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$Al/AlO_x/Al$-layer systems are frequently used for Josephson junction-basedsuperconducting devices. Although much work has been devoted to theoptimization of the superconducting properties of these devices, systematicstudies on influence of deposition conditions combined with structural analyseson the nanoscale are rare up to now. We have focused on the optimization of thestructural 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 systemwas used and the effect of substrate pretreatment, different Al-depositiontemperatures and Al-deposition rates was studied. Transmission electronmicroscopy 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 boundarytypes and the morphology of the $Al/AlO_x$ interface. We show that thestructural properties of the lower Al layer are decisive for the structuralquality of the whole $Al/AlO_x/Al$-layer system. Optimum conditions yield anepitaxial Al(111) layer on a Si(111) substrate with an Al-layer thicknessvariation of only 1.6 nm over more than 10 $\mu m$ and large lateral grainsizes up to 1 $\mu m$. Thickness fluctuations of the $AlO_x$-tunnel barrier areminimized on such an Al layer which is essential for the homogeneity of thetunnel current. Systematic variation of the Al-deposition rate and depositiontemperature allows to develop an understanding of the growth mechanisms.

Optimization of $Al/AlO_x/Al$-Layer Systems for Josephson Junctions from a Microstructure Point of View