Characterization of high-T_c superconducting

YBCO thin films deposited by sputtering

High-Tc superconducting YBa₂Cu₃O_7-x (YBCO) thin films deposited by sputtering were characterized by SEM, HRTEM, AFM, XRD and EDX. Oxide outgrowths are always present on the surface of YBCO films deposited by sputtering. This is due to the relatively high pressure (³ 40 Pa for off-axis and ³ 250 Pa for on-axis sputtering geometry) and high substrate temperature (» 800^oC) during deposition, which cause local non-stoichiometry of the condensing species flux through back diffusion and resputtering/desorption. SEM/energy dispersive X-ray analysis showed that these surface imperfections were Cu-O crystals. AFM, surface and cross-section SEM images presented visualize the shape and size of the outgrowths. Their size depended on the growing time of the crystals. They continuously start growing during the deposition and hence their dimensions range from zero to a certain maximum size. The biggest crystals observed had started to form with the onset of dislocation-initiated film spiral growth, usually when the film thickness reaches 20-30 nm. It was found that the places in which crystals start to grow were in the middle of the spiral. New spiral islands start to form around new dislocations during the deposition. That’s why AFM images showed Cu-O outgrowths with different sizes on the film surface. For 200-nm films the height of the biggest crystals was nearly twice the thickness of the film. X-ray analisis showed high c-axis orientation of the YBCO films despite of the presence of Cu-O outgrowths. Phi and Psi scans showed very small amount of a-/b-film orientation. At lower substrate temperature (under » 700 ^oC), when a-/b-film orientation is preferred, the outgrowing crystals had a needle shape lying parallel to the c-axis of the film. Investigations of the electrical parameters of the films suggest that these outgrowths can play a role of pinning centers for the magnetic vortices. For single layer devices it is more important to have higher density of pinning centers in order to enhance the film critical current, which is necessary for low frequency device applications. On the other hand, a high density of grain boundaries in the YBCO films is an obstacle, for example, for high performance microwave component fabrication.

The interface between YBCO film and the compatible perovskite structure, as is La_0.3Sr_0.7MnO₃ (LSMO), was investigated by HRTEM. The cross-section SEM and electron diffraction patterns showed that the interface was sharp and the number of defects was low.

# Corresponding author. Fax: +359 2 975-3201, E-mail address: tdonchev@ie.bas.bg