bluuebunny/crossref_metadata_2025 huggingface dataset:
Room-temperature perpendicular magnetic anisotropy in Ni/Pd (111) multilayers
Authors: Jeong Jong-Ryul , Shin Sung-Chul
Published: July 2002

We report the experimental observation of room-temperature perpendicular magnetic anisotropy (PMA) in Ni/Pd (111) multilayer films having the thickness range of 5–11 Å Ni and 4–11 Å Pd sublayers, prepared by direct current magnetron sputtering on glass substrates at Ar sputtering pressure of 7 mTorr. Perpendicular magnetic anisotropy was found to be sensitively dependent on both Ni- and Pd-sublayer thicknesses, and a maximum anisotropy energy of 5.6×105 erg/cm3 was obtained for the (5 Å Ni/6 Å Pd)30 multilayer. The magnetoelastic anisotropy, quantitatively determined from in situ stress and ex situ magnetostriction coefficient measurements, was found to play an important role for the observed PMA in this system, together with the surface anisotropy.
Enhancement of magnetic coercivity and macroscopic quantum tunneling in monodispersed Co/CoO cluster assemblies
Authors: Peng D. L. , Sumiyama K. , Hihara T. , Yamamuro S.
Published: July 2002

Magnetic properties have been measured for monodisperse-sized Co/CoO cluster assemblies prepared by a plasma-gas-condensation-type cluster beam deposition technique. The clear correlation obtained between exchange bias field and coercivity suggests the enhancement of uniaxial anisotropy owing to the exchange coupling between the ferromagnetic Co core and antiferromagnetic CoO shell, and magnetic disorder at the core–shell interface. A nonthermal magnetic relaxation observed below 8 K, being referred to as macroscopic quantum tunneling of the magnetization, is ascribed to the enhanced uniaxial anisotropy.
Nonlinear optical response of GaN layers on sapphire: The impact of fundamental beam interference
Authors: Kravetsky I. V. , Tiginyanu I. M. , Hildebrandt Ralf , Marowsky Gerd , Pavlidis D. , Eisenbach A. , Hartnagel H. L.
Published: July 2002

GaN layers grown by metalorganic chemical-vapor deposition were characterized by optical second- and third-harmonic generation techniques. The angular dependence of the second-harmonic intensity in transmission showed a c-textured growth of the GaN layers on the sapphire substrates. The measured ratios d33/d15 and d33/d31 are equal to −2.02 and −2.03, respectively, which is indicative of a wurzite structure of the GaN layers. The measured d33 is 33 times that of the d11 of quartz. Fine oscillations were observed in the measured second- and third-harmonic angular dependencies that are explained by taking into account the interference of the fundamental beam in the GaN/sapphire structure.
Shallow electron traps at the 4H–SiC/SiO2 interface
Authors: Afanas’ev V. V. , Stesmans A. , Bassler M. , Pensl G. , Schulz M. J.
Published: July 2002

Low-temperature electrical measurements and photon-stimulated electron tunneling experiments reveal the presence of a high density of interface states at around 0.1 eV below the conduction band of 4H–SiC at its interface with thermally grown SiO2. These states, related to defects in the near-interfacial oxide layer, trap a considerable density of electrons from the SiC, and are likely responsible for the severe degradation of the electron mobility observed in the surface channel of 4H–SiC/SiO2 devices. The negative impact of the observed defects can be minimized by using SiC modifications (e.g., 6H, 15R, 3C) with a larger conduction band offset with the oxide than 4H–SiC leading to a largely reduced density of electrons trapped in the oxide.
Hardening of fullerite C60 during temperature-induced polymerization and amorphization under pressure
Authors: Lyapin A. G. , Brazhkin V. V. , Gromnitskaya E. L. , Popova S. V. , Stal’gorova O. V. , Voloshin R. N. , Bayliss S. C. , Sapelkin A. V.
Published: July 2002

We report a detailed study of Vicker’s hardness and ultrasonic elastic moduli for carbon phases prepared by heating fullerite C60 at pressures of 3.5, 5, and 8 GPa. It is shown that the transformation of two-dimensional C60 polymers into graphite-like amorphous carbon is accompanied by an increase in hardness of 100–200 times, as well as an increase in bulk and shear moduli by 4–5 and 2–3 times, respectively, with no density jump taking place. It is proposed that the high hardness (up to 40 GPa) of the disordered phases synthesized is caused by the three-dimensional ordering of sp2-based network. It was found that, in the 3.5–8 GPa interval, the mechanical properties of the phases obtained depend basically on the temperature rather than on the pressure of synthesis.
Influence of buffer-layer surface morphology on the self-organized growth of InAs on InP(001) nanostructures
Authors: González L. , García J. M. , García R. , Briones F. , Martínez-Pastor J. , Ballesteros C.
Published: July 2002

We have studied the influence of InP buffer-layer morphology in the formation of InAs nanostructures grown on InP(001) substrates by solid-source molecular-beam epitaxy. Our results demonstrate that when InP buffer layers are grown by atomic-layer molecular-beam epitaxy, InAs quantum dot-like structures are formed, whereas InP buffer layers grown by MBE produce quantum-wire-like structures. The optical properties of these corrugated structures make them potential candidates for their use in light-emitting devices at 1.55 μm.
Direct determination of the Andreev reflection probability by means of point contact spectroscopy
Authors: Jakob M. , Stahl H. , Knoch J. , Appenzeller J. , Lengeler B. , Hardtdegen H. , Lüth H.
Published: July 2002

In this letter, we describe a technique for determining the Andreev reflection probability of electrons impinging on a semiconductor–superconductor interface. A two-dimensional electron gas (2DEG) in an InGaAs/InP heterostructure is linked to a niobium superconductor. A point contact in the 2DEG emits ballistic electrons and detects the reflected carriers. The vast majority of the detected carriers are retroreflected holes because of our specific sample setup. We have found an Andreev reflection probability of up to 20%. The experimental results are compared with the predictions of two theoretical models.
Conduction band offset in InAs/GaAs self-organized quantum dots measured by deep level transient spectroscopy
Authors: Ghosh S. , Kochman B. , Singh J. , Bhattacharya P.
Published: July 2002

The heterostructure conduction band offset, ΔEc, in InAs/GaAs self-organized quantum dots has been measured by deep level transient spectroscopy. Measurements were made with Au–Al0.18Ga0.82As Schottky diodes in which the multilayer dots are embedded in the ternary layer. The estimated value of the band offset ΔEc=341±30 meV.
Highly monodisperse polymer-capped ZnO nanoparticles: Preparation and optical properties
Authors: Guo Lin , Yang Shihe , Yang Chunlei , Yu Ping , Wang Jiannong , Ge Weikun , Wong George K. L.
Published: July 2002

We report the preparation of highly monodisperse ZnO nanoparticles using poly(vinyl pyrrolidone) (PVP) as the capping molecules. The surface-modified ZnO nanoparticles were found to be remarkably stable. The optical absorption shows distinct excitonic features. Markedly enhanced near-band-edge ultraviolet photoluminescence and significantly reduced defect-related green emission were also observed. We attribute this observation to the nearly perfect surface passivation of the ZnO nanoparticles by the PVP molecules. The third-order nonlinear optical response of these PVP-capped ZnO nanoparticles in a dilute solution was found to be significantly larger (by at least two orders of magnitude) than that of the bulk ZnO.
Functional separation of biasing and sustaining voltages in two-frequency capacitively coupled plasma
Authors: Kitajima T. , Takeo Y. , Petrović Z. Lj. , Makabe T.
Published: July 2002

Separation of the effects of rf sources used for biasing the wafer and for sustaining the plasma is studied by measuring the space profiles of net excitation rate of Ar(3p5) for a two-frequency capacitively coupled plasma as a representation of a typical oxide etcher. Measurements were performed in Ar and in CF4/Ar mixtures. For biasing supply operating at low frequency, 700 kHz, it was shown that the effect of the voltage becomes significantly smaller as the sustaining voltage is changed from high frequency, 13.56 MHz, to very high frequency (VHF), 100 MHz, and it even disappears for pulsed operation in mixtures. This is the result of the low dc self-bias at the VHF electrode that allows the high energy secondary electrons to leave the plasma without excessive contribution to ionization and dissociation.
A perturbation of CHSH inequality induced by fluctuations of ensemble distributions
Authors: Khrennikov Andrei
Published: July 2002

We reconsider the theory of hidden variables under the assumption that the conjecture on the ensemble (experiment run) independence of the distribution of hidden variables (which was indirectly used by J. Bell and his followers) is violated. Ensemble fluctuations imply perturbations of Bell’s inequality and its generalizations. We study (by experimental reasons) CHSH (Clauser, Horne, Shimony, Holt) inequality and obtain its modification. This modified inequality is not in disaccord with the predictions of quantum formalism. The deviation from the standard CHSH inequality depends on the magnitude of ensemble fluctuations. We find these magnitude for fluctuating families of Gaussian distributions. We found that if the dimension of the space of hidden variables is very high, then to obtain a contradiction between the local realism and quantum formalism, we must be sure there is no even negligibly small deviations in probability distributions of hidden variables corresponding to different runs of the experiment (in particular, the efficiency of detectors must be equal to one).
Laser rapid prototyping in nonlinear medium
Authors: Daneshvar K. , Raissi M. , Bobbio S. M.
Published: July 2002

We have measured the second harmonic generation (SHG) using a 1.06 μm Q-switched pump beam in powdered nonlinear crystals suspended in a photopolymeric solution. The powdered crystals used include lithium niobate, LiNbO3, and potassium titanyl phosphate. Measurements were made as a function of the grain size and the number of grains per unit volume. The photopolymeric solution is transparent at the pump beam frequency while it is highly absorbing at the SH and it is the SH which transforms the photopolymer from liquid to solid. The results show that relatively intense SH incoherent light is produced in powder after the pump intensity exceeds a certain threshold. SHG does not depend on the grain size up to a critical grain size beyond which SHG falls off rapidly. Furthermore, the emission increases linearly as a function of the number of grains per unit volume and eventually levels off and stays constant. A practical focus of these studies has been in the areas of laser induced rapid prototyping and three dimensional real time image formation. This study clearly demonstrates the application of a nonlinear optical medium in high resolution rapid prototyping. Furthermore, because efficient SHG occurs for very small powder grain size this technique may provide a way of realizing high resolution three dimensional imaging in which the feature size is only a few microns.
Polyparaphenylene-based low-temperature carbons studied by transmission electron microscopy
Authors: Hayashi T. , Endo M. , Dresselhaus M. S.
Published: July 2002

A comparison of the microtexture and structure of the low temperature heat-treated polyparaphenylene (PPP)-based carbons which are promising for the Li ion battery has been performed using high-resolution transmission electron microscopy and electron energy loss spectroscopy. It is found that the PPP-based carbon prepared by Kovacic method remains in suitable microtexture for high Li storage over a wider temperature range relative to that obtained by Yamamoto method. This characteristic of the Kovacic PPP-based carbon could lead to a more controlled microtexture for obtaining a higher Li storage capacity for Li ion battery applications.
Compact fiber-optic fluorosensor using a continuous-wave violet diode laser and an integrated spectrometer
Authors: Gustafsson Ulf , Pålsson Sara , Svanberg Sune
Published: July 2002

A compact fluorosensor with a fiber-optic measurement probe was developed, employing a continuous-wave violet diode laser as an exciting source and an integrated digital spectrometer for the monitoring of fluorescence signatures. The system has the dimensions 22×13×8 cm3, and features 5 nm spectral resolution and an excellent detectivity. Results from measurements on vegetation and human premalignant skin lesions are reported, illustrating the potential of the instrument.
Charged species profiles in oxygen plasma
Authors: Berezhnoj S. V. , Shin C. B. , Buddemeier U. , Kaganovich I.
Published: July 2002

The charged species profiles have been studied in oxygen plasma of asymmetrically driven capacitively coupled rf discharge. The electron (ne), negative (n), and positive (p) ion densities have been measured by Langmuir probe in a wide range of pressures (3–30 Pa) and interelectrode distances (2–10 cm). At small powers (ne<1015 m−3) it was obtained that the electronegativity is high (n/ne∼10) and the electron density profile is flat in plasma bulk. The negative ion density profile is nearly flat in the ion–ion core, where n≫ne, and falls off towards the plasma-sheath boundary with n≈0 with the spatial scale πDn/(2γd), determined by the negative ion diffusion coefficient Dn and detachment frequency γd. As the pressure decreases, this spatial scale exceeds half of the interelectrode distance and the negative ion density profile tends to a parabolic one. The obtained spatial distributions of charged particles are compared with the results of self-consistent modeling by use of a fluid approach for ions, and kinetic equation for electrons. Numerical simulation results agree well with both theory predictions and measurement data.
High-efficiency organic electrophosphorescent devices with tris(2-phenylpyridine)iridium doped into electron-transporting materials
Authors: Adachi Chihaya , Baldo Marc A. , Forrest Stephen R. , Thompson Mark E.
Published: July 2002

We demonstrate high-efficiency organic light-emitting devices employing the green electrophosphorescent molecule, fac tris(2-phenylpyridine)iridium [Ir(ppy)3], doped into various electron-transport layer (ETL) hosts. Using 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole as the host, a maximum external quantum efficiency (ηext) of 15.4±0.2% and a luminous power efficiency of 40±2 Im/W are achieved. We show that very high internal quantum efficiencies (approaching 100%) are achieved for organic phosphors with low photoluminescence efficiencies due to fundamental differences in the relationship between electroluminescence from triplet and singlet excitons. Based on the performance characteristics of single and double heterostructures, we conclude that exciton formation in Ir(ppy)3 occurs within close proximity to the hole-transport layer/ETL:Ir(ppy)3 interface.
Diffusion coefficient for disordered systems with coupled coherent and incoherent transport in one dimension
Authors: Pfluegl Wolfgang , Palenberg Markus A. , Silbey Robert J.
Published: July 2002

We present a calculation of the diffusion coefficient from a Hamiltonian for coupled coherent and incoherent transport with site-diagonal and off-diagonal stochastic fluctuations, which is not restricted to the usual white noise expression for the correlation between the fluctuating variables. Our result is applicable to any specific correlation and allows for long correlation times. We give examples for both monotonically decreasing and oscillating correlations. In the case of an exponential decay of the correlations the diffusion coefficient can be expressed by complete elliptic integrals.
Thermal conductivity of diamond and related materials from molecular dynamics simulations
Authors: Che Jianwei , Çağın Tahir , Deng Weiqiao , Goddard William A.
Published: July 2002

Based on the Green–Kubo relation from linear response theory, we calculated the thermal current autocorrelation functions from classical molecular dynamics (MD) simulations. We examined the role of quantum corrections to the classical thermal conduction and concluded that these effects are small for fairly harmonic systems such as diamond. We then used the classical MD to extract thermal conductivities for bulk crystalline systems. We find that (at 300 K) C12 isotopically pure perfect diamond has a thermal conductivity 45% higher than natural (1.1% C13) diamond. This agrees well with experiment, which shows a 40%–50% increase. We find that vacancies dramatically decrease the thermal conductivity, and that it can be described by a reciprocal relation with a scaling as nv−α, with α=0.69±0.11 in agreement with phenomenological theory (α=1/2 to 3/4). Such calculations of thermal conductivity may become important for describing nanoscale devices. As a first step in studying such systems, we examined the mass effects on the thermal conductivity of compound systems, finding that the layered system has a lower conductivity than the uniform system.
Magnetic properties and structure evolution of amorphous Co–C nanocomposite films prepared by pulsed filtered vacuum arc deposition
Authors: Wang H. , Wong S. P. , Cheung W. Y. , Ke N. , Wen G. H. , Zhang X. X. , Kwok R. W. M.
Published: July 2002

The magnetic properties and structure evolution of CoxC1−x (x=44, 65, 71 at. %) nanocomposite films prepared by pulsed filtered vacuum arc deposition were studied. The as-deposited films consist of amorphous Co clusters in a-C and are soft ferromagnets. The low coercivity in the amorphous films is due to the very low anisotropy of the amorphous Co clusters. After annealing in vacuum at 350 °C for 1 h, the Co clusters crystallize and these films become magnetically harder. The magnetic hardening is attributed to the enhanced magnetic anisotropy of the nanocrystals. Their saturation magnetization reduces slightly. This is attributed to the reduction of the density of states at the Fermi level caused by the crystallization of the amorphous Co clusters.
Ab initio modeling of boron clustering in silicon
Authors: Liu Xiang-Yang , Windl Wolfgang , Masquelier Michael P.
Published: July 2002

We present results of ab initio calculations for the structure and energetics of boron-interstitial clusters in Si and a respective continuum model for the nucleation, growth, and dissolution of such clusters. The structure of the clusters and their possible relationship to boron precipitates and interstitial-cluster formation are discussed. We find that neither the local-density approximation nor the generalized-gradient approximation to the density-functional theory result in energetics that predict annealing and activation experiments perfectly well. However, gentle refitting of the numbers results in a model with good predictive qualities.
Magnetothermal instabilities in type II superconductors: The influence of magnetic irreversibility
Authors: Chabanenko V. V. , D’yachenko A. I. , Zalutskii M. V. , Rusakov V. F. , Szymczak H. , Piechota S. , Nabialek A.
Published: July 2002

Complete magnetization and magnetostriction loops for type II superconductors were calculated in terms of the following pinned critical state models which incorporate the flux jump instability criterion: the Kim–Anderson model, the exponential model, the linear model, and the model with peak effect. These results were used for constructing magnetic field–temperature (H–T) diagrams of instability of these critical state models. Magnetothermal instabilities of superconductors with peak effect were investigated experimentally and theoretically. Regions in the H–T diagram where flux jumps occur were built and compared with experimental M(H) hysteresis loops. We have explained the phenomenon of “island” jumps. The influence of flux trapping on the flux jump regions of magnetic fields was studied.
An International Standard Formulation for the Thermodynamic Properties of 1,1,1-Trifluoroethane (HFC-143a) for Temperatures From 161 to 450 K and Pressures to 50 MPa
Authors: Lemmon Eric W. , Jacobsen Richard T
Published: July 2002

A new formulation is presented for the thermodynamic properties of refrigerant 143a (1,1,1-trifluoroethane, CH3–CF3) based upon available experimental data. The formulation can be used for the calculation of density, heat capacity, speed of sound, energy, and saturation properties using an equation of state explicit in Helmholtz energy. Ancillary equations are given for the ideal gas heat capacity, the vapor pressure, and for the saturated liquid and vapor densities as functions of temperature. Comparisons to available experimental data are given that establish the accuracy of calculated properties using this equation of state. The estimated uncertainties of properties calculated using the new equation are 0.1% in density, 0.5% in heat capacities, 0.02% in the speed of sound for the vapor at pressures less than 1 MPa, 0.5% in the speed of sound elsewhere, and 0.1% in vapor pressure, except in the critical region. The equation is valid for temperatures from the triple point temperature (161.34 K) to 450 K and pressures up to 50 MPa, and can be extrapolated to 650 K. It has been accepted as an international standard formulation for the properties of R-143a by the International Energy Agency-Annex 18. © 2001 by the U.S. Secretary of Commerce on behalf of the United States. All rights reserved.
Intramolecular vibrational redistribution in aromatic molecules. I. Eigenstate resolved CH stretch first overtone spectra of benzene
Authors: Callegari A. , Merker U. , Engels P. , Srivastava H. K. , Lehmann K. K. , Scoles G.
Published: July 2002

We have used infrared–infrared double resonance spectroscopy to record a rovibrational eigenstate resolved spectrum of benzene in the region of the CH stretch first overtone. This experiment is the first of a series aimed at investigating intramolecular vibrational energy redistribution (IVR) in aromatic molecules. The experiment has been carried out in a supersonic molecular beam apparatus using bolometric detection. A tunable resonant cavity was used to enhance the on-beam intensity of the 1.5 μm color center laser used to pump the overtone, and a fixed frequency [R(30)] CO213 laser was used to saturate the coinciding ν18 rQ(2) transition of benzene. After assigning the measured lines of the highly IVR fractionated spectrum to their respective rotational quantum number J, analysis of the data reveals that the dynamics occurs on several distinct time scales and is dominated by anharmonic coupling with little contribution from Coriolis coupling. After the fast (∼100 fs) redistribution of the energy among the previously observed “early time resonances” [R. H. Page, Y. R. Shen, and Y. T. Lee, J. Chem. Phys. 88, 4621 (1988) and 88, 5362 (1988)], a slower redistribution (10–20 ps) takes place, which ultimately involves most of the symmetry allowed vibrational states in the energy shell. Level spacing statistics reveal that IVR produces a highly mixed, but nonstatistical, distribution of vibrational excitation, even at infinite time. We propose that this nonintuitive phenomenon may commonly occur in large molecules when the bright state energy is localized in a high-frequency mode.
Observation of excimer luminescence from electron-excited liquid xenon
Authors: Schüssler A. S. , Burghorn J. , Wyder P. , Lembrikov B. I. , Baptist R.
Published: July 2002

This letter presents an experimental study of vacuum-ultraviolet luminescence from low-energy electron-excited liquid xenon. Our experimental results provide strong support for the theory by [E. B. Gordon, V. V. Khmelenko and O. S. Rzhevskii, Chem. Phys. Lett. 217, 605 (1994)] which considers the rare-gas-atom excitations to be the main reason for the electron drift-velocity saturation occurring at high-electric fields in the condensed rare gases. Furthermore, our results prove the existence of hot electrons in liquid xenon and show that electron-excited liquid xenon could serve as the active medium for a novel type of excimer laser as proposed by [E. B. Gordon, O. S. Rzhevsky, and V. V. Khmelenko, Quantum Electron. 24, 227 (1994)].
Magnetic entropy change in Fe-based compound LaFe10.6Si2.4
Authors: Zhang X. X. , Wen G. H. , Wang F. W. , Wang W. H. , Yu C. H. , Wu G. H.
Published: July 2002

A large magnetic entropy change has been observed in an intermetallic compound LaFe10.6Si2.4. The maximum −ΔSM≈3.2 J/kg K was found at its Curie temperature, ∼242 K, upon a 2 T magnetic field change. Although the entropy change is slightly smaller than that of pure Gd metal, such Fe-rich compounds still appear to be very attractive candidates since (a) the raw materials are much cheaper than pure Gd metal; (b) the Curie temperature can be easily shifted by tuning the composition; (c) the materials are much more chemically stable than pure Gd metal.
Formation of metal nanowires on suspended single-walled carbon nanotubes
Authors: Zhang Y. , Dai Hongjie
Published: July 2002

Suspended single-wall carbon nanotubes are used as substrates for deposition of various metals by evaporation. Deposition of many types of metals normally forms discrete particles on nanotubes due to a weak interaction between the metals and nanotubes. However, continuous nanowires of virtually any metal are obtained by using titanium, a metal with strong interaction with carbon, as a buffer layer on nanotubes. The metal nanowires can be ⩽10 nm wide with continuous length up to tens of microns.
Nanoindentation of epitaxial GaN films
Authors: Kucheyev S. O. , Bradby J. E. , Williams J. S. , Jagadish C. , Toth M. , Phillips M. R. , Swain M. V.
Published: July 2002

Wurtzite GaN films grown on sapphire substrates are studied by nanoindentation with a spherical indenter. No systematic dependence of the mechanical properties of GaN epilayers on the film thickness (at least for thicknesses from 1.8 to 4 μm) as well as on doping type is observed. Slip is identified as one of the physical mechanisms responsible for plastic deformation of GaN and may also contribute to the “pop-in” events observed during loading. No visible material cracking is found even after indentations at high loads (900 mN), but a pronounced elevation of the material surrounding the impression is observed.
Substrate-site selective growth of aligned carbon nanotubes
Authors: Zhang Z. J. , Wei B. Q. , Ramanath G. , Ajayan P. M.
Published: July 2002

We report highly substrate-site selective growth of carbon nanotubes by chemical vapor deposition from precursors of ferrocene and xylene mixtures. The technique allows us to grow well-aligned multiwalled carbon nanotubes preferentially on the SiO2 regions of patterned SiO2/Si substrates prepared by conventional lithography. This eliminates the catalyst predeposition step in the fabrication process. This simple approach may also be applied to build large-scale networks of organized nanotubes on planar substrates.
Crystal and magnetic structure of YCo4−xFexB
Authors: Chacon Cyril , Isnard Olivier
Published: July 2002

YCo 4−x Fe x B compounds have been investigated by means of neutron powder diffraction. Substitution of Co with Fe causes substantial modifications of the crystal structure. Iron occupies preferentially one (2c) of the two cobalt sites (2c and 6i). The Curie temperature of the material increases with Fe content from Tc=380 K for YCo4B, and concurrently a spin reorientation is suppressed. The magnetic phenomenology is explained in terms of the preferential occupation of Fe, which carries in the 2c sites an ordered magnetic moment almost double of that in the 6i sites.
Long-range Coulomb interactions in small Si devices. Part I: Performance and reliability
Authors: Fischetti M. V. , Laux S. E.
Published: July 2002

In the ever smaller silicon metal–oxide–semiconductor field-effect transistors of the present technology, electrons in the conductive channel are subject to increasingly stronger long-range Coulomb interactions with high-density electron gases present in the source, drain, and gate regions. We first discuss how two-dimensional, self-consistent full-band Monte Carlo/Poisson simulations can be tailored to reproduce correctly the semiclassical behavior of a high-density electron gas. We then employ these simulations to show that for devices with channel lengths shorter than about 40 nm and oxides thinner than about 2.5 nm, the long-range Coulomb interactions cause a significant reduction of the electron velocity, and so a degradation of the performance of the devices. In addition, the strong “thermalization” of the hot-electron energy distribution induced by Coulomb interactions has an effect on the expected reliability of the transistors.
Photonic-crystal-based beam splitters
Authors: Bayindir Mehmet , Temelkuran B. , Ozbay E.
Published: July 2002

We proposed and demonstrated two different methods to split electromagnetic waves in three-dimensional photonic crystals. By measuring transmission spectra, it was shown that the guided mode in a coupled-cavity waveguide can be splitted into the coupled-cavity or planar waveguide channels without radiation losses. The flow of electromagnetic waves through output waveguide ports can also be controlled by introducing extra defects into the crystals. Our results may have an important role in the design of efficient power splitters in a photonic circuit.
Investigation of surface treatments for nonalloyed ohmic contact formation in Ti/Al contacts to n-type GaN
Authors: Lin Yow-Jon , Lee Ching-Ting
Published: July 2002

To investigate the possibility of preparing a nonalloyed ohmic contact to n-type GaN, the effects of GaN surface treatments on the metal/GaN interface were studied using x-ray photoelectron spectroscopy. A specific contact resistance of 5.0×10−5 Ω cm2 for the Ti/Al nonalloyed ohmic contacts to (NH4)2Sx-treated n-type GaN can be obtained. The nonalloyed ohmic contact formation would be impeded by the native oxide and the hydroxyl induced from the surface treatment of chemical solutions.
Detection of up to 20 THz with a low-temperature-grown GaAs photoconductive antenna gated with 15 fs light pulses
Authors: Kono Shunsuke , Tani Masahiko , Gu Ping , Sakai Kiyomi
Published: July 2002

We report on the ultrabroadband coherent detection of radiation in wavelengths spanning from far to midinfrared with a low-temperature-grown GaAs photoconductive dipole antenna gated with 15 fs light pulses. The detected spectral frequency exceeds 20 THz.
Microcalorimetric measurements of the heat of adsorption of Pb on well-defined oxides: MgO(100) and p(2×1)-oxide on Mo(100)
Authors: Starr D. E. , Bald D. J. , Musgrove J. E. , Ranney J. T. , Campbell C. T.
Published: July 2002

The energetics of Pb film grown at 300 K on two well-defined oxides [a MgO(100) thin film and a p(2×1)-oxide on Mo(100)] have been measured using single-crystal adsorption microcalorimetry. The evolution of the film morphology was followed using Auger electron spectroscopy (AES). An initial heat of adsorption of 103 kJ/mol is observed for the Pb/MgO(100) system. Auger results indicate that this is due to the formation of an ∼20 atom Pb island in the first pulse of Pb gas incident on the MgO(100) surface. This allows the extraction of a Pb–MgO(100) bond energy of ∼32±2 kJ/mol in the small two-dimensional particles formed in the first pulse. As more Pb is deposited onto the MgO(100) surface, the Pb forms three-dimensional islands. The integral of the heats of adsorption up to high coverage indicates an adhesion energy of 76.5±∼20 μJ/cm2 for large 3D Pb particles to the MgO(100) substrate. This indicates a Pb–MgO(100) bond energy of 49±13 kJ/mol at the 3D interface. Similar analyses of the Pb/p(2×1)-oxide on Mo(100) surface give an initial heat of adsorption of 146.2 kJ/mol and an adhesion energy of 82.5±20 μJ/cm2 for thick Pb films. This indicates a bond energy of 52±12 kJ/mol for a Pb atom to this surface at the interface of a thick Pb film. The metal’s sticking probabilities as a function of coverage and the coverage-dependent changes in optical reflectivity were measured. For the Pb/MgO(100) system, an initial sticking probability of 0.70 at 300 K was observed, whereas for the Pb/p(2×1)-oxide on Mo(100) surface unit sticking probability was observed. The relationship between these quantities and the adsorption energetics is discussed.
Ultralarge capacitance–voltage hysteresis and charge retention characteristics in metal oxide semiconductor structure containing nanocrystals deposited by ion-beam-assisted electron beam deposition
Authors: Kim Yong , Park Kyung Hwa , Chung Tae Hun , Bark Hong Jun , Yi Jae-Yel , Choi Won Chel , Kim Eun Kyu , Lee Ju Wook , Lee Jeong Yong
Published: July 2002

Amorphous silicon films are deposited by ion-beam-assisted electron beam deposition and subsequently oxidized by a rapid thermal oxidation process. The oxidized film contains a large density of nanocrystals specifically localized at a certain depth from the Si/SiOx interface, whereas no evidence of nanocrystals is found for oxidized films deposited without ion beam assistance. Such a marked contrast resulted from the enhancement of nucleation rate by ion beam irradiation. The metal-oxide-semiconductor structure utilizing the film shows an ultralarge capacitance–voltage hysteresis whose width is over 20 V. In addition capacitance–time measurement shows a characteristic capacitance transient indicating nondispersive carrier relaxation. The retention time shows a dependence on applied bias and the maximum time of ∼70 s is obtained near midgap voltage. The retention time dependence on applied bias and large capacitance–voltage hysteresis are attributed to direct tunneling of trapped charges in the deep traps of nanocrystals to the interface states.
Thin interphase/imperfect interface in conduction
Authors: Hashin Zvi
Published: July 2002

Thin interphase between two media is modeled in terms of imperfect interface conditions in the context of potential theory, thus for conduction, dielectric behavior, and permeability. There is no limitation on the magnitude of interphase material conductivity. The imperfect interface conditions involve interface discontinuities of potential and normal component of flux. The theory is tested by application to the boundary value problem of a coated inclusion and to evaluation of effective conductivity of composites with coated spherical particles. Very accurate agreement with three phase solutions of those problems is obtained.
Dominant role of the piezoelectric field in the pressure behavior of InGaN/GaN quantum wells
Authors: Vaschenko G. , Patel D. , Menoni C. S. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: July 2002

We show that the emission characteristics of InGaN/GaN quantum wells under hydrostatic pressure are strongly influenced by the built-in piezoelectric field. The dominant role of the piezoelectric field is established from the dramatic increase of the photoluminescence decay time with pressure and the dependence of the linear pressure coefficient of the photoluminescence peak energy on Si doping in the barriers and excitation intensity. A nonlinear increase of the piezoelectric field with hydrostatic pressure determined from these experiments is explained as being due to a significant dependence of the InGaN piezoelectric constants with strain.
Hydrogen bond radii for the hydrogen halides and van der Waals radius of hydrogen
Authors: Mandal Pankaj K. , Arunan E.
Published: July 2002

In this article, the effective size of hydrogen in the hydrogen halides forming hydrogen bonded complexes is estimated. The scheme proposed by Bhadane and Gadre [J. Chem. Phys. 107, 5625 (1997)] for estimating the size of hydrogen in HF is extended to the other hydrogen halides (HCl and HBr) and HCN. It is noted that the radius of H atom in HF, HCl, HBr, and HCN are, respectively, 0.55±0.07, 0.74±0.08, 0.80±0.11, and 0.93±0.07 Å. The radii found for HF, HCl, and HBr show a strong inverse correlation with the dipole moment of the HX. From this correlation the radius of H atom in HI is estimated to be 0.90±0.11 Å. By extrapolating to zero dipole moment, the van der Waals radius of H atom is determined to be 1.0±0.1 Å, reasonably close to the value proposed by Pauling, 1.2 Å.
Fermi electron wave packet interference images on carbon nanotubes at room temperature
Authors: Hassanien A. , Tokumoto M. , Umek P. , Mihailovic D. , Mrzel A.
Published: July 2002

We report on the structure and electronic properties of single wall carbon nanotubes tips with atomically spatial resolution. Scanning tunneling microscopy shows topographic images of closed tips with a variety of geometrical structure; these include round, conical, as well as tips with a messy shape. Standing wave pattern of the charge density is observed at the tube cap which is formed due to constructive interference between the electronic states and its reflection on the nanotube tips. Atomically resolved images show asymmetry in the charge density that decay out within 6 nm away from the cap. These distinctive tip states do not exist elsewhere on the tube and are related to the presence of topological defects at tube ends.
A density-functional study of the structures and electronic properties of C59Ni and C60Ni clusters
Authors: Alemany M. M. G. , Diéguez O. , Rey C. , Gallego L. J.
Published: July 2002

We report ab initio calculations of C59Ni and C60Ni clusters using a fully self-consistent density-functional method that employs linear combinations of atomic orbitals as basis sets, standard nonlocal, norm-conserving pseudopotentials, and a generalized gradient approximation to exchange and correlation. Our results for C59Ni show that the replacement of a C atom of the C60 fullerene cage by a Ni atom yields (on relaxation) a stable substitutionally doped fullerene, in keeping with both recent density-functional calculations using the local spin density approximation and the results of mass spectrometry. For C60Ni, our calculations show that the most favorable site for the Ni atom is a bridge site over a C–C double bond.
Crystalline zirconia oxide on silicon as alternative gate dielectrics
Authors: Wang S. J. , Ong C. K. , Xu S. Y. , Chen P. , Tjiu W. C. , Chai J. W. , Huan A. C. H. , Yoo W. J. , Lim J. S. , Feng W. , Choi W. K.
Published: July 2002

Epitaxial crystalline yittria-stabilized zirconia (YSZ) oxide films were grown on silicon wafers by the laser molecular beam epitaxy technique. The interface of crystalline YSZ film in contact with silicon was found to be atomically sharp and commensurately crystallized without an amorphous layer. An x-ray photoelectron spectroscopy depth profile and transmission electron microscopy investigation showed that no SiO2 formed at the interface. For a film with electrical equivalent oxide thickness (teox) 14.6 Å, the leakage current is about 1.1×10−3 A/cm2 at 1 V bias voltage. The hysteresis and interface state density in this film are measured to be less than 10 mV and 2.0×1011 eV−1 cm−2, respectively.
Probing spin polarization with Andreev reflection: A theoretical basis
Authors: Mazin I. I. , Golubov A. A. , Nadgorny B.
Published: July 2002

Andreev reflection at the interface between a ferromagnet and a superconductor has become a foundation of a versatile technique of measuring the spin polarization of magnetic materials. In this article we will briefly outline a general theory of Andreev reflection for spin-polarized systems and arbitrary Fermi surface in two limiting cases of ballistic and diffusive transport.
Mechanisms for O2 dissociation during pulsed-laser ablation and deposition
Authors: Camposeo A. , Cervelli F. , Fuso F. , Allegrini M. , Arimondo E.
Published: July 2002

We investigate different dissociation mechanisms for O2 gas during pulsed-laser ablation and deposition. Mesaurements are carried out by using an in situ diagnostics based on absorption spectroscopy of oxygen gas, with space- and time-resolved capabilities, during laser ablation of a metal alloy target in the presence of an oxygen environment. Data, analyzed as a function of ablation parameters, indicate that two different mechanisms, involving electron collisions and formation of a high-density, high-temperature shock layer, play an important role in producing atomic oxygen which can subsequently react with the ablated species.
Measurements of the negative ion density in SF6/Ar plasma using a plane electrostatic probe
Authors: Shindo Masako , Uchino Satoshi , Ichiki Ryuta , Yoshimura Shinji , Kawai Yoshinobu
Published: July 2002

A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.
Fullerene C80: Are there still more isomers?
Authors: Furche Filipp , Ahlrichs Reinhart
Published: July 2002

A theoretical investigation of fullerene C80 using density functional theory is presented. Following the isolated pentagon rule (IPR), seven different topologies are considered. Calculated energy differences are less than 30 kJ/mol except for Ih topology which is more unstable. Isomers of D5d, C2v(I), and D2 topology are found to have singlet ground states. Calculated nuclear magnetic resonance and electronic absorption spectra allow to identify the two known isomers as D2 and D5d, in agreement with previous assignments. The absolute configuration of the chiral D2 isomer is predicted by means of the calculated electronic circular dichroism spectrum. The results further suggest that the C2v(I) isomer might be accessible from soots. All other IPR isomers are either diradicals or have nearly zero singlet–triplet splittings. These species are expected to polymerize, forming insoluble solids. The classification of fullerenes according to their gap is analyzed, and computational methods for treating small gap cases are discussed.
Generic degradation mechanism for 980 nm InxGa1−xAs/GaAs strained quantum-well lasers
Authors: Chu S. N. G. , Chand N. , Joyce W. B. , Parayanthal P. , Wilt D. P.
Published: July 2002

We have observed In out diffusion from strained InxGa1−xAs quantum wells into the adjacent GaAs barriers in degraded 980-nm-wavelength strained quantum-well lasers. A previous calculation on misfit stress-induced compositional instability indicates that this material system is stable with respect to misfit strain. Therefore, the out diffusion of In from an InxGa1−xAs quantum well is mainly driven by the compositional discontinuity across the well/barrier heterointerfaces, and is believed to be activated by the nonradiative recombination of injected carriers.
Microstructure of ultrananocrystalline diamond films grown by microwave Ar–CH4 plasma chemical vapor deposition with or without added H2
Authors: Jiao S. , Sumant A. , Kirk M. A. , Gruen D. M. , Krauss A. R. , Auciello O.
Published: July 2002

Ultrananocrystalline diamond (UNCD) films, grown using microwave plasma-enhanced chemical vapor deposition with gas mixtures of Ar–1%CH4 or Ar–1%CH4–5%H2, have been examined with transmission electron microscopy (TEM). The films consist of equiaxed nanograins (2–10 nm in diameter) and elongated twinned dendritic grains. The area occupied by dendritic grains increases with the addition of H2. High resolution electron microscopy shows no evidence of an amorphous phase at grain boundaries, which are typically one or two atomic layer thick (0.2–0.4 nm). Cross-section TEM reveals a noncolumnar structure of the films. The initial nucleation of diamond occurs directly on the Si substrate when H2 is present in the plasma. For the case of UNCD growth from a plasma without addition of H2, the initial nucleation occurs on an amorphous carbon layer about 10–15 nm thick directly grown on the Si substrate. This result indicates that hydrogen plays a critical role in determining the nucleation interface between the UNCD films and the Si substrate. The relation between diamond nuclei and Si is primarily random and occasionally epitaxial.
Equations of state for fluids: The Dieterici approach revisited
Authors: Sadus Richard J.
Published: July 2002

In 1873, van der Waals proposed a simple equation of state that qualitatively described the phase behavior of fluids. Since then, the principles behind the van der Waals equation have been used and refined countless times in the quest for an accurate equation of state. Despite the enormous amount of work reported, the goal of a simple and accurate equation of state for even relatively simple systems, such as monatomic or polyatomic gases, has proved elusive. Therefore, the analysis of phase equilibria with equations of state is more often than not an exercise in data fitting rather than genuine prediction. In this work, we revisit the early work of Dieterici. When coupled with modern developments in equations of state, this little used approach has the potential of generating useful equations of state. To illustrate the usefulness of the Dieterici formula, we use it to derive a simple equation of state based on the Carnahan–Starling hard-sphere term and van der Waals interactions. This simple approach yields more accurate predictions of the phase co-existence of fluids than the traditional additive approach used to develop equations of state.
The formation and characterization of silver clusters in zirconia
Authors: Govindaraj R. , Kesavamoorthy R. , Mythili R. , Viswanathan B.
Published: July 2002

Silver clusters with size ranging from a few nanometers to several nanometers are formed in zirconia matrix and have been characterized by low frequency Raman scattering (LFRS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies. Surface acoustic vibrational modes of the Ag nanoclusters are observed in the LFRS measurements and the estimated size of silver clusters is of the order of a few nm. XRD measurements indicate the presence of silver clusters in the matrix having a mean size of 20 nm. TEM observations on the zirconia powder indicate the presence of silver clusters as deduced by selected area diffraction pattern analysis. The microscopic structure of the reference ZrO2 sample indicates oxygen related disorder which might act as the heterogeneous nucleating centers of silver clusters as studied by time differential perturbed angular correlation. Based on the experimental results in the present study, it is proposed that the diffusion of Ag atoms through grain boundaries and pore walls could be the dominant mechanism leading to the formation of Ag clusters in the zirconia matrix.
High-energy-resolution scintillator: Ce3+ activated LaBr3
Authors: van Loef E. V. D. , Dorenbos P. , van Eijk C. W. E. , Krämer K. , Güdel H. U.
Published: July 2002

The scintillation properties of LaBr3 doped with 0.5% Ce3+ are presented. Under optical and gamma ray excitation, Ce3+ emission is observed to peak at 356 and 387 nm. The scintillation light output is 61 000±5000 photons/MeV at 662 keV. More than 90% is emitted with a decay time of 35 ns. An energy resolution (full width at half maximum over the peak position) of 2.85%±0.05% was observed for the 662 keV full absorption peak. A time resolution of 385 ps was obtained using BaF2 as second scintillator and gamma rays of Co60.