All MgB2 tunnel junctions with Al2O3 or MgO tunnel barriers

Abstract

All MgB2 thin film tunnel junctions with Al2O3 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 Al2O3 tunnel barrier, we observed both superconducting gaps of MgB2, 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 MgB2 films: the film grown on Al2O3 was polycrystalline whereas the film grown on MgO was c-axis oriented.