Superconducting properties of very high quality NbN thin films grown by pulsed laser deposition

In this work, we study the effect of the various substrates on the growth and superconducting properties of NbN thin films grown by using pulsed laser ablation in a N₂ + 1%H₂ atmosphere on MgO, Al₂O₃ and Si substrates. Structural and superconducting analyses of the films demonstrate that using MgO and Al₂O₃ substrates can significantly improve the film properties compared to Si substrate. The X-ray diffraction data indicate that MgO and Al₂O₃ substrates produce highly oriented superconducting NbN films with large coherent domain size in the out-of plane direction on the order of layer thickness and with a superconducting transition temperature of 13.1 K and 15.2 K, respectively. On the other hand, the NbN film grown on the Si substrate exhibits random polycrystalline orientation. Together with the smallest coherent domain size it leads to the lower critical temperature of 8.3 K. Finally, by using a passivation surface layer we are able to improve superconducting properties of NbN thin film and we observe superconducting transition temperature 16.6 K, the one of the highest value reported so far for 50 nm thick NbN film on sapphire.