National Institute Of Materials Physics - Romania

In this communication we propose a further improvement of the cavity model for microstrip antennas, given by Richards et al. [1]. The main attention is paid to the calculation of the radiated fields E-0, E-phi and to the variation of the input impedance as a function of the frequency and the feed point. In our model we have suppose that the magnetic walls are not perfect and the patch acts as a cavity with the length corresponding to its resonance frequency. The validity of the proposed model was proved by comparison of theoretical computations and experimental results.

The transport properties of nanocrystalline silicon films are presented. The studies were performed on fresh and stabilized nanocrystalline porous silicon (nc-PS) and on Si/SiO2 nanocomposite films. Current-voltage characteristics and the temperature dependence of the dark current were measured on both systems. From the current-voltage characteristics of the oxidized nc-PS, the Si-SiO2 potential barrier was determined (2.2 eV). The Si/SiO2 measurements proved that the main conduction mechanism for the nanodot systems is the field-assisted tunneling under Coulomb blockade. The quantum confinement was evidenced by the activation energies observed in the current-temperature characteristics and it was modeled by means of an infinite quantum well. The trapping-detrapping processes were studied by using the Optical Charging Spectroscopy (OCS) method: the traps are filled by illumination with a suitable wavelength at low temperature and the detrapping is produced by heating at a constant rate. The experimental investigations evidenced six trapping levels in nc-PS, four located at the surface/interface and two deep ones located in the bulk. The deepest level was observed only in oxidized samples. The modeling of the OCS trapping-detrapping processes allows for the determination of the characteristic parameters that cannot be directly obtained from the measurements.

Nanocrystalline PbS thin films were grown on glass substrates using Chemical Bath Deposition (CBD) method. The presence of nanocrystals was revealed by optical absorption and photocurrent measurements. We have compared the absorption threshold between large size and nanocrystalline PbS. Our study was performed in order to demonstrate that the size of the PbS thin films crystallites affects the photoelectric properties of the material. Using the Scanning Electron Microscopy (SEM) we have determined that the grains inside large size PbS crystallites are 300 nm. From the optical absorption measurements we have determined that the size of PbS nanocrystals is between 5-50 nm. The photoconduction measurements show that large size PbS has a maximum in infrared situated at 2.4 mu m, and in PbS nanocrystalline thin film this maximum is shifted toward smaller wavelengths towards visible and near infrared.

A novel method based on thermo-ionic vacuum arc plasma was developed to grow metal-carbon zero stress films for MEMS applications. The method chosen to prepare metal-carbon films uses an electron beam emitted by an externally heated cathode accelerated by a high anodic voltage. Applying a high voltage (1-6 kV) between cathode and anode bright plasma was ignited in carbon atoms. In order to minimize the internal stress and to get films with increased adherence, interlayer and graded interlayer with Fe, Cr, Al and Ni was deposited. The prepared films were characterized by XPS, XRD, AFM and ellipsometry.

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.