Fabrication of NbTiN Tunnel Junctions Using Hf Overlayers

We have fabricated NbTiN tunnel junctions by introducing a Hf overlayer to a junction barrier layer. Two types of junctions consisting of NbTiN/Hf/Al- AlNx/NbTiN and NbTiN/Hf-HfOx/NbTiN were fabricated on thermally oxidized Si substrates. The electrical characteristics for junctions with various thickness of the Hf layer were evaluated at 4.2 K and compared. The results indicated that the diffusion of nitrogen from the base NbTiN layer into the normal layer was suppressed. In addition, the normal coherence length in the Hf layer was estimated to be about 13 nm.