The influence of Al morphology on quality in Nb/Al/AlO/sub x//Al/Nb epitaxial base layer junctions

Nb/Al/AlO/sub x//Al/Nb devices with single-crystal base electrodes and low subgap current leakage have applications for particle detectors. To increase the quality and reproducibility in these devices, the authors sought to characterize and improve the microstructure of the first Al layer, from which a better-quality AlO/sub x/ barrier can then be formed. Using a liquid-nitrogen-cooled stage, they have established a temperature range over which it is possible to grow ultra-high-purity Al epitaxially on single-crystal Nb layers. They compared the effect of epitaxial and nonepitaxial layers on device quality, and the effect of varying the Al deposition temperature and therefore the nucleation rate at the start of Al layer growth. The main factor in improving V/sub m/ was the lower Al deposition temperature. The authors have seen an improvement of 20-30% in V/sub m/ values at 4.2 K, using single-crystal Nb base electrodes, across a range of J/sub c/ (critical current) from 50 to 9000 A cm/sup -2/.<>