Effect of annealing in oxygen on the structure formation of Bi-Sr-Ca-Cu-O thin films

The structure, bonding, and $T_c$ of Bi-Sr-Ca-Cu-O superconducting films, prepared by rf magnetron sputtering, changed systematically upon varying the annealing conditions in oxygen. The spacing between the double Bi-O layers decreased from 22 to 18 Å and the degree of c-axis orientation became higher with the annealing. No change in the Bi core-level electron spectra with the annealing was observed. The Cu core-level electron spectra indicated an increase of the Cu valence with the annealing. The core-level electron spectra of Sr and Ca showed the possibilities of random distribution of the Ca and Sr atoms between the double Bi-O layers prior to annealing and exchanging their lattice sites in the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ crystal structure after annealing. The zero-resistance temperature of the films was 10 K before annealing, and rose up to 96 K (midpoint 105 K) after annealing. The annealing favors both oxidation of Cu and ordering of Sr and Ca atoms within the unit cell of the 2:2:2:3 crystal structure.