Ultrathin epitaxial Al2O3 films grown on Nb(110)∕sapphire(0001) investigated by tunneling spectroscopy and microscopy

Structural as well as electronic properties of ultrathin epitaxial Al2O3 films prepared on Nb(110)∕sapphire(0001) were analyzed in situ by applying scanning tunneling microscopy (STM) and spectroscopy as well as ultraviolet photoelectron spectroscopy, cathode luminescence, and low-energy electron diffraction. According to these experiments, the niobium base film is protected from oxidation, while the ultrathin Al film deposited onto the Nb is fully oxidized and (0001)-oriented with a very smooth surface. The STM-imaged topography of the oxide films in most cases reflects monatomic steps of the underlying Nb(110) film. In some cases (10% of all samples with low tunneling barriers) additional ∼0.4-nm-high steps are observed characteristic of monatomic Al2O3 steps. Furthermore, for growing tunneling voltages (>1V), the STM-imaged topographies reveal an increasing density of small hillocks, which are attributed to localized defect states such as oxygen vacancies still present within the oxide layer.