Surface and interface study on MBE-grownNd1.85Ce0.15CuO4thin films by photoemission spectroscopy and tunnel spectroscopy

${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_4$ (NCCO) surfaces and metal (Au, Ag, and Pb)/NCCO interfaces have been extensively investigated by x-ray and ultraviolet photoelectron spectroscopies using films grown by molecular beam epitaxy (MBE). The photoelectron spectra obtained in situ on the surfaces of MBE-grown NCCO films are free from any dirt peak and show a fine structure with a Fermi edge in the valence region. Experiments were performed on the prepared clean surfaces, focusing on the evolution of the surface or interface electronic structure with oxygen nonstoichiometry at the surface or interface regions. The results show (1) a systematic spectral change due to the oxygen content at the surface, and (2) a redox phenomena at the metal/NCCO interface. In oxygenated surfaces or insufficiently reduced surfaces, excess oxygens seem to occupy the interstitial (apical) oxygen sites, which compensates Ce doping, eventually leading to a nonmetallic surface. In excessively reduced surfaces, oxygen deficiencies seem to occur at regular oxygen sites, leading to a metallic but not a superconducting surface. The photoelectron spectroscopy data are discussed in comparison with complementary tunnel spectroscopy data. Both sets of data indicate that precise control of oxygen stoichiometry at NCCO surface is essential to obtain an intrinsic (i.e., superconducting) NCCO surface, which is indispensable in obtaining reliable data using surface sensitive experiments and in fabricating tunnel junctions and superlattices with desirable characteristics.