HTS quasiparticle injection devices with large current gain at 77 K

Recent progress on the development of planar QP-injection devices using YBC O and STO as an epitaxial injection barrier will be discussed. The main pro blem for HTS injection devices is to grow reliably a well defined, ultra-th in tunneling barrier suitable for QP tunneling. For this purpose, we used i nverted cylindrical magnetron sputtering to first optimize the smoothness o f our YBCO films by controlling tightly all relevant sputtering conditions. We are able to prepare smooth (001) YBCO films on (001) STO substrates on a routine basis with an average roughness varying between 1 and 2 nm. With these net YBCO films both planar as web as grain boundary junctions were fa bricated using epitaxial STO barriers between 2 and 8 nm thick and a 50 nm of Au counter electrode. Planar junctions with 6 nm STO barriers were in mo st cases fully insulating, in some cases, a current gain of up to 7.4 at 77 K was obtained. For 3 nn STO barriers, the highest current gain was 15 at 81 K. The injection results also show a scaling behavior with junction size . Based on the present materials development and device understanding, we c onsider a current gain of up to 20 at 77 K possible.