Effect of target composition on properties of NdBa2Cu3O7−δ superconducting thin films grown by pulsed laser deposition: A comparative study

Abstract

High quality epitaxial NdBa2Cu3O7−δ (NBCO) superconducting thin films have been deposited on SrTiO3 (100), LaAlO3 (100), and NdGaO3 (110) substrates by pulsed laser deposition using slightly Ba-rich Nd1−xBa2+xCu3O7−δ (x=0.00, 0.03, 0.05, and 0.10) targets. The influence of target composition on the properties of thin films, including superconductivity, epitaxial structure, and surface morphology have been systematically investigated. In both four-probe and dc magnetization measurements, the Tc0 of 91–93 K is routinely achieved from targets with x=0.00, 0.03, and 0.05. The highest is 94 K for the films corresponding to the target of Nd0.97Ba2.03Cu3O7−δ. The critical current density Jc in a 1000 Å thick, 5 μm wide bridge is 3.8×106 A/cm2 at 77 K in zero magnetic field. X-ray diffraction (XRD) analysis demonstrates that the structure of films sensitively depends on the substrate deposition temperature. When the substrate deposition temperature decreases from 900 to 800 °C, the orientation of NBCO films gradually changes from pure c-axis to a-axis, which is similar to the results observed for YBa2Cu3O7−δ films. For pure c-axis oriented films deposited at 850–900 °C, the omega scan x-ray rocking curve of the (005) reflection gives the very small value (∼0.05°) of the full width at half maximum. In the films deposited from the targets with x=0.05 and 0.10, very weak peaks corresponding to BaO, BaCuO2, and Ba2CuO3 are detected by XRD, while only (123) phase is observed in the films deposited from the targets with less Ba-rich compositions. The surface morphology of the film was studied by high-resolution scanning electron microscope and atomic force microscope (AFM) in detail. It was found that the surface morphology, featuring mainly square-shaped out-growths, is sensitive to the deposition parameters, especially the substrate temperature. Smooth nearly out-growth-free surface with relatively low density (∼106/cm2) of droplets has been obtained by optimizing the deposition parameters. AFM images demonstrate that with the increase of the barium content in the target the surface morphology of films changes from very smooth to somewhat rough.