Annealing properties of high quality Nb/Al-AlO/sub x//Nb tunnel junctions

High quality, low current density Josephson tunnel junctions are obtained from Nb/Al-AlO/sub x//Nb trilayers, using a SNAP process to define the junction area. A large number of junctions has been annealed in air for several hours at temperatures as high as 300/spl deg/C. The most relevant effects observed were: a) a marked decrease of the junction current density and a related increase of the normal-state resistance; b) under suitable conditions, a barrier quality improvement measured in terms of the subgap current and of the quality factor V/sub m/; the most remarkable result is a V/sub m/ as large as 2.5 V at T=1.2 K for our best sample. We believe that oxygen diffusion through the Nb grain boundaries is the main mechanism responsible for the normal-state resistance increase and that device quality variation is closely related to the modification of the barrier interfaces.<>