National Institute Of Materials Physics - Romania

The alkoxide route and the spinning deposition were used to prepare sol gel ITO films. The morphology and crystalline structure were investigated by Cross-section Transmission Electron Microscopy and Atomic Force Microscopy. The inside of the ITO layer has three regions of different porosity and the basic crystalline structure is that of the In2O3 lattice. Three types of surface morphological structures were found from AFM images.

Dark-brown crystalline powders and selected single crystals of cubic boron nitride grown by the HP-HT method, with boron excess, have been studied in a broad temperature range by electron spin resonance (ESR) in the X and W microwave frequency bands, respectively. The X-band spectra consist of two superimposed lorentzian components attributed to two types of related paramagnetic defects called D1 and D2, respectively. According to the W-band data, the D I centre exhibits local axial < 111 > symmetry and ground spin state S = 1/2, with g(parallel to) = 2,0033 and g(perpendicular to) = 2.0094 at T = 10 K. The broad D2 line, observed only in the X-band, at g = 2.0084, at higher temperatures, seems to result from transitions inside the excited levels of another boron related defect.

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or gamma-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial [001] symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl- ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl- ions, compressed along an[001] direction.

Traps in Multi-Quantum Wells (nc-Si/CaF2)(50) nanostructures using Optical Charging Spectroscopy method were investigated. Three maxima and a final abrupt increase were found in the discharge current versus temperature characteristics. The relative contributions of the tunneling and displacement currents through the insulating calcium fluoride were analyzed.

High-quality epitaxial strontium-hexaferrite (SrFe12O19) thin films were grown by pulsed laser deposition (PLD) on c-cut sapphire using KrF* excimer laser at a fluency of 2 J/cm(2) and substrate temperature of 800 degreesC in 100 mTorr oxygen environment. The X-ray diffraction (XRD) and morphology analyzes showed films with excellent crystalline structure and Aat surface. The thickness was found to influence considerably the surface morphology and magnetic properties of the as-deposited films. The highest orientation and the best morphology with smooth surface and fine grain structure was obtained for the him having a thickness of 750 nm. The highest coercive force of 1453 Oe was measured for this film in perpendicular to the plane direction. (C) 2000 Elsevier Science B.V. All rights reserved.

We present a method for processing hydroxyapatite (HA) thin films on Ti-5Al-2.5Fe substrates. The films were grown by pulsed laser deposition (PLD) in vacuum at room temperature, using a KrFdouble dagger excimer laser. The amorphous as-deposited HA films were recrystallized in ambient air by a thermal treatment at 550 degreesC. The best results have been obtained when inserting a buffer layer of ceramic materials (TiN, ZrO2 or Al2O3) The films were characterized by complementary techniques: grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), cross-section transmission electron microscopy (XTEM), SAED, energy dispersive X-ray spectroscopy (EDS) and nanoindentation. The samples with buffer interlayer preserve the stoichiometry are completely recrystallized and present better mechanical characteristics as compared with that without buffer interlayer. (C) 2000 Published by Elsevier Science B.V.

The changes produced by anodic and natural oxidation upon the trap parameters in nanocrystalline porous silicon were compared. To put them in evidence, we used optical charging spectroscopy The same trapping It levels (with the same activation energies) were observed after both oxidation processes. In comparison with flesh samples, a new trapping level (the deepest one) appears and the concentrations of the surface traps strongly diminish.

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.

The influence of the mesa size on the structural and luminescence properties of Ge island layers deposited by low-pressure chemical vapour deposition was studied. Diode structures containing one Ge island layer were grown selectively on patterned wafers with variable area. The analysis shows that the electroluminescence spectra could be separated into three contributions lying at low-, medium and high-energy positions, respectively. The absolute peak intensity of each signal depends on the mesa size and on the injected current density. The signal at the low-energy position was assigned to an effect due to the facets of the diodes; the two other signals originate from islands on the (001) plane having different Si contents or strain states on the (001) plane.

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H-2 gas flow rates, keeping a constant [CH4]:[H-2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H-2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic atomic species, is directly related to the concentration of the extended lattice defects. (C) 2000 American Institute of Physics. [S0021- 8979(00)08611-4].