National Institute Of Materials Physics - Romania

Publications

860 articles found

741. Temperature dependence of the electron paramagnetic resonance spectra of Mn2+, impurity ions in PbWO4 single crystals

Authors: Stefan, M; Nistor, SV; Goovaerts, E; Nikl, M; Bohacek, P

Published: FEB 2 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 728, DOI: 10.1088/0953-8984/17/4/014

The temperature variation of the fine and hyperfine parameters of Mn2+ in single crystals of PbWO4 in the low temperature range reveals the presence of a resonant mode of frequency omega = 8.8 x 10(12) rad s(-1). Moreover, above 60 K, where the temperature induced broadening becomes dominant, a T-2 variation of the relaxation time was inferred from the analysis of the temperature dependence of the Mn2+ linewidth. This variation is attributed to a Raman relaxation process due to the coupling with the same local resonant mode.

742. Magnetic properties of iron-modified amorphous carbon

Authors: Yastrebov, SG; Ivanov-Omskii, VI; Pop, V; Morosanu, C; Slav, A; Voiron, J

Published: 2005, SEMICONDUCTORS, 39, 844, DOI: 10.1134/1.1992645

The structure and magnetic properties of films of iron-modified amorphous carbon (a-C:Fe) prepared by magnetron cosputtering of iron and graphite targets are studied. X-ray diffraction measurements show that iron enters the samples in the form of Fe nanocrystals that are typically about 20 nm in size and also forms nanocrystals of hexagonal iron carbide. The temperature dependences of the magnetization, measured under cooling in zero and nonzero magnetic fields, are studied. At temperatures T less than or similar to 8 K, a magnetic transition, which provides evidence for the onset of magnetic ordering in the material, is observed to occur. The magnetization isotherms obtained in the 8- to 20-K temperature range are in agreement with this observation. It is shown that a modified version of Langevin's formalism adequately describes the observed features of a-C:Fe film magnetization. (c) 2005 Pleiades Publishing, Inc.

743. Quantum confinement in the photoluminescence of nanocrystalline porous silicon

Authors: Stavarache, I; Ciurea, ML; Iancu, V

Published: 2005, CAS 2005: INTERNATIONAL SEMICONDUCTOR CONFERENCE, 1-2, 58, DOI:

The paper presents the photoluminescence of nanocrystalline porous silicon. Two maxima were observed for rather fresh samples, one located at the infrared edge of the visible range and the other one in red. After ageing, the first maximum vanishes, suggesting its relation with the surface states, while the red one undergoes a significant blue shift. A simple quantum confinement model allows to correlate the photon energy of the red maximum with a transition between two confinement levels and to interpret the blue shift in terms of size reduction by oxidation. These results are in good agreement with previous microstructure measurements.

744. Deposition of hydroxyapatite thin films by Nd : YAG laser ablation: a microstructural study

Authors: Nistor, LC; Ghica, C; Teodorescu, VS; Nistor, SV; Dinescu, M; Matei, D; Frangis, N; Vouroutzis, N; Liutas, C

Published: NOV 2 2004, MATERIALS RESEARCH BULLETIN, 39, 2101, DOI: 10.1016/j.materresbull.2004.07.006

Hydroxyapatite (HA) thin films has been successfully deposited by Nd:YAG laser ablation lambda = 532 nm. The morphology and microstructure of the deposited layers was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Polycrystalline HA films were directly obtained with the substrate at 300 degreesC and without introducing water vapors in the deposition chamber. Electron paramagnetic resonance (EPR) measurements show that the oxygen stoichiometry in the HA films is also maintained. Depositions performed at lambda = 335 nm laser wavelength and 300 degreesC substrate temperature resulted in polycrystalline layers of mixed composition of HA and tricalciumphosphate (TCP). (C) 2004 Elsevier Ltd. All rights reserved.

745. Two-dimensional arrays of nanometre scale holes and nano-V-grooves in oxidized Si wafers for the selective growth of Ge dots or Ge/Si hetero-nanocrystals

Authors: Olzierski, A; Nassiopoulou, AG; Raptis, I; Stoica, T

Published: NOV 2004, NANOTECHNOLOGY, 15, 1700, DOI: 10.1088/0957-4484/15/11/056

Two-dimensional (2D) arrays of nanometre scale holes were opened in thin SiO2 layers on silicon by electron beam lithography and chemical etching. Oxidized silicon wafers with a 5 nm thick SiO2 layer on top were used in this respect. Pattern transfer involved either only removal of SiO2 or a two-step process of oxide removal and anisotropic silicon chemical etching to form nanometre scale silicon V-grooves. The size of the holes in the photoresist layer varied in the range 40-80 nm, depending on the exposure dose used. The smallest holes in the oxide were about 50 nm in diameter, while in V-grooves the smallest width was approximate to70 nm. 2D arrays of Ge dots or Ge/Si hetero-nanocrystals were selectively grown on these patterned silicon wafers. In small windows only one Ge island per hole was nucleated.

746. High frequency ESR of native point defects in beryllium doped c-BN single crystals

Authors: Goovaerts, E; Nistor, SV; Ghica, D; Taniguchi, T

Published: SEP 2004, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 201, 2598, DOI: 10.1002/pssa.200405195

High frequency (95 GHz) ESR measurements have been performed down to 5 K, on three cubic boron nitride (c-BN) single crystals doped with beryllium. The measured samples exhibit at low temperature different ESR spectra, which are sensitive to low temperature in situ illumination using a series of Kr+- and Ar+-laser lines. The analysis of the ESR spectra resulted in the identification of several native paramagnetic centers responsible for the observed component spectra lines. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

747. ESR characterization of point defects in amber colored c-BN super abrasive powders

Authors: Nistor, SV; Ghica, D; Stefan, M; Bouwen, A; Goovaerts, E

Published: SEP 2004, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 201, 2590, DOI: 10.1002/pssa.200405194

Cubic boron nitride (c-BN) crystalline superabrasive powder (Borazon** CBN 400), consisting of 200-300 microns sized amber colored crystallites prepared by HP/HT synthesis, has been examined from 2.1 K to 293 K by X-band ESR spectroscopy. The observed spectrum consists of a component line A1, visible in the whole temperature range, and two component lines A2 and A3, visible at high and low temperatures, respectively. The A1 and A3 lines originate from transitions inside S = 1/2 ground states of distinct paramagnetic species and A2 from transitions inside an excited state of another paramagnetic center. The intensity of the A1 and A3 lines changes differently during in situ low temperature illumination in the UV-VIS range. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

748. X- and Q-band ENDOR study of the Fe+(II) center in chlorinated SrCl2 : Fe crystals

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

Published: JUL 2004, PHYSICAL REVIEW B, 70, DOI: 10.1103/PhysRevB.70.024105

The <001> axially symmetric Fe+(II) center observed in SrCl2:Fe2+ crystals has been studied by the electron nuclear double resonance (ENDOR) technique in the microwave X and Q bands. This center is produced only in crystals grown in chlorine atmosphere and x-ray irradiated at low temperature (below 100 K). The analysis of the ENDOR spectra unambiguously confirms the structural model earlier suggested by the electron paramagnetic resonance (EPR) study, as a Fe+ ion strongly displaced off center along an <001> axis, almost in the center of the square determined by the four nearest Cl- ligands. The unpaired spin densities f(3s) and f(3p) in the 3s and 3p chlorine orbitals have been determined from the resulting ENDOR parameters.

749. Absence of island-island interaction during formation of isolated Ge islands in small windows

Authors: Stoica, T; Vescan, L; Sutter, E

Published: JUN 15 2004, JOURNAL OF APPLIED PHYSICS, 95, 7711, DOI: 10.1063/1.1736312

Isolated Ge islands, i.e., islands not connected by a wetting layer can be obtained by selective epitaxial growth in voids of ultrathin oxides of thickness 1-2 nm. Voids of 30-600 nm size were created before epitaxy during a high temperature anneal of the ultrathin oxide. The formation of one island per window was investigated at 700 degreesC as a function of Ge thickness and void size. Islands nucleate mainly at the edge of the void and for this reason they have an anisotropic shape. In voids smaller than 300 nm only one island is nucleated. Islands form only in voids greater than a critical size (30-80 nm) which depends on the total amount of Ge deposited. We observe that height, width, and aspect ratio of isolated islands increase with void size for a given Ge thickness. A metastable state of Ge in small windows was observed. Moreover, the Si interdiffusion is strongly reduced with decreasing island size (i.e., with void size) reaching only similar to10% in comparison with similar to50% in islands on large areas. (C) 2004 American Institute of Physics.

750. Spectroellipsometric study of the sol-gel nanocrystalline ITO multilayer films

Authors: Stoica, TF; Gartner, M; Losurdo, M; Teodorescu, V; Blanchin, M; Stoica, T; Zaharescu, M

Published: MAY 1 2004, THIN SOLID FILMS, 455, 512, DOI: 10.1016/j.tsf.2003.11.251

Tin-doped indium oxide (ITO) thin films have been deposited by sol-gel process using 'sols' of indium and tin isopropoxides. The thickness of one deposited ITO layer is approximately 50 nm. The desired thickness was obtained by 1-5 successive depositions. The XTEM cross-sectional view of an ITO sample with five depositions showed a clear delimitation of the layers with an alternating structure dense/porous ITO layers. The void fraction in porous regions varies between 20 and 25%. Cubic bixbyite In2O3 nanocrystals with size of 10-20 nm and no phases separation of tin oxide were observed. The optical properties of the films have been investigated by optical transmission and spectroscopic ellipsometry. Reliable optical constants and porosity are obtained only with the model of internal structure based on XTEM results. (C) 2003 Elsevier B.V. All rights reserved.