スパッタ法によるＮｂＮ薄膜の製作と高磁界超電導特性に関する研究

Abstract

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, 000A. 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 ρn 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 ρn. The Hc2 increases with ρn in a range of below 500μΩcm. High values of Hc2 are obtained in films with ρn 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 ρn of -1, 000μΩcm maintain high Jc in fields higher than 10T. The highest values of Jc at 15T reach 1.0×105A/cm2 for the parallel field direction and 5×105A/cm2 for the perpendicular field direction.