National Institute Of Materials Physics - Romania

Publications

490 articles found

471. Radiological impact of C-14

Authors: Bucur, C; Olteanu, M; Olteanu, C; Pavelescu, M

Published: APR 2006, REVISTA DE CHIMIE, 57, 367, DOI:

Near surface disposal is considered a suitable option for the disposal of short-lived low and intermediate level waste (LILW) containing radionuclides which decay to insignificant radiation levels within a few decades or centuries. LILW containing limited concentrations of long-lived radionuclides may also be disposed of in near surface facilities. The main radionuclides in short lived LILW are Cs-137 and Sr-90 and therefore, a period of 300 years for the post-closure institutional controls would correspond to about ten half-lives of these radionuclides. The longer-lived radionuclides, such as C-14, I-129, Tc-99 will continue to be released even after that period due to the engineering barriers degradation. The paper presents the post closure radiological impact of LIL waste disposal due to the C-14, one of the long life radionuclides presented in the LILW generated at Cernavoda Nuclear Power Plant.

472. XPS study of Ti/oxidized GaAs interface

Authors: Grita, RV; Logofatu, C; Negrila, C; Manea, AS; Cernea, M; Ciupina, V; Lazarescu, MF

Published: FEB 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 36, DOI:

X-Ray Photoelectron Spectroscopy (XPS) analysis was performed on Ti/oxidized GaAs. This interface is prepared by chemical etching in a mixture of reagents, followed by a vacuum deposition of a nanometer Ti layer. The surface sensitivity of XPS (typically 40-100 angstrom), makes the technique ideal for the measurements of oxidation states and oxide layer thickness on III-V compounds. The study of Ti/oxidized GaAs put into evidence a lack of stoichiometry at the semiconductor surface due to the presence of a volatile element (As). At the interface Ti/Oxide/n-GaAs common oxides are present: Ga2O3, As2O3 and As2O5, together with titanium oxide. The XPS spectra recorded on the sample surface sputtered with 5 keV Ar+ ions (in two steps: t = 15 min, t = 45 min) were analyzed using SDP subroutines program facilities. The As signal at the interface after t = 15 min plasma etching is related to GaAs, As2O3 and over 40% from a GaAs oxidation complex or due to intermetallic oxide of Ti. After the last plasma etching at the surface of GaAs the stoichiometry was demonstrated: the signal of As and Ga arises commonly from GaAs. It was remarked a broader titanium line profile, which arises from titanium oxide and from different intermediate states at the interface.

473. Physical characterization of CdMnS nanocrystalline thin films grown by vacuum thermal evaporation

Authors: Iacomi, F; Salaoru, I; Apetroaei, N; Vasile, A; Teodorescu, CM; Macovei, D

Published: FEB 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 270, DOI:

CdMnS ternary alloys have attracted much attention because they permit to tune the energy gap, the effective mass and the lattice constant by varying the concentration of the magnetic material. Growth and physical characterization of Cd1-xMnxS thin films, prepared in different conditions on glass and silicon substrates by thermal evaporation technique is presented. The starting powders, CdS and Mn, were mixed in a crucible in various molar ratios. The samples were characterized by XRD, AFM and XPS. The blue shift of band gap of thin films might be ascribed to the size effect of the small grains and to the fact that Cd was substituted by Mn in the CdS structure.

474. Studies of ohmic contact and Schottky barriers on Au-Ge/GaAs and Au-Ti/GaAs

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

Published: DEC 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 3037, DOI:

Gallium Arsenide (GaAs) has increasingly become an important compound-semiconductor material suited for high speed digital circuits, microwaves and detectors for high energy physics. For the fabrication of ohmic contacts on GaAs (semi-insulating- SI) wafers with characteristics as: Cr doped, (100) oriented, and p similar to 10(6)similar to 10(7) Omega cm, has been used alloyed contact Au-Ge. The ohmic film was deposited in a high vacuum chamber (10(-8) Torr), on a plasma etched surface of GaAs, followed by a rapid thermal annealing (RTA) at 450 degrees C in low vacuum. For the fabrication of the rectifying contact-respectively the Schottky diode structure, we have used the same GaAs substrates. The wafers were degreased in a standard cleaning procedure, followed by a chemical etching. The Au-Ti contact has been deposited by thermal evaporation in vacuum (10(-6) torr) followed by a RFA procedure at 300-320 degrees C. The carrier transport mechanisms through M/S interface are strongly influenced by the doping concentration in the semiconductor and temperature. There are presented the experimental I-V characteristics for selected samples in dark and illumination conditions. Schottky barriers height on Au-Ti/GaAs was 0.7325 V in accord with mean values on n and p type GaAs (0.5-0.8 V). The aim of this work is to fabricate a competitive X-ray detector.

475. Surface chemistry of plasma deposited ZrC hard coatings

Authors: Balaceanu, M; Braic, M; Braic, V; Vladescu, A; Negrila, CC

Published: OCT 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2560, DOI:

X-ray Photoelectron Spectroscopy (XPS) was used to investigate the chemical state of ZrC coatings deposited by cathodic arc method. A chemistry model of the ZrC films has been proposed. It was pointed out that on the films surface a highly oxidized passive layer, containing also carbon and oxygen as contaminants, was formed. The film bulk was composed by a mixture of ZrC (the dominant phase), ZrO2 and free carbon.

476. Microchemical and mechanical characteristics of arc plasma deposited TiAlN and TiN/TiAlN coatings

Authors: Braic, M; Braic, V; Balaceanu, M; Pavelescu, G; Vladescu, A; Tudor, I; Popescu, A; Borsos, Z; Logofatu, C; Negrila, CC

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 676, DOI:

Single layer TiAlN and alternate TiN/TiAlN multilayered hard coatings were deposited on Si, plain carbon steel and high-speed steel substrates by the cathodic are technique. Chemical composition, phase composition, texture, Vickers microhardness, adhesion, bilayer period, sliding friction coefficient and wear resistance were investigated by XPS, EDX and XRD analyses, microhardness measurements, scratch and tribological tests.

477. Heusler bulk materials as targets for pulsed laser deposition: growth and characterisation

Authors: Manea, AS; Monnereau, O; Notonier, R; Guinneton, F; Logofatu, C; Tortet, L; Garnier, A; Mitrea, M; Negrila, C; Branford, W; Grigorescu, CEA

Published: FEB 15 2005, JOURNAL OF CRYSTAL GROWTH, 275, E1792, DOI: 10.1016/j.jcrysgro.2004.11.213

This work concerns the preparation and characterisation of homogeneous bulk Co2Mn (Si, Ge, Ga, Sn Sb0.8Sn0.2) to be used as targets for pulsed laser deposition of magnetic contacts for spintronic devices. The samples have been prepared by direct synthesis from elemental powders followed by vertical gradient freeze. Characterisation of their structural properties and compositional homogeneity was performed by X-ray diffraction and energy-dispersive X-ray spectroscopy combined with scanning electron microscopy. The results show one-phase for the Heusler alloys. The Curie temperatures of the samples have been measured by both thermogravimetric analysis and differential scanning calorimetry and are in a good agreement with the previously published values. The saturation magnetisation values are close to the values for single crystals. (C) 2004 Elsevier B.V. All rights reserved.

478. Structural and magnetic investigations of nickel clusters in C-60 matrices

Authors: Teodorescu, CM; Macovei, D; Lungu, A

Published: DEC 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 1285, DOI:

Nanostructured Ni/C-60 thin films, produced by co-evaporation of Ni and fullerene, have been investigated by both X-ray diffraction (XRD) and X-ray magnetic circular dichroism (XMCD). X-ray diffraction has shown the co-existence of diffraction specific peaks for the fullerene face-centered cubic crystal structure and diffraction peaks of Ni nanoparticles in the face-centered cubic structure, which are broadened by nanoparticule finite dimensions. Analysis of peak widths shows that the Ni nanocrystallite size ranges from 4.7 to 19.2 nm. The XMCD signals for a sample series, preparated on relatively low-temperature substrates during the co-evaporation, can be interpreted as having distinct contributions due to both bulk and surface Ni atoms. These two kind of atoms are antiferromagnetic coupled. The Ni atoms which are at the interface with the fullerite matrix show ail enhanced orbital magnetic moment, while the orbital magnetic moment of Ni atoms in the bulk has very low values, as it can be expected for a crystalline field with a cubic symmetry.

479. 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.

480. Bar-configuration in hall measurements with GaAs

Authors: Ghita, RV; Logofatu, C; Negrila, C; Lazarescu, MF; Manea, AS; Ciupina, V

Published: MAR 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 363, DOI:

The electrical and transport properties of GaAs crystals such as resistivity (rho), mobility (mu) and carrier concentration (n) depend on the purity and defects of a particular crystal. These parameters are normally obtained by performing a simple Hall effect analysis in a bar configuration. The technical procedure is connected with the design of appropriate contacts for Hall bar configuration. In order to ensure good ohmic contacts oil GaAs, thin metal layers of Au-Ge-Ni have been deposited in high vacuum followed by heat treatment in low vacuum. Hall effect measurement is important for checking the quality of the GaAs wafers. We analyzed various GaAs wafers with (100) and (110), orientation obtained by LEC and HB methods. The resistivity of the wafers varied from low resistivity, n-type doped crystals (n = 10(18) cm(-3)), to high resistivity crystals (n = 10(14) cm(-3)). This paper presents the data of Hall characterization of wafers cut from GaAs crystals grown by different methods (LEC, HB).