X-ray photoelectron spectroscopy study for Nb Josephson junctions with overlayer structure

We studied metal/Nb structures with Al, Zr, Ta, Hf, V, and Ti as thin overlayers (all transition elements except Al) using x-ray photoelectron spectroscopy. The metal thickness sufficient to protect Nb from the formation of Nb native oxide was 2.0 nm. In structures with Al, Zr, Ta, and Hf overlayers, the oxide peaks in the photoelectron spectra, which resulted from native oxide grown on the metal surface, shifted toward higher binding energies with increasing metal overlayer thickness. These shifts were attributed to the metal coverage and the insulating oxide layers. The oxide insulating properties of the native oxides and the metal coverage of the Nb surface were studied at the same time, nondestructively, using XPS. These are essential factors for fabricating Nb junctions with excellent current-voltage characteristics.