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

Abstract

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 is fully oxidized without pinholes over areas of some mm2. The tunneling barrier heights range from 1.0 to 1.5 eV according to MIM tunneling and STS.