Improvement of the critical temperature of NbTiN films on III-nitride substrates

Abstract

In this paper, we study the impact of using III-nitride semiconductors (GaN,\nAlN) as substrates for ultrathin (11 nm) superconducting films of NbTiN\ndeposited by reactive magnetron sputtering. The resulting NbTiN layers are\n(111)-oriented, fully relaxed, and they keep an epitaxial relation with the\nsubstrate. The higher critical superconducting temperature (Tc = 11.8 K) was\nobtained on AlN-on-sapphire, which was the substrate with smaller lattice\nmismatch with NbTiN. We attribute this improvement to a reduction of the NbTiN\nroughness, which appears associated to the relaxation of the lattice misfit\nwith the substrate. On AlN-on-sapphire, superconducting nanowire single photon\ndetectors (SNSPDs) were fabricated and tested, obtaining external quantum\nefficiencies that are in excellent agreement with theoretical calculations.\n