Superconducting transition metal nitride films for THz SIS mixers

The development of sensitive THz SIS mixers requires a low-loss superconduc ting stripline material with a transition temperature above 15 K. In this p aper we examine the properties of (Nb,Ti)N, NbN, and (NbZr)N thin films spu ttered at ambient substrate temperature on glass wafers, The best propertie s 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 dep osited 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 micro-wave behavior of the strip-line. We observe that (N b,Ti)N films deposited on silicon, sapphire and glass wafers hive much wors e homogeneity compared to the films deposited on the MgO wafers.