National Institute Of Materials Physics - Romania

GaN thin films were deposited by laser ablation of liquid Ga target in nitrogen reactive atmosphere. An Nd-YAG laser (lambda = 1.06 mu m, tau(FWHM) = 10 ns) of 0.35 J energy/pulse was used as laser source. The nitrogen pressure was varied in the range of 10(-2) to 10(-1) mbar. As substrates, we used (0001) sapphire plates and (100) Si wafers, coated or uncoated with ZnO as buffer layers, and heated below 300 degrees C. Different analysis techniques evidenced the characteristics of the deposited films. SIMS profiles corresponding to N and Ga in-depth distribution carried out the presence of layers of the order of 130-150 nm, with uniform distribution of Ga and N inside the layer. XPS studies evidenced the Ga-N bonding. The Nls signal contains as main peak the one centered at 397.3 eV and corresponding to Ga-N bond. From the distance between the photoelectron Ga 3d peak and the Auger Ga LMM peak, the calculated Auger parameter of 1083.9 eV corresponds to the one reported in literature for GaN compound (1084.05 eV). Both techniques evidenced an oxygen contamination below 5%. XRD recorded spectra show the presence of a peak assigned to (002) GaN crystalline orientation. Optical absorption spectroscopy studies in the UV-visible range evidenced a high transparency (over 80% transmission) for the deposited films. The energy band gap obtained from the absorption spectra was found to be larger than 3.6 eV. (C) 1998 Elsevier Science B.V.

Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl2 single crystals. Besides Pb-2(3+) self-trapped electron (STEL) centers the PbCl2 : Tl crystals exhibit trapped-electron (PbTl)(+)-type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl-2 divided by centers the holes are self trapped at pairs of neighbouring Cl- anions resulting in V-k type centers with various orientation and length of the Cl-Cl axis. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

The absorption spectrum of CuGeO3 crystals is measured in the transmittance region and at the fundamental edge, The reflection spectrum is studied in the range 3-6 eV, The identified transitions are interpreted in the context of band-structure calculations.

High-resolution synchrotron-radiation powder diffraction experiments were performed to observe structural changes induced by hydrogen loading in rapidly-quenched Ti-Zr-Ni alloy ribbons with dominant icosahedral character. Lattice expansion effects due to hydrogen storage in Ti-Zr-Ni quasicrystals as well as phonon and phason disorder coefficients are obtained from an analysis of diffraction linewidths.

The gallophosphate cloverite has been investigated because of its applicability to act as a host for larger molecules. 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB) liquid crystals were used as guest molecules. The results show that the adsorption of liquid crystals from the gas phase proceeds without destruction of the cloverite structure. The typical phase properties of the liquid crystals remain unchanged after adsorption on cloverite: however, they appear with lower intensity in the DSC curve. A new smectic phase seems to be formed inside the cloverite pore system. FTIR measurements show the interaction of 5CB with the Lewis and Bronsted acid sites of cloverite. Desorption of liquid crystals out of the pore system proceeds slowly under liberation of the POH groups of the lattice. In summary, the results show that the gallophosphate cloverite is a suitable host for large molecules. (C) 1998 Elsevier Science B.V. All rights reserved.

The molecular structures for N-phenyl-N'-acetylurea, N-phenyl-N'-capronylthiourea, and N - phenyl - N' - benzoyl thiourea have been characterized by X-ray diffraction and NMR spectroscopy. The relative positions of atoms in the parent molecular fragment, common to all three analyzed compounds, are discussed. (C) 1998 Elsevier Science B.V.

Co, Ni, Mn, Cr, and Cu substituted magnetites were prepared by the hydrothermal method at 300 degrees C, with concentrations ranging from 8.2% to 12.5%. Transmission electron microscopy determined the average particle diameter (Phi) to be in the hundred of nm range and the morphological modifications induced by the various substitutions employed, Hysteresis loop measurements were performed to determine the coercive field H-c, and saturation magnetic moment m(s). While H-c decreased with increasing (Phi), the particle shape was found to play an important role in explaining the dependence of m(s) on (Phi). Transmission Mossbauer spectroscopy was used to determine the site preference of the substitutions and their effect on the hyperfine magnetic fields. The room-temperature Mossbauer spectra were analyzed assuming a random distribution of substitutents using the binomial distribution from the ionic crystal point of view. Superparamagnetic particles were observed at room temperature in the case of Cu and Cr substituted magnetites. (C) 1998 American Institute of Physics.

The practical importance of these studies comes from the fact that, the local degradation of dielectric layers due to the treeing discharge leads to the increment of energy losses and finally to the electric breakdown of the films. The thickness of the deposition layers was 1000 Angstrom. For the application of high voltage, we used the needle-plane system. The Mirror Electron Microscopy (MEM) investigations in polyethylene films without the application of high voltage have shown electrical inhomogeneities. After the application of high voltage (4 to 12 kV), thin polyethylene films have no electrical inhomogeneities because the sites corresponding to local conduction filaments are the starting points for pre-breakdown phenomenon. (C) 1998 Elsevier Science S.A.

The non-ionising energy loss and the concentration of primary radiation defects induced by charged pions, in the energy range 50-500 MeV, in bulk diamond have been calculated in order to characterise its radiation resistance. The mechanisms by which the pion imparts energy to the diamond lattice have been analysed and their contribution has been evaluated starting from the nuclear data on pion-carbon interaction. The energy partition of the primary recoil nuclei between ionisation and displacements has been calculated in the frame of the Lindhard theory. It has been found that diamond, from the point of view of concentration of primary defects, is approximately one order of magnitude harder than silicon in pion fields, in the energy range of the Delta(33) resonance, and about two orders of magnitude harder than GaAs in the same energy region. (C) 1998 Elsevier Science B.V. All rights reserved.

Models are presented which are compatible with the Rietveld method for the anisotropic broadening of the diffracted peaks produced by crystallite size and microstrain. The models, derived from general crystallographic considerations, are different for different Laue groups.