Hydrogenated amorphous silicon barriers for niobium-niobium Josephson junctions

We report on further studies of the effects of hydrogenation of sputtered amorphous silicon barriers upon the current-voltage (I-Y) characteristics of Nb-Nb Josephson tunnel junctions. For composite trilayer barriers (a-Si/a-Si:H/a-Si) which are deposited using 8 mT of Ar, we find that there is an abrupt improvement in device chararteristics when the central hydrogenated layer is deposited using a hydrogen partial pressure which exceeds about 0.5 mT. We attribute this to the reduction in the density of localized states in the a-Si:H layer. We have observed excellent I-Y characterisitics with trilayer barrier devices whose central hydrogenated layer is only about 1/7 of the thickness of the entire barrier. This observation suggests that localized states near the geometric center of the barrier are the most significant in degrading device characteristics. Annealing experiments and published data on the diffusion of deuterium in a-Si suggest that the composite barriers will be extremely stable during processing and storage. Zero bias anomalies in device I-Y characteristics and spin density in the a-Si and a-Si:H layers have been measured. Suggestions for future experiments are made.