Effect of Quenching Immediately after Sputtering on Superconducting NbN Films

Abstract

In order to stabilize the metastable NaCl structure (the δ phase) of NbN at low temperatures, NbN films sputtered on sapphire substrates were thermally quenched immediately after deposition by thermal conduction from a water-or liq. N₂-cooled anode container, or by direct spraying of liq. N₂ onto the as-sputtered film. When the cooling rate Q _r was increased, the superconducting transition temperature T _c and the resistance ratio β(=R _300K/R _20K) changed from a slightly downward trend to an upward one near Q _r =10⁴°C/min. Above this value of Q _r , X-ray analysis indicated that the growth of the δ phase was enhanced, while that of the ε phase was depressed. The broad transition width Δ T _c which appeared at a value of Q _r between 2×10³ and 10⁴°C/min is considered to be closely related to the coexistence of the δ and ε phases.