National Institute Of Materials Physics - Romania

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5781 articles found

4301. X-ray photoelectron spectroscopy study on n-type GaAs

Authors: Ghita, RV; Negrila, C; Manea, AS; Logofatu, C; Cernea, M; Lazarescu, MF

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 863, DOI:

A surface characterization has been performed on n-type GaAs (Si: GaAs) samples by using X-ray photoelectron spectroscopy (XPS) technique in order to get information on the degree of surface contamination during the device processing steps. The GaAs samples were etched by "in situ" ion sputtering (Ar plasma) before and after carrying out the measurements. According to XPS measurements the native oxide layer on as-received surface contains As2O3, AS(2)O(5) and Ga2O3. Large amounts of C and O are also present at surface before plasma cleaning of surface: C-C bonds, chemisorbed oxygen atoms. The XPS spectra recorded on the surface sample sputtered with 5 keV Ar+ ions were analyzed using SDP subroutines program facilities. After the first sputtering step it was observed different shifts of the principal peaks of GaAs bonding and compounds together with a modification of the relative peak intensities. The C1s and O1s peak size decreased but remained still significant. The following sputtering steps lead to a drastically decrease of C and 0 peaks down to the noise level.

4302. BPSCCO superconductor with addition of Ag2O synthesized under electrical field

Authors: Aldica, G; Groza, JR; Badica, P; Bunescu, MC

Published: DEC 2003, JOURNAL OF SUPERCONDUCTIVITY, 16, 1005, DOI: 10.1023/A:1026295408106

Ag2O-doped (1.2% wt.) nitrate freeze-dried powders (Bi : Pb : Sr : Ca : Cu = 1.7 : 0.3 : 2 : 2.5 : 3.5) were processed under an external electrical field and 17.5 MPa pressure at 800 degreesC, for 4 min in vacuum. Final heat treatments (HT) were applied at 835 - 850 degreesC for 70 h. in air (Bi, Pb)(2)Sr2CaCu2Ox (2212-phase) was formed by electrical field processing in just 4 min. Electrical field application enhanced (Bi, Pb)(2)Sr2Ca2Cu3Oy (2223-phase) formation during final HT. Ag2O additions to field sintered BSCCO ceramics increased the amount of 2223-phase and the zero resistance critical temperature (T-e(R=0)) by similar to 4 k.

4303. Bi-2223 freeze-dried ceramic: Specific features, related problems and search for new solutions

Authors: Badica, P; Aldica, G

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 1039, DOI:

The paper presents a review of our data and the current status on synthesis and processing of Bi-2223 superconducting ceramic, by spray frozen, freeze drying method (SF-FD). By this method powders of 200-300 nm were obtained and processed by several different routes to the final superconducting product. Optimization process and comparative analysis with the literature data revealed intrinsic problems associated to the specific nature (i.e high degree of mixing and large surface area relative to the volume) of the SF-FD material. The paper focuses on the effects induced by the specific features of the SF-FD material, sometimes opposite to those observed in the conventional materials produced by solid-state method. It resulted that in order to take full advantage of the specific features for a nano SF-FD material, it is necessary to consider a special, non-conventional, processing approach and some examples in this regard will be presented.

4304. Magnetism and anisotropy in core-shell nanoparticles

Authors: Crisan, O; Angelakeris, M; Flevaris, NK; Filoti, G

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 962, DOI:

The subject of magnetic anisotropy in nanostructured materials is of considerable interest from both experimental and theoretical points of view. Systems that consist of either isolated or interacting nanoparticles are shown to exhibit different magnetic properties of the nanoparticle surface than those of the bulk. This altering of magnetic behavior is mainly related to surface spins structure cot-related with finite size effects, providing that the nanoparticles are sufficiently small so that the surface-to-volume atomic ratio would be high enough. This work reports on both the simulation and experimental approach on the magnetic behavior of ferromagnetic nanoparticles. We investigate the competition between surface and bulk magnetocrystalline anisotropy in small magnetic particles. The experimentally obtained magnetic data for ferromagnetic core-shell-type AgCo nanoparticles are interpreted consistently with the results of Monte Carlo simulation of the magnetic behavior of single domain nanoparticles.

4305. Near-infrared emission spectra of negative metal ions aggregates

Authors: Topa, V; Apostol, E

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 833, DOI:

In this review paper we attempt to summarize the achievements of the near infrared emission studies of negative metal ions (Me-) obtained by nonstandard electrolytical colouring. We present the emission characteristics of Pb--ions aggregates in KCl crystals at 300 and 4.2 K by excitation in the UV and visible range of different laser lines. These centres perturbed by Li+, Na+, Rb+, Ca2+, Sr(2+)and Ba2+ cations, Tl- and Tl- + Ca2+- ions aggregates and the negative aggregates of ln(-) and Ga- in KCl crystals are, also, discused.

4306. Densification of In2O3: Sn multilayered films elaborated by the dip-coating sol-gel route

Authors: Daoudi, K; Canut, B; Blanchin, MG; Sandu, CS; Teodorescu, VS; Roger, JA

Published: NOV 24 2003, THIN SOLID FILMS, 445, 25, DOI: 10.1016/S0040-6090(03)01061-7

Indium Tin Oxide (ITO) thin films have been deposited by the Sol-Gel Dip-Coating technique, the starting solutions being prepared from chlorides. These multilayered films were crystallized by means of a classical heat treatment at temperatures ranging from 500 to 600 degreesC. Five stacked layers are necessary to obtain a global electrical resistivity value of 2.9 x 10(-3) Omega cm, for 500 degreesC annealed film. The paper focuses on the study of the structure of such multilayered deposits, and on the densification process, using transmission electron microscopy, Rutherford Back-scattering Spectrometry and electrical resistivity measurements. This analysis reveals structural inhomogeneities and different crystallite growth processes as a function of annealing temperature and number of deposited layers. (C) 2003 Elsevier B.V. All rights reserved.

4307. Bulk damage effects in standard and oxygen-enriched silicon detectors induced by Co-60-gamma radiation

Authors: Fretwurst, E; Lindstrom, G; Stahl, J; Pintilie, I; Li, Z; Kierstead, J; Verbitskaya, E; Roder, R

Published: NOV 21 2003, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 514, 8, DOI: 10.1016/j.nima.2003.08.077

The influence of oxygen in silicon on bulk damage effects induced by Co-60-gamma irradiation has been studied in a dose range between 0.2 and 900 Mrad. The detector processing and oxygen enrichment were carried out in a common project by the Institute of Micro-sensors CiS using n-type high-resistivity FZ silicon (3-6 kOmega cm) with <111> and <100> orientation. Different oxygen concentrations were achieved by diffusion at 1150degreesC for 24, 48 and 72 h. This report on bulk damage effects is focussed on the observed changes in the reverse current, the effective space charge density N-eff extracted from C/V measurements and investigations using the transient current technique. A substantial improvement of radiation hardness concerning the development of the macroscopic properties was found for detectors manufactured on oxygenated material compared to standard material. It will be demonstrated that the change of the effective space charge density as well as the increase of the reverse current can be attributed to the creation of two deep acceptor levels and a shallow donor level. (C) 2003 Elsevier B.V. All rights reserved.

4308. Microscopic modelling of defects production and their annealing after irradiation in silicon for HEP particle detectors

Authors: Lazanu, S; Lazanu, I; Bruzzi, M

Published: NOV 21 2003, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 514, 17, DOI: 10.1016/j.nima.2003.08.078

In this contribution, the production of defects in radiation fields and their evolution toward equilibrium in silicon for detector uses has been modelled. In the quantitative model developed, the generation rate of primary defects is calculated starting from the projectile-silicon interaction and from recoil energy redistribution in the lattice. Vacancy-interstitial annihilation, interstitial migration to sinks, divacancy and vacancy-impurity complex (VP, VO, V2O, CiOi and CiCs) formation are considered. The results of the model support the experimental available data. The correlation between the initial material parameters, temperature, irradiation and annealing history is established. The model predictions could be a useful clue in obtaining harder materials for detectors at the new generation of accelerators or for space missions. (C) 2003 Elsevier B.V. All rights reserved.

4309. Flux-creep annealing in Bi2CaCu2O8+delta and La2-xSrxCuO4 single crystals at low temperatures

Authors: Miu, L; Tamegai, T; Tokunaga, M

Published: NOV 21 2003, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 399, 80, DOI: 10.1016/S0921-4534(03)01300-5

We performed flux-creep annealing experiments on disordered (overdoped) Bi2Sr2CaCu2O8+delta and La2-xSrxCuO4 single crystals in remnant magnetic field at T = 10 K, where, as generally accepted, the vortex creep process is thermally activated, and at T = 2 and 1.8 K, in the putative quantum creep regime. A simple data analysis shows that the variation of the creep barrier U with the current density J remains qualitatively the same, regardless of T. In the case of La2-xSrxCuO4 single crystals in remnant field U(J) diverges in the low-J limit, and flux-creep annealing considerably reduces the normalized relaxation rate S at low T. We emphasize that the relatively large "residual" S values appearing below a certain crossover temperature in "standard" magnetization relaxation experiments are mainly due to the usually fixed relaxation time window in the conditions of a strong U(J) nonlinearity. (C) 2003 Published by Elsevier B.V.

4310. Results on defects induced by Co-60 gamma irradiation in standard and oxygen-enriched silicon

Authors: Pintilie, I; Fretwurst, E; Lindstrom, G; Stahl, J

Published: NOV 21 2003, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 514, 24, DOI: 10.1016/j.nima.2003.08.079

Radiation-induced defects in silicon diodes were investigated after exposure to high doses of Co-60 gamma irradiation, using Deep Level Transient Fourier Spectroscopy and Thermally Stimulated Current methods. The main focus was on differences between standard and oxygen-enriched material and the impact of the observed defect generation on the diode properties. Two close to mid gap trapping levels and a bi-stable donor level have been found to be responsible for the main macroscopic changes both in standard and oxygen-enriched float zone diodes. (C) 2003 Elsevier B.V. All rights reserved.