Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits

摘要

This work is concerned with Al/Al-oxide(AlO$_{x}$)/Al-layer systems which areimportant for Josephson-junction-based superconducting devices such as quantumbits. The device performance is limited by noise, which has been to a largedegree assigned to the presence and properties of two-level tunneling systemsin the amorphous AlO$_{x}$ tunnel barrier. The study is focused on thecorrelation of the fabrication conditions, nanostructural and nanochemicalproperties and the occurrence of two-level tunneling systems with particularemphasis on the AlO$_{x}$-layer. Electron-beam evaporation with two differentprocesses and sputter deposition were used for structure fabrication, and theeffect of illumination by ultraviolet light during Al-oxide formation iselucidated. Characterization was performed by analytical transmission electronmicroscopy and low-temperature dielectric measurements. We show that thefabrication conditions have a strong impact on the nanostructural andnanochemical properties of the layer systems and the properties of two-leveltunneling systems. Based on the understanding of the observed structuralcharacteristics, routes are derived towards the fabrication ofAl/AlO$_{x}$/Al-layers systems with improved properties.