Microwave Loss in High-Q Titanium Nitride Resonators

We have measured the loss in a superconducting thin film titanium nitride microwave resonator from 20 mK to 1 K and at different stored microwave powers. The titanium nitride film had a superconducting transition temperature of Tc = 5.5 K and was grown in an MBE by reactively reacting evaporated titanium in a nitrogen plasma. The measured internal loss of the resonator is found to decrease by approximately a factor of 10 from Q^-1 = 2.5 x 10^-6 at low applied microwave powers and low temperatures to Q^-1 = 2.5 x 10^-7 at large applied powers and a temperature of 600 mK. We compare the measured Q data to a model based on loss from the interaction of the superconducting resonator with lossy two-level systems and separately to a model we have developed based on non-equilibrium quasiparticles accumulating in regions of the TiNx film with a lower superconducting gap (D). To distinguish between these competing models, we will also discuss results where we apply superconducting pair-breaking infrared light directly to the resonator device and measure the loss.