National Institute Of Materials Physics - Romania

Thermally stimulated current (TSC) measurements performed in the 100 K-400 K temperature range on Bi4Ti3O12 (BiT) thin films annealed at 550 degrees C and 700 degrees C had revealed two trapping levels having activation energies of 0.55 eV and 0.6 eV. The total trap concentration was estimated at 10(15) cm(-3) for the samples annealed at 550 degrees C and 3 x 10(15) cm(-3) for a 700 degrees C annealing and the trap capture cross-section was estimated about 10(-18) cm(2). From the temperature dependence of the dark current in the temperature range 20 degrees C-120 degrees C the conduction mechanism activation energy was found to be about 0.956-0.978 eV. The electrical conductivity depends not only on the sample annealing temperature but also whether the measurement is performed in vacuum or air. The results on the dark conductivity are discussed considering the influence of oxygen atoms and oxygen vacancies.

Soft type piezoceramics with stoichiometric composition Pb(Nb0.02Li0.007Zr0.51Ti0.463)O-3 were prepared by the usual ceramic technique. Several samples with lead oxide in excess of up to 4% wt, were prepared. Careful measurements of the density of the sintered disc-shaped materials and the piezoelectric properties (piezoelectric voltage constant, g(31), piezoelectric strain constant, d(31), dielectric permittivity,epsilon(r), electromechanical planar coupling coefficient, k(p), mechanical quality factor, Q(m)) were carried out. All piezoelectric properties were spectacularly enhanced by addition of small amount of lead oxide. The maximum values were reached for samples containing 2-3 wt.% PbO, regardless the sintering temperature. The experimental results were discussed in terms of the two competitive processes: structural rearrangement and dissolution-reprecipitation, which are active in the sintering process in the presence on the excess PbO liquid phase. The last process seems to be dominant and explains the improvement on the piezoelectric properties of PZT materials.

Cadmium sulphide (CdS) thin films were deposited on glass substrate by precipitation from aqueous solution technique. The films were doped with copper using the direct method consisting in the addition of a copper salt (CuCl(2)) in the deposition bath of CdS. The doped films were annealed in air, at 300 degrees C, for 1 h. We report some structural, optical, electrical and photoelectrical properties of CdS thin films before and after Cu doping, correlated with investigation of trapping levels by Thermally Stimulated Currents (TSC) method. (C) 1999 Elsevier Science S.A. All rights reserved.

The effect of niobium and lithium doping on the structure and piezoelectric properties of some samples with the following compositions PbZr0.051 Nb3Li)(x)Ti0.49-xO3, with 0.015 less than or equal to x less than or equal to 0.050, was investigated. The materials were prepared by the usual ceramic technique using high purity raw materials. Disc shaped samples of each composition were sintered at temperatures between 1200 and 1300 degrees C, for 4 h. The samples structure was determined by X-ray diffractometry and the piezoelectric properties were determined by means of resonance-antiresonance method, using a HP 4194A impedance gain phase analyser. A morphotropic phase boundary (MPB) with the optimum piezoelectric properties was found around the compositions with x=0020. The optimum values for relative dielectric constant epsilon(r), planar electromechanical coupling factor k(p), and mechanical quality, factor Q(m) of about 1700, 0.59 and 75, respectively, were found for these compositions. The lattice constants a and c of rhombohedral and tetragonal phases were determined by means of an X-ray CuK alpha diffractometer. The results are discussed in terms of the domain easy switching phenomena and high poling degree, brought about by the A-vacancies produced by the NbLi pairs doping. (C) 1999 Elsevier Science Limited. All rights reserved.

Three ceramic magnetic compositions within the CuZnTi ferrite system, having the Curie temperatures centered around 60, 80 and 100 degrees C, respectively, were investigated in order to determine the time-temperature stability of their main magnetic properties. Permeability and the slope of permeability around their Curie points were determined on ring shaped samples, before and after being subjected to long term (over 6000 h) ageing at their Curie temperature and for a shorter time of about 50h at higher temperatures up to 700 degrees C. Rates of changes of 33, 66 and 100 ppm h-l of the maximum slopes of permeability were found for the samples aged at 60, 80 and 100 degrees C, respectively. The samples aged at temperatures higher than 200 degrees C show a rather sudden decrease of maximum slope of permeability, but no shift of the Curie temperature and the working point, corresponding to the temperature where the slope has the maximum value. This is the most interesting result as concerns the use of such magnetic temperature sensors for the construction of highly sensitive temperature controllers, for example ultrathermostats. The results are discussed in terms of the migration processes of the cations, especially Cu2+,from metastable positions on which they were frozen during the rapid cooling of the sample from the sintering temperature, to the more stable ones, namely the octahedral sites into the spinel lattice. (C) 1999 Elsevier Science Limited. All rights reserved.

Ferroelectric Bi4Ti3O12 (BiT) thin films of different thicknesses were deposited on p-type Si substrates using the Chemical Solution Deposition (CSD) method. The films were crystallized by the conventional thermal annealing for 30 min at temperatures in the 500-700 degrees C range. It was found that the shape of the photoconductive signal spectral distribution is dependent on the film thickness. For thin films (150 nm) four peaks were observed (400, 500, 860 and 1075 nm) and the photoconductive signal occurs only if the Si substrate is negatively biased. For thicker films (500 nm) only two peaks were observed (370 and 1075 nm) and the photoconductive signal occurs no matter the polarity of the applied voltage. (C) 1999 Elsevier Science Limited. All rights reserved.

Superconductor-ferroelectric heterostructures were prepared by deposition of ferroelectric doped-PZT type material on the top of the superconductor YBCO thin films. Both, ferroelectric and superconductor thin films were obtained by off-axis RF magnetron sputtering method with a single target. On these heterostructures, measurements were made in order to evidence the influence of contact between the two materials on the electrical and thermal parameters. X-ray diffraction patterns of superconductor thin film show a good crystallization c-axis oriented perpendicular to the substrate plane, and a perovskite structure without traces of pyrochlore at 550 degrees C the ferroelectric thin film deposited by RF magnetron sputtering (off axis method). Important variations of the critical temperature of superconductor films have been obtained, due to the increased dielectric permittivity (epsilon).

Photoconductive properties of SrBi2Ta2O9 thin films in the 250-400 nm wavelength range were investigated. The films were deposited on Pt/SiO2/Si substrates using the Chemical Solution Deposition and were crystallized by conventional thermal annealing at 850 degrees C. One sensitivity maximum was observed in the spectral distribution of the photoconductive signal and was attributed to band-to-band generation in the film. The wavelength corresponding to this maximum was found to be dependent on the applied voltage, on the delay time, defined as the time between the application of light on the film and the reading of the generated photocurrent, and on the wavelengths sweeping direction tug or down). The gap value was estimated to be around 3.94-4 eV. (C) 1999 Elsevier Science Limited. All rights reserved.

Bismuth titanate ferroelectric thin films were deposited on platinum coated silicon using chemical solution deposition. Photoconductive and photovoltaic effects were observed both in continuous and modulated light. With the increasing of the annealing temperature the maximum of the photoconductive signal spectral distribution is moving toward longer wavelengths, from 340 nm to 370 nm. The existence of the photovoltaic effect could be due to some potential barriers existing at the grain boundaries and me enlargement of its spectral distribution toward longer wavelengths could be due to the presence of some traps at the grain boundaries.

An analytical formula for the optical charging spectroscopy (OCS) discharging current in the case of homogeneous semiconductors is deduced. The computation shows that the OCS current is proportional with the square of the initially trapped carrier concentrations. This result leads to a higher sensitivity in case of the OCS method compared with the thermally stimulated current method. The OCS current formula obtained for one trapping level is generalized in the case of many trapping levels. From the generalized formula it can be seen that the OCS current can change the sign if both electron and hole traps exist in the sample. (C) 1998 American Institute of Physics.