bluuebunny/crossref_metadata_2025 huggingface dataset:
Highly sensitive open-flat coil magnetometer for the λ(H,T) measurements in plate-like high-Tc cuprates
Authors: Gevorgyan S. G. , Kiss T. , Movsisyan A. A. , Shirinyan H. G. , Hanayama Y. , Katsube H. , Ohyama T. , Takeo M. , Matsushita T. , Funaki K.
Published: July 2002

The “LC resonator” method was greatly improved for high resolution measurements of magnetic-field penetration depth, λ, of radio frequency into thin films or plate-like high-Tc superconductors by replacing a solenoid testing coil by an open-flat coil (OFC) driven by a tunnel diode oscillator of a low power and a highly stable frequency. The assembled OFC magnetometer showed new technical advantages such as simplicity, convenience, and high resolution of Δλ∼1–3 Å (Δλ/λ∼10−6), ability of reliable operation in wide ranges of temperature (4.2–300 K) and magnetic field (up to 12 T), etc. This enables us to use it for studying peculiarities of the vortex dynamics with weak signal in small sample volume high-Tc cuprates. As a first test of the assembled 23 MHz measuring setup, the temperature dependence of the upper critical field, HC2(T), is determined from the magnetic transition curves at different temperatures for Y1Ba2Cu3O7−δ thin-film ring. The HC2(T) data obey the formula: [1−(T/Tc)2]β with β=1.22±0.03 at temperatures close to Tc. The methods to increase the resolution of the OFC magnetometer by several orders of magnitude are suggested. It is believed to be feasible to reach the level of detection of the suppression of superconductivity in samples of size of a few micrometers by penetration of a few flux quanta by decreasing the size of the open-flat coil.
A Josephson field effect transistor using an InAs-inserted-channel In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure
Authors: Akazaki Tatsushi , Takayanagi Hideaki , Nitta Junsaku , Enoki Takatomo
Published: July 2002

A Josephson field effect transistor (JOFET) was coupled with a two-dimensional electron gas in a strained InAs quantum well inserted into an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure. The characteristics of this JOFET are much improved over previous devices by using a high electron mobility transistor (HEMT)-type gate instead of the usual metal-insulator- semiconductor (MIS)-type gate. The superconducting critical current as well as the junction normal resistance are completely controlled via a gate voltage of about −1 V; this provides voltage gain over 1 for a JOFET.
Defect-related versus excitonic visible light emission from ion beam synthesized Si nanocrystals in SiO2
Authors: Min K. S. , Shcheglov K. V. , Yang C. M. , Atwater Harry A. , Brongersma M. L. , Polman A.
Published: July 2002

Two sources of room temperature visible luminescence are identified from SiO2 films containing ion beam synthesized Si nanocrystals. From a comparison of luminescence spectra and photoluminescence decay lifetime measurements between Xe+-implanted SiO2 films and SiO2 films containing Si nanocrystals, a luminescence feature attributable to defects in the SiO2 matrix is unambiguously identified. Hydrogen passivation of the films selectively quenches the matrix defect luminescence, after which luminescence attributable to Si nanocrystals is evident, with a lifetime on the order of milliseconds. The peak energy of the remaining luminescence attributable to Si nanocrystals ‘‘redshifts’’ as a function of different processing parameters that might lead to increased nanocrystal size and the intensity is directly correlated to the formation of Si nanocrystals. Upon further annealing hydrogen-passivated samples at low temperatures (<500 °C), the intensity of nanocrystal luminescence increases by more than a factor of 10.
Oxide-free blue photoluminescence from photochemically etched porous silicon
Authors: Mizuno Hiroyuki , Koyama Hideki , Koshida Nobuyoshi
Published: July 2002

It has been shown that the blue photoluminescence (PL) of porous silicon (PS) can be obtained simply from postanodization illumination by white light without any oxidation processes. The PS samples were formed on 5–6 Ω cm p-type (100) Si wafers. The anodization was carried out in an ethanoic HF solution at a current density of 50 mA/cm2 for 5 min in the dark. After the anodization, the samples were illuminated under the open-circuit condition by a 500 W tungsten lamp for several minutes. The PL measurements of prepared PS samples were carried out in vacuum. With increasing the postanodization illumination time, the PL band exhibited a continuous shift from red to blue. During this blue shift, fourier transform infrared spectra did not show any signs of the growth of the surface oxide. In addition, the emission energy dependence of the PL decay time for each PL band behaved in accordance with a universal curve. These results strongly suggest that there is an oxide-free mechanism in the PL emission from PS throughout the visible range.
Imaging the local electrical properties of metal surfaces by atomic force microscopy with conducting probes
Authors: Houzé F. , Meyer R. , Schneegans O. , Boyer L.
Published: July 2002

A promising technique capable of performing localized resistance measurements over a surface is presented using a modified commercial atomic force microscope with a conducting probe. Its overall purpose is to obtain simultaneous cartographies of surface roughness and local resistance within a given microscopic area of a sample with nanometer scale resolution. Although an elaboration of suitable probes remains an ongoing problem, convincing images of some metal surfaces that reveal occasionally surprising features have already been obtained. Calculations performed from measurements have allowed us to clarify the mechanical nature of the tip/surface nanocontact and hence to determine the most probable transport process according to the range of resistance considered.
Nonalloyed Ti/Al Ohmic contacts to n-type GaN using high-temperature premetallization anneal
Authors: Lester L. F. , Brown J. M. , Ramer J. C. , Zhang L. , Hersee S. D. , Zolper J. C.
Published: July 2002

On Si-implanted n-type GaN, a nonalloyed Ti/Al metallization has been found to form an Ohmic contact that has a specific contact resistance as low as 1.0×10−5 Ω cm2. The Ohmic character is believed to be caused by the 1120 °C implant activation anneal which generates nitrogen vacancies that leave the surface heavily n type. This theory is indirectly confirmed on unimplanted n-type GaN by comparing the rc of nonalloyed Ti/Al on unannealed GaN with that of nonalloyed Ti/Al on 1120 °C annealed GaN. The former has rectifying electrical characteristics, while the latter forms an Ohmic contact with an rc=1.3×10−3 Ω cm2.
Field-dependence of microscopic probes in magnetic force microscopy
Authors: Babcock K. L. , Elings V. B. , Shi J. , Awschalom D. D. , Dugas M.
Published: July 2002

We present a technique for characterizing the magnetic state of a magnetic force microscopy (MFM) probe as a function of uniform external magnetic field H. A local magnetic field is generated by micron-scale current carrying conductors and directly imaged by MFM. As H alters the magnetic state of the probe, changes in image contrast yield componentwise measures of the tip’s net magnetic moment m, tip hysteresis loops and coercivities, and possible orientations (vertical vs lateral) of remanent states mr used for most MFM imaging. Results are presented for a variety of thin-film probes.
Quantum-size effects in the titanosilicate molecular sieve
Authors: Borello Enzo , Lamberti Carlo , Bordiga Silvia , Zecchina Adriano , Areán Carlos Otero
Published: July 2002

The recently synthesized Engelhard titanosilicate (ETS-10) represents a material which contains in the structure well defined atomic ⋯O–Ti–O–Ti–O⋯ quantum wires embedded in a highly insulating siliceous matrix. We report and discuss the UV-Vis spectrum of this material and compare the experimentally determined optical band gap with the results predicted by simple modeling of a titanium oxide semiconductor wire unidimensionally confined by an infinite potential barrier.
Room-temperature continuous-wave lasing from stacked InAs/GaAs quantum dots grown by metalorganic chemical vapor deposition
Authors: Heinrichsdorff F. , Mao M.-H. , Kirstaedter N. , Krost A. , Bimberg D. , Kosogov A. O. , Werner P.
Published: July 2002

We report on quantum dot (QD) lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (dl⩾4×1010 cm−2) was achieved in a narrow growth parameter window. The room-temperature photoluminescence (PL) intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In0.3Ga0.7As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.
Inducing a large second-order optical nonlinearity in soft glasses by poling
Authors: Garcia F. C. , Carvalho I. C. S. , Hering E. , Margulis W. , Lesche B.
Published: July 2002

Electrothermal poling of soft glasses (soda lime and borosilicate glass) at relatively high currents led to the creation of a second-order optical nonlinearity. The second-harmonic generation efficiency decays in time, and the decay rate can be accelerated by exposure to intense infrared radiation.
Diamondlike carbon deposition on silicon using radio-frequency inductive plasma of Ar and C2H2 gas mixture in plasma immersion ion deposition
Authors: Lee D. H. , He X. M. , Walter K. C. , Nastasi M. , Tesmer J. R. , Tuszewski M. , Tallant D. R.
Published: July 2002

Diamondlike carbon (DLC) was deposited on silicon using a plasma immersion ion deposition (PIID) method. Inductive radio-frequency plasma sources were used to generate Ar and C2H2 plasmas at low gas pressures ranging from 0.04 to 0.93 Pa. The film stress and hardness were sharply dependent upon bias voltage at an operating pressure of 0.04 Pa. A maximum hardness of 30 GPa and compressive stress of 9 GPa was observed at a pulsed bias of −150 V bias (carbon energy of 80 eV). The mechanical properties of DLC films are correlated with UV Raman peak positions which infer sp3-bonded carbon contents.
Characterization of catastrophic optical damage in Al-free InGaAs/InGaP 0.98 μm high-power lasers
Authors: Park K. H. , Lee J. K. , Jang D. H. , Cho H. S. , Park C. S. , Pyun K. E. , Jeong J. Y. , Nahm S. , Jeong J.
Published: July 2002

Catastrophic optical damage (COD) in Al-free InGaAs/InGaP 0.98 μm lasers has been investigated using real-time electroluminescence (EL) and transmission electron microscopy (TEM). From EL images, we observed that multiple bright spots initiated from one of the facets and then propagated to the center of the cavity during the COD process. It is clarified by the TEM analysis that the propagation of bright spots resulted in 60-nm-wide Moiré fringe along the cavity and the crystalline phase of the active area became polycrystalline. Highly nonradiative polycrystalline phase of the active area is the major cause of COD failure in the Al-free 0.98 μm lasers.
Effect of three-dimensional strain states on magnetic anisotropy of La0.8Ca0.2MnO3 epitaxial thin films
Authors: Nath T. K. , Rao R. A. , Lavric D. , Eom C. B. , Wu L. , Tsui F.
Published: July 2002

Magnetic anisotropy of La0.8Ca0.2MnO3 (LCMO) epitaxial thin films grown on (001) SrTiO3 and LaAlO3 a substrates exhibits strong correlation with substrate-induced strain states as determined by normal and grazing incidence x-ray diffraction. In a 250 Å thick LCMO (001)T film grown on SrTiO3 substrate, an in-plane biaxial magnetic anisotropy is observed, and it is accompanied by a substrate-induced in-plane biaxial tensile strain. In contrast, the observed magnetic easy axis for a 250 Å (110)T film grown on LaAlO3 substrate is perpendicular to the film plane, and the corresponding in-plane strain is biaxial compressive. In both cases the magnetic easy axes are along the crystallographic directions under tensile strain, indicating the presence of a positive magnetostriction. In thicker films (∼4000 Å) grown on both substrates that are nearly strain relaxed, the magnetic easy axis lies in the film plane along the [110] direction of the (001) substrate.
Epitaxial thin films of ordered double perovskite Sr2FeMoO6
Authors: Manako Takashi , Izumi Makoto , Konishi Yoshinori , Kobayashi Kei-Ichiro , Kawasaki Masashi , Tokura Yoshinori
Published: July 2002

Epitaxial thin films of Sr2FeMoO6, that are ordered double perovskite with half-metallic nature, have been successfully prepared on SrTiO3 (001) and (111) substrates by pulsed laser deposition in a narrow window of the temperature and oxygen pressure. From the surface morphology analysis for the atomic scale step-and-terrace structures, the film growth is concluded to take place with the chemical formula as the growth unit when the ordering direction is normal to the surface. The films showed metallic conduction with ferromagnetic transition temperature above 400 K and intergrain tunneling type magnetoresistance even at room temperature.
Polarization modal noise and dichroism in vertical-cavity semiconductor lasers
Authors: van Exter M. P. , Willemsen M. B. , Woerdman J. P.
Published: July 2002

We show quantitatively how the limited polarization stability of vertical-cavity semiconductor lasers is related to the small dichroism and large quantum noise in these lasers. We introduce several techniques to measure this dichroism and noise. Suitable polarization projections are shown to yield either the heterodyne beat or the partition noise of the two polarization modes.
Spin filtering by ultrathin ferromagnetic films
Authors: Upadhyay Shashi K. , Louie Richard N. , Buhrman R. A.
Published: July 2002

Spin filtering of ballistic electrons by ultrathin cobalt films of thicknesses ranging from 0.2 to 3.5 nm has been studied experimentally using nonmagnetic metal–ferromagnet–superconductor nanocontacts. In such systems the flow of electrons with energies below the superconducting gap is very sensitive to any net spin polarization of the electron current. This effect was used to quantitatively measure the transmission rates of up and down spin electrons passing through an individual ferromagnetic layer of nanometer thickness.
High-quality near-field optical probes by tube etching
Authors: Stöckle Raoul , Fokas Christian , Deckert Volker , Zenobi Renato , Sick Beate , Hecht Bert , Wild Urs P.
Published: July 2002

A method called tube etching for the fabrication of near-field optical probes is presented. Tip formation occurs inside a cylindrical cavity formed by the polymer coating of an optical fiber which is not stripped away prior to etching in hydrofluoric acid. The influence of temperature, etchant concentration, and fiber type on the tip quality is studied. A tip formation mechanism for the given geometry is proposed. The procedure overcomes drawbacks of the conventional etching techniques while still producing large cone angles: (i) tips with reproducible shapes are formed in a high yield, (ii) the surface roughness on the taper is drastically reduced, and (iii) the tip quality is insensitive to vibrations and temperature fluctuations during the etching process. After aluminum coating, optical probes with well-defined apertures are obtained. Due to the smooth glass surface the aluminum coating is virtually free of pinholes.
Quantum ferroelectricity in SrTiO3 induced by oxygen isotope exchange
Authors: Itoh Mitsuru , Wang Ruiping
Published: July 2002

Oxygen isotope exchange results in a transition of SrTiO3 from quantum paraelectric to quantum ferroelectric. In the vicinity of the composition for the quantum limit, partially isotope-exchanged SrTiO3 shows a high dielectric constant ∼172 000, whereas unexchanged or fully exchanged SrTiO3 shows lower values. The composition dependence of partially oxygen-exchanged SrTiO3 can be explained by the quantum mechanical “vector model” proposed by Schneider et al. [Phys. Rev. B 13, 1123 (1976)].
Laser-induced phase transformations in semiconductor quantum dots
Authors: Yakovlev V. V. , Lazarov V. , Reynolds J. , Gajdardziska-Josifovska M.
Published: July 2002

We report laser-induced phase transitions in CdS nanocrystals under radiation of 7 ns laser pulses with a wavelength of 532 nm. The observed change in size distribution of the nanocrystals, as well as the change of their crystal structure from orthorombic to cubic, are uncovered by high-resolution transmission electron microscopy and selected area transmission electron diffraction.
Partially crystallized La0.5Sr0.5MnO3 thin films by laser ablation and their enhanced low-field magnetoresistance
Authors: Liu J.-M. , Li J. , Huang Q. , You L. P. , Wang S. J. , Ong C. K. , Wu Z. C. , Liu Z. G. , Du Y. W.
Published: July 2002

Amorphous, partial-crystallized, and epitaxial La0.5Sr0.5MnO3 thin films have been deposited at various temperatures of 200–650 °C on (001) SrTiO3 substrates using pulsed-laser deposition. The x-ray diffraction and high-resolution transmission electron microscopy indicate complete (001) orientation of the crystalline structures in these films. Enhanced low-field magnetoresistance effect has been observed for the partial-crystallized thin films where the nanosized ferromagnetic crystals are embedded in nonferromagnetic amorphous matrix. It is argued that the amorphous layer separating the neighboring nanocrystals behaves as the barrier for the spin-polarized tunneling and/or spin-dependent scattering, resulting in enhanced magnetoresistance at low magnetic field.
From N isoelectronic impurities to N-induced bands in the GaNxAs1−x alloy
Authors: Klar P. J. , Grüning H. , Heimbrodt W. , Koch J. , Höhnsdorf F. , Stolz W. , Vicente P. M. A. , Camassel J.
Published: July 2002

GaN x As 1−x samples with x<3% grown by metalorganic vapor phase epitaxy were studied by low-temperature photoluminescence under hydrostatic pressure and photomodulated reflectance spectroscopy. The transformation from N acting as an isoelectronic impurity to N-induced band formation takes place at x≈0.2%. The N level does not shift with respect to the valence band edge of GaNxAs1−x. Concentration as well as hydrostatic-pressure dependence of the GaNxAs1−x bands can be described by a three band kp description of the conduction band state E− and E+ and the valence band at k=0. The model parameters for T<20 and T=300 K were determined by fitting the model to the experimental data. Modeling the linewidth of the E− transition by combining the kp model and ion statistics leads to the conclusion that the electron-hole pairs are strongly localized.
The benzene molecule as a molecular resonant-tunneling transistor
Authors: Di Ventra M. , Pantelides S. T. , Lang N. D.
Published: July 2002

Experiments and theory have so far demonstrated that single molecules can form the core of a two-terminal device. Here we report first-principles calculations of transport through a benzene-1, 4-dithiolate molecule with a third capacitive terminal (gate). We find that the resistance of the molecule rises from its zero-gate-bias value to a value roughly equal to the quantum of resistance (12.9 kΩ) when resonant tunneling through the π* antibonding orbitals occurs.
Estimation of interwell terahertz gain by photon-assisted tunneling measurement in triple-barrier resonant tunneling diodes
Authors: Asada M. , Oguma Y. , Sashinaka N.
Published: July 2002

Terahertz (THz) gain due to electron transition between adjacent quantum wells was estimated from a measurement of current change in triple-barrier resonant tunneling diodes under the THz irradiation. Measured current change was separated into stimulated emission and absorption components, and the gain coefficient was estimated from the difference between these components. For a sample with relatively thick barriers and peak current density of 70 A/cm2, the room-temperature peak gain coefficient was about 0.15 cm−1 at 2.24 THz. The estimated gain coefficient is in reasonable agreement with an approximate theory. High gain is expected by increasing the current density with thin barriers.
Soliton dynamics of symmetry-endowed two-soliton solutions of the nonlinear Schrödinger equation
Authors: Panoiu N.-C. , Mihalache D. , Mazilu D. , Crasovan L.-C. , Mel’nikov I. V. , Lederer F.
Published: July 2002

A comprehensive analysis is presented of the propagation of symmetry-endowed two-soliton solutions under the influence of various perturbations important in nonlinear optics. Thus, we begin by introducing the analytical expressions of these two-soliton solutions. Then, by considering perturbations which preserve the initial symmetry of the two-soliton solutions, the dependence of the soliton parameters on the propagation distance is determined by using an adiabatic perturbation method. As perturbations of this kind, important for soliton-based communication systems, we consider the bandwidth-limited amplification, nonlinear amplification, and amplitude and phase modulation. Moreover, the results obtained by the adiabatic perturbation method are compared with those obtained by direct numerical simulations of the corresponding governing differential equations.
Electron-beam induced degradation in CdTe photovoltaics
Authors: Harju R. , Karpov V. G. , Grecu D. , Dorer G.
Published: July 2002

We used electron beam induced current (EBIC) to measure degradation of CdTe photovoltaic cells. We have observed that: (i) the EBIC signal shows a considerable, continuous degradation depending on the electron-beam current, scan area, energy, and sample treatment; (ii) the characteristic degradation time fluctuates between different spots on the same sample; and (iii) grain boundary regions are the most effective collectors of the electron-beam generated charge carriers. Our phenomenological model relates the observed degradation to defects caused by the electron-beam generated electrons and holes.
Maximized sp3 bonding in carbon nitride phases
Authors: Rodil S. E. , Milne W. I. , Robertson J. , Brown L. M.
Published: July 2002

Carbon nitride films were deposited using a low pressure, dual ion beam system consisting of a filtered cathodic vacuum arc and a plasma beam source for carbon and nitrogen ions, respectively. This system maintains highly ionized beams even at high nitrogen fluxes, unlike in single beam systems. Film composition and bonding were measured by electron energy loss spectroscopy. Films with nitrogen to carbon atom ratios (N/C) up to 0.5 are produced. The carbon bonding is found to change gradually from sp3 to sp2, rather than sharply above a critical N content, as found previously. This indicates that N atoms form individual C=N bonds rather than causing a reversion of the entire C network to sp2. This allows us to maintain C sp3 bonding to the highest N contents so far achieved.
Energetic frustration and the nature of the transition state in protein folding
Authors: Shea Joan-Emma , Onuchic José N. , Brooks Charles L.
Published: July 2002

The amino acid sequence and the folding motif are essential in determining the protein folding mechanism. The interplay between energetic (sequence-dependent) and topological (motif dependent) frustrations is investigated for two model beta-barrel proteins of the same native fold but with different interaction Hamiltonians. The nature of the folding transition state ensemble for both models is probed. The extent of structure in the transition state is determined by performing point mutations and recording their effect on the stability of the transition state through their φ values. The transition state shows more structural heterogeneity for the more frustrated sequence, a reflection of the increased roughness of the funneled energy landscape which restricts the number of pathways to the native state. The validity of the φ-analysis approach was assessed to be critically dependent on the degree of frustration of the model. The interpretation of φ values as a measure of the structure of the transition state breaks down for sequences with higher levels of frustration (lower cooperativity) in which a Kramers’ description of the folding reaction is no longer appropriate.
Analytical unitary bounds on quantum dynamics: Design of optimum NMR experiments in two-spin-12 systems
Authors: Untidt Thomas S. , Nielsen Niels Chr.
Published: July 2002

We report analytical solutions to the unitary bound problem for coherence/polarization transfer in IS two-spin-12 systems by means of unitary operations. Theoretical upper bounds for the transfer efficiency along with the associated optimum transformation operators are obtained analytically by decomposing the unitary operator as a product of exponentials in the special unitary Lie group SU(4). Addressing NMR spectroscopy as a specific example, the method is demonstrated for the non-Hermitian transfers I−→S− and 2I−Sz→S− being relevant for heteronuclear single-quantum coherence (HSQC) experiments as well as the double- to single-quantum transfer I−S−→I−Sβ+IβS− being representative for coherence-order and spin-state-selective transfer in INADEQUATE CR experiments. Furthermore, using a Lagrangian function approach it is demonstrated how the method enables analytical description of two-dimensional bounds for Iz→Sz cross polarization.
In
x
Ga
1−x
N
light emitting diodes on Si substrates fabricated by Pd–In metal bonding and laser lift-off
Authors: Wong W. S. , Sands T. , Cheung N. W. , Kneissl M. , Bour D. P. , Mei P. , Romano L. T. , Johnson N. M.
Published: July 2002

Indium–gallium nitride (InxGa1−xN) single-quantum-well (SQW) light emitting diodes (LEDs), grown by metalorganic chemical vapor deposition on sapphire, were transferred onto Si substrates. The thin-film InxGa1−xN SQW LED structures were first bonded onto a n+-Si substrate using a transient-liquid-phase Pd–In wafer-bonding process followed by a laser lift-off technique to remove the sapphire growth substrate. Individual, 250×250 μm2, LEDs with a backside contact through the n+-Si substrate were then fabricated. The LEDs had a typical turn-on voltage of 2.5 V and a forward current of 100 mA at 5.4 V. The room-temperature emission peak for the InxGa1−xN SQW LEDs was centered at 455 nm with a full width at half maximum of 19 nm.
Temperature dependence analysis of the thermo-optic effect in silicon by single and double oscillator models
Authors: Della Corte Francesco G. , Esposito Montefusco Maurizio , Moretti Luigi , Rendina Ivo , Cocorullo Giuseppe
Published: July 2002

The thermo-optic coefficient (dn/dT) of crystalline silicon has been critically analyzed in the temperature range 300–600 K, at the fiber optic communication wavelength of 1.5 μm. The temperature dependence has been attributed to the variation of the interband transition energies at some critical points of the silicon band structure. The experimental data have been fitted using single and double oscillator models. In particular, the double oscillator model, which is physically correlated to the silicon band structure, has been exploited to extrapolate the temperature dependence of the interband transition energies at some points (critical points) of the combined density of states. The extracted parameters are in good agreement with the data reported in the literature. Finally, in connection with both of the oscillator approximations, an analysis based on thermodynamic considerations is carried out, and electron–hole formation entropy and specific heat are calculated. The consistency of the obtained results validate the reliability of the proposed analysis.
Irreversible bimolecular reactions of Langevin particles
Authors: Bicout D. J. , Berezhkovskii A. M. , Szabo Attila
Published: July 2002

The reaction A+B→B is studied when the reactants diffuse in phase space, i.e., their dynamics is described by the Langevin equation. The steady-state rate constants are calculated for both the target (static A and mobile B’s) and trapping (mobile A and static B’s) problems when the reaction is assumed to occur at the first contact. For Brownian dynamics (i.e., ordinary diffusion), the rate constant for both problems is a monotonically decreasing function of the friction coefficient γ. For Langevin dynamics, however, we find that the steady-state rate constant exhibits a turnover behavior as a function of γ for the trapping problem but not for the target problem. This turnover is different from the familiar Kramers turnover of the rate constant for escape from a deep potential well because the reaction considered here is an activationless process.
Dynamic conductance of mesoscopic waveguides
Authors: Yip Man-Kit , Kwok Wing-Chung , Wang Jian , Guo Hong
Published: July 2002

We report a theoretical investigation of dynamic conductance G(ω), for general ac frequency ω, of two-dimensional mesoscopic waveguides whose transport is characterized by antiresonances. We calculate G(ω) by numerically evaluating nonequilibrium Green’s functions. By tuning the ac frequency we observe photon-assisted resonant transport as well as a gradual smearing out of the antiresonances. The antiresonance causes the dynamic response to vary between capacitive-like behavior to that of the inductive-like behavior.
Heat capacity and turbidity near the critical point of succinonitrile–water
Authors: Nowicki A. W. , Ghosh Madhujit , McClellan S. M. , Jacobs D. T.
Published: July 2002

Both the heat capacity and the turbidity of the liquid–liquid mixture succinonitrile–water near its upper critical consolute point were measured and two amplitude relations were tested. Using an adiabatic calorimeter to measure the heat capacity and the transmitted light intensity to determine the turbidity, precise and reproducible data determined the critical exponents α, ν, and γ consistent with theoretical predictions. The correlation length ξ0=0.168±0.004 nm was determined from the turbidity experiment while the heat capacity amplitudes were A+=0.0543±0.0004 J/(cm3 K) in the one- and A−=0.1013±0.0004 J/(cm3 K) in the two-phase region. The amplitude ratio A+/A−=0.536±0.005 was consistent with other experimental determinations in liquid–liquid mixtures or liquid–vapor systems, and with recent theoretical predictions. The two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.017 and 0.020, was determined to be 0.0187±0.0013.
Electrical isolation of GaN by MeV ion irradiation
Authors: Boudinov H. , Kucheyev S. O. , Williams J. S. , Jagadish C. , Li G.
Published: July 2002

The evolution of sheet resistance of n-type GaN epilayers exposed to irradiation with MeV H, Li, C, and O ions is studied in situ. Results show that the threshold dose necessary for complete isolation linearly depends on the original free electron concentration and reciprocally depends on the number of atomic displacements produced by ion irradiation. Furthermore, such isolation is stable to rapid thermal annealing at temperatures up to 900 °C. In addition to providing a better understanding of the physical mechanisms responsible for electrical isolation, these results can be used for choosing implant conditions necessary for an effective electrical isolation of GaN-based devices.
Liquid impedance matching system for ion cyclotron heating
Authors: Saito K. , Torii Y. , Kumazawa R. , Mutoh T. , Seki T. , Shimpo F. , Nomura G. , Yokota M. , Watari T. , Cattanei G. , Zhao Yangping
Published: July 2002

Ion cyclotron heating has been established as one of the heating schemes in nuclear fusion research and its use in steady state plasma heating in various devices is being considered. The optimal technology for steady state ion cyclotron range of frequency heating has not been firmly established. This article reports on the liquid stub tuner which was newly developed in research and development activities on the large helical device. It demonstrated high performance in real use in experiments. Two different impedance-matching systems based on the liquid stub tuner are studied: one is a triple liquid stub tuner system and the other is a single stub tuner system with a liquid phase shifter. The characteristics of the two systems are compared from the points of view of how wide a frequency range is covered, and how great the reduction of the voltage in the transmission line is.
Influence of disorder on the electron transport properties in fluorinated copper-phthalocyanine thin films
Authors: Schön Jan Hendrik , Bao Zhenan
Published: July 2002

Electron transport in polycrystalline thin films of fluorinated copper phthalocyanine is investigated by space charge limited current, photoconductivity, and field-effect transistor measurements in the temperature range from 200 to 300 K. In this region the conductivity follows the Meyer–Neldel rule. The Meyer–Neldel energy EMN is connected to the characteristic energy of the exponential tail state distribution of localized states determined by voltage-dependent space charge limited current spectroscopy. Moreover, a correlation of the conductivity prefactor and EMN is observed revealing a maximum energy barrier of 0.52 eV for the relaxation of trapped electrons.
Origins of threading dislocations in GaN epitaxial layers grown on sapphire by metalorganic chemical vapor deposition
Authors: Narayanan V. , Lorenz K. , Kim Wook , Mahajan S.
Published: July 2002

The origins of threading dislocations (TDs) in GaN epitaxial layers grown on (0001) sapphire have been investigated by examining different stages of high-temperature (HT) GaN growth on low-temperature GaN nucleation layers (NLs) by transmission electron microscopy. Results indicate that after 20 s of HT growth, GaN islands were free of TDs. After 75 and 120 s of growth, most of the islands contained pure screw (c type) and pure edge (a type) TDs with an interspersion of mixed (c+a type) TDs. Most of the TDs originated from faulted regions located within NLs. TDs move toward the island top surface (c type) or curve toward island side facets (a, c+a type). Coalescence of HT GaN islands did not give rise to either a, c, or c+a type TDs. After 240 s of growth, most TDs were predominantly of a type and could result from climb and glide of basal plane (BP) dislocations that form by the dissociation of Shockley partials located within the faulted regions. BP dislocations are also observed attached to the side facets of islands away from the faulted regions and their possible origins are discussed. c and c+a type TDs form primarily by the coalescence of Frank partials near the GaN/sapphire interface.
Carrier transport in amorphous SiC/crystalline silicon heterojunctions
Authors: Nazarov A. N. , Vovk Ya. N. , Lysenko V. S. , Turchanikov V. I. , Scryshevskii V. A. , Ashok S.
Published: July 2002

Charge carrier transport in chemical vapor-deposited amorphous SiC/p-type crystalline Si heterostructures has been studied over the temperature range 80–400 K, using current–voltage (I–V), current–temperature (I–T), capacitance–voltage (C–V), and capacitance relaxation (C–t) characteristics. These heterojunctions exhibit high breakdown voltages (230 V) and a diode rectification ratio of 103 at ±0.5 V. At low temperatures (80–120 K) the a-SiC behaves like a dielectric, and the interface built-in voltage can be determined from the capacitance–voltage plot. The corresponding low forward bias current flow is limited by variable-range electron hopping conductivity at Fermi level in the a-SiC layer. At increasing temperature and forward bias voltage, an additional hole current component is found with the transport governed by a multistep tunneling hole emission process through the a-SiC/c-Si heterobarrier. At still higher forward bias voltages (>0.8 V), space-charge-limited hole conduction in the presence of traps in the a-SiC bulk limits transport.
Orientation dependence of ferroelectricity of epitaxially grown Pb(ZrxTi1−x)O3 thin films prepared by metalorganic chemical vapor deposition
Authors: Nagashima Kuniharu , Aratani Masanori , Funakubo Hiroshi
Published: July 2002

Epitaxial Pb(ZrxTi1−x)O3 films with (001) and (100), (101) and (110), and (111) orientations were grown on (100)SrRuO3//(100)SrTiO3, (110)SrRuO3//(110)SrTiO3, and (111)SrRuO3//(111)SrTiO3 substrates, respectively. The two composition films with Zr/(Zr+Ti) ratios of 0.42 and 0.68 were prepared with the Pb/(Pb+Zr+Ti) ratio of 0.5. Well saturated and good square shape hysteresis loops with large remanent polarization (Pr) above 40 μC/cm2 were observed for all films. The Pr increased in the following order: (101), (111), and (001) orientations for the film with the Zr/(Zr+Ti) ratio of 0.42 and (100), (110), and (111) orientations for the Zr/(Zr+Ti) ratio of 0.68. On the other hand, the Ec value mainly depended on the Zr/(Zr+Ti) ratio and not on the orientation of the film; the Ec value of the film with a Zr/(Zr+Ti) ratio of 0.42 was larger than that of 0.68. The saturation behavior did not strongly depend on the orientation, especially for the films with the Zr/(Zr+Ti) ratio of 0.42. The frequency dependence of the dielectric constant was small regardless of the orientation of the films with a Zr/(Zr+Ti) ratio of 0.42. On the other hand, for 0.68, it was also small for the (100) and (110) orientations, but increased by about 15% from 103 to 106 Hz for the (111) orientation due to the relative large leakage. The (101)-oriented film with the Zr/(Zr+Ti) ratio of 0.42 and the (100)- and (111)-oriented films with 0.68 did not show deterioration up to 1010 switching cycles.
Brownian dynamics simulations of flexible polymers with spring–spring repulsions
Authors: Kumar Satish , Larson Ronald G.
Published: July 2002

We develop a method which incorporates spring–spring repulsions into Brownian dynamics simulations of flexible polymers. The distance of closest approach between two springs is computed, and a repulsive force is then applied based on this distance. Repulsive potentials of the exponential and power-law forms are considered. We demonstrate that our method is capable of accounting for excluded-volume effects in start-up of extensional flow. Equilibrium simulations indicate that spring-spring repulsions can be used to prevent the passage of two springs through each other, and thus maintain the topological integrity of polymer molecules. The method developed here is expected to be useful for simulating entanglement phenomena in both single and multichain systems.
Growth and characterization of GaAs epitaxial layers on Si/porous Si/Si substrate by chemical beam epitaxy
Authors: Saravanan S. , Hayashi Y. , Soga T. , Jimbo T. , Umeno M. , Sato N. , Yonehara T.
Published: July 2002

The initial growth of GaAs films on a Si/porous Si/Si (SPS) substrate has been investigated using reflection high-energy electron diffraction. The morphology and the thickness have been examined by a Nomarski optical microscope and scanning electron microscope, respectively. The results of the low temperature photoluminescence studies have shown that a significant reduction in the residual thermal tensile stress can be achieved with reduced growth temperature. The 77 K photoluminescence spectra for GaAs/Si show a strain-induced splitting between the heavy and light hole valence bands which corresponds to a biaxial tensile stress of 2.45 kbar acting on the GaAs layer where the same for GaAs/SPS grown at 450 °C is 1.69 kbar. The results have shown that a SPS substrate with the combination of low temperature growth is a promising candidate for obtaining GaAs films with low stress.
Effect of Néel coupling on magnetic tunnel junctions
Authors: Tegen S. , Mönch I. , Schumann J. , Vinzelberg H. , Schneider C. M.
Published: July 2002

We have studied the effect of the dipolar magnetic coupling (also known as Néel coupling or “orange-peel” coupling) in tunneling magnetoresistive (TMR) elements. With an in situ scanning tunneling microscope we directly accessed the roughness of the films and found a close correspondence between the values for the coupling fields determined by the magneto-optical Kerr effect and the ones computed on the basis of the measured morphology parameters. We confirm an increase of the dipole coupling between the magnetic layers with decreasing barrier thickness as predicted by the model. Deviations from the theoretical predictions are observed for the case of thinner soft magnetic layers, which can be explained by reduced magnetization in very thin films. We demonstrate the importance of dipolar coupling for understanding the magnetic behavior of TMR elements by comparing TMR curves for optimized and nonoptimized structures.
Hybrid interfaces of poly(9,9-dioctylfluorene) employing thin insulating layers of CsF: A photoelectron spectroscopy study
Authors: Greczynski G. , Fahlman M. , Salaneck W. R.
Published: July 2002

The recent discovery that thin insulating layers inserted between organic layer and metal cathode can significantly improve performance and lifetime of organic-based light-emitting devices is the reason for detailed studies of related interfaces. In this work we investigate the role of thin (∼5 Å) interfacial layer of CsF between poly(9,9-dioctylfluorene) (PFO) polymer film and aluminum electrode. A number of sandwich-type interfaces prepared in situ have been studied using x-ray photoelectron spectroscopy including CsF/poly(9,9-dioctylfluorene). Al/CsF/poly(9,9dioctylfluorene), Al/CsF/Au, CsF/Al, CsF/AlxOy, and CsF/Au. We found that CsF does not decompose when in contact with polymer film. There is, however, evidence that for coverage higher than 3 Å, molecules are oriented with the fluorine atom pointing downwards. Deposition of aluminum on such structures causes CsF to dissociate with cesium n-doping the polymer at the interface and fluorine likely reacting with aluminum to form AlF3. When deposited onto either a sputter-cleaned Al surface or Al with native oxide layer (AlxOy), CsF also was found to dissociate at the interface but remain in the CsF form away from the Al surface. In the case of a clean Al surface, cesium left after decomposition resides at the interface, presumably in the form of metal and metal oxide, whereas there is evidence for cesium oxide only for the CsF/AlxOy interface. Thickness of the converted CsF layer has been found to be approximately 5 Å in all cases where decomposition takes place. The lack of CsF decomposition observed for Au/CsF/PFO and CsF/Au interfaces emphasizes the crucial role aluminum plays in the whole process. The observed decomposition of CsF when in contact with Al and following n-doping of the surface region of PFO likely enhances injection of electrons into the organic layer, improving device performance. Since CsF dissociation is independent of the underlying material, the Al/CsF/emissive-material structure could be effective for almost all types of polymer/organic-based electronic devices.
Theoretical study on the outer- and inner-valence ionization spectra of H2O, H2S and H2Se using the SAC-CI general-R method
Authors: Ehara M. , Ishida M. , Nakatsuji H.
Published: July 2002

The outer- and inner-valence ionization spectra of the Group VI hydrides H2O, H2S and H2Se below the double-ionization threshold were studied by the SAC-CI (symmetry-adapted-cluster configuration-interaction) general-R method. The SAC-CI method quite accurately reproduced the experimental spectra of these hydrides and gave detailed characterizations of the shake-up states. Several unknown satellite peaks were predicted. The shake-up state which includes excitations to the Rydberg orbitals was found to be very important for describing the satellite peaks of these hydrides. A detailed inner-valence satellite spectrum of H2Se is theoretically proposed prior to any experimental observation.
Nanoanalysis of Co/Cu/NiFe thin films by tomographic atom probe
Authors: Schleiwies J. , Schmitz G. , Heitmann S. , Hütten A.
Published: July 2002

Offering the possibility of improving data storage and magnetic sensoric, applications of giant magnetoresistance (GMR) in thin metallic films are of great interest. In order to study thermal reactions in such layered structures, atom probe tomography is used, which has been proven to perform a real three-dimensional analysis on the relevant length scale of several angstroms only. Co/Cu/Ni79Fe21 layered structures were deposited on tungsten substrate tips by ion beam sputtering and analyzed in the as-prepared state and after suitable heat treatments. After annealing at 250 °C for 30 min, Fe segregation at the Co/Cu interface inside the Co layer is clearly observed. This effect may be interpreted as an interface dusting potentially increasing the GMR. After annealing at 350 °C for 30 min, an additional Ni segregation inside Cu grain boundaries is observed. It is suggested that this segregation path forms the initial stage of pinhole formation and finally causes ferromagnetic bridges through the paramagnetic coupling layer.
Over 1.5 μm light emission from InAs quantum dots embedded in InGaAs strain-reducing layer grown by metalorganic chemical vapor deposition
Authors: Tatebayashi Jun , Nishioka Masao , Arakawa Yasuhiko
Published: July 2002

We demonstrated the 1.52 μm light emission at room temperature from self-assembled InAs quantum dots embedded in the In0.45Ga0.55As strain-reducing layer. By capping InAs quantum dots with an InGaAs strain-reducing layer instead of GaAs, the photoluminescence peak of InAs quantum dots can be controlled by changing the indium composition of the InGaAs strain-reducing layer. The full width at half maximum is as narrow as 22 meV. The wavelength of 1.52 μm is the longest wavelength so far achieved in self-assembled InAs quantum dots, which would be promising to quantum-dot lasers on GaAs substrate for application to light sources in long-wavelength optical communication systems.
Hole injection barriers at polymer anode/small molecule interfaces
Authors: Mäkinen A. J. , Hill I. G. , Shashidhar R. , Nikolov N. , Kafafi Z. H.
Published: July 2002

A photoemission study of the interface between spin-cast films of a conducting polymer blend consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4′-bis(carbazol-9-yl)biphenyl, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1-1′biphenyl-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′- biphenyl-4,4′-diamine reveals a hole injection barrier between 0.5 and 0.9 eV at the glycerol-modified PEDOT-PSS/HTL interface. The measured energy barriers imply a reasonable charge injection, which is very encouraging for further development of the novel anode structures based on a conducting polymer/small molecule interface to be utilized in electro-optic applications such as organic light-emitting devices.