Publications

Transmission electron microscopy-selected area electron diffraction studies were performed on as-pyrolyzed sol-gel-derived Pb(Zr1-xTix)O-3 films deposited on Pt electrode layers to elucidate the structural cause behind the large effects of pyrolysis conditions on orientation selection [L. F, G.J. Norga, H.E. Maes, and G. Maes, J. Mater. Res.. 16, 2499 (2001)]. The crystallinity of the intermediate pyrochlore phase, which forms during pyrolysis, was found to strongly depend oil pyrolysis conditions. Specifically, pyrolysis for 10 s at 350 degreesC was seen to result in conversion of the film to a well-crystallized, oxygen-deficient pyrochlore phase with the fluorite crystal structure (disordered pyrochlore). We speculate that formation of the metastable fluorite phase is favored by the reduced oxygen partial pressure, caused by burnoff of residual acetates. Longer pyrolysis times and/or higher pyrolysis temperatures result in a quasi-amorphous intermediate phase. The presence of the well-crystallized fluorite phase in the pyrolyzed film is seen to result in (111)-oriented films after crystallization, while pyrolyzed films consisting of a quasi-amorphous intermediate phase turn out (100) or mixed (100)/(111) oriented after crystallization. An explanation for the observed orientation effects, on the basis of the different surface energetics of fluorite versus perovskite structure oxides, is proposed.

Tl2Ba2CaCu2Oy (Tl-2212) and Tl(Ba,Sr)(2)Ca2Cu3Oy (Tl-1223) superconductor thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process namely, amorphous phase epitaxy (APE) method. In the case of Tl-1223, an amorphous layer of TlSr2CaCu2Oy (TlSr-1212) was deposited on top Of CeO2 layer to avoid chemical reaction between Tl-1223, containing Ba, and CeO2 layers. All films are epitaxial with smooth surface and excellent crystallinity. Superconducting transition temperatures (T-c) of Tl-2212 and 1223 are around 95 and 104 K, respectively. Critical current density (J(c)) at 77 K measured on 5 x 5 mm. films by inductive technique is as high as 2 MA/cm(2). Surface resistance R-s of Tl-2212 films measured by a dielectric resonator technique at 38 GHz is comparable to the best YBCO films. (C) 2003 Elsevier Science B.V. All rights reserved.

In this paper we report correlations between the structure, the photoluminescence and the transport properties of luminescent porous silicon. These correlations combined with the observed temperature dependence of tunneling characteristics yield quite a wholesome (pea-pod-like) model for this system. (C) 2002 Elsevier Science B.V. All rights reserved.

Samples of (Cu0.6C0.4)Ba2Ca3Cu4Oy ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes' the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba2Can-1CunOy series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T-c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J(c) has shown a fish-tail effect. (C) 2003 Elsevier Science B.V. All rights reserved.

Annealing effect of the irreversibility fields (H-irr) in (Cu,C)Ba2Ca2Cu3Oy ((Cu,C)-1223) and (Cu,C)Ba2Ca3Cu4Oy ((Cu,C)-1234) have been investigated. The samples prepared under high pressure were post-annealed in nitrogen gas in the temperature range of 400-500degreesC. H-irr of (Cu,C)-1234 was improved after annealing at 450degreesC while T-c remained almost constant. This suggests that carrier density of five-coordinated CuO2 planes or conductivity of charge reservoir layers is changed by annealing while keeping that of four-coordinated CuO2 planes almost constant. Both samples show H-irr similar or equal to 4 T at 90 K by the post-annealing, which is quite attractive for practical application. (C) 2003 Elsevier Science B.V. All rights reserved.

To enhance or improve critical currents density (J(c)) and irreversibility field (H-irr) of (Cu,C)Ba2Ca3Cu4O10.5-delta ((Cu,C)-1234), pinning centers were introduced by heavy-ion irradiation. The polycrystalline samples were irradiated with An(15+) ions (240 MeV energy) at various fluence of 1 x 10(11), 2.5 x 10(11) and 5 x 10(11) ions/cm(2). J(c) and H-irr were determined for the irradiated samples as well as unirradiated samples. J(c) (77 K, 1 T) increase from 3.9 x 10(4) A/cm(2) to 4.1 x 10(6) A/cm(2) for at fluence of 1 x 10(11) ions/cm(2) of heavy-ion irradiated sample and decreases with further increase of fluence. These results indicate the possibility of further enhancement of Jc and of achieving a very high H-irr of (Cu,C)-1234 below a fluence of 1 x 10(11) ions/cm(2). (C) 2003 Elsevier Science B.V. All rights reserved.

Complex semiconducting ceramic compositions were prepared. Structure, crystallite morphology, hardness and electrical conduction were investigated. After annealing at 150 degreesC a distinct crystalline phase was obtained and the ceramic became compact, hard, of higher resistivity, while preserving the basic semiconducting properties (C) 2002 Elsevier Science Ltd. All rights reserved.

Thin films of (Ca1-xBax)CuO2 (x = 0-1) have been grown on (0 0 1) SrTiO3 by metal organic chemical vapor deposition. These films are potential candidates as barrier layer for SIS and/or SNS Josephson junctions. For this purpose, the films should be flat and smooth. In this regard we have investigated the possibility of decreasing the films roughness by the control of the Ba/Ca ratio, the substrate temperature and oxygen partial pressure during the growth of the film. The minimum mean square root roughness and regular morphology are obtained for the x = 0.3-0.5. In the samples with x = 0.5 the highest intensity of the X-ray diffraction peaks for the (Ca0.5Ba0.5)CuO2 phase and the lowest for the impurity phases were attained at a substrate temperature of 750-780 degreesC and oxygen partial pressure of 15 Torr. (C) 2002 Published by Elsevier Science B.V.

Superconducting (0 0 1) thin films of SmBa2Cu3Oy have been grown by MOCVD on (0 0 1) SrTiO3 and (0 0 1) LaAlO3 substrates. The films on both substrates are smooth with RMS of 1.428 and 1.037 nm respectively, but have different morphology (revealed by atomic force microscopy) for the same synthesis conditions. The films grown on SrTiO3 have shown rectangular- or square-type terraces. The height between the neighboring terraces was corresponding to the c-axis length of one unit dell. This type of morphology is due to the two-dimensional nucleation and growth mechanism. In the case of the films on the LaAlO3 the surface morphology consisted of irregular grains (close to spherical). For both situations we have not found any evidence for a spiral growth mechanism. Therefore, our films are suitable for electronics applications of integrate type and not only. The as-grown films were oxygen deficient and due to this exhibited rather poor superconducting properties. Oxygen annealing at temperatures of 500-600 degreesC increased T-c(R=0) up to 87.8 K. (C) 2002 Published by Elsevier Science B.V.

Asymmetric metal-ferroelectric-metal (MFM) structures were manufactured by sol-gel deposition of a lead zirconate-titanate (PZT with Zr/Ti ratio 65/35) film on Pt-coated Si, with a Au top electrode. The average remnant polarization of 9 muC/cm2 and the coercive field of 39 kV/cm were obtained from the hysteresis loop measurements. A detailed analysis of the polarization-electric field (P-E), capacitance-voltage (C-V), and current-voltage (I-V) measurement results allowed us to estimate the near-electrode space-charge region thickness (roughly half of the film thickness at zero voltage), net doping concentration (around 10(18) cm(-3)), built-in potential (in the 0.4-0.8 V range, depending on the injecting electrode), and dynamic dielectric constant (5.2). The current logarithm-voltage dependence for the field-enhanced Schottky emission obeys a "1/4" law. The spectral distribution of the short circuit current measured under continuous light illumination in the 290-800 nm range exhibits a cutoff wavelength at 370 nm and a maximum sensitivity at about 340 nm. The estimated band-gap energy of the PZT material is 3.35 eV. The MFM structure is discussed in terms of two back-to-back Schottky diodes with a ferroelectric material in between. It is concluded that the semiconductor properties of the films are not negligible and, in certain conditions, are dominating over the ferroelectric ones. (C) 2003 American Institute of Physics.