Effect of Annealing Atmosphere on the Contact Resistivity, Superconductivity, and Phase Transformation in the Al / BiSrCaCuO System

Abstract

The contact resistivity, superconductivity, and phase transformation in the A1/BiSrCaCuO(BSCCO) system annealed in air, argon, and vacuum, respectively, were studied. The BSCCO films were deposited by dc sputtering on MgO substrates. For the A1/BSCCO films, capped with gold and annealed at 750~ in air, the contact resistivity of 7 x 10 .7 ~-cm 2 at 77 K can be obtained, whereas for the Al/BSCCO films annealed at above 500~ either in argon or in a vacuum of 6 x 10 .6 Torr, the contact resistivities show semiconducting behavior due to degraded superconductivity of the BSCCO film. During annealing in argon or a vacuum of 6 x 10 .6 Torr the melting temperatures of the 2212 phase are about 750 and 675~ respectively. These are considerably lower than that in air, 875-895~ The 2201 phase is the most stable phase in comparison with the 2212 and Bi2Sr2Ca~Cu30= phases during annealing in the oxygen-free atmosphere. The reversible change of the Tc of the BSCCO film has an intimate relationship with the decomposition and recrystallization of the 2212 phase, depending on the annealing atmosphere. Two of the decomposition products of the 2212 phase for the BSCCO film annealed in the oxygen-free atmosphere are identified to be 3-Bi203 and Cu20. In this study, the phase transformation and reversible change of the Tc for the BSCCO film annealed in various atmospheres can be explained in terms of the desorption and absorption of oxygen in the film.The discovery of high Tc superconductors (1, 2) has given a great impetus to applications of superconductors in many technical fields such as electronic devices, power transmission lines, and Josephson junctions. The essential requirement for these applications is the preparation of low-resistance electrical contacts to the superconductors. Numerous studies on silver and gold contacts to the superconductors have been performed (3-8). Aluminum is widely used in electronic devices, and it has been reported 5 that the substitution of aluminum into the BiSrCaCuO (BSCCO) system up to 20 atom percent (a/o) of the cations seems to have no significant effect on T~s (9-10). Therefore, 2.4 it is suggested that aluminum may be chosen for the electrode contacts on the BSCCO superc6nductor. Our previo 1.8 ous results have shown that for the Au/A1/BSCCO system W annealed in air a contact resistivity as low as 7 • 10 .7 ~-cm 2 can be obtained (11). However, in semiconductor processing technology, the aluminum contact is generally an-~ 1.2 0) nealed in an oxygen-free atmosphere to protect it from oxidation. Moreover, several authors have reported that for BSCCO samples annealed in various oxygen pressure the 0.6 Tc increases with decreasing oxygen content (12-17). Therefore, it is very interesting to study further the effect 0 of oxygen-free annealing on the AI/BSCCO contact system. In this study the contact resistivity, superconducting behavior, and the phase transformation of the A1]BSCCO and BSCCO films after annealing in argon and vacuum were studied and compared with the results obtained after anne...

Electrical study of an unusual phase transformation in a Bi2Sr2Ca2Cu3O10+xwhisker at room temperature

“…was already proposed in the studies by Rubin et al [1] and by MacManus-Driscoll et al [2] on polycrystalline samples at oxygen partial pressures of about 10 −5 bar in the temperature range 600-900 • C. In spite of the much lower temperature of our experiment, we consider that this reaction might be favoured by the very low oxygen partial pressure of the present study (∼2 × 10 −6 bar). Actually, an analogous lowering of the 2223 → 2212 phase transformation temperature, induced by vacuum or argon atmosphere, has already been observed, during postannealing treatments of BSCCO films [39].…”