National Institute Of Materials Physics - Romania

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860 articles found

811. Investigation of the W-74 L emission spectra and satellites

Authors: Vlaicu, AM; Tochio, T; Ishizuka, T; Ohsawa, D; Ito, Y; Mukoyama, T; Nisawa, A; Shoji, T; Yoshikado, S

Published: NOV 1998, PHYSICAL REVIEW A, 58, 3551, DOI: 10.1103/PhysRevA.58.3544

The L alpha and L beta spectra of tungsten were measured using a high-resolution single-crystal x-ray spectrometer. The observed spectra were fitted into Lorentzians. The fit residuals of the L alpha and L beta spectra indicate the presence of satellites in the vicinity of each spectrum, which are originated from Coster-Kronig transitions. Linewidths, energies, and intensities were estimated for each diagram line and compared with previous data. The L beta(2) satellite line observed by Salgueiro er al. [J. Phys. (Paris), Colloq 48, C9-609 (1987)] was confirmed on the high energy side of the L beta(2) line. The origin of this satellite is discussed experimentally and theoretically. [S1050-2947(98)02011-3].

812. Quantum confinement model for electric transport phenomena in fresh and stored photoluminescent porous silicon films

Authors: Iancu, V; Ciurea, ML

Published: OCT 1998, SOLID-STATE ELECTRONICS, 42, 1896, DOI: 10.1016/S0038-1101(98)00160-9

A quantum confinement model is proposed to explain the electrical transport properties in fresh and stored porous silicon (PS) films. In the present paper, the model is verified for the temperature dependence of the dark current. The studied samples are Formed mainly by a network of nanowires, suggesting the separation of the electron Hamiltonian into a longitudinal and a transversal part. The last one can be well described by a two-dimensional (2D) cylindrical infinite quantum well? whose levels determine the dark current activation energies. The storage oxidation induces modifications both in the number and the values of the activation energies, which are in excellent agreement with our model. (C) 1998 Elsevier Science Ltd. All rights reserved.

813. Detailed structure of a Pb-doped Bi2Sr2CuO6 superconductor

Authors: Ito, Y; Vlaicu, AM; Mukoyama, T; Sato, S; Yoshikado, S; Julien, C; Chong, I; Ikeda, Y; Takano, M; Sherman, EY

Published: AUG 1 1998, PHYSICAL REVIEW B, 58, 2858, DOI: 10.1103/PhysRevB.58.2851

The crystal structure of (Bi1.82Pb0.18) (ST1.84Pb0.16) CuO6 was determined from single-crystal intensity data measured on a four-circle diffractometer. The structure is orthorhombic, space group Cccm (No. 66); the lattice parameters: a=5.392(3) Angstrom, b=24.603(5) Angstrom, c=5.300(3) Angstrom, V=703.2 (5) Angstrom(3) (the space group can be denoted as Amaa by a cyclic transformation of the cell constants). The R value was 0.048 for 418 unique reflections with I>3.00 sigma(I). One of the O atoms bonded to Bi ions is disordered at two sites with a half occupancy and B-eq=3.6(6) Angstrom(2); although the thermal parameter is not so large as that reported by Torardi et al. [Phys. Rev. B 38, 225 (1988)], the disorder suggests that the crystal has a lower symmetry. The valence state of the Cu ion in the present compound was evaluated to be trivalent based on the Cu-O distances [Asbrink and Norrby, Acta Crystallogr. B 26, 8 (1970)] and the valence of Bi was estimated in comparison with that in other compounds. The O(3) sublattice dynamics in these types of superconductors is discussed based on the structural investigations.

814. Electrical behaviour of fresh and stored porous silicon films

Authors: Ciurea, ML; Baltog, I; Lazar, M; Iancu, V; Lazanu, S; Pentia, E

Published: JUL 18 1998, THIN SOLID FILMS, 325, 277, DOI: 10.1016/S0040-6090(98)00429-5

We have measured I-V and C-V characteristics, the temperature dependence of dark currents, and thermally stimulated depolarisation currents on fresh and stored samples of photoluminescent porous silicon, By storage in ambient, the low rectifying I-V curves become strong rectifying, and C-V curves become MIS-like. I-T characteristics for fresh samples have only one activation energy, in the 0.49-0.55 eV range. After storage, a slightly modified value, of about 0.50-0.60 eV is observed at low temperatures only. At about 280 K, the activation energy suddenly changes to 1.20-1.80 eV. Also, both the number and the positions of maxima in thermally stimulated depolarisation currents change by storage. The annealing at about 50 degrees C induces small reversible changes in I-T characteristics and strong irreversible ones in thermally stimulated depolarisation currents, both for fresh and stored samples. A simplified quantum confinement model is proposed to explain the main aspects of the electrical behaviour of porous silicon films. The surface and/or interface contributions are observed especially in thermally stimulated depolarisation currents. The changes induced by storage are attributed to the oxidation process of the internal surface of porous silicon films. (C) 1998 Elsevier Science S.A. All rights reserved.

815. Characterization of carbon nitride thin films deposited by a combined RF and DC plasma beam

Authors: Dinescu, G; Aldea, E; Musa, G; van de Sanden, MCM; de Graaf, A; Ghica, C; Gartner, M; Andrei, A

Published: JUL 18 1998, THIN SOLID FILMS, 325, 129, DOI: 10.1016/S0040-6090(98)00509-4

Thin carbon nitride films have been deposited on silicon(100) substrates downstream of a nitrogen plasma beam generated in a combined RF (13.56 MHz, 40-50 W) and DC (voltage +/-200 V, power 1-10 W) discharge between a graphite electrode and a graphite nozzle. By combining the RF and DC sources the capability of rf field to create extended plasmas is used together with the enhanced sputtering and biasing effect of the DC source. The plasma characteristics (electron temperature, presence of molecular species) have been studied by optical emission spectroscopy. Deposition rates of 2.5-3 nm/s are obtained at the centre of the plasma beam and at a few centimetres distance from the nozzle. The films have been investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, scanning and transmission electron microscopy, and microhardness measurements. The films have an overall N:C ratio of 0.28 but the distribution of different nitrogen bonds depends upon the DC bias conditions. In the spectral range 0.3-0.7 mu m the refractive index increases slightly from 1.5 to 2.2. The films are amorphous, with morphology consisting of a columnar structure. The columns have a diameter of about 20 nm. A hardness of 24 GPa has been measured. (C) 1998 Elsevier Science S.A. All rights reserved.

816. Electron and hole trapping in PbCl2 and PbCl2 : Tl crystals

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

Published: MAY 1998, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 141, 541, DOI: 10.1016/S0168-583X(98)00051-2

Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl2 single crystals. Besides Pb-2(3+) self-trapped electron (STEL) centers the PbCl2 : Tl crystals exhibit trapped-electron (PbTl)(+)-type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl-2 divided by centers the holes are self trapped at pairs of neighbouring Cl- anions resulting in V-k type centers with various orientation and length of the Cl-Cl axis. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

817. The molecular structure of some urea and thiourea derivatives

Authors: Bally, I; Simion, C; Mazus, MD; Deleanu, C; Popa, N; Bally, D

Published: APR 20 1998, JOURNAL OF MOLECULAR STRUCTURE, 446, 68, DOI: 10.1016/S0022-2860(97)00400-6

The molecular structures for N-phenyl-N'-acetylurea, N-phenyl-N'-capronylthiourea, and N - phenyl - N' - benzoyl thiourea have been characterized by X-ray diffraction and NMR spectroscopy. The relative positions of atoms in the parent molecular fragment, common to all three analyzed compounds, are discussed. (C) 1998 Elsevier Science B.V.

818. Electroluminescence of strained SiGe/Si selectively grown above the critical thickness for plastic relaxation

Authors: Stoica, T; Vescan, L; Goryll, M

Published: MAR 15 1998, JOURNAL OF APPLIED PHYSICS, 83, 3373, DOI: 10.1063/1.367104

Electroluminescence of strained Si0.80Ge0.20Si(001) pin diodes has been investigated experimentally and by quantitative modeling. The key aspect of this investigation was that by selective epitaxial growth the experimental critical thickness for plastic relaxation (80 nm at T-epi=700 degrees C and large areas) could be increased in finite pads. SiGe layers with thickness of 60, 72 or 370 nm have been grown within the intrinsic i region of pin structures. Samples free of misfit dislocations revealed electroluminescence with the SiGe no-phonon peak and its transversal optical-phonon replica corresponding to interband transitions. It was found that by increasing the thickness of the SiGe layer the drop in the electroluminescence with increasing temperature could be shifted to higher temperature, so that for the 370 nm thick SiGe sample the emission was observed to persist still at 300 K. Modeling based on drift-diffusion and carrier recombination equations was used to simulate the current-voltage characteristics of the pin diodes and their band gap electroluminescence. It was found that the modeling results can account for the temperature and thickness dependence of the electroluminescence. Hole and electron Shockley-Read-Hall recombination times could be evaluated. (C) 1998 American Institute of Physics.

819. Selective epitaxial growth of strained SiGe/Si for optoelectronic devices

Authors: Vescan, L; Stoica, T; Goryll, M; Grimm, K

Published: FEB 27 1998, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 51, 169, DOI: 10.1016/S0921-5107(97)00253-5

Two aspects of the selective epitaxial growth of Si and Si(1-x)Ge(x) will be discussed. First the facet formation as dependent on the oxide wall orientation and the lateral size of oxide openings. Besides the often cited {111}, {113} and {110} planes additional planes were observed, the {119} and the {0 1 12} planes. Second, the reduction of misfit dislocation density by reducing the area of the pads allows strained Si(1-x)Ge(x) layers to grow much thicker than the critical thickness. As an application for the latter the electroluminescence of forward biased PIN diodes with strained Si(0.80)Ge(0.20)/Si(001) will be discussed as being dependent on the thickness of the SiGe layer. It was found that in thicker strained samples the band edge electroluminescence persists up to room temperature. Quantitative modelling of the electroluminescence could explain the temperature and SiGe thickness dependence of the electroluminescence. (C) 1998 Elsevier Science S.A. All rights reserved.

820. X-ray-absorption features from multielectron excitations above Xe L edges

Authors: Ito, Y; Vlaicu, AM; Tochio, T; Mukoyama, T; Takahashi, M; Emura, S; Azuma, Y

Published: FEB 1998, PHYSICAL REVIEW A, 57, 878, DOI: 10.1103/PhysRevA.57.873

The x-ray-absorption cross section above the L edges in xenon gas has been measured using synchrotron radiation. Multielectron excitation effects are investigated over a few hundred eV region from the Xe L-I, L-II, and L-III edges and the contributions from the effects of shake-up and shake-off to the photoabsorption spectrum are elucidated. Several multielectron excitation features were detected and analyzed by the theoretical energy of the shake-up process. Previous observations of the [2(s,p)5(s,p)] and [2(s,p)4d] transitions have been confirmed. [2(s,p)4p] transitions are clearly identified in the present study. It is very difficult to detect the [2s3s] multielectron transition with the sensitivity of similar to 2.0 x 10(-4) at the L edge jump. The [2s3d] edge was distinguished using the first derivative of the absorption curve. A new transition is found between [2(s,p)4p] and [2(s,p)4d] transitions and identified as a three-electron transition [2(s,p)4d5p]. Moreover, the difference in the behavior of the multielectron excitation as a function of x-ray energy among the L-I and L-II, or L-III edges is confirmed and investigated. [S1050-2947(98)01802-2].