ANNEALING PROPERTIES OF HIGH-QUALITY NB/AL ALOX/NB TUNNEL-JUNCTIONS

High quality, low current density Josephson tunnel junctions are obtained from Nb/Al-AlOx/ 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 degrees 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 Vm; the most remarkable result is a Vm as large as 2.5 V at T = 1.2 K, for our best sample. We believe that oxygen diffusion through the Nb gain 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.