Relationships between x-ray photoelectron spectra and superconductivity in Bi-O thin films

Abstract

Detailed X-ray photoelectron spectra (XPS) have been obtained from BiSrCaCuO thin films without lead doping (T c = 86 °K, 2212 phase) and from thin films with lead doping ( T c = 107 °K, mainly 2223 phase). Films were prepared by single target sputtering and post-annealed to synthesize superconducting films 1,2. The spectral features of the Cu 2p, O 1s, Sr 3p, Ca 2p and Bi 4f are considered in some detail. It is found that in general the XPS signatures of the Cu 2p peaks are consistent with the presence of a mixed oxide environment with a main contribution from a binding energy similar to that for CuO but also with a significant contribution from a component with a binding energy shifted up to 1.7 eV for the low T c phase and to 2 eV for the high T c phase consistent with some trivalent state of copper for lead doped films. The Cu 2p structure exhibits the main line and satellite peaks familiar from screening of the 2p hole by the two valence-band configurations 2p 3d 10 and 2p 3d 9 in the intensity ratio ~2:1. Line-shape decomposition of the O 1s emission suggests that the main line corresponds to emission from oxygen bonded to copper in the Cu-O planes while the satellite peak with the higher binding energy corresponds to oxygen bonded to bismuth in Bi-O planes. No oxygen species bonded to Ca and Sr is clearly identified by line-shape analysis. Such explanations agree with angular-resolved photoemission spectra on our c axis oriented thin films. The Ca 2p and Sr 3d core-level spectra do not exhibit two sets of doublets as reported in other papers but their corresponding binding energies are shifted in the opposite trend of that observed for oxides. The bismuth ionization state in unleaded films seems inert under different annealing conditions and is very similar to Bi 2O 3. It is equally shown that the valence band structures are quite sensitive to small changes of the surface conditions and lead doping. In essence, this strongly suggests that valence band spectra may be able to be used as a finger print for the surface of the superconducting films.