Publications

Experimental data indicating an anti-Stokes Raman emission, reminiscent of coherent anti-Stokes Raman scattering (CARS), that originates in a wave-mixing process between the incident laser light (omega(1)) and Stokes Raman light (omega(s)) generated by a surface enhanced Raman scattering (SERS) mechanism are reported. The variation of anti-Stokes SERS spectra of single-walled carbon nanotubes (SWNTs) and copper phthalocyanine (CuPc) as a function of the film thickness, laser excitation intensity and numerical aperture of the microscopic objective used for collecting the scattered light are presented as experimental arguments.

The influence of preliminary treatment in hydrogen plasma on elimination of radiation defects and formation of thermal donors has been studied in detector structures made of standard float zone silicon. The detectors were irradiated with 3.5MeV electrons and annealed at temperatures of 50-350 degrees C. It has been found that preliminary hydrogenation at 300 degrees C leads to disappearance of divacancies and vacancy-oxygen complexes at lower annealing temperatures. The annealing of hydrogenated and irradiated crystals is accompanied by hydrogen redistribution and formation of hydrogen-related donors. (c) 2005 Elsevier B.V. All rights reserved.

The proposed luminosity upgrade of the Large Hadron Collider (S-LHC) at CERN will demand the innermost layers of the vertex detectors to sustain fluences of about 10(16) hadrons/cm(2). Due to the high multiplicity of tracks, the required spatial resolution and the extremely harsh radiation field new detector concepts and semiconductor materials have to be explored for a possible solution of this challenge. The CERN RD50 collaboration "Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders" has started in 2002 an R&D program for the development of detector technologies that will fulfill the requirements of the S-LHC. Different strategies are followed by RD50 to improve the radiation tolerance. These include the development of defect engineered silicon like Czochralski, epitaxial and oxygen-enriched silicon and of other semiconductor materials like SiC and GaN as well as extensive studies of the microscopic defects responsible for the degradation of irradiated sensors. Further, with 3D, Semi-3D and thin devices new detector concepts have been evaluated. These and other recent advancements of the RD50 collaboration are presented and discussed. (c) 2005 Elsevier B.V. All rights reserved.

Thin epitaxial layers grown on Cz substrates (Epi) and high resistivity Cz diodes have been irradiated with fluences of 2 x 10(14) cm(-2) 24 GeV protons. It is shown that the differences in the changes observed in the effective doping concentration (N-eff) after irradiation of Epi silicon can be explained by the balance between the formation of two type of defects-a deep acceptor (the I center) and a shallow donor (the BD complex). The BD concentration in Epi material is evaluated to be similar to 1.3 x 10(12) cm(-3). (c) 2005 Elsevier B.V. All rights reserved.

Various types of silicon material and silicon p(+)n diodes have been treated to increase the concentration of the oxygen dimer (02) defect. This was done by exposing the bulk material and the diodes to 6 MeV electrons at a temperature of about 350 degrees C. FTIR spectroscopy has been performed on the processed material confirming the formation of oxygen dimer defects in Czochralski silicon pieces. We also show results from TSC characterization on processed diodes. Finally, we investigated the influence of the dimer enrichment process on the depletion voltage of silicon diodes and performed 24 GeV/c proton irradiations to study the evolution of the macroscopic diode characteristics as a function of fluence. (c) 2005 Elsevier B.V. All rights reserved.

We present the nonlinear I-V characteristics of a nanoscale metal-oxide-semiconductor field-effect transistor in the Landauer-Buttiker formalism. In our three-dimensional ballistic model the gate, source, and drain contacts are treated on an equal footing. As in the drift-diffusion regime for ballistic transport a saturation of the drain current results. We demonstrate the quantum mechanism for the ballistic drain current saturation. As a specific signature of ballistic transport we find a specific threshold characteristic with a close-to-linear dependence of the drain current on the drain voltage. This threshold characteristic separates the ON-state regime from a quasi-OFF-state regime in which the device works as a tunneling transistor. Long- and short-channel effects are analyzed in both regimes and compared qualitatively with existing experimental data by Intel [B. Doyle , Intel Technol. J. 6, 42 (2002)]. (c) 2005 American Institute of Physics.

X- and Q-band electron paramagnetic resonance (EPR) investigation of different crystalline Alq(3) (tris(8-hydroxyquinoline)aluminium (III)) fractions formed by a train sublimation method are reported. Several paramagnetic defect centres corresponding to 1/2, 1, and 3/2 spin are observed at room temperature. Their intensity is dependent on the temperature, nature of the crystalline phase, and preparation conditions. Spectra simulation and analysis based on the spin Hamiltonian appropriate to a high spin system (S >=, 1) suggest the existence of randomly oriented triplets and quartets in annealed Alq(3) fractions. The crystalline Alq(3) phases responsible for the EPR powder spectra have been identified by transmission electron microscopy measurements performed on these sample fractions.

Addition of diamond like carbon (DLC) phase to Ag-base overlay prepared by ECR-DC hybrid sputtering was found to reduce its coefficient of friction compared to that of the substrate material in dry sliding. The coefficient of friction, tested in dry conditions, of the Ag matrix overlay decreased by sp(3)-rich C formation. It decreased from 0.72 +/- 0.05 to 0.25 +/- 0.05 by increasing the graphite/Ag target area ratio from 1/9 to 515 at low sliding speed (0.01 ms(-1)), 5 N load in dry sliding. Its drastic decrease was found when the grain size of the prepared films was lower than 10 mn as measured by transmission electron microscopy (TEM). The prepared coatings were characterized also by Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and atomic force microscopy (AFM). (c) 2005 Elsevier B.V. All rights reserved.

Fe-sheathed tapes of MgB2 with addition of SiC have been prepared by the powder-in-tube (PIT) method using a powder mixture mechanically milled in an argon or hydrogen atmosphere. The reactivity of the component powders in the mixtures, Mg, B and SiC, was significantly enhanced by mechanical milling, so that MgB, formed faster than in tapes fabricated from a hand-milled mixture. At the same time the number of impurity phases was higher, and the impurity phases of Mg2Si and Fe2B were observed only in these tapes. Mechanical milling also induced a lower quality of MgB2, especially for tapes produced from powders milled in a hydrogen atmosphere, with a significant influence on the values of the critical temperature T-C and critical field, J(C). (c) 2005 Published by Elsevier B.V.

Simultaneous discharges of Fe and Cu metal sources by thermoionic vacuum arc (TVA) were used as a new procedure for preparation of granular Fe-Cu thin films with sensibly enhanced room temperature giant magnetoresistive effects. The possibilities to produce thin films with a wide range of structural and magneto-transport characteristics are emphasized. The structural aspects of granular Fe-Cu thin films deposited onto Kapton are investigated via X-ray diffractometry, atomic force microscopy and Mossbauer spectroscopy. The magneto-transport properties are strongly related to composition, structure and magnetic interactions inside the films. (c) 2005 Elsevier B.V. All rights reserved.