Publications

We demonstrate the application of the Landauer-Buttiker formalism (LBF) to MIS (metal-insulator-semiconductor)-type quantum nanostructures and analyze their static and dynamical response properties. For the time-dependent problem, we employ the random phase approximation (RPA) and calculate the irreducible polarization from the one-particle scattering wave functions in the Hartree approximation. For their calculation, we employ the R-matrix approach which we have developed over the past years. We find in the low frequency limit a reasonable agreement with the static Hartree calculations and experimental capacitance data. (C) 2003 Published by Elsevier B.V.

Recent developments of lithographic techniques as well as improved chemical synthesis methods allow researchers to engineer novel nanostructured materials consisting of arrays of self-organized nanocrystals and multilayers grown as patterns on different substrates. In our case, the magnetic nanostructures consist either of multilayers directly deposited on pre-patterned substrates to form regular arrays of stripes and grooves or colloidal solutions of self-organized bimetallic Ag/Co nanoparticles on patterned and nonpatterned substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed in order to study the surface morphology of the 2D patterning arrays and the 3D nanostructures. The development of periodic arrays of magnetic patterns of micrometer size is strongly dependent on technological parameters such as: film, thickness, distances, size and shape of the patterns. Moreover, it is shown that the substrate morphology significantly affects the colloidal crystallization of magnetic nanoparticles and leads to different growth modes. This will ultimately affect the overall magnetic behavior of the nanostructures. Consequently, the combination of self-assembly and patterning allows for the controlled fabrication of the novel magnetic nanostructures at a macroscopic level and the study of fundamental aspects in magnetism such as quantum tunneling magnetization and magneto-transport properties along well-defined nanosized patterns. (C) 2003 Elsevier B.V. All rights reserved.

For the LHC upgrade (fluences up to 10(16) p/cm(2)) epi-Si devices are shown to be a viable solution. No type inversion was measured up to 1.3 x 10(15) 24 GeV/c protons/cm(2) and the charge collection efficiency (CCE) remained close to 100%. For reactor neutrons CCE was measured to be 60% at 8 x 10(15) n/cm(2). Annealing measurements have shown that only moderate cooling during beam off periods would be necessary. As a tentative explanation for the superior quality of these devices, we assume that radiation-induced donor generation leads to compensation effects of deep acceptors. In the future, we will extend the experiments to fluences up to 10(16) p/cm(2) and use also different variants of the epi-Si material and device geometry. (C) 2003 Elsevier B.V. All rights reserved.

Two types of solid solutions (i.e, Sn1-xCux/3Sb2x/3O2 and Cu1-xSb2(1-x)Sn3xO6 mixed crystals), have been experimentally evidenced over a large concentration range, in the SnO2 - CuSb2O6 binary system, For these two types of solid solutions, the measured values of the unit cell parameters obey Vegard's rule. The electric and magnetic properties of the Sn0.50Cu0.167Sb0.33O2 and Cu0.923Sb1.836Sn0.231O6 mixed crystals, with compositions placed at the maximum solid solubility limit, on either side, were evaluated over 77 to 1123 K and 2 to sample exhibited semiconducting behaviour, with 300 K, respectively. Sn0.50Cu0.167Sb0.33O2 sample exhibited semiconducting behaviour, with activation energy E-d (eV) and Seebeck coefficient (muV/grd) values close to the values reported for metals, while the Cu0.923Sb1.836Sn0.231O6 sample can be treated as insulators. Short - range magnetic ordering has been evidenced at about 60 K for the CuSb2O6 binary compound and Cu0.923Sb1.836Sn0.231O6 mixed crystal, wiib calculated magnetic moments in good agreement with literature data. In contrast, the Sn0.50Cu0.167Sb0.33O2 mixed crystal presented only a paramagnetic behaviour.

Films of (119) Bi-2223 high-T-c superconductor, potentially useful for future sandwich-stacked structures exhibiting Josephson effect have been prepared by MOCVD on (100) NdGaO3 and (110) SrTiO3 flat and vicinal substrates with different off-angles. It is shown that off-angle is a key parameter in growth control mechanism of the films. In-plane aligned films, with regular morphology and low roughness as well as having maximum zero-resistivity critical temperature: of T-c0=67.2K and T-c0=74K when "single" and "two"-temperature growth routes are used, respectively, have been obtained for high of-angles (20 degrees). Growth mechanism is changing from a two-dimensional type for the flat substrate to a step-flow one for vicinal substrates.

Field effect assisted Thermally Stimulated Currents (TSC) measurements were used to differentiate between electron and hole traps in CdS thin films deposited on SiO2/Si substrate. The electrode configuration was designed as for a pseudo-MOS structure, with drain and source contacts on the CdS film surface and using the Si substrate as gate electrode. Electron or hole traps will be favored to fill-up depending on the polarity of the gate voltage applied during illumination at low temperatures. Collection of the thermally released electrons or holes in the drain circuit will depend also on the polarity of the gate voltage applied during heating at constant rate. Comparing the results with those obtained in case of US films deposited on glass, it is possible to differentiate between SiO2/CdS interface traps and bulk CdS ones.

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.

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.

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.