bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural properties of hot wire a-Si:H films deposited at rates in excess of 100 Å/s
Authors: Mahan A. H. , Xu Y. , Williamson D. L. , Beyer W. , Perkins J. D. , Vanecek M. , Gedvilas L. M. , Nelson B. P.
Published: July 2002

The structure of a-Si:H, deposited at rates in excess of 100 Å/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content CH as probed by infrared spectroscopy. As the film deposition rate Rd is increased from 5 to >140 Å/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 Å/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in the film electronic structure (Urbach edge, as-grown defect density) to the increase in the SAXS signals. We also note the invariance of the saturated defect density versus Rd, measured after light soaking, and discuss possible reasons why the increase in the microvoid density apparently does not play a role in the Staebler–Wronski effect for this type of material.
A dual-wavelength indium gallium nitride quantum well light emitting diode
Authors: Ozden I. , Makarona E. , Nurmikko A. V. , Takeuchi T. , Krames M.
Published: July 2002

We have designed and implemented a monolithic, dual-wavelength blue/green light emitting diode (LED) consisting of two active indium gallium nitride/gallium nitride (InGaN/GaN) multiple-quantum-well segments. The segments are part of a single vertical epitaxial structure in which a p++/n++ InGaN/GaN tunnel junction is inserted between the LEDs, emitting in this proof-of-concept device at 470 nm and 535 nm, respectively. The device has been operated as a three-terminal device with independent electrical control of each LEDs to a nanosecond time scale.
Transition events in butane simulations: Similarities across models
Authors: Zuckerman Daniel M. , Woolf Thomas B.
Published: July 2002

From a variety of long simulations of all-atom butane using both stochastic and fully solved molecular dynamics, we have uncovered striking generic behavior that also occurs in one-dimensional systems. We find an apparently universal distribution of transition event durations, as well as a characteristic speed profile along the reaction coordinate. An approximate analytic distribution of event durations, derived from a one-dimensional model, correctly predicts the asymptotic behavior of the universal distribution for both short and long durations.
Observation of the Mott–Gurney law in tris (8-hydroxyquinoline) aluminum films
Authors: Kiy M. , Losio P. , Biaggio I. , Koehler M. , Tapponnier A. , Günter P.
Published: July 2002

We show that tris (8-hydroxyquinoline) aluminum (Alq3) thin films produced and characterized under ultrahigh vacuum conditions present a well-defined squared-law dependence of the injected current on the applied voltage at applied electric fields of the order of 0.25–1 MV/cm. From this, one derives an electric-field-independent electron mobility of the order of 10−7 cm2/(V s), with a variation between different samples of about one order of magnitude. Observations of current–voltage characteristics with clear indications of trap-filling and space-charge-limited conduction at high fields in Alq3 excludes the existence of traps with an exponential distribution of trap energies, as is commonly assumed in amorphous materials.
The dielectric tensor of simple-pole distribution functions in magnetized plasmas
Authors: Tjulin Anders , André Mats
Published: July 2002

The dielectric tensor of a magnetized plasma described by a simple-pole particle distribution function is presented. Such model distributions give a new tool for understanding waves in collisionless non-Maxwellian plasmas. These distributions can model high-energy tails of the distribution in a way that is not possible with Maxwellians. An advantage over Lorentzian distributions is that an expression for the elements of the dielectric tensor can be obtained that does not involve any integrals over the perpendicular velocity. As an example, a simple-pole model of a magnetized space plasma is studied numerically and it is found that particle populations with the same density and mean particle energy, but with somewhat different distribution functions, correspond to different propagation properties that should be observable.
Kinetic simulations of ion temperature measurements from retarding field analyzers
Authors: Valsaque F. , Manfredi G. , Gunn J. P. , Gauthier E.
Published: July 2002

Retarding field analyzers (RFA) provide an integral of the ion velocity distribution in tokamak edge plasmas, leading, in principle, to an estimate of the ion temperature. However, the presence of the RFA itself perturbs the ambient plasma, such that the measured distribution is distorted with respect to the unperturbed one far from the probe. Here, collisionless kinetic modeling is employed to investigate the modification of the plasma characteristics (temperature, particle flux, density, and electric potential) in the presheath of the RFA. The kinetic equations are solved independently by means of two different numerical methods, which provide a reliable check of their results. Moreover, they are interpreted in light of a simplified kinetic analytical model. Systematic numerical studies are performed for a large range of values of the ion-to-electron temperature ratio and the parallel drift speed. In the same way that a Mach probe measures upstream–downstream asymmetries of ion saturation current in flowing plasmas, RFAs are expected to measure important asymmetries of sheath potential and ion temperature. These asymmetries can be used to estimate accurately the ion temperature in the absence of the probe perturbation.
Electron energy distribution function and electron characteristics of conventional and micro hollow cathode discharges
Authors: Petrov G. M. , Zhechev D.
Published: July 2002

The conventional hollow cathode discharge, micro hollow cathode discharge, and the transition between them have been analyzed. The time independent and spatially averaged electron energy distribution function, electron density, mean electron energy, excitation, and ionization rates have been calculated and compared. The direct comparison showed substantial differences between the conventional and micro hollow cathode discharges, particularly in absorbed power per unit volume, degree of ionization, and excitation and ionization rates.
Drift wave instability near a magnetic separatrix
Authors: Myra J. R. , D’Ippolito D. A. , Xu X. Q.
Published: July 2002

It is well known that the pure drift-Alfvén wave (DW) (i.e., in the absence of curvature and toroidal coupling effects) is stabilized by magnetic shear in circular flux surface geometry when the drift frequency is constant radially [P. N. Guzdar, L. Chen, P. K. Kaw, and C. Oberman, Phys. Rev. Lett. 40, 1566 (1978)] as is implicit in a local ballooning analysis. In the edge plasma near a magnetic separatrix, X-point geometry is important and the circular flux surface model does not apply. Using several numerical codes and analytical models, it is found that the DW is robustly unstable in this case. Physically, instability is driven by wave reflection from the steep profile of k⊥ near the X-points, due to magnetic shear and the local minimum of the poloidal magnetic field. It is concluded that a complete set of dimensionless parameters describing edge turbulence must include DW parameters that embody the physics of X-point effects and plasma shaping.
Blocking reactions between indium-tin oxide and poly (3,4-ethylene dioxythiophene):poly(styrene sulphonate) with a self-assembly monolayer
Authors: Wong K. W. , Yip H. L. , Luo Y. , Wong K. Y. , Lau W. M. , Low K. H. , Chow H. F. , Gao Z. Q. , Yeung W. L. , Chang C. C.
Published: July 2002

In the fabrication of polymeric electroluminescent devices with indium-tin oxide (ITO) as anode, indium contamination of the polymers can greatly degrade the device performance. In the present study, we have used x-ray photoelectron spectroscopy to measure indium incorporation in poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate), referred to as PEDOT:PSS, which were spincast on bare ITO and encapsulated ITO. We found that the deposition of a self-assembled monolayer of alkylsiloxanes on ITO prior to spincasting PEDOT:PSS was effective and practical in blocking the reactions between ITO and PEDOT:PSS.
The symmetric broadening of the water relaxation peak in polymer–water mixtures and its relationship to the hydrophilic and hydrophobic properties of polymers
Authors: Ryabov Yaroslav E. , Feldman Yuri , Shinyashiki Naoki , Yagihara Shin
Published: July 2002

The dielectric relaxation of water molecules in polymer–water mixtures is discussed. The memory function approach and scaling relationships are used as a basis for the model of symmetric dielectric spectrum broadening. The correspondence between the relaxation time, the geometrical properties, the self-diffusion coefficient, and the Cole–Cole exponent is established. The relationship between the hydrophilic and hydrophobic properties of the polymers and the dielectric relaxation parameters is discussed.
Complex dynamics and multistability in a damped harmonic oscillator with delayed negative feedback
Authors: Campbell Sue Ann , Bélair Jacques , Ohira Toru , Milton John
Published: July 2002

A center manifold reduction and numerical calculations are used to demonstrate the presence of limit cycles, two-tori, and multistability in the damped harmonic oscillator with delayed negative feedback. This model is the prototype of a mechanical system operating with delayed feedback. Complex dynamics are thus seen to arise in very plausible and commonly occurring mechanical and neuromechanical feedback systems.
Resistively shunted dc SQUID coupled to an input coil
Authors: Enpuku K. , Yoshida K.
Published: July 2002

A theoretical study has been made of a resistively shunted dc SQUID coupled to an input coil. The effect of a circulating current is taken into account. An analytical expression for the circulating current is obtained by replacing the nonlinear Josephson current with a linear ac current generator. It is shown that in the conventional SQUID, the circulating current changes considerably the input impedance seen from the input coil, and that the input impedance becomes nonlinear. As a result, previous optimization conditions of amplifiers and magnetometers, which were obtained by neglecting the circulating current, lead to errors amounting to 40%. In the case of a resistively shunted SQUID, where a damping resistance is attached in parallel with an inductance of the SQUID, it is shown that the circulating current can be suppressed significantly with the damping resistance. Therefore, the change of the input impedance caused by the circulating current can be eliminated in this case.
Photoluminescence of InAsBi and InAsSbBi grown by organometallic vapor phase epitaxy
Authors: Fang Z. M. , Ma K. Y. , Cohen R. M. , Stringfellow G. B.
Published: July 2002

Infrared photoluminescence (PL) from InAsBi and InAsSbBi epitaxial layers grown by atmospheric pressure organometallic vapor phase epitaxy has been studied. The PL from ternary InAsBi was investigated for Bi concentrations of ≤2.3 at. %. The peak energy decreases at a rate of 55 meV/at. % Bi with increasing Bi concentration. A study of the transmission spectra of these Bi-containing alloys confirms the above result. The PL peak is assigned to near band edge emission for InAsBi. The value of dEg/dx=−55-meV/at. % Bi is more than double the previously reported theoretical prediction for the band gap of InAsBi. The PL for the quaternary layer of InAsSbBi is also studied for Sb concentrations of <10 at. % and Bi concentrations of ≤1.5 at. %. Bi incorporation in InAs1−xSbx(0.07<x<0.10) reduces the PL peak energy at a rate of 46-meV/at. % Bi. These results imply that incorporation of only a few percent of Bi is required in InAs0.35Sb0.65 to achieve a band gap of 0.1 eV, equivalent to a wavelength of 12 μm, desired for infrared detector applications.
Plasma density dependence of the oxidation rate of Si by in situ during process rapid ellipsometry
Authors: Kuroki H. , Shinno H. , Nakamura K. G. , Kitajima M. , Kawabe T.
Published: July 2002

Oxidation of silicon in a radio frequency discharge oxygen plasma was studied using in situ during process rapid ellipsometry (1 s resolution and interval). Plasma characteristics were also determined by the Langmuir probe method and emission spectroscopy. From the measurement of in situ during process rapid ellipsometry, oxidized film thickness of silicon increased steeply right after starting rf discharge and slowly after ≊3000 s. It was found that thickness change rate measured by in situ during process rapid ellipsometry immediately after starting the rf discharge was strongly proportional to O2+ ions density.
Analysis of pressure dependence of electrical conductivity in polypyrrole
Authors: Maddison D. S. , Tansley T. L.
Published: July 2002

Pressure dependence of the electrical conductivity of a range of polypyrrole samples doped to various levels is interpreted in terms of variable-range hopping theory. All samples are well described by a model that treats the density of localized states around the Fermi level as the pressure-dependent commodity. The presumption of invariance of wave-function inverse localization length is vindicated by samples with a range of mean hopping lengths all being described by a single state-compressibility term. This term is defined by the elastic constants of polypyrrole and measurements of macroscopic elasticity confirm the value deduced from the pressure dependence of conductivity versus temperature curves.
Thin films of barium ferrite with perpendicular magnetic anisotropy produced by laser ablation deposition
Authors: Masterson H. J. , Lunney J. G. , Coey J. M. D. , Atkinson R. , Salter I. W. , Papakonstantinou P.
Published: July 2002

Barium ferrite thin films with perpendicular magnetic anisotropy have been prepared on heated (001) sapphire substrates by laser ablation deposition in an oxygen atmosphere using a KrF excimer laser. X-ray diffraction patterns reveal highly oriented films of the magnetoplumbite phase with a c-axis angular spread of 1.8°. Scanning electron micrographs reveal that these films are polycrystalline with a crystallite size of 0.4 μm. There is no surface particulate contamination. The saturation magnetization was similar to bulk barium ferrite, and the coercivity was 145 mT. The magneto-optical properties of the films are compared with those derived from the optical and magneto-optical constants of the bulk material.
Damage and strain in epitaxial GexSi1−x films irradiated with Si
Authors: Lie D. Y. C. , Vantomme A. , Eisen F. , Vreeland T. , Nicolet M.-A. , Carns T. K. , Arbet-Engels V. , Wang K. L.
Published: July 2002

The damage and strain induced by irradiation of both relaxed and pseudomorphic GexSi1−x films on Si(100) with 100 keV 28Si ions at room temperature have been studied by MeV 4He channeling spectrometry and x-ray double-crystal diffractometry. The ion energy was chosen to confine the major damage to the films. The results are compared with experiments for room temprature Si irradiation of Si(100) and Ge(100). The maximum relative damage created in low-Ge content films studied here (x=10%, 13%, 15%, 20%, and 22%) is considerably higher than the values obtained by interpolating between the results for relative damage in Si-irradiated single crystal Si and Ge. This, together with other facts, indicates that a relatively small fraction of Ge in Si has a significant stabilizing effect on the retained damage generated by room-temperature irradiation with Si ions. The damage induced by irradiation produces positive perpendicular strain in GexSi1−x, which superimposes on the intrinsic positive perpendicular strain of the pseudomorphic or partially relaxed films. In all of the cases studied here, the induced maximum perpendicular strain and the maximum relative damage initially increase slowly with the dose, but start to rise at an accelerated rate above a threshold value of ∼0.15% and 15%, respectively, until the samples are amorphized. The pre-existing pseudomorphic strain in the GexSi1−x film does not significantly influence the maximum relative damage created by Si ion irradiation for all doses and x values. The relationship between the induced maximum perpendicular strain and the maximum relative damage differs from that found in bulk Si(100) and Ge(100).
Defect recognition in conductive materials by local magnetic-field measurement
Authors: Doi T. , Hayano S. , Marinova I. , Ishida N. , Saito Y.
Published: July 2002

Among the various nondestructive testing methods, the electric potential method requires relatively simple device and measurement. However, because of the high electrical contact resistance, sometimes it is difficult to measure the correct local electric potentials by direct contact. In order to overcome this difficulty, method is proposed which involves substituting the local magnetic field for electric potential measurements. The comparison with the conventional electric potential method demonstrates the usefulness this method, especially for materials with low resistivity.
Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 μm using polarized light interference
Authors: Fischer D. W. , Ohmer M. C. , Schunemann P. G. , Pollak T. M.
Published: July 2002

The birefringence (Δn) of ZnGeP2 has been measured directly from polarized light interference spectra obtained in transmittance over the 0.66–12 μm wavelength range from samples of six different thicknesses. The Δn values were determined from the positions of fringe maxima (Δn=kλ/t) and then compared to previously published data which were obtained by a different technique. It was found that the interference fringe method results in values of Δn accurate to ±0.00005. The data are shown to exhibit much less scatter as a function of wavelength than previous results and can lead to more accurate calculations of phase-matching angles for second-harmonic generation applications.
Preparation and characterization of LiNbO3 thin films produced by chemical-vapor deposition
Authors: Sakashita Yukio , Segawa Hideo
Published: July 2002

Lithium niobate [LiNbO3 (LN)] thin films with good surface morphology and crystallinity have been epitaxially grown on lithium tantalate [LiTaO3 (LT)] substrates by chemical-vapor deposition using lithium dipivaloylmethane [Li(C11H19O2)] and niobium pentaethoxide [Nb(OC2H5)5] without postannealing. The Li/Nb ratio of LN thin films was easily controlled by adjusting the Nb source temperature, and it is possible to prepare LN thin films with stoichiometric composition (Li/Nb=1). The crystallinity of LN thin films greatly depends on substrate temperature and surface orientation of the LT substrate. It becomes better with increasing substrate temperature and is equivalent to a single crystal at 700 °C. It also becomes better in the order of (012), (110), and (001) planes of the LT substrate. The properties of surface acoustic wave propagation can be improved by preparing the LN thin film on the LT substrate. The electromechanical coupling factor k2 of LN/(012)LT is 0.52%, and is greater than that of the (012)LT substrate alone, 0.081%. The sound velocity of LN/(012)LT calculated from the central resonant frequency is 3200 m/s, and this value is greater than that of the (012)LT substrate by itself, 3140 m/s.
Yielding and phase transition under shock compression of yttria-doped cubic zirconia single crystal and polycrystal
Authors: Mashimo Tsutomu , Nakamura Akira , Kodama Masao , Kusaba Keiji , Fukuoka Kiyoto , Syono Yasuhiko
Published: July 2002

Inclined-mirror Hugoniot measurements of yttria (Y2O3) -doped (9.6 and 8.0 mol %) cubic zirconia single crystal and polycrystal were performed in the pressure range up to 120 GPa to study yielding and phase transition. The Hugoniot-elastic limit (HEL) stresses parallel to the 〈100〉 and 〈110〉 axes were approximately 14 and 25 GPa, respectively, while that of the polycrystal was approximately 13 GPa. Above the HELs the Hugoniot data parallel toward the 〈100〉 and 〈110〉 axes converged on each other, and showed large relief to an isotropic compression state, while those of the polycrystal preserved a considerably larger shear strength. A phase transformation took place at approximately 53 GPa (both 〈100〉 and 〈110〉 axis directions), and was completed by about 70 GPa. The phase transition pressure was much higher than those of the monoclinic- or tetragonal-orthorhombic II phase transitions under static compression. The shock velocity Us versus particle velocity Up relation of the final phase of the single crystal was given by Us=2.19+2.35Up km/s. The equation of state of the final phase was analyzed by using a program based on the Debye model, the Mie–Grüneisen equation, and the Birch–Murnaghan equation. The volume change between the cubic phase and the final phase at zero pressure was estimated to be 20%, which was larger than that expected for the cubic-orthorhombic II transition. The bulk moduli of the final phase at zero pressure were estimated to be 510 GPa.
On the role of atomic oxygen in the etching of organic polymers in a radio-frequency oxygen discharge
Authors: Collart E. J. H. , Baggerman J. A. G. , Visser R. J.
Published: July 2002

The kinetics of rise and decay of ground-state atomic oxygen was investigated in a low-pressure O2 radio-frequency discharge. The ground state of O was monitored using laser-induced-fluorescence spectroscopy. The discharge was operated in two modes: a continuous mode and a pulsed mode. Typical discharge settings are rf power of 100 W, a pressure of 10 Pa, and a gas flow of 10 sccm. Using a pulsed mode the behavior of the O density was determined in a pure O2 discharge with no reaction products, and in a discharge during etching of organic polymers. Production and loss mechanisms for O in a pure O2 discharge are discussed. For an O2 discharge during etching of resist it is found that under the given experimental conditions atomic oxygen does not play a significant role in the chemical etch mechanisms involved.
A self-consistent fluid model for radio-frequency discharges in SiH4–H2 compared to experiments
Authors: Nienhuis G. J. , Goedheer W. J. , Hamers E. A. G. , van Sark W. G. J. H. M. , Bezemer J.
Published: July 2002

A one-dimensional fluid model for radio-frequency glow discharges is presented which describes silane/hydrogen discharges that are used for the deposition of amorphous silicon (a-Si:H). The model is used to investigate the relation between the external settings (such as pressure, gas inlet, applied power, and frequency) and the resulting composition of the gas and the deposition rate. In the model, discharge quantities such as the electric field, densities, and fluxes of the particles are calculated self-consistently. Look-up tables of the rates of the electron impact collisions as a function of the average electron energy are obtained by solving the Boltzmann equation in a two term approximation for a sequence of values of the reduced electric field. These tables are updated as the composition of the background neutral gas evolves under the influence of chemical reactions and pumping. Pumping configuration and gas inlet are taken into account by adding source terms in the density balance equations. The effect of pumping is represented by an average residence time. The gas inlet is represented by uniformly distributed particle sources. Also the radial transport of neutrals from the discharge volume into the discharge-free volume is important. As the fluid model is one dimensional, this radial transport is taken into account by an additional source term in the density balance equations. Plasma–wall interaction of the radicals (i.e., the growth of a-Si:H) is included through the use of sticking coefficients. A sensitivity study has been used to find a minimum set of different particles and reactions needed to describe the discharge adequately and to reduce the computational effort. This study has also been used to identify the most important plasma-chemical processes and resulted in a minimum set of 24 species, 15 electron-neutral reactions, and 22 chemical reactions. In order to verify the model, including the chemistry used, the results are compared with data from experiments. The partial pressures of silane, hydrogen, disilane, and the growth rate of amorphous silicon are compared for various combinations of the operating pressure (10–50 Pa), the power (2.5–10 W), and the frequency (13.56–65 MHz). The model shows good agreement with the experimental data in the dust free α regime. Discharges in the γ′ regime, where dust has a significant influence, could not be used to validate the model.
Correlation between electron spin resonance and dc conductivity data of polypyrrole, poly(N-methyl pyrrole-pyrrole), and poly(N-methyl pyrrole)
Authors: Singh Ramadhar , Narula Amarjeet K.
Published: July 2002

The dc conductivity, dielectric constant, and electron spin resonance measurements have been performed on samples of polypyrrole, poly(N-methyl pyrrole-pyrrole), and poly(N-methyl pyrrole) having different BF4− concentrations in the temperature range 77–350 K. Different parameters evaluated from these measurements support the existence of Mott’s variable range hopping mechanism in these systems.
Chemical kinetics in low pressure acetylene radio frequency glow discharges
Authors: Doyle James R.
Published: July 2002

The product gas yields for a 30 mTorr C2H2 13.56 MHz glow discharge were measured using mass spectrometry. The volatile products are H2, C4H2, and C6H2. A reaction mechanism has been proposed that accounts quantitatively for the yields of C4H2 and C6H2, and film deposition rate, as a function of discharge power. The product gases C4H2 and C6H2 are depleted at high rates in the plasma by electron collisional dissociation, and by reaction with H atoms and C2H radicals. Under high power conditions suitable for diamondlike carbon deposition it is proposed that the film growth is dominated by the radicals C4H3, C6H3, and C2H. These results are compared with the kinetics of methane rf glow discharges under similar conditions.
Chemical etching of ion tracks in LiF crystals
Authors: Trautmann C. , Schwartz K. , Geiss O.
Published: July 2002

In LiF, the formation of defects upon electronic excitations was studied using various heavy ions with energies up to 11.4 MeV/u. The induced damage was revealed by the technique of selective chemical etching. Track etching was successful only in those cases where the linear energy transfer of the ions had surpassed a critical threshold of about 1 keV/Å. Annealing tests after ion irradiation show that track etching is possible up to a temperature of 450 °C, i.e., the thermal stability of the etchable damage in ion tracks is much higher than simple defects such as color centers. It is concluded that the etchability of tracks in LiF is strongly related to the creation of defect aggregates.
Low-frequency noise in single electron tunneling transistor
Authors: Tavkhelidze A. N. , Mygind J.
Published: July 2002

The noise in current biased aluminium single electron tunneling (SET) transistors has been investigated in the frequency range of 5 mHz<f<30 Hz. A refined high frequency (HF) shielding including resistive coaxial lines, that prevents spurious electromagnetic radiation and especially high energy photons emitted by the 4.2 K environment from reaching the sample, allows us to study a given background charge configuration for many hours below ≈100 mK. The noise at relatively high frequencies originates from internal (presumably thermal equilibrium) charge fluctuations. For f⩾10 Hz, we find the same input charge noise, typically QN=5×10−4 e/Hz1/2 at 10 Hz, with and without the HF shielding. At lower frequencies, the noise is due to charge trapping, and the voltage noise pattern superimposed on the V(Vg) curve (voltage across transistor versus gate voltage) strongly depends on the background charge configuration resulting from the cooling sequence and eventual radio frequency (rf) irradiation. The measured noise spectra which show both 1/f and 1/f1/2 dependencies and saturation for f<100 mHz can be fitted by two-level fluctuators with Debye–Lorentzian spectra and relaxation times of order seconds. In some cases, the positive and negative slopes of the V(Vg) curve have different overlaid noise patterns. For fixed bias on both slopes, we measure the same noise spectrum, and believe that the asymmetric noise is due to dynamic charge trapping near or inside one of the junctions induced when ramping the junction voltage. Dynamic trapping may limit the high frequency applications of the SET transistor. Also reported on are the effects of rf irradiation and the dependence of the SET transistor noise on bias voltage.
Anisotropic third-order magneto-optical Kerr effect
Authors: Petukhov A. V. , Rasing Th. , Katayama T. , Nakajima N. , Suzuki Y.
Published: July 2002

The magnitude of the polar magneto-optical Kerr effect in reflection from thick iron epitaxial films is experimentally found to strongly depend on the crystallographic orientation of the film, with variations of more than 20%. Similar results are obtained on the (001) and (110) faces of a bulk Fe crystal. It is shown that this anisotropy can be described as a third-order effect in the magnetization M on the optical response ε(ω,M). Our analysis can also be used to understand the transversal Kerr effect at normal incidence recently observed by Gridnev et al.
Small-signal deep level transient spectroscopy on hydrogenated amorphous silicon based metal/insulator/semiconductor structures
Authors: Thurzo Ilja , Nádaždy Vojtech , Teramura Satoshi , Durný Rudolf , Kumeda Minoru , Shimizu Tatsuo
Published: July 2002

The high-temperature tail of small-signal deep level transient spectroscopy (DLTS) spectra of hydrogenated amorphous silicon (a-Si:H) based metal/insulator/semiconductor structures has been analyzed in terms of oxidation of some negatively charged mobile hydrogen species at the Al/a-Si:H interface, the excitation pulses of an amplitude of 0.1 V being applied repetitively between the top Al and the bottom (gate) electrodes. The concentration of the mobile hydrogen in the a-Si:H layer has been estimated to an amount up to ≈2×1020 cm−3, taking the earlier published data on the diffusion coefficient. Having chosen an appropriate polarity of the excitation pulses, one could observe a DLTS peak moving with applied bias on the temperature axis. The latter is pointing to an energy distribution of states at the Al/a-Si:H interface. The interface state density increases with increasing energy depth of the states. At the ultimate experimentally observable energy of E≈0.95 eV below the electron mobility edge Ec a corresponding areal density Na(E) of these states amounted up to 1.5×1011 cm−2 eV−1.
Reorientation of the W′ domain walls in ferroelastic silver niobate crystal
Authors: Miga S. , Dec J.
Published: July 2002

Using a polarizing microscope the temperature dependence of orientation of W′-type domain walls in a ferroelastic AgNbO3 crystal was determined. Results obtained are discussed within the framework of the Sapriel theory in relation to temperature dependence of lattice unit cell parameters. Inversion of the W′-domain wall orientation was observed.
Ageing of aluminum electrical contacts to porous silicon
Authors: Martı́n-Palma R. J. , Pérez-Rigueiro J. , Guerrero-Lemus R. , Moreno J. D. , Martı́nez-Duart J. M.
Published: July 2002

Electrical contacts to porous silicon (PS) were formed by depositing aluminum onto its surface. The corresponding Al/PS/Si structures show a rectifying behavior, even after prolonged times in contact to the atmosphere. The series resistance and the ideality factor as a function of time of storage in ambient air have been determined by employing a variation of the Norde method. We have also studied the influence of the electrolytic formation parameters in the process of aging of the Al/PS/Si structures. The results show that longer anodization times and higher formation current densities of the PS layer lead to a faster diminution of the current flowing through the Al/PS/Si structure as a result of its exposition to the atmosphere. However, when the surface of the PS layer is chemically etched, the diminution of the current is significantly slower than in the case of untreated samples.
Acoustic quality factor of copper alloys from 50 mK to 300 K
Authors: Duffy William
Published: July 2002

The amplitude-independent mechanical quality factors (reciprocal internal frictions) of the binary alloys 98Cu–2Be and 94Cu–6Be, and the commercial alloys Cu–Be “10” (C17 500), phosphor bronze “A” (C51 000), and silicon bronze “A” (C65 500) were measured in the temperature range 50 mK–300 K. This study was motivated by the need to identify materials with very low acoustic losses, which could be used for the fabrication of resonant-mass detectors of gravitational radiation. Data were obtained from measurements of 1 kHz torsional modes of mechanical resonators configured to minimize extraneous internal friction effects. Flexural mode measurements at 9 kHz gave substantially the same quality factor for the 94Cu–6Be as seen at 1 kHz from 50 mK to 10 K. The measured quality factor of this alloy of about 40 million at 50 mK is the largest that has ever been observed in a copper alloy. High resolution resonator frequency data below 10 K are shown to exhibit a temperature dependence which has been associated with dislocation motion in pure elemental metals.
Widely tunable intersubband energy spacing of self-assembled InAs/GaAs quantum dots due to interface intermixing
Authors: Wang X. C. , Xu S. J. , Chua S. J. , Zhang Z. H. , Fan W. J. , Wang C. H. , Jiang J. , Xie X. G.
Published: July 2002

In this article, we showed the significant reduction of the energy spacing between ground state and excited state emissions from InAs/GaAs quantum dots (QDs) due to interface interdiffusion induced by thermal treatment. In addition, the strong narrowing of the luminescence linewidth of the ground state and excited state emissions from the InAs dot layers for the annealed samples indicates an improvement of the size distribution of the QDs. Large blueshift of the energy positions of both emissions was also observed. High resolution x-ray diffraction experiments give strong evidence of the interface atom interdiffusion in the annealed samples. This work shows the ability to tune the wavelength for applications like infrared detectors and lasers based on intrasubband transitions of self-assembled QDs.
Comprehensive study of the film surface temperature and plasma thermokinetics during La1.85Sr0.15CuO4 deposition by laser ablation
Authors: Buzea Cristina , Wang Huabing , Nakajima Kensuke , Kim Sang-Jae , Yamashita Tsutomu
Published: July 2002

We report a comprehensive study of the film surface temperature (TS) variation during La1.85Sr0.15CuO4 deposition by laser ablation as a function of the process parameters (oxygen pressure and target-substrate distance). For high O2 pressures, TS (measured with an infrared pyrometer) follows a decreasing exponential law, while for lower oxygen pressures the experimental data suffer a departure from the theoretical curve. The observed deviation suggests the existence of nonsteady thermal processes in the plume during deposition. The modifications of plume kinetics along with the characterization of deposited films provide important information and a better understanding of the pulsed-laser deposition process.
Effects of topological patterning on bubble domain behavior in magnetic garnet
Authors: Morrissey R. P. , Fedyunin Y. , Ruane M. F. , Humphrey F. B.
Published: July 2002

Topological patterning of magnetic thin films has a substantial effect on domain behavior. Bubble domains located underneath a groove in patterned material display an ovoid profile with the major axis along the groove. The major axis was measured to have a diameter up to twice that along the minor axis. Measurements of the collapse field indicate that it can vary by up to 3% with changes in the groove depth, and by up to 5% with changes in the pitch of a groove array.
Influence of defects on magnetostrictive performance in Terfenol-D
Authors: Zhao X. G. , Lord D. G. , Faunce C. A.
Published: July 2002

A large remanent strain has been normally observed in free-standing float zone-grown Tb0.3Dy0.7Fe2 (Terfenol-D) following the application of magnetic field or compressive stress. Such a strain is considered to be composed of two parts, leading to reversible and irreversible domain contributions to the magnetostriction, in which the irreversible part corresponds to the magnetostrictive jump effect belonging to non-180° domain wall motion, and the reversible part contributes to the slower changes of magnetostriction. With no stress applied, the specimen shows a jump field of 21 and 53 Oe for 180° and non-180° domain wall motion, respectively. The jump field, distribution width and hysteresis field for non-180° domain wall motion are all increased with increasing stress, indicating complicated interactions between magnetic domains and defects. Stacking faults are found by electron microscopy to often act as domain-wall pinning centers, and scanning probe microscopy shows specific domain configurations are formed around excess rare earth, twin boundaries, and grain boundaries due to induced stresses and demagnetization effects. The influence of such interactions on magnetostrictive performance is discussed.
Domain structures and temperature-dependent spin reorientation transitions in c-axis oriented Co–Cr thin films
Authors: Kusinski Greg J. , Krishnan Kannan M. , Thomas Gareth , Nelson E. C.
Published: July 2002

Highly c-axis oriented Co95Cr5 films with perpendicular anisotropy were grown epitaxially on Si (111), using an Ag seed layer, by physical vapor deposition. Films were characterized by x-ray diffraction, transmission electron microscopy (TEM), selected area electron diffraction, and Lorentz microscopy in a TEM. The following epitaxial relationship was confirmed: (111)Si∥(111)Ag∥(0001)CoCr;[2̄20]Si∥[2̄20]Ag∥[1̄100]CoCr. Magnetic domain structures of these films were observed as a function of thickness; t, in the range, 200 Å<t<700 Å using a wedge-shaped sample, and temperature-dependent measurements were carried out by in situ resistance heating. Thickness was measured locally by electron energy loss spectroscopy. At room temperature, below a critical thickness, tc≈300 Å, the magnetization was found to be effectively in-plane of the film, and above tc a regular, stripe-like domain pattern with a significant, alternating in sign, perpendicular component was observed. The spin reorientation transitions of the stripe domains to the in-plane magnetization were studied dynamically by observing the domains as a function of temperature by in situ heating up to 350 °C. The critical transition thickness, tc, which is a function of Ku and magnetostatic energy, was found to increase with increasing temperature. The stripe-domain period, L observed at room temperature was found to increase gradually with thickness; L=90 nm at t=300 Å, and L=110 nm at t=700 Å.
Collision-induced transitions between molecular hyperfine levels: Quantum formalism, propensity rules, and experimental study of CaBr(X 2Σ+)+Ar
Authors: Alexander Millard H. , Dagdigian Paul J.
Published: July 2002

The general quantum treatment of collisions of a 2Σ+ molecule with hyperfine structure is presented. The recoupling technique introduced by Corey and McCourt into the field of molecular collisions [J. Phys. Chem. 87, 2723 (1983)] allows us to represent hyperfine-state-resolved tensor opacities, and hence cross sections, in terms of the corresponding nuclear- and also electron-spin-free quantities. The formalism also predicts (independent of the dynamical limit) that the largest F→F′ cross sections will be those for which ΔF=ΔJ, a rule well known for radiative transitions. Hyperfine-state-resolved scattering involving collisions of CaBr(X 2Σ+) with Ar is also studied here experimentally by electric quadrupole state selection and cw dye laser fluorescence detection. The relative final F′ distributions were determined for the N=3,e→N=5,e and N=2,e→N=1,e collisional transitions. These results clearly exhibit the ΔF=ΔJ propensity rule. Moreover, the F′ distributions were predicted with nearly quantitative accuracy using our previously determined CaCl(X 2Σ+)-Ar tensor opacities. By contrast, the MJ- randomization model, first proposed to treat the influence of hyperfine structure in atomic collisions, is shown to disagree with both our experimental data and theoretical predictions.
A classical fluid-like approach to the density-functional formalism of many-electron systems
Authors: Ghosh Swapan K. , Berkowitz Max
Published: July 2002

Within the framework of a new local thermodynamic transcription [Ghosh, Berkowitz, and Parr, Proc. Natl. Acad. Sci. U.S.A. 81, 8028 (1984)] of ground state density-functional theory, the electron cloud is interpreted as an inhomogeneous fluid at an effective local temperature. The correlation functions are defined and integral equations are introduced for this electron fluid along the lines of the theory of inhomogeneous classical liquids. An equation of state and a local compressibility equation are also developed. Extension to a mixture of two fluid components is explored as a means for investigating spin-polarized cases. The formalisms broaden the scope of the hydrodynamic analogy to quantum mechanics and provide an alternative integral equation method for calculation of the electron density of many-electron systems.
Solution of the Hartree–Fock equations by a pseudospectral method: Application to diatomic molecules
Authors: Friesner Richard A.
Published: July 2002

An improved version of the pseudospectral method is used to accurately solve the Hartree–Fock equations for the neon atom and the diatomic molecules H2 and Li2. Because the method eliminates two-electron integrals and is amenable to efficient vectorization on a supercomputer, order of magnitude reductions in computation time can be expected for large polyatomic molecules (for which a straightforward generalization of the procedure described here exists). The present work demonstrates that accuracy comparable to conventional methods can be achieved by the pseudospectral technique.
On the interaction between order and a moving interface: Dynamical disordering and anisotropic growth rates
Authors: Harrowell Peter R. , Oxtoby David W.
Published: July 2002

The way in which the velocity of a propagating interface determines the degree of surface disorder and the anisotropy of such interfacial velocities due to a crystalline lattice are examined using steady state solutions of the time dependent Landau–Ginzburg or Cahn–Hilliard equations. In the case of an interface described by two weakly coupled order parameters a divergence of the thickness of the surface disorder at a critical interfacial velocity is described. It is demonstrated that even for two surfaces with the same surface tension the growth rates may differ significantly due to a geometric factor arising from the underlying crystal lattice.
Theoretical study of the N+2 Meinel system
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

The potential curves for the X 2Σ+g and A 2Πu states of N+2 are determined using generally contracted atomic natural orbital Gaussian basis sets and large CASSCF/multireference CI wave functions. The dependence of the X 2Σ+g– A 2Πu electronic transition moment on internuclear distance is accurately determined, and is shown to differ significantly from previous determinations. However, the theoretically determined lifetimes agree with the experimental time-of-flight data of Peterson and Moseley and of Holland and Maier within the stated experimental error bars.
Collision energy dependence of product branching in Penning ionization: He*(2 1S, 2 3S) + H2, D2, and HD
Authors: Martin D. W. , Weiser C. , Sperlein R. F. , Bernfeld D. L. , Siska P. E.
Published: July 2002

Relative ionization cross sections for the title systems with articulation of all product ion channels have been measured in the collision energy range 1.5–4.0 kcal/mol using crossed supersonic molecular beams; the H2 results have been extended down to 0.5 kcal/mol by the use of a 10% H2/Ar seeded beam. The data are interpreted with a microscopic two-step model that assumes ionization near the turning point in the excited state, a centrifugal barrier criterion for ionic complex formation, and statistical partitioning of flux among the possible ionic products, i.e., phase-space theory. A full statistical calculation underestimates the amount of rearrangement ionization He*+H2→HeH++H+e− by a factor of 2, but one which excludes antiparallel coupling of orbital and rotational angular momenta in the H+2 channel is in better accord with the data. A substantial isotope effect favoring HeD+ over HeH+ in the HD reactions by a factor of 1.9±0.2 is well represented by the model.
Vibrational structure of an electrostatically bound ion–water complex
Authors: Lessen D. E. , Asher R. L. , Brucat P. J.
Published: July 2002

Supersonically cooled VH2O+ is resonantly one-photon dissociated in the visible region. An excited state vibrational progression in the V+(OH2) stretching mode is observed with a frequency W′e=339 cm−1 and an anharmonicity We X′e =4.5 cm−1. Analysis of the spectra of isotopically substituted species places an estimate of the ground state ion–water stretch at 420±75 cm−1. The electronic origin of the upper state places a strict upper limit to the adiabatic binding energy of this complex at 1.97 eV.
Nonlinear resonance and torsional dynamics: Model simulations of HOOH and CH3OOCH3
Authors: Schranz Harold W. , Collins Michael A.
Published: July 2002

Simple models of the vibrational dynamics of HOOH and CH3OOCH3 are investigated by classical trajectory methods. Nonlinear resonances due to kinematic coupling between the torsional motion and symmetric bond bending are found to have significant dynamical effects in some cases. The time scales and magnitudes of these energy-transfer processes are examined.
Coadsorbate induced compression of sulfur overlayers on Re(0001) and Pt(111) by CO
Authors: Dunphy J. C. , McIntyre B. J. , Gomez J. , Ogletree D. F. , Somorjai G. A. , Salmeron M. B.
Published: July 2002

Low coverages of sulfur chemisorbed on the rhenium(0001) and platinum(111) surfaces were studied in UHV by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). On both of these surfaces of triangular symmetry the lowest coverage (≊0.25 monolayers) ordered structure is p(2×2). Exposure of this ordered sulfur overlayer on either surface to low pressures (10−9 Torr) of CO induces compression of the sulfur layer to a structure associated with a higher local coverage and CO chemisorbs in the holes created in the sulfur layer. The reordering was observed by both a change in the LEED pattern and by real space STM imaging of the surface. On the Re surface the new overlayer has (3√3×3√3)R30° symmetry, while on the Pt surface it has (√3×√3)R30° symmetry. There was no increase in the amount of sulfur on the surface during this reordering. On both surfaces the overlayers could be returned to the original p(2×2) by annealing for several seconds at 600 °C, during which CO desorbs and sulfur atoms reoccupy the vacant metal sites. This phenomenon of the compression of atoms in a strongly chemisorbed layer upon coadsorption of another molecule provides a mechanism for carrying out catalytic reactions on metal surfaces that are covered with strongly chemisorbed layers that do not participate in the reaction.