Fabrication of DC Sputtered NbN Thick Film with High Upper Critical Field of Above 400 kOe

We report detailed study on control of sputtering parameters for NbN thin film superconductor. The NbN films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique i.e., deposition of Nb in presence of reactive N-2 (purity 99.9%) gas. After several runs, the partial gas ratio of Ar:N-2 and deposition time are optimized respectively at 80:20 and 10 minutes. The optimized film possesses T-c (R=0) in zero and 140 kOe fields at 14.3 K and 8.8 K respectively. The fabricated relatively thick film (600 nm) is crystallized in cubic structure, with small quantity of Nb/NbOx embedded in main NbN phase. Our efforts to reduce the thickness of the film failed because 5 minutes deposited film resulted in hexagonal phase with non-superconducting behavior. The upper critical field Hc(2)(0) of the optimized superconducting film is evaluated from the magneto-transport measurements in applied of up to 140 kOe. The Hc(2)(0) is found to be above 400 kOe, which to our knowledge is best reported value of Hc(2)(0) yet for thin/thick film or bulk NbN in literature