Study of inhomogenous superconducting gap in titanium nitride (TiN) films measured by low-temperature STM

Titanium nitride films with low loss and high kinetic inductance show great promise for use in superconducting qubits and resonators. Here we report STM measurements performed at 500 mK on 25 nm and 50 nm thick films of TiN that have been prepared with oxygen. After gentle Ar-ion sputter cleaning of the surface, we observe a topographically rough surface with tip-sample current-voltage (I-V) characteristics that show a superconducting gap. We use both the Dynes model and the Blonder-Tinkham-Klapwijk (BTK) model to fit the I-V data. The Dynes model fits depend upon the superconducting gap and a quasiparticle lifetime (broadening) parameters, while the BTK model depends upon the gap, the temperature and the barrier transparency. Surprisingly, the two models yield strikingly different results and we present evidence that in these samples the BTK model yields the correct physical interpretation.