Ultrathin epitaxial Al 2 O 3 films grown on Nb ( 110 ) ∕ sapphire ( 0001 ) investigated by tunneling spectroscopy and microscopy

• Published in: Journal of applied physics 2005-04, Vol.97 (8), p.083515-083515-6
• Main Authors: Dietrich, Ch, Koslowski, B., Ziemann, P.
• Format: Article
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.1876580

Structural as well as electronic properties of ultrathin epitaxial Al 2 O 3 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 Al 2 O 3 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.