Study of laser-deposited Bi₂Sr₂CaCu₂O_{8 + δ} thin films by rutherford backscattering, X-ray photoemission and X-ray Auger spectroscopies

Combining Rutherford backscattering spectroscopy (RBS), X-ray photoemission spectroscopy (XPS) and X-ray Auger spectroscopy, we have studied thin films of Bi₂Sr₂CaCu₂O_{8 + δ} deposited by laser ablation on oriented MgO(100) single crystals. RBS and XPS indicate the absence of interdiffusion into the substrate and continuity of the films. On heating the films in vacuum up to the fusion temperature (880°C), oxygen losses are observed and correlated with the disappearance of formal Cu³⁺, with the strong decrease in oxidized (hole-doped) oxygen and with a metal- semiconductor phase transition. An appreciable percentage of formal Cu³⁺ (about 24%) is estimated from the analysis of the Auger Cu L₃VV spectrum in contrast to the ceramic case where formal trivalent copper is not present. This is likely because the thin film (textured) structure allows the retention of more excess oxygen than the surface of the ceramic grains. We suggest that Cu³⁺ appears under the form of 3d⁹L and 3d¹⁰L² configurations, instead of 3d⁸, consistently with the formation of p holes (L, L²) in the valence band.