Publications

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.

Thin C-60 films were deposited by vacuum sublimation of soot on single-crystal and amorphous substrates. The absence of higher fullerenes was confirmed by IR, Raman and UV-VIS spectroscopy. X-ray diffraction revealed a high density of stacking faults (probabilities in the range 10(-2)), correlated with lattice distortions. Analysis of the W-VIS absorption bands yielded values of the h(u)-->t(1g) and h(u)-->t(1u) optical gaps.

The octahedral complex [PbX6](4,) (X=Cl, Br) in natrium/potassium chloride or bromide monocrystals as well as in the concentrated solutions of these alkali halides, impurified by lead, shows a characteristic absorption in the region 250-300 nm. Using the height of this absorption band, a quantitative method for determination of lead in the range 8'10(14) - 3.2'10(16) atoms/cm(3) of halide monocrystals or 0.5 20 mg/ml of solutions respectively, was elaborated. Standard graphs of the peak extinction of [PbX6](4) in natrium/potassium chloride and bromide as function of the lead concentration have been obtained.

ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-standing polycrystalline diamond films grown by microwave plasma enhanced CVD from CH4/H-2 mixtures under variable deposition conditions. An exponential decrease in the concentration of the N-0 centres with increase in the H-2 flow rate was found. Neither H1 nor H2 centres could be detected.

The local electric properties at K and Zn sites in the normal, incommensurate and commensurate phases of K2ZnCl4, as derived from a numerical computation of the lattice contributions to the electric potential V(r), electric field intensity E(r) and electric field gradient tensor V-alpha beta(r) are reported. The numerical data obtained at each cationic position were correlated with the experimental K-39 NMR, Cu2+ and Mn2+ EPR and Fe-57 Mossbauer results in pure and doped K2ZnCl4. A proportionality between crystal field and zero-field splitting was taken into account for Mn2+, whereas for K+, Cu2+ and Fe3+ ions the electric field gradient is directly related to the crystal field parameter. By this comparison, on computations done in the ionic fractional charge and relaxed lattice approximations, the insertion of probe-species of iron, copper and manganese ions on off-center Zn sites is proposed. The K-39 electric field gradient tensor calculations in the incommensurate phase fit well with the NMR data reported recently.

Two Fe+ centres have been identified by ESR after X-ray irradiation at 80 K of chlorinated SrCl2 : Fe2+ crystals. Both centres, with tetragonal symmetry, consist of an Fe+ ion with a neighbouring charged defect along the [001] symmetry axis. The substitutional Fe+ ion of the Fe+(I) centre is strongly off-centre displaced in the Fe+(II) centres, in the middle of a square of four nearest neighbour chlorine ligands.