  $Al/AlO_x/Al$-layer systems are frequently used for Josephson junction-based
superconducting devices. Although much work has been devoted to the
optimization of the superconducting properties of these devices, systematic
studies on influence of deposition conditions combined with structural analyses
on the nanoscale are rare up to now. We have focused on the optimization of the
structural properties of $Al/AlO_x/Al$-layer systems deposited on Si(111)
substrates with a particular focus on the thickness homogeneity of the
$AlO_x$-tunnel barrier. A standard high-vacuum electron-beam deposition system
was used and the effect of substrate pretreatment, different Al-deposition
temperatures and Al-deposition rates was studied. Transmission electron
microscopy was applied to analyze the structural properties of the
$Al/AlO_x/Al$-layer systems to determine the thickness homogeneity of the
$AlO_x$ layer, grain size distribution in the Al layers, Al-grain boundary
types and the morphology of the $Al/AlO_x$ interface. We show that the
structural properties of the lower Al layer are decisive for the structural
quality of the whole $Al/AlO_x/Al$-layer system. Optimum conditions yield an
epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness
variation of only 1.6 nm over more than 10 $\mu m$ and large lateral grain
sizes up to 1 $\mu m$. Thickness fluctuations of the $AlO_x$-tunnel barrier are
minimized on such an Al layer which is essential for the homogeneity of the
tunnel current. Systematic variation of the Al-deposition rate and deposition
temperature allows to develop an understanding of the growth mechanisms.
