Publications

The carrier transport mechanisms in as-grown LaAlO3 and La2Hf2O7 high-k insulator layers deposited on n- and p-Si were deduced from temperature dependent C-V and I-V characteristics correlated with photoelectric measurements. Large, parallel shifts in the high frequency C-V curves are explained by the presence of a large density of interface states and an approximate analytical formula relating the density of states to the C-V shift is deduced. The space charge limited current is explained by the existence of impurity channels situated energetically near the conduction or valence band of silicon.

Fatigue is investigated in epitaxial Pb(Zr,Ti)O-3 films grown on SrRuO3/SrTiO3 substrates with Pt or SrRuO3 (SRO) top electrodes. It was experimentally determined that fatigue occurs irrespective of whether the top electrode is Pt or SRO. The fatigue behavior is strongly dependent on the frequency. A polarization recovery was observed for both types of top electrodes, but the recovery is almost complete for a SRO top electrode and only about 40% from the initial polarization value for Pt top electrodes. The results are tentatively explained by the frequency response of the deep traps and by migration of oxygen vacancies.

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.