Ultrathin epitaxial Al2O3 films grown on Nb(110)/sapphire(0001) investigated by tunneling spectroscopy and microscopy -: art. no. 083515

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 similar to 0.4-nm-high steps are observed characteristic of monatomic Al2O3 steps. Furthermore, for growing tunneling voltages (>1 V), 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. (C) 2005 American Institute of Physics.