The effects of oxidation and air exposure on RbxBa1−xBiO3 superconducting thin films

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Using x-ray photoelectron spectroscopy (XPS), we have investigated the effects of oxidation on RbxBa1−xBiO3, the degradation of its surface on exposure to air, and the feasibility of cleaning techniques to reverse this degradation. The sample was a superconducting thin film grown on MgO by molecular-beam epitaxy. XPS spectra of the initial, air-exposed surface show two O 1s peaks. The main O 1s peak increases in intensity upon oxidation at 300 °C for 10 min in 100 Torr O2, and decreases upon air exposure. Also, this peak shifts, together with the Ba and Bi peaks, to lower binding energy upon oxidation. These shifts to lower binding energy upon gain of oxygen (and to higher binding energy upon oxygen loss) can be explained by a rise in the conductivity of the film with rising oxygen content. Carbon and chlorine are observed on the initial air-exposed surface. Higher binding energy contributions in the Rb XPS peaks indicate that impurity compounds containing Rb are also present. Air-exposed surfaces do not show any XPS features at the Fermi level. However, removal of C and Cl by Ar+ sputtering at 1 kV, followed by oxidation, results in the appearance of intensity at the Fermi level. Hence sputter/oxidation treatments may prove useful in restoring the surfaces of films exposed to air (during patterning, for example) prior to overlayer deposition.