National Institute Of Materials Physics - Romania

Designs of new compact filters using cross-coupled resonators are presented. The square shape resonators require only 40% of the area of folded open loop half-wavelength resonators. The filters were designed for the 900 MHz band of the mobile communications systems (GSM, GPRS, etc.). The cross coupling provides a better filter skirt for a higher selectivity. The effect of the resonator finite quality factors was confirmed for the cross-coupled filters.

We study the transport properties of metal-insulator-semiconductor-type quantum devices with a back gate located close to the field induced two-dimensional electron gas (2DEG). These devices allow for a detailed analysis of the coupling between the quantum system and the contacts (top- and back gate) which we consider as classical particle reservoirs with infinite conductivity. The transport properties are described in the Landauer-Buttiker formalism and the Hartree approximation is used for the self-consistent calculations of the ground state. Decreasing the width of the insulator barrier, at low temperatures a leakage tunneling current starts to flow. The states of the 2DEG provide a resonant tunneling level for the vertical transport between back- and top gate. We analyze the effect of the resonance on the current and on the capacitance of the system. Analytical calculations based on our previous theory on resonant transport allow for a better numerical implementation of the model and for a detailed understanding of the electronic states. (C) 2003 Elsevier Ltd. All rights reserved.

Two series of PT-type materials doped with Eu+3 with the formula PbxEuyTi0.98Mn0.02O3 (x=0.98, 1.00 and y=0.00, 0.02, 0.04, 0.08, 0.12) were prepared by conventional ceramic technique. The compounds with compensated and uncompensated valences were sintered between 1150 and 1220degreesC. The two types of compounds and the content of the donor ion, Eu+3 , influence the mechanical and dielectric properties of sintered samples. The spectra and data obtained on Eu-151 Mossbauer measurements are discussed.

It is shown that self-organization can be revealed in non-crystalline materials. Experimental and modelling studies are discussed. The importance of the Boolchand intermediary phase for the chalcogenide network glasses with self-organization is pointed out. The basical principles of self-organization are the key for the production of nano-materials with new properties.

Hybrid polymer based on selenium-sulphur doped by paraffins has been deposited by pulsed laser deposition. The properties of the thin films were investigated. Sample stabilization was carried out by annealing. A nano-crystalline fraction was obtained in the films annealed at 100degreesC. The structure of the nanocrystals corresponds to the Se3S5 crystallographic form. The films exhibit optical memory properties and are suggested for use in optical phase change memories.

The thermal stability of amorphous Al2O3 films (similar to8 and 80 nut thick) deposited by atomic layer deposition on HF-last and thin SiO2 covered (001) Si substrates is studied by transmission electron microscopy. The layers are in- and ex-situ annealed in the same temperature range.

(Ba1-xSrx)TiO3 solid solution (BST), with different molar compositions (x = 0.25, 0.5, 0.75, 0.9) Were prepared by conventional solid-state reaction from raw materials. Structural parameters, crystallite sizes and bulk densities were determined. Perovskite type polycrystalline structure of the BST ceramics was revealed by X-ray diffraction (XRD) data. The dependence of permittivity and losses at low frequency (1 kHz) was analyzed. The microwave investigations at room temperature revealed dielectric constant around 1000 and loss smaller than 1% at 1.1 GHz. The results indicate that some BST dielectric ceramics are suitable in paraelectric phase for microwave devices. The dielectric constant can be adjusted by using a DC-bias field and such materials are appropriate for manufacturing electric field controlled components such as resonators, phase shifters etc.

Starting from the idea that energies of adsorption, dissociation and surface reactions, involved in gas interaction with SnO2 chemoresistive sensors, are gas-specifics, we present a new method for improving selectivity, which consist in combining the variations of two physical parameters: thermal effect Delta T and electrical resistance R.

As-grown 4H-SiC epitaxial layers were investigated by deep-level transient spectroscopy to study the formation of the well-known Z(1,2) defect with energy levels normally detected at about E-C-0.7 eV. Chemical vapor deposition, applying various nitrogen-doping concentrations and C/Si ratios (1.2-3) in the gas phase, was used to prepare the samples. The Z(1,2) defect concentration was observed to increase with the incorporated nitrogen concentration. The dependence was linear for medium C/Si ratios (1.5-2.5). The highest and lowest applied C/Si ratios (3 and 1.2) enhanced and suppressed the Z(1,2) defect formation, respectively. This behavior tentatively suggests a complex of nitrogen with interstitial carbon atoms or, less probably, silicon vacancies. In particular, the correlation between the Z(1,2) defect formation and the nitrogen incorporation was clearly shown in the present investigation, in contradiction to conclusions of other authors. Previously reported negative-U properties of the Z(1,2) deep-level defects could be confirmed. A 1:1 relation between the concentrations of Z(1) and Z(2) was obtained for the present as-grown epitaxial layers. (C) 2002 American Institute of Physics.

The DyNi5-xAlx compounds crystallize in CaCu5-type structure for an aluminum content xless than or equal to1.5 and in HoNi2.6Ga2.4 type for xgreater than or equal to2. The saturation moments per formula unit extrapolated at 0 K, in composition range x<2 are similar to9.2 mu(B) in agreement with x-ray photoelectron spectroscopy studies which show negligible crystal field effects. For xgreater than or equal to2 a mictomagnetic type contribution to magnetic ordering develops which becomes dominant only for the DyNi2Al3 compound. Band structure calculations on DyNi5-xAlx show that the Dy 5d band is polarized parallel to the 4f moment. For DyNi5 the nickel moments at 2c and 3g sites are antiparallel oriented to that of Dy. The nickel contributions to magnetization decrease when increasing aluminum content and are near nil for xgreater than or equal to2. Above the Curie points the reciprocal susceptibilities follow linear dependencies. The effective nickel moments decrease from 2.11 mu(B)/atom (x=0) to 0.87 mu(B)/atom (x=2.5). The magnetic behavior of nickel is analyzed in models which take into account the electron correlation effects in d bands. (C) 2002 American Institute of Physics.