RECOVERY OF SUPERCONDUCTIVITY AND RECRYSTALLIZATION OF ION-DAMAGED YBA2CU3O7-X FILMS AFTER THERMAL ANNEALING TREATMENT

We have systematically studied the annealing effects of high-quality e pitaxial YBa2CuO7-x (YBCO) films on (100) SrTiO3 substrates implanted with 100 keV Ar ions at different fluences. The mechanisms for T-c deg radation induced by ion implantation have been discussed. After anneal ing in oxygen atmosphere in a temperature range from 450 to 950 degree s C, R(T) curve measurements show a partial or complete recovery of su perconductivity. The recrystallization of ion-implanted YBCO films hav e been investigated using X-ray diffraction (XRD) and Rutherford backs cattering spectrometry with channeling. It is found that the annealing at 450 degrees C indeed results in the movement of oxygen defects and ordering of the oxygen sublattice in the sample implanted with low to moderate fluences, however, only a small partial recovery of supercon ductivity is observed. The results suggest that ion implantation induc ed T-c degradation is mainly caused by disordering of the cation subla ttice not the oxygen sublattice. After annealing at 850 degrees C, the recovery of superconductivity and recrystallization of implanted YBCO films are related to the fluence. For YBCO films implanted with low t o moderate fluences (1X10(11)-1X10(14)/cm(2)), the superconductivity c an be nearly completely recovered, and the pure c-axis orientation of the samples is observed in the XRD spectra. However, recrystallization of amorphous YBCO films implanted with high fluences (1X10(15)-1X10(1 6)/cm(2)) is quite different. The orientation of the samples depends o n the annealing temperature. With the increasing of the annealing temp erature from 750 to 950 degrees C, the regrowth orientation gradually changes from a- to c-axis orientation. After annealing at temperatures of 750 and 800 degrees C, the samples have the nearly pure a-axis ori entation, At 850 degrees C, the sample has a mixed a- and c axis orien ted structure but the a-axis orientation is still dominant. Above 900 degrees C the structure of the annealed sample has the pure c-axis ori entation. In the case of amorphous YBCO films caused by high-fluence A r ion implantation, the T-co is lower than 77 K in the whole annealing temperature range due to the polycrystalline structure. The experimen tal results indicate that the amorphous YBCO films formed by ion impla ntation regrow homogeneously rather than epitaxially and thus have the polycrystalline structure. This effect can be used to make a-axis ori ented YBCO films and Josephson junctions or weak links by ion implanta tion with a narrow bridge mask or focused ion beam technique.