Mechanisms controlling the in-situ formation and superconducting properties of Bi-Sr-Ca-Cu-O films

The authors report on the in-situ formation of superconducting films of Bi₂Sr₂CaCu₂O_x and Bi₂Sr₂Ca₂Cu₃O_x using composite-target sputtering with substrate temperatures, T_s , significantly less than those required for postannealed films. Specifically, superconducting films of BSCCO 2212 and 2223 can be sputtered onto single-crystal MgO substrates by an in-situ processing technique with T_czero´s as high as 64 K and a very pronounced c-axis orientation without the need for epitaxy. Bi loss in the films can be reduced by biasing the substrate negative or increasing the total system pressure. This should provide greater control over film properties and the possibility of sputter deposition at higher substrate temperatures to see if T_czero´s close to bulk can be achieved. The microstructure generally reveals a smooth matrix, with submicron to micron-sized particles protruding from it. These particles increase in size and frequency with increasing T_s and appear to be a Bi-Sr-Ca-oxide mixture. It is suggested that oxygen resputtering of the film may play a role similar to that reported for YBa₂Cu₃O_7-x (YBCO)