bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrostatic force characterization on InAs dot-covered n-type (001) GaAs surfaces by contact-mode atomic force microscopy with a conductive tip
Authors: Takahashi T. , Kawamukai T. , Kamiya I.
Published: July 2002

We performed atomic force microscopy in contact mode while applying an ac bias voltage between a conductive tip and a sample to characterize near surface band structures of InAs-covered n-type (001) GaAs, where self-assembled dot structures were formed. Electrostatic force of less than 10 pN was detectable, and clear electrostatic force images and topographic images were simultaneously obtained with lateral resolution higher than 20 nm. The electrostatic force images from single and double frequency components reveal that the gap width between the tip and the conductive region under the dot-covered area of the sample is smaller and is less modulated by the bias voltage than under the wetting layer. The results indicate that surface depletion is more suppressed beneath the dots.
Nonvolatile ferroelectric-gate field-effect transistors using SrBi2Ta2O9/Pt/SrTa2O6/SiON/Si structures
Authors: Tokumitsu Eisuke , Fujii Gen , Ishiwara Hiroshi
Published: July 2002

We report fabrication and characterization of p-channel metal–ferroelectric-metal–insulator–semiconductor (MFMIS) field-effect transistors (FETs) using the Pt/SrBi2Ta2O9/Pt/SrTa2O6/SiON/Si structures. It is shown that SrTa2O6/SiON stacked layer is suitable as an “I” layer in the MFMIS structure, because the SrTa2O6/SiON/Si structure has low leakage current and good interface properties. We also show that by using a small MFM capacitor on a large MIS structure, a large memory window of 3.0 V can be obtained. Furthermore, it is found that the data retention characteristics have been improved for Pt/SrBi2Ta2O9/Pt/SrTa2O6/SiON/Si MFMIS-FETs.
Normal-incidence infrared photoconductivity in Si p-i-n diode with embedded Ge self-assembled quantum dots
Authors: Yakimov A. I. , Dvurechenskii A. V. , Proskuryakov Yu. Yu. , Nikiforov A. I. , Pchelyakov O. P. , Teys S. A. , Gutakovskii A. K.
Published: July 2002

Room-temperature infrared photoconductivity in a single layer of Ge self-assembled quantum dots incorporated into silicon p-i-n diode is reported. An in-plane polarized photocurrent response with two bias controlled maxima at 2.9 μm and at 1.7 μm wavelength has been observed. The two kinds of absorption driven by reverse bias in opposite ways are ascribed to the intraband hole bound-to-continuum transition and to the interband excitonic transition.
Room-temperature photoluminescence excitation spectroscopy of Er3+ ions in Er- and (Er+Yb)-doped SiO2 films
Authors: Kozanecki A. , Przybylinska H. , Jantsch W. , Palmetshofer L.
Published: July 2002

We apply photoluminescence excitation spectroscopy to study the efficiency of excitation at 960–990 nm of the 1.54 μm emission of erbium implanted into SiO2 films thermally grown on silicon for a range of Er and Yb concentrations. We show that, in silica films doped solely with Er, the concentration of Er should be lower than 1020 cm−3 to prevent efficient concentration quenching of the emission. It is shown that the Yb/Er concentration ratio of 1–2 is optimum for the dopant densities of 1–2×1020 cm−3, whereas 1021 cm−3 results in quenching of the emission due to energy losses within the Yb subsystem.
High quality GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia
Authors: Nikishin S. A. , Faleev N. N. , Antipov V. G. , Francoeur S. , Grave de Peralta L. , Seryogin G. A. , Temkin H. , Prokofyeva T. I. , Holtz M. , Chu S. N. G.
Published: July 2002

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.
Improved device performance using a semi-transparent p-contact AlGaN/GaN heterojunction positive-intrinsic-negative photodiode
Authors: Collins C. J. , Li T. , Beck A. L. , Dupuis R. D. , Campbell J. C. , Carrano J. C. , Schurman M. J. , Ferguson I. A.
Published: July 2002

We report on the improved device performance of GaN-based ultraviolet heterojunction photodiodes using a semi-transparent p-contact device structure. At a reverse bias of 10 V, these photodiodes exhibit a low dark current density of 0.3 nA/cm2. The external quantum efficiency is 38% at the band edge, with only a slight decrease at the shorter wavelengths. The forward current is >10 mA at Vf=5 V. Fitting of the forward current–voltage data to the diode equation yields a very low series resistance (Rs=62Ω), which results in a very fast decay of the time response. The improved performance afforded by the thin, semi-transparent, p-contact layer is due to an increase in the uniformity of the lateral field distribution.
Synchrotron-radiation-induced anisotropic wet etching of GaAs
Authors: Ma Qing , Mancini Derrick C. , Rosenberg Richard A.
Published: July 2002

A room-temperature photoenhanced chemical wet etching process for n-type GaAs using x rays from a synchrotron radiation source is described. HNO3:H2O was used as the etching solution. This process produces smoothly etched surfaces on n-GaAs with a root-mean-square surface roughness of 0.7–2.0 nm, which compares favorably to the unetched surface roughness (0.4 nm). Dependence of the etching rate on x-ray intensity and energy, solution concentration, and semiconductor doping type are reported.
Photostimulated luminescence in a rare earth-doped fluorobromozirconate glass ceramic
Authors: Edgar A. , Spaeth J.-M. , Schweizer S. , Assmann S. , Newman P. J. , MacFarlane D. R.
Published: July 2002

Photostimulated luminescence at room temperature was observed in a fluorozirconate glass which was doped with 1% Eu2+ or 1% Ce3+ and 5% Br− ions. Small crystals of BaBr2 (high-pressure phase) form in the glass on cooling. The Eu2+ and Ce3+ luminescence spectra comprised two emission lines each: for Eu2+ one line at 413 nm and a broader band centered at 485 nm, and for Ce3+ one line at 320 nm and a broader band at 425 nm. The luminescence is attributed to the 5d14fn−1→4fn emissions of the rare earth ions. In both cases only one of the emission bands showed photostimulated luminescence. The stimulation band for both rare earth ions was centered at 570 nm.
Carrier pocket engineering applied to “strained” Si/Ge superlattices to design useful thermoelectric materials
Authors: Koga T. , Sun X. , Cronin S. B. , Dresselhaus M. S.
Published: July 2002

The concept of carrier pocket engineering is applied to strained Si/Ge superlattices to obtain a large thermoelectric figure of merit ZT. In this system, the effect of the lattice strain at the Si/Ge interfaces provides another degree of freedom to control the conduction band structure of the superlattice. We explore various geometries and structures to optimize ZT for the whole three-dimensional superlattice. The resultant ZT, calculated for a symmetrized Si(20 Å)/Ge(20 Å) superlattice grown on a (111) oriented Si0.5Ge0.5 substrate, is 0.96 at 300 K and is shown to increase significantly at elevated temperatures. Such a superlattice can be grown using molecular beam epitaxy.
Nanocrystallization of ZrTiCuNiBeC bulk metallic glass under high pressure
Authors: Wang Wei Hua , He D. W. , Zhao D. Q. , Yao Y. S. , He M.
Published: July 2002

Nanocrystallization of Zr41Ti14Cu12.5Ni9Be22.5C1 bulk metallic glass (BMG) under high pressure is investigated. It is found that the nanocrystallization is pressure assisted, and the primary nanocrystallization temperature decreases as the applied pressure increases. Pressure annealing of the BMG in the supercooled liquid region produces a composite with dispersion of very fine nanocrystallites in the amorphous matrix. A fully nanocrystallization is obtained by pressure annealing under 6 GPa at 723 K. The pressure also controls the phase selection during the crystallization. The mechanism for the pressure-assisted nanocrystallization is discussed.
Effect of indirect minima carrier population on the output characteristics of AlGaInP light-emitting diodes
Authors: Patel D. , Pikal J. M. , Menoni C. S. , Thomas K. J. , Kish F. A. , Hueschen M. R.
Published: July 2002

We show that carrier transfer to the indirect X level in the confining layer is responsible for most of the substantial decrease in the efficiency of AlGaInP light-emitting diodes (LEDs) operating at short wavelengths. Carrier transfer to the confining X level was obtained by reducing the separation between the AlGaInP direct Γ minimum and the X levels by varying the Al composition in the active region and by the application of hydrostatic pressure. Carrier transfer to the confining X level appeared as an additional peak in the electroluminescence (EL) and resulted in a significant decrease of the LED efficiency. A simple model of the EL emission that takes into account carrier population in the X minima was found to be in excellent agreement with the measured EL behavior.
Energy dissipation in tapping-mode scanning force microscopy with low quality factors
Authors: Tamayo Javier
Published: July 2002

The phase angle of the cantilever oscillation in tapping mode scanning force microscopy can be related to the energy dissipated per oscillation period through an analytical model that assumes a sinusoidal movement of the cantilever [J. Tamayo and R. Garcı́a, Appl. Phys. Lett. 73, 2926 (1998); J. P. Cleveland, B. Anczykowski, E. Schmid, and V. Elings, Appl. Phys. Lett. 72, 2613 (1998)]. In this work, numerical calculations of the oscillation of cantilevers with quality factors lower than 10 show a significant contribution of higher harmonics (∼5%–20%). This contribution can lead to a significant error in the energy dissipated deduced by using the model cited above. Thus, an extended relationship between the phase shift and the energy dissipated is presented, that takes into account the higher harmonics of the oscillation. These results determine the conditions for the measurement of energy dissipation in a liquid.
Observation of dark-pulse formation in gain-clamped semiconductor optical amplifiers by cross-gain modulation
Authors: Selbmann P. E. , Hessler T. P. , Pleumeekers J. L. , Dupertuis M.-A. , Deveaud B. , Dagens B. , Emery J.-Y.
Published: July 2002

We have measured the ultrafast simultaneous cross-gain and laser mode dynamics in a gain-clamped semiconductor amplifier perturbed by an intense detuned 150 fs pump pulse. Besides relaxation oscillations, we demonstrate the instantaneous formation of a dark pulse in the laser mode that repeats itself with a period given by the cavity round-trip time. The dark pulse sequence subsequently decays into two-mode beating and is shown to weakly cross modulate the amplifier gain. To describe dark-pulse formation a time- and spatially dependent model based on rate equations is necessary. The experimental results are in reasonable agreement with numerical simulations.
Growth control of MnAs on GaAs(001) by reflection high-energy electron diffraction
Authors: Schippan F. , Kästner M. , Däweritz L. , Ploog K. H.
Published: July 2002

The MnAs(1̄100) surface is investigated during growth by reflection high-energy electron diffraction (RHEED). (1×2), (1×1), (2×1) and (4×1) RHEED patterns have been observed by varying the growth conditions, indicating various stoichiometry dependent reconstructions. A phase diagram showing the dependence of the reconstructions on the growth parameters is presented. RHEED intensity oscillations have been found, evidencing layer-by-layer growth of MnAs.
Current-phase relationship of YBa2Cu3O7−x ramp-edge Josephson junctions
Authors: Il’ichev E. , Zakosarenko V. , Schultze V. , Hoenig H. E. , Meyer H.-G. , Subke K. O. , Burkhardt H. , Schilling M.
Published: July 2002

The current-phase relation (CPR) of high-Tc ramp-edge Josephson junctions (YBa2Cu3O7−x/PrBa2Cu3O7/YBa2Cu3O7−x) was investigated experimentally in the temperature range 65<T<85 K. The investigated junctions were incorporated into a washer-shaped superconducting loop with inductance L≈80 pH forming a single-junction interferometer. The CPR was obtained from the measurement of the impedance of the phase-biased junction. We show that, for junctions with sufficiently small critical current Ic, the CPR is sinusoidal. In the case of larger critical current, deviations of the apparent CPR from sinusoidal dependence were found. These deviations can be described by thermal fluctuations of the magnetic flux in the loop. So we conclude that, for all investigated junctions, the “true” CPR is sinusoidal.
Nonvolatile programmable two-terminal diodes using a ferroelectric semiconductor
Authors: Okano Motochika , Watanabe Yukio
Published: July 2002

The conductance of diodes formed by epitaxial (Pb, La)(Zr, Ti)O3 on SrTiO3 doped with Nb is programed using the relaxation semiconductor characteristics of a ferroelectric. Namely, a three-terminal device function is given to a two-terminal device by time-domain control. The conductance modulation programed by a short-voltage pulse is perfectly retained for ten days at room temperature and is nondestructively read. The relaxation current at very low bias is also programmable. When the current is regulated by a metal/ferroelectric contact, the pulse modulation is obscure and is not retained. This implies that the surface layer at the metal/ferroelectric contact is negligibly thin or thinner than the tunneling distance.
Reverse-poling effects on charge retention in Pb(Zr,Ti)O3(001)/LaNiO3(001) heterostructures
Authors: Jo W. , Kim D. C. , Hong J. W.
Published: July 2002

We report charge retention in preferentially c-axis oriented ferroelectric Pb(Zr,Ti)O3 (PZT) thin films on LaNiO3 by electrostatic force microscopy. The surface charge density of the PZT films was observed as a function of time in a selected area where a region is single poled and another region is reverse poled. Retention behaviors of the regions are very different: the single-poled region shows a declined response and the reverse-poled region reveals a retained characteristic. Decay and retention mechanisms of the regions are explained by space-charge redistribution and trapping of defects in the films.
Picosecond photoacoustics using common-path interferometry
Authors: Nikoonahad Mehrdad , Lee Shing , Wang Haiming
Published: July 2002

A pump-probe technique based on the detection of optical phase changes at the surface is described. A common-path interferometer in which the reference and probe pulses interrogate the surface immediately (∼500 ps) before and after the pump pulse is demonstrated. The primary application of this system is film thickness measurement in integrated circuit processing. Pulses 60 to 100 fs wide, from a Ti-sapphire laser at 800 nm, are brought to a 5 μm focus on the surface, resulting in picosecond acoustic pulses in the film. Echoes from film interfaces, upon arrival at the surface, lead to a phase change in the probe beam which is measured. Results from tungsten, aluminum, and copper film structures are presented.
Fine crystal lattice fringes observed using a transmission electron microscope with 1 MeV coherent electron waves
Authors: Kawasaki T. , Yoshida T. , Matsuda T. , Osakabe N. , Tonomura A. , Matsui I. , Kitazawa K.
Published: July 2002

A transmission electron microscope with a 1 MeV cold field-emission electron source has been developed for coherent and penetrating electron waves. We confirmed the coherence and overall stability of the microscope by observing Au(337̄) lattice fringes. These fringes have a 0.498 Å spacing.
Self-frequency doubling in Yb3+ doped periodically poled LiNbO3:MgO bulk crystal
Authors: Capmany J. , Montoya E. , Bermúdez V. , Callejo D. , Diéguez E. , Bausá L. E.
Published: July 2002

Continuous-wave laser action from an Yb3+ doped periodically poled LiNbO3:MgO bulk crystal at 1.06 μm is reported. In addition, efficient and stable self-frequency-doubled laser action at 531 nm was obtained by quasiphase matching. Up to 10.5 mW of green output power is obtained from a total laser output power of 58 mW. The experiments were carried out by end pumping with a Ti:sapphire laser, as a surrogate source for a diode laser, at 980 nm. Laser operation was stable at room temperature. The results are compared with those corresponding to single-domain Yb-doped crystals.
Evolution of sp2 bonding with deposition temperature in tetrahedral amorphous carbon studied by Raman spectroscopy
Authors: Chhowalla M. , Ferrari A. C. , Robertson J. , Amaratunga G. A. J.
Published: July 2002

Two transitions in the bonding are found in tetrahedral amorphous carbon (ta-C) films as a function of deposition temperature. The total sp3 fraction shows a sharp decrease at a transition temperature of order 250 °C. In contrast, visible Raman finds that the sp2 sites show a gradual ordering into the graphitic clusters through the sharp bonding transition. The optical gap and resistivity show a similar, gradual transition. This indicates that the sp2 cluster size determines the optical gap, even when the sp2 content does not change. The Raman I(D)/I(G) peak ratio is found to vary inversely with the square of the gap.
Self-assembled InGaN quantum dots grown by molecular-beam epitaxy
Authors: Adelmann C. , Simon J. , Feuillet G. , Pelekanos N. T. , Daudin B. , Fishman G.
Published: July 2002

Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.
Significant photoinduced refractive index change observed in porous silicon Fabry–Pérot resonators
Authors: Takahashi Morio , Toriumi Yuichi , Matsumoto Takahiro , Masumoto Yasuaki , Koshida Nobuyoshi
Published: July 2002

It is shown that a porous silicon Fabry–Pérot resonator device exhibits a significant nonlinear optical transmission behavior at wavelengths in the near-infrared region. The input–output (transmission–incident power) response curve shows superlinear or sublinear behavior dependent on excitation wavelengths around the resonant position. These characteristics are interpreted as a result of nonlinear refractive index change, including by enhanced optical power in the Fabry–Pérot cavity configuration and carrier accumulation in silicon nanocrystallites. The result leads to further progress of nanocrystalline silicon into functional photonic device applications.
Dynamic behavior of hot-electron–hole plasma in highly excited GaN epilayers
Authors: Juršėnas S. , Kurilčik G. , Tamulaitis G. , Žukauskas A. , Gaska R. , Shur M. S. , Khan M. A. , Yang J. W.
Published: July 2002

The room-temperature spontaneous luminescence of electron–hole plasma was investigated in GaN epilayers under extremely high quasi-steady-state photoexcitation. The photoluminescence spectra were measured for excitation power densities up to 200 MW/cm2 both under quasiresonant and off-resonant excitation conditions. High carrier temperatures up to 1000 K were observed under off-resonant excitation. A nonmonotonous dependence of the luminescence band peak position Ep on the excitation power density was observed. We attribute this nonmonotonous behavior of Ep to two competing mechanisms: (i) band-gap shrinkage due to carrier screening effects (redshift); and (ii) nonequilibrium carrier heating (blueshift). The obtained results are in a good agreement with finite-temperature theory of the band-gap renormalization.
Stabilization of strained alloy film growth by a difference in atomic mobilities
Authors: Spencer B. J. , Voorhees P. W. , Tersoff J.
Published: July 2002

We analyze the development of compositional and surface nonuniformities during the growth of strained alloy films, for the case of different surface mobilities of the alloy components. We find that a difference in surface mobilities can completely suppress the stress-driven morphological instability. The stabilization occurs under compressive misfit when one atomic species is both large and fast relative to the other, and similarly under tensile misfit when one species is small and fast.
Large electrically induced transmission changes of GaAs/AlGaAs quantum-cascade structures
Authors: Eickemeyer F. , Kaindl R. A. , Woerner M. , Elsaesser T. , Barbieri S. , Kruck P. , Sirtori C. , Nagle J.
Published: July 2002

Using a tunable midinfrared light source we study optical transmission changes of an electrically driven GaAs/AlGaAs quantum-cascade structure without resonator. For forward bias, we observe a transmission increase in the spectral range around the electroluminescence maximum which is due to resonant optical amplification. The observed transmission increase is enhanced up to 20-fold with respect to a bare active region by the internal field enhancement in the quantum-cascade structure and by interference effects in the midinfrared beam. Model calculations account quantitatively for this behavior.
Room-temperature far-infrared emission from a self-organized InGaAs/GaAs quantum-dot laser
Authors: Krishna Sanjay , Qasaimeh Omar , Bhattacharya Pallab , McCann Patrick J. , Namjou Khosrow
Published: July 2002

Far-infrared spontaneous emission at 300 K and lower temperatures, due to intersubband transitions in self-organized In0.4Ga0.6As/GaAs quantum dots, has been characterized. Measurements were made with a multidot layer near-infrared (∼1 μm) interband laser. The far-infrared signal, centered at 12 μm, was enhanced after the interband transition reached threshold at 300 K. The results are explained in terms of the carrier dynamics in the dots.
Strained InGaAs/AlGaAs/GaAs-quantum cascade lasers
Authors: Gianordoli S. , Schrenk W. , Hvozdara L. , Finger N. , Strasser G. , Gornik E.
Published: July 2002

We report on the realization of a quantum cascade laser based on strained In0.04Ga0.96As/Al0.33Ga0.67As/GaAs grown on GaAs substrate using molecular beam epitaxy. Lasing at 10.40 μm and at 9.45 μm was achieved with a good temperature performance showing a T0,2=112 K between 125 and 200 K and a maximum working temperature exceeding T=200 K. Between 78 and 130 K a considerably higher T0,1 of 291 K is found. The decreasing T0 with higher temperatures is due to misalignment of the injector with the upper laser level at elevated temperatures, thermal activation of tunneling of carriers above 130 K, and increasing carrier leakage from the injector into the continuum.
Photon scanning tunneling microscopy of tailor-made photonic structures
Authors: Peeters C. , Flück E. , Otter A. M. , Balistreri M. L. M. , Korterik J. P. , Kuipers L. , van Hulst N. F.
Published: July 2002

Optical field distributions around individually fabricated subwavelength scatterers mapped with a photon scanning tunneling microscope are presented. The photonic structures are produced from ridge waveguides using focused-ion-beam milling. This flexible technique allows us to make single holes and slits of sizes down to 30 nm. A quantitative analysis of the observed optical pattern due to interference between incoming and reflected light yields insight about subwavelength scatterers in waveguides. We conclude that light scattering into high-loss modes of the waveguide occurs.
Double-hump diffusion profiles of copper and nickel in germanium wafers yielding vacancy-related information
Authors: Giese A. , Stolwijk N. A. , Bracht H.
Published: July 2002

Diffusion of Cu and Ni into Ge was investigated between 700 and 900 °C with the aid of rapid isothermal lamp annealing and spreading-resistance profiling. Using low-dislocation-density single-crystal Ge wafers with a backside gold layer, we observed typical double-hump diffusion profiles of both Cu and Ni. These profiles can be described within the dissociative model by taking into account that the front surface acts as source for both vacancies (V) and Cu or Ni while the back surface combines the V-source feature with a Cu, Ni-sink property. Profile fitting yields data regarding the V-assisted Ge self-diffusion coefficient and the equilibrium concentration of vacancies as a function of temperature.
Selection rules for nuclear spin modifications in ion-neutral reactions involving H3+
Authors: Cordonnier M. , Uy D. , Dickson R. M. , Kerr K. E. , Zhang Y. , Oka T.
Published: July 2002

We present experimental evidence for nuclear spin selection rules in chemical reactions that have been theoretically anticipated by Quack [M. Quack, Mol. Phys. 34, 477 (1997)]. The abundance ratio of ortho-H3+ (I=3/2) and para-H3+ (I=1/2), R=[o-H3+]/[p-H3+], has been measured from relative intensities of their infrared spectral lines in hydrogen plasmas using para-H2 and normal-H2 (75% o-H2 and 25% p-H2). The observed clear differences in the value of R between the p-H2 and n-H2 plasmas demonstrate the spin memory of protons even after ion-neutral reactions, and thus the existence of selection rules for spin modifications. Both positive column discharges and hollow cathode discharges have been used to demonstrate the effect. Experiments using pulsed plasmas have been conducted in the hollow cathode to minimize the uncertainty due to long-term conversion between p-H2 and o-H2 and to study the time dependence of the o-H3+ to p-H3+ ratio. The observed R(t) has been analyzed using simultaneous rate equations assuming the nuclear spin branching ratios calculated from Quack’s theory. In p-H2 plasmas, the electron impact ionization followed by the ion-neutral reaction H2++H2→H3++H produces pure p-H3+, but the subsequent reaction between p-H3+ and p-H2 scrambles protons. While the proton hop reaction (rate constant kH) maintains the purity of p-H3+, the hydrogen exchange reaction (rate constant kE) produces o-H3+ and acts as the gateway for nuclear spin conversion. The value of R(t), therefore, depends critically on the ratio of their reaction rates α=kH/kE. From observed values of R(t), the ratio has been determined to be α=2.4. This is in approximate agreement with the value reported by Gerlich using isotopic species.
Resonantly forced inhomogeneous reaction-diffusion systems
Authors: Hemming C. J. , Kapral R.
Published: July 2002

The dynamics of spatiotemporal patterns in oscillatory reaction-diffusion systems subject to periodic forcing with a spatially random forcing amplitude field are investigated. Quenched disorder is studied using the resonantly forced complex Ginzburg–Landau equation in the 3:1 resonance regime. Front roughening and spontaneous nucleation of target patterns are observed and characterized. Time dependent spatially varying forcing fields are studied in the 3:1 forced FitzHugh–Nagumo system. The periodic variation of the spatially random forcing amplitude breaks the symmetry among the three quasi-homogeneous states of the system, making the three types of fronts separating phases inequivalent. The resulting inequality in the front velocities leads to the formation of “compound fronts” with velocities lying between those of the individual component fronts, and “pulses” which are analogous structures arising from the combination of three fronts. Spiral wave dynamics is studied in systems with compound fronts.
Effect of temperature on the optical poling process in a side-chain polymer system
Authors: Liu Xuchun , Xu Gang , Si Jinhai , Ye Peixian , Li Zhao , Shen Yuquan
Published: July 2002

An improvement to the Fiorini and Nuzi’s theory for all-optical poling is proposed. The influence of cis is taken into account in the photoinduced process. An analytical expression for the dynamic optical poling process is derived. Contrary to Fiorini and Nuzi’s theory which is based on unique process, three processes exist in the time evolution of the photoinduced second-order susceptibility χ(2) during the optical poling: two of which are fast and the other one which is slow. Furthermore we discuss the influence of temperature on the poling process in a side-chain polymer system with a high glass transition temperature. It is predicted that the two fast processes will become even faster and their relative contribution to the magnitude of χ(2) will decrease with the increase of temperature. The prediction is verified by our experiment with disperse red 19-functionalized polyimide polymer.
The magnetostriction of Tb(Fe0.9MnxAl0.1−x)2 alloys
Authors: Shih Jhy-Chau , Hsu Shu-Yang , Chao Ling-Jiun , Chin Tsung-Shune
Published: July 2002

Small amounts of Al and Mn were introduced to partly substitute for Fe in polycrystalline Terfenol alloy TbFe2 as Tb(Fe0.9MnxAl0.1−x)2(x=0–0.1). Their effects on structure, magnetic properties, magnetostriction, and hardness were studied. X-ray diffraction shows that the studied alloys remain cubic Laves phase. The saturation magnetization increases with Mn content. Curie temperature dramatically decreases with Mn addition and is further lowered due to the Al replacement. A magnetostriction of 1890 ppm at 19 kOe was obtained in the modified alloy x=0.06. Magnetostriction value at low field is distinctly greater, especially when x=0.04, than that of the TbFe2 or the Mn/Al. At 1 kOe, the magnetostriction of the x=0.04 alloy is 250 ppm compared to, 22 ppm in the base alloy. Addition of both Mn and Al improves the maximum value of dynamic strain coefficient d33(max) and lowers the corresponding field for Tb(Fe0.9MnxAl0.1−x)2 alloys as compared with the base alloy. When x=0.04, d33(max) is 360×10−9 Oe−1, 1.7 times higher than that of the base alloy, while the corresponding field is 0.562 kOe, 3.1 times lower than that of the base alloy.
Blue photoluminescence in ZnGa2O4 thin-film phosphors
Authors: Lee Yong Eui , Norton David P. , Park Chan , Rouleau Christopher M.
Published: July 2002

The growth and properties of luminescent ZnGa2O4 thin films using pulsed laser ablation has been investigated. As-deposited films on glass and (100) MgO substrates exhibit blue-white photoluminescence with a broad emission band under ultraviolet excitation. In situ epitaxial films obtained on single crystal (100) MgO substrates possess enhanced luminescent intensity as compared to polycrystalline films on glass substrates. The enhanced luminescence in epitaxial films presumably reflects lower defect densities due to growth on low energy surfaces.
Tracer surface diffusion at high pressures: Molecular-dynamics study
Authors: Zeiri Yehuda
Published: July 2002

Molecular-dynamics simulations were employed to investigate the influence of high pressure on tracer surface diffusion. A model potential was used to describe the interaction among the various species in the system. The different binding energy values and masses used in this model simulation correspond to surface diffusion of N2 on Ru(001) surface under pressure of Ar. A pronounced enhancement in the magnitude of the diffusion coefficients was observed when pressure increased from P=0 to P=200 atm. The relationship between diffusion coefficient and three parameters that characterize the system was explored. It was found that the gas temperature and the nature of gas–adsorbate interaction (i.e., attractive or repulsive) have only a negligible influence on the diffusion coefficient. However, a marked variation in the diffusion coefficient was observed when the magnitude of gas-substrate binding energy was altered. The temperature dependence of the surface diffusion coefficient exhibits an Arrhenius behavior for all cases investigated. The relationship between the pressure and both pre-exponential factor and activation energy for surface diffusion was discussed based on a detailed analysis of the diffusion mechanism. The diffusion mechanism was deduced by careful examination of large number of individual trajectories.
On the relationships among special temperatures for supercooled liquids: A configuration space analysis
Authors: Mohanty Udayan , Diezemann Gregor , Fourkas John T.
Published: July 2002

The relationships among the Vogel–Tamman–Fulcher, the glass transition, and the crossover temperatures proposed by Odagaki is rederived without recourse to configurational entropy arguments and is compared with experimental data on several glass-forming liquids.
Two-phase flow analysis of unstable fluid mixing in one-dimensional geometry
Authors: Saltz David , Lee Wonsuck , Hsiang Tien-Ruey
Published: July 2002

A two-phase flow model for an acceleration-driven compressible fluid mixing layer is applied to an initially planar/cylindrical/spherical fluid configuration. A conservative form of the one-dimensional compressible equations is derived under the assumption that the fluid concentration is continuous. With a hyperbolic conservation law for the concentration gradient, the model supports traveling discontinuities in this quantity. The primary examples of this wave type are the moving boundaries of a finite mixing layer, which determine the instability growth rate. Constitutive laws for interfacial averages, previously derived for planar incompressible mixing, are reinterpreted and shown to be applicable to other one-dimensional mixing problems of interest. The equations of motion for an incompressible mixing layer in planar, cylindrical, or spherical geometry are solved exactly, up to a history integral of a function of the edge trajectories, and without assuming incompressible flow outside the layer. Full solutions are obtained by numerically integrating a coupled system of ordinary differential equations for the volume fraction characteristics. Results for self-similar Rayleigh–Taylor mixing in planar geometry are compared to the work of others. This comparison suggests that the shape of the fluid concentration profile is primarily a consequence of mass conservation, parameterized by the expansion ratio of the mixing zone edges.
Full configuration interaction and multiconfigurational spin density in boron and carbon atoms
Authors: Pak Michael V. , Gordon Mark S.
Published: July 2002

The reliability of spin polarization method results for atomic spin densities, obtained with several widely used Gaussian basis sets, is examined by comparison with the results of full configuration interaction (FCI) calculations. The spin densities obtained with these basis sets using the spin polarization model and some other methods disagree with the FCI treatment. Since the FCI wave function is exact for a given basis, it is not clear that the spin polarization model will be generally reliable. A large active space multiconfigurational (CASSCF) calculation is shown to be inadequate as an alternative to FCI treatment. The importance of accounting at least to some extent for excitations to all orbitals in the complete space of basis functions is illustrated by very slow convergence of CASSCF results with increasing size of active space. The FCI results reported here can be used as benchmarks to test various approaches to spin density calculation.
Thermochemistry of gas phase CF2 reactions: A density functional theory study
Authors: Lau Kenneth K. S. , Gleason Karen K. , Trout Bernhardt L.
Published: July 2002

Enthalpies of formation and enthalpies of reaction at 298 K for a set of fluorocarbon species derived from the pyrolysis of hexafluoropropylene oxide (HFPO) were computed using the B3LYP (Becke three-parameter Lee–Yang–Parr) density functional theory. Total energies were calculated at the B3LYP/6-311+G(3df )//B3LYP/6-31G(d) level. Zero-point energies and thermal corrections were calculated using B3LYP/6-31G(d)//B3LYP/6-31G(d) vibrational frequencies scaled by a factor of 0.96. The average absolute deviation of enthalpies of formation and reaction were 2.33 and 1.42 kcal/mol, respectively. The pyrolysis of HFPO to produce difluorocarbene, CF2, and trifluoroacetyl fluoride, CF3CFO, was predicted to be endothermic at 23.6 kcal/mol. The singlet state of CF2 was predicted to be more stable than its triplet state by 52.4 kcal/mol. CF2 polymerization through the addition of a singlet CF2 to an existing unterminated chain was calculated to be more favorable than through the addition of a singlet CF2 to an existing perfluoroalkene molecule. For the former pathway, a linear relationship was found between the enthalpy of formation of the CF2 chains and the number of chain carbons (n) for n⩾2. The reaction enthalpy for each successive CF2 extension was found to be −48.7 kcal/mol, and the carbon–carbon bond dissociation energy was found to be 75.5 kcal/mol. For the latter pathway, the stability of CF2=CF2 hindered chain polymerization but provided theoretical support to CF2=CF2 as a primary product in HFPO pyrolysis experiments.
Improved quantum mechanical study of the potential energy surface for the bithiophene molecule
Authors: Duarte Hélio A. , Dos Santos Hélio F. , Rocha Willian R. , De Almeida Wagner B.
Published: July 2002

The potential energy surface (PES) for the 2,2′-bithiophene molecule was investigated using Hartree–Fock, correlated MP2, MP4(SDQ), CCSD, and density functional theory levels. Distinct basis sets ranging from double-zeta to triple-zeta quality, with polarization functions added on all atoms, were employed as well as the Dunning correlated consistent polarized valence double-zeta (cc-pVDZ) basis set. Single point configuration interaction CISD calculations were also performed using the cc-pVDZ basis set. Harmonic frequency calculations were performed for the unambiguous characterization of the stationary points located on the PES and also to calculate thermal Gibbs free energy corrections. Regarding the structural predictions we found that the B3LYP/6-311G** and MP2/cc-pVDZ fully optimized geometries exhibit the best agreement with the gas phase electron diffraction data. The calculated B3LYP/6-311G**, MP2/cc-pVDZ and experimental torsional angle for the syn-gauche structure are, respectively, 37.4° (B3LYP), 39.9° (MP2), and 36°±5° (expt.) with the corresponding values for the anti-gauche form being, respectively, 150.3° (B3LYP), 146.0° (MP2), and 148°±3° (expt.). The relative energy between the two minima and torsional barriers are sensitive both to the size of the basis set and the level of the quantum mechanical method used. Therefore, larger basis sets are needed to assess the ability of the DFT approach for describing torsional barriers. The MP4(SDQ) and CCSD relative energy results, reported in this work, can be considered as the most reliable torsional potential data available for the 2,2′-bithiophene molecule. Our results indicate that the experimentally estimated relative energy value for the two equilibrium structures present on the PES for the bithiophene molecule, and consequently the relative abundance of the anti-gauche species, is somewhat underestimated. By comparison with MP4(SDQ) and CCSD results we have shown that single point DFT/6-311G** calculations using HF/6-31G* geometries is the most computationally efficient procedure to study bithiophene like systems, with energy barriers agreeing within 2 kJ/mol.
Preparation of highly transparent superconductor-semiconductor contacts
Authors: Knoch J. , Appenzeller J. , Lengeler B.
Published: July 2002

In this report we present a procedure to fabricate highly transmissive superconducting Nb-In0.77Ga0.23As contacts. A combination of a sulphur passivation of the etched semiconductor prior to the deposition of Nb and an annealing step is used. To quantitatively classify the transparency of the contacts, transport measurements of the differential conductance are carried out at 300 mK and compared with a model given by Blonder, Tinkham, and Klapwijk [Phys. Rev. B 25, 4515 (1981)]. The procedure yields almost ideal superconductor-semiconductor contacts. Additionally, a high reproducibility of the contact transparency is achieved. The results are interpreted in terms of diffusion of In in both the niobium and the In0.77Ga0.23As.
Crystal morphology and phase purity of diamond crystallites during bias enhanced nucleation and initial growth stages
Authors: Sun C. , Zhang W. J. , Wang N. , Chan C. Y. , Bello I. , Lee C. S. , Lee S. T.
Published: July 2002

The crystal morphologies and phase composition of diamond crystallites during bias enhanced nucleation and initial growth stages in microwave plasma chemical vapor deposition were investigated. Diamond nuclei were first formed in the central regions of substrates and then propagated to the sample edges. During the course of bias nucleation, excessive ion bombardment induced secondary nucleation sites on the already formed nuclei. The secondary nucleation deteriorated the overall alignment of the growing crystals. Hence, the elimination of secondary nucleation and homogeneous nucleation over substrates are fundamental requirements for the deposition of large-area uniformly oriented diamond films. Decreasing reactant pressure was found to be effective for improving plasma homogeneity and consequently nucleation uniformity. The results of bias enhanced nucleation within a pressure range from 8 to 20 Torr showed that the lower pressure of reactants enlarged the area of oriented diamond films. However, the optimum bias and duration of nucleation was found to be specific for each pressure.
Photoelectric phenomena in polymer-based composites
Authors: Shik A. , Ruda H. , Sargent E. H.
Published: July 2002

We consider photoelectric phenomena in a composite medium in which inclusions form a type-II heterojunction with a host matrix. The model describes, in particular, polymer-based photosensitive structures with different types of semiconducting or insulating inclusions. It is shown that the separation on nonequilibrium carriers due to interface electric fields increases the hole component of photoconductivity, decreases the electron component, and results in a substantial increase of the total photoconductivity. The effect is observed experimentally in a wide range of polymer-based composites with disparate inclusions. For photodiode structures, this same phenomena either does not change, or slightly decrease, the quantum yield.
Optical properties of undoped and Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 single crystals
Authors: Goh Jae-Mo , Kim Wha-Tek , Jin Moon-Seog , Choe Sung-Hyu , Kim Hyung-Gon , Park Tae-Young
Published: July 2002

The optical energy band gaps of BaAl2S4 and BaAl2Se4 single crystals at 300 K were found to be 3.98 and 3.35 eV, respectively, and the optical energy band gaps of Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 single crystals were smaller than those of the undoped single crystals. Photoluminescence spectra peaked at 459 and 601 nm in the BaAl2S4 and at 486 and 652 nm in the BaAl2Se4. The photoluminescence emission peaks were attributed to donor–acceptor pair recombinations. Photoluminescence spectra of the Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 at 5 K were measured in the wavelength range of 400–900 nm. Sharp emission peaks due to Ho3+, Er3+, and Tm3+ ions were observed and their transition mechanisms were proposed.
Magnetic properties of CoII mesoclusters
Authors: Sato Michi , Takada Syozo , Kohiki Shigemi , Babasaki Tomohiro , Deguchi Hiroyuki , Oku Masaoki , Mitome Masanori
Published: July 2002

Clusters of cobalt (II) ions dispersed in a mesoporous silicate, prepared by calcination of the MCM-41 molecular sieve soaked in CoCl2 0.01 mol/l aqueous solution, showed no x-ray diffraction peaks except one from the molecular sieve. X-ray photoelectron spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy revealed that the mesoclusters consisted of CoII ions in tetrahedral coordination. We have observed temperature and frequency dependences of the alternating current susceptibility, and no divergent peaks of the nonlinear susceptibility due to superparamagnetism. Spins derived from the field dependent magnetization above the blocking temperature with the Langevin equation fairly agree with those at the cluster surface estimated from the ionic radii of Co2+ and O2−.
Microwave near-field imaging of conducting objects of a simple geometric shape
Authors: Golosovsky M. , Lann A. F. , Davidov D. , Frenkel A.
Published: July 2002

We study imaging properties of a 90 GHz microwave scanning probe consisting of a thin slit aperture (width of 10–100 μm) in the waveguide endwall. We perform vector measurements of the near-field reflectivity of test samples (conducting dot, strip, half plane, plane) at various probe–sample separations and orientations. Experimental results for small objects agree fairly well with analytical calculations and computer simulations and can be described by a quasistatic model. Experimental results for extended conducting objects show pronounced deviations from the quasistatic model due to excitation of the surface waves. Our experimental results demonstrate several peculiar features which have not been properly addressed in the context of near-field imaging, namely: (i) strong collimation of the fields away from the probe up to a distance equal to the probe width, (ii) very weak phase dependence on distance in the extreme near field, and (iii) excitation of surface waves above conducting surfaces.
Sample size effect in photoelectrochemical etching of n-GaAs
Authors: Ma Qing , Moldovan Nicolaie , Mancini Derrick C. , Rosenberg Richard A.
Published: July 2002

A sample size effect on the etching rate in the photoelectrochemical etching of n-GaAs is demonstrated using synchrotron radiation x rays as the light source. It is shown that the etching rate increases significantly with the ratio of sample size to x-ray illuminated area. The rate-limiting effect on the charge transfer across the semiconductor–electrolyte junction is found to account for the phenomenon. It is also found that the etching rate relates to the nonilluminated area with a rather simple function.
Harmonic current-phase relation in Nb–Al-based superconductor/insulator/normal conductor/insulator/superconductor-type Josephson junctions between 4.2 K and the critical temperature
Authors: Götz M. , Khanin V. V. , Schulze H. , Zorin A. B. , Niemeyer J. , Il’ichev E. , Chwala A. , Hoenig H. E. , Meyer H.-G.
Published: July 2002

For a study of the supercurrent–phase relationship, we have fabricated Nb/Al/AlOx/Al/AlOx/Al/Nb-based Josephson junctions of low critical current density. At 4.2 K, junctions of a nominal area of 10×10 μm2 showed a nonhysteretic dc current–voltage characteristic with a critical current of about 10 μA. Similar junctions were part of washer-type rf superconductor quantum interference devices. These samples were inductively coupled to high-quality tank circuits for measurements at 4.2 K or at higher temperatures up to values close to the critical temperature of niobium. In this range, the current–phase relation, as deduced from impedance measurements of the phase-biased junction, did not deviate significantly from the sinusoidal shape.