bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-dimensional electro-optic imaging of THz beams
Authors: Wu Q. , Hewitt T. D. , Zhang X.-C.
Published: July 2002

We report on a novel electro-optic sampling system for real-time terahertz (THz) imaging applications. By illuminating a 6×8 mm2 ZnTe crystal with a 300 μW optical sampling beam and detecting the beam with a digital CCD camera, we achieved time-resolved images of pulsed far-infrared radiation emitted from an unbiased GaAs wafer. At the focal point of the peak far-infrared field, the THz beam diameter is approximately 0.75 mm (full width at half-maximum). The temporal and spatial resolutions of this imaging system are mainly limited by the laser pulse duration and the diffraction limit of the THz beam, respectively.
Rectification properties and interface states of heterojunctions between solid C60 and n-type GaAs
Authors: Chen K. M. , Zhang Y. X. , Qin G. G. , Jin S. X. , Wu K. , Li C. Y. , Gu Z. N. , Zhou X. H.
Published: July 2002

Solid C60/n-GaAs heterojunctions have been fabricated by deposition of solid C60 film on the (100)-oriented epitaxial n-type GaAs substrates and their electrical characteristics have been measured. The rectification ratio is greater than 106 at a bias of ±1 V. The current for a fixed forward bias is an exponential function of reciprocal temperature, from which the effective potential barrier height of the heterojunction is determined to be 0.58 eV. A trap with an energy level 0.35 eV below the conduction band of GaAs at the C60/GaAs interface has been observed by the deep level transient spectroscopy technique.
Determining band offsets using surface photovoltage spectroscopy: The InP/In0.53Ga0.47As heterojunction
Authors: Leibovitch M. , Kronik L. , Mishori B. , Shapira Yoram , Hanson C. M. , Clawson A. R. , Ram Prakhya
Published: July 2002

A direct technique for determining band offsets at semiconductor heterojunctions is presented, which is applicable to at least any type I heterojunction, where the top layer doping is sufficiently low. The technique is based on surface photovoltage spectroscopy measurements as a function of overlayer thickness. A numerically simulated example shows that the band offset is a very strong function of the critical overlayer thickness, at which the overlayer contribution to the surface photovoltage spectrum appears. The method is applied to the technologically important InP/InGaAs heterojunction and is shown to yield the commonly accepted band offset value.
Monitoring surface stoichiometry with the (2×2) reconstruction during growth of hexagonal-phase GaN by molecular beam epitaxy
Authors: Hacke P. , Feuillet G. , Okumura H. , Yoshida S.
Published: July 2002

Reflection high-energy electron diffraction is used to study the stability of the (2×2) reconstruction on the (0001) surface of hexagonal-phase GaN as a function of growth parameters. The relationship between the critical conditions for existence of the reconstruction, which corresponds to a unique surface stoichiometry, is used to show the interdependency and scalability of growth parameters. A model is proposed to describe the stoichiometric balance of the species arriving on the surface at the critical conditions for observation of the reconstruction. Hall mobility of the GaN epitaxial layers was improved by growing under these conditions.
Three-layered multicolor organic electroluminescent device
Authors: Yoshida Masayoshi , Fujii Akihiko , Ohmori Yutaka , Yoshino Katsumi
Published: July 2002

We have realized a three-layered multicolor organic electroluminescent device utilizing poly (9,9-dihexylfluorene) and perylene derivatives as emission layers, with diamine derivatives as an electron blocking layer. Emission colors could be selected either blue or red by changing the polarity of the applied voltage. By driving with an ac biased electric field, both emission colors were observed and the intensity ratio of each emission color was modulated by the frequency of the applied ac field.
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band-gap energy and lattice constant of ternary Zn1−xMgxSe as functions of composition
Authors: Jobst B. , Hommel D. , Lunz U. , Gerhard T. , Landwehr G.
Published: July 2002

The E0 band gap energies and the lattice constants of zinc-blende Zn1−xMgxSe alloys grown by molecular beam epitaxy in the composition range of 0≤x≤0.95 are determined. A nonlinear dependence on the composition is observed for both the band-gap energies and the lattice con- stants of the ternary alloys. To our knowledge this is an initial report of a bowing in the lattice constant of a ternary II–VI alloy. Considering the bowings, the band-gap energy and the lattice constant of zinc-blende MgSe are extrapolated to be about 4.0 eV and 5.91 Å, respectively.
Single-mode vertical cavity surface emitting laser by graded-index lens spatial filtering
Authors: Koch Barry J. , Leger James R. , Gopinath Anand , Wang Zhong , Morgan Robert A.
Published: July 2002

4.5 mW of power in a single spatial mode has been obtained from a vertical cavity surface emitting laser (VCSEL) in an external cavity setup, using a graded index (GRIN) lens with a spatial filtering high reflecting aperture deposited on its endface. The spatial filter on the GRIN lens endface forces a larger single transverse mode in the VCSEL than is obtained without it. A brightness of 5.1×105 W/cm2 str is demonstrated, which is 91% of the maximum achievable.
Using an elastomeric phase mask for sub-100 nm photolithography in the optical near field
Authors: Rogers John A. , Paul Kateri E. , Jackman Rebecca J. , Whitesides George M.
Published: July 2002

Bringing an elastomeric phase mask into conformal contact with a layer of photoresist makes it possible to perform photolithography in the near field of the mask. This technique provides an especially simple method for forming features with sizes of 90–100 nm in photoresist: straight lines, curved lines, and posts, on both curved and planar surfaces. It combines experimental convenience, new optical characteristics, and applicability to nonplanar substrates into a new approach to fabrication. Nanowire polarizers for visible light illustrate one application for this technique.
Cobalt doping in BaTiO3 thin films by two-target pulsed KrF laser ablation with in situ laser annealing
Authors: Ito Atsushi , Machida Akihiko , Obara Minoru
Published: July 2002

Cobalt doped BaTiO3 thin films have been fabricated using two-target pulsed laser ablation with in situ laser annealing. KrF laser ablated simultaneously both BaTiO3 and Co3O4 sintered targets and the cobalt concentration could simply be controlled at the concentration range of 0.08%–5% by adjusting laser fluences, off-axis deposition angles, and laser-irradiated area for Co ablation. All Co:BaTiO3 films were epitaxially grown on MgO(100) substrate. Optical absorption coefficients were measured at visible region of the spectrum with increasing Co dopant concentrations.
Bulk amorphous alloys with high mechanical strength and good soft magnetic properties in Fe–TM–B (TM=IV–VIII group transition metal) system
Authors: Inoue Akihisa , Zhang Tao , Takeuchi Akira
Published: July 2002

New bulk amorphous alloys exhibiting a wide supercooled liquid region before crystallization were found in Fe–(Co,Ni)–(Zr,Nb,Ta)–(Mo,W)–B systems. The Tg is as high as about 870 K and the supercooled liquid region reaches 88 K. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys with diameters up to 6 mm. These bulk amorphous alloys exhibit a high compressive strength of 3800 MPa, high Vickers hardness of 1360, and high corrosion resistance. Besides, the amorphous alloys exhibit a high magnetic-flux density of 0.74–0.96 T, low coercivity of 1.1–3.2 A/m, high permeability exceeding 1.2×104 at 1 kHz, and low magnetostriction of about 12×10−6.
Fabrication of waveguides in glasses by a poling procedure
Authors: Margulis W. , Laurell F.
Published: July 2002

Single mode channel waveguides were fabricated in soda lime glass by poling the samples at 280 °C with 2.5 kV bias voltage. The refractive index of the glass was reduced by 1.5% due to the depletion of sodium ions, making possible the formation of waveguides in a depressed cladding configuration.
Pulsed laser deposition of Cu:Al2O3 nanocrystal thin films with high third-order optical susceptibility
Authors: Ballesteros J. M. , Serna R. , Solı́s J. , Afonso C. N. , Petford-Long A. K. , Osborne D. H. , Haglund R. F.
Published: July 2002

Nanocomposite films comprising metal Cu nanocrystals embedded in an Al2O3 matrix were deposited by alternating pulsed laser ablation from metallic Cu and ceramic Al2O3 targets. The films were grown in vacuum on glass substrates held at room temperature. The as-grown films contain 4 nm Cu nanocrystals in an amorphous Al2O3 matrix, with a total thickness of 190 nm. The films show a substantial third-order susceptibility with an electronic nonlinear refractive index of (2.93±1.08)⋅10−10 cm2 W−1 and a nonlinear saturation of −(2.34±0.18)⋅10−5 cm W−1.
Theoretical analysis of laser-induced periodic structures at silicon-dioxide/silicon and silicon-dioxide/aluminum interfaces
Authors: Lu Y. F. , Yu J. J. , Choi W. K.
Published: July 2002

An analytical model was established to predict the laser-induced periodic structures at silicon-dioxide/silicon and silicon-dioxide/aluminum interfaces. The freezing of surface waves is considered the dominant mechanism for ripple formation. The model precisely predicts a linear relationship between the interface periodicity and the silicon dioxide thickness. The ripple periodicity in the substrates can hence be adjusted by varying the thickness of SiO2 overlayer. This process is expected to be useful in laser microtexturing for magnetic media of high storage density, which requires microtextures to be well controlled within a certain roughness to prevent a stiction failure. The theoretical calculation has a good agreement with the experimental results.
Accumulation layer profiles at InAs polar surfaces
Authors: Bell G. R. , Jones T. S. , McConville C. F.
Published: July 2002

High resolution electron energy loss spectroscopy, dielectric theory simulations, and charge profile calculations have been used to study the accumulation layer and surface plasmon excitations at the In-terminated (001)-(4×1) and (111)A-(2×2) surfaces of InAs. For the (001) surface, the surface state density is 4.0±2.0×1011 cm−2, while for the (111)A surface it is 7.5±2.0×1011 cm−2, these values being independent of the surface preparation procedure, bulk doping level, and substrate temperature. Changes of the bulk Fermi level with temperature and bulk doping level do, however, alter the position of the surface Fermi level. Ion bombardment and annealing of the surface affect the accumulation layer only through changes in the effective bulk doping level and the bulk momentum scattering rate, with no discernible changes in the surface charge density.
Optically induced rotation of a trapped micro-object about an axis perpendicular to the laser beam axis
Authors: Higurashi E. , Sawada R. , Ito T.
Published: July 2002

A strongly focused Nd:YAG laser beam has been used experimentally to achieve optical trapping and simultaneous rotation of a micro-object about an axis perpendicular to the trapping laser beam axis. The micro-object (12 μm in diameter), which has shape anisotropy in its interior, can be produced by oxygen reactive ion etching of a fluorinated polyimide film (refractive index n=1.53). This optically induced rotation of the micro-object in ethanol (n=1.36) is accomplished by utilizing the net radiation pressure force arising from the momentum change of light scattered at the micro-object’s inner walls.
The influence of mechanical stress on the dielectric breakdown field strength of thin SiO2 films
Authors: Jeffery Steve , Sofield Carl J. , Pethica John B.
Published: July 2002

We describe a surface probe technique for measuring the electrical properties of thin films while simultaneously controlling the local mechanical stress. We have used this technique to measure the dielectric breakdown voltage of thin SiO2 films, and we present results which show that there exists a correlation between the dielectric breakdown strength and the mechanical stress applied to the oxide. We propose a model, in which the mechanical strength of the material provides the criterion for breakdown, which accounts well for this behavior. Apart from the possible importance in the breakdown mechanism, the results suggest that residual stresses in insulating films may influence their effectiveness as dielectrics.
CuAu-type ordering in epitaxial CuInS2 films
Authors: Su D. S. , Neumann W. , Hunger R. , Schubert-Bischoff P. , Giersig M. , Lewerenz H. J. , Scheer R. , Zeitler E.
Published: July 2002

Ordering of Cu and In atoms in near-stoichiometric CuInS2 epitaxial films grown on Si (111) by molecular beam epitaxy was studied by transmission electron microscopy. Nonchalcopyrite ordering of the metal atoms in CuInS2 is observed, which is identified as CuAu-type ordering. Sharp spots in electron diffraction patterns reveal the ordered Cu and In atom planes alternating along the [001] direction over a long range. High-resolution electron microscopy confirms this ordering. The CuAu-ordered structure coexists with the chalcopyrite ordered structure, in agreement with theoretical prediction.
Effects of ion irradiation on the residual stresses in Cr thin films
Authors: Misra A. , Fayeulle S. , Kung H. , Mitchell T. E. , Nastasi M.
Published: July 2002

Cr films sputtered onto {100} Si substrates at room temperature were found to be under residual tension, as revealed by wafer curvature measurements. A 150 nm thick Cr film was bombarded with 300 keV Ar ions after deposition. The intrinsic residual tensile stress increased slightly and then decreased with further increase in the ion dose. For ion doses >1×1015 ions/cm2, the stress in the film became compressive and increased with increasing dose. Transmission electron microscopy revealed that the grain boundaries in as-deposited Cr have columnar porosity. A Cr film, ion irradiated to a dose of 5×1015 ions/cm2, showed no grain boundary porosity. The changes in the residual stress during ion irradiation are explained by considering Ar incorporation in the film and the manner in which irradiation may change the interatomic distances and forces.
Photoluminescence from GaAs nanocrystals fabricated by Ga+ and As+ co-implantation into SiO2 matrices
Authors: Okamoto Shinji , Kanemitsu Yoshihiko , Min Kyu Sung , Atwater Harry A.
Published: July 2002

We have fabricated GaAs nanocrystals by means of Ga+ and As+ co-implantation into SiO2 matrices and applied selective excitation spectroscopy to clarify the origin of photoluminescence (PL) from GaAs nanocrystals in SiO2 matrices. Under blue laser excitation, broad PL spectra with multipeaks are observed in the visible spectral region. Under selective excitation at energies within a certain band, fine structures are observed at low temperatures and the size-dependent PL structures are attributed to quantum confinement states in GaAs nanocrystals. The origin of visible PL from GaAs/SiO2 nanocomposites is discussed.
Giant effective pyroelectric coefficients from graded ferroelectric devices
Authors: Jin F. , Auner G. W. , Naik R. , Schubring N. W. , Mantese J. V. , Catalan A. B. , Micheli A. L.
Published: July 2002

Effective pyroelectric coefficients as large as 5 μC/cm2 °C, with peak responsivities at approximately 50 °C, were obtained from compositionally graded barium strontium titanate ferroelectric thin film devices formed on silicon using unbalanced magnetron sputter deposition. These effective pyroelectric coefficients are nearly two orders of magnitude larger than those observed from conventional pyroelectric thin film ferroelectric detectors.
Modulation of second harmonic generation in photochromic materials by the application of electric fields and low intensity light
Authors: Large M. C. J. , Kajzar F. , Raimond P.
Published: July 2002

The simultaneous application of low intensity optical and static electric fields to photoisomerizable electro-optic materials is shown to allow manipulation of the refractive index. A dc electric field was applied to the sample by means of a corona poling mechanism, which was illuminated within the absorption band using a low power laser. The application of the optical field caused a reduction in polar order in the material, with a corresponding change in the refractive index. The change of polar order was monitored by optical second harmonic generation. For typical experimental values, the variation in refractive index was calculated to be about 1%, which is sufficient to change waveguiding conditions. Electroded samples also showed the effect.
Electrical noise in hysteretic ferromagnet–insulator–ferromagnet tunnel junctions
Authors: Nowak E. R. , Weissman M. B. , Parkin S. S. P.
Published: July 2002

Low frequency noise has been measured in magnetic tunnel junctions that have Al2O3 tunnel barriers and magnetoresistance values up to 35% at 295 K. Fluctuations in voltage were found to cross over from Johnson noise to shot noise at low bias voltages, in quantitative agreement with theories of noise in quantum ballistic systems. 1/f resistance noise, where f is frequency, predominates at larger biases and is proportional to the mean current squared. This noise is attributed to trapping processes and it depends sensitively on the relative position of the oxide edge and the ferromagnet–Al interface.
Temperature dependence of the thermo-optic coefficient in crystalline silicon between room temperature and 550 K at the wavelength of 1523 nm
Authors: Cocorullo G. , Della Corte F. G. , Rendina I.
Published: July 2002

The temperature dependence of the thermo-optic coefficient for crystalline silicon has been measured in the temperature range between room temperature and 550 K at the wavelength of 1523 nm by means of an interferometric technique. This technique, which requires a very simple experimental setup, is based on the observation of the fringe pattern produced by temperature changes in a Fabry–Perot resonator. Measurement results indicate that the thermo-optic coefficient is independent on the sample doping and crystal plane orientation. The experimental data appear to be in agreement with the few values reported to date at this important wavelength. The temperature dependence of the excitonic band gap is also calculated by fitting these data with a recently introduced model of ∂n/∂T.
Negative electron affinity at the Cs/AlN(0001) surface
Authors: Wu C. I. , Kahn A.
Published: July 2002

The effects of cesium (Cs) adsorption on band bending and electron affinity at the AlN(0001)-1×1 surface are investigated via ultraviolet and x-ray photoemission spectroscopy. The movement of the Fermi level indicates an initial interaction between Cs and empty surface states, followed by an increase in band bending presumably linked to metallization. The electron affinity, χ, of the clean AlN surface is positive and equal to 1.9±0.3 eV. The Cs-surface dipole layer decreases χ by 2.6±0.3 eV, leading to evidence of true negative electron affinity at the surface of this important material.
Non-Arrhenius temperature dependence of reliability in ultrathin silicon dioxide films
Authors: DiMaria D. J. , Stathis J. H.
Published: July 2002

The non-Arrhenius temperature dependence observed in the charge-to-breakdown data in thin oxides is related to the temperature dependence of the defect buildup in the same films. For each temperature, this defect buildup is studied as a function of the defect generation probability and the total number of defects at breakdown. Each of these quantities is shown to have its own unique temperature dependence, which when combined gives the results observed for the charge-to-breakdown data. As the oxide layers are made thinner, the temperature dependence of the defect generation probability dominates these observations.
Oxidation study of GaN using x-ray photoemission spectroscopy
Authors: Watkins N. J. , Wicks G. W. , Gao Yongli
Published: July 2002

The oxidation of GaN at room temperature was investigated using x-ray photoemission spectroscopy. The onset of oxidation is observed at 105 Langmuir (L=10−6 Torr s) of oxygen exposure. This is seen both in changes in the percent composition of oxygen and in shifts in the core spectrum of Gallium. The oxidation saturates at an exposure of 108 L. Detailed core level analysis shows that at this exposure most of the surface Ga atoms have been oxidized. The results indicate that the oxidation of GaN is a kinetically limited process restricted to the surface and the underlying bulk is not strongly perturbed. This is in sharp contrast with GaAs where oxidation begins at the same level of exposure and then continues for all further exposures as subsurface layers are oxidized.
Excitonic transitions and exchange splitting in Si quantum dots
Authors: Reboredo F. A. , Franceschetti A. , Zunger A.
Published: July 2002

In a quantum dot made of an indirect gap material such as Si, the electron–hole Coulomb interaction alone can give rise to “dark” excitons even in the absence of exchange interaction. We present the predicted excitonic spectra for hydrogen-passivated Si dots and find very good agreement with the recent experiment of Wolkin, Jorne, Fauchet, Allan, and Delerue [Phys. Rev. Lett. 82, 197 (1999)]. The calculated splitting between dark and bright excitons, arising from Coulomb and exchange interactions, agrees very well with the optical data of Calcott, Nash, Canham, Kane, and Brumhead [J. Phys Condens. Matter 5, L91 (1993)].
Activation field and fatigue of (Pb, La)(Zr, Ti)O3 thin films
Authors: Chen I-Wei , Wang Y.
Published: July 2002

For a class of (Pb, La)(Zr, Ti)O3 ferroelectric thin films, the Merz equation adequately describes the dynamics of switching resistance. A single parameter, activation field, can be used to relate the coercive field throughout the polarization fatigue process. We suggest that the increased switching resistance causes fatigue, and that it could originate from the dielectric degradation of the film. The constitutive equation for the coercive field and its correlation with the switchable polarization offer a tool for fatigue characterization and life prediction.
Current perpendicular to plane giant magnetoresistance of multilayered nanowires electrodeposited in anodic aluminum oxide membranes
Authors: Evans P. R. , Yi G. , Schwarzacher W.
Published: July 2002

Co–Ni–Cu/Cu multilayered nanowires were prepared by electrodeposition using nanoporous aluminum oxide membranes rather than the more usual track-etched polycarbonate membranes as templates. Very large values of the current perpendicular to plane giant magnetoresistance (CPP-GMR) were recorded: 55% at room temperature and 115% at 77 K. The use of aluminum oxide membranes also made possible a study of the effects of annealing on the CPP-GMR.
Bipolar transport organic light emitting diodes with enhanced reliability by LiF doping
Authors: Choong Vi-En , Shi Song , Curless Jay , So Franky
Published: July 2002

An electrode contact scheme based on the use of an organic LiF alloy is investigated. The performance of organic light emitting diodes (OLED) with this contact scheme in both heterojunction and bipolar transport/emitting layer (BTEL) OLED structures are compared with their counterparts with LiF buffer layers. The organic LiF contact scheme improved device reliability of BTEL OLEDs by 32% to 92 500 h while adversely affecting device reliability of heterojunction OLEDs.
Experimental measurements of the strength of metals approaching the theoretical limit predicted by the equation of state
Authors: Moshe E. , Eliezer S. , Henis Z. , Werdiger M. , Dekel E. , Horovitz Y. , Maman S. , Goldberg I. B. , Eliezer D.
Published: July 2002

The approach to the ultimate strength of metals is determined experimentally. The ultimate strength of metals was calculated using a realistic wide-range equation of state. The strength of metals was measured using shock waves created in aluminum and copper foils with a short- (20–100 ps) pulse high-power laser. The strength of the materials was determined from the free-surface-velocity time history, which was measured with an optically recording velocity interferometer system. The strain rates in these experiments were in the range (1.5–5)×108 s−1.
Collective resonance and form factor of homogeneous broadening in semiconductors
Authors: Zaitsev S. V. , Gordeev N. Yu. , Karachinsky L. Ya. , Kopchatov V. I. , Novikov I. I. , Tarasov I. S. , Pikhtin N. A. , Ustinov V. M. , Kop’ev P. S.
Published: July 2002

The superradiance model was recently successfully applied to describe semiconductor quantum well luminescence spectra at low temperature (77 K). In the present work, we expand and develop this approach for room-temperature spectra and compare results with optical autocorrelation data obtained earlier. Also, the convenience of the superradiance model has been proven for precise description of cryogenic spectra of quantum dot heterostructures. Numerical analysis of quantum dot spectra allows us to conclude that homogeneous line broadening dominates over inhomogeneous one.
Low-resistance ohmic contacts to p-type GaN
Authors: Li Y.-L. , Schubert E. F. , Graff J. W. , Osinsky A. , Schaff W. F.
Published: July 2002

The specific contact resistance of two types of ohmic contacts to p-type GaN is analyzed. First, an ohmic contact formed by a metal electrode deposited on a highly doped p-type GaN layer. Second, an ohmic contact formed by a metal electrode deposited on a thin GaN layer with an internal electric field caused by polarization effects. It is shown that contacts mediated by polarization effects can result, for typical materials parameters, in low contact resistances comparable or better than contacts mediated by dopant-induced surface fields. A type of contact is proposed and demonstrated. These contacts employ polarization charges to enhance tunneling transport as well as high doping. Experimental results on Ni contacts to p-type AlxGa1−xN/GaN doped superlattices are presented. The contacts have linear current–voltage characteristics with contact resistances of 9.3×10−4 Ω cm2, as inferred from linear transmission-line method measurements. The influence of annealing at temperatures ranging from 400 to 500 °C on the contact resistance is studied.
Fabrication of high-magnetoresistance tunnel junctions using Co75Fe25 ferromagnetic electrodes
Authors: Han Xiu-Feng , Oogane Mikihiko , Kubota Hitoshi , Ando Yasuo , Miyazaki Terunobu
Published: July 2002

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature and applied voltage dependence of the TMR ratio and resistance at room temperature for a tunnel junction, Ta (5 nm)/Ni79Fe21 (3 nm)/Cu (20 nm)/Ni79Fe21 (3 nm)/Ir22Mn78 (10 nm)/Co75Fe25 (4 nm)/Al (0.8 nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta(5 nm), were investigated. TMR ratio, effective barrier height and width, and breakdown voltage of the junction can be remarkably enhanced after annealing at 300 °C for an hour. High TMR ratio of 49.7% at room temperature and 69.1% at 4.2 K were observed. The value of spin polarization of Co75Fe25, P=50.7%, deduced from the TMR ratio at 4.2 K was corresponding well to the experimental data measured at 0.2 K in a spin polarized tunneling experiment using a superconductor/insulator/ferromagnet tunneling junction.
Characterization of nitrogen-containing radical products from the photodissociation of trimethylamine using photoionization detection
Authors: Forde Nancy R. , Butler Laurie J. , Ruscic Branko , Sorkhabi Osman , Qi Fei , Suits Arthur
Published: July 2002

The outcome of neutral and photoionized N(CH3)2 primary products of trimethylamine photodissociation at 193 nm is determined by combining photoionization detection with supporting G3 theoretical calculations. N(CH3)2 primary products with very little internal energy show an experimentally observed ionization onset of 9.1±0.2 eV, but do not appear at the parent ion (m/e=44). Instead, the parent ion is unstable and easily fragments to m/e=42, where the signal is observed. N(CH3)2 radicals with higher internal energies undergo H-atom loss from the neutral to give CH2NCH3, which has an observed ionization onset at parent (m/e=43) of <9.3 eV. At slightly higher ionization energies, these secondary products also appear at m/e=42 (where their appearance energy is roughly 9.8–9.9 eV, uncorrected for internal energy). Finally, N(CH3)2 radicals with the highest internal energy in this study appear to undergo H2 loss as neutrals, giving rise to a species whose parent ion has m/e=42. The ionization onset of this species at m/e=42 is found to be in the range of 9.5–9.6 eV.
Functionals of Gegenbauer polynomials and D-dimensional hydrogenic momentum expectation values
Authors: Van Assche W. , Yáñez R. J. , González-Férez R. , Dehesa Jesús S.
Published: July 2002

The system of Gegenbauer or ultraspherical polynomials {Cnλ(x);n=0,1,…} is a classical family of polynomials orthogonal with respect to the weight function ωλ(x)=(1−x2)λ−1/2 on the support interval [−1,+1]. Integral functionals of Gegenbauer polynomials with integrand f(x)[Cnλ(x)]2ωλ(x), where f(x) is an arbitrary function which does not depend on n or λ, are considered in this paper. First, a general recursion formula for these functionals is obtained. Then, the explicit expression for some specific functionals of this type is found in a closed and compact form; namely, for the functionals with f(x) equal to (1−x)α(1+x)β, log(1−x2), and (1+x)log(1+x), which appear in numerous physico-mathematical problems. Finally, these functionals are used in the explicit evaluation of the momentum expectation values 〈pα〉 and 〈log p〉 of the D-dimensional hydrogenic atom with nuclear charge Z⩾1. The power expectation values 〈pα〉 are given by means of a terminating F45 hypergeometric function with unit argument, which is a considerable improvement with respect to Hey’s expression (the only one existing up to now) which requires a double sum.
The velocity distribution of barotropic turbulence
Authors: Bracco Annalisa , LaCasce Joe , Pasquero Claudia , Provenzale Antonello
Published: July 2002

We study the statistical properties of the velocity and velocity gradient distributions in barotropic turbulence. At large enough Reynolds number, the velocity distribution becomes non-Gaussian outside the vortex cores, and its characteristics are completely determined by the properties of the far field induced by the coherent vortices. The velocity gradients are always non-Gaussian inside coherent vortices, due to the spatial velocity correlations associated with the ordered flow in the vortex cores, and become non-Gaussian also in the background turbulence at large enough Reynolds number.
Hydrodynamic approach to time-dependent density functional theory; Response properties of metal clusters
Authors: Banerjee Arup , Harbola Manoj K.
Published: July 2002

Performing electronic structure calculations for large systems, such as nanoparticles or metal clusters, via orbital based Hartree–Fock or Kohn–Sham theories is computationally demanding. To study such systems, therefore, we have taken recourse to the hydrodynamic approach to time-dependent density-functional theory. In this paper we develop a variation-perturbation method within this theory in terms of the particle and current densities of a system. We then apply this to study the linear and nonlinear response properties of alkali metal clusters within the spherical jellium background model.
Interference fringe-free transmission spectroscopy of amorphous thin films
Authors: Li Tong , Kanicki Jerzy , Kong Wei , Terry Fred L.
Published: July 2002

Based on optical fundamentals, we present in this article a practical method to obtain an interference fringe-free transmission spectrum for hydrogenated amorphous solid thin films. From this spectrum, reliable optical properties, such as the Urbach edge and optical band gap of the thin films, can be extrapolated directly. In terms of the Brewster angle accuracy, the margins of error of the proposed method due to material dispersion are less than ±1% for hydrogenated amorphous silicon and less than ±1.2% for hydrogenated amorphous silicon nitride. These figures are less than the detectable limit of the proposed method.
Theory of the anisotropy of the electron Hall mobility in n-type 4H– and 6H–SiC
Authors: Iwata Hisaomi , Itoh Kohei M. , Pensl Gerhard
Published: July 2002

A theoretical model for the calculation of the anisotropy in the electron Hall mobility is reported for n-type bulk single crystals of 4H– and 6H–SiC for the three distinct Hall measurement configurations: (a) [B∥c, j⊥c], (b) [B⊥c, j⊥c], and (c) [B⊥c, j∥c], where B, j, and c are the directions of the magnetic field, current flow, and c axis of the hexagonal unit cell, respectively. Comparison with experimental results shows that the anisotropy of the electron transport in both 4H– and 6H–SiC can be explained solely by the anisotropy in the effective electron mass tensors.
Phonon-assisted anti-Stokes excitation of the fluorescence of Mn2+ ions in the CsMnCl3⋅2H2O crystal
Authors: Eremenko V. V. , Fomin V. I. , Kumosov V. S.
Published: July 2002

We demonstrate that anti-Stokes excitation of Mn2+ ions in the CsMnCl3⋅2H2O crystal can occur through multiphonon addition to the energy of the photons of exciting light. The deficit dE between the energy of the Mn2+ 4T1 exciton and the photon energy of the He–Ne laser used as the source of illumination is compensated by simultaneous annihilation of several thermally activated lattice phonons. The absence of two-photon activation of the fluorescence was verified by measurement of the dependence of the fluorescence intensity on the intensity of the exciting light, which appears linear at a fixed energy mismatch dE.
Direct determination of electron effective mass in GaNAs/GaAs quantum wells
Authors: Hai P. N. , Chen W. M. , Buyanova I. A. , Xin H. P. , Tu C. W.
Published: July 2002

Electron effective mass (me*) in GaNxAs1−x/GaAs quantum wells (QWs) is investigated by the optically detected cyclotron resonance technique. The me* values of 0.12m0 and 0.19m0 are directly determined for the 70-Å-thick QWs with N composition of 1.2% and 2.0%, respectively. This sizable increase in the electron effective mass is consistent with the earlier theoretical predictions based on the strong interaction of the lowest conduction band states with the upper lying band states or impurity band induced by the incorporation of N.
Plasma-induced damage to n-type GaN
Authors: Choi H. W. , Chua S. J. , Raman A. , Pan J. S. , Wee A. T. S.
Published: July 2002

The effects of plasma etching on 1/f noise and photoluminescence (PL) characteristics of n-GaN have been investigated. A reduction of 1/f noise was observed after plasma exposure, a result of enhanced passivation of the reactive surface. This is attributed to the removal of carbon and the creation of a Ga-rich surface by the etching process. Nevertheless, the formation of nonradiative recombination centers impaired the PL intensity. Reconstruction of a stoichiometric surface was achieved by annealing. This induced the incorporation of carbon into GaN, deteriorating the PL performance further, but it could be restored by a chemical treatment of 10:1 HF:H2O.
Small-world Rouse networks as models of cross-linked polymers
Authors: Jespersen S. , Sokolov I. M. , Blumen A.
Published: July 2002

We use the recently introduced small-world networks (SWNs) to model cross-linked polymers, as an extension of the linear Rouse chain. We study the SWN dynamics under the influence of external forces. We focus on the (structurally and thermally averaged) stretching of the SWN, which we determine numerically through diagonalization and analytically using an approximate expression for the SWN density of states. We show that stretching is related to the probability of a random walker over the network to return to its origin. We compare our SWN results to the corresponding ones for Cayley trees.
Simple and high-yield method for synthesizing single-crystal GaN nanowires
Authors: Tang C. C. , Fan S. S. , Dang H. Y. , Li P. , Liu Y. M.
Published: July 2002

A simple and high-yield method involving vapor-liquid-solid wire-like growth mechanism was developed for the synthesis of GaN nanowires. In this process, the mixture of Ga and SiO2 reacted with ammonia in the presence of the Fe2O3 catalyst supported by Al2O3. The x-ray powder diffraction measurement and transmission electron microscopy observations confirmed that the synthesized GaN nanowires are single-crystal hexagonal wurtzite structure with diameters ranged from 10 to 50 nm and lengths up to several micrometers. Based on the fact that a small Fe dominant particle attached to one end of some nanowires, a growth model of the GaN nanowires was proposed.
Relationship between fluence gradient and lateral grain growth in spatially controlled excimer laser crystallization of amorphous silicon films
Authors: Lee Minghong , Moon Seungjae , Hatano Mutsuko , Suzuki Kenkichi , Grigoropoulos Costas P.
Published: July 2002

In order to clarify the relationship between excimer laser fluence gradient and the length of lateral grain growth, the excimer laser fluence used for crystallization is modulated by a beam mask. The crystallized 50-nm-thick a-Si films are secco etched in order to observe the lateral grain growth by scanning electron microscope. The fluence distribution across the pattern is measured by a negative photoresist that has a linear relationship between laser fluence and resist thickness after development. This mapped fluence distribution is utilized to deduce the fluence gradient for each laser energy output. It is shown that lateral growth length increases and the directionality of the grains improves as the fluence gradient increases. Lateral growth length as long as 1.5 μm can be driven in a 50-nm-thick a-Si film by a single excimer laser pulse without any substrate heating. Electrical conductance measurement is used to probe the solidification dynamics. The lateral solidification velocity is estimated to be 7 m/s. A model is proposed to explain the formation of grain microstructure.
Landau free energy curves for melting of quantum solids
Authors: Chakravarty C. , Lynden-Bell R. M.
Published: July 2002

Path integral Monte Carlo simulations are used to study quantum Lennard-Jones solids and to examine the dependence of the melting temperature on the degree of quantum delocalization. To determine the solid–liquid coexistence conditions and metastability limits, an umbrella sampling strategy is adopted to generate Landau free energy curves. Simulation results for the melting point dependence on the particle mass are shown to be reasonable in comparison with experimental data on the melting curves of solid H2 and D2. Both the enthalpy and entropy of melting are shown to decrease with increasing strength of quantum effects. For solid molecular hydrogen and its isotopomers, zero-point energy effects are shown to be important even at melting temperatures.
Optical characterization of CdS nanocrystals in Al2O3 matrices fabricated by ion-beam synthesis
Authors: Matsuura D. , Kanemitsu Y. , Kushida T. , White C. W. , Budai J. D. , Meldrum A.
Published: July 2002

We have studied optical properties of CdS nanocrystals formed by sequential Cd+ and S+ ion implantation into Al2O3 matrices. Two bands related to free excitons in the wurtzite CdS are clearly observed in the absorption spectrum at low temperatures. Efficient photoluminescence (PL) appears near the absorption edge. At high temperatures, the band edge PL band consists of two components. One is the free-exciton emission with a short lifetime (several hundreds of picoseconds), while the other is the bound exciton emission at shallow localized states with a long lifetime (several nanoseconds). The temperature dependence of the band gap energy has been determined for wurtzite CdS nanocrystals. Spectroscopic analysis shows that high-quality compound semiconductor nanocrystals are fabricated by the ion-beam synthesis technique.
Ferroelectric phase transition and maximum dielectric permittivity of displacement type ferroelectrics (BaxSr1−xTiO3)
Authors: Vendik O. G. , Zubko S. P.
Published: July 2002

The main interest in the area of practical applications of ferroelectrics, particularly at the microwave frequency, is oriented now to room temperature. In this connection, ferroelectrics like BaxSr1−xTiO3 should be carefully studied. Such materials are characterized by the second order phase transition. In a perfect ferroelectric crystal, the phase transition takes place at temperature TC, which is called the Curie temperature. Real (defected) crystals and ceramic samples are characterized by a presence of built-in electric field and mechanical strains. In the case of the real crystal, the temperature of the phase transition TC′, is displaced to lower temperature and the temperature of the maximum of ε(T)Tm is displaced to higher value with respect to TC. Thus, for the real ferroelectric sample (not for an incipient ferroelectric), one has TC′<TC<Tm. Some investigators suppose by default that TC′=TC=Tm. Such a supposition is wrong and can lead to an incorrect treatment of experimental results. In the case of a thin film sample, the phase transition and the dielectric response of a ferroelectric sample are affected by the size of the sample, what is treated as a size effect. Experimental data obtained as a result of measurement of the dielectric constant as a function of temperature can be used for finding the Curie temperature and other parameters of the material. For this procedure, a suitable model of the dielectric response of the ferroelectric sample should be used.
Photodissociation of H2O at 121.6 nm: A state-to-state dynamical picture
Authors: Harich Steven A. , Hwang Dennis W. H. , Yang Xuefeng , Lin Jim J. , Yang Xueming , Dixon Richard N.
Published: July 2002

Photodissociation dynamics of H2O at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D00(H–OH) is determined to be 41151±5 cm−1. Two clear alternations in the OH(X,v=0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products (up to v=9) have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in O–H–H geometries. The effect of parent rotational excitation on the OH(A) product state distribution and anisotropy distribution was observed for the first time. Experimental results also show clear evidence for the triple dissociation channel, O(3P)+2H. Accurate branching ratios of different product channels have been determined. Results of detailed QCT calculations agree well with the experimental results in this work.
Transformation kinetics of nanoicosahedral phase from a supercooled liquid region in Zr70Pd30 binary glassy alloy
Authors: Saida J. , Matsushita M. , Inoue A.
Published: July 2002

The formation of a nanoicosahedral phase in the diameter range below 20 nm from the melt-spun Zr70Pd70 binary glassy alloy was confirmed, and the kinetics of the precipitation upon isothermal annealing in the supercooled liquid region was examined by differential scanning calorimetry. Based on the kinetic analysis, it is clarified that the precipitation in the supercooled liquid region takes place by a diffusion-controlled growth with increasing nucleation rate. The Arrhenius plot between effective time lag, τ, of nucleation and isothermal annealing temperature yields a single linear relation, in which the activation energy for nucleation is evaluated to be 267 kJ mol−1. The activation energy in the present alloy is much lower than that of the Zr65Al7.5Ni10Cu7.5Pd10 alloy, which is due to the difference in the magnitude as well as the number of atoms for the rearrangements in the nucleation stage. It is concluded that the formation of a nanoscale icosahedral quasicrystalline phase is attributed to the transformation mode of an increase of nucleation rate, which is different from that of Zr65Al7.5Ni10Cu7.5Ag10 quasicrystal arising from an interfacial controlled growth with a steady-state nucleation rate. The formation of a nanoicosahedral phase in the Zr70Pd30 binary glassy alloy implies the existence of the icosahedral short-range order in the glassy state.
Random-field model for ferroelectric domain dynamics and polarization reversal
Authors: Viehland Dwight , Chen Yun-Han
Published: July 2002

A new concept of polarization reversal and domain dynamics in modified ferroelectrics is presented. Accordingly, clusters with a reversed polarization within ferroelectric domains are nucleated under applied field in the vicinity of random fields around quenched defects. The random fields significantly lower the activation barrier required for nucleation, thus controlling the kinetics of polarization reversal and domain dynamics. In this model, polarization switching occurs by the breakdown of ferroelectric long-range order, through a polar cluster state.
Hole transport through single and double SiGe quantum dots
Authors: Cain Paul A. , Ahmed Haroon , Williams David A. , Bonar Janet M.
Published: July 2002

We report on measurements of hole transport through a double quantum dot structure formed by trench isolation from a SiGe:Si heterostructure. A period change in the Coulomb oscillations is observed upon changing a gate bias, which is attributed to the lowering of one of the tunnel barriers, effectively changing the device to a single quantum dot. Accompanying the period change is a significant change in the level of noise associated with the oscillations. This is explained by a carrier energy filtering effect in the double dot compared with the single dot, caused by the slight difference in energy level spacing in each quantum dot.
Surface states and surface oxide in GaN layers
Authors: Shalish I. , Shapira Yoram , Burstein L. , Salzman J.
Published: July 2002

Surface photovoltage spectroscopy, photoluminescence, Auger electron spectroscopy and x-ray photoelectron spectroscopy were used to correlate the chemical changes induced by HCl etching of GaN surface to changes in the yellow luminescence related states, through their manifestation in surface photovoltage. The results show a correlation between a removal of the gallium oxide from the surface and a reduction of the yellow luminescence related transition in the surface photovoltage spectra. Based on this observation, it is suggested that the well known yellow luminescence is emitted from surface states associated with the gallium oxide that decorates the free surface and possibly also the substrate interface and internal grain boundaries.