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 deg. showed superconductivity. A best TC onset of 36 K with a sharp transition width of -1 K was obtained 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. XPS revealed that 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 (DOS) at EF is low and the major components of the valence band are located between 5 and 11 eV.