Synthesis and photoemission study of as-grown superconducting MgB₂ thin films

As-grown superconducting thin films of MgB₂ were prepared by molecular beam epitaxy (MBE), and studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Only films prepared at temperatures between 150 and 320 °C showed superconductivity. The best T_c^onset of 36 K was obtained with a sharp transition width of ∼ 1 K although the film crystallinity was poor. The in situ photoelectron spectra obtained on the surfaces of the MBE grown MgB₂ films were free from dirt peaks. The XP spectra revealed the binding energy of the Mg 2p levels in MgB₂ is close to that of metallic Mg and the binding energy of B 1s is close to that of transition-metal diborides. The valence UP spectra showed a clear Fermi edge although the density of states at E_F is low and the major components of the valence band are located between 5 and 11 eV.