National Institute Of Materials Physics - Romania

Publications

847 articles found

761. The influence of the h-BN morphology and structure on the c-BN growth

Authors: Nistor, L; Teodorescu, V; Ghica, C; Van Landuyt, J; Dinca, G; Georgeoni, P

Published: MAR-JUL 2001, DIAMOND AND RELATED MATERIALS, 10, 1356, DOI: 10.1016/S0925-9635(00)00377-0

The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI <5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-EN ponders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp(3), as in the case of c-BN, (C) 2001 Elsevier Science B.V. All rights reserved.

762. Point defects in cubic boron nitride crystals

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Bouwen, A; Schoemaker, D; Dinca, G

Published: MAR-JUL 2001, DIAMOND AND RELATED MATERIALS, 10, 1411, DOI: 10.1016/S0925-9635(00)00396-4

The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S = 1/2, local symmetry axis along one of the crystal [111] axes and principal g values g(parallel to) = 2.0032 +/- 0.0009 and g(perpendicular to) = 2.0094 +/- 0.0005 at T = 10 K. (C) 2001 Elsevier Science B.V. All rights reserved.

763. Atomic 6p-Tl-0 centers in ferroelectric Rb2ZnCl4 crystals

Authors: Stefan, M; Nistor, SV; Schoemaker, D

Published: 2001, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 155, 377, DOI: 10.1080/10420150108214140

Several Tl-0 (6s(2)6p(1))-type paramagnetic centers, produced by low temperature X-ray irradiation, were observed and studied by electron spin resonance (ESR) in the orthorhombic ferroelectric phase of thallium doped Rb2ZnCl4 crystals. The centers were formed by electron trapping at Tl+ ions localized substitutionally at Rb+ sites. The number and properties of the observed centers account for the tripling of the unit cell in the ferroelectric phase.

764. Internal structure of the nanosized sol-gel ITO thin films

Authors: Stoica, TF; Teodorescu, VS; Blanchin, MG; Stoica, TA; Gartner, M; Zaharescu, M

Published: 2001, 2001 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOL 1 & 2, PROCEEDINGS, 66, DOI:

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.

765. Multifrequency ESR studies of paramagnetic point defects in cubic boron nitride crystals

Authors: Nistor, SV; Stefan, M; Schoemaker, D; Goovaerts, E; Dinca, G

Published: 2001, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 156, 194, DOI: 10.1080/10420150108216892

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.

766. X and Q-band ENDOR study of the Fe+(I) center in chlorinated Srcl(2) single crystals

Authors: Vrielinck, H; Ghica, D; Callens, F; Nistor, SV; Schoemaker, D

Published: 2001, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 155, 111, DOI: 10.1080/10420150108214101

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.

767. Traps in (nc-Si/CaF2)(50) nanostructures

Authors: Draghici, M; Jdira, L; Iancu, V; Ioannou-Sougleridis, V; Nassiopoulou, A; Ciurea, ML

Published: 2001, CAS: 2002 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, PROCEEDINGS, 122, DOI:

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.

768. Sr-ferrite thin films grown on sapphire by pulsed laser deposition

Authors: Koleva, ME; Zotova, S; Atanasov, PA; Tomov, RI; Ristoscu, C; Nelea, V; Chiritescu, C; Gyorgy, E; Ghica, C; Mihailescu, IN

Published: DEC 15 2000, APPLIED SURFACE SCIENCE, 168, 113, DOI: 10.1016/S0169-4332(00)00612-7

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.

769. Pulsed laser deposition of hydroxyapatite thin films on Ti-5A1-2.5Fe substrates with and without buffer layers

Authors: Nelea, V; Ristoscu, C; Chiritescu, C; Ghica, C; Mihailescu, IN; Pelletier, H; Mille, P; Cornet, A

Published: DEC 15 2000, APPLIED SURFACE SCIENCE, 168, 131, DOI: 10.1016/S0169-4332(00)00616-4

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.

770. Oxidation-induced modifications of trap parameters in nanocrystalline porous silicon

Authors: Draghici, M; Miu, M; Iancu, V; Nassiopoulou, A; Kleps, I; Angelescu, A; Ciurea, ML

Published: NOV 2000, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 182, 243, DOI: 10.1002/1521-396X(200011)182:1<239::AID-PSSA239>3.0.CO;2-K

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.