Publications

A phenomenological model was developed to explain quantitatively, without free parameters, the production of primary defects in silicon after particle irradiation, the kinetics of their evolution toward equilibrium and their influence on detector parameters. The type of the projectile particle and its energy is considered in the evaluation of the concentration of primary defects. Vacancy-interstitial annihilation, interstitial migration to sinks, vacancy-impurity complexes (VP, VO, V2O), and divacancy (V-2) formation are taken into account in different irradiation conditions, for different concentrations of impurities in the semiconductor material, for 20 and 0degreesC. The model can be extended to include other vacancy and interstitial complexes. The density of the reverse current in the detector after irradiation is estimated. Comparison with experimental measurements is performed. A special application considered in the paper is the modelled case of the behaviour of silicon detectors operating in the pion field estimated for the LHC accelerator, under continuum generation and annealing.

The cohesive energy of the Al-Mn Mackay icosahedron (MI) was estimated for different variants of chemical order, using pair-bonding energies. This problem is related to the nucleation of i-AlMn phase in sequentially deposited thin films. The chemically ordered MI comes out as the thermodynamically most stable configuration. The role of kinetic factors during deposition is also discussed.

A slice of the (Cu,C)Ba2Ca3Cu4O12-y bulk superconductor grown by high-pressure synthesis was polished down to a thickness of 50 mum, and then irradiated with high-energy (240 MeV) Au15+, ions (10 11 cm(-2)). Since the length of the ion tracks was estimated to be 12.2 mum, our sample consists of two different types of grain, with and without columnar defects, having very different critical current densities. Using the third-harmonic response of the sample, we were able to separate the small dissipation in the irradiated grains from the much larger dissipation in the non-irradiated grains, in a certain experimental range of temperature, field and current density. The critical current density was estimated from critical state models. For temperatures in the range of 85-105 K, we observed a peak effect at high dc magnetic fields, suggesting the possibility of a record irreversibility field at 77.3 K.

Effect of 1-4 wt.% excess PbO, added to the calcined powder, on the structure and dielectric and piezoelectric properties of Bi-doped lead titanate ceramics was investigated. The ceramics containing 4, 6 and 8 mol% Bi were prepared by the usual ceramic technique, using high purity oxides. The calcined powder of each composition was next intimately mixed with 1, 2, 3 and 4 wt.% PbO and then sintered at 1200-1300 degreesC for 3 h. The dielectric piezoelectric and electromechanical properties were determined as a function of excess PbO. Addition of PbO up to 2 wt.% was found to improve the densification, by the presence of the PbO liquid phase during sintering. For all Bi doping levels, the dielectric constant and the thickness coupling factor reach maxima (representing an increase of 2.5-8% for dielectric constant and 7-8% for thickness coupling factor) at about 2 wt.% PbO addition. The planar coupling factor is very low, less than 0.01, regardless the PbO content. The disipation factor slightly decreases with increasing PbO content and the mechanical quality factor of the thickness mode was about 300. The experimental results were discussed in terms of the mechanisms of densification in the presence of the liquid phase and of lead vacancy concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L-2,L-3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk alpha-Fe (2.2 mu(B)) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices.

The thermochemical behavior of the coordination compound [Fe(urea)(6)](NO3)(3) was studied by simultaneous CG-TG-DTG-DTA and mass spectrometry methods non-isothermal conditions. The compound decomposes at -200 degreesC into a mixture of spinel-type oxides and hematite. The nature and particle size of the final decomposition products are strongly associated with the conditions during the thermal treatments, in particular the heating rate and the calcination temperature. A certain fraction of the products are formed as nanometric particles; they show superparamagnetic behavior at room temperature. The comparably low temperature of the calcination treatments of this compound is a promising perspective to attain small sized magnetic powders. (C) 2002 Elsevier Science B.V. All rights reserved.

The deep level transient spectroscopy method was applied on standard and oxygenated float-zone silicon detectors exposed to high doses of Co-60-gamma irradiation. We have detected and characterized a close to midgap trapping level having an ionization energy of E-C-(0.545+/-0.005) eV and electron/hole capture cross sections of sigma(n)=(1.7+/-0.2)x10(-15) cm(2)/sigma(p)=(9+/-1)x10(-14) cm(2) respectively. This level has a strong impact on the detector performance being responsible for more than 90% of the change in the effective doping concentration. The defect is strongly oxygen related and a possible connection with the V2O complex is discussed. (C) 2002 American Institute of Physics.

The structure of several silver alloyed arsenic chalocgenide has been determined by X-ray diffraction. For low silver doping the disordered layer structure, characteristic to the glassy As2S3; is retained as demonstrated by the well developed first sharp diffraction peak in the X-ray diffraction pattern. For high amount of silver introduced in the As2S3; matrix, the disoredered layer configurations disappear, as shown by the diminishing and even disappearance of the first sharp diffraction peak in the X-ray patterns. A three-dimensional structure based on Ag2S-type configuration is formed.

In this paper we report modifications of the surface enhanced Raman scattering (SERS) spectra with film thickness of carbon single-walled nanotubes (SWNTs). The decrease of film thickness reveals an increased degree of disorder, which is interpreted in terms of degradation of the single-walled nanotubes and the formation of particles similar to highly oriented pyrolytic graphite (HOPG), C-60 and amorphous carbon. The degradation of the nanotubes, dependent on the metal substrate type and its morphology, is explained on the basis of interactions between the nanotubes and the metal substrate. The D band, which is an indicator of the degree of disorder, increases its intensity when the film thickness is decreased. SERS spectra recorded at different excitation wavelengths reveal that metallic nanotubes are primarily affected. A mutual correlation was found between the variations in the relative intensity of the D band and the Breit-Wigner-Fano component due to metallic tubes displayed by Raman bands situated in the 1500-1600 cm(-1) range. A direct interaction between C-60 and nanotubes is revealed by comparative SERS spectra on SWNTs and (SWNTs+C-60) films. At lambda(exc)=676.4 nm the narrowing in the lower energy side of the broad band associated to tangential vibration modes indicates that the metallic tubes are involved in the formation of a [(SWNT)C-+(60)-] complex type.

In the present work, the bulk degradation of SiC in hadron (pion and proton) fields, in the energy range between 100 MeV and 10 GeV. is characterised theoretically by means of the concentration of primary defects per unit fluence. The results are compared to the similar ones corresponding to diamond, silicon and GaAs. (C) 2001 Elsevier Science B.V. All rights reserved.