Electronic properties of laser-deposited Bi₂Sr₂CaCu₂O_8+δ thin films by X-ray photoemission and X-ray Auger spectroscopies

Abstract

Using a combination of X-ray photoemission and Auger core level spectroscopies, we have investigated the surface layer of bulk ceramic and thin films of the Bi₂Sr₂CaCu₂O_8+δ superconductor compound. Films deposited by laser ablation on MgO single crystal substrates and annealed at 880°C in 1 atm of pure oxygen can exhibit retention of excess oxygen with respect to the surface of the ceramic grains. Then the Cu2p core levels are characterized by a low satellite-to-main peak intensity ratio (0.25). We suggest that the 3d⁸ two-holes final state components of the Auger Cu L₃VV spectrum can be explained by the presence of an appreciable amount of formal Cu³⁺. Oxygen losses, observed upon heating the films under vacuum up to 635°C, are correlated to the disappearance of formal Cu³⁺, to the strong decrease of hole-doped oxygen and to a metallic-insulator transition. The 3d core levels of the Sr ions are extremely sensitive to changes in the electronic structure of the material. Formal trivalent copper is not present in bulk samples. We suggest that Cu³⁺ appears in the form of 3d⁹L̲ and 3d¹⁰L̲² configurations, instead of 3d⁸, consistently with the formation of p holes (L̲, L̲²) in the oxygen valence band.