On the use of low barrier potential materials to improve high frequency coupling to Josephson tunnel junctions

Abstract

Improved high frequency coupling to Josephson tunneling junctions must involve impedance transformation techniques or the fabrication of junctions with more appropriate impedance values. Two kinds of calculations about efficiency of junction coupling with a cavity, one with an antenna and another with an inductive loop show clearly the importance of the term Ic r0/Γ where Γ is the capacitance, Ic the critical current and r0 a dimension related to the size of the junction. Several ways may be proposed to increase this « figure of merit ». It appears that, when considering only tunneling phenomena, there is an advantage to decrease the barrier potential height. A number of people have employed semiconductors as barriers because it is possible to use either a naturally narrow forbidden band, or to use the metal-semiconductor interface properties, to get small barrier heights. This paper is devoted to a review of experimental works on junctions using low barrier materials. Attempts have been made with various deposited materials such as Te, PbTe, CdS, CdSe, Ge, InSb, etc., ... But at the present time these trials have not yet been clearly successful. Many problems are related to the use of these materials, some of the more important being : presence of surface states, variation of doping with time, diffusion near the interface, pinholes and amorphous-polycrystalline behaviour. Moreover, these various phenomena can lead to several possible mechanisms for explaining conduction in superconductor-semiconductor-superconductor sandwiches. Some of these mechanisms are not compatible with good high frequency coupling