National Institute Of Materials Physics - Romania

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5975 articles found

4961. Hall effect analysis in irradiated silicon samples with different resistivities

Authors: Borchi, E; Bruzzi, M; Dezillie, B; Lazanu, S; Li, Z; Pirollo, S

Published: AUG 1999, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 46, 838, DOI: 10.1109/23.790687

The changes induced by neutron irradiation in n- and p-type silicon samples with starting resistivities from 10 Omega-cm up to 30 K Omega-cm, grown, using different techniques, as Float-Zone (FZ), Czochralski (CZ) and epitaxial, have been analyzed by Van der Pauw and Hall effect measurements. Increasing the fluence, each set of samples evolves toward a quasi-intrinsic p-type material. This behavior has been explained in the frame of a two-level model, that considers the introduction during irradiation of mainly two defects. A deep acceptor and a deep donor,probably related to the divacancy and to the CiOi complex, are placed in the upper and lower half of the forbidden gap, respectively. This simple model explains quantitatively the data on resistivity and Hall coefficient of each set of samples up to the fluence of approximate to 10(14) n/cm(2).

4962. Diamond degradation in hadron fields

Authors: Lazanu, S; Lazanu, I; Borchi, E

Published: AUG 1999, NUCLEAR PHYSICS B-PROCEEDINGS SUPPLEMENTS, 78, 688, DOI: 10.1016/S0920-5632(99)00624-6

The energy dependence of the concentration of primary displacements induced by protons and pions in diamond has been calculated in the energy range 50 MeV - 50 GeV, in the frame of the Lindhard theory. The concentrations of primary displacements induced by protons and pions have completely different energy dependencies: the proton degradation is very important at low energies, and is higher than the pion one in the whole energy range investigated, with the exception of the Delta(33) resonance region. Diamond has been found, theoretically, to be one order of magnitude more resistant to proton and pion irradiation in respect to silicon.

4963. Off-center displacement of Fe+ ions in irradiated SrCl2 : Fe crystals grown in chlorine

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

Published: AUG 1999, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 214, 236, DOI: 10.1002/(SICI)1521-3951(199908)214:2<229::AID-PSSB229>3.0.CO;2-H

Chlorinated SrCl2:Fe2+ crystals exhibit, after X-ray irradiation, two trapped-electron Fe+(I) and Fe+(II) centers both with axial [001] symmetry, but different EPR spectrum parameters and production properties. The analysis of the experimental data strongly suggests a substitutional eight-fold coordination of the Fe+ ion in the Fe+(I) center and a strong [001] off-center displacement to a fourfold coordinated site in the Fe+(II) center resulting in S = 3/2 and S = 1/2 ground states, respectively.

4964. Design for a focusing high-resolution neutron crystal diffractometer

Authors: Ionita, I; Stoica, AD; Popovici, M; Popa, NC

Published: JUL 21 1999, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 431, 520, DOI: 10.1016/S0168-9002(99)00280-6

A new concept of high-resolution focusing configuration begins to be accepted as an alternative solution to the existing conventional configurations. Among the earliest work performed in this direction is that performed at the Institute for Nuclear Research, Pitesti. These results are presented below. The experimentally determined resolution properties for two focusing configurations obtained at TRIGA reactor Pitesti and at VVRS reactor Bucharest are given in order to be compared with those obtained for the conventional ones. The principles to get focusing in crystal neutron diffractometry are presented. The main characteristics for a focusing instrument are given. (C) 1999 Elsevier Science B.V. All rights reserved.

4965. Nanostructure and phase distributions in (Cu0.80Fe0.20)(100-x)B-x melt spun ribbons studied by Mossbauer spectroscopy

Authors: Kuncser, V; Rosenberg, M; Yavari, AR; Filoti, G

Published: JUL 20 1999, JOURNAL OF ALLOYS AND COMPOUNDS, 289, 276, DOI: 10.1016/S0925-8388(99)00163-2

The analysis of the Mossbauer spectra of (Cu0.8Fe0.2)(100-x))B-x the melt spun ribbons taken at several temperatures between 4.2 K and room temperature allowed us to identify the occurring Fe-containing phases: alpha-Fe, gamma-Fe and <(Cu)under bar>Fe solid solutions. The presence of B inhibits the phase transformation from fee to bcc Fe. For x = 0 and 1, the amount of Fe in the <(Cu)under bar>Fe solid solution is close to 3 at.% solubility limit, while for x=3 besides crystalline Fe3B, a supersaturated phase of 9.7 at.% Fe in the <(Cu)under bar>Fe solid solution was found, showing a strong deviation from the binary equilibrium phase diagram. (C) 1999 Elsevier Science S.A. All rights reserved.

4966. Structural aspects of CVD diamond wafers grown at different hydrogen flow rates

Authors: Nistor, L; Van Landuyt, J; Ralchenko, V

Published: JUL 16 1999, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 174, 9, DOI: 10.1002/(SICI)1521-396X(199907)174:1<5::AID-PSSA5>3.0.CO;2-C

Microstructural modifications of thick diamond films grown by microwave plasma CVD at different reactant gas now rates have been studied by transmission electron microscopy. A strong degradation of the diamond quality was observed at low hydrogen flow rates, while the layer grown at the highest hydrogen flow rate was almost defect free. The abundance of various defects observed, mostly microtwins, stacking faults, dislocations and even amorphous regions, clearly correlates with the gas flow rate. Illustrations of typical defects are given and, in particular, planar defect intersections are characterized at atomic resolution in specimens where their density was high.

4967. Colloidal sol-gel ITO films on tube grown silicon

Authors: Stoica, TF; Stoica, TA; Vanca, V; Lakatos, E; Zaharescu, M

Published: JUL 6 1999, THIN SOLID FILMS, 348, 278, DOI: 10.1016/S0040-6090(99)00136-4

Electrical and optical properties of colloidal PTO sol-gel films on glass substrates have been investigated. The optical gap of 3.14 eV and room temperature conductivity of 4 Ohm(-1) cm(-1) were obtained on ITO layers of about 500 Angstrom thickness. Heterostructures of ITO/n-Si have been obtained by colloidal sol-gel deposition of ITO. In this deposition method, the oxidation of silicon during the heterostructure formation can be avoided. The silicon substrates were cut out of silicon tubes. Electrical and photoelectric properties of these structures have been studied in comparison with those of p-n tube-Si junctions obtained by thermal boron diffusion. On ITO/n-Si heterojunctions, the spectral quantum efficiency has a maximum value of about two times higher and a blue shift of the maximum position from 0.75 to 0.6 mu m, than those for p-n junctions. A theoretical investigation of the I-V curves for ITO/n-Si heterojunctions under various illumination levels has been performed and contribution of the tunneling and series resistance phenomena were evaluated. (C) 1999 Elsevier Science S.A. All rights reserved.

4968. On the confinement of liquid crystals in molecular sieves: dielectric measurements

Authors: Frunza, S; Frunza, L; Schoenhals, A; Zubowa, HL; Kosslick, H; Carius, HE; Fricke, R

Published: JUL 2 1999, CHEMICAL PHYSICS LETTERS, 307, 176, DOI: 10.1016/S0009-2614(99)00523-0

The molecular dynamics of 4-octyl-4'-cyanobiphenyl and 4-pentyl-4'-cyanobiphenyl, confined to the mesopores with diameters less than 25 Angstrom of the molecular sieve AlMCN-41, was investigated over a wide temperature range using broadband dielectric spectroscopy. As expected, the phase transitions of the bulk liquid crystals cannot be detected for the confined molecules. A new, slow relaxational process occurs over the whole temperature range due to the motions of the molecules confined in the pores; the temperature dependence of the characteristic frequency obeys a Vogel-Fulcher-Tamman (VFT) law associated with a glassy state. The orientational order within these small pores is mostly dominated by the local environment. (C) 1999 Elsevier Science B.V. All rights reserved.

4969. Trap characterization for Bi4Ti3O12 thin films by thermally stimulated currents

Authors: Pintilie, L; Pintilie, I; Petre, D; Botila, T; Alexe, M

Published: JUL 1999, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 69, 109, DOI: 10.1007/s003390050980

Thermally stimulated current (TSC) measurements performed in the 100 K-400 K temperature range on Bi4Ti3O12 (BiT) thin films annealed at 550 degrees C and 700 degrees C had revealed two trapping levels having activation energies of 0.55 eV and 0.6 eV. The total trap concentration was estimated at 10(15) cm(-3) for the samples annealed at 550 degrees C and 3 x 10(15) cm(-3) for a 700 degrees C annealing and the trap capture cross-section was estimated about 10(-18) cm(2). From the temperature dependence of the dark current in the temperature range 20 degrees C-120 degrees C the conduction mechanism activation energy was found to be about 0.956-0.978 eV. The electrical conductivity depends not only on the sample annealing temperature but also whether the measurement is performed in vacuum or air. The results on the dark conductivity are discussed considering the influence of oxygen atoms and oxygen vacancies.

4970. Structural characterization of FeCrP-type mesoscopic system

Authors: Mihalca, I; Morariu, M; Lita, M; Bibicu, I; Ercuta, A; Luca, D

Published: JUL 1999, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 201, 57, DOI: 10.1016/S0304-8853(99)00169-9

The paper describes the structural properties of some Fe80-xCrxP20 (1 less than or equal to x less than or equal to 7) mesoscopic (similar to 1 mu m) particles produced by mechanical alloying. The influence of the atomic concentration of Cr and that of the milling time are examined. The hyperfine held distribution obtained from the room temperature Mossbauer spectra of the as-milled samples exhibits three well separated peaks, associated with different chemical short range order, and depending on the atomic concentration of Cr atoms. (C) 1999 Elsevier Science B.V. All rights reserved.