Recovery of superconductivity and recrystallization of ion-damaged YBa 2Cu 3O 7− x films after thermal annealing treatment

Abstract

We have systematically studied the annealing effects of high-quality epitaxial YBa 2Cu 3O 7− x (YBCO) films on (100) SrTiO 3 substrates implanted with 100 keV Ar ions at different fluences. The mechanisms for T c degradation induced by ion implantation have been discussed. After annealing in oxygen atmosphere in a temperature range from 450 to 950°C, R( T) curve measurements show a partial or complete recovery of superconductivity. The recrystallization of ion-implanted YBCO films have been investigated using X-ray diffraction (XRD) and Rutherford backscattering spectrometry with channeling. It is found that the annealing at 450°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 superconductivity is observed. The results suggest that ion implantation induced T c degradation is mainly caused by disordering of the cation sublattice not the oxygen sublattice. After annealing at 850°C, the recovery of superconductivity and recrystallization of implanted YBCO films are related to the fluence. For YBCO films implanted with low to moderate fluences (1 × 10 11−1 × 10 14/cm 2), the superconductivity can 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 (1 × 10 15−1 × 10 16/cm 2) is quite different. The orientation of the samples depends on the annealing temperature. With the increasing of the annealing temperature from 750 to 950°C, the regrowth orientation gradually changes from a- to c-axis orientation. After annealing at temperatures of 750 and 800°C, the samples have the nearly pure a-axis orientation. At 850°C, the sample has a mixed a- and c-axis oriented structure but the a-axis orientation is still dominant. Above 900°C the structure of the annealed sample has the pure c-axis orientation. In the case of amorphous YBCO films caused by high-fluence Ar ion implantation, the T c0 is lower than 77 K in the whole annealing temperature range due to the polycrystalline structure. The experimental results indicate that the amorphous YBCO films formed by ion implantation regrow homogeneously rather than epitaxially and thus have the polycrystalline structure. This effect can be used to make a-axis oriented YBCO films and Josephson junctions or weak links by ion implantation with a narrow bridge mask or focused ion beam technique.