bluuebunny/crossref_metadata_2025 huggingface dataset:
Cavity-Q-driven spectral shift in a cylindrical whispering-gallery-mode microcavity laser
Authors: Moon Hee-Jong , Chough Young-Tak , Kim Jung Bog , An Kyungwon , Yi Jonghoon , Lee Jongmin
Published: July 2002

Cavity-Q-driven spectral shift of lasing was observed in a cylindrical microcavity formed by rhodamine6G-doped quinoline in a capillary. The envelope of lasing spectrum showed a blueshift induced by the decreasing cavity Q of whispering gallery modes as the pump fluence increases. The thermally induced refractive index changes were measured from the shifts of individual lasing modes. The observed cavity-Q-driven spectral shift was well described by a simple dye laser model, which accounts for the dependence of cavity Q on the thermally induced refractive index change.
High hole mobility in Si0.17Ge0.83 channel metal–oxide–semiconductor field-effect transistors grown by plasma-enhanced chemical vapor deposition
Authors: Höck G. , Kohn E. , Rosenblad C. , von Känel H. , Herzog H.-J. , König U.
Published: July 2002

We report on effective hole mobility in SiGe-based metal–oxide–semiconductor (MOS) field-effect transistors grown by low-energy plasma-enhanced chemical vapor deposition. The heterostructure layer stack consists of a strained Si0.17Ge0.83 alloy channel on a thick compositionally-graded Si0.52Ge0.48 buffer. Structural assessment was done by high resolution x-ray diffraction. Maximum effective hole mobilities of 760 and 4400 cm2/Vs have been measured at 300 and 77 K, respectively. These values exceed the hole mobility in a conventional Si p-MOS device by a factor of 4 and reach the mobility data of conventional Si n-MOS transistors.
An historical review of application of optical solitons for high speed communications
Authors: Hasegawa Akira
Published: July 2002

The history of application of optical solitons for high speed communications presents an interesting example of how an abstract mathematical concept has been put into practical use. The historical development introduced in this manuscript is divided into five stages: (1) historical background and the discovery of the optical soliton, (2) the idea and demonstration of all optical soliton transmission systems, (3) identification of possible problems, (4) demonstration of soliton control and (5) discovery of dispersion managed optical solitons (DMS). Although there are many interesting related subjects such as modulational instability, dark solitons, and spatial solitons, the manuscript focuses only on the subject of primarily theoretical developments of soliton based communications in fibers.
Ordinary optical dielectric functions of anisotropic hexagonal GaN film determined by variable angle spectroscopic ellipsometry
Authors: Yan C. H. , Yao H. , Van Hove J. M. , Wowchak A. M. , Chow P. P. , Zavada J. M.
Published: July 2002

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angle of incidence. Thus the ordinary optical dielectric functions (E⊥〈c〉) are precisely determined by the isotropic mode VASE measurements at angles of incidence between 20° and 40° in the range of 0.75–6.5 eV. The VASE data were analyzed by a model dielectric function based on the GaN critical point structure, which allows for a nonzero extinction coefficient k below the band gap. The thicknesses of these GaN films are accurately determined via the analysis as well.
Anisotropy measurements in mesoscopic magnets by magneto-optical torque
Authors: Armelles G. , Costa-Krämer J. L. , Martin J. I. , Anguita J. V. , Vicent J. L.
Published: July 2002

The development of a versatile tool to quantify magnetic anisotropies in continuous and patterned magnetic thin films is reported. The technique involves measuring the magnetooptic response to a rotating magnetic field. Similarly to mechanical torque techniques, a single measurement obtains both the anisotropy constants and their symmetry axes distribution. The technique has been applied to analyze arrays of submicrometer stripe-shaped Fe (001) elements with different interelement separations (s). For s larger than 1 μm, the anisotropy associated with the stripes is independent on separation, with a value of the effective uniaxial anisotropy constant very consistent with theoretical estimations for these systems.
Interfacial chemistry of Alq3 and LiF with reactive metals
Authors: Mason M. G. , Tang C. W. , Hung L.-S. , Raychaudhuri P. , Madathil J. , Giesen D. J. , Yan L. , Le Q. T. , Gao Y. , Lee S.-T. , Liao L. S. , Cheng L. F. , Salaneck W. R. , dos Santos D. A. , Brédas J. L.
Published: July 2002

The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure.
Low-resistance magnetic tunnel junctions by in situ natural oxidation
Authors: Boeve H. , De Boeck J. , Borghs G.
Published: July 2002

Spin-dependent tunneling has received much attention in recent years. In this study, low-resistance exchange-biased magnetic tunnel junctions (down to 2 μm2) were fabricated in a self-aligned way. The insulating barrier was achieved by in situ natural oxidation of very thin Al layers (<1.3 nm). The resistance and magnetoresistance of tunnel junctions is studied as a function of the aluminum thickness for different oxidation times, leading to resistance-area products from 0.1 to 5 kΩ μm2, and tunnel magnetoresistance (TMR) ratios up to 20%. The decrease in TMR for the lowest resistance barriers in the data set is explained by the nonuniformity of the initial Al layer, which has been characterized using different methods. In this case, the optimal oxidation state cannot be reached. The barrier oxidation state can be improved by thermal treatment, by which an increase in TMR is observed for temperatures up to 275 °C.
Enhanced tunnel magnetoresistance in granular nanobridges
Authors: Yakushiji K. , Mitani S. , Takanashi K. , Takahashi S. , Maekawa S. , Imamura H. , Fujimori H.
Published: July 2002

We have fabricated granular nanobridge structures consisting of electrodes separated by a nanometer-sized gap in which a thin insulating CoAlO granular film is filled, and measured the current–bias voltage characteristics in a magnetic field to investigate the spin-dependent transport. The Coulomb blockade with a clear threshold voltage (Vth) is observed at 4.2 K. Tunnel magnetoresistance (TMR) is enhanced by fabricating nanobridges. TMR shows a maximum exceeding about 30% at the voltage slightly above Vth. This enhancement is explained by the orthodox theory of single electron tunneling in ferromagnetic multiple tunnel junctions.
Presence of oxygen in the lattice of CdTe thin films
Authors: Picos-Vega A. , Arizpe-Chávez H. , Zelaya-Angel O. , Ramı́rez-Bon R. , Espinoza-Beltrán F. J.
Published: July 2002

Polycrystalline CdTe thin films, with oxygen concentrations (x) in the range of 0.01–15 at. %, were grown at room temperature on 7059 Corning glass by means of the rf sputtering method. For low oxygen concentrations (x⩽0.3 at. %) the CdTe develops compressive stress during growth, making the films mechanically unstable. The stress normally relaxes when films are exposed to air, making stable films. In some cases, in films with low oxygen concentration, the stress relaxes abruptly immediately after they are exposed to air, and this results in either some fractures or in a loud explosive cracking of the film. In the latter case, the film is reduced to small pieces and violently dispersed over a wide area. Surface images of the fractures, obtained by atomic force microscopy, show detachment and liftup of the films in the regions adjacent to fractures. This indicates that the films were originally under compressive stress. Values of x in the bulk of the films were measured by means of Auger mass spectroscopy. The functional dependence of the crystalline structure, interplanar distance, grain size and band gap of the films on the oxygen content is studied.
Enhanced initial susceptibility in small amorphous disks
Authors: Aharoni Amikam
Published: July 2002

A highly enhanced initial susceptibility is predicted theoretically for thin, amorphous ferromagnetic films when they are patterned as separate disks of appropriate dimensions. The relation between the required disk radius and the film thickness is given as a function of the exchange constant and the saturation magnetization of the amorphous material.
Evidence of exciton recombination at very high temperature in InGaN
Authors: Wang Xiaojun , Choa Fow-Sen , Liu Feng , Worchersky T. L.
Published: July 2002

Time-resolved electroluminescence measurements are carried out on the blue light emitting diodes with InGaN active layer at temperatures from 30 to 530 K. The decay mechanisms of the ultraviolet optical pulses corresponding to the band-to-band recombinations are investigated. The exciton-related recombination is found mainly responsible for this band-edge radiative recombination from 250 to 425 K. A thermal equilibrium model, in which the exciton dissociation process is taken into account, is used to fit the experimental results. The fitted exciton binding energy is about 48.3 meV. This high exciton binding energy is attributed to the indium-related localization effect in InGaN. Moreover, it is also found that the nonradiative lifetimes in these samples are quite long. This is ascribed to the suppression of the nonradiative recombination centers because of the incorporation of indium in GaN material.
Wavelength control from 1.25 to 1.4 μm in InxGa1−xAs quantum dot structures grown by metal organic chemical vapor deposition
Authors: Passaseo A. , Maruccio G. , De Vittorio M. , Rinaldi R. , Cingolani R. , Lomascolo M.
Published: July 2002

This letter reports on the realization of long-wavelength InGaAs quantum dots (QDs) fabricated by metal organic chemical vapor deposition. By controlling the In incorporation in the QD layers and/or in the barrier embedding the QDs, we are able to tune the wavelength emission continuously from 1.25 to 1.4 μm at room temperature. Efficient stacking of dots emitting at 1.3 μm is also demonstrated.
Formation of a stratified lanthanum silicate dielectric by reaction with Si(001)
Authors: Copel M. , Cartier E. , Ross F. M.
Published: July 2002

We have characterized the structure and electrical properties of lanthanum silicate layers formed on Si(001) by reaction of lanthanum oxide with the substrate. Postoxidation of the deposited films results in the formation of a stacked dielectric with a lanthanum silicate layer atop an interfacial layer of SiO2. This structure combines the interfacial properties of SiO2 with the large permittivity of lanthanum silicate. Although the resulting film has leakage properties far superior to an equivalent thickness of SiO2, there is evidence of significant quantities of ionic charge that must be eliminated before use in electronic applications.
Impedance spectroscopy evidence of the phase separation in La0.3Pr0.4Ca0.3MnO3 manganite
Authors: Souza J. A. , Jardim R. F. , Muccillo R. , Muccillo E. N. S. , Torikachvili M. S. , Neumeier J. J.
Published: July 2002

We have measured electrical resistance R(T), magnetization M(T), and for the first time impedance spectroscopy Z(T,ω) of polycrystalline samples of La0.3Pr0.4Ca0.3MnO3 compounds. The combined results suggest the coexistence of two metallic ferromagnetic phases below the Curie temperature of the system. The R(T) data exhibit two important features: the occurrence of a metal–insulator (MI) transition at temperatures close to TMI≈170 K and a large thermal hysteresis below TMI. The first feature was found to be associated with the development of ferromagnetism and takes place when the magnetization of the samples becomes about 15% of its saturated value at low temperatures. The second feature suggests two contributions to R(T) below the Curie temperature TC. The Z(T,ω) data taken from 77 to 300 K and frequency varying from 5 to 107 Hz are much more valuable. These results reveal two well defined bulk contributions to the transport properties of these manganites below ∼170 K: one occurring at high frequencies ∼4×106 Hz and a second one at low frequencies, typically on the order of 1.5×105 Hz. An analysis of the Z(T,ω) data suggests that these contributions are related to the spin lattice relaxation rate of the two distinct phases. Such a result suggests a phase separation below TC in the compound in complete agreement with recent muon spin relaxation and neutron spin echo measurements performed on manganites.
Voltage-induced barrier-layer damage in spin-dependent tunneling junctions
Authors: Rao D. , Sin K. , Gibbons M. , Funada S. , Mao M. , Chien C. , Tong H.-C.
Published: July 2002

The effect of a dc stress voltage on the junction resistance and magnetoresistance (MR) of spin-dependent tunneling (SDT) junctions with naturally oxidized barriers was investigated. There is a threshold voltage at which irreversible resistance change begins. Beyond this threshold, device resistance decreases gradually over a transition period prior to breakdown of the tunneling barrier. The onset voltage of irreversible resistance change is much higher than the optimum operating voltage of SDT heads having the precursor aluminum thicknesses here investigated (5–11 Å). The MR ratio decreased with increasing stress voltage in a pattern similar to that of the junction resistance.
Thickness dependence of surface roughness and transport properties of La2/3Ca1/3MnO3 epitaxial thin films
Authors: Bibes M. , Balcells Ll. , Valencia S. , Sena S. , Martı́nez B. , Fontcuberta J. , Nadolski S. , Wojcik M. , Jedryka E.
Published: July 2002

In this work, we report on the impact of distinct growth parameters that affect the roughness and surface morphology of La2/3Ca1/3MnO3 epitaxial thin films grown by rf sputtering, namely, the film thickness and the deposition temperature. Data for films with thicknesses ranging from 2.4 to 108 nm will be presented. A correlation with transport measurements is also reported: whereas films thicker than 6 nm show the typical metal-to-insulator transition, the thinner film is insulating. The resistivity is strongly enhanced when decreasing film thickness. Nuclear magnetic resonance measurements have been used to monitor the relative concentration of the localized Mn4+ and delocalized Mn3+/4+ states. It is found that the relative intensity of the delocalized Mn3+/4+ configuration (I3+/4+) progressively lowers when reducing film thickness. Of significance could be the observation that I3+/4+ remains finite for the thicknesses corresponding to insulating films, thus suggesting that an electrically inhomogeneous state is formed in a region close to the interface with the substrate.
Analysis of nanocrystalline diamond powder by scanning transmission electron microscopy
Authors: Peng J. L. , Fehlhaber R. P. , Bursill L. A. , McCulloch D. G.
Published: July 2002

Theoretical studies [R. P. Fehlhaber and L. A. Bursill, Phys. Rev. B 60, 14147 (1999)] of small nanocrystalline diamond spheres, using the classical electrodynamic formalism with a well defined impact parameter and including all multipole modes, are used to analyze and discuss electron energy loss spectra recorded with a high resolution scanning transmission electron microscope. The theory is used to predict spectra for spheres of different radii with and without surface coatings. Due to the relatively large damping factor for diamond the surface, interface and bulk plasmon peaks overlap; but it is shown that the theory is nevertheless capable to allow particle diameter and coating thickness to be determined accurately. The theory is also applied to make some deductions concerning particle morphology by analysis of low-loss spectra from particles of different thickness and shape.
Temperature dependent resistance of magnetic tunnel junctions as a quality proof of the barrier
Authors: Rüdiger U. , Calarco R. , May U. , Samm K. , Hauch J. , Kittur H. , Sperlich M. , Güntherodt G.
Published: July 2002

Tunnel junctions of Co(10 nm)/AlOx (nominally 2 nm)/Co(20 nm) have been prepared by molecular beam epitaxy applying a shadow mask technique in conjunction with an UV light-assisted oxidation process of the AlOx barrier. The quality of the AlOx barrier has been proven by x-ray photoelectron spectroscopy and temperature dependent tunneling magnetoresistance (TMR) measurements. Optimum-oxidized tunnel junctions show a TMR of 20% at 285 K and up to 36% at 100 K. At 285 K the TMR values as a function of oxidation time are not symmetric about the optimum time. For underoxidized junctions the TMR is reduced more strongly than for overoxidized junctions. The temperature dependence of the junction’s resistance is a clear and reliable indicator whether pinholes (or imperfections) contribute to the conduction across the barrier.
Screw dislocations in GaN: The Ga-filled core model
Authors: Northrup John E.
Published: July 2002

First-principles total energy calculations performed for [0001] screw dislocations in GaN with |b|=c indicate that a model with a helical Ga-filled core is more stable than the hollow core model in Ga-rich conditions. This model gives rise to electronic states dispersed throughout the band gap. Such a dislocation is therefore expected to be a very strong center for nonradiative recombination and a pathway for current leakage.
Enhancement of metal–semiconductor barrier height with superthin silicon dioxide films deposited on gallium arsenide by liquid phase deposition
Authors: Huang C. J.
Published: July 2002

This study presents a method for surface passivation using silicon dioxide (SiO2). The proposed method has shown great effectiveness on metal–semiconductor barrier height enhancement. A high quality SiO2 layer is developed via liquid phase deposition, a method which naturally leaves a doping-level fluorine residue in the SiO2. The addition of fluorine to enhance the Schottky barrier height (SBH) is first discussed. Experimental results are presented. It is found that this fluorine addition enhances the Schottky barrier height, which allows a larger positive gate bias for enhancement mode metal–semiconductor field-effect transistors, thus permitting the fabrication of digital logic circuits with improved noise margins and relaxed tolerance with regard to device threshold voltage uniformity. The SBH to n-gallium arsenide (GaAS) is found to be approximately 0.7 eV. Finally, the effective barrier height of the metal–insulator–semiconductor structure reached 1.03 eV after annealing. The enhancement of SBH has been attributed to the formation of these stable interface layers. A model for fluorine-enhanced SiO2 SBH enhancement is also presented.
Variations of the infrared transmission properties with the metal–insulator transition in thin films of the yttrium-hydride system
Authors: Lee M. W. , Kuo C. Y. , Lin H. C. , Wang H. C.
Published: July 2002

This work investigates the variations of the infrared transmission spectra of yttrium-hydride films YHx during the hydrogen loading process for the frequency range 500–4000 cm−1. The results indicate that the transmittance slightly decreases in the dihydride phase, followed by a significant increase in the trihydride phase. In addition, the carrier concentration decreases, whereas the carrier relaxation time increases with hydrogen content. The hydrogen vibration modes at interstitial sites are completely screened in the dihydride phase. The screening effect decreases as the system goes through the metal–insulator transition. Moreover, the screening effect can be continuously tuned by simply varying the hydrogen content in the yttrium-hydride system. Analysis indicates that the absorption intensity of the vibration mode depends on the carrier concentration. This effect can be used as a diagnostic tool for estimating the carrier concentration and hydrogen content in rare-earth hydrides.
Dislocation dynamics of strain relaxation in epitaxial layers
Authors: Wang T. C. , Zhang Y. W. , Chua S. J.
Published: July 2002

Many experimental observations have clearly shown that dislocation interaction plays a crucial role in the kinetics of strain relaxation in epitaxial thin films. A set of evolution equations are presented in this article. The key feature of the equations is the incorporation of dislocation interactions into the kinetic process by introducing a resistance term. The resistance to threading dislocation gliding is characterized by a hardening function, which depends only on the relaxed plastic strain. The evolution equations are tested on the GexSi1−x/Si(100) materials system. Existing fundamental parameters are incorporated into the present model. The evolution equations successfully reproduce a wide range of experimental data on strain relaxation in GexSi1−x/Si heterostructures.
Origin and annealing of deep-level defects in p-type GaAs/Ga(As,N)/GaAs heterostructures grown by molecular beam epitaxy
Authors: Krispin P. , Spruytte S. G. , Harris J. S. , Ploog K. H.
Published: July 2002

Deep-level defects in p-type GaAs/Ga(As,N)/GaAs heterostructures grown by molecular beam epitaxy are investigated by deep-level transient Fourier spectroscopy. Depth-resolved distributions of hole traps are measured in as-grown and annealed heterojunctions in order to identify the defects, which lead to the degradation of the Ga(As,N) properties. Four defects are recognized in the heterostructures studied. Two dominant hole traps are found in Ga(As,N) at energies of about 0.35 and 0.45 eV above the valence band edge. These midgap levels originate from copper- and iron-related defects, the formation of which is connected with operation of the nitrogen plasma cell during Ga(As,N) growth. Both traps, which are removed by annealing, are discussed as the possible nonradiative centers that deteriorate the optical properties. Two other hole traps of intrinsic origin are related to the GaAs growth conditions close to the Ga(As,N)-on-GaAs interface, where the GaAs growth is affected by the nitrogen plasma despite a closed shutter. As far as electronic levels in the lower half of the band gap are concerned, the Ga(As,N) layers and GaAs-on-Ga(As,N) interfaces become practically defect free after rapid thermal annealing.
Effective magnetic anisotropy of amorphous and nanocrystalline Fe71.5Al2Cu1Nb3Si13.5B9 alloy ribbon
Authors: Aranda G. R. , Miguel C. , Garcı́a-Tello P. , González J.
Published: July 2002

The effective magnetic anisotropy of the Fe71.5Al2Cu1Nb3Si13.5B9 alloy with different nanostructures has been investigated. The nanostructured phase was produced by means of thermal treatment (520–600 °C during 1 h) to develop the devitrification of the precursor amorphous material. The effective magnetic anisotropy was evaluated from the measurement of the bias field dependence of the transverse susceptibility. Values of the effective anisotropy field, Hk, from 20 Oe (as-quenched sample) down to 7 Oe (optimum nanocrystalline state) were obtained. Such values of Hk are close to those obtained in the classical nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy (8–10 Oe) alloy. The Al could enhance the intergranular coupling, leading to the ultrasoft magnetic character of this nanocrystalline Al-containing alloy.
Magnetic tunnel junctions with tantalum oxide barriers displaying a magnetoresistance ratio of up to 10% at room temperature
Authors: Gillies M. F. , Kuiper A. E. T. , van Zon J. B. A. , Sturm J. M.
Published: July 2002

The magnetoresistance (MR) of magnetic tunnel junctions containing tantalum oxide barrier materials is studied as a function of oxidation time. The barriers are characterized both electrically and via in situ x-ray photoelectron spectroscopy. The highest MR ratio of ≈10% is achieved when the Ta is completely oxidized to form Ta2O5. Barriers are also studied where oxidation not only occurs from O plasma but also via “solid state”, i.e., where the bottom Co electrode is first partially oxidized before depositing the Ta layer. Annealing experiments were performed on all these samples but no further increase in MR ratio was observed.
Ionic and electronic dark decay of holograms in LiNbO3:Fe crystals
Authors: Yang Yunping , Nee Ingo , Buse Karsten , Psaltis Demetri
Published: July 2002

The lifetimes of nonfixed holograms in LiNbO3:Fe crystals with doping levels of 0.05, 0.138, and 0.25 wt % Fe2O3 have been measured in the temperature range from 30 to 180 °C. The time constants of the dark decay of holograms stored in crystals with doping levels of 0.05 and 0.25 wt % Fe2O3 obey an Arrhenius-type dependence on absolute temperature T, but yield two activation energies: 1.0 and 0.28 eV, respectively. For these crystals, two different dark decay mechanisms are prevailing, one of which is identified as proton compensation and the other is due to electron tunneling between sites of Fe2+ and Fe3+. The dark decay of holograms stored in crystals with the doping level of 0.138 wt % Fe2O3 is the result of a combination of both effects.
Nitrogen-related complexes in gallium arsenide
Authors: Lowther J. E. , Estreicher S. K. , Temkin H.
Published: July 2002

A first-principles pseudopotential method has been used to study some potentially important metastable defects in N-doped GaAs. Formation energies have been obtained and related to those of As and Ga vacancies in the intrinsic material. Of the structures considered, two are identified that crucially depend on overall material stoichiometry. These are N on a Ga site (NGa) or a NN dimer on an As site (NNAs). NGa has a partly shallow energy-level structure with a fully occupied level lying near the valence-band edge, whereas NNAs has localized character with a midgap deep level. Both defects may be responsible for the onset of a reduction in the apparent band gap that has been recently observed in heavily N-doped GaAs prior to alloying.
Lie algebras associated with the exponential solutions of nonautonomous linear differential equations
Authors: Suárez Rodolfo , Sáenz Leonardo
Published: July 2002

A new free Lie algebra structure generated by a continuous set of operators is introduced. The Friedrichs’ criterion, which is known for a finite (countable) number of generators, is extended to the continuous case. Based on Friedrichs’ criterion, we obtain an explicit expression for the coefficients of the Magnus expansion, which is central for one to get approximately the exponential solutions of nonautonomous linear differential equations.
Trapping of gold by nanocavities induced by H+ or He++ implantation in float zone and Czochralski grown silicon wafers
Authors: Périchaud I. , Yakimov E. , Martinuzzi S. , Dubois C.
Published: July 2002

In silicon, implantation of He++ or H+ ions and subsequent annealing can lead to the formation of nanocavities below the implanted surface of the wafers. These nanocavities, which behave as trapping sites for metallic impurities, can be located near the devices in integrated circuits in order to induce a proximity gettering. In this article, we investigate, in float zone (FZ) and Czochralski (Cz) wafers, the trapping of gold by nanocavities formed by implantation of He++ or H+ ions at 250 keV and at a dose of 3×1016 cm−2 followed by subsequent annealing(s) at 750  °C for 1 h. Deep level transient spectroscopy profiles show that substitutional gold concentration decreases near the cavity band in FZ and Cz samples. Gold profiles obtained by secondary ion mass spectroscopy show that there is a strong trapping of gold in the cavity band in all samples. In the case of He++ implanted wafers, this trapping also occurs in the region between the implanted surface and the cavities, and the higher the oxygen concentration, the higher this trapping is. This could be explained by a higher density of implantation induced defects in He++ implanted samples and by the formation of oxygen-vacancy complexes in this region. In H+ implanted samples, the hydrogen passivation of such defects reduces their efficiency as trapping sites. The results suggest that gold trapping is related to gold precipitation which could inject self-interstitials in the bulk and then decreases the substitutional gold concentration near the cavity band even though there is a constant source of gold atoms in our experiments.
Intermartensitic transformation and magnetic-field-induced strain in Ni52Mn24.5Ga23.5 single crystals
Authors: Wang W. H. , Wu G. H. , Chen J. L. , Gao S. X. , Zhan W. S. , Wen G. H. , Zhang X. X.
Published: July 2002

We have found a complete thermoelastic intermartensitic transformation between modulated and unmodulated martensite in single-crystal Ni52Mn24.5Ga23.5. This intermartensitic transformation provides a much larger strain than that of the martensitic transformation. A giant switching-like strain of ±5.0% can be achieved by a small magnetic field of 0.2 T upon the intermartensitic transformation. In the modulated martensite, a large recoverable magnetic-field-induced strain of up to 1.2% has been obtained.
Effect of an AlN buffer layer on the epitaxial growth of InN by molecular-beam epitaxy
Authors: Lu Hai , Schaff William J. , Hwang Jeonghyun , Wu Hong , Koley Goutam , Eastman Lester F.
Published: July 2002

The effect of an AlN buffer layer on the epitaxial growth of InN by molecular-beam epitaxy (MBE) is studied. Using an AlN buffer layer can significantly improve the structural and electrical properties of InN. With increasing thickness of the AlN buffer layer, the Hall mobility of InN will monotonically increase while the electron carrier concentration decreases. The surface morphology of the film also improves. A Hall mobility of more than 800 cm2/V s with a carrier concentration of 2–3×1018 cm−3 at room temperature can be routinely obtained on ∼0.1 μm InN film. More importantly, it is found that under optimum growth conditions, by using an AlN buffer layer, InN films with comparable quality can be achieved by the conventional MBE technique compared to InN grown by migration-enhanced epitaxy. Increasing InN thickness also increases Hall mobility.
Vapor Pressures, Critical Parameters, Boiling Points, and Triple Points of Halomethane Molecular Substances
Authors: Xiang Hong Wei
Published: July 2002

A simple general corresponding-states principle for vapor pressure incorporated with a simple vapor-pressure equation is presented to describe within and extend beyond the range of available experimental data for 25 halomethane substances. Apparatus type, measurement method, and precision and uncertainties are listed for the experimental vapor-pressure data sets. The experimental data with the corresponding ranges have been reviewed to test the present method in representing the vapor-pressure behavior. Comparisons with available data show that the extended corresponding-states principle for vapor pressure has very good accuracies with different data sets. The substance-dependent characteristic parameters such as critical temperature, critical density, critical pressure, and acentric factor are given from reliable literature sources along with the general method. The values of the pressures, along with their first and second derivatives as function of temperature over the entire region from the triple point to the critical point, are tabulated and recommended for scientific and practical uses.
Current injection into high temperature superconductors: Does spin matter?
Authors: Gim Y. , Kleinsasser A. W. , Barner J. B.
Published: July 2002

Recent comparative studies of quasiparticle injection from colossal magnetoresistance (CMR) materials and normal metals into high transition temperature superconductors (HTS) have led several investigators to conclude that spin-polarized injection from CMR electrodes is far more efficient than ordinary injection in suppressing superconductivity. On the other hand, some ordinary quasiparticle injection experiments have shown greater suppression of superconductivity than was claimed in any of the published spin-injection experiments. Careful analysis and numerical simulations lead us to argue that all spin-injection experiments to date have been hampered by inadequate control samples and that the reported results can be explained by ordinary (unpolarized) quasiparticle injection or simple current summation. We conclude that there is no conclusive evidence that spin-polarized injection into HTS behaves differently from ordinary quasiparticle injection and we discuss criteria for future experiments that could differentiate between the two types of injection.
Pairs of pseudopure states for 4- and 5-qubit nuclear magnetic resonance systems
Authors: Fung B. M.
Published: July 2002

The use of pairs of pseudopure states (POPS) as the basis for nuclear magnetic resonance (NMR) quantum computing is a method recently introduced. The advantage of this approach is its experimental simplicity: each POPS can be prepared in a single experiment with two steps, the first step having two pulses and the second having only one. The POPS method is now demonstrated for a 4-qubit and a 5-qubit system, for which the F19 and H1 spectra are first order and all the peaks are completely resolved. The characteristics of the POPS spectra are examined in detail, and the spectra of one of the five available sets of POPS for the 5-qubit system are shown as an illustration. The result of applying a c4-NOT quantum logic gate (controlled-NOT gate conditional upon the state of four other qubits) to another set of POPS in the 5-qubit system is also shown.
Bulk ferroelectric domain nucleation in KTiOPO4 crystals
Authors: Urenski P. , Molotskii M. , Rosenman G.
Published: July 2002

Ferroelectric domain inversion in KTiOPO4 crystals has been studied for two different states of these crystals: superionic (at room temperature), and dielectric (at low temperature, when the ion transport is frozen). The structure of the ferroelectric domain inverted at room temperature demonstrates a dramatic domain broadening due to a bulk domain nucleation. The observed effect is ascribed to highly mobile potassium ions that contribute to the minimization of the depolarizing field and the surface energy of domain walls of new domain nuclei.
A general method to obtain well localized Wannier functions for composite energy bands in linear combination of atomic orbital periodic calculations
Authors: Zicovich-Wilson Claudio M. , Dovesi Roberto , Saunders Victor R.
Published: July 2002

A method for obtaining spatially localized crystalline orbitals starting from delocalized Bloch functions is proposed. The method, that has been implemented in the LCAO CRYSTAL code, is intrinsic and general for nonconducting systems, and provides a set of well localized Wannier functions that can be used for applications that take advantage of their localized character. Examples are given that illustrate the performances and efficiency of the proposed scheme.
Strain dependence on barrier material and its effect on the temperature stability of photoluminescence wavelength in self-assembled GaInAs quantum wires
Authors: Wohlert D. E. , Change K. L. , Pickrell G. W. , Hsieh K. C. , Cheng K. Y.
Published: July 2002

The strain-induced lateral-layer ordering process was used to fabricate GaInAs quantum wire (QWR) heterostructures on InP whose QWR layers are separated by different barrier materials. Using cross-sectional transmission electron microscopy the microstructure of the different QWR samples was studied. It was found that GaInAs QWR heterostructures with nominally lattice matched AlGaInAs barriers resulted in heavily strained QWR regions relative to the same structure with nominally lattice matched AlInAs or InP barriers. Furthermore, photoluminescence studies demonstrate that these heavily strained QWR samples with AlGaInAs barriers have a wavelength-shift rate of 0.2 to 1.0 Å/°C in the 293–360 K range. This is in comparison to GaInAs QWR samples with ternary or binary barriers and a GaInAs quantum well control sample that have wavelength-shift rates of ∼2.5 and ∼6.0 Å/°C, respectively. It is shown empirically that a minimum amount of strain in the GaInAs QWR/AlGaInAs barrier region is required to achieve wavelength-shift rates below 1 Å/°C.
Polarization effects of imperfections in conducting and dielectric samples imaged with polarization-sensitive scanning near-field optical microscopy
Authors: Eggers G. , Rosenberger A. , Held N. , Güntherodt G. , Fumagalli P.
Published: July 2002

The influence of intrinsic birefringence and of surface imperfections on the polarization of near-field light is investigated in thin films by polarization-sensitive scanning near-field optical microscopy in transmission mode. The experimental results will be discussed and a simple simulation algorithm is proposed.
Spin-filter specular spin valves
Authors: Lu Z. Q. , Pan G. , Jibouri A. A. , Zheng Yaunkai
Published: July 2002

Both a thin free layer and high magnetoresistance (MR) ratio are required in spin valves for high magnetic density recording heads. In traditional spin valve structures, reducing the free layer normally results in a reduction in MR. We report here on a spin-filter specular spin valve with structure Ta 3.5 nm/NiFe 2 nm/IrMn 6 nm/CoFe 1.5 nm/Nol/CoFe 2 nm/Cu 2.2 nm/CoFe tF/Cu tSF/Nol2/Ta 3 nm, which is demonstrated to maintain MR ratio higher than 12% even when the CoFe free layer is reduced to 1 nm. The semiclassical Boltzmann transport equation was used to simulate MR ratio. An optimized MR ratio of ∼14.5% was obtained when tF was about 1.5 nm and tSF about 1.0 nm as a result of the balance between the increase in electron mean free path difference and current shunting through conducting layer. It is found that the Cu enhancing layer not only enhances the MR ratio but also improves soft magnetic properties of CoFe free layer due to the low atomic intermixing observed between Co and Cu. The CoFe free layer of 1–4 nm exhibits a low coercivity of ∼3 Oe even after annealing at 270 °C for 7 h in a field of 1 kOe. Furthermore, the interlayer coupling field Hint between free layer and pinned layer can be controlled by balancing the Rudermann–Kittel–(Kasuya)–Yosida and magnetostatic coupling. Such a thin soft CoFe free layer is particularly attractive for high density read sensor application.
Faceting evolution during self-assembling of InAs/InP quantum wires
Authors: Gutiérrez H. R. , Cotta M. A. , de Carvalho M. M. G.
Published: July 2002

The self-assembling of InAs quantum wires on (001) InP substrates during chemical beam epitaxy has been studied. The samples were characterized by reflection high-energy electron diffraction (RHEED), atomic force microscopy, and high-resolution transmission electron microscopy (HRTEM). By monitoring the RHEED chevron structures along the [11̄0] direction, we studied the facets formation during the initial states of InAs growth. The facets angles measured by HRTEM are in perfect agreement with the angles between chevron streaks. A time dependence of the chevron streaks angles is reported and correlated to the wire formation. These results can be interpreted using nonequilibrium models existing in literature.
Effect of current direction on the lifetime of different levels of Cu dual-damascene metallization
Authors: Gan C. L. , Thompson C. V. , Pey K. L. , Choi W. K. , Tay H. L. , Yu B. , Radhakrishnan M. K.
Published: July 2002

Electromigration in the lower metal (M1) and the upper metal (M2) of Cu dual-damascene interconnections has been studied. The failure times of M2 test structures are significantly longer than those of identical M1 structures. It is proposed that this asymmetry is the result of a difference in the location of void formation and growth, which is believed to be related to the ease of electromigration-induced void nucleation and growth at the Cu/Si3N4 interface. Asymmetric via reliability is therefore an intrinsic characteristic of current Cu interconnect technology.
Micromagnetic study of subnanosecond magnetic switching in perpendicular multilayers
Authors: Lyberatos A. , Ju G. , van de Veerdonk R. J. M. , Weller D.
Published: July 2002

Measurements and micromagnetic modeling results are presented of the subnanosecond magnetization reversal process of perpendicularly magnetized (CoCr/Pt) multilayers. The switching time of the magnetization is primarily dependent on the field strength and field bias angle to the perpendicular direction. The intergranular exchange coupling results in a domino-type magnetization reversal, significant delay in the media response far from the gap edge, and an asymmetry between the maximum upper and lower magnetization levels attained in response to a periodically varying head field. The thermal excitations of the magnetization assist the switching of the magnetization, however, their effect is small and can be neglected, to first order, in studies of high-speed switching of the magnetization in response to a change in the head field polarity. The gyromagnetic switching in the multilayered film appears to be heavily damped.
Thermal stability of spin-valves incorporating amorphous CoNbZr under and capping layers
Authors: Cho Ho Gun , Kim Young Keun , Lee Seong-Rae
Published: July 2002

We introduce amorphous CoNbZr-based spin-valves (SVs) where CoNbZr substituted Ta layers that have been traditionally used for under and capping layers. The purpose of this study was to investigate thermal stability of the new structure at elevated temperatures. A comparison was also made with a similarly structured Ta-based SV. Magnetoresistance (MR) ratio and exchange bias field (Hex) of CoNbZr-based SV increased up to about 50% (from 4.5% to 6.8%) and 52% (from 294 Oe to 448 Oe), respectively, after annealing at 300 °C for 240 min. The root-mean-square (rms) surface roughness values for CoNbZr and Ta single layers were 0.07 nm and 0.43 nm, respectively, indicating that an amorphous CoNbZr offers much smoother surface than the Ta layer. Because CoNbZr-based SVs possess fine and dense microstructures, they were able to resist interdiffusion at elevated temperature, resulting in an improvement in thermal stability. In addition, Mn diffusion into the pinned CoFe layer might be reduced probably due to the fact that CoNbZr capping layer provided almost equal chemical potential for Mn diffusion into the pinned CoFe layer.
Magnetic anisotropy and magnetoresistance of sputtered [(FeTaN)/(TaN)]n multilayers
Authors: Nie H. B. , Xu S. Y. , Li J. , Ong C. K. , Wang J. P.
Published: July 2002

We studied the in-plane magnetic anisotropy of rf (radio frequency) sputtered [(FeTaN)/(TaN)]n multilayers synthesized on Si substrates. In the multilayers where n=5, the FeTaN thickness is fixed at 30 nm and the thickness of TaN, tTaN, is varied from 0 to 6.0 nm, we observed a clear trend that, with increasing tTaN, the values of coercivity, grain size, and amplitude of maximum magnetoresistance (MR) of the samples all decrease first and then increase after reaching a minimum when tTaN is around 2.0–4.0 nm. This trend is also associated with an evolution of in-plane magnetic anisotropy, where the multilayers change from uniaxial anisotropy to biaxial at tTaN around 4.0 nm and above. We attribute the phenomena to the interlayer coupling effect of FeTaN films as a function of the coupling layer (TaN) thickness, rather than to the thickness dependence observed in single-layered FeTaN films, where the direction of easy axis switches 90° when the film is thicker than 300 nm. The in-plane anisotropy of the [(FeTaN)/(TaN)]n multilayers also shows signs of oscillation when the number of coupling layers varies. The MR effects observed are mainly due to anisotropy MR (AMR), while the grain size and exchange coupling may also contribute to the change of maximum MR ratios in the multilayers with changing tTaN.
Magnetotransport properties of a room temperature rectifying tunnel junction made of electron and hole doped manganites
Authors: Mitra C. , Köbernik G. , Dörr K. , Müller K.-H. , Schultz L. , Raychaudhuri P. , Pinto R. , Wieser E.
Published: July 2002

The hole-doped (p) manganite La0.7Ca0.3MnO3 and the electron-doped (n) manganite La0.7Ce0.3MnO3 undergo an insulator-to-metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have fabricated an epitaxial trilayer (La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3), where SrTiO3 is an insulator. At room temperature, i.e., in the semiconducting regime, it exhibits asymmetric current–voltage (I–V) characteristics akin to a p–n diode. The observed asymmetry in the I–V characteristics disappear at low temperatures where both the manganite layers are metallic. The I–V curves exhibit an intriguing temperature dependence in the presence of magnetic field. At room temperature, i.e., above the ordering temperature, we have a negative magnetoresistance (MR) and at low temperature we have a positive MR, indicative of a minority spin carrier band in La0.7Ce0.3MnO3. A possible mechanism for the observed effects are discussed.
Electron paramagnetic resonance study of La0.7Ca0.3−xBaxMnO3 lanthanum manganites
Authors: Ulyanov A. N. , Yu S. C. , Min S. G. , Levchenko G. G.
Published: July 2002

Electron paramagnetic resonance (ESR) study of La0.7Ca0.3−xBaxMnO3 manganites (x=0; 0.15; 0.3) is presented. Experimentally observed exponential decreasing of line intensity on temperature is in agreement with the deduced one and indicates the spin-lattice relaxation in manganites near the Curie temperature Tc. The obtained dependence of ESR linewidth on the x value shows the decreasing of strength of spin-lattice interaction with the increasing of Tc (caused by the increasing of the x value).