Secondary ion mass spectrometry study for Josephson junction with Nb/AlOx-Al/Nb structure

The interface changes of Nb Josephson-junction structures (Nb/Al /Nb and Nb/AlO2-Al/Nb) were studied using secondary ion mass spectrometry (SIMS). Using a Cs exp + primary beam, CsM exp + ions (M = hydrogen, oxygen, OH, Nb, and Al) were monitored for Cs bombardment. In the SIMS depth profile, the CsH exp + and CsO exp + intensity indicated accumulation at junction interfaces, where these intensities were larger for the Al/Nb base electrode interface than for the Nb counterelectrode/Al interface. A layer of absorbed water vapor with a thickness of a few nm, is formed on the junction interface, and is attributed to chamber outgassing. In the Nb/Al/Nb structure, the annealing changes for the Al/Nb base electrode interface were smaller than those for the Nb counterelectrode/Al interface. The absorbed water vapor layer plays an important role in the junction structure, that is, it acts as a barrier for grain boundary diffusion between Nb and Al.