Structural and Nanochemical Properties of AlOx Layers in Al/AlO_x/Al-Layer Systems for Josephson Junctions

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 γ-Al_2O3 layers were grown by electron-beam evaporation of Al_2O3 and reactive sputter deposition.