  The structural and nanochemical properties of thin $AlO_x$ layers are
decisive for the performance of advanced electronic devices. For example, they
are frequently used as tunnel barriers in Josephson junction-based
superconducting devices. However, systematic studies of the influence of
oxidation parameters on structural and nanochemical properties are rare up to
now, as most studies focus on the electrical properties of $AlO_x$ layers. This
study aims to close this gap by applying transmission electron microscopy in
combination with electron energy loss spectroscopy to analyze the structural
and nanochemical properties of differently fabricated $AlO_x$ layers and
correlate them with fabrication parameters. With respect to the application of
$AlO_x$ as tunnel barrier in superconducting Josephson junctions,
$Al/AlO_x/Al$-layer systems were deposited on Si substrates. We will show that
the oxygen content and structure of amorphous $AlO_x$ layers is strongly
dependent on the fabrication process and oxidation parameters. Dynamic and
static oxidation of Al yields oxygen-deficient amorphous $AlO_x$ layers, where
the oxygen content ranges from x = 0.5 to x = 1.3 depending on oxygen pressure
and substrate temperature. Thicker layers of stoichiometric crystalline
$\gamma-Al_2O_3$ layers were grown by electron-beam evaporation of $Al_2O_3$
and reactive sputter deposition.
