National Institute Of Materials Physics - Romania

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847 articles found

721. Quantum confinement in nanocrystalline silicon

Authors: Ciurea, ML

Published: OCT 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2346, DOI:

Quantum confinement effects in different kinds of nanocrystalline silicon systems are experimentally and theoretically investigated. Porous silicon structured as a nanowire network and silicon nanodots embedded in amorphous silicon dioxide are studied. The main quantum confinement effect in both cases is represented by the appearance of new energy levels in the silicon band gap. The corresponding energies can be experimentally determined from the current - temperature characteristics, which show an Arrhenius-like behavior. The curves present several activation energies between liquid nitrogen temperature and room temperature. The energy levels can be evaluated from a quantum well model. The fundamental level is located at the top of the valence band. The change of the activation energy is then related with the filling of the levels. The ratios of the consecutive activation energies in the current - temperature characteristics prove that the excitation undergoes the angular momentum conservation law imposed by the applied electric field. The estimation of the mean size of the nanocrystals from the values of the activation energies is in good agreement with the microstructure investigations performed on the samples. The confinement levels are also in good agreement with the photoluminescence measurements..

722. Properties of high-porosity sol-gel derived indium-tin oxide films

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

Published: OCT 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2358, DOI:

Indium-tin oxide (ITO) sol-gel films have been obtained with the void concentration up to 50%. The films are nanostructured with nanocrystals of In2O3:Sn and nanovoids. The information obtained from derivative thermo-gravimetry was used to design the annealing program for ITO film formation with a high void concentration. Multilayer films were obtained by successive deposition. The thickness of one layer was about 9 nm. By successive depositions, the void density of the film is reduced. Quantitative analysis of the void density has been performed by spectroscopic ellipsometry. The conductivity of the films can be varied in a large range by annealing in vacuum or in air, at temperature higher than 200 degrees C.

723. Nanostructure and properties of Pb(Zr,Ti)O-3-Pb(Ni-1/3 Nb-2/3)O-3 piezoceramics

Authors: Dimitriu, E; Craciun, F; Ghica, C; Ramer, R

Published: SEP 2005, JOURNAL DE PHYSIQUE IV, 128, 143, DOI: 10.1051/jp4:2005128022

Piezoceramics with composition 0.46 Pb(Zr0.3Ti0.7)O-3 - 0.53 Pb(Ni1/3Nb2/3)O-3 have been synthesized by solid-state reaction at different temperatures. The density varied between 8.05-8.1 g/cm(3). The sintered samples have been chemically electroded with Ni and poled at 220C under an electric field of 3 kV/cm. The XRD spectra confirmed the formation of tetragonal crystalline phase with a tetragonal distortion c/a = 1.0125 which was decreased from a value of about 1.042 that corresponds to pure PZT with y = 0.3. Some minor traces of non-reacted precursor oxides such as Nb2O5 have been also found. TEM results confirmed the formation of tetragonal phase and revealed crystal grains with domains of different size, slightly disoriented from each other. Dielectric measurements performed between 20 degrees C and 300 degrees C at different frequencies between 100 Hz and 1 MHz revealed a broad dielectric anomaly at T-m = 118 degrees C (200Hz) which shifts to 127 degrees C at I MHz driving frequency. A dependence of T-m on sintering temperature has been observed. A thermal hysteresis is also found on heating and cooling. This shows that the samples present both relaxor and ferroelectric characteristics. High values of the dielectric constant are found both at room temperature (about 5000) and at T-m (about 18000). The dependence of dielectric and piezoelectric constants on sintering temperature has been found.

724. 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.

725. 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.

726. 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.

727. 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.

728. 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.

729. 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.

730. 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.