Andrea Cavallaro

High quality YBa2Cu3O7 thin films grown by trifluoroacetates metalorganic deposition

Chemical solution growth of epitaxial YBa2Cu3O7 thin films on single crystalline substrates has been investigated using trifluoroacetate precursors. The concentration of the starting solution was selected to achieve a final film thickness of 250 nm. First, it is shown that high quality films can only be achieved if an homogeneous nanocrystalline film is obtained after the spin coating deposition and pyrolysis steps. Secondary phases such as BaCuO2, Y2BaCuO5 and CuO remain after the high temperature growth process, when macrosegregation is detected after these initial processing steps. A local degradation of texture associated with macrosegregation is detected by μ-Raman spectroscopy. The influence of growth temperature on the film quality has been analysed and the reaction times have been optimized at each temperature according to the reaction kinetics. In-situ fluoride analysis and μ-Raman spectroscopy have been used to determine the advancement of the formation reaction. The most apparent microstructural modification of these thin film samples with optimized annealing times has been found to be an enhanced porosity for low processing temperatures. The influence of porosity on the normal state resistivity and the critical currents has been evidenced. Optimized processing parameters lead to samples with very high critical currents (Jabc = 3.2 × 106 A cm−2 at 77 K and 2.7 × 107 A cm−2 at 5 K) which demonstrates the capability of the trifluoroacetate metalorganic deposition method for thin film and coated conductor preparation.

Influence of porosity on the critical currents of trifluoroacetate-MOD YBa/sub 2/Cu/sub 3/O/sub 7/ films

The influence of porosity on the superconducting properties have been investigated on YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on LaAlO/sub 3/ [100] substrates by the so-called Trifluoroacetate (TFA) route. Micro-Raman spectroscopy have been used to determine the concentration of c-axis grains /spl delta/ in different samples and their influence on the final film porosity as observed from SEM imaging. This has been compared with measurements of resistivity and critical currents in the same samples. We prove that this /spl delta/ fraction is the main parameter controlling the porosity and hence the normal-state resistivity of the thin films. The optimization of the microstructure of these YBa/sub 2/Cu/sub 3/O/sub 7/ TFA films allow to have high critical currents : J/sub c/ = 3 /spl times/ 10/sup 6/ A/cm/sup 2/ at 77 K.

Mechanisms of nanostructural and morphological evolution of CeO2 functional films by chemical solution deposition

Tuning the interfacial area is essential for the optimization of nanostructured CeO2 in a variety of applications. In this work, we investigate the microstructural mechanism of an oxidation-induced transition from a partially oriented, granular and porous nanostructure to a dense epitaxial one in CeO2 films deposited from chemical solutions. Crystallization under reducing atmospheric conditions results in a nanometric granular microstructure with a high fraction of C decorating grain boundaries and interstitial cavities. Annealing in oxidizing conditions removes C impurities and promotes grain growth, resulting in a fully epitaxial film, as well as stabilizing the otherwise energetically prohibitive polar (001) planes. A mechanism that considers an impurity induced grain boundary blocking mechanism and the stabilization of (001) planes via surface oxidation is proposed.

Chemistry and structure of homoepitaxial SrTiO3 films and their influence on oxide-heterostructure interfaces

The properties of single-crystal SrTiO3 substrates and homoepitaxial SrTiO3 films grown by pulsed laser deposition have been compared, in order to understand the loss of interfacial conductivity when more than a critical thickness of nominally homoepitaxial SrTiO3 is inserted between a LaAlO3 film and a SrTiO3 substrate. In particular, the chemical composition and the structure of homoepitaxial SrTiO3 investigated by low-energy ion-scattering and surface X-ray diffraction show that for insulating heterointerfaces, a Sr-excess is present between the LaAlO3 and homoepitaxial SrTiO3. Furthermore, an increase in the out-of-plane lattice constant is observed in LaAlO3, indicating that the conductivity both with and without insertion of the SrTiO3 thin film originates from a Zener breakdown associated with the polar catastrophe. When more than a critical thickness of homoepitaxial SrTiO3 is inserted between LaAlO3 and SrTiO3, the electrons transferred by the electronic reconstruction are trapped by the formation of a Sr-rich secondary phase and Sr-vacancies. The migration of Sr towards the surface of homoepitaxial SrTiO3 and accompanying loss of interfacial conductivity can be delayed by reducing the Sr-content in the PLD target.

Chemical solution techniques for epitaxial growth of oxide buffer and YBa2Cu3O7 films

Abstract Chemical solution techniques have been investigated for the growth of both, oxide buffer layers suitable for coated conductors and YBa 2 Cu 3 O 7 thin films, on single crystal substrates. Growth conditions have been optimised for CeO 2 and BaZrO 3 buffer layers, leading to high quality epitaxial films (misorientation spread typically below 1°). YBa 2 Cu 3 O 7 films (thickness: 250 nm) have been grown from trifluoroacetate precursors. The kinetic hindrances for the formation of single phases have been investigated by means of Raman spectroscopy and fluorine analysis. After optimisation of the deposition and growth conditions very high critical currents have been achieved (J c ab =3.2×10 6 A/cm 2 at 77 K and 2.7×10 7 A/cm 2 at 5 K).

Heteroepitaxial orientation control of YSZ thin films by selective growth on SrO-, TiO2-terminated SrTiO3 crystal surfaces

Epitaxial integration of dissimilar materials with a large mismatch and different crystalline structures is of increasing interest to generate heterostructures with new or enhanced functional properties. In this study, fluorite yttria-stabilised zirconia (YSZ) thin films have been prepared by pulsed laser deposition on atomically-flat (100)-oriented perovskite SrTiO3 (STO) single crystal surfaces with different controlled SrO/TiO2 termination ratios (from 0 to 100%) tailored by using different strategies (thermal and chemical treatments, or the use of proper buffer layers). We show important changes in the film orientation and morphology that are directly related to the substrate termination. The SrO termination favours the epitaxial growth of atomically-flat films consisting of (100)-oriented YSZ, which corresponds to the cube-on-cube arrangement with a 45° in-plane rotation, whereas the TiO2 termination induces island growth of four 10–15° tilted (111)-YSZ domains. Under appropriate conditions we have achieved films with either full (100)YSZ or tilted-(111)YSZ orientations. However, both morphologies and orientations coexist in substrates with mixed terminations. The selectivity of the oriented growth of the first YSZ nuclei on the substrate areas with different STO terminations has been associated to changes in their corresponding interfacial energies.

Preparation of MZrO/sub 3/ (M=Ba,Sr) buffer layers on surface oxidized Ni/NiO templates by PLD and MOD

The preparation of cube textured NiO buffer layers on biaxially textured Ni tapes using surface oxidation epitaxy (SOE) offers a cheap and scalable route for the production of long-length YBCO coated conductors. A second buffer layer is necessary to ensure epitaxial growth of the YBCO as well as to prevent Ni contamination of the superconducting layer. Zirconate buffer layers offer a close lattice misfit to NiO as well as a high stability during the YBCO deposition. BaZrO/sub 3/ and SrZrO/sub 3/ buffers were successfully deposited on NiO using pulsed laser deposition. Additionally, BaZrO/sub 3/ films were prepared on NiO using metal-organic decomposition. All buffers show a high quality epitaxial growth with an in-plane orientation similar to the underlying NiO. The subsequent deposition of YBCO on top of these buffers resulted in epitaxial layers with a T/sub c/ above 85 K and j/sub c/ up to 1 MA/cm/sup 2/ on NiO if a thin intermediate SrTiO/sub 3/ layer is used. Microstructural investigations showed that the roughness and the surface topography of the buffer layers is mainly determined by the quality of the NiO layer.

Controlling the surface termination of NdGaO3 (110): the role of the gas atmosphere

In this work the effect of gas atmosphere on the surface termination reconstruction of single crystal NdGaO3 (110) (NGO) during thermal annealing was analyzed. Using Low Energy Ion Scattering (LEIS) it has been possible to study the chemical composition of the first atomic layer of treated NGO single crystal samples. NGO has been analyzed both as-received and after a specific thermal treatment at 1000 °C under different gas fluxes (argon, nitrogen, static air, synthetic air, nitrogen plus 5% hydrogen and wet synthetic air respectively). Thermal annealing of perovskite single crystals, as already reported in the literature, is used to obtain a fully A-cation surface termination. Nevertheless the effect of the gas-atmosphere on this process has not been previously reported. By the use of sequential low energy Ar(+) sputtering combined with the primary ion LEIS analysis, the reconstruction of the outermost atomic layers has allowed the clarification of the mechanism of NGO neodymium surface enrichment. It is proposed that the gallium at the surface is submitted to a reduction/evaporation mechanism caused by low oxygen partial pressure and/or high water pressure in the vector gas. Below the first surface atomic layers of an as-received NGO single-crystal a gallium-rich phase has also been observed.

High J/sub c/ YBCO thin films and multilayers grown by chemical solution deposition

Chemical solution deposition (CSD) has emerged as a very competitive technique to obtain superconducting films of high quality. However, there is still few knowledge about how underlying microstructure can affect the performances of the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films. Here we will summarize our recent progress in this subject. We have grown YBCO thin films by the so called trifluoroacetate route on top of buffer layers also grown by CSD. By modifying the growth conditions of SrTiO/sub 3/ and BaZrO/sub 3/ buffer layers we have investigated the influence of surface roughness and grain size. The role of lattice parameter mismatch has been studied by producing multibuffered architectures such as SrTiO/sub 3//BaZrO/sub 3//LAO. It has been proved that YBCO thin films with critical currents in excess of 1 MA/cm/sup 2/ at 77 K in self field can be achieved, thus demonstrating the abilities of the CSD technique.

Fabrication and characterization of yttria-stabilized zirconia membranes for micro solid oxide fuel cells

The present study is devoted to analyze the compatibility of yttria-stabilized zirconia thin films prepared by pulsed laser deposition technique for developing new silicon-based micro devices for micro solid oxide fuel cells applications. Yttriastabilized zirconia free-standing membranes with thicknesses from 60 to 240 nm and surface areas between 50x50 μm2 and 820x820 μm2 were fabricated on micromachined Si/SiO2/Si3N4 substrates. Deposition process was optimized for deposition temperatures from 200ºC to 800ºC. A complete mechanical study comprising thermomechanical stability, residual stress of the membranes and annealing treatment as well as a preliminary electrical characterization of ionic conductivity was performed in order to evaluate the best processing parameters for the yttria-stabilized zirconia membranes.