Improvement of the critical temperature of superconducting NbTiN and NbN thin films using the AlN buffer layer

Thin superconducting NbTiN and NbN films with a few nm thickness are used in various device applications including in hot electron bolometer mixers. Such thin films have lower critical temperature (T(c)) and higher resistivity than corresponding bulk materials. In an effort to improve them, we have investigated an effect of the AlN buffer layer between the film and the substrate (quartz or soda lime glass). The AlN film is deposited by DC magnetron sputtering, and the process condition is optimized so that the x-ray diffraction intensity from the 002 surface of wurtzite AlN becomes the highest. By use of this well-characterized buffer layer, T(c) and the resistivity of the NbTiN film with a few nm thickness are remarkably increased and decreased, respectively, in comparison with those without the buffer layer. More importantly, the AlN buffer layer is found to be effective for NbN. With the AlN buffer layer, T(c) is increased from 7.3 to 10.5 K for the 8 nm NbN film. The improvement of T(c) and the resistivity originates from the good lattice matching between the 002 surface of AlN and the 111 surface of NbTiN or NbN, which results in better crystallization of the NbTiN or NbN film. This is further confirmed by the x-ray diffraction measurement.