Abstract: A Ti target was submitted to laser ablation in low ambient pressure N-2. Electron microscopy examination of the cross-section of the crater zone forming on the Ti target, and XPS analyses, indicate that there is a small effect on the nitridation processes taking place on and in the vicinity of the target. The studies show a zone influenced by the multipulse laser treatment extending beneath the crater down to a depth of the same order of magnitude as the crater depth (i.e. similar to 10 mu m). In this zone, TiN could be identified as being present only in traces, while the whole zone exhibited a layer structure with differences in morphology and mechanical wear.

Abstract: We optimized the deposition of YBa2(Cu1-xZnx)(3)O-7-delta thin-films using inverted cylindrical magnetron sputtering and report here a detailed structural study, especially in relation to crystal growth, associated surface morphology, Y2O3 precipitation and other secondary phases important for flux pinning. We find that the epitaxial quality of the Zn-substituted YBa2Cu3O7-delta films is decreased compared with high-quality pure YBa2Cu3O7-delta films prepared under identical conditions. The pure films have smoother surfaces, while those of Zn-substituted films contain pinholes and outgrowths. Secondary phases and a-axis grains were observed in the Zn-substituted films. Y2O3 precipitates with typical dimensions of 50-100 Angstrom have been found in both pure and Zn-substituted samples. However, their density of about 10(23) m(-3), observed in the pure films, is significantly reduced in the Zn-substituted films when increasing the Zn concentration up to 4%.

Abstract: A transmission electron microscopy study of epitaxial C-60 and C-70 films grown on a GeS (001) surface is presented. The relationship between the orientation of the substrate and the films and structural defects in the films, such as grain boundaries, unknown in bulk C-60 and C-70 crystals, are studied. Small misalignments of the overlayers with respect to the orientation of the substrate, so-called epitaxial rotations, exist mainly in C-70 films, but also sporadically in the C-60 overlayers. A simple symmetry model, previously used to predict the rotation of hexagonal overlayers on hexagonal substrates, is numerically tested and applied to the present situation. Some qualitative conclusions concerning the substrate-film interaction are deduced. (C) 1996 American Institute of Physics.

Abstract: Synchrotron radiation, high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction, and Raman spectroscopy were used to study the structure and thermal behavior of malayaite, CaSnSiO5. No indications of deviation from A2/a symmetry and no structural transitions were observed between 100 and 870 K. HRTEM revealed that the material is free of domains and antiphase boundaries. However, the lattice constants, cell volume, and Raman-active phonons show a thermal discontinuity near 500 K, which is possibly related to variation of the coordination sphere around the highly anisotropic Ca position.

Abstract: A combination of CeO2 and PtO2 doping has been studied in melt-processed YBa2Cu3O7-x. This study was carried out using an optimized well established MTG process. The cerium-platinum doped samples exhibit a high fishtail effect with a J(c) of 4.3 x 10(4) A/cm(2) under an applied field of 1 T. Microstructural and nanostructural studies have been performed. The 211 and BaCeO3 inclusions are a few micrometers wide and the formation of a metastable nanocrystalline phase (Y4Ba4)Cu-8-x(Ce,Pt)(x)O-20+delta related to the perovskite is detected. The 123 grains show no specific extended defect generated by the doping process. The relationship between structure and properties are discussed.

Abstract: Recently discovered series of high Tc superconductors, characterized by the existence of two types of cations within the same layer, are presented. The first family concerns the mercury based cuprates, Hg(1-x)M(x)A(2)Ca(m-1)Cu(m)O(2m+2+delta), with A = Ba and/or Sr, which exhibit structures closely related to that of the thallium cuprates TlBa2Cam-1CumO2m+3. They differ from the thallium cuprates by a high oxygen deficiency at the level of the mercury layer. It is shown that cations such as M = Cu, Pb, Tl, Bi, Ce, Pr, Cr, V, Mo, W, Ti, Sr, Ca,... can partially substitute for mercury ions, stabilizing the structures. The cationic composition of the layer depends indeed on the nature of the M cation but also on that of the alkaline earth A. For given A and M cations, the a: value remains unchanged even when the number of copper layers varies. M and Hg cations are either statistically distributed over the same site or ordered. Different types of ordering have been detected. Another way of generating mixed layers is to shear periodically the structure, leading to the formation of the so called ''collapsed phase''. In the collapsed bismuth cuprates, bismuth and copper segments, a few octahedra long, alternate in strongly waving layers. In the collapsed oxycarbonates, carbonate groups and M cations are ordered within the intermediate layer so that they can be simply described from a partial and ordered substitution of carbon for Hg,TI, Bi and other M cations building the intermediate layer. The oxycarbonitrates (Y1-xCax)(n)Ba2nCu3n-1(C,N)O3O7n-3 can also be described as an ordered substitution of carbon for copper in the 123 matrix. The different families of superconducting materials which are generated by such mechanisms are described as well as the way the different species are distributed within the mixed layers. Their influence on the physical properties are discussed.

Abstract: A two-dimensional model is presented that describes the behavior of argon metastable atoms, copper atoms, and copper ions in an argon direct. current glow discharge, in the standard cell of the VG9000 glow discharge mass spectrometer for analyzing flat samples. The model is combined with a previously developed model for the electrons, argon ions, and atoms in the same cell to obtain an overall picture of the glow discharge, The results of the present model comprise the number densities of the described plasma species, the relative contributions of different production and loss processes for the argon metastable atoms, the thermalization profile of the sputtered copper atoms, the relative importance of the different ionization mechanisms for the copper atoms, the ionization degree of copper, the copper ion-to-argon ion density ratio, and the relative roles of copper ions, argon ions, and atoms in the sputtering process. All these quantities are calculated for a range of voltages and pressures, Moreover, since the sticking coefficient of copper atoms on solid surfaces is not well-known in the literature, the influence of this parameter on the results is briefly discussed.

Abstract: We present a theoretical study of the electronic structure of a heavily Si delta-doped layer in a GaAs/AlxGa1-xAs/GaAs quantum barrier. In this class of structures the effect of DX centers on the electronic properties can be tuned by changing the AlxGa1-xAs barrier width and/or the Al concentration, which leads to a lowering of the DX level with respect to the Fermi energy without disturbing the wave functions much. A self-consistent approach is developed in which the effective confinement potential and the Fermi energy of the system, the energies, the wave functions, and the electron densities of the discrete subbands have been obtained as a function of both the material parameters of the samples and the experimental conditions. The effect of DX centers on such structures at nonzero temperature and under an external pressure is investigated for three different models: (1) the DX(nc)(0) model with no correlation effects, (2) the d(+)/DX(0) model, and (3) the d(+)/DX(-) model with inclusion of correlation effects. In the actual calculation, influences of the background accepters, the discontinuity of the effective mass of the electrons at the interfaces of the different materials, band nonparabolicity, and the exchange-correlation energy of the electrons have been taken into account. We have found that (1) introducing a quantum barrier into delta-doped GaAs makes it possible to control the energy gaps between different electronic; subbands; (2) the electron wave functions are mon spread out when the repellent effect of the barriers is increased as compared to those in delta-doped GaAs; (3) increasing the quantum-barrier height and/or the application of hydrostatic pressure are helpful to experimentally observe the effect of the DX centers through a decrease of the total free-electron density; and (4) the correlation effects of the charged impurities are important for the systems under study.

Abstract: 6H SiC single crystals were implanted al room temperature with 1 MeV He+ up to a fluence of 2 x 10(17) at./cm(2) RBS-channeling analysis with a 2 MeV He+ beam indicated the formation of extended defects or the generation of point defects at a constant concentration over a depth of about 1 mu m. Electron microscopy characterisation revealed the presence of two amorphous buried layers at depths of about 1,75 and 4.8 mu m. They an due to the implantation and to the analysing RES beam, respectively, No extended planar or linear faults were found in the region between the surface and the first amorphous layer. However, at the surface, a 50 nm thick amorphous layer was observed in which crystalline inclusions were embedded. Electron diffraction and HREM data of the inclusions were typical for diamond, These inclusions were even found in the crystalline SiC material below this layer, however at a reduced density.

Abstract: We have studied tunnelling through Si donors incorporated in the quantum well of double barrier resonant tunnelling devices. In addition to a resonance associated with the ground state of a single donor (1s level), a novel donor-related resonance at a smaller binding energy is observed in high magnetic fields where it becomes dominant over the Is resonance. We attribute this novel feature to a D-minus state of a shallow donor.