National Institute Of Materials Physics - Romania

The room-temperature compressibility and stability of single-phase icosahedral Ti-Zr-Ni alloys were investigated for pressures up to 30 GPa, using synchrotron radiation energy dispersive diffraction. The icosahedral Ti-Zr-Ni single phase is stable over this pressure range, no phase transition being noticed. The pressure-volume data were least-squares fitted to a Birch-Murnaghan equation of state to obtain the zero-pressure bulk modulus B-0 = (173+/-5) GPa and its pressure derivative B-0' = 2.3+/-0.5.

Photodarkening relaxation under light exposure of a-As2Se3 amorphous films doped with 0.5 at.% of metals Sn, Mn, Sm or Dy and a-AsSe films doped with Sn up to 10.0 at.% Sn was studied in dependence on the impurity and thermal treating. Both factors reduce photodarkening and the degree of reduction depends on the type of impurity. The relaxation process may be described by a stretched exponential with the dispersion parameter 0.4 less than or equal to alpha 1 and time constant increasing with embedding of impurity or thermal annealing. The results are discussed in the frame of the recently forwarded "slip-motion" model of photodarkening in chalcogenide glasses.

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ce islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 degrees C), therefore the diffusion length of adatoms is high (similar to 100 mu m) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38-1.55 mu m. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up Bo 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed.

Sodium borate (Na2O.4B(2)O(3)) and copper doped glasses were investigated with respect to the effects of thermal annealing below the softening temperature. X-ray diffraction, optical spectroscopy and electron spin resonance methods were used. Annealing of the glass network results in a partial healing of the structural defects and a redistribution of the borate structural units, so that copper atoms situated in tetrahedral coordination in as-quenched samples adopt a distorted octahedral configuration after annealing.

Amorphous C-Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown on quartz substrate by magnetron sputter deposition. A new digital signal processor-based atomic force microscope (AFM) system has been involved in investigating the surface topography of the final surface of the multilayer structure as well as the substrate. In the first step, roughness measurements involving AFM-tips with different radius curvature indicate the C-Ni multilayer surface profile as being in the nanometer and sub-nanometer range, representing a high quality ended surface of the reflectors. In the second step, a study of the roughness distribution using power spectral density functions (PSD) calculated from AFM measurements was performed. The analysis of the amplitudes of the PSD spectra for the multilayer and the substrate surface provided the evidence of the optimized deposition parameters. The shape of the PSD curves for multilayer and substrate reveals fractal profiles with the same distribution of the roughness for both of them, attesting the influence of the substrate on the quality of the reflectors. (C) 2000 Elsevier Science B.V. All rights reserved.

Relations between the concentrations of neutral (N-0) and charged (N+) single-substitutional nitrogen and of nitrogen-vacancy (N-V) complexes in chemical vapour deposited diamond films of approximate to 0.2 mm thickness with nitrogen impurity concentration levels of 10 ppm are studied. For this purpose the films were subjected to 8 MeV electron irradiation at room temperature and subsequent annealing at 800 degrees C. The samples were analysed by micro-photoluminescence; visible and IR absorption, and Electron Spin Resonance techniques. It was found that the concentration of nitrogen in the (N-V) and N+ forms, in as-grown films, is less than 0.1% and 10% of the neutral substitutional nitrogen N-0, respectively.

Optical absorption evolution is studied for 150-800 mu m thick diamond films grown by microwave plasma enhanced CVD technique and annealed in vacuum at temperatures 900-1600 degrees C. A noticeable internal degradation (darkening) of the films takes place at T > 1300 degrees C. Changes in the concentration of different CH, groups are revealed from the IR spectra starting at T > 1000 degrees C. Up to 25% of the total amount of hydrogen in as-grown films is found to be unbonded. An interesting effect of reduced absorption in UV range due to the formation of new C-H bonds by reaction of primarily unbonded hydrogen is observed. The diamond darkening correlating with the bonded hydrogen loss process, is ascribed to the appearance of a graphite-like phase (being under a high stress) presumably at grain boundaries, that is revealed by the appearance of a specific absorption peak in UV spectra.

We synthesized Fe3-xCoxO4 (x=0-0.6) using the hydrothermal method at 300 degrees C in order to demonstrate substitution effects on the hyperfine magnetic fields and populations of the tetrahedral (A) and octahedral (B) sites of magnetite. Our Mossbauer spectroscopy results were first analyzed using the site model, which provided direct evidence for the presence of the Co substitution in the (B) sublattice. This was accompanied by a systematic increase of the hyperfine magnetic field at (B) sites. The mechanism we propose relies on the substitution of Fe2+ by Co2+ in the (B) sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites on the content x of cobalt substitution. Finally, the Mossbauer spectroscopy results were also analyzed using the hyperfine magnetic field distribution model for direct comparison. For the first time, it was demonstrated that the held values provided by the site model fall within the standard deviation obtained from the distribution model. (C) 2000 Elsevier Science Ltd. All rights reserved.

In this review are presented the main results of a complex research project devoted to preparation and investigations of physical properties of [(BaCuO2)(n)/(CaCuO2)(m)](N) artificial superconducting superlattices. The paper starts with the layer-by-layer pulsed laser deposition technique developed for this project and discuss the structure of this new material. The superconducting properties and the effects of structural disorder on them are also presented.

Thermal relaxation and the relaxation induced by UV radiation in thin amorphous films of composition Ge(x)As(40.x)Se(60) (0 less than or equal to x less than or equal to 40) have been investigated by diffraction de rayons X. The structural results allowed to suggest the models for the atomic scale transformations.