NbN films with B1 structure have been investigated on their superconducting critical temperature
Tc, upper critical field
Hc2 and field dependent critical current density
Jc. NbN has been formed on quartz and sapphire substrates heated up to a temperature between 200 and 700°C in an argon-nitrogen atmosphere by rf sputtering from a Nb disk target. The film thickness ranges from 1, 500 to 6, 000Å. The
Tc of films is strongly influenced by one of deposition parameters, the nitrogen partial pressure
PN2. With increasing
PN2 from 1m Torr, the
Tc rapidly increases. The maximum
Tc in excess of 16K is obtained around
PN2=4m Torr. A further increase of
PN2 provides a slight decrease of
Tc and causes an appreciable increase of the normal state resistivity ρ
n which is closely related to the
Hc2 of films. Most films deposited at
PN2 higher than 4m Torr show a single B1 phase and exhibit a strong anisotropy of
Hc2. The upper critical fields perpendicular to the film plane (
Hc2⊥) are lamer than those parallel (
Hc2_??_). Ratios of
Hc2_??_ to
Hc2⊥ are in the range from -0.72 to -0.85 and are smaller for films deposited on quartz. Both kinds of films deposited on sapphire and quartz show similar relationships between
Hc2 and ρ
n. The
Hc2 increases with ρ
n in a range of below 500μΩcm. High values of
Hc2 are obtained in films with ρ
n above 500μΩcm and with
Tc of 14-15K. The maximum values of
Hc2⊥ and
Hc2_??_ are to -30T and -25T, respectively. NbN films with a ρ
n of -1, 000μΩcm maintain high
Jc in fields higher than 10T. The highest values of
Jc at 15T reach 1.0×10
5A/cm
2 for the parallel field direction and 5×10
5A/cm
2 for the perpendicular field direction.
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