Characterization of ultrathin insulating Al2O3 films grown on Nb(110)/sapphire(0001) by tunneling spectroscopy and microscopy

Various structural as well as chemical properties of ultrathin Al2O3 films prepared on Nb(110)/sapphire(0001) were analyzed. For this purpose, in a first step, 40-nm-thick Nb(110) films are grown epitaxially by sputtering on top of sapphire(0001). The Nb(110) films are (1×1) reconstructed and exhibit the epitaxial relations Nb(110)‖Al2O3(0001), Nb[001]‖Al2O3[1_010] and equivalents as determined by x-ray diffraction. In a second step, a 1-nm-thick Al film is evaporated on top of the Nb(110) and oxidized ex situ in an oxygen plasma resulting in a polycrystalline Al2O3 film of thickness ∼1.3 nm. This oxide film was characterized by scanning tunneling microscopy and spectroscopy (STS) as well as by x-ray photoelectron spectroscopy. Additionally, the ultrathin Al-oxide films were tested as insulators in large area metal-insulator-metal (MIM) or superconductor-insulator-metal tunneling contacts. According to these experiments, the niobium film underneath is completely protected from oxidation, while the Al film...