Improved critical temperature of superconducting plasma-enhanced atomic layer deposition of niobium nitride thin films by thermal annealing

The efficiency of the superconducting radio frequency cavities composed of Nb required the deposition of thickness-controlled multilayer coatings of superconductor-insulator-superconductor (S-I-S) on the internal surfaces of the cavities. Herein, we report the plasma-enhanced atomic layer deposition of carbon-free NbN (50 µm thick), followed by a thermal treatment, to obtain the superconducting layer in the S-I-S structure. Using (tert-butylimido)-tris(diethylamino)-niobium as the niobium precursor and H2 and NH3 plasma as reactive gasses, the deposition and annealing parameters were optimized by studying their effects on the film properties (crystallinity, density, and composition). We demonstrated that the superconducting critical temperature (Tc) can be improved after thermal annealing up to 13.8 K, a value compatible with the targeted application. In addition to the expected densified layers and increased grain size, we observed a partial transformation of the oxide present in the as-deposited layer into niobium oxynitride, which could indicate the origin of the improvement of the superconducting properties. With low carbon and oxygen impurity concentrations in the films, this study contributes to the understanding of the relationship between the structure, composition, and superconductivity in NbN.