their nature. Finally, a complete compensation of the material can be obtained, which demonstrates the feasibility of producing semi-insulating layers by irradiation.
Photoluminescence study of InxAl1−xAs-GaAs strained-layer superlattices
Authors: Kato H. , Iguchi N. , Chika S. , Nakayama M. , Sano N.
Published: July 2002

We present the strain-induced effects of the InxAl1−xAs-GaAs strained-layer superlattices grown by molecular beam epitaxy. The evaluation of the effects of biaxial strain in the planes perpendicular to the [001] superlattice direction was made by conventional photoluminescence measurements. The observed optical transition energies were evaluated by a Kronig-Penny model involving strain-induced band structure. Comparison between the observed transition energies and the calculated energies suggests that the optical transition of strained-layer superlattices is explained by the band-gap shift and the valence-band splitting, which are induced by the biaxial strain.