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 Qr was increased, the superconducting transition temperature Tc and the resistance ratio β(=R300K/R20K) changed from a slightly downward trend to an upward one near Qr=104°C/min. Above this value of Qr, 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 ΔTc which appeared at a value of Qr between 2×103 and 104°C/min is considered to be closely related to the coexistence of the δ and ε phases.
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