Electronic properties of laser-deposited Bi2Sr2CaCu2O8+delta thin films by x-ray photoemission and x-ray Auger spectroscopies

Using a combination of XPS and Auger core level spectroscopies, we have investigated the surface layer of bulk ceramic and thin films of tle BSCCO 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 Cu 2p 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 ot the Auger CuL_3 VV 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 the bulk samples. We suggest that Cu3+ appears in the form of 3d9L and 3d10L2 configurations, instead of 3d8, consistently with the formation of p holes (L,L2)in the oxygen valence band.