Nb/Al-AlOx/Nb Junction Properties\u27 Variations Due to Storage and Mounting

We report studies of room temperature aging and annealing of Nb/Al-AlO x /Nb tunnel junctions with a 2. . . 3 sq.\ub5msize. We observed a noticeable drop of the junction normal resistance Rn unusually combined with increase of subgap resistance Rj as a result of aging. Changes of Rj occur at sufficiently shorter time scale than that of Rn. Variation of both Rn and Rj depend on the junction size. An effect of aging history on the junction degradation after consequent annealing was discovered. We suggest that the observed junction aging andannealing behavior could be explained by diffusional ordering and structural reconstruction in the tunnel AlOx barrier. The diffusion driving such structural ordering and reconstruction of the AlO x tunnel layer is enhanced due to the intrinsic stress relaxation (creep) processes in the underlying Al layer. Also, we discuss the influence of dicing the wafer into the single mixer chip on the junction aging behavior