Transition temperatures and upper critical fields of NbN thin films fabricated at room temperature

NbN thin films were deposited on thermally oxidized Si substrate at room temperature by using reactive magnetron sputtering in an Ar-N2 gas mixture. Total sputtering gas pressure was fixed while varying N2 flow rate from 1.4 sccm to 2.9 sccm. X-ray diffraction pattern analysis revealed dominant NbN(200) orientation in the low N2 flow rate but emerging of (111) orientation with diminishing (200) orientation at higher flow rate. The dependences of the superconducting properties on the N2 gas flow rate were investigated. All the NbN thin films showed a small negative temperature coefficient of resistance with resistivity ratio between 300 K and 20 K in the range from 0.98 to 0.89 as the N2 flow rate is increased. Transition temperature showed non-monotonic dependence on N2 flow rate reaching as high as 11.12 K determined by the mid-point temperature of the transition with transition width of 0.3 K. On the other hand, the upper critical field showed roughly linear increase with N2 flow rate up to 2.7 sccm. The highest upper critical field extrapolated to 0 K was 17.4 T with corresponding coherence length of 4.3 nm. Our results are discussed with the granular nature of NbN thin films.