National Institute Of Materials Physics - Romania

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858 articles found

741. Strain mapping around dislocations in diamond and cBN

Authors: Willems, B; Nistor, LC; Ghica, C; Van Tendeloo, G

Published: SEP 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 2228, DOI: 10.1002/pssa.200561923

Strain fields around dislocations in diamond and cBN have been quantitatively characterized at a sub nanometer scale. The combination of experimental high resolution electron microscopy (HREM) and image analysis using the geometric phase method provides an accurate determination of the local deformations and strain. The results of the strain analysis of the 60 degrees perfect dislocation in diamond and cBN are discussed. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

742. Properties of ZnO thin films prepared by radio-frequency plasma beam assisted laser ablation

Authors: Scarisoreanu, N; Matei, DG; Dinescu, G; Epurescu, G; Ghica, C; Nistor, LC; Dinescu, M

Published: JUL 15 2005, APPLIED SURFACE SCIENCE, 247, 525, DOI: 10.1016/j.apsusc.2005.01.140

Zinc oxide thin films were obtained by laser ablation of a Zn target in oxygen reactive atmosphere, the oxygen being supplied either by a standard gas inlet valve or from a radio-frequency (rf) oxygen plasma. Pt-coated silicon and MgO were used as substrates. The influence of the deposition parameters as laser wavelength (266, 355, 1064 nm), laser fluence (1.5-20 J/cm(2)) and oxygen pressure (1-60 Pa) was studied. The influence of the rf plasma beam addition on the morphological proprieties of zinc oxide films was particularly investigated, simultaneously with several configurations of the direction of the ablation plasma, the rf plasma beam and the substrate. The obtained films, with thicknesses in the range of 50 nm to 1 mu m have been characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM). (c) 2005 Elsevier B.V. All rights reserved.

743. Growth and characterization of a-axis textured ZnO thin films

Authors: Nistor, LC; Ghica, C; Matei, D; Dinescu, G; Dinescu, M; Van Tendeloo, G

Published: APR 15 2005, JOURNAL OF CRYSTAL GROWTH, 277, 31, DOI: 10.1016/j.jcrysgro.2004.12.162

Smooth a-axis oriented ZnO thin films are grown on (001) MgO substrates by pulsed laser deposition assisted by a very directional radio frequency oxygen plasma. The structure of the film is examined by X-ray diffraction, electron diffraction and high resolution electron microscopy. The film grows with vertical columns and is highly textured. Two preferential orientation relations between the ZnO film and the MgO substrate are observed: (10010)(ZnO) vertical bar vertical bar (002)(MgO); [0002](ZnO)vertical bar vertical bar[2 0 0](MgO) and (1010)(ZnO) vertical bar (002)(ZnO) vertical bar vertical bar [200](MgO) [1210](MgO). The film/substrate interface is flat and strained because of the rather large lattice mismatch between the (1010) plane of ZnO and the (002) plane of MgO. (c) 2005 Elsevier B.V. All rights reserved.

744. Piezoresponse force microscopy for imaging of GaN surfaces

Authors: Calarco, R; Meijers, R; Stoica, T; Luth, H

Published: APR 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 789, DOI: 10.1002/pssa.200461298

Surfaces of GaN films have been investigated by Atomic Force Microscopy (AFM) with implemented Piezoelectric Force Microscopy (PFM) technique. The GaN layers were grown by molecular beam epitaxy (MBE) on Si(111) substrates with an AlN buffer layer. Simultaneous imaging of surface morphology in the AIM and PFM response was performed. The PFM imaging is sensitively dependent on measurement conditions. By increasing the ac voltage the contrast of the PFM images is increased. Using the Voltage-Modulated Force Microscopy (VMFM) regime, with the frequency close to the first resonant frequency of the piezoelectric signal, PFM images with high contrast and high resolution on the nanometer scale were obtained. Regions of opposite piezoresponse were observed and explained by the presence of inversion domains. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

745. 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.

746. 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.

747. 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.

748. 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.

749. 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.

750. 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.