反復スパッタ法によるBi系薄膜の作製と超伝導特性

抄録

Thin films of the Bi system have been prepared on MgO substrates by a repeat-sputtering method from a single sintered target. Sputtering was carried out in a mixed gas of argon and oxygen with a oxygen concentration of 20% without heating the substrate. The deposition rate was about 2.3 nm/min and the film thickness ranged from 400 to 600 nm. The films were crystallized though annealing in a muffule furnace.
Our procedure for preparation of the superconducting films by low-temperature annealing are as follows: About 150 nm-thick film was deposited on MgO substrate. After annealing, we superimposed an amorphous film with the same thickness on the annealed film, and then the film was annealed similarly. The cycle of deposition and annealing was repeated until the desired thickness is obtained. This repeat-sputtering method is useful for preparation of the higher-T_c films. Moreover, the film surface is relatively smooth. The T_c^zero depends on the value of resistivity at 300 K, ρ (300 K), and the T_c^zero increases with decreasing ρ (300 K). And, metallic behaviour is observed above the superconducting on the set temperature.
When a BiPbSrCaCuO target was used, we covered the amorphous film with MgO substrate and placed it in the furnace together with Pb₃O₄ to suppress the loss of Pb during the annealing. It is confirmed that the Pb promotes crystallization of the 110 K phase. The target with composition Bi_1.8Pb_xSr_1.0Ca_1.2Cu_1.8O_y yields the maximum volume fraction of the 110 K phase at x=0.2. The Ag addition into the BiPbSrCaCuo target raises T_c^zero. By the repeat-sputtering method, we obtained the thin films with T_c^zero≥105 K in a shorter annealing time than by the conventional method.