Publications

The axially symmetric Fe+(II) center observed in SrCl2:Fe2+ crystals has been studied by the electron nuclear double resonance (ENDOR) technique in the microwave X and Q bands. This center is produced only in crystals grown in chlorine atmosphere and x-ray irradiated at low temperature (below 100 K). The analysis of the ENDOR spectra unambiguously confirms the structural model earlier suggested by the electron paramagnetic resonance (EPR) study, as a Fe+ ion strongly displaced off center along an axis, almost in the center of the square determined by the four nearest Cl- ligands. The unpaired spin densities f(3s) and f(3p) in the 3s and 3p chlorine orbitals have been determined from the resulting ENDOR parameters.

Lead zirconate-titanate (PZT) thin films with Zr/Ti ratio of 92/8 were deposited by sol-gel on Pt coated silicon and single crystal MgO(100) substrates. In case of the films deposited on Pt the perovskite structure is established only after oxygen annealing, while for the films deposited on MgO annealing in air is enough. The films are polycrystalline with preferred orientation that is substrate dependent ((110) on Pt and (100)/(200) on MgO). Average values of 2.10 muC/cm(2) and 78 kV/cm were obtained for remnant polarization and coercive field, respectively. The I-V characteristic at low-to-intermediate electric fields is controlled by thermionic emission, but the Schottky representation is Ln(current)-(voltage)(1/4), implying that the film behaves like a semiconductor and not like an insulator. From transmission-reflectance measurements a value of 3.85 +/- 0.05 eV was estimated for the band gap, while the refraction index in the transparency region was found to be 2.6. A pyroelectric signal was detected on as grown films, making them attractive for infrared detection. (C) 2003 Elsevier B.V. All rights reserved.

Investigations on barium neodymium titanate (BNT) prepared by solid state reaction techniques are presented in this paper. The samples were sintered at temperatures, T-s, in a range 1200-1300degreesC. High density values and adequate microwave parameters were obtained by adding 0.5 mol PbO substitutional for BaO. A decrease of the Curie temperature with the increase of the Nd2O3 concentration was confirmed. For the sample with 28 wt.% Nd2O3, 0.5 mol PbO pyroelectric data indicate Curie temperature around 75 K, therefore, at room temperatures, the materials are in paraelectric phase. BNT ceramics with high Nd2O3 concentration (28 wt.%), sintered at 1250degreesC for 2 h appear to be suitable for dielectric resonators for frequencies up to 3 GHz (epsilon(r)=80-90 and Q = 2000). (C) 2003 Elsevier B.V. All rights reserved.

Pb(Zr, Ti)O-3 (PZT) thin films with Zr/Ti ratio of 92/8 and doped with up to 4 at.% Nb were deposited by sol-gel on Pt coated Si substrates. The scope was to investigate the influence of Nb additive on the structural, optical and electrical properties of Zr-rich PZT phase. it was found that a residual pyrochlore phase stabilizes with increasing the Nb content. The remnant polarization, the coercive field, and the dielectric losses increase with increasing Nb content. The presence of the pyroelectric effect was also evidenced on as-deposited layers. The films have potential for light detection in the infrared region of the electromagnetic spectrum (pyroelectric effect). (C) 2003 Elsevier B.V. All rights reserved.

Isolated Ge islands, i.e., islands not connected by a wetting layer can be obtained by selective epitaxial growth in voids of ultrathin oxides of thickness 1-2 nm. Voids of 30-600 nm size were created before epitaxy during a high temperature anneal of the ultrathin oxide. The formation of one island per window was investigated at 700 degreesC as a function of Ge thickness and void size. Islands nucleate mainly at the edge of the void and for this reason they have an anisotropic shape. In voids smaller than 300 nm only one island is nucleated. Islands form only in voids greater than a critical size (30-80 nm) which depends on the total amount of Ge deposited. We observe that height, width, and aspect ratio of isolated islands increase with void size for a given Ge thickness. A metastable state of Ge in small windows was observed. Moreover, the Si interdiffusion is strongly reduced with decreasing island size (i.e., with void size) reaching only similar to10% in comparison with similar to50% in islands on large areas. (C) 2004 American Institute of Physics.

In this paper, electrical investigations performed on complex metal-oxide-semiconductor (MOS) structures with amorphous LaAlO3 deposited as gate oxide on a rapid thermal processed SiO2 buffer layer are presented. Current-voltage (I-V) measurements at different temperatures and high frequency room-temperature capacitance-voltage (C-V measurements indicate the presence of a non-equilibrium state in p-type Si (100) substrate under reverse polarization of the structure. The asymmetry in I-V characteristics is opposite to the expected one for the high-kappa/interfacial layer stack behavior. Different indications for the non-equilibrium MOS state are inferred from the experimental data, and the extraction of MOS physical parameters by using the classical C-V method is shown to be unreliable. For small and large applied forward voltages defect-related charge transport and tunneling currents dominate, respectively. (C) 2003 Elsevier B.V. All rights reserved.

Cadmium sulphide (CdS) thin films were deposited by chemical bath deposition (CBD) method on SiC2/Si (n-type) substrates. Approximately, 70 nm thick nano-crystalline CdS layers were obtained. Thin film field effect transistors were realised by deposition of two coplanar electrodes of Au (drain and source) on the US surface. The gate contact is aluminium deposited on the backside of the Si substrate. The drain current-drain voltage characteristics (I-d - V-d) were performed in dark. Normal field effect transistor characteristics are obtained in case of positive gate and drain voltages, the device acting as an n-channel transistor in the accumulation mode. For negative drain voltages the characteristic is dominated by space charge limited currents (SCLC). An on/off current ratio of about 10(2) is reported, this being limited in our case by geometry. (C) 2003 Elsevier B.V. All rights reserved.

The puzzling disappearance of the second magnetisation peak below a certain temperature T-0 in the case of Bi2Sr2CaCu2O8+delta single crystals is explained by considering the dynamic ordering of the vortex system caused by the current induced in the sample during standard magnetisation measurements. Another ingredient is the limitation of the second magnetisation peak to field values lower than the characteristic field B-0 for the 3D-quasi-2D dimensional crossover. It was found that due to the inherent increase of the inborn driving force with decreasing T the dynamic response of the vortex system for T less than or equal to T-0 is elastic on both sides of the limiting field B-0. Consequently, there will be no jump in the current-voltage characteristics across B-0 and the second magnetisation peak disappears. (C) 2004 Elsevier B.V. All rights reserved.

Second harmonic generation from the interface between GaAs(111)-B and an aqueous electrolyte has been investigated at a long wavelength (1064 nm) and at two shorter ones (865 and 815 nm). The signal showed the symmetry of the crystal; the isotropic amplitude was found to vary substantially with the electrode potential, indicating that a major part of the signal is generated near the interface. The potential dependence was stronger at shorter wavelengths. Impedance spectroscopy revealed the existence of surface states, which may affect the magnitude of the signal. (C) 2004 Elsevier B.V. All rights reserved.

Barium-strontium titanate (BST), ternary ceramic compounds with the molar formula (Ba1-xSrx)TiO3, were prepared by solid-state reaction from raw materials. Three compositions with x = 0.25, 0.50 and 0.75, have been checked. Samples with two type of thermal treatment at 1230 and 1260degreesC have been manufactured and analyzed. Perovskite type, polycrystalline structure of the BST ceramics was revealed by the X-ray diffraction data at room temperature. Strontium ion (Sr2+) enters substitutionally on the Ba2+ site. A small decrease in the unit cell volume with the Sr content increase was observed. Dielectric measurements have been performed at low and high frequencies. The temperature dependence of low frequency permittivity shows large interval of ferroelectric transition, which appears to be merely due to dispersion of crystallites dimensions. The increasing concentration of Sr ion leads to a shift of the Curie point below room temperature. Experimental data show a linear decrease of the Curie point T-C (degreesC); approximate to 120-360x, versus x Strontium fraction. Room temperature permittivity and dielectric loss have been measured in microwaves by using the Hakki-Coleman method. BST samples prepared using water as a milling medium exhibit lower porosity and lower dielectric loss in microwaves than samples prepared using acetone as a milling medium. Dielectric loss Of 10(-3) at 1 GHz shows that this ceramic material is quite suitable for high frequency device applications. (C) 2003 Elsevier B.V. All rights reserved.