All MgB_2 tunnel junctions with Al_2O_3 or MgO tunnel barriers

摘要

All MgB_2 thin film tunnel junctions with Al_2O_3 or MgO tunnel barriers were fabricated in situ on Si substrates in a molecular beam epitaxy system and their tunneling characteristics were investigated. In the quasiparticle tunneling spectra of the junction with Al_2O_3 tunnel barrier, we observed both superconducting gaps of MgB_2, while only a small gap was seen with MgO tunnel barrier. Using a microscopic structural analysis, we found that the difference in the spectra is due to the crystal orientation difference of the MgB_2 films: the film grown on Al_2O_3 was polycrystalline whereas the film grown on MgO was c-axis oriented.