Publications

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.

The lead zirconate-titanate (PZT) epitaxial thin films are analyzed as being p-type, wide-gap semiconductors with standard Schottky contacts. The results of the hysteresis, capacitance-voltage (C-V), and current-voltage (I-V) measurements are explained in the frame of the classical model for metal-semiconductor Schottky contacts, in which the effect of the ferroelectric polarization on band-bending was considered. The presence of the deep traps was also considered and their effect on the measured quantities was discussed. The free carrier concentration was estimated to about 3 x 10(18) cm(-3), with a remnant polarization of 40 mu C/cm(2), and with an effective density of the fixed charges in the depletion region of about 1.8 x 10(19) cm(-3).

Mg-Ni-Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg88Ni11Fe1 sample (A) was prepared by melt spinning Mg-Ni-Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH2)(88)Ni11Fe1 sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH2 powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas-solid reaction controller at temperatures in the range 520 divided by 590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mossbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mossbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg2Ni grain boundaries, with catalytic effect on the gas-solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.

Irradiation of YBa(2)Cu(3)(Li)O(7-delta) ceramics with thermal and epithermal neutrons at fluences higher than 10(17) neutrons/cm(2) has led to a spectacular improvement of the intragranular properties. The critical current density has shown a monotonous dependence. The suppression of the critical temperature was below 2% but shows an increase at 9.8x10(17) neutrons/cm(2). This behaviour is consistent with the appearance of self-organized population of defects at high fluence. An analysis of the nature of the defect distribution, which is responsible for these effects, is provided.

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.

Irradiation of YBa2Cu3(Li)O7-delta ceramics with thermal and epithermal neutrons at fluences higher than 10(17) neutrons/cm(2) has led to a spectacular improvement of the intragranular properties. The critical current density has shown a monotonous dependence. The suppression of the critical temperature was below 2% but shows an increase at 9.8x10(17) neutrons/cm(2). This behaviour is consistent with the appearance of self-organized population of defects at high fluence. An analysis of the nature of the defect distribution, which is responsible for these effects, is provided.

A microscopic theory is used to study the optical properties of semiconductor quantum dots. The dephasing of a coherent excitation and line shifts of the interband transitions due to carrier-carrier Coulomb interaction and carrier-phonon interaction are determined from a quantum kinetic treatment of correlation processes. We investigate the density dependence of both mechanisms and clarify the importance of various dephasing channels involving the localized and delocalized states of the system.

Investigations on compact devices developed for the second (2G) and the third (3G) mobile communications systems are presented in this paper. Four-pole and six-pole filters were designed by cross-coupling new compact planar resonators. The coupling constants were investigated as functions of coupling gaps. Several models of the pass-band filters were manufactured and characterized. They exhibit steeper responses due to the cross-couplings, which introduce transmission zeros on each side of the pass-band. Moreover, due to their shapes, the resonators exhibit a first,high order response at more than twice the fundamental frequency, therefore the filters show an enlarged rejection band. The measured filter responses are in a good agreement with the simulated responses. The developed filters are compact, cost-effective and suitable to be designed on high dielectric constant ceramic substrates, or for high temperature superconducting (HTS) circuits.

This paper presents a study of the nonhomogeneities in doped meta-dinitrobenzene (m-DNB) crystalline films. These growth defects lying in the film plane with dimensions smaller than the films thickness are correlated with the effect of the dopant on the crystallization regime in the dendritic growth conditions. To evaluate the dimension of the growth nonhomogeneities we have used a model based on the Kubelka-Munk theory, emphasising the general constraints imposed to the sample and the experimental set-up, and developing a more complex model through a step-by-step relaxation of these constraints, that ensures a better approximation for our experimental configuration. We have deduced higher scattering coefficients in m-DNB doped with 1,3 dihydroxybenzene (resorcinol) than in m-DNB doped with 8-hydroxyquinoline (oxine) films prepared by a rapid thermal solidification process. We also have emphasised that for the films of m-DNB doped with oxine the size of the scattering centers is larger than the wavelength of the radiation and in films of m-DNB doped with resorcinol the radiation scattering corresponds to a multiple scattering process leading to the superposition of different scattering mechanisms on centers with different dimensions.

Thin films of MgB2 on r-cut Al2O3 substrates have been grown by pulsed-laser deposition (PLD) using a Nd-YAG laser (fourth harmonic-266 nm) instead of the popular KrF excimer laser. The growth window to obtain superconducting films is laser energy 350-450 mJ and vacuum pressure with Ar-buffer gas of 1-8/10 Pa (initial background vacuum 0.5-1 x 10(-3) Pa). Films were deposited at room temperature and post-annealed in situ and ex situ at temperatures of 500-780 degrees C and up to 1 h. Films are randomly oriented with maximum critical temperature (offset of resistive transition) of 27 K. SEM/TEM/EDS investigations show that they are mainly composed of small sphere-like particles (<= 20 nm), and contain oxygen and some carbon, uniformly distributed in the flat matrix, but the amount of Mg and/or oxygen is higher in the aggregates-droplets (100-1000 nm) observed on the Surface of the film's matrix. Some aspects of the processing control and dependences on film characteristics are discussed. The technique is promising for future development of coated conductors.