Fabrication of Atomically Flat Bi₂Sr₂CaCu₂O_8+δ Films on MgO Substrates by Pulsed Laser Deposition and a Post-annealing Process

Abstract

We have developed a method for preparing atomically flat high-quality Bi₂Sr₂CaCu₂O_8+δ (Bi2212) thin films on MgO (001) substrates by using a combination of pulsed laser deposition (PLD) and a post-annealing process. Before depositing Bi2212 films, annealing of a MgO substrate was performed in a tube furnace. MgO substrates were annealed at 1073-1103 K under 8% of oxygen atmosphere. Thermal annealing of MgO substrates significantly contributed to the morphologies, crystallographic orientations, and superconducting properties of the Bi2212 films. The Bi2212 films were deposited by a PLD system using a KrF excimer laser. Only the Bi2212[100]//MgO[110] epitaxial relationship was observed in the Bi2212 films grown at lower substrate temperatures. Although the as-grown films have an extremely smooth surface, the grain size was limited to ∼100 nm, and the superconducting transition temperature (T_c) of the films were considerably lower than the reported values of Bi2212 bulk materials. In order to improve the superconducting properties of films, post-annealing was subsequently carried out in a tube furnace under various oxygen pressures. The surface morphologies and the in-plane orientation relationships between MgO substrates and Bi2212 films were drastically changed by post-annealing. After post-annealing, we found that the in-plane orientations of the Bi2212 grains thoroughly rotated to the Bi2212[100]//MgO[100]±12.5° epitaxial relationships. According to the AFM images of post-annealed Bi2212 films, periodic step and terrace structures were clearly observed. The step height corresponded to the half unit cell height of Bi2212, and the width of atomically flat terraces was larger than 500 nm. The post-annealing process also affected the superconducting properties of the films as the T_c over 80 K improved with sharp superconducting transitions. We have successfully fabricated high-quality Bi2212 thin films on MgO substrates, whose atomically flat surface morphology and superconducting properties as comparable to those of bulk materials. This technique offers a great technological advantage for high T_c superconducting device fabrications.