National Institute Of Materials Physics - Romania

The (Zr0.8Sn0.2)TiO4 material (ZST), has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1260-1320 degrees C for 2 h. The effects of sintering parameters like sintering temperature (T-s) and MgO addition (0.2 wt.%) on structural and dielectric properties were investigated. Bulk density increases from 4900 to 5050 kg/m(3) with the increase of sintering temperature. The effect of MgO addition is to lower the sintering temperature in order to obtain well sintered samples with high value of bulk density. The material exhibits a dielectric constant epsilon(r) similar to 37 and high values of the Q x f product, greater than 45,000, at microwave frequencies. The dielectric properties make the ZST material very attractive for microwave applications such as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc. (c) 2006 Elsevier B.V. All rights reserved.

Solid solutions Ba-1 (-) xSrxTiO3 (BST) are of high technological importance, particularly in microwave domain. Barium titanate has "naturally" three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (0) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 degrees C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range (+/- 200 degrees C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic-tetragonal-rhombohedral, phase orthorhombic being excluded. Compositions with x >= 63%, Sr shows only one peak value corresponding to a genuine transition cubic-rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x >= 80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported. (c) 2006 Elsevier B.V. All rights reserved.

Shallow defect levels in floating zone (FZ) and diffusion oxygenated FZ (DOFZ) silicon, before and after irradiation with a Co-60 gamma-source up to 300 Mrad, have been studied by thermally stimulated currents (TSC) and deep level transient spectroscopy (DLTS) in the temperature range 4.2-110 K. Besides vacancy oxygen (VO) and interstitial-substitutional carbon (CiCs) emissions, several TSC peaks have been observed. A trap with an activation energy of 11 meV has been observed at 6 K only in irradiated DOFZ. Two hole traps at 80 meV and 95 meV have been observed both in irradiated FZ and DOFZ, while a trap at 100 meV, related to an interstitial-oxygen (IO2) complex, has been revealed only in irradiated DOFZ. A TSC peak close to 24 K has been resolved into two components, whose concentrations are independent of irradiation fluence: a trap at 55 meV and a level which remains charged after emission at 80 meV. Our measurements confirm the formation, only in DOFZ, of a radiation induced donor at 230 meV. It appears to be responsible for the improved radiation hardness of oxygenated Si together with the suppression of deep acceptors, since no shallower radiation-induced donors have been detected in DOFZ samples.

PbZr1-XTiXO3 (PZT) film was prepared by addition of polymer polyvinylpirrolidone (PVP) in the PZT precursor and hydrolysis of the PZT precursor sol. Porous thin films were prepared by spin coating on Pt-coated Si substrate. The structure of the films was investigated by X-ray diffraction (XRD). (100)-oriented PZT 20180 film was obtained by hydrolysis of sol. PVP addition leads to the (110)-orientation of the film. The surface microstructure of the films was characterised with Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The surface profile indicates a films roughness of 5 nm. No micro-cracks were observed on film surface. The average remnant polarization and the coercive field were obtained from the hysteresis loop measurements. The ferroelectric behavior has been evidenced by the presence of the butterfly shape of the capacitance-voltage characteristics. Experimental results show that the dielectric constant of the PZT porous film is lower than that of the dense film.

Solid solutions of the Ba1-xSrxTiO3 (BST) type are very attractive for applications in information technology, but also in microwaves for such electrically controlled devices as phase shifters, tunable filters, steerable antennas, varactors, etc. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard powder technology-solid state reaction and sintered at 1230 degrees C and 1260 degrees C. Dielectric permittivity and loss at low (1 kHz) and high frequency (1 divided by 2 GHz) were measured on a large temperature range - 200 degrees C divided by + 200 degrees C. The influence of Sr content and of I wt.% MgO and I wt.% MnO2 doping on the complex dielectric constant was studied. The dielectric permittivity at high frequencies and room temperature considerably decreases with the increase of Sr content. Similarly, low frequency measurements showed a severe and almost linear decrease of the Curie Point with the increase of Sr fraction. At low Sr content, the measured ceramic densities slowly decrease with Sr fraction and represent about 92% of the X-ray density rho(g/cm(3)) approximate to 5.99-0.99x. At x=90% Sr concentration, very low-density ceramics were obtained with both sintering temperatures. (c) 2005 Elsevier B.V All rights reserved.

Band-pass filters with dielectric resonators of (Zr-0.8, Sn-0.2)TiO4 ceramic materials (ZST) were developed for X-band applications. The ZST materials exhibit a dielectric constant of about 36.8 and low loss in microwave range. A Qxf product higher than 50,000 was achieved for the ZST samples with 0.2 % wt NiO sintering addition. The external coupling was performed using microstrip lines on a substrate of 9.22 dielectric constant and 0.635 mill height. Filters with a minimum insertion loss between 0.46 dB and 1.74 dB, the bandwidth between 40 MHz and 170 MHz and with the central frequency around 10 GHz were manufactured.

Epitaxial Pb(Zr,Ti)O-3 (PZT) thin films with thicknesses in the range of 50-200 nin and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD). The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0)SrTiO3 (STON). SrRuO3 (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the ext,ended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance-voltage (C-V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT. (c) 2005 Elsevier B.V. All rights reserved.

La2/3Ca1/3MnO3 (LCMO) nanowires were prepared using a sol-gel process and an anodically oxidized aluminum template. Scanning electron microscopy (SEM) shows that the nanowires are almost parallel and their diameter is around 40 nm. The temperature dependence of the electron spin resonance (ESR) linewidth is similar to that observed for the case of nanostructured LCMO. From this dependence, the determined magnetic transition temperature, T-C, of the LCMO nanowires, is of the order of 120-130 K.

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.