Synthesis and ion beam modification of the Bi system superconducting ultrathin films

Superconducting Bi-Sr-Ca-Cu-O ultrathin films with thicknesses of 300, 150, 70 and 40 Angstrom were synthesized by using single-target magnetron sputtering and by optimizing the heat treatment conditions. Ultrathin films thus obtained were highly oriented with the c axis perpendicular to the film plane. These him specimens were irradiated with 100 keV Ar ions at low temperatures, 10-20 K, and subsequently annealed at a relatively low temperature around 730 degrees C. For a him thickness of 300 A, the zero-resistance transition temperature T-c,T-o was raised to 108 K, which is equivalent to the maximum value so far observed for the bulk system. The critical current density J(c) was 104 A cm(-2) at 77 K in zero magnetic field. Even for 40 Angstrom thick ultrathin film, the initial T-c,T-o of 78 K was improved to 88 K, approaching T-c,T-o of the high T-c phase with a significant increase in J(c) of the order of 10(3) A cm(-2) at 77 K. The role of 100 keV range Ar ions in the thin him modification was discussed in terms of a selective creation of ''unit cell'' scales of the low energy collision cascades and ion channelling in thin crystalline films.