Publications

Cubic boron nitride (c-BN) crystals synthesised at high pressures and temperatures are analysed by optical microscopy, transmission electron microscopy, electron diffraction and electron spin resonance. For various growth conditions, the results of these studies indicate that the c-BN crystals contain defects and impurities. This is the first time that dislocation cores have been revealed in c-BN at the atomic level. Atomic resolution at extended dislocations allows us to determine the stacking-fault energy in c-BN, yielding a mean value of 191+/-15 mJ m(-2). This value, which is reported for the first time for c-BN, is of the same order of magnitude as in diamond. (C) 1999 Elsevier Science S.A. All rights reserved.

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

CxByNz layers have been prepared by laser-assisted chemical vapour deposition (CVD) in a gas atmosphere containing C2H4, B2H6 and NH3 where the starting composition ratio could vary in a large range. The characterization of these C-B-N materials was made by XRD, EPMA, XPS, optical and scanning electron microscopy. The chemical composition of C-B-N layers tended to a composition C1B6N3 on the line BN-'B3C'. The turbostratic structure of C-B-N layers could be influenced and modified as a function of composition. XPS investigations confirmed the single-phase nature and the existence of bonding between all the elements. Some planar structures, containing especially CB2N groups, were suggested for the 'unit cell' of these C-B-N solid solutions, in agreement with EPMA and XPS analysis. (C) 1999 Elsevier Science S.A. All rights reserved.

We calculate energy spectra; of a two-dimensional electron system in a perpendicular magnetic field and periodic potentials of short periods. The Coulomb interaction is included within a screened Hartree-Fock approximation. The electrostatic screening is poor, and the exchange interatcion amplifies the energy dispersion We obtain, by numerical iterations, self-consistent solutions that have a hysteresislike property. With increasing amplitude of the external potential, the energy dispersion and the electron density become periodic, and they remain stable when the external potential is reduced to zero. We explain this property in physical terms, and speculate that a real system could memorize short-range potential fluctuations after the potential has been turned off.

In this paper, we present some transport measurements on artificially layered (BaCuO2)(2)/(CaCuO2)(2) superlattices, engineered by the pulsed laser deposition (PLD) technique, and having critical temperatures (zero resistance) of about 80 K. The experimental current-voltage characteristics are discussed in the frame of current-induced unbinding of thermally created vortex-antivortex pairs (in zero-field) and in the frame of vortex glass (VG) transition for fields up to about 10 kG. The transport critical current density is estimated and compared to values reported from magnetisation measurements. The field dependence of the activation energy of vortices in the vortex liquid (VL) phase shows a strong effective pinning, due to the high disorder intrinsic to these artificially layered superconductors, at low magnetic fields, and a transition to a thermally activated plastic motion of an entangled VL in higher fields. From the temperature and field orientation dependencies of the vortex melting field, an anisotropy factor of about 20 is estimated for these superlattices. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

A low-confinement asymmetric GaAs-AlGaAs double-quantum-well molecular-beam-epitaxy grown laser diode structure with optical trap layer is characterized. The value of the internal absorption coefficient is as low as 1.4 cm(-1), while keeping the series resistance at values comparable with symmetrical quantum-well gradient index structures in the same material system. Uncoated devices show COD values of 35 mW/mu m. If coated, this should scale to about 90 mW/mu m, The threshold current density is about 1000 A/cm(2) for 2-mm-long devices and a considerable part of it is probably due to recombination in the optical trap layer. Fundamental mode operation is limited to 120-180 mW for 6.5-mu m-wide ridge waveguide uncoated devices and to 200-300 mW for 13.5-mu m-wide ones, because of thermal waveguiding effects, These values are measured under pulsed conditions, 10 mu s/l ms.

The technological process to achieve high quality oxides on GsAs and AlGaAs is more difficult than to achieve native oxides on silicon due to the lack of stable oxides on GaAs and related compounds. This paper presents technological conditions to a grow large area of native oxides on AlGaAs epilayers, to be used in laser diodes technology. Experimental results from scanning electron microscopy (SEM) and electron dispersion spectroscopy (EDS) measurements reveals the presence of two native oxides forms, namely GaAsO and AlGaAsO on a 430-mu m(2) surface (twice as large a lateral growth than found in the literature). The proposed oxides structures are Ga0.5As0.5O and Al0.1Ga0.4As0.5O2. These oxide forms have dielectric behaviour on an AlGaAs epilayer. (C) 1999 Elsevier Science S.A. All rights reserved.

The H-graphite interaction is studied for the first time in the framework of density functional theory. A coronene-like model of the (0001) graphite surface is considered. LSD and LSD + GGA results are presented for both a planar substrate and for considering substrate relaxation. Two adsorption regions separated by a barrier are found: a physisorption region around 3 Angstrom from the surface and a chemisorption region around 1.5 Angstrom. The former is site-independent and compatible with a high mobility of the H atoms parallel to the surface. The latter is located exclusively on top of a carbon atom and requires a significant surface relaxation. (C) 1999 Elsevier Science B.V. All rights reserved.

Based on the multiple scattering technique [K. F. Freed and M. Muthukumar, J. Chem. Phys. 69, 2657 (1978); 68, 2088 (1978); M. Muthukumar and K. H. Freed, J. Chem. Phys. 70, 5875 (1979)] previously applied to the study of suspensions of spheres and polymers, we propose an approach to the computation of the effective elastic properties of a composite material containing rigid, mono-sized, randomly dispersed, spherical particles. Our method incorporates the many-body, long-range elastic interactions among inclusions. The effective medium equations are constructed and numerically solved self-consistently. We have calculated the effective shear mu' and Young E' moduli, as well as the effective Poisson ratio sigma', as functions of the particle volume fraction Phi and of the Poisson ratio a of the continuous phase. Comparisons with two sets of experimental data-glass beads in a polymer matrix and tungsten carbide particles in a cobalt matrix (Wc/Co)-and to a previous theoretical solution, are also presented. Our model can predict the effective Poisson ratio of the Wc/Co system for Phi less than or equal to 1 and for the glass/polymer system for Phi less than or equal to 0.5. In particular, the present work describes accurately composites with a high volume fraction of inclusions, where a percolation transition occurs. Very good agreement with the experimental data are obtained for E' and mu' when Phi less than or equal to 0.4, for both systems. (C) 1999 American Institute of Physics. [S0021-9606(98)50148-1].

Pb(Zr,Ti)O-3 (PZT) oriented films deposited by laser ablation on Bu(lll)/Si(lll) have been tested by different techniques for their physico-chemical homogeneity. The samples have been divided in different zones, in order to verify the existence of chemical and structural differences between different regions. Few techniques like X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been employed for characterization. XRD analyses showed differences regarding the crystallographic content and the degree of orientation between zones subjected to the plasma arriving at different angles of incidence on the sample surface. In the same zones XPS studies have been performed by a VG ESCALAB 210 Spectrometer, using a non-monochromatic AlKalpha X-ray source (300 W) in the five channel hemispherical analyzer. Wide scans in the binding energy scale 0-1200 eV, at 50 eV analyzer pass energy, were collected both from the as received surface and after sputter cleaning in order to put into evidence all the constituents of the film. Narrow scans of Ti 2p, Zr 3d, Pb 4f and O 1s is were also acquired at 20 eV and 0.1 eV/channel pass energy and 100 ms of dwell time in order to give a better insight into the chemical bonds form and a semiquantitative analyze of the present chemical species. The piezoelectric properties of different zones were also measured, (C) 1999 Published by Elsevier Science B.V. All rights reserved.