Superconducting transition metal nitride films for THz SIS mixers

The development of sensitive THz SIS mixers requires a low-loss superconducting strip-line material with a transition temperature above 15 K. In this paper we examine the properties of (Nb,Ti)N, NbN, and (Nb,Zr)N thin films sputtered at ambient substrate temperature on glass wafers. The best properties are observed for the (Nb,Ti)N films, sputtered from an NbTi alloy target with 30 at.% Ti. A similar picture is observed for the epitaxial films deposited on the MgO wafers. We have also examined the homogeneity of the (Nb,Ti)N films versus film thickness and in plane since this factor is clearly important for the microwave behavior of the strip-line. We observe that (Nb,Ti)N films deposited on silicon, sapphire, and glass wafers have much worse homogeneity compared to the films deposited on the MgO wafers.