NbN based Josephson junctions with silicon barriers deposited at 700°C

The authors have fabricated NbN/Si/Nb tunnel junctions where the barriers were deposited at substrate temperatures to 700°C. The device characteristics for those tunnel junctions with V_m (figure of merit) values exceeding 15 mV were studied as a function of the barrier deposition temperature. A reduced NbN energy gap was observed for barriers deposited at high substrate temperatures, possibly due to an interaction of the silicon with the NbN or an off-stoichiometric surface resulting from the high temperature sputter etch of the base electrode. The zero-bias conductivity as a function of temperature was measured for thick barriers and found to follow an activated conduction process at the high temperatures and Mott hopping conduction at the lower temperature. Some initial measurements on germanium barriers deposited at 250°C were made and found to follow a similar behavior except at a much lower temperature, possibly due to a lower activation energy