Publications

The time evolution of optically excited carriers in semiconductor quantum wells and quantum dots is analyzed for their interaction with LO phonons. Both the full two-time Green's function formalism and the one-time approximation provided by the generalized Kadanoff-Baym ansatz are considered, in order to compare their description of relaxation processes. It is shown that the two-time quantum kinetics leads to thermalization in all the examined cases, which is not the case for the one-time approach in the intermediate-coupling regime, even though it provides convergence to a steady state. The thermalization criterion used is the Kubo-Martin-Schwinger condition.

Samples of RFe3 (R = Dy, Sm, Y) were doped with Si, Al and Ti and studied by Mossbauer spectroscopy and X-ray diffraction. Most Mossbauer spectra were analyzed with three sextets, corresponding to the 6c, 3b and 18 h inequivalent lattice sites for iron. A quadrupole split doublet was also present in some spectra, which became dominant for the case of A] substitution. The collapse of the hyperfine magnetic structure is a magnetic effect due to substitutions: the X-ray diffraction patterns yielded a particle size of about 100 nm, thus precluding the occurrence of superparamagnetism in these systems. (c) 2005 Elsevier Ltd. All rights reserved.

Detailed knowledge of the type and strength of pair interactions between high-spin metal ions is paramount to the understanding and design of molecular magnetic materials. In this work, the anisotropic magnetic interactions in a beta-diketonate-alkoxide iron(III) dimer compound, [Fe-2(OCH3)(2)(dbM)(4), Hdbm = diberizoyhriethane] (Fe-2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95 GHz). The diamagnetic Substitution method. was employed using the isomorphous gallium(III)-based compound doped with iron(III) to produce Ga-Fe dimers(GaFe). The single-ion zero-field splitting(ZFS) tensor could be separately determined in GaFe with the iron ion in a local environment quasi-identical to the one in Fe2. Its principal directions are found to point in arbitrary directions, uncorrelated with the Fe-O bonds. The Fe2 EPR spectra consist of transitions within the lowest multiplet states S = 1, 2, 3, which were analyzed using the full Spill Hamiltonian description of all exchange coupled pair of s = 5/2 spins. The anisotropic spin-spin interaction tensor of Fe2 possesses a principal axis close to the Fe-Fe direction and was shown to arise both from through-space (dipolar) and through-bond (anisotropic exchange) contributions. The latter involves ail rhombic component J(E) = (J(X) - J(Y))/2 approximate to 0.093 cm(-1) of magnitude comparable. to the dipolar interaction, and even to the rhombic part of the single-ion ZFS (E = 0.097 cm(-1)). Our results show that the anisotropic exchange, usually neglected for S-type ions, is significant for the anisotropic interactions in exchange-coupled iron(III) clusters, including the Fe-4 and Fe-8 families of single-molecule magnets and the anti Ferrornagnetic iron wheels. (c) 2005 Elsevier Inc. All rights reserved.

The crystal field disorder in some tetragonal and trigonal Ca-gallo-germanates, i.e., A(2)Fe(2)ZO(7) and A(2)LnFe(3)Ge(3)O(14), with ACe=CeBa or Sr, LnCe=CeLa or Nd and ZCe=CeGe or Si, has been studied by Fe-57 Mossbauer spectroscopy. The observed distribution of the hyperfine fields is analysed from a theoretical model based on the additive perturbation approximation of crystal fields, a rigid lattice of ionic point charges and the random substitution of Fe3+/Z(4+) and A(2+)/Ln(3+).

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).