Effect of surface modification ofCeO2 bufferlayers on Jc and defect microstructures of large-area YBCO thin films

Abstract

High-quality CeO2 buffer layers are requisite for the successful growth of YBCO thin films with excellent propertieson sapphire substrates. In this study, we evaluated the effect of surface modification of theCeO2 layers on the properties of the YBCO thin films prepared by large-areapulsed laser deposition (PLD), in particular the critical current densityJc and defect microstructure. High-temperature annealing(1050 °C) has been found to significantly smoothen the very rough and granular surfaces of the as-grownCeO2 layers (surfaceroughness rms∼5–10 nm)to atomic flatness (rms∼0.5 nm). However, a rather unique characteristic of theCeO2 layers deposited by large-area PLD is the development of pores when subjected to prolongedhigh-temperature annealing. For very short annealing periods (10–20 min), the surface morphologybecomes atomically flat, along with the appearance of a high density of ‘nanopores’ that are∼40–100 nm indiameter and ∼3–5 nm in depth. Extending the annealing period to 60 min or moreresults in the development of a surface subtended with enlarged pores∼0.2–0.5 µm in diameter. Compared with the YBCO thin films deposited on as-grownCeO2, YBCO thin films onannealed CeO2 exhibitedbetter homogeneity of Jc and better crystalline texture. Among the YBCO thin films deposited on annealedCeO2, higher self-fieldand in-field Jc was obtained for YBCO thin films deposited onCeO2 with smooth surfaces but interspersed with nanopores. Investigation of the defectmicrostructure via the etch pit method in conjunction with atomic force microscopy (AFM)of the YBCO thin films revealed a high density of linear defects in the form of screw andedge dislocations, which correlated well with a high density of nanopores on annealedCeO2. Transmission electron microscopy (TEM) further confirmed the presence of threadingdislocations clearly emanating from the nanopore sites. Angular dependence ofJc revealed enhanced flux pinning for YBCO thin films deposited onCeO2 containing a high density of nanopores.