We prepared Bi2Sr2CaCu2Oy superconducting single crystals by a modified vertical Bridgman method using the Curich melt with the compositions of Bi : Sr : Ca : Cu = 2 : 2 : 1 : x (x = 2.0 and 3.0), and investigated their chemical states with a high-resolution X-ray photoelectron spectroscopy (XPS).
From the XPS results, we found that the shape of the Cu-2p3/2satellite peak from the surface of the single crystal grown with x = 3.0 has changed in comparison with that of the single crystal grown with x = 2.0, which indicates that the chemical bonding nature in CuO2 plane has slightly changed from covalent to ionic bonding. The slight change of the chemical bonding nature gave us the consistent understanding of the semiconductive behavior at the normal state and low Tc = 74 K in resistance-temperature characteristic of the single crystal grown with x = 3.0.