Publications

6H-SiC bulk single crystals grown by physical vapor transport (PVT) were investigated by deep-level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). One of the observed deep level defects was identified as isolated tungsten on Si sites by EPR. The electron spin of 1 could be explained by W5+ (5d(1)). This is equivalent to the single positive charge state of a double donor when taking into account the Fermi level position in the n-type samples. The interpretation is also consistent with the DLTS detection of a W related deep level which showed a behavior of the capture of electrons and holes that hints at a double donor. In addition a tantalum related deep level is tentatively discussed. W and Ta were incorporated on electrically active sites in 6H-SiC only in low concentrations (2-4 x 10(4) cm(-3)) during crystal growth by PVT. (C) 2001 Elsevier Science B.V. All rights reserved.

La2O3-doped barium titanate (BaTiO3) thin films have been obtained by pulsed laser deposition. The structure and quality of deposited films were characterized by X-ray diffraction and scanning electron microscopy. A smooth surface was obtained when the films were deposited in 30 Pa ambient oxygen. The composition (elements and oxides) of the thin films were close to those of the target. The layer obtained by pulsed laser deposition from a target of BaTiO3 doped with 0.5 at.% La exhibits good dielectric characteristics: capacitance 88 pF, Curie temperature 59 degreesC and dielectrics loss, tan delta, 0.084.

It is presented a study of PbSe1-xTex thick films electrodeposition on glassy carbon and ITO electrodes in acid solutions (0.1M HNO3), using cyclic voltammetry and rotating-disk techniques. Electrochemical impedance spectroscopy study of PbSe1-xTex film in 0.1M HNO3 is reported.

We present a structural and morphological study on La0.6Y0.07Ca0.33MnO3-delta (LYCMO) thin films deposited by laser ablation onto MgO-buffered silicon wafers using two deposition configurations named on-axis and off-axis. All thin films have been deposited at high temperatures in an oxygen atmosphere. A comparative study between the on-axis and off-axis samples is performed by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The films are polycrystalline and show a columnar morphology. The off-axis sample shows a better crystalline texture, although slight compositional variations have been found. (C) 2001 Elsevier Science Ltd. All rights reserved.

In spite of difficult working conditions and with very low financial support, many groups from Romania are involved in emerging fields, such as the nanoscale science and technology. Until the last years, this activity was developed without a central coordination and without many interactions between these research groups. In the year 2000, some of the institutes and universities active in the nanotechnology field in Romania founded the MICRONANOTECH network. The aim of this paper is to emphasize the main activities and results of the Romanian groups working in this novel domain. Most of the groups are deal with the nanomaterial technology anti only few of them have activities in nanostructure science and engineering, in new concepts and device modeling and technology. This paper describes the nanotechnology research development in two of the most significant institutes from Romania: Centre for Nanotechnologies from National Institute for Research and Development in Microtehnologies (IMT-Bucharest) and from National Institute for Research and Development in Materials Physics (INCD-FM), Magurele. The Romanian research results in nanotechnology field were presented in numerous papers presented in international conferences or published in national and international journals. They are also presented in patents, international awards and fellowships. The research effort and financial support are outlined. Some future trends of the Romanian nanoscale science and technology research are also described.

We report the synthesis, magnetization and ac susceptibility of the triangular mu (3)-oxo-bridged heterometallic complex [Fe2CoO(CH3COO)(6)(3-Cl-Py)(3)], in powder form. The model Hamiltonian is elaborated involving isotropic exchange interactions Fe(III)-Fe(III) and Co(II)-Fe(III) dealing with the real spins of high-spin Co(II) and Fe(III) ions, and also spin-orbit coupling and low-symmetry crystal field (C-2v) acting within the T-4(1g) ground manifold of Co(II) ion. The outlined general model takes into account strong anisotropic orbital contribution to the magnetic characteristics of the whole system arising from Co(II) ion with unquenched orbital angular momentum. The treatment of the Hamiltonian is performed with the use of the irreducible tensor operator technique. The influence of the exchange and spin-orbital interactions on the magnetic properties of Fe-2(III)Co(II) clusters is revealed. The result of calculations in the framework of the isotropic model (neglecting low-symmetry crystal field) are in a good agreement with the experimentally observed magnetization versus magnetic field, as well as with the unusual temperature behavior of the magnetic susceptibility of the powder samples (1.8-250 K). The states of the system are enumerated by the quantum number of the total angular momentum J, these hybrid states are combined from the sets of full and intermediate spins of Fe2Co triangle and orbital angular momentum of Co(II), (with fictious L=1). The best fit to the experimental data is achieved for the exchange parameters J(Fe-Fe)=-87.5 cm(-1), J(Fe-Co)=-17 cm(-1) and orbital reduction factor for Co(II) ion kappa =0.93. The ground state and the first excited one are found to possess total angular momentum J=1/2, Effective g-factors for the ground- and first-excited states calculated using the hybrid spin-orbital functions prove to be 4.3 and 0.75, that are distinctly different from the pure spin value 2 peculiar for the Heisenberg model. The comparison of the magnetic measurements with the theory clearly demonstrates the insufficiency of a simple Heisenberg spin-coupling model as evidenced by the discovered strong orbital magnetic contribution. (C) 2001 American Institute of Physics.

The luminescence spectra of fully oxidized porous silicon show two broad bands and one vibronic structure between 2 and 3.3 eV, all excited at 340 or 275 nm (3.65 and 4.51 eV). Two of these structures have not been described previously. Interestingly, these emissions are found in other forms of amorphous or disordered silica with high specific surfaces: frosted silica glass, silica gel, even various natural opals. These emissions disappear when the surface is passivated. They are, therefore, attributed to surface-related defects. The vibronic structure involves a nearly free SiO4 tetrahedron. Another vibronic structure found only in some opals is attributed to the uranyl group. (C) 2001 American Institute of Physics.

The relaxation of the irreversible magnetization of disordered YBa2Cu3O7-x crystals measured in the "flux-creep annealing" regime reveals that across the second magnetization peak (SMP) the barriers against flux motion remain finite at low current densities, which supports the existence of a crossover to a dissipation process involving the plastic deformation of the vortex system, In our experiments, the vortex creep process appears to be exclusively controlled by collective pinning barriers (diverging at low current densities) only below the onset of the SMP, where the vortex system is stable against dislocation formation. The (elastic) collective pinning barriers observed for magnetic field values close to the onset of the SMP (where the plastic barriers are high) could be related to the recently proposed collective pinning of individual dislocations. The proliferation of dislocations across the SMP leads to liquid-like behavior of the disordered vortex phase in the vicinity and above the peak field. (C) 2001 Elsevier Science B.V. All rights reserved.

In situ decomposition of silicon nitride films was observed by high-resolution electron microscopy. The films, which were produced by reactive pulsed laser deposition from Si wafer targets at 50-100 Pa ammonia pressure, had a prevalent content of hydrogen-doped, amorphous, non-stoichiometric silicon nitride. A layered morphology of the film, consisting of a density variation in the amorphous structure, developed under electron beam irradiation. This morphology became evident only in cross-sectional observation and was related to the stress-relaxation effect, based on a rearrangement of the bonds, We presume that the stress field anisotropy in the amorphous structure must be related to the presence of some bond texture in the as-grown, amorphous film. (C) 2001 Elsevier Science B.V. All rights reserved.